Development of Functional Mesocrystalline

Materials and Ferroelectric Perovskites

Wenxiong Zhang

March 2019

KAGAWA UNIVERSITY

1

Table of Contents

Chapter I General Introduction ..........................................................................................3

1.1 Overview on mesocrystals ...................................................................................... 4

1.1.1 Structural and formation principles of Mesocrystals ........................................ 4

1.1.2 Characteristic properties of mesocrystals ......................................................... 7

1.1.3 Recent advances and future outlook in mesocrystal materials ....................... 10

1.2 Metal oxides and complex compounds mesocrystals ........................................... 13

1.2.1 TiO2 mesocrystals ........................................................................................... 13

1.2.2 CaCO3 mesocrystals ....................................................................................... 16

1.2.3 SrTiO3 mesocrystals ....................................................................................... 17

1.2.4 Ferroelectric perovskite mesocrystals ............................................................ 20

1.3 Ferroelectric perovskites................................................................................... 20

1.3.1 Inorganic metal oxide ferroelectric perovskites ............................................. 20

1.3.2 Halide perovskites .......................................................................................... 26

1.4 Lattice strain engineering ...................................................................................... 32

1.5 Topochemical synthesis ........................................................................................ 37

1.5.1 Approach of topochemical synthesis .............................................................. 37

1.5.2 Soft chemical process for mesocrystalline nanocomposites ........................... 40

1.6 Purpose of present study ....................................................................................... 41

1.7 Reference .............................................................................................................. 44

Chapter II .........................................................................................................................56

Anomalous Piezoelectric Response of Ferroelectric Mesocrystalline

BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain Engineering ......................56

2.1 Introduction ........................................................................................................... 56

2.2 Experimental ......................................................................................................... 59

2.2.1 Sample Preparation ......................................................................................... 59

2.2.2 Physical Properties Analysis ........................................................................... 60

2.3 Results and discussion .......................................................................................... 62

2.3.1 Synthesis of mesocrystalline BT/HTO nanocomposite .................................. 62

2.3.2 Synthesis of mesocrystalline BT/BNT nanocomposite .................................. 66

2

2.3.3 Formation reaction mechanism of mesocrystalline BT/BNT nanocomposite 71

2.3.4 Ferroelectric and piezoelectric responses of mesocrystalline BT/BNT

nanocomposite ......................................................................................................... 73

2.3.5 Dielectric responses of mesocrystalline BT/BNT nanocomposite ................. 84

2.4 Conclusion ............................................................................................................ 89

2.5 References ............................................................................................................. 89

Chapter Ⅲ ........................................................................................................................94

Ferroelectric Mesocrystalline BaTiO3/BaBi4Ti4O15 Nanocomposite: Formation

Mechanism, Nanostructure, and Anomalous Ferroelectric Response .............................94

3.1 Introduction ........................................................................................................... 94

3.2 Experimental ......................................................................................................... 97

3.2.1 Sample Preparation ......................................................................................... 97

3.2.2 Physical Properties Analysis ........................................................................... 98

3.3 Results and discussion ........................................................................................ 100

3.3.1 Synthesis of BT/BBT nanocomposites and BBT mesocrystals .................... 100

3.3.2 Nanostructural analysis for BT/BBT nanocomposite ................................... 107

3.3.3 Formation mechanism of BT/BBT nanocomposite ...................................... 108

3.3.4 Ferroelectric, dielectric and piezoelectric responses of BT/BBT

nanocomposite ........................................................................................................ 113

3.4 Conclusions ......................................................................................................... 123

3.5 References ........................................................................................................... 124

Chapter Ⅳ .....................................................................................................................129

Compelling Evidences for Antiferroelectric to Ferroelectric Transition of MAPbI3-xClx

Perovskite in Perovskite Solar Cells..............................................................................129

Chapter V Summary ......................................................................................................131

Publications ...................................................................................................................137

Publications in Journals ......................................................................................... 137

Publications in Conferences .................................................................................. 138

Acknowledgment ...........................................................................................................140

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Chapter I General Introduction

The study of nanocrystals has become a major interdisciplinary research area and

drawn increasing attentions in the past decades because nanocrystals can be used as

building units to fabricate hierarchically structured solid materials.1 Solids organized by

nanocrystals open up the opportunities of fabricating new materials and devices, which

not only exhibit the properties from individual nanocrystals but also exhibit collective

properties produced via their interactions.2, 3 In these solids, assemblies constructed

from crystallographically oriented nanocrystals have single-crystal-like structures and

much higher porosity than conventional single crystals. Subsequently, the assemblies

have been defined as Mesocrystal, a new class of material, which is a polycrystal

constructed from oriented nanocrystals or microcrystals.3-5 In particular, for the material

chemistry, mesocrystals can offer unique new opportunities for the design of materials,

and be applied to catalysis, sensing, and energy storage and conversion.6-8 Hence, they

have attracted considerable attention of physicists and chemists in recent years, and

become a hot research field. However, the understanding of mesocrystals is still very

limited, such as the preparation approaches, the formation mechanisms, microstructures,

and characteristics, as well as the developments for the various types and

high-performance applications.6

On the other hand, the metal oxide perovskites and perovskite-related halides are

important materials as they possess a number of interesting properties, such as

ferroelectric; piezoelectric; electron-acceptor behavior; a large optical transmission

domain; high resistivity; antiferromagnetic; exceptional magnetic; photoluminescent

properties; anionic conductivity over a wide temperature range.9 Some halide

4

perovskites have the semiconducting property, which has been widely applied to

photovoltaic areas.10 Still, the ferroelectricity is also possible in this kind of halide

perovskites, hence, it has quite interesting and debatable behaviors.11, 12 Therefore,

figuring out the connection between the semiconductor and ferroelectric behavior in the

halide perovskite materials is significant.13

In this chapter, structural and formation principles, characteristic properties, the

applications of conventional mesocrystals, recent advances and future outlook in

mesocrystalline materials are described. Furthermore, the structure, characteristic

properties and the application of the perovskite materials have also been presented. In

addition, the purposes of this dissertation are also clarified.

1.1 Overview on mesocrystals

1.1.1 Structural and formation principles of Mesocrystals

Mesocrystal, as an abbreviation for mesoscopically structured crystalline materials,

represents a structure composed of nanocrystals aligned in a crystallographic pattern but

separated by porosity or a second phase. The first observation of mesocrystal traces

back to 1969 when a porous intermediate structure of BaSO4 was reported by Petres and

co-workers.14 In 2003, Cölfen et al. established some principles and concepts that

illustrate the mesoscale self-assembly of nanocrystals contributing another mechanism

for growing the single crystals.3 In such mesoscale self-assembly (Fig. 1.1, the red

route), the crystal growth by the ordered aggregation results in organized crystals with

iso-oriented directions, which is totally different from the classical crystallization

process (Fig. 1.1, the blue route). For the classical crystallization, including the

5

nucleation and the growth processes, the crystallized primary nanoparticles grow via

ion-by-ion (or molecule-by-molecule) attachment to fulfill the formation of the single

crystal. Until 2005, Cölfen et al. created the word mesocrystal as a replacement for the

iso-oriented crystals and proposed that mesocrystals are kinetically metastable

intermediates, in which the primary units can be still identified.4 Hence, it raises a

challenge to the well-known classification of solids as amorphous, polycrystalline, or

single crystalline. Moreover, mesocrystals show much higher crystallinity than

polycrystalline materials, and in some case even exhibit many characteristic properties

of a conventional single crystal.3, 15, 16 In 2010, Song et al. contributed a more precise

definition that mesocrystals are superstructured crystals consisting of mesoscopically

scaled (1-1000 nm) crystalline subunits that are aligned in the same crystallographic

direction.17

Fig. 1.1 Schematic presentation of classical crystallization (blue route) via ion-by-ion addition

versus single crystal formation (red route) by a mesocrystal intermediate made of nanoparticles.

Although the structure and formation mechanism of nanocrystal superstructures and

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other nanostructured materials have been investigated for several decades, the

systematic research on mesocrystals only started about 10 years ago. Yet, very little is

known for mesocrystal formation processes, which are fundamental for the

understanding of the fabrication of mesocrystal structure and the control of its synthesis

process.6 Up to now, a large number of mesocrystals have been successfully synthesized,

whereas the growth mechanism of mesocrystal is still a big problem due to the large

range of involved time and length scale during the growth of mesocrystals.5 Recently,

Sturm and Cölfen demonstrated seven scenarios of different mesocrystal formation

pathways, as shown in Fig. 1.2: (a) alignment by organic matrix; (b) alignment by

physical forces; (c) crystalline bridges, epitaxial growth, and secondary nucleation; (d)

alignment by spatial constraints; (e) alignment by oriented attachment; (f) alignment by

face-selective molecules and (g) topotactic (epitaxial) solid phase transformation.6 Each

of these mechanisms involves different driving forces of chemical and physical origin

directing the formation of the mesocrystalline materials, in which (a) to (f) can be

classified to self-assembled materials (red route in Fig. 1.1) and (g) is obtained from

topochemical process (purple route in Fig. 1.1). The topotactic transformation in solids

is well-known and has been quite extensively studied for many classes of solid-state

materials. In other words, if the initial material is single crystalline, the transformation

induces the formation of a new phase in a specific crystallographic orientation, as

shown in Fig. 1.3. It can be obviously seen that the obtained mesocrystals still maintain

the morphologies of the original precursors after one or multi-step transformation,

suggesting that the morphologies of the mesocrystals are inherited from the original

precursors. Usually, this reaction occurs via accompanying the ion exchange,

intercalation, de-intercalation, and topochemical micro/nanocrystal conversion.6, 18

7

Fig. 1.2 Simplified schematic illustration of main formation pathways of mesocrystals. (a)

alignment by organic matrix; (b) alignment by physical forces; (c) crystalline bridges, epitaxial

growth and secondary nucleation; (d) alignment by spatial constraints; (e) alignment by oriented

attachment; (f) alignment by face selective molecules and (g) topotactic (epitaxial) solid phase

transformation. Reproduced with permission.6 Copyright 2017, MDPI.

Fig. 1.3 Various dimensional schematic diagrams for 1 and/or n-step in-situ topotactic

conversion reaction for formations of mesocrystals from original precursors.

1.1.2 Characteristic properties of mesocrystals

It is well-known that the nanocrystal building units with structural multiplicity and

nanoscale size can provide additional opportunities for self-assembly.5 A variety of

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self-assemblies or topochemical bridge connections for the formations of the

mesocrystals can offer new possibilities for superstructure formations, giving rise to the

mesocrystals presenting some different characteristic properties. The mesocrystals from

functional materials are highly attractive due to the emergent properties of

mesocrystalline materials, such as single-crystal-like behavior, high crystallinity, high

porosity and inner connection bridged by organic components and/or inorganic

nanocrystals.19, 20 Hence, the mesocrystals can exhibit the following characteristic

properties.

Firstly, the mesocrystal is a polycrystal constructed from iso-oriented nanocrystals,

which exhibits a single crystal behavior in X-ray scattering and electron diffraction. For

instance, the octahedron-shaped magnetite (Fe3O4) mesocrystal shows a

single-crystal-like SAED pattern (Fig. 1.4(e)).21 In addition, all the nanocrystals for the

construction of the mesocrystal exhibit the same direction of the interplanar spacing.

These are due to that the mesocrystals are constructed from nanocrystals, and each

nanocrystal is crystal-axis-oriented with each other.

Fig. 1.4 (a, b) TEM images of the 2D monolayer assembly of 21 nm magnetite octahedra in a

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magnetic field. (c) HRTEM image of one single nanoparticle in the assembly (solid red triangle:

(111) plane at the top; dashed red triangle: (111) plane at the bottom); solid blue triangles:

interparticle spaces. (d) Model of the magnetite octahedra viewed along the [111] zone axis. (e)

Fast Fourier transformation (FFT) pattern and (f) selected area electron diffraction (SAED)

pattern of the 2D mesocrystals shown in (b). Reproduced with permission.21Copyright 2010,

American Chemical Society.

Secondly, the mesocrystals show some special properties of well-aligned and oriented

crystalline assembly, which is unrivalled for the amorphous, polycrystalline, and

single-crystalline materials. Namely, some of desired properties can be satisfied by

using the mesocrystalline superstructure rather than using the same material in

amorphous, polycrystalline aggregate, and single-crystalline. As the case in Fig. 1.4,

magnetic mesocrystals with non-spherical shapes demonstrate more appealing

anisotropic magnetic properties.22

Thirdly, because of the primary crystallites sharing a common crystallographic

orientation, the mechanical properties of the mesocrystals are unusual.23 They can

exhibit higher ductility and toughness than the corresponding single crystalline

materials, and almost all the mesocrystals exhibit the fracture surfaces like amorphous

glasses, but unlike the single crystals.24

Finally, the mesocrystal simultaneously present over two kinds of functional

properties and prevail on the materials having a large improvement in some application

areas. For instance, mechanical toughness and dielectric dissipation, or optical and

magnetic properties can be combined in one system. It is said that these properties

would never be mixed on this nanoscale.5 They combine the high crystallinity with

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small crystal size, high surface area and high porosity of the mesocrystal as well as

good handling because the size range of mesocrystal is in the nanometer to micrometer.

The existence of the superlattice structure is also the main reason of high attraction due

to the emergent properties of mesocrystals. A self-assembly process for mesocrystals

does not occur by an ion-by-ion manner, however, ionic strength and species of the

solution are still important variables in controlling crystallization to form mesocrystals.

In particular for surfactant phases and microemulsion involved crystallization processes,

the phase equilibrium and physical characteristics of the product can strongly depend on

ionic strength and species, especially if the cationic lyotropic phases are applied.17

1.1.3 Recent advances and future outlook in mesocrystal materials

Recently, the researches on mesocrystals are drawing much attention, not only by

fundamental research but also by its applications, and an increasing number of the

mesocrystals preparations for applications to a wide range of the functional materials

have been reported.21, 25, 26 In addition, the development of the techniques used for the

characterization of the mesocrystals will further allow for the observation of the

mechanisms of mesocrystal formation, and understanding of their structural principles.

The aforementioned techniques mainly include the in-situ techniques like AFM, TEM,

and High-Resolution TEM or other in-situ microscopic and diffraction techniques,

namely SAXS/WAXS, Grazing-Incidence Small-Angle Scattering

(GISAXS)/Grazing-Incidence Wide-Angle Scattering (GIWAXS).

In the research area of the formation mechanism, the carbonates mesocrystals were

studied at the earliest and most. The initial demonstration for the formation of

mesocrystal was originated from the carbonates chemistry, which is a fundamental

11

source of the mesocrystal theory today.27 However, one of the earliest referring

indications of the mesocrystal intermediates was not derived from CaCO3 but the

BaSO4 crystal with the porous structure.28 After exploration of some intermediate metal

oxides mesocrystals, the mesocrystals with even higher definition were reported for

CaCO3 made in silica gels in 1986.29 Therefore, CaCO3 mesocrystals are increasingly

studied and developed via controlling the polymorph and morphology. However, very

little is known that all the carbonate mesocrystals are involved with the

high-performance applications. Subsequently, the study on the metal oxides

mesocrystals not only becomes attracting but also becomes the hottest. Especially in

ZnO and TiO2 mesocrystals, the preparation approaches, the formation mechanisms,

microstructures, and the high-performance applications have been getting increased

attention. At present, the ZnO and TiO2 mesocrystals have been used to catalysis,

sensing, and energy storage and conversion.23, 26 The perovskite metal oxides

mesocrystals have been also developed unsubstantially, they are likely to widely apply

to the high-performance electro-optic field, ferroelectric materials, and other functional

composite materials. Very recently, Hu et al. have developed the platelike

mesocrystalline BaTiO3/SrTiO3 and BaTiO3/CaTiO3 nanocomposite via the

topochemical process with highly elevated dielectric, ferroelectric and piezoelectric

responses.30, 31

Although an increasing number of mesocrystals have been developed, the formation

mechanisms of the mesocrystals are still limited and the mesocrystals are still a new

study field for the solid materials. For example, the metal oxide and carbonate

mesocrystals are predominantly investigated, and they are still understood limitedly,

furthermore, their formation mechanisms are still very difficult to be clear. In addition,

12

the varieties of the mesocrystals are not enough, the application studies of the other

mesocrystals are rarely found out. In the following, the carbonates, metal oxides, and

perovskite mesocrystals are briefly introduced and summarized, respectively. The goal

is to further understand the approach, formation mechanism, and functional application

of the mesocrystals, and is ready for the development and application of new kinds of

functional mesocrystals.

The above contents already reveal the tremendous potential of mesocrystals. Hence,

application-driven research will be increasing, and more applications will be reported

without a doubt. In the near future, the new mesocrystal applications will be

investigated. In the meantime, the development of the aforementioned in-situ techniques

will be achieved for the observation of the formation mechanisms and understanding of

the structuration of the mesocrystals in much more detail than it was possible before. In

addition, given that a large number of optimizations of mesocrystal formation processes

are done by trial and error, so that the introduction of new methodological methods for

characterization and interpretation of the mesocrystal structures is extremely needed. Up

to now, mesocrystals have been applied to Li-ion batteries, catalysts or sensors, and

solar cells like quantum dot based solar cell etc.25, 32, 33 Also, for plasmonic materials,

the two different surface plasmon resonance bands for metal nanorods can lead to

interesting directional couplings in metal nanoparticle mesocrystals, and therefore,

further exploitation of metal nanoparticle mesocrystals can be anticipated in the future.

Furthermore, mesocrystals from magnetic nanoparticles where coupling of the magnetic

fields can also be anticipated. Besides, an untouched area in the mesocrystal research is

mesocrystals of organic nanocrystals, which have a great potential in the pharmaceutical

formulations.6 Therefore, a large number of exciting developments can be foreseen in

13

the field of mesocrystals.

1.2 Metal oxides and complex compounds mesocrystals

1.2.1 TiO2 mesocrystals

As described above, the mesocrystal materials exhibit specific properties in

comparison to its single crystal, which have wide application in sensing, catalysis,

energy storage, and conversion etc. As far as we know, TiO2 (titanium dioxide) is

among the most widely investigated metal oxides materials for its functional properties

and many promising applications in environment, energy, photocatalysts, and sensing

areas.25, 26, 34, 35 Crystal structures for TiO2, including TiO2 (B), TiO2 (Ⅱ), brookite,

anatase, rutile etc. have been widely studied. Among them, the rutile is a

thermodynamic stable phase and others are metastable phases. In addition, anatase is the

most stable phase in the metastable phases.

TiO2 nanocrystals have been increasing investigated and developed into the functional

materials. In addition, the effectiveness of TiO2 in practical applications varied

considerably with its specific surface area and mesoporosity,36 compositions,37

crystallinity38, 39 and, importantly, the morphology and texture of the material.40

Well-defined structural TiO2 materials obtained from the controlled synthesis with, such

as single crystals, ordered mesoporous thin films, nanotubes, and spherical particles, has

attracted much attention in recent decade. For instance, the anatase single crystals

containing high percentage of reactive facets have been fabricated via solvothermal

process with adding the fluorine species. Thermally stable mesoporous TiO2 thin films

with uniformly distributed pores can be fabricated via an evaporation-induced

self-assembly process in the presence of various block copolymers.41 TiO2 nanotubes

14

with different aspect ratios can be synthesized via anodization of titanium foil with

variable potential and electrolyte composition.42 Furthermore, 3D interconnected TiO2

networks with high crystallinity and controllable macropores have been successfully

obtained from preformed templates via a sol-gel nanocasting process.43

Excellent candidates for the applications in sensing, photocatalysis, solar cells and

lithium-ion batteries have been manifested for nanoporous TiO2 materials with large

surface areas. It is noted that anatase TiO2 mesocrystals were first reported by Ye et al.

via the topotactic conversion from NH4TiOF3 mesocrystals, which were fabricated with

the assistance of non-ionic surfactants.44 In such transformation process, the NH4TiOF3

mesocrystal serves as a crystallographically matched template for the subsequent

growth of the TiO2 mesocrystals. Furthermore, the direct mesoscale assembly of TiO2

mesocrystals and their photocatalytic properties have been attracted much attentions. In

the meantime, solid templates and organic additives were gradually introduced during

these mesoscale transformation processes.

Liu et al. have demonstrated that the rutile TiO2 hollow spheres mesocrystals can be

synthesized through the hydrothermal reaction process by using L-serine and

N,N′-dicyclohexylcarbodiimide (DCC) as biologic additives.45 Subsequently, Zhang et

al. have illustrated the formation of the photocatalytically active rutile TiO2

mesocrystals without the help of surfactants or additives, which exhibited the BET

specific surface area of only 16 m2/g. To explore the additive-free approaches for TiO2

mesocrystals with high porosity and high crystallinity, Ye et al. have reported the first

additive-free fabrication of nanoporous anatase TiO2 mesocrystal by using tetrabutyl

titanate as the titanium source and acetic acid as the solvent, as shown in Fig. 1.5.46 In

this formation process, organic titanium firstly reacts with organic acid by a

15

hydrolytic/nonhydrolytic condensation reaction to from amorphous fiber-like titanium

acetate complex precursors with Ti-O-Ti bonds. After two times of continuing

condensation processes, the other crystalline spherical-like precursors come into being

at the expense of the amorphous precursor. Subsequently, the crystalline spherical-like

precursors gradually release soluble titanium including the nucleation and growth of

anatase nanocrystals. Finally, the formed anatase nanocrystals gather along the [001]

direction, accompanying with the entrapment of in situ produced butyl acetate, giving

rise to the formation of the spindle shaped anatase mesocrystals elongated along the

[001] direction. The acetic acid molecules played multiple key roles during the

nonhydrolytic processing of the [001]-oriented anatase mesocrystals. The obtained

anatase mesocrystals with nanoporous exhibits remarkable crystalline stability and

improved performance as anode materials for lithium-ion batteries. In this regard, TiO2

mesocrystals with tunable architectures are promising for a wide range of applications.

16

Fig. 1.5 Schematic illustration of the formation mechanism of nanoporous anatase TiO2

mesocrystals without additives. Reproduced with permission.46 Copyright 2010, American

Chemical Society.

1.2.2 CaCO3 mesocrystals

CaCO3 (calcium carbonate), as one of the most abundant natural existing minerals in

such as the sedimentary rocks47 and the biological skeletons and tissues,3 has been

drawn much attention. There are three kinds of crystalline polymorphs for CaCO3,

including: calcite (dominated phase at lower temperature), aragonite (dominated phase

at higher temperature), and vaterite (at higher supersaturation).48 There are some new

methods for the development of the CaCO3 with the controlling morphologies and

crystallization. Besides, the various inorganic or organic–inorganic composite materials,

superstructure materials, and improved functional materials can be fabricated

accordingly.49, 50 A number of studies on CaCO3 materials have effectively promoted the

development of the mesocrystal chemistry and provided theoretical basis for the

mesocrystal chemistry. A similar gel-sol reaction or a block copolymerization reaction

has been widely used to prepare CaCO3 mesocrystals.

By using the self-assemble in a polyacrylamide gel, a typical calcite mesocrystal with

a pseudo-octahedron morphology formed from rhombohedral nanocrystals has been

fabricated (Fig. 1.6).51 It is obvious that the calcite mesocrystal was obtained by a

precipitation process from an aqueous solution containing Ca2+ and sulfide. It can be

clearly seen that crystallographic block with rough surfaces are constructed from

well-oriented nanocrystalline building units. According to the TEM image (Fig. 1.6b),

the tightly connected nanocrystals and the organic matrix between the individual

17

nanocrystal interspaces can be observed. The inserted SAED pattern reveals a

single-crystal-like crystallographic block, which means the formation of a mesocrystal

structure. In addition, the calcite mesocrystal can be also fabricated by adding the CO2

vapor diffusion into a Ca(OH)2 solution. Since the CO2 vapor diffusion approach has the

advantage of avoiding the interference of the extraneous ions, minimizing ionic strength

and approaching a pH close to biological conditions at the end of the crystallization

reaction, this approach is beneficial to the growth of calcite mesocrystals. As a result,

the vapor diffusion approach gives a better understanding of the driving forces for the

oriented and/or self-assembling of nanocrystals to mesocrystals.

Fig. 1.6 (a) SEM and (b) TEM images of calcite (CaCO3) aggregate with characteristic

pseudo-octahedral morphology obtained from polyacrylamide gel. (Inserted in (b): SAED

pattern of calcite mesocrystal). Reproduced with permission.51 Copyright 2003, American

Mineralogical Society.

1.2.3 SrTiO3 mesocrystals

SrTiO3 (strontium titanate, ST) with a cubic crystal structure has been drawn much

18

attention in the field of solar energy conversion systems and electromagnetic devices.52

In regard to further cutting down human energy usage and controlling environmental

pollution, heterogeneous solid photocatalysts have been anticipated to show promising

solar-to-chemical energy conversion from water since 1972.

For the development of more active ST-based photocatalysts, it is crucial to seek a

versatile route for the structure and property design. It may be expected that the

selectivity and efficiency of the photocatalytic reactions of ST with tailored crystal

facets and morphologies can be achieved for making its mesocrystals. However, there

remain challenges that are relevant to the fabrication of the organized assembly of such

structure-controlled nanoparticles up to the micrometer scale. Only a few of ST

mesocrystals have been reported. Calderone et al. reported the formation of the ST

mesocrystal with cubic morphology by precipitation approach from a suspension of

hydrolyzed TiOCl2 aqueous gel.53 The obtained ST mesocrystal was formed via an

epitaxial self-assembly of nanocrystals with a size of 4-5 nm. Furthermore, the

formation process is a non-additive spontaneous process. In addition, Kalyani et al.

demonstrated that ST mesocrystals with a [010] zone axis along the direction of the

wire-like morphology can be fabricated via an oriented topochemical conversion

approach. In this formation process, the H2Ti3O7 nanowire precursor was firstly

thermally reacted to form the anatase nanowire. The solvothermal and hydrothermal

treatments of the anatase nanowire in Sr(OH)2 solutions were carried out to form the

mesocrystalline ST. The formation of mesocrystals is achieved from a topochemical

reaction.

Very recently, TiO2 mesocrystals, which consist of nanocrystal building blocks, have

showed highly improved performance compared to that of the disorder system due to

19

their efficient charge transfer and separation along the neighboring nanoparticles. The

epitaxial intergrowth of ST from TiO2 has drawn much attention due to the scientific

and technology importance of oxide interface engineering. Zhang et al. reported that the

ST mesocrystals with enhanced photocatalytic activity were produced by topotactic

epitaxial transformation from anatase TiO2 mesocrystals via a facile hydrothermal

process (Fig. 1.7).54 Compared with the conventional disordered system, the material

exhibits the three-fold photocatalytic efficiency (Fig. 1. 7(c)) for the hydro evolution

reaction of water splitting in alkaline aqueous solution because of the ordered

mesocrystalline structure (Fig. 1.7 (d-g))

Fig. 1.7 (a) Schematic presentation of the formation of SrTiO3 mesocrystals via topotactic

epitaxial transformation of TiO2. (b) Structural model of interface between SrTiO3 and TiO2,

suggesting the epitaxial intergrowth of both phases. (c) Simplified scheme of a SrTiO3

mesocrystal showing the photogeneration of electrons and holes and anisotropic electron

transport form the inside to the outside. (d) SEM image of typical SrTiO3 mesocrystals, (e)

zoomed image inside the red frame marked in (d). (f) TEM image and SAED pattern of SrTiO3

mesocrystals. (g) High resolution TEM image of the area marked with red-square in (f).

Reproduced with permission.54 Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA,

20

Weinheim.

1.2.4 Ferroelectric perovskite mesocrystals

Ferroelectric materials have been paid much attention to scientific and technological

fields due to their wide-applications in sensors, actuators, and energy harvesters.55, 56

The ferroelectric mesocrystals with specific morphology can be fabricated via the

bottom-up self-assembly or the topochemical process.57, 58 In our previous research, the

BaTiO3 mesocrystals with platelike morphology have been fabricated by a hydrothermal

soft chemical process from the H1.07Ti1.73O4 (HTO) precursor with a lepidocrocite-like

layered structure.58 Such perovskite mesocrystals with complex chemical compositions

are very difficult to be prepared via the conventional methods. In addition, the platelike

Ba1−xCaxTiO3, Ba0.5(Bi0.5K0.5)0.5TiO3 and Bi0.5Na0.5TiO3 mesocrystals have also been

developed from the same HTO precursor via the topochemical process.59-61 Furthermore,

all of these platelike mesocrystals are promising for making the oriented ceramics with

improved dielectric and ferroelectric properties.62 Very recently, KNbO3 mesocrystals

with different morphologies have been prepared via the self-assembly without polymer

additives by our group.57

1.3 Ferroelectric perovskites

1.3.1 Inorganic metal oxide ferroelectric perovskites

Perovskite is a calcium titanium oxide mineral (calcium titanate, CaTiO3). The

mineral was discovered in the Ural Mountains of Russia by Gustav in 1839 and is

named after Russian mineralogist Lev Perovski. Its name is lent to the class of

21

compounds which have the same type of crystal structure as CaTiO3, known as the

perovskite structure. Generally, perovskites are materials described by the formula

ABX3, where X is an anion and A and B are cations of different sizes (A being larger

than B), and the crystal structure is described in Fig. 1.8(a). Namely, the BX6

octahedrons connect with each other via corner-sharing, forming the 3 dimension space.

The structure was first described by Victor Goldschmidt in 1926, in his work on

tolerance factors.63 Generally, the phase stability of the perovskite materials can be

confirmed through the tolerance factor t, which is defined as below:

t =γA + γX

√2(γ𝐵 + γ𝑋)

Where γA, γB and γX represent for the ionic radii of the A-site cation, B-site cation and

X-site anion in the perovskite ABX3 structure, respectively. A t value between 0.8 and

1.0 is favorable for perovskite structure with rhombohedral or orthorhombic structure,

and larger (>1) one tends to form the tetragonal structure, whereas the smaller (<0.8)

values of tolerance factor usually result in non-perovskite structures.64, 65

22

Fig. 1.8 (a) ABX3 perovskite structure and (b) P-E hysteresis loop of the ferroelectric perovskite

materials. (c) Piezoelectric effect and inverse piezoelectric effect of ferroelectric perovskite

materials.

Ferroelectrics are a section of a much larger class of substances called pyroelectrics.

A pyroelectric has the property that the single crystal with no surface changes is polar;

polarity is masked by the twining or by surface charges and is only revealed by the

heating treatment. A ferroelectric has an additional property that the polarization

direction can be reversed by applying the external electric-field, therefore it exhibits

hysteresis as shown in Fig. 1.8(b).66 Since the discovery of ferroelectricity in

single-crystal materials (Rochelle salt) in 1921 and its subsequent extension into the

realm of polycrystalline ceramics (BaTiO3) during the early to mid-1940s, there has

23

been a continuous succession of new materials and technology developments that have

led to a significant number of industrial and commercial applications.67 All the

ferroelectric materials exhibit an invariably piezoelectric nature, which have

piezoelectric and inverse piezoelectric effects as shown in Fig. 1.8(c). Up to now, the

inorganic metal oxide perovskite ferroelectric materials, such as Pb(Zr1-xTix)O3, BaTiO3,

Bi0.5Na0.5TiO3 etc., have been widely applied to actuators, sensors and energy

harvesters.55, 56, 68

Pb(Zr1-xTix)O3 perovskite ferroelectric. Lead zirconate titanate, namely

Pb(Zr1-xTix)O3 (PZT), was discovered in 1955 by B. Jaffe et al.,69 which has much more

excellent piezoelectric performance than that of BaTiO3 ceramics.55 PZT and related

perovskite compositions have been the mainstream for the high performance actuators

and transducers, owing to their excellent dielectric, piezoelectric, and electromechanical

coupling coefficients.69

PZT is a solid solution constructed from PbZrO3 (PZ) and PbTiO3 (PT) where the x =

0.48 of PT, and its ceramic lies near a morphotropic phase boundary (MPB) separating

the tetragonal and rhombohedral phases. The MPB can usually exhibit anomalously

high dielectric and piezoelectric responses.70-72 The enhancements of the piezoelectric

and dielectric responses are associated with lattice strains derived from the lattice

mismatches between the two different crystalline symmetries with slightly different

lattice constants at their interface.72 Dopants are always used to improve the property of

the PZT materials. With the adding of different kinds of dopants, the PZT material is

categorized into two classes of “soft” and “hard” PZT, respectively, which can be

applied to different areas.55

Nonetheless, lead has gradually been expelled from many commercial applications

24

and materials due to the concerns regarding its poisonousness. The replacement of the

lead-based piezoelectric ceramics represented by PZT has fueled the excitement and

spurned wide-spread scientific activity to find the alternatives in the last decade due to

the publication by Saito et al..70 Since PZTs contain more than 60 weight percent lead,

the work of Saito et al.70 got attention for identifying a mixture of morphotropic and

polymorphic phase transition region in a (K, Na)NbO3-based system (KNN) with

PZT-like values of piezoelectric coefficients in the texture ceramics. Finally, they

achieved a comparable piezoelectric response with PZTs.

The lead-free piezoelectric materials can be categorized into two general groups: one

competes for the same application as PZT and another excels in properties that are

outside the range where PZT can be used. As for the first group (in competition with

PZT), it contains KNN, Bi0.5Na0.5TiO3 (BNT) and (Ba,Ca)(Zr,Ti)O3 (BCZT) based

ceramics. These materials are inferior to PZT in some sense but are superior in another.

The second group includes materials with properties with which PZT cannot compete.

Examples include: BaTiO3, LiNbO3 (single crystal), Bi-based layered compounds with

Aurivillius structure, and other high temperature piezoelectric materials.73

(K, Na)NbO3 (KNN) perovskite ferroelectric. KNN-based ceramics have attracted

considerable attention due to the high Curie temperature, large piezoelectric response

and strong ferroelectricity. These ceramics are supposed to be suitable alternatives for

lead-based PZT.74 Reported in 1955, a morphotropic phase boundary (MPB) separating

two orthorhombic ferroelectric phases was identified at x = 0.5 ((K1-xNax)NbO3).

Inherent to MPBs, a maximum in remnant polarization (Pr) and minimum in coercive

field (Ec) were reported.75 The KNN-based ceramics obtained by solid state reaction

process cannot possess a good piezoelectric and ferroelectric properties as compared to

25

the lead-based PZT systems76 due to that the phase stability of KN is limited to 1040 oC

and KNN is limited to 1140 oC.77 Until now, the high density KNN ceramics can be

only synthesized by hot pressing techniques.78 It is important to note that KNN material

prepared by spark plasma sintering reported significantly higher dielectric and

piezoelectric properties than those synthesized by hot pressing, solid state and molten

salt reaction process, with εr = 700 and d33 = 148 pC/N.79, 80 It is indicated that the

enhancement is the result of extrinsic contributions to the polarizability associated with

submicron grain sizes, similar to that found in fine grain BaTiO3 ceramics.81

Bi0.5Na0.5TiO3 perovskite ferroelectric. Bi0.5Na0.5TiO3 (bismuth sodium titanate, or

BNT) is a well-known lead-free perovskite ferroelectric material. It has a perovskite

structure with a rhombohedral R3c space group (a = 38.91 nm; α= 89.6o) at room

temperature.82 In the perovskite structure with ABO3 formula, where one half of A-site

is filled with Na+ and the other half with Bi3+, and the B-site is filled with Ti4+. The

BNT has also been regarded as one of the most promising candidate materials for the

development of lead-free piezoelectric materials due to its relatively large remnant

polarization (Pr = 38 μC/cm2) and high Curie temperature (Tc = 320 oC).83 However, a

large coercive field and a high conductivity of the un-doped BNT result in difficult

poling, which then directed the exploration of BNT toward BNT-based materials, such

as Bi0.5Na0.5TiO3/BaTiO3 (BNT/BT), Bi0.5Na0.5TiO3/Bi0.5K0.5TiO3 (BNT/BKT),

Bi0.5Na0.5TiO3/BiAlO3 (BNT/BA), Bi0.5Na0.5TiO3/SrTiO3 (BNT/ST) etc., and the

piezoelectric properties along with the electrical resistivity have been greatly

improved.84-86 Processing and properties of the binary BNT-based ceramics have been

extensively reported.87, 88

BaTiO3 perovskite ferroelectric. One of the well-known lead-free ferroelectrics is

26

BaTiO3 (barium titanate, or BT), which exhibits a tetragonal symmetry of perovskite

structure at room temperature having large piezoelectricity and excellent dielectric

properties,89 which make it the most important compound applied in the composition of

ceramic capacitors, especially for the manufacture of multilayer ceramic capacitors

(MLCC).90, 91 Since the performance of the BT ceramics significantly depends on the

microstructure of the calcined body, much attention has been focused on the synthesis

of BT nanoparticles. In recent years, wet-chemistry synthesis techniques, including

sonochemical synthesis,92 sol-gel,93 hydrothermal,58 solvothermal94 and chemical

coprecipitation,95 have been investigated to prepare BT nanoparticles. However, it is

still a challenge to synthesize well dispersive BT nanoparticles with controlled

morphology. In addition, the molten salt method has also been used to prepare many

ceramic materials.96 Above the melting point of the chosen salt, the molten salt forms a

liquid phase to act as a solvent for reactant dissolution, diffusion and precipitation. To

achieve the competition to the lead-based ferroelectric materials, two kinds of effective

approaches, i.e. domain engineering and oriented engineering, have been widely used to

improve the piezoelectric performance for the ferroelectric materials.97-99 The domain

engineering is to reduce the domain size of the ferroelectric materials, while the oriented

engineering is to optimize their orientation direction. The piezoelectric response of

BaTiO3 (BT) can be improved from 200 to 788 pC/N using the combined domain and

oriented engineering, the value is larger than that of 500 pC/N for the commercially

available PZTs.89

1.3.2 Halide perovskites

Researches on the earth-abundant metal halide-based perovskite for high-efficiency

27

photovoltaics have demonstrated this class of materials to be excellent semiconductors

for optoelectronic devices.9, 100, 101 For halide perovskite, perovskite compounds based

on metal halides adopt the ABX3 perovskite structure, which has existed for more than a

century. This structure consists of a network of corner-sharing BX6 (B = Pb(II), Sn(II),

Ge(II); X = Cl, Br, I) octahedra, while the A cation is selected to balance the total charge

and it can even be a Cs+ or a small organic ions (CH3NH3+ or MA, HC (NH2)2

+ or FA).

MAPbX3 perovskites. Examples of insulating, semiconducting, and superconducting

perovskite structured materials are known; they represent a unique system class across

solid-state chemistry and condensed matter physics.9 Particularly, they have phase

transitions with accessible monoclinic, trigonal, orthorhombic, tetragonal and cubic

polymorphs relying on the rotation and tilting of the BX6 octahedra in the lattice.

Stimuli, like temperature, electric field and pressure, can give rise to the reversible

phase transitions. In fact, the Cl-, Br- and I- with different ionic radii have been widely

applied to establish the MAPbX3 (X: Cl, Br, I) perovskite structures, and it has been

found that a smaller ion radius for X- tends to form the cubic phase, such as MAPbCl3

and MAPbBr3, while the MAPbI3 exhibits a tetragonal structure.102 MAPbI3, with a

bandgap of about 1.5-1.6 eV and a light absorption spectrum up to a wavelength of 800

nm (Fig. 1.9), has been extensively used as a light harvester in solar cells.103 In addition,

the ferroelectricity of the MAPbI3, has been confirmed by some researches, in which the

spontaneous polarization of the ferroelectric property gives rise to a more efficient

charge-separation, then improve the power conversion efficiencies (PCEs) of perovskite

solar cells (PSCs).12, 104-110 On the other hand, the MAPbCl3 and MAPbBr3 exhibit

paraelectric property due to their room temperature cubic structure. But, the MAPbBr3

has an advantage of larger band gap of 2.32 eV, which gives rise to a larger voltage

28

potential, whereas the low current density impedes its further improvement of the

PCE.111 In order to fulfill a high open circuit voltage in a solar cell, it is necessary to

combine a suitable energy band structure of the constituent materials with good charge

transfer kinetics. Therefore, the MAPbI-based (MAPbI3-xBrx, or MAPbI3-xClx)

perovskites have been widely studied.112-115 The advantages of utilizing a perovskite

material as the main ingredient are ascribed to its large absorption coefficients, high

carrier mobility, ambipolar transporting properties, and low-cost solution-based

fabrication process.116

Chronologically, Miyasaka et al. demonstrated the formation of the MAPbBr3 and

MAPbI3 in dye-sensitized solar cells (DSSCs) and the efficiency of 2.6 % and 3.8 %,

respectively in 2006 and 2009. Subsequently, Park et al. reported an efficiency of 6.5 %

by creating a cell architecture similar to the extremely thin absorber (ETA) DSSC. In

2012, Park et al. replaced the liquid-based hole transport layer with solid-state

spiro-MeOTAD and immersed perovskite into the TiO2 scaffold because of the

corrosion problems for the liquid electrolytes, in the meantime an efficiency of 9.7 %

was fulfilled. Since then, the researches on the perovskite solar cells show the explosive

growth.10, 117, 118 In 2015, Park reported the MAPbI3 PSCs with a PCE of 19 %.119 Many

studies and investigations on the performances of perovskite solar cells are still ongoing

in the effort to surpass this PCE of 20%, as well as to establish a stable performance for

PSCs, in order to eliminate costly silicon PV cells.120

29

Fig. 1.9 Tuning perovskite bandgap by replacing the A cation. (a), The ABX3 perovskite crystal

structure. (b) The atomic structure of the three A site cations explored. (c) UV-Vis spectra for the

APbI3 perovskites formed, where A is either methylammonium (MA), formamidinium (FA) or

caesium (Cs). Reproduced with permission.121 Copyright 2014, Royal Society of Chemical.

FAPbI3 perovskites. The chemical modification of the X site anions for MAPbI3

(substitution of I for Br or Cl) has been achieved and was shown to increase the

bandgap while modulating the PCEs of PSCs. Similarly, the substitution of the MA+

with a longer chain C2H5NH3+ was reported and gave rise to a larger band gap with a

lower PCE (2.4 %). Therefore, the exploration of the alternative perovskites with

attractive bandgaps is necessary, which can be formed into efficient solar cells.

30

It is known that the optimal bandgap for a single-junction solar cell is between 1.1

and 1.4 eV, currently beyond the range of the MAPbI3 system. Koh et al. have

demonstrated the introduction of a new metal-halide perovskite based on FAPbI3, which

displays a favorable bandgap and exhibits a broader absorption compared to MAPbI3.122

This is because that the FA cation has shown a slightly larger ionic radius than the MA

group, which is expected to result in an increase in perovskite tolerance factor (t).65 An

increase in the tolerance factor t while maintaining the FAPbI3 perovskite structure

generally leads to an increase in symmetry, with an expected reduction in electronic

bandgap (1.47 eV) (Fig. 1.9). This bandgap for FAPbI3 is closer to the optimum value of

∼1.4 eV and presents FAPbI3 as an appealing candidate for photovoltaic applications, as

it displays an extended absorption of light compared to the MAPbI3 analogue.123 Eperon

et al. have reported that the short-circuit currents of FAPbI3 PSCs achieved > 23

mA/cm2, giving rise to PCE of up to 14.2 %.121 Hence, FAPbI3 perovskite is promising

as a new candidate for this class of solar cell.

However, the photovoltaic performance of FAPbI3 has been reported lower than that

of MAPbI3.121 In addition, a black perovskite polymorph (α-phase: stable at temperature

above 160 oC) was discovered to transformed into a yellow FAPbI3 polymorph (δ-phase:

non-perovskite) in an ambient humid atmosphere.122 Given the suitable bandgap that is

lower than that in MAPbI3, the performance of FAPbI3 solar cells can be considerably

improved by stabilizing the perovskite structure of the FAPbI3 phase. Based on this,

Pellet et al. reported an improved PCE by using mixed cation lead iodide perovskites by

gradually substituting MA with FA cations, which increases the absorption range by red

shift, allowing for a higher current density, but the performance was still dominated by

MAPbI3 rather than FAPbI3.124 Therefore, Jeon et al. proposed a strategy for extending

31

the absorption range of solar light by replacing MAPbI3 with FAPbI3 in the combined

composition of FAPbI3 and MAPbBr3.103 Consequently, the (FAPbI3)0.85(MAPbBr3)0.15

perovskite with a trigonal structure at room temperature was fabricated, confirming that

the co-substitution of MA to FA and I to Br can efficiently stabilize the perovskite phase.

Furthermore, the fabricated FAPbI3-based PSCs exhibited a maximum PCE greater than

20%.125

CsPbX3 perovskite. Although all top-performing photovoltaic cells used MA or FA

as the A cation, it is attracting that whether an inorganic A cation would also form light

absorbers with comparable properties to the MA ones. Studies on the all-inorganic

halide perovskites have revealed that these materials have great potential in

optoelectronic applications.126, 127 To find out whether the organic nature or anisotropic

geometry of the A cation is essential for the high performance of the hybrid PSCs, the

CsPbBr3 was chosen because, unlike the CsPbI3 compound, this compound occurs at

standard temperature and pressure in the perovskite structure and exhibits very good

charge transport properties.128 Beal et al. reported that by replacing the volatile MA

cation with cesium, it is possible to synthesize a mixed halide absorber material with

improved optical and thermal stability, a stabilized PCE of 6.5%, and a bandgap of 1.9

eV.129

Although the PCE of PSCs has a tremendous potential to exceed the silicon solar

cells (25 %), conclusive charge separation mechanism is still missing for PSCs because

of the lack of the fundamental understanding of the structure-property-performance

relationship of the perovskite properties, which hampers the optimization and

development of high-performance PSCs. Up to now, a traditional p-n junction charge

separation has been employed to PSCs, however, there are some mysterious behaviors

32

like the strong current-voltage (J-V) hysteresis, yet remains unexplained reasonably.13,

130 And the J-V response of the PSCs could lead to unfaithful estimation of the solar cell

device efficiency, where the reverse scan and forward scan exhibit the overestimated

and underestimated PCEs.131 Therefore, some possible theories have been proposed to

explain the origin of the hysteresis in PSCs, involving ferroelectricity,106

vacancy-assisted ionic migration,132 charge carrier trapping,133 and capacitive effect.13

1.4 Lattice strain engineering

Two-dimension (2D) materials have been paid much attention in the past decade due

to their extraordinary properties and great potential in a wide range of applications.

Strain engineering is regarded as a powerful tool to modulate the properties of 2D

materials because of its direct impact on lattice structure, and thus alters electronic

structure.135 There are five possible methods, like deforming a substrate,136 creating

wrinkles,137 employing a pre-patterned substrates,138 deforming a suspended

membrane139 and lattice mismatch,140 which have been reported in the literatures, as

shown in Fig. 1.10. Among these methods, the lattice mismatch has been widely applied

to ferroelectric 2D thin film materials for enhancing their dielectric and ferroelectric

responses.140-143

33

Fig. 1.10 Schematic presentation of methods to introduce strains to 2D materials. (a) Deforming

a substrate; (b) Creating wrinkles; (c) Employing a pre-patterned substrate; (d) Deforming a

suspended membrane. (e) Lattice mismatch.

Enormous strains can appear in thin film materials when one material is epitaxially

grew on another, originating from differences in the lattice constants of crystals and the

different thermal expansion behaviors between the film and the underlying substrate or

the defects formed during the film deposition. Therefore, some properties of thin film

can be found remarkable different from the corresponding unstrained bulk materials. In

the meantime, the suitable strains are needed for the enhancement of properties of a

chosen material in thin film form, namely lattice strain engineering. It has been widely

applied to 2D thin film materials with superlattice structure, correspondingly causing

the 2D heteroepitaxial interface, as shown in Fig. 1.11(a).

34

Fig. 1.11 Schematic illustrations of local environments for (a) the 2D heteroepitaxial interface in

the superlattice nanocomposite obtained by the MBE approach and (b) the possible 3D

heteroepitaxial interface in the mesocrystalline nanocomposite.

Heteroepitaxial growth has been established as a powerful technique to create

single-crystalline thin films or artificial low-dimensional quantum systems. Furthermore,

it gives the possibility to combine crystalline materials, which can be very different in

symmetry, bonding and lattice constant, respectively. The atomic arrangement at the

interface is the most responsible for the specific epitaxial alignment, and also for the

development of the resulting microstructure. Residual strains as well as extended

defects affect the desired physical properties and, thus, strongly decide the quality of

functionality of heterosystem. Hence, it is possible to synthesize heterosystems with

tailored electrical, optical or mechanical properties.144 For the ferroelectric materials,

the spontaneous polarization direction around the heteroepitaxial interface can be

changed to the other directions due to the lattice distortion originated from different

lattice constants (Fig. 1.12). Therefore, a sloping spontaneous polarization structure can

35

be imported into the interfaces of the

Fig. 1.12 Schematic diagrams of imported heteroepitaxial interfaces from (a)

ferro-tetragonal-ferro-trigonal composite with polarization direction along [001] and [111]

respectively, (b) ferro-tetragonal-ferro-orthorhombic composite with polarization direction

along [001] and [110] respectively, (c) ferro-tetragonal-para-cubic composite.

different substances, which results in producing the polarization reversal sustaining.145

The artificial interfaces such as in artificial superlattices can produce an obviously

significant enhancement of piezoelectric and dielectric constants.140, 141, 143, 146, 147

As mentioned above, the replacement of lead-based Pb(Zr1−xTix)O3 (PZT)

ferroelectric materials has become quite significant for the environmental protection.

However, the reported lead-free materials exhibit quit lower piezoelectric performance

than that PZTs. Although, the piezoelectric response of BaTiO3 (BT) can be improved

from 200 to 788 pC/N using the both domain engineering and oriented engineering, the

value is larger than that of 500 pC/N for the commercially available PZTs.89 However,

the piezoelectric application of BT is restrained in a temperature range of below 130 °C

36

of its Curie temperature (Tc). Therefore, the elevated Tc is also important for the

development of high performance lead-free ferroelectric materials. Haeni et al. first

reported the improvement of Curie temperature (Tc) in SrTiO3 thin film by introducing

the substrate-induced epitaxial strain, in the meantime Choi et al. demonstrated the

same manner in BaTiO3 thin film.140, 141 However, the increase of Tc by means of

substrate control is limited to film materials with only tens of nanometers thick, whereas

many ferroelectric devices need much thicker films. Therefore, a BaTiO3/Sm2O3

composite thick film has been reported with a highly improved Tc of BaTiO3, where the

BaTiO3 lattice bears a tensile strain with +2.35 % lattice mismatch at the BaTiO3/Sm2O3

interface by Harrington et al.143

At present, the most studies on the superlattices of ferroelectric materials are in

regard to 2D BaTiO3/SrTiO3 superlattice structure prepared by the

molecular-beam-epitaxy (MBE) process.141 But the MBE process is inefficient and high

cost, and the industrialization production is difficult to achieve by MBE process.142

Furthermore, 3D superlattice structures (Fig. 1.11(b)) which can achieve a high density

interface are difficult to be fabricated via the MBE process.30, 31, 148 Hence, the

exploration of a new approach to develop the superior superlattice composite materials

is expected. The development of the ferroelectric mesocrystalline nanocomposites with

the 3D superlattice structure by a facile low cost process is a significant subject and will

attract much attention in the ferroelectric materials field.149 In our former research, the

mesocrystalline ferroelectric BaTiO3/SrTiO3 (BT/ST) and BaTiO3/CaTiO3

nanocomposites constructed from the different nanocrystals with same crystal-axis

direction exhibit an both elevated dielectric and ferroelectric responses via the

introduction of the strains originated from the lattice mismatches between BT and ST or

37

BT and CT, which have different lattice constants.30, 31

1.5 Topochemical synthesis

Topochemical synthesis is a quite useful and classical approach for the fabrication of

the targeted particles with the desired morphologies.54, 150 Compared with other

reactions, the topochemical conversion reaction can be regarded as special phase

transformations of the parent crystals into the daughter crystals and is driven by the

crystal structures rather than by the chemical nature of the reactants. Therefore, the

crystallographic directions of parent and daughter crystals have some certain topological

correspondences. Some mesocrystals can be prepared by the topochemical syntheses

method as described above. The conventional synthesis approaches, such as

hydrothermal/solvothermal process, molten salt process, and solid-state reaction process

etc., can be employed for the topochemical synthesis.

1.5.1 Approach of topochemical synthesis

The hydrothermal/solvothermal process is a liquid chemical reaction process under a

higher pressure than 1 atm and a higher temperature than boiling point of the solvent

used. When an aqueous solution is used as the solvent, it is called hydrothermal process.

When other solvents rather than aqueous solvent, such as organic solvents, organic and

aqueous mixed solvents, are used, it is called solvothermal process. These processes are

widely applied to prepare the ceramics powders. The basic mechanism of crystal

nucleation and growth under the hydrothermal and solvothermal conditions is the

dissolution-deposition reactions. The particle size is controlled by the crystal growth

rate, reaction time, and reaction temperature. The particle morphology is dependent on

the crystal growth direction or the non-classical self-assemble direction that is not easy

38

to be controlled in the normal cases. The advantages of the hydrothermal/solvothermal

process are preparations of the products with a controllable morphology, a controllable

crystal facet, a uniform size distribution, a small crystal size at a relatively low

temperature. The hydrothermal/solvothermal process is a potential method for the

topochemical synthesis.

Because of the advancements in modern technology, the study of molten salt

synthesis has achieved considerable progress, and lots of molten salts have been used

for this process. The molten salt process is usually carried out in a low-temperature

molten salt as a reaction medium, and the molten salt can also act as a reagent.151 Salt

melts have a long history as a solvent in research as well as in industry due to their low

toxicity, low cost, low vapor pressure, abundant availability, high heat capacity, large

electrochemical window, and high ionic conductivity. The crystal growth occurs easily

in the salt melt medium, the product particles usually have its original crystal

morphology, uniform and large particle size. The precursor host particles can react

easily with the guest ion or molecule species in the molten-salt via host-guest

mechanism to obtain the desired composition and morphology of the products.

Therefore, the molten salt process is suitable for the topochemical synthesis.

For the normal solid-solid reaction process, the ball-milled precursor powders with

desired compositions should be annealed at high temperatures. This reaction occurs

simply via solid-state diffusion at a high temperature. The obtained products usually

have the characteristics of isometric morphology such as cubic or spherical, large

particle size, and compositional inhomogeneity.152 Nonetheless, the solid-state process

can still be used for the topochemical synthesis.153

39

1.5.2 Layered protonated titanate HTO as precursor for topochemical synthesis

Fig. 1.13 Schematic diagrams of HTO (H1.07Ti1.74O4·H2O) crystal with lepidocrocite structure. (a)

[100] zone axis structure and (b) three-dimensional (3D) structure.

Layered titanates with variety 2D structures have recently been drawn much attention

because of their interesting interlayer chemistry. One of the most studied layered

titanate is lepidocrocite (γ–FeOOH)-type protonated titanate, which has a composition

of H1.07Ti1.73O4·H2O (HTO) and exhibits excellent ion-exchange/intercalation

reactivities, and can be readily exfoliated/delaminated into its structure unit single

sheets with a distinctive 2D morphology and a thickness of about 1 nm.58, 154 In the

HTO crystal structure, the TiO6 octahedrons are combined with each other via angle and

edge-sharing to form a 2D TiO6 octahedral sheet, as illustrated in Fig. 1.13. The host

sheets are stacked with a basal spacing of about 8.82 Å in a body-centered orthorhombic

system (a = 3.7831 Å, b = 17.6413 Å, c = 2.9941 Å), accommodation H2O and H3O+

40

between the octahedral sheets (Fig. 1.13). Approximately 52% of the interlayer sites are

occupied by H3O+ and remaining by H2O. The positive charge of H3O

+ is balanced with

minus one of the host TiO6 octahedral sheets arising from the Ti site vacancies.

In our previous works, we have used the HTO crystal as a precursor to prepare the

various perovskite titanate mesocrystals and the TiO2 platelike mesocrystals,18 and

furthermore as a template to fabricate oriented ceramics by a reactive template grain

growth method.62 Very recently, the phase transition mechanism of the HTO crystal to

anatase TiO2 under supercritical water has been reported.18 These results suggest that

the HTO crystal is an excellent precursor for the preparations of the titanate

mesocrystals and titanium oxides mesocrystals by the topochemical conversion reaction

mechanisms.

1.5.2 Soft chemical process for mesocrystalline nanocomposites

The solvothermal soft chemical process is a useful and unique method for preparation

and design of functional inorganic materials.18, 31, 57, 148, 155 Advantages of the

hydrothermal/solvothermal process are suitable for the soft chemical synthesis,

especially in effectively maintaining the precursor morphologies in the synthesis

process. The solvothermal soft chemical process typically comprises two steps: the first

step is the preparation of a framework precursor with layered structure and insertion of

structural directing-agents (template ions or molecules) into its interlayer space by a soft

chemical reaction; the second step is the structural transformation of the structural

directing-agent- inserted precursor into a desired structure by a solvothermal reaction.

The crystal structure of the product can be controlled by the structural directing-agent

used, and the product particle morphology is dependent on the precursor morphology

41

used. This process has been utilized for the synthesis and design of metal oxides

mesocrystals and organic-inorganic nanocomposites with controlled structure,

morphology, and chemical composition.20, 54, 156 As described above, the platelike

mesocrystalline nanocomposites perovskite mesocrystals, such as BaTiO3/SrTiO3 and

BaTiO3/CaTiO3 nanocomposites, have been successfully fabricated by using the facile

two-step solvothermal soft chemical process, which give rise to highly elevated

dielectric and ferroelectric properties.30, 31

1.6 Purpose of present study

As described above, up to now, studies on the mesocrystals have been reported

mainly on the syntheses and formation mechanisms; however, the understandings on

mesocrystal properties, formation mechanisms, and especially the potential application

possibilities are still not enough. Further development of the functional mesocrystals,

investigations of the mesocrystal performances and the formation mechanisms are

necessary in current nanomaterial research fields. The development of the ferroelectric

perovskite mesocrystals has drawn much attention since the potential improvements on

dielectric, ferroelectric and piezoelectric responses of the mesocrystalline materials. In

addition, the approaches, like oriented engineering, domain engineering and

nanocomposition engineering, can be applied to improve the piezoelectric property of

the ferroelectric materials by using the mesocrystalline materials. However,

simultaneously elevated Curie temperature and piezoelectric responses have not yet

been reported, in which the application temperature ranges of the ferroelectric materials

are restrained by their Curie temperatures.

As described above, the strain engineering can be applied to elevate Curie

42

temperature of the ferroelectric materials by introduction of 2D heteroepitaxial interface.

But for bulk ferroelectric ceramic materials, the strain engineering for simultaneous

improvement of both piezoelectric response and Curie temperature has not been

reported, as far as we know, since the 3D heteroepitaxial interface is very difficult to be

introduced into bulk materials. Therefore, in the present study, the 3D heteroepitaxial

interface is challenged to introduce into the functional mesocrystalline nanocomposites

through a two-step soft chemical process via the in situ topochemical conversion

mechanism. In the meantime, both piezoelectric response and Curie temperature of the

mesocrystalline ferroelectric nanocomposites can be highly improved.

On the other hand, the perovskite compounds possess a number of interesting

properties, such as electron-acceptor behavior; a large optical transmission domain;

antiferromagnetic; exceptional magnetic; piezoelectric; photoluminescent properties;

anionic conductivity over a wide temperature range. The semiconducting properties of

perovskite-related halides have been widely applied to photovoltaic areas. Still, the

ferroelectricity is also possible in perovskite-related halides, which would exhibit quite

interesting and debatable behaviors in the PSCs. Therefore, figuring out the

semiconductor and ferroelectric behaviors in the perovskite-related halides is

significant.

In Chapter II, the ferroelectric mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)

nanocomposite synthesized from a layered titanate H1.07Ti1.73O4 (HTO) by an ingenious

two-step topochemical process is introduced. The BT/BNT nanocomposite is

constructed from well-aligned BT and BNT nanocrystals with the same crystal-axis

orientation. The BT/BNT heteroepitaxial interface in the nanocomposite is promising

for the enhanced piezoelectric performance by using the lattice strain engineering,

43

which gives a giant piezoelectric response with a d*33 value of 408 pm/V. The

introduced lattice strain at the BT/BNT heteroepitaxial interface causes transitions of

pseudo-paraelectric BT and BNT nanocrystals to the ferroelectric nanocrystals in the

mesocrystalline nanocomposite, which enlarges ferroelectric, piezoelectric and

dielectric responses. The lattice strain also results in the elevated Curie temperatures

(Tc) of BT and BNT and a new intermediate phase transition.

In Chapter III, the ferroelectric mesocrystalline BaTiO3/BaBi4Ti4O15 (BT/BBT)

nanocomposite synthesized from the layered titanate HTO by a two-step topochemical

process is exhibited. The BT/BBT nanocomposite is constructed from well-aligned BT

and BBT nanocrystals oriented along the [110] and [11-1] crystal-axis directions

respectively. The lattice strain is introduced into the nanocomposite by the formation of

the BT/BBT heteroepitaxial interface, which causes a greatly elevated Curie

temperatures from 400 to 700 °C and an improved piezoelectric response with d*33=130

pm/V. In addition, the BT/BBT nanocomposite is stable up to a high temperature of

1100 oC, therefore the mesocrystalline ceramic can be fabricated as a high-performance

ferroelectric material.

In Chapter IV, the ferroelectric and semiconducting properties of the

CH3NH3PbI3-xClx perovskite are studied by structural analysis, measurements of the

ferroelastic behavior, the ferroelectric hysteresis loops, the piezoelectric response and

conductivity. The results reveal that the CH3NH3PbI3-xClx perovskite exhibits the

anti-ferroelectric and semiconducting natures, and the anti-ferroelectricity can be

switched to ferroelectricity by poling treatment, which gives a solid evidence for the

argument between the non-ferroelectric and ferroelectric nature for this perovskite and

paves the way for the fabrication of high-performance perovskite solar cells.

44

In Chapter V, the main points concluded in each chapter are summarized. In addition,

on the basis of the results of the present study, the future prospects for the applications

of these results are mentioned.

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56

Chapter II

Anomalous Piezoelectric Response of Ferroelectric

Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites

Designed by Strain Engineering

2.1 Introduction

Ferroelectric materials have drawn much attention to scientific and technological

fields due to their wide-applications in sensors, actuators, and energy harvesters.1-3

Pb(Zr1-xTix)O3 (PZT) is an excellent and wide-applied ferroelectric material, and a

successful example of application of the morphotropic phase boundary (MPB) around x

= 0.48 with maximized ferroelectric response,4 that occupies the major share of the

piezoelectric ceramic material market. However, the lead-based PZT piezoelectric

ceramics contain more than 60 wt% of poisonous lead component. Recently, the

replacement of the PZTs has fueled the excitement and spread scientific activity to find

the lead-free alternatives, since Saito et al. reported high performance lead-free

piezoelectric materials using MPB and oriented ceramics.5 Nevertheless, the

performances of the lead-free piezoelectric materials are still not comparable to PZTs.

6-8

Some approaches concerning improving the performance of the lead-free

piezoelectric materials have been proposed, such as MPB, oriented ceramics, domain

engineering, and strain engineering.4, 5, 9-21 Among these approaches, the MPB is one of

the effective and common approaches to further elevate the piezoelectric performance

57

of ferroelectric perovskites. The ferroelectric perovskites exhibit elevated piezoelectric

and dielectric responses at MPB due to a lattice strain originating from lattice

mismatching between the two different phases with different symmetrical systems and

little different lattice constants at their interface.4, 5, 14 Then the ferroelectric polarization

rotation of the distorted lattice at the interface becomes sensitive to an applied-bias.22, 23

However, the application of MPB is restrained due to the narrow composition range,

and it is also hard to obtain a temperature-independent MPB.24

The strain engineering is also a promising approach to enhance the piezoelectric and

dielectric responses, and expected to overcome the disadvantages of MPB.9, 10, 12, 15, 16, 18,

19 An artificial superlattice constructed from two kinds of crystals has been employed to

elevate ferroelectric responses, for example large dielectric constant and remanent

polarization have been achieved in BaTiO3/SrTiO3 (BT/ST) artificial superlattice with a

heteroepitaxial interface between the ferroelectric BT and paraelectric ST phases.16, 17,

25-28 The enhanced dielectric and ferroelectric responses are attributed to the horizontal

strain originated from the two-dimensional (2D) BT/ST heteroepitaxial interface, where

little different lattice constants of BT and ST causes lattice strain at the heteroepitaxial

interface.16, 17, 27

Recently, the in-plane biaxial strain regarded as three-dimensions (3D) has drawn

much attention because of its potential for constructing a high density heteroepitaxial

interface.12, 15, 18, 20 Mimura, et al. have fabricated BT/ST heteroepitaxial interface by

self-assembling BT and ST nanocubes to improve piezoelectric response.20 The

enhanced piezoelectric responses have been reported by constructing a 3D BT/KNbO3

(BT/KN) heteroepitaxial interface.12 However, the expected large piezoelectric

responses have not been achieved by employing the 3D heteroepitaxial interfaces. The

58

main reason is attributed to that the perfect 3D heteroepitaxial interfaces are not easy to

be constructed by the bottom-up process by assembling the nanocubes and the surface

coating process using sol-gel or solvothermal method. Therefore, a perfect 3D

heteroepitaxial interface is expected for the further enlarged piezoelectric response.

Mesocrystal is a polycrystal constructed from the nanocrystals with the same

crystal-axis orientation.29, 30 The mesocrystals not only have some potential properties

based on the individual nanocrystals but also exhibit unique collective properties of

nanocrystal ensembles.31 It has become a fascinating research area as a new class of

material for catalysis, sensing, and energy storage and conversion in the past

decade.32-35 Recently, we have reported some ferroelectric perovskite mesocrystals,36-40

and demonstrated that ferroelectric mesocrystalline BT/ST and BT/CaTiO3 (BT/CT)

nanocomposites 15, 18 with the 3D heteroepitaxial interfaces between ferroelectric BT

phase and paraelectric ST phase, and ferroelectric BT phase and ferroelectric CT phase,

respectively, show a highly elevated piezoelectric responses comparing with those of 3D

heteroepitaxial interfaces fabricated by using other methods.19, 20, 41 The results suggest

the potential application of the mesocrystals to the improvement of piezoelectric

performance for the lead-free piezoelectric materials.

In the lead-free piezoelectric families, bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT)

is a promising candidate for the replacement of PZT materials because of its relatively

large remanent polarization (Pr) of 38 µC/cm2 and high Curie temperature (Tc) of 320

○C. The BNT-based Bax(Bi0.5Na0.5)1-xTiO3 (BBNT) materials with a MPB composition

around x = 0.07 have been extensively investigated.14, 42-46 The piezoelectric constant

has been improved from 78 pC/N of BNT to 160 pC/N for non-oriented piezoelectric

BNT-BT bulk ceramic by introducing the MPB.14, 47 Although the BNT-based material

59

is one of the most promising lead-free piezoelectric candidate, but no any strain

engineering approach has been reported on this material.

Herein, we challenge on a mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)

nanocomposite for the first time because the large remanent polarization and high Curie

temperature of BNT are expected for the improved ferroelectric behavior of the

mesocrystalline nanocomposite. The BT/BNT nanocomposite constructed from

well-aligned BT and BNT nanocrystals with the same crystal-axis orientation is

synthesized using an ingenious two-step topochemical process. A giant piezoelectric

response of d*33 = 408 pm/V and elevated Tc of 380 °C were achieved by successfully

introducing BT/BNT heteroepitaxial interface. The giant piezoelectric response can be

explained by an optimized combination of Ferro-Tetragonal/Ferro-Rhombohedral

crystal systems with a lattice mismatching of about 2.5 % for the heteroepitaxial

interface. These results will give a trend toward the high performance piezoelectric

materials using the strain engineering.

2.2 Experimental

2.2.1 Sample Preparation

H1.07Ti1.73O4·nH2O (n = 1, abbreviated as HTO) powder sample was prepared from

K0.80Ti1.73Li0.27O4 (KTLO) as reported in our previous study.34 For the synthesis of the

platelike particle sample of mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)

nanocomposite, a two-step reaction process, namely the solvothermal reaction and

solid-state reaction, was used. In the first step, the platelike HTO crystals (0.4 g) and

Ba(OH)2·8H2O (mole ratio of Ba/Ti = 0.5) were solvothermally treated in 30 mL

60

distilled water under the stirring conditions at 150 C for 12 h. After the solvothermal

treatment, the obtained sample was washed with distilled water and dried at room

temperature to obtain mesocrystalline BaTiO3/HTO (BT/HTO) nanocomposites. In the

second step, the mesocrystalline BT/HTO sample (0.8 g) was mixed with 20 % mole

excess of Bi2O3 and 40 % mole excess of Na2CO3 in ethanol solvent by ball-milling

with a speed of 50 r/min for 12 h at room temperature, and then the mixture was dried at

60 C for 6 h. Finally, the mixed powders were heated at a desired temperature for 3 h

to obtain mesocrystalline BT/BNT nanocomposites.

A pellet sample of BT/HTO-Bi2O3-Na2CO3 mixture was fabricated by pressing

BT/HTO-Bi2O3-Na2CO3 mixture powder sample using a pellet press mold with a

diameter of 10 mm at 30 M Pa for 3 min. Subsequently, the cold isostatic press (CIP)

was employed with a pressure of 200 M Pa. The obtained pellet samples were heated at

desired temperatures for 3 h to obtain a BT/BNT pellet samples. The obtained pellet

samples were polished with diamond slurry, and cut using a crystal cutter to sizes of

4×4×0.5 mm3. Gold electrodes were printed on the top and bottom surfaces with an area

of 4×4 mm2. A BT mesocrystal sample was prepared by solvothermal treatment of HTO

(0.4 g) in Ba(OH)2 solution (mole ratio of Ba/Ti = 1.2) at 200 oC for 12 h, and then heat

treated at a desired temperature for 3 h. A BNT mesocrystal sample was prepared by

heat-treament of a HTO-TiO2-Bi2O3-Na2CO3 mixture at a desired temperature for 3 h.40

The samples obtained by heat-treatment are designated as BT/BNT-X, BT-X, and

BNT-X, respectively, where X represents for the heating temperature.

2.2.2 Physical Properties Analysis

The structures of powder samples were investigated using a powder X-ray

61

diffractometer (Shimadzu, XRD-6100) with Cu Kα (λ = 0.15418 nm) radiation. The

particle size and morphology of the samples were observed using scanning electron

microscopy (SEM) (JEOL, JSM-5500S) and field emission scanning electron

microscopy (FE-SEM) (Hitachi, S-900). Transmission electron microscopy (TEM)

observation and selected-area electron diffraction (SAED) were performed on a JEOL

Model JEM-3010 system at 300 kV, and the powder sample was supported on a Cu

microgrid.

Piezoelectric responses of the mesocrystal individuals were detected by using a

scanning probe microscopy system (SPM) (SPA-400/Nano Navi Station, SII,

NanoTechnology Inc.) combining atomic force microscopy (AFM) and piezoresponse

force microscopy (PFM). The sample dispersed on an Au-coated silicon substrate (40

nm thickness of Au-film), and an individual platelike particle dispersed on the Si

substrate surface was scanned using the AFM probe tip with a conductive Rh-coated Si

cantilever probe (SI-DF3-R, spring constant: 1.2 N/m) in the contact mode. The

generated strain of the platelike particle was detected using Z/V mode in AFM system

after a DC bias from -10 V to 10 V was employed on the surface of the platelike particle.

And the converse piezoelectric constant d*33 can be calculated by equation (1).

d*33 = D/Va (1)

where D is the displacement (pm), Va is the applied bias (V).48, 49

The polarization-electric field (P-E) loop of the pellet sample was measured using a

ferroelectric testing system (Toyo Corporation, FCE3-4KVSYS) at room temperature.

The dielectric response of the pellet sample was measured using an LCR meter (Agilent

E4980A) in a frequency range of 1 to 2 M Hz. For the measurement of

temperature-dependent dielectric response, the pellet sample was heated in a

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temperature controller with a heating rate of 3 C/min from room temperature to 460 C

during the measurement.

2.3 Results and discussion

2.3.1 Synthesis of mesocrystalline BT/HTO nanocomposite

In the present study, the platelike mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 (BT/BNT)

nanocomposites are prepared using a two-step reaction process. In the first step, the

platelike H1.07Ti1.73O4·nH2O (HTO) crystals were solvothermally reacted with Ba(OH)2

to obtain a BaTiO3/HTO (BT/HTO) nanocomposite. Fig. 2.1 presents the XRD patterns

of the HTO precursor and BT/HTO nanocomposite obtained by solvothermal treatment

in Ba(OH)2 solution at 150 °C for 12 h with mole ratio of Ba/Ti = 0.5. The HTO

precursor has a lepidocrocite-like (γ–FeOOH) layered structure with a basal spacing of

0.882 nm corresponding to (020) crystal plane.15 After the solvothermal reaction in

Ba(OH)2 solution, except the unreacted HTO phase, a BT phase can be confirmed,

namely, the HTO precursor is partially transformed into the BT phase. The basal

spacing of the HTO crystal decreases slightly from 0.882 to 0.871 nm because of the

H+/Ba2+ ion-exchange in the HTO interlayers. To confirm the nanostructure of BT/HTO

sample and the distribution of the BT in the sample, BT/HTO sample was treated with a

2 M HCl solution to remove BT in the sample. It could be obviously seen that the BT

phase disappeared and a small amount of anatase-type TiO2 phase was found after the

acid treatment (Fig. 2.1(c)). This result reveals that BT phase is dissolved and

transformed to the TiO2 phase after the acid treatment, where the BT in the BT/HTO

nanocomposite formed by the topochemical process have a uniform distribution in the

63

BT/HTO particle, which is a prerequisite for the formation of well-aligned BT/BNT

nanocomposite.

Fig. 2.1 XRD patterns of (a) H1.07Ti1.73O4·nH2O (HTO) and (b) BT/HTO sample obtained by

solvothermal treatment of HTO-Ba(OH)2 mixture with mole ratio of Ba/Ti = 0.5 at 150 ºC for

12 h, and (c) sample obtained by the treatment of BT/HTO with 2 M HCl solution for 12 h.

The nanostructure of BT/HTO sample was investigated by using TEM and FE-SEM.

Fig. 2.2 shows TEM images and SAED spots patterns of HTO, BT/HTO, and

acid-treated BT/HTO samples. HTO has a platelike particle morphology with smooth

surface (Fig. 2.2(a)). The SAED pattern reveals that the platelike HTO particle is a

single crystal located along the [010] zone axis of orthorhombic system, where the

[010] direction (b-axis direction) is perpendicular to the basal plane of the platelike

HTO crystal (Fig. 2.2(b)). BT/HTO retains the platelike particle morphology of HTO

64

precursor, where many nanocrystals with a size of about 40 nm which corresponds to

BT phase are observed in the platelike particle of BT/HTO (Fig. 2.2(c) and Fig. 2.3(c)).

The two sets of SAED spots patterns corresponding to layered HTO phase and BT

perovskite phase, respectively, are simultaneously detected in one platelike crystal,

indicating that the HTO and BT phases coexist in one platelike particle (Fig. 2.2(d)).

This result demonstrates that the BT/HTO nanocomposite can be obtained under the

solvothermal conditions at 150 °C. It is notable that although the platelike particle of

BT/HTO nanocomposite is a polycrystal particle, only one set of SAED pattern is

observed for BT and HTO phases, respectively. The result reveals that all the BT and

HTO nanocrystals in one platelike particle show the same crystal-axis direction,

respectively, namely a mesocrystalline BT/HTO nanocomposite is formed.15 The [200]

and [002] directions of the HTO phase correspond to the [002] and [1-10] directions of

the BT phase, respectively, which reveals the BT phase is formed by a topochemical

structural transformation reaction from HTO phase.

After the acid treatment of the BT/HTO, a porous platelike particle with a pore size of

about 50 nm is generated (Fig. 2.2(e)). The pores are formed by the dissolution of BT

nanocrystals in the BT/HTO nanocomposite. Some nanoparticles with a size of 10 nm

are observed on the surface of the porous platelike particle, which correspond to anatase

TiO2 phase (Fig. 2.3(f)). The acid-treated BT/HTO platelike particle shows a SAED

pattern of single crystal HTO phase (Fig. 2.2(f)), namely, a porous single crystal can be

obtained by the dissolution of BT nanocrystals in the BT/HTO nanocomposite. The

above results indicate that the mesocrystalline BT/HTO nanocomposite is formed by

solvothermal reaction of HTO in Ba(OH)2 solution, and the BT nanocrystals uniformly

distribute in the mesocrystalline BT/HTO nanocomposite particle. An artificial porous

65

single crystal of HTO can be obtained by removing BT nanocrystals with acid treatment.

The formation process of the porous HTO can be confirmed also by FE-SEM

observation results (Fig. 2.3).

Fig. 2.2 (a, c, e) TEM images and (b, d, f) SAED patterns of (a, b) HTO single crystal precursor,

(c, d) BT/HTO sample obtained after solvothermal treatment of HTO in Ba(OH)2 solution with

a mole ratio of Ba/Ti = 0.5 at 150 °C for 12 h, and (e, f) sample obtained by acid treatment of

BT/HTO with 2 M HCl solution.

66

Fig. 2.3 (a, b, c) FE-SEM and (d, e, f) TEM images of (a) HTO precursor, (b) BT/HTO

nanocomposite obtained by solvothermal treatment of HTO-Ba(OH)2 mixtures with mole ratio of

Ba/Ti = 0.5 at 150 °C for 12 h, (c, d, e, f) sample obtained by acid-treatment of BT/HTO

nanocomposite with 2 M HCl solution for 12 h. (f) HRTEM image derived from white pane in TEM

image (d).

2.3.2 Synthesis of mesocrystalline BT/BNT nanocomposite

In the second step of the two-step reaction process for the synthesis of

mesocrystalline BT/BNT nanocomposite, a mixture of BT/HTO, Bi2O3 and Na2CO3 was

calcined. Under the condition of stoichimetric mole ratio of BT/HTO, Bi2O3 and

Na2CO3 for formation of BT/BNT, impurity is formed due to the losses of Bi and Na

contents by evaporation during the calcination at elevated temperature (Fig. 2.4). When

20 and 40 % mole excess of Bi2O3 and Na2CO3, respectively, were introduced into the

67

(BT/HTO)-Bi2O3-Na2CO3 reaction system, high purity BT/BNT can be obtained, and

the XRD patterns of the products prepared at different temperatures are shown in Fig.

2.5. After heat-treatment at 500 °C, no obvious new phase is observed. The basal

spacing of the HTO decreases from 0.871 nm to 0.726 nm because of the dehydration of

its interlayer water. Except BT phase, all other starting material phases disappear when

the temperature was elevated to 600 ºC. The main crystalline phase of the product is the

BT phase, and a small amount of Bi12TiO20 phase that can be well identified by JCPDS

File No. 34-0097 is also found.40 At 700 °C, a mixture of BT and BNT phases are

formed. These results indicate that the HTO firstly reacts with Bi2O3 to form Bi12TiO20,

and then Bi12TiO20 reacts with Na2CO3 to form BNT in the reaction system.

Subsequently, the BT and BNT phases react together gradually to form

Ba0.5Bi0.25Na0.25TiO3 (BBNT) solid solution with the increasing temperature above 800

C, and finally the formation reaction of BBNT almost completes at 1000 C.

Fig. 2.4 XRD patterns of the samples obtained by heating treatments of (BT/HTO)-Bi2O3-Na2CO3

68

mixture with (a) stoichimetric mole ratio, (b) 10 % mole excess of Bi2O3 and Na2CO3, (c) 20 % mole

excess of Bi2O3 and Na2CO3, and (d) 20 % mole excess of Bi2O3 and 40 % mole excess of Na2CO3

for formation of BT/BNT nanocomposite, respectively, at 700 ºC for 3 h.

Fig. 2.5 XRD patterns of (a) (BT/HTO)-Bi2O3-Na2CO3 mixture and samples obtained by

heat-treatments of the mixture at (b) 500, (c) 600, (d) 700, (e) 800, (f) 900, and (g) 1000 ºC for 3 h,

respectively.

In the (BT/HTO)-Bi2O3-Na2CO3 reaction system, the platelike morphology of BT/HTO

retains up to 800 °C, where the mixture of BT and BNT phases is formed, and almost

destroyed at 1000 °C, where the formation reaction of BBNT solid solution is completed

(Fig. 2.6). The formation reaction of BNT in the (BT/HTO)-Bi2O3-Na2CO3 system is

investigated in detail using TEM and SAED, and the results are shown in Fig. 2.7. It can

be clearly seen that the platelike particles constructed from the nanoparticles are formed

at 600 and 700 ºC (Fig. 2.7(a, e)). At 600 ºC, the SAED spots patterns corresponding to

69

the BT phase and the Bi12TiO20 phase respectively can be observed in one platelike

particle (Fig. 2.7 (b)), indicating that BT phase and Bi12TiO20 phase coexist in one

platelike particle. This result is consistent with the XRD result in Fig. 2.5. The

nanocrystals with a size of about 5 nm, which are distributed uniformly on the surface

of the platelike particle, can be confirmed to be Bi12TiO20 phase by HRTEM and FFT

pattern (Fig. 2.7(c, d)). These results suggest that the Bi12TiO20 nanocrystals are formed

on the BT nanocrystal surface by the heteroepitaxial growth mechanism.

70

Fig. 2.6 SEM images of samples obtained by heat-treatments of the (BT-HTO)-Bi2O3-Na2CO3

mixture at (a) 500, (b) 600, (c) 700, (d) 800, (e) 900, (f) 1000, (g) 1100, and (h) 1200 °C for 3 h,

respectively.

Fig. 2.7 (a, e) TEM images and (b, f) SAED patterns of samples obtained by heat-treatments of

(BT/HTO)-Bi2O3-Na2CO3 mixture at (a, b) 600 oC and (e, f) 700 oC for 3 h, respectively. (c)

HRTEM image is an enlarged image derived from white pane in TEM image (a) and (d) the FFT

pattern is obtained from the whole region of the HRTEM (c).

71

At 700 ºC, while two sets of single crystal-like SAED spots patterns corresponding to

the BT phase and the BNT phase are observed simultaneously in one platelike particle

(Fig. 2.7 (f)), revealing that the platelike particle is constructed from the BT and BNT

nanocrystals which have the same crystal-axis orientation direction. Namely, the

mesocrystalline BT/BNT nanocomposite is formed at 700 ºC. The SAED result also

reveals that the BT and BNT nanocrystals in the mesocrystalline BT/BNT

nanocomposite have the same crystal-axis orientation direction in [110]-zone axis, and

lattice constant of BT phase is slightly larger than that of BNT phase, which also agrees

well with the XRD result in Fig. 2.5. This result suggests the formation of a

heteroepitaxial interface between BT and BNT nanoparticles in the mesocrystalline

BT/BNT nanocomposite.

2.3.3 Formation reaction mechanism of mesocrystalline BT/BNT nanocomposite

According to the results described above, a schematic representation for the

formation reaction mechanism of the mesocrystalline BT/BNT nanocomposite from

HTO by the two-step reaction process is given in Fig. 2.8. In the first step, firstly, Ba2+

ions are intercalated into the HTO bulk crystal through its interlayer pathway by a

H+/Ba2+ exchange reaction, subsequently, the Ba2+

ions react with the TiO6 octahedral

layers of HTO to form the BT nanocrystals in the crystal bulk by the topochemical

structural conversion reaction under the solvothermal conditions.36 In the topochemical

solvothermal reaction, about 50% of the HTO phase is transformed to the BT

nanocrystals due to 0.5 of Ba/Ti mole ratio in the reaction system, in which the BT

nanocrystals are uniformly distributed in the HTO platelike particle (Fig. 2.2). The

72

formation of BT phase by a dissolution-deposition reaction is also possible on the

platelike particle surface.39 In the present case, the topochemical conversion reaction is

predominant, owing to the low concentration of Ba(OH)2 and low reaction

temperature.15 There is a definite corresponding relationship between the crystal-axis

directions of HTO precursor and formed BT product in the topochemical reaction

system, in which [200] and [002] directions of HTO phase correspond to [002] and

[1-10] directions of BT phase, respectively, as shown in Fig. 2.2(d). Therefore, all the

formed BT nanocrystals in one platelike crystal of the BT/HTO nanocomposite present

the same crystal-axis orientation in the [110]-zone direction which is consistent with the

[010]-zone direction of the HTO matrix crystal, as shown in Fig. 2.2(d).

Fig. 2.8. Schematic representation for the formation mechanism of the mesocrystalline BT/BNT

nanocomposite from the layered HTO single crystal by a two-step reaction process.

In the second step, firstly the Bi3+ ions immigrate into the HTO bulk crystal of the

BT/HTO via the interlayer pathway of HTO, and react with the TiO6 octahedral layers

73

of HTO framework to form the Bi12TiO20 nanocrystals on the BT nanoparticle surface,

which causes the formation of the BT/Bi12TiO20 nanocomposite, as shown in Fig. 2.7(b).

Secondly, the Bi12TiO20 nanocrystals in the BT/Bi12TiO20 nanocomposite react with

Na2CO3 to form the BNT nanocrystals by a heteroepitaxial growth mechanism, which

results in formation of mesocrystalline BT/BNT nanocomposite, where all BT and BNT

nanocrystals show the same crystal-axis orientation in the [110]-zone direction, as

shown in Fig. 2.7(f). In the mesocrystalline BT/BNT nanocomposite, the heteroepitaxial

interface between BT and BNT nanocrystals is formed, where the BT phase and BNT

phase have little different lattice constants. The BT and BNT nanoparticles in the

mesocrystalline BT/BNT nanocomposite can react together to form BBNT solid

solution at the heteroepitaxial BT/BNT interface over 900 °C, and finally the BT and

BNT nanocrystals are transformed completely to BBNT solid solution over 1000 °C.

2.3.4 Ferroelectric and piezoelectric responses of mesocrystalline BT/BNT

nanocomposite

To figure out the ferroelectric properties of the mesocrystalline BT/BNT

nanocomposite, the pellet samples of the BT/BNT nanocomposite were prepared by

heat-treatment of the (BT/HTO)-Bi2O3-Na2CO3 mixture pellets at different temperatures

for ferroelectric studies. The pellet samples with lower leakage currents can be obtained

by the cold isostatic pressing (CIP) treatment (Fig. 2.9). The samples prepared in a

temperatures range from 600 to 900 oC show the closed ferroelectric-like P-E hysteresis

loops (Fig. 2.10), revealing ferroelectricity of the mesocrystalline nanocomposites. The

dependence of remanent polarization (Pr) on the fabrication heating temperature for the

74

mesocrystalline nanocomposite is exhibited in Fig. 2.11(a). It is interesting that with

elevating the calcination temperature, the Pr value increases, reaches a maximum value

at 700 oC, and then decreases in the studied temperature range. BT/BNT-700 exhibites a

larger ferroelectric response with a remanent polarization Pr value of 2.4 μC/cm2 at an

applied-electric field of 6 kV/cm than the mesocrystalline BT/ST nanocomposite with a

Pr value of 0.6 μC/cm2 at an applied-electric field of 17 kV/cm.15

Fig. 2.9 Plots of leakage current densities against time for the ferroelectric BT/BNT-700 pellet

samples (a) with and (b) without CIP treatment at applied voltage of 6 kV/cm.

75

Fig. 2.10 P-E hysteresis loops of the pellet samples obtained by heat-treatments of

(BT/HTO)-Bi2O3-Na2CO3 mixture at different temperatures measured at 100 Hz.

Fig. 2.11 (a) Variations of remanent polarization and relative permittivity measured at 1 k Hz of

frequency for pellet samples obtained by heat-treatments of (BT/HTO)-Bi2O3-Na2CO3 mixture

at different temperature for 3 h. (b) Nanostructural models of the heteroepitaxial BT/BNT

interface at diferent fabrication heating temperatures.

76

The ferroelectric behavior of the mesocrystalline BT/BNT nanocomposites prepared

at different heating temperatures can be explained by the variation of the nanostructure

at BT/BNT interface in the nanocomposite, as shown in Fig. 2.11(b), based on the TEM

results at the interface (Fig. 2.12). The enhancement of the Pr value from 600 to 700 oC

is attributed to formation of mesocrystalline BT/BNT nanocomposite from

BT/Bi12TiO20 nanocomposite, which generates the heteroepitaxial BT/BNT interface

(Fig. 2.12). Around the heteroepitaxial BT/BNT interface, the BT lattice shrinks and

BNT lattice expands because the lattice constant of BT is slightly larger than that of

BNT with a lattice mismatching of 2.6 % (Table 2.1), which introduces a lattice

distortion around the BT/BNT interface (Fig. 2.12(b)), therefore, the pseudo-cubic

lattices of BT and BNT nanocrystals with paraelectric or weak ferroelectric responses

are distorted to ferroelectric tetragonal and rhombohedral lattices by increasing or

reducing the lattice constants in the direction paralleled to the interface, respectively.

The direction of ferroelectric spontaneous polarization around the interface become

unstable and very sensitive to the applied-bias, which generates an enlarged ferroelectric

response.15, 23, 28

77

Table 2.1. Piezoelectric constants of ferroelectric nanocomposites and single phase of

nanostructured materials, and lattice mismatching in nanocomposites.

Nanocomposite Nanostructured single phase

Material * d*

33

(pm/V)

Lattice

mismatching

(%)

Compound d*

33

(pm/V)

Polarization

direction

BT/ST-C 20

BT/ST-M 15

BT/BNT-M **

BT/KN-S 12

BT/CT-M 18

59

306

408

136

208

2.2

2.2

2.6

0.6

4.3

BaTiO3 (BT) 51

SrTiO3 (ST)

CaTiO3 (CT) 18

Bi0.5Na0.5TiO3 (BNT) 53

KNbO3 (KN) 54

28.0

-

40.9

18.0

19.5

[001]

-

[110]

[111]

[110]

* -C, -M, and -S represent nanocomposites prepared by nanocubes, mesocrystals, and surface coating,

respectively.

** Result of present study.

78

Fig. 2.12 (a, d, g) TEM images and (b, e, h) HRTEM images of BT/BNT-700, BT/BNT-800, and

BT/BNT-1000, respectively. (b, e, h) HRTEM images are an enlarged image derived from white

pane in (a, d, g) TEM images, respectively, and (c, f, i) FFT pattern obtained from the whole

region of HRTEM (b, e, h), respectively.

With further elevating the heating temperature to 800 oC, BBNT solid solution phase

is formed at the BT/BNT interface by diffusing Ba2+ ions of BT phase to BNT phase

and Na+, Bi3+ ions of BNT phase to BT phase, then a BT/BBNT/BNT interface is

formed (Fig. 2.12(e, f)). The lattice distorton effect of the BT/BBNT/BNT interface on

the ferroelectric response is less than that of the BT/BNT interface because the

differences of the lattice constants of BT and BBNT, and BBNT and BNT are less than

that of BT and BNT. The diminution of the lattice distortion causes the dropping of

ferroelectric response. The BT/Bi12TiO20 nanocomposite sample prepared at 600 °C

shows a weak ferroelectric response (Fig. 2.10), which may be ascribed to the formation

of heteroepitaxial interface between Bi12TiO20 and BT nanocrystals, which causes the

lattice distortion in the pseudo-cubic lattice of the BT nanocrystals.

In comparison with the mesocrystalline nanocomposite sample of BT/BNT-700, the

BT single phase mesocrystal sample of BT-700 and the BNT single phase mesocrystal

sample of BNT-700 exhibit a much weaker ferroelectric response (Fig. 2.13(a)) because

BT-700 and BNT-700 mesocrystals are constructed from small BT or BNT nanocrystals

with a size of 60 or 50 nm (Fig. 2.14), where without lattice distortion at nanocrystals

interface. It is well know that the BT and BNT prepared by the low temperature process

have the pseudo-cubic structure due to their small crystal sizes or low crystallinities.15,

50-52 These pseudo-cubic lattices of BT and BNT nanocrystals exhibit the paraelectric or

79

weak ferroelectric responses. The BT and BNT nanocrystals in BT-700 and BNT-700

mesocrystals also have the pseudo-cubic structure and exhibit the weak ferroelectric

response. To further confirm the effect of the heteroepitaxial interface of BT/BNT

nanocomposite on ferroelectric response, the ferroelectric BT-1250 and BNT-1050

samples were also prepared. The BT-1250 and BNT-1050 exhibit ferroelectirc responses

(Fig. 2.13(b)). However, the BT/BNT-700 sample shows an amazing ferroelectirc

response compared to the ferroelectirc BT-1250 and BNT-1050. These results suggest

that the the lattice distortion at BT/BNT interface is highly useful for strengthening

ferroelectric response in the BT/BNT nanocomposite.

Fig. 2.13 P-E hysteresis loops of the pellet samples of (a) BT-700, BNT-700 and BT/BNT-700

as well as (b) BT/BNT-700, BNT-1050 and BT-1250 measured at 100 Hz.

80

Fig. 2.14. (a, c) TEM images and (b, d) SAED spots patterns of (a, b) BT single phase

mesocrystal sample BT-700 and (c, d) BNT single phase mesocrystal sample BNT-700.

The piezoelectric response of the mesocrystalline nanocomposite BT/BNT-700 that

shows the largest ferroelectric response was investigated by using piezoresponse force

microscopy (PFM), and compared with the single phase mesocrystals of BT-700 and

BNT-700 prepared at same temperature. The Displacement-applied voltage (D-V) loops

and d*33-applied voltage (d*

33-V) loops are presented in Fig. 2.15, where converse

piezoelectric constant (d*33) is calculated from D/V value of the D-V loop. It is noted

that the d*33 value of 408 pm/V at 10 V of applied voltage for the BT/BNT-700

nanocomposite is much larger than those of 60 and 50 pm/V for BT-700 and BNT-700

mesocrystals, respectively. The d*33 value of BT/BNT-700 nanocomposite is one order

of magnitude larger than d*33 values of a nanostructured BaTiO3 (28 pm/V)51 and a

81

highly oriented BNT thin film (25 pm/V) fabricated by PLD method, 52 and even higher

than that of a high performance oriented BBNT ceramic (332 pC/N).14 This remarkably

enhanced piezoelectric response can be attributed to the introduced lattice distortion at

the heteroepitaxial BT/BNT interface, which makes the polarization rotation sensitive.22,

23

Fig. 2.15. Displacement-applied voltage loops and d*33-applied voltage loops for (a) BT-700, (b)

BNT-700, and (c) BT/BNT-700 mesocrystalline samples.

82

To impove piezoelectric responses of lead-free piezoelectric materials, some

challenges on the enhanced piezoelectric response using the lattice distortion at

heteroepitaxial interfaces of nanostructured nanocomposites have been reported, and

some d*33 values are summarized in Table 2.1 for the comparison. Most studies have

focused on the BaTiO3/SrTiO3 (BT/ST) heteroepitaxial interface because Harigai et al.

have reported a giant dielectric response of BT/ST heteroepitaxially stacked thin film.27

We think except the nanostructure of the heteroepitaxial interface, the combinations of

the nanocomposite for build-up of heteroepitaxial interface, such as the combinations of

ferroelectric/paraelectric phases and ferroelectric/ferroelectric phases, the combinations

between tetragonal, cubic, rhombohedral, orthorhombic crystal systems, and their lattice

mismatchings at the heteroepitaxial interface, will also affect piezoelectric response.

In the BT/ST combination nanocomposite system, Mimura et al. have reported a

BT/ST nanocomposite (BT/ST-C in Table 2.1) constructed by self-assembling

nanocubes of BT and ST, and it only gives a d*33 value of 59 pm/V.20 The largest d33

value of 306 pm/V in the BT/ST system have been achieved using the mesocrystalline

BT/ST nanocomposite (BT/ST-M in Table 1) by our group.15 The larger piezoelectric

response of mesocrystalline BT/ST nanocomposite can be attributed to the formation of

a perfect and higher density BT/ST heteroepitaxial interface than those in other cases.

Therefore, it can be concluded that the nanostructure of the mesocrystalline

nanocomposite is advantageous for the enhancement of piezoelectric response by the

lattice strain engineering.

In the mesocrystalline nanocomposites, the piezoelectric response increases in a order

of BT/CT-M (208 pm/V) < BT/ST-M (306 pm/V) < BT/BNT-M (408 pm/V) (Table 1).

83

In this case, the difference of the piezoelectric responses can be attributed to the

combinations of the heteroepitaxial interfaces. The BT, CT, ST, and BNT are tetragonal

ferroelectric, orthorhombic ferroelectric, cubic paraelectric, and rhombohedral

ferroelectric phases, respectively, at room temperature. Therefore, the BT/CT, BT/ST,

and BT/BNT heteroepitaxial interfaces can be assigned to Tetra-Ferro/Orth-Ferro

combination with a lattice missmatching of 4.3 %, Tetra-Ferro/Cub-Para combination

with a lattice missmatching of 2.2 %, Tetra-Ferro/Rho-Ferro combination with a lattice

missmatching of 2.6 %, respectively. The BT/ST combination exhibits a larger

piezoelectric response than that of BT/CT, which may be ascribed to that the lattice

mismatching of the BT/CT is too large for the formation of a stable heteroepitaxial

interface in the nanocomposite. We think the much larger piezoelectric response of

BT/BNT combination than that of BT/ST combination can be attributed to the

optimized lattice mismatching of about 2.6 % and the Tetra-Ferro/Rho-Ferro interface

combination. It is well known that PZT exhibits excellent piezoelectric performance at

the morphotropic phase boundary (MPB), in which a Tetra-Ferro/Rho-Ferro interface is

formed also.4 The combination of the polarization directions at the

Tetra-Ferro/Rho-Ferro interface, namely [001]/[111], is important to obtain a large

piezoelectric response. Therefore, we think BT/BNT interface is one of optimized

combination for the enhanced piezoelectric performance.

Wada et al. have reported a BT/KNbO3 (BT/KN-S in Table 1) nanocomposite with a

d33 value of 136 pC/N, although the nanocomposite is fabricated by a simple method,

namely, coating BT grain surface in a porous ceramic with KN layer.12 The BT/KN-S

nanocomposite exhibits a larger d33 value than that BT/ST nanocomposites, except the

mesocrystalline BT/ST nanocomposite, even with its small lattice mismatching of 0.6 %.

84

It hints that the Tetra-Ferro/Orth-Ferro ([001]/[110]) combination may be also a

promising system for the enhanced piezoelectric performance if the lattice mismatching

is appropriate.

2.3.5 Dielectric responses of mesocrystalline BT/BNT nanocomposite

The relative permittivities (εr) of the mesocrystalline BT/BNT nanocomposite pellet

samples prepared at different temperatures were measured using the LCR meter in a

frenquency range of 1 kHz to 2 MHz. The samples prepared above 700 oC with the

ferroelectric-like behavior present large frenquency dependences in the low frenquency

range (Fig. 2.16). The BT/BNT-700 and -800 samples exhibit much larger εr values than

those of mesocrystalline BT-700 and BNT-700 single phases in the frenquency range

measured, because of the enhancing effect of the mesocrystalline nanocomposite on the

dielectric response. The enhancing effect on the dielectric response is strongly

dependent on fabrication heating temperature of the BT/BNT nanocomposite, as shown

in Fig. 2.11(a). With increasing the heating temperature, εr value increases, reaches the

maximum at 700 C, and then gradually decreases above 700 C. This behavior fits well

with the ferroelectric Pr behavior, and can be explained by enhanced ferroelectricity due

to the lattice distortion around the heteroepitaxial BT/BNT interface, where the

polarization directions are tilted gradually with enlarging the lattice distortion, as shown

in Fiure 7(b), which makes the polarization rotations become sensitive.22, 23 With further

increasing the heating temperature above 800 oC, the gradual transformation of the

BT/BNT nanocomposite into the BBNT solid solution at the interface causes a less

lattice distortion, thereby degrading the dielectric response until the complete formation

85

to the BBNT solid solution, similar to the behavior of the ferroelectric response. These

results indicate that the εr value can be enhanced by introducing the heteroepitaxial

BT/BNT interface.

Fig. 2.16. Variations of relative permittivities for the pellet samples prepared by heat-treatment

of (BT/HTO)-Bi2O3-Na2CO3 mixture at different temperatures for 3 h, and BT and BNT pellet

samples prepared by heat-treatments at 700 oC for 3 h, respectively.

86

Fig. 2.17. Temperature dependences of the relativie permittivity (εr) of BT/BNT-700,

BT/BNT-800 and BT-BNT-900 pellet samples at 10 kHz.

The temperature dependences of the dielectric response of the mesocrystalline

BT/BNT nanocomposites prepared at different temperatures were also measured and

compared with those of BT and BNT single phases. The BT-1250 and BNT-1050

samples prepared at 1250 and 1050 °C exhibit a maximum εr value at around 130 and

320 °C (Fig. 2.18(a)), as like their normal bulk crystals, respectively.55-57 These

temperatures correspond to their Curie temperatures (Tc) or phase transitions from

ferroelectric phase to paraelectric phase. However, no obvious phase transitions can be

observed for the both mesocrystalline BT-700 and BNT-700 single phase samples (Fig.

2.18(b)). These samples exhibit the paraelectric or weak ferroelectric responses because

their small crystal sizes of about 60 and 50 nm, respectively (Fig. 2.14), as mentioned

above.

87

Fig. 2.18. Temperature dependences of the relativie permittivity (εr) of (a) BT-1300 and

BNT-1050, and (b) BT-700, BNT-700, and BT/BNT-700 pellet samples at 10 kHz.

Fig. 2.17 exhibits the temperature dependences of the relative permittivities for the

BT/BNT nanocomposites prepared in the temperature range of 700 to 900 oC.

Surprisingly, the BT/BNT-700 nanocomposite exhibits three εr peaks at around 160, 270,

and 380 °C, respectively. The εr peaks at around 160 and 380 °C can be assigned to the

Tc of BT and BNT phases in the nanocomposite, respectively, although they shift to

higher temperatures. Such high-temperature shifts of Tc have been observed by

introducing the lattice strain originated from the heteroepitaxial interface, since the

lattice strain causes the lattice mismatches for BT and BNT, in which restrain the

transformation of the tetragonal to cubic for BT and the rhombohedral to cubic for BNT

with elevated heating temperature, respectively, namely the increasing of the Curie

88

temperature. Furthermore, Haeni et al. have demonstrated hundreds of degrees

increasing in the Tc of ST by introducing the epitaxial lattice strain, resulting in a

room-temperature ferroelectric ST thin film.10 Subsequently, Choi et al. have presented

an elevated Tc for BT thin film with the same method.9 Suzuki et al. have reported a 3D

compressive stress induced mesostructured BT/ST composite film with largely elevated

Tc by using a surfactant-substrate sol-gel method.19

It is notable that the third phase transition at around 270 °C was observed for the first

time in the mesostructured nanocomposite. We think this third phase transition can be

assigned to BT/BNT interface or a distorted BBNT phase between BT and BNT phases

as shown in Fig. 2.11(b) and 2.12(b). Nevertheless, with further increasing fabrication

heating temperature of the BT/BNT nanocomposite, Tc at 270 oC for the third phase

disappears above 900 oC, in fact, as the solid solution process proceeds with the

increasing temperature, the peak of the BBNT solid solution phase peak should appear

but not disappear according to the XRD result in Fig. 2.5, therefore, we speculate the

peak at around 270 °C for BT/BNT-700 sample should be the phase of the BT/BNT

heteroepitaxial interface. Also, the high density of BT/BNT interface in the

mesocrytstalline nanocomposite may be the reason why a distinct third phase is

observed. In addition, the low-temperature shifts and broadings of Tc for the BT and

BNT phases were also observed, this is due to the phase transformation of the BT/BNT

nanocomposite to BBNT solid solution, in which the lattice mismatches between BT

and BBNT or BBNT and BNT become smaller (Fig. 2.14(b)). This behavior

corresponds to the transformation of BT and BNT phases to BBNT solid solution at

high temperature, as shown in Fig. 2.5.

89

2.4 Conclusion

A two-step topochemical process is an effective approach for the synthesis of the

mesocrystalline BT/BNT nanocomposite constructed from well-aligned BT and BNT

nanocrystals with the same crystal-axis orientation. The formation of the

mesocrystalline nanocomposite is attributed to the topochemical transformation

reactions from HTO to BT and HTO to BNT in the two-step process. The BT/BNT

nanocomposite exhibits enlarged ferroelectric, piezoelectric and dielectric responses by

introducing the lattice strain at BT/BNT heteroepitaxial interface. The giant

piezoelectric response suggests the BT/BNT heteroepitaxial interface is one of

optimized combination for the enhanced piezoelectric performance by using the lattice

strain engineering. The introduced lattice strain causes transitions of paraelectric BT and

BNT nanocrystals to the ferroelectric nanocrystals in the mesocrystalline

nanocomposite, and the elevated Tc for BT and BNT, which can expand application

temperature range of this promising lead-free piezoelectric material.

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Chapter Ⅲ

Ferroelectric Mesocrystalline BaTiO3/BaBi4Ti4O15

Nanocomposite: Formation Mechanism, Nanostructure, and

Anomalous Ferroelectric Response

3.1 Introduction

Ferroelectric materials with coupled mechanical-electrical and thermal-electrical

responses have drawn much attentions due to the wide applications to sensors, actuators

and energy harvesters.1-3 Although the Pb(Zr1−xTix)O3 (PZT) is an excellent and widely

employed ferroelectric material, it contains more than 60 % toxic lead component,

therefore, the replacement of the PZTs with lead-free alternatives has become quite

significant for the environmental protection.4-6 However, the reported lead-free

materials exhibit quit lower piezoelectric performance than that PZTs.4 Two kinds of

effective approaches, i.e. domain engineering and oriented engineering, have been

widely used to improve the piezoelectric performance for the ferroelectric materials.

The domain engineering is to reduce the domain size of the ferroelectric materials,

while the oriented engineering is to optimize their orientation direction.5, 7-10 The

piezoelectric response of BaTiO3 (BT) can be improved from 200 to 788 pC/N using the

combined domain and oriented engineering, the value is larger than that of 500 pC/N for

the commercially available PZTs.11 However, the piezoelectric application of BT is

restrained in a temperature range of below 130 °C of its Curie temperature (Tc).

95

Therefore, the elevated Tc is also important for the development of high performance

lead-free ferroelectric materials.12

Another approach is the lattice strain engineering by applying an lattice strain at the

heteroepitaxial interface constructed using two kinds of crystals with little different

lattice parameters, which can improve some specific properties of the ferroelectric

materials, including the Tc, piezoelectric and dielectric responses.13-21 The

improvements of the Tc have been achieved by using the lattice strain engineering, but

mainly limited in the thin film or low dimensional quantum systems.14, 22, 23 Choi et al.

have presented a (001)-oriented BT thin film epitaxially grown on a (110) DyScO3

substrate with little different lattice parameters, which results in a large Tc improvement

of BT from 130 to 540 °C.14 It has reported that a mesostructured BT/SrTiO3 (BT/ST)

composite film exhibits an elevated Tc of 230 °C for BT but without obvious

improvement for the piezoelectric response.23 An astonishing dielectric response has

been achieved by construction of an artificial BT/ST superlattice using a molecular

beam epitaxy process.15 Furthermore, the improvement for the piezoelectric response

has been achieved also using BT/KNbO3 (BT/KN) heteroepitaxial interface but without

increase of Tc.24 As far as we know, the improvement has been limited in the individual

effect, such as Tc, piezoelectric response or dielectric response, however the

multi-improvement effect has not been reported yet by using the lattice strain

engineering.

Mesocrystal is a polycrystal constructed from the nanocrystals with the same

crystal-axis orientation.25, 26 The mesocrystals not only have some potential properties

based on the individual nanocrystals but also exhibit unique collective properties of

nanocrystal ensembles.27 It has become a fascinating research area as a new class of

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material for catalysis, sensing, and energy storage and conversion in the past

decade.27-31 Until recently, we have found that the mesocrystalline nanocomposite is a

promising material for the strain engineering to improve the piezoelectric response

because it has high density of the heteroepitaxial interface, and the piezoelectric

responses of the mesocrystalline nanocomposites of BT/ST and BT/CaTiO3 (BT/CT)

can be improved greatly by the introduction of the lattice strain engineering.32, 33

Furthermore, a mesocrystalline BT/Bi0.5Na0.5TiO3 (BT/BNT) nanocomposite exhibits an

anomalous ferroelectric response with a very large piezoelectric response of 408 pm/V

and an elevated Tc effect.34 However, these mesocrystalline nanocomposites are

constructed from two kinds of materials with the same perovskite structure, which are

easily transformed to their solid solution at high temperature, namely disappearances of

the heteroepitaxial interface and the lattice strain effect on the ferroelectric response.32,

34 Therefore, the search for a mesocrystalline nanocomposite that can endure the high

temperature is essential for the practical application of the mesocrystalline

nanocomposites.

Herein, we describe a new challenge on a mesocrystalline BaTiO3/BaBi4Ti4O15

(BT/BBT) nanocomposite constructed from two kinds of nanocrystals with different

crystal structures. BaBi4Ti4O15 (BBT) is a bismuth-layered (Aurivillius) ferroelectric

constructed by stacking the bismuth oxide layers ([Bi2O2]2+) and pseudo-perovskite

layers ([BaBi2Ti4O13]2-), which is different from the normal perovskite structure of BT.35

BBT has been studied as temperature-stable ferro-piezoelectrics with a relatively high

Tc of around 400 °C. 36, 37 The high Curie temperature for BBT and large piezoelectric

response for BT are expected for a simultaneous improvement of the Tc and the

piezoelectric response of the mesocrystalline BT/BBT nanocomposite. Furthermore, the

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different structures of BBT and BT are expected to inhibit formation of their solid

solution at high temperature. The BT/BBT nanocomposite constructed from

well-aligned BT and BBT nanocrystals with the crystal-axis orientation along the [110]

and [11-1] respectively was synthesized using a facile two-step topochemical process. A

highly elevated Tc of BBT from 400 to 700 °C and an enhanced piezoelectric response

of d*33 = 130 pm/V were achieved by successfully introducing BT/BBT heteroepitaxial

interface into the BT/BBT nanocomposite. In addition, the mesocrystalline BT/BBT

nanocomposite can be further made into the bulk ceramic without the formation of the

solid solution at high temperature, which will open the gate to the fabrication of the

mesocrystalline ceramics for the high-performance ferroelectric materials using the

lattice strain engineering.

3.2 Experimental

3.2.1 Sample Preparation

H1.07Ti1.73O4·H2O (HTO) powder sample was prepared from K0.80Ti1.73Li0.27O4

(KTLO) as reported in our previous study.38 For the synthesis of the platelike particle

sample of mesocrystalline BaTiO3/BaBi4Ti4O15 (BT/BBT) nanocomposite, a facile

two-step reaction process was used. In the first step, the platelike HTO crystals (0.4 g)

and Ba(OH)2·8H2O (mole ratios of Ba/Ti = 0.25, 0.5 and 0.75) were hydrothermally

treated in 30 mL distilled water under the stirring conditions at 150 C for 12 h. After

the hydrothermal treatment, the obtained sample was washed with distilled water and

dried at room temperature to obtain mesocrystalline BaTiO3/HTO (BT/HTO)

nanocomposites. The obtained BT/HTO samples are designated as BT/HTO-X, where

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X represents for the Ba/Ti mole ratio in the reaction system. In the second step, the

mesocrystalline BT/HTO-X sample (0.8 g) was mixed with stoichiometric Bi2O3 in

ethanol solvent by ball-milling with a speed of 50 r/min for 12 h at room temperature,

and then the mixture was dried at 60 C for 6 h. Finally, the mixed powders were heated

at a desired temperature for 3 h to obtain mesocrystalline BT/BBT nanocomposites. The

samples obtained by heat-treatment of BT/HTO-X (X = 0.25, 0.5 or 0.75) are

designated as BT/BBT-X-Y, where Y represents for the heating temperature.

Pellet samples of BT/HTO-Bi2O3 mixture was fabricated by pressing a

BT/HTO-Bi2O3 mixture powder sample using a pellet press mold with a diameter of 10

mm at 30 M Pa for 3 min. Subsequently, the cold isostatic press (CIP) was employed

with a pressure of 200 M Pa. The obtained pellet sample was heated at desired

temperatures for 3 h to obtain a BT/BBT pellet sample. A BT mesocrystal powder

sample was prepared by hydrothermal treatment of HTO (0.4 g) in a Ba(OH)2 solution

(mole ratio of Ba/Ti = 1.2) at 200 oC for 12 h. The BT pellet sample was fabricated

using the BT mesocrystal powder sample by analogous manner as BT/BBT pellet

sample at a desired temperature for 3 h. The obtained pellet samples were polished with

diamond slurry, and cut using a crystal cutter to sizes of 4×4×0.5 mm3. The silver paste

was screen-printed on the top and bottom surfaces of the pellet sample with an area of

4×4 mm2 and heated at 500 oC for 30 min.

3.2.2 Physical Properties Analysis

The structures of powder samples were investigated using a powder X-ray

diffractometer (Shimadzu, XRD-6100) with Cu Kα (λ = 0.15418 nm) radiation. The

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particle size and morphology of the samples were observed using field emission

scanning electron microscopy (FE-SEM) (Hitachi, S-900). Transmission electron

microscopy (TEM) observation and selected-area electron diffraction (SAED) were

performed on a JEOL Model JEM-3010 system at 300 kV, and the powder sample was

supported on a Cu microgrid. Raman spectra were detected by a JASCO NRS-3100

Raman spectrometer with a scanning step of 1 cm−1 at an excitation wavelength of 532

nm.

Piezoelectric response of the platelike mesocrystal particle was detected by using a

scanning probe microscopy system (SPM) (SPA-400/Nano Navi Station, SII,

NanoTechnology Inc.) combining atomic force microscopy (AFM) and piezoresponse

force microscopy (PFM). The platelike particles dispersed on an Au-coated silicon

substrate (40 nm thickness of Au-film), and an individual platelike particle dispersed on

the Si substrate surface was scanned using the AFM probe tip with a conductive

Rh-coated Si cantilever probe (SI-DF3-R, spring constant: 1.2 N/m) in the contact mode.

The generated strain of the platelike particle was detected using Z/V mode in AFM

system after a DC bias from -10 V to 10 V was employed on the surface of the platelike

particle. And the converse piezoelectric constant d*33 can be calculated by equation (1).

d*33 = D/Va (1)

where D is the displacement (pm), Va is the applied bias (V).27, 32, 39, 40

The polarization-electric field (P-E) loop of the pellet sample was measured using a

ferroelectric testing system (Toyo Corporation, FCE3-4KVSYS) at room temperature.

The dielectric response of the pellet sample was measured using an LCR meter (Agilent

E4980A) in a frequency range of 1 to 2 M Hz. For the measurement of

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temperature-dependent dielectric response, the pellet sample was heated in a

temperature controlled chamber with a heating rate of 3 C/min from room temperature

to 700 C during the measurement.

3.3 Results and discussion

3.3.1 Synthesis of BT/BBT nanocomposites and BBT mesocrystals

Fig. 3.1 XRD patterns of (a) H1.07Ti1.73O4·H2O (HTO) and BT/HTO samples obtained by

hydrothermal treatment of HTO-Ba(OH)2 mixture with Ba/Ti mole ratios of (b) 0.25, (c) 0.5 and

(d) 0.75 at 150 °C for 12 h, respectively.

The facile two-step process was employed to synthesize the mesocrystalline

BaTiO3/BaBi4Ti4O15 (BT/BBT) nanocomposites. In the first step, the layered titanate

H1.07Ti1.73O4·H2O (HTO) with platelike particle morphology was hydrothermally treated

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with Ba(OH)2 at 150 °C for 12 h. The formation of the BT/HTO nanocomposites can be

achieved when the Ba/Ti mole ratios in the reaction system are 0.5 and 0.75, as shown

in Fig. 3.1, where the formed BT is a cubic perovskite phase (JCPDS File No.74-1964).

However, the sample obtained at mole ratio of Ba/Ti = 0.25 exhibits the layered titanate

structure and without the BT phase is observed, which will be discuss later. Based on

the results of our former research, we conclude that the platelike HTO crystals are

partially transformed into the BT by a topochemical reaction, which results in the

formation of the mesocrystalline BT/HTO nanocomposite at Ba/Ti mole ratios of 0.5

and 0.75,32-34 namely the BT/HTO-0.5 and BT/HTO-0.75 samples are mesocrystalline

BT/HTO nanocomposites. The formation of the platelike mesocrystalline BT/HTO

nanocomposite can be confirmed also from their TEM results, as shown in Fig. 3.2.

Fig. 3.2 (a, c, e, g) TEM images and (b, d, f, h) SAED patterns of (a, b) HTO, (c, d)

BT/HTO-0.25, (e, f) BT/HTO-0.5 and (g, h) BT/HTO-0.75.

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Fig. 3.3 XRD patterns of sample obtained by heat-treatment of (a) (BT/HTO-0.5)-Bi2O3 mixture

at (b) 500, (c) 600, (d) 700, (e) 800, (f) 900, (g) 1000 and (h) 1100 °C for 3 h, respectively.

In the second step, the BT/HTO nanocomposite was mixed with stoichiometric Bi2O3

and calcined at different temperatures for 3 h, where the XRD results of the

BT/BBT-0.5-Y samples prepared from BT/HTO-0.5 are shown in Fig. 3.3. With

increasing the heating temperature, the BT phase and a new phase of Bi12TiO20 were

observed when the heating temperature reached 600 °C, and the blended phases of BT,

Bi12TiO20 and BaBi4Ti4O15 (BBT) (JCPDS File No.73-2184, tetragonal system) were

observed simultaneously at 700 °C. And then the Bi12TiO20 phase gradually disappeared

and was transformed completely into the BBT phase at 800 °C, resulting in the

formation of a mixture of BT and BBT phases. Judging from the FE-SEM results in Fig.

3.4, the BT/BBT-0.5-Y samples exhibit the platelike particle morphology constructed

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from the nanocrystals with uniform crystal size below 900 °C, namely the platelike

particle morphology of BT/HTO is retained. With further elevating the heating

temperature, the nanocrystals constructed the platelike particle continue to grow up and

finally lose the platelike morphology over 1000 °C.

Fig. 3.4 FE-SEM images of the samples obtained by the heat-treatment of the

(BT/HTO-0.5)-Bi2O3 mixture at different temperatures for 3 h.

The mixed BT and BBT phases were observed in the heating temperature range from

800 to 1100 °C (Fig. 3.3). In our previous researches, though the mesocrystalline

BT/Bi0.5Na0.5TiO3, BT/SrTiO3 and BT/CaTiO3 nanocomposites can be successfully

obtained, these perovskite mesocrystalline nanocomposites are transformed to their

solid solution perovskites, respectively, at high temperature because they have the same

perovskite structure, which causes the degradation in ferroelectric and piezoelectric

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responses.32-34 In the present study, BBT owns a layered perovskite structure

constructed by stacking the bismuth oxide layers ([Bi2O2]2+) and pseudo-perovskite

layers ([BaBi2Ti4O13]2-),35 which is different from the perovskite structure of the BT

phase. Therefore, BBT and BT cannot react to forms their solid solution phase, which

reduces the reactivity between BBT and BT nanocrystals in the mesocrystalline

BT/BBT nanocomposite. We think this is the reason why the nanostructure of the

mesocrystalline BT/BBT nanocomposite can be retained even at the high temperature of

1100 °C, which is different from the mesocrystalline BT/Bi0.5Na0.5TiO3, BT/SrTiO3 and

BT/CaTiO3 nanocomposites where solid solution phases are formed respectively at the

high temperature.32-34

Fig. 3.5 XRD patterns of samples obtained by heat-treatments of (a) BT mesocrystal at 800 °C

for 3 h and (BT/HTO-0.75)-Bi2O3 mixture at (b) 600, (c) 700, (d) 800 (e) 900 and (f) 1100 °C

for 3 h.

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Fig. 3.6 (a, c) TEM images and (b, d) SAED spot patterns of samples obtained by

heat-treatment of (a, b) the BT mesocrystal and (c, d) (BT/HTO-0.75)-Bi2O3 mixture at 800 °C

for 3 h, respectively.

When the (BT/HTO-0.75)-Bi2O3 mixture was heat-treated, the BT/BBT

nanocomposites can be obtained also in the temperature range of 700 to 1100 °C (Fig.

3.5), similar to the case of (BT/HTO-0.5)-Bi2O3 mixture except higher BT content in the

BT/BBT nanocomposite. The sample keeps the platelike particle morphology of

BT/HTO-0.75 after transformation to the BT/BBT nanocomposite by heat-treatment at

800 °C (Fig. 3.6(c)). However, a single BBT phase was obtained when the

(BT/HTO-0.25)-Bi2O3 mixture was heat-treated in a temperature range of 800 to

1100 °C (Fig. 3.7). The formation of the Bi12TiO20 intermediate phase can be detected

also at 600 °C. The BBT sample obtained at 800 °C keeps the platelike particle

morphology of the BT/HTO-0.25 precursor, and loses the platelike particle morphology

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at above 900 °C (Fig. 3.8), similar to the case of (BT/HTO-0.5)-Bi2O3 reaction system

(Fig. 3.4).

Fig. 3.7 XRD patterns of samples obtained by heat-treatment of (a) (BT/HTO-0.25)-Bi2O3 mixture at

(b) 500, (c) 600, (d) 700, (e) 800, (f) 900 and (g) 1100 °C for 3 h, respectively.

Fig. 3.8 FE-SEM images of samples obtained by heat-treatment of (BT/HTO-0.25)-Bi2O3

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mixture at different temperatures for 3 h.

3.3.2 Nanostructural analysis for BT/BBT nanocomposite

To figure out the phase transition reactions in the formation process of the BT/BBT

nanocomposite, a nanostructural study was carried out on the products in the synthesis

process of the BT/BBT nanocomposites by employing TEM observation. Fig. 3.9

presents the TEM images, HRTEM images, and SAED patterns of the

BT/BBT-0.25-800 (BBT-800), BT/BBT-0.5-700, and -800 samples. The BBT and

BT/BBT samples inherited the platelike morphology from the HTO precursor.34 It is

noted that the platelike BT/BBT-0.25-800 particle is a BBT polycrystal, but exhibits a

single-crystal like SAED spot pattern with a [11-1] crystal-axis orientation direction

(Fig. 3.9(b)), namely the formation of the platelike BBT mesocrystal by a topochemical

mechanism from the layered HTO precursor. Such platelike mesocrystals have the

potential applications in the fabrications of oriented ceramics.9, 41-43

Fig. 3.9 (a, c, g) TEM images and (b, d, h) SAED patterns of (a, b) BT/BBT-0.25-800, (c, d)

BT/BBT-0.5-700, and (g, h) BT/BBT-0.5-800 samples. (e, f) HRTEM images obtained from (c)

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and (e) of BT/BBT-0.5-700 sample, respectively.

Three sets of SAED spots corresponding to BT, BBT and Bi12TiO20 phases are

observed simultaneously in one platelike particle of BT/BBT-0.5-700 (Fig. 3.9(d)),

which corresponds to the XRD result in Fig. 3.3. And the BT and BBT phases in the

nanocomposite particle show the crystal-axis orientation along the [110] and [11-1]

directions, respectively. The HRTEM image shows lattice fringes of BT, BBT and the

intermediate phase of Bi12TiO20 (Fig. 3.9(f)), revealing that the platelike particle is

constructed from BT, BBT and Bi12TiO20 nanocrystals. The interface between the BT

and BBT nanocrystals, namely the area between two orange lines marked in Fig. 3.9(f),

can be confirmed clearly in the HRTEM image. The platelike particle of

BT/BBT-0.5-800 exhibits simultaneously two sets of SAED spots corresponding to BT

and BBT phases, that is to say, the platelike particle is a mesocrystalline BT/BBT

nanocomposite. The BT and BBT nanocrystals in the platelike mesocrystalline

nanocomposite show the crystal-axis orientations along the [110] and [11-1] directions,

respectively, which is the same as in the BT/BBT-0.5-700. The result reveals that the

mesocrystalline BT/BBT nanocomposite is formed from the BT/HTO precursor by a

topochemical reaction.34

3.3.3 Formation mechanism of BT/BBT nanocomposite

Based on the results described above, we give a reaction mechanism for the formation

of BT/BBT nanocomposite by the two-step topochemical process, as shown in Fig. 3.10.

In the first step under hydrothermal conditions, the Ba2+ ions in the solution are

intercalated into the HTO bulk crystal by an ion-exchange reaction with H3O+ ions in

the interlayer spaces to form a Ba2+-form HTO that can be expressed with

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BaxH1.07-2xTi1.73O4. Furthermore, the Ba2+ ions in the interlayer spaces can react with the

TiO6 octahedra of the layered titanate to form BaTiO3 nanocrystals in the bulk crystal by

a topotactic structural transformation reaction.34 In the present study, the HTO precursor

is partially transformed into BT phase, and then BT/HTO nanocomposite is obtained at

the mole ratios of Ba/Ti=0.5 and 0.75. However, when the mole ratio of Ba/Ti is less

than 0.25, BT phase cannot be formed because the concentration of Ba2+ is too low in

the hydrothermal reaction system for the formation of the BT phase, where the

Ba2+-form layered titanate BaxH1.07-2xTi1.73O4 is formed.

In the second step, when (BT/HTO)-Bi2O3 mixture is heat-treated, firstly the Bi2O3

phase reacts with the HTO phase to form the intermediate Bi12TiO20 phase on the HTO

surface, and then a BT/HTO/Bi12TiO20 nanocomposite is formed as shown in Fig. 3.9.

Secondly, the Bi12TiO20 phase reacts with the Ba2+-form HTO (BaxH1.07-2xTi1.73O4) to

form BBT nanocrystals in the nanocomposite. Since all the reactions in the

transformation processes of the HTO precursor to the BT/BBT nanocomposite are

topochemical reactions, finally the mesocrystalline BT/BBT nanocomposite is formed.

In the mesocrystalline BT/BBT nanocomposite, the BT and the BBT nanocrystals show

the crystal-axis orientation in the [110]-zone and [11-1]-zone directions, respectively, as

shown in Fig. 3.9(h). Similarly when the mixture of Ba2+-form HTO and Bi2O3 is

heat-treated, the Ba2+-form HTO is transformed to the BBT mesocrystal with the

crystal-axis orientation in the [11-1]-zone direction by the same topochemical

mechanism.

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Fig. 3.10 Schematic representation of formation mechanism of mesocrystalline BT/BBT

nanocomposite by two-step topochemical reactions from HTO single crystal precursor.

The formation of a heteroepitaxial interface between the BT and BBT nanocrystals in

the mesocrystalline BT/BBT nanocomposite is expected similar to the cases of the

mesocrystalline BT/Bi0.5Na0.5TiO3, BT/SrTiO3 and BT/CaTiO3 nanocomposites in our

previous studies.32-34 The SAED result of the BT/BBT nanocomposite in Fig. 3.9(h)

reveals that the (00-1) facet of the BT nanocrystals faces to the (011) facet of the BBT

nanocrystals in the nanocomposite. The atoms arrangements on the (00-1) facet of BT

and the (011) facet of BBT are presented in Fig. 3.11. The four oxygens constitute a

quadrangular lattice with a side length of 4.001 Å on the (00-1) facet of BT, and a

rectangle lattice with a length of 3.864 Å and a width of 3.732 Å on the (011) facet of

BBT. This result suggests that it is possible to form a heteroepitaxial interface between

the (00-1) facet of BT and the (011) facet of BBT with the lattice mismatches of 3.4 and

6.7 % along the [0-11] and [100]-directions of the BBT lattice cell, respectively, as

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shown in Fig. 3.11(c). Hayashi et al. have reported a heteroepitaxial interface in the 2D

(Sr,Ba)TiO3/(Ca,Sr)TiO3 superlattice with an effective lattice mismatch of 2.5-5.5 %.44

Trampert has demonstrated a MnAs/GaAs heteroepitaxial system with a lattice

mismatch of 7.5 %.19 Although the lattice mismatch of 6.7 % along [100]-direction of

the BT/BBT interface is relatively large for the formation of the heteroepitaxial

interface, we think the BT/BBT heteroepitaxial interface is possible in the lattice

mismatch range of 3.4 to 6.7 %, especially for the nanocrystals because the lattice strain

is relatively easy to be mitigated at nanocrystals interface than that at large crystals

interface.

Fig. 3.11 Atoms arrangements on (a) (00-1) facet of BT and (b) (011) facet of BBT, and (c)

schematic presentation of lattice mismatches between (00-1) facet of BT and (011) facet of BBT

at BT/BBT heteroepitaxial interface.

Fig. 3.12 shows the Raman spectra of the mesocrystals of BT and BBT, and the

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BT/BBT nanocomposites. The bands at 303, 515 and 714 cm-1 in Fig. 3.12(a) can be

assigned to the characteristic bands of the BT phase, in which the sharp peak at 303

cm-1 illustrates the formation of the tetragonal BT phase for BT-800 mesocrystal, which

has a nanocrystal size of about 75 nm (Table 3.1).32, 45, 46 The characteristic bands of

BBT are observed at 280, 540 and 890 cm-1 in the spectrum of the BBT-800 mesocrystal

(Fig. 3.12(b)), 37, 47 which further confirms the formation of BBT by the heating

treatment of the (BT/HTO-0.25)-Bi2O3 mixture. In the spectrum of the BT/BBT-0.5-800

nanocomposite, the bands corresponding to the cubic BT phase at 515 and 714 cm-1 and

those corresponding to tetragonal BBT phase at 280, 540 and 890 cm-1 are confirmed

(Fig. 3.12(c)). It is noted that the tetragonal BT phase is formed in the BT-800

mesocrystal, whereas the cubic BT phase is formed in the BT/BBT-0.5-800

nanocomposite. The formation of the cubic BT phase in the BT/BBT-0.5-800 can be

ascribed to the smaller size of the BT nanocrystals (40 nm) in the nanocomposite than

that (75 nm) in the BT-800 mesocrystal (Table 3.1).48, 49

In addition, it is quite interesting that some new bands appear at 222, 248, 262, 355,

413, 676 and 802 cm-1, which cannot be assigned to the single BT and BBT phases. We

think these new bands may be ascribed to the introduction of lattice strain at the

BT/BBT heteroepitaxial interface. The spectrum of the BT/BBT-0.75-800

nanocomposite reveals that the tetragonal phases of BT and BBT are formed in the

BT/BBT nanocomposite (Fig. 3.12(d)). The formation of the tetragonal BT phase in the

BT/BBT-0.75-800 nanocomposite can be ascribed to relatively large size of the BT

nanocrystals (70 nm) in the BT/BBT-0.75-800 nanocomposite (Table 3.1), namely the

crystal size of BT increases with increasing its content in the nanocomposite.

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Fig. 3.12 Raman spectra of (a) BT mesocrystal heat-treated at 800 °C, (b) BBT mesocrystal of

BT/BBT-0.25-800 (BBT-800), BT/BBT nanocomposites of (c) BT/BBT-0.5-800 and (d)

BT/BBT-0.75-800.

3.3.4 Ferroelectric, dielectric and piezoelectric responses of BT/BBT

nanocomposite

To study the ferroelectric behavior of the BT/BBT nanocomposites, the P-E

hysteresis measurement was employed as shown in Fig. 3.13. It can be clearly seen that

the BT-1100 and BBT-1100 (BT/BBT-0.25-1100) samples exhibit the largest and the

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smallest remanent polarizations (Pr), respectively, in these samples. The larger

ferroelectric response of BT than that of BBT is consistent with the results of their

normal ceramic samples reported.14, 37 It is noteworthy that although BT/BBT-0.5-1100

has a lower BT content than BT/BBT-0.75-1100, the remanent polarization of

BT/BBT-0.5-1100 (Pr = 2.8 μC/cm2) is much larger than that of BT/BBT-0.75-1100 (Pr =

1.1 μC/cm2). The larger Pr value of BT/BBT-0.5-1100 than that of BT/BBT-0.75-1100

can be explained by higher density of the BT/BBT heteroepitaxial interface which can

cause a lattice strain around the interface and enhance the ferroelectric response.16, 32, 34

According to the SEM-EDS results, the BT/BBT mole ratios are about 2 in

BT/BBT-0.5-1100 and about 10 in BT/BBT-0.75-1100, respectively (Fig. 3.14). This

result indicates that the chemical composition of BT/BBT-0.5-1100 closes to the

optimum condition for the high density of the BT/BBT heteroepitaxial interface to

enhance the ferroelectric response by using the lattice strain engineering.

Fig. 3.13 P-E hysteresis loops of the BT-1100, BBT-1100, BT/BBT-0.5-1100 and

BT/BBT-0.75-1100 pellet samples.

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Fig. 3.14 SEM-EDS spectra for (a) BBT-1100, (b) BT/BBT-0.5-1100 and (c)

BT/BBT-0.75-1100 samples.

The relative permittivities (εr) measurement was also employed to understand the

influence of the BT/BBT heteroepitaxial interface on the dielectric behavior, as shown

in Fig. 3.15. The εr values of the BT/BBT-0.5 nanocomposite are improved greately

when the heating temeperature increases from 600 to 800 oC due to formation of the

BT/BBT nanocomposite, and then improved slowly from 800 to 1100 oC maybe due to

the enhancemences of dennsity and crystallinity (Fig. 3.16). The εr results for BT/BBT,

BT, BBT samples exhibit the same tendency as their P-E hysteresis results, in which εr

value increases in an order of BBT-1100 < BT/BBT-0.75-1100 < BT/BBT-0.5-1100 <

BT-1100. Generally, the BT-based composites exhibit the enhancing ferroelectric and

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dielectric responses with increasing BT contents,14, 37, 50 whereas the εr value of

BT/BBT-0.5-1100 is about 1.3 times larger than that of BT/BBT-0.75-1100, namely the

opposite result in the case of the BT/BBT nanocomposite because the higher density of

the BT/BBT heteroepitaxial interface in BT/BBT-0.5-1100 than that in the

BT/BBT-0.75-1100, which corresponds the Raman spectrum results (Fig. 3.12) and the

P-E hysteresis (Fig. 3.13) results.

Fig. 3.15 Variations of relative permittivities (εr) with frequency for BT-1100, BBT-1100,

BT/BBT-0.5-1100 and BT/BBT-0.75-1100 pellet samples.

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Fig. 3.16 Variations of relative permittivity (εr) with frequency for BT/BBT-0.5 samples

obtained at different temperatures.

Fig. 3.17 shows the temperature dependences of the relatively permittivity (εr) for the

BT-1100, BBT-1100, and BT/BBT-0.5-1100 pellet samples. The BT-1100 pellet sample

exhibits a phase transition peak around 130 °C that corresponds to the Curie temperature

(Tc) of the normal ferroelectric BT phase (Fig. 3.17(a)).49, 51 The BBT-1100 pellet

sample shows a phase transition peak around 400 °C that corresponds to the Tc of the

normal ferroelectric BBT phase (Fig. 3.17(b)).52 It is interesting that two phase

transition peaks are observed for the BT/BBT-0.5-1100 nanocomposite at around 40 and

635 °C, which can be assigned to the Tc of BT and BBT nanocrystals in the

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nanocomposite, respectively. We think the anomalous enhancement of the Tc for the

BBT phase can be attributed to the introduced lattice stain at the BT/BBT

heteroepitaxial interface due to their lattice mismatch.13, 14, 23, 34

Fig. 3.17 Temperature dependences of the relative permittivity (εr) for (a) BT-1100, (b)

BT/BBT-0.25-1100 (BBT-1100) and (c) BT/BBT-0.5-1100 nanocomposite at measurement

frequency of 10 kHz.

By considering the larger crystal lattice of BT phase than that of BBT phase (Fig.

3.11), at around the heteroepitaxial BT/BBT interface, BT lattice will bear an in-plane

compressive strain and an out-of-plane tensile strain, while BBT lattice will bear an

in-plane tensile strain and an out-of-plane compressive strain. Haeni et al. have firstly

demonstrated hundreds of degrees increasing in the Tc of SrTiO3 (ST) thin film

deposited on DyScO3 substrate by introducing the in-plane tensile strain with +1 %

119

lattice mismatch.13 A BaTiO3/Sm2O3 composite thick film has been reported with a

highly improved Tc of BT, where the BT lattice bears an in-plane tensile strain with

+2.35 % lattice mismatch at the BT/Sm2O3 interface.12 This result reveals that the

in-plane tensile strain can result in the elevated Tc of the ferroelectric phase. Therefore,

we think the elevated Tc of BBT phase in the BT/BBT nanocomposite can be assigned

to the introduced in-plane tensile strain.

However, it is very interesting that the Tc for BT in the BT/BBT-0.5-1100

nanocomposite was lowered from 130 to 40 °C. We think the decreased Tc of BT in the

BT/BNT nanocomposite could not be ascribed to the introduced in-plane compressive

strain. Choi et al. have presented an elevated Tc for BT thin film epitaxially grown on

the DyScO3 substrate where the BT film undergoes a compressive strain of -1.7% lattice

mismatch because the lattice constant of BT is larger than that of DyScO3 substrate. 14

Suzuki et al. have reported a mesostructured BT/ST composite film with elevated Tc of

BT by introducing an in-plane compressed strain with -4 % lattice mismatch to BT

lattice.23 In our former research, the mesocrystalline ferroelectric BaTiO3/Bi0.5Na0.5TiO3

(BT/BNT) nanocomposite exhibits an elevated Tc for both BT and BNT in the

mesocrystalline nanocomposite, where BNT has a smaller lattice constant than that of

BT, namely, BNT lattice bears an in-plane tensile strain and BT lattice bears an in-plane

compressive strain.34

The above results suggest that both tensile and compressed strains can result in the

elevated Tc of ferroelectric phases, which may be due to that an in-plane tensile strain

accompanies an out-of-plane compressed strain and an in-plane compressed strain

accompanies also an out-of-plane tensile strain. The lattice strain situation in the 3D

system should be quite complicated. To understand lattice strain situation and its effect

120

on ferroelectric phase in the 3D system, a further detail study is necessary, and the phase

field calculation can be an effective approach.53 Another possible reason for the lowered

Tc of BT is due to its small crystal size in the BT/BBT nanocomposite (Table 3.1).

Sánchez-Jiménez and co-workers have reported a lowered Tc of BT to around 80 °C in a

BT-Ni nanocomposite with a crystal size of BT about 45 nm.54

Table 3.1 Sizes of BT nanocrystals in BT/BBT-0.5 nanocomposites obtained at different

temperatures, and in BT/BBT-0.75-800 nanocomposite and BT-800 mesocrystal.

Sample Size of BT nanocrystal*

800 °C 900 °C 1000 °C 1100 °C

BT/BBT-0.5 40 nm 50 nm 55 nm 70 nm

BT/BBT-0.75 70 nm - - -

BT mesocrystal 75 nm - - -

* Scherrer equation (also referred to as the Debye–Scherrer equation) was applied to estimate the

sizes of BT nanocrystals in BT/BNT nanocomposites and BT mesocrystal, as shown followed:

D(2θ) 110 = Kλ/(Bcosθ110)

Where D is the average nanocrystal size, K is a constant (K = 0.89), λ is the wavelength of the X-ray

source (λ = 0.154056 nm), B is the value of the full width at half maximum (FWHW) of the

diffraction peak of plane (110) and θ is the Bragg angle.

The Curie temperatures of the BT and BBT phases in the BT/BBT-0.5 nanocomposite

are dependent on the preparation temperature, as shown in Fig. 3.18. The

BT/BBT-0.5-800 exhibits a BBT phase transition peak at around 700 oC, and an unclear

peak of the BT phase transition at around 40 °C. With increasing the heating temperature,

the Tc of BBT shifts slightly to the lower temperature, while a sharp and clear phase

transition peak of BT is observed at around 40 °C. The low-temperature-shifting Tc of

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BBT may be ascribed to the destruction of the BT/BBT heteroepitaxial interface due to

the crystal growth of the BBT nanocrystals with increasing heating temperature.6 The

appearance of the sharp peak for the BT phase can be ascribed to the transition from the

pseudo-cubic structure to the tetragonal one with increasing heating temperature from

800 to above 900 °C due to the lattice mismatch stress at the BT/BBT heteroepitaxial

interface.34, 49 The Tc of BT is almost constant in the temperature rang of 900 to 1100 °C

because the crystal size of the BT phase is almost constant in this temperature range

(Table 3.1), namely the crystal growth of the BT nanocrystals is limited by the

neighboring BBT nanocrystals.

Fig. 3.18 Temperature dependences of relativie permittivities (εr) for (a) BT/BBT-0.5-800, (b)

BT/BBT-0.5-900, and (c) BT/BBT-0.5-1100 at measurement frequency of 10 kHz.

122

The piezoelectric responses of the mesocrystalline BT/BBT-0.5 nanocomposites

obtained at different temperatures were investigated by using piezoresponse force

microscopy (PFM). The Displacement-applied voltage (D-V) loops and d*33-applied

voltage (d*33-V) loops are presented in Fig. 3.19, where converse piezoelectric constant

(d*33) is calculated from D/V value of the D-V loop. The d*

33 value of 130 pm/V at 10 V

of applied voltage for BT/BBT-0.5-800 is much larger than those of 40 and 60 pm/V for

BT/BBT-0.5-600 and BT/BBT-0.5-700, respectively. The sudden increase of the d*33

value with the increase of heating temeprature from 700 to 800 °C can be ascribed to the

formation of the mesocrystalline BT/BBT nanocomposite above 800 °C (Fig. 3.3),

which introduces the lattice stain at the BT/BBT heteroepitaxial interface. The d*33

value of BT/BBT-0.5-800 nanocomposite is about 5 and 6 times larger than d*33 values

of a nanostructured BT (28 pm/V)55 and BBT ceramic (23 pm/V).37 The result reveals

that the lattice strain engineering is an effective aproach for enhancing the piezoelectric

response.

123

Fig. 3.19 Displacement-applied voltage loops and d*33-applied voltage loops for BT/BBT-0.5

nanocomposites obtained by heat-treatement at (a) 600, (b) 700 and (c) 800 °C, respectively.

3.4 Conclusions

The mesocrystalline BT/BBT nanocomposite can be fabricated by a facile two-step

124

process, including the first step of hydrothermal process and second step of solid state

reaction process. The construction of mesocrystalline BT/BBT nanocomposite using BT

and BBT nanocrystals with different types of crystal structures can improve the stability

of the nanocomposite at high temperature due to no formation of their solid solution,

which provides an opportunity to fabricate a high density of the mesocrystalline

nanocomposite ceramic sample. The introduction of the BT/BBT heteroepitaxial

interface into the mesocrystalline BT/BBT nanocomposite results in the greatly elevated

Curie temperature of the BBT phase, which broadens the application temperature range

as a ferroelectric material. The BT/BBT heteroepitaxial interface also enhances greatly

the ferroelectric, piezoelectric, and dielectric responses of the BT/BBT nanocomposites,

which exposes its potential for the replacement of the lead-based piezoelectric materials.

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Chapter Ⅳ

Compelling Evidences for Antiferroelectric to Ferroelectric

Transition of MAPbI3-xClx Perovskite in Perovskite Solar

Cells

Abstract

Perovskite solar cells based on organic-inorganic perovskites have allured massive

scientific attention due to their excellent photovoltaic performances. The power

conversion efficiencies of CH3NH3PbI3 (MAPbI3) based PSCs have demonstrated

extremely high efficiency of 25.2 %. However, a conclusive charge separation

mechanism is still missing for PSCs because of the lack of the fundamental

understanding of organic-inorganic perovskite properties, which hampers the

optimization and development of high-performance perovskite solar cells. Up to now, a

traditional p-n junction charge separation mechanism has been employed to perovskite

solar cells, however, there are some mysterious behaviors like the strong current-voltage

(J-V) hysteresis, yet remains unexplained reasonably. And the J-V response of the

perovskite solar cells could lead to an unfaithful estimation of the efficiency, where the

reverse scan and forward scan exhibit the overestimated and underestimated power

conversion efficiency. Some possible theories have been proposed to explain the origin

of the hysteresis in PSCs, involving ferroelectricity, vacancy-assisted ionic migration,

charge carrier trapping, and capacitive effect.

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To figure out the aforementioned problems, in this chapter, we explore the ferroelectric

(FE) behavior of MAPbI3-xClx perovskite by the structural analysis and the

measurements of the piezoelectric, ferroelectric, dielectric and ferroelastic responses.

Antiferroelectric (AFE) nature of MAPbI3-xClx perovskite (I4/mcm space group) was

first experimentally proved by piezoelectric force microscopy (PFM) measurements,

which also undoubtedly demonstrates the transformation of the AFE phase to the FE

phase by applying an electric field at the room temperature, and the FE phase can be

returned to the AFE phase after heat treatment at above its Curie temperature and then

cooling down to room temperature, namely being reversibly tunable between AFE and

FE phases. XRD results reveal that the spontaneous polarization direction of this

perovskite can be switched by applying an electric field and mechanic press. Based on

the AFE behavior, we propose a possible FE semiconductor charge separation

mechanism for perovskite solar cells, which might help us to understand the origin of

J/V hysteresis in the MAPbI3-based perovskite solar cells.

131

Chapter V Summary

In the present study, the application of the lattice strain engineering to a

newly-designed lead-free ferroelectric mesocrystalline nanocomposite has been

demonstrated. Some specific properties of the ferroelectric materials, including the

Curie temperature (Tc), piezoelectric and dielectric responses can be improved by an

enormous strain in the heteroepitaxial interface constructed by two kinds of crystals

with different lattice parameters. Generally, the lattice strain engineering is mainly

applied to the thin film materials because the heteroepitaxial interfaces are relatively

easy to be fabricated by heteroepitaxial crystal growth of the film materials, but it is a

high cost for the fabrication of thin film materials.

The mesocrystals not only have some potential properties based on the individual

nanocrystals, but also exhibit unique collective properties of nanocrystal ensembles. The

mesocrystalline nanocomposite constructed by two kinds of nanocrystals is a promising

material for the lattice strain engineering to improve the ferroelectricity because it has

high density of the heteroepitaxial interface and is low-cost. It is noteworthy that the

mesocrystalline nanocomposites exhibit both improved Tc and piezoelectric response,

which cannot be achieved simultaneously in the thin film materials or bulk materials

without mesocrystalline nanostructure, as far as I know. Therefore, this approach

provides a new concept to design the high-performance lead-free piezoelectric

materials.

In addition, there has been a recent surge of interest in perovskite solar cells (PSCs)

due to soaring power conversion efficiencies (PCEs). However, the fundamental

understanding of organic-inorganic halide perovskites employed as the absorber in the

132

PSCs is still limited. Consequently, systematic studies on further improvements of the

materials and device structures for the commercialization have been severely hampered.

In the present study, the relationship between structure and ferroelectricity of

MAPbI3-xClx perovskite has been investigated by the structural analysis and the

measurements of the piezoelectric, ferroelectric, dielectric and ferroelastic responses.

The transformation from antiferroelectric MAPbI3-xClx phase to its ferroelectric phase

by the poling treatment has been uncovered for the first time.

The swapping behavior between antiferroelectric and ferroelectric phases of the

MAPbI3-based perovskites suggest that the ferroelectricity would affect charge

separation performance, and the ferroelectric phase can possess a higher charge

separation effect than that of the non-ferroelectric phase in the PSCs; hence,

current-voltage (J-V) hysteresis behaviors for the PSCs can be well explained based on

this behavior. The results conclude that the J-V hysteresis is one of the solid evidence

for the exhibition of higher charge separation effect of the ferroelectric perovskites than

that of non-ferroelectric perovskites and the reverse scan J-V curve should be used to

evaluate PSCs of the antiferroelectric or ferroelectric perovskites because it corresponds

to the ferroelectric semiconductor charge separation effect.

The main results and points of the present study are summarized as follow:

In Chapter I, some reviews on the synthesis, the formation mechanisms,

characterizations, and the applications of conventional mesocrystals were described.

The general introduction for the topochemical synthesis, and the layered protonated

titanate as a precursor for the topochemical synthesis of the mesocrystals were

mentioned. In addition, the perovskite and perovskite-related halides were described

also as they possess several interesting properties, such as electron-acceptor behavior, a

133

large optical transmission domain and piezoelectric etc. Furthermore, the purposes of

the present study were clarified.

In Chapter II, the ferroelectric mesocrystalline BT/BNT nanocomposite synthesized

from a layered titanate H1.07Ti1.73O4 (HTO) by a facile two-step topochemical process,

namely first-step solvothermal process and second-step solid-state process, was

introduced. The BT/BNT nanocomposite is constructed from well-aligned BT and BNT

nanocrystals with the same crystal-axis orientation. The BT/BNT heteroepitaxial

interface in the nanocomposite is promising for the enhanced piezoelectric performance

by using the lattice strain engineering, which gives a giant piezoelectric response with a

d*33 value of 408 pm/V. The introduced lattice strain at the BT/BNT heteroepitaxial

interface causes transitions of pseudo-paraelectric BT and BNT nanocrystals to the

ferroelectric nanocrystals in the mesocrystalline nanocomposite, which enlarges

ferroelectric, piezoelectric and dielectric responses. The lattice strain also results in the

elevated Curie temperatures (Tc) of BT and BNT and a new intermediate phase

transition.

In Chapter III, the ferroelectric mesocrystalline BT/BBT nanocomposite synthesized

from the layered titanate HTO by a facile two-step topochemical process, namely

first-step solvothermal process and second-step solid state process, was exhibited. The

BT/BBT nanocomposite is constructed from well-aligned BT and BBT nanocrystals

oriented along the [110] and [11-1] crystal-axis directions respectively. The lattice strain

is introduced into the nanocomposite by the formation of the BT/BBT heteroepitaxial

interface, which causes a greatly elevated Curie temperatures from 400 to 700 °C and an

improved piezoelectric response with d*33=130 pm/V. In addition, the BT/BBT

nanocomposite is stable up to a high temperature of 1100 oC, hence, the mesocrystalline

134

ceramic can be fabricated as a high-performance ferroelectric material.

In Chapter IV, the ferroelectric and semiconducting properties of the

CH3NH3PbI3-xClx perovskites were studied by structural analysis, measurements of the

ferroelastic behavior, the ferroelectric hysteresis loops, the piezoelectric response and

conductivity. The results reveal that the CH3NH3PbI3-xClx perovskite exhibits the

antiferroelectric and semiconducting natures, and the antiferroelectricity can be

switched to ferroelectricity by poling treatment, which gives a solid evidence to put an

end to the heated argument between the non-ferroelectric and ferroelectric nature for the

MAPbI3-based perovskites and paves the way for the fabrication of high-performance

perovskite solar cells by using ferroelectric and antiferroelectric phases.

The results described above conclude that the in situ topochemical mesocrystal

conversion reaction process is an attractive approach. This approach can be employed to

the development of the platelike functional titanate ferroelectric mesocrystalline

nanocomposite. The nanocrystal size, morphology, structure, and composition of the

mesocrystalline nanocomposite can be controlled by adjusting the reaction conditions in

the in situ topochemical mesocrystal conversion reaction process. These mechanisms

will serve also as a guide to develop the topochemical syntheses of other materials in

the solvothermal processes and solid-state processes. Therefore, both the solvothermal

chemical processes and solid-state processes accompanying with the in situ

topochemical conversion reaction are of notable significance for the fundamental

research, and can provide important knowledge for controlling the chemical reaction

process to achieve the materials with advanced functions.

The study of mesocrystalline nanomaterials constructed from well-aligned oriented

nanocrystals has increasingly become an intense and major interdisciplinary research

135

area in the recent decade owing to their potential applications to catalysis, sensing,

ferroelectric, and energy storage and conversion. In addition, strain engineering has

been used to alter the electronic structure of materials, which can greatly change a series

of physical properties of the materials, since its impact is delivered directly on the

lattice. Up to now, the strain engineering has been widely applied to 2D materials with

simple 2D heteroepitaxial interface, while its application to 3D bulk materials have been

rarely reported. In the 3D systems, strain can be effectively introduced to a bulk

material by either pulling or squeezing the lattice. However, the 3D heteroepitaxial

interface is very difficult to be constructed in the 3D bulk materials. Therefore, the

application of the strain engineering to a newly-designed mesocrystalline

nanocomposite with a 3D heteroepitaxial interface is a big breakthrough for making 3D

bulk materials with newly excellent properties. The success in developing these

mesocrystalline nanocomposites not only expand mesocrystalline nanomaterials

chemistry and offers a good opportunity to understand the formation process of this

unique mesocrystalline nanocomposite structure, but also paves a way for the

application of the mesocrystalline materials to improve ferroelectric, piezoelectric, and

dieletric nanomaterials via the strain engineering.

On the other hand, our findings for the MAPbI3-xClx perovskite not only put an end to

the heated argument between its non-ferroelectric and ferroelectric natures, but also

pave a new avenue toward the fabrication of high-performance PSCs using the

antiferroelectric and ferroelectric semiconductor perovskites with optimizing the cell

performances in future developments for the commercialization.

In our next challenges, firstly, figuring out the connection between the atomic

arrangement structure and the anomalous ferroelectric behavior of the mesocrystalline

136

nanocomposite is significant. Of course, this requires the help of some state-of-art

technologies, such as the scanning transmission electron microscopy (STEM), the

energy-dispersive X-ray spectroscopy (EDS) and the electron energy loss spectroscopy

(EELS). Given that ferroelectric mesocrystalline nanocomposites constructed

from the different nanocrystals with the same perovskite structure tend to transform into

the solid solution phase under high heating temperature, therefore, the application of the

mesocrystalline nanocomposite to its ceramic counterpart is scarcely possible. Therefore,

the formation of its film materials or/and polymer-based composite applied to

nanoelectronic devices is promising.

As mentioned above, the lattice strain engineering has been mainly applied to the

ferroelectric super-structured film materials with 2D heteroepitaxial interfaces, which

has been widely studied. Whereas, the ferroelectric mesocrystalline nanocomposite film

materials should exhibit much more complicated 3D heteroepitaxial interface, which

needs much more refined STEM image analysis and other assistant methods to find out

the atomic arrangements near the interfaces and its connection to the anomalous

ferroelectric behavior. Although, some preliminary works have been done, the further

study is still needed.

Besides, the further research on the ferroelectricity of other types of halide

perovskites used for PSCs and discussing its connection to the power conversion

efficiency are meaningful. The ferroelectric semiconductor charge separation

mechanism would not be limited in the halide perovskites, and can be applied also to

other ferroelectric semiconductors, such as metal oxides and metal sulfides. It could

offer a better avenue for the development of the high-performance PSCs with the help

of piezoresponse force microscopy (PFM) technique based on atomic force microscopy

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(AFM) or the strain-electric-field (S-E) texting system.

Publications

Publications in Journals

1. Wenxiong Zhang; Hao Ma; Sen Li; Dengwei Hu; Xinggang Kong; Shinobu

Uemura; Takafumi Kusunose; Qi Feng. Anomalous piezoelectric response of

ferroelectric mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 nanocomposites designed by

strain engineering. Nanoscale 2018, 10, (17), 8196-8206.

2. Wenxiong Zhang; Sen Li; Hao Ma; Dengwei Hu; Xinggang Kong; Shinobu

Uemura; Takafumi Kusunose; Qi Feng. Ferroelectric Mesocrystalline

BaTiO3/BaBi4Ti4O15 Nanocomposite: Formation Mechanism, Nanostructure, and

Anomalous Ferroelectric Response. Nanoscale 2019, DOI: 10.1039/C8NR07587E

3. Dengwei Hu; Wenxiong Zhang; Yasuhiro Tanaka; Naoshi Kusunose,; Yage Peng;

Qi Feng. Mesocrystalline Nanocomposites of TiO2 Polymorphs: Topochemical

Mesocrystal Conversion, Characterization, and Photocatalytic Response. Crystal

Growth & Design 2015, 15, (3), 1214-1225.

4. Dengwei Hu; Xiaomei Niu; Hao Ma; Wenxiong Zhang; Galhenage A. Sewvandi;

DesuoYang; Xiaoling Wang; Hongshei Wang; Xinggang Kong; Qi Feng.

Topological relations and piezoelectric responses of crystal-axis-oriented

BaTiO3/CaTiO3 nanocomposites. RSC Adv. 2017, 7, (49), 30807-30814.

5. Dengwei Hu; Wenxiong Zhang; Fangyi Yao; Fang Kang; Hualei Cheng; Yan Wang;

Xinggang Kong; Puhong Wen; Qi Feng. Structural and morphological evolution of

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an octahedral KNbO3 mesocrystal via self-assembly-topotactic conversion process.

CrystEngComm 2018, 20, (6), 728-737.

6. Ma, H.; Wenxiong Zhang; Xinggang Kong; Shinobu Uemura.; Takafumi

Kusunose.; Qi Feng. BaTi4O9 mesocrystal: Topochemical synthesis, fabrication of

ceramics, and relaxor ferroelectric behavior. Journal of Alloys and Compounds 2019,

777, 335-343.

7. Wenxiong Zhang; Galhenage A. Sewvandi; Sen Li; Xinggang Kong; Dengwei Hu;

Shinobu Uemura.; Takafumi Kusunose.; Qi Feng. Compelling Evidences for

Antiferroelectric to Ferroelectric Transition of MAPbI3-xClx Perovskite in Perovskite

Solar Cells. (In the submission)

Publications in Conferences

1. Wenxiong Zhang, Qi Feng. Topochemical Synthesis of BaTiO3 Platelike

Mesocrystals from Layered Titanate by Flux Method. 第 21 回ヤングセラミスト

ミーティング in 中四国, p103-104, Shimane, 2014/11/15.

2. Wenxiong Zhang, Hao Ma, Qi Feng. Synthesis and Characterization of

Ferroelectric Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3 Nanocomposites. The 54th

Symposium on Basic Science of Ceramics, p82, Saga, 2016/01/07-08.

3. Wenxiong Zhang, Hao Ma, Qi Feng. Fabrication and Characterization of

Ferroelectric Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3 Nanocomposites. 日本化学会

中国四国支部大会, ポスター, 2016/11/05-06.

4. Wenxiong Zhang, Qi Feng. Fabrication of Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3

Nanocomposites and Their Ferroelectric Behavior. The 55th Symposium on Basic

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Science of Ceramics, p109, Okayama, 2017/01/12-13.

5. Wenxiong Zhang, Qi Feng. Fabrication of Mesocrystalline BaTiO3-Bi0.5Na0.5TiO3

Nanocomposites and Their Ferroelectric Behavior. International Symposium on

Advanced Materials: Golden Era in Hydrothermal Research, p36-37, Kochi,

2017/03/27-30.

6. Wenxiong Zhang, Qi Feng. Anomalous Piezoelectric Response of Ferroelectric

Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain

Engineering. The 56th Symposium on Basic Science of Ceramics, p9, Tsukuba,

2018/01/11-12.

7. Wenxiong Zhang, Qi Feng. Anomalous Piezoelectric Response of Ferroelectric

Mesocrystalline BaTiO3/Bi0.5Na0.5TiO3 Nanocomposites Designed by Strain

Engineering. 2018 ISAF-FMA-AMF-AMEC-PFM Joint Conference, p49, Hiroshima,

2018/05/25-06/01.

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Acknowledgment

I could not have completed this dissertation without the help of many people that

have influenced and supported me scientifically, financially and socially over the past 5

years at Kagawa University, Japan.

I would like to firstly thank my supervisor, Prof. Qi Feng, for his kind guidance,

cultivation, and continuous supervision, excellent advice and continuous encouragement

towards the completion of this present research successfully in time. His rigorous and

pragmatic academic attitude is worth studying for my life. Besides the fundamental

knowledge he had taught me, more importantly, he realized early that I have a strong

desire to research, anything, and you gave me the freedom to develop. And I’m also

appreciated for Associate Prof. Lin Yu (Okayama Shoka University), for her much

supports and patience on our research supervisor team, for her precious guidance and

perspectives on my daily life.

I would also like to express my thanks to my vice supervisors Prof. Takafumi

Kusunose and Associate Prof. Shinobu Uemura for their kind advice, valuable

suggestion, necessary support, and enthusiastic assistances to my Ph. D study. In

addition, I would like to express my thanks to Prof. Chengling Pan (Anhui University of

Science and Technology) for recommending me to study in Japan. Grateful

acknowledgements are to Senior Dengwei Hu (Baoji University of Arts and Science),

Changdong Chen (Liaoning Shihua University), Yien Du (Jinzhong University) and

Sewvandi Asha Galhenage (University of Moratuwa) for their valuable suggestion,

enthusiastic assistances and life experience.

I would like to acknowledge the former and current administrative staff at Kagawa

University. Thank you very much for making my life easier in Japan: Especially, I

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would like to thank Sakamoto Ai who helped me out with my application for the

scholarship. In addition, I also want to give my appreciation to Ms. Yuka Kojima for

teaching me Japanese and helping me prepare the interview of the scholarship.

I would like to thank the scientists, who introduced me to the facilities at Kagawa

University. Special thanks to the three scientists, who are the responsible for my favorite

instruments: Associate Prof. Shinobu Uemura (AFM), Mr. Toshitaka Nakagawa

(FE-TEM) and Ms. Ayami Nishioka (TEM). I greatly enjoyed your expertise with the

electron microscopes. Thank you for giving me so many ideas to solve my problems.

I also owe my sincere gratitude to my friends and fellow classmates in our research

group, who gave me many helps and much pleasures on my life and study.

Lastly, I would like to acknowledge my family for their love, understanding and

support throughout my research and life in Japan, in particular, I sincerely appreciate

my lovely girlfriend, Hui Liu, for her endless support and encouragement, as well as

accompanying me through a lot of hard times.

.

Wenxiong Zhang

Feng Lab, Department of Advanced Materials Science,

Faculty of Engineering and Design

Kagawa University, Kagawa, Japan

January, 2018