Electrodeposition of ZnO Nanorod Arrays on Transparent Conducting Substrates–a Review

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Electrochimica Acta 127 (2014) 467–488

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me page: www.elsev ier .com/ locate /e lec tac ta

lectrodeposition of ZnO Nanorod Arrays on Transparent Conductingubstrates–a Review

agdalena Skompska ∗, Kamila Zarebskaaboratory of Electrochemistry, Faculty of Chemistry, Biological and Chemical Research Center, University of Warsaw, Pasteura 1, 02 093 Warsaw, Poland

r t i c l e i n f o

rticle history:eceived 4 November 2013eceived in revised form 10 February 2014ccepted 11 February 2014vailable online 25 February 2014

eywords:nO nanorods

a b s t r a c t

ZnO nanorods (NRs) are promising components in a wide range of nanoscale devices for future applica-tions in photocatalysis, solar cells, optical devices and biochemical sensing. The nanorods in the form ofarrays vertically oriented to the substrate may be obtained by electrochemical deposition but morpho-logy of the film is very sensitive to the synthesis conditions. This article provides a comprehensive reviewon various electrosynthesis procedures developed to obtain the nanorods of desired structure, diameter,density on the substrate.

We discuss the growth mechanisms and influence of different parameters such as the type and concen-2+ −

lectrodepositiontructure modifiershotoluminescencelectrical properties, ZnO nanorodspplications

tration of Zn and OH precursors, the value of applied potential or current density on the morphologyof obtained ZnO films and the role of various structural modifiers on the shape of ZnO nanostructures. Wepresent a brief analysis of the influence of electrosynthesis conditions and postannealing of the sampleson optical and electrical properties of ZnO nanowires deposited on the conducting substrate. A shortsummary of the practical applications of ZnO nanorods is also provided.

© 2014 Elsevier Ltd. All rights reserved.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4682. Basic information on stability of Zn(OH)2 and ZnO–thermodynamic data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4683. Crystallographic structure of ZnO nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4694. General rules for electrodeposition of ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

4.1. Pretreatment of the substrate–seeding process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4714.2. Electrochemical deposition of ZnO nanorod arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472

4.2.1. Molecular oxygen precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4724.2.2. Nitrate precursor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

5. Modification of the ZnO structure by different solution additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4765.1. The influence of KCl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4765.2. The influence of hexamethylenetetramine (HMT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4765.3. The influence of ethylenediamine (EDA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

6. Optical properties of ZnO nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4807. Electrical properties of electrodeposited ZnO nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4828. Applications of ZnO nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

8.1. Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4838.2. Biosensors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4838.3. Light emitting diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483
8.4. Solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +48 22 8220211, fax: +48 22 8225996.E-mail address: [email protected] (M. Skompska).

ttp://dx.doi.org/10.1016/j.electacta.2014.02.049013-4686/© 2014 Elsevier Ltd. All rights reserved.

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68 M. Skompska, K. Zarebska / Elec

. Introduction

ZnO is a wide bandgap semiconductor with a direct bandgapf about 3,37 eV and relatively high exciton binding energy (60eV) at room temperature. ZnO has attracted increasing interest

ue to its unique ability to form a variety of nanostructures suchs nanowires, nanoribbons, nanobelts, nanocombs, nanospheres,anofibers, nano-tetrapods. A special attention is focused on thenO in the form of nanorods (NRs) and nanorod arrays verticallyrranged with respect to the substrate due to their unique pro-erties. The ordered ZnO nanostructures are expected to enhanceerformance of various technologically important devices such ashort-wavelength lasers [1,2], electroluminescent devices [3–6],ensors [7–11], photocatalytic systems [12–16] and third genera-ion of solar cells [17–23]. In the latter case, the ZnO NRs are usuallyeposited on a transparent conducting oxide (TCO) substrate i.e.lass plate covered either with a thin layer of indium-tin oxideITO) or F-doped SnO2 (FTO) and the length, diameter and den-ity of nanorods are important parameters influencing the solarell efficiency.

Over past decades, many methods have been developed forreparation of ZnO of various morphologies and size, includingol–gel processes [24–28], chemical vapor deposition [29–34],ydrothermal [35–39] and electrochemical methods [40–45]. Elec-rodeposition provides several advantages in comparison withther methods, like low cost, large-scale deposition and possibilityf morphology control of resultant films, from rough deposits toell aligned nanorods arrays. The electrochemical synthesis from

queous solution containing Zn2+ ions and source of OH-, consistedn electroreduction of hydroxide precursor, followed by precipita-ion of Zn(OH)2 and dehydration to ZnO, was firstly reported in996 independently by Izaki et al. [40] and by Peulon and Lincot41]. A numerous papers published up to date have shown that thehape and the properties of obtained films may be varied by thehange of parameters such as the type and concentrations of Zn
nd O precursors, temperature, type of a substrate and method ofts pretreatment as well as by manipulation with electrochemicalarameters (current density or electrodeposition potential). A fewtudies have been also performed in non-aqueous solutions, such as
ig. 1. Comparison of the phase stability diagrams for ZnO (a) �-Zn(OH)2 (b) in aqueous

n equilibrium between Zn(OH)3− and the solid phases is out of the range. Reproduced fr

mica Acta 127 (2014) 467–488

propylene carbonate [46], dimethylsulfoxide [47] and ionic liquids[48–50].

The aim of this review is to sum up the recent advances inelectrochemical deposition of ZnO nanorods on the transparentconducting substrates from aqueous solutions. In the following sec-tions we discuss the stability diagrams of ZnO, processes involvedin the nucleation and growth of ZnO on the electrodes and analyzecorrelations between the electrosynthesis conditions and morphol-ogy of the nanostructures, their density on the substrate, diameterand length as well as the crystal structure, optical and electricalproperties.

2. Basic information on stability of Zn(OH)2 andZnO–thermodynamic data

A fundamental reaction in electrodeposition of ZnO is based onreduction of an oxygen precursor to OH-, followed by reaction withZn2+ ions, according to the scheme:

Zn2++2OH-⇔ Zn(OH)2 (1)

The zinc hydroxide formed in this reversible reaction is trans-formed at elevated temperature into zinc oxide:

Zn(OH)2temp.−→ ZnO + H2O (2)

Among several different forms of Zn(OH)2 denoted by �-�-, �-,�- and �-Zn(OH)2 [51–53] the latter one is the most stable and thisform will be considered further in the text.

One of the crucial aspects which should be considered in ZnOdeposition is limited pH stability of Zn(OH)2 and ZnO, as it is illus-trated in the stability diagrams presented in Fig. 1. The solid linesrepresent solubility of the solid phases as a function of pH, i.e. indi-cate the total concentration of the Zn(II) soluble species, while thedashed lines denote the thermodynamic equilibria between solublespecies and the solid phases [54]. It means, that in aqueous solu-
tion at the temperature of 25 ◦C the ZnO and Zn(OH)2 are formedat concentrations of Zn (II) above 10−5.9 and 10−5.6, respectively. Inthe solution containing for example 10−4 M Zn (II), both Zn(OH)2and ZnO are thermodynamically stable over the pH ranging from 8
solutions at 25 ◦C. Concentration in y-axis corresponds to all soluble Zn(II) species.om [54] with permission of The Royal Society of Chemistry.

M. Skompska, K. Zarebska / Electrochimica Acta 127 (2014) 467–488 469

Fig. 2. Effect of temperature on the solubility of Zn(II) in the presence of 0.1 M Cl− . S’ relates to all zinc species in the solution. Solid lines correspond to ZnO(s), whereas dashedlines to �-Zn(OH)2(s): full diagram (A), enlarged view (B) at: 25 ◦C (a), 50 ◦C (b), 70 ◦C (c) and 90 ◦C (d) (the arrows show the effect of increasing temperature). Reproducedf

tiZ

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ob[atvhatZod

plZ1

ai

enodhs

rom [55] with permission of Elsevier.

o 13, whereas outside this range Zn(II) is present in the solutionn the form of Zn2+ ions or soluble complex Zn(OH)+ at pH < 8 orn(OH)4

2− at pH > 13.The influence of temperature on the solubility of ZnO and

n(OH)2 in the solution containing Cl- ions has been reported byincot et al. [55]. As it is illustrated in solubility-pH diagrams pre-ented in Fig. 2, the increase of temperature leads to the shift of theolubility curves towards lower pH and the shift is stronger for zincxide than for zinc hydroxide. The difference between the V-shapef solubility curves presented in Fig. 2 and U-shape reported by thisroup in the previous paper [53] (similar to that presented in Fig. 1)as been explained as a result of the use of another set of data.

The studies of distribution of Zn(II) in different forms in aque-us solutions at pH > 6.7 at several temperatures from the rangeetween 12.5 ◦C and 75 ◦C have been carried out by Reichle et al.56]. The solubility data of zinc hydroxide obtained from atomicbsorption spectrophotometric measurements were used by themo evaluate the solubility products of Zn(OH)2 at different pH andarious temperatures and the equilibrium constants of differentydroxide complexes of zinc. The results presented in ref. [56]llow calculating the fractions of Zn(II) existing in the solution inhe form of Zn2+

(aq), Zn(OH)+(aq), Zn(OH)2(aq), Zn(OH)3

-(aq), andn(OH)4

2-(aq). These data were used by us to construct the plots

f distribution of different forms of Zn(II) as a function of pH atifferent temperatures, presented in Fig. 3.

The obtained results are consistent with solubility-pH diagramsresented in Fig. 2 and demonstrate the shift of the curves towards

ower pH values with the increase of temperature. Namely, then(OH)2 is the major soluble species in the pH range from 9 to1.2 at the temperature of 25 ◦C and from pH 7.8 to 10.5 at 75 ◦C.

The dependence of temperature on the formation of Zn(OH)2nd ZnO films during electrochemical deposition will be discussedn more detail further in the text.

The data presented in Fig. 3 may be useful in estimation of iso-lectric point (IP) of ZnO, i.e. the pH value corresponding to zeroet surface charge of ZnO. The establishment of a surface charge
n the metal oxide in water is a result of two effects: amphotericissociation of surface M-OH groups and/or adsorption of metalydroxo complexes formed as the hydrolysis products of dissolvedpecies [57,58]. Both phenomena may explain qualitatively the pH
dependence of the surface charge. The isoelectric point of ZnOnanoparticles reported in the literature ranges from 8.7 to 10.3([57] and references therein) and its high value is very useful forimmobilization of enzymes by electrostatic interactions (see moredetails in Section 8).

3. Crystallographic structure of ZnO nanorods

The ZnO crystal in hexagonal wurtzite-type structure, whichis thermodynamically stable at ambient conditions, may be con-sidered as a number of alternating planes composed of four-foldcoordinated O2− and Zn2+ ions stacked alternately along the c-axis[59–61], as shown in Fig. 4a. It is characteristic that hexagonalZnO has polar and nonpolar planes. The top, Zn-terminated (0001)plane is positively charged and exhibits high surface energy. There-fore, the anions from the solution (OH-, Zn(OH)3

-, Zn(OH)42−) may

be adsorbed on this polar phase, resulting in fast growth of along[0001] direction. The second polar plane is negatively charged,O-terminated (0001) basal plane. Beside these two primary polarplanes in the wurtzite crystal structure one can distinguish manyother secondary non-polar planes, for example (2110) or (0110)side planes with 3-fold coordinated atoms [62], as well as othernon-polar surfaces, as pyramidal (0110) plane [35] (see Figs 4band 4c).

According to the theoretical models concerning the ZnO crystalgrowth under hydrothermal conditions presented in the literature[35,63], the fastest growth rate is in direction [0001], which leads tothe increase of the surface area of the (2110) or (0110) facets, whichis consistent with observed growth habit. According to the modelproposed Li et al. [35], the crystal growth rates along differentdirections decrease are in the order: [0001] > (0111) > (0110) >(0111) > (0001). However, in practice, the presence of selectedcrystal face is strongly dependent on the growth conditions, suchas pH, presence of ligands and concentration of Zn2+ [2,61]. Inturn, in the case of electrochemical growth the main parameters
influencing the shape of ZnO nanocrystals and its orientation withrespect to the substrate are: the type and concentrations of ZnOprecursors, electric parameters of deposition (potential or currentdensity), pretreatment of the substrate as well as the presence of

470 M. Skompska, K. Zarebska / Electrochimica Acta 127 (2014) 467–488

131211109870.0

0.2

0.4

0.6

0.8

1.0

Zn(OH)

Zn(OH)

Zn(OH)

Zn(OH)

Zn

Fraction

pH13121110987

0.0

0.2

0.4

0.6

0.8

1.0Zn(OH) Zn(OH)

Zn(OH)

ZnFraction

pH

Zn(OH)

131211109870.0

0.2

0.4

0.6

0.8

1.0Zn(OH)

Zn(OH)

Zn(OH)

Zn(OH)

Zn

Fraction

pH

(a) (b) (c)

Fig. 3. Distribution of different species: Zn2+(aq), Zn(OH)+

(aq), Zn(OH)2(aq), Zn(OH)3−

(aq) and Zn(OH)42−

(aq) as a function of pH at different temperatures 25 ◦C (a) 50 ◦C (b) and75 ◦C (c) obtained on the base of data reported by Reichle et al. [56].

Fig. 4. Unit cell of ZnO crystal structure (the yellow spheres correspond to oxygen and the blue ones to zinc atoms) (a) (© National Institute for Materials Science (NIMS).R n behalf of NIMS), typical morphology of hexagonal ZnO nanowire (b) with indicationo from [60] by permission of IOP Publishing. All rights reserved) and ZnO nanowire withp

d(tech

na

ds(trooo

a(so

eproduced from [61] under a CC BY-NC-SA licence by IOP Publishing Limited of corresponding facets and growth directions (c) (© IOP Publishing. Reproduced

yramidal end (d) (reproduced from [35] with permission of Elsevier).

ifferent organic or inorganic additives, such as ethylenediamineEDA), hexamethylenetetramine (HMT) or Cl-, considered as struc-ure modifiers. By the change of these parameters, ZnO may belectrodeposited in the form of compact film, hexagonal parti-les, hexagonal plates, hexagonal rods, flower-like rod bundles andexagonal nanowire arrays.

The preferred crystallographic orientation of the deposit inano/microscale may be determined by analysis of its XRD pattern,s it has been illustrated by Xu et al. [64] (see Fig. 5).

The crystal growth of hexagonal nanowire arrays along [0001]irection with their c-axis perpendicularly oriented to the sub-trate results in highly intensive peak at 2�=34.4o corresponding to0002) plane (Fig. 5e). The relative intensities of two other impor-ant diffraction peaks:

(1010

)at 31.7o and

(1011

)at 36.3o with

espect to the (0002) signal depend strongly on the crystal shapef ZnO and its orientation on the substrate. For example, formationf vertically arranged ZnO plates results in high relative intensityf the

(1010

)and

(1011

)peaks. The other signals at 47.5◦, 56.6◦( )

nd 62.8◦ corresponding respectively to the crystal planes 1012 ,

1120)

and(

1013)

are of lower intensities or are absent in thepectrum of nanorods and they are less important in determinationf the ZnO crystal shape from XRD spectra.

Fig. 5. XRD patterns of the ZnO structures with different crystal shapes: hexagonalparticles (a) hexagonal plates (b) hexagonal rods (c) flower-like rod bundles (d) andhexagonal nanowire arrays (e) Reproduced from [64] with permission of Elsevier.


Fig. 6. Chronoamperometric curves obtained for potentiostatic polarization of the rotating FTO electrode (at � = 300 rpm) at -0.75 V vs. standard hydrogen electrode (SHE)i atured permi

bT

T

oori

4

4

tpcipoao

t[cc

psiiTp

ctffwep

n the solution of 5 mM ZnCl2, saturated with molecular oxygen, at various temperifferent stages for ZnO film formation above 34 ◦C (b). Reproduced from [55] with

A quantitative analysis of the preferred orientation may be doney determination of a texture coefficient for considered orientation,C(hkl), by means the equation [65]:

C(hkl) = I(hkl)/I0(hkl)N−1

∑I(hkl)/I0(hkl)

(3)

where I is the measured relative intensity, Io is relative intensityf corresponding plane given in the JCPDS data and N is the numberf diffraction peaks. The texture coefficient approaches unity for aandomly distributed powder sample, while it is larger than unityf the (hkl) plane is preferentially oriented.

. General rules for electrodeposition of ZnO

.1. Pretreatment of the substrate–seeding process

The first step in ZnO synthesis is nucleation occurring whenhe solution in the electrode vicinity achieves supersaturation. Thisrocess is promoted by the presence of underlying thin ZnO film, soalled seed layer, previously deposited on the substrate and play-ng a role of nucleation centers. Heterogeneous nucleation takinglace on the substrate in most cases has lower free energy barrierf activation than homogeneous nucleation and therefore, it occurst lower supersaturation level than that for the process in the bulkf the solution [66].

In electrochemical deposition the seeding stage may be omit-ed but in general it enhances formation of the ZnO nanorods42–44,67–69], improves alignment of ZnO nanowires and allowsontrolling their diameter and density. All these effects will be dis-ussed in more detail in the next sections.

The seed layer may be formed by chemical methods, for exam-le by thermal decomposition of zinc acetate deposited on theubstrate [70,71], sol-gel method [67] as well as by electrochem-cal deposition [43,44,68,72]. The advantage of the latter methods quantitative control of seeding by the electrodeposition charge.he seed layer should be thin and uniform to provide a homogenousostdeposition of vertically aligned ZnO nanorods.

The electrochemical seeding is usually performed in the solutionontaining soluble zinc salt of concentration ranging from 0.005 Mo 0.5 M, at constant potential or constant current density. In theormer case, the value of the potential applied should not be very
ar from that commonly required for the electrodeposition of ZnO,hich is usually in the range -1.0 V ÷ −1.2 V vs. saturated calomel
lectrode (SCE). It was found that the increase of the pretreatmentotential to -1.4 V resulted in the increase of the number of ZnO

s (a): 22 ◦C (1), 28 ◦C (2), 34 ◦C (3), 40 ◦C (4), 50 ◦C (5) and schematic illustration ofssion of Elsevier.

crystallites and in effect the ZnO nanorods grown on a such layerformed a very dense film [43,72]. The similar effect may be obtainedby application of high current density in the galvanostatic pre-treatment. According to the results presented by Wei et al. [68],the increase of the cathodic current density from j = −1.3 mA cm−2

to -3 mA cm−2 resulted in the increase of nucleation centers den-sity. This may lead to decrease of the size of postdeposited ZnOnanorods, higher compactness of the film up to the coalescence ofthe grains at some critical current density.

A comparison of two different methods of seeding, spraypyrolysis and galvanostatic deposition at the current density−0.13 mA cm−2 was reported by Elias et al. [44]. The ZnO nanowiresdeposited on both types of buffer layers revealed higher density(up to a factor of 6) in comparison to that formed on naked TCOsubstrate. It was also found that the morphology of the seed layerinfluenced on the diameter of nanowires, which varied from 45 nmfor arrays deposited on the substrate seeded electrochemically upto 160 nm for those grown on the substrate pretreated by spraying.On the other hand, in the latter case a dispersion of ZnO nanowirediameters was very large.

Another important parameter which also influences the mor-phology of the seed layer electrodeposited on the substrate istemperature of the process. Lincot et al. [55] on the base ofchronoamperometric results obtained on a rotating FTO electrodeunder potentiostatic conditions (shown in Fig. 6a), have pro-posed a general mechanism for thin film deposition, illustrated inFig. 6b.

According to this mechanism, the polarization of FTO in thesolution of ZnCl2 saturated with molecular oxygen, at the temper-atures below 34 ◦C leads to a slow electrode passivation with a thinlayer of Zn(OH)2. This results in a monotonic decrease of the cur-rent density (curves 1,2 in Fig. 6a). Transformation of Zn(OH)2 intoZnO takes place at higher temperatures due to slow dehydrationof hydroxide layer initially formed on the electrode. This tran-sition process occurs after some induction period which lengthsdecreases with increasing temperature [55]. The XRD studies [69] ofthe film electrodeposited from the solution of Zn(NO3)2 at low tem-perature (25 ◦C) indicated however, that amorphous zinc hydroxidedeposited in the form of uniform layer on ITO electrode alreadycontains the zones of ZnO (of wurzite structure), which act as thenucleation centers in transformation of Zn(OH)2 to ZnO.

Dehydration of Zn(OH)2 at higher temperatures leads to for-mation of polycrystalline islands (see the scheme in Fig. 6b andSEM picture D in Fig. 7). The process seems to occur by curling theZn(OH)2 fibrils into grains which then play the role of the growing


Fig. 7. A comparison of chronoamperometric curves obtained for polarization of ITO at the potential of −1.2 V vs. Ag/AgCl,Cl− reference electrode in the solution of 0.05 MZ ◦ ◦ ◦ ◦ ◦ f seed ◦

B perm

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n(NO3)2 at different temperatures: 25 C, 40 C, 50 C, 60 C, 80 C and SEM images o-E). The arrows indicate the end of deposition process. Reproduced from [69] with

enters (Fig. 7 B–E). It was found that the full coverage with ZnO ischieved at the deposition charge density above 100 mC cm−2 butt the same time a transparency of the sample strongly decreases69]. In contrast, annealing of the sample seeded at room temper-ture leads to transformation of zinc hydroxide to oxide withouthanges in the film coverage but with the increase of transparencynd therefore, this seeding procedure may be very useful in prepa-ation of ZnO rod arrays for solar cell application. A compact andransparent buffer layer predeposited on TCO substrate prevents

direct contact of electrolyte or solid hole transporting materialnd TCO and hinders the charge carrier recombination (see Section.4).

The influence of the bath temperature and concentration of KCl0.1 M or 1 M) added to the solution of 5 mM ZnCl2, on morphol-gy, composition and transparency of thin layers deposited at theharge density of 800 mC cm−2, have been studied by Ivanova et al.73]. It was found that at higher concentration of KCl the thin lay-rs are amorphous regardless of the bath temperature and probablyontain hydrochlorides (Zn(OH)xCly). Lowering the KCl concentra-ion to 0.1 M leads to various film morphology, dependent of theeposition temperature.

.2. Electrochemical deposition of ZnO nanorod arrays

In electrochemical deposition of ZnO, the interfacial concen-ration of OH− ions may be adjusted by reduction of differentrecursors, such as molecular oxygen (O2) [41,42,45,53] nitrate

ons (NO3−) [74–76] or hydrogen peroxide (H2O2) [77–81]. How-

ver, according to the literature, deposition of ZnO from the
olution of H2O2 precursor leads to formation of more or less porouslms but not nanorods vertically orientated to the substrate. There-
ore, in this review we focus only on the results obtained in theresence of O2 and NO3

− precursors.

ed substrates obtained at different stages of deposition carried out at 60 C (imagesission of Elsevier.

4.2.1. Molecular oxygen precursorThe presence of molecular oxygen in the solution may be

provided by bubbling of oxygen or O2/Ar mixture of different vol-umetric ratio to control O2 concentration in the solution from zeroto saturation value, which in aqueous solution is about 10−3 M atroom temperature and 8 × 10−4 M at 80 ◦C [82].

Electroreduction of oxygen to hydroxide ions is a four-electronreaction:

O2+2H2O + 4e → 4OH-. (4)

Although the standard potential of this process, derived fromthermodynamic data is 0.40 V vs. SHE, at 25 ◦C [83], the reactiontakes place on ITO electrode at the potentials more negative than-0.65 V vs. SCE (i.e. -0.41 V vs. SHE), in the solution of 0.1 M KClelectrolyte [42].

In the presence of Cl− ions, the reaction (4) may be accompaniedby a parallel 2-electron process leading to H2O2 byproduct:

O2+2H2O + 2e → H2O2 + 2OH- (5)

with the rate depending on the Cl− concentration and the elec-trode material [84–86].

A combined measurements of the current intensity and interfa-cial pH performed by Peulon and Lincot [53] in aqueous solution of0.1 M KCl without Zn2+ ions indicated that electroreduction of oxy-gen upon scanning of the electrode potential from -0.6 V to -0.9 Vvs. SCE leads to the increase of pH in the vicinity of the electrodefrom the initial value of 6 to about 10. In the presence of ZnCl2,the OH− ions generated at the electrode are consumed in pre-
cipitation of ZnO and therefore, the interfacial pH remains nearlyconstant (6.5 ± 0.2) within the same polarization range. Thermody-namic calculations have indicated that in the solutions containingZnCl2 at pH ranging from 0 to 6.5 and temperature above 50 ◦C, the


Fig. 8. Plan-view SEM images of ZnO nanowire arrays formed on seeded FTO by electrodeposition at constant potential of −1.0 V vs. SCE in aqueous solutions of 0.1 M KClc M (c),a e charf

ps

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o

ctbc

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ontaining different concentration of ZnCl2: 5 × 10−5 M (a), 1 × 10−4 M (b), 5 × 10−4

s a function of ZnCl2 concentration (e), for the samples obtained after passing of throm [87] with permission of Elsevier.

redominant soluble species is ZnCl+ [55]. Thus, the electrosynthe-is of ZnO may occur via two alternative schemes:

n2+ + 2OH−→ ZnO + H2O (6)

rZnCl+ + 2OH−→ ZnO + H2O + Cl− (7)

The studies performed by many groups, in a wide range of ZnCl2oncentrations (from 5 × 10−5 M to 0.02 M), at different polariza-ion potentials have shown that the morphology of the deposit maye changed from a compact film to nanowires, depending on bothhemical and electrochemical parameters.

The mechanism of electrodeposition of ZnO nanowires on theTO substrate seeded with a thin buffer layer of nanocrystallinenO has been discussed by Elias et al. [87] and Tena-Zaera et al. [88].ccording to their reasoning, the precipitation of ZnO is kinetically

ast and therefore, the aspect ratio of growing nanowires (the ratiof their length to the width) depends on relative concentration ofH− and Zn2+ ions at the substrate. When the electrode reaction is

he only source of hydroxide ions, the [OH−]/[Zn2+] ratio is mainlyetermined by the relative rates of OH− generation and diffusionf Zn2+ to the electrode. From this point of view it is important tonow that diffusion coefficient of Zn2+ (6.2–7.4 × 10−6 cm2 s−1 [89]n aqueous solution of ZnCl2) is much lower than that of molecularxygen, 3.4 × 10−5 cm2 s−1 at 75 ◦C and 2.1 × 10−5 cm2 s−1 at 25 ◦C90]. If production of hydroxide ions is much faster than the trans-ort of Zn2+ ions, the majority of Zn2+ ions arriving at the electrodere consumed in reaction with OH− ions adsorbed on nanowireips, hindering the lateral growth of nanowires and promoting therowth along longitudinal axis [87]. However, under these condi-ions the deposition efficiency, defined as the ratio of OH− ionsonsumed in reaction with Zn2+ and the total amount of OH− ge-erated in electroreduction of O2, is very low (below 17%) and maye improved by the increase of ZnCl2 concentration or by decreasef the current density [88]. It has been also found that significantncrease of the deposition efficiency (above 40%) may be achievedy the increase of KCl concentration to the value of 3.4 M. This effectay be explained by modification of the oxygen reduction mecha-

ism, from four-electron reaction (4) to two-electron process (5) atigh Cl− concentration, leading to decrease of the current density.ome effect may be also ascribed to the lowered solubility of O2 inhe chloride-containing solution.

1 × 10−3 M (d) and nanowire dimensions (diameter (squares) and length (circles))ge density of 20 C cm−2 (filled symbols) and 70 C cm−2 (open symbols). Reproduced

Since the oxygen reduction rate is dependent on the potentialor current density applied to the electrode and the rate of Zn2+ iontransport to the electrode may be controlled by ZnCl2 concentra-tion in the solution (namely, concentration gradient of Zn2+ acrossthe diffusion layer), the growth process is a complex interplayof electrical and chemical parameters. However, there are somethreshold values for deposition of ZnO in the form of nanorods.The lowest concentration limit of ZnCl2 below which ZnO is notdeposited on the electrode is 5 × 10−5 M, whereas at concentra-tions above 5 × 10−3 M the close-packed ZnO nanorods of large dia-meter (>200 nm) undergo to coalescence leading to formation of acompact layer on the electrode [91].

On the other hand, it was found that the films obtained at theconcentration of Zn2+ higher than 5 × 10−2 M in the presence ofKCl may exhibit the sheet-like morphology [92,93] and this trans-formation in the growth habit was explained by adsorption anddirectional effect of Cl− ions. This effect will be discussed in moredetail in Section 5.1.

For the reasons stated above, the studies of the influence of ZnCl2concentration on morphology and the size of ZnO nanorods areusually performed in the concentration range from 5 × 10−5 M to5 × 10−3 M, as illustrated in Fig. 8.

According to the data presented by Elias et al. [87], thenanowire dimensions are the most sensitive to concentrationat [ZnCl2] < 5 × 10−4 M (see Fig. 8e). At the lowest concentra-tion (5 × 10−5 M) the mean diameter of electrodeposited nanorods(∼25 nm) matched fairly well with the size of the crystallites ofthe nanocrystalline ZnO buffer layer which are considered as thenucleation sites. Moreover, the nanorod diameter, in contrast tothe length, is only slightly dependent on the charge density passedduring deposition, i.e. on the deposition time.

The nanorods of smaller diameter (18–20 nm) were obtained byGuo et al. [91] under similar deposition conditions, i.e. in aqueoussolutions of ZnCl2 in the presence of 0.1 M KCl supporting elec-trolyte, at deposition potential of −1.0 V vs. SCE and temperatureof 70 ◦C. Since the only difference in the procedures applied by thesetwo groups was the method of seeding of the substrate, namely a
spin coating of colloidal zinc acetate followed by annealing [91], orelectrodeposition [87], this indicates that the pretreatment of thesubstrate is of crucial importance for the width of the nanorods. Astrong influence of the morphology, thickness and grain size of the

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eed layer on the nanorods diameter has been also confirmed byvanova et al. [73].

Several attempts were also done to prepare the film of wellligned ZnO nanorods from ZnCl2 solution on non-seeded sub-trate [44,89,94,95]. Although the nanorods were formed, theirensity in the film was 6 times lower than density of nanorodsrown on the substrate seeded electrochemically (1 × 109 cm−2 and

× 109 cm−2, respectively) [44]. According to Li et al. [89], the ini-ial 2 min of applied bias are the most important for determininghe diameter of ZnO nanowires because this initial time corre-ponds to formation of nuclei for further growth. However, sincehe nucleation and growth of individual nanowires tends to work in

cooperative fashion, it results in proximate nanowires coalescingnto a pseudo-film on non-seeded substrate even at ZnCl2 concen-ration of 2.8 × 10−3 M.

The mechanism of ZnO electrodeposition on non-seeded FTOubstrate in the chloride medium has been discussed by Belghitit al. [94]. They considered two different regimes, depending onhe pH in the vicinity of the electrode. In the classical deposi-ion conditions, at pH close to neutral, the Zn(II) is present in theorm of Zn2+ ions and zinc complexes, ZnCl+ and Zn(OH)+, which

ay be adsorbed on the negatively polarized electrode. These ionsapidly react with generated OH-, giving rise to the ZnO growthoth in lateral and vertical directions. This situation usually occurs

n the initial deposition period. If zinc ions are in excess in the solu-ion, this growth regime occurs during long time period, leading tooalescence of the growing crystals and formation of a continuous,ense film. If after the initial period the pH of the solution increasesbove 9 (OH− ions are generated at the electrode in excess), theydroxide complexes Zn(OH)3

− and Zn(OH)42− become predomi-

ant soluble species. These ions are not attracted by the negativelyharged electrode and in effect the lateral growth is completelyuenched. However, the growth of nanorods is not fully stoppedut is continued along c-axis due to higher reactivity of the (0001)lane of ZnO [94] (see Section 3).

.2.2. Nitrate precursorThe nitrate precursor of hydroxide ions is used in the form of

n(NO3)2 salt, well soluble in water. It seems to be advantageoushat it can act as the precursor of both reagents, Zn2+ and OH−

ons. Cox and Brajter have shown [96] that the reduction of nitrates catalyzed by the presence Zr(IV) and La(III) ions adsorbed on thelectrode surface. According to Izaki [72] and Yoshida et al. [75], theame effect is responsible for the shift of the reduction potential ofO3

− ions towards less negative values and the mechanism of ZnOlectrodeposition from the solution of Zn(NO3)2 has been proposeds follows:

O−3 + H2O + 2e → NO−

2 + 2OH−, (8)

n2+ + 2OH−temp−→ZnO + H2O (9)

Since Zn2+ plays the role both of the catalyst and reagent, theeaction occurs more likely in one step and it may be written in theorm:

n2+ + NO−3 + 2e

temp−→ZnO + NO−2 (10)

However, Zn2+ ions adsorbed on the electrode may also medi-te the charge transfer in the reduction of nitrate to ammonia,ccording to the scheme:

O−3 + 6H2O + 8e → NH3 + 9OH− (11)

but it is not clear whether this process may have a negative effect
n morphology of electrodeposited ZnO.
Another effect which should be also considered is aging of thetock solution of Zn(NO3)2 which causes that electrosynthesis ofnO from the solution stored for a few days is failed.

mica Acta 127 (2014) 467–488

Since ZnO electrodeposition is usually performed in aerobic con-ditions, the second precursor of OH− is molecular oxygen dissolvedin the deposition bath.

The two-step procedure of formation of ZnO films revealingwurzite structure with (0001) preferred orientation, from 0.1 MZn(NO3)2 precursor has been developed by Izaki [72]. In the firststep performed at -1.2 V the substrate was covered with granularZnO particles, whereas the film grown in the second step at thepotential of -0.7 V was composed of the aggregates of hexagonalcolumnar grains. Under such conditions a high concentration ofZn2+ at the electrode provides immediate consumption of gener-ated OH− and the ZnO is deposited in the form of the film witha smooth surface. The kinetic studies performed by Yoshida et al.[76] with the use of rotating disc electrode in the solution of 0.1 MZn(NO3)2 have shown the rate constant of NO3

− reduction is verylow (5.98 × 10−10 cm2 s−1) and therefore, this reaction is a ratedetermining step in electrodeposition of ZnO. Thus, in order toobtain ZnO in the form of well separated nanorods, one shoulddecrease the Zn(NO3)2 concentration to the value below 5 × 10−3 Mor manipulate with the concentration ratio of Zn2+ to OH− ionsat the electrode by addition of NaNO3 electrolyte or/and by theincrease of deposition potential to speed up the rate of the chargetransfer at the electrode/solution interface [97,98].

The results of systematic studies on the influence of differentparameters on the growth of ZnO nanorods from the solution ofZn(NO3)2 have been reported recently by Tena-Zaera et al. [99].Similarly to the process carried out in the presence of O2, twodifferent regimes of electrodeposition, controlled either by Zn2+ dif-fusion or OH− generation were distinguished. If diffusion of Zn2+ issignificantly slower than generation rate of OH− ions, the growthof nanowires takes place mainly along longitudinal axis (Fig. 9a).When the rates of these two processes are of the same order, thegrowth occurs both along longitudinal and transversal axis leadingto nanorods of larger width (Fig. 9b).

Since the diffusion rate of Zn2+ ions depends on the concen-tration gradient across the diffusion layer, the diameter of ZnOnanorods may be controlled by the change of Zn(NO3)2 concen-tration in the bulk of solution, at constant and high concentrationof NaNO3 [99], as it is illustrated in Fig. 10. The observed tendencyof the increase of nanorod’s diameter is similar to that reportedfor ZnO deposition from the solution containing ZnCl2 and O2 [87],discussed earlier in the text (Section 4.2.1) (see also Fig. 8).

It is worth noting that the results reported above were obtainedon the FTO substrate seeded with ZnO thin layer. Electrodepositionof ZnO in the form of nanorods from the solution of low Zn(NO3)2concentration (10−4 M) is also possible on the bare substrate (with-out predeposited seed layer) but the nanorod density in a such filmis very low [100] (see Fig. 11 a,b).

The studies on the influence of Zn(NO3)2 concentration onthe average nanostructure diameter and surface coverage ratioperformed by Sun et al. [101] have demonstrated that at[Zn(NO3)2] < 1 mM only 30% of the whole non-seeded substrate wascovered with ZnO nanowires (Fig. 12). The increase of Zn(NO3)2concentration led to the improvement of the substrate coveragewith nanorods and to the increase of their diameter but on the otherhand, a dispersion of nanorods diameters also increased (Fig. 11c).

A strong influence of Zn(NO3)2 concentration on the quality ofnanorods grown on non-seeded ITO substrate is also confirmed bythe results presented by Lin et al. [97]. A film of vertically orientedrods with the lengths of about 4 m and diameter ∼450 nm wasobtained after 4 h deposition from the solution of 5 mM Zn(NO3)2 atconstant current density of -0.9 mA cm−2 and temperature of 70 ◦C
(see Figs 13 a,b). However, the increase of zinc nitrate concentrationabove 10−2 M resulted in the growth of the rods of greater diame-ter, randomly oriented to the surface with tendency to coalescence(Fig. 13c).


Fig. 9. Schematic view of the growth mechanism of ZnO NWs by electrodeposition from nitrate-based solutions in the case when diffusion of Zn2+ ions is significantly slowerthan OH− generation rate (a) and the OH− generation rate and Zn2+ diffusion rate are of the same order (b). Reproduced from [99] with permission of Elsevier.

F ed su5 −0.25o

ccocet8

wZrsMp0

Fc

ig. 10. Top-view SEM micrographs of ZnO nanowires electrodeposited on the seed × 10−3 M (c). The other deposition parameters were kept constant (q = 2 C cm−2, j =

f Elsevier.

Lin et al. [97] have also investigated the influence of depositionurrent density on the nanorod diameter and length, at constantoncentration of Zn(NO3)2 (0.005 M). It was found that the increasef the current density from −0.9 mA cm−2 to −2 mA cm−2, and inonsequence the increase of concentration of OH− generated at thelectrode, resulted in the increase of both dimensions in such wayhat the aspect ratio of nanorods also increased from about 4.6 to.4.

The ZnO nanorods vertically arranged on non-seeded substrateere also successfully obtained from the solution of 0.005 M

n(NO3)2 by potentiostatic deposition at the potentials from theange between -0.6 V and −1.1 V [102]. However, the rods were of
ubmicron width and the diameter distribution was rather large.oreover, the size of nanorods was very sensitive to deposition
otential. Namely, the thinnest nanorods, of average diameter.24 m and aspect ratio 5.4, were obtained at the deposition

ig. 11. Plan-view SEM images of the ZnO nanowires electrodeposited on naked ITO

oncentration: 10−4 M (a), 5 × 10−4 M (b) and 10−3 M (c), at 70 ◦C. Reproduced from [100]

bstrates at different concentrations of Zn(NO3)2: 5 × 10−5 M (a), 5 × 10−4 M (b) and mA cm−2, 0.1 M NaNO3, temperature 80 ◦C). Reproduced from [99] with permission

potential -1.0 V but the increase of the potential to −1.1 V resultedin significant thickening of nanorods to 0.73 m and their coales-cence. A broad distribution of the NR diameters was also observedafter decrease of Zn(NO3)2 concentration to 10−4 M (at the depo-sition potential −1.0 V vs. SCE). The influence of concentration ofthe second precursor, NO3

-, on diameter of ZnO nanorods grownunder potentiostatic conditions on non-seeded ITO electrode hasbeen demonstrated by Xue et al. [98]. It was found that one cancontrol the average diameter of the nanorods by addition an extraamount of NaNO3 to the deposition bath, keeping other parame-ters of the process constant ([Zn2+] = 0.005 M and deposition time1800 s at the potential of −1.0 V), as it is presented in Fig. 14.
It is worth noting that morphology, size and density of the ZnONRs obtained under these conditions were similar to those of thenanorods formed on the seeded substrate [99] (for comparison seeFig. 10).
substrate at the potential of −1.1 V vs. SCE in solution of Zn(NO3)2 of different with permission of Journal of Ceramic Processing Research.

476 M. Skompska, K. Zarebska / Electrochi

Fig. 12. Mean values of nanorod diameter and coverage ratio as a function of[Zn(NO ) ]. Electrodeposition was carried out at the constant potential of -1.1 V vs.Ao

htrtcitt

5a

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growth of ZnO rod arrays [37,60,66,106]. However, the mecha-

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3 2

g/AgCl, for 30 min at temperature of 80 ◦C. Reproduced from [101] with permissionf Elsevier.

As it has been discussed above, irrespectively of the type ofydroxide precursor, the small changes in interfacial concentra-ion of Zn2+ and OH− ions lead to strong changes of quality, aspectatio and arrangement of electrodeposited nanorods with respecto the surface, making difficult the precise control of growth pro-ess. Therefore, intensive studies have been undertaken towardsmprovement of the quality of ZnO nanostructures and modifica-ion of their shape by addition of organic or inorganic compoundso the deposition bath.

. Modification of the ZnO structure by different solutiondditives

.1. The influence of KCl

Since ZnCl2 salt is used in electrodeposition of ZnO from theolutions containing oxygen precursor, the chloride ion is a naturalomponent of the deposition bath. Another source of Cl− ions isCl or NaCl very often added to the deposition bath to provide aood conductivity of the solution at low concentration of Zn2+. Itas been found however, that Cl− ion due to strong adsorption onnO nanocrystals acts as a capping agent blocking certain faces andnhancing the growth of other facets [103,104]. The scheme of therocess, in which the preferable adsorption of Cl− onto positively
harged, Zn-terminated (0001) plane improves the growth along1010) direction leading to plate-shaped ZnO, is illustrated in Fig. 1564].
ig. 13. Plan-views (a, c) and cross-sectional (b) SEM micrographs of ZnO nanorod arrays

.005 M ((a) and (b)) and 0.05 M (c). Other parameters: temperature 70 ◦C, current densilsevier.

mica Acta 127 (2014) 467–488

It should be stressed however, that the results referred abovewere obtained on non-seeded ITO substrate, in the solution of rel-atively high Zn2+concentration (0.05 M) which is likely too high toform the ZnO nanorods.

The studies reported by Taena-Zera et al. [86,88] carried outon the seeded ITO at concentration of ZnCl2 two orders lower(5 × 10−4 M) have demonstrated that the arrays of vertically ori-ented ZnO nanorods were formed even in the solution saturatedwith KCl (3.4 M) (see Section 4.2.1). As it has been already dis-cussed, a high concentration of chlorides results in slowing downthe rate of generation OH− ions due to the change of the mecha-nism of O2 reduction from 4-electron process to 2-electron reactionwith formation of H2O2 intermediate species at high [Cl−]. In effect,the concentration ratio [Zn2+]/[OH−] at the electrode becomes suit-able for efficient growth of ZnO and formation of longer nanowires[86]. However, at [KCl] >1 M a significant increase of the nanowirethickness (from 80 nm up to 300 nm) was also observed which wasexplained by preferential adsorption of Cl− on the (0001) plane ofZnO which stabilizes of the (0001) face and favors the lateral growthof nanowires.

Cui et al. [105] have studied the growth of ZnO nanowiresfrom the solution containing 2.5 mM Zn(NO3)2 in the presenceof equimolar concentration of hexamethylenetetramine (2.5 mM)and different concentration of NH4Cl (from the range 0-50 mM).Their results confirm that the increase of Cl− concentration sup-presses the growth along c-axis and promotes the lateral growth ofnanowires and therefore, the growth rate decreased from 2.5 mh-1 in the absence of NH4Cl, to 1.3 m h-1 in the presence of 50 mMNH4Cl. The presence of the peak at 2.63 keV in EDX spectrum andthe increase of its high with increasing amount of NH4Cl in thedeposition bath led them to the conclusion that chlorine is incor-porated into ZnO nanowires. The increase of ammonium chlorideresulted also in the shift of the diffraction peaks in the XRD spec-trum to lower angles which was explained by lattice expansionof ZnO nanowires due to substitution of O2− (of the size 1.4 A) bylarger Cl− ions (1.81 A).

5.2. The influence of hexamethylenetetramine (HMT)

One of the most popular organic compounds added to the depo-sition bath to support the growth of ZnO nanorods of requiredmorphology is methenamine, also known as hexamethylenete-tramine (HMT) or hexamine. This non-ionic, cyclic tertiary amine,highly soluble in water has been widely used in hydrothermal

nism which governs the deposition process in the presence ofHMT is still not clear. It is known that in acidic aqueous solu-tion at elevated temperature HMT hydrolyses to formaldehyde and

electrodeposited on naked ITO from aqueous solution of Zn(NO3)2 of concentrationty 0.9 mA cm−2 and deposition time 4 h. Reproduced from [97] with permission of


F − ions

at

(

N

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ig. 14. SEM images of ZnO nanorods deposited at different concentration of NO3

ubstrate. Reproduced from [98] with permission of Elsevier.

mmonia, which undergoes to acid-base equilibrium, according tohe schemes:

CH2)6N4 + 6H2OH+�OH

6COH2 + 4NH3 (12)

H3 + H2O ⇔ NH+4 + OH− (13)

Thus, HMT acts as an additional source of OH-, promoting the
ucleation and growth of ZnO. Besides, it is supposed that HMTlays a role of buffer because the rate of its hydrolysis decreasesith increasing pH and vice versa [66]. Different approach has beenresented by Sugunan et al. [107] who proposed that HMT, being a
ig. 15. SEM image of plate-shaped ZnO electrodeposited from aqueous solutions contcheme of corresponding out-of-plane growth of ZnO plates vertically oriented to the ITO

s: 0.005 M (a), 0.01 M (b), 0.05 M (c), 0.1 M (d), at [Zn2+] = 0.005 M on non-seeded

nonpolar chelating agent is preferentially attached to the nonpolar(1010) and (1011) facets of the ZnO crystal, thereby cutting off theaccess of Zn2+ ions to their surfaces, leaving only the polar (0001)face for epitaxial growth along [0001] direction. However, it seemsthat this mechanism may be taken into account at higher pH whenhydrolysis of HTM is strongly inhibited.

The shaping properties of HTM has been also studied with
respect to ZnO nanowires obtained by electrodeposition. It has beendemonstrated that addition of equimolar concentration of HTM tothe solution of 0.03 M Zn(NO3)2 led to significant improvementof crystallinity and alignment along c-axis of the ZnO nanorods
aining 0.05 M Zn(NO3)2 + 0.1 M KCl at constant potential of -1.1 V vs. SCE (a) and substrate (b). Reproduced from [64] with permission of Elsevier.


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ig. 16. Comparison of SEM images (a,b) and XRD pattern (c) of ZnO nanorods electrithout (a) and in the presence (b) of 0.03 M HMT. Reproduced from [108] with pe

eposited on FTO substrate seeded with a thin layer of TiO2 [108]Fig. 16).

The electrodeposition of aligned compact ZnO arrays on thelass slides covered with thin Ag film from the solution contain-ng equimolar concentrations (0.03 M) of Zn(NO3)2 and HMT wastudied by Chandler et al. [109]. Although the layer of Ag provided

good electrical conductivity of the substrate, a distribution of dia-eters of electrodeposited ZnO nanorods was very large, from

0 nm up to 500 nm.The HMT–assisted procedure has been also applied for elec-

rodeposition of large-scale and dense ZnO nanorod arrays on amooth ITO without any seeding [110].

A comparison of morphology and XRD patterns of ZnO filmslectrodeposited on a bare substrate from the solution of 0.01 Mn(NO3)2 of pH 5.5, in the presence of HMT and without it, pre-ented in Fig. 17, indicate that addition of equimolar amount ofTM improves orientation of ZnO nanorods with respect to the

ubstrate, prevents from their coalescence and provides uniformistribution of nanorods diameters (200–300 nm).

The influence of excessive amount of HTM on the morphologynd optical properties of ZnO grown under potentiostatic regime in
he solution of 6.3 mM Zn(NO3)2 has been studied by Cui [111]. Itas found that the increase of HTM concentration above 25 mM up
o 43 mM resulted in the formation of corn-like shape ZnO nanorodsith a rough surface, in contrast to smooth surface obtained at

sited on FTO/TiO2 substrate at −0.7 V vs Ag/AgCl in the solution of 0.03 M Zn(NO3)2

on of Springer.

equimolar concentration of Zn2+ and HTM. This effect was inter-preted as a consequence of microscopic variation of hexamineattachment to the nonpolar facets of nanorods, leading to inho-mogeneous growth of ZnO.

5.3. The influence of ethylenediamine (EDA)

EDA is a strong bidentate chelating agent, which complexes zincions with high stability constant (higher than those of ammonia andOH−). At elevated temperature zinc-EDA complex decomposes andreleased EDA molecules undergo to hydrolysis. This results in theformation of NH+

3 (CH2)2NH+3 , bearing two positive charges:

[Zn(NH2(CH2)2NH2)3]2+ → Zn2+ + 3NH2(CH2)2NH2 (14)

NH2(CH2)2NH2 + 2H2O → NH+3 (CH2)2NH+

3 + 2OH− (15)

The influence of EDA on morphology of ZnOnano/microstructures electrodeposited on non-seeded sub-strate from the solution of 0.05 M Zn(NO3)2 has been reported byand F. Xu et al. [64] and L. Xu et al. [104]. As it is illustrated inFig. 18a, addition of EDA to the deposition bath allows formation
of hexagonal taper-like nanostructures even at relatively highconcentration of Zn(NO3)2. The tapers of the length 2 m arerandomly oriented to the substrate and their diameter decreasegradually from 100–500 nm at the base to several nm at the top.


F ited o(

ttnasKsEi

FE

ig. 17. Plan-view SEM images (a, b) and XRD pattern (c) of ZnO films electrodeposb) 0.01 M HMT. Reproduced from [110] with permission of Springer.

It was also found that in the presence of 0.06 M KCl in the elec-rodeposition bath containing 0.05 M Zn(NO3)2 and 0.01 M EDA,he obtained ZnO film consisted of tightly arranged hexagonalanorods of diameter 100–300 nm (instead of tapers), verticallyligned on the ITO substrate (Fig. 18 b). In addition, it was pos-ible to diminish the aspect ratio of nanorods by the increase of
Cl concentration or decrease of EDA concentration [104]. Thistructure-modifying effect was explained by adsorption of neutralDA molecules on the lateral (1010) face and negatively charged Cl−
ons on the polar (0001) plane. In the absence of Cl− ions a strong

ig. 18. SEM images of the ZnO nanostructures obtained by electrodeposition at constantDA (a) and 0.01 M EDA + 0.06 M KCl (b). Reproduced with permission from [104]. Copyri

n non-seeded ITO from aqueous solution of 0.01 M Zn (NO3)2 without (a) and with

coordination ability of EDA to zinc limits the growth along the sidedirection and enhances the crystal growth along [0001] direction.As a result, the (0001) face contracts and even disappears to forma hexagonal taper shape. In the presence of both capping agents,a simultaneous adsorption of EDA on lateral

(1010

)plane and Cl−

on (0001) face, results in the decrease of the contracting rate of the
(0001) plane and in effect the ZnO grows in the form of hexagonalnanorods.
The change of morphology from the taper-like ZnO nanostruc-tures to nanorods was also observed by F. Xu et al. [64] upon

potential of -1.1 V vs. SCE from the solution of 0.05 M Zn(NO3)2 containing 0.013 Mght 2005 American Chemical Society.


F of -1d lutiong Elsevi

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6

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ig. 19. SEM images of the ZnO structures electrodeposited at constant potentialifferent growth stage (a,b), 0.05 M Zn(NO3)2 + 0.03 M EDA (c), and a schematic evoradually increasing addition of EDA (d). Reproduced from [64] with permission of

ncrease of concentration of EDA in the electrodeposition bath of.05 M Zn(NO3)2 (in the absence of Cl−). In contrast to the resultsiscussed above, the taper-like structures obtained at low EDAoncentration (0.005 M) were arranged upside-down, as shown inig. 19a.

This special arrangement of the tapers on the substrate wasxplained by adsorption of positively charged NH+

3 (CH2)2NH+3

roduct of EDA hydrolysis on the negatively charged cathode inhe initial stage of the process. A gradual increase of EDA concen-ration to 0.03 M resulted in the change of ZnO morphology fromaper via short-fat and finally to long-thin rods (see the scheme inig. 19 d). It has been postulated that at longer time scale the tapersransform into the short-fat ZnO rods. The formation of longer andhinner rods at higher concentration of EDA was explained by totaloverage of lateral

(1010

)planes of ZnO crystal by adsorbed EDA

olecules which enhances the growth in [0001] direction [64].Preferential adsorption of EDA molecules and OH− anions on dif-

erent crystal faces (on the lateral and (0001) plains, respectively)as also used to achieve a selective dissolution of the ZnO nanorods

112]. According to the postulated mechanism, the adsorbed EDAolecule provides the electrons to Zn atoms, changing the charge

istribution between Zn and O atoms, and strengthening the Zn- bond. As a result, the ZnO dissolution rate by OH− ions at thedge of the (0001) surface becomes much slower than in the centereading to formation of nanotubes. The quality of nanotubes maye additionally improved by decrease of etching rate by applicationf a small positive potential to the nanotubes.

. Optical properties of ZnO nanorods

Practical applications of ZnO nanorods in optoelectronic devices,uch as short-wavelength lasers, light-emitting diodes and hybridolar cells, require the nanocrystals of high quality. Therefore, thereparation procedures including synthesis of nanostructures and
heir later treatment (such as annealing) should be optimized to
inimize the structural defects which may influence the deviceerformance. One of the methods applied to investigate the opticalroperties of ZnO is fluorescence spectrophotometry.

.1 V vs. SCE from aqueous solution containing: 0.05 M Zn(NO3)2 + 0.005 M EDA at of ZnO morphology from taper-like crystals to short-fat and long-thin ZnO rod byer.

The photoluminescence (PL) spectrum of ZnO at room tempe-rature usually shows a near band-edge UV emission peak centeredat 3.26 eV (380 nm) and at least one broad band extending between400 nm and 750 nm, i.e. in the whole visible region. The broadnessof the band results from overlapping of several visible emissionslocalized in green (centered around 2.4 eV), yellow (about 2.2 eV)and red or orange (around 2.07 eV) ranges [113,114].

At low temperature of 4 K, the UV near band edge peak ofelectrodeposited ZnO is shifted to higher energy and splits intotwo bands, one centered at 3.366 eV attributed to band-to-bandtransitions and the second one, at 3.328 eV, ascribed to recombi-nation process including surface states [115]. The contribution ofthe latter peak in the PL spectrum increased with the decrease ofthe nanorod diameter due to rising surface-to-volume ratio. Theimprovement of the PL spectrum (sharpening of exciton-relatedtransition peaks) was achieved by thermal annealing of the samplefor 60 min at 500 ◦C leading to decrease of donor density by a factorof 20 (from about 8 × 1019 cm−3 to 4 × 1018 cm−3). In the presenceof high donor densities, the overlapping of the wave functions ofneighboring donors results in formation of a band of donor states[116] and then a recombination processes takes place between thisband and the valence band.

The UV emission peak is expected to be a dominant signal in thePL spectrum of ZnO nanorods of good crystallinity, free of struc-tural defects. Some shift of its position in the spectrum as well asthe change of the shape may be ascribed to the presence of ZnOdoping [105,117] or/and quantum confinement effect (blue-shift)[102]. The broad visible band, known also as deep-level emission(DLE) peak is usually ascribed to intrinsic “native” defects in ZnO,such as zinc or oxygen vacancies and interstitials or antisite oxy-gen [114,118]. However, according to the literature, Zn antisites,O antisites and O interstitials have high formation energies andunder normal conditions they are probably not present in largeconcentration [119]. Thus, the oxygen and Zn vacancies as well asZn interstitials seem to be the predominant ionic defects and the
growth environment controls their concentration in ZnO. Thesedefects introduce the levels in the band gap of the semiconduc-tor, responsible for transitions between different charge states.Although the exact assignment of the visible emission peaks is still


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ig. 20. The influence of deposition temperature (a) (Reproduced with permissionhe deposition bath (b) (Reproduced with permission from [105]. Copyright 2008 Aere obtained at excitation wavelengths of 266 nm and 325 nm, respectively.

matter of debate, it is generally accepted that the green lumi-escence band centered around 510 nm (2.4–2.5 eV) results fromecombination of electrons in singly ionized oxygen vacancy withhotogenerated holes in the valence band [120] and/or electronransition from conduction band to zinc vacancies level (Vzn) situ-ted 0.9 eV above the valance band [121].

The relative intensity of UV exciton emission to visible DLE bandIUV/Ivis or Iexc/IDLE) is usually the way of quality evaluation of differ-nt ZnO samples [122]. It should be stressed however, that this ratioay be also dependent on excitation efficiency of the photons with

ifferent energies and excitation area [105,123,124] and therefore,he spectra used for comparison of different samples should bebtained under the same excitation conditions.

A systematic study on the influence of the main growth para-eters of ZnO electrodeposition (temperature, Zn2+ concentration,

verpotential) on the optical parameters of ZnO nanorods has beeneported by Pauporte et al. [125]. The deposition was performedn non-seeded FTO, from the solution of Zn2+ at low concentration0.2 and 5 mM), saturated with molecular oxygen as the source ofH−. It has been demonstrated that the increase of deposition tem-erature from 70 ◦C to 88 ◦C resulted in significant increase of UVxciton emission and quenching the visible band (Fig. 20 a). Thiseans that electrodeposition temperature is a key parameter in the

rowth of high quality ZnO nanorods containing low concentrationf structural defects.

It was found [125] that the excitonic band also increased andecame more narrow when ZnCl2 precursor was replaced byn(ClO4)2, which indicates that the presence of chlorides enhanceshe formation of defects in the ZnO structure. In contrast, Cui et al.105] have observed an increase of relative intensity of the UV andisible peaks (IUV/Ivis) with the increase of amount of ammoniumhloride in deposition bath (Fig. 20 b), which was interpreted byeduction of the oxygen vacancies in ZnO nanowires. Tena-Zaerat al. [126] analyzed the evolution of the lattice parameters atncreasing KCl concentration and concluded that the formation ofhe structural defects is a result of the change of local compositionf the electrolyte around the nanowires.

According to the results of Cao et al. [127], the intensity ratioexc/IDLE is also dependent on the deposition current density whichontrols the growth rate along the c-axis (see Section 4). The most
avorable ratio (about 15), obtained for the noncompact and well-aceted ZnO nanorod array films deposited at the current density of1.5 mA/cm2, was similar to that of the film prepared by plasma-ssisted MBE method on sapphire substrate [128]. Very strong and
[125]. Copyright 2009 American Chemical Society) and concentration of NH4Cl incan Chemical Society) on photoluminescence spectra of ZnO NR films. The spectra

sharp UV emission peak and a weak green emission signal werealso obtained for well-aligned nanoneedle arrays galvanostaticallydeposited on the silicon substrate coated with an oriented gold film(Au/Si) [124].

The increase of the intensity ratio Iexc/IDLE was also observed byCui et al. [105] upon the increase of ZnO deposition potential of andthis effect was ascribed to improvement of crystalline quality andreduction of defect density in the nanowires. The concentration ofthe defects, controlled by the change of hexamine in the electrode-position bath, was found to play an important role in the excitonemission from ZnO nanorods. A decrease of the exciton emissionat high concentration of the defects in ZnO grown at low concen-tration of HTM, has been ascribed to the exciton scattering by thedefects [111].

Another important parameter enhancing the relative intensi-ties of UV and visible emission peaks is seeding of the substratewith a thin ZnO film and it was found that the intensity ratio wasdependent on the seeding current density [68]. This confirms thatthe nucleation centers formed on the substrate during seeding playimportant role in improvement of the structural properties of theZnO nanowires.

The improvement of the crystallographic structure of ZnOnanorods may be also achieved by annealing of the sample. Tena-Zaera et al. [126] have explained this effect by reduction of defectsdensity due to out-diffusion of the Zn interstitials at high tem-perature. The enhancement of the PL spectrum may also resultfrom transformation of remaining traces of amorphous Zn(OH)2into ZnO, especially in the case of the samples electrodeposited atrelatively low temperature [129]. The removal of Zn(OH)2 traces ineffect of annealing at 380 ◦C leads also to the increase of transmit-tance of the sample in the visible wavelength range as well as to theshift of absorption edge towards lower energies due to decrease ofthe band gap energy to the value typical of ZnO (3.2–3.4 eV).

The influence of doping of ZnO nanorods by metal ions (Cu, Co,Al) via electrodeposition on electroluminescence spectra have beenstudied by Seipel et al. [130]. Incorporation of metal ions, confirmedby SIMS analysis, resulted in a shift of a broad defect luminescencepeak, from 600 nm for undoped ZnO nanowires towards longerwavelengths. Although the ionic radius of the host ion is largerthan the ionic radii of the dopants, no clear trend toward decreasing
lattice constants was observed.
A vertical arrangement of nanorods with respect to the substrateand formation of dense nanorod arrays result in a strong light scat-tering which is important feature in terms of application of ZnO


Fig. 21. Schematic energy diagram in the radial direction of a nanowire indicating the depletion layer at the surface and the quasi-neutral region of radius x in the center.E sity (( oncen2

isgaa(ip4isoarsb

7

adhiAorAolo(

V

waa

tm

C

c is the lower edge of the conduction band and EF is the Fermi level (a). Donor denfilled squares) and annealed (open squares) ZnO nanowires as a function of KCl c008 American Chemical Society.

n the solar cells. Tena Zaera et al. [23] have analyzed a relation-hip between dimension of ZnO NRs and light scattering with theoal to optimize the optical configuration of the nanorod arrays andttain efficient solar light absorption. It was found that for smallnd constant NR diameter the maximum of total ZnO reflectancemaximum of scattering) increased from 8.6% to 20.4% with increas-ng nanorod length (from 0.5 to 2.0 m, respectively). The mostronounced enhancement was observed at the wavelength of00 nm. On the contrary, for a constant NRs length (1.5 m), the

ncrease of their diameter (from 105 nm to 330 nm) gave rise to con-iderable redshift of the reflectance maximum without the changef its value. This means that the control of the nanorod dimensionsllows monitoring of the light scattering over a wide wavelengthange. These results are important in terms of the increase of theolar light harvesting in nanostructural solar cells. This aspect wille discussed further in Section 8.

. Electrical properties of electrodeposited ZnO nanorods

Determination of electric properties of ZnO nanorod arrays, suchs charge carrier density and flatband potential, is a challenging taskue to particular morphology of the arrays. Mora-Sero et al. [131]ave adapted electrochemical impedance spectroscopy to derive

nformation on electronic properties of the entire nanowire surface. cylindrical Mott-Schottky model, taking into account geometryf nanowires, has been developed by them to determine the car-ier density in ZnO nanorods electrodeposited on FTO substrate.ccording to this model, each nanowire is described as a cylinderf radius R with axial symmetry and donor density ND. A circu-ar depletion layer is formed from the surface towards the centerf nanowire, as illustrated in Fig. 21a and the potential differenceVsc) across the space-charge region was defined as [131]:

SC = −qND

2ε

[12

(R2 − x2

)+ R2 ln

(x

R

)], (16)

here ε is the dielectric constant of ZnO, q is the elementary chargend x is a radius of quasi-neutral region, dependent on the potentialpplied.

It was also found [126] that the dependence of the total capaci-ance of the nanowire array on the radius of the neutral region (x)

ay be expressed by the equation:

= 2�εLDnwS

ln(

R/x) (17)

Nd) determined from electrochemical impedance spectroscopy (b) of as-depositedtration in the deposition bath. Reproduced with permission from [126]. Copyright

where Dnw is the nanowire density per flat surface unit, L is thelength of nanowire, while S is the total flat surface.

The impedance spectra obtained at different potentials werefitted to a simple equivalent circuit with a resistance (corre-sponding to a sum of resistances of the solution, ohmic contacts,etc.) in series with the loop containing parallel resistance (cor-responding to charge transfer resistance) and capacitance. Theextracted capacitance was then used in Mott-Schottky analy-sis.

The influence of the chloride concentration (KCl) ranging from0.05 M to 3.4 M in the deposition bath (containing also 5 × 10−5 MZnCl2 and saturated with O2) and annealing of resultant sampleson the donor density in ZnO nanowires have been studied by Tena-Zaera et al. [88,126]. A linear Mott-Schottky plots (1/C2 in a functionof applied potential), typically observed for flat samples [132], wereachieved for as-grown nanorods. The donor density in these sam-ples was found to increase from 5 × 1018 cm−3 to 2.4 × 1020 cm−3

when the KCl concentration in the deposition bath was changedfrom 0.05 M to 3.4 M, respectively (Fig. 21 b). These values are sig-nificantly higher than those obtained for ZnO nanowires formedby vapor phase techniques (1016–1018 cm−3) [133,134]. Higherdonor density in the nanorods deposited at higher concentra-tion of KCl was ascribed to the decrease of the formation energyof ZnO intrinsic defects (oxygen vacancies and Zn interstitials)in reach [Zn2+] conditions [88]. The latter one is a consequenceof the increase of [Zn2+]/[OH−] ratio because of low generationrate of OH− at high chloride concentration (see Sections 4.2.1and 5.1). It was also found that a thermal annealing of electrode-posited ZnO nanowires at 450 ◦C (for 1 hour) results in the curvedMott-Schottky plots and leads to the lowering of donor densityto 1017–1018 cm−3 [88,131]. The decrease of donor density uponannealing was explained by the out-diffusion of the Zn intersti-tials.

The spatially resolved insight in the free-carrier distribution inZnO nanorods electrodeposited from the solution containing 3.4 MKCl was presented by Stiegler et al. [135]. As-grown and annealedsamples were studied by means of scattering-type scanning near-field optical microscopy at infrared frequency (infrared s-SNOM).In this technique a metalized tip is illuminated by a focused IRlaser beam and back scattered radiation is recorded simultaneouslywith topography, yielding nanoscale resolved images [136]. Due to
interactions between the tip and the sample, the light scattered bythe tip is modified in both its amplitude and its phase, dependingon the local dielectric properties of the sample. The optical reso-lution of s-SNOM is only limited by the tip’s radius of curvature,

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sually 10-20 nm. From the nanoscale-resolved IR amplitude andhase images, one can conclude on the local structural properties,aterial composition [137,138] and the free-carrier concentration

n doped semiconductor nanowires [139]. The results presentedy Steigler et al. [135] revealed a large amount of free carriersleading to high conductivity) due to the point defects in the shellegion of as-deposited ZnO nanorods. Lower conductivity in theore of nanowires was explained by trapping or scattering of free-arriers at the grain boundaries. It was also found that annealingesults in the reduction of free carriers concentration in the shellithout structural changes in this region. Moreover, the annealing

mproved crystallinity of the nanowire core.

. Applications of ZnO nanorods

The unique properties of ZnO nanostructures and develop-ent of various cheap methods of their synthesis have triggered

great interest in practical industrial and medical applicationsf ZnO nanostructures. Over previous several years there haveeen published a number of reviews on application of ZnO inifferent morphological forms (nanorods, nanotubes, nanobelts,anosheets, nanostars, etc.) [60,140–144]. In this section we focusnly on selected applications of the nanorods and discuss thespects related to their properties discussed in the previous sec-ions. Although the nanorods in the references quoted below were

ainly formed by hydrothermal methods, the considerations coulde likely extended to ZnO nanorods obtained by electrochemicaleposition.

.1. Sensors

ZnO in the form of nanorods is attractive material for gas sensorsith great possibility for overcoming the fundamental limitations

wing to their ultrahigh surface-to-volume ratio and electronicarameters strongly influenced by surface processes. ZnO nanorodsemonstrate high sensitivity even at room temperature, whereas ahin-film gas sensors often need to be operated at elevated tem-eratures [141]. Therefore, application of the ZnO nanorod (oranowire) arrays for sensing various gases, including H2, NO2, O2,O, and NH3, have been reported extensively over the previousears [145–149]. The sensing process is related to crystallographictructure of ZnO and/or oxygen vacancies in the ZnO nanorods. Thedsorption of gas molecules on Zn-terminated (0001) polar surfacend/or on the structural defects influence the electrical behavior ofanorods. Namely, the adsorption of electron accepting molecules,uch as O2 results in withdrawing of electrons, leading to formationf O2

− and decrease of conductivity of ZnO [150]. On the other hand,hen ZnO is exposed to the target gas of reducing molecules, theseolecules could react with oxygen ions adsorbed on the surface and

elease the trapped electrons back to the conduction band, resultingn the increase of conductivity [141]. The real problem is gas selec-ivity and sufficient sensitivity. These parameters may be improvedy deposition of Pd clusters on ZnO nanorods [146], manipulationf the nanorod diameter or modulation by the voltage.

A metal–semiconductor junction (Schottky diode), consisted ofnO nanorods electrodeposited on FTO substrate with a silver con-act formed on the top of nanorods, has been used as a roomemperature sensor for relative humidity [151]. The sensing pro-ess in this device is based on the adsorption of water moleculesn the nanorod surface and decrease of concentration of majo-ity carriers in the conduction band of semiconductor, according
o the relation associated to the work-function of Schottky barrier.he response time was around 5 s and the sensitivity showed aogarithmic relation in the range between 20% and 75% of relativeumidity.
mica Acta 127 (2014) 467–488 483

8.2. Biosensors

ZnO nanorods with high isoelectric point, ranging from 8.7 to10.3, are attractive substrate for loading of enzymes of low iso-electric point, such as cholesterol oxidase (IP = 4.7) [142], glucoseoxidase (IP = 4.2) [152,153] and uricase (IP = 4.3) [154], throughelectrostatic interactions. Important advantage is also excellentbiological compatibility of ZnO helping enzyme to retain its bioac-tivity. Besides, it provides a direct electron transfer betweenenzyme’s active site and the electrode and therefore, it can beemployed for developing implantable biosensors. A glassy carbonelectrode modified with Sb-doped ZnO nanowires has been usedfor determination of L-cysteine by it electrochemical oxidation[155]. Such electrode exhibited a high reproducible sensitivity, alow limit of detection, favorable stability and high resistance tointerference. ZnO-based biosensors have been developed in vari-ous configurations, depending on the type of answer, such as fieldeffect transistors (FET), ion-sensitive field effect transistors (ISFET),optical devices, piezoelectric devices and electrochemical trans-ducers. Recent advances in application of ZnO nanostructures inbiosensor application have been summarized in the review papers[11,142].

8.3. Light emitting diodes

ZnO is promising material for short-wavelength luminescentand lasing devices due to its characteristic direct and wide bandgapand large excitation binding energy of 60 meV. The p-n hete-rojunction electroluminescent device based on ZnO nanowiresshowed a high current density and strong luminescence even at areverse bias voltage of 3 V [3]. The electroluminescent diode, con-sisting of a p-type conducting polymer and electrodeposited ZnOnanorods, embedded in an insulating polystyrene layer and sand-wiched between transparent SnO2 films, has been constructed byKönenkamp et al. [156,157]. As-grown nanorods showed a broadelectroluminescence band over the visible spectrum but anneal-ing at moderate temperature of 300 ◦C increased the emission andstrongly raised the excitonic contribution at 390 nm.

Light emitting diode with tunable emission wavelength, basedon Cd-alloyed ZnO (n-Zn1-xCdxO) electrodeposited on (0001) orien-ted p-GaN has been developed by Pauporte et al. [158,159]. Theincrease of the Cd content in the deposition bath resulted in theredshift of the room temperature UV emission peak, with a widthat half-maximum of 10 nm, from 397 nm for pure ZnO(NRs)/p-GaNto 417 nm for the LED of Zn0.92Cd0.08O(NRs)/p-GaN structure. Theeffect was ascribed to expansion of the lattice parameters withincreasing Cd content.

8.4. Solar cells

Applications of ZnO nanorods, obtained by different syntheticmethods, in the dye- and semiconductor sensitized solar cells havebeen reported in several review papers [160–163]. In this Sectionwe only refer to the main problems connected with relatively lowefficiency of the ZnO-based solar cells and focus on the last achieve-ments in the field.

The design and performance of dye-sensitized solar cells (DSSCs)based on ZnO nanowires grown on FTO by chemical vapor depo-sition and hydrothermal method have been reported for the firsttime by Baxter et al. [164,165]. However, the described solar cells,sensitized with the ruthenium dye N719, revealed very low power
efficiency (0.5%). Although the device performance increased withincreasing nanowire length, the efficiency still remained muchlower than that of the cells based on mesoporous TiO2. This wasexplained by low light harvesting efficiency resulting from a small

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urface area of the nanowires as compared to that of mesoporouslms.

The studies with dense and long ZnO NRs synthesized byydrothermal method, performed by Law et al. [166] have shownhat the longest nanorods reaching 20–25 m with a diameteranging from 130 to 200 nm have the surface area only one-fifthf the nanoparticle film.

Since even a very dense array of long nanowires is not capa-le to permit a large amount of adsorbed dye and, in consequence,he solar light harvesting is not sufficient, an interesting option ispplication of hierarchical ZnO nanostructures [167–169] insteadf non-branched nanowires. An attractive approach also consistsn combining nanowires and nanoparticles. It was found that spe-ific surface area of hierarchical structures of ZnO covered withanoparticles increased by more than one order of magnitudeompared to uncoated nanowires and larger amount of dye wasccommodated [170–172].

The experiments performed with a dye of high extinction coef-cient has recently pointed out that the main limitation for highfficiency of the cells may also arise from high recombinationate due to high surface states concentration in ZnO nanorodsbtained in wet synthesis [173]. Coating the ZnO nanorods withnO nanocrystalline layer (ZnO core-shell) significantly slowedown the recombination dynamics and led to enhancement of pho-ovoltage (of more than 250 mV) with respect to the device basedn bare nanorods [174].

However, so far, the dye-sensitized solar cells utilized ZnOanowires have relatively low power conversion efficiency (below%) in comparison with the systems based on mesoporous TiO2efficiency up to 12%) [175]. Among the reasons of limited perfor-

ance in ZnO-based DSSCs is instability of ZnO in acidic dye due toissolution of ZnO surface, resulting in the formation of Zn2+/dyegglomerates, retarding the electron injection from the dye to ZnO176].

To overcome these problems, the dye sensitizer may be replacedy semiconductor quantum dots (QDs) or extremely thin absorberlm (in ETA cells). Semiconductor QDs (CdS, CdSe, CdTe, PbS,bSe) offer several significant advantages over dyes, such as higherxtinction coefficient, easily tuned band gap from infrared to ultra-iolet by the control of QD size and composition, multiple excitoneneration owing to utilization hot electrons in QDs [177]. Most ofhe papers in this area concern the solar cells based on mesoporousiO2 but recently many attempts have been also done towardsnO NR-based systems. Despite of very promising features, theD semiconductor-sensitized solar cells (QD-SSSCs) reveal rather

ow power conversion efficiency and the main limiting factor isrobably recombination of the charge carriers due to slow electronransfer from semiconductor to the electron acceptor (ZnO or TiO2).ccording the Chen et al. [178], a suppression of the charge car-ier recombination at the semiconductor surface may be achievedy incorporation of graphene thin film into the ZnO NR/CdSe solarell. This enhanced the electron transport and improved efficiencylmost twice with respect to the device without graphene layer,hereas fill factor, 62%, was one of the highest reported so far for

he QD-SSSCs based on ZnO nanorods.Levy-Clément and Elias have recently showed that the increase

f the solar cell efficiency may be also achieved by the light scat-ering over a large wavelength range, close to the CdSe absorberandgap, leading to significant enhancement in the effective lightbsorption [179]. This was realized by the change of ZnO nanorodsiameter at constant NR length, as it has been reported in ref [22]see discussion in Section 6)

Another way of performance improvement of the QDs solar cellss broadening the absorption spectral region by co-sensitizationf the nanorods by two semiconductors of different size or type,or example CdS/CdSe [180–183] or CdS/CdTe [184]. Seol et al.

mica Acta 127 (2014) 467–488

[182] have demonstrated that the CdS interlayer between ZnOnanorods (of the lengths 10 m and diameter 100 nm) and CdSeQDs deposited on the top, passivates the ZnO suppressing thecharge carrier recombination and in effect, the power conversionefficiency of the cell increased to 4.15%.

ZnO nanowire arrays have been also applied as electron trans-port building blocks in organic-inorganic hybrid solar cells, inwhich a conducting polymer (for example poly(3-hexylthiophene))and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) play the roleof electron donor and electron acceptor, respectively. Improvementof efficiency of the cells with configuration ITO/ZnO/P3HT:PCBM/Agfrom about 3% to 3.9% was achieved by incorporation of vana-dium oxide which acts as an electron-blocking layer betweenP3HT:PCBM and the Ag electrode, enhancing the electron collectionvia ZnO nanorods [185].

Further increase of efficiency of the hybrid ZnO-based solar cells(to 4.1%) has been achieved by covering of ZnO nanorods with ZnOnanoparticles, leading to enlargement of interfacial surface area (asin the case of DSSCs) and improvement of the charge lifetime [22].

The recent studies on the improvement of efficiency and long-term stability of hybrid solar cells based on TiO2 and ZnO arefocused on utilization of organic-inorganic perovskites, of thetype CH3NH3PbX3 (X = Cl, Br, I), as the light harvesting materials[186,187]. The lead perovskite is a direct band semiconductor, havea large absorption coefficient and a high carrier mobility. The use ofCH3NH3PbI3 absorber in combination with ZnO nanorod arrays andspiro-MeOTAD as a hole transporting material has been reportedrecently [188,189]. It was shown that by employing the electrode-posited ZnO compact layer as a hole blocking layer preventing therecombination of electrons in FTO with the holes in spiro-MeOTAD,the power conversion efficiency of perovskite ZnO-based solar cellsmay be increased even up to 8.9% [189]. The unique nature of thesevery promising organometal halide light absorbers is still not clearand therefore, further systematic investigations are expected.

9. Conclusions

In this review we summarized the achievements on electro-chemical synthesis of ZnO nanorod arrays on the transparentconducting electrodes. We have attempted to highlight the fac-tors crucial for formation of uniform films of nanorods, verticallyoriented to the substrate of desired length, diameter, density andoptoelectronic properties.

We have discussed the mechanisms of the ZnO deposition inthe solutions containing two different precursors of hydroxide ions(molecular oxygen and nitrate ions) and advantageous or disad-vantageous aspects of the use of each of them. The third potentialprecursor, hydrogen peroxide, is not suitable because it does notlead to formation of ZnO nanorods.

Since transformation of Zn(OH)2 into ZnO is achieved via dehy-dration of hydroxide, the electrodeposition should be performed atthe temperature above 65 ◦C. Further increase of deposition tem-perature even up to 88 ◦C is recommended because of significantreduction of structural defects in ZnO, supported by the increase ofrelative intensity of UV and visible emission peaks (IUV/Ivis) in thephotoluminescence spectrum of ZnO nanorods.

Irrespectively of the chosen OH− precursor (O2 or NO3−),

one of the main parameters which decides on the form ofZnO deposit (compact film or nanorods) is concentration ofZn2+ ions. In general, formation of nanorods is possible at5 × 10−5 M ≤ [Zn2+] < 5 × 10−3 M. The upper range is not so strict
because the quality of nanorods depends also on the additivespresent in deposition bath, such as Cl− ions, hexamethylenete-tramine and ethylenediamine. However, at concentrations of Zn2+
above 5 × 10−3 M the ZnO nanorods have tendency to coalescence.

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A diameter of nanorods grown on the electrode may be con-rolled by the concentration ratio of Zn2+ and OH− ions in theicinity of the electrode. Namely, a decrease of [Zn2+]/[OH−] ratioesults in the formation of ZnO nanowires of greater diameter. Thisay be achieved by:

manipulation of the electrical parameters, i.e. potential and cur-rent density applied to the electrode, which control the rate ofOH− electrogeneration,

addition of extra amount of NaNO3 supporting electrolyte, if elec-trodeposition is performed from nitrate precursor,

addition of KCl into the deposition bath because of preferen-tial adsorption of Cl− ions onto positively Zn-terminated (0001)plane, which favors the lateral growth of nanorods.

However, addition of high amount of Cl− ions, slows down theate of electrogeneration of OH− from oxygen precursor due to thehange of mechanism of O2 reduction from four-electron to two-lectron reaction, leading in consequence to the formation of longeranowires.

The increase of the nanowire length may be obtained by addi-ion of equimolar amount of hexamethylenetetramine (HMT) to theolution of Zn2+ as a result of attachment of this chelating agent toonpolar facets of ZnO crystal and its preferential growth along c-xis. However, an excessive amount of HTM results in the formationnO nanorods with corn-like surface due to microscopic variationf HTM attachment to ZnO.

The change of local composition of the electrolyte aroundrowing nanowires, for example due to the presence of high con-entration of additives (Cl− or HTM), is responsible for formationf structural defects. Annealing of the samples at 450 ◦C improvesrystallinity of the nanorods and leads to decrease of structuralefects density.

The improvement of morphology and density of ZnO nanorodsn the electrode may be achieved by seeding of the substrate beforehe NRs deposition. It consists in formation of the thin ZnO layer byhemical or electrochemical methods. The grains of the seed layerct as nucleation centers and their size influences on the diameterf post-deposited nanorods. The deposition of ZnO nanorods is alsoossible on non-seeded substrate, but then the parameters, suchs orientation, density and diameter of nanorods are much moreensitive to electrodeposition conditions than in the case of therowth on the seeded substrate.

ZnO nanorods are attractive candidates for applications in var-ous devices like solar cells, sensors, biosensors or fluorescentevices. Nowadays the studies in this direction are very advancednd it is expected that in the next few years the ZnO nanorod arraysill be widely used in different branches of industry.

cknowledgements

The authors gratefully acknowledge partial support of thisork by NCN under grants DEC-2011/03/N/ST5/04712 and DEC-

012/07/B/ST5/02431.

eferences

[1] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He,H.-J. Choi, Controlled growth of ZnO nanowires and their optical properties,Advanced Functional Materials 12 (2002) 323.

[2] K. Govender, D.S. Boyle, P. O’Brien, D. Brinks, D. West, D. Coleman, Room tem-perature lasing observed from ZnO nanocolumns grown by aqueous solutiondeposition, Advanced Materials 14 (2002) 1221.

[3] W.I. Park, G.C. Yi, Electroluminescence in n-ZnO Nanorod Arrays VerticallyGrown on p-GaN, Advanced Materials 16 (2004) 87.

[4] J.J. Hassan, M.A. Mahdi, A. Ramizy, H. Abu Hassan, Z. Hassan, Fabrica-tion and characterization of ZnO nanorods/p-6H-SiC heterojunctionLED by

mica Acta 127 (2014) 467–488 485

microwave-assisted chemical bath deposition, Superlattices and Micro-structures 53 (2013) 31.

[5] S.J. An, G. Yi, Near ultraviolet light emitting diode composed of n-GaN/ZnOcoaxial nanorod heterostructures on a p-GaN layer, Applied Physics Letters91 (2007) 123109.

[6] T. Hirate, H. Tanaka, S. Sasaki, M. Ozawa, W. Li, T. Satoh, Synthesis of alignedZnO hexagonal nanorods and its application to ZnS based DC electrolumi-nescent devices, Materials Research Society Symposium - Proceedings 799(2003) 311.

[7] W.W. Guo, T.M. Liu, L. Huang, H.J. Zhang, Q. Zhou, W. Zeng, HMT assistedhydrothermal synthesis of various ZnO nanostructures: Structure, growth andgas sensor properties, Physica E-Low-Dimensional Systems & Nanostructures44 (2011) 680.

[8] C. M. Chang, M. H. Hon, I. C. Leu, Effect of seed layer on the growth and theconsequent gas sensing characteristics of ZnO nanorod arrays using aqueouschemical growth, Journal of Sol-Gel Science and Technology (2013) 1. DOI:10.1007/s10971-013-3126-2.

[9] M. Kashif, M.E. Ali, S.M.U. Ali, U. Hashim, Sol-gel synthesis of Pd doped ZnOnanorods for room temperature hydrogen sensing applications, CeramicsInternational 39 (2013) 6461.

[10] R. Ahmad, N. Tripathy, Y. Hahn, High-performance cholesterol sensor basedon the solution-gated field effect transistor fabricated with ZnO nanorods,Biosensors and Bioelectronics 45 (2013) 281.

[11] S.K. Arya, S. Saha, J.E. Ramirez-Vick, V. Gupta, S. Bhansali, S.P. Singh, Recentadvances in ZnO nanostructures and thin films for biosensor applications:Review, Analytica Chimica Acta 737 (2012) 1.

[12] Y. Ni, J. Zhu, L. Zhang, J. Hong, Hierarchical ZnO micro/nanoarchitectures:hydrothermal preparation, characterization and application in the detectionof hydrazine, CrystEngComm 12 (2010) 2213.

[13] Y.Y. Zhang, J. Mu, One-pot synthesis, photoluminescence, and photocatalysisof Ag/ZnO composites, Journal of Colloid and Interface Science 309 (2007)478.

[14] M. Pirhashemi, A. Habibi-Yangjeh, Simple and large scale one-potmethod for preparation of AgBr-ZnO nanocomposites as highly effi-cient visible light photocatalyst, Applied Surface Science (2013) DOI:10.1016/j.apsusc.2013.07.079.

[15] X. Li, J. Wang, J. Yang, J. Lang, J. Cao, F. Liu, Y. Jiang, Size-controlled fabricationof ZnO micro/nanorod arrays and their photocatalytic performance, MaterialsChemistry and Physics 141 (2013) 929.

[16] O. Yayapao, T. Thongtem, A. Phuruangrat, S. Thongtem, Sonochemical syn-thesis of Dy-doped ZnO nanostructures and their photocatalytic properties,Journal of Alloys and Compounds 576 (2013) 72.

[17] C. Xu, P. Shin, L. Cao, D. Gao, Preferential Growth of Long ZnO Nanowire Arrayand Its Application in Dye-Sensitized Solar Cells, Journal of Physical ChemistryC 114 (2010) 125.

[18] J.A. Anta, E. Guillén, R. Tena-Zaera, ZnO-based dye-sensitized solar cells, Jour-nal of Physical Chemistry C 116 (2012) 11413.

[19] H. Woo Choi, K. Lee, N. David Theodore, T.L. Alford, Improved performance ofZnO nanostructured bulk heterojunction organic solar cells with nanowire-density modified by yttrium chloride introduction into solution, Solar EnergyMaterials and Solar Cells 117 (2013) 273.

[20] S. Sarkar, A. Makhal, T. Bora, K. Lakhsman, A. Singha, J. Dutta, S.K. Pal,Hematoporphyrin-ZnO nanohybrids: Twin applications in efficient visible-light photocatalysis and dye-sensitized solar cells, ACS Applied Materials andInterfaces 4 (2012) 7027.

[21] J.H. Lee, J. Shin, J.Y. Song, W. Wang, R. Schlaf, K.J. Kim, Y. Yi, Interface formationbetween ZnO nanorod arrays and polymers (PCBM and P3HT) for organic solarcells, Journal of Physical Chemistry C 116 (2012) 26342.

[22] J. Ajuria, I. Etxebarria, E. Azaceta, R. Tena-Zaera, N. Fernández-Montcada, E.Palomares, R. Pacios, Novel ZnO nanostructured electrodes for higher powerconversion efficiencies in polymeric solar cells, Physical Chemistry ChemicalPhysics 13 (2011) 20871.

[23] R. Tena-Zaera, J. Elias, C. Lévy-Clément, ZnO nanowire arrays: Optical scat-tering and sensitization to solar light, Applied Physics Letters 93 (2008)233119.

[24] L. Znaidi, Sol-gel-deposited ZnO thin films: A review, Materials Science andEngineering B: Solid-State Materials for Advanced Technology 174 (2010) 18.

[25] S.A. Kamaruddin, K.Y. Chan, H.K. Yow, M. Zainizan Sahdan, H. Saim, D. Knipp,Zinc oxide films prepared by sol-gel spin coating technique, Applied PhysicsA: Materials Science and Processing 104 (2011) 263.

[26] Y. Liao, X. Zhou, X. Xie, Q. Yu, The effects of solvents on the highly oriented ZnOfilms prepared using sol-gel method, Journal of Materials Science: Materialsin Electronics 24 (2013) 4427.

[27] V. Kumar, N. Singh, R.M. Mehra, A. Kapoor, L.P. Purohit, H.C. Swart, Role offilm thickness on the properties of ZnO thin films grown by sol-gel method,Thin Solid Films 539 (2013) 161.

[28] Z.N. Ng, K.Y. Chan, T. Tohsophon, Effects of annealing temperature on ZnO andAZO films prepared by sol-gel technique, Applied Surface Science 258 (2012)9604.

[29] F.T.J. Smith, Metalorganic chemical vapor deposition of oriented ZnO filmsover large areas, Applied Physics Letters 43 (1983) 1108.

[30] J.J. Wu, S.C. Liu, Low-temperature growth of well-aligned ZnO nanorods bychemical vapor deposition, Advanced Materials 14 (2002) 215.

[31] S.W. Kim, S. Fujita, S. Fujita, Self-organized ZnO quantum dots on SiO2/Si sub-strates by metalorganic chemical vapor deposition, Applied Physics Letters 81(2002) 5036.

http://refhub.elsevier.com/S0013-4686(14)00351-X/sbref0005



















































































































































































































































































































































































































































































































































































































































































































































































































































4 trochi
[32] D.N. Montenegro, A. Souissi, C. Martínez-Tomás, V. Munoz-Sanjosé, V. Sallet,Morphology transitions in ZnO nanorods grown by MOCVD, Journal of CrystalGrowth 359 (2012) 122.

[33] H. Fujisawa, C. Kobayashi, S. Nakashima, M. Shimizu, Two-step growth ofZnO nanorods by using MOCVD and control of their diameters and surfacedensities, Journal of the Korean Physical Society 62 (2013) 1164.

[34] A.Y. Kim, S. Jang, D.H. Lee, S.Y. Yim, D. Byun, Effects of temperature onZnO hybrids grown by metal-organic chemical vapor deposition, MaterialsResearch Bulletin 47 (2012) 2888.

[35] W.J. Li, E.W. Shi, W.Z. Zhong, Z.W. Yin, Growth mechanism and growth habitof oxide crystals, Journal of Crystal Growth 203 (1999) 186.

[36] J. Yu, B. Huang, X. Qin, X. Zhang, Z. Wang, H. Liu, Hydrothermal synthesis andcharacterization of ZnO films with different nanostructures, Applied SurfaceScience 257 (2011) 5563.

[37] K. Govender, D.S. Boyle, P.B. Kenway, P. O’Brien, Understanding the factorsthat govern the deposition and morphology of thin films of ZnO from aqueoussolution, Journal of Materials Chemistry 14 (2004) 2575.

[38] J. Song, S. Baek, J. Lee, S. Lim, Role of OH- in the low temperature hydrothermalsynthesis of ZnO nanorods, Journal of Chemical Technology and Biotechnol-ogy 83 (2008) 345.

[39] A. Rivera, J. Zeller, A. Sood, M. Anwar, A comparison of ZnO nanowires andnanorods grown using MOCVD and hydrothermal processes, Journal of Elec-tronic Materials 42 (2013) 894.

[40] M. Izaki, T. Omi, Transparent zinc oxide films prepared by electrochemicalreaction, Applied Physics Letters 68 (1996) 2439.

[41] S. Peulon, D. Lincot, Cathodic Electrodeposition from Aqueous Solution ofDense or Open-Structured Zinc Oxide Films, Advanced Materials 8 (1996)166.

[42] B. Canava, D. Lincot, Nucleation effects on structural and optical properties ofelectrodeposited zinc oxide on tin oxide, Journal of Applied Electrochemistry30 (2000) 711.

[43] X.D. Gao, F. Peng, X.M. Li, W.D. Yu, J.J. Qiu, Growth of highly oriented ZnO filmsby the two-step electrodeposition technique, Journal of Materials Science 42(2007) 9638.

[44] J. Elias, R. Tena-Zaera, C. Lévy-Clément, Electrodeposition of ZnO nanowireswith controlled dimensions for photovoltaic applications: Role of buffer layer,Thin Solid Films 515 (2007) 8553.

[45] T. Pauporté, D. Lincot, B. Viana, F. Pellé, Toward laser emission of epitaxialnanorod arrays of ZnO grown by electrodeposition, Applied Physics Letters89 (2006) 233112.

[46] B. O’Regan, V. Sklover, M. Grätzel, Electrochemical Deposition of Smoothand Homogeneously Mesoporous ZnO Films from Propylene Carbonate Elec-trolytes, Journal of the Electrochemical Society 148 (2001) C498.

[47] D. Gal, G. Hodes, D. Lincot, H.W. Schock, Electrochemical deposition of zincoxide fims from non-aqueous solution: a new buffer/window process for thinfilm solar cells, Thin Solid Films 361–362 (2000) 79.

[48] E. Azaceta, R. Tena-Zaera, R. Marcilla, S. Fantini, J. Echeberria, J.A. Pomposo,H. Grande, D. Mecerreyes, Electrochemical deposition of ZnO in a room tem-perature ionic liquid: 1-Butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, Electrochemistry Communication 11 (2009) 2184.

[49] M. Tulodziecki, J.-M. Tarascon, P.L. Taberna, C. Guéry, ElectrodepositionGrowth of Oriented ZnO Deposits in Ionic Liquid Media, Journal of the Elec-trochemical Society 159 (2012) D691.

[50] E. Azaceta, J. Idigoras, J. Echeberria, A. Zukal, L. Kavan, O. Miguel, H.J. Grande,J.A. Anta, R. Tena-Zaera, ZnO–ionic liquid hybrid films: electrochemical syn-thesis and application in dye-sensitized solar cells, Journal of MaterialsChemistry A 1 (2013) 10173.

[51] B. Beverskogt, I. Puigdomenech, Revised Pourbaix diagrams for zinc at 25-300 (C, Corrosion Science 39 (1997) 107.

[52] A. E. Martell, R. M. Smith, in Critical Stability Constants, Vol. 4, Plenum Press,New York (1976).

[53] S. Peulon, D. Lincot, Mechanistic study of cathodic electrodeposition of zincoxide and zinc hydroxychloride films from oxygenated aqueous zinc chloridesolutions, Journal of the Electrochemical Society 145 (1998) 864.

[54] S. Yamabi, H. Imai, Growth conditions for wurtzite zinc oxide films in aqueoussolutions, Journal of Materials Chemistry 12 (2002) 3773.

[55] A. Goux, T. Pauporte, J. Chivot, D. Lincot, Temperature effects on ZnO elec-trodeposition, Electrochimica Acta 50 (2005) 2239.

[56] R. Reichle, G. McCurdy, L. Hepler, Zinc Hydroxide: Solubility Product andHydroxy-complex stability Constants from 12-75 ◦C, Canadian Journal ofChemistry 53 (1975) 3841.

[57] G.A. Parks, The isoelectric points of solid oxides, solid hydroxides, and aqueoushydroxo complex systems, Chemical Reviews 65 (1965) 177.

[58] A. Degen, M. Kosec, Effect of pH and impurities on the surface charge of zincoxide in aqueous solution, Journal of the European Ceramic Society 20 (2000)667.

[59] X.Y. Kong, Z.L. Wang, Spontaneous polarization-induced nanohelixes,nanosprings and nanorings of piezoelectric nanobelts, Nano Letters 3 (2003)1625.

[60] Z. L. Wang, Zinc oxide nanostructures: growth, properties and applications,Journal of Physics: Condensed Matter 16 (2004) R829.

[61] S. Baruah, J. Dutta, Hydrothermal growth of ZnO nanostructures, Science andTechnology of Advanced Materials 10 (2009) 013001.

[62] L. Vayssieres, K. Keis, A. Hagfeldt, S.E. Lindquist, Three-dimensional array ofhighly oriented crystalline ZnO microtubes, Chemistry of Materials 13 (2001)4395.

mica Acta 127 (2014) 467–488

[63] R.A. Laudise, A.A. Ballman, Hydrothermal synthesis of zinc oxide and zincsulfide, Journal of Physical Chemistry 64 (1960) 688.

[64] F. Xu, Y. Lu, Y. Xie, Y. Liu, Controllable morphology evolution of electrode-posited ZnO nano/micro-scale structures in aqueous solution, Materials andDesign 30 (2009) 1704.

[65] E.A. Dalchiele, P. Giorgi, R.E. Marotti, F. Martin, J.R. Ramos-Barrado, R. Ayouci,D. Leinen, Electrodeposition of ZnO thin films on n-Si(100), Solar EnergyMaterials & Solar Cells 70 (2001) 245.

[66] L. Vayssieres, K. Keis, S.-E. Lindquist, A. Hagfeldt, Purpose-Built AnisotropicMetal Oxide Material: 3D Highly Oriented Microrod Array of ZnO, Journal ofPhysical Chemistry B 105 (2001) 3350.

[67] Z. Liu, J. Ya, E. Lei, Effects of substrates and seed layers on solution growingZnO nanorods, Journal of Solid State Electrochemistry 14 (2010) 957.

[68] S. Wei, J. Lian, X. Chen, Q. Jiang, Effects of seed layer on the structure andproperty of zinc oxide thin films electrochemically deposited on ITO-coatedglass, Applied Surface Science 254 (2008) 6605.

[69] K. Zarebska, M. Kwiatkowski, M. Gniadek, M. Skompska, Electrodepositionof Zn(OH)2, ZnO thin films and nanosheet-like Zn seed layers and influenceof their morphology on the growth of ZnO nanorods, Electrochimica Acta 98(2013) 255.

[70] L.E. Greene, M. Law, D.H. Tan, M. Montano, J. Goldberger, G. Somorjai, P. Yang,General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds,Nano Letters 5 (2005) 1231.

[71] C.-T. Hsieh, S.-Y. Yang, J.-Y. Lin, Electrochemical deposition and superhy-drophobic behavior of ZnO nanorod arrays, Thin Solid Films 518 (2010)4884.

[72] M. Izaki, Preparation of Transparent and Conductive Zinc Oxide Films byOptimization of the Two-Step Electrolysis Technique, Journal of the Electro-chemical Society 146 (1999) 4517.

[73] S. Sanchez, C. Levy-Clement, V. Ivanova, Electrochemical Deposition of ZnOThin Films and Nanowires for Photovoltaic Applications, Journal of The Elec-trochemical Society 159 (2012) D705.

[74] M. Izaki, T. Omi, Electrolyte optimization for cathodic growth of zinc oxidefilms, Journal of the Electrochemical Society 143 (1996) L53.

[75] T. Yoshida, M. Tochimoto, D. Schlettwein, D. Wo, T. Sugiura, H. Minoura, Self-Assembly of Zinc Oxide Thin Films Modified with Tetrasulfonated Metal-lophthalocyanines by One-Step Electrodeposition, Chemistry of Materials 11(1999) 2657.

[76] T. Yoshida, D. Komatsu, N. Shimokawa, H. Minoura, Mechanism of cathodicelectrodeposition of zinc oxide thin films from aqueous zinc nitrate baths,Thin Solid Films 451–452 (2004) 166.

[77] T. Pauporté, D. Lincot, Hydrogen Peroxide Oxygen Precursor for Zinc OxideElectrodeposition. I. Deposition in Perchlorate Medium, Journal of the Elec-trochemical Society 148 (2001) C310.

[78] T. Pauporté, D. Lincot, Hydrogen peroxide oxygen precursor for zincoxide electrodeposition II—Mechanistic aspects, Journal of ElectroanalyticalChemistry 517 (2001) 54.

[79] K. Laurent, B.Q. Wang, D.P. Yu, Y. Leprince-Wang, Structural and opticalproperties of electrodeposited ZnO thin films, Thin Solid Films 517 (2008)617.

[80] D. Dimova-Malinovska, P. Andreev, M. Sendova-Vassileva, H. Nichev, K. Star-bova, Preparation of ZnO nanowires by electrochemical deposition, EnergyProcedia 2 (2010) 55.

[81] A. El Hichou, N. Stein, C. Boulanger, L. Johann, Structural and spectroscopicellipsometry characterization for electrodeposited ZnO growth at differenthydrogen peroxide concentration, Thin Solid Films 518 (2010) 4150.

[82] K. Kinoshita in Electrochemical Oxygen Technology Wiley&Sons, New York,Electrochemical Society, 1992.

[83] A.J. Bard, L.R. Faulkner–Electrochemical methods. Fundamentals and applica-tions, 2nd ed., Wiley&Sons, Inc., 2001.

[84] S.P. Jiang, C.Q. Cui, A.C.C. Tseung, Reactive Deposition of Cobalt Electrodes -V Mechanistic Studies of Oxygen Reduction in Unbuffered Neutral SolutionsSaturated with Oxygen, Journal of the Electrochemical Society 138 (1991)3599.

[85] S. Strabac, R.R. Adzic, The influence of OH- chemisorptions on the catalyticproperties of gold single crystal surfaces for oxygen reduction in alkalinesolutions, Journal of Electroanalytical Chemistry 403 (1996) 169.

[86] R. Tena-Zaera, J. Elias, G. Wang, C. Levy-Clement, Role of chloride ions on elec-trochemical deposition of ZnO nanowire Arrays from O2 reduction, Journalof Physical Chemistry C 111 (2007) 16706.

[87] J. Elias, R. Tena-Zaera, C. Lévy-Clément, Electrochemical deposition of ZnOnanowire arrays with tailored dimensions, Journal of ElectroanalyticalChemistry 621 (2008) 171.

[88] R. Tena-Zaera, J. Elias, C. Lévy-Clément, I. Mora-Seró, Y. Luo, J. Bisquert, Elec-trodeposition and impedance spectroscopy characterization of ZnO nanowirearrays, Physica Status Solidi (A) Applications and Materials Science 205 (2008)2345.

[89] B.B. Li, U. Philipose, C.F. De Souza, H.E. Ruda, Role of electro-deposition para-meters on preparing tailored dimension vertically-aligned ZnO nanowires,Journal of the Electrochemical Society 158 (2011) D282.

[90] A. Goux, T. Pauporté, D. Lincot, Oxygen reduction reaction on electrodeposited
zinc oxide electrodes in KCl solution at 70 ◦C, Electrochimica Acta 51 (2006)3168.
[91] M. Guo, C. Yang, M. Zhang, Y. Zhang, T. Ma, X. Wang, X. Wang, Effects ofpreparing conditions on the electrodeposition of well-aligned ZnO nanorodarrays, Electrochimica Acta 53 (2008) 4633.







































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































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[92] H. Kim, J.Y. Moon, H.S. Lee, Effect of ZnCl2 concentration on the growth of ZnOby electrochemical deposition, Current Applied Physics 12 (2012) 35.

[93] M.R. Mahmoudian, W.J. Basirun, Y. Alias, M. Ebadi, Facile fabrication ofZn/Zn5(OH)8Cl2 H2O flower-like nanostructure on the surface of Zn coatedwith poly (N-methyl pyrrole), Applied Surface Science 257 (2011) 10539.

[94] H. El Belghiti, T. Pauporté, D. Lincot, Mechanistic study of ZnO nanorod arrayelectrodeposition, Physica Status Solidi (a) 205 (2008) 2360.

[95] Y. Hames, Z. Alpaslan, A. Kösemen, S.E. San, Y. Yerli, Electrochemically grownZnO nanorods for hybrid solar cell applications, Solar Energy 84 (2010) 426.

[96] J.A. Cox, A. Brajter, Mechanisms of Zr(IV) and La (III) catalysis of the reductionof nitrate at mercury, Electrochimica Acta 24 (1979) 517.

[97] Y. Lin, J. Yang, X. Zhou, Controlled synthesis of oriented ZnO nanorod arraysby seed-layer-free electrochemical deposition, Applied Surface Science 258(2011) 1491.

[98] B. Xue, Y. Liang, L. Donglai, N. Eryong, S. Congli, F. Huanhuan, X. Jingjing, J.Yong, J. Zhifeng, S. Xiaosong, Electrodeposition from ZnO nano-rods to nano-sheets with only zinc nitrate electrolyte and its photoluminescence, AppliedSurface Science 257 (2011) 10317.

[99] M.R. Khajavi, D.J. Blackwood, G. Cabanero, R. Tena-Zaera, New insight intogrowth mechanism of ZnO nanowires electrodeposited from nitrate-basedsolutions, Electrochimica Acta 69 (2012) 181.

100] H.Y. Yang, T.W. Kim, Dependence of the structural and optical properties onthe zinc nitrate concentration in ZnO nanowires formed on indium-tin-oxide-coated glass substrates, Journal of Ceramic Processing Research 11 (2010)769.

101] S. Sun, S. Jiao, K. Zhang, D. Wang, S. Gao, H. Li, J. Wang, Q. Yu, F. Guo, L. Zhao,Nucleation effect and growth mechanism of ZnO nanostructures by electrode-position from aqueous zinc nitrate baths, Journal of Crystal Growth 359 (2012)15.

102] F. Xu, Y. Lu, L. Xia, Y. Xie, M. Dai, Y. Liu, Seed layer-free electrodepositionof well-aligned ZnO submicron rod arrays via a simple aqueous electrolyte,Materials Research Bulletin 44 (2009) 1700.

103] Y. Mastai, D. Gal, G. Hodes, Nanocrystal-size control of electrodepositednanocrystalline semiconductor films by surface capping, Journal of the Elec-trochemical Society 147 (2000) 1435.

104] L. Xu, Y. Guo, Q. Liao, J. Zhang, D. Xu, Morphological control of ZnO nano-structures by electrodeposition, Journal of Physical Chemistry B 109 (2005)13519.

105] J.B. Cui, Y.C. Soo, T.P. Chen, U.J. Gibson, Low-temperature growth and charac-terization of Cl-doped ZnO nanowire arrays, Journal of Physical Chemistry C112 (2008) 4475.

106] M. Andres-Verges, A. Mifsud, C.J. Serna, Formation of rod-like zinc oxidemicrocrystals in homogeneous solutions, Journal of the Chemical Society,Faraday Transactions 86 (1990) 959.

107] A. Sugunan, H.C. Warad, M. Boman, J. Dutta, Zinc oxide nanowires in chemicalbath on seeded substrates: Role of hexamine, Journal of Sol-Gel Science andTechnology 39 (2006) 49.

108] J.S. Lin, W. Fu, L.Q. Jing, Y.C. Qu, Z.J. Li, Electrochemical deposition of nano-structured ZnO on the nanocrystalline TiO2 film and its characterization,Science China Chemistry 53 (2010) 1732.

109] R. Chander, A.K. Raychaudhuri, Electrodeposition of aligned arrays of ZnOnanorods in aqueous solution, Solid State Communications 145 (2008) 81.

110] F. Xu, Y. Lu, Y. Xie, Y. Liu, Seed layer-free electrodeposition and characte-rization of vertically aligned ZnO nanorod array film, Journal of Solid StateElectrochemistry 14 (2010) 63.

111] J. Cui, Defect control and its influence on the exciton emission of electrode-posited ZnO nanorods, Journal of Physical Chemistry C 112 (2008) 10385.

112] L. Xu, Q. Liao, J. Zhang, X. Ai, D. Xu, Single-crystalline ZnO nanotube arrays onconductive glass substrates by selective disolution of electrodeposited ZnOnanorods, Journal of Physical Chemistry C 111 (2007) 4549.

113] S.A. Studenikin, N. Golego, M. Cocivera, Fabrication of green and orange pho-toluminescent, undoped ZnO films using spray pyrolysis, Journal of AppliedPhysics 84 (1998) 2287.

114] X. Liu, X.H. Wu, H. Cao, R.P.H. Chang, Growth mechanism and properties ofZnO nanorods synthesized by plasma-enhanced chemical vapor deposition,Journal of Applied Physics 95 (2004) 3141.

115] T. Voss, C. Bekeny, J. Gutowski, R. Tena-Zaera, J. Elias, C. Lévy-Clément, I. Mora-Seró, J. Bisquert, Localized versus delocalized states: Photoluminescence fromelectrochemically synthesized ZnO nanowires, Journal of Applied Physics 106(2009) 054304.

116] C. Bekeny, T. Voss, H. Gafsi, J. Gutowski, B. Postels, M. Kreye, A. Waag, Originof the near-band-edge photoluminescence emission in aqueous chemicallygrown ZnO nanorods, Journal of Applied Physics 100 (2006) 104317.

117] M. Ahmad, J. Zhao, J. Iqbal, W. Miao, L. Xie, R. Mo, J. Zhu, Conductivityenhancement by slight indium doping in ZnO nanowires for optoelectronicapplications, Journal of Physics D: Applied Physics 42 (2009) 165406.

118] B. Liu, Z. Fu, Y. Jia, Green luminescent center in undoped zinc oxide filmsdeposited on silicon substrates, Applied Physics Letters 79 (2001) 943.

119] M.D. McCluskey, S.J. Jokela, Defects in ZnO, Journal of Applied Physics 106(2009) 071101.

120] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade,
Mechanisms behind green photoluminescence in ZnO phosphor powders,Journal of Applied Physics 79 (1996) 7983.
121] M. Willander, O. Nur, J.R. Sadaf, M.I. Quadir, S. Zaman, A. Zainelabdin, N. Bano,I. Hussain, Luminescence from Zinc Oxide Nanostructures and Polymers andtheir Hybrid Devices, Materials 3 (2010) 2643.

mica Acta 127 (2014) 467–488 487

[122] G. She, X. Zhang, W. Shi, X. Fan, J.C. Chang, Electrochemical/chemical syn-thesis of highly-oriented single-crystal ZnO nanotube arrays on transparentconductive substrates, Electrochemistry Communications 9 (2007) 2784.

[123] W.S. Shi, B. Cheng, L. Zhang, E.T. Samulski, Influence of excitation density onphotoluminescence of zinc oxide with different morphologies and dimen-sions, Journal of Applied Physics 98 (2005) 083502.

[124] B.Q. Cao, Y. Li, G.T. Duan, W.P. Cai, Growth of ZnO Nanoneedle Arrays withStrong Ultraviolet Emissions by an Electrochemical Deposition Method, Crys-tal Growth & Design 6 (2006) 1091.

[125] T. Pauporté, E. Jouanno, F. Pellé, B. Viana, P. Aschehoug, Key growth parame-ters for the electrodeposition of ZnO films with an intense UV-light emissionat room temperature, Journal of Physical Chemistry C 113 (2009) 10422.

[126] R. Tena-Zaera, J. Elias, C. Levy-Clement, C. Bekeny, T. Voss, I. Mora-Sero, J.Bisquert, Influence of the Potassium Chloride Concentration on the Physi-cal Properties of Electrodeposited ZnO Nanowire Arrays, Journal of PhysicalChemistry C 112 (2008) 16318.

[127] B. Cao, W. Cai, H. Zeng, G. Duan, Morphology evolution and photoluminesce-nce properties of ZnO films electrochemically deposited on conductive glasssubstrates, Journal of Applied Physics 99 (2006) 073516.

[128] Y.F. Chen, D.M. Bagnall, H.J. Koh, K.T. Park, K. Hiraga, Z.Q. Zhu, T. Yao, Plasmaassisted molecular beam epitaxy of ZnO on c-plane sapphire: Growth andcharacterization, Journal of Applied Physics 84 (1998) 3912.

[129] Q.T. Wang, G.Z. Wang, J.S. Jie, X.H. Han, B. Xu, J.G. Hou, Annealing effect onoptical properties of ZnO films fabricated by cathodic electrodeposition, ThinSolid Films 492 (2005) 61.

[130] B. Seipel, A. Nadarajah, B. Wutzke, R. Könenkamp, Electrodeposition of ZnOnanorods in the presence of metal ions, Materials Letters 63 (2009) 736.

[131] I. Mora-Seró, F. Fabregat-Santiago, B. Denier, J. Bisquert, R. Tena-Zaera, J.Elias, Cl. Lévy-Clément, Determination of carrier density of ZnO nanowiresby electrochemical techniques, Applied Physics Letters 89 (2006) 203117.

[132] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., John Wiley and Sons,New York, 1981, pp. p79.

[133] Q.H. Li, Y.X. Liang, Q. Wang, T.H. Wang, Oxygen sensing characteristics ofindividual ZnO nanowire transistors, Applied Physics Letters 85 (2004) 6389.

[134] J. J. Wu, D. K.-P. Wong, Fabrication and Impedance Analysis of n-ZnONanorod/p-Si Heterojunctions to Investigate Carrier Concentrations in Zn/OSource-Ratio-Tuned ZnO Nanorod Arrays, Advanced Materials 19 (2007)2015.

[135] J.M. Stiegler, R. Tena-Zaera, O. Idigoras, A. Chuvilin, R. Hillenbrand, Correl-ative infrared–electron nanoscopy reveals the local structure–conductivityrelationship in zinc oxide nanowires, Nature Communications 3 (2012) 1131.

[136] A. Cvitkovic, N. Ocelic, J. Aizpurua, R. Guckenberger, R. Hillenbrand, Infraredimaging of single nanoparticles via strong field enhancement in a scanningnanogap, Physical Review Letters 97 (2006) 060801.

[137] A.J. Huber, A. Ziegler, T. Kock, R. Hillenbrand, Infrared nanoscopy of strainedsemiconductors, Nature Nanotechnology 4 (2009) 153.

[138] G. Wollny, E. Brundermann, Z. Arsov, L. Quaroni, M. Havenith, Nanoscaledepth resolution in scanning near-field infrared microscopy, Optics Express16 (2008) 7453.

[139] J.M. Stiegler, A.J. Huber, S.L. Diedenhofen, J. Gomez Rivas, R.E. Algra, E.P.A.M.R.Bakkers, Hillenbrand Nanoscale free-carrier profiling of individual semicon-ductor nanowires by infrared near-field nanoscopy, Nano Letters 10 (2010)1387.

[140] G.-C. Yi, C. Wang, W.I. Park, ZnO nanorods: synthesis, characterization andapplications, Semiconductor Science and Technology 20 (2005) S22.

[141] L. Schmidt-Mende, J.L. Mac-Manus-Driscoll, ZnO- nanostructures, defects anddevices, Materials Today 10 (2007) 40.

[142] M. Ahmad, J. Zhu, ZnO based advanced functional nanostructures: Synthesis,properties and applications, Journal of Materials Chemistry 21 (2011) 599.

[143] Y.-B. Hahn, Zinc oxide nanostructures and their applications, Korean Journalof Chemical Engineering 28 (2011) 1797.

[144] B. Weintraub, Z. Zhou, Y. Li, Y. Deng, Solution synthesis of one-dimensionalZnO nanomaterials and their applications, Nanoscale 2 (2010) 1573.

[145] L.-J. Bie, X.-N. Yan, J. Yin, Y.-Q. Duan, Z.-H. Yuan, Nanopillar ZnO gas sensor forhydrogen and ethanol, Sensors and Actuators, B: Chemical 126 (2007) 604.

[146] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J. Pearton,J. Lin, Hydrogen-selective sensing at room temperature with ZnO nanorods,Applied Physics Letters 86 (2005) 243503.

[147] T.-J. Hsueh, Y.-W. Chen, S.-J. Chang, S.-F. Wang, C.-L. Hsu, Y.-R. Lin, T.-S. Lin, I.-C. Chen, ZnO nanowire-based CO sensors prepared at various temperatures,Journal of the Electrochemical Society 154 (2007) J393.

[148] J.X. Wang, X.W. Sun, Y. Yang, H. Huang, Y.C. Lee, O.K. Tan, L. Vayssieres,Hydrothermally grown oriented ZnO nanorod arrays for gas sensing appli-cations, Nanotechnology 17 (2006) 4995.

[149] L. Liao, H.B. Lu, J.C. Li, H. He, D.F. Wang, D.J. Fu, C. Liu, W.F. Zhang, Size depend-ence of gas sensitivity of ZnO nanorods, Journal of Physical Chemistry C 111(2007) 1900.

[150] Z. Fan, D. Wang, P.-C. Chang, W.-Y. Tseng, J.G. Lu, ZnO nanowire field-effecttransistor and oxygen sensing property, Applied Physics Letters 85 (2004)5923.

[151] J. Herrán, I. Fernández, R. Tena-Zaera, E. Ochoteco, G. Cabanero, H. Grande,
Schottky diodes based on electrodeposited ZnO nanorod arrays for humiditysensing at room temperature, Sensors and Actuators B 174 (2012) 274.
[152] A. Wei, X.W. Sun, J.X. Wang, Y. Lei, X.P. Cai, C.M. Li, Z.L. Dong, W. Huang, Enzy-matic glucose biosensor based on ZnO nanorod array grown by hydrothermaldecomposition, Applied Physics Letters 89 (2006) 123902.






















































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































4 trochi

Efficient and stable CH3NH3PbI3-sensitized ZnO nanorod array solid-state


[153] T. Kong, Y. Chen, Y. Ye, K. Zhang, Z. Wang, X. Wang, An amperometric glu-cose biosensor based on the immobilization of glucose oxidase on the ZnOnanotubes, Sensors and Actuators B 138 (2009) 344.

[154] F. Zhang, X. Wang, S. Ai, Z. Sun, Q. Wan, Z. Zhu, Y. Xian, L. Jin, K. Yamamoto,Immobilization of uricase on ZnO nanorods for a reagentless uric acid biosen-sor, Analytica Chimica Acta 519 (2004) 155.

[155] M. Ahmad, C. Pan, J. Zhu, Electrochemical determination of L-Cysteine by anelbow shaped, Sb-doped ZnO nanowire-modified electrode, Journal of Mate-rials Chemistry 20 (2010) 7169.

[156] R. Könenkamp, R.C. Word, C. Schlegel, Vertical nanowire light-emitting diode,Applied Physics Letters 85 (2004) 6004.

[157] R. Könenkamp, R.C. Word, M. Godinez, Ultraviolet Electroluminescence fromZnO/Polymer Heterojunction Light-Emitting Diodes, Nano Letters 5 (2005)2005.

[158] O. Lupan, T. Pauporte, Le.T. Bahers, I. Ciofini, B. Viana, High Aspect RatioTernary Zn1-x CdxO Nanowires by Electrodeposition for Light-Emitting DiodeApplications, Journal of Physical Chemistry C 115 (30) (2011) 14548.

[159] T. Pauporte, O. Lupan, B. Viana, Tunable electroluminescence fromlow-threshold voltage LED structure based on electrodeposited Zn1-xCdxO-nanorods/p-GaN Heterojunction, Physica Status Solidi (A) 209 (2012) 359.

[160] L. Li, T. Zhai, Y. Bando, D. Golberg, Recent progress of one-dimensional ZnOnanostructured solar cells, Nano Energy 1 (2012) 91.

[161] Q. Zhang, G. Cao, Nanostructured photoelectrodes for dye-sensitized solarcells, Nano Today 6 (2011) 91.

[162] I. Gonzalez-Valls, M. Lira-Cantu, Vertically-aligned nanostructures of ZnO forexcitonic solar cells: a review, Energy and Environmental Science 2 (2009)19.

[163] M. Yu, Y.-Z. Long, B. Sun, Z. Fan, Recent advances in solar cells based on one-dimensional nanostructure arrays, Nanoscale 4 (2012) 2783.

[164] J.B. Baxter, E.S. Aydil, Nanowire-based dye-sensitized solar cells, AppliedPhysics Letters 86 (2005) 053114.

[165] J.B. Baxter, A.M. Walker, K. van Ommering, E.S. Aydil, Synthesis and charac-terization of ZnO nanowires and their integration into dye-sensitized solarcells, Nanotechnology 17 (2006) S304.

[166] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nanowire dye-sensitized solar cells, Nature Materials 4 (2005) 455.

[167] C. Cheng, Y. Shi, C. Zhu, W. Li, L. Wang, K.K. Fung, N. Wang, ZnO hierar-chical structures for efficient quasi-solid dye-sensitized solar cells, PhysicalChemistry Chemical Physics 13 (2011) 10631.

[168] S. Haller, T. Suguira, D.T. Lincot, Yoshida ZnO hierarchical structures forefficient quasi-solid dye-sensitized solar cells, Physica Status Solidi (A) Appli-cations and Materials Science 207 (2010) 2252.

[169] S.H. Ko, D. Lee, H.W. Kang, K.H. Nam, J.Y. Yeo, S.J. Hong, C.P. Grigoropoulos,H.J. Sung, Nanoforest of Hydrothermally Grown Hierarchical ZnO Nanowiresfor a High Efficiency Dye-Sensitized Solar Cell, Nano Letters 11 (2011) 666.

[170] S. Yodyingyong, Q. Zhang, K. Park, C.S. Dandeneau, X. Zhou, D. Triampo, G.Cao, ZnO nanoparticles and nanowire array hybrid photoanodes for dye-sensitized solar cells, Applied Physics Letters 96 (2010) 073115.

[171] V.M. Guérin, T. Pauporté, From nanowires to hierarchical structures of
template-free electrodeposited ZnO for efficient dye-sensitized solar cells,Energy and Environmental Science 4 (2011) 2971.
[172] V.-M. Guérin, J. Rathousky, T. Pauporté, Electrochemical design of ZnO hierar-chical structures for dye-sensitized solar cells, Solar Energy Materials & SolarCells 102 (2012) 8.

mica Acta 127 (2014) 467–488

[173] E. Guillén, E. Azaceta, L.M. Peter, Z. Arnost, R. Tena-Zaera, J.A. Anta, ZnOSolar Cells with an Indoline Sensitizer: a Comparison Between Nanoparticu-late Films and Electrodeposited Nanowire Arrays, Energy and EnvironmentalScience 4 (2011) 3400.

[174] E. Guillén, E. Azaceta, A. Vega-Poot, J. Idígoras, J. Echeberría, J.A. Anta,R. Tena-Zaera, ZnO/ZnO core-shell nanowire array electrodes: Blockingof recombination and impressive enhancement of photovoltage in dye-sensitized solar cells, Journal of Physical Chemistry C 117 (2013) 13365.

[175] A. Yella, H.-W. Lee, H.N. Tsao, C. Yi, A.K. Chandiran, K. Nazeeruddin Md.,E.W.-G. Diau, C.-Y. Yeh, S.M. Zakeeruddin, M. Gratzel, Porphyrin-SensitizedSolar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12% Efficiency,Science 334 (2011) 629.

[176] Q. Zhang, C.S. Dandeneau, X. Zhou, C. Cao, ZnO nanostructures for dye-sensitized solar cells, Advanced Materials 21 (2009) 4087.

[177] J.B. Sambur, T. Novet, B.A. Parkinson, Multiple Exciton Collection in a Sensi-tized Photovoltaic System, Science 330 (2010) 63.

[178] J. Chen, C. Li, G. Eda, Y. Zhang, W. Lei, M. Chhowalla, W.I. Milne, W.Q. Deng,Incorporation of graphene in quantum dot sensitized solar cells based on ZnOnanorods, Chemical Communications 47 (2011) 6084–6086.

[179] Cl. Lévy-Clément, J. Elias, Optimization of the Design of ExtremelyThin Absorber Solar Cells Based on Electrodeposited ZnO Nanowires,ChemPhysChem 14 (2013) 2321.

[180] R. Zhang, Q.-P. Luo, H.-Y. Chen, X.-Y. Yu, D.-B. Kuang, C.-Y. Su, CdS/CdSeQuantum Dot Shell Decorated Vertical ZnO Nanowire Arrays by Spin-Coating-Based SILAR for Photoelectrochemical Cells and Quantum-Dot-SensitizedSolar Cells, ChemPhysChem 13 (2012) 1435.

[181] J. Chen, J. Wu, W. Lei, J.L. Song, W.Q. Deng, X.W. Sun, Co-sensitized quantumdot solar cell based on ZnO nanowire, Applied Surface Science 256 (2010)7438.

[182] M. Seol, H. Kim, Y. Tak, K. Yong, Novel nanowire array based highly efficientquantum dot sensitized solar cell, Chemical Communications 46 (2010) 5521.

[183] X. Yin, W. Que, D. Fei, H. Xie, Z. He, G. Wang, Strategies to prepare an effi-cient photoanode for ZnO nanowires-based CdS–CdSe co-sensitized solarcells, Electrochimica Acta 89 (2013) 561.

[184] M.-J. Jin, X.-Y. Chen, Z.-M. Gao, T. Ling, X.-W. Du, Improve photo-electronconversion efficiency of ZnO/CdS coaxial nanorods by p-type CdTe coating,Nanotechnology 23 (2012) 485401.

[185] K. Takanezawa, K. Tajima, K. Hashimoto, Efficiency enhancement of polymerphotovoltaic devices hybridized with ZnO nanorod arrays by the introductionof a vanadium oxide buffer layer, Applied Physics Letters 93 (2008) 063308.

[186] H.S. Kim, C.R. Lee, J.H. Im, K.B. Lee, T. Moehl, A. Marchioro, S.J. Moon, R.Humphry-Baker, J.H. Yum, J.E. Moser, M. Gratzel, N.G. Park, Lead iodide pero-vskite sensitized all-solid-state submicron thin film mesoscopic solar cellwith efficiency exceeding 9%, Scientific Reports 2 (2012) 591.

[187] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Efficient hybridsolar cells based on meso-superstructured organometal halide perovskites,Science 338 (2012) 643.

[188] D. Bi, G. Boschloo, S. Schwarzmüller, L. Yang, E.M.J. Johansson, A. Hagfeldt,

solar cells, Nanoscale 5 (2013) 11686.[189] M.H. Kumar, N. Yantara, S. Dharani, M. Graetzel, S. Mhaisalkar, P.P. Boix,

N. Mathews, Flexible, low-temperature, solution processed ZnO-based per-ovskite solid state solar cells, Chemical Communications 49 (2013) 11089.






























































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































Electrodeposition of ZnO Nanorod Arrays on Transparent Conducting Substrates–a Review

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Transcript of Electrodeposition of ZnO Nanorod Arrays on Transparent Conducting Substrates–a Review