Optimization of CeO2–TiO2 composition for fast switching kinetics and improved Li ion storage...

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Solid State Ionics 171 (2004) 81–90

Optimization of CeO2–TiO2 composition for fast switching kinetics and

improved Li ion storage capacity

Amita Vermaa, S.B. Samantaa, A.K. Bakhshib, S.A. Agnihotrya,*

aNational Physical Laboratory, Electronic Material Division, Dr. K.S. Krishnan Marg, New Delhi 110012, IndiabUniversity of Delhi, New Delhi 110007, India

Received 7 January 2004; received in revised form 12 April 2004; accepted 19 April 2004

Abstract

Thin films of CeO2 and CeO2–TiO2 having a wide range of compositions prepared following sol–gel spin coating technique involving a

Ce (III) salt and titanium propoxide in ethanol fired at 500 jC were investigated by different characterization techniques to obtain information

on thermal, structural, optical and electrochemical aspects of the films. The influence of the structure on the kinetics of electrochemical

insertion/extraction of lithium ions was evaluated with the outcome of above results emphasizing the importance of TiO2 in relation to

improvement of coloration–bleaching kinetics of electrochromic material configured with CeO2–TiO2 films, rendering them suitable for

electrochromic windows. The XRD investigations showed the prevalence of amorphicity in the films constituting 50% or lower mole

percentage of CeO2. The compositions with higher CeO2 contents exhibited nanocrystallinity and were distinguished by the existence of

diffraction peaks assigned to cerianite and the mixed compound, CeO1.6�2TiO2. In all the compositions, presence of TiO2 in the amorphous

phase was clearly evident, which led to the enhanced ion insertion capacity of the films. The highest cathodic charge density (CCD) of 23

mC/cm2 was observed for the film containing 75% TiO2. The AFM images clearly show the reduced level of roughness and the grain size of

CeO2 in the films with the increased content of TiO2. The best performance characteristics achieved for the electrochromic device comprising

WO3 and 50% CeO2 electrodes conveys the practical utility of the latter in the transmissive electrochromic devices.

D 2004 Elsevier B.V. All rights reserved.

PACS: 68.55-a; 81.15.Lm

Keywords: Sol–gel; Spin coating; CeO2–TiO2; Passive counter electrode; Electrochromism

1. Introduction

The performance of electrochromic windows strongly

depends upon the choice of the counter electrode. The

counter electrodes are accepted in two forms, one involving

an electrochromic layer complementary to the selected

primary electrochromic material, e.g. combination of WO3

with NiOxHy [1,2] or Prussian Blue and the other alternative

is an optically passive counter electrode, remaining trans-

parent in both reduced and oxidized states. Several materials

have been studied and reported to exhibit properties render-

ing them suitable candidates as passive counter electrodes,

which include V2O5, CeO2, etc. The disadvantages inherent

in V2O5 are the partial reversibility of the intercalation

0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2004.04.015

* Corresponding author. Tel.: +91-11-25742610/2283; fax: +91-11-

25726938.

E-mail address: [email protected] (S.A. Agnihotry).

reaction and low transmission in the bleached state. CeO2

has good reversibility for lithium ion intercalation and has

complete optically passive behavior. To overcome the slow

reaction kinetics in CeO2 films, Baudry et al. [3] suggested

the replacement of some Ce atoms by an element of lower

ionic radius such as Ti. Mixed CeO2–TiO2 thin films also

display no coloration during insertion and extraction,

extending their use as a prospective material for optically

passive counter electrode. Several workers have attempted

the synthesis of mixed CeO2–TiO2 [4–13] coatings follow-

ing different sol–gel routes. Baudry et al. [3] prepared these

films using ceric ammonium nitrate and titanium isoprop-

oxide as the precursor materials and reported the charge

inserted and extracted from the material as 10 mC/cm2 at a

sweep rate of 10 mV/s, while cerium chloride and titanium

isopropoxide were the starting materials in the method

proposed by Makishima et al. [4–6]. Keomany et al. [9]

used the method involving only metal alkoxides, Ce(OBu)4and Ti(OBu)4 in BuOH.

A. Verma et al. / Solid State Ionics 171 (2004) 81–9082

The present study focuses on the synthesis of CeO2 and

mixed oxide CeO2–TiO2 coatings based on the sol–gel

spin coating method using CeCl3�7H2O in combination

with Ti(OPr)4, same as used by Makishima et al.; however,

unlike Makishima et al., the present studies cover a wide

range of Ce/Ti compositions. The films displayed excellent

chemical and mechanical stability in addition to high

adhesion with the substrates. The electrochemical, struc-

tural and optical studies of the films suggest their plausible

application in transmissive electrochromic devices and

emphasize the importance of TiO2 in relation to improve-

ment of kinetics of Li ion intercalation in CeO2 films and

also to enhanced ion storage capacity of CeO2–TiO2 films

with increased proportion of titanium oxide. Also, the

comparison with other reported studies clearly bring out

the influence of precursors, additives used, and the thermal

treatment parameters on the properties of the films and the

xerogels.

2. Experimental

2.1. Preparation of coating sols

Starting solutions were prepared with an ethanolic

solution of CeCl3�7H2O (0.22 M) containing Ti(OPr)4 in

quantities so as to obtain sols with different Ce/Ti compo-

sitions. The compositions of the studied sols are presented

in Table 1. Aging of the sols resulted in gels, with the

gelling time decreasing regularly with the amount of

titanium propoxide. The gels were dried under ambient

conditions leading to the formation of xerogels. The

xerogels were studied as such and after thermal treatment

at 500 jC for 5 min.

2.2. Preparation of CeO2–TiO2 thin films

Acetone washed and dried transparent conducting oxide

(SnO2:F) coated glass substrates were spin coated by the

coating solutions at 3000 rpm for 35 s followed by drying in

air at room temperature for 5 min. Subsequently, the films

were fired in air for 5 min at 500 jC. Crack-free, pale

yellow, hard films highly adherent to the substrates with

Table 1

Compositions of the Ce/Ti sols

Sol CeO2 (mol%) TiO2 (mol%)

C1 100 –

CT1 80 20

CT2 66 34

CT3 57 43

CT4 50 50

CT5 37 63

CT6 33 67

CT7 28 72

CT8 25 75

T9 – 100

even up to 1 Am thickness could be obtained. The excellent

chemical and mechanical stability of the films reflected in

their high resistance to etch in acidic/alkaline medium

bestowed them with additional advantage over other counter

electrodes.

2.3. Device fabrication

Two types of transmissive ‘‘all solid state electrochro-

mic devices’’ were constructed using either (i) sol–gel

deposited WO3 films or (ii) electrodeposited Prussian Blue

as the primary electrochromic electrodes and the 1 M

LiClO4 + PC+ PMMA+SiO2 as the gel polymeric electro-

lyte. The passive counter electrode incorporated into these

devices was a 50% CeO2 film. The devices thus constructed

had the following configurations:

ECD1 : SnO2 : F=CeO2 � TiO2=1 M LiClO4 þ PC

þ PMMA ð10 wt:%Þ þ SiO2 ð5 wt:%Þ=WO3

=SnO2 : F

ECD2 : SnO2 : F=CeO2 � TiO2=1 M LiClO4 þ PC

þ PMMA ð10 wt:%Þ þ SiO2ð5 wt:%Þ=Prussian Blue=SnO2 : F

2.4. Characterization of films and xerogels

Transmission spectra of the films in the range of 300–

800 nm were recorded using UV 3101 PC Shimadzu

Spectrophotometer. Transmission profiles of the films were

recorded in the visible spectral region after intercalation and

deintercalation of lithium ions with platinum as the counter

electrode in 1 M LiClO4/propylene carbonate with the

voltage F 1.5 V applied for 60 s. Transmission in colored

and bleached states for electrochromic devices based on

WO3 and Prussian Blue films were recorded using a

Keithley 224 programmable current source. XRD patterns

of the films and thermally treated xerogels were recorded in

the 2h range from 5j to 70j with a D8 Advanced Bruker

Diffractometer. Thermal studies (DTA) on xerogels were

performed at a heating rate of 5 jC/min in an inert

atmosphere in the temperature range of room temperature

to 600 jC using a Rigaku Thermoflex PTC-10A. FTIR

spectra of the films and thermally treated xerogels were

recorded in the wave number range of 400–4000 cm� 1 on a

Perkin Elmer FT-IR Spectrometer SPECTRUM 2000 Spec-

trophotometer. While spectra of films were recorded in the

reflectance mode, the spectra for xerogels were recorded in

the transmission mode in the form of KBr pellets. Electro-

chemical measurements on the (i) Ce/Ti electrodes versus

platinum/WO3/Prussian Blue in the liquid electrolyte and

(ii) electrochromic devices were performed on a computer-

controlled setup consisting of an He–Ne laser source

(k = 632.8 nm), an Si photodetector together with a versatile

micro-controller based ECD characterization unit. Multiple

Fig. 2. Transmission profiles of 50% CeO2 film deposited after different

A. Verma et al. / Solid State Ionics 171 (2004) 81–90 83

step potential cycling was performed by applying a square

wave potential of amplitude F 1.5 V at a fixed frequency of

0.0011 Hz. Cyclic voltammetric studies were performed on a

computer-controlled OMNI potentiostat. All measurements

were performed in an electrolyte of 1 M LiClO4 in propylene

carbonate (PC) in a three electrode arrangement comprising

CeO2–TiO2 film as theworking electrode, a platinum counter

electrode and Ag/AgCl/KCl serving as the reference elec-

trode. The CVexperiments were conducted using scan rate of

20 mV/s and potential was swept between cathodic (1.0 V) to

anodic (1.0 V) versus Ag/AgCl/KCl. The surface morpholo-

gy of the films was observed using scanning electron micros-

copy on a JEOL JSM 840 Scanning ElectronMicroscope and

atomic force microscopic images for the films were obtained

on a Nanoscope II instrument.
aging times: (. . .) aging time (2 days), (—) aging time (1 day) and (- - - -)
aging time (As-prepared).

3. Results and discussion

3.1. Optical properties of the coatings

The optical transmission spectra of CeO2–TiO2 films on

SnO2:F deposited glass substrates as a function of added

TiO2 are presented in Fig. 1. Addition of 50 mol% TiO2

achieved films with the highest transmission. The cut-off in

transmission is observed in the range of 350–400 nm. Fig. 2

illustrates the 50% CeO2 composition films deposited from

sols after different aging periods. As can be seen, the highest

transmission is attained by the films prepared from sols aged

to the extent of just reaching the gel point. The enhanced

transmission in all likelihood appears to be related to

microstructural changes due to increased degree of hydro-

lysis with aging.

A special mention needs for the transmission of the films

immersed in a liquid electrolyte. Observed visually, all the

films, irrespective of their composition, showed noticeable

enhancement in transmission. This observation is further

discussed in the section on the morphological studies.

Fig. 1. Transmission spectra of CeO2–TiO2 coatings with different CeO2

mole percentages: 80% (. . .), 66% (—. . .—), 57% (—..—), 50% (—), 37%

(- - -), 33% (— - —), 28% (— - - —) and 25% (—.—).

An insight into the spectral performance of the 50%

CeO2 composition film subjected to lithium ion intercala-

tion/deintercalation (by the application of F 1.5 V versus

platinum for 60 s) can be obtained from Fig. 3. Almost

identical properties on Li ion intercalation and deintercala-

tion exemplify the passive nature of the films.

3.2. Structural characterization

X-ray diffractograms of the films of various composi-

tions are illustrated in Fig. 4. Both CeO2 and TiO2 films

(Fig. 4a and e) show crystallinity. Peaks of CeO2 cubic

cerianite structure and TiO2 tetragonal anatase [14,15] are

observed. Intermediate compositions, however, show dif-

ferent characteristics. One common feature of all the

compositions is a weak and broad hump in the low 2hregion between 15j and 35j, characteristic of the short-

range order or amorphicity. Overlapped on this broad

hump, not all but only compositions with 80% and 66%

CeO2 show well defined peaks indicating presence of

Fig. 3. Spectral optical transmittance for a 50% CeO2 film. Data are given

for as deposited (—), intercalated (- - - -) at � 1.5 V versus Pt and

deintercalated (- — -) state at 1.5 V versus Pt.

Fig. 4. X-ray diffraction patterns of films with different CeO2 mole

percentages. (a) 0%, (b) 50%, (c) 66%, (d) 80% and (e) 100%. Symbol (.)denotes the diffraction peaks of CeO1.6�2TiO2.

Fig. 5. X-ray diffraction patterns of thermally treated xerogels with different

CeO2 mole percentages. (a) 25%, (b) 33%, (c) 37%, (d) 50% and (e) 57%.


some crystalline phase/s. The analysis shows that these

peaks can be identified to two different phases. One of

them being the cubic cerianite phase as observed in CeO2

films and the other corresponding to an oxygen deficient

compound CeO1.6�2TiO2 [16]. No defined compound has

been earlier observed in films deposited using different

precursor materials. The cubic cerianite CeO2 phase gives

its characteristic peaks signifying orientation along (111),

(200), (220) and (311) planes. The crystallite size calcu-

lated with 80% and 66% CeO2 along the (111) plane using

the Scherrer formula amount to 14 and 9 nm, respectively.

For 50% CeO2 composition, no well-defined peaks either

of CeO2 or the mixed compound are present, only a broad

hump is present. This could be attributed to the XRD

amorphous nature of the films with 50% CeO2. The

amorphicity is seen to prevail (XRD patterns not shown

here) in compositions constituting CeO2 at and below

50%. In other words increasing TiO2 seems to have

decreased CeO2 crystallite size. Similar observations were

reported by Keomany et al. [9], although the crystallite

sizes were far too smaller than the present observations

and can be explained due to limited crystal growth during

the thermal treatment with increasing TiO2 content. Some

other differences also stem out by comparing the results of

the present studies with earlier reports. Firstly, TiO2 films

derived from butoxide precursor stabilized by acac-H by

Keomany et al. were shown to be completely amorphous

as against the crystalline films obtained in the present

studies. Secondly, in none of the films reported earlier,

was there coexistence of compound like CeO1.6�2TiO2

along with CeO2 for any intermediate composition as

has been observed by us for 80% and 66% CeO2 compo-

sition. These observations bring out clearly the strong

dependence of crystallinity/amorphicity/crystallite size in

films on the precursor materials, stabilizer used as well as

other experimental conditions like temperature and dura-

tion of thermal treatment.

X-ray diffractograms of xerogels heated at 500 jC for 5

min of various Ce/Ti compositions are shown in Fig. 5.

None of the compositions had the presence of any well-

Fig. 6. FT-IR transmission spectra of xerogels heated at 500 jC with

different CeO2 mole percentages. (a) 25%, (b) 28%, (c) 33%, (d) 37%, (e)

50%, (f) 57%, (g) 66% and (h) 80%.

Fig. 7. Differential thermal analysis of the xerogels with different CeO2

mole percentages. (Heating rate: 5j/min, nitrogen atmosphere.) (a) 0%, (b)

25%, (c) 28%, (d) 33%, (e) 37%, (f) 50%, (g) 57%, (h) 66% and (i) 80%.


defined Ce/Ti compound as in the films. Although all the

compositions showed well-formed CeO2 crystallites, TiO2

crystallites could be identified/detected in the compositions

at and below 37% CeO2. With increased content of TiO2,

the crystallite size of CeO2 decreased and that of TiO2

increased. The CeO2 crystallite sizes evaluated by Scherrer

equation for the compositions equal to 50%, 37%, 33% and

25% CeO2 were 5.9, 5.1, 4.1 and 3.9 nm, respectively along

the (111) plane. This decrease is a resultant of diminished

crystal growth of cerianite due to increase in the crystallite

size of anatase (in consequence of increased TiO2 content),

which inhibits the growth of the crystalline cerium oxide.

The TiO2 crystallite size for the compositions, 63%, 67%

and 75% TiO2 amounted to 2.4, 2.5 and 3.1 nm, respectively

along the (101) plane, showing increase in the crystallite

size of TiO2 with the increased proportion of TiO2 in the

thermally treated xerogels. Similar study on the heat-treated

xerogels was also carried out by Makishima et al. [4]. Their

results showed the existence of both cerianite and anatase

phases in the heat-treated gelled samples containing Ce/Ti in

1/2, 1/1 and 3/2 proportions. Makishima et al. thermally

treated the samples for 3 h. The variation observed in our

studies is the consequence of the duration of the thermal

treatments given to the gelled samples.

3.3. FTIR spectroscopy data

Fig. 6 shows the FTIR spectra of xerogels containing

different Ce/Ti compositions, thermally treated at 500 jC in

the transmission mode. All the curves are characterized by a

broad band extending in the range of 3394–3439 cm� 1,

arising from the m(O–H) stretching vibration confirming the

presence of water even in the thermally treated xerogels. A

weak medium band assigned to d(O–H) mode at f 1654

cm� 1 is also seen. The wave number region between 400

and 1000 cm� 1 contains bands typical of metal oxygen

bondings. For the compositions containing CeO2 between

80% and 37%, a medium intensity band appears in the wave

number range of 530–571 cm� 1. This is the wave number

region encompassing the characteristic bands of both m(Ce–OH) [17] and m(Ti–O) [18]. In addition, the band at f 790

cm� 1 is due to the m(Ti–O) vibrational mode [19]. The

continuous reduction in the intensity of the band at f 500

cm� 1 with the enhanced proportion of TiO2 is substantiated

by the assumption that the same band in all probability is

assigned to m(Ce–OH) mode of vibration. This assumption

is further supported by the observation that there is an


increase in the intensity of the band assigned to m(Ti–O) atf 790 cm� 1 with the increased amount of TiO2 in the

thermally treated xerogels. A band at 458 cm� 1 appears for

the 25% CeO2 composition xerogel, which is most likely

assigned to m(Ti–O–Ti) mode of vibration. The increased

growth of the Ti–O–Ti polymeric chain due to increased

proportion of TiO2 is forwarded as a possible explanation

for the assignment of the band at 458 cm� 1. Infrared and X-

Fig. 8. AFM images of the CeO2–TiO2 films with different C

ray diffraction studies together prove the existence of CeO2

and TiO2 as independent entities, ruling out the possibility

of formation of any mixed compound of cerium oxide and

titanium oxide except in compositions constituting 80% and

66% CeO2. FT-IR spectrum in the reflectance mode of a

50% CeO2 composition film fired at 500 jC is characterized

by a broad band at 3393 cm� 1, which is consistent with the

m(O–H) stretching mode of water (either coordinated, hy-

eO2 mole percentages. (a) 80%, (b) 50% and (c) 25%.

Fig. 9. SEM micrographs of the CeO2–TiO2 films with different CeO2

mole percentages. (a) 80%, (b) 50% and (c) 25%.


drogen bonded or free). A broad absorption band observed

at 780 cm� 1 is assigned to the m(Ti–O) stretching vibration.

The existence of water even in the annealed films is an

important feature for the electrochemical activity of these

films.

3.4. Thermal analysis of the xerogels

Fig. 7 depicts the DTA patterns of the xerogels with

different compositions. For a xerogel of pure TiO2, an

endotherm appears at around 100 jC attributed to the

removal of water. In addition, sharp exothermic peaks

centered at 259 and 398 jC, respectively are due to

combustion of remnant organic moieties and transition of

amorphous TiO2 to anatase phase [9]. Thermograms of

xerogels containing different mole percentages of CeO2

are characterized by two endothermic peaks positioned

between room temperature and 300 jC; these are assigned

to the release of physisorbed, chemisorbed water and

decomposition of organic groups. Additionally, the exo-

therm in the 50% CeO2 composition xerogel at 397 jC is

ascribed to the transition from amorphous to crystalline

cubic phase of cerianite in conformity with the results of

thermal studies on xerogels constituting more than 50%

CeO2. The assignment of the exotherm to crystallization of

CeO2 is substantiated by the appearance of exotherms in the

thermograms of all the compositions with 50% CeO2 and

above. The exothermic peaks for the xerogels containing

80% and 66% CeO2 appeared at f 428 and f 425 jC,respectively. These exotherms are ascribable to the crystal-

lization of cerium oxide and CeO1.6�2TiO2. The variation in

the exotherm position of 50% CeO2 composition xerogel to

a relatively lower temperature can be explained by its

assignment to crystallization to cerianite phase alone. Fur-

ther the X-ray diffraction results of the corresponding

thermally treated xerogel show the existence of only CeO2

crystallites thereby confirming the exotherm’s assignment.

3.5. Morphological properties

The topographical studies on the CeO2–TiO2 films with

different Ce/Ti compositions were performed by AFM as

shown in Fig. 8. All the AFM images show the existence of

(i) nanosized CeO2 in well-defined crystalline phase and (ii)

prominent grain boundaries. The films are also characterized

by finite roughness in the nanorange. The general trend

observed is that the increase in TiO2 content decreases the

average CeO2 grain size and also the roughness of the films.

In particular, the films with 50% CeO2 composition exhibit

a grain size of 89 nm, a grain boundary width around 2.6 nm

and roughness of the order of 9.67 nm. Presence of these

nanograins throughout the film is of tremendous importance

as they aid enormously in enhancing the electrochemical

activity of the electrode when used in an electrochromic

device. This is also reflected in the high ion storage capacity

of the film. The enhanced transmission of the films im-

mersed in a liquid electrolyte stated in Section 3.1 is a

manifestation of the rough surface of the films.

Fig. 9 shows the SEM micrographs of the CeO2–TiO2

films with different compositions exhibiting no cracks are

indicative of the critical role of the preparation conditions

and thickness of the film as the parameters deciding the

morphology of the films. Agglomerates of varying dimen-

sions in the nanorange characterize the films with 170 nm

being the average size for the 50% CeO2 composition film.

The grain size appears to reduce with enhanced TiO2

proportion. The film containing 25% CeO2 showed no

grains in the SEM micrograph proving amorphous nature

of the film, as was earlier established on the basis of XRD

investigations. Also a network kind of pattern observed in

this composition differentiated it from the other Ce/Ti

Table 2

Cathodic (Qinserted) charge density as a function of added TiO2 and aging

period of the sol

TiO2

(mol%)

Qinserted

(mC/cm2)

(as-prepared sol)

Qinserted

(mC/cm2)

(aged sol)

Coloration

time (tc, s)

Bleaching

time (tb, s)

0 – 13.48 – –

20 6.2 14.04 60 40

36 7.19 14.70 – –

43 7.67 18.85 90 80

50 9.29 18.36 46 10

63 11.04 19.29 – –

67 11.11 20.50 – –

72 11.13 23.00 50 20

75 11.7 23.10 196 30

100 – 6.18 – –

Also shown are the coloration–bleaching times of the devices based on

WO3 and Ce/Ti films with different compositions.


composition films. The pore size in the films was observed

to reduce with the enhanced proportion of CeO2. All the

parameters discussed above will have direct bearing on the

electrochemical properties and consequently their electro-

chromic activity.

3.6. Electrochemical investigations

To investigate the usefulness of CeO2 and CeO2–TiO2

spin coated films as counter electrodes for transmissive

electrochromic devices, cyclic voltammetry (CV) was

employed. The technique measures the reversibility capa-

bility of the films to intercalate/deintercalate the lithium

ions. Fig. 10 presents voltammograms obtained for differ-

ent film compositions. The absence of both anodic and

cathodic peaks is evident from the voltammograms. The

most symmetrical pattern is obtained for the 25% CeO2

composition having cathodic and anodic wave potentials,

respectively at � 0.5 and + 0.25 V. Although the voltam-

mograms presented in this figure do not correspond to

films of comparable geometrical areas, better comparison

is possible only for three compositions (a, d, h) with

almost equivalent area. The peak current values (imax) are

clearly seen to be increasing proportionately with the TiO2

content in the film. This result is corroborated by the

chronoamperometric measurements mentioned in Table 2

showing the enhanced ion insertion capacity in the films

in consequence of increased TiO2 proportion. Generally

observed differences in the voltammograms of crystalline

and amorphous films are not evident in the study. The

reason may be the very small crystallites embedded in

amorphous matrix especially for the compositions with

80% and 66% CeO2. Another important point emerging in

Fig. 10. Cyclic voltammograms of films with different CeO2 mole percentages at

57%, (d) 50%, (e) 37%, (f) 33%, (g) 28% and (h) 25%.

the present study is the calculated current density value

being higher by an order than that reported by Keomany

et al. [8], reflecting superior electrochemical response of

the films. This difference clearly brings out the strong

dependence of the properties of the films on the precursor

material and the other process parameters, more impor-

tantly the liquid electrolyte in which the voltammetric

measurements are performed.

In multiple step chronoamperometric studies, two differ-

ent sets of experiments were carried out. In one to evaluate

the charge capacity of the films with different CeO2 con-

tents, the films were used as working electrodes in 1 M

LiClO4 + PC solution against platinum counter electrode. In

the other to investigate the impact of these films on the

kinetics of the electrochromic electrodes like WO3 or

Prussian Blue, the platinum counter electrode was replaced

a potential scan rate of 20 mV/s in 1 M LiClO4/PC. (a) 80%, (b) 66%, (c)

Fig. 11. Transmittance variation of the Prussian Blue film working in

conjunction with films with different CeO2 mole percentages. (a) 50% and

(b) 100%.


by the corresponding electrochromic electrode. The meas-

urements were carried out under an applied square wave

potential of amplitude F 1.5 V at a fixed frequency of

0.0011 Hz.

The cathodic charge density (mC/cm2) for the films with

different compositions is presented in Table 2. In contrast to

the reports by Keomany et al., showing the highest charge

density for 50% CeO2, our observations clearly show that

the charge capacity linearly increases with TiO2 content

attaining a maximum value of about 23 mC/cm2 for film

with 75% TiO2. This could be explained as due to small

crystallite size of CeO2 in amorphous TiO2 matrix for

composition with low TiO2 content and highly disordered

nature of the films with higher TiO2 content, which is

responsible for ease in Li ion diffusion into the films.

Similar conclusions have been arrived at for sputtered

CeO2–TiO2 films by Granqvist et al. [20]. The improved

cathodic charge density in the films deposited from the aged

Fig. 12. Optical response (a) and chronoamperogram (b) of 50% CeO2 film for

electrolyte.

sols as shown in Table 2 is explained on the basis of

conducive microstructural changes induced.

Considering TiO2 films to have 1000 times higher

diffusion of lithium than in CeO2 [8] and films with

50% CeO2 composition accounting for 10 times higher

apparent diffusion coefficient of lithium in relation to CeO2

films a fast response in terms of coloration and bleaching

time for the electrochromic electrode is expected and is

indeed obtained. Fig. 11 gives an overview of optical

response in terms of sensor current of Prussian Blue film

serving as primary electrochromic working electrode in

combination with films having (i) 50% CeO2 and (ii) 100%

CeO2 composition functioning as the counter electrodes.

The result obtained above is an experimental proof for the

slow reaction kinetics in CeO2 films in agreement with

Baudry et al. [3]. The calculated coloration and bleaching

times for Prussian Blue film against CeO2 and CeO2–

TiO2, respectively are, tc = 64 s, tb = 107 s and tc = 20 s,

tb = 34 s. Both the switching times for the Prussian Blue

film working in combination with CeO2–TiO2 film are

faster by a factor more than 3 in comparison with the CeO2

film alone, as expected on the basis of reported higher

diffusion coefficient for CeO2–TiO2 films. Fig. 12 illus-

trates the electrical and optical response of the film having

50% CeO2 composition as a function of time. No variation

in optical response is supported by the optical data with the

latter showing an insignificant optical modulation of 1% at

632.8 nm. To assess the suitability of the mixed CeO2–

TiO2 films with different compositions for transmissive

electrochromic devices, five different compositions were

configured along with sol–gel deposited WO3 films in the

form of a device. The switching characteristics of these

devices are also included in Table 2. It is evident from tc, tband the cathodic charge density (CCD) values that the high

CCD of the counter electrode is not the only prerequisite

for better performance in terms of kinetics of coloration–

bleaching reactions, although the reversibility of electro-

chemical reaction causing coloration/bleaching is main-

tained in a superior manner. For the best choice of the

counter electrode, a good agreement between the CCD and

a potential step between 1.5 and � 1.5 V versus Pt in a 1 M LiClO4/PC

Fig. 13. Optical transmission variation of the complete transmissive device,

glass/SnO2:F/CeO2 – TiO2/1 M LiClO4 + PC + PMMA + SiO2/WO3/

SnO2:F/glass in the visible spectral region on applied voltage of F 1.5 V.

The inset figure depicts the transmission variation of the device at k= 632.8nm as a function of time.


the fast kinetics needs to be fulfilled. In view of this,

CeO2–TiO2 films with 50% CeO2 content appear to be the

most promising counter electrodes for transmissive electro-

chromic devices. For window applications additionally

highest modulation is very much necessary. The transmis-

sion characteristics of the device based on WO3 and 50%

CeO2 films as the primary and counter electrodes, respec-

tively separated by a composite gel electrolyte (1 M

LiClO4 + PC+ PMMA+SiO2) over the whole visible spec-

tral region are illustrated in Fig. 13. Such a device offers

optical modulation of 53% and a coloration efficiency of

37.3 cm2/C at 550 nm. Although the highest intercalated

charge is observed for the composition containing 25%

CeO2 but the fastest coloration and bleaching is observed

for the WO3 film in combination with the 50% CeO2

composition implying appropriate microstructural changes

induced by the addition of TiO2 in an amount equaling to

that of CeO2. The optical transmission spectrum in the

visible region of the equimolar CeO2–TiO2 film in com-

bination with electrodeposited Prussian Blue in the colored

and bleached state demonstrates an optical modulation of

f 35% at k= 632.8 nm and these spectrophotometric

results of the devices prove the practical utility of

CeO2–TiO2 films in transparent electrochromic devices.

4. Conclusions

The films prepared by sol–gel spin coating process

involving organic– inorganic precursors {Ti(OPr)4 and

CeCl3�7H2O in ethanol} are highly adherent, transparent,

hard and homogeneous, with excellent chemical and me-

chanical stability over the wide range of compositions

studied, varying from pure CeO2 to pure TiO2. Introduction

of TiO2 induces amorphicity in films for CeO2 content less

than 57% contributing to the improvement of intercalation

kinetics of lithium ions and ion storage capacity. For the

films comprising 80% and 66% CeO2, coexistence of two

kinds of crystalline phases, i.e. face centered cubic cerianite

phase of CeO2 and CeO1.6�2TiO2 was evident from the X-

ray diffraction patterns. The optical passivity in the films

towards Li ion intercalation/deintercalation was also ob-

served. Of all the compositions, the mixed CeO2–TiO2

(50% CeO2) film working in conjunction with the primary

electrochromic electrode (WO3) contributed to its fastest

response in terms of coloration and bleaching times in

addition to being the most transparent and an amorphous

film.

Acknowledgements

The financial support from Council of Scientific and

Industrial Research (AV) and Ministry of Non-Conventional

Energy Sources (MNES), Government of India and the

characterization facility extended by the University Science

Instrumentation Center, University of Delhi is highly

acknowledged.

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Optimization of CeO2–TiO2 composition for fast switching kinetics and improved Li ion storage...

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