structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process

Among the numerous thin films of transition metal oxides, tungsten oxide was recognized as a promising candidate for the optically active electrodes.. Compared with the non-templated samp

Structural and electrochromic properties of tungsten oxide prepared by

surfactant-assisted process

The Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences (CAS), 1295 Dingxi Road, Shanghai 200050, China

a r t i c l e i n f o

Article history:

Received 30 June 2008

Received in revised form

18 January 2009

Accepted 6 February 2009

Available online 25 March 2009

Keywords:

Nanostructured

Electrochromic

Tungsten oxide

a b s t r a c t

By virtue of gemini surfactant template, nanostructured tungsten oxides thin films were prepared from the modified tungsten hexachloride sol–gel techniques Temperature was varied as it is an important factor for crystallization, surface morphology and microstructure of tungsten oxides, from the studies of X-ray diffractions, scanning electron microscopy and transmission electron microscopy The mesopor-ous sample calcined at 300 1C has tri-dimensional vermicular mesopores with nanocrystallites embedded in the pore wall, while such uniform structure would be destroyed by higher calcination temperature of about 400 1C X-ray photoelectron spectroscopy was used for analyzing the surface-binding states and the stoichiometry for the oxides Electrochromic characterization was implemented

by simultaneous voltametric and spectrophotometric measurements of tungsten oxides/indium tin oxide (ITO) electrodes The investigation results showed that organized pore-wall nanostructure has strong effects on the electrochemical and chromogenic properties depending on the specific surface area and the impacts from the evolved crystallization

&2009 Elsevier B.V All rights reserved

1 Introduction

Electrochromic material was known to undergo change in their

optical properties upon applied voltages, which could be used for

smart windows, displays, antiglare mirrors and advanced glazing

[1–3] Among the numerous thin films of transition metal oxides,

tungsten oxide was recognized as a promising candidate for the

optically active electrodes Much research on electrochromic WO3

thin films was based on the preparation of vacuum evaporation,

laser ablation or sputting methods, etc.[4–6] On the other hand,

wet deposition could be an alternate technique to study the

preparation of tungsten oxide thin films [7,8] Lately, some

template-mediated methodology has been proven feasible to

bring up pore organization into tungsten oxide thin films

including the Poly(acrylic acid) (PAA)/WO3route for microporous

coatings[9], the two-step molding process using porous anodic

alumina and poly(methyl methacrylate) (PMMA) templates[10]

and electrochemical deposition containing sodium dodecyl sulfate

surfactant template [11] From the tetraisopropyl titanate

pre-cursors system, Zhao et al [12]have presented that

nanostruc-tured titania with mesoporosity could be prepared by using

nonionic Pluronic P123 surfactant template At the same time, the

evaporation-induced self assembly (EISA) approach has also

offered new opportunities for the preparation of

nanoarchitec-tured metal oxides with a considerable high surface area from the modified sol–gel processing [13] A number of mesoporous transition metal oxides have been successfully prepared using self-assembled nonionic surfactants templates into chloro-alkoxide sol–gel precursors systems [14–16] Mesoporous vana-dium oxide has been synthesized from ethanolic solution of metal chlorides (VOCl3 or VCl4) and monomeric nonionic surfactant Brij56 or Brij58 templates[17] Some efforts about the study of WCl6 sol–gel associated with complexing agents were also investigated [18,19] Acetylacetone was used into WCl6 sol–gel

by Bechinger et al.[20]for the fabrication of microcontact-printed nanostructure In addition, Bell et al [21] reported that the introduction of epoxide additive could improve the chromogenic quality of coatings

The preparation method undertaken in the current work follows the concepts of controlled EISA [22], as schematically shown by the proposed mechanism ofFig 1 The gemini nonionic surfactant template used here mainly consisted of TMDD micella The major feature of TMDD (2,4,7,9-tetra-methyl-5-decyn-4,7-diol) was that it has two dendritic branched hydrophobic groups and two hydrophilic hydroxyl groups, which makes it highly efficient with lower critical micelle concentration (CMC) com-pared with monomeric surfactant[23] For essential concern of homogeneousness and transparency, the full hydrolysis-conden-sation was constrained by one controlled low relative humidity (RH), which was permitted to increase after volatile evaporations and cooperative self-assembly of the tungsten-oxo/TMDD meso-phase Finally, the hybrid was subjected to thermal treatment to 0927-0248/$ - see front matter & 2009 Elsevier B.V All rights reserved.



Corresponding author Tel.: +86 21 52412359; fax: +86 21 52414993.

E-mail addresses: yzzhang@mail.sic.ac.cn (Y Zhang) , jgyuan@live.cn (J Yuan)

remove the organic template and build pore-wall framework

mesostructuation

The scope of this paper was centred on the characterization of

microstructural and electrochromic properties of the

nanostruc-tured tungsten oxides with mesopores from the TMDD

surfactant-assisted process The crystallization status, surface morphology

and microstructure characteristics of the materials were

investi-gated with X-ray diffractions (XRD), scanning electron microscopy

(SEM) and transmission electron microscopy (TEM), respectively

The surface-binding states and stoichiometry of the films were

studied by X-ray photoelectron spectroscopy (XPS)

Moreover, the influence of nanostructure upon electrochromic

behaviours for tungsten oxide thin films was discussed The

principle that was found was that mesopores organization

has strong effects on the electrochemical and chromogenic

performance of tungsten oxide thin films Compared with the

non-templated sample, the nanostructured oxide with a higher

specific surface area has better electrochromic response and

dynamic optical quality On the other hand, more evolved

crystallization of the monoclinic phase would have impacts to

destroy this nanostructured pore-wall organization and induce

inferior properties

2 Experimental

2.1 Preparation of tungsten oxide thin films

Primarily, all chemical reagents are commercially available As

a consequence, the preparation process of mesoporous tungsten

oxides was reported as described elsewhere[24] Briefly, a certain

amount of TMDD C14H24EO20 (HO(EO)xC14(EO)yOH, Mw 1250,

AR, Aldrich) was dissolved in 10 ml of ethanol (C2H5OH, AR 99.5%,

SCRC Co.) to form the surfactant template solution Then 1 g of

anhydrous tungsten hexachloride (WC16, 99.9%, Aldrich) was

slowly added into this solution After stirring for further 2 h at

room temperature in air, the precursor was aged at 40 1C for about

24 h to prepare the coating solution Mesoporous tungsten oxides

were coated onto substrates in a chamber under a controlled low

relative humidity of about 30–50% at room temperature, followed

by drying at 120 1C in an oven for 1 h to evaporate the solvents

and to consolidate the network of the self-assembled hybrid

Hereafter, the as-dried films were exposed to the other controlled

high RH of 80–100% for about 1 min to force the extended

hydrolysis–condensation and obtain the as-synthesized samples

When electrochemical electrodes were fabricated, the tungsten

oxides thin films were dip-coated onto indium tin oxide (ITO)

coated conducting transparent glass with a sheet resistance of

10O/& Finally, the as-synthesized sample (named as sample B

with thickness about 240 nm) was subjected to a series of

temperatures in air at a ramp of 2 1C/min and calcined for 2 h,

where sample C was named for 300 1C with a thickness of about

200 nm and sample D for 400 1C with a thickness of about 180 nm

Additionally, besides the mesoporous samples, non-templated

tungsten oxide (named as sample A with thickness about 205 nm calcined at 300 1C) as analog has also been prepared under the same conditions as above, except for the service of the TMDD surfactant template

2.2 Characterization and measurement methods X-ray diffractions (XRD) measurement was carried out on a D/max 2550 V diffractometer with a Cu-Karadiation The surface morphology of tungsten oxides thin film was examined by a field-emission scanning electron microscope of a JEOL JSM-6700F The thickness measurements of the films studied in this work were performed on a Talystep profilometer (Rank Taylor Hobson) and subsequently refined by cross-sectional SEM images Transmis-sion electron microscopy image and selected area electron diffraction (ED) pattern were taken with JEOL JEM-2100 high-resolution electron microscope operated with an accelerating voltage of 200 kV The surface chemical composition and binding states were analyzed by X-ray photoelectron spectroscopy (XPS) using a VG Microlab 310F instrument with Mg-Ka radiation All regional XPS spectra were calibrated with the binding energy of C 1s peak (284.6 eV), and a Shirley-type background was subtracted Electrochromic behaviour of the sample was studied with simultaneous voltametric and spectrophotometric measurements Electrochemical instrument procedure was carried out in a standard three-electrode cell configuration as demonstrated previously [25] The cell consisted of a working electrode of tungsten oxide/ITO films samples, a Pt electrode used as the counter electrode and a saturated calomel electrode (SCE) reference electrode Cyclic voltammogram (CV) for the working electrode was performed in 0.1 N H2SO4 aqueous electrolyte solution The spectroelectrochemical property of the electrochro-mic electrode was studied by means of in-situ transmittance spectra, and the dynamic transmittance modulation was recorded

at a wavelength of 633 nm

3 Results and discussion 3.1 XRD

The XRD patterns of sample A, sample C and sample D deposited on ITO glass substrates are shown inFig 2 It could be seen that the (2 2 2), (4 0 0) and (4 4 0) peaks located at about 30.21, 35.11 and 50.51, respectively in all patterns belonged to the substrate of ITO glass From the illustrations ofFigs 2a and b, no obvious WO3crystallized phase was found for both sample A and sample C However, a new strong XRD peak at 24.21 appeared in the illustration ofFig 2c with the (2 0 0) reflection, accompanied with 34.01 (2 0 2), 44.31 (1 2 3) and 23.11 (0 0 2) in high resolution

Fig 2d According to JCPDS 43-1035, it is evident that sample D calcined at a high temperature of 400 1C was crystallized with monoclinic phase

Fig 1 Simplified schematic representation of the mechanism proposed for the formation of mesoporous tungsten oxides.

3.2 Surface morphology

Figs 3a, b and c show the surface scanning electron

micro-scopy (SEM) top-view images of sample A, sample C and sample D,

respectively It could be seen that all of the tungsten oxides

thin films are crack-free On one side, some shallow concaves

are interspersed in the surface of non-templated sample A from

Fig 3a, which would be a very common phenomenon for generic

sol–gel deposited films.Fig 3b of mesoporous sample C calcined

at 300 1C shows a very smooth surface morphology without any

detected remarkable features, which was very different from the

surface observation of sample A As expected, the nanograins of

WO3were ultra-fine, and the mesoporosity arised from the scale

of microstructural features such as zeolite-like level other than

textural features According toFig 3c of sample D calcined at a

high temperature of 400 1C, the former morphology of sample C

changed rimous and rough Such surface mophology of sample D

could be due to the agglomerated growth of grains and the

formation of good crystallinity Typical cross-sectional image of

3.3 Microstructure

In order to explore the inner microstructural characteristics of the nanostructured samples, high-resolution transmission elec-tron microscopy images were used.Figs 4a, b and c show the TEM images of nanostructured tungsten oxides of as-synthesized sample B and sample C calcined at 300 1C and sample D calcined

at 400 1 Significant tri-dimentional vermicular mesoporous structures were found inFigs 4a and b for sample B and sample

C The white spots and channels correspond to the interpenetrat-ing pore-wall channels A similar type of the mesoporous nanostructure has been found in some other transition metal oxides systems previously[26] Compared with sample B

of the uncalcined, the contrast under TEM for sample C calcined

at 300 1C was improved greatly It could be due to the removal of the TMDD surfactant template The average mesopore size for sample C is about 3–4 nm, which is in good agreement with the results of nitrogen adsorption–desorption isotherms studies According to the black representing tungsten oxide walls in

Fig 4b, the most distributed nanocrystallite skeletons size of mesoporous sample C is about 7 nm The inset ofFig 4b shows the electron diffraction (ED) pattern of the corresponding selected area, indicating semi-crystallized wall structures by several diffuse electron diffraction rings As described inFig 1, such type

of vermicular mesostructure originated from the cooperative self-assemblies of the inorganic tungsten oxo-oligomers and the organic TMDD surfactant through the process of controlled evaporation of solvents

However, the organized pore structure could not be well recognized in Fig 4c for sample D calcined at a higher temperature of 400 1C, and the wall crystalline was aggregated The growth of the grain size has led to the partial disruption of the mesopores and decreased the BET surface area to 33.8 m2/g and as about 0.24 times of sample C identified by nitrogen adsorption– desorption isotherms studies

3.4 X-ray photoelectron spectroscopy XPS was conducted to determine the surface oxidation states of the tungsten oxide thin film sample.Fig 5a shows survey scans spectra recorded within a range of 0 and 1000 eV at a takeoff angle

of 901.Fig 5b shows W 4f high-resolution XPS spectra, and the inserted was the O 1s level peak analysis of sample C It is confirmed that only W, O and C elements are found in all of the films and Cl element could not be found As an internal reference

to determine the binding energy, the C 1s peak of surface contamination was used From the analysis of high-resolution spectra, the W 4f core level spectrum exhibited doublet components associated with W 4f7/2 and W 4f5/2 spin–orbit split InTable 1, the resolved binding energies of sample A and sample C for W 4f7/2 and W 4f5/2 are consistent with the characteristic Eb of W 4f level in the oxidation state of +6

[27,28] The O 1s peak of sample C was situated at 530.4 eV above the W 4f7/2 core level line, which corresponds to the W ¼ O in the mesopore oxide wall A small tail toward lower energy could be attributed to the component of hydroxyl groups adsorbed on the sample surface [29] On the basis of the XPS Fig 2 XRD patterns for sample A (a), sample C (b) calcined at 300 1C, and sample

D (c,d) at 400 1C.

Fig 3 SEM images of sample A (a), nanostructured sample C (b) calcined at 300 1C and sample D calcined at 400 1C (c), cross-sectional SEM image of sample C (d).

measurements, the obtained quantitative stoichiometry of

oxygen to tungsten atomic ratio was summarized inTable 1 It is

revealed that sample A and sample C were very close to

the stoichiometric formulation of WO3, but the surface of

sample D seems oxygen deficient The oxygen vacancies of

sample D may be caused by higher temperature calcination,

whilst the surface hydroxyl oxygen escaped in excess and

resulted in partial reduction of the oxide Sample D could be

composed of tungsten oxides, in which the oxidation state

of tungsten ranges from +6 like in WO3 with a small amount

of WO

300 1C and sample D at 400 1C was measured during the electrochromic processes as shown in illustrations ofFig 6c Superior proton intercalation performance was noted for sample

C because of the more opened nanostructure On the other hand, the electrochemical kinetic property of sample D reduced greatly under the same measurement condition according to mesoporous sample C It is in expected reasonable agreement with the results monitored by specific surface area The transient current density deduction could be due to the mesopores loss induced by wall crystallization shrinkage The charge densities of sample A, sample

C and sample D were derived during a full 5th cycle The intercalation result of mesoporous sample C was about 2 times that

of the non-templated sample A.Table 2 exhibited the calculated deintercalation rate (u %), which was the ratio of the anodic deintercalation charge density to the cathodic intercalation charge density The enhancement of reversibility is observed for mesopor-ous sample C This result is in good accordance with the structural characteristics The original effect of the increase for the deinterca-lation rate of sample C to sample A was related to the mesopores framework interface of sample C The mesoporosity associated with more surface nanograin boundaries may be provided as a larger number of electrochemical active sites, which contributes to smooth proton diffusion

3.6 Spectroelectrochemical and monochromatic

Fig 6b shows the spectroelectrochemical spectra of mesopor-ous sample C calcined at 300 1C The colored state of the tungsten oxide thin film was achieved by reduction at 1 V vs SCE, and the bleached state was obtained by oxidization at +1 V vs SCE The response of color change to the potential on the working electrode was evaluated by the transmittance at the wavelength of 633 nm The results show that all of the samples have the range for optical modulation, and the final transmittance variations (DT) at the wavelength of 633 nm are reported inTable 2 The in-situ dynamic optical density variations of sample A, sample C, and sample D at monochromatic 633 nm during the CV are shown inFig 6d, while the optical density (OD) was defined as OD ¼ lg(1/T) In the cases

of sample A and sample D, the bleaching process was slower than coloring, but the result is just the contrary for mesoporous sample

C The analysis of the optical density variation indicates sample C had the sharpest slope during the proton deintercalation, where accelerated bleaching occurs from opaque state to transparent state By the way, the transmittance modulation as a function of time for monoclinic phase sample D was also radically different from those results of the sample A and sample C calcined at

300 1C The coloring of sample D was the slowest and the transmittance variation was gradually approaching a final range

of about 30% in the 5th cycle due to surface oxygen deficiency Electrochromic coloration efficiency (CE) could be defined as the change in the optical density divided by the intercalation charge density, which was CE ¼D(OD)/DQ The CE of mesoporous sample

C at the wavelength of 633 nm was 63.7 cm2/C, which is higher than the results of sample A and sample D It is interesting that this CE result at the wavelength of 633 nm of sample C was also slightly higher than that of the studies using lithium/PC electro-lyte solution[30]

Fig 5 Survey scan XPS spectra (a) and W 4f high-resolution XPS spectra (b) for

tungsten oxide films of sample A, sample C and sample D, and O 1s level peak

analysis of sample C (inserted in Fig 5b).

Table 1

XPS binding energy (E b ) and assessed O/W ratio of non-templated sample A,

nanostructured sample C calcined at 300 1C and sample D calcined at 400 1C.

Sample E b (W 4f 7/2 ) E b (W 4f 5/2 ) E b (O 1s) O/W ratio

3.7 Discussion

The electrochromic coloration occurred during proton M+/e

intercalation into tungsten oxide, and can be generally written as

a result of

WO3ðtransparencyÞþ

xMþ

þxe2MxWO3ðblueÞ

The studies of cyclic voltammograms and spectrophotometric

methods have shown the effects of mesoporous nanostructure on

the characteristics of electrochemical charge density, electro-chromic optical modulation and coloration efficiency, other than just nanosize in the continuity of tungsten oxide frameworks According to the previous study of electrochromic oxides [31], such type of nanostructure containing mesopores in tungsten oxides thin films may play some important roles to promote surface electrochemical reactions for electrochromics In other words, the mesoporous nanostructure generated more hierarch-ical nanograin boundaries in tungsten oxides thin film, which could facilitate surface chromogenic reactions

4 Conclusions The properties of tungsten oxides thin film prepared by the WCl6sol–gel method were strongly dependent on the condition of the precursor sol system Since the TMDD gemini surfactant was used as a structural-directed template, mesoporous tungsten oxide was obtained by the modified sol–gel route The micro-structural properties of the as-prepared tungsten oxides thin films

Fig 6 Typical CV in the range of 1 to +1 V vs SCE (a) and spectroelectrochemical spectra (b) of nanostructured sample C, transient current density (c) and corresponding monochromatic optical density variations at 633 nm (d) of non-templated sample A, nanostructured sample C calcined at 300 1C and sample D calcined at 400 1C.

Table 2

Deintercalation rate (u %), final transmittance variation (DT) and coloration

efficiency (CE) at 633 nm of non-templated sample A, nanostructured sample C

calcined at 300 1C and sample D calcined at 400 1C.

surfactant-assisted process method If both high specific surface

area and tuned crystallization could be taken into account in

future, the produced crystallized tungsten oxide with good

mesostructuation would vary within the design applications

Associated preparation applications for transition metal oxides

may be also expected in the study for advanced electrochromic

devices, in which the switching, modulation or coloration

efficiency may be substantially desired

Acknowledgement

This work was supported by Natural Science Foundation

of China for primary under NSFC Grant no 59932040-2 and

National Key Basic Research Development Plan (9 7 3) Project

(2009CB939900)

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