Structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process Yuzhi Zhang à , Jiaguo Yuan, Jun Le, Lixin Song, Xingfang Hu The Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences (CAS), 1295 Dingxi Road, Shanghai 200050, China article info 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 abstract 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 s tudies of X-ray diffractions, scanning electron microscopy and transmission electron microscopy. The mesopor- ous sample calcined at 300 1C has tri-dimensional vermicular mesop ores 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 WO 3 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)/WO 3 route 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 (VOCl 3 or VCl 4 ) and monomeric nonionic surfactant Brij56 or Brij58 templates [17]. Some efforts about the study of WCl 6 sol–gel associated with complexing agents were also investigated [18,19]. Acetylacetone was used into WCl 6 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 of Fig. 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 ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ -see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.02.016 à Corresponding author. Tel.: +86 2152412359; fax: +86 2152414993. E-mail addresses: yzzhang@mail.sic.ac.cn (Y. Zhang), jgyuan@live.cn (J. Yuan). Solar Energy Materials & Solar Cells 93 (2009) 1338–1344 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 C 14 H 24 EO 20 (HO(EO) x C 14 (EO) y OH, Mw $1250, AR, Aldrich) was dissolved in 10 ml of ethanol (C 2 H 5 OH, AR 99.5%, SCRC Co.) to form the surfactant template solution. Then 1 g of anhydrous tungsten hexachloride (WC1 6 , 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 80–10 0% 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 10 O /&. 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-K a radiation. The surface morphology of tungsten oxides thin film was examined by a field- emission scanning electron microscope of a JEOL JSM-670 0F. 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-K a 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 H 2 SO 4 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 in Fig. 2. It could be seen that the (2 2 2), (40 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 of Figs. 2a and b, no obvious WO 3 crystallized phase was found for both sample A and sample C. However, a new strong XRD peak at 24.21 appeared in the illustration of Fig. 2c with the (2 0 0) reflection, accompanied with 34.01 (2 0 2), 44.31 (12 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. ARTICLE IN PRESS Fig. 1. Simplified schematic representation of the mechanism proposed for the formation of mesoporous tungsten oxides. Y. Zhang et al. / Solar Energy Materials & Solar Cells 93 (2009) 1338–1344 1339 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 WO 3 were ultra-fine, and the mesoporosity arised from the scale of microstructural features such as zeolite-like level other than textural features. According to Fig. 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 the samples is shown in Fig. 3d, and the confirmed thickness of the processed films was 190 nm and close to the profilometer result. The interfaces of ITO to glass and WO 3 to ITO of the sample can be seen. However, it is not suitable for observing the internal microstructure here by the thin section due to the inferior discrimination, and so the microstructure property was still unknown by the SEM image. 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 in Figs. 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 of Fig. 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 in Fig. 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 m 2 /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 4f 7/2 and W 4f 5/2 spin–orbit split. In Table 1, the resolved binding energies of sample A and sample C for W 4f 7/2 and W 4f 5/2 are consistent with the characteristic E b 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 4f 7/2 core level line, which corresponds to the W ¼ Oin 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 ARTICLE IN PRESS Fig. 2. XRD patterns for sample A (a), sample C (b) calcined at 300 1C, and sample D (c,d) at 400 1C. Y. Zhang et al. / Solar Energy Materials & Solar Cells 93 (2009) 1338–13441340 ARTICLE IN PRESS 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). Fig. 4. Representative TEM images of nanostructured tungsten oxides of as-synthesized sample B (a), sample C calcined at 300 1C (b), and sample D calcined at 400 1C (c). Y. Zhang et al. / Solar Energy Materials & Solar Cells 93 (2009) 1338–1344 1341 measurements, the obtained quantitative stoichiometry of oxygen to tungsten atomic ratio was summarized in Table 1.Itis revealed that sample A and sample C were very close to the stoichiometric formulation of WO 3 , 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 WO 3 with a small amount of WO x . 3.5. Cyclic voltammogram Typical cyclic voltammogram (CV) of sample C is shown in Fig. 6a, which was obtained under trigonometric sweeping potentials recorded at a scan rate of 100 mV/s. Successive cycles are superimposed and almost a single curve was obtained. The corresponding transient current density (J, mA/cm 2 ) of the non- templated sample A, the nanostructured sample C calcined at 300 1C and sample D at 400 1C was measured during the electrochromic processes as shown in illustrations of Fig. 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 5 th 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 À1 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 ( D T) at the wavelength of 633 nm are reported in Table 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 in Fig. 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 ¼ D (OD)/ D Q. The CE of mesoporous sample C at the wavelength of 633 nm was 63.7 cm 2 /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]. ARTICLE IN PRESS 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 A 35.2 37.3 530.3 2.99 C 35.5 37.6 530.4 3.05 D 35.1 37.2 529.8 2.58 Y. Zhang et al. / Solar Energy Materials & Solar Cells 93 (2009) 1338–13441342 3.7. Discussion The electrochromic coloration occurred during proton M + /e À intercalation into tungsten oxide, and can be generally written as a result of WO 3 ðtransparencyÞ þ xM þ þ xe À 2M x WO 3 ðblueÞ. 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 WCl 6 sol–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 ARTICLE IN PRESS Fig. 6. Typical CV in the range of À1 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. Sample u (%) DT (%) CE (cm 2 /C) A 71.35 0.45 33.6 C 94.78 0.85 63.7 D 89.54 0.30 12.7 Y. Zhang et al. / Solar Energy Materials & Solar Cells 93 (2009) 1338–1344 1343 were characterized as the studies of transmission electron microscopy, which rendered the demonstrable proof that this surfactant-assisted process method has yielded the preferentially vermicular mesoporosity with tri-dimensional accessible pores. From the results of the cyclic voltammograms, the spectro- electrochemical and monochromatic studies presented above, it is identified that the electrochemical and the electrochromic properties of WO 3 can be tailored by fine microstructure with the 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. 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Zhang, J. Le, L.X. Song, X.F. Hu, Study on preparation and performance of electrochromic mesoporous tungsten trioxide films, Acta Chim. Sin. 63 (2005) 1884–1888. [31] X. Xiao, X. Hu, X. Chen, Interfacial structure of nano-granular thin films, Thin Solid Films 375 (2000) 151–154. ARTICLE IN PRESS Y. Zhang et al. / Solar Energy Materials & Solar Cells 93 (2009) 1338–13441344 . Structural and electrochromic properties of tungsten oxide prepared by surfactant-assisted process Yuzhi Zhang à , Jiaguo. 2009 Keywords: Nanostructured Electrochromic Tungsten oxide abstract By virtue of gemini surfactant template, nanostructured tungsten oxides thin films were prepared from the