Electrochromic properties of WO3 thin film onto gold nanoparticles modified indium tin oxide electrodes

Applied Surface Science 257 (2011) 5903–5907

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Electrochromic properties of WO3 thin film onto gold nanoparticles modified indium tin oxide electrodes Jiajia Deng, Ming Gu, Junwei Di ∗ College of Chemistry, Chemical Engineering and Material Science, Soochow University, Suzhou Industrial Park, Suzhou, Jiangsu 215123, PR China

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Article history: Received 13 December 2010 Received in revised form 30 January 2011 Accepted 30 January 2011 Available online 25 February 2011 Keywords: Tungsten oxide Electrochromic Gold nanoparticles Electrodeposition

a b s t r a c t Gold nanoparticles (GNPs) thin films, electrochemically deposited from hydrogen tetrachloroaurate onto transparent indium tin oxide (ITO) thin film coated glass, have different color prepared by variation of the deposition condition. The color of GNP film can vary from pale red to blue due to different particle size and their interaction. The characteristic of GNPs modified ITO electrodes was studied by UV–vis spectroscopy, scanning electron microscope (SEM) images and cyclic voltammetry. WO3 thin films were fabricated by sol–gel method onto the surface of GNPs modified electrode to form the WO3 /GNPs composite films. The electrochromic properties of WO3 /GNPs composite modified ITO electrode were investigated by UV–vis spectroscopy and cyclic voltammetry. It was found that the electrochromic performance of WO3 /GNPs composite films was improved in comparison with a single component system of WO3 . © 2011 Elsevier B.V. All rights reserved.

1. Introduction Electrochromism is defined as reversible changes of the optical properties of materials under applied voltages. Electrochromic material could be used for smart windows, displays and antiglare mirrors [1–3]. Among the numerous thin films of transition metal oxides, tungsten oxide (WO3 ) has been extensively studied because it has fast response time and high coloration efficiency. Therefore, it is of great interest to improve the electrochromic performance of WO3 for practical applications. Recently, WO3 composite materials containing noble metal nanoparticles have attracted great attention due to their unique chemical and physical properties, which result in the enhancement of the efficiency as well as the lifetime of the electrochromism of transition-metal oxides [4–11]. For example, Pt–WO3 [4] and Au–WO3 [4–7] nanostructrure thin films obtain a rapid response time compared with pure WO3 thin film. On the other hand, Pang et al. [7,8] reported that cathodic electrodeposition WO3 thin film onto the Ag nanoparticles modified ITO electrode improved the electrochromic properties relative to pure WO3 thin film. He et al. [9] reported that the electrochromic performance of the WO3 thin film was also improved greatly by surface modification with gold nanoparticles (GNPs). Among all kinds of nanoparticles, GNPs have been extensively used in modification of various electrodes due to their excellent inertness, stability and conductivity. The spectrum

∗ Corresponding author. Fax: +86 512 65880089. E-mail address: [email protected] (J. Di). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.137

of GNPs displays the characteristic peak of the local surface plasmon resonance (LSPR) at 520–550 nm, which appear red color. However, the blue coating may be more suitable for architectural application than that the red coating obtained with colloidal GNPs [12]. By comparison with Ag nanoparticles, Au nanoparticles are more chemical stable and less intense LSPR bands. It is also well known that the surface plasmon of GNPs is strongly dependent on the size, shape, aggregation, and local environment of the nanostructures [13,14]. Therefore, we can turn the surface plasmon band of GNPs by controlling the nanostructures. Some advantages of electrochemical methods over other GNPs synthesis approaches are the high purity of products and easy control of size, shape, and morphology of the nanostructured materials [15–17]. In this paper, GNP thin films with different colors were fabricated by electrochemical deposition method. GNPs/WO3 composite films were prepared by sol–gel method onto the surface of GNPs modified indium tin oxide (ITO) coated electrodes. The electrochromic properties of the resulting composite films were investigated and compared with a single component system of WO3 film.

2. Experimental 2.1. Materials and apparatus Hydrogen tetrachloroaurate (HAuCl4 ) was products of Guoyao Chemical Regent Co., Ltd., Na2 WO4 ·2H2 O was obtained from Shanghai Chemical Regent Plant. A 0.1% polyvinyl alcohol (PVA) solution was prepared by dissolving PVA (PVA-124, average degree of polymerization was 2400–2500, Shanghai Chemical Reagent Plant

J. Deng et al. / Applied Surface Science 257 (2011) 5903–5907

imported from Japan) in water under heating to near boiling. ITO glass (1.1 mm thickness, 80–100 ) was purchased from Suzhou NSG Electronics Co., Ltd. (Suzhou, PR China). The buffer solution (pH 8.0) was prepared by Na2 HPO4 and KH2 PO4 . All electrochemical experiments were performed on CHI830B electrochemical workstation (Shanghai Chenhua Apparatus Corporation, China) with a conventional three-electrode system. A UV–visible spectrophotometer (U-2810, Hitachi) was used to investigate the optical properties of the films with 1 nm resolution. The measurements were against a bare ITO glass as a reference. The morphology of gold nanoparticle films was characterized by an S-570 scanning electron microanalyzer (Hitachi, Japan) at an acceleration voltage of 15 kV. X-ray diffraction (XRD) analysis was measured by X’PertPro MPD (Holand). All chemicals were of analytical grade. Twice-distilled water was used throughout all experiments.

0.10

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Absorbrance

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0.04

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2.2. Electrodeposition of gold nanoparticles on ITO substrate

2.3. Preparation of precursor sol and WO3 films The preparation of precursor sol and WO3 films were described previously [18]. A 1.0 mL of 1 mol L−1 Na2 WO4 flowed though a cation exchange column. The effluent solution was added 2 mL of ethanol and 0.2 mL of 0.1% PVA and then diluted to total volume of 10 mL. Within 45–90 min, the clean bare and GNPs modified ITO electrodes were dipped in the mixed solution about 5 min and stored at room temperature for about 12 h. Finally, the as-deposited films were annealed at 100 ◦ C in air for 2 h. 2.4. Electrochromic properties The electrochromic effects were demonstrate by a cyclic voltammogram in an electrochemical workstation. The WO3 composition films on ITO coated glass were used as the working electrode in the electrochromic cell. A saturated calomel electrode (SCE) was used as reference electrode and a Pt wire as counter electrode. The reversibility and stability of the electrochromic process was assured at each applied potential by coloring and bleaching the film several times before recording the electrochemical and optical measurement reported here. 3. Results and discussion 3.1. Characterization of the GNP films electrodeposited on ITO substrate The deposition of GNPs on the ITO substrate surface can be observed by changing of the color and characterized by UV–vis spectroscopy. Fig. 1 shows UV–vis spectra of the GNPs dispersion. The gold thin film displayed red-violet color at the electrodeposition of 20 cycles and the surface plasmon absorption band peak was located at about 550 nm (curve a in Fig. 1). With an increase in electrodeposition to 50 cycles, the color of gold thin film changed to blue and a very broad surface plasmon absorption band at about

700

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Fig. 1. Absorption spectra of red GNP (a) and blue GNP (b) thin films electrodeposited onto the ITO substrate.

640 nm (curve b in Fig. 1). The red-shift and broad peak of the surface plasmon band is due to the connection of nanoparticles on the substrate surface or decreased interparticle distance as more and larger particles are packed into the surface bring the particles in close proximity [19,20]. By contrast, it is difficult to obtain high coverage and largely red-shift SPR band of GNPs immobilized on ITO surface by self-assembly method because of the repulsive force between surface-confined nanoparticles and free nanoparticles in solution [21,22]. This suggests that it is easy to turn the color of gold thin films by controlling the condition of the electrochemical deposition. In order to get surface information about the electrodeposited Au nanoparticles on ITO film coated glass electrodes, cyclic voltammograms are usually recorded in 0.05 mol/L H2 SO4 solution [23]. The peak potentials for Au or its oxide redox are almost not change at low scan rates in this acid solution. The cyclic voltammograms of GNPs prepared under different deposition cycles exhibit the characteristic oxidation and subsequent reduction peaks of gold (Fig. 2). The peak current of the GNPs are also observed to increase with electrodeposition cycle. The nanostructure of the gold film deposited on the ITO electrode surface was further investigated by the scanning electron microscopy (SEM) as shown in Fig. 3. As can be seen, the deposition of gold nanoparticles on both ITO electrode surfaces is confirmed.

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The ITO glass (1 cm × 5 cm) was washed with NH3 –H2 O (1:20), ethanol, and distilled water for 10 min consequently in an ultrasonic bath. Then, the ITO electrode was immersed into the solution of 0.16 mmol L−1 KAu(CN)2 in phosphate buffer solution (pH 8.0). The mixed solution was kept in water bath at 50 ± 2 ◦ C and deaerated with N2 for about 10 min. The electrodeposition was performed by cyclic voltammetric mode in the potential range of −0.3 to −1.2 V at scan rate 50 mV s−1 for 20 and 50 cycles, respectively. The obtained GNPs modified ITO electrodes display red violet and blue, respectively. Therefore, we defined red violet of GNPs as red GNPs and blue color of GNPs as blue GNPs.

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E/V vs SCE Fig. 2. Cyclic voltammograms of red GNPs (a) and blue GNPs (b) modified ITO electrodes in N2 -saturated 0.05 mol/L H2 SO4 solution. Scan rate: 50 mV/s.

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Intensity

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(200) (220) (311)

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30

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2 q (degree) Fig. 4. XRD pattern of blue GNPs deposited on ITO surface.

peak was shifted positively. The peak potential separation of the redox waves were reduced to 0.3 and 0.19 V for red GNPs and blue GNPs modified ITO electrode, respectively. It is indicated that the reversibility of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− redox is effectively improved by the deposited GNPs on ITO substrate. This is consistent with the report that the charge transfer resistance of the ITO electrode is substantially reduced after the immobilization of the GNPs on its surface [25]. 3.2. Electrochromic properties of WO3 /GNPs composite films

However, the morphological structures of red GNPs and blue GNPs on the ITO substrate are obviously different. On red GNPs modified ITO electrode surface, the nanoparticles were mostly isolated spherical or spheroidal in an average size of about 40 nm formed. In contrast, on blue GNPs modified ITO electrode surface, most Au nanospheres were connected in an average diameter of about 60 nm. The images shows that with an increase in electrodeposition cycles the larger sized particles and much more aggregation are formed, which is consistent with the UV–vis spectrometric data. The crystalline structure of the GNPs was characterized by XRD measurement. The XRD pattern of the electrodeposited GNPs is shown in Fig. 4. Four peaks can be observed in the XRD pattern at 38.1◦ , 44.3◦ , 64.5◦ and 77.4◦ , which are assigned to the (1 1 1) (2 0 0) (2 2 0) and (3 1 1) reflections, respectively. The results suggested that the surface of nanoparticles is primarily composed of (1 1 1) facets. The redox behavior of a reversible couple, Fe(CN)6 4− /Fe(CN)6 3− , in 0.1 mol/L KCl is useful for probing the electrochemical properties of an electrode [24]. Since the thin film of indium tin oxide is semiconductor, the scan rate may markedly influence the difference of the peak potential in cyclic voltammogram. 50 mV/s scan rate is generally selected for evaluation of the bilayers on the GNPs/ITO modified electrode. As shown in Fig. 5 are the cyclic voltammograms of various GNPs/ITO modified electrode with a bare ITO electrode in 5 mmol/L K3 Fe(CN)6 /K4 Fe(CN)6 aqueous solution containing 0.1 mol/L KCl at a scan rate of 50 mV/s. The anodic and cathodic peaks of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− couple on the bare ITO electrode appeared with the peak potential separation of 0.41 V. At the Au nanoparticles modified ITO electrodes, the anodic peak of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− was shifted negatively while the cathodic

The WO3 /GNPs composite films were prepared by sol–gel method on the red GNPs and blue GNPs modified ITO electrodes. Fig. 6 shows the typical SEM image of WO3 film deposited on red GNPs modified ITO electrode surface. The SEM micrograph shows well adherent, smooth and uniform film surface without cracks and pinholes. It is also indicated that the WO3 film was completely covered over the Au nanoparticles modified electrode surface. During the repeated redox cycles, all the samples showed reversible color changes between intense blue at the cathodic limit and bleached state at the anodic limit. Fig. 7 shows the transmittance spectra of the films at the bleached and colored state, 0.8

c

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Fig. 3. SEM images of red GNP (a) and blue GNPs (b) films on ITO substrate.

0.0 -0.2 -0.4 -0.6 -0.8 -0.4

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E/V (vs SCE) Fig. 5. Cyclic voltammograms of 5 mmol/L K3 Fe(CN)6 in 0.1 mol/L KCl at bare ITO electrode (a), and red GNPs (b) and blue GNPs (c) modified ITO electrodes. Scan rate: 50 mV/s.

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J. Deng et al. / Applied Surface Science 257 (2011) 5903–5907

3

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E/ V (vs SCE) Fig. 6. SEM image of WO3 film on the red GNPs modified ITO substrate.

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respectively, in the region between 400 and 800 nm. It can be seen that all WO3 and WO3 /GNPs thin films appear deep blue after they have been polarized cathodically for a short period of time. After the anodic polarization, all samples for bleached state are found to be highly transparent (∼80%) in the entire visible range. The electrochromic properties of the WO3 /GNPs composite films were also investigated by a cyclic voltammetry. The cyclic voltammograms were performed on these films after a few initial stabilization cycles, which avoided the interference of the surface tension between solid and liquid in the pores, and the limitation of the ion mobility. Voltages between −1.0 and +1.0 V relative to SCE were applied with a scan rate of 0.1 V/s because the color change rate of electrochromic devices is an important factor. For the evaluation of electrochromic characteristic, there are two main kinds of supported solution: LiClO4 + propylene carbonate (PC) solution or HCl aqueous solution [9]. The cyclic votammograms of the WO3 and WO3 /GNPs thin films electrode in HCl solution is shown in Fig. 8 for convenient. The WO3 was optically colored when reduced at negative potential, while it was bleached when oxidized at positive potential. The cathodic current, starting at approximately +0.1 V, is associated with the coloring process for all electrodes. It is interesting that the anodic peak was shifted negatively from about 0.5 V at pure WO3 electrode to 0.2 V at WO3 /GNPs/ITO electrodes. Moreover, after the initial stabilization, we find that the successive cycles

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Fig. 8. Cyclic voltammograms of WO3 (A), WO3 /red GNPs (B) and WO3 /blue GNPs (C) modified ITO electrodes in 0.01 mol L−1 HCl solution. Scan rate is 0.1 V s−1 .

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are almost superimposed from many bleached/colored cycles. This suggests that the films have promising durability characteristics essential for tandem cell operation. In general, the accepted electrochromic reaction of WO3 films can be described by the following equations [3,26]:

colored

20

WO3 + xM+ + xe ↔ Mx WO3

0 400

500

600

700

(1)

800

Wavelength/nm Fig. 7. Typical transmittance spectra of the WO3 (a), WO3 /red GNP (b) and WO3 /blue GNP (c) thin films monitored at bleached and colored states, respectively.

VI

V

VI

V

W (A) + W (B) ↔ W (B) + W (A)

(2)

where M+ is H+ or Li+ ions. When a negative bias is applied, electron (e− ) from the electrode and cations (H+ ) from the electrolyte

J. Deng et al. / Applied Surface Science 257 (2011) 5903–5907

are injected into WO3 film simultaneously. A tungsten bronze is thus formed. The blue color is caused by the optical intervalence change transfer. With the reverse (bleaching) process, electrons and cations exit from the WO3 film. It has been known that the electrical conductivity of the substrate can influence the electrochromic properties of WO3 films [27,28]. Therefore, the electrochromic performance of the WO3 thin film is improved due to the decrease of charge transfer resistance of the ITO electrode modified by Au nanoparticles. 4. Conclusion In summary, WO3 thin films have been prepared on the Au nanoparticles modified electrodes using sol–gel method. The Au nanoparticles were electrodeposited directly onto the transparent ITO electrode surface. Coatings comprised of isolated Au nanoparticles showed the plasmon resonance band at about 550 nm on the ITO substrate and were pale red-violet. Furthermore, the absorption peak was broad and red-shifted to approximately 640 nm due to particles–particle interaction, which could be developed by increasing the deposition cycles. It was also found that the electrochromic performance of the WO3 thin films would be improved by ITO electrode surface modification with Au nanoparticles due to increase of the charge transfer ability. It suggests that the WO3 /GNPs/ITO electrodes are potential materials for fabrication of electrochromic devices. Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 20675055, 21075086) and pre-research project of Soochow University.

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