Electrochimica Acta 50 (2005) 4690–4693
Electrochromic properties of Au–WO3 nanocomposite thin-film electrode Kyung-Won Park ∗ Department of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, PA 16802, USA Received 4 November 2004; received in revised form 26 February 2005; accepted 1 March 2005 Available online 31 March 2005
Abstract We observed proton transfer phenomenon of WO3 in Au–WO3 nanocomposite thin-film electrode prepared by sputtering deposition method. The Au–WO3 nanocomposite electrode formed using both the Au and WO3 targets consisted of a nano-sized Au crystalline phases and a tungsten oxidative phase, indicating the formation of crystalline Au nanophases, as confirmed by X-ray diffraction analysis and X-ray photoelectron spectroscopy. In particular, due to Au metallic nanophases, the modified electrochromic and electrochemical properties of WO3 were observed. The Au–WO3 electrode showed a reverse optical modulation with respect to applied potential compared to that of WO3 electrode. © 2005 Elsevier Ltd. All rights reserved. Keywords: Proton transfer; Electrochromism; WO3 ; Metallic nanophases; Nanocomposite
1. Introduction Most transition metal oxide structures can be electrochemically switched to redox state that has an intense electronic absorption band. In particular, an electrochromism of transition metal oxide is defined as a reversible change of the optical properties of materials under an applied electric field [1–3]. The electrochromic behaviour can be described in terms of double injection or extraction of electrons and ions. In other words, during an electrochromic process, electrons are injected or extracted depending on an applied voltage and, at the same time, ions are moved uniformly into or out of the electrochromic materials to balance charge neutrality. Since an electrochromic (EC) reaction involves electron conduction and ion diffusion similar to typical electrochemical process, the electronic conductivity and ionic diffusivity in electrochromic materials are clearly critical factors. In general, WO3 has been extensively studied due to a fast response time and high coloration efficiency compared to other electrochromic materials [4–7]. ∗
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Recently, since metallic nanophases show size-dependent effects such as decreased melting point, optical absorbance spectra, and high surface-to-volume ratio, they are of particular interest for many applications including catalysis, photochemistry, and chemical–biological sensors [8,9]. In particular, an EC reaction for electrochromic electrode with metallic nanophases, i.e. nanocomposite electrode, can be described as following equations : WO3 + nH+ + ne− ↔ Hn WO3
M(metal) + nH+ + ne− ↔ M–Hn
However, the proton transfer phenomena competitively occur in the nanocomposite electrode, which can be mutually affected under electrochemical potentials. In other words, this means that conducting nanophases in the electrode could affect or modify both electrochromic and electrochemical properties of WO3 . In this paper, we reported proton transfer phenomenon of WO3 in the Au–WO3 nanocomposite electrode prepared by sputtering deposition method. In particular, due to Au metallic nanophases in the electrode, the modified electrochromic and electrochemical properties of WO3 were observed.
K.-W. Park / Electrochimica Acta 50 (2005) 4690–4693
2. Experimental WO3 and Au–WO3 electrodes were grown using an RF magnetron sputtering system, which WO3 and Au were used as the target materials [11,12]. Indium tin oxides coated transparent glasses were used as the substrate. Cu grids were also used as substrates for analysis by transmission electron microscopy (TEM). The sputtering was performed under an atmosphere of inert Ar gas at 40 sccm at room temperature (RT). The Au–WO3 electrode was deposited at RF powers of 20 and 180 W on the Au and WO3 target, respectively, and was compared with WO3 sputtered at an RF power of 180 W. All the same thickness of thin-film electrodes was adjusted. Electrochromic properties such as optical intensity modulation with response time were evaluated by switching a pulse potential wave between +0.7 and −0.1 V versus normal hydrogen electrode with a duration time of 30 s. The change of optical intensity was in situ measured during potential cycling using 633 nm He–Ne laser [13,14]. The nanocomposite electrodes were prepared using an RF magnetron sputtering system with two guns consisting of gold metal and tungsten oxide target. Sputtering was carried out under an atmosphere of inert Ar gas at room temperature. The transmission electron microscopy (TEM) investigation was carried out using a Phillips CM20T/STEM Electron Microscope at an accelerating voltage of 200 kV. XRD analyses of as-prepared electrode were used to analyze the nanocomposite structure. In order to analyze chemical states of the samples, XPS was carried out using a VG Scientific photoelectron spectrometer. The X-ray source was Al K␣ with 1486.6 eV operating at 150 kV and 150 W. The base pressure of the system was 2 × 10−9 Torr. To evaluate the electrochemical properties of the nanocomposite electrode, cyclic voltammetry (CV) was examined using an electrochemical system consisting of sputtered thin-film electrodes, a Pt gauze, and Ag/AgCl as the working, counter, and reference electrode, respectively, at 25 ◦ C. All potentials are reported with respect to normal hydrogen electrode (NHE).
3. Results and discussion Fig. 1 shows optical intensity modulation curves for the WO3 and Au–WO3 electrodes as a function of pulse potential. The WO3 as a representative cathodic coloration material is optically colored with low transmission power when reduced at negative potential, while WO3 is bleached with high optical intensity when oxidized at positive potential. To our surprise, however, the Au–WO3 electrode exhibits reverse optical modulation compared to that of WO3 electrode. For example the Au–WO3 electrode becomes colored at negative reduction potential and bleached at positive oxidation potential. This indicates that the addition of Au nanophase cause the modification of electrochromic prop-
Fig. 1. Optical intensity modulation curves of the WO3 and Au–WO3 electrodes as a function of pulse potential in 0.5 M H2 SO4 .
erty of the WO3 electrode. In addition, the Au–WO3 shows fast response time to reach maximum or minimum intensity during transition of potentials. In the case of the Au–WO3 , the response time is ∼5 s while a response time of the WO3 is over 20 s. Fig. 2 shows TEM and transmission diffraction (TED) images of WO3 and Au–WO3 electrodes fabricated by the sputtering system. The WO3 electrode is typical amorphous phase formed when only a WO3 target is used as shown in Fig. 2(a). In contrast, as shown in Fig. 2(b), the Au–WO3 nanocomposite electrode formed using both the Au and WO3 targets consists of a Au crystalline phases of ∼4 nm in size (HRTEM image of the Fig. 2(b) inset) and a tungsten oxidative phase. Moreover, Au nanophases seem to be relatively well dispersed in the oxide matrix despite a little agglomeration of nanoparticles. The XRD diffraction pattern in the inset of the Fig. 2(b) confirms the formation of crystalline Au (2θ = 38.1◦ and 43.8◦ ) in contrast to the amorphous oxide having no XRD patterns. In addition, XPS spectra of Fig. 3 show chemical states of tungsten and gold in the Au–WO3 . The W 4f XPS peaks at the binding energy of ∼35.7 and ∼37.2 eV corresponds to W 4f7/2 and 4f5/2 of WO3 , respectively. In the Au 4f spectra, the Au 4f7/2 and 4f5/2 lines appear at ∼84.1 and ∼87.7 eV, respectively, with the theoretical ratio of peak areas of 4–3. The comparison of the binding energies shows that Au is present in the zero-valent metallic state, that is, the oxidative peaks were not found. From the chemical composition analysis of XPS and EDX data, the atomic ratios of W to Au in the WO3 electrodes incorporated by Au nanophases is (40:60). The possible origins of the Au metallic nanophases in an amorphous WO3 may be as follows: (1) thermodynamically stable phase separation between Au and WO3 , (2) prevention of migration of deposited metal adatoms by oxide matrix, and (3) excellent unreactivity of Au during sputtering at room temperature. Generally, the spectrum of Au nanoparticles display the characteristic peak of the surface plasmon resonance (SPR) at 520–540 nm [15–17]. As shown in Fig. 4 of optical absorp-
K.-W. Park / Electrochimica Acta 50 (2005) 4690–4693
Fig. 3. XPS spectra of (a) W 4f and (b) Au 4f in Au–WO3 electrode.
Fig. 5(a) shows the typical electrochemical cyclic voltammogram (CV) of the WO3 electrode in 0.5 M H2 SO4 solution. However, as shown in the Fig. 5(b), the CV of the Au–WO3 electrode exhibits Au-related electrochemical reactions. The Fig. 2. TEM and transmission diffraction (TED) images of (a) WO3 and (b) Au–WO3 electrodes fabricated by the sputtering system. The insets of right top and bottom show high-resolution TEM image of crystalline Au phase and X-ray diffraction pattern of the Au–WO3 electrode, respectively.
tion spectra of WO3 and Au–WO3 electrode, the Au–WO3 electrode shows a broad peak at 583 nm with the full width at half maximum of ∼253 nm. It is likely that such a shift of SPR could be caused by an agglomeration of Au nanostructures in the Au–WO3 nanocomposite electrode [18,19]. In addition, compared to optical absorption edge of pure WO3 , the edge of the Au–WO3 was shifted to longer wavelength, which a new energy level is produced in the band gap of WO3 by metallic conducting nanophases. Accordingly, the SPR and optical absorption edge shift of the nanocomposite electrode are clear evidences of existence of gold nanophases in tungsten oxide.
Fig. 4. Optical absorption spectra of WO3 and Au–WO3 electrode.
K.-W. Park / Electrochimica Acta 50 (2005) 4690–4693
4. Conclusions We reported proton transfer phenomenon of WO3 in Au–WO3 nanocomposite electrode consisting of a nanosized Au crystalline phases and a tungsten oxidative phase. The Au–WO3 electrode showed optical modulation reverse to that of WO3 electrode with respect to applied potential. We found that the modified electrochromic and electrochemical properties of WO3 were observed because of conducting nanophases dispersed in the WO3 .
Fig. 5. Cyclic voltammograms (CVs) of (a) WO3 and (b) Au–WO3 electrode in 0.5 M H2 SO4 solution.
CV of the Au–WO3 electrode displays crystalline Au-based electrochemical characteristics. It is likely that the hydrogen transfer phenomenon in the nanocomposite electrode could be modified by the Au nanophases. However, to fully understand reverse optical properties of Au–WO3 , electrochromic properties of tungsten oxide including Au nanophases should be considered as function of amount of Au-doping or its electronic structure.
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