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Enhanced electrochromic performance of macroporous WO3 films formed by anodic oxidation of DC-sputtered tungsten layers
Electrochimica Acta 55 (2010) 6953–6958
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced electrochromic performance of macroporous WO3 films formed by anodic oxidation of DC-sputtered tungsten layers J. Zhang, X.L. Wang ∗ , X.H. Xia, C.D. Gu, Z.J. Zhao, J.P. Tu ∗ State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 20 April 2010 Received in revised form 26 June 2010 Accepted 29 June 2010 Available online 19 July 2010 Keywords: Tungsten oxide Thin films Anodic oxidation Macroporous structure Electrochromism
a b s t r a c t Self-organized macroporous tungsten trioxide (WO3 ) films are obtained by anodic oxidation of DCsputtered tungsten (W) layers on 10 mm × 25 mm indium tin oxide (ITO)-coated glass. Under optimized experimental conditions, uniformly macroporous WO3 films with a thickness of ca. 350 nm are formed. The film shows a connected network with average pore size of 100 nm and a pore wall thickness of approximately 30 nm. The anodized film becomes transparent after annealing without significant change in macroporous structure. In 0.1 M H2 SO4 , the macroporous WO3 films show enhanced electrochromic properties with a coloration efficiency of 58 cm2 C−1 . Large modulation of transmittance (∼50% at 632.8 nm) and a switching speed of about 8 s are also achieved with this macroporous film. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten trioxide (WO3 ) receives extensive attention because of its potential applications in electrochromic (EC) devices [1–3], gas sensors [4,5], photocatalysis [6–8] and batteries [9,10]. Electrochromic devices are based on the change of optical properties when a voltage is applied [1]. There are many transition metal oxides that show EC properties, such as the oxides of tungsten, nickel, iridium, vanadium, titanium, cobalt and molybdenum [11–17]. The initiation of EC is attributed to the intercalation of ions (H+ , Li+ , Na+ ) to the films [1], thus the process is believed to occur first at the film surface [18]. Therefore, to achieve fast and sufficient intercalation of ions, EC films with high surface ratios, such as porous films are required. Conventional approaches for synthesizing porous WO3 films are based on templating methods that use structure-directing agents, such as polyethylene glycol (PEG 300, 600, etc.) [19,20] and polystyrene (PS) microspheres [21], to create pores and channels within the film. Anodization provides a template-free method for forming porous oxide layers on metallic surfaces. Anodization has been widely used to synthesize porous alumina, namely, anodic aluminum oxide (AAO) [22–26], which can be used as a template to form nanostructures of various materials. Beyond AAO, many other metals and alloys have been found to form macroporous or nanotube structures by anodic oxidation, including titanium
[27–31], magnesium [32,33], niobium [34], and tantalum [35]. Anodic macroporous WO3 structures have been reported for tungsten and tungsten-containing alloy foils [8,36–40]. Dipaola et al. [40] found that two different EC mechanisms occur on anodically formed tungsten oxide films – namely, coloration occurs due to double injection of electrons and H+ as well as due to the formation of the blue oxide hydrate (WO3−x ·H2 O). Schmuki and co-workers [37] reported that TiO2 –WO3 composite nanotubes formed by anodic oxidation of Ti/W alloys (Ti 0.2–9%) showed enhanced EC properties. However, metal foils are not suitable for fabrication of EC devices, which generally need a transparent conductive substrate such as indium tin oxide (ITO)-coated glass. As a result, transparent films with nanostructures, for example, AAO, anodic TiO2 nanotubes and macroporous films [27,28,41–43] have been developed. However, few reports exist on the anodization of transparent WO3 films with macroporous structures on ITOcoated glass [44]. To the best of our knowledge, the EC properties of transparent WO3 macroporous film have not been reported in the literature. In this work, metallic W layers were deposited on ITOglass substrates through a well-controlled DC-sputtering process. Macroporous WO3 films were then prepared by anodic oxidation of the W layers. The EC properties of the macroporous WO3 films were investigated. Optical transmittance modulation ranges (T) and coloration efficiencies (CE) of anodized macroporous WO3 films were obtained for the first time. 2. Experimental
∗ Corresponding authors. Tel.: +86 571 87952856; fax: +86 571 87952856. E-mail addresses: [email protected] (X.L. Wang), [email protected] (J.P. Tu). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.082
The ITO-coated glass slides measuring 10 mm × 25 mm were used as substrates. The W layers were deposited by means of a
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J. Zhang et al. / Electrochimica Acta 55 (2010) 6953–6958
DC magnetron sputterer using a metallic W (99.99%) target in an Ar atmosphere at a pressure as high as 6.67 Pa (base pressure 5.0 × 10−3 Pa) with a deposition speed of 0.22 nm s−1 at a power of 30 W and a substrate rotation speed of 5 rpm. The distance between substrate and target was 8 cm. During the deposition process, a bias of −100 V and a temperature of 400 ◦ C were applied to the substrate. All parameters were specially selected to deposit highly compact W layers that were strongly adhered to the substrate. In this work, a deposition time of 60 min was used to obtain a W layer 0.8 m thick. Anodization treatment of the sputtered W layer was performed using an electrochemical cell with a conventional anode–cathode configuration, where the working electrode was W/ITO-glass and the counter electrode was a platinum foil. The W/ITO-glass electrodes were anodized in a 0.2 wt% NaF solution from the opencircuit potential (OCP) to 60 V with a sweep rate of 1000 mV s−1 . The electrodes were then held at 60 V for various durations (i.e., 10, 20, 30, 40, and 60 min). After anodization, the samples were washed with deionized water and dried in a vacuum oven. Transparent WO3 films were obtained by annealing the as-anodized samples at 400 ◦ C for 4 h in air. For comparison, a compact WO3 film was deposited on an ITOglass substrate by means of a DC magnetron sputtering using a metallic W (99.99%) target in an Ar (80%) + O2 (20%) atmosphere at a pressure of 0.6 Pa (base pressure 5.0 × 10−3 Pa). To obtain a crystalline WO3 film, the substrate was heated at 200 ◦ C during the deposition process. The sputtering power was 120 W and the distance between substrate and target was 8 cm. The morphology and microstructure of the anodized and sputtered films were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4700) and X-ray diffraction (XRD, PANalytical X’Pert PRO diffractometer and Cu K␣ radiation, = 1.54056 Å). Electrochemical and electrochromic properties were assessed in an aqueous solution of 0.1 M H2 SO4 with a 1 cm × 1 cm Pt foil as the counter electrode and a Ag/AgCl electrode as the reference electrode using a CHI660c electrochemical workshop. Optical transmittance was measured with a SHIMADZU UV-240 spectrophotometer. 3. Results and discussion During anodization, the evolution of surface morphology and cross-sections were recorded by ex situ SEM. The sputtered W layer exhibits a compact structure and good adhesion to the ITO-coated glass (Fig. 1a). The surface morphologies of the samples anodized for different times are shown in Fig. 1b–d. For the 10 min-anodized film, pore formation can be observed only in specific areas. With the continuous anodization, a highly organized macroporous structure is formed. The film that was anodized for 40 min shows a connected network with an average pore size of 100 nm and a pore wall thickness of approximately 30 nm. After 60 min of anodization, the layer is partially etched and the network is destroyed. These changes in the surface morphology were also observed in the cross-section view with SEM (Fig. 2). It can be seen that as the anodization time is increased, the unoxidized metallic W layers become thinner in the cross-section view. A comparison of XRD patterns of sputtered W layers on an ITO substrate and 40 min-anodized films both before and after annealing are shown in Fig. 3a. The broadened peaks of -W (JCPDS Cards 47-1319) are observed for the metallic W layer. The patterns of as-anodized films contained only peaks corresponding to the ITO substrate. Furthermore, the grazing incidence XRD pattern (data not shown) did not show any characteristic peaks except an amorphous shoulder indicating that the as-anodized films are amorphous (pattern b). Fig. 4a shows an image of a sputtered W film on an ITO substrate. As shown in Fig. 4b, most of the W layer is
Fig. 1. SEM images of (a) sputtered W layers and films anodized for (b) 10 min; (c) 40 min; and (d) 60 min.
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Fig. 3. (a) XRD patterns of (A) sputtered metallic W on ITO-glass; (B) as-anodized film; and (C) annealed film. Insets show the top-view SEM images of as-anodized and annealed WO3 film. (b) XRD pattern of sputtered WO3 film.
To evaluate the electrochromic properties of macroporous WO3 films, cyclic voltammograms (CVs) were conducted in a 0.1 M H2 SO4 electrolyte solution between −0.5 V and 1.0 V (vs. Ag/AgCl) with a scan rate of 50 mV s−1 . For chronoamperometry (CA), a voltage step between −0.5 V (30 s) and +1.0 V (30 s) was used and the corresponding in situ transmittance switching was set at 632.8 nm. For comparison, a compact crystalline WO3 film (10 mm × 25 mm) deposited by reactive sputtering is also presented. To deposit masses equal to that of the anodized WO3 film, the thickness of the sputtered WO3 film was set to 220 nm. The anodized macroporous WO3 films switched from a transparent (bleached) state to a deep blue (colored) state as the applied voltage was adjusted from +1.0 V to −0.5 V (Fig. 5). WO3 films in the colored and bleached states are shown in Fig. 4d and e. Transmittance spectra for WO3 film of both states are shown in Fig. 4f. This process can be attributed to the following reversible electrochemical reaction: Fig. 2. Evolution of ex situ cross-sectional SEM images of films anodized over a range of times.
anodized and no characteristic peaks were found. The as-anodized films also show poor transparency suggesting that the metallic W layer is not fully oxidized. After annealing, as shown in pattern c of Fig. 3a, the film represents an orthorhombic WO3 crystalline structure (JCPDS Cards 71-0131). The remnant W is totally oxidized during annealing resulting in a highly transparent film (Fig. 4c). The top-view SEM images indicate that the surface morphology does not change significantly after annealing (insets in Fig. 3a).
WO3 + xH+ + e− ⇔ Hx WO3 Transparent
Deep blue
As evidenced from the typical CVs shown in Fig. 5, the anodized macroporous WO3 films attain a steady state within 20 cycles, whereas sputtered WO3 films require several hundred cycles. We hypothesize that the porous structure of anodized WO3 films provide sufficient tunnels for H+ ions to intercalate/deintercalate. At the steady states, the peak current densities for macroporous WO3 films are higher than that of compact films. For the 1000th cycle of both CVs, the cathodic charge (Qc ) and anodic charge (Qa ) calculated from the CVs are Qc = 13.1 mC cm−2 and Qa = 10.5 mC cm−2 for the
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Fig. 5. Cyclic voltammograms (CV) performed between −0.5 V and 1.0 V with a scan rate of 50 mV s−1 in 0.1 M H2 SO4 for (a) 40 min-anodized and (b) sputtered WO3 films (arrows denote scan direction).
Fig. 4. Images of (a) sputtered metallic W films on ITO-glass; (b) 40 min-anodized film; (c) anodized film after annealing; and color changes of the WO3 film between (d) colored at −0.5 V and (e) bleached at +1.0 V, and (f) corresponding transmittance spectra of films in the colored state and bleached state.
larger modulation range of visible light (up to 50%), although the anodized films exhibits a lower transparency than the sputtered film in its bleached state. Switching times, including both the coloration and bleaching times, are defined as the time required for a 90% change in the whole transmittance modulation at 632.8 nm. Coloration efficiency (CE) is a characteristic parameter for comparing the electrochromic property of different materials and is defined as: CE() =
anodized WO3 film and Qc = 10.7 mC cm−2 and Qa = 7.4 mC cm−2 for the sputtered WO3 film. These results suggest that larger amounts of ions can intercalate into the anodized WO3 film due to the highly macroporous surface. The reversibility during the electrochemical reactions can be estimated by the ratio of charge densities (Qa /Qc ). Reversibility ratios for the anodized WO3 films and sputtered WO3 films were 80% and 69%, respectively, indicating that the reactions in the macroporous WO3 films are more reversible than those in the compact WO3 films. Both films exhibit excellent cycle stability within the 2000 CVs cycles; however, the anodized WO3 film declines rapidly after 2000 cycles. This instability may result from the dissolution of WO3 film into sulfuric acid solution. The porous surface of the anodized WO3 film with an open network would accelerate the dissolution. The chronoamperometry (CA) curves and corresponding in situ transmittance at 632.8 nm are shown in Fig. 6a and b. The macroporous WO3 film has shorter switching times (about 8 s) and a
OD Q
OD() = log
(1) Tb Tc
(2)
where OD is optical density of the film in its colored (Tc ) and bleached (Tb ) states at a certain wavelength (), and Q is the corresponding injected (or ejected) charge density per unit area, which is the integration of the current within the time shown in Fig. 6a. The calculated CE values, shown in Fig. 6c, of the macroporous WO3 film and the compact film are 58 cm2 C−1 and 25 cm2 C−1 , respectively. To the best of our knowledge, this is the first report of the coloration efficiency of anodized macroporous WO3 films on ITO-coated glass. The comparatively high CE is attributed to the macroporous structure of the WO3 film, which provides more surface area to allow for the process of ion intercalation (or deintercalation). EC properties of anodized macroporous and sputtered compact WO3 films are listed in Table 1.
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4. Conclusions In summary, self-organized macroporous WO3 films were grown by an anodic oxidation in a NaF electrolyte from a DC-sputtered tungsten layer on ITO-glass. Well-structured films adhered to the entire surface under optimized experimental conditions. The as-anodized films show an amorphous structure and are poorly transparent. After annealing, the films adopt an orthorhombic structure and become transparent. The anodized WO3 films on ITO-glass with macroporous structure exhibit excellent electrochromic properties, including faster switching speed and larger color contrast. The work presented here provides an approach to fabricate transparent metal oxide films with macroporous structures for optical applications such as electrochromism and photocatalysis by anodic oxidation of sputtered metal layers on ITO-coated glass. Acknowledgment The authors would like to acknowledge financial support from Zhejiang University K.P. Chao’s High Technology Development Foundation (Grant No. 2008ZD001). References
Fig. 6. (a) Chronoamperometry (CA) with voltage step from −0.5 V (30 s) to +1.0 V (30 s); (b) corresponding in situ transmittance curves for sputtered and 40 minanodized WO3 at 632.8 nm and (c) optical density variation with respect to the charge density. Table 1 EC properties of anodized macroporous and sputtered compact WO3 films. Sample
T/% Tc /s
Anodized 50 Sputtered 40
8.0 18.6
Tb /s Reversibility ratios/%
Cycle stability
Coloration efficiency (cm2 /mC)
8.1 80 10 69
∼2000 >3000
58 25
T denotes the transmittance modulation range at ∼632.8 nm. Tc and Tb denote the response time of switching from bleached state to colored state and colored state to bleached state, respectively.
[1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] S.K. Deb, Sol. Energy Mater. Sol. Cells 92 (2008) 245. [3] J. Zhang, J.P. Tu, X.H. Xia, Y. Qiao, Y. Lu, Sol. Energy Mater. Sol. Cells 93 (2009) 1840. [4] O.U. Nimittrakoolchai, S. Supothina, Mater. Chem. Phys. 112 (2008) 270. [5] E. Gyorgy, G. Socol, I.N. Mihailescu, C. Ducu, S. Ciuca, J. Appl. Phys. 97 (2005) 093527. [6] M. Sadakane, K. Sasaki, H. Kunioku, B. Ohtani, W. Ueda, R. Abe, Chem. Commun. (2008) 6552. [7] M. Shibuya, M. Miyauchi, Adv. Mater. 21 (2009) 1373. [8] S. Berger, H. Tsuchiya, A. Ghicov, P. Schmuki, Appl. Phys. Lett. 88 (2006) 203119. [9] A. Hauch, A. Georg, U.O. Krasovec, B. Orel, J. Electrochem. Soc. 149 (2002) A1208. [10] W.J. Li, Z.W. Fu, Appl. Surf. Sci. 256 (2010) 2447. [11] M.J. Wang, G.J. Fang, L.Y. Yuan, H.H. Huang, Z.H. Sun, N.S. Liu, S.H. Xia, X.Z. Zhao, Nanotechnology 20 (2009) 185304. [12] J. Backholm, E. Avendano, A. Azens, G. de M. Azevedo, E. Coronel, G.A. Niklasson, C.G. Granqvista, Sol. Energy Mater. Sol. Cells 92 (2008) 91. [13] X.H. Xia, J.P. Tu, J. Zhang, X.H. Huang, X.L. Wang, W.K. Zhang, H. Huang, Electrochem. Commun. 10 (2008) 1815. [14] J. Backholm, G.A. Niklasson, Sol. Energy Mater. Sol. Cells 92 (2008) 1388. [15] S.Y. Kim, M. Taya, C.Y. Xu, J. Electrochem. Soc. 156 (2009) E40. [16] C.S. Hsu, C.C. Chan, H.T. Huang, C.H. Peng, W.C. Hsu, Thin Solid Films 516 (2008) 4839. [17] X.H. Xia, J.P. Tu, J. Zhang, X.L. Wang, W.K. Zhang, H. Huang, Nanotechnology 19 (2008). [18] C.G. Granqvist, G.A. Niklasson, A. Azens, Appl. Phys. A 89 (2007) 29. [19] Y. Djaoued, P.V. Ashrit, S. Badilescu, R. Bruning, J. Sol–Gel Sci. Technol. 28 (2003) 235. [20] M. Deepa, M. Kar, D.P. Singh, A.K. Srivastava, S. Ahmad, Sol. Energy Mater. Sol. Cells 92 (2008) 170. [21] S. Badilescu, P.V. Ashrit, Solid State Ionics 158 (2003) 187. [22] P. Bocchetta, C. Sunseri, G. Chiavarotti, F. Di Quarto, Electrochim. Acta 48 (2003) 3175. [23] J.P. Tu, C.X. Jiang, S.Y. Guo, M.F. Fu, Mater. Sci. Eng. A 398 (2005) 241. [24] K. Lee, Y. Tang, M. Ouyang, Nano Lett. 8 (2008) 4624. [25] G.K. Singh, A.A. Golovin, I.S. Aranson, Phys. Rev. B 73 (2006) 205422. [26] M. Fogazza, M. Santamaria, F. Di Quarto, S.J. Garcia-Vergara, I. Molchan, P. Skeldon, G.E. Thompson, H. Habazaki, Electrochim. Acta 54 (2009) 1070. [27] A.Z. Sadek, H.D. Zheng, K. Latham, W. Wlodarski, K. Kalantar-Zadeh, Langmuir 25 (2009) 509. [28] S. Berger, A. Ghicov, Y.C. Nah, P. Schmuki, Langmuir 25 (2009) 4841. [29] F.M.B. Hassan, H. Nanjo, H. Tetsuka, M. Kanakubo, T. Aizawa, M. Nishioka, T. Ebina, A.M. Bond, J. Electrochem. Soc. 156 (2009) K227. [30] H. Tsuchiya, T. Akaki, J. Nakata, D. Terada, N. Tsuji, Y. Koizumi, Y. Minamino, P. Schmuki, S. Fujimoto, Electrochim. Acta 54 (2009) 5155. [31] Z.B. Wu, S. Guo, H.Q. Wang, Y. Liu, Electrochem. Commun. 11 (2009) 1692. [32] L.J. Zhang, J.J. Fan, Z. Zhang, F.H. Cao, J.Q. Zhang, C.N. Cao, Electrochim. Acta 52 (2007) 5325. [33] P.C. Howlett, T. Khoo, G. Mooketsi, J. Efthimiadis, D.R. MacFarlane, M. Forsyth, Electrochim. Acta 55 (2010) 2377. [34] A.I. Mardare, A. Savan, A. Ludwig, A.D. Wieck, A.W. Hassel, Electrochim. Acta 54 (2009) 5973.
6958
J. Zhang et al. / Electrochimica Acta 55 (2010) 6953–6958
[35] H.A. El-Sayed, V.I. Birss, Nano Lett. 9 (2009) 1350. [36] H. Tsuchiya, J.M. Macak, I. Sieber, L. Taveira, A. Ghicov, K. Sirotna, P. Schmuki, Electrochem. Commun. 7 (2005) 295. [37] Y.C. Nah, A. Ghicov, D. Kim, S. Berger, P. Schmuki, J. Am. Chem. Soc. 130 (2008) 16154. [38] Y.C. Nah, A. Ghicov, D. Kim, P. Schmuki, Electrochem. Commun. 10 (2008) 1777. [39] M. Yang, N.K. Shrestha, P. Schmuki, Electrochem. Commun. 11 (2009) 1908.
[40] A. Dipaola, F. Diquarto, C. Sunseri, J. Electrochem. Soc. 125 (1978) 1344. [41] H.D. Zheng, A.Z. Sadek, M. Breedon, D. Yao, K. Latham, J. du Plessis, K. KalantarZadeh, Electrochem. Commun. 11 (2009) 1308. [42] A. Ghicov, S.P. Alba, J.M. Macak, P. Schmuki, Small 4 (2008) 1063. [43] A. Ghicov, M. Yamamoto, P. Schmuki, Angew. Chem.-Int. Ed. 47 (2008) 7934. [44] H.D. Zheng, A.Z. Sadek, K. Latham, K. Kalantar-Zadeh, Electrochem. Commun. 11 (2009) 768.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhanced electrochromic performance of macroporous WO3 films formed by anodic oxidation of DC-sputtered tungsten layers J. Zhang, X.L. Wang ∗ , X.H. Xia, C.D. Gu, Z.J. Zhao, J.P. Tu ∗ State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 20 April 2010 Received in revised form 26 June 2010 Accepted 29 June 2010 Available online 19 July 2010 Keywords: Tungsten oxide Thin films Anodic oxidation Macroporous structure Electrochromism
a b s t r a c t Self-organized macroporous tungsten trioxide (WO3 ) films are obtained by anodic oxidation of DCsputtered tungsten (W) layers on 10 mm × 25 mm indium tin oxide (ITO)-coated glass. Under optimized experimental conditions, uniformly macroporous WO3 films with a thickness of ca. 350 nm are formed. The film shows a connected network with average pore size of 100 nm and a pore wall thickness of approximately 30 nm. The anodized film becomes transparent after annealing without significant change in macroporous structure. In 0.1 M H2 SO4 , the macroporous WO3 films show enhanced electrochromic properties with a coloration efficiency of 58 cm2 C−1 . Large modulation of transmittance (∼50% at 632.8 nm) and a switching speed of about 8 s are also achieved with this macroporous film. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Tungsten trioxide (WO3 ) receives extensive attention because of its potential applications in electrochromic (EC) devices [1–3], gas sensors [4,5], photocatalysis [6–8] and batteries [9,10]. Electrochromic devices are based on the change of optical properties when a voltage is applied [1]. There are many transition metal oxides that show EC properties, such as the oxides of tungsten, nickel, iridium, vanadium, titanium, cobalt and molybdenum [11–17]. The initiation of EC is attributed to the intercalation of ions (H+ , Li+ , Na+ ) to the films [1], thus the process is believed to occur first at the film surface [18]. Therefore, to achieve fast and sufficient intercalation of ions, EC films with high surface ratios, such as porous films are required. Conventional approaches for synthesizing porous WO3 films are based on templating methods that use structure-directing agents, such as polyethylene glycol (PEG 300, 600, etc.) [19,20] and polystyrene (PS) microspheres [21], to create pores and channels within the film. Anodization provides a template-free method for forming porous oxide layers on metallic surfaces. Anodization has been widely used to synthesize porous alumina, namely, anodic aluminum oxide (AAO) [22–26], which can be used as a template to form nanostructures of various materials. Beyond AAO, many other metals and alloys have been found to form macroporous or nanotube structures by anodic oxidation, including titanium
[27–31], magnesium [32,33], niobium [34], and tantalum [35]. Anodic macroporous WO3 structures have been reported for tungsten and tungsten-containing alloy foils [8,36–40]. Dipaola et al. [40] found that two different EC mechanisms occur on anodically formed tungsten oxide films – namely, coloration occurs due to double injection of electrons and H+ as well as due to the formation of the blue oxide hydrate (WO3−x ·H2 O). Schmuki and co-workers [37] reported that TiO2 –WO3 composite nanotubes formed by anodic oxidation of Ti/W alloys (Ti 0.2–9%) showed enhanced EC properties. However, metal foils are not suitable for fabrication of EC devices, which generally need a transparent conductive substrate such as indium tin oxide (ITO)-coated glass. As a result, transparent films with nanostructures, for example, AAO, anodic TiO2 nanotubes and macroporous films [27,28,41–43] have been developed. However, few reports exist on the anodization of transparent WO3 films with macroporous structures on ITOcoated glass [44]. To the best of our knowledge, the EC properties of transparent WO3 macroporous film have not been reported in the literature. In this work, metallic W layers were deposited on ITOglass substrates through a well-controlled DC-sputtering process. Macroporous WO3 films were then prepared by anodic oxidation of the W layers. The EC properties of the macroporous WO3 films were investigated. Optical transmittance modulation ranges (T) and coloration efficiencies (CE) of anodized macroporous WO3 films were obtained for the first time. 2. Experimental
∗ Corresponding authors. Tel.: +86 571 87952856; fax: +86 571 87952856. E-mail addresses: [email protected] (X.L. Wang), [email protected] (J.P. Tu). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.06.082
The ITO-coated glass slides measuring 10 mm × 25 mm were used as substrates. The W layers were deposited by means of a
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DC magnetron sputterer using a metallic W (99.99%) target in an Ar atmosphere at a pressure as high as 6.67 Pa (base pressure 5.0 × 10−3 Pa) with a deposition speed of 0.22 nm s−1 at a power of 30 W and a substrate rotation speed of 5 rpm. The distance between substrate and target was 8 cm. During the deposition process, a bias of −100 V and a temperature of 400 ◦ C were applied to the substrate. All parameters were specially selected to deposit highly compact W layers that were strongly adhered to the substrate. In this work, a deposition time of 60 min was used to obtain a W layer 0.8 m thick. Anodization treatment of the sputtered W layer was performed using an electrochemical cell with a conventional anode–cathode configuration, where the working electrode was W/ITO-glass and the counter electrode was a platinum foil. The W/ITO-glass electrodes were anodized in a 0.2 wt% NaF solution from the opencircuit potential (OCP) to 60 V with a sweep rate of 1000 mV s−1 . The electrodes were then held at 60 V for various durations (i.e., 10, 20, 30, 40, and 60 min). After anodization, the samples were washed with deionized water and dried in a vacuum oven. Transparent WO3 films were obtained by annealing the as-anodized samples at 400 ◦ C for 4 h in air. For comparison, a compact WO3 film was deposited on an ITOglass substrate by means of a DC magnetron sputtering using a metallic W (99.99%) target in an Ar (80%) + O2 (20%) atmosphere at a pressure of 0.6 Pa (base pressure 5.0 × 10−3 Pa). To obtain a crystalline WO3 film, the substrate was heated at 200 ◦ C during the deposition process. The sputtering power was 120 W and the distance between substrate and target was 8 cm. The morphology and microstructure of the anodized and sputtered films were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4700) and X-ray diffraction (XRD, PANalytical X’Pert PRO diffractometer and Cu K␣ radiation, = 1.54056 Å). Electrochemical and electrochromic properties were assessed in an aqueous solution of 0.1 M H2 SO4 with a 1 cm × 1 cm Pt foil as the counter electrode and a Ag/AgCl electrode as the reference electrode using a CHI660c electrochemical workshop. Optical transmittance was measured with a SHIMADZU UV-240 spectrophotometer. 3. Results and discussion During anodization, the evolution of surface morphology and cross-sections were recorded by ex situ SEM. The sputtered W layer exhibits a compact structure and good adhesion to the ITO-coated glass (Fig. 1a). The surface morphologies of the samples anodized for different times are shown in Fig. 1b–d. For the 10 min-anodized film, pore formation can be observed only in specific areas. With the continuous anodization, a highly organized macroporous structure is formed. The film that was anodized for 40 min shows a connected network with an average pore size of 100 nm and a pore wall thickness of approximately 30 nm. After 60 min of anodization, the layer is partially etched and the network is destroyed. These changes in the surface morphology were also observed in the cross-section view with SEM (Fig. 2). It can be seen that as the anodization time is increased, the unoxidized metallic W layers become thinner in the cross-section view. A comparison of XRD patterns of sputtered W layers on an ITO substrate and 40 min-anodized films both before and after annealing are shown in Fig. 3a. The broadened peaks of -W (JCPDS Cards 47-1319) are observed for the metallic W layer. The patterns of as-anodized films contained only peaks corresponding to the ITO substrate. Furthermore, the grazing incidence XRD pattern (data not shown) did not show any characteristic peaks except an amorphous shoulder indicating that the as-anodized films are amorphous (pattern b). Fig. 4a shows an image of a sputtered W film on an ITO substrate. As shown in Fig. 4b, most of the W layer is
Fig. 1. SEM images of (a) sputtered W layers and films anodized for (b) 10 min; (c) 40 min; and (d) 60 min.
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Fig. 3. (a) XRD patterns of (A) sputtered metallic W on ITO-glass; (B) as-anodized film; and (C) annealed film. Insets show the top-view SEM images of as-anodized and annealed WO3 film. (b) XRD pattern of sputtered WO3 film.
To evaluate the electrochromic properties of macroporous WO3 films, cyclic voltammograms (CVs) were conducted in a 0.1 M H2 SO4 electrolyte solution between −0.5 V and 1.0 V (vs. Ag/AgCl) with a scan rate of 50 mV s−1 . For chronoamperometry (CA), a voltage step between −0.5 V (30 s) and +1.0 V (30 s) was used and the corresponding in situ transmittance switching was set at 632.8 nm. For comparison, a compact crystalline WO3 film (10 mm × 25 mm) deposited by reactive sputtering is also presented. To deposit masses equal to that of the anodized WO3 film, the thickness of the sputtered WO3 film was set to 220 nm. The anodized macroporous WO3 films switched from a transparent (bleached) state to a deep blue (colored) state as the applied voltage was adjusted from +1.0 V to −0.5 V (Fig. 5). WO3 films in the colored and bleached states are shown in Fig. 4d and e. Transmittance spectra for WO3 film of both states are shown in Fig. 4f. This process can be attributed to the following reversible electrochemical reaction: Fig. 2. Evolution of ex situ cross-sectional SEM images of films anodized over a range of times.
anodized and no characteristic peaks were found. The as-anodized films also show poor transparency suggesting that the metallic W layer is not fully oxidized. After annealing, as shown in pattern c of Fig. 3a, the film represents an orthorhombic WO3 crystalline structure (JCPDS Cards 71-0131). The remnant W is totally oxidized during annealing resulting in a highly transparent film (Fig. 4c). The top-view SEM images indicate that the surface morphology does not change significantly after annealing (insets in Fig. 3a).
WO3 + xH+ + e− ⇔ Hx WO3 Transparent
Deep blue
As evidenced from the typical CVs shown in Fig. 5, the anodized macroporous WO3 films attain a steady state within 20 cycles, whereas sputtered WO3 films require several hundred cycles. We hypothesize that the porous structure of anodized WO3 films provide sufficient tunnels for H+ ions to intercalate/deintercalate. At the steady states, the peak current densities for macroporous WO3 films are higher than that of compact films. For the 1000th cycle of both CVs, the cathodic charge (Qc ) and anodic charge (Qa ) calculated from the CVs are Qc = 13.1 mC cm−2 and Qa = 10.5 mC cm−2 for the
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Fig. 5. Cyclic voltammograms (CV) performed between −0.5 V and 1.0 V with a scan rate of 50 mV s−1 in 0.1 M H2 SO4 for (a) 40 min-anodized and (b) sputtered WO3 films (arrows denote scan direction).
Fig. 4. Images of (a) sputtered metallic W films on ITO-glass; (b) 40 min-anodized film; (c) anodized film after annealing; and color changes of the WO3 film between (d) colored at −0.5 V and (e) bleached at +1.0 V, and (f) corresponding transmittance spectra of films in the colored state and bleached state.
larger modulation range of visible light (up to 50%), although the anodized films exhibits a lower transparency than the sputtered film in its bleached state. Switching times, including both the coloration and bleaching times, are defined as the time required for a 90% change in the whole transmittance modulation at 632.8 nm. Coloration efficiency (CE) is a characteristic parameter for comparing the electrochromic property of different materials and is defined as: CE() =
anodized WO3 film and Qc = 10.7 mC cm−2 and Qa = 7.4 mC cm−2 for the sputtered WO3 film. These results suggest that larger amounts of ions can intercalate into the anodized WO3 film due to the highly macroporous surface. The reversibility during the electrochemical reactions can be estimated by the ratio of charge densities (Qa /Qc ). Reversibility ratios for the anodized WO3 films and sputtered WO3 films were 80% and 69%, respectively, indicating that the reactions in the macroporous WO3 films are more reversible than those in the compact WO3 films. Both films exhibit excellent cycle stability within the 2000 CVs cycles; however, the anodized WO3 film declines rapidly after 2000 cycles. This instability may result from the dissolution of WO3 film into sulfuric acid solution. The porous surface of the anodized WO3 film with an open network would accelerate the dissolution. The chronoamperometry (CA) curves and corresponding in situ transmittance at 632.8 nm are shown in Fig. 6a and b. The macroporous WO3 film has shorter switching times (about 8 s) and a
OD Q
OD() = log
(1) Tb Tc
(2)
where OD is optical density of the film in its colored (Tc ) and bleached (Tb ) states at a certain wavelength (), and Q is the corresponding injected (or ejected) charge density per unit area, which is the integration of the current within the time shown in Fig. 6a. The calculated CE values, shown in Fig. 6c, of the macroporous WO3 film and the compact film are 58 cm2 C−1 and 25 cm2 C−1 , respectively. To the best of our knowledge, this is the first report of the coloration efficiency of anodized macroporous WO3 films on ITO-coated glass. The comparatively high CE is attributed to the macroporous structure of the WO3 film, which provides more surface area to allow for the process of ion intercalation (or deintercalation). EC properties of anodized macroporous and sputtered compact WO3 films are listed in Table 1.
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4. Conclusions In summary, self-organized macroporous WO3 films were grown by an anodic oxidation in a NaF electrolyte from a DC-sputtered tungsten layer on ITO-glass. Well-structured films adhered to the entire surface under optimized experimental conditions. The as-anodized films show an amorphous structure and are poorly transparent. After annealing, the films adopt an orthorhombic structure and become transparent. The anodized WO3 films on ITO-glass with macroporous structure exhibit excellent electrochromic properties, including faster switching speed and larger color contrast. The work presented here provides an approach to fabricate transparent metal oxide films with macroporous structures for optical applications such as electrochromism and photocatalysis by anodic oxidation of sputtered metal layers on ITO-coated glass. Acknowledgment The authors would like to acknowledge financial support from Zhejiang University K.P. Chao’s High Technology Development Foundation (Grant No. 2008ZD001). References
Fig. 6. (a) Chronoamperometry (CA) with voltage step from −0.5 V (30 s) to +1.0 V (30 s); (b) corresponding in situ transmittance curves for sputtered and 40 minanodized WO3 at 632.8 nm and (c) optical density variation with respect to the charge density. Table 1 EC properties of anodized macroporous and sputtered compact WO3 films. Sample
T/% Tc /s
Anodized 50 Sputtered 40
8.0 18.6
Tb /s Reversibility ratios/%
Cycle stability
Coloration efficiency (cm2 /mC)
8.1 80 10 69
∼2000 >3000
58 25
T denotes the transmittance modulation range at ∼632.8 nm. Tc and Tb denote the response time of switching from bleached state to colored state and colored state to bleached state, respectively.
[1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] S.K. Deb, Sol. Energy Mater. Sol. Cells 92 (2008) 245. [3] J. Zhang, J.P. Tu, X.H. Xia, Y. Qiao, Y. Lu, Sol. Energy Mater. Sol. Cells 93 (2009) 1840. [4] O.U. Nimittrakoolchai, S. Supothina, Mater. Chem. Phys. 112 (2008) 270. [5] E. Gyorgy, G. Socol, I.N. Mihailescu, C. Ducu, S. Ciuca, J. Appl. Phys. 97 (2005) 093527. [6] M. Sadakane, K. Sasaki, H. Kunioku, B. Ohtani, W. Ueda, R. Abe, Chem. Commun. (2008) 6552. [7] M. Shibuya, M. Miyauchi, Adv. Mater. 21 (2009) 1373. [8] S. Berger, H. Tsuchiya, A. Ghicov, P. Schmuki, Appl. Phys. Lett. 88 (2006) 203119. [9] A. Hauch, A. Georg, U.O. Krasovec, B. Orel, J. Electrochem. Soc. 149 (2002) A1208. [10] W.J. Li, Z.W. Fu, Appl. Surf. Sci. 256 (2010) 2447. [11] M.J. Wang, G.J. Fang, L.Y. Yuan, H.H. Huang, Z.H. Sun, N.S. Liu, S.H. Xia, X.Z. Zhao, Nanotechnology 20 (2009) 185304. [12] J. Backholm, E. Avendano, A. Azens, G. de M. Azevedo, E. Coronel, G.A. Niklasson, C.G. Granqvista, Sol. Energy Mater. Sol. Cells 92 (2008) 91. [13] X.H. Xia, J.P. Tu, J. Zhang, X.H. Huang, X.L. Wang, W.K. Zhang, H. Huang, Electrochem. Commun. 10 (2008) 1815. [14] J. Backholm, G.A. Niklasson, Sol. Energy Mater. Sol. Cells 92 (2008) 1388. [15] S.Y. Kim, M. Taya, C.Y. Xu, J. Electrochem. Soc. 156 (2009) E40. [16] C.S. Hsu, C.C. Chan, H.T. Huang, C.H. Peng, W.C. Hsu, Thin Solid Films 516 (2008) 4839. [17] X.H. Xia, J.P. Tu, J. Zhang, X.L. Wang, W.K. Zhang, H. Huang, Nanotechnology 19 (2008). [18] C.G. Granqvist, G.A. Niklasson, A. Azens, Appl. Phys. A 89 (2007) 29. [19] Y. Djaoued, P.V. Ashrit, S. Badilescu, R. Bruning, J. Sol–Gel Sci. Technol. 28 (2003) 235. [20] M. Deepa, M. Kar, D.P. Singh, A.K. Srivastava, S. Ahmad, Sol. Energy Mater. Sol. Cells 92 (2008) 170. [21] S. Badilescu, P.V. Ashrit, Solid State Ionics 158 (2003) 187. [22] P. Bocchetta, C. Sunseri, G. Chiavarotti, F. Di Quarto, Electrochim. Acta 48 (2003) 3175. [23] J.P. Tu, C.X. Jiang, S.Y. Guo, M.F. Fu, Mater. Sci. Eng. A 398 (2005) 241. [24] K. Lee, Y. Tang, M. Ouyang, Nano Lett. 8 (2008) 4624. [25] G.K. Singh, A.A. Golovin, I.S. Aranson, Phys. Rev. B 73 (2006) 205422. [26] M. Fogazza, M. Santamaria, F. Di Quarto, S.J. Garcia-Vergara, I. Molchan, P. Skeldon, G.E. Thompson, H. Habazaki, Electrochim. Acta 54 (2009) 1070. [27] A.Z. Sadek, H.D. Zheng, K. Latham, W. Wlodarski, K. Kalantar-Zadeh, Langmuir 25 (2009) 509. [28] S. Berger, A. Ghicov, Y.C. Nah, P. Schmuki, Langmuir 25 (2009) 4841. [29] F.M.B. Hassan, H. Nanjo, H. Tetsuka, M. Kanakubo, T. Aizawa, M. Nishioka, T. Ebina, A.M. Bond, J. Electrochem. Soc. 156 (2009) K227. [30] H. Tsuchiya, T. Akaki, J. Nakata, D. Terada, N. Tsuji, Y. Koizumi, Y. Minamino, P. Schmuki, S. Fujimoto, Electrochim. Acta 54 (2009) 5155. [31] Z.B. Wu, S. Guo, H.Q. Wang, Y. Liu, Electrochem. Commun. 11 (2009) 1692. [32] L.J. Zhang, J.J. Fan, Z. Zhang, F.H. Cao, J.Q. Zhang, C.N. Cao, Electrochim. Acta 52 (2007) 5325. [33] P.C. Howlett, T. Khoo, G. Mooketsi, J. Efthimiadis, D.R. MacFarlane, M. Forsyth, Electrochim. Acta 55 (2010) 2377. [34] A.I. Mardare, A. Savan, A. Ludwig, A.D. Wieck, A.W. Hassel, Electrochim. Acta 54 (2009) 5973.
6958
J. Zhang et al. / Electrochimica Acta 55 (2010) 6953–6958
[35] H.A. El-Sayed, V.I. Birss, Nano Lett. 9 (2009) 1350. [36] H. Tsuchiya, J.M. Macak, I. Sieber, L. Taveira, A. Ghicov, K. Sirotna, P. Schmuki, Electrochem. Commun. 7 (2005) 295. [37] Y.C. Nah, A. Ghicov, D. Kim, S. Berger, P. Schmuki, J. Am. Chem. Soc. 130 (2008) 16154. [38] Y.C. Nah, A. Ghicov, D. Kim, P. Schmuki, Electrochem. Commun. 10 (2008) 1777. [39] M. Yang, N.K. Shrestha, P. Schmuki, Electrochem. Commun. 11 (2009) 1908.
[40] A. Dipaola, F. Diquarto, C. Sunseri, J. Electrochem. Soc. 125 (1978) 1344. [41] H.D. Zheng, A.Z. Sadek, M. Breedon, D. Yao, K. Latham, J. du Plessis, K. KalantarZadeh, Electrochem. Commun. 11 (2009) 1308. [42] A. Ghicov, S.P. Alba, J.M. Macak, P. Schmuki, Small 4 (2008) 1063. [43] A. Ghicov, M. Yamamoto, P. Schmuki, Angew. Chem.-Int. Ed. 47 (2008) 7934. [44] H.D. Zheng, A.Z. Sadek, K. Latham, K. Kalantar-Zadeh, Electrochem. Commun. 11 (2009) 768.