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High electrochromic performance of co-sputtered vanadium–titanium oxide as a counter electrode
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 2069–2074
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
High electrochromic performance of co-sputtered vanadium–titanium oxide as a counter electrode Ju Wan Lim, Sung Jong Yoo, Sun Ha Park, Seong Uk Yun, Yung-Eun Sung School of Chemical and Biological Engineering, Seoul National University, San 56-1, Silim-dong, Gwanak-gu, Seoul, Republic of Korea
a r t i c l e in fo
abstract
Article history: Received 30 September 2008 Received in revised form 2 March 2009 Accepted 7 March 2009 Available online 10 April 2009
This study examined the material and electrochromic properties of vanadium–titanium oxides (V–Ti oxides) as a counter electrode material in electrochromic devices. These oxides were deposited on an ITO substrate using a co-sputtering method at different levels of RF power. Electrochemical experiments of these oxides were carried out using half-cell and semi full-cell tests which are good methods for measuring the potential applied to each electrode. The change in electrochromic properties after 1000 cycles of a semi full-cell test was examined. Reversibility and durability of an electrochromic device were improved by increasing the titanium content in V–Ti oxides. & 2009 Elsevier B.V. All rights reserved.
Keywords: Vanadium oxide Vanadium–titanium oxide Electrochromism Counter electrode Electrode potential
1. Introduction Materials with optical properties that can be changed by external stimulation (light, change of temperature and electric potential) are called chromogenic. The first report of an electrochromic material, tungsten oxide, was published approximately 40 years ago by Deb [1]. Initially, the optical absorption of a visible ray can be changed broadly by the absorption and emission of an electrical charge. Electrochromism has attracted considerable attention in research. In the mid-1980s, there was some interest in placing large windows on the ceiling of buildings. The interest in electrochromism was increased significantly by the concept of a ‘smart window’, which was invented to reduce energy consumption in buildings [2]. For electrochromic materials, there are many species, such as organic, inorganic and metal organic complexes. Among them, the electrochromic materials used in electrochromic windows are generally inorganic materials owing to the thermal stability they possess. In addition, inorganic electrochromic materials can generally be divided into two parts: cathodic coloring materials and anodic coloring materials. Cathodic coloring materials are oxides of W, Nb, Mo, Ti, Ta, etc. and anodic coloring materials are oxides of Cr, Mn, Fe, Co, Rh, Ir and Ni. Among them, tungsten trioxide (WO3) is the material most studied. Tungsten trioxide has a similar crystal structure to perovskite [3]. The electrochromic
Corresponding author. Tel.: +82 2 880 1889; fax: +82 2 888 1604.
E-mail address: [email protected] (Y.-E. Sung). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.03.008
properties (color change, response time and reversibility of lithium intercalation) of crystal tungsten trioxide are poor. However, the electrochromic properties of amorphous tungsten trioxide are superior to any other metal oxide examined [4]. Vanadium oxide (V2O5) has been actively studied for use in secondary batteries [5] due to a high charge capacity. Currently, vanadium oxide is used primarily as an anodic material for electrochromic devices. However, the demerits of pure vanadium oxide (durability and transparency) make it difficult for use as a counter electrode material in an EC device. Therefore, many research groups have mixed vanadium oxide with other materials, such as titanium oxide, cerium oxide, zirconium oxide, iron oxide and nickel oxide [6–13]. Among the demerits of vanadium oxide, cycling durability is the main problem preventing commercialization. Generally, the materials used to enhance the cycling durability of a material are titanium oxide and cerium oxide [14,15]. Cerium oxide is used as an additive material for the counter electrodes of EC devices due to its transparency. Its charge capacity, however, is very low. Therefore, it is difficult to maintain a sufficient charge when the charge capacity of the working electrode is high. Titanium oxide is used as an additive material to enhance electrical conductivity and prevent crystallization of a material during cycling. Therefore, it can be used to improve cycling durability. In this study, V–Ti oxides were deposited by RF magnetron co-sputtering. The composition of vanadium and titanium was varied to control the power ratio of the two targets. The electrochemical and optical properties of these samples were measured using a half-cell test, and their cycling durability was measured using a semi full-cell test. In the case of the semi full-
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cell test, it is easy to determine the level of electrode degradation. Moreover, the potential applied to each electrode could be measured using two electrode experiments.
Table 1 Abbreviations of the V–Ti oxides, compositions of the V–Ti oxides by EDS and film thicknesses of the V–Ti oxides by FE-SEM. Abbreviations
RF power of V2O5 (W)
RF power of TiO2 (W)
Ti:V atomic ratio
Film thickness (nm)
V2O5 VTi01 VTi07 VTi17 VTi33 VTi44
300 300 300 300 300 300
0 50 80 130 200 300
0:100 1:99 7:93 17:83 33:67 44:56
270 163 177 158 186 202
2. Experiment The V–Ti oxides and tungsten oxide were deposited by RF magnetron sputtering (Korea Vacuum Tech.) onto a glass precoated with indium–tin oxide (ITO) with a resistance of 10 O/m. The base pressure of the deposition chamber was 5 106 Torr. Pre-sputtering prior to deposition was carried out for more than 30 min under the same deposition conditions to clean the targets and ensure steady state deposition. The Ar pressure was maintained at 10 mTorr during pre-sputtering and sputtering. Substrate rotation (11 rpm) assured film uniformity, and the sputtering angle to the substrate was 521. Vanadium oxide thin film was prepared using a V2O5 target. Sputtering deposition was carried out at 300 W RF power for 2 h. The V–Ti oxide thin films were prepared by co-sputtering V2O5 and TiO2 targets. Co-sputtering deposition was carried out for 2 h with 300 W RF power to the V2O5 target, and with 80, 130, 200 and 300 W RF power to the TiO2 target. Tungsten oxide thin films used as the counter electrodes of the full-cell were also prepared by RF magnetron sputtering using a WO3 target with 150 W RF power for 52 min. The deposition conditions and film thickness of the tungsten oxide films (400 nm) were optimized in previous work [16]. The compositions of the deposited films were measured by energy dispersive X-ray spectroscopy (EDS, JSM 6700F) and electron probe X-ray micro analyzer (EPMA, JXA-8900R). Field emission scanning electron microscopy (FE-SEM, JSM 6700F) was used to measure film thickness and surface structure. The structures of the films were examined by X-ray diffraction (XRD, D-MAX2500-PC). The electrochemical and electrochromic properties and optical properties of the deposited films were measured using a potentiostat/galvanostat (AUTOLAB PGSTAT30) and He/Ne laser (l ¼ 632.8 nm), respectively. The electrochemical techniques used in this study were the cyclic voltammetry (CV) and the chronocoulometry (CC), which measured the Li+ intercalation/ deintercalation potential and charge capacity of the films, respectively. CC was also used to test the operation for 1000 cycles. In these electrochemical experiments, 1 M LiClO4 in propylene carbonate was used as the electrolyte. In the case of the half-cell test, the working electrode was connected to V–Ti oxide. The reference and counter electrodes were connected to Ag/AgCl (sat. KCl) and Pt wire, respectively. In the semi full-cell test (the two electrode experiments), the working and counter electrodes were tungsten oxide and V–Ti oxide thin films, respectively. In this case, with the Ag/AgCl (sat. KCl) not connected to the potentiostat, it was immersed in the electrolyte, and a digital voltmeter was used to measure the voltage between the Ag/AgCl and the working electrode (tungsten oxide). The in-situ optical properties were determined during the electrochemical experiment using a He–Ne laser.
3. Results and discussion Table 1 shows the atomic ratios as measured by EDS and EPMA and film thicknesses as measured by FE-SEM of the V–Ti oxides. The titanium content in the V–Ti oxide films increased with the increasing sputtering power to the titanium oxide target but the tendency of the increase was not linear. The Ti/V ratio of the sample with the same sputtering power applied to both targets
(300 W) was 0.8. Therefore, the co-sputtering yields of vanadium oxide and titanium oxide can be considered similar. Abbreviations of the samples are shown in Table 1. Fig. 1 shows the surface FE-SEM images of VTi01, VTi17 and VTi44. The figure also shows the surface morphology of the V–Ti oxides and the boundaries of the oxide lumps. These surface morphologies may allow the lithium ions to intercalate into the oxide film. However, the boundaries of the VTi44 oxide lumps appeared faint, and the surface morphology of the VTi44 was denser than that of the VTi01. The FE-SEM images suggest that the intercalating rate of lithium ions into the VTi01 film would be faster than it was into the VTi44 film. Fig. 2 shows the XRD patterns of the V–Ti oxides. Every XRD pattern showed ITO peaks because the film thickness was at most 270 nm and the substrate was ITO glass. With the exception of ITO, none of the patterns of the V2O5 and V–Ti oxides showed crystal peaks. Therefore, these results showed that V2O5 and V–Ti oxides have an amorphous structure. Fig. 3(a) shows the results of cyclic voltammetry. The current density in the cyclic voltammograms of the samples containing a large amount of vanadium was high due to the charge capacity. In addition, the anodic current peaks were quite broad. These broad peaks were shown in typical cyclic voltammograms of V2O5 when the scan rate was relatively fast (20 mV/s) [17]. Fig. 3(b) shows the electrochromic properties measured using the half-cell test (charge capacity and optical transmittance modulation). In the present study, the charge capacity was measured by chronocoulometry. Therefore, the charge capacity expressed in this paper is not the precise meaning of ‘charge capacity’ but, rather, it is the ‘degree of ease’ of Li+ ion intercalation/ deintercalation. The charge capacities of the V–Ti oxides were maintained at more than 37.6 mC/cm2 until the titanium content was 33 at%, and an abrupt decrease in charge capacity was observed at a titanium content of 44 at%. The aspect of DT at different titanium contents was similar to that observed for the charge capacity. The only difference was an abrupt decrease in DT at low titanium content. Originally, both vanadium oxide and titanium oxide showed cathodic coloration. These transmittance modulations are undesirable when used as a counter electrode for tungsten oxide in an EC device. Therefore, in V–Ti oxide, even a small amount of titanium is desirable for use as a counter electrode. An electrochromic full-cell test was carried out using V–Ti oxide and a semi full-cell test was carried out using tungsten oxide. After conducting the half-cell test of V–Ti oxide, the tungsten oxide was fully reduced, containing lithium ions, and the V–Ti oxide was fully oxidized. The semi full-cell was constructed as shown in Fig. 4. The tungsten and V–Ti oxides were connected to the working and counter electrodes, respectively. The initial semi full-cell test was operated for 5 cycles by applying +1.5 V for 30 s and 1.5 V for 30 s. During this test, the optical transmittance
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Fig. 2. XRD patterns of V2O5 and V–Ti oxides.
Fig. 1. Surface FE-SEM images of (a) VTi01, (b) VTi17 and (c) VTi44. Fig. 3. (a) Cyclic voltammograms and (b) charge capacities and transmittance modulations of V–Ti oxides.
modulation of V–Ti oxide was measured using a He–Ne laser. After an initial 5-cycle test, another 5-cycle test was carried out by measuring the optical transmittance modulation of tungsten oxide. The same set of cycling tests was then operated for 1000 cycles. After the 1000-cycle test, final-state tests of the transmittance modulation measurement were carried out using the same methodology. During the initial and final 10-cycle tests, the applied electrode potential vs. Ag/AgCl (to working electrode
and counter electrode) was measured using a voltmeter. In this experiment, the measured gaps of the applied potential values between the working electrode and the counter electrode were +1.50 and 1.50 V. This means that the conductivity of the electrolyte was high enough to ignore the potential drop that could be caused by the resistance electrolyte.
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Fig. 4. The illustration of the semi full-cell test.
The makeup of the semi full-cell test was more complicated than that of a conventional full device. There are advantages in using a semi full-cell test compared with a conventional full device test: (1) it is possible to measure the potential applied to each electrode, as well as the variations in the potential; (2) it can prevent a short circuit; (3) there is no dismantling process needed to characterize the samples after the cycling operation; and (4) it is possible to know which electrode is responsible for the performance degradation of the full-cell. The initial and final transmittance of the semi full-cell was calculated by multiplying the transmittance of tungsten and V–Ti oxides. The results and charge capacity degradation are plotted in Fig. 5(a) and (b), respectively. In Fig. 5(a), the initial transmittance modulations of the WO3/V–Ti oxide samples were smaller than those of WO3/V2O5, with the exception of the WO3/VTi01 sample. However, the final transmittance modulations of the WO3/V–Ti oxide samples were larger than those of WO3/V2O5. In addition, the initial charge capacities of all WO3/V–Ti oxide samples were lower than that of the WO3/V2O5 sample but the final charge capacities of all the WO3/V–Ti oxide samples were higher than that of the WO3/V2O5 sample. The degradation of charge capacity and transmittance modulation decreased with increasing titanium content, and an activation of transmittance modulation after the cycling test was detected. This shows that the WO3/V–Ti oxide samples were more reversible than the WO3/V2O5 samples. Therefore, for use in electrochromic devices, the V–Ti oxide samples were better counter electrodes for WO3 than for V2O5. Fig. 5(c) shows the response times, which is the time taken to achieve 80% coloration from the bleached state (tc), as well as bleaching from the colored state (tb) of the WO3/V–Ti oxide samples. The initial tb (tbi) of all samples was higher than the initial tc (tci). Bleaching occurred when the lithium ions
Fig. 5. The degradation of (a) the transmittance modulation, (b) charge capacity and (c) response time of WO3/V–Ti oxide samples before and after the 1000 cycling test in the semi full-cell. Tbi/Tci and Tbf/Tcf are the transmittances of the initial bleached/colored state and the final bleached/colored state, respectively. tbi/tci and tbf/tcf are the response times of the initial bleaching/coloring and the final bleaching/coloring processes, respectively.
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deintercalated from tungsten oxide and intercalated into the V–Ti oxide. Generally, the lithium ion intercalation process is slower than the deintercalation process. Therefore, lithium ion intercalation into V–Ti oxide was the slowest process in the initial cycle. This is because the lithium intercalation rate into vanadium oxide is higher than it is into titanium oxide. This phenomenon has previously been reported [18]. However, in the final cycle tb (tbf) became either similar to or less than tc (tcf). According to these results, the cycling operation enhanced the lithium intercalation rate into V–Ti oxide. In the semi full-cell test, potentials applied to each electrode were measured using a voltmeter. During operation, the potentials detected by the voltmeter fluctuated up to the saturation state of the intercalation/deintercalation reaction. Therefore, the measured potentials were the saturated potentials. Fig. 6(a) and (b) shows the changes in the electrode potentials of WO3/V2O5 and WO3/VTi44 before and after the 1000 cycling operation, respectively. The solid lines show the change in the applied potential to the WO3 electrode. In Fig. 6(a), the potential applied to WO3 ranged from 2.01 to 2.19 V during 1000 cycling, and in Fig. 6(b), the potential applied to WO3 ranged from 0.74 to 1.32 V during the 1000 cycling. If the charge capacity for each electrode was different, the range of applied electrode potential to the electrode, which has a low charge capacity, was expected to be wider than that to the other electrode. The number of reactions
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occurring in both electrodes should be equal in an electrochemical cell. Therefore, the electrode material with the low charge capacity required a more positive potential than the other electrode material to deintercalate the Li+ ion from the material and a more negative potential to intercalate the Li+ ion into the material. In the half-cell test, the charge capacity of V2O5 (51 mC/cm2) was higher than that of WO3 (31 mC/cm2), and that of VTi44 (2.5 mC/cm2) was lower than that of WO3. These charge capacities are consistent with the initial results of WO3/V2O5 and WO3/VTi44. In both cases, the potential ranges applied to WO3 increased (2.01–2.19, 0.74–1.32 V), and that to the counter electrode (V2O5 and VTi44) decreased (0.99–0.81, 2.26–1.68 V). Furthermore, in the case of WO3/V2O5, the difference in the electrode potential range applied to each electrode increased from 1.02 to 1.38 V. However, in the case of WO3/VTi44, the difference in the electrode potential range applied to the working and counter electrodes decreased from 1.52 to 0.36 V. This showed that there was a larger difference in the charge capacities of each electrode in the WO3/V2O5 and a lower difference in WO3/VTi44. Therefore, it is believed that the degradation of WO3/V2O5 was due to the widened difference in charge capacities of each electrode, and the slight degradation of WO3/VTi44 was due to the lower difference in charge capacity of each electrode. In the case of WO3/VTi44 in Fig. 5(a) and (b), the charge capacity of WO3/VTi44 was not changed but the transmittance of WO3/VTi44 was enhanced after the 1000 cycling. Fig. 6(b) suggests a reason for this abnormal result. WO3 is the main electrochromic material in WO3/VTi44. After 1000 cycles, the potential range applied to the WO3 electrode was increased from 0.74 to 1.32 V. Therefore, the transmittance modulation caused by WO3 coloring/bleaching was enhanced after the 1000 cycling. In addition, the initial small change in color was due to the narrow potential range applied to WO3. In the case of WO3/V2O5, the potential range applied to WO3 was wide, but the transmittance modulation was decreased abruptly after the 1000 cycling. Applying an excessive potential to an electrode material causes early degradation. The potential range applied to the WO3 was too wide during the 1000 cycling operation. Moreover, in the half-cell test, WO3 film can be damaged easily when the potential applied to WO3 is 4+1.0 V or o1.0 V vs. Ag/AgCl (sat. KCl). Therefore, this was due to the charge capacity degradation of WO3.
4. Conclusion In the present study, V–Ti oxides with various compositions were prepared by co-sputtering and characterized by FE-SEM and XRD. The electrochromic properties of these oxides were then measured using an electrochemical half-cell test and a semi fullcell test. The V–Ti oxides with various compositions deposited by co-sputtering were all amorphous. In the half-cell test, the V–Ti oxides showed lower charge capacities and transmittance modulations than V2O5. The full-cell tests for V–Ti oxides with tungsten oxide were carried out by forming a semi fullcell instead of assembling a full device. In this experiment, the WO3/VTi44 sample containing the largest amount of titanium showed the highest electrochromic performance and the most durable performance of the cycling tests. Overall, the reversibility and the durability of the electrochromic devices were improved by increasing the titanium content in V–Ti oxides. Fig. 6. Applied electrode potentials of (a) WO3/V2O5 and (b) WO3/VTi44 before (initial) and after (final) the 1000 cycling test in the semi full-cell. The filled and empty points show that positive and negative potentials were applied to the WO3 electrode, respectively. The square points show the electrode potentials applied to WO3 and the triangular points show the electrode potential applied to V2O5 and VTi44.
Acknowledgements This work was supported by the Division of Advanced Batteries in the NGE (next generation engine) Program (Project no.
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10028960-2007-11) and by Korea Research Foundation (Grant # KRF-2004-005-D00064). References [1] S.K. Deb, A novel electrophotographic system, Appl. Opt. Suppl. 3 (1969) 192–195. [2] J.S.E.M. Svensson, C.G. Granqvist, Electrochromic coatings for "smart windows", Sol. Energy Mater. 12 (1985) 391–402. [3] L.F. Reyes, S. Saukko, A. Hoel, V. Lantto, C.G. Granqvist, Structure engineering of WO3 nanoparticles for porous film applications by advanced reactive gas deposition, J. Eur. Ceram. Soc. 24 (2004) 1415–1419. [4] X. Wei, P.K. Shen, Electrochromics of single crystalline WO3–H2O nanorods, Electrochem. Commun. 8 (2006) 293–298. [5] K. West, B. Zauchau-Christiansen, S.V. Skaaruo, Lithium insertion in sputtered vanadium oxide film, Solid State Ionics 57 (1992) 41–47. [6] M. Rubin, K. von Rottkay, S.-J. Wen, N. Ozer, J. Slack, Optical indices of lithiated electrochromic oxides, Sol. Energy Mater. Sol. Cells 54 (1998) 49–57. [7] E. Masetti, F. Varsano, F. Decker, Sputter-deposited cerium vanadium mixed oxide as counter-electrode for electrochromic devices, Electrochim. Acta 44 (1999) 3117–3119. [8] A.M. Salvi, F. Decker, F. Varsano, G. Speranza, Use of XPS for the study of cerium–vanadium (electrochromic) mixed oxides, Surf. Interface Anal. 31 (2001) 255–264.
[9] E. Avendano, A. Azens, G.A. Niklasson, C.G. Granqvist, Proton diffusion and electrochromism in hydrated NiOy and Ni1–xVxOy thin films, J. Electrochem. Soc. 152 (2005) F203–F212. [10] I. Turhan, F.Z. Tepehan, G.G. Tepehan, Effect of V2O5 content on the optical, structural and electrochromic properties of TiO2 and ZrO2 thin films, J. Mater. Sci. 40 (2005) 1359–1362. [11] A. Turkovic, M. Pavlovic, M. Ivanda, M. Gaberscek, Z.C. Orel, Influence of intercalated lithium on structural and electrical properties of V2O5, mixed V/Ce oxide, and Fe2O3, J. Electrochem. Soc. 153 (2006) A122–A126. [12] Y. Iida, Y. Kaneko, Y. Kanno, Fabrication of pulsed-laser deposited V2O5 thin films for electrochromic devices, J. Mater. Process. Technol. 197 (2008) 261–267. [13] K.-C. Cheng, F.-R. Chen, J.-J. Kai, V2O5 nanowires as a functional material for electrochromic device, Sol. Energy Mater. Sol. Cells 90 (2006) 1156–1165. [14] M.S. Burdis, Properties of sputtered thin films of vanadium–titanium oxide for use in electrochromic windows, Thin Solid Films 311 (1997) 286–298. [15] Z.C. Orel, New counter electrode prepared as vanadium oxide and V/Ce oxide films: preparation and characterization, Solid State Ionics 116 (1999) 105–116. [16] S.J. Yoo, J.W. Lim, Y.-E. Sung, Improved electrochromic devices with an inorganic solid electrolyte protective layer, Sol. Energy Mater. Sol. Cells 90 (2006) 477–484. [17] N. Ozer, C.M. Lampert, Electrochromic characterization of sol–gel deposited coatings, Sol. Energy Mater. Sol. Cells 54 (1998) 147–156. [18] F. Bellenger, C. Chemarin, D. Deroo, S. Maximovitch, A.S. Vuk, B. Orel, Insertion of lithium in vanadium and mixed vanadium–titanium oxide films, Electrochim. Acta 46 (2001) 2263–2268.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
High electrochromic performance of co-sputtered vanadium–titanium oxide as a counter electrode Ju Wan Lim, Sung Jong Yoo, Sun Ha Park, Seong Uk Yun, Yung-Eun Sung School of Chemical and Biological Engineering, Seoul National University, San 56-1, Silim-dong, Gwanak-gu, Seoul, Republic of Korea
a r t i c l e in fo
abstract
Article history: Received 30 September 2008 Received in revised form 2 March 2009 Accepted 7 March 2009 Available online 10 April 2009
This study examined the material and electrochromic properties of vanadium–titanium oxides (V–Ti oxides) as a counter electrode material in electrochromic devices. These oxides were deposited on an ITO substrate using a co-sputtering method at different levels of RF power. Electrochemical experiments of these oxides were carried out using half-cell and semi full-cell tests which are good methods for measuring the potential applied to each electrode. The change in electrochromic properties after 1000 cycles of a semi full-cell test was examined. Reversibility and durability of an electrochromic device were improved by increasing the titanium content in V–Ti oxides. & 2009 Elsevier B.V. All rights reserved.
Keywords: Vanadium oxide Vanadium–titanium oxide Electrochromism Counter electrode Electrode potential
1. Introduction Materials with optical properties that can be changed by external stimulation (light, change of temperature and electric potential) are called chromogenic. The first report of an electrochromic material, tungsten oxide, was published approximately 40 years ago by Deb [1]. Initially, the optical absorption of a visible ray can be changed broadly by the absorption and emission of an electrical charge. Electrochromism has attracted considerable attention in research. In the mid-1980s, there was some interest in placing large windows on the ceiling of buildings. The interest in electrochromism was increased significantly by the concept of a ‘smart window’, which was invented to reduce energy consumption in buildings [2]. For electrochromic materials, there are many species, such as organic, inorganic and metal organic complexes. Among them, the electrochromic materials used in electrochromic windows are generally inorganic materials owing to the thermal stability they possess. In addition, inorganic electrochromic materials can generally be divided into two parts: cathodic coloring materials and anodic coloring materials. Cathodic coloring materials are oxides of W, Nb, Mo, Ti, Ta, etc. and anodic coloring materials are oxides of Cr, Mn, Fe, Co, Rh, Ir and Ni. Among them, tungsten trioxide (WO3) is the material most studied. Tungsten trioxide has a similar crystal structure to perovskite [3]. The electrochromic
Corresponding author. Tel.: +82 2 880 1889; fax: +82 2 888 1604.
E-mail address: [email protected] (Y.-E. Sung). 0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.03.008
properties (color change, response time and reversibility of lithium intercalation) of crystal tungsten trioxide are poor. However, the electrochromic properties of amorphous tungsten trioxide are superior to any other metal oxide examined [4]. Vanadium oxide (V2O5) has been actively studied for use in secondary batteries [5] due to a high charge capacity. Currently, vanadium oxide is used primarily as an anodic material for electrochromic devices. However, the demerits of pure vanadium oxide (durability and transparency) make it difficult for use as a counter electrode material in an EC device. Therefore, many research groups have mixed vanadium oxide with other materials, such as titanium oxide, cerium oxide, zirconium oxide, iron oxide and nickel oxide [6–13]. Among the demerits of vanadium oxide, cycling durability is the main problem preventing commercialization. Generally, the materials used to enhance the cycling durability of a material are titanium oxide and cerium oxide [14,15]. Cerium oxide is used as an additive material for the counter electrodes of EC devices due to its transparency. Its charge capacity, however, is very low. Therefore, it is difficult to maintain a sufficient charge when the charge capacity of the working electrode is high. Titanium oxide is used as an additive material to enhance electrical conductivity and prevent crystallization of a material during cycling. Therefore, it can be used to improve cycling durability. In this study, V–Ti oxides were deposited by RF magnetron co-sputtering. The composition of vanadium and titanium was varied to control the power ratio of the two targets. The electrochemical and optical properties of these samples were measured using a half-cell test, and their cycling durability was measured using a semi full-cell test. In the case of the semi full-
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cell test, it is easy to determine the level of electrode degradation. Moreover, the potential applied to each electrode could be measured using two electrode experiments.
Table 1 Abbreviations of the V–Ti oxides, compositions of the V–Ti oxides by EDS and film thicknesses of the V–Ti oxides by FE-SEM. Abbreviations
RF power of V2O5 (W)
RF power of TiO2 (W)
Ti:V atomic ratio
Film thickness (nm)
V2O5 VTi01 VTi07 VTi17 VTi33 VTi44
300 300 300 300 300 300
0 50 80 130 200 300
0:100 1:99 7:93 17:83 33:67 44:56
270 163 177 158 186 202
2. Experiment The V–Ti oxides and tungsten oxide were deposited by RF magnetron sputtering (Korea Vacuum Tech.) onto a glass precoated with indium–tin oxide (ITO) with a resistance of 10 O/m. The base pressure of the deposition chamber was 5 106 Torr. Pre-sputtering prior to deposition was carried out for more than 30 min under the same deposition conditions to clean the targets and ensure steady state deposition. The Ar pressure was maintained at 10 mTorr during pre-sputtering and sputtering. Substrate rotation (11 rpm) assured film uniformity, and the sputtering angle to the substrate was 521. Vanadium oxide thin film was prepared using a V2O5 target. Sputtering deposition was carried out at 300 W RF power for 2 h. The V–Ti oxide thin films were prepared by co-sputtering V2O5 and TiO2 targets. Co-sputtering deposition was carried out for 2 h with 300 W RF power to the V2O5 target, and with 80, 130, 200 and 300 W RF power to the TiO2 target. Tungsten oxide thin films used as the counter electrodes of the full-cell were also prepared by RF magnetron sputtering using a WO3 target with 150 W RF power for 52 min. The deposition conditions and film thickness of the tungsten oxide films (400 nm) were optimized in previous work [16]. The compositions of the deposited films were measured by energy dispersive X-ray spectroscopy (EDS, JSM 6700F) and electron probe X-ray micro analyzer (EPMA, JXA-8900R). Field emission scanning electron microscopy (FE-SEM, JSM 6700F) was used to measure film thickness and surface structure. The structures of the films were examined by X-ray diffraction (XRD, D-MAX2500-PC). The electrochemical and electrochromic properties and optical properties of the deposited films were measured using a potentiostat/galvanostat (AUTOLAB PGSTAT30) and He/Ne laser (l ¼ 632.8 nm), respectively. The electrochemical techniques used in this study were the cyclic voltammetry (CV) and the chronocoulometry (CC), which measured the Li+ intercalation/ deintercalation potential and charge capacity of the films, respectively. CC was also used to test the operation for 1000 cycles. In these electrochemical experiments, 1 M LiClO4 in propylene carbonate was used as the electrolyte. In the case of the half-cell test, the working electrode was connected to V–Ti oxide. The reference and counter electrodes were connected to Ag/AgCl (sat. KCl) and Pt wire, respectively. In the semi full-cell test (the two electrode experiments), the working and counter electrodes were tungsten oxide and V–Ti oxide thin films, respectively. In this case, with the Ag/AgCl (sat. KCl) not connected to the potentiostat, it was immersed in the electrolyte, and a digital voltmeter was used to measure the voltage between the Ag/AgCl and the working electrode (tungsten oxide). The in-situ optical properties were determined during the electrochemical experiment using a He–Ne laser.
3. Results and discussion Table 1 shows the atomic ratios as measured by EDS and EPMA and film thicknesses as measured by FE-SEM of the V–Ti oxides. The titanium content in the V–Ti oxide films increased with the increasing sputtering power to the titanium oxide target but the tendency of the increase was not linear. The Ti/V ratio of the sample with the same sputtering power applied to both targets
(300 W) was 0.8. Therefore, the co-sputtering yields of vanadium oxide and titanium oxide can be considered similar. Abbreviations of the samples are shown in Table 1. Fig. 1 shows the surface FE-SEM images of VTi01, VTi17 and VTi44. The figure also shows the surface morphology of the V–Ti oxides and the boundaries of the oxide lumps. These surface morphologies may allow the lithium ions to intercalate into the oxide film. However, the boundaries of the VTi44 oxide lumps appeared faint, and the surface morphology of the VTi44 was denser than that of the VTi01. The FE-SEM images suggest that the intercalating rate of lithium ions into the VTi01 film would be faster than it was into the VTi44 film. Fig. 2 shows the XRD patterns of the V–Ti oxides. Every XRD pattern showed ITO peaks because the film thickness was at most 270 nm and the substrate was ITO glass. With the exception of ITO, none of the patterns of the V2O5 and V–Ti oxides showed crystal peaks. Therefore, these results showed that V2O5 and V–Ti oxides have an amorphous structure. Fig. 3(a) shows the results of cyclic voltammetry. The current density in the cyclic voltammograms of the samples containing a large amount of vanadium was high due to the charge capacity. In addition, the anodic current peaks were quite broad. These broad peaks were shown in typical cyclic voltammograms of V2O5 when the scan rate was relatively fast (20 mV/s) [17]. Fig. 3(b) shows the electrochromic properties measured using the half-cell test (charge capacity and optical transmittance modulation). In the present study, the charge capacity was measured by chronocoulometry. Therefore, the charge capacity expressed in this paper is not the precise meaning of ‘charge capacity’ but, rather, it is the ‘degree of ease’ of Li+ ion intercalation/ deintercalation. The charge capacities of the V–Ti oxides were maintained at more than 37.6 mC/cm2 until the titanium content was 33 at%, and an abrupt decrease in charge capacity was observed at a titanium content of 44 at%. The aspect of DT at different titanium contents was similar to that observed for the charge capacity. The only difference was an abrupt decrease in DT at low titanium content. Originally, both vanadium oxide and titanium oxide showed cathodic coloration. These transmittance modulations are undesirable when used as a counter electrode for tungsten oxide in an EC device. Therefore, in V–Ti oxide, even a small amount of titanium is desirable for use as a counter electrode. An electrochromic full-cell test was carried out using V–Ti oxide and a semi full-cell test was carried out using tungsten oxide. After conducting the half-cell test of V–Ti oxide, the tungsten oxide was fully reduced, containing lithium ions, and the V–Ti oxide was fully oxidized. The semi full-cell was constructed as shown in Fig. 4. The tungsten and V–Ti oxides were connected to the working and counter electrodes, respectively. The initial semi full-cell test was operated for 5 cycles by applying +1.5 V for 30 s and 1.5 V for 30 s. During this test, the optical transmittance
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Fig. 2. XRD patterns of V2O5 and V–Ti oxides.
Fig. 1. Surface FE-SEM images of (a) VTi01, (b) VTi17 and (c) VTi44. Fig. 3. (a) Cyclic voltammograms and (b) charge capacities and transmittance modulations of V–Ti oxides.
modulation of V–Ti oxide was measured using a He–Ne laser. After an initial 5-cycle test, another 5-cycle test was carried out by measuring the optical transmittance modulation of tungsten oxide. The same set of cycling tests was then operated for 1000 cycles. After the 1000-cycle test, final-state tests of the transmittance modulation measurement were carried out using the same methodology. During the initial and final 10-cycle tests, the applied electrode potential vs. Ag/AgCl (to working electrode
and counter electrode) was measured using a voltmeter. In this experiment, the measured gaps of the applied potential values between the working electrode and the counter electrode were +1.50 and 1.50 V. This means that the conductivity of the electrolyte was high enough to ignore the potential drop that could be caused by the resistance electrolyte.
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Fig. 4. The illustration of the semi full-cell test.
The makeup of the semi full-cell test was more complicated than that of a conventional full device. There are advantages in using a semi full-cell test compared with a conventional full device test: (1) it is possible to measure the potential applied to each electrode, as well as the variations in the potential; (2) it can prevent a short circuit; (3) there is no dismantling process needed to characterize the samples after the cycling operation; and (4) it is possible to know which electrode is responsible for the performance degradation of the full-cell. The initial and final transmittance of the semi full-cell was calculated by multiplying the transmittance of tungsten and V–Ti oxides. The results and charge capacity degradation are plotted in Fig. 5(a) and (b), respectively. In Fig. 5(a), the initial transmittance modulations of the WO3/V–Ti oxide samples were smaller than those of WO3/V2O5, with the exception of the WO3/VTi01 sample. However, the final transmittance modulations of the WO3/V–Ti oxide samples were larger than those of WO3/V2O5. In addition, the initial charge capacities of all WO3/V–Ti oxide samples were lower than that of the WO3/V2O5 sample but the final charge capacities of all the WO3/V–Ti oxide samples were higher than that of the WO3/V2O5 sample. The degradation of charge capacity and transmittance modulation decreased with increasing titanium content, and an activation of transmittance modulation after the cycling test was detected. This shows that the WO3/V–Ti oxide samples were more reversible than the WO3/V2O5 samples. Therefore, for use in electrochromic devices, the V–Ti oxide samples were better counter electrodes for WO3 than for V2O5. Fig. 5(c) shows the response times, which is the time taken to achieve 80% coloration from the bleached state (tc), as well as bleaching from the colored state (tb) of the WO3/V–Ti oxide samples. The initial tb (tbi) of all samples was higher than the initial tc (tci). Bleaching occurred when the lithium ions
Fig. 5. The degradation of (a) the transmittance modulation, (b) charge capacity and (c) response time of WO3/V–Ti oxide samples before and after the 1000 cycling test in the semi full-cell. Tbi/Tci and Tbf/Tcf are the transmittances of the initial bleached/colored state and the final bleached/colored state, respectively. tbi/tci and tbf/tcf are the response times of the initial bleaching/coloring and the final bleaching/coloring processes, respectively.
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deintercalated from tungsten oxide and intercalated into the V–Ti oxide. Generally, the lithium ion intercalation process is slower than the deintercalation process. Therefore, lithium ion intercalation into V–Ti oxide was the slowest process in the initial cycle. This is because the lithium intercalation rate into vanadium oxide is higher than it is into titanium oxide. This phenomenon has previously been reported [18]. However, in the final cycle tb (tbf) became either similar to or less than tc (tcf). According to these results, the cycling operation enhanced the lithium intercalation rate into V–Ti oxide. In the semi full-cell test, potentials applied to each electrode were measured using a voltmeter. During operation, the potentials detected by the voltmeter fluctuated up to the saturation state of the intercalation/deintercalation reaction. Therefore, the measured potentials were the saturated potentials. Fig. 6(a) and (b) shows the changes in the electrode potentials of WO3/V2O5 and WO3/VTi44 before and after the 1000 cycling operation, respectively. The solid lines show the change in the applied potential to the WO3 electrode. In Fig. 6(a), the potential applied to WO3 ranged from 2.01 to 2.19 V during 1000 cycling, and in Fig. 6(b), the potential applied to WO3 ranged from 0.74 to 1.32 V during the 1000 cycling. If the charge capacity for each electrode was different, the range of applied electrode potential to the electrode, which has a low charge capacity, was expected to be wider than that to the other electrode. The number of reactions
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occurring in both electrodes should be equal in an electrochemical cell. Therefore, the electrode material with the low charge capacity required a more positive potential than the other electrode material to deintercalate the Li+ ion from the material and a more negative potential to intercalate the Li+ ion into the material. In the half-cell test, the charge capacity of V2O5 (51 mC/cm2) was higher than that of WO3 (31 mC/cm2), and that of VTi44 (2.5 mC/cm2) was lower than that of WO3. These charge capacities are consistent with the initial results of WO3/V2O5 and WO3/VTi44. In both cases, the potential ranges applied to WO3 increased (2.01–2.19, 0.74–1.32 V), and that to the counter electrode (V2O5 and VTi44) decreased (0.99–0.81, 2.26–1.68 V). Furthermore, in the case of WO3/V2O5, the difference in the electrode potential range applied to each electrode increased from 1.02 to 1.38 V. However, in the case of WO3/VTi44, the difference in the electrode potential range applied to the working and counter electrodes decreased from 1.52 to 0.36 V. This showed that there was a larger difference in the charge capacities of each electrode in the WO3/V2O5 and a lower difference in WO3/VTi44. Therefore, it is believed that the degradation of WO3/V2O5 was due to the widened difference in charge capacities of each electrode, and the slight degradation of WO3/VTi44 was due to the lower difference in charge capacity of each electrode. In the case of WO3/VTi44 in Fig. 5(a) and (b), the charge capacity of WO3/VTi44 was not changed but the transmittance of WO3/VTi44 was enhanced after the 1000 cycling. Fig. 6(b) suggests a reason for this abnormal result. WO3 is the main electrochromic material in WO3/VTi44. After 1000 cycles, the potential range applied to the WO3 electrode was increased from 0.74 to 1.32 V. Therefore, the transmittance modulation caused by WO3 coloring/bleaching was enhanced after the 1000 cycling. In addition, the initial small change in color was due to the narrow potential range applied to WO3. In the case of WO3/V2O5, the potential range applied to WO3 was wide, but the transmittance modulation was decreased abruptly after the 1000 cycling. Applying an excessive potential to an electrode material causes early degradation. The potential range applied to the WO3 was too wide during the 1000 cycling operation. Moreover, in the half-cell test, WO3 film can be damaged easily when the potential applied to WO3 is 4+1.0 V or o1.0 V vs. Ag/AgCl (sat. KCl). Therefore, this was due to the charge capacity degradation of WO3.
4. Conclusion In the present study, V–Ti oxides with various compositions were prepared by co-sputtering and characterized by FE-SEM and XRD. The electrochromic properties of these oxides were then measured using an electrochemical half-cell test and a semi fullcell test. The V–Ti oxides with various compositions deposited by co-sputtering were all amorphous. In the half-cell test, the V–Ti oxides showed lower charge capacities and transmittance modulations than V2O5. The full-cell tests for V–Ti oxides with tungsten oxide were carried out by forming a semi fullcell instead of assembling a full device. In this experiment, the WO3/VTi44 sample containing the largest amount of titanium showed the highest electrochromic performance and the most durable performance of the cycling tests. Overall, the reversibility and the durability of the electrochromic devices were improved by increasing the titanium content in V–Ti oxides. Fig. 6. Applied electrode potentials of (a) WO3/V2O5 and (b) WO3/VTi44 before (initial) and after (final) the 1000 cycling test in the semi full-cell. The filled and empty points show that positive and negative potentials were applied to the WO3 electrode, respectively. The square points show the electrode potentials applied to WO3 and the triangular points show the electrode potential applied to V2O5 and VTi44.
Acknowledgements This work was supported by the Division of Advanced Batteries in the NGE (next generation engine) Program (Project no.
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