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Electrochromism of vanadium oxide films doped by rare-earth (Pr, Nd, Sm, Dy) oxides
Journal of Electroanalytical Chemistry 559 (2003) 83 /86 www.elsevier.com/locate/jelechem
Electrochromism of vanadium oxide films doped by rare-earth (Pr, Nd, Sm, Dy) oxides Wenqing Chen *, Yoshikazu Kaneko Center for Crystal Science and Technology, Faculty of Engineering, Yamanashi University, Miyamae 7, Kofu, Yamanashi 400-8155, Japan Received 30 April 2002; received in revised form 9 October 2002; accepted 17 October 2002
Abstract To enhance the electrochromism of vanadium oxide films used as a counter electrode of tungsten oxide (WO3) films in electrochromic (EC) devices, V/M (Pr, Nd, Sm, Dy) oxide films were synthesized by vacuum evaporation and annealed at 400 8C. The effects of 25% rare earth addition on electrochromism, structure and optical properties of the vanadium oxide films were investigated. By using cyclic voltammetry, it was found that all V /M (Pr, Nd, Sm, Dy) oxide films heated at 400 8C for 30 min showed ten times better cycling stabilities in PC solution than vanadium oxide films. In situ UV /vis /NIR spectroelectrochemical measurements confirmed that all V /M oxide films had higher transmittances than vanadium oxide film under the colored state in vis and NIR. Of the V/M oxide films, V/Sm oxide film showed a very small coloration efficiency (CE; 0.6) in vis and NIR, and showed good ionic and electronic conduction, implying its potential application as a counter electrode in EC devices. From XRD results, the formation of orthovanadate SmVO4 and DyVO4 was testified in V /Sm and V /Dy oxide films, respectively. Moreover, a close relationship between the doping effects of M3 and its radius was found. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochromism; Film; Rare earth oxides; V2O5
1. Introduction As a counter electrode of WO3 in electrochromic (EC) devices, vanadium oxide films have been widely investigated [1]. To date vanadium oxide films have generally been prepared by vacuum evaporation, sol /gel processes, sputtering and chemical vapor deposition. Vanadium oxide films prepared by these methods exhibited less reversibility than tungsten oxide films (beyond 1200 cycles), for which, cyclic reversibility has been demonstrated through FT-IR by Macek and Orel [2]. Another drawback of vanadium oxide films is that they show mixed anodic/cathodic electrochromism because of the combined effects of bandgap widening and the appearance of a polaron absorption extending from vis to near and mid-IR [3]. In order to solve these problems, researchers have attempted to minimize near infrared coloration of reduced V2O5 films by adding dopants
* Corresponding author. Tel./fax: /81-55-220-8610. E-mail address: [email protected] (W. Chen).
such as Ce [4], Fe [5], Ti [6], Nb and Cr [7]. According to the study of Opara et al. [4], the CeVO4 films synthesized by sol /gel processes show good electrochromism. They reported that the CeO8 units in the CeVO4-w formed prevent direct kinematic coupling of different ions in the D4h unit, consequently there is a VO3 4 negligible cathodic coloring in vis and NIR. Because the rare-earth orthovanadates MVO4 except LaVO4 have the same structure as CeVO4 and can be formed easily, vanadium oxide films can be expected to have a negligible cathodic coloring in vis and NIR by doping rare-earth ions. The chemical characteristics of lanthanide are very similar in their special f orbits, even if the number of 4f electrons is different. So it is very interesting to examine the influence of lanthanide ions with different radii on the electrochromism of vanadium oxide films. In this study, rare-earth ions Pr3, Nd3, Sm3 and Dy3 were added into vanadium oxide films separately to improve their reversibility and to decrease their coloration in vis and NIR. We also wanted to investigate the formation of MVO4 in V /M oxide films. A
0022-0728/03/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(02)01283-4
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further objective was to explore the relationship between the doping effect and the radius of M3 by analyzing the electrochromism and structures as well as optical properties of V /M oxide films.
2. Experiment The commercial reagents (99.99%) of V2O5 and rareearth oxides were used. V2O5 powder was mixed with Pr2O3, Nd2O3, Sm2O3 and Dy2O3 powders with a V /M molar ratio of 1:0.25, respectively. Then they were heated at 900 8C for 2 h. After being ground they were evaporated onto indium tin oxide (ITO) glass substrates. The vacuum in the chamber was maintained at 5 /10 4 Torr during the evaporation. The films formed were annealed in a furnace at 400 8C for 0.5 and 2 h, respectively. As references, V2O5, Pr2O3, Nd2O3, Sm2O3 and Dy2O3 films were prepared in the same way. Molar ratios of V to M were analyzed by EDX and ICP. The thickness and surface morphology of V /M oxide films were investigated by SEM. Cyclic voltammetry was performed at voltages between /2.0 and /2.0 V relative to Ag j AgCl with a sweep rate of 200 mV s 1. In situ spectroelectrochemical measurements were carried out using a three-electrode cell with two Pt wires as counter and reference electrodes installed in a compartment of the scanning spectrophotometer (Shimadzu RF-5300PC). The spectral region used in this work was 300/1050 nm. The microstructures of these films were analyzed by XRD. The photoluminescence spectra in the wavelength range of 300 /650 nm were recorded under 320 nm photoexcitation by a spectrofluorophotometer (Shimadzu UV-3100PC).
3. Results and discussion According to the results of SEM and EDX about the surface of the V /M oxide films, it was verified that uniform V /Pr, V /Nd, V /Sm and V /Dy oxide films could be synthesized by vacuum evaporation. The ICP result showed that the molar ratio of V to M was about 1:0.25. Films as-evaporated were brown, then became yellow /green after being heated at 400 8C. Such a color change, using the same experimental method was also observed by Guan et al. [8]. They pointed out that this is because there are large amounts of V3 in as-evaporated films, and they are oxidized to V4 and V5 after being heated at 400 8C. Fig. 1 gives the XRD patterns of the V /M oxide films obtained (thickness about 700 nm), revealing the existence of V5O9 in V /Pr oxide film, V6O11 in V /Nd oxide film, SmVO4 in V /Sm oxide film, and DyVO4 in V /Dy oxide film. Comparing the lattice constant of these crystals with the radii of Pr3, Nd3, Sm3 and
Fig. 1. XRD patterns of V /Pr (a), V /Nd (b), V /Sm (c), V /Dy (d) oxide films heated at 400 8C for 2 h (thickness about 700 nm).
Dy3, it was found that the lattice constant became smaller with decreasing radius of M3, and the smaller M3 ions enabled MVO4 to be formed. It is probable that smaller M3 ions would insert more easily into the vanadium oxide molecules to form M /V bonds than larger ions. The relationship between the ionic radius and lattice constant is shown in Fig. 2. UV /vis /NIR spectra of V /M oxide films and V oxide film heated at 400 8C for 30 min are shown in Fig. 3, from which it can be observed that in vis and NIR, V /M oxide films have absorptions similar to that of the V oxide film. These absorptions of V /M oxide films may be principally attributed to the V oxides. A strong absorption in the wavelength range of 400 /668 nm signifies a V2O5 optical bandgap, and a weak broad absorption in NIR is caused by V2O5 polaron absorption, whose magnitude depends on the density of V4 ions. It is also observed that the absorption peaks of V / M oxide films in vis and NIR shift to shorter wavelength with decreasing M3 ionic radius. The blue shift in vis is probably due to the effect of M3 ion on the V2O5 optical bandgap, and the V2O5 optical bandgap becomes larger with insertion of M3 ion. As stated, it is easier for the smaller M3 ions to insert into the vanadium
Fig. 2. The relationship between the absorption wavelength of V /M oxide films and the M3 ionic radius.
W. Chen, Y. Kaneko / Journal of Electroanalytical Chemistry 559 (2003) 83 /86
Fig. 3. Absorption spectra of V (a), V /Pr (b), V /Nd (c), V /Sm (d), V /Dy (e) oxide films heated at 400 8C for 0.5 h.
oxide molecules, so the smaller M3 ions enlarge the V2O5 optical bandgap much more than bigger ones. In the same way, the smaller M3 ions give more effect on polaron absorption than larger ions. In the same way, the smaller M3 have more effect on polaron absorption than larger ions. The relationships between wavelength and ionic radius are illustrated in Fig. 4. In situ spectroelectrochemical measurements show that the transmittances of V /M oxide films in both the colored and the bleached state are much higher than that of V2O5 film in the wavelength range of 500 /1050 nm (Fig. 5). A decrease in coloration was also observed in Ti/La and Ti/Pr oxide films by Azens and Kullman [9]. This may be attributed to dilution effects causing an absence of polaron absorption in both the colored and bleached state. The spectral coloration efficiency (CE) in Table 1 is defined as CE /(DD /Q ) /log(Tb/Tc)/Q . Tb and Tc are averaged spectral transmittances between 500 and 1050 nm in the bleached (Tb) and colored states (Tc). Small CEs of SmVO4 (0.6) and NdVO4 (1.1) oxide films suggest their application as a counter electrode in EC devices. V /Pr, V /Nd and V /Dy oxide films, which correspond to the V2O5 film, show yellow /green in the
Fig. 4. The relationship between the absorption wavelength of V /M oxide films and the M3 ionic radius.
85
Fig. 5. In situ UV /vis spectroelectrochemical responses of V (a), V / Pr (b), V/Nd (c), V /Sm (d), V /Dy (e) oxide films charged at /1.0 V (---) and discharged at /2.0 V ( /) vs. Ag j AgCl. Table 1 Coloration efficiencies of V /M oxide films Film 2
CE/cm C
1
V /Pr
V /Nd
V/Sm
V/Dy
4.2
1.1
0.6
3.4
colored state and black /green in the bleached state. The color of V /Sm oxide film changes from yellow /green in the cathodic state to red/brown in the anodic state. The voltammograms of M2O3 films appear to show little ionic and electronic conduction in PC solution. Thus it may be concluded that M3 is stable, and is hard to reduce to M2. In order to compare film stability, the films were cycled many times. Fig. 6 shows cyclic voltammograms of V /M oxide films and vanadium oxide film. A large decay is found in the vanadium
Fig. 6. Cyclic voltammogram of V (a), V /Pr (b), V /Nd (c), V /Sm (d), V /Dy (e) oxide films heated at 400 8C for 0.5 h.
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W. Chen, Y. Kaneko / Journal of Electroanalytical Chemistry 559 (2003) 83 /86
absorption peak of the V /O double bond. The blue shift of the photoluminescence absorption peak of the V /O double bond may be caused by reduction effects of M3 on the V /O double bond, and this reduction effect increases with decreasing M3 ionic radius.
4. Conclusion V /M (Pr, Nd, Sm and Dy) oxide films were successfully prepared by vacuum evaporation. It can be concluded that:
Fig. 7. Luminescence spectral of ITO glass (f) and V (a), V/Pr (b), V / Nd (c), V/Sm (d), V/Dy (e) oxide films heated at 400 8C for 2 h (thickness about 700 nm).
oxide film cycled ten times, in V /Pr film cycled 90 times, in V /Nd film cycled 150 times, in V /Sm film cycled 140 times and in V /Dy film cycled 140 times, respectively. Consequently, it is supposed that V /M oxide films showed about ten times better cycling stability in PC solution than vanadium oxide film. V /Sm and V /Dy oxide films exhibited better ionic and electronic conduction than V /Pr and V /Nd oxide films. This could be due to the formation of SmVO4 and DyVO4. It is also found that the ionic and electronic conductivities of V / Sm and V /Dy oxide films are close to those of V oxide film, but the reversibility of the former two films is much better than that of V oxide film. The photoluminescence of V and V /M oxide films was measured. As shown in Fig. 7, ITO glass, as a substrate of the film, shows photoluminescence peaks between 450 and 490 nm, so peaks between 450 and 490 nm in all film spectra might be caused by ITO glass. Apart from V /Sm oxide film, all the films show photoluminescence peaks near 520 nm. It is also observed that the photoluminescence peaks of V /M oxide films shift to shorter wavelength, and their intensities become weaker with decreasing ionic radius of M3. Because only a small quantity of M3 was added, peaks near 520 nm in V /M oxide films are assigned to the V /O double bonds which have been reported by Anpo et al. [10]. It has been elucidated that a change of the environment around the V /O double bond can cause a shift of the photoluminescence
1) V /Sm oxide film showed a very small CE (0.6) in vis and NIR, and showed good ionic and electronic conduction, implying its application as a counter electrode in EC devices. 2) The formation of MVO4 in V /M oxide films is helpful to improve their ionic and electronic conductivities and reversibility. 3) The lattice constants in V /M oxide films become smaller with decreasing M3 ionic radius. 4) The absorption peaks of V /M oxide films in vis and NIR are found to shift to shorter wavelengths with decreasing M3 ionic radius. 5) With decreasing M3 ionic radius, the photoluminescence peaks of V /M oxide films shift to shorter wavelengths, and the intensities of these peaks become weaker.
References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] M. Macek, B. Orel, Tr. J. Chem. 22 (1998) 67. [3] U. Opara Krasovec, B. Orel, A. Surca, N. Bukovec, R. Reisfeld, Solid State Ionics 118 (1999) 195. [4] U. Opara Krasovec, B. Orel, A. Surca, N. Bukovec, R. Reisfeld, Solid State Ionics 118 (1999) 195. [5] M. Macek, Ph.D. thesis, University of Ljubljana, Slovenia, 1997. [6] H. Hiroshima, S. Kamimura, Mater. Res. Soc. Symp. Proc. 121 (1988) 779. [7] S.F. Cogan, R.D. Rauch, N.M. Nguyen, T.D. Plante, J.D. Westwood, J. Electrochem. Soc. 140 (1993) 112. [8] Z.S. Guan, J.N. Yao, Y.A. Yang, B.H. Loo, J. Electroanal. Chem. 443 (1998) 175. [9] A. Azens, L. Kullman, Solar Energy Mater. Solar Cells 56 (1999) 193. [10] M. Anpo, I. Tanahashi, Y. Kubokawa, J. Phys. Chem. 84 (1980) 3441.
Electrochromism of vanadium oxide films doped by rare-earth (Pr, Nd, Sm, Dy) oxides Wenqing Chen *, Yoshikazu Kaneko Center for Crystal Science and Technology, Faculty of Engineering, Yamanashi University, Miyamae 7, Kofu, Yamanashi 400-8155, Japan Received 30 April 2002; received in revised form 9 October 2002; accepted 17 October 2002
Abstract To enhance the electrochromism of vanadium oxide films used as a counter electrode of tungsten oxide (WO3) films in electrochromic (EC) devices, V/M (Pr, Nd, Sm, Dy) oxide films were synthesized by vacuum evaporation and annealed at 400 8C. The effects of 25% rare earth addition on electrochromism, structure and optical properties of the vanadium oxide films were investigated. By using cyclic voltammetry, it was found that all V /M (Pr, Nd, Sm, Dy) oxide films heated at 400 8C for 30 min showed ten times better cycling stabilities in PC solution than vanadium oxide films. In situ UV /vis /NIR spectroelectrochemical measurements confirmed that all V /M oxide films had higher transmittances than vanadium oxide film under the colored state in vis and NIR. Of the V/M oxide films, V/Sm oxide film showed a very small coloration efficiency (CE; 0.6) in vis and NIR, and showed good ionic and electronic conduction, implying its potential application as a counter electrode in EC devices. From XRD results, the formation of orthovanadate SmVO4 and DyVO4 was testified in V /Sm and V /Dy oxide films, respectively. Moreover, a close relationship between the doping effects of M3 and its radius was found. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochromism; Film; Rare earth oxides; V2O5
1. Introduction As a counter electrode of WO3 in electrochromic (EC) devices, vanadium oxide films have been widely investigated [1]. To date vanadium oxide films have generally been prepared by vacuum evaporation, sol /gel processes, sputtering and chemical vapor deposition. Vanadium oxide films prepared by these methods exhibited less reversibility than tungsten oxide films (beyond 1200 cycles), for which, cyclic reversibility has been demonstrated through FT-IR by Macek and Orel [2]. Another drawback of vanadium oxide films is that they show mixed anodic/cathodic electrochromism because of the combined effects of bandgap widening and the appearance of a polaron absorption extending from vis to near and mid-IR [3]. In order to solve these problems, researchers have attempted to minimize near infrared coloration of reduced V2O5 films by adding dopants
* Corresponding author. Tel./fax: /81-55-220-8610. E-mail address: [email protected] (W. Chen).
such as Ce [4], Fe [5], Ti [6], Nb and Cr [7]. According to the study of Opara et al. [4], the CeVO4 films synthesized by sol /gel processes show good electrochromism. They reported that the CeO8 units in the CeVO4-w formed prevent direct kinematic coupling of different ions in the D4h unit, consequently there is a VO3 4 negligible cathodic coloring in vis and NIR. Because the rare-earth orthovanadates MVO4 except LaVO4 have the same structure as CeVO4 and can be formed easily, vanadium oxide films can be expected to have a negligible cathodic coloring in vis and NIR by doping rare-earth ions. The chemical characteristics of lanthanide are very similar in their special f orbits, even if the number of 4f electrons is different. So it is very interesting to examine the influence of lanthanide ions with different radii on the electrochromism of vanadium oxide films. In this study, rare-earth ions Pr3, Nd3, Sm3 and Dy3 were added into vanadium oxide films separately to improve their reversibility and to decrease their coloration in vis and NIR. We also wanted to investigate the formation of MVO4 in V /M oxide films. A
0022-0728/03/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0728(02)01283-4
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W. Chen, Y. Kaneko / Journal of Electroanalytical Chemistry 559 (2003) 83 /86
further objective was to explore the relationship between the doping effect and the radius of M3 by analyzing the electrochromism and structures as well as optical properties of V /M oxide films.
2. Experiment The commercial reagents (99.99%) of V2O5 and rareearth oxides were used. V2O5 powder was mixed with Pr2O3, Nd2O3, Sm2O3 and Dy2O3 powders with a V /M molar ratio of 1:0.25, respectively. Then they were heated at 900 8C for 2 h. After being ground they were evaporated onto indium tin oxide (ITO) glass substrates. The vacuum in the chamber was maintained at 5 /10 4 Torr during the evaporation. The films formed were annealed in a furnace at 400 8C for 0.5 and 2 h, respectively. As references, V2O5, Pr2O3, Nd2O3, Sm2O3 and Dy2O3 films were prepared in the same way. Molar ratios of V to M were analyzed by EDX and ICP. The thickness and surface morphology of V /M oxide films were investigated by SEM. Cyclic voltammetry was performed at voltages between /2.0 and /2.0 V relative to Ag j AgCl with a sweep rate of 200 mV s 1. In situ spectroelectrochemical measurements were carried out using a three-electrode cell with two Pt wires as counter and reference electrodes installed in a compartment of the scanning spectrophotometer (Shimadzu RF-5300PC). The spectral region used in this work was 300/1050 nm. The microstructures of these films were analyzed by XRD. The photoluminescence spectra in the wavelength range of 300 /650 nm were recorded under 320 nm photoexcitation by a spectrofluorophotometer (Shimadzu UV-3100PC).
3. Results and discussion According to the results of SEM and EDX about the surface of the V /M oxide films, it was verified that uniform V /Pr, V /Nd, V /Sm and V /Dy oxide films could be synthesized by vacuum evaporation. The ICP result showed that the molar ratio of V to M was about 1:0.25. Films as-evaporated were brown, then became yellow /green after being heated at 400 8C. Such a color change, using the same experimental method was also observed by Guan et al. [8]. They pointed out that this is because there are large amounts of V3 in as-evaporated films, and they are oxidized to V4 and V5 after being heated at 400 8C. Fig. 1 gives the XRD patterns of the V /M oxide films obtained (thickness about 700 nm), revealing the existence of V5O9 in V /Pr oxide film, V6O11 in V /Nd oxide film, SmVO4 in V /Sm oxide film, and DyVO4 in V /Dy oxide film. Comparing the lattice constant of these crystals with the radii of Pr3, Nd3, Sm3 and
Fig. 1. XRD patterns of V /Pr (a), V /Nd (b), V /Sm (c), V /Dy (d) oxide films heated at 400 8C for 2 h (thickness about 700 nm).
Dy3, it was found that the lattice constant became smaller with decreasing radius of M3, and the smaller M3 ions enabled MVO4 to be formed. It is probable that smaller M3 ions would insert more easily into the vanadium oxide molecules to form M /V bonds than larger ions. The relationship between the ionic radius and lattice constant is shown in Fig. 2. UV /vis /NIR spectra of V /M oxide films and V oxide film heated at 400 8C for 30 min are shown in Fig. 3, from which it can be observed that in vis and NIR, V /M oxide films have absorptions similar to that of the V oxide film. These absorptions of V /M oxide films may be principally attributed to the V oxides. A strong absorption in the wavelength range of 400 /668 nm signifies a V2O5 optical bandgap, and a weak broad absorption in NIR is caused by V2O5 polaron absorption, whose magnitude depends on the density of V4 ions. It is also observed that the absorption peaks of V / M oxide films in vis and NIR shift to shorter wavelength with decreasing M3 ionic radius. The blue shift in vis is probably due to the effect of M3 ion on the V2O5 optical bandgap, and the V2O5 optical bandgap becomes larger with insertion of M3 ion. As stated, it is easier for the smaller M3 ions to insert into the vanadium
Fig. 2. The relationship between the absorption wavelength of V /M oxide films and the M3 ionic radius.
W. Chen, Y. Kaneko / Journal of Electroanalytical Chemistry 559 (2003) 83 /86
Fig. 3. Absorption spectra of V (a), V /Pr (b), V /Nd (c), V /Sm (d), V /Dy (e) oxide films heated at 400 8C for 0.5 h.
oxide molecules, so the smaller M3 ions enlarge the V2O5 optical bandgap much more than bigger ones. In the same way, the smaller M3 ions give more effect on polaron absorption than larger ions. In the same way, the smaller M3 have more effect on polaron absorption than larger ions. The relationships between wavelength and ionic radius are illustrated in Fig. 4. In situ spectroelectrochemical measurements show that the transmittances of V /M oxide films in both the colored and the bleached state are much higher than that of V2O5 film in the wavelength range of 500 /1050 nm (Fig. 5). A decrease in coloration was also observed in Ti/La and Ti/Pr oxide films by Azens and Kullman [9]. This may be attributed to dilution effects causing an absence of polaron absorption in both the colored and bleached state. The spectral coloration efficiency (CE) in Table 1 is defined as CE /(DD /Q ) /log(Tb/Tc)/Q . Tb and Tc are averaged spectral transmittances between 500 and 1050 nm in the bleached (Tb) and colored states (Tc). Small CEs of SmVO4 (0.6) and NdVO4 (1.1) oxide films suggest their application as a counter electrode in EC devices. V /Pr, V /Nd and V /Dy oxide films, which correspond to the V2O5 film, show yellow /green in the
Fig. 4. The relationship between the absorption wavelength of V /M oxide films and the M3 ionic radius.
85
Fig. 5. In situ UV /vis spectroelectrochemical responses of V (a), V / Pr (b), V/Nd (c), V /Sm (d), V /Dy (e) oxide films charged at /1.0 V (---) and discharged at /2.0 V ( /) vs. Ag j AgCl. Table 1 Coloration efficiencies of V /M oxide films Film 2
CE/cm C
1
V /Pr
V /Nd
V/Sm
V/Dy
4.2
1.1
0.6
3.4
colored state and black /green in the bleached state. The color of V /Sm oxide film changes from yellow /green in the cathodic state to red/brown in the anodic state. The voltammograms of M2O3 films appear to show little ionic and electronic conduction in PC solution. Thus it may be concluded that M3 is stable, and is hard to reduce to M2. In order to compare film stability, the films were cycled many times. Fig. 6 shows cyclic voltammograms of V /M oxide films and vanadium oxide film. A large decay is found in the vanadium
Fig. 6. Cyclic voltammogram of V (a), V /Pr (b), V /Nd (c), V /Sm (d), V /Dy (e) oxide films heated at 400 8C for 0.5 h.
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absorption peak of the V /O double bond. The blue shift of the photoluminescence absorption peak of the V /O double bond may be caused by reduction effects of M3 on the V /O double bond, and this reduction effect increases with decreasing M3 ionic radius.
4. Conclusion V /M (Pr, Nd, Sm and Dy) oxide films were successfully prepared by vacuum evaporation. It can be concluded that:
Fig. 7. Luminescence spectral of ITO glass (f) and V (a), V/Pr (b), V / Nd (c), V/Sm (d), V/Dy (e) oxide films heated at 400 8C for 2 h (thickness about 700 nm).
oxide film cycled ten times, in V /Pr film cycled 90 times, in V /Nd film cycled 150 times, in V /Sm film cycled 140 times and in V /Dy film cycled 140 times, respectively. Consequently, it is supposed that V /M oxide films showed about ten times better cycling stability in PC solution than vanadium oxide film. V /Sm and V /Dy oxide films exhibited better ionic and electronic conduction than V /Pr and V /Nd oxide films. This could be due to the formation of SmVO4 and DyVO4. It is also found that the ionic and electronic conductivities of V / Sm and V /Dy oxide films are close to those of V oxide film, but the reversibility of the former two films is much better than that of V oxide film. The photoluminescence of V and V /M oxide films was measured. As shown in Fig. 7, ITO glass, as a substrate of the film, shows photoluminescence peaks between 450 and 490 nm, so peaks between 450 and 490 nm in all film spectra might be caused by ITO glass. Apart from V /Sm oxide film, all the films show photoluminescence peaks near 520 nm. It is also observed that the photoluminescence peaks of V /M oxide films shift to shorter wavelength, and their intensities become weaker with decreasing ionic radius of M3. Because only a small quantity of M3 was added, peaks near 520 nm in V /M oxide films are assigned to the V /O double bonds which have been reported by Anpo et al. [10]. It has been elucidated that a change of the environment around the V /O double bond can cause a shift of the photoluminescence
1) V /Sm oxide film showed a very small CE (0.6) in vis and NIR, and showed good ionic and electronic conduction, implying its application as a counter electrode in EC devices. 2) The formation of MVO4 in V /M oxide films is helpful to improve their ionic and electronic conductivities and reversibility. 3) The lattice constants in V /M oxide films become smaller with decreasing M3 ionic radius. 4) The absorption peaks of V /M oxide films in vis and NIR are found to shift to shorter wavelengths with decreasing M3 ionic radius. 5) With decreasing M3 ionic radius, the photoluminescence peaks of V /M oxide films shift to shorter wavelengths, and the intensities of these peaks become weaker.
References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] M. Macek, B. Orel, Tr. J. Chem. 22 (1998) 67. [3] U. Opara Krasovec, B. Orel, A. Surca, N. Bukovec, R. Reisfeld, Solid State Ionics 118 (1999) 195. [4] U. Opara Krasovec, B. Orel, A. Surca, N. Bukovec, R. Reisfeld, Solid State Ionics 118 (1999) 195. [5] M. Macek, Ph.D. thesis, University of Ljubljana, Slovenia, 1997. [6] H. Hiroshima, S. Kamimura, Mater. Res. Soc. Symp. Proc. 121 (1988) 779. [7] S.F. Cogan, R.D. Rauch, N.M. Nguyen, T.D. Plante, J.D. Westwood, J. Electrochem. Soc. 140 (1993) 112. [8] Z.S. Guan, J.N. Yao, Y.A. Yang, B.H. Loo, J. Electroanal. Chem. 443 (1998) 175. [9] A. Azens, L. Kullman, Solar Energy Mater. Solar Cells 56 (1999) 193. [10] M. Anpo, I. Tanahashi, Y. Kubokawa, J. Phys. Chem. 84 (1980) 3441.