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Structural and electrochemical properties of opaque sol–gel deposited WO3 layers
Applied Surface Science 218 (2003) 275–280
Structural and electrochemical properties of opaque sol–gel deposited WO3 layers George Leftheriotisa, Spiros Papaefthimioua,*, Panayiotis Yianoulisa, Angeliki Siokoub, Dimitris Kefalasb a Department of Physics, University of Patras, 26500 Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation of Research and Technology Hellas, Stadiou St., Platani Achaias 26500, Greece b
Received 28 January 2003; received in revised form 17 April 2003; accepted 18 April 2003
Abstract The preparation of thick and opaque WO3 layers by a simple sol–gel method is presented. The structure and morphology of these films has been assessed by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The sol–gel WO3 layers were found to have oxygen replaced by a significant amount of hydroxyls and water incorporated in their structure. They consist of particles with dimensions of several microns, have high porosity and increased surface area, which renders them suitable for gas sensing applications. Their color changes reversibly from yellowish green to dark blue upon insertion of lithium ions. They are electrochemically stable and can withstand more than 1000 voltammetric coloration–bleaching cycles. # 2003 Elsevier Science B.V. All rights reserved. PACS: 81.20.F (sol–gel processing); 79.60 (X-ray photoelectron spectra: surface analysis); 81.15 (thin films: deposition methods) Keywords: Electrochemical methods; Scanning electron microscopy; X-ray photoelectron spectroscopy; Tungsten oxide
1. Introduction Tungsten oxide is a material with a large potential for use in many practical applications such as ‘‘smart’’ electrochromic windows, switchable devices (displays or mirrors) and gas (NOx) sensors [1]. Some of these applications (e.g. displays and sensors) do not require transparent WO3 films. In that case, WO3 layers can be prepared by a simple and inexpensive sol–gel method, as presented in this work. The struc*
Corresponding author. Tel.: þ30-2610-997446; fax: þ30-2610-997472. E-mail address: [email protected] (S. Papaefthimiou).
ture, composition and morphology of such films were analyzed using surface sensitive spectroscopies, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and electrochemical measurements.
2. Experimental For the preparation of the WO3 films, 0.33 g of WO3 powder (Aldrich, 99.99%) were dissolved into 20 ml of 15% H2O2. With vigorous agitation a yellow-greenish solution was obtained. The solution was unstable and precipitation occurred when the agitation stopped.
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00616-0
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G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
Heating at 80 8C was used to increase the viscosity of the solution. This solution was then drop-casted on various types of substrates: SnO2:F coated glass sheets (Pilkington K-glass), uncoated microscope slides, and c-Si wafers (for the FTIR measurements). The substrates were heated at 80 8C for 1 h and a yellow-green WO3 film resulted. Heating at higher temperatures to increase the water evaporation rate also caused ‘‘evaporation’’ of the solvate from the substrate. This was due to the water in the solution, which upon evaporation carried along the oxide particles. Drop-casting and heating was repeated several times to obtain thick, homogeneous films. The film thickness was estimated from the SEM photographs to 1–5 mm. The adherence of the WO3 films on all the substrates is satisfactory: they can withstand the scotch tape test, but can be scratched away by a sharp metal object. The method described above is simple and inexpensive, it does not require specialized equipment and can be used to coat large areas. The atomic composition and the chemical state of the films were studied by X-ray photoelectron spectroscopy (XPS) using a SPECS LH-10 hemispherical analyzer. The unmonochromatized Mg Ka line at 1253.6 eV and an analyzer pass energy of 97 eV, giving a full-width at half-maximum (FWHM) of 1.7 eV for the Au 4f7/2 peak, were used in all measurements. The electron binding energies (BE) were referenced to the Au 4f7/2 peak of a clean gold ribbon at 84:07 0:05 eV. During the photoemission studies, sample charging was observed in some occasions. The binding energy scale was then calibrated by assigning the C 1s signal at 284:6 0:1 eV. Thermal annealing of the films was carried out in situ, and the XP spectra were recorded after the sample was cooled down to room temperature. Bulk WO3 powder pressed into a pellet has been used as internal reference for the calculation of the film stoichiometry. Scanning electron microscopy pictures of the films were taken using a JEOL 6300 microscope and transmission spectra in the far IR were measured by a PerkinElmer Paragon 1000 FTIR spectrometer. Cyclic voltammetry experiments were carried out using an AMEL model 2053 potensiostat–galvanostat driven by an AMEL model 586 function generator. A purpose-built electrochemical cell [2] has been used in the threeelectrode configuration. During the cyclic voltammetry
tests, the potential between the working (WO3/K-glass) and the reference (Pt) electrode was varied linearly, at a rate of 50 mV/sec. That potential and the resulting current flowing through the counter (Pt) electrode were measured and analyzed with use of a PC computer.
3. Results and discussion A SEM picture of the sol–gel WO3 film appears in Fig. 1, revealing its granular nature. The grain size varies from 300 to 1400 nm. The film has high porosity and increased surface area, compared to similar films prepared by electron gun deposition, that appear homogeneous with no observable features when viewed by SEM [3]. The high porosity of the sol– gel WO3 films renders them suitable for gas sensing applications. Indeed, it has been reported that the NOx gas sensitivity increases with increasing grain size and porosity of the WO3 films [1]. The stoichiometry of the films was studied by XPS at elevated temperatures. Fig. 2 shows the evolution of the W 4f peak during the thermal treatment. The first spectrum corresponds to room temperature and it is analyzed, after a Shirley background subtraction, to one doublet, constrained by the W 4f7/2–W 4f5/2 spinorbit separation being 2.15 eV. The ratio of the two peaks is 0.75, in accordance to the data from the WO3 reference sample. The binding energies of the peaks are 35.6 and 37.7 eV for W 4f7/2 and W 4f5/2, respectively and their FWHM is 1:70 0:05 eV. The energy position of this doublet corresponds to W6þ [3].
Fig. 1. SEM picture of a sol–gel WO3 film.
G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
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Fig. 2. Evolution of the W 4f XPS peak during annealing a sol– gel WO3 film in UHV. The fitted doublets are shown for each spectrum.
Fig. 3. Evolution of the O 1s XPS peak during annealing a sol–gel WO3 film in UHV. The fitted components are shown for each spectrum.
Fig. 3 shows the XPS O 1s peak at temperatures up to 300 8C. Each peak has been analyzed into three components after a Shirley background subtraction. Their FWHM is 1:9 eV 0:05 and they have a mixed 70% Gaussian–30% Lorenzian character. The binding energy of the first component is 530.5 eV and it is assigned to the oxygen atoms that form the strong W¼O bonds in the oxide [3]. The second peak is at about 531.7 eV. This peak has also been observed at evaporated WO3 films and has been attributed to O atoms in sub-stoichiometric WOx structures [4]. Nevertheless, the oxygen component that corresponds to OH groups in several metal hydroxides has been reported at the same binding energy [5]. The third O 1s peak, at about 532.5 eV, corresponds to oxygen in water molecules bound in the film structure or adsorbed on the sample surface [3,4]. The stoichiometry of the film is evaluated from the intensity ratio of the XPS peaks W 4f and O 1s. By using only the O 1s component at 530.5 eV (W¼O), the stoichiometry at room temperature is found to be x ¼ 2:3 0:07. This result seems to contradict the
fact that the W 4f peak consists only of one doublet assigned perfectly to W6þ. With a stoichiometry of x ¼ 2:3 one should also detect the doublets that correspond to W5þ and W4þ [6]. If the O 1s components at binding energies 530.5 and at 531.7 eV are both used for the stoichiometry calculation, then x ¼ 3:10 0:07. This result in combination with the fact that there is no spectroscopic evidence of sub-stoichiometric tungsten species, indicates that the O 1s peak at BE ¼ 531:7 eV corresponds to oxygen atoms that participate in the oxide stoichiometry as well, probably in the form of hydroxyl groups attached to the tungsten atoms. Measurements at elevated temperatures, that are presented next also support this conclusion. When the sample is heated to 100 8C the shape and energy of the W 4f peak does not change (Fig. 2) indicating that it still consists of one kind of tungsten species (W6þ). However, the intensity of the component corresponding to water (532.5 eV) at the O 1s spectrum (Fig. 3), decreases due to desorption of water that was adsorbed on the sample surface. The other two components of the O 1s spectrum remain
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G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
unchanged. At 200 8C a new doublet appears at the W 4f spectra at BE ¼ 36:8 eV for W 4f5/2 and 34.7 for W 4f7/2 which corresponds to W5þ [3,6]. Even at this temperature, the intensity of the O 1s component corresponding to oxygen in W¼O bonds (530.5 eV) remains unchanged. On the other hand, the intensity of the peak at 531.7 eV decreases. The oxide stoichiometry at 300 8C is reduced to x ¼ 2:85 0:07 from x ¼ 3:10 0:07 at room temperature. Analysis of the W 4f XPS peak shows that at this temperature 31% of the peak corresponds to W5þ and 69% to W6þ. Apparently, at temperatures higher than 100 8C part of the hydroxyls which are bonded to the tungsten atoms desorb resulting to partial reduction of the oxide. Additional information on the water content of the films at room temperature was obtained by FTIR spectroscopy. In the region around 3400 cm1, porous films exhibit a strong absorption band, as a result of various O–H stretching modes of water molecules adsorbed or incorporated in the film structure. The hydroxylation vibration modes are associated with wavenumbers in the region 3400–3500 cm1 and the hydration modes appear between 2950 and 3050 cm1. A third mode, attributed to the surface water appears in the region 3200–3250 cm1 [7]. The FTIR transmission (T) spectrum of a freshly prepared
WO3 film was measured and the corresponding absorption was calculated (as A ¼ 1 T). The various peaks that appear were identified, as shown in Fig. 4, and the spectrum was decomposed in the three O–H stretching modes described above (see inset of Fig. 4). The area of each mode is proportional to the number of the corresponding water molecules present in the film. Thus, by comparison of the areas of different modes one can assess the proportion of H2O molecules in the various states. The sol–gel films were found to have a large amount of structural water which is in agreement with the XPS findings. They are highly hydroxylated and to a lesser extent hydrated (44 and 10% of the total H2O, respectively). The adsorbed water amounts to 46% of the total H2O. A direct quantitative comparison of the percentages obtained by the IR measurements with the XPS results is not possible, first of all because the O 1s binding energy of adsorbed and structural water is the same and thus they cannot be discriminated and quantified. Furthermore, the high surface sensitivity of X-ray photoelectron spectroscopy would lead to an overestimation of the species adsorbed on the surface. The sol–gel WO3 films exhibit reversible electrochromic properties: placing them within a Liþ
Fig. 4. FTIR spectrum of a sol–gel WO3 film.
G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
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Fig. 5. Cyclic voltammograms of a sol–gel WO3 film.
electrolyte (1 M LiClO4 dissolved in PC) and with the application of a dc voltage in the range of 1–3 V, their color changes reversibly from yellow to green to dark blue within a few seconds. In order to assess the endurance of the films in continuous cycling, extended cyclic voltammetry testing has been carried out. The cyclic voltammograms resulting from these tests, appear in Fig. 5. Their shape is typical of amorphous films [8–11] and the voltammogram area is proportional to the charge density (Qin) of the Li ions intercalated into the film. Qin is 3.88 mC/cm2 during the first cycle and decreases gradually by 1.3% upon the 500th cycle and by 8.4% on the 1000th cycle. A possible cause of the observed reduction of Qin is the irreversible trapping of Li ions into the oxide matrix. Various charge trapping mechanisms have been proposed associated with impurities incorporated into the films during preparation [12–14]. Gradual degradation of WO3 sol–gel films has been reported by others [12], while some extremely unstable layers could not withstand more than 300 cycles [15,16]. Compared to this kind of performance, the layers presented in this study can be considered stable and insensitive to continuous cycling. Their properties and stability are adequate for the applications they are designed for.
4. Conclusions A simple and inexpensive sol–gel method has been developed to prepare thick and opaque WO3 layers. The structural and electrochemical properties of these films were investigated. The sol–gel films have oxygen replaced by a significant amount of hydroxyl groups and water is incorporated in their structure. They have high porosity, enhanced surface area and they are electrochemically stable. They are suitable for gas sensing and switching applications.
Acknowledgements The authors would like to acknowledge the EU for the financial support of this work in the frames of the EESD program (project name ELEVAG, Contract No. ENK6-CT-2001-00547). References [1] D.S. Lee, K.H. Nam, D.D. Lee, Thin Solid Films 375 (2000) 142. [2] S. Papaefthimiou, G. Leftheriotis, P. Yianoulis, Electrochim. Acta 46 (13–14) (2001) 2145.
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G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
[3] G. Leftheriotis, S. Papaefthimiou, P. Yiannoulis, A. Siokou, Thin Solid Films 384 (2001) 298. [4] H.Y. Wong, C.W. Ong, R.W. Kwok, K.W. Wong, S.P. Wong, W.Y. Cheung, Thin Solid Films 376 (2000) 131. [5] J. Kelber, G. Seshadri, Surf. Interface Anal. 31 (2001) 431. [6] A. Siokou, G. Leftheriotis, S. Papaefthimiou, P. Yianoulis, Surf. Sci. 482 (5) (2001) 294. [7] I. Porqueras, E. Bertran, Thin Solid Films 377–378 (2000) 8. [8] C.O. Avelladena, P.R. Bueno, R.C. Faria, L.O.S. Bulhoes, Electrochim. Acta 46 (2001) 1977. [9] P.R. Bueno, C.O. Avelladena, R.C. Faria, L.O.S. Bulhoes, Electrochim. Acta 46 (2001) 2113.
[10] A. Bessiere, J.C. Badot, M.C. Certiat, J. Livage, V. Lucas, N. Baffier, Electrochim. Acta 46 (2001) 2251. [11] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [12] J. Livage, D. Ganguli, Solar Energy Mater. Solar Cells 68 (2001) 365. [13] Z. Wang, X. Hu, Electrochim. Acta 46 (2001) 1951. [14] J. Bludska, J. Vondrak, I. Jakubec, P. Krtil, Solar Energy Mater. Solar Cells 31 (1993) 415. [15] X. Luo, B. Zhu, C. Xia, G.A. Niklasson, C.G. Granqvist, Solar Energy Mater. Solar Cells 53 (1998) 341. [16] B. Zhu, C. Xia, X. Luo, G. Niklasson, Thin Solid Films 385 (2001) 209.
Structural and electrochemical properties of opaque sol–gel deposited WO3 layers George Leftheriotisa, Spiros Papaefthimioua,*, Panayiotis Yianoulisa, Angeliki Siokoub, Dimitris Kefalasb a Department of Physics, University of Patras, 26500 Patras, Greece Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation of Research and Technology Hellas, Stadiou St., Platani Achaias 26500, Greece b
Received 28 January 2003; received in revised form 17 April 2003; accepted 18 April 2003
Abstract The preparation of thick and opaque WO3 layers by a simple sol–gel method is presented. The structure and morphology of these films has been assessed by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The sol–gel WO3 layers were found to have oxygen replaced by a significant amount of hydroxyls and water incorporated in their structure. They consist of particles with dimensions of several microns, have high porosity and increased surface area, which renders them suitable for gas sensing applications. Their color changes reversibly from yellowish green to dark blue upon insertion of lithium ions. They are electrochemically stable and can withstand more than 1000 voltammetric coloration–bleaching cycles. # 2003 Elsevier Science B.V. All rights reserved. PACS: 81.20.F (sol–gel processing); 79.60 (X-ray photoelectron spectra: surface analysis); 81.15 (thin films: deposition methods) Keywords: Electrochemical methods; Scanning electron microscopy; X-ray photoelectron spectroscopy; Tungsten oxide
1. Introduction Tungsten oxide is a material with a large potential for use in many practical applications such as ‘‘smart’’ electrochromic windows, switchable devices (displays or mirrors) and gas (NOx) sensors [1]. Some of these applications (e.g. displays and sensors) do not require transparent WO3 films. In that case, WO3 layers can be prepared by a simple and inexpensive sol–gel method, as presented in this work. The struc*
Corresponding author. Tel.: þ30-2610-997446; fax: þ30-2610-997472. E-mail address: [email protected] (S. Papaefthimiou).
ture, composition and morphology of such films were analyzed using surface sensitive spectroscopies, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and electrochemical measurements.
2. Experimental For the preparation of the WO3 films, 0.33 g of WO3 powder (Aldrich, 99.99%) were dissolved into 20 ml of 15% H2O2. With vigorous agitation a yellow-greenish solution was obtained. The solution was unstable and precipitation occurred when the agitation stopped.
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00616-0
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G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
Heating at 80 8C was used to increase the viscosity of the solution. This solution was then drop-casted on various types of substrates: SnO2:F coated glass sheets (Pilkington K-glass), uncoated microscope slides, and c-Si wafers (for the FTIR measurements). The substrates were heated at 80 8C for 1 h and a yellow-green WO3 film resulted. Heating at higher temperatures to increase the water evaporation rate also caused ‘‘evaporation’’ of the solvate from the substrate. This was due to the water in the solution, which upon evaporation carried along the oxide particles. Drop-casting and heating was repeated several times to obtain thick, homogeneous films. The film thickness was estimated from the SEM photographs to 1–5 mm. The adherence of the WO3 films on all the substrates is satisfactory: they can withstand the scotch tape test, but can be scratched away by a sharp metal object. The method described above is simple and inexpensive, it does not require specialized equipment and can be used to coat large areas. The atomic composition and the chemical state of the films were studied by X-ray photoelectron spectroscopy (XPS) using a SPECS LH-10 hemispherical analyzer. The unmonochromatized Mg Ka line at 1253.6 eV and an analyzer pass energy of 97 eV, giving a full-width at half-maximum (FWHM) of 1.7 eV for the Au 4f7/2 peak, were used in all measurements. The electron binding energies (BE) were referenced to the Au 4f7/2 peak of a clean gold ribbon at 84:07 0:05 eV. During the photoemission studies, sample charging was observed in some occasions. The binding energy scale was then calibrated by assigning the C 1s signal at 284:6 0:1 eV. Thermal annealing of the films was carried out in situ, and the XP spectra were recorded after the sample was cooled down to room temperature. Bulk WO3 powder pressed into a pellet has been used as internal reference for the calculation of the film stoichiometry. Scanning electron microscopy pictures of the films were taken using a JEOL 6300 microscope and transmission spectra in the far IR were measured by a PerkinElmer Paragon 1000 FTIR spectrometer. Cyclic voltammetry experiments were carried out using an AMEL model 2053 potensiostat–galvanostat driven by an AMEL model 586 function generator. A purpose-built electrochemical cell [2] has been used in the threeelectrode configuration. During the cyclic voltammetry
tests, the potential between the working (WO3/K-glass) and the reference (Pt) electrode was varied linearly, at a rate of 50 mV/sec. That potential and the resulting current flowing through the counter (Pt) electrode were measured and analyzed with use of a PC computer.
3. Results and discussion A SEM picture of the sol–gel WO3 film appears in Fig. 1, revealing its granular nature. The grain size varies from 300 to 1400 nm. The film has high porosity and increased surface area, compared to similar films prepared by electron gun deposition, that appear homogeneous with no observable features when viewed by SEM [3]. The high porosity of the sol– gel WO3 films renders them suitable for gas sensing applications. Indeed, it has been reported that the NOx gas sensitivity increases with increasing grain size and porosity of the WO3 films [1]. The stoichiometry of the films was studied by XPS at elevated temperatures. Fig. 2 shows the evolution of the W 4f peak during the thermal treatment. The first spectrum corresponds to room temperature and it is analyzed, after a Shirley background subtraction, to one doublet, constrained by the W 4f7/2–W 4f5/2 spinorbit separation being 2.15 eV. The ratio of the two peaks is 0.75, in accordance to the data from the WO3 reference sample. The binding energies of the peaks are 35.6 and 37.7 eV for W 4f7/2 and W 4f5/2, respectively and their FWHM is 1:70 0:05 eV. The energy position of this doublet corresponds to W6þ [3].
Fig. 1. SEM picture of a sol–gel WO3 film.
G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
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Fig. 2. Evolution of the W 4f XPS peak during annealing a sol– gel WO3 film in UHV. The fitted doublets are shown for each spectrum.
Fig. 3. Evolution of the O 1s XPS peak during annealing a sol–gel WO3 film in UHV. The fitted components are shown for each spectrum.
Fig. 3 shows the XPS O 1s peak at temperatures up to 300 8C. Each peak has been analyzed into three components after a Shirley background subtraction. Their FWHM is 1:9 eV 0:05 and they have a mixed 70% Gaussian–30% Lorenzian character. The binding energy of the first component is 530.5 eV and it is assigned to the oxygen atoms that form the strong W¼O bonds in the oxide [3]. The second peak is at about 531.7 eV. This peak has also been observed at evaporated WO3 films and has been attributed to O atoms in sub-stoichiometric WOx structures [4]. Nevertheless, the oxygen component that corresponds to OH groups in several metal hydroxides has been reported at the same binding energy [5]. The third O 1s peak, at about 532.5 eV, corresponds to oxygen in water molecules bound in the film structure or adsorbed on the sample surface [3,4]. The stoichiometry of the film is evaluated from the intensity ratio of the XPS peaks W 4f and O 1s. By using only the O 1s component at 530.5 eV (W¼O), the stoichiometry at room temperature is found to be x ¼ 2:3 0:07. This result seems to contradict the
fact that the W 4f peak consists only of one doublet assigned perfectly to W6þ. With a stoichiometry of x ¼ 2:3 one should also detect the doublets that correspond to W5þ and W4þ [6]. If the O 1s components at binding energies 530.5 and at 531.7 eV are both used for the stoichiometry calculation, then x ¼ 3:10 0:07. This result in combination with the fact that there is no spectroscopic evidence of sub-stoichiometric tungsten species, indicates that the O 1s peak at BE ¼ 531:7 eV corresponds to oxygen atoms that participate in the oxide stoichiometry as well, probably in the form of hydroxyl groups attached to the tungsten atoms. Measurements at elevated temperatures, that are presented next also support this conclusion. When the sample is heated to 100 8C the shape and energy of the W 4f peak does not change (Fig. 2) indicating that it still consists of one kind of tungsten species (W6þ). However, the intensity of the component corresponding to water (532.5 eV) at the O 1s spectrum (Fig. 3), decreases due to desorption of water that was adsorbed on the sample surface. The other two components of the O 1s spectrum remain
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unchanged. At 200 8C a new doublet appears at the W 4f spectra at BE ¼ 36:8 eV for W 4f5/2 and 34.7 for W 4f7/2 which corresponds to W5þ [3,6]. Even at this temperature, the intensity of the O 1s component corresponding to oxygen in W¼O bonds (530.5 eV) remains unchanged. On the other hand, the intensity of the peak at 531.7 eV decreases. The oxide stoichiometry at 300 8C is reduced to x ¼ 2:85 0:07 from x ¼ 3:10 0:07 at room temperature. Analysis of the W 4f XPS peak shows that at this temperature 31% of the peak corresponds to W5þ and 69% to W6þ. Apparently, at temperatures higher than 100 8C part of the hydroxyls which are bonded to the tungsten atoms desorb resulting to partial reduction of the oxide. Additional information on the water content of the films at room temperature was obtained by FTIR spectroscopy. In the region around 3400 cm1, porous films exhibit a strong absorption band, as a result of various O–H stretching modes of water molecules adsorbed or incorporated in the film structure. The hydroxylation vibration modes are associated with wavenumbers in the region 3400–3500 cm1 and the hydration modes appear between 2950 and 3050 cm1. A third mode, attributed to the surface water appears in the region 3200–3250 cm1 [7]. The FTIR transmission (T) spectrum of a freshly prepared
WO3 film was measured and the corresponding absorption was calculated (as A ¼ 1 T). The various peaks that appear were identified, as shown in Fig. 4, and the spectrum was decomposed in the three O–H stretching modes described above (see inset of Fig. 4). The area of each mode is proportional to the number of the corresponding water molecules present in the film. Thus, by comparison of the areas of different modes one can assess the proportion of H2O molecules in the various states. The sol–gel films were found to have a large amount of structural water which is in agreement with the XPS findings. They are highly hydroxylated and to a lesser extent hydrated (44 and 10% of the total H2O, respectively). The adsorbed water amounts to 46% of the total H2O. A direct quantitative comparison of the percentages obtained by the IR measurements with the XPS results is not possible, first of all because the O 1s binding energy of adsorbed and structural water is the same and thus they cannot be discriminated and quantified. Furthermore, the high surface sensitivity of X-ray photoelectron spectroscopy would lead to an overestimation of the species adsorbed on the surface. The sol–gel WO3 films exhibit reversible electrochromic properties: placing them within a Liþ
Fig. 4. FTIR spectrum of a sol–gel WO3 film.
G. Leftheriotis et al. / Applied Surface Science 218 (2003) 275–280
279
Fig. 5. Cyclic voltammograms of a sol–gel WO3 film.
electrolyte (1 M LiClO4 dissolved in PC) and with the application of a dc voltage in the range of 1–3 V, their color changes reversibly from yellow to green to dark blue within a few seconds. In order to assess the endurance of the films in continuous cycling, extended cyclic voltammetry testing has been carried out. The cyclic voltammograms resulting from these tests, appear in Fig. 5. Their shape is typical of amorphous films [8–11] and the voltammogram area is proportional to the charge density (Qin) of the Li ions intercalated into the film. Qin is 3.88 mC/cm2 during the first cycle and decreases gradually by 1.3% upon the 500th cycle and by 8.4% on the 1000th cycle. A possible cause of the observed reduction of Qin is the irreversible trapping of Li ions into the oxide matrix. Various charge trapping mechanisms have been proposed associated with impurities incorporated into the films during preparation [12–14]. Gradual degradation of WO3 sol–gel films has been reported by others [12], while some extremely unstable layers could not withstand more than 300 cycles [15,16]. Compared to this kind of performance, the layers presented in this study can be considered stable and insensitive to continuous cycling. Their properties and stability are adequate for the applications they are designed for.
4. Conclusions A simple and inexpensive sol–gel method has been developed to prepare thick and opaque WO3 layers. The structural and electrochemical properties of these films were investigated. The sol–gel films have oxygen replaced by a significant amount of hydroxyl groups and water is incorporated in their structure. They have high porosity, enhanced surface area and they are electrochemically stable. They are suitable for gas sensing and switching applications.
Acknowledgements The authors would like to acknowledge the EU for the financial support of this work in the frames of the EESD program (project name ELEVAG, Contract No. ENK6-CT-2001-00547). References [1] D.S. Lee, K.H. Nam, D.D. Lee, Thin Solid Films 375 (2000) 142. [2] S. Papaefthimiou, G. Leftheriotis, P. Yianoulis, Electrochim. Acta 46 (13–14) (2001) 2145.
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[3] G. Leftheriotis, S. Papaefthimiou, P. Yiannoulis, A. Siokou, Thin Solid Films 384 (2001) 298. [4] H.Y. Wong, C.W. Ong, R.W. Kwok, K.W. Wong, S.P. Wong, W.Y. Cheung, Thin Solid Films 376 (2000) 131. [5] J. Kelber, G. Seshadri, Surf. Interface Anal. 31 (2001) 431. [6] A. Siokou, G. Leftheriotis, S. Papaefthimiou, P. Yianoulis, Surf. Sci. 482 (5) (2001) 294. [7] I. Porqueras, E. Bertran, Thin Solid Films 377–378 (2000) 8. [8] C.O. Avelladena, P.R. Bueno, R.C. Faria, L.O.S. Bulhoes, Electrochim. Acta 46 (2001) 1977. [9] P.R. Bueno, C.O. Avelladena, R.C. Faria, L.O.S. Bulhoes, Electrochim. Acta 46 (2001) 2113.
[10] A. Bessiere, J.C. Badot, M.C. Certiat, J. Livage, V. Lucas, N. Baffier, Electrochim. Acta 46 (2001) 2251. [11] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [12] J. Livage, D. Ganguli, Solar Energy Mater. Solar Cells 68 (2001) 365. [13] Z. Wang, X. Hu, Electrochim. Acta 46 (2001) 1951. [14] J. Bludska, J. Vondrak, I. Jakubec, P. Krtil, Solar Energy Mater. Solar Cells 31 (1993) 415. [15] X. Luo, B. Zhu, C. Xia, G.A. Niklasson, C.G. Granqvist, Solar Energy Mater. Solar Cells 53 (1998) 341. [16] B. Zhu, C. Xia, X. Luo, G. Niklasson, Thin Solid Films 385 (2001) 209.