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Micro-arc oxidized S-TiO2 nanoporous layers: Cationic or anionic doping?
Materials Letters 64 (2010) 2215–2218
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Micro-arc oxidized S-TiO2 nanoporous layers: Cationic or anionic doping? M.R. Bayati a,b,⁎, A.Z. Moshfegh c, F. Golestani-Fard a,b a b c
School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box: 16845-161, Tehran, Iran Center of Excellence for Advanced Materials, Iran University of Science and Technology, P.O. Box: 16845-195, Tehran, Iran Department of Physics, Sharif University of Technology, P.O. Box: 11155-9161, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 27 April 2010 Accepted 4 July 2010 Available online 13 July 2010 Keywords: Micro arc oxidation TiO2 Sulfur Doping Nanomaterials Porosity
a b s t r a c t S-doped TiO2 layers were grown on titanium substrates by MAO process. SEM results revealed a porous morphology with a pore size of 40–100 nm. Our XRD analysis showed that the anatase relative content reached its maximum value at the voltage of 500 V. The existence of sulfur in the states of S4+ and S6+ which substituted Ti4+ in the titania crystalline lattice was confirmed by XPS results; meanwhile, no S2− was detected. That is, a cationic doping was observed. EDS results showed that sulfur concentration in the layers increased with the voltage. The band gap energy was also calculated as 2.29 eV employing a UV–Vis spectrophotometer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the band gap energy of titania is wide, considerable efforts have been directed to extend the absorption edge of TiO2 toward the visible part of the spectrum in the last three decades. One approach for acquiring a visible response in titanium dioxide is introducing defects into its lattice through doping with metallic [1–3] and non-metallic species [4–6] and loading other semiconductors [7–10] on the surface or into the crystal lattice of titania. Several works concerning sulfur doping have been reported. Ohno reported that S cation-doped TiO2 powder absorbed visible light more strongly than N, C and the S anion doped TiO2 powders and showed photocatalytic activity under visible light. They demonstrated that the substitution of Ti4+ by S4+ was responsible to the visible light absorbance [11–13]. Umebayashi et al. who synthesized S-TiO2 photocatalyst using the ion implantation method, indicated that S is doped in TiO2 as an anion and replace the lattice oxygen in TiO2 [14–16]. Sulfur doping has been achieved under high temperature process or using expensive precursors or preparation instruments [12,13] which limits investigations on S-doped titania catalysts. In contrast, Micro Arc Oxidation (MAO) is a simple and promising approach for fabrication of different categories of oxide layers. It is an electrochemical technique for formation of anodic films by spark/arc micro-
⁎ Corresponding author. School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box: 16845-161, Tehran, Iran. Tel./fax: +98 21 77240291. E-mail address: [email protected] (M.R. Bayati). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.010
discharges which move rapidly on the vicinity of the anode surface [17–20]. It is characterized by high productivity, economic efficiency, ecological friendliness, high hardness, good wear resistance, and excellent bonding strength with the substrate [21–23]. The present study sheds light on the effect of the applied on surface morphology, phase structure, stoichiometry, and optical properties of the MAO-grown S:TiO2 layers. How sulfur is introduced into the TiO2 crystalline lattice, synthesized via MAO process, is investigated. 2. Experimental Details of the employed experimental arrangement can be found in our previous works [7,8,24–26]. Na2S2O3.5H2O with a concentration of 10 g l-1 was used as electrolyte whose temperature was fixed at 70 ± 3 °C employing a water circulating system. DC mode of the rectifier was selected, and different voltages were applied in a range of 350 V to 550 V with + 50 V intervals. Meanwhile, the growth time for each run was same and considered as 3 minutes. Surface morphology of the layers was evaluated by scanning electron microscopy (TESCAN, Vega II). X-ray diffraction (Philips, PW3710), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 X-ray source, using AlKα = 1486.6 eV) techniques were respectively used in order to study phase structure, elemental composition, and stoichiometry of the synthesized layers. A UV-Vis spectrophotometer (Carry 500) was used to measure the band gap energy of the layers as well. Details of the calculation method can be found in our other works [25,26].
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3. Results and discussion SEM top-views of the S:TiO2 layers grown under different voltages are depicted in Fig. 1 where a porous microstructure can be observed. The electrical sparks, which are responsible for structural pores formation, are weak at low voltages due to low electrical current passing through the electrochemical cell; therefore, the pores are small at low voltages. In contrast, the electrical current increases at higher voltages resulting in formation of larger pores due to the stronger electrical sparks taking place on the vicinity of the anode. At very high voltages (550 V), such strong electron avalanches can partially destroy the structure (Fig. 1e). XRD patterns, exhibited in Fig. 2, show that the layers consisted of anatase and rutile phases with a varying fraction depending on the applied voltage. It is obvious that the WA/WR parameter increases and, then, decreases with the voltage. Applying higher voltages results in increasing the circuit current; hence, stronger avalanches take place on the vicinity of the anode. This phenomenon warms up the anode and the growing oxide layer. Under such conditions, the anatase metastable phase transforms to rutile phase which is thermodynamically stable at higher temperatures. A small shift toward higher diffraction angles is observed in characteristic peaks of the anatase and the rutile phases when sulfur is doped into the titania. This shows that the lattice parameter decreases when S is incorporated in the TiO2 lattice. It should be noted that the ionic radius of Ti4+ (0.068 nm) is greater than that of S4+(0.037 nm) and S6+(0.029 nm). This shift was not observed in the pure MAO-grown titania layers fabricated and characterized in our previous works [25]. EDS technique was also employed in order to measure the sulfur content of the grown layers. The sulfur contents were determined as 0.14, 0.54, 0.71, 1.09, and 1.23 (wt.%) for the applied voltages of 350, 400, 450, 500, and 550 V. In other words, the sulfur content increased with the applied voltage because the electrical field between anode and cathode intensifies at higher voltages which results in increasing the concentration of S2O23 anions surrounding the anode, and, hence, the sulfur weight percent in the layers. To determine how sulfur atoms incorporate in the crystalline lattice of titania, XPS technique was employed. All binding energies were referenced to C(1 s) core level with the binding energy of 285.0 eV. The S(2p) peaks, shown in Fig. 3, can be resolved into two components using original software with Gaussian rule demonstrating that there are two kinds of S-binding states in the layers. The peak A, located at the binding energy of 167.5 to 167.6 eV, is assigned to the S4+ state, while the peak B, located at the binding energy of 168.5 eV, represents the sulfur in the form of S6+. No peak corresponding to S2- state was observed. These results show that the sulfur ions occupy the Ti4+ sites. Fig. 3 also illustrates the O(1 s) peak which can be deconvoluted into five components. The peak A, located at the binding energy of 529.5 to 529.6 eV, is assigned to the crystal lattice oxygen (Ti–O). This binding energy is lower than that in the pure TiO2 layers. The peaks B and C, located at the binding energies of 530.4–530.6 eV and 531.3–531.4,
Fig. 2. XRD patterns of the S:TiO2 layers synthesized under various voltages.
respectively, represent the oxygen in the S–O–Ti and S–O–S groups. Thus, it is further confirmed that sulfur occupies the Ti sites. The peak D corresponds to the oxygen in the hydroxyl groups (-OH). Finally, the peak E is assigned to the O-bonding in water molecules which is usual for porous layers grown in aqueous solutions. Diffuse reflectance spectra (DRS) of the S-doped titania layers are shown in Fig. 4. It can be observed that the absorption edge of the layers shifted toward the visible region when sulfur was doped into the TiO2 lattice. These results reveal that the sulfur dopants alter titania crystal and electronic structures. It has been suggested that doping of a foreign element into TiO2 lattice makes donor or acceptor levels in the forbidden band. Doping of the S atom perturbs CB or VB (or both of them), and produces states in the band gap of TiO2 that absorb visible light [11,14]. Using DRS results, the band gap energies of the grown S:TiO2 layers were calculated and the results are presented in Fig. 4. It is observed that the absorption edge shifts to longer wavelengths with increasing the voltage because the amount of doped sulfur increased with the voltage, as elucidated earlier. Another reason is that the fraction of rutile phase, which has a narrower band gap (3.0 eV) than anatase phase (3.2 eV), increases with the voltage.
4. Conclusions Narrow band gap (~2.29 eV) S:titania layers were synthesized via MAO process. The layers had a porous structure (40–100 nm) with a rough surface. They consisted of anatase and rutile phases whose fraction depended on the applied. Based on the obtained XRD and XPS results, it was revealed that Ti4+ ions were substituted by sulfur S4+ and S6+. That is, a cationic doping was observed.
Fig. 1. SEM top-view of the S-doped TiO2 layers grown under voltage of (a) 350, (b) 400, (c) 450, (d) 500, and (e) 550 V.
M.R. Bayati et al. / Materials Letters 64 (2010) 2215–2218
Fig. 3. XPS core level binding energies of the S:TiO2 layers fabricated under different voltages: (a) 350, (b) 450, and (c) 550 V.
Fig. 4. DRS and dα/dλ as a function of the wavelength.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Yang Y, Wang H, Li X, Wang C. Mater Lett 2009;63:331. Lin Z, Chen G, Zhou X, Zeng W, Zhang H. Mater Lett 2009;63:2277. Yang Y, Zhang C, Xu Y, Wang H, Li X, Wang C. Mater Lett 2010;64:147. Wu G, Wen J, Wang J, Thomas DF, Chen A. Mater Lett 2010;64:1728. Yuan JJ, Li HD, Gao SY, Sang DD, Li LA, Lu D. Mater Lett 2010;64:2012. Dong L, Ma Y, Wang Y, Tian Y, Ye G, Jia X, Cao G. Mater Lett 2009;63:1598. Bayati MR, Golestani-Fard F, Moshfegh AZ. Catal Lett 2010;134:162. Bayati MR, Moshfegh AZ, Golestani-Fard F. Appl Surf Sci 2010;256:2903. Bayati MR, Golestani-Fard F, Moshfegh AZ. J Appl Catal A General 2010;382:322. Beyers E, Biermans E, Ribbens S, Witte KD, Mertens M, Meynen V, et al. Appl Catal B Environmental 2009;88:515. [11] Ohno T, Akiyoshi M, Umebayashi T, Asai K, Mitsui T, Matsumura M. Appl Catal A Gen 2004;265:115. [12] Ohno T, Mitsui T, Matsumura M. Chem Lett 2003;32:364.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Ohno T. Water Sci Technol 2004;49:159. Umebayashi T, Yamaki T, Itoh H, Asai K. Appl Phys Lett 2002;81:454. Umebayashi T, Yamaki T, Tanala S, Asai K. Chem Lett 2003;32:330. Umebayashi T, Yamaki T, Yamamoto S, Miyashita A, Tanala S, Sumita T, et al. Appl Phys 2003;93:5156. Wan L, Li JF, Feng JY, Sun W, Mao ZQ. Mater Sci Eng B 2007;139:216. Yerokhin AL, Leyland A, Matthews A. Surf Coat Technol 2002;200:172. Sun X, Jiang Z, Xin S, Yao Z. Thin Solid Films 2005;471:194. Gupta P, Tenhundfeld G, Daigle EO, Ryabkov D. Surf Coat Technol 2007;201:8746. Yerokhin AL, Nie X, Leyland A, Matthews A. Surf Coat Technol 2000;130:195. Xue W, Wang C, Chen R, Deng Z. Mater Lett 2002;52:435. Xue W, Deng Z, Chen R, Zhang T. Thin Solid Films 2000;372:114. Bayati MR, Golestani-Fard F, Moshfegh AZ. Electrochim Acta 2010;55:2760. Bayati MR, Golestani-Fard F, Moshfegh AZ. Mater Chem Phys 2010;120:582. Bayati MR, Moshfegh AZ, Golestani-Fard F. Electrochim Acta 2010;55:3093.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Micro-arc oxidized S-TiO2 nanoporous layers: Cationic or anionic doping? M.R. Bayati a,b,⁎, A.Z. Moshfegh c, F. Golestani-Fard a,b a b c
School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box: 16845-161, Tehran, Iran Center of Excellence for Advanced Materials, Iran University of Science and Technology, P.O. Box: 16845-195, Tehran, Iran Department of Physics, Sharif University of Technology, P.O. Box: 11155-9161, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 27 April 2010 Accepted 4 July 2010 Available online 13 July 2010 Keywords: Micro arc oxidation TiO2 Sulfur Doping Nanomaterials Porosity
a b s t r a c t S-doped TiO2 layers were grown on titanium substrates by MAO process. SEM results revealed a porous morphology with a pore size of 40–100 nm. Our XRD analysis showed that the anatase relative content reached its maximum value at the voltage of 500 V. The existence of sulfur in the states of S4+ and S6+ which substituted Ti4+ in the titania crystalline lattice was confirmed by XPS results; meanwhile, no S2− was detected. That is, a cationic doping was observed. EDS results showed that sulfur concentration in the layers increased with the voltage. The band gap energy was also calculated as 2.29 eV employing a UV–Vis spectrophotometer. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the band gap energy of titania is wide, considerable efforts have been directed to extend the absorption edge of TiO2 toward the visible part of the spectrum in the last three decades. One approach for acquiring a visible response in titanium dioxide is introducing defects into its lattice through doping with metallic [1–3] and non-metallic species [4–6] and loading other semiconductors [7–10] on the surface or into the crystal lattice of titania. Several works concerning sulfur doping have been reported. Ohno reported that S cation-doped TiO2 powder absorbed visible light more strongly than N, C and the S anion doped TiO2 powders and showed photocatalytic activity under visible light. They demonstrated that the substitution of Ti4+ by S4+ was responsible to the visible light absorbance [11–13]. Umebayashi et al. who synthesized S-TiO2 photocatalyst using the ion implantation method, indicated that S is doped in TiO2 as an anion and replace the lattice oxygen in TiO2 [14–16]. Sulfur doping has been achieved under high temperature process or using expensive precursors or preparation instruments [12,13] which limits investigations on S-doped titania catalysts. In contrast, Micro Arc Oxidation (MAO) is a simple and promising approach for fabrication of different categories of oxide layers. It is an electrochemical technique for formation of anodic films by spark/arc micro-
⁎ Corresponding author. School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box: 16845-161, Tehran, Iran. Tel./fax: +98 21 77240291. E-mail address: [email protected] (M.R. Bayati). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.010
discharges which move rapidly on the vicinity of the anode surface [17–20]. It is characterized by high productivity, economic efficiency, ecological friendliness, high hardness, good wear resistance, and excellent bonding strength with the substrate [21–23]. The present study sheds light on the effect of the applied on surface morphology, phase structure, stoichiometry, and optical properties of the MAO-grown S:TiO2 layers. How sulfur is introduced into the TiO2 crystalline lattice, synthesized via MAO process, is investigated. 2. Experimental Details of the employed experimental arrangement can be found in our previous works [7,8,24–26]. Na2S2O3.5H2O with a concentration of 10 g l-1 was used as electrolyte whose temperature was fixed at 70 ± 3 °C employing a water circulating system. DC mode of the rectifier was selected, and different voltages were applied in a range of 350 V to 550 V with + 50 V intervals. Meanwhile, the growth time for each run was same and considered as 3 minutes. Surface morphology of the layers was evaluated by scanning electron microscopy (TESCAN, Vega II). X-ray diffraction (Philips, PW3710), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 X-ray source, using AlKα = 1486.6 eV) techniques were respectively used in order to study phase structure, elemental composition, and stoichiometry of the synthesized layers. A UV-Vis spectrophotometer (Carry 500) was used to measure the band gap energy of the layers as well. Details of the calculation method can be found in our other works [25,26].
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3. Results and discussion SEM top-views of the S:TiO2 layers grown under different voltages are depicted in Fig. 1 where a porous microstructure can be observed. The electrical sparks, which are responsible for structural pores formation, are weak at low voltages due to low electrical current passing through the electrochemical cell; therefore, the pores are small at low voltages. In contrast, the electrical current increases at higher voltages resulting in formation of larger pores due to the stronger electrical sparks taking place on the vicinity of the anode. At very high voltages (550 V), such strong electron avalanches can partially destroy the structure (Fig. 1e). XRD patterns, exhibited in Fig. 2, show that the layers consisted of anatase and rutile phases with a varying fraction depending on the applied voltage. It is obvious that the WA/WR parameter increases and, then, decreases with the voltage. Applying higher voltages results in increasing the circuit current; hence, stronger avalanches take place on the vicinity of the anode. This phenomenon warms up the anode and the growing oxide layer. Under such conditions, the anatase metastable phase transforms to rutile phase which is thermodynamically stable at higher temperatures. A small shift toward higher diffraction angles is observed in characteristic peaks of the anatase and the rutile phases when sulfur is doped into the titania. This shows that the lattice parameter decreases when S is incorporated in the TiO2 lattice. It should be noted that the ionic radius of Ti4+ (0.068 nm) is greater than that of S4+(0.037 nm) and S6+(0.029 nm). This shift was not observed in the pure MAO-grown titania layers fabricated and characterized in our previous works [25]. EDS technique was also employed in order to measure the sulfur content of the grown layers. The sulfur contents were determined as 0.14, 0.54, 0.71, 1.09, and 1.23 (wt.%) for the applied voltages of 350, 400, 450, 500, and 550 V. In other words, the sulfur content increased with the applied voltage because the electrical field between anode and cathode intensifies at higher voltages which results in increasing the concentration of S2O23 anions surrounding the anode, and, hence, the sulfur weight percent in the layers. To determine how sulfur atoms incorporate in the crystalline lattice of titania, XPS technique was employed. All binding energies were referenced to C(1 s) core level with the binding energy of 285.0 eV. The S(2p) peaks, shown in Fig. 3, can be resolved into two components using original software with Gaussian rule demonstrating that there are two kinds of S-binding states in the layers. The peak A, located at the binding energy of 167.5 to 167.6 eV, is assigned to the S4+ state, while the peak B, located at the binding energy of 168.5 eV, represents the sulfur in the form of S6+. No peak corresponding to S2- state was observed. These results show that the sulfur ions occupy the Ti4+ sites. Fig. 3 also illustrates the O(1 s) peak which can be deconvoluted into five components. The peak A, located at the binding energy of 529.5 to 529.6 eV, is assigned to the crystal lattice oxygen (Ti–O). This binding energy is lower than that in the pure TiO2 layers. The peaks B and C, located at the binding energies of 530.4–530.6 eV and 531.3–531.4,
Fig. 2. XRD patterns of the S:TiO2 layers synthesized under various voltages.
respectively, represent the oxygen in the S–O–Ti and S–O–S groups. Thus, it is further confirmed that sulfur occupies the Ti sites. The peak D corresponds to the oxygen in the hydroxyl groups (-OH). Finally, the peak E is assigned to the O-bonding in water molecules which is usual for porous layers grown in aqueous solutions. Diffuse reflectance spectra (DRS) of the S-doped titania layers are shown in Fig. 4. It can be observed that the absorption edge of the layers shifted toward the visible region when sulfur was doped into the TiO2 lattice. These results reveal that the sulfur dopants alter titania crystal and electronic structures. It has been suggested that doping of a foreign element into TiO2 lattice makes donor or acceptor levels in the forbidden band. Doping of the S atom perturbs CB or VB (or both of them), and produces states in the band gap of TiO2 that absorb visible light [11,14]. Using DRS results, the band gap energies of the grown S:TiO2 layers were calculated and the results are presented in Fig. 4. It is observed that the absorption edge shifts to longer wavelengths with increasing the voltage because the amount of doped sulfur increased with the voltage, as elucidated earlier. Another reason is that the fraction of rutile phase, which has a narrower band gap (3.0 eV) than anatase phase (3.2 eV), increases with the voltage.
4. Conclusions Narrow band gap (~2.29 eV) S:titania layers were synthesized via MAO process. The layers had a porous structure (40–100 nm) with a rough surface. They consisted of anatase and rutile phases whose fraction depended on the applied. Based on the obtained XRD and XPS results, it was revealed that Ti4+ ions were substituted by sulfur S4+ and S6+. That is, a cationic doping was observed.
Fig. 1. SEM top-view of the S-doped TiO2 layers grown under voltage of (a) 350, (b) 400, (c) 450, (d) 500, and (e) 550 V.
M.R. Bayati et al. / Materials Letters 64 (2010) 2215–2218
Fig. 3. XPS core level binding energies of the S:TiO2 layers fabricated under different voltages: (a) 350, (b) 450, and (c) 550 V.
Fig. 4. DRS and dα/dλ as a function of the wavelength.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Yang Y, Wang H, Li X, Wang C. Mater Lett 2009;63:331. Lin Z, Chen G, Zhou X, Zeng W, Zhang H. Mater Lett 2009;63:2277. Yang Y, Zhang C, Xu Y, Wang H, Li X, Wang C. Mater Lett 2010;64:147. Wu G, Wen J, Wang J, Thomas DF, Chen A. Mater Lett 2010;64:1728. Yuan JJ, Li HD, Gao SY, Sang DD, Li LA, Lu D. Mater Lett 2010;64:2012. Dong L, Ma Y, Wang Y, Tian Y, Ye G, Jia X, Cao G. Mater Lett 2009;63:1598. Bayati MR, Golestani-Fard F, Moshfegh AZ. Catal Lett 2010;134:162. Bayati MR, Moshfegh AZ, Golestani-Fard F. Appl Surf Sci 2010;256:2903. Bayati MR, Golestani-Fard F, Moshfegh AZ. J Appl Catal A General 2010;382:322. Beyers E, Biermans E, Ribbens S, Witte KD, Mertens M, Meynen V, et al. Appl Catal B Environmental 2009;88:515. [11] Ohno T, Akiyoshi M, Umebayashi T, Asai K, Mitsui T, Matsumura M. Appl Catal A Gen 2004;265:115. [12] Ohno T, Mitsui T, Matsumura M. Chem Lett 2003;32:364.
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Ohno T. Water Sci Technol 2004;49:159. Umebayashi T, Yamaki T, Itoh H, Asai K. Appl Phys Lett 2002;81:454. Umebayashi T, Yamaki T, Tanala S, Asai K. Chem Lett 2003;32:330. Umebayashi T, Yamaki T, Yamamoto S, Miyashita A, Tanala S, Sumita T, et al. Appl Phys 2003;93:5156. Wan L, Li JF, Feng JY, Sun W, Mao ZQ. Mater Sci Eng B 2007;139:216. Yerokhin AL, Leyland A, Matthews A. Surf Coat Technol 2002;200:172. Sun X, Jiang Z, Xin S, Yao Z. Thin Solid Films 2005;471:194. Gupta P, Tenhundfeld G, Daigle EO, Ryabkov D. Surf Coat Technol 2007;201:8746. Yerokhin AL, Nie X, Leyland A, Matthews A. Surf Coat Technol 2000;130:195. Xue W, Wang C, Chen R, Deng Z. Mater Lett 2002;52:435. Xue W, Deng Z, Chen R, Zhang T. Thin Solid Films 2000;372:114. Bayati MR, Golestani-Fard F, Moshfegh AZ. Electrochim Acta 2010;55:2760. Bayati MR, Golestani-Fard F, Moshfegh AZ. Mater Chem Phys 2010;120:582. Bayati MR, Moshfegh AZ, Golestani-Fard F. Electrochim Acta 2010;55:3093.