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Controlled growth of MoO3 nanorods on transparent conducting substrates
Materials Letters 136 (2014) 146–149
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled growth of MoO3 nanorods on transparent conducting substrates Ying Ma a,b, Xia Zhang a,b, Min Yang a, Yanxing Qi a,n a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 June 2014 Accepted 23 July 2014 Available online 1 August 2014
MoO3 nanorods with well-defined crystalline structure have been grown in situ on Fluorine doped Tin Oxide glass (FTO) by magnetron sputtering and subsequent oxidation treatment. Moreover, the morphologies and the crystalline structures of MoO3 products could be rationally tailored by adjusting the annealing temperature. More specifically, the calcination operated at 500 1C for 6 h leads to the formation of uniform MoO3 nanorods with an average diameter of 200 nm, and length of up to 800 nm, whereas only irregular nanoparticles or nanoplates have been obtained when the temperature was higher or lower than 500 1C. & 2014 Elsevier B.V. All rights reserved.
Keywords: MoO3 nanorods Sputtering Oxidation morphology UV–vis
1. Introduction Transition metal oxide materials have recently received numerous attentions because of their promising optical, electrical, magnetic and ionic-transport properties. Among them, MoO3 has been extensively investigated ranging from catalysts, chemical sensor, photochromic and electrochromic device to lithium ion battery due to its layered structure and wide band-gap [1–3]. It is well known that the physical and chemical characteristics could be significantly enhanced by tailoring the dimensions as well as morphologies. Thereby, great efforts have been devoted to the synthesis of MoO3 with various nanostructures. Among them, the one dimensional (1D) MoO3 nanostructures, especially nanorods, have become of particular interests as a result of their novel confinement and crystal anisotropy effects. Up to now, MoO3 nanorods have been synthesized via acidification under hydrothermal [4]. Patzake et al. obtained MoO3 nanorods by a flexible one-step solvothermal reaction [5]. However, note that the asreported synthetic approaches cannot be utilized to grow MoO3 nanorods on planar substrates, which greatly restricts their future applications in the fabrication of nano-devices. Thus, the exploration of simple and efficient method for the large-scale synthesis of MoO3 nanorods on planar substrates is still a great challenge. Recently, several approaches have been available for preparing MoO3 film on planar substrates such as sputtering [6], electro-
n
Corresponding author. E-mail addresses: [email protected] (Y. Ma), [email protected] (Y. Qi).
http://dx.doi.org/10.1016/j.matlet.2014.07.143 0167-577X/& 2014 Elsevier B.V. All rights reserved.
deposition [7] and vacuum evaporation [8]. Yeong et al. [9] have prepared nanocracked and porous MoO3 films by electrodeposition and thermal annealing at high temperature. Song et al. [10] have fabricated h- and α-MoO3 films on glass substrates by ion exchange and chemical deposition. Gesheva et al. [11] have obtained MoO3 films on Si wafer and conductive glass by atmospheric pressure Chemical Vapor Deposition (CVD) method using Mo(CO)6 as precursor. Li et al. [12] have synthesized nanobelts on a Si wafer by heating a Mo foil at 850 1C. However, there are still some limitations inherent in the above process. More specifically, the obtained nanoproducts usually consisted of irregular MoO3 nanoparticles, while 1D nanorods have been rarely reported. Furthermore, the hard synthesis conditions, such as high temperature, greatly restricted their practical application in growing MoO3 nanorods on glass substrates. Herein, we demonstrate that the MoO3 nanorods with welldefined crystalline structure could be in-situ grown on FTO substrates by a simple two-step strategy. Furthermore, the morphologies and the crystalline structures of MoO3 products could be rationally tailored by adjusting the reaction parameters.
2. Experiment The molybdenum film was deposited on FTO by magnetron sputtering from Mo metallic targets in a load-locked high vacuum chamber sputter system. The molybdenum deposited FTO (Mo/ FTO) was heated in muffle furnace at 300 1C, 400 1C, 500 1C and 600 1C for 6 h with a heating rate of 1 1C/min, which were named
Y. Ma et al. / Materials Letters 136 (2014) 146–149
Mo-300, Mo-400, Mo-500 and Mo-600, respectively. MoO3 nanorods were obtained when the Mo/FTO was annealing at 500 1C for 6 h which was schematically shown in Scheme 1. The crystalline structure of the films on FTO was analyzed by X-ray diffraction (XRD) by a Philips X’pert MPD instrument. Field-emission scanning electron microscope (FESEM) patterns were recorded on a JEOL-JSM6701F instrument at an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) experiments were conducted with a VG ESCALAB MK Π spectrometer in an ion-pumped chamber. UV vis absorption spectra of the films were recorded using a UV–vis spectrophotometer (JASCOV 550) in the spectral range of 250–800 nm.
3. Results and discussion The molybdenum film was deposited on FTO by sputtering. As shown in Fig. 1a, the typical scanning electron microscopy (SEM) image clearly demonstrates that the molybdenum film is well dispersed on FTO. The Fig. 1b gives a panoramic picture of Mo-500 consisting entirely of nanorods lying randomly with cross-link and intersection. Many smooth nanorods with average diameter of 200 nm, and length of up to 800 nm are shown in Fig. 1c. The
Scheme 1. The growth schematic diagram of sample annealing at 773 K for 6 h.
147
compositions and crystalline structure of obtained Mo/FTO and Mo-500 were characterized by powder X-ray diffraction analysis (Fig. 1d). It can be clearly observed that the XRD pattern of Mo/FTO consists of Mo and SnO2. As shown in the XRD pattern of the Mo500, the disappearance of the most strong diffraction peak at 2θ ¼40.51 devoted to Mo and the occurrence of peaks indexed to orthorhombic MoO3 phase (α-MoO3) (JCPDS no.05-0508) indicates that the molybdenum is oxidized to Mo(VI) entirely. In order to obtain more information between morphology and annealing temperature, the products annealed at 300, 400 and 600 1C were characterized by SEM (Fig. 2). The Mo-300 consists of uniform nanoparticles with diameter of 50 nm, while Mo-400 shows irregular particles with length of 100 nm. When subjected to 600 1C, the belts with 25 μm length, 5 μm width are clearly dispersed on the surface, proving that the product tend to form large size at high temperature. Therefore it is worth mentioning that the temperatures have a significant influence on the morphologies of products. When annealing at 300 1C, the diffraction peaks (Fig. 2d) are identical with Mo/FTO except the weak (111) of α-MoO3, proving that the main composition is Mo. When the temperature increases to 400 1C, the diffraction peaks consist of Mo, α-MoO3 and βMoO3. The diffraction peaks located at 2θ ¼23.31, 27.31 and 45.71 are indexed as (110), (021) and (200) of MoO3 with orthorhombic system (JCPDS no.05-0508), while one peak at 2θ ¼29.21 corresponds to the (101) plane of β-MoO3. The Mo-600 exhibits (020), (040) and (060) diffraction peaks with the strongest intensities and no other peaks are observed, suggesting its anisotropic property [13]. The UV–vis absorption spectra are exhibited in Fig. 3a. The original sample (Mo/FTO) shows no absorption band, and the Mo300 exhibits two strong absorption bands between 250 nm and 400 nm corresponding to the sphere-shape of sample. Besides,
Fig. 1. SEM image of (a) Mo/FTO, (b,c) Mo-500 and (d) XRD pattern (Mo/FTO and Mo-500).
148
Y. Ma et al. / Materials Letters 136 (2014) 146–149
Fig. 2. SEM images of (a) Mo-300, (b) Mo-400, (c) Mo-600, and (d) XRD pattern (Mo/FTO, Mo-300, Mo-400, Mo-600).
Fig. 3. (a) UV–vis spectra (Mo/FTO, Mo-300, Mo-400, Mo-500 and Mo-600), and (b) XPS pattern (Mo-300, Mo-400, Mo-500 and Mo-600).
an adsorption band between 650 nm and 800 nm may be resulted from the existence of MoO3 x suboxides. Both Mo-400 and Mo500 demonstrate an adsorption band between 250 nm and 400 nm, which originate from the charge transfer of the Mo–O band in the MoO66 octahedron [14]. The plot of (αhν)2 versus the energy of light afford band gap energy of 2.92 eV and 3.01 eV for the films annealed at 400 1C and 500 1C, which shows blue-shift for the films compared to that of bulk MoO3 ( 2.9 eV) [15]. The enhanced band gap can be attributed to the nanostructured nature of the films, reduction of oxygen deficiency and the stoichiometric approach of film composition, which results in an increase in band-gap energy [16]. The increase in band gap with particle size is also due to the enhanced band bending effect at the particle
boundaries [17]. While Mo-600 shows scarcely any absorption in the UV range which may be induced by the small amount of products on the substrate at high-temperature. Photoelectron spectroscopy (XPS) was used to examine the products collected at different temperatures to gain insight into the chemical transformation mechanism. The binding energy (232.38 and 235.35 eV) of Mo-300 is lower than the standard Mo (VI) which represents chemically shifted (ΔEa ¼0.4 7 0.1 eV) due to molybdenum and MoO3 x suboxides [18]. Except Mo-300, all products exhibit intense doublet at EB (Mo3d5/2) ¼232.65 eV and EB (Mo3d3/2)¼ 235.85 eV correspond to the Mo6 þ oxidation state from α-MoO3, illustrating that the surface in a few nanometers can be oxidized to Mo(VI) at 400 1C and above. In all
Y. Ma et al. / Materials Letters 136 (2014) 146–149
samples, only Mo-500, the corresponding integral areas ratio of Mo3d5/2/Mo3d3/2 is about 3:2 and the energy gap between the two doublets is 3.2 eV which is in a good agreement with the previous report [19]. This is another powerful evidence to confirm that pure MoO3 with high crystalline were obtained at 500 1C. In addition, the growth mechanism is also speculated on the basis of the growth process in the Supporting Information (SI). 4. Conclusions This study provides a simple, controllable way of fabricating MoO3 nanostructures of interest on FTO based on the oxidation of Mo on FTO by oxygen. It has been found that the Mo film can be entirely oxidized and uniformly nanorods can be obtained at 500 1C for 6 h. On the basis of above results, a clear correlation between the oxidation conditions and the characters of the samples. The band gap energies of these products at different temperature are size-dependent, and blue-shifting and redshifting phenomena are observed from the UV spectra. Moreover, due to the nanorods grown on the FTO, many photoelectric or chemical electric characters can be further checked. Acknowledgments This work is supported by the Chinese Academy of Sciences and Technology Project (XBLZ-2011-013) and the Technologies R&D Program of Gansu Province (No. 1104FKCA156).
149
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.07.143.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Christodoulakis A, Heracleous E, Lemonidou AA. J Catal 2006;242:16–25. Wang KK, Wang FX, Liu YD, Pan GB. Mater Lett 2013;102:8–11. Wang ZY, Madhavi S, Lou XW. J Phys Chem C 2012;116:12508–13. Lou XW, Zeng HC. Chem Mater 2002;14:4781–9. Patzke GR, Michailovski A, Krumeich F. Chem Mater 2004;16:1126–34. Mutschall D, Holzner K, Obermeier E. Sens Actuator B 1996;36:320–4. Nagirnyi VM, Apostolova RD, Shembel EM. Russ J Appl Chem 2006;79: 1438–42. Yao JN, Hashimoto K, Fujishima A. Nature 1992;355:624–6. Ko YH, Raju GSR, Kim S. Phys Status Solidi A 2012;209:2161–6. Song JM, Li YP, Zhu XM. Mater Lett 2013;95:190–2. Ivanova T, Surtchev M, Gesheva K. Mater Lett 2002;53:250–7. Li YB, Bando Y, Golberg D. Appl Phys Lett 2002;81:5048–50. Chen JS, Cheah YL, Madhavi Srinivasan, Lou XW. J Phys Chem C 2010;114: 8675–8. Neeraj S, Kijima N, Cheetham AK. Chem Phys Lett 2004;387:2–6. Toyoda T, Nakanishi H, Endo S, Irie T. J Phys D Appl Phys 1985;18:747–51. Kale RB, Lokhande CD. Semicond Sci Technol 2005;20:1–9. Srikant V, Speck JS, Clark DR. J Appl Phys 1997;81:6357–64. Gulino A, Condorelli GG, Fragala I. J Mater Chem 1996;6:1335–8. Zhou J, Deng SZ, Xu NS, Chen J. Appl Phys Lett 2003;83:2653–5.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled growth of MoO3 nanorods on transparent conducting substrates Ying Ma a,b, Xia Zhang a,b, Min Yang a, Yanxing Qi a,n a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 June 2014 Accepted 23 July 2014 Available online 1 August 2014
MoO3 nanorods with well-defined crystalline structure have been grown in situ on Fluorine doped Tin Oxide glass (FTO) by magnetron sputtering and subsequent oxidation treatment. Moreover, the morphologies and the crystalline structures of MoO3 products could be rationally tailored by adjusting the annealing temperature. More specifically, the calcination operated at 500 1C for 6 h leads to the formation of uniform MoO3 nanorods with an average diameter of 200 nm, and length of up to 800 nm, whereas only irregular nanoparticles or nanoplates have been obtained when the temperature was higher or lower than 500 1C. & 2014 Elsevier B.V. All rights reserved.
Keywords: MoO3 nanorods Sputtering Oxidation morphology UV–vis
1. Introduction Transition metal oxide materials have recently received numerous attentions because of their promising optical, electrical, magnetic and ionic-transport properties. Among them, MoO3 has been extensively investigated ranging from catalysts, chemical sensor, photochromic and electrochromic device to lithium ion battery due to its layered structure and wide band-gap [1–3]. It is well known that the physical and chemical characteristics could be significantly enhanced by tailoring the dimensions as well as morphologies. Thereby, great efforts have been devoted to the synthesis of MoO3 with various nanostructures. Among them, the one dimensional (1D) MoO3 nanostructures, especially nanorods, have become of particular interests as a result of their novel confinement and crystal anisotropy effects. Up to now, MoO3 nanorods have been synthesized via acidification under hydrothermal [4]. Patzake et al. obtained MoO3 nanorods by a flexible one-step solvothermal reaction [5]. However, note that the asreported synthetic approaches cannot be utilized to grow MoO3 nanorods on planar substrates, which greatly restricts their future applications in the fabrication of nano-devices. Thus, the exploration of simple and efficient method for the large-scale synthesis of MoO3 nanorods on planar substrates is still a great challenge. Recently, several approaches have been available for preparing MoO3 film on planar substrates such as sputtering [6], electro-
n
Corresponding author. E-mail addresses: [email protected] (Y. Ma), [email protected] (Y. Qi).
http://dx.doi.org/10.1016/j.matlet.2014.07.143 0167-577X/& 2014 Elsevier B.V. All rights reserved.
deposition [7] and vacuum evaporation [8]. Yeong et al. [9] have prepared nanocracked and porous MoO3 films by electrodeposition and thermal annealing at high temperature. Song et al. [10] have fabricated h- and α-MoO3 films on glass substrates by ion exchange and chemical deposition. Gesheva et al. [11] have obtained MoO3 films on Si wafer and conductive glass by atmospheric pressure Chemical Vapor Deposition (CVD) method using Mo(CO)6 as precursor. Li et al. [12] have synthesized nanobelts on a Si wafer by heating a Mo foil at 850 1C. However, there are still some limitations inherent in the above process. More specifically, the obtained nanoproducts usually consisted of irregular MoO3 nanoparticles, while 1D nanorods have been rarely reported. Furthermore, the hard synthesis conditions, such as high temperature, greatly restricted their practical application in growing MoO3 nanorods on glass substrates. Herein, we demonstrate that the MoO3 nanorods with welldefined crystalline structure could be in-situ grown on FTO substrates by a simple two-step strategy. Furthermore, the morphologies and the crystalline structures of MoO3 products could be rationally tailored by adjusting the reaction parameters.
2. Experiment The molybdenum film was deposited on FTO by magnetron sputtering from Mo metallic targets in a load-locked high vacuum chamber sputter system. The molybdenum deposited FTO (Mo/ FTO) was heated in muffle furnace at 300 1C, 400 1C, 500 1C and 600 1C for 6 h with a heating rate of 1 1C/min, which were named
Y. Ma et al. / Materials Letters 136 (2014) 146–149
Mo-300, Mo-400, Mo-500 and Mo-600, respectively. MoO3 nanorods were obtained when the Mo/FTO was annealing at 500 1C for 6 h which was schematically shown in Scheme 1. The crystalline structure of the films on FTO was analyzed by X-ray diffraction (XRD) by a Philips X’pert MPD instrument. Field-emission scanning electron microscope (FESEM) patterns were recorded on a JEOL-JSM6701F instrument at an accelerating voltage of 5 kV. X-ray photoelectron spectroscopy (XPS) experiments were conducted with a VG ESCALAB MK Π spectrometer in an ion-pumped chamber. UV vis absorption spectra of the films were recorded using a UV–vis spectrophotometer (JASCOV 550) in the spectral range of 250–800 nm.
3. Results and discussion The molybdenum film was deposited on FTO by sputtering. As shown in Fig. 1a, the typical scanning electron microscopy (SEM) image clearly demonstrates that the molybdenum film is well dispersed on FTO. The Fig. 1b gives a panoramic picture of Mo-500 consisting entirely of nanorods lying randomly with cross-link and intersection. Many smooth nanorods with average diameter of 200 nm, and length of up to 800 nm are shown in Fig. 1c. The
Scheme 1. The growth schematic diagram of sample annealing at 773 K for 6 h.
147
compositions and crystalline structure of obtained Mo/FTO and Mo-500 were characterized by powder X-ray diffraction analysis (Fig. 1d). It can be clearly observed that the XRD pattern of Mo/FTO consists of Mo and SnO2. As shown in the XRD pattern of the Mo500, the disappearance of the most strong diffraction peak at 2θ ¼40.51 devoted to Mo and the occurrence of peaks indexed to orthorhombic MoO3 phase (α-MoO3) (JCPDS no.05-0508) indicates that the molybdenum is oxidized to Mo(VI) entirely. In order to obtain more information between morphology and annealing temperature, the products annealed at 300, 400 and 600 1C were characterized by SEM (Fig. 2). The Mo-300 consists of uniform nanoparticles with diameter of 50 nm, while Mo-400 shows irregular particles with length of 100 nm. When subjected to 600 1C, the belts with 25 μm length, 5 μm width are clearly dispersed on the surface, proving that the product tend to form large size at high temperature. Therefore it is worth mentioning that the temperatures have a significant influence on the morphologies of products. When annealing at 300 1C, the diffraction peaks (Fig. 2d) are identical with Mo/FTO except the weak (111) of α-MoO3, proving that the main composition is Mo. When the temperature increases to 400 1C, the diffraction peaks consist of Mo, α-MoO3 and βMoO3. The diffraction peaks located at 2θ ¼23.31, 27.31 and 45.71 are indexed as (110), (021) and (200) of MoO3 with orthorhombic system (JCPDS no.05-0508), while one peak at 2θ ¼29.21 corresponds to the (101) plane of β-MoO3. The Mo-600 exhibits (020), (040) and (060) diffraction peaks with the strongest intensities and no other peaks are observed, suggesting its anisotropic property [13]. The UV–vis absorption spectra are exhibited in Fig. 3a. The original sample (Mo/FTO) shows no absorption band, and the Mo300 exhibits two strong absorption bands between 250 nm and 400 nm corresponding to the sphere-shape of sample. Besides,
Fig. 1. SEM image of (a) Mo/FTO, (b,c) Mo-500 and (d) XRD pattern (Mo/FTO and Mo-500).
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Y. Ma et al. / Materials Letters 136 (2014) 146–149
Fig. 2. SEM images of (a) Mo-300, (b) Mo-400, (c) Mo-600, and (d) XRD pattern (Mo/FTO, Mo-300, Mo-400, Mo-600).
Fig. 3. (a) UV–vis spectra (Mo/FTO, Mo-300, Mo-400, Mo-500 and Mo-600), and (b) XPS pattern (Mo-300, Mo-400, Mo-500 and Mo-600).
an adsorption band between 650 nm and 800 nm may be resulted from the existence of MoO3 x suboxides. Both Mo-400 and Mo500 demonstrate an adsorption band between 250 nm and 400 nm, which originate from the charge transfer of the Mo–O band in the MoO66 octahedron [14]. The plot of (αhν)2 versus the energy of light afford band gap energy of 2.92 eV and 3.01 eV for the films annealed at 400 1C and 500 1C, which shows blue-shift for the films compared to that of bulk MoO3 ( 2.9 eV) [15]. The enhanced band gap can be attributed to the nanostructured nature of the films, reduction of oxygen deficiency and the stoichiometric approach of film composition, which results in an increase in band-gap energy [16]. The increase in band gap with particle size is also due to the enhanced band bending effect at the particle
boundaries [17]. While Mo-600 shows scarcely any absorption in the UV range which may be induced by the small amount of products on the substrate at high-temperature. Photoelectron spectroscopy (XPS) was used to examine the products collected at different temperatures to gain insight into the chemical transformation mechanism. The binding energy (232.38 and 235.35 eV) of Mo-300 is lower than the standard Mo (VI) which represents chemically shifted (ΔEa ¼0.4 7 0.1 eV) due to molybdenum and MoO3 x suboxides [18]. Except Mo-300, all products exhibit intense doublet at EB (Mo3d5/2) ¼232.65 eV and EB (Mo3d3/2)¼ 235.85 eV correspond to the Mo6 þ oxidation state from α-MoO3, illustrating that the surface in a few nanometers can be oxidized to Mo(VI) at 400 1C and above. In all
Y. Ma et al. / Materials Letters 136 (2014) 146–149
samples, only Mo-500, the corresponding integral areas ratio of Mo3d5/2/Mo3d3/2 is about 3:2 and the energy gap between the two doublets is 3.2 eV which is in a good agreement with the previous report [19]. This is another powerful evidence to confirm that pure MoO3 with high crystalline were obtained at 500 1C. In addition, the growth mechanism is also speculated on the basis of the growth process in the Supporting Information (SI). 4. Conclusions This study provides a simple, controllable way of fabricating MoO3 nanostructures of interest on FTO based on the oxidation of Mo on FTO by oxygen. It has been found that the Mo film can be entirely oxidized and uniformly nanorods can be obtained at 500 1C for 6 h. On the basis of above results, a clear correlation between the oxidation conditions and the characters of the samples. The band gap energies of these products at different temperature are size-dependent, and blue-shifting and redshifting phenomena are observed from the UV spectra. Moreover, due to the nanorods grown on the FTO, many photoelectric or chemical electric characters can be further checked. Acknowledgments This work is supported by the Chinese Academy of Sciences and Technology Project (XBLZ-2011-013) and the Technologies R&D Program of Gansu Province (No. 1104FKCA156).
149
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.07.143.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Christodoulakis A, Heracleous E, Lemonidou AA. J Catal 2006;242:16–25. Wang KK, Wang FX, Liu YD, Pan GB. Mater Lett 2013;102:8–11. Wang ZY, Madhavi S, Lou XW. J Phys Chem C 2012;116:12508–13. Lou XW, Zeng HC. Chem Mater 2002;14:4781–9. Patzke GR, Michailovski A, Krumeich F. Chem Mater 2004;16:1126–34. Mutschall D, Holzner K, Obermeier E. Sens Actuator B 1996;36:320–4. Nagirnyi VM, Apostolova RD, Shembel EM. Russ J Appl Chem 2006;79: 1438–42. Yao JN, Hashimoto K, Fujishima A. Nature 1992;355:624–6. Ko YH, Raju GSR, Kim S. Phys Status Solidi A 2012;209:2161–6. Song JM, Li YP, Zhu XM. Mater Lett 2013;95:190–2. Ivanova T, Surtchev M, Gesheva K. Mater Lett 2002;53:250–7. Li YB, Bando Y, Golberg D. Appl Phys Lett 2002;81:5048–50. Chen JS, Cheah YL, Madhavi Srinivasan, Lou XW. J Phys Chem C 2010;114: 8675–8. Neeraj S, Kijima N, Cheetham AK. Chem Phys Lett 2004;387:2–6. Toyoda T, Nakanishi H, Endo S, Irie T. J Phys D Appl Phys 1985;18:747–51. Kale RB, Lokhande CD. Semicond Sci Technol 2005;20:1–9. Srikant V, Speck JS, Clark DR. J Appl Phys 1997;81:6357–64. Gulino A, Condorelli GG, Fragala I. J Mater Chem 1996;6:1335–8. Zhou J, Deng SZ, Xu NS, Chen J. Appl Phys Lett 2003;83:2653–5.