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Effect of sulfur dopants on the porous structure and electrical properties of mesoporous TiO2 thin films
Materials Letters 106 (2013) 401–404
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of sulfur dopants on the porous structure and electrical properties of mesoporous TiO2 thin films Chang-Sun Park, Uzma K.H. Bangi, Hyung-Ho Park n Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2013 Accepted 18 May 2013 Available online 27 May 2013
In this work, sulfur-doped mesoporous TiO2 films were formed by a chemical solution deposition technique to investigate their structure and electrical conductivity. Phase formation and crystallization of TiO2 was retarded and the optical band gap was reduced due to the presence of the sulfur dopants. As a result, enhancements in the microstructural stability and electrical conductivity of mesoporous TiO2 could be obtained by sulfur doping. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mesoporous TiO2 Sulfur doping Porous structure Electrical properties
1. Introduction Mesoporous materials are materials containing pores with diameters between 2 and 50 nm [1]. They have long been a popular research topic and research in this field has grown steadily since the 1970s [2]. Mesoporous metal oxides are most commonly used for thermal stabilization, catalyst support, and sensor applications due to their low cost and thermal, chemical, and mechanical stability [3]. However, mesoporous materials have narrow industrial applications because of the majority of studies on the preparation of mesoporous materials appear to be focused on silica. A limited body of research has been carried out on mesoporous transition metal oxides due to instability of the pore structure of transition metal oxides during calcination [4], the high reactivity of the common precursors and the relatively soft chemical conditions needed in the initial stages of the assembling of the mesoporous networks [5]. Especially this pore structural instability is because crystallization or phase transformation of transition metal oxides can induce a modification of the mesoporous wall structure and degradation of the overall pore structure, which is not the case in amorphous silica. Micro-structural stability at high temperature is one of the most important factors considered in the application of mesoporous materials. Recently, the application of mesoporous metal oxides as thermoelectric materials has also been studied due to the excellent thermal insulation offered by the mesoporous structure [6,7]. A good thermoelectric material typically has high electrical conductivity and low thermal conductivity. A high temperature annealing may
n
Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 312 5375. E-mail address: [email protected] (H.-H. Park).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.091
be necessary for mesoporous metal oxide to form the desired phase state. However, high temperature thermal treatments may also lead to the collapse of the mesoporous structure. For this reason, it is necessary to enhance the micro-structural stability of mesoporous metal oxides during high temperature annealing to maintain its phonon-scattering property [8]. Titanium dioxide (TiO2) has been actively investigated due to its wide range of possible applications. Addition of sulfur (S) to TiO2 induces a decrease in its band gap and inhibits grain growth in TiO2 [9]. In general, decrease in band gap and increase in inhibition of grain growth correlate with increase in electrical conductivity and enhancement in structural pore stability. In this study, the effects of S-doping on the pore structural stability and electrical conductivity of mesoporous TiO2 was investigated to control both the thermal and electrical conductivities. 2. Experimental Materials and method: Sulfur-doped mesoporous TiO2 thin films were synthesized from titanium tetraisopropoxide [Ti(OPr)4, TTIP, Aldrich, 97%], a triblock copolymer Pluronic P-123 (EO20PO70EO20, Aldrich, MW 5800), thiourea (CH4N2S, Aldrich, 99.0%), 1-propanol (Duksan, 99.5%), and hydrochloric acid (HCl, Duksan), used as TiO2 precursor, surfactant, dopant, solvent and catalyst, respectively. The reactant mix had a composition of TTIP:P-123:HCl:1-propanol in a molar ratio of 1:0.015:3.2:13.2. The S was introduced as thiourea in the amount of 0.24 g (an overall concentration of 2.3 at % S in TiO2 in film); these doped and undoped mesoporous TiO2 samples were called T and TS, respectively. Spin-coating was performed at 3000 rpm for 20 s to form thin films approximately 250 nm thick. The as-prepared thin films were aged under fixed
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C.-S. Park et al. / Materials Letters 106 (2013) 401–404
conditions (at 8 1C) for 120 h. After aging, the as-prepared films were annealed in a vacuum chamber at 450 1C or 600 1C for 3 h. Characterization: X-ray diffraction (Ultima IV, Rigaku) using Cu Kα radiation (λ ¼1.5418 Å) was performed to study the pore ordering and crystalline structure of mesoporous S-doped TiO2 films. Small- and wide-angle patterns were recorded in the 2θ ranges between 11 and 51 and 201 and 451, respectively. The film composition was determined with X-ray photoelectron spectroscopy. The mesostructure of S-doped TiO2 films was investigated using grazing incidence small angle X-ray scattering (GISAXS) with the 3C beamline (λ ¼1.24 Å and ΔE/E ¼2 10−4) of the Pohang Light Source (PLS) in Republic of Korea. The surface morphology was measured with surface image scanning electron microscopy (JEOL, JSM 7001F, Tokyo, Japan). Optical transmittance measurements were performed using an ultraviolet–visible-near infrared (UV–vis-NIR, V-570, Jasco) spectrophotometer (V-570, JASCO). Hall measurement equipment was used to measure the electrical resistivity of the mesoporous S-doped TiO2 films.
3. Results and discussion In order to confirm the crystal structure of the synthesized mesoporous TiO2 films, XRD analysis was performed in a wide angle range, between 201 and 451, as shown in Fig. 1(a). To remove the surfactant templates and form a mesoporous structure, the spin-coated films were annealed at 450 1C for 3 h. At this temperature, we found that neither the T nor TS samples were crystallized. The diffraction peaks of anatase and rutile TiO2 phases were not found until annealing the T sample at 600 1C; on the other hand, almost no diffraction peaks were observed in the TS sample. The retardation of the crystallization of TiO2 and the inherent increases in the phase formation temperature could be associated to the presence of the sulfur, which is known to form sulfated TiO2 with consequently surface stabilization effect [10]. This effect can be also observed on the particle/grain growth of TiO2 as shown in Fig. 1(b), microstructure of T and TS samples after anneal at 450 and 600 1C, respectively. It was revealed that the microstructures of T samples (450 and 600 1C) seemed to be more and better crystallized than those of TS samples (450 and 600 1C) due to S-doping in TiO2. Calcination of TiO2 mesoporous structures at high temperature is usually undesirable because of the collapse
of the mesoporous framework due to crystallization and subsequent crystal growth [4]. Small angle XRD analysis was performed to investigate the pore arrangement in the S-doped TiO2 mesoporous structure after high temperature anneal; results are presented in Fig. 2(a). As shown in the figure, the diffraction peak intensity of ordered mesoporous TiO2 decreased with increasing anneal temperature because of the collapse of the ordered mesoporous structure. The diffraction peak intensity of a TS sample annealed at a given temperature was found to be higher than that of the corresponding T sample. From these results, it can be said that the incorporation of S in TiO2 reduced the grain growth due to a presence of sulfated TiO2 [10] and increases the stability of the mesoporous structure. To clarify the S-doping dependent change in mesostructure, GISAXS analysis was carried out; the data from which is presented in Fig. 2(b). The relative GISAXS pattern of 450 1C shows a wing-type pattern, the intensity of which was increased with TS samples. It has been known that this wing-type pattern is correlated to the presence of ordered pore arrangements in mesoporous structures [11]. In the GISAXS patterns of samples annealed at 600 1C, this wing-type pattern was no longer observed due to the collapse of the ordered pore arrangement at high temperature. However, a triangular prism-type pattern was observed due to an order-disorder pore arrangement transition [12]; the intensity of the pattern was higher in the TS samples because S-doping induces an increase in the micro-structural stability of the mesoporous TiO2 structure. We analyzed the optical band gap of S-doped TiO2 mesoporous films to investigate the effect of S dopants on the TiO2 band gap. The absorption coefficient was calculated using Lambert's formula, α ¼ ð1=tÞ½lnð1=T r Þ where Tr and t are the transmittance and thickness of the film, respectively [13]. The indirect band gaps of S-doped and undoped TiO2 could be estimated from the (αhv)1/2 vs. hv (Tauc relation) plot by extrapolating the linear fit to α¼0, as shown in Fig. 3(a). The estimated optical band gap values for T and TS (450, 600 1C) samples were approximately 3.47 eV and 3.41–3.38 eV, respectively. The optical band gap decreased slightly with S-doping. The indirect optical band gap of the TiO2 films varied from 3.0 to 3.75 eV [14,15]. Reason for the reduction in the optical band gap of S-doped TiO2 films is an increase in the valence band width due to orbital mixing with the S 3p valence state [16]. We measured the
Fig. 1. (a) Wide angle XRD spectra and (b) surface image of T and TS films.
C.-S. Park et al. / Materials Letters 106 (2013) 401–404
403
Fig. 2. (a) Small angle XRD spectra and (b) GISAXS patterns of T and TS films.
Fig. 3. (a) Plot for the evaluated optical band gap and (b) electrical resistivity of T and TS films.
electrical resistivity to confirm the effect of the narrow optical band gap and presented the results in Fig. 3(b). The figure shows clearly that the electrical resistivity of S-doped mesoporous TiO2 decreased with increasing anneal temperature and by S-doping. As a result, S-doping of TiO2 mesoporous films strengthened pore structure and increased electrical conductivity due to a possible integration of the sulfur to the TiO2 network. This structural and electrical property improvement could be useful when applying mesoporous TiO2 to thermoelectric devices.
4. Conclusions The mesoporous structure, optical band gap, and electrical properties of S-doped mesoporous TiO2 films were investigated. When sulfur was doped into mesoporous TiO2, the stability of the mesoporous structure was enhanced by the retardation of new phase formation and crystallization and the electrical resistivity was decreased by the narrowing of the bandgap.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A2A2A01011014). Experiments at PLS were supported in part by MEST and POSTECH.
References [1] Rouquerol J, Avnir D, Fairbridge CW, Everett DH, Haynes JM, Pernicone N, et al. Recommendations for the characterization of porous solids (Technical Report). Pure Appl Chem 1994;66:1739. [2] Chiola V, Ritsko JE, Vanderpool CD. Process for producing low-bulk density silica. U.S. Patent 3556725 1971. [3] Misra C. Industrial alumina chemicals, ACS monograph series.. Washington DC: American Chemical Society; 1986. [4] Elder SH, Gao Y, Li X, Liu J, McCready DE, Windisch Jr CF, et al. Zirconiastabilized 25-Å TiO2 anatase crystallites in a mesoporous structure. Chem Mater 1998;10:3140–5. [5] Grosso D, Cagnol F, Soler-Illia GJ, Crepaldi EL, Amenitsch H, Brunet-Bruneau A, et al. Adv Funct Mater 2004;14:309–22.
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C.-S. Park et al. / Materials Letters 106 (2013) 401–404
[6] Ha TJ, Park HH, Jung SY, Yoon SJ, Kim JS, Jang HW. Effect of porosity on the Seebeck coefficient of mesoporous TiO2 thin films. Thin Solid Films 2010;518:7196–8. [7] Jung SY, Ha TJ, Seo WS, Lim YS, Shin S, Cho HH, et al. Thermoelectric properties of Nb-doped ordered mesoporous TiO2. J Electron Mater 2011;40:652–6. [8] Ha TJ, Jung SY, Bae JH, Lee HL, Jang HW, Yoon SJ, et al. Analysis of heat transfer in ordered and disordered mesoporous TiO2 films by finite element analysis. Microporous Mesoporous Mater 2011;144:191–4. [9] Periyat P, Pillai SS, McCormack DE, Colreavy J, Hinder SJ. Improved hightemperature stability and sun-light-driven photocatalytic activity of sulfurdoped anatase TiO2. J Phys Chem C 2008;112:7644–52. [10] Colon G, Sanchez-Espana JM, Hidalgo MC, Navio JA. Effect of TiO2 acidic pretreatment on the photocatalytic properties for phenol degradation. J Photochem Photobiol A: Chem 2006;179:20–7.
[11] Lancelle-Beltran E, Prené P, Boscher C, Belleville P, Buvat P, Lamberti S, et al. Nanostructured hybrid solar cells based on self-assembled mesoporous titania thin films. Chem Mater 2006;18:6152–6. [12] Casanova JR, Heredia EA, Bojorge CD, Cánepa HR, Kellermann G, Craievich AF. Structural characterization of supported nanocrystalline ZnO thin films prepared by dip-coating. Appl Surf Sci 2011;257:10045–51. [13] Tan ST, Chen BJ, Sun XW, Fan WJ, Kwok HS, Zhang XH, et al. Blueshift of optical band gap in ZnO thin films grown by metal-organic chemical-vapor deposition. J Appl Phys 2005;98:013505–9. [14] Park YR, Kim KJ. Structural and optical properties of rutile and anatase TiO2 thin films: effects of Co doping. Thin Solid Films 2005;484:34–8. [15] Tang H, Prasad K, Sanjines R, Schmid PE, Levy F. Electrical and optical properties of TiO2 anatase thin films. J Appl Phys 1994;75:2042–7. [16] Umebayashi T, Yamaki T, Itoh H, Asai K. Band gap narrowing of titanium dioxide by sulfur doping. Appl Phys Lett 2002;81:454–6.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of sulfur dopants on the porous structure and electrical properties of mesoporous TiO2 thin films Chang-Sun Park, Uzma K.H. Bangi, Hyung-Ho Park n Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2013 Accepted 18 May 2013 Available online 27 May 2013
In this work, sulfur-doped mesoporous TiO2 films were formed by a chemical solution deposition technique to investigate their structure and electrical conductivity. Phase formation and crystallization of TiO2 was retarded and the optical band gap was reduced due to the presence of the sulfur dopants. As a result, enhancements in the microstructural stability and electrical conductivity of mesoporous TiO2 could be obtained by sulfur doping. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mesoporous TiO2 Sulfur doping Porous structure Electrical properties
1. Introduction Mesoporous materials are materials containing pores with diameters between 2 and 50 nm [1]. They have long been a popular research topic and research in this field has grown steadily since the 1970s [2]. Mesoporous metal oxides are most commonly used for thermal stabilization, catalyst support, and sensor applications due to their low cost and thermal, chemical, and mechanical stability [3]. However, mesoporous materials have narrow industrial applications because of the majority of studies on the preparation of mesoporous materials appear to be focused on silica. A limited body of research has been carried out on mesoporous transition metal oxides due to instability of the pore structure of transition metal oxides during calcination [4], the high reactivity of the common precursors and the relatively soft chemical conditions needed in the initial stages of the assembling of the mesoporous networks [5]. Especially this pore structural instability is because crystallization or phase transformation of transition metal oxides can induce a modification of the mesoporous wall structure and degradation of the overall pore structure, which is not the case in amorphous silica. Micro-structural stability at high temperature is one of the most important factors considered in the application of mesoporous materials. Recently, the application of mesoporous metal oxides as thermoelectric materials has also been studied due to the excellent thermal insulation offered by the mesoporous structure [6,7]. A good thermoelectric material typically has high electrical conductivity and low thermal conductivity. A high temperature annealing may
n
Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 312 5375. E-mail address: [email protected] (H.-H. Park).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.05.091
be necessary for mesoporous metal oxide to form the desired phase state. However, high temperature thermal treatments may also lead to the collapse of the mesoporous structure. For this reason, it is necessary to enhance the micro-structural stability of mesoporous metal oxides during high temperature annealing to maintain its phonon-scattering property [8]. Titanium dioxide (TiO2) has been actively investigated due to its wide range of possible applications. Addition of sulfur (S) to TiO2 induces a decrease in its band gap and inhibits grain growth in TiO2 [9]. In general, decrease in band gap and increase in inhibition of grain growth correlate with increase in electrical conductivity and enhancement in structural pore stability. In this study, the effects of S-doping on the pore structural stability and electrical conductivity of mesoporous TiO2 was investigated to control both the thermal and electrical conductivities. 2. Experimental Materials and method: Sulfur-doped mesoporous TiO2 thin films were synthesized from titanium tetraisopropoxide [Ti(OPr)4, TTIP, Aldrich, 97%], a triblock copolymer Pluronic P-123 (EO20PO70EO20, Aldrich, MW 5800), thiourea (CH4N2S, Aldrich, 99.0%), 1-propanol (Duksan, 99.5%), and hydrochloric acid (HCl, Duksan), used as TiO2 precursor, surfactant, dopant, solvent and catalyst, respectively. The reactant mix had a composition of TTIP:P-123:HCl:1-propanol in a molar ratio of 1:0.015:3.2:13.2. The S was introduced as thiourea in the amount of 0.24 g (an overall concentration of 2.3 at % S in TiO2 in film); these doped and undoped mesoporous TiO2 samples were called T and TS, respectively. Spin-coating was performed at 3000 rpm for 20 s to form thin films approximately 250 nm thick. The as-prepared thin films were aged under fixed
402
C.-S. Park et al. / Materials Letters 106 (2013) 401–404
conditions (at 8 1C) for 120 h. After aging, the as-prepared films were annealed in a vacuum chamber at 450 1C or 600 1C for 3 h. Characterization: X-ray diffraction (Ultima IV, Rigaku) using Cu Kα radiation (λ ¼1.5418 Å) was performed to study the pore ordering and crystalline structure of mesoporous S-doped TiO2 films. Small- and wide-angle patterns were recorded in the 2θ ranges between 11 and 51 and 201 and 451, respectively. The film composition was determined with X-ray photoelectron spectroscopy. The mesostructure of S-doped TiO2 films was investigated using grazing incidence small angle X-ray scattering (GISAXS) with the 3C beamline (λ ¼1.24 Å and ΔE/E ¼2 10−4) of the Pohang Light Source (PLS) in Republic of Korea. The surface morphology was measured with surface image scanning electron microscopy (JEOL, JSM 7001F, Tokyo, Japan). Optical transmittance measurements were performed using an ultraviolet–visible-near infrared (UV–vis-NIR, V-570, Jasco) spectrophotometer (V-570, JASCO). Hall measurement equipment was used to measure the electrical resistivity of the mesoporous S-doped TiO2 films.
3. Results and discussion In order to confirm the crystal structure of the synthesized mesoporous TiO2 films, XRD analysis was performed in a wide angle range, between 201 and 451, as shown in Fig. 1(a). To remove the surfactant templates and form a mesoporous structure, the spin-coated films were annealed at 450 1C for 3 h. At this temperature, we found that neither the T nor TS samples were crystallized. The diffraction peaks of anatase and rutile TiO2 phases were not found until annealing the T sample at 600 1C; on the other hand, almost no diffraction peaks were observed in the TS sample. The retardation of the crystallization of TiO2 and the inherent increases in the phase formation temperature could be associated to the presence of the sulfur, which is known to form sulfated TiO2 with consequently surface stabilization effect [10]. This effect can be also observed on the particle/grain growth of TiO2 as shown in Fig. 1(b), microstructure of T and TS samples after anneal at 450 and 600 1C, respectively. It was revealed that the microstructures of T samples (450 and 600 1C) seemed to be more and better crystallized than those of TS samples (450 and 600 1C) due to S-doping in TiO2. Calcination of TiO2 mesoporous structures at high temperature is usually undesirable because of the collapse
of the mesoporous framework due to crystallization and subsequent crystal growth [4]. Small angle XRD analysis was performed to investigate the pore arrangement in the S-doped TiO2 mesoporous structure after high temperature anneal; results are presented in Fig. 2(a). As shown in the figure, the diffraction peak intensity of ordered mesoporous TiO2 decreased with increasing anneal temperature because of the collapse of the ordered mesoporous structure. The diffraction peak intensity of a TS sample annealed at a given temperature was found to be higher than that of the corresponding T sample. From these results, it can be said that the incorporation of S in TiO2 reduced the grain growth due to a presence of sulfated TiO2 [10] and increases the stability of the mesoporous structure. To clarify the S-doping dependent change in mesostructure, GISAXS analysis was carried out; the data from which is presented in Fig. 2(b). The relative GISAXS pattern of 450 1C shows a wing-type pattern, the intensity of which was increased with TS samples. It has been known that this wing-type pattern is correlated to the presence of ordered pore arrangements in mesoporous structures [11]. In the GISAXS patterns of samples annealed at 600 1C, this wing-type pattern was no longer observed due to the collapse of the ordered pore arrangement at high temperature. However, a triangular prism-type pattern was observed due to an order-disorder pore arrangement transition [12]; the intensity of the pattern was higher in the TS samples because S-doping induces an increase in the micro-structural stability of the mesoporous TiO2 structure. We analyzed the optical band gap of S-doped TiO2 mesoporous films to investigate the effect of S dopants on the TiO2 band gap. The absorption coefficient was calculated using Lambert's formula, α ¼ ð1=tÞ½lnð1=T r Þ where Tr and t are the transmittance and thickness of the film, respectively [13]. The indirect band gaps of S-doped and undoped TiO2 could be estimated from the (αhv)1/2 vs. hv (Tauc relation) plot by extrapolating the linear fit to α¼0, as shown in Fig. 3(a). The estimated optical band gap values for T and TS (450, 600 1C) samples were approximately 3.47 eV and 3.41–3.38 eV, respectively. The optical band gap decreased slightly with S-doping. The indirect optical band gap of the TiO2 films varied from 3.0 to 3.75 eV [14,15]. Reason for the reduction in the optical band gap of S-doped TiO2 films is an increase in the valence band width due to orbital mixing with the S 3p valence state [16]. We measured the
Fig. 1. (a) Wide angle XRD spectra and (b) surface image of T and TS films.
C.-S. Park et al. / Materials Letters 106 (2013) 401–404
403
Fig. 2. (a) Small angle XRD spectra and (b) GISAXS patterns of T and TS films.
Fig. 3. (a) Plot for the evaluated optical band gap and (b) electrical resistivity of T and TS films.
electrical resistivity to confirm the effect of the narrow optical band gap and presented the results in Fig. 3(b). The figure shows clearly that the electrical resistivity of S-doped mesoporous TiO2 decreased with increasing anneal temperature and by S-doping. As a result, S-doping of TiO2 mesoporous films strengthened pore structure and increased electrical conductivity due to a possible integration of the sulfur to the TiO2 network. This structural and electrical property improvement could be useful when applying mesoporous TiO2 to thermoelectric devices.
4. Conclusions The mesoporous structure, optical band gap, and electrical properties of S-doped mesoporous TiO2 films were investigated. When sulfur was doped into mesoporous TiO2, the stability of the mesoporous structure was enhanced by the retardation of new phase formation and crystallization and the electrical resistivity was decreased by the narrowing of the bandgap.
Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A2A2A01011014). Experiments at PLS were supported in part by MEST and POSTECH.
References [1] Rouquerol J, Avnir D, Fairbridge CW, Everett DH, Haynes JM, Pernicone N, et al. Recommendations for the characterization of porous solids (Technical Report). Pure Appl Chem 1994;66:1739. [2] Chiola V, Ritsko JE, Vanderpool CD. Process for producing low-bulk density silica. U.S. Patent 3556725 1971. [3] Misra C. Industrial alumina chemicals, ACS monograph series.. Washington DC: American Chemical Society; 1986. [4] Elder SH, Gao Y, Li X, Liu J, McCready DE, Windisch Jr CF, et al. Zirconiastabilized 25-Å TiO2 anatase crystallites in a mesoporous structure. Chem Mater 1998;10:3140–5. [5] Grosso D, Cagnol F, Soler-Illia GJ, Crepaldi EL, Amenitsch H, Brunet-Bruneau A, et al. Adv Funct Mater 2004;14:309–22.
404
C.-S. Park et al. / Materials Letters 106 (2013) 401–404
[6] Ha TJ, Park HH, Jung SY, Yoon SJ, Kim JS, Jang HW. Effect of porosity on the Seebeck coefficient of mesoporous TiO2 thin films. Thin Solid Films 2010;518:7196–8. [7] Jung SY, Ha TJ, Seo WS, Lim YS, Shin S, Cho HH, et al. Thermoelectric properties of Nb-doped ordered mesoporous TiO2. J Electron Mater 2011;40:652–6. [8] Ha TJ, Jung SY, Bae JH, Lee HL, Jang HW, Yoon SJ, et al. Analysis of heat transfer in ordered and disordered mesoporous TiO2 films by finite element analysis. Microporous Mesoporous Mater 2011;144:191–4. [9] Periyat P, Pillai SS, McCormack DE, Colreavy J, Hinder SJ. Improved hightemperature stability and sun-light-driven photocatalytic activity of sulfurdoped anatase TiO2. J Phys Chem C 2008;112:7644–52. [10] Colon G, Sanchez-Espana JM, Hidalgo MC, Navio JA. Effect of TiO2 acidic pretreatment on the photocatalytic properties for phenol degradation. J Photochem Photobiol A: Chem 2006;179:20–7.
[11] Lancelle-Beltran E, Prené P, Boscher C, Belleville P, Buvat P, Lamberti S, et al. Nanostructured hybrid solar cells based on self-assembled mesoporous titania thin films. Chem Mater 2006;18:6152–6. [12] Casanova JR, Heredia EA, Bojorge CD, Cánepa HR, Kellermann G, Craievich AF. Structural characterization of supported nanocrystalline ZnO thin films prepared by dip-coating. Appl Surf Sci 2011;257:10045–51. [13] Tan ST, Chen BJ, Sun XW, Fan WJ, Kwok HS, Zhang XH, et al. Blueshift of optical band gap in ZnO thin films grown by metal-organic chemical-vapor deposition. J Appl Phys 2005;98:013505–9. [14] Park YR, Kim KJ. Structural and optical properties of rutile and anatase TiO2 thin films: effects of Co doping. Thin Solid Films 2005;484:34–8. [15] Tang H, Prasad K, Sanjines R, Schmid PE, Levy F. Electrical and optical properties of TiO2 anatase thin films. J Appl Phys 1994;75:2042–7. [16] Umebayashi T, Yamaki T, Itoh H, Asai K. Band gap narrowing of titanium dioxide by sulfur doping. Appl Phys Lett 2002;81:454–6.