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Effect of plasma treatment on chemical bonding states of porous TiO2 thin films prepared from polymer-blended solution
Thin Solid Films 519 (2011) 6916–6919
Contents lists available at ScienceDirect
Thin Solid Films 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 / t s f
Effect of plasma treatment on chemical bonding states of porous TiO2 thin films prepared from polymer-blended solution J.P. Kim a, H.U. Lee b, J.E. Yang a, J.S. Bae a, E.S. Park a, J.H. Yoon a, J.M. Kim a, C.R. Cho b,⁎ a b
Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea College of Nanoscience and Nanotechnology, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Available online 22 April 2011 Keywords: Plasma treatment Chemical bonding states Porous TiO2 Polymer Photocatalysis Surface energy
a b s t r a c t Dense TiO2 (D-TiO2) thin films and porous TiO2 (P-TiO2) thin films were prepared by using a polymer-blended solution. The film porosity decreased gradually or disappeared with an increase in the polyethylene glycol (PEG) or TiO2 content of the solution. To modify their surface properties, the thin films were treated with atmospheric pressure (AP) plasma by using a reactive gas. The surface morphologies of the O2-plasma-treated TiO2 (O-TiO2) thin films were smooth and did not change significantly. The decolorization efficiency of the P-TiO2 thin films was found to be enhanced when compared to that of the D-TiO2 thin films. The enhancement was due to an increase in the specific surface area and the number of hydroxyl groups, and a decrease of Ti2O3 states. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2), which is a transition metal oxide, has received considerable attention its use in many applications, e.g., photocatalysis, solar energy cells, and gas sensors. TiO2 occurs naturally in the minerals rutile, anatase, and brookite. The rutile and anatase phases have been intensively studied and have significant technological uses owing to their optical properties. TiO2 is a largebandgap semiconductor with a catalytically active surface. The surface of transition metal oxides plays an important role in a variety of technological applications, ranging from electronic devices to catalyst development [1]. Recently, a large number of studies have been carried out to investigate surface structures of TiO2 and to devise TiO2 preparation methods in which the surface structure can be controlled [2,3]. The properties of TiO2 nanostructures with large surface area, such as nanorods, nanowires, and nanofibers have been reported [4,5]. The use of TiO2 nanoparticles encapsulated with polymers, which can be synthesized by using an emulsification process, has been shown to improve the efficiency of various optoelectronic devices [6]. Porous TiO2 thin films prepared from a polymer-blended TiO2 solution are considered as better photocatalytic materials than films having a dense morphology because the former films are capable of absorbing a large amount of visible light and have a high specific surface area [7]. A photoelectron conductive TiO2 film usually suffers the loss of a small amount of oxygen [8], which results in the formation of dominant surface defect states. These surface states are known to be located in
⁎ Corresponding author. Tel.: + 82 55 350 5297; fax: + 82 55 353 1314. E-mail address: [email protected] (C.R. Cho). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.053
the interband region, and they lead to slow charge transport in the film owing to the trapping and detrapping of electrons [9]. The oxygen vacancy states have been found to exist about 0.75–1.18 eV below the conduction band edge of TiO2 [10] and can thus be excited from the valence band by visible wavelengths of around 506 and 614 nm. Many studies have focused on the surface properties related to the photocatalytic behavior of TiO2 materials [11]. However, the effect of atmospheric pressure (AP) plasma treatment on the surface properties and decolorization efficiency of polymer-mediated porous TiO2 thin films has not been studied. In this study, dense TiO2 thin films and porous TiO2 thin films were prepared on silicon and glass substrates by using a polymer-blended solution. The surface properties of the TiO2 thin films after O2 plasma treatment (O-TiO2) were examined. The photocatalytic behavior of the films was also determined by examining the decolorization of the dye solution. 2. Experimental details Polymer-blended TiO2 solution was synthesized at room temperature in the following manner: (C5H8O2)2TiO (Ti-oxyacetylacetonate) was dissolved in a mixture solution of 10 ml of 2-methoxyethanol (2-MOE) and 0.12 g of polyethylene glycol (PEG). As is well known, PEG is a nonionic surfactant and has a long chain with a zigzag structure; it is also known that in the molecule, \O\ is hydrophilic and \CH2\CH2\ is hydrophobic [12]. After stirring the mixture solution for 2 h, Ti-acylacetate was added and the solution was continually stirred for 2 h; transparent polymer-blended TiO2 solution was obtained. A spin-coating technique was used to coat the polymerblended solution on glass and Si substrates. The annealing temperature
J.P. Kim et al. / Thin Solid Films 519 (2011) 6916–6919
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S-4200). The chemical bonding states of the films were determined by X-ray photoelectron spectroscopy (XPS; VG Scientific, ESCALAB250) that was performed with a hemispherical electrostatic energy analyzer and an Al Kα (1486.6 eV) X-ray source. The photocatalytic characteristics and optical transmittance of the films were examined by using an ultraviolet–visible spectrophotometer (Varian, Cary5E). The surface wettability was determined from the contact angle (CA; Dataphysics, OCA100). 3. Results and discussion
Fig. 1. GIXRD patterns of TiO2 thin films: (a) dense film, (b) porous film, (c) dense film treated with O2 plasma, and (d) porous film treated with O2 plasma. A denotes anatase phase of the TiO2 films.
was set at 450 °C because the phase transition from anatase to rutile starts around this temperature [13]. Dense TiO2 (D-TiO2) thin films were prepared, and they were then coated with TiO2 solution without PEG polymer and annealed. Porous TiO2 (P-TiO2) thin films that were coated with the polymer-blended solution and annealed showed porous surface morphology because of the evaporation of the PEG polymer. Then, to improve the surface properties of the D-TiO2 thin films and P-TiO2 thin film, the films were treated with O2 plasma (the treated films are hereafter referred to as OD-TiO2 and OP-TiO2 films, respectively). Atmospheric plasma (AP) treatment was carried out in air using an AP treatment system (FemtoScience Co., Plasmaflux™). Argon (99.9% purity) was used as the carrier gas and O2 (99.9% purity) was used as the reactive gas. The films were treated under AP treatment conditions (Ar/O2 flow ratio: 100; power: 30 W; treatment time: 5 min) and showed high hydrophilicity and low damage [14,15]. The crystallinity and crystal orientation of the films were investigated by using an X-ray diffractometer (XRD; Philips, X'Pert Pro), and all samples were scanned from 20° to 50° (2θ) in the continuous mode. The morphology and thickness of the films were determined by using a scanning electron microscope (SEM; Hitachi,
Fig. 1 shows glancing incident X-ray diffraction (GIXRD) patterns of the films coated on substrates and annealed at 450 °C for photocatalytic evaluations. The GIXRD patterns of the D-TiO2, P-TiO2, OD-TiO2, and OP-TiO2 films show the presence of broad peaks. These peaks indicate that the TiO2 nanograins contain very small crystallites. All the diffraction peaks correspond to the anatase crystalline phase (JCPDS 21-1272) of titanium dioxide. Fig. 2 shows SEM images of dense and porous TiO2 films before and after O2 plasma treatment. The morphologies of the D-TiO2 and OD-TiO2 films showed nanoparticles and compact surfaces even after plasma treatment. The thickness of the films was about 100 nm. The formation of the P-TiO2 films is surmised to be as follows: PEG-containing titanium oligomers (solid phase) are isolated from the solvent (2-MOE), leading to the formation of pores [16]. The P-TiO2 film was composed of two layers, a dense thin layer and a porous thin layer, in the absence of lithography and patterning processes. The pore size depends on that of PEG and is 80–140 nm (Fig. 2c and d). The porosity disappears or gradually decreases in size with an increase in the PEG or TiO2 content of the solvent. The optical bandgaps of the D-TiO2, P-TiO2, OD-TiO2, and OP-TiO2 films on glass substrates were measured to be about 3.38, 3.29, 3.32, and 3.24 eV, respectively (not shown here). The bandgap of O-TiO2 films decreased to a greater extent than those of pristine TiO2 films due to the formation of defect levels during plasma bombardment. The water contact angles were measured to determine the hydrophilicity of the TiO2 film surface (Fig. 3). When the surfaces of the D-TiO2 and P-TiO2 films were exposed to AP plasma, the water contact angles of the OD-TiO2 (18.5°) and OP-TiO2 (13.5°) films slightly decreased to 6.5° and 3°, respectively. The increase in the surface energy can be attributed to the increase in the number of hydroxyl groups on the film surface after the O2 plasma treatment.
Fig. 2. SEM images of TiO2 thin films: (a) dense film, (b) porous film, (c) dense film treated with O2 plasma, and (d) porous film treated with O2 plasma.
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Fig. 3. Contact angles of dense and porous TiO2 (D-TiO2 and P-TiO2) thin films and dense and porous TiO2 (OD-TiO2 and OP-TiO2) thin films treated with O2 plasma.
Fig. 4 shows the decolorization efficiency of various TiO2 films before and after O2 plasma treatment. The TiO2 photocatalyst was immersed in 100 ml of dye solution (Reactive Black 5), and the suspension was irradiated with UV (254 nm) light. Aliquots were taken at different time intervals (1, 2, 3, and 5 h); the aliquots were taken with a syringe and filtered using a 0.45-μm Millipore filter. The absorption spectra of the dye solution were recorded, and the rate of decolorization was observed in terms of the change in the intensity of the dye at λmax. The decolorization efficiency (%) was calculated as follows: Decolorization efficiency (%) = (C0 − C)/C0 × 100, where C0 is the initial concentration of the dye and C is the concentration of the dye after UV irradiation. From the measurement results, the order of decolorization efficiency of the various photocatalysts was found to be D-TiO2 films b P-TiO2 films b OD-TiO2 films b OP-TiO2 films. This behavior is considered to be a consequence of the production of hydroxyl groups through the breakage of the triple bonds of nitrogen in the dye solution upon plasma treatment. Thus, decolorization is found to increase with photocatalytic activity. XPS measurements were carried out in order to investigate the valence states of Ti and O in the TiO2 films. Before carrying out the XPS measurements, the film surface was etched at a rate of 0.05 nm/s for 30 s. The measurement area had 200 × 200 μm2. All the peaks were calibrated with respect to the C 1s peak at 284.6 eV. Fig. 5 shows the Ti 2p and O 1s core-level XPS spectra of the TiO2 films before and after O2
Fig. 5. Ti 2p and O1s core-level XPS spectra of TiO2 thin films: (a) dense film, (b) porous film, (c) dense film treated with O2 plasma, and (d) porous film treated with O2 plasma. The O1s peak was deconvoluted to three peaks.
plasma treatment. The Ti 2p peaks show a symmetric shape and can be well fitted by Gaussian curves. However, the O 1s peak shows a slightly asymmetric shape and can be deconvoluted by three asymmetric Gaussian curves. The deconvoluted oxygen peaks near 530 eV (529.8–530.1 eV, 531.1–531.5 eV, and 531.9–532.5 eV) can be ascribed to the number of oxygen atoms in TiO2 (Ot), the number of hydroxyl groups (Oh), and the number of oxygen atoms in Ti2O3 (Od), respectively. Upon comparing the peak area ratios of Oh to Ot and Od to Ot in the TiO2 films subjected to O2 plasma treatment and those not subjected to the treatment, we observed that Oh/Ot and Od/Ot for the O-TiO2 films are higher and lower than those of pristine TiO2 films, respectively; the reason for this observation may be the hydrophilicity and decolorization due to the production of a hydroxyl group and the decrease in oxygen defects [7,17] in the case of OD-TiO2, as shown in Figs. 3 and 4. 4. Conclusions Dense TiO2 and porous TiO2 films were prepared from a polymerblended solution. The porosity of the films was controlled by varying the amount of PEG and solvent. Films annealed at 450 °C were found to be in the anatase crystalline phase. The CA and bandgap of TiO2 films treated with AP plasma and oxygen gas decreased slightly. The photocatalytic characteristics of the OP-TiO2 films improved due to an increase in the specific surface area and the number of hydroxyl groups, and a decrease of Ti2O3 oxygen state. These results demonstrate that OP-TiO2 films show strong decolorization efficiency in Reactive Black 5 dye, thus showing promise as a photocatalytic material. Acknowledgments This work was supported by KBSI Grant (T30619) to J.P. Kim. References [1] [2] [3] [4] [5] [6]
Fig. 4. Decolorization efficiencies at different reaction times for dense and porous TiO2 (D-TiO2 and P-TiO2) thin films and dense and porous TiO2 (OD-TiO2 and OP-TiO2) thin films treated with O2 plasma.
[7] [8] [9] [10]
A. Fujishima, K. Honda, Nature 37 (1972) 238. U. Diebold, J.F. Anderson, K. Ng, D. Vanderbilt, Phys. Rev. Lett. 77 (1996) 1322. R.A. Bennett, P. Stone, N.J. Price, M. Bowker, Phys. Rev. Lett. 82 (1999) 3831. D.S. Zhang, T. Yoshida, K. Furuta, H. Minoura, J. Photoch. Photobio., A 164 (2004) 159. I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Nano Lett. 6 (2006) 2009. T. Kietzke, B. Stiller, K. Landfester, R. Montenegro, D. Neher, Synthetic. Met. 152 (2005) 101. L.Y. Yu, H.M. Shen, Z.L. Xu, J. Appl. Polym. Sci. 74 (1999) 2220. V.E. Henrich, R.L. Kurts, Phys. Rev. B 23 (1981) 6280. H. Wang, J. He, G. Boschloo, H. Lindstrom, A. Hagfeldt, S.E. Lindquist, J. Phys. Chem. B 105 (2001) 2529. D.C. Cronemeyer, Phys. Rev. B 113 (1959) 1222.
J.P. Kim et al. / Thin Solid Films 519 (2011) 6916–6919 [11] [12] [13] [14]
S.K. Kansal, A.H. Ali, S. Kapoor, Desalination 259 (2010) 147. Z. Liu, J. Ya, E. Lei, Y. Xin, W. Zhao, Mater. Chem. Phys. 120 (2010) 277. Y. Li, G.P. Demopoulos, Hydrometallugy 90 (2008) 26. K. Ahn, Y.S. Jeong, H.U. Lee, S.Y. Jeong, H.S. Ahn, H.S. Kim, S.G. Yoon, C.R. Cho, Thin Solid Films 518 (2010) 4066.
6919
[15] Y.S. Jeong, H.U. Lee, K. Ahn, J.H. Jeon, S.Y. Jeong, C.R. Cho, W.J. Lee, J. Kor. Phys. Soc. 54 (2009) 944. [16] Z.F. Liu, J.W. Li, J. Ya, Y. Xin, Z.G. Jin, Mater. Lett. 62 (2008) 1190. [17] H. Yoshiki, T. Saito, J. Vac. Sci. Technol. A 26 (2008) 338.
Contents lists available at ScienceDirect
Thin Solid Films 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 / t s f
Effect of plasma treatment on chemical bonding states of porous TiO2 thin films prepared from polymer-blended solution J.P. Kim a, H.U. Lee b, J.E. Yang a, J.S. Bae a, E.S. Park a, J.H. Yoon a, J.M. Kim a, C.R. Cho b,⁎ a b
Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea College of Nanoscience and Nanotechnology, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Available online 22 April 2011 Keywords: Plasma treatment Chemical bonding states Porous TiO2 Polymer Photocatalysis Surface energy
a b s t r a c t Dense TiO2 (D-TiO2) thin films and porous TiO2 (P-TiO2) thin films were prepared by using a polymer-blended solution. The film porosity decreased gradually or disappeared with an increase in the polyethylene glycol (PEG) or TiO2 content of the solution. To modify their surface properties, the thin films were treated with atmospheric pressure (AP) plasma by using a reactive gas. The surface morphologies of the O2-plasma-treated TiO2 (O-TiO2) thin films were smooth and did not change significantly. The decolorization efficiency of the P-TiO2 thin films was found to be enhanced when compared to that of the D-TiO2 thin films. The enhancement was due to an increase in the specific surface area and the number of hydroxyl groups, and a decrease of Ti2O3 states. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2), which is a transition metal oxide, has received considerable attention its use in many applications, e.g., photocatalysis, solar energy cells, and gas sensors. TiO2 occurs naturally in the minerals rutile, anatase, and brookite. The rutile and anatase phases have been intensively studied and have significant technological uses owing to their optical properties. TiO2 is a largebandgap semiconductor with a catalytically active surface. The surface of transition metal oxides plays an important role in a variety of technological applications, ranging from electronic devices to catalyst development [1]. Recently, a large number of studies have been carried out to investigate surface structures of TiO2 and to devise TiO2 preparation methods in which the surface structure can be controlled [2,3]. The properties of TiO2 nanostructures with large surface area, such as nanorods, nanowires, and nanofibers have been reported [4,5]. The use of TiO2 nanoparticles encapsulated with polymers, which can be synthesized by using an emulsification process, has been shown to improve the efficiency of various optoelectronic devices [6]. Porous TiO2 thin films prepared from a polymer-blended TiO2 solution are considered as better photocatalytic materials than films having a dense morphology because the former films are capable of absorbing a large amount of visible light and have a high specific surface area [7]. A photoelectron conductive TiO2 film usually suffers the loss of a small amount of oxygen [8], which results in the formation of dominant surface defect states. These surface states are known to be located in
⁎ Corresponding author. Tel.: + 82 55 350 5297; fax: + 82 55 353 1314. E-mail address: [email protected] (C.R. Cho). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.053
the interband region, and they lead to slow charge transport in the film owing to the trapping and detrapping of electrons [9]. The oxygen vacancy states have been found to exist about 0.75–1.18 eV below the conduction band edge of TiO2 [10] and can thus be excited from the valence band by visible wavelengths of around 506 and 614 nm. Many studies have focused on the surface properties related to the photocatalytic behavior of TiO2 materials [11]. However, the effect of atmospheric pressure (AP) plasma treatment on the surface properties and decolorization efficiency of polymer-mediated porous TiO2 thin films has not been studied. In this study, dense TiO2 thin films and porous TiO2 thin films were prepared on silicon and glass substrates by using a polymer-blended solution. The surface properties of the TiO2 thin films after O2 plasma treatment (O-TiO2) were examined. The photocatalytic behavior of the films was also determined by examining the decolorization of the dye solution. 2. Experimental details Polymer-blended TiO2 solution was synthesized at room temperature in the following manner: (C5H8O2)2TiO (Ti-oxyacetylacetonate) was dissolved in a mixture solution of 10 ml of 2-methoxyethanol (2-MOE) and 0.12 g of polyethylene glycol (PEG). As is well known, PEG is a nonionic surfactant and has a long chain with a zigzag structure; it is also known that in the molecule, \O\ is hydrophilic and \CH2\CH2\ is hydrophobic [12]. After stirring the mixture solution for 2 h, Ti-acylacetate was added and the solution was continually stirred for 2 h; transparent polymer-blended TiO2 solution was obtained. A spin-coating technique was used to coat the polymerblended solution on glass and Si substrates. The annealing temperature
J.P. Kim et al. / Thin Solid Films 519 (2011) 6916–6919
6917
S-4200). The chemical bonding states of the films were determined by X-ray photoelectron spectroscopy (XPS; VG Scientific, ESCALAB250) that was performed with a hemispherical electrostatic energy analyzer and an Al Kα (1486.6 eV) X-ray source. The photocatalytic characteristics and optical transmittance of the films were examined by using an ultraviolet–visible spectrophotometer (Varian, Cary5E). The surface wettability was determined from the contact angle (CA; Dataphysics, OCA100). 3. Results and discussion
Fig. 1. GIXRD patterns of TiO2 thin films: (a) dense film, (b) porous film, (c) dense film treated with O2 plasma, and (d) porous film treated with O2 plasma. A denotes anatase phase of the TiO2 films.
was set at 450 °C because the phase transition from anatase to rutile starts around this temperature [13]. Dense TiO2 (D-TiO2) thin films were prepared, and they were then coated with TiO2 solution without PEG polymer and annealed. Porous TiO2 (P-TiO2) thin films that were coated with the polymer-blended solution and annealed showed porous surface morphology because of the evaporation of the PEG polymer. Then, to improve the surface properties of the D-TiO2 thin films and P-TiO2 thin film, the films were treated with O2 plasma (the treated films are hereafter referred to as OD-TiO2 and OP-TiO2 films, respectively). Atmospheric plasma (AP) treatment was carried out in air using an AP treatment system (FemtoScience Co., Plasmaflux™). Argon (99.9% purity) was used as the carrier gas and O2 (99.9% purity) was used as the reactive gas. The films were treated under AP treatment conditions (Ar/O2 flow ratio: 100; power: 30 W; treatment time: 5 min) and showed high hydrophilicity and low damage [14,15]. The crystallinity and crystal orientation of the films were investigated by using an X-ray diffractometer (XRD; Philips, X'Pert Pro), and all samples were scanned from 20° to 50° (2θ) in the continuous mode. The morphology and thickness of the films were determined by using a scanning electron microscope (SEM; Hitachi,
Fig. 1 shows glancing incident X-ray diffraction (GIXRD) patterns of the films coated on substrates and annealed at 450 °C for photocatalytic evaluations. The GIXRD patterns of the D-TiO2, P-TiO2, OD-TiO2, and OP-TiO2 films show the presence of broad peaks. These peaks indicate that the TiO2 nanograins contain very small crystallites. All the diffraction peaks correspond to the anatase crystalline phase (JCPDS 21-1272) of titanium dioxide. Fig. 2 shows SEM images of dense and porous TiO2 films before and after O2 plasma treatment. The morphologies of the D-TiO2 and OD-TiO2 films showed nanoparticles and compact surfaces even after plasma treatment. The thickness of the films was about 100 nm. The formation of the P-TiO2 films is surmised to be as follows: PEG-containing titanium oligomers (solid phase) are isolated from the solvent (2-MOE), leading to the formation of pores [16]. The P-TiO2 film was composed of two layers, a dense thin layer and a porous thin layer, in the absence of lithography and patterning processes. The pore size depends on that of PEG and is 80–140 nm (Fig. 2c and d). The porosity disappears or gradually decreases in size with an increase in the PEG or TiO2 content of the solvent. The optical bandgaps of the D-TiO2, P-TiO2, OD-TiO2, and OP-TiO2 films on glass substrates were measured to be about 3.38, 3.29, 3.32, and 3.24 eV, respectively (not shown here). The bandgap of O-TiO2 films decreased to a greater extent than those of pristine TiO2 films due to the formation of defect levels during plasma bombardment. The water contact angles were measured to determine the hydrophilicity of the TiO2 film surface (Fig. 3). When the surfaces of the D-TiO2 and P-TiO2 films were exposed to AP plasma, the water contact angles of the OD-TiO2 (18.5°) and OP-TiO2 (13.5°) films slightly decreased to 6.5° and 3°, respectively. The increase in the surface energy can be attributed to the increase in the number of hydroxyl groups on the film surface after the O2 plasma treatment.
Fig. 2. SEM images of TiO2 thin films: (a) dense film, (b) porous film, (c) dense film treated with O2 plasma, and (d) porous film treated with O2 plasma.
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Fig. 3. Contact angles of dense and porous TiO2 (D-TiO2 and P-TiO2) thin films and dense and porous TiO2 (OD-TiO2 and OP-TiO2) thin films treated with O2 plasma.
Fig. 4 shows the decolorization efficiency of various TiO2 films before and after O2 plasma treatment. The TiO2 photocatalyst was immersed in 100 ml of dye solution (Reactive Black 5), and the suspension was irradiated with UV (254 nm) light. Aliquots were taken at different time intervals (1, 2, 3, and 5 h); the aliquots were taken with a syringe and filtered using a 0.45-μm Millipore filter. The absorption spectra of the dye solution were recorded, and the rate of decolorization was observed in terms of the change in the intensity of the dye at λmax. The decolorization efficiency (%) was calculated as follows: Decolorization efficiency (%) = (C0 − C)/C0 × 100, where C0 is the initial concentration of the dye and C is the concentration of the dye after UV irradiation. From the measurement results, the order of decolorization efficiency of the various photocatalysts was found to be D-TiO2 films b P-TiO2 films b OD-TiO2 films b OP-TiO2 films. This behavior is considered to be a consequence of the production of hydroxyl groups through the breakage of the triple bonds of nitrogen in the dye solution upon plasma treatment. Thus, decolorization is found to increase with photocatalytic activity. XPS measurements were carried out in order to investigate the valence states of Ti and O in the TiO2 films. Before carrying out the XPS measurements, the film surface was etched at a rate of 0.05 nm/s for 30 s. The measurement area had 200 × 200 μm2. All the peaks were calibrated with respect to the C 1s peak at 284.6 eV. Fig. 5 shows the Ti 2p and O 1s core-level XPS spectra of the TiO2 films before and after O2
Fig. 5. Ti 2p and O1s core-level XPS spectra of TiO2 thin films: (a) dense film, (b) porous film, (c) dense film treated with O2 plasma, and (d) porous film treated with O2 plasma. The O1s peak was deconvoluted to three peaks.
plasma treatment. The Ti 2p peaks show a symmetric shape and can be well fitted by Gaussian curves. However, the O 1s peak shows a slightly asymmetric shape and can be deconvoluted by three asymmetric Gaussian curves. The deconvoluted oxygen peaks near 530 eV (529.8–530.1 eV, 531.1–531.5 eV, and 531.9–532.5 eV) can be ascribed to the number of oxygen atoms in TiO2 (Ot), the number of hydroxyl groups (Oh), and the number of oxygen atoms in Ti2O3 (Od), respectively. Upon comparing the peak area ratios of Oh to Ot and Od to Ot in the TiO2 films subjected to O2 plasma treatment and those not subjected to the treatment, we observed that Oh/Ot and Od/Ot for the O-TiO2 films are higher and lower than those of pristine TiO2 films, respectively; the reason for this observation may be the hydrophilicity and decolorization due to the production of a hydroxyl group and the decrease in oxygen defects [7,17] in the case of OD-TiO2, as shown in Figs. 3 and 4. 4. Conclusions Dense TiO2 and porous TiO2 films were prepared from a polymerblended solution. The porosity of the films was controlled by varying the amount of PEG and solvent. Films annealed at 450 °C were found to be in the anatase crystalline phase. The CA and bandgap of TiO2 films treated with AP plasma and oxygen gas decreased slightly. The photocatalytic characteristics of the OP-TiO2 films improved due to an increase in the specific surface area and the number of hydroxyl groups, and a decrease of Ti2O3 oxygen state. These results demonstrate that OP-TiO2 films show strong decolorization efficiency in Reactive Black 5 dye, thus showing promise as a photocatalytic material. Acknowledgments This work was supported by KBSI Grant (T30619) to J.P. Kim. References [1] [2] [3] [4] [5] [6]
Fig. 4. Decolorization efficiencies at different reaction times for dense and porous TiO2 (D-TiO2 and P-TiO2) thin films and dense and porous TiO2 (OD-TiO2 and OP-TiO2) thin films treated with O2 plasma.
[7] [8] [9] [10]
A. Fujishima, K. Honda, Nature 37 (1972) 238. U. Diebold, J.F. Anderson, K. Ng, D. Vanderbilt, Phys. Rev. Lett. 77 (1996) 1322. R.A. Bennett, P. Stone, N.J. Price, M. Bowker, Phys. Rev. Lett. 82 (1999) 3831. D.S. Zhang, T. Yoshida, K. Furuta, H. Minoura, J. Photoch. Photobio., A 164 (2004) 159. I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Nano Lett. 6 (2006) 2009. T. Kietzke, B. Stiller, K. Landfester, R. Montenegro, D. Neher, Synthetic. Met. 152 (2005) 101. L.Y. Yu, H.M. Shen, Z.L. Xu, J. Appl. Polym. Sci. 74 (1999) 2220. V.E. Henrich, R.L. Kurts, Phys. Rev. B 23 (1981) 6280. H. Wang, J. He, G. Boschloo, H. Lindstrom, A. Hagfeldt, S.E. Lindquist, J. Phys. Chem. B 105 (2001) 2529. D.C. Cronemeyer, Phys. Rev. B 113 (1959) 1222.
J.P. Kim et al. / Thin Solid Films 519 (2011) 6916–6919 [11] [12] [13] [14]
S.K. Kansal, A.H. Ali, S. Kapoor, Desalination 259 (2010) 147. Z. Liu, J. Ya, E. Lei, Y. Xin, W. Zhao, Mater. Chem. Phys. 120 (2010) 277. Y. Li, G.P. Demopoulos, Hydrometallugy 90 (2008) 26. K. Ahn, Y.S. Jeong, H.U. Lee, S.Y. Jeong, H.S. Ahn, H.S. Kim, S.G. Yoon, C.R. Cho, Thin Solid Films 518 (2010) 4066.
6919
[15] Y.S. Jeong, H.U. Lee, K. Ahn, J.H. Jeon, S.Y. Jeong, C.R. Cho, W.J. Lee, J. Kor. Phys. Soc. 54 (2009) 944. [16] Z.F. Liu, J.W. Li, J. Ya, Y. Xin, Z.G. Jin, Mater. Lett. 62 (2008) 1190. [17] H. Yoshiki, T. Saito, J. Vac. Sci. Technol. A 26 (2008) 338.