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Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering
SCT-21679; No of Pages 5 Surface & Coatings Technology xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering Yu-Xiang Zhao a, Sheng Han b, YiHan Lin c, Chung-Hsuan Hu c, Li-Yu Hua c, ChinTan Lee c, Ying Chun Hung d, Ko Wei Weng c,⁎ a
Department of Computer Science and Information Engineering, National Quemoy University, 1 Daxue Road, Jinning Township, Kinmen 89250, Taiwan ROC Center for General Education, National Taichung University of Science and Technology, 129 San-min Road, Section 3, Taichung 40401, Taiwan ROC Department of Electronic Engineering, National Quemoy University, 1 Daxue Road, Jinning Township, Kinmen 89250, Taiwan ROC d Department of Urban Planning and Landscape, National Quemoy University, 1 Daxue Road, Jinning Township, Kinmen 89250, Taiwan ROC b c
a r t i c l e
i n f o
Article history: Received 15 August 2016 Revised 1 October 2016 Accepted in revised form 12 October 2016 Available online xxxx Keywords: TiO2 thin films Cr implamtation fluence Pulsed magnetron sputtering
a b s t r a c t TiO2 thin films were deposited by the bipolar pulsed magnetron sputtering of a Ti target at low working pressure (1 mTorr) and various Ar and O2 flow ratios. (The Ar flow rate was fixed at 30 sccm.) The effects of the Cr implantation fluence on the crystal structure, surface microstructure, and optical and photocatalytic properties of Cr-implanted TiO2 films were investigated. The structure and composition of the films were characterized by grazingincidence X-ray diffraction and X-ray photoelectron spectroscopy. The surface morphology of the films was characterized by field emission scanning electron microscopy. The optical properties were determined by UV–VIS transmission spectroscopy. The photocatalytic properties of the films were analyzed by the de-colorization of methylene blue with UV and visible light. Experimental results show that the implantation of a moderate amount of Cr with a fluence of 5 × 1015 in the TiO2 film preserves the anatase crystal structure and modifies the film in a manner that favors its photocatalytic properties. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Titanium dioxide (TiO2) films are known for their hydrophilic and photocatalytic characteristics. Increasing their specific surface area and doping can enhance their photocatalytic activity and hydrophilicity [1–4]. Self-cleaning titanium dioxide films exhibit excellent photocatalytic and hydrophilic properties owing to their wide band gap [5,6]. Only the ultraviolet fraction of solar irradiation is active in photo-excitation processes in pure TiO2. A great deal of effort is now being made to enhance the photocatalytic and hydrophilic properties of pure TiO2 films, mainly by widening their range of wavelength sensitization and by ensuring stability of the properties of their properties under severe conditions [7,8]. The presence of a foreign element in the matrix of a pure metal oxide can greatly modify its structural, acid–base and catalytic properties, which therefore can be modulated by carefully selecting of the species and loading of the foreign element as well as the method of preparation [9–11]. This study investigates the photocatalytic properties of Cr-implanted TiO2 films. Moderate Cr implantation at a fluence of 5 × 1015 atoms/cm2 optimizes the photocatalytic property of such films.
TiO2 films were prepared using a hybrid physical vapor deposition (PVD) system that involved magnetron sputtering deposition (MS) and metal plasma ion implantation (MPII). The films were deposited on float glass and Si wafer. Anatase-TiO2 films were prepared by the reactive sputtering of a Ti target using a bipolar pulsed magnetron sputter system, followed by the implantation of Cr as a third element with various fluences. In this work, 1 × 1015, 5 × 1015, 1 × 1016, 5 × 1016, 1 × 1017, and 2 × 1017 atoms/cm2 of Cr3+ were implanted into the TiO2 films by MPII during the final step of deposition. The acceleration voltage was fixed at 20 kV, so the ion energy of Cr3+ was 60 keV. Table 1 lists in detail the experimental parameters. X-ray diffraction (XRD, X'pert MRD diffractometer with Cu Kα (λ = 1.542 nm) radiation) and X-ray photoelectron spectroscopy (XPS, PHI 1600 using Al Kα radiation hν = 1253.6 eV) were used to elucidate the crystallization, microstructure and valence state of the TiO2/Cr films. The surface morphology and cross-sectional microstructures were observed using a scanning electron microscope (SEM, JOEL HR FESEM JSM-7000F). The photocatalytic properties of the film were estimated by the de-colorization of methylene blue using a spectrophotometer and a UV–visible spectrometer (Jasco V-570) with wavelengths in the range 300–900 nm.
⁎ Corresponding author. E-mail address: [email protected] (K.W. Weng).
http://dx.doi.org/10.1016/j.surfcoat.2016.10.034 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
2
Y.-X. Zhao et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Table 1 Experimental conditions of the deposition parameters. Parameter
Value
Sputter target Substrate Ar flow rate (sccm) O2 flow rate (sccm) Work pressure (torr) Pulsed power (W) Implantation fluence (atoms/cm2) Deposition time (min)
Ti ITO glass 20 60 1.1 × 10−2 200 1 × 1015,5 × 1015,1 × 1016,5 × 1016, 1 × 1017, 2 × 1017 120
3. Results and discussion Fig. 1 shows the evolution of the crystallinity of TiO2 films in which is implanted Cr at different implantation fluences. The crystalline phase of the as-deposited film is an anatase phase and oriented along TiO2 (101). The intensity of A (101) decreased as the implantation fluence increases from 1 × 1015 to 1 × 1016 atoms/cm2. A high implantation fluence that exceeds 5 × 1016 atoms/cm2 results in the formation of amorphous TiO2 and a related chromium oxide phase (Cr2O3). However, the amorphous TiO2 structure and a Cr2O3 phase do not favor the photocatalytic property of TiO2 films, as will be revealed below. Figs. 2 and 3 show XPS spectra of Ti and O in the as-deposited TiO2 and TiO2 films with implanted Cr. In Fig. 2, the binding energy of Ti 2p3/2 in all TiO2 films is 459.8–458.6 eV, indicating the presence of a TiO2 phase. The binding energy of Ti 2p3/2 in the as-deposited TiO2 films is 459.55 eV while that of Ti 2p3/2 in TiO2 films with implanted Cr shifts toward lower energy as the implantation fluence increases. The binding shift in the energy suggests that the binding state transforms from TiO2 to TiO, presumably owing to the formation of Cr2O3 in the TiO2 films when Cr is implanted. In Fig. 3, O1s in all films is observed at 531–528.1 eV, indicating the presence of titanium oxide. From the XPS database, the individual peaks of Ti4+ 2p3/2 and Ti2 + 2p3/2 (indicating the presence of TiO2 and TiO) can be identified, and the proportion of each binding state can be estimated by analyzing the oxygen peak signals in Fig. 3. This binding state transition suggests that implanting Cr changes the affinity of Ti and oxygen in the films. Fig. 4 shows the surface morphologies of pure and Cr-implanted TiO2 films. The as-deposited TiO2 film exhibits a sputtered surface with an equi-axed and crack-free homogeneous structure, revealing a uniform distribution of clusters of grains. Fig. 4 (b) shows cross-sectional SEM micrographs of the as-deposited TiO2 films that were prepared with fixed parameters (as listed in Table 1) with a thickness of 390 ± 10 nm. The images of the surfaces of the TiO2 films reveal a change from granular clusters to rice-shaped grains that become longer as the
implantation fluences is increased from 1 × 1015 to 1 × 1016 atoms/cm2. When the implantation fluence reaches 5 × 1016 atoms/cm2, the riceshape grains tend to integrate and form a brain-like structure. The changes in surface morphology are caused by energetic plasma bombardment (energy and heat) during the Cr ion implantation process, which affect the surface of the film, cause a re-organization of the surface atoms, and form a dense and coarse film structure. Cr-implanted TiO2 films with high implantation fluences of greater than 1 × 1017 atoms/cm2 have a rather smooth and dense surface structure. The XRD results reveal the formation of amorphous TiO 2 and Cr2O3. Fig. 5 shows the surface roughness of the TiO2 films at various implantation fluences. The roughness values of the implanted TiO2 films that were obtained as the implantation fluence increased from 1 × 10 15 to 2 × 1017 atoms/cm 2 were 21, 21, 29, 35, 12 and 14, indicating that a higher implantation fluence associated with higher plasma energy and heat accumulation, yields a smoother morphology. This result is consistent with both SEM observations and XRD analysis. The optical bandgap (Eg) of the as-deposited and Cr-implanted TiO2 films that are shown in Fig. 6 can be obtained using the equations that were presented in our prior research [11]. The as-deposited TiO2 film has a bandgap of 3.39 eV. The implantation of Cr into TiO2 films reduces its bandgap to an extent that increases with the fluence, shifting it toward the visible region. Implantation therefore changes the optical bandgap to one consistent with solar visible light without the application of a UV light source, and it also causes the film to exhibit a photocatalytic property. The implantation fluence of Cr markedly affects the rate of decomposition of methylene blue (MB) by the films. All films were immersed in methylene blue for 9 h and were used in de-colorization tests using
Fig. 1. Crystallinity evolution of a-TiO2 films implanted with Cr.
Fig. 3. XPS spectra for O1s in the TiO2 films.
Fig. 2. XPS spectra for Ti2p in the TiO2 films.
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
Y.-X. Zhao et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
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Fig. 4. SEM microstructures for as-deposited TiO2 film (a,b) and Cr implanted TiO2 films with varying fluence (c) 1 × 1015, (d) 5 × 1015, (e) 1 × 1016, (f) 5 × 1016, (g) 1 × 1017 and (h) 2 × 1017 atoms/cm2.
visible and UV light to evaluate their absorption behavior. A lower absorption ratio corresponds to better photocatalytic activity. Fig. 7(a) shows de-colorization tests of the as-deposited and Cr-implanted TiO2 films using UV light. The absorption ratios of the Cr-implanted TiO2 films were lower than 0.15 at fluences of 5 × 1015 and 1 × 1016 atoms/cm2, revealing that they exhibited strong photocatalysis. Fig. 7(b) shows the absorption rate under illumination by visible light; an implantation fluence in the range of 1 × 1015 to 1 × 1016 atoms/cm2 yields better photocatalytic properties than those of the as-deposited TiO2 films. XRD
analysis shows the influence of implantation fluence on the intensity (or FWHM) of the (101) anatase-TiO2 peak and the photocatalytic activity of TiO2 films, which is determined by using them in the decomposition of MB solution. As the implantation fluence increases from 1 × 1015 to 1 × 1016 atoms/cm2, the FWHM is dramatically increased. This result can probably be explained by a reduction of crystallization upon bombardment of the growing film by energetic particles. The roughness of the film increases with implantation. Furthermore, its photocatalytic property depends on the lifetime of the electron hole pairs. The grain boundaries
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
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Y.-X. Zhao et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Fig. 5. Surface roughnesses of the TiO2 films with varying implantation fluence.
(G.B.) are potential recombination centers for electron hole pairs, and so reduce the carrier lifetime, weakening the photocatalytic property. The decomposition rate is associated with the surface roughness. A lager surface area enables the simultaneous adsorption of more molecules. In this investigation, factors that influence the photocatalytic property include crystal structure, optical response, the surface chemicals present and morphology. Further investigation, involving band gap engineering, pole figures and TEM, is needed to confirm the factors that affect the photocatalytic property.
4. Summary and conclusions This study investigates the effect of Cr implantation on bipolar pulsed magnetron sputtered TiO2 thin films with a view to improving their photocatalytic properties. The strong photocatalytic property of such a film depends on the presence of the anatase phase. Its photocatalytic activity also strongly depends on its roughness and microstructure. The implantation fluence between 1 × 1015 and 2 × 1017 atoms/cm2 affects the roughness and the free surface area for the adsorption of molecules. At a flounce of 1 × 1015 to 1 × 1016 atoms/cm2, the energy of the particles
Fig. 7. Decomposition rate of MB for TiO2 films after (a) UV and (b) Visible light sources.
that impinge upon the substrate is increased, increasing the mobility of surface particles, promoting the formation of crystalline TiO2 films. As the fluence is increased above 5 × 1016 atoms/cm2, the accumulation of impact energy and heat exceeds. Therefore, only amorphous films of TiO2 that was mixed with chromium oxide, which had relatively smooth surface, were deposited. TiO2 following implantation at the optimal fluence of 5 × 1015 atoms/cm2 exhibited the strongest photocatalytic property of all Cr-implated TiO2 films herein. References
Fig. 6. Energy gaps of the as-deposited and Cr implanted TiO2 films with varying fluence.
[1] A.G.G. Toh, R. Cai, D.L. Butler, The influence of surface topography on the photocatalytic activity of electrophoretically deposited titanium dioxide thin films, Wear 266 (2009) 585–588. [2] K.-W. Weng, C.-H. Hua, L.-Y. Huaa, C.-T. Leea, Y.-X. Zhaod, J. Changc, S.-Y. Yangc, S. Han, Photocatalytic activity of bipolar pulsed magnetron sputter deposited TiO2/ TiWOx thin films, Nucl. Instr. Meth. B 381 (2016) 6–10. [3] J.T. Chang, Y.F. Lai, J.L. He, Photocatalytic performance of chromium or nitrogen doped arc ion plated-TiO2 films, Surf. Coat. Technol. 200 (2005) 1640–1644. [4] C.-C. Pan, J.C.S. Wu, Visible-light response Cr-doped TiO2-xNx photocatalysts, Mater. Chem. Phys. 100 (2006) 102–107. [5] D.Y. Lee, J.-H. Park, Y.-H. Kim, M.-H. Lee, N.-I. Cho, Effect of Nb doping on morphology, crystal structure, optical band gap energy of TiO2 thin films, Curr. Appl. Phys. 14 (2014) 421–427. [6] J. Yu, M. Zhou, H. Yu, Q. Zhang, Y. Yu, Enhanced photoinduced super-hydrophilicity of the sol-gel-derived TiO2 thin films by Fe-doping, Mater. Chem. Phys. 95 (2006) 193–196.
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
Y.-X. Zhao et al. / Surface & Coatings Technology xxx (2016) xxx–xxx [7] K.O. Awitor, A. Rivaton, J.-L. Gardette, A.J. Down, M.B. Johnson, Photo-protection and photo-catalytic activity of crystalline anatase titanium dioxide sputter-coated on polymer films, Thin Solid Films 516 (2008) 2286–2291. [8] G.S. Chen, C.C. Lee, H. Niu, W. Huang, R. Jann, T. Schütte, Sputter deposition of titanium monoxide and dioxide thin films with controlled properties using optical emission spectroscopy, Thin Solid Films 516 (2008) 8473–8478. [9] M.A. Henderson, A surface science perspective on photocatalysis, Surf. Sci. Rep. 66 (2011) 185–297.
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[10] M. Okada, K. Tajima, Y. Yamada, K. Yoshimura, Photocatalytic performance of very thin TiO2/SnO2 stacked-film prepared by magnetron sputtering, Vacuum 83 (2008) 688–690. [11] K.-W. Weng, S. Han, Y.-C. Chen, H.-C. Chuang, Effects of MPII-implanted titanium on the electrochromic properties of tungsten trioxide, Nucl. Instr. Meth. B 266 (2008) 1069–1073.
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering Yu-Xiang Zhao a, Sheng Han b, YiHan Lin c, Chung-Hsuan Hu c, Li-Yu Hua c, ChinTan Lee c, Ying Chun Hung d, Ko Wei Weng c,⁎ a
Department of Computer Science and Information Engineering, National Quemoy University, 1 Daxue Road, Jinning Township, Kinmen 89250, Taiwan ROC Center for General Education, National Taichung University of Science and Technology, 129 San-min Road, Section 3, Taichung 40401, Taiwan ROC Department of Electronic Engineering, National Quemoy University, 1 Daxue Road, Jinning Township, Kinmen 89250, Taiwan ROC d Department of Urban Planning and Landscape, National Quemoy University, 1 Daxue Road, Jinning Township, Kinmen 89250, Taiwan ROC b c
a r t i c l e
i n f o
Article history: Received 15 August 2016 Revised 1 October 2016 Accepted in revised form 12 October 2016 Available online xxxx Keywords: TiO2 thin films Cr implamtation fluence Pulsed magnetron sputtering
a b s t r a c t TiO2 thin films were deposited by the bipolar pulsed magnetron sputtering of a Ti target at low working pressure (1 mTorr) and various Ar and O2 flow ratios. (The Ar flow rate was fixed at 30 sccm.) The effects of the Cr implantation fluence on the crystal structure, surface microstructure, and optical and photocatalytic properties of Cr-implanted TiO2 films were investigated. The structure and composition of the films were characterized by grazingincidence X-ray diffraction and X-ray photoelectron spectroscopy. The surface morphology of the films was characterized by field emission scanning electron microscopy. The optical properties were determined by UV–VIS transmission spectroscopy. The photocatalytic properties of the films were analyzed by the de-colorization of methylene blue with UV and visible light. Experimental results show that the implantation of a moderate amount of Cr with a fluence of 5 × 1015 in the TiO2 film preserves the anatase crystal structure and modifies the film in a manner that favors its photocatalytic properties. © 2016 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Titanium dioxide (TiO2) films are known for their hydrophilic and photocatalytic characteristics. Increasing their specific surface area and doping can enhance their photocatalytic activity and hydrophilicity [1–4]. Self-cleaning titanium dioxide films exhibit excellent photocatalytic and hydrophilic properties owing to their wide band gap [5,6]. Only the ultraviolet fraction of solar irradiation is active in photo-excitation processes in pure TiO2. A great deal of effort is now being made to enhance the photocatalytic and hydrophilic properties of pure TiO2 films, mainly by widening their range of wavelength sensitization and by ensuring stability of the properties of their properties under severe conditions [7,8]. The presence of a foreign element in the matrix of a pure metal oxide can greatly modify its structural, acid–base and catalytic properties, which therefore can be modulated by carefully selecting of the species and loading of the foreign element as well as the method of preparation [9–11]. This study investigates the photocatalytic properties of Cr-implanted TiO2 films. Moderate Cr implantation at a fluence of 5 × 1015 atoms/cm2 optimizes the photocatalytic property of such films.
TiO2 films were prepared using a hybrid physical vapor deposition (PVD) system that involved magnetron sputtering deposition (MS) and metal plasma ion implantation (MPII). The films were deposited on float glass and Si wafer. Anatase-TiO2 films were prepared by the reactive sputtering of a Ti target using a bipolar pulsed magnetron sputter system, followed by the implantation of Cr as a third element with various fluences. In this work, 1 × 1015, 5 × 1015, 1 × 1016, 5 × 1016, 1 × 1017, and 2 × 1017 atoms/cm2 of Cr3+ were implanted into the TiO2 films by MPII during the final step of deposition. The acceleration voltage was fixed at 20 kV, so the ion energy of Cr3+ was 60 keV. Table 1 lists in detail the experimental parameters. X-ray diffraction (XRD, X'pert MRD diffractometer with Cu Kα (λ = 1.542 nm) radiation) and X-ray photoelectron spectroscopy (XPS, PHI 1600 using Al Kα radiation hν = 1253.6 eV) were used to elucidate the crystallization, microstructure and valence state of the TiO2/Cr films. The surface morphology and cross-sectional microstructures were observed using a scanning electron microscope (SEM, JOEL HR FESEM JSM-7000F). The photocatalytic properties of the film were estimated by the de-colorization of methylene blue using a spectrophotometer and a UV–visible spectrometer (Jasco V-570) with wavelengths in the range 300–900 nm.
⁎ Corresponding author. E-mail address: [email protected] (K.W. Weng).
http://dx.doi.org/10.1016/j.surfcoat.2016.10.034 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
2
Y.-X. Zhao et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Table 1 Experimental conditions of the deposition parameters. Parameter
Value
Sputter target Substrate Ar flow rate (sccm) O2 flow rate (sccm) Work pressure (torr) Pulsed power (W) Implantation fluence (atoms/cm2) Deposition time (min)
Ti ITO glass 20 60 1.1 × 10−2 200 1 × 1015,5 × 1015,1 × 1016,5 × 1016, 1 × 1017, 2 × 1017 120
3. Results and discussion Fig. 1 shows the evolution of the crystallinity of TiO2 films in which is implanted Cr at different implantation fluences. The crystalline phase of the as-deposited film is an anatase phase and oriented along TiO2 (101). The intensity of A (101) decreased as the implantation fluence increases from 1 × 1015 to 1 × 1016 atoms/cm2. A high implantation fluence that exceeds 5 × 1016 atoms/cm2 results in the formation of amorphous TiO2 and a related chromium oxide phase (Cr2O3). However, the amorphous TiO2 structure and a Cr2O3 phase do not favor the photocatalytic property of TiO2 films, as will be revealed below. Figs. 2 and 3 show XPS spectra of Ti and O in the as-deposited TiO2 and TiO2 films with implanted Cr. In Fig. 2, the binding energy of Ti 2p3/2 in all TiO2 films is 459.8–458.6 eV, indicating the presence of a TiO2 phase. The binding energy of Ti 2p3/2 in the as-deposited TiO2 films is 459.55 eV while that of Ti 2p3/2 in TiO2 films with implanted Cr shifts toward lower energy as the implantation fluence increases. The binding shift in the energy suggests that the binding state transforms from TiO2 to TiO, presumably owing to the formation of Cr2O3 in the TiO2 films when Cr is implanted. In Fig. 3, O1s in all films is observed at 531–528.1 eV, indicating the presence of titanium oxide. From the XPS database, the individual peaks of Ti4+ 2p3/2 and Ti2 + 2p3/2 (indicating the presence of TiO2 and TiO) can be identified, and the proportion of each binding state can be estimated by analyzing the oxygen peak signals in Fig. 3. This binding state transition suggests that implanting Cr changes the affinity of Ti and oxygen in the films. Fig. 4 shows the surface morphologies of pure and Cr-implanted TiO2 films. The as-deposited TiO2 film exhibits a sputtered surface with an equi-axed and crack-free homogeneous structure, revealing a uniform distribution of clusters of grains. Fig. 4 (b) shows cross-sectional SEM micrographs of the as-deposited TiO2 films that were prepared with fixed parameters (as listed in Table 1) with a thickness of 390 ± 10 nm. The images of the surfaces of the TiO2 films reveal a change from granular clusters to rice-shaped grains that become longer as the
implantation fluences is increased from 1 × 1015 to 1 × 1016 atoms/cm2. When the implantation fluence reaches 5 × 1016 atoms/cm2, the riceshape grains tend to integrate and form a brain-like structure. The changes in surface morphology are caused by energetic plasma bombardment (energy and heat) during the Cr ion implantation process, which affect the surface of the film, cause a re-organization of the surface atoms, and form a dense and coarse film structure. Cr-implanted TiO2 films with high implantation fluences of greater than 1 × 1017 atoms/cm2 have a rather smooth and dense surface structure. The XRD results reveal the formation of amorphous TiO 2 and Cr2O3. Fig. 5 shows the surface roughness of the TiO2 films at various implantation fluences. The roughness values of the implanted TiO2 films that were obtained as the implantation fluence increased from 1 × 10 15 to 2 × 1017 atoms/cm 2 were 21, 21, 29, 35, 12 and 14, indicating that a higher implantation fluence associated with higher plasma energy and heat accumulation, yields a smoother morphology. This result is consistent with both SEM observations and XRD analysis. The optical bandgap (Eg) of the as-deposited and Cr-implanted TiO2 films that are shown in Fig. 6 can be obtained using the equations that were presented in our prior research [11]. The as-deposited TiO2 film has a bandgap of 3.39 eV. The implantation of Cr into TiO2 films reduces its bandgap to an extent that increases with the fluence, shifting it toward the visible region. Implantation therefore changes the optical bandgap to one consistent with solar visible light without the application of a UV light source, and it also causes the film to exhibit a photocatalytic property. The implantation fluence of Cr markedly affects the rate of decomposition of methylene blue (MB) by the films. All films were immersed in methylene blue for 9 h and were used in de-colorization tests using
Fig. 1. Crystallinity evolution of a-TiO2 films implanted with Cr.
Fig. 3. XPS spectra for O1s in the TiO2 films.
Fig. 2. XPS spectra for Ti2p in the TiO2 films.
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
Y.-X. Zhao et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
(a)
3
(b)
250nm
250nm
(d)
(c)
250nm
250nm
(e)
(f)
250nm
250nm
(h)
(g)
250nm
250nm
Fig. 4. SEM microstructures for as-deposited TiO2 film (a,b) and Cr implanted TiO2 films with varying fluence (c) 1 × 1015, (d) 5 × 1015, (e) 1 × 1016, (f) 5 × 1016, (g) 1 × 1017 and (h) 2 × 1017 atoms/cm2.
visible and UV light to evaluate their absorption behavior. A lower absorption ratio corresponds to better photocatalytic activity. Fig. 7(a) shows de-colorization tests of the as-deposited and Cr-implanted TiO2 films using UV light. The absorption ratios of the Cr-implanted TiO2 films were lower than 0.15 at fluences of 5 × 1015 and 1 × 1016 atoms/cm2, revealing that they exhibited strong photocatalysis. Fig. 7(b) shows the absorption rate under illumination by visible light; an implantation fluence in the range of 1 × 1015 to 1 × 1016 atoms/cm2 yields better photocatalytic properties than those of the as-deposited TiO2 films. XRD
analysis shows the influence of implantation fluence on the intensity (or FWHM) of the (101) anatase-TiO2 peak and the photocatalytic activity of TiO2 films, which is determined by using them in the decomposition of MB solution. As the implantation fluence increases from 1 × 1015 to 1 × 1016 atoms/cm2, the FWHM is dramatically increased. This result can probably be explained by a reduction of crystallization upon bombardment of the growing film by energetic particles. The roughness of the film increases with implantation. Furthermore, its photocatalytic property depends on the lifetime of the electron hole pairs. The grain boundaries
Please cite this article as: Y.-X. Zhao, et al., Photocatalytic properties of TiO2 films prepared by bipolar pulsed magnetron sputtering, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.034
4
Y.-X. Zhao et al. / Surface & Coatings Technology xxx (2016) xxx–xxx
Fig. 5. Surface roughnesses of the TiO2 films with varying implantation fluence.
(G.B.) are potential recombination centers for electron hole pairs, and so reduce the carrier lifetime, weakening the photocatalytic property. The decomposition rate is associated with the surface roughness. A lager surface area enables the simultaneous adsorption of more molecules. In this investigation, factors that influence the photocatalytic property include crystal structure, optical response, the surface chemicals present and morphology. Further investigation, involving band gap engineering, pole figures and TEM, is needed to confirm the factors that affect the photocatalytic property.
4. Summary and conclusions This study investigates the effect of Cr implantation on bipolar pulsed magnetron sputtered TiO2 thin films with a view to improving their photocatalytic properties. The strong photocatalytic property of such a film depends on the presence of the anatase phase. Its photocatalytic activity also strongly depends on its roughness and microstructure. The implantation fluence between 1 × 1015 and 2 × 1017 atoms/cm2 affects the roughness and the free surface area for the adsorption of molecules. At a flounce of 1 × 1015 to 1 × 1016 atoms/cm2, the energy of the particles
Fig. 7. Decomposition rate of MB for TiO2 films after (a) UV and (b) Visible light sources.
that impinge upon the substrate is increased, increasing the mobility of surface particles, promoting the formation of crystalline TiO2 films. As the fluence is increased above 5 × 1016 atoms/cm2, the accumulation of impact energy and heat exceeds. Therefore, only amorphous films of TiO2 that was mixed with chromium oxide, which had relatively smooth surface, were deposited. TiO2 following implantation at the optimal fluence of 5 × 1015 atoms/cm2 exhibited the strongest photocatalytic property of all Cr-implated TiO2 films herein. References
Fig. 6. Energy gaps of the as-deposited and Cr implanted TiO2 films with varying fluence.
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