Influence of heat treatment process on photocatalytic activity of photocatalyst TiO 2 /TiC x O y coatings during heat treatment in carbon powder

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1 J Mater Sci: Mater Electron (2016) 27: DOI /s Influence of heat treatment process on photocatalytic activity of photocatalyst TiO 2 /TiC x O y coatings during heat treatment in carbon powder Sujun Guan 1 Liang Hao 2 Hiroyuki Yoshida 3 Hiroshi Asanuma 1 Fusheng Pan 4 Yun Lu 1 Received: 24 March 2016 / Accepted: 3 June 2016 / Published online: 14 June 2016 Springer Science+Business Media New York 2016 Abstract Photocatalyst TiO 2 /TiC x O y coatings on alumina (Al 2 O 3 ) balls had been successfully fabricated by mechanical coating technique with titanium (Ti) powder and subsequent heat treatment in carbon powder. The influence of heat treatment conditions in carbon powder on the formed compounds, surface morphology and photocatalytic activity of photocatalyst TiO 2 /TiC x O y coatings was investigated. XRD results show that the compounds of TiC x O y and TiO 2 were formed on the surface of Ti coatings during heat treatment in carbon powder. The relative amounts of TiC x O y and TiO 2 remarkably varies with changing the heat treatment temperature and/or holding time. The formed nano-bump structure during heat treatment in carbon powder at 1073 or 1173 K, could increase the accessible surface area. The formation of Ti 3? and generated oxygen vacancies confirmed by XPS measurement, are in favor of narrowing band gap to enhance the visible-light photocatalytic activity of photocatalyst TiO 2 / Electronic supplementary material The online version of this article (doi: /s ) contains supplementary material, which is available to authorized users. & Yun Lu luyun@faculty.chiba-u.jp Graduate School and Faculty of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba , Japan College of Mechanical Engineering, Tianjin University of Science and Technology, No. 1038, Dagu Nanlu, Hexi District, Tianjin , People s Republic of China Chiba Industrial Technology Research Institute, , Tendai, Inage-ku, Chiba , Japan College of Materials Science and Engineering, Chongqing University, No. 174 Shazhengjie, Shapingba, Chongqing , People s Republic of China TiC x O y coatings. The photocatalytic activity of the sample by heat treatment in carbon powder at 1173 K for 15 h shows highest. The fabrication strategy provides us a facile preparation procedure of visible-light responsive photocatalyst coatings. 1 Introduction Titanium dioxide (TiO 2 ) is well-known for its extensive applications in various fields, such as water and air purification, decontamination, the production of carbonaceous solar fuels, and the self-cleaning surfaces, due to its suitable band edge positions, excellent stability, and low cost [1 3]. However, the application of TiO 2 is limited by its wide band gap, and fast electron hole recombination [4 6]. Therefore, an amount of research has been dedicated to enhancing the visible-light activity of TiO 2 photocatalysts, by narrowing the band gap, and sensitizing with the semiconductors of narrow band gap [7 10]. Recently, it has been shown that manipulating structure morphology and electronic structure are effective approaches to improve the separation and transportation of charge carriers [11 14]. Recently, titanium oxycarbide (TiC x O y ) has been attracting much interest for the possible use in catalysis and electrocatalysis, both as catalysts and supports [15 17]. TiC x O y is a kind of solid solution of titanium carbide (TiC) and titanium monoxide (TiO), and a large amount of research has been conducted on the structure of TiC x O y [18]. It is reasonable for us to believe that TiC x O y must contain some vacancies at both Ti and C/O sites, as the Ti atoms occupy the 4a (0 0 0) sites, and C/O atoms randomly occupy the 4b ( ) sites [19, 20]. The mixed crystal of TiC x O y and TiO 2 fabricated by heat treatment in carbon

2 10400 J Mater Sci: Mater Electron (2016) 27: powder, has proven to be an effective method to enhance the photocatalytic activity [21]. However, the influence of heat treatment process on crystal, surface morphology during heat treatment in carbon powder need to be further studied. In this work, photocatalyst TiO 2 /TiC x O y coatings were fabricated with various heat treatment conditions in carbon powder. The influence of heat treatment process on crystal, surface morphology and photocatalytic activity of photocatalyst TiO 2 /TiC x O y coatings was investigated and characterized. The relationship among the photocatalytic activity and the surface morphology as well as the narrowed band gap was discussed. 2 Experimental 2.1 Preparation of photocatalyst TiO 2 /TiC x O y coatings Ti coatings were formed on Al 2 O 3 balls by mechanical coating using a planetary ball mill (Type: P6, Fritsch, Germany). Ti powder (average diameter of 30 lm, Osaka Titanium technologies, Japan) was used as the coating material. Al 2 O 3 balls (average diameter of 1 mm, Nikkato, Japan) were used as the substrates. Ti powder (40 g) and Al 2 O 3 balls (60 g) were first charged into an Al 2 O 3 bowl with a volume of 250 ml. The rotation speed of the planetary ball mill was set at 480 rpm and the operation was held for 10 h. The detailed procedure has been described in our previous work [22]. After the mechanical coating operation, Ti coatings adhered to Al 2 O 3 balls were prepared. Then, the Al 2 O 3 balls coated with Ti coatings were immersed in carbon powder (average diameter of 150 lm, Kishu charcoal, Japan) and then heat treated in an electric furnace [21], under the temperature of x K and holding for y h, and named as C-xKyh. After heat treatment process, the samples had been cooled to room temperature in the furnace. 2.2 Characterization X-ray diffractometer (XRD, JDX-3530, JEOL) with Cu-Ka radiation at 30 kv and 20 ma was used to determine the composition and crystal structure. The surface morphology was observed by scanning electron microscopy (SEM, JSM-5300, JEOL). To determine the chemical composition and valence state on the surface, X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) measurements were carried out. Photocatalytic activity of the C-xKyh sample was evaluated for the photodecomposition of methylene blue (MB) solution (C 0 : 10 lmol/l, 35 ml) under visible light irradiation at room temperature, according to ISO [23]. The C-xKyh sample was first dried under UV irradiation (FL20S BLB) holding for 24 h, then absorbed MB solution (20 lmol/l, 35 ml) in dark for 18 h, to reach complete adsorption desorption equilibrium. Two 20 W fluorescent lamps (FL20SSW/18) were used as the visible light irradiation source (irradiance of 5000 lx), with a UV cut-off filter (L42) to ensure that only visible light irradiation (k [ 420 nm) could reach the sample. 3 Results and discussion 3.1 Crystal structure Figure 1 shows XRD patterns of the C-xKyh sample by heat treatment in carbon powder. From the C-1073K15h sample (Fig. 1a), it can be seen that the compounds of titanium oxycarbide (TiC x O y ) and rutile TiO 2 formed on the surface of Ti coatings [22], due to the diffraction peaks at 36.1,42.0 and 60.9 could be attributed to TiC x O y, and 27.4, 41.3 and 54.3 could be indexed to rutile TiO 2, respectively. With decreasing the temperature to 973 K, the crystal of the C-973K15h sample shows slight TiC x O y and rutile TiO 2 (41.3 and 54.3 ). With increasing the temperature to 1173 K, the crystal becomes only TiC x O y, indicating that the possible formed TiO 2 had been reduced to be TiC x O y [24, 25]. The peak intensity of TiC x O y in the sample is stronger than that of the C-1073K15h sample. While increasing the heat treatment temperature to 1273 K, the crystal shows that TiC x O y increases and Ti decreases, which hints that the relative amount of crystal structure of TiC x O y becomes large. Further, with changing the heat treatment time at 1173 K, the crystal shows slight change, as shown in Fig. 1b. From the C-1173K2h sample, it can be seen that the crystal of TiC x O y formed on the surface of Ti coatings. It means that the temperature of 1173 K plays a key role on the formation of TiC x O y. Also, the influence of heat treatment time at 1073 K on the rutile TiO 2 is significant (Supplementary Fig. S1). 3.2 Surface morphology The surface morphology of the C-xK15h sample fabricated with different heat treatment temperatures for 15 h is shown in Fig. 2. In general, it clearly shows the size of surface morphology increases, with increasing the temperature. From Fig. 2a, any reaction compound is not detected from the surface of the C-973K15h sample, which hardly changes compared with that of Ti coatings [23]. With increasing the temperature to 1073 K, the reaction compounds show the nano-bump structure (Fig. 2b). While

3 J Mater Sci: Mater Electron (2016) 27: Fig. 1 XRD patterns of the C-xKyh sample by heat treatment in carbon powder. a C-xK15h, b C-1173Kyh C-1273K15h C-1173K30h TiO 2 rutile TiC x O y Ti C-1073K15h C-973K15h C-1173K2h θ (deg) θ (deg) (c) (d) Fig. 2 Comparison of surface morphology of the C-xK15h sample. a C-973K15h, b C-1073K15h, c, d C-1273K15h the sample also shows the nano-bump structure, increasing in size and number (Fig. 2c). However, the reaction compounds become to micro structure (Fig. 2d), with increasing the temperature to 1273 K. Therefore, the significant difference of surface morphology could be attributed to the difference in the crystal structure and the reaction process of Ti with the involved carbon oxide and its concentration, under different temperatures. While Fig. 3 further shows the surface morphology of the C-1173Kyh sample at 1173 K with different heat treatment times. The size of surface morphology significantly increases, with increasing the time under 1173 K. When the heat treatment time is too short, the reaction compound is difficult to be detected from the C-1173K2h sample (Fig. 3a). With extending the time to 15 h, the reaction compounds increase in size and number and the sample shows the nano-bump structure (Fig. 3b). With extending the time to 30 h, the reaction compounds significantly increase in size and the C-1173K30h sample shows micro structure (Fig. 3c). Also,

4 10402 J Mater Sci: Mater Electron (2016) 27: (c) Fig. 3 Comparison of surface morphology of the C-1173Kyh sample. a C-1173K2h, b, c C-1173K30h the influence of the heat treatment time at 1073 K on the surface morphology of the C-1073Kyh sample is similar to that of the C-1173Kyh sample (Supplementary Fig. S2). 3.3 Bonding environment XPS are used to investigate the samples of C-1073K15h and, as shown in Fig. 4. For the O 1 s highresolution XPS spectra, the peak usual locates at corresponds to the Ti O banding for TiO 2 coatings [22]. Figure 4a shows a significant shift of the O 1 s peak to ev of the C-1073K15h sample and ev of the sample, respectively. The shift of O 1 s XPS spectra is related to the replacement of oxygen to form the oxygen vacancies [16, 26, 27]. The Ti 2p XPS spectra also shows the peak of Ti 3? (Fig. 4b), which further reveals the formed oxygen vacancies. To investigate the carbon states in the photocatalyst, C 1 s XPS spectra has been measured and shown in Fig. 4c. Since the peak nearby at ev, which have previously been found to result from Ti C bonds, was significantly observed from the sample. It means that carbon has doped into mix crystal under heat treatment in carbon powder [25, 28]. O1s Ti 2p TiO 2 (c) C 1s C-C Ti 3+ Ti-C Ti-O Ti-C C-1073K15h Binding Energy (ev) Binding Energy (ev) Binding Energy (ev) Fig. 4 XPS spectra of the C-1073K15h and samples. a O 1 s XPS spectra, b Ti 2p XPS spectra, c C 1 s XPS spectra

5 J Mater Sci: Mater Electron (2016) 27: Fig. 5 Comparison of the photocatalytic activity towards the degradation of MB solution under visible light irradiation. a C-xK15h, b C-1173Kyh 10 MB solution concentration ( L -1 ) 9 8 MB solution C-973K15h C-1073K15h C-1273K15h 10 MB solution concentration ( L -1 ) 9 8 MB solution C-1173K2h C-1173K30h Visible light irradiation time (min) Visible light irradiation time (min) 3.4 Photocatalytic activity The photocatalytic activity of the C-xKyh sample is evaluated by degradation of methylene blue (MB) solution under visible light irradiation at room temperature, as shown in Fig. 5. The concentration of the blank (marked with MB solution) hardly decreased, which hints the MB solution did not degrade. All of the C-xKyh sample show the photocatalytic activity, according to the change of concentration of MB solution. Compared with the influence on photocatalytic activity with different heat treatment temperature, the photocatalytic activity of the sample is far higher than that of the C-1173Kyh sample, as shown in Fig. 5a. With increasing the temperature from 973 to 1273 K, Fig. 5a shows the photocatalytic activity first increases and then decreases, which hints that photocatalytic activity is related with the crystal structure (Fig. 1a), the accessible surface area (Fig. 2), and the doped carbon and the generated oxygen vacancy (Fig. 4). With extending the time from 2 to 30 h at 1173 K, Fig. 5b shows the photocatalytic activity of the C-1173K15 sample is highest. It clearly shows the relationship among the photocatalytic activity and relative intensity of TiC x O y in the crystal structure (Fig. 1b) as well as the accessible surface area (Fig. 3). 4 Conclusions The photocatalyst TiO 2 /TiC x O y coatings on alumina (Al 2 O 3 ) balls were successfully fabricated with various heat treatment conditions in carbon powder for Ti coatings. The compounds of TiC x O y and TiO 2 were formed on the surface of Ti coatings during heat treatment in carbon powder. The relative amounts of TiC x O y and TiO 2 remarkably varies with increasing the heat treatment temperature from 973 to 1273 K by heat treatment in carbon powder, while they are relatively stable with extending the heat treatment time from 2 to 30 h at 1173 K. The size of surface morphology increases in size and number, with increasing the temperature for 15 h and extending the time at 1173 K. The formed nano-bump structure during heat treatment in carbon powder could increases the accessible surface area. The formation of Ti 3? and generated oxygen vacancies in the lattice of crystal are in favor of generating more photo-induced electrons and holes to enhance the photocatalytic activity. The photocatalytic activity of photocatalyst TiO 2 /TiC x O y coatings has been effectively enhanced by heat treatment in carbon powder, and the sample fabricated at 1173 K for 15 h shows highest. Acknowledgments This work is supported by the National Nature Science Foundation of China (No ). References 1. A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63, (2008) 2. S. Kumar, L. Devi, J. Phys. Chem. A 115, (2011) 3. K. Prabakaran, S. Mohanty, S. Nayak, J. Mater. Sci. Mater. El. 25, (2014) 4. S. Anandan, T. Rao, M. Sathish, D. Rangappa, I. Honma, M. Miyauchi, ACS Appl. Mater. Interface 5, (2013) 5. J. Dong, J. Han, Y. Liu, A. Nakajima, S. Matsushita, S. Wei, W. Gao, ACS Appl. Mater. Interface 6, (2014) 6. X. Ma, X. Ni, J. Mater. Sci. Mater. Electron. 26, (2015) 7. R. Daghrir, P. Drogui, D. Robert, Ind. Eng. Chem. Res. 52, (2013) 8. L. Kong, Z. Jiang, C. Wang, F. Wan, Y. Li, L. Wu, J. Zhi, X. Zhang, S. Chen, Y. Liu, ACS Appl. Mater. Interface 7, (2015)

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