Fabrication and characterization of photocatalyst coatings by heat treatment in carbon powder for TiC coatings

Transcription

1 Solid State Phenomena Vol. 263(2017) pp (2017) Trans Tech Publications, Switzerland doi: / Fabrication and characterization of photocatalyst coatings by heat treatment in carbon powder for coatings Sujun Guan 1, a, Liang Hao 2,b,Hiroyuki Yoshida 3,c, Hiroshi Asanuma 4, d, Fusheng Pan 5,e and Yun Lu 4,f,* 1 Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo , Japan 2 College of Mechanical Engineering, Tianjin University of Science and Technology, No.1038, Dagu Nanlu, Hexi District, Tianjin , P.R. China 3 Chiba Industrial Technology Research Institute, , Tendai, Inage-ku, Chiba , Japan 4 Graduate School and Faculty of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba , Japan 4 College of Materials Science and Engineering, Chongqing University, No. 174 Shazhengjie, Shapingba, Chongqing , P.R. China a guansujun1222@gmail.com, b haoliang@tust.edu.cn, c h.yshd14@pref.chiba.lg.jp d asanuma@faculty.chiba-u.jp, e fspan@cqu.edu.cn, f,* luyun@faculty.chiba-u.jp Keywords: titanium carbide, TiO 2, surface morphology, carbon powder, photocatalytic activity Abstract. Photocatalyst coatings on alumina (Al 2 O 3 ) balls had been successfully fabricated by mechanical coating technique, with titanium carbide () powder and subsequent heat treatment in carbon powder. The effect of heat treatment conditions in carbon powder on the formed compounds, surface morphology and photocatalytic activity of photocatalyst coatings was investigated. XRD results show that the formed compounds change with increasing the heat treatment temperature in carbon powder, and rutile TiO 2 on the surface of coatings at 1073 K and 1173 K. The generated oxygen vacancies confirmed by XPS measurement, are in favor of narrowing band gap to enhance the visible-light photocatalytic activity of photocatalyst coatings. The photocatalytic activity of photocatalyst coatings has been effectively enhanced, and the samples fabricated at 1073 K and 1173 K for 2 h show higher activity. The fabrication strategy provides us a facile preparation procedure of visible-light responsive photocatalyst coatings. Introduction The search for suitable semiconductors as photocatalysts for solving energy and environmental issues is one of the major objectives of material science. The most widely used material for photocatalytic applications is TiO 2, because of its excellent chemical stability, low-cost and high photocatalytic activity [1-3]. However, the photoreaction efficiency of TiO 2 is severely limited by its wide band gap (>3 ev) and fast electron-hole recombination due to a high density of trap states [3-6]. Therefore, an enormous amount of research has been devoted to enhancing the visible-light absorption of TiO 2, mainly by narrowing the band gap via elemental doping and sensitization with semiconductors possess narrowed band gap [7-10]. Accordingly, much effort has been dedicated to the introduction of oxygen vacancies into the lattice of TiO 2, in order to form extra electron energy levels and narrow the band gap [11,12]. Recently, transition-metal carbides exhibit chemical stability and catalytic activity, and their surface has been proposed as excellent support for the dispensability. Transition-metal carbides also often show chemical and electronic properties similar to those of Pt-group metals [13,14]. Among different transition-metal carbides, titanium carbide () has attracted much interest because of its superior chemical stability. Moreover, as a catalyst support, has been reported to enhance the activity of catalyst [15-17].

2 138 Functional Materials and Metallurgy In this work, photocatalyst coatings were fabricated with different heat treatment temperatures for 2 h in carbon powder for coatings. The effect of heat treatment conditions on phase structure, surface morphology and photocatalytic activity of photocatalyst coatings was investigated and characterized. The relationship between the photocatalytic activity and the formed compounds as well as the generated oxygen vacancies is discussed. Experimental Fabrication of photocatalyst coatings. coatings were coated on alumina (Al 2 O 3 ) balls (diameter: 1 mm) by mechanical coating operation [18], with powder (diameter: 2-5 μm) and named as " coatings". Then coatings were subjected to heat treatment in carbon powder (average diameter: 150 μm) at x K holding for 2 h [19], and named as "C-xK2h". Characterization and photocatalytic activity. The crystal phase of the formed compounds was analyzed by X-ray diffraction (XRD, D8 Advance) equipped with Cu-Kα radiation. The surface morphology was examined by scanning electron microscopy (SEM, JSM-5300). The spectra of O 1s, Ti 2p and C 1s were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250Xi). The samples were first dried under UV irradiation (FL20S BLB) for 24 h, then absorbed with 35 ml of 20 μmol/l MB solution in dark for 18 h, to reach complete adsorption-desorption equilibrium. Then the photocatalytic activity of the samples was evaluated for photodecomposition of methylene blue (MB) solution under visible light and ultraviolet (UV) irradiation, according to ISO Results and discussion Phase structure. Fig. 1 shows XRD patterns of the C-xK2h samples by heat treatment in carbon powder and the schematic of change process between TiO 2 and. From the sample, it can be found that rutile TiO 2 formed on the surface of coatings, with the evidence of the diffraction peaks at 27.4, 41.3, and With decreasing the temperature to 973 K, the spectrum of the C-973K2h sample shows small percentage of (35.9 and 41.7 ) without rutile TiO 2. With increasing the temperature to 1173 K, the spectrum is similar to that of the, but the peak intensity of rutile TiO 2 is stronger. While increasing the temperature to 1273 K, the content of rutile TiO 2 becomes too low to be detected, indicating that the possible formed TiO 2 had been reduced to be [20]. Based on the results, the possible schematic of formation process of TiO 2 from is also proposed in Fig. 1. TiO 2 rutile Al 2 O 3 C-973K2h coatings TiO θ (deg) Fig. 1. XRD patterns of the C-xK2h samples and schematic of change process between TiO 2 and. Surface morphology. The surface morphology of the C-xK2h samples is shown in Fig. 2. In general, it shows the size of surface morphology increases, with increasing the temperature. When the temperature

3 Solid State Phenomena Vol lower than 1073 K, the C-xK2h samples does not show obvious change compared with coatings (Fig. 2a-c). While the temperature is higher than 1073 K, the formed compounds show significant growth in size and number (Fig. 2d and 2e). However, the formed compounds of the sample coarsened to micron meter scale (Fig. 2e). Therefore, the significant difference of surface morphology could be attributed to the difference in the formed compounds and the reaction process of with the involved oxocarbon and its concentration, under different temperatures [21]. (a) (b) (c) (d) 1 μm (e) Fig. 2. Comparison of surface morphology of the C-xK2h samples. (a) coatings, (b) C-973K2h, (c), (d), (e). (a) O 1s (b) Ti 2p (c) C 1s 2p Ti-C 3/ TiO 2 coatings p 1/ Binding Energy (ev) Binding Energy (ev) Binding Energy (ev) Fig. 3. XPS spectra of the C-xK2h samples fabricated at 1073 K, 1173 K, 1273 K. (a) O 1s. (b) Ti 2p. (c) C 1s. Bonding environment. XPS are used to investigate the bonding environment of the C-xK2h samples, as shown in Fig. 3. For the O 1s XPS spectra of the typical absorbance of TiO 2, the peak usual locates at corresponds to the Ti-O-Ti linkages in TiO 2 [19]. Fig. 3a shows a significant shift of the O 1s peak to ev of the sample and the sample, ev of the sample, respectively. The shift of O 1s XPS spectra could be related to the replacement of oxygen to generate the oxygen vacancies [11,12]. However, Fig. 3b shows that the Ti 2p peaks are almost identical, indicating that Ti atoms have a similar bonding environment [12]. Fig. 3c shows the results of the C 1s peak, 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 the formed TiO 2 had been reduced to be under 1273 K, which is matched with that of XRD results. Photocatalytic activity. The photocatalytic activity of the C-xK2h samples is evaluated by degradation of MB solution under visible light and UV irradiation at room temperature, as shown in Fig. 4. The C-xK2h samples show the photocatalytic activity expect the C-973K2h sample, according to the change of

4 140 Functional Materials and Metallurgy concentration of MB solution. Compared with the influence on photocatalytic activity with different heat treatment temperatures, the photocatalytic activity of the C-xK2h samples fabricated at 1073 K and 1173 K is higher, as shown in Fig. 5. With increasing the temperature from 973 K to 1273 K, the photocatalytic activity first increases and then decreases, which hints that photocatalytic activity is related with the formed compounds (Fig. 1), the suitable accessible surface (Fig. 2), and the generated oxygen vacancies (Fig. 3). (a) (b) C / C C / C MB solution coatings C-973K2h Visible light irradiation time (min) UV irradiation time (min) Fig. 4. Comparative photocatalytic degradation of MB solution of the C-xK2h samples. (a) Under visible light irradiation. (b) Under UV irradiation. Summary The photocatalyst coatings were successfully fabricated with different heat treatment temperatures for 2 h in carbon powder for coatings. The formed rutile TiO 2 changes with increasing the heat treatment temperature from 973 K to 1273 K. The size of surface morphology increases in size and number, with increasing the temperature. The oxygen vacancies generated in the lattice of rutile TiO 2 are in favor of generating more photo-induced electrons and holes to enhance the photocatalytic activity. The photocatalytic activity of photocatalyst coatings has been effectively enhanced with increasing the temperature, and the samples fabricated at 1073 K and 1173 K for 2 h show higher. References [1] S.G. Kumar and L.G. Devi. J. Phys. Chem. A 115 (2011) [2] A. Fujishima, X. Zhang and D.A. Tryk. Surf. Sci. Rep. 63 (2008) [3] M.V. Dozzi and E. Selli. J. Photoch. Photobio. C 14 (2013) [4] X. Chen, S. Shen, L. Guo and S.S. Mao. Chem. Rev. 110 (2010) [5] Y. Li and J. Zhang. Laser Photonics Rev. 4 (2010) [6] Z. Chen and K. Zhou. Surf. Coat. Technol. 263 (2015) [7] J. Hensel, G. Wang, Y. Li and J. Zhang. Nano. Lett. 10 (2010) [8] Y. Gai, J. Li, S. Li, J. Xia and S. Wei. Phys. Rev. Lett. 102 (2009) [9] M. Xu, P. Da, H. Wu, D. Zhao and G. Zheng. Nano Lett. 12 (2012) [10] L. Kong, C. Wang, H. Zheng, X. Zhang and Y. Liu. J. Phys. Chem. C 119 (2015) [11] C. Chen, J. Shieh, S. Hsieh, C. Kuo and H. Liao. Acta. Mater. 60 (2012) [12] X. Chen, L. Liu, P. Yu and S. Mao. Science 331 (2011) [13] Y. Shao, J, Liu, Y. Wang and Y. Lin. J. Mater. Chem. 19 (2009) [14] H. Hwu and J. Chen. Chem. Rev. 105 (2005) [15] A. Ignaszak, C. Song, W. Zhu, J. Zhang, A. Bauer, R. Baker, V. Neburchilov, S. Ye and S. Campbell. Electrochim. Acta 69 (2012) [16] Y. Li and T. Ishigaki. Chem. Phys. Lett. 367 (2003)

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