Properties of hydrogenated titanium dioxide thin films

Siam Physics Congress 2015 20-22 May 2015

Properties of hydrogenated titanium dioxide thin films prepared by sparking method for photocatalytic application Porntipa Pooseekheaw1, and Pisith Singjai2* 1

Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai-50200 (Thailand) 2 Materials Science Research Center, Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai-50200 (Thailand) E-mail: [email protected]

Abstract Titanium dioxide (TiO2) is a widely used photocatalyst in water splitting process for hydrogen production. In this study, hydrogenation of titanium dioxide thin films was investigated in order to examine the photocatalytic properties and to improve the performance for photocatalytic application. Titanium dioxide thin films were prepared by sparking method, then annealed at 500 °C in hydrogen atmosphere for 2 h to get hydrogenated titanium dioxide thin films (H:TiO2). The result of scanning electron microscopy and atomic force microscopy show that the film was a very high porosity with high specific area for better photocalytic activity. The structure of films were found to be a mixture of anatase and rutile phase. UV-Vis-NIR spectrometer measurement indicated that the absorbance from 300 to 700 nm of H:TiO2 was more than that of TiO2. Furthermore, energy gap of H:TiO2 thin film was found to be lower than that of TiO2. Keywords: photocatalytic, hydrogenated titanium dioxide, sparking method, thin films Research Material Ltd.) and quartz sheets (1×10×10 mm3, Ted Pella,Inc., Redding,CA) were cleaned in acetone, deionized water and ethanol by ultrasonication, respectively.

Introduction Hydogen production from water photocatalytic process is an interesting choice for energy shortage problem. It is produced from clean and renewable energy sources. Thus, its life cycles is clean and renewable [1-3]. TiO2 is a widely used photocatalyst because of low cost and environmentally friendly [46]. It is well known there are three structure forms of TiO2: anatase, rutile and brookite. Anatase phase has the grater photocatalytic activity than that of rutile phase. However, a mixture phase of anatase and rutile is a better choice [7-10]. The energy gap of TiO2 is 3.2 eV with respond in ultraviolet region at a small part of solar spectra (5%). The wide energy gap of TiO2 limits the use of sunlight as excitation energy of photo-generate electron-hole pairs. In order to enhance efficiency of photocatalytic activity, reducing energy gap of TiO2 was investigated. One of a promising method is based on the disorder in surface of TiO2 through hydrogenation [11-17]. In this paper, we report on a hydrogenation of TiO2, prepared by sparking method and annealed at 500 °C and compared the photocatalytic properties.

The sparking apparatus equipped with 9 pairs of titanium tips were sparked in air at room temperature. The tips were placed horizontally at 1 mm spacing at 1 mm above the surface of quartz substrate. The power supply for a sparking voltage of 3 kV with limited current of 3 mA was connected to apparatus.TiO2 film was prepared by sparking nanoparticle (NPs) of Ti on quartz substrate for 100 times with a substrate speed of 1.5 cm/min as show in figure 1 and then annealed in air at 500 °C. For hydrogenation sample, hydrogen gas was generated by water electrolysis process (1 M KOH solution, voltage of 15 volt, stainless steel electrodes) and flowed into the quartz tube in a diameter 3.81 cm and a length of 100 cm. The sample prepared by sparking for 100 times was placed inside before the quartz tube was heated to heat to 500 °C for 2 h, air inside the tube was replace for 40 minutes by H2 as mention above. Characterization The surface morphology, cross-section and roughness of films were observed by SEM (JEUL JSM-635F) and AFM in the tapping mode (Digital Instruments, Inc., Santa Barbara, CA) Equipped with a standard Si tip and operated at a scan size 10×10

Materials and Methods Preparation sample Titanium tips (φ 0.25 mm, purity 99.5%, Advent

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µm2 in air at room temperature. The phase structure of films were observed by Raman spectrometer at room temperature with 514.5 nm argon iron laser (JOBIN YVON HORIBA T64000). The optical property of films were measured by UV-Vis-NIR spectrometer (Perkin Elmer Instruments) in absorption mode.

modes indicate that TiO2 and H:TiO2 thin films were defected at surface and in crystal structure [19-20].

Figure 1. Diagram of sparking apparatus

Results and Discussion Surface morphology of films Figures 2 (a) and (b) display the top view SEM image of TiO2 and H:TiO2, respectively. The surface of films are very porous. A morphology of films that annealed in H2 atmosphere (H:TiO2) is more dense than that of TiO2. The roughness and surface morphology of TiO2 and H:TiO2 are characterized by three dimension plots AFM as show in figures 2 (c) and (d), respectively. From the figures 2 (c) and (d) show that the granular morphology of films are different, H:TiO2 is smaller. The difference of roughness between TiO2 and H:TiO2 is not significant. Figures 2 (e) present the cross-section SEM image of films. The thickness of films was found to be 2 µm. Phase structure of films The structural properties of TiO2 and H:TiO2 that annealed at 500 °C are characterized by Raman spectrometer and compared with those pure anatase phase and rutile phase of TiO2 as show in figure 3. It wsa found that the phase structure of TiO2 and H:TiO2 annealed at 500 °C are anatase-rutile mixture phase [18]. Five Raman active mode were assigned to pure anatase phase of TiO2 (144, 200, 401, 520 and 643 cm-1) and three for pure rutile (240, 450 and 610 cm1 ). In the case of TiO2, two bands of anatase phase (150 and 523 cm-1) and three band of rutile phase (271, 444 and 616 cm-1) were observed. For the case of H:TiO2, only one band of anatase phase (141 cm-1) and three band of rutile phase (263, 440 and 610 cm-1) were observed . There are shifted in comparison to pure antase and rutile phase. The shifted of Raman

Figure 2. Top view SEM image show the surface morphology of TiO2 film (a) and H:TiO2 film (b), three-dimensional AFM image of TiO2 film (c) and H:TiO2 film (d), cross-section SEM image (e)

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Figure 3. Raman spectra of TiO2 and H:TiO2 thin films annealed at 500 °C as compared with those pure anatase phase and rutile phase of TiO2

Figure 5. Absorbance spectra for the direct electronic transition (αhν)2 versus hν (eV) of the synthesized TiO2 and H:TiO2 at 500 °C

Optical property

Table 1. Comparison of root-mean-square roughness, absorption band edge (λ) and the energy gap of TiO2 and H:TiO2 thin films

Figure 4 display the UV-Vis NIR diffuse refactance absorption spectra of TiO2 and H:TiO2 at 500 °C. The garph show that TiO2 and H:TiO2 are strong absorption in UV region and weak absorption in visible region. When compare between TiO2 and H:TiO2 found that H:TiO2 exhibit stronger absorpbance than TiO2. The result confirm that hydrogenation on TiO2 can enhance solar absorption. Figure 5 display (αhν)2 versus hν (eV) plots for direct transition of TiO2 and H:TiO2, where α is absorption coefficience, hν is energy of photon (Ephot) by Ephot = (1240/λ) eV, λ is wavelengh in nanometer. From equation (αhν)2 = (Ephot-Eg), the value of the direct energy gap estimated from the α =0 exptrapolation as 3.35 eV for the TiO2 film and 2.98 eV for H:TiO2 film. The energy gap of films was demonstrated that it can be reduced by hydrogenation [11].

Sample

Roughness, nm

Absorption Band Edge (λ), nm

Energy Gap (Eg), eV

TiO2 H:TiO2

185 176

370 416

3.35 2.98

The root-mean-square roughness, absorption band edge (λ) and the energy gap of TiO2 and H:TiO2 were show in table 1. The root-mean-square roughness of the TiO2 and H:TiO2 film were determined to be 185 nm and 176 nm by AFM, respectively, no significant difference. The absorption spectra consists of a single broad at 370 nm for TiO2 and 416 nm for H:TiO2 can be attributed to chargetransfer from valence band to conduction band.

Conclusions Titanium dioxide (TiO2) and hydrogenated titanium dioxide (H:TiO2) mixture anatase-rutile phase thin films were successfully prepared by sparking method and annealed at 500 °C for 2 h in air and in H2 atmosphere, respectively. The surface morphology of films have more porous and high roughness, as well as high specific area for photocatalytic activity. H:TiO2 increase UV-Vis-NIR absorption for 20.7%. Direct energy gap of TiO2 and H:TiO2 were determined to be 3.35 eV and 2.98 eV, respectively. Figure 4. Absorbance spectra of TiO2 and H:TiO2 films

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12. G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R. C. Fitzmorris, C. Wang, J.Z. Zhang, and Y. Li ,“Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting”, Nano

Acknowledgments Science achievement scholarship of Thailand (SAST) and National Research University Project Thailand’s office of the Commission on Higher Education (CHE) were thanked for financial support.

Lett., 2011, 11 (7), pp 3026–3033.

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