Nagoya Institute of Technology, Japan
Hadi Razavi-Khosroshahi has obtained his PhD in Materials Physics and Chemistry from Kyushu University, Fukuoka, Japan, in 2013. He is currently an Assistant Professor in Nagoya Institute of Technology, Japan. His main research interests are production of nanostructured metal oxides both from top-down and bottom-up approaches. He is recently working on narrowing the band gap of semiconductors by means of severe plastic deformation (SPD), and in particular by high-pressure torsion (HPT) method. His research results based on strain-induced phase transformations have revealed that high-pressure phases of various semiconductors are stabilized at ambient pressure are capable of being excellent environmental and energy materials like visible-light-active photocatalysts.
Titanium dioxide (TiO2) is a well-known semiconductor with superior photocatalytic properties. TiO2 has three polymorphs (anatase, rutile, and brookite) among which anatase exhibits the best photocatalytic properties because of its large electron effective mass, which leads to a low mobility of charge carriers. Despite its good photocatalytic features, application of pure anatase as a photocatalyst has been limited to the UV range of sunlight due to its wide bandgap (3.2 eV). Narrowing the bandgap of pure TiO2 using dopant-free approach has been in the spotlight in recent years. Theoretical studies suggest that high-pressure phases of TiO2 have narrower bandgaps which can coincide with the visible light. However, these phases are stable only under high pressures. High-pressure torsion (HPT) method is an effective technique for stabilizing high-pressure phases at ambient pressure. In this method, high pressure and large plastic strain are simultaneously applied to a sample between two rotating anvils. In this study, the HPT process was conducted at room temperature on pure (99.8%) anatase powder with an average particle size of ~150 nm. Photocatalytic hydrogen evolution was examined under UV light and under visible light. Moreover, X-ray diffraction, Raman spectroscopy, differential scanning calorimetry, photoluminescence spectroscopy, electron paramagnetic resonance analysis, electron energy loss spectroscopy, Fourier transform infrared spectroscopy, and UV-V diffuse reflectance spectroscopy are used for characterization of the material. Photocatalytic hydrogen generation by water splitting was examined as a measure of photocatalytic activity of TiO2.
Photocatalytic hydrogen generation as a function of time under UV and visible lights are shown in Figure1. Hydrogen generation rate under UV light is slower for the HPT-processed sample comparing with that for the anatase powder. However, the hydrogen generation rate is faster for the HPT-processed sample when illuminated by visible light. Hydrogen generation rate both under UV and visible lights significantly improves when the HPT-processed sample is annealed at 500°C.