Visible-Light-Driven Photocatalytic Hydrogen Generation on Nanosized TiO₂-II Stabilized by High-Pressure Torsion

Abstract

Photocatalytic activity of pure TiO2 is limited to ultraviolet (UV) light due to the wide bandgap of anatase and rutile phases. The bandgap of high-pressure phases of TiO2 can theoretically coincide with visible light, but these phases are unstable at ambient pressure. In this work, the high-pressure TiO2-II (columbite) phase with large fractions of oxygen vacancies was stabilized by inducing plastic strain to anatase under 6 GPa. The material could absorb visible light as a consequence of bandgap narrowing by ∼0.7 eV. Formation of nanosized TiO2-II enhanced the hydrogen generation efficiency under visible light, and the efficiency improved after removing the oxygen vacancies by annealing.

Review on Recent Advancements in Severe Plastic Deformation of Oxides by High‐Pressure Torsion (HPT)

“…Figure a shows the occurrence of compaction in Al 2 O 3 powders when they are processed by HPT . Analyzing the surface area using the pulsed nuclear magnetic resonance (NMR) and Brunauer‐Emmett‐Teller (BET) methods, as summarized in Figure b for TiO 2 , ZnO and Al 2 O 3 , confirms that the surface area is reduced by HPT processing which is a clear indication for partial consolidation. This partial consolidation is unfavorable, when large surface area are required for an application such as photocatalysis .…”

“…Analyzing the surface area using the pulsed nuclear magnetic resonance (NMR) and Brunauer‐Emmett‐Teller (BET) methods, as summarized in Figure b for TiO 2 , ZnO and Al 2 O 3 , confirms that the surface area is reduced by HPT processing which is a clear indication for partial consolidation. This partial consolidation is unfavorable, when large surface area are required for an application such as photocatalysis . However, this partial consolidation can lead to better sintering at lower temperature, as was shown for Al 2 O 3 , BaTiO 3 , and some kind of ceramic‐based composites …”

“…a) Appearance of α ‐Al 2 O 3 powders before and after HPT processing (reproduced with permission . 2010, Elsevier); b) surface area of anatase‐TiO 2 , wurtzite‐ZnO, and γ ‐Al 2 O 3 measured using pulsed NMR and BET methods before and after HPT processing.…”

“…The continuous formation of vacancies up to supersaturated levels in HPT‐processed metals is currently an accepted fact, which is believed to contribute to fast diffusivity in UFG metals and alloys . Similar to metallic materials, formation of oxygen vacancies by HPT processing is another general features that can be observed in different oxides such as BaTiO 3 , TiO 2 , ZnO, and Al 2 O 3 . Bridgman's pioneering studies roughly indicated the reduction of some oxides by HPT processing, although he did not have access to advanced experimental tools to prove this issue .…”

“…In 2010, our group started working on SPD processing of oxides to develop nanostructured functional oxides. The first publication on α‐Al 2 O 3 , together with a few following studies on other oxides such ZrO 2 , BaTiO 3 , TiO 2 , Y 2 O 3 , ZnO, and γ‐Al 2 O 3 proved significant effect of HPT processing on microstructure, phase transformation, and functional properties of oxides. In this paper, after a brief introduction of some old publications on the application of HPT to oxides, recent advancements reported by the author's group in Kyushu University are reviewed and compared with metals by focusing on six main issues: 1) partial consolidation, 2) nanograin formation and dislocation activity, 3) oxygen vacancy formation, 4) phase transformation and size effect, 5) bandgap narrowing, and 6) functionality enhancement.…”