Global environmental pollution and energy supply demand have been regarded as important concerns in recent years. Metal oxide semiconductor photocatalysts is a promising approach to apply environmental remediation as well as fuel generation from water splitting and carbon dioxide reduction. ZnO nanostructures have been shown promising photocatalytic activities due to their non-toxic, inexpensive, and highly efficient nature. However, its wide band gap hinders photo-excitation for practical photocatalytic applications under solar light as an abundant, clean and safe energy source. To overcome this barrier, many strategies have been developed in the last decade to apply ZnO nanostructured photocatalysts under visible light. In this review, we have classified different approaches to activate ZnO as a photocatalyst in visible-light spectrum. Utilization of various nonmetals, transition metals and rare-earth metals for doping in ZnO crystal lattice to create visible-light-responsive doped ZnO photocatalysts is discussed. Generation of localized energy levels within the gap in doped ZnO nanostructures have played an important role in effective photocatalytic reaction under visible-light irradiation. The effect of dopant type, ionic size and its concentration on the crystal structure, electronic property and morphology of doped ZnO with a narrower band gap is reviewed systematically. Finally, a comparative study is performed to evaluate two classes of metals and nonmetals as useful dopants for ZnO nanostructured photocatalysts under visible light.
In the present research, we investigate the synergistic effects of Ru-doping and Ar/H 2 reduction treatment on the photoelectrochemical water splitting performance and hydrogen evolution rate of TiO 2 nanotube array photoelectrodes. The Ti−Ru alloy with 0.2 at. % Ru was used to grow anodic self-organized Ru-doped TiO 2 nanotube layers. An ideal synergy between Ar/H 2 reduction treatment and Ru-doping results in the extended absorption toward the visible light region and improved photoelectrocatalytic activity. The black Rudoped TiO 2−x photoanode's water splitting rate improves remarkably (∼ninefold) compared to the black TiO 2−x sample (∼twofold). Moreover, the black Rudoped TiO 2−x photoanode shows a considerable increase in the H 2 production rate (1.91 μmol h −1 cm −2 ) compared with the pristine TiO 2 nanotube layer (0.044 μmol h −1 cm −2 ). We have also developed an ab initio model of pristine, Ru-doped, and Ru-doped hydrogenated TiO 2 nanotubes to rationalize our experimental findings from the theoretical perspective.
In this research, we investigate the effect of alkali metal cations including Li, Na and Cs in hydrothermal solution on the morphology, stability, and photoactivity of nanostructured TiO 2 nanoflakes as a photoanode. The TiO 2 nanoflakes are formed through hydrothermal treatment of Ti foil in 1.0 M LiOH, NaOH or CsOH at 100°C for 3 h. By subsequent thermal reduction of the structure in an optimized Ar/H 2 environment, conductive TiO 2 nanoflakes were formed. The reduction treatment remarkably improves the photocurrent density of the TiO 2 nanoflakes and has the highest impact on the sample treated in the NaOH alkali solution. For the nanoflakes produced in NaOH alkali solution, the bandgap is shifted to a lower value and the structure shows the most stable morphology after thermal treatment compared to nanoflakes formed in other alkali solutions. Such reduced hydrothermally treated nanoflakes formed in NaOH can generate a photocurrent density of approximately 1 mA/cm À 2 vs. Ag/AgCl in 1.0 M KOH solution, which is six times higher than for pristine TiO 2 .[a] E.
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