Enhanced photocurrent in bimetallic/TiO<sub>2</sub> nanotube composite structures

In recent years, due to the shortage of fossil energy and environmental pollution, the harvesting and conversion of solar energy using semiconductors has attracted more and more attention. Among many kinds of traditional semiconductor photocatalysts, the titanium dioxide (TiO₂) has become one of the most popular semiconductor photocatalysts because of its low cost, good stability and environmental friendliness. However, TiO₂ has a relatively wide band gap (i.e., 3.2 eV for anatase and 3.0 eV for rutile), which can only absorb ultraviolet light with a wavelength less than 387 nm, and has a low utilization rate of sunlight. Moreover, the photo-generated electron-hole pairs in the system of TiO₂ film and particle are easy to recombine, which makes the photocatalytic efficiency of the material relatively low. In order to solve these problems, TiO₂ structure with porous (such as nanotube) is used to restrain the recombination of photo-generated electron-hole pairs due to its large specific surface area and good charge transfer characteristics, thereby improving its photocatalytic efficiency. In addition to changing the structure of traditional semiconductor materials, using the surface plasmon resonance effect of metal micro-nano structures to improve its photoelectric conversion efficiency has many potential applications in the fields of photovoltaic, photocatalysis and photoelectric detection. In this paper, we prepare different metallic nanoparticles loaded TiO₂ nanotube composite structures by atomic layer deposition method and electron beam thermal evaporation technology and investigate the photocatalytic properties of the composite structures. It is shown that comparing with the pure TiO₂ nanotubes, the photocurrent of TiO₂ nanotubes loaded with Au nanoparticles increases by about 400% under 568 nm visible light irradiation; the photocurrent of TiO₂ nanotubes loaded with Al nanoparticles increases by about 50% under 365 nm ultraviolet (UV) irradiation; the photocurrent of TiO₂ nanotubes loaded with bimetallic Au and Al nanoparticles increases by about 50% in the whole UV-visible light region: it is significantly enhanced. Based on the fact that the surface plasmon resonance frequency of Au and Al nanoparticles are complementary, not only is the optical absorption of TiO₂ nanotubes enhanced and broadened, but also the photocurrent is enhanced from ultraviolet to visible light. We believe that these results will contribute to the further development of photocurrent in semiconductor nanotubes.