Preparation And Photocatalytic Properties Of Porous Black Titanium Dioxide And Its Composites

Porous TiO₂ can increase the probability of light reflection and refraction within the channels effectively due to its large specific surface area, opening pore, high pore volume and narrow pore size distribution, which is not merely beneficial to the interaction and diffusion between reactants and catalysts, but also can improve the efficiency of photo-induced electronic-hole separation and migration and shorten the relative migrating distance from the bulk to surface. Therefore it can produce more active sites for participating reactions, which leads to wide applications in the area of photocatalysis. However, the traditional TiO₂ has a large band gap and only absorb the UV light, which results in a low utilization of the sunlight. Furthermore, photo-induced carrier separation efficiency of single-component TiO₂ is still low. So how to improve the photo-induced carrier separation efficiency and enhance the photocatalytic performance is still a great challenge. Based on the above consideration, in this paper, we establish various structures of porous black TiO₂ nanomaterials with surface hydrogenation strategy, which control their band gaps for extending the light absorption to visible light light regions. The low-valence Ti forming in porous backbone of our products promotes the separation of photo-generated carriers, which significantly improve photocatalytic performance under the sunlight. And as a host, it was used to combine nanometer silvers in its channels, which improves the utilization of the visible light with the resonance surface plasma of the noble metals. In this paper, we mainly investigate the relationship between the synthesis of different porous black TiO₂ and their composites and photocatalytic performance. The paper is consisted of the following four portions:1) Mesoporous black anatase TiO₂ nanosheets were synthesized via earth-abundant cost-free biotemplate method combined with ethanediamine encircling process, and subsequent high-temperature calcinations and surface hydrogenation. Small molecule amine-coated is able to protect the mesoporous frameworks and restrain the phase transformation from anatase to rutile. Mesoporous black anatase TiO₂ nanosheets have high specific surface area of 74 m2·g^-1 and narrow bandgap energy of 2.85 e V. It also can extend the light absorption from UV to visible light region. Photocatalytic hydrogen production(165 μmol·h^-1·0.05 g^-1) of mesoporous black anatase TiO₂ nanosheets is about twice than that of pristine mesoporous anatase TiO₂ nanosheets, which is due to the synergistic effect of the Ti³⁺ in frameworks and the surface amorphous layers, and the high crystallinity.2) 3D flower-like nanostructures are synthesized through solvothermal approach, which is assembled by the N-doped TiO₂ porous black nanosheets with a method of in-situ nitrogen-doping and self-assembly, combining with high-temperature surface hydrogenation. Tripolycyanamide plays the roles of both nitrogen source and structure-directing. Comparing with the traditional TiO₂ nanosheets, the flower-like nanostructures constituted by porous nanosheets are more beneficial to the diffusion of H2. The introduction of Ti³⁺ successfully narrows the band gap. Photocatalytic hydrogen production(1500 μmol·g^-1·h^-1) under the simulated sunlight is about twice than that of the sample of pristine one(690 μmol·g^-1·h^-1). The improvement of photocatalytic hydrogen production is the introduction of Ti³⁺ formed more activity sites to enhance the absorption of visible light and accelerate the separation of photo-induced electron-hole pairs and the migration capabilities.3) Utilizing the adhesive and flocculation effect of polyacrylamide to small particles, we synthesize carrier-free black TiO₂ wafers which have lipostomous structure and floating ability with a method of auxiliary freeze-dried and high-pressure casting technology combining with high temperature hydrogen reduction, which improve the active sites in contact with organics as well as decrease the energy consumption of light in medium. The floating black TiO₂ wafers improve the capacity of light absorption and the separation efficiency of photo-induced electron-hole pairs. The photo-degradation ratio of the resulting product for organic pollutants（hexadecane） floating on the surface of the water is up to 98.2% under the visible-light irradiation and simulated natural ecological environment. And photo-degradation efficiency for high-toxic organic pollutants has got a significant improvement.4) Black floating Ag/TiO₂ composites are prepared by wet-impregnation approach and high temperature surface hydrogenation. Based on the synergistic effect of the resonance surface plasma of nano-silvers and the formation of Ti³⁺, the light absorption is extended from UV to visible light region. The photo-degradation ratio of the composites of 3% Ag/TiO₂ composites to the persistent organic pollutants thiobencarb has reached 97.1%. The photo-degradation ability of the resulting water soluble contaminants has been further improved. This novel carrier-free floating photocatalyst will have potential applications in fields of environment.