Photocatalysis is a promising catalytic process for solar energy utilization, which can underpin the development of critical renewable energy and environment technologies. The key of photocatalysis technology is to research and develop highly efficient photocatalytic materials under solar light irradiation. Doping is indispensible for extending light responsive range and improving photocatalysis efficiency of photocatalysts. However, the detailed investigation targeting on doping in well-defined crystal facets and the effects of dopant configurations and distribution on electronic structure and surface structure of materials is extremely rare, which thus restricts the design and controllable synthesis of highly efficient photocatalysts. This dissertation focuses on doping in anatase TiO2 crystals with dominant (001) facets and subnanometer titania nanosheets, controlling dopant configurations and distribution and their role in developing highly efficient titania based photocatalysts.
A new route is designed to prepare phase-controllable TiO2, oxygen deficient anatase TiO2 with dominant (001) facets, and nitrogen/sulfur doped anatase TiO2 with dominant (001) facets. The key in this route is to employ crystal compounds (TiB2, TiN or TiS2) hard to dissolve where both Ti and dopants are containing in a single precursor for TiO2. By simply adjusting mineralizers such as Cl-、SO42- or NO3-, the phase tunable (rutile/anatase/brookite) titania with different morphologies (rutile nanocones, anatase bipyramids and brookite spherical nanoparticles) can be obtained. TiB2 hydrolyzing in HF solution can form oxygen deficient anatase TiO2 with dominant (001) facets, where surface reconstruction induced by oxygen vacancies can substantially enhance photoactivity. Nitrogen/sulfur doped anatase TiO2 with dominant (001) facets is prepared by employing TiN/TiS2 as precursor in HF solution under hydrothermal conditions. The developed nitrogen/sulfur doped TiO2 shows visible light absorption and corresponding visible light photoactivity.
It is indicatively evidenced that the TiO2 surface structures containing B-N bonds can exhibit bifunctionality in promoting photocatalysis, 1) supplying partially occupied localized states attributed to B-N coupling with spectral distribution that is advantageous for enhancing visible light absorption as well as 2) acting as photocatalytic ‘hot sites’ to support localization and separation of charge carriers at the surface. On the other hand, surface iodine doped TiO2 with coexisting atomic configurations of iodine dopant, I-O-I and I-O-Ti structures, exhibits extended absorption edge up to 800 nm. Furthermore, photocatalytic activity investigations confirm the efficient generation of important oxidative species •OH radicals in photocatalysis oxidation processes beyond 600 nm. It is theoretically found that iodine atoms prefer to be doped within the surface due to the strong I-O repulsion. I-doping in surface leads to little change in its intrinsic bandgap, but the distribution and occupation of localized states of iodine strongly depends on the surface iodine configurations. The latter is responsible for the wider range visible light response exhibited in anatase TiO2 with the coexistence of I-O-I and I-O-Ti structures. The coexistence of I-O-I and I-O-Ti structures can distinctly change the surface structure due to the release of local strain energies. These results offer important implications for designing highly efficient photocatalysts based on co-doping strategies.
In contrast to most nonmetal doped titania photocatalysts with some localized states in the intrinsic bandgap and small visible light absorption shoulders induced by inhomogeneous nitrogen doping near the particle surface, the homogenous substitution of O by N is realized in the whole particles of layered titanates. The resultant materials Cs0.68Ti1.83O4-xNx and H0.68Ti1.83O4-xNx exhibit extraordinary band-to-band excitation in the visible-light ranging up to blue light. The upward shift of valence band maximum by N 2p states is concluded as the cause of the band-to-band visible light excitation. The holes generated upon visible light excitation in the newly formed valence bands of Cs0.68Ti1.83O4-xNx and H0.68Ti1.83O4-xNx have strong oxidation ability in oxidizing OH- into active •OH radicals in photocatalysis. These findings are the clear evidence for the substantial role of homogenous nitrogen doping in obtaining band-to-band visible-light photon excitation in layered titanates. The new physical insights into the electronic structure of homogeneous substitutional N in layered titanates gained here may have important implications for developing other efficient visible light photocatalysts by nonmetal doping.
Semiconducting metal oxide nanosheets (NSs) with extremely small thickness in subnano- to nanometer scale can be derived from delamination of layered compounds. By exfoliating layered H0.68Ti1.83O4-xNx with homogeneous nitrogen doping, visible light responsive Ti0.91O2-xNx superthin nanosheets with a thickness of 0.75 nm are developed. Furthermore, by using this new type of nanosheets as building blocks, multilayer thin films via the layer-by-layer (LBL) self-assembly exhibit the capability of photoelectrochemically splitting water as photoanodes under visible light irradiation.
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