Construction Of Metal-Oxide-Based Photocatalysts With High Charge-Separation Ability

Increasing the utilization of solar energy is significant for reducing the emission of greenhouse gases such as CO2. Among various ways of utilizing solar energy, photocatalysis, which can convert solar energy into storable chemical energy (for example, photocatalytic water-splitting to produce hydrogen), is considered to be a promising technology. The key to promoting practical application of photocatalysis is developing photocatalytic materials with high energy conversion efficiency. However, photocatalysts for solar fuels suffer from the bottleneck of low photocatalysis activity. Photocatalysis is synergistically determined by three steps, namely solar absorption, separation of photo-generated charge carriers and their surface transfer. The charge separation, which connects the other two steps, is considered a central step in photocatalysis. Controlling the property of interlace beween different components of photocatalysts or properties of different micro-regions of single component represents an important direction to improve the charge separation. In this thesis, three typical photocatalysts with unique interface properties or modulated micro-region properties were constructed with purpose of improving charge separation and thus photocatalysis activity. They are tantalum boride/tantalum oxide core/shell particles with strong interface contact, Ti3+/Ti4+ crystalline/amorphous core/shell TiO2 particles, and rutile TO2 crystals with their surface consisting of both low energy and. high energy facets modified with fluorine. The results obtained in thesis could provide some guidelines for designing efficient photocatalyts on the basis of charge separation.Construction of tantalum boride/tantalum oxide core/shell particles with strong interface contact. It is also necessary to enable the interface contact between light absorber and co-catalyst in photocatalyst in order to promote the charge transfer between them. With this in mind, a novel strategy featured by using metal boride co-catalyst as the precursor of in situ growing its metal oxide light absorber on it was developed. Specifically, TaB2 was chosen as a model system to demonstrate this idea. By treating micron-sized TaB2 powder under gaseous hydrothermal conditions, the growth of Ta2O5 nanorods on TaB2 particles was conducted to construct TaB2/Ta2O5 core/shell heterostructure particles. As a consequence of strong interface contact, two features were introduced. One is the redshift of the intrinsic absorption edge of Ta2O5 caused by lattice distortions at the interface. The other is the formation of additional visible light absorption band of Ta2Os caused by interfacial doping of boron from TaB2. Owing to these favorable features together with catalytic activity of TaB2 itself for hydrogen evolution, the core/shell particles without noble metal co-catalyst gave a much superior photocatalytic activity in hydrogen generation from the mixture of water/methanol to reference Pt/Ta2O5 photocatalyst under UV-visible light. Moreover, the core/shell particles showed visible light photocatalytic activity while the reference was inactive.Construction of TP+/Ti4+ crystalline/amorphous core/shell particles. The electrons in metal oxides/sulfides/nitrides usually have a smaller effective mass than holes so that the population of electrons reaching photocatalyst surface is always larger than that of holes. While photocatalysis is dominantly controlled by holes involved half reaction. To address this intrinsic restriction, a strategy for reversing the population of electrons and holes reaching surface by modulating the band alignments of core and shell regions of single particle was developed. Taking rutile TiO2 as an example, the TP+/Ti4+ crystalline/amorphous core/shell TiO2 particles were prepared as follows. Micron-sized TiO2 particles were first heated in gaseous hydrogen atmosphere to introduce Ti3+ within the whole bulk of the particles. The subsequent hydrothermal treatment in hydrofluoric acid solution led to the formation of Ti3+-free amorphous TiO2 layer with the thickness of several nanometers. The resultant core/shell TiO2 particles have the unique band alignments, where both conduction and valence band edges of the TiO3+-rich core region are lower than that of the Ti3+-free shell. These band alignments set a barrier for electrons to overcome during the transport of the electrons from the bulk to surface so that the probability of electrons reaching surface can be lowered. As a consequence, the populations of electrons and holes reaching surface are reversed, which leads to the improvement of photocatalytic hydrogen generation from water splitting by two orders of magnitude.Fluorine modification induced charge separation between{110} and{111} facets of rutile TiO2 crystals. The difference in energy levels of two facets of crystals may lead to the separation of photogenerated charge carriers between different facets. However, such difference is not always large enough to drive the charge separation between the facets. To address this challenge, a strategy of enlarging the energy level difference between the exposed facets by modifying the facets with fluorine atoms was proposed. The basis of this strategy is the different percentages of unsaturated cations on each facet. Well-defined rutile TiO2 crystals with their surface consisting of low-energy{110} and high-energy {111} facets were chosen as starting material to demonstrate the effectiveness of this strategy. To modify crystal surface with fluorine, the faceted crystals were subjected to the hydrothermal treatment in a hydrofluoric acid solution. Owing to different surface atomic structures of {110} and {111} facets, the concentration of fluorine on {111} facets is higher than that on {110} facets. The separation of photogenerated electrons and holes between {110} and {111} facets of the crystals with surface fluorine modification was confirmed by monitoring the distribution of depositions of photo-reduction and photo-oxidation reactions. In contrast, no such separation was observed in pristine faceted rutile TiO2 crystals. The analysis of fine electronic structure reveals the enlarged energy level difference between fluorine-modified {110} and {111} facets. Moreover, the study of the dynamic behaviors of photogenerated electrons further suggests the electron transfer between two facets proceeds through the defect related shallow trap states below the conduction band. The enabled separation of charge carriers between {110} and {111} facets greatly promotes the improvement of photocatalytic hydrogen and oxygen evolution from water splitting.