Crystal Facet Effect On Semiconductor-based Photocatalysts

It is well known that the photo-induced charge separation has a critical impact on the efficiency of photocatalytic reactions based on the semiconductors. The crucial steps of charge separation and surface reaction can be principally tuned by semiconductor’s interface and surface engineering. For example, fabrication of heterojunction, p-n junction, phase junction and the charge separation on the different facets have been well adopted for achieving efficient charge separation. Therefore, it would be interesting to investigate how photogenerated electrons and holes are separated on the semiconductor crystals with equivalent facets and nonequivalent facets.(1) Based on the cubic structured NaTaO3 with six equivalent facet, photodeposition of noble metals and metal oxides show no preferential deposition, indicating that no spatial charge separation occurs among the six equivalent facets of NaTaO3. However, observation of efficient overall water splitting reaction upon loading of well-known cocatalyst NiO on the NaTaO3 clearly demonstrates that photogenerated electrons and holes could still be well-separated. Insitu formation of Ni and NiO cocatalysts during the water splitting process was revealed by XPS and XAS analysis, confirming the role of dual cocatalysts Ni/NiO, where nickel serves as an electron trap(catalytic sites for water reduction) and NiO serves as a hole trap(catalytic sites for water oxidation). Such kind of vicinal charge separation by dual cocatalysts can also lead to efficient overall water splitting.(2) Based on the semiconductor photocatalysts with nonequivalent facets, the rutile TiO2 crystals with regulable ratios of(110) and(111) factets were prepared. The origin of the preferential deposition of noble metal cocatalysts on the particular facet of(110) was carefully studied. Our results clearly show that the noble metals could be photo-deposited exclusively on the(110) facet. This implies that origin of the noble metal preferential deposition on the(110) facet of rutile TiO2 is due to the reduction of the noble metal precursors by the photoexcited electrons enriched on the particular facet of(110). Rutile TiO2 samples with cocatalyst selectively deposited on the(110) facets have much higher photocatalytic H2 production activities than rutile samples with noble metal cocatalysts randomly deposited on the entire surface. This demonstrates that the facet charge separation upon photoexcitation may be a unique route for efficient photocatalytic systems.(3) Layered perosvskite BaLa4Ti4O15 photocatalyst shows the unique selective photodeposition between the edge and basal facets. It is found that the selective depositon of Pt and Au on the edge of BaLa4Ti4O15 can improve the activity of water splitting, but when selective depositon of MnOx, CoOx, PbO2 occurs on the basal of BaLa4Ti4O15, the activities have not been improved.The other part of this thesis is controlled growth, optical properties and electrochemical catalysis of hybrid PdAg alloy-Au nanorods.