Photocatalytic Water Splitting Over Modified SiC Under Visible Light

The conversion of solar power into hydrogen energy by photocatalytic water splitting is highly pursued by researchers due to energy and environmental issues. SiC is promising for H2 evolution from water reduction because of its suitable band gap, abundant source, as well as environmental-friendly properties.5-9 In this thesis, we modify the SiC catalyst through depositing BiVO4, graphene, quantum sized BiVO4 (QD-BiVO4) and Pt on SiC surface. The photocatalytic water splitting over modified SiC has been studied under visible light. And, the transfer pathway of the photo-generated electrons and holes in these composites was deeply investigated. The specific research results are as follows:(1) The photocatalytic activity of SiC can be greatly enhanced by compositing BiVO4 to form well-connected interfaces via crystal engineering method. The rate of photocatalytic oxygen evolution is leading to 659 μmol h-1 g-1 observed in 1:1 (weight ratio) SiC/BiVO4 composites under visible-light irradiation. The apparent quantum efficiency (AQE) reaches 1.04% at 420 nm. The transfer pathway of the photo-generated electrons between the composite is facilitated through a Z-scheme contact between BiVO4 and SiC, which is further evidenced by using H2PtCl6 photo-reduction as a molecular probe.(2) The graphene was introduced in BiVO4/SiC composites acted as a photo-generated electrons carrier. The lifetime in this composite was prolonged and the transfer rate of photo-generated electrons was enhanced. The rate of photocatalytic O2 evolution is further enhanced to 988.2 μmol h-1 g·1 observed in SiC/GO-1%/BiVO4 composites under the same conditions. The AQE reaches 1.96% at 420 nm.(3) The QD-BiVO4 was controlled deposition selectively on the C-determined face of SiC via a hydro-thermal method. The lifetime in this composite was further prolonged. The distance of transfer pathway for photo-generated electrons was shorted. The rate of photocatalytic O2 evolution is further enhanced to 2069 μmol h-1 g-1 observed in SiC/QD-BiVO4 (1:0.1) composites under the same conditions. The AQE reaches 3.1% at 420 nm.(4) The Pt was controlled deposition selectively on the Si sites of a micro-SiC photocatalyst surface via in-situ photo-depositing. The Pt-Si bond forming on the interface constructs an excellent channel, which is responsible for accelerating photo-electron transfer from SiC to Pt and then reducing water under visible-light. The hydrogen production is1376 μL h-1 g-1.The AQE reaches 1.81% at 420 nm.(5) The efficient charge separation that can be achieved on Si-determined and C-determined facets of SiC is investigated, as evidenced by the reduction-oxidation reaction with photogenerated electrons and holes. The reduction (Pt) and oxidation (MnO2) cocatalysts are selectively deposited on the Si-determined and C-determined facets respectively, resulting in much higher activity in photocatalytic water reduction reactions and oxidation reactions, compared with the photocatalyst with randomly distributed cocatalysts. For Pt/SiC/MnO2 catalyst, the rate of photocatalytic hydrogen evolution is leading to 2640 μL h-1 g-1, and the rate of photocatalytic oxygen evolution is 245.7 μL h-1 g-1.(6) We present the micro-sized SiC powder applying to overall water splitting under visible light irradiation without adding any sacrificial compounds. The stoichiometric amounts of H2 and O2 in 2:1 obtain when using WO3 nanoparticle and Pt-loaded micro-SiC grains as the O2 and H2 evolution photocatalysts, respectively. The relatively low efficiency step for splitting water reaction is the H+ reduction in the solution. Under optimal condition, the apparent quantum efficiency reaches up to 0.021% at 420 nm.