Factors in the Metal Doping of BiVO4 for Improved Photoelectrocatalytic Activity as Studied by Scanning Electrochemical Microscopy and First-Principles Density-Functional Calculation

The semiconductor properties important in the design of photocatalysts required for a photosynthetic system, e.g., a photoelectrochemical cell to split water to hydrogen and oxygen, have largely been taken to be the band gap energy, Eg, and the band-edge locations, Ec and Ev (usually with considerations of factors affecting stability). These affect the amount of solar energy absorbed in the material and the energies of electrogenerated carriers (holes and electrons). However, many other factors affect the performance and efficiency of the photocatalyst, such as the depth of penetration of the incident radiation, the carrier mobility, the depth of the electric field at the surface, the rate of recombination of the photogenerated carriers in the bulk and on the surface, and the carrier transfer rate to solution species. Clearly, both the photocatalyst composition and its structure are important. Combinatorial screening of the photocatalysts has been used to find new photocatalysts including scanning electrochemical microscopy (SECM) based on scanning with a fiber optic and robotic fabrication of arrays; these have been useful in finding photocatalysts with the desired composition. 10 The rapid screening by SECM of metal oxide and metal sulfide semiconductors has been useful in finding highly active photocatalysts. For example, we recently developed W-doped BiVO4, which has several times higher photoactivity for water oxidation than undoped BiVO4. 9 Solid solutions of ZnxCd1 xSeyS1 y were also developed using the SECM screening method to find the optimum values of x and y to tune the band gap and photoelectrochemical activity for polysulfide oxidation. Development of an excellent photocatalyst with high conversion efficiency for light, along with electrocatalysts to drive the desired reactions, is essential in the design of the overall photoelectrochemical system to achieve the photosynthetic production of a fuel, such as hydrogen fromwater. SECM as described above has been proven to be a very useful tool for discovering a complicated composition with optimal properties. In the nearly 40 years since Fujishima and Honda suggested the possibility of water splitting in a photoelectrochemical cell, there have been enormous efforts to find a photocatalyst and electrocatalysts for this reaction. However, to date none of these investigated photocatalysts has fulfilled all of the necessary design requirements for practical photosynthetic water splitting. BiVO4 is a promising material that can adopt a wide range of dopants with doping levels of 10 atomic percent (at %) or more to tune its optical and chemical properties. BiVO4 has a band gap of 2.4 2.5 eV which harvests visible light, and it has been studied as a water oxidation photocatalyst. 21 Furthermore, the photocatalytic activity of BiVO4 also depends on its crystal