Photoelectrochemical Water Splitting
The direct conversion of solar to chemical energy can be achieved photoelectrohemistry. In this approach, we place light absorbing semiconductor materials in an aqueous solution to drive the photochemical oxidation of water and reduction of water to produce hydrogen. Such a system is very complex and involves light harvesting, charge separation, and catalysis simultaneously. Finding one (or more) stable materials that can do this efficiently is a challenge that combines physics, chemistry, and engineering. Our group focuses on the fabrication and characterization of thin film and nanostructured electrodes, with an emphasis on defect engineering bulk properties and mitigating interfacial charge recombination losses. In particular, we use BiVO4 and amorphous SiC as the platforms to study both water oxidation and reduction, respectively.
Improving Charge Separation and Transport
The separation and transport of photoexcited electrons and holes are essential to understand and optimize for an efficient photoelectrode. In our lab, we have taken several approaches to mitigate recombination losses within the bulk of various semiconductors to be able to extract as many photogenerated charge carriers as possible. We have primarily used non-isovalent doping in order to increase carrier concentrations (Fe-doped CuWO4) and gradient doping profiles (a-SiC photocathodes) to improve the charge carrier separation efficiency and lower the amount of potential required to extract usable current.
(D. Bohra and W.A. Smith, PCCP, 17, 9857 (2015), L. Han et al., J. Mat. Chem. A, 3, 4155 (2015))
(D. Bohra and W.A. Smith, PCCP, 17, 9857 (2015), L. Han et al., J. Mat. Chem. A, 3, 4155 (2015))
Interfacial Band Edge Energetic Control
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The semiconductor-liquid junction is the defining characteristic of a photoelectrochemical system. Here, the electronic structure of the semiconductor must come into equilibrium with the electrolyte to allow efficient charge transfer to convert electrical to chemical energy. To optimize the charge transfer efficiency, and to minimize recombination losses at the interface, our lab focuses on creating front surface field layers (such as n-TiO2 on p-i a-SiC) to maximize the photovoltage created in the semiconductor, while also allowing effective charge extraction at the semiconductor-liquid junction.
(I. Digdaya et al., Energy Environ. Sci., 8, 1585 (2015), W.A. Smith et al., Energy Environ. Sci., 8, 2851 (2015), I. Digdaya et al., J. Mat. Chem. A, 4, 6842 (2016)) |
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Surface Functionalization with Metal Nanoparticles
It is very difficult to simultaneously manage the optical, electronic, and catalytic properties of a single semiconductor material. Therefore, functionalizing a semiconductor with other materials to enhance one or more meaningful property at a time is advantageous, but difficult to achieve. In our group, we study the effect of placing metal nanoparticles on the surface of different semiconductors (BiVO4 and CuWO4), and monitor the changes in the opto-electronic, catalytic, and photoelectrochemical performance.
(M. Valenti et al., J. Phys. Chem. C, 119, 2096 (2015), M. Valenti et al., ChemNanoMat, in press (2016))
In-situ Characterization and Optimization of Photoelectrodes
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It is essential to be able to probe photoelectrochemical systems both in the dark and under solar illumination conditions. We have used in-situ techniques (such as UV-Vis spectroscopy, X-Ray absorption spectroscopy, ATR-FTIR) to study the optoelectronic properties of BiVO4 photoanodes during photoelectrochemical water splitting reactions. We have also developed a new technique to improve the charge separation, transport, and catalytic efficiencies of BiVO4 photoanodes through a so-called photocharging experiment, which has resulted in record performance for this material without any external dopants or surface catalyst added.
(B.J. Trzesniewski and W.A. Smith, J. Mat. Chem. A, 4, 2919 (2016)) |
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