Materials to Harvest Sunlight

The search for cheap, Earth abundant materials for solar cells, and for photo-electrodes for water splitting, calls for detailed investigations of the efficiency of light absorption in materials and nanostructures. Theoretical frameworks and efficient computer simulations are used to help interpret a growing body of complex measurements, and to predict optimal systems for harvesting sun light.

Catalysts for water splitting

The conversion of water to oxygen and hydrogen molecules is essential for a variety of renewable energy technologies. Nickel–iron (NiFe) oxyhydroxide is an important, earth-abundant electrocatalyst for the oxygen evolution reaction. A combined experimental and computational study of pure Ni oxyhydroxide and mixed NiFe oxyhydroxide thin films elucidates the chemistry governing their different electrochemical and optical properties. The Ni and Fe oxidation states in each system are assigned as a function of applied potential based on quantum-mechanical calculations, cyclic voltammetry, and UV-visible spectroscopy. In the more catalytically active NiFe system, oxidation to Fe4+ coincides with the onset of oxygen evolution. Synergy between experiment and theory provides a detailed, atomistic understanding of this robust catalyst.

Light absorbers for water oxidation

The desirable properties of water-splitting photoanode and/or photocathode materials include: (i) Efficient absorption of visible light. The optimum value of the band gap should be larger than 1.9 eV and smaller than 3.1 eV, so as to fall within the visible range of the solar spectrum. (ii) High chemical stability in the dark and under illumination. (iii) Band edge positions that straddle the water reduction and oxidation potentials.

We are studying the opto-electronic properties of metal oxide and nitride semiconductors that are promising, stable materials for water oxidation, in particular WO3 and solid solutions of copper tungstanates and molibdates, BiVO4, and Ta3N5.

Functionalized Si surfaces

Semiconductor/liquid interfaces are promising platforms for solar fuels production, and hence for solar energy storage. The efficiency of such systems depends critically on the alignment of the semiconductor band edges with the Nernst potentials for fuel production, e.g., with the reduction and oxidation half-reactions involved with water-splitting and/or CO2 reduction. We have carried out first principles calculations of functionalized Si surfaces to understand and interpret spectroscopic measurements. We have also investigated the effect of water on these surfaces and how band edges are shifted by solvation effects.

Interfaces between catalysts and photoelectrodes

The design of optimal interfaces between photoelectrodes and catalysts is a key challenge in building photoelectrochemical cells to split water. Using first-principles mechanical calculations, we investigated the structural and electronic properties of tungsten trioxide (WO3) surfaces interfaced with an IrO2 thin film. We found that, upon full coverage of WO3 by IrO2, the two oxides form undesirable Ohmic contacts. However, our calculations predicted that if both oxides are partially exposed to water solvent, the relative position of the absorber conduction band and the catalyst Fermi level favors charge transfer to the catalyst and hence water splitting. We propose that, for oxide photoelectrodes interfaced with IrO2, it is advantageous to form rough interfaces with the catalyst, e.g., by depositing nanoparticles, instead of sharp interfaces with thin films. Our study highlights the importance of catalysts and interface morphology in designing optimal photoelectrochemical cells.