Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting.

n-Type bismuth vanadate has been identified as one of the most promising photoanodes for use in a water-splitting photoelectrochemical cell. The major limitation of BiVO4 is its relatively wide bandgap (∼2.5 eV), which fundamentally limits its solar-to-hydrogen conversion efficiency. Here we show that annealing nanoporous bismuth vanadate electrodes at 350 °C under nitrogen flow can result in nitrogen doping and generation of oxygen vacancies. This gentle nitrogen treatment not only effectively reduces the bandgap by ∼0.2 eV but also increases the majority carrier density and mobility, enhancing electron-hole separation. The effect of nitrogen incorporation and oxygen vacancies on the electronic band structure and charge transport of bismuth vanadate are systematically elucidated by ab initio calculations. Owing to simultaneous enhancements in photon absorption and charge transport, the applied bias photon-to-current efficiency of nitrogen-treated BiVO4 for solar water splitting exceeds 2%, a record for a single oxide photon absorber, to the best of our knowledge.

Energetics and Solvation Effects at the Photoanode/Catalyst Interface: Ohmic Contact versus Schottky Barrier.

The design of optimal interfaces between photoelectrodes and catalysts is a key challenge in building photoelectrochemical cells to split water. Iridium dioxide (IrO2) is an efficient catalyst for oxygen evolution, stable in acidic conditions, and hence a good candidate to be interfaced with photoanodes. Using first-principles quantum mechanical calculations, we investigated the structural and electronic properties of tungsten trioxide (WO3) surfaces interfaced with an IrO2 thin film. We built a microscopic model of the interface that exhibits a formation energy lower than the surface energy of the most stable IrO2 surface, in spite of a large lattice mismatch, and has no impurity states pinning the Fermi level. 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.

Raman Spectra of Liquid Water from Ab Initio Molecular Dynamics: Vibrational Signatures of Charge Fluctuations in the Hydrogen Bond Network.

We report the first ab initio simulations of the Raman spectra of liquid water, obtained by combining first principles molecular dynamics and density functional perturbation theory. Our computed spectra are in good agreement with experiments, especially in the low frequency region. We also describe a systematic strategy to analyze the Raman intensities, which is of general applicability to molecular solids and liquids, and it is based on maximally localized Wannier functions and effective molecular polarizabilities. Our analysis revealed the presence of intermolecular charge fluctuations accompanying the hydrogen bond (HB) stretching modes at 270 cm(-1), in spite of the absence of any Raman activity in the isotropic spectrum. We also found that charge fluctuations partly contribute to the 200 cm(-1) peak in the anisotropic spectrum, thus providing insight into the controversial origin of such peak. Our results highlighted the importance of taking into account electronic effects in interpreting the Raman spectra of liquid water and the key role of charge fluctuations within the HB network; they also pointed at the inaccuracies of models using constant molecular polarizabilities to describe the Raman response of liquid water.

Thermally stable N2-intercalated WO3 photoanodes for water oxidation.

We describe stable intercalation compounds of the composition xN(2)·WO(3) (x = 0.034-0.039), formed by trapping N(2) in WO(3). The incorporation of N(2) significantly reduced the absorption threshold of WO(3); notably, 0.039N(2)·WO(3) anodes exhibited photocurrent under illumination at wavelengths ≤640 nm with a faradaic efficiency for O(2) evolution in 1.0 M HClO(4)(aq) of nearly unity. Spectroscopic and computational results indicated that deformation of the WO(3) host lattice, as well as weak electronic interactions between trapped N(2) and the WO(3) matrix, contributed to the observed red shift in optical absorption. Noble-gas-intercalated WO(3) materials similar to xN(2)·WO(3) are predicted to function as photoanodes that are responsive to visible light.

Nanoporous Si as an efficient thermoelectric material.

Room-temperature thermoelectric properties of n-type crystalline Si with periodically arranged nanometer-sized pores are computed using a combination of classical molecular dynamics for lattice thermal conductivity and ab initio density functional theory for electrical conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity. The electrical conductivity is found to decrease by a factor of 2-4, depending on doping levels, compared to that of bulk due to confinement. The Seebeck coefficient S yields a 2-fold increase for carrier concentrations less than 2 x 10(19) cm(-3), above which S remains closer to the bulk value. Combining these results with our calculations of lattice thermal conductivity, we predict the figure of merit ZT to increase by 2 orders of magnitude over that of bulk. This enhancement is due to the combination of the nanometer size of pores which greatly reduces the thermal conductivity and the ordered arrangement of pores which allows for only a moderate reduction in the power factor. We find that while alignment of pores is necessary to preserve power factor values comparable to those of bulk Si, a symmetric arrangement is not required. These findings indicate that nanoporous semiconductors with aligned pores may be highly attractive materials for thermoelectric applications.