Photoelectrochemical energy conversion involving transition metal d-states and intercalation of layer compounds

Transient Surface Photovoltage Spectroscopy of (NH4)2Mo3S13/WSe2 Thin-Film Photocathodes for Photoelectrochemical Hydrogen Evolution

Hydrogen produced from solar energy has the potential to replace petroleum in the future. To this respect, there is a need in the abandoned and efficient materials that can continuously split water molecules using solar energy. In this report, an ammonium thiomolybdate (ATM: (NH₄)₂Mo₃S₁₃) is evaluated as a p-type semiconductor film photocathode for hydrogen evolution reaction. The ATM thin films are prepared by spin-coating on fluorine-doped tin oxide substrates, and their structural, morphological, optical, photoelectrical, and photoelectrochemical (PEC) properties are studied. Transient surface photovoltage (TSPV) spectroscopy and spectroscopic ellipsometry indicate the band gap E_g = 1.9 eV for the ATM thin films. Furthermore, the photovoltage of the ATM thin films measured by TSPV is correlated to the photocurrents measured by the PEC characterization that can be used to evaluate the material potential for hydrogen generation. The films exhibit a low photocurrent density of 46 μA cm^-2 at 0 V_RHE. However, its combination with WSe₂ thin-film photocathodes results in a significant increase in photocurrent density up to 4.6 mA cm^-2 at 0 V_RHE (100 times). The reason for such a strong charge carrier transfer effect for ATM/WSe₂ heterojunction photocathodes is studied by TSPV spectroscopy that allows a comprehensive evaluation of potential photovoltaic materials toward PEC hydrogen production. Furthermore, the photovoltage generated by a WSe₂ thin film is 30 times lower than that of its single crystal, which indicates that the quality of WSe₂ thin films should be improved for faster PEC hydrogen evolution.

Iron sulfide (FeS) nanotubes using sulfurization of hematite nanowires

We report the phase transformation of hematite (α-Fe2O3) single crystal nanowires to crystalline FeS nanotubes using sulfurization with H2S gas at relatively low temperatures. Characterization indicates that phase pure hexagonal FeS nanotubes were formed. Time-series sulfurization experiments suggest epitaxial growth of FeS as a shell layer on hematite. This is the first report of hollow, crystalline FeS nanotubes with NiAs structure and also on the Kirkendall effect in solid-gas reactions with nanowires involving sulfurization.

Modeling the environmental stability of FeS2 nanorods, using lessons from biomineralization

Previous experimental studies have indicated that the controlled formation of anisotropic pyrite nanoparticles, such as nanorods or nanowires, is dependent on the right combination of solution chemistry and temperature. Similarly, the morphology of the individual nanocrystals during intracellular biomineralization of single nanocrystals has been attributed to the local environmental conditions, as well as the species of the micro-organism. Although there are obvious similarities, using the lessons from biomineralization to assist the laboratory synthesis of anisotropic pyrite nanostructures, and in the anticipation of environmental stability, requires a more detailed understanding of the role played by individual environmental parameters. In the present study we use a multi-scale thermodynamic model, combined with parameters obtained from first principles calculations, to investigate the formation and stability of pyrite nanorods as a function of temperature and chemical environment. The results of our systematic modeling of parameter space predict that the morphology of pyrite nanorods grown in the laboratory, or associated with biomineralization, is more likely to be a function of surface ligands and the biology of the organisms than a function of simpler environmental parameters such as temperature, pressure, concentration of sulfur and adsorption of water.