Transient Surface Photovoltage Spectroscopy of (NH 4 ) 2 Mo 3 S 13 /WSe 2 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: (NH 4 ) 2 Mo 3 S 13 ) 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 WSe 2 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/WSe 2 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 WSe 2 thin film is 30 times lower than that of its single crystal, which indicates that the quality of WSe 2 thin films should be improved for faster PEC hydrogen evolution.