Beyond Mono-, Di-, and Trisulfides: Synthesizing vanadium tetrasulfide (VS4) films for energy conversion

Vanadium tetrasulfide (VS4, called patronite as a mineral) is a one-dimensional compound with promising properties for energy conversion applications. However, it has been scarcely investigated because of its complex synthesis. In this work, we report a detailed investigation about the formation mechanism of VS4 (V4+(S22–)2) as well as its structural, transport, and photoelectrochemical properties. To this aim, VS4 films were grown by a solid–gas reaction process between vanadium films and sulfur at temperatures between 350 and 450 °C during different reaction times. Film characterization (X-ray diffraction, energy-dispersive analysis of X-ray, micro-Raman spectroscopy, and scanning electron microscopy) reveals the formation of monoclinic VS4 nanorods (I2/C) as single crystalline phase in very short reaction times (t < 5 h). Optical characterization was carried out by reflectance and transmittance measurements to obtain the optical absorption coefficient (α = 104 cm–1 at photon energies higher than 1.6 eV). From these measurements, a direct optical band gap of 1.35 ± 0.05 eV is obtained. Additionally, VS4 films were used as photoanodes of a photoelectrochemical cell (PEC) with a platinum foil as counter electrode and a Ag/AgCl reference electrode to characterize the VS4/electrolyte (aqueous 0.5 M Na2SO3) interface. Finally, the evolved hydrogen under 200 mW/cm2 white light illumination over the VS4/interface at 0.3 V (Ag/AgCl) bias potential was quantified by a quadrupole mass spectrometer (QMS) reaching fluxes of ∼20 μmol/h.The authors are thankful for the technical support from Mr. F. Moreno. E.F. acknowledges the Mexican National Council for Science and Technology (CONACyT) for providing the funding necessary to complete his Ph.D. Authors from the MIRE Group acknowledge financial support by MINECO-FEDER (MAT2015-65203R). M.M.-G. and O.C.-C. acknowledge financial support by INFANTE (PIE 201550E072) and MINECO-FEDER (MAT2017-86450-C4-3-R) projects.Peer reviewe