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

Abstract

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.