Ternary transition titanium-niobium trisulfide as photoanode for assisted water splitting

Abstract

Ternary titanium-niobium trisulfide, [TixNb1-xS3 (Nb rich) and NbxTi1-xS3 (Ti rich), x<(1-x)], is explored as photoanode in photoelectrochemical cells (PEC) and compared with their respectives binary sulfides, TiS3 and NbS3. Investigated samples were successfully synthesized by solid-gas reaction of metal and sulfur vapor in a vacuum-sealed ampoule at 550 °C during 20h. They were characterized by X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDX), Raman spectroscopy (RS) and scanning electron microscopy (SEM). Whereas samples of binary trisulfides were univocally identified as polycrystalline nanoribbons of monoclinic TiS3 (P21/m) and triclinic NbS3 (P1¯2), respectively, the structure of the ternary trisulfide depends on the dominant presence of one of the metals: Ti (monoclinic (P21/m)) or Nb (triclinic (P1¯)). Optical absorption coefficients were determined from reflectance and transmittance measurements. The TixNb1-xS3 and NbxTi1-xS3 direct optical bandgap energies obtained are 1.0±0.1 eV. Photoelectrochemical measurements in 0.5 M Na2SO3 with a platinum counter electrode have been carried out to characterize the semiconductor/electrolyte interface. The flat band potentials (TiS3-Vfb = -0.64 ± 0.02V NHE, NbS3-Vfb = -0.73 ± 0.02V NHE, NbxTi1-xS3-Vfb = -0.63 ± 0.05V NHE and TixNb1-xS3V-fb = -0.72 ± 0.05V NHE) have been measured by Electrochemical Impedance Spectroscopy (EIS) and they have been used, beside the bandgap energy, to depict the energy band levels scheme. Hydrogen fluxes (photogenerated under 200 mW/cm2 white light illumination in a PEC at 0.3 V bias potential) have been quantified as a function of illumination time by quadrupole mass spectrometry (QMS) reaching values up to 2.2±0.1 μmolH2/min⋅cm2 in the best scenario.