Improvement of the photoelectrochemical oxidation of halides by platinization of metal dichalcogenide photoanodes

Electrochemical deposition of Pt onto the surface of n-MoS₂ or n-WS₂ electrodes results in significant improvement in the efficiency of the photoelectrochemical oxidation of Cl^- or Br^- in aqueous or nonaqueous media. The optimum amount of Pt is in the 10^-8-10^-7 mol/cm² range where electron microscopy and Auger and X-ray photoelectron spectroscopy show that the Pt incompletely covers the MS₂ surfaces. Phenomenologically, the deposited Pt behaves as a catalyst for the two-electron oxidation of X^- and does not affect the interfacial energetics of the n-MS₂/liquid electrolyte. Improvement in efficiency for X^- oxidation results from enhanced photovoltages and fill factors. Improvement in the oxidation of Cl^- in 15 M LiCl is most notable; for one n-MoS₂ electrode the platinization improved the energy conversion efficiency from 0.7% to 9.8% for 632.8 nm (36 mW/cm²) and for one n-WS₂ electrode the efficiency was improved from 7.4% to 13.4% for 632.8 nm (15 mW/cm²). The improved efficiency can be maintained at ∼10 mA/cm² for periods of ∼1 h, but the Pt catalyst is slowly oxidatively dissolved and the output eventually declines to that associated with naked n-MS₂ photoanodes.