Electrochemical Characterization of p-Type Semiconducting Tungsten Disulfide Photocathodes: Efficient Photoreduction Processes at Semiconductor/Liquid Electrolyte Interfaces

Single-crystal p-type WS₂ (£g = 1.3 eV) has been synthesized and characterized as an electrode material in CH₃CN/0.1 M [n-Bu₄N]C₁₀₄ and in aqueous electrolytes containing a variety of one-electron redox reagents having different E1/2 values. In either CH₃CN or H₂0 solvent the flat-band potential, EFB, is measured to be –1-0.95 V vs. SCE. In aqueous l3/l- the £FB is shifted more negative by at least 0.3 V as is found for n-type WS2 photoanodes. Capacitance measurements of the WS2/electrolyte interface to determine eFB accord well with electrochemical measurements. For E1/2 more negative than £FB the p-type WS2 behaves as a photocathode giving an open-circuit photovoltage, Ev(oc), up to ~0.8 V depending on E1/2. For E1/2 between +1.3 and -0.1 V vs. SCE, Ev(oc) varies as expected: for E1/2 more positive than £ra the p-type WS2 behaves as a metallic electrode while for E1/2 more negative than £FB we find Ev(oc) = |E1/2 - EFB|. It appears that for negative redox couples carrier inversión results at the p-WS2 surface, but for E1/2 more negative than -0.1 V vs. SCE Ev(oc) declines, a result associated with junction breakdown at sufficiently negative Potentials. p-Type WS₂-based photoelectrochemical cells can be used to effect the sustained conversion of visible light (632.8 nm) to electricity in H₂0 or CH₃CN with efficiencies of up to ~7%. In H₂0 the photochemical reduction to H₂ can be effected by illumination of p-type WS₂ modified by depositing ~ 10~7 mol/cm² of Pd(0) or Pt(0) onto the surface as an H₂ evolution catalyst. Efficiency for H₂ evolution from 6 M H₂SO₄ is typically 6-7% for 632.8 nm (50 mW/cm²) intensity.