Characterization of n-Type Semiconducting Tungsten Disulfide Photoanodes in Aqueous and Nonaqueous Electrolyte Solutions: Photo-oxidation of Halides with High Efficiency

Synthetic, single crystal, n-type semiconducting WS2 (bandgap ≈ 1.3 eV) has been characterized as a photoanode in aqueous and nonaqueous electrolyte media. The WS2 was synthesized from the elements by bromine and chlorine transport to yield plates up to 3 × 3 mm in dimension. Interface characterization includes (i) cyclic voltammetry in the presence of a large number of fast, one-electron redox couples in CH3CN/0.1M [n-Bu4N]C1O4 solutions; (ii) steady-state photocurrent-voltage properties in aqueous and nonaqueous solutions of X− (X− = CI−, Br−, I−); (iii) tests of durability; (iv) wavelength dependence of photocurrent and photovoltage; and (v) high resolution (~5 μm) laser mapping of the surface to reveal surface inhomogeneity with respect to output photovoltage. Highlights of the results are: (i) n-type WS2 is durable in aqueous electrolytes containing high concentrations of X− to yield efficient visible light-assisted oxidation of X−; e.g., CI− → 1/2 Cl2 has up to 6.9% and Br− → 1/2 Br2 up to 12% efficiency at a 632.8 nm input power of 16 mW/cm2; (ii) in aqueous, but not nonaqueous, solutions I− adsorbs such that the onset of photocurrent is shifted several hundred millivolts as for other metal dichalcogenide photoanodes; the shift is sufficient that visible light can be used to sustain the conversion of 2HI to H2 and I2 with no other energy input; (iii) cyclic voltammetry in CH3CN/0.1M [n-Bu4N]C104 for a number of redox couples shows that a photovoltage of up to ~0.7V is possible; photovoltage varies from 0.0 to ~0.7V for redox couples having E½ from ~0.0Vvs. SCE to ~+0.8V while the photovoltage is fixed at ~0.7V for E½’s more positive than ~ +0.8V vs. SCE; (iv) efficiency for halogen generation in aqueous solutions generally exceeds efficiency in CH3CN solutions; and (v) the diffusion length of holes parallel to the surface is ~200 μm which explains the dramatic influence of the steps on the recombination of carriers on layered compounds.