Flat-Band Potential of n-Type Semiconducting Molybdenum Disulfide by Cyclic Voltammetry of Two-Electron Reductants: Interface Energetics and the Sustained Photooxidation of Chloride

Cyclic voltammetry has been used to locate the band edges of n-type MoS₂ in CH₃CN/ and EtOH/[n-Bu₄N]ClO₄ solutions. The crucial experiments concern the study of the cyclic voltammetry of biferrocene (BF) and N,N,N',N'-tetra-methyl-p-phenylenediamine (TMPD) each of which has two, reversible, one-electron waves at Pt. At MoS₂, the first oxidation is reversible in the dark, whereas the second oxidation is observed only upon illumination of the MoS₂. The dark oxidation BF → BF+ and the photoanodic BF+ → BF²⁺ are separated by only ~150 mV, allowing us to assign an uncommonly accurate flat-band potential of +0.30 + 0.05 V vs. SCE to MoS₂. This flat-band potential reveals that the valence band edge is at ca. +1.9 V vs. SCE showing that photooxidations workable at TiO₂ are thermodynamically possible at illuminated MoS₂ as well. As an example of the ruggedness of MoS₂, we demonstrate the ability to effect the sustained oxidation of Cl^- at illuminated n-type MoS₂. Conclusions from BF are fully supported by those from TMPD and one-electron systems ferrocene, acetylferrocene, 1,1'-diacetylferrocene, and [Ru(2,2'-bipyridine)₃]²⁺. Oxidation of [Ru(2,2'-bipyridine)₃]²⁺ can be effected >0.5 V more negative than at Pt by illumination of MoS₂.