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

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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.



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