Halogen Adsorption on n‐MoSe2 (0001) van der Waals Faces: Simulation of Electrochemical Junctions in UHV

Adsorption of halogens (Cl2, Br2, I2) on semiconducting n‐MoSe2 surfaces and changes due to the adsorption are investigated and related to photoelectrochemical results. Clean (0001) faces are prepared by cleaving the crystals in UHV and are characterized by UPS, XPS and LEED. At low doses the photoelectron spectra indicate adsorption of halogens via charge transfer. Changes of the energy correlation at the interface due to the adsorbates are observed in the He I photoelectron energy distribution curves. For this n‐type semiconductor a depletion layer (band bending) as well as increased electron affinities are obtained. The induced band bending can partly be reversed by white light illumination creating a surface photovoltage, which was found to be dependent on the surface ideality. The values determined for the work function, electron affinity and band bending (approximately equivalent to the observed photopotential) are very similar to values obtained in electrochemical junctions of nonreacting organic electrolytes. Evidently the electronic conditions of the semiconductor/adsorbate junction in UHV is comparable to the semiconductor/electrolyte junction containing the respective X2/X− (X = Cl, Br, I) redox couple. The results indicate the possibility to model semiconductor/electrolyte junctions in UHV in such adsorption experiments which allow the spectroscopic identification of surface species and in addition the investigation of their electronic influence.