Characterization of MoSe2(0001) and ion-sputtered MoSe2 by XPS

Molybdenum dichalcogenides (MoS2, MoSe2, MoTe2) have attracted significant attention because of their primary importance as catalysts [1–3] and photovoltaic materials [4–6]. These materials have a hexagonal closed packed (hcp) structure that results from the stacking of weakly bonded X-Mo-X layers (X = S, Se, Te) and gives rise to highly anisotropic properties [7]. Of these, molybdenum disulfide (MoS2) has been widely studied primarily due to its relevance as a hydrotreating catalyst to remove sulfurand nitrogencontaining compounds from various oil fractions. The large basal plane (0001) of MoS2 is relatively catalytically inactive, while the edge planes (S-edge,1010; Moedge, 1010) are generally accepted to be the active sites due to the presence of coordinatively unsaturated (cus) Mo (or promoter) sites. The MoS2(0001) surface has been shown to be catalytically inactive toward thiophene [8]; however, additional studies with ethanethiol have indicated slight reactivity suggesting the presence of cus sites on the freshly cleaved basal surface. Wiegenstein and Schulz [9] demonstrated the density of cus Mo sites can be increased using ion bombardment, thereby increasing the reactivity of methanethiol. Their XPS measurements of MoS2(0001) indicated that the Mo 3d peaks of a (0001) surface are consistent with Mo(IV), and no significant change in binding energy or intensity was observed after annealing to 573 K. However, following ion bombardment the Mo 3d peaks shifted to a higher binding energy consistent with a concentration of additional Mo oxidation states on the defective basal surface. Consequently, the MoS2(0001) surface can be made reactive using controlled ion bombardment to increase the density of cus sites. In addition to the widespread application in the photovoltaic industry, MoSe2 may also have the potential to be used as a nanostructured hydrotreating catalyst. For example, the mechanism of C N bond cleavage in the hydrodenitrogenation (HDN) of aliphatic nitrogen-containing molecules has been suggested to be Hofmann-type elimination or nucleophilic substitution [10–14]. Both of these mechanisms require a pair of sites: an acid site to react with the nitrogenmolecule through the nitrogen atom, making the amine group ready to leave from the molecule; and a basic site to abstract the β-hydrogen in the elimination mechanism, or a nucleophile to attack the α-carbon in the nucleophilic substitution mechanism. It has been previously suggested that on a promoted (Ni) edge of MoS2 (1010), the uncovered promoter atoms can ac-