Angle-resolved X-ray photoelectron spectroscopy of in situ deposited Li on MoS2(0002)

A contrasting behavior in alkali/MoS2 composites is demonstrated between a reductive reconstruction in the Li/MoS2 system and previously reported pure electron transfer in Cs/MoS2. The interaction of in situ cleaved 2H-MoS2(0002) with Li was investigated using. angle-resolved X-ray photoelectron spectroscopy (ARXPS). The high-resolution XPS in the core level regions revealed that the deposited Li reduced MoS2(0002) into Li2S/Mo(II)S via the reaction 2Li + MoS2 --> Li2S + MoS, evidenced by the binding energies and the stoichiometry of Mo, S, and Li. The ARXPS data of the Li 1s, Mo 3d, and S 2p core levels further indicated that the deposited Li together with the reduced MoS2 surface formed a highly disordered surface region, which extended approximately 12 Angstrom deep into the MoS2 bulk. The valence band XPS also revealed a clearly different interaction of Li with MoS2 from that of Cs. The supravalence electron density of states in Li/MoS2 appeared at ca. 0.3 eV above the MoS2 valence band maximum (VBM), whereas that from Cs/MoS2 was previously observed at 1.25 eV above the VBM, very close to the conduction band minimum of MoS2. Thus, the interaction of Li with MoS2 cannot be characterized by a rigid band model unlike that which made a good approximation for the Cs/MoS2 system. The Li/MoS2 specimens were also exposed to molecular oxygen, and the formation of surface peroxide is reported herein.