Angle-resolved X-ray photoelectron spectroscopy of in situ deposited Li onMoS2(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(0 002) with Li was investigated using. angle-resolved X-ray photoelectron spe ctroscopy (ARXPS). The high-resolution XPS in the core level regions reveal ed that the deposited Li reduced MoS2(0002) into Li2S/Mo(II)S via the react ion 2Li + MoS2 --> Li2S + MoS, evidenced by the binding energies and the st oichiometry 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 reduc ed MoS2 surface formed a highly disordered surface region, which extended a pproximately 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 char acterized 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 molecul ar oxygen, and the formation of surface peroxide is reported herein.