XPS spectra and electronic structure of Group IA sulfates

The results of systematic XPS measurements of Group IA (H, Li, Na, K, Rb and Cs) sulfates together with NaHSO4, KHSO4 and (NH4)2SO4 are presented. The experiments on the alkali metal salts were preformed on ground powders at both liquid nitrogen and room temperatures; concentrated sulfuric acid was measured as a fast-frozen liquid drop. Spectra from grounded and floated samples were compared, and no significant difference relating to charging effects was observed. The influence of grinding on surface chemistry of the powders is described. Such a mechanical activation produces mainly monohydrates on the surface of all sulfates. In the case of Na2SO4, an additional NaHSO4 surface phase seems to form that is not stable in vacuum even at liquid nitrogen temperatures. It was found that the binding energies (O 1s and S 2p) of sulfate ion decrease down the group. The shifts are discussed and related to ionicity of the metal–sulfate bond. The structure of XPS valence band spectra is in good agreement with cluster calculations of SO42− and HSO4− [A.A. Audi, P.M.A. Sherwood, Surf. Interface Anal. 29 (2000) 265]. While the energies of bisulfate bands are not influenced by the cation (H+, Na+ or K+), the sulfate ones experience an increase in bond ionicity and demonstrate the same binding energy shifts as the core levels.