XPS investigation of chemical states in monolayers

Abstract

Abstract The paper presents new results on the adsorption of 2-mercaptobenzothiazole (MBT) and 2-mercaptobenzoxazole (MBO) on galena (PbS) and pyrite (FeS 2 ). Adsorption of MBT and MBO on galena and pyrite surfaces from 10 −5 M aqueous solution results in monolayers of chemisorbed deprotonated molecules bound to the surface via the thiolate group. A secondary interaction between the lone pair of the nitrogen and the sulphide surface may be possible. From the unequal intensity of the two S2p emissions in MBT an upright orientation of the molecule can be assumed. Adsorption of MBT from 10 −4 M aqueous solution leads to the formation of 2,2′-dithiobis(benzothiazole) as an oxidation product of MBT. Oxidation of MBO is not observed which may be the result of its 100 mV higher redox potential. Optical activation of the MBT oxidation on CdS and FeS 2 in contrast to PbS is suggested from the increase of the S2p signals attributed to the bridging –S–S– group of BBTD during illumination. We conclude that a surface photovoltage reducing the band bending may be responsible for the higher oxidation rate at the pyrite/electrolyte interface during illumination with respect to the dark. Oxidation of sulphide surfaces by UV light in air produces lead(II) sulphate soluble in aqueous solution. Dissolved Pb 2+ ions lead to the formation and precipitation of a Pb(mbt) 2 complex in alkaline MBT solution.