XPS investigation of chemical states in monolayers - Recent progress in adsorbate redox chemistry on sulphides

The paper presents new results on the adsorption of 2-mercaptobenzothiazole (MBT) and 2-mercaptobenzoxazole (MBO) on galena (PbS) and pyrite (FeS2). A dsorption of MBT and MBO on galena and pyrite surfaces from 10(-5) M aqueou s solution results in monolayers of chemisorbed deprotonated molecules boun d to the surface via the thiolate group. A secondary interaction between th e 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 orientatio n of the molecule can be assumed. Adsorption of MBT from 10(-4) M aqueous s olution leads to the formation of 2,2'-dithiobis(benzothiazole) as an oxida tion product of MET. Oxidation of MBO is not observed which may be the resu lt of its 100 mV higher redox potential. Optical activation of the MBT oxidation on CdS and FeS2 in contrast to PbS is suggested from the increase of the S2p signals attributed to the bridgin g -S-S- group of BBTD during illumination. We conclude that a surface photo voltage reducing the band bending may be responsible for the higher oxidati on rate at the pyrite/electrolyte interface during illumination with respec t to the dark. Oxidation of sulphide surfaces by UV Light in air produces lead(II) sulphat e soluble in aqueous solution. Dissolved Pb2+ ions lead to the formation an d precipitation of a Pb(mbt)(2) complex in alkaline MBT solution. (C) 1999 Elsevier Science B.V. All rights reserved.