Thiolate layers on metal sulfides characterised by XPS, ToF-SIMS and NEXAFS spectroscopy
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Surface spectroscopic characterisation of some Cu and Ag thiolate multilayers on metal and metal sulfide substrates was undertaken to establish unequivocally the composition and possible orientation of the multilayer species. This information was sought to attempt to explain the undiminished floatability of sulfide minerals observed for collector coverage exceeding a monolayer. The thiol collectors investigated were dithiophosphate and 2-mercaptobenzothiazole (MBT), and bulk CuMBT and AgMBT complexes were prepared for comparison with the corresponding multilayers. Surface optimised synchrotron X-ray photoelectron spectra and partial electron yield near-edge X-ray absorption fine structure (NEXAFS) spectra confirmed that the Cu dithiolate, detected by secondary ion mass spectroscopy (SIMS), was no more than a minor constituent of the corresponding multilayer. The photoelectron spectra for multilayer CuMBT and AgMBT were similar to those for the corresponding bulk complexes. NEXAFS spectroscopy detected some CuII in bulk CuMBT prepared from cupric ions but not cuprous. The SIMS data were consistent with multilayer patches or islands on top of a chemisorbed monolayer and hence continued exposure of the monolayer in the presence of the multilayer. For each multilayer investigated, the SIMS data provided no evidence to support a multinuclear cluster structure as is present in the corresponding bulk thiolate, but nor could they exclude such a possibility. Angle-dependent NEXAFS spectroscopy at the N K-edge confirmed that MBT monolayers were aligned and revealed that the metal thiolate multilayer was not aligned relative to the substrate, but might nevertheless have been ordered in a cluster structure. It was surmised that undiminished floatability of sulfide minerals with multilayer collector coverage could probably be attributed to the patch-wise nature of the multilayer.