X-ray Photoelectron Spectroscopy Studies of Solute Depth Distributions at the Liquid/Vapor Interface of Aqueous Solutions and Surface Structure of Pyrite Thin Films

Ion behavior at liquid/vapor interfaces of aqueous NaCl and RbCl solutions was investigated by using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We have shown both solutions exhibited small chloride surface enhancement but no cation size effect on the chloride surface enhancement was observed. We found that Cl/O depth profile was a more informative measure of chloride surface enhancement. Integrated density profiles of the anion/cation ratios and Cl/O ratios calculated from MD simulations were consistent with AP-XPS results. We concluded that different cation behavior in the two solutions could mask differences in the chloride behavior from the anion/cation ratios.;AP-XPS was also used to explore the ion behavior at the liquid/vapor interface of an aqueous KI solution in the presence of organic isopropanol or acetone. We observed that anion/cation atomic composition ratios at the interface were substantially suppressed for both organic surfactant conditions. The extent of the suppressed anion/cation atomic composition ratios at the interface was similar for both conditions. Furthermore, we observed that the amount of interfacial isopropanol is larger than acetone, which might be attributed to the interaction between interfacial hydroxyl functional groups in isopropanal and interfacial iodide ions in the solution. Surface structure of substrate-dependent pyrite thin films was investigated by XPS. We have shown that Na diffusion from glass substrates plays significant role in growing high-quality pyrite thin films. Four different types of sulfur components on the surface of substrate-dependent pyrite thin films were resolved. Except for three common sulfur components which were observed in pyrite crystals, the additional sulfur component changed with respect to substrates and process procedures. For an annealed pyrite thin film on Si, the additional sulfur component was found to be associated with the mixture of residual elemental sulfur and polysulfide. For a deposited pyrite thin film on glass, the additional sulfur component was associated with residual sulfur precursor. However, the additional sulfur components for an annealed pyrite thin film on glass were associated with not only the mixture of residual elemental sulfur and polysulfide but also sodium sulfide due to the reaction of elemental sulfur vapor and surface sodium on glass.