No abstract

Galena oxidation was investigated by AFM in acetate buffer under potentiostatic control and by photoelectron spectroscopy on potentiostatically pretreated specimens. At +236 mV (SHE) formation of sulfur protrusions could be observed with AFM. XPS showed the formation of elemental sulfur to start at potentials more anodic than +161 mV (SHE). Elemental sulfur could only be retained on the galena surface if sample cooling was started before the beginning of the evacuation in the spectrometer entry chamber. Sulfur-oxygen species could not be detected on galena samples oxidized in acetate buffer even when investigated with synchrotron-excited X-ray photoelectron spectroscopy. AFM images showed two important features: Oxidation starts with a roughening of the sample surface. At slightly more anodic potentials oxidation products are present on the samples as protrusions of 10-200 nm in height and with mutual distances of several hundred nanometers. Two types of sulfur deposits are formed differing in the emergence potential, size, and mutual distance. The formation of such protrusions can only be understood if the reactants for the depositions reach the growing protrusion by diffusion in the liquid phase. Therefore, it is proposed that the process causing the surface roughening is a dissolution of PbS to lead(II) ions and hydrosulfide ions while the deposition reaction is the electrochemical oxidation of hydrosulfide ions to elemental sulfur. By removal of the hydrosulfide ion from the aqueous solution, further dissolution becomes possible at other sample regions. The sulfur formation occurs at distinct points which are not preferentially located at steps. It is likely that the sulfur formation starts at impurity locations. Different impurities may be responsible for different rates of deposit formation, leading to protrusions of different size which however cannot be distinguished by XPS.

No abstract

The CPP (combined polymerization and pyrolysis) preparation route, in its enhanced liquid-precursor-based version, was combined with consecutive soft milling. For studies of temperature- and size-dependent structural changes occurring in ferroelectric lead titanate, this combined route yields a nanopowder series covering the relevant particle-size region at target quality. This material basis enables consistent SEM, TEM, XRD, Raman, EPR and dielectric measurements, which furnish a comprehensive picture of the cooperation between temperature rise and size reduction to eliminate tetragonality and concomitant ferroelectricity. Our previous original EPR studies on nanosized barium titanate are now extended to the lead titanate case. Furthermore, as compared to the pertinent literature standard, the materials basis is extended to powder samples of smaller mean particle sizes, comprising the critical size at which a PbTiO3 particle undergoes a transition into cubic paraelectric phase. Thus, the size-driven phase transition can be observed in a direct way (at 7 nm, which compares to 40 nm for BaTiO3), and the EPR data suggest a much less spacious gradient shell at the particle surfaces (thickness ≈ 2 nm) than in previous analogous investigations on BaTiO3 (15 nm).

The nitridation of GaAs, InAs, and InSb by low-energy N2+ ion bombardment at room temperature was studied by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy measurements. The formation of thin surface nitride layers, consisting mostly of GaN or InN but also containing minor amounts of mixed nitrides, was observed. Besides the nitride-related features, sharp peaks in the NEXAFS due to π* resonance at 401.0 eV and correlated peaks at 403.8 eV in N 1s core level spectra were detected. Both spectral features could be assigned to the presence of interstitial nitrogen, most likely molecular nitrogen. It was found that the amount of interstitial nitrogen in the surface layer strongly depends on the AIII–BV semiconductor system and may be affected by modification of the conditions during low energy ion bombardment.