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The electrocatalytic performance of transition metal sulfide (TMS)− graphene composites has been simply regarded as the results of high conductivity and the large surface/volume ratio. However, unavoidable factors such as degree of oxidation of TMSs have been hardly considered for the origin of this catalytic activity of TMS−graphene composites. To accomplish the reliable application of TMS-based electrocatalytic materials, a clear understanding of the thermodynamic stability of TMS and effects of oxidation on catalytic activity is necessary. In addition, the mechanism of charge transfer at the TMS−graphene interface must be studied in depth to properly design composite materials. Herein, we report a comprehensive study of the physical chemistry at the junction of a Co 1−x Ni x S 2 −graphene composite, which is a prototype designed to unravel the mechanisms of charge transfer between TMS and graphene. Specifically, the thermodynamic stability and the effects of oxidation of TMSs during the oxygen evolution reaction (OER) on the reaction mechanism are systematically investigated using density functional theory (DFT) calculations and experimental observations. Cobalt atoms anchored on pyridinic N sites in the graphene support form metal−semiconductor (SC) junctions, and the internal band bending at these junctions facilitates electron transfer from TMSs to graphene. The junction enables fast sinking of the excess electron from OH − adsorbate. Partially oxidized amorphous TMS layers formed during the OER can facilitate adsorption and desorption of OH and H atoms, boosting the OER performance of TMS−graphene nanocomposites. From the DFT calculations, the enhanced electrocatalytic activity of TMS−graphene nanocomposites originates from two important factors: (i) increased internal band bending and (ii) parallelized OER pathways at the interface of pristine and oxidized TMSs.

Single-particle characterization of summertime Arctic aerosols is useful to understand the impact of air pollutants on the polar atmosphere. In the present study, a quantitative single particle analytical technique, low-Z particle electron probe X-ray microanalysis, was used to characterize 8100 individual particles overall in 16 sets of aerosol samples collected at Ny-Alesund, Svalbard, Norway on 25-31 July, 2007. Based on their X-ray spectral and secondary electron image data of individual particles, 13 particle types were identified, in which particles of marine origin were the most abundant, followed by carbonaceous and mineral dust particles. A number of aged (reacted) sea salt (and mixture) particles produced by the atmospheric reaction of genuine sea-salts, especially with NO(x) or HNO(3), were significantly encountered in almost all the aerosol samples. They greatly outnumbered genuine sea salt particles, implying that the summertime Arctic atmosphere, generally regarded as a clean background environment, is disturbed by anthropogenic air pollutants. The main sources of airborne NO(x) (or HNO(3)) are probably ship emissions around the Arctic Ocean, industry emission from northern Europe and northwestern Siberia, and renoxification of NO(3)(-) within or on the melting snow/ice surface.