In general, wurtzite CdS exhibits a higher photocatalytic activity than zinc-blende CdS. However, the underlying physicochemical reasons responsible for the differences of photocatalytic activity between the wurtzite and zinc-blende CdS are still unclear. In this work, the structural characteristics, band structures, density of states, bond populations, optical properties and charge carrier effective mass of wurtzite and zinc-blende CdS were investigated based on first-principle theoretical calculations. The calculated results indicate that the distortion of CdS4 tetrahedron units results in the formation of internal electric field in wurtzite CdS, which is beneficial for the efficient separation and diffusion of photogenerated charge carriers. Contrarily, the internal electric field is absent in zinc-blende CdS. Moreover, the effective masses of photogenerated charge carriers of wurtzite CdS are smaller than those of zinc-blende CdS, implying faster migration of photogenerated charge carriers to perform photocatalytic reactions on wurtzite CdS surfaces. All the above factors result in the lower recombination rate of photogenerated charge carriers within wurtzite CdS. Therefore, it is not surprising that wurtzite CdS usually shows a higher photocatalytic activity than zinc-blende CdS. This investigation will provide some new understanding on the difference of photocatalytic activity between wurtzite and zinc-blende CdS.

New understanding on the different photocatalytic activity of wurtzite and zinc-blende CdS

Methylmercury is the environmental form of neurotoxic mercury that is biomagnified in the food chain. Methylation rates are reduced when the metal is sequestered in crystalline mercury sulfides or bound to thiol groups in macromolecular natural organic matter. Mercury sulfide minerals are known to nucleate in anoxic zones, by reaction of the thiol-bound mercury with biogenic sulfide, but not in oxic environments. We present experimental evidence that mercury sulfide forms from thiol-bound mercury alone in aqueous dark systems in contact with air. The maximum amount of nanoparticulate mercury sulfide relative to thiol-bound mercury obtained by reacting dissolved mercury and soil organic matter matches that detected in the organic horizon of a contaminated soil situated downstream from Oak Ridge, TN, in the United States. The nearly identical ratios of the two forms of mercury in field and experimental systems suggest a common reaction mechanism for nucleating the mineral. We identified a chemical reaction mechanism that is thermodynamically favorable in which thiol-bound mercury polymerizes to mercury−sulfur clusters. The clusters form by elimination of sulfur from the thiol complexes via breaking of mercury−sulfur bonds as in an alkylation reaction. Addition of sulfide is not required. This nucleation mechanism provides one explanation for how mercury may be immobilized, and eventually sequestered, in oxygenated surface environments.

Formation of Mercury Sulfide from Hg(II)−Thiolate Complexes in Natural Organic Matter

Plant leaves serve both as a sink for gaseous elemental mercury (Hg) from the atmosphere and a source of toxic mercury to terrestrial ecosystems. Litterfall is the primary deposition pathway of atmospheric Hg to vegetated soils, yet the chemical form of this major Hg input remains elusive. We report the first observation of in vivo formation of mercury sulfur nanoparticles in intact leaves of 22 native plants from six different species across two sampling areas from China. The plants grew naturally in soils from a mercury sulfide mining and retorting region at ambient-air gaseous-Hg concentrations ranging from 131 ± 19 to 636 ± 186 ng m–3 and had foliar Hg concentration between 1.9 and 31.1 ng Hg mg–1 dry weight (ppm). High energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy shows that up to 57% of the acquired Hg is nanoparticulate, and the remainder speciated as a bis-thiolate complex (Hg(SR)2). The fractional amount of nanoparticulate Hg is not correlated with Hg concentratio...

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Biogenesis of Mercury-Sulfur Nanoparticles in Plant Leaves from Atmospheric Gaseous Mercury.

Cd1-xMnxS (x = 0.1-0.3) nanowires were synthesized by using the chemical vapor deposition method. They all consist of a single-crystalline wurtzite CdS structure with a [010] or [011] growth direction. The X-ray diffraction pattern reveals the contraction of the lattice constants due to the incorporation of Mn. The Mn2+ emission at approximately 2.15 eV, originating from the d-d (4T1 --> 6A1) transition, appears below 50-80 K. Its decay time is in the range of 0.55-1 ms, showing a decrease with increasing Mn content. The Mn doping reduces significantly the decay time of band-edge emission from 590 ps to 20-30 ps. Upon applying magnetic field (up to 7 T), the Mn2+ emission is suppressed and donor-acceptor pair emission becomes dominant, suggesting the energy transfer from the band electrons to the Mn2+ ions.

Per Önnerud

Papers

Similarity of structure properties of Hg1− x Mnx S and Cd1 − x Mnx S (structure properties of HgMnS and CdMnS)

The crystal and magnetic structures of ordered cubic Pd3MnD0.7

The magnetic structure of ordered cubic Pd3Mn

A magnetic structure investigation of tetragonal (Fe1 − xCox)2 As

The ferromagnetic structure of hexagonal (Fe1−xCox)2As