Aqueous-Phase Synthesis of Size-Tunable PbSe Nanocubes at Room Temperature for Optical Property Characterization

Growth of Size-Tunable Ag2O Polyhedra and Revelation of Their Bulk and Surface Lattices

By primarily adjusting the reagent amounts, particularly the volume of AgNO3 solution introduced, Ag2O cubes with decreasing sizes from 440 to 79 nm, octahedra from 714 to 106 nm, and rhombic dodecahedra from 644 to 168 nm are synthesized. 733 nm cuboctahedra are also prepared for structural analysis. With in-house X-ray diffraction (XRD) peak calibration, shape-related peak shifts are recognizable. Synchrotron XRD measurements at 100 K reveal the presence of bulk and surface layer lattices. Bulk cell constants also deviate slightly. They show a negative thermal expansion behavior with shrinking cell constants at higher temperatures. The Ag2O crystals exhibit size- and facet-dependent optical properties. Bandgaps red-shift continuously with increasing particle sizes. Optical facet effect is also observable. Moreover, synchrotron XRD peaks of a mixture of Cu2O rhombicuboctahedra and edge- and corner-truncated cubes exposing all three crystal faces can be deconvoluted into three components with the bulk and the [111] microstrain phase as the major component. Interestingly, while the unheated Cu2O sample shows clear diffraction peak asymmetry, annealing the sample to 450 K yields nearly symmetric peaks even when returning the sample to room temperature, meaning even moderately high temperatures can permanently change the crystal lattice.

Synthesis of Zinc Blende-Phased CdSe Nanocrystals with Size-Tunable Optical Properties and Adjustable Valence Band Positions

CdSe nanocrystals with average sizes of 15, 24, and 32 nm have been synthesized from an aqueous solution of Na₂SeSO₃, HCl, and cadmium nitrate at 15, 45, and 70 °C, respectively, for about 1 h. Mixing aqueous CdCl₂, HNO₃, and Na₂SeSO₃ at 22 °C for 4 h yields 44 nm CdSe nanocrystals. X-ray and electron diffraction analyses indicate the possession of a zinc blende crystal structure for all the samples. Despite the large particle dimensions, their absorption band red-shifts significantly from 520 to 570 nm with increasing particle sizes, and band gap values decrease from 2.03 eV for 15 nm particles to 1.68 eV for 44 nm crystals. Although these nanocrystals are not emissive, introduction of the cetyltrimethylammonium chloride surfactant during crystal growth can restore their photoluminescence attributed to the improved crystal quality, and the similarly sized CdSe nanocrystals have an emission band red-shifting from 544 nm for 15 nm particles to 583 nm for 47 nm crystals. A band diagram was constructed for these CdSe nanocrystals using information from Mott-Schottky plots. While they have close conduction band positions, the notable size-related band gap variation means that their valence band energies differ considerably with implications of electrochemical and photocatalytic properties. The 44 nm CdSe particles also show the smallest electrochemical charge-transfer resistance.

Simultaneous removal of lead and selenium through biomineralization as lead selenide by anaerobic granular sludge

This study demonstrated the simultaneous removal of lead (Pb) and selenium (Se) as lead selenide biomineralization using anaerobic granular sludge. The microbial community of the granular sludge was first enriched for 140 days in the presence of Pb(II) only, selenate and selenite only, Pb(II)+selenate, and Pb(II)+selenite. In the absence of Se, removal of Pb(II) mainly occurred via biosorption and deposited on the biomass as lead oxide and lead carbonate. The Pb removal efficiency (94% of initial 50 mg L^-1) was reduced to 90% and 86% in the presence of selenate and selenite, respectively, due to biosorption. Addition of Pb(II) didn't exert any toxic effect on the Se-reducing microbial community, on the contrary: Pb(II) addition improved the Se removal efficiency for selenate from 85% to 90%, but did not affect selenite removal after 14 d of incubation. The bioreduction of the Se-oxyanions produced elemental Se (Se(0)) and selenide, which later interacted with Pb(II) to produce lead selenide (PbSe). Adsorption of Pb(II) onto the Se(0) nanoparticles and precipitation as the Se(0)-Pb complex might also have contributed to the simultaneous removal of Pb and Se. XPS and XRD analysis further confirmed the immobilization of Pb as PbSe, PbO and PbCO₃ in the biomass.