Probing the Local Atomic Structure of In and Cu in Sphalerite by XAS Spectroscopy Enhanced by Reverse Monte Carlo Algorithm

The distortion of atomic structure around In and Cu dopants in sphalerite ZnS was explored by extended X-ray absorption fine structure (EXAFS) spectroscopy enhanced by reverse Monte Carlo (RMC) simulation (RMC-EXAFS method). These data were complemented with quantum chemical Density Functional Theory (DFT) calculations and theoretical modeling of X-ray absorption near edge spectroscopy (XANES) spectra. The RMC-EXAFS method showed that in the absence of Cu, the In-bearing solid solution is formed via the charge compensation scheme 3Zn2+↔2In3+ + □, where □ is a Zn vacancy. The coordination spheres of In remain undistorted. Formation of the solid solution in the case of (In, Cu)-bearing sphalerites follows the charge compensation scheme 2Zn2+↔Cu+ + In3+. In the solid solution, splitting of the interatomic distances in the 2nd and 3rd coordination spheres of In and Cu is observed. The dopants’ local atomic structure is slightly distorted around In but highly distorted around Cu. The DFT calculations showed that the geometries with close arrangement (clustering) of the impurities—In and Cu atoms, or the In atom and a vacancy—are energetically more favorable than the random distribution of the defects. However, as no heavy In atoms were detected in the 2nd shell of Cu by means of EXAFS, and the 2nd shell of In was only slightly distorted, we conclude that the defects are distributed randomly (or at least, not close to each other). The disagreement of the RMC-EXAFS fittings with the results of the DFT calculations, according to which the closest arrangement of dopants is the most stable configuration, can be explained by the presence of other defects of the sphalerite crystal lattice, which were not considered in the DFT calculations

Synthesis of ZnO Nanoparticles Doped with Cobalt Using Bimetallic ZIFs as Sacrificial Agents

We report here a simple two-stage synthesis of zinc–cobalt oxide nanoparticles. We used Zn/Co-zeolite imidazolate framework (ZIF)-8 materials as precursors for annealing and optional impregnation with a silicon source for the formation of a protective layer on the surface of oxide nanoparticles. Using bimetallic ZIFs allowed us to trace the phase transition of the obtained oxide nanoparticles from wurtzite ZnO to spinel Co3O4 structures. Using (X-ray diffraction) XRD and (X-ray Absorption Near Edge Structure) XANES techniques, we confirmed the incorporation of cobalt ions into the ZnO structure up to 5 mol.% of Co. Simple annealing of Zn/Co-ZIF-8 materials in the air led to the formation of oxide nanoparticles of about 20–30 nm, while additional treatment of ZIFs with silicon source resulted in nanoparticles of about 5–10 nm covered with protective silica layer. We revealed the incorporation of oxygen vacancies in the obtained ZnO nanoparticles using FTIR analysis. All obtained samples were comprehensively characterized, including analysis with a synchrotron radiation source

The State of Trace Elements (In, Cu, Ag) in Sphalerite Studied by X-Ray Absorption Spectroscopy of Synthetic Minerals

The oxidation state and local atomic environment of admixtures of In, Cu, and Ag in synthetic sphalerite crystals were determined by X-ray absorption spectroscopy (XAS). The sphalerite crystals doped with In, Cu, Ag, In–Cu, and In–Ag were synthesized utilizing gas transport, salt flux, and dry synthesis techniques. Oxidation states of dopants were determined using X-ray absorption near edge structure (XANES) technique. The local atomic structure was studied by X-ray absorption fine structure spectroscopy (EXAFS). The spectra were recorded at Zn, In, Ag, and Cu K-edges. In all studied samples, In was in the 3+ oxidation state and replaced Zn in the structure of sphalerite, which occurs with the expansion of the nearest coordination shells due to the large In ionic radius. In the presence of In, the oxidation state of Cu and Ag is 1+, and both metals can form an isomorphous solid solution where they substitute for Zn according to the coupled substitution scheme 2Zn2+ ↔ Me+ + In3+. Moreover, Ag K-edges EXAFS spectra fitting, combined with the results obtained for In- and Au-bearing sphalerite shows that the Me-S distances in the first coordination shell in the solid solution state are correlated with the ionic radii and increase in the order of Cu < Ag < Au. The distortion of the atomic structure increases in the same order. The distant (second and third) coordination shells of Cu and Ag in sphalerite are split into two subshells, and the splitting is more pronounced for Ag. Analysis of the EXAFS spectra, coupled with the results of DFT (Density Function Theory) simulations, showed that the In–In and Me+–In3+ clustering is absent when the metals are present in the sphalerite solid solution. Therefore, all studied admixtures (In, Cu, Ag), as well as Au, are randomly distributed in the matrix of sphalerite, where the concentration of the elements in the “invisible” form can reach a few tens wt.%

Enhanced Sorption of Radionuclides by Defect-Rich Graphene Oxide

Extremely defect graphene oxide (dGO) is proposed as an advanced sorbent for treatment of radioactive waste and contaminated natural waters. dGO prepared using a modified Hummers oxidation procedure, starting from reduced graphene oxide (rGO) as a precursor, shows significantly higher sorption of U(VI), Am(III), and Eu(III) than standard graphene oxides (GOs). Earlier studies revealed the mechanism of radionuclide sorption related to defects in GO sheets. Therefore, explosive thermal exfoliation of graphite oxide was used to prepare rGO with a large number of defects and holes. Defects and holes are additionally introduced by Hummers oxidation of rGO, thus providing an extremely defect-rich material. Analysis of characterization by XPS, TGA, and FTIR shows that dGO oxygen functionalization is predominantly related to defects, such as flake edges and edge atoms of holes, whereas standard GO exhibits oxygen functional groups mostly on the planar surface. The high abundance of defects in dGO results in a 15-fold increase in sorption capacity of U(VI) compared to that in standard Hummers GO. The improved sorption capacity of dGO is related to abundant carboxylic group attached hole edge atoms of GO flakes as revealed by synchrotron-based extended X-ray absorption fine structure (EXAFS) and high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy

New insights into the mechanism of graphene oxide and radionuclideinteraction

The sorption of U(VI), Am(III)/Eu(III) and Cs(I) radionuclides by graphene oxides (GOs) synthesized byHummers’s, Brodie’s and Tour’s methods was studied through a combination of batch experiments withcharacterization by microscopic and spectroscopic techniques such as X-ray photoelectron spectroscopy(XPS), attenuated total reflection fourier-transform infrared spectroscopy (ATR-FTIR), high-energy resolutionfluorescence detected X-Ray absorption spectroscopy (HERFD-XANES), extended X-ray absorptionfine structure (EXAFS) and high resolution transmission electron microscopy (HRTEM). Remarkablydifferent sorption capacity and affinity of radionuclides was found towards GOs synthesized by Hummers’sand Brodie’s methods reflecting different structure and oxidation state of these materials.Mechanism underlying GO e radionuclide interaction is determined using variety of experimentaltechniques. For the first time it is shown here that GO - radionuclides interaction takes place on the smallholes or vacancy defects in the GO sheets. Mechanism of GO’s interaction with radionuclides wasanalyzed and specific functional groups responsible for this interaction were identified. Therefore, a newstrategy to produce improved materials with high capacity for radionuclides suggests the use perforatedand highly defected GO with a larger proportion of carboxylic functional groups

Study of the Cathode Pt-Electrocatalysts Based on Reduced Graphene Oxide with Pt-SnO₂ Hetero-Clusters

A complex study of the structure, morphology, and electrochemical properties of the Pt20/SnO210/RGO electrocatalyst is presented. The advantage of the chemical synthesis of reduced graphene oxide (c-RGO) compared to thermal methods (t-RGO) is due to the formation of graphene plates with amorphous carbon black agglomerates and the chemical composition of the surface. The nature of the interaction between platinum and tin dioxide particles and a conclusion about the formation of heterostructures Pt-SnO2 with the surface interaction of lattices excluding the formation of hetero phases has been established. This achieves high dispersity during the formation of platinum particles without significant agglomeration and increases the electrochemical surface area (ESA) of platinum to 85 m2 g−1 vs. carbon black. In addition, the surface interaction of particles and the formation of hetero-clusters Pt-SnO2 can cause the improved activity and stability of the Pt20/SnO210/c-RGO electrocatalyst

Oxidation and Nanoparticle Formation during Ce(III) Sorption onto Minerals

The sorption of Ce(III) on three abundant environmental minerals (goethite, anatase, and birnessite) was investigated. Batch sorption experiments using a radioactive 139Ce tracer were performed to investigate the key features of the sorption process. Differences in sorption kinetics and changes in oxidation states were found in the case of the sorption of Ce(III) on birnessite compared to that on other minerals. Speciation of cerium onto all of the studied minerals was investigated using spectral and microscopic methods: high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), and X-ray absorption spectroscopy (XAS) in conjunction with theoretical calculations. It was found that during the sorption process onto birnessite, Ce(III) was oxidized to Ce(IV), while the Ce(III) on goethite and anatase surfaces remained unchanged. Oxidation of Ce(III) by sorption on birnessite was also accompanied by the formation of CeO2 nanoparticles on the mineral surface, which depended on the initial cerium concentration and pH value