X-ray Near-edge Structure Spectra Research Articles

Characterization, X-ray Absorption Spectroscopic Analysis and Photocatalytic Activity of Co/Zn Co-Doped TiO2 Nanoparticles Synthesized by One-Step Sonochemical Process

A novel one-step preparation of sonochemical method was applied to synthesize Co/Zn co-doped TiO2 nanoparticles using a sonicator of 750 W, 20 kHz for 30 min at room temperature. The formation of the anatase TiO2 phase for all as-prepared samples was observed from XRD results with a crystalline size in nanoscale. The use of ultrasound allowed for the successful doping of both Co and Zn into the TiO2 lattice, which was confirmed by Synchrotron light including X-ray near edge structure (XANES) and Extended X-ray absorption fine structure (EXAFS) spectroscopy. Ti K-edge, Co K-edge, and Zn K-edge XANES spectra exhibited the dominating +4, +2, and +2 valence state of Ti, Co, and Zn in as-prepared samples, respectively. A detailed XANES and EXAFS data analysis give strong evidence that the Co/Zn dopants partially replace the Ti atom of the TiO2 host. The Co/Zn co-doping extends the light absorption of the host to the visible region and restricts the e+/h+ recombination. The photocatalytic activity of samples was tested for degradation of Rhodamine B dye solution under visible light irradiation. The as-synthesized of the co-doped catalyst was presented as highly efficient, with 2.5 and 5 times dye degradation compared with single-doped and bare TiO2.

In Situ XAS Study on Growth of PVP‐Stabilized Cu Nanoparticles

AbstractCu nanoparticles have been synthesized from copper acetate using NaBH4 as a reducing agent and poly(N‐vinyl‐2‐pyrrolidone) (PVP) as a stabilizing agent under an inert ambience. In‐situ time‐resolved X‐ray absorption spectroscopy (XAS) measurement, which include both X‐ray near edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) techniques, have been carried out in a specially designed measurement cell during the formation of Cu nanoparticles using a specialized computer controlled flow setup. Principal Component Analysis (PCA) and Multivariate Curve Resolution with Alternating Least Squares (MCR‐ALS) analysis have been successfully applied to the time‐resolved Cu K‐edge XANES data recorded during the growth of Cu nanoparticles. PCA of the XANES spectra predicted three chemical species present in the reaction solution including an intermediate species apart from the initial and the product species. From MCR‐ALS analysis of the Cu K‐edge XANES spectra, the oxidation state of the reaction intermediate species and the concentration profile of the constituents during the reactions have been determined. Finally the EXAFS spectra of the intermediate species has been extracted and analyzed to predict the local structure of Cu cations in this species.

Local structure investigation of Ni doped ZnO thin films by X-ray absorption spectroscopy

Radio Frequency magnetron sputtering technique has been used to prepare Ni doped ZnO thin films with varying Ni doping concentration of upto 10at.%. Grazing Incidence X-ray Diffraction results show wurtzite structure of the samples throughout the Ni concentration range while X-ray Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure results at Zn and Ni K edges show that Ni is substituting Zn in ZnO lattice and no other extra phase is present in the samples. The substitution of Zn by Ni is further confirmed by O K-edge XANES measurements. Photoluminescence measurements on the samples and theoretical simulations of the XANES spectra at Ni and O K-edges have been used to explain the room temperature ferromagnetic character on the basis of the presence of oxygen vacancies in the samples.

Local Structure Investigation of Mn‐ and Co–Doped TiO2 Thin Films by X‐Ray Absorption Spectroscopy

AbstractMn and Co doped TiO2 thin films with varying doping concentrations have been prepared by r.f magnetron sputtering technique. Preliminary structural characterization of the samples have been carried out by Grazing Incidence X‐ray diffraction (GIXRD) measurements. Detail studies by X‐ray Absorption Spectroscopy (XAS), which comprises of both X‐ray Near Edge Structure (XANES) and Extended X‐ray Absorption Fine Structure (EXAFS) techniques, have been carried out on the samples at dopants (Mn,Co) and host (Ti) K‐edges to have an overall information on the local structures around the above atomic sites of the samples. The information obtained above have been corroborated by O K‐edge XANES, metal L‐edge XAS measurements and theoretical simulations of XANES spectra also. Finally room temperature ferromagnetism observed in the doped TiO2 samples has been explained on the basis of the above structural information.

Insight into growth of Au-Pt bimetallic nanoparticles: an insitu XAS study.

Au-Pt bimetallic nanoparticles have been synthesized through a one-pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous insitu measurement of X-ray absorption spectroscopy (XAS) and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at Indus-2 SRS at RRCAT, Indore, India. Insitu XAS spectra, comprising both X-ray near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L3-edges. While the XANES spectra of the precursors provide real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core-shell-type configuration of a bimetallic alloy in a one-pot synthesis method. The core-shell-type structure of the nanoparticles has also been confirmed by exsitu energy-dispersive spectroscopy line-scan and X-ray photoelectron spectroscopy measurements with insitu ion etching on fully formed nanoparticles.

The electronic structure of homogeneous ferromagnetic (Ga, Mn)N epitaxial films

X-ray Absorption Fine Structure (XAFS) techniques, namely, X-ray Near Edge Structure (XANES), Extended XAFS (EXAFS), and Anomalous X-ray Diffraction (AXRD) were used to investigate the local atomic and electronic structure of (Ga, Mn)N magnetic layers with Mn concentrations of up to 10% grown by Molecular Beam Epitaxy. The XANES and AXRD analysis prove the Mn incorporation on substitutional GaN lattice sites. EXAFS results indicate the good quality of the structure under examination, although 0.5 nitride atom vacancies were found. The Wien2k code was applied to interpret the XANES spectra quantitatively, i.e., to determine the electronic structure of the Mn atoms. It was shown that accounting for the core-hole effect is necessary to reconstruct effectively the XANES spectra. Conducted charge density analysis based on DFT calculations identified the valency of Mn atom to be of 2.4+.

Interpretation of X-ray absorption spectra of As(III) in solution using Monte Carlo simulations.

We performed X-ray absorption spectroscopy measurements on the arsenic K-edge of As(III) in solution under acidic conditions. Extended X-ray absorption fine structure (EXAFS) and X-ray near edge structure (XANES) spectra were compared with theoretical calculations which use local atomic structure configurations, either derived from density functional theory (DFT) energy minimization (EM) calculations or based on classical Monte Carlo (MC) simulations, for a As(OH)3 cluster surrounded by water molecules. The nearest arsenic-oxygen distances obtained from the fit of the XAFS spectra are consistent with the distances present in configurations derived from Monte Carlo simulations but not with those obtained from DFT-EM calculations. Calculations of XANES using either DFT-EM or the average configuration obtained from MC simulations do not reproduce the XANES spectra in the vicinity of the absorption edge. However, specific local atomic structural configurations of the As(OH)3 and water molecules, obtained from MC simulations, which show some ordering of water molecules up to 5 Å from the arsenic, reproduce qualitatively the experimental spectra. These results highlight the capability of XANES to yield information about hydration of ions in solution.

On preferred Mn site in multiferroic BiFeO3: A view by synchrotron x-ray absorption near edge structure spectroscopy

Synchrotron X-ray Near Edge Structure (XANES) experiments were performed on Mn-doped BiFeO3 samples and compared with ab initio XANES calculations. The Fe and Mn K-edge XANES measurements were carried out on Mn-doped BiFeO3 powders (with 5–40 Mn mole%). Both XANES spectra were obtained in a fluorescent mode at ambient temperature with a Ge(111) double crystal monochromator. From the XANES results, it was clearly identified that the oxidation state of both Fe and Mn ions in BiFeO3 structure was +3. The features of the measured Mn K-edge XANES were consistent with ab initio XANES of Mn on the Fe site and inconsistent with Mn on other sites. The clear agreement between measured and ab initio theoretical XANES spectra was the strongest evidence of Mn substituting for Fe in BiFeO3 structure for low Mn content. More interestingly, at higher Mn content, the presence of a second phase precipitation of BiMnO3 and BiMn2O5 was evident. This clearly indicated the Mn solubility limit in the BiFeO3 structure, while the two trace compounds could also be responsible for the relevant properties reported in Mn-doped BiFeO3 materials.

A Novel in-Situ Synchrotron XANES Technique to Study All Vanadium Redox Flow Battery

Vanadium redox flow battery (VRB) is one of the most suitable energy storage systems for large-scale intermittently renewable energy storage systems1-2. In the VRB, positive and negative electrolytes play important roles to store the energy3-4. Consequently, in order to develop new electrolyte solutions for the VRBs with higher energy and power density, as well as longer cycling life and better safety characteristics, the in-depth understanding of the relationship between the oxidation state /local structure changes of the electrolytes and electrochemical performance is critically important. Synchrotron radiation based X-ray adsorption fine structure (XAFS) could be very informative when applied to probe the environment around atoms of all states of matter5. Also the X-ray near-edge structure (XANES) could be especially powerful to determine the average oxidation state and local symmetry of the elements in all phases. With these considerations in mind, we carried out in situ XANES measurements during cycling of an all vanadium redox flow battery (VRB). In this study, the home-made VRB single cell was charged at a constant current density up to a 1.65 V, then the cell was discharged at the constant current density until the voltage reach 0.85 V3. In situXANES characterization was performed at the K-edge of the vanadium to monitor the change of the valence state of V in both positive and negative electrolyte solutions. To better illustrate the XANES spectra evolution, 2D contour plots of XANES spectra for both positive and negative electrolytes have been presented in Fig 1(a) and (b). In addition, the VRB voltage profile in the first charge/discharge process is displayed on the side of the XANES spectra. Obviously, the electrochemical process for both sides is well correlated with the evolution of the XANES data. For example, the inflection point at 1.65 V in the voltage profile coincides well the critical point at contour plot of XANES spectra. The spectra of vanadium XANES during the first charge process is shown in Fig 1.(c) and (d). During the first charge process, the electrolytes were charged at a constant current density until the V4+ was converted to V2+ (negative electrode electrolyte) and V5+(positive electrode electrolyte). Hence, in the positive half-cell, the K-edge of the vanadium continuously shifts toward the higher binding energy. This shift indicates an increase in the average valence state of vanadium in the electrolyte (Fig.1c), while the K-edge of vanadium will shifts back to the lower binding energy, which indicate an decrease in the average valence state of vanadium in the electrolyte (not shown in this abstract). Meanwhile, in the negative half-cell, the K-edge of the vanadium continuously shifts toward to the lower binding energy during the charge process (Fig. 1d), while it will shift back during the discharge process (not shown in this abstract). In summary, a new synchrotron based in situ XANES technique has been applied to study the average oxidation state of the electrolytes in the all vanadium redox flow battery (VRB) under realistic cycling conditions. [1] Zhenguo Yang, et al. Chemical reviews., 2011,111, 3577-3613.[2] M. Skyllas-Kazacos, et al. J. Electrochem.Soc., 2011, 158, R55-R79.[3] C. K. Jia, et al. J. Power Sources., 2010,195, 4380-4383.[4] Y Qin, et al. J. Chem. Eng. Data., 2010, 55, 1276-1279.[5] Mansour, et al. Electrochimica Acta., 2002, 47, 3151-3161.