A call for a round robin study of XAFS stability and platform dependence at synchrotron beamlines on well defined samples

Round robin studies have been used across fields of science for quality control testing and to investigate laboratory dependencies and cross-platform inconsistencies as well as to drive forward the improvement of understanding of experimental systems, systematic effects and theoretical limitations. Here, following the Q2XAFS Workshop and Satellite to IUCr Congress 2017 on `Data Acquisition, Treatment, Storage - quality assurance in XAFS spectroscopy', a mechanism is suggested for a suitable study across XAFS (X-ray absorption fine-structure) beamlines and facilities, to enable each beamline to cross-calibrate, provide representative test data, and to enable collaborative cross-facility activities to be more productive.

An X-ray absorption fine structure spectroscopy study of metal sorption to graphene oxide

Remediation and prevention of environmental contamination by toxic metals is an ongoing issue. Additionally, improving water filtration systems is necessary to prevent toxic metals from circulating through the water supply. Graphene oxide (GO) is a highly sorptive material for a variety of heavy metals under different ionic strength conditions over a wide pH range, making it a promising candidate for use in metal adsorption from contaminated sites or in filtration systems. We present X-ray absorption fine structure (XAFS) spectroscopy results investigating the binding environment of Cd (II), U(VI) and Pb(II) ions onto multi-layered graphene oxide (MLGO). This study shows that the binding environment of each metal onto the MLGO is unique, with different behaviors governing the sorption as a function of pH. For Cd sorption to MLGO, the same mechanism of electrostatic attraction between the MLGO and the Cd⁺² ions surrounded by water molecules prevails over the entire pH range studied. The U(VI), present in solution as the uranyl ion, shows only subtle changes as a function of pH, likely due to the varied speciation of uranium in solution. The adsorption of the U to the MLGO is through a covalent, inner-sphere bond. The only metal from this study where the dominant adsorption mechanism to the MLGO changes with pH is Pb. In this case, under lower pH conditions, Pb is bound onto the MLGO through dominantly outer-sphere, electrostatic adsorption, while under higher pH conditions, the bonding changes to be dominated by inner-sphere, covalent adsorption. Since each of the metals in this study show unique binding properties, it is possible that MLGO could be engineered to effectively adsorb specific metal ions from solution and optimize environmental remediation or filtration for each metal.

Peptide-Directed PdAu Nanoscale Surface Segregation: Toward Controlled Bimetallic Architecture for Catalytic Materials

Bimetallic nanoparticles are of immense scientific and technological interest given the synergistic properties observed when two different metallic species are mixed at the nanoscale. This is particularly prevalent in catalysis, where bimetallic nanoparticles often exhibit improved catalytic activity and durability over their monometallic counterparts. Yet despite intense research efforts, little is understood regarding how to optimize bimetallic surface composition and structure synthetically using rational design principles. Recently, it has been demonstrated that peptide-enabled routes for nanoparticle synthesis result in materials with sequence-dependent catalytic properties, providing an opportunity for rational design through sequence manipulation. In this study, bimetallic PdAu nanoparticles are synthesized with a small set of peptides containing known Pd and Au binding motifs. The resulting nanoparticles were extensively characterized using high-resolution scanning transmission electron microscopy, X-ray absorption spectroscopy, and high-energy X-ray diffraction coupled to atomic pair distribution function analysis. Structural information obtained from synchrotron radiation methods was then used to generate model nanoparticle configurations using reverse Monte Carlo simulations, which illustrate sequence dependence in both surface structure and surface composition. Replica exchange with solute tempering molecular dynamics simulations were also used to predict the modes of peptide binding on monometallic surfaces, indicating that different sequences bind to the metal interfaces via different mechanisms. As a testbed reaction, electrocatalytic methanol oxidation experiments were performed, wherein differences in catalytic activity are clearly observed in materials with identical bimetallic composition. Taken together, this study indicates that peptides could be used to arrive at bimetallic surfaces with enhanced catalytic properties, which could be leveraged for rational bimetallic nanoparticle design using peptide-enabled approaches.

How Does a SILAR CdSe Film Grow? Tuning the Deposition Steps to Suppress Interfacial Charge Recombination in Solar Cells

Successive ionic layer adsorption and reaction (SILAR) is a popular method of depositing the metal chalcogenide semiconductor layer on the mesoscopic metal oxide films for designing quantum-dot-sensitized solar cells (QDSSCs) or extremely thin absorber (ETA) solar cells. While this deposition method exhibits higher loading of the light-absorbing semiconductor layer than direct adsorption of presynthesized colloidal quantum dots, the chemical identity of these nanostructures and the evolution of interfacial structure are poorly understood. We have now analyzed step-by-step SILAR deposition of CdSe films on mesoscopic TiO2 nanoparticle films using X-ray absorption near-edge structure analysis and probed the interfacial structure of these films. The film characteristics interestingly show dependence on the order in which the Cd and Se are deposited, and the CdSe-TiO2 interface is affected only during the first few cycles of deposition. Development of a SeO2 passivation layer in the SILAR-prepared films to form a TiO2/SeO2/CdSe junction facilitates an increase in photocurrents and power conversion efficiencies of quantum dot solar cells when these films are integrated as photoanodes in a photoelectrochemical solar cell.

Effects of the microbial siderophore DFO-B on Pb and Cd speciation in aqueous solution

This study investigates the complexation environments of aqueous Pb and Cd in the presence of the trihydroxamate microbial siderophore, desferrioxamine-B (DFO-B) as a function of pH. Complexation of aqueous Pb and Cd with DFO-B was predicted using equilibrium speciation calculation. Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy at Pb L(III) edge and Cd K edge was used to characterize Pb and Cd-DFO-B complexes at pH values predicted to best represent each of the metal-siderophore complexes. Pb was not found to be complexed measurably by DFO-B at pH 3.0, but was complexed by all three hydroxamate groups to form a totally "caged" hexadentate structure at pH 7.5-9.0. At the intermediate pH value (pH 4.8), a mixture of Pb-DFOB complexes involving binding of the metal through one and two hydroxamate groups was observed. Cd, on the other hand, remained as hydrated Cd2+ at pH 5.0, occurred as a mixture of Cd-DFOB and inorganic species at pH 8.0, and was bound by three hydroxamate groups from DFO-B at pH 9.0. Overall, the solution species observed with EXAFS were consistent with those predicted thermodynamically. However, Pb speciation at higher pH values differed from that predicted and suggests that published constants underestimate the binding constant for complexation of Pb with all three hydroxamate groups of the DFO-B ligand. This molecular-level understanding of metal-siderophore solution coordination provides physical evidence for complexes of Pb and Cd with DFO-B, and is an important first step toward understanding processes at the microbial- and/or mineral-water interface in the presence of siderophores.

Exciton recombination dynamics in CdSe nanowires: bimolecular to three-carrier Auger kinetics

Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density. At high electron-hole pair densities above 10(19) cm(-3) the dominant process for carrier cooling is the "bimolecular" Auger recombination of one-dimensional (1D) excitons. However, below this excitation level an unexpected transition from a bimolecular (exciton-exciton) to a three-carrier Auger relaxation mechanism occurs. Thus, depending on excitation intensity, electron-hole pair relaxation dynamics in the nanowires exhibit either 1D or 0D (quantum dot) character. This dual nature of the recovery kinetics defines an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates in either regime.

Probing photochemical transformations at TiO2∕Pt and TiO2∕Ir interfaces using x-ray absorption spectroscopy

Structural transformations at the TiO2Pt and TiO2Ir interfaces during UV-irradiation have been probed by X-ray absorption spectroscopy. Oxidation by the photogenerated holes results in the intercalation of Pt and Ir into the Titania matrix. The structural transformations observed with Pt and Ir nanoparticles anchored on TiO2 is different than the clustering of gold atoms observed in the TiO2/Au system. Implications of such structural transformations on the photocatalytic activity of semiconductor photocatalyts are discussed.

Size-dependent spontaneous alloying of Au-Ag nanoparticles

We report on systematic studies of size-dependent alloy formation of silver-coated gold nanoparticles (NPs) in aqueous solution at ambient temperature using X-ray absorption fine structure spectroscopy (XAFS). Various Au-core sizes (2.5-20 nm diameter) and Ag shell thicknesses were synthesized using radiolytic wet techniques. The equilibrium structures (alloy versus core-shell) of these NPs were determined in the suspensions. We observed remarkable size dependence in the room temperature interdiffusion of the two metals. The interdiffusion is limited to the subinterface layers of the bimetallic NPs and depends on both the core size and the total particle size. For the very small particles (< or =4.6 nm initial Au-core size), the two metals are nearly randomly distributed within the particle. However, even for these small Au-core NPs, the interdiffusion occurs primarily in the vicinity of the original interface. Features from the Ag shells do remain. For the larger particles, the boundary is maintained to within one monolayer. These results cannot be explained either by enhanced self-diffusion that results from depression of the melting point with size or by surface melting of the NPs. We propose that defects, such as vacancies, at the bimetallic interface enhance the radial migration (as well as displacement around the interface) of one metal into the other. Molecular dynamics calculations correctly predict the activation energy for diffusion of the metals in the absence of vacancies and show an enormous dependence of the rate of mixing on defect levels. They also suggest that a few percent of the interfacial lattice sites need to be vacant to explain the observed mixing.

XAFS and X-ray reflectivity study of III-V compound native oxide/GaAs interfaces

Fluorescence-mode XAFS has been used to study the local environment about chosen atomic species such as Ga and As in bulk oxide Al(1-x)Ga(x)As (x=0.96) and at the interface between thin (300 A) oxidized Al(1-x)Ga(x)As (x=0.94) film and GaAs substrate in total external-reflection mode. X-ray reflectivity experiments have also been employed to investigate the density profile of the oxide film on a GaAs substrate revealing the density profile as a function of depth. It is important to find out how the As is incorporated at the interface, the interfacial strain, and related local structural parameters for understanding that may be central in developing high performance III-V MOSFET devices.

XAFS determination of the bacterial cell wall functional groups responsible for complexation of Cd and U as a function of pH

Bacteria, which are ubiquitous in near-surface geologic systems, can affect the distribution and fate of metals in these systems through adsorption reactions between the metals and bacterial cell walls. Recently, Fein et al. (1997) developed a chemical equilibrium approach to quantify metal adsorption onto cell walls, treating the sorption as a surface complexation phenomenon. However, such models are based on circumstantial bulk adsorption evidence only, and the nature and mechanism of metal binding to cell walls for each metal system have not been determined spectroscopically. The results of XAFS measurements at the Cd K-edge and U L3-edge on Bacillus subtilis exposed to these elements show that, at low pH, U binds to phosphoryl groups while Cd binds to carboxyl functional groups.

X-ray absorption fine-structure spectroscopy studies of Fe sites in natural human neuromelanin and synthetic analogues

X-ray absorption fine-structure spectroscopy is used to study the local environment of the iron site in natural (human) neuromelanin extracted from substantia nigra tissue and in various synthetic neuromelanins. All the materials show Fe centered in a nearest neighbor sixfold (distorted) oxygen octahedron; the Fe-O distances, while slightly different in the natural and synthetic neuromelanin, are both approximately 2.0 A. Appreciable differences arise, however, in the second (and higher) coordination shells. In this case the synthetic melanin has the four planar oxygens bound to carbon rings with Fe-C distances of approximately 2.82 and 4.13 A; the human sample does not show the 2.82 A link but instead indicates a double shell at approximately 3.45 and 3.78 A.