Introduction The deposition of noble metals on oxide surfaces is a commonly adopted method in het... more Introduction The deposition of noble metals on oxide surfaces is a commonly adopted method in heterogeneous catalysis. Noble metal deposited on semiconductor nanoparticles is often found to be beneficial for maximizing the efficiency of photocatalytic reactions [1]. Semiconductor-metal composite nanoparticles facilitate charge rectification and promote interfacial charge transfer processes. Recently, attention has been drawn to the modification of oxide surfaces with gold nanoparticles. Improved photoelectrochemical performance was observed in the case of gold-coated TiO2 films [2]. The improved performance of the nanostructured composite film was attributed to the charging of metal nanoparticles followed by Fermi-level equilibration between the metal and semiconductor in the composite film [3]. However, when the ultraviolet (UV) irradiation was extended for a longer duration (30-60 min), these composite films exhibited a deterioration in photoelectrochemical performance [2]. The po...
The basic design considerations for computer hardware and software are reviewed, applicable not o... more The basic design considerations for computer hardware and software are reviewed, applicable not only to laboratory EXAFS facilities, but also to synchrotron installations. Uniformity and standardization of both hardware configurations, as well as program packages for data collection and analysis, are heavily emphasized. Specific recommendations are made with respect to choice of computers, peripherals and interfaces, and also guide-lines for the development of software packages. A description of two working computer-interfaced EXAFS facilities is presented which can serve as prototypes for future developments.
X-ray absorption fine structure measurements of the Sr and La K edges of the solid solution La(1-... more X-ray absorption fine structure measurements of the Sr and La K edges of the solid solution La(1-x)SrxMnO3 reveal a consistent deviation from a random distribution of Sr at the La/Sr sites for x less than or similar to 0.3. Local structural disorder on the cation sublattice in the low-x samples is also observed to differ in the vicinity of the La-rich and Sr-rich clusters. The local clustering and structural disorder establish an intrinsic chemical as well as structural inhomogeneity on the nanometer scale, which may provide a mechanism for the nucleation of magnetoelectronic phase separation.
... Rev. Lett. 33, 1527 (1974); T. Baer and I. D. Abella, Phys. Lett. 59A, 371 (1976). R. Bonifac... more ... Rev. Lett. 33, 1527 (1974); T. Baer and I. D. Abella, Phys. Lett. 59A, 371 (1976). R. Bonifacio and L. A. Lugiato, Phys. Bev. A 11, 1507 {1975),and 12, 587 (1975). Beats in SF or superradiance are discussed in J.H. Eberly, Lett. Nuovo Cimento 1, 182 (1971). ...
Abstract Using the extended x-ray absorption fine structure (EXAFS) technique, we have investigat... more Abstract Using the extended x-ray absorption fine structure (EXAFS) technique, we have investigated the local structure of the quaternary alloy series Ga x In 1− x As y Sb 1− y . Our measurements reveal the types of atoms about the “average” As atom and their nearest-neighbor distances. We observed the deviation from random site occupation with Ga-As and In-Sb pairs being clearly preferred. This result was compared with a recent theoretical model. The near-neighbor distances were found to be nearly constant, in agreement with the results found for different ternary and quarternary alloys.
The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20n... more The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20nm mean diameter) in an aqueous solution has been investigated. Size dependent alloying was observed upon deposition of Ag on Au core. Ag forms distinct layers around the 20 nm Au nanoparticles. In contrast, random mixing is found for Ag deposited on 3.5nm Au particles.
An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute... more An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute samples at energies above 13 keV. The analyzer is a bent Laue crystal of logarithmic spiral shape. The K fluorescence lines can be collected with a large solid angle and ...
We report studies of bimetallic nanoparticles with 15%-16% atomic crystal parameters size mismatc... more We report studies of bimetallic nanoparticles with 15%-16% atomic crystal parameters size mismatch. The degree of alloying was probed in a 2-nm Pt core ͑smallest attainable core size͒ of Pt-Ag nanoparticles ͑completely immiscible in bulk͒ and 20-nm-diameter Pd-Ag nanowires ͑completely miscible in bulk͒. Particles were synthesized radiolytically, and depending on the initial parameters, they assume spherical or cylindrical ͑nanowire͒ morphologies. In all cases, the metals are seen to follow their bulk alloying characteristics. Pt and Ag segregate in both spherical and wire forms, which indicates that strain due to crystallographic mismatch overcomes the excess surface free energy in the small particles. The Pd-Ag nanowires alloy similar to previously reported spherical Pd-Ag particles of similar diameter and composition.
The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, ... more The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, redox, and precipitation reactions. Previous work demonstrated the formation of nanometer-sized hydrogen uranyl phosphate (abbreviated as HUP) crystals on the cell walls of Bacillus subtilis, a non-U VI-reducing, Gram-positive bacterium. The current study examined the reduction of this biogenic, cell-associated HUP mineral by three dissimilatory metal-reducing bacteria, Anaeromyxobacter dehalogenans strain K, Geobacter sulf urreducens strain PCA, and Shewanella putrefaciens strain CN-32, and compared it to the bioreduction of abiotically formed and freely suspended HUP of larger particle size. Uranium speciation in the solid phase was followed over a 10-to 20-day reaction period by X-ray absorption fine structure spectroscopy (XANES and EXAFS) and showed varying extents of U VI reduction to U IV. The reduction extent of the same mass of HUP to U IV was consistently greater with the biogenic than with the abiotic material under the same experimental conditions. A greater extent of HUP reduction was observed in the presence of bicarbonate in solution, whereas a decreased extent of HUP reduction was observed with the addition of dissolved phosphate. These results indicate that the extent of U VI reduction is controlled by dissolution of the HUP phase, suggesting that the metal-reducing bacteria transfer electrons to the dissolved or bacterially adsorbed U VI species formed after HUP dissolution, rather than to solid-phase U VI in the HUP mineral. Interestingly, the bioreduced U IV atoms were not immediately coordinated to other U IV atoms (as in uraninite, UO 2) but were similar in structure to the phosphate-complexed U IV species found in ningyoite [CaU(PO 4) 2 •H 2 O]. This indicates a strong control by phosphate on the speciation of bioreduced U IV , expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.
The atomic environment around the iron site in the nonheme iron sulfur protein rubredoxin was stu... more The atomic environment around the iron site in the nonheme iron sulfur protein rubredoxin was studied by the extended X-ray absorption fine structure (EXAFS) technique. Within experimental error, the Fe-S bonds in oxidized Clostridiwn pasteurianum rubredoxin are the same as in the analogue anion [Fe(S2-o-xyl)2]synthesized by Holm. The average Fe-S bond length is 2.267 i 0.003A and the root mean square deviation about this average due to structural disorder is 0.032 + 0°032 Receivedfor publication II March 1977.
An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute... more An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute samples at energies above 13 keV. The analyzer is a bent Laue crystal of logarithmic spiral shape. The K fluorescence lines can be collected with a large solid angle and ...
The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20n... more The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20nm mean diameter) in an aqueous solution has been investigated. Size dependent alloying was observed upon deposition of Ag on Au core. Ag forms distinct layers around the 20 nm Au nanoparticles. In contrast, random mixing is found for Ag deposited on 3.5nm Au particles.
We have measured the ability of nonmetabolizing cells of the bacterial species Bacillus subtilis;... more We have measured the ability of nonmetabolizing cells of the bacterial species Bacillus subtilis; Sporosarcina ureae, and Shewanella putrefaciens to reduce aqueous Cr(VI) to Cr(III) in the absence of externally supplied electron donors. Each species can remove signi cant amounts of Cr(VI) from solution, and the Cr(VI) reduction rate is strongly dependent on solution pH. The fastest reduction rates occur under acidic conditions, with decreasing rates with increasing pH. XANES data demonstrate that Cr(VI) reduction to Cr(III) occurs within the experimental systems. Control experiments indicate that the Cr removal is not a purely adsorptive process. Reduction appears to occur at the cell wall, and is not coupled to the oxidation of bacterial organic exudates. Detailed kinetic data suggest that the reduction involves at least a two-stage process, involving an initial rapid removal mechanism followed by a slower process that follows rst-order reaction kinetics. Due to the prevalence of nonmetabolizing cells and cell wall fragments in soils and deeper geologic environments, our results suggest that the observed nonmetabolic
Introduction The deposition of noble metals on oxide surfaces is a commonly adopted method in het... more Introduction The deposition of noble metals on oxide surfaces is a commonly adopted method in heterogeneous catalysis. Noble metal deposited on semiconductor nanoparticles is often found to be beneficial for maximizing the efficiency of photocatalytic reactions [1]. Semiconductor-metal composite nanoparticles facilitate charge rectification and promote interfacial charge transfer processes. Recently, attention has been drawn to the modification of oxide surfaces with gold nanoparticles. Improved photoelectrochemical performance was observed in the case of gold-coated TiO2 films [2]. The improved performance of the nanostructured composite film was attributed to the charging of metal nanoparticles followed by Fermi-level equilibration between the metal and semiconductor in the composite film [3]. However, when the ultraviolet (UV) irradiation was extended for a longer duration (30-60 min), these composite films exhibited a deterioration in photoelectrochemical performance [2]. The po...
The basic design considerations for computer hardware and software are reviewed, applicable not o... more The basic design considerations for computer hardware and software are reviewed, applicable not only to laboratory EXAFS facilities, but also to synchrotron installations. Uniformity and standardization of both hardware configurations, as well as program packages for data collection and analysis, are heavily emphasized. Specific recommendations are made with respect to choice of computers, peripherals and interfaces, and also guide-lines for the development of software packages. A description of two working computer-interfaced EXAFS facilities is presented which can serve as prototypes for future developments.
X-ray absorption fine structure measurements of the Sr and La K edges of the solid solution La(1-... more X-ray absorption fine structure measurements of the Sr and La K edges of the solid solution La(1-x)SrxMnO3 reveal a consistent deviation from a random distribution of Sr at the La/Sr sites for x less than or similar to 0.3. Local structural disorder on the cation sublattice in the low-x samples is also observed to differ in the vicinity of the La-rich and Sr-rich clusters. The local clustering and structural disorder establish an intrinsic chemical as well as structural inhomogeneity on the nanometer scale, which may provide a mechanism for the nucleation of magnetoelectronic phase separation.
... Rev. Lett. 33, 1527 (1974); T. Baer and I. D. Abella, Phys. Lett. 59A, 371 (1976). R. Bonifac... more ... Rev. Lett. 33, 1527 (1974); T. Baer and I. D. Abella, Phys. Lett. 59A, 371 (1976). R. Bonifacio and L. A. Lugiato, Phys. Bev. A 11, 1507 {1975),and 12, 587 (1975). Beats in SF or superradiance are discussed in J.H. Eberly, Lett. Nuovo Cimento 1, 182 (1971). ...
Abstract Using the extended x-ray absorption fine structure (EXAFS) technique, we have investigat... more Abstract Using the extended x-ray absorption fine structure (EXAFS) technique, we have investigated the local structure of the quaternary alloy series Ga x In 1− x As y Sb 1− y . Our measurements reveal the types of atoms about the “average” As atom and their nearest-neighbor distances. We observed the deviation from random site occupation with Ga-As and In-Sb pairs being clearly preferred. This result was compared with a recent theoretical model. The near-neighbor distances were found to be nearly constant, in agreement with the results found for different ternary and quarternary alloys.
The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20n... more The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20nm mean diameter) in an aqueous solution has been investigated. Size dependent alloying was observed upon deposition of Ag on Au core. Ag forms distinct layers around the 20 nm Au nanoparticles. In contrast, random mixing is found for Ag deposited on 3.5nm Au particles.
An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute... more An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute samples at energies above 13 keV. The analyzer is a bent Laue crystal of logarithmic spiral shape. The K fluorescence lines can be collected with a large solid angle and ...
We report studies of bimetallic nanoparticles with 15%-16% atomic crystal parameters size mismatc... more We report studies of bimetallic nanoparticles with 15%-16% atomic crystal parameters size mismatch. The degree of alloying was probed in a 2-nm Pt core ͑smallest attainable core size͒ of Pt-Ag nanoparticles ͑completely immiscible in bulk͒ and 20-nm-diameter Pd-Ag nanowires ͑completely miscible in bulk͒. Particles were synthesized radiolytically, and depending on the initial parameters, they assume spherical or cylindrical ͑nanowire͒ morphologies. In all cases, the metals are seen to follow their bulk alloying characteristics. Pt and Ag segregate in both spherical and wire forms, which indicates that strain due to crystallographic mismatch overcomes the excess surface free energy in the small particles. The Pd-Ag nanowires alloy similar to previously reported spherical Pd-Ag particles of similar diameter and composition.
The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, ... more The mobility of uranium (U) in subsurface environments is controlled by interrelated adsorption, redox, and precipitation reactions. Previous work demonstrated the formation of nanometer-sized hydrogen uranyl phosphate (abbreviated as HUP) crystals on the cell walls of Bacillus subtilis, a non-U VI-reducing, Gram-positive bacterium. The current study examined the reduction of this biogenic, cell-associated HUP mineral by three dissimilatory metal-reducing bacteria, Anaeromyxobacter dehalogenans strain K, Geobacter sulf urreducens strain PCA, and Shewanella putrefaciens strain CN-32, and compared it to the bioreduction of abiotically formed and freely suspended HUP of larger particle size. Uranium speciation in the solid phase was followed over a 10-to 20-day reaction period by X-ray absorption fine structure spectroscopy (XANES and EXAFS) and showed varying extents of U VI reduction to U IV. The reduction extent of the same mass of HUP to U IV was consistently greater with the biogenic than with the abiotic material under the same experimental conditions. A greater extent of HUP reduction was observed in the presence of bicarbonate in solution, whereas a decreased extent of HUP reduction was observed with the addition of dissolved phosphate. These results indicate that the extent of U VI reduction is controlled by dissolution of the HUP phase, suggesting that the metal-reducing bacteria transfer electrons to the dissolved or bacterially adsorbed U VI species formed after HUP dissolution, rather than to solid-phase U VI in the HUP mineral. Interestingly, the bioreduced U IV atoms were not immediately coordinated to other U IV atoms (as in uraninite, UO 2) but were similar in structure to the phosphate-complexed U IV species found in ningyoite [CaU(PO 4) 2 •H 2 O]. This indicates a strong control by phosphate on the speciation of bioreduced U IV , expressed as inhibition of the typical formation of uraninite under phosphate-free conditions.
The atomic environment around the iron site in the nonheme iron sulfur protein rubredoxin was stu... more The atomic environment around the iron site in the nonheme iron sulfur protein rubredoxin was studied by the extended X-ray absorption fine structure (EXAFS) technique. Within experimental error, the Fe-S bonds in oxidized Clostridiwn pasteurianum rubredoxin are the same as in the analogue anion [Fe(S2-o-xyl)2]synthesized by Holm. The average Fe-S bond length is 2.267 i 0.003A and the root mean square deviation about this average due to structural disorder is 0.032 + 0°032 Receivedfor publication II March 1977.
An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute... more An analyzer for detection of fluorescence radiation was developed for fluorescence XAFS of dilute samples at energies above 13 keV. The analyzer is a bent Laue crystal of logarithmic spiral shape. The K fluorescence lines can be collected with a large solid angle and ...
The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20n... more The x-ray absorption fine structure (XAFS) of colloidal Au and Au/Ag nanoparticles (3.5nm and 20nm mean diameter) in an aqueous solution has been investigated. Size dependent alloying was observed upon deposition of Ag on Au core. Ag forms distinct layers around the 20 nm Au nanoparticles. In contrast, random mixing is found for Ag deposited on 3.5nm Au particles.
We have measured the ability of nonmetabolizing cells of the bacterial species Bacillus subtilis;... more We have measured the ability of nonmetabolizing cells of the bacterial species Bacillus subtilis; Sporosarcina ureae, and Shewanella putrefaciens to reduce aqueous Cr(VI) to Cr(III) in the absence of externally supplied electron donors. Each species can remove signi cant amounts of Cr(VI) from solution, and the Cr(VI) reduction rate is strongly dependent on solution pH. The fastest reduction rates occur under acidic conditions, with decreasing rates with increasing pH. XANES data demonstrate that Cr(VI) reduction to Cr(III) occurs within the experimental systems. Control experiments indicate that the Cr removal is not a purely adsorptive process. Reduction appears to occur at the cell wall, and is not coupled to the oxidation of bacterial organic exudates. Detailed kinetic data suggest that the reduction involves at least a two-stage process, involving an initial rapid removal mechanism followed by a slower process that follows rst-order reaction kinetics. Due to the prevalence of nonmetabolizing cells and cell wall fragments in soils and deeper geologic environments, our results suggest that the observed nonmetabolic
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