Showing papers on "Extended X-ray absorption fine structure published in 2015"
TL;DR: The trivalent europium ion (Eu3+) is well known for its strong luminescence in the red spectral region, but this ion is also interesting from a theoretical point of view as mentioned in this paper.
1,906 citations
TL;DR: A ‘radiation-damage-free’ structure of PSII from Thermosynechococcus vulcanus in the S1 state is reported, and it is expected that this structure will provide a blueprint for the design of artificial catalysts for water oxidation.
Abstract: Photosynthesis converts light energy into biologically useful chemical energy vital to life on Earth. The initial reaction of photosynthesis takes place in photosystem II (PSII), a 700-kilodalton homodimeric membrane protein complex which catalyses photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been solved by X-ray diffraction (XRD) at 1.9-angstrom (A) resolution, which revealed that the OEC is a Mn4CaO5-cluster coordinated by a well-defined protein environment1. However, extended X-ray absorption fine structure (EXAFS) studies showed that the manganese cations in the OEC are easily reduced by X-ray irradiation2, and slight differences were found in the Mn–Mn distances between the results of XRD1, EXAFS3–7 and theoretical studies8–14. Here we report a ‘radiation-damage-free’ structure of PSII from Thermosynechococcus vulcanus in the S1 state at a resolution of 1.95 A using femtosecond X-ray pulses of the SPring-8 angstrom compact free-electron laser (SACLA) and a huge number of large, highly isomorphous PSII crystals. Compared with the structure from XRD, the OEC in the X-ray free electron laser structure has Mn–Mn distances that are shorter by 0.1–0.2 A. The valences of each manganese atom were tentatively assigned as Mn1D(III), Mn2C(IV), Mn3B(IV) and Mn4A(III), based on the average Mn–ligand distances and analysis of the Jahn–Teller axis on Mn(III). One of the oxo-bridged oxygens, O5, has significantly longer Mn–O distances in contrast to the other oxo-oxygen atoms, suggesting that it is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as one of the substrate oxygen atoms. These findings provide a structural basis for the mechanism of oxygen evolution, and we expect that this structure will provide a blueprint for design of artificial catalysts for water oxidation.
978 citations
TL;DR: In this article, the synthesis, characterization, and catalytic performance for gas phase propane dehydrogenation of single-site Co2+ ions supported on silica was reported.
160 citations
TL;DR: In this paper, a comparative study of isolated FeII, iron oxide particles, and metallic nanoparticles on silica for non-oxidative propane dehydrogenation was performed.
Abstract: We report a comparative study of isolated FeII, iron oxide particles, and metallic nanoparticles on silica for non-oxidative propane dehydrogenation. It was found that the most selective catalyst was an isolated FeII species on silica prepared by grafting the open cyclopentadienide iron complex, bis(2,4-dimethyl-1,3-pentadienide) iron(II) or Fe(oCp)2. The grafting and evolution of the surface species was elucidated by 1H NMR, diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopies. The oxidation state and local structure of surface Fe were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure. The initial grafting of iron proceeds by one surface hydroxyl Si–OH reacting with Fe(oCp)2 to release one diene ligand (oCpH), generating a SiO2-bound FeII(oCp) species, 1-FeoCp. Subsequent treatment with H2 at 400 °C leads to loss of the remaining diene ligand and formation of nanosized iron oxide clusters, 1-C. Dispersion ...
139 citations
TL;DR: The results indicate that in a rigid structure, the charge deformation provides an efficient way to tune chromaticity, but the band gaps and crystal defects must be controlled by comprehensively accounting for luminescence thermal stability and efficiency.
Abstract: The deficiency of Y3Al5O12:Ce (YAG:Ce) luminescence in red component can be compensated by doping Gd3+, thus lead to it being widely used for packaging warm white light-emitting diode devices. This article presents a systematic study on the photoluminescence properties, crystal structures and electronic band structures of (Y1-xGdx)(3)Al5O12:Ce3+ using powerful experimental techniques of thermally stimulated luminescence, X-ray diffraction, X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and ultraviolet photoelectron spectra (UPS) of the valence band, assisted with theoretical calculations on the band structure, density of states (DOS), and charge deformation density (CDD). A new interpretation from the viewpoint of compression deformation of electron cloud in a rigid structure by combining orbital hybridization with solid-state energy band theory together is put forward to illustrate the intrinsic mechanisms that cause the emission spectral shift, thermal quenching, and luminescence intensity decrease of YAG:Ce upon substitution of Y3+ by Gd3+, which are out of the explanation of the classic configuration coordinate model. The results indicate that in a rigid structure, the charge deformation provides an efficient way to tune chromaticity, but the band gaps and crystal defects must be controlled by comprehensively accounting for luminescence thermal stability and efficiency.
111 citations
TL;DR: In this paper, five types of MnO2 nanostructres (nanowires, nanotubes, nanoparticles, nanorods, and nanoflowers) were synthesized with a fine control over their α-crystallographic form by hydrothermal method.
Abstract: In this work, five types of MnO2 nanostructres (nanowires, nanotubes, nanoparticles, nanorods, and nanoflowers) were synthesized with a fine control over their α-crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined toward oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS), and electrochemical properties (Tafel, Koutecky–Levich plots and % of H2O2 produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO2 coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nanoshapes and follow an order nano...
101 citations
TL;DR: In this paper, the authors present a comprehensive study on low hydration Ir/IrO2 electrodes, made of an Ir core and an IrO2 shell, that are designed and synthesized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir active centers.
Abstract: In this paper, we present a comprehensive study on low hydration Ir/IrO2 electrodes, made of an Ir core and an IrO2 shell, that are designed and synthesized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir–O active centers. Three materials with different hydration degrees have been deeply investigated in terms of structure and microstructure by means of transmission electron microscopy (TEM) and synchrotron radiation techniques such as high-resolution (HR) and pair distribution function (PDF) quality X-ray powder diffraction (XRPD), X-ray absorption spectroscopy (XAS), and for what concerns their electrochemical properties by means of cyclic voltammetry and steady-state I/E curves. The activity of these materials is compared and discussed in the light of our most recent results on hydrous IrOx. The main conclusion of this study is that the Ir core is noninteracting with the IrOx shell, the latter being able to easily accommodate Ir in different oxidation states, as pr...
98 citations
TL;DR: The absorption coefficient of nominally transparent undoped, congruently grown lithium niobate for ordinarily and extraordinarily polarized light in the wavelength range from 390 to 3800 nm is measured and shows good agreement.
Abstract: We employ three highly sensitive spectrometers: a photoacoustic spectrometer, a photothermal common-path interferometer and a whispering-gallery-resonator-based absorption spectrometer, for a comparative study of measuring the absorption coefficient of nominally transparent undoped, congruently grown lithium niobate for ordinarily and extraordinarily polarized light in the wavelength range from 390 to 3800 nm. The absorption coefficient ranges from below 10−4 cm−1 up to 2 cm−1. Furthermore, we measure the absorption at the Urbach tail as well as the multiphonon edge of the material by a standard grating spectrometer and a Fourier-transform infrared spectrometer, providing for the first time an absorption spectrum of the whole transparency window of lithium niobate. The absorption coefficients obtained by the three highly sensitive and independent methods show good agreement.
93 citations
TL;DR: In this paper, the degradation of LiNi 0.8 Co 0.15 Al 0.05 O 2 (LNCAO), a cathode material in lithium-ion-batteries, was studied using in situ powder diffraction and in situ Ni K edge X-ray absorption spectroscopy.
91 citations
TL;DR: In this article, a modified co-precipitation method with three different schemes of synthesis was used to synthesize CuO nanoparticles (NPs) and Cu2O/CuO and CuO/TiO2 nanocomposites (NCs).
Abstract: CuO nanoparticles (NPs) and Cu2O/CuO and CuO/TiO2 nanocomposites (NCs) have been synthesized by using a modified co-precipitation method with three different schemes of synthesis. The crystal structures and morphologies of the samples have been investigated using synchrotron X-ray diffraction and transmission electron microscopy, respectively. The detailed local electronic structures of the NPs and NCs have been determined using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. O K-, Cu K- and Cu L-edge XANES spectra revealed a dominant +2 valence state of Cu in the case of the CuO NPs and CuO/TiO2 NCs, although Cu+1 was dominant in the Cu2O/CuO NCs. A comparison of the local atomic structure around the Cu sites revealed shorter Cu–O bond distances in the as-synthesized samples with respect to the bulk CuO or Cu2O. The Ti K-edge EXAFS fittings for the CuO/TiO2 NCs revealed that the local anatase TiO2 phase was formed, with a Ti–O bond distance of 1.98 A. We further demonstrated that the CuO NPs, and Cu2O/CuO and CuO/TiO2 NCs can serve as effective photocatalysts towards the degradation of two novel water pollutants, (i) methyl orange (MO) and (ii) potassium dichromate (PD), under visible light irradiation. It was found that the Cu2O/CuO NCs exhibit a higher photocatalytic activity towards the degradation of MO and PD than the CuO NPs or CuO/TiO2 NCs. The mechanism of the photodegradation of MO and PD is also discussed in terms of possible chemical reactions, along with the electronic structure and surface properties of the samples.
82 citations
TL;DR: The implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles is demonstrated by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers.
Abstract: Removing effects of sample heterogeneity through single-molecule and single-particle techniques has advanced many fields. While background free luminescence and scattering spectroscopy is widely used, recording the absorption spectrum only is rather difficult. Here we present an approach capable of recording pure absorption spectra of individual nanostructures. We demonstrate the implementation of single-particle absorption spectroscopy on strongly scattering plasmonic nanoparticles by combining photothermal microscopy with a supercontinuum laser and an innovative calibration procedure that accounts for chromatic aberrations and wavelength-dependent excitation powers. Comparison of the absorption spectra to the scattering spectra of the same individual gold nanoparticles reveals the blueshift of the absorption spectra, as predicted by Mie theory but previously not detectable in extinction measurements that measure the sum of absorption and scattering. By covering a wavelength range of 300 nm, we are furth...
TL;DR: In this article, the conditions that favor trace metal adsorption on the edge surfaces of birnessite nanoparticles were identified by using Ni as a probe ion for Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and geometry optimizations based on density function theory.
TL;DR: In this paper, a mechanism for Li-S battery operation with a composite electrode and an adsorption additive obtained by using operando ultraviolet/visible (UV/vis) spectroscopy and X-ray absorption spectrographs confirms the role of the additive and reflects the conversion mechanism of sulfur into Li2S.
Abstract: A mechanism for Li–S battery operation with a composite electrode and an adsorption additive obtained by using operando ultraviolet/visible (UV/vis) spectroscopy and X-ray absorption spectroscopy confirms the role of the adsorption additive and reflects the conversion mechanism of sulfur into Li2S. Operando UV/vis spectroscopy shows a reversible appearance of the long-chain polysulfides in the separator in the fifth cycle, whereas the appearance of mid- and short-chain polysulfides suggests a polysulfide shuttle mechanism. By using a nonsulfur-containing electrolyte, a high-precision analysis of sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra is possible. The XANES analysis shows that polysulfides reach the maximum concentration at the end of the high-voltage plateau, and the low-voltage plateau is characteristic of the polysulfides/Li2S equilibrium. The relative amount of Li2S increases linearly until the end of discharge and reac...
TL;DR: Results indicate that sulfate exists as both inner- and outer-sphere complexes in schwertmannite, and that the drying effect suggests that the tunnel contains readily exchangeable H2O molecules in addition to sulfate ions.
Abstract: Schwertmannite, a nanocrystalline ferric oxyhydroxy-sulfate mineral, plays an important role in many environmental geochemical processes in acidic sulfate-rich environments. The sulfate coordination environment in schwertmannite, however, remains unclear, hindering our understanding of the structure, formation, and environmental behavior of the mineral. In this study, sulfur K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses in combination with infrared spectroscopy were used to determine the sulfate local atomic environment in wet and air-dried schwertmannite samples after incubation at various pHs and ionic strengths. Results indicate that sulfate exists as both inner- and outer-sphere complexes in schwertmannite. Regardless of the sample preparation conditions, the EXAFS-determined S–Fe interatomic distances are 3.22–3.26 A, indicative of bidentate-binuclear sulfate inner-sphere complexes. XANES spectroscopy shows that the pro...
TL;DR: It is found that the first high-resolution structural model of the oxygen-evolving complex of photosystem II that is free of radiation-induced manganese reduction does not match extended X-ray absorption fine structure and QM/MM data for the S1 state.
Abstract: A recent femtosecond X-ray diffraction study produced the first high-resolution structural model of the oxygen-evolving complex of photosystem II that is free of radiation-induced manganese reduction (Protein Data Bank entries 4UB6 and 4UB8). We find, however, that the model does not match extended X-ray absorption fine structure and QM/MM data for the S1 state. This is attributed to uncertainty about the positions of oxygen atoms that remain partially unresolved, even at 1.95 A resolution, next to the heavy manganese centers. In addition, the photosystem II crystals may contain significant amounts of the S0 state, because of extensive dark adaptation prior to data collection.
TL;DR: High signal-to-noise, in situ synchrotron X-ray diffraction was used quantitatively, in addition to in situ diffuse-reflectance infrared Fourier transform spectroscopy and extendedX-ray absorption fine structure to follow the structural changes in the catalyst as a function of reduction temperature to report the structural evolution of Pd-Zn alloys.
Abstract: We report the structural evolution of Pd–Zn alloys in a 3.6% Pd–12% Zn/Al2O3 catalyst which is selective for propane dehydrogenation. High signal-to-noise, in situ synchrotron X-ray diffraction (XRD) was used quantitatively, in addition to in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) and extended X-ray absorption fine structure (EXAFS) to follow the structural changes in the catalyst as a function of reduction temperature. XRD in conjunction with DRIFTS of adsorbed CO indicated that the β1-PdZn intermetallic alloy structure formed at reduction temperatures as low as 230 °C, likely first at the surface, but did not form extensively throughout the bulk until 500 °C which was supported by in situ EXAFS. DRIFTS results suggested there was little change in the surfaces of the nanoparticles above 325 °C. The intermetallic alloy which formed was Pd-rich at all temperatures but became less Pd-rich with increasing reduction temperature as more Zn incorporated into the structure. In addition to the β1-PdZn alloy, a solid solution phase with face-center cubic structure (α-PdZn) was present in the catalyst, also becoming more Zn-rich with increasing reduction temperature.
TL;DR: The present results set the foundation for understanding advanced solvent extraction processes for the separation of the minor, tripositive actinides from the Ln(3+) ions in terms of the local structure of Eu( 3+) in a solid state coordination complex with TODGA.
Abstract: The synthesis, stoichiometry, and structural characterization of a homoleptic, cationic europium(III) complex with three neutral tetraalkyldiglycolamide ligands are reported. The tri(bismuth tetrachloride)tris(N,N,N′,N′-tetra-n-octyldiglycolamide)Eu salt, [Eu(TODGA)3][(BiCl4)3] obtained from methanol was examined by Eu L3-edge X-ray absorption spectroscopy (XAS) to reveal an inner-sphere coordination of Eu3+ that arises from 9 O atoms and two next-nearest coordination spheres that arise from 6 carbon atoms each. A structural model is proposed in which each TODGA ligand with its OCa–Cb–O–Cb–CaO backbone acts as a tridentate O donor, where the two carbonyl O atoms and the one ether O atom bond to Eu3+. Given the structural rigidity of the tridentate coordination motif in [Eu(TODGA)3]3+ with six 5-membered chelate rings, the six Eu–Ca and six Eu–Cb interactions are readily resolved in the EXAFS (extended X-ray absorption fine structure) spectrum. The three charge balancing [BiCl4]− anions are beyond the cationic [Eu(TODGA)3]3+ cluster in an outer sphere environment that is too distant to be detected by XAS. Despite their sizeable length and propensity for entanglement, the four n-octyl groups of each TODGA (for a total of twelve) do not perturb the Eu3+ coordination environment over that seen from previously reported single-crystal structures of tripositive lanthanide (Ln3+) complexes with tetraalkyldiglycolamide ligands (of the same 1 : 3 metal-to-ligand ratio stoichiometry) but having shorter i-propyl and i-butyl groups. The present results set the foundation for understanding advanced solvent extraction processes for the separation of the minor, tripositive actinides (Am, Cm) from the Ln3+ ions in terms of the local structure of Eu3+ in a solid state coordination complex with TODGA.
TL;DR: In this article, the authors used differential X-ray absorption spectroscopy, an elementally specific and spatially sensitive method, to detect changes in Gd-doped ceria, recently shown to exhibit giant electrostriction.
Abstract: Studying electric field-induced structural changes in ceramics is challenging due to the very small magnitude of the atomic displacements. We used differential X-ray absorption spectroscopy, an elementally specific and spatially sensitive method, to detect such changes in Gd-doped ceria, recently shown to exhibit giant electrostriction. We found that the large electrostrictive stress generation can be associated with a few percent of unusually short Ce-O chemical bonds that change their length and degree of order under an external electric field. The remainder of the lattice is reduced to the role of passive spectator. This mechanism is fundamentally different from that in electromechanically active materials currently in use.
01 Jan 2015
TL;DR: The coexistence of different surface complexes or the predominance of one sorption mechanism over others was directly related to surface loading, with bidentate mononuclear, bidentates binuclear and monodentate Mononuclear surface complexes forming at the goethite/water interface under the studied conditions.
Abstract: To investigate the effect of P surface loading on the structure of surface complexes formed at the goethite/water interface, goethite was reacted with orthophosphate at P concentrations of 0.1, 0.2, and 0.8 mmol L −1 at pH 4.5 for 5 days. The P concentrations were chosen to ensure that P loadings at the surface would allow one to follow the transition between adsorption and surface precipitation. Extended X-ray Absorption Fine Structure (EXAFS) spectra were collected in fluorescence mode at the P K-edge at 2150 eV. The structural parameters were obtained through the fits of the sorption data to single and multiple scattering paths using Artemis. EXAFS analysis revealed a continuum among the different surface complexes, with bidentate mononuclear ( 2 E), bidentate binuclear ( 2 C) and monodentate mononuclear ( 1 V) surface complexes forming at the goethite/water interface under the studied conditions. The distances for P–O (1.51–1.53 A) and P–Fe (3.2–3.3 A for bidentate binuclear and around 3.6 A for mononuclear surface complexes) shells observed in our study were consistent with distances obtained via other spectroscopic techniques. The shortest P–Fe distance of 2.83–2.87 A was indicative of a bidentate mononuclear bonding configuration. The coexistence of different surface complexes or the predominance of one sorption mechanism over others was directly related to surface loading.
TL;DR: It is shown that a triple-layer metal-insulator-metal (MIM) structure has spectrally selective IR absorption, while an ultra-thin metal film has non-selective absorption in the near infrared wavelengths, and it is possible to engineer IR emissive properties of these ultra- thin nanocavities by controlling the electric field localization with proper designs.
Abstract: We show that a triple-layer metal-insulator-metal (MIM) structure has spectrally selective IR absorption, while an ultra-thin metal film has non-selective absorption in the near infrared wavelengths. Both sub-wavelength scale structures were implemented with an ultra-thin 6 nm Cr top layer. MIM structure was demonstrated to have near perfect absorption at λ = 1.2 μm and suppressed absorption at λ = 1.8 μm in which experimental and simulated absorptions of the thin Cr film are even higher than the MIM. Occurrence of absorption peaks and dips in the MIM were explained with the electric field intensity localization as functions of both the wavelength and the position. It has been shown that the power absorption in the lossy material is a strong function of the electric field intensity i.e. the more the electric field intensity, the more the absorption and vice versa. Therefore, it is possible to engineer IR emissive properties of these ultra-thin nanocavities by controlling the electric field localization with proper designs.
TL;DR: In this paper, the authors investigated the sorption of Cu by δ-MnO 2, an analog for natural birnessite (layer-type Mn oxide) that is characterized by randomly stacked and curled nanosheets, a low to moderate vacancy content, and variable amounts of layer and interlayer Mn 3+.
TL;DR: There is no evidence of Al coordination modification after aging in water, and sites are hypothesized to be blocked due to reprecipitation of silica dissolved during framework hydrolysis in the aging procedure.
Abstract: The location and stability of Bronsted acid sites catalytically active in zeolites during aqueous phase dehydration of alcohols were studied on the example of cyclohexanol. The catalytically active hydronium ions originate from Bronsted acid sites (BAS) of the zeolite that are formed by framework tetrahedral Si atom substitution by Al. Al K-edge extended X-ray absorption fine structure (EXAFS) and 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopies in combination with density functional theory (DFT) calculations are used to determine the distribution of tetrahedral Al sites (Al T-sites) both qualitatively and quantitatively for both parent and HBEA catalysts aged in water prior to catalytic testing. The aging procedure leads to partial degradation of the zeolite framework evidenced from the decrease of material crystallinity (XRD) as well as sorption capacity (BET). With the exception of one commercial zeolite sample, which had the highest concentration of framework silanol-def...
TL;DR: The results of adsorption edge, ζ potential, and surface complexation modeling suggest that coexistence of As(III) and Cd(2+) enhanced their synergistic adsorbed on TiO2 and, consequently, resulted in the formation of a ternary surface complex, which facilitated the immobilization of the heavy metals.
Abstract: Insights from molecular-level mechanisms of arsenite [As(III)] and cadmium (Cd) co-adsorption on TiO2 can further our understanding of their synergistic removal in industrial wastewaters. The motivation for our study is to explore the interfacial interactions of neutrally charged As(III) and cationic Cd2+ on nanocrystalline TiO2 using multiple complementary techniques. The results of adsorption edge, ζ potential, and surface complexation modeling suggest that coexistence of As(III) and Cd2+ enhanced their synergistic adsorption on TiO2 and, consequently, resulted in the formation of a ternary surface complex. This ternary surface complex, in turn, inhibited the metal release into the aqueous phase and, therefore, facilitated the immobilization of the heavy metals. Our in situ flow-cell attentuated total reflectance Fourier transform infrared (ATR–FTIR) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy evidence showed that, regardless of the order of contact, As(III) was prefer...
TL;DR: In this paper, the structural, local structural and magnetic properties of sol-gel derived Zn1−xFexO (0 ≤ x ≤ 0.06) nanoparticles were studied.
Abstract: In this paper we have studied the structural, local structural and magnetic properties of sol–gel derived Zn1−xFexO (0 ≤ x ≤ 0.06) nanoparticles. The crystalline structure and crystallite size have been estimated by X-ray diffraction with Rietveld refinement and high-resolution transmission electron microscopy (HRTEM). Other structural and local structural properties have been studied by extended X-ray absorption fine structure (EXAFS)-, X-ray absorption near edge structure (XANES)- and Raman-analysis. Weak ferromagnetism is observed at room temperature and magnetization increases with increasing Fe-concentration. The oxygen vacancy assisted bound magnetic polarons (BMPs) and possibly the grain boundaries are responsible for this room temperature ferromagnetism. Variation of resistivity with temperature has also been studied. Appearance of ferromagnetism in ZnO:Fe nanoparticles may open the potential in bio-imaging and drug-delivery applications.
TL;DR: A large effect on the photoluminescent properties is observed: the intensity of the defect luminescence (DL) emission decreases by more than 2 orders of magnitude, theintensity of the near band edge (NBE) emission increases by 20%, and the NBE wavelength decreases with increasing cysteine concentration corresponding to a blue shift of about 35 nm due to the Burstein-Moss effect.
Abstract: Zinc oxide (ZnO) nanoparticles (NPs) were stabilized in water using the amino acid l-cysteine. A transparent dispersion was obtained with an agglomerate size on the level of the primary particles. The dispersion was characterized by dynamic light scattering (DLS), pH dependent zeta potential measurements, scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray absorption fine structure (EXAFS, XANES) spectroscopy. Cysteine acts as a source for sulfur to form a ZnS shell around the ZnO core and as a stabilizer for these core–shell NPs. A large effect on the photoluminescent properties is observed: the intensity of the defect luminescence (DL) emission decreases by more than 2 orders of magnitude, the intensity of the near band edge (NBE) emission increases by 20%, and the NBE wavelength decreases with increasing cysteine concentration corresponding to a blue shift of about 3...
TL;DR: The solvation structure of Zn( 2+) and Cu(2+) in acetonitrile has been determined by a combined approach using both X-ray absorption near edge structure (XANES) and the extended X- Ray absorption fine structure (EXAFS) spectroscopy.
Abstract: The solvation structure of Zn2+ and Cu2+ in acetonitrile has been determined by a combined approach using both X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) spectroscopy. For the former cation, an octahedral geometry of the acetonitrile solvate complex has been found with a Zn–N distance of 2.12(1) A. For the Cu2+ solvates the EXAFS technique has been found to be not able to provide a conclusive determination of the coordination numbers and polyhedral environment, while the analysis of the XANES spectra unambiguously shows the existence of an axially elongated square pyramidal coordination, ruling out the previously proposed octahedral Jahn–Teller (JT) distorted geometry. The Cu–N distances obtained are 2.00(1) and 2.28(2) A for the equatorial and axial ligands, respectively, and the EXAFS and XANES techniques find values of the bond distances in good agreement. The XANES technique has proven to be extremely powerful in providing a reliable resolutio...
TL;DR: [Cu(H2O)5](2+) as the dominant core structure for Cu(II) in water solution is established, and this study is the first to both empirically resolve multiple extended solution structures for fluxional [Cu( H2O)-2+) and to provide direct evidence for second shell dynamics.
Abstract: High-resolution EXAFS (k = 18 A−1) and MXAN XAS analyses show that axially elongated square pyramidal [Cu(H2O)5]2+ dominates the structure of Cu(II) in aqueous solution, rather than 6-coordinate JT-octahedral [Cu(H2O)6]2+. Freezing produced a shoulder at 8989.6 eV on the rising XAS edge and an altered EXAFS spectrum, while 1s → 3d transitions remained invariant in energy position and intensity. Core square pyramidal [Cu(H2O)5]2+ also dominates frozen solution. Solvation shells were found at ∼3.6 A (EXAFS) or ∼3.8 A (MXAN) in both liquid and frozen phases. However, MXAN analysis revealed that about half the time in liquid solution, [Cu(H2O)5]2+ associates with an axially non-bonding 2.9 A water molecule. This distant water apparently organizes the solvation shell. When the 2.9 A water molecule is absent, the second shell is undetectable to MXAN. The two structural arrangements may represent energetic minima of fluxional dissolved aqueous [Cu(H2O)5]2+. The 2.9 A trans-axial water resolves an apparent conflict of the [Cu(H2O)5]2+ core model with a dissociational exchange mechanism. In frozen solution, [Cu(H2O)5]2+ is associated with either a 3.0 A axial non-bonded water molecule or an axial ClO4− at 3.2 A. Both structures are again of approximately equal presence. When the axial ClO4− is present, Cu(II) is ∼0.5 A above the mean O4 plane. This study establishes [Cu(H2O)5]2+ as the dominant core structure for Cu(II) in water solution, and is the first to both empirically resolve multiple extended solution structures for fluxional [Cu(H2O)5]2+ and to provide direct evidence for second shell dynamics.
TL;DR: The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co–Co pairs, likely related to the precipitation of metallic Co in nanoparticles.
Abstract: Zinc oxide nanopowders doped with 1–15 mol % cobalt were produced by the microwave solvothermal synthesis (MSS) technique. The obtained nanoparticles were annealed at 800 °C in nitrogen (99.999%) and in synthetic air. The material nanostructure was investigated by means of the following techniques: X-ray diffraction (XRD), helium pycnometry density, specific surface area (SSA), inductively coupled plasma optical emission spectrometry (ICP-OES), extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and with magnetometry using superconducting quantum interference device (SQUID). Irrespective of the Co content, nanoparticles in their initial state present a similar morphology. They are composed of loosely agglomerated spherical particles with wurtzite-type crystal structure with crystallites of a mean size of 30 nm. Annealing to temperatures of up to 800 °C induced the growth of crystallites up to a maximum of 2 μm in diameter. For samples annealed in high purity nitrogen, the precipitation of metallic α-Co was detected for a Co content of 5 mol % or more. For samples annealed in synthetic air, no change of phase structure was detected, except for precipitation of Co3O4 for a Co content of 15 mol %. The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co–Co pairs. After annealing in synthetic air, the samples remained paramagnetic and samples annealed under nitrogen flow showed a magnetic response under the influences of a magnetic field, likely related to the precipitation of metallic Co in nanoparticles.
TL;DR: A method for quantifying the accuracy of extended X-ray absorption fine structure (EXAFS) fitting models and a combination of experimental EXAFS analysis with candidate models from DFT is a promising strategy for a more accurate determination of nanoparticle structures.
Abstract: We present a method for quantifying the accuracy of extended X-ray absorption fine structure (EXAFS) fitting models. As a test system, we consider the structure of bare Au147 nanoparticles as well as particles bound with thiol ligands, which are used to systematically vary disorder in the atomic structure of the nanoparticles. The accuracy of the fitting model is determined by comparing two distributions of bond lengths: (1) a direct average over a molecular dynamics (MD) trajectory using forces and energies from density functional theory (DFT) and (2) a fit to the theoretical EXAFS spectra generated from that same trajectory. Both harmonic and quasi-harmonic EXAFS fitting models are used to characterize the first-shell Au–Au bond length distribution. The harmonic model is found to significantly underestimate the coordination number, disorder, and bond length. The quasi-harmonic model, which includes the third cumulant of the first-shell bond length distribution, yields accurate bond lengths, but incorrec...
TL;DR: In this paper, the optical absorption edge and near infrared absorption of SrTiO were measured at temperatures from 4 to 1703 K and extrapolated to approximately 1.2 eV at the melting point.
Abstract: The optical absorption edge and near infrared absorption of SrTiO were measured at temperatures from 4 to 1703 K. The absorption edge decreases from 3.25 eV at 4 K to 1.8 eV at 1703 K and is extrapolated to approximately 1.2 eV at the melting point (2350 K). The transmission in the near IR decreases rapidly above 1400 K because of free carrier absorption and is about 50% of the room temperature value at 1673 K. The free carriers are generated by thermal excitation of electrons over the band gap and the formation of charged vacancies. The observed temperature-dependent infrared absorption can be well reproduced by a calculation based on simple models for the intrinsic free carrier concentration and the free carrier absorption coefficient. The measured red shift of the optical absorption edge and the rising free carrier absorption strongly narrow the spectral range of transmission and impede radiative heat transport through the crystal. These effects have to be considered in high temperature applications of SrTiO-based devices, as the number of free carriers rises considerably, and in bulk crystal growth to avoid growth instabilities.
Temperature dependent optical absorption edge of SrTiO3, measured, fitted, and extrapolated to the melting point.