Din-Goa Liu

A small/wide-angle X-ray scattering instrument for structural characterization of air-liquid interfaces, thin films and bulk specimens

At the National Synchrotron Radiation Research Center, a small/wide-angle X-ray scattering (SAXS/WAXS) instrument has been installed at the BL23A beamline with a superconducting wiggler insertion device. This beamline is equipped with double Si(111) crystal and double Mo/B4C multilayer monochromators, and an Si-based plane mirror that can selectively deflect the beam downwards for grazing-incidence SAXS (GISAXS) studies of air–liquid or liquid–liquid interfaces. The SAXS/WAXS instrument, situated in an experimental hutch, comprises collimation, sample and post-sample stages. Pinholes and slits have been incorporated into the beam collimation system spanning a distance of ∼5 m. The sample stage can accommodate various sample geometries for air–liquid interfaces, thin films, and solution and solid samples. The post-sample section consists of a 1 m WAXS section with two linear gas detectors, a vacuum bellows (1–4 m), a two-beamstop system and the SAXS detector system, all situated on a motorized optical bench for motion in six degrees of freedom. In particular, the vacuum bellows of a large inner diameter (260 mm) provides continuous changes of the sample-to-detector distance under vacuum. Synchronized SAXS and WAXS measurements are realized via a data-acquisition protocol that can integrate the two linear gas detectors for WAXS and the area detector for SAXS (gas type or Mar165 CCD); the protocol also incorporates sample changing and temperature control for programmable data collection. The performance of the instrument is illustrated via several different measurements, including (1) simultaneous SAXS/WAXS and differential scanning calorimetry for polymer crystallization, (2) structural evolution with a large ordering spacing of ∼250 nm in a supramolecular complex, (3) SAXS for polymer blends under in situ drawing, (4) SAXS and anomalous SAXS for unilamellar lipid vesicles and metalloprotein solutions, (5) anomalous GISAXS for oriented membranes of Br-labeled lipids embedded with peptides, and (6) GISAXS for silicate films formed in situ at the air–water interface.

Effect of Al-substitution on the stability of LiMn2O4 spinel, synthesized by citric acid sol–gel method

Abstract Spinel LiMn2O4 and LiAlxMn2−xO4 (x=0.05, 0.15) are synthesized by a sol–gel method using citric acid as a chelating agent. The effect of calcination temperature on the purity of the Al-substituted spinel is examined by X-ray diffraction measurements which suggests that pure material is obtained by calcination at 800°C. Cyclic voltammetry is employed to characterize the reactions of lithium insertion into and extraction from the spinel materials. Surface morphology changes are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that Al-substitution enhances sintering of the spinel LiMn2O4. Charge–discharge cycling studies show that Al-substitution improves substantially the capacity retention of the spinel LiMn2O4.

Effect of various synthetic parameters on purity of LiMn2O4 spinel synthesized by a sol–gel method at low temperature

Abstract Spinel lithium manganese oxide, LiMn 2 O 4 , prepared by the sol–gel method using citric acid as a chelating agent under different (i) pH conditions, (ii) molar ratio of citric acid to total metal ion, (iii) amount of water, (iv) calcination temperature, and (vi) starting materials. The effects of various synthetic parameters on the purity of this oxide are analysed by means of X-ray diffraction measurements. The results show that pure LiMn 2 O 4 can be prepared from nitrate salts as starting materials at a low temperature of 600°C. The optimum pH and molar ratio of chelating agent to total metal ions are 6.0 and 1.0, respectively.

Architecture of Pd-Au bimetallic nanoparticles in sodium bis(2-ethylhexyl)sulfosuccinate reverse micelles as investigated by X-ray absorption spectroscopy.

In this study, we demonstrate the unique application of X-ray absorption spectroscopy (XAS) as a fundamental characterization tool to help in designing and controlling the architecture of Pd-Au bimetallic nanoparticles within a water-in-oil microemulsion system of water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane. Structural insights obtained from the in situ XAS measurements recorded at each step during the formation process revealed that Pd-Au bimetallic clusters with various Pd-Au atomic stackings are formed by properly performing hydrazine reduction and redox transmetalation reactions sequentially within water-in-oil microemulsions. A structural model is provided to explain reasonably each reaction step and to give detailed insight into the nucleation and growth mechanism of Pd-Au bimetallic clusters. The combination of in situ XAS analysis at both the Pd K-edge and the Au L(III)-edge and UV-vis absorption spectral features confirms that the formation of Pd-Au bimetallic clusters follows a (Pd(nuclei)-Au(stack))-Pd(surf) stacking. This result further implies that the thickness of Au(stack) and Pd(surf) layers may be modulated by varying the dosage of the Au precursor and hydrazine, respectively. In addition, a bimetallic (Pd-Au)(alloy) nanocluster with a (Pd(nuclei)-Au(stack))-(Pd-Au(alloy))(surf) stacking was also designed and synthesized in order to check the feasibility of Pd(surf) layer modification. The result reveals that the Pd(surf) layer of the stacked (Pd(nuclei)-Au)(stack) bimetallic clusters can be successfully modified to form a (Au-Pd alloy)(surf) layer by a co-reduction of Pd and Au ions by hydrazine. Further, we demonstrate the alloying extent or atomic distribution of Pd and Au in Pd-Au bimetallic nanoparticles from the derived XAS structural parameters. The complete XAS-based methodology, demonstrated here on the Pd-Au bimetallic system, can easily be extended to design and control the alloying extent or atomic distribution, atomic stacking, and electronic structure to construct many other types of bimetallic systems for interesting applications.

Kinetically Controlled Autocatalytic Chemical Process for Bulk Production of Bimetallic Core–Shell Structured Nanoparticles

Although bimetallic core@shell structured nanoparticles (NPs) are achieving prominence due to their multifunctionalities and exceptional catalytic, magnetic, thermal, and optical properties, the rationale underlying their design remains unclear. Here we report a kinetically controlled autocatalytic chemical process, adaptable for use as a general protocol for the fabrication of bimetallic core@shell structured NPs, in which a sacrificial Cu ultrathin layer is autocatalytically deposited on a dimensionally stable noble-metal core under kinetically controlled conditions, which is then displaced to form an active ultrathin metal-layered shell by redox-transmetalation. Unlike thermodynamically controlled under-potential deposition processes, this general strategy allows for the scaling-up of production of high-quality core-shell structured NPs, without the need for any additional reducing agents and/or electrochemical treatments, some examples being Pd@Pt, Pt@Pd, Ir@Pt, and Ir@Pd. Having immediate and obvious commercial potential, Pd@Pt NPs have been systematically characterized by in situ X-ray absorption, electrochemical-FTIR, transmission electron microscopy, and electrochemical techniques, both during synthesis and subsequently during testing in one particularly important catalytic reaction, namely, the oxygen reduction reaction, which is pivotal in fuel cell operation. It was found that the bimetallic Pd@Pt NPs exhibited a significantly enhanced electrocatalytic activity, with respect to this reaction, in comparison with their monometallic counterparts.

X-ray beamlines for structural studies at the NSRRC superconducting wavelength shifter

Using a superconducting-wavelength-shifter X-ray source with a photon flux density of 10(11)-10(13) photons s(-1) mrad(-1) (0.1% bandwidth)(-1) (200 mA)(-1) in the energy range 5-35 keV, three hard X-ray beamlines, BL01A, BL01B and BL01C, have been designed and constructed at the 1.5 GeV storage ring of the National Synchrotron Radiation Research Center (NSRRC). These have been designed for structure-related research using X-ray imaging, absorption, scattering and diffraction. The branch beamline BL01A, which has an unmonochromatized beam, is suitable for phase-contrast X-ray imaging with a spatial resolution of 1 microm and an imaging efficiency of one frame per 10 ms. The main beamline BL01B has 1:1 beam focusing and a medium energy resolution of approximately 10(-3). It has been designed for small-angle X-ray scattering and transmission X-ray microscopy, used, respectively, in anomalous scattering and nanophase-contrast imaging with 30 nm spatial resolution. Finally, the branch beamline BL01C, which features collimating and focusing mirrors and a double-crystal monochromator for a high energy resolution of approximately 10(-4), has been designed for X-ray absorption spectroscopy and high-resolution powder X-ray diffraction. These instruments, providing complementary tools for studying multiphase structures, have opened up a new research trend of integrated structural study at the NSRRC, especially in biology and materials. Examples illustrating the performances of the beamlines and the instruments installed are presented.

Structural and Electronic Effects of Carbon‐Supported PtxPd1−x Nanoparticles on the Electrocatalytic Activity of the Oxygen‐Reduction Reaction and on Methanol Tolerance

We report a systematic investigation on the structural and electronic effects of carbon-supported Pt(x)Pd(1-x) bimetallic nanoparticles on the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in acid electrolyte. Pt(x)Pd(1-x)/C nanocatalysts with various Pt/Pd atomic ratios (x=0.25, 0.5, and 0.75) were synthesized by using a borohydride-reduction method. Rotating-disk electrode measurements revealed that the Pt(3)Pd(1)/C nanocatalyst has a synergistic effect on the ORR, showing 50% enhancement, and an antagonistic effect on the MOR, showing 90% reduction, relative to JM 20 Pt/C on a mass basis. The extent of alloying and Pt d-band vacancies of the Pt(x)Pd(1-x)/C nanocatalysts were explored by extended X-ray absorption fine-structure spectroscopy (EXAFS) and X-ray absorption near-edge structure spectroscopy (XANES). The structure-activity relationship indicates that ORR activity and methanol tolerance of the nanocatalysts strongly depend on their extent of alloying and d-band vacancies. The optimal composition for enhanced ORR activity is Pt(3)Pd(1)/C, with high extent of alloying and low Pt d-band vacancies, owing to favorable O-O scission and inhibited formation of oxygenated intermediates. MOR activity also shows structure dependence. For example, Pt(1)Pd(3)/C with Pt(rich-core)Pd(rich-shell) structure possesses lower MOR activity than the Pt(3)Pd(1)/C nanocatalyst with random alloy structure. Herein, extent of alloying and d-band vacancies reveal new insights into the synergistic and antagonistic effects of the Pt(x)Pd(1-x)/C nanocatalysts on surface reactivity.

Local structure transformation of nano-sized Al-doped LiMn2O4 sintered at different temperatures

LiMn2O4 and LiAl0.15Mn1.85O4 were synthesized via the sol–gel process using citric acid as the chelating agent, followed by sintering at various temperatures. The electronic and atomic structures of LiMn2O4 and LiAl0.15Mn1.85O4 powders were probed by means of Mn K-edge X-ray absorption spectroscopy (XAS). Al-doping was found to promote the sintering of spinel LiMn2O4 so that the degree of structural disorder around Mn atoms in LiAl0.15Mn1.85O4 becomes lower than that of LiMn2O4, leading to an excellent capacity retention of this cathode material for lithium battery in charge–discharge cycle. # 2003 Published by Elsevier Science B.V.

Relating Structural Aspects of Bimetallic Pt3Cr1/C Nanoparticles to Their Electrocatalytic Activity, Stability, and Selectivity in the Oxygen Reduction Reaction

Two methods were used to prepare bimetallic Pt(3)Cr(1)/C nanocatalysts with similar composition but different alloying extent (structure). We investigated how these differences in alloying extent affect the catalytic activity, stability and selectivity in the oxygen reduction reaction (ORR). One method, based on slow thermal decomposition of the Cr precursor at a rate that matches that of chemical reduction of the Pt precursor, allows fine control of the composition of the Pt(3)Cr(1)/C alloy, whereas the second approach, using the ethylene glycol method, results in considerable deviation (>25 %) from the projected composition. Consequently, these two methods lead to variations in the alloying extent that strongly influence the Pt d-band vacancy and the Pt electroactive surface area (Pt ESCA). This relationship was systematically evaluated by transmission electron microscopy, X-ray absorption near edge structure spectroscopy, and electrochemical analysis. The ORR activity depends on two effects that nullify each other, namely, the number of active Pt sites and their activity. The Pt-site activity is more dominant in governing the ORR activity. The selectivity of the nanocatalyst towards the ORR and the competitive methanol oxidation reaction (MOR) depend on these two effects acting in cooperation to give enhanced ORR activity with suppressed MOR. The number of active Pt sites is associated with the Pt ESCA value, while Pt-site activity is associated with the alloying extent and Pt d-band vacancy (electronic) effects. The presence of Cr atoms in Pt(3)Cr(1)/C enhances stability during electrochemical treatment. Overall, the Pt(3)Cr(1)/C catalyst prepared by controlled-composition synthesis was shown to be superior in ORR activity, selectivity and stability owing to its favorable alloying extent, Pt d-band vacancy, and Pt ESCA.

Anomalous small- and wide-angle X-ray scattering and X-ray absorption spectroscopy for Pt and Pt-Ru nanoparticles

We have characterized the structures of two kinds of catalytic nanoparticles of Pt and Pt–Ru, using anomalous small-angle X-ray scattering (ASAXS), anomalous wide-angle X-ray scattering (AWAXS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. With several X-ray energies near the Pt LIII edge, the AWAXS data reveal a face-centered cubic (f.c.c.) crystalline structure for Pt nanoparticles supported on carbon black, whereas the ASAXS data characterize the monometallic nanoparticles as polydisperse spheres with a mean size of 23 A and a size distribution of 20%. With similar X-ray energies, ASAXS and AWAXS data for the Pt–Ru nanoparticles indicate that they have a mean size of ~29 A and a slightly Pt-rich core that can be characterized by an f.c.c. crystalline structure similar to that of the pure Pt nanoparticles. The coordination numbers of the bimetallic nanoparticles extracted from the EXAFS data, collected at the Pt LIII edge and Ru K edge, also reveal a consistent structure of largely, but not completely, intermixed Pt and Ru atoms in the nanoparticles.