Jyh-Fu Lee

Structure and Electronic Configuration of an Iron(II) Complex in a LIESST State: A Pump and Probe Method

Two polymorphs of mononuclear six-coordinate iron(II) spin-crossover complex trans-[Fe(tzpy)(2)(NCS)(2)] (tzpy = 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) (1) were isolated and structurally characterized. According to the thermally dependent magnetic measurements, polymorph A undergoes a gradual spin transition from a paramagnetic high-spin state ((5)T(2), S = 2, HS-1) above 200 K to a diamagnetic low-spin state ((1)A(1), S = 0, LS-1) below 120 K, whereas polymorph B shows an abrupt spin transition with T(1/2) at 102 K. Molecular and crystal structures of polymorph A in the HS-1 and LS-1 states were studied at 300 and 40 K, respectively. Significant differences in Fe-N distances and coordination geometries of Fe were found between the two spin states, as expected. Light-induced excited spin state trapping (LIESST) was observed upon irradiating the crystal with 532 nm laser light at 40 K, whereupon a metastable high-spin state (HS-2) was formed; the molecular and crystal structure of this metastable state were investigated by a pump and probe method because of its relatively fast relaxation. The electronic configuration of the Fe center in the HS-1, LS-1, and LIESST (HS-2) states were further confirmed by Fe K- and L-edge absorption spectroscopy. In addition, the C[triple bond]N stretching frequency on the ligand can also be followed through the spin transition. The excitation and relaxation process concerning such metastable state were followed by the C[triple bond]N stretching frequency and magnetic susceptibility measurements in the temperature ranges 15-55 K and 5-80 K, respectively. The structure and electronic configuration of the LIESST state of polymorph A were firmly established by X-ray diffraction, X-ray absorption, infrared absorption, and magnetic measurements. A single-crystal-to-single-crystal transition through irradiation was demonstrated. The changes in structure and electronic configuration as a result of the spin transition are believed to occur concurrently.

Electronic structure of GaN nanowire studied by x-ray-absorption spectroscopy and scanning photoelectron microscopy

X-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been employed to obtain information on the electronic structures of the GaN nanowires and thin film. The comparison of the XANES spectra revealed that the nanowires have a smaller (larger) N (Ga) K edge XANES intensity than that of the thin film, which suggests an increase (decrease) of the occupation of Nu200a2pu200a(Gau200a4p) orbitals and an increase of the N (Ga) negative (positive) effective charge in the nanowires. The SPEM spectra showed that the Gau200a3d band for the nanowires lies about 20.8 eV below the Fermi level and has a chemical shift of about −0.9 eV relative to that of the thin film.

Platinum-Decorated Ruthenium Nanoparticles for Enhanced Methanol Electrooxidation

A promising electrocatalyst based on the reduction of Pt2+ ions on the surface of hexagonally close‐packed (hcp) Ru core nanoparticles has been prepared by a redox–transmetalation process. This simple synthetic process generates a Pt‐on‐Ru catalyst with a lower Pt content than commercially available Ptuf8ffRu electrocatalysts and with a long‐range ordered hcp structure, which can significantly reduce the Pt loading. X‐Ray absorption spectroscopy of the Pt‐on‐Ru catalyst reveals pronounced electronic modifications when compared to the commercial Ptuf8ffRu black catalyst. The Pt‐on‐Ru catalyst exhibits a higher mass‐specific current than the Ptuf8ffRu black catalyst in solution in 0.5u2009M H2SO4 with 10u2005vol.u2009% CH3OH under the conditions of rotating disk experiments. Further optimization of this synthetic procedure may yield even more active electrocatalysts with a significant reduction in noble metal loadings.

Evolution of local electronic and atomic structure of Co-doped LiMn2O4 cathode material for lithium rechargeable batteries

Abstract In situ X-ray absorption spectroscopy (XAS) techniques are performed to investigate the evolution of the local electronic and atomic structure of a spinel cathode material LiCo0.2Mn1.8O4 in the 4xa0V range. Changes in the oxidation state, bond distance, and local disorder of Mn and Co absorbers in the Co-doped LiMn2O4 are measured as a function of potentials. The X-ray absorption near edge spectra shows that charging (Li deintercalation) leads to the oxidation of manganese to Mn4+. On the other hand, Co atoms are also oxidized but the edge energy shift is small, which suggests that Co is also electroactive in LiCo0.2Mn1.8O4 in the 4xa0V range. The change in the Debye–Waller factor of the Co–O shell is smaller than that of Mn–O shell. Furthermore, from Debye–Waller factor and bond distance values, it appears that the Co environment fluctuates markedly during discharging.

Depth profile of alloying extent and composition in bimetallic nanoparticles investigated by in situ x-ray absorption spectroscopy

The authors report a general methodology for probing the depth profile of alloying extent and composition in bimetallic nanoparticles (NPs) by a combined in situ x-ray absorption spectroscopy (XAS) and electrochemical strategy. The XAS results of Pt-richcore–Ru-richshell NPs during methanol electro-oxidation revealed that the alloying extent of Pt (JPt) and Ru (JRu) is higher in the core region compared to those in the shell region. An average decrease in the JPt and JRu is observed when the cluster undergoes reduction. By controlling the cluster oxidation degree it is possible to probe the depth profile of the alloying extent in bimetallic NPs.

Pellet Vanadia Catalysts for Oxidative Destruction of 1,2-Dichlorobenzene: Roles of the Grafted TiO2 in Vanadia Morphology and Catalytic Reaction

AbstractnVanadia redox catalysts, V2O5/SiO2, V2O5/TiO2, and V2O5/TiO2–SiO2 having different structures were prepared by incorporating vanadium oxytripropoxide on granular SiO2, TiO2 nano-particles, and TiO2-grafted-SiO2 pellets, respectively. In order to accelerate the catalyst deactivation, the catalysts were tested for oxidation of 1,2-dichlorobenzene (o-DCB) with temperature elevated from 200 to 550xa0°C. Using EXAFS and XRPD, the structural changes in the accelerated aging tests were characterized to assess the catalyst stability and the role of the grafted TiO2 in catalysis. The correlation of catalyst structures with catalytic reaction results indicated that: (1) the grafted TiO2 helps anchoring and dispersing vanadia in the catalyst preparation; (2) monomeric vanadia species with umbrella geometry, polymeric VO4, and TiVO4 coexisting with V2O5 clusters were formed on TiO2–SiO2 pellet, granular SiO2, and TiO2 nano-particles, respectively; (3) oxidative destruction of o-DCB induces the aggregation of vanadia species on the supports leading to a decrease of catalytic activity; (4) lower total oxidation selectivity for V2O5/TiO2 as opposed to the other two catalyst samples could be due to the presence of higher Brønsted-to-Lewis acid sites ratio; (5) a decrease of vanadium-atoms valence charge induced by TiO2–V2O5 interactions alleviates strong adsorption of oxygen-containing intermediates on vanadia sites, thereby increasing the reaction rate; and (6) hydration of vanadia in reaction could lead to aggregation of the vanadia species and catalyst deactivation.