Browsing by Author "Chen, Jin-Ming"

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Carbon nanobeads as an anode material on high rate capability lithium ion batteries
(2009-11-30)
Chang, Jian-Chia
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Tzeng, Yu-Fen
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Chen, Jin-Ming
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Chiu, Hsin-Tien
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Lee, Chi-Young
Carbon nanobeads (CNBs) were prepared by reacting cyclohexachlorobenzene with dispersed sodium metal at 200 degrees C for 4 h. The CNBs prepared in this manner formed uniform nanobeads, with sizes ranging from 100 to 300 nm. Heating resulted in a reduction in the size of the CNBs, and improvements in their degree of crystallinity. The nanosized carbon materials considerably increased the surface area of the powder, reducing the distance of the intercalation/deintercalation pathway, substantially improving the charge capacity of the lithium ion battery at a high charging rate. The charge capacity of CNBs was found to be 238 mAh g(-1), while that of commercial MCMB reached only 36 mAh g(-1), when the charging rate was 1C (372 mAh g(-1)). As the charging rate was further increased to 2C (744 mAh g(-1)) and 3C (1116 mAh g(-1)), the charge capacities of CNBs dropped to 173 and 111 mAh g(-1), respectively. The cyclic performance of the CNBs was measured and found to be significantly improved in comparison to other carbonaceous materials, for up to 100 cycles. Although cyclic performance did result in a gradual reduction in capacity, the CNBs still greatly exceeded the capacity of MCMB. These results clearly demonstrate the potential role of CNBs as anodes for high capacity Li ion batteries for use in the automobile industry. (C) 2009 Elsevier Ltd. All rights reserved.
Characterization of magnetic properties of Sr2CuWO6 and Sr2CuMoO6
(2014-04-21)
Vasala, Sami
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Saadaoui, Hassan
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Morenzoni, Elvezio
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Chmaissem, Omar
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Chan, Ting-Shan
In this work we examine the low-temperature magnetic properties of the two double-perovskite compounds Sr2CuWO6 and Sr2CuMoO6 using magnetic susceptibility, muon spin rotation and relaxation, and neutron powder diffraction measurements. Additionally, the most relevant spin exchange interaction constants are derived from ab initio electronic structure calculations, aided by x-ray absorption spectroscopy. The compounds exhibit quasi-two-dimensional magnetic properties, with broad maxima at T-max = 83 and 95 K for Sr2CuWO6 and Sr2CuMoO6, respectively. However, three-dimensional long-range order takes place below T-N = 24(1) and 28(2) K for Sr2CuWO6 and Sr2CuMoO6, respectively. Our results show that the low-dimensional magnetic correlations are mainly due to the significant next-nearest-neighbor interactions in the ab plane of the double-perovskite structure, whereas three-dimensional long-range magnetic order is caused by weaker next-nearest-neighbor interactions along the c axis. Next-nearest-neighbor interactions are also slightly frustrated by weaker nearest-neighbor interactions within the ab plane. Based on these results we predict the low-temperature magnetic structure in these compounds to be type-II antiferromagnetic order of the double-perovskite lattice.
Characterization of magnetic properties of Sr2CuWO6 and Sr2CuMoO6
(2014-04-21)
Vasala, Sami
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Saadaoui, Hassan
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Morenzoni, Elvezio
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Chmaissem, Omar
;
Chan, Ting-Shan
In this work we examine the low-temperature magnetic properties of the two double-perovskite compounds Sr2CuWO6 and Sr2CuMoO6 using magnetic susceptibility, muon spin rotation and relaxation, and neutron powder diffraction measurements. Additionally, the most relevant spin exchange interaction constants are derived from ab initio electronic structure calculations, aided by x-ray absorption spectroscopy. The compounds exhibit quasi-two-dimensional magnetic properties, with broad maxima at T-max = 83 and 95 K for Sr2CuWO6 and Sr2CuMoO6, respectively. However, three-dimensional long-range order takes place below T-N = 24(1) and 28(2) K for Sr2CuWO6 and Sr2CuMoO6, respectively. Our results show that the low-dimensional magnetic correlations are mainly due to the significant next-nearest-neighbor interactions in the ab plane of the double-perovskite structure, whereas three-dimensional long-range magnetic order is caused by weaker next-nearest-neighbor interactions along the c axis. Next-nearest-neighbor interactions are also slightly frustrated by weaker nearest-neighbor interactions within the ab plane. Based on these results we predict the low-temperature magnetic structure in these compounds to be type-II antiferromagnetic order of the double-perovskite lattice.
Crucial Role of Site Disorder and Frustration in Unusual Magnetic Properties of Quasi-2D Triangular Lattice Antimonate Na4FeSbO6
(2015-10-01)
Zvereva, Elena A.
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Presniakov, Igor A.
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Whangbo, Myung-Hwan
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Koo, Hyun-Joo
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Frantsuzenko, Tatyana V.
The effect of antisite disorder in the layered sodium iron antimonate Na4FeSbO6 was examined both experimentally and theoretically. The magnetic susceptibility and specific heat measurements show that Na4FeSbO6 does not undergo a long-range antiferromagnetic order, unlike its structural analog Li4FeSbO6. The electron spin resonance yields the complicated picture of coexistence of two magnetic subsystems corresponding to two different Fe cation positions in the lattice (regular and antisite) and driving the magnetic properties of the Na4FeSbO6. This conclusion found perfect confirmation from both the Mossbauer and X-ray absorption data which show the presence of two kinds of Fe3+ ions being in high-spin Fe3+ (S = 5/2) and low-spin Fe3+ (S = 1/2) states. These findings arise from the antisite disorder between the Fe3+ and Sb5+ ions in the (NaFeSbO6)(3-) layers of Na4FeSbO6. Our density functional calculations show that the Fe3+ ions located at the Sb5+ sites exist as low-spin Fe3+ ions, and that the spins of each Fe3+ (S = 5/2)-Fe3+ (S = 1/2)-Fe3+ (S = 5/2) trimer generated by the antisite disorder has a ferrimagnetic arrangement Fe(3+)a dagger aEuro"Fe(3+)a dagger"aEuro"Fe(3+)a dagger, which enhances the magnetization of Na4FeSbO6 and leads to an apparently positive Curie-Weiss temperature.
Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0001)
(2019-12-04)
Liu, Wei-Rein
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Pao, Chih-Wen
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Yu, Doris Weijyun
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Chin, Yi-Ying
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Wu, Wen-Bin
We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0001) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L-2(,3) edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0005) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L-3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (V-Zn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous V-Zn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density V-Zn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.
Efficacies of dopants in thermoelectric BiOCuSe
(2016-07-01)
Chou, Ta-Lei
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Tewari, Girish C.
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Srivastava, Divya
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Yamamoto, Atsushi
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Chan, Ting-Shan
Moderately-doped BiOCuSe-based materials are actively investigated worldwide as new promising candidates for intermediate-temperature thermoelectric power generation. Here we explain for two differently doped BiOCuSe materials, i.e. Cu-deficient BiOCu1-xSe and Ca-for-Bi substituted (Bi1-yCay) OCuSe systems, the differences in their efficacies to enhance the thermoelectric performance of the pristine BiOCuSe material using theoretical calculations and x-ray absorption spectroscopy (XAS) at various edges. In the Cu-deficient system the doping dominantly induces an oxide layer-to-selenide layer charge transfer process. On the other hand, the Ca-for-Bi substitution not only gives lower electrical resistivity but also better thermal stability thanks to the increased ionic bonding character in the selenide block. Density of states nearby the Fermi level suggests that even though the Bi-Se distance is much longer than the Bi-O bond, its contribution cannot be ruled out. The Se-Bi-O chain contributes to the hole states at the VBM with the Cu 3d - Se 4p antibonding state. (C) 2016 Elsevier B.V. All rights reserved.
Electrochemical and in situ X-ray spectroscopic studies of MnO2/reduced graphene oxide nanocomposites as a supercapacitor
(2016-07-28)
Chang, Han-Wei
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Lu, Ying-Rui
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Chen, Jeng-Lung
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Chen, Chi-Liang
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Lee, Jyh-Fu
Electrochemical and in situ X-ray absorption spectroscopy (XAS) measurements of various MnO2-coated carbon materials (MnO2/acid-functionalized carbon nanotubes (C-CNT), MnO2/reduced graphene oxide (RGO), and MnO2/RGO-Au electrodes) were conducted to evaluate the supercapacitive performances and electronic structures. MnO2 was deposited on the surface of C-CNT, RGO, and RGO-Au via a spontaneous redox reaction to facilitate the growth of the bulk form of MnO2/C-CNT and the surface forms of MnO2/RGO-based materials. Various forms of MnO2 on the carbon materials exhibited different charge/discharge behaviors. The specific capacitances of the MnO2/RGO and MnO2/RGO-Au electrodes at a current density of 1 A g(-1) were about 433 and 469 F g(-1), respectively; these values are about 1.5 times that of the MnO2/C-CNT (259 F g(-1)) electrode. Specific capacitances of 220 and 281 F g(-1) with retention rates of about 50-60% were obtained from MnO2/RGO and MnO2/RGO-Au, respectively, even at a high current density of 80 A g(-1). Experimental results revealed that the long-term electrochemical stability of the MnO2/RGO-based electrodes (with similar to 90% retention) exceeded that of the MnO2/C-CNT electrode (with similar to 60% retention) after 1000 cycles at a high scan rate of 80 A g(-1). This finding indicates that MnO2/RGO-based electrodes feature excellent cycling stability and rate capacity retention performance. To elucidate the atomic/electronic structures of the MnO2/C-CNT, MnO2/RGO, and MnO2/RGO-Au electrodes during the charge/discharge process, in situ XAS of the Mn K-edge was performed. The MnO2/RGO-based electrodes exhibited the least variations in the pre-peak intensity of the Mn K-edge during the charge/discharge process because a nano-network of MnO2 is homogeneously decorated on the outer surfaces of RGO-based electrodes to facilitate the growth of surface forms of MnO2/RGO and MnO2/RGO-Au. Analytical results further revealed suppression of changes in tunnel size and promotion of insertion/extraction behavior. This work, particularly the combination of cyclic voltammetry with in situ XAS measurements, will be of general value in the fields of nanomaterials and nanotechnology, and in their use in energy storage.
Electrochemically Activated Reduced Graphene Oxide Used as Solid-State Symmetric Supercapacitor: An X-ray Absorption Spectroscopic Investigation
(2016-10-06)
Chang, Han-Wei
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Lu, Ying-Rui
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Chen, Jeng-Lung
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Chen, Chi-Liang
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Chen, Jin-Ming
A solid-state symmetric supercapacitor is successfully fabricated by assembling the sulfuric acidpoly(vinyl alcohol) (H2SO4-PVA) gel electrolyte between the two pieces of electrochemically activated reduced graphene oxide (aRGO) electrodes. The electrochemical and electronic properties of reduced graphene oxide (RGO) and aRGO were characterized by ex situ X-ray absorption spectroscopy (XAS). The aRGO exhibits better electrochemical supercapacitive performances than RGO, owing to the conjunction of the electrochemical double-layer capacitance (EDLC) and the pseudocapacitance. The specific capacitance increases with the oxygen-containing functional groups content in the electrochemically activated aRGO than RGO, increasing the pseudocapacitive contribution. The aRGO20//aRGO20 solid-state symmetric supercapacitors (SSC) exhibit an energy density of 4.7 Wh kg(-1) at a power density of 402 W kg(-1) and 4 Wh kg(-1) at a power density of 1989 W kg(-1), which is competitive with the commercially available supercapacitors. To elucidate the atomic and electronic structures of the RGO and aRGO electrodes in the charge/discharge process, ex situ XAS at the C and O K-edge were performed. Both RGO and aRGO electrodes exhibit a reversible energy shift of the O K-edge, owing to the reversible redox pseudocapacitance close to the surface of graphene-based materials. However, the absorption edge of O K-edge for aRGO electrodes shifts more significantly than that for RGO electrodes during the charge/discharge process. The results indicate that aRGO electrodes exhibit rapid and more active redox reactions for pseudocapacitance than the RGO electrodes. The results herein suggest potential use in high-performance symmetric supercapacitors.
Ex-situ soft X-ray absorption spectroscopic investigation of NiCo2O4 annealed in different gases for hydrogen generation by electrolysis of urea
(2019-06-14)
Chang, Han-Wei
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Dong, Chung-Li
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Lu, Ying-Rui
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Huang, Yu-Cheng
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Chen, Chi-Liang
Electrochemical and ex-situ X-ray absorption spectroscopic (XAS) measurements of NiCo2O4 that have been annealed in different gases (Ar and air) were made to evaluate the efficiency of its electrocatalysis of hydrogen generation by the electrolysis of urea. The electrochemical and XAS measurements demonstrated that NCO-Ar contain significant amounts of Co2+ in octahedral coordination was more favorable than NCO-air for urea electrolysis owing to the effective exposure of electrochemical active sites to participate in the Faradic redox reaction. Further insight into the electrocatalytic mechanism of NiCo2O4-based materials during urea electrolysis was obtained using ex-situ soft XAS. The results of ex-situ soft XAS revealed that the variation of the octahedral and tetrahedral coordinations of Co2+ in NCO-Ar and NCO-air importantly contributes to electrocatalytic behavior. NCO-Ar exhibits excellent electrocatalytic activity towards the Co redox couples in a repetitive cycling process owing to the creation of highly efficient active sites (octahedral Co2+). This study presents importantly x-ray spectroscopic determination of the change in electronic structure of catalyst for hydrogen production. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Facile electrochemical synthesis of 3D nano-architectured CuO electrodes for high-performance supercapacitors
(2014-08-28)
Deng, Ming-Jay
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Wang, Cheng-Chia
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Ho, Pei-Jung
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Lin, Chih-Ming
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Chen, Jin-Ming
With a simple electrochemical process, we prepared nano-architectured CuO electrodes with a 3D hierarchically porous structure and an excellent supercapacitive performance. These nano-architectured CuO electrodes were processed through co-deposition of a Ni-Cu layer on Ni foam, selective etching of Cu from the Ni-Cu film (leaving tentacle-like nanoporous Ni), and anodic deposition of CuO nanoribbons (NRs) on the tentacle-like nanoporous Ni/Ni foam substrate. Because of its unique nano-architecture, the prepared CuO nanoribbon-on-Ni-nanoporous/Ni foam (CNRNP) electrode shows exceptional performance of energy storage relative to a conventional version of the electrode. The CNRNP electrode has also a superior kinetic performance relative to CuO nanoflake-on-Ni foam (CNFNF) and flake-like CuO (FLC) electrodes. Besides its excellent cyclic stability, an exceptionally large specific capacitance of 880 and 800 F g(-1) (deducting the substrate capacitance from the total) for the CNRNP electrode is obtained at scan rates of 10 and 200 mV s(-1), respectively. The excellent pseudocapacitive characteristics of CNRNP electrodes associated with the variation of the Cu oxidation state during charge and discharge cycles were elucidated with in situ X-ray absorption near-edge structure (XANES) spectra.
Local geometric and electronic structures of gasochromic VOx films
(2014)
Jang, Wei-Luen
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Lu, Yang-Ming
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Chen, Chi-Liang
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Lu, Ying-Rui
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Dong, Chung-Li
VOx films were deposited by radio-frequency reactive magnetron sputtering from a vanadium target at room temperature. Local atomic and electronic structures of the films were then modified by thermal annealing. The oxidation state and structural and gasochromic properties of the films were elucidated by X-ray absorption spectroscopy. Analytical results indicate that the as-deposited VOx films were amorphous with mixed V4+ and V5+ valences. The amorphous VOx had a disordered and expanded lamellar structure resembling that of polymer-intercalated V2O5 gels. VOx films were crystallized into orthorhombic V2O5 at 300 degrees C, and the lamellar structure was eliminated at 400 degrees C. Additionally, the gasochromic reaction reduced the vanadium valence via intervalence transitions between V5+ and V3+. Moreover, removing the lamellar structure reduced the gasochromic rate, and the gasochromic reaction transformed the V2O5 crystalline phase irreversibly into an H1.43V2O5 phase. Based on the results of this study, amorphous VOx with a lamellar structure is recommended for use in H-2 gas sensors.
Low Cost Facile Synthesis of Large-Area Cobalt Hydroxide Nanorods with Remarkable Pseudocapacitance
(2015-05-06)
Deng, Ming-Jay
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Song, Cheng-Zhao
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Wang, Chien-Chia
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Tseng, Yuan-Chieh
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Chen, Jin-Ming
Large-area Co(OH)(2)-based supercapacitor electrodes composed of nanotube arrays grown on a 3D nickel-foam (CONTA) electrode and sucker-like nanoporous films grown on a 3D nickel-foam (COSNP) electrode were prepared with a facile electrochemical method for applications in energy storage. These nanoporous Co(OH)(2) electrodes were fabricated with the codeposition of Cu/Ni film on the nickel foam, then etching of Cu from the Cu/Ni layer to form Ni nanotube arrays and sucker-like Ni nanoporous layers, and further cathodic deposition of Co(OH)(2) on the prepared nanoporous Ni substrates. The CONTA and COSNP electrodes exhibited specific capacitances of 2500 and 2900 F/g in a voltage range of 0.65 V (capacitance of the substrates deducted from the total) at 1 A/g in a three electrode cell, respectively. The COSNP electrode demonstrated an excellent supercapacitive performance with specific capacitances 1100 F/g at 1 A/g and 850 F/g at 20 A/g in a voltage range of 1.2 V in a two electrode cell. The remarkable performance of COSNP electrodes correlated with a large conversion of the Co oxidation state during the charge/discharge cycling were examined by in situ X-ray absorption near edge structure (XANES).
Nanoflaky MnO2/functionalized carbon nanotubes for supercapacitors: an in situ X-ray absorption spectroscopic investigation
(2015-01-01)
Chang, Han-Wei
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Lu, Ying-Rui
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Chen, Jeng-Lung
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Chen, Chi-Liang
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Lee, Jyh-Fu
The surfaces of acid- and amine-functionalized carbon nanotubes (C-CNT and N-CNT) were decorated with MnO2 nanoflakes as supercapacitors by a spontaneous redox reaction. C-CNT was found to have a lower edge plane structure and fewer defect sites than N-CNT. MnO2/C-CNT with a highly developed surface area exhibited favorable electrochemical performance. To determine the atomic/electronic structures of the MnO2/functionalized CNTs (MnO2/C-CNT and MnO/N-CNT) during the charge/discharge process, in situ X-ray absorption spectroscopy (XAS) measurements were made at the Mn K-edge. Both C-CNT and N-CNT are highly conductive. The effect of the scan rate on the capacitance behavior was also examined, revealing that the pi* state of CNT and the size of the tunnels in pseudo-capacitor materials (which facilitate conduction and the transport of electrolyte ions) are critical for the capacitive performance, and their role depends on the scan rate. In the slow charge/discharge process, MnO2/N-CNT has a more symmetrical rectangular cyclic voltammetry (CV) curve. In the fast charge/discharge process, MnO2/C-CNT with a highly developed surface provides fast electronic and ionic channels that support a reversible faradaic redox reaction between MnO2 nanoflakes and the electrolyte, significantly enhancing its capacitive performance over that of MnO2/N-CNT. The MnO2/C-CNT architecture has great potential for supercapacitor applications. The information that was obtained herein helps to elucidate CNT surface modification and the design of the MnO2/functionalized CNT interface with a view for the further development of supercapacitors. This work, and especially the combination of CV with in situ XAS measurements, will be of value to readers with an interest in nanomaterial, nanotechnology and their applications in energy storage.
NiCo2O4/graphene quantum dots (GQDs) for use in e fficient electrochemical energy devices: An electrochemical and X-ray absorption spectroscopic investigation
(2020-05-15)
Chang, Han-Wei
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Fu, Ji-Xuan
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Huang, Yu-Cheng
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Lu, Ying-Rui
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Kuo, Chun-Hong
NiCo2O4/graphene quantum dots (GQDs) were synthesized by the incorporation of GQDs into NiCo2O4 as electrode materials for use in electrochemical energy devices. The electronic structure and the electrochemical performance of NiCo2O4/GQDs were investigated by X-ray absorption spectroscopy (XAS) and electrochemical measurements. The rich redox active sites were revealed on the NiCo2O4/GQDs, which were formed by the strong CeOeNi/Co interaction between NiCo2O4 and GQDs. The synergetic double-layer capacitive and pseudocapacitive properties were suggested to provide favorable electrochemical characteristics. In/ex situ XAS elucidates local electronic and chemical environments and provides insights into the energy storage/conversion mechanisms. Analytical results demonstrate that the NiCo2O4/GQDs have strong oxygen-carbon bridges and have more Ni active sites than does NiCo2O4, favoring metal-to-ligand charge transfer at the interface between NiCo2O4 and GQDs, critically influencing electrochemical performance. This work contributes importantly to the field by elucidating an energy storage/conversion mechanism for electrochemical energy devices.
Operando X-ray spectroscopic observations of modulations of local atomic and electronic structures of color switching smart film
(2017-06-14)
Lu, Ying-Rui
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Wu, Tzung-Zing
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Chang, Han-Wei
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Chen, Jeng-Lung
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Chen, Chi-Liang
Smart windows, which change color in response to external stimuli, are extensively studied owing to their potential technological applications in sensors and their ability to reduce the energy consumed by buildings. Most related studies focus on the optical properties of smart color switching films that can control the transmission of light and that of heat independently. This study examines the vanadium pentoxide thin film as a model system of a color switchable window. A gasochromic thin film of V2O5 is fabricated using sol-gel spin coating. In operando soft X-ray absorption spectroscopy (XAS) at the V L-edge is used to determine the evolutions of the electronic and atomic structures of V2O5 thin film under gasochromic color switching. Analysis of the V K-edge with respect to crystalline structural symmetry and valence requires many reference samples, whereas the V L-edge, which involves V 3d orbitals of various symmetries, can provide information about the atomic/electronic structures without many reference samples. A new gas reaction in situ cell was developed to collect the total-electron-yield XAS. The total-electron-yield signal can provide more accurate information about atomic and electronic structures than can the fluorescence-yield signal, which typically exerts a saturation effect. Analytical results reveal that the gasochromic reaction changes the charge state and causes a local atomic structural deformation of the film. The suggestion has been made that in the reaction, the central vanadium atom within the octahedron moves closer to the basal plane such that the apical V-O bond becomes more symmetrical than the film before gasochromic coloration. Unlike the cell that is used for hard XAS, and for which only cation sites can be studied, this in situ gas cell enables the real-time studies of atomic/electronic structures at gas-solid interfaces from viewpoints of both cation and anion sites.
Semiconducting BiOCuSe Thermoelectrics and Its Metallic Derivative Bi2YO4Cu2Se2
(2015-05-01)
Chou, Ta-Lei
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Tewari, Girish C.
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Chan, Ting-Shan
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Hsu, Ying-Ya
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Chen, Jin-Ming
ZrCuSiAs-type mixed-anion oxychalcogenides and oxypnictides form a massive family of functional materials readily tailored to host diverse transport properties. Here we investigate semiconducting BiOCuSe, which has been considered as a promising candidate for intermediate-temperature thermoelectric applications, and its intergrowth derivative Bi2YO4Cu2Se2, by using X-ray absorption spectroscopy. Stronger anisotropy and bond covalency are revealed for the latter phase, and in contrast to the simple charge-balance scenario of [Bi3+O2-](+)[Cu+Se2-](-) in the former phase, mixed-valence Cu+/Cu2+ with a single coordination sphere is seen in the selenide block of Bi2YO4Cu2Se2, which gives rise to a metallic electrical conductivity.
Spin-State Transition, Magnetism and Local Crystal Structure in Eu1-xCaxCoO3-delta
(2013-04-01)
Vasiliev, Alexander N.
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Vasilchikova, Tatyana M.
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Volkova, Olga S.
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Kamenev, Anton A.
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Kaul, Andrey R.
The doping series Eu1-xCaxCoO3-delta provides a rather peculiar way to study the spin-state transitions in cobalt-based complex oxides since partial substitution of Eu3+ ions by Ca2+ ions does not increase the mean valence of cobalt but is accompanied by appearance of oxygen vacancies in the ratio delta similar to x/2. In the parent compound EuCoO3, the low spin (LS)-intermediate spin (IS) or high spin (HS) transition takes place at temperatures so high that the chemical decomposition prevents its direct observation. The substitution of Eu3+ by Ca2+ shifts the spin state transition to lower temperatures and the associated energy gap Delta in octahedrally-coordinated Co3+ ions changes from 1940K in EuCoO3 to 1540K in Eu0.9Ca0.1CoO2.95 and 1050K in Eu0.8Ca0.2CoO2.9. Besides, each O2- vacancy reduces the local coordination of two neighboring Co3+ ions from octahedral to pyramidal thereby locally creating magnetically active sites which couple to dimers. These dimers form another gapped magnetic system with a very different energy scale, D-dim similar to 3 K, on the background of the intrinsically non-magnetic lattice of octahedrally-coordinated low-spin Co3+ ions.
Visible light-induced electronic structure modulation of Nb- and Ta-doped alpha-Fe2O3 nanorods for effective photoelectrochemical water splitting
(2018-02-09)
Chang, Han-Wei
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Fu, Yanming
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Lee, Wan-Yi
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Lu, Ying-Rui
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Huang, Yu-Cheng
The photoelectrochemical (PEC) water splitting activity of Nb and Ta-doped hematite (alpha-Fe2O3) nanorods was investigated with reference to electronic structures by in situ synchrotron x-ray absorption spectroscopy (XAS). Current density-potential measurements demonstrate that the PEC activity of alpha-Fe2O3 nanorods depends strongly on the species and concentrations of dopants. The doping of alpha-Fe2O3 nanorods with a low level of Nb or Ta can improve their electrical conductivity and thereby facilitate charge transport and reduced electron-hole recombination therein. The photoconversion effects of Nb and Ta-doped alpha-Fe2O3 by in situ XAS in the dark and under illumination revealed opposite evolutions of the spectral intensities of the Fe L-edge and Nb/Ta L-edge, indicating that charge transfer and a conduction pathway are involved in the photoconversion. Analytic in situ XAS results reveal that the alpha-Fe2O3 that is doped with a low level of Nb has a greater photoconversion efficiency than that doped with Ta because Nb sites are more active than Ta sites in alpha-Fe2O3. The correlation between PEC activity and the electronic structure of Nb/Ta-doped alpha-Fe2O3 is examined in detail using in situ XAS and helps to elucidate the mechanism of PEC water splitting in terms of the electronic structure.
X-ray Absorption Spectroscopic Study on Interfacial Electronic Properties of FeOOH/Reduced Graphene Oxide for Asymmetric Supercapacitors
(2017-04-01)
Chang, Han-Wei
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Dong, Chung-Li
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Lu, Ying-Rui
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Huang, Yu-Cheng
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Chen, Jeng-Lun
The effects of growth time and interface between the iron oxyhydroxide (FeOOH) and carbon materials (carbon nanotubes (CNT) and reduced graphene oxide (RGO)) to form an asymmetric supercapacitor was studied by X-ray absorption spectroscopy (XAS) and electrochemical measurements. FeOOH/CNT (FCNT) and FeOOH/RGO (FRGO) were successfully synthesized by a simple spontaneous redox reaction with FeC13. The RGO functions as an ideal substrate, providing rich growth sites for FeOOH, and it is believed to facilitate the transport of electrons/ions across the electrode/electrolyte interface. FRGO has been identified as a supercapacitor and found to exhibit significantly greater capacitance than FCNT. To gain further insight into the effects of growth times and the interface of FeOOH for FCNT and FRGO, the electronic structures of FCNT and FRGO with various FeOOH growth times were elucidated by XAS. The difference between the surface electronic structures of CNT and RGO yields different nucleation and growth rates of FeOOH of FeOOH. RGO with excellent interface properties arises from a high degree of covalent functionalization, and/or defects make it favorable for FeOOH growth. FRGO is therefore a promising electrode material for use in the fabrication of asymmetric supercapacitors. In this work, coupled XAS and electrochemical measurements reveal the electronic structure of the interface between FeOOH and the carbon materials and the capacitance performance of asymmetric supercapacitors, which are very useful in the fields of nanomaterials and nanotechnology, especially for their applications in storing energy.