Browsing by Author "Chou, Wu Ching"

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Control of crystal phase of BiVO4 nanoparticles synthesized by microwave assisted method
(2016-06)
Nguyen Dang Phu
;
Luc Huy Hoang
;
Pham Khac Vu
;
Kong, Meng-Hong
;
Chen, Xiang-Bai
Different crystalline phases BiVO4 nanoparticles (tetragonal-zircon, monoclinic-scheelite, and tetragonal-zircon/monoclinic-scheelite heterophase) have been prepared by fast microwave assisted method and annealing treatment. For the heterophase, the ratio of tetragonal-zircon and monoclinic-scheelite phases can be well controlled by controlling the annealing temperature. Furthermore, a tight interface junction has been formed between tetragonal-zircon BiVO4 and monoclinic-scheelite BiVO4 in a nanosize level. This tight interface junction can modify the electronic structure of BiVO4 nanoparticles, which would be very helpful for achieving high photocatalytic activity.
Electrochemical and in situ X-ray spectroscopic studies of MnO2/reduced graphene oxide nanocomposites as a supercapacitor
(2016-07-28)
Chang, Han-Wei
;
Lu, Ying-Rui
;
Chen, Jeng-Lung
;
Chen, Chi-Liang
;
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
;
Lu, Ying-Rui
;
Chen, Jeng-Lung
;
Chen, Chi-Liang
;
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.
Growth of free-standing GaN layer on Si(111) substrate
(2009-03-15)
Yang, Tsung Hsi
;
Ku, Jui Tai
;
Chang, Jet-Rung
;
Shen, Shih-Guo
;
Chen, Yi-Cheng
This investigation demonstrates the epitaxial growth of a free-standing GaN layer on Si(1 1 1) using a funnel-like GaN nano-rod buffer structure. The funnel-like GaN nano-rods were directly grown on Si substrates by RF-plasma molecular beam epitaxy. Free-standing GaN layers were achieved through the coalescence of funnel-like GaN nano-rods by the metalorganic chemical vapor deposition. This study examines the structure, optical characteristics and stress of GaN nano-rods and free-standing GaN layers. The c-axis lattice constant of the strain-free Ga-face GaN layer on Si is 5.1844 angstrom, as determined by high-resolution X-ray diffraction. The fully relaxed band edge at 3.468 eV without deep-level emission around 2.3 eV, was revealed in a free-standing GaN layer on Si, using photoluminescence. (C) 2008 Elsevier B.V. All rights reserved.
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
;
Fu, Ji-Xuan
;
Huang, Yu-Cheng
;
Lu, Ying-Rui
;
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.
Optical and Magnetic Properties of Zn1-xMnxO Grown by Plasma-Assisted Molecular Beam Epitaxy
(2014-06-01)
Chou, Wu Ching
;
Tzou, An-Jye
;
Chen, Hsuan-Shao
;
Chien, Kun-Feng
;
Fan, Wen-Chung
Zn1-xMnxO films (0 <= x <= 0.070) were grown by plasma-assisted molecular beam epitaxy on Si substrates with AlN buffer layers. For low Mn concentration samples, x <= 0.02, photoluminescence spectra show strong near band edge emission. Strong phonon and exciton coupling-induced resonant Raman scattering was observed. Paramagnetic, antiferromagnetic, and ferromagnetic phases were observed. Room temperature ferromagnetism for all samples was attributed to the bound magnetic polaron effect.
Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Ag doping and Ag nanoparticles modification
(2020-05-25)
Nguyen Dang Phu
;
Luc Huy Hoang
;
Pham Van Hai
;
Tran Quang Huy
;
Chen, Xiang-Bai
Bi2WO6 nanoparticles doped with Ag, and modified with Ag nanoparticles, were successfully prepared using a two-step microwave assisted and hydrothermal synthesis. With Ag doping, the Bi ions were substituted by the dopant Ag ions in the host lattice structure of Bi2WO6. With Ag nanoparticles modification, the Ag nanoparticles grew on the surface of the Bi2WO6 nanoparticles without changing the host lattice structure. The photocatalytic activities of the samples were studied by visible light irradiation of rhodamine B. Both the Ag doping and the Ag nanoparticles modification enhanced the photocatalytic activity of the Bi2WO6 nanoparticles. Moreover, Ag nanoparticles modification produced a more than two times higher enhancement of the photocatalytic activity than the Ag doping method. Furthermore, a mechanism of the significant photocatalytic activity enhancement by Ag nanoparticles modification is proposed. (C) 2020 Elsevier B.V. All rights reserved.
Photocatalytic activity enhancement of Bi2WO6 nanoparticles by Gd-doping via microwave assisted method
(2017-08-01)
Luc Huy Hoang
;
Nguyen Dang Phu
;
Pham Do Chung
;
Guo, Peng-Cheng
;
Chen, Xiang-Bai
Gd-doped Bi2WO6 nanoparticles were successfully prepared through microwave assisted synthesis. The incorporation of Gd3+ ions into Bi2WO6 crystallite was analyzed by X-ray diffraction, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The optical properties of the nanoparticles were characterized by UV-Vis absorption and photoluminescence spectroscopy. The photocatalytic activity of the nanoparticleswas investigated by degradation of Rhodamine B under visible-light irradiation. Our results showed that Gd-doping plays an important role for enhancing the visible light photocatalytic activity, and the enhanced photocatalytic activity of Gd-doped Bi2WO6 nanoparticles would be mainly correlated with the effective decrease of recombination rate of photogenerated electron-hole pairs.
Rashba spin splitting in parabolic quantum dots
(2006-06-01)
Lee, Johnson
;
Spector, Harold N.
;
Chou, Wu Ching
;
Chu, Chon Saar
;
電子物理學系
We have investigated the effect of the Rashba spin splitting and a magnetic field on the energy levels of electrons in parabolic quantum dots. We find that with an increase in the Rashba parameter, the spin-orbit interaction mixes states of higher angular momentum together. In the absence of a magnetic field, the energy levels of the electrons are doubly degenerate and decrease as the Rashba parameter increases. In the presence of a magnetic field, this degeneracy is removed and the energy splitting of the spin states increases with the increase in both the Rashba parameter and the magnetic field. The Fermi energy level as a function of the magnetic field shows oscillatory behavior due to the crossings between the energy levels of the system. The magnetization of the electron gas is investigated and shows strong oscillations with the magnetic field for large values of the Rashba parameter. (c) 2006 American Institute of Physics.
Study of photocatalytic activities of Bi2WO6 nanoparticles synthesized by fast microwave-assisted method
(2015-10-25)
Nguyen Dang Phu
;
Hoang, Luc Huy
;
Chen, Xiang-Bai
;
Kong, Meng-Hong
;
Wen, Hua-Chiang
We present a study of photocatalytic activities of Bi2WO6 nanoparticles synthesized by fast microwave-assisted method. The photocatalytic activities of the nanoparticles were evaluated by the decolorization of methylene-blue under visible-light-irradiation. Our results show that the surface area of Bi2WO6 nanoparticles plays a major role for improving photocatalytic activity, while visible-light absorption has only a weak effect on photocatalytic activity. This suggests efficient transportation of photo-generated electrons and holes to the oxidation active sites on the surface of nanoparticles, indicating Bi2WO6 nanoparticles synthesized by fast microwave-assisted method are promising for achieving high photocatalytic activity under visible-light-irradiation. (C) 2015 Elsevier B.V. All rights reserved.
Study of photocatalytic activities of Bi2WO6/BiVO4 nanocomposites
(2017-09-01)
Nguyen Dang Phu
;
Luc Huy Hoang
;
Guo, Peng-Cheng
;
Chen, Xiang-Bai
;
Chou, Wu Ching
The Bi2WO6/BiVO4 nanocomposites were successfully synthesized using fast microwave assisted method. The photocatalytic activities of Bi2WO6/BiVO4 nanocomposites were evaluated by the degradation of Rhodamine B under visible irradiation. The optimal BiVO4 content of 30% was observed for improving the photocatalytic activity of Bi2WO6/BiVO4 nanocomposites, and the mechanism of photocatalytic activity enhancement was investigated. Our results suggest that for the Bi2WO6/BiVO4 nanocomposites, particle surface area plays the most important role for improving photocatalytic activity, and electron-hole recombination rate plays more important role than amount of light absorbed by nanocomposites for improving photocatalytic activity. [GRAPHICS]
Synthesis and characterization of MnWO4 nanoparticles encapsulated in mesoporous silica SBA-15 by fast microwave-assisted method
(2015-02-01)
Luc Huy Hoang
;
Pham Van Hanh
;
Nguyen Dang Phu
;
Chen, Xiang-Bai
;
Chou, Wu Ching
The MnWO4 nanoparticles encapsulated in mesoporous silica (MnWO4/SBA-15) was successfully synthesized by a fast microwave-assisted method. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen absorption desorption isotherm, and Fourier transform infrared spectroscopy (FTIR). Our results showed that the MnWO4/SBA-15 nanocomposites have the ordered hexagonal meso-structure of SBA-15, indicating MnWO4 nanoparticles were successfully distributed into the channels of SBA-15. The size of MnW04 nanoparticles in SBA-15 is significantly smaller than the size of MnWO4 nanoparticles prepared without SBA-15, indicating that the MnWO4/SBA15 nanocomposites would be very promising for improving photocatalytic activity of MnWO4 nanoparticles. (C) 2014 Elsevier Ltd. All rights reserved.
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
;
Fu, Yanming
;
Lee, Wan-Yi
;
Lu, Ying-Rui
;
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.