Browsing by Author "Shen, Shaohua"

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A novel hybrid artificial photosynthesis system using MoS2 embedded in carbon nanofibers as electron relay and hydrogen evolution catalyst
(2017-08-01)
Niu, Fujun
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Dong, Chung-Li
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Zhu, Changbao
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Huang, Yu-Cheng
;
Wang, Miao
Inspired by photosynthesis in nature, artificial photosynthesis (AP) systems have been widely investigated in the context of energy and environmental research. Here we report a noble-metal-free AP system for visible-light-driven H-2 generation from aqueous solutions consisting of fluorescein (FL) as photosensitizer, single-layer ultrasmall MoS2 nanoplates embedded in carbon nanofibers (CNF) as electron relay and redox catalyst, and triethanolamine (TEOA) as sacrificial electron donor. This CNF-MoS2/FL system exhibits outstanding H-2 evolution performance, with an H-2 generation rate that exceeds not only both MoS2/FL (by 100%) and CNF/FL (by 1100%), but also the Pt/FL system (by 40%). The excellent photocatalytic activity of this CNF-MoS2/FL system can be ascribed to the synergistic effects of CNF and MoS2 coupling: (1) the simultaneous presence of MoS2 with its delocalized and increased Mo 4d unoccupied states and of CNF with increased graphitic characteristics enables electron transfer from FL* to MoS2 via CNF electron relay; (2) the single-layered ultrasmall MoS2 nanoplates with short effective lengths for electron transfer and high density of reactive S-edges effectively catalyze the H-2 evolution reaction (HER). The presented work successfully fabricated a highly efficient AP system for solar H-2 production from a fully aqueous solution and indicated CNF-MoS2 as a promising candidate to replace Pt for solar fuel conversion. (C) 2017 Elsevier Inc. All rights reserved.
Bifunctional cobalt phosphide nanoparticles with convertible surface structure for efficient electrocatalytic water splitting in alkaline solution
(2019-03-01)
Wang, Miao
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Dong, Chung-Li
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Huang, Yu-Cheng
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Shen, Shaohua
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電子物理學系
Highly efficient non-noble materials for water splitting are essential for renewable energy application. Herein, cobalt phosphide nanoparticles with high P/Co ratio are synthesized and display overpotentials of 348 mV (or 343 mV) for oxygen evolution reaction (OER) with surface oxidized active species formed and 200 mV (or 184 mV) for hydrogen evolution reaction (HER) in 1 M NaOH (or 1 M KOH) at 10 mA cm(-2). The improved OER could be ascribed to the smaller particle size for increased active site number and more importantly the improved intrinsic activity probably assisted by more phosphates formed during electrocatalytic reaction; while the improved HER performance is mainly attributed to the increased active sites in the form of Co-P bonds. In a two-electrode configuration, the catalysts exhibit excellent overall water splitting performance, with 10 mA cm(-2) at 1.58 V and a long-time reaction stability without attenuation during 10 h operation. (C) 2019 Elsevier Inc. All rights reserved.
Electronic Structure Evolution in Tricomponent Metal Phosphides with Reduced Activation Energy for Efficient Electrocatalytic Oxygen Evolution
(2018-08-29)
Wang, Miao
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Dong, Chung-Li
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Huang, Yu-Cheng
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Li, Yanrui
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Shen, Shaohua
Non-noble metal catalysts for high-active electrocatalytic oxygen evolution reaction (OER) are essential in large-scale application for water splitting. Herein, tricomponent metal phosphides with hollow structures are synthesized from cobalt-contained metal organic frameworks (MOFs), i.e., ZIF-67, by tailoring the feeding ratios of Ni and Fe, followed by a high-temperature reduction and a subsequent phosphidation process. Excellent OER activity and long-time stability are achieved in 1 M NaOH aqueous solution, with an overpotential of 329 mV at 10 mA cm(-2) and Tafel slope of 48.2 mV dec(-1), even superior to the noble metal-based catalyst. It is evidenced that the formed (oxyhydr)oxide/phosphate species by in situ electrochemical surface oxidation are responsible for active OER. Accordingly, the simultaneous introduction of external Ni and Fe elements significantly influences the electronic structures of the parent metal phosphides, leading to the in situ electrochemical formation of surface active layer with decreased OER activation energy for greatly improved water oxidation performance. This electronic structure tuning strategy by introducing multicomponent metals demonstrates a versatile method to use MOFs as precursors for synthesizing high-efficient water splitting electrocatalysts.
Electronic structures associated with enhanced photocatalytic activity in nanogap-engineered g-C3N4/Ag@SiO2 hybrid nanostructures
(2020-06-01)
Huang, Yu-Cheng
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Arul, K. Thanigai
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Chen, Chi-Liang
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Chen, Jeng-Lung
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Chen, Jie
Graphitic carbon nitride (g-C3N4) has attracted considerable attention with regard to its use in photocatalytic solar hydrogen production by the splitting of water. High charge carrier recombination critically limits the photocatalytic activity of g-C3N4. Plasmonic metal nanoparticles that can generate localized surface plasmon resonance (LSPR) have been suggested to enhance the harvesting of visible light and to improve water splitting efficiency. However, direct contact between metal nanoparticles and g-C3N4 reduces the hydrogen generation efficiency owing to energy loss by Forster resonance energy transfer (FRET), which competes with plasmon resonance energy transfer (PRET). Decorating g-C3N4 with Ag@SiO2 core-shell plasmonic nanoparticles increases its photocatalytic ability. Tuning the size of the SiO2 nanogap can optimize the photocatalytic performance of g-C3N4/Ag@SiO2, which involves a trade-off between PRET and FRET. X-ray absorption spectroscopy (XAS) is utilized to investigate the electronic structure of g-C3N4 and its modulation with Ag@SiO2. In situ XAS reveals the dynamics of the charge carriers under solar illumination. Analytic results suggest charge redistribution, shifting of the conduction band, modification of the unoccupied states, and consequent improvement in photocatalytic activity by solar illumination. This work sheds light on the effect of LSPR on this photocatalyst with reference to its electronic structure.
In situ evolution of highly dispersed amorphous CoOx clusters for oxygen evolution reaction
(2017-09-07)
Chen, Dawei
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Dong, Chung-Li
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Zou, Yuqin
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Su, Dong
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Huang, Yu-Cheng
Electrocatalytic water splitting is a key technique to produce hydrogen fuels, which can be considered as an efficient strategy to store renewable energy. Oxygen evolution reaction (OER) that occurs at the anode side requires a four-electron transfer under highly oxidizing conditions. OER has a large overpotential and therefore determines the overall efficiency. Certain electrocatalysts can efficiently help to improve the reaction kinetics. Owing to the high cost of precious metals such as Pt, Ru, and Ir, non-precious metal oxide catalysts have been vigorously investigated under alkaline conditions. Herein, we synthesized novel highly dispersed amorphous CoOx for the first time in the form of a cluster favorable to enhance the OER activity using a facile method via the air dielectric barrier discharge (DBD) plasma. Compared with the pristine biopolymer-cobalt complex, the amorphous CoOx cluster exhibits a much higher current density and a lower overpotential for OER, e.g., the overpotential of 290 mV at 10 mA cm(-2) and the overpotential of only 350 mV at 300 mA cm(-1). The excellent electrocatalytic OER activity was attributed to the unsaturated catalytic sites on the amorphous CoOx cluster. In addition, we studied the reaction mechanism, and it was observed that pure O-2 DBD plasma could lead to the evolution of crystalline CoOx; however, the presence of N-2 and O-2 in DBD plasma could ensure the facile evolution of amorphous CoOx clusters. This study provides a new strategy to design amorphous materials for electrocatalysis and beyond.
Interlayer interaction in ultrathin nanosheets of graphitic carbon nitride for efficient photocatalytic hydrogen evolution
(2017-08-01)
Zhao, Daming
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Chen, Jie
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Dong, Chung-Li
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Zhou, Wu
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Huang, Yu-Cheng
Two-dimensional graphitic carbon nitride (g-C3N4) nanosheets (CNNS) have attracted intense interest in photocatalysis, given their small thickness and high specific surface area favoring charge transfer and surface reactions. Herein, a facile strategy of breaking and following repolymerizing the heptazine units in bulk g-C3N4 (BCN) is developed to synthesize ultrathin CNNS with thickness of 1 nm in relatively high product yield (similar to 24%). The as-prepared 1 nm-thick CNNS show significantly enhanced photocatalytic performance for hydrogen evolution than BCN and even the 3 nm-thick CNNS acquired by thermal oxidation etching of BCN. It is evidenced that the disordered layer structure of the obtained ultrathin CNNS causes strong interlayer C-N interaction, tunneling electron transport between the C-N layers. Meanwhile, the broken in-plane C-N bonds create more unsaturated N sites in the 1 nm-thick CNNS, facilitating the electron excitation from the occupied states in g-C3N4 to its unoccupied states for water reduction reaction. (C) 2017 Elsevier Inc. All rights reserved.
Nb-Doped Hematite Nanorods for Efficient Solar Water Splitting: Electronic Structure Evolution versus Morphology Alteration
(2016-07)
Fu, Yanming
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Dong, Chung-Li
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Lee, Wan-Yi
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Chen, Jie
;
Guo, Penghui
In this study, Nb-doped alpha-Fe2O3 nanorod photoanodes were prepared via a facile solution-based process by directly adding NbCl5 in the aqueous solution. The morphology and electronic structure of Nb-doped alpha-Fe2O3 films depended strongly on the dopant concentrations. Compared with the undoped sample, the optimal Nb-doped alpha-Fe2O3 film showed an approximately 4-fold photocurrent increase under solar light at 1.0 V versus Ag/AgCl, and the incident photon-to-current conversion efficiency was increased by 2.5 times, reaching 13.7% at 350 nm and 1.23 V versus RHE. The enhancement in PEC activity, as induced by moderate Nb doping, was attributed to the increased charge carrier density for promoted charge transfer ability as well as the smaller nanorod diameter for shortened charge transfer distance. However, superfluous Nb dopants would destroy the nanorod structure and greatly reduce the thickness of alpha-Fe2O3 films, leading to poor optical absorption and thus decreased photoelectrochemical performance.
Operando Spectral and Electrochemical Investigation into the Heterophase Stimulated Active Species Transformation in Transition-Metal Sulfides for Efficient Electrocatalytic Oxygen Evolution
(2020-02-07)
Wang, Miao
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Dong, Chung-Li
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Huang, Yu-Cheng
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Shen, Shaohua
;
交大名義發表
In-depth understanding of electrocatalytically active species transformation during oxygen evolution reaction (OER) is highly guidable to design effective electrocatalysts for water splitting. Herein, cobalt sulfide (CoSx) nanovesicles decorated with iron sulfide (FeSx) heterophases were successfully synthesized by using a metal-organic framework precursor through a solvothermal method. The obtained CoSx/FeSx exhibited much elevated OER activity as compared to pristine CoSx and even most previously reported cobalt-based electrocatalysts, with the overpotential of 304 mV at 10 mA cm(-2) in 1 M KOH. As revealed by operando Raman and X-ray absorption spectroscopy measurements, the electrocatalytically active species transformation in CoSx nanovesicles could be stimulated by the incorporated FeSx heterophases due to the increased average valence state of cobalt and lowered coordination of cobalt sites. As a result, the electrocatalytically active cobalt oxyhydroxides could be more easily formed on the catalyst surface during OER and thus contributed to the highly improved performance. This real-time spectral and electrochemical insight into the electrocatalytically active species transformation may guide the rational design of highly efficient electrocatalysts for water splitting from the viewpoint of electronic structure-electrocatalytic activity relationship.
Synergy of Dopants and Defects in Graphitic Carbon Nitride with Exceptionally Modulated Band Structures for Efficient Photocatalytic Oxygen Evolution
(1970-01-01)
Zhao, Doming
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Dong, Chung-Li
;
Bin Wang
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Chen, Chao
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Huang, Yu-Cheng
Electronic structure greatly determines the band structures and the charge carrier transport properties of semiconducting photocatalysts and consequently their photocatalytic activities. Here, by simply calcining the mixture of graphitic carbon nitride (g-C3N4) and sodium borohydride in an inert atmosphere, boron dopants and nitrogen defects are simultaneously introduced into g-C3N4. The resultant boron-doped and nitrogen-deficient g-C3N4 exhibits excellent activity for photocatalytic oxygen evolution, with highest oxygen evolution rate reaching 561.2 mu mol h(-1) g(-1), much higher than previously reported g-C3N4. It is well evidenced that with conduction and valence band positions substantially and continuously tuned by the simultaneous introduction of boron dopants and nitrogen defects into g-C3N4, the band structures are exceptionally modulated for both effective optical absorption in visible light and much increased driving force for water oxidation. Moreover, the engineered electronic structure creates abundant unsaturated sites and induces strong interlayer C-N interaction, leading to efficient electron excitation and accelerated charge transport. In the present work, a facile approach is successfully demonstrated to engineer the electronic structures and the band structures of g-C3N4 with simultaneous introduction of dopants and defects for high-performance photocatalytic oxygen evolution, which can provide informative principles for the design of efficient photocatalysis systems for solar energy conversion.
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.