Browsing by Author "Chen, Hao Ming"

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Heterojunction of Zinc Blende/Wurtzite in Zn1-xCdxS Solid Solution for Efficient Solar Hydrogen Generation: X-ray Absorption/Diffraction Approaches
(2015-10-14)
Hsu, Ying-Ya
;
Suen, Nian-Tzu
;
Chang, Chung-Chieh
;
Hung, Sung-Fu
;
Chen, Chi-Liang
In the past decade, inorganic semiconductors have been successfully demonstrated as light absorbers in efficient solar water splitting to generate chemical fuels. Pseudobinary semiconductors Zn1-xCdxS (0 <= x <= 1) have exhibited a superior photocatalytic reactivity of H-2 production from splitting of water by artificial solar irradiation without any metal catalysts. However, most studies had revealed that the extremely high efficiency with an optimal content of Zn1-xCdxS solid solution was determined as a result of elevating the conduction band minimum (CBM) and the width of bandgap. In addition to corresponding band structure and bandgap, the local crystal structure should be taken into account as well to determine its photocatalytic performance. Herein, we demonstrated the correlations between the photocatalytic activity and structural properties that were first studied through synchrotron X-ray diffraction and X-ray absorption spectroscopy. The crystal structure transformed from zinc blende to coexisted phases of major zinc blende and minor wurtzite phases at a critical point. The heterojunction formed by coexistence of zinc blende and wurtzite phases in the Zn1-xCdxS solid solution can significantly improve the separation and migration of photoinduced electron hole pairs. Besides, X-ray absorption spectra and UV-vis spectra revealed that the bandgap of the Zn0.45Cd0.55S sample extended into the region of visible light because of the incorporation of Cd element in the sample. These results provided a significant progress toward the realization of the photoelectrochemical mechanism in heterojunction between zinc blende and wurtzite phases, which can effectively separate the charge-carriers and further suppress their recombination to enhance the photocatalytic reactivity.
Heterojunction of zinc Blende/Wurtzite in Zn1-xCdxS solid solution for efficient solar hydrogen generation: X-ray absorption/diffraction spectroscopy approach
(2016-03-13)
Hsu, Ying-Ya
;
Suen, Nian-Tzu
;
Hong, Song-Fu
;
Chan, Ting-Shan
;
Chen, San-Yuan
Plasmon Inducing Effects for Enhanced Photoelectrochemical Water Splitting: X-ray Absorption Approach to Electronic Structures
(2012-08-01)
Chen, Hao Ming
;
Chen, Chih Kai
;
Chen, Chih-Jung
;
Cheng, Liang-Chien
;
Wu, Pin Chieh
Artificial photosynthesis using semiconductors has been investigated for more than three decades for the purpose of transferring solar energy into chemical fuels. Numerous studies have revealed that the introduction of plasmonic materials into photochemical reaction can substantially enhance the photo response to the solar splitting of water. Until recently, few systematic studies have provided clear evidence concerning how plasmon excitation and which factor dominates the solar splitting of water in photovoltaic devices. This work demonstrates the effects of plasmons upon an Au nanostructure-ZnO nanorods array as a photoanode. Several strategies have been successfully adopted to reveal the mutually independent contributions of various plasmonic effects under solar irradiation. These have clarified that the coupling of hot electrons that are formed by plasmons and the electromagnetic field can effectively increase the probability of a photochemical reaction in the splitting of water. These findings support a new approach to investigating localized plasmon-induced effects and charge separation in photoelectrochemical processes, and solar water splitting was used herein as platform to explore mechanisms of enhancement of surface plasmon resonance.
Reversible adapting layer produces robust single-crystal electrocatalyst for oxygen evolution
(2015-08-01)
Tung, Ching-Wei
;
Hsu, Ying-Ya
;
Shen, Yen-Ping
;
Zheng, Yixin
;
Chan, Ting-Shan
Electrochemically converting water into oxygen/hydrogen gas is ideal for high-density renewable energy storage in which robust electrocatalysts for efficient oxygen evolution play crucial roles. To date, however, electrocatalysts with long-term stability have remained elusive. Here we report that single-crystal Co3O4 nanocube underlay with a thin CoO layer results in a high-performance and high-stability electrocatalyst in oxygen evolution reaction. An in situ X-ray diffraction method is developed to observe a strong correlation between the initialization of the oxygen evolution and the formation of active metal oxyhydroxide phase. The lattice of skin layer adapts to the structure of the active phase, which enables a reversible facile structural change that facilitates the chemical reactions without breaking the scaffold of the electrocatalysts. The single-crystal nanocube electrode exhibits stable, continuous oxygen evolution for 41,000 h. This robust stability is attributed to the complementary nature of defect-free single-crystal electrocatalyst and the reversible adapting layer.
The synergistic effect of a well-defined Au@Pt core-shell nanostructure toward photocatalytic hydrogen generation: interface engineering to improve the Schottky barrier and hydrogen-evolved kinetics
(2016)
Hung, Sung-Fu
;
Yu, Ya-Chu
;
Suen, Nian-Tzu
;
Tzeng, Guan-Quan
;
Tung, Ching-Wei
A well-defined co-catalyst system TiO2 nanotube-Au (core)-Pt (shell) was demonstrated to be the combination of the localized surface plasmon effect of gold and excellent proton reduction nature of platinum. Furthermore, surface engineering by the descending Fermi energies of gold and platinum was beneficial to electron transfer.
Valence- and element-dependent water oxidation behaviors: in situ X-ray diffraction, absorption and electrochemical impedance spectroscopies
(2017-04-07)
Hsu, Chia-Shuo
;
Suen, Nian-Tzu
;
Hsu, Ying-Ya
;
Lin, Hsuan-Yu
;
Tung, Ching-Wei
Metal oxides of the spinel family have shown great potential towards the oxygen evolution reaction (OER), but the fundamental OER mechanism of spinel oxides is still far from being completely understood, especially for the role of the metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface conditions (morphology and defects), it is a great challenge to have a fair assessment of the electrocatalytic performance of spinel systems. Herein, we demonstrated a series of MFe2O4 (M = Fe, Co, Ni, Zn) with a well-controlled morphology to achieve a comprehensive study of electrocatalytic activity toward OER. By utilizing several in situ analyses, we could conclude a universal rule that the activities for OER in the metal oxide systems were determined by the occurrence of a phase transformation, and this structural transformation could work well in both crystallographic sites (Td and Oh sites). Additionally, the divalent metal ion significantly dominated the formation of oxyhydroxide through an epitaxial relationship, which depended on the atomic arrangement at the interface of spinel and metal oxyhydroxide, while trivalent metal ions remained unchanged as a host lattice. The metal oxyhydroxide was formed during a redox reaction rather than being formed during OER. The occurrence of the redox reaction seems to accompany a remarkable increase in resistance and capacitance might result from the structural transformation from spinel to metal oxyhydroxide. We believe that the approaching strategies and information obtained in the present study can offer a guide to designing a promising electrocatalytic system towards the oxygen evolution reaction and other fields.