Browsing by Author "Tsai, Kai-An"

Now showing 1 - 13 of 13

Au-Cd1-xZnxS core-alloyed shell nanocrystals: boosting the interfacial charge dynamics by adjusting the shell composition
(2018-09-28)
Pu, Ying-Chih
;
Chen, Wei-Ta
;
Fang, Mei-Jing
;
Chen, Yu-Lin
;
Tsai, Kai-An
Au-Cd1-xZnxS core-shell nanocrystals (NCs) with controllable shell compositions (from x = 0 to x = 0.37) were synthesized by using the hot-injection method. By increasing the mole fraction of ZnS in the Cd1-xZnxS shell, the conduction band of Cd1-xZnxS can be cathodically shifted to cause an increase of energy difference between the conduction band level of the Cd1-xZnxS shell and the Fermi level of the Au core, thereby enlarging the driving force of interfacial electron transfer to enhance the photoelectrochemical (PEC) efficiency. The interfacial charge dynamics of the samples were examined by time-resolved photoluminescence (TRPL) spectroscopy. The results showed that the interfacial electron transfer rate constant (k(et)) from the Cd1-xZnxS shell to the Au core was increased by 2 orders of magnitude, from 2.41 x 10(7) to 4.91 x 10(9) s(-1) as the mole fraction of ZnS increased from 0 to 0.37. Furthermore, PEC characterization, including Mott-Schottky analysis and photovoltage decay measurements, illustrated that gradually introducing ZnS into the shell composition of Au-Cd1-xZnxS NCs can modify the band structure and enhance the effectiveness of interfacial ET for advancing the PEC properties. The practical use of Au-Cd1-xZnxS NCs in PEC methanol oxidation was also demonstrated, revealing their promising potential as viable photoelectrodes for various PEC applications. The present study delivers an alternative approach of modulating the interfacial ET dynamics of core-shell MSNs by means of shell composition adjustment. The illustrations provide an empirical guideline to the intelligent design of core-shell metal-semiconductor nanoheterostructures (MSNs) for the desired PEC applications.
Complex Oxide-Noble Metal Conjugated Nanoparticles
(2013-04-11)
Guo, Jun-Ling
;
Chiou, Yao-De
;
Liang, Wen-I
;
Liu, Heng-Jui
;
Chen, Ying-Jiun
Hybrid nanoparticles (NPs) composed of multiple components offer new opportunities for next-generation materials. In this study, a paradigm for the noble metal/ternary complex oxide hybrid NPs is reported by adopting pulsed laser ablation in liquids. As model hybrids, gold-spinel heterodimer (Au-CoFe2O4) and gold-pervoskite heterodimer (Au-SrTiO3) NPs are investigated. This work has demonstrated the diverse playgroup of NP conjugation enlarged by complex oxides.
Flexible Heteroepitaxy Photoelectrode for Photo-electrochemical Water Splitting
(2018-08-01)
Le Thi Quynh
;
Chien Nguyen Van
;
Tzeng, W. Y.
;
Huang, Chun-Wei
;
Lai, Yu-Hong
MICAtronics represents a new research direction by taking the advantages of good mechanical flexibility and optical transparency of muscovite mica, leading to a new platform to use oxide heteroepitaxy for novel transparent soft technology. Here we report a model flexible photoelectrode for photo-electrochemical (PEC) water splitting based on Fe2O3/ZnO/mica heteroepitaxy. The heteroepitaxy was confirmed by a combination of X-ray diffraction and transmission electron microscopy. The PEC performance of this flexible and semitransparent photoelectrode under various bending states was investigated. We found that the photocurrent of the heteroepitaxial system was enhanced by three times relative to pure ZnO and Fe2O3 under visible-light irradiation, and the heteroepitaxial photoelectrodes retain their photocurrent after continuous bending in cycling (>3000 cycles) with a smallest bending radius of 3.5 mm. The energy band alignment and charge dynamics under light excitation were characterized to understand the mechanism of the enhanced PEC performance. This study provides a new platform for design and fabrication of a flexible transparent photoelectrode in the harvest of solar energy based on oxide heteroepitaxy.
Graphene quantum dots mediated charge transfer of CdSe nanocrystals for enhancing photoelectrochemical hydrogen production
(2015-03-01)
Tsai, Kai-An
;
Hsu, Yung-Jung
;
材料科學與工程學系
;
Department of Materials Science and Engineering
We demonstrated the use of CdSe/graphene quantum dot (QD) nanoheterostructures as the photoanode for remarkable photoelectrochemical hydrogen production. By employing a delicate hydrothermal cutting approach, reduced graphene oxide (RGO) sheets with the lateral size in a desirable range can be obtained, from micrometer size (micro-RGO), to 30-100 nm (nano-RGO), and to 2-4 nm (QD-RGO). Because of the significant zigzag edge effect, nano-RGO and QD-RGO possessed well-defined band structure which enabled efficient light absorption and distinctive photoluminescence emission. Time-resolved photoluminescence spectra showed that nano-RGO and QD-RGO surpassed micro-RGO in enhancing the charge separation efficiency of CdSe. According to the cyclic voltammetry data, a type-II vectorial charge transfer model was considered for CdSe/nano-RGO and CdSe/QD-RGO nanoheterostructures, fundamentally different from the unidirectional electron transfer mechanism of CdSe/micro-RGO. Among the three CdSe/RGO samples tested, CdSe/QD-RGO achieved the highest photocurrent generation in the photoelectrochemical cell, which exceeded 5 times the value of CdSe. The incident photon-to-electron conversion efficiency (IPCE) spectra suggested that the significantly enhanced photoactivity of CdSe/QD-RGO originated from the type-II vectorial charge transfer feature, which not only promoted charge carrier separation but also improved the overall light harvesting. Furthermore, no appreciable decay of photocurrent was found for CdSe/QD-RGO after continuously used in the photoelectrochemical cell for over 2 h, revealing its substantially high stability during the water reduction process. The demonstrations from this work may facilitate the use of graphene QDs in semiconductor-based photocatalysis, in which the efficient light harvesting and high chemical inertness of graphene QDs can be well employed. (C) 2014 Elsevier B.V. All rights reserved.
Heteroepitaxial approach to explore charge dynamics across Au/BiVO4 interface for photoactivity enhancement
(2015-07-01)
Chien Nguyen Van
;
Chang, Wei Sea
;
Chen, Jhih-Wei
;
Tsai, Kai-An
;
Tzeng, Wen-Yen
Heterostructure provides a powerful route in manipulating electrical transport, optical response, electrolytic water splitting and water treatment of complex oxides. As a model for noble metal/ complex oxide heterostructures, we have successfully prepared Au/BiVO4 (BVO) heterostructures in which the Au nanoparticles (NPs) with various sizes and densities were uniformly deposited on the {001} facets of epitaxial BVO thin films. The heterostructures exhibit significantly enhanced photoactivities in both dye degradation and electrolytic water splitting. By employing X-ray photoelectron spectroscopy, the energy band alignment of Au/BVO heterojunction suggests a charge separation at their interfaces, that can manipulate the photoexcited electron-hole pairs and photocatalytic efficiency of the heterostructures. Photogenerated carrier injection, which mainly affects the photoactivity of photocatalysis, was detected across Au/BVO interfaces by ultrafast dynamics spectroscopy. This study delivers a general approach to probe and understand the photochemistry of noble metal-complex oxide heterostructures for photoconversion applications. (C) 2015 Elsevier Ltd. All rights reserved.
High-quality Quantum-Dot-Based Full-Color Display Technology by Pulsed Spray Method
(2013)
Chen, Kuo-Ju
;
Chen, Hsin-Chu
;
Tsai, Kai-An
;
Lin, Chien-Chung
;
Tsai, Hsin-Han
We fabricated the colloidal quantum-dot light-emitting diodes (QDLEDs) with the HfO2/SiO2-distributed Bragg reflector (DBR) structure using a pulsed spray coating method. Moreover, pixelated RGB arrays, 2-in. wafer-scale white light emission, and an integrated small footprint white light device were demonstrated. The experimental results showed that the intensity of red, blue, and green (RGB) emissions exhibited considerable enhancement because of the high reflectivity in the UV region by the DBR structure, which subsequently increased the use in the UV optical pumping of RGB QDs. In this experiment, a pulsed spray coating method was crucial in providing uniform RGB layers, and the polydimethylsiloxane (PDMS) film was used as the interface layer between each RGB color to avoid cross-contamination and self-assembly of QDs. Furthermore, the chromaticity coordinates of QDLEDs with the DBR structure remained constant under various pumping powers in the large area sample, whereas a larger shift toward high color temperatures was observed in the integrated device. The resulting color gamut of the proposed QDLEDs covered an area 1.2 times larger than that of the NTSC standard, which is favorable for the next generation of high-quality display technology.
Nitrogen-Doped Graphene Quantum Dots for Remarkable Solar Hydrogen Production
(2020-06-22)
Tsai, Kai-An
;
Hsieh, Ping-Yen
;
Lai, Ting-Hsuan
;
Tsao, Chun-Wen
;
Pan, Hui
We synthesized nitrogen (N)-doped graphene quantum dots (N-GQDs) using a top-down hydrothermal cutting approach. The concentration of N dopants was readily controlled by adjusting the concentration of the N source of urea. When N dopants were incorporated into GQDs, visible absorption was induced by C-N bonds, which created another pathway for generating photoluminescence (PL). Time-resolved PL data revealed that the carrier lifetime of GQDs was increased upon doping with the optimized N concentration. The photoelectrochemical properties of N-GQDs toward water splitting were studied, and the results showed that 2N-GQDs prepared with 2 g of urea produced the highest photocurrent. The photocatalytic activity of 2N-GQDs powder photocatalyst for hydrogen production was also examined under AM 1.5G illumination, showing substantial enhancement over that of pristine GQDs. Electrochemical impedance spectroscopy data further revealed a significant improvement in charge dynamics and reaction kinetics and an increased carrier concentration as a result of N doping.
Resonant-Enhanced Full-Color Emission of Quantum-Dot-Based Display Technology Using a Pulsed Spray Method
(2012-12-19)
Chen, Kuo-Ju
;
Chen, Hsin-Chu
;
Tsai, Kai-An
;
Lin, Chien-Chung
;
Tsai, Hsin-Han
Colloidal quantum-dot light-emitting diodes (QDLEDs) with the HfO2/SiO2-distributed Bragg reflector (DBR) structure are fabricated using a pulsed spray coating method. Pixelated RGB arrays, 2-in. wafer-scale white light emission, and an integrated small footprint white light device are demonstrated. The experimental results show that the intensity of red, green, and blue (RGB) emission exhibited considerable enhancement because of the high reflectivity in the UV region by the DBR structure, which subsequently increases the use in the UV optical pumping of RGB QDs. A pulsed spray coating method is crucial in providing uniform RGB layers, and the polydimethylsiloxane (PDMS) film is used as the interface layer between each RGB color to avoid cross-contamination and self-assembly of QDs. Furthermore, the chromaticity coordinates of QDLEDs with the DBR structure remain constant under various pumping powers in the large area sample, whereas a larger shift toward high color temperatures is observed in the integrated device. The resulting color gamut of the proposed QDLEDs covers an area 1.2 times larger than that of the NTSC standard, which is favorable for the next generation of high-quality display technology.
Self-Assembled BiFeO3-epsilon-Fe2O3 Vertical Heteroepitaxy for Visible Light Photoelectrochemistry
(2016-09-21)
Le Thi Quynh
;
Chien Nguyen Van
;
Bitla, Yugandhar
;
Chen, Jhih-Wei
;
Thi Hien Do
Self-assembled vertical heterostructure with a high interface-to-volume ratio offers tremendous opportunities to realize intriguing properties and advanced modulation of functionalities. Here, a heterostructure composed of two visible-light photocatalysts, BiFeO3 (BFO) and epsilon-Fe2O3 (epsilon-FO), is designed to investigate its photoelectrochemical performance. The structural characterization of the BFO-FO heterostructures confirms the phase separation with BFO nanopillars embedded in the epsilon-FO matrix. The investigation of band structure of the heterojunction suggests the assistance of photoexcited carrier separation, leading to an enhanced photoelectrochemical performance. The insights into the charge separation are further revealed by means of ultrafast dynamics and electrochemical impedance spectroscopies. This work shows a delicate design of the self-assembled vertical heteroepitaxy by taking advantage of the intimate contact between two phases that can lead to a tunable charge interaction, providing a new confi guration for the optimization of photoelectrochemical cell.
Superior photoelectrochemical activity of self-assembled NiWO4-WO3 heteroepitaxy
(2016-05)
Thi Hien Do
;
Chien Nguyen Van
;
Tsai, Kai-An
;
Le Thi Quynh
;
Chen, Jhih-Wei
Self-assembled nanocomposites with a high interface-to-volume ratio offer tremendous opportunities of tailoring functionalities by a proper combination of components. In this study, we report the first fabrication of self-assembled NiWO4-WO3 heteroepitaxy and its excellent performance on photoel-ectrochemical water splitting. The heteroepitaxy is evidenced by a combination of x-ray diffraction and transmission electron microscopy. The energy band diagram of NiWO4-WO3 heterostructure probed by x-ray photoelectron spectroscopy suggests an efficient charge separation at their interface by injecting photo -generated electrons from NiWO4 to WO3 and holes from WO3 to NiWO4. Therefore, a great improvement in the photoelectrochemical activity of self -assembled NiWO4-WO3 heteroepitaxy was achieved (1400 mu A cm(-2) at 0.7 V vs. Ag/AgCl). This work delivers a new category of composite materials for promising photoelectrode in photoelectrochemical cells. (C) 2016 Elsevier Ltd. All rights reserved.
Tailor magnetic order and spin-polarized gap states of opto-spintronic compounds by carrier mediation
(2018-08-15)
Yang, Chao-Yao
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Chiu, Yi-Hsuan
;
Tsai, Kai-An
;
Hsu, Hua-Shu
;
Lin, Jun-Xiao
We investigate the role of carriers in the presence of oxygen vacancies (Vo) in mediating the ferromagnetic order and spin-polarized gap states of Cu-doped ZnO nanoparticles. We demonstrate the tuning of magnetic order via post-annealing under Ar/O-2. Carrier (Vo)-rich condition (Ar-treatment) is found to favor higher magnetic order of the nanoparticles with enhanced thermal stability and wide spin-polarized gap states from ultra-violet (UV) to visible energy range. Conversely, carrier (Vo)-poor condition (O-2-treatment) reduces resistance to thermal fluctuations and quenched spin-polarization due to lack of carrier mediation. The proposed method is effective to tailor the magnetic order as well as the wide polarized band-gap states, of oxide opto-spintronic compounds by finding the physical equivalency between charge carriers and spin electrons. (C) 2018 Elsevier B.V. All rights reserved.
Tunable photoelectrochemical performance of Au/BiFeO3 heterostructure
(2016)
Huang, Yen-Lin
;
Chang, Wei Sea
;
Chien Nguyen Van
;
Liu, Heng-Jui
;
Tsai, Kai-An
Ferroelectric photoelectrodes, other than conventional semiconductors, are alternative photo-absorbers in the process of water splitting. However, the capture of photons and efficient transfer of photo-excited carriers remain as two critical issues in ferroelectric photoelectrodes. In this work, we overcome the aforementioned issues by decorating the ferroelectric BiFeO3 (BFO) surface with Au nanocrystals, and thus improving the photoelectrochemical (PEC) performance of BFO film. We demonstrate that the internal field induced by the spontaneous polarization of BFO can (1) tune the efficiency of the photo-excited carriers\' separation and charge transfer characteristics in bare BFO photoelectrodes, and (2) modulate an extra optical absorption within the visible light region, created by the surface plasmon resonance excitation of Au nanocrystals to capture more photons in the Au/BFO heterostructure. This study provides key insights for understanding the tunable features of PEC performance, composed of the heterostructure of noble metals and ferroelectric materials.
WO3 mesocrystal-assisted photoelectrochemical activity of BiVO4
(2017-03-01)
Chien Nguyen Van
;
Thi Hien Do
;
Chen, Jhih-Wei
;
Tzeng, Wen-Yen
;
Tsai, Kai-An
Self- assembled nanocomposites have gained much attention over the past decade due to their intriguing properties and functionalities. In this work, we developed a self-assembled nanocomposite photoanode composed of an epitaxial BiVO4 matrix embedded with WO3 mesocrystals for photoelectrochemical (PEC) applications in the visible- light regime. The orientation of the crystal facet and interface provides a superior template to understand the intimate contact between the two constituent phases. We demonstrate that the interfacial coupling of the mesocrystal and matrix improves the separation of photoexcited carriers and the properties of charge transfer, resulting in a greatly enhanced PEC performance compared with their parent compounds. The current study demonstrates that the utilization of the interface-to-volume ratio to optimize charge interactions in the nanocomposite is essential for the advanced design of novel mesocrystal-embedded nanocomposite photoelectrodes.