Browsing by Author "Chien Nguyen Van"

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Energy Band Gap Modulation in Nd-Doped BiFeO3/SrRuO3 Heteroepitaxy for Visible Light Photoelectrochemical Activity
(2019-01-09)
Tan, Kok Hong
;
Chen, Yun-Wen
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Chien Nguyen Van
;
Wang, Hongliang
;
Chen, Jhih-Wei
The ability of band offsets at multiferroic/metal and multiferroic/electrolyte interfaces in controlling charge transfer and thus altering the photoactivity performance has sparked significant attention in solar energy conversion applications. Here, we demonstrate that the band offsets of the two interfaces play the key role in determining charge transport direction in a downward self-polarized BFO film. Electrons tend to move to BFO/electrolyte interface for water reduction. Our experimental and first-principle calculations reveal that the presence of neodymium (Nd) dopants in BFO enhances the photoelectrochemical performance by reduction of the local electron hole pair recombination sites and modulation of the band gap to improve the visible light absorption. This opens a promising route to the heterostructure design by modulating the band gap to promote efficient charge transfer.
Flexible Heteroepitaxy Photoelectrode for Photo-electrochemical Water Splitting
(2018-08-01)
Le Thi Quynh
;
Chien Nguyen Van
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Tzeng, W. Y.
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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.
Heteroepitaxial approach to explore charge dynamics across Au/BiVO4 interface for photoactivity enhancement
(2015-07-01)
Chien Nguyen Van
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Chang, Wei Sea
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Chen, Jhih-Wei
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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.
Microstructure evolution determined by the crystalline phases competition in self-assembled WO3-BiVO4 hetero nanostructures
(2018-02-28)
Song, Haili
;
Li, Chao
;
Chien Nguyen Van
;
Dong, Wenxia
;
Qi, Ruijuan
A series of self-assembled WO3-BiVO4 nanostructured thin films were grown on the (001) yttria-stabilized zirconia (YSZ) substrate at the substrate temperatures of 400 degrees C, 500 degrees C, 550 degrees C, 600 degrees C, 650 degrees C and 700 degrees C by a pulsed laser deposition method. The microstructures including crystalline phases, epitaxial relationships, surface morphologies and interface structures were investigated by a combination of x-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The sample grown at 400 degrees C was amorphous due to the low driving forces for nucleation and diffusion. For the samples made at 500 degrees C, 550 degrees C and 600 degrees C, the monoclinic BiVO4 matrix epitaxially grew on YSZ, forming the matrix, where the WO3 nanopillars were embedded in with a specific orientation relationship among BiVO4, WO3 and YSZ. However, in thin films deposited at 650 degrees C and 700 degrees C, the WO3 grains randomly grew on the YSZ substrate, which dominated the microstructures of the resultant thin films. Quantitative analyses of the microstructures revealed that the lateral grain sizes of BiVO4 and WO3 increased and the volume fraction of BiVO4 in the thin films decreased with the increase of the deposition temperature. A three-regime growth mechanism of the WO3-BiVO4 composite thin film was proposed based on the growth dynamics determined by the competition between BiVO4 and WO3. Published by AIP Publishing.
Microstructure evolution with composition ratio in self-assembled WO3-BiVO4 hetero nanostructures for water splitting
(2017-07-01)
Song, Haili
;
Li, Chao
;
Chien Nguyen Van
;
Liu, Heng-Jui
;
Qi, Ruijuan
A series of self-assembled WO3-BiVO4 nanostructured thin films with 17, 25, 50, 67, and 100 mol% WO3 were grown on the (001) yttria-stabilized zirconia (YSZ) substrate by pulsed laser deposition method. The microstructures including crystalline phases, epitaxial relationship, interface structures, and chemical composition distributions were investigated by a combination of various electron microscopy techniques including scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectroscopy. The monoclinic BiVO4 formed the matrix, in which WO3 nanopillars were embedded with specific epitaxial relationships. In BiVO4-rich sample, orthorhombic Bi2WO6 was formed. However, metastable hexagonal WO3 phase and orthorhombic WO3 phase coexisted in other composite samples. The thin amorphous layer at the film/substrate interface indicated that the mismatch strain between films and substrate is released. The hydrostatic tensile strain due to thermal expansion mismatch between BiVO4 and WO3 as well as the diffusion of Bi into the WO3 stabilized the metastable h-WO3. A WO3-BiVO4 pseudobinary phase diagram was proposed based on the magnitude of the thermal expansion mismatch and the distance of Bi diffusion, which can be applied to design the microstructures of WO3-BiVO4 heterojunctions and optimize their photoelectrochemical properties.
Role of indium tin oxide electrode on the microstructure of self-assembled WO3-BiVO4 hetero nanostructures
(2017-11-07)
Song, Haili
;
Li, Chao
;
Chien Nguyen Van
;
Dong, Wenxia
;
Qi, Ruijuan
Self-assembled WO3-BiVO4 nanostructured thin films were grown on a (001) yttrium stabilized zirconia (YSZ) substrate by the pulsed laser deposition method with and without the indium tin oxide (ITO) bottom electrode. Their microstructures including surface morphologies, crystalline phases, epitaxial relationships, interface structures, and composition distributions were investigated by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray energy dispersive spectroscopy. In both samples, WO3 formed nanopillars embedded into the monoclinic BiVO4 matrix with specific orientation relationships. In the sample with the ITO bottom electrode, an atomically sharp BiVO4/ITO interface was formed and the orthorhombic WO3 nanopillars were grown on a relaxed BiVO4 buffer layer with a mixed orthorhombic and hexagonal WO3 transition layer. In contrast, a thin amorphous layer appears at the interfaces between the thin film and the YSZ substrate in the sample without the ITO electrode. In addition, orthorhombic Bi2WO6 lamellar nanopillars were formed between WO3 and BiVO4 due to interdiffusion. Such a WO3-Bi2WO6-BiVO4 double heterojunction photoanode may promote the photo-generated charge separation and further improve the photoelectrochemical water splitting properties. Published by AIP Publishing.
Self-Assembled BiFeO3-epsilon-Fe2O3 Vertical Heteroepitaxy for Visible Light Photoelectrochemistry
(2016-09-21)
Le Thi Quynh
;
Chien Nguyen Van
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Bitla, Yugandhar
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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
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Chien Nguyen Van
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Tsai, Kai-An
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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.
Tunable photoelectrochemical performance of Au/BiFeO3 heterostructure
(2016)
Huang, Yen-Lin
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Chang, Wei Sea
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Chien Nguyen Van
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Liu, Heng-Jui
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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.