Browsing by Author "Lu, Yi-Hsuan"

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A facile green antisolvent approach to Cu2+-doped ZnO nanocrystals with visible-light-responsive photoactivities
(2014-08-07)
Lu, Yi-Hsuan
;
Lin, Wei-Hao
;
Yang, Chao-Yao
;
Chiu, Yi-Hsuan
;
Pu, Ying-Chih
An environmentally benign antisolvent method has been developed to prepare Cu2+-doped ZnO nanocrystals with controllable dopant concentrations. A room temperature ionic liquid, known as a deep eutectic solvent (DES), was used as the solvent to dissolve ZnO powders. Upon the introduction of the ZnO-containing DES into a bad solvent which shows no solvation to ZnO, ZnO was precipitated and grown due to the dramatic decrease of solubility. By adding Cu2+ ions to the bad solvent, the growth of ZnO from the antisolvent process was accompanied by Cu2+ introduction, resulting in the formation of Cu2+-doped ZnO nanocrystals. The as-prepared Cu2+-doped ZnO showed an additional absorption band in the visible range (400-800 nm), which conduced to an improvement in the overall photon harvesting efficiency. Time-resolved photoluminescence spectra, together with the photovoltage information, suggested that the doped Cu2+ may otherwise trap photoexcited electrons during the charge transfer process, inevitably depressing the photoconversion efficiency. The photoactivity of Cu2+-doped ZnO nanocrystals for photoelectrochemical water oxidation was effectively enhanced in the visible region, which achieved the highest at 2.0 at% of Cu2+. A further increase in the Cu2+ concentration however led to a decrease in the photocatalytic performance, which was ascribed to the significant carrier trapping caused by the increased states given by excessive Cu2+. The photocurrent action spectra illustrated that the enhanced photoactivity of the Cu2+-doped ZnO nanocrystals was mainly due to the improved visible photon harvesting achieved by Cu2+ doping. These results may facilitate the use of transition metal ion-doped ZnO in other photoconversion applications, such as ZnO based dye-sensitized solar cells and magnetism-assisted photocatalytic systems.
A facile green antisolvent approach to Cu2+-doped ZnO nanocrystals with visible-light-responsive photoactivities
(2014-08-07)
Lu, Yi-Hsuan
;
Lin, Wei-Hao
;
Yang, Chao-Yao
;
Chiu, Yi-Hsuan
;
Pu, Ying-Chih
An environmentally benign antisolvent method has been developed to prepare Cu2+-doped ZnO nanocrystals with controllable dopant concentrations. A room temperature ionic liquid, known as a deep eutectic solvent (DES), was used as the solvent to dissolve ZnO powders. Upon the introduction of the ZnO-containing DES into a bad solvent which shows no solvation to ZnO, ZnO was precipitated and grown due to the dramatic decrease of solubility. By adding Cu2+ ions to the bad solvent, the growth of ZnO from the antisolvent process was accompanied by Cu2+ introduction, resulting in the formation of Cu2+-doped ZnO nanocrystals. The as-prepared Cu2+-doped ZnO showed an additional absorption band in the visible range (400-800 nm), which conduced to an improvement in the overall photon harvesting efficiency. Time-resolved photoluminescence spectra, together with the photovoltage information, suggested that the doped Cu2+ may otherwise trap photoexcited electrons during the charge transfer process, inevitably depressing the photoconversion efficiency. The photoactivity of Cu2+-doped ZnO nanocrystals for photoelectrochemical water oxidation was effectively enhanced in the visible region, which achieved the highest at 2.0 at% of Cu2+. A further increase in the Cu2+ concentration however led to a decrease in the photocatalytic performance, which was ascribed to the significant carrier trapping caused by the increased states given by excessive Cu2+. The photocurrent action spectra illustrated that the enhanced photoactivity of the Cu2+-doped ZnO nanocrystals was mainly due to the improved visible photon harvesting achieved by Cu2+ doping. These results may facilitate the use of transition metal ion-doped ZnO in other photoconversion applications, such as ZnO based dye-sensitized solar cells and magnetism-assisted photocatalytic systems.
Au nanoplates as robust, recyclable SERS substrates for ultrasensitive chemical sensing
(2014-03-15)
Lin, Wei-Hao
;
Lu, Yi-Hsuan
;
Hsu, Yung-Jung
;
材料科學與工程學系
;
Department of Materials Science and Engineering
With the structural advantages of being sharp and straight, Au nanoplates may work as a promising surface-enhanced Raman scattering (SERS) platform for detection of Raman-sensitive analytes. However, the utilization of Au nanoplates as realistic SERS substrates is still not widely investigated, especially in the practical detection of environmentally persistent pollutants. This work delivers the first successful demonstration of using Au nanoplate platform in practical SERS sensing toward a typical polycyclic aromatic hydrocarbons pollutant of pyrene. The samples were prepared using an environmentally benign seed-mediated growth approach without the post-purification treatment. It was found that Au nanoplates exhibited significantly enhanced SERS activities (enhancement factor = 7.30 x 10(7)) and achieved an extremely low detection limit (5 x 10(-10) M) toward pyrene molecules. Furthermore, the SERS activity of Au nanoplates can be fully recovered after repeatedly used and recycled in pyrene detection. These results manifest that the present Au nanoplates can serve as robust, recyclable SERS substrates that allow rapid detection of trace levels of analytes with a high degree of sensitivity and stability. The findings from this work may facilitate the use of Au nanoplate SERS substrates in more realistic applications such as biomolecule sensing and environmental monitoring. (C) 2013 Elsevier Inc. All rights reserved.
First demonstration of rainbow photocatalysts using ternary Cd1-xZnxSe nanorods of varying compositions
(2014-04-22)
Nguyen, An T.
;
Lin, Wei-Hao
;
Lu, Yi-Hsuan
;
Chiou, Yao-De
;
Hsu, Yung-Jung
We demonstrated for the first time that the assembly of Cd1-xZnxSe nanorods (NRs) with five compositions (x= 0, 0.35, 0.54, 0.61,1) may absorb the whole visible spectrum in a gradient fashion for photoconversion applications. The samples were prepared by conducting cation exchange reactions on Ag2Se NRs with excess Cd2+ and Zn2+ ions. By modulating the molar ratio of Cd2+ to Zn2+ employed, the composition of the resulting Cd1-xZnxSe NRs can be delicately controlled. Because of the tunability of band edge with stoichiometry, Cd1-xZnxSe NRs of varying compositions absorbed light at different wavelength regions, which spanned almost the entire visible spectrum. As compared to the individual constituent NRs, the NRs assembly exhibited greatly improved photoactivity in photoelectrochemical water splitting as well as superior photocatalytic performance toward methylene blue degradation under white light illumination. This superiority emanates from the composition-gradient configuration that significantly improves the light harvesting efficiency by absorbing almost the whole visible spectrum of the incident white light. The full visible photon harvesting of the NRs assembly was validated by the photocurrent action spectrum which showed spectral accordance with the absorption spectrum. The recycling test manifests that the NRs assembly displayed substantially high stability during its use as photocatalyst. Furthermore, the result of performance evaluation under natural sunlight reveals that the present NRs assembly can be used as practical rainbow photocatalysts which may effectively harvest energy from sunlight. The demonstration from this work may facilitate the use of sophisticated assembly of semiconductor nanocrystals in relevant photoconversion processes where the effectiveness of photon harvesting is determinant. (C) 2014 Elsevier B.V. All rights reserved.
Structural imperfections and attendant localized/itinerant ferromagnetism in ZnO nanoparticles
(2014-08-30)
Yang, Chao-Yao
;
Lu, Yi-Hsuan
;
Lin, Wei-Hao
;
Lee, Min-Han
;
Hsu, Yung-Jung
Using synchrotron-based x-ray magnetic spectroscopy, we report a study focusing on the local symmetry of Cu-dopant and resultant structural imperfections in mediating Cu-doped ZnO nanoparticles\' ferromagnetism (FM). Prepared by an antisolvent method, Cu appeared to preferably populate on the basal plane of ZnO with a local symmetry of [CuO4]. This unique symmetry was antiferromagnetic in nature, while electronically and structurally coupled to surrounded oxygen vacancies (V-o) that yielded a localized FM, because of a strong dependency on the number/location of the [CuO4] symmetry. Surprisingly, the FM of undoped but oxygen-deficient ZnO appeared to be more itinerant and long-range, where V-o percolated the FM effectively and isotropically through oxygen\'s delocalized orbital. By adopting the approach of structural imperfection, this study clearly identifies V-o\'s (defect\'s) true characters in mediating the FM of magnetic semiconductors which has been thought of as a long-standing debate, and thus provides a different thinking about the traditional extrinsic ferromagnetic-tuning in the semiconductors. It even illuminates recent research concerning the intrinsic FM of low-dimensional systems that contain defects but non-magnetic elements.
Supercritical CO2-Assisted Electrochemical Deposition of ZnO Mesocrystals for Practical Photoelectrochemical Applications
(2013-12-05)
Lin, Wei-Hao
;
Chang, Tso-Fu Mark
;
Lu, Yi-Hsuan
;
Sato, Tatsuo
;
Sone, Masato
We have successfully developed an effective supercritical CO2 (sc-CO2) emulsion-assisted electrochemical approach for cathodic deposition of ZnO mesocrystals. The sc-CO2 was introduced along with a nonionic surfactant in the process to form emulsified electrolyte, which significantly increased the supersaturation degree and promoted the molecular diffusion to affect the crystal growth of ZnO. The deposition involved the generation of primary nanocrystals with substantially high surface energy, followed by the preferred attachment of the primary nanocrystals along an energetically favorable orientation to form ZnO mesocrystals. The as-deposited ZnO mesocrystals exhibited remarkable optical properties at room temperature in terms of prominent near band-edge emission and substantially long exciton lifetime, attributable to the highly oriented crystallinity of mesocrystals that effectively suppressed the nonradiative charge recombination to extend the exciton decay dynamics. As compared to the structures prepared without the addition of surfactant or sc-CO2, ZnO mesocrystals from sc-CO2 emulsion displayed greatly improved photoactivity toward photoelectrochemical water oxidation, revealing their promising potential as photoanodes in relevant photoelectrochemical processes. The current study delivers the first demonstration of directly depositing ZnO mesocrystals on conductive substrates, which paves the way for utilization of mesocrystals as practical electrodes in technologically important energy conversion fields, such as electrochemical cells, photovoltaic devices, as well as photoelectrochemical water splitting, where the advantageous structural characteristics of mesocrystals, i.e., high crystallinity and abundant porosity, can be fully exploited.