Browsing by Author "Liu, Ru-Shi"

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An integrated cobalt disulfide (CoS2) co-catalyst passivation layer on silicon microwires for photoelectrochemical hydrogen evolution
(2015-01-01)
Chen, Chih-Jung
;
Chen, Po-Tzu
;
Basu, Mrinmoyee
;
Yang, Kai-Chih
;
Lu, Ying-Rui
An integrated cobalt disulfide (CoS2) co-catalyst passivation layer on Si microwires (MWs) was used as a photocathode for solar hydrogen evolution. Si MWs were prepared by photolithography and dry etching techniques. The CoS2Si photocathodes were subsequently prepared by chemical deposition and thermal sulfidation of the Co(OH)(2) outer shell. The optimized onset potential and photocurrent of the CoS2Si electrode were 0.248 V and -3.22 mA cm(-2) (at 0 V), respectively. The best photocatalytic activity of the CoS2Si electrode resulted from lower charge transfer resistances among the photoabsorber, co-catalyst, and redox couples in the electrolyte. X-ray absorption near edge structure was conducted to investigate the unoccupied electronic states of the CoS2 layer. We propose that more vacancies in the S-3p unoccupied states of the CoS2Si electrode were present with a lower negative charge of S-2(2-) to form weaker S-H bond strength, promoting water splitting efficiency. Moreover, the CoS2 co-catalyst that completely covered underlying Si MWs served as a passivation layer to prevent oxidation and reduce degradation during photoelectrochemical measurements. Therefore, the optimal CoS2Si electrode maintained the photocurrent at about -3 mA cm-2 (at 0 V) for 9 h, and its hydrogen generation rate was approximately 0.833 mu mol min(-1).
(Ba,Sr)Y(2)Si(2)Al(2)O(2)N(5):Eu(2+): a novel near-ultraviolet converting green phosphor for white light-emitting diodes
(2011-01-01)
Liu, Wei-Ren
;
Yeh, Chiao-Wen
;
Huang, Chien-Hao
;
Lin, Chun Che
;
Chiu, Yi-Chen
A highly intense green-emitting phosphor BaY(2)Si(2)Al(2)O(2)N(5) : Eu(2+) (BYN : Eu(2+)) peaking at 511 nm was synthesized by a solid state reaction. The structure refinement, luminescence properties of BYN : Eu(2+) phosphors as well as their thermal quenching and the fabrication of white-light-emitting diodes (W-LEDs) were firstly investigated. By partially substituting Ba by Sr, namely (Ba, Sr)Y(2)Si(2)Al(2)O(2)N(5) : Eu(2+) (BSYN : Eu(2+)), the emission peak was shifted to 565 nm, giving a yellow-orange color, due to the splitting of the 5d energy level. By utilizing a mixture of blue-emitting BaMgAl(10)O(17) : Eu(2+), green-emitting BYN : Eu(2+) and red-emitting CaAlSiN(3) : Eu(2+) phosphors as light converters, an intense white GaN-based n-UV-LED (380 nm) was fabricated to a exhibit good color-rendering index R(a) of 84.5 at a correlated color temperature of 5089 K and CIE coordinates of (0.3425, 0.3478). Based on these results, we are currently evaluating the potential application of BYN : Eu(2+) as a green-emitting near-UV convertible phosphor.
(Ba,Sr)Y2Si2Al2O2N5:Eu2+: a novel near-ultraviolet converting green phosphor for white light-emitting diodes
(2011-01-01)
Liu, Wei-Ren
;
Yeh, Chiao-Wen
;
Huang, Chien-Hao
;
Lin, Chun Che
;
Chiu, Yi-Chen
A highly intense green-emitting phosphor BaY2Si2Al2O2N5 : Eu2+ (BYN : Eu2+) peaking at 511 nm was synthesized by a solid state reaction. The structure refinement, luminescence properties of BYN : Eu2+ phosphors as well as their thermal quenching and the fabrication of white-light-emitting diodes (W-LEDs) were firstly investigated. By partially substituting Ba by Sr, namely (Ba, Sr)Y2Si2Al2O2N5 : Eu2+ (BSYN : Eu2+), the emission peak was shifted to 565 nm, giving a yellow-orange color, due to the splitting of the 5d energy level. By utilizing a mixture of blue-emitting BaMgAl10O17 : Eu2+, green-emitting BYN : Eu2+ and red-emitting CaAlSiN3 : Eu2+ phosphors as light converters, an intense white GaN-based n-UV-LED (380 nm) was fabricated to a exhibit good color-rendering index R-a of 84.5 at a correlated color temperature of 5089 K and CIE coordinates of (0.3425, 0.3478). Based on these results, we are currently evaluating the potential application of BYN : Eu2+ as a green-emitting near-UV convertible phosphor.
Eu(2+)-activated silicon-oxynitride Ca(3)Si(2)O(4)N(2): a green-emitting phosphor for white LEDs
(2011-05-09)
Chiu, Yi-Chen
;
Huang, Chien-Hao
;
Lee, Te-Ju
;
Liu, Wei-Ren
;
Yeh, Yao-Tsung
The green-emitting phosphor Ca(3)Si(2)O(4)N(2):Eu(2+) was synthesized using a solid-state reaction. The luminescence properties, diffuse reflection spectrum, and thermal quenching were firstly studied, and a white light-emitting diode (wLED) was fabricated using the Eu(2+)-activated Ca(3)Si(2)O(4)N(2) phosphor. Eu(2+)-doped Ca(3)Si(2)O(4)N(2) exhibited a broad green emission band centered between 510 and 550 nm depending on the concentration of Eu(2+). The optimal doping concentration of Eu(2+) in Ca(3)Si(2)O(4)N(2) was 1 mol%. The energy transfer between Eu(2+) ions proceeds by an electric multipolar interaction mechanism, with a critical transfer distance of approximately 30.08 angstrom. A wLED with an color-rendering index R(a) of 88.25 at a correlated color temperature of 6029 K was obtained by combining a GaN-based n-UV LED (380 nm) with the blue-emitting BaMgAl(10)O(17):Eu(2+), green-emitting Ca(3)Si(2)O(4)N(2): Eu(2+), and red-emitting CaAlSiN(3):Eu(2+) phosphors. The results present Ca(3)Si(2)O(4)N(2):Eu(2+) as an attractive candidate for use as a conversion phosphor for wLED applications. (C) 2011 Optical Society of America
Eu2+-activated silicon-oxynitride Ca3Si2O4N2: a green-emitting phosphor for white LEDs
(2011-05-09)
Chiu, Yi-Chen
;
Huang, Chien-Hao
;
Lee, Te-Ju
;
Liu, Wei-Ren
;
Yeh, Yao-Tsung
The green-emitting phosphor Ca3Si2O4N2:Eu2+ was synthesized using a solid-state reaction. The luminescence properties, diffuse reflection spectrum, and thermal quenching were firstly studied, and a white light-emitting diode (wLED) was fabricated using the Eu2+-activated Ca3Si2O4N2 phosphor. Eu2+-doped Ca3Si2O4N2 exhibited a broad green emission band centered between 510 and 550 nm depending on the concentration of Eu2+. The optimal doping concentration of Eu2+ in Ca3Si2O4N2 was 1 mol%. The energy transfer between Eu2+ ions proceeds by an electric multipolar interaction mechanism, with a critical transfer distance of approximately 30.08 angstrom. A wLED with an color-rendering index R-a of 88.25 at a correlated color temperature of 6029 K was obtained by combining a GaN-based n-UV LED (380 nm) with the blue-emitting BaMgAl10O17:Eu2+, green-emitting Ca3Si2O4N2: Eu2+, and red-emitting CaAlSiN3:Eu2+ phosphors. The results present Ca3Si2O4N2:Eu2+ as an attractive candidate for use as a conversion phosphor for wLED applications. (C) 2011 Optical Society of America
Plasmon Inducing Effects for Enhanced Photoelectrochemical Water Splitting: X-ray Absorption Approach to Electronic Structures
(2012-08-01)
Chen, Hao Ming
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Chen, Chih Kai
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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.
Silicon microwire arrays decorated with amorphous heterometal-doped molybdenum sulfide for water photoelectrolysis
(2017-02-01)
Chen, Chih-Jung
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Yang, Kai-Chih
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Liu, Chi-Wei
;
Lu, Ying-Rui
;
Dong, Chung-Li
Silicon is a promising photocathode material for solar hydrogen evolution because of its small band gap, negative conduction band position, and ideal theoretical current density. In this study, p-type Si microwire (p-Si MW) arrays were prepared as photocathodes because of the large surface area and high light-harvesting capability. However, Si MWs suffered from low photocatalytic activity because of slow photo-induced carriers during driving of water-splitting reaction. Therefore, molybdenum sulfide (MoS2) with appropriate band alignment with p-Si material was employed for surface modification to function as a co-catalyst for collecting photo-generated minority carriers and reducing recombination possibility. The onset potential and current density at 0 V versus reversible hydrogen electrode (RHE) of Si@MoS2 MWs were + 0.122 V and -8.41 mA cm(-2). Heterometal atoms were employed to dope MoS2 co-catalyst and expose more sulfurterminated active sites to further boost photoelectrochemical performance. Optimal activity of Si@MMoSx (M = Fe, Co, Ni) was achieved by doping Co heteroatoms, and its turn-on voltage and photocurrent density at 0 V (vs. RHE) were respectively increased to + 0.192 V and -17.2 mA cm(-2). X-ray absorption spectroscopy was applied to demonstrate that Fe ions of FeMoSx were dichalcogenide materials, forming a composite with MoS2 and contributing better photoelectrolytic efficiency. By contrast, two-valent heteroatoms of CoMoSx and NiMoSx substituted the Mo4+ ions in MoS2. For charge compensation, more defects and edges were revealed as active sites of solar hydrogen production by adding Co or Ni dopants in MoS2 co-catalyst, which led to lower overpotential.
Synergistic-Effect-Controlled CoTe2/Carbon Nanotube Hybrid Material for Efficient Water Oxidation
(2016-12-15)
Lu, Tzu-Hsiang
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Chen, Chih-Jung
;
Lu, Ying-Rui
;
Dong, Chung-Li
;
Liu, Ru-Shi
In anode, electrocatalytic water splitting involves oxygen evolution reaction (OER), which is a complex and sluggish reaction, and thus the efficiency to produce hydrogen is seriously limited by OER. We report that CoTe2 exhibits optimized OER activity for the first time. Multiwalled carbon nanotube (MWCNT) is utilized to support CoTe2 in generating a synergistic effect to enhance OER activity and improve stability by tuning different loading amounts of CoTe2 on CNT. In 1.0 M KOH, bare CoTe2 needed overpotential of 323 mV to produce 10 mA/ cm(2) with Tafel slope of 85.1 mV/dec, but CoTe2/carbon nanotube (CNT) with optimized loading amount of CoTe2 required only 291 mV to produce10 mA/cm(2) with Tafel slope of 44.2 mV/dec. X-ray absorption near edge structure (XANES) was applied to prove that an electron transfer from e(g) band of CoTe2 to CNT caused a synergistic effect. This electron transfer modulated the bond strength of oxygen-related intermediate species on the surface and optimized OER performance. In situ XANES was used to compare CoTe2/CNT and pristine CoTe2 during OER. It proved the transition state of CoOOH more easily existed by adding CNT in hybrid material during OER to enhance the efficiency of OER. Moreover, bare CoTe2 is unstable under OER, but the CoTe2/CNT hybrid materials exhibited improved and exceptional durability by time dependent potentiostatic electrochemical measurement for 24 h and continuous cyclic voltammetry for 1000 times. Our result suggests that this new OER electrocatalyst for OER can be applied in various water-splitting devices and can promote hydrogen economy.
Wide Range pH-Tolerable Silicon@Pyrite Cobalt Dichalcogenide Microwire Array Photoelectrodes for Solar Hydrogen Evolution
(2016-03-02)
Chen, Chihjung
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Yang, Kai-Chih
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Basu, Mrinmoyee
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Lu, Tzu-Hsiang
;
Lu, Ying-Rui
This study employed silicon@cobalt dichalcogenide micro wires (MWs) as wide range pH-tolerable photocathode material for solar water splitting. Silicon microwire arrays were fabricated through lithography and dry etching technologies. Si@Co(OH)(2) MWs were utilized as precursors to synthesize Si@CoX2 (X = S or Se) photocathodes. Si@CoS2 and Si@CoSe2 MWs were subsequently prepared by thermal sulfidation and hydrothermal selenization reaction of Si@Co(OH)(2), respectively. The CoX2 outer shell served as cocatalyst to accelerate the kinetics of photogenerated electrons from the underlying Si MWs and reduce the recombination. Moreover, the CoX2 layer completely deposited on the Si surface functioned as a passivation layer by decreasing the oxide formation on Si MWs during solar hydrogen evolution. Si@CoS2 photocathode showed a photocurrent density of -3.22 mA cm(-2) at 0 V (vs RHE) in 0.5 M sulfuric acid electrolyte, and Si@CoSe2 MWs revealed moderate photocurrent density of -2.55 mA cm(-2). However, Si@CoSe2 presented high charge transfer efficiency in neutral and alkaline electrolytes. Continuous chronoamperometry in acid, neutral, and alkaline solutions was conducted at 0 V (vs RHE) to evaluate the photoelectrochemical durability of Si@CoX2 MWs. Si@CoS2 electrode showed no photoresponse after the chronoamperometry test because it was etched through the electrolyte. By contrast, the photocurrent density of Si@CoSe2 MWs gradually increased to -5 mA cm(-2) after chronoamperometry characterization owing to the amorphous structure generation.