Daniel H. C. Chua

National University of Singapore, Tumasik, Singapore

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Publications (112)303.41 Total impact

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    Wei Qin · Taiqiang Chen · Ting Lu · Daniel H.C. Chua · Likun Pan
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    ABSTRACT: Layered nickel sulfide (NS)-reduced graphene oxide (RGO) composites are prepared via a simple microwave-assisted method and subsequent annealing in N2/H2 atmosphere. A detailed array of characterization tools are used to study their morphology, structure and electrochemical performance. It was found that these composites exhibit significantly improved sodium-ion storage ability as compared with pure NS under galvanostatic cycling at a specific current of 100 mA g-1 in a potential limitation of 0.005-3.0 V. Furthermore, the composite with the RGO content of 35 wt.% achieves a high maximum reversible specific capacity of about 391.6 mAh g-1 at a specific current of 100 mA g-1 after 50 cycles. These results prove that NS-RGO composites are highly promising when applied directly as anode materials in sodium-ion batteries.
    Full-text · Article · Jan 2016 · Journal of Power Sources
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    ABSTRACT: Catalyst degradation is one major challenge preventing the worldwide commercialization of the Proton Exchange Membrane Fuel Cells. In this study, we investigate the development of a novel hierarchical carbonaceous support for the platinum catalysts, called graphene-carbon nanotube hybrids (GCNT), and its degradation behavior during an accelerated degradation test. The carbon support is fabricated by growing graphene directly onto carbon nanotubes to form a unique all-carbon nanostructure possessing both an ultra-high density of exposed graphitic edges of graphene and a porous structure of carbon nanotubes. The GCNT-supported platinum catalyst exhibits a higher intrinsic catalytic activity than a carbon black-supported platinum catalyst, and much higher than a CNT-supported platinum catalyst. The enhanced catalytic activity of the GCNT-supported platinum catalyst is explained by the high graphitic edge density which promotes the catalytic reactions on platinum catalyst. The GCNT-supported platinum catalyst also exhibits a superior electrochemical stability over that of the carbon black-supported platinum catalyst, explained by the high crystallinity of the GCNT support. The superior stability is expressed by a lower loss in polarization performance, a smaller increase in charge transfer resistance, a lower loss in the platinum electrochemical surface area, a lower rate of carbon corrosion, and a more stable catalyst microstructure.
    Full-text · Article · Jan 2016 · Journal of The Electrochemical Society
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    Tamie A. J. Loh · Daniel H. C. Chua · Andrew T. S. Wee
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    ABSTRACT: Atomically thin tungsten disulfide (WS2) has attracted much attention in recent years due its indirect-to-direct band gap transition, band gap tunability, and giant spin splitting. However, the fabrication of atomically thin WS2 remains largely underdeveloped in comparison to its structural analogue MoS2. Here we report the direct fabrication of highly crystalline few-layer WS2 on silver substrates by pulse laser deposition at the relatively low temperature of 450 °C. The growth takes places by conventional epitaxy, through the in-situ formation of nearly lattice-matching Ag2S on the silver surface. Intriguingly, it was observed that the resulting film was composed of not only the usual semiconducting 2H-WS2 structure but also the less common metallic 1T-WS2. Modifications of the synthesis parameters allow for control over the crystalline quality, film thickness and crystal phase composition of the resulting WS2 film.
    Preview · Article · Dec 2015 · Scientific Reports
  • Tamie Ai-Jia Loh · Daniel H.C. Chua
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    ABSTRACT: Partial 2H-to-1T phase transition in two-dimensional (2D) WS2 grown by pulsed laser deposition is discovered through X-ray photoelectron spectroscopy. The resulting hybrid structure is observed only on substrates of the noble metals Ag and Au, whereas other metals such as Ni and Al produced only 2H-WS2. The origin of the 1T-phase is attributed to electron doping of the WS2 lattice by Ag or Au atoms, in a mechanism reminiscent of Li in the intercalation of WS2 and the related MoS2 structure. The doping process is viable as both Ag and Au atoms are able to donate one electron each, leading to the formation of a stable d10 shell and a phase transformation in the WS2 structure from 2H-to-1T. In the case of Ni, however, the formation of nickel oxides at the metal surface inhibits its ability to donate electrons to the WS2 lattice, whereas Al is unable to act as electron donor for WS2. Thus, the 1T-phase is absent in either of these substrates.
    No preview · Article · Nov 2015 · The Journal of Physical Chemistry C
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    Xingtao Xu · Yong Liu · Ting Lu · Zhuo Sun · Daniel H. C. Chua · Likun Pan
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    ABSTRACT: Capacitive deionization (CDI) is an emerging technology offering a green and efficient route to obtain clean water. Up to now, the key of CDI technology is still focused on the exploration of electrode materials with a rationally designed structure and excellent performance, because the electrosorption performance of the carbon-based electrodes reported by now cannot meet the demand of practical application of CDI. Herein, novel graphene/carbon nanotubes (CNTs) hybrid sponge (GNS) structures were designed and fabricated via directly freeze-drying graphene oxide/CNTs mixed solution followed by annealing in nitrogen atmosphere. The morphology, structure and electrochemical performance of GNS were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, cyclic voltammetry and electrochemical impedance spectroscopy. The results show that GNS with 20 wt% CNTs shows a maximum specific surface area of 498.2 m2 g-1 and a highest specific capacitance of 203.48 F g-1 among all the samples. When used as CDI electrode, it exhibits an ultrahigh electrosorption capacity of 18.7 mg g-1, and to our knowledge, this value outperforms other carbon electrodes reported recently. GNS should be a promising electrode material for high performance CDI.
    Full-text · Article · Jul 2015 · Journal of Materials Chemistry A
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    ABSTRACT: In this article, we introduce an electrochemical doping method of amorphous selenium (a-Se) using NaCl(aq). Recently, an a-Se photovoltaic device fabricated using this method [I. Saito, W. Miyazaki, M. Onishi, Y. Kudo, T. Masuzawa, T. Yamada, A. Koh, D. Chua, K. Soga, M. Overend, M. Aono, G. A. J. Amaratunga, and K. Okano, Appl. Phys. Lett. 98, 152102 (2011)] has been announced and opened up the potential of a new impurity doping method. This study will further explore its possibilities by doping chlorine (Cl) and sodium (Na) and aim to fabricate a p–n junction by reversing the applied voltage during the electrolysis. The device is characterized through photoelectric measurements. The I–V characteristics show rectification under light illumination.
    No preview · Article · Jul 2015 · Physica Status Solidi (A) Applications and Materials
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    Xingtao Xu · Zhuo Sun · Daniel H C Chua · Likun Pan
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    ABSTRACT: As water shortage has become a serious global problem, capacitive deionization (CDI) with high energy efficiency and low cost, is considered as a promising desalination technique to solve this problem. To date, CDI electrodes are mainly made up of porous carbon materials. However, the electrosorption performance obtained by now still cannot meet the demand of practical application. Therefore, a rationally designed structure of electrode materials has been an urgent need for CDI application. Here, a novel nitrogen-doped graphene sponge (NGS), with high specific surface area and rationally designed structure was fabricated, and used as CDI electrodes for the first time. The results show that NGS exhibits an ultrahigh electrosorption capacity of 21.0 mg g(-1) in ∼500 mg L(-1) NaCl solution, and to our knowledge, it is the highest value reported for carbon electrodes in similar experimental conditions by now. NGS in this work is expected to be a promising candidate as CDI electrode material.
    Full-text · Article · Jun 2015 · Scientific Reports
  • Yuan Chen · Zhe Tang · Shengyuan Yang · Yihua Wang · Daniel Chua
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    ABSTRACT: All-solid-state thin-film lithium-ion batteries are deemed a promising replacement for liquid electrolyte-based Li-ion batteries owing to their superior cycle life, compact structure, high energy and power densities with an extremely low self-discharge rate. However, the state-of-art all-solidstate Li-ion batteries mainly use thermal-evaporated lithium metal as anode, which considerably undermines their cost-effectiveness and also raises safety concerns of Li explosion. In this study, a spinel Li-Ni-Mn-O/lithium phosphorus oxynitride (LiPON)/Si-based all-solid-state Li-ion battery has been successfully developed using sputter-deposition technique. This Li-free all-solid-state Li-ion battery exhibited a high cut-off voltage of 4.55 V and superior cycling stability in a wide potential range from 3.0 to 4.55 V. The highest specific capacity of the spinel Li-Ni-Mn-O/LiPON/ Si-based all-solid-state Li-ion battery achieved so far is 40 μAh cm-2 and further optimisation is currently under way.
    No preview · Article · May 2015 · Materials Technology
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    Yong Liu · Ting Lu · Zhuo Sun · Daniel H. C. Chua · Likun Pan
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    ABSTRACT: Ultra-thin carbon nanofiber networks (bc-CNFs) were prepared from the natural based bacterial-cellulose pellicle through freeze drying and subsequently carbonization at different temperatures. The morphology, structure and electrochemical performance of bc-CNFs were characterized by field emission scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, Fourier transform infrared spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. Their electrosorption performance in NaCl solution was studied and compared with carbon nanotubes (CNTs) and electrospun carbon nanofibers (e-CNFs). The results show that bc-CNFs treated at 800 °C exhibit excellent desalination performance with an electrosorption capacity of 12.81 mg g-1 in 1000 mg l-1 NaCl solution, much higher than those of CNTs (3.78 mg g-1) and e-CNFs (6.56 mg g-1). The excellent performance of bc-CNFs is ascribed to their high specific surface area, low charge transfer resistance and superior hydrophility.
    Full-text · Article · Apr 2015 · Journal of Materials Chemistry A
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    Yong Liu · Xingtao Xu · Ting Lu · Zhuo Sun · Daniel H. C. Chua · Likun Pan
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    ABSTRACT: A nitrogen-doped electrospun reduced graphene oxide–carbon nanofiber composite (NG–CNF) was fabricated via electrospinning by adding graphite oxide into a precursor solution and subsequent thermal treatment under an ammonia atmosphere. The morphology, structure and electrochemical performance of the composite were characterized by scanning electron microscopy, nitrogen adsorption–desorption, cyclic voltammetry and electrochemical impedance spectroscopy, and their capacitive and electrosorption performances in NaCl solution were studied. The NG–CNF composite electrode shows excellent specific capacitance (337.85 F g−1) and electrosorption capacity (3.91 mg g−1), much higher than those of pure carbon nanofibers (171.28 F g−1 and 3.13 mg g−1) and the reduced graphene oxide–carbon nanofiber composite (264.32 F g−1 and 3.60 mg g−1). The enhanced performance of the NG–CNF is ascribed to the nitrogen doping and the formation of an effective “plane-to-line” conducting network in the composite, which facilitates the electron transfer and ion transport as well as increases the specific surface area.
    Full-text · Article · Apr 2015 · RSC Advances
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    Yong Liu · Taiqiang Chen · Ting Lu · Zhuo Sun · Daniel H.C. Chua · Likun Pan
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    ABSTRACT: Nitrogen-doped porous carbon spheres (NPCSs) were prepared through a fast microwave-assisted approach using sucrose as the precursor in a microwave system and subsequent thermal treatment in ammonia atmosphere at different temperatures. The morphology, structure and electrochemical performance of the NPCSs were characterized by scanning electron microscopy, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy, and their electrosorption performance in NaCl solution was studied. The results show that NPCSs treated at 1000 °C exhibit an extremely high electrosorption capacity of 14.91 m g g−1 when the initial NaCl concentration is 1000 mg l−1, which shows great improvement compared with their undoped counterpart. The nitrogen doping is suggested to be a very effective method to improve the electrosorption performance, and the NPCSs should be a very promising candidate as electrode material for CDI application.
    Full-text · Article · Mar 2015 · Electrochimica Acta
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    Xingtao Xu · Likun Pan · Yong Liu · Ting Lu · Zhuo Sun · Daniel H C Chua
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    ABSTRACT: Capacitive deionization (CDI) is an effective desalination technique offering an appropriate route to obtain clean water. In order to obtain excellent CDI performance, a rationally designed structure of electrode materials has been an urgent need for CDI application. In this work, a novel graphene sponge (GS) was proposed as CDI electrode for the first time. The GS was fabricated via directly freeze-drying graphene oxide solution followed by annealing in nitrogen atmosphere. The morphology, structure and electrochemical performance of GS were characterized by scanning electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The electrosorption performance of GS in NaCl solution was studied and compared with pristine graphene (PG). The results show that due to the unique 3D interconnected porous structure, large accessible surface area and low charge transfer resistance, GS electrode exhibits an ultrahigh electrosorption capacity of 14.9 mg g(-1) when the initial NaCl concentration is ~500 mg L(-1), which is about 3.2 times of that of PG (4.64 mg g(-1)), and to our knowledge, it should be the highest value reported for graphene electrodes in similar experimental conditions by now. These results indicate that GS should be a promising candidate for CDI electrode.
    Full-text · Article · Feb 2015 · Scientific Reports
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    Yong Liu · Likun Pan · Taiqiang Chen · Xingtao Xu · Ting Lu · Zhuo Sun · Daniel H.C. Chua
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    ABSTRACT: Porous carbon spheres (PCSs) were fabricated through a fast microwave-assisted approach using sucrose as the precursor in a microwave system and subsequent thermal treatment at 600, 800 and 1000 °C. The morphology, structure and electrochemical performance of the PCSs were characterized by scanning electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, cyclic voltammetry and electrochemical impedance spectroscopy. Their electrosorption performance in NaCl solution was studied and compared with activated carbon, carbon nanotubes, reduced graphene and carbon aerogels. The results show that due to their high specific surface area and low charge transfer resistance, PCSs treated at 1000 °C exhibit high electrosorption capacity of 5.81 m g g−1 when the initial solution concentration is 500 mg l−1, which is higher than those of other carbon materials.
    Full-text · Article · Jan 2015 · Electrochimica Acta
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    Yong Liu · Likun Pan · Xingtao Xu · Ting Lu · Zhuo Sun · daniel h. c. chua
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    ABSTRACT: Carbon nanorods (CNRs) were fabricated from natural based nanocrystalline cellulose through simple thermal treatment at 800, 1000 and 1200 °C. The morphology, structure and electrochemical performance of CNRs were characterized by atomic force microscopy, Raman spectroscopy, nitrogen adsorption-desorption, cyclic voltammetry and electrochemical impedance spectroscopy. Their electrosorption performance in NaCl solution was studied. The results show that CNRs treated at 1200 °C exhibit the highest specific capacitance of 264.19 F g-1 and electrosorption capacity of 15.12 mg g-1 with the initial NaCl concentration is 500 mg l-1, due to their high specific surface area and low charge transfer resistance.
    Full-text · Article · Oct 2014
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    ABSTRACT: Molybdenum sulfide has recently attracted much attention due to its low cost and excellent catalytical effects in the application of hydrogen evolution reaction (HER). To improve the HER efficiency, many researchers have extensively explored various avenues such as material modification, forming hybrid structures or modifying geometric morphology. In this work, we reported a significant enhancement in the electrocatalytic activity of the MoSx via growing on Tetracyanoquinodimethane (TCNQ) treated carbon cloth, where the MoSx was synthesized by thermolysis from the ammonium tetrathiomolybdate ((NH4)2MoS4) precursor at 170 oC. The pyridinic N- and graphitic N-like species on the surface of carbon cloth arising from the TCNQ treatment facilitate the formation of Mo5+ and S22- species in the MoSx, especially with S22- serving as an active site for HER. In addition, the smaller particle size of the MoSx grown on TCNQ-treated carbon cloth reveals a high ratio of edge sites relative to basal plane sites, indicating the richer effective reaction sites and superior electrocatalytic characteristics. Hence, we reported a high hydrogen evolution rate for MoSx on TCNQ-treated carbon cloth of 6408 mLg-1cm-2h-1 (286 mmolg-1cm-2h-1) at an overpotential of V = 0.2 V. This study provides the fundamental concepts useful in the design and preparation of transition metal dichalcogenide catalysts, beneficial in the development in clean energy.
    Full-text · Article · Sep 2014 · ACS Applied Materials & Interfaces
  • Aditya P. Murawala · Tamie A.J. Loh · Daniel H.C. Chua
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    ABSTRACT: We report the fabrication of a three-dimensional forest of highly crystalline two-dimensional (2D) molybdenum disulfide (MoS2) nano-petals encapsulating vertically aligned carbon nanotubes (CNT) in a core-shell configuration. Growth was conducted via magnetron sputtering at room temperature and it was found that the nano-petal morphology was formed only when a critical threshold in sputter deposition time was reached. Below this threshold, an amorphous tubular structure composed of mainly molybdenum oxides dominates instead. The presence of the MoS2 nano-petals was shown to impart photoluminescence to the CNTs, in addition to significantly enhancing their electron emission properties, where the turn-on field was lowered from 2.50 Vμm−1 for pristine CNTs to 0.80 Vμm−1 for MoS2-CNT heterostructures fabricated at 30 min sputter deposition time. Photoluminescence was detected at wavelengths of approximately 684 nm and 615 nm, with the band at 684 nm gradually blue-shifting as sputter time was increased. These results demonstrate that it is possible to synthesize 2D MoS2 layers without the need for chemical routes and high growth temperatures.
    No preview · Article · Sep 2014 · Journal of Applied Physics
  • Tamie A J Loh · Daniel H C Chua
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    ABSTRACT: Pulsed laser deposition (PLD) on metal substrates has recently been discovered to present an alternative method for producing highly crystalline few-layer MoS2. However, not every metal behaves in the same manner during film growth, and hence, it is crucial that the ability of various metals to produce crystalline MoS2 be thoroughly investigated. In this work, MoS2 was deposited on metal substrates, Al, Ag, Ni, and Cu, using a pulsed laser. Highly crystalline few-layer MoS2 was successfully grown on Ag, but is absent in Al, Ni, and Cu under specific growth conditions. This discrepancy was attributed to either excessively strong or insufficient adlayer-substrate interactions. In the case of Al, the effects of the strong interface interactions can be offset by increasing the amount of source atoms supplied, thereby producing semicrystalline few-layer MoS2. The results show that despite PLD being a physical vapor deposition technique, both physical and chemical processes play an important role in MoS2 growth on metal substrates.
    No preview · Article · Sep 2014 · ACS Applied Materials & Interfaces
  • Tamie A.J. Loh · Daniel H. C. Chua
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    ABSTRACT: Precise control of the thickness of few-layer molybdenum disulphide (MoS2) is still a challenge for commonly utilized physical exfoliation and chemical synthesis methods. Herein, we report the successful fabrication of highly crystalline few-layer MoS2 on silver substrates by pulse laser deposition at the relatively low temperature of 500 degrees C. The growth process is facilitated by the in situ formation of nearly lattice-matching Ag2S on the silver surface. The number of layers is easily controlled by selection of appropriate laser energy and deposition time. This synthetic approach is straightforward and easily adapted to other transition metal dichalcogenides to create heterostructures with alternating layers.
    No preview · Article · Aug 2014 · Chemical Physics Letters
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    ABSTRACT: Graphene has been known for its superior electronic properties ever since its discovery in 2004. The high aspect ratio and ballistic transport properties exhibited by this one-dimensional material are especially useful for electron emission applications. However, they are typically grown horizontally and excess efforts, such as the use of transfer techniques, is required to orientate them before effective electron emission from the graphene edges can occur. These transfer techniques have been shown to lead to additional defects to the as-grown graphene structure, thereby degrading its properties. Here, we present an approach to directly fabricate graphene onto metal nano-sized spindt tips (or nanocones) using the solid-state transformation of carbon deposited from a pulsed laser system at low temperature. Besides providing a layer of chemical and mechanical protection for the metal nanocones, the graphene-on-metal nanocones gave enhanced emission properties compared to bare metal nanocones. This was due to the reduction of effective field emission tunneling barrier, which was a result of graphene-metal charge transfer interactions. Controlling the metal nanocones density was also an important factor in determining the field emission performance, as electron screening from neighboring cones should be minimized.
    Full-text · Article · Aug 2014 · Advanced Materials Interfaces
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    ABSTRACT: In the present study, high-sensitivity photodetection has been demonstrated using an amorphous selenium (a-Se) based photodetector driven by nitrogen-doped diamond cold cathode. The emission current - applied voltage characteristics are compared between different lighting conditions, and their sensitivities are evaluated in terms of nominal quantum efficiency. The estimated nominal quantum efficiency was 10-40 for visible and up to 1000 for ultraviolet, proving a successful photodetection utilizing carrier multiplication in a-Se.
    No preview · Article · Jul 2014 · Canadian Journal of Physics