RESEARCH

Our group combines materials science and nanotechnology approaches to develop functional nanostructures for advanced energy storage, catalysis, water purification, and biosensor applications. Our research activities involve nanomaterials design and fabrication, surface modification, coating, arrays and thin-film fabrications, engineering of nanodefects, 3D printing, upcycling of plastics wastes to high-value carbon, materials characterization and device design.      

3D Printed Graphene-based Materials 
 

Graphene-based materials are key enablers in water-energy nexus technologies like water purification and energy storage, owing to their beneficial properties such as facile surface modification of functional group, adjustable interlayer pore channel for solvent transportation, excellent mechanical, and electrical conductivity. Massive efforts are being made to improve their chemical and physical properties, including combining them with other materials, and so on. However, one of the major challenges of implementing graphene-based materials (GBM) for rechargeable batteries and membrane filtration applications is the lack of structural design and controllable dimensions, which limit their optimum and consistency of performances. The rise of 3D printing technology has revolutionized the fabrication of GBM for advanced rechargeable batteries and membrane filtration. This technique enables accurate control of the shape and thickness of the GBM which can greatly influence the performances of the rechargeable batteries and membrane filtration. The benefits of using advanced 3D printing technology include the fact that it is automated, scalable, reduces manufacturing time/manpower costs by nearly 40%, complex geometries can be fabricated without the use of molds and different geometries, and multiple parts can be fabricated in a single build with precise dimensions. In this work, his team has developed various GBM ink formulations (with different functionalities and properties) and demonstrated that by customizing GBM into various 3D morphologies (such as an array, mesh, and film) using an aerosol jet printing technique, the specific capacity of rechargeable Zinc batteries can be enhanced by around 15% to 25% as compared to the traditional hot-press process and the permeance of water can be improved by 2X with similar rejection rates when compared to the conventional interfacial polymerization process. This amazing work has resulted in 3X provisional patents. These works were supported by a funding agency from the national additive manufacturing innovation cluster (NAMIC).      

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Figure 1: 3D printed graphene-based Materials for Rechargeable Batteries and Membrane Filtration

[1] “3D-Printing Membrane for Dye Removal from Wastewater” Singapore Patent Application No. 10202112559R (Filed on 11th Nov 2021)

[2] “Graphene-CNT-MnO2 Ink Formulation for 3D Printing Rechargeable Zinc-Air Batteries” Singapore Patent Application No. 10202105941S (Filed on 3rd Jun 2021)​

[3] “Graphene-CNT Ink Formulation for 3D Printing Rechargeable Zinc-Ion Batteries” Singapore Patent Application No. 10202105356S (Filed on 21st Mar 2021)

Energy Storage
 

Rechargeable lithium-oxygen batteries are emerging as next-generation batteries due to their high theoretical specific energy densities (up to 2−3 kWh/kg) compared to other existing rechargeable batteries. One of the grand challenges of lithium-oxygen batteries is that it suffers from sluggish kinetics reactions, leading to poor energy efficiency. Here, we focus on rational-design of electrodes that are able to lower the charging potential of the batteries (from d to f in Figure 2a) via a shorter electron pathway (Figure 1b). This key concept plays an important role in lithium-oxygen batteries. 

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Figure 2: (a) Full discharge-charge profile of Li-O2 battery. (b) Schematic of the proposed mechanisms.

 

Chemistry of Materials. 2016, 28, 5743-5752. Link

Journal of Power Sources, 2015, 294, 377-385. Link

Electrocatalysis
 

Hydrogen generated by the electrolysis of water has become an increasingly attractive energy carrier due to its high energy density. Electrocatalysts used for the hydrogen evolution reaction (HER) are important and key components for water splitting. It is well known that noble metals, such as Pt, are the most efficient HER electrocatalyst due to their fast reaction kinetics and low overpotential to drive the HER reaction. However, its high cost and low natural abundance hamper its wide applications. Therefore, alternative electrocatalysts such as metal carbide materials are being explored because they are low cost, have good stability, and
high catalytic activity towards HER. In addition, we also examine the doping effect of active metals into metal carbide to further enhance the electronic properties as shown in Figure 3a-b.

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Figure 3: (a) Atomistic configuration of Ni-W2C nanosheet at high hydrogen adsorption coverage. (b) Free-energy diagrams for HER of W2C and Ni-W2C high hydrogen adsorption coverage.

Science Partner Journal, 2019, DOI: 10.34133/2019/4029516. Link

Small, 2016, 12, 2859–2865. Link

Small, 2015,47, 6278–6284. Link

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2D-Based Membrane

 

Membrane-based technology has been demonstrated as an effective alternative to traditional separation methods (i.e. distillation and adsorption) for purifying water because of its high separation efficiency, low energy consumption, and ecofriendliness. The commercial nanofiltration (NF) membrane made of poly(ether sulfone) suffers from slow water permeance (ca. 10 L m−2 h−1 bar−1) because it partly operates at high pressure (up to 40 bar), leading to large energy consumption and subsequently high operational cost. Therefore, alternative membrane materials are required to remedy this issue. We use 2D nanosheets as elementary blocks to construct membranes which are used as a new-generation material for water purification because of their unique intrinsic structures (i.e., in-plane pores/defects and interlayer spacing) could render effective nanosieving, enabling selective transport of molecules to permeate through the nanochannels of the scaffold as shown in Figure 4a. Moreover, he can also engineer defects into 2D nanosheets to create additional channels for ultrafast solvent permeation (Figure 4b-c).

Figure 4: (a) Schematic illustration of solvent (in this case water) permeate through the 2D-based membrane (periodic pore system of the metal-organic framework) but rejecting the unwanted solutes. (b) Engineering defects into 2D nanosheets.

Journal of Membrane Science, 2019, 591, 117318. Link

Journal of Materials Chemistry A, 2017, 5, 20598-20602. Link

ACS Applied Materials & Interfaces, 2017, 9, 28079-28088. Link

Solar-Assisted Membrane Distillation

 

The demand for potable water has become increasingly important due to the exponentially increasing global population, which motivates tremendous efforts towards water sustainability. To this end, membrane distillation (MD), which is a thermal-driven process using a porous hydrophobic membrane, is an emerging technique. MD is promising vis-à-vis other more mainstay membrane technologies (such as reverse osmosis and nanofiltration) due to its low-pressure operation, high rejection of non-volatile solutes, and the capability to recover low-grade waste heat from the process industry. One of the key impediments to the more widespread use of MD is the energy cost in heating up and maintaining the feed temperature. This can be circumvented by the use of green energy from the sun to supply the required thermal energy, and is often employed in solar desalination to bridge the gap between fossil fuel and renewables while reducing the environmental impact. We focus on spacer materials for solar-assisted membrane distillation for water desalination (as shown in Figure 5a-c). 

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Figure 5: (a) Overview of the energy balance of the plasmonic MD system. (b) Thermal image of pristine NF (top) and Pt-MBT@Ag NSs/NF (bottom) immersed in pure water. (c) Recorded solar heat flux versus time over 24 h (from 0:00 to 24:00), and the corresponding MD distillate flux versus time using real seawater and the Pt-MBT@Ag NSs/NF spacer.

Journal of Materials Chemistry A, 2019, 7. 10206-10211. Link

Journal of Membrane Science, 2019, 572, 171-183. Link

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Biosensor

 

Colloidal semiconductor nanocrystals (NCs) have found applications in biological fluorescence labelling. NCs with tunable dual-colour and multiphoton emission could be useful for biolabeling. However, most of the NCs had a large size distribution, broad emission, low stability, and low optical quality at high doping levels (>1%). Herein, we explore the structural characteristics of heterostructure and used these water-soluble NCs for the labelling of cancerous HeLa and noncancerous Chinese hamster ovary (CHO) cells.

Figure 6: (a) Low-magnification of TEM image of uniform-sized NCs. (b) HR-TEM image of ZnS-CuS heterostructure. (c-d) Transmission and fluorescence images of HeLa cells with ZnS:0.25Cu@silica.

ACS Applied Nano Materials, 2020, 3, 3088-3096. Link

ChemPhysChem, 2016, 17, 2489-2495. Link

RECENT PUBLICATIONS

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"Solar 2D Membrane"

We are the first to utilize solar radiation and water for cleaning fouled OSN membrane and recover up to 99% of solvent permeance after photo-assisted cleaning. This unique cleaning technique of fouled OSN membrane using green energy could replace the use of precious organic solvents for membrane cleaning, thereby opening up a new path for making a smart and sustainable OSN membrane.