Short Description

Cascade energy transfer for cosensitizing DSSC with double core-shell structure

A second technology that has been developed for DSSC is the bilayer nanofiber photoanode. Instead of using a layer of nanofibers with small diameter (SNF) for light harvesting, a second layer of nanofibers with larger diameter (BNF) adjacent to the SNF layer is used concurrently for light scattering so that light can be scattered and trapped in the photoanode. The PCE can reach 9.5% with this technology. The work was published in Advanced Materials with high citations.

Bilayer Photoanode DSSC

A third technology that research our group have developed on DSSC was to insert carbon nanotube (CNT) completely in the truncated nanofibers (see diagram c below) such that they can be highly conductive. As such, photogenerated electrons can be transported efficiently to the electrode reducing the chance of recombination via (a) electrons recombining with sensitized dye, (b) electrolyte in the photoanode, or (c) at the TiO₂ particle-particle interface. The PCE can reach a high value of 10.24%. The work was published in Advanced Materials again with high citations.

TiO2 nanorods with embedded CNT for DSSC

Alternatively, graphene can also be used and inserted in the TiO2 nanofibers with diameter 60-70nm. The PCE registered between 9 and 10%, which is 22% higher than the conventional device without graphene. Further, a thicker photoanode of over 23 micrometers can be realized as compared to conventional which is typically no more than 13-15 micrometers. This facilitates thicker photoanode for use in DSSC that can harvest more light as well as electrolytes that are more suitable for use in thicker photoanode.

TiO2 nanofibers with embedded graphene for thicker photoanode DSSC

Nano-Environment

Nanofiber Filter for air filtration

Filtration of nano-aerosols using novel nanofiber filters (from CNN News and New York Times reports)

• CNN News

• New York Times

Pollutants are largely due to vehicular emissions, power plant emissions, and atmospheric reaction. Recent measurements made in Hong Kong near the cross-harbor tunnel revealed a large amount of submicron aerosols, especially aerosols less than 100 nm (also known as nano-aerosols or ultrafine particles) which are attributed to vehicular emissions and atmospheric photochemical reactions. This is consistent with testing that our group have performed in the lab also on nano-aerosols predominantly less than 100nm with aerated sodium chloride solution.

Nano-aerosols by virus of their small size can be inhaled easily into our lung from which they enter our cardiovascular system leading to various respiratory and cardioascular problems. Filters made of

 

nanofiber from polymeric materials (left) can effectively filter the nano-aerosols (see right figure) up to 90%. However, the pressure drop can be very large. We have developed multilayer nanofiber technology wherein the same amount of nanofibers to achive a given eefficiency has been redistributed from a single layer into multiple layers. This ensures the pores in the fiber mat to be wide open. This is especially advantageous for producing wearable face mask that stops infiltration of viruses from infectious diseases (from common cold to more severe viruses SARS, H1N1, H5N1 etc.) as well as nanoparticle pollutants from diesel/LPG vehicle emissions. This can also be used in indoor air filter to remove submicron and ultrafine particles as well as cabin filter of motor vehicles protecting health of passengers. By engineering the nanofiber technology (design, configuration, material) accordingly, the capture target (i.e. spec filtration efficiency) of a given submicron particle size under a given challenging velocity can be achieved, see efficiency vs. size for different amount of nanofibers installed in the filter.

Novel technologies developed:

Multilayer Nanofiber technology – A quasi-3D structure with wide open pores to achieve high efficiency and low pressure drop

Charged Electret Nanofiber – Charged electret nanofiber filter that provides high efficiency while reducing pressure drop, high quality factor exceeding 0.1Pa^-1 across all challenging aerosols size. Charges stay in filter for at least 3 months in humid environment without decay.

Portable filter testing – Portable filter test developed that proves nanofiber can effectively captured real nano-aerosols from vehicular emissions and atmospheric photochemical reactions

Composite filter – Filter can capture nano-aerosols over extended period with high capacity, low pressure drop, and high efficiency even during initial period.

Nanofiber photocatalysts for breaking-down harmful gases in air and harmful organics in water

Photocatalyst function

As the photocatalyst, primarily TiO₂ with wide-band gap, is being irradiated by light, electrons are excited and jump from the valence band to the conduction band. Electrons can combine with the oxygen in air to form super-anions, while the holes in the valence band can combine with the water vapor in air to form hydroxyl radical. Both the superanions and hydroxyl radicals break down by oxidation the harmful adsorbed onto the photocatalyst.

A suite of nanofiber-based photocatalysts have been developed targeting at effectiveness, reduction of recombinations, and operating under visible light:

TiO2 composite nanofibers with hierarchy structure reducing separated electrons-holes from recombination

Harvest UV and vis light unlike TiO2 which harvest only UV light (~5% light spectrum)

Photocatalyst integrated into flexible and rigid surfaces, washable and cleanable

3X more effective than conventional gold standard TiO2 nanoparticles of 25nm (P25) in formaldehyde breakdown

7X at least more effective than conventional gold standard TiO2 nanoparticles of 25nm (P25) in NOx conversion

Breakdown poisonous 0-xylene gas

Can be used for face mask and air purifier

Breakdown harmful organics in water (proven extremely effective for Methylene Blue dye and Rhodamine dye) much more effective than TiO2 nanoparticles of 25nm (P25)

Nano-Innovations

Nanofibers of composite semiconductor with embedded Graphene

We have developed semiconductor nanofibers with embedded graphene that can be used for photonic devices such as solar cells and photocatalyst applications.

(B) Rotating Microfluidics Platform

Microfluidics can simulate many chemical and biological processes taking place at a micro-scale. This brings the lab processes to a more controlled environment requiring very small sample. It is convenient for point-of-use diagnosis. By rotating the microfluidic platform in a certain way, we can carry out further separation by centrifugal acceleration and mixing by secondary flow thus intensifying the process. When mixing is intense, this facilitates cell culture and even cell lysing. We are interested in both continuous as well as batch mixing in this research. Fig. 5a shows mixing in a rotating chamber due to continuous angular acceleration of a chamber generating complicated vortices in a 3-D manner providing effective mixing.

 

(Left) Batch mixing in microfluidics, (Right) mixing in continuous feed through a T-channel.

(C) Centrifugation

Centrifugal separation of a solid from a fluid involves rotating the bowl at high rotation speed generating centrifugal acceleration acting on the heavier phase, which gravitates toward the bowl displacing the light phase toward the axis. Unexpected phenomena and flow pattern can result in a centrifuge with complicated fluid dynamics. By understanding these phenomena, better centrifuge design and operation can be developed with improved process results.

 

Thin boundary layer observed on rotating pool from special illumination.

Our interest in centrifugation encompasses the following:

We are interested in innovations in water and wastewater :

Water Treatment:

Forward & Reverse osmosis

• Nanofiltration
• Ultrafiltration
• Adsorption
• Oxidation
• Lamella Sedimentation

Waste Water Treatment:

• Membrane Bioreactor
• High-Solids dewatering by centrifugation and Filter Press

We are interested to develop healthcare devices and software that can help patients and elderly through product enabled services and process innovations. This may include but not limited to intention-driven robots, electrical stimulation devices based on pain sensing, decision support system, innovative wound dressing, etc.

Selected Recent Publications

1. Y Li, WWF Leung*, “Introduction of graphene nanofibers into the perovskite layer of perovskite solar cells,” Chemsuschem, doi.org/10.1002/cssc.201800758, 2018.
2. Y Li, WWF Leung*, “Conditioning Lead Iodide with Dimethylsulfoxide And Hydrochloric Acid to Control Crystal Growth Improving Performance of Perovskite Solar Cell”, Solar Energy, Vol 157, pp 328-334, Nov 2017, doi:10.1016/j.solener.2017.08.011.
3. KKS Lo, WWF Leung*, Dye-Sensitized Solar Cells with Shear-Exfoliated Graphene Solar Energy, Solar Energy, 180, 16-24, 2019.
4. LJ Yang, WWF Leung*, “Electrospun TiO₂ Nanorod with Carbon Nanotube for Efficient Electron Collection in Dye Sensitized Solar Cell”, Advanced Materials, 25, #12, 1792-1795, Mar 25, 2013
5. LJ Yang, WWF Leung*, “Application of a Bilayer TiO₂ Nanofiber Photoanode for Optimization of Dye-Sensitized Solar Cells”, Advanced Materials, 23, #39, 4559-4562, Oct 18, 2011
6. LJ Yang, WWF Leung*, JC Wang, “Improvement of Light Harvesting in Dye Sensitized Solar Cell Based on Cascade Charge Transfer,” Nanoscale, 2013, 5 (16), 7493-7498 DOI:10.1039/C3NR01868G, 2013
7. LJ Yang, JC Wang, WWF Leung*, “Lead Iodide Thin Film Crystallization Control for High-Performance and Stable Solution-Processed Perovskite Solar Cells”, ACS Applied Materials and Interfaces, 2015, 7 (27), pp 14614–14619.
8. QQ Sun and WWF Leung*, “Charged PVDF multi-layer filters with enhanced filtration performance for filtering nano-aerosols”, Sep. and Purif. Tech. J., doi.org/10.1016/j.seppur.2018.11.063, 212, pp854-876, 2019.
9. WWF Leung*, Yuen Ting Chau, “Experiments on filtering nano-aerosols from vehicular and atmospheric pollutants under dominant diffusion using nanofiber filter”, Sep. and Purif. Tech. J., 10.1016/j.seppur.2018.12.021. 213, 186-198, 2019.
10. WWF Leung*, WY Hau, HF Choy, “Microfiber-nanofiber composite filter for high-efficiency and low pressure drop under nano-aerosol loading”, Sep. & Purif. Tech. J, 206 (2018) 26-38, https://doi.org/10.1016/j.seppur.2018.05.033
11. WWF Leung*, HF Choy, “Transition from depth to surface filtration for a low-skin effect filter subject to continuous loading of nano-aerosols”, Sep & Puri Tech, 190, 202-210, 2018.
12. WWF Leung*, HF Choy, “Transition from depth to surface filtration for a high-efficiency, high-skin effect, nanofiber filter under continuous nano-aerosol loading”, Chem. Eng. Sci., 182, 67-76, 2018.
13. WWF Leung*, WY Hau, “Skin layer in cyclic loading-cleaning of a nanofiber filter in filtering nano- aerosols”, Sep & Puri Tech, 188, 367-378, 2017.
14. WY Hau, WWF Leung*, Experimental investigation of backpulse and backblow cleaning of nanofiber filter loaded with nano-aerosols, Sep & Puri Tech, 163, 30-38, 2016.
15. WWF Leung*, WY Hau, “A model of backpulse and backblow cleaning of nanofiber filter loaded with nano-aerosols”, Sep & Puri Tech J., 169, 171-178, 2016; doi: 10.1016/j.seppur.2016.06.007
16. WWF Leung*, CH Hung, “Skin effect in nanofiber filtration of submicron aerosols”, Separation and Purification Technology, 92, 174-180, 2012.
17. CH Hung, WWF Leung*, “Investigating the filtration of nano-aerosol using nanofiber filter under low Peclet number regime”, Separation Purification Technology, Vol 79, Issue 1, May 2011, 34-42.
18. WWF Leung*, CH Hung, and PT Yuan, “Effect of face velocity, nanofiber packing density and thickness on filtration performance of filters with nanofibers coated on a substrate” Separation and Purification Technology, 71, 30-37, 2010
19. WWF Leung*, CH Hung, and PT Yuan, “Continuous filtration of sub-micron aerosols by filter composed of multi-layers including a nano-fiber layer” J. Aerosol Science and Technology, 43:1174–1183, 2009.
20. WWF Leung* and CH Hung, “Investigation on pressure drop evolution of fibrous filter operating in aerodynamic slip regime under continuous loading of sub-micron aerosols”, Separation and Purification Technology, 63, pp 691-700, 2008.
21. M. Kanjwal*, WWF Leung*, Titanium based composite-graphene nanofibers as high-performance photocatalyst for formaldehyde gas purification, Ceramics International, doi.org/10.1016/j.ceramint.2018.12.022
22. CC Pei, KKS Lo, WWF Leung*, “Titanium-Zinc-Bismuth Oxides-Graphene Composite Nanofibers as High-Performance Photocatalyst for Gas Purification,” Sep. & Purif. Tech. J, Vol 184, pp 205-212, 2017.
23. CC Pei, WWF Leung*, “Enhanced photocatalytic activity of electrospun TiO2/ZnO nanofibers with optimal anatase/rutile ratio“, Catalyst Comm., 37 (2013) 100–104, DOI:10.1016/j.catcom.2013.03.029
24. CC Pei, WWF Leung*, “Photocatalytic degradation of Rhodamine B by TiO₂/ZnO nanofibers under visible light irradiation,” J. of Sep. & Purif. Tech, 114 (2013) 108–116, DOI:10.1016/j.seppur.2013.04.032, 2013
25. CC Pei, WWF Leung*, “Solar photocatalytic oxidation of NO by electronspun TiO2/ZnO composite nanofiber mat for enhancing indoor air quality,” J. of Chemical Technology and Biotechnology, 89, #11, 2014, 1646-1652, DOI 10.1002/jctb.4506
26. CC Pei, WWF Leung*, “Photocatalytic oxidation of nitrogen monoxide and o-xylene byTiO2/ZnO/Bi2O3nanofibers: Optimization, kinetic modeling and mechanisms,” Applied Catalysis B: Environmental, 174-175 (2015), 515-525, DOI 10.1016/j.apcatb.2015.03.021
27. Y Ren, WWF Leung*, “Multiplexing Diagnostic Assays Using Novel Centrifugal Microfluidic Emulsification and Separation Platform”, Micromachines, 2016, 7, 17; doi:10.3390/mi7020017
28. Y Ren, WWF Leung*, “Flow and mixing in zigzag channel”, Chemical Engineering, 215-216, pp 561–578, 2013
29. Y Ren, WWF Leung, “Experimental and numerical investigation on flow and mixing in batch-mode rotating microfluidics”, International Journal of Heat and Mass Transfer, 60, 95–104, 2013
30. Y Ren, LMC Chow, WWF Leung*, “Pichia pastoris culture on centrifugal microfluidic platform,” Biomedical Microdevice, Volume 15, Issue 2, pp 321-337, April 2013
31. Y Ren, WWF Leung*, “Vortical flow and mixing in rotating microfluidics,” Computers and Fluids, 79 (2013),
32. WWF Leung* and Y Ren, “Crossflow and mixing in obstructed and width-constricted rotating radial microchannel,” International Journal of Heat and Mass Transfer, 64 (2013) 457–467, DOI: 10.1016/j.ijheatmasstransfer.2013.04.064, 2013
33. WWF Leung* and Y Ren, “Scale-up on mixing in rotating microchannel under subcritical and supercritical modes“, International Journal of Heat and Mass Transfer, 77 (2014) 157-172.
34. SC Fu, WWF Leung, R. So, “A Lattice Boltzmann and Immersed Boundary Scheme for Model Blood Flow in Constricted Pipes: Part 1 – Steady Flow,” in press, Comm. in computational Physics,  doi: 10.4208/cicp.171011.180712a, Vol. 14, No. 1, pp. 126-152, July 2013
35. SC Fu, R. So, WWF Leung, “A Lattice Boltzmann and Immersed Boundary Scheme for Model Blood Flow in Constricted Pipes: Part 2 – Pulsatile Flow,”, Comm. in computational Physics, doi: 10.4208/cicp.171011.190712a, Vol. 14, No. 1, pp. 153-173, July 2013
36. SC Fu, R. So, WWF Leung, “Linearized-Boltzmann-Type-Equation-Based Finite Difference Method for Thermal Incompressible Flow”, Computers and Fluids, Vol 69, pp 67-80, Oct 30, 2012.
37. SC Fu, R. So, WWF Leung, “A discrete Flux Scheme for Aerodynamics and Hydrodynamics flows”, Comm. in computational Physics, 9, pp. 1257-1283, 2011.
38. SC Fu, R. So, WWF Leung, “Stochastic Finite Difference Lattice Boltzmann Method for Steady Incompressible Flows.” J. Computational Physics, 229, 17, pp 6084-6103, 2010.
39. SC Fu, WWF Leung, R So, “A lattice Boltzmann Method base numerical scheme for microchannel flows” J. Fluid Engineering, vol. 131, Issue 8, Aug. 2009.
40. WWF Leung, “Measurement of low yield-stress materials“, J. Chinese Inst. of Chem. Eng., vol. 39, 107-115, 2008.

Handbook Chapters

1. Scale-up of Solid-Liquid Separation Equipment by R. Wakeman R. and S. Tarleton, Sedimenting Centrifuges, Elsevier, 2005.

2. Plant Design Handbook for Mineral Processing, pp. 1262-1288, published by Soc. of Min. Eng., 2002.

3. Perry’s Chemical Engineers Handbook, 7 ed., pp. 18-106 to 18-125, McGraw-Hill, New York, NY, 1997.

4. Handbook on Separation Techniques for Chemical Engineers, pp. 4-63 to 4-96, McGraw-Hill, New York, NY, 1997.

Industrial Centrifugation Technology, McGraw-Hill, NY, 1998	Centrifugal Separation in Biotechnology, Academic Press/Elsevier, UK, 2007