ORCID Profile
0000-0003-1688-4931
Current Organisation
The Hong Kong Polytechnic University
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Publisher: Elsevier BV
Date: 08-2021
Publisher: Wiley
Date: 08-04-2014
Abstract: Nanocomposites that contain reinforcements with preferred orientation have attracted significant attention because of their promising applications in a wide range of multifunctional fields. Many efforts have recently been focused on developing facile methods for preparing aligned graphene sheets in solvents and polymers because of their fascinating properties including liquid crystallinity and highly anisotropic characteristics. Self-aligned in situ reduced graphene oxide (rGO) olymer nanocomposites are prepared using an all aqueous casting method. A remarkably low percolation threshold of 0.12 vol% is achieved in the rGO/epoxy system owing to the uniformly dispersed, monolayer graphene sheets with extremely high aspect ratios (>30000). The self-alignment into a layered structure at above a critical filler content induces a unique anisotropy in electrical and mechanical properties due to the preferential formation of conductive and reinforcing networks along the alignment direction. Accompanied by the anisotropic electrical conductivities are exceptionally high dielectric constants of over 14000 with 3 wt% of rGO at 1 kHz due to the charge accumulation at the highly-aligned conductive filler/insulating polymer interface according to the Maxwell-Wagner-Sillars polarization principle. The highly dielectric rGO/epoxy nanocomposites with the engineered structure and properties present high performance electromagnetic interference shielding with a remarkable shilding efficiency of 38 dB.
Publisher: Royal Society of Chemistry (RSC)
Date: 2013
DOI: 10.1039/C3TC31497A
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/D0MH00922A
Abstract: A novel multidimensional strain sensor mimicking human skin is developed. In-plane and pressure subsensors are integrated into a sensor with exceptional sensitivity and selectivity capable of detecting stimuli from 3D directions.
Publisher: Springer Science and Business Media LLC
Date: 02-02-2022
DOI: 10.1007/S40820-022-00797-6
Abstract: With the mandate of worldwide carbon neutralization, pursuing comfortable living environment while consuming less energy is an enticing and unavoidable choice. Novel composite aerogels with super thermal insulation and high sunlight reflection are developed for energy-efficient buildings. A solvent-assisted freeze-casting strategy is used to produce boron nitride nanosheet olyvinyl alcohol (BNNS/PVA) composite aerogels with a tailored alignment channel structure. The effects of acetone and BNNS fillers on microstructures and multifunctional properties of aerogels are investigated. The acetone in the PVA suspension enlarges the cell walls to suppress the shrinkage, giving rise to a lower density and a higher porosity, accompanied with much diminished heat conduction throughout the whole product. The addition of BNNS fillers creates whiskers in place of disconnected transverse ligaments between adjacent cell walls, further ameliorating the thermal insulation transverse to the cell wall direction. The resultant BNNS/PVA aerogel delivers an ultralow thermal conductivity of 23.5 mW m −1 K −1 in the transverse direction. The superinsulating aerogel presents both an infrared stealthy capability and a high solar reflectance of 93.8% over the whole sunlight wavelength, far outperforming commercial expanded polystyrene foams with reflective coatings. The anisotropic BNNS/PVA composite aerogel presents great potential for application in energy-saving buildings.
Publisher: Wiley
Date: 21-05-2019
Publisher: American Chemical Society (ACS)
Date: 15-03-2023
Publisher: American Chemical Society (ACS)
Date: 09-02-2018
Abstract: Unidirectional graphene aerogels (UGAs) with tunable densities, degrees of alignment, and electrical conductivities are prepared by varying the average size of precursor graphene oxide (GO) sheets between 1.1 and 1596 μm
Publisher: Elsevier BV
Date: 08-2015
Publisher: Elsevier BV
Date: 08-2021
Publisher: Springer Science and Business Media LLC
Date: 04-05-2021
DOI: 10.1007/S40820-021-00615-5
Abstract: Flexible multidirectional strain sensors are crucial to accurately determining the complex strain states involved in emerging sensing applications. Although considerable efforts have been made to construct anisotropic structures for improved selective sensing capabilities, existing anisotropic sensors suffer from a trade-off between high sensitivity and high stretchability with acceptable linearity. Here, an ultrasensitive, highly selective multidirectional sensor is developed by rational design of functionally different anisotropic layers. The bilayer sensor consists of an aligned carbon nanotube (CNT) array assembled on top of a periodically wrinkled and cracked CNT–graphene oxide film. The transversely aligned CNT layer bridge the underlying longitudinal microcracks to effectively discourage their propagation even when highly stretched, leading to superior sensitivity with a gauge factor of 287.6 across a broad linear working range of up to 100% strain. The wrinkles generated through a pre-straining/releasing routine in the direction transverse to CNT alignment is responsible for exceptional selectivity of 6.3, to the benefit of accurate detection of loading directions by the multidirectional sensor. This work proposes a unique approach to leveraging the inherent merits of two cross-influential anisotropic structures to resolve the trade-off among sensitivity, selectivity, and stretchability, demonstrating promising applications in full-range, multi-axis human motion detection for wearable electronics and smart robotics.
Publisher: American Chemical Society (ACS)
Date: 02-03-2017
Abstract: Ultralight, high-performance electromagnetic interference (EMI) shielding graphene foam (GF) oly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites are developed by drop coating of PEDOT:PSS on cellular-structured, freestanding GFs. To enhance the wettability and the interfacial bonds with PEDOT:PSS, GFs are functionalized with 4-dodecylbenzenesulfonic acid. The GF/PEDOT:PSS composites possess an ultralow density of 18.2 × 10
Publisher: Elsevier BV
Date: 07-2019
Publisher: Elsevier BV
Date: 10-2014
Publisher: Elsevier BV
Date: 10-2019
Publisher: Elsevier BV
Date: 02-2021
Publisher: American Chemical Society (ACS)
Date: 11-07-2018
Abstract: Hexagonal boron nitride (h-BN) has tremendous potential for dielectric energy storage by rationally assembling with graphene. We report the fabrication of microlaminate composites consisting of alternating reduced graphene oxide (rGO) and h-BN nanosheets embedded in a polyurethane (PU) matrix using a novel, two-step bidirectional freeze casting process. Porous, highly-aligned rGO-PU aerogels having ultrahigh dielectric constants with relatively high dielectric losses and low dielectric strengths are fabricated by initial freeze casting. The losses are suppressed, whereas the dielectric strengths are restored by assembling the porous rGO-PU skeleton with electrically insulating BN-PU tunneling barrier layers in the second freeze casting routine. The ligaments bridging the conductive rGO-PU layers are effectively removed by the BN-PU barrier layers, eliminating the current leakage in the transverse direction. The resultant rGO-PU/BN-PU microlaminate composites deliver a remarkable dielectric constant of 1084 with a low dielectric loss of 0.091 at 1 kHz. By virtue of synergy arising from both the rGO-PU layers with a high dielectric constant and the BN-PU barrier layers with a high dielectric strength, the microlaminate composites present a maximum energy density of 22.7 J/cm
Publisher: American Chemical Society (ACS)
Date: 04-12-2012
DOI: 10.1021/NN303904Z
Abstract: This study demonstrates that large-size graphene oxide (GO) sheets can impart a tremendous positive impact on self-alignment, electrical conductivity, and mechanical properties of graphene papers. There is a remarkable, more than 3-fold improvement in electrical conductivity of the papers made from ultralarge GO sheets (with an average area of 272.2 μm(2)) compared to that of the small GO counterpart (with an average area of 1.1 μm(2)). The corresponding improvements in Young's modulus and tensile strength are equally notable, namely 320% and 280%, respectively. These improvements of bulk properties due to the large GO sheets are correlated to multiscale elemental and structural characteristics of GO sheets, such as the content of carboxyl groups on the GO edge, C/O ratio and Raman D/G-band intensity ratio of GO on the molecular-scale, and the degree of dispersion and stacking behavior of GO sheets on the microscale. The graphene papers made from larger GO sheets exhibit a closer-stacked structure and better alignment as confirmed by the fast Fourier transform analysis, to the benefits of their electrical conductivity and mechanical properties. The molecular dynamics simulation further elucidates that the enhanced intersheet interactions between large GO sheets play a key role in improving the Young's modulus of GO papers. The implication is that the said properties can be further improved by enhancing the intersheet stress transfer and electrical conduction especially through the thickness direction.
Publisher: Elsevier BV
Date: 2021
Publisher: Royal Society of Chemistry (RSC)
Date: 2018
DOI: 10.1039/C7MH00984D
Abstract: A CVD-grown high-density 3D multilayer graphene web (MGW) is used as the filler for polymer nanocomposites, delivering exceptional electrical and thermal conductivities with outstanding fracture toughness.
Publisher: Elsevier BV
Date: 08-2013
Publisher: Elsevier BV
Date: 10-2017
Publisher: Wiley
Date: 24-03-2018
Publisher: Elsevier BV
Date: 08-2021
Publisher: Royal Society of Chemistry (RSC)
Date: 2023
DOI: 10.1039/D2TA09983G
Abstract: A highly anisotropic boron nitride composite aerogel is obtained at a low freezing temperature, exhibiting excellent thermal insulation and solar reflectance for energy efficient cooling.
Publisher: Wiley
Date: 12-03-2020
Publisher: Wiley
Date: 05-02-2022
Abstract: Solar‐powered water evaporation is a straightforward, practical approach to use solar energy for water desalination. Solar absorbers made from photothermal materials capable of effectively confining heat and pumping water to the evaporation surface are essential for a high energy efficiency. However, separate designs of water transport routes and thermal insulating layers are required to simultaneously achieve desired water and thermal managements. This work reports an integrated design for efficient multifunctional capabilities through rational assembly of spectrally modified Ti 3 C 2 T x (SM‐Ti 3 C 2 T x ) nanosheets and polyvinyl alcohol (PVA) into a multiscale 3D aerogel with a feather‐like microstructure. The aerogel contains longitudinal struts with transversely parallel ligaments developed at an angle of ≈ 60 ° from the struts, resembling the microstructure of down feathers in penguins and thus leading to excellent thermal insulation. The hydrophilic porous ligaments serve as upward water transport channels, pumping the water to the evaporation surface while confining it within the ligaments to avoid oversaturation. These functional features endow the composite aerogel with a high energy efficiency of 88.52% and an evaporation rate of 0.92 kg m −2 h −1 at a weak solar irradiance of 0.5‐sun, indicating its great potential for practical solar‐powered water desalination under natural sunlight.
Publisher: Royal Society of Chemistry (RSC)
Date: 2012
DOI: 10.1039/C2JM34870E
Publisher: Wiley
Date: 31-05-2021
Abstract: The electrode–electrolyte interfaces play a critical role in influencing the cyclic stability, Coulombic efficiency, and safety of rechargeable batteries. Although there are many recent efforts for investigating the solid electrolyte interface formed on anodes, much less attention has been paid to examine the cathode–electrolyte interface (CEI) established on cathodes. Understanding of the chemistry, morphology, and structure of CEI layers is still illusive requiring further in‐depth characterization. The cryogenic electron microscopy is used to reveal a 1.1 nm thick CEI layer formed on a flower‐shaped, carbon‐coated Na 3 V 2 (PO 4 ) 3 /C (NVP/C) cathode in ether‐based electrolyte for Na‐ion batteries. The rationally designed NVP/C cathode delivers cyclic stability with a capacity retention of over 88% at 50 mA g −1 after 1600 cycles and an excellent high‐rate capability at up to 3200 mA g −1 . These findings may shed new light on the design of CEI layers to achieve high energy and power densities in rechargeable Na‐ion/metal batteries.
Publisher: Elsevier BV
Date: 12-2014
Publisher: American Chemical Society (ACS)
Date: 27-05-2014
DOI: 10.1021/NN500590G
Abstract: Cellular-structured graphene foam (GF)/epoxy composites are prepared based on a three-step fabrication process involving infiltration of epoxy into the porous GF. The three-dimensional (3D) GF is grown on a Ni foam template via chemical vapor deposition. The 3D interconnected graphene network serves as fast channels for charge carriers, giving rise to a remarkable electrical conductivity of the composite, 3 S/cm, with only 0.2 wt % GF. The corresponding flexural modulus and strength increase by 53 and 38%, respectively, whereas the glass transition temperature increases by a notable 31 °C, compared to the solid neat epoxy. The GF/epoxy composites with 0.1 wt % GF also deliver an excellent fracture toughness of 1.78 MPa·m(1/2), 34 and 70% enhancements against their "porous" epoxy and solid epoxy counterparts, respectively. These observations signify the unrivalled effectiveness of 3D GF relative to 1D carbon nanotubes or 2D functionalized graphene sheets as reinforcement for polymer composites without issues of nanofiller dispersion and functionalization prior to incorporation into the polymer.
Publisher: Elsevier BV
Date: 06-2013
Publisher: Elsevier BV
Date: 06-2020
Publisher: Wiley
Date: 05-10-2021
Abstract: A rational approach is proposed to design soft multifunctional sensors capable of detection and discrimination of different physical stimuli. Herein, a flexible multifunctional sensor concurrently detecting and distinguishing minute temperature and pressure stimuli in real time is developed using electrospun carbon nanofiber (CNF) films as the sole sensing material and electrical resistance as the only output signal. The stimuli sensitivity and discriminability are coordinated by tailoring the atomic‐ and device‐level structures of CNF films to deliver outstanding pressure and temperature sensitivities of − 0.96 kPa −1 and − 2.44% ° C −1 , respectively, enabling mutually exclusive sensing performance without signal cross‐interference. The CNF multifunctional sensor is considered the first of its kind to accomplish the stimulus discriminability using only the electrical resistance as the output signal, which is most convenient to monitor and process for device applications. As such, it has distinct advantages over other reported sensors in its simple, cost‐effective fabrication and readout system. It also possesses other invaluable traits, including good bending stability, fast response time, and long‐term durability. Importantly, the ability to simultaneously detect and decouple temperature and pressure stimuli is demonstrated through novel applications as a skin‐mountable device and a flexible game controller.
Publisher: Elsevier BV
Date: 2014
Publisher: Springer Science and Business Media LLC
Date: 22-09-2022
DOI: 10.1038/S41467-022-33234-8
Abstract: Cooling in buildings is vital to human well-being but inevitability consumes significant energy, adding pressure on achieving carbon neutrality. Thermally superinsulating aerogels are promising to isolate the heat for more energy-efficient cooling. However, most aerogels tend to absorb the sunlight for unwanted solar heat gain, and it is challenging to scale up the aerogel fabrication while maintaining consistent properties. Herein, we develop a thermally insulating, solar-reflective anisotropic cooling aerogel panel containing in-plane aligned pores with engineered pore walls using boron nitride nanosheets by an additive freeze-casting technique. The additive freeze-casting offers highly controllable and cumulative freezing dynamics for fabricating decimeter-scale aerogel panels with consistent in-plane pore alignments. The unique anisotropic thermo-optical properties of the nanosheets combined with in-plane pore channels enable the anisotropic cooling aerogel to deliver an ultralow out-of-plane thermal conductivity of 16.9 mW m −1 K −1 and a high solar reflectance of 97%. The excellent dual functionalities allow the anisotropic cooling aerogel to minimize both parasitic and solar heat gains when used as cooling panels under direct sunlight, achieving an up to 7 °C lower interior temperature than commercial silica aerogels. This work offers a new paradigm for the bottom-up fabrication of scalable anisotropic aerogels towards practical energy-efficient cooling applications.
Publisher: American Chemical Society (ACS)
Date: 24-12-2018
Abstract: Advanced wearable strain sensors with high sensitivity and stretchability are an essential component of flexible and soft electronic devices. Conventional metal- and semiconductor-based strain sensors are rigid, fragile, and opaque, restricting their applications in wearable electronics. Graphene-based percolative structures possess high flexibility and transparency but lack high sensitivity and stretchability. Inspired by the highly flexible spider web architecture, we propose semitransparent, ultrasensitive, and wearable strain sensors made from an elastomer-filled graphene woven fabric (E-GWF) for monitoring human physiological signals. The highly flexible elastomer microskeleton and the hierarchical structure of a graphene tube offer the strain sensor with both excellent sensing and switching capabilities. Two different types of E-GWF sensors, including freestanding E-GWF and E-GWF olydimethylsiloxane (PDMS) composites, are developed. When their structure is controlled and optimized, the E-GWF strain sensors simultaneously exhibit extraordinary characteristics, such as a high gauge factor (70 at 10% strain, which ascends to 282 at 20%) in respect to other semitransparent or transparent strain sensors, a broad sensing range up to 30%, and excellent linearity. The E-GWF/PDMS composite sensor shows a unique reversible switching behavior at a high strain level of 30-50%, making it a suitable material for fast and reversible strain switching required in many early warning systems. With a view to real-world applications of these sensors and switches, we demonstrate human motion detection and switch controls of light-emitting-diode l s and liquid-crystal-display circuits. Their unique structure and capabilities can find a wide range of practical applications, such as health monitoring, medical diagnosis, early warning systems for structural failure, and wearable displays.
Publisher: Royal Society of Chemistry (RSC)
Date: 2018
DOI: 10.1039/C7NH00147A
Abstract: We report dual-functional wearable strain sensors and switches with tunable sensitivities and switching capabilities for wearable wireless health monitoring.
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D1MH00907A
Abstract: This review summarizes strategies for achieving high thermal conductivities of polymer composites by tailoring orientation of fillers that is hardly achieved in conventional particulate-filled composites for emerging thermal management applications.
Publisher: IOP Publishing
Date: 16-06-2020
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D1MH00018G
Abstract: A flexible temperature sensor is developed using electrospun aligned carbon nanofiber films by tuning their atomic structures governed by carbonization temperature. It shows exceptional sensitivity of 1.52% °C −1 and good stimuli discriminability.
Publisher: American Chemical Society (ACS)
Date: 19-11-2021
Abstract: Mechanochromic smart membranes capable of optical modulation have great potential in smart windows, artificial skins, and camouflage. However, the realization of high-contrast optical modulation based on light scattering activated at a low strain remains challenging. Here, we present a strategy for designing mechanochromic scattering membranes by introducing a Young's modulus mismatch between the two interdigitated polydimethylsiloxane phases with weak interfaces in a periodic three-dimensional (3D) structure. The refractive index-matched interfaces of the nanocomposite provide a high optical transparency of 93%. Experimental and computational studies reveal that the 3D heterogeneity facilitates the generation of numerous nanoscale debonds or "nanogaps" at the modulus-mismatching interfaces, enabling incident light scattering under tension. The heterogeneous scatterer delivers both a high transmittance contrast of >50% achieved at 15% strain and a maximum contrast of 82%. When used as a smart window, the membrane demonstrates effective diffusion of transmitting sunlight, leading to moderate indoor illumination by eliminating extremely bright or dark spots. At the other extreme, such a 3D heterogeneous design with strongly bonded interfaces can enhance the coloration sensitivity of mechanophore-dyed nanocomposites. This work presents insights into the design principles of advanced mechanochromic smart membranes.
No related grants have been discovered for Xi Shen.