Yulong Ying

Ultrafast viscous water flow through nanostrand-channelled graphene oxide membranes

Pressure-driven ultrafiltration membranes are important in separation applications. Advanced filtration membranes with high permeance and enhanced rejection must be developed to meet rising worldwide demand. Here we report nanostrand-channelled graphene oxide ultrafiltration membranes with a network of nanochannels with a narrow size distribution (3-5 nm) and superior separation performance. This permeance offers a 10-fold enhancement without sacrificing the rejection rate compared with that of graphene oxide membranes, and is more than 100 times higher than that of commercial ultrafiltration membranes with similar rejection. The flow enhancement is attributed to the porous structure and significantly reduced channel length. An abnormal pressure-dependent separation behaviour is also reported, where the elastic deformation of nanochannels offers tunable permeation and rejection. The water flow through these hydrophilic graphene oxide nanochannels is identified as viscous. This nanostrand-channelling approach is also extendable to other laminate membranes, providing potential for accelerating separation and water-purification processes.

Graphene oxide nanosheet: an emerging star material for novel separation membranes

Advanced membranes that enable ultrafast permeance are very important for processes such as water purification and desalination. Ideally, an efficient ultrafast membrane should be as thin as possible to maximize the permeance, be robust enough to withstand the applied pressure and have a narrow distribution of pore size for excellent selectivity. Graphene oxide nanosheets offer an encouraging opportunity to assemble a brand new class of ultrathin, high-flux and energy-efficient sieving membranes because of their unique two-dimensional and mono-atom thick structure, outstanding mechanical strength and good flexibility as well as their facile and large-scale production in solution. The current state-of-the-art in graphene oxide membranes will be reviewed based on their exceptional separation performance (gas, ions and small molecules). We will focus on the structure of nanochannels within the graphene oxide membranes, the permeance and rejection rate, and the interactions between graphene oxide sheets. The separation performance of graphene oxide membranes can be easily influenced by the state of oxygen-containing groups on the graphene oxide sheets, which provides much more straightforward strategies to tune the pore size of graphene oxide nanochannels when compared to other filtration membranes. We will illustrate in the review theoretical calculations to elucidate the potential of precisely controlling the ionic and small molecular sieving and water transport behaviour through graphene oxide nanochannels. This review will serve as a valuable platform to fully understand how the ions, small molecules and water are transported through the laminar graphene oxide membrane as well as the latest progress in graphene oxide separation membranes.

Flexible CuO nanosheets/reduced-graphene oxide composite paper: binder-free anode for high-performance lithium-ion batteries.

Flexible free-standing CuO nanosheets (NSs)/reduced graphene oxide (r-GO) hybrid lamellar paper was fabricated through vacuum filtration and hydrothermal reduction processes. A unique three-dimensional nanoporous network was achieved with CuO NSs homogeneously embedded within the r-GO layers. This hybrid lamellar composite paper was examined as a binder-free anode for lithium ion batteries, and demonstrated excellent cyclic retention with the specific capacity of 736.8 mA h g(-1) after 50 cycles. This is much higher than 219.1 mA h g(-1) of the pristine CuO NSs and 60.2 mA h g(-1) of r-GO film at the same current density of 67 mA g(-1). The high capacitance and excellent cycling performance were generated from the integrated nanoporous structure compose of CuO NSs spaced r-GO layers, which offered an efficient electrically conducting channels, favored electrolyte penetration, and buffered to the volume variations during the lithiation and delithiation process. These outstanding electrochemical capabilities of CuO NSs/r-GO paper holds great promise for flexible binder-free anode for lithium ion batteries.

Two-Dimensional Titanium Carbide for Efficiently Reductive Removal of Highly Toxic Chromium(VI) from Water

Two dimensional (2-D) Ti3C2Tx nanosheets are obtained by etching bulk Ti3C2Tx powders in HF solution and delaminating ultrasonically, which exhibit excellent removal capacity for toxic Cr(VI) from water, due to their high surface area, well dispersibility, and reductivity. The Ti3C2Tx nanosheets delaminated by 10% HF solution present more efficient Cr(VI) removal performance with capacity of 250 mg g(-1), and the residual concentration of Cr(VI) in treated water is less than 5 ppb, far below the concentration (0.05 ppm) of Cr(VI) in the drinking water standard recommended by the World Health Organization. This kind of 2-D Ti3C2Tx nanosheet can not only remove Cr(VI) rapidly and effectively in one step from aqueous solution by reducing Cr(VI) to Cr(III) but also adsorb the reduced Cr(III) simultaneously. Furthermore, these reductive 2-D Ti3C2Tx nanosheets are generally explored to remove other oxidant agents, such as K3[Fe(CN)6], KMnO4, and NaAuCl4 solutions, by converting them to low oxidation states. These significantly expand the potential applications of 2-D Ti3C2Tx nanosheets in water treatment.

Ultrafast Molecule Separation through Layered WS2 Nanosheet Membranes

Two-dimensional layered materials have joined in the family of size-selective separation membranes recently. Here, chemically exfoliated tungsten disulfide (WS2) nanosheets are assembled into lamellar thin films and explored as an ultrafast separation membrane for small molecules with size of about 3 nm. Layered WS2 membranes exhibit 5- and 2-fold enhancement in water permeance of graphene oxide membranes and MoS2 laminar membranes with similar rejection, respectively. To further increase the water permeance, ultrathin nanostrands are used as templates to generate more fluidic channel networks in the WS2 membrane. The water permeation behavior and separation performance in the pressure loading-unloading process reveal that the channels created by the ultrathin nanostrands are cracked under high pressure and result in a further 2-fold increase of the flux without significantly degrading the rejection for 3 nm molecules. This is supported by finite-element-based mechanical simulation. These layered WS2 membranes demonstrate up to 2 orders of magnitude higher separation performance than that of commercial membranes with similar rejections and hold the promising potential for water purification.

CuO nanosheets/rGO hybrid lamellar films with enhanced capacitance

CuO nanosheets (NSs)/reduced graphene oxide (rGO) hybrid lamellar films were prepared by vacuum filtration of CuO NSs/GO composite dispersions, followed by hydrothermal reduction. The CuO NSs/GO composite dispersions were assembled electrostatically by mixing a negatively charged GO sheets aqueous solution with a positively charged CuO NSs aqueous dispersion at room temperature. The prepared CuO NSs/rGO hybrid lamellar films exhibited a specific capacitance of 163.7 F g(-1), which is much higher than the 69.7 F g(-1) of CuO NSs and 66.0 F g(-1) of rGO. The effective specific capacitance was 82.5 F g(-1) after 1000 cycles, which was more than two times the 32.7 F g(-1) of CuO NSs electrodes. The synergistic redox activity of the CuO NSs, in combination with the high electronic conductivity of the rGO and the unique CuO NSs spaced sandwich-like porous structures, dominated the excellent capacitance of CuO NSs/rGO hybrid lamellar films. The sandwiched, lamellar, porous structures not only provide plenty of paths for electrolyte-ion access to the CuO NSs but also expose the rGO sheets to the electrolyte as much as possible. This process provides a potential way to synthesise metal oxide/GO composite electrodes for capacitors.

General incorporation of diverse components inside metal-organic framework thin films at room temperature

Porous metal-organic frameworks (MOFs) demonstrate great potential for numerous applications. Although hetero-functional components have been encapsulated within MOF crystalline particles, the uniform incorporation of functional species with different sizes, shapes and functions in MOF thin films with dual properties, especially at room temperature and without the degradation of the MOF framework, remains a significant challenge towards further enriching their functions for various purposes. Here we report a general method that can rapidly encapsulate diverse functional components, including small ions, micrometre-sized particles, inorganic nanoparticles and bioactive proteins, in MOF thin films at room temperature via a metal-hydroxide-nanostrand-assisted confinement technique. These functional component-encapsulated MOF composite thin films exhibit synergistic and size-selective catalytic, bio-electrochemical, conductive and flexible functionalities that are desirable for thin film devices, including catalytic membrane reactors, biosensors and flexible electronic devices.

Pressure-Assisted Synthesis of HKUST‑1 Thin Film on Polymer Hollow Fiber at Room Temperature toward Gas Separation

The scalable fabrication of continuous and defect-free metal-organic framework (MOF) films on the surface of polymeric hollow fibers, departing from ceramic supported or dense composite membranes, is a huge challenge. The critical way is to reduce the growth temperature of MOFs in aqueous or ethanol solvents. In the present work, a pressure-assisted room temperature growth strategy was carried out to fabricate continuous and well-intergrown HKUST-1 films on a polymer hollow fiber by using solid copper hydroxide nanostrands as the copper source within 40 min. These HKUST-1 films/polyvinylidenefluoride (PVDF) hollow fiber composite membranes exhibit good separation performance for binary gases with selectivity 116% higher than Knudsen values via both inside-out and outside-in modes. This provides a new way to enable for scale-up preparation of HKUST-1/polymer hollow fiber membranes, due to its superior economic and ecological advantages.

Polystyrene Sulfonate Threaded through a Metal–Organic Framework Membrane for Fast and Selective Lithium‐Ion Separation

Extraction of lithium ions from salt-lake brines is very important to produce lithium compounds. Herein, we report a new approach to construct polystyrene sulfonate (PSS) threaded HKUST-1 metal-organic framework (MOF) membranes through an in situ confinement conversion process. The resulting membrane PSS@HKUST-1-6.7, with unique anchored three-dimensional sulfonate networks, shows a very high Li+ conductivity of 5.53×10-4  S cm-1 at 25 °C, 1.89×10-3  S cm-1 at 70 °C, and Li+ flux of 6.75 mol m-2  h-1 , which are five orders higher than that of the pristine HKUST-1 membrane. Attributed to the different size sieving effects and the affinity differences of the Li+ , Na+ , K+ , and Mg2+ ions to the sulfonate groups, the PSS@HKUST-1-6.7 membrane exhibits ideal selectivities of 78, 99, and 10296 for Li+ /Na+ , Li+ /K+ , Li+ /Mg2+ and real binary ion selectivities of 35, 67, and 1815, respectively, the highest ever reported among ionic conductors and Li+ extraction membranes.

Binder-free three-dimensional porous Mn3O4 nanorods/reduced graphene oxide paper-like electrodes for electrochemical energy storage

This research demonstrates novel flexible and binder-free Mn3O4 nanorods (NRs)/reduced graphene oxide (rGO) hybrid papers with unique three-dimensional nanoporous networks were fabricated by filtration and a hydrothermal reduction process, where rGO acts not only as a flexible substrate but also as an electron conductor. The three-dimensional nanoporous networks were generated by the homogeneous intercalation of Mn3O4 NRs into the lamellar rGO layers, which exhibited excellent mechanical stability and provided electrically conducting channels to promote electrolyte penetration when used as electrodes for Li-ion batteries (LIBs) and supercapacitors. The prepared Mn3O4 NRs/rGO hybrid lamellar papers demonstrated excellent cyclic retention with the specific capacity of 669.6 mA h g−1 after 100 cycles in LIBs, which is 9 times higher than 65.8 mA h g−1 of γ-MnOOH/Mn3O4 mixed phase nanorods. Additionally, the three-dimensional porous hybrid Mn3O4 NRs/rGO papers also exhibit superior specific capacitance of 204.2 F g−1, two times higher than that of γ-MnOOH/Mn3O4 mixed phase nanorods, and only decreases by 10% after 2000 cycles in the supercapacitor. These Mn3O4 NRs/rGO papers hold promising potential for flexible electrochemical energy storage devices.