Luwei Sun

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.

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.

A facile fluorine-mediated hydrothermal route to controlled synthesis of rhombus-shaped Co3O4 nanorod arrays and their application in gas sensing

We have successfully synthesized rhombus-shaped Co3O4 nanorod (NR) arrays via a facile fluorine-mediated hydrothermal route involving the formation of Co(OH)F as precursor and then thermal conversion to porous Co3O4. The hydrothermal temperature was critical to the rhombic shape of the nanostructures. At low temperatures, the Co2+ ions interacted with F− anions to form CoF+ complexes and then reacted with OH− to form rhombus-shaped Co(OH)F. Above 100 °C, the disproportionation of formaldehyde from hydrolysis of hexamethylenetetramine (HMT) hindered the formation of Co(OH)F precursor. In this process, CO32− anions direct the dissolution–recrystallization process instead of F− anions due to stronger affinity of CO32− to Co2+. During the transformation from the precursor to the oxide, various annealing temperatures affect the gas sensing performance. The synthesized rhombus-shaped Co3O4 NR arrays gas sensor annealed at 450 °C showed the highest sensitivity to ethanol because of good contact, porous structure, good crystallinity, high surface-to-volume ratio and open space.

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.

In-plane mesoporous graphene oxide nanosheet assembled membranes for molecular separation

In-plane mesoporous graphene oxide (GO) sheets were prepared by a re-oxidation process and subsequently assembled into lamellar membranes. The in-plane pores significantly shortened the mass transport paths, resulting in 2–3 times higher water permeance than that of the pristine GO membrane, but continued to reject 3 nm molecules.

Defects induced ferromagnetism in ZnO nanowire arrays doped with copper

We report a preparation method of vertically aligned Cu-doped ZnO nanowire arrays by chemical vapor transportation combining with simple thermal diffusion. Copper has been incorporated into the nanowires. Analyses through Raman, photoluminescence and EPR indicate that besides the induced CuZn defects there are another two kinds of intrinsic defects, i.e. Zni and Vo, with their density varying with the thermal treatments. The strong green emission might be caused by a joint action of three different reasons: CuZn, Vo and surface defects. Intrinsic room-temperature ferromagnetism is observed in Cu-diffused samples, which is attributed to an indirect double-exchange mechanism associated with dual-donor (Zni and Vo) and variable-valence Cu ions. The valence state of Cu seems to have a strong connection both with ferromagnetism and optical response which suggests the possibility of engineering opto-magnetic switches based on a single nanowire. Furthermore, the diffusion method can be used to prepare room-temperature Cu-doped ZnO nanowire arrays for future spintronic devices.

Enhanced gas separation through well-intergrown MOF membranes: seed morphology and crystal growth effects

Cuboctahedral and octahedral HKUST-1 seeds were quickly prepared on a porous anodic alumina oxide support at room temperature within one hour, from water–ethanol and ethanol solvents. The cuboctahedral seeds were favourable for the synthesis of continuous and well intergrown HKUST-1 membranes. However, the octahedral seeds could not produce high quality HKUST-1 membranes. Interestingly, both the shape of the seeds and growth habits of the HKUST-1 seed and crystals in different solvents also determines the final quality of the HKUST-1 membranes. The gas separation performance of the well intergrown HKUST-1 membrane is good and beyond the Knudsen selectivity.

Tuning the photoluminescence of porous silicon nanowires by morphology control

A systematic study was carried out to investigate the morphology and photoluminescence (PL) of porous silicon nanowire (Si NW) arrays fabricated by metal-assisted chemical etching with diverse etching conditions and starting Si wafers. The morphology of the Si NWs, including the orientation and porosity, could be well controlled. For Si NWs prepared from p-Si-(111) wafers, the orientation switched from the 〈100〉 to the 〈111〉 direction as the hydrogen peroxide (H2O2) concentration was increased. In addition, the NW porosity could be adjusted by changing the H2O2 concentration and the substrate doping elements. The PL intensity of the NWs correlated directly with their porosity. The PL intensity was strongly dependent on the oxidizer concentration, substrate orientation, and doping element of the starting Si wafer. The maximum PL enhancement ratio reached 14.5 when the concentration of H2O2 was doubled. The photoluminescence of p-Si-(100) was 1.15, 1.39 and 1.45 times stronger than that of p-Si-(111) with 0.25, 0.5, and 0.75 M H2O2, respectively. The morphology-controllable fabrication and tunable optical properties of such porous Si NWs will promote research on the application of semiconductor optical nanodevices.

Zinc hydroxide nanostrands: unique precursors for synthesis of ZIF-8 thin membranes exhibiting high size-sieving ability for gas separation

Well-intergrown ZIF-8 membranes are prepared directly from zinc hydroxide nanostrands without any modification of the substrate in ethanol–water at room temperature in a short time and exhibit high molecular sieving performance for gas separation after secondary growth. This strategy exhibits excellent reproducibility and versatility and is suitable for large-scale production.

Indium-doped ZnO nanowires with infrequent growth orientation, rough surfaces and low-density surface traps

Indium-doped ZnO nanowires have been prepared by vapor transport deposition. With increasing In content, the growth orientation of the nanowires switches from [101_0] to infrequent [022_3] and the surface becomes rough. No surface-related exciton emission is observed in these nanowires. The results indicate that large surface-to-volume ratio, high free electron concentration, and low density of surface traps can be achieved simultaneously in ZnO nanowires via In doping. These unique properties make In-doped ZnO nanowire a potential material for photocatalysis application, which is demonstrated by the enhanced photocatalytic degradation of Rhodamine B.