Hubiao Huang

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.

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.

Mesoporous separation membranes of {[Cu(BTC–H2)2·(H2O)2]·3H2O} nanobelts synthesized by ultrasonication at room temperature

Copper and 1,3,5-benzenetricarboxylic acid complex {[Cu(BTC–H2)2·(H2O)2]·3H2O} nanobelts were successfully prepared in aqueous solution at low temperature by mixing ultra-thin, highly positively charged copper hydroxide nanostrands with 1,3,5-benzenetricarboxylic acid for the first time. These nanobelts have a thickness of about 15 nm, width of 200–300 nm, and length of several micrometers. We found that the copper hydroxide nanostrands play the key role for the formation of these nanobelts. The effects of the synthesis parameters were investigated in detail. Due to their thin thickness, high aspect ratio and nice dispersion in solution, these {[Cu(BTC–H2)2·(H2O)2]·3H2O} nanobelts were successfully assembled into mesoporous membranes by filtration techniques. These {[Cu(BTC–H2)2·(H2O)2]·3H2O} nanobelts membranes demonstrated 96% rejection for 5 nm nanoparticles with flux of 4506 L m−2 h−1 bar−1 and were stable at pH 3. The separation performance of these membranes is 5 to 10 times higher than that of the commercial membranes with similar rejections. This method may provide a method for the synthesis of metal–organic complex nanostructures by using metal hydroxide nanostrands as precursors and extend their application for nanoparticle separation from water.

Superior separation performance of ultrathin gelatin films

Ultrathin gelatin films were prepared by filtering gelatin on a porous nanofibrous scaffold and cross-linked by glutaraldehyde. The gelatin thin films were obtained by using a copper hydroxide nanostrand scaffold after removal of the scaffold. At relative high pressure, the rejection properties of these gelatin films became worse due to mechanical fouling. However, the gelatin/carbon nanotube composite thin films prepared on a single-walled carbon nanotube scaffold could sustain a pressure up to 18 bars with more than 90% rejection for 2–3 nm molecules. The carbon nanotube layer dramatically increased the mechanical properties of the gelatin layer. The pressure dynamically controlled the network structure of the gelatin layer. The corresponding separation performances were investigated in detail. The gelatin films could be as thin as 62 nm. These gelatin films can separate 865 Dalton Direct Yellow 50 molecules at a rate of magnitude one to two orders higher than that of commercially available membranes with similar rejections.

Facile synthesis of highly fluorescent gelatin/Si nanocrystals composite thin films for optical detection of amines in water

We demonstrate a facile method to prepare gelatin/Si nanocrystal composite thin films via a filtration technique using metal hydroxide nanostrands as a sacrificial layer. This film is free-standing and demonstrates fast and reversible detection of amines in water.

Hetero-metal hydroxide nanostrand assisted synthesis of MIL-110 nanorod arrays on porous substrate

Oriented MIL-110 nanorod arrays on porous anodic aluminum oxide membrane were successfully prepared through a hetero-metal hydroxide nanostrand-assisted growth process in H3BTC aqueous solution. This process provides an environmentally friendly method for synthesis of MIL-110 nanorod arrays without using toxic metal ionic sources and organic solvents.