pH-dependent water permeability switching and its memory in MoS2 membranes

Intelligent transport of molecular species across different barriers is critical for various biological functions and is achieved through the unique properties of biological membranes1-4. Two essential features of intelligent transport are the ability to (1) adapt to different external and internal conditions and (2) memorize the previous state5. In biological systems, the most common form of such intelligence is expressed as hysteresis6. Despite numerous advances made over previous decades on smart membranes, it remains a challenge to create a synthetic membrane with stable hysteretic behaviour for molecular transport7-11. Here we demonstrate the memory effects and stimuli-regulated transport of molecules through an intelligent, phase-changing MoS2 membrane in response to external pH. We show that water and ion permeation through 1T' MoS2 membranes follows a pH-dependent hysteresis with a permeation rate that switches by a few orders of magnitude. We establish that this phenomenon is unique to the 1T' phase of MoS2, due to the presence of surface charge and exchangeable ions on the surface. We further demonstrate the potential application of this phenomenon in autonomous wound infection monitoring and pH-dependent nanofiltration. Our work deepens understanding of the mechanism of water transport at the nanoscale and opens an avenue for the development of intelligent membranes.

Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation. However, their use is limited to aqueous solutions because GO membranes appear impermeable to organic solvents, a phenomenon not yet fully understood. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from large (10-20 μm) flakes. Without modification of sieving characteristics, these membranes can be made exceptionally thin, down to ∼10 nm, which translates into fast water and organic solvent permeation. We attribute organic solvent permeation and sieving properties to randomly distributed pinholes interconnected by short graphene channels with a width of 1 nm. With increasing membrane thickness, organic solvent permeation rates decay exponentially but water continues to permeate quickly, in agreement with previous reports. The potential of ultrathin GO laminates for organic solvent nanofiltration is demonstrated by showing >99.9% rejection of small molecular weight organic dyes dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification and filtration technologies.

Tunable sieving of ions using graphene oxide membranes

Graphene oxide membranes show exceptional molecular permeation properties, with promise for many applications. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of ∼9 Å (ref. 4), which is larger than the diameters of hydrated ions of common salts. The cutoff is determined by the interlayer spacing (d) of ∼13.5 Å, typical for graphene oxide laminates that swell in water. Achieving smaller d for the laminates immersed in water has proved to be a challenge. Here, we describe how to control d by physical confinement and achieve accurate and tunable ion sieving. Membranes with d from ∼9.8 Å to 6.4 Å are demonstrated, providing a sieve size smaller than the diameters of hydrated ions. In this regime, ion permeation is found to be thermally activated with energy barriers of ∼10-100 kJ mol^-1 depending on d. Importantly, permeation rates decrease exponentially with decreasing sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for the entry of water molecules and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.

Zn2SnO4 nanowires versus nanoplates: electrochemical performance and morphological evolution during Li-cycling

Zn2SnO4 nanowires have been synthesized directly on stainless steel substrate without any buffer layers by the vapor transport method. The structural and morphological properties are investigated by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The electrochemical performance of Zn2SnO4 nanowires is examined by galvanostatic cycling and cyclic voltammetry (CV) measurements in two different voltage windows, 0.005-3 and 0.005-1.5 V vs Li and compared to that of Zn2SnO4 nanoplates prepared by hydrothermal method. Galvanostatic cycling studies of Zn2SnO4 nanowires in the voltage range 0.005-3 V, at a current of 120 mA g(-1), show a reversible capacity of 1000 (±5) mAh g(-1) with almost stable capacity for first 10 cycles, which thereafter fades to 695 mAh g(-1) by 60 cycles. Upon cycling in the voltage range 0.005-1.5 V vs Li, a stable, reversible capacity of 680 (±5) mAh g(-1) is observed for first 10 cycles with a capacity retention of 58% between 10-50 cycles. On the other hand, Zn2SnO4 nanoplates show drastic capacity fading up to 10 cycles and then showed a capacity retention of 80% and 70% between 10 and 50 cycles when cycled in the voltage range 0.005-1.5 and 0.005-3 V, respectively. The structural and morphological evolutions during cycling and their implications on the Li-cycling behavior of Zn2SnO4 nanowires are examined. The effect of the choice of voltage range and initial morphology of the active material on the Li-cycleabilty is also elucidated.

Interconnected network of CoMoO₄ submicrometer particles as high capacity anode material for lithium ion batteries

Interconnected networks of CoMoO(4) submicrometer particles are prepared by thermolysis of polymer matrix based metal precursor solution. The material exhibited a high reversible capacity of 990 (±10) mAh g(-1) at a current density of 100 mA g(-1), with 100% capacity retention between 5 and 50 cycles. The improved electrochemical performance of CoMoO(4) submicrometer particles with interconnected network like morphology makes it promising as a high-capacity anode material for rechargeable lithium ion batteries.