Layer-by-layer Assembly of Nanosheets with Matching Size and Shape for More Stable Membrane Structure than Nanosheet-Polymer Assembly.

Layer-by-layer (LbL) assembly of oppositely charged materials has been widely used as an approach to make two-dimensional (2D) nanosheet-based membranes, which often involves 2D nanosheets being alternately deposited with polymer-based polyelectrolytes to obtain an electrostabilized nanosheet-polymer structure. In this study, we hypothesized that using 2D nanosheets with matching physical properties as both polyanions and polycations may result in a more ordered nanostructure with better stability than a nanosheet-polymer structure. To compare the differences between nanosheet-nanosheet vs nanosheet-polymer structures, we assembled negatively charged molybdenum disulfide nanosheets (MoS2) with either positively charged graphene oxide (PrGO) nanosheets or positively charged polymer (PDDA). Using combined measurements by ellipsometer and quartz crystal microbalance with dissipation, we discovered that the swelling of MoS2-PrGO in ionic solutions was 60% lower than that of MoS2-PDDA membranes. Meanwhile, the MoS2-PrGO membrane retained its permeability upon drying, whereas the permeability of MoS2-PDDA decreased by 40% due to the restacking of MoS2. Overall, the MoS2-PrGO membrane demonstrated a better filtration performance. Additionally, our X-ray photoelectron spectroscopy results and analysis on layer density revealed a clearer transition in material composition during the LbL synthesis of MoS2-PrGO membranes, and the X-ray diffraction pattern suggested its resemblance to an ordered, layer-stacked structure. In conclusion, the MoS2-PrGO membrane made with nanosheets with matching size, shape, and charge density exhibited a much more aligned stacking structure, resulting in reduced membrane swelling under high salinity solutions, controlled restacking, and improved separation performance.

Tuning phase compositions of MoS2 nanomaterials for enhanced heavy metal removal: performance and mechanism.

Two-dimensional MoS2 nanosheets have shown great potential in heavy metal remediation due to their unique properties. MoS2 has two primary phases: 1T and 2H. Each has different physiochemical properties, but the impact of these differences on the overall material's heavy metal removal performance and associated mechanisms is rarely reported. In this study, we synthesized morphologically similar but phase-distinct MoS2 samples via hydrothermal synthesis, which comprised dominantly either a metallic 1T phase or a semiconducting 2H phase. 1T-MoS2 samples exhibited higher removal capacities for Ag+ and Pb2+ cations relative to 2H-MoS2. In particular, an eight-fold increase in the Pb2+ adsorption capacity was observed in the 1T-MoS2 samples (i.e. ∼632.9 mg g-1) compared to the 2H-MoS2 samples (∼81.6 mg g-1). The mechanisms driving the enhanced performance of 1T-MoS2 were investigated through detailed characterization of metal-laden MoS2 samples and DFT modelling. We found that 1T-MoS2 intrinsically had a larger interlayer spacing than 2H-MoS2 because water molecules were retained between the hydrophilic 1T nanosheets during hydrothermal synthesis. The widened interlayer spacing in 1T-MoS2 allowed the diffusion of heavy metal ions into the nanochannels, increasing the number of adsorption sites and total removal capacities. On the other hand, DFT modelling revealed the energy-favorable adsorption complex of Ag+ and Pb2+ for 1T-MoS2, in which each metal atom was bonded with three S atoms leading to much higher adsorption energies relative to 2H-MoS2 for Ag+ and Pb2+. This study unravels the underlying mechanisms of phase-dependent heavy metal remediation by MoS2 nanosheets, providing an important guide for the use of 2D nanomaterials in environmental applications which include heavy metal removal, contaminant sensing, and membrane separation.

Interfacial Solar Evaporation by a 3D Graphene Oxide Stalk for Highly Concentrated Brine Treatment.

In this work, we demonstrate a 3-dimensional graphene oxide (3D GO) stalk that operates near the capillary wicking limit to achieve an evaporation flux of 34.7 kg m-2 h-1 under 1 sun conditions (1 kW/m2). This flux represents nearly a 100 times enhancement over a conventional solar evaporation pond. Interfacial solar evaporation traditionally uses 2D evaporators to vaporize water using sunlight, but their low evaporative water flux limits their practical applicability for desalination. Some recent studies using 3D evaporators demonstrate potential for more efficient water transfer, but the flux improvement has been marginal because of a low evaporation area index (EAI), which is defined as the ratio of the total evaporative surface area to the projected ground area. By using a 3D GO stalk with an ultrahigh EAI of 70, we achieved nearly a 20-fold enhancement over a 2D GO evaporator. The 3D GO stalk also exhibited additional advantages including omnidirectional sunlight utilization, a high evaporation flux under dark conditions from more efficient utilization of ambient heating, a dramatic increase of the evaporation rate by introducing wind, and scaling resistance in evaporating brines with a salt content of up to 17.5 wt %. This performance makes the 3D GO stalk well suited for the development of a low-cost, reduced footprint technology for zero liquid discharge in brine management applications.

Interfacial solar vapor generation for desalination and brine treatment: Evaluating current strategies of solving scaling.

Interfacial solar vapor generation, an efficient, sustainable, and low-cost method for producing clean water, has attracted great interest for application in solar desalination and wastewater treatment. Although recent studies indicated significant enhancement of overall performance by developing photothermal materials and constructing different dimensional systems, stable evaporation performance and long-term operation of the evaporator are hindered by severe scaling issues. In this critical review, we present the latest strategies in reducing salt accumulation on the evaporator for solar desalination and brine treatment. We first demonstrate the consequences of salt accumulation, and then discuss various self-cleaning methods based on bio-inspired concepts and other strategies such as physical cleaning, ion rejection and exchange, fast ion diffusion, and controlled crystallization, etc. Importantly, we discuss and address the rational design of the evaporator via establishing a relationship model between its porosity, thickness, and thermal conductivity. Lastly, we evaluate salt-resistance strategies, evaporation performance, and possibilities of real application in different evaporation systems with scaling-resistant abilities.

Surface slip on rotating graphene membrane enables the temporal selectivity that breaks the permeability-selectivity trade-off.

Membrane separation technology is dictated by the permeability-selectivity trade-off rule, because selectivity relies on membrane pore size being smaller than that of hydrated ions. We discovered a previously unknown mechanism that breaks the permeability-selectivity trade-off in using a rotating nanoporous graphene membrane with pores of 2 to 4 nanometers in diameter. The results show that the rotating membrane exhibits almost 100% salt rejection even when the pore size is larger than that of hydrated ions, and the surface slip at the liquid/graphene interface of rotating membrane enables concurrent ultra-selectivity and unprecedented high permeability. A novel concept of "temporal selectivity" is proposed to attribute the unconventional selectivity to the time difference between the ion's penetration time through the pore and the bypass time required for ion's sliding across the pore. The newly discovered temporal selectivity overcomes the limitation imposed by pore size and provokes a novel theory in designing high-performance membranes.

Superselective Removal of Lead from Water by Two-Dimensional MoS2 Nanosheets and Layer-Stacked Membranes.

Point-of-use (POU) devices with satisfactory lead (Pb2+) removal performance are urgently needed in response to recent outbreaks of lead contamination in drinking water. This study experimentally demonstrated the excellent lead removal capability of two-dimensional (2D) MoS2 nanosheets in aqueous form and as part of a layer-stacked membrane. Among all materials ever reported in the literature, MoS2 nanosheets exhibit the highest adsorption capacity (740 mg/g), and the strongest selectivity/affinity toward Pb2+ with a distribution coefficient Kd that is orders of magnitude higher than that of other lead adsorption materials (5.2 × 107 mL/g). Density functional theory (DFT) simulation was performed to complement experimental measurements and to help understand the adsorption mechanisms. The results confirmed that the cation selectivity of MoS2 follows the order Pb2+ > Cu2+ ≫ Cd2+ > Zn2+, Ni2+ > Mg2+, K+, Ca2+. The membrane formed with layer-stacked MoS2 nanosheets exhibited a high water flux (145 L/m2/h/bar), while effectively decreasing Pb2+ concentration in drinking water from a few mg/L to less than 10 µg/L. The removal capacity of the MoS2 membrane is a few orders of magnitude higher than that of other literature-reported membrane filters. Therefore, the layer-stacked MoS2 membrane has great potential for POU removal of lead from drinking water.

Correlating Interlayer Spacing and Separation Capability of Graphene Oxide Membranes in Organic Solvents.

Membranes synthesized by stacking two-dimensional graphene oxide (GO) hold great promise for applications in organic solvent nanofiltration. However, the performance of a layer-stacked GO membrane in organic solvent nanofiltration can be significantly affected by its swelling and interlayer spacing, which have not been systematically characterized. In this study, the interlayer spacing of the layer-stacked GO membrane in different organic solvents was experimentally characterized by liquid-phase ellipsometry. To understand the swelling mechanism, the solubility parameters of GO were experimentally determined and used to mathematically predict the Hansen solubility distance between GO and solvents, which is found to be a good predictor for GO swelling and interlayer spacing. Solvents with a small solubility distance (e.g., dimethylformamide, N-methyl-2-pyrrolidone) tend to cause significant GO swelling, resulting in an interlayer spacing of up to 2.7 nm. Solvents with a solubility distance larger than 9.5 (e.g., ethanol, acetone, hexane, and toluene) only cause minor swelling and are thus able to maintain an interlayer spacing of around 1 nm. Correspondingly, GO membranes in solvents with a large solubility distance exhibit good separation performance, for example, rejection of more than 90% of the small organic dye molecules (e.g., rhodamine B and methylene blue) in ethanol and acetone. Additionally, solvents with a large solubility distance result in a high slip velocity in GO channels and thus high solvent flux through the GO membrane. In summary, the GO membrane performs better in solvents that are unlike GO, i.e., solvents with large solubility distance.

Dew Point Measurement Using a Carbon-Based Capacitive Sensor with Active Temperature Control.

Laser-induced graphene (LIG) has both good electrical conductivity and three-dimensional porous structures. Here, porous graphene interdigital electrodes (IDE) were constructed as a capacitive sensor from commercial polymer films by the laser ablation process and transferred to the polydimethylsiloxane (PDMS) substrate. The graphene oxide (GO) adsorption layer was electrosprayed as a humidity sensing structure, and a Peltier device was used to control the temperature to produce the condensation of water vapors. The dew point was identified by the equilibrium state of the capacitor when the adsorption layer and the surface air reached the saturation equilibrium. The performances of the hydrophilic dew point sensing system under different environmental conditions were investigated. The results show that the precision of the carbon-based dew point sensor of ≤±0.8 °C DP with good stability and repeatability is better than those of other dew point instrument based on electrical sensing parameters at ±1.0 °C DP.

Removal and Recovery of Heavy Metal Ions by Two-dimensional MoS2 Nanosheets: Performance and Mechanisms.

We investigated the removal of heavy metals from water by two-dimensional MoS2 nanosheets suspended in aqueous solution, and restacked as thin film membranes, respectively. From these studies we elucidated a new heavy metal ion removal mechanism that involves a reduction-oxidation (redox) reaction between heavy metal ions and MoS2 nanosheets. Ag+ was used as a model species and MoS2 nanosheets were prepared via chemical exfoliation of bulk powder. We found that the Ag+ removal capacity of suspended MoS2 nanosheets was as high as ∼4000 mg/g and adsorption accounted for less than 20% of removal, suggesting the reduction of Ag+ to metallic silver as a dominant removal mechanism. Furthermore, we demonstrated that MoS2 membranes were able to retain a similar high removal capacity, and attribute this capability to the formation of a conductive, permeable multilayer MoS2 structure, which enables a corrosion-type reaction involving electron transfer from a MoS2 site inside the membrane (anode) to another site on membrane surface (cathode) where heavy metal ions are reduced to metallic particles. The membrane surface remains active to efficiently recover metallic particles, because the primary oxidation products are soluble, nontoxic molybdate and sulfur species, which do not form an insulating oxide layer to passivate the membrane surface. Therefore, MoS2 membranes can be used effectively to remove and recover precious heavy metals from wastewater.

2D graphene oxide channel for water transport.

Layer-stacked graphene oxide (GO) membranes, in which unique two-dimensional (2D) water channels are formed between two neighboring GO nanosheets, have demonstrated great potential for aqueous phase separation. Subjects of crucial importance are to fundamentally understand the interlayer spacing (i.e. channel height) of GO membranes in an aqueous environment, elucidate the mechanisms for water transport within such 2D channels, and precisely control the interlayer spacing to tune the membrane separation capability for targeted applications. In this investigation, we used an integrated quartz crystal mass balance (QCM-D) and ellipsometry to experimentally monitor the interlayer spacing of GO, reduced GO and crosslinked GO in aqueous solution and found that crosslinking can effectively prevent GO from swelling and precisely control the interlayer spacing. We then used molecular dynamics simulations to study the mass transport inside the 2D channels and proved that the chemical functional groups on the GO plane dramatically slow down water transport in the channels. Our findings on GO structure and water transport provide a necessary basis for further tailoring and optimizing the design and fabrication of GO membranes in various separation applications.

Partially reduced graphene oxide and chitosan nanohybrid membranes for selective retention of divalent cations.

A tremendous quantity of brackish water with a high proportion of divalent cations is in great need of water softening. Layer-stacked graphene oxide membranes show potential in membrane processing due to their molecular sieving properties, but show poor selective retention of cations due to unstable interlayer spacing and electrostatic interaction. In this study, a partially reduced graphene oxide (prGO) and chitosan (CS) nanohybrid membrane (prGO-CS) was fabricated to achieve the selective retention of divalent cations by adjusting the configuration and controlling the surface charge. The prGO-CS membrane, which included a CS skin and embedded prGO sheets, showed a performance boost of 98.0% rejection of Mg2+ and 95.5% rejection of Ca2+ when compared with a CS membrane. The membrane showed good water softening performance for brackish water under low operation pressure with a high Na+/Mg2+ selectivity of 33.8. The excellent performance was attributed to the dense structure and positive charge of prGO-CS.

Understanding the Aqueous Stability and Filtration Capability of MoS2 Membranes.

Membranes made of layer-stacked two-dimensional molybdenum disulfide (MoS2) nanosheets have recently shown great promise for water filtration. At present, the reported water fluxes vary significantly, while the accountable structure and properties of MoS2 nanochannels are largely unknown. This paper aims to mechanistically relate the performance of MoS2 membranes to the size of their nanochannels in different hydration states. We discovered that fully hydrated MoS2 membranes retained a 1.2 nm interlayer spacing (or 0.9 nm free spacing), leading to high water permeability and moderate-to-high ionic and molecular rejection. In comparison, completely dry MoS2 membranes had a 0.62 nm interlayer spacing (or 0.3 nm free spacing) due to irreversible nanosheet restacking and were almost impermeable to water. Furthermore, we revealed that the interlayer spacing of MoS2 membranes in aqueous solution is maintained by comparable van der Waals and hydration forces, thereby ensuring the aqueous stability of MoS2 membranes without the need of cross-linking. In addition, we attributed the high water flux (30-250 L m-2 h-1 bar-1) of MoS2 membranes to the low hydraulic resistance of smooth, rigid MoS2 nanochannels. We also concluded that compaction of MoS2 membranes with a high pressure helps create a more neatly stacked nanostructure with minimum voids or looseness, leading to stable water flux and separation performance. Besides, this paper systematically compares MoS2 membranes with the widely studied graphene oxide membranes to highlight the uniqueness and advantages of MoS2 membranes for water-filtration applications.

Dual-Channel, Molecular-Sieving Core/Shell ZIF@MOF Architectures as Engineered Fillers in Hybrid Membranes for Highly Selective CO2 Separation.

A novel core/shell porous crystalline structure was prepared using a large pore metal organic framework (MOF, UiO-66-NH2, pore size, ∼ 0.6 nm) as core surrounded by a small pore zeolitic imidazolate framework (ZIF, ZIF-8, pore size, ∼ 0.4 nm) through a layer-by-layer deposition method and subsequently used as an engineered filler to construct hybrid polysulfone (PSF) membranes for CO2 capture. Compared to traditional fillers utilizing only one type of porous material with rigid channels (either large or small), our custom designed core/shell fillers possess clear advantages via pore engineering: the large internal channels of the UiO-66-NH2 MOFs create molecular highways to accelerate molecular transport through the membrane, while the thin shells with small pores (ZIF-8) or even smaller pores generated at the interface by the imperfect registry between the overlapping pores of ZIF and MOF enhance molecular sieving thus serving to distinguish slightly larger N2 molecules (kinetic diameter, 0.364 nm) from smaller CO2 molecules (kinetic diameter, 0.33 nm). The resultant core/shell ZIF@MOF and as-prepared hybrid PSF membranes were characterized by transmission electron microscopy, X-ray diffraction, wide-angle X-ray scattering, scanning electron microscopy, Fourier transform infrared, thermogravimetric analysis, differential scanning calorimetry, and contact angle tests. The dependence of the separation performance of the membranes on the MOF/ZIF ratio was also studied by varying the number of layers of ZIF coatings. The integrated PSF-ZIF@MOF hybrid membrane (40 wt % loading) with optimized ZIF coating cycles showed improved hydrophobicity and excellent CO2 separation performance by simultaneously increasing CO2 permeability (CO2 permeability of 45.2 barrer, 710% higher than PSF membrane) and CO2/N2 selectivity (CO2/N2 selectivity of 39, 50% higher than PSF membrane), which is superior to most reported hybrid PSF membranes. The strategy of using dual-channel molecular sieving core/shell porous crystals in hybrid membranes thus provides a promising means for CO2 capture from flue gas.

Synthetic Graphene Oxide Leaf for Solar Desalination with Zero Liquid Discharge.

Water vapor generation through sunlight harvesting and heat localization by carbon-based porous thin film materials holds great promise for sustainable, energy-efficient desalination and water treatment. However, the applicability of such materials in a high-salinity environment emphasizing zero-liquid-discharge brine disposal has not been studied. This paper reports the characterization and evaporation performance of a nature-inspired synthetic leaf made of graphene oxide (GO) thin film material, which exhibited broadband light absorption and excellent stability in high-salinity water. Under 0.82-sun illumination (825 W/m2), a GO leaf floating on water generated steam at a rate of 1.1 L per m2 per hour (LMH) with a light-to-vapor energy conversion efficiency of 54%, while a GO leaf lifted above water in a tree-like configuration generated steam at a rate of 2.0 LMH with an energy efficiency of 78%. The evaporation rate increased with increasing light intensity and decreased with increasing salinity. During a long-term evaporation experiment with a 15 wt % NaCl solution, the GO leaf demonstrated stable performance despite gradual and eventually severe accumulation of salt crystals on the leaf surface. Furthermore, the GO leaf can be easily restored to its pristine condition by simply scraping off salt crystals from its surface and rinsing with water. Therefore, the robust high performance and relatively low fabrication cost of the synthetic GO leaf could potentially unlock a new generation of desalination technology that can be entirely solar-powered and achieve zero liquid discharge.

Swelling of Graphene Oxide Membranes in Aqueous Solution: Characterization of Interlayer Spacing and Insight into Water Transport Mechanisms.

Graphene oxide (GO) has recently emerged as a promising 2D nanomaterial to make high-performance membranes for important applications. However, the aqueous-phase separation capability of a layer-stacked GO membrane can be significantly limited by its natural tendency to swell, that is, absorb water into the GO channel and form an enlarged interlayer spacing (d-spacing). In this study, the d-spacing of a GO membrane in an aqueous environment was experimentally characterized using an integrated quartz crystal microbalance with dissipation and ellipsometry. This method can accurately quantify a d-spacing in liquid and well beyond the typical measurement limit of ∼2 nm. Molecular simulations were conducted to fundamentally understand the structure and mobility of water in the GO channel, and a theoretical model was developed to predict the d-spacing. It was found that, as a dry GO membrane was soaked in water, it initially maintained a d-spacing of 0.76 nm, and water molecules in the GO channel formed a semiordered network with a density 30% higher than that of bulk water but 20% lower than that of the rhombus-shaped water network formed in a graphene channel. The corresponding mobility of water in the GO channel was much lower than in the graphene channel, where water exhibited almost the same mobility as in the bulk. As the GO membrane remained in water, its d-spacing increased and reached 6 to 7 nm at equilibrium. In comparison, the d-spacing of a GO membrane in NaCl and Na2SO4 solutions decreased as the ionic strength increased and was ∼2 nm at 100 mM.

Environmental Applications of 2D Molybdenum Disulfide (MoS2) Nanosheets.

In an era of graphene-based nanomaterials as the most widely studied two-dimensional (2D) materials for enhanced performance of devices and systems in numerous environmental applications, molybdenum disulfide (MoS2) nanosheets stand out as a promising alternative 2D material with many excellent physicochemical, biological, and mechanical properties that differ significantly from those of graphene-based nanomaterials, potentially leading to new environmental phenomena and novel applications. This Critical Review presents the latest advances in the use of MoS2 nanosheets for important water-related environmental applications such as contaminant adsorption, photocatalysis, membrane-based separation, sensing, and disinfection. Various methods for MoS2 nanosheet synthesis are examined, and their suitability for different environmental applications is discussed. The unique structure and properties of MoS2 nanosheets enabling exceptional environmental capabilities are compared with those of graphene-based nanomaterials. The environmental implications of MoS2 nanosheets are emphasized, and research needs for future environmental applications of MoS2 nanosheets are identified.