Self-passivated nanoporous phosphorene as a membrane for water desalination
Introduction
Water crisis predominantly for human consumption is one of the largest global risks in terms of potential impact over the next coming years according to World Economic Forum [1]. This urges a need to purify the inexhaustible source and abundantly available i.e. earth's salt water in seas, oceans and ice at poles [2] using economically and friendly techniques. Although, numerous desalination technologies are available but even the most energy-efficient technique, reverse osmosis (RO) has low efficiency and demands high capital cost [[3], [4], [5]]. Advances in nanotechnology open up opportunities to design energy efficient membrane for water desalination [6,7]. Among these, two dimensional (2D) materials offer promising water desalination e.g. graphene, carbon nanotubes (CNTs), zeolites etc. [[8], [9], [10], [11], [12]]. Use of these materials greatly reduces the energy consumption as water flux scales inversely with the membrane thickness.
Graphene, one atomic thick membrane (0.34 nm) has been extensively explored for water desalination through simulations [[13], [14], [15]] and experiments [16,17]. Tanugi and Grossman [14] suggested that water flux through pristine and OH-functionalized pores is 2–3 times faster than diffusive RO membranes at equal pressure drop. In molybdenum disulphide (MOS2) membrane the nanopores with only molybdenum atoms on their edges lead to higher flux, which is ≈70% greater than graphene [18]. The presence of charged functional groups blocks the ions effectively but its steric effects reduce water diffusion [19,20]. Yang et al. [21] showed higher water permeability of inherently nanoporous g-C2N membrane than graphene filters. The salt water rejection capability was attributed to steric hindrance and electrostatic interactions on salt ions and water molecules under confinement. Filter membranes based on CNTs are limited by low salt rejection rates and difficulty of producing highly aligned and dense CNT arrays [22].
Moreover, existing materials possess certain limits such as high chemical reactivity of graphene so, it should be protected with functional groups and lower salt rejection in case of MOS2 [18,23]. The chemical functionalization of these materials can provide hydrophilic sites at the edges of the pore, gives rise to the attraction of water molecules and enhanced water flux (for example, adding hydroxyl groups to graphene). But complex fabrication is involved in adding the precise functional groups to the edge of the nanopores [24].
To overcome these issues, for the first time we have explored the desalination performance of phosphorene which has a honeycomb lattice and puckered geometry. Single layer of phosphorene has a thickness of ~0.85 nm which is less than MOS2 layer (~1.0 nm) [18,25]. The striking feature of nanoporous phosphorene is that the edge atoms of the pore can rebuild bonds with each other and becomes self passivated which makes it distinctly convenient to use phosphorene as a water desalination membrane without any additional protection. Additionally, the nature of phosphorene provides a large energy barrier for the healing of defect, therefore defective phosphorene is more stable [26]. Defects can be quite easily created in phosphorene compared with graphene and silicone [27]. Furthermore, due to the anisotropic nature of phosphorene it can sustain 30% and 20% tensile strain along armchair and zigzag directions respectively. Phosphorene has Young's modulus of 44–146 GPa and fracture strength 2.6–4.5 ± 0.1 GPa [28]. These limits are pertinent enough as only few MPa pressure is required for desalination purposes. However, the membranes used for desalination are rarely freestanding but composite, which including dense layer for separation (phosphorene studied here) and porous substrate for improving the composites mechanical stability. These composite layers offer lower transport resistance and better permeability. Xue et al. [29] demonstrated that pervoporation desalination thin-film composite membranes crosslinked with aliphatic compounds exhibits best hydrostability, mechanical properties, and desalination performance.
Here, we investigated the desalination performance of nanoporous phosphorene at the atomistic level. Nanoporous phosphorene serve as an ideal membrane for rejecting salt ions from water and its desalination performance is sensitive to the pore dimensions, shape and pressure drop. Interestingly, phosphorene showed complete ions rejection even at higher pore areas and pressures. This study will be supportive for experimentalist in designing the most efficient phosphorene based membranes for water purification.
Section snippets
Model and simulation details
The snapshot of the simulation domain consists of a feed reservoir on the left and a permeate reservoir on the right as shown in Fig. 1(a). A nanoporous phosphorene membrane with a pore area ~ 41 Å2 divides the two reservoirs. Both ends of the simulation domain is bounded with specular reflection boundaries which were initially located at z = 0.0 Å and z = 75 Å, respectively. The feed side of membrane is surrounded by saltwater and the permeate side by pure water along the z direction. Fig. 1
Results and discussion
First, nanopores inside the phosphorene membrane center were created by removing even number of atoms (n = 6, 8, 10, 12, 14, 16, and 18) and were named as D6, D8, D10, D12, D14, D16, and D18 respectively (Supplementary Fig. S1). These pores were broadly classified into three categories: narrow (D6 and D8), medium (D10, D12, and D14) and wide pores (D16 and D18). The accessible pore areas were considered here range from 4 Å2 to 41 Å2. The details of nanopore creation are provided in
Conclusions
Nanoporous phosphorene membrane displayed unprecedented salt rejection across all nanopore sizes and applied pressures simulated in our work. The increased ionic radii and inert P atoms of the nanopore without any bonding sites cause weak interaction between P atoms and ions lead to complete salt rejection. For rectangular shaped nanopores, we observed higher water flux with increase in pressure in contrast to oval shape, hence shape, size of the nanopore has greatest effect on water flow rate.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge financial support from Department of Science and Technology, New Delhi for our respective grant nos. SR/WOS-A/PM-36/2017 and SR/WOS-A/PM-30/2017 to carry out this research work. Computational facilities from the Center for Development of Advance Computing (C-DAC), Pune are also gratefully acknowledged
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Both the authors contributed equally to this work.