2D Materials for Potable Water Application: Basic Nanoarchitectonics and Recent Progresses

Abstract

Water polluted by toxic chemicals due to waste from chemical/pharmaceuticals and harmful microbes such as E. Coli bacteria causes several fatal diseases; and therefore, water filtration is crucial for accessing clean and safe water necessary for good health. Conventional water filtration technologies include activated carbon filters, reverse osmosis, and ultrafiltration. However, they face several challenges, including high energy consumption, fouling, limited selectivity, inefficiencies in removing certain contaminants, dimensional control of pores, and structural/chemical changes at higher thermal conditions and upon prolonged usage of water filter. Recently, the advent of 2D materials such as graphene, BN, MoS₂, MXenes, and so on opens new avenues for advanced water filtration systems. This review delves into the nanoarchitectonics of 2D materials for water filtration applications. The current state of water filtration technologies is explored, the inherent challenges they face are outlines, and the unique properties and advantages of 2D materials are highlighted. Furthermore, the scope of this review is discussed, which encompasses the synthesis, characterization, and application of various 2D materials in water filtration, providing insights into future research directions and potential industrial applications.

This review explores the innovative use of 2D materials in water filtration systems. It focuses on the nanoarchitectonics of these materials, showcasing their ability to remove contaminants efficiently. Various 2D material types, their properties, and applications in creating advanced filtration systems are discussed, highlighting their potential in addressing global water purification challenges.

1. Introduction

The quality of the water we drink correlates with our overall quality of life. Of the 1386 million km³ of water on the Earth, only 10.63 million km³ is freshwater, and just 30.1% of that freshwater is groundwater.^{[ 1 ]} Unfortunately, water is susceptible to several contaminants that can adversely affect its safety for consumption. Contaminations by waterborne pathogens and the resulting diseases is a major global issue impacting water quality.^{[ 2 ]} Beyond biological pollutants, impurities such as dust, fine sand, mud, and soil also contaminate water. Rainwater, streams, and boreholes often contain acid and metallic rust constituents. Many drinking water systems also contain chlorine and excessive calcium. Due to its composition of hydrogen and oxygen, water can dissolve many substances it encounters. Ingesting contaminated water can lead to diseases and medical conditions such as typhoid, malaria, cholera, dracunculiasis, and diarrhea.^{[ 3 ]}

Every year, a startling 3.4 million people die from waterborne infections caused by consuming tainted water. Rivers contaminated with harmful microorganisms due to poor sanitation, inadequate wastewater treatment, and catastrophic flooding lead to widespread illness and death. The severity of this issue has led the Centers for Disease Control and Prevention (CDC) to establish a specific division dedicated to addressing tainted water in the United States. Biological pollutants in water include pathogens, bacilli, viruses, protozoa, parasites, algae, and their cysts (eggs), contributed by living beings and microbes in the environment.^{[ 4 ]} These microorganisms, often referred to as germs, can proliferate at alarming rates (Figure 1 ). Colloidal pollutants in water, such as amino acids and organic matter, arise when water in lakes, rivers, and streams interacts with suspended particles and materials such as sand, boulders, and biological matter, rendering the water impure and unfit for drinking. Despite the environmental protection measures implemented in the previous century, they did not adequately support the immense industrial revolution worldwide. Many cities are located near rivers, which serve as transportation hubs and water sources. Unfortunately, these rivers are often exploited as sewage and wastewater dumping grounds. Pathogens are found in rivers for various reasons, including low sanitation standards and inefficient sewerage systems, which are common in many towns in underdeveloped nations. However, even in developed nations, extreme flooding can lead to pathogen contamination of rivers. Due to global warming, such floods following heavy rains are becoming increasingly frequent.

Figure 1.

Pathogenic diseases aggravated by climatic hazards.^{[ 5 ]}

Petroleum, commonly known as “rock oil,” is a fossil fuel that has formed beneath the Earth's surface over millions of years. It originates from the remains of extinct marine organisms, including bacteria, plants, and algae. Under specific geological conditions, overburden pressure and subterranean temperature have transformed these organic materials into hydrocarbon molecules such as natural gas and crude oil. Petroleum is a complex mixture of various constituents, including monocyclic, straight chained, branched, cyclic, and polycyclic aromatic hydrocarbons. Oil pollution can significantly harm ecosystems, affecting both plants and animals, and contaminating water supplies. The toxicity levels of oil in water can be assessed by determining the hazardous potency of its constituents at water solvency.^{[ 6 ]} Offshore areas worldwide, with significant economic potential, are being explored and developed for petroleum resources. These regions include New Zealand, Alaska, Western Australia, Malaysia, the Caribbean Sea, Venezuela, the North Atlantic Ocean, Mexico, West Africa, the North Sea, Brazil, the Caspian Sea, California, the Gulf of Mexico, Indonesia, Newfoundland, and the South China Sea. The petroleum industry is actively investing in the development and exploration of oil and gas fields in these locations.

Arsenic (As) and fluoride are the two most common pollutants in the affected areas and populations.^{[ 7 ]} In many nations, As and fluoride naturally occur in groundwater. Due to their highly toxic nature and detrimental health effects even at low concentrations, arsenic and fluoride contamination of groundwater has become a major global environmental issue. Arsenic concentrations in drinking water exceed World Health Organization (WHO) guidelines, affecting ≈200 million people worldwide. However, the most common groundwater contaminant is fluoride. Fluorosis, a condition caused by excessive fluoride intake, affects over 200 million individuals in 25 countries, including China, India, and Bangladesh, with South‐East Asian nations being particularly hard hit.^{[ 8 ]} In India, inhabitants of the middle and lower Gangetic plains, as well as regions in Central and South India with hard rock topography, are especially impacted by arsenic and fluoride poisoning.

Microplastics, including those originating from the wear and tear of car tires,^{[ 9 ]} have emerged as a significant environmental contaminant, especially in water systems. Tire wear particles, which are a form of microplastic, are generated during the use of vehicles and are composed of synthetic rubber, polymers, and additives that can release toxic chemicals into the environment. These particles are small enough to enter waterways through runoff, eventually contaminating both freshwater and marine ecosystems. Studies have shown that microplastics can absorb and transport harmful pollutants, exacerbating their environmental impact and posing risks to aquatic life and human health when present in drinking water sources.^{[ 10 ]} Due to their persistent nature and widespread occurrence, the removal of microplastics, including those from car tires, is a growing focus in water treatment technologies.

Purified water is essential for everyone on Earth for drinking, cooking, and domestic tasks such as bathing, brushing teeth, and washing clothes. Maintaining the hygiene of our environment and promoting healthier lives is crucial. Despite its seemingly clean appearance, tap water contains various germs and viruses that can harm our well‐being, as well as contaminants such as fluorine compounds, lead, pesticides, chlorine, mercury, microplastics and other waste products, primarily from various industries (for example textile and pharmaceutical). Drinking polluted water can lead to severe health problems, including polio, dysentery, typhoid, cholera, and diarrhea, causing up to 502 000 diarrhea‐related deaths annually.^{[ 11} , ^{12 ]} Water purification is vital for maintaining a healthy and safe environment.^{[ 13 ]} Removing chlorine from water makes it safe to drink, and eliminating lead, which is particularly harmful to human health, improves digestion. Using filtered water regularly can also help with skin problems and improve air quality. Despite the critical need for clean water, billions of people worldwide still lack access to it. Approximately 2.2 billion people do not have access to safe drinking water, 4.2 billion lack sanitation facilities, and 3 billion lack basic hand‐hygiene facilities. Children and families in underdeveloped and rural areas are the most at risk. The analysis shows that eight out of ten individuals living in rural regions lack access to essential services, and in one out of every four countries, access to basic facilities among the wealthiest individuals is at least twice as high as among the poorest. Those who are underprivileged or marginalized due to gender, age, nationality, or religious beliefs are also more likely to lack adequate sanitation and water.

Having purified drinking water at home eliminates the need to purchase expensive bottled water, significantly reducing monthly expenses. Water filtration equipment is a one‐time investment that lasts a long time, making the cost of a water purifier much lower than the ongoing expense of buying water.

Various tried‐and‐true techniques^{[ 14 ]} (Figure 2 ) that are highly effective for water purification include: a) Boiling: This is the simplest method, where water is boiled for a considerable amount of time to kill microbes and viruses. However, boiling doesn't remove all contaminants, so the water must be passed through a microporous screen to fully purify it.^{[ 15 ]} b) Bleach Solution: For emergency treatment, a weak bleach solution with 5% chlorine can be added to water. This acts as an oxidant, quickly disinfecting the water and making it safe for consumption.^{[ 16 ]} c) Iodine: Available in liquid or tablet form, iodine is potent at eradicating viruses and bacteria. However, it imparts a bad taste and can be harmful in large amounts, so it should only be used when other filtration methods are unavailable.^{[ 17 ]} d) Solar Purification: This is a method that uses the sun's ultraviolet light to purify water. Water is placed in a plastic container, shaken to release oxygen, and left in direct sunlight, effectively killing viruses and bacteria.^{[ 18 ]} e) Distillation: This method involves collecting condensed water after evaporation. While effective, it is less efficient than RO filters due to the time required and the removal of essential minerals.^{[ 18 ]} f) Clay Vessel Filters: This is an ancient method still used in some rural areas, where clay pots filter contaminated water by removing muck and allowing pure, drinkable water to pass through. This method can also be used for desalination.^{[ 19 ]} g) Reverse Osmosis (RO) Purifiers: One of the most effective methods, RO purifiers force water through a semipermeable membrane to remove impurities.^{[ 20 ]} h) Electric Water Purifiers: Common in today's households, these purifiers use a multistage process that includes UV and UF filtration, carbon blocking, and modern purification technologies to produce the purest drinking water.^{[ 21 ]} These methods vary in efficiency and practicality, but all contribute to making water safe for drinking and other domestic uses, improving health and hygiene for households worldwide. Traditional water filtration technologies, including activated carbon filters, reverse osmosis (RO), and ultrafiltration (UF), have been instrumental in providing potable water.^{[ 22 ]} However, these technologies come with significant drawbacks. Activated carbon filters, while effective in removing organic contaminants, have limited efficacy against heavy metals and pathogens. Reverse osmosis, known for its ability to desalinate water, is energy‐intensive and prone to membrane fouling. Ultrafiltration, although useful for removing particulates and some pathogens, struggles with removing dissolved salts and organic molecules.

Figure 2.

Water purification techniques.

The emergence of 2D materials, such as graphene, graphene oxide, and transition metal dichalcogenides (TMDs), has garnered significant attention in the field of water filtration. These materials possess unique structural and chemical properties, including high surface area, tunable porosity, and exceptional mechanical strength, making them promising candidates for advanced filtration technologies. The ability to engineer these materials at the nanoscale—referred to as nanoarchitectonics—allows for the design of highly efficient and selective filtration systems. In this review, we provide a comprehensive overview of the nanoarchitectonics of 2D materials for water filtration. We begin by examining the current landscape of water filtration technologies and the challenges they face. We then delve into the specific advantages of 2D materials, such as their high permeability, anti‐fouling properties, and the potential for functionalization to target specific contaminants. The scope of this review includes the synthesis methods, structural characterization, and application of various 2D materials in water filtration. Keeping track of the recent advancements and identifying future research directions, we aim to provide a roadmap for the development of next‐generation water filtration technologies leveraging the unique properties of 2D materials.

2. Evolution of 2D Materials and their Heterostructures

Owing to the commercial realization of 2D material‐based products and the constant unmatched effort from the researchers, 2D material research is in its infancy. Therefore, it is crucial to understand the existing flatland members and investigate their role in membrane technology. The advent of graphene in 2004, consisting of a hexagonally arranged atomically thick monolayer sheet of carbon^{[ 23} , ²⁴ , ^{25 ]} in the flatland, brought revolutionary changes in material science and condensed matter physics. The exfoliation of graphene, an elemental sheet of carbon with exceptional physical and chemical properties, triggers curiosity amongst scientists and researchers to look for other unexplored graphene cousins. The recently found elemental analogs of graphene are from the group III to V, known as Xenes.^{[ 26 ]} The family of Xenes (X = Si, Ge, Sn, etc.) consists of an atomically thick layer of silicon (silicone), germanium (germanene), tin (stanine), lead (plumbene), phosphorus (phosphorene), and boron (borophene).^{[ 27 ]} Compared to planar graphene with sp ² hybridized carbon atoms and no dangling bonds, pseudo planar Xenes with sp ²–sp ³ hybridization (electron–electron correlations) are promising candidates for covalent functionalization owing to atoms or buckled (out of plane) atoms.^{[ 28 ]}

Recently, compound 2D materials, a new class of 2D materials, has been found and termed MXene^{[ 27} , ²⁸ , ^{29 ]} by Gogotsi^{[ 30 ]} and termed MXene.^{[ 27} , ^{28 ]} It consists of early transition metals (M) bound with carbon and nitrogen (X) and edge or surface terminated atoms (O, OH, F, and Cl). A typical MXene sheet can be represented with M _{n +1}X _n Tx,^{[ 27} , ^{28 ]} where M _{n + 1} belongs to early transition metals, and n varies from one to three layers. Notably, it was found that MXene was prepared using a precursor called the MAX phase, in which the M─A bond was metallic and chemically stronger than M─X bonds. This unique layer property offers selective etching of the elemental layer (A). 2D material chalcogenide comes under another class of flatlands, in which the layer of the hexagonal plane is made up of transition metals (groups IV–VII) and is sandwiched between layers of sulfur, selenium, or tellurium by covalent bonding. Akin to graphene, the properties of its cousins, such as electrical, optical, thermal, mechanical, and optoelectronic, are determined by the number of layers, synthesis routes, and defect density. A list of 2D family members that the scientific community has widely accepted has been summarized in Figure 3 .^{[ 31 ]} The inception of 2D hybrids or heterostructures and the post graphene era open many unexplored properties in 2D materials. Heterostructures are categorized into two types: a) lateral heterostructures (LHs) and b) vertical heterostructures (VHs).^{[ 32 ]} LH is an elongation of the conventional heterostructure concept in the 2D regime. It consists of different 2D crystal bonding in a single atomic layer. LH is limited to a few 2D materials as it allows marginal lattice mismatch in a single atomic crystalline sheet. For example, graphene and h‐BN have the same crystal structure and a lattice mismatch of 1.7%, and together, they form LH in a single atomic layer. LH occurs in transition metal di‐chalcogenides (TMDCs) as its entire variant has similar structures and almost the same lattice constant. However, the evidence of LH via vapor phase deposition is limited, which has been recently demonstrated by Huang et al.^{[ 33 ]} Heterostructures of 2D materials are an exploratory field and materials such as MoS₂–WS₂, MoS₂–MoSe₂, MoTe₂–WTe₂, MoSe₂–MoTe₂, MoS₂–WS₂, and MoSe₂–WSe₂ are the most promising amongst 2D materials for LH. Understanding and realizing LH‐based membranes and their properties would also be interesting. However, at present, no studies or research has been initiated in this direction; thus, research in this field is yet to be started to realize TMDc‐based LH membranes.

Figure 3.

The family of 2D materials.^{[ 29 ]}

VH comprises weak interlayer van der Waals bonding (out‐of‐plane) and strong in‐plane intralayer covalent bonding. Compared to LH, VH gives more freedom to stacked 2D materials even though they belong to different families, for example, graphene, hBN, TMDs, nitrides, and oxides. G–hBN, hBN–G, G–TMD, TMD–TMD, and G–hBN stack superlattices are some examples of VH, which have been investigated and shown great potential for applications in optoelectronics, thermal, and so on. Mayorov et al.^{[ 34 ]} demonstrated ballistic transport over tens of µm in the encapsulated h‐BN–graphene‐based heterostructure systems at room temperature. The state‐of‐the‐art device showed carrier fluctuations of ≈5 × 10⁹ cm⁻² and ballistic transport at a zero magnetic field owing to the noninteraction of the graphene with the polymer. Cao et al.^{[ 35 ]} proved that an air‐sensitive 2D material, such as NbSe₂, can be protected from degradation using a heterostructured system.

The superior properties of heterostructures result in unprecedented and enhanced physical and chemical properties as well as technological changes in membrane technology. However, their synthesis at the industrial or commercial level using existing techniques, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD), is in its infancy. Therefore, realizing these 2D material‐based heterostructures containing optimal ingredients in a single material for novel device applications is challenging and appealing. Hence, it is imperative to study the engineering or synthesis of 2D materials and their heterolayered systems via a top–down or bottom–up approach for manufacturing nanoporous membranes, as well as the associated challenges (synthesis of a high‐quality atomically thick crystalline sheet of 2D materials, transfer of the 2D materials on to a substrate, control over high temperature, size, defects, and pore size distribution).^{[ 36} , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ^{43 ]}

3. Synthesis of 2D Materials

Bottom–up techniques, including CVD, atomic layer deposition (ALD), molecular beam epitaxy (MBE), and top–down micromechanical exfoliation techniques, are also widely used in industries for synthesizing atomically thick 2D materials. Synthesizing graphene, TMDCs, h‐BN, and so on, as a continuous layer via CVD has already been reported. However, the first report on the large‐area synthesis of a uniform graphene sheet on a copper foil by Li et al.^{[ 44 ]} in 2009 brings up the revolutionary change in realizing the potential application of graphene in electronics industries. This breakthrough innovation was soon utilized by companies such as Samsung^{[ 45 ]} and Sony,^{[ 46 ]} where graphene sheets with a length of 30 in. and 100 m were fabricated, respectively, via roll‐to‐roll transfer technology. CVD provides high‐quality, impervious, and large graphene domains with a high growth rate and reproducibility. However, CVD utilizes toxic and corrosive gases, leading to a wrinkled graphene sheet if the temperature is not controlled introducing defects while transferring the film onto a substrate, thus making it difficult for industries to synthesize 2D materials commercially.^{[ 47 ]} The next advanced version of the CVD is metalorganic CVD (MOCVD), presently being used with high scientific interest owing to the ease in fabricating 2D materials.^{[ 48} , ⁴⁹ , ⁵⁰ , ⁵¹ , ⁵² , ^{53 ]} MBE and ALD are other techniques that can produce a large atomistic graphene sheet or other 2D materials. However, MBE involves a hydrocarbon decomposition reaction and offers various substrates, including metal, semiconductors, and insulators, which CVD lags. ALD deposits ultrathin, high‐quality substrates with atomic thickness using four steps: precursor exposure, purging, counter‐reactant exposure, and purging. The frequent exposure of the precursor to counter reactant leads to a chemical reaction at the surface of the substrate; thus, synthesis in ALD depends on the surface chemical reaction and reactive surface sites.

Recently, Mannix et al.^{[ 56 ]} deposited the first‐of‐its‐kind 2D sheet of boron, namely borophene, via PVD. In a follow‐up study, Feng et al.^{[ 57 ]} deposited borophene via MBE. The typical 2D material synthesis via different methods, including ALD and MBE is shown in Figure 4a,b.

Figure 4.

a) 2D material synthesis via ALD. Reproduced with permission.^{[ 54 ]} Copyright 2020, Elsevier. b) MBE. Reproduced with permission.^{[ 55 ]} Copyright 2019, Wiley.

The synthesis of 2D materials involves a range of methods, each offering unique advantages and challenges. Among these, chemical vapor deposition (CVD) is one of the most widely used techniques due to its scalability and the high purity of the materials produced (Figure 5 ). While molecular beam epitaxy (MBE) and physical vapor deposition (PVD) can also create 2D atomic sheets, CVD offers greater advantages in terms of quality and scalability. Traditionally, top–down techniques, such as exfoliation, were used to synthesize single atomic layers, but these methods are limited by their inability to control material quality and structure, particularly when creating nanopores. In contrast, bottom–up techniques like CVD offer greater control over the synthesis process, enabling the creation of 2D materials with tunable properties and providing platforms for nanopore formation using chemicals. However, one limitation remains: achieving control over the pore size in covalently bonded 2D sheets is still a challenge with bottom–up techniques.

Figure 5.

Chemical vapor deposition (CVD) technique reported for various 2D. Reproduced with permission.^{[ 58 ]} Copyright 2021, American Chemical Society.

A classic example of a nanoporous 2D sheet is graphene oxide (GO) and its hybrid forms, often combined with polymers to enhance conductivity, mechanical strength, and flexibility. The synthesis of GO typically involves the oxidation of graphite in an acidic medium, resulting in functionalization with various chemical groups such as hydroxyl, carbonyl, and ether. Methods developed by Brodie, Hummers, Hirata, Marcano, and others have been extensively used to produce GO,^{[ 59} , ⁶⁰ , ⁶¹ , ^{62 ]} particularly for membrane filtration applications due to its cost‐effectiveness. However, GO's poor mechanical strength, limited control over functional groups, and the release of toxic gases during synthesis pose significant challenges for industrial scalability. In addition, the ecotoxicological effects and fouling issues limit its broader use in filtration membranes.

High material quality and crystallinity are usually achieved via CVD or MOCVD. However, the transfer method of 2D materials on the substrate (for supporting membrane) creates defects, such as wrinkles and folding,^{[ 63} , ⁶⁴ , ⁶⁵ , ^{66 ]} owing to the dry or wet transfer methods. The transfer techniques involve dry stamping or coating with the polymer,^{[ 67} , ^{68 ]} which allows 2D materials to stick on the polymer, and then, followed by lift‐off or through careful pealing; the polymer will get etched by the solvents.^{[ 67} , ^{68 ]} This process leads to inevitable contamination^{[ 69 ]} of the 2D materials and makes 2D materials prone to functionalization. To overcome such challenges, CVD techniques are currently used to deposit atomic 2D sheets directly on the porous substrate.^{[ 70 ]} The creation of 2D compound materials by CVD has made significant strides so far. The nonmetal and metal precursors, substrate architecture, temperature, and gas flux are all important factors that affect the CVD growth operation. To realize the controlled fabrication of 2D compound materials, ranging from binary and ternary to even more complex materials, it is crucial to have a thorough grasp of these factors and the growth mechanism. The probability of using the nanoporous membrane for filtration is in the bottom–up technique owing to its cost‐effectiveness, but it has many limitations, such as no control over functionalization, pore size, and density. Therefore, a proper mixture of top–down (high selectivity, enhanced permeability, less fouling, chemical stability, and overall cost‐to‐benefit) and bottom–up techniques for synthesizing 2D materials may be used for scalable synthesis and control over porosity.

Beyond CVD, other bottom–up synthesis methods include liquid‐phase exfoliation,^{[ 71 ]} laser‐based synthesis,^{[ 72 ]} hydrothermal/solvothermal synthesis,^{[ 73 ]} mechanical exfoliation,^{[ 74 ]} electrochemical exfoliation,^{[ 75 ]} and microwave exfoliation,^{[ 76 ]} each capable of producing high‐quality 2D materials. While CVD or metal–organic CVD (MOCVD) methods typically produce materials with high crystallinity and material quality, transfer techniques such as dry stamping or polymer‐assisted wet transfer introduce defects like wrinkles and contamination.^{[ 63} , ⁶⁴ , ⁶⁵ , ^{66 ]} These imperfections often require post‐synthesis functionalization, which complicates the process.

To overcome the limitations of each technique, a combination of top–down and bottom–up approaches may offer a balanced pathway for the scalable synthesis of 2D materials. For instance, top–down methods provide high selectivity, enhanced permeability, and chemical stability, while bottom–up approaches offer cost‐effectiveness and control over the material's atomic structure. However, this blended technique has yet to be fully realized and developed for practical applications. CVD remains the most promising method for depositing 2D atomic sheets directly onto porous substrates, especially for filtration membranes, but further refinement is needed to control porosity and functionality.

4. Properties of 2D Materials

The conventional membrane and techniques (boiling, sedimentation, distillation, and chemical disinfectants) for water purification have become obsolete due to the ever‐growing demand for pure water or potable water.^{[ 77 ]} Membrane technology working on pressure‐driven mechanisms such as ultra or nanofiltration techniques, respectively as well as on reverse osmosis is deemed to fulfill the potable water demand.^{[ 78 ]}

The inception of graphene and its analog in the last decade opened up a new potential opportunity for 2D materials in the use of membrane technology. However, the synthesis techniques for only the graphene‐based membranes have been extensively explored and its application in membrane technologies has been practically realized for water desalination and filtration (Figure 6 ). A typical graph representing the average diameter of the most commonly found bacteria in water flavobacterium, micro bacterium to lactobacillus and cytophaga, and diameter of the some of the common toxic ions such as potassium, sodium, aluminum, and calcium, is are represented in Figures 6a and 6b, respectively. To realize pore size at the nanoscale, heavy‐ion bombardment (IB) and etching (plasma or ozone treatment) techniques have been utilized but it leads to the random generation of pore size of various diameters.^{[ 79} , ⁸⁰ , ^{81 ]} In contrast to IB, the focused ion beam (FIB)^{[ 82} , ⁸³ , ⁸⁴ , ⁸⁵ , ⁸⁶ , ^{87 ]} and the transmission electron microscope (TEM)^{[ 79} , ⁸⁰ , ⁸⁸ , ^{89 ]} produce fine, accurate, and desired size of pores. TEM is the most used technique to achieve the pore size in 2D materials as it allows to kick out of a single atom or remove atom by atom in an in situ experiment. This technique opens up the prospect of realizing graphene‐based nanoporous membranes but fails to deliver the same to other 2D materials. The other 2D materials (especially MoS₂, BN, etc.) are more sensitive to electron irradiation and get contaminated due to the generation of carbon atoms as ad‐atoms in the in situ experiments.^{[ 90} , ^{91 ]} Contamination of other 2D materials^{[ 92} , ^{93 ]} is a huge challenge to overcome, therefore, this technique has lost its glory for becoming a front‐runner technique for the industrial production of nanoporous membranes. The other technique which has been used for making nanopores is the ion‐bombardment technique.^{[ 94} , ^{95 ]} However, ion‐bombardment techniques have very poor control over pore sizes diameter, density, and arrangements; thus, the use of ion‐bombardment techniques for making nanoporous membranes gets obsolete. Focused ion beam (FIB) is the next promising technique used for making nanoporous membranes, and indeed, it delivers the promise it holds. It uses various (Ga⁺, Ar⁺, or He⁺ atoms) atoms to dislocate or remove the atoms in the atomic arrangement of 2D materials.^{[ 96 ]} It has been used in the low‐energy regime (in comparison to TEM) and primarily uses a different size of atoms to generate desired pore size. A helium ion microscope has been used to achieve the pore size of 5–30 nm in graphene.^{[ 89 ]} It has also been tested to create nanopores of less than 5 nm in MoS₂ and BN. This technique has been deemed to be the next potential method for making nanopores in membranes at an industrial scale. The next technique for generating nanopores at the nanoscale is lithography; however, creating a pore size of less than 16 nm is a challenge and has not been achieved to date. Some hybrids technique has been used for making nanopores less than 16 nm and involves the use of polymer mask and e‐beam lithography, followed by dry etching.^{[ 97 ]} In addition to this, the pore sizes in membranes created by sputtering or plasma etching have also been tuned by defocused ion irradiation.^{[ 92} , ^{93 ]} However, this all leads to an increase in surface charge; thus, decreasing membrane strength and stability. The dielectric breakdown has also been used vastly as another technique for making nanopores using voltages to the thin membranes in a solution of an electrolyte.^{[ 98} , ^{99 ]} It gains popularity due to the creation of sub‐nm pore size at laboratory scale experiments, but it lags greatly in controlling the position of the pores and their geometry. It should be noted down that the use of the different methods for making nanopores should be implemented after having an in‐depth understanding of the 2D materials, their composition, toxicity, and performance‐dependent relation of the 2D nanoporous membrane in accordance to mechanical strength, surface energy, and hydrophobicity. The above‐mentioned features of 2D materials are of utmost importance to prevent their further deterioration and to enhance their efficiency as a membrane. It is therefore imperative to study the physical properties of 2D materials and their toxicity. Besides the ideal pore size, the surface energy of the material also plays a crucial role in removing fouling from the water because fouling is a phenomenon that is inevitable and can be realized everywhere a bacteria can be found. Therefore, any kind of perturbation or defect in 2D materials can lead to the accumulation of foreign analytes and decrease the performance of the membrane (low permeability, short membrane life, etc.). The foulants are very less likely to adhere on a surface that has low friction, roughness, and less hydrophobicity (Figure 6c). Fouling (such as biological, colloidal, organic, and scaling) severely destroys the performance of the membrane and increases the need for chemical cleaning, which after enhancing the permeance, degrades or shortens the membrane life, while simultaneously increasing the maintenance cost and energy consumption of membranes. The realization of low fouling material is always the need of the hour; a comparison between 2D materials and their heterostructures surface energy has been done (Figure 6d) and the prospect of heterostructures‐based membranes due to low surface energy was proposed to have a plethora of exciting and emerging opportunities in the realization of the nanoporous membrane (considering the control over lattice mismatch and strain). An elemental sheet of TMDCs heterostructures such as MoS₂/WS₂, MoS₂/WS₂/BN, WS₂/BN, and MoS₂/BN on SiO₂/Si as a free‐standing sheet is proposed to have an anisotropic arrangement of atoms, however, its deposition on a substrate leads to a significant decrease in the surface energy when compared to BN, graphene, BP, MoS₂, MoSe₂, WS₂, and so on monolayer atomic sheet. The next surprising and most advantageous feature of having heterostructures is realized in the case of surface energy graphene sheets on SiO₂/Si and individual graphene sheets. It is found that a 3232‐graphene sheet possesses a surface energy of nearly ≈75 mJ m⁻²; while, its heterostructures have a surface energy of ≈575 mJ m⁻²; this phenomenon is attributed to relaxation in strain and to the lattice match of the sheet with the substrate. It is evident from the figure that boron nitride has the lowest surface energy followed by black phosphorus and graphene. The surface energy of TMDc is almost three times in comparison to atomic sheets of BN and BP. However, heterostructures outperform the 2D materials family by enormous margins and mark their presence for potential application in upcoming future membrane technologies. The mechanical stability of the 2D membrane (Figure 6e) varies with the thickness of the sheet (Figure 6f). The mechanical strength of 2D materials varies in two directions, that is, Y _a and Y _b. It is therefore crucial to understand the mechanical and breaking strength of the atomic sheets in both directions (Y _a and Y _b) to withstand abrasion and high pressure. Graphene, the first entrant into the flatland family, has the highest Young's modulus of 338 Gpa and sustains the strain of ≈25%.^{[ 100 ]} Pores and defects lead to a decrease in the mechanical strength of graphene atomic sheets with many orders of magnitudes.^{[ 101} , ^{102 ]} However, graphene on a supporting substrate has still good enough mechanical strength to withhold high pressure. It is calculated that an atomic sheet of supported (on a 1 µm pores substrate) graphene nanoporous membrane can stand at a pressure of 570 bar.^{[ 103 ]} Analogous to graphene, the 2D family also possesses sufficiently very high mechanical strength and has been compared. It is evident from Figure 6 that Xenes (germanene, silicone) possess almost equivalent mechanical strength in both directions. However, phosphorene (another family member of Xene) has two striking different values of Young's modulus and its value varies three times (almost) when compared to each direction of Young's modulus. Boron nitride, another member of the graphene family, and also known as white graphene has almost the same mechanical strength in comparison to graphene. Moreover, the next big upcoming challenge for the 2D nanoporous membrane is toxicity as to find immense application in water filtration, they need to have a minimal toxic nature to humans as well as to the environment. It is, therefore, imperative to investigate its cytotoxic nature to humans and manage the risk of health hazards or biological systems or ecosystems. However, little knowledge and research are available on the potential adversity and toxicity of 2D materials to humans. It is therefore vital to summarize and understand its cytotoxic nature, thus playing safe by choosing a potential preventive approach (Table 1 ).^{[ 104} , ¹⁰⁵ , ¹⁰⁶ , ¹⁰⁷ , ¹⁰⁸ , ¹⁰⁹ , ¹¹⁰ , ¹¹¹ , ¹¹² , ¹¹³ , ^{114 ]} Transition metal dichalcogenides, graphene, boron nitride, and black phosphorus toxicity have been discussed based on their synthesis protocol. It is evident that the chemical route has a much larger impact on human health in comparison to mechanical or liquid phase exfoliation techniques. Moreover, amongst all 2D families being compared, graphene poses no toxic behavior, which may be attributed to the carbon constituents. Thus, it can be easily forecasted that graphene‐based membrane or their heterostructures are the future materials for membranes used in water filtration technology.

Figure 6.

List of a) Hydrated toxic ions along with their diameter and b) bio‐fouling causing bacterial species with their average diameter respectively. c) The contact angle and d) surface energy of various 2D materials and heterostructures. e) 2D materials with their Young's modulus and f) breaking strength and thickness of 2D materials.

Table 1.

Toxicity of 2D materials.

Materials	Synthesis	Exposure method	Toxicity	Species/cell type	Refs.
MoS2	Chemical	Cell culture	≈50% viability for A549 cell exposed to 400 ppm nanosheets (400 × ≈4.5)	Human lung carcinoma epithelial cells (A549)	[104, 105, 106, 107]
MoS2	Liquid phase exfoliation	Cell culture	≥80% viability up to 50 µg mL−1	Human bronchial cells (BEAS‐2B) and the human (THP‐1)	[104]
MoS2	Mechanical exfoliation	Cell culture	≥80% viability up to 48 h	Human epithelial kidney cells (HEK293F)	[108]
MoS2	Chemical vapor deposition	Cell culture	≥80% viability up to 48 h	Human epithelial kidney cells (HEK293F)	[108]
WS2	Mechanical exfoliation	Cell culture	≥80% viability up to 48 h	Human epithelial kidney cells (HEK293F)	[108]
WS2	Liquid phase exfoliation	Cell culture	No loss of viability up to 100 µg mL−1	HeLa (human cervical cancer cell line), 4t1 (mouse breast cancer cell line) and (human embryonic kidney) cell	[109]
WS2	High temperature solution phase exfoliation	Cell culture	≥70% viability up to 200 µg mL−1	Reticuloendothelial system (RES) such as liver and spleen in mice	[110]
TiS2	High temperature solution phase exfoliation	Cell culture	≥75% viability up to 200 µg mL−1	Reticuloendothelial system (RES) such as liver and spleen in mice	[111]
h‐BN	Liquid phase exfoliation	Cell culture	No loss of viability up to 100 µg mL−1	HeLa (human cervical cancer cell line), 4t1 (mouse breast cancer cell line), and (human embryonic kidney) cell	[109]
BP	Bulk	Cell culture	8% viability (WST‐8) 34% viability (MTT) at 50 µg mL−1	Cancer cells	[112]
BP	Liquid phase exfoliation	Cell culture	≥80% viability up to 48 h	Skin keratinocytes	[113]
Graphene	Few‐layer graphene	Cell culture	No significant effects were observed after 24h	Tumor‐bearing mice	[114]
TiS2	Nanosheet (bottom–up solution‐phase method)	Cell culture	Mouse mammary gland cancer cells (4T1)	Tumor‐bearing mice	[111]

5. Types of Membrane

Membranes are fine‐knitted thin barriers that selectively permit desired species to transit (permeate) and concurrently restrict undesired species (selectivity) from passing through them.^{[ 115} , ¹¹⁶ , ^{117 ]} Concentration gradient, pressure difference, and temperature change are some factors that drive permeance. An evident change and advancement in membrane technology (increased throughput, energy efficiency, low toxicity, compactness, low cost, and environment friendliness)^{[ 117 ]} have been observed over the past few decades, implementing membrane technology in industries such as pharmaceuticals, petrochemical, water desalination, gas separation, water filtration, wastewater treatment, hemodialysis, bioprocessing, biotechnology, food processing, energy harvesting, fuel cells, and nitrogen generation.^{[ 118} , ¹¹⁹ , ¹²⁰ , ¹²¹ , ¹²² , ¹²³ , ¹²⁴ , ¹²⁵ , ¹²⁶ , ¹²⁷ , ¹²⁸ , ¹²⁹ , ¹³⁰ , ¹³¹ , ¹³² , ¹³³ , ¹³⁴ , ¹³⁵ , ¹³⁶ , ¹³⁷ , ¹³⁸ , ¹³⁹ , ¹⁴⁰ , ¹⁴¹ , ¹⁴² , ¹⁴³ , ¹⁴⁴ , ^{145 ]} However, some of the challenges that persist in membrane technology hinder its applications in industry, including difficulty in controlling the sensitivity and selectivity of the membrane material, reduction in fouling, less lifetime, high cost, robust performance in extreme conditions, and toxicity of the membrane material.

The following Table 2 provides a comprehensive overview of various membrane materials, their typical pore sizes, and their impact on water filtration efficiency. It includes traditional polymeric membranes as well as recent developments in 2D materials, highlighting how these advances are shaping the future of water purification technologies.

Table 2.

Pore sizes across membrane materials for water filtration.

Filtration type	Pore size	Materials	Filtration efficiency	Refs.
Microfiltration	0.1–10 µm	PP, PES, PVDF, PTFE	Removes large particulates, bacteria, and protozoa. Allows passage of viruses and dissolved salts.	[147, 148]
Ultrafiltration	0.01–0.1 µm	PAN, PS, cellulose acetate, PVDF	Removes viruses, proteins, and colloids. Permits passage of smaller organic molecules and ions.	[149, 150, 151]
Nanofiltration	0.001–0.01 µm	TFC, polyamide, 2D graphene oxide (GO)	Removes multivalent ions (Ca2⁺, Mg2⁺), small organic compounds (herbicides). Limited removal of monovalent ions.	[152, 153, 154]
Reverse osmosis	<0.001 µm	TFC, polyamide, 2D molybdenum disulfide (MoS₂), 2D COFs	Removes nearly all dissolved solids, salts, heavy metals, and small organic molecules. It is ideal for desalination.	[155, 156, 157]
Forward osmosis	<0.001 µm	Cellulose triacetate (CTA), TFC, 2D MXenes	Similar to RO, but energy‐efficient due to osmotic pressure. There is high selectivity for salts and small organics.	[158, 159]

Varieties of atomically thin sheets have been developed, nicknamed as 2D materials, and they exhibit quantum nature in several domains.^{[ 160} , ¹⁶¹ , ¹⁶² , ¹⁶³ , ¹⁶⁴ , ¹⁶⁵ , ¹⁶⁶ , ¹⁶⁷ , ¹⁶⁸ , ¹⁶⁹ , ¹⁷⁰ , ^{171 ]} While the most explored 2D material is graphene, which is semi‐metallic in nature and exhibits several superlative physical and chemical properties,^{[ 172} , ¹⁷³ , ¹⁷⁴ , ¹⁷⁵ , ^{176 ]} electrically insulating boron nitride is structurally and chemically robust. These two materials have now been commercialized and available at industrial scale at economic cost. They provide an opportunity to overcome the challenges faced by bulk materials because in their pristine state, they debar the analytes with a size bigger than a proton to pass through them owing to their unique atomic arrangements in their lattice (d < ≈5–10 Å).^{[ 55} , ⁵⁶ , ⁵⁷ , ^{58 ]} Monoelemental atomic sheets (Xenes) such as metallic sheets of borophene,^{[ 177} , ¹⁷⁸ , ^{179 ]} molybdenene,^{[ 180 ]} berylline,^{[ 181 ]} goldene,^{[ 182 ]} phosphorene,^{[ 183 ]} silicene,^{[ 184 ]} germanene,^{[ 185 ]} transition metal oxides,^{[ 163} , ¹⁸⁶ , ¹⁸⁷ , ¹⁸⁸ , ¹⁸⁹ , ¹⁹⁰ , ¹⁹¹ , ¹⁹² , ^{193 ]} and 2D Nitrides^{[ 168} , ^{194 ]} have been developed. These 2D materials have been atomistically engineered via 3D Straining,^{[ 195 ]} 2D–2D hybridization,^{[ 196} , ¹⁹⁷ , ^{198 ]} and substitutional doping.^{[ 199} , ²⁰⁰ , ^{201 ]} These 2D materials have been employed for several frontier applications.^{[ 169} , ²⁰² , ²⁰³ , ²⁰⁴ , ²⁰⁵ , ²⁰⁶ , ²⁰⁷ , ^{208 ]} Anti‐oxidative nature^{[ 209 ]} due to chemical inertness and cooling features^{[ 210 ]} arising from high thermal conductivity are significant ones to mention.

Unlike ceramics, zeolites, and polymers, which limit designing defect control at bulk, these 2D materials offer precise defect control, frictionless transport, high permeability, high sensitivity, selectivity, low surface energy, and freedom to tailor dangling bonds (for attachment of tunable functionalities) at the atomic level; thus, making facile synthesis of nanoporous 2D‐membranes possible. Zeolites, while widely used in filtration due to their highly ordered porous structures and effective ion‐exchange capabilities, are limited in tunability compared to 2D materials.^{[ 211 ]} They excel at removing contaminants such as heavy metals and ammonia from water but lack the atomic‐level precision that allows for more versatile and customizable filtration. Industrial‐scale 2D‐material membranes seemed challenging when 2D materials were first discovered. In 2013, Lockheed Martin discovered a revolutionary membrane technology (at the commercial level) based on graphene called Perforene. It was used in the desalination process for obsolete salts and ions (sodium, chlorine, etc.) from seawater, resulting in fresh drinking water.^{[ 212 ]} Thereafter, the same technology was realized in the purification of oil wastewater. This experimental observation, once addressed, evokes the ability of graphene cousins (yet limited) to provide a more facile fabrication of nanoporous membranes technology. Later on, Arvia technology (UK‐based firm) and Nyex (Manchester University) stepped up to make membranes to eliminate other challenges such as organic compounds and fouling properties.^{[ 212 ]} According to IUPAC, the pore sizes (d) <0.2 nm, 0.2–2 nm, and >50 nm have been categorized as nanoporous, mesoporous, and macroporous membranes, respectively.^{[ 213} , ²¹⁴ , ^{215 ]} Moreover, based on the filtration process type, membrane technology is further classified into reverse osmosis (0.1–1 nm), nanofiltration (1–10 nm), ultrafiltration (2–100 nm), and microfiltration (0.1–10 µm).^{[ 214} , ^{215 ]} Therefore, to develop a perspective for pressure‐driven water‐based filtration technologies (Figure 7 ), a relationship between the target species and corresponding pore size has been summarized. As the working principle of water filtration and gas separation is the same, the membranes designed for water filtration in industries are often used for separating the admixture of gas molecules. Therefore, we limited the scope of this review only to water filtration, and common terminologies between water and gas filtration are defined as needed.

Figure 7.

Basic features of commonly used water treatment membrane.^{[ 146 ]}

Presently, graphene rules the nanoporous membrane industries; soon its cousins or analogs will challenge it. Few seminal review articles (in 2D materials nanoporous membrane technology) have been reported for understanding the functionality, mechanism, and fabrication of the nanoporous membrane technology, but a void has been realized from the perspective of scientific challenges (synthesis, physical, and chemical properties) faced by the new member of the 2D family as well as the role of blended 2D materials/hybrids in water filtration. A number of review papers based on 2D materials membranes for water filtration has been documented.^{[ 216} , ²¹⁷ , ²¹⁸ , ²¹⁹ , ²²⁰ , ^{221 ]} The effect of filter pore size, 2D material crystallographic structure, and electrostatic/van der Waals interactions between pathogens/contaminants and filter materials on filtration efficiency is depicted in Figure 8 . Moreover, it envisages the role of flatland (graphene [planar], Xenes [pseudoplanar], their cousins) members and the potential role of flatland hybrids in water filtration. A summary of the current technological development in scalable synthesis and intrinsic properties (such as surface energy, toxicity, high mechanical strength, hydrophobicity, and the dependency of breaking strength on the thickness and pore size) of the flatland members have been reported. It also emphasizes the significant progress, problems, and challenges made in the development of state‐of‐the‐art membrane technology for nanoporous atomically thin membranes. Moreover, in the end, a perspective has been presented to foster a solution to numerous futuristic engineered nanoporous membranes.

Figure 8.

Crucial parameters determining filtration efficiency.

In addition to commonly studied 2D materials such as graphene and MXenes, silicates^{[ 222 ]} and 2D zeolites^{[ 223 ]} also represent an important class of materials with significant potential in water purification applications. Natural and synthetic silicates, due to their unique layered structures and ion exchange capabilities, are highly effective in removing a wide range of contaminants from water.^{[ 224 ]} Their porous frameworks allow for selective filtration, making them ideal for trapping heavy metals and organic pollutants. Similarly, 2D zeolites, which are derived from traditional zeolites but exist as ultrathin sheets, exhibit enhanced surface area and permeability, making them efficient for applications such as molecular sieving and desalination.^{[ 225} , ^{226 ]} These materials possess a high degree of tunability, allowing for modifications in pore size and surface chemistry to optimize their performance for specific contaminants. Their natural abundance, chemical stability, and environmentally friendly nature further underscore their importance in developing sustainable water filtration technologies.

The filtration properties of membranes depend on porosity (pore size and pore length), material constituents, surface energy, mechanical strength, and hydrophobicity.^{[ 213} , ^{227 ]} Mass resistance to molecules was often achieved in conventional membranes via membrane structure and porosity. High permeance is controlled by tuning the porosity density. However, some analyte dissolves and passes through the pore length, limiting the use of the conventional membrane.^{[ 228} , ²²⁹ , ^{230 ]} The traditional membrane water filtering method worked well for larger pathogens, such as bacteria and protozoa. However, owing to the miniature size of the viruses 20–100 nm range, the traditional membrane filters could not remove them. Thus, the adsorbing surface was employed to sieve the tainted water to eliminate viruses using electrostatic forces, including hydrophobic interactions, electrostatic forces, and van der Waals forces. This virus is responsible for many water‐borne illnesses. As the adsorption interface of the filtering screen is positively charged, it attracts negatively charged viruses. In this manner, polluted water is filtered out. Enhancing the electrostatic interaction, chemical affinity, and mechanical strength in the membrane are ways to restrict the analyte flow through the membrane.^{[ 212} , ^{213 ]} Contrary to the conventional membrane, in the nanoporous membrane with pores sizes of 2–100 nm, diffusivity depends on the molecular or solution gradient; selectivity is controlled by diffusion rate, layer thickness, surface charge, membrane porosity, structure, chemical bonding, steric effects, electrostatic interactions, and dielectric effects.^{[ 213} , ^{227 ]} Various membranes have been synthesized and developed for filtration (Figure 9 ).^{[ 231} , ²³² , ²³³ , ²³⁴ , ²³⁵ , ²³⁶ , ²³⁷ , ^{238 ]} Polymeric membranes (PMs) (first membrane generation) are vital in water purification (reverse, microfiltration, nanofiltration, and ultrafiltration), fuel cells, and batteries owing to their control over the selective transport of ions or molecules, low cost, and flexibility. The most often used polymeric membranes are cellulose acetate (CA), cellulose triacetate (CTA), polysulfone (PSf), polydimethylsiloxane (PDMS), polyamides (PA), polycarbonate (PC), polyaniline (PANI), and so on.^{[ 239} , ^{240 ]} The exact composition of molecules or ions in transport is often ambivalent and relies on the complexity of the PM structure and the pressure dynamics that facilitate transport. Water molecules do not attack PMs, which do not have thermal stability and are prone to fouling and hydrophobicity loss. Merkel et al.^{[ 241 ]} proposed a nanocomposite (polymer and silica) matrix membrane with a reverse‐selective mechanism for high permeance. Moreover, the polymeric matrix was hybridized with 2D materials (Li et al., Putz et al., and An et al.)^{[ 242} , ²⁴³ , ^{244 ]} to overcome the drawbacks of plasticization and thermal stability. However, Robenson et al.^{[ 245} , ^{246 ]} proposed that for PM, there is a trade‐off between permeance and selectivity; as permeance increases, selectivity decreases and vice‐versa, limiting PM in membrane technology. Thus, the aim was to search for a suitable membrane that can overcome the trade‐off between permeance and selectivity of PM. Carbon molecular sieves (CMS), being an inorganic material, have demonstrated the potential to overcome the upper limit and attenuate the challenges (plasticization and deformation at high temperature and pressure)^{[ 247 ]} faced by PM, which is proposed to be the front‐runner in nanoporous membrane filtration technology.^{[ 248 ]} CMS is synthesized via carbonizing polymeric precursors in a vacuum at high temperatures (773.15–1273.15 K, pyrolysis), effusing all gaseous contents in polymers. Upon cooling, micropores or ultra‐micropores appear (naturally) with a dimensional shrinkage and considerable weight loss. In short, CMS constitutes microporous or ultra‐microporous pores on amorphous carbon membranes. The production cost of CMS membrane is nearly three times that of PM, and CMS is brittle in nature.^{[ 249} , ²⁵⁰ , ^{251 ]}

Figure 9.

Existing membrane technology concerning the structure of membranes.

Often, oxidation is a curse to materials, but sometimes it can be a blessing. The natural oxidation of aluminum (thickness = 2–3 nm) is a blessing rather than a curse as it prevents aluminum from getting further oxidized. Buff et al.^{[ 252 ]} reported the first synthesis of electrochemically oxidized alumina, which is called anodization.^{[ 253} , ^{254 ]} In practice, anodization occurs in the presence of an aqueous electrolyte. The chemical composition of the electrolyte decides the structure and morphology of the alumina. If the used electrolyte has a pH of 5–7, such as borate, citrate, and adipate,^{[ 255} , ^{256 ]} it will lead to nonporous type barrier aluminum oxide. Contrary to this, a porous oxide film is formed if the pH is acidic, such as phosphoric, malonic, tartaric, or chromic.^{[ 256} , ^{257 ]} Porous anodized* aluminum oxide (AAO) membrane is the next exploratory membrane technology (besides PM and CMS). The aspect ratio of pore diameter to pore length in AAO can reach up to 1:1000. AAO offers precise pore size retention, pore density, and pore length and does not have any extractable organics. It also offers stability to various organic solvents. However, its filtration properties heavily rely on interpore distance and diameters, layer thickness, breakdown of glowing oxides, surface defects, the electrolyte used for synthesis, and more.^{[ 258} , ²⁵⁹ , ^{260 ]} AAO often loses its permeance owing to degradation influenced by the pH of the solvent, which increases in basic media (major obstacle); hence, it needs further development.

The upcoming potential membrane materials for water filtration are carbon nanotube membranes (CNT), and they offer many unique and interesting properties, such as mechanical, thermal, electrical, and chemical properties, as well as of its large surface area, ease of functionalization, high aspect ratio, and fast water transport. CNT is a hollow cylindrical graphene sheet. It may or may not be limited to monolayer folding; thus, it has been categorized into single‐wall CNT (SWCNT) and multi‐wall CNT (MWCNT). MWCNTs comprise multiple graphene layers rolled in a hollow cylindrical shape. Dai et al., Li et al., Nasrabadi et al., and Joseph et al.^{[ 261} , ²⁶² , ²⁶³ , ^{264 ]} have extensively used SWCNTs and MWCNTs to desalinate seawater and brackish water. CNTs offer enhanced antifouling behavior, strength, rejection, and permeability as fillers in various membranes.^{[ 265 ]} However, the major drawbacks of CNT membranes are the synthesis of vertically aligned CNT membranes at an industrial scale and the coagulation of CNTs owing to van der Waals's forces in bucky paper membranes. Dumee et al.^{[ 266 ]} proposed the first bucky paper CNT concept in 2010 and analyzed its potential performance by blending CNTs with polymers to enhance filtration and increase permeance.^{[ 267 ]} However, successful fine particle filtration was demonstrated with CNT membranes by Viadero Jr et al.^{[ 268 ]} Commercial availability, cost reduction, toxicity, control over suitable pore size, and distribution challenges further enhance the obstacle of CNT membranes for water filtration. The failure of CNTs marled the rising of metal–organic framework (MOF) membranes. The MOF framework comprised metal ions and bridged organic ligands.^{[ 233} , ²⁶⁹ , ²⁷⁰ , ^{271 ]} Compared to other membranes (PM, CMS, CNT, and AAO), MOFs offer pore size tunability, superlative selectivity, high permeability, and increased active adsorption sites.^{[ 272 ]} MOFs are divided into three categories: a) metal carboxylate, b) metal azolate, and c) functionalized MOFs.^{[ 273} , ^{274 ]} The first two MOFs are composed of hard acid metal ions, such as Cr³⁺, Al³⁺, Fe³⁺, and Zr⁴⁺,^{[ 275} , ²⁷⁶ , ^{277 ]} while the latter comprises soft base azolate ligands (imidazolates, pyrazolates, triazolates, and tetrazolates) and soft acid metal ions, such as Zn²⁺, Cu²⁺, Ni²⁺, Mn²⁺, and Ag⁺.^{[ 278 ]} Despite having water‐stable membranes (wastewater treatment and water regeneration), MOFs lag in design and synthesis strategies, defect control, recyclability, low production cost, and environmental toxicity.

Carbon nanomembranes are yet another membrane filtration technology (after CMS) that utilizes amorphous carbon (diamond‐like) and have been implemented in membrane technology owing to their unprecedented mechanical strength and chemical inertness. Although Karan et al.^{[ 279 ]} demonstrated carbon‐based nanomembrane for filtration, the real‐time application of carbon nanomembranes (at the laboratory scale) remains challenging as it requires high‐energy, sophisticated, and mega equipment. In 2010, Nair et al.^{[ 280 ]} demonstrated superior filtration properties of another carbon‐based functionalized 2D sheet, namely graphene oxide (GO) membranes that can filter ions and molecules. GO‐based membranes were well accepted and technologically implemented owing to their ease in synthesis, processing, and making uniform pore sizes.^{[ 280 ]} Moreover, the freedom of interlayer spacing is another added advantage in real‐time applications.^{[ 281} , ^{282 ]} However, the durability and stability of the membranes are still a big challenge to overcome. The only problem with the GO‐based membrane, which limits its use in industries, is thermal stability.

In principle, superior membranes with ultrafast permeability (economical, nontoxic, and environmentally friendly) should be at the thinnest while retaining mechanical strength, solvent resistance, maximum permeability, and energy. Over an era, it's been realized that every membrane technology needs further improvement or research and development to implement it on an industrial scale. Therefore, it is time to explore something that can give out‐of‐the‐box solutions. 2D materials and their hybrids membranes arrived well in time and are currently being explored for their filtration properties with great scientific interest. Fabricating a nanoporous 2D membrane provides one unique solution to overcome all existing challenges in filtration technology. The synthesis of 2D materials or their heterostructures and the fabrication of the nanoporous membrane from 2D materials or flatlands for water filtration at an industrial scale is the next important fact to understand, which we now discuss.

2D membranes have gained significant interest due to their potential to deliver precise filtration, challenging current desalination platforms. Sapkota et. al. demonstrated that molybdenum disulfide (MoS₂) laminar membranes have superior stability in aqueous environments compared to extensively studied graphene‐based membranes. However, challenges such as low ion rejection for high salinity water, low water flux, and limited stability over time hinder their adoption as a viable technology. Composite laminate multilayer MoS₂ membranes with stacked heterodimensional one‐ to two‐layer‐thick, porous nanosheets, and nanodisks have been reported. These membranes feature a multimodal porous network with tunable surface charge, pore size, and interlayer spacing. In forward osmosis, these membranes reject more than 99% of salts at high salinities, and in reverse osmosis, they efficiently filter small‐molecule organic dyes and salts. In addition, these membranes operate stably for over a month, suggesting their potential for commercial water purification applications (Figure 10a–d)^{[ 283 ]} A comprehensive comparison of water permeability and ion rejection rates has been studied in detail by Cao et. al. across various 2D materials, including MoS₂, graphene, phosphorene, boron nitride, and MoSe₂. (Figure 10e)^{[ 284 ]} Molecular dynamics simulations demonstrated that, among 2D materials with the same pore size, single‐layer MoS₂ consistently outperformed graphene by 27%, phosphorene by 38%, BN by 35%, and MoSe₂ by 20% in water permeability while maintaining an ion rejection rate greater than 99%. The outstanding performance of MoS₂ was attributed to a combination of water structure and dynamics near the membrane surface, energy barriers, and water packing and velocity inside the nanopore.

Figure 10.

2D materials‐based membranes employing a–d) MoS₂. e) Simulation studies on membranes made up of various 2D materials.^{[ 283} , ^{284 ]}

Salt rejection in membrane filtration is governed by several key factors that influence the membrane's ability to block salt ions while allowing water to pass through. One of the primary factors is pore size and distribution, where membranes with smaller, uniform pores, such as those in reverse osmosis and nanofiltration systems, are more effective at rejecting salts.^{[ 286 ]} The surface charge of the membrane also plays a significant role as negatively charged membranes repel anions like Cl−, enhancing rejection through electrostatic interactions. The hydrated radius of ions, such as Na⁺ and Cl⁻, determines their effective size in water, and membranes must be designed to filter out these larger hydrated ions.^{[ 287 ]} Membrane thickness impacts both mechanical strength and permeability, with thinner membranes often providing better water flow without compromising salt rejection.^{[ 288 ]} Operating pressure further affects performance, where higher pressures increase water permeability but may allow more salt to pass through if the membrane is not well‐optimized.^{[ 289 ]} Concentration polarization, where salt ions accumulate near the membrane surface, can reduce rejection efficiency, requiring effective flow design to mitigate this issue. In addition, there is often a trade‐off between permeability and selectivity; highly selective membranes may have lower water permeability, while more permeable membranes might sacrifice salt rejection.^{[ 290 ]} Feed water composition, including the concentration and type of salts, also influences rejection rates, with higher salt concentrations or multivalent ions impacting the membrane's performance.^{[ 291 ]} By addressing these factors, advanced membrane materials such as graphene oxide and MXenes can optimize both selectivity and permeability for effective salt rejection in water filtration systems.

With respect to the abbreviations used in Figure 11 , TFN is used to define thin‐film nanocomposite membranes; CNT is used for carbon nanotube membranes; SWRO, BWRO, and HFRO represent sea‐water reverse osmosis, brackish reverse osmosis, and high‐flux water reverse osmosis, respectively; while NF, as well as MFI, are used for nanofiltration and zeolites respectively. It can be referred from Figure 11, Boretti et al.^{[ 285 ]} that the maximum permeability for salt rejection has been observed for graphene‐based membranes as ≈10⁻⁹m Pa^{− 1} s^{− 1}. This, however, is the starting point to realize and synthesize other 2D material‐based membranes (graphene cousins in flatland) through the technique used to synthesize graphene, so that, it can certainly boost their performance and application in water filtration.

Figure 11.

A comparison between the permeability and salt rejection of the commercially available reverse osmosis membranes and nanostructured membranes technology for operation in high‐saline and low‐saline sea as well as brackish water respectively. Reproduced with permission.^{[ 285 ]} Copyright 2018, Springer Nature.

The following Table 3 provides a comparative overview of several 2D materials, highlighting their key properties and potential applications in water purification technologies. The materials listed include graphene, graphene oxide, MoS₂, phosphorene, hexagonal boron nitride (h‐BN), and MXenes, each of which exhibit unique characteristics such as water permeability, mechanical strength, antifouling properties, and selectivity. These properties directly influence their performance in critical applications, such as desalination, heavy metal removal, and the separation of organic contaminants.

Table 3.

Summary of 2D materials: properties and potential applications in water filtration.

Material	Water permeability	Mechanical strength	Antifouling properties	Selectivity	Key applications	Refs.
Graphene	Moderate	Very high	Moderate	High	Desalination, organic contaminants	[280, 292, 293]
Graphene oxide	High (up to 10 L m− 2·h−1·bar−1)	High	High	Tunable	Desalination, heavy metals, organic pollutants	[294, 295, 296]
MoS2	Moderate	High	High	Moderate	Heavy metals, catalytic degradation	[297, 298]
Phosphorene	Moderate	Moderate	Moderate	High	Heavy metals, dyes, nano‐filtration	[299]
Hexagonal BN	Low	High	High	Low	Oil–water separation, organic solvents	[300, 301, 302]
MXenes	High	High	High	High	Heavy metals, organic contaminants, and desalination	[303, 304, 305]

6. Scalable Membrane Fabrication

In the era of water filtration technology for large‐scale and industrial manufacture, the preparation method of 2D materials faces many challenges at this stage. As for the preparation method above, these methods often fail to meet the requirements of industrial preparation. Therefore, it is an essential step for developing large scale, reliable, inexpensive printing/coating processes. The methods such as roll‐to‐roll, inkjet printing, and laser processing have shown great feasibility in large scale fabrication methods for 2D membrane, which has great potential for water filtration in large scale (Figure 12 ).

Figure 12.

Large scale fabrication 2D material method. a) R2R method for preparing graphene membrane.^{[ 116 ]} b) Inkjet printing as a viable method for large‐area fabrication of graphene devices.^{[ 122 ]} c) Laser‐induced porous graphene.^{[ 130 ]}

Roll‐to‐roll (R2R) processing is widely used in industry and known to be cost‐effective and scalable. Roll‐to‐roll (R2R) manufacturing and techniques have been used for the growth and transfer of graphene, two important steps in graphene manufacturing (Figure 12a). By combining CVD and R2R process, large‐scale production of 2D material has proved feasible and has the potential to be an economic success in the near future.^{[ 306 ]} The first report on the large‐area synthesis of a uniform graphene sheet on a copper foil was from Li et al.^{[ 44 ]} in 2009, and it brought up the revolutionary change in realizing the potential application of graphene in electronics industries. This breakthrough innovation was soon utilized by companies such as Samsung^{[ 45 ]} and Sony^{[ 46 ]} where a graphene sheet of 30 in. and 100 m long was fabricated, respectively via roll‐to‐roll transfer technology. In 2015, S. Polsen presented the design of a reactor for roll‐to‐roll chemical vapor deposition (CVD) on flexible substrates for application to continuous production of graphene on copper foil. Meanwhile, the substrate translation speed varied from 25 to 500 mm min⁻¹, with a good uniformity and coverage of graphene.^{[ 307 ]} R. Esfahani discussed the scaled fabrication and characterization of reduced graphene oxide (rGO) nanofiltration membranes by slot die coating on a roll‐to‐roll (R2R) with integrated vacuum filtration. These R2R‐rGO membranes retained the same flux and stability properties as those fabricated previously using the vacuum filtration process.^{[ 308 ]} Lim et al. reported a facile methodology for the large‐scale production of layer‐controlled MoS₂ layers on an inexpensive substrate involving a simple coating of single source precursor with subsequent roll‐to‐roll‐based thermal decomposition developed.^{[ 309 ]} In the same way, WS₂ and the WS₂/graphene heterostructures could be fabricated on a large‐area.^{[ 310 ]}

Inkjet printing is a useful large‐area membrane manufacturing method, which has the advantages of fast processing steps and mass production (Figure 12b).^{[ 311} , ^{312 ]} For 2D material membrane, inkjet printing is a digital non‐contact printing technique vastly used in both research and industry, where ink droplets are jetted and deposited in fast succession onto a heated substrate to produce predesigned patterns without requiring a mask.^{[ 311} , ^{313 ]} This technology is a step forward toward the development of industrial‐scale, reliable, inexpensive printing/coating processes. It can provide an attractive route to the fabrication of complex heterostructures with high resolution, low cost, and large‐scale advantages. As a result, graphene and other 2D crystals are emerging as promising functional materials in ink formulations. For example, Li et al. introduced an efficient and mature inkjet printing technique for mass production of high‐quality graphene with high resolution. oration. In addition, the technology facilitated the fabrication of a variety of graphene, which only involved printing and annealing process.^{[ 313 ]} Torrisi et al. proposed the graphene demonstrated by inkjet printing with mobilities up to ≈95 cm² V⁻¹ s⁻¹, as well as transparent and conductive patterns, with ≈80% transmittance and ≈30 kΩ □⁻¹ sheet resistance.^{[ 314 ]} Juntunen et al. reported inkjet‐printed large‐area few‐layer graphene, and it was demonstrated that the inkjet‐printed graphene had a thermoelectric performance similar to that of state‐of‐the‐art graphene‐conductive polymer nanocomposites.^{[ 315 ]} Based on the high performance advantages of inkjet printing graphene, Fathizadeh et al. demonstrated a low‐cost, simple, rapid, and scalable method for the deposition of ultra‐thin (7.5–60 nm) homogeneous graphene oxide (GO) nanofiltration membranes on polymer supports for efficient water purification.^{[ 316 ]} In the same way, a thin film composite (TFC) NF membrane was modified by coating a binding agent polydopamine (PDA) and graphene oxide (GO) using a simple and scalable inkjet printing process showing great anti‐fouling properties and chlorine resistance.^{[ 317 ]} In 2020, Hu et al. presented an experimental study of the drying mechanism of a binary solvent ink formulation, which could naturally suppress the capillary flows that give rise to the coffee‐ring effect, enabling them for industrial‐level additive manufacturing.^{[ 318 ]} In addition to graphene, other 2‐D materials can also be printed at scale through using inkjet printing. In 2014, Withers et al. reported that graphene, WS₂, MoS₂, and BN are produced by drip casting, inkjet printing, and vacuum filtration achieving heterogeneous structure.^{[ 319 ]} In 2017, McManus et al. introduced a general approach to achieve inkjet‐printable, water‐based, 2D crystal formulations, which also provided optimal film formation for multi‐stack fabrication. In addition, a suitable heterostructure fabrication due to the re‐mixing of different 2D crystal led to a good membrane performance.^{[ 320 ]} Li et al. introduced a simple and efficient 2D layer material molybdenum disulfide (MoS2) ink‐jet printing technique, which benefited the massive and cost‐effective manner, while retaining the unique properties of MoS₂.^{[ 321 ]}

In recent years, laser synthesis of 2D materials has also developed into a potential large‐scale preparation process, such as laser‐induced graphene, which has been widely reported (Figure 12c). Conventional methods for preparing micro/nano graphene materials mainly include the assembly of graphene nanosheets or in situ growth by vapor‐phase deposition or sputtering deposition. These methods involve expensive raw materials or complex preparation processes that consume large amounts of energy and solvents, which limit the further applications of the graphene. James Tour et al. reported a facile, solvent‐free, scalable, and economical graphene preparation process, which was obtained by laser inscription on commercial polymers.^{[ 322 ]} In addition to the advantages mentioned above, laser induced graphene(LIG)has the potential to be prepared on a large scale and has already been used in a variety of scenarios for water purification.^{[ 323} , ³²⁴ , ^{325 ]} LIG can be applied into multiple water pollution and has a place in the field of graphene membrane synthesis due to its excellent expandability. In some research, the LIG was used to desalinate sea water, which was beneficial to the impressive sieving performance.^{[ 324} , ^{326 ]} Apart from membrane filtration for sea water treatment, LIG has also been explored for water pollutant adsorption and detection.^{[ 327} , ^{328 ]} Besides, LIG has antimicrobial and antifouling surface effects mainly because of its electrochemical properties and texture, and LIG‐based water filters have been used for the inactivation of bacteria.^{[ 329} , ^{330 ]} Except from polyimide, the porous graphene obtained by treating MOF material exhibits excellent solar‐driven desalination and shows great sea water filtration application potential.^{[ 331 ]} When considering large‐scale production, methods such as solution synthesis and roll‐to‐roll manufacturing offer scalability advantages over more complex methods such asCVD, which are limited by production speed and cost.

7. Fabrication of Nanopores in 2D Materials

For 2D materials, the micro–mesoporous surface has great influence on water filtration performance. At present, the methods for the fabrication of 2D porous materials are versatile, mainly chemical etching, physical field etching, and phase transition methods. Meanwhile, the porous 2D materials can also be obtained from self‐assembly reactions between atoms or molecules, mainly including template free methods and template‐assisted methods. Ultraviolet‐induced oxidative etching,^{[ 230 ]} chemical/plasma etching,^{[ 332} , ³³³ , ³³⁴ , ^{335 ]} and ion irradiation^{[ 336} , ³³⁷ , ^{338 ]} are the techniques being pursued to introduce nanopores in graphene and other 2D materials. Several research efforts to synthesize porous graphene through physical field etching method (using photo, electron beam, and oxygen plasma‐etching methods) have been reported.^{[ 339 ]} For example, Bai et al. used the poly(styrene‐block‐methyl methacrylate) (P(S‐b‐MMA)) block copolymer thin film as the etching template, followed by employing oxygen plasma etch to punch holes into the graphene layer.^{[ 340 ]} P. Koenig et al. introduced that ultraviolet‐induced oxidative etching can create pores in µm‐sized graphene membranes, and the resulting membranes can be used as molecular sieves.^{[ 341 ]} Mechanical fields can be used to introduce pores in the same way; for example, Li et al. synthesized 2D molybdenum disulfide by combining ball milling and ultrasonic process.^{[ 342 ]} Lin et al. reported that the laser induced porous graphene is affected by reducing gases, and plasmas during processing also have randomly distributed micropores and mesoporous pores.^{[ 322 ]} By chemically etching the framework of materials through chemical etchants, the porous 2D materials can be easily obtained. For example, multi‐layered RGO were refluxed in HNO₃ over several hours to control the pore‐size through a chemical etching process.^{[ 343 ]} In the same way, few‐layered GO sheets were refluxed with a hydrogen peroxide solution, and the broad pore size distribution between 2 and 70 nm.^{[ 344 ]} Ren et al. developed a chemical etching method to produce porous 2D Ti3C2Tx MXenes at room temperature in aqueous solutions. In addition, this membrane had larger specific surface areas and more open structures.^{[ 345 ]} Pore size drives and determines the efficiency and performance of a membrane. For an ideal membrane, a 7–15 nm pore (diameter) size^{[ 79} , ^{88 ]} is considered to be the best for filtration. It is believed that bio‐fouling bacteria^{[ 346 ]} and toxic hydrated ions present in the water^{[ 347 ]} can be eliminated by controlling the pore size between 7 and 15 nm.

8. E‐Beam Fabrication of 2D Materials‐Based Membrane

Only a few 2D materials derived from van der Waals layered bulk materials have been realized, confirming their 2D freestanding structure is challenging. Despite this, many metals have been predicted to exist as 2D systems. Ta et. al. reviewed recent progress in synthesizing and characterizing these 2D metals or metallenes and their oxide forms using transmission electron microscopy (TEM) and scanning TEM (STEM). (Figure 13 )^{[ 348 ]} Two primary approaches for forming freestanding monoatomic metallic membranes were identified: suspending the metallene or metallene oxide in graphene pores and electron‐beam sputtering for selective etching of metal alloys. The data show a growing number of confirmed 2D metals/metallenes and 2D metal/metallene oxides, indicating a bright future for further discoveries. 2D materials such as MoS₂, WS₂, and coated on copper substrate can thus be e‐beam irradiated, and chalcogen atoms can selectively be evaporated, leaving the traces of transition metals (Mo/W etc). Inter‐atomic distances in these metallenes are ≈3.5 Å, which is way larger than the diameter of water molecules. Thus, this technique can be implemented for efficient water filtration, filtering out all undesirable ions.

Figure 13.

E‐beam thinning of 2D materials. E‐beam‐driven‐formation of 2D Au membrane through thinning of Au–Ag alloy. a) Schematic of fabrication of monolayer Au membrane by in situ dealloying of Au–Ag alloy. Au and Ag atoms are shown as yellow and blue spheres, respectively. b) HRSTEM image of monolayer Au membrane framed in bulk Au–Ag along ⟨001⟩ zone axis. c,d) Depiction of structural changes in Mo nanoribbons upon electron beam irradiation until the formation of a single atom Mo chain. e‐beam fabrication of monolayer Mo membranes. e) Schematic illustration of fabrication of monolayer Mo membrane from freestanding monolayer MoSe₂ film. f) STEM‐ADF image of Mo membrane embedded in monolayer MoSe₂ film. Mo membrane regions are highlighted in false green.^{[ 348 ]}

9. Design of Ångström‐Scale Capillaries

Ångström‐scale capillaries, created using 2D materials, represent a cutting‐edge technique in water filtration technology. These capillaries, with dimensions comparable to typical ions, leverage the unique properties of 2D materials to achieve high selectivity and efficiency in separating ions and molecules. Unlike traditional nanoporous membranes that rely primarily on the size and charge of hydrated ions, ångström‐scale capillaries offer a new mechanism for ion selectivity. Recent studies have demonstrated that ångström‐scale channels, assembled via van der Waals forces, can distinguish between ions with the same charge and similar hydrated diameters. This selectivity is attributed to the positions that ions occupy within the layered structure of nanoconfined water, which depend on the ion‐core size. Such a mechanism allows for selective ion separation beyond simple steric sieving, providing a pathway to more sophisticated filtration systems (Figure 14 ).^{[ 349 ]}

Figure 14.

a) Cartoons of ions with similar hydrated diameters (DH) but different ionic diameters (DI) relative to the 6.8 Å channel height. Right: schematic of Å‐channel device and setup. Inset: cross‐sectional view of the three‐layer Å channel. b) I–V characteristics for various salts (color coded) measured for a hBN‐channel device (w  ≈  120 nm, h  ≈  6.8 Å, N  ≈  400, L  ≈  6 µm, and C  =  0.1 m). Inset: resistance per channel versus L for six devices. c) I–V curves for 0.01 and 0.10 m salt solutions. Top inset: schematic of drift–diffusion experiment. Bottom inset: E _m measured for the salts.^{[ 349 ]}

In addition, these capillaries have shown the capability of complete steric exclusion of even the smallest hydrated ions such as Na⁺ and Cl⁻, while allowing water to permeate with little resistance. This remarkable selectivity is akin to biological channels such as aquaporins and is achieved by fabricating capillaries through the removal of a single atomic plane from bulk crystals.^{[ 350 ]} Notably, these channels permit the transport of protons (H⁺), highlighting their potential in applications requiring selective ion transport such as proton‐conducting membranes. Another intriguing aspect of water confined in ångström‐scale capillaries is its anomalously low dielectric constant. Experimental evidence indicates that water molecules in these confined spaces exhibit significantly restricted rotational freedom, resulting in a dielectric constant as low as ≈2 compared to ≈80 in bulk water. This electrically “dead” layer of water, a few molecules thick, provides crucial insights into the behavior of interfacial water and has implications for theories on water‐mediated surface interactions and the design of other fluidic systems under extreme confinement.^{[ 351 ]} Moreover, studies on ion transport through these ultra‐narrow slits reveal that ions with hydrated diameters larger than the slit size can still pass through, albeit with reduced mobility. This phenomenon underscores the role of hydration shell deformation and ion‐surface interactions in confined spaces. Interestingly, an asymmetry has been observed between anions and cations of the same diameter, which is essential for the development of advanced nanofluidic devices and precise molecular separation technologies.^{[ 352 ]}

In summary, the development of ångström‐scale capillaries using 2D materials has significantly advanced the field of water filtration by providing a platform for highly selective and efficient ion separation. These findings pave the way for innovative applications in various domains, including energy storage, nanofluidics, and beyond. As research progresses, the understanding of transport mechanisms at the atomic scale will further enhance the design and functionality of these advanced filtration systems.

10. Sunlight‐Powered 2D Materials‐Based Water Purifier

Natural vascular plants utilize osmotic pressure, transpiration, and guttation to produce clean water using sunlight. Inspired by this natural process, a sunlight‐driven purifier for efficient water purification is reported. This device features a negative temperature response poly(N‐isopropylacrylamide) hydrogel (PN) anchored onto a superhydrophilic melamine foam skeleton, with a PNIPAm‐modified graphene (PG) filter membrane. Molecular dynamics simulations and experiments demonstrate that the superhydrophilic melamine skeleton accelerates the swelling and deswelling rate of the PNPG‐F purifier. Under one sun, Geng et. al. demonstrated that this structure collected 4.2 kg m⁻² h⁻¹ and achieved >99% ionic rejection from brine, offering great potential for diverse water treatments (Figure 15 ).^{[ 354 ]}

Figure 15.

Schematic of water treatment of the sunlight‐powered purifier (PNPG‐F). a) Microstructure of the sunlight‐driven purifier. The PNPG‐F purifier consists of PNIPAm chains growing on the surface of melamine skeleton and PG membrane coated outside. Melamine provides a water suction channel via capillary force and inhibits bulk volume collapse of the purifier. b) Water purification procedure based on the PNPG‐F purifier. The coated PG demonstrates solar–thermal conversion and pollute rejection. After being immersed in pollute water, PNPG‐F adsorbs a large amount of clean water. Under sunlight irradiation, the PG membrane converts sunlight into thermal energy and heats PN to the temperature above LCST, facilitating hydrophilization; thus, clean water can be produced via transpiration and guttation.^{[ 353 ]}

11. Conclusion 

The progress in the development of 2D nanoporous membranes, particularly those based on graphene and its derivatives, has shown tremendous potential for water filtration applications. These membranes leverage the exceptional properties of 2D materials, including mechanical strength, chemical robustness (absence of dangling bonds), low surface energy, and antifouling characteristics, making them ideal candidates for filtering contaminants from water. Frontline materials, such as boron nitride‐graphene heterostructures, various TMDCs, phosphorene, borophene, and MXenes, have demonstrated promise due to their unique structural and surface properties. Despite these promising properties, challenges in scaling up production, achieving precise pore size control, and maintaining membrane longevity still hinder their widespread industrial use. Graphene, in particular, has garnered significant attention, but its monolayer hexagonal structure is not naturally effective at blocking all pathogens or soluble ions due to the interatomic distances between carbon atoms. To address this, introducing defects in the graphene structure has been explored as a means to enhance filtration capabilities without sacrificing water flow efficiency. Heterostructured materials have been particularly promising in increasing the strength and durability of membranes, overcoming some of the limitations of monolayer graphene membranes. In addition, inducing strain in graphene to adjust interatomic distances is an area of ongoing theoretical research, though practical applications remain limited.

Key challenges (Figure 16 ) include managing defects during membrane synthesis, maintaining membrane stability during fluid flow, and ensuring long‐term performance under real‐world filtration conditions. In addition, addressing the toxicity and environmental impact of these materials remains an important consideration. Efforts to improve packaging, reduce synthesis costs, and extend the operational lifespan of these membranes continue to be areas of active research.

Figure 16.

Challenges faced by 2D material nanoporous membrane during water filtration.

Notably, sunlight‐powered 2D materials‐based water purifiers have emerged as a sustainable solution, harnessing solar energy for high‐efficiency water purification. These systems are especially suited for off‐grid or remote applications where traditional energy sources may not be available. This innovative approach reduces energy consumption and environmental impact, making it a promising alternative for clean energy‐driven filtration technologies.

In summary, while considerable progress has been made in the field of 2D nanoporous membranes, several technical, economic, and environmental hurdles remain before they can be widely adopted for industrial applications. The field is still in its early stages, with ongoing research necessary to refine fabrication techniques, improve membrane properties, and enable large‐scale production. Continued efforts in defect engineering, scalability, and material optimization are critical to realizing the full potential of these advanced membranes for global water purification challenges.

12. Perspective

Looking ahead, the future of 2D materials in water filtration is promising, with several cutting‐edge techniques and innovations expected to drive the field forward. Among these, the integration of lasers for precise material manipulation and the use of nanoparticles during chemical vapor deposition (CVD) growth are two areas of high potential. Nanoparticles such as aluminum oxide, silicon, and titanium oxide can inhibit carbon growth on copper substrates, allowing for selective etching. This technique is still being developed, but it shows great promise for enabling the precise assembly of nanopores in 2D materials, potentially overcoming some of the limitations of current approaches such as ion bombardment, plasma etching, and chemical etching.

Another key advancement lies in the design of ångström‐scale capillaries, which represent a significant leap forward in filtration technology. These capillaries, with their precisely controlled dimensions, are capable of filtering out contaminants at the molecular level while maintaining efficient water flow. This level of precision, combined with the antifouling properties and chemical robustness of 2D materials, makes them highly attractive for next‐generation filtration systems.Sunlight‐powered 2D materials‐based water purifiers also present a highly innovative and sustainable solution. By harnessing solar energy, these systems can purify water without the need for external power sources, making them particularly useful in remote or off‐grid areas. This approach not only reduces energy consumption but also contributes to environmental sustainability by providing a clean energy source for filtration processes. The application of such systems is still in the early stages but holds immense potential for future deployment in various settings.

While significant progress has been made, several technical challenges remain to be addressed. These include controlling pore size and distribution, managing defects during membrane fabrication, improving support membranes, and optimizing real‐time filtration performance. The scalability of these materials and the development of cost‐effective production methods are critical for their widespread adoption. Further, addressing the cytotoxicity and environmental impact of these materials will be important to ensure that they are safe for large‐scale use. Research into the synthesis of new multifunctional 2D/0D and 2D/1D hybrid nanocomposites is another promising avenue. These nanocomposites, which incorporate various 2D materials such as borophene, phosphorene, and their heterostructures, have the potential to outperform conventional membranes in terms of filtration efficiency, durability, and scalability. Their development could lead to novel thin‐film nanocomposite membranes that significantly enhance performance while reducing synthesis costs.

Ultimately, the future of water filtration technology will likely be shaped by these emerging techniques and materials. Ångström‐scale capillaries, laser‐based nanopore assembly, and sunlight‐powered purification systems offer exciting opportunities for advancing membrane performance and addressing global water contamination challenges. Continued research and development will be essential to optimize these technologies and realize their full potential in providing sustainable and effective water purification solutions. These advancements could revolutionize the field, offering cleaner, more efficient, and environmentally friendly filtration options for both industrial and commercial applications.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

P.R. and Z.L. contributed equally to this work. P.R. and Z.L. primarily wrote the manuscript. The outline was finalized in consultation with E.H.S.S. and P.K. A.A., S.A., and M.A.S. contributed to collecting data for making analytic and informative graphs, as well as figures and have equal second author contributions. Consultations with S.P. were made during manuscript preparation. P.R., A.V., and P.K. revised it. All authors have given approval for the final version of the manuscript.

Biographies

Pranay Ranjan is presently an assistant professor at the Indian Institute of Technology Jodhpur. He got a Ph.D. (Physics) from the Indian Institute of Technology Patna in Feb 2019. He then carried out his post‐doctoral research at the College of Science, UAE University where he worked until he got an offer from IIT Jodhpur. His research thrust includes exploring novel large‐scale synthetic approaches for various 2D materials (including graphene, graphene oxide, borophene, borophene oxide, alpha lead oxide, white lead, and their hybrids) and their frontline applications in molecular as well as gas sensing, wettability manipulation, electronics/optoelectronics/spintronic devices, and environmental remediation.

Zhixuan Li is a Ph.D. candidate at the Global Innovative Centre for Advanced Nanomaterials, the University of Newcastle. She received her Bachelor's degree in Chemistry from the University of Science and Technology of China, and Master's degree in Chemical Engineering from the Monash University. Currently, she is working on the development of 2D materials for energy applications.

Shashikant P. Patole is an assistant professor (Physics) at the Khalifa University of Science and Technology (KUST). He earned his Ph.D. from the Sungkyunkwan University (SKKU), Suwon, South Korea in 2010. He then carried out postdoctoral research at the King Abdulla University of Science and Technology (KAUST). His research interests embrace condensed matter physics, nano science and technology, quantum materials, development and commercialization of advanced materials, structural composites, membranes, energy, optoelectronics, electron field emission, photovoltaics, electron microscopy, and artificial intelligence.

Gary J. Cheng works at the School of Industrial Engineering and Birck Nanotechnology Centre at Purdue University, USA. He received his Ph.D. from the Columbia University. He has received the Young Investigator Award from the Office of Naval Research, the CAREER award from the National Science Foundation, and the Outstanding YOUNG Manufacturing Engineer award from the Society of Manufacturing Engineers. His research includes laser materials processing, hybrid manufacturing systems, rapid manufacturing of 3D bulk structural nanomaterials, multilayer functional coating, and mechanical/physical property enhancement of metals.

El Hadi S. Sadki obtained his Ph.D. degree in Physics from the Cavendish Laboratory at Cambridge University in 2002. He worked as a postdoctoral researcher at both the National Institute for Materials Science (NIMS) in Tsukuba, Japan, and the Physics Department at Harvard University. In 2008, he joined the Physics Department at UAE University as an Assistant Professor and got promoted to an Associate Professor in 2015. Dr. Sadki's current research interests are in the synthesis, characterization, and device applications of nanomaterials, including carbon nanotubes, graphene, and superconducting nanowires.

Ajayan Vinu is the Director of GICAN at the University of Newcastle. He was previously working as a full professor and ARC Future Fellow at the University of South Australia and the University of Queensland. Before coming to Australia, he had been working as a research group leader at the National Institute for Materials Science in Japan. His research is mainly focused on developing new approaches to create nanoporosity in carbon nitrides, conducting polymers, metal nitrides, metal silicates, graphenes, silicas, sulfides, fullerenes, and biomolecules with tunable structures and pore diameters and their potential applications in energy, environmental, biomedical and catalysis technology.

Prashant Kumar works at University of Newcastle (Australia) from Aug 2021. Awarded Ph.D. (Physics, 2009), he had postdoctoral stints with world‐renowned scientist Prof. C.N.R. Rao at JNCASR Bangalore (2009–2012) and Prof. Gary Cheng at Purdue University (USA). Being awarded the prestigious “The Ramanujan Fellowship” in 2015, he returned to India and joined IIT Patna, where he worked for 6 years. He has grossly been involved in crystal growth of 2D materials under extreme thermodynamic conditions, exotic quantum states in undoped/doped/hybrid atomic sheets and their applications in electronics/spintronics/excitonics/straintronics, ultrafast sensing, energy generation/storage, catalysis, water filtration, bacterial killing, and brain imaging.

Ranjan P., Li Z., Ansari A., Ahmed S., Siddiqui M. A., Zhang S., Patole S. P., Cheng G. J., Sadki E. H. S., Vinu A., Kumar P., 2D Materials for Potable Water Application: Basic Nanoarchitectonics and Recent Progresses. Small 2024, 20, 2407160. 10.1002/smll.202407160