Potential of nanoporous graphene and functionalized nanoporous graphene derived nanocomposites for environmental membranes – a review

Abstract

Nanoporous graphene own high surface area and unique structural and physical properties due to nano-sized pores in graphene nanosheet. Nanoporous graphene has been modified by the introduction of surface functional groups, doping, or nanoparticle modification and applied for nanocomposites. The overview basically highlights design, properties, and potential of functionalized nanoporous graphene derived nanocomposites in advanced membranes (desalination/gas/oil separation), thermal management, electrocatalysts, and micro-supercapacitors. According to literature, pore size of neat nanoporous graphene is 3–11 Å and hydroxyl functional nanoporous graphene also had nanopore size of <10 Å. Functionalized nanoporous graphene revealed 100% salt rejection, while hydroxyl functional nanoporous graphene had 89% salt rejection due to altered surface properties. Both neat and functionalized nanoporous graphene has high gas permeance of ∼10³–10⁵ GPU towards separation CO₂, CH₄, N₂, etc. Few attempts on micro-supercapacitor like molybdenum disulfide nanoporous graphene resulted in volumetric capacitance of 55 F cm⁻³ and capacitance retention of 82%. Forthcoming research on functionalized nanoporous graphene may overcome design/performance challenges leading to superior features and large-scale utilizations.

Keywords:

• Desalination
• functionalized
• membranes
• micro-supercapacitors
• nanocomposites
• nanoporous graphene
• oil recovery
• thermal management

1. Introduction

Graphene is a unique form of carbon at nanoscale having sp² hybridized carbon nanostructure [1]. Enormous structural and physical characteristics of graphene have been investigated and found useful for range of technical applications [2,3]. An essential utilization of graphene has been observed in the field of nanocomposite nanomaterials [4,5]. Nano-sized pores in graphene nanosheet have been observed to enhance the surface area, semiconductivity, and permeability features [6]. Unlike graphene, nanoporous graphene has band or energy gap of about ∼1 eV [7]. The porous graphene is widely referred as nanoporous graphene in literature [8]. Formation of pores in graphene nanosheet can be attained using sophisticated top down and bottom up strategies [9]. Studies on nanopore sizes, distance among nanopores, pore density, and nature of pores have been investigated in literature [10]. Nanoporous graphene oxide [11–13], reduced graphene oxide [14], and functional graphene oxide membranes [15] have been reported. These nanocomposite membranes have been efficiently applied for ionic or molecular sieving [16], water permeability [17], lead and arsenic separations [18], removal of heavy metals [19,20], dye removal [21,22], organic separations [23,24], and desalination [25–27]. Consequently, the nanoporous graphene has been explored as multipurpose semiconducting nanomaterial for the fabrication of sensors and molecular species [28]. Research has led to the formation of functionalized nanoporous graphene, which further enhanced the structural and physical properties of porous nanosheets [29]. Carefully engineered nanoporous graphene may lead to high performance water purification and gas separation applications [30,31]. Graphene or graphene oxide derived permeation membranes and systems have already been reported for the separation of water and gaseous species [2]. Gas or water molecules can only cross the graphene nanosheets having defects in the nanostructure because pristine graphene nanosheet is impervious to gaseous or aqueous molecular transportations [32–34]. Here, the interlayer gaps among graphene nanosheets and mechanisms of molecular transportation have been investigated to explore the potential of these nanomaterials [35,36]. In addition, nanoporous graphene based membranes have been used in important environmental applications such as desalination and water treatment purposes [37–39]. Nanoporous graphene based ion selective membranes have been used for the separation of toxic ions such as lead. Thus, using nanoporous graphene and functional forms of nanoporous graphene has enormously enhanced the essential properties and technical opportunities of the resultant nanocomposites [40]. Ongoing research towards the formation of high performance nanoporous graphene derived nanocomposites may lead to future advancements covering the design complications, large-scale fabrication, and commercialization [41].

Durability of nanoporous graphene based nanomaterials has been studied in terms of radiation exposure, heat, mechanical forces, and other environmental factors. According to molecular dynamics simulations, damage in nanoporous graphene was tested through irradiation with 18–23 MeV copper ion beam [42]. Elastic collisions have been observed between the carbon atoms of nanoporous graphene and ions. Exposure of the nanomaterial with heavy ion beams of high energies did not cause any damage or ion tracks. It has been observed that almost 40% of radiations energy is safely dissipated without causing any damage to the material. Mechanical studies on the nanoporous graphene based materials have been performed using the molecular dynamics simulations [43]. Consequently, the effect of nanopore sizes and spacing on mechanical features of nanoporous graphene material has been analyzed. Impact of axial tensile loading on nanoporous graphene was studied. The presence of nanopores weakened the nanomaterial structure upon impact exposure. Increase in the nanopore size or decrease in the spacing between the nanopores decreased the mechanical features of the nanomaterials. Consequently, lessening in Young’s modulus and ultimate strength of nanoporous graphene was perceived with the increase in nanopores in the nanostructure. In addition, the durability features depend upon the structural features and interactions in the matrix-nanofiller.

There are several advantages and significance of using nanoporous graphene and derived nanomaterials. Nanoporous graphene has various superior features, relative to the traditional graphene or porous materials. Nanoporous graphene has been recognized as a striking material for characterizing, sensing, or localizing molecules like proteins [44]. Compared with the solid-state nanoporous nanomaterials, nanoporous graphene based materials own atom thick layer formation, fine durability, and electrical conduction and sensitivity [45]. Nanoporous graphene own remarkably high surface area and electron mobilization while maintaining optimum heat conduction and strength characters. Precisely, defined sized nanoporous graphene has been applied for molecular sieving. This feature has been found advantageous for the passage of gas or liquid molecules. The nanopores actually act as channels to promote ultrafast molecular permeation for the filtration of molecules. On the other hand, pristine graphene is non-permeable.

For superior performance of nanoporous graphene based membranes, even sized nanopores, high porosity, and low thickness have been found desirable to attain mechanical robustness and superior permeability and flux properties [46]. Only finely controlled pore architectures may separate the molecules precisely. The existing nanoporous graphene based membranes have still not been processed for the large-scale manufacturing [47]. Here, the challenges of controlled and homogeneously formed nanopore sizes, measured thickness, and commercial level utilizations need to be overcome for progress in this field. Subsequently, nanofabrication technique can be promising to form well-ordered monodisperse nanopores, large-scale system issues, and performance challenges.

This state-of-the-art overview highlights the design, synthesis, features, as well as technical strength of appropriately functionalized nanoporous graphene and derived nanocomposite nanomaterials. Functionalization has enormously improved the unique features and utilizations of nanoporous graphene especially the fields of separation membranes, electronics, electrocatalysts, etc. To the best of the knowledge, literature research and reviews have been reported on the nanoporous graphene nanomaterials. Novelty of the manuscript depends upon the systematic breakdown of the topics into nanoporous graphene, functional nanoporous graphene, nanoporous graphene filled nanocomposites, and then functional nanoporous graphene nanocomposite membranes leading to identification of several advances and challenges in the field. Important areas identified for membranes, electronics, and heat management have been discussed. In terms of nanoporous graphene nanocomposites, such precise review has not been seen before; although research reports and technical analysis on specific systems have been reported. In the case of functionalized nanoporous graphene, important increasing literature reports (2016–2023) have been observed (Figure 1). However, no comprehensive reviews have been seen on functionalized nanoporous graphene in literature, so far. Consequently, this review article is certainly a revolutionary contribution in the field of nanoporous graphene towards progressive industrial applications. Motivation behind this comprehensive attempt is to offer a novel article on nanoporous graphene derived nanocomposites while throwing appropriate light on previous, current, and predictable future progresses for the field professionals. In our view, future progress in this arena is not possible for interested researchers before getting prior knowledge of useful and very recent literature on these unique nanomaterials. Thus, following this review can lead to considerable progressions in high performance nanoporous graphene materials for future engineering applications. Hence, future progressions in the field of functionalized nanoporous graphene have not been possible without acquiring prior knowledge of the compiled literature in this field. Forthcoming efforts in the field of functional nanoporous graphene nanocomposites may unfold several hidden application areas of these nanomaterials.

Figure 1. Publication trend in graphene from 2000 to 2023.

2. Nanoporous graphene and functionalized nanoporous graphene

Graphene is a technically efficient nanomaterial for various applications [48]. Graphene was initially discovered in 2004 and opened several technical doors to the fields of materials sciences [49]. After that, numerous top down and bottom up methods have been applied to form this unique nanocarbon nanostructure [50]. Graphene is a one atom thick two-dimensional nanosheet of sp² hybridized carbon. It has zero bandgap and semiconducting features [51–53]. Graphene had electron mobility of 200,000 cm² V⁻¹ s⁻¹, at room temperature. Graphene also has charge carrier density of about 10¹³ cm⁻². In addition to electron conduction, graphene has heat conductivity, strength, heat stability, optical transmittance, etc. [54,55]. Significant uses of graphene have been observed for wide ranging fields such as electronics, sensors, catalysis, energy devices, nanocomposite systems, etc. [56,57].

Nanoporous graphene is a unique graphene derived nano form [58]. It is simply a graphene nanosheet having nano-sized pores in structure. It has compact consistent network of graphene nanoribbons or connected graphene nanostructures [59]. Nanoporous graphene has large surface area than pristine graphene [60]. Unlike graphene, nanoporous graphene possess optimum band gap to form better semiconducting materials [61]. Important structural parameters of nanoporous graphene are neck width, center-to-center distance among adjacent nanopores, and periodicities [62]. Numerous effective methodologies have been industrialized for the formation of nanoporous graphene nano-architectures [63]. For example, the electron beam technique [64], surface-mediated molecular precursor method [65], block copolymer/nanosphere lithography [66,67], chemical vapor deposition [68], chemical etching [69], and other physical and chemical methods can be stated. Chevron-type nanoporous graphene has been used for the functional nanoporous graphene [70]. Jacobse et al. [71] developed the functionalized Chevron-type nanoporous graphene. The functionalized chevron-type nanoporous graphene was formed using the surface-mediated molecular precursors method. Two molecular precursors were used to form the functionalized nanoporous graphene (Figure 2). The functional surface had methyl and methylene groups in the precursors. The precursors were deposited on the gold substrate at 180 °C and annealed to 400 °C. The nanopore cross sectional area of 20–40 nm was observed. The bond-resolved scanning tunneling microscopy revealed the formation of 4 nm area having three interconnected ribbons. Functional chevron-type nanoporous graphene was formed through the covalent linking. According to high-resolution bond-resolved scanning tunneling microscographs, a rubicene-type interface was observed (Figure 3).

Figure 2. Schematic of the bottom up formation of chevron-type nanoporous graphene (CNPG) from molecular precursors with methyl 1 and methylene 2 groups [71]. Reproduced with permission from ACS.

Figure 3. Synthesis of chevron-type nanoporous graphene (CNPG): (A) Scanning tunneling microscopy (STM) image of 0.25 monolayer coverage of CNPG on Au(1 1 1) after deposition of methylene functional configuration 2 on Au(1 1 1) held at T₁ = 180 °C and subsequent annealing to T₂ = 400 °C; (B) STM topographic image of 0.75 monolayer coverage of CNPG on Au(1 1 1) after deposition of methyl functional configuration 1 on Au(1 1 1) held at 24 °C and subsequent annealing to T₂ = 400 °C; (C) STM topographic image of 0.75 monolayer coverage of CNPG on Au(1 1 1) after deposition of 2 onto Au(1 1 1) held at T₁ = 180 °C and subsequent annealing to T₂ = 400 °C; (D) Bond-resolved STM image of a region containing three fused ribbons; (E) High-resolution bond-resolved STM image of the region indicated in (D); and (F) Schematic representation of rubicene-type interface imaged in (D, E) [71]. Reproduced with permission from ACS.

Block copolymer lithography has been applied as a successful method to form nanoporous graphene and functionalized nanoporous graphene on large scale for important device applications [72–74]. Kim et al. [72] used the block copolymer lithography technique for the formation of semiconducting nanoporous graphene. Here, the spin coating technique was applied to form the polystyrene-block-poly(methyl methacrylate) and large area functional nanoporous graphene based thin film. Figure 4 shows the process of surface modification with the large area graphene film. Initially, substrate was irradiated with UV irradiation and nitrogen for cleaning. Afterwards, the thin film of graphene oxide was formed using the spin coating method. Here, graphene oxide was obtained using Hummer’s method [75]. Spin coating deposited consistent and wrinkle free graphene oxide film on the substrate [76]. Then, a 80 nm thin film was deposited on the substrate and thermally annealed at high temperature of 200–280 °C to develop the semiconducting nanoporous graphene film having few hydroxyl and oxygen functionalities. Bai et al. [77] prepared the nanoporous graphene using block copolymer lithographic technique. The resulting nanoporous graphene own neck width of about 5 nm in addition to varying periodicities. As compared to the pristine graphene, the nanoporous graphene was suggested to enhance the performance of field-effect transistors by more than 100 time [78]. Further advancements in the neck width may lead to advanced applications in semiconducting devices and circuits [79].

Figure 4. Schematic representation of graphene film surface modification: (a) Spin coating of graphene oxide thin film on various substrates assisted by N₂ gas blowing; (b) Thermal or chemical reduction; and (c) Spin-casting a PS-b-PMMA thin film and thermal annealing for self-assembly [72]. PS-b-PMMA = polystyrene-block-poly(methyl methacrylate). Reproduced with permission from ACS.

Like block copolymer lithography, the nanosphere lithography has also been used for the fabrication of nanoporous graphene [80]. In the block copolymer lithography, nanoporous graphene with 100 nm periodicities has been attained. In the nanosphere lithography, the colloidal microspheres are used as a substitute of block copolymers [81]. This approach promotes two times more periodicities than the block copolymer lithographic method [82]. The nanoporous graphene periodicities were found dependent on the colloidal microsphere sizes, whereas the neck widths rely on the etching time. Liang et al. [83] applied the nanoimprint lithography by using block copolymer self-assembling for the formation of functional or pristine nanoporous graphene. Here, the template was used with hexagonal pillars to imprint the polystyrene resist. Oxygen based reactive ion etching was used for the etching of nanoporous graphene and resist residual layer thickness [84]. By removing the polystyrene resist, the nanoporous graphene nanosheet was obtained [85,86].

The doped nanoporous graphene has been considered as a functional nanoporous graphene form. Daryabari et al. [87] prepared various type of functionalized nanoporous graphene such as hydrogen doped nanoporous graphene, boron doped nanoporous graphene, nitrogen doped nanoporous graphene, and fluorine doped nanoporous graphene. The functional doped nanoporous graphene nanostructures were used as the selective membranes for helium gas diffusion from mixture with natural gas [79]. Figure 5 shows the formation of doped nanoporous graphene nanostructure. In the simple nanoporous graphene, central carbon ring was removed at 2.76 eV to form the hydrogen doped nanostructure. The three edge atoms were replaced by boron at 2.69 eV to attain boron doped nanoporous graphene. Similarly, three edge atoms were replaced by nitrogen atoms at 2.67 eV. Floro groups were also introduced at edge atoms at 3.25 eV. Using these modified nanoporous graphene nanostructures, helium selectivity from natural gas was observed in the range of 300–1000 K. The functionalized nanoporous graphene has been suggested for the separation of helium from natural gas in oil wells. Further progressions in the fabrication techniques of nanoporous graphene may lead to applications in supercapacitors [88], transistors or field-effect transistors [89,90], sensors [91], nucleic acid sensing [92], molecular sieves [93], as well as water decontamination [94].

Figure 5. The fully relaxed defected structure: (a) HG; (b) BG; (c) NG; and (d) FG membrane used to simulate permeation of passing natural gas molecules [87]. HG = hydrogen doped nanoporous graphene; BG = boron doped nanoporous graphene; NG = nitrogen doped nanoporous graphene; FG = fluorine doped nanoporous graphene. Reproduced with permission from Elsevier. Reduced with permission from ACS.

3. Nanoporous graphene filled nanocomposites

Graphene is a unique and widely used nanocarbon nanostructure owing to low cost, facile processing, and superior physical features [95–97]. Graphene derived nanocomposites have been studied for the electron conducting, strength, heat stability, and flame resistance properties [98–100]. The superior characteristics have been achieved due to synergic effects of the matrix and nanofiller [101,102]. Inclusion of modified graphene has also been considered in addition to graphene for the enhancement of the nanomaterial properties [103–105]. Nanoporous graphene and three dimensional nanoporous graphene have been observed as essential nanocarbon forms [106–108]. Consequently, the advanced application areas of graphene and derived nanomaterials have been focused [108–110]. Nanoporous graphene has very high surface area, low weight, conducting, super elastic, and compressibility properties [111]. However, the presence of interconnecting nanosheet nanostructure affected the mechanical features of nanoporous graphene [112,113]. Polymers usually have very low mechanical strength properties, therefore amalgamation with nanoporous graphene has been used to enhance the mechanical strength and Young’s modulus for the technical applications [114,115]. Nevertheless, weakly interacted nanoporous graphene may lead to low mechanical performance. Here, nanocompositing techniques have been used to enhance the dispersion and features of nanoporous graphene [116]. Naderi et al. [117] developed the epoxy and nanoporous graphene derived nanocomposites. The microstructure, heat stability, mechanical robustness, and corrosion resistance characteristics of the epoxy/nanoporous graphene nanocomposites have been studied. Inclusion of up to 1 wt.% nanoporous graphene contents has enhanced the thermal, mechanical, and corrosion resistance properties of the nanocomposites [118]. Kashani et al. [119] formed the poly(dimethyl sulfoxide) and nanoporous graphene derived nanocomposite membranes. Three dimensional nanoporous graphene was prepared form benzene (carbon source) and nickel template based chemical vapor deposition method [120]. The microstructures of the three-dimensional nanoporous graphene and nanocomposites have been studied [121]. The nanocomposites have low weight and high flexibility and strength features. Due to three-dimensional nanostructure, the nanocomposites revealed high electrical conductivity and electromagnetic shielding properties [122]. The electromagnetic interference shielding effectiveness was observed in the range of 51–83 dB. Zheng et al. [123] designed the three-dimensional poly(vinylidene fluoride) and nanoporous graphene derived nanomaterials. Inclusion of nanoporous graphene to poly(vinylidene fluoride) matrix has found to improve the mechanical and electro-mechanical properties. In addition, nanofiller has been found to induce the formation of crystalline β phase in the matrix [124]. The improved physical features were observed due to the interaction between the –CF₂ functionalities of poly(vinylidene fluoride) and the –C = O functional groups of nanoporous graphene leading to the compatible matrix–nanofiller interface formation [125].

For water remediation, the nanoporous membrane development has been considered for desired ion removal [126]. Experimental as well as molecular dynamics studies have been reported for the nanoporous graphene derived membranes [127]. Suk et al. [128] studied the nanoporous graphene with pore size of 1 nm allowing the water permeation. Sint et al. [129] developed the nanoporous graphene and used as ionic sieves for the selective separation of ions. The theoretical studies on the nanoporous graphene for salt rejection have also been observed in literature [130]. According to these studies, nanoporous graphene based membranes reveal salt rejection at flow rate of 10–100 L/cm²/day/MPa. Cohen-Tanugi et al. [131] developed the nanoporous graphene derived thin film membranes. The nanomaterial was used for the reverse osmosis membranes through the water desalination process [132]. Figure 6 displays the nanoporous graphene derived thin film membranes. The average pore size, pore structure, as well as nanochannels in the thin film membrane were investigated [132]. The pressure and mechanical loading were studied on the nanoporous membrane during the reverse osmosis process. Applied pressure of 5 MPa was used on the membrane. Through the membrane, nanopore size varied from 1.7 to 2 nm, whereas the average pore size of about 10 nm was attained. The uniformity of nanoporous graphene, nanopore size, and nanochannel formation facilitated the membrane process [133]. The nanoporous graphene nanostructure maintained the durability and structural robustness during reverse osmosis process [134].

Figure 6. (a) Schematic representation of the arrangement of a nanoporous graphene (NPG) membrane. The NPG sheet is supported by a substrate with average pore radius, that is, R; (b) Mechanical loading on a patch of NPG due to applied pressure in a reverse osmosis (RO) system. The NPG layer is approximately uniform and isotropic at the length scale of the substrate; and (c) Atomic-scale visualization of NPG with nanopore radius, that is, a. [131]. Reproduced with permission from ACS.

High performance polymer/nanoporous membranes have been fabricated and studied [135,136]. Jang et al. [137] performed the theoretical multi-scale modeling of nanoporous graphene for diffusion of solute particles and osmotic water flux. Nanoporous graphene was found efficient as nanofiltration membranes leading to high water and monovalent ions permeability. On the other hand, the nanofiltration membranes were capable of rejecting small molecules and salts. Table 1 portrays the pore density of neat nanoporous graphene and polycarbonate track etch/nanoporous graphene nanocomposite membranes. The pore density of nanocomposite membranes was reduced relative to the pristine nanoporous graphene [138]. The transport properties were studied through the pore size, distribution, and density well-defined the performance of nanocomposite membranes [139]. Cohen-Tanugi et al. [140] performed the simulation studies on the single layer freestanding functionalized nanoporous graphene and polysulfone derived membranes for the water desalination. The free standing functional nanoporous membranes were found effective for separating NaCl from water. The nanopores size of around 40–160 Å was formed in the graphene membrane. These membranes were capable of bearing 500 MPa upward pressure. Figure 7 shows the nanopore nanostructures of the hydrogenated and hydroxylated nanopores according to oxygen density maps. The hydrogenated nanopore had size of 23 Å and hydroxylated nanopore was of size 28 Å. The hydroxylated nanopores have large cross sectional area for the passage of water molecules by 25%. The water permeability was decreased from 69% to 113%. The hydroxyl functionalities on the nanoporous graphene promoted the water permeation and separation of desired ions from the water during reverse osmosis process [141].

Figure 7. Oxygen density maps at inside a hydrogenated (left) and hydroxylated pore (right), with open pore areas of 23 and 28 Å, respectively [140]. Reproduced with permission from ACS.

Table 1. Pore density of pristine nanoporous graphene and polycarbonate track etch/nanoporous graphene nanocomposite membrane [97].

Sample	Pore density (cm^−2)	Water permeable pore density (cm^−2)	Water permeable pore fraction (%)
Pristine nanoporous graphene	1.59 × 10^13	0.151 × 10^13	18.45
Polycarbonate track etch/nanoporous graphene	0.098 × 10^13	0.026 × 10^13	42.86

Yuan et al. [142] designed the nanoporous graphene from graphene oxide through hydrothermal treatment. The nanoporous graphene was filled in ethyl cellulose matrix to form mixed matrix membranes. The ethyl cellulose/nanoporous graphene membranes have been applied for C₃H₆/C₃H₈ separations. Inclusion of ∼1.13 wt.% nanoporous graphene nanofiller was found to enhance the C₃H₆ permeability from 57.9 to 89.95 Barrer. In addition, the ideal selectivity of the membranes for C₃H₆/C₃H₈ was improved from 3.45 to 10.42 Barrer. It was observed that the high surface area and dispersion of nanoporous graphene led to the formation of interlinking paths in the matrix for facilitation of permeability and selectivity for C₃H₆/C₃H₈ separatione [143]. Hence, various designs of nanoporous graphene derived nanocomposites have been developed and studied for the morphology, structural, and physical properties.

4. Functional nanoporous graphene filled nanocomposite membranes

Functional nanoporous graphene derived membranes have been explored for the advantageous water purification application [144]. Han et al. [145] designed the functionalized nanoporous graphene membranes for the separation of methanol from water. Here, membrane performance of nanoporous graphene with capacitive deionization system was studied through the molecular dynamics simulations. The nanoporous graphene membranes were also studied under electric field [146]. Application of electric field has been found to enhance the membrane permeability and removal of methanol from water. In addition, the electrostatic or hydrogen binding interactions as well as interaction energy between methanol and membrane have been studied [147]. Moreover, the self-diffusion coefficients of methanol and water were analyzed. The water density near the membrane was also considered. The functionalized membranes were also suggested successful for the removal of the organic molecules from water. Wang et al. [148] explored the functionalized nanoporous graphene derived desalination membranes using the molecular dynamics simulations. The hydrogen and hydroxyl functional nanoporous membranes have been considered for desalination. It was observed that the external pressure facilitated the desalination process [149]. In addition, the external pressure and pore diameter directly affected the water flux permeation [150]. The small pore sizes promoted 100% salt rejection due to least permeation of Na⁺ and Cl⁻ ions. The functionalized membranes revealed water permeance of 785.6 L per m²·h·bar. The functionalized membranes have been suggested for the development of nanofiltration and reverse osmosis membranes.

Hosseini et al. [151] applied the molecular dynamics simulations to study the functionalized nanoporous graphene oxide membrane. The hydroxyl functionalized nanoporous graphene derived membrane was explored for water desalination. The nanopore size was found in the range of 2.9–4.5 Å. The membrane had salt rejection of about >89%. The water flux of reverse osmosis functionalized nanoporous graphene based membrane was found as 77%. Figure 8(A) depicts the molecular structure of functionalized nanoporous graphene oxide. Two types of nanopores were observed, that is, hydrophilic nanopores (P2 and P4) have epoxy and hydroxyl functionalities, while hydrophobic or hydrogenated nanopores (P1 and P3) were also formed. The nanopore radius of hydrophobic P1 and P3 was ∼2.9 Å, while hydroxylated P2 and P4 had pore radius of ∼3.1 Å. Figure 8(B) shows the molecular dynamics simulated cell with functionalized nanoporous graphene oxide membrane. The nanosheet acted as a barrier for Na⁺ or Cl⁻ ions as well as water molecules. The system attained equilibrium in I ns and temperature of about 298 K. The hybrid Nosé−Hoover Langevin piston technique [152] was used to sustain the pressure at 1 bar. Hence, the molecular dynamics simulation technique has been effectively used for the formation of functional nanoporous graphene oxide membrane.

Figure 8. (A). Nanoporous graphene oxide and different pore architectures (a) hydrogenated nanoporous graphene oxide (NPGO) with epoxy and hydroxyl groups on the surface; (P1) hydrogenated NPGO pore, pore radius ∼2.9 Å; (P2) hydroxylated NPGO pore, pore radius ∼3.1 Å; (P3) hydrogenated NPGO pore, pore radius ∼4.5 Å; and (P4) hydroxylated NPGO pore, pore radius ∼4.1 Å; (B) A snapshot of the simulated cell containing nanoporous graphene oxide (NPGO) nanosheet as a membrane, a graphene sheet as a barrier, water molecules, and ions (Na⁺ and Cl⁻). The NPGO was placed in the center of box and water molecules and ions were added to the one side of box [151]. Reproduced with permission from Elsevier.

Jafarzadeh et al. [153] studied the hydrogenated and fluorinated nanoporous graphene membranes using molecular dynamics simulations. A simulation box with ionic solution of Na⁺ and Cl⁻ ions was investigated with functionalized graphene membranes. Applied pressure of about 10–100 MPa was considered for the water transportation and desalination. The presence of functional groups on nanoporous graphene surface was found to enhance the water permeability at low pressure [132]. Azamat et al. [154] designed the hydrogenated and fluorinated nanoporous graphene membranes for the separation of trihalomethanes from water. Molecular dynamics simulations were performed to study the performance of the functional system. Smaller pores in the functional membranes did not promote the separation of trihalomethanes from water. On the other hand, functionalized nanoporous graphene having large diameter pores revealed better permeability for trihalomethanes separations [155]. Hence, the process was found dependent upon the pore size and pressure applied. Golchoobi et al. [156] applied the molecular dynamics simulation method to study the functionalized nanoporous graphene based membranes. The nanopores structure of pristine nanoporous membrane as well as functionalized nanoporous graphene has been explored. Consequently, the desalination, permeability, and salinity properties of the membranes were analyzed. Graphene nanosheet has nanopore size of about <10 Å. Figure 9 depicts six hydroxyl functional groups and six hydrogens on the nanopores structure. The area of nanopore was adjusted at 48 Å. The hydroxylated nanopore revealed better water permeability and salt rejection. The 9% porosity was observed for the nanoporous graphene. Figure 10 reveals the normalized water permeability of the functionalized nanoporous graphene derived membrane vs. salinity. The permeability was observed in the range of 83.5–131 L/cm²/day/MPa for 33.4 g/L salinity. Hence, a linear relationship was observed between the pore area and water permeability according to the classical fluid dynamics model [157].

Figure 9. Pristine graphene nanosheet with a pore, nanosheet dimension of about 27 Å, unit cell dimension was set 30 Å (top left); lateral view of the graphene nanosheet functionalized with OH and H (top center); top view of hydroxylated graphene showing average pore diameter of about 11 Å (top right); schematic representation of desalination in a simulation box with 800 H₂O molecules and eight ions of Na⁺ and Cl⁻ with salinity of about 68 g/L (bottom) [156]. Reproduced with permission from Elsevier.

Figure 10. Permeability of water through nanoporous graphene membrane plotted against salinity [156]. Reproduced with permission from Elsevier.

Chemical enhanced oil recovery of nanoporous graphene nanocomposite membranes has been studied [158]. AfzaliTabar et al. [159] developed the nanoporous graphene and silica nanohybrids using the Pickering emulsion method. The nanocomposite revealed fine stability at 1% salinity. For oil separation process, the nanoporous graphene/silica nanohybrid has capability to decrease the interfacial tension and promote the separation process [160]. The temperature of oil reservoir was monitored between 25 and 120 °C, whereas the pH levels were between 7 and 10. Here, the rheological behavior of the nanomaterial has been found important to investigate for better oil separations.

Gas permeability and selectivity applications of modified nanoporous graphene derived nanocomposites have derived lots of research interest [161]. Du et al. [162] theoretically investigated the transportation of N₂/H₂ mixture through modified nanoporous graphene membrane. In this regard, the pore sizes and permeation rate of N₂/H₂ mixture were studied. It has been observed that the N₂ permeation rate was better than that of H₂ due to adsorption of N₂ molecules. Shan et al. [163] studied the nitrogen functional porous graphene membranes. The nitrogen functional pores have been found to enhance the selectivity of CO₂ molecules due to electrostatic interactions. Wang et al. [164] studied the functionalized nanoporous graphene based gas separation membranes. Pristine nanoporous graphene based membranes as well as nitrogen or fluorine functional nanoporous graphene have been considered. The nanopore sizes were studied using the density functional theory [165]. The functional nanoporous graphene membranes possess suitable nanopore sizes for the gas permeance in the range of ∼10³–10⁵ GPU, during the selective separations of CO₂/N₂, CO₂/CH₄, and N₂/CH₄ gas mixtures. These membranes have been found advantageous comparable to the traditional polymeric membranes having gas permeance of just about 100 GPU [166]. Further variation of functional groups on the nanoporous graphene surface and exploration of mass transportation mechanism may offer theoretical basis for designing the nanoporous graphene membranes [167].

Few attempts on the functionalized nanoporous graphene for thermal management systems have been reported [168–170]. Jia et al. [171] studied the application of nanoporous graphene in thermal management systems. The thermal management functions were studied using the simulated temperature profiles. The heat energy was observed to cross the specific areas and reach the thermal concentrator (Figure 11). The direction of heat flux was analyzed. Helical structures with isothermal lines were seen in the simulated temperature profile. The anisotropy in the isothermal and temperature gradient profiles were investigated. Due to the spatial geometry, the heat flux vector was rotated at 45° towards the inner area. The thermal properties of the nanoporous graphene metamaterials have been observed superior than the pristine graphene as well as the three dimensional materials.

Figure 11. (a) Schematics of a thermal rotator device; (b) simulated temperature distribution profile with white isothermal lines; and (c) simulated heat flux distribution, where the arrows and color indicate the direction and value of heat flux, respectively [171]. Reproduced with permission from Wiley.

The bifunctional electrocatalysts have been developed using the nanoporous graphene and related materials. Qiu et al. [172] designed the bifunctional electrocatalysts having high activities and durability towards oxygen evolution and oxygen reduction reaction for rechargeable metal–air batteries. For this purpose, three-dimensional nanoporous graphene was formed and used for the catalyst towards batteries [173]. In addition, the nanoporous graphene was doped with N and Ni dopants. According to density functional theory, synergistic effects of N and Ni dopants have been observed on the electrocatalytic activity.

The nanoporous graphene has been applied in the micro-supercapacitor application and wearable electronics [174]. Kim et al. [175] developed the nanoporous graphene and molybdenum disulfide based film for micro-supercapacitors. Spin coating technique was used to cast the films [176]. The modified nanoporous graphene possess high volumetric capacitance around 55 F cm⁻³ and capacitance retention of 82% (20,000 cycles). The electrode also had energy density and power density of 7.64 mW h cm⁻³ and 9.96 W cm⁻³, respectively. The functional nanoporous graphene electrodes revealed high flexibility, durability, and potential for wearable electronics.

5. Scenarios and encounters of functionalized nanoporous graphene derived nanocomposite membranes

Nanoporous graphene offers the functional opportunities for the engineered nanocomposites [177]. Nanoporous graphene own high surface area, nano-level porosity, semiconductivity, and permeability features for the development of advanced nanocomposites [178]. Exclusive nanoporous nanostructures have capability to entrap the matrix particles through fine interactions leading to the formation of nanocomposites [179]. The matrix–nanofiller interactions and interface effects play essential role to substantially improve the conductivity, thermal, mechanical, and physical characteristics of the nanomaterials.

In addition to the nanoporous graphene, the functional nanoporous graphene has been focused in recent literature [180]. Like nanoporous graphene, the functionalized nanoporous graphene has also been designed through numerous top down and bottom up strategies. Like pristine nanoporous graphene, the functionalized nano form has also been investigated for the varying pore sizes, neck widths, periodicities, etc. [181]. Modified nanoporous graphene has been explored for the ionic or molecular separations in aqueous as well as gaseous media. Including modified nanoporous graphene in nanocomposites revealed elasticity, strength, toughness, heat stability, and conducting properties. Research performed up till now has pointed towards the utilization of functionalized nanoporous graphene in the fields of water permeation, gas separations, chemically enhanced oil recovery, thermal managements, bifunctional electrocatalysts, micro-supercapacitors, and others. Essential features and uses of important functionalized nanoporous graphene derived nanocomposite systems are shown in Table 2. Present research on functionalized nanoporous graphene face encounters of appropriate surface engineering to form unvarying porosity, optimum pore sizes, desired modification, and large-scale processing. However, more functional forms of nanoporous graphene need to be discovered in addition to the hydroxyl functionalization or doping. The challenges related to the nanocomposites of these unique nanostructures include dispersion in matrix, matrix–nanofiller interactions, interface generation, compatibility, controlling low percolation threshold to assure high electron conductivity, structure–property relationships, and commercial level synthesis or processing [182]. In the nanocomposite form, physical or covalent interactions with the matrices formed miscible nanostructure. Due to the current challenges, the functionalized nanoporous graphene and derived nanocomposites have not been observed for numerous technical applications. In addition to the above mentioned potential, these functionalized nanostructures can be used in the fabrication of fuel cell membranes, transistors, electronic circuits, and biomedical systems [183,184]. Here, the comprehensive computational, theoretical, and simulation studies need to be performed on the functional nanoporous graphene in order to overcome the related challenges and to achieve the desired advanced application areas.

Table 2. Specifications of nanoporous graphene derived nanocomposites.

Nanomaterial	Method/technique	Features/application	Ref.
Functionalized nanoporous graphene	Molecular dynamics simulations	Desalination membranes; water permeance 785.6 L per m^2·h·bar; 100 % salt rejection	[148]
Hydroxyl functionalized nanoporous graphene oxide	Molecular dynamics simulations	Reverse osmosis membrane; salt rejection >89%; water flux 77%	[151]
Hydrogenated and fluorinated nanoporous graphene	Molecular dynamics simulations	Water transportation/desalination for applied pressure ∼10–100 MPa	[153]
Hydrogenated and fluorinated nanoporous graphene	Molecular dynamics simulations	Aqueous separation of trihalomethanes	[154]
Hydroxyl functional nanoporous graphene	Molecular dynamics simulations	Nanopore size <10 Å; nanopore area ∼48 Å; permeability 83.5–131 L/cm^2/day/MPa (33.4 g/L salinity)	[156]
Silica functional nanoporous graphene	Pickering emulsion method	Oil separation; oil reservoir temperature 25–120 °C; pH 7 − 10	[159]
Functionalized nanoporous graphene	Density functional theory	Gas separation; gas permeance of ∼10^3–10^5 GPU; selective separations of CO2/N2; gas permeance of 100 GPU	[164]
Functionalized nanoporous graphene	Simulated temperature profiles	Thermal management; helical structures with isothermal lines; heat flux vector rotated at 45°	[171]
N and Ni doped nanoporous graphene	Density functional theory	Bifunctional electrocatalysts; synergistic effects of N and Ni dopants on electrocatalytic activity	[172]
Molybdenum disulfide modified nanoporous graphene	Spin coating	Micro-supercapacitors; volumetric capacitance 55 F cm^−3; capacitance retention 82 % (20,000 cycles); energy density 7.64 mW h cm^−3; power density 9.96 W cm^−3	[175]
Poly(dimethyl sulfoxide)/nanoporous graphene	Nickel template based chemical vapor deposition method	Electromagnetic interference shielding effectiveness 51–83 dB	[119]

6. Conclusions

In summary, this cutting-edge manuscript discussed the functional forms of nanoporous graphene and derived nanocomposites. These unique nanoporous graphene derivatives have been investigated for the remarkable nanostructure, pore sizes, periodicities, neck width, and physical features such as the microstructure, electrical conductivity, thermal conductivity, capacitance, permeability, etc. The advancements in the field of functional nanoporous graphene revealed several remarkable features and potential applications. Specifically, the fields of desalination membranes, gas separations, oil separations, heat management, electrocatalysts, and energy storage devices have been focused. In future, the investigations on the modification of nanoporous graphene, dispersibility, interactions with matrix, interface formation, enhanced conductivity, and permeability may resolve the current challenges to further expand this field.

Author contributions

Conceptualization, Kausar, A.; Data curation, Kausar, A.; Writing of original draft preparation, Kausar, A.; Review and editing, Kausar, A.; Ahmad, I.; Aldaghri, O.; Ibnaouf, K.H.; Eisa, M.H.; Lam, T.D. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research. The authors also appreciate the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) for supporting and supervising this project.

Disclosure statement

The authors declare no conflict of interest.

Additional information

Funding

The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research through the project number IFP-IMSIU-2023132. The authors also appreciate the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) for supporting and supervising this project.

Notes on contributors

Ayesha Kausar

Ayesha Kausar is currently affiliated with NPU-NCP joint international research center on Advanced Nanomaterials and Defects engineering, Northwestern Polytechnical University Xi’an, China and UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, iThemba LABS, Somerset West, South Africa. Her current research interests include design, fabrication, characterization, and structure-property relationship/potential explorations of nanocomposites, polymeric nanocomposites, hybrid materials, nanoparticles, carbon nanoparticles (graphene, fullerene, carbon nanotube, nanodiamond, etc.), nanofibers, nano-foams, and related nanostructures/nanomaterials. Dr. Kausar has contributed 7 books (authored monographs), 90 book chapters, and more than 500 publications to the field of nano/materials sciences and technology, so far.

Ishaq Ahmad

Ishaq Ahmad is Professor affiliated with NPU-NCP Joint International Research Center on Advanced Nanomaterials and Defects Engineering, Northwestern Polytechnical University, Xi’an, China, UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, iThemba LABS, Somerset West 7129, South Africa, and NPU-NCP Joint International Research Center on Advanced Nanomaterials and Defects engineering, National centre for Physics, Islamabad Pakistan.

Osamah Aldaghri

Osamah Aldaghri is Professor affiliated with Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Saudi Arabia.

Khalid H. Ibnaouf

Khalid H. Ibnaouf works as senior Professor in Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Saudi Arabia.

M. H. Eisa

M. H. Eisa is currently Professor in Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Saudi Arabia.

Tran Dai Lam

Tran Dai Lam is a scientist and professor from Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Viet Nam.