1George S. Ansell Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA
2Department of Precision Engineering, The University of Tokyo, Tokyo, 113-8656, Japan
3State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China
Proceedings of the Nature Research Society, Volume 2, Article Number 02003, 2018
Published Online: 05 February 2018 (Review)
Nanoporous structures exhibit great application potential in the biosensing, energy storage and other nanoelectronics. As a newly discovered 2D material with extraordinary properties, graphene presents the ability to hold a nanoporous structure which could overcome the inherent shortages for most of the existing nanoporous materials. While how to efficiently synthesize the nanoporous structures in graphene and how to further expand the applications of nanoporous graphene (NPG) are still big challenges. In this paper, we reviewed the recent advancements related to the synthesis and potential applications of NPGs. By analyzing the different approaches to fabricate the NPGs, and the research trends for the application realization of NPG, we aim at stimulating further research on this subject.
Nanopore originally refers to a vital biological feature which is generally existed in cellular membranes for recognition and transport of ions and molecules between compartments within the cell and between the cell interior and outside environment . After being successfully created in biological and solid-state materials, nanoporous structures have the potentials to be used in biosensing [2-4], diagnostics [5, 6], separation [7-10] and so on. In general, the nanopore is broadly divided into two categories: biological nanopore and solid-state nanopore. Though it was proved that most of the reported biological and solid-state nanoporous structures can offer the alternative approaches for many applications, they are still limited by some inherent shortages. For example, the biological nanopores have short life time, intrinsic instability and strict requirement, which are not favored for long-term operations. The solid-state nanopores usually have robust chemical and thermal characteristics, while it is hard for them to discriminate molecules with similar sizes but different biological characteristics . What’s more, the thickness of these nanopores is usually far larger than the length of nucleotide, which makes them hard to read single nucleotide information from a long chain of DNA. The sensitivity of the nanopore technology used in biosensing also needs to be further improved. Graphene, as a newly discovered material made up of single layer carbon atoms, holds a subnanometer thickness as 0.34 nm, which is comparable to the spatial interval between neighboring DNA nucleotide. So a nanoporous structure created in graphene could offer the possibility of gene sequencing at a single-base resolution. Besides, graphene was demonstrated to exhibit amazing mechanical [12-14], electrical properties [15-17], and the nanoporous graphene (NPG) could inherit most of the unique properties and even further improve them, which could promise NPG to be outstanding among the nanoporous materials for many applications. For example, the nanomesh structure created in graphene sheet could open its zero bandgap, which can be used to synthesize field effect transistors (FETs). The NPG-based FETs are able to provide saturated currents 100 times stronger than the devices based on graphene nanoribbon . NPG is also superior in the applications of single-molecule sequencing because of its excellent mechanical stability, and electrical sensitivity [19, 20]. Moreover, owing to the single-atom thickness of graphene itself, the transport rates of molecules through the NPG membranes are expected to be extremely high, which proves that NPG could achieve high permeability and selectivity for gas separations and water purification [ 21- 25]. Furthermore, processing graphene into NPG could improve the performances of graphene-based electrochemical capacitors by providing much larger specific double layer capacitance . To realize and expand the potential applications of NPGs, the exploration of an efficient nanopore synthesis technology is also essential. There are lots of techniques proposed in the recent years to produce and control NPG structures, like the high energy beam irradiation [27, 28], mask-based lithography [ 18, 29- 30], and chemical vapor deposition . Though some of the synthesis methods are still under development, they can offer reliable materials for the construction of the NPG-based devices.
By aiming at the realization of the NPG-based devices in the market, a lot of studies have been conducted to investigate the synthesis approaches and application expanding of NPGs. In this paper, the recent research advancements on the synthesis of NPG materials and their applications in different areas were summarized, by which we hope to stimulate the further research in this topic.
2 Synthesis of NPG
The research of NPG synthesis mainly focuses on how to obtain high quality and high throughput array holes. Based on the different stages that the pore structures are generated, the techniques used to synthesize NPG can be categorized into top-down and bottom-up methods. Among them, the top-down methods, which mainly include focused beam irradiation method, block copolymer lithography (BCL), nano-particle lithography (NPL), nano-imprint lithography (NIL) and oxygen plasma etching, refer to the processing of NPG after the preparation of single or multi-layer graphene. The bottom-up methods refer to the generation of nanoporous structures in the graphene material during its preparation process. The studies of bottoms-up methods are limited at present and focused on the chemical vapor deposition (CVD) based methods.
2.1 Top-down methods
With the improvement of the technique, the ion beam spot can be now focused into nanometer scale, and the electron beam spot can even be focused into sub-nanometer. So the focused beam irradiation methods have the potential to fabricate nanoporous structures in graphene. In 2008, Drndić et al. first demonstrated the fabrication of nanometer-scale pores in suspended multilayer graphene under the irradiation of focused electron beam in a transmission electron microscope (TEM). The structures they generated are stable and don’t evolve over time. Then, based on their work, some follow-up studies were conducted to further control the size of the pore structure and drilled the nanopore in TEM under various temperatures [ 32- 34]. It was shown that the size of graphene nanopore can be regulated by thermal heat. In 2012, Lu et al.  explored the possibility of shrinkage and expansion of graphene nanopores under electron beam irradiation by setting different temperature conditions in the apparatus. Under the temperature range of 400-1200 ˚C, they found the shrinkage of nanpores of various sizes, and the nanopores with small diameters could be fully closed. This nanopore shrinking process can be stopped by blocking the electron beam. In 2013, Xu et al.  also reported the fabrication of sub-nanometers graphene nanopore under various temperatures. Their results indicated that the high temperature conditions and direct thermal heating treatment can be beneficial to fabricate the NPGs with high crystallization, and promote the size control. Figure 1(a) depicts the typical nanopores created in graphene by electron beam irradiation, and the corresponding size control by temperatures. Due to the self-repairing of carbon materials at high temperature [35,36], the drilling rate decreased when raising the temperature. The thermal heating process could also offer an alternative method for closed edge nanopore formation in graphene, which can improve the stability of nanopores for a wide range of applications . With higher energy transfer efficiency, focused ion beam were also successfully applied in the NPG fabrication. But due to the large spot size, a mask needs to be used for this technique. The current fabrication methods include Ar+ [ 16, 27, 37], Ga+ [38, 39] and Helium  ion beam irradiation. It was revealed that the Ar+ etching could reach the pore sizes of around 80 nm, and the Ga+ focused ion beam has the capacity to fabricate holes about 10 nm in diameter, both of which are much larger than the request in the applications of DNA sequencing. The Helium ion irradiation could obtain nanopores below 3 nm. Though the technique of Helium ion microscope is not yet widely available, it shows great promise in NPG fabrication.
The focused beam irradiation methods have limited throughputs and cannot be applied in patterning large-area graphene, which are not suitable for industrial production. Then lithography methods were introduced to synthesize large-area NPG membranes. The BCL approach is intrinsically scalable and could allow for the rational design and fabrication of NPG-based devices. In 2010, Bai et al.  reported the first study using the BCL method to fabricate NPG. As shown in Figure 1(b), there are several steps involving in the fabrication process. First, the preparation of graphene and transfer of the as-synthesized graphene onto silicon oxide substrate. Then, covering the graphene flake with SiOx layer and a thin film of spin-coated block-copolymer. Third, the block-copolymer film is annealed and developed, leaving the porous matrix. Fourth, using the reactive ion etching to generate the SiOx nanomesh hard mask. Fifth, etching away the exposed graphene by O2 plasma. Sixth, removing the oxide mask, and finally the free-standing NPG membranes can be obtained after etching away the underlying silicon oxide. By using these procedures, the authors fabricated NPGs with variable periodicities and neck widths down to 5 nm. Also, by using the BCL method, NPG based FETs were fabricated . The as-prepared NPG has a sub-20 nm neck width with quantum confinement, which could lead to a bandgap opening of around 0.08 eV. In 2014, Cagliani et al.  used a spherical block copolymer etch mask to generate the nanomesh patterning structure in graphene. They found the nanopatterned graphene can detect NO2 concentration as low as 300 ppt, and the ultimate detection limit could be tens of ppt, which is the smallest reported value for non-UV illuminated graphene chemiresistive NO2 gas sensors. Besides the BCL approach, NSL is also an alternative method for making large-scale NPGs. NSL can be considered as a complementary approach for BCL in terms of achieving NPGs with different periodicities . The fabrication steps for making the NPGs by NSL are shown in Figure 1(c). First, the graphene was prepared and transferred onto Si substrates with 200 nm SiO2 layer. Then mask 1 (SiO2, 10nm) was deposited on the graphene covered substrates, followed by self-assembly of colloidal spheres on a vertical substrate. Third, RIE 1 (CF4) was used to define ~10 nm gaps between the spheres. Fourth, mask 2 (Au, 10 nm) was deposited and then the colloidal spheres were removed. Fifth, RIE 2 (CF4:O2=1:1) was used to remove the exposed graphene. Sixth, the masks 1 and 2 were removed, and finally the fabrication of NPG devices by e-beam lithography. The use of the NSL method could yield graphene nanostructures with promising electronic properties featuring high conductivities and ON-OFF ratios up to 10. In 2011, Safron et al.  synthesized NPG structure by using NSL, and proved the as-synthesized structures behave as semiconductors with a field-effect conductance modulation of up to 450 and charge mobilities of ~1 cm2 V-1 s-1. In addition, the NIL, when combined with the block copolymer self-assembly, can also be used to fabricate the NPGs . Figure 1(d) schematically shows the fabrication route of the NPGs by NIL: (1) preparation and transfer of graphene; (2) polymeric imprint resist by spin-coating; (3) nanoimprinting by using a template with pillar features; (4) removing the imprint template; (5) etching away the resist residual layer thickness; (6) removing the imprint resist. Such a fabrication route could obtain hexagonal graphene nanomeshes with sub-10 nm ribbon width, and the ON/OFF of the NPG-based device is close to 100 at room temperature when ribbon width is narrower than 100 nm.
NPGs can also be synthesized by many other top-down approaches, like the ultraviolet-induced oxidative etching [46, 47], oxygen plasma , photocatalysts , MnO2 etching , metal nanoparticles assisted catalysis [50, 51], and so on. Though various methods have been proposed to synthesize NPGs efficiently, it is still a challenge to find out the pathways for low-cast, high throughput production of NPGs with critical dimensions down to subnanometer and are compatible with wafer-scale applications.
2.2 Bottom-up methods
The NPGs prepared by top-down methods usually exist with rough and disordered pore edges, which may interact with the transported molecules, increase the scattering of electrons, decrease the carrier mobility and reduce the sensitivity. To overcome these weaknesses, the bottom-up methods are then expected. According to the synthesis methods of graphene material, CVD-based methods were widely used in the fabrication of NPGs. In 2012, Safron et al.  firstly reported the usage of barrier-guided chemical vapor deposition (BG-CVD) method to pattern graphene. BG-CVD relies on the self-terminating growth processes rather than the harsh chemical etchants to abruptly define edges, and also can create the rationally-designed nanopore patterns. Figure 2(a) schematically shows the fabrication of NPG of the BG-CVD process. First, the aluminum oxide is deposited on the Cu substrate and patterned. Then methane decomposes on the exposed Cu to produce C. Graphene is nucleated with the diffusion and accumulation of C. After the nucleation, the graphene crystallites grow up to the Cu/barrier edge-boundary. The pattered barrier layer terminated the growth of graphene in the exposed Cu, leaving a NPG structure with superior edges of less disorder. Figure 2(b) and Figure 2(c) depict the aluminum oxide barrier on Cu and the nanoperforated graphene. By using the BG-CVD method, the large-area NPG can be fabricated. The similar distribution of dot-size and hole size revealed that the oxide barriers are stable and could terminate the growth of graphene with atomic exactness. Based on this work, a lot follow-up CVD-based studies were conducted to synthesize NPG. In 2015, Pourmand et al.  prepared the NPG by CVD method using porous zinc oxide as the catalyst. They used the NPG as nanosorbent and predicted a maximum sorption capacity of this nanosorbent for benzene, toluene and xylenes as 118.83, 123.45 and 125.36 g/g nanosorbent, respectively. By using the nanoporous copper as a catalytic substrate, Tu et al. also synthesized the NPG films based on a CVD method. They proved that the nanoporosity of graphene significantly improves the surface enhanced Raman scattering efficiency of graphene as a substrate when compared to planar graphene. In 2016, Zhan et al.  fabricated the 3D NPG by using the plasma-enhanced CVD on a nanoporous anodic alumina substrate. The plasma-enhanced graphitization was demonstrated to produce better quality NPG than thermal graphitization. The CVD-based methods were also used to fabricate the NPG for the applications in oil spill removal  and supercapacitor electrodes .
The researches related to the bottom-up synthesis methods are limited. Based on the various synthesis approaches of graphene material, like the redox method [58, 59], silicon carbide epitaxial growth method [60, 61], much more efforts should be done to expand the bottom-up methods, and figure out the most efficient way for the fabrication of high quality wafer-scale NPGs.
3 The potential applications of NPG
As noted above, the single-atom thickness and the extraordinary properties of NPG promise it as attractive materials in a lot of applications. The applications of NPG cover many areas. In this section, we will focus on the four main application potentials, which include the single-molecule sequencing, gas separation, seawater desalination and energy storage.
3.1 Single-molecule sequencing
Since the first nanopore paper published in PNAS in 1996 , nanopore-based devices have emerged as one of the most powerful and promising technologies for single molecule sequencing. As a label-free and amplification-free single-molecule approach, nanopore was demonstrated experimentally and numerically to be capable of DNA sequencing [ 63- 67], RNA sequencing [ 68- 70], and protein detection [ 71- 74]. The working concept of sequencing of nanopore is straightforward (taking DNA as the example): a DNA molecule is electrophoretically driven through a nanoscale pore in a membrane separating two electrolytes. The changing of ionic current signal corresponds to the transport of DNA chain through the pore, which could provide the information to identify the specific nucleotide. It is required that the membranes used in the single-molecule sequencing are robust, durable under the fluid pressure, and the thickness should be as thin as possible for sequencing the nucleinic acid at a single base resolution. Graphene is a single layer of graphite with extraordinary electrical, mechanical and thermal properties, which is an ultimate choice for the DNA sequencing. In 2010, Drndić , Dekker  and Golovchenko  groups experimentally reported the DNA sensing by graphene based nanopores respectively, and all of their results suggested the remarkable durability and insulating capability of single graphene layer in high-ionic-strength electrolyte. Figure 3(a) gives the typical setup of the graphene nanopore device. Few layer graphene is suspended over silicon nitride (SiN) membrane with a 1 μm diameter hole. SiN memebrane is suspended over a silicon chip with SiO2 layer. The nanopore structure is inserted into a PDMS cell with microfluidic channels in each side of the chip. A bias is applied to drive the ionic current through the nanopore. The typical TEM image of the graphene nanopore is shown in Figure 3(b). By using this setup, the current trace showing DNA translocation through the nanopore is given in Figure 3(c). The histogram of block currents and the inset (Figure 3(d)) indicate the unfolded, singly folded and doubly folded entries at the current values of 0.45, 0.9, 1.35 nA. It demonstrates the great promise of graphene nanopore for future genomic screening. Several parameters affect the performance of a NPG. First is the DNA conformation. Because the magnitude of the blocked ionic current indicates to what the extend the molecule obstructs the current flow through the nanopore, then when DNA translocate through the nanopore in different macroscopic configurations, the current signal would be different. The dips in Figure 3(c) correspond to the translocation of DNA under fully or partially folded conditions. Second is the nanopore geometry. Garaj et al.  experimentally investigated the size effects of the graphene nanopore and they found that in nanopores with diameters greater than 4 nm, dsDNA molecules translocate through the pore either as an extended linear molecule or as a folded molecule. While in nanopores with diameters smaller than 4 nm, only unfolded molecules were observed, which means these nanopores are too small to easily admit folded dsDNA molecules. Third is the sensitivity of the nanopores. The vibrations of membrane in the solution may reach a level comparable to the thickness of the membrane , which would decrease the sensitivity. The mechanical stability of the graphene could be improved by inducing some mechanical stress, increasing the salt concentration  and increasing the working temperature . Fourth is the interaction between DNA and graphene. The DNA molecule may stick strongly (hydrophobic interaction) on the membrane and finally clog the nanopore. Schneider et al.  confirmed the strong adsorption of DNA on the surface using AFM mapping of a graphitic surface exposed to the solution containing single stranded DNA. To minimize this interaction, several methods were proposed, like the deposition of hydrophilic TiO2 , increasing the pH of the solution , non-covalently functionalization . Due to the difficulties in conducting the experimental test, atomics simulations were widely applied in the investigation of DNA sequencing by graphene nanopores. In 2010, Nelson et al.  first used ab initio density functional theory to study the DNA sequence detecting by a NPG-based device. Their results could offer the theoretical support for the using of NPG in ultrafast, low-cost DNA sequencing. Molecule dynamics (MD) simulations were also proved to be effective in this type of study. In 2011, Sathe et al.  used MD to investigate the DNA sequencing by NPG under the consideration of the effects of applied voltage, DNA conformation, pore charge, and pore size. Figure 3(e) described the MD model. By using this model, they demonstrated that A-T and G-C base pairs could be discriminated in dsDNA using the NPG (Figure 3(f)). Some other simulations work were also conducted [ 81- 85], which could further promote the realization of this approach. Besides the applications of NPG in DNA sequencing, there is much excitement surrounding the possibility of applying the NPG to sequence protein. In the last couple of decades a typical application of interest of nanopore was NDA analysis, while only recently, the technique was applied to proteins and polypeptides. In 2015, Bonome et al.  demonstrated a multistep current signal during the translocation of protein through graphene nanopores. In 2016, Wilson et al.  also found that the transport of peptides could produce stepwise modulations of the ionic current correlated with the type of amino acids presenting in the nanopore. Both their results suggested that protein sequencing by measuring ionic current blockades may be possible. Protein translocation presents two crucial differences with respect to DNA translocation: proteins are not uniformly charged and hence they are not easily imported into the pore by the bias voltage; also the proteins need to partially or completely unfold to cross the pore. Though the application of NPG in protein detection and sequencing is just as the beginning, the research of DNA sequencing could offer instructive information in this area.
3.2 Gas separation and seawater desalination
Separation, as a technique that converts a mixture of substances into distinct pure mixtures/fractures, is an important indispensable process in industry for gas separation, hydrogen production and seawater desalination, etc. Compared to traditional separation methods such as distillation  and pressure-swing adsorption , membrane separation is emerging as a new method with low energy usage, flexible arranged process conductions, safety, low cost and less mechanical complexity [90, 91]. To be effective in the separation, the membrane should be as thin as possible to maximize the flow rate, mechanically strong and chemically inert to ensure its stability. Graphene, possessing a single-atom-thick sheet of sp2-hybridized carbon atoms arrayed in a honeycomb pattern, is considered as the ideal candidate for membrane materials, due to its high durability, withstanding high applied working pressures and excellent selectivity [ 92- 94]. However, it was demonstrated that the pristine graphene is impermeable to any molecules, even the smallest helium atoms [95, 96]. Consequently, NPG has been proposed as a very promising size-selective separation membrane based on molecular sieving effects, which were widely studied in the applications of gas separation and seawater desalination.
In 2009, Jiang et al.  first proposed the usage of NPG membrane as one-atom-thin, highly efficient, and highly selective membranes for gas separation. By using first principle calculation, they found high selectivity on the order of 108 for H2/CH4 with a high H2 permeance for a nitrogen-functionalized pore, and extremely high selectivity on the order of 1023 for H2/CH4 for an all-hydrogen passivated pore. Afterwards, several theoretical works further proved the capability of NPG in the application of gas separation for different types of gas. For example, in 2011, Du et al.  used the NPG membrane to separate the hydrogen and nitrogen gases. In 2010, Blankenburg et al.  adopted the first principle calculations to confirm that the high selectivity of H2 and He among other atmospheric gases, such as Ne, O2, N2, CO, CO2, NH3, and Ar by using NPGs. By functionalizing the pore with nitrogen atoms, Hauser et al.  further confirmed that the NPGs have the ability to separate 3He from 4He efficiently. NPGs were also predicted to be able to be applied for the purification of methane , and the separation of CO2/N2 . There are also other theoretical works [ 101- 104], which could further promote to uncover the mechanism existed in the separation process. Figure 4(a) shows the simulation model for the CO2/N2 separation through NPG . By using this sandwich-like model, it was found that within the simulation timeframe, CO2 can readily permeate through the nanopores, while N2 never passes through (Figure 4(b)), confirming the experimental finding of the CO2/N2 selective separation. Meanwhile, the selectivity remains high under different pressures (Figure 4(c)). Few experimental studies have been reported due to the difficulties in conducting the test. In 2012, Koenig et al. launched the first measurement of the transport of a range of gases (H2, CO2, Ar, N2, CH4, and SF6) through NPGs. Figure 4(d-g) show their measurement system. By using this system, they predicted a H2 leak rate on Bi-3.4 Å as 4.5x10-23 mols-1Pa-1 for a hydrogen-passivated pore in graphene consisting of two missing benzene at room temperature, which is similar to the simulation results by Blankenburg . Their results show the first experimental realization of graphene gas separation membranes by molecular sieving, and represent an important step towards the realization of macroscopic, size-selective NPG membranes. In 2014, Celebi et al.  further reported the highly efficient mass transfer across physically perforated double-layer graphene, with pore diameters ranging from less than 10 nm to 1μm. Based on the theoretical prediction, there will be more and more experiment tests conducting related to this topic in the following years.
As for the seawater desalination, the NPG was demonstrated to be selectively impassable to certain solvated ions , while the nanopores as small as 0.75 nm behave high water permeability . Then in 2012, Grossman and colleagues  first theoretically predicted that single-layer NPG can effectively separate NaCl from salty water. Figure 5(a-c) depicts their simulation model. They considered the effect of the pore size, chemical functionalization and applied pressure, and found that the NPG membrane’s capability for water desalination depends critically on pore diameter with adequately sized pores allowing for water molecule passage while blocking ions. The NPG based water permeability is several orders of magnitude higher than conventional reverse osmosis membranes (Figure 5(d)). Their results indicate that NPG could act as a high-permeability desalination membrane. Then, by using the MD simulations, Konatham et al.  further researched the influence of the pore functionalities on the transport of water and ions through NPGs. They demonstrated that narrow graphene pores functionalized with hydroxyl groups remain effective at excluding Cl– ions even at moderate solution ionic strength. In 2016, Grossman et al.  proposed the usage of multilayer NPG membranes for water desalination. They established an atomic-level understanding of the effects of NPG layers, layer separation, and pore alignment on desalination performance, which could provide useful guidelines for the design of multilayer NPG membranes. Furthermore, it was theoretically demonstrated that the performance of water desalination could be improved by using the functionalized NPG oxide membrane . The experimental work is limited. In 2014, O’Hern et al.  experimentally measured the selective ionic transport through controlled, high-density, subnanometer diameter pores in single-layer graphene membranes. They demonstrated the ability of tuning the selectivity of graphene through controlled generation of subnanometer pores. Subsequently, in 2015, Surwade et al.  demonstrated the first experimental evidence of the use of single-layer NPG as a desalination membrane. The as-synthesized NPG membrane exhibits high rejection of salt ions and rapid water transport, thus functioning as an efficient water desalination membrane. The application of NPG in water desalination is a promising and meaningful topic, for which more investigations are expected.
3.3 Energy storage
NPG materials are furthermore attractive for energy storage because of the atom-thick 2D structure, high conductivity, large specific surface area, good electrochemical stability and processability. The applications of NPGs in energy storage may include lithium-ion batteries (LIBs), lithium-sulfur (Li-S) batteries, supercapacitors, the dye-sensitized solar cells (DSSCs), and fuel cells. Here we will talk about the applications of LIBs and supercapacitors.
In 2007, Tskamura et al.  found that when nanopores were created in the graphene layers, the performance of graphitized materials as anodes for Li charge/discharge would be improved. Then in 2011, the hierarchically NPG was proposed as a lithium-air battery electrode to further improve its performance . Figure 6(a-b) shows the schematic structure of a functionalized graphene sheet with an ideal bimodal porous structure. Through this hierarchical setup, the authors predicted an exceptionally high discharge capacity of 15000 mAh/g in lithium-O2 batteries (Figure 6(c)), which is the highest value ever reported for nonaqueous Li-O2 batteries [115, 116]. In 2013, Fan et al.  synthesized the NPG structure by using CVD methods with MgO sheets as template, and the NPGs were demonstrated to exhibit a high reversible capacity (1732 mAh/g), excellent high-rate capability and cycling stability for Li-ion batteries. Further, Zhu et al.  synthesized nanostructured Fe2O3-NPG composites and used them as anodes for LIBs. It was found that this composite could behave much higher capacity than the corresponding values for the pure NPG electrode. In 2015, Zhang et al.  prepared large scale NPG sheets through thermal annealing procedure using composites of ferrocene nanoparticles and graphene oxide sheets as precursors. The anodes prepared by the NPG sheets exhibit higher specific lithium ion storage capacity, better discharge/charge rate capacity and higher cycling stability than anodes made up of graphene sheets. Recently, Zhou et al.  used the oxidation of graphene oxide by H2O2 under high-power UV irradiation with a subsequent reduction process to synthesize NPGs, and they found that the as-synthesized NPGs present higher specific capacity and better charge/discharge rate capability compared with chemically reduced graphene sheets for use as an anode material in lithium ion batteries. All of the above studies demonstrate the great potential of NPG as an electrode material for the lithium-ion battery.
Some studies have also been done on the use of NPGs for the realization of supercapacitors [121, 122]. In 2012, Fan et al.  reported the easy synthesis of NPGs using etching of graphene sheets by MnO2, and they proved that the NPGs-based electrode exhibits a specific capacitance much higher than graphene nanosheets (154 F/g vs. 67 F/g). Moreover, NPGs show better capacitive performances at high rates when compared to the active carbon electrodes due to the mesopores structure (pore size 2-50 nm) . In 2013, Zhu et al.  prepared a Co3O4-NPG composite layer which could deliver an ultra-high capacity (1543 mAh/g at 150 mA/g) and excellent rate capability (1075 mAh/g at 1000 mA/g) with good cycling stability. In addition, in 2017, Chini et al. prepared the hydrothermally reduced NPG-polyaniline (PANI) nanofiber composites as supercapacitor. Figure 6(d) illustrated the process for preparation of NPG-PANI composites. It was found that a maximum power density of 71.8 W/kg and energy density of 49.5 W h/kg was obtained for NPGs–PANI nano composites (Figure 6(f)). While for NPGs, the power density and energy density achieved are 48.9 W/kg and 25.7 Wh/kg, respectively. Based on Figure 6(e), it can also be derived that the specific capacitance of NPGs-PANI composites is 357 F/g, which is much larger than the value of NPGs (185 F/g). By using a scalable NPGs synthesis method involving an annealing process in hydrogen, Yang et al.  demonstrated the supercapacitors with highly NPG electrodes capable of achieving a high power density (41 kW/kg), a Coulombic efficiency (97.5%), a high energy density (148.75 Wh/kg), a high specific gravimetric and volumetric capacitance (306.03 F/g and 64.27 F/cm3). These works highlight the advantages and improvements made by the use of NPG as electrodes in the field of energy storage, and encourage more investigations on this topic.
4 Conclusion and prospect
In this paper, we reviewed the different synthesis methods of NPGs and focused on the potential applications of this promising material. NPGs can be synthesized through either top-down or bottom-up approaches. Though a lot of efficient methods have emerged, a cheaper method that permits large-scale production of porous graphene is still highly desirable. The NPGs have extraordinary properties, which promise them enormous application potential in the area of single-molecule sequencing, gas separation, seawater desalination and energy storage. A lot of theoretical studies have been conducted to demonstrate the feasibility of application of NPG in these areas and uncover the underlying mechanisms, while the experimental studies are limited by the current technique and technology. So for the coming years, there will be more and more experimental works proposed to realize the application potential of NPGs.
The authors are grateful for the helpful discussions with professor Honwei Zhu and valuable comments from anonymous referees.
The authors declare no competing financial interest.
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