石墨炔孔结构:设计、合成和应用

詹舒辉，赵亚松，杨乃亮，王 丹

（1.中国科学院过程工程研究所生化工程国家重点实验室,北京100190；2.中国科学院大学,北京100049）

1 Introduction

The diversity of hybridization states in carbon atoms forms a plentiful carbon material family［1］.In the past 40 years，with the rapid development of material science，the emergence of new materials such as fullerenes，carbon nanotubes，and graphene has promoted the development of electronics，optics，catalysis，biology，and other research fields［2—4］.Exploring new carbon allotropes and applying them to practice is always the dream of scientists.

Until 2010，graphdiyne（GDY）was first synthesized by Chinese scientistsviaanin situGlaser coupling reaction of hexaethynylbenzene（HEB，the monomer）on copper substrate［5］.The successful synthesis of GDY further stimulates property and application research.

GDY is composed ofsp-andsp2-hybridized carbon atoms.In detail，the well-known two-dimensional（2D）carbon network is constructed by a periodic array of diyne linkages and benzene rings，featured with uni⁃form pores，and highlyπ-conjugated structure.Its high chemical stability，thermal stability，and semiconduc⁃tor property make it a rising star after fullerene，carbon nanotube，and graphene［6—10］.GDY can be applied in⁃to energy，catalysis，optoelectronic，separation，sensor，and so on.Besides，through pore structure design，its application can be further extended.Thus，we briefly review the recent progress in the theoretical and ex⁃perimental researches of GDY’s pore structure design and its corresponding application.For the convenience of following discussion，GDY’s pore structures are classified into two scales as shown in Fig.1.（1）Molecular pores：the pores consist of adjacent carbon atoms on the molecular scale；（2）piled pores：pores formed by piling of GDY.Currently，the naturally existed bulk GDY has not been found，therefore，the synthesis strate⁃gies are all based on the bottom-up method，i.e.，in the liquid or gas phase，GDY is gradually formed through terminal alkyne coupling reactions of HEB on metal or other surfaces.

Up to now，more and more researchers have designed GDY’s pore structures by controlling several fac⁃tors mentioned above.For instance，Huanget al.［11］fabricated fluorine-substituted monomer to prepare fluo⁃rine-substituted GDY（F-GDY）and the diameter of molecular pores ranges from 0.54 nm to 1.3 nm.Further⁃more，GDY with larger and hierarchical piled pores can be obtained by appropriate templates and solvents system［12—14］.

Fig.1 Classification of GDY’s pore structures

Thanks to the pore structure design，the GDY with various pore sizes have shined in various fields.For example，GDY membrane with reasonable molecular pore size can be used in efficient gas separation and wa⁃ter purification.The large in-plane cavities also make it a good electrode material to reduce ion transport resis⁃tance and provide Li ion storage sites［16—20］.The piled pores formed in GDY can efficiently reduce the surface loss caused byπ-πstacking，significantly improve mass transfer and enhance absorption and utilization of light and wave［12，15，21］.Pore structure design has gradually revealed its advantages in material synthesis.How⁃ever，there is still a lack in comprehensive comparison and guideline on application-oriented design of GDY with ideal structure.

As shown in Fig.2，this review aims to analyze and discuss how to design target materials according to specific applications.Firstly，the influence of four ma⁃jor factors on GDY’s pore structure including mono⁃mers，catalysts，templates，and solvents will be dis⁃cussed in detail respectively.Secondly，the specific relationships between GDY’s pore structures and ad⁃vanced properties in some typical energy-related fields including water treatment，lithium-ion batteries，and catalysis are described.Finally，insights into the chal⁃lenges and perspectives for three-dimensional（3D）macroporous GDY are provided.

Fig.2 Schematics of GDY’s pore structures design

2 GDY’s Pore Structure Design and Its Influence Factors

Manipulating the pore structure on different scales（i.e.molecular pores or piled pores）involves different factors to be controlled.Additionally，the independent adjustment of molecular pores or piled pores will change GDY’s properties in different ways.Various pore sizes and special electronic cloud distribution will be endowed by the design of molecular pores.However，regulating sizes and morphologies of piled pores will in⁃duce changes in optical properties，mass transfer and energy transfer.Fig.3 presents performance required in the application and its corresponding structure design strategies.Rational control over the molecular pore size and its atomic components are essential for GDY separation membrane to acquire high efficiency and selectivi⁃ty.In catalysis field，appropriate molecular pore size should be managed to ensure activity of catalytic center and intermediate stability.Besides，high surface area and well catalysts dispersion endowed by piled pore de⁃sign will boost mass transfer and stability of reaction in catalytic process.As for energy storage/conversion，constructing larger molecular pore makes it an excellent electrode material for lithium/sodium battery which can significantly reduce resistance for ion transport.And the introduced halide atoms can provide more lithium/sodium ions storage sites.Thus，high capacity of GDY-based battery can be realized.Similar to its applications in catalysis，some favorable piled pore design principles can also be applied to achieving improved performance in energy storage/conversion.As a cathode material of lithium metal battery，the 3D porous carbon serves as a lithium container to avoid lithium dendrite and ensure security.For energy conver⁃sion material，constructing 3D hierarchical structure will lead to significant improvement in mass transfer ab⁃sorption and utilization of energy.Therefore，it is of great significance to synthesize GDY with desirable struc⁃tures and targeted functionalities based on the relationship among synthesis condition，structure，and proper⁃ties.This part summarizes the influence factors and provides guidance for design of pore structures with different scales.

Fig.3 Application⁃oriented design of GDY

2.1 Molecular Pores

According to bottom-up method，monomers are the key factor for constructing molecular pores.Herein，the monomer，structure，size，and synthesis conditions are summarized in Table 1.Fabricating various mono⁃mers by introducing heteroatoms and changing the number of terminal alkynes can manipulate its pore size and even the electron cloud distribution.As shown in Table 1，after the acetylenic groups are substituted by halide atoms，the sub-nanometer pore（0.54 nm）will be increased to nanoscale（1.3 nm，1.6 nm）.Meanwhile，the molecular pores and heteroatoms are evenly distributed on GDY.The molecular pore with appropriate size can be used for anchoring catalysts or optimizing the ion transport rate.So far，there are various monomers havebeen synthesized，which enriches family members of GDY.

Table 1 Molecular pore structures and sizes of GDY prepared by different monomers

Continued

Organic synthesis technology is highly required in monomer design.HEB，the original monomer is ob⁃tained by multi-step substitution on the aromatic core，i.e.six bromine atoms are all substituted by trimethylsilylacetylene（TMSA）.Usually，this reaction lasts 3—7 days［5，28］.However，when there are other groups like—H，—F，—NH2on the aromatic ring，the electron structure of benzene ring will be changed due to the different electronegativity，which will affect the substitution efficiency and even further affect the polymeriza⁃tion process［29—31］.Therefore，we should investigate the influence of electric effect and steric hindrance effect caused by introduced groups when designing monomers.

2.2 Piled Pores

Compared with molecular pores，the structures of piled pores are more macroscopic，and their morpholo⁃gy are more complex.These pores are formed by assembly of GDY，usually presenting typical 3D structure.Until now，GDYs with nanowall structure，nanowire accumulation structure，copper foam skeleton structure，and more complex diatomite structure have been successfully prepared［12—14，32］.Such structural designs lead to significant increase in specific surface area，mass transfer，and light/wave absorption.Therefore，structural design of piled pores is an effective way to enhance performance in catalysis and energy conversion.

The essence of GDY growth is a coupling reaction of terminal alkynes，which is called the Glaser cou⁃pling reaction developed by Glaser［33］.About 100 years later，Eglinton and Hay［34，35］modified this reaction.In Glaser-Eglinton reaction，the coupling reaction is catalyzed by copper acetate，and pyridine acted as base，li⁃gand，and solvent.The other modified reaction（Glaser-Hay reaction）is catalyzed by a catalytic amount of CuCl andN，N，N′，N′-tetramethyl-ethylenediamine（TMEDA）in acetone solvent.In this reaction，TMEDA acts as base and ligand.At present，GDY synthetic methods are mainly based on these two modified reactions.The reaction mechanism is shown in Fig.4，the deprotonated terminal acetylene is oxidized by cop⁃per ions to form a radical intermediate in the Glaser-Eglinton reaction［Fig.4（A）］，and then two radicals form a diacetylene linkage［7］.Compared with Glaser-Eglinton reaction，Hay modified reaction undergoes a Cu（Ⅰ）/Cu（Ⅲ）/Cu（Ⅱ）/Cu（Ⅰ）catalytic cycle［Fig.4（B）］.Dioxygen activation is the key step for the cycle.In this cy⁃cle，TMEDA acted as a base can grab protons on a terminal alkyne.Then two Cu（Ⅰ）species bind a deprotonated acetylide and reaction with O2to form a dicopper-dioxo complex，which is an intermediate for the Cu（Ⅲ）species.Finally the subsequent collapse of the intermediate leads to diacetylene linkage［36］.When compared with the original Glaser reaction，these two modified processes are safer and more efficient［37］.

Fig.4 Mechanism of Glaser⁃Eglinton reaction(A)and Glaser⁃Hay reaction(B)

The templates，catalysts/ligands，and solvents have great impact on GDY’s morphology.For templates，acetylenic coupling reaction takes place at the interface between the target templates and solution.On this ba⁃sis，Zhang’s group［38，39］has developed a general method（copper envelope strategy）to realize growth of struc⁃ture-controlled GDY on arbitrary substrates.That is，GDY will replicate the profile of template.As shown in Fig.5，the template here can be appropriate for any interface such as gas/liquid，solid/liquid，gas/solid，and liquid/liquid.Therefore，the target surface will determine the structure of GDY.

Fig.5 Growth of GDY on gas/liquid interface(A)[40],solid(graphene)/liquid(solvent)interface(B)[41],silver through CVD method(C)[42]and liquid/liquid interface(D)[40]

In addition，the quality of GDY depends on many critical factors，including interaction between template surface and monomer molecules，ability of catalyzing coupling reactions，and lattice matching between tem⁃plate and GDY.It is molecule diffusion，rather than intermolecular coupling，better for construction of regular 2D network［41，43，44］.Most substrates used in GDY synthesis are Cu，because the cross-coupling reaction is modulated by copper complexes［45，46］.However，the surface-assisted coupling reaction is not efficient on a Cu sub⁃strate，due to side reactions.On contrast，Ag substrate is more efficient for a surface-assisted coupling reac⁃tion，which provides less side reactions［42］.Generally，GDY’s piled pores design does not request the high quality.Gaoet al.［38］prepared GDY with 3D porous structure by using copper foam as template.In their later research，they got the GDY 3D hierarchical structure by pretreating the copper foam［15］.

The catalyst and ligand are necessary for the reaction［47］.Generally，catalyst will combine with ligand to form complex，and catalyze the cross-coupling reactions.Ligand plays two roles in reaction process：（1）as a base，it grabs proton from terminal alkyne；（2）after forming the complex，it binds a deprotonated acetylide to produce intermediate（as shown in Fig.5）.Tanget al.［37］proposed that TMEDA can facilitate the reaction by enhancing the solubility of intermediate.Therefore，only a catalytic amount of CuCl and TMEDA are needed in the Hay modified reaction.This catalyst with low amount and high activity has great influence on the struc⁃ture of GDY.Under the initial synthesis conditions（copper foil，pyridine solution system），layered GDY will be acquired［5，48］.However，Zhouet al.［13］fabricated GDY nanowalls on the Cu foil by adding catalytic amount of the organic base（pyridine and TMEDA），and proposed that the slow release of catalyst concentration and the rapid reaction rate are the key to form nanowall.In addition to these two most common ligands（pyridine，TMEDA），piperidine，triethylamine，and TMEDA derivatives，etc.，are also used and tested in coupling re⁃action.Among these，piperidine is considered to be a good candidate for its sufficient alkalinity to deprotonate and enhance solubility of intermediate［36，49］.Therefore piperidine may also be used in the synthesis of GDY，and form a morphology different from that of pyridine and TMEDA.

There are few studies on the influence of solvents，which also play important role in cross-coupling reac⁃tion.The solvents have two main effects.（1）According to Houghes-Ingold transition state theory，charge and configuration will change during the reactant/transition state/products process.Fig.5 shows the terminal alkyne undergoes significant charge and configuration changes during the process to form diyne linkages.Solvation effect of the activated complex is stronger than that of the original reactants due to the ionic charge gene-rated from intermediate.Therefore，increasing the polarity of the solvent can reduce the energy of the transition state，and further reducing the activation energy of the reaction to speed up the reaction process［42，50］.（2）Sol⁃vents can affect the stereo structure of products.The covalent bond betweensp2hybridized carbon atom andsphybridized atom can rotate freely during the cross-coupling reaction.Therefore，the final GDY may not be an ideal plane，and the crystallinity of GDY may also be affected.Electrostatic interactions orπ-πstacking be⁃tween solvent molecules and reactants can restrict free rotation of covalent bond.Thus，the growth and crystal⁃linity of GDY will be controlled by choosing rational solvent［50，51］.

After a well understanding of the influence factors on the GDY’s structure and the demands of targeted ap⁃plications，application-oriented design and synthesis of GDY can be realized.

3 Performance Enhanced by Pore Structure of GDY

Performance of material is usually determined by its structure.Unique design of GDY’s pore structure on different scales will lead to a significant performance improvement.

3.1 Benefits from GDY’s Molecular Pores

3.1.1 Separation

As 2D materials，GDY and its derivatives with pores in nanometer or sub-nanometer scales have a broad application prospect in separation.The size of channel depends on the monomers.Different number of alkynyl on the monomer results in varied molecular pore size.Specifically，size of molecular pores formed after polymerization increase with the decrease in alkynyl number in monomer.When alkynyls in positions 1，3，5 of HEB are substituted by halide heteroatoms（—F，—Cl，—H，etc.），the original uniformly distributed 18Chexagon rings will be extended to form the bigger 42C ring，thus offering a way of tuning their permeability as membrane［29—31］.GDY with different pores can be applied to selective separation of small molecules or atoms.Kimet al.［52］have simulated the process of sieving transition metal atoms byγ-graphyne based on density func⁃tional theory.It revealed that even in a hypothetical aqueous solution，Pd and Au atoms would spontaneously trespassγ-graphyne layer without H2O，rather than anchoring on theγ-graphyne low diffusion energy barrier.Cranfordet al.［53］have proved that GDY with single molecular thickness can effectively separate hydrogen from syngas at ambient temperature and pressure based on molecular dynamics simulation.It takes only 0.11—0.03 eV for a single H2to pass through the GDY’molecular pore.

The selectivity of GDY varies with molecular pore size.Using both first-principle density functional theo⁃ry and molecular dynamic simulations，Zhaoet al.［54］found that GDY substituted one-third diacetylenic linkages with H，F，O atoms（Fig.6）can excellently sepa⁃rate CO2and N2from CH4in a wide temperature range，and even the CO2/N2mixture.The GDY-H has relative⁃ly low energy barriers for hindering CO2，N2，and CH4，while GDY-F and GDY-O exhibit significantly high energy barriers for the CH4.The ability of GDY-O to selective separation of CO2can be ascribed to the elec⁃trostatic attraction between the modified O atoms in the pore and the C atom in CO2，which largely stabilizes the transition state.However，there are no obvious in⁃teractions in GDY-H and GDY-F for CO2penetration process.

Fig.6 Structures(A—C)and pore electron den⁃sity isosurfaces(D—F)of GDY⁃H(A,D),GDY⁃F(B,E)and GDY⁃O(C,F)mono⁃layers[54]

GDY with designed molecular pores can also be used in sea water desalination.A number of theoretical and computational studies predict that graphyne with certain pore structure［such as graphyne-3，showed in Fig.7（A）］possesses high permeability and high selec⁃tivity，allowing water molecules to pass through while blocking ions.Fig.7（B）shows the schematic diagram of graphyne used as membrane material for seawater desalination.Driven by pressure，water molecules can pass through the uniformly distributed molecular pores of monolayer graphyne，while the ions are blocked on the other side，achieving effective seawater desalination.According to simulation results obtained by Xueet al.［22］，α-garphyne，β-garphyne and graphyne-3 are potential candidates for seawater desalination.With complete ion rejection and water transport at higher speed［Fig.7（C）and（D）］，their performances are all better than reverse osmosis（RO）membranes.

3.1.2 Catalysis

As a promising carrier of a single atom，graphyne can be used in various specific catalytic reac⁃tions［55—58］.Metal atoms such as Ni，Fe and Cu tend to anchor at the corner of GDY’s triangle molecular pore near thesp-C atoms through strong chemisorption.By loading Ni and Fe single atom on GDY，Xueet al.［55］have first realized high active single atoms catalysis（SAC）with zero valence single atoms.In their following research，this group constructed zero valence Cu single atoms by anchoring Cu atoms on GDY，which has high activity and good selectivity for hydrogen evolution reaction（HER）.More importantly，they have found that the zero valence state of the atomic-scaled transition metal catalyst attributed to strongp-dcoupling induced charge compensation［56］.Thus it is believed that structure of supports will largely determine the activity of cata⁃lytic sites due to its influence on the coordination environment around single atoms and the stability of interme⁃diates at the active site［59］.The molecular pore size of graphyne substrate has great impact on the coordination number of metal atoms and the stability of intermediates generated in carbon dioxide reduction process.Niet al.［60］explored the role of supports in tuning the CO2electrocatalytic activity through density functional theory.Fig.8 shows the results that support skeletons with different pore sizes could greatly impact the coordi⁃nation configuration of metal atoms and the steric repulsion of support skeletons to intermediates.On the one hand，with the increase of pore size，the coordination number of Cu in the plane decreases，leading to the im⁃provement of catalytic activity.On the other hand，the increase of pore size results in the reduce of repulsion between intermediate*COOH and the carbon skeleton，which leads to a constant decrease of free energy level and enables intermediates to become more and more stable.Therefore，efficiency of the reaction will be im⁃proved，and onset potentials of reduction products will be reduced.

Fig.7 Structures ofα⁃garphyne,β⁃garphyne andγ⁃garphyne(A),schematic diagram of graphyne used for seawater desalination(B),performance comparison between graphyne and commercial reverse osmo⁃sis membranes(C)and single⁃pore flow rates ofα⁃garphyne,β⁃garphyne and graphyne⁃3 at different pressures(D)[22]

Fig.8 Coordination number of Cu atoms and the positions of intermediates vary with molecular different pore sizes(A)and onset potentials of the products of CO2 reduction on Cu⁃Graphynes(B)[60]

3.1.3 Energy Storage

According to the calculating results，single atomic hollow structure of graphynes made up ofsp-andsp2-hybridized carbon atoms enhances not only the ions（Na+，Li+，etc.）storage capacity but also the ions（Na+，Li+，etc.）diffusion.Therefore，as 2D porous material，graphynes make themselves promising candidates for the anode material used in energy storage devices.It was proved in some simulation research that the maxi⁃mum Na/Li storage concentration is far exceeding the upper limit of Na/Li insertion into graphite［61，62］.Based on that，researchers continuously improve device performance by changing molecular pores size and the substi⁃tuted atoms around the pores.Liet al.［5］synthesized bulk GDY for the first time and applied it in lithium ion batteries.The obtained GDY-based battery showed a reversible capacity of 520 mA·h/g after 400 cycles at a current density of 500 mA/g.After that，in 2017，Li’s team［26］fabricated new monomer with chlorine atoms substituting in positions 1，3，5 of HEB to synthesize chlorine-substituted GDY，which had a larger pore size.To our knowledge，reported size characterization analysis on GDY’s molecular pore is mainly relied on nitro⁃gen adsorption-desorption isotherms combine with density functional theory（DFT）calculation.According to DFT calculation results，the pore size of chlorine-substituted GDY，isca.1.6 nm.The modified GDY with a larger pore size provides lithium ions larger transport channel along the perpendicular direction of the plane and more lithium storage sites［Fig.9（A）］.Therefore，the capacity can be significantly improved［Fig.9（B）］.A capacity of 1150 mA·h/g at a current density of 50 mA/g was obtained.Subsequently，the performance of lithium batteries was further improved with the successful synthesis of GDY substituted by F［11］.According to nitrogen adsorption-desorption isotherms and DFT calculation results，the main pore size distribution for F-GDY isca.0.55 and 1.3 nm based on two different kinds of stacking model.The structure of F-GDY and storage sites of lithium ions were shown in Fig.9（C）.Lithium storage in C-F semi-ionic bonds and molecular pores ensure the high energy and power density.The reversible capacity can reach 1700 mA·h/g at a current density of 50 mA/g.

Fig.9 Storage sites of lithium ions in Cl⁃substituted GDY(gray sphere:C,green sphere:Cl,orange sphere:Li)(A),the cycling performance of Cl⁃substituted GDY in lithium batteries(B)[26],storage sites of lithium ion in F⁃substituted GDY(gray sphere:C,yellow sphere:F,purple sphere:Li)(C)and cycling performance of F⁃substituted GDY in lithium batteries(D)[11]

3.2 Benefits from the Piled Pores of GDY

For the better application of bulk GDY，it will escape from the surface loss caused by layer stacking of GDY during growth process［63］.The piled pores design，i.e.construction of 3D structure，resulted in better pore connectivity，which can avoid the surface area loss，extend mass transfer and surface exposure，leading to improvement in catalytic activity and energy conversion efficiency［15，64，65］.Compared with the studies on gra⁃phene［66—68］，the research on the piled pores of 3D graphyne is just started，but it shows a very promising appli⁃cation prospect.

3.2.1 Energy Storage

Fig.10 Three⁃dimensional GDY nanosheets on copper foil substrates prepared by a modified Glaser⁃Hay cou⁃pling reaction(A),the cycle performance of sodium batteries based on 3D GDY nanosheets at the current density of 1 A/g(B)[69],schematic for weaving the network of ultrathin GDY nanosheets on the Si anode(C),SEM images of ultrathin GDY nanosheets on the Si anode(D)and the performance of the as⁃prepared sample at a high current density of 2 A/g(E)[23]

Wanget al.［69］have prepared porous 3D GDY by a modified Glaser-Hay coupling reaction.As shown in Fig.10（A），this 3D porous structure enables to connect more active sites and enhance mass transfer，promoting the rapid transfer of ions between different layers and facilitating ions migration and diffusion.When it was used in sodium batteries，a capacity of 405 mA·h/g at a current density of 1 mA/g was obtained［Fig.10（B）］.Shanget al.［23］have directly constructed a 3D GDY networks with excellent mechanical properties and electrical conductivity on the silicon negative electrode.The structure of this electrode material is showed in Fig.10（C）and（D）.The seamless contact between all-carbon interface and electrode modules can effectively inhibit the damage of the conductive network and electrode interface，caused by the huge volume change of the silicon negative electrode during the cycling process，and give full play to its high specific capacity.The asprepared lithium battery showed a reversible capacity of 4122 mA·h/g at a current density of 0.2 mA/g and even retained high specific capacity of 1503 mA·h/g after 1450 cycles at a current density of 2 mA/g［Fig.10（E）］.

In addition to inhibiting surface loss and promoting mass transfer，the piled pores can also serve as storage sites.For example，Shanget al.［24］have prepared GDY on the surface of independent 3D copper nanowires［Fig.11（A）］and applied it to lithium metal batteries.The super lithiophilicity ofsp-hybridization carbon atoms in GDY allows lithium to deposit on piled pores［Fig.11（B）］，which can inhibit the formation of lithium dendrite，thus improving the safety，lifespan，and performance of the battery［Fig.11（C）］.

Fig.11 3D copper nanowires@GDY(A),lithium deposited in the pores of 3D copper nanowires@GDY(B)and stability of lithium metal batteries based on 3D copper nanowires@GDY(C)[24]

Efficient absorption and utilization of light and wave can be realized by multi-reflection and refraction effect of 3D porous structure，which provides an effective way to achieve high-efficient energy conversion［70，71］.Gaoet al.［15］designed and synthesized GDY with a 3D hierarchical structure using copper foam as template.Fig.12（A）—（C）shows the structure of 3D material Cu@GDY.Mechanism of enhancing light adsorption is illustrated in Fig.12（F），the 3D material Cu@GDY can promote the absorption of light through multi-reflec⁃tion and refraction and realize high light to heat conversion efficiency.Under 1 kW/m2illumination，the mate⁃rial Cu@GDY can achieve energy absorption of 94%and water evaporation rate of 1.55 kg·m−2·h−1.

3.2.2 Catalysis

The design of a 3D porous catalyst carrier can not only effectively prevent the aggregation effect of the catalyst，but also promote mass transfer and improve the catalytic performance by increasing the contact surface［72］.Liet al.［12］have prepared high-quality freestanding 3D GDY by using hierarchical porous diatomite as template［Fig.13（A）and（B）］.When loaded with rhodium nanoparticles［Fig.13（C）—（E）］，the highly dispersed 3D structure can effectively prevent aggregation of rhodium nanoparticles，which displays good activity and stability in the catalysis of p-nitrophenol hydrogenation.3.2.3 Others

Fig.12 SEM images of 3D GDY at different scales(A—C),water evaporation rate(D),evaporation stability(E)and schematic diagram of promoting light absorption(F)[15]

Fig.13 SEM images of 3D diatomite shaped GDY(A,B),schematic illustration of 4⁃nitrophenol reduction cata⁃lyzed by Rh@3DGDY(C),TEM image of Rh@3DGDY(D)and time⁃dependent UV⁃Vis absorption spec⁃tra recorded during the 4⁃nitrophenol reduction catalyzed by Rh@3DGDY(E)[12]

In addition to examples mentioned above，GDY with piled pores can be also used into other fields.GDY nanotubes that own excellent field emission performance have been successfully preparedviaan aluminum oxide template strategy［14］.Gaoet al.［73］coated polydimethylsiloxane on the as-prepared GDY nanowall to make it super hydrophobic，which can be used for efficient oil-water separation.Liuet al.［74］have applied GDY with 3D porous structure to removed lead ions from lead-polluted water，thus achieving the purpose of water purification.

4 Summary and Outlooks

GDY has been hugely developed since its first synthesis.Its diversification in abundant structural design and broad application attract considerable attention.Exploring the relationship between structure and the target application is of significance for realizing application-oriented design.Herein，four major factors for GDY’s pore structural design including monomers，catalysts，templates，and solvents are discussed in detail in this review.In terms of demands in target application，the guideline of how to design materials is proposed.

By fabricating new monomers based on HEB，the size of molecular pores can be adjusted directly，which has made a figure in the field of energy storage.However，molecular pores design towards catalysis and separa⁃tion has just started.Theoretical research results have predicted a promising future of GDY’s molecular pores design.In addition，synthesis of single or few-layer GDY with high quality is the cornerstone to link theoreti⁃cal simulation and practical application.At present，researchers have devoted much effort in synthesizing high-quality GDY and scaling up its production，such as few-layer single-crystalline GDY obtained by van der Waals epitaxial strategy［41］，few-layer GDY by oxidation exfoliation［48］，few-layer GDY by microwave-assist growth（thickness＜2 nm）on the surface of sodium chloride［75］，monolayer GDY by exfoliation with the assis⁃tance of Li2SiF6［76］.However，scientists still need to spare no efforts to reinforce the cornerstone.Besides，characterization analysis methods are also important to GDY’s molecular pores and their applications.It’s necessary to find more direct characterization techniques to characterize GDY’s molecular pore structures and their interaction with molecules when in practical applications.It will be of great significance to GDY related research field.

In addition，the construction of GDY with 3D structure has shown a very charming application prospect in catalysis and energy conversion.Although just started，the structural advantages have received much atten⁃tion.For design of GDY’s molecular pore，scientists have prepared a series of monomers，however，for piled pore，there are only a few 3D templates with unique structure are reported.Thus，It’s necessary to design templates with advanced structures in the future to achieve the large effective surface area and enhanced mass transfer.Considering the bottom-up method，the structure of GYD can be linked with arbitrary complex templates.In summary，manipulating pore structure from molecular pore to nano-micron pore will deepen understanding of material transporting in GDY and give full play to its structural characteristics and advantages.

This work is supported by the National Key Research and Development Program of China（No.2018YFA0703504）and the National Natural Science Foundation of China（Nos.51932001，21971244）.