Controlled Synthesis of Graphdiyne-Based Multiscale Catalysts for Energy Conversion

Abstract

Graphdiyne (GDY) science is a new and rapidly developing interdisciplinary field that touches on various areas of chemistry, physics, information science, material science, life science, environmental science, and so on. The rapid development of GDY science is part of the trend in development of carbon materials. GDY, with its unique structure and fascinating properties, has greatly promoted fundamental research toward practical applications of carbon materials. Many important applications, such as catalysis and energy conversion, have been reported. In particular, GDY has shown great potential for application in the field of catalysis. Scientists have precisely synthesized a series of GDY-based multiscale catalysts and applied them in various energy conversion and catalysis research, including ammonia synthesis, hydrogen production, CO₂ conversion, and chemical-to-electrical energy conversion. In this paper, we systematically review the advances in the precisely controlled synthesis of GDY and aggregated structures, and the latest progress with GDY in catalysis and energy conversion.

1. Introduction

Graphdiyne (GDY) is a rapidly rising star on the horizon of materials science and has shown exceptionally unique chemical and electronic structures and fascinating properties, such as rich carbon chemical bonds, naturally distributed pores, unevenly distributed surface charge, incomplete charge transfer, highly conjugated and large π structures, etc. These are superior characteristics that traditional carbon materials do not have. GDY is the only carbon material that can be controllably grown on any substrate and can “do chemistry”, providing new opportunities for developing advanced materials and opening broad spaces for the development of GDY-based materials.^1,2

Graphdiyne research has accelerated exponentially since 2010, when Li and co-workers realized the controlled synthesis of GDY for the first time through a facile chemical synthesis method.³ The sp and sp² co-hybridized two-dimensional (2D) one-atom-thick all-carbon network endows it with attractive chemical and electrical structures, versatile properties, and great potential in many fields, including nitrogen reduction reaction (NRR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), CO₂ reduction reaction (CO₂RR), etc. GDY is currently one of the most favored carbon materials, and “Graphdiyne Research” was selected and listed in the Top 10 research areas in the 2020 Research Frontiers report jointly released by the Institutes of Science and Development of the Chinese Academy of Sciences (CASISD), the National Science Library of CAS, and Clarivate.⁴⁻⁷

The present Review first introduces the main structure and properties of GDY, followed by a discussion of the advances in the precisely controlled synthesis of GDY and GDY-based multiscale catalysts, including the GDY-based zerovalent atom catalysts, quantum dot catalysts, heterostructured catalysts, and metal-free catalysts. Finally, we highlight the latest progress with GDY in catalysis and energy conversion.

2. Structure and Properties of Graphdiyne

Graphdiyne is a new allotrope of carbon with a 2D highly conjugated structure consisting of sp² (benzene rings) and sp (acetylenic linkers) carbon atoms (Figure 1a), presenting four types of carbon–carbon bonds: C_sp²–C_sp² bonds (1.41 Å) of benzene rings, C_sp²–C_sp single bonds (1.40 Å), C_sp–C_sp triple bonds (1.24 Å), and C_sp–C_sp single bonds (1.33 Å).^5,7 The d-spacing of the (110) plane in a GDY monolayer was predicted to be 0.474 nm. The optimized crystal parameters of GDY were calculated by Liu et al., where a, b, c, and θ were 9.38 Å, 9.38 Å, 3.63 Å, and 120°, respectively.⁸ There are three stable stacking modes—AA, AB, and ABC—for GDY.⁹ As evidenced by high-resolution transmission electron microscopy (HRTEM) images, the ABC stacking mode was the only form present for all GDY nanosheets prepared experimentally (Figure 1b–d).^10,11

Figure 1.

(a) Schematic diagram of graphdiyne (GDY). (b) GDY model with ABC stacking mode. (c) Simulated SAED pattern of the ABC stacking model. (d) HRTEM images of crystalline GDY. Reproduced from ref (19). Copyright 2018 Springer Nature. (e) Simulated model of ABC-stacked GDY. (f, g) HRTEM and (h) SAED images of all-crystalline GDY. Reproduced from ref (18). Copyright 2023 Elsevier Ltd. (i) TEM and (j) magnified HRTEM images of GDY. (k, l) SAED patterns and HRTEM image. Reproduced from ref (20). Copyright 2023 John Wiley and Sons. (m) TEM image of eGDY films, with the corresponding SAED pattern inset. (n) HRTEM and (o) STEM images of GDY. (p) AFM image of single-layered GDY. Reproduced from ref (27). Copyright 2018 John Wiley and Sons.

Graphdiyne possesses a direct natural band gap that was calculated to be from 0.46 to 1.10 eV, based on different calculation principles.^5,12 Remarkably, it is easy to controllably tune the band gaps of GDY by various strategies, including topologically driven modulation,^13,14 application of tensile strain,¹⁵ chemical modification,¹⁶ and so on. For example, Casari et al. reported that, by varying the sp-C/sp²-C ratio, the relative energies and the band structure of GDY could be easily changed.¹³ In addition, GDY possesses excellent carrier transport ability, with both high in-plane electron mobility and high hole mobility at ambient conditions.⁷ Because of its outstanding electrical characteristics and natural semiconductor band gaps, GDY shows great potential for applications in semiconductor and optoelectronic devices.

3. Controlled Synthesis of Graphdiyne and Derivatives

3.1. Controlled Synthesis of Graphdiyne

The successful synthesis of a large-area 2D GDY film was first realized by Li and co-workers through a cross-coupling strategy, in which hexaethynylbenzene (HEB) was used as monomer and copper foil served as both substrate and catalyst.³ Briefly, copper ions were released from the Cu foil in the presence of pyridine and rapidly formed a pyridine–Cu complex to catalyze the cross-coupling of HEB on the surface of the Cu foil to grow the GDY film with large area. Guided by this pioneering work, great efforts have been made toward the controlled synthesis of well-defined GDY with various morphologies, optimized crystalline structure, and few layers.^5−7,17 Li’s group reported a surface-induced assembly coupling strategy for the controlled synthesis of GDY films on the surface of carbon fibers.¹⁸ HRTEM and selected area electron diffraction (SAED) results confirmed the all-crystalline nature of the synthesized GDY samples (Figure 1e–h) with a typical hexagonal lattice, in which the d spacing of 0.45 nm corresponded to the (11–20) reflections of GDY. Crystalline GDY can also be obtained through slowly dropping HEB monomer into a mixture of pyridine, acetone, and TMEDA, followed by in situ growth on Cu foil.¹⁹ The experimental results corresponded well to the simulated model with ABC stacking mode. Similarly, Luan et al. reported the controlled growth of a GDY film with a well-defined crystalline structure on Zn foil (Figure 1i–l).²⁰ Fu et al. reported the controlled in situ growth of crystalline GDY nanosheets array on a 3D melamine sponge.²¹ The interface-induced growth method was another useful strategy to synthesize crystalline GDY. A gas/liquid interfacial synthesis was carried out by Nishihara et al. in which only a few HEB monomers were contained in the gas phase and Cu²⁺ ions existed in the liquid phase.²² Uniform single-crystalline GDY nanosheets were generated at the gas/liquid interface at ambient conditions. Mao et al. constructed a liquid/liquid interface of microemulsions by vigorously stirring two immiscible liquid phases which contained HEB and Cu²⁺ ions, respectively, where crystalline GDY hierarchical hollow microspheres were grown.²³ Moreover, solution-phase van der Waals epitaxial growth was also an effective strategy for the synthesis of ultrathin crystalline GDY using graphene as the template.¹⁰

Recently, the controlled synthesis of few-layer GDY has been reported.^10,24−27 The solution-phase van der Waals epitaxy strategy reported by Gao and Zhou was an effective strategy for the synthesis of ultrathin crystalline GDY films in which graphene was used as the template.^10,24 The absorption and Eglinton coupling reaction preferentially occurred on the surface of graphene rather than GDY because HEB possessed a higher binding energy on graphene, especially at low coverage, which resulted in the formation of a few-layer single-crystalline GDY film. A microwave-induced temperature gradient strategy was used by Yin et al. to synthesize ultrathin GDY at a solid/liquid interface.²⁵ In this method sodium chloride was used as the solid substrate due to its ability to absorb microwave energy, while a mixture of toluene and hexane was selected as the non-microwave-absorbing solvent. With the effect of the temperature gradient generated around the interface between the solution and NaCl substrate under microwave radiation, the growth of HEB was restricted on the surface of NaCl, resulting in the formation of cuboidal GDY boxes. Consequently, ultrathin crystalline GDY was synthesized successfully through this rapid and catalyst-free method. Chemical/electrochemical exfoliation is another effective strategy to obtain few-layer GDY. For example, a lithium-intercalation process was carried out by Parvin et al. to exfoliate bulk GDY into few-layer GDY nanosheets.²⁶ A facile damage-free liquid-phase expansion and exfoliation method was reported by Mao et al. for the preparation of few-layer and even monolayer GDY films from GDY bulk with the effect of Li₂SiF₆ (Figure 1m–p).²⁷ As shown in Figure 1n, a typical lattice fringe of 0.455 nm was observed, indicating the crystalline structure of as-prepared GDY flakes. The thickness of the GDY flakes was measured by atomic force microscopy (AFM), which indicated the GDY flakes were in the few-layer form. In addition, monolayer GDY films can be synthesized through a chemical vapor deposition (CVD) method with HEB as the precursor at low temperature.²⁸ Zhang et al. proposed an electric double layer (EDL)-confined strategy for the synthesis of ultrathin wafer-scale GDY films with highly crystalline structure and large area.²⁹ In a general procedure, copper ions released from the cathode of Cu foil reacted with organic amines to form the catalytic Cu complex, which was confined on the surface of the cathode and served as the EDL. As a result, the cross-coupling of HEB only occurred in ultrathin EDL, and thus a few-layered GDY film was obtained.

3.2. Precisely Controlling the Morphologies of Graphdiyne

Morphology can significantly affect a variety of properties of materials, and thus morphology-controlled growth is of great importance to achieve precise synthesis of materials. Examples of GDY with various well-defined morphologies have recently been reported, including 2D films/nanosheeets,^{22,24,28,30−33} 1D nanowires/nanotubes,³⁴⁻³⁶ 0D nanospheres,³⁷ and so on. The controlled synthesis of 2D GDY films has been realized through various strategies following the pioneering work of Li et al. For example, a catalytic pre-growth and solution polymerization method was developed by Gong et al. for precisely controlling the structure and thicknesses of as-prepared GDY films on various substrates with the deposition of Cu catalytic sites.³⁰ Kong et al. reported the rapid and controlled synthesis of GDY thin films with adjusted thicknesses and large-scale uniform morphology.³¹ The GDY films were grown rapidly on the constructed liquid/liquid interface under the catalysis of Cu(II)-TMEDA at room temperature, in which the thickness of GDY films was effectively adjusted from 4 to 50 nm by adjusting the concentration of HEB and the reaction time. An interface-confined strategy was conducted by Nishihara et al. for the controlled growth of multilayer GDY nanosheets with well-defined crystalline structure through a successive alkyne–alkyne homocoupling process at liquid/liquid or gas/liquid interfaces.²² Moreover, ultrathin GDY nanosheet (∼1.0 nm in thickness) arrays could be controllably synthesized through precisely adjusting the concentration of HEB and reaction time (Figure 2a–d).³⁸ By using a solution-phase van der Waals epitaxy strategy, Gao et al. achieved the synthesis of thin GDY films using graphene as the growing template (Figure 2e–h).^10,24 Zhang et al. reported an effective approach to synthesize well-defined GDY nanowalls.³² In their work, additional organic alkali TMEDA was applied to promote the reaction rate of Glaser–Hay coupling and adjust the amount of copper ions dissolved in the solution. Therefore, with the gradual increase in the concentration of copper ions, HEB monomers grew vertically at catalytic sites on copper substrate, and thus GDY nanowalls formed. Another facile method was developed to synthesize well-defined GDY nanosheet arrays on arbitrary substrates at room temperature.^33,39 In a general procedure, the substrate enveloped by Cu foil was immersed in the HEB solution, followed by reaction under dark conditions at room temperature for the growth of the GDY nanosheets array. Xue et al. reported the synthesis of Cu@GDY nanowires array using a Cu nanowire as the substrate.³⁴ Mao et al. reported the controlled preparation of GDY hollow microspheres through a facile interface-induced growth strategy.²³ Through vigorously stirring and then standing in a dark environment at room temperature, the coupling reaction of HEB monomers was carried out in the microemulsion formed at the interface between aqueous and organic phases, which contained Cu²⁺ ions and HEB, respectively. As a result, dark-brown GDY hierarchical hollow microspheres were obtained (Figure 2i,j). Yu et al. successfully synthesized GDY nanospheres through a template confined growth strategy.³⁷ Generally, the HEB monomers were absorbed in the mesoporous channels of Pd/mSiO₂, and then the GDY nanospheres were in situ grown at once when a slight explosion procedure was carried out (Figure 2k,l). It is worth noting that the as-prepared GDY nanospheres exhibited excellent wettability and electronic modification ability, which matched well with the catalysts for gas–liquid–solid triphase reactions. Qi et al. reported the synthesis of nanowire-structured heterogeneous MnCo₂O₄/GDY arrays on carbon cloth (Figure 2m,n).⁴⁰ Additionally, a vapor–liquid–solid (VLS) growth process was applied for the controlled synthesis of GDY nanowires.³⁵ The as-prepared GDY nanowires exhibited a defect-free surface with lengths ranging from 0.6 to 1.8 mm. The synthesis of GDY nanotubes was first reported by Li and co-workers via an anodic aluminum oxide template method.³⁶ HEB monomers were adsorbed and cross-coupled on an anodic aluminum oxide substrate in the presence of a pyridine–Cu complex, followed by selective etching of the template in NaOH solution, and as a result GDY nanotubes were obtained. The wall thickness of as-prepared GDY nanotubes would decrease from about 40 nm to 15 nm after annealing. Cuboidal GDY boxes were synthesized by Yin through a rapid and catalyst-free method with sodium chloride as the solid substrate.²⁵ NaCl@GDY boxes were synthesized, in which the in situ growth of GDY was restricted on the surface of NaCl by the effects of a temperature gradient (Figure 2o,p).

Figure 2.

Controlled synthesis of graphdiyne with different morphologies. (a, b) SEM images of an ultrathin GDY nanosheets array. (c) AFM image of an ultrathin GDY nanosheet. (d) TEM image of an ultrathin GDY nanosheet. Reproduced from ref (38). Copyright 2021 John Wiley and Sons. (e) Schematic illustration of a GDY film grown on graphene. (f) Optical microscopy image and (g) SEM image of GDY/graphene films. (h) AFM image of a GDY film grown on graphene. Reproduced from ref (24). Copyright 2019 American Chemical Society. (i) Schematic diagram and (j) TEM image of GDY hierarchical hollow microspheres. Reproduced from ref (23). Copyright 2023 Springer Nature. (k) Schematic diagram and (l) TEM image of GDY nanospheres@Pd/mSiO₂. Reproduced from ref (37). Copyright 2022 John Wiley and Sons. (m, n) Low- and high-magnification SEM images for NW-MnCo₂O₄/GDY. Reproduced from ref (40). Copyright 2021 John Wiley and Sons. (o) Schematic diagram and (p) TEM image of cuboidal GDY boxes. Reproduced from ref (25). Copyright 2020 John Wiley and Sons.

3.3. Controlled Synthesis of Graphdiyne Derivatives

Because GDY has the important property that it can “do chemistry”, GDY could serve as the ideal platform for precise synthesis of various derivatives with desired structures and properties by simply changing the length of alkyne linkers, heteroatoms (e.g., hydrogen, nitrogen, oxygen, halogen, etc.), and functional groups (e.g., triphenylamine-, tetraphenylmethane-, triphenylene-cored, etc.) of the precursors.⁴¹⁻⁴³ Zhang et al. realized the synthesis of graphyne through a dynamic covalent synthetic approach, in which two types of modified monomers were used as the co-monomers.⁴⁴ Graphtetrayne (GTTY) with a highly ordered crystalline structure was controllably synthesized by Pan et al. through the cross-coupling reaction of hexa(buta-1,3-diyn-1-yl)benzene monomers on copper substrate.⁴⁵ Lu et al. synthesized a 3D pyrenyl-modified GDY with 1,3,6,8-tetraethynylpyrene as the monomer.⁴⁶ Further, by utilizing heteroatom-substituted HEB as the monomer, various heteroatom-doped GDY derivatives with well-defined structures can be precisely synthesized. For example, Li et al. reported the synthesis of fluorographdiyne nanosheets through in situ cross-coupling of 1,3,5-triethynyl-2,4,6-trifluorobenzene on carbon cloth.⁴⁷ Wang et al. controllably synthesized the 2D phosphine–graphdiyne with a well-defined crystalline structure (Figure 3a–c).⁴⁸ It was noted that the strong noncovalent interaction between phosphine–graphdiyne layers leads to the dynamic assembly behavior of p−π conjugation, which was important for the realization of reversible Li ion transport with high responsiveness. Hydrogen-substituted GDY (HsGDY) could serve as an ideal material for the development of novel devices in energy storage and microelectronics because of its excellent stability and conductivity.⁴⁹⁻⁵¹ Cai et al. reported the controlled preparation of HsGDY films through the cross-coupling of 1,3,5-triethylbenzene in a CVD process (Figure 3d,e).⁴⁹ Chlorine-substituted GDY⁵² and boron-GDY⁵³ were designed and synthesized through a similar bottom-up procedure (Figure 3f,g). With their modified band gap and optimized electronic structure, heteroatom-substituted GDY derivatives were widely applied in the field of catalysis and energy conversion. Moreover, Qiu et al. comprehensively studied the synthesis of GDY nanostructures through a homocoupling method in which 1,3-bis(2-bromoethynyl)benzene and Au(111) were used as the monomer and substrate, respectively.⁵⁴ It was found that high-yield production of GDY macrocycles could be realized by stepwise demetallization and cyclization at optimized conditions (Figure 3h–j).

Figure 3.

(a) Schematic representation of the assembly and dynamic conversion of phosphine–graphdiyne. (b) Schematic diagram of phosphine–graphdiyne. (c) HRTEM image of phosphine–graphdiyne. Reproduced from ref (48). Copyright 2023 American Chemical Society. (d) Schematic diagram of the synthesis process of hydrogen-substituted GDY (HsGDY). (e) AFM image of the as-prepared HsGDY film. Reproduced from ref (49). Copyright 2022 American Chemical Society. (f) Schematic diagram of chlorine–graphdiyne. Reproduced from ref (52). Copyright 2017 John Wiley and Sons. (g) Schematic representation of boron–graphdiyne. Reproduced from ref (53). Copyright 2018 John Wiley and Sons. (h, i) AFM images of GDY macrocycles. (j) STM image of the close-packed GDY macrocycles and a few demetallized organometallic rings. Reproduced from ref (54). Copyright 2018 American Chemical Society.

4. Controlled Synthesis of Graphdiyne-Based Multiscale Catalysts

4.1. Graphdiyne-Based Zerovalent Atom Catalysts

Atomic catalysts constructed by precisely anchoring zerovalent metal atoms on appropriate substrate materials display unique structures and superior properties, which have been widely applied in many fields of catalysis and energy conversion.⁵⁵ Atomic catalysts could serve as the ideal platform for gaining in-depth understanding of the anchoring behavior of metal atoms on support, the interactions between anchored metal atoms and support, the energy, electron transfer, and conversion behaviors, and the relationship between electronic structure and catalytic performance. However, traditional carbon materials cannot fulfill these requirements due to the weak stabilization effect as well as uncertain charge-transfer behavior of anchored metal atoms.^7,17 Li et al. reported the first zerovalent atom catalyst, made by anchoring zerovalent Ni and Fe atoms on GDY.⁵⁶ Benefiting from the unique acetylene-rich porous structure of GDY, as well as the incomplete charge transfer and spatial-confined effects between GDY and Fe/Ni atoms, the zerovalent Fe/Ni atoms were firmly anchored and uniformly dispersed on GDY, which could be clearly observed in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 4a–d). There was no signal of metal–metal bonds and metal aggregates in X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and X-ray photoelectron spectroscopy (XPS), confirming the only presence of single isolated metal atoms. Notably, Figures 4b and 4d indicate that Ni (Fe) atoms in Ni⁰/GDY (Fe⁰/GDY) exist in the metallic zerovalent state, stabilized through the strong incomplete charge transport between metal atoms and adjacent alkyne bonds. This electrochemical synthesis method was fast, scalable, controlled, and efficient, and thus it was universally applied for the precise synthesis of many other GDY-based atomic catalysts, including Ir⁰/GDY, Pt⁰/GDY, Mn⁰/GDY, and so on.⁵⁷⁻⁶⁰

Figure 4.

(a) HAADF-STEM image of Ni/GDY. (b) Ex situ EXAFS spectra of Ni/GDY and Ni foil at the Ni K-edge. (c) HAADF-STEM image of Fe/GDY. (d) Ex situ EXAFS spectra of Fe/GDY and Fe foil at the Fe K-edge. Reproduced from ref (56). Copyright 2018 Springer Nature. (e) HAADF-STEM image of Mo⁰/GDY. (f–h) Configurations of Mo atoms anchored on GDY. Reproduced from ref (61). Copyright 2019 American Chemical Society. (i) Atomic-resolution HAADF-STEM image and (j) elemental mapping of atomic catalyst Pd₁/GDY/G-O. Reproduced from ref (62). Copyright 2019 John Wiley and Sons. (k) HAADF-STEM image of Ru SAs/GDY/G. (l) EXAFS of Ru SAs/GDY/G and corresponding fitting curves. Reproduced from ref (63). Copyright 2023 American Chemical Society. (m–p) HAADF-STEM images of CuEr-GDY. Reproduced from ref (18). Copyright 2023 Elsevier Ltd.

It is of great significance to replace the traditional Haber–Bosch process with facile electrocatalytic production of ammonia (NH₃), in which the development of highly effective catalysts plays an important role. To address this challenge, our group reported the first GDY-based zerovalent Mo atomic catalysts (Mo⁰/GDY; mass loading up to 7.5 wt%) for electrocatalytic synthesis of NH₃ at room temperatures and ambient pressures (Figure 4e–h).⁶¹ Mo⁰/GDY exhibited excellent selectivity and activity in both the electrochemical nitrogen reduction reaction (ECNRR) and HER at ambient temperatures and pressures, being superior to previously reported ECNRR catalysts. By dropwise addition of a solution of Pd(NO₃)₂ to the GDY suspension in an alkaline environment under heated conditions, single Pd atoms were successfully anchored on a well-designed graphdiyne/graphene heterostructure (Figure 4i,j) reported by Zhang, which showed excellent electrocatalytic performance for aromatic nitroreduction.⁶² Ru metal atoms were anchored on graphdiyne/graphene substrate through a similar impregnation method (Figure 4k,l).⁶³ Moreover, the results reported by Yin demonstrated that the coordination environment of GDY-based Pt single-atom catalysts (SAs) can be precisely engineered by employing appropriate preparation methods.⁶⁴ The five-coordinated C₁-Pt-Cl₄ was transferred into a four-coordinated C₂-Pt-Cl₂ structure after annealing at 200 °C under an argon atmosphere, and the latter showed superior catalytic activity for HER. Compared with single-metal atomic catalysts, bimetal atomic catalysts have specific advantages and properties for multiple-step catalytic processes. Our group first reported the controlled synthesis of CuEr/GDY bimetal atomic catalysts by anchoring Cu and Er bimetal atoms on GDY simultaneously.¹⁸ HAADF-STEM images (Figure 4m–p) together with elemental mapping results proved that Cu and Er atoms were simultaneously anchored on GDY in the form of atomic pairs. Further, theoretical calculations have been widely applied for the design and screening of high-performance GDY-based atomic catalysts in the past several years.⁶⁵⁻⁶⁸ For example, Li et al. predicted a large potential range for the precise synthesis of GDY-based atomic catalysts through theoretical simulation.⁶⁵ It was found that single metal atoms could remain during the electrochemical oxidation process because of their strong interaction with GDY, while larger metal species were all etched. Based on this result, a facile method was experimentally developed for the synthesis of GDY-based atomic catalysts. Huang et al. comprehensively screened and discussed the construction of highly stable GDY-based diatomic catalysts utilizing machine learning.⁶⁸ It was indicated that the strong f–d orbital coupling in GDY-based diatomic catalysts consisting of a transition metal and a lanthanide metal could effectively enhance the activity and stability due to the modification of the d-band center and electronic structure, which was of great significance for the synthesis of GDY-based diatomic catalysts.

4.2. Graphdiyne-Based Quantum Dot Catalysts

Quantum dot (QD) catalysts with extremely small size and quantum effects bring infinite active sites for various catalytic reactions.⁶⁹ However, several issues still hinder the application of QD catalysts, including harsh synthetic conditions, poor stability, and so on.⁷⁰ The fascinating structures and properties of GDY afford opportunities for the precisely controlled synthesis of GDY-based metal QD catalysts.⁷¹⁻⁷⁸ The well-defined rhodium QDs, with a high density of atomic defects, were controllably synthesized by Gao et al. through a facile method in aqueous solutions (Figure 5a), in which HCOOH and GDY were used as the reduction agent and stabilizing support, respectively.⁷¹Figure 5b–e indicates that there were large amounts of atomic steps efficiently induced by GDY on the faces of hexahedral Rh QDs in HAADF-STEM, resulting in excellent performance for hydrogen production with large current densities at low overpotentials from seawater. Additionally, it was reported by Li et al. that the single-crystal Pd(111) QDs were controllably in situ grown on GDY without addition of additional reductive agents.⁷² The strong interaction between Pd QDs and GDY could facilitate charge transfer and enhance electric conductivity, resulting in excellent electrocatalytic performance for HER with large current density. Through a facile reduction process of PdCl₂ on oxidized GDY (GDYO), Dai et al. synthesized a Pd/GDYO QDs catalyst which displayed excellent catalytic performance for the oxidation of H₂O₂.⁷³ Moreover, a facile electrodeposition approach was applied for the epitaxial growth of Au(111) QDs with Pt-Cl₄ single interfacial sites on porous GDY, in which atomic platinum chlorine species were used as the seeds.⁷⁴ The multicomponent PtCl₂ Au(111) QDs with a size of 2.37 nm were uniformly dispersed on porous GDY, which displayed excellent catalytic performance for both methanol oxidation reactions and ethanol oxidation reactions.

Figure 5.

Controlled synthesis of graphdiyne-based QD catalysts. (a) Schematic diagram of the in situ growth process of Rh nanocrystals. (b, c) High-resolution HAADF-STEM images of Rh/GDY. (d, e) HAADF-STEM images of the (111) plane in Rh/GDY. Reproduced from ref (71). Copyright 2022 Springer Nature. (f–h) High-resolution TEM and (i) HAADF-STEM images of HOCo/GDY. Reproduced from ref (82). Copyright 2024 John Wiley and Sons. (j) High-resolution TEM and (k–m) HAADF-STEM images of Cu_xO/GDY. Reproduced from ref (86). Copyright 2023 John Wiley and Sons.

Due to the advantage that GDY can controllably grow in situ on the surface of arbitrary substrates, in situ growth of GDY on as-prepared metal QDs is another effective strategy for the synthesis of GDY-based metal QD catalysts. Cu QDs/GDY was synthesized by Zhang et al. through the pyrolysis of a ZIF-8 precursor, followed by the in situ growth of GDY.⁷⁵ These results demonstrate that GDY is an excellent support for precise synthesis of well-defined metal QDs as well as showing the potential to achieve large-scale preparation. Metal oxide QDs are also precisely synthesized on GDY and serve as efficient catalysts for HER, OER, CO₂RR, and so on. The hydrothermal method is an effective strategy for the in situ growth of metal oxide QDs on GDY.⁷⁹⁻⁸¹ For example, a catalyst composed of GDY-rhodium oxide QDs-GDY (GDY/RhO_x/GDY) with a bilayer heterointerface was reported by Li et al., formed through a surface-induced growth strategy.⁷⁹ As shown in Figure 5f, Rh³⁺ absorbed on ultrathin GDY nanosheets was reduced to anchored Rh atoms under hydrothermal conditions, followed by the process of confined nucleation and growth of RhO_x. Finally, GDY was in situ grown on the as-synthesized RhO_x/GDY again to form more heterogeneous interfaces between metal oxide and GDY, which generated large amounts of active sites for electrocatalytic seawater electrolysis. It was characterized by TEM and HAADF-STEM that RhO_x QDs were uniformly dispersed on GDY nanosheets with an average size of 2.29 nm and showed significant lattice distortion induced by GDY. The GDY/RhO_x QDs/GDY catalyst exhibited excellent performance for overall seawater electrolysis at a large current density. Du et al. reported the synthesis of OsO_x QDs/GDY through a facile hydrothermal method.⁸¹ GDY not only plays an important role for the in situ growth of defect-rich OsO_x QDs but also induces charge transfer for the stabilization of high-coordination osmium (Os⁴⁺). As a result, OsO_x QDs/GDY shows excellent photo-electrocatalytic activity and stability for HER under alkaline conditions. A HOCo/GDY QDs catalyst was controllably synthesized by Zheng et al. through assembling cobalt acetylacetonate on GDY substrate, followed by a thermal decomposition procedure (Figure 5f–i).⁸² In addition, RuO_x QDs were controllably anchored on GDY nanoboxes.⁸³ CoO_x QDs were grown on GDY through an in situ growth–deposition strategy as reported by Liu et al.⁸⁴ Briefly, GDY was used as the working electrode to perform potentiostatic and cyclic voltammetry tests sequentially. CoO_x QDs/GDY showed ultrahigh photocatalytic performance for fixation nitrogen reaction due to the rapid conversion of chemical states. Strong electronic oxide–support interactions are rarely reported in QD catalysts supported on traditional carbon materials due to their chemical inertness. Recently, based on the specific properties and structure of GDY, a GDY-based Cu₂O QDs catalyst has been reported by Cao et al. to show the novel electronic oxide–GDY strong interaction.⁸⁵ The synthesis procedure for Cu₂O QDs/GDY showed that GDY was formed through a one-step Eglinton coupling reaction with HEB as the precursor and copper acetate as the catalyst, while Cu⁺ ions reduced by the alkyne through electron transfer were anchored and grown to form Cu₂O on the GDY network. It was indicated that Cu₂O QDs were uniformly distributed on GDY. Moreover, the presence of a strong oxide–support electronic interaction was proved in Cu₂O QDs/GDY, which significantly improved the catalytic activity of Cu₂O QDs/GDY in the azide–alkyne cycloaddition reaction. Xing et al. obtained Cu_xO QDs/GDY with a uniform size distribution through a facile method in which Cu²⁺ was gently reduced and then grown into Cu_xO QDs on GDY.⁸⁶ As shown in Figure 5j–m, there were abundant atomic steps on the surface of as-prepared Cu_xO QDs/GDY, generating various active sites for the selective hydrogenation of olefins.

4.3. Graphdiyne-Based Heterostructured Catalysts

Heterostructured catalysts are of great importance in many fields of energy conversion and storage.^87,88 However, the development of heterostructured catalysts is still hindered by harsh synthetic conditions, low catalytic activity, and low stability, which cannot fulfill the requirements of industry.⁸⁹ Benefiting from the infinite active sites, high intrinsic activity, and excellent conductivity of GDY, GDY-based heterostructures are expected to have high activity, high selectivity, and high stability. Inspired by this, many efforts have been made for the controlled synthesis of 2D/2D heterostructured catalysts constructed from GDY and 2D materials including layered double hydroxide (LDH),⁹⁰⁻⁹⁵ transition metal dichalcogenides (TMDs),⁹⁶⁻⁹⁸ metal oxide or nitride nanosheets,^99,100 and so on.^101,102 Multimetal LDHs drew widespread attention due to their great intrinsic activity, but the applications of traditional LDHs were still hindered by low conductivity and poor stability. Constructing LDH/GDY heterostructures is an effective strategy to solve these problems. For example, Li et al. realized the controlled synthesis of a GDY-exfoliated and -sandwiched FeCo LDH nanosheets array.⁹⁰ As shown in Figure 6a, HEB monomers entered the LDH gallery first through replacing the interlayer anions, followed by in situ cross-coupling to form GDY films, during which process the interlayer stress/deformation generated and gradually increased due to intimate contact between the layers. Consequently, bulk LDHs were exfoliated into LDH nanosheets, and GDY-sandwiched FeCo LDH nanosheet arrays were synthesized (Figure 6b,c). The FeCo LDHs/GDY nanosheet arrays showed greatly enhanced performance for electrocatalytic overall water splitting due to the interaction between LDHs and GDY with more catalytically active sites and stronger ability to prevent corrosion. In addition, the heterostructures of GDY-encapsulated CeNi LDH nanosheets were controllably synthesized by Zhao et al. through the growth of GDY on an as-prepared LDH nanosheets array (Figure 6d,e).⁹¹ The growth of LDH nanosheet arrays on GDY was another strategy to construct LDH/GDY heterostructures. Yuan et al. reported a GDY/NiFe LDH heterostructure formed through electrodeposition using GDY as the substrate.⁹² TMDs are another typical class of 2D materials that exhibit great potential in the fields of energy conversion and storage.^103,104 Recently, some research has been reported into constructing GDY/TMD heterostructures to serve as highly efficient catalysts.⁹⁶⁻⁹⁸ The heterostructure of a GDY/MoS₂ nanosheets array was reported by Hui et al., showing excellent HER activity and stability at all pH values.⁹⁶ A MoS₂ nanosheets array was first synthesized through hydrothermal method on the 3D carbon fiber network, followed by growing ultrathin GDY films on as-prepared MoS₂. The SEM images showed the morphology of both MoS₂ and GDY/MoS₂ were ordered nanosheets array, while the surface became rough after GDY grown (Figure 6f,g). Yu et al. realized the controlled synthesis of NGDY/MoS₂ nanosheets through a facile hydrothermal method.⁹⁷ The NGDY/MoS₂ heterostructure possessed obvious advantages for facilitating the charge transport behavior and improving the HER kinetics due to strong interactions between NGDY and MoS₂, resulting in greatly enhanced catalytic activity and stability for HER. Moreover, a well-designed MoS₂/GDYO heterostructure was reported by Wang et al., formed through an electrostatic self-assembly method, in which positively charged CTAB-GDYO was adsorbed and electrostatically self-assembled on a MoS₂ monolayer exfoliated from bulk MoS₂.⁹⁸ GDY can also effectively improve the intrinsic catalytic activity of transition metal oxide and nitrides.⁹⁹⁻¹⁰¹ For example, SnO_x/GDY nanosheets were controllably synthesized by Li et al. by an in situ reduction method at ambient conditions.³³ SEM images (Figure 6h,i) indicate that the 3D porous structure was maintained with the decreased pore size. Fang et al. reported the construction of a Co₂N/GDY heterostructure by growing GDY on a Co₂N nanosheets array.⁹⁹ The heterostructure of ultrathin GDY encapsulated on the Co₂N nanosheet can be clearly observed in the SEM and TEM images (Figure 6j,k). Zhang et al. controllably constructed Co-NiO_x@GDY nanosheet arrays with multiple heterointerfaces, which presented excellent selectivity and activity for urea production.¹⁰⁵ Du et al. reported the preparation of a Bi/GDY nanoflowers heterostructure through the in situ reduction of Bi³⁺ on GDY, which exhibited high activity for CO₂RR to produce HCOOH.¹¹⁰

Figure 6.

(a) Schematic diagram of the synthetic process of e-ICLDH@GDY/NF heterostructures. (b) SEM image of ICLDH/NF. (c) SEM image of e-LDH@GDY/NF. Reproduced from ref (90). Copyright 2018 Springer Nature. (d, e) SEM images of Ce_0.6Ni_0.4LDH@GDY/NF. Reproduced from ref (91). Copyright 2021 Royal Society of Chemistry. (f) SEM image of MoS₂ NS/CF. (g) SEM image of GDY-MoS₂ NS/CF. Reproduced from ref (96). Copyright 2019 Elsevier Ltd. High-magnification SEM images of (h) GDY electrode and (i) SnO_x/GDY electrode. Reproduced from ref (33). Copyright 2023 Elsevier Ltd. (j, k) High-magnification TEM images of Co₂N/GDY NS. Reproduced from ref (99). Copyright 2020 John Wiley and Sons. (l) EXAFS spectra of IVR-FO/GDY at the Fe K-edge. (m) SEM and (n) HRTEM images of IVR-FO/GDY. Reproduced from ref (106). Copyright 2021 John Wiley and Sons. (o) TEM image of Pd-UNs/Cl-GDY. (p) HAADF-STEM image of Pd-UNs/Cl-GDY. Reproduced from ref (108). Copyright 2023 John Wiley and Sons.

2D/1D GDY-based heterostructures have also been precisely synthesized through various strategies and served as highly efficient catalysts recently.^34,39,106 The heterostructure consisting of GDY film and Fe₃O₄ nanoneedles was formed by Fang et al. through a mild co-precipitation method, in which Fe²⁺ and Fe³⁺ ions were absorbed on as-prepared GDY substrate and then co-precipitated in an alkaline environment.¹⁰⁶Figure 6m,n presents the morphology and structure of the as-prepared IVR-FO/GDY heterostructure, in which there was visible lattice distortion in the HRTEM image of GDY-incorporated Fe₃O₄. Besides, large amounts of iron vacancies were indicated by the visibly weaker amplitudes of Fe-Fe shell peaks and increased Debye–Waller factor in EXAFS analysis (Figure 6l), resulting in enhanced performance for efficient nitrogen conversion. Xue et al. reported the growth of a Cu@GDY nanowires array with a highly active heterointerface.³⁴ In general, the Cu nanowires array was prepared through electrochemical anodization of Cu foam followed by a chemical reduction procedure, and then the cross-coupling reaction of HEB occurred on as-prepared Cu nanowires to form a Cu@GDY heterostructure. It is also important to realize the precisely controlled construction of GDY/nanoparticles heterostructured catalysts with highly active heterogeneous interfaces, which has drawn increasing attention from researchers.^107−109 Li et al. reported the GDY@Janus magnetite heterostructure with excellent performance for photocatalytic nitrogen fixation.¹⁰⁷ In their work, GDY was used as the substrate to absorb Fe²⁺ ions, followed by a nucleation and growth process of FeO_x nanoparticles, in which GDY played an important role to change the morphology, coordination environment, and valence states of FeO_x. Additionally, Pd nanoparticles were in situ electrodeposited on GDY by Zhang et al. using a similar strategy to form a Pd-UNs/Cl-GDY heterostructured catalyst.¹⁰⁸ As shown in Figure 6o,p, the defect-rich branched structure was observed with various atomic steps and low coordination number, which greatly enhanced the catalytic activity for methanol oxidation reaction.

4.4. Graphdiyne-Based Metal-Free Catalysts

Metal-free catalysts are drawing increasing attention due to the advantages such as lower cost, higher selectivity, enhanced durability, and environmental friendliness compared with conventional metal catalysts.^111−113 Recently, a novel class of carbon-based metal-free catalysts has been researched with efficient catalytic performance.¹¹⁴ However, the development of traditional carbon-based metal-free catalysts is still hindered by various problems including harsh synthetic conditions, poor intrinsic activity, ambiguous active sites, and unclear catalytic mechanisms. Thanks to the extremely inhomogeneous distribution of surface charge, GDY exhibits high intrinsic activity for catalysis. Moreover, the structure and properties of GDY can be easily and precisely regulated through chemical modification, and thus it is possible to obtain metal-free catalysts with well-defined structure and catalytic mechanism.¹¹⁵ Zhang et al. reported that GDY could be directly applied as the metal-free cathode catalyst in reversible Li-CO₂ batteries.¹¹⁶ The rich diacetylenic units and large amounts of pores in GDY play important roles for promoting the chemical adsorption of CO₂ and diffusion of Li⁺, resulting in outstanding performance.

Heteroatom doping is widely used for the controlled synthesis of GDY-based metal-free catalysts with modified band gap and charge distribution. Bottom-up synthesis using heteroatom-substituted HEB as monomer is one of the universal strategies to obtain doped GDY with well-defined structure.⁴⁷⁻⁵³ For example, the controlled synthesis of fluorographdiyne (FGDY) nanosheets was reported by Li et al. (Figure 7a), in which 1,3,5-triethynyl-2,4,6-trifluorobenzene was used as the monomer and in situ grown on carbon cloth through a Glaser coupling reaction.⁴⁷ As shown in Figure 7b–e, homogeneous distribution of C and F over the whole nanosheet was observed in the TEM and EDS results. Besides, only C and F peaks existed in the XPS spectra, with the new C–F peak emerging, indicating the successful synthesis of FGDY (Figure 7f,g). The as-prepared FGDY/CC could serve as an excellent metal-free catalyst for both HER and OER in wide pH ranges. He et al. synthesized a 2D HsGDY film through cross-coupling reaction of triethynylbenzene on Cu foil.⁵⁰ Recently, an improved approach has been proposed by Yang et al. using Zn rather than Cu as the substrate, and consequently the scalable and controlled synthesis of a uniform HsGDY film (Figure 7j) with well-defined structure was realized.⁵¹ The AFM images (Figure 7k,l) showed that the average thickness of HsGDY films was about 0.72 nm. Moreover, this bottom-up strategy was also applied for the controlled synthesis of other heteroatom-substituted GDY, such as phosphine–graphdiyne⁴⁸ and chlorine–graphdiyne.⁵² Wang et al. reported the controlled synthesis of ultrathin N-doped GDY in which nitrogen atoms were in the form of sp hybridization at acetylenic sites (Figure 7h), which displayed outstanding electrocatalytic performance for ORR.¹¹⁷ Few-layer oxidized graphdiyne (FLGDYO) was selected as the precursor, and melamine served as the nitrogen source. It was proposed and proved by TG-DTA-MS that, during the calcination process, FLGDYO was reduced into few-layer graphdiyne (FLGDY) through releasing CO₂⁺ fragments, while NHCNH₂⁺ fragments released from melamine were absorbed on FLGDYO, followed by a pericyclic reaction to replace the sp-C atoms with nitrogen atoms (Figure 7i). TEM and EELS mapping (Figure 7m,n) indicated the uniformly distributed C and N atoms on ultrathin films, while the existence of sp-hybridized nitrogen was confirmed by XPS and XANES (Figure 7o,p). Further, the mechanism for the synthesis of sp-N-doped GDY was comprehensively studied, with an intermediate-like molecule obtained.¹¹⁸ The stereo-defined co-doping of both sp-N and other elements (S, B) in GDY was also realized by Zhao et al. through a similar procedure.^119,120 It was noted that, through increasing the concentration of N and S atoms, NSFLGDY displayed enhanced catalytic performance for OER. Moreover, the annealing strategy can also be used for selectively doping the sp²-C atoms of GDY.^121,122 For example, Huang et al. synthesized pyridinic N-doped GDY through annealing under an NH₃ atmosphere, which could serve as an effective bifunctional catalyst for zinc air batteries.¹²¹

Figure 7.

(a) Schematic diagram of p-FGDY for HER and OER. (b, c) TEM images and (d, e) corresponding elemental mapping of p-FGDY. (f) Raman spectrum and (g) C 1s XPS spectra of p-FGDY/CC. Reproduced from ref (47). Copyright 2019 John Wiley and Sons. (h) Geometries of N atoms in NFLGDY. (i) Schematic illustration of the synthetic mechanisms of NFLGDY. Reproduced from ref (117). Copyright 2018 Springer Nature. (j) TEM, (k) AFM, and (l) height–length images of the HsGDY synthesized on Zn substrate. Reproduced from ref (51). Copyright 2020 Elsevier Ltd. (m) TEM image of NFLGDY-900c. (n) HAADF-STEM and EELS mapping of NFLGDY-900c. (o) N K-edge XANES of different samples of NFLGDY. (p) High-resolution N 1s spectra of different samples of NFLGDY. Reproduced from ref (117). Copyright 2018 Springer Nature.

5. Applications of GDY-Based Multiscale Catalysts

5.1. NH₃ Production

As an ideal carrier of H₂, NH₃ plays an important role in agricultural and industrial production. However, to increase the reaction rate and reaction conversion, harsh conditions including high temperatures and pressures are needed for the synthesis of NH₃ in industrial production, resulting in significant energy losses. Therefore, it is important to develop new strategies for efficient NH₃ synthesis at ambient temperature and pressure. Various zerovalent metal atoms in situ anchored on GDY were reported to be efficient catalysts for NRR to produce NH₃.¹²³ Mo⁰/GDY displayed high electrocatalytic performance for NH₃ production from N₂ (NH yield:⁵³ 145.4 μg h^–1 mg_cat^–1, Faradaic efficiency: 21%) (Figure 8a,b).⁶¹ Li et al. synthesized a novel GDY-based atomic catalyst consisting of GDY-anchored zerovalent Pd atoms (Pd⁰/GDY) for NH₃ production by a simple self-reduction strategy, which exhibited the highest NH₃ yield of 4.45 mg_NH3 mg_Pd^–1 h^–1 (Figure 8c,d).¹²³ Recently, Li et al. prepared Mn⁰/GDY for ECNRR via an electrochemical deposition strategy, exhibiting high NH₃ yield and Faradaic efficiency, with values of 45.35 ± 1.38 mg_NH3 mg_cat^–1 h^–1 and 38.08 ± 2.26%, respectively.⁵⁹ Duan et al. reported the use of densely anchored metal atoms (Rh, Ru, and Co) on GDY in a steric confinement-induced strategy for ECNRR at high pressure.¹²⁴ Rh⁰/GDY exhibits both high NH₃ yield and Faradaic efficiency at 55 atm. Duan et al. further developed a generalized in situ coordination method to successfully anchor a series of metal atoms (W, Mo, Re, Mn) on GDY. Further, Ru single atoms anchored on a graphdiyne/graphene structure (Figure 8e) were reported by Zhang et al. to be a highly efficient electrocatalyst for NH₃ synthesis at a low potential.⁶³ Notably, a hydrogen radical-transfer mechanism was proposed to effectively activate N₂ to generate NNH radicals, which were further hydrogenated on Ru single atoms to form NH₃. Li et al. successfully loaded CoO_x QDs (GDY@CoO_x QDs) on GDY nanosheets by an electrochemical growth–deposition strategy.⁸⁴ The high-energy orbitals of Co were adapted to the π* orbitals of N₂, which could adsorb and stabilize N₂ and weaken N≡N via d−π* orbital interactions. The GDY@CoO_x QDs exhibited photocatalytic nitrogen fixation to NH₃ performances that far exceeded those of conventional catalysts. The average NH₃ yield could reach 19583 μmol_NH3 g_cat^–1 h^–1 (Figure 8f,g). Besides, the GDY/Fe₃O₄ heterostructure reported by Fang et al. exhibited excellent catalytic performance for NH₃ synthesis by photocatalysis, in which GDY played an important role to modify the chemical state and coordination structure of Fe₃O₄.¹⁰⁷ The GDY@Fe–B heterostructure displayed transformative photocatalytic activity with an average NH₃ yield (Y_NH3) of 1762.35 μmol h^–1 g_cat^–1. It is worth noting that the record-high performance for both electrocatalytic and photocatalytic NRR were achieved by GDY-based catalysts. Besides, various GDY-based multiscale catalysts displayed excellent catalytic performance for N₂ fixation to produce NH₃, which indicated the superiority of GDY in catalytic synthesis of NH₃.

Figure 8.

Applications for NH₃ production. (a) Schematic structure of Mo⁰/GDY in the catalysis process of HER and NRR. (b) ECNRR energetic pathway on the Mo⁰/GDY. Reproduced from ref (61). Copyright 2019 American Chemical Society. (c) Yield of NH₃ and corresponding Faradaic efficiency applied with different potentials. (d) Synthesis and electrocatalysis process of Pd/GDY for the production of NH₃. Reproduced from ref (123). Copyright 2021 Oxford University Press. (e) Schematic representation of NRR to product NH₃ on Ru/GDY/G atomic catalyst. Reproduced from ref (63). Copyright 2023 American Chemical Society. (f) Schematic diagram of the ECNRR on CoO_x QDs/GDY. (g) Independent results of yield of NH₃ on GDY@CoO_x QD catalyst. Reproduced from ref (84). Copyright 2021 Elsevier Ltd.

5.2. Hydrogen Production

Electrochemical water splitting (EWS), comprising a hydrogen precipitation reaction and an oxygen precipitation reaction, is evaluated as one of the most ideal technologies for hydrogen production. Unfortunately, the low energy conversion efficiency and sluggish kinetics of the EWS have greatly limited its practical application. Nowadays, various effective strategies are proposed to modify the HER kinetics and greatly enhanced the performance of HER, including adjusting the alkali metal cations in electrolyte,¹²⁵ tailoring the local chemical environment with an amorphous nickel hydroxide shell,¹²⁶ utilizing hydrazine to assist water electrolysis,¹²⁷ and so on. GDY-based atomic catalysts are also ideal catalyst model systems based on their unique properties and have exhibited excellent electrocatalytic performance for HER.^56,128,129 Li et al. achieved the first in situ anchoring of zerovalent transition metal atoms (Ni⁰ and Fe⁰) on the GDY surface, which exhibited the best acidic hydrogen precipitation performance with a minimum onset overpotential of 9 mV at a current density of 10 mA cm^–2.⁵⁶ Besides, the preparation of Pd⁰/GDY was achieved by Li et al. through a facile electroreduction procedure, exhibiting excellent activity and stability for HER.¹²⁸

Constructing precious metal QD catalysts is another efficient strategy to increase the utilization of precious metal atoms with reduced cost. GDY/RhO_x/GDY QDs constructed by Li et al. possessed a bilayer interface of sp-C∼O–Rh with high reactive activity.⁷⁹ As a result, the overall seawater electrolysis achieved by GDY/RhO_x/GDY exhibited a low cell voltage of 1.52 V at 500 mA cm^–2. Nb_yRhO_x QDs/GDY was reported by Gao et al. which also exhibited excellent catalytic performance for HER due to the construction of highly active heterogeneous interfaces (Figure 9a–c).⁸⁰ Li et al. controllably constructed Rh nanocrystals with high-density atomic defects (Rh/GDY) on the surface of GDY using formic acid as a reducing agent.⁷¹ HAADF-STEM images confirmed the successful growth of high-density atomic defective Rh on the GDY surface. Theoretical calculations showed that the highly active stepped surface exposed by Rh nanocrystals and the GDY-induced d-band modulation at the active site significantly facilitated the water decomposition and hydrogen binding process of HER. The current density of Rh/GDY could reach 1000 mA cm^–2 at the overpotential of 65 mV with ultrafast reaction kinetics (Figure 9d,e), which is superior to those achieved with the reported electrocatalysts and commercial Pt/C electrocatalyst. Li et al. developed a scheme for low-temperature controllable induced single-crystal growth of Pd(111) on the surface of GDY.⁷² Theoretical calculations showed that the significant d–p hybridization in GDY-Pd1 weakened the over-absorption of H* on the surface of Pd(111) , resulting in a significant reduction of the free energy of hydrogen precipitation. GDY-Pd exhibited a low overpotential of 261 mV at 1000 mA cm^–2 and high stability. GDY/MoO₃ was reported by Guo for efficient electrocatalytic water splitting.¹³⁰ Due to the rapid charge transfer originating from “sp-C–O–Mo hybridization”, the dissociation process of H₂O molecules was accelerated, which thus led to the remarkable HER activity at high current density (Figure 9f,g). GDY-based Pt atom catalysts (Pt-GDY-1 and Pt-GDY-2) were prepared by Lu et al. through spontaneous chemical reduction between GDY and [PtCl₄]^2– and further annealing treatment, respectively, which showed different C–Pt coordination structures. Pt-GDY-2 exhibited excellent HER performance with higher mass activity than commercial Pt/C (Figure 9h,i).⁶⁴ Li et al. experimentally prepared a GDY-based Ru atom bifunctional catalyst for HER and OER under acidic conditions which provides a new idea for designing and synthesizing new catalysts.¹²⁹ Wu et al. constructed the CdSe QDs/GDY photocathode by loading CdSe QDs functionalized on the surface of the GDY film as a hole-transferring layer in a photo-electrochemical water-cleavage cell.¹³¹ The strong interaction between the GDY and the CdSe QDs enabled efficient hole transport. Jin et al. prepared GDY/CQD Z-type heterojunctions by spontaneous electrostatic self-assembly of GDY with Co₃O₄ QDs.¹³² The constructed Z-type heterojunctions could greatly improve the charge transfer as well as uniformly disperse and stabilize the Co₃O₄ QDs. The maximum photocatalytic H₂ release rate of GDY/CQDs was 1500.85 μmol h^–1 g^–1.

Figure 9.

(a) Schematic diagram of Nb_yRhO_x/GDY for high-performance HER. (b) Polarization curves of Nb_yRhO_x/GDY for HER. (c) Stability tests of Nb_yRhO_x/GDY for HER. Reproduced from ref (80). Copyright 2022 John Wiley and Sons. (d) Schematic illustration of Rh/GDY QDs catalyst for efficient H₂ production from seawater. (e) Comparison of HER performance of Rh/GDY with other catalysts. Reproduced from ref (71). Copyright 2022 Springer Nature. (f) Schematic representation of efficient hydrogen reduction reaction on GDY/MoO₃. (g) HER polarization curves of GDY/MoO₃. Reproduced from ref (130). Copyright 2021 American Chemical Society. (h) HER mass activities at an overpotential of 0.1 V for commercial Pt/C, Pt-GDY1, and Pt-GDY2. (i) Polarization curves for Pt/C, Pt-GDY1, and Pt-GDY2. Reproduced from ref (64). Copyright 2018 John Wiley and Sons.

5.3. Conversion of CO₂ into Value-Added Chemicals

The sustainable generation of energy is severely hindered by excessive emission of CO₂, which nowadays has become the consensus of the international community. Electric/light/thermal field-driven CO₂ reduction and efficient cycloaddition reactions of CO₂ are important to realize efficient CO₂ catalytic conversion.¹³³ However, the slow kinetics and poor product selectivity during the catalytic conversion limited the development of this field due to the multiple electron-transfer process during CO₂ conversion. GDY-based catalysts with well-defined structure and efficient catalytic activity can be the key materials to realize efficient catalytic conversion of CO₂. Zerovalent Co atoms were successfully in situ anchored on GDY by Li et al. via a simple pre-adsorption–electrochemical reduction strategy.¹³⁴ The strong charge transfer between the zerovalent Co and sp-C atoms significantly improved the surface adsorption and achieved efficient CO₂ fixation. Therefore, Co⁰/GDY realized the highly selective conversion of CO₂ for epoxide cycloaddition to cyclic carbonate with nearly 100% selectivity. Cu-based materials have been widely studied by researchers and displayed excellent performance for CO₂RR. Recently, an efficient bimetallic atomic catalyst CuEr-GDY was reported by Li et al. for artificial photosynthesis converting CO₂, in which all-crystalline GDY served as the substrate for the precise anchoring of Cu and Er atoms (Figure 10a).¹⁸ Because of the obvious charge transfer between GDY and Cu/Er atoms as well as the synergistic effects of heteronuclear metals, CuEr-GDY demonstrated excellent catalytic performance, with CO selectivity up to 97.6% and a high yield of CO. Cu SAs/GDY was reported by Yuan et al. through in situ anchoring of Cu atoms on GDY for CO₂ conversion into methane (Figure 10b,c).¹³⁵ It was illustrated that the *OCHO intermediate generated in the catalytic process was of great benefit for CH₄ production. As a result, Cu SAs/GDY displayed high selectivity and activity for CO₂RR to produce methane.

Figure 10.

Applications in CO₂RR and the conversion of small organic molecules. (a) Schematic diagram of artificial photosynthesis on CuEr/GDY. Reproduced from ref (18). Copyright 2023 Elsevier Ltd. (b) Schematic illustration of CO₂RR process on Cu SAs/GDY. (c) Faradaic efficiency of Cu/GDY atomic catalyst at different potentials. Reproduced from ref (135). Copyright 2022 John Wiley and Sons. (d) Schematic diagram of CO₂RR on SnO_x/GDY to form HCOOH. (e) Faradaic efficiency of SnO_x/GDY at various potentials in 0.5 M CO₂-saturated KHCO₃ solution. Reproduced from ref (33). Copyright 2023 Elsevier Ltd. (f) Stability test of CoPc/GDY/G and Faradaic efficiency of CO with time. (g) FE_CO and FE_H2 of CoPc/GDY/G and reference samples applied with different potentials. Reproduced from ref (136). Copyright 2021 American Chemical Society. (h) Schematic illustration of the formation process of Pd1/GDY/G and catalysis for 4-NP reduction. (i) UV–vis absorption spectra in the reduction process 4-NP catalyzed by Pd1/GDY/G. Reproduced from ref (62). Copyright 2019 John Wiley and Sons. (j) Mass activity of Pd/mSiO₂ and GDY@Pd/mSiO₂ for benzaldehyde conversion. (k) Time-dependent conversion of benzaldehyde. Reproduced from ref (37). Copyright 2022 John Wiley and Sons. (l) Schematic diagram of the charge transfer in the Co-NiO_x@GDY heterostructure. (m) Yield rates of urea on different catalysts applied with various potentials. Reproduced from ref (105). Copyright 2023 Oxford University Press. (n) Schematic representation and substrate scope of azide–alkyne cycloaddition reaction catalyzed by Cu₂O NCs/GDY. (o) Time-dependent yield on different Cu(I) catalysts. Reproduced from ref (85). Copyright 2023 American Chemical Society.

A heterostructured electrocatalyst, SnO_x/GDY, was reported by Li et al. for CO₂ reduction with high HCOOH selectivity.³³ It is notable that GDY could effectively regulate the phase structure and valence states of Sn, as well as promote charge transfer. Consequently, SnO_x/GDY showed superior performance for CO₂ conversion at room conditions (Figure 10d,e). Moreover, Zhang et al. constructed the CoPc/GDY/G heterostructure for highly selective CO₂ reduction to CO.¹³⁶ A HAADF-STEM image indicated that CoPc was anchored monodispersed on the 2D GDY/graphene heterostructure. Due to the obvious charge transfer and enhanced electron conductivity, CoPc/GDY/G displayed distinguished catalytic performance for CO₂RR (Figure 10f,g). The conversion of some small organic molecules can also be realized by using GDY-based multiscale catalysts. For example, Li et al. synthesized a Pd1/GDY/G atomic catalyst through anchoring palladium atoms on a graphene/graphene heterostructure for highly efficient reduction of 4-nitrophenol to 4-aminophenol (Figure 10h,i).⁶² In addition, it was indicated by Yu et al. that GDY could serve as the wettability modifier to enhance the catalytic performance of the hydrogenation reaction in GDY@Pd/mSiO₂ heterostructures.³⁷ According to XPS results, there are strong d−π interactions between GDY nanospheres and Pd nanoparticles which significantly changed the electronic structure. As a result, the performance for hydrogenation was obviously improved compared with that using pure Pd/mSiO₂ (Figure 10j,k). Further, the Co-NiO_x@GDY reported by Zhang et al. could serve as a highly efficient catalyst for the reduction of carbon dioxide and nitrite to generate urea (Figure 10l,m).¹⁰⁵ The record Faradaic efficiency was 64.3%, and yield rates of urea could reach 913.2 μg h^–1 mg^–1. Moreover, Cu₂O NCs/GDY reported by Yu et al. displayed high efficiency for the azide–alkyne cycloaddition reaction with broad reactant scope (Figure 10n,o).⁸⁵

5.4. Energy Conversion

GDY, with its unique porous and acetylene-rich structure, provides numerous storage sites, rapid diffusion channels, and a low ion diffusion barrier of interlayer insertion/extraction and surface absorption/desorption, which are of great significance for energy storage and conversion. Based on these unique properties of GDY, our group defined the concept of “alkyne–alkene transition” and illustrated the new mechanism from the aspect of lithium fast charge. For example, self-expanding Li-ion transport channels were constructed based on GDY and applied for fast-charging Li-ion batteries with excellent performance.¹³⁷ It was demonstrated that Li⁺ reversibly bound with acetylenic bonds in GDY, resulting in the alkyne–alkene transition (Figure 11a,b). RuO_x QDs were controllably anchored on GDY nanoboxes by Wang et al. which could greatly promote the performance of Li-S batteries (Figure 11c).⁸³ The d-electron structures of Ru were effectively regulated, and the d–p orbital hybridization was greatly promoted because of the significant interaction between RuO_x and GDY; thus, the catalytic performance of RuO_x QDs/GDY was significantly enhanced. GDY derivatives with larger pore size fabricated by substituent groups could promote the diffusion and transfer of Li ions. Huang et al. synthesized FGDY as the free-stranding electrode.¹³⁸ The high capacity and stability of FGDY were derived from the reversible transition from C–F covalent bonds to ionic bonds. Gao et al. constructed an IV-GDY-FO heterostructure as the anode of LIBs that showed remarkable performance (Figure 11d).¹³⁹ It was demonstrated that Fe with high valence was reduced and stabilized by GDY, and plenty of Fe vacancies were generated, which could regulate the charge distribution, thus facilitating electronic transportation and Li-ion diffusion. In addition, GDY could serve as an efficient electrode in rechargeable aqueous Zn-ion batteries. Based on the in-depth research on the anchoring, nucleation, and growth behavior of metal atoms on GDY, Luan et al. reported the evolution and de-intercalating process of Zn-ion plating on GDY, during which no Zn dendrites are formed and there is no side reaction (Figure 11e,f).²⁰

Figure 11.

(a) Schematic diagram of the self-expanded Li-ion transport channels on GDY. (b) Comparison of the electrochemical performance of GDY and other anodes. Reproduced from ref (137). Copyright 2022 John Wiley and Sons. (c) Schematic representation of RuO_xQDs/GDY for Li-S full batteries with high energy density. Reproduced from ref (83). Copyright 2023 John Wiley and Sons. (d) Schematic diagram of the kinetic process of Li-ion transport in IV-GDY-FO heterostructure with Fe vacancies. Reproduced from ref (139). Copyright 2023 John Wiley and Sons. (e) Optical and SEM images and schematic diagram of Zn/GDY electrode. (f) Comparison of performance between Zn/GDY and other reported electrodes. Reproduced from ref (20). Copyright 2023 John Wiley and Sons. (g) Schematic diagram of the interfacial behavior of GSMB. Reproduced from ref (21). Copyright 2023 American Chemical Society. (h) Schematic diagram of GDY-based artificial synapse (GAS). Reproduced from ref (140). Copyright 2021 Springer Nature. (i) Schematic diagram of device structure. (j) Current density–voltage curve of the devices constructed with CN or GCl. Reproduced from ref (142). Copyright 2020 John Wiley and Sons. (k) Schematic representation of the BHJ and LbL devices treated with GOMe. (l) Current density–voltage curves of BHJ and LbL devices treated with or without GOMe. Reproduced from ref (143). Copyright 2022 Elsevier Ltd.

Further, GDY-based energy conversion systems have been developed into next-generation multifunctional intelligent devices based on the unique structure and properties of GDY. Fu et al. constructed a solid magnesium cell with both continuous humidity and sunlight response using a GDY nanosheets array as the electrode (Figure 11g).²¹ The constructed device can efficiently capture and transfer water molecules, as well as absorb and convert solar energy, and so it was utilized as a highly efficient humidity monitor with rapid response time and high sensitivity. Utilizing the highly conjugated π-extended structure and superior ion shuttle characteristics of GDY, Xu et al. constructed GDY-based artificial synapses (GASs, Figure 11h).¹⁴⁰ It was demonstrated that the superior ion diffusion kinetics of GASs could be well maintained under relatively harsh conditions.

Extensive efforts in the field of solar cells have also been based on GDY and its derivatives.¹⁴¹ For example, Li et al. tuned the morphology and improved the device efficiency of organic solar cells through adding chlorine-substituted GDY, which displayed a remarkable efficiency of 17.3%, while the short-circuit current simultaneously increased with fill factor.¹⁴² Notably, due to the nonvolatile property of GCl, batch-to-batch variations were dramatically reduced and mass production was enhanced after the addition of GCl (Figure 11i,j). Sun et al. constructed an organic solar cell containing only small molecules through layer-by-layer (LbL) deposition followed by treatment with methoxy-substituted GDY, which greatly enhanced both the carrier generation and transport ability (Figure 11k,l).¹⁴³

6. Conclusion and Perspectives

Graphdiyne (GDY) has characteristics of one-atom-thick two-dimensional layers comprising sp- and sp²-hybridized carbon atoms. In contrast to traditional carbon materials with sp² hybridization, GDY shows many unique chemical and electronic structure features and fascinating properties, such as rich chemical bonds, distributed pores, and inhomogeneity of charge distribution. GDY brings an opportunity to pursue fundamental and applied research on carbon materials and overcomes significant challenges to achieve transformative results in many interdisciplinary research fields, such as catalysis and energy.

As a transformative material, GDY provides an ideal platform for developing new materials for catalysis, electrodes, and energy conversion with accurate structures. Realizing the efficient application of GDY for interdisciplinary research and as a key material for developing new technologies will release the power of GDY to accelerate scientific progress and provide new opportunities for achieving transformative breakthroughs in different fields.

The authors declare no competing financial interest.