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Graphynes for Photocatalytic and Photoelectrochemical Applications

Xiaoqing Ma

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1042 -1060.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (5) : 1042-1060 DOI: 10.7536/PC210636
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Graphynes for Photocatalytic and Photoelectrochemical Applications
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Abstract

Long-term and efficient utilization of solar energy is an eternal issue for sustainable development. Among the solar energy conversion techniques, photo(electro)catalysis plays an overwhelming role in clean energy production and pollutant treatment. Carbon-based catalysts have been studied as promising candidates achieving low cost, high energy conversion efficiency and environmental friendliness. Very recently, the family of graphynes (GYs) is rising as a superb new star of carbon allotrope. It consists of merely sp- and sp2- hybridized carbon atoms, constructing a huge conjugated network and expanded two-dimensional porous structure. The unique topological structures endow GYs distinctive semiconductor and optical properties, excellent charge mobility and intrinsic band gap. Therefore, a broad application in the conversion and utilization of solar energy is expected. Since graphdiyne, one of the graphyne-family members has been firstly synthesized in 2010, many efforts have been made in the fields of photo(electro)catalysis. The enhanced photocatalytic or photoelectrocatalytic efficiencies of these carbon allotropes either alone or combined with other photocatalysts are reported. Generally, according to current reports, the enhancements are mainly attributed to the high carrier mobility promoting charge transfer, the natural pore structure which is conductive to mass transport and more exposed active sites for catalysis. However, as a newly-emerged family of carbon allotrope, the essential potential of GYs-based photocatalysts are expected to be further explored. In this review, the synthesis of GYs with different morphology and the structural characterization methods are briefly introduced. Then the photocatalysts based on GYs for specific chemical reactions are comprehensivey included. The synthesis, performances and mechanisms of these photocatalysts are elucidated systematically in the applications of polution degradation, water splitting, CO2 reduction and photoelectrolysis as well as nitrogen fixation and bacterial disinfection (noted in few reports). Furthermore, some problems existing in the current research are put forward, and the perspectives and challenges are presented aiming at photo(electro)catalytic efficiency elevations.

Contents

1 Introduction

2 Synthesis and structures of graphynes

2.1 First synthesis of two-dimensional graphdiyne films

2.2 Preparation of graphdiyne with controlled morphology

2.3 Mechanochemical synthesis ofγ-graphyne and its derivatives

2.4 Characterization of graphynes

3 Preparation and applications of GYs-based photocatalysts

3.1 Water treatment

3.2 Photoelectrode materials

3.3 Photocatalytic water splitting

3.4 Photocatalytic CO2reduction

4 Conclusions and outlook

Graphical abstract

Keywords

graphynes / synthesis / photocatalysis / photoelectrocatalysis / mechanisms

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Xiaoqing Ma. Graphynes for Photocatalytic and Photoelectrochemical Applications. Progress in Chemistry, 2022, 34(5): 1042-1060 DOI:10.7536/PC210636

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1 Introduction

Fossil fuels have always been the main energy sources in the world. After two centuries of exponential economic growth, mankind is facing two major consequences of climate change and environmental degradation, while fossil energy is likely to be exhausted in 50 to 100 years. Global emissions of carbon dioxide from burning fossil fuels currently exceed 33 billion tons, twice the natural rate at which carbon dioxide is absorbed by land and oceans. Therefore, all countries in the world comprehensively promote the strategic goal of "carbon neutrality", that is, to achieve the positive and negative offset of carbon emissions and achieve relatively "zero emissions". One of the important means to achieve this goal is "energy substitution". Solar energy is the most abundant energy source, which can meet the future energy needs of mankind. The efficient conversion of solar radiation into chemical energy, or energy-dense liquid energy carriers, is the key to achieving large-scale solar energy output at the gigaton scale[1]. Solar water splitting to hydrogen, carbon dioxide reduction and nitrogen fixation technologies based on semiconductor photocatalysis and photoelectrocatalysis have realized the direct conversion of dispersed and discontinuous solar energy into stable chemical energy storage, which is an important direction to alleviate environmental problems and solar fuel research[2⇓⇓⇓~6]. However, due to the constraints of solar energy absorption, the separation efficiency of photogenerated carriers and the effectiveness of catalytic reaction, the light energy conversion efficiency achieved by current research is low[7,8]. Because the lifetime of photogenerated electrons and holes is often in the nanosecond and picosecond range, it is very easy to recombine and lose energy, so the key to improve the efficiency is to suppress the recombination of photogenerated electrons and holes and achieve effective charge separation. The continuous search for efficient and inexpensive semiconductor photocatalytic materials and the construction of systems that facilitate the separation of photogenerated electron-hole pairs are particularly important for improving the photoconversion efficiency.
Carbon materials play an important role in daily life. With the advantages of low cost, high energy conversion efficiency and environmental friendliness, carbon-based catalysts have become an important research direction[9⇓⇓~12]. Different ways of hybridization endow carbon materials with diversity. With the successive discovery of fullerenes, carbon nanotubes and graphene, it has been proposed whether there are new materials with mixed hybrid carbon. The discovery of graphyne has achieved a great breakthrough in carbon materials. Simply speaking, it is formed by inserting acetylenic bonds between benzene rings, with sp and sp2 hybrid carbon, two-dimensional planar structure similar to graphene, and diverse chemical structures. First-principles calculations show that the unique structure not only makes it highly π-conjugated and good conductivity, but also has a tunable natural band gap, which is a semiconductor material[13,14]. More importantly, graphyne has a unique porous framework structure, and its atomic arrangement is quite different from the compact structure of other carbon materials. This is very beneficial to the diffusion and transfer of ions and gases. Meanwhile, the high specific surface area originated from the pore structure provides abundant active sites for the storage of electrons or ions. The high charge density near the acetylene bond in graphyne provides the adsorption ability for ions and gases, and also provides the catalytic ability to some extent[15]. In addition, the mild synthetic route endows graphyne with functional tunability and good film-forming ability. Therefore, in the past decade, graphyne materials have gained extensive attention and research in the fields of optics, electronic information, energy conversion and storage, catalysis, environment and biomedicine (Figure 1)[16⇓⇓⇓⇓⇓⇓⇓~24]. In particular, the strong motivation and enthusiasm of researchers for the development of graphyne has led to its rapid development in the field of photoinduced catalytic chemical reactions (Fig. 2). In this paper, the application of graphyne and graphyne-based photocatalytic materials synthesized in experiments in solar energy conversion is introduced.

2 Synthesis and characterization of graphyne

2.1 Discovery and synthesis of two-dimensional graphdiyne film

Baughman et al. First proposed a theoretical model of graphyne in 1987, which is a series of new two-dimensional carbon allotropes composed of sp and sp2 hybridized carbon atoms[25]. Apparently, sp and sp2 hybridized carbon atoms can be interconnected by covalent bonds according to certain rules, resulting in a wide variety of 2D structures. Thus, "graphyne" is a common name for a class of structures in which two adjacent sp2 hybridized carbon atoms are connected by n "— C ≡ C —" bonds. We can classify such materials simply according to the number of "— C ≡ C —". They are referred to as graphitic monoyne, graphitic diyne, and graphitic nyne, as shown in Figure 3A. In the original work, Baughman named it α, β, γ-graphyne according to the number of carbon atoms on the graphyne ring, where α and β refer to the number of carbon atoms on the smallest ring and the number on the adjacent smallest ring, respectively, and α and β rings are connected by C(sp2)C(sp)C(sp)C(sp2)[25]. The index γ is the number of carbon atoms of the third ring, connected to the β ring by C(sp2)C(sp)C(sp)C(sp2). For example, Figure 3D is named 6,6,12-graphyne according to this nomenclature. Since then, for convenience, several commonly used graphyne have been named with Greek letters, known as the customary nomenclature, which has been widely used to this day[26]. For example, 12,12,12-graphyne is named β-graphyne (Figure 3C). Fig. 3 shows several γ-graphyne (fig. 3A), α-graphyne (fig. 3B), and β-graphite (fig. 3C) named according to the conventional nomenclature.
The different arrangement rules of sp and sp2 hybrid carbons in graphyne determine their different structural characteristics, thus bringing about the diversity of properties. In general, as the proportion of alkyne-linked units in the graphyne network increases, its stability decreases[14]. So the most stable one is γ-graphyne (concentration of acetylene bond 33.33%), followed by β-graphyne (concentration of acetylene bond 66.67%) and α-graphyne (100%). The acetylenic bond ratios of 6,6,12-graphyne and graphdiyne are 41.67% and 50%, respectively. The theoretical results show that γ-graphyne is a high mobility semiconductor, while α- and β-graphyne show different numbers and positions of Dirac points[27⇓~29]. Wang et al. Compared the fracture stress, strain and Young's modulus of different types of graphyne, and found that the fracture stress and Young's modulus of graphyne decreased with the increase of the proportion of alkynyl linkage unit[30]. This is attributed to the decrease of atomic density and weak carbon-carbon single bond brought by "— C ≡ C —". Moreover, first-principles calculations have been used to reveal elastic constants and strain-tunable band gaps of the graphyne family[31]. Different carbon network structures bring about the diversity and variability of band structure, electrical properties, mechanical properties, optical properties, magnetic properties and thermal properties, and have extremely high structure and size dependence[13,32⇓⇓⇓⇓⇓⇓ ~39].
Most of the predictions on the structure and properties of different graphynes remain in the theoretical stage, among which γ-graphyne has attracted the attention of scientists by virtue of its natural tunable band gap and high stability. A great deal of prior work has been devoted to the synthesis of monomers or oligomers of graphyne[26]. Until 2010, Li Yuliang's research group of the Institute of Chemistry of the Chinese Academy of Sciences used hexaalkynylbenzene as the precursor to synthesize and obtain a large-area (3.61 cm2)γ-type graphite diyne film on the surface of a copper sheet through a cross-coupling reaction, which was named "graphdiyne" in English (Figure 4A), or GDY for short (the graphite diyne mentioned in the article refers to this kind of γ-type graphite diyne)[40]. This work has achieved zero breakthrough in the synthesis of graphyne laboratory and has become a leading discovery of Chinese native scientists in the field of world science.
The synthesis method of graphdiyne film is shown in Fig. 4. Based on the successful preparation of hexaalkynylbenzene (product 2), the graphyne (product 1) film was obtained on the copper film by alkyne-acetylene coupling. The key of this reaction is the use of copper film, which can play the role of "catalysis" and "planar template" at the same time. On the one hand, in pyridine solution, a small amount of copper ions will be produced on the surface of the copper film, which will form a complex with pyridine and become a catalyst to promote the ordered alkyne coupling reaction; On the other hand, the planar structure of the copper film allows graphyne to grow along the surface of the copper film, thus forming a graphyne film with 3.6 cm2. In order to realize the controllable growth of graphite diacetylene with different layers, in 2015, Li Yuliang's group realized the preparation of GDY films with different layers on zinc oxide nanorod arrays by vapor-liquid-solid (VLS) method[41]. Using graphyne powder as vapor source, two-dimensional nanostructured GDY thin films were prepared by combining the reduction process of ZnO with the autocatalytic VLS growth, in which the reduced Zn droplets from ZnO acted as both the catalyst and the adsorption sites of GDY during the VLS growth. Large area ordered semiconductor films with high surface quality and high conductivity (2800 S/cm) have been realized by this method. The synthesis method on the surface of copper foil fully illustrates the importance of confined reaction space in the controlled synthesis of graphdiyne.
Subsequently, Nishihara's group realized the preparation of graphdiyne films at liquid-liquid and vapor-liquid interfaces, respectively, through the method of interface construction[42]. As shown in fig. 5A, B, in the liquid-liquid interface synthesis method, the monomer molecule hexaalkynylbenzene is dissolved in the lower dichloromethane, and the catalyst copper acetate and pyridine are placed in the upper aqueous phase. Under the protection of Ar atmosphere, the catalytic coupling reaction continued for 24 H at room temperature, and the graphdiyne multilayer film (24 nm thick) was formed at the liquid/liquid interface. The two solutions were separately replaced with pure solvent to clean the film and then transferred to a flat substrate. Thinner and higher quality graphdiyne can be obtained at the gas-liquid interface (Fig. 5C ~ f). A very small amount of hexabromobenzene precursor was dissolved in a mixed solution of dichloromethane and toluene and gently placed on the surface of an aqueous solution containing copper catalyst. Under the inert atmosphere, the organic solvent was rapidly volatilized, and the catalytic polymerization reaction lasted for 24 H at the gas/liquid interface, resulting in the formation of GDY nanosheets floating on the interface. The nanosheet is not visible to the naked eye, but can be characterized by Langmuir-Sch Schäfer transfer onto a planar substrate. Graphite diacetylene single crystal nanosheets with uniform thickness (3. 0 nm), uniform lateral size (1. 5 μm) and regular hexagonal structure were obtained by this method. At the same time, the stacking model of multilayer graphite diacetylene is analyzed. This study opens up a new method for the synthesis of graphdiyne by solution polymerization, and also lays a foundation for the synthesis and fine structure characterization of high-quality few-layer or single-layer graphdiyne crystals.

2.2 Morphology-controlled preparation of graphite diacetylene

After the synthesis of graphdiyne films, the in-depth study of two-dimensional graphdiyne and graphdiyne has laid a foundation for the development of low-dimensional aggregation structure of sp-sp2 hybrid carbon materials, and a series of progress has been made in the controlled synthesis of graphdiynes. Zero-dimensional, one-dimensional, and three-dimensional graphdiyne structures were successively synthesized[43⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~58]. Huang et al. Prepared graphdiyne nanoparticles of about 10 nm from top to bottom using 2D graphdiyne powder as raw material by simple manual grinding combined with ultrasonic method[59]. In 2019, the research group of Nie Guangjun of the National Center for Nanoscience successfully prepared graphite diacetylene quantum dots (GDQDs) with uniform size and high crystallinity and applied them to biological imaging[49]. The preparation method is as follows: firstly, hexaalkynylbenzene is used as a raw material to synthesize GDY on the surface of copper, and then graphyne oxide (GDYO) is prepared by a modified Hummer method. The GDYO/water suspension was mixed with ammonia and sonicated, and then subjected to hydrothermal reaction. The purified GDQDs were obtained by ultrafiltration and purification of the product.
Shortly after the synthesis of two-dimensional graphite diacetylene, Li Yuliang's research group successively synthesized one-dimensional structures such as nanotubes and nanowires of graphite diacetylene[46,50]. Graphite diacetylene nanotube arrays (GDNT) were prepared using an anodic aluminum oxide template under copper foil catalysis (Figure 6A)[46]. Graphyne nanowires (GDNWs) were fabricated on ZnO nanorod arrays by VLS method, with excellent conductivity and mobility (Fig. 6 B)[50]. One-dimensional GDY nanowires can also be obtained by surface confined covalent coupling on the silver surface[54]. Xu et al. Of Tongji University also used the surface synthesis method to prepare graphyne nanowires with alkynyl groups connected to benzene rings by two different types of dehalogenation homocoupling reactions in steps (Fig. 6C)[53]. The preparation of one-dimensional graphyne realizes the band control of graphyne, verifies the theoretical prediction, and broadens the application prospect of graphyne in electronic devices[32-33].
The synthesis of graphyne with specific morphology is a major challenge. The choice of catalyst and the control of monomer concentration are essential for the structural control of the polymerization product. Inspired by this, Liu Zhongfan, Zhang Jin and Li Yuliang developed an improved Glaser-Hay coupling reaction method based on the solution phase surface polymerization method, and successfully synthesized a new graphdiyne nanowall structure (Fig. 7A)[54]. Hexaalkynylbenzene and copper plate were still used as precursor monomer and substrate respectively in the synthesis process. By adjusting the addition ratio of the organic base to the hexaalkynylbenzene, a proper amount of copper ions are firstly dissolved to form catalytic reaction sites. The Glaser – Hay coupling reaction occurs immediately at these sites. GDY first grows vertically at these sites as monomers are slowly added. Then as the reaction proceeds, more copper ions dissolve, forming uniform and compact GDY nanowalls (GDNWs) on the surface of the copper substrate. The structure has abundant tips, which can realize the preparation of graphite diacetylene hierarchical structure. In order to overcome the problem of limited substrate selection caused by the dependence on copper substrate, Liu Zhongfan, Zhang Jin and Wu Lizhu jointly developed a new way to synthesize GDY using copper cladding catalysis, which has become a general and simple method to prepare GDY nanowalls on any substrate[43]. As shown in Fig. 7 B, this method borrows from the copper foil encapsulation concept in graphene CVD growth to put the target substrate into a copper foil "envelope" to ensure that a sufficient concentration of catalyst can diffuse to the interface between the target substrate and the solution to induce the in situ growth of GDY. The prepared copper film can be used not only as a catalyst reservoir, but also as a container for any substrate, realizing a variety of multi-dimensional substrates such as 1D (Si nanowires), 2D (Au, Ni, W foils and quartz) and even 3D (stainless steel mesh and graphene foam). Inspired by the alkyne-acetylene coupling reaction in solution and on the surface of noble metals, Zuo et al. Developed the "explosion method" to prepare graphite diacetylene in large quantities in the atmosphere (Fig. 7c)[55]. The cross-coupling reaction of hexaalkynylbenzene was controlled by three thermal treatments, and the highest yield was 98%. In order to further reduce the preparation cost, Liu Zhongfan and Zhang Jin cooperated to successfully prepare unsupported three-dimensional GDY using natural abundant and cheap diatomite as a template (Fig. 7d)[48]. In addition to the inherent characteristics of GDY, the as-prepared 3D GDY has good mechanical properties, porous structure and high specific surface area, which enrich the application of GDY. The controlled preparation of different aggregation States and morphologies of graphite diacetylene is complementary to the theoretical calculation research, which not only broadens the application range of graphite diacetylene and improves the application performance from multiple dimensions, but also broadens and optimizes the synthesis method, which is conducive to the rational design and synthesis of more targeted applications of graphite diyne[26,39,60,61].

2.3 Mechanochemical synthesis of γ-graphyne and its derivatives

With the in-depth study of the synthesis and application of graphite diacetylene, the known preparation methods have the disadvantages of complex steps, high cost, unstable required monomers and cumbersome preparation process. Inspired by the preparation of acetylene carbon materials by ball milling, Cui Xiaoli's research group of Fudan University prepared γ-graphite monoyne (γ-GY, Graphyne) with a single alkynyl connected to benzene ring for the first time through a simple mechanochemical synthesis method in 2018, which added a new member to the synthetic Graphyne family and made a breakthrough on the basis of theoretical calculation[62][63⇓~65]. As shown in Fig. 8A, the method uses calcium carbide and hexabromobenzene/benzene as raw materials, uses the interfacial mechanochemical reaction under high-energy ball milling to connect the alkyne group with the benzene ring, and combines with the subsequent heat treatment and acid washing purification to obtain γ-graphite monoalkyne. The yield of the prepared graphite monoacetylene is as high as nearly 2g/tank, and the laboratory macro preparation in gram can be realized. Its optical band gap and semiconductor properties were systematically studied, and it was confirmed to be a p-type semiconductor with a band gap of about 2.58 eV, and the valence and conduction band positions were at 2.53 and -0.05 V (vs NHE), respectively (Fig. 8 B, C)[63]. The research group further prepared nitrogen-doped graphite monoacetylene (NGY) by adding nitrogen precursor in the ball milling process and atmosphere heat treatment, which broadened the synthesis path of graphite monoacetylene materials and their applications in electrocatalysis, lithium-ion batteries and photocatalysis (Fig. 8 d ~ f)[64⇓⇓~67]. Compared with the liquid phase coupling method for preparing graphite diacetylene, the mechanical ball milling method has the advantages of low cost, simple procedure, no by-product and large-scale application.

2.4 Characterization methods and techniques of graphyne

It is important to explore the synthesis of high-quality graphyne materials and reveal the relationship between their structure and properties, in which the characterization and identification of the structure is the key. The characterization methods of graphyne reported so far can be divided into two categories: imaging and spectroscopy. Among them, imaging techniques include scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The spectra mainly include Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) absorption spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS). SEM and AFM were mainly used to characterize the morphology of graphyne, while TEM, EELS, EDS, Raman, XPS, XRD, FT-IR, UV-vis, NMR and XAS were used to characterize the crystal structure, chemical and bonding characteristics.
SEM can provide deep focus and high resolution images, making complex and rough surfaces or topographies clearly imaged. Generally speaking, for graphyne based materials, SEM is used in the following cases: first, direct imaging of various morphologies and sizes; The second is to characterize the thickness and cross-section of the two-dimensional film (Fig. 9 a, B)[54,69,70]. AFM is another common method to reveal the morphology of materials. The advantage over SEM is that the sample can be imaged in air or liquid for high-precision measurement of surface roughness and thickness of ultrathin two-dimensional graphyne films (Figure 9 C)[40,42,43,54]. TEM and its related electron diffraction, simulation, EDS and EELS are one of the most powerful methods to characterize the morphology, crystal structure, composition, thickness and chemical valence of graphyne[56,65,68,71]. The crystal structure and lamellar spacing of graphyne can be characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) (Fig. 9 d ~ I)[40,64,72,73]. The bonding of carbon can be qualitatively and quantitatively analyzed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, solid-state nuclear magnetic resonance (13C NMR) spectroscopy and ultraviolet-visible (UV-Vis) absorption spectroscopy[69,74,75]. 13C NMR and XAS can analyze the structural information of different carbon species in graphyne more precisely, but they are not common compared with other characterization methods because of the high requirements for equipment[76⇓⇓~79]. The following is a detailed introduction to two main means of characterizing graphyne carbon bonding, which can be distinguished from graphene carbon structure.
Raman spectroscopy is one of the most commonly used, rapid and non-destructive and high-resolution techniques for the characterization of carbon materials. For graphyne, Raman spectroscopy can qualitatively identify the degree of structural order and detect the presence of acetylenic bonds. The theoretical calculation shows that there are six strong Raman peaks in the Raman spectrum of graphite diacetylene (Fig. 10a)[80]. The B peak is at 956 cm-1, there is also a G ′ peak at 1364 cm-1, a G ′ peak at 1478 cm-1 and a G peak at 1521 cm-1. These are attributed to breathing vibrations of the alkyne-related and benzene rings, shear vibrations of atoms in the benzene ring, and vibrations of the C — C bond between the tricoordinated carbon atom and the dicoordinated ortho site and stretching of aromatic bonds, respectively. The Y peak at 2142 cm-1 is from the synchronously contracted/stretched triple bond. The Y 'peak is at UNK 1, which is another stretching mode of acetylenic triple bond. The experimental results of graphyne are shown in Fig. 10B, which is the Raman spectrum of the first synthetic graphdiyne film[40]. The four prominent peaks are at 1382.2 cm-1, 1569.5 cm-1, 1926.2 cm-1, 2189.8 cm-1, which correspond to the D band, G band and the stretching vibration of the conjugated diyne bond, respectively. The D band is a disordered band related to structural defects, and the smaller the ratio of D band to G band is, the less defects and the better the crystalline state of graphyne is. The D/G intensity ratio in fig. 10B is about 0.729, indicating that the prepared graphyne film is highly ordered and multilayer. Fig. 10c shows the Raman spectrum of graphyne first prepared by Cui et al[63]. The G band around 1583 cm-1 comes from the stretching vibration of the hybridized carbon of the benzene ring sp2. The D band at around 1360 cm-1 originates from defects and disordered structure. The G 'band near 2686 and 3019 cm-1 is due to the double-resonance intervalley Raman scattering process. The weak peaks at 2095 and 2250 cm-1 are attributed to the vibration of the conjugated ethynyl group caused by the sp-hybridized carbon. The results of Raman spectroscopy confirmed sp and sp2 hybridized carbon.
X-ray photoelectron spectroscopy (XPS) can be used to qualitatively and quantitatively analyze the bonding mode of carbon in graphyne. Fig. 10 d ~ e show the XPS spectra and C1s elemental spectra of graphite diacetylene[40]. The peak separation results show that the peaks at 284.5, 285.2, 286.9, and 288.5 eV correspond to the C 1s orbitals of C—C(sp2), C — C (sp), and C — O, C = O, respectively. The area ratio of sp/sp2 is 2, which confirms that the benzene ring is connected with other rings through two alkynyl groups in the prepared graphyne film, which is consistent with the structural model of graphdiyne. The area ratio of (Fig. 10f),sp/sp2 in the C 1s XPS spectrum of graphdiyne is about 1, which confirms the carbon network structure with benzene rings connected by a single ethynyl group[63].

3 Preparation and Application of Graphene Based Photocatalytic Materials

Graphyne is considered to be a good electron acceptor because of its huge highly conjugated structure, and it has a large number of free electrons and good electron donating characteristics, so it is called "electron sponge" and shows significant electron transfer enhancement characteristics[81]. In addition, γ-graphyne has a uniform subnanometer pore structure, which can provide a large number of available active sites and improve the effective mass transport. According to the calculation, the unique electronic characteristics make it have a tunable band gap of 0.46 ~ 2.23 eV[26]. Therefore, with its large surface area, high carrier mobility and tunable electronic band gap, compared with other carbon-based materials, graphyne is considered to be a better promoter to promote the separation of photogenerated carriers, and has great potential in improving the efficiency of photocatalysis. In the following sections of this review, several major applications of graphyne based materials in photocatalysis will be discussed: (1) degradation of pollutants in water; (2) photoelectrocatalytic electrode materials; (3) water cracking to produce hydrogen and oxygen; (4) photoreduced CO2. Table 1 comprehensively summarizes the reported applications and performances of GYs-based photocatalysts.

3.1 Sewage treatment

The surface freshwater resources available to human beings account for only 0.01% of all the water on the earth, and human beings discharge a large amount of sewage into the environment every year. How to purify water at low cost is one of the important problems faced by production and life. Using solar energy to decompose bacteria and pollutants in water is one of the important applications of photocatalysis. Carbon-based materials with π-conjugated system can often induce the synergistic effect between metal oxides and carbon, and achieve the purpose of enhancing the effect by expanding the optical absorption range or inhibiting the recombination of electron-hole pairs[109,110]. The application of graphyne has achieved the removal and degradation of different pollutants in the laboratory.
Wang Dan's research group prepared graphite diacetylene (GDY)-P25 TiO2 composite photocatalyst by hydrothermal method, which has higher degradation efficiency of methylene blue than pure TiO2, TiO2- carbon nanotubes and TiO2- graphene under both ultraviolet and visible light (Fig. 11 a)[82]. They believe that the 2p orbital of carbon is inserted into the TiO2 band gap as an impurity level, so that the electrons of the complex can jump from the valence band to the impurity level, and then from the impurity level to the conduction band more easily, thus broadening the absorption range of the material and promoting the interface electron transport. In order to further understand the surface and interface charge transport mechanism of TiO2 and graphite diacetylene composite, the research group designed and synthesized TiO2 with exposed crystal faces, and obtained TiO2(001)-GDY and TiO2(001)-GR photocatalysts by compounding with GDY and graphene (GR) through hydrothermal method (Fig. 11b)[111]. The results of functional theory calculations show that the adsorption activity of GDY on each crystal plane of TiO2 is higher, forming strong C — O bonds, and at the same time, it has stronger surface charge trapping ability than TiO2- graphene, which is beneficial to the electron transport at the composite interface. Experimental and theoretical studies have shown that TiO2(001)-GDY has stronger charge separation ability, abundant impurity energy levels and higher oxidation ability, so it shows a degradation rate constant of TiO2(001)1.63 times and TiO2(001)-GR 1.27 times. Feng et al. Prepared graphite-diacetylene hybrid exposed (001) facet nitrogen-doped TiO2 nanosheets (GD-NTNS) by hydrothermal method and used as photocatalyst to improve the photodegradation efficiency of rhodamine B under visible light[84]. The enhanced photoactivity is attributed to the synergistic effect of GDY and nitrogen, in which GDY acts as an "electron pool" and plays an important role in promoting the separation of photogenerated electron-hole pairs (Figure 11 C). Other graphyne materials also showed good photocatalytic performance. Using tetraethynylethylene as a precursor, Liu et al. Artificially synthesized a smooth and continuous β-graphyne film with a conductivity of 3.47×10-6S/m and a work function of 5.22 eV on copper foil by a modified Glaser-Hay crosslinking reaction[83]. TiO2@β-GDY prepared by hydrothermal method was used for photodegradation of methylene blue with higher catalytic activity than TiO2@γ-GDY and TiO2. This may be due to the higher content of acetylene bonds in β-GDY than γ-GDY (67% vs 50%), which has a stronger π conjugation. A large number of cyclic diacetylene bonds are beneficial to the formation of carbon-metal bonding, which has a stronger synergistic effect with TiO2 and is conducive to the advantages of electron transport layer.
This synergistic enhancement is also reflected in the combination with other semiconductor photocatalysts. Thangavel et al. Hydrothermally prepared graphdiyne-zinc oxide composite nanomaterials[85]. After ZnO was hybridized with GDY, the recombination of electron-hole pairs was greatly suppressed, and the rate constants for photodegradation of methylene blue and rhodamine B were nearly twice that of ZnO nanoparticles (Fig. 11d). Mao et al. And Yang et al. Prepared photocatalysts by combining γ-graphite diacetylene and graphite monoacetylene with Ag3PO4, respectively[87][86]. By simply mixing hydrophilic Ag3PO4 and hydrophobic GDY, a stable water-in-oil Pickering emulsion was formed, which exhibited a methylene blue degradation rate constant three times higher than that of Ag3PO4-CNT and Ag3PO4- graphene under visible light[87]. The Ag3PO4/GDY-based emulsion also showed higher photocatalytic water oxidation characteristics. The mechanism study shows that the valence band position of GDY is at 1.05 V (vs NHE), which is located in the band gap of Ag3PO4, which makes it easy for GDY to capture photoelectrons on the conduction band of Ag3PO4. In addition, the abundant butadiene bond (— C = C — C = C —) forms a large π conjugated system in GDY, making GDY both an electron donor and an electron acceptor. It efficiently traps electrons photoexcited from Ag3PO4, thereby suppressing electron-hole pair recombination. This result indicates that GDY can narrow the band gap of Ag3PO4, thereby promoting charge separation and improving the photocatalytic activity of the composite. Yang et al. Prepared γ-graphyne by mechanochemical method, and used a facile electrostatic self-assembly method to prepare Ag3PO4@γ-GY photocatalyst (Fig. 11e)[86]. In the visible light degradation of norfloxacin, 2-hydroxynaphthalene and benzene, the apparent rate constant is 9 ~ 20 times higher than that of the Ag3PO4. The significantly enhanced photocatalytic activity is attributed to the construction of an efficient γ-GY electron transport layer. In addition, Chen et al. Designed a novel functional hybrid material based on a variety of advanced carbon materials[89]. Graphene oxide (GO) was used as a bifunctional coupling agent to construct a novel graphite-diacetylene hybrid Ag/AgBr photocatalyst. As shown in Fig. 11f, the activities of Ag/AgBr, Ag/AgBr/GO, Ag/AgBr/GDY and Ag/AgBr/GO/GDY for visible light degradation of methyl orange were compared, respectively. The degradation rate of Ag/AgBr was increased by 8 times and the visible photocurrent density was increased by 22 times by the synergistic effect of GO as the functional component of the binder and GDY as the effective electron separation additive.

3.2 Photoelectrocatalytic electrode

The energy conversion technology of photoelectrocatalysis (PEC) using semiconductors has attracted much attention[112,113]. As a new type of nonmetallic catalyst, graphyne has great advantages in electrocatalytic oxygen reduction[73,114⇓ ~116]. Some research progress has been made in the field of photoelectrocatalysis[117,118].
The research group of Wu Lizhu, Institute of Physics and Chemistry, Chinese Academy of Sciences, first realized the application of graphdiyne GDY as a photogenerated charge transport material[91]. CdSe quantum dots (QDs)/GDY composites were prepared by using the strong π-π interaction between 4-mercaptopyridine modified CdSe QDs and graphite diacetylene, as shown in Fig. 12 a and B. Since the valence band potential of CdSe QDs (∼ 1.23 V vs NHE) is more positive than that of graphdiyne (∼ 1.05 V vs NHE), the photogenerated holes can be conducted from the QDs to graphdiyne. Using CdSe QDs/GDY as the photoelectrode material, the photocurrent density generated under Xe lamp irradiation can reach 70μA·cm-2. This is because of the matching energy levels and the superior hole transfer ability of GDY, which enables the photogenerated holes to be rapidly extracted from the QDs to GDY and then to the counter electrode, while maintaining a faradaic efficiency of (90 ± 5)% in the photoelectrocatalytic hydrogen production test. After that, the researchers realized the modification of GDY on the BiVO4 photoanode. For example, on the porous BiVO4 electrode, the GDY nanowall is modified by the copper-coated catalytic strategy, and by virtue of the effective extraction of photogenerated holes by GDY and the unique vertical nanowall structure, most of the photogenerated holes can reach the electrode/electrolyte interface to participate in the water oxidation reaction, and the photoelectrocatalytic water splitting efficiency is significantly improved (Fig. 12 C, d)[43]. Wu Lizhu and Zhang Jin's research group innovatively improved the hydrophilic properties of GDY by air plasma treatment, and then assembled CoAl-LDH nanosheets on the surface to obtain a good OER electrocatalyst[92]. It was further extended to the BiVO4 photoanode, and the super-hydrophilic CoAl-LDH/GDY/BiVO4 photoanode with high efficiency was fabricated. Superhydrophilic GDY, which plays a key role, has a stronger interaction with the catalyst and adsorbs water molecules around the catalyst, which can greatly promote the mass/electron transport at the interface. The super-hydrophilic GDY has obvious advantages in the photoelectrocatalytic water splitting system, and provides an idea for the super-wetting electrode for gas reaction.
g-C3N4 is a typical nonmetallic photocatalyst, and GDY has a suitable valence band position and a similar conjugated π structure. They form an effective heterojunction through π-π stacking interaction, which can greatly reduce the carrier recombination rate while maintaining the two-dimensional ultra-thin structure, and has become the focus of researchers. The research group of Lu Tong prepared g-C3N4/GDY 2D/2D heterojunction photoelectrocatalytic materials by immersing copper sheets with 3D GDY nanosheet arrays in NMP solution of g-C3N4 through a low-temperature solvothermal method[93]. Benefiting from the excellent hole transfer ability of GDY and the strong interaction of 2D/2D heterojunction, the electron lifetime of the g-C3N4/GDY is prolonged by 7 times and the photocurrent density is increased by 3 times compared with the g-C3N4. The resulting energy level structure is shown in Figure 12E, indicating that GDY can rapidly extract the photo-induced holes of g-C3N4, thus preventing the recombination of photo-induced carriers. Considering that it is the OER reaction that limits the surface dynamics in PEC, Si et al. Further combined the NiFe LDH OER photocatalyst with the layered NiFe LDH OER catalyst to form a stable and efficient g-C3N4/GDY/NiFe LDH photoanode (Figure 12F), constructing a coupling system for efficient photoelectrocatalytic oxidation of water[94]. Among them, GDY and g-C3N4 form a built-in electric field, which is beneficial for hole transfer to GDY. At the same time, strong bonds are formed between Ni2+, Fe2+ and acetylenic bonds, which promotes the further transfer of holes from GDY to NiFe LDH layer. GDY acts as a high-speed channel for holes, so that the holes are quickly transferred from the g-C3N4 to the NiFe LDH, and the water oxidation reaction occurs. In this study, the water oxidation efficiency of PEC was significantly improved by combining interface regulation, charge transport design and energy band engineering. In addition, GDY can also be introduced into a silicon heterojunction Photoelectrochemical cell[96]. With the assistance of magnetron sputtering NiOx, the saturated photocurrent density of SiHJ/GDY/NiOx with optimized NiOx thickness as photoanode can reach 39.1 mA·cm-2.2-fold higher than that of SiHJ/NiOx, illustrating the catalytic effect of GDY itself and the synergistic effect of GDY with NiOx. The results confirm that GDY is a promising photoelectrode material that can achieve efficient PEC performance, which lays a foundation for further research. In recent research work, Zhang et al. Designed and compared three kinds of graphdiyne materials through molecular engineering, and found that Pyr-GDY nanofibers synthesized by coupling reaction with 1,3,6,8-tetraethylpyrene (TEP) monomer showed wider visible light absorption range, improved charge separation/transport efficiency and more active sites for water reduction[98]. The results showed that Pyr-GDY alone exhibited excellent PEC performance, with a cathodic photocurrent density of about 138μA·cm-2(-0.1 V vs NHE), which was 12 times higher than that of the commonly used GDY(12μA·cm-2) coupled with hexabromobenzene. This work provides a new strategy for constructing graphyne-based metal-free photoelectrocatalysts with stronger photoelectrocatalytic performance.
After graphyne was synthesized, Sun et al. First synthesized a series of γ-graphyne /TiO2 nanotube arrays (GY/TNT) heterostructures by drop-coating method[99,119]. The characterization results show the formation of C — O — Ti bonds between γ-graphyne and TiO2 nanotube arrays. The maximum instantaneous photocurrent and photopotential of the TiO2 nanotube array were enhanced by 2.2 and 1.3 times, respectively, after γ-graphyne modification. The efficiency of photoelectrocatalytic degradation of levofloxacin and rhodamine B was increased by 3.64 and 1.35 times, respectively. The heterostructure also exhibited excellent photoelectrocatalytic nitrogen fixation and oxygen evolution ability (Figure 12 G). This is attributed to the strong interaction and matched energy level structure between γ-GY and TNT, as well as the sensitization of γ-GY in the heterojunction. This work realizes the design and application of γ-graphyne in photoelectrochemistry and photoelectrocatalysis.

3.3 Photocatalytic water splitting

In addition to PEC, photocatalytic water splitting to produce green hydrogen is a low-cost, effective and pollution-free energy conversion technology, which is favored by scientists[120]. The research group of Lu Tong prepared CdS/GDY heterojunction by hydrothermal method, which showed much higher activity than CdS and GDY[101]. When the content of GDY was 2.5 wt%, the best hydrogen production activity was 4.1 mmol·g-1. As shown in fig. 13A, the introduction of GDY not only improves the stability of CdS by forming a CdS/GDY heterojunction, but also promotes the separation of photogenerated carriers as a hole transfer material. Si et al. Constructed a Z-type heterojunction Ag3PO4/GDY/g-C3N4 nanocomposite fully water-splitting photocatalyst by electrostatic self-assembly method using the π-π stacking interaction of GDY and g-C3N4 nanosheets (Fig. 13 B)[100]. Exhibited an ultrahigh oxygen evolution activity 753.1 mmol·g-1·h-1, which was 12.2 times higher than that of Ag3PO4 nanoparticles. Among them, GDY not only acts as a charge transfer medium to improve the reaction kinetics, but also acts as an ideal substrate material to stabilize Ag3PO4. Inspired by the synthesis method of GDY copper film, Jin et al. Slowly added the pyridine solution containing monomer hexaalkynylbenzene to the pyridine solution containing CuI dropwise, and reacted at low temperature for 48 H to obtain GDY-CuI photocatalyst in one step[103]. Graphite diacetylene was grown in pyridine solution with CuI as the catalyst carrier, and the prepared composite catalyst was directly used for photocatalytic hydrogen production, which realized the innovation of the method. Under visible light irradiation, the maximum hydrogen production rate of GDY-CuI was 465. 95 μmol/5 H in the solution system with Pt as cocatalyst and dye EY as sensitizer, which was 15. 8 times and 3. 0 times higher than that of GDY and CuI, respectively. The prepared GDY-CuI has a wider light absorption range and enhanced light absorption ability, and the anchoring of monovalent copper ions is the main reason for the photocatalytic activity of GDY. As shown in Fig. 13C and d, in this photocatalytic system, GDY-CuI is a good electron acceptor, which can quickly transfer electrons from EY-· to Pt active sites, thus participating in the reaction of proton reduction to hydrogen. Photoelectrochemical measurements further show that the GDY-CuI hybrid structure prepared in one step can effectively accelerate electron migration. This innovative synthesis method confirms that the monovalent copper ion can also be used as the co-catalytic ion of GDY, which undoubtedly provides a new reference for the further synthesis and application of GDY. In addition, Li et al. Recently reported the synthesis and self-assembly (PDBA) of a crystalline dehydrobenzoannulene-based three-dimensional graphite diyne[104]. The typical helical fiber structure was formed by polymerization-induced self-assembly, which showed good photocatalytic hydrogen production rate in the solution system with Pt as co-catalyst and triethanolamine as sacrificial agent, and the apparent quantum yield was up to 4. 68% at 340μmol·h-1·g-1,420 nm. This discovery opens new avenues for the development of novel functional carbon materials by controlling the size and shape of acetylenes.
Graphite monoacetylene (γ-GY) was recently synthesized by Cui Xiaoli's group and successfully applied to hydrogen production by water photolysis[66,67]. As shown in Fig. 14 a ~ C, the TiO2/γ-GY binary hybrid photocatalyst was first prepared by a one-step hydrothermal method, and the optimized hydrogen production rate reached 77.6 μmol within 4 H, which was P25 TiO28.4 times higher[66]. This is attributed to the construction of type II semiconductor heterojunction, which improves the visible light absorption while suppressing the recombination of photoexcited electron-hole pairs. The ternary TiO2/MoSe2/γ-GY nanocomposite was further prepared (Fig. 14 d ~ f) and proved its excellent photocatalytic hydrogen evolution activity[67]. The hydrogen production rate of the optimal sample is 16μmol·h-1, which is 3.2 times that of binary TiO2/MoSe2 and 6.2 times that of TiO2. The enhancement of photocatalytic activity was achieved by the transfer effect and synergistic effect driven by the hierarchical linkage of the composite heterojunction. MoSe2 and γ-GY act as an electron collector and a hole transport channel, respectively, while improving the light absorption ability, which can effectively separate the photogenerated electron-hole pairs and inhibit their recombination, thus enhancing the photocatalytic activity. The TiO2/MoSe2/γ-GY multicomponent heterojunction constructed by improving the light-harvesting ability and providing more active edge sites and charge transfer channels provides a practical method for band engineering to design non-noble metal photocatalysts for total water splitting. The results show that γ-GY has good stability and semiconductor properties, which is beneficial to the improvement of photocatalytic performance. At the same time, the surface structure, morphology and dosage of γ-GY have a great impact on the photocatalytic performance, which provides a broad research and development space for its application in the direction of artificial photosynthesis.

3.4 Photoreduced CO2

The use of fossil fuels and the continuous emission of carbon dioxide have caused serious problems of energy shortage and environmental pollution. In order to achieve the global strategic goal of "carbon neutrality", the reduction and conversion of carbon dioxide are important research topics. Photochemical conversion of carbon dioxide to solar fuels is a sustainable strategy that is clean, "carbon neutral" and simultaneously mitigates global energy and environmental issues.
The research group of Yu Jiaguo first prepared TiO2 nanofibers and GDY composite photocatalyst for CO2 reduction through a simple electrostatic self-assembly method, and proposed the concept of GDY as a "co-catalyst" to act as an electron "trough" and adsorption site in the photocatalytic process (Fig. 15 a, B)[105]. The experimental results showed that the CO production increased significantly with the increase of GDY content, and the highest was 50.53 mmol·h-1·g-1. On the contrary, the formation rate of CH4 was slightly increased, indicating the strong selectivity of GDY for product CO. The enhancement mechanism mainly includes the following aspects: 1) The delocalized electrons in GDY can hybridize with the unoccupied orbitals in TiO2, forming an interfacial built-in electric field (IEF) from GDY to TiO2 to promote carrier separation; 2) GDY can effectively adsorb and activate CO2 molecules, which is beneficial to the subsequent photoreduction process; 3) Photoinduced electron transfer to and accumulation on GDY, which catalyzes CO2 photoreduction, preferably CO; 4) The photothermal effect of GDY can heat the local environment and promote the adsorption of CO2 molecules. Therefore, the TiO2/GDY heterostructure has an electron-directed transfer path and improved charge separation efficiency, an enhanced CO2 adsorption activation effect, and an accelerated catalytic reaction, thereby significantly improving the photoreduction efficiency of CO2 with obvious selectivity. This work shows the great application potential of GDY in catalytic CO2 reduction. Furthermore, the research group used CdS as a model semiconductor to grow CdS nanocrystals on the surface of GDY in situ by solvothermal method, and prepared CdS/GDY photocatalysts with different mass ratios, which showed high activity, selectivity and stability in the gas-phase CO2 photoreduction reaction without sacrificial agents (Fig. 15C)[106]. The experimental and theoretical results confirm that the chemical bonding between GDY and CdS is stronger than that of graphene/CdS. Sulfur vacancies in CdS as well as electron-deficient acetylenic bonds in graphite diacetylene provide sufficient adsorption sites for CO2. These are conducive to efficient electron transfer and subsequent carbon dioxide storage and reduction reactions. This study further illustrates the excellent performance of graphite diacetylene and its broad application prospect as a new material in the field of solar energy conversion.
Huang Baibiao's research group recently reported a g-C3N4 nanotube (CNtb)/graphite diacetylene (GDY) composite for photocatalytic reduction of CO2 (Fig. 15d)[107]. The composite photocatalyst was prepared by self-assembly of g-C3N4 nanotubes and layered graphite diacetylene in N-methylpyrrolidone solvent. The test results under visible light showed that the production rates of CO and CH4 could reach 29.33 μmol/G and 7.71 μmol/G, respectively. Compared with pure CNtb, the CNtb is improved by 18 to 20 times. This is mainly attributed to the enhancement of visible light absorption by GDY and the promotion of electron-hole pair separation by high carrier mobility.

4 Conclusion and prospect

In this paper, the synthesis and structure of graphyne, as well as its application in the field of photo/photoelectrocatalysis, including sewage treatment, water splitting to produce hydrogen and oxygen, photoelectrocatalysis, nitrogen fixation and carbon dioxide reduction, are reviewed. As a new type of synthetic carbon allotrope, the electronic structure of sp2 and sp hybridization and the characteristics of layered two-dimensional plane endow graphyne with rich chemical bonds, large conjugated system, wide interplanar spacing, uniformly dispersed pore configuration, controllable band structure, high hole/electron mobility and inhomogeneous electronic structure. These properties make graphyne a new star to be developed urgently, and it has important application prospects in the fields of electronic information, energy storage, catalysis and optoelectronics. At present, γ-graphyne, β-graphyne and other graphynes and their derivatives have been successfully synthesized in the laboratory, and have been applied to the field of photocatalysis and photoelectrocatalysis, among which γ-graphdiyne (GDY) is the most widely studied. The special structural characteristics of graphyne make it have stronger affinity with other catalysts, especially metal-based catalysts. When graphyne is combined with metal nanoparticles or semiconductors, it can greatly improve the separation and transport of charges in the process of photoinduced catalytic conversion, and provide good electrical contact and low interfacial resistance for the combination between semiconductors or metals, which has great potential in the preparation of multi-component catalysts. In addition, the doping sites are rich, and the chemical customization of the alkynyl-containing monomer is strong, so that the graphyne can be used as a high-performance non-metallic single-component catalyst. However, the research on graphyne materials has just started, and there are many kinds of graphyne materials, and the preparation technology and basic theory are still in the stage of development, improvement and accumulation. The research in the field of photocatalysis and photoelectrocatalysis is still in its infancy, which can be further explored and developed from the following aspects.
(1) Low-cost, high-quality, high-volume preparation methods are the cornerstone of basic research and practical applications. At present, there is still much room for improvement in the synthesis of graphite diacetylene. In addition, it is a challenging development direction to adjust the number of acetylenic bonds and the ratio of carbon skeletons, optimize the electronic structure and chemical characteristics, design new graphyne and its derivatives for high photocatalytic activity, and explore their preparation methods. In addition, there are only a very few examples of graphdiene-based catalysts for carbon dioxide reduction and nitrogen fixation compared with photodegradation and water splitting, and therefore, more emphasis should be placed on the development of graphdiene-based catalysts for CO2 reduction and nitrogen fixation.
(2) At present, a large number of research results confirm that graphyne mainly acts as a charge transport layer in heterogeneous catalysts, which can promote the separation of electron-hole pairs and the improvement of photocatalytic efficiency. This takes advantage of the excellent conductivity of graphyne. However, as an intrinsic semiconductor different from graphene, the semiconductor type, carrier concentration and mobility, band gap and energy level position of graphyne are closely related to its optical absorption characteristics and catalytic ability. At present, the research in this direction remains at the stage of theoretical calculation, and the lack of this basic research system has seriously affected the rational design and promotion of graphdiene-based photocatalytic materials, so such basic issues as the use of graphdiene-based semiconductors for photocatalytic reactions need to be systematically studied.
(3) In the construction of photo- (electro-) catalysts, more future research should focus on the following points: (I) construction of hierarchical graphyne based structures for enhanced light absorption; (ii) Construct more kinds of heterojunctions, establish directional charge transfer channels, and explore the separation mechanism of photoinduced holes and electrons; (iii) assembly of co-catalyst to further accelerate that kinetics of surface reaction on graphyne; (iv) There is a strong chemisorption between metal atoms and graphyne, and the sp-hybridized alkynyl carbon in the hexagonal pores of graphyne provides an ideal site for fixing the dispersed noble metal single atoms, which is an excellent support for single-atom dispersed catalysts. Single atom photocatalyst based on graphyne has a huge exploration space in improving the photocatalytic performance.
(4) The high activity of the acetylene bond unit in graphyne provides a good platform for its chemical modification and doping. At present, although a large number of doped graphite diacetylenes have been found and applied in electrocatalytic oxygen reduction, little research has been devoted to photocatalysis and photoelectrocatalysis of various reactions. Heteroatom doping will change a series of physical and chemical properties of graphyne, such as molecular structure, energy band, active site and charge mobility. Therefore, systematic studies on the controlled synthesis, electronic structure and band tuning, carrier density and mobility, and reactive active sites of heteroatom-doped graphyne and its derivatives play an important role in their application in a variety of photocatalytic and photoelectrocatalytic reactions.

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Funding

National Natural Science Foundation of China(52002241)

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