1 Introduction
With the development of society, energy shortage and environmental pollution have become the focus of attention
[1]. Photocatalysis, as a green advanced oxidation technology, has a wide application prospect in the field of environmental pollution control and energy conversion. Photocatalytic technology can convert low-density solar energy into chemical fuels in the form of hydrogen and hydrocarbons, thus solving the problem of energy shortage
[2⇓⇓⇓⇓⇓~8]. Can degrade or mineralize toxic and harmful organic and inorganic environmental pollutants into low-toxicity and non-toxic substances, thereby reducing environmental pollution; It can also reduce CO
2 into low-carbon hydrocarbons used as fuel, reduce greenhouse effect and solve energy crisis
[9⇓~11][12,13]. Therefore, photocatalytic technology has great development potential in the future production and life
[14⇓⇓~17].
As a kind of widely used photocatalytic materials, inorganic semiconductors have the advantages of high carrier mobility and stable physical and chemical properties, but their absorption bands are narrow and fixed, and the process of band engineering to meet the broad spectral response is more complex, and because of their inherent hardness and brittleness, it is difficult to be made into flexible devices
[18][19]. Compared with inorganic semiconductors, organic optoelectronic materials are rich in variety, easy to tailor their molecular structures, easy to adjust their band structures, and have a wider and more flexible spectral response range
[20]. More importantly, its electronic structure and optoelectronic properties can be easily adjusted by molecular design
[21]. At present, many organic compounds with semiconductor behavior and good stability have been widely used in the field of photocatalysis. Common small molecular organic semiconductor materials include fullerene (C
60), perylene diimide (PDIs), porphyrin (Pors), phthalocyanine (Pcs) and so on
[22][23][24⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~35]. Polymer organic semiconductor materials include polyacetylene, polyaromatic ring and copolymer, such as polypyrrole (PPy), graphitic carbon nitride (g-C
3N
4) and so on
[36,37]. In addition, metal-organic frameworks (MOFs), which are assembled by organic ligands and metal ions, have attracted a lot of attention in the field of optoelectronics because of their excellent properties. However, organic optoelectronic materials have the disadvantage of short lifetime of photogenerated electron-hole pairs, which usually recombine within a few nanoseconds, which greatly reduces the photocatalytic efficiency. Heterostructures composed of organic photocatalysts and other organic/inorganic semiconductors, metals, carbon, etc., can expand the range of light absorption, reduce the recombination rate of electron-hole pairs, and improve the photocatalytic efficiency. Therefore, the design and preparation of organic photocatalysts with high catalytic efficiency and good stability are the focus of research
[38].
Homogeneous catalysis has always been the main mode of action of organic photocatalysts. However, considering the practical problems such as continuous and repeated utilization, heterogeneous organic photocatalysts have gradually become a research hotspot and have been widely used in environmental remediation, such as water and air purification
[39][40⇓⇓⇓~44]. At present, the most basic means of heterogenization is to graft organic photocatalyst molecules onto inorganic carriers including carbon tubes, porous TiO
2, molecular sieves, silica gels and so on, but the grafting process is complicated.It is necessary to consider not only the carrier shadowing effect to facilitate the free diffusion of organic molecules into the pores, but also the selection of appropriate chemical treatment or solvent to ensure the effective loading of organic photocatalyst molecules. With the continuous development and progress of nanotechnology, the assembly of organic photocatalysts into heterogeneous nanostructures is expected to effectively solve the above problems. With the decrease of the size and dimension of nanostructures, traditional materials show new functions and effects, such as stronger Photoelectrochemical activity, higher Photoelectrochemical stability, higher electron mobility and so on. Therefore, nano-organic photocatalysts will be of great significance in efficient and stable photocatalytic applications.
Due to the lack of systematic research in this field, various organic nano-photocatalysts in recent years are reviewed in this paper. The physical and chemical properties of organic semiconductors and their photocatalytic processes are briefly introduced. The effects of structure and photoelectric properties on the photocatalytic performance, as well as their applications in the fields of solving environmental pollution and hydrogen production by water splitting are emphatically introduced. Finally, some challenges and prospects for the future development of organic nano-photocatalysts are proposed.
2 Physicochemical Properties of Organic Semiconductor
There are many kinds of organic semiconductors, which are organic compounds with semiconductor properties. The main components of organic semiconductors, such as carbon and hydrogen, exist in the form of molecules. Organic semiconductors have weak intermolecular interactions, narrow energy bands, wide optical absorption range (Fig. 1), conductivity in the range of 10
-10~100 s·cm
-1, and very few thermally excited carriers
[45]. Organic semiconductors are essentially low-temperature materials that can be processed from the gas phase or solution at temperatures below 100-150 ° C into thin films with a low density of electronic defects. Amorphous organic semiconductors have low carrier mobility (10
-5~10
-3cm
2·V
-1·s
-1) and low current density. Polycrystalline organic semiconductors are capable of providing field effect mobilities higher than 1 cm
2·V
-1·s
-1, such as organic semiconductors in crystalline films and polymer films with mobilities exceeding 5 cm
2·V
-1·s
-1 and 1 cm
2·V
-1·s
-1[46], respectively.
In organic semiconductors, the exciton radius is small and localized, the diffusion length is short (LD = 3-40 nm), and the binding ability between electrons and holes is strong. Weak van der Waals forces between organic molecules tend to result in lower molecular orbital overlap and low dielectric constant ε
r(3~4), favoring the diffusion of photogenerated excitons to the interface of heterostructures with sufficient band offsets to reduce the recombination rate
[47]. At the same time, organic semiconductors have significant size effect, and the limitation of molecular exchange force will lead to the enhancement of nonlinear optical properties. Under the action of π-π conjugation, electrostatic and hydrogen bonding, self-assembly structures are formed between molecules.
Organic semiconductor molecules generally have a typical two-dimensional sheet structure, and π-stacking only exists in the direction perpendicular to the molecular conjugation plane, so three-dimensional organic semiconductors show anisotropy in the crystal π-stacking
[48]. Organic semiconductors are a promising class of organic photocatalysts due to their excellent charge transport properties, high photoluminescence efficiency, and ability to cover a wide emission range in the visible spectrum.
3 Photocatalytic process
The photocatalytic process of organic semiconductors is similar to that of inorganic semiconductors, which can be roughly divided into three processes: photon absorption, photogenerated electron-hole separation or recombination, and photogenerated charge transfer (Figure 2)
[49,50].
I) Photon absorption process: When a semiconductor is irradiated by light with photon energy equal to or higher than its forbidden band width (hν ≥ Eg), some electrons in the Highest Occupied Molecular Orbital (HOMO) will be excited to jump to the Lowest Unoccupied Molecular Orbital (LUMO) to form photogenerated electrons (Highest Occupied Molecular Orbital) and leave the same number of positively charged photogenerated holes (Lowest Unoccupied Mole cular Orbital) in the valence band.
ii) Separation or recombination of photogenerated electrons and holes: After the semiconductor is excited by light, part of the photogenerated electrons and holes migrate to the surface of the semiconductor first, and then recombine and release energy. The time required for this process is 10 ~ 15 s. The shorter the diffusion distance, the less the number of charge recombination and the larger the photocurrent generated.
iii) Charge transfer: After the semiconductor is excited by light, some photogenerated electrons and holes migrate to the surface of the semiconductor. Because of the strong reduction ability of photogenerated electrons and the strong oxidation ability of photogenerated holes, they can react with the substances adsorbed on the surface of semiconductor catalysts to complete the conversion from light energy to chemical energy. Whether the photogenerated charge can be effectively transferred to the surface material depends on the quality (surface functional groups) and quantity (specific surface area) of the active sites on the catalyst surface.
4 Self-assembly of organic molecular
Self-assembly is a process in which the same or different molecules are spontaneously assembled into molecular aggregates with specific structure, morphology and function by means of one or more weak non-covalent interactions between molecules, such as hydrogen bonding, dipolarization, π-π stacking, van der Waals force, hydrophobic interaction, electrostatic interaction, metal-ligand interaction and so on.
Organic semiconductor molecules mostly have conjugated heterocycles and various chromophores, the conjugated length of which can be changed by introducing unsaturated groups such as ethylenic bonds and benzene rings around the chromophores, and the affinity between molecules can be adjusted by introducing hydroxyl, amino, carboxyl and other groups
[51,52]. The introduction of large π-conjugated structures and functional groups makes it possible for organic semiconductor molecules to self-assemble through non-covalent interactions. Molecular aggregates show rich adaptability and responsiveness to external visible light stimuli, and most of the self-assembled structures are ordered at the molecular and nanoscale. Therefore, in the ordered aggregates, the building units are arranged in order, and different dimensional structures will be formed, such as zero-dimensional nanoparticles, one-dimensional nanowires, two-dimensional nanosheets and so on
[53⇓⇓⇓⇓~58].
Nowadays, the research on organic semiconductors has extended from small organic molecules to organic macromolecules and various composite structures. Structural units with different structures, components and functions form heterojunctions through self-assembly technology, which can realize the complementary advantages and synergistic enhancement of each component. Therefore, it is a simple and efficient way to prepare nano-organic photocatalysts by self-assembly. The common nano-assembly method of organic semiconductors is shown in Figure 3, which has a wide range of applications in photocatalytic degradation of pollutants, water decomposition and hydrogen production.
5 Modification Strategies of Organic Nanometer Photocatalyst
The weak coupling between organic semiconductor molecules makes the organic photocatalyst have narrow HOMO and LUMO, and low carrier mobility. However, most organic materials have a high absorption coefficient (>10
5cm
-1), and a very thin organic layer can absorb a large amount of sunlight
[59]. Organic semiconductor photocatalysts can be modified by some chemical means, such as doping, forming heterojunctions, loading appropriate cocatalysts, etc., which can make organic semiconductor photocatalysts have higher photocatalytic efficiency
[60].
5.1 Ingle organic nano photocatalyst
5.1.1 Zero-dimensional organic nano photocatalyst
Zero-dimensional organic nanomaterials are generally assembled by non-polar organic molecules due to their unique photoluminescence properties such as multifunctional fluorescence narrowing and multi-color tunable emission, so more and more work has extended the study of nanostructured materials to the organic small molecule region
[39]. At present, the zero-dimensional nanostructures widely reported include metal nanoparticles, nanoclusters, carbon quantum dots and semiconductor nanoparticles.
Re-precipitation is the most common method for preparing zero-dimensional organic nanoparticles, which induces the nucleation and growth of molecules into nanoparticles by exposing them to poor solvent environment in a very short time. Nakanishi et al. First prepared zero-dimensional organic nanoparticles by reprecipitation method
[61]. Following the pioneering work of this method, perylene nanoparticles were also prepared and the emission of free excitons and self-trapped excitons was observed
[62]. At present, a series of organic nanoparticles, such as perylene nanocrystals and pyrene nanoparticles (PeNPs), have been successfully prepared by the above methods
[63].
Based on the principle of reprecipitation, Zhong et al. Developed an acid-base neutralization method to prepare zinc tetrakis (4-pyridyl) porphyrin (ZnTPP) nanostructures, and achieved the control of morphology and catalytic performance by adjusting pH and surfactant content (myristyltrimethylammonium bromide, MTAB)
[64]. As shown in fig. 4 (a), amorphous nanoparticles, single crystal square nanorods, hexagonal nanorods, hexagonal porous nanodisks and other structures can be obtained by controlling the pH and MTAB concentration. By comparing the photodegradation efficiency of methyl orange (MO) (Fig. 4 (B)), the catalytic efficiency was ranked as single crystal nanodisks (90%, 80 min) > single crystal hexagonal rods > single crystal tetragonal rods > amorphous particles. It can be inferred that the photocharge transport rate of organic nano-single crystals is higher than that of amorphous nanoparticles due to the ordered molecular arrangement and strong intermolecular forces. The specific surface areas of nanodisks, hexagonal nanorods and tetragonal nanorods were 457, 301 and 32.8 m
2·g
-1, respectively. The higher the specific surface area, the higher the active adsorption and reaction sites, and thus the higher the corresponding catalytic efficiency. It can be seen that the assembly of single crystal structure and the improvement of specific surface area are of great significance to the improvement of photocatalytic performance.
In addition to the reprecipitation process, chemical reactions can also be used to generate organic nanoparticles, and size control of organic nanoparticles has been achieved. Kang et al. prepared PeNPs using the reduction of pyrene perchlorate by Br
- in the presence of cetyltrimethylammonium bromide (CTA
+Br
-) in acetonitrile, as shown in Figure 5
[65]. By changing the growth parameters, the size can be controlled between 25 and 90 nm. Similarly, C
60 nanotubes with a diameter of 300 ~ 500 nm and a length of tens of microns and nanospheres with irregular morphology can be prepared by colloidal precipitation method.
In 2017, Kakhki et al. Prepared phthalhydrazide nanoparticles (NPHD) by ultrasonic treatment for photodegradation of a series of azo dyes, including important organic and inorganic pollutants such as MO, methyl red, Congo red, 4-nitrophenol and permanganate
[66]. Recently, Yuan et al. used three different methods to prepare PPy structures: (I) uniform spherical nanostructures (PPy-NS-c) with an average size of 40 nm were obtained by chemical oxidation of the hexagonal mesophase (soft template) (Fig. 6a, B); (ii) template-free method,Nanospheres (PPy-NS-γ) with an average size of 400 nm were synthesized directly by γ irradiation (Figure 6 C, d); iii) template-free method, bulk PPy bulk was synthesized by chemical oxidation, resulting in amorphous particles connected in a beaded structure (Figure 6 e, f)
[67]. PPy-NS-c shows a small absorption in the visible region (≥ 400 nm). In the PPy-NS-γ spectrum, there is a high absorption intensity from the visible region to the near-infrared region (700 ~ 1 000 nm), which is the characteristic of polaron and bipolaron transitions. By comparison, among the three PPy structures, PPy-NS-c had the highest phenol photodegradation activity under UV light, with a phenol degradation rate of about 100% after 4.5 H (Fig. 6G), but the UV reactivity was still lower than that of commercial P25 TiO
2 and TiO
2(Ag-TiO
2) modified with silver nanoparticles (AgNPs). However, PPy-NS-γ showed high photocatalytic activity under visible light, with a degradation rate of 20% after 5 H, which was much higher than that of PPy-NS-c (10%) and PPy-bulk (5%), and its visible-light activity was close to that of Ag-TiO
2 (Figure 6h), so it had excellent visible-light photocatalytic performance.
5.1.2 One-dimensional organic nano-photocatalyst
One-dimensional organic nanomaterials have a large π-π conjugated structure in one direction, and electrons are transmitted directionally in the one-dimensional direction, which is conducive to the separation of photogenerated charges. One-dimensional structures are often prepared by liquid-phase self-assembly. Wang et al. Prepared porphyrin nanotubes by self-assembly of two oppositely charged porphyrin ions in aqueous solution
[68]. Zhao et al. induced the self-assembly of organic small molecule 2,4,5-triphenylimidazole (TPI) in water at different temperatures to obtain single crystal nanotubes and submicron tubes
[69]. In addition to liquid phase self-assembly, one-dimensional organic nanoparticles can also be prepared by organic gel self-assembly, solvent evaporation self-assembly, vapor deposition, template method and so on
[70][71][72][73].
A large number of papers have shown that PDIs are easy to self-assemble, have good environmental stability, strong visible light absorption ability and adjustable molecular orbital energy levels, and have attracted wide attention in the one-dimensional field of photocatalysis. Organic photocatalysts with supramolecular structure formed by self-assembly of PDIs are special n-type organic semiconductor materials. It has high electron affinity and charge carrier mobility. Wang et al. Proposed that one-dimensional supramolecular organic nanofibers formed by H-type α-β self-assembly of PDI molecules can act as powerful and effective photocatalysts for organic pollutant degradation and water oxidation under visible light through π-π stacking and hydrogen bonding without adding metal cocatalysts
[74]. The phenol degradation and oxygen evolution rates of this photocatalyst were one order of magnitude higher than those of unmodified PDI and g-C
3N
4. Most importantly, this class of photocatalysts is very stable to cycling under acidic conditions, which is crucial for industrial applications.
In order to optimize the photocatalytic performance of amorphous or semi-crystalline g-C
3N
4, Li et al. Prepared high-crystalline carbon nitride nanorods (HCCN) by molten salt method
[75]. The excellent crystallinity can reduce the surface defects and hydrogen bonds in the prepared samples, thereby increasing the mobility of photo-induced carriers and reducing the recombination sites of electron-hole pairs. In addition, the K
+ inserted into the HCCN interlayer can also increase the electron delocalization and expand the π-conjugated system, thus promoting the photoinduced electron transfer (Fig. 7). Therefore, the photocatalytic hydrogen evolution rate (HER) of HCCN is about 2 and 10 times higher than that of crystalline carbon nitride (CCN) and bulk carbon nitride (BCN), respectively, when the HCl concentration is 0.1 mol·L
-1.
5.1.3 Two-dimensional organic nano-photocatalyst
The preparation of the organic small molecule material is simple, and the large-area and high-quality two-dimensional photocatalyst material can be obtained by considering the growth of the organic small molecule film in the two-dimensional direction on the basis of successfully obtaining nanoscale and micron-scale crystals. Generally, there are three mechanisms for the preparation of two-dimensional organic small molecule films: self-assembly, domain alignment, and molecular epitaxy
[76][77,78][79]. Seidel et al. studied the epitaxial growth of small organic molecule pyrene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on metal Ag substrate, and successfully obtained monolayer PTCDA by deposition
[80]. However, the development of two-dimensional organic small molecule crystals still faces many challenges, such as the formation of polycrystalline structure in the preparation process of two-dimensional thin films, and many restrictions on the control process of thin film thickness
[81⇓~83].
In recent years, two-dimensional nano-photocatalysts based on macromolecules, such as graphene, g-C3N4 and MOFs, have attracted a lot of attention and research due to their high surface area, porous structure and abundant surface active sites, and have been applied to a variety of photocatalytic reactions.
g-C
3N
4 is a typical two-dimensional polymer with unique electronic band structure and high chemical stability, which can be used as an efficient visible-light active photocatalyst
[84]. g-C
3N
4 nanosheets can be synthesized on a large scale by "green" liquid exfoliation of g-C
3N
4 bulk materials in water, and thermal oxidation exfoliation, ultrasonic exfoliation, chemical exfoliation and other methods have gradually matured
[60][85,86][87][88][89,90]. In the preparation process, Zhao et al. Found that its microstructure and photocatalytic activity depend on the quality of the precursor
[91]. With the same surface area, the visible-light photocatalytic ability of g-C
3N
4 nanosheets for gaseous NO was significantly enhanced when the thiourea mass was reduced from 20 G to 2 G. After 30 min, the removal rate of NO can reach 48. 3%. Xu et al. Obtained g-C
3N
4 nanosheets with monoatomic layer structure (0. 4 nm) by concentrated H
2SO
4 chemical exfoliation method, and their photocurrent and hydrogen evolution rate were 4 times and 2. 6 times higher than those of bulk g-C
3N
4, respectively
[92].
In order to further optimize the photocatalytic performance of g-C
3N
4, it can be modified. Element doping can change the surface properties, electronic structure and light absorption properties of g-C
3N
4, thus affecting their photocatalytic activity. For example, Fe-doped Fe-g-C
3N
4 photocatalyst can directly oxidize benzene to phenol using hydrogen peroxide
[93]. Recently, Li et al. successfully prepared a Cu/g-C
3N
4 photocatalyst by incorporating Cu
2+ into the g-C
3N
4 structure, and the g-C
3N
4 still maintained a two-dimensional sheet-like structure (Figure 8 a)
[94]. After 105 min of visible light irradiation, the mass ratio of Cu/g-C
3N
4-2(Cu(Ac)
2 to g-C
3N
4 was 1: 8.3) showed the highest photocatalytic activity, and the removal efficiency of rhodamine B (RhB) reached 93.6% (Fig. 8 B). In general, holes, hydroxyl radical (· OH), and superoxide radical (
) can be trapped by disodium ethylenediaminetetraacetate (EDTA-2Na), tert-butyl alcohol (TBA), and benzoquinone (BQ), respectively. Therefore, it can be seen from fig. 8 (C) that the
plays a major role in the photodegradation process, and · OH and holes play an auxiliary role. The introduction of Cu
2+ successfully reduces the band gap of organic semiconductors and reduces the recombination efficiency, thereby improving the separation efficiency of electron-hole pairs.
Compared with conventional porous materials, the inherent porosity of 2D MOFs facilitates the diffusion of organic pollutants and products through their pores, which is essential for improving the efficiency of photocatalytic reactions. And the electronic structure of the two-dimensional MOFs photocatalyst can be adjusted through component modification at the molecular level. He et al. Prepared ultrathin zirconium porphyrin MOF nanosheets with a thickness of less than 1.5 nm, which exposed a large area of catalytic active surface during rapid mass transfer and showed excellent photocatalytic performance
[95]. Xiao et al. Synthesized two different two-dimensional metal-tetra (4-carboxyphenyl) porphyrin (M-TCPP) nanosheets (M = Cu, Zn), in which the Cu-TCPP MOF showed excellent catalytic performance for styrene oxidation due to the exposure of surface metal sites and the low diffusion barrier of substrate molecules, with a conversion efficiency of up to 94.0%, showing great potential in the application of heterogeneous photocatalysts
[96].
Therefore, by comparing and analyzing the above g-C3N4 and MOFs as typical examples of two-dimensional photocatalysts, it can be concluded that when the π-π conjugated structure in two directions is larger, it is easier to assemble into two-dimensional nano-organic photocatalysts, and the two-dimensional nano-sheet structure is more conducive to electron transport. Meanwhile, the photocatalytic performance of the two-dimensional nano organic photocatalyst can be more excellent by changing the components or modification means in the preparation process.
To sum up, nano-organic photocatalysts with single structure (zero-dimensional, one-dimensional, two-dimensional) have high aspect ratio or relatively large specific surface area, which can effectively capture visible light and broaden the range of light absorption. The single ordered microstructure plays a role in promoting the directional transport and effective separation of carriers. At the same time, the same material can also be controlled by conditions to obtain nano-organic photocatalysts with different dimensions, and the photocatalytic efficiency is also different with different structures. However, single material is still limited by poor charge transport ability and low electron-hole separation efficiency, so organic semiconductor materials need to be compounded with other materials to construct organic nano-heterostructures, delay charge recombination and enhance light utilization.
5.2 Binary composite organic nano photocatalyst
In recent years, the research focus of efficient and stable visible photocatalysis has shifted from single-phase materials to multi-phase materials, that is, a new semiconductor heterojunction is constructed by combining two or more different semiconductor materials with appropriate conduction band and valence band positions. Two semiconductors with matched electronic energy level structures are combined to form a heterointerface, and the main purpose is to combine narrow-band-gap semiconductors to sensitize wide-band-gap semiconductors so that the wide-band-gap semiconductors can respond to visible light under illumination; Secondly, it can effectively inhibit the recombination of photogenerated electrons and holes, and improve the photocatalytic activity through the two-photon process. For widely used semiconductor heterojunctions, there are three types of photocatalytic processes (Fig. 9): straddle type (type I), staggered type (type II), and gapped type (type III)
[97]. Among them, the organic photocatalytic process is usually of the staggered gap type (type II). In photocatalysis, the formation of interface/heterojunction by semiconductor can effectively improve the photocatalytic activity.
5.2.1 Organic/organic composite nano organic photocatalyst
Chen et al. Obtained one-dimensional organic single crystal p-n nanoheterostructure (type II) by vapor deposition method, which was composed of p-type tetraphenylporphyrin (H
2TPP) and n-type N, N- (dicyclohexyl) perylene-3,4,9,10 tetracarboxylic diimide (CH-PTCDI)
[98]. The composite structure combines the absorption of the two and can effectively increase the utilization rate of sunlight. At the same time, this composite structure has a large p-n interface, which can increase the charge separation and expose the maximum surface of photogenerated charge carriers to the photocatalytic reaction. When CH-PTCDI recombines with H
2TPP, there is a large quenching, which can reflect the separation and recombination of photo-induced charge carriers (Figure 10 a). After 4 H of visible light irradiation, the composite had the highest degradation rate of methylene blue (MB), reaching 72% (Fig. 10B). The above indicates that the efficient separation of holes and electrons is caused by the p-n heterojunction, which generates a large number of hydroxyl radicals, resulting in a significant increase in the degradation rate (Fig. 10c). Therefore, the H
2TPP-CH-PTCDI organic/organic composite nanoheterojunction showed excellent photocatalytic activity for MB degradation under visible light.
Based on the same principle, Mendori et al. prepared an organic p-n bilayer composed of 3,4,9,10-perylenetetracarboxylic acid-bisbenzimidazole (PTCBI, n-type) and lead phthalocyanine (PbPc, p-type) (Fig. 11A) and examined it using a photoelectrode in an aqueous phase
[99]. As shown in Fig. 11b, the PTCBI-PbPc bilayer can respond to a broad solar spectrum (400 ~ 1100 nm) compared to the PTCBI-H
2Pc (metal-free phthalocyanine) bilayer. When the PTCBI-PbPc bilayer is used as a photoanode, the reactant (Fe
Ⅱ(CN)
64-) on the surface of PbPc will be oxidized to generate photocurrent. Therefore, ITO (indium tin oxide) -PTCBI-PbPc was used as a photoanode for hydrogen production. As shown in Fig. 11c, the yield of H
2 increases linearly with the number of cycles (1 cycle = 1 H), and its original performance is maintained even after several cycles. In addition, due to the oxidative and reductive abilities of PbPc and PTCBI, the organic bilayer can also induce IR-responsive photocatalytic reactions.
In addition to the recombination between the above small organic molecules, heterojunctions can also be constructed between small organic molecules and polymers to enhance the separation of photogenerated charges. Chen et al. compounded Co-2-methylimidazole (Co-MOF) with g-C
3N
4 to prepare a photocatalyst, which showed excellent photocatalytic reduction of CO
2 under visible light irradiation
[100]. The introduction of Co-MOF not only broadens the absorption range of visible light, but also enhances the charge separation, thereby increasing the photocatalytic activity of g-C
3N
4 to a higher level. Wang et al. Prepared a PDI-doped g-C
3N
4-based composite photocatalyst g-C
3N
4-PDI(gCP), which effectively realized the synergistic redox conversion of arsenic (III) and chromium (VI) in acidic medium
[101]. Compared with P25 TiO
2, gCP exhibited excellent durability due to its non-adsorption to the oxidation product arsenic (Ⅴ). Compared with the pristine g-C
3N
4, the gCP
2 with the mass ratio of g-C
3N
4 to PDI of 1 ∶ 2 showed the greatest catalytic activity for H
2O
2, improving the photocatalytic production of H
2O
[102].
5.2.2 Organic/carbon composite nano organic photocatalyst
Among the large family of carbon-based nanomaterials, C
60 has a band gap of 1.7 ~ 1.9 eV, which is known as an excellent electron acceptor. Its molecule is a closed-shell configuration consisting of 30 bonded molecular orbitals and 60 p electrons, which is conducive to electron transport. In addition, this unique structure endows C
60 with many other interesting properties, including excellent exciton mobility, high thermal stability, low density and strain-tunable semiconductor properties. Li et al. discussed in detail the interfacial effects of C
60/g-C
3N
4 nanocomposites on the electronic properties, charge transfer, and optical response
[103]. The valence band maximum and conduction band minimum of the C
60/g-C
3N
4 heterostructure are dominated by unsaturated nitrogen atoms and C
60 molecules, respectively, which interact strongly with each other, resulting in strong charge transfer between the two relevant components. Meanwhile, the C
60/g-C
3N
4 heterojunction shows more sensitive ultraviolet light absorption and low-energy visible light absorption, and has a wider UV-visible absorption spectrum response range, which can meet the demand of water splitting.
In addition, Deng et al. Successfully prepared carbon /g-C
3N
4 composite photocatalyst (CCN) by high temperature decomposition method using cheap absorbent cotton as carbon precursor, which proved that the carbon content had a significant impact on the performance of CCN
[104]. When the mass ratio of carbon to g-C
3N
4 is 1 wt%, the degradation rate constant (K) of the composite CCN1. 0 for RhB is about 5. 5 times that of g-C
3N
4. Isopropyl alcohol (IPA), ammonium oxalate (AO), and BQ were used as the capturing agents for · OH, h
+, and ·O
2-, as shown in fig.It can be seen that h
+ and ·O
2- radicals are the main active species, but the role of ·O
2- is more important than that of h
+. The improved catalytic performance of CCN is mainly attributed to the good interfacial contact between the coupled carbon material and g-C
3N
4, which is conducive to the conduction of electrons and improves the separation efficiency of photogenerated carriers and visible light absorption (Fig. 12b). The contact interface between 2D carbon and 2D g-C
3N
4 with similar structure is the largest, and the network structure is thinner, which is conducive to the rapid transfer of electrons. Therefore, the CCN catalyst has high catalytic activity and good chemical stability, and is a promising photocatalyst with high efficiency in the removal of organic pollutants and the utilization of solar energy.
As a two-dimensional carbon material, rGO has excellent electron acceptance and transport properties, which is helpful to separate photogenerated excitons. Patra et al. prepared a one-dimensional nanostructured composite of porphyrin nanorods (TCPP NR, Fig. 13A) and rGO
[105]. In the composite system, the emission peak position remained unchanged (666 nm), and the photoluminescence quenching efficiency of the TCPP composite with 240 μg of rGO was 89% (Fig. 13b), which means that there may be energy transfer or electron transfer from TCPP NR to rGO. The rGO-TCPP NR composite system showed a 1.9-fold photocurrent enhancement compared to the pure TCPP NR (fig. 13C). This novel porphyrin-based composite structure provides the possibility for the emerging applications of solar energy conversion and photocatalysis.
In addition to the composite one-dimensional structure of rGO surface, Ong et al. Used electrostatic self-assembly method to carry out 2D-2D recombination of rGO with protonation g-C
3N
4(PCN,HCl protonation) at a relatively low temperature (Figure 14 a)
[106][107]. The optimized 15 wt% rGO/PCN (15rGO/PCN) heterojunction photocatalyst exhibited the highest CH
4 yield of 13.93μmol·g
-1 with a quantum yield of 0.560% after irradiation for 10 H, which was 1.7 and 5.4 times that of 15rGO/CN and PCN, respectively (Figure 14 B). The results show that a compact 2D-2D interfacial coupling can be formed between rGO and PCN through electrostatic self-assembly, which facilitates charge transfer between rGO/PCN heterojunctions and suppresses electron-hole recombination, resulting in excellent photocatalytic activity for the reduction of CO
2 to CH
4[108].
5.2.3 Organic/metal composite nano organic photocatalyst
When metal and organic semiconductor are in contact, due to their different work functions, there will be a potential difference (Schottky barrier), which will dissociate excitons into free charge carriers. Therefore, the combination of metal particles and organic semiconductors can effectively inhibit the recombination of photogenerated charge carriers and improve the overall photocatalytic performance. Among them, noble metals (Au, Ag, Pb, etc.) Have excellent optical properties and are widely used in the field of photocatalysis.
Metal nanoparticles composite is the most typical organic modification approach. For example, the combination of AgNPs and organic compounds can combine the advantages of both, improve the aggregation of AgNPs and obtain ideal catalytic activity. Jiang et al. prepared a composite structure (S-g-C
3N
4) of AgNPs and monatomic layer g-C
3N
4[109]. Due to the strong interaction, the AgNPs were uniformly distributed on the edges and surfaces of the S-g-C
3N
4 nanosheets. Khan et al. prepared Ag@g-C
3N
4 photocatalyst by depositing AgNPs on g-C
3N
4 sheet-like nanostructures
[110]. The interaction between the spherical AgNPs and the g-C
3N
4 of the π-conjugated structure can stabilize the composite material, effectively inhibit the photocorrosion in the photocatalytic reaction process and improve the photodegradation process. Meanwhile, the AgNPs enhanced the antibacterial activity, prolonged the response to visible light, inhibited the charge recombination, and eventually generated reactive oxygen species such as e
-, ·O
2-, · OH, and h
+, thereby improving the efficiency of photocatalytic dye degradation. Under visible light irradiation, the degradation rates of MB and RhB were about 100% and 89%, respectively, by Ag@g-C
3N
4 within 210 min and 250 min. The invention provides a novel, high-stability visible light-induced and environment-friendly photocatalyst, and opens up a new way for preparing high-efficiency intelligent nanocomposite materials with various purposes.
Tong et al. Have studied noble metal (Pt, Pd, Au)/g-C
3N
4) photocatalytic systems
[111]. The loading of metal atoms will significantly reduce the band gap of g-C
3N
4, thereby increasing its optical absorption in the visible region. Pt and Pd atoms are embedded in the g-C
3N
4 interlayer and act as bridges to facilitate the transfer of interlayer charge carriers due to the conduction band offset. Au atoms can only adsorb on the g-C
3N
4 surface and transport the limited charge carriers excited in the N-conjugated aromatic pores on the g-C
3N
4 surface, which provides inspiration for the development of efficient single-atom cocatalysts supported on two-dimensional layered materials. In addition to Pt nanoparticles, Wang et al also studied the composites of Pt-based transition metal (Co, Ni, Fe) alloys (Pt-M) and g-C
3N
4[112]. When the Co content is increased to an atomic ratio of Pt
2.5Co(Pt/Co of 2.5), the H
2 yield of the composite rises to 4.63μmol·h
-1, reaching 1.6 times that of pure PtCN (Fig. 15).
In addition, metal atoms can be quantized to obtain a stable, efficient and noble metal-free photocatalyst, which greatly avoids the disadvantages of noble metal atoms. Liang et al. modified CdS quantum dots (QDs) on phosphorus-doped g-C
3N
4 nanotubes (P-CNT) by in-situ oil bath method to obtain 0D/1D heterojunction photocatalyst (CdS QDs/P-CNT)
[113]. Phosphorus-doped 0D/1D heterojunction broadens the optical absorption, reduces the band gap, activates the H
2O molecules, and promotes the separation of space charges. Due to the advantages of phosphorus doping, quantum effect, one-dimensional hollow channel and n-n type heterojunction, the CdS QDs/P-CNT has large specific surface area, excellent photocatalytic hydrogen production rate and high stability.
5.2.4 Organic/inorganic composite nano organic photocatalyst
Inorganic semiconductors have high electron mobility, high electron affinity and good thermal stability. Organic/inorganic hybrid nanomaterials usually combine organic and inorganic components into heterojunction devices with efficient charge transfer. Organics are used as donors to absorb sunlight and transport holes, while inorganic semiconductors act as acceptors to transport electrons. Nanostructured inorganic semiconductors can also provide large area for efficient exciton dissociation when mixed with soluble organics.
Organic sensitizing dyes loaded on the surface of inorganic substrates are the most commonly used methods
[114]. Min et al. Loaded four different modified porphyrin dyes (PORPC) on the surface of TiO
2, as shown in fig. 16A: two containing phenyl carboxylic acid moieties, one metal-free (PORPC-1) and one metallated (PORPC-2)
[115]; The other two are asymmetric, with each porphyrin containing two meso positions bearing a di-tert-butyl substituted phenyl ring and the remaining two positions being substituted by phenyl carboxylic acids (PORPC-3 and PORPC-4). The composite structure can effectively increase the absorption range of visible light (fig. 16b). More than 70% of MB was degraded by the composite photocatalyst within 30 min of UV-visible light irradiation, and the degradation rate of TiO
2-PORPC-4 was the highest (Fig. 16c). This is because the 3,5-di-tert-butylphenyl group contained in it has efficient porphyrin protection and electron-donating ability, and also effectively inhibits π-π stacking aggregation, so it shows the best photocatalytic activity.
In addition to being used as a sensitizer, polar organic molecules, such as thiophene electron-withdrawing groups, can also be used to modify the semiconductor surface to construct a polarizing electric field
[116]. The BiO (HCOO) (BFM) sample modified with 4-substituted thiophene ester
-S-C
6H
4Z(4CBT) has a strong absorption of visible light due to the formation of Bi-S bond between them, and the photogenerated electrons transfer to 4CBTs, which greatly inhibits the recombination of photogenerated electrons and holes. Under the same conditions, the composite sample 4CBT @ BFM can decompose nearly 100% of RhB, which is much higher than that of pure BFM. It is found that the modification of the semiconductor surface by polar organic anions, such as 4-Z-C
6H
4S
-, results in easier and simpler operation, facilitating more efficient separation of photogenerated charge carriers
[117]. The photocatalytic activity caused by the surface modification can be maximized by minimizing the size of the nanoparticles.
In addition, organic/inorganic composite materials can also be used to obtain organic photocatalysts with core/shell structure. Li et al. prepared a zinc oxide-polyimide (ZnO-PI) core/shell photocatalyst by calcination, which showed significantly enhanced photocatalytic activity for MB decolorization under visible light irradiation
[118]. The efficient interfacial charge transfer between ZnO and PI suppresses the recombination of photoexcited electron-hole pairs and improves the photocatalytic efficiency. Peng et al. First formed MOF @ COF core-shell hybrid photocatalyst (NH
2-MIL-68@TPA-COF) by complexing MOF(InO
4(OH)
2-2- amino terephthalic acid, NH
2-MIL-68) with covalent organic framework (COF, TPA-COF), which showed high photocatalytic activity with a degradation constant of 0.077 min
-1 for RhB, and opened the way for the application of MOF @ COF composites in the field of photocatalysis
[119].
The construction of p/n heterojunction can reduce the charge transfer resistance and promote the separation of photoinduced carriers, so Yang et al. Constructed an effective 0D/1D p/n Ag
2S/PDI heterojunction with full spectral response
[120]. Ag
2S QDs improved the well-organized π-π stacking degree of the self-assembled PDI, contributed to the quasi-one-dimensional π-π stacking migration of photogenerated electrons along the PDI, and enhanced the light absorption of Ag
2S/PDI.
The construction of 2D/2D surface heterojunction can highlight the characteristics of large surface area and short electron transmission distance. In 2020, Monga et al. prepared 2D/2D MoS
2/g-C
3N
4 composites (MSC) with different weight ratios (1 ∶ 1, 1 ∶ 3, 3 ∶ 1 and 5 ∶ 1) by a rapid (30 min) one-step microwave-assisted method
[121]. Among them, MSC (5 ∶ 1) had the best photocatalytic activity, and the degradation rate of MB was about 94% under visible light. This may be due to the higher specific surface area and lower recombination rate of MSC (5:1). Sun et al. grew BiOCl nanosheets on g-C
3N
4 nanosheets in situ by hydrothermal method and obtained the ordered 2D/2D composite BiOCl/g-C
3N
4[122]. When the amount of g-C
3N
4 was 0. 26 G, the composite photocatalyst showed the best photocatalytic efficiency for tetracycline (TC), and the photocatalytic efficiency of TC reached 97. 1% within 60 min. The main reason is that the contact area of two-dimensional materials is large, and the charge can be transferred quickly in the BiOCl/g-C
3N
4.
5.3 Multi-element composite organic nano photocatalyst
In recent years, in addition to the construction of binary photocatalysts by compounding organic semiconductors, multi-element novel heterostructure composite photocatalysts which can efficiently separate photogenerated carriers are also a very active research field.
Azzam et al. Encapsulated carbon nanotubes (CNTs) with TiO
2 nanoparticles and compounded with AgNPs to obtain an effective photocatalyst TiO
2@CNTs/AgNPs for the degradation of MB dyes
[123]. The band gap energy of the composite was reduced to 2. 25 eV after the C
10 treatment with cationic surfactant. Among them, CNTs enhanced the surface and electronic properties of TiO
2, and AgNPs significantly reduced the recombination of electron-hole pairs in the composites, showing great photocatalytic activity under visible light irradiation.
Song et al. Synthesized a novel C
60/ graphene /g-C
3N
4 composite photocatalytic material for efficient hydrogen production, which has an ultrathin sheet shape similar to dragonfly wings, a large specific surface area, and more active sites exposed, contributing to the improvement of the photocatalytic ability of g-C
3N
4[124]. Similarly, Putri et al. Prepared 0D-2D-2D CdSeQD/B-rGO/O-gC
3N
4 photocatalyst, and there was efficient charge transfer at the ternary multi-level heterointerface
[125]. When loaded, the maximum hydrogen evolution (1435μmol·h
-1·g
-1) can be achieved even without the help of noble metal additives. The enhanced photoactivity is attributed to the synergistic effect of p-type B-rGO and n-type O-gC
3N
4 on CdSe sensitization and p-n junction.
Akbarzadeh et al. synthesized a novel Ag
3VO
4/Cu-MOF/rGO nano-photocatalyst (Figure 17 a)
[126]. Its degradation rate constant (0.008 min
-1) for AB92 dye was 2.5 and 4.7 times higher than that of pure Ag
3VO
4(0.0032 min
-1) and Cu-MOF/rGO(0.0017 min
-1), respectively (Figure 17 B). The Ag
3VO
4 nanoparticles were composited with Cu-MOF/rGO, and the photogenerated electrons could migrate between Ag
3VO
4 and Cu-MOF through the rGO nanosheets, and the photocatalytic activity was greatly improved (Fig. 17c). Based on the excellent optoelectronic properties of rGO, Zheng et al. First prepared rGO/Bi
2O
2CO
3/BiOCl ternary photocatalyst (rBB) composed of self-assembled nanoplates by hydrothermal method
[127]. Due to the unique structure and synergistic effect between the nanoplates, the visible-light-driven catalytic activity of the composite is greatly improved, and the photocatalytic performance for the degradation of TC, MB and RhB is also good, so the composite is a promising broad-spectrum photocatalyst.
Guan et al. Used zeolite imidazole ester skeleton material ZIF-8(Zn(Hmim)
2,Hmim=2- methylimidazole) to prepare Ag/AgCl/ZIF-8/TiO
2 composite photocatalyst, which was assembled on the surface of cotton fabric
[128]. The degradation rate of MB on cotton fabric coated with Ag/AgCl/ZIF-8/TiO
2 can reach 98. 5% within 105 min of visible light irradiation. Good photocatalytic activity and good cycle performance make it a good material for the treatment of dye wastewater.
To sum up, multi-component nano-organic photocatalysts are usually based on binary organic composite materials, and co-catalytic materials with similar electronic structure are selected for doping or compounding. Different dimensions and different types of nano-photocatalytic materials are assembled through synergy, and the formation of heterostructures effectively promotes the separation of photogenerated electron-hole pairs, thus hindering the recombination of carriers. After the multi-component material is compounded, the photocatalytic advantages of each component can be more effectively exerted under the condition that the heterojunction and the synergistic effect coexist, so that the photocatalytic process is effectively driven, and the photocatalytic performance of the nano organic material is remarkably improved.
6 Application of Organic Nanometer Photocatalyst
As photocatalyst materials, nano-organic semiconductors can directly use solar energy to generate active radicals with strong oxidizing properties, such as hydroxyl and
, at room temperature and atmospheric pressure. These two free radicals and photogenerated holes (h
+) can degrade organic pollutants into low-toxic or non-toxic small molecules, and even mineralize them into CO
2 and water
[129⇓⇓⇓⇓⇓⇓~136]. After irradiation, the photogenerated electrons and holes produced by organic semiconductors have strong oxidizability and reducibility, and can degrade most of the organic matter in sewage, such as dyes (including RhB, MO, MB, bisphenol A (BPA), benzyl alcohol, tetracycline hydrochloride, etc.), nitrogen-containing organic matter, phenol, etc. Some literatures even show that they can be used for air pollution control and soil remediation
[137]. Zhao et al. First prepared 4-PySH@TiO
2 by the interaction between the thiol group of 4-mercaptopyridine (4-PySH) and the group on the surface of TiO
2, and then compounded with the zirconium cluster (PCN-222 (Zn)) linked by ZnTCPP under certain conditions to obtain TP-222 (Zn)
[138]. The broad and strong visible light absorption of PCN-222 (Zn) expands the visible light region of the composite (fig. 18 a). The pyridine nitrogen atom in the 4-mercaptopyridine has a strong coordination force with the porphyrin center metal zinc, and the band gaps of TiO
2 and PCN-222 (Zn) are about 3.29 and 1.74 eV, respectively (Fig. 18b). Therefore, in this complex, 4-PySH has a strong coordination force with the porphyrin center metal zinc through the interaction force between the sulfhydryl group and TiO
2, and between the pyridine nitrogen atom and the porphyrincenter metal zinc, respectively.The TiO
2 and the PCN-222 (Zn) are more closely compounded together, so that the TiO
2 is more uniformly and effectively dispersed in a pore canal cavity of the PCN-222 (Zn), the effective separation of photogenerated electron-hole is promoted through a mutual cooperation mechanism, and the efficiency of photocatalytic degradation of RhB is improved. Meng et al. Also prepared a zirconium-porphyrin MOF (PCN-222), which produced
under light conditions that effectively degraded BPA
[139]. After dark adsorption, the photodegradation of BPA was completed within 10 min of visible light irradiation.
At the same time, nano-organic semiconductor photocatalytic materials also have many applications in the field of energy catalysis, such as photocracking water to produce hydrogen and oxygen
[140]. Graphene, carbon nanotubes, carbon quantum dots, carbon black and carbon fibers, which have high work functions, are used as cocatalysts to form Schottky heterojunctions, which further improve the surface catalytic activity of metal-based cocatalysts
[141,142]. Xu et al. enhanced the g-C
3N
4 material activity by hybridizing the C-N co-doped Ni
2P and Fe
2P cocatalysts (Figure 19 a)
[143]. Sensitized by eosin Y (EY), the H
2 generation rate of the photocatalyst EY-g-C
3N
4-NiFeP can reach 13.81 mmol·h
-1g
-1, which is 10 and 5 times higher than that of EY-g-C
3N
4 and EY-g-C
3N
4-FeP, respectively (Fig. 19 B). Compared with pure Fe
2P, the hybrid material of Ni
2P and Fe
2P cocatalyst can significantly improve the charge carrier lifetime of g-C
3N
4, which indicates that more photoexcited electrons are generated during the photocatalytic reaction, leading to the H
2 effect. The photocurrent results further confirmed the improved charge carrier transfer of the Ni
2P and Fe
2P mixed cocatalysts (Figure 19D).
In addition to photocatalytic hydrogen production, photocatalytic reduction of CO
2 to hydrocarbon fuels is also one of the means to solve the energy problem
[144]. Nano-organic semiconductors can reduce CO
2 to methane, formic acid, formaldehyde, methanol and other products through photocatalytic process
[145⇓⇓~148]. Cao et al. Found that semiconductor heterojunction g-C
3N
4/In
2O
3 can photocatalytically reduce CO
2 to CH
4, thus turning CO
2 into fuels that can be directly utilized
[149]. Liang et al. Prepared Au@g-C
3N
4/SnS shell heterostructure with Au as electron mediator. This unique structure is the main factor to improve the efficiency of charge separation and enhance the photocatalysis, thus improving the conversion of photocatalytic CO
2[150]. Au@g-C
3N
4/SnS exhibited the best CO
2 redox catalytic performance (Fig. 20). When the content of SnS was 41.5%, the highest yields of CH
4, CO, and CH
3OH were 3.8, 17.1, and 5.3μmol·g
-1, respectively.
7 Conclusion and prospect
In summary, the recent progress of organic nanoheterostructured photocatalysts is reviewed. Organic semiconductors have become one of the cornerstones in the field of photocatalysis due to their excellent photoelectric properties such as wide band gap, wide range of visible light absorption and easy adjustment. Organic semiconductor materials are assembled into nanostructured photocatalysts, which have significantly improved catalytic activity and efficiency and excellent photocatalytic performance. In this paper, the preparation of nano-organic photocatalysts is discussed from two aspects: single structure and composite structure, in which single structure is divided into three types: zero-dimensional, one-dimensional and two-dimensional, and composite structure includes two types: binary (organic semiconductor/organic semiconductor, organic semiconductor/inorganic semiconductor, organic semiconductor/carbon, organic semi-conductor/metal) and multiple. Assembly of nanoheterostructures is very important to improve the structural properties and photocatalytic performance of materials. Using different photocatalytic mechanisms to effectively enhance light absorption, charge transfer and carrier transport can develop efficient and stable nano-organic photocatalysts. Therefore, it is of great significance to promote the development of photocatalytic industry in the fields of energy and environment, such as degradation of organic pollutants and hydrogen production from water decomposition (Table 1).
Although great progress has been made in the study of nano-sized organic photocatalytic materials, there are still several challenges. First of all, organic nanostructures are mainly stacked through π-π conjugation, and the interaction is weak, which is not conducive to the perfect stacking of molecules, so it is difficult to obtain regular organic nanocrystals, and electron transport may be hindered. Secondly, most of the existing photocatalytic research focuses on the degradation of single component pollutants, but the actual production and living sewage is complex, generally in the form of multi-component, so in order to truly and efficiently use solar energy to solve the actual pollution problem, it is necessary to develop in the direction of multi-component degradation. Thirdly, the loss and solidification of active components in the use of nano-organic catalysts put forward higher requirements for the service life of catalysts and recycling technology. At the same time, heavy metal ions can not be treated in time after the pollutants are solved. Therefore, in the future development process, if the above problems can be effectively solved, organic nano-photocatalysts will achieve energy saving, high efficiency, safety and non-toxicity in solving the actual environmental pollution and energy problems. Besides catalytic degradation of pollutants and decomposition of water, it also has potential application value in the transformation of organic matter and the treatment of heavy metal ions.
National Natural Science Foundation of China(51503014)
Fundamental Research Funds for the Central Universities of China(230201818-001A3)
Scientific and Technological Innovation Foundation of Shunde Graduate School, USTB(BK19AE027)
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