English

• 1. INTRODUCTION

  Big data analysis can transform the abstract and complex data into vivid geometric figures, and the relationship and evolution rules of knowledge points are obvious. CiteSpace is information visualization software developed by Chaomei Chen, a famous Chinese American scholar, which is mainly used to measure and analyze the data of scientific literature. Java has been widely used in various disciplines at home and abroad, because it displays visually the information panorama of scientific knowledge in the form of scientific knowledge map, conducts all-round, correct and real-time information analysis, also explores easily and quickly the key literature, hot research and frontier direction in a scientific field. In the work, the related literature was depicted and analyzed with CiteSpace software through big data visualization map. We expect to grasp quickly the present situation of the year's literature research to show its hot spot and front dynamic from the macro and provide references in future.

  Photocatalyst is one of the most potential and important solar energy conversion materials with a very broad application prospect in the environment, energy, chemical industry, building materials and so on, and has been recognized internationally as a hotspot at present. The basic contents of photocatalytic materials mainly include: (1) modification of traditional ones such as the regulation of morphology and microstructure, element doping, semiconductor composite, photosensitization and co-catalyst modification; (2) the development of new ones, which are designed as new efficient materials with the help of theoretical calculation combined with experimental means; (3) basic application research, including the deactivation and regeneration of ones, immobilization of powder ones, engineering, etc. Big data quantitative analysis of the core research direction and hot spots of photocatalytic materials in 2020 will provide scientific basis for domestic and foreign scholars to summarize timely research objectives and search for breakthroughs in follow-up research, so as to better achieve the goal of promoting the development of discipline and serving the national needs.

  2. METHODS FOR DATA COLLECTION AND ANALYSIS

  2.1 Data collection

  Bibliometrics, which integrates mathematics, statistics and philology, is an important means for statistical and visual analysis of literature. Web of Sciences is an important database for the access to global academic information. It contains more than 12, 400 authoritative and high-impact academic journals from all over the world, covering natural sciences, engineering, biomedical, social sciences, arts and humanities and other fields. It has a strict screening mechanism, according to Garfield's law of literature concentration in bibliometrics. It only includes important academic journals in various disciplines, which can well reflect the development trends of the subject frontier. In this work, 4147 papers and records were retrieved as the data support based on the Web of Sciences as the data pool and "photocatalytic materials" as the main topic in the papers published in 2020.

  2.2 Analytical methods

  The indexed English bibliotheca named "download_××.txt" was imported into CiteSpace 5.6.R5 for format conversion. The data obtained from Web of Sciences were performed visual analysis with the CiteSpace software with Time Slicing at 2020, 1 year of "Years Per Slice", and the default selection of "Term Source". The node types are used in "Author, Institute, Country and Keyword", respectively, and "g-index" = 25 for Selection criteria. The maps were done with the pruning algorithm including Pathfinder, Pruning sliced network and Pruning the merged network to make the map clearer.

  3. ANALYSIS OF THE LITERATURE SOURCES FROM COUNTRIES/REGIONS, RESEARCH INSTITUTIONS AND AUTHORS

  3.1 National/regional analysis

  4147 papers were analyzed for national/regional sources with the CiteSpace data and Excel table. As shown in Fig. 1, the top 10 countries/territories in terms of number of posts are in turn the Peoples R China, India, USA, South Korea, Saudi Arabia, Spain, Iran, Germany, Pakistan and Japan. China has the largest number of published papers with a total of 1, 810, which is far ahead of India (590 papers) at the second place, suggesting China attaches great importance and pays wide attention to the photocatalytic materials. Fig. 2 shows the co-occurrence map with "country" as object in countries/regions. The node in the figure represents each country/region, and the font size represents the number of published articles. The more published articles, the larger the country's font. The connection between nodes represents the cooperative relationship between countries. Countries with relatively strong, direct, close or positive cooperative relations show the close distance between nodes, forming a cohesive node group. As illustrated in Fig. 2, the number of nodes is 52, which means that researchers from 52 countries have published relevant articles in the field of photocatalysis in 2020. The node connections are 156, representing the cooperative research between different countries. The network density of the shared map is 0.1176, indicating there is frequent cooperation between different countries, and photocatalytic materials have become a hotspot of international cooperation. Among them, Germany, Spain and Saudi Arabia have a relatively obvious cluster of nodes, suggesting there are more transnational cooperation, especially close contact and cooperation with neighboring countries. Peoples R China and India have the largest number of publications, however, their cooperation with other countries is relatively unitary, and they need to enhance the internationalization research.

  Figure 1

  

  Figure 1.  Histogram of the top 10 countries/regions in the published number in 2020

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  Figure 2

  

  Figure 2.  Co-occurrence map in different countries/regions

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  3.2 Institutional analysis

  Institution refers specifically to the institution of the author. Fig. 3 shows the institutions of 4147 literatures. It can be seen the top 10 institutions are: Chinese Academy of Sciences, Jiangsu University, Zhengzhou University, University of Chinese Academy of Sciences, Tianjin University, Fuzhou University, King Saudi Arabia University, Hunan University, Duy Tan University, King Abdulaziz University. Furthermore, the largest number is 149 papers published from the Chinese Academy of Sciences, reflecting its leading position. Seven Chinese institutions, including Chinese Academy of Sciences, Jiangsu University, Zhengzhou University, University of Chinese Academy of Sciences, Tianjin University, Fuzhou University and Hunan University, are among the top 10 published papers, indicating Chinese academic community has established some key centers and achieved relatively fruitful results.

  Figure 3

  

  Figure 3.  Histogram of top 10 institutions in 2020

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  According to Price's law M = 0.749 (Nmax½), the core institution of a discipline field can be calculated, that is, the key institution is more important and outstanding in the discipline field. Nmax is the number of papers published by the institution with the highest number of papers, and M is the criterion for being selected as the core institution. In 2020, the Chinese Academy of Sciences published the highest number of papers (149), so the Nmax was 149, and the M value was 10.592. In other words, the institutions with more than or equal to 11 papers published are the core institutions. In the work, 134 institutions reach at the value, which comes in 41.8% (320, total number). The co-occurrence analysis of institutions was carried out with the CiteSpace software. The co-occurrence map was obtained: the network node was set as institution, and the frequency (FREQ), namely the number of published papers, was set to display the top 10 core institutions of published papers. In Fig. 4, the node represents each institution. The font size is related to the number of articles. The more articles, the larger the font size of the institution. The lines between nodes represent a collaborative relationship between various institutions. As can be seen from the figure, there are many connections between Chinese institutions, reflecting they have more cooperation.

  Figure 4

  

  Figure 4.  Co-occurrence map of top 10 core institutions in 2020

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  3.3 Author analysis

  The top 10 authors are shown in Fig. 5. They are in turn Huaming Li, Danlian Huang, Yang Yang, Elson Longo, Jing Xu, Jiajie Fan, and Guangming Zeng, Yang Liu, Jiaguo Yu, Wei Wang. Among them, Huaming Li reached the maximum value at 22.

  Figure 5

  

  Figure 5.  Histogram of the top 10 authors in 2020

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  The co-occurrence of the authors was analyzed using CiteSpace software with the network node as the author to obtain the co-occurrence map (Fig. 6). To some extent, the academic research ability of scholars can be reflected by the number of published papers. Nodes in the graph represent authors' publishing frequency, and the more authors publish, the more nodes are obvious. The line between nodes indicates collaboration between two authors. As shown in Fig. 6, the number of network nodes is 173 and the connections one is 265, indicating there is a certain cooperative relationship between authors. It is particularly noted the nodes of several high-frequency authors are interrelated, indicating a relatively rich cooperation network has been formed among high-yield authors, such as Guangming Zeng and Danlian Huang of Hunan University, Jiajie Fan and Jiaguo Yu of Wuhan University of Technology, as well as Elson Longo of Federal University of San Carlos in Brazil and Huaming Li of Jiangsu University. Based on the papers cited by the authors of relevant cooperative network from the top quality and influential SCI Region I and II journals during 2017-2019, and combined with the results of 2020, we can obtain the current research focus and direction of the relevant authors and the cooperative relationships.

  Figure 6

  

  Figure 6.  Co-occurrence map of photocatalytic materials

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  Considering the highly cited papers of Guangming Zeng and Danlian Huang 2017-2019, it can be found outstanding achievements are as follows: the unique flower-like Ag/ZnO samples with high visible light catalytic activity and stability were successfully prepared with simple hydrothermal method^[1]; BiVO₄/Ag/Cu₂O, a double-Z type photocatalyst with the transfer ability of efficient photoelectrons from vacancies and strong REDOX capacity, was synthesized by applying the surface plasmon resonance of Ag to BiVO₄/Cu₂O^[2]; A novel and efficient Z-type Ag₂CO₃/Ag/WO₃ photocatalyst was prepared by simple deposition and photochemical reduction process, or microwave polymerization and surface imprinting technology. In addition, Ag₂CO₃/Ag/WO₃ has better photocatalytic activity and selectivity than Ag₂CO₃ rods or WO₃ nanoparticles^{[3, 4]}. A novel metal-free heterostructured photocatalyst composed of boron nitride quantum dots (BNQDs) and ultrathin porous g-C₃N₄ (UPCN) was successfully synthesized^[5]. In 2020, there are two related cooperative studies. Firstly, the persulfate (PS) activation system with bone char (BBC) can achieve efficient degradation of 2, 4-dichlorophenol (2, 4-DCP)^[6], and secondly, a novel BiVO₄/Ag₃PO₄/PANI photocatalyst combining planar engineering and heterojunction was obtained by depositing Ag₃PO₄ on the highly active crystal plane (040) of BiVO₄ and then introducing polyaniline (PANI)^[7].

  There are seven outstanding achievements from Jiajie Fan and Jiaguo Yu 2017-2019. The rutile TiO₂ nanocrystalline films were obtained on fluorine-doped tin oxide substrates with simple chemical bath deposition method^[8]; A onedimensional Z-type TiO₂/WO₃/Pt heterostructure was prepared by electrospinning^[9]; Texture modification and surface functionalization of g-C₃N₄ were performed by a step-by-step NH₃-mediated heat-peelable method^[10]; A direct Z-type g-C₃N₄/Ag₂WO₄ photocatalyst was prepared using g-C₃N₄ as support and AgNO₃ as precursor with in-situ precipitation method^[11]; a novel ultrathin nanosheet heterojunction 2D-Ti₃C₂/2D-Bi₂WO₆ was successfully synthesized ultrathin nanosheets Bi₂WO₆ grown in situ on the surface of Ti₃C₂ ultrathin nanosheets^[12]; TiO₂/WO₃@MoS₂ (TWM) composites exhibit good photocatalytic activity in the absence of precious metal co-catalyst^[13]; Ultrathin 2D/2D WO₃/g-C₃N₄ stepped composite heterojunction photocatalyst can be prepared using ultra-thin WO₃ and g-C₃N₄ nanosheets as raw materials by electrostatic self-assembly^[14]. In 2020, the group reported grapheme with a good application prospect due to its unique 3D frame structure and performance advantage such as hierarchical network, large specific surface area, multiple pore distribution, excellent light absorption capability and excellent electrical conductivity^[15].

  Huaming Li group has made outstanding achievements from 2017 to 2019. They summarized various strategies for the synthesis and performance improvement of layered bismuth halide materials, and proposed their applications in energy and environment^[16], introduced systematically the strategy of preparing 2D materials with atomic layer thickness, analyzed the relationship between structure and photocatalytic activity from the light response, charge separation and interface reaction^[17], and introduced in detail how to design the 2D photocatalysts with surface defects to improve the photo-catalytic performance with the anionic vacancy, cationic vacancy, vacancy association, pit, distortion or disorder methods in recent years^[18]. The group obtained metal 1T-WS₂ co-catalyst with high activity by solvothermal method^[19] and synthesized Mn³⁺ active site MnO₂/monolayer g-C₃N₄ composite photo-catalyst with partial defects by hydrothermal reaction^[20]. In addition, they constructed a defect-regulated single crystal cell Bi₃O₄Br nano-sheet and clarified the effect of surface defect on improving the efficiency of charge separation and promoting the improvement of photo-catalytic reaction performance^[21]. In 2020, Li group classified common strategies for improving the photo-catalytic decomposition efficiency of g-C₃N₄ from surface modification, functionalization and assembly^[22], and have synthesized successfully a novel MoS₂/Bi₅O₇I nano-rod composites with good photo-catalytic performance and photo-stability by solvothermal method assisted by ionic liquid^[23].

  Elson Longo group has also shown the outstanding achievements based on the cited highly papers from 2017 to 2019. They studied the energy band structure and density of states of ZnS nanoparticles and elucidated the relationship between luminescence, photocatalytic activity and electronic structure of ZnS nanoparticles^[24]; the effects of temperature on crystal structure, morphology and optical properties of ZnWO₄ nanocrystals have been researched^[25]; Ag₃PO₄: Mo microcrystal has been synthesized to degrade the Rhodamine B (RhB) under visible light by chemical precipitation method^[26]. A series of α-(Ag_1.97Re_0.01)WO₄ nano-phosphors (Re = Pr, Sm, Eu, Tb, Dy and Tm) were prepared by co-precipitation method, and the luminescence mechanism was described^[27]. In 2020, Elson Longo group obtained zinc tungstate (ZnWO₄) nanoparticles with microwave-assisted hydrothermal method^[28]. Hydroxyapatite/α-silver vanadate (HA/α-AgVO₃) composites were synthesized to inhibit the growth of fungi and photocatalytic degradation of organic compounds by chemical precipitation method^[29]. They also studied the effects of Sr²⁺ and Sn⁴⁺ sites in Eu³⁺ doping pairs on the crystal structure, electronic structure and photocatalytic performance of SrSnO₃^[30].

  4. HIGH-FREQUENCY KEYWORDS AND CLUSTER ANALYSIS OF PHOTOCATALYTIC MATERIALS IN 2020

  The keyword frequency analysis was carried out on the 4147 literatures through CiteSpace data. The frequency was set as the number of keywords in the paper. The bar chart is shown in Fig. 7, and the top 10 keywords are degradation, photocatalysis, nanoparticle, performance, water, TiO₂, composite, nanocomposite, photocatalytic activity and photocatalytic degradation.

  Figure 7

  

  Figure 7.  Histogram of top 10 high-frequency keywords in 2020

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  Co-occurrence analysis takes keywords as network nodes and forms keyword co-occurrence map composed of nodes and lines after cropping, which can be used for in-depth digging of the situation. In the work, a total of 904 nodes and 2712 lines were obtained, with each node representing a keyword, and the lines representing the relationships between keywords. On the basis of keyword co-occurrence network, the "Keywords" algorithm is used to extract tags and generate keyword clustering knowledge map. Cluster analysis was carried out in the 4147 literatures to generate four hotspots, and the cluster map of keyword is exhibited in Fig. 8. The photocatalyst in 2020 includes mainly g-C₃N₄ (graphite-like carbon nitnitide), Mxene, Metal-Organic Frameworks (MOFs), and Titanium Dioxide (TiO₂). Furthermore, based on the topic of correlation clustering and the theme of "photocatalytic materials", the paper was searched in Web of Sciences again for further refinement, sorted according to the number of citations in SCI regions I and II with high citation quantity and high quality.

  Figure 8

  

  Figure 8.  Key words cluster map of photocatalytic materials in 2020

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  4.1 g-C₃N4 (graphite-like carbon nitride)

  Compared with traditional TiO₂, g-C₃N₄ as a novel nonmetallic photocatalyst can effectively activate molecular oxygen to produce superoxide free radicals with a wide light absorption range. g-C₃N₄ is used widely in the photocatalytic transformation of organic matter and the photocatalytic degradation of organic pollutants. Che H prepared UF resin-modified g-C₃N₄ nanosheet with heat treatment method. The composite has not only the advantages of large specific surface area, good crystallinity and stability, but also fast carrier separation rate, and is able to respond to visible and near-infrared lights, and has excellent photolysis performance of hydrogen in the water^[31]. Dong H loaded rhodium phosphide (RhP_x) on g-C₃N₄ nanosheets as a co-catalyst, and the RhP_x could significantly promote the migration and separation of photoelectron-hole pairs, thus improving the performance of photocatalytic hydrogen production^[32]. Song B prepared the C₃N₄/B-C-Dots catalyst modified by B-doped carbon dots (B-C-dots) with large specific surface area, strong visible light response, fast transfer of photo-generated carriers and low recombination rate using the surface deposition method^[33]. Ou B has synthesized magnetically recoverable CoFe-LDH/g-C₃N₄ catalyst with co-precipitation method, which can remove Cr^VI through synergistic action of adsorption and photocatalysis^[34]. Jin Z loaded successfully Ni–Cu bimetallic nanoparticles onto g-C₃N₄ nanosheets with a simple heat treatment, obtaining a highly efficient visible light catalyst Ni-Cu/g-C₃N₄, where a tight Schottky interface was formed between the metal and semiconductor, thus enhancing the hydrogen production performance and optimizing the oxidation kinetics of TEOA^[35]. Graphite carbon nitride (GCN-HC) was synthesized by one-step rapid polymerization for the first time by Wang L, which has excellent visible light catalytic hydrogen production performance^[36]. Yi J classified the strategies to improve the photo-hydrolysis efficiency of g-C₃N₄ from the aspects of surface modification, functionalization and assembly, and discussed in detail the latest progress of g-C₃N₄ surface modification strategies, revealing the internal relationship between photocatalytic performance and surface modification strategies^[37].

  Hao Q identified the problems needed to be solved for g-C₃N₄ to apply in industrial energy and environment fields: (1) regulating quantum size effect by precisely controlling the size of quantum dots; (2) to explore the synthesis strategy of 1D g-C₃N₄ to solve the problems of high energy consumption and high pollution in the synthesis process; (3) to inhibit the interlayer accumulation and improve the stability of 2D nanosheets; (4) to collect and convert the end gas (ammonia) produced in the synthesis process; (5) to study the novel 3D g-C₃N₄ hydrogel^[38].

  4.2 MXene

  MXenes refers to low dimensional transition metal carbides and nitridesas energy storage and conversion materials. Liu X synthesized Mxene, and then prepared ZnO nanorods/Mxene (ZnO-MX) composite with ultrasonic vibration method. The degradation rate of RHB by ZnO-MX under visible light irradiation was 3.2 times that of pure ZnO nanorods, indicating Mxene can be used as a suitable carrier for photocatalyst^[39]. The conclusion is that MXenes is a potential embedded pseudocapacitive electrode material for supercarpacitors with extremely high theoretical capacitance. Hu M et al. proposed to achieve the theoretical capacitance limit through interlayer engineering, which not only provided "highways" (layer spacing) conducive to rapid ion diffusion, but also served as "trucks" (REDOX active sites) for heteroatoms with low electronegativity to accelerate charge transfer, resulting in high capacitance. Following this concept, Ti₃C₂ Mxene can be prepared efficiently^[40]. Nguyen V summarized the comprehensive strategy of using Ti₃C₂TX as catalyst to improve the performance of solar hydrogen evolution, including surface functional group engineering, structural modification, co-catalyst coupling, etc^[41]. Zhang H prepared lamellar (α-Fe₂O₃/ZnFe₂O₄@Ti₃C₂MXene) composite by self-assembly method under ultrasonic condition, and the powerful combination of magnetic α-Fe₂O₃/ZnFe₂O₄ component and high conductivity Ti₃C₂ Mxene one makes the material exhibit excellent photocatalytic degradation of RhB and reduction of Cr(Ⅵ) under visible light, as well as the catalyst has good cycling stability^[42]. Cheng L prepared 2D Ti₃C₂ nanosheets by HF etching and ultrasonic stripping, and then grew CdLa₂S₄ nanoparticles in situ on the surface of Ti₃C₂ nanosheets to obtain CdLa₂S₄/Ti₃C₂ nanocomposite, which has a good photocatalytic activity of hydrogens in water under visible light^[43]. Cao Y used a novel CuFe₂O₄/Mxene(CFO/Ti₃C₂) heterojunction photocatalyst to photodegrade the antibiotic sulfamethazine (SMZ) under visible light, and found the synergistic and coupling effects of the two components in the heterojunction system. The carrier lifetime and dynamic behavior were studied by time-resolved photoluminescence (TRPL) and transient surface photovoltage (TPV) spectra. It was found that Ti₃C₂ could prolong the carrier lifetime and inhibit the recombination of photogenerated electron holes. High performance liquid chromatography-mass spectrometry (HPLC-MS) and total organic carbon (TOC) techniques were used to identify the organic intermediates and their salinity. The results showed that the erosion of OH at the central point played a leading role in the breaking of S–N bond, the oxidation of aniline and the deamination^[44].

  Zuo G employed Ti₃C₂T_x nanosheets (MnS) as substrate for in-situ growth of ultrathin ZnIn₂S₄ nanosheets (UZNs) to obtain the sandwich-like UZNs-MnS-UzNs layered heterostructures (MnZIS). Furthermore, proper lateral epitaxy of UZnS on MnS surface can improve the specific surface area, pore size and hydrophilicity of MnZIS, which is conducive to improving its photocatalytic activity. Due to the Schottky junction of UZnS and MnS and the 2D ultra-thin structure, MnZIS can effectively inhibit the photogenerated electronhole recombination, and promote the photogenerated charge transfer and separation, which exhibits good photocatalytic hydrogen evolution performance (6.6 times higher than pure ZnInS4) and good stability^[45]. Li J has reported g-C₃N₄/Ti₃C₂ with unique 2D/3D structure in situ with the one-step calcination methods, and its rate of photocatalytic hydrogen production was more than 6 times that of pure g-C₃N₄ under visible light irradiation^[46]. Wang W synthesized a novel Cu₂O/MXene nanosheet, which can effectively prevent the electron-hole pair recombination of Cu₂O, and has a good antibacterial activity against Staphylococcus aureus and Pseudomonas aeruginosa^[47]. Chen J thought the research on MXenes was still in preliminary stage, and there were still the following problems to be solved: (1) HF etching method has great limitations, resulting in a large amount of acidic gas and waste liquid during synthesis; (2) Surface properties of MXene-based materials have not been well studied; (3) Toxicity and effects of MXenes and its complexes on the environment and humans have not been systematically studied, and the mechanism of toxicity remains unclear^[48]. Zhan X proposed that the application of MXenes needs to be further studied, such as the need to further explore the catalytic and electrocatalytic mechanism of MXene-based composites, which is crucial for the design of new efficient catalysts, which may open up a new way for clean energy conversion. In addition, the mechanism of MXene's high sensitivity to NH₃ is still unclear, which limits the practical application of MXene gas sensor^[49].

  4.3 Metal-organic frameworks (MOFs)

  MOFs are metal-organic frameworks with high porosity, low density, large specific surface area, regular pore sizes, adjustable pore sizes, diverse topological structures and portability. These compounds can be used in gas storage, chemical separation, catalysis and other fields. Bavykina A introduced in detail the latest progress of MOFs for CO₂ photoreduction, suggesting MOF can be directly used as a photoactive conversion for CO₂ to chemical fuels, and can also be used as a unit of mixed catalytic unit to improve the CO₂ conversion rate. Functionalization of MOFs is an effective way to improve their photocatalytic activity. The main methods to improve the visible light absorption of MOF-like are the modification structure and the adjustment of organic ligands (reactive functional groups, metal nodes with higher reduction potential, or both). MOF can also be used as a co-catalyst with other photocatalysts to produce MOF based composites with enhanced CO₂ photoreduction^[50]. Jiang ZW prepared successfully a series of 2D porphyrin-based lanthanide MOF (Ln-TCP, Ln = Ce, Sm, Eu, Tb, Yb, TCP = tetrakis (4-carboxyphenyl) porphyrins with different thickness with a household microwave oven. These 2D-Ln-TCPP nanosheets exhibited thickness dependent photocatalytic activity for the photooxidation of 1, 5-dihydroxynaphthalene (1, 5-DHN) to juglone, and in particular, Yn-TCPP exhibited excellent photokinetic activity^[51]. Huang H first reported Ti-MOF can combine Ti³⁺ active site, functionalized ligands with visible light absorption to realize visible-assisted photocatalytic nitrogen fixation. They combined light collection and catalytic components into a single material, providing a new way for MOF efficient nitrogen fixation^[52]. Yun-Pei Z synthesized 1D titanium phosphate MOF with hydrothermal method, where the uniform addition of organic phosphine linker made the band gap smaller and expanded the visible light response range. Moreover, the MOF enhances the transmission and separation of photogenerated carriers due to its unique 1D nanowire topology, and has significantly enhanced photocatalytic activity of hydrogen evolution under visible light and simulated full spectrum light irradiation^[53]. Li Y. H. synthesized NH₂-MIL-125@TAPB-PDA nanocomposites with core-shell structure with seed growth method, where the TAPB-PDA COF shell layer and NH₂-MIL-125 coating are connected by the covalent bonds. This not only improved the inherent electronic properties of the materials, but also reduced the composite efficiency of photogenerated electron-hole pairs. Meanwhile, the COF shell also promotes the absorption of visible light, which shows high activity and stability in the photooxidation of aromatic alcohols^[54]. Lv S first synthesized two novel MOFs@COFs composites (MIL-101-NH₂@TpMA and UiO-66-NH₂@TpMA) by a step-by-step assembly method. The MOF and COF can greatly improve the visible light absorption range. Meanwhile, the heterojunction interface can effectively promote the separation and transfer of photogenerated electron-hole pairs, so the solar /MOFs@COFs/PS system has strong ability to degrade bisphenol A^[55]. Wang Q summarized the recent developments of MOF's light photocatalytic technology under visible in environmental restoration areas, including waste water treatment, air purification and disinfection, and designed some strategies to modify and adjust the original MOF in order to enhance the photocatalytic properties, such as the ligand functionalization, mixed metal/binder strategy, metal ions/ligand immobilized, the dye sensitization, metal nanoparticle loading, carbon decoration, semiconductor coupling, MOF/COF coupling, carrier loading, magnetic recovery, etc^[56]. According to the current characteristics, researchers have made some prospects of MOFs. Yan Y believed the study of Ti-MOFs was still in the initial stage, and their photocatalytic applications were mainly limited to MIL-125 and NH2-MIL-125. There was an urgent need to further develop and utilize other Ti-MOFs, especially those based on infinitely long Ti-oxo chains and sheets. It should be noted that the pore size and structure of MOFs play an important role in gas adsorption and separation, but the application of Ti-MOFs in gas adsorption and separation remains to be explored. Some or all of the above challenging problems can be solved, which can effectively promote the practical application of Ti-MOFs^[57].

  4.4 Titanium dioxide (TiO₂)

  TiO₂, the most common photocatalyst, has attracted extensive attention because of green environmental protection with high efficiency, low toxicity and low cost. He F prepared graphene-modified WO₃/TiO₂ heterojunction (S-heterojunction) with hydrothermal method. The synergistic effect of S-type heterojunction formed between WO₃ and TiO₂ and Schottky heterojunction formed between TiO₂ and graphene sheets could not only effectively inhibit the recombination of electrons and holes, but also enhance the light capture. Therefore, the novel ternary WO₃/TiO₂/RGO shows significantly enhanced photocatalysis^[58]. Basavarajappa P S described the recent progress of TiO₂ nanostructure in the visible light photocatalysis: (1) enhancing the performance of TiO₂ by studying the mechanism of photogenerated charge separation; (2) design and synthesis of non-metallic doped/co-doped TiO₂ and nano-TiO₂ based composite photocatalyst^[59]. Qin Y designed a novel CdS/Ti³⁺/N-TiO₂(TNTC) composite by combining evaporation-induced self-assembly (EISA) with in-situ hydrothermal method. The doping of Ti³⁺/O_V and N effectively reduced the TiO₂ band gap and improved the light utilization efficiency. Meanwhile, the heterojunction structure of TNTC promoted the charge separation. Moreover, Ti³⁺ can form a hydrogenation layer, which reduces the activation barrier of H₂ and improves the hydrogen evolution ability. Therefore, TNTC shows efficient hydrogen production performance under visible light irradiation^[60]. Wang Y synthesized N-TiO₂/CNO_NV composite composed of N-doped TiO₂ and O-doped N-vacancy g-C₃N₄ by solvothermal method, which showed good photocatalytic activity and chemical stability in the coexistence of tetracycline hydrochloride (TC-HCl) and Cr^VI[61]. Chen J prepared uniform lamellar TiO₂/C nanocomposites using Lavoisier-125 (Ti) (Mil-125 (Ti)) as raw material with two crystal types (rutile and anatase) and large specific surface area (438 m²g^-1), which shows good activity and stability of photocatalytic degradation of tetracycline under visible light^[62]. Wang C designed a Z-type Au/A-TiO₂@g-C₃N₄ core-shell photocatalyst with high activity and selectivity for visible light catalytic CO₂ conversion^[63]. Hu X introduces the latest progress of TiO₂ to ciprofloxacin (CIP) visible light catalytic degradation and the factors that affect the photocatalytic performance, and put forward the future direction including photocatalytic degradation of the intermediate ecological toxicology effect, thermodynamics and kinetics mechanism of photocatalytic degradation process and the prospect for application of multiphase TiO₂ light catalyst etc^[64].

  5. CONCLUSION

  In this paper, the achievements of photocatalyst in 2020 were summarized and analyzed. The visualization software CiteSpace was used to draw the knowledge map. Through big data, the hot spots, major countries, institutions and researchers were identified. The results show China is the country with the largest number of articles. The research institutions have extensive and close academic cooperation. There are close cooperative relationships among middle and high yield authors, and a fruitful cooperative network in related topics has been formed, such as the cooperative network of Guangming Zeng and Danlian Huang, Jiajie Fan and Jiaguo Yu, and Elson Longo and Huaming Li; the focus of photocatalyst mainly includes g-C₃N₄, Mxene, MOFs and TiO₂.

  
  
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  Figure 1  Histogram of the top 10 countries/regions in the published number in 2020

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  Figure 2  Co-occurrence map in different countries/regions

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  Figure 3  Histogram of top 10 institutions in 2020

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  Figure 4  Co-occurrence map of top 10 core institutions in 2020

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  Figure 5  Histogram of the top 10 authors in 2020

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  Figure 6  Co-occurrence map of photocatalytic materials

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  Figure 7  Histogram of top 10 high-frequency keywords in 2020

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  Figure 8  Key words cluster map of photocatalytic materials in 2020

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