Photocatalytic Applications of g-C₃N₄ Based on Bibliometric Analysis

Abstract

To further understand the application of g-C₃N₄ in the field of photocatalysis, this study focuses on the visualization and analysis of articles in this field using VOSviewer and Citespace. These articles were analyzed in terms of number of articles, journals, authors, countries and keywords, respectively. The results show that there is little collaboration among the core authors in this field and insufficient cross-directional communication; the current applications of g-C₃N₄ are concentrated on hydrogen evolution, CO₂ reduction and water treatment. The developing trend is in the direction of constructing Z-scheme structures, regulating the separation of photogenerated carriers and reducing the recombination rate, to which more and more attention is being paid. In the future, cross-directional communication among scholars can be strengthened to promote faster development of the field of photocatalytic applications of g-C₃N₄.

1. Introduction

With current industrial development, the global environmental pollution and energy demand problems are increasingly becoming the center of people’s concerns [1]. However, how to better deal with contamination problems and reduce the energy consumption and cost of the treatment process have become hot research topics. Photocatalysis is considered as a green, clean and promising technology due to its effective use of sunlight and high reaction efficiency [2]. In a photocatalytic reaction system, the performance of the catalyst determines the catalytic efficiency and the final degradation behavior. Therefore, the search for a catalyst that can capture visible light efficiently with low cost, stable performance and simple synthesis has become a hot research topic. As a traditional photocatalyst, TiO₂ is still sought after by many researchers because of its unique photoelectric properties, good chemical stability and low manufacturing cost, but its inability to be excited by visible light limits its large-scale commercial application [3]. Therefore, g-C₃N₄ has gained wide attention owing to the characteristics of non-metallic catalyst, simple synthesis and abundant reserves of precursors. The g-C₃N₄ is a layered structure with adjustable condensation and a unique tri-s-triazine units structure that gives it good physical and chemical stability, leading to a wide range of applications in hydrogen production, CO₂ reduction, and the removal of pollutants from water and air atmospheres [4]. Due to its low band gap, it has the ability to harvest sunlight and act as a visible light-driven photocatalyst to degrade pollutants [5,6]. Unfortunately, pure g-C₃N₄ suffers from low visible light utilization, small specific surface area and high recombination rate of photogenerated carriers, thus leading to its reduced photocatalytic efficiency [7]. In addition, its photocatalytic applications are inhibited by the stacked layered structure and the lower efficiency of charge transfer [8].

In the past, to solve these problems, various researchers have tried to improve the catalytic performance of g-C₃N₄ with different methods, such as loading metal atoms (e.g., Pt [9], K [10], and Ag [11]), preparing heterojunctions with other semiconductors (e.g., TiO₂ [12], MnO₂ [13], and Ni₂P [14]), and fabricating microstructures. In addition, the loading of non-metallic elements (e.g., S [15,16,17], O [18,19,20,21] and N [22,23,24]) on g-C₃N₄ improves the catalytic activity by changing the structure, band gap and morphology of the catalyst. Compared with pure g-C₃N₄, the loading of heteroatoms improves the separation efficiency of photogenerated carriers and reduces the recombination rate, and the construction of heterojunctions increases the utilization of light energy, and the additional introduced active sites play a crucial role in the degradation of the reactants.

Hence, the main objective of this paper is to systematically analyze the applications of g-C₃N₄ in photocatalysis that have been indexed in the Web of science (WoS) core collection to 31 December 2021. In order to achieve the expected purpose, a bibliometric study of articles on the application of g-C₃N₄ in photocatalysis with the aid of Citespace and VOSviewer is conducted to analyze the key words of g-C₃N₄ in photocatalysis with its focus on the current research hotspots and major applications to help researchers better understand the latest research hots.

2. Results and Discussion

2.1. Analysis of the Number of Articles Issued over the Years

Statistics on the annual publication of g-C₃N₄ in the field of photocatalysis in WoS can be obtained from the number of articles published in the past years. As can be seen in Figure 1, g-C₃N₄ has been used in photocatalysis for about 15 years so far, and it was only after 2009 that the number of articles started to increase slowly. This is because the photocatalytic properties of nonmetallic semiconductor g-C₃N₄ have attracted a lot of attention after the observation demonstrated in 2009 that the catalytic production of hydrogen from water could be achieved under light [25]. The volume of articles on g-C₃N₄ photocatalysis has continued to grow each year from 2013 to 2021, indicating that g-C₃N₄ is gradually known and started to be investigated.

2.2. Co-Citation Analysis on Cited References

To further understand the citation of the literature on g-C₃N₄, we performed a co-citation analysis of the cited literature based on previous bibliometric studies [26] and obtained four clusters (Figure 2). A total of 104,925 citations were involved in this research, and when the threshold value of citation was set to 50, this meant that a cited document was cited a minimum of 50 times before it was counted, and a total of 641 documents were eventually used for co-citation analysis. The most frequently cited references were Wang Xinchen and Maeda Kazuhiko (2009) (1837 times), Ong Weejun and Tan Lingling (2016) (1036 times), Niu Ping and Zhang Lili (2012) (687 times), etc.

After the literature co-citation analysis using VOSviewer, the 641 cited documents that were screened could finally be divided into four clusters, each color representing a cluster. Cluster 1 is marked in red in Figure 2, which focuses on the modification of g-C₃N₄ by loading single atoms to improve its light absorption capacity and catalytic activity. In this cluster, the most cited is the publication by Wang’s group [25] (2009) on hydrogen generation from water by g-C₃N₄ under visible light irradiation, which confirms for the first time that g-C₃N₄ as a nonmetallic catalyst can produce hydrogen in the presence of light. Important articles include recent progress related to the design and modification of g-C₃N₄-based materials by Ong et al. (2016) [27]. The work on the modification of g-C₃N₄ at the atomic or molecular level and a summary of progress of its application in various fields were most cited by Cao et al. (2015) [28]. The loading of Ni and NiS onto g-C₃N₄ to enhance the photocatalytic activity to improve the efficiency of hydrogen evolution was most cited by Wen et al. (2017) [29].

Cluster 2 is marked in green in Figure 2, which focuses on the formation of cocatalyst with other semiconductor materials. The most influential articles in this group are related to load silver (Ag) atoms to form co-catalysis with g-C₃N₄, which can enhance the photocatalytic performance of g-C₃N₄ to strengthen the degradation of methyl orange (Yan et al., 2009) [30], and boron atom doping on g-C₃N₄ to enhance the degradation of organic pollution in wastewater (Yan et al., 2010) [31]. Another predominant literature is published by Zhao et al. (2015) [32], related to the fabrication methods and applications of several different heterojunction systems formed with g-C₃N₄ and further elaboration of the enhanced the mechanism of g-C₃N₄ based composites in photocatalysis. Xiang et al. (2011) [33] investigated a series of g-C₃N₄ composites containing Pt as a co-catalyst in aqueous methanol solution for hydrogen production performance tests and proposed the detailed mechanism for enhanced photocatalytic activity.

Cluster 3 in Figure 2, marked in blue, focuses mostly on the modification of g-C₃N₄ using different precursors and different synthesis methods. An important reference is that Niu et al. (2012) [34] easily obtained nanosheet materials using thermal oxidation etching bulk g-C₃N₄ in air, which further improved the generation of ·OH and significantly improved the photocatalytic hydrogen production. Yang et al. (2013) [35] studied the evolution of hydrogen in visible light using simple liquid-phase exfoliation of the bulk material g-C₃N₄ to obtain nanosheets, which was the aim. The effects of different precursors and synthesis conditions on the properties of g-C₃N₄ were investigated by Thomas et al. (2008) [36], who explored different structures and morphologies of g-C₃N₄ for different applications.

Cluster 4, marked in yellow, mainly consists of some review articles on photocatalytic materials. The most important article in this cluster is the pioneering discovery by Fujishima et al. (1972) [37] that water can be decomposed into hydrogen and oxygen using TiO₂ under the action of UV light. Kudo et al. (2009) [38] summarized the basic and experimental points of photocatalytic water splitting and investigated the photocatalytic materials used to split water.

2.3. Collaborative Mapping Analysis

Collaboration mapping can reveal the social relationships between scholars, institutions and countries in a certain field, providing a new perspective for evaluating the academic influence of researchers, institutions and countries, and facilitating the identification of which researchers and institutions are worthy of attention.

2.3.1. Author Collaboration Map

The graph of published authors presents information about the core authors in the field and the strength of the links between each author [39]. According to Price’s theorem (Equation (1)), the minimum number of publications for core authors in a field is 5.95. Therefore, authors with more than or equal to 6 publications are defined as core authors in the field.

$$m=0.749\times \sqrt{n_{\max }}$$

(1)

where m is the minimum number of publications by core authors, and n_max is the number of papers by the most prolific authors in the field. In this paper, a total of 15,198 authors were analyzed, of which 748 had more than 6 articles, as shown in

Table 1. Author publications and citations information.

Serial Number	Author	Documents	Citations	Average Number of Citations
1	Wang X	63	15,613	247.8
2	Zeng G	61	8257	135.4
3	Dong F	58	6189	106.7
4	Yu J	49	12,008	245.1
5	Li H	46	2467	53.6

, which lists the top five core authors.

According to the data, the largest number of articles was published by Wang X (63), followed by Zeng G (61), Dong F (58), Yu J (49) and Li H (46). The most cited authors on average are Wang Xi (247.8), followed by Yu J (245.1), Zeng G (135.4), Dong F (106.7) and Li H (53.6), which together make Wang X the most influential author in the field. As shown in Figure 3, the size of the circles indicates the number of publications, and the thickness of the connecting lines indicates the strength of the connection, which clearly shows that there are more connections between Zeng G and other authors, and the more closely connected authors are Huang D, Zhou C, Yang Y, etc.

2.3.2. Journal Mapping Analysis

Generally speaking, when a journal publishes more articles, as well as more citations, then the journal’s influence is higher [40]. Therefore, in this paper, a total of the top 10 journals in terms of number of publications were summarized, and their citations as well as the average number of citations were counted (

Table 2. Journal publication volume and information.

Serial Number	Journal	Documents	Citations	Average Number of Citations	IF(2021)
1	Applied Catalysis B-Environmental	407	41,668	102.4	19.503
2	Applied Surface Science	275	13,177	47.9	6.707
3	Chemical Engineering Journal	215	8860	41.2	13.273
4	Journal of Colloid and Interface Science	165	5677	34.4	8.128
5	International Journal of Hydrogen Energy	102	2981	29.2	5.816
6	Journal of Alloys and Compounds	98	2649	27.0	5.316
7	Journal of Hazardous Materials	96	5091	53.0	10.588
8	ACS Applied Materials & Interfaces	93	9969	107.2	9.229
9	ACS Sustainable Chemistry & Engineering	87	4694	54.0	8.198
10	Chemosphere	75	1448	19.3	7.086

).

The top three journals in terms of number of publications are Applied Catalysis B-Environmental (407), Applied Surface Science (275) and Chemical Engineering Journal (215). As can be seen in

Table 3. National publication volume and citation information.

Serial Number	Country	Documents	Citations	Average Number of Citations
1	China	3624	187120	51.6
2	India	307	9740	31.7
3	USA	220	20233	92.0
4	Australia	185	14654	79.2
5	Germany	163	14372	88.2
6	South Korea	151	4979	33.0
7	Saudi Arabia	132	12502	94.7

, the two journals with the highest average citations are ACS Applied Materials & Interfaces (107.2) and Applied Catalysis B-Environmental (102.4). This indicates that the articles published in these two journals are of high quality and have received much attention in the field of g-C₃N₄ photocatalysis.

For the data analyzed in this paper, there are 422 journals, and we decided to use a cut-off point of 25 or more publications, which means we mainly visualize the top 46 journals in terms of number of publications. As a result, we obtained a journal analysis graph as shown in Figure 4. The circle in the graph indicates the number of citations of the journal, and the larger the circle, the more citations of the journal; the line between the circle and the circle indicates the citation relationship between the two journals, and the thicker the connection line indicates the closer the citation relationship. As can be seen in Figure 4, the connection line between Applied Catalysis B-Environmental and Chemical Engineering Journal is the thickest, which indicates that the citation relationship between these two journals is the most frequent.

2.3.3. Most Influential Countries

In order to understand which countries have made more prominent contributions to the photocatalytic applications of g-C₃N₄, this study analyzed the number of publications for 76 countries. Firstly, the countries with more than or equal to 10 publications were visualized using Vosviewer as shown in Figure 5. In Figure 5, each dot represents a country, and the size of the dot indicates the number of articles issued by the country; the connecting line between each dot indicates the degree of association between countries, and the thicker the line is, the stronger the association is. It can be seen from Figure 5 that China has published the most articles in the field of g-C₃N₄ photocatalysis, and China has closer ties with countries such as the United States, Australia and India.

For further information on the specific number of publications and citations for each country, the top 7 countries in terms of number of publications are listed in Table 3. As can be seen, the three countries with the highest number of published articles are China (3624), India (307) and the United States (220). However, in terms of average citations, the top three countries are Saudi Arabia (94.7), the United States (92.0) and Germany (88.2), which indicates that these countries are publishing at a higher level and receiving more attention.

2.4. Clustering and Co-Occurrence of Keywords

The keywords represent the core and research focus of an article, and the analysis of keywords can be used to understand the research hotspots in a certain field. A total of 6911 author keywords from 4754 articles were analyzed in this paper, in which keywords with frequencies of 20 and above were visualized using VOSviewer, and the results are shown in Figure 6. The larger size of the circle indicates the more times the keyword appears, representing the hotspot of the field; the connecting line between the nodes represents the strength of association, and the thicker the line means the more times both appear in the same article; the different colors of the nodes represent different clusters, which means different research topics.

As can be seen in Figure 6, all the keywords are divided into 6 clusters. Cluster 1 is labeled in red and focuses on g-C₃N₄ applications in hydrogen evolution and hydrogen peroxide production. Hydrogen has a pivotal role in modern industrial applications and the search for sustainable energy sources, but the scarcity and high cost of catalysts have severely hindered its large-scale industrial application. Numerous scholars have started to explore low-cost catalysts for hydrogen production, such as the use of sulfur (S) vacancy engineering [41] and molybdenum disulfide (MoS₂) [42] modified with tin sulfide (SnS₂) quantum dots, as well as the use of g-C₃N₄ catalysts to reduce the cost of hydrogen production. Yuan et al. [43] used g-C₃N₄ nanosheets prepared using a simple probe ultrasound-assisted liquid exfoliation method to construct 2D-2D MoS₂/g-C₃N₄ for photocatalytic H₂ production. The results demonstrated that the catalysts prepared by this method showed a large catalytic efficiency enhancement over the pristine catalysts in terms of hydrogen evolution. Liu et al. [44] prepared an ultrathin g-C₃N₄ nanoplate to be introduced into the highly yield production of H₂O₂, and the prepared catalyst had a larger surface area, more active sites, smaller charge transfer length and stronger redox ability than the pristine g-C₃N₄.

Cluster 2, labeled in green in Figure 6, is mainly about g-C₃N₄ under visible light, and modification of g-C₃N₄ to enhance photocatalytic activity. Chen et al. [45] aimed to improve the catalyst activity by modifying the structure of the photocatalyst to regulate its electronic and physicochemical properties. In this study, they co-doped nonmetallic element (S) and metal element (K) into g-C₃N₄ to prepare a catalyst with more significant activity for hydrogen production under visible light. Starukh et al. [46] systematically described the modulation of the morphology and composition of g-C₃N₄ using chemical doping of oxygen (O), sulfur (S), phosphorus (P), nitrogen (N), carbon (C) and vacancies of nitrogen and carbon in recent years and summarized the main design directions for doping of nonmetallic elements into g-C₃N₄. The blue part in Figure 6 is group 3, which focuses on enhancing the photocatalytic activity of g-C₃N₄ by forming heterojunction structures with other semiconductor materials and further explaining the changes at the microscopic level using first-principles calculations. Yu et al. [47] synthesized a self-hybridized 2D/2D g-C₃N₄ catalyst using different precursors, and the synthesized catalyst improved the interfacial electron transfer and increased the photocatalytic active site, thus enhancing its photocatalytic activity. Li et al. [48] doped K and I onto the prepared g-C₃N₄ spheres, using density functional theory calculations to investigate the respective roles of the doping elements during the reaction.

Cluster 4 is marked in yellow in Figure 6, and there are mainly studies on the effectiveness of g-C₃N₄ for practical applications. Chen et al. [49] prepared porous oxygen-doped nanoscale catalysts using urea and sodium oleate with thermal copolymerization and promoted their visible photocatalytic efficacy by altering the electronic structure to promote light absorption and separation. Lam et al. [50] summarized that the photocatalytic effect of g-C₃N₄ in wastewater treatment was improved by using different synthetic methods to compound g-C₃N₄ with different semiconductor materials. Zhang et al. [51] revealed new prospects for the role of direct drug production by constructing a powerful photocatalytic system, thus providing a method that can efficiently, consistently and cost-effectively achieve controlled isotopic labeling in n-alkylated amines. The Purple part in Figure 6 is Cluster 5, in which the clusters are mainly related to CO₂ reduction and the formation of co-catalytic materials with other catalysts. Chen et al. [52] introduced CO₄ into g-C₃N₄ to form a co-catalyst, and the results showed that the introduction of CO₄ not only promoted the charge transfer of g-C₃N₄ but also greatly improved its surface catalytic oxidation ability and further enhanced the reduction of CO₂. Shown in the light blue section of Figure 6 is Cluster 6, in which the main concern is the modification of g-C₃N₄ through nanostructures to enhance its catalytic activity. Ishap et al. [53] decorated Ag nanoparticles on g-C₃N₄ substrate and incorporated carbon nanotubes (CNTs) in Ag/g-C₃N₄ bulk, with which the light-generated e-h pairs could be effectively separated and transferred, thus improving their catalytic activity. Ismael et al. [3] summarized in detail the different composite systems such as metal oxides (O), sulfides (S) and ferrite compounded with g-C₃N₄ by increasing the specific surface area and improving the efficiency of electron-hole separation, which in turn enhance their photocatalytic effect.

2.5. Co-Occurrence and Burst Mapping of Keywords

In order to clearly identify the temporal patterns of inflection points and frontiers of disciplinary development, we have arranged the keyword co-occurrence mapping in a time series, thus showing the distribution of research hotspots within each time period. In this study, the nodes are set to Keyword, the Slice Length is set to 2, and the Selection Criteria is set to Top 25 per slice, meaning that each time slice is ranked 25 to generate a keyword time zone map, as shown in Figure 7. Each time period in this time zone mapping corresponds to a vertical time axis, the keyword appearing on the time axis indicates when the keyword first appeared, the size of the node indicates the frequency of the keyword, and the line between the keywords indicates the co-occurrence relationship.

As can be seen in Figure 7, the photocatalytic applications of g-C₃N₄ are constantly evolving and are analyzed in two parts under synthesis. Firstly, the first stage was from 2007 to 2012, when g-C₃N₄, one of the oldest synthetic compounds, started to come into the public eye after it was discovered to produce hydrogen gas under light by Prof. Wang Xinchen’ group in 2009 [25]. For example, g-C₃N₄, synthesized by Zhang et al. [54] using thiourea as a precursor, exhibited a higher H₂ production rate than g-C₃N₄ prepared from dicyandiamide or urea, and this activity could be further enhanced by increasing the condensation temperature. Ge et al. [55] prepared a new polymeric g-C₃N₄ photocatalyst loaded with precious metal Ag nanoparticles by a facile heating method, and the hydrogen evolution rate exceeded that of pure g-C₃N₄ by more than 11.7 times due to the synergistic interaction between Ag and g-C₃N₄. The second part focuses on improving the specific surface area, light absorption capacity and reducing the recombination rate of photogenerated carriers by loading, constructing heterojunctions and preparing nanostructures to improve the activity of the catalyst. Liang et al. [56] heat-treated bulk g-C₃N₄ under NH₃ atmosphere to obtain hollow g-C₃N₄ with abundant in-plane pores. The formed in-plane pores not only can greatly accelerate the charge transfer but also provide more boundaries, reducing the aggregation and increasing the specific surface area. Jiang et al. [57] successfully synthesized direct solid-state bis-Z-scheme WO₃/g-C₃N₄/Bi₂O₃ photocatalyst using tungstate, melamine and bismuth nitrate pentahydrate as precursors by a one-step co-calcination method, which improved the absorption capacity of visible light, increased the surface area and improved the separation efficiency of photogenerated electron-hole pairs.

Further, we combined all data from 2007 to 2021 for the detection of emergent keywords. These keywords not only show how the research hotspots have changed over time in recent years but also reflect the current research trends in the field. For the top 24 keywords of outbreak intensity are shown in

Table 4. The top 24 keywords with the highest outbreak intensity.

No.1	Keywords	Strength	Begin	End	2007–2021
1	Titanium dioxide	8.30	2007	2015
2	Visible light irradiation	34.20	2010	2018
3	Electronic structure	23.26	2010	2017
4	Selective oxidation	8.52	2011	2016
5	Methyl orange	21.00	2012	2016
6	Visible light photocatalysis	18.39	2012	2017
7	C3N4	18.39	2012	2017
8	Polymer	11.29	2012	2016
9	Composite photocatalyst	32.64	2013	2017
10	Photocatalysis	20.08	2013	2017
11	Semiconductor	20	2013	2016
12	Hybrid	13.66	2013	2016
13	Photoreactivity	10.37	2013	2016
14	Catalysis	16.85	2014	2016
15	Carbon nitride nanosheet	12.23	2015	2017
16	Photocatalytic activity	8.17	2015	2018
17	Irradiation	18.58	2016	2018
18	Artificial photosynthesis	16.68	2017	2019
19	Enhancement	10.60	2018	2019
20	Z scheme photocatalyst	19.88	2019	2021
21	Charge transfer	13.05	2019	2021
22	Wastewater	12.7	2019	2021
23	Charge separation	10.83	2019	2021
24	Metal organic framework	9.19	2019	2021

. As can be seen, about the preparation of Z-type photocatalysts, charge transfer and degradation of pollutants in wastewater are the directions that have received much scholarly attention in the field of g-C₃N₄ photocatalysis since 2019. For Z-scheme photocatalysts, Raizada et al. [58] prepared Ag/AgBr/V₂O₅/PCN ternary Z-scheme catalysts with heterojunction by a single-step hydrothermal method, which showed advanced photocatalytic efficacy thanks to the good electron storage and transport between PCN, AgBr and VO and the reduction of photo-induced charge carrier recombination. Another keyword with high explosive intensity is charge transfer. Wang et al. [59] synthesized Ag-bridged 2D/2D Bi₅FeTi₃O₁₅/ultrathin g-C₃N₄ Z-scheme heterojunction photocatalysts with powerful interfacial charge transfer by a simple sonication and photoinduction strategy. Using characterization and density functional theory, it was verified that the matched band structure of Bi₅FeTi₃O₁₅ and g-C₃N₄ could induce an ultra-fast Z-scheme interfacial charge transfer path. In addition, the bridged silver nanoparticles in the 2D/2D heterojunction extend the light absorption range and prolong the lifetime of the Bi₅FeTi₃O₁₅ induced photogenerated electron holes. The destructive environmental impact of active pharmaceutical ingredients (APIs) in surface water and drinking water has become a hot concern for researchers in recent decades, and graphite nitride, a non-metallic semiconductor photocatalyst, is an emerging hot nanomaterial that has been widely recognized for its practical utility in water purification [60]. Lin et al. [61] synthesized a ternary heterostructured photocatalyst by co-immobilizing graphitic carbon nitride quantum dots (CNQDs) and nitrogen-doped carbon quantum dots (NCDs) on the surface of BiVO₄ microspheres, and the synthesized catalyst showed good photocatalytic activity for both Rhodamine B and tetracycline degradation under visible light. It is worth mentioning that some of the highlighted words in Table 4 show that they have been discontinued in previous years, but it does not mean that no researchers are still working in this direction at present; it is just that the source of the data in this paper does not cover all the databases.

3. Materials and Methods

3.1. Data Source

The selected literature data were retrieved from the Science Citation Index Expanded (SCI-EXPANDED) and the Social Science Citation Index (SSCI) in Web of Science Core Collection. SCI-EXPANDED and SSCI is a globally well-known index database of scientific papers, collecting world’s leading scientific and technical journals by strict selection criteria. This ensured a high quality in my choice of papers on the use of g-C₃N₄ in photocatalysis. In this paper, the keywords “g-C₃N₄”, “graphitic carbon nitride” and “photocatalysis” were searched, and the three were matched by AND or OR, and the article type was selected as “article” and “review”, select “English” as the language. To ensure the reliability of the analyzed data, the years of the papers were chosen from articles published from 2007 to 2021. The search results were saved in plain text format with the cited references, and a total of 4547 relevant publications were obtained.

3.2. Analysis Method

Bibliometrics uses statistical methods to quantitatively analyze papers and visualize the research situation and emerging trends in a field, helping researchers to better understand the hot spots and directions of research in the field [62]. In this paper, we use CiteSpace and VOSviewer to analyze the obtained data for posting volume, co-occurrence analysis, research hotspot analysis and co-citation analysis. Co-citation analysis indicates the relationship between two or more authors (journals, references) that are cited simultaneously by the same author (journal, reference) [63]. The visualization of co-citation analysis facilitates the interpretation of the data and can make the results more intuitive and comprehensive. In co-occurrence and cluster analysis networks, nodes represent specific key terms, such as countries, institutions, or keywords, where the larger the node size, the higher the frequency. The color of the node indicates the year; the larger the circle, the more important the node; and the thicker the line, the closer the collaboration relationship. Additionally, the cluster network was formed by grouping closely connected keywords into one category based on VOSviewer software. Keywords reflect the core and essence of an article, and the visual analysis of keyword co-occurrence mapping can provide a more intuitive view of the research hotspots in a certain field.

4. Conclusions and Future Prospects

A total of 4547 articles were analyzed, and the main findings are as follows. First, the application of g-C₃N₄ photocatalysis has started to gain attention from many scholars since 2009. Second, after years of research, several well-known scholars have been formed in this field. Thirdly, core journals that publish papers in this field have been established, mainly Applied Catalysis B-Environmental and ACS Applied Materials & Interfaces, etc. Fourth, Chinese scholars are far ahead in the field in terms of contribution of articles published, but in terms of average citations per article, the articles published by scholars from Saudi Arabia are more recognized in the field. Fifth, the co-occurrence and clustering analysis of keywords reveals that several stable research themes have been developed in this field, such as in hydrogen evolution, CO₂ reduction and wastewater treatment.

With the development of science and technology, more and more high technology can be combined with traditional research, which can greatly improve the research progress. For example, in today’s digital age, the judicious use of some new technological tools may lead to greater development of current basic research. Currently, many scholars have used artificial intelligence to replace manual work to complete some basic experimental investigations, in order to achieve the effect of saving a lot of human and material resources. Backed by a large amount of high-quality data, the use of numerical drivers allows the screening of desired materials or the prediction of material properties without experiments [64]. For example, Mahmood et al. [65] used machine learning techniques to design efficient solar cell materials quickly and cheaply, significantly reducing the time and material costs required for traditional experiments. Zhao et al. [66] used machine learning to probe the effect of a large number of single atoms loaded onto g-C₃N₄ on the performance of formic acid dehydrogenation, and then screened the scheme with excellent effect on formic acid dehydrogenation. We hope that more new technologies can be combined with traditional experiments in the subsequent development to promote social progress.