An Exploration of the Four‐Decade‐Long History of Photocatalytic Water Treatment: Analysis of Key Advancements, Underlying Relationships, and Failures along the Way

This study explores the decade‐long history of photocatalytic water treatment using a novel integration of the Delphi method and the preferred reporting items for systematic reviews and meta‐analyses approach, along with bibliometric analysis, to examine publication trends, author and country coauthorship networks, keyword co‐occurrence, citation analysis, and catalyst/contaminant evolution over time. The key findings include a steady increase in annual publications, a shift in dominant disciplines toward Environmental Sciences and Chemical Engineering, the role of specific authors in country productivity and citation rankings, and a growing need for international collaboration. TiO2‐ and TiO2‐based materials remain dominant, but a clear trend of abandonment in favor of novel materials is observed. Solar light is gaining importance for the photocatalytic elimination of aquatic pollutants, shifting simple organic compounds to priority substances. The evolution of this field is highlighted by tracing advancements in catalyst diversity and complexity, process optimization, and intensification. However, despite important milestone achievements, persistent knowledge gaps, discrepancies in comparability among studies, a lack of understanding of the fundamentals, and low funding incentives for transitioning to market hinder large‐scale applications. This study aims to provide a comprehensive guide for researchers by summarizing previous findings, unsuccessful approaches, and possibilities for improving photocatalytic applications.


Introduction
Photocatalysis has attracted considerable interest in water treatment systems because of its superior characteristics, resulting in a significant decrease in energy and chemical requirements while achieving better treatment efficiency. [1]These unique characteristics allow fast degradation of contaminants and efficient treatment of water due to the high electron conductivity of the catalyst surface. [2]ver the past four decades, photocatalysis has been implemented in numerous laboratory-scale pilot water treatment plants worldwide.The results of using photocatalysts in water treatment have been presented in many book chapters, conference proceedings, reviews, and research articles in Scopus, Web of Science, and PubMed databases.The many documents may be overwhelming, making identifying current trends and research gaps difficult.Moreover, numerous publications on the application of photocatalysts in water treatment indicate that this research can attract more attention in the This study explores the decade-long history of photocatalytic water treatment using a novel integration of the Delphi method and the preferred reporting items for systematic reviews and meta-analyses approach, along with bibliometric analysis, to examine publication trends, author and country coauthorship networks, keyword co-occurrence, citation analysis, and catalyst/contaminant evolution over time.The key findings include a steady increase in annual publications, a shift in dominant disciplines toward Environmental Sciences and Chemical Engineering, the role of specific authors in country productivity and citation rankings, and a growing need for international collaboration.TiO 2-and TiO 2 -based materials remain dominant, but a clear trend of abandonment in favor of novel materials is observed.Solar light is gaining importance for the photocatalytic elimination of aquatic pollutants, shifting simple organic compounds to priority substances.The evolution of this field is highlighted by tracing advancements in catalyst diversity and complexity, process optimization, and intensification.However, despite important milestone achievements, persistent knowledge gaps, discrepancies in comparability among studies, a lack of understanding of the fundamentals, and low funding incentives for transitioning to market hinder large-scale applications.This study aims to provide a comprehensive guide for researchers by summarizing previous findings, unsuccessful approaches, and possibilities for improving photocatalytic applications.
future and overcome bottlenecks in the processes that impede their field applicability.
It has been found that a bibliometric analysis of relevant databases can demonstrate the patterns of worldwide scientific literature and trends in any field of science. [3][14][15][16] In particular, this coupling methodology has recently been used to identify thematic patterns in research and highlight recent advances in the field of Fenton and persulfate advanced oxidation (AOP) processes. [17]ther insightful approaches include the Delphi method and the preferred reporting items for systematic reviews and metaanalyses (PRISMA) method, which have been widely used across a range of disciplines for conducting systematic reviews and meta-analyses as well as to develop and gain consensus among a panel of experts. [18,19]The PRISMA technique ensures that research is conducted and reported rigorously, transparently, and completely, essential for producing high-quality, reliable, and trustworthy research. [20]Bias was minimized, and all relevant studies were identified; therefore, a systematic evaluation of the body of literature was performed.Identifying relevant studies is essential for providing evaluation questions (EQ), as it ensures accuracy, builds a strong foundation, avoids duplication, informs decision-making, and enhances credibility throughout the evaluation process. [21]Qs developed by the Delphi method can better inform decision-makers and support effective processes. [22]A key characteristic of the classic Delphi method is identifying EQs. [23]his characteristic enhances the validity and reliability of the study design for identifying knowledge organization and discovering hidden relationships and trends in the research topic. [24] key distinction between the two methods is that the Delphi technique was used to identify the criteria and EQs in the literature.In contrast, PRISMA was used to select relevant studies. [25,26]herefore, this study aims to provide a comprehensive view of the distribution of research papers on photocatalysis in terms of authors, sources, countries, and citations.Collaboration among countries was also highlighted in this study.Moreover, keyword correlation and co-occurrence are elucidated, followed by a discussion of current and potential hot topics in photocatalytic research.This study analyzed the available publications on photocatalysis from the Scopus database, whereby the available data were analyzed using a combined Delphi-PRISMA approach.This study used the Delphi method as a consensus group tool to frame a wide range of EQs on critical bibliometric issues related to the study topic, fueled by data scavenged by the PRISMA method.This study will benefit researchers, academics, policymakers, and industries in discovering this field's future perspectives and potential.Further extension to other fields can be achieved by implementing this framework.Subject area analysis was used to identify the document topics.Figure S1, Supporting Information, shows the subject areas of the photocatalysis documents for water treatment.Chemistry (n = 12 414; 21.20%),Chemical Engineering (n = 10 693; 18.26%), and Environmental Science (n = 9574, 16.35%) were the most common, accounting for >16% of the documents.However, it is well known that Scopus categories are not mutually exclusive, with some documents being assigned to multiple categories.Still, there has been a recent shift toward the Materials Science, Energy, and Biochemistry categories.Among these works, almost 90% are research papers, and only 10% are reviews, conference papers, etc.The Scopus documenttype classification of the scientific documents is presented in Table S1, Supporting Information.

Results
Figure 2 presents an overview of global research productivity.China, India, Japan, the U.S.A., Spain, South Korea, Italy, France, Iran, and the UK were the top ten most productive countries, producing 61.76%(16 582/26 847) of all documents, with China alone contributing 43.02%.Table S2, Supporting Information, lists the top 50 countries (out of 132) based on the published documents.Scopus indexes more than 150 journals that published studies on photocatalysis for water treatment, revealing the high diversity and interest of the scientific community.The top 20 journals published between 243 and 9487 documents during the 1912-2023 period (Figure S2 and Text S1, Supporting Information).

PRISMA Results
Delphi panelists performed all stages of the PRISMA technique with expertise in photocatalysis for water treatment.A total of 1626 documents published in 22 subject areas and 159 sources, with 5667 authors, were identified during the initial literature search (Table S3, Supporting Information).Review articles, letters to the editor, books, and book chapters were also excluded (9.8%).Although reviews are important for consolidating knowledge in a field, they do not offer new research data, as defined within the scope of this study.Moreover, an important challenge among the panel members was disregarding research areas unrelated to photocatalysis.This step excluded 32 documents published in irrelevant subject areas, such as hydrogen production.After completing the screening process, 1328 research papers (original articles and conference papers) met the criteria for inclusion in the bibliographic analysis.

Analysis of Author and Countries' Co-Authorship Network
In co-authorship analysis, the relatedness of items (authors, organizations, or countries) was determined based on the number of coauthored documents.Figure 3 shows the coauthorship network of the overlay authors based on the total strength of the coauthorship links between a given author and other authors.This figure is divided into two categories: the average publication year (Figure 3a) and the average number of citations (Figure 3b) received by the author.It involves analyzing the coauthorship network, which connects authors collaborating on one or more publications.The results in Figure 3a,b indicate that Liu (122), Wang (120), Wang (109), Li (102), Zhang (97), Liu (96), Zhang (88), Wang (87), Wang (85), and Domen (76) were the authors with the highest total link strength.In other words, these authors were identified as central authors with several collaborations in the field.
As shown in Figure 3a, most coauthorship links among authors in this field began in 2010.Most likely, advances in materials and techniques have sparked increased research on photocatalysis for water treatment, which resparked attention in 2010, leading to the collaboration of more researchers.Before 2010, based on the author's total link strength, Guillard (43,  2001), Inoue (53, 2002), Yamashita (50, 2006), and Domen (76, 2006) had the most coauthorship links among the authors.Between 2010 and 2015, Dionysiou (73, 2011), Mantzavinos (36,  2012), and Li (58, 2014) had the highest coauthorship links among authors.The citation trend (Figure 3b) follows the tendency described in Figure 3a; however, incorporating total link strength into bibliometric analysis enhances our understanding of an author's impact, collaboration, and influence within their field.Considering the cumulative weight of an author's connections and citations enables a quantitative assessment of key researchers, an evaluation of research output, and its overall significance.
Similarly, Figure S3, Supporting Information, shows the countries' coauthorship networks, as detailed in Text S3, Supporting Information.Besides the nonsurprising first place for China due to the sheer number of scientists, it is worth noting that the top 10 countries in citations include less populous countries (e.g., Tunisia, Cyprus, the UAE, Qatar, and Switzerland), which underpins the significant milestone works produced.The case of Switzerland is interesting: despite having fewer connections than China (with Japan, China, Colombia, and Spain), and the fact that China has more international links and has published impactful articles on photocatalysis in water treatment, Switzerland has more average citations than China.In searching for a common denominator of this effect, Cesar Pulgarin (Switzerland) and its connections with Ricardo Antonio Torres-Palma (Colombia) and Sixto Malato (Spain) have been influential.Scopus data on institutions may not be harmonized; [27] therefore, although their presentation and description are available, this part will not be highlighted further (more information on the statistics for this section is provided in Figure S4 and Text S4, Supporting Information).

Keyword Co-Occurrence Analysis
The document information page contains two types of keywords: author and indexed.Scopus does not influence either the Author or Indexed keywords because it is determined by a third party. [28]e author and indexed keyword networks are shown in Figure 4.
Author keywords were chosen by the author(s) who, in their opinion, best reflect the content of their document.This conveys important information essential for research, and their analysis can reveal field trends and differences. [29]Hence, keyword cooccurrence analysis has been used to measure connections between keywords in many documents.Examining keyword co-occurrence reveals a field's core composition and structure and identifies research frontiers. [30]This study analyzed the author's keyword co-occurrence in PRISMA documents to uncover hidden relationships and future trends in photocatalysis for water treatment.Thirteen clusters of keywords were found, the most frequent of which is presented in Table 1.The dominant keyword was "photocatalysis" (612 occurrences), indicating the joint objective of the scientific community and its popularity as a research domain.
The indexed keywords were chosen by content suppliers and standardized based on publicly available vocabulary.The most frequently used terms are listed in Table 1.Notably, there were significantly fewer clusters, but they were more populated in more general areas, such as Materials, Irradiation, Physicochemical Processes, Chemical Pollutants, and Biological Contaminants (more information is provided in Text S5, Supporting Information).Despite this observation, having fewer clusters owing to the more general classification from Scopus validated the approach and established a hierarchy in terms of frequency.

Citation Analysis
Citation analysis was used to generate a knowledge domain map of the main authors on photocatalysis for water treatment.Table 2 lists the 37 most-cited studies (top research papers with more than 200 citations) on photocatalysis for water treatment.
Concerning the outlets of preference for authors, the weight of the overlay visualization was the number of citations for each journal, and the average publication per year for each journal was considered to show the time trends.The top five most popular journals with the most citation links publishing photocatalytic Our analysis showed that photocatalysis for water treatment is not at the forefront of Environmental Science, comparing the impact factors (IF) of the latter with those of the former popular outlets (for example, JECE-2021 IF:7.968 vs APCATB-2021 IF:24.319).Interestingly, in absolute numbers, the field now encompasses a higher number of authors working on the topic, citing previous works, as seen by comparing the relevant IFs, such as Applied Catalysis B: Environmental, for example, 10 years ago (APCATB-2011 IF:5.625), and the Journal of Environmental Chemical Engineering today.

Photocatalysts Co-Occurrence Analysis
Data were obtained from the Scopus bibliographic database to illustrate the photocatalyst trends in the literature.TiO 2 was first used in 1983 in heterogeneous photo-assisted catalysis to remove trichloroethylene from aqueous solutions. [31]Since then, several photocatalysts have been developed.Table 3 lists the photocatalysts widely used in water treatment and shows that TiO 2 (437 occurrences), ZnO (79 occurrences), and graphene (44 occurrences) are the most commonly used photocatalysts for water treatment.Association strength analysis (a graphic representation in Figure S6, Supporting Information) showed that TiO 2 had 54 relationships with other photocatalysts.ZnO (21), Fe (12), and polymers (9) are the materials most commonly used in combination with TiO 2 for photocatalysis in water.In addition, metal oxide-based photocatalysts have been utilized more frequently than graphene oxide-based photocatalysts in water photocatalysis.
Over time, significant changes were observed in the occurrence of photocatalysts during water treatment.In recent decades, some of the most commonly studied and utilized photocatalysts include TiO 2 , ZnO, and various metal-organic frameworks.However, research and development in this field  are constantly evolving, leading to the emergence of new photocatalysts with improved performance and efficiency.For example, researchers have explored using carbon-based materials, such as graphene and graphitic carbon nitride (g-C 3 N 4 ), as photocatalysts for water treatment because of their unique properties.

Contaminants Co-Occurrence Analysis
A term co-occurrence map was created from the title text data to identify the most researched contaminants for photocatalytic water treatment.PRISMA titles included papers that provided 624 terms, as summarized in Table 4.
In contrast, Figure 5 presents the co-occurrence of these targets, indicating clusters of interest for researchers (more information can be found in Text S7, Supporting Information).Organic contaminants dominate the field, with dyes being the most commonly encountered target (due to their simplicity in monitoring at the laboratory scale).Furthermore, recalcitrant or nonbio-treatable compounds such as pharmaceuticals and biocides have gained interest over the last decade, making photocatalysis a possible remediation method in the context of environmental protection.

Key Historical Facts and Influential Figures in Photocatalytic Water Treatment Research
A total of 1626 publications indexed in Scopus (1980-2022) were identified, averaging 48.10 per year (Figure 1).The first was a conference paper by Francis Fong at the Energy from Biomass and Waste 4 Symposia. [32]The first research paper, Photocatalytic decomposition of phenol in the presence of titanium dioxide was published in 1984 by Kawaguchi in Environmental Technology Letters.Adding Cu to titanium dioxide increased phenol degradation in aqueous solutions. [33]Ollis et al. used heterogeneous photocatalysis to transform reactants onto a photocatalyst surface, a key study in the 20th century.Before 1980, many researchers proposed heterogeneous photocatalysis; however, Dionysiou et al. were the first to publish experimental evidence.In the early 21st century, Dionysiou et al. studied the effects of ionic strength (KNO 3 ) on 4-CBA adsorption and photocatalytic degradation of Degussa P-25, Ishihara ST-01, and Ishihara ST-21 TiO 2 powders [34] Exponential regression (R 2 = 0.93) estimates that by 2022, more than 3100 papers on this topic will be published.However, the second most published resource on photocatalysis in water treatment is review papers and not books.Notably, many research papers have been published compared to review papers (ratio of 12), indicating that the field of water treatment by photocatalysis is consolidated but is still evolving.To give better context to this value, Giannakis et al. [17] found a 19 and 69 "research paper-to-literature reviews" ratio for Fenton and persulfate processes over the last 20 years.More recent discoveries have revealed a high production rate, which is not true for photocatalysis.Like other fields, scientific production includes the forefathers and key players.Figure 3b shows Inoue, Domen, Maeda, and Saito had high average citations per document.Further analysis of the highly cited authors (Y.Inoue, K. Domen, K. Maeda, and N. Saito) revealed they were listed in 16 articles in the Scopus database.These four authors have been involved in developing novel and the most-cited photocatalysts.For example, they developed an advanced catalyst for the overall splitting of water under visible light using a novel gallium and zinc nitrogen oxide (G a1Àx Zn x )(N 1Àx O x ) catalyst modified with nanoparticles of a mixed oxide of rhodium and chromium. [35]Another developed photocatalyst was a solid solution of GaN and ZnO with a bandgap of 2.58À2.76eV modified with RuO 2 nanoparticles. [36]h/Cr 2 O 3 (core/shell) nanoparticles supported on a (Ga 1Àx Zn x )(N 1Àx O x ) solid solution were used as a promoter for overall water splitting upon visible-light irradiation (λ > 400 nm) by four authors. [35]nother indisputable fact is the influence of Dionysios on the development of photocatalytic water treatment science.In a highly cited study, he showcased the reactive oxygen species generated with visible light-active TiO 2 and revealed various mechanisms of photoactivation compared with UV light.In addition, visible light-active TiO 2 is more effective than conventional TiO 2 under visible light for removing persistent contaminants of emerging concern in the water treatment. [37]To date, Dionysiou has contributed to the development of many photocatalysts, such as TiO 2 /Al 2 O 3 composite membranes, [38] SnS 2 nanocrystals, [39] nanocrystalline TiO 2 membranes with hierarchical mesoporous multilayers, [40] SnS 2 /SnO 2 nanocomposites, [41] visible light-activated N-F-co-doped TiO 2 nanoparticles, [42] mesoporous nitrogen-doped TiO 2 , [43] and Bi 2 MoO 6 nanocrystals. [44]2.Basics of Photocatalysis: Using Solar Light and TiO2 in Plain and Composite Water Treatment Methods Solar photocatalysis decomposes and mineralizes organic compounds under UV and visible light.[45] Solar light is a renewable energy source used in water treatment technologies, mainly through two approaches: 1) developing photocatalysts to reduce the bandgap energy and 2) optimizing photocatalytic oxidation processes.[46] The most frequently used keywords related to photocatalysis were "titanium dioxide" (113), "TiO 2 " (101), "water treatment" (72), "degradation" (66), "adsorption" (61), "photocatalytic degradation" (55), "photocatalyst" (54), "visible light" (44), and "photodegradation" (209).
Photocatalysis is often associated with water treatment processes such as adsorption, ozonation, and AOP.For instance, adsorption removes pollutants, and photocatalysis further eliminates them, converting them into harmless substances. [47]This synergistic effect may have been caused by the electrostatic attraction of the adsorbent and the increased separation efficiency of the photocatalyst electron-hole pairs. [48]Ozonation eliminates organic and inorganic pollutants from the water by generating ozone and hydroxyl radicals. [49]Photocatalytic ozonation (Figure 4) was mentioned fewer times than photocatalysis but appeared more advantageous. [50]Sonolysis is another AOP that can be combined with photocatalysis.Sonophotocatalysis combines ultrasonic sound waves, UV radiation, and photocatalysts. [51]It has been utilized in recent studies on the degradation of several pollutants, such as Cr(VI), [52] water disinfection, [53] tetracycline, [54] and azo dyes. [55]Sonocatalysis has 13 links and six occurrences, but it only causes structural breakage and not mineralization, similar to the sonophotocatalysis. [56]igure 4a,b shows solar light is the most common radiation source.Although its efficacy is influenced by external factors, such as weather and time of day, [57] solar light has gained popularity because of its accessibility and cost-effectiveness.1) It is renewable, sustainable, and provided "free" from the sun.
2) It provides UV, visible, and IR wavelengths.3) It is ecofriendly, with no greenhouse gas emissions. [58]itanium dioxide, the most widely used photocatalyst for water treatment, is a wide-bandgap semiconductor photocatalyst that can be excited to generate electron-hole pairs after exposure to light. [59]However, titanium dioxide can only be excited by UV light (shorter than 387.5 nm), which utilizes only a small part of the solar spectrum (%3% on Earth and %10% in outer space). [59]To enhance photocatalytic applications in the future, TiO 2 -based catalysts should be active under "visible light" due to LEDs' low cost and applicability.
In terms of applications, based on the information shown in Figure 4, various objectives were discovered in water, including splitting (79 occurrences), disinfection (36 occurrences), adsorption (197 occurrences), degradation (396 occurrences), and pollutant removal (196 occurrences).As shown, the degradation of pollutants is the most important application of photocatalysts in water.The co-occurrence results (both author and index keywords) showed that the adsorption process (197 occurrences) has been of great interest in photocatalysis studies; therefore, researchers may need to pay more attention to other side chemical reactions such as absorption (29 occurrences), hydrolysis (28 occurrences), and redox reactions (10 occurrences).
Finally, the keywords "band gap", "band gap energy", "band gaps", "band structure", "valence band holes", and "wide band gap semiconductor" were used to refer to the bandgap issue.Despite these advances, researchers have paid little attention (31 occurrences) to bandgap measurements and evaluations.

Evolution of Photocatalytic Water Treatment: Innovations and Trends in Target Contaminant or Catalyst Selection
An analysis of the keywords (Section 2.7) showed that photocatalytic applications included the removal of hazardous materials produced by industries, such as dyes (137 occurrences), herbicides (55 occurrences), pesticides (54 occurrences), pharmaceutical and personal care products (31 occurrences), insecticides (13 occurrences), plastics (six occurrences), and glass (six occurrences) How has the process progressed over time?First, as shown in Table 1, the term "photoassisted catalysis" has been used instead of photocatalysis in 1983.In 1991, the term "photooxidative degradation" was used to refer to photocatalysis.In 2001, photocatalysis was first used in a highly cited article by Ikarashi et al.Furthermore, the types of photocatalysts reported in Table 1 are diverse regarding target contaminants for catalytic activity.For example 1983, chlorocarbon contaminants were degraded from the water via heterogeneous photo-assisted catalysis using TiO 2 . [31]In 2018, tetracycline antibiotics were removed from natural water using 3D polymeric carbon nitride foam. [63]Hence, the evolution of the work, divided into 5 year segments, can be summarized as follows.1) <2000: As shown in Table 2, before 2000, Ollis [64] "Contaminant degradation in water" was the most cited paper (453 citations).In this study, a heterogeneous photocatalytic process, including TiO 2 powder at wavelengths <360 nm, completely degraded chloromethane, bromomethanes, chloroethanes, chloroethylenes, bromoethylenes, chlorobenzene, and chloroacetic acids in dilute aqueous solutions. [64]) Between 2000 and 2005, Houas et al. [64] received the most citations for their paper titled "Photocatalytic degradation pathway of methylene blue in water", in which the degradation of methylene blue was investigated in an aqueous environment using a TiO 2 / UV-based photocatalysis process.They attempted to demonstrate important concepts related to the photocatalytic degradation of methylene blue, such as the adsorption of methylene blue on TiO 2 , photocatalytic nature of the reaction, kinetics of methylene blue, total organic carbon catalytic and chemical oxygen demand reduction, quantum yield, ion evolution in the solution during photocatalytic degradation, influence of pH, identification of intermediate products, and the photocatalytic degradation pathway of methylene blue.[65] 3) Between 2005 and 2010, a study by Hariharan [66] entitled "Photocatalytic degradation of organic contaminants in water by ZnO nanoparticles: Revisited" attracted much attention because of detailed comparisons among ZnO nanoparticles, bulk ZnO, and commercially available degussa TiO 2 for the degradation of organic contaminants in water. 5) Since 2015, Cai (ref.[68]; 262 citations), Wang ( ref. [62]; 233 citations), and Ke (ref.[69]; 222 citations) have been the most cited papers in the field of photocatalytic water treatment.Cai et al. investigated the possible reaction mechanism, stability of ZnFe 2 O 4 , toxicity of the treated solution, main reaction intermediates, and a plausible degradation pathway of Orange II using a visible light-assisted heterogeneous Fenton reaction with ZnFe 2 O 4 .[68] This indicates that effluent toxicity is an important aspect of photocatalytic studies.Wang et al. [68] published a highly cited study on the visible light-driven removal of tetracycline antibiotics and the reclamation of hydrogen energy from natural water matrices and wastewater using polymeric carbon nitride foam.They comprehensively addressed the practical challenges of high cost, low efficiency, secondary pollution, and the influence of water matrices.[63] Ke et al. fabricated a novel Bi 2 O 3 / Bi2S 3 /MoS 2 n-p heterojunction system to enhance photocatalytic water oxidation and pollutant degradation for the first time.They reported that creating Bi 2 S 3 and n-p heterojunctions is a significant characteristic for improving photocatalytic performance.This can provide deep insight into the design and fabrication of superior photocatalysts in the future.[69] As a partial conclusion and food for further thought, we can suggest that this overview clearly shows the evolution of photocatalytic water treatment and the slow distancing of the scientific community from TiO 2 -based processes.

Discrepancies in Understanding Fundamental Principles are a Scientific Bottleneck in the Progress of Photocatalytic Water Treatment
The scientific community has identified the limitations of photocatalysis, demonstrated by efforts to modify TiO 2 or conceive new catalysts (Table 2) in highly cited studies, and the manuscript's focus has changed since 2001.TiO 2 and BiVO 4 were the first photocatalysts whose names have been mentioned in the article titles.This indicates that research in this field has shifted from mentioning the general photocatalytic process to highlighting the type of catalyst used and focusing on developing a suitable material rather than discovering its fundamental aspects.
However, despite the intensification of efforts to improve photocatalytic water treatment using these materials, we cannot support the fact that significant advances have been made, judging by the absence of widespread real-world applications.For instance, Rengifo-Herrera et al. reported that many studies had been conducted on synthesizing N-, C-, S-, and B-doped TiO 2 with visible light response since 2001.N-doped TiO 2 is the most investigated approach for nonmetallic modification of the TiO 2 . [110]Although they searched the WOS database and benchmarked Scopus, further analysis of the clusters in Figure 4 showed that most doping was performed on TiO 2 compared to the other photocatalysts discussed in this study.Moreover, the N-doped form is the most studied approach and has been indexed as nitrogen-doped, nitrogen-doped, and 'n-doped TiO 2 .However, the key message of this study is that visible light-active N-doped materials, which are the dominant materials, cannot outperform those obtained over the entire solar spectrum.Hence, we suggest that this inconsistency indicates discrepancies in the field of photocatalytic water treatment, which ultimately impede progress.
Finally, a recent study by Rengifo-Herrera and Pulgarin [111] weighed the absence of TiO 2 photocatalytic water remediation applications, further generalizing the problem above.Specifically, the root of the problem is the lack of application of fundamental photocatalysis principles, such as the intrinsic properties of the materials, type of reactor, and mode of application (slurry vs. supported), and the vague idea that by modifying the material's physical or chemical properties, a magical improvement will take place.The scientific bet of many manuscripts manifests that modifying the crystal structure will inflict notable changes in performance and allow its application in water treatment.On the contrary, material modifications have been more related to toxicity issues due to material instability.
In conclusion, it is the author's conviction that there were high hopes for semiconductor photocatalytic applications that were not met, partially because of the ease of modification of the material to achieve interesting results, and that somewhere in the process, TiO 2 research on water lost its way and got caught in a caged environment, losing the perspective of its initial vision and environmental remediation.

Money Talks: Progress in Photocatalytic Water Treatment Is Directly Related to Investment in Science
Having presented various aspects and ways to analyze the available data pool in the literature, some initial tendencies and conclusions have been drawn.We further assessed apparent contradictions or aspects not explained by the data.For example, China dominates the field regarding the number of publications generated, followed by India, Japan, the United States, Spain, South Korea, Italy, France, Iran, and the UK (Figure 2).Understandably, China and India have more documents in this field because they are more populated than other countries.However, addressing secondary financial factors can change the degree of preferences in these countries.As observed in this study, economic analyses have received little attention from researchers (Figure 4).This concept has been observed in a few articles (29 of 1328) using the keywords related to "costs", "analysis", and "efficiency" (not effectiveness/efficacy).Therefore, researchers in this field should consider economic issues further.
We firmly believe that expenditure on research and development (R&D) is a key indicator of government and private sector efforts to obtain a competitive advantage in science and technology.We assumed the distribution among the countries above was similar; no country favors photocatalytic water treatment by default.Gross domestic expenditure on research and development (R&D) is a percentage of GDP.In 2018, the global share of R&D expenditure was %2.2% of GDP. [111]This indicator was for India (0.65), Japan (3.28), the United States (2.83), Spain (1.24),South Korea (4.53), Italy (1.39), France (2.19), Iran (0.83), and the UK (1.70).By comparing R&D expenditures with the results of this study, it can be suggested that Iran and India, which had the lowest R&D expenditures, were more successful than other countries in publications on photocatalytic water treatment.However, whether this translates into a successful application remains unclear.

Conclusions
The bibliometric parameters of TiO 2 -based water treatment technologies have not yet been exploited to better understand the evolution of this field.To address this gap, an integrated Delphi and PRISMA method was used to find the most relevant literature publications on photocatalysis in water treatment indexed in Scopus from 1980 to 2022 and to identify EQs based on bibliometric parameters, including coauthorship, co-occurrence, and citation analysis.Some key conclusions can be drawn despite some evident drawbacks of our study, such as using Scopus as a unique database and the inevitable exclusion of sources due to a lack of bibliographic data. 1) The number of articles published annually has increased steadily.Articles related to photocatalysis in water treatment have shifted from Physics and Chemistry to Environmental Sciences and Chemical Engineering.2) China was found to be the most productive country during the study period.China and the United States have the highest coauthorship rates in photocatalysis for water treatment.However, China has fewer average citations than other countries, such as France, the United States, and Japan.3) Coauthorship has recently increased because of the need for multidisciplinary water treatment and the impossibility of achieving this within a single group.Country wise, Japanese, Korean, Vietnamese, Australian, American, Chinese, and Greek institutions have significant shares of coauthorship in water photocatalysis.4) The results of this study validate that titanium dioxide is the most widely studied photocatalyst in the water treatment field.In contrast, the application of solar light is highly regarded because of the decomposition and mineralization of organic compounds compared to the UV-or single-vis light spectrum.TiO 2 has been associated with 54 photocatalysts, most of which are ZnO, Fe, or polymers.The sol-gel method is the most commonly used for photocatalyst preparation; however, it has been slowly abandoned or significantly modified to overcome some limitations.5) Finally, in terms of sources of organic pollutant emissions, dyes are the predominant organic pollutants studied in the field of water treatment by photocatalysis because of the need for the standardization of new processes, followed by pharmaceuticals and herbicides, which have dominated the efforts of previous years to validate treatments in real-life Environmental Science problems.
Numerous researchers worldwide have contributed to the advancement of photocatalytic water treatment by exploring various aspects, such as catalyst design, light sources, reactor systems, and contaminant removal kinetics.Plain photocatalysis uses TiO 2 alone as the catalyst.In contrast, composite photocatalysis involves incorporating additional materials (e.g., metal oxides and carbon-based nanomaterials) to enhance the photocatalytic activity or target-specific contaminants.The evolution of photocatalytic water treatment has resulted in advancements in contaminant coverage, catalyst selection, and process optimization to enhance its efficiency and applicability in addressing diverse and emerging water pollution challenges.By focusing on doping, modification, and alternative catalytic materials, researchers have aimed to overcome the limitations of TiO 2 and develop catalysts with enhanced performance, efficiency, and applicability in various fields, including environmental remediation, energy production, and chemical synthesis.
The findings of this study provide a comprehensive historical summary of literature published over the past four decades.In addition, it highlights the most relevant and productive authors and seminal publications, providing a handbook for determining which topics are of interest, countries or institutions that conduct high-level research, and suitable journals for submitting their findings.

Perspectives and Outlook
Overall, photocatalytic water treatment appears to be a vibrant, evolving, rich, and fruitful field of study, as demonstrated by the objective analysis of the Delphi and PRISMA methods and our detailed assessment.Nonetheless, at this point, it would be a great omission not to mention that despite decades of research, there are still significant knowledge gaps in understanding the fundamentals of photocatalytic water treatment.These gaps may include uncertainties in reaction mechanisms, catalyst behavior, design criteria, and optimization strategies.
There is an ongoing effort to overcome these issues, which has been demonstrated in the available scientific literature but has not materialized into scientific breakthroughs.
Furthermore, despite the rampant growth in the available literature and significant advancements in photocatalytic water treatment studies, a lack of large-scale applications is evident.Investigating the reasons behind this limitation can uncover barriers such as cost-effectiveness, scalability issues, and regulatory challenges that must be addressed for successful implementation.Consequently, analyzing the existing landscape and identifying potential barriers or gaps can help inform policymakers and funding agencies about the need for increased investment in this area to encourage the pilot or large-scale implementation of notable proof-of-concept studies.
The authors highlight that funding has always played a role in science.Funding allotment and ease of acquisition and distribution are the main reasons for the uneven growth of knowledge versus application practices in TiO 2 photocatalytic research.For instance, funding bodies that use the number of articles as the sole metric of scientists' activities have led to a distorted view of excellence and, consequently, awarded grants.As the present study discusses, the percentage of investment per capita in research as a percentage of the GDP is notably low.Hence, since there are not enough means for the core funding of research groups, the battle for financial support did not initiate a healthy scientific competition.Still, there was a run on a hamster wheel among peers to secure resources, attained by overcompensating with research papers that ultimately deviated from the goal of high importance for humanity (i.e., water remediation).
However, the authors did not intend to prevent further research by presenting a one-sided outlook on TiO 2 -mediated water treatment.In contrast, with this work, we hope to provide a complete guide to new scholars entering the field and establish researchers responsible for training the new generation of scientists to consult on what has already been attempted.Adopting successful strategies for improving photocatalytic studies and applications and modifying the paths of previous photocatalytic research activities are key to future improvements.

Experimental Section
Delphi Method: The EQs were identified and selected by in-house expert panel members from the participating institutions in this study, who had highly related expertise in photocatalysis in water treatment.This panel evaluated all papers extracted from the Preferred Reporting Items for Systematic Reviews and Meta-Analyses section for co-occurrence and coauthorship analyses.The proposed EQs for photocatalysis in water treatment were as follows.1) What are the global trends in publications?2) What are the subject areas of these publications?3) What are the document types of publications?4) What is the global distribution of publications?5) What are the dominant journals in publications?6) What is the network of authors' coauthorships?7) What is the network of countries' coauthorship?8) What is the network of institutional coauthorships? 9) How is the network of author keyword co-occurrence?10) How is the network of index keyword co-occurrence?11) What is the citation analysis of the research papers?12) What is the citation analysis of journals that publish research papers?13) What are the most well-known photocatalysts in the literature?14) What are the most studied contaminants in research papers?15) What are the irregularities, imbalances, and outliers in literature?
PRISMA Approach: The literature was reviewed using the PRISMA framework to answer the EQs.Data were obtained from Scopus, a bibliometric analysis tool used by scholars for quality analysis. [112,113]116] Bibliometric Analysis: The overall literature overview was performed by a search query including the keyword "photocatalysis" in Title, Abstract, or Keywords and the terms "water", "degradation", "removal", "decomposition", "aqueous", and "solution" in Title.
Network analysis is an integrated approach to display the relationships between factors in a scientific field. [117]It is used to show academic collaborations among authors, countries, and organizations and to visualize co-occurring keywords in bibliometric studies. [118]VOSviewer, a bibliometric analysis software, extracts key parameters from numerous scientific publications to create and display coauthorship, cocitation, and co-occurrence networks. [119]PRISMA was used to obtain the dataset, which was then analyzed using VOSviewer to create the visualization networks.Coauthorship analysis was used to identify the most influential authors, countries, and institutions in the field of photocatalysis for water treatment.Keyword co-occurrence analysis examined the potential link between two keywords in the same paper.VOSviewer generates the best-related network maps, including overlay and density visualizations.This software tool builds and visualizes bibliometric networks and offers text mining to construct and visualize co-occurrence networks of key terms from the literature. [120]The VOSviewer and VOS mapping/clustering techniques are described in ref. [121].

Disclaimer
The views expressed by S.S. are purely those of the author and may not under any circumstances be regarded as stating an official position of the European Union.

2. 1 .Figure 1
Figure1shows the cumulative number of documents published on photocatalysis in water treatment as ofAugust 19, 2023.As depicted in the figure, the total number of published documents was 26 847 from 1912 to 2024.The number of documents published on photocatalysis in water treatment increased annually from 37 in 2007 (1.75% of the total research papers) to 307 in 2021 (16.79%).The number of papers published annually increased by over 40% over 3 years within the study (100% in 2012, 41% in 2014, and 73% in 2018).The average number of papers published before 2006 was 8.4 per year, as there were multiple years with fewer than three publications.Subject area analysis was used to identify the document topics.FigureS1, Supporting Information, shows the subject areas of the photocatalysis documents for water treatment.Chemistry (n = 12 414; 21.20%),Chemical Engineering (n = 10 693; 18.26%), and Environmental Science (n = 9574, 16.35%) were the most common, accounting for >16% of the documents.However, it is well known that Scopus categories are not mutually exclusive, with some documents being assigned to multiple categories.Still, there has been a recent shift toward the Materials Science, Energy, and Biochemistry categories.Among these works, almost 90% are research papers, and only 10% are reviews, conference papers, etc.The Scopus documenttype classification of the scientific documents is presented in TableS1, Supporting Information.Figure2presents an overview of global research productivity.China, India, Japan, the U.S.A., Spain, South Korea, Italy, France, Iran, and the UK were the top ten most productive countries, producing 61.76%(16 582/26 847) of all documents, with China alone contributing 43.02%.TableS2, Supporting Information, lists the top 50 countries (out of 132) based on the published documents.Scopus indexes more than 150 journals that published studies on photocatalysis for water treatment, revealing the high diversity and interest of the scientific community.The top 20 journals published between 243 and 9487

Figure 2 .
Figure 2. The world map of the worldwide research productivity between 1912 and 2023.

Figure 3 .
Figure 3. a) Authors' co-uthorship network visualization map based on the average publication year of the documents published by an author and b) the average number of citations received by an author .The size of the circles refers to the total link strength and the color of the circles refers to custom score attributes (see Text S2, Supporting Information, for more details).

Figure 4 .
Figure 4. a) Author keywords co-occurrence network.The total link strength for each of the 136 keywords is split into 13 clusters with 2375 links.b).Index keywords co-occurrence network of all keywords map.The map shows 6 clusters with 118 003 links.For either subgraph, the circles are sized according to the number of occurrences for each term.

Figure 5 .
Figure 5. Co-occurrence analysis of top contaminants removed by photocatalysis.

Mohsen
Ansari holds a B.Sc. in environmental health engineering from Tehran University of Medical Sciences and a Ph.D. from Shahid Sadoughi University of Medical Sciences, Iran.He now serves as an assistant professor at Qazvin University of Medical Sciences.Recognized as the National Distinguished Student in 2019, he is a member of Iran's National Elites Foundation.His current research interests include plasma-advanced oxidation processes, catalysis, and environmental pollution control.Gholamreza Moussavi holds a Ph.D. in environmental health engineering, in 2005, from the Tehran University of Medical Sciences, Iran, where he became specialized in environmental pollution control techniques.He is currently working as a full professor at the Tarbiat Modares University, Tehran, Iran.His work has been widely recognized and published in top-tier scientific journals, establishing him as an expert in his field.Sofia Samoili earned a Ph.D. in civil and environmental engineering from Ecole Polytechnique Fédérale de Lausanne (CH).Her research focused on stochastic modeling and dynamic optimization in traffic flow management operations under economic constraints.Before joining the European Commission's JRC, she worked on automated driving behavior modeling.Dr. Samoili has (co)authored 60þ works, including articles, reviews, policy briefs, white papers, and conference presentations, and has an h-index of 11.Her current research lines involve technology and data mining, AI applications mapping, modeling emerging technologies as part of complex systems, and text modeling.Stefanos Giannakis holds a double Ph.D. in environmental engineering from the Aristotle University of Thessaloniki (GR) and Universitat Politècnica de Catalunya (ES).After graduation, he joined the Ecole Polytechnique Fédérale de Lausanne (CH), where he investigated the photochemical and photobiological processes occurring in surface waters and worked on applications of (waste)water treatment projects in resource-poor contexts.He is (co)author of over 130 manuscripts, with h-index of 43.After 3 years as "Ramon y Cajal" research fellow at Universidad Politécnica de Madrid (ES), he is now associate professor, focusing on disinfection and decontamination fundamentals and abating their emerging challenges.

Table 1 .
Author and indexed keyword co-occurrence.

Table 2 .
A Chronological review of the highly cited works on photocatalysts in water treatment.Environmental (APCATB:2010, 26 publications) are the oldest journals on photocatalytic water treatment.Colloids and Surfaces A: Physicochemical and Engineering Aspects (COLSUA, 2020, 20 publications) and the Journal of Environmental Chemical Engineering (JECE, 2019, 79 publications) are emerging journals that may continue to receive more citations and influence the field.
268 Photooxidative degradation of colored organics in water using supported catalysts TiO 2 on sand Ohko et al. [123] 381 Degradation of Bisphenol A in Water by TiO 2 Photocatalyst Houas et al. [65] 2,346 Photocatalytic degradation pathway of methylene blue in the water Vautier et al. [124] 450 Photocatalytic Degradation of Dyes in Water: Case Study of Indigo and Indigo Carmine Lachheb et al. [125] 1,400 Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania Grzechulska and Morawski [126] 284 Photocatalytic decomposition of azo-dye acid black 1 in water over modified titanium dioxide Kamat et al. [127] 279 A "Sense and Shoot" approach for photocatalytic degradation of organic contaminants in water Oshikiri et al. [128] 242 Electronic structures of promising photocatalysts InMO 4 (M = V, Nb, Ta) and BiVO 4 for water decomposition in the visible wavelength region Dunlop et al. [129] 245 The photocatalytic removal of bacterial pollutants from drinking water Ikarashi et al. [130] 233 Photocatalysis for water decomposition by RuO 2 -dispersed ZnGa 2 O 4 with d 10 Configuration Daneshvar et al. [131] 850 Photocatalytic degradation of azo dye acid red 14 in water: investigation of the effect of operational parameters Yamashita et al. [132] 403 Photocatalytic degradation of organic compounds diluted in water using visible light-responsive metal ionimplanted TiO 2 catalysts: Fe ion-implanted TiO 2 Kaneko et al. [133] 238 Optimization of solar photocatalytic degradation conditions of bisphenol A in water using titanium dioxide [139] 236 Heterogeneous photocatalytic degradation of pharmaceuticals in water by using polycrystalline TiO 2 and a nanofiltration membrane reactor Salvador [140] 232 On the nature of photogenerated radical species active in the oxidative degradation of dissolved pollutants with TiO 2 aqueous suspensions: a revision in the light of the electronic structure of adsorbed water Tayade et al. [141] 272 Photocatalytic degradation of dyes and organic contaminants in water using nanocrystalline anatase and rutile TiO 2 water treatment works are (a graphic representation is shown in Figure S5 and Text S6, Supporting Information) as follows.1) Applied Catalysis B: Environmental (86 link strengths, 13.27%).2) Journal of Hazardous Materials (55 link strengths, 8.48%).3) Chemical Engineering Journal (54 link strengths, 8.33%).4) Chemosphere (50 link strengths, 7.7%).5) Journal of Photochemistry and Photobiology A: Chemistry (50 links, 7.7%).Journal of Photochemistry and Photobiology A: Chemistry (JPPA:2008, 89 publications) and Applied Catalysis B:

Table 2 .
Continued.Study on the photocatalytic degradation of methyl orange in water using Ag/ZnO as a catalyst by liquid chromatography-electrospray ionization ion-trap mass spectrometry

Table 3 .
Top occurrences of photocatalysts in the field of study.

Table 4 .
Top contaminant occurrences in photocatalytic water treatment.