A State‐of‐Art Review of the Metal Oxide‐Based Nanomaterials Effect on Photocatalytic Degradation of Malachite Green Dyes and a Bibliometric Analysis

Abstract A wide range of hard contaminants in wastewater is generated from different industries as byproducts of the organic compound. In this review, various metal oxide‐based nanomaterials are employed for the photocatalytic removal of malachite green (MG) dye from wastewater. Some cost‐effective and appropriate testing conditions are used for degrading these hard dyes to get higher removal efficiency. The effects of specific parameters are considered such as how the catalyst is made, how much dye is in the solution at first, how much nanocatalyst is needed to break down the dye, the initial pH of the dye solution, the type of light source used, the year of publications, and how long the dye has to be exposed to light to be removed. This study suggests that Scopus‐based core collected data employ bibliometric methods to provide an objective analysis of global MG dye from 2011 to 2022 (12 years). The Scopus database collects all the information (articles, authors, keywords, and publications). For bibliometric analysis, 658 publications are retrieved corresponding to MG dye photodegradation, and the number of publications increases annually. A bibliometric study reveals a state‐of‐art review of metal oxide‐based nanomaterials' effects on photocatalytic degradation of MG dyes (12 years).


DOI: 10.1002/gch2.202300001
industries, like dye manufacturing, apparel, paper pulp mill, reed mat, tanneries, and printing significantly threaten our natural resources. [2] The textile industries release a considerable amount of waste, that is, the organic dyes, chemicals, heavy metals, and oil in water bodies create a mass disaster for the environment because of their nasty nature and act as an agent for cancer disease. [3] Textile dyes are highly soluble in water; due to this high release of dark dye, the entry of sunlight is blocked, causing severe damage to aquatic organisms and humans in and around the areas of these industries. [4] These dyes are ≈10 000 in number. Viewing large-scale usage, the azo dye comprises immense and critical damage. [5] The malachite green (MG) dye "Nmethylated diaminotriphenylmethane" is an organic compound containing a "green" color crystal having mostly metallic luster used in various dyestuff industries and is one of the most effective chemicals worldwide. [6] It is highly toxic; therefore, it must be purified before being released into the environment. It poisons the liver and causes damage to kidneys, gills, intestines, and damage to the mammalian cell. It may lead to cancer when inhaled; causes skin irritation with pain when it comes into contact. [7,8] The direct discharge of MG dye in the hydrosphere causes an imbalance in the environment. This untreated water used for irrigation purposes might result in reducing the quality of crop production. [9] These dyes are banned in many countries but are still in use due to their low cost, accessibility, and potency. [10] Treating these dye wastes before discharging them into water bodies is the most crucial need to be performed. One in eight people on the earth lacks domestic and clean drinking water. Around 1 million population has no access to pure water for drinking purposes. In developing countries, 3.5 million people die annually from inadequate sanitation and hygiene. Around 1.5 million children die because of water-borne diseases. In 2025, around 1.8 billion populations will live in countries or areas where there is not enough water to go around. This water scarcity problem will make a worldwide systematized hazard. [11] Researching inappropriate materials and perfect treatment methods/devices should be urgently implemented to overcome this issue.
This review contains the process in literature from 2011 to now (2022) for the degradation of MG dye by a photocatalytic technique using a variety of metal oxide nanoparticles (NPs).
A bibliometric study is a quantitative study that uses manuscripts as a database. [12] This work used bibliometric methods to examine the current state of MG dye research from 2011 to 2022 including research fields, key researchers, levels, significant institutions, and countries-wise research work. To sum up, the search results on the subject from the most reliable search engine, that is, Scopus, were summarized, and bibliometric studies were employed from the data.
In this current research, title search was done as part of a bibliometric analysis between 2011 and 2022 using VOSviewer with R-studio and CiteSpace software to analyze a Scopus CSV and RIS file for countries, keywords, inter-country coauthor networks, journals, and including co-occurrence visualizations. Professor Chaomei Chen created the CiteSpace software at the start of 2004. The "Co-occurrence" network maps of "keywords," "authors," "nations," "institutions," and subject categories, including co-citation networks of "cited references," "cited authors," and "cited journals," characterize it. [12,13]

Malachite Green Dye Remediation Methods
Both potable and wastewater should be treated for the benefit of society and to address the issue of insufficient drinking water supply. Diverse physical, chemical, and biological methods have been used to eliminate water pollutants (Figure 1). The electrocoagulation process, [14] adsorption, enzyme degradation, [15] membrane filtration, ion exchange, chemical precipitation, chemical oxidation, flocculation, [16] bacterial decomposition, electrochemical decolorization, ozonization [17] nano photocatalysis, ceramic nanofiltration membrane, biofilms, [18] organic resin, electrolysis, reverse osmosis, hybrid materials, oxidation, electrolysis, and biofouling. [19] The qualities of these hazardous dyes reduce the efficacy of conventional decolonization techniques. However, the light-based adsorbent (photocatalysis) approach has proven highly effective at degrading these dyes.

Remediated by Photocatalysis of Malachite Green Dye
The photocatalytic treatment uses heterogeneous photocatalysis, a semiconducting absorbent that breaks down many pollutants when exposed to light including organic pollutants in the atmosphere. The photocatalysts can completely degrade dye in a short amount of time, even at room temperature. More importantly, no toxic residues are left behind, and the organic pollutants are completely broken down into water and carbon dioxide, which are not dangerous byproducts. [20] The efficiency of degradation is also affected by the conditions of the photocatalysis reaction, such as the light source used for irradiation (sunlight is a natural light source), the time in which light irradiation is, the pH value of the dye solution, the amount of catalyst needed for efficient degradation, the distance between the light source and the dye, the temperature during the reaction, and the concentration of the dye solution. [21] At first, activated carbon, carbon nanotubes (CNTs), mesoporous silica, and chitosan beads, which are all porous and smaller than a nanometer, were used as adsorbents. Some drawbacks of these materials were that they were hard to use, did not work as well, were expensive, and needed a lot of energy. As a photocatalyst strongly suggested, different materials were compensated for the abovementioned problems. [22]

Nanomaterials
In material science and chemistry, nanotechnology is an essential technique. Consequently, the results of advanced nanotechnology led to the invention of nanomaterials (NMs) with exclusive and unexplored features. [23] Several NMs, including TiO 2 metal oxide, Ag nanoparticles, carbon nanotubes, and discreet aqueous fullerene (nC 60 ), have demonstrated potential antibacterial activity and water treatment. This study revealed that antimicrobial NMs are biocompatible. [24] In Figure 2, the top ten nanotechnology-based publications and their relative contribution percentages from 1975 to 2020: a) "carbonaceous nanostructure," b) "nanoparticle," and c) "nanocomposite." Carbonaceous nanostructure, nanocomposite, wastewater, nanoparticle, and treatment were selected as keywords in Web of Science (WOS) databases. [25] Various nanomaterials include carbon nanostructures, oxide nanoparticles, metal nanoparticles, and semiconducting nanoparticles (Figure 3a). Nanoparticles have the potential to cause damage to microbial cells through a variety of mechanisms, including oxidizing cell components, inhibiting transmembrane electron transmission, interfering with or entering the cell envelope, and producing secondary products as reactive oxygen species (ROS) or aqueous heavy metal ions. [26] The photocatalysis mechanism involves light energy to accelerate chemical . Top ten manuscript writing nations in nanomaterials and their compared contribution % to one another during 1975-2020: a) "carbonaceous nanostructure," b) "nanoparticle," and c) "nanocomposite." Some keywords were chosen "carbonaceous nanostructures," "nanoparticles," "nanocomposites," "wastewater," and "treatment" (databases of WOS). Reproduced with permission. [25] Copyright 2019, Royal Society of Chemistry.
reactions, producing reactive species that can degrade pollutants or synthesize useful products. [27] In the past, "nano-metals," such as Zn, Ag, Mg, and Cu, were utilized to treat diseases; by manipulating the physicochemical features of these NMs, nontoxic and effective antibacterial medicines for human health can be manufactured. [28] The remediation of aquatic environments using five primary pathways is also significant, as shown in Figure 3a.

Carbonaceous Nanostructures
Carbon is one of the essential elements that may utilize to synthesize various carbon products; it is also one of the most abundant elements in the universe. The adsorption capacity of typical adsorbents like activated carbon is limited; to overcome this problem, scientists are trying to synthesize carbonaceous nanostructures. Additionally, carbon contributes to the development of carbon-based nanostructures, including fullerenes (0D), "carbon nanotubes" (1D), and "graphene" (2D). Carbonaceous nanostructures are a deterministic base in water treatment because of their functional properties. These characteristics include high mechanical strength, a remarkable aspect ratio, strong thermal and electrical properties, impressive "hydrophobicity adsorption," application simplicity, and separation characteristics. [29,30] Fossil fuels based on hydrocarbons, such as ethane and methane, are the primary source of these elements, [31,32] as described below in Figure 3b.

Nanoparticles
One of the essential categories of NMs is NPs, which are synthesized atomic and molecular clusters found in metals and the ox-ides of certain metals. Most of these atoms are quasi; as a result, they have a strong propensity to engage in chemical reactions with other atoms. According to the findings, 40% of the atoms are found within 10 nm above the surface; however, 20% are only found within 20 nm inside. [33] This results in bioremediation, high adsorption capacity for metal ions, degradation, and reduction of pollutants from aqueous solution. Nanoparticles have become popular for various environmental contaminants cleaning purposes [34,35] such as nitrate, [36,37] pesticides, [38] heavy metals, MG dyes, hydrocarbons, [39,40] radioactive elements, and salts. [41] The two major categories of nanoparticles are metal oxides and oxy acids, as shown in Figure 3b

Nanocomposites
Solid structure nanocomposites are classified into two fundamental categories: I) an inorganic solid core surrounded by organic shells or vice versa, and II) a mixture of two or more inorganic/organic phases. [42,43] Combinations of both inorganic and organic phases are characterized as the third form of solid structure nanocomposites. Impregnating metal nanoparticles into porous substrates (such as polymers and C 3 N 4 ) was utilized to synthesize composite adsorbents. [44] In recent years, combining nanoparticles into a single nanocomposite facility beneficial changes in their functionality (synergistic association effects) has gained much importance. [45,46] Generally, nanocomposites are obtained by combining metals, metal oxides, and oxy acids in the following combinations: metal-metal, metal oxides-metal oxides, metal-metal oxide, metal oxides-oxy, acidsoxy acids, and acids. Different classifications of nanocomposites are represented in Figure 3b. . a) Common ways to remediation water, groundwater, and wastewater. b) Sorting out of the various NMs utilized to treat water and 150 wastewater goals. Reproduced with permission. [25] Copyright 2019, Royal Society of Chemistry.

Metal Oxide Nanoparticle as Photocatalyst
Different catalysts have been synthesized to improve photocatalytic activity. Metal oxide nanoparticles are less toxic and easily modified into hydroxides or oxides. Another important property is its band gap between observing the oxidation and reduction process. The band gap of nanomaterial affects their photocatalytic activity, with a smaller band gap corresponding with higher photocatalytic activity. An increase in absorption accompanied redshift, manifesting that photocatalyst is time reliable and costeffective. [47] Photocatalysts are often synthesized from nanoparticles of metal oxide such as TiO 2 , ZnO, Bi 2 O 3 , Fe 2 O 4 , [48] WO 3 , CuO, Cu 2 O, [49] SnO 2 , BiVO 4 , InTaO 4 , CeO 2 , Bi 2 WO 6 , ZnAl 2 O 4 , ZnGaNO, and Zn 1.7 GeN 1.8 O. [50] These metal oxides have many shapes, such as nanoparticles, nanospheres, nanofibers, nanotubes, nanoribbons, and sheets. [51] Table 1 shows how different metal oxide nanoparticles degrade MG dye and the conditions of the experiment.  and increases the number of electron-hole pairs involved in photodegradation. Various functional groups and the high surface-tovolume ratio of metal oxide-based nanocomposites improve the interaction with dye molecules during photocatalytic dye degradations. The generated electrons (e − ) interact with the dissolved oxygen molecule to form superoxide ion radicals (O 2 ▪ − ). The hydroxyl radicals (▪OH+H + ) are formed when positively charged holes (h + ) combine with H 2 O. The dye degradation is usually caused by the active participation of ROS such as hydroxyl rad-ical (OH•), superoxide radical anion (O 2 • − ), and electrons-holes pairs. [27] When the photon energy was larger than or equal to its band gap, [52,53] these electron-hole pairs shifted to the surface from the NMs. They interacted with the MG dye molecules to contribute to redox processes, and the dye molecules were transformed into harmless intermediates, H 2 O, and CO 2 . Under solar irradiation, this synergistic effect causes more electrons to promote from the valence band (VB) to the conduction band (CB). [53,54]  Meanwhile, more electron vacancies are generated in VB, and thus the recombination process starts. As a result, with the help of the NMs photocatalyst, higher state (CB) electrons transfer to MG dye molecules and breakdown the MG dye (pollutant) to harmless intermediates H 2 O and CO 2 molecules (Figure 4). The following equations describe the numerous photocatalytic degradation processes conceivable through the NMs catalyst

Reaction Mechanism and Kinetics
Dye + h + (VB) → oxidation products (8) Dye + e − (CB) → reduction products (9) Porous structure distribution of material with a high surface area can offer substantial electro-active sites and provides fast ion diffusion pathways; both are essential for photocatalytic applications. [55] Additionally, the reactive sites on the surface of magnetic recyclable photocatalysts may considerably boost the effectiveness of decomposing any complicated dye molecules utilizing the photocatalyst. [56]

Recyclability and Stability Study
Recyclability and stability were understood by an example of the dye used under the nanocomposite. [27] Reusability without any loss in catalytic efficiency is one of the most crucial factors for wastewater treatment to reduce the catalyst cost. [56,57] Several  cycles of photocatalytic degradation were used to test the generated CdS-G nanocomposite's stability and reusability ( Figure 5). The nanocomposites were removed from the solution by centrifugation in each cycle, washed with methanol, and then dried at 70°C in a vacuum oven. The dye degradation under sunlight was satisfactorily tested on all nanocomposites for up to four cycles. From Figure 5, it can be seen that the nanocomposite efficacy in preventing dye degradation remained steady up until the third cycle and that it has thereafter steadily decreased. After four consecutive cycles, the catalytic activity has maintained over 80% of its initial value, demonstrating the produced RE-CdS-G nanocatalyst's good stability and reusability up to four www.advancedsciencenews.com www.global-challenges.com cycles within 90 min. Even after four cycles, Ce-CdS-G and La-CdS-G demonstrated considerable efficiency, with 87% and 85%, respectively. With an 80% degradation rate even after five cycles, [57][58][59] reported the Ce-TiO 2 nanocomposites for degrading the dye under UV radiation.

Bibliometric Analysis
Bibliometric analysis studies publications in a particular field that provides current information about the subject. The investigation explains co-citation, bibliographic coupling, research trends, Global Challenges. 2023, 7, 2300001 www.advancedsciencenews.com www.global-challenges.com co-authorship networks, subjects, journals, and the growth of new fields. This review examines the most-cited articles, the most-used keywords, the evolution of publications, and the coauthorship network. VOSviewer, R-studio packages, and Citespce software were used. The statistical examination of published articles shows that connections between papers about a specific research topic or field by examining other published works have cited them on the same subject or in the same field. [142,143] The pertinent information (including publication year, nation, author, journal, keywords, citations, and abstracts) for 658 articles has been downloaded. The CSV raw and RIS files were acquired from Scopus (as of 28 October 2022). A dataset consisting of the CSV raw and RIS files was employed to input the analysis findings into the Bibliometric software. The technical route map is depicted in Figure 6.
Basic information about "Malachite green dye" data with their description and results were collected from Scopus (As of 28 October 2022). Within 12 years (2011Within 12 years ( -2022, data were filtered from 658 documents, 2 review articles, and 594 articles, with an average citation per doc of 20.97, international co-authorship of 22.34%, references 26 843, author keywords (DE) of 658 proceedings as shown in Figure 7. The main information data is collected from the Scopus search database from 2011-2022 ( Figure 7).

Keywords Co-Occurrence
The outcome of the cited journals, most-cited articles, influential institutions, and cluster analysis was sorted and analyzed as part of the search strategy and method. There are several articles about how metal oxides affect the photocatalytic degradation of MG dyes. This is a significant trend in wastewater treatment, materials science, and metal oxide research. The most repeated keywords about MG dye degradation are shown in Figure 8a,b, comprising about 5 total clusters; 435 items, 27 965 total links; 71 155 total link strengths. Cluster: 1 has 155 items, 407 total links, 2516 total link strength, and 128 occurrences; cluster 2 has 117 items, 434 total links, 5434 total link strength, and 402 occurrences; cluster 3 has 97 items, 361 total links, 2219 total link strength, and 90 occurrences, cluster 4 have 62 items, 357 total links, 2086 total link strength and 111 occurrences and cluster 5 have 4 items, 113 total links, 174 total link strength, and 5 occurrences.
The most-important clusters (density of visualizations is centered) are the "red," "green," and "blue clusters." The "red cluster" contains 155 total items; the most important items have occurrences (total links strength) as follows; scanning electron   Figure 9. Citations and publications report on photocatalytic degradation of malachite Green dyes from 2011 to 2022, retrieved from Scopus with the keywords "Malachite green dye" and photocatalysis.   16(110) and textile 11 (134). Finally the last purple color cluster have surface area 16(200), nitrogen 11 (138), electron 5 (113) and zeta potential 5 (92). The importance of keywords for research is because they recognize and show the most important part of the research field. [144] Table 2 shows the top 30 most prominent keywords in publications about this topic. These results will help future authors choose the Keywords that will facilitate research to find the information already published about a certain topic.

Year-Wise Publication and Citations Trend
The number of publications and citations reported on "Malachite green dye" from the Scopus database (Figure 9) indicates a developed interest in research areas, years-wise publications, and citations. This demonstrated an increase in the number of research publications produced each year in this field. Consequently, the total number of citations reached more than 2474 in 2021, which has since decreased to more than 2097 in October 2022.
The results of publications and citations between 2011 and 2022 in the research area were not very famous until 2019. Consequently, the total publications and citations have increased several times than the previous. In recent years, photocatalytic activity has gained excellent economic and environmental viability and widespread acknowledgment from the scientific community. The significant development in investigating potential industrial wastewater replacements has gained worldwide attention. [145] Within 12 years (2011-2022), data were filtered from 658 documents, 2 review articles, and 594 articles, average citation per doc of 20.97, international co-authorship of 22.34%, references of 26 843, author keywords (DE) of 658 proceedings, as shown in Figure 9. The most productive authors in the field of photocatalytic degradation of MG dye with total citations and a total number of citations per year ( Table 3). Figure 10a,b shows both the keyword co-occurrence network and the keyword co-occurrence cluster on maps. With a Q-value of 0.8415, 12 clusters were found and a silhouette value was 0.9276. In Table 4, the 12 largest groups are shown, and the timeline view shows that almost every year, there were new keywords (Figure 10b).

Keyword Co-Occurrence Timeline View
We use a "cluster network" for all of the articles (from 2011 to 2022) in the field of "effect of metal oxides on photocatalytic degradation of MG dyes" (Figure 10). The co-occurrence keyword knowledge of the graph shows which keywords are essential and used a lot. The size of each node is based on how references have been made recently. Co-occurrence keywords can be divided into 12 sub-clusters, including #0 aqueous solution, #1 adsorption removal, #2 using paper aminobenzoic acid, #3 artificial neural, #4 malachite green, #5 coal-associated soil, #6 domestic wastewa- ter, #7 wastewater treatment, #9 using Cinnamomum camphora sawdust, and #11 ion 42. Figure  10b shows different Citespace timeline visualization. [146] 12 clusters, with the number of citations in each and the names of the groups as shown in Figure 8b. When Citespace is observed, the co-references only pick the 45 most-used articles. All the references in Figure 10b were published between 2011 and 2022, which means there were not enough co-references in terms of when they were published. The node's position shows the time it was published, the ring color shows the time it was cited, the ring thickness is based on how often it was cited, and the lines between the nodes show co-references. With the spending of time, the color changes from green to orange and then changes to red.
A timeline view shows research trends have changed in various fields, and the timeline view observed that cluster nodes are connected with horizontal lines. The timeline view makes it clear how many nodes are in each cluster. More nodes mean that the field is more important. The most important articles in certain sub-field show how to research topics that rise to popularity and are the most cited article. The timeline view shows the advancement of cluster 6 (domestic waste efflux) occurs first, indicating the previous studies of the photocatalytic effect of metal oxides. Cluster 0 (aqueous solution) and cluster 7 (wastewater treatment) developed later because they show how the early ideas improved (clusters 1, 2, and 3). [147] Figure 11 shows that the nodes are the countries, the lines are the productive countries that worked together, and 65 nodes and 164 linkages in international partners formed a network. The colors showed the year the articles were published, and the same rules apply to the colors of the lines. Figure 11 indicates that the publications of India, Saudi Arabia, Iran, China, Malaysia, Egypt, South Korea, and Turkey have progressed in the last 12 years. In this area, India, Saudi Arabia, Iran, and China often worked together with other countries. Table 5 shows the 15 most productive countries in terms of publications. First, India has cited 247 publications, making it an essential part of this field. Saudi Arabia and Iran are second and third, with 62 and 58 publications, respectively. The quantity analysis of the publications shows two countries' research work led significantly. Iran, China, Malaysia, Egypt, and other countries at the end of the list have about the same number of articles.

Conclusion
This review article showed a widely used photocatalytic degradation approach to address the industrial wastewater issues, which Figure 11. The joint map of the productive "countries." Each node on the map stands for a different country. The different colors in the nodes showed when the articles were published. The more publications there are, the larger the area of the node. The more nodes there are in clustering, the smaller the number. proceedings. The top 15 most productive countries corresponding with articles were reported in this review. India is responsible for 247 publications and is a significant part of this field, and 62 and 58 records, Saudi Arabia and Iran are second and third, respectively.