Air quality in outdoor and indoor environments remains the primary concern for healthy lifestyle (Dinh et al., 2021). In recent years, the use of fuels indoors, construction of high-end office buildings, and continuous upgradation of interior furniture are leading to an increase in the deterioration of indoor air quality (Sheehan et al., 2017; Grigoryan et al., 2018; Perez-Padilla, 2018; Luo, 2020). Due to poor microclimatic conditions and considerable use of air conditioners, the closed office buildings cause sick building syndrome because of several volatile organic compounds such as ethanol, propanol, acetone, toluene, formaldehyde, etc.(Lv et al., 2016; Sun et al., 2019). These have led to a surge in many health-related issues including but not limited to respiratory diseases, low immunity, allergies, and cancer (Sheehan et al., 2017; Grigoryan et al., 2018). According to World Health Organization (WHO), approximately 3 billion people cook using kerosene, biomass, and coal-fueled stove, which causes indoor air pollution (Simkovich et al., 2019). These result in the premature death of 4 million people every year globally (Devien et al., 2018). Several techniques have been brought in to deal with the situation. Some studies have demonstrated the efficacies of advanced oxidation processes in degrading organic pollutants. Among them, the photocatalytic oxidation technique that emerged in the 1980s has gained momentum recently because of its promising features, which include low-cost and complete mineralization of pollutants. A photocatalyst is a material that accelerates the reaction when activated under the exposure of light without being consumed. The bandgap, defined as the energy difference between the valence band (measure oxidation potential) and conduction band (measure reduction potential), determines if the material is a metal, a semiconductor, or an insulator (Pawar et al., 2018). When light energy (equal to or higher than bandgap) is incident on the surface of a semiconductor, electron/hole pair is formed, unlike in metals and insulators. Therefore, semiconducting metal oxides are suitable materials for photocatalysis.
Both oxidation and reduction reaction occurs at the surface of semiconducting metal oxide (Madima et al., 2020). The leading role is played by the catalyst which decomposes the toxic agents into less toxic products. Fujishima et al. (1972) first demonstrated titanium dioxide as a photo electro-catalyst for water splitting (Liet al., 2020). Since then, primitive titanium dioxide (TiO2) based materials remain the most exhaustively studied photocatalysts mainly because of their low cost, chemical stability and environmental compatibility. However, to obtain more effective photocatalysts, TiO2 has also been doped with either cationic or anionic counterparts by the researchers (Basavarajappa et al., 2020). Researchers have also explored ZnO and other simple and complex metal oxides as photocatalysts (Lee et al., 2016).
Although initially TiO2 photocatalysts were applied for water treatment; in recent years, it has been shown that the photocatalytic detoxification of organic compounds is generally more efficient in the gas phase compared to the liquid phase(Carpet al., 2004). Thus, attention to applying this technology for air treatments has increased. The organic compounds degraded include alkanes (methane, ethane, hexane), alkenes (ethylene, propylene, butylene), alkynes (acetylene, methylacetylene), aromatics (benzene, toluene, xylene), alcohols (methanol, ethanol), aldehydes (formaldehyde, acetaldehyde), or ketones (acetone, methyl-ethyl-ketone) and sulphur containing compounds (hydrogen sulphide, SOx, diethyl / dimethyl sulphide) and nitrogen-containing compounds (NOx, diethyl amine, propylamine or pyridine) (Portela and Hernández-alonso, 2013).
Significant volumes of literature are mostly devoted to the laboratory-based photocatalytic performances of materials. The basic principles and underlying science of photocatalysts can easily be found in peer-reviewed journals like Nature, Energy and Buildings, Environmental Research, Environmental Pollution and many more (Paz, 2010; Carpet al., 2004). These articles are produced by researchers who have contributed towards studying various photocatalysts and their application for pollution treatment. Predicting the current research trend and gaining knowledge on the utility and prospects of this interdisciplinary research field seems complicated until the enormous number of scattered literatures are collected and analysed systematically. Despite the large number of papers on photocatalysts, only few (Zhanget al., 2010; Quiet al., 2020; Yue et al., 2020) have made attempts to compile and analyse systematically the world scientific publications towards understanding the current research trend and future directions on photocatalysts through the manual review process which scans through only limited literature. Limited study exists on bibliometric analysis which aggregates the large volume of existing information to be analysed in one place for better understanding. The scientific community widely uses this tool for statistical analysis of scientific research publications(Long et al., 2020). Bibliometric tools can include various types of data like academic research publications, book and proceedings etc. for analysis (Moral-muñoz et al., 2020). Some of these tools are Bibexcel, CitNetExplorer, SciMAT,Sci2Tool, VOSviewer, etc. (Sangam and Mogali, 2014). The data is extracted from the several databases with reference to the relevant keywords. These tools are then used to carry out the bibliometric analysis. Databases such as Web of Science, Scopus, PubMed etc. and various reference managers such as EndNote, Mendeley, etc. can be used. The data is extracted in the formats like txt, .csv, ris etc. which has to be supported by the respective tool.
In the present work, bibliometric analysis has been carried out to understand and assess the current trend in the research activities related to photocatalysts for air treatment. The data from the Scopus database for the 2000–2020 year on the topichas been used for the purpose. The research trends, problem statements, adopted strategies, and materials explored on the topic over recent years throughout the globe could be insightful to the researchers and eventually will be beneficial to formulate new research strategies.