Introduction
The 20th-century industrial revolution has transformed the lifestyle but deteriorated the environment by polluting the air and aquifers. Industrial wastewaters containing harmful and non-biodegradable organic pollutants such as dyes and pigments (Berradi et al., 2019; Gürses et al., 2021; Orozco et al., 2022), drugs and pharmaceuticals (Akkari et al., 2018; Al-Areqi et al., 2022), etc. Contaminate surface and groundwater sources leading to health risks. Also, the prevalence of organic compounds in the air contaminates surfaces such as textiles and fabrics, glass windows, doors, and other installations, which is often hazardous due to their perilous effects on human health and wellbeing.
To reduce the impact of organic pollutants on the environment and health, photocatalysis-light-induced chemical reactions to oxidize and decompose organic molecules-is developed (Ameta et al., 2018; Zhang et al., 2018). The photoactive nanomaterials can be used as heterogeneous catalysts as well as self-cleaning coatings to remove hazardous organic molecules from the wastewaters (Purcar et al., 2021; Shahid et al., 2021, 2022) or contaminated surfaces (Afzal et al., 2021; Tănase et al., 2021). A variety of nanostructured materials have been developed for the photocatalytic degradation of organic pollutants such as those based on carbon nanostructures (Khan, 2021), titania (Chen et al., 2020; Varma et al., 2020), or zinc oxide (Qi et al., 2017; Majumder et al., 2020).
ZnO is classified as active photocatalysts due to wide absorption range and high photostability (Kołodziejczak-Radzimska and Jesionowski, 2014; Laurenti et al., 2017). Besides, ZnO is harmless and inexpensive, and ZnO nanostructures can be synthesized via different methods (Khan et al., 2019a, 2019b; Raha and Ahmaruzzaman, 2022). Qi et al. (Qi et al., 2017) reviewed the photocatalytic applications of ZnO and suggested that the photocatalytic performance of ZnO nanomaterials could be improved by: 1) doping with metals or non-metals, 2) constructing heterojunctions, 3) coupling with carbon nanostructures, or 4) deposition of noble metals. For instance, Ag doping is shown to improve the photocatalytic degradation of both cationic and anionic dyes under simulated solar irradiation by Ag@ZnO nanocomposites (Sharwani et al., 2022).
However, these methods generally involve the addition of other active components in ZnO to enhance its photoactivity. Herein, a simple and effective method is used to improve the photocatalytic properties of ZnO nanoparticles that employs a mineral acid treatment to protonate ZnO surfaces and improve dye adsorption and degradation. Previously, acid treatment of TiO2-based photocatalyst and its influence on the degradation of organic dyes have been studied (Park and Shin, 2014). For instance, Dhandole et al. (Dhandole et al., 2017) suggested that acid treatment of Co2O3-TiO2 nanorods exhibited a higher efficiency for the photocatalytic degradation of orange-II dye. However, contrary to these findings, Onoda et al. (Onoda, 2019) recently reported that phosphoric acid treatment was detrimental to the photocatalytic activity of ZnO.
This work is aimed at studying the effects of a mineral acid treatment of ZnO on its photocatalytic properties. For this purpose, ZnO nanoparticles are prepared and treated with 1.0 M HCl to form ZnO/Zn(OH)2 hybrid nanoparticles and coatings, which are subsequently used for the degradation of organic pollutants in deionized water. As a model organic compound, sunset yellow dye is chosen because of its carcinogenicity and non-biodegradability (Sharma et al., 2020). ZnO/Zn(OH)2 nanoparticles and coatings are characterized and tested for their photocatalytic activity. Hexagonal wurtzite ZnO/Zn(OH)2 coatings exhibit excellent photocatalytic properties by oxidizing and removing sunset yellow. The coatings are stable over several cycles of photocatalytic measurements in aqueous solutions.
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