Recent Advances in Photocatalysis for Environmental Applications

The most important resource for the survival of all living beings is water. Access to clean water is essential for a healthy life. Even though water covers more than two-thirds of the Earth’s surface, only about 3% is usable, and most of it is frozen in polar ice caps and glaciers [1]. In light of this, there is a lack of available water that can meet our daily consumption needs, and, as a result, clean water is becoming increasingly scarce. One of the most significant environmental issues of the twenty-first century is the lack of clean water. Furthermore, numerous chemicals have recently been detected in various water sources, primarily due to population growth, globalization, and extensive industrialization [2,3], which exacerbates the problem.

Among the challenges associated with chronic pollution, the most common are the contamination of drinking water, the discharge of dyes and pharmaceutical waste, and the leaching of heavy metals into waterbodies. According to the UN’s World Water Development Agenda 2030, changes in the water cycle will also have detrimental effects on energy production, agricultural output, human and animal health, and economic advancement, jeopardizing the achievement of the Sustainable Development Goals [4]. Contaminants remain in landfills and the environment for a long time because of their poor biodegradability. Aquatic ecosystems may eventually be impacted by the eutrophication of waterbodies, which is a major environmental consequence that could result from the buildup of these pollutants. When contaminants enter waterbodies, they potentially cause significant toxicity and oxygen demand, which can degrade water quality and damage aquatic life. Remediation is necessary to lessen the detrimental long-term effects these toxins have on environmental and human health [5]. Modern, environmentally safe, affordable, and effective wastewater treatment techniques are, therefore, essential.

Numerous methods have been developed to treat the pollutants found in wastewater. These processes include, but are not limited to, membrane processes, flocculation, coagulation, biological treatment, absorption/adsorption, sedimentation, and physical and chemical treatments. Unfortunately, organic contaminants and heavy metals cannot be eliminated from wastewater using conventional treatment methods; the procedures require extensive systems, infrastructure, and engineering expertise, in addition to being chemically and operationally demanding, which makes them complicated, inefficient, costly, and slow [6,7].

Substances such as colors, antibiotics, analgesics, herbicides, insecticides, and stimulants—referred to as emerging contaminants (ECs)—are important sources of water pollution. Numerous biological, physicochemical, filtration, absorption, and oxidation treatments have been created in order to address these problems; however, they have several major disadvantages, including high energy consumption, low treatment efficiency, a tendency to pollute, and the ability to only successfully eliminate a certain pollutant. The sophisticated process of photocatalytic degradation is receiving increased interest in scholarship due to its unique and promising mechanisms that allow for a range of ECs to be completely broken down into simpler chemicals [8,9].

Photocatalysis is expected to play a crucial part in this process by converting ECs into non-toxic compounds without producing waste. This differs from other separation-based water treatment methods that produce waste that requires cleanup or disposal. To prevent metal leaching and the need for essential raw materials, the catalysts must be stable [10,11,12].

This Special Issue, ‘Recent Advances in Photocatalysis for Environmental Applica-tions’, gathers original research papers and reviews relating to the synthesis, characterization, and usage of novel or known photocatalytic nanomaterials, as well as self-cleaning materials, photocatalytic hydrogen generation, and CO₂ reduction.

In Contribution 1, Li Chen and coworkers investigated different means of enhancing charge separation efficiency, accelerating reaction kinetics, and lowering photocatalytic water splitting (PWS) energy barriers. To address these issues, the authors constructed donor-acceptor covalent triazine-based organic frameworks (CTFs), such as CTF-BP, CTF-DBT, and CTF-DBTS, using biphenyl (BP), benzothiophene (DBT), and benzothiophene sulfone (DBTS) as basic units. Their findings highlight the importance that sulfone groups have for DBTS’s ability to adjust electrical characteristics, improve charge separation, and lower PWS reaction barriers.

The authors in Contribution 2 proposed a nanocomposite with different concentrations of g-C₃N₄ into ZnO for methylene blue (MB) degradation. After extensive characterization, the results indicate excellent photocatalytic performance, good stability, and the reusability of the nanocomposite catalysts.

Contribution 3 reports the synthesis, characterization, and performance of a visible-active ternary nanocomposite, Cu₄O₃/ZrO₂/TiO₂, prepared hydrothermally alongside its binary (Cu₄O₃/ZrO₂) and rutile TiO₂ counterparts for the photocatalytic removal of the textile dye Everzol Yellow 3RS (reported for the first time). This study illustrates the overall potential of multioxide heterojunctions to treat industrial wastewater and provides a workable and affordable method for treating sunlight-driven dye-enriched textile effluents.

Another composite catalyst, nitrogen-doped TiO₂/TiN bilayer, was examined in Contribution 4. The obtained results show enhanced visible-light photocatalytic activity through controlled nitrogen diffusion and gold functionalization. Nitrogen doping significantly improved photocatalytic activity, as measured by MB degradation. By adding a 5 nm gold layer to the sample, the authors were able to study the synergistic effects of nitrogen doping and gold inclusion, which led to increased photocatalytic activity.

Contribution 5 studied the correlation between photoluminescence and photocatalysis using a down-conversion luminescent material as a catalyst for the degradation of Rhodamine B. After just four hours, the photocatalytic degradation of particular dyes led to a notable decrease in dye concentration. It is evident from the data that the samples with higher luminescence intensities exhibited superior photocatalytic activity, meaning that novel multifunctional materials with encouraging properties have applications in a wide range of fields.

In addition to the abovementioned original papers, this Special Issue publishes two review papers. Contribution 6 reviews recent advances in metal-sulfide-based photocatalysts, which represent a promising and sustainable strategy for addressing dye-induced water pollution. The significant advancements in green synthesis techniques, especially those that use plant extracts, offer the dual benefits of environmental sustainability and improved photocatalytic activity.

A significant concern that has received little attention in the literature is related to the transformation products (TPs) produced by the degradation of emerging contaminants. Their potential risks to health and the environment are examined in Contribution 7. Monitoring the TPs produced in breakdown reactions is challenging due to limited analytical standards, compound variability, and low concentrations. Based on the results presented in the study, it can be deduced that TPs can impact photocatalytic performance, result in questionable findings, be more harmful than the original pollutant, and provide valuable insights into the mechanisms of degradation.

Considering the papers published in this Special Issue, future studies on the use of catalytic processes to control persistent organic pollutants are anticipated. However, the conditions under which this research is conducted should be as close to real-world conditions as possible. There are certain properties that a catalyst must fulfill—such as being very active and stable, as well as preventing metal leakage from the catalyst—which can pose considerable environmental risks. Catalysts must also be highly selective, preventing the production of byproducts that are potentially more harmful than the original contaminants, and they must also operate at ambient temperature and atmospheric pressure in order to save energy.

I would like to offer my deepest appreciation to all of the contributors for their invaluable efforts; without them, this Special Issue would not be possible. I hope that the review articles and original research papers included in this Special Issue will contribute to resolving the many difficulties currently faced in the field of photocatalysis. Also, I would like to extend my utmost gratitude to the MDPI Editorial team and the Editors of the Catalysts journal for the opportunity to serve as Guest Editor. Last but not least, my special thanks go to the Assistant Editor, Ms. Rita Lin, for working diligently with me to publish this Special Issue.