Photocatalytic Nanomaterials for Environmental Purification

The accelerating pace of industrialization and continuous population growth have made environmental pollution, particularly of water and air, a severe challenge to global ecosystems and human health. Conventional water treatment and air purification methods often suffer from limitations such as limited efficiency, high energy consumption, or the generation of secondary pollution. Consequently, the development of efficient, sustainable, and environmentally friendly purification technologies is urgently needed [1,2,3].

Among various strategies developed to address environmental pollution, photocatalytic technology has emerged as a robust and promising solution for water and air purification. This technology utilizes light energy to drive redox reactions that degrade harmful pollutants into harmless byproducts, thereby offering a green, efficient, and sustainable pathway for environmental remediation [4,5,6]. In the field of environmental purification applications utilizing solar energy, nanostructured photocatalysts have attracted increasing research attention. Various photocatalytic nanomaterials demonstrate superior photocatalytic activity compared to their bulk counterparts, owing to their enhanced interfacial charge separation efficiency and larger surface area (which provides more active sites) [7,8].

This Special Issue, Photocatalytic Nanomaterials for Environmental Purification, brings together a collection of original research articles that highlight recent advances in the synthesis, characterization, and application of nanoscale photocatalysts for environmental purification. The contributions reflect the diversity and depth of ongoing efforts to address real-world pollution challenges through innovative material design and process optimization.

In Contribution 1, the study evaluates UVC photolysis with type C ultraviolet radiation (UVC) and UVC/TiO₂ photocatalysis of a mixture of four pharmaceuticals-atenolol (ATL), acetaminophen (ACM), clofibric acid (CLA), and antipyrine (ANT)-commonly found in treated urban wastewater. A comprehensive kinetic model was developed to describe their degradation, taking into account the generation of reactive oxygen species (ROS): hydroxyl, superoxide ion radicals, and singlet oxygen, along with their reactions with both the pharmaceuticals and dissolved organic matter. The model reproduces the inhibitory effect of natural organic matter in secondary effluent and, in most cases, treated effluent, accurately predicts the concentration profiles of the pharmaceuticals. This validated model provides a useful tool for understanding the degradation mechanisms of pharmaceutical mixtures and for supporting the design of effective water strategies based on photochemical processes.

Zhang Mei and Wang Xu (Contribution 2) present a green and eco-friendly egg white (EW)/TiO₂ hydrogel synthesized using waste eggs nearing expiration, designed as a multifunctional bio-adsorbent for wastewater treatment. The material exhibits a high adsorption capacity of 333.172 mg·g⁻¹ for methylene blue (MB) following the Langmuir model. Under visible light irradiation, the 5 wt% TiO₂-doped hydrogel achieves 99% photocatalytic degradation of MB within 120 min. Additionally, it demonstrates significant antibacterial activity, achieving a 90.4% inactivation efficiency against Escherichia coli through synergistic effects of natural lysozymes in EW and reactive oxygen species generated by TiO₂. The hydrogel also shows low cytotoxicity, highlighting its potential as a sustainable, cost-effective, and reusable material for simultaneous organic dye removal and microbial disinfection in industrial wastewater.

In Contribution 3, Zhu et al. conducted a first-principles study to address the critical limitation of monolayer MoS₂—rapid electron–hole recombination—by designing four van der Waals heterojunctions: MoS₂/WSe₂, MoS₂/MoSe₂, MoS₂/AlN, and MoS₂/ZnO. All the heterojunction systems exhibit stable Type-II band alignments, which generate internal electric fields that effectively suppress charge recombination and reduce the bandgap by 0.2–0.5 eV compared to pristine MoS₂. Notably, chalcogenide-based heterojunctions (MoS₂/WSe₂ and MoS₂/MoSe₂) significantly enhance visible light absorption, with MoS₂/MoSe₂ demonstrating the highest photoresponsivity (absorption coefficient > 10⁵ cm⁻¹). In contrast, non-chalcogenide systems (MoS₂/AlN and MoS₂/ZnO) exhibit comparatively weaker optical performance. These findings establish MoS₂/MoSe₂ as the optimal photocatalyst for optoelectronic applications, demonstrating that strategic pairing of chalcogenide elements can maximize charge separation efficiency and solar energy utilization.

Bobirică et al. (Contribution 4) focused on developing a novel photocatalytic system for the removal of estrogenic pollutants (estradiol valerate/norgestrel) from wastewater. A plug-flow reactor operating under UV-A light was employed, with TiO₂, ZnO, and TiO₂/ZnO composite photocatalysts immobilized on glass beads. These beads were strung on stainless-steel wires and arranged modularly within the reactor. The photocatalysts were synthesized via sol–gel, coated onto the glass supports thermally, and calcined at 500 °C for 2 h. Results showed that degradation efficiency strongly depends on catalyst loading, which can be conveniently tuned by adding or removing photocatalytic modules. The TiO₂/ZnO composite exhibited optimal performance, achieving near-complete mineralization within 120 min using only two modules.

In Contribution 5, Mehta et al. synthesized pure CdO nanoparticles, magnetic Fe₃O₄ nanoparticles, and Fe₃O₄-CdO nanocomposites via a solution combustion method using cetyltrimethylammonium bromide (CTAB) as a template. Under optimized conditions, the photocatalytic performance of the Fe₃O₄-CdO nanocomposites was evaluated for the degradation of methylene blue dye. Under visible light irradiation, the Fe₃O₄-CdO nanostructures exhibited efficient photocatalytic activity, achieving 92% degradation of methylene blue, and demonstrated excellent stability over multiple reuse cycles

In Contribution 6, Dong et al. fabricated Pt-M/WO₃ (M = Cu, Co, Ni) thin films through a process involving homogeneous precursor sol preparation, spin-coating, toluene etching, and calcination. Their microstructural, chemical, and electrochemical properties were systematically compared with those of pure WO₃. The Pt-M/WO₃ films exhibited significantly enhanced photocatalytic degradation of methylene blue (MB) under visible light. Based on experimental evidence, the authors proposed detailed charge transfer pathways and an improved photocatalytic mechanism. The large work function difference between Pt-M alloys and WO₃ induces interfacial band bending, while transition metal doping further adjusts the electronic structure of Pt-M/WO₃, effectively promoting charge carrier separation.

In Contribution 7, the study presented a facile one-pot refluxing strategy to synthesize a 2D/2D CdIn₂S₄/In₂S₃ (CISI) heterojunction photocatalyst for efficient visible light-driven Cr(VI) reduction. The intimate interfacial contact between CdIn₂S₄ and In₂S₃ nanosheets significantly enhanced charge carrier separation and transfer, effectively suppressing recombination. The optimized 0.5 CISI composite demonstrated superior Cr(VI) photoreduction performance, outperforming its individual components. Mechanistic analysis confirmed the heterojunction’s role in accelerating electron–hole separation and promoting interfacial redox reactions. This work illustrated a scalable approach to designing high-efficiency photocatalysts for toxic heavy metal remediation, highlighting the critical impact of 2D/2D heterostructure engineering on environmental photocatalysis.

In Contribution 8, this study reported the microwave solvothermal synthesis of two-dimensional CeO₂/Bi₂O₃ layered composites for visible light-driven photocatalytic oxidative desulfurization (PODS). Optimized at a 1:2 Ce/Bi molar ratio, the composite achieved the highest desulfurization efficiency for dibenzothiophene (DBT) in fuel. Active species trapping experiments identified hydroxyl radicals (•OH) as the dominant reactive species driving DBT oxidation. A mechanistic pathway was proposed in which the CeO₂/Bi₂O₃ interface facilitated electron transfer under visible light, enabling efficient conversion of DBT to sulfones for deep desulfurization. This work provided a sustainable, hydrogen-free strategy for fuel purification with potential industrial applications.

In Contribution 9, Zhang et al. fabricated isostructural Sillén–Aurivillius oxyhalides, Bi₇Fe₂Ti₂O₁₇X (X = Cl, Br, I; denoted as BFTOX), for the first time for CO₂ photoreduction and degradation of organic pollutants. Density functional theory (DFT) calculations revealed that the valence band maximum (VBM) of BFTOC and BFTOB was dominated by the delocalized 2p orbitals of oxygen atoms, contributing to their narrow bandgap (E_g) and potentially enhancing stability against self-decomposition. The photocatalytic activities of BFTOX materials were significantly influenced by the halogen type (Cl, Br, I), with BFTOCl exhibiting the highest performance (3.74 μmol·g⁻¹·h⁻¹). This enhancement was attributed to efficient charge carrier separation, a small optical bandgap, and broadened light absorption. This study provided a useful reference for developing efficient and stable photocatalysts based on Sillén–Aurivillius layered oxyhalide materials.

In Contribution 10, Zhou et al. successfully constructed BiPO₄/Ov-BiOBr heterojunction composites by a two-step solvothermal method, in which bismuth phosphate (BiPO₄) with various molar ratios was in situ grown on the surface of oxygen-vacancy-rich bismuth oxybromide (Ov-BiOBr). By establishing a novel type-I high–low junction structure between semiconductor BiPO₄ and Ov-BiOBr, the system effectively retained more strongly oxidizing holes or reducing electrons, thereby enhancing the redox performance of the photocatalyst. Among these, the composite catalyst containing 10 mol% BiPO₄ exhibited the highest degradation efficiency for tetracycline (TC) while also demonstrating optimal photocatalytic activity for hydrogen peroxide (H₂O₂) production. The enhanced performance is attributed to suitable band alignment between BiPO₄ and Ov-BiOBr, an efficient electron transfer channel formed via bismuth bridges, and effective charge separation.

The studies presented in this Special Issue reflect the vibrant progress in photocatalytic nanomaterials for environmental purification. Through innovative material design, interface engineering, and process optimization, researchers are continuously enhancing the efficiency, stability, and applicability of photocatalysts. Future efforts should focus on scaling up synthesis methods, improving solar energy utilization, and advancing mechanistic understanding to facilitate the real-world deployment of photocatalytic technologies. Interdisciplinary collaboration will be key to addressing complex environmental challenges and achieving sustainable development goals.