Photocatalytic Materials and Photocatalytic Reactions

1. Introduction

This Special Issue, titled “Photocatalytic Materials and Photocatalytic Reactions”, focuses on designing advanced photocatalysts, understanding their structure-dependent properties, and seeking to exploit them in the fields of energy conversion, pollutant degradation, artificial photosynthesis, organic synthesis, etc. As early as 1912, in the age of coal, Giacomo Ciamician [1] proposed the theory that photochemistry could be a potential and promising strategy to realize the harmonious development of human society and nature. However, the pioneering work on photosynthesis was a study on photo-electrochemically water splitting, published in 1972 by Fujishima and Honda [2]. Throughout the next few years, photocatalysis was mainly studied in regard to the degradation of toxic compounds. In recent decades, photocatalysis has attracted extensive and ongoing attention, because it exhibits great potential for applications in artificial photosynthesis, including H₂ production and CO₂ reduction, organic synthesis, pollutant degradation, N₂ fixation, precious metal recovery, H₂O₂ photosynthesis, life science and medical research, space exploration, and other related fields (Figure 1) [3,4,5,6,7,8,9]. Meanwhile, photocatalysts have evolved from inorganic substances to new nanomaterials such as graphitic carbon nitride (g-C₃N₄), polymers, piezoelectric materials, ferroelectric materials, metal–organic frameworks (MOFs), covalent organic frameworks (COFs), single-atom catalysts (SACs), high-entropy alloys (HEAs), supramolecules, superlattices, topological insulators, localized surface plasmon resonance (LSPR), and diverse composite materials/heterojunctions, among other things (Figure 2) [6,7,8,9,10,11,12,13,14,15,16].

Our search results indicated a recent boom in photocatalysis research (Figure 3). About 243,000 studies on this topic have been published in the last 25 years. A total of 146 research areas and 212 countries/regions are involved. Photocatalysis is notable because it is driven by inexhaustible solar energy under mild reaction conditions. The design of advanced photocatalytic materials with good performance and the exploration of green photocatalytic reactions with carbon neutrality are significant for sustainability.

This Special Issue contains 23 original research articles related to photocatalytic materials, including metal oxides, metal sulfides, metal nitrides, metallo-organic compounds, g-C₃N₄, clusters, LSPR, and heterojunction/composite materials. Their applications included H₂ production, CO₂ reduction, organic synthesis, environmental remediation, disinfection, toxicity, and dual-function photoredox reactions.

2. An Overview of the Published Articles

In the Special Issue’s first contribution, Li et al. synthesized a new magnetic nanocomposite, Ag₂O/Fe₃O₄, to achieve the photocatalytic degradation of methyl orange (MO) under visible light irradiation. They observed that 99.5% of MO could be degraded by the Ag₂O/Fe₃O₄ (10%) photocatalyst within 15 min. In addition, the designed Ag₂O/Fe₃O₄ photocatalyst also exhibited broad applicability and stability. Their work demonstrated successful photocatalysis–Fenton coupling and overcame the challenges involved in a difficult catalyst recovery process and regarding low photocatalytic efficiency.

Next, in the study by Wang et al. (Contribution 2), a multicomponent composite MoP/a-TiO₂/Co-ZnIn₂S₄ was prepared through multiple hydrothermal processes. The effects of Co dopants, i.e., amorphous TiO₂ and MoP, increased visible light absorption, improving the separation of photoexcited charge carriers via heterojunction and hydrogen production sites, respectively. Thus, the high efficiency of photocatalytic H₂ production was realized on MoP/a-TiO₂/Co-ZnIn₂S₄. Compared to the pristine ZnIn₂S_4, the H₂ production of MoP/a-TiO₂/Co-ZnIn₂S₄ was enhanced by about three times.

Furthermore, in the research work by Meng et al. (Contribution 3), nanostructured polymeric carbon nitride (g-C₃N₄, PCN) was synthesized using a one-step thermal polymerization process with the assistance of hot water vapor. Vapor has a dual function used to prepare nanostructured PCN: besides being a green etching reagent, it can act as a gas bubble template. Moreover, reaction times, temperatures, mechanisms were also studied in the precursors of PCN. The H₂ production of nanostructured PCN increased by about four times in contrast to bulk PCN. This study offers a new and versatile strategy for fabricating nanostructured g-C₃N₄ with high photocatalytic performance.

Wang et al. (Contribution 4) outlined a visible light-induced regioselective cascade and the sulfonylation–cyclization of a cascade of 1,5-dienes under mild conditions. An array of 3-sulfonylated pyrrolin-2-one derivatives was constructed through lower catalyst loading and achieved good to excellent yields at room temperature. Importantly, this protocol can be used in large-scale synthesis.

Meanwhile, in the research paper by Meng et al. (Contribution 5), a series of Zn_mIn₂S_3+m photocatalysts (m = 1, 2, 3, 4 and 5) was prepared and applied in dual-function photoredox reactions: the selective oxidation of alcohols and the reduction of CO₂ in one reaction system. In Zn₅In₂S₈, Zn₄In₂S₇, Zn₃In₂S₆, Zn₂In₂S₅, and ZnIn₂S₄, structures and properties was studied using experiments and theoretical calculation. The morphology, light absorption, and band structures were tuned by changing the Zn/In molar ratio. Moreover, the selectivity of gas products (H₂ and CO) and liquid products (hydrobenzoin and benzaldehyde) could also be regulated.

In the research work by Chen et al. (Contribution 6), Ag/PW12/TiO₂ composed of TiO₂, polyoxometalates (POMs) [H₃PW₁₂O₄₀] (PW12), and Ag nanoparticles was fabricated through successive electrospinning and photoreduction processes. Ag/PW12/TiO₂ exhibited high degradation efficiencies for methyl orange (MO, 99.29%), enrofloxacin (ENR, 93.65%), and tetracycline (TC, 78.19%). Moreover, for TC degradation and TC concentration, the Ag/PW12/TiO₂ dosage, the pH of the TC solution, and the intermediates and toxicities of the products were also investigated in detail.

Furthermore, in the study by Zhou et al. (Contribution 7), the Bi₂S₃-ZnO/CA film was fabricated by assembling Bi₂S₃, ZnO, and cellulose acetate (CA). In the Bi₂S₃-ZnO/CA film, the addition of Bi₂S₃ improved cavity density and uniformity. On the other hand, the addition of ZnO enabled the formation of heterojunctions with Bi₂S₃ to promote the separation and migration of photogenerated electron–hole pairs, thus improving the photocatalytic activity and stability of Bi₂S₃. This is evidenced by the fact that the RhB (rhodamine B) degradation efficiencies of ZnO/CA, 4Bi2S3/CA, and 4Bi₂S₃-ZnO/CA were about 26.51%, 50.26%, and 90.2%, respectively.

Shahid et al. (Contribution 8) prepared a 2D I-FeWO₄/GO composite photocatalyst by combining halogen-doped FeWO₄ (I-FeWO₄) and graphene oxide (GO). In the designed I-FeWO₄/GO composite, GO acted as a supporter, halogen facilitated H₂O₂ production, and the interface of the I-FeWO₄/GO heterostructure promoted charge separation and migration. Thus, the I-FeWO₄ displayed good photocatalytic performance for the degradation of methylene blue (MB). Under sunlight irradiation for 120 min, 97.0% of MB could be degraded by the I-FeWO₄.

Furthermore, Ai et al. (Contribution 9) fabricated 2D heterojunction g-C₃N₄@CdS using a water bath method. Interestingly, tiny CdS nanorods were grown in situ in the gaps between the 2D g-C₃N₄ nanosheets. Due to the band-band transfer mechanism of the g-C₃N₄@CdS photocatalyst, the charge carriers could be separated efficiently, and thus excellent H₂ production with the assistance of visible light and lactic acid was observed. The H2 production rate of G-CdS-3 could reach up to 1611.4 μmol·g⁻¹·h⁻¹, which was about 10 times that of CdS and 76 times that of g-C₃N₄.

In the research work by Wang et al. (Contribution 10), a 2D–3D hybrid junction In₂S₃/CdS/N-rGO was synthesized using a one-step pyrolysis method. The rational 2D–3D In₂S₃/CdS/N-rGO hybrid junctions not only provided more active sites but also formed multiple tight interfaces, which facilitated charge separation and migration. Thus, a high H₂ evolution rate (10.9 mmol·g⁻¹·h⁻¹) could be obtained with the assistance of visible light and Na₂S/Na₂SO₃ aqueous solution.

To remove high levels of toxic 4-nitrophenol (4-NP), Ma et al. (Contribution 11) designed the alkaline earth metal ion-doped photocatalyst Ca²⁺-doped AgInS₂. The charge recombination and inactive production of superoxide radicals over AgInS₂ could be improved by doping Ca²⁺. Thus, 63.2% of 4-NP was degraded under visible light for 120 min. In addition, capturing tests demonstrated that photoexcited holes and hydroxyl radicals were the main active species.

In the research work by Lu et al. (Contribution 12), cobalt phosphate (Co-Pi) was developed to modify ZnIn₂S₄ (ZnIn₂S₄/Co-Pi), aiming to suppress its charge recombination. Due to the transfer of photoexcited holes, ZnIn₂S₄/Co-Pi exhibited sustainable H₂ production with assistance of visible light and triethanolamine (TEOA). Specifically, 3593 μmol·g⁻¹·h⁻¹ of H₂ production was reached using ZnIn₂S₄/5%Co-Pi.

To remove hexavalent chromium, Guo et al. (Contribution 13) prepared a 2D g-C₃N₄/MoS₂ nanocomposite using an ultrasonic method. Due to the Z-scheme transfer mechanism, photoexcited charges could not only be separated efficiently at the g-C₃N₄/MoS₂ heterojunction but they also retained strong redox abilities. Thus, hexavalent chromium could be removed with high photocatalytic efficiency whenever it was exposed to UV light, visible light, or sunlight.

Furthermore, Garcia et al. (Contribution 14) fabricated a PdIn/TiO₂ hybrid photoelectrocatalyst for wastewater treatment with simultaneously clean energy production. In the reaction system, on the one hand, paracetamol was degraded by oxidation at the photoanode; on the other hand, hydrogen was produced through reduction at the photocathode. Thus, a dual-function redox reaction system was established.

To use sunlight for CO₂ reduction, Wang et al. (Contribution 15) designed TiO₂/CuPc heterojunctions by combining TiO₂ microspheres and copper phthalocyanines (CuPc). Benefiting from the heterojunction effect and the additional light-absorbing properties of TiO₂ and CuPc, a good reduction rate in 32.4 μmol·g⁻¹·h⁻¹ of CO₂ was achieved with TiO₂/CuPc at about 3.7 times that of the pristine TiO₂.

Unlike the experiments above, Gao et al. (Contribution 16) utilized the extended broken symmetry (EBS) method to investigate the low-lying spin states of the [Fe₃S₄] cluster, which is important for photosynthetic H₂O splitting. The results indicated that the EBS results matched well with the experimental data. The weaknesses of the BS method could be compensated for through the developed EBS method.

Meanwhile, Tang et al. (Contribution 17) investigated the Sc₂CX₂/Sc₂CY₂ (X, Y = F, Cl, Br) Janus heterojunction for photocatalysis and photovoltaics using first-principles calculations. The calculated results indicate that these Janus heterojunctions possess type-II band structures and direct Z-scheme transfer mechanisms, which thus facilitates their application in photocatalysis and photovoltaics.

In their research paper, P. Ávila-Torres et al. (Contribution 18) prepared metal-yielded coordination compounds Cu-g-C₃N₄, Ni-g-C₃N₄, and Mn-g-C₃N₄. The structural properties of disinfected E. coli bacteria were investigated. The results indicate that the textural property is a key characteristic.

Liu et al. (Contribution 19) studied hydroxyl groups on the g-C₃N₄ (HCN) for O₂ activation and pollutant degradation. The results show that hydroxyl groups can increase hydrophilicity and surface area, decrease interlayer distances, and promote the charge separation and transportation of the pristine g-C₃N₄, thus improving rhodamine B degradation.

Furthermore, A. Pavlatou et al. (Contribution 20) developed a new photocatalyst, namely magnesium oxide (MgO). MgO has a wide band gap and can produce hydroxyl radicals. It showed selectivity for rhodamine 6G and rhodamine B degradation under UV light. A total of 100% of rhodamine B could be degraded over MgO under UV light for 180 min.

In the research paper by Narkiewicz et al. (Contribution 21), triethylamine (TEA), diethylamine (DEA), and ethylenediamine (EDA) were used to modify TiO₂. The effect of amines and temperature on the photocatalytic reduction of CO₂ was investigated in detail. The results demonstrated that TEA-TiO₂ treated in the microwave reactor exhibited the highest activity.

Furthermore, in the research by Wang et al. (Contribution 22), the 2D GaN/g-C₃N₄ heterojunction was investigated using first-principle calculations. The calculated results indicate that the GaN/g-C₃N₄ heterojunction possesses type-II band structures, broad light absorption capabilities, and direct Z-scheme transfer mechanisms, and thus has potential applications in the field of photocatalysis.

In the final research paper, Gerken et al. (Contribution 23) investigated the interplay between photocatalytic growth and the chemical dissolution of gold structures on TiO₂/ITO patterns as a novel approach to mimic axonal dynamic connections. By optimizing gold growth and dissolution parameters, we demonstrated the potential for the precise control of the formation and removal of conductive pathways. This work bridges photocatalytic materials research and bio-inspired system development, offering new insights into the application of photodeposition and chemical etching in dynamic, adaptive systems.

3. Conclusions

In the face of increasingly severe environmental problems and resource challenges, green and sustainable development has become a global consensus and a guide for action. Photocatalysis is one of the most promising strategies for addressing the severe issues facing the environment and energy production and has attracted extensive and ongoing attention due to its inexhaustible, green, safe and economically viable characteristics. However, there are still many challenges involved in the practical application of photocatalysis, such as quantum efficiency, stability and reusability, selectivity, output-to-input ratio, and scaling-up. Fortunately, as a result of decades of hard work, photocatalysis has progressed to a new stage. Various and ingenious photocatalytic materials and photocatalytic reactions have been developed. Photocatalytic materials include but are not limited to nonmetallic LSPR materials, MOFs, COFs, SACs, SCCs, HEAs, heterojunctions, and composite materials. Photocatalytic reactions are involved in the fields of environmental, life science, medical research, space exploration, agriculture and food, energy, etc. However, designing advanced photocatalytic materials with good performance and exploring green photocatalytic reactions with carbon neutrality will require further progress.