Guest Editorial: Solar Energy Utilization

With the fast development of the social economy and the improvement of people's living standards, energy and environmental issues are attracting more and more attention. In the future, the great challenge for mankind is to shift energy supply from fossil energy to renewable energy. Solar energy is the most important renewable energy on Earth. However, low energy density and intermittency limit its practical application. Photocatalysis has broad application prospects in solar energy utilization. Photocatalysis can utilize solar energy to decompose water to produce hydrogen, reduce carbon dioxide to synthesize solar fuel, and degrade pollutants to purify the environment. However, the low photocatalytic efficiency limits its practical application. Thus, from the viewpoint of practical utilization, the improvement in methods and new photocatalysts are highly required. A total of 16 papers have been published in this issue, covering H2 production, CO2 reduction, H2O2 synthesis, and pollutant degradation. Among them, there are 10 papers about hydrogen production and 6 papers related to S-scheme heterojunction photocatalysts. We would like to express our sincere thanks to all the authors who submitted their interesting works to this special issue. A summary of all 18 accepted papers is provided as follows. Firstly, in article number 2200394, the authors present different functional ligands or metals incorporated into the parent metal-organic frameworks (MOFs) to enhance the photocatalytic performance of multivariate MOFs. The synthesis methods and unique advantages of multivariate MOFs-based photocatalysts are discussed. The recent advance in three multivariate MOFs for solar-to-chemical energy conversion are summarized according to mixed-metal MOFs, mixed-metal and mixed-ligand MOFs, and mixed-ligand MOFs. Finally, future perspectives and challenges in CO2 conversion and H2 evolution over Multivariate MOFs-based photocatalysts are discussed. In article number 2200364, Liu and colleagues reported enhanced CO2 photoreduction over Ni(OH)2-x/WO3 nanofibers, which were prepared by in situ growth of freestanding oxygen-vacancy Ni(OH)2-x nanosheets on WO3 nanofibers. The Ni(OH)2-x/WO3 nanofibers exhibit an enhanced CO production rate with respect to WO3 (54.4 vs 8.1 µmol g−1 h−1). The 13CO2 isotope tracing experiment confirmed that the CO product originated from the input CO2. The article with number 2200189 presents n-type CoP2 semiconductors as one of the main active components for efficient hydrogen evolution obtained from bulk P-CoV-LDH (layered double hydroxide). To achieve oriented control of carrier migration, the ZnxCd1−xS solid solution is effectively combined with P-CoV-LDH to synthesize a highly efficient and stable S-scheme heterojunction photocatalyst. The best P-CoV-LDH/ZnxCd1−xS 30% composite has a hydrogen evolution rate of 1244.3 µmol without noble metal additives, which is 6.4 times more than ZnxCd1−xS. Li and co-workers, in article number 2200143, reported g-C3N4 with edge grafting of 4-(1H-imidazol-2-yl) benzoic acid and NiS cocatalysts fabricated via a one-pot chemical condensation of monomers with urea and subsequent photodeposition. The obtained composites exhibit greatly enhanced visible-light photocatalytic performance for H2 evolution, in comparison with the undoped g-C3N4. The synergistic effect of bimetallic sulfide is discussed in article number 2200139, which reports the composite bimetallic sulfide ZnCo2S4 and CdS with excellent photocatalytic hydrogen evolution capability. The synergistic effect of zinc ions and cobalt ions enriches the redox-active sites, which provides favorable conditions for the photocatalytic hydrogen evolution reaction. The synergistic effect of bimetallic ions as the main driving force for the accelerated hydrogen precipitation reaction is analyzed by fluorescence and electrochemical characterization. The results of hydrogen production experiments show that the hydrogen evolution amount of ZnCo2S4/CdS is about 10 times that of single CdS. Article number 2200134 describes carbon nanotubes in situ grown onto g-C3N4 nanosheets via a chemical vapor deposition process, catalyzed by Au nanoparticles pre-deposited on g-C3N4 surface via deposition-precipitation. Systematic characterizations, in particular femtosecond transient absorption spectroscopy and time-resolved photoluminescence, prove that carbon nanotubes can efficiently extract the localized electrons in the tri-s-triazine units of g-C3N4, thereby enhancing charge carrier diffusion and separation. In article number 2200130, a donor–acceptor modified g-C3N4 conjugated copolymer is fabricated via facile thermal copolymerization of 2-aminobenzimidazole (abIM) and urea. The experimental results demonstrate that the abIM units are successfully incorporated into the framework of g-C3N4 and the main chemical structure of g-C3N4 is still preserved. These abIM units can serve as electron acceptors, extending the π-conjugated system and inducing the intramolecular charge transfer via an internal electric field. As a result, the construction of D–A structure not only improves the optical utilization efficiency but also facilitates the intramolecular migration of electrons and holes, leading to enhanced photocatalytic hydrogen evolution (2566 µmol g−1 h−1) as compared to pristine g-C3N4. Article number 2200113 presents a novel S-scheme heterojunction photocatalyst g-C3N4/PDA comprised of ultrathin g-C3N4 and polydopamine (PDA) constructed by in situ self-polymerization. The optimal photocatalyst presents an excellent H2O2 production rate of 3801.25 µmol g−1 h−1 under light irradiation, which is about 2 and 11 times higher than that of pure g-C3N4 and PDA, respectively, and exceeds most of the reported C3N4-based photocatalysts. The improvement of photocatalytic activity is ascribed to the synergistic effect of improved light absorption and promoted charge separation and transfer induced by the S-scheme heterojunction. In article number 2200030, a novel quaternary CdIn2S4-xSex solid-solution nanocrystal photocatalyst was prepared by one-step hydrothermal synthesis. The bandgap structure of CdIn2S4-xSex nanocrystals can be adjusted from 2.42 to 1.87 eV by varying the molar ratio of Se/S. Compared with pure CdIn2S4, the CdIn2S4-xSex solid-solution photocatalyst clearly represents excellent photocatalytic hydrogen production performance, while the CdIn2S4-xSex (x = 0.4) solid-solution nanocrystal exhibits the optimal hydrogen-production efficiency of 314.24 µmol h−1, which is 3.3 times superior to that of CdIn2S4 (94.83 µmol h−1). In article number 2200027, ZnS/TiO2 S-scheme heterojunction photocatalysts were constructed by in situ depositing ZnS nanoparticles on TiO2 nanofibers via hydrothermal method. A highly improved photocatalytic H2 evolution rate is achieved for the ZnS/TiO2 heterojunction as compared to the mono-component ZnS and TiO2. Remarkably, the TiO2/ZnS-5 sample possesses the highest H2 evolution rate of 5503.8 µmol g–1 h–1, which is 4.8 times of ZnS and 38.8 times of TiO2, respectively. In article number 2200009, highly dispersed Ni sites are planted on C3N5, an N-rich carbon nitride, by a facial two-step annealing method to construct a Ni-C3N5 material. The incorporation of Ni sites can significantly enhance the e–/h+ separation efficiency of C3N5 under light irradiation and promote the activation of O2 to produce reactive oxygen species. Compared with pristine C3N5 (with NO removal ratio of ≈35%), the as-prepared 0.1- or 0.25-Ni-C3N5 material can remove ≈54% continuous-flowing NO (initial concentration: 600 ppb) quickly in less than 25 min under white LED light irradiation. A novel sandwich-like hierarchical heterostructure of Ti3C2 MXene/WO3 is created by in situ growth of ultrathin WO3 nanosheets onto the surface of few-layer Ti3C2 nanosheets via a one-pot solvothermal synthesis strategy (article number 2100507). The resultant Ti3C2/WO3 heterostructure holds a large interface contact area, an intimate electronic interaction, and a short carrier migration distance, which is beneficial for bulk-to-surface and interfacial charge transfer. As expected, the as-prepared Ti3C2/WO3 nanohybrids exhibit superior visible-light-driven photoactivity and stability toward tetracycline hydrochloride decomposition. Article number 2100498 presents an S-scheme of Mn0.2Cd0.8S-diethylenetriamine/porous g-C3N4 heterojunction designed, which accelerates the charge transfer at the interface of Mn0.2Cd0.8S-diethylenetriamine and porous g-C3N4, and provides electrons for photocatalytic hydrogen production. Under the same light conditions, the hydrogen production efficiency of the composite is 11.42 mmol h–1 g–1, which is 30 times higher than that of porous g-C3N4. The paper “Porous Zn conformal coating on dendritic-like Ag with enhanced selectivity and stability for CO2 electroreduction to CO” (article number 2200374) presents uniform porous Zn conformal coating on high-curvature dendritic Ag nanoneedles by vacuum thermal evaporation. As the surface sacrificial shell, the dissolution and reconstruction of Zn protect the inner Ag core, thus enhancing the CO2 reduction stability of the composited samples. In article number 2200402, a step-scheme heterojunction consisting of thin TiO2 nanosheets and few-layered MoO3 structures is reported. With a decoration of a low dose of MoO3 layer by ball milling method, TiO2 shows a 3-fold increase in the hydrogen evolution rate. The presence of MoO3 promotes the electron-hole pair separation via the Step-scheme mechanism. Finally, in article number 2200381, a novel MOFs-derived In2O3/ZnO tubular S-scheme heterojunction photocatalyst for CO2 photoreduction is reported. Because of Fermi level difference and electron transfer, an internal electric field is produced at In2O3/ZnO heterojunction interfaces, which results in the formation of S-scheme heterojunctions. The CO2 photoreduction follows a *COOH-intermediate mechanism and the CO production rate (12.6 µmol g−1) with nearly 100% selectivity is obtained over In2O3/ZnO S-scheme photocatalyst. The authors declare no conflict of interest. Jiaguo Yu is a professor in the Faculty of Materials Science and Chemistry at the China University of Geosciences. He received his BS and MS degrees in chemistry from Central China Normal University and Xi'an Jiaotong University, respectively, and his PhD degree in materials science in 2000 from Wuhan University of Technology. In 2000, he became a Professor at Wuhan University of Technology. In 2021, he moved to the China University of Geosciences (Wuhan). His research interests include photocatalysis, adsorption, electrocatalysis and so on. He is a Foreign Member of Academia Europaea (2020), a Foreign Fellow of the European Academy of Sciences (2020), and a KIA Laureate of the 35th Khwarizmi International Award (2022). Kai Dai is a professor at the College of Physics and Electronic Information, Huaibei Normal University, Huaibei, China. He received Ph.D. degree from Shanghai University in 2007 and then worked as an assistant professor in Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences. He joined Huaibei Normal University in 2010 and his research interests mainly focus on semiconductor photocatalysis. Chuanbiao Bie obtained his Ph.D. degree in Materials Science and Engineering from Wuhan University of Technology (2021). He is now a postdoctoral researcher working in the Laboratory of Solar Fuel, Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). His research interests are focused on semiconductor photocatalysis, including H2 evolution, CO2 reduction, H2O2 production, and organic synthesis.