Synergistic Photocatalytic Hydrogen Evolution of NiCeO x Bimetallic Oxide Nanosheets and Cadmium Sulfide Heterojunction

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.

Enhanced visible light activated hydrogen evolution activity over cadmium sulfide nanorods by the synergetic effect of a thin carbon layer and noble metal-free nickel phosphide cocatalyst

Preparation of CdS nanorods on silicon nanopillars surface by hydrothermal method

Designing low-cost, environment friendly, and highly active photocatalysts for water splitting is a promising path toward relieving energy issues. Herein, one-dimensional (1D) cadmium sulfide (CdS) nanorods are uniformly anchored onto two-dimensional (2D) NiO nanosheets to achieve enhanced photocatalytic hydrogen evolution. The optimized 2D/1D NiO/CdS photocatalyst exhibits a remarkable boosted hydrogen generation rate of 1,300 μmol h−1 g−1 under visible light, which is more than eight times higher than that of CdS nanorods. Moreover, the resultant 5% NiO/CdS composite displays excellent stability over four cycles for photocatalytic hydrogen production. The significantly enhanced photocatalytic activity of the 2D/1D NiO/CdS heterojunction can be attributed to the efficient separation of photogenerated charge carriers driven from the formation of p-n NiO/CdS heterojunction. This study paves a new way to develop 2D p-type NiO nanosheets-decorated n-type semiconductor photocatalysts for photocatalytic applications.

It is the first timethat cadmium sulfide (CdS) nanorods have beenfabricated on silicon (Si) pyramid surface by the hydrothermal reactionmethod. In our work, the Si pyramid morphology is able to increasethe adhesion between the CdS seed layer and Si wafer. Hence, it iscritical for CdS nanorods to grow successfully. During the fabricationprocess, the glutathione is used as the complexing agent for the formationof the CdS nanorods. By continuously adjusting the experimental conditions,the thickness of the CdS seed layer, the concentration of the glutathione,and the temperature and time of the hydrothermal reaction, the optimalcondition for CdS nanorods growth on Si pyramid surface is 80 nm seedlayer, 0.2–0.3 mmol glutathione, 200 °C, and 1.5 h. The Cd and S elements have a ratioof 1:1.03 from the energy-dispersive spectroscopy test, which is inagreement with the stoichiometric composition of CdS. The CdS nanorodshave a bandwidth of 2.22 eV through the optical absorption spectra.The photosensitivity response test results reveal these CdS nanorodson the Si pyramid structure have an obvious photosensitive effect.From the analysis, the CdS nanorods can grow on any morphologicalSi surface if the adhesion between the CdS seed layer and the Si surfaceis strong enough.

In this study, cadmium sulphide (CdS) nanorods doped ferroelectric liquid crystal (FLC) sample cells have been prepared and studied. A memory effect has been observed in CdS nanorods (≤0.3 wt%) doped FLC mixture and confirmed by textures, dielectric and optical studies. The addition of nanorods increases the memory behaviour and efficiency. The occurrence of memory behaviour has been explained due to charge transfer from liquid crystal molecules to CdS nanorods and exists there for 5–15 min in 0.1–0.3 wt% CdS nanorods doped samples. An improvement in polarization, switching time, threshold voltage and rise time parameters has also been noticed in CdS nanorods doped FLC samples.

Methods are presented for synthesizing nanocrystal heterostructures comprised of two semiconductor materials epitaxially attached within individual nanostructures. The chemical transformation of cation exchange, where the cations within the lattice of an ionic nanocrystal are replaced with a different metal ion species, is used to alter the chemical composition at specific regions ofa nanocrystal. Partial cation exchange was performed in cadmium sulfide (CdS) nanorods of well-defined size and shape to examine the spatial organization of materials within the resulting nanocrystal heterostructures. The selectivity for cation exchange to take place at different facets of the nanocrystal plays an important role in determining the resulting morphology of the binary heterostructure. The exchange of copper (I) (Cu<sup>+</sup>) cations in CdS nanorods occurs preferentially at the ends of the nanorods. Theoretical modeling of epitaxial attachments between different facets of CdS and Cu<sub>2</sub>S indicate that the selectivity for cation exchange at the ends of the nanorods is a result of the low formation energy of the interfaces produced. During silver (I) (Ag<sup>+</sup>) cation exchange in CdS nanorods, non-selective nucleation of silver sulfide (Ag<sub>2</sub>S), followed by partial phase segregation leads to significant changes in the spatial arrangement of CdS and Ag<sub>2</sub>S regions at the exchange reaction proceeds through the nanocrystal. A well-ordered striped pattern of alternating CdS and Ag<sub>2</sub>S segments is found at intermediate fractions of exchange. The forces mediating this spontaneous process are a combination of Ostwald ripening to reduce the interfacial area along with a strain-induced repulsive interaction between Ag<sub>2</sub>S segments. To elucidate why Cu<sup>+</sup> and Ag<sup>+</sup> cation exchange with CdS nanorods produce different morphologies, models for epitaxial attachments between various facets of CdS with Cu<sub>2</sub>S or Ag<sub>2</sub>S lattices were used to calculate interface formation energies. The formation energies indicate the favorability for interface nucleation at different facets of the nanorod and the stability of the interfaces during growth of the secondary material (Cu<sub>2</sub>S or Ag<sub>2</sub>S) within the CdS nanocrystal. The physical properties of the CdS-Ag<sub>2</sub>S and CdS-Cu2S binary nanorods are discussed in terms of the electronic structure of their components and the heterostructure morphology.

Shape effects of CdS photocatalysts on hydrogen production

One-dimensional cadmium sulfide (CdS) nanostructures by the solvothermal process: Controlling crystal structure and morphology aided by different solvents

Developing non-noble metal photocatalysts for efficient photocatalytic hydrogen evolution is crucial for exploiting renewable energy. In this study, a photocatalyst of Ni2P/CdS nanorods consisting of cadmium sulfide (CdS) nanorods (NRs) decorated with Ni2P nanoparticles (NPs) was fabricated using an in-situ solvothermal method with red phosphor (P) as the P source. Ni2P NPs were tightly anchored on the surface of CdS NRs to form a core-shell structure with a well-defined heterointerface, aiming to achieve a highly efficient photocatalytic H2 generation. The as-synthesized 2%Ni2P/CdS NRs photocatalyst exhibited the significantly improved photocatalytic H2 evolution rate of 260.2 μmol∙h−1, more than 20 folds higher than that of bare CdS NRs. Moreover, the as-synthesized 2%Ni2P/CdS NRs photocatalyst demonstrated an excellent stability, even better than that of Pt/CdS NRs. The photocatalytic performance enhancement was ascribed to the core-shell structure with the interfacial Schottky junction between Ni2P NPs and CdS NRs and the accompanying fast and effective photogenerated charge carriers’ separation and transfer. This work provides a new strategy for designing non-noble metal photocatalysts to replace the noble catalysts for photocatalytic water splitting.

The partial transformation of ionic nanocrystals through cation exchange has been used to synthesize nanocrystal heterostructures. We demonstrate that the selectivity for cation exchange to take place at different facets of the nanocrystal plays an important role in determining the resulting morphology of the binary heterostructure. In the case of copper(I) (Cu(+)) cation exchange in cadmium sulfide (CdS) nanorods, the reaction starts preferentially at the ends of the nanorods such that copper sulfide (Cu(2)S) grows inward from either end. The resulting morphology is very different from the striped pattern obtained in our previous studies of silver(I) (Ag(+)) exchange in CdS nanorods where nonselective nucleation of silver sulfide (Ag(2)S) occurs (Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L.-W.; Alivisatos, A. P. Science 2007, 317, 355-358). From interface formation energies calculated for several models of epitaxial connections between CdS and Cu(2)S or Ag(2)S, we infer the relative stability of each interface during the nucleation and growth of Cu(2)S or Ag(2)S within the CdS nanorods. The epitaxial attachments of Cu(2)S to the end facets of CdS nanorods minimize the formation energy, making these interfaces stable throughout the exchange reaction. Additionally, as the two end facets of wurtzite CdS nanorods are crystallographically nonequivalent, asymmetric heterostructures can be produced.

Single crystalline CdS nanorods fabricated by a novel hydrothermal method

Constructing V2O5/CdS/CuS multi-level heterojunction for efficient photocatalytic hydrogen evolution

Improved hybrid solar cells consisting of vertical aligned cadmium sulfide (CdS) nanorod arrays and interpenetrating polythiophene (P3HT) have been achieved via modification of CdS nanorod surface by using conjugated N719 dye. The complete infiltration of P3HT between CdS nanorods interspacing was verified by scanning electron microscopy. By employing absorption and photoluminescence spectra, and current-voltage characterization the interaction between N719 molecules and CdS nanorods/P3HT interface was explored, and the role of N719 dye on the improvement of device performance was discussed.

A very novel phenomenon of morphological variations of cadmium sulfide (CdS) nanorods under the transmission electron microscopy (TEM) beam was observed without structural phase transformation. Environmentally stable and highly crystalline CdS nanorods have been obtained via a chemical bath method. The energy of the TEM beam is believed to have a significant influence on CdS nanorods and may melt and transform them into smaller nanowires. Morphological variations without structural phase transformation are confirmed by recording selected area electron diffraction at various stages. The prepared CdS nanorods have been characterized by X-ray powder diffraction, TEM, UV-Vis spectroscopy, and photoluminescence spectroscopy. The importance of this phenomenon is vital for the potential application for CdS such as smart materials.