In-situ growth of CdS quantum dots on g-C3N4 nanosheets for highly efficient photocatalytic hydrogen generation under visible light irradiation

Well dispersed CdS quantum dots were successfully grown in-situ on g-C₃N₄ nanosheets through a solvothermal method involving dimethyl sulfoxide. The resultant CdS–C₃N₄ nanocomposites exhibit remarkably higher efficiency for photocatalytic hydrogen evolution under visible light irradiation as compared to pure g-C₃N₄. The optimal composite with 12 wt% CdS showed a hydrogen evolution rate of 4.494 mmol h⁻¹ g⁻¹, which is more than 115 times higher than that of pure g-C₃N₄. The enhanced photocatalytic activity induced by the in-situ grown CdS quantum dots is attributed to the interfacial transfer of photogenerated electrons and holes between g-C₃N₄ and CdS, which leads to effective charge separation on both parts.     

In situ tuning of band gap of Sn doped composite for sustained photocatalytic hydrogen generation under visible light irradiation

The significance of Sn dopant on the photocatalytic performance of Iron/Titanium nanocomposite towards photocatalytic hydrogen generation by water splitting reaction is investigated. Iron/Titanium nanocomposite modified by Sn⁴⁺ dopant acts as a suitable photocatalyst for induced visible light absorption facilitating pronounced charge separation efficiency. Various characterization techniques reveal the heterojunction formation of hematite Fe₂O₃ with anatase - rutile mixed phase of TiO₂ employing Sn doping, where Sn⁴⁺ dopant accomplishes the phase transformation of anatase to rutile, entering into the TiO₂ lattice. This extended the lifetime of photogenerated charge carriers and enhanced the quantum efficiency of the photocatalyst. The band gap of the nanocomposite is tuned to ~2.4 eV, favoring visible light absorption. A hydrogen generation activity of 1102.8 μmol, approximately five times higher than the lone system (216.5 μmol) is achieved for the 5% Sn doped system for an average of 5 h. Heterojunctions of hematite with anatase-rutile mixed phase, generated as a consequence of tin doping facilitated the enhanced hydrogen generation activity of photocatalyst.     

g-C3N4/B doped g-C3N4 quantum dots heterojunction photocatalysts for hydrogen evolution under visible light

Developing high activity and eco-friendly photocatalysts for water splitting is still a challenge in solar energy conversion. In this paper, B doped g-C₃N₄ quantum dots (BCNQDs) were prepared via a facile molten salt method using melamine and boron oxide as precursors. By introducing BCNQDs onto the surface of g-C₃N₄, g-C₃N₄/BCNQDs heterojunction was constructed via hydrothermal treatment. The resulting g-C₃N₄/BCNQDs heterojunction exhibits enhanced hydrogen evolution performance for water splitting under visible light irradiation. The mechanism underlying the improved photocatalytic activity was explored and discussed based on the formation of heterojunction between g-C₃N₄ and BCNQDs with well-matched band structure.     

Bandgap modulation of polyimide photocatalyst for optimum H2 production activity under visible light irradiation

A series of polyimide photocatalysts with different polymerization degrees were prepared by thermal condensation of melamine and pyromellitic dianhydride at different condensation temperatures between 250 °C and 350 °C. Fourier transform infrared (FT-IR) spectra, X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) revealed that the polymerization degree increased with increasing condensation temperature. UV–Vis absorption spectra showed that the bandgap energy was lineally decreased with the increase of temperature. The theoretical calculations indicated that the level of conduction band downshifted and valence band upshifted upon the bandgap narrowing. Photocatalytic results displayed that the sample with moderate bandgap exhibited the highest H₂ evolution activity, which adapted the balance among many factors such as light absorption, band potentials, crystallinity, particle size and surface area.     

Photocatalytic hydrogen production over modified SiC nanowires under visible light irradiation

β-SiC nanowires were synthesized by simple carbothermal reduction of a mixture composed of low-cost water glass and starch, and characterized by XRD, SEM, TEM, BET, UV–visible absorption, FT-IR spectrometer and X-ray photoelectron spectrometer. The results show that the β-SiC nanowires can absorb visible light and exhibit excellent photocatalytic hydrogen evolution performance from pure water under visible light irradiation. The hydrogen evolution rate can reach more than 60 μL/g·h. SiC nanowires by simple modification can greatly enhance the efficiency of H₂ production. Average H₂ production rate over the modified SiC is 76.1% higher than that of unmodified SiC. The enhanced H₂ production is mainly due to stronger hydrophilic ability of the modified SiC. In addition, O₂ is detectable in the experiment, indicating that water has been decomposed into H₂ and O₂.     

Metalloid Ni2P and its behavior for boosting the photocatalytic hydrogen evolution of CaIn2S4

The development of high-efficiency and low-cost photocatalysts in photocatalytic H₂ evolution systems from water remains challenging. The substitution of a noble metal as the co-catalyst is still one of the important and meaningful issues in this field. Herein, we report a series of CaIn₂S₄ catalysts combined with Ni₂P, which acts as the co-catalyst, for boosting photocatalytic hydrogen evolution under visible light. The integrated system of the Ni₂P/CaIn₂S₄ composite exhibited high efficiency and durability, which were even higher than those of Pt decorated catalysts. The promoting effect of Ni₂P can be ascribed to its excellent reductive ability and analogous metallic character, which can accelerate the transfer and consumption of the photo-generated electrons. Moreover, based on the surface photo-voltage technique and electrochemical tests, the unique mechanism of Ni₂P for the movement of photo-generated charges during the photocatalysis process is proposed for the first time.     

One-pot synthesis of CdS-MoS2/RGO-E nano-heterostructure with well-defined interfaces for efficient photocatalytic H2 evolution

Quality of interfaces is a key factor determining photoexcited charge transfer efficiency, and in turn photocatalytic performance of heterostructure photocatalysts. In this paper, we demonstrated CdS-MoS₂/RGO-E (RGO-E: reduced graphene oxide modified by ethylenediamine) nanohybrid synthesized by using a facile one-pot solvethermal method in ethylenediamine, with CdS nanoparticles and MoS₂ nanosheets intimately growing on the surface of RGO. This unique high quality heterostructure facilitates charge separation and transportation, and thus effectively suppressing charge recombination. As a result, the CdS-MoS₂/RGO-E exhibits a state-of-the-art H₂ evolution rate of 36.7 mmol g^?1 h^?1 and an apparent quantum yield of 30.5% at 420 nm, which is the advanced performance among all the same-type photocatalysts (see Table S1), and far exceeding that of bare CdS by higher than 104 times. This synthesis strategy gives an inspiration for the synthesis of other compound catalysts, and higher performance photocatalyst may be obtained.     

Efficient photocatalytic hydrogen evolution on amorphous MoSx-modified CdS/KTaO3 heterojunctions under visible light irradiation

Constructing heterostructures with efficient charge separation is a promising route to improve photocatalytic hydrogen production. In this paper, MoS_x/CdS/KTaO₃ ternary heterojunction photocatalysts were successfully prepared by a two-step method (hydrothermal method and photo deposition method), which improved the photocatalytic hydrogen evolution activity. The results show that the rate of hydrogen evolution for the optimized photocatalyst is 2.697 mmol g^?1·h^?1under visible light, which is 17 times and 2.6 times of the original CdS (0.159 mmol g^?1 h^?1) and the optimal CdS/KTaO₃(1.033 mmol g^?1 h^?1), respectively, and the ternary photocatalyst also shows good stability. The improvement on photocatalytic hydrogen evolution performance can be attributed to the formation of heterojunction between the prepared composite materials, which effectively promotes the separation and migration of photo-generated carriers. Amorphous MoS_x acts as an electron trap to capture photogenerated electrons, providing active sites for proton reduction. This provides beneficial enlightenment for hydrogen production by efficiently utilizing sunlight to decompose water.     

Photocatalytic hydrogen production with nickel oxide intercalated K4Nb6O17 under visible light irradiation

Nickel oxide nanoclusters were intercalated to layered niobate, K₄Nb₆O₁₇, to improve the photocatalytic hydrogen production for water splitting under visible light irradiation. A K₄Nb₆O₁₇–SSRx (Ni/Nb ratio range of 0.8–5%) series of nickel oxide intercalated layered niobates was prepared by a two-step solid-state reaction and characterized by Extended X-ray Absorption Fine Structure (EXAFS), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometer (EDS), X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet–visible spectroscopy (UV–vis). The photocatalytic reaction was carried out in a quartz reactor irradiated under a 500-W halogen lamp. The K₄Nb₆O₁₇–SSR0.2 catalyst exhibited a much higher photocatalytic activity for water splitting than unloaded K₄Nb₆O₁₇ catalyst and NiO_y/K₄Nb₆O₁₇ catalyst prepared by the conventional impregnation method. The high catalytic performance was attributed to the well dispersed nickel oxide nanoclusters intercalated into the bulk structure of K₄Nb₆O₁₇ catalyst and the lack of NiO particles on the external particle surface.     

Microwave-assisted hydrothermal synthesis of transition-metal doped ZnIn2S4 and its photocatalytic activity for hydrogen evolution under visible light

Aiming at the enhancement of photocatalytic activity for hydrogen evolution over ZnIn₂S₄, different transition metals (Cr, Mn, Fe, Co) are doped into the lattices of ZnIn₂S₄ to narrow the band gap. The doped ZnIn₂S₄ is characterized by XRD, Raman, UV-vis spectra, photoluminescence spectra, SEM and XPS techniques. The photocatalytic evaluation shows that Mn-doped ZnIn₂S₄ performs photocatalytic activity 20% higher than undoped ZnIn₂S₄, while Cr-, Fe-, and Co-doped ZnIn₂S₄ perform poorer activities in an order of Cr > Fe > Co. Based on the combined characterization results, the band structures of doped ZnIn₂S₄ are schematically depicted, which illustrates the different effects of transition-metal doping on the photocatalytic activity for hydrogen evolution. For Mn-doped ZnIn₂S₄, the enhancement of photocatalytic activity could be due to narrowed band gap induced by Mn doping. However, for Cr-, Fe-, and Co-doped ZnIn₂S₄, the suppressed photocatalytic activities should be attributed to the dopant-related impurity energy levels localizing the charge carriers or acting as non-radiative recombination centers for photoexcited electrons and holes. Hence, this study indicates that it is of great importance to make the in-depth investigation on the effects of band structures on the photocatalytic activity, especially for the doped semiconducting photocatalysts.     

WO3 cocatalyst improves hydrogen evolution capacity of ZnCdS under visible light irradiation

Semiconductor photocatalysts can convert solar energy into clean pollution-free hydrogen energy and thus are a novel technology to alleviate the energy crisis. To acquire catalysts with higher photocatalytic hydrogen production efficiency, we synthesized ZnCdS catalysts from a hydrothermal method and the WO₃ cocatalyst through temperature-programmed reduction. The surface morphology and optical properties of the catalysts were characterized via X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV–Vis spectroscopy, which proved the successful synthesis of the WO₃/ZnCdS compound catalysts. The effects of WO₃ dosage on the photocatalytic activity of ZnCdS were studied, and in particular, the hydrogen production activity of the 35 wt% WO₃/ZnCdS was the highest to 98.68 μmol/mg, about 9.6 times that of pure ZnCdS (10.28 μmol/mg). After 5 cycles, it yet had high repeatability and preserved high hydrogen production activity after 100 h of photocatalytic tests. The underlying mechanism was explored via photoluminescence and photocurrent assays. It was found the 35 wt% WO₃/ZnCdS generated higher photocurrent than pure ZnCdS, indicating WO₃ could facilitate electron transfer to involve more electrons in hydrolysis reactions, thereby increasing the photoelectron use efficiency and photocatalytic hydrogen production activity.     

In-situ La doped Co3O4 as highly efficient photocatalyst for solar hydrogen generation

Photocatalytic hydrogen production via water splitting using metal oxide semiconductors has attract great interests because of the two electrons on the kinetics. Pristine Co₃O₄ was widely studied as efficient photocatalyst, but prefers to produce oxygen due to its lower band-edge positions with regard to water redox potentials. In this work, high efficient photocatalyst basing on non-noble La doped Co₃O₄ on graphene, i.e., La_xCo_3-xO₄/G, were first reported and prepared by the microwave hydrothermal synthesis. In this newly developed hybrids, La and Co ions were adsorbed on the surface of graphene (G) and subsequently reacted with ammonia to yield the La_xCo_3-xO₄/G nanohybrid by in-situ chemical deposition methods. The activity for hydrogen generation of the nanohybrid exhibits 2 times higher than undoped Co₃O₄/G under visible light irradiation. The H₂ evolution of nanohybrid reaches 6.543 mmol g^?1 h^?1 when the molar ratio of La/Co is 10% in the nanohybrid. Our experimental results indicate the incorporation of La doped in the Co₃O₄ crystal lattice not only forms the lattice defects, resulting in provision for capture trap and the separation of electrons and holes, but also changes the band structure to eventually improve the photocatalytic activity under visible light. Therefore, non-noble La is a promising substitute to prepare highly efficient hydrogen photocatalyst and can be extendedly applied to the other metal oxide semiconductors for solar hydrogen production.     

TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation

The CdS/TiO₂ composites were synthesized using titanate nanotubes (TiO₂NTs) with different pore diameters as the precursor by simple ion change and followed by sulfurization process at a moderate temperature. Some of results obtained from XRD, TEM, BET, UV–vis and PL analysis confirmed that cadmium sulfide nanoparticles (CdSNPs) incorporated into the titanium dioxide nanotubes. The photocatalytic production of H₂ was remarkably enhanced when CdS nanoparticles was incorporated into TiO₂NTs. The apparent quantum yield for hydrogen production reached about 43.4% under visible light around λ = 420 nm. The high activity might be attributed to the following reasons: (1) the quantum size effect and homogeneous distribution of CdSNPs; (2) the synergetic effects between CdS particles and TiO₂NTs, viz., the potential gradient at the interface between CdSNPs and TiO₂NTs.     

Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructures for highly efficient photocatalytic hydrogen evolution under solar light irradiation

Designing of noble-metal free and morphologically controlled advanced photocatalysts for photocatalytic water splitting using solar light is of huge interest today. In the present work, novel polygonal Zn₂TiO₄ (ZTO) nanostructures have been synthesized by citricacid assisted solid state method for the first time and synthesized nanostructures were characterized by using various techniques like PXRD, UV-Vis-DRS, PL, FT-IR, BET, FE-SEM and TEM for their structural, optical, chemical, surface and morphological properties. The PXRD and UV-Vis-DRS analysis show the existence of cubic and tetragonal phases. FE-SEM and TEM results confirm the formation of polygonal ZTO nanostructures. Synthesised ZTO nanostructures have been potentially applied for solar light-driven photocatalytic hydrogen evaluation from water splitting and compare the photocatalytic activity with synthesized conventional Zn₂TiO₄ and commercially available TiO₂, ZnO photocatalysts. A high rate of 529 μmolh^?1g^?1 solar light-driven photocatalytic H₂ evolution has been achieved by using a small amount (5 mg) of polygonal Zn₂TiO₄ nanostructures from glycerol-water solution. The enhanced photocatalytic performance of the polygonal Zn₂TiO₄ nanostructures compare to conventional Zn₂TiO₄ under solar light irradiation is due to the large surface area and low recombination rate. However having the same bandgap, the polygonal Zn₂TiO₄ nanostructures have shown enhanced photocatalytic performance than that of commercially available TiO₂, ZnO photocatalysts.     

Enhanced photocatalytic hydrogen evolution using a novel in situ heterojunction yttrium-doped Bi4NbO8Cl@Nb2O5

The novel in situ Z-scheme heterostructure materials Y-doped Bi₄NbO₈Cl@Nb₂O₅ (Bi_4-xY_xNbO₈Cl, x = 0, 1, 1.33, 2, 2.67, 3) have been synthesized successfully via a solid-state method. The as-prepared samples were characterized by XRD, Raman spectrum, SEM, EDS, element mapping, HRTEM, XPS and UV–vis spectrum to explore the structures, morphologies and optical properties. Photocatalytic activities were evaluated for hydrogen generation using the Pt as the co-catalysts. HRTEM results indicated the Pt particles were deposited on the surface of the Bi₄NbO₈Cl. Photocatalytic activities were evaluated by hydrogen generation. While photocatalytic results showed that BiY₃NbO₈Cl composites exhibited the best performance of hydrogen production under the full-range irradiation (λ > 300 nm) while the Y-doped Bi₄NbO₈Cl@Nb₂O₅ with Y:Bi molar ratio 1:1 obtained the highest efficiency with ultraviolet light eliminated. The H₂ production was 1.35 mmol and 0.9 mmol in 8 h, respectively. Furthermore, a direct Z-scheme mechanism with enhanced hydrogen evolution competent for accelerating the separation of photogenerated carries has been presented and proved by electrochemical impedance spectroscopy (EIS). Finally, considering the conclusions of the electron spin-resonance spectroscopy (EPR), ·OH radicals served as an active species played an important role in the hydrogen production. Mechanisms about the action of the ·OH radicals were also proposed.     

Solar hydrogen generation with H2O/ZnO/MnFe2O4 system

The hydrogen generation reaction in the H₂O/ZnO/MnFe₂O₄ system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H₂. At 1273 K, the mixture of ZnO and MnFe₂O₄ reacted with water to generate H₂ gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn^II (divalent manganese ion) in the A-site of MnFe₂O₄ was substituted with Zn^II (divalent zinc ion) and that chemical formula of the solid product was estimated to be Zn_0.58Mn^II_0.42Mn^III_0.39Fe_1.61O₄ (Mn^III: trivalent manganese ion). Its lattice constant was smaller than that of the MnFe₂O₄ (one of the two starting materials). From the chemical composition, the reaction mechanism of the H₂ generation with this system was discussed. Since the Mn ions in the product solid after the H₂ generation reaction are oxidized to Mn³⁺, which can readily release the O²⁻ ions as O₂ gas around 1300 K, the two-step of H₂ generation and O₂ releasing seem to be cyclic.     

Well-defined FeP/CdS heterostructure construction with the assistance of amine for the efficient H2 evolution under visible light irradiation

The photocatalyst is a crucial factor in determining solar-to-H₂ efficiency for solar-driven water splitting. Here, the FeP/CdS well-defined heterostructure was elaborately designed and successfully constructed in-situ to achieve efficient water splitting by using a simple and green solvothermal approach. In the synthetic process, the ethylenediamine plays an important role in the construction of intimate contact interface between FeP and CdS. This good quality FeP/CdS heterostructure can efficiently promote charge separation and transportation, and therefore the charge recombination of CdS was significantly suppressed. As a result, the as-synthesized FeP/CdS heterostructure showed excellent photocatalytic performance under visible-light irradiation with an optimal hydrogen evolution rate of 37.92 mmol g⁻¹ h⁻¹ and an apparent quantum yield of 31.50% at 420 nm far exceeding that of pristine CdS by more than 122 folds. This rate, to the best of our knowledge, outperforms other similar catalytic systems.     

Photocatalytic H2 evolution on a novel CaIn2S4 photocatalyst under visible light irradiation

A novel visible-light-driven photocatalyst CaIn₂S₄ was synthesized using a facile hydrothermal method followed by a post-calcination process. The influence of the calcination temperature and time on the activities of the photocatalyst was investigated. CaIn₂S₄ exhibits optical absorption predominantly in visible region with an optical band gap of 1.76 eV. Considerable activity for hydrogen evolution from pure water was observed without any sacrificial agents or cocatalysts under visible light irradiation. The maximum hydrogen evolution rate achieved was 30.92 μmol g⁻¹ h⁻¹ without obvious deactivation of the photocatalytic activity for four consecutive runs of 32 h.     

Photocatalytic hydrogen generation in the presence of glucose over ZnS-coated ZnIn2S4 under visible light irradiation

ZnS coated ZnIn₂S₄ (ZnS–ZnIn₂S₄) photocatalysts were prepared in methanol by a facile solvothermal process. The photocatalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), UV–Vis diffusive reflectance spectroscopy (DRS), BET, and electrochemistry measurements. ZnS–ZnIn₂S₄ photocatalysts have hexagonal crystal phase and complex morphology such as micro-spheres, micro-tubes and micro-ribbons. Using glucose as an electron donor, photocatalytic hydrogen generation over Pt/ZnS–ZnIn₂S₄ was investigated. The results show that photoactivity of hydrogen generation over Pt/ZnS–ZnIn₂S₄ was improved notably with simultaneous degradation of glucose. The factors which affect photocatalytic hydrogen generation, such as composition of ZnS-ZnIn₂S₄, initial concentration of glucose and concentration of NaOH, were investigated. The prepared ZnS–ZnIn₂S₄ photocatalysts exhibit better activity for hydrogen generation than pure ZnIn₂S₄, which may be attributed to enhancement of the adsorption of glucose by ZnS on the ZnIn₂S₄ surface. The effect of glucose concentration on the hydrogen generation rate is consistent with a Langmuir model. The basic condition is favorable for the photocatalytic hydrogen generation. A large number of ·OH radicals generated in ZnS–ZnIn₂S₄ system, have been tested by a TA-FL (terephthalic acid-fluorescence) method. A possible mechanism was discussed.