Fabrication of the SnS2/ZnIn2S4 heterojunction for highly efficient visible light photocatalytic H2 evolution

A series of SnS₂/ZnIn₂S₄ (x-SS/ZIS) photocatalysts with different mass ratios of SnS₂ were prepared by a hydrothermal method. The resulted composites were used for photocatalytic hydrogen evolution under visible light excitation. All the SS/ZIS composites exhibited significantly enhanced photocatalytic activity for H₂ evolution. Obviously, the highest H₂ evolution rate of 769 μmol g^?1 h^?1 was observed over 2.5-SS/ZIS, which was approximately 10.5 times that of the ZnIn₂S₄ (73 μmol g^?1 h^?1). The enhanced photocatalytic performance was attributed to the successful construction of SnS₂/ZnIn₂S₄ heterojunctions, leading to rapid charge separation and fast transfer of the photo-generated electrons and holes under light irradiation. On the basis of PL, electrochemical impedance spectroscopy (EIS), photocurrent measurements and the H₂ evolution tests, a plausible photocatalytic mechanism was proposed.     

Enhanced photocatalytic hydrogen evolution over semi-crystalline tungsten phosphide

Increasing the separation efficiency and transfer rate of photogenerated charges is the dominant factor for improving photocatalytic activity. Herein, we successfully prepared semi-crystalline WP (SC-WP) with good optical properties and as a cocatalyst to modify CdS nanorods (CdS NRs) to construct SC-WP/CdS (PD) composite catalyst by simple electrostatic self-assembly method for photocatalytic hydrogen evolution. Two high-efficiency and stable photocatalytic hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10 vol% lactic acid solution as sacrificial agents, respectively. Surprisingly, the maximum photocatalytic H₂ production rate of 15446.21 μmol h⁻¹ g⁻¹ is obtained over 10PD composite, which is 10.58 times greater than that of pure CdS. The improved photocatalytic activity can be attributed to the fact that the SC-WP nanoparticles provides a large number of exposed active sites on the surface of CdS for hydrogen evolution reaction, which can efficiently capture photogenerated electrons from CdS nanorods and promotes the transport and separation of light-induced charges. And the introduction of SC-WP nanoparticles with excellent optical properties can efficiently improve the visible light absorption range and the utilization rate of the absorbed light of the PD composite. In addition, the SC-WP nanoparticles show semi-crystalline state, which is also conducive to enhancing the photocatalytic activity.     

g-C3N4 doping TiO2 recovered from spent catalyst for H2 evolution from water

Spent catalysts of selective catalytic reduction (SCR) contain a high content of TiO₂ (>70 wt%). The effective recovery of TiO₂ from spent SCR catalysts and its reuse in photocatalytic hydrogen production is of great importance for environmental protection. In this study, the recovered TiO₂ from the spent SCR catalyst was recovered by the alkali washing method, and the purity of the recovered TiO₂ reached 94.7%. g-C₃N₄ as a co-catalyst and enhanced the separation efficiency of the photogenerated electron-hole pairs of the TiO₂ photocatalyst. The composite photocatalyst R–TiO₂/g-C₃N₄ prepared by directly mixing the recovered TiO₂ with g-C₃N₄ significantly improved the photocatalytic activity. The experimental design of the photocatalyst synthesis was optimized using the Design Expert software. The results showed that the recovered TiO₂ was 0.334 g when the g-C₃N₄ was 0.046 g and the ultrasonic time was 163 min. Moreover, the hydrogen production rate reached 443.105 μmol g⁻¹ h⁻¹ within 4 h.     

Giant enhancement of photocatalytic H2 production over KNbO3 photocatalyst obtained via carbon doping and MoS2 decoration

This paper was designed for the first time to improve the photocatalytic activity of KNbO₃ via carbon doping and MoS₂ decoration simultaneously. The efficient photocatalytic hydrogen production was realized on the MoS₂/C-KNbO₃ composite under simulated sunlight irradiation in the present of methanol and chloroplatinic acid. The optimal composite presents a H₂ production rate of 1300  μmol·g^?1·h^?1, which reaches 260 times that of pure KNbO₃. Characterization results of the synthesized composite indicates that the introduction of a small amount of carbon into the KNbO₃ lattice greatly hinders the recombination of electron-hole pairs. The decoration of MoS₂ further induces the separation of charge carriers via trapping the electron in the conduction band of C-KNbO₃, which is proven by the EIS and transient photocurrent response analyses. The remarkably enhanced separation efficiency of electron-hole pairs is believed to be the origin of the excellent photocatalytic performance, though other changes in surface area and optical property may also contribute the photocatalytic process. This study provides a feasible way for the design and preparation of novel photocatalysts with high efficiency.     

Enhanced photocatalytic hydrogen evolution from aqueous solutions on Ag2S/Ag heteronanostructure

A new approach for synthesis of hybrid Ag₂S/Ag heteronanostructure based on hydrochemical bath deposition with the using of Trilon BD has been suggested. The synthesized Ag₂S/Ag heteronanostructure has shown higher catalytic activity in the hydrogen evolution from the aqueous solution of Na₂S/Na₂SO₃ under irradiation by light with wavelength of 450 nm than nanostructured Ag₂S photocatalyst. The main reason for the enhanced photoactivity of Ag₂S/Ag heterostructure as compared with Ag₂S catalyst is a presence of Ag, and also nonstoichiometry of silver sulfide in this heteronanostructure which leads to the improved charge separation as compared with silver sulfide catalyst.     

2D CoP supported 0D WO3 constructed S-scheme for efficient photocatalytic hydrogen evolution

For heterojunction composite photocatalyst, intimate contact interface is the key to the carrier transfer separation conditions. Due to the interface contact, the electron transfer rate between catalysts can be increased during photocatalytic hydrogen production, therefore, we design the close contact of 0D/2D heterojunction, which greatly enhanced the photocatalytic hydrogen production activity of the composite catalyst. The composite catalyst WO₃/CoP was obtained by simple high temperature in situ synthesis. Moreover, it was proved by photoelectric chemistry and fluorescence tests that appropriate conduction band and valence band locations of WO₃ and CoP provided a favorable way for thermodynamic electron transfer. In addition, fluorescence results showed that WO₃ load effectively promoted photoelectron-hole transfer and increased electron lifetime. The formation of S-scheme heterojunctions can make more efficient use of useful photogenerated electrons and prevent the photogenerated electron-hole recombination of CoP itself, further promote the liveness of photocatalytic H₂ evolution. Meanwhile, the study of Metal-organic frameworks (MOFs) materials further promoted the application of MOFs derivatives in the field of photocatalytic hydrogen evolution, and provided a reference for the rational design of composite catalysts for transition metal phosphide photocatalysts.     

Fabrication of CdS/Zn2GeO4 heterojunction with enhanced visible-light photocatalytic H2 evolution activity

CdS/Zn₂GeO₄ (CG) composites were synthesized through the simple hydrothermal process. The crystal structure, morphology and light absorption property of the products were studied in detail. The CG composites showed excellent photocatalytic hydrogen production performance upon visible light illumination. Especially, the CG-3 composite displayed the highest H₂ evolution rate of 1719.8 μmol h⁻¹ g⁻¹, which was about 3.80 and 4.28 times higher than the pure CdS and Zn₂GeO₄. Besides, the cyclic stability of the CG-3 composite was also excellent. The PL, photocurrent response and EIS spectra results testified that the efficient separation and transfer of photoinduced charge carriers achieved between CdS and Zn₂GeO₄, which could result in the promotion of photocatalytic performance. Moreover, a possible mechanism of H₂ generation over CdS/Zn₂GeO₄ heterojunction was discussed. The practicable way to construct heterojunction composites would be helpful for the design of other systems with excellent photocatalytic property.     

Sulfur vacancy and CdS phase transition synergistically boosting one-dimensional CdS/Cu2S/SiO2 hollow tube for photocatalytic hydrogen evolution

Constructing an efficient photoelectron transfer route to improve carrier separation efficiency is crucial for photocatalytic hydrogen evolution. In this work, CdS/Cu₂S/SiO₂ heterostructure with one-dimensional hollow tube morphology was designed by the solvothermal method using CuO/SiO₂ hollow tube as carrier. The hexagonal phase CdS and sulfur vacancies were adjusted simultaneously by the reduction strategy of NaBH₄ aqueous solution. CdS/CuS/SiO₂ with cubic phase CdS was synthesized in the absence of NaBH₄ aqueous solution. CdS/Cu₂S/SiO₂ was characterized by SEM, TEM, XRD, XPS, SPV and so on. The results showed that hexagonal CdS and sulfur vacancies benefited the separation of photo-generated carriers. As a consequence, the CdS/Cu₂S/SiO₂-10 composite exhibited a high photocatalytic hydrogen production rate (1196.98 μmol/g/h), and its performance almost 7.18 times than that of CdS/CuS/SiO₂. Moreover, CdS/Cu₂S/SiO₂-10 showed an excellent cyclic stability. This was attributed to the strong electron interaction of CdS/Cu₂S/SiO₂ heterostructure and the sulfur vacancy acted as an electron trap, enhancing the separation of photo-induced electrons and holes.     

Enhanced photocatalytic hydrogen evolution and CO2 to CH4 selectivity of Co3O4/Ti3+-TiO2 hollow S-scheme heterojunction via ZIF-67 self-template and Ti3+/Ov

The potential would be an important issue for enhancing photocatalytic HER and CO₂ photoreduction selectivity. Herein, the Co₃O₄/Ti³⁺-TiO₂ hollow S-scheme heterojunction is fabricated via continuous light modification-chemical-annealing-reduction method. Evaluated by photocatalytic performance, the as-prepared Co₃O₄/Ti³⁺-TiO₂ hollow S-scheme heterojunction exhibits remarkable photocatalytic performance enhancement than single TiO₂, including hydrogen evolution (∼396.16 μmol/g·h, ∼15 folds) and CO₂ photoreduction (H₂/CH₄/CO: ∼20.32/80.57/9.85 μmol/g·h, ∼20 folds), and achieves CO₂ photoreduction to CH₄ selectivity enhancement, which can be mainly ascribed to the synergism of Ti³⁺/O_v and S-scheme heterojunction. There, Ti³⁺/O_v can not only increase the solar efficiency, but also decrease the energy barrier of H⁺ and 1CO photoreduction to enhance HER and CO₂ to CH₄ selectivity, including promote the H⁺ diffusion and CO₂ absorption. Additionally, the formed Co₃O₄/Ti³⁺-TiO₂ S-scheme heterojunction can promote the photo-generated carrier separation/transportation. Also, the hollow 3D structure obtained by ZIF-67 self-template can increase solar efficiency and stability.     

The ZnIn2S4/CdS hollow core-shell nanoheterostructure towards enhanced visible light photocatalytic H2 evolution via bimetallic synergism

The ZnIn₂S₄/CdS hollow core-shell nanoheterostructure with bimetallic synergism is synthesized via a hybrid chemical method. As revealed, the ZnIn₂S₄/CdS hollow core-shell nanoheterostructure (ZnIn₂S₄/CdS-3) exhibits remarkable visible light photocatalytic hydrogen evolution (～5209.43 μmol·g^?1·h^?1, AQE of ～20.26%) than that of single CdS (～40 folds) and single ZnIn₂S₄ (～12 folds), and achieves decent photocatalytic stability (average HER performance of ～5056.80 μmol·g^?1·h^?1), which is mainly ascribed to that, the formed ZnIn₂S₄/CdS heterostructure with appropriate potential gradient and Zn/In bimetallic synergism can improve carrier transportation, including increasing carrier transportation, prolonging lifetime and decreasing recombination, the hollow core-shell nanostructure can provide abundant active sites and increase solar efficiency, while can maintain a photocatalytic stability.     

In-situ synthesis of novel plate-like Co(OH)2 co-catalyst decorated TiO2 nanosheets with efficient photocatalytic H2 evolution activity

Efficient separation of photo-generated electrons and holes is a crucial aspect for photocatalytic hydrogen evolution. Herein, novel plate-like Co(OH)₂ decorated TiO₂ nanosheets for photocatalytic water splitting were synthesized by a facile in-situ synthetic method. The results of X-ray diffractometry (XRD), transmission electron microscope (TEM), UV–Vis diffusion reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) indicate the successfully incorporation of Co(OH)₂ co-catalysts onto the surface of TiO₂ nanosheet photocatalysts. Further photocatalytic hydrogen evolution experiments illustrate that all Co(OH)₂ decorated TiO₂ samples show higher rate of hydrogen production performance than pure TiO₂ sample and the composite sample with Co(OH)₂ loading amount of 0.5mol% presents the highest photocatalytic hydrogen production activity of 746.93 μmol g^?1·h^?1. It is indicated that plate-like Co(OH)₂ particle act as an electron collector, which leads to photo-generated electrons transfer from TiO₂ to Co(OH)₂, and therefore enhance the photocatalytic activity. Based on above results, a possible mechanism is proposed and further verified by surface photovoltage spectra (SPV).     

Dye-sensitized black phosphorus nanosheets decorated with Pt cocatalyst for highly efficient photocatalytic hydrogen evolution under visible light

Although black phosphorous (BP) and its derived materials have shown great potential for application in photocatalytic H₂ evolution reaction (HER), their HER activity and stability still remains unsatisfied mainly due to the insufficient charge separation, the lack of surface active sites, and the defect-riched nature of BP. Herein, we report that BP nanosheets decorated with in situ grown Pt (BP NSs/Pt) could act as a highly efficient catalyst for photocatalytic H₂ evolution in an Erythrosin B (ErB)-sensitized system under visible light irradiation (≥450 nm) in the presence of triethanolamine (TEOA) as sacrificial electron donor. It is found that BP NSs can provide large surface area for the confined growth of Pt nanoparticles with a high dispersion and a reduced size but also stabilize the loaded Pt nanoparticles by covalent bonds at the BP NSs/Pt interfaces. Moreover, BP NSs offer a fast electron transfer pathway to facilitate the photocatalytic HER over in situ grown Pt catalyst. As a result, BP NSs/Pt catalyst exhibits ∼6 times higher H₂ evolution activity than free Pt nanoparticles and an apparent quantum yield (AQY) of 0.57% at 500 nm irradiation in ErB-TEOA system. This work indicates the potential of BP NSs as an effective 2D matrix to construct numerous high performance photocatalysts and photocatalytic systems.     

Boosted photogenerated carriers separation in Z-scheme Cu3P/ZnIn2S4 heterojunction photocatalyst for highly efficient H2 evolution under visible light

Developing low-cost, highly efficient and robust photocatalystic hydrogen evolution system is a promising solution to environmental and energy crisis. Herein, a Z-scheme Cu₃P/ZnIn₂S₄ heterojunction photocatalyst was successfully constructed for the first time via a facile solution-phase hybridization method. The optimized Cu₃P/ZIS composite exhibited the highest H₂ production rate of 2561.1 μmol g⁻¹ h⁻¹ under visible light irradiation (>420 nm), which was 5.2 times greater than that of bare ZnIn₂S₄ and even exceeded the photocatalytic performance of Pt/ZIS composite. The apparent quantum yield of 10 wt% Cu₃P/ZnIn₂S₄ can reach 22.3% at 420 nm. The huge boost of photocatalytic hydrogen evolution activity is ascribed to the formation of heterojunction with the built in electric field within Cu₃P/ZnIn₂S₄ and Z-scheme charge carriers transfer pathway, which result in efficient separation and migration of charge carriers. In addition, both experimental and theoretical calculation confirmed that the charge-carriers transfer pathway of Cu₃P/ZnIn₂S₄ photocatalyst follows the Z-scheme mechanism instead of conventional type-Ⅱ heterojunction mechanism. This work is considered helpful for getting a great deal of insight into constructing high-activity and cost-effective transition metal phosphides (TMPs) based photcatalytic hydrogen production system and rationally designing Z-scheme heterojunction photocatalyst.     

Potential Co-catalyst application of novel M3P2 (M=Zn,Cd) in photocatalytic hydrogen evolution reaction from water splitting

New noble-metal-free co-catalysts based on transition metal phosphides, Zn₃P₂ and Cd₃P₂, were fabricated and loaded on hetero-structure of BiFeO₃–CdWO₄ with the aim of promoting hydrogen evolution from water splitting without using sacrificial agent. The opto-electrochemical properties of the photocatalysts were investigated by various characterization techniques and it was observed that the co-catalyst loaded BiFeO₃–CdWO₄ exhibited better light harvestability, charge separation efficiency and charge mobility. The photocatalytic hydrogen productivity reached up to 572.81 μmol h⁻¹.g_cat⁻¹ h by loading 12 wt% Zn₃P₂ and 488.25 μmol h⁻¹.g_cat⁻¹ by loading 9 wt% Cd₃P₂ over BiFeO₃–CdWO₄. Zn₃P₂ and Cd₃P₂ have shown light absorption in visible to near IR region, thus they may also have the additional role of a photocatalyst other than being active sites for the photocatalytic reduction half-reaction. Loading the co-catalysts also resulted in multiplication of specific surface area which means an increase in the number of surface active sites. We observed a higher hydrogen productivity with lower photocatalytic rate by loaded Zn₃P₂ in compared with Cd₃P₂. This is attributed to the different photoresponsivity and band edge energy of the co-catalysts. The details of charge transfer mechanism between the host hetero-structure photocatalyst and loaded co-catalysts has been discussed.     

CuInS2-based photocatalysts for photocatalytic hydrogen evolution via water splitting

The realization of efficient photocatalytic hydrogen evolution (PHE) significantly depends on the development of durable and effective semiconductor photocatalysts. Copper indium sulfide (CuInS₂) is an emerging ternary chalcogenide semiconductor material for solar-to-chemical energy application, because it possesses a suitable bandgap, environment-friendly elements, and a low melting point. CuInS₂-based semiconductor photocatalysts have been investigated for PHE via water splitting, but current PHE performance still has difficulty in meeting commercial application requirements and needs to be further improved. In this review, the basic semiconductor properties of CuInS₂, including its crystal and band structures, are introduced, and its PHE mechanism is discussed in detail. The PHE performance of CuInS₂-based photocatalysts is systematically discussed, with a focus on morphology, engineered structure, and heterojunction construction. Finally, issues and challenges currently encountered in the PHE application of CuInS₂-based photocatalysts and their possible solutions are presented.     

In situ oxidation of ultrathin Ti3C2Tx MXene modified with crystalline g-C3N4 nanosheets for photocatalytic H2 evolution

With photoconductor being transferred and separated, interface contact plays a crucial role in developing composites photocatalyst. In the present work, 2D crystalline g-C₃N₄ (called as CCN) with 2D TiO₂ nanosheets (called as TO) obtained in situ oxidized single-layered Ti₃C₂T_x MXene is designed by an electrostatic self-assembly technology. This CCN/TO nanosheets system with a few TiO₂ nanosheets distributed to the surface is not only prolonging lifetime of photoelectron but also stimulating photogenerated carriers transferred in contact interface. The electron transfer mechanism of CCN/TO is further proved by Pt photo-deposition method. Therein, the optimal CCN-TO-0.6 exhibits excellent performance of H₂ generation compared with single photocatalyst of CCN. The result shows that the crystalline g-C₃N₄ photocatalysts introduced TiO₂ with interfacial effect favorably reduce H⁺ to H₂ and enhance photocatalytic activity.     

Construct organic/inorganic heterojunction photocatalyst of benzene-ring-grafted g-C3N4/CdSe for photocatalytic H2 evolution

Photocatalytic water splitting to produce hydrogen is a vital research direction for alleviating the energy crisis. Herein, benzene-ring grafted g-C₃N₄ nanotubes (Ph-g-C₃N₄) were prepared skillfully and coupled with CdSe nanoparticles which was realized efficiently hydrogen production. The addition of CdSe nanoparticles enhanced the stability of the catalytic system dispersion in water, and the absorbance of the composites catalyst CdSe/Ph-g-C₃N₄ (CPG) was enhanced. In addition, the CPG had been characterized to have low resistance and efficient photogenerated electron separation efficiency. The Ph-g-C₃N₄ nanotubes with a three-dimensional structure can provide an anchoring platform for CdSe nanoparticles and effectively prevent the agglomeration of CdSe. The constructed composites catalyst achieved the efficient transfer of photogenerated electrons as known from photoluminescence spectroscopy test analysis. When CdSe nanoparticles were anchored to Ph-g-C₃N₄, the electron transfer rate of the constructed composite was about twice that of the Ph-g-C₃N₄, which facilitates the hydrogen evolution reaction. The character and electron transfer pathways of the photocatalysts were investigated theoretically by performing density functional calculations. The finding provides a new idea for the doping of photocatalysts and the design of organic/inorganic heterojunction composites photocatalyst to achieve an efficient hydrogen production system.     

Hierarchical ZnIn2S4: A promising cocatalyst to boost visible-light-driven photocatalytic hydrogen evolution of In(OH)3

Efficient separation of electrons and holes, associated with the reduction and oxidation, is of great importance in a photocatalytic reaction. 3D hierarchical core-shell-like ZnIn₂S₄@In(OH)₃ microspheres have been fabricated by a facile hydrothermal method via controlling the sulfur source. The marigold-like spherical ZnIn₂S₄ induced the in situ growth of cubic In(OH)₃ nanosheets as the outer shell, which efficiently transferred the photogenerated electrons and achieved efficient charge separation efficiency for highly photocatalytic H₂ production. Moreover, the intimate interfacial contact between ZnIn₂S₄ core and In(OH)₃ shell offered rectified charge transfer directions, which further boosted the charge separation. In consequence, the photocatalytic H₂ evolution under visible light irradiation was achieved on wide-gap In(OH)₃ owing to ZnIn₂S₄ as a cocatalyst, and a prominent photocatalytic H₂ production of 2088 μmol g⁻¹ was obtained on core-shell-like ZnIn₂S₄@In(OH)₃ structure with an apparent quantum efficiency of 1.45% (400 nm), which was nearly 2-folds higher of H₂ production rate than the pristine ZnIn₂S₄. This work provides a prototype material for high efficiency of hydrogen evolution, and gives a new insight for the development of efficient heterojunction photocatalysts.     

MOF-derived carbon-containing Fe doped porous CoP nanosheets towards hydrogen evolution reaction and hydrodesulfurization

In order to explore the effect of Fe doped CoP nanosheets on both hydrogen evolution reaction (HER) and hydrodesulfurization (HDS), FeCoP/C nanosheets have been successfully synthesized by the solvothermal process with Fe doped ZIF-67/C for the growth of FeCo-hydroxides nanosheets, as well as a following low-temperature phosphorization process. The performance evaluation results show that the FeCoP/C nanosheet exhibits an excellent acidic HER performance and an ordinary HDS performance compared to CoP/C and FeP/C catalysts. Experimental characterization and density functional theory results show that the high HER activity of the FeCoP/C nanosheet can be attributed to the fact that the nanosheet structure facilitates the exposure of active sites and the doped Fe atoms make CoP have a suitable hydrogen adsorption energy, while the ordinary HDS activity of the FeCoP/C nanosheet can be attributed to the fact that the doped Fe atoms can inhibit the cleavage of C–S bonds by CoP.     

Electronic structure and photocatalytic properties of copper-doped CaTiO3

Perovskite oxides, CaTi_1−xCu_xO₃, with the density of copper ions, x, ranging from 0.01 to 0.04, were prepared by sol-gel method coupled with ultrasonic technique for the first time. The determination by X-ray diffraction pattern of crystal structure and UV–visible light adsorption studied by ultraviolet-visible absorption spectroscopy (UV–vis) were reported. Electronic structures were investigated by density functional theory (DFT). It has been found that CaTiO₃-doped with 2 mol% Cu²⁺ exhibits the highest activity to the photocatalytic decomposition of water. Photocatalytic activity of doped CaTiO₃ powder for hydrogen evolution under UV light is increased dramatically about 8 times than that of pure CaTiO₃ powder when the NiO_x is used for cocatalyst. The results of DFT calculation illuminate that absorption of visible light is mainly due to the transition from the donor levels formed by Cu²⁺ to the conduction band of copper-doped CaTiO₃.