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.     

Hydrogen evolution using CdWO4 modified by BiFeO3 in the presence of potassium iodide; a combination of photocatalytic and non-photocatalytic water splitting

Novel heterogeneous structure of BiFeO₃–CdWO₄ with different molar ratios was applied for the photocatalytic hydrogen evolution in a self-designed externally UV/visible irradiated photoreactor in the presence of potassium iodide. The photocatalysts were synthesized by simple hydrothermal method and characterized by XRD, FE-SEM-mapping, TEM, UV–Vis DRS, PL, EIS, transient photocurrent and Mott-schottky techniques to identify the structural, optical and photoelectrochemical properties. The slope of Mott-schottky plots confirmed the p-type and n-type conductivity of the synthesized BiFeO₃ and CdWO₄, respectively. The p-n heterojunctions exhibited more efficiently light absorption, charge separation and electron mobility relative to the pure photocatalysts. We observed that coupling 40 mol% BiFeO₃ with CdWO₄ provided the best photocatalytic performance of hydrogen evolution, 268.90 μmol h⁻¹.g_cat⁻¹ from distilled water and 379.43 μmol h⁻¹.g_cat⁻¹ from 0.05 M KI aqueous solution. Iodine species increased H₂ evolution efficiency because of taking part in the charge transfer processes, either by scavenging excited holes or by direct reduction of H⁺ to H^• under UV irradiation. Fermi level equilibrium in the p-n heterojunction suggests the best interparticle charge transfer mechanism explaining how photoinduced electrons with superior energy states and desirable lifetime can be supplied to reduce H⁺ to H^•.     

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.     

Enhanced visible light photocatalytic hydrogen evolution of Ta2O5 nanohoneycomb induced by plasmonic Au nanoparticle array

Polystyrene nanospheres in the sizes of 400, 200, and 100 nm were employed as a template to form hexagonal close-packed Ta₂O₅ nanohoneycombs (nHCs) by means of solution-based nanosphere lithography. Moreover, Au nanoparticle arrays with various diameters were further deposited on the Ta₂O₅ nHCs to achieve surface plasmon resonance (SPR) and enhance visible-light photocatalytic hydrogen evolution. Finite difference time domain (FDTD) simulation was performed and the result showed that the 100-nm Ta₂O₅ nHCs coupled with smaller Au nanoparticles exhibited more effective localized SPR effect to enclose the Ta₂O₅ photocatalyst. The photocatalytic hydrogen evolution results were consistent with those of FDTD simulation and photoelectrochemical tests. A concept of effective yield ratio is proposed to explain how the Au nanoparticle size and spacing affect the hydrogen evolution efficiency.     

Promoted photocatalytic hydrogen evolution via double-electron migration in Ag@g-C3N4 heterojunction

The unsaturated edge Ag introduced on the surface of photocatalysts plays an important role in boosting photo-excited electrons for the photoreduction H₂O reaction. However, a moderate and tractable strategy to efficiently expose edge Ag remains an enormous challenge. For this purpose, a core skeleton with ‘Ag conductive nanosphere array’ inside and a two-dimensional sheet structure with g-C₃N₄ layer outside are formed. The introduction of Ag nanospheres into g-C₃N₄ not only enlarges the distance between g-C₃N₄ nanolayers, but also increases the exposure of Ag nanospheres. When Ag intimately contacts with g-C₃N₄, the electrons of g-C₃N₄ voluntarily flow to the Ag. Consequently, a built-in electric field (IEF) has been formed at interface of Ag@g-C₃N₄ heterojunction, which prevents the continuous flow of electrons from g-C₃N₄ to Ag. Under irradiation, the e⁻ accumulated in Ag tend to recombine with the h⁺ in the valence band of g-C₃N₄ which is driven by Coulomb interaction and IEF. Ag nanospheres are fabricated as a co-catalyst to decorate the g-C₃N₄ nanolayer, which hinder the conglomeration of g-C₃N₄ nanolayers. Moreover, Ag@g-C₃N₄ heterojunction provides polyunsaturated edge Ag as active sites, inducing prolonged lifetime of photogenerated electrons and formed the unique charge transfer channels. In addition, abundant nitrogen vacancies are formed, which strengthens the chemisorption of H₂O. As a result, supreme Ag@g-C₃N₄ realizes a high H₂ evolution of 312.5 μmol and preserves a good sustainability. This paper emphasizes the importance of unique electron transfer pathway and chemisorption of water for photoreduction H₂O.     

Single-atom catalysts for photocatalytic hydrogen evolution: A review

The application of hydrogen energy is significant to meet the challenge of fossil energy depletion and carbon emission limitation. The photocatalytic hydrogen evolution has been considered as a green and clean strategy for obtaining hydrogen. However, due to the high cost and limited efficiency, photocatalysis is only considered as one of the candidate methods for hydrogen production. Recently, several researchers have devoted to develop the single-atom catalysts (SACs) as promising cocatalysts and found great potential applications that are distributed across the fields of energy and environmental science. SACs exhibit several advantages, including abundant active sites, efficient photo-generated carrier recombination, atom-economic reaction mechanism, etc. In this synthetic review, we have summarized the advances of SACs in photocatalytic hydrogen evolution. Firstly, the synthesis strategies and characterization methods of SACs have been introduced. Then, the approaches for immobilizing prepared SACs on various supports have been elucidated. Finally, the photocatalytic activity of representative SACs-loaded supports has been analyzed, as well as the modification routes for enhancing performance. The review aims to record the development and applications of SACs in the field of photocatalytic hydrogen evolution. More studies are still required to clarify the mechanism of SACs based photocatalytic reactions, thus guiding the design of SACs-support system.     

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.     

Design and synthesis of Ti-peroxo/phosphorus heterostructures for enhanced photocatalytic hydrogen evolution

For the heterojunction composite photocatalyst, the contact interface is the key to charge carrier separation conditions. In order to present novel research through the interaction between these interfaces, the Ti based peroxo complex (TP)/red phosphorus (RP) composite system was introduced and designed to improve carrier separation and transport properties during photocatalytic hydrogen evolution. In this study, we have successfully synthesized TP/RP by facile solution process through stirring at room temperature and pressure. Regarding the specific surface area, which is one of the important factors in the photocatalytic activity, it was confirmed that the specific surface area of the TP (166.4 m²/g) and TP/RP (281.4 m²/g) samples was dramatically improved as the particle surface was oxidized based on TiH₂ (0.613 m²/g), the precursor. And the photo-induced charge carrier life time of the TP/RP was extended by approximately 60% compared to the conventional TP. Finally, excellent research results were obtained in which the photocatalytic hydrogen evolution efficiency (17.05 μmol/h) under visible-light irradiation (200 W Xenon lamp) was improved by about 3 times than that of the conventional TP sample (5.26 μmol/h).     

Facile preparation and enhanced photocatalytic hydrogen evolution of cation-exchanged zeolite LTA supported TiO2 photocatalysts

Photocatalytic hydrogen evolution (PHE) is an attractive green way to produce clean energy. Zeolite-supported titanium dioxide (TiO₂) offers a cost-efficient pathway to prepare an efficient and recyclable photocatalyst for PHE. As known, zeolite has an excellent cation exchange capacity, and the exchangeable cations show considerable influence on its properties. However, the effect of exchangeable cations within zeolite-supported TiO₂ photocatalysts on their PHE activity remains unclear. Here, we prepared a series of zeolite LTA-supported TiO₂ photocatalysts (TXA, X = Na, K, Ca) with various cations (Na⁺, K⁺, Ca²⁺) inside the zeolite and investigated the effect of cations on their PHE activity. The results demonstrated that all TXA photocatalysts exhibit significantly enhanced PHE activity, and TCaA shows the highest hydrogen evolution rate, which is 3.84 times higher than that of P25. According to experimental results, TCaA possesses a higher solid-acid concentration and a larger surface area, which provides more proton sources and active sites for PHE, facilitates charge separation, and reduces photogenerated electron-hole pair recombination. We believe this work will offer a new exploration strategy for regulating the PHE performance of zeolite-supported TiO₂-based photocatalysts and shed new light on the rational design of cost-efficient PHE catalysts.     

CuS nanosheet-induced local hot spots on g-C3N4 boost photocatalytic hydrogen evolution

An ideal model composite made of CuS nanosheet (photothermal material) and g-C₃N₄ was constructed through in-situ assembly procedure, along with integration of dual photochemical effect and photothermal effect. Consequently, optimized CN/CuS composite approaches remarkable photocatalytic performance improved by 44.5 times with regarding to that of pristine CN. This superiority can be assignable to inherent characteristic of CuS as photochemical component, with improved charge separation and enriched surface active-sites. Additionally, the critical contribution of photothermal effect in boosting water photosplitting was also experimentally validated. CuS nanosheet as hot spots enables rapid temperature increment around photocatalysts under visible/near-infrared (NIR) light irradiation. The heat converted from solar is conducive to increase carrier density, accelerate carrier mobility, alleviate onset potential and facilitate surface redox kinetics, so as to promote photocatalytic activity. It is believed that synergetic incorporation of photothermal and photochemical conversion could be expanded to other photocatalytic systems towards effective solar energy conversion.     

Decoration of nicale phosphide nanoparticles on CdS nanorods for enhanced photocatalytic hydrogen evolution

Manipulation of the co-catalyst plays an important role in charge separation and reactant activation to enhance the activity of CdS based photocatalysts. Transition-metal phosphides have aroused widespread interest in catalysis owing to their special structure and catalytic behavior. Herein, Ni₂P as a cocatalyst coupled with CdS for efficient photocatalytic hydrogen evolution with a rate of 483.25 mmol g^?1.h^?1, which was nearly 525 and 1.92 times higher than that of CdS (0.92 mmol g^?1.h^?1) and 1 wt% noble metal Pt modified CdS (251.29 mmol g^?1.h^?1), respectively. Its apparent quantum yield reaches 70% at 420 nm. Based on data analysis, Schottky heterostructure was constructed by combining Ni₂P with CdS. The Schottky junction provides a convenient way for photoinduced electrons to transfer and promotes the effective separation of photoinduced carriers.     

Self-assembled CdS@BN core-shell photocatalysts for efficient visible-light-driven photocatalytic hydrogen evolution

CdS@BN NRs core-shell photocatalysts for hydrogen evolution were synthesized by a solvothermal and chemical adsorption method. CdS NRs coated by 5 wt% boron nitride (BN) shell exhibited remarkably visible-light photocatalytic hydrogen evolution activity of up to 30.68 mmol g⁻¹ h⁻¹, nearly 6.79 times higher than that of pure CdS NRs, and the apparent quantum efficiency at 420 nm was 7.5%. Transmission electron microscopy showed the CdS NRs were coated with a thin (~5 nm) BN layer, which together with the hydrogen evolution results proved the photocatalytic ability of CdS NRs was significantly improved. The hydrogen evolution rate of CdS NRs coated by 5 wt% BN remained at 91.4% after four cycles, indicating the photocorrosion of CdS NRs was effectively alleviated. Moreover, the large and close coaxial interfacial contact between the CdS core and the BN shell was beneficial to the separation and transfer of photogenerated electron-hole pairs.     

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.     

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.     

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.     

Optimal preparation of molybdenum phosphide cocatalyst for efficient dye-sensitized photocatalytic hydrogen evolution

Searching for non-noble-metal cocatalyst for hydrogen evolution in photocatalytic water–splitting has attracted much attention. Herein, molybdenum phosphide (MoP) as an efficient and stable cocatalyst was prepared in a facile phosphorization process at relatively low temperatures under N₂ atmosphere, and the effect of preparation temperature (300–500 °C) was studied. Using Eosin Y (EY) and the prepared sample as catalyst (sensitizer) and cocatalyst, respectively, the photocatalytic activity for hydrogen evolution was investigated in aqueous trimethylamine (TMA) solution under visible light irradiation (λ ≥ 420 nm). MoP prepared at 400 °C (MoP-400) exhibits the highest sensitization activity and superior stability, and the maximal apparent quantum yield (AQY) for hydrogen evolution is up to 48.0% at 420 nm, much higher than the most reported data for MoP-based photocatalysts. The highest activity can be attributed to the highest P content in MoP-400 and the rapidest electron transfer between photoexcited EY and MoP-400. The possible mechanism was discussed.     

Facial synthesis of dandelion-like g-C3N4/Ag with high performance of photocatalytic hydrogen production

Nowadays, energy shortage is one of the major problems in the world. Photocatalytic hydrogen production is a new type of energy technology with good application prospect. As a new type of photocatalytic semiconductor material, g-C₃N₄ has attracted much attention as a photocatalyst. By ultrasonic treatment of a mixed solution of g-C₃N₄ and bovine serum albumin, followed by adding a certain amount of silver nitrate solution and then directly hydrothermal treatment, a special dandelion-like g-C₃N₄/Ag (D-g-C₃N₄/Ag) was prepared. The scanning electronic microscopy, transmission electronic microscopy, X-ray diffraction, Fourier transform infrared, fluorescence and physicochemical adsorption methods were used to characterize the morphology and structure of D-g-C₃N₄/Ag. In addition, the photocatalytic H₂ production of D-g-C₃N₄/Ag with different Ag loadings or in different sacrificial agents and different pH conditions were investigated. The results indicated that when triethanolamine was used as sacrificial agent, photocatalytic hydrogen efficiency was the best, and the rate of photocatalytic hydrogen production reached 862 μmol g⁻¹ h^{− 1} as the Ag loading was 4%.     

A hydrothermal route for constructing reduced graphene oxide/TiO2 nanocomposites: Enhanced photocatalytic activity for hydrogen evolution

A series of reduced graphene oxide/TiO₂ (RGO/TiO₂) nanowire microsphere composites were synthesized with a facile one-step hydrothermal method using TiCl₃ and graphene oxide (GO) as the starting materials, during which the formation of TiO₂ and the reduction of GO occur simultaneously. The obtained nanocomposites were characterized with X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, respectively. UV–vis absorption spectra showed that the absorption edges of TiO₂ were extended into visible light region with the addition of RGO. The photocatalytic activities of the samples with and without Pt as cocatalysts were evaluated by hydrogen evolution from water photo-splitting under UV–vis light illumination. Enhanced photocatalytic properties were observed for the as-prepared RGO/TiO₂ nanocomposites. The amount of hydrogen evolution from the optimized photocatalyst reached to 43.8 μmol h⁻¹, which was about 1.6 times as high as that of bare TiO₂. The results shown here indicate a convenient and applicable approach to further exploitation of high activity materials for photocatalytic water splitting applications.     

Interfacial optimization of CeO2 nanoparticles loaded two-dimensional graphite carbon nitride toward synergistic enhancement of visible-light-driven photoelectric and photocatalytic hydrogen evolution

A novel series of CeO₂ nanoparticles (CeNP) loaded two-dimensional (2D) graphite carbon nitride nanosheet (CeNP/g-C₃N₄) composites which possess heterojunction are prepared with different proportions of CeNP by a reliable and straightforward method. The samples were employed to degrade the rhodamine B (RhB) and produce hydrogen. The microstructure, morphology, composition and surface chemical states of the samples are analyzed, and the ability of the photoelectric response is characterized. The characterization ensures uniform loading of CeNP over the g-C₃N₄ surface and a co-existence of Ce³⁺/Ce⁴⁺ in the CeNP/g-C₃N₄ composite. The FT-IR spectra have revealed the changes in the local dipole moment of amino groups, vibration mode of the constituent functional group and electronegativity, indicating the electric interaction between CeNP and g-C₃N₄. The photocatalytic hydrogen evolution efficiency of the CeNP/g-C₃N₄ increased initially and then decreased with the increasing of loaded CeNP. The CeNP/g–C₃N₄–C with 20 mg of CeNP was found to be the optimum proportion, which exhibited outstanding separation efficiency of the photo-generated carriers. The electron spin-resonance (ESR) spectra exhibit that the production of superoxide free radicals (?O₂^?) was much higher than that of hydroxyl free radicals (?OH) indicating that ?O₂^? species play a predominant role in the photocatalytic action. The mechanism for enhanced photocatalytic activity of the CeNP/g-C₃N₄ is attributed to the interfacial optimization, which improved the photo-generated carrier separation.     

Enhanced photocatalytic activity towards H2 evolution over NiO via phosphonic acid surface modification with different functional groups

Enhancing the photocatalytic activity of NiO towards hydrogen generation (6 times) is achieved by phenyl phosphoric acid (PPA) surface modification. The photocatalytic activity can be further improved by changing PPA to 4-aminophenyl phosphonic acid (PPA-NH₂), about 10 times, suggesting the functional group plays an important role in improving the activity. A chemical bond (NiOP) between NiO and organic phosphoric acid was proved by FT-IR spectra. XPS and LSV analysis suggested organic phosphonic acid modification increased the electron density of Ni, which promoted the transformation from Ni²⁺ to Ni⁰. Mott-Schottky (M − S) measurement revealed that PPA-NH₂ modified NiO processed a larger the space charge layer thickness (d_sc), which enhanced ability towards H₂ evolution from dynamical aspect.