NiS modified CdS pyramids with stacking fault structures: Highly efficient and stable photocatalysts for hydrogen production from water

NiS modified three-dimensional pyramidic CdS with stacking fault structures were successfully synthesized by using the one-step method and an ammonia aqueous solvent as the hydrothermal solvent. Thus, CdS showed superior photocatalytic activities for hydrogen evolution from water under visible light irradiation (λ ≥ 420 nm), which could achieve a hydrogen evolution rate of 49.2 mmol g^?1 h^?1, with an extremely high apparent quantum yield (AQY = 74.6%) at 420 nm. To our knowledge, this value is the highest reported efficiency value for NiS_x modified CdS photocatalysts. CdS exhibited a three-dimensional pyramid structure with large specific surface areas, which may provide more active sites for the photocatalytic reaction. Stacking fault structures were observed in CdS by transmission electron microscopy (TEM). P-type NiS nanoparticles were highly dispersed on the surface of n-type CdS pyramids, forming p-n junctions at the interface. The stacking fault structures and junctions strengthened the separation of photo-carriers near the interface, which may greatly enhance the activity of photocatalytic hydrogen production for CdS. The catalyst also showed perfect stability, and the photoactivity showed no significant degradation during continuous hydrogen production over nearly 120 h, which has not been reported in the literature.     

Self-sacrificing template synthesis of CdS quantum dots/Cd-Hap composite photocatalysts for excellent H2 production under visible light

Well dispersive CdS quantum dots (QDs) were successfully in-situ grown on cadmium hydroxyapatite (Cd₅(PO₄)₃OH, Cd-Hap) assembled rods through a self-sacrificing hydrothermal method. No any nocuous organic ligands were used in such self-sacrificing route, allowing for a green approach to prepare CdS QDs with clean surfaces and enough active sites. The deposition of CdS QDs onto Cd-Hap surfaces led to a dramatically enhanced performance in H₂ production under visible light irradiation as compared to bulk CdS nanoparticles. The optimal CdS QDs/Cd-Hap composite displayed a H₂ evolution rate of 14.1 μmol h^?1 without using any noble metal cocatalyst, which was about 4.2 times higher than that of pristine CdS. The apparent quantum efficiency for CdS QDs/Cd-Hap composite was up to 18%. It was also found that CdS QDs/Cd-Hap composite can continuously generate H₂ from water in the presence of electron donors for more than 125 h. The enhanced photocatalytic performance of CdS QDs/Cd-Hap composites could be attributed to the high charge separation efficiency resulting from the efficient capture of photoinduced electrons by oxygen vacancies in Cd-Hap rods and the quantum confinement effect of CdS QDs with strong redox capacity as well as the increased active sites.     

The first catalytic hydrolysis of ethylenediamine bisborane with hydrogel-supported metallic nanoparticles

Ethylene diamine bisborane (EDB) was synthesized in a single step as the hydrogen storage material. The synthesized compound was firstly used in the literature for the production of hydrogen gas by catalytic hydrolysis reaction. Cu, Co and Ni nanoparticles with average sizes of 75–150 nm formed in p(acrylicacid-co-vinylimidazole) hydrogel network structures were used as catalysts for the hydrolysis reaction. The effect of the parameters such as catalyst type, EDB concentration, catalyst concentration, temperature and solvent environment on the catalytic hydrolysis reaction of EDB was investigated. In the activity tests for the catalyst, it was determined that the catalyst had a loss of only 15% in activity even at the end of 5 cycles. The activation energies of hydrolysis reaction were calculated as 39.42 kJmol^–1, 44.77 kJmol^–1 and 47.48 kJmol^–1 for Cu, Co and Ni hydrogel composite catalyst, respectively.     

Photocatalytic hydrogen generation with simultaneous organic degradation by composite CdS–ZnS nanoparticles under visible light

A visible light-driven CdS–ZnS photocatalyst in the form of nanoparticles with a heterogeneous structure was synthesized using the stepped microemulsion method. The composite CdS–ZnS was capable of simultaneous photocatalytic hydrogen production and organic degradation under visible light. The ZnS deposition on CdS helped to suppress the recombination of electron/hole pairs generated on the more reactive CdS, leading to faster hydrogen production and improved stability of the CdS–ZnS in comparison to the bare CdS catalyst. Deposition of Ru on the catalyst surface further increased its photo-reactivity by about 4 times for hydrogen production. The heterostructured nanoparticles were effective in photocatalytic hydrogen production together with the degradation of model organic substances, including formic acid, methanol, and ethanol. The highest hydrogen production rate was achieved by the (CdS–ZnS)/Ru catalyst at 266 mmol/m²-h in the formic acid solution with an energy conversion efficiency of 3.05% in visible light, and the corresponding organic degradation rate in terms of the removal of chemical oxygen demand (COD) was estimated at 4272 mg COD/m²-h.     

Greatly enhanced photocatalytic hydrogen production on CdS/(Pt/g-C3N4) via dual functions of Pt on semiconductors interface

CdS and g-C₃N₄ are famous semiconductors in photocatalytic hydrogen evolution, however, their low efficiencies limit their further application. Here, a highly efficient ternary catalyst CdS/(Pt/g-C₃N₄) was reported and its photocatalytic hydrogen production activity reached up to 1465.9 μmol/h/g, which is 5.3 times of Pt/CdS and 4.0 times of Pt/g-C₃N₄, respectively. TEM and HRTEM images demonstrate the Pt nanoparticles exists on the interface of between CdS and g-C₃N₄ acting as a cocatalyst for hydrogen evolution. SPV spectra and electrochemical tests demonstrate that Pt as bridge between CdS and g-C₃N₄ also accelerates the electrons transforming which benefits for the inhibition of the recombination of photoexcited electrons and holes. This study demonstrated the dual roles of interface Pt and provides a new method to design a highly efficient photocatalyst.     

Increased photocatalytic hydrogen evolution and stability over nano-sheet g-C3N4 hybridized CdS core@shell structure

A novel visible-light-active CdS@g-C₃N₄ photocatalyst was synthesized via a chemisorption method. This core@shell structure catalyst exhibited enhanced photocatalytic H₂ production activity under visible-light (λ ≥ 420 nm) irradiation. The nano-sheet g-C₃N₄ was successfully coated on CdS nanoparticles with intimate contact. When the content of g-C₃N₄ in the hybridized composite is 3 wt. %, the hydrogen-production rate of the CdS@g-C₃N₄ is 2.5 and 2.2 times faster than pure CdS and bulk g-C₃N₄, respectively. Superior stability was also observed in the cyclic runs. The improvement in stability and activity result from the ability of the π-conjugated g-C₃N₄ material in transporting photo-induced holes. The core@shell structure promoted separation of the photo-generated electron-hole pair and accelerated the emigration speed of the hole from the valence band of CdS. This effect also results in a greatly improved amount of hydrogen production. The possible mechanism for the photocatalytic activity and stability of CdS@g-C₃N₄ are tentatively proposed.     

Designing large-sized cocatalysts for fast charge separation towards highly efficient visible-light-driven hydrogen evolution

A key challenge in photocatalysts is their rapid recombination of photo-generated electron-hole pairs. To decrease the recombination probability, significant studies have been devoted to increasing potential active sites of photocatalytic systems for fast charge separation. Here we demonstrate a strategy to boost active sites that is different from conventional approaches, in which large-sized Mxene nanosheets were employed as cocatalysts, while small-sized CdS nanosheets were used as photocatalysts forming effective nanocomposites. Based on our designed strategy, the enrichment of transfer routes guides the fast separation of electrons from CdS nanosheets and transferred onto large-sized Mxene nanosheets. The visible-light-driven hydrogen production of our innovatively designed composites has been improved from 2.8 mmol/g of pure CdS nanosheets to 17.5 mmol/g of the CdS@Mxene optimized composite. The obvious improvement of hydrogen production verifies the feasibility of our design strategy as a unique way to design photocatalytic systems for fast charge separation and highly efficient photocatalytic properties.     

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.     

Self-defective ZnS mediated charge transfer for bran-new inter-step mode with boosted photoactivity and enhanced photostability

The appropriate interfacial contact and charges transfer mode of heterojunction photocatalysts were critical for high-efficiency hydrogen production. Inter-step mode heterojunction composite had advantages of enhanced visible-light response, improved charge space separation rate, increased electron utilization, which could also protect catalyst anode from photocorrosion. Zinc-vacancy-rich ZnS decorated CdS heterojunction photocatalyst with inter-step mode was constructed in order to fundamentally enhance photocatalytic performance and overcome photocorrosion of CdS. The charge transfer mode was modulated from pervasive type-II to bran-new inter-step mode by defect engineering. Zinc vacancies functioned as acceptor level for charge separation and up-shifted conduction and valance band energy of ZnS. The defective engineered CdS/ZnS heterojunction displayed a reduced over-potential and enhanced photocatalytic activity. The optimal photocatalytic hydrogen production rate for CdS/ZnS reached 42.1 mmol?g^?1 under visible light without any co-catalyst. An apparent quantum yield (AQY) of 38.75% at 450 nm was achieved, which was 269.3 and 71.9 times higher than pristine zinc-vacancy-rich ZnS and CdS, respectively. Meanwhile, holes aggregated on the surface of CdS were blocked and the oxidation corrosion process was suppressed. The charge transfer mechanism and kinetics of charge transfer and separation in inter-step mode heterojunction photocatalysts were investigated and discussed. This work will accelerate practical applications of photocatalysis with inter-step mode and give deep insights into understanding how inherent acceptor levels play a role in designing defect-engineered semiconductor with enhanced photocatalytic performance.     

Enhanced photocatalytic hydrogen production of CdS embedded in cationic hydrogel

In this study, we describe the successful fabrication of CdS in ionic hydrogels by an in situ growth method and demonstrate that the as-prepared CdS in hydrogels (CdS/HGel) can be used as cost-effective and recyclable catalysts for photocatalytic hydrogen generation. The structure and morphology of CdS/HGels were characterized by various techniques including X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, and Fourier transform infrared spectroscopy. The resultant CdS in cationic hydrogel (CdS/HGel_PDAM2) showed the best performance of photocatalytic hydrogen production and the hydrogen production rate was up to 10.35 or 7.70 mmol h⁻¹ g⁻¹ when triethanolamine or Na₂S–Na₂SO₃ was used as sacrificial agent. However, CdS in anionic hydrogel (CdS/HGel_PAAM) showed poor photocatalytic hydrogen production performance under the same conditions. The solution pH and sacrificial agent type are also indispensable factors that affect the photocatalytic hydrogen production. The enhancement of hydrogen production comes from interaction between polymer chains and Cd²⁺, high dispersibility of CaS nanoparticles in hydrogels, high hydrophilic and swelling ability of hydrogel, high diffusion rate of reactant in hydrogel, and inhibited binding possibility of photogenerated electron-hole pairs. Since CdS/HGel_PDAM has excellent hydrogen production efficiency and ease recovery property, it will be a potential photocatalyst for photocatalytic reactions.     

One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light

The development of efficient and stable noble-metal-free photocatalysts is crucial for hydrogen evolution from water splitting as clean energy. This study reports uniform CdS/NiS spherical nanoparticles through a simple one-pot hydrothermal method with the aid of KOH. The prepared CdS/NiS composites show superior photocatalytic activities toward the water splitting under visible light. A suitable amount of KOH in the synthesis benefits to form CdS/NiS photocatalysts with the improved activity. The CdS/NiS composite including 10 mol% metal percentage of Ni exhibits the highest photocatalytic activity. The high hydrogen evolution rate of 24.37 mmol h^?1 g^?1 is achieved over the CdS/NiS composite photocatalyst. The CdS/NiS photocatalyst has good photocatalytic stability in the recycling uses. The present CdS/NiS as a noble-metal-free photocatalyst provides the superior visible-light driven catalytic activity for hydrogen evolution.     

Investigation of surface interaction in rGO-CdS photocatalyst for hydrogen production: An insight from XPS studies

Formation of heterojunction in supported-photocatalyst plays an important role in photocatalysis for hydrogen production. However, study of this formation is needed to be more. Reduced graphene supported-cum-doped Cadmium sulphide (rGO-CdS) is widely employed for the production of hydrogen by photocatalysis splitting of water. The Formation of surface interaction between rGO and CdS was explained by detailed analysis by using x-ray photoelectron spectroscopy with depth profiling, Fourier-transform infrared spectroscopy and electrochemical impedance spectroscopy. The charge transfer phenomena through heterojunction are also explicated. The higher conductivity of rGO further facilitated the transmission of electrons to the interface of solid-liquid. Thus, the restriction of charge recombination and greater mobility of electron yielded superior activity to the CdS/rGO catalyst which was prepared by impregnation method followed by the H₂S gas reaction with high temperature.     

Development of high quantum efficiency CdS/ZnS core/shell structured photocatalyst for the enhanced solar hydrogen evolution

Development of co-catalyst free, core/shell structured photocatalyst with ultra-thin shell is of great importance towards the stable and continuous hydrogen (H₂) production, where the shell prevents photo-corrosion of the core for longer stability with continuous H₂ generation. Accordingly, herein, we report a one-step, surfactant free hydrothermal process for synthesis of high-efficient CdS/ZnS core/shell structured catalyst for H₂ evolution under natural solar light. The structural and morphological characterizations using XRD and TEM techniques revealed the formation of phase pure CdS/ZnS system, with core and shell thickness of 395 and 15 nm, respectively. XPS studies revealed that the constituted elements in system exist in their native oxidation states, which indicated the stable structural integrity of the individual phase in the core/shell structure. The synergistic optical properties of CdS/ZnS showed the absorption edge around 500 nm and the decreased PL intensity indicated the improved charge recombination resistance in the system. The parametric studies such as synthesis time, core diameters and shell thickness optimization were conducted to study the formation kinetics of the core/shell structure and their photocatalytic efficiencies. Accordingly, the optimized core/shell catalyst showed around 763 and 2.4 folds superior activity when compared to the pristine CdS and ZnS, respectively. Further, the catalyst showed excellent stability for over 100 h with quantum efficiency of 8.78% under the irradiation of 20 W LED light at 420 nm. Based on the obtained results, the observed improved photocatalytic quantum efficiency could be ascribed to their synergistic effects of CdS and ZnS towards increased charge separation and spatial distributions of the carriers due to their core/shell configuration of the materials.     

Hollow tubular Co9S8 grown on In2O3 to form S-scheme heterojunction for efficient and stable hydrogen evolution

In this work, an innovative Co₉S₈/In₂O₃ composite photocatalyst was prepared by an in-situ grown method. After the composite catalyst loaded with Co₉S₈ on In₂O₃ effectively solved the self-agglomeration effect of In₂O₃ prepared by the method in this work and significantly improved the efficiency of hydrogen production. The useless electrons and holes in Co₉S₈/In₂O₃ are removed more effectively than in the other catalyst, so that more highly reducible electrons are involved in the reduction reaction of hydrogen evolution, which is due to the formation of S-scheme heterojunction. The cyclic hydrogen production experiment confirmed that the stability of the composite photocatalyst is excellent, and its 5 h hydrogen production amount reached 277.77 μmol. In this work, a simple synthetic method for the synthesis of composite photocatalysts and new photocatalysts for the photocatalytic hydrogen evolution of dye-sensitized systems (Eosin Y) is presented.     

Creation of heterojunction in CdS supported on N,S-rGO for efficient charge separation for photo-reduction of water to hydrogen

An efficient CdS supported on nitrogen and sulfur-doped reduced graphene oxide (N,S-rGO) has been prepared by a high-temperature gas-solid reaction for the first time. This catalyst was about ten times more efficient compared to pristine CdS in the dissociation of water to hydrogen. The high-temperature gas-solid reaction promotes chemical interaction between CdS and reduced graphene oxide (rGO) forming a heterojunction at the interface for transfer of photogenerated electrons from CdS to rGO. Doping of rGO with N and S enhances electron mobility and charge carrier density on the surface of the catalyst. The synergy between the efficient electron transfer, the enhanced electron mobility and the high charge carrier density results in a high activity of the prepared catalyst for photo-reduction of water to hydrogen. A detailed microstructural analysis for understanding the interaction amongst preparation technique, microstructure and activity, is also presented.     

Photocatalytic system with water soluble nickel complex of S,S′-bis(2-pyridylmethyl)-1,2-thioethane over CdS nanorods for hydrogen evolution from water under visible light

In this paper, we report a new nickel complex, [(bpte)NiCl₂] (bpte = S,S′-bis(2-pyridylmethyl)-1,2-thioethane) that can serve as a catalyst both for electrochemical and photochemical driven hydrogen production from water. As an electrocatalyst, [(bpte)NiCl₂] can electrocatalyze hydrogen generation from a neutral buffer with a turnover frequency (TOF) of 555.78 mol of hydrogen per mole of catalyst per hour (mol H₂/mol catalyst/h) at an overpotential (OP) of 837.6 mV. Together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H₂A) as a sacrificial electron donor, the nickel complex also can photocatalyze hydrogen evolution in heterogeneous environments and can work for 107 h. Under an optimal condition, the photocatalytic system can afford 24900 mol of H₂ per mole of catalyst during 83 h irradiation, with a TOF of 300H₂ per catalyst per hour. The average value of apparent quantum yield (AQY) is ～24% at 420 nm.     

Facile preparation of Pt/CdS photocatalyst by a modified photoreduction method with efficient hydrogen production

A series of 0.75 wt% Pt/CdS photocatalysts were successfully synthesized via a modified photoreduction process, with the assistance of a protective agent (polyvinylpyrrolidone, PVP) and/or structural inducer (NaI, NaBr, and NaCl). The physicochemical properties of the obtained 0.75 wt% Pt/CdS photocatalysts were characterized in more detailed. Their photocatalytic efficiencies were evaluated by visible-light photocatalytic hydrogen production. The results show that the photocatalytic activities of 0.75 wt% Pt/CdS photocatalysts for H₂ production mainly depend on the type of structural inducer. Furthermore, a suitable ratio of PVP/NaI is necessary to optimize the photocatalytic performance of Pt/CdS composites. Notably, 0.75 wt% Pt/CdS (PVP/NaI = 4:1) gains the highest hydrogen production activity with a rate of 1155.8 μmol h^?1, which is 1.8 times higher than that of 0.75 wt% Pt/CdS obtained from the traditional photoreduction method (640.9 μmol h^?1) and 17.3 times higher than that of the bare CdS sample (66.9 μmol h^?1). The as-prepared 0.75 wt% Pt/CdS photocatalyst (PVP/NaI = 4:1) also exhibits a good stability. An optimum ratio of PVP/NaI not only causes a decrease in particle size of Pt nanoparticles but also leads to an increase in BET specific surface area of Pt/CdS and an enhanced electron transfer capability of Pt nanoparticles, which should be responsible for the enhanced photocatalytic performance.     

Reduced graphene oxide as an efficient support for CdS-MoS2 heterostructures for enhanced photocatalytic H2 evolution

Cadmium sulphide nanorods-reduced graphene oxide-molybdenum sulphide(CdS-rGO-MoS₂) composites were successfully synthesized using hydrothermal process for enhancing the interfacial contact between CdS nanorods and MoS₂ layer. The good contact between CdS and MoS₂ is important for improving the photocatalytic hydrogen (H₂) evolution. The morphological and structural studies showed the production of highly pure CdS phase with nanorod-like structure dispersed on rGO-MoS₂ layer. X-ray photoelectron spectroscopy (XPS) and Raman results confirmed the reduction of graphene oxide (GO) into reduced graphene oxide (rGO). The higher photocurrent density of CdS-rGO-MoS₂ composites compared to CdS/MoS₂ and the fluorescence quenching observed for this composite provided some evidence for an inhibition of electron-hole recombination, which leads to a longer life time of the photogenerated carriers. Fast electron transfer can occur from CdS nanorods by the bidimensionnel rGO area to MoS₂ layer due to the intimate interfacial contact. Composite CdS-rGO-MoS₂ with 20 wt% rGO was found to be the most effective photocatalyst for H₂ evolution (7.1 mmol h^?1g^?1). The good photocatalytic performance arose from the positive synergistic effect between CdS, rGO and MoS₂ elements.     

Preparation,characterization and photocatalytic activity of novel CeO2 loaded porous alkali-activated steel slag-based binding material

In the present study, CeO₂ loaded porous alkali-activated steel slag-based photocatalyst (CeO₂-PASSP) as a new catalyst for photocatalytic water-splitting of hydrogen production was prepared via impregnation method. The BET result showed that adding pore-forming agent changed the pore structure of the alkali-activated steel slag-based binding material and the mesoporous volume of photocatalyst carrier increased by 70%. The XRD, TEM and HRTEM results indicated that calcium silicate hydrate was mainly mineral phase in the alkali-activated steel slag-based binding material. CeO₂ nanoparticles with particle size about 10 nm were highly dispersed on the surface of photocatalyst carrier. The photocatalyst loaded 8 wt% CeO₂ showed the weakest photoluminescence intensity. 8CeO₂-PASSP sample exhibited the highest photocatalytic hydrogen production activity (68.64 mmol/g) and hydrogen generation rate (13.73 mmol/(g?h)) in the simulated solar light irradiation for 5 h, and was quite stable after exposure to photocatalytic hydrogen production for a long time. The excellent activity of hydrogen production for 8CeO₂-PASSP specimen was ascribed to the co-action of the high S_BET, large mesoporous volume and the active components of the CeO₂ and FeO existed in photocatalyst carrier.     

Cu-MOF assisted synthesis of CuS/CdS(H)/CdS(C): Enhanced photocatalytic hydrogen production under visible light

CuS/CdS(H)/CdS(C) photocatalysts were synthesized via the hydrothermal method by employing thiourea, Cd(CH₃COO)₂·3H₂O and copper 1,4-benzenedicarboxylate MOF (CuBDC). The photocatalysts were characterized by XRD, XPS, BET, TEM and UV–vis diffuse reflectance spectra. Interestingly, hexagonal CdS (CdS(H)) and cubic CdS (CdS(C)) were formed with phase junctions in one step when CuBDC was introduced in the synthesis process, in addition, CuS nanoparticles were deposited on CdS. However, only hexagonal CdS was obtained without CuBDC. It demonstrated that CuBDC was not only the precursor of CuS but also the structural modifier for CdS. With the reduction of re-combination of photo-induced electrons and holes caused by phase junctions and the enhancement of visible-light absorptions due to the loading of CuS, all CuS/CdS(H)/CdS(C) photocatalysts had higher photocurrent densities under visible-light irradiation, and consequently the higher rates of H₂ production than pure CdS(H). Typically, the catalyst with 2.89 wt% of Cu showed a highest rate of H₂ evolution at 2042 μmol/g/h.