Engineering Zn1-xCdxS/CdS Heterostructures with Enhanced Photocatalytic Activity

The Zn Vacancy-Mediated De-Accumulation Based Process for Hydrogen Production Performance Promotion of 1D Zn─Cd─S Nanorods

Due to their anisotropy, 1D semiconductor nanorod-based materials have attracted much attention in the process of hydrogen production by solar energy. Nevertheless, the rational design of 1D heterojunction materials and the modulation of photo-generated electron-hole transfer paths remain a challenge. Herein, a ZnxCd1-xS@ZnS/MoS2 core-shell nanorod heterojunction is precisely constructed via in situ growth of discontinuous ZnS shell and MoS2 NCs on the Zn─Cd─S nanorods. Among them, the Zn vacancy in the ZnS shell builds the defect level, and the nanoroelded MoS2 builds the electron transport site. The optimized photocatalyst shows significant photocatalytic activity without Platinum as an auxiliary catalyst, mainly due to the new interfacial charge transfer channel constructed by the shell vacancy level, the vertical separation and the de-accumulation process of photo-generated electrons and photo-generated holes. At the same time, spectral analysis, and density functional theory (DFT) calculations fully prove that shortening difference of speed between the photogenerated electron and hole movement process is another key factor to enhance the photocatalytic performance. This study provides a new path for the kinetic design of enhanced carrier density by shortening the carrier retention time of 1D heterojunction photocatalysts with improved photocatalytic performance.

Improving photocatalytic hydrogen production by switching charge kinetics from type-I to Z-scheme via defective engineering

By providing the spatial separation of the active sites and retaining high oxidative and reducing capacity, the direct Z-scheme heterostructure is considered the most potential structure for yielding photo-electric response. However, challenges still exist in the directional transfer of charge carriers between two semiconductors in direct Z-scheme structures. In this regard, by constructing the Vzn defect and p-n junction, a direct Z-scheme ZnxCd1-xS@ZnS-NiS heterostructure was obtained for the regulated electronic structure, which ensured high-yield hydrogen properties. The Zn vacancy in the partially-coated ZnS shell led to the Vzn energy level, and the addition of NiS led to the p-n structure, which caused a drastic downshift of the band edge potentials in comparison to that of pristine CdS. This variation gave rise to a staggered band edge alignment between ZnxCd1-xS and NiS, resulting in the variation of charge transfer kinetics from type-I to direct Z-scheme. Through careful characterization, it was found that the optimal photocatalytic hydrogen precipitation activity reached 16 683.6 μmol g-1 h-1, which was 70 times that of CdS, and this improvement was considered to form a spatial barrier, providing a clear direction and path for carrier transmission.

Graphitic Carbon Nitride Nanosheets Decorated with Zinc-Cadmium Sulfide for Type-II Heterojunctions for Photocatalytic Hydrogen Production

In this study, we fabricated graphitic carbon nitride (g-C3N4) nanosheets with embedded ZnCdS nanoparticles to form a type II heterojunction using a facile synthesis approach, and we used them for photocatalytic H2 production. The morphologies, chemical structure, and optical properties of the obtained g-C3N4-ZnCdS samples were characterized by a battery of techniques, such as TEM, XRD, XPS, and UV-Vis DRS. The as-synthesized g-C3N4-ZnCdS photocatalyst exhibited the highest hydrogen production rate of 108.9 μmol·g-1·h-1 compared to the individual components (g-C3N4: 13.5 μmol·g-1·h-1, ZnCdS: 45.3 μmol·g-1·h-1). The improvement of its photocatalytic activity can mainly be attributed to the heterojunction formation and resulting synergistic effect, which provided more channels for charge carrier migration and reduced the recombination of photogenerated electrons and holes. Meanwhile, the g-C3N4-ZnCdS heterojunction catalyst also showed a higher stability over a number of repeated cycles. Our work provides insight into using g-C3N4 and metal sulfide in combination so as to develop low-cost, efficient, visible-light-active hydrogen production photocatalysts.

In Situ Growth Facilitating the Piezo-Photocatalytic Effect of Zn1-xCdxS/ZnO Nanorods for Highly Efficient H2 Production

Photocatalytic H₂ production holds promise for alleviating energy and environmental issues. The separation of photoinduced charge carriers plays vital roles in enhancing the activity of photocatalytic H₂ production. The piezoelectric effect has been proposed to be effective in facilitating the separation of charge carriers. However, the piezoelectric effect is usually restricted by the noncompact contact between the polarized materials and semiconductors. In this study, Zn_1-xCd_xS/ZnO nanorod arrays on stainless steel for piezo-photocatalytic H₂ production are fabricated by an in situ growth method, achieving an electronic-level contact between Zn_1-xCd_xS and ZnO. The separation and migration of photogenerated charge carriers in Zn_1-xCd_xS are significantly improved by the piezoelectric effect induced by ZnO under mechanical vibration. Consequently, under solar and ultrasonic irradiation, the H₂ production rate of Zn_1-xCd_xS/ZnO nanorod arrays achieves 20.96 μmol h^-1 cm^-2, which is 4 times higher than that under solar irradiation. Such a performance can be attributed to the synergies of the piezoelectric field of bent ZnO nanorods and the built-in electric field of the Zn_1-xCd_xS/ZnO heterostructure, which efficiently separate the photoinduced charge carriers. This study provides a new strategy to couple polarized materials and semiconductors for highly efficient piezo-photocatalytic H₂ production.

1D/1D W18O49/Cd0.9Zn0.1S S-scheme heterojunction with spatial charge separation for high-yield photocatalytic H2 evolution

Semiconductor photocatalytic water splitting is a green way to convert solar energy into chemical energy, but the recombination of electron and hole pairs and the low utilization of sunlight restrict the development of photocatalytic technology. By comparing the morphologies and hydrogen production properties of different proportions of solid solutions (Cd_xZn_1-xS), one-dimensional (1D) Cd_0.9Zn_0.1S nanorods (NRs) with the best photocatalytic properties are obtained. In addition, 1D W₁₈O₄₉ nanowires are assembled on the surface of 1D Cd_0.9Zn_0.1S NRs to construct a novel 1D/1D step-scheme (S-scheme) W₁₈O₄₉/Cd_0.9Zn_0.1S heterojunction photocatalyst. The W₁₈O₄₉/Cd_0.9Zn_0.1S heterojunction expands the optical absorption capacity of Cd_0.9Zn_0.1S NRs to provide more energy for the photoexcitation of electrons. The optimal hydrogen production rate of W₁₈O₄₉/Cd_0.9Zn_0.1S NRs with W₁₈O₄₉ content of 9 wt% is as high as 66.3 mmol·h^-1·g^-1, which is 5.7 times and 1.6 times higher than that of Cd_0.9Zn_0.1S NRs and 1 wt% Pt/Cd_0.9Zn_0.1S NRs. The apparent quantum efficiency (AQE) of 9 wt% W₁₈O₄₉/Cd_0.9Zn_0.1S reaches 56.0 % and 25.9 % under light wavelength irradiation at 370 and 456 nm, respectively. After the 20 h cycle stability test, the activity of photocatalytic hydrogen evolution does not decrease, due that the severe photo-corrosion of Cd_0.9Zn_0.1S NRs is efficiently inhibited. This work not only provides a simple and controllable synthesis method for the preparation of heterojunction structure, but also opens up a new way to improve the hydrogen evolution activity and stability of sulfur compounds.

ZIF-8 metal organic framework composites as hydrogen evolution reaction photocatalyst: A review of the current state

In the past decade, extensive research has been devoted to synthesis of ZIF-8 materials for catalytic applications. As physico-chemical properties are synthesis-dependent, this review explores different synthesis strategies based the solvent and solvent-free synthesis of zeolitic imidazole framework. Accordingly, the effect of several parameters on the ZIF-8 synthesis were discussed including solvent, deprotonating agents, precursors ratio is delivered. Additionally, the advantages and disadvantages of each synthesis have been discussed and assessed. ZIF-8 textural and structural properties justify its wide use as a stable high surface area MOF in aqueous catalytic reactions. This review includes the applicatios of ZIF-8 materials in photocatalytic hydrogen evolution reaction (HER). The efficiency of the reviewed materials was fairly assessed. Finally, Limitations, drawbacks and future challenges were fully debated to ensure the industrial viability of the ZIFs.

Graphene Quantum Dots Improved "Caterpillar"-like TiO2 for Highly Efficient Photocatalytic Hydrogen Production

Photocatalytic water splitting for hydrogen production via heterojunction provides a convenient approach to solve the world crises of energy supply. Herein, graphene quantum dots modified TiO₂ hybrids (TiO₂-GQDs) with a “caterpillar”-like structure exhibit stronger light absorption in the visible region and an enhanced hydrogen production capacity of about 3.5-fold compared to the pristine TiO₂ caterpillar. These results inferred that the addition of GQDs drastically promotes the interfacial electron transfer from GQDs to TiO₂ through C–O–Ti bonds via the bonding between oxygen vacancy sites in TiO₂ and in-plane oxygen functional groups in GQDs. Using a “caterpillar”-like structure are expected to provide a new platform for the development of highly efficient solar-driven water splitting systems based on nanocomposite photocatalyst.

Hot injection-induced synthesis of ZnCdS-rGO/MoS2 heterostructures for efficient hydrogen production and CO2 photoreduction

A highly efficient hybrid ZnCdS-rGO/MoS₂ heterostructure is successfully synthesized through a hot injection method and control loading of rGO/MoS₂. The synergism provides an unprecedently high H₂-generation rate of 193.4 mmol H₂ g^-1 h^-1 from water under full arc solar radiation and MeOH production (5.26 mmol g^-1 h^-1, AQY of 14.6% at λ = 420 ± 20 nm) from CO₂ reduction.

Metal chalcogenide-based core/shell photocatalysts for solar hydrogen production: Recent advances, properties and technology challenges

Metal chalcogenides play a vital role in the conversion of solar energy into hydrogen fuel. Hydrogen fuel technology can possibly tackle the future energy crises by replacing carbon fuels such as petroleum, diesel and kerosene, owning to zero emission carbon-free gas and eco-friendliness. Metal chalcogenides are classified into narrow band gap (CdS, Cu₂S, Bi₂S_3, MoS_2, CdSe and MoSe₂) materials and wide band gap materials (ZnS, ZnSe and ZnTe). Composites of these materials are fabricated with different architectures in which core-shell is one of the unique composites that drastically improve the photo-excitons separation, where chalcogenides in the core can be well protected for sustainable uses. Thus,the core-shell structures promote the design and fabrication of composites with the required characteristics. Interestingly, the metal chalcogenides as a core-shell photocatalyst can be classified into type-I, reverse type-I, type-II and S-type nanocomposites, which can effectively influence and significantly enhance the rate of hydrogen production. In this direction, this review is undertaken to provide a comprehensive overview of the advanced preparation processes, properties of metal chalcogenides, and in particular, photocatalytic performance of the metal chalcogenides as a core-shell photocatalysts for solar hydrogen production.

A facile approach to synthesize CdS-attapulgite as a photocatalyst for reduction reactions in water

At room temperature, a facile approach has been utilized for preparing novel CdS–attapulgite (CdS–ATP) composites and the composites were applied in photocatalytic reduction of p-nitrophenol and Cr(vi). The effect of ATP on the photocatalytic activity of the CdS–ATP composites were studied by controlling the mass ratio of attapulgite. The results showed that the CdS–20%ATP composite has an excellent photocatalytic activity. In order to figure out the key to improve the photocatalytic efficiency, the prepared composites were characterized by Brunauer–Emmett–Teller (BET) specific surface area, UV-vis diffuse reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS). The superior photocatalytic performance of the CdS–20%ATP composite can be ascribed to the existence of the ATP which can fix the CdS and prevent agglomeration. The interaction between ATP and CdS in the composites facilitates the electron transfer and also promoted their photocatalytic performance. This work provides us with some significant guidance in the development of CdS–ATP composite photocatalysts.

The application of CdS–attapulgite composites in photocatalytic reduction of p-nitrophenol and Cr(vi) demonstrated that the attapulgite could overcome the limitations of CdS.

Porous cauliflower-like molybdenum disulfide/cadmium sulfide hybrid micro/nano structure: Enhanced visible light absorption ability and photocatalytic activity

Micro-/nanostructured materials can control the diffraction and propagation of light, thereby providing new optical properties that can be exploited to enhance photocatalytic processes. In this work, a series of the cauliflower-like MoS₂/CdS hybrid micro-/nanostructures is synthesized. These structures contain numerous cracks and pores that can enhance the absorption and utilization of light as well as shorten the distance for transferring photogenerated electrons to the catalyst surface. The results of ultraviolet-visible diffuse reflectance absorption spectra show that the composite material has enhanced absorption in the visible light region. Further investigation of the optical characteristics of the synthesized materials using a finite-difference time-domain (FDTD) simulation reveals that the cauliflower-like micro-/nanostructure increases the optical absorption intensity at the MoS₂/CdS interface. Notably, the MoS₂/CdS hybrid micro-/nanostructures exhibits high photocatalytic hydrogen production activity (9.5 mmol g^-1 h^-1) and long-lasting cycle stability. This work helps us to further understand the enhancement mechanism of light absorption and utilization by porous structural materials.

Design of earth-abundant Z-scheme g-C3N4/rGO/FeOOH ternary heterojunctions with excellent photocatalytic activity

A ternary Z-scheme g-C₃N₄/rGO/FeOOH heterostructure photocatalyst for H₂ production was designed and fabricated, which exhibited photocatalytic H₂ production of 124.9 and 869.8 μmol h⁻¹ g⁻¹ under visible and UV-Vis light irradiation, respectively.

Constructing Built-in Electric Field in Ultrathin Graphitic Carbon Nitride Nanosheets by N and O Codoping for Enhanced Photocatalytic Hydrogen Evolution Activity

Codoping of N and O in ultrathin graphitic carbon nitride (g-C₃ N₄ ) nanosheets leads to an inner electric field. This field restrains the recombination of photogenerated carriers and, thus, enhances hydrogen evolution. The layered structure of codoped g-C₃ N₄ nanosheets (N-O-CNNS) not only provides abundant sites of contact with the reaction medium, but also decreases the distance over which the photogenerated electron-hole pairs are transported to the reaction interface. Quantum confinement in the ultrathin structure results in an increased bandgap and makes the photocatalytic reaction more favorable than bulk g-C₃ N₄ . Under visible light irradiation, N-O-CNNS with 3 wt% Pt achieves a hydrogen evolution rate of 9.2 mmol g^-1 h^-1 and a value of 46.9 mmol g^-1 h^-1 under AM1.5 with 5 wt% Pt. Thus, this work paves the way for designing efficient nanostructures with increased separation/transfer efficiency of photogenerated carriers and, hence, increased photocatalytic activities.

Concept of Stagnant Capillarity Water in the Nanoporous SiO2@(Zn,Ni)(O,S) Nanocomposite Photocatalyst as a Strategy to Improve Hydrogen Evolution

(Zn,Ni)(O,S) nanoparticles were uniformly deposited on nanoporous SiO₂ spheres to form SiO₂@(Zn,Ni)(O,S) nanocomposites (NCs). To obtain optimum deposition of (Zn,Ni)(O,S) on the SiO₂ spheres for the hydrogen evolution reaction (HER), different amounts of 0.25, 0.5, 1, and 1.5 mmol zinc precursor for (Zn,Ni)(O,S) were deposited on SiO₂ to obtain different SiO₂@(Zn,Ni)(O,S) NCs. All the as-prepared catalysts were examined with X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, electrochemical impedance spectroscopy, photocurrent response, photoluminescence spectral studies. Finally, the HER performance was evaluated with SiO₂@(Zn,Ni)(O,S). The best SiO₂@(Zn,Ni)(O,S)-0.5 surprisingly reached 41.1 mmol/gh for generating H₂, which was about a 840% increase as compared to that of the SiO₂ sphere-free one. The great improvement in the HER rate was due to the utilization of nanoporous SiO₂ spheres. The concept of stagnant capillarity water, adopted from the leaf vein system, was applied to explain the enhanced HER reaction.

Growth of Zn0.5Cd0.5S/α-Ni(OH)2 heterojunction by a facile hydrothermal transformation efficiently boosting photocatalytic hydrogen production

A rationally designed/constructed catalyst with α-Ni(OH)₂ over Zn–Cd–S shows enhanced photocatalytic hydrogen evolution attributed to the interface providing a fast charge transfer channel and reducing the photo-generated carrier recombination probability.

High-performance TiO2 photocatalyst produced by the versatile functions of the tiny bimetallic MOF-derived NiCoS-porous carbon cocatalyst

Bimetallic zeolitic imidazolate framework (NiCo-ZIF)-templated NiCoS-porous carbon (PC) at only 0.2 at% exhibited versatile effects on the morphology as well as the photocatalytic hydrogen performance of TiO₂ nanocrystals.

Hybrid VS2 cocatalyst and phosphorus dopant towards both surface and bulk modification of ZnCdS/CdS heterostructures

The bulk phosphorus doping and the surface VS₂ modification over ZnCdS–CdS show synergistic effects for improving its photocatalytic activity.

Enhanced photocatalytic hydrogen evolution over bimetallic zeolite imidazole framework-encapsulated CdS nanorods

Through doping the Zn ion with the transition metal, this work fabricated the bimetallic ZnM-ZIF (M = Ni, Cu, Co) encapsulated CdS nanorod heterostructure for the first time.

Structural modulation of CdS/ZnO nanoheterojunction arrays for full solar water splitting and their related degradation mechanisms

Higher PEC performance for full water splitting was realized for a CoPi–CdS/ZnO NHA via structural modulation, and degradation in the performance was elucidated.

Enhanced photocatalytic H2 production of cadmium-free rGO-mediated ZnS/CuS heterojunction derived from a MOF

The MOF-templated porous ZnS/CuS heterojunction with rGO as the charge carrier mediator exhibits enhanced photocatalytic hydrogen production under visible light illumination.

Fabrication of the ternary heterojunction Cd0.5Zn0.5S@UIO-66@g-C3N4 for enhanced visible-light photocatalytic hydrogen evolution and degradation of organic pollutants

This article reports a novel ternary heterojunction Cd_0.5Zn_0.5S@UIO-66@g-C₃N₄ that exhibits enhanced photocatalytic hydrogen production and MO degradation.

A porous triptycene-based covalent polymer stabilized binary metal sulfide for enhanced hydrogen evolution under visible light

A novel triptycene-based covalent polymer (TCP) with a high surface area was constructed through the Suzuki coupling reaction.

Enhanced photogenerated carrier separation in CdS quantum dot sensitized ZnFe2O4/ZnIn2S4 nanosheet stereoscopic films for exceptional visible light photocatalytic H2 evolution performance

CdS quantum dot sensitized ZnFe₂O₄/ZnIn₂S₄ nanosheet stereoscopic films were synthesized through two sequential solvothermal processes and the ionic layer adsorption-reaction method. The hydrophilic ZnIn₂S₄ nanosheet stereoscopic film was pre-prepared to act as a suitable host material, and then ZnFe₂O₄ nanoparticles and CdS quantum dots were uniformly decorated on the surface of the ZnIn₂S₄ nanosheet stereoscopic film to form a ternary heterostructure stereoscopic film. The band structure difference in the ternary heterostructure can promote the spatial separation and transport efficiency of photogenerated charge carriers. Meanwhile, the composite stereoscopic film has significant structural advantages and can provide a large amount of reaction active sites and outstanding visible light utilization. The superhydrophilic surface contributes to interface contact of catalyst/solution and gas detachment. These positive factors led to a significantly enhanced photocatalytic H₂ evolution activity of the CdS/ZnFe₂O₄/ZnIn₂S₄ ternary heterostructure film in comparison with the pristine ZnIn₂S₄ and binary heterostructure film photocatalysts. The optimized CdS/ZnFe₂O₄/ZnIn₂S₄ ternary heterostructure film demonstrates the highest H₂ production rate of 79.0 μmol h^-1, which surpasses that of ZnIn₂S₄ by more than 3.2 times. This synthesis strategy can be applicable for the facile synthesis of other visible-light-driven composite film catalysts.

Co-Doped Zn1−xCdxS nanocrystals from metal–organic framework precursors: porous microstructure and efficient photocatalytic hydrogen evolution

Nanoporous Co-doped Zn_1−xCd_xS fabricated with ZnCo-ZIFs as templates afford a high H₂-production rate, corresponding to 45.2 and 422.2 times larger than those of Zn_0.5Cd_0.5S and CdS without using ZIF as template.