Heteroepitaxial growth of core-shell ZnO/CdS heterostructure for efficient and stable photocatalytic hydrogen generation

Solar-driven water splitting provides a promising way to generate clean and renewable hydrogen. The efficiency and stability are of importance for practical application of this technology. Herein, colloidal ZnO/CdS heterostructure catalysts were obtained via a two-step heteroepitaxial growth strategy. It was discovered that the anion exchange step plays an important role in construction of core-shell ZnO/CdS heterostructure and photo-induced electron-hole separation is promoted by reducing the charge carriers transfer distance in prepared ultra-small heterostructure. By optimizing the molar ratio of the ZnO and CdS components, the highest hydrogen generation rate of 669.6 μmol/h (100 mg) was achieved without any cocatalyst loading in the presence of S^2? and SO₃^2? as sacrificial reagents. Furthermore, the heterostructure displayed excellent photocatalytic stability for ～72 h. The photocatalytic performance of as-prepared nanoscale ZnO/CdS is superior than that of the well-studied bulk ZnO/CdS heterostructures, demonstrating its great potential in practical application of photocatalytic water splitting.     

Noble-metal-free CuS/CdS composites for photocatalytic H2 evolution and its photogenerated charge transfer properties

CuS/CdS composites have been successfully prepared by a simple hydrothermal and cation exchange method. Even without noble-metal cocatalyst, the prepared CuS/CdS composites exhibited enhanced photocatalytic H₂ evolution activity. CuS content had a great influence on photocatalytic activity and an optimum amount of CuS was determined to be ca. 3 mol%, at which the CuS/CdS displayed the highest photocatalytic activity, giving an H₂ evolution rate of 332 μmol g⁻¹ h⁻¹, exceeding that of pure CdS by 3.5 times. The results of SPV (surface photovoltage) and SPC (surface photocurrent) revealed that photogenerated electrons were captured by CuS loaded. TPV (transient photovoltage techniques) indicated that photogenerated charges lifetime in CdS, was prolonged with CuS loaded. Those are the main reasons for the improvement of photocatalytic H₂ evolution.     

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

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

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.     

Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S + Na2SO3 solution

The photocatalytic hydrogen production in the sacrificial S²⁻–SO₃²⁻ anions was investigated with ZnO in the addition of metal sulfides containing Ag₂S, CuS, Fe₂S₃, and NiS. In the absence of metal sulfides, the photocatalytic H₂ evolution using ZnO was observed with 255 μmol g⁻¹. The CuS amount and the concentrations of S²⁻ and SO₃²⁻ ions were optimized. It was found that ternary component semiconductor CuS/ZnS/ZnO was formed during the photocatalytic hydrogen production in the aqueous Na₂S + Na₂SO₃ solution. The photocatalytic hydrogen evolution with CuS/ZnS/ZnO in the 0.4 M Na₂S–0.4 M Na₂SO₃ solution was more than about 8.5 times better compared with those obtained with only ZnO. The CuS clusters on the surface of ZnS/ZnO seem to play an important role on the separation for electron–hole pair and the enhancement of H₂ production. Nano-sized ZnS/ZnO photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

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.     

Constructing V2O5/CdS/CuS multi-level heterojunction for efficient photocatalytic hydrogen evolution

Multi-level heterojunction can effectively promote charge separation and transfer to improve photocatalytic hydrogen evolution activity. Based on the successful preparation of CdS/CuS heterojunction by one-pot hydrothermal method, V₂O₅ is introduced through the thermal decomposition of NH₄VO₃ for constructing V₂O₅/CdS/CuS(VCU) multi-level heterojunction. In this heterostructure, CdS and CuS are closely combined as mixed nanoparticles, which can boost the electron transfer (ET) process between them, and the introduction of V₂O₅ can increase the light absorption of the whole catalyst system. The hydrogen evolution test shows that VCU has the optimal performance with the hydrogen production rate of 1475 μmol/g/h, which is 16.4 times higher than pure CdS. According to the analysis of the binary composite structures (V₂O₅/CuS and V₂O₅/CdS), the probable ET process of VCU has been given, unraveling the internal catalytic mechanism. The present work expands the approaches for photocatalyst mechanism analysis and demonstrates the dramatic improvement in photocatalytic hydrogen production by the multi-level heterostructure.     

Novel FeVO4/CuS heterojunction nanocomposite as a high-performance visible-light-active photocatalyst for ibuprofen degradation in aquatic media

Herein, we report the fabrication of type-II FeVO₄/CuS heterojunction nanocomposite by a versatile reflux-assisted co-precipitation procedure. The hybrid FeVO₄/CuS heterostructure with a band gap of 1.95 eV, demonstrated excellent degradation efficiency of ibuprofen antibiotic (～95%) after 90 min of visible-light irradiation, which displayed remarkably enhanced photocatalytic degradation (1.75 fold higher) in comparison to pristine FeVO₄ structure. The superior photocatalytic performance of the FeVO₄/CuS nanocomposite is associated with hierarchical flower-like architectures, great visible-light harvesting, and poor electron-hole recombination due to the fabrication of type-II heterojunction. In addition, we introduced an obvious photocatalytic destruction mechanism, which indicated that superoxides (^?O₂^?) and holes (h⁺) were the invasive species in antibiotic degradation on the FeVO₄/CuS photocatalyst. Consequently, the as-prepared FeVO₄/CuS heterojunction photocatalyst is a promising candidate for the development of future photocatalysts towards the elimination of antibiotics under sun light irradiation at short process time.     

Noble-metal-free CdS@MoS2 core-shell nanoheterostructures for efficient and stabilized visible-light-driven H2 generation

Photocatalytic water splitting is considered to be a green H₂ generation approach and has potential to be applied in the future. As a photocatalytic active material for H₂ evolution, CdS is a good candidate. However, the pristine CdS still suffers from low efficiency and poor stability. To address those issues, we developed noble-metal-free CdS@MoS₂ core-shell nanoheterostructures which exhibit outstanding photocatalytic H₂ evolution performance thus far with rate of 62.55 mmol g⁻¹ h⁻¹, which exceeds that of pristine CdS by a factor of 148. Meanwhile, the photocatalytic stability can be well retained with no deterioration of activity in 24 h reaction. The excellent performance can be reasonably attributed to the low crystallinity of MoS₂ with numerous active sites provided, and the band alignment of CdS and MoS₂ as determined by valence band-XPS and Mott-Schottky plots analysis, which significantly promotes charge transportation and separation. The enhanced photocatalytic stability here should be ascribed to the intimate growth of MoS₂ shells which significantly passivate the surface trap states of CdS cores and thus the photocorrosion is remarkably retarded. This novel strategy will inspire the fabrication of other photocatalytic systems, and may high-efficient photocatalysts be obtained.     

Sonochemistry synthesis of Bi2S3/CdS heterostructure with enhanced performance for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution from water splitting is an efficient, eco-friendly method for the conversion of solar energy to chemical energy. A great number of photocatalysts have been reported but only a few of them can respond to visible-light. Metal sulfides, a class of visible-light response semiconductor photocatalysts for hydrogen evolution and organic pollutant degradation, receive a lot of attention due to their narrow band gaps. Herein, we report the sonochemical synthesis of Bi₂S₃/CdS nanocrystal composites with microsphere structure at mild temperature. The phases of Bi₂S₃ and CdS can be observed obviously in HRTEM image. The heterostructure consisting of the two species of nanocrystals plays a key role in separating photo-generated charge carriers. Photocatalytic activities for water splitting are investigated under visible-light irradiation (λ > 400 nm) and an enhanced photocatalytic activity is achieved. The initial rate of H₂ evolution is up to 5.5 mmol h⁻¹ g⁻¹ without resorting to any cocatalysts.     

Interfacing CdS particles on Ni foam as a three-dimensional monolithic photocatalyst for efficient visible-light-driven H2 evolution

Solar photocatalytic water splitting using particulate semiconductors has been valued as a potentially scalable way for the production of clean H₂ energy, yet the performances of the powder-suspension systems are constrained by insufficient utilization of light energy and tedious recycling of photocatalyst particles. Here, we present a high-performance photocatalytic H₂ evolution using a visible-light-driven CdS-based monolithic photocatalyst with three-dimensional (3D) heterostructure. The monolithic photocatalyst is fabricated by firmly growing CdS microspheres on a Ni(OH)₂ nanosheet-modified Ni foam (NF) (denoted as CdS-NiS_x/NF) via a simple hydrothermal process. The structure and component synergy endows the monolithic CdS-NiS_x/NF photocatalyst advantageous features including high-density CdS microspheres for visible light harvesting, multiple heterojunction interfaces for efficient electron-hole separation, and abundant interfacial NiS_x active sites for efficient H₂ evolution reaction (HER). Upon visible light irradiation, the monolithic CdS-NiS_x/NF photocatalyst exhibits an outstanding photocatalytic H₂ evolution activity with an enhanced rate of 6.2 mmol·h⁻¹ g⁻¹_CdS, which is 6 times higher than that of the suspended CdS powder. In addition, the structural integrity of the CdS-NiS_x/NF enables a good stability for H₂ evolution over a 30 h reaction. This monolithic photocatalyst is scalable in preparation and compatible for device fabrication, which offers great potentials for applications in solar cells, photoelectrocatalysis, and electrocatalysis.     

Improved photocatalytic efficiency and stability of CdS/ZnO shell/core nanoarrays with high coverage and enhanced interface combination

Photocatalytic hydrogen production of CdS/ZnO shell/core nanoarrays were investigated by combining the sensitization and calcining techniques. Long single crystal ZnO nanoarrays hydrothermally grown on FTO were fully covered with CdS using an optimized chemical bath deposition method. Heating treatment not only improved the interface connection and CdS crystallinity but also formed a (Cd_0.8Zn_0.2)S buffer layer between ZnO and CdS. The CdS/ZnO shell/core arrays showed gradually enhanced photocatalytic activity with raising the calcining temperature. This is predominantly attributed to the improved CdS crystallinity and the resultant (Cd_0.8Zn_0.2)S. The (Cd_0.8Zn_0.2)S buffer layer formed by calcining shows dramatic effect on the photocatalytic activity and stability. The CdS/ZnO shell/core arrays calcined at 550 °C exhibits the optimized photocurrent density of 5.1 mA cm^?2 and a photocatalytic stability over 12 h under visible-light irradiation.     

Hierarchical nanohybrid of CuS/NiS2/Ti3C2 heterostructure with boosting charge transfer for efficient photocatalytic hydrogen evolution

Fabricating heterostructure photocatalysts with co-catalysts can improve the separation and transfer of photo-induced electrons and holes for photocatalysis reactions. Herein, Ti₃C₂T_x nanosheets are obtained by chemical etching via the hydrothermal route and serve as a template for growing photocatalysts. NiS₂ nanoparticles and CuS nanoneedles are deposited sequentially on the surface of Ti₃C₂T_x nanosheets to form “Type II” CuS/NiS₂/Ti₃C₂T_x hierarchical heterostructure via the solvothermal method. The enormous nanoneedles morphology provides enlarged active sites for the photocatalytic processes. The fabricated CuS/NiS₂/Ti₃C₂T_x heterostructure delivers an increased hydrogen generation rate of 32.66 mmol g⁻¹ h⁻¹, which is higher than that of pure CuS (2.38 folds), NiS₂ (1.93 folds), and NiS₂/Ti₃C₂T_x (1.71 folds). CuS/NiS₂/Ti₃C₂T_x heterostructure also performs a superior hydrogen evolution retention of 97.7% after 4 cycles (one cycle lasts 4 h), implying its decent structural stability and light corrosion resistance. The reasons are ascribed to the constructed “Type II” heterostructure of CuS/NiS₂ with higher active sites, improved conductivity, and efficient separation of electrons and holes. DFT calculation and Mott-Schottky plots results elucidate the formation mechanism of CuS/NiS₂/Ti₃C₂T_x “Type II” structure. CuS/NiS₂/Ti₃C₂T_x heterostructure also obtains a reduced bandgap with increased light absorption. The van der Waals force between 2D materials enhances the transfer of photo-generated electrons. This work demonstrates that designing hierarchical co-catalyst heterostructure without non-noble can effectively promote water splitting in the solar-to-chemical system.     

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

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

Lollipop-shaped Co9S8/CdS nanocomposite derived from zeolitic imidazolate framework-67 for the photocatalytic hydrogen production

The unlimited consumption of traditional fossil fuels prompted an urgent need for the new H₂ production strategies to provide green and sustainable energy resources. In this context, Co₉S₈ has emerged as a potent photocatalyst due to its narrow bandgap. Herein, we describe the synthesis of a recyclable lollipop-shaped Co₉S₈/CdS nanocomposite via direct annealing of zeolitic imidazolate framework-67 (ZIF-67) and CdS, as the template and sulfur source, respectively. The close contact of these two materials promotes the electron transfer and enhances the number of active sites toward the desired redox reaction. The special structure of Co₉S₈/CdS results in a high photocatalytic H₂ production rate of 1852 μmol h⁻¹ g⁻¹ under visible-light illumination, which is ~200 times higher than that of pure ZIF-67 and Co₉S₈. Therefore, this study provides a new method for the preparation of Co₉S₈ that concomitantly improves the photocatalytic performance of pure ZIF-67 in the H₂ production technology.     

High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2

Photocatalysts CuS/TiO₂ for hydrogen production were synthesized by hydrothermal method at high temperature and characterized by XRD, UV–visible DRS, XPS, EDX, SEM and TEM. When TiO₂ was loaded with CuS, it showed photocatalytic activities for water decomposition to hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the one with 1 wt% CuS-loaded TiO₂ showed the maximum photocatalytic activity for water splitting, which indicated CuS could improve the separation ratio of photoexcited electrons and holes. What's more, the amounts of the produced hydrogen was about 570 μmol h⁻¹, which had exceeded pure titania (P25) 32 times. In the present paper, it is proven that CuS can act as an effective co-catalyst to enhance the photocatalytic H₂ production activity of TiO₂.     

Nanoarchitectonics of CdS/ZnSnO3 heterostructures for Z-Scheme mediated directional transfer of photo-generated charges with enhanced photocatalytic performance

The construction of heterostructure is an effective strategy to synergetically couple wide-band-gap with the narrow-band-gap semiconductor with a mediate optical property and charge transfer capability. Herein, the Z-Scheme CdS/ZnSnO₃ (CdS/ZSO) heterostructures were constructed by anchoring CdS nanoparticles on the surface of double-shell hollow cubic ZnSnO₃ via the hydrothermal method. The direct recombination of excited electrons in the conduction band (CB) of ZSO and holes in the valence band (VB) of CdS via d-p conjugation at the interface greatly accelerated the internal electric field (IEF). The transfer mode follows the Z-Scheme mechanism, where CdS/ZSO synergistically facilitates the efficient charges transfer from CdS to ZnSnO₃ through the intimate interface. Here, ZnSnO₃ and CdS serve as an oxidation photocatalyst (OP) and reduction photocatalyst (RP), respectively. Thus, it can promote synergistically the oxidation half-reaction and reduction half-reaction of H₂ evolution. The density-functional theory (DFT) calculation further confirms the charges transfer from CdS to ZnSnO₃. The hydrogen evolution of 5% CdS/ZSO heterostructure reached 1167.3 μmol g^?1, which was about 8 and 3 folds high compared to pristine ZSO (141.9 μmol g^?1) and CdS (315.5 μmol g^?1), during 3 h of reaction respectively. Furthermore, the CdS/ZSO heterostructures could suppress the photo corrosion of CdS, resulting in its high stability. This work is expected to enlighten the rational design of heterostructure for OP and RP to promote the hybrid heterostructures photocatalytic H₂ evolution.     

Deposition of CdS and Au nanoparticles on TiO2(B) spheres towards superior photocatalytic performance

TiO₂(B)/CdS/Au and TiO₂(B)/Au/CdS heterostructures were synthesized to investigate the effect of the selected deposition of CdS and Au nanoparticles (NPs) on H₂ generation. TiO₂(B) spheres (phase B) consisted of nanosheets were synthesized via a hydrothermal reaction. The deposition of CdS and Au NPs were carried out using wet-chemical method and a reduction reaction, respectively. The size and amount of Au and CdS NPs were adjusted to optimize the resulting properties and discuss the change of band gap. Two kinds of heterogeneous revealed different photocatalytic hydrogen generation which indicated the position of Au NPs affect the transfer of photogenerated carriers. The hydrogen production rate of TiO₂(B)/CdS/Au heterostructures reached up to 12100 μmol g⁻¹ h⁻¹, which is about 3.8 times of that of pure TiO₂(B) spheres. This is ascribed to the structure of heterostructures. CdS NPs increase the separation of photogenerated electrons and Au NPs accelerated the transfer of the electrons. The result provided a utilizable strategy for efficient photocatalysis H₂ generation.     

The CdS/CaTiO3 cubic core-shell composite towards enhanced photocatalytic hydrogen evolution and photodegradation

The CdS/CaTiO₃ cubic core-shell composite is synthesized via a hydrothermal-chemical method. The CdS/CaTiO₃ cubic core-shell composite (CdS/CTO-2) exhibits remarkable photocatalytic HER activity (∼1025.27 μmol·g⁻¹ h⁻¹) and photodegradation enhancement than that of single CaTiO₃ (∼21 folds of HER, ∼19 folds of photodegradation) and single CdS (∼15 folds of HER, ∼15 folds of photodegradation), and a decent stability. There, CdS/CaTiO₃ composite with appropriate potential gradient and CdS with better visible light response can improve carrier efficiency, including increasing carrier transportation, prolonging lifetime and decreasing recombination. Additionally, cubic core-shell microstructure can increase active sites, while maintaining photocatalytic stability.