Fabrication of 1D/2D CdS/CoSx direct Z-scheme photocatalyst with enhanced photocatalytic hydrogen evolution performance

In this work, we fabricate a 1D/2D heterojunction photocatalyst composed of n-type CdS nanorods and p-type CoS_x nanoflake. This photocatalyst achieves a hydrogen evolution rate of 9.47 mmol g^?1 h^?1, which is 13.7 times higher than that of pure CdS nanorods. Scanning Kelvin Probe, Mott-Schottky plots, UV–Vis absorption spectra and surface photocarrier orienting reaction results indicate that the enhanced photocatalytic performance of CdS/CoS_x is owing to the fabrication of direct Z-Scheme heterojunction system which greatly improves the utilization, migration and separation rate of photo-generated carriers. To the best of our knowledge, this work is the first time to describe a CdS/CoS_x direct Z-scheme system with 1D/2D nanostructure, which can expedite the transfer process of photogenerated carriers with strong redox energy to participate in photocatalytic reactions.     

CuS-modified ZnO rod/reduced graphene oxide/CdS heterostructure for efficient visible-light photocatalytic hydrogen generation

Solar energy utilization is a promising strategy for the photocatalytic generation of H₂ from water. Herein, a CuS-modified ZnO rod/reduced graphene oxide (rGO)/CdS heterostructure was fabricated via Cu-induced electrochemical growth with Zn powder at room temperature. The resulting powder revealed good interfacial bonding and promoted photoexcited carrier transport. The CuS nanoparticles played a pivotal role in enhancing visible-light responses and demonstrated excellent catalytic performance. A high visible-light photocatalytic H₂ generation rate of 1073 μmol h⁻¹ g⁻¹ was obtained from the CuS–ZnO/rGO/CdS heterostructure containing 0.23% CuS and 1.62% CdS. Increased photoexcited electron lifetimes, improved carrier transport rates, and decreased fluorescence intensities confirmed the synergistic effects of each of the components of the heterostructure. This study provides an innovative strategy for constructing multi-component heterostructures to achieve efficient visible-light H₂ evolution.     

Direct Z-Scheme NiWO4/CdS nanosheets-on-nanorods nanoheterostructure for efficient visible-light-driven H2 generation

Exploiting efficient catalysts is of interest for solar-driven water splitting. Herein, a novel NiWO₄/CdS nanosheets-on-nanorods direct Z-Scheme heterostructure was developed by using a facile in-situ approach. The optimized NiWO₄/CdS heterostructure shows a H₂ evolution rate of 26.43 mmol g^?1 h^?1 exceeding that of bare CdS by more than 75 folds. Systematic investigations reveal the nanostructures with numerous active sites, intimate contact interface, and enhanced charge separation rate synergistically account for the outstanding performance of the NiWO₄/CdS. Moreover, the band structures were detailedly analyzed and the tentative photocatalytic mechanism was proposed, which could contribute to deeply understanding the catalytic process and guide the synthesis of the efficient heterostructure. These findings and strategies may have great significance in promoting the development of highly efficient and low-cost photocatalysts.     

Construction of porous Ni2P cocatalyst and its promotion effect on photocatalytic H2 production reaction and CO2 reduction

Due to superior light absorption abilities, porous materials are suitable to be served in photocatalytic reactions. In this study, porous Ni₂P is target-constructed from porous Ni(OH)₂ nanoflower. Promotion effect of the porous Ni₂P as cocatalyst is confirmed on photocatalytic performance of Ni₂P/CdS composite. The constructed porous Ni₂P/CdS photocatalyst shows much higher photocatalytic H₂ evolution rate (111.3 mmol h⁻¹ g⁻¹) from water and much higher CO (178.0 μmol h⁻¹ g⁻¹) and CH₄ (61.2 μmol h⁻¹ g⁻¹) evolution rates from CO₂ reduction than non-porous Ni₂P/CdS photocatalyst. Characterizations including UV-Vis diffuse reflectance, photoluminescence, transient photocurrent response, electrochemical impedance and electron paramagnetic resonance are conducted to verify the role of porous Ni₂P cocatalyst. The slow photon effect derived from porous structure Ni₂P is found to improve light path and increase the absorption utilization of light. The enhanced photocurrent intensity and the lowered resistance of porous Ni₂P/CdS due to the formed heterojunctions indicate much rapid isolation of photogenerated electron-hole pairs and rapid charge transfer of electrons. The higher signal of ⋅O₂- radicals is detected in porous Ni₂P/CdS than non-porous Ni₂P/CdS, which result in the remarkable photocatalyst activities of porous Ni₂P/CdS. Reaction mechanisms over Ni₂P/CdS photocatalyst are illustrated with a Z-scheme charge transfer path.     

Nanoflower-like Ti3CN@TiO2/CdS heterojunction photocatalyst for efficient photocatalytic water splitting

Solar energy to hydrogen production is an effective way to solve the energy crisis. Here, we report a Ti₃CN@TiO₂/CdS photocatalyst with highly efficient photocatalytic performance. Ti₃CN@TiO₂ materials with nanoflower morphology or lamellar morphology were obtained from Ti₃AlCN by controlling the etching time, and then loaded CdS nanoparticles to improve the photocatalytic efficiency. The physical and chemical properties of the catalyst were characterized by various characterization techniques. Ti₃CN@TiO₂/CdS photocatalyst shows an enhanced photocatalytic activity of 3393.4 μmol g^?1h^?1, much higher than that of CdS and Ti₃CN@TiO₂.^.     

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.     

Efficient photocatalytic H2 evolution over Cu and Cu3P co-modified TiO2 nanosheet

Schottky junction and p-n heterojunction are widely employed to enhance the charge transfer during the photocatalysis process. Herein, Cu and Cu₃P co-modified TiO₂ nanosheet hybrid (Cu–Cu₃P/TiO₂) was fabricated using an in situ hydrothermal method. The ternary composite achieved the superior H₂ evolution rate of 6915.7 μmol g^?1 h^?1 under simulated sunlight, which was higher than that of Cu/TiO₂ (4643.4 μmol g^?1 h^?1) and Cu₃P/TiO₂ (6315.8 μmol g^?1 h^?1) and pure TiO₂ (415.7 μmol g^?1 h^?1). The enhanced activity can be attributed to the collaboration effect of Schottky junction and p-n heterojunction among Cu/TiO₂ and Cu₃P/TiO₂, which can harvest the visible light, reduce the recombination of charge carriers and lower the overpotential of H₂ evolution, leading to a fast H₂ evolution kinetics. This work develops a feasible method for the exploration of H₂ evolution photocatalyst with outstanding charge separation properties.     

Facile synthesis of anatase/rutile TiO2/g-C3N4 multi-heterostructure for efficient photocatalytic overall water splitting

Rational design of high-efficiency heterostructure photocatalyst is an effective strategy to realize photocatalytic H₂ evolution from pure water, but remains still a considerable challenge. Herein, an anatase/rutile TiO₂/g-C₃N₄ (A/R/CN) multi-heterostructure photocatalyst was prepared by a facile thermoset hybrid method. The combination of two type-II semiconductor heterostructures (i.e., A/R and R/CN) significantly improve the separation and transfer efficiency of photogenerated carriers of anatase TiO₂, rutile TiO₂ and g-C₃N₄, and A/R/CN photocatalyst with high activity is obtained. The optimal A/R/CN photocatalyst exhibits significantly increased photocatalytic overall water splitting activity with a rate of H₂ evolution of 374.2 μmol g⁻¹h⁻¹, which is about 8 and 4 times that of pure g-C₃N₄ and P25. Moreover, it is demonstrated to be stable and retained a high activity (ca. 91.2%) after the fourth recycling experiment. This work comes up with an innovative perspective on the construction of multi-heterostructure interfaces to improve the overall photocatalytic water splitting performance.     

Efficient photocatalytic hydrogen evolution on amorphous MoSx-modified CdS/KTaO3 heterojunctions under visible light irradiation

Constructing heterostructures with efficient charge separation is a promising route to improve photocatalytic hydrogen production. In this paper, MoS_x/CdS/KTaO₃ ternary heterojunction photocatalysts were successfully prepared by a two-step method (hydrothermal method and photo deposition method), which improved the photocatalytic hydrogen evolution activity. The results show that the rate of hydrogen evolution for the optimized photocatalyst is 2.697 mmol g^?1·h^?1under visible light, which is 17 times and 2.6 times of the original CdS (0.159 mmol g^?1 h^?1) and the optimal CdS/KTaO₃(1.033 mmol g^?1 h^?1), respectively, and the ternary photocatalyst also shows good stability. The improvement on photocatalytic hydrogen evolution performance can be attributed to the formation of heterojunction between the prepared composite materials, which effectively promotes the separation and migration of photo-generated carriers. Amorphous MoS_x acts as an electron trap to capture photogenerated electrons, providing active sites for proton reduction. This provides beneficial enlightenment for hydrogen production by efficiently utilizing sunlight to decompose water.     

Refined Z-scheme charge transfer in facet-selective BiVO4/Au/CdS heterostructure for solar overall water splitting

Artificial Z-scheme systems that mimic natural photosynthesis are well applicable to photocatalytic overall water splitting for hydrogen (H₂) production free of electricity. However, it commonly confronts low efficiency with huge challenge of steering charge transfer between H₂ evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP). Here we report an all-solid-state Z-scheme system with facet-selective construction that favors charge spatial separation toward HEP and OEP for high efficient solar overall water splitting. Based on the spontaneous separation of photogenerated electrons and holes on the different crystal facets of BiVO₄ decahedra, we successively implemented the selective depositions of Au and CdS nanoparticles (NPs) onto the electron-rich {010} facets, to fortify the Z-scheme charge transfer between BiVO₄ and CdS across Au mediators upon two-step photoexcitation. In-situ photoelectron dynamics ascertains Z-scheme model of resultant BiVO₄/Au/CdS, which enables an impressive overall water splitting with stoichiometric H₂ and O₂ evolution rates of 281 and 138 μmol g^?1 h^?1, respectively, under 1 sun irradiation (100 mW cm^?2, AM 1.5G) without using any sacrificial agents and external bias. This work not only presents a refined Z-scheme overall water splitting system, but also gains insights into photo-induced charge transfer dynamics.     

Highly efficient Bi2O3/MoS2 p-n heterojunction photocatalyst for H2 evolution from water splitting

The design of p-n heterojunction photocatalysts to overcome the drawbacks of low photocatalytic activity that results from the recombination of charge carriers and narrow photo-response range is promising technique for future energy. Here, we demonstrate the facile hydrothermal synthesis for the preparation of Bi₂O₃/MoS₂ p-n heterojunction photocatalysts with tunable loading amount of Bi₂O₃ (0–15 wt%). The structure, surface morphology, composition and optical properties of heterostructures were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV–visible absorption spectroscopy, Brunauer-Emmett-Teller (BET) surface area, photoluminescence (PL), electrochemical impedance spectroscopy (EIS). Compare to pure Bi₂O₃ and MoS_2, the Bi₂O₃/MoS₂ heterostructures displayed significantly superior performance for photocatalytic hydrogen (H₂) production using visible photo-irradiation. The maximum performance for hydrogen evolution was achieved over Bi₂O₃/MoS₂ photocatalyst (10 μmol h⁻¹g⁻¹) with Bi₂O₃ content of 11 wt%, which was approximately ten times higher than pure Bi₂O₃ (1.1 μmol h⁻¹g⁻¹) and MoS₂ (1.2 μmol h⁻¹g⁻¹) photocatalyst. The superior performance was attributed to the robust light harvesting ability, enhanced charge carrier separation via gradual charge transferred pathway. Moreover, the increased efficiency of Bi₂O₃/MoS₂ heterostructure photocatalyst is discussed through proposed mechanism based on observed performance, band gap and band position calculations, PL and EIS data.     

Designing of a novel Mn0.2Cd0.8S@ZnO heterostructure with Type-II charge transfer path for efficient photocatalytic hydrogen evolution reaction

The development of photocatalysts with efficient hydrogen evolution activity has been the goal for sustainable hydrogen production. In this work, heterojunction composite photocatalyst is formed by hydrothermal coupling of ZnO and Mn_0.2Cd_0.8S. Compared with pure ZnO and Mn_0.2Cd_0.8S, the composite photocatalyst has the ability to provide more abundant active sites and better photogenerated carriers separation efficiency. The optimized composite photocatalyst shows a 9.36-fold increase in hydrogen evolution activity (4297.99 μmol g^?1 h^?1) compared to Mn_0.2Cd_0.8S (459.31 μmol g^?1 h^?1) and exhibits excellent cycling stability. Density functional theory calculations identifies Type-II charge transfer path in the composite photocatalyst, achieving effective separation in space of photogenerated electrons from holes and suppressing recombination within the semiconductor. The results show that the construction of Type-II heterojunction in this work achieves a significant enhancement of the hydrogen evolution activity of the photocatalyst by constructing carrier transport channels at the contact interface of the heterojunction.     

Novel 2D Zn-porphyrin metal organic frameworks revived CdS for photocatalysis of hydrogen production

The main challenge of photocatalysis is how to improve the coefficient of utilization and conversion rate for solar energy. Herein, we report a composite photocatalyst related to a novel porphyrin metal organic frameworks (MOFs), in which cadmium sulfide nanoparticles (CdS NPs) are grown in situ on the surface of two-dimensional (2D) zinc porphyrin nanosheets (Zn-TCPP NSs) by hydrothermal method. Interestingly, Zn-TCPP NSs and CdS NPs form a Type II heterojunction structure, which reduces the photogenerated electron-hole recombination rate of CdS. Moreover, in the near-infrared region, the photo-excited electrons generated by Zn-TCPP NSs are transmitted to CdS NPs, so that cadmium sulfide can realize both visible light and near-infrared light for photocatalytic hydrogen production. The Zn-TCPP NSs not only has excellent light absorption capacity, but also has a unique frame design that effectively reduces the recombination rate of photoinduced electron hole pairs, thus improving the conversion rate of solar energy. As expected, the photocatalytic performance of the porphyrin MOFs modified materials is significantly enhanced compared to CdS NPs. The hydrogen production rate of the Pt@CdS NPs/Zn-TCPP NSs(C-Z-T) composite material in the visible light region is about 15.3 mmol g^?1 h^?1, which is 11 times for Pt@CdS NPs. Furthermore, the Pt@CdS NPs/Zn-TCPP NSs(C-Z-T) also has a considerable hydrogen production rate in the near-infrared region, such as 200 μmol g^?1 h^?1 at 600 nm, 90 μmol g^?1 h^?1 at 765 nm and 20 μmol g^?1 h^?1 at > 800 nm.     

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.     

Oxygen vacancy rich α-MnO2 @B/O-g-C3N4 photocatalyst: A thriving 1D-2D surface interaction effective towards photocatalytic O2 and H2 evolution through Z-scheme charge dynamics

Herein, we have prepared a 1D-2D junction based Oxygen Vacancy rich α-MnO₂@B/O-g-C₃N₄ photocatalyst by using 1D α-MnO₂ nanorods over 2D nanosheets of boron, oxygen co-doped exfoliated graphitic carbon nitride (BOCN). The important feature of the above composite material is the availability of oxygen vacancy as well as the presence of dual oxidation state of Mn (Mn⁴⁺, Mn³⁺) which enhances surface activity as well as chemical reaction output of the material. Among the synthesized materials α-MnO₂@B/O-g-C₃N₄ (MBOCN-20) shows best photocatalytic activity by following Z-Scheme charge dynamics towards water oxidation (295.1 μmol h^?1) and reduction (560.1 μmol h^?1) reactions in presence of methanol (hole scavenger) and AgNO₃ (electron scavenger) as sacrificial agent. However, 44.2 and 86.5 μmol h^?1 of O₂ and H₂ evolution was observed in absence of any sacrificial agent. This analysis will confer a valuable blue-print to construct stimulating photocatalysts to achieve the paramount performance towards photocatalytic water redox reaction.     

Separation of hot electrons and holes in Au/LaFeO3 to boost the photocatalytic activities both for water reduction and oxidation

Construction of plasmon-based nanostructures is an effective way to enhance the photocatalytic activities of semiconductor photocatalysts for water-splitting. However, the synergistic effect of plasmon-related hot electrons and holes for water splitting in the plasmon-hybrid photocatalyst is rarely considered. Herein, we construct a plasmon-based Au/LaFeO₃ composite photocatalyst to investigate the complex roles of hot electrons and holes for solar water splitting. Benefiting from the formation of Schottky junction and surface plasmon resonance effect of the Au nanoparticles, the synthesized photocatalyst exhibits an excellent photocatalytic activity for each half-reaction of water splitting, and the rates for H₂ and O₂ generation are obtained as high as 202 μmol g⁻¹ h⁻¹ and 23 μmol g⁻¹ h⁻¹, respectively. Moreover, an in-depth investigation reveals that the improved hydrogen evolution is caused by the hot electron injection from Au to LaFeO₃, and the hot holes in Au induced by the separation of hot charges can initiate the water oxidation directly on the surface of gold. Thus, this work provides a new insight into the synergistic effect of plasmon-related hot electrons and holes for boosting the photocatalytic reactions.     

In2Se3/CdS nanocomposites as high efficiency photocatalysts for hydrogen production under visible light irradiation

Hydrogen energy is an important clean energy. Using visible light to produce hydrogen by semiconductor photocatalysts is one of the current research hotspots. In this work, In₂Se₃/CdS nanocomposite photocatalysts with different mass content of CdS are prepared. The In₂Se₃/CdS photocatalyst with 85.25% CdS mass content exhibits the optimal photocatalytic hydrogen evolution activity (1.632 mmol g^?1 h^?1), which is much higher than that of CdS (0.715 mmol g^?1 h^?1) and In₂Se₃ (trace). Moreover, the In₂Se₃/CdS photocatalyst still maintains a high hydrogen evolution rate after five cycles. The high photocatalytic activity and stability of the In₂Se₃/CdS nanocomposite is due to the formation of heterojunction between In₂Se₃ and CdS. The existence of heterojunction is confirmed by high resolution transmission electron microscopy image and X-ray photoelectron spectra. Theoretical calculations and experimental results indicate that the electron transfer route at the heterojunction is step-scheme. The step-scheme helps the separation of photogenerated electrons and holes, and maximize the hydrogen evolution activity. This work provides a high efficiency step-scheme photocatalyst for hydrogen production.     

Efficient and long-term photocatalytic H2 evolution stability enabled by Cs2AgBiBr6/MoS2 in aqueous HBr solution

Lead-free Cs₂AgBiBr₆ (CABB) double perovskite as a new-type photocatalytic material alternative to lead halide perovskites holds promise to implement the solar-H₂ conversion, but the interior recombination of photo-generated carriers and thus low photocatalytic hydrogen evolution reaction (HER) rate of CABB restrict its further industrial applications. Herein, we report the composite fabrication of MoS₂/CABB heterostructure for high-efficiency and durable photocatalytic HER by anchoring non-noble MoS₂ onto CABB via a facile dissolution-recrystallization method. The optimized MoS₂/CABB performs a visible-light HER rate of 87.5 μmol h^?1 g^?1 in aqueous HBr solution, ca. 20-fold compared to that of pure CABB (4.3 μmol h^?1 g^?1), and presents a discontinuous 500-h photocatalytic HER stability with no evident loss. The superb performance of MoS₂/CABB can be ascribed to the kinetics-facilitated heterostructure consisting of stable CABB and MoS₂. This work proposes a facile and versatile tactic to construct a low-cost Cs₂AgBiBr₆-based heterostructure for efficient and long-term photocatalytic HER.     

Surface polar charge induced Ni loaded CdS heterostructure nanorod for efficient photo-catalytic hydrogen evolution

Designing of artificial heterostructure photo-catalysts to crop solar energy for H₂ evolution from water is of great importance nowadays. The ultrafine Ni (0.5, 1.0, 2.0 and 5.0 wt%) particles loaded CdS nanorods were synthesized by a simple chemical process. XRD shows the crystalline phase of CdS with increase in size from 17 to 28 nm with 10.19% and 10.06% enhancement in the lattice strain and the dislocation density for Ni (0.5–5.0 wt%). The XPS peaks observed at 854.88 eV and 861.07 eV for Ni²⁺ with energy separation of 6.18 eV confirmed the existence of NiO on Ni surface. The Raman bands for pure CdS and Ni (1.0 wt%)-CdS nanorods were observed at 300 cm^?1 and 293 cm^?1 for 1LO phonon and 601 cm^?1 and 586 cm^?1 for 2LO phonon, respectively. The Ni loading tuned the CdS band gap from 2.36 to 2.20 eV. The eight fold enhancement in the CdS specific surface area i.e., from 4.19194 m² g^?1 to 34.8343 m² g^?1 was achieved. After Ni loading, the synergetic effect of efficient electron separation and transportation was observed by the continuous quenching of luminescence emission intensity and the reduction of charge transfer resistance from 706 Ω for CdS to 484 Ω of CdS. The Ni (1.0 wt%)@ NiO optimal loading on CdS results highest photo-catalytic H₂ evolution of 9.0 mmol at rate of 1.8 mmol h^?1, which is about 50 times higher than that of 180 μmol at rate of 36 μmol h^?1 for pure CdS. A thin layer of NiO on plasmonic Ni surface could be the promising system for photo-catalytic H₂ evolution due to visible light photo-activity.     

Carboxymethyl cellulose gel membrane loaded with nanoparticle photocatalysts for hydrogen production

Carboxymethyl cellulose (CMC) gel membranes were prepared by a chemical crosslinking method and an in situ method was used to load CdS nanoparticles with an average size of about 3 nm into the CMC gel. The negative ion groups in the CMC serve as strong binding sites for the metal ions and help prevent aggregation of the CdS nanoparticles during their growth process. This results in a CMC gel matrix loaded with stable and well-dispersed CdS nanoparticles (CdS-CMC). Pt co-catalyst particles were also loaded into the gel membrane to give CdS/Pt-CMC and both materials were used as photocatalysts for the production of hydrogen. The CdS/Pt-CMC catalyst with 5 wt % Pt exhibited a H₂ evolution rate of 1365 μmol h⁻¹ g⁻¹, which is 81 times that of pure CdS. This improvement can be attributed to the separation of the CdS photo-generated electron-hole pairs. The photo-electrons are transferred to Pt and the formed aqueous H⁺ ions are then rapidly converted to H₂. The incorporation of the photocatalyst into a gel matrix makes the photocatalyst easily recoverable which can help avoid environmental pollution caused by free CdS nanoparticles.