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.     

Ti3C2 MXene coupled with CdS nanoflowers as 2D/3D heterostructures for enhanced photocatalytic hydrogen production activity

As a novel co-catalyst, Ti₃C₂ MXene has an excellent prospect in the field of photocatalysis. Herein, the 2D/3D Ti₃C₂ MXene@CdS nanoflower (Ti₃C₂@CdS) composite was successfully synthesized by a hydrothermal method. The combination of 2D Ti₃C₂ MXene and 3D CdS nanoflowers can promote carrier transfer and separation, which can improve the performance of CdS. Compared to pure CdS nanoflowers, Ti₃C₂@CdS composite presents lower photoluminescence intensity, longer fluorescence lifetime, higher photocurrent density and smaller electrochemical impedance. The Ti₃C₂@CdS composite with 15 wt% Ti₃C₂ adding amount presents high photocatalytic hydrogen evolution activity (88.162 μmol g^?1 h^?1), 91.57 times of pure CdS. The improved photocatalytic activity of Ti₃C₂@CdS composite is ascribed to the addition of lamellar Ti₃C₂ MXene, which improves the electrical conductivity of the photocatalytic system and effectively accelerates the excited electrons transfer from CdS to Ti₃C₂ MXene.     

Study on photocatalytic performance for hydrogen evolution over CdS/M-MCM-41 (M = Zr, Ti) composite photocatalysts under visible light illumination

A series of CdS/M(x)-MCM-41 (M = Zr, Ti, x stands for molar ratio of M/Si) photocatalysts were preprared by hydrotherm, ion-exchange and sulfidation process. The catalysts were characterized by X-ray diffraction, UV-vis spectra and N₂ adsorption-desorption isotherm et al. The characterization results shown that Zr or Ti was successfully doped into the mesoporous of MCM-41, and CdS was also successfully incorporated into such modified mesoporous. The results of photocatalytic performance for hydrogen production shown that CdS/Zr(0.005)-MCM-41 and CdS/Ti(0.02)-MCM-41 had the highest hydrogen evolution activity in triethanolamine aqueous solution under visible light (λ > 430 nm) irradiation, which can be explained by the diffusion velocity of the reactants and resultants and the protection which MCM-41 provided for CdS.     

Well-designed NiS/CdS nanoparticles heterojunction for efficient visible-light photocatalytic H2 evolution

Heterojunction photocatalysts based on semiconducting nanoparticles show excellent performance in many photocatalytic reactions. In this study, 0D/0D heterojunction photocatalysts containing CdS and NiS nanoparticles (NPs) were successfully synthesized by a chemical precipitation method. The NiS NPs were grown in situ on CdS NPs, ensuring intimate contact between the semiconductors and improving the separation efficiency of hole-electron pairs. The obtained NiS/CdS composite delivered a photocatalytic H₂ evolution rate (7.49 mmol h^?1 g^?1), which was 39.42 times as high as that of pure CdS (0.19 mmol h^?1 g^?1). This study demonstrates the advantages of 0D/0D heterojunction photocatalysts for visible light-driven photocatalytic hydrogen production.     

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.     

Construction of hollow tubular Co9S8/ZnSe S-scheme heterojunctions for enhanced photocatalytic H2 evolution

Herein, ZnSe nanoparticles with good visible-light response were in-situ deposited on the surface of the hollow tubular Co₉S₈ to form compact Co₉S₈/ZnSe heterojunctions via hydrothermal and solvothermal methods. This architecture is beneficial to expose more active sites due to the uniform dispersion of ZnSe particles. Under visible light irradiation, the composites at the optimum Co₉S₈ amount (5 wt%) take on notably higher hydrogen evolution activity, 967.8 μmol/g/h, which is 3.1 times that of independent ZnSe (314.2 μmol/g/h). A series of tests manifested that the Co₉S₈–ZnSe heterojunction significantly promotes the separation of photo-induced electron-hole pairs, notably improves hydrogen evolution kinetics and reduces the electron transfer resistance, which is responsible for the enhanced photocatalytic activity of the composites. Furthermore, the photocatalytic mechanism of the S-scheme heterojunction was proposed based on the measured energy band potentials. This work provides a strategy in constructing inexpensive heterojunction photocatalysts for enhancing the hydrogen evolution performance.     

Design and preparation of CdS/H-3D-TiO2/Pt-wire photocatalysis system with enhanced visible-light driven H2 evolution

In recent years, tremendous efforts have been devoted to develop new photocatalyst with wide spectrum response for H₂ generation from water or aqueous solution. In this work, CdS nanoparticles (NPs) have been immobilized on hydrogenated three-dimensional (3D) branched TiO₂ nanorod arrays, resulting in a highly efficient photocatalyst, i.e, CdS/H-3D-TiO₂. In addition, electrochemical reduction of H⁺ ion is identified as a limiting step in the photocatalytic generation of H₂ at this catalyst, while here a Pt wired photocatalysis system (CdS/H-3D-TiO₂/Pt-wire) is designed to overcome this barrier. Without the application of potential bias, visible light photocatalytic hydrogen production rate of CdS/H-3D-TiO₂/Pt-wire is 18.42 μmol cm^?2 h^?1, which is 11.2 times that of CdS/H-3D-TiO₂ without Pt (1.64 μmol cm^?2 h^?1). The Pt wire acts as an electron super highway between the FTO substrate and H⁺ ions to evacuate the generated electrons to H⁺ ions and catalyze the reduction reaction and consequently generate H₂ gas. This work successfully offers a novel direction for dramatic improvement in H₂ generation efficiency in photocatalysis field.     

Fabrication of CdS/Zn2GeO4 heterojunction with enhanced visible-light photocatalytic H2 evolution activity

CdS/Zn₂GeO₄ (CG) composites were synthesized through the simple hydrothermal process. The crystal structure, morphology and light absorption property of the products were studied in detail. The CG composites showed excellent photocatalytic hydrogen production performance upon visible light illumination. Especially, the CG-3 composite displayed the highest H₂ evolution rate of 1719.8 μmol h⁻¹ g⁻¹, which was about 3.80 and 4.28 times higher than the pure CdS and Zn₂GeO₄. Besides, the cyclic stability of the CG-3 composite was also excellent. The PL, photocurrent response and EIS spectra results testified that the efficient separation and transfer of photoinduced charge carriers achieved between CdS and Zn₂GeO₄, which could result in the promotion of photocatalytic performance. Moreover, a possible mechanism of H₂ generation over CdS/Zn₂GeO₄ heterojunction was discussed. The practicable way to construct heterojunction composites would be helpful for the design of other systems with excellent photocatalytic property.     

Co3O4/CeO2 p-n heterojunction construction and application for efficient photocatalytic hydrogen evolution

The construction of p-n type heterojunction is an effective way to enhance the efficiency of photocatalytic hydrogen evolution. In this work, Co₃O₄/CeO₂ p-n heterojunction was construct by a simple hydrothermal method. This heterojunction mainly uses the internal electric field formed and accelerate the separation of electrons and holes in the opposite direction. In addition, according to SEM and TEM characterization, it was found that the granular cobalt oxide nanoparticles prepared by in-situ hydrothermal method were firmly and uniformly dispersed in cerium oxide, which effectively increased the active sites of hydrogen evolution. And combined with the BET results, it shows that the growth of cobalt oxide effectively increases the specific surface area and increases the active sites for hydrogen evolution. By exploring the hydrogen evolution capacity of different ratios of the complex, the test results showed that in all different ratios of the catalyst, CC-0.16 showed the best performance, and the hydrogen production efficiency reached 2298.52 μmol g⁻¹h⁻¹, which was 71 times that of nanobelt CeO₂ and 2.72 times that of Co₃O₄. According to the characterization results, the photocatalytic water splitting mechanism of the p-n heterojunction was proposed, and the charge transfer mechanism in the process was discussed in depth.     

Hydrogenated CdS nanorods arrays/FTO film: A highly stable photocatalyst for photocatalytic H2 production

To improve the photocorrosion of CdS nanorod arrays (CdS NRAs), we have designed a simple and facile treatment method of in-situ hydrogenation to fabricate CdS@SnS/SnO₂ heterostructure on fluorine-doped tin oxide glass, which is a highly photostable hydrogenated CdS-based film photocatalyst (CdS NRAs-H₂). Over a 25-h long time irradiation, the total photocatalytic hydrogen production of hydrogenated CdS NRAs is almost 2.0 times higher than that of un-hydrogenated CdS NRAs. Moreover, the average hydrogen production rate of CdS NRAs-H₂ can steadily maintain at 23.75 μmol cm^?2 h^?1 with 102% of retention rate after 5 reaction cycles, while they are only 6.13 μmol cm^?2 h^?1 with 30% of retention rate for un-hydrogenated common CdS NRAs. The photocatalytic mechanism on enhanced activity and stability for hydrogenated CdS NRAs photocatalyst is also investigated and discussed in detail.     

Fe2P nanoparticles as highly efficient freestanding co-catalyst for photocatalytic hydrogen evolution

Design of non-noble-metal artificial photosynthesis system that can split water with high apparent quantum yield (AQY) and robust stability remains a fundamental challenge. Here we report that a physical mixture of Fe₂P nanopaticles (NPs) and CdS nanosheets (NSs) can gives AQY of photocatalytic hydrogen production as high as 90% at 420 nm monochromatic light with ethanol as electron donor at strong alkaline conditions. The highest rate for hydrogen production reached about 220 mmol g^?1 h^?1. In this hybrid photocatalyst system, free standing Fe₂P NPs act as efficient and robust noble-metal-free co-catalysts and ultrathin CdS NSs are used as the photosensitizer. PL and TRPL results demonstrate that photoexcited electron can transfer from the conduction band of the excited CdS to Fe₂P, which aided charge separation and enhanced the hydrogen evolution activity. Femtosecond transient absorption result reveals that the time-averaged interfacial electron transfer (ET) rate constant (<k_ET>) from CdS NSs to Fe₂P is about 7.4 × 10⁹ s^?1 under the guarantee of the scavenging of photoexcited hole immediately, which is one order faster than the electron relaxation rate in pure CdS NSs.     

The interfacial charge transfer in triphenylphosphine-based COF/PCN heterojunctions and its promotional effects on photocatalytic hydrogen evolution

In this study, the novel triphenylphosphine-based covalent organic frameworks (P–COF-1) were firstly introduced into polymeric carbon nitride (PCN) to fabricate P–COF-1/PCN heterojunctions via intermolecular π-π interaction. The photocatalytic H₂ production rate over the 9% P–COF-1/PCN heterojunctions is ca. 12 times as much as that of pure PCN. The photoelectrochemical measurements and theoretical calculation results show that due to the well-matched band structure between P–COF-1 and PCN, the photo-generated electrons tend to migrate from P–COF-1 into the conduction band of PCN through the interface of heterostructures. In addition to the π→π1 electron transition of conjugated tri-s-triazine units in the 9% P–COF-1/PCN with band gap energy of 2.53 eV, the lone pair electrons of P transition to the π1 orbitals of P–COF-1 (n→π1) with lower band gap energy of 1.82 eV results in the effective separation of photo-generated carriers and more visible light absorption, and thus enhanced the photocatalytic hydrogen evolution.     

A 2D/1D TiO2 nanosheet/CdS nanorods heterostructure with enhanced photocatalytic water splitting performance for H2 evolution

TiO₂ with exposed (001) facets were composited with CdS nanorods to construct 2D/1D heterojunction. As comparison, P25 with mainly exposed (101) facets were employed to combine with CdS nanorods. The 2D/1D heterojunction of TiO₂ nanosheets and CdS nanorod displayed 3.7 times higher hydrogen generation than that of P25/CdS composites. The results indicated that TiO₂ with exposed (001) facets were favorable for enhancing the photocatalytic activity of CdS via optimizing the heterojunction between TiO₂ and CdS. Photoluminescence and photoelectrochemical characteristics results demonstrated that the 2D-TiO₂/1D-CdS heterojunction exhibits higher separation efficiency of photoinduced carriers and superior electron transfer ability. This work exemplifies that heterojunction modification is an effective strategy to improve the efficiency of the photocatalyst composites.     

Metalloid Ni2P and its behavior for boosting the photocatalytic hydrogen evolution of CaIn2S4

The development of high-efficiency and low-cost photocatalysts in photocatalytic H₂ evolution systems from water remains challenging. The substitution of a noble metal as the co-catalyst is still one of the important and meaningful issues in this field. Herein, we report a series of CaIn₂S₄ catalysts combined with Ni₂P, which acts as the co-catalyst, for boosting photocatalytic hydrogen evolution under visible light. The integrated system of the Ni₂P/CaIn₂S₄ composite exhibited high efficiency and durability, which were even higher than those of Pt decorated catalysts. The promoting effect of Ni₂P can be ascribed to its excellent reductive ability and analogous metallic character, which can accelerate the transfer and consumption of the photo-generated electrons. Moreover, based on the surface photo-voltage technique and electrochemical tests, the unique mechanism of Ni₂P for the movement of photo-generated charges during the photocatalysis process is proposed for the first time.     

Fabrication of CdS/NiFe LDH heterostructure for improved photocatalytic hydrogen evolution from aqueous methanol solution

2D CdS/NiFe LDH (short for layered double hydroxide) heterostructures were designed and fabricated by following a facile in-situ growth method. The CdS nanoparticles are well dispersed on the surface of NiFe LDH to form nanoscale heterojunctions, as suggested from the TEM and elemental mapping images. The composites with optimum CdS amount (15 wt%) take on notably higher hydrogen evolution activity (469 μmol h^?1 g^?1) than the independent CdS and NiFe LDH from aqueous methanol solution under xenon lamp irradiation. The nano-heterojunction notably promotes the H₂ evolution kinetics and greatly suppresses the recombination of photo-induced electrons and holes, which is responsible for the enhanced photocatalytic activity of the composites, as demonstrated by the reducing onset potential and increasing photocurrent of the composites in the photoelectrochemical experiments. The possible photocatalytic mechanism is proposed on the basis of the defined position of energy band edges.     

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.     

In-situ synthesis of ternary heterojunctions via g-C3N4 coupling with noble-metal-free NiS and CdS with efficient visible-light-induced photocatalytic H2 evolution and mechanism insight

The two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) nanosheets based composites are prepared in the form of the NiS/g-C₃N₄, CdS/g-C₃N₄ and CdS/NiS/g-C₃N₄ using a facile and reliable method of chemical deposition. The TEM and HRTEM images demonstrated a spectacular representation of the 2D lamellar microstructure of the g-C₃N₄ with adequately attached CdS and NiS nanoparticles. The changes in crystallinity and the surface elemental valence states of composites with the incorporation of two metal sulphides are studied, which confirmed the formation of composites. The photocatalytic response of the composites was estimated by photodegradation of Rhodamine B (C₂₈H₃₁ClN₂O₃–RhB), and the ternary composite CdS/NiS/g-C₃N₄ samples exhibited the superior photocatalytic performance. Further, the free radical capture and electron paramagnetic resonance (EPR) spectroscopy experiments identified the main active species that contributed to the photocatalytic reaction. Besides, the samples’ photocatalytic performance was evaluated by photocatalytic hydrogen production. The stability of the performance-optimized composite was determined by employing cyclic experiments over five cycles. The CdS/NiS/g-C₃N₄ showed the highest efficiency of hydrogen production i.e. about 423.37 μmol.g^?1.h^?1, which is 2.89 times that of the pristine g-C₃N₄. Finally, two types of heterojunction structures were proposed to interpret the enhanced photocatalytic efficiency.     

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.     

A novel pathway toward efficient improvement of the photocatalytic activity and stability of CdS-based photocatalyst for light driven H2 evolution: The synergistic effect between CdS and SrWO4

It has been a research hot spot how to efficiently heighten the photocatalytic activity and stability of CdS-based photocatalysts for H₂ evolution. Here, SrWO₄/CdS nanoparticles which contained CdS/SrWO₄ heterojunctions were prepared. Meanwhile, their photocatalytic performance and stability were investigated in detail for H₂ evolution. At last, the photocatalytic mechanism of the SrWO₄/CdS nanoparticles was discussed roughly. The results show that the photocatalytic performance of CdS can be heightened significantly due to introduction of SrWO₄. The fastest evolution rate of H₂ over the SrWO₄/CdS nanoparticles is 392.5 μmol g⁻¹ h⁻¹, which is 5.8 times as high as that over the pure CdS nanomaterial. More interestingly, the SrWO₄/CdS nanoparticles possess excellent stability. The evolution rate of H₂ over the photocatalyst used 10 times can be up to 473 μmol g⁻¹ h⁻¹, which is the same as that over the once used sample, even is 37% higher than that over of the fresh one. In contrast, after used five times, the photocatalytic activity of the pure CdS nanomaterial is only 57% of that of the fresh sample. This study will supply a new idea for the design and development of highly stable and efficient CdS-based photocatalysts for H₂ evolution in the future.     

High-efficient C3N4-viologen charge transfer systems for promoting photocatalytic H2 evolution through band engineering

Accelerating the charge transfer (CT) capability of photocatalysts is an efficient way to improve the overall photocatalytic performance, yet the precise regulation of CT in photocatalyst systems is still lacking. In this paper, a series of hybrid photocatalysts composed of graphitic carbon nitride (CN) and various viologens (V) were prepared for the photocatalytic hydrogen evolution (PHE) from water splitting under visible-light irradiation. Considering the fixed energy structure of CN, the different electron-withdrawing substituents were introduced to engineer the band structure of V delicately and modulate the CT process between CN and V. It was shown that all the hybrid photocatalysts CN-x%V_y exhibited higher photocatalytic performance, of which CN–1%V₃, possessing the strongest electron withdrawing group (-NO₂), demonstrated the best PHE performance (3572.3 μmol g⁻¹ h⁻¹), exceeding 29 times over the unmodified CN. It was proposed that the introduction of V can optimize the interfacial photogenerated electron transfer (CN→V→Pt) of the whole photocatalytic system effectively. We highlighted the V as an efficient chemical segment to modify semiconductors toward enhanced activity due to the following unique characteristics: (i) the unique redox ability, (ii) the easy synthetic methods for controlling the band structures precisely, and (iii) the inherent positively charged feature. This work provides a deep understanding of CT for the rational design of high-performance photocatalysts through band engineering.