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.     

Synthesis and photochemical performance of morphology-controlled CdS photocatalysts for hydrogen evolution under visible light

Novel CdS nanomaterials were synthesized by a simple “one-pot” hydrothermal biomolecule-assisted method using glutathione (GSH) as the sulfur source and structure-directing reagent. Various morphologies of CdS photocatalysts, such as solid nanospheres (s-CdS), hollow nanospheres (h-CdS) and nanorods (r-CdS), were obtained by controlling only the hydrothermal temperatures. The X-ray diffraction patterns confirmed that all of the samples were typical hexagonal wurtzite CdS. It was found that the absorption edge of s-CdS was at 465 nm with a greater blue shift compared to that of h-CdS and r-CdS. The photocatalytic activity of s-CdS was superior to that of h-CdS and r-CdS under visible light. Photoluminescence measurements revealed their different photogenerated electron/hole recombination ability, which was in accordance with the order of s-CdS < h-CdS < r-CdS. The excellent photocatalytic activity of s-CdS was ascribed to the small sizes of sub-nanocrystallites, which make it easy for photoinduced electrons and holes on the solid sphere to migrate to the surface and react with water and the sacrificial agent quickly. It was crucial to control the temperature for preparing CdS photocatalysts via hydrothermal methods. The formation mechanism of different morphology might be due to complexation, S-C bond rupture, spherical aggregation and Ostwald ripening processes.     

Synergistic effects of multiple heterojunctions significantly enhance the photocatalytic H2 evolution rate CdS/La2Ti2O7/NiS2 ternary composites

Novel CdS/La₂Ti₂O₇/NiS₂ ternary composite photocatalysts without noble metal were successfully constructed by a simple hydrothermal method. Under visible light irradiation (λ > 400 nm), the optimal CdS/La₂Ti₂O₇/NiS₂ composite produced H₂ at a rate of about 12.77 mmol g⁻¹ h⁻¹, which was 84 times as high as that of pure CdS. This performance enhancement can be attributed to the formation of multiple heterojunctions (including CdS/NiS₂, CdS/La₂Ti₂O₇ and CdS/La₂Ti₂O₇/NiS₂ interface structures) between the three components in the as-prepared CdS/La₂Ti₂O₇/NiS₂ composites. The formed multiple heterojunctions help to separate electron-hole pairs more quickly and efficiently, thus greatly increasing the photocatalytic activity of the CdS/La₂Ti₂O₇/NiS₂ composites. This is very important for the utlization of solar energy for water splitting.     

g-C3N4/B doped g-C3N4 quantum dots heterojunction photocatalysts for hydrogen evolution under visible light

Developing high activity and eco-friendly photocatalysts for water splitting is still a challenge in solar energy conversion. In this paper, B doped g-C₃N₄ quantum dots (BCNQDs) were prepared via a facile molten salt method using melamine and boron oxide as precursors. By introducing BCNQDs onto the surface of g-C₃N₄, g-C₃N₄/BCNQDs heterojunction was constructed via hydrothermal treatment. The resulting g-C₃N₄/BCNQDs heterojunction exhibits enhanced hydrogen evolution performance for water splitting under visible light irradiation. The mechanism underlying the improved photocatalytic activity was explored and discussed based on the formation of heterojunction between g-C₃N₄ and BCNQDs with well-matched band structure.     

In-situ synthesis of well dispersed CoP nanoparticles modified CdS nanorods composite with boosted performance for photocatalytic hydrogen evolution

Development of photocatalysts with characters of low-cost, environment friendliness, visible light response and good performance is vital for the transformation of solar energy into hydrogen fuel. Here, we constructed CoPCdS nanorods hybrid composites via a novel two-step in-situ growth method for the first time. The obtained CoPCdS composites exhibited remarkably enhanced photocatalytic performance and excellent stability in comparison with bare CdS nanorods. Notably, the optimum H₂ evolution rate of 1 wt%CoPCdS was 9.11 times higher than that of pristine CdS. The apparent quantum efficiency of the photocatalyst was calculated to be 11.6%. The superior activity of this material could be attributed to the role of well dispersed CoP nanoparticles and the intimate interface between CoP cocatalysts and CdS nanorods, which efficiently accelerated the separation and transfer of photogenerated electrons. This work provided a new in-situ growth method for the preparation of transition metal phosphides coated photocatalysts with boosted photocatalytic activity of hydrogen evolution.     

MoS2 grown in situ on CdS nanosheets for boosted photocatalytic hydrogen evolution under visible light

A novel nano-heterojunction photocatalysts of CdS/MoS₂ with appropriate interfacial contact was successfully obtained by the facile two-step hydrothermal synthesis. The MoS₂ ultrathin layer was well combined with CdS nanosheets and formed the interaction, which facilitated the transfer and separation of charges. The CdS/MoS₂ 15 wt% possessed much higher H₂ evolution photocatalytic performance (35.24 mmol h^?1 g^?1), exhibiting an 85.95 times enhancement as compared to that of pure CdS (0.41 mmol h^?1 g^?1). Moreover, the photochemical stability of CdS/MoS₂ heterojunctions was excellent, which showed no significant decrease in activity after four cycles of experiments. The finding provides a novel method to integrate the structure of MoS₂ with CdS, which exhibits great potential in solar energy conversion.     

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.     

TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation

The CdS/TiO₂ composites were synthesized using titanate nanotubes (TiO₂NTs) with different pore diameters as the precursor by simple ion change and followed by sulfurization process at a moderate temperature. Some of results obtained from XRD, TEM, BET, UV–vis and PL analysis confirmed that cadmium sulfide nanoparticles (CdSNPs) incorporated into the titanium dioxide nanotubes. The photocatalytic production of H₂ was remarkably enhanced when CdS nanoparticles was incorporated into TiO₂NTs. The apparent quantum yield for hydrogen production reached about 43.4% under visible light around λ = 420 nm. The high activity might be attributed to the following reasons: (1) the quantum size effect and homogeneous distribution of CdSNPs; (2) the synergetic effects between CdS particles and TiO₂NTs, viz., the potential gradient at the interface between CdSNPs and TiO₂NTs.     

Biomass-derived hierarchical porous CdS/M/TiO2 (M = Au,Ag, pt,pd) ternary heterojunctions for photocatalytic hydrogen evolution

We demonstrate a general method for the synthesis of biomass-derived hierarchical porous CdS/M/TiO₂ (M = Au, Ag, Pt, Pd) ternary heterojunctions for efficient photocatalytic hydrogen evolution. A typical biomass—wood are used as the raw sources while five species of wood (Fir, Ash, White Pine, Lauan and Shiraki) are chosen as templates for the synthesis of hierarchical porous TiO₂. The as-obtained products inherited the hierarchical porous features with pores ranging from micrometers to nanometers, with improved photocatalytic hydrogen evolution activity than non-templated counterparts. Noble metals M (M = Pt, Au, Ag, Pd) and CdS are loaded via a two-step photodeposition method to form core (metal)/shell (CdS) structures. The photocatalytic modules—CdS(shell)/metal (core)/TiO₂ heterostructures, have demonstrated to increase visible light harvesting significantly and to increase the photocatalytic hydrogen evolution activity. The H₂ evolution rates of CdS/Pd/TiO₂ ternary heterostructures are about 6.7 times of CdS/TiO₂ binary heterojunctions and 4 times higher than Pd/CdS/TiO₂ due to the vertical electron transfer process. The design of such system is beneficial for enhanced activity from morphology control and composition adjustment, which would provide some new pathways for the design of promising photocatalytic systems for enhanced performance.     

Effects of Pt3Ni alloy polyhedral and its de-alloying on CdS's performance in hydrogen evolution from water-splitting under visible light

In this paper, Pt₃Ni alloy polyhedral was synthesized through solvothermal method and loaded on the surface of CdS by photo-induced electrons. Under visible light irradiation, the photocatalytic activity for hydrogen evolution from solar water splitting was performed, Pt₃Ni/CdS showed the hydrogen evolution rate about 40.0 mmol/h/g (QE = 44.90%, λ = 420 nm), which was 1.8 times higher than that of Pt/CdS, indicating that Pt₃Ni NPs could effectively improve the hydrogen production activity of CdS. Next, the influence of de-alloyed Pt₃Ni NPs on the activity of CdS for water-splitting under visible light was investigated, the hydrogen evolution rate of de-alloyed Pt₃Ni NPs modified CdS was 46.1 mmol/h/g (QE = 52.70%, λ = 420 nm), which was 1.2 times as much as that of Pt₃Ni/CdS and 2.1 times as much as that of Pt/CdS, suggesting that de-alloyed Pt₃Ni NPs could further enhance the hydrogen production activity of CdS. In addition, the improved photocatalytic activity was mainly due to the surface unsaturation of Pt atoms in a metastable structure after de-alloying, which will expose more surface active sites of Pt, thus the fast electron hole charge transfer at the interface of CdS and de-alloyed Pt₃Ni NPs.     

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

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

Noble metal free efficient photocatalytic hydrogen generation by TaON/CdS semiconductor nanocomposites under natural sunlight

Tantalum oxynitride have narrow band gap and its band potentials are suitable for visible light induced hydrogen generation. However, due to fast electron-hole recombination, the efficiency of photocatalytic hydrogen evolution reaction is very low. Herein, we have synthesized semiconductor heterojunction photocatalyst, i.e., TaON/CdS with suitable band positions by a simple precipitation method. Ratio between two semiconductors is optimized to obtain maximum hydrogen evolution. XRD, XPS and TEM analysis demonstrate the formation of heterojunction between these semiconductors. Among the synthesized catalysts, 3% TaON/CdS heterostructure exhibits the highest hydrogen evolution activity with H₂ production rate of 7.5 mmol h⁻¹ under natural solar light, whereas the rate is 11 mmol h⁻¹ under the visible light generated by xenon (Xe) lamp without the addition of any noble metal as the co-catalyst. The CdS and 3% TaON/CdS nanomaterials show an AQE of 5.1% and 12.2%, respectively. Combination of Mott-Schottky, UPS and DR UV–visible spectroscopy studies revealed the formation of S scheme semiconductor heterojunction between these nanomaterials with valence, conduction band positions, i.e., 1.46, −0.78 eV for CdS and 2.19, −0.66 eV for TaON, respectively. These band positions help in efficient e-h pair separation to produce hydrogen from water.     

Kinetics of reduction of water to hydrogen by visible light on alumina supported Pt–CdS photocatalysts

The kinetics of hydrogen production from photolysis of water on alumina supported Pt–CdS catalyst using visible light has been studied. An induction period with negligible rate was observed upon illumination of catalyst. The rate gradually increases and again falls down. The rate has been observed to be proportional to the sulfide ions adsorbed on the surface of CdS. The induction period has been related to the re-establishment of adsorption equilibrium of sulfide ions on the catalyst surface under illumination. The decrease in rate is due to the deactivation of catalyst by hydrogen. A power law type rate expression for hydrogen production has been proposed which takes into account the deactivation of the catalyst.     

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.     

New insights into the mechanism of photocatalytic hydrogen evolution from aqueous solutions of saccharides over CdS-based photocatalysts under visible light

A new multiphase photocatalyst – 1%Pt/Cd_0.6Zn_0.4S/Cd_0.1Zn_0.9S – was synthesized by a simple hydrothermal treatment of as-prepared Cd_0.3Zn_0.7S solid solution and thoroughly characterized by different methods. The activity was tested in a sustainable process of hydrogen evolution from aqueous solutions of two saccharides (glucose and xylose) under visible light. The mechanism of the photocatalytic hydrogen evolution from complex organic substrates is still controversial. In this research it was shown that optimizing the experimental conditions (pH, substrate concentration) leads to a significant increase in the photocatalytic activity for both saccharides. A kinetic equation based on a Langmuir model and including the degree of dissociation of the substrate was proposed and verified for the first time. The highest activities during hydrogen evolution for glucose and xylose were achieved in strongly alkaline media. The photocatalyst 1%Pt/CdZnS120 possessed an activity equal to 3.4 mmol H₂ h⁻¹ g⁻¹ (glucose, apparent quantum efficiency 8.4%), that exceeds recently reported values.     

Photocatalytic hydrogen production over modified SiC nanowires under visible light irradiation

β-SiC nanowires were synthesized by simple carbothermal reduction of a mixture composed of low-cost water glass and starch, and characterized by XRD, SEM, TEM, BET, UV–visible absorption, FT-IR spectrometer and X-ray photoelectron spectrometer. The results show that the β-SiC nanowires can absorb visible light and exhibit excellent photocatalytic hydrogen evolution performance from pure water under visible light irradiation. The hydrogen evolution rate can reach more than 60 μL/g·h. SiC nanowires by simple modification can greatly enhance the efficiency of H₂ production. Average H₂ production rate over the modified SiC is 76.1% higher than that of unmodified SiC. The enhanced H₂ production is mainly due to stronger hydrophilic ability of the modified SiC. In addition, O₂ is detectable in the experiment, indicating that water has been decomposed into H₂ and O₂.     

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.     

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.     

Synthesis of MOF/MoS2 composite photocatalysts with enhanced photocatalytic performance for hydrogen evolution from water splitting

With the shortage of global fossil energy and the increasing crisis of environmental deterioration, hydrogen energy has become an environmentally benign alternative as a clean energy source. In most studies on photocatalytic hydrogen production, novel photocatalytic material has played an important role to enhance the hydrogen production rate. In this study, the optimal conditions for the synthesis of MoS₂ were established through series of characterizations with 190 °C calcination temperature and 1 wt% PEG surfactant addition. The best conditions for synthesizing MOF include copper nitrate as the copper precursor, 30% ultrasonic amplitude, and 240 °C calcination temperature. After adding 1 wt% MOF in MOS₂, a flower-like structure with small particle size, uniform distribution, regularity, and large surface pores, has been formed, where its unit is modified with many rough, porous, and high specific surface area octahedral structures. In addition, 1MOF/MOS₂ has the most negative conduction band edge (?0.135 V), the smallest charge transfer resistance (R_ct = 1.78 Ω), the largest photo current (11.1 mA/cm²), the lowest PL spectral peak intensity, and excellent photocatalytic stability. The above morphological features and optical properties can significantly form more active sites, enhance the electron transfer rate, and inhibit the electron-hole recombination, thus making the MOF/MOS₂ composite photocatalyst achieve the maximum hydrogen production capacity (626.3 μmol g^?1 h^?1).     

Design and synthesis of cyclometalated iridium-based polymer dots as photocatalysts for visible light-driven hydrogen evolution

The organic semiconducting polymer dots (Pdot) or nanoparticles exhibited a promising efficiency as photocatalyst for hydrogen production. This study reported a new Pdot-based photocatalyst constructed in the form of Donor-Acceptor-Metal cocatalyst (D-A-M_cat). These D-A-M_cat Pdots systems consisting of fluorene (F) as the donor, thienyl-benzo-dithiophene-dione (TBDD) as the acceptor and [Ir (TPy)₂ (acac)] as the metal complex, and their structural, thermal, electrochemical and photophysical properties were systematically demonstrated for the first time. Introducing the cyclometalated iridium (III) complex comonomer into the polymer chain resulting in PFTBDD-IrTPy Pdots showed enhancement of the hydrogen evolution rates, which is over 20-times higher than those of pure polymer (PFTBDD Pdots) under otherwise identical conditions. In addition, the optical and electrical measurements indicated that the cyclometalated iridium (III) part have an important function for inhibiting the charge recombination of the PFTBDD-IrTPy Pdots during the photocatalytic reaction. The result strongly implies that inserted catalytic amount of Ir(III)-complex into the D-A polymers is beneficial for the enhancement of photocatalytic hydrogen evolution, which can inspire further optimization and greater molecular design strategies at a low-cost are highly desirable for the development of high-performance in photocatalysis.