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.     

TiO2 nanoparticles modified with 2D MoSe2 for enhanced photocatalytic activity on hydrogen evolution

As an emerging two-dimensional (2D) nanomaterial, 2D MoSe₂ nanosheets has the advantages of wide light response and rapid charge migration ability. In this work, 2D MoSe₂/TiO₂ nanocomposites were successfully synthesized through a simple hydrothermal method. The microstructure and photocatalytic activity of the nanocomposites were systematically investigated and determined. The corresponding Raman peaks and crystal planes of MoSe₂ were analysed by Raman spectroscopy and transmission electron microscopy respectively, demonstrating the successful combination of the MoSe₂ nanosheets and TiO₂ nanoparticles. UV-vis diffused reflectance spectra demonstrated that the introduction of MoSe₂ did increase the light absorption ability of the nanocomposites. A lower recombination of electrons and holes was demonstrated for the MoSe₂/TiO₂ heterojunction from photoluminescence results. The photocatalytic hydrogen evolution test showed that the hydrogen production rate was 4.9 μmol h⁻¹ for the sample with 0.1 wt.% MoSe₂, 2 times higher than that of bare TiO₂. This work provides a novel strategy for improving the photocatalytic properties of semiconductor photocatalyst.     

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.     

MoS2/CdS rod-like nanocomposites as high-performance visible light photocatalyst for water splitting photocatalytic hydrogen production

This study demonstrates a high-performance visible-light-driven photocatalyst for water splitting H₂ production. CdS nanorods (30 nm in diameters) with shorter radial transfer paths and fewer defects were prepared by a solvothermal method. To mitigate the recombination of electrons and holes, MoS₂ nanosheets with rich active sites were modified on the surface of CdS nanorods by a room-temperature sonication treatment. The photocatalytic water splitting tests show that the MoS₂/CdS nanocomposites exhibit excellent H₂ evolution rates. The highest H₂ evolution rates (63.71 and 71.24 mmol g^?1h^?1 in visible light and simulated solar light irradiation) was found at the 6% MoS₂/CdS nanocomposites, which was 14.61 times and 13.39 times higher than those of the corresponding pristine CdS nanorods in visible light and simulate solar light irradiation, respectively. The apparent quantum efficiency (AQE) of the 6% MoS₂/CdS nanocomposites at 420 nm was calculated to be 33.62%. The electrochemistry tests reveal that the enhanced photocatalytic activity is a result of extra photogenerated charge carries, greatly enhanced charge separation and transfer ability of the MoS₂/CdS composites. This study may give new insights for the rational design and facile synthesis of high-performance and cost-effective bimetallic sulfide photocatalysts for solar-hydrogen energy conversion.     

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.     

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.     

Noble metal-free cobalt oxide (CoOx) nanoparticles loaded on titanium dioxide/cadmium sulfide composite for enhanced photocatalytic hydrogen production from water

In this present paper, cobalt oxide (CoO_x) is studied as an effective cocatalyst in a photocatalytic hydrogen production system. CoO_x-loaded titanium dioxide/cadmium sulfide (TiO₂/CdS) semiconductor composites were prepared by a simple solvothermal method and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). Photocatalytic hydrogen production was studied using the as-synthesized photocatalysts in aqueous solution containing sodium sulfide (Na₂S)/sodium sulfite (Na₂SO₃) as hole scavengers under visible light irradiation (λ > 400 nm). The optimal cobalt content in CoO_x-loaded TiO₂/CdS composite is determined to be 2.1 wt% and the corresponding rate of hydrogen evolution is 660 μmol g⁻¹ h⁻¹, which is about 7 times higher than TiO₂/CdS and CdS photocatalysts under the same condition. Visible light-driven photocurrents of the semiconductor composites were further measured on a photoelectrochemical electrode, revealing that the photocorrosion of CdS can be prevented due to the presence of TiO₂–CoO_x.     

CuS,NiS as co-catalyst for enhanced photocatalytic hydrogen evolution over TiO2

TiO₂ photocatalysts loaded CuS and NiS as co-catalyst were prepared by hydrothermal approach and characterized by XRD, UV–visible DRS, BET, XPS, SEM and TEM. When TiO₂ was loaded MS as co-catalyst, it showed higher photocatalytic activities for splitting water into hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the maximum evolution of H₂ obtained from 5 wt% CuS–5 wt% NiS–TiO₂ sample was about 800 μmol h⁻¹, which was increased up to about twenty-eight times than that of TiO₂ alone. It was proven that CuS, NiS can act as effective dual co-catalysts to enhance the photocatalytic H₂ production activity of TiO₂.     

Hydrogen production performance of active Ce/N co-doped SrTiO3 for photocatalytic water splitting

An efficient visible light responsive photocatalyst Ce/N co-doped SrTiO₃ was prepared via a hydrothermal method for hydrogen production. The phase structure, morphology, contents and valence states of the dopant elements, specific surface area, optical properties, and photocatalytic activity of the samples were characterized. The transient photocurrent response and electrochemical impedance spectra under visible light illumination indicated that Ce/N co-doped SrTiO₃ possessed a more intense photo-current response and lower surface resistance than N–SrTiO₃ and Ce–SrTiO₃. The water splitting rate of Ce/N-co-doped SrTiO₃ is 4.28 mmol/g/h, which is 84.49 times higher than that of pure SrTiO₃. The enhanced photocatalytic performance is due to the narrowing of the band gap of SrTiO₃ by Ce ion and N ion impurities.     

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.     

Visible-light photo-assisted synthesis of GO-TiO2 composites for the photocatalytic hydrogen production

The main aim of this study is to propose a fast and simple synthesis method for GO-TiO₂ nanocomposites, which present self-tuning optoelectronic properties, advantageous features, for their application towards the photocatalytic hydrogen production. Graphene oxide (GO) was prepared through a modification of the Tour method from powder graphite by applying a microwave pretreatment. GO-TiO₂ nanocomposites were synthesized by visible-light assisted anchoring, while the study of morphology, crystalline structure, size, surface area and optical characterization of the material was carried out by SEM, XRD, TEM, BET and UV-Vis spectroscopy, respectively. Photocatalytic activity evaluation of the composite material (GO-TiO₂) towards hydrogen production through the water splitting reaction from water under visible light was followed by gas chromatography, reaching a production of 6500 μmolH₂/g, thus showing an enhancement effect compared to the conventional H₂ production using TiO₂ only.     

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.     

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.     

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.     

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.     

Alloyed PdNi hollow nanoparticles as cocatalyst of CdS for improved photocatalytic activity toward hydrogen production

Photocatalytic hydrogen evolution has been regarded as an efficient method for H₂ production, in which the cocatalysts play a crucial role. In this work, two-dimensional (2D) snow-flake CdS was synthesized via a solvothermal method. PdNi hollow alloy with different compositions were synthesized by a galvanic replacement method, and decorated on CdS surface. The structural microscopic and spectroscopic analysis demonstrated that the formation of PdNi hollow nanoparticles (HNPs) and the decoration of PdNi HNPs on CdS (PdNi/CdS). Photocatalytic hydrogen evolution reaction was performed under visible light irradiation (λ ≥ 420 nm). Pd₁Ni₁/CdS exhibited higher photocatalytic H₂ generation rate about 54 mmol/h/g with a quantum efficiency of 63.97% at 420 nm, which was 1.7-fold higher than that of Pd/CdS (32.4 mmol/h/g). The high photocatalytic performance for Pd₁Ni₁/CdS was mainly attributed to the strong interaction between Pd₁Ni₁ HNPs and CdS, and the formation of unique hollow structure of PdNi alloy with porous nature which provided more active sites for H₂ evolution. Additionally, the synergistic effect between Pd and Ni, as well as the 2D morphology of CdS enhance the mobility of photo-generated charge carriers which minimize their recombination in turn enhance the photocurrent and photocatalytic performance of solar water splitting reaction.     

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.     

Effect of reaction atmosphere on photodeposition of Pt nanoparticles and photocatalytic hydrogen evolution from SrTiO3 suspension system

We systematically investigated the impact of reaction atmosphere on the Pt cocatalyst photodeposition and hydrogen evolution. A long induction period which is hardly found in close-system appears in the presence of oxygen, and is affected by the concentration of chloroplatinic acid. The Pt cocatalyst is metal state and contributes to the electronic states near Fermi level judging from the valence band X-ray photoelectron spectrum. The cocatalyst metal Pt will reduce the surface photovoltage signals due to the metal feature. The hydrogen evolution rate increased and reached a platform with increasing the Pt coverage on the surface of photocatalyst which is characterized by the Pt:Ti atomic ratio.     

Prickly Ni3S2 nanowires modified CdS nanoparticles for highly enhanced visible-light photocatalytic H2 production

The photocatalytic water splitting strategy is one of the most promising ways to achieve clean and renewable solar-to-hydrogen energy conversion. In this study, a highly enhanced photocatalytic H₂ production system has been achieved, using CdS nanoparticles (NPs) decorated on prickly Ni₃S₂ nanowires (NWs) as the light-driven photocatalyst. The photocatalyst was prepared by a co-precipitated method in which spiky Ni NWs were employed as starting material for prickly Ni₃S₂ NWs. Characterization analysis (XRD, TEM, XPS, etc.) show the high purity of Ni₃S₂/CdS hybrid structures and the well deposition of CdS NPs on prickly Ni₃S₂ NWs. Besides, the as synthesized Ni₃S₂/CdS photocatalyst exhibit reduced photoluminescence peak intensity, which means the Ni₃S₂ NWs functions as electron collector and transporter to quench the photoluminescence of CdS. This prickly Ni₃S₂/CdS nanocomposite demonstrates a 70 times higher H₂ production rate than that of pure CdS and a quantum efficiency of 12.3% at the wavelength of 400 nm in the absence of noble metals. This enhanced H₂ production activity is better than the one of CdS loaded with 0.5 wt% Pt. Our findings highlight the potential application of Ni₃S₂/CdS hybrid structures for visible light photocatalytic hydrogen yielding in the energy conversion field.