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.     

A visible-light-driven heterojunction for enhanced photocatalytic water splitting over Ta2O5 modified g-C3N4 photocatalyst

The photocatalytic water splitting for generation of clean hydrogen energy has received increasingly attention in the field of photocatalysis. In this study, the Ta₂O₅/g-C₃N₄ heterojunctions were successfully fabricated via a simple one-step heating strategy. The photocatalytic activity of as-prepared photocatalysts were evaluated by water splitting for hydrogen evolution under visible-light irradiation (λ > 420 nm). Compared to the pristine g-C₃N₄, the obtained heterojunctions exhibited remarkably improved hydrogen production performance. It was found that the 7.5%TO/CN heterojunction presented the best photocatalytic hydrogen evolution efficiency, which was about 4.2 times higher than that of pure g-C₃N₄. Moreover, the 7.5%TO/CN sample also displayed excellent photochemical stability even after 20 h photocatalytic test. By further experimental study, the enhanced photocatalytic activity is mainly attributed to the significantly improve the interfacial charge separation in the heterojunction between g-C₃N₄ and Ta₂O₅. This work provides a facile approach to design g-C₃N₄-based photocatalyst and develops an efficient visible-light-driven heterojunction for application in solar energy conversion.     

Two-dimensional C3N/WS2 vdW heterojunction for direct Z-scheme photocatalytic overall water splitting

The development and design of efficient photoelectric catalysts is of great significance for environmental friendliness. This paper is devoted to finding a new two-dimensional van der Waals heterojunction to realize hydrogen production from water splitting. Based on first-principles calculations, a direct type-Z C₃N/WS₂ heterojunction was successfully designed by combining highly active two-dimensional transition metal dichalcogenides (TMDs) with C₃N similar in structure to graphene. The staggered band structure of the direct Z-scheme and high carrier mobility facilitates the efficient separation of photogenerated electron and hole pairs. More importantly, the C₃N/WS₂ direct Z-type heterojunction can perfectly realize total water splitting from pH = 0 to pH = 7. What's more, the Gibbs free energy and overpotential demonstrate the excellent hydrogen evolution capability and the oxygen evolution capacity of the material. In summary, these studies provide new ideas for designing high-performance photoelectric catalysts for visible-light water splitting.     

Hierarchical spheres assembled from large ultrathin anatase TiO2 nanosheets for photocatalytic hydrogen evolution from water splitting

We report the synthesis of TiO₂ hierarchical spheres (THS) with large specific surface area via a facile one-pot solvothermal method. The as-prepared THS are self-assembled by ultrathin TiO₂ nanosheets with thickness of several nanometers and they show a uniform spherical morphology with an average size of 500–700 nm. However, the as-prepared light yellow THS exhibit inferior photocatalytic activity for hydrogen evolution from water splitting due to the poor crystallization of TiO₂ and the existence of oxygen vacancies. Significantly, a subsequent thermal treatment improves the crystallinity of THS, reduces the oxygen vacancies, and thereby enhances the photocatalytic performance. It demonstrates that the sample annealed at 550 °C (THS550) exhibits the highest photocatalytic activity, about 5 times higher than that of commercial TiO₂ nanoparticles (CTiO₂). Moreover, the THS550 sample loaded with 1 wt% Pt exhibits an hydrogen evolution rate as high as 17.9 mmol h^?1g^?1, and the corresponding apparent quantum efficiency has been determined to be 28.46% under 350 nm light irradiation.     

Ti3C2 MXene nanoparticles modified metal oxide composites for enhanced photoelectrochemical water splitting

MXene, an emerging family of two-dimensional (2-D) material, has shown outstanding electronic properties and promise for the applications on energy storage and conversion. In this paper, Ti₃C₂ MXene nanoparticles were synthesized by a facile solvent exfoliation method and used to construct metal oxide/Ti₃C₂ heterostructures. When these heterostructures were used as photoanodes for photoelectrochemical water splitting, significantly improved photoactivity and stability were achieved. Compared to pristine TiO₂, 6-fold enhanced applied bias photon-to-current efficiency (ABPE) was achieved for TiO₂/Ti₃C₂ heterostructures. According to the electron spin resonance, electrical impedance spectroscopic and Mott-Schottky measurements, the enhanced photoelectrochemical performance was ascribed to the presence of Ti₃C₂ as oxygen evolution cocatalysts and the strong interfacial interactions between metal oxide and Ti₃C₂. Therefore, our research provides a new way to design MXene-based heterostructures for solar energy conversion applications.     

Recent developments of strontium titanate for photocatalytic water splitting application

The high efficiency of SrTiO₃ in the reaction of heterogeneous photocatalysis needs a suitable architecture that maximises photon absorption and minimises electron loss during excitation state. In order to further enhance the migration of charge carriers during excitation state, considerable effort has to be exerted to further develop the heterogeneous photocatalysis of this SrTiO₃ under UV, visible, and solar illumination. Currently, unique and interesting features of binary photocatalyst system have gained more attention by researchers and it became a favourite research topic among various groups of scientists around the world. It was noticed that the binary photocatalyst system properties primarily depends on the nature of the surface properties, surface morphologies, as well as the role of optimum dopants amount incorporated into the SrTiO₃. Thus, this article presents a critical review of recent achievements in the photocatalytic activity of the SrTiO₃ for water splitting H₂ generation technology.     

Sustainable and stable hydrogen production over petal-shaped CdS/FeS2 S-scheme heterojunction by photocatalytic water splitting

One-dimensional CdS nanorods have garnered interest because of their highly visible light response, narrow bandgap, and negative potential at the conduction band edge, which are suitable for proton reduction. However, their poor charge separation and surface photocorrosion remain unresolved. In this study, CdS was synthesized with a 3D dendrite-like morphology to reduce its surface instability and junctioned with inexpensive FeS₂ particles to extend the absorption region toward visible light and improve its photoactivity. The photocurrent density was increased 8.3 times, and the photoluminescence reduced by half in petal-shaped CdS/15% FeS₂ compared with pure CdS. The petal-shaped CdS/15% FeS₂ heterojunction catalyst exhibited significantly enhanced photostability and photocatalytic activity; when 10% lactic acid was used as a hole scavenger, the hydrogen generation rate was 22.91 mL g^?1 for 10 h in pure CdS particles and 107.56 mL g^?1 in the petal-shaped CdS/15% FeS₂ particles. Moreover, the amount of hydrogen generated was maintained until 8th recycling experiments. The Cd and S ions eluted via photocorrosion were not detected after the reaction was complete. This was attributed to the petal-shaped CdS/FeS₂ heterojunction system which protected the unstable CdS surface owing to its controlled morphology. The FeS₂ junction improved visible light absorption facilitating the separation of photogenerated charges.     

Synthesis and high photocatalytic hydrogen production of SrTiO3 nanoparticles from water splitting under UV irradiation

Cubic SrTiO₃ powders were synthesized by three methods: the polymerized complex (PC) method, the solid state reaction, and the milling assistant method. The samples obtained were characterized by X-ray diffraction (XRD), UV–vis spectroscopy (UV–vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The mean diameters of the as-synthesized SrTiO₃ particles were 30 nm by the polymerized complex method, 140 nm by the solid state reaction, and 30 nm by the milling assistant method. The photocatalytic activity of hydrogen evolution from water splitting over SrTiO₃ powders by the polymerized complex method is higher than that by the solid state reaction and the milling assistant method. Particle size, uniformity of components, and particle aggregation extent affect the photocatalytic activity of SrTiO₃ for hydrogen evolution. The best rate of photocatalytic hydrogen evolution over SrTiO₃ by the polymerized complex method under UV illumination is as high as 3.2 mmol h⁻¹ g⁻¹.     

Two-dimensional g-C6N6/SiP-GaS van der Waals heterojunction for overall water splitting under visible light

The energy crisis caused by the decrease of fossil fuels and the environmental pollution problems related to combustion can be resolved through the green technology of photocatalytic water splitting to produce hydrogen. A novel g-C₆N₆/SiP-GaS van der Waals heterojunction is designed and its structural, optoelectronic and photocatalytic properties are systematically investigated by using the first-principles method. The results indicate that the g-C₆N₆/SiP-GaS heterojunction is a type-Ⅱ heterojunction and the band edge straddles the redox potential of water splitting. The g-C₆N₆/SiP-GaS heterojunction exhibits good optical absorption in the visible-light range. It is worth noting that the optoelectronic properties and thermodynamic feasibility of the heterojunction can be modulated by the biaxial strain. Interestingly, all the optical absorption, OER and HER can be effectively improved under the strain of ?4%. The work supplies a strategy for the design and making of novel heterojunction for a highly effective photocatalyst in water splitting.     

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).     

A novel Pd-Cr2O3/CdS photocatalyst for solar hydrogen production using a regenerable sacrificial donor

Noble metals and transition-metal oxides are needed as cocatalysts for facilitating H₂ production over photocatalysts. Such cocatalysts are typically deposited as nanoparticles on the catalyst surface using impregnation or photodeposition methods, for which an activation treatment process is necessary. In this paper, we describe a facile method that utilizes a Pd-Cr₂O₃ nanocomposite cocatalyst at room temperature for the efficient production of H₂ without the need of an activation treatment process. The Pd-Cr₂O₃/CdS photocatalyst shows a higher hydrogen evolution rate than that of a plain Pd metal loaded catalyst. Formation of a composite structure appears to be an important reason for the increased performance of Pd-Cr₂O₃/CdS photocatalyst.     

Oxygen vacancies mediated in-situ growth of noble-metal (Ag,Au, Pt) nanoparticles on 3D TiO2 hierarchical spheres for efficient photocatalytic hydrogen evolution from water splitting

Defect engineering is effective to extend the light absorption range of TiO₂. However, the oxygen vacancy defects in TiO₂ may serve as recombination centers, hampering the separation and transfer of photo-generated charges. Here, we present a strategy of in-situ depositing noble-metal (M = Ag, Au or Pt) nanoparticles (NPs) on defective 3D TiO₂ hierarchical spheres (THS) with large surface area through the redox reaction between metal ions in solution and the electrons trapped at oxygen vacancies in THS. The oxygen vacancies at the THS surface are consumed, resulting in direct contact between TiO₂ and noble-metal NPs, while the other oxygen vacancies in the bulk are retained to promote visible light absorption. The noble-metal NPs with well-controlled size and distribution throughout the porous hierarchical structure not only facilitate the generation of electron-hole pairs in THS due to the effect of surface plasmon-induced resonance energy transfer (SPRET) from noble-metal NPs to TiO₂, but also expediate the electron transfer from TiO₂ to noble-metal NPs due to the Schottky junction at the TiO₂/M interface. Therefore, THS-M shows improved photocatalytic performance in water splitting compared to THS. The optimum performance is achieved on THS-Pt (13.16 mmol h⁻¹g⁻¹) under full-spectrum (UV–Vis) irradiation but on THS-Au (1.49 mmol h⁻¹g⁻¹) under visible-light irradiation. The underlying mechanisms are proposed from the surface plasmon resonance of noble-metal NPs as well as the Schottky junction at the TiO₂/M interface.     

CdS@Mg(OH)2 core/shell composite photocatalyst for efficient visible-light photocatalytic overall water splitting

Coating a protective agent or promoter on the surface of the photocatalyst is a proven good strategy to realize photocatalytic hydrogen production from pure water, but remains still a considerable challenge. Herein, a novel CdS@Mg(OH)₂ core/shell composite nanorods photocatalyst was synthesized by coating Mg(OH)₂ on CdS surface by hydrothermal and precipitation processes. The coated-Mg(OH)₂ layer did not change the structure of CdS, and the photocatalytic overall water splitting performance of the CdS@Mg(OH)₂ under visible light irradiation was improved obviously. After loading nano-Pt via the photodeposited method, the hydrogen production rate and stability of Pt/CdS@Mg(OH)₂ were 3.3 and 2.4 times that of the Pt/CdS under the visible light irradiation, respectively. The surface Mg(OH)₂ layer improved the hydrophilicity and stability of the core/shell composites and increased the amount of active sites, thus improving the photocatalytic properties. It is believed that Mg(OH)₂ can be used as a new co-catalyst to enhance the performance of photocatalytic overall water splitting.     

Butterfly wing architecture assisted CdS/Au/TiO2 Z-scheme type photocatalytic water splitting

Inspired by natural Z-scheme photosynthesis and black butterfly wing's antireflection morphology, we used the wings of butterfly Papilio nephelus Boisduva as templates to synthesize CdS/Au/TiO₂ with butterfly wing architecture. This combination of artificial Z-scheme photosystem and butterfly wing's hierarchical architecture was expected to enhance the light harvesting and water splitting efficiency. The finite-difference time-domain (FDTD) simulation was applied to demonstrate the optical function of the architecture inherited from butterfly wing theoretically, UV–vis spectra and photocatalytic H₂ evolution rates were further recorded to experimentally demonstrate the coupled effect of butterfly wing architecture and CdS/Au/TiO₂ Z-scheme components. The FDTD simulation shows that the architecture of the wing scale TiO₂ effectively reduced the UV light reflection by about 40%. Meanwhile, the wing scale architecture model exhibited lower UV reflection and transmission in water than those in air, which can be attributed to the stronger diffuse reflection in water. UV–vis spectra and photocatalytic H₂ evolution experiments confirmed that the combination of the wing scale architecture and CdS/Au/TiO₂ Z-scheme components contributed to the enhancement of the light harvesting ability and improved the water-splitting efficiency by 200% compared to the plate architecture TiO₂. Inspired by Nature, we present a promising way for constructing efficient photocatalysts for hydrogen evolution.     

Light-assisted synthesis of Au/TiO2 nanoparticles for H2 production by photocatalytic water splitting

A series of Au/TiO₂ photocatalysts was synthesized via the light assistance through the photo-deposition for H₂ production by photocatalytic water splitting using ethanol as the hole scavenger. Effect of solution pH in the range of 3.2–10.0 on the morphology and photocatalytic activity for H₂ production of the obtained Au/TiO₂ photocatalysts was explored. It was found that all Au/TiO₂ photocatalysts prepared in different solution pH exhibited comparable anatase fraction (～0.84–0.85) and crystallite size of TiO₂ (21–22 nm), but showed different quantity of deposited Au nanoparticles (NPs) and other properties, particularly the particle size of the Au NPs. Among all prepared Au/TiO₂ photocatalysts, the Au/TiO₂ (10.0) photocatalyst exhibited the highest photocatalytic activity for H₂ production, owning to its high metallic state and small size of Au NPs. Via this photocatalyst, the maximum H₂ production of 296 μmol (～360 μmol/g?h) was gained at 240 min using the 30 vol% ethanol as the hole scavenger at the photocatalyst loading of 1.33 g/L under the UV light intensity of 0.24 mW/cm² with the quantum efficiency of 61.2% at 254 nm. The loss of the photocatalytic activity of around 20% was observed after the 5th use.     

Factors affecting the production of H2 by water splitting over a novel visible-light-driven photocatalyst GaFeO3

A d¹⁰ photocatalyst, GaFeO₃ having a band gap of ∼2.7 eV, exhibits significant activity for the overall splitting of water under visible light (>395 nm) irradiation, in the absence of sacrificial reagent or a noble metal co-catalyst. The doping of an anion led to considerable enhancement in activity, the S-doped catalysts displaying better activity compared to the samples containing nitrogen. Even though the H₂/O₂ yields were affected by preparation-dependent grain morphology, no direct relationship was observed between the photoactivity of a sample and its specific surface area. The techniques of HRTEM, SEM, XPS, Laser Raman, UV–visible and photoluminescence spectroscopy have enabled to demonstrate that, besides the grain morphology, certain lattice imperfections and microstructure may also play a crucial role in water splitting activity of a photocatalyst. The factors responsible for catalyst deactivation are examined.     

Alternate synthetic strategy for the preparation of CdS nanoparticles and its exploitation for water splitting

Cadmium sulphide nanoparticles (6–12 nm) are prepared by a precipitation process using different zeolite matrices as templates. The nanoparticles were characterized by UV-Vis, XRD, SEM, TEM and sorptometric techniques. XRD study shows the presence of hexagonal and cubic phases for the nanoparticles whereas in case of the bulk samples only the hexagonal phase is observed. These nanomaterials have been used as catalysts for the photocatalytic decomposition of water. The nanoparticles show a higher hydrogen evolution rate compared to the bulk samples which correlates well with the particle size and surface area. Noble metal (Pt, Pd, Rh, Ru)-loaded samples were subsequently prepared and tested for hydrogen evolution reaction. The presence of Pt metal is found to enhance the hydrogen production rate whereas the hydrogen production rate is retarded in the presence of Ru metal. This has been explained on the basis of metal hydrogen bond, redox potential and work function of the noble metal.     

TiO2/CeO2 core/shell heterojunction nanoarrays for highly efficient photoelectrochemical water splitting

In this paper, novel TiO₂/CeO₂ core/shell heterojunction nanorod (NR) arrays were synthesized as photoanode for photoelectrochemical (PEC) water splitting via a simple and facial two-step hydrothermal approach. This synthesis route can obtain different amount of CeO₂ nanoparticles by controlling the hydrothermal time and eventually achieve uniform TiO₂/CeO₂ core/shell nanostructures. The uniform TiO₂/CeO₂ core/shell heterojunction nanoarrays exhibit a markedly enhanced photocurrent density of 5.30 mA·cm^?2 compared to that of pristine TiO₂ NR 1.79 mA·cm^?2 at 1.23 V vs. RHE in 1 M KOH solution. The superior PEC performance of the TiO₂/CeO₂ core/shell heterojunction is primarily due to much enhanced visible light absorption and appropriate gradient energy gap structure. This work not only offers the synthesis route for the novel TiO₂/CeO₂ core/shell heterojunction, but also suggests that this new core/shell heterojunction has a great potential application for efficient PEC water splitting devices.