CuO@NiO core-shell nanoparticles decorated anatase TiO2 nanospheres for enhanced photocatalytic hydrogen production

Core-shell structured co-catalyst has been created much attention in photocatalytic hydrogen production due to their efficient electron-hole pair separation, suppression of surface back reaction and long term stability. Here, we report the preparation of CuO@NiO hierarchical nanostructures as a co-catalyst deposited on TiO₂ nanospheres for enhanced photocatalytic hydrogen generation. The formation of ultrathin NiO shell over the CuO core was confirmed by TEM analysis. Fabricated core-shell nanostructured CuO@NiO over TiO₂ nanospheres was studied for hydrogen evolution under the direct solar light and it showed a high rate of H₂ production of 26.1 mmol. h⁻¹. g⁻¹_cat. It was scrutinized that the rate of hydrogen production was improved with shell thickness and co-catalyst loading. Systematic investigation on CuO@NiO co-catalyst loading, pH of the medium and glycerol concentration for augmented H₂ production. The recorded rate of hydrogen production is almost six folds greater than that of pristine TiO₂. In the view of largescale synthesis for alternative energy storage applications, the composited photocatalyst was made of by simple mixing method, which could be scaled up without any loss in photocatalytic activity. Further, the stability test of photocatalyst for continuous use found that 82% of initial photocatalytic activity is retained even after three days.     

Cation exchange synthesis of hollow-structured cadmium sulfide for efficient visible-light-driven photocatalytic hydrogen evolution

Efficient solar absorption and photoinduced charge separation are extremely important for solar-energy conversion on semiconductor photocatalysts. To advance the photocatalytic performance, we developed a general templated-assisted reverse cation exchange strategy to successfully synthesize hollow-structured CdS semiconductors with the textile structural surface. The crystal phase, particle morphology, optical/electrical properties, and photocatalytic performance of the as-syntheszied sample are investiagted by XRD, SEM, TEM, XPS, DSR, PL, ESR photoelectrochemical measurements, and Photocatalytic H₂ evolution test. The final CdS sample exhibits an enhanced photocatalytic hydrogen evolution rate of up to 965 μmol·g⁻¹ h⁻¹, 2.8 times higher than the reported CdS nanorods. Based on the experimental and characterization results, the improved photocatalytic activity of the cadmium sulfide semiconductor can be ascribed to the special hollow cubic structure with a thin shell, which can enhance the light-harvesting ability and provide abundant photocatalytic active sites for facilitating the separation of photogenerated electron/hole pairs. This synthetic strategy may pave a new path for the rational design of efficient sulfur-based semiconductor photocatalysts for solar driven H₂ production.     

Self-assembled CdS@BN core-shell photocatalysts for efficient visible-light-driven photocatalytic hydrogen evolution

CdS@BN NRs core-shell photocatalysts for hydrogen evolution were synthesized by a solvothermal and chemical adsorption method. CdS NRs coated by 5 wt% boron nitride (BN) shell exhibited remarkably visible-light photocatalytic hydrogen evolution activity of up to 30.68 mmol g⁻¹ h⁻¹, nearly 6.79 times higher than that of pure CdS NRs, and the apparent quantum efficiency at 420 nm was 7.5%. Transmission electron microscopy showed the CdS NRs were coated with a thin (~5 nm) BN layer, which together with the hydrogen evolution results proved the photocatalytic ability of CdS NRs was significantly improved. The hydrogen evolution rate of CdS NRs coated by 5 wt% BN remained at 91.4% after four cycles, indicating the photocorrosion of CdS NRs was effectively alleviated. Moreover, the large and close coaxial interfacial contact between the CdS core and the BN shell was beneficial to the separation and transfer of photogenerated electron-hole pairs.     

0D, 1D and 2D nanomaterials for visible photoelectrochemical water splitting. A Review

Global energy problems of the 21st century have led to the search for alternative energy sources, among which is hydrogen produced via photoelectrochemical solar water splitting. Photo-electrochemical water splitting using semiconductor nanostructured materials is a progressive method for producing hydrogen. The unique electronic, mechanical, surface and optical properties of nanomaterials make it possible to create photocatalysts with complex structures of energy zones, allowing the use of a wide range of sunlight and exerting a positive effect on absorption and scattering of sunlight. This review contains a detailed analysis of current studies aimed at improving the efficiency of photocatalytic systems by using 0D, 1D and 2D nanostructures. Special attention is paid to the mechanisms of photocatalytic water splitting to produce hydrogen with the help of various nanostructures.     

Facile construction of porous C-PDA@CN-ms core-shell heterostructures with superior photocatalytic H2 evolution

Building carbon nitride (CN)-based core shell heterostructures is an effective strategy to enhance the photocatalytic performance and stability by optimize the interface area and protect the CN core, respectively. Moreover, by fabricating the porous structures in core shells can further optimize the light absorption, charge separation, and mass transfer. Herein, we have constructed porous C-PDA–CN–ms core-shell heterostructures through a facile green molten salt (ms) sculpture the polydopamine (PDA) derived carbon (C-PDA) shells with CN core. In which, the C-PDA-CN core-shells arise from in situ polymerization of dopamine (DA) on the surface of melamine to form PDA@melamine coatings followed by thermal polycondensation. The molten salts at high-temperature act as a green fluid immersing in and out of C-PDA-CN core-shells to further produce porous structures. The 1 wt% C-PDA–CN–ms with porous core-shell structures display photocatalytic H₂ evolution rate of 3830 μmol h⁻¹ g⁻¹, which is 20.8 times enhancement of 1 wt% C-PDA-CN core-shells, even 73.6 times higher than that of pristine CN. It reveals that the porous and core-shell heterostructures endow C-PDA–CN–ms enhanced light absorption, various charge transport channels for improved charge carrier separation and transfer, contributing to the superior photocatalytic H₂ evolution performance. Our work opens a new window for the green construction of porous core-shell heterostructures of CN-based photocatalysts.     

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

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

High-efficiency of novel hierarchical 3D macroporous g-C3N4 material on solar-driven photocatalytic water-splitting for hydrogen evolution

The use of multi-pore nanostructured g-C₃N₄ photocatalysts is an efficient approach to separate photogenerated charge carriers and increase visible light photocatalytic performance. Recent progress has yielded nanostructured material through hard templating, which limits potential applications. Integrating the 2D building block into multidimensional porous structures remains a significant challenge in scalable production. Herein, a novel technique based on P407 bubble clusters templating and fixation by freezing is described for the first time to fabricate a 3D opened-up macroporous g-C₃N₄ nanostructures for photocatalytic H₂ evolution. Three-dimensional hierarchical nanostructures provide more contact active sites and synergistically promote the creation of heterogeneous catalytic interfaces. This feature is very useful for understanding the transfer path of photoinduced charges as well as the origins of the high charge separation efficiency in photocatalytic reactions, thus yielding a remarkable visible light-induced H₂ evolution rate of 4213.6 μmol h⁻¹ g⁻¹, which is nearly 5.6 times (716 μmol h⁻¹ g⁻¹) higher than that of lamellar bulk g-C₃N₄. This newly developed approach offers a promising alternative to synthesize broad-spectral response 3D hierarchal g-C₃N₄ nanostructures and can be extended to assemble other functional nanomaterials as building blocks into macroscopic configurations coupled with electronic modulation strategy simultaneously.     

Pd/PdO and hydrous RuO2 difunction-modified SiO2@TaON@Ta3N5 nano-photocatalyst for efficient solar overall water splitting

A novel Pd/PdO and hydrous RuO₂ difunction-modified SiO₂@TaON@Ta₃N₅ core-shell structured nano-photocatalyst was synthesized successfully, which displayed excellent photocatalytic activity for overall water splitting into H₂ (473.52 μmol⁻¹·g⁻¹·h⁻¹), about 2.86 times higher than unmodified SiO₂@TaON@Ta₃N₅ (165.74 μmol⁻¹·g⁻¹·h⁻¹), under the visible-light irradiation with the wavelength ≥420 nm, without any sacrificial agent, as well as excellent stability against photocorrosion. The apparent quantum yield (AQY) reaches to 0.253% under irradiation intensity of 12 mW cm⁻² at 420 nm. The spatially separated Pd, PdO and RuO₂ clusters were decorated on the Ta₃N₅ surface to construct local multi-heterojunctions, which were confirmed to enhance the light absorption capability, drive efficient separation of charge carriers and directional transfer, and promote surface redox reaction kinetics of HER and OER. The trace modification of metallic Pd clusters and TaN could mainly contribute to the significant decrease in the HER overpotential, while PdO exhibited a stronger contribution than RuO₂ for OER catalytic activity. The synergetic mechanism of enhanced photocatalytic overall water splitting for hydrogen production was discussed in detail. Thus the combination of core-shell heterojunction construction and surface difunction modification provides a promising strategy for develop efficient all-in-one photocatalysts for solar overall water splitting.     

Facile preparation of ZnS/CdS core/shell nanotubes and their enhanced photocatalytic performance

In this paper, ZnS/CdS core/shell nanotubes were successfully synthesized by combining hydrothermal treatment and ion exchange conversion, and the significant influence of CdS content in the shell on photo absorption and photocatalytic activity was also investigated. The core/shell nanotubes structure of CdS deposition on both sides of ZnS nanotube was confirmed by scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The room temperature PL spectra of ZnS/CdS core/shell nanotubes indicated that CdS on the shell can reduce the recombination of photon-generated electron and hole. The photocatalytic activity tests prove that ZnS/CdS nanotubes have much higher photocatalytic hydrogen production activity than ZnS nanotube and CdS nanotube. Under the irradiation of visible light, the highest photocatalytic hydrogen production rate of 110 μmol h⁻¹ g⁻¹ is observed over the ZnS/CdS core/shell nanotubes with CdS/ZnS molar ratio of 1:4, which is about 11.02 and 5.56 times more active than ZnS nanotube and CdS nanotube, respectively. The improved performance of ZnS/CdS samples can be due to the strong photo response in the visible light region and the efficient separation of electron–hole pairs.     

Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructures for highly efficient photocatalytic hydrogen evolution under solar light irradiation

Designing of noble-metal free and morphologically controlled advanced photocatalysts for photocatalytic water splitting using solar light is of huge interest today. In the present work, novel polygonal Zn₂TiO₄ (ZTO) nanostructures have been synthesized by citricacid assisted solid state method for the first time and synthesized nanostructures were characterized by using various techniques like PXRD, UV-Vis-DRS, PL, FT-IR, BET, FE-SEM and TEM for their structural, optical, chemical, surface and morphological properties. The PXRD and UV-Vis-DRS analysis show the existence of cubic and tetragonal phases. FE-SEM and TEM results confirm the formation of polygonal ZTO nanostructures. Synthesised ZTO nanostructures have been potentially applied for solar light-driven photocatalytic hydrogen evaluation from water splitting and compare the photocatalytic activity with synthesized conventional Zn₂TiO₄ and commercially available TiO₂, ZnO photocatalysts. A high rate of 529 μmolh^?1g^?1 solar light-driven photocatalytic H₂ evolution has been achieved by using a small amount (5 mg) of polygonal Zn₂TiO₄ nanostructures from glycerol-water solution. The enhanced photocatalytic performance of the polygonal Zn₂TiO₄ nanostructures compare to conventional Zn₂TiO₄ under solar light irradiation is due to the large surface area and low recombination rate. However having the same bandgap, the polygonal Zn₂TiO₄ nanostructures have shown enhanced photocatalytic performance than that of commercially available TiO₂, ZnO photocatalysts.     

Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

TiO2 hollow spheres with separated Au and RuO2 co-catalysts for efficient photocatalytic water splitting

Hollow mesoporous TiO₂ photocatalysts with dual co-catalysts, located at specific positions, were prepared using Polystyrene (PS) as sacrificial templates. Au nanoparticles (NPs) were in situ loaded on the surface of PS spheres and the resulting nanocomposites were coated with TiO₂ shell using sol-gel reaction. The outer surface of core-shell spheres was impregnated with Ru and the subsequent calcination produced hollow anatase spheres with Au and RuO₂ dual co-catalysts. The hollow mesoporous spheres of Au@TiO₂@RuO₂ were proved by various techniques such as TEM, EDX, and SEM images. Photocatalysts were applied for hydrogen generation from water splitting and that with dual co-catalysts showed efficient catalytic activity under simulated solar light. The catalytic activity of photocatalysts with both oxidation and reduction co-catalysts (Au@TiO₂@RuO₂) showed hydrogen evolution (3165 μmol g⁻¹) almost two times more than that Au@TiO₂ and TiO₂@RuO₂ with single co-catalysts. And the hydrogen evolved is more than three times as compared to TiO₂ (935 μmol g⁻¹) without any co-catalyst. Hollow mesoporous morphology with different co-catalysts on inner and outer surfaces is believed to enhance photocatalytic activity which is due to better separation of photo-generated charges.     

Nanostructured materials for the visible-light driven hydrogen evolution by water splitting: A review

The conversion of abundantly available photonic energy into useful chemical energy is considered to be a greener protocol for addressing the energy shortage. Recently, since most of the emphasis has been centralized on the semiconductor-based photocatalysis; the designing and fabrication of the novel semiconductor photocatalytic material is happening at a blistering rate. Recently, the nanostructured materials have attracted ever-growing research attention as photocatalytic material for hydrogen generation reaction by dissociation of water. Such photocatalytic nanomaterials are known to exhibit superior activity than their corresponding bulk counter-parts because of the improved interfacial charge separation and the broad surface area providing sufficient active sites. However, the improvement in the efficiency and selectivity towards hydrogen production reaction under solar or visible light radiation always remains a challenging assignment. In the present review, the segregation of the so far reported nanostructured photocatalysts into different categories, based on their dimensionality such as 0-D, 1-D and 2-D materials, is implemented. Furthermore, their synthetic route and the photocatalytic hydrogen evolving efficiencies are explored and briefly summarized. Moreover, the methodology of development of nanocomposite materials leading to the construction of heterojunctions including Type-I, Type-II, Type-III, Z-Scheme and S-Scheme system is also discussed. In addition, an in-depth investigation on the charge carrier's generation, separation and their transportation is also reviewed. Finally, the future perspectives regarding the designing of an efficient, stable and economic photoactive nano-architecture material for the efficient hydrogen production via photocatalytic dissociation of water are also pointed.     

A review of TiO2 nanostructured catalysts for sustainable H2 generation

Hydrogen is an attractive alternative to fossil fuels that addresses several environmental and energy shortage issues. Nano-sized TiO₂-based photocatalysts with unique structural and functional properties are the most extensively studied photocatalytic nanomaterials for hydrogen production and pollutant degradation. However, titania is hampered by a wide band gap, low utilization of solar light and a rapid recombination of electron/hole pairs. These issues limit its photocatalytic performance. In this review, we present the latest developments in the fabrication of different higher dimensional TiO₂ nanostructured materials that aim to address these inherent limitations to an otherwise very promising material. Specifically, we will look into critical engineering strategies to enlarge the active surface area, enhance visible light absorption and suppress the recombination of electrons/holes that benefit their photo/photoelectric-catalytic water splitting activity. Finally, the current challenges and perspectives for TiO₂ nanostructures are also discussed. Continuous efforts are necessary to endow TiO₂-based materials with novel advanced functionality and commercialization potential in the coming years.     

Enhanced surface electron transfer by fabricating a core/shell Ni@NiO cluster on TiO2 and its role on high efficient hydrogen generation under visible light irradiation

A Ni@NiO core/shell cluster was fabricated on TiO₂ surface (Ni@NiO/TiO₂) and its roles on surface electron transfer and the enhancement on hydrogen evolution under visible light irradiation were investigated. For a comparison, the Ni/TiO₂ and NiO/TiO₂ catalysts were fabricated, respectively. By photosensitization using Eosin Y as an antenna molecule, (1.6 wt%)Ni@NiO/TiO₂ exhibited the highest activity (364.1 μmol h⁻¹) in comparison with (1.6 wt%)Ni/TiO₂ and (1.6 wt%)NiO/TiO₂ and the corresponding apparent quantum efficiency reached 28.6% at 460 nm. The photoluminescence spectra and photoelectrochemical characterization results confirmed that the Ni@NiO core/shell structure could promote the photogenerated electrons transferring from TiO₂ conduction band to Ni@NiO clusters, resulting in the quicker separation of electron–hole pairs. In addition, part of NiO shell can be reduced into metallic Ni during the photoreaction and vice versa. Cyclic voltammogram characterization verified that the transformation between Ni and NiO was a dynamic balance process, which can not only provide reacting channels for electrons and protons but also ensure the photocatalytic hydrogen evolution proceeding continuously. This study discloses structure-dependent effect of non-noble metal cocatalyst on semiconductor photocatalysts in photocatalytic water reduction, and gives an insight into designing high-efficient non-noble metal/semiconductor hybrid photocatalysts.     

Graphitic carbon nitride tetragonal hollow prism with enhanced photocatalytic hydrogen evolution

Graphitic carbon nitride tetragonal hollow prism (GCN-THP) with nitrogen vacancies was prepared by a simple two-step calcination method. Based on the characterizations of the as-prepared GCN-THP and the intermediate precursor, a possible mechanism was proposed for the formation of GCN-THP. The as-prepared GCN-THP exhibits superior activity and excellent stability during photocatalytic hydrogen evolution under visible light irradiation. The photocatalytic hydrogen evolution rate of GCN-THP was measured to be 1990 μmol g⁻¹ h⁻¹, which is 6.2 times as that of GCN. The enhanced photocatalytic activity could be attributed to unique 1D tetragonal hollow prism morphology and the presence of nitrogen vacancies in the as-prepared GCN-THP, which could increase the surface area, expand the visible light absorption, and promote the charge separation during photocatalytic hydrogen evolution. Our work could provide a new route to synthesize highly efficient photocatalysts with 1D hollow structures.     

(In,Cu) Co-doped ZnS nanoparticles for photoelectrochemical hydrogen production

In and Cu co-doped ZnS nanoparticles were successfully synthesized in DI water and ethanol solvent by a sonochemical approach using citric acid as surfactants in aqueous medium. FESEM micrographs show that In and Cu co-doped ZnS crystallites have a rough surface nanostructure and the as-synthesized photocatalysts were tested for the photocatalytic hydrogen evolution from water splitting via the irradiation of simulated sunlight. Among In and Cu co-doped ZnS products, 4In4CuZnS photocatalyst can achieve the maximum hydrogen production rate (752.7 μmol h⁻¹ g⁻¹) in 360 min under simulated sunlight illumination. Meanwhile, we separated the hydrogen and oxygen cells using an ion exchange membrane. Both electrodes (working electrode and Pt electrode) are dipped into each cell containing an aqueous solution containing 0.1 M Na₂S at pH 3 to convert water into hydrogen and oxygen under solar irradiation. As expected, the photoelectrochemical water splitting cells could significantly improve the photocatalytic activity, where the 4In4CuZnS nanoparticles shows the photoelectrochemical performance with photocurrent density of 12.2 mA cm⁻² at 1.1 V and hydrogen evolution rate of 1189.4 μmol h⁻¹ g⁻¹.     

In-situ construction of graphene oxide in microsphere ZnS photocatalyst for high-performance photochemical hydrogen generation

Exploring high-efficiency photocatalysts for hydrogen generation from water splitting has recently spurred enormous scientific interest. Herein, the formation of Pt/GO-ZnS photocatalysts via a step-wise strategy and the mechanism of improved hydrogen production efficiency are systematically studied. Thus-prepared optimal Pt/GO-ZnS composite (with the ratio of Pt and GO are 1 and 2 wt%, respectively) exhibits excellent photocatalytic hydrogen production performance of 1082 μmol h⁻¹ g⁻¹ under simulated sunlight irradiation, which stays 2.8 and 2.6 times higher than that of ZnS/GO (2 wt%) and ZnS/Pt (1 wt%), respectively. Exhaustive experimental studies reveal that the outstanding activity of Pt/GO-ZnS is attributed to the efficient interfacial charge transfer and enhanced visible light harvesting caused by the synergetic effects of GO and Pt. In further contexts, the photocatalytic mechanism is explored: Photo-generated electrons in the conduction band of ZnS can be immediately trapped through GO and rapidly transferred onto Pt nanoparticles thanks to the tight interface. This study represents an effective and facile strategy to rationally design advanced photocatalytic system in solar energy conversion.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

A current perspective for photocatalysis towards the hydrogen production from biomass-derived organic substances and water

Recently, an increasing interest has been devoted to produce chemical energy – hydrogen (H₂) by converting sustainable sunlight energy via water splitting and reforming of renewable biomass-derived organic substances. These photocatalytic processes are very promising, sustainable, economic, and environment-friendly. Herein, this article gives a concise overview of photocatalysis to produce H₂ as solar fuel via two approaches: water splitting and reforming of biomass-derived organic substances. For the first approach – photocatalytic water splitting, there are two reaction types have been used, including photoelectrochemical (PEC) and photochemical (PC) cell reactions. For the second approach, biomass-derived oxygenated substrates could undergo selective photocatalytic reforming under renewable solar irradiation. Significant efforts to date have been made for photocatalysts design at the molecular level that can efficiently utilize solar energy and optimize the reaction conditions, including light irradiation, type of sacrificial reagents. Critical challenges, prospects, and the requirement to give more attention to photocatalysis for producing H₂ are also highlighted.