Recent advances in Co-based co-catalysts for efficient photocatalytic hydrogen generation

Recent progress in photocatalytic hydrogen generation reaction highlights the critical role of co-catalysts in enhancing the solar-to-fuel conversion efficiency of diverse band-matched semiconductors. Because of the compositional flexibility, adjustable microstructure, tunable crystal phase and facet, cobalt-based co-catalysts have stimulated tremendous attention as they have high potential to promote hydrogen evolution reaction. However, a comprehensive review that specifically focuses on these promising materials has not been reported so far. Therefore, this present review emphasizes the recent progress in the pursuing of highly efficient Co-based co-catalysts for water splitting, and the advances in such materials are summarized through the analysis of structure-activity relationships. The fundamental principles of photocatalytic hydrogen production are profoundly outlined, followed by an elaborate discussion on the crucial parameters influencingthe reaction kinetics. Then, the co-catalytic reactivities of various Co-based materials involving Co, Co oxides, Co hydroxides, Co sulfides, Co phosphides and Co molecular complexes, etc, are thoroughly discussed when they are coupled with host semiconductors, with an insight towards the ultimateobjective of achieving a rationally designed photocatalyst for enhancing water splitting reaction dynamics. Finally, the current challenge and future perspective of Co-based co-catalysts as the promising noble-metal alternative materials for solar hydrogen generation are proposed and discussed.

Photo-augmented PHB production from CO2 or fructose by Cupriavidus necator and shape-optimized CdS nanorods

Particulate photocatalysts developed for the solar energy-driven reduction of the greenhouse gas CO₂ have a small product range and low specificity. Hybrid photosynthesis expands the number of products with photocatalysts harvesting sunlight and transferring charges to microbes harboring versatile metabolisms for bioproduction. Besides CO₂, abiotic photocatalysts have been employed to increase microbial production yields of reduced compounds from organic carbon substrates. Most single-reactor hybrid photosynthesis systems comprise CdS assembled in situ by microbial activity. This approach limits optimization of the morphology, crystal structure, and crystallinity of CdS for higher performance, which is usually done via synthesis methods incompatible with life. Here, shape and activity optimized CdS nanorods were hydrothermally produced and subsequently applied to Cupriavidus necator for the heterotrophic and autotrophic production of the bioplastic polyhydroxybutyrate (PHB). C. necator with CdS NR under light produced 1.5 times more PHB when compared to the same bacterium with suboptimal commercially-available CdS. Illuminated C. necator with CdS NR synthesized 1.41 g PHB from fructose over 120 h and 28 mg PHB from CO₂ over 48 h. Interestingly, the beneficial effect of CdS NR was specific to C. necator as the metabolism of other microbes often employed for bioproduction including yeast and bacteria was negatively impacted. These results demonstrate that hybrid photosynthesis is more productive when the photocatalyst characteristics are optimized via a separated synthesis process prior to being coupled with microbes. Furthermore, bioproduction improvement by CdS-based photocatalyst requires specific microbial species highlighting the importance of screening efforts for the development of performant hybrid photosynthesis.

Enhanced Photocatalytic H2 -Production Activity of CdS Quantum Dots Using Sn2+ as Cocatalyst under Visible Light Irradiation

Herein, oil-soluble CdS quantum dots (QDs) are first prepared through a solvent-thermal process. Then, oil-soluble CdS QDs are changed into water-soluble QDs via ligand exchange using mercaptopropionic acid as capping agent at pH 13. The photocatalytic performance is investigated under the visible light irradiation using glycerol as sacrificial agent and Sn²⁺ as cocatalyst. No H₂ -production activity is observed for oil-soluble CdS QDs. Water-soluble CdS QDs exhibit significantly enhanced hydrogen evolution rate. When the concentration of cocatalyst Sn²⁺ increases to 0.2 × 10^-3 m, the rate of hydrogen evolution reaches 1.61 mmol g^-1 h^-1 , which is 24 times higher than that of the pristine water-soluble CdS QDs. The enhanced H₂ -production efficiency is attributed to the adsorption of Sn²⁺ ions on the surface of CdS QDs that are further reduced to Sn atoms by photogenerated electrons. The in situ generated Sn atoms serve as photocatalytic cocatalyst for efficient hydrogen generation.

Fabrication of Hybrid Catalyst ZnO Nanorod/α-Fe2O3 Composites for Hydrogen Evolution Reaction

This report presents the synthesis of ZnO nanorod/α-Fe2O3 composites by the hydrothermal method with different weight percentages of α-Fe2O3 nanoparticles. The as-synthesized nanorod composites were characterized by different techniques, such as X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), field emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). From our results, it was found that the ZnO/α-Fe2O3 (3 wt%) nanorod composites exhibit a higher hydrogen evolution reaction (HER) activity when compared to other composites. The synergetic effect between ZnO and (3 wt%) of α-Fe2O3 nanocomposites resulted in a low onset potential of −125 mV, which can effectively produce more H2 than pure ZnO. The H2 production rate over the composite of ZnO/α-Fe2O3 (3 wt%) clearly shows a significant improvement in the photocatalytic activity in the heterojunction of the ZnO nanorods and α-Fe2O3 nanoparticles on nickel foam.

New Versatile Synthetic Route for the Preparation of Metal Phosphate Decorated Hydrogen Evolution Photocatalysts

Photocatalytic hydrogen generation will benefit from the realization of more active but less expensive cocatalysts compared with noble metal counterparts. Herein we developed a universal vapor deposition method that selectively uses the thermal decomposition products of sodium hypophosphite as a phosphorus source for the fabrication of inexpensive and highly efficient metal phosphate (MPi) modified CdS nanorods. We find that the modification with a bimetal phosphate (i.e., 5 wt % NiCoPi) leads to an activity enhancement by a factor of approximately 52 in boosting visible-light-driven hydrogen evolution relative to the pristine CdS nanorods. The photocatalyst exhibits a high hydrogen generation rate of 13.44 mmol·g^-1·h^-1, which is much higher than that of its single metal counterparts (NiPi, 8.70 mmol·g^-1·h^-1; CoPi, 5.79 mmol·g^-1·h^-1) and 1 wt % Pt modified CdS (1.33 mmol·g^-1·h^-1). Its apparent quantum efficiency reaches 23.5% at 420 nm. Furthermore, it also shows remarkable photostability for eight consecutive cycles of photocatalytic activity tests with total reaction time of 24 h. The excellent photocatalytic performance of the photocatalyst is believed to be associated with the in situ formed Ni^ICoP and NiCo^IIIPi cocatalysts, which not only play an important role in photogenerated charge separation but also provide highly active catalytic reaction sites for the corresponding hydrogen evolution reaction and the sacrificial agent oxidation reaction.

In situ addition of Ni salt onto a skeletal Cu7S4 integrated CdS nanorod photocatalyst for efficient production of H2 under solar light irradiation

Development of earth-abundant, low cost, skeletal-type copper sulfide superstructures and in situ addition of Ni salts plays a prominent role to enhance the activity of CdS semiconductor nanostructures for photocatalytic H₂ production.

PtNixCoy concave nanocubes: synthesis and application in photocatalytic hydrogen generation

PtNi_xCo_y alloy concave nanocubes coupled with CdS nanorods were applied in the H₂ evolution reaction under visible light irradiation.

Photocatalytic Hydrogen Production: Role of Sacrificial Reagents on the Activity of Oxide, Carbon, and Sulfide Catalysts

Photocatalytic water splitting is a sustainable technology for the production of clean fuel in terms of hydrogen (H2). In the present study, hydrogen (H2) production efficiency of three promising photocatalysts (titania (TiO2-P25), graphitic carbon nitride (g-C3N4), and cadmium sulfide (CdS)) was evaluated in detail using various sacrificial agents. The effect of most commonly used sacrificial agents in the recent years, such as methanol, ethanol, isopropanol, ethylene glycol, glycerol, lactic acid, glucose, sodium sulfide, sodium sulfite, sodium sulfide/sodium sulfite mixture, and triethanolamine, were evaluated on TiO2-P25, g-C3N4, and CdS. H2 production experiments were carried out under simulated solar light irradiation in an immersion type photo-reactor. All the experiments were performed without any noble metal co-catalyst. Moreover, photolysis experiments were executed to study the H2 generation in the absence of a catalyst. The results were discussed specifically in terms of chemical reactions, pH of the reaction medium, hydroxyl groups, alpha hydrogen, and carbon chain length of sacrificial agents. The results revealed that glucose and glycerol are the most suitable sacrificial agents for an oxide photocatalyst. Triethanolamine is the ideal sacrificial agent for carbon and sulfide photocatalyst. A remarkable amount of H2 was produced from the photolysis of sodium sulfide and sodium sulfide/sodium sulfite mixture without any photocatalyst. The findings of this study would be highly beneficial for the selection of sacrificial agents for a particular photocatalyst.

In situ photodeposition of amorphous NixP on CdS nanorods for efficient visible-light photocatalytic H2 generation

Noble metal-free amorphous Ni_xP modified CdS with superior photocatalytic H₂ evolution activity has been successfully fabricated.

Mini-review on an engineering approach towards the selection of transition metal complex-based catalysts for photocatalytic H2 production

Advances in transition-metal (Ru, Co, Cu, and Fe) complex-based catalysts since 2000 are briefly summarized in terms of catalyst selection and application for photocatalytic H₂ evolution.

Photochemical route for synthesizing Co-P alloy decorated ZnIn2S4 with enhanced photocatalytic H2 production activity under visible light irradiation

A series of amorphous Co-P alloy modified ZnIn2S4 composite samples were synthesized through a one-step photochemical method. The as-prepared samples were systematically characterized and the photocatalytic activity for H2 production under visible-light irradiation was investigated. It was found that the Co-P/ZnIn2S4 composite samples exhibited higher photocatalytic activity, which is 44 times higher than that of pure ZnIn2S4 and higher than that of the 1 wt% Pt/ZnIn2S4 sample. After modifying with amorphous Co-P alloy, the composite samples showed enhanced photocurrent, reduced photoelectric impedance, weakened fluorescence intensity and extended fluorescence lifetime, which accelerate the separation and transfer of photoinduced charge effectively, thus improving the catalytic activity of the composite sample. This work could provide a new strategy for the design and synthesis of low-cost, high-efficiency composite materials for photocatalytic hydrogen evolution and is promising for energy conversion and utilization.

An amorphous CoSx modified Mn0.5Cd0.5S solid solution with enhanced visible-light photocatalytic H2-production activity

Amorphous noble metal-free CoS_x modified Mn_0.5Cd_0.5S with excellent photocatalytic hydrogen production activity has been successfully synthesized.