A homogeneous Cu-based polyoxometalate coupled with mesoporous TiO2 for efficient photocatalytic H2 production

High-performance heterometallic photocatalysts afforded by polyoxometalate synthons for efficient H2 production

MoS2-based materials have emerged as photoelectric semiconductors characterized by a narrow band gap, high capacity for absorbing visible light, and reduced H2 adsorption energy comparable to Pt. These attributes render them appealing for application in photocatalytic hydrogen production. Despite these advantages, the widespread adoption of MoS2-based materials remains hindered by challenges associated with limited exposure to active sites and suboptimal catalytic hydrogen production efficiency. To address these issues, we have designed and synthesized a new class of highly dispersed bimetallic/trimetallic sulfide materials. This was achieved by developing polyoxometalate synthons containing Ni-Mo elements, which were subsequently reacted with thiourea and CdS. The resulting Ni3S2-MoS2 and Ni3S2-MoS2-CdS materials achieve photocatalytic hydrogen production rates of 2770 and 2873 μmol g-1h-1, respectively. Notably, the rate of 2873 μmol g-1h-1 for Ni3S2-MoS2-CdS surpassed triple (3.23 times) the performance of CdS and nearly sextuple (5.77 times) that of single MoS2. These materials outperformed the majority of MoS2-based photocatalysts. Overall, this study introduces a straightforward methodology for synthesizing bimetallic/trimetallic sulfides with enhanced photocatalytic H2 evolution performance. Our findings underscore the potential of transition metal sulfide semiconductors in the realm of photocatalysis and pave the way for the development of more sustainable energy production systems.

Molten salt synthesis of 1T/2H mixed phase MoS2 for boosting photocatalytic H2 evolution via Schottky junction under EY-sensitized system

Clean H2 fuel obtained from the photocatalytic water splitting to hydrogen reaction could efficiently alleviate current energy crisis and the concomitant environmental pollution problems. Therefore, it is desirable to search for a highly efficient photocatalytic system to decrease the energy barrier of water splitting reaction. Herein, the 1T/2H mixed phase MoS2 sample with Schottky junction between contact interfaces is developed through molten salt synthesis for photocatalytic hydrogen production under a dye-sensitized system (Eosin Y-TEOA-MoS2) driven by the visible light. In mixed phase MoS2 sample, the photogenerated electrons of 2H-phase MoS2 migrated to the 1T-phase MoS2 are difficult to jump back because of the existence of Schottky barrier, which greatly suppresses the quenching of EY and therefore results in an enhanced hydrogen evolution performance. Therefore, the optimized MoS2 sample (MoS2-350) has an initial hydrogen evolution rate of 213 μmol h-1 and corresponding apparent quantum yield of 36.1 % at 420 nm, far higher than those of pure Eosin Y. It is strongly confirmed by the steady-state/time-resolved photoluminescence (PL) spectra and transient photocurrent response experiments. With the assistance of Density functional theory (DFT) calculation, the function of Schottky junction in photocatalytic hydrogen evolution reaction is well explained. In addition, a new and universal method (SVM curve) of judging oxidation or reduction quenching for photosensitizers is proposed.

P-doped Mn0.5Cd0.5S coupled with cobalt porphyrin as co-catalyst for the photocatalytic water splitting without using sacrificial agents

Photocatalytic water splitting over semiconductors is an important approach to solve the energy demand of human beings. Most photocatalytic H2 generation reactions are conducted in the presence of sacrificial agent. However, the use of sacrificial reagents increases the cost of hydrogen generation. Realizing photocatalytic water splitting for hydrogen production without the addition of sacrificial agents is a major challenge for photocatalysts. The porphyrin MTCPPOMe and P doped MnxCd1-xS make a significant contribution in facilitating the MnxCd1-xS photocatalytic pure water splitting to H2 reaction. Herein, a novel MTCPPOMe/P-MnxCd1-xS (M = 2H, Fe, Co, Ni) composite catalyst which can efficiently split pure water without using sacrificial agents is developed. As a result, the H2 generation rate of CoTCPPOMe/P-Mn0.5Cd0.5S is as high as 2.10 μmol h-1, which is 9.1 and 4.2 times higher than that of Mn0.5Cd0.5S (MCS) and P-Mn0.5Cd0.5S (P-MCS), respectively. P doped MnxCd1-xS inhibits the recombination of photogenerated carriers, and introduction of MTCPPOMe as co-catalyst enhances the reduction capacity. In summary, an efficient and economical photocatalystis prepared for pure water splitting to prepare hydrogen.

Metalloporphyrin based MOF-545 coupled with solid solution ZnxCd1-xS for efficient photocatalytic hydrogen production

The growing concern over the rapid consumption of fossil fuels has spurred scientists to seek environment-friendly and sustainable energy alternatives. The utilization of photocatalytic water splitting shows great potential in generating environmentally friendly hydrogen energy. Zirconium-based porphyrin MOF-545 has drawn extensive attention in photocatalysis since it has strong light absorption and fast electron transfer capability. A series of Zr-based metalloporphyrins containing different metals, MOF-545M (M = Co, Ni, Cu, Zn), were synthesized and assembled with solid solution ZnxCd1-xS (ZCS) nanoparticle to develop the corresponding of hybrid photocatalysts, ZCS/MOF-545M. Various characterizations show that introducing the porphyrin MOF-545Co not only improve the visible light response range of Zn0.5Cd0.5S (ZCS-0.5) but also enhance the carrier separation efficiency of ZCS-0.5. The carrier transfer direction during the reaction was confirmed by surface photovoltage spectroscopy and shows the formation of S-scheme. Under visible light irradiation, the hybrid photocatalyst ZCS-0.5/MOF-545Co exhibits a high H2 evolution rate of 148 μmol h-1, with increases of 52.8, 22.9 and 6.5 times than those of bare ZnS, CdS and ZCS-0.5, respectively. The present work gives a strategy for erecting S-scheme heterojunction to inhibit the photocorrosion of metal sulfides and enhancement the carrier separation efficiency.

Applications of BiOX in the Photocatalytic Reactions

BiOX (X = Cl, Br, I) families are a kind of new type of photocatalysts, which have attracted the attention of more and more researchers. The suitable band gaps and their convenient tunability via the change of X elements enable BiOX to adapt to many photocatalytic reactions. In addition, because of their characteristics of the unique layered structure and indirect bandgap semiconductor, BiOX exhibits excellent separation efficiency of photogenerated electrons and holes. Therefore, BiOX could usually demonstrate fine activity in many photocatalytic reactions. In this review, we will present the various applications and modification strategies of BiOX in photocatalytic reactions. Finally, based on a good understanding of the above issues, we will propose the future directions and feasibilities of the reasonable design of modification strategies of BiOX to obtain better photocatalytic activity toward various photocatalytic applications.

Mononuclear ruthenium (II) complex covalently anchored on melem and g-C3N4 as efficient heterogeneous catalysts for chemical water oxidation

Ru-melem and Ru-C₃N₄ were synthesized by a simple and facile strategy to construct a novel covalently anchoring by introducing easily synthesized amide bond as a bridge connecting the Ru-terpy and melem or g-C₃N₄, respectively. The covalent anchoring of Ru complex on melem or C₃N₄ not only makes these materials exhibit water oxidation activity under Ce^IV-driven (Ce^IV = Ce(NH₄)₂(NO₃)₆) reaction condition, but also makes the obtained heterogeneous catalysts show higher catalytic activity than the corresponding homogeneous catalysts, which reveals that the covalent anchoring strategy of Ru complex is beneficial to improve the catalytic activity of homogeneous Ru catalysts. The synthetic method of hybrid catalysts offers an insightful strategy for enhancing water oxidation activity of molecular catalysts.

Incorporating Ni-Polyoxometalate into the S-Scheme Heterojunction to Accelerate Charge Separation and Resist Photocorrosion for Promoting Photocatalytic Activity and Stability

The emerging polyoxometalate (POM) nanomaterials are transition metal oxygen anion clusters with d⁰ electronic configurations, which could be attractive and potential photocatalysts. Hence, a nickel (Ni)-substituted polyoxometalate K₆Na₄[Ni₄(H₂O)₂(PW₉O₃₄)₂]·32H₂O (Ni₄POM)-incorporating step (S)-scheme heterojunction was developed to promote photocatalytic activity and stability in H₂ and H₂O₂ production. The multielectron transfer through variable valence metal centers in Ni₄POM would facilitate the recombination of invalid charges through the S-scheme pathway. Moreover, incorporating Ni₄POM into the S-scheme heterojunction can broaden the light absorption range and meanwhile lead to resistance to photocorrosion to promote the optical and chemical stability of Cd_0.5Zn_0.5S (CZS). The optimized CZSNi-70 exhibited a H₂ evolution rate of 42.32 mmol g^-1 h^-1 under visible-light irradiation with an apparent quantum yield of 32.27% at 420 nm and a H₂O₂ production rate of 295.4 μmol L^-1 h^-1 under 420 nm light-emitting diode irradiation. This work can provide a new view for the development of transition metal-substituted POM-based stable and efficient S-scheme photocatalysts.

Ultrathin tungsten-doped hydrogenated titanium dioxide nanosheets for solar-driven hydrogen evolution

Ultrathin tungsten-doped hydrogenated TiO2 (W-h-TiO2) nanosheets are highly efficient for photocatalytic hydrogen production by water splitting without a noble metal cocatalyst.

Rational design of all-solid-state TiO2-x/Cu/ZnO Z-scheme heterojunction via ALD-assistance for enhanced photocatalytic activity

Poor visible light utilization and charge separation efficiency of TiO₂ restrict its extensive application in the photocatalytic field. Herein, a specific Z-scheme TiO_2-x/Cu/ZnO heterojunction was successfully constructed by atomic layer deposition (ALD) technique and spray pyrolysis technology. Benefited from the surface plasmon resonance (SPR) effect of Cu and Z-scheme heterojunction, the visible light absorption capacity was greatly enhanced. Meanwhile, ZnO nanolayer coating, prepared by ALD technique, protects Cu element to hinder its oxidation, thus enhancing the separation efficiency of photogenerated carriers. Therefore, the photocatalytic hydrogen production performance was significant improved, exhibiting a maximum value of 342.0 µmol·g^-1·h^-1 for the optimal B-T-0.1C-10Z (black TiO₂/0.1Cu/10 nm ZnO) sample without any noble-metal cocatalyst, which is higher than pure TiO₂ (310.7 µmol·g^-1·h^-1, with 3 wt% Pt) synthesized by spray pyrolysis method under equal conditions. In addition, a possible mechanism for the enhanced performance was briefly discussed based on the experimental results.

Efficient hydrogen generation of vector Z-scheme CaTiO3/Cu/TiO2 photocatalyst assisted by cocatalyst Cu nanoparticles

Herein, the CaTiO₃/Cu/TiO₂ all-solid-state Z-scheme heterojunction is successfully designed via Cu nanoparticles situating at the interface between CaTiO₃ and TiO₂ with a new synthesis route. Interestingly, TiO₂ nanosheets are generated in-situ on the surface of CaTiO₃ in the second step hydrothermal reaction. The lifetimes of photoexcited carriers, photoluminescence emission spectra and transient photocurrent response tests have confirmed that the efficient Z-scheme charge transmission path of the CaTiO₃/Cu/TiO₂ is beneficial to facilitate the separation of photogenerated carriers and reduce their recombination efficiency. As expected, the hydrogen generation rate of CaTiO₃/Cu/TiO₂ is increased to 23.550 mmol g^-1h^-1 with the appropriate amount of copper loading, which is about 981 times and 93 times higher than that of pristine CaTiO₃ (0.024 mmol g^-1h^-1) and CaTiO₃/TiO₂ (0.253 mmol g^-1h^-1), respectively. Furthermore, the CaTiO₃/Cu/TiO₂ sample shows good stability in cycle experiments. Particularly, experimental results show that the non-noble metal Cu nanoparticles can be an effective electron mediator. And these merits strongly demonstrate that the CaTiO₃/Cu/TiO₂ composites have potential application in photocatalytic field. This study can provide fundamental guidance for designing rationally efficient non-noble metal vector Z-scheme system photocatalysts with outstanding photocatalytic H₂ generation performance.

ZIF-8 derived ZnO/TiO2 heterostructure with rich oxygen vacancies for promoting photoelectrochemical water splitting

Due to the serious recombination of electron-hole and weak photoresponse ability, achieving highly efficient photoelectrochemical (PEC) water splitting activity for TiO₂ photoelectrode has become a key issue. In this paper, we reported a new method for preparing ZnO/TiO₂ photoelectrodes by electrostatic adsorption from zeolitic imidazolate framework-8 (ZIF-8) as the precursor. ZIF-8 was combined with TiO₂ nanorods (NRs) through electrostatic interaction and then calcined to obtain ZnO/TiO₂ heterojunction photoelectrodes with abundant oxygen vacancies (O_vac). The introduced ZnO with O_vac provides a large number of active sites which enhanced the electrical conductivity and charges separation of ZnO/TiO₂ photoelectrode. The optimal photocurrent density of ZnO/TiO₂ photoelectrodes at 1.23 V versus (vs.) reversible hydrogen electrode (RHE) under illumination (100 mW/cm²) has reached 1.76 mA/cm², almost 2.75 times that of the pure TiO₂. Meanwhile, the incident photon-to-electron conversion efficiency (IPCE) of the best photoelectrode has increased to 58.2% at 390 nm, the charge injection (η_injection) and separation (η_separation) efficiency have reached to 93.53% and 51.62% (1.23 V vs. RHE), respectively.