Exploring the Mechanisms of Charge Transfer and Identifying Active Sites in the Hydrogen Evolution Reaction Using Hollow C@MoS2-Au@CdS Nanostructures as Photocatalysts

Plasmonic metal-semiconductor nanocomposites are promising candidates for considerably enhancing the solar-to-hydrogen conversion efficiency of semiconductor-based photocatalysts across the entire solar spectrum. However, the underlying enhancement mechanism remains unclear, and the overall efficiency is still low. Herein, a hollow C@MoS2-Au@CdS nanocomposite photocatalyst is developed to achieve improved photocatalytic hydrogen evolution reaction (HER) across a broad spectral range. Transient absorption spectroscopy experiments and electromagnetic field simulations demonstrate that compared to the treated sample, the untreated sample exhibits a high density of sulfur vacancies. Consequently, under near-field enhancement, photogenerated electrons from CdS and hot electrons generated by intra-band or inter-band transitions of Au nanoparticles are efficiently transferred to the CdS surface, thus significantly improving the HER activity of CdS. Additionally, in situ, Raman spectroscopy provided spectral evidence of S─H intermediate species on the CdS surface during the HER process, which is verified through isotope experiments. Density functional theory simulations identify sulfur atoms in CdS as the catalytic active sites for HER. These findings enhance the understanding of charge transfer mechanisms and HER pathways, offering valuable insights for the design of plasmonic photocatalysts with enhanced efficiency.

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

Polygonal gold nanocrystal induced efficient phase transition in 2D-MoS2for enhancing photo-electrocatalytic hydrogen generation

Plasmonic nanocrystals (NCs) assisted phase transition of two-dimensional molybdenum disulfide (2D-MoS₂) unlashes numerous opportunities in the fields of energy harvesting via electrocatalysis and photoelectrocatalysis by enhancing electronic conductivity, increasing catalytic active sites, lowering Gibbs free energy for hydrogen adsorption and desorption, etc. Here, we report the synthesis of faceted gold pentagonal bi-pyramidal (Au-PBP) nanocrystals (NC) for efficient plasmon-induced phase transition (from 2 H to 1 T phase) in chemical vapor deposited 2D-MoS₂. The as-developed Au-PBP NC with the increased number of corners and edges showed an enhanced multi-modal plasmonic effect under light irradiations. The overpotential of hydrogen evolution reaction (HER) was reduced by 61 mV, whereas the Tafel slope decreased by 23.7 mV/dec on photoexcitation of the Au-PBP@MoS₂hybrid catalyst. The enhanced performance can be attributed to the light-induced 2H to 1 T phase transition of 2D-MoS₂, increased active sites, reduced Gibbs free energy, efficient charge separation, change in surface potential, and improved electrical conductivity of 2D-MoS₂film. From density functional theory (DFT) calculations, we obtain a significant change in the electronic properties of 2D-MoS₂(i.e. work function, surface chemical potential, and the density of states), which was primarily due to the plasmonic interactions and exchange-interactions between the Au-PBP nanocrystals and monolayer 2D-MoS₂, thereby enhancing the phase transition and improving the surface properties. This work would lay out finding assorted routes to explore more complex nanocrystals-based multipolar plasmonic NC to escalate the HER activity of 2D-MoS₂and other 2D transition metal dichalcogenides.

Regulating the charge carrier transport rate via bridging ternary heterojunctions to enable CdS nanorods' solar-driven hydrogen evolution

Solar-driven hydrogen generation using single-semiconductor photocatalysts for hydrogen evolution seems to be challenging due to their poor solar to fuel conversion efficiency because of their fast charge carrier recombination. The ternary heterostructure was prepared by an advanced approach to suppress the recombination of photogenerated charge carriers and has contributed a new platform for designing highly efficient photocatalytic systems. Herein, we fabricated a ternary heterojunction with ultrathin WS₂-SnS₂ nanosheets and CdS nanorods, and the photocatalytic activity was studied. The optimized CdS/SnS₂-WS₂ (6 wt%) nanostructures were found to be highly stable and exhibited the highest hydrogen evolution rate of 232.45 mmol g^-1 h^-1, which was almost 93-fold higher than that of the pristine CdS nanorods. Also, Density Functional Theory (DFT) calculations confirmed that the favorable band alignment for charge transport and superior catalytic activity of the newly fabricated ternary nanostructures make them a potential candidate for solar-driven hydrogen production.

Rapid Charge Separation Boosts Solar Hydrogen Generation at the Graphene-MoS2 Junction: Time-Domain Ab Initio Analysis

Transition metal dichalcogenides and graphene hybrids hold great promise for photovoltaics and photocatalysts. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we investigate the interplay between forward and backward electron transfer (ET), as well as energy relaxation in a van der Waals graphene-MoS₂ heterojunction. We demonstrated that built-in potential formed at the polarized interface produces charge separation upon photoexcitation. The electron left on graphene is injected into MoS₂ on an ultrafast time scale, which is notably faster than energy losses to heat regardless of the initial state energy. Once the electron is relaxed to the conduction band edge state of MoS₂, it transfers back and recombines with the hole remaining on graphene on ultrafast time scales by considering quantum transitions among multiple k points. The obtained time scales for ET, back-ET, and energy relaxation agree well with experimental data. The study reveals that ET that is faster than energy loss makes the graphene-MoS₂ heterojunction efficient for optoelectronic applications.

Facile photo-ultrasonic assisted synthesis of flower-like Pt/N-MoS2 microsphere as an efficient sonophotocatalyst for nitrogen fixation

Semiconductor photocatalytic technology is a sustainable and less energy consuming one for nitrogen (N₂) reduction to produce ammonia (NH₃). In this study, flower-like hierarchical N doped MoS₂ (N-MoS₂) microsphere was synthesized as a photocatalyst by one-step solvothermal method, which was assembled by numerous interleaving nanosheets petals with thin thickness. Besides, Pt nanoparticles were loaded on the surface of N-MoS₂ via photo-ultrasonic reduction method. The as-prepared Pt/N-MoS₂ photocatalyst exhibited higher N₂ fixation ability than that over pure MoS₂ and N-MoS₂, which can be attributed to that the N doping narrows the band gap, and the Schottky barrier due to the existence of Pt nanoparticles improves the charge transfer and carrier separation. The reduction of N₂ with ultrasonic irradiation was also investigated under visible light irradiation to evaluate the sonophotocatalytic activity of the Pt/N-MoS₂ microsphere. The results showed that the N₂ reduction rate of sonophotocatalysis (133.8 µmol/g_(cat)h) was higher than that of sonocatalysis and photocatalysis, which can be ascribed to the synergistic effect of ultrasound and visible light irradiation. The effects of catalyst dosage, ultrasonic power and ultrasonic pulse on the photocatalytic efficiency were also studied. Meanwhile, a possible mechanism for improved sonophotocatalytic performance was also proposed.

Tunable Pt-MoS x Hybrid Catalysts for Hydrogen Evolution

Platinum (Pt)-based materials are inevitably among the best-performing electrocatalysts for hydrogen evolution reaction (HER). MoS₂ was suggested to be a potent HER catalyst to replace Pt in this reaction by theoretical modeling; however, in practice, this dream remains elusive. Here we show a facile one-pot bottom-up synthesis of Pt-MoS _x composites using electrochemical reduction in an electrolytic bath of Pt precursor and ammonium tetrathiomolybdate under ambient conditions. By modifying the millimolar concentration of Pt precursors, composites of different surface elemental composition are fabricated; specifically, Pt_1.8MoS₂, Pt_0.1MoS_2.5, Pt_0.2MoS_0.6, and Pt_0.3MoS_0.8. All electrodeposited Pt-MoS _x hybrids showcase low overpotentials and small Tafel slopes that outperform MoS₂ as an electrocatalyst. Tantamount to electrodeposited Pt, the rate-limiting process in the HER mechanism is determined to be the Heyrovsky desorption across Pt-MoS _x hybrids and starkly swings from the rate-determining Volmer adsorption step in MoS₂. The Pt-MoS _x composites are equipped with catalytic performance that closely mirrors that of electrodeposited Pt, in particular the HER kinetics for Pt_1.8MoS₂ and Pt_0.1MoS_2.5. This work advocates electrosynthesis as a cost-effective method for catalyst design and fabrication of competent composite materials for water splitting applications.

Largely enhanced photocatalytic activity of Au/XS2/Au (X = Re, Mo) antenna-reactor hybrids: charge and energy transfer

An antenna-reactor hybrid coupling plasmonic antenna with catalytic nanoparticles is a new strategy to optimize photocatalytic activity. Herein, we have rationally proposed a Au/XS₂/Au (X = Re, Mo) antenna reactor, which has a large Au core as the antenna and small satellite Au nanoparticles as the reactor separated by an ultrathin two-dimensional transition-metal dichalcogenide XS₂ shell (∼2.6 nm). Due to efficient charge transfer across the XS₂ shell as well as energy transfer via coupling of the Au antenna and Au reactor, the photocatalytic activity has been largely enhanced: Au/ReS₂/Au exhibits a 3.59-fold enhancement, whereas Au/MoS₂/Au exhibits a 2.66-fold enhancement as compared to that of the sum of the three individual components. The different enhancement in the Au/ReS₂/Au and Au/MoS₂/Au antenna-reactor hybrid is related to the competition and cooperation of charge and energy transfer. These results indicate the great potential of the Au/XS₂/Au antenna-reactor hybrid for the development of highly efficient plasmonic photocatalysts.

Synergistic effect of rare earth metal Sm oxides and Co1−xS on sheet structure MoS2 for photocatalytic hydrogen evolution

The novel composite Sm₂O₃@Co_1−xS/MoS₂ has high photocatalytic activity and stability, the electron transfer and the charge separation were obviously improved with the synergistic effects of Sm₂O₃ and Co_1−xS on the surface of MoS₂.

Sulfur vacancy induced high performance for photocatalytic H2 production over 1T@2H phase MoS2 nanolayers

Mo-precursor has great impact on the morphology, surface chemistry and photocatalytic activity of MoS₂ nanostructure.