Red-Light-Driven Water Splitting by Au(Core)–CdS(Shell) Half-Cut Nanoegg with Heteroepitaxial Junction

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Solution-phase vertical growth of aligned NiCo2O4 nanosheet arrays on Au nanosheets with weakened oxygen-hydrogen bonds for photocatalytic oxygen evolution

Vertical heterojunctions of two-dimensional (2D) semiconducting materials have attracted more and more research interest recently due to their unique optical, electrical, and catalytic properties and potential applications. Although great progress has been made, vertical integration of the layered materials formed by 2D semiconductor nanosheets and 2D plasmatic metal nanosheets remains a huge challenge. Here, we demonstrate for the first time a solution-phase growth of vertical plasmatic metal-semiconductor heterostructures in which aligned NiCo2O4 nanosheet arrays vertically grow on a single Au nanosheet, forming vertically aligned NiCo2O4-Au-NiCo2O4 sandwich-type heterojunctions with hierarchical open channels. Such vertical NiCo2O4-Au-NiCo2O4 heterojunctions can effectively promote the separation and transfer of a photoinduced charge. Density functional theory (DFT) studies and time-resolved transient absorption spectroscopy show that electrons transfer from NiCo2O4 to Au, and the formation of the heterojunction weakens the H-O bond of H2O. Due to the unique structure and superiority of the component, the vertical NiCo2O4-Au-NiCo2O4 heterojunctions exhibit significant activity with an O2 production rate of up to 33 μmol h-1 and long-term stability for photocatalytic water oxidation. We calculated the apparent quantum efficiency (AQE) to be 21.9% for NiCo2O4-Au-NiCo2O4 heterojunctions at the wavelength λ = 450 ± 10 nm, which is higher than that of NiCo2O4 nanosheets (10.9%), Au nanosheets (0.96%) and other photocatalysts. The present study paves the way for the controlled synthesis of novel vertical heterojunctions based on 2D semiconductor nanosheets and 2D metal nanosheets for efficient photocatalysis.

Multi-interfacial plasmon coupling in multigap (Au/AgAu)@CdS core-shell hybrids for efficient photocatalytic hydrogen generation

Plasmon coupling induced intense light absorption and near-field enhancement have vast potential for high-efficiency photocatalytic applications. Herein, (Au/AgAu)@CdS core-shell hybrids with strong multi-interfacial plasmon coupling were prepared through a convenient strategy for efficient photocatalytic hydrogen generation. Bimetallic Au/AgAu cores with an adjustable number of nanogaps (from one to four) were primarily synthesized by well-controlled multi-cycle galvanic replacement and overgrowth processes. Extinction tests and numerical simulations synergistically revealed that the multigap Au/AgAu hybrids possess a gap-dependent light absorption region and a local electric field owing to the multigap-induced multi-interfacial plasmon coupling. With these characteristics, hetero-photocatalysts prepared by further coating of CdS shells on multigap Au/AgAu cores exhibited a prominent gap-dependent photocatalytic hydrogen production activity from water splitting under light irradiation (λ > 420 nm). It is found that the hydrogen generation rates of multigap (Au/AgAu)@CdS have an exponential improvement compared with that of pure CdS as the number of nanogaps increases. In particular, four-gap (Au/AgAu)@CdS core-shell catalysts displayed the highest hydrogen generation rate, that is 96.1 and 47.2 times those of pure CdS and gapless Au@CdS core-shell hybrids. These improvements can be ascribed to the strong plasmon absorption and near-field enhancement induced by the multi-interfacial plasmon coupling, which can greatly improve the light-harvesting efficiency, offer more plasmonic energy, and boost the generation and separation of electron-hole pairs in the multigap catalysts.

Highly efficient charge transfer in CdS-covalent organic framework nanocomposites for stable photocatalytic hydrogen evolution under visible light

A facile and effective impregnation combined with photo-deposition approach was adopted to deposit cadmium sulfide (CdS) nanoparticles on CTF-1, a covalent triazine-based frameworks (CTFs). In this system, CTF-1 not only acted as supporter but also served as photocatalyst and electron donor. The performance of the obtained CdS deposited CTF-1 (CdS-CTF-1) nanocomposite was evaluated by H₂ evolution reaction under visible light irradiation. As a result, CdS-CTF-1 exhibited high H₂ production from water, far surpassing the CdS/CTF-1 nanocomposite, in which CdS was deposited via solvothermal method. The high activity of CdS-CTF-1 was attributed to the confined CdS nanoparticles with small size, leading to expose more active sites. In addition, time-resolved spectroscopy indicated that the superior performance of CdS-CTF-1 also can be ascribed to the fast electron transfer rate and injection efficiency (K_ET = 0.18 × 10⁹ s^-1, η_inj = 39.38%) between CdS and CTF-1 layers, which are 3.83 times faster and 4.84 times higher than that of CdS/CTF-1 nanocomposite. This work represents the first example on using covalent organic frameworks (COFs) as a support and electron-donor for fabricating novel CdS-COF nanocomposite system and its potential application in solar energy transformations.

A redox interaction-engaged strategy for multicomponent nanomaterials

The review article focuses on the redox interaction-engaged strategy that offers a powerful way to construct multicomponent nanomaterials with precisely-controlled size, shape, composition and hybridization of nanostructures.

Plasmonic nanoparticle-film-assisted photoelectrochemical catalysis across the entire visible-NIR region

Low solar light absorption and high electron-hole pair recombination are still the main challenges for solar energy conversion. Here, we design a plasmonic nanoparticle (NP)-film with a unique structure combining the advantages of a Au NP and film, which exhibits strong broadband absorption from the visible to near-infrared (NIR) wavelength range. In addition, the high density of sub-1 nm inter-particle gaps in the Au NP-film supports electromagnetic field enhancement of several orders of magnitude that greatly promotes the generation and separation of electron-hole pairs. Accordingly, the plasmonic NP-film-assisted photocatalyst (TiO₂/90Au/TiO₂) leads to an 88-fold increase in the photocurrent density at 0.75 V vs. RHE in 25% methanol solution under visible-NIR light irradiation (λ > 420 nm) compared to a TiO₂ film, which is higher than those of the ever reported Au/TiO₂ photocatalysts in the entire visible-NIR range. Our finding indicates a promising way to explore full solar spectrum photocatalysts, which can be easily extended to other energy conversion applications.

Overall water splitting and hydrogen peroxide synthesis by gold nanoparticle-based plasmonic photocatalysts

Plasmonic photocatalysts driven by the localized surface plasmon resonance excitation of gold nanoparticles (Au NPs) can be efficient solar-to-chemical converters due to their wide spectral response. This review article highlights recent studies on plasmonic water splitting and H₂O₂ synthesis from water and oxygen (O₂) with a particular emphasis placed on the electrocatalysis of Au NPs. The Introduction (Section 1) points to the importance of the establishment of solar hydrogen and oxygen cycles involving hydrogen (H₂) and hydrogen peroxide (H₂O₂) as the key compound, respectively, for realizing a "sustainable society". Section 2 deals with the basic action mechanisms of Au NP-based plasmonic photocatalysts. Section 3 treats the electrocatalytic activity of Au NPs for the half-reactions involved in the reactions. Section 4 describes recent advances in the plasmonic overall water splitting (4.1) and H₂O₂ synthesis (4.2). Finally, a summary is presented with the possible development direction in Section 5.

Plasmon-Enhanced Deuteration under Visible-Light Irradiation

Deuteration has found important applications in synthetic chemistry especially for pharmaceutical developments. However, conventional deuteration methods using transition-metal catalysts or strong bases generally involve harsh reaction conditions, expensive deuterium source, insufficient efficiency, and poor selectivity. Herein, we report an efficient visible-light-driven dehalogenative deuteration of organic halides using plasmonic Au/CdS as photocatalyst and D₂O as deuterium donor. Electron transfer from Au to CdS, which has been confirmed by surface-enhanced Raman spectroscopy, plays a decisive role for the plasmon-mediated dehalogenation. The deuteration is revealed to proceed via a radical pathway in which substrates are first activated by the photoinduced electron transfer to generate aryl radicals, and the radicals are further trapped by D₂O to give deuterated products. Under visible-light irradiation, excellent deuteration efficiency is achieved with high functional group tolerance and a wide range of substrates at room temperature. Compared with bare CdS, the photocatalytic activity increases ∼18 times after the loading of plasmonic Au nanoparticles. This work sheds light on the interfacial charge transfer between plasmonic metals and semiconductors as an important criterion for rational design of visible-light photocatalysts.

Shockley Partial Dislocation-Induced Self-Rectified 1D Hydrogen Evolution Photocatalyst

Photocatalytic stability and efficient charge separation are key factors to photocatalytic performance for visible-light-driven H₂ evolution from water. Here, we report a whole novel self-rectified photocatalyst constructed from the Shockley partial dislocation-induced multiple faults, using a ternary chalcogenide, that is, Cd_0.8Zn_0.2S nanorod as a model material. The introduction of multiple faults, which are typical planar defects, constructs a nanorectifier that aligns along the axial direction and constitutes a relatively ordered superstructure. The band bending and Fermi-level flattening at the nanorectifier would cause the photogenerated charge carriers to be transferred reversely at the axial direction on account of the charge type and then realize the separation of the charge carriers.

Size, shape and interface control in gold nanoparticle-based plasmonic photocatalysts for solar-to-chemical transformations

The key parameters in gold nanoparticle-based plasmonic photocatalysts are the gold particle size and shape, and the state of junction between the components. This frontier article highlights our recent advances in the structural design of plasmonic photocatalysts, and their application to solar-to-chemical transformations. As representatives, gold nanoparticle-loaded TiO2 and CdS systems are dealt with in this review. However, the contents would be widely applicable to the design of other systems.

Oxygen-Doped MoS2 Nanospheres/CdS Quantum Dots/g-C3N4 Nanosheets Super-Architectures for Prolonged Charge Lifetime and Enhanced Visible-Light-Driven Photocatalytic Performance

Oxygen-doped MoS₂ nanospheres/CdS quantum dots/g-C₃N₄ nanosheets are synthetized through hydrothermal and chemical bath deposition-calcination processes. The prepared materials are characterized by X-ray diffraction transient-state photoluminescence spectra, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical experiment. These results show that the ternary composite material has longer lifetime of photogenerated carriers and more active sites, thereby enhancing photocatalytic performance. CdS quantum dots act as a bridge between the intermediate transport charges in the ternary composite. The oxygen defect engineering prolongs the lifetime of carriers obviously, which is confirmed by transient-state photoluminescence. Moreover, the photocatalytic H₂ evolution and photodegradation of bisphenol A for MoS₂/CdS/g-C₃N₄ is up to 956 μmol h^-1 g^-1 and 95.2% under visible-light irradiation, respectively. Furthermore, excellent photocatalytic activity can be ascribed to the synergistic effect of defect engineering and formation of ternary heterostructures, which is with broad-spectrum response, longer lifetime of photo-induced electron-holes, and more surface active sites.

The dual-defective SnS2 monolayers: promising 2D photocatalysts for overall water splitting

Photocatalytic water splitting on the dual-defective SnS₂ monolayer is a promising way to produce hydrogen fuel from solar energy.

In Situ Shape Change of Au Nanoparticles on TiO2 by CdS Photodeposition: Its Near-Field Enhancement Effect on Photoinduced Electron Injection from CdS to TiO2

Hemisphere-like gold nanoparticles (NPs) were loaded on TiO₂ (Au/TiO₂) by the deposition-precipitation method. Subsequent photodeposition of CdS on the Au surface of Au/TiO₂ at 50 °C yields Au(core)-CdS(shell) hybrid quantum dots with a heteroepitaxial (HEPI) junction on TiO₂ (Au@#CdS/TiO₂), whereas nonHEPI Au@CdS/TiO₂ was formed by CdS photodeposition at 25 °C. In the HEPI system, the shape of the Au core changes to an angular shape, whereas it remains in a hemisphere-like shape in the nonHEPI system. The hot photodeposition technique was applied to the Au NP-loaded mesoporous TiO₂ nanocrystalline film (Au/mp-TiO₂). Using Au@CdS/mp-TiO₂ and Au@#CdS/mp-TiO₂ as the photoanodes, two-electrode quantum dot-sensitized photoelectrochemical cells were fabricated for hydrogen (H₂) generation from water, and the performances of the cells were evaluated under illumination of simulated sunlight. In the photocurrent and the rate of H₂ evolution, the Au@#CdS/mp-TiO₂ photoanode cell surpasses the CdS/mp-TiO₂ and Au@CdS/mp-TiO₂ ones. Three-dimensional finite-difference time-domain calculations for the model systems indicated that the angular shape Au core generates an intense electric field at the corners and edges, extending the electric field distribution over the Au core-CdS shell interface. The striking shape effect on the cell performances can originate from the promotion of the CdS excitation and charge separation due to the near-field enhancement by the deformed Au core.

Noble metal-free 0D–1D NiSx/CdS nanocomposites toward highly efficient photocatalytic contamination removal and hydrogen evolution under visible light

The heterostructures formed between 1D CdS nanorods and 0D NiS_xnanoclusters were prepared and showed high photocatalytic activity.

Controlled design of PtPd nanodendrite ornamented niobium oxynitride nanosheets for solar-driven water splitting

A facile road-map is developed for one-pot synthesis of PtPd nanodendrite ornamented niobium oxynitride nanosheets for efficient solar-driven water splitting.