Plasmon-Induced Hot Electrons in Nanostructured Materials: Generation, Collection, and Application to Photochemistry

Plasmon refers to the coherent oscillation of all conduction-band electrons in a nanostructure made of a metal or a heavily doped semiconductor. Upon excitation, the plasmon can decay through different channels, including nonradiative Landau damping for the generation of plasmon-induced energetic carriers, the so-called hot electrons and holes. The energetic carriers can be collected by transferring to a functional material situated next to the plasmonic component in a hybrid configuration to facilitate a range of photochemical processes for energy or chemical conversion. This article centers on the recent advancement in generating and utilizing plasmon-induced hot electrons in a rich variety of hybrid nanostructures. After a brief introduction to the fundamentals of hot-electron generation and decay in plasmonic nanocrystals, we extensively discuss how to collect the hot electrons with various types of functional materials. With a focus on plasmonic nanocrystals made of metals, we also briefly examine those based upon heavily doped semiconductors. Finally, we illustrate how site-selected growth can be leveraged for the rational fabrication of different types of hybrid nanostructures, with an emphasis on the parameters that can be experimentally controlled to tailor the properties for various applications.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Enhancement of Cu+ stability under a reducing atmosphere by the long-range electromagnetic effect of Au

In conventional thermocatalytic reactions under a reducing atmosphere, stabilization of the active Cu⁺ component and inhibition of over-reduction into metallic Cu⁰ are extremely challenging. In this study, Au@Cu₂O core-shell nano-catalysts with different Cu₂O shell thicknesses were synthesized, and the effect of the Au nano-core on Cu⁺ stability under a reducing atmosphere and the catalytic performance of Cu⁺ in the ethynylation of formaldehyde were investigated. The Au nano-core facilitates Cu₂O dispersion and leads to an increase of 0.2-0.5 eV in electron binding energies of Cu₂O and Cu₂C₂ in the range of 27-55 nm, attributed to the long-range electromagnetic effect of Au NPs. Specifically, active Cu⁺ centers exhibit high stability under a reducing atmosphere due to the long-range electromagnetic effect of the Au nano-core. In the ethynylation of formaldehyde as a probe reaction, Cu⁺/(Cu⁰ + Cu⁺) on Au@Cu₂O catalysts remained at 88-91%. The catalytic performance in the ethynylation of formaldehyde revealed that the introduction of an Au nano-core into Cu-based catalysts increased the TOF from 0.37 to 0.7 h^-1, and decreased the activation energy from 42.6 to 38.1 kJ mol^-1. Additionally, the Cu⁺/(Cu⁰ + Cu⁺) ratios and the catalytic performance in the ethynylation of formaldehyde (BD yield = 65%, BD selectivity = 95%) on Au@Cu₂O catalysts remained constant after nine cycles, while pure Cu₂O readily deactivated due to the dramatically reduced Cu⁺/(Cu⁰ + Cu⁺) ratios and carbyne deposition. In summary, Cu⁺ in Cu-based catalysts showed high catalytic activity and stability during the ethynylation of formaldehyde due to the long-range electromagnetic effect of the Au nano-core.

Yolk-shell nanostructures: synthesis, photocatalysis and interfacial charge dynamics

Solar energy has long been regarded as a promising alternative and sustainable energy source. In this regard, photocatalysts emerge as a versatile paradigm that can practically transform solar energy into chemical energy. At present, unsatisfactory conversion efficiency is a major obstacle to the widespread deployment of photocatalysis technology. Many structural engineering strategies have been proposed to address the issue of insufficient activity for semiconductor photocatalysts. Among them, creation of yolk-shell nanostructures which possess many beneficial features, such as large surface area, efficient light harvesting, homogeneous catalytic environment and enhanced molecular diffusion kinetics, has attracted particular attention. This review summarizes the developments that have been made for the preparation and photocatalytic applications of yolk-shell nanostructures. Additional focus is placed on the realization of interfacial charge dynamics and the possibility of achieving spatial separation of charge carriers for this unique nanoarchitecture as charge transfer is the most critical factor determining the overall photocatalytic efficiency. A future perspective that can facilitate the advancement of using yolk-shell nanostructures in sophisticated photocatalytic systems is also presented.

Regulation and enhancement of the nonlinear optical properties of reduced graphene oxide through Au nanospheres and Au@CdS core-shells

Regulating nonlinear optical (NLO) absorption and refraction properties of graphene is significant in nonlinear photonics. In this work, the functionalizing strategy of quantum dots (QDs) was used for the regulation of the NLO properties of reduced graphene oxide (rGO) with Au nanospheres and Au@CdS core-shells. The third-order NLO properties of rGO-Au and rGO-Au@CdS nanocomposites were investigated by Z-scan technique with 38 ps laser pulses at 532 nm wavelength. The saturation absorption (SA) of rGO was weakened when it was combined with Au nanospheres and switched to reverse saturation absorption (RSA) when combined with Au@CdS core-shells as irradiance intensity increased. The strength of RSA was tunable with the size of Au@CdS core-shells. Meanwhile, the nonlinear susceptibility of rGO was weakened with the complex Au but was tunable with the complex Au@CdS and realized two times enhancement. The effect of charge transfer mechanism was proposed to explain the regulated effect.

Sensitive Detection of Separated Charges in Nanohybrids by Laser Excitation Mass Spectrometry with Tetrabutylammonium Cationic Probe

Charge separation lays the foundation for photocatalysis and photovoltaics, in which the catalytic/voltaic efficiency is primarily related to the amount of separated charges generated. Yet, direct experimental approaches for the quantification of separated charges are very limited, especially for nanostructures in small quantities. Here, by laser excitation mass spectrometry with tetrabutylammonium as a sensitive probe, the separated charges in gold-metal sulfide core-shell nanostructures are determined and correlated with the bandgap of the semiconductor shell. Moreover, the separated charges formed can already be detected unambiguously in only an attomole-level of nanoparticles (i.e., 1 × 10⁸ NPs).

Ultrathin and Isotropic Metal Sulfide Wrapping on Plasmonic Metal Nanoparticles for Surface Enhanced Ram Scattering-Based Detection of Trace Heavy-Metal Ions

A facile and general strategy is presented for homogenous and ultrathin metal sulfide wrapping on plasmonic metal (PM) nanoparticles (NPs) based on a thiourea-induced isotropic shell growth. This strategy is typically implemented just via adding the thiourea into pre-formed PM colloidal solutions containing target metal ions. The validity of this strategy is demonstrated by taking the wrapped NPs with Au core and CuS shell or Au@CuS NPs as an example. They are successfully fabricated via adding the thiourea and Cu²⁺ solutions into pre-formed Au NP colloidal solution. The CuS shell layer is highly homogenous (<10% in relative standard deviation of shell thickness), regardless of the NPs' shape or curvature. The shell thickness can be controlled from tens down to 0.5 nm just by the addition of different amounts of shell precursors. The formation of the shell layer on the Au NPs can be attributed to the alternative deposition of Cu²⁺ and S^2- ions on the thiourea-modified surface of Au NPs in the solution, which induces the isotropic shell growth. Further, this strategy is of good universality. Many other sulfide-wrapped PM NPs, such as Ag@CuS, Au@PtS₂, Au@HgS, Ag@Ag₂S NPs, and Ag@CuS nanorods, have been successfully obtained with homogeneous and ultrathin shells. Importantly, such ultrathin sulfide-wrapped PM NPs can be used for surface enhanced Raman scattering (SERS)-based detection of trace heavy-metal ions with strong anti-interference via the ion exchange process between the metal sulfide shell and heavy-metal ions. This study provides a simple and controllable route for wrapping the homogenous and ultrathin sulfide layers on the PM NPs, and such wrapped NPs have good practical applications in the SERS-based detection of trace heavy-metal ions.

Rare-Earth-Based MIS Type Core-Shell Nanospheres with Visible-Light-Driven Photocatalytic Activity through an Electron Hopping-Trapping Mechanism

A bilayered rare-earth-based metal-insulator-semiconductor, Dy₂O₃@SiO₂@ZnO core-shell nanospheres, was synthesized by a stepwise synthesis for enhanced visible photocatalytic activity. The prepared material was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, field-emission scanning electron microscopy, energy-dispersive spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, and electron paramagnetic resonance techniques. Dy₂O₃@SiO₂@ZnO core-shell nanospheres were found be in a spherically arranged cauliflower-like morphology (40-60 nm). The high-resolution transmission electron microscopy analysis proved the core-shell morphology of the prepared material with a single Dy₂O₃ core and two shells comprising SiO₂ and ZnO. The material possessed a surface roughness of 4. 98 nm (2 × 2 μm area) and a band gap energy of 2.82 eV. The in situ generation of OH radicals was confirmed by electron paramagnetic resonance. Electron hopping through the SiO₂ layer from ZnO to Dy₂O₃ played a major role in trapping electrons in the f-shells of lanthanides, thus, preventing the recombination of electron-hole pair. X-ray photoelectron spectroscopy studies proved the band alignment of the material. Brunauer-Emmett-Teller analysis further showed the core-shell surface area was 14 m²/g. The visible photocatalytic activity was tested against 2,4-D (2,4-dichlorophenoxyacetic acid), an endocrine disruptor. The kinetic studies showed that the photocatalytic degradation process followed a pseudo-first-order pathway. The photocatalyst was found to be reusable even up to the third cycle.

Carbon quantum dot sensitized Pt@Bi2WO6/FTO electrodes for enhanced photoelectro-catalytic activity of methanol oxidation

Carbon quantum dots (CQDs) sensitized Pt@Bi₂WO₆/FTO electrodes (simplified as CQDs-Pt@Bi₂WO₆/FTO) were prepared by loading platinum particles onto Bi₂WO₆ nanoplates via a photo-deposition method and sensitized CQDs via a dip-coating method.

Metal-Particle-Decorated ZnO Nanocrystals: Photocatalysis and Charge Dynamics

Understanding of charge transfer processes is determinant to the performance optimization for semiconductor photocatalysts. As a representative model of composite photocatalysts, metal-particle-decorated ZnO has been widely employed for a great deal of photocatalytic applications; however, the dependence of charge carrier dynamics on the metal content and metal composition and their correlation with the photocatalytic properties have seldom been reported. Here, the interfacial charge dynamics for metal-decorated ZnO nanocrystals were investigated and their correspondence with the photocatalytic properties was evaluated. The samples were prepared with a delicate antisolvent approach, in which ZnO nanocrystals were grown along with metal particle decoration in a deep eutectic solvent. By modulating the experimental conditions, the metal content (from 0.6 to 2.3 at%) and metal composition (including Ag, Au, and Pd) in the resulting metal-decorated ZnO could be readily controlled. Time-resolved photoluminescence spectra showed that an optimal Au content of 1.3 at% could effectuate the largest electron transfer rate constant for Au-decorated ZnO nanocrystals, in conformity with the highest photocatalytic efficiency observed. The relevance of charge carrier dynamics to the metal composition was also inspected and realized in terms of the energy level difference between ZnO and metal. Among the three metal-decorated ZnO samples tested, ZnO-Pd displayed the highest photocatalytic activity, fundamentally according with the largest electron transfer rate constant deduced in carrier dynamics measurements. The current work was the first study to present the correlations among charge carrier dynamics, metal content, metal composition, and the resultant photocatalytic properties for semiconductor/metal heterostructures. The findings not only helped to resolve the standing issues regarding the mechanistic foundation of photocatalysis but also shed light on the intelligent design of semiconductor/metal composite systems to consolidate their utility in photocatalytic fields.

High Efficiency Photoelectrocatalytic Methanol Oxidation on CdS Quantum Dots Sensitized Pt Electrode

A cadmium sulfide quantum dots sensitized Pt (Pt-CdS) composite was synthesized using a solvothermal method and characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectroscopy. The catalytic properties of the as-prepared electrode for methanol oxidation were evaluated by cyclic voltammetry (CV), chronoamperometry, electrochemical impedance spectrum (EIS) and photocurrent responses. The as-prepared Pt-CdS electrode displayed a significant enhancement in the electrocatalytic activity and stability for methanol oxidation in the presence of visible light irradiation. The synergistic effect of both the electro- and photocatalytic reaction contributes to this enhanced catalytic performance. Our result suggests a new paradigm to construct photoelectrocatalysts with high performance and good stability for direct methanol fuel cells with the assistance of visible-light illumination.

Plasmonic Au nanoparticles embedding enhances the activity and stability of CdS for photocatalytic hydrogen evolution

A semiconductor fabricated by embedding plasmonic Au nanoparticles into CdS exhibits a high activity and stability for photocatalytic H₂evolution. This composite structure could take full advantage of electromagnetic fields at the surface of the Au nanoparticles under visible light illumination.

Ambient synthesis of a multifunctional 1D/2D hierarchical Ag–Ag2S nanowire/nanosheet heterostructure with diverse applications

A new type of unique 1D/2D hierarchical Ag–Ag₂S hybrids is fabricated by an extremely simple solution route under ambient conditions. The diffusion and Ostwald ripening processes dominate the evolution of heterostructure.

ZnSe·0.5N2H4 hybrid nanostructures: a promising alternative photocatalyst for solar conversion

As the molecular precursor of ZnSe, ZnSe·0.5N2H4 inorganic-organic hybrids have received relatively less attention due to the feasibility of their further processing and decomposition into pure-phase ZnSe. Here we demonstrated that ZnSe·0.5N2H4 hybrid nanostructures, which were prepared using a facile hydrazine-assisted hydrothermal method, may practically harvest solar energy for photoconversion applications. By modulating the volume ratio of hydrazine hydrate to deionized water employed in the synthesis, the morphology of the grown ZnSe·0.5N2H4 can be varied, which included nanowires, nanobelts and nanoflakes. With the relatively long exciton lifetime and highly anisotropic structure, ZnSe·0.5N2H4 nanowires performed much better in the photodegradation of rhodamine B than the other two counterpart products. As compared to pure ZnSe nanoparticles and single-phase ZnSe nanowires obtained from further processing ZnSe·0.5N2H4, the ZnSe·0.5N2H4 hybrid nanowires exhibited superior photocatalytic performance under visible light illumination. The hybrid nanowires were further decorated with Au particles to endow them with structural and compositional diversities. Time-resolved photoluminescence spectra suggested that almost 40% of the photoexcited electrons in ZnSe·0.5N2H4 nanowires can be transported to the decorated Au, which enabled a fuller extent of participation of charge carriers in the photocatalytic process and thus conduced to a significant enhancement in the photocatalytic activity. The demonstrations from this work illustrate that ZnSe·0.5N2H4 hybrid nanostructures can serve as a versatile photocatalyst platform for advanced photocatalytic applications.

Metal/Semiconductor hybrid nanostructures for plasmon-enhanced applications

Hybrid nanostructures composed of semiconductor and plasmonic metal components are receiving extensive attention. They display extraordinary optical characteristics that are derived from the simultaneous existence and close conjunction of localized surface plasmon resonance and semiconduction, as well as the synergistic interactions between the two components. They have been widely studied for photocatalysis, plasmon-enhanced spectroscopy, biotechnology, and solar cells. In this review, the developments in the field of (plasmonic metal)/semiconductor hybrid nanostructures are comprehensively described. The preparation of the hybrid nanostructures is first presented according to the semiconductor type, as well as the nanostructure morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then elucidated. Lastly, possible future research in this burgeoning field is discussed.

EFFECT OF Au AND Pt DEPOSITION AND THERMAL TREATMENT ON THE PHOTOCATALYTIC ACTIVITY OF AS-PREPARED ZnS NANOROD

The wurtzite phase of ZnS -nanorods (NRs) (length 100–120 nm and width 10–15 nm) has been prepared by solvothermal method and 1 wt.% Au and Pt nanoparticles (2–5 nm) were photodeposited and sintered at 500°C for 2 h. ZnS -NR exhibits an exciton band at 283 nm and absorption onset at 304 nm (bandgap = 4.06 eV) as compared to 336 nm (bandgap = 3.77 eV) of bulk ZnS , indicating its quantum size effect. The absorption is red shifted to ~350 nm and ~366 nm after Au and Pt deposition, and also sintering led to quench the ZnS absorption band in UV region due to the exciton–plasmon interactions. Photoluminescence (PL) of ZnS -NRs (band at 366–377 nm) is significantly reduced after Au or Pt deposition and sintering. The photocatalytic activity of p-nitrophenol (PNP) degradation has been appreciably improved (rate constants k = 3.6 × 10-3 min-1 and k = 3.6 × 10-3 min-1) after 1 wt.% Pt loading on ZnS followed by sintering at 500°C for 2 h as compared to bare (k = 1.2 × 10-3 min-1) and sintered ZnS (k = 3.3 × 10-3 min-1) catalyst, respectively. Similarly, Au loading and sintering led to increased the k = 2.7 × 10-3 and 4.5 × 10-3 min-1 relative to bare ZnS -NR and direct photolysis (k = 0.3 × 10-3 min-1) of PNP under UV light (10.4 mWcm-2) irradiation. Sintering appreciably improves the ZnS (surface area = 58.9 m2g-1) photoactivity due to better crystallinity and removal of defect sites.