Visible-light-driven photochemistry has continued to attract heightened interest due to its capacity to efficiently harvest solar energy and its potential to solve the global energy crisis. Plasmonic nanostructures boast broadly tunable optical properties coupled with catalytically active surfaces that offer a unique opportunity for solar photochemistry. Resonant optical excitation of surface plasmons produces energetic hot electrons that can be collected to facilitate chemical reactions. This review sums up recent theoretical and experimental approaches for understanding the underlying photophysical processes in hot electron generation and discusses various electron-transfer models on both plasmonic metal nanostructures and plasmonic metal/semiconductor heterostructures. Following that are highlights of recent examples of plasmon-driven hot electron photochemical reactions within the context of both cases. The review concludes with a discussion about the remaining challenges in the field and future oppor...

Surface-Plasmon-Driven Hot Electron Photochemistry

Semiconductor photocatalysis is a trustworthy approach to harvest clean solar light for energy conversions, while state-of-the-art catalytic efficiencies are unsatisfactory because of the finite light response and/or recombination of robust charge carriers. Along with the development of modern material characterization techniques and electronic-structure computations, oxygen vacancies (OVs) on the surface of real photocatalysts, even in infinitesimal concentration, are found to play a more decisive role in determining the kinetics, energetics, and mechanisms of photocatalytic reactions. This Review endeavors to clarify the inherent functionality of OVs in photocatalysis at the surface molecular level using 2D BiOCl as the platform. Structure sensitivity of OVs on reactivity and selectivity of photocatalytic reactions is intensely discussed via confining OVs onto prototypical BiOCl surfaces of different structures. The critical understanding of OVs chemistry can help consolidate and advance the fundamental theories of photocatalysis, and also offer new perspectives and guidelines for the rational design of catalysts with satisfactory performance.

Oxygen Vacancy-Mediated Photocatalysis of BiOCl: Reactivity, Selectivity, and Perspectives.

The challenge in the artificial photosynthesis of fossil resources from CO2 by utilizing solar energy is to achieve stable photocatalysts with effective CO2 adsorption capacity and high charge-separation efficiency. A hierarchical direct Z-scheme system consisting of urchin-like hematite and carbon nitride provides an enhanced photocatalytic activity of reduction of CO2 to CO, yielding a CO evolution rate of 27.2 µmol g-1 h-1 without cocatalyst and sacrifice reagent, which is >2.2 times higher than that produced by g-C3 N4 alone (10.3 µmol g-1 h-1 ). The enhanced photocatalytic activity of the Z-scheme hybrid material can be ascribed to its unique characteristics to accelerate the reduction process, including: (i) 3D hierarchical structure of urchin-like hematite and preferable basic sites which promotes the CO2 adsorption, and (ii) the unique Z-scheme feature efficiently promotes the separation of the electron-hole pairs and enhances the reducibility of electrons in the conduction band of the g-C3 N4 . The origin of such an obvious advantage of the hierarchical Z-scheme is not only explained based on the experimental data but also investigated by modeling CO2 adsorption and CO adsorption on the three different atomic-scale surfaces via density functional theory calculation. The study creates new opportunities for hierarchical hematite and other metal-oxide-based Z-scheme system for solar fuel generation.

A Hierarchical Z‑Scheme α-Fe2O3/g-C3N4 Hybrid for Enhanced Photocatalytic CO2 Reduction

Defect engineering in photocatalytic materials

Solar-driven reduction of dinitrogen (N2 ) to ammonia (NH3 ) is severely hampered by the kinetically complex and energetically challenging multielectron reaction. Oxygen vacancies (OVs) with abundant localized electrons on the surface of bismuth oxybromide-based semiconductors are demonstrated to have the ability to capture and activate N2 , providing an alternative pathway to overcome such limitations. However, bismuth oxybromide materials are susceptible to photocorrosion, and the surface OVs are easily oxidized and therefore lose their activities. For realistic photocatalytic N2 fixation, fabricating and enhancing the stability of sustainable OVs on semiconductors is indispensable. This study shows the first synthesis of self-assembled 5 nm diameter Bi5 O7 Br nanotubes with strong nanotube structure, suitable absorption edge, and many exposed surface sites, which are favorable for furnishing sufficient visible light-induced OVs to realize excellent and stable photoreduction of atmospheric N2 into NH3 in pure water. The NH3 generation rate is as high as 1.38 mmol h-1 g-1 , accompanied by an apparent quantum efficiency over 2.3% at 420 nm. The results presented herein provide new insights into rational design and engineering for the creation of highly active catalysts with light-switchable OVs toward efficient, stable, and sustainable visible light N2 fixation in mild conditions.

Light-Switchable Oxygen Vacancies in Ultrafine Bi 5 O 7 Br Nanotubes for Boosting Solar-Driven Nitrogen Fixation in Pure Water

Selective organic transformation under mild conditions constitutes a challenge in green chemistry, especially for alcohol oxidation, which typically requires environmentally unfriendly oxidants. Here, we report a new plasmonic catalyst of Au supported on BiOCl containing oxygen vacancies. It photocatalyzes selective benzyl alcohol oxidation with O2 under visible light through synergistic action of plasmonic hot electrons and holes. Oxygen vacancies on BiOCl facilitate the trapping and transfer of plasmonic hot electrons to adsorbed O2, producing •O2– radicals, while plasmonic hot holes remaining on the Au surface mildly oxidize benzyl alcohol to corresponding carbon-centered radicals. The hypothesized concerted ring addition between these two radical species on the BiOCl surface highly favors the production of benzaldehyde along with an unexpected oxygen atom transfer from O2 to the product. The results and understanding acquired in this study, based on the full utilization of hot charge carriers in a pla...

New Reaction Pathway Induced by Plasmon for Selective Benzyl Alcohol Oxidation on BiOCl Possessing Oxygen Vacancies.

Engineering light–matter interactions up to the strong-coupling regime at room temperature is one of the cornerstones of modern nanophotonics. Achieving this goal could enable new platforms for potential applications such as quantum information processing, quantum light sources, and even quantum metrology. Layered materials like transition metal dichalcogenides (TMDCs) and, in particular, tungsten disulfide (WS2), possess strong dipole moments which are comparable to semiconductor-based quantum dots, but the former also exhibit large exciton binding energies, thereby making TMDCs suitable candidates for exploring light–matter interactions at ambient conditions. Furthermore, the combination of TMDCs with plasmonic nanocavities, which tightly confine light down to nanometer scale, has recently emerged as a suitable platform for achieving strong coupling between plasmons and excitons at room temperature. Here, we use ultrathin single-crystalline gold nanodisks featuring large in-plane electric dipole moments...

https://backend.orbit.dtu.dk/ws/files/196720847/Single_crystalline_gold_nanodisks_on_WS_2.pdf

Single-Crystalline Gold Nanodisks on WS2 Mono- and Multilayers for Strong Coupling at Room Temperature

Engineering light-matter interactions up to the strong-coupling regime at room temperature is one of the cornerstones of modern nanophotonics. Achieving this goal will enable new platforms for potential applications such as quantum information processing, quantum light sources and even quantum metrology. Materials like transition metal dichalcogenides (TMDC) and in particular tungsten disulfide (WS$_2$) possess large transition dipole moments comparable to semiconductor-based quantum dots, and strong exciton binding energies allowing the detailed exploration of light-matter interactions at room temperature. Additionally, recent works have shown that coupling TMDCs to plasmonic nanocavities with light tightly focused on the nanometer scale can reach the strong-coupling regime at ambient conditions. Here, we use ultra-thin single-crystalline gold nanodisks featuring large in-plane electromagnetic dipole moments aligned with the exciton transition-dipole moments located in monolayer WS$_2$. Through scattering and reflection spectroscopy we demonstrate strong coupling at room temperature with a Rabi splitting of $\sim$108 meV. In order to go further into the strong-coupling regime and inspired by recent experimental work by St\"uhrenberg et al., we couple these nanodisks to multilayer WS$_2$. Due to an increase in the number of excitons coupled to our nanodisks, we achieve a Rabi splitting of $\sim$175 meV, a major increase of 62%. To our knowledge, this is the highest Rabi splitting reported for TMDCs coupled to open plasmonic cavities. Our results suggest that ultra-thin single-crystalline gold nanodisks coupled to WS$_2$ represent an exquisite platform to explore light-matter interactions.

Single-crystalline gold nanodisks on WS$_2$ mono- and multilayers: Strong coupling at room temperature

Circular Gold Nanodisks with Synthetically Tunable Diameters and Thicknesses

A number of plasmonic devices and applications, such as chemical and biological sensors, plasmon-enhanced solar cells, optical nanoantennas, metamaterials and metasurfaces, require the deposition of plasmonic metal nanocrystals on various substrates. Because the localized plasmon resonance modes, energies and strengths are strongly dependent on the dielectric function of the surrounding environment, the substrate is expected to largely affect the plasmonic properties of supported metal nanocrystals. Therefore, understanding the effects of the substrate on the plasmonic properties of metal nanocrystals and the roles of the involved factors will be crucial for designing various plasmonic devices that are made of metal nanocrystals deposited on different substrates. Herein we report on our study and results of the effects of substrates with distinct dielectric functions on the plasmonic properties of three types of Au nanocrystals. A combination of experiments and numerical simulations shows that the presence of a substrate causes plasmonic shifts as well as the appearance of new plasmon modes. The plasmonic shifts and the emergence of new plasmon modes are found to be dependent on the particle shape of Au nanocrystals and in turn on the fractional particle surface area that is in contact with the supporting substrate. For Au nanospheres and nanorods, plasmonic shifts, less than 100 nm, are observed on the scattering spectra by changing the supporting substrate from indium tin oxide to silicon. In comparison, a giant spectral shift of more than 300 nm is obtained for Au nanoplates. Moreover, silicon substrates induce the emergence of an out-of-plane quadrupolar mode of Au nanoplates, which interacts with an out-of-plane octupolar mode to give rise to a distinct Fano resonance. The Fano resonance is found to become stronger as the thickness of Au nanoplates is decreased. These results are of great importance for understanding the plasmonic properties of noble metal nanocrystals supported on various substrates, and for designing novel plasmonic nanostructures with desired optical properties and functions.

Ximin Cui

Papers

New Reaction Pathway Induced by Plasmon for Selective Benzyl Alcohol Oxidation on BiOCl Possessing Oxygen Vacancies.

Single-Crystalline Gold Nanodisks on WS2 Mono- and Multilayers for Strong Coupling at Room Temperature

Single-crystalline gold nanodisks on WS$_2$ mono- and multilayers: Strong coupling at room temperature

Circular Gold Nanodisks with Synthetically Tunable Diameters and Thicknesses

Role of shape in substrate-induced plasmonic shift and mode uncovering on gold nanocrystals.