Construction of Gold/Rhodium Freestanding Superstructures as Antenna-Reactor Photocatalysts for Plasmon-Driven Nitrogen Fixation.

Precisely controlling the architecture and spatial arrangement of plasmonic heterostructures offers unique opportunities to tailor the catalytic property, whereas the lack of a wet-chemistry synthetic approach to fabricating nanostructures with high-index facets limits their practical applications. Herein, we describe a universal synthetic strategy to construct Au/Rh freestanding superstructures (SSs) through the selective growth of ordered Rh nanoarrays on high-index-faceted Au nanobipyramids (NBPs). This synthetic strategy works on various metal nanocrystal substrates and can yield diverse Au/Rh and Pd/Rh SSs. Especially, the obtained Au NBP/Rh SSs exhibit high photocatalytic activity toward N2 fixation as a result of the spatially separated architecture, local electric field enhancement, and the antenna-reactor mechanism. Both theoretical and experimental results reveal that the Au NBPs can function as nanoantennas for light-harvesting to generate hot charge carriers for driving N2 fixation, while the Rh nanoarrays can serve as the active sites for N2 adsorption and activation to synergistically promote the overall catalytic activity in the Au NBP/Rh SSs. This work offers new avenues to rationally designing and constructing spatially separated plasmonic photocatalysts for high-efficiency catalytic applications.

A Pd-based plasmonic photocatalyst for nitrogen fixation through an antenna-reactor mechanism.

Plasmonic metal nanocrystals (e.g., Au, Ag, and Cu) hold great promise for driving photocatalytic reactions, but little is known about the plasmonic properties of Pd nanocrystals. Herein, we constructed a plasmonic Pd/Ru antenna-reactor photocatalyst through the controllable growth of a Ru nanoarray 'reactor' on a Pd nano-octahedron 'antenna' and demonstrated a plasmonic Pd-driven N2 photofixation process. The plasmonic properties of Pd nano-octahedrons were verified using finite-difference time-domain (FDTD) simulations and refractive index sensitivity tests in water-glycerol mixtures. Notably, the constructed plasmonic antenna-reactor nanostructures exhibited superior photocatalytic activities during N2 photofixation, with a maximum ammonia production rate of 117.5 ± 15.0 µmol g-1 h-1 under visible and near-infrared (NIR) light illumination. The mechanism can be attributed to the ability of the plasmonic Pd nanoantennas to harvest light to generate abundant hot electrons and the Ru nanoreactors to provide active sites for adsorption and activation of N2. This work paves the way for the development of Pd-based plasmonic photocatalysts for efficient N2 photofixation and sheds new light on the optimal design and construction of antenna-reactor nanostructures.

Steric hindrance-induced selective growth of rhodium on gold nanobipyramids for plasmon-enhanced nitrogen fixation.

The construction of an antenna-reactor plasmonic photocatalyst that is composed of a plasmonic and a catalytically active metal holds great promise in driving N2 photofixation, but its photocatalytic performance is highly dependent on the spatial distribution of the two components. Up to now, the fabrication of dumbbell-shaped nanostructures featuring spatially separated architecture has remained challenging. Herein, we develop a facile synthetic strategy for the site-selective growth of a Rh nanocrystal 'reactor' on two tips of an Au nanobipyramid (NBP) 'antenna' through the precise manipulation of steric hindrance toward Rh overgrowth. The obtained Au NBP/tip-Rh nanodumbbells (Au NBP/tip-Rh NDs) can function as an excellent antenna-reactor plasmonic photocatalyst for N2 photofixation. In this scenario, the Au nanoantenna harvests light and generates hot electrons under plasmon resonance, meanwhile the hot electrons are transferred to the active sites on Rh nanocrystals for N2 reduction. In comparison with that of classical core@shell nanostructures, the spatially separated architecture of the Au NBP/tip-Rh NDs facilitates charge separation, greatly improving the photocatalytic activity. This study sheds new light on the structure-function relationship for N2 photofixation and benefits the design and construction of spatially separated plasmonic photocatalysts.

Enhanced Photothermal Steam Generation and Gold Using the Efficiency of Ultralight Gold Foam with Hierarchical Porosity.

Controlling the optical properties of metal plasma nanomaterials through structure manipulation has attracted great attention for solar steam generation. However, realizing broadband solar absorption for high-efficiency vapor generation is still challenging. In this work, a free-standing ultralight gold film/foam with a hierarchical porous microstructure and high porosity is obtained through controllably etching a designed cold-rolled (NiCoFeCr)99Au1 high-entropy precursor alloy with a unique grain texture. During chemical dealloying, the high-entropy precursor went through anisotropic contraction, resulting in a larger surface area compared with that from the Cu99Au1 precursor although the volume shrinkage is similar (over 85%), which is beneficial for the photothermal conversion. The low Au content also results in a special hierarchical lamellar microstructure with both micropores and nanopores within each lamella, which significantly broadens the optical absorption range and makes the optical absorption of the porous film reach 71.1-94.6% between 250 and 2500 nm. In addition, the free-standing nanoporous gold film has excellent hydrophilicity, with the contact angle reaching zero within 2.2 s. Thus, the 28 h dealloyed nanoporous gold film (NPG-28) exhibits a rapid evaporation rate of seawater under 1 kW m-2 light intensity, reaching 1.53 kg m-2 h-1, and the photothermal conversion efficiency reaches 96.28%. This work demonstrates the enhanced noble metal gold using efficiency and solar thermal conversion efficiency by controlled anisotropic shrinkage and forming a hierarchical porous foam.

Symmetry-breaking synthesis of Janus Au/CeO2 nanostructures for visible-light nitrogen photofixation.

Precise manipulation of the reactive site spatial distribution in plasmonic metal/semiconductor photocatalysts is crucial to their photocatalytic performance, but the construction of Janus nanostructures through symmetry-breaking synthesis remains a significant challenge. Here we demonstrate a synthetic strategy for the selective growth of a CeO2 semi-shell on Au nanospheres (NSs) to fabricate Janus Au NS/CeO2 nanostructures with the assistance of a SiO2 hard template and autoredox reaction between Ag+ ions and a ceria precursor. The obtained Janus nanostructures possess a spatially separated architecture and exhibit excellent photocatalytic performance toward N2 photofixation under visible-light illumination. In this scenario, N2 molecules are reduced by hot electrons on the CeO2 semi-shell, while hole scavengers are consumed by hot holes on the exposed Au NS surface, greatly promoting the charge carrier separation. Moreover, the exposed Au NS surface in the Janus structures offers an additional opportunity for the fabrication of ternary Janus noble metal/Au NS/CeO2 nanostructures. This work highlights the genuine superiority of the spatially separated nanoarchitectures in the photocatalytic reaction, offering instructive guidance for the design and construction of novel plasmonic photocatalysts.

Construction of Spatially Separated Gold Nanocrystal/Cuprous Oxide Architecture for Plasmon-Driven CO2 Reduction.

Plasmonic hot electrons have shown great potential in photocatalysis, but little is known about the hot hole-driven chemical reactions due to the lack of desired plasmonic metal/p-type semiconductor architectures. Herein, we describe a general and robust strategy for the site-selective growth of a p-type semiconductor, Cu2O on Au nanocrystals (NCs), to produce diverse spatially separated Au/Cu2O heterostructures. The preferential growth of Cu2O on the tips/ends/edges of Au NCs is directed by the sparse coverage of the surfactant molecules at the high-curvature sites of Au NCs. The obtained dumbbell-shaped nanostructures serve as the ideal platforms for probing the hot-hole-mediated CO2 reduction reaction. Benefiting from the hot-hole injection, a new reaction pathway is unlocked, and the C2 product activity and selectivity are significantly improved. This study demonstrates the genuine superiority of the dumbbell-shaped nanostructures in photocatalysis, offering a new unique avenue to explore the underlying mechanism of hot-hole-mediated chemical reactions.

(Plasmonic gold core)@(ultrathin ruthenium shell) nanostructures as antenna-reactor photocatalysts toward nitrogen photofixation.

Ruthenium (Ru) is known as the optimal metal catalyst for ammonia (NH3) synthesis, but the poor light-harvesting capability restricts its application in photocatalysis. Herein, we construct an antenna-reactor nanostructure through the controllable growth of an ultrathin Ru nanocluster shell with desired catalytic activity on the plasmonic gold (Au) nanoantennas. In this nanostructure, Au nanoantennas interact strongly with light to generate hot carriers, meanwhile Ru nanoclusters adsorb and activate N2, leading to the reduction of N2 to NH3 by the generated hot electrons. This antenna-reactor plasmonic photocatalyst exhibits shell-thickness-dependent photocatalytic activity toward nitrogen (N2) photofixation under visible and near-infrared light illumination.

Janus silver/ternary silver halide nanostructures as plasmonic photocatalysts boost the conversion of CO2 to acetaldehyde.

Photocatalytic conversion of carbon dioxide (CO2) to liquid product acetaldehyde (CH3CHO) remains a great challenge due to the involvement of a complex 10-electron reduction process and a sluggish C-C coupling reaction. Herein, we report the synthesis of Janus silver/ternary silver halide (Ag/AgClBr) nanostructures through precisely manipulating the growth kinetics and its function as a plasmonic photocatalyst to boost the conversion of CO2 to CH3CHO. The obtained Janus nanostructures featuring both spatially separated architecture and broad light-harvesting capability facilitate the photocatalytic reduction of CO2 under solar illumination. The photocatalytic CO2 reduction with the characteristics of high activity and good selectivity can generate a 10-electron reduction product CH3CHO with a generation rate of 209.3 ± 9.5 µmol h-1 g-1 and a selectivity of 96.9%, which are rarely achieved in previously reported photocatalytic CO2 reduction systems. The excellent photocatalytic performance can be ascribed to the plasmonic effect of Ag nanocrystals and the favorable active sites on the catalyst surface. This research demonstrates for the first time the utilization of the Janus Ag/AgClBr nanostructures to generate the value-added C2 liquid product through photocatalytic CO2 reduction, paving the way for the design and construction of novel plasmonic photocatalysts.

Nanoporous Al-Ni-Co-Ir-Mo High-Entropy Alloy for Record-High Water Splitting Activity in Acidic Environments.

Ir-based binary and ternary alloys are effective catalysts for the electrochemical oxygen evolution reaction (OER) in acidic solutions. Nevertheless, decreasing the Ir content to less than 50 at% while maintaining or even enhancing the overall electrocatalytic activity and durability remains a grand challenge. Herein, by dealloying predesigned Al-based precursor alloys, it is possible to controllably incorporate Ir with another four metal elements into one single nanostructured phase with merely ≈20 at% Ir. The obtained nanoporous quinary alloys, i.e., nanoporous high-entropy alloys (np-HEAs) provide infinite possibilities for tuning alloy's electronic properties and maximizing catalytic activities owing to the endless element combinations. Particularly, a record-high OER activity is found for a quinary AlNiCoIrMo np-HEA. Forming HEAs also greatly enhances the structural and catalytic durability regardless of the alloy compositions. With the advantages of low Ir loading and high activity, these np-HEA catalysts are very promising and suitable for activity tailoring/maximization.

Site-Selective Growth of Crystalline Ceria with Oxygen Vacancies on Gold Nanocrystals for Near-Infrared Nitrogen Photofixation.

Site-selective growth of crystalline semiconductors on gold nanocrystals remains a great challenge because of the difficult control of both nucleation and growth dynamics as well as the easy agglomeration and deformation of gold nanocrystals at high temperatures of 400-1000 °C. Here we report a facile wet-chemistry route for the selective growth of crystalline ceria at the ends of gold nanorods (Au NRs) in the presence of a small amount of bifunctional K2PtCl4. Due to the smaller steric hindrance at the ends than at the side surface, K2PtCl4 may preferentially adsorb at the ends of Au NRs, triggering the autoredox reaction with the ceria precursor to obtain crystalline CeO2 at the ends. Notably, the surface of grown ceria is rich in oxygen vacancies (OVs) that facilitate the adsorption and activation of N2 molecules. The unique structure, the plasmon-induced hot carriers and the OVs make the obtained Au/end-CeO2 an excellent catalyst for nitrogen photofixation under near-infrared (NIR) illumination.

Understanding the roles of plasmonic Au nanocrystal size, shape, aspect ratio and loading amount in Au/g-C3N4 hybrid nanostructures for photocatalytic hydrogen generation.

Hybrid photocatalysts containing plasmonic metal and semiconductor building blocks can alleviate charge carrier recombination and broaden the range of light absorption of the semiconductor. In this work, plasmonic Au nanocrystals of different sizes and shapes (spheres and rods) are attached on graphitic carbon nitride (g-C3N4) nanosheets through electrostatic attraction. The effects of the morphology and loading amount of the Au nanocrystals are carefully studied for understanding and optimizing the hybrid photocatalysts. The optimized 18 nm-sized Au nanospheres/g-C3N4 photocatalyst exhibits a superior activity for H2 evolution at a rate of 540 µmol g-1 h-1 under visible light (λ > 420 nm), exceeding those produced over larger-sized Au nanospheres/g-C3N4, Au nanorods/g-C3N4 and photodeposited Au nanoparticles/g-C3N4 photocatalysts. The excellent activity for H2 evolution is attributed to the electron sink and plasmonic effects of the Au nanocrystals in different spectral regions, as evidenced by photocurrent measurements. The introduced plasmonic Au nanocrystals not only enhance the photocatalytic activity, but they also endow the hybrid photocatalysts with an extended light absorption range. Our results and understanding will be useful for the design of efficient plasmonic photocatalysts for solar to fuel energy conversion as well as for other plasmon-driven chemical reactions.

Aerosol-Sprayed Gold/Ceria Photocatalyst with Superior Plasmonic Hot Electron-Enabled Visible-Light Activity.

Integration of nanoscale plasmonic metals with semiconductors is a promising strategy for utilizing visible and near-infrared light to enhance chemical reactions. Here we report on the preparation of Au/CeO2 microsphere photocatalysts through aerosol spray and the study of their photocatalytic activity toward the aerobic oxidation of 1-phenylethanol under visible light. The microsphere catalysts exhibit a remarkable photocatalytic performance with their turnover frequency values reaching 108 h-1, which is more than 23 times that of (Au core)@(CeO2 shell) nanostructures and much larger than those obtained previously for the visible-light photocatalytic oxidation of 1-phenylethanol. In addition, the Au/CeO2 catalyst shows the best performance among eight types of oxide semiconductor supports. Moreover, the photocatalytic mechanism of the Au/CeO2 catalyst is systematically investigated. This study offers insights for plasmonic hot electron-enabled photocatalysis, which will be valuable for the design of various efficient (plasmonic metal)/semiconductor photocatalysts.

Porous Pt Nanoparticles with High Near-Infrared Photothermal Conversion Efficiencies for Photothermal Therapy.

Plasmonic nanostructures are of potential in acting as a type of optical agents for cancer photothermal therapy. To effectively function as photothermal therapy agents, plasmonic nanostructures are strongly desired to have good biocompatibility and high photothermal conversion efficiencies. In this study, poly(diallyldimethylammonium chloride)-coated porous Pt nanoparticles are synthesized for photothermal therapy. The Pt nanoparticles possess broadband near-infrared light absorption in the range from 650 to 1200 nm, therefore allowing for selecting different laser wavelengths for photothermal therapy. The as-prepared Pt nanoparticles exhibit remarkable photothermal conversion efficiencies under 809 and 980 nm laser irradiation. In vitro studies indicate that the Pt nanoparticles display good biocompatibility and high cellular uptake efficiencies through an endocytosis pathway. Photothermal heating using 808 nm laser irradiation (>7.0 W cm-2 , 3 min) leads to notable cytotoxic effect, and more than 70% of cells are photothermally ablated after 3 min irradiation at 8.4 W cm-2 . Furthermore, simultaneous application of photothermal therapy synergistically enhances the cytotoxicity of an anti-cancer drug doxorubicin. Therefore, the porous Pt nanoparticles have great potential as an attractive photothermal agent for cancer therapy.

Highly enhanced transverse plasmon resonance and tunable double Fano resonances in gold@titania nanorods.

Gold nanorods have attracted intensive interest owing to their localized surface plasmon resonance properties and enormous potential applications. The transverse plasmon of Au nanorods is usually weaker than the longitudinal one, hampering certain plasmonic applications. Herein we report on the intensification of the transverse plasmon resonance by coating TiO2 onto Au nanorods. The transverse plasmon mode of the resultant Au@TiO2 nanorods with a sufficiently thick shell can be comparable to or even stronger than the longitudinal one in intensity. Moreover, both the transverse and longitudinal plasmon resonances of the Au@TiO2 nanorods exhibit an asymmetric line shape on their scattering spectra. Electrodynamic simulations and analyses based on a coupled oscillator model suggest that the asymmetric line shape originates from the coupling between the Au core and TiO2 shell. Apart from the shell thickness, the plasmonic properties of the Au@TiO2 nanorods can also be tuned by the dimension of the Au nanorod core. In addition, the polarization-dependent light scattering from the individual Au@TiO2 nanorods has also been investigated. These results will be of high importance for understanding the interactions between noble metals and semiconductors in plasmonic hybrid nanosystems, and for designing novel plasmonic nanostructures with desired optical properties and functions.

Synthesis of Absorption-Dominant Small Gold Nanorods and Their Plasmonic Properties.

Absorption-dominant small Au nanorods with diameters of less than 10 nm are prepared using a facile seed-mediated growth method. The diameters of the small gold nanorods range from 6 to 9 nm, and their lengths vary from 16 to 45 nm. Their aspect ratios can be tailored from 2.7 to 4.7. As a result, the longitudinal plasmon resonance wavelengths are readily tunable from ∼720 nm to ∼830 nm by changing the seed-to-Au(III) molar ratio in the growth solution. The fractions of the scattering in the total extinction of the small Au nanorods are found to be in the range of 0.005 to 0.025 with finite-difference time-domain simulations, confirming that the extinction values of these small Au nanorods are dominantly contributed to by the light absorption. Moreover, the small Au nanorod sample is coated with a dense silica layer for photothermal therapy with three cell lines. It shows improved photothermal therapy performance compared to a large Au nanorod sample for the same cellular Au contents. Our study suggests that small Au nanorods are promising light absorbers and photothermal therapy agents.

Cellular uptake behaviour, photothermal therapy performance, and cytotoxicity of gold nanorods with various coatings.

With the development of Au nanorods for a number of biomedical applications, understanding their cellular responses has become increasingly important. In this study, we systematically evaluated the cellular uptake behaviour and cytotoxicity of Au nanorods with various surface coatings, including organic cetyltrimethylammonium bromide (CTAB), poly(sodium 4-styrenesulfonate) (PSS), and poly(ethylene glycol) (PEG), and inorganic mesoporous silica (mSiO2), dense silica (dSiO2), and titanium dioxide (TiO2). The cellular behaviour of Au nanorods was found to be highly dependent on both the surface coating and the cell type. CTAB-, PSS-, and mSiO2-coated Au nanorods exhibit notable cytotoxicity, while PEG-, dSiO2-, and TiO2-coated Au nanorods do not induce cell injury. Optical imaging studies indicated that the cell type plays a preferential role in Au nanorod cellular uptake. Higher cellular uptake of Au nanorods was seen in U-87 MG, PC-3, MDA-MB-231, and RAW 264.7 cells, as opposed to HepG2 and HT-29 cells. In addition, Au nanorod cellular uptake is also highly affected by serum protein binding to the surface coating. mSiO2-, dSiO2-, and TiO2-coated Au nanorods show significantly higher cellular uptake than PSS- and PEG-coated ones, which results in a better photothermal ablation effect for Au nanorods with the inorganic surface coatings. Our study provides valuable insights into the effects of the surface modification on the biocompatibility, cellular uptake, as well as biomedical functions of Au nanorods.