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

Mass-Based Photothermal Comparison Among Gold Nanocrystals, PbS Nanocrystals, Organic Dyes, and Carbon Black

Gold nanocrystals have attractive plasmon-enabled photothermal conversion properties, which have been widely employed for photothermal therapy and solar energy harvesting. For practical applications, the mass-normalized photothermal conversion performance is often desired to be known for Au nanocrystals with different shapes and sizes and for different nanomaterials. We study the photothermal conversion performances of differently shaped and sized Au nanocrystals and compare them with those of PbS nanocrystals, carbon black, and organic dyes at the same mass concentrations. Both the mass-normalized extinction cross section and the photothermal conversion efficiency of Au nanocrystals decrease as their size is increased. The photothermal conversion performance of carbon black is comparable to that of relatively small Au nanocrystals, while the photothermal conversion performance of organic dyes and PbS nanocrystals is inferior to that of Au nanocrystals. Our results are useful for the design of Au nanocrystals and the choice of nanomaterials for photothermal applications

Correlating the Plasmonic and Structural Evolutions during the Sulfidation of Silver Nanocubes

Ag/Ag₂S hybrid nanostructures have recently received much attention, because of their synthetically tunable plasmonic properties and enhanced chemical stability. Sulfidation of pregrown Ag nanocrystals is a facile process for making Ag/Ag₂S nanostructures. Understanding the sulfidation process can help in finely controlling the compositional and structural parameters and in turn tailoring the plasmonic properties. Herein we report on our study of the structural and plasmonic evolutions during the sulfidation process of Ag nanocubes, which is carried out at both the ensemble and single-particle levels. Ensemble extinction measurements show that sulfidation first causes the disappearance of the high-order triakontadipolar plasmon modes, which have electric charges located on the sharp vertices and edges of Ag nanocubes, suggesting that sulfidation starts at the vertices of Ag nanocubes. As sulfidation goes on, the dipolar plasmon peak gradually red-shifts, with its intensity first decreasing and then increasing. Electron microscopy characterizations reveal that sulfidation progresses from the outer region to the center of Ag nanocubes. The cubic shape is maintained throughout the sulfidation process, with the edge length being increased gradually. Single-particle scattering measurements show that the dipolar plasmon peak red-shifts and decreases in intensity during sulfidation. An additional scattering peak appears at a shorter wavelength at the late stage of sulfidation. The difference in the sulfidation behavior between ensemble and single-particle measurements is understood with electrodynamic simulations. During ensemble measurements, the Ag core is increasingly truncated, and it becomes a nanosphere eventually. Sulfidation stops at an intermediate stage. During single-particle measurements, Ag nanocubes are completely transformed into Ag₂S, leading to the observation of the shorter-wavelength scattering peak

High-Efficiency “Working-in-Tandem” Nitrogen Photofixation Achieved by Assembling Plasmonic Gold Nanocrystals on Ultrathin Titania Nanosheets

The fixation of atmospheric N₂ to NH₃ is an essential process for sustaining life. One grand challenge is to develop efficient catalysts to photofix N₂ under ambient conditions. Herein we report an all-inorganic catalyst, Au nanocrystals anchored on ultrathin TiO₂ nanosheets with oxygen vacancies. It can accomplish photodriven N₂ fixation in the “working-in-tandem” pathway at room temperature and atmospheric pressure. The oxygen vacancies on the TiO₂ nanosheets chemisorb and activate N₂ molecules, which are subsequently reduced to NH₃ by hot electrons generated from plasmon excitation of the Au nanocrystals. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to NH₃ is higher than those reported so far. Optimizing the absorption across the overall visible range with the mixture of Au nanospheres and nanorods further enhances the N₂ photofixation rate by 66.2% in comparison with Au nanospheres used alone. This work offers a new approach for the rational design of efficient catalysts toward sustainable N₂ fixation through a less energy-demanding photochemical process compared to the industrial Haber–Bosch process

Unraveling the Mechanism of the Zn-Improved Catalytic Activity of Pd-Based Catalysts for Water–Gas Shift Reaction

The water–gas shift (WGS) reaction plays a key role in hydrogen economy. Owing to the exothermic nature of the reaction, low-temperature WGS catalysts are highly desired. Zn-modified Pd-based catalysts are promising candidates for low-temperature WGS. Herein, the effect of Zn addition on the WGS catalysis is systematically studied by using the Pd(111) and PdZn(111) surface as models. Owing to the addition of Zn, the electron-accepting ability of the catalyst is weakened, while the electron-donating ability is increased. As a result, the adsorptions of electron-donor adsorbates, including H₂O, CO, H, <i>cis</i>-COOH, <i>trans</i>-COOH, and H₂, are weakened, while the adsorptions of electron-acceptor adsorbates, including O and OH, are strengthened. The same most favorable reaction path is found on Pd(111) and PdZn(111), which is the associative mechanism with the carboxyl dehydrogenation assisted by adsorbed OH. Although the most favorable path is the same, the weakening of CO adsorption makes the rate-determining step change from the association of CO and OH forming <i>cis</i>-COOH on Pd(111) to the dissociation of H₂O on PdZn(111). The rate-determining step on PdZn(111) has an energy barrier lower than the rate-determining step on Pd(111). The promotion mechanism of the PdZn alloy for WGS is therefore attributed to the fact that the addition of Zn weakens the adsorption of CO and thereby alters the rate-determining step

Realization of Red Plasmon Shifts up to ∼900 nm by AgPd-Tipping Elongated Au Nanocrystals

The synthesis of metal nanostructures with plasmon wavelengths beyond ∼1000 nm is strongly desired, especially for those with small sizes. Herein we report on a AgPd-tipping process on Au nanobipyramids with the resultant red plasmon shifts reaching up to ∼900 nm. The large red plasmon shifts are ascribed to the deposition of the metal at the tips of Au nanobipyramids, which is verified by electrodynamic simulations. The method has been successfully applied to Au nanobipyramids and nanorods with different longitudinal dipolar plasmon wavelengths, demonstrating that the plasmon wavelengths of these Au nanocrystals can be extended to the entire near-infrared region. Pt can also induce the tipping on Au nanobipyramids and nanorods to realize red plasmon shifts, suggesting the generality of our approach. We have further shown that the metal-tipped Au nanobipyramids possess a high photothermal conversion efficiency and good photothermal therapy performance. This study opens up a route to the construction of Au nanostructures with plasmon resonance in a broad spectral region for plasmon-enabled technological applications

A Chemical Approach To Break the Planar Configuration of Ag Nanocubes into Tunable Two-Dimensional Metasurfaces

Current plasmonic metasurfaces of nanocubes are limited to planar configurations, restricting the ability to create tailored local electromagnetic fields. Here, we report a new chemical strategy to achieve tunable metasurfaces with nonplanar nanocube orientations, creating novel lattice-dependent field localization patterns. We manipulate the interfacial behaviors of Ag nanocubes by controlling the ratio of hydrophilic/hydrophobic molecules added in a binary thiol mixture during the surface functionalization step. The nanocube orientation at an oil/water interface can consequently be continuously tuned from planar to tilted and standing configurations, leading to the organization of Ag nanocubes into three unique large-area metacrystals, including square close-packed, linear, and hexagonal lattices. In particular, the linear and hexagonal metacrystals are unusual open lattices comprising nonplanar nanocubes, creating unique local electromagnetic field distribution patterns. Large-area “hot hexagons” with significant delocalization of hot spots form in the hexagonal metacrystal. With a lowest packing density of 24%, the hexagonal metacrystal generates nearly 350-fold stronger surface-enhanced Raman scattering as compared to the other denser-packing metacrystals, demonstrating the importance of achieving control over the geometrical and spatial orientation of the nanocubes in the metacrystals

Design of Palladium-Doped <i>g</i>‑C₃N₄ for Enhanced Photocatalytic Activity toward Hydrogen Evolution Reaction

Graphitic carbon nitride (<i>g</i>-C₃N₄) has been believed to be a promising photocatalyst for water splitting due to its right band gap and band edges. However, the kinetics of hydrogen evolution on <i>g</i>-C₃N₄ is very slow. Cocatalysts are usually needed to improve the catalytic kinetics. Herein, palladium-doped graphitic carbon nitride (<i>g</i>-C₃N₄–Pd) is designed by virtue of the tenacious coordination of Pd atoms with the pyridinic nitrogen atoms of six-fold cavities in <i>g</i>-C₃N₄. The introduction of Pd does not affect the structure and morphology of <i>g</i>-C₃N₄. Palladium is found to exist as Pd ions in <i>g</i>-C₃N₄–Pd catalysts. <i>g</i>-C₃N₄–Pd catalysts exhibit clearly higher hydrogen evolution activities than <i>g</i>-C₃N₄. The highest hydrogen evolution activity on <i>g</i>-C₃N₄–Pd is 15.3 times that of <i>g</i>-C₃N₄. The improvement of hydrogen evolution activity is found to arise from both the alternation of the electron excitation manner and the acceleration of hydrogen evolution kinetics induced by Pd doping. Our findings provide a promising way to improve the photocatalytic performance for hydrogen evolution and pave a new avenue for the development of highly efficient and cost-effective photocatalysts for water splitting

Plasmonic Harvesting of Light Energy for Suzuki Coupling Reactions

The efficient use of solar energy has received wide interest due to increasing energy and environmental concerns. A potential means in chemistry is sunlight-driven catalytic reactions. We report here on the direct harvesting of visible-to-near-infrared light for chemical reactions by use of plasmonic Au–Pd nanostructures. The intimate integration of plasmonic Au nanorods with catalytic Pd nanoparticles through seeded growth enabled efficient light harvesting for catalytic reactions on the nanostructures. Upon plasmon excitation, catalytic reactions were induced and accelerated through both plasmonic photocatalysis and photothermal conversion. Under the illumination of an 809 nm laser at 1.68 W, the yield of the Suzuki coupling reaction was ∼2 times that obtained when the reaction was thermally heated to the same temperature. Moreover, the yield was also ∼2 times that obtained from Au–TiO_<i>x</i>–Pd nanostructures under the same laser illumination, where a 25-nm-thick TiO_<i>x</i> shell was introduced to prevent the photocatalysis process. This is a more direct comparison between the effect of joint plasmonic photocatalysis and photothermal conversion with that of sole photothermal conversion. The contribution of plasmonic photocatalysis became larger when the laser illumination was at the plasmon resonance wavelength. It increased when the power of the incident laser at the plasmon resonance was raised. Differently sized Au–Pd nanostructures were further designed and mixed together to make the mixture light-responsive over the visible to near-infrared region. In the presence of the mixture, the reactions were completed within 2 h under sunlight, while almost no reactions occurred in the dark

Highly Compressible Carbon Sponge Supercapacitor Electrode with Enhanced Performance by Growing Nickel–Cobalt Sulfide Nanosheets

The development of compressible supercapacitor highly relies on the innovative design of electrode materials with both superior compression property and high capacitive performance. This work reports a highly compressible supercapacitor electrode which is prepared by growing electroactive NiCo₂S₄ (NCS) nanosheets on the compressible carbon sponge (CS). The strong adhesion of the metallic conductive NCS nanosheets to the highly porous carbon scaffolds enable the CS–NCS composite electrode to exhibit an enhanced conductivity and ideal structural integrity during repeated compression–release cycles. Accordingly, the CS–NCS composite electrode delivers a specific capacitance of 1093 F g^–1 at 0.5 A g^–1 and remarkable rate performance with 91% capacitance retention in the range of 0.5–20 A g^–1. Capacitance performance under the strain of 60% shows that the incorporation of NCS nanosheets in CS scaffolds leads to over five times enhancement in gravimetric capacitance and 17 times enhancement in volumetric capacitance. These performances enable the CS–NCS composite to be one of the promising candidates for potential applications in compressible electrochemical energy storage devices