Hot carrier extraction from plasmonic-photonic superimposed heterostructures.

Plasmonic nanostructures have been exploited in photochemical and photocatalytic processes owing to their surface plasmon resonance characteristics. This unique property generates photoinduced potentials and currents capable of driving chemical reactions. However, these processes are hampered by low photon conversion and utilization efficiencies, which are issues that need to be addressed. In this study, we integrate plasmonic photochemistry and simple tunable heterostructure characteristics of a dielectric photonic crystal for the effective control of electromagnetic energy below the diffraction limit of light. The nanostructure comprises high-density Ag nanoparticles on nanocavity arrays of SrTiO3 and TiO2, where two oxides constitute a chemical heterojunction. Such a nanostructure is designed to form intense electric fields and a vectorial electron flow channel of Ag → SrTiO3 → TiO2. When the plasmonic absorption of Ag nanoparticles matched the photonic stopband, we observed an apparent quantum yield of 3.1 × 10-4 e- per absorbed photon. The contributions of light confinement and charge separation to the enhanced photocurrent were evaluated.

Isotope Effects in Plasmonic Photosynthesis.

The photoexcitation of plasmonic nanoparticles has been shown to drive multistep, multicarrier transformations, such as the conversion of CO2 into hydrocarbons. But for such plasmon-driven chemistry to be precisely understood and modeled, the critical photoinitiation step in the reaction cascade must be identified. We meet this goal by measuring H/D and 12 C/13 C kinetic isotope effects (KIEs) in plasmonic photosynthesis. In particular, we found that the substitution of H2 O with D2 O slows hydrocarbon production by a factor of 5-8. This primary H/D KIE leads to the inference that hole-driven scission of the O-H bond in H2 O is a critical, limiting step in plasmonic photosynthesis. This study advances mechanistic understanding of light-driven chemical reactions on plasmonic nanoparticles.

The Chemical Potential of Plasmonic Excitations.

By the photoexcitation of localized surface plasmon resonances of metal nanoparticles, one can generate reaction equivalents for driving redox reactions. We show that, in such cases, there is a chemical potential contributed by the plasmonic excitation. This chemical potential is a function of the concentration of light, as we determine from the light-intensity-dependent activity in the plasmon-excitation-driven reduction of CO2 on Au nanoparticles. Our finding allows the treatment of plasmonic excitation as a reagent in chemical reactions; the chemical potential of this reagent is tunable by the light intensity.

Plasmonic photosynthesis of C1-C3 hydrocarbons from carbon dioxide assisted by an ionic liquid.

Photochemical conversion of CO2 into fuels has promise as a strategy for storage of intermittent solar energy in the form of chemical bonds. However, higher-energy-value hydrocarbons are rarely produced by this strategy, because of kinetic challenges. Here we demonstrate a strategy for green-light-driven synthesis of C1-C3 hydrocarbons from CO2 and H2O. In this approach, plasmonic excitation of Au nanoparticles produces a charge-rich environment at the nanoparticle/solution interface conducive for CO2 activation, while an ionic liquid stabilizes charged intermediates formed at this interface, facilitating multi-step reduction and C-C coupling. Methane, ethylene, acetylene, propane, and propene are photosynthesized with a C2+ selectivity of ~50% under the most optimal conditions. Hydrocarbon turnover exhibits a volcano relationship as a function of the ionic liquid concentration, the kinetic analysis of which coupled with density functional theory simulations provides mechanistic insights into the synergy between plasmonic excitation and the ionic liquid.

Plasmonic Control of Multi-Electron Transfer and C-C Coupling in Visible-Light-Driven CO2 Reduction on Au Nanoparticles.

Artificial photosynthesis relies on the availability of synthetic photocatalysts that can drive CO2 reduction in the presence of water and light. From the standpoint of solar fuel production, it is desirable that these photocatalysts perform under visible light and produce energy-rich hydrocarbons from CO2 reduction. However, the multistep nature of CO2-to-hydrocarbon conversion poses a significant kinetic bottleneck when compared to CO production and H2 evolution. Here, we show that plasmonic Au nanoparticle photocatalysts can harvest visible light for multielectron, multiproton reduction of CO2 to yield C1 (methane) and C2 (ethane) hydrocarbons. The light-excitation attributes influence the C2 and C1 selectivity. The observed trends in activity and selectivity follow Poisson statistics of electron harvesting. Higher photon energies and flux favor simultaneous harvesting of more than one electron from the photocharged Au nanoparticle catalyst, inducing the C-C coupling required for C2 production. These findings elucidate the nature of plasmonic photocatalysis, which involves strong light-matter coupling, and set the stage for the controlled chemical bond formation by light excitation.

Tuning the Structural Color of a 2D Photonic Crystal Using a Bowl-like Nanostructure.

Structural colors of the ordered photonic nanostructures are widely used as an effective platform for manipulating the propagation of light. Although several approaches have been explored in attempts to mimic the structural colors, improving the reproducibility, mechanical stability, and the economic feasibility of sophisticated photonic crystals prepared by complicated processes continues to pose a challenge. In this study, we report on an alternative, simple method for fabricating a tunable photonic crystal at room temperature. A bowl-like nanostructure of TiO2 was periodically arranged on a thin Ti sheet through a two-step anodization process where its diameters were systemically controlled by changing the applied voltage. Consequently, they displayed a broad color distribution, ranging from red to indigo, and the principal reason for color generation followed the Bragg diffraction theory. This noncolorant method was capable of reproducing a Mondrian painting on a centimeter scale without the need to employ complex architectures, where the generated structural colors were highly stable under mechanical or chemical influence. Such a color printing technique represents a potentially promising platform for practical applications for anticounterfeit trademarks, wearable sensors, and displays.

Interfacial Adsorption and Redox Coupling of Li4Ti5O12 with Nanographene for High-Rate Lithium Storage.

Despite the many efforts to solve the problem associated with lithium storage at high rates, it is rarely achieved up until now. The design with experimental proof is reported here for the high rate of lithium storage via a core-shell structure composite comprised of a Li4Ti5O12 (LTO) core and a nanographene (NG) shell. The LTO-NG core-shell was synthesized via a first-principles understanding of the adsorption properties between LTO and NG. Interfacial reactions are considered between the two materials by a redox coupling effect. The large interfacial area between the LTO core and the NG shell resulted in a high electron-conducting path. It allowed rapid kinetics to be achieved for lithium storage and also resulted in a stable contact between LTO and NG, affording cyclic performance stability.

Hot-electron-transfer enhancement for the efficient energy conversion of visible light.

Great strides have been made in enhancing solar energy conversion by utilizing plasmonic nanostructures in semiconductors. However, current generation with plasmonic nanostructures is still somewhat inefficient owing to the ultrafast decay of plasmon-induced hot electrons. It is now shown that the ultrafast decay of hot electrons across Au nanoparticles can be significantly reduced by strong coupling with CdS quantum dots and by a Schottky junction with perovskite SrTiO3 nanoparticles. The designed plasmonic nanostructure with three distinct components enables a hot-electron-assisted energy cascade for electron transfer, CdS→Au→SrTiO3, as demonstrated by steady-state and time-resolved photoluminescence spectroscopy. Consequently, hot-electron transfer enabled the efficient production of H2 from water as well as significant electron harvesting under irradiation with visible light of various wavelengths. These findings provide a new approach for overcoming the low efficiency that is typically associated with plasmonic nanostructures.

A combination of two visible-light responsive photocatalysts for achieving the Z-scheme in the solid state.

The light reaction in natural photosynthesis is generally recognized as one of the most efficient mechanisms for converting solar energy into other energy sources. We report herein on a novel strategy for generating H(2) fuel via an artificial Z-scheme mechanism by mimicking the natural photosynthesis that occurs in green plants. Designing a desirable photocatalyst by mimicking the Z-scheme mechanism leads to a conduction band that is sufficiently high to reduce protons, thus decreasing the probability of charge recombination. We combined two visible light sensitive photocatalysts, CdS and carbon-doped TiO(2), with different band structures. The used of this combination, that is, CdS/Au/TiO(1.96)C(0.04), resulted in the successful transfer of photogenerated electrons to a higher energy level in the form of the letter 'Z'. The system produced about a 4 times higher amount of H(2) under irradiation by visible light than CdS/Au/TiO(2). The findings reported herein describe an innovative route to harvesting energy by mimicking natural photosynthesis, and is independent of fossil fuels.