Surface Plasmon-Assisted Solar Energy Conversion

Single-nanoparticle spectroelectrochemistry studies enabled by localized surface plasmon resonance

This review describes recent progress of spectroelectrochemistry (SEC) analysis of single metallic nanoparticles (NPs) which have strong surface plasmon resonance properties. Dark-field scattering (DFS), photoluminescence (PL), and electrogenerated chemiluminescence (ECL) are three commonly used optical methods to detect individual NPs and investigate their local redox activities in an electrochemical cell. These SEC methods are highly dependent on a strong light-scattering cross-section of plasmonic metals and their electrocatalytic characteristics. The surface chemistry and the catalyzed reaction mechanism of single NPs and their chemical transformations can be studied using these SEC methods. Recent progress in the experimental design and fundamental understanding of single-NP electrochemistry and catalyzed reactions using DFS, PL, and ECL is described along with selected examples from recent publications in this field. Perspectives on the challenges and possible solutions for these SEC methods and potential new directions are discussed.

Electric Field Assisted Femtosecond Laser Preparation of Au@TiO2 Composites with Controlled Morphology and Crystallinity for Photocatalytic Degradation

TiO₂ is popular in photocatalytic degradation dye pollutants due to its abundance and its stability under photochemical conditions. Au loaded TiO₂ can achieve efficient absorption of visible light and deal with the problem of low conversion efficiency for solar energy of TiO₂. This work presents a new strategy to prepare Au nanoparticles-loaded TiO₂ composites through electric−field−assisted temporally−shaped femtosecond laser liquid-phase ablation of Au³⁺ and amorphous TiO₂. By adjusting the laser pulse delay and electric field parameters, gold nanoparticles with different structures can be obtained, such as nanospheres, nanoclusters, and nanostars (AuNSs). AuNSs can promote the local crystallization of amorphous TiO₂ in the preparation process and higher free electron density can also be excited to work together with the mixed crystalline phase, hindering the recombination between carriers and holes to achieve efficient photocatalytic degradation. The methylene blue can be effectively degraded by 86% within 30 min, and much higher than the 10% of Au nanoparticles loaded amorphous TiO₂. Moreover, the present study reveals the crystallization process and control methods for preparing nanoparticles by laser liquid ablation, providing a green and effective new method for the preparation of high-efficiency photocatalytic materials.

Plasmonic Oxidation of Glycerol Using Au/TiO2 Catalysts Prepared by Sol-Immobilisation

Abstract

Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile sol-immobilisation method and investigated for the surface plasmon-assisted glycerol oxidation under base-free conditions. The Au/TiO2 samples were characterized by UV–vis spectroscopy and transmission electron microscopy. Catalysts were prepared using polyvinyl alcohol as stabiliser as well as in the absence of polymer stabiliser. Both the conversion and the reaction selectivity are affected by the plasmon-assisted oxidation and there is an interplay between the presence of the stabiliser and the Au nanoparticle size.

Graphic Abstract

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Plasmonic oxidation of glycerol using AuPd/TiO2 catalysts

AuPd nanoparticles supported on P25 TiO₂ (AuPd/TiO₂) were prepared by a facile sol-immobilisation method and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions.

Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis

We use plasmonic Au-TiO₂ aerogels as a platform in which to marry synthetically thickened particle-particle junctions in TiO₂ aerogel networks to Au∥TiO₂ interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H₂) generation under both broadband (i.e., UV + visible light) and visible-only excitation. In doing so, we elucidate the dual functions that incorporated Au can play as a water reduction cocatalyst and as a plasmonic sensitizer. We also photodeposit non-plasmonic Pt cocatalyst nanoparticles into our composite aerogels in order to leverage the catalytic water-reducing abilities of Pt. This Au-TiO₂/Pt arrangement in three dimensions effectively utilizes conduction-band electrons injected into the TiO₂ aerogel network upon exciting the Au SPR at the Au∥TiO₂ interface. The extensive nanostructured high surface-area oxide network in the aerogel provides a matrix that spatially separates yet electrochemically connects plasmonic nanoparticle sensitizers and metal nanoparticle catalysts, further enhancing solar-fuels photochemistry. We compare the photocatalytic rates of H₂ generation with and without Pt cocatalysts added to Au-TiO₂ aerogels and demonstrate electrochemical linkage of the SPR-generated carriers at the Au∥TiO₂ interfaces to downfield Pt nanoparticle cocatalysts. Finally, we investigate visible light-stimulated generation of conduction band electrons in Au-TiO₂ and TiO₂ aerogels using ultrafast visible pump/IR probe spectroscopy. Substantially more electrons are produced at Au-TiO₂ aerogels due to the incorporated SPR-active Au nanoparticle, whereas the smaller population of electrons generated at Au-free TiO₂ aerogels likely originate at shallow traps in the high surface-area mesoporous aerogel.

Oxidation-stable plasmonic copper nanoparticles in photocatalytic TiO2 nanoarchitectures

Ultraporous copper/titanium dioxide (Cu/TiO₂) aerogels supporting <5 nm diameter copper nanoparticles are active for surface plasmon resonance (SPR)-driven photocatalysis. The extended nanoscale Cu‖TiO₂ junctions in Cu/TiO₂ composite aerogels-which arise as a result of photodepositing copper at the surface of the nanoparticulate-bonded TiO₂ aerogel architecture-stabilize Cu against oxidation to an extent that preserves the plasmonic behavior of the nanoparticles, even after exposure to oxidizing conditions. The metallicity of the Cu nanoparticles within the TiO₂ aerogel is verified by aberration-corrected scanning transmission electron microscopy, electron energy-loss spectroscopy, and infrared spectroscopy using CO binding as a probe to distinguish Cu(0) from Cu(i). In contrast, photoreduction of Cu(ii) at a commercial nanoscale anatase TiO₂ powder with primary particle sizes significantly larger than those in the aerogel results in a copper oxide/TiO₂ composite that exhibits none of the plasmonic character of Cu nanoparticles. We attribute the persistence of plasmonic Cu nanoparticles without the use of ligand stabilizers to the arrangement of Cu and TiO₂ within the aerogel architecture where each Cu nanoparticle is in contact with multiple nanoparticles of the reducing oxide. The wavelength dependence of the photoaction spectra for Cu/TiO₂ aerogel films reveals visible-light photocatalytic oxidation activity initiated by an SPR-driven process-as opposed to photo-oxidation initiated by excitation of narrow-bandgap copper oxides.

Photoinduced Force Mapping of Plasmonic Nanostructures

The ability to image the optical near-fields of nanoscale structures, map their morphology, and concurrently obtain spectroscopic information, all with high spatiotemporal resolution, is a highly sought-after technique in nanophotonics. As a step toward this goal, we demonstrate the mapping of electromagnetic forces between a nanoscale tip and an optically excited sample consisting of plasmonic nanostructures with an imaging platform based on atomic force microscopy. We present the first detailed joint experimental-theoretical study of this type of photoinduced force microscopy. We show that the enhancement of near-field optical forces in gold disk dimers and nanorods follows the expected plasmonic field enhancements with strong polarization sensitivity. We then introduce a new way to evaluate optically induced tip-sample forces by simulating realistic geometries of the tip and sample. We decompose the calculated forces into in-plane and out-of-plane components and compare the calculated and measured force enhancements in the fabricated plasmonic structures. Finally, we show the usefulness of photoinduced force mapping for characterizing the heterogeneity of near-field enhancements in precisely e-beam fabricated nominally alike nanostructures - a capability of widespread interest for precise nanomanufacturing, SERS, and photocatalysis applications.

Quantum Mode Selectivity of Plasmon-Induced Water Splitting on Gold Nanoparticles

Plasmon induced water splitting is a promising research area with the potential for efficient conversion of solar to chemical energy, yet its atomic mechanism is not well understood. Here, ultrafast electron-nuclear dynamics of water splitting on gold nanoparticles upon exposure to femtosecond laser pulses was directly simulated using real time time-dependent density functional theory (TDDFT). Strong correlation between laser intensity, hot electron transfer, and reaction rates has been identified. The rate of water splitting is dependent not only on respective optical absorption strength, but also on the quantum oscillation mode of plasmonic excitation. Odd modes are more efficient than even modes, owing to faster decaying into hot electrons whose energy matches well the antibonding orbital of water. This finding suggests photocatalytic activity can be manipulated by adjusting the energy level of plasmon-induced hot carriers, through altering the cluster size and laser parameter, to better overlap adsorbate unoccupied level in plasmon-assisted photochemistry.

Plasmonic Au/TiO2nanostructures for glycerol oxidation

Visible light irradiation of the reaction volume results in boosted conversion for glycerol oxidation due to plasmonic properties of Au/TiO₂.