After more than a decade, it is still unknown whether the plasmon-mediated growth of silver nanostructures can be extended to the synthesis of other noble metals, as the molecular mechanisms governing the growth process remain elusive. Herein, we demonstrate the plasmon-driven synthesis of gold nanoprisms and elucidate the details of the photochemical growth mechanism at the single-nanoparticle level. Our investigation reveals that the surfactant polyvinylpyrrolidone preferentially adsorbs along the nanoprism perimeter and serves as a photochemical relay to direct the anisotropic growth of gold nanoprisms. This discovery confers a unique function to polyvinylpyrrolidone that is fundamentally different from its widely accepted role as a crystal-face-blocking ligand. Additionally, we find that nanocrystal twinning exerts a profound influence on the kinetics of this photochemical process by controlling the transport of plasmon-generated hot electrons to polyvinylpyrrolidone. These insights establish a molecular-level description of the underlying mechanisms regulating the plasmon-driven synthesis of gold nanoprisms.

Water reduction under two different visible-light ranges (λ > 400 nm and λ > 435 nm) was investigated in gold-loaded titanium dioxide (Au-TiO2) heterostructures with different sizes of Au nanoparticles (NPs). Our study clearly demonstrates the essential role played by Au NP size in plasmon-driven H2O reduction and reveals two distinct mechanisms to clarify visible-light photocatalytic activity under different excitation conditions. The size of the Au NP governs the efficiency of plasmon-mediated electron transfer and plays a critical role in determining the reduction potentials of the electrons transferred to the TiO2 conduction band. Our discovery provides a facile method of manipulating photocatalytic activity simply by varying the Au NP size and is expected to greatly facilitate the design of suitable plasmonic photocatalysts for solar-to-fuel energy conversion.

Surface plasmon resonance (SPR)-induced photothermal heating has garnered a substantial amount of research interest across various disciplines. The first applications of SPR-induced light-to-heat energy conversion were in biological systems to photothermally ablate cancer cells in vivo. More recently, this spatially localized and highly tunable heating technique has been extensively used for a variety of chemical reactions and other associated applications. This feature article highlights the recent developments in surface plasmon-mediated photothermal chemistry. We review the current theoretical and experimental work toward estimating the photothermal heating-induced surface temperatures of plasmonic nanostructures. From a mechanistic perspective, we show how this local heating can activate reactant molecules and boost numerous types of chemical reactions. We also discuss the physical changes occurring in a surrounding solvent, such as water, during the photothermal process. Finally, we extend the scope of SPR-induced photothermal chemical reactions by manipulating the plasmonic nanostructure to facilitate nanomaterial fabrication, paving the way for a wide range of applications based on SPR-mediated photothermal chemistry. This perspective establishes a framework for the current applications, potential uses, and remaining challenges associated with harnessing SPR-induced photothermal heating.

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Solar water splitting provides a mechanism to convert and store solar energy in the form of stable chemical bonds. Water-splitting systems often include semiconductor photoanodes, such as n-Fe 2 O 3 and n-BiVO 4 , which use photogenerated holes to oxidize water. These photoanodes often exhibit improved performance when coated with metal-oxide/(oxy)hydroxide overlayers that are catalytic for the water-oxidation reaction. The mechanism for this improvement, however, remains a controversial topic. This is, in part, due to a lack of experimental techniques that are able to directly track the flow of photogenerated holes in such multicomponent systems. In this Perspective, we illustrate how this issue can be addressed by using a second working electrode to make direct current/voltage measurements on the catalytic overlayer during operation in a photoelectrochemical cell. We discuss examples where the second working electrode is a thin metallic film deposited on the catalyst layer, as well as where it is the tip of a conducting atomic-force-microscopy probe. In applying these techniques to multiple semiconductors (Fe 2 O 3 , BiVO 4 , Si) paired with various metal-(oxy)hydroxide overlayers (e.g., Ni(Fe)O x H y and CoO x H y ), we found in all cases investigated that the overlayers collect photogenerated holes from the semiconductor, charging to potentials sufficient to drive water oxidation. The overlayers studied thus form charge-separating heterojunctions with the semiconductor as well as serve as water-oxidation catalysts.

Sub-15 nm Au nanoparticles have been fabricated on a nanostructured Ag surface at room temperature via a liquid-phase chemical deposition upon excitation of the localized surface plasmon resonance (SPR). Measurement of the SPR-mediated photothermal local heating of the substrate surface by a molecular thermometry strategy indicated the temperature to be above 230 °C, which led to an efficient decomposition of CH(3)AuPPh(3) to form Au nanoparticles on the Ag surface. Particle sizes were tunable between 3 and 10 nm by adjusting the deposition time. A surface-limited growth model for Au nanoparticles on Ag is consistent with the deposition kinetics.

Oxide/(oxy)hydroxide overlayers such as cobalt (oxy)hydroxide phosphate (CoPi) enhance the performance of BiVO 4 water-spitting photoanodes, but the mechanism of this enhancement remains unclear. We show that if the BiVO 4 layer is thin and incompletely covers an underlying conductive glass, the performance dramatically decreases as CoPi loading is increased. This is consistent with direct contact between the CoPi and conducting glass that leads to "shunt" recombination of photogenerated holes accumulated in the CoPi. For thicker BiVO 4 layers that completely cover the conducting glass, these shunt pathways are blocked. We then use a nanoelectrode atomic force microscopy probe to measure, in operando, the electrochemical potential of CoPi on thick BiVO 4 films under illumination. We find that CoPi is charged to a potential necessary to drive water oxidation at a rate consistent with the measured photocurrent. CoPi acts as a hole collector and is the principal driver of water oxidation on BiVO 4 .

Electrocatalysts improve the efficiency of light-absorbing semiconductor photoanodes driving the oxygen evolution reaction, but the precise function(s) of the electrocatalysts remains unclear. We directly measure, for the first time, the interface carrier transport properties of a prototypical visible-light-absorbing semiconductor, α-Fe2O3, in contact with one of the fastest known water oxidation catalysts, Ni0.8Fe0.2Ox, by directly measuring/controlling the current and/or voltage at the Ni0.8Fe0.2Ox catalyst layer using a second working electrode. The measurements demonstrate that the majority of photogenerated holes in α-Fe2O3 directly transfer to the catalyst film over a wide range of conditions and that the Ni0.8Fe0.2Ox is oxidized by photoholes to an operating potential sufficient to drive water oxidation at rates that match the photocurrent generated by the α-Fe2O3. The Ni0.8Fe0.2Ox therefore acts as both a hole-collecting contact and a catalyst for the photoelectrochemical water oxidation process. Separate measurements show that the illuminated junction photovoltage across the α-Fe2O3|Ni0.8Fe0.2Ox interface is significantly decreased by the oxidation of Ni2+ to Ni3+ and the associated increase in the Ni0.8Fe0.2Ox electrical conductivity. In sum, the results illustrate the underlying operative charge-transfer and photovoltage generation mechanisms of catalyzed photoelectrodes, thus guiding their continued improvement.