Ligands are the quintessential synthesis tool in the preparation of colloidal metal catalysts, allowing for the rational design of nanostructured surfaces with high activity and selectivity. These same agents can, however, strongly influence the catalytic performance of metal nanostructures in aqueous media. In this regard, the current literature describing the influence of ligands on the model catalytic reaction that sees 4-nitrophenol reduced to 4-aminophenol by borohydride is highly fragmented in that the understanding of reaction rate, induction time, and ligand desorption phenomena is disconnected and, at times, contradictory. Herein, we present a study in which three chemically distinct ligands are applied to bare gold catalysts followed by their exposure to aqueous solutions of relevance to 4-nitrophenol reduction while simultaneously tracking the ligand whereabouts. It is shown that the exposure of ligands to borohydride leads to their near-complete removal from the gold catalyst. This, in turn, gives rise to severe disruptions to the rate of 4-nitrophenol reduction for the scenario where the aqueous reactants are purged of dissolved oxygen and ligand desorption times are slow. In sharp contrast, the reaction rate is little affected when the same reactants contain dissolved oxygen because the resulting induction period provides ample time for the ligands to desorb prior to the onset of the reaction. Moreover, strongly bound ligands are shown to give rise to an induction-time-like feature that is only observable when the reactants are free of dissolved oxygen. Collectively, these findings advocate procedures for catalytic benchmarking that differ from current best practices and underscore the point that a fundamental understanding of 4-nitrophenol catalysis must adopt a holistic approach that accounts for ligand–nanostructure interdependencies.

The seed-mediated growth of noble metal nanostructures with planar geometries requires the use of seeds lined with parallel stacking faults so as to provide a break in symmetry in an otherwise isotropic metal. Although such seeds are now routinely synthesized using colloidal pathways, equivalent pathways have not yet been reported for the fabrication of substrate-based seeds with the same internal defect structures. The challenge is not merely to form seeds with planar defects but to do so in a deterministic manner so as to have stacking faults that only run parallel to the substrate surface while still allowing for the lithographic processes needed to regulate the placement of seeds. Here, we demonstrate substrate-imposed epitaxy as a viable synthetic control able to induce planar defects in Au seeds while simultaneously dictating nanostructure in-plane alignment and crystallographic orientation. The seeds, which are formed in periodic arrays using nanoimprint lithography in combination with a vapor-phase assembly process, are subjected to a liquid-phase plasmon-mediated synthesis that uses light as an external stimuli to drive a reaction yielding periodic arrays of hexagonal Au nanoplates. These achievements not only represent the first of their kind demonstrations but also advance the possibility of integrating wafer-based technologies with a rich and exciting nanoplate colloidal chemistry.

A comprehensive mechanistic framework is key to the effective utilization of model catalytic reactions. Although the reduction of 4-nitrophenol by borohydride has emerged as one of the most widely used model reactions for accessing the catalytic activity of nanostructures, there still exist knowledge gaps. The cause of the induction time, which is a period at the beginning of the reaction where no reaction seemingly occurs, has long been the subject of debate. Recent work provides compelling experimental evidence that links the induction time to the consumption of dissolved oxygen within the aqueous reactants and provides a mechanistic understanding based on a previously unknown side reaction. A concern has, however, been raised that the proposed mechanism is unable to account for prior work showing the induction time to be independent of the borohydride concentration. Here, a systematic study is presented that re-examines this dependency where reactions are simultaneously monitored using spectroscopy and an in situ dissolved oxygen probe. The dependency is shown to be far more involved than prior studies suggest because it varies with the amount of catalyst added. The understanding obtained is consistent with the previously proposed mechanism for the induction time and resolves perceived inconsistencies with earlier work.

The fundamental understanding of liquid-phase catalytic reactions is unavoidably complicated when the catalyst is prone to leaching since questions inevitably arise as to the true nature of the catalyst. While the catalytic reduction of 4-nitrophenol by borohydride is widely accepted as a trusted model reaction, it has faced little scrutiny concerning the potential impact of leached species or the appropriateness of assigning catalytic activity to the inserted nanostructures without rigorous experimental verification. Here, we present results from a spectroscopically monitored split test in which supported silver catalysts are physically separated from the reactants midway through the reaction. It is unambiguously demonstrated that the influence of leaching is far from benign, instead acting to extinguish the catalytic activity of the inserted nanostructures while giving rise to an unsupported heterogeneous catalyst that is the true catalytic entity. With only submonolayer quantities of silver leached from the supported structures, the unsupported species must be exceedingly catalytic. Moreover, it is shown that leaching is inherent to aqueous media containing dissolved oxygen, without which the supported nanostructures remain catalytically active. With the same nanomaterial being able to act either as a heterogeneous catalyst or as a reservoir from which leached metal is derived, such influences have undoubtedly compromised prior studies. We, nevertheless, capitalize on the sensitivity of 4-nitrophenol reduction to leached species by using it as a reaction-based indicator able to quantitatively determine the time dependence of the leaching process and enhancements to oxidative etching when silver, copper, palladium, platinum, and gold are exposed to chloride ions.

Understanding the dynamics of the micro-sized surface bubbles produced by plasmonic heating can benefit a wide range of applications like microfluidics, catalysis, micro-patterning and photo-thermal energy conversion. Usually, surface plasmonic bubbles are generated on plasmonic nano-structures pre-deposited on the surface subject to laser heating. In our studies, we have investigated the growth dynamics and movement mechanism of surface microbubbles generated in plasmonic nanoparticle (NP) suspension. In the first section, we observe much faster bubble growth rates compared to those in pure water with surface plasmonic structures. Our analyses show that the volumetric heating effect around the surface bubble due to the existence of NPs in the suspension is the key to explain this difference. In the second section, we demonstrate that surface bubbles on a solid surface are directed by a laser to move at high speeds (> 1.8 mm/s), and we elucidate the mechanism to be the de-pinning of the three-phase contact line (TPCL) by rapid plasmonic heating of NPs deposited in-situ during bubble movement.