Plasmonic nanostructures, particularly those creating sub-diffraction limited optical field confinement in plasmonic gaps, are increasingly utilized to study catalytic reactions. The enhanced fields boost reaction efficiency and sensitivity, allowing for molecular-scale pathway tracking. Common plasmonic metals like Au and Ag, while providing strong field enhancements, are chemically inert. To overcome this limitation, the 'antenna-reactor' concept combines plasmonic and catalytic metals in hetero-nanostructures. However, these structures often lack precision, leaving active surface sites poorly defined. This research addresses this issue by employing a precision approach to construct a nanoreactor, enabling a mechanistic understanding. The ability to modify metallic surfaces with atomic layers of catalytically active metals like Pd, Pt, or Cu offers a route to high chemical selectivity and efficiency. This work utilizes atomic monolayer coatings of Pd on Au nanoparticles and mirrors, exploiting their surface-sensitive catalytic activity towards specific reactions, particularly the Suzuki-Miyaura C-C cross-coupling reaction. The Suzuki-Miyaura reaction, traditionally relying on soluble organometallic Pd catalysts, offers an excellent model system for investigating surface-mediated heterogeneous catalysis due to its high chemo- and stereo-selectivity. Previous efforts to enhance plasmonic catalysis for Suzuki coupling have focused on nanostructure assemblies, but a detailed understanding of molecular mechanisms and surface interactions remained lacking. This study aims to bridge this gap by employing precisely assembled molecular monolayers within nanogaps of a single-particle and a metallic mirror configuration. This system will allow us to directly study the reaction mechanism while retaining the plasmonic properties of Au to provide thermal energy and hot electrons that boost the reaction, and simultaneously tracking the reaction with surface-enhanced Raman scattering (SERS) spectroscopy.

The literature demonstrates the increasing use of plasmonic nanostructures in catalysis, particularly the antenna-reactor approach combining plasmonic and catalytic metals. However, a significant challenge has been the lack of precision in controlling the active surface sites within these structures. Previous studies have shown that atomically precise monolayer deposition of catalytic metals such as Pd, Pt, and Cu onto plasmonic surfaces enhances the catalytic efficiency and selectivity. Research on the Suzuki-Miyaura reaction, a key palladium-catalyzed C-C cross-coupling reaction, has focused on soluble organometallic catalysts. Recent works have explored plasmonic enhancement of the Suzuki coupling reaction using various nanostructures; however, a detailed understanding of the molecular mechanisms and the role of nearby surfaces remained elusive. This research builds upon these existing studies by employing a precision approach to create nanogap reactors that allows to control and track the reaction mechanism with unprecedented accuracy.

The study utilizes a nanoparticle-on-mirror (NPOM) platform to construct nanogap reactors. Colloidal Au nanoparticles (80 nm diameter) serve as the top surface, with a Pd monolayer deposited via chemical reduction of Pd2+ in solution, resulting in Au@Pd core-shell nanoparticles. Atomically smooth Au surfaces, prepared by template stripping, provide well-defined binding sites for thiol self-assembled monolayers (SAMs). These surfaces can be pre-coated with a Pd monolayer via electrochemical underpotential deposition (UPD). A SAM of 4-bromothiophenol (4-BTP) is adsorbed onto the Au mirror, forming the reactant layer. Four different NPOM types are created: Au NP on Au mirror (Au-on-Au), Au NP on Au@Pd mirror (Au-on-Pd), Au@Pd NP on Au mirror (Pd-on-Au), and Au@Pd NP on Au@Pd mirror (Pd-on-Pd). The Suzuki-Miyaura reaction is initiated by illuminating the NRs in a liquid flow cell containing 4-cyanophenylboronic acid (CPBA) and K2CO3. The progress of the reaction is dynamically tracked using time-series SERS spectroscopy. Dark-field spectroscopy and SEM imaging confirm the formation of the NRs. SERS spectroscopy provides real-time monitoring of reactant (4-BTP) and product (4-cyanobiphenylthiol; NC-BPT) molecules. Density functional theory (DFT) calculations are used to simulate Raman spectra, confirming the identification of reactant and product molecules. To compare, monolayer aggregates (MLaggs) of Au or Au@Pd NPs are also prepared using a liquid-liquid interfacial self-assembly method. The reaction kinetics are analyzed by fitting the SERS peak intensities over time to a two-component kinetic model: an initial exponential increase and subsequent linear increase. The initial relative catalytic yields and rate constants are extracted and compared for different NR types and MLaggs. DFT calculations of the projected density of states (PDOS) help elucidate the electronic properties of the surfaces. SEM and TEM measurements characterize the nanoparticle morphology.

The study reveals significant differences in catalytic efficiency and reaction kinetics among the four different NPOM types. The Au-on-Au NR shows no reaction, confirming the crucial role of Pd in catalyzing the Suzuki-Miyaura reaction. All other NRs exhibit the reaction, but with varying rates and yields. The Pd-on-Pd NR shows the fastest reaction, suggesting a synergistic effect of having Pd on both the nanoparticle and mirror. The initial yield is significantly higher for Pd NR surfaces, doubling when two Pd surfaces are present. This is attributed to the efficient conversion of plasmonic energy into chemical energy due to the precise molecular alignment. The reaction proceeds in two steps: a fast catalytic surface-driven step and a diffusion-limited step. The Pd monolayer on the nanoparticle facilitates direct contact between the catalytic site and the Br atom of 4-BTP, accelerating the reaction. The Pd monolayer on the mirror influences the polarization of the nanogap, affecting the diffusion of CPBA and K2CO3 into the gap. The MLaggs exhibit significantly lower catalytic activity compared to NRs, likely due to the random orientation of 4-BTP molecules within the nanogaps. The study further reveals that the precise molecular alignment in NPOMs creates a dipole moment that influences mass transfer and reaction kinetics. DFT calculations support the interpretation of experimental findings, showing the electron-donating nature of the Pd ML on Au. The study shows that ~2-3 molecules per second diffuse into the gap of the Pd-on-Au NR, resulting in a saturation time of ~24 minutes.

The findings demonstrate that the precise control of molecular orientation and surface chemistry in the nanogap significantly impacts catalytic efficiency. The observed two-step reaction kinetics highlight the interplay between surface-mediated catalysis and mass transfer. The superior performance of NPOM-based NRs compared to MLaggs underscores the importance of precise molecular alignment for optimal reaction kinetics. The results indicate that not only plasmonic heating and hot electrons but also the molecular arrangement and surface dipole moments strongly influence the catalytic process. This level of control over reaction kinetics and mass diffusion has not been previously achieved in traditional plasmonic catalysis experiments. The precise alignment of specific molecular sites with catalytic surfaces opens new avenues for designing efficient and selective plasmonic photocatalysis.

This study successfully developed highly efficient nanogap reactors based on precisely controlled plasmonic nanocavity constructs. By controlling the surface chemistry with atomic monolayers of Pd, and the precise orientation of reactants, it has been shown how to enhance the photocatalytic Suzuki-Miyaura coupling reaction. The study reveals a two-step reaction mechanism with distinct roles for the nanoparticle and mirror surfaces. The findings offer valuable design rules for plasmonic catalysis, highlighting the importance of precise molecular alignment and surface dipole control. Future research can explore various catalytic reactions and expand applications of these nanogap reactors to single-molecule catalysis, nano-constrained environments, and metasurface engineering for industrial applications.

The study focuses on a specific model reaction and specific nanoparticle sizes. The generalizability of the findings to other reactions or nanoparticle sizes requires further investigation. The mechanistic understanding of the plasmonic enhancement is not fully elucidated and requires more detailed investigations. The analysis relies on SERS spectroscopy, which may have limitations in terms of quantification and the potential for surface selection effects, although efforts were made to mitigate this.