Design rules for catalysis in single-particle plasmonic nanogap reactors with precisely aligned molecular monolayers

Abstract

Plasmonic nanostructures can both drive and interrogate light-driven catalytic reactions. Sensitive detection of reaction pathways is achieved by confining optical fields near the active surface. However, effective control of the reaction kinetics remains a challenge to utilize nanostructure constructs as efficient chemical reactors. Here we present a nanoreactor construct exhibiting high catalytic and optical efficiencies, based on a nanoparticle-on-mirror (NPoM) platform. We observe and track pathways of the Pd-catalysed C-C coupling reaction of molecules within a set of nanogaps presenting different chemical surfaces. Atomic monolayer coatings of Pd on the different Au facets enable tuning of the reaction kinetics of surface-bound molecules. Systematic analysis shows the catalytic efficiency of NPoM-based nanoreactors greatly improves on platforms based on aggregated nanoparticles. More importantly, we show Pd monolayers on the nanoparticle or on the mirror play significantly different roles in the surface reaction kinetics. Our data provides clear evidence for catalytic dependencies on molecular configuration in well-defined nanostructures. Such nanoreactor constructs therefore yield clearer design rules for plasmonic catalysis.

Fig. 1. Nanogap reactor construct for Suzuki-coupling catalytic reaction.

a Monolayer (ML) of Pd deposited onto 80 nm Au NPs. b ML of Pd electrochemically reduced via UPD onto the mirror of template-stripped Au substrate. c Molecular-thick (d = 1.2 nm) nanogap formed by SAM of 4-BTP sandwiched between NP and mirror from (a, b). Four NR types are 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). d Light-driven Suzuki–Miyaura C–C coupling reaction of 4-BTP and CPBA in the NR, resulting in C–C coupled NC-BPT as a product (Au: yellow, Pd: grey).

Fig. 2. Plasmonic activity of the four NR types.

Histograms of the dominant coupled mode wavelengths from dark-field spectra of >200 NRs and average spectra from each bin of a Au-on-Au, b Au-on-Pd, c Pd-on-Au, and d Pd-on-Pd NRs. Black dotted curves are Gaussian fits of histograms, and the centre wavelength (λ_centre) from each fit is noted at the top. e Representative dark-field spectrum of each single NR. Normalized SERS spectra of f 4-BTP and g NC-BPT measured over 5 min from each NR under ambient conditions without second reactant, for 100 µW laser. From bottom to top: Au-on-Au (orange), Pd-on-Au (darker orange), Au-on-Pd (brown), Pd-on-Pd (grey), and DFT-simulated spectrum of isolated molecule (dashed).

Fig. 3. SERS spectra from each NR type loaded with 4BPT in solution of CPBA and K₂CO₃.

a Colourmaps of repeated SERS spectra (300 frames @ 1 s integration times). b Initial (t = 1 s, blue) and final (t = 5 min, red) SERS spectra from colourmaps in (a). c SERS spectra normalized by the maximum intensity in solution over 5 min for each NR type. d SERS spectra of blue-shaded region in (c). From bottom to top: Au-on-Au (orange), Pd-on-Au (darker orange), Au-on-Pd (brown), and Pd-on-Pd (grey).

Fig. 4. Progress of SERS peak intensity ratio during Suzuki–Miyaura coupling reaction, comparing NRs to monolayer NP aggregates (MLaggs).

a, b Dynamics of peak intensity ratio $R_{21}$ of 1572 cm⁻¹ NC-BPT mode (${\nu }_2$) to 1532 cm⁻¹ mode of 4-BTP (${\nu }_1$) for a NRs and b MLaggs (note multiplied by ×5). c, d Peak intensity ratio of 2255 cm⁻¹ NC-BPT mode (${\nu }_3$) to ${\nu }_1$ for c NRs and d MLaggs. e Laser-driven catalytic conversion yields at t = 1 s (darker bars) and t = 5 min (for NRs) or t = 30 min (MLaggs) (lighter hatched bars). f, g Slow rate constants ($k_{\mathrm{slow}}$) fitted from a–d using two-step kinetics. h Schematic molecular alignment in NPoM and MLagg geometries, showing thiols bound to one or both facet surfaces. Labels 1–6 as defined in (a, b).

Fig. 5. Molecular schematics of NRs and MLaggs.

Schematics show molecular alignments in the nanogap and gap dipole moment strengths of a Au-on-Au, b Au-on-Pd, c Pd-on-Au, d Pd-on-Pd NRs, e Au and f Pd MLaggs.