Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

Summary

A protocol for anisotropic photodeposition of Pd onto aqueously-suspended Au nanorods via localized surface plasmon excitation is presented.

Abstract

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot electrons upon SPR irradiation drive reductive deposition of Pd on colloidal AuNR in the presence of [PdCl₄]^2-. Plasmon-driven reduction of secondary metals potentiates covalent, sub-wavelength deposition at targeted locations coinciding with electric field “hot-spots” of the plasmonic substrate using an external field (e.g., laser). The process described herein details a solution-phase deposition of a catalytically-active noble metal (Pd) from a transition metal halide salt (H₂PdCl₄) onto aqueously-suspended, anisotropic plasmonic structures (AuNR). The solution-phase process is amenable to making other bimetallic architectures. Transmission UV-vis monitoring of the photochemical reaction, coupled with ex situ XPS and statistical TEM analysis, provide immediate experimental feedback to evaluate properties of the bimetallic structures as they evolve during the photocatalytic reaction. Resonant plasmon irradiation of AuNR in the presence of [PdCl₄]^2- creates a thin, covalently-bound Pd⁰ shell without any significant dampening effect on its plasmonic behavior in this representative experiment/batch. Overall, plasmonic photodeposition offers an alternative route for high-volume, economical synthesis of optoelectronic materials with sub-5 nm features (e.g., heterometallic photocatalysts or optoelectronic interconnects).

Introduction

Guiding metal deposition onto plasmonic substrates via plasmonic hot carriers generated from a resonant external field could support 2-step formation of heterometallic, anisotropic nanostructures at ambient conditions with new degrees-of-freedom^1,2,3. Conventional redox chemistry, vapor deposition, and/or electrodeposition approaches are ill-suited for high-volume processing. This is primarily due to excess/sacrificial reagent waste, low throughput 5+ step lithography processes, and energy intensive environments (0.01-10 Torr and/or 400-1000 °C temperatures) with little ....

Protocol

1. Allocation of Au nanorods

NOTE: Cetyltrimethylammonium bromide (CTAB)-covered AuNR may be synthesized by wet-chemistry (step 1.1) or purchased commercially (step 1.2) according to the reader’s preference, with each yielding similar results. Results in this work were based on commercially-sourced, AuNR with penta-twinned crystal structure. Impact of AuNR seed crystal structure (i.e., monocrystalline vs. penta-twinned) on ultimate morphology of the secondary metal shell remains unclear within the scope of plasmonic photodeposition, but has been of keen interest in both wet-^14,15

Results

Transmission UV-vis spectra, X-ray photoelectron spectroscopy (XPS) data, and transmission electron microscopy (TEM) images were acquired for the CTAB-covered AuNR in the presence/absence of H₂PdCl₄ in dark and under resonant irradiation at their longitudinal SPR (LSPR) to catalyze nucleation/growth of Pd. Transmission UV-vis spectra in Figure 1 and Figure 2 provide insights into the reaction dynamics according to changes in: (a) precursor .......

Discussion

Monitoring changes in optical absorbance using transmission UV-vis spectroscopy is useful to assess status of the photocatalytic reaction, with particular attention to the LMCT features of H₂PdCl₄. Wavelength maxima of LMCT features after injection of H₂PdCl₄ at step 2.3.1 (going from solid black to solid blue in Figure 1) provide insights into the local “environment” of the [PdCl₄]^2- molecules¹

Materials

Name	Company	Catalog Number	Comments
Aspheric Condenser Lens w/ Diffuser	Thorlabs	ACL5040U-DG15	f=40 mm, NA=0.60, 1500 grit, uncoated
Deuterium + Tungsten-Halogen Lightsource	StellarNet	SL5
Gold Nanorods, AuNR	NanoPartz	A12-40-808-CTAB	CTAB surfactant, 808 nm LSPR, 40 nm diameter
Ground Glass Diffuser	Thorlabs	DG20-1500	1500 grit, N-BK7
Hydrochloric acid, HCl	J.T. Baker	9539-03	concentrated, 37%
Low Profile Magnetic Stirrer	VWR	10153-690
Macro Disposable Cuvettes, UV Plastic	FireFlySci	1PUV	10 mm path length
Methanol, MeOH	J.T. Baker	9073-05	≥99.9%
Palladium (II) chloride, PdCl2	Sigma Aldrich	520659	≥99.9%
Plano-Convex Lens	Thorlabs	LA1145	f=75 mm, N-BK7, uncoated
Quartz Tungsten-Halogen Lamp	Thorlabs	QTH10
UV-vis Spectrometer	Avantes	ULS2048L-USB2-UA-RS	AvaSpec-ULS2048L