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 or no direct control over resultant material characteristics. Immersion of a plasmonic substrate (e.g., Au nanoparticle/seed) into a precursor environment (e.g., aqueous Pd salt solution) under illumination at the localized surface plasmon resonance (SPR) initiates externally-tunable (i.e., field polarization and intensity) photochemical deposition of the precursor via plasmonic hot electrons and/or photothermal gradients^3,4. For example, protocol parameters/requirements for plasmonically-driven photothermal decomposition of Au, Cu, Pb, and Ti organometallics and Ge hydrides onto nanostructured Ag and Au substrates have been detailed^5,6,7,8,9. However, utilization of femtosecond plasmonic hot electrons to directly photoreduce metal salts at a metal-solution interface remains largely undeveloped, absent processes employing citrate or poly(vinylpyrrolidone) ligands acting as intermediary charge relays to direct nucleation/growth of the secondary metal^2,10,11,12. Anisotropic Pt-decoration of Au nanorods (AuNR) under longitudinal SPR (LSPR) excitation was recently reported^1,13 where the Pt distribution coincided with the dipole polarity (i.e., the assumed spatial distribution of hot carriers).

The protocol herein expands upon recent Pt-AuNR work to include Pd and highlights key synthesis metrics that can be observed in real-time, showing the reductive plasmonic photodeposition technique is applicable toward other metal halide salts (Ag, Ni, Ir, etc.).

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 and similar photo-chemical¹² syntheses. Alternative surfactants to CTAB may be employed so long as Zeta-potential is positive, although final Pd morphology could change.

1. Synthesis Techniques: Synthesize aqueously-dispersed AuNR at 0.5 mM Au using the silver-assisted method by Nikoobakht et al.^16,17 (yielding monocrystalline structure) or the surfactant-assisted method by Murphy et al.^18,19 (yielding penta-twinned crystal structure). Wash the AuNR via centrifugation^20,21 to remove excess, free CTAB to a final concentration of 1-10 mM.
2. Commercial Sources: Purchase aqueous AuNR dispersions at 0.5 mM Au with the following specifications: 40 nm diameter, 808 nm LSPR, and CTAB ligand (5 mM concentration) in DI water. Wash the AuNR via centrifugation^20,21 to remove excess, free CTAB if the CTAB concentration exceeds 1-10 mM upon receipt.
   NOTE: Aqueous AuNR dispersions with CTAB surfactant at a variety of sizes, aspect ratios, and particle number densities may be purchased from many commercial vendors and used successfully in this protocol.

2. Plasmonic photodeposition of Pd onto Au nanorods

1. Preparation of Pd precursor
   1. Prepare a 20 mM HCl solution. First, make 0.1 M HCl by diluting 830 μL of stock concentrated HCl (37%, 12 M) with water to 100 mL. Second, make 0.02 M HCl by diluting 4 mL of 0.1 M HCl with water to 20 mL.
   2. Pipette 10 mL of 20 mM HCl into appropriate glassware and place in a bath sonicator (no sonication) with water temperature set to 60 °C.
   3. Add 0.0177 g of PdCl₂ into the 10 mL of 20 mM HCl and mix via sonication until all PdCl₂ is dissolved. The resultant 10 mM H₂PdCl₄ solution should exhibit a dark orange color.
2. Preparation of photodeposition reaction mixture
   NOTE: The procedure described assumes a 3 mL total volume for use in a cuvette to allow real-time feedback into plasmonic photodeposition process. The cited masses/volumes were selected for compatibility with typical chemicals/materials/reagents while allowing facile washing/recovery of the Pd-decorated AuNR. It is anticipated that similar results may be achieved if scaled to other volumes and/or alternative reaction vessels are used (e.g., glass beaker).
   1. Degas stock AuNR solution and methanol (MeOH) in a bath sonicator for 30 min.
   2. Pipette 2.5 mL of aqueously-suspended AuNR (from step 2.2.1) into a 1 cm path length, macrovolume cuvette with a magnetic stir bar. Place the cuvette on a stir plate.
      NOTE: Typical volume of a macrovolume cuvette is 3.5 mL. Quartz may be substituted with UV-transparent plastics.
   3. Pipette 475 μL of degassed MeOH (from step 2.2.1) into the cuvette while gently stirring for approximately 15-30 min. Periodically remove any bubbles by gently tapping the bottom of the cuvette against a rigid surface as needed; removing solvated gasses can prolong the stability of the metal halide salt.
   4. Pipette 5 μL of stock concentrated HCl (37%, 12 M) into the cuvette and let mix for 15 min.
      NOTE: Tuning concentration of HCl support could influence final morphology/rate of Pd deposition, but concentrations less than 20 mM in the reaction mixture will allow H₂PdCl₄ to progressively hydrolyze and oxolate, leading to eventual PdO_x formation after ~3 h.
3. Plasmonic photoreduction of [PdCl₄]^2- onto AuNR^1,13
   1. Inject 25 μL of 10 mM H₂PdCl₄ into the reaction mixture for a 1:5 Pd:Au atomic ratio. Let the solution complex in dark for 1 h while stirring.
      NOTE: This quantity may be adjusted according the desired Pd:Au ratio as the expense of altering the final molarities of Au, [PdCl₄]^2-, HCl, and MeOH of the reaction mixture. Reference²² illustrates example Pt-AuNR morphologies at different Pt:Au ratios- similar results can be expected with Pd.
   2. Irradiate the reaction mixture with an un-polarized, 715 nm long-pass filtered tungsten-halogen lamp at 35 mW/cm² intensity for 24 h.
      NOTE: Different light filters (or sources, e.g., laser) may be chosen according to unique LSPR wavelength for different Au nanostructure seeds. For example, a 420 nm long-pass filter may be used for plasmonic seed structures exhibiting LSPR at 450 nm. Light intensity may be decreased with neutral density filtration at the expense of a slower [PdCl₄]^2- reduction rate, leading to a longer total reaction time. Light intensity may be increased to reduce reaction time at the expense of potential for thermal reduction of [PdCl₄]^2- (onset is ~360 °C via Reference²³). An appropriate intensity can be calculated a priori to mitigate thermal reduction via calculation of nanoparticle surface temperature in isolation and/or collective ensembles²⁴. Effects on ultimate Pd-AuNR morphology from varying irradiation intensity have not been explored.
   3. Wash the residual chemicals/reagents from the Pd-AuNR two times, each by: centrifugation at 9,000 x g, removing the supernatant with a pipette, re-suspending the Pd-AuNR pellet in water, and immersing the vial into a bath sonicator for 1-2 min to disperse^20,21.

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