This work was sponsored by the Army Research Laboratory and was accomplished under USARL Cooperative Agreement Number W911NF&#8208;17&#8208;2&#8208;0057 awarded to G.T.F. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

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&#8217;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 wi...

<ol>
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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 &#8220;environment&#8221; of the [PdCl4]2- molecules1</...

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-freedom1,2,3. 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 &#176;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 gradients3,4. 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 detailed5,6,7,8,9. 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 metal2,10,11,12. Anisotropic Pt-decoration of Au nanorods (AuNR) under longitudinal SPR (LSPR) excitation was recently reported1,13 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.).

<table>
	<tbody>
		<tr>
			<td>Aspheric Condenser Lens w/ Diffuser</td>
			<td>Thorlabs</td>
			<td>ACL5040U-DG15</td>
			<td>f=40 mm, NA=0.60, 1500 grit, uncoated</td>
		</tr>
		<tr>
			<td>Deuterium + Tungsten-Halogen Lightsource</td>
			<td>StellarNet</td>
			<td>SL5</td>
			<td></td>
		</tr>
		<tr>
			<td>Gold Nanorods, AuNR</td>
			<td>NanoPartz</td>
			<td>A12-40-808-CTAB</td>
			<td>CTAB surfactant, 808 nm LSPR, 40 nm diameter</td>
		</tr>
		<tr>
			<td>Ground Glass Diffuser</td>
			<td>Thorlabs</td>
			<td>DG20-1500</td>
			<td>1500 grit, N-BK7</td>
		</tr>
		<tr>
			<td>Hydrochloric acid, HCl</td>
			<td>J.T. Baker</td>
			<td>9539-03</td>
			<td>concentrated, 37%</td>
		</tr>
		<tr>
			<td>Low Profile Magnetic Stirrer</td>
			<td>VWR</td>
			<td>10153-690</td>
			<td></td>
		</tr>
		<tr>
			<td>Macro Disposable Cuvettes, UV Plastic</td>
			<td>FireFlySci</td>
			<td>1PUV</td>
			<td>10 mm path length</td>
		</tr>
		<tr>
			<td>Methanol, MeOH</td>
			<td>J.T. Baker</td>
			<td>9073-05</td>
			<td>&#8805;99.9%</td>
		</tr>
		<tr>
			<td>Palladium (II) chloride, PdCl2</td>
			<td>Sigma Aldrich</td>
			<td>520659</td>
			<td>&#8805;99.9%</td>
		</tr>
		<tr>
			<td>Plano-Convex Lens</td>
			<td>Thorlabs</td>
			<td>LA1145</td>
			<td>f=75 mm, N-BK7, uncoated</td>
		</tr>
		<tr>
			<td>Quartz Tungsten-Halogen Lamp</td>
			<td>Thorlabs</td>
			<td>QTH10</td>
			<td></td>
		</tr>
		<tr>
			<td>UV-vis Spectrometer</td>
			<td>Avantes</td>
			<td>ULS2048L-USB2-UA-RS</td>
			<td>AvaSpec-ULS2048L</td>
		</tr>
	</tbody>
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photodeposition of pd onto colloidal au nanorods by surface plasmon excitation

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 &#8220;hot-spots&#8221; 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).

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 ...

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Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

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

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot ...

A reductive plasmonic photodeposition protocol potentiates the ability to guide sub-five nanometer covalent metal assembly at targeted locations with optimal interfacial characteristics at ambient processing conditions. The main advantage of this technique is the ability to use an external field such as a laser to control the thickness and location of epitaxial metal growth. We have already demonstrated plasmonic photodeposition of metallic platinum and silver, and have confidence that this method can be extended to many others for a variety of catalytic applications.Demonstrating the procedure will be myself and Jonathan Boltersdorf, a chemist at the U.S.Army Research Laboratory. To begin this procedure, dilute 830 microliters of stock concentrated hydrochloric acid with water to 100 milliliters to obtain a 0.1 molar hydrochloric acid solution. Dilute 4 milliliters of this solution with water to 20 milliliters to create a 20 millimolar hydrochloric acid solution.Pipette 10 milliliters of this 20 millimolar hydrochloric acid solution into appropriate glassware. Next, add 0.0177 grams of palladium-two chloride to the hydrochloric acid and mix via sonication until all of the palladium(ii)chloride is dissolved. The resulting solution of dihydrogen tetrochloropallidate should have a concentration of 10 millimolar, and should exhibit a dark orange color.Then, pipette 2.5 milliliters of aqueously suspended gold nanorod solution with one to 10 millimolar cetyltrimethyl ammonium bromide surfactant into a 1 centimeter path length macro volume cuvette with a magnetic stir bar. Place the cuvette on a stir plate. Pipette 475 microliters of degassed methanol into the cuvette while gently stirring.Continue stirring for approximately 15 to 20 minutes while periodically removing any bubbles by gently tapping the bottom of the cuvette against a rigid surface as needed. After this, pipette five microliters of stock concentrated hydrochloric acid into the cuvette and let it mix for 15 minutes. Next, inject 25 microliters of 10 millimolar dihydrogen tetrochloropallidate into the reaction mixture for an atomic ratio of palladium to gold of one to five.Let the solution complex in the dark for one hour while stirring. Then irradiate the reaction mixture with an unpolarized 715 nanometer long past filtered tungsten-halogen lamp at an intensity of 35 milliwatts per centimeter squared for 24 hours. The next day, wash the residual chemicals and reagents from the palladium-gold nanorods by centrifuging at 9000 times G.Use a pipette to remove the supernatant, and re-suspend the pellet in water.Immerse the vial into a bath sonicator for one to two minutes to disperse the nanorods and complete the wash. Repeat this entire wash process once more. In this study, UV vis absorbent spectra are temporally recorded to elucidate changes upon adding each component comprising the reaction mixture beginning with the gold nanorods.The addition of methanol and hydrochloric acid decrease absorbance magnitude via dilution. The addition of dihydrodgen tetrochloropallidate causes UV lag and metal charge transfer features to emerge. After equilibrating in the dark for one hour, the palladium(ii)molecules exhibit charge transfer bands at 247 and 310 nanometers.Upon irradiation at the gold nanorod longitudinal surface plasmon resonance, the charge transfer bands quickly blueshift to 230 and 277 nanometers respectively. Absorbance magnitude of the dominant charge transfer peak decreases from 1.7 to approximately 0.47 over a 24 hour period due to photoreduction of absorbed palladium(ii)by plasmonic hot electrons. The two SPR modes are then analyzed before and after resonant irradiation.The longitudinal SPR redshifts from 807 to 816 nanometers. With 7 nanometer line width expansion, the transverse SPR remains unchanged. XPS confirms the presence of metallic palladium by the emergence of palladium-3D lines at binding energies of 335 and 340 electron volts.TEM reveals the morphologies of the gold nanorods mixed with dihydrogen tetrochloropallidate in the dark and after resonant irradiation. Resonant illumination yields sharp-tipped nanorods with roughened radial interfaces. Such characteristics are not observed in the dark.Distribution of nanorod lengths indicates that mean length grew from 127 to 129 nanometers. This is due to the presence of photodeposited palladium at the ends of the rods, as confirmed in an EDS map. It is important that CTAB concentration does not exceed one to 10 millimolar in the stock gold nanorod solution.Otherwise, precursor transport to the gold surface will be inhibited. This procedure can easily be adapted to photoreduce other catalytic metals such as platinum or iridium onto plasmonically active seed structures to synthesize a wide range of photocatalysts. We now have an architecture where plasmonic energy can be passed directly to a catalytic surface without any interfacial barrier which enables us to investigate plasmonic photocatalysis more effectively.Hydrochloric acid is a corrosive, strong acid and should be handled in a fume hood with proper personal protective equipment such as goggles and gloves.

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7.3K vistas.  U.S. Army Research Laboratory. Un protocolo de fotodeposición plasmónica reductiva potencia la capacidad de guiar el ensamblaje de metal covalente de sub cinco nanómetros en lugares específicos con características interfaciales óptimas en condiciones de procesamiento ambiental. La principal ventaja de esta técnica es la capacidad de utilizar un campo externo como un láser para controlar el grosor y la ubicación del crecimiento del metal epitaxial. Ya hemos demostrado fotodeposición plasmónica de platino metálic...