Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation

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.