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RSC Advances PAPER View Article Online / Journal Homepage / Table of Contents for this issue RSC Advances Dynamic Article Links Cite this: RSC Advances, 2012, 2, 3968–3977 www.rsc.org/advances PAPER Palladium nanoshells coated with self-assembled monolayers and their catalytic properties Jun-Hyun Kim,*a Joon-Seo Park,b Hae-Won Chung,c Brett W. Bootea and T. Randall Lee*c Received 13th October 2011, Accepted 6th February 2012 DOI: 10.1039/c2ra00883a This report describes the preparation and characterization of palladium nanoshells protected with alkanethiol self-assembled monolayers (SAMs) and their application as efficient catalysts. Monodispersed silica core particles (y100 nm in diameter) were prepared and coated with a thin layer of palladium (y20 nm in thickness). Subsequently, the palladium nanoshells were treated with two separate alkanethiol adsorbates having different alkyl chain lengths: dodecanethiol (C12SH) and hexadecanethiol (C16SH). The optical properties, morphology, and chemical structure/composition of these nanoshells were thoroughly examined by ultraviolet-visible spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy. Additional studies revealed that these SAM-coated palladium nanoshells possessed enhanced colloidal stability in nonpolar solvents and in the solid state. Further, palladium nanoshells modified with C16SH SAM coatings were employed in the Suzuki coupling of phenylboronic acid with iodobenzene in organic solvents. Notably, these SAM-coated nanoshells afforded a greater conversion yield than that of related bare palladium nanoshells. Downloaded on 19 February 2013 Introduction palladium nanoparticles as catalysts depend largely on their size, shape, surrounding medium, and dispersion ability in organic The controlled fabrication of nanoscale metallic particles offers the solvents, which facilitates their manipulation and incorporation 1–5 opportunity to develop novel catalysts. Palladium nanoparticles 16–19 Published on 13 March 2012 http://pubs.rsc.org | doi:10.1039/C2RA00883A into various inert supports. have received particular attention due to their useful catalytic Palladium nanoparticles with sizes up to 10 nm are routinely 1,3,6–9 y activities. A number of palladium complexes have been prepared by the following methods: sonochemical, gas evapora- developed as homogeneous catalysts for various reactions and tion, liquid deposition, refluxing alcohol reduction, chemical have shown good activities and selectivities when compared to reduction, and electrochemical.6,20–23 Although these small 2,10–12 heterogeneous analogs. However, for large-scale applications nanoparticles exhibit excellent catalytic properties, they suffer in the solution phase, heterogeneous catalysts are much more 2,11 from a problematic separation from the reaction mixture and are attractive because of their ease of separation and recyclability. thus not readily amenable to recycling.3,18,24–26 In contrast, a Consequently, the crafting of palladium atoms into unique limited number of chemical reduction methods afford large nanoscale metallic particles has been widely studied for the palladium nanoparticles with diameters as large as 50 nm. purpose of generating heterogeneous catalysts with unique Typically, these methods involve the reduction of palladium capabilities.11–13 Due to their large surface-to-volume ratio, metal ions in the presence of heterogeneous supports or nanoscale palladium particles catalyze a diverse scope of reactions, stabilizers such as organic molecules, polymers, and other including the hydrogenation of olefins and dienes, the hydration of surfactants.4,7,18,22,27,28 acrylonitrile to acrylamide, the photogeneration of hydrogen from An alternative strategy employs core–shell structures, where the water, the reduction of carbon dioxide, and a variety of coupling size of palladium nanoparticles can be tuned to greater than reactions.2,4,8,12,14,15 Interestingly, the reactivity and selectivity of 100 nm in diameter in an aqueous solution without using any stabilizers.3,29 Given the stability and dispersity, nanoshells can aDepartment of Chemistry, Illinois State University, Normal, potentially possess excellent catalytic activity in aqueous or polar IL 61790-4160, USA. E-mail: [email protected]; Fax: +1-309-438-5538; organic solutions (e.g.,ethanol).3,9 Additionally, the employment Tel: +1-309-438-2604 bDepartment of Chemistry, Eastern University, 1300 Eagle Road, of silica and alumina supports (e.g., cores) for palladium-based St. Davids, PA, 19087-3696, USA. Fax: +1-610-341-1466; catalysts has shown enhanced catalytic properties (due to their Tel: +1-610-341-1485 excellent chemical/thermal stability, good accessibility, and poros- cDepartment of Chemistry and the Texas Center for Superconductivity, University of Houston, Houston, TX, 77204-5003, USA. ity) and the easy recovery of catalysts from a reaction mixture by E-mail: [email protected]; Fax: +1-281-754-4445; Tel: +1-713-743-2724 simple filtration, centrifugation, and decantation.24,25,30–33 This 3968 | RSC Adv., 2012, 2, 3968–3977 This journal is ß The Royal Society of Chemistry 2012 View Article Online straightforward workup process and simple handling of catalysts is All glassware and equipment used in the experiments were first extremely practical for industrial-scale catalysis. For versatile cleaned in an aqua regia solution (3 : 1, HCl:HNO3), rinsed in Milli- catalytic applications, these palladium nanoparticles/shells on the Q water, immersed in a base bath (saturated KOH in isopropyl supports can be further protected with organic molecules to alcohol), and then rinsed again in Milli-Q water before use. enhance their dispersity in various solvents.8,33–36 Specifically, we propose that coating palladium nanoshells with alkanethiols will Amine-functionalized silica nanoparticles enhance their stability and dispersity in organic solvents that can Preparation of the amine-functionalized silica nanoparticles allow for their practical use in numerous catalytic reactions. (y100 nm in diameter) was achieved by the modification of Previous research in our laboratory demonstrated that the the well-known Sto¨ber method.37 Specifically, ammonium seeded-growth method can be used to generate palladium shells hydroxide (6 mL, 28–30% NH ) was added to 100 mL of on silica cores having a variety of diameters.29 Silica nanopar- 3 absolute ethanol in a 250 mL two-necked round-bottomed flask. ticles are ideal templates for this methodology because of their Tetraethylorthosilicate (3.4 mL, TEOS) was quickly added into small polydispersity (less than 1%) and their ease of surface the mixture after the solution was warmed to 30 uC. The color of functionalization.37 While our studies did not emphasize the the mixture changed from colorless to milky white within colloidal stability of the palladium nanoshells, we noted that the 30 min. The mixture was stirred overnight, and excess APTMS particles failed to undergo rapid aggregation in an aqueous (0.125 mL) was then added to the mixture. The mixture was solution for several weeks. In contrast, it is known that vigorously stirred with a football-shaped magnetic stirring bar palladium nanoparticles deteriorate rapidly in the solid state, for an additional 6–8 h, and boiled at 88 uC for 1 h to ensure the particularly when exposed to reactive gases (e.g., hydro- covalent bonding of APTMS on the surface of the silica gen).28,38,39 nanoparticles.50–52 About 4–5 mL of absolute ethanol was added One widely used strategy to stabilize metal nanoparticles is to drop-wise during the refluxing step to maintain the same volume coat their surface with a self-assembled monolayer (SAM).8,36,39–41 of the mixture. The final solution containing amine-functiona- In particular, SAMs derived from alkanethiols are densely packed lized silica particles was centrifuged in a RC-3B Refrigerated and defect-free. Consequently, they have the capacity to isolate the Centrifuge (Sorvall Instruments) at 3000 rpm (revolutions per particles from the surrounding environment so they have been minute) for 1 h and re-dispersed in 100 mL of ethanol twice to widely used as protectants for metallic nanoparticles.19,27,34,35,41–44 remove any side products, unattached APTMS, and small silica In the present study, we sought to examine the stabilization of nanoparticles. palladium nanoshells by the adsorption of alkanethiols on their surfaces as well as their catalytic efficiency in the Suzuki–Miyaura cross coupling reaction in organic solvents. While it is widely THPC-reduced Au nanoparticle seeds on amine-functionalized silica nanoparticles Downloaded on 19 February 2013 known that good quality SAMs form on flat (2D) palladium 34,40,45,46 substrates, there are, to the best of our knowledge, A solution of small gold seeds was prepared by the reduction of relatively few studies of SAMs on palladium nanoparti- hydrogen tetrachloroaurate(III) trihydrate with tetrakis(hydrox- 19,27,28,35,41,47,48 Published on 13 March 2012 http://pubs.rsc.org | doi:10.1039/C2RA00883A cles, and the particles are quite small (, 10 nm in ymethyl)phosphonium chloride (THPC) using the modification diameter) compared to those reported here. Notably, these species of a Duff et al. method.53,54 A sodium hydroxide solution and related SAM-coated platinum nanoparticles have been shown (1.0 mL of 1 M NaOH), THPC solution (2 mL of water to be catalytically
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