Uniform, close-packed monolayer and bilayer arrays of alkanethiol-coated gold nanoparticles have been used as “ink” for microcontact printing (μCP) following the technique of Xia and Whitesides (see Xia, Y.; Whitesides, G. M. Polym. Mater. Sci. Eng. 1997, 77, 596). The process is accomplished in two steps. First, a uniform monolayer of the nanoparticles is self-assembled on a water surface and is transferred intact to a patterned poly(dimethylsiloxane) (PDMS) stamp pad by the Langmuir−Schaefer (LS) method. In the case of multilayer printing, this “inking” step is repeated as many times as desired. Because multilayer arrays are assembled on the stamp pad layer-by-layer, adjacent layers may be made up of the same or different particles. The nanoparticles are transferred to a solid substrate by conformal contact of the stamp pad and the substrate. The technique has been used to print patterned monolayer and bilayer arrays on both hydrophobic and hydrophilic substrates. The quality of the transferred arrays has been verified optically and by transmission electron microscopy (TEM). This new μCP technique should be applicable to any particles that can be spread as a monolayer on a water surface and promises to be useful for nanofabrication.

Five nanometer diameter gold particles encapsulated by alkanethiol molecules have been self-assembled into well-ordered monolayers on a water surface, and these nanoparticle films have been transferred intact onto various solid substrates. The method involves spreading a thin layer of an organic solvent containing the gold nanoparticles on a water subphase that has a controlled surface curvature. As the solvent evaporates, a nanoparticle monolayer free of microscopic cracks and voids nucleates at the center of the apparatus and grows radially outward until it covers practically the entire water surface. This monolayer is transferred from the water surface to a polydimethylsiloxane (PDMS) stamp pad by the Langmuir-Schaefer (LS) technique and is applied to the solid substrate by microcontact printing (µCP). Uniform centimeter-scale films have been produced with this method. The quality of the transferred films has been verified by transmission electron microscopy (TEM). The nanoparticle monolayer is a hexagonal close-packed array with a center-to-center spacing that is approximately equal to the sum of the diameter of the gold particles and twice the height of a self-assembled monolayer (SAM) of the alkanethiol molecules on Au(111). The transferred films are free of multilayer regions and of microscopic voids and grain boundaries over their entire area and exhibit crystalline order across the openings in the TEM grid (∼4000 µm 2 ).

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We report a simple and rapid process for the roomtemperature synthesis of gold nanoparticles using tannic acid, a green reagent, as both the reducing and stabilising agent. We systematically investigated the effect of pH on the size distribution of nanoparticles synthesized. Based on induction time and -potential measurements, we show that particle size distribution is controlled by a fine balance between the rates of reduction (determined by the initial pH of reactants) and coalescence (determined by the pH of the reaction mixture) in the initial period of growth. This insight led to the optimal batch process for size-controlled synthesis of 2-10 nm gold nanoparticles -slow addition (within 10 minutes) of chloroauric acid into tannic acid.

Gold-core platinum-shell (Au@Pt) nanoparticles with ultrathin platinum overlayers, ranging from submonolayer to two monolayers of platinum atoms, were prepared at room-temperature using a scalable, wet-chemical synthesis route. The synthesis involved the reduction of chloroauric acid with tannic acid to form 5 nm (nominal dia.) gold nanoparticles followed by addition of desired amount of chloroplatinic acid and hydrazine to form platinum overlayers with bulk Pt/Au atomic ratios (Pt surface coverages) corresponding to 0.19 (half monolayer), 0.39 (monolayer), 0.58 (1.5 monolayer) and 0.88 (2 monolayers). The colloidal particles were coated with octadecanethiol and phase-transferred into chlroformhexane mixture to facilitate sample preparation for structural characterization. The structure of the resultant nanoparticles were determined to be Au@Pt using HRTEM, SAED, XPS, UV−vis and confirmed by cyclic voltammetry (CV) studies. Monolayers of octadecanethiol coated Au@Pt nanoparticles were self-assembled at an air−water interface and transfer printed twice onto a gold substrate to form bilayer films for electrochemical characterization. Electrochemical activity on such films was observed only after the removal of the octadecanethiol ligand coating the nanoparticles, using a RF plasma etching process. The electrochemical activity (HOR, MOR studies) of Au@Pt nanoparticles was found to be highest for particles having a two atom thick platinum overlayer. These nanoparticles can significantly enhance platinum utilization in electrocatalytic applications as their platinum content based activity was three times higher than pure platinum nanoparticles. ■ INTRODUCTIONNanoparticles with reduced platinum loadings are sought after for electrocatalytic applications in varied fields related to energy and environment. 1 For instance, polymer electrolyte (or proton exchange) membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are efficient, high power density, electrochemical energy devices, wherein platinum nanoparticles are used to catalyze the oxygen reduction reaction (ORR) occurring at cathode, and hydrogen oxidation reaction (HOR)/ methanol oxidation reaction (MOR) occurring at anode in PEMFC/DMFC fuel cells, respectively. Recently, bimetallic nanoparticles have attracted interest not only because of their use as vehicles for maximizing precious metal utilization 2−5 but also because of enhanced electrocatalytic activity, which is attributed to structural and electronic interactions of the constituent metals. 6−9 Typically, bimetallic nanoparticles are comprised of a precious, catalytically active metal like platinum or palladium and a lower-cost, stable/noble metal like gold, ruthenium, cobalt, iron, etc.Bimetallic nanoparticles can be broadly classified as alloy and core−shell nanoparticles. The performance of alloy nanoparticles decays over time, due to loss of electroactive surface area caused by particle agglomeration or by leaching of the alloying nonprecious metal to the surrounding electrolyte. 10 Core−she...