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J. Phys. Chem. C 2007, 111, 14320-14326
Core/Shell Formation of Gold Nanoparticles Induced on Exposure to N,N-Dimethylformamide: Chemical and Morphological Changes De-Quan Yang and Edward Sacher* Regroupement Que´ be´ cois de Mate´ riaux de Point, De´ partement de Ge´ nie Physique, EÄ cole Polytechnique, C.P. 6079, succursale Centre-Ville, Montre´ al, Que´ bec H3C 3A7, Canada ReceiVed: February 7, 2007; In Final Form: July 5, 2007
Au nanoparticles, ∼15 nm in diameter, were deposited onto both Au and Si substrates and were exposed to droplets of N,N-dimethyl formamide (DMF), which were permitted to dry (∼24 h). When drying was complete, nanoparticle coalescence was found to have occurred in both cases, giving nanoparticles ranging from tens to hundreds of nm in diameter. DMF drying on the Au substrate surface was seen to have occurred by contact line pinning/depinning, to give concentric drying rings, whose edges contained coalesced nanoparticles. No such drying rings were seen for Au nanoparticles after DMF drying on the Si substrate; instead, coalesced Au nanoparticles were found to be uniformly distributed. All Au surfaces, both substrate and nanoparticle, were found to have been covered with a ∼3 nm C,O-containing shell, which formed through the reaction of DMF with Au in the presence of air; the heat produced by this reaction aided the coalescence process, resulting in the larger nanoparticle sizes. The shell was not wet by DMF.
Introduction
TABLE 1: Attributions of Photoacoustic FTIR Peaks of the 100 nm Au Surface after DMF Droplet Drying
The drying of a spilled drop of coffee on a solid surface, which leaves a dense, ringlike deposit along its perimeter, has been studied both theoretically and experimentally,1-9 because this phenomenon is involved in some important applications, such as (a) printing, washing, painting, and coating,3,10 (b) the modification of surface microstructuring,6 and (c) the selfassembly of nanoparticle patterns.11-13 The deposit along the drying perimeter is concentrated from aggregated particles initially dispersed through the entire drop and is commonly found on the evaporation of drops containing dispersed solids. The ringlike appearance on evaporation is related to contact line pinning/depinning,14,15 occurring over and over during the evaporation process, for which several substantially different reasons have been offered. During the evaporation of droplets at a soluble surface,16 microwells are formed at the droplet site, which can be used, for example, as chemical microreactors or as masks for molding microlenses. Besides capillary forces,1 several other reasons have been proposed for the concentric ring formation, among them (1) evaporation rate effects,17 (2) particle-particle interactions,18 (3) particle-substrate interactions,19 and (4) a combination of particles size and substrate surface morphology.20 While these imply strong solvent interaction and, perhaps, even chemical reaction, no such likelihood was previously mentioned. We were, therefore, surprised to find that such chemical reaction actually occurs when a drop of N,Ndimethylformamide (DMF), a commonly used solvent in Au peparation,21 is permitted to evaporate from a surface containing Au nanoparticles: all Au exteriors, nanoparticles as well as substrates, are found to be covered with an ∼3 nm * To whom correspondence should be addressed. E-mail.
[email protected]. Tel.: (514) 340-4711, ext. 4858. Fax: (514) 340-3218.
peak posn (cm-1)
attribution
3500 2800-3000 ∼1700 (shoulder) 1600 1460 1000-1200
ν(O-H) ν(C-Hn) ν(CdO) ν(CdC) δ(C-Hn) ν(C-O-C), δ(C-OH)
C,O-containing shell. Here, we explore both the nanoparticle size increase and the shell formation occurring during DMF drying. Experimental Section A schematic for the “dropping and drying” processing is shown in Figure 1. Commercial n-type Si(100) wafers were used as substrates. In the first case, used in preparing a Au substrate surface, a 10 nm Ti adhesion promoter layer film was first deposited by e-beam evaporation, followed by the evaporation of 100 nm of Au, deposited at a rate of 0.1 nm/s, at a pressure of 2 × 10-7 Torr; the deposition produced a smooth, continuous Au film, whose surface was found to be covered with ∼15 nm Au nanoparticles, as seen in Figure 2a. In the second case, used in preparing Au nanoparticles on a bare Si surface (with ∼2 nm native SiOx) for comparison purposes, the Si sample was used without the Ti adhesion promoter layer. The evaporation of 5 nm of Au formed ∼8 nm nanoparticles, near enough in size to the 15 nm nanoparticles in the first case for the comparative evaluation of their behaviors. Thus, the two cases differ in that, in the first case, Au nanoparticles exist on a smooth Au surface and, in the second case, on a Si surface. About 25 µL of DMF was dropped onto both types of substrates, using a glass pipet. In neither case did the DMF spread spontaneously but formed an ∼8 mm diameter hemi-
10.1021/jp071070w CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007
Core/Shell Formation of Gold Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 39, 2007 14321
Figure 1. Illustration of the DMF dropping and drying process.
spheric drop on the substrates, which took ∼24 h to dry. Following drying on the 100 nm thick Au surface, concentric ringlike deposits were visible; such deposits were absent when drying on the Si substrate. Fast drying was carried out by placing the sample in a mechanical vacuum for a period of ∼3 h. The surface morphology was studied by scanning fieldemission electron microscopy (FEI FBI-DB235), with the magnification parameters given on the photomicrographs (see Figure 2). XPS was carried out in the analysis chamber of a VG ESCALAB 3 Mark II, using non-monochromated Mg KR X-rays (1253.6 eV), at a base pressure of
