Size and shape evolution of core–shell nanocrystals C. J. Zhong,* W. X. Zhang, F. L. Leibowitz and H. H. Eichelberger Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, USA. E-mail: cjzhong@binghamton.edu Received (in Columbia, MO, USA) 20th April 1999, Accepted 19th May 1999 The findings of an investigation of temperature-manipu- lated size and shape evolution for pre-formed decanethio- late–encapsulated gold nanocrystals are described. Monolayer-encapsulated metal nanoparticles are of consider- able technological interest because of the potential electronic, optical, magnetic, catalytic and sensing applications emerging from the core–shell combinations. 1,2 Part of our motivation stems from the opportunities of manipulating such structural properties as interfacial building blocks towards fine-tunable electrode nanomaterials. 3 One important advance emerging from various synthetic strategies 4–16 is the two-phase method, which was first reported by Schiffrin and coworkers 4,5 and has been extensively utilized. 1,4–8 This relatively simple method, which involves transferring an aqueous tetrachloroaurate (AuCl 4 2 ) precursor into an organic phase followed by reduction in the presence of thiols (RSH), has been proven very effective for synthesizing core sizes ranging from 1.5 to 5 nm by controlling the RSH/Au ratio and reaction temperature. 4–8 Narrow-size preparation and shape control are the ultimate synthetic challenges. Relatively little is, however, known about shape control for such composite nanomaterials, 12–16 though examples have recently been demonstrated for platinum nanoparticles by manipulating precursor ratios, 14 or introducing ‘shape-inducing reagent’. 15 In view of the molecular and crystal natures of such core–shell systems, size and shape are inherently a dynamic process and an evolution may occur as a result of changes in chemical potentials of the particles. Such a process is the basis of crystallography involving nucleation, dissolution and growth, 18,19 but to our knowledge has not been studied for the core–shell type systems. Here, we describe the preliminary results of an investigation of such a process of pre- formed thiolate-encapsulated gold nanoparticles by manipulat- ing temperatures. Thiolate-encapsulated gold nanoparticles were prepared first by the two-phase method. 4,6 Briefly for decanethiolate-encap- sulated nanoparticles, after transferring an aqueous AuCl 4 2 by tetraoctylammonium bromide (98%) into a toluene phase, decanethiol (DT, 96%) was added at a 2+1 ratio of DT/Au, and followed by adding an excess (12 3) of an aqueous reducing agent (NaBH 4 , 99%). The reaction produced a dark-brown solution of the DT-encapsulated nanoparticles, which was then subjected to two types of sample handling procedures. In the first procedure, the solvent was removed by rotary evaporator (at ca. 50 °C) yielding a black and waxy product, DT/Au(1). In the second procedure, the solution was brought to ca. 110 °C under reduced pressure for ca. 30 min. Upon this treatment, the solution showed a color change from brownish to pinkish. We note here two observations for the heating treatment. First, simply heating the solution at temperatures between 50 and 110 °C did not lead to any color change. Secondly, a color change occurred under conditions of the concentration rising to ca. 10-fold and the temperature to 100–115 °C. A systematic study of the related factors is in progress. This treatment was performed after the formation of the encapsulated nanoparticles, a procedure different from temperature control during the synthesis. 8 After removing solvent, the gray and powdery product, DT/Au(2), was collected. Both 1 and 2 were subjected to subsequent cycles of suspension in ethanol and acetone and centrifugation for at least four times to ensure a complete removal of non-nanoparticle materials. These two samples, DT/Au(1) and DT/Au(2), were first examined by transmission electron microscopy (TEM). Fig. 1 presents two representative TEM images from the two samples. For the DT/Au(1) sample [Fig. 1(a)], the result displays an average core size of ca. 2.0 nm with ca. 80% populations within the range 1.5–2.5 nm. The crystal shapes, though not clearly identifiable with the TEM resolution, appear non-uniform. These microscopic features are in good agreement with those reported previously for samples prepared under similar condi- tions. 8 A previous study has determined that the most likely shape for such a core size is a truncated octahedron. 12 In contrast to the results shown for the DT/Au(1) sample, two striking features are evident for the DT/Au(2) sample [Fig. 1(b)]. First, it displays an increased core size with a narrow distribution in which more than 90% populations are within the range 4.7–5.7 nm (average 5.2 nm). Second, a close examina- tion of the particle shapes indicates a ‘hexagon’ outline for an observable percentage of the particles. Clearly, an evolution in both size and shape of the particles has occurred from DT/Au(1) to DT/Au(2) samples. While not reported for thiolate-encapsu- lated nanocrystals, temperature-induced changes in size and morphology, including quasimelting, coalescence and sintering of fine particles, have indeed been reported for metal or metal oxide crystals or nanocrystals. 18,19 The size and shape evolution of our encapsulated nanoparticles in the solution may involve a Fig. 1 TEM micrographs of two DT-encapsulated Au nanocrystal samples: (a) DT/Au(1) and (b) DT/Au(2) on carbon-coated TEM grids. The insert represents an enlarged view of the indicated area. Chem. Commun., 1999, 1211–1212 1211 Published on 01 January 1999. Downloaded on 27/10/2014 16:51:09. View Article Online / Journal Homepage / Table of Contents for this issue