Article pubs.acs.org/JPCC
Low-Temperature Synthesis of Au/Polyaniline Nanocomposites: Toward Controlled Size, Morphology, and Size Dispersity Yu-Fong Huang,† Young Il Park,† ChengYu Kuo,† Ping Xu,†,∥ Darrick J. Williams,‡ Jian Wang,§ Chi-Wen Lin,⊥ and Hsing-Lin Wang*,† †
C-PCS, Chemistry Division, ‡Center for Integrated Nanotechnology, MPA, and §Structure/Properties Relations, MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ∥ Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China ⊥ Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin, Taiwan, Republic of China S Supporting Information *
ABSTRACT: Varying the concentration and molar ratio between aniline and AuCl3 leads to a series of Au/polyaniline (PANI) nanocomposites with a wide range of morphologies ranges from nanosphere, nanorod, to complex nanosheet assemblies. These nanocomposites consist of an ensemble of very small Au nanoparticles held together by a PANI matrix, which arise from the oxidation of aniline by metal ions. Using a steric stabilizer such as poly(vinyl pyrrolidone), we have demonstrated control over size dispersity and morphology to achieve monodispersed nanocomposites. These nanoparticles embedded in the PANI matrix are coupled electronically and thus lead to a shift of plasmonic absorption from 500 nm down to 900 nm depending on the Au nanoparticle size and morphology. PANI that is bound to the nanoparticles can be washed off with N-methyl-2-pyrrolidone (NMP) to release individual nanoparticles to form a stable Au nanoparticle solution. On the basis of the above results, we propose a possible formation mechanism of hybrid nanocomposites encompassing Au nanoparticles in a PANI matrix.
I. INTRODUCTION Novel nanocomposites with metal and/or metal oxide nanoparticles that are either decorated on the polymer surface1,2 or embedded in a polymer matrix3−7 are a very important class of composite materials that exhibit collective properties combining the polymer and nanostructured metals/ metal oxides. The emergent properties of these nanocomposites are typically dominated by how these two materials come together and the assembly manner of nanoparticles in the polymer matrices. The unique electronic and optical properties of these nanocomposites are also impacted by the size and morphology of these nanoparticles as well as the distance between individual particles. Of particular interest are the polyaniline (PANI)/Au nanocomposites, as they show application potential for DNA detection,8 glucose sensing,9 catalysis,10−12 and nonvolatile memory devices.10,13 Developing a new method of making nanocomposites involving PANI and gold nanoparticles has been an area of strong research for the past few decades due to potential emergent properties that can be used to enable applications that have not been realized. Very recently, Wang et al. prepared novel dandelion-like Au/PANI nanocomposites and demonstrated their application as surfaceenhanced Raman spectroscopy (SERS) nanosensors.14 Au/ PANI nanocomposites prepared by the interfacial polymer© 2012 American Chemical Society
ization approach render either nanorod structure or nanosphere structure, and the size of the Au nanoparticles embedded within the polymer matrix is estimated to be 2−50 nm.15,16 In terms of the synthesis of Au nanoparticle and PANI composites, a very common approach is to simply immerse a PANI nanofiber in aqueous AuCl3 solution, which allows the PANI to reduce the Au3+ and form nanoparticles typically immobilized on the fiber to form composites.17 In contrast to recent studies where aniline molecules are oxidized by AuHCl4 results in the formation of very small nanoparticles immobilized on the surface of PANI nanofibers,18,19 Li et al. used the microemulsion polymerization method to create a PANI/Au nanocomposite whose size is ∼50 nm, and the Au nanoparticles scarely populated in the nanocomposites had an average size distribution of 2−3 nm.20 Very recently, Wang et al. showed a very interesting yet simple (template-free) method of making PANI/Au nanocomposites by mixing hydrochloroaurate with aniline monomer at room temperature to create nanocomposites with Au nanoparticle size ranges from 10 to 50 nm. The dandelion-like composites reveal strong SERS Received: April 12, 2012 Revised: May 1, 2012 Published: May 3, 2012 11272
dx.doi.org/10.1021/jp303538z | J. Phys. Chem. C 2012, 116, 11272−11277
The Journal of Physical Chemistry C
Article
response toward detection of Hg2+.14 Despite the intense efforts of making all sorts of PANI and metal nanoparticle composites, controlled synthesis of functional nanocomposites with desired structure and functionality remains a topic of strong interest, as they possess properties strongly related to size, morphology, and structures. In this paper, we report a low-temperature synthesis that leads to the formation of Au/PANI nanocomposites with PANI serving as a binder of the densely populated Au nanoparticles; however, these nanoparticles do not form aggregates, yet the distance between Au nanoparticles is close enough such that the electronic interactions between individual Au nanoparticles are coupled. The collective interactions among Au nanoparticles result in strong plasmonic absorption at a wavelength that is morphology- and size-dependent. These Au nanoparticles embedded in the PANI matrix appear to show very good size dispersity and are distributed very homogeneously throughout the composite particle. We also find that the presence of PVP steric stabilizer allows for fine-tuning the size and size distribution of the Au/PANI nanocomposites. Moreover, we have shown the formation of Au/PANI nanocomposites with a wide range of morphologies ranging from nanosphere to nanorod to nanosheet assemblies comprised of Au nanoparticles whose size is dependent on the AuCl3 concentration, as manifested by X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies.
III. RESULTS AND DISCUSSION With a fixed concentration of aniline, as we increase the concentration of AuCl3 from 1 mM to 50 mM, the morphology of the final nanocomposite changes from nanorod (
