Topotactic Thermal Oxidation of Sn Nanowires: Intermediate Suboxides and Core-Shell Metastable Structures Andrei Kolmakov,* Youxiang Zhang, and Martin Moskovits Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93117 Received May 17, 2003; Revised Manuscript Received June 11, 2003 ABSTRACT An easily generalizable method is reported for converting metal nanowires topotactically to their stoichiometric oxides. Because many such metal oxides are the active semiconductor elements in sensors, this method is potentially useful in preparing nanowire-based sensor elements. The process is illustrated by converting Sn nanowires fabricated electrochemically in porous alumina templates to SnO 2 in a manner that preserves the wire’s nanostructure. The kinetically controlled oxidation process, which is initially fed by molten tin at the nanowire’s core, gives rise to a number of distinct, coaxial core-shell metastable phases. The process can easily be extended to fabricate free-standing arrays of parallel metal oxide nanowires with possible sensor and optoelectronic applications that are structurally compatible with planar technologies. Quasi-1D nano-objects such as nanowires and nanotubes have been the subject of a growing body of literature. (See, for example, refs 1-4 and references therein.) Carbon nanotubes and metal and compound semiconductor nano- wires grown by a variety of synthetic routes have been reported in the context of their putatively novel function as electronic and optoelectronic device elements, as catalysts and photocatalysts, and as solar cells and sensors. 5-6 Far fewer instances of metal oxide nanowires have been reported, 7-9 although they are potentially attractive objects with new nanometer-scale properties that stand in juxtaposi- tion to the broad array of material traits and applications that macroscopic oxide systems exhibit in much the same way that semiconductor nanowires relate to their bulk counterparts. In particular, metal oxide nanowires are anticipated to have an impact on our understanding of fundamental quasi-1D electronic and transport properties of oxides and their relation to the surface chemistry of the nanowire. These sorts of studies are only now beginning to appear, largely because of the experimental challenge of fabricating and character- izing metal oxide nanowire systems. Very recently, high- yield synthetic routes leading to a variety of oxide nanostruc- tures such as vapor-grown nanowires 10-13 and nanobelts 14-16 or template-synthesized nanowires, 17,18 nanorods, and nano- tubules 19-23 have been reported. In this letter, we describe a high-yield fabrication method for producing metal oxide nanowires using a controlled structure-preserving thermal oxidation of metal nanowires released from porous anodic alumina films. This method is applicable to the synthesis of a wide range of metal oxides. The synthesis of SnO 2 nanowires is used here to illustrate the paradigm. The oxidation process is shown to be highly kinetically controlled, permitting a number of distinct phases to coexist in a roughly coaxial core-shell configuration. The metastable structures are interesting objects in their own right as, for example, nanocables and capacitors. 1,24-26 Although not explicitly illustrated here, the fabrication method, which depends initially on the synthesis of a high-density, ordered nanowire array, may potentially lead to a process that converts the entire array of metal nanowires, topotactically, to their stoichiometric oxides, resulting in free-standing arrays of parallel metal oxide nanowiressstructures inherently com- patible with currently used planar fabrication technologies. Templates consisting of hexagonal close-packed, 2D arrays of nanopores in anodic aluminum oxide (PAO) with pore densities of ∼10 11 cm -2 , pore diameters ranging from 30 to 70 nm, and lengths of ∼50 µm were produced using the two-step anodization technique pioneered by Masuda. 27 Anodization was carried out in 0.3 M oxalic acid solution at 40 V DC and 15 °C. Sn nanowires were electrochemically grown in the nanopores of the PAO (Figure 1) using AC electrodeposition (80 V peak-to-peak, 200 Hz) out of an electrolyte consisting of 0.05 M SnCl 2 ‚2H 2 O acidified to pH ≈ 1 using 36.5% HCl. The as-prepared nanowires were released from their oxide matrix either by etching the template in 0.1 M aqueous NaOH solution or by cracking the template several times and then briefly agitating the broken template ultrasonically in high-purity methanol to release the nanowires from the oxide. The latter method was * Corresponding author. E-mail: akolmakov@chem.ucsb.edu. Phone (805) 893-3700. Fax: (805) 893-4120. NANO LETTERS 2003 Vol. 3, No. 8 1125-1129 10.1021/nl034321v CCC: $25.00 © 2003 American Chemical Society Published on Web 07/02/2003