NANO LETTERS
Topotactic Thermal Oxidation of Sn Nanowires: Intermediate Suboxides and Core−Shell Metastable Structures
2003 Vol. 3, No. 8 1125-1129
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 SnO2 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 nanowires 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 juxtaposition 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 characterizing metal oxide nanowire systems. Very recently, highyield synthetic routes leading to a variety of oxide nanostructures such as vapor-grown nanowires10-13 and nanobelts14-16 or template-synthesized nanowires,17,18 nanorods, and nanotubules19-23 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 * Corresponding author. E-mail:
[email protected]. Phone (805) 893-3700. Fax: (805) 893-4120. 10.1021/nl034321v CCC: $25.00 Published on Web 07/02/2003
© 2003 American Chemical Society
alumina films. This method is applicable to the synthesis of a wide range of metal oxides. The synthesis of SnO2 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 compatible 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 ∼1011 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 SnCl2‚2H2O 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
Figure 1. SEM cross-sectional image of a porous alumina template showing the electrochemically grown Sn metal nanowires inside the nanopores. The white bar indicates 1 µm.
used to form the nanowire suspensions out of which single tin nanowire devices were fabricated for this study. Depending on the type of experiment envisioned, the metal nanowires were produced either in the form of free-standing arrays in which each nanowire was supported by its neighbors in the manner of cornstalks in a heavily planted field, the entire assembly resting on the aluminum substrate on which they were formed, or as ropes or cables of nanowires lying on a convenient substrate such as a SiO2covered Si wafer (Figure 2) or a flat polycrystalline Pt surface. The thermal oxidation process used to convert the metallic tin nanowires to nanowires of tin oxide without the loss of structure was carried out in a process approximating rheotaxial growth and thermal oxidation28 (RGTO), which is widely used to prepare dispersed SnO2 films out of compacted tin mesoparticles,29 the major difference being that deliberate and crucial steps were taken to maintain the shape and integrity of the Sn nanowire during its conversion to tin oxide (which, of course, occurred with the requisite diameter increase corresponding to the density change on going from metallic tin to the oxide). This form-retaining step is especially important for low melting point metals (mp ) 232 °C for tin). Hence, before carrying out the complete oxidation (a process that covers a temperature range beyond 550 °C28), a thin supporting oxide skin was grown on the surface of the tin nanowires by annealing the sample in air at ∼200 °C for 2 h. In situ temperature-programmed X-ray diffraction (TPXRD) analysis (Scintage-X2 powder diffractometer, Cu KR Xrays: λ ) 0.154 nm) was used to follow the compositional and structural changes of the metal nanowires upon oxidation. 1126
Figure 2. SEM image of a topotactically oxidized individual Sn nanowire. Inset on the left shows the as-prepared metal Sn nanowire deposited on the surface of an oxidized (300-nm SiO2) Si wafer. The same nanowire after its stoichiometric oxidation to SnO2 is shown on the right. The nanowire is outfitted with Au/Ti electrodes for electrical measurements.
Signals of measurable intensity were obtained by carrying out the measurements on nanowire bundles. Oxidation was performed in one of two ways: fast or slow. In the fast mode, the Sn nanowires were subjected, in air, to a sequence of temperature steps (100 °C/step) from room temperature to 550 °C with a heating rate of 0.5 °C/s between steps. X-ray diffraction measurements were carried out immediately upon reaching the programmed temperature at each step. The typical fast XRD run required ∼10 min at each temperature step, the entire process taking in excess of 1 h to complete. In the slow-oxidation mode, the same series of temperature steps were used; however, on reaching a desired temperature, the sample was held at that temperature for 2 h before X-ray diffraction measurements were performed, and the acquisition time per spectrum was increased to 45 min. In the slowoxidation mode, a run required ∼10 h to complete, permitting kinetically limited chemical and structural transformations to proceed further toward completion. The dynamic range provided by the two time scales allowed the effect of kinetics to be assessed and compared. A typical TPXRD result showing the product evolution on oxidizing a Sn nanowire bundle using the slow-oxidation approach is shown in Figure 3 as a function of temperature. Figure 4a summarizes the intensity changes as a function of temperature of the major X-ray diffraction peaks associated with the three observed tin-containing phases.28 The observed, room-temperature diffraction pattern of β-Sn has an anomalously intense (200) reflection when compared with a polycrystalline β-Sn standard.30 This is due to the preferential crystalline growth of the tin nanowires in the pores Nano Lett., Vol. 3, No. 8, 2003
Figure 3. Temperature-programmed X-ray diffraction spectra of an array of Sn nanowires liberated from the alumina matrix in which it was synthesized and deposited onto a Pt support. Sn (200) reflections dominate the diffraction pattern up to the melting point of tin (the ordinate has an exponential scale). As-deposited free Sn nanowires are covered by a thin SnO2 skin indicated by observed (weak) SnO2 (110), (200), and (101) reflections even at room temperature. The data were normalized to a Pt substrate peak and offset for clarity of presentation.
along the [100] direction. On the basis of a Debye-Scherer analysis of the width (fwhm) of the (200) peak,31 the crystalline domain size along the axis of the nanowire was evaluated to be ∼102-103 nm. Such high crystallinity and orientationally coherent growth in nanowires electrochemically grown in PAO templates had been previously reported and ascribed to the effect of confinement on nucleus formation and growth.32 When the melting point of β-Sn is reached and surpassed (T > 232 °C) (Figures 3 and 4a), the associated reflections decrease in intensity, and those of an intermediate phase, romarchite SnO, appear and grow.33 The X-ray reflections associated with this new phase do not show the preferential orientation of its Sn precursor, and the size of its crystallites is reduced, indicating that the new phase is nucleating and growing incoherently out of the now-molten tin. The relatively slow growth of the tin suboxide and dioxide peaks implies that the oxidation process is kinetically controlled. Because the Sn nanowire was not originally a single crystal spanning the whole length of the nanowire but a number of rather large single-crystal domains presumably separated by grain boundaries, one expects the most rapid oxide growth to occur at grain boundaries and at the defects (Figure 4b). When molten, the tin forms a liquid pool confined to and structurally determined by the outer oxide layer. At first, the molten tin appears to extend in a connected region covering the entire nanowire. When the rapid oxidation at the grain boundaries proceeds sufficiently completely for the oxide to transect the nanowire, the molten tin will become confined to the interior of the regions that were originally the single-crystal grains. Subsequent oxide growth proceeds at a rate presumably determined by ion (likely oxygen via a vacancy mechanism) diffusion through the Nano Lett., Vol. 3, No. 8, 2003
Figure 4. (a) Relative intensities of the major peaks for three observed phases (Sn, SnO, and SnO2) as a function of temperature obtained using the slow-oxidation mode. Rapid formation of tin suboxide and dioxide begins near the melting point of tin. Both oxides are observed to coexist in the temperature range of 200450 °C. The numbers correspond, approximately, to the temperature ranges where the prominent morphological changes depicted in b take place. The curves connecting the experimental points were drawn as a guide to the eye. (b) Schematic presentation of the major observed structural and compositional changes occurring during the thermal oxidation of a tin nanowire. A nanowire segment containing two grain boundaries is shown (1). The unevenness in the thickness of the newly formed oxide films (2-4) is assumed to be due to the enhanced oxidation rate along grain boundaries and other crystal defects.
various oxide skins surrounding the molten tin regions. On average, therefore, the tin, tin suboxide, and tin dioxide layers form approximately coaxial regions (Figure 4b). At temperatures near and slightly above the melting point of tin, all three phases are found to coexist. Thereafter, pairs of phases coexist until all of the tin is consumed. For example, at 200 °C, all three phasessSn, SnO and SnO2 (casseterite34)scoexist, but at 500 °C, coexisting SnO and SnO2 are visible. Annealing in oxygen for 2 h at 600 °C leads to the complete oxidation of the tin nanowires into casseterite SnO2 nanowires with average crystallite dimensions in the submicrometer, range. The existence of SnO2 features at even the lowest temperatures whose intensities remain unchanged until ∼250 °C is surpassed, suggests that the initially formed oxide coat that presumably required a minimum of ion diffusion to form was primarily composed of SnO2 and that subsequent oxidation was kinetically controlled by ion diffusion through the growing oxide layer, 1127
Figure 5. Reappearance of the Sn (200) and (101) reflections upon recooling tin nanowires that had been oxidized using the fastoxidation mode demonstrates the fact that (while it lasts) the tin for the oxidation process is provided by molten tin at the core of the topotactically oxidized nanowire.
as is the case for most metal oxidation reactions. It is this kinetic control that first gives rise to SnO and subsequently to SnO2 when the system is heated to a high enough temperature for a sufficient time. Some peak broadening implies a reduction in crystallinity of the resultant SnO2 nanowires. As with the suboxide, there is no preferential orientation of the SnO2; the formation of the initial oxide skin ensures that the overall nanowire morphology is preserved during the oxidation (Figure 2). Running the oxidation in the fast-oxidation mode produces similar results with one major difference. On cooling the sample at the end of the experiment, metallic Sn peaks reappear (but with significantly reduced intensity) (Figure 5). Because the morphological integrity and shape of nanowires remain intact after oxidation (Figure 2) and no segregation of tin was detected by TEM, the reappearance of X-ray reflections belonging to Sn metal confirms a mechanism in which molten tin is wholly encased within a coherent oxide skin during the oxidation, at least until all of the metallic tin is oxidized. (That is, the reappearance of metallic Sn X-ray reflections on cooling implies that the reaction did not proceed to completion when fast oxidation was carried out.) This also illustrates that oxygen diffusioncontrolled metal oxidation processes (whose rates are normally limited by vacancy migration35) can take many hours to complete even in so narrow a nanowire. (The mechanism outlined above has also been proposed for RGTO-grown submicrometer tin particles and films.36-38) The observations reported in this letter are, therefore, consistent with a kinetically controlled oxidation mechanism leading to the formation of sequential, approximately coaxial core-shell structures formed and stabilized over the entire length of the nanowire. Recently, apart from the diffusion, O2 dissociative adsorption was proposed to be the ratelimiting step in the oxidation kinetics of tin.39,40 The length of time we observed for complete oxidation to occur is consistent with oxygen ion diffusion through the various oxide layers being the rate-determining process, as has been postulated for the oxidation of tin particles and films of more conventional forms to tin oxide. Moreover, the time required to complete the oxidation of a tin nanowire is consistent with known diffusion constants for oxygen diffusion in these 1128
oxides. Assuming a diffusion time of t ≈ d2/D (where D, the diffusion constant35 of oxygen in SnO2 at 400-600 °C, is ∼10-15-10-14 cm2 s-1 and d, the diameter of the nanowire, is ∼60 nm), one estimates several hours for t, in accordance with what is observed. In summary, we have illustrated an easily generalizable strategy for topotactically converting metal nanowires to their oxides. The tin nanowires used in this demonstration were fabricated electrochemically in porous anodic oxide templates. The preservation of the shape of individual nanowires as well as the morphology of an entire array of parallel nanowires upon oxidation suggests a promising route for fabricating parallel nanowire-array structures for sensor or optoelectronic applications. Tin can exists in two stable oxidation states (Sn(II) and Sn(IV)), and indeed, the oxidation of the nanowire proceeds by first forming the suboxide SnO and then the dioxide SnO2. Tin suboxide, SnO, is a p-type semiconductor. Its metastability at certain temperatures makes that material interesting in its own right.41 A crucial aspect of the structure-preserving oxidation process is the formation of a thin oxide skin through a low-temperature process before the full oxidation is carried out in temperature steps ending at a temperature elevated enough to form the stoichiometric, thermodynamically stable oxide (in this case, SnO2). The oxidation process was shown to proceed by a kinetically controlled process in which several metastable core-shell approximately coaxial structures are formed, of which, initially, the inner core is molten tin. Acknowledgment. We thank Dr. J. P. Zhang for his help with HRTEM. This work made extensive use of the MRL Central Facilities at UCSB supported by the National Science Foundation under award no. DMR96-32716. References (1) Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Nature 2002, 420, 57-61. (2) Cobden, D. H. Nature 2001, 409, 32-33. (3) Lieber, C. M. Solid State Commun. 1998, 107, 607-616. (4) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353389. (5) Avouris, P.; Hertel, T.; Martel, R.; Schmidt, T.; Shea, H. R.; Walkup, R. E. Appl. Surf. Sci. 1999, 141, 201-209. (6) Fukuoka, A.; Higashimoto, N.; Sakamoto, Y.; Inagaki, S.; Fukushima, Y.; Ichikawa, M. Microporous Mesoporous Mater. 2001, 48, 171-179. (7) (a) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. Angew. Chem., Int. Ed. 2002, 41, 2405-2408. (b) Comini, E.; Faglia, G.; Sberveglieri, G.; Pan, Z. W.; Wang, Z. L. Appl. Phys. Lett. 2002, 81, 18691871. (8) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659-663. (9) Li, C.; Zhang, D. H.; Liu, X. L.; Han, S.; Tang, T.; Han, J.; Zhou, C. W. Appl. Phys. Lett. 2003, 82, 1613-1615. (10) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1274-1279. (11) Yang, P. D.; Yan, H. Q.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Choi, H. J. AdV. Funct. Mater. 2002, 12, 323-331. (12) Li, C.; Zhang, D. H.; Han, S.; Liu, X. L.; Tang, T.; Zhou, C. W. AdV. Mater. 2003, 15, 143. (13) Gao, P. X.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1265312658. Nano Lett., Vol. 3, No. 8, 2003
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(31) Debye-Scherer. The grain size was evaluated using the equation L ) 0.94 l (B cos q)-1, where l ) 0.154 nm (the wavelength of Cu KR radiation), B is the measured full width at half-maximum of the (200) Sn reflection in radians, and q is the corresponding diffraction angle. Because instrumental effects and inherent broadening also contribute to the fwhm of the reflection, the 100-nm domain size that is determined is a lower bound, confirmed later by HRTEM micrographs of resultant SnO2 nanowires, which regularly exhibit grain sizes larger than 1 µk. (32) Routkevitch, D.; Bigioni, T.; Moskovits, M.; Xu, J. M. J. Phys. Chem. 1996, 100, 14037-14047. (33) SnO. romarchite (JCPDS 6-395). (34) SnO2. casseterite (JCPDS 21-1250). (35) Hellmich, W.; Boschvonbraunmuhl, C.; Muller, G.; Sberveglieri, G.; Berti, M.; Perego, C. Thin Solid Films 1995, 263, 231-237. (36) Huh, M. Y.; Kim, S. H.; Ahn, J. P.; Park, J. K.; Kim, B. K. Nanostruct. Mater. 1999, 11, 211-220. (37) Dieguez, A.; Romano-Rodriguez, A.; Morante, J. R.; Sangaletti, L.; Depero, L. E.; Comini, E.; Faglia, G.; Sberveglieri, G. Sens. Actuators, B 2000, 66, 40-42. (38) Pan, X. Q.; Fu, L. J. Appl. Phys. 2001, 89, 6048-6055. (39) Weaver, J. F.; Cambell, T. J.; Hoflund, G. B.; Salaita, G. N. J. Electron Spectrosc. Relat. Phenom. 2000, 106, 81-91. (40) Kamp, B.; Merkle, R.; Maier, J. Sens. Actuators, B 2001, 77, 534542. (41) Calderer, J.; Molinas, P.; Sueiras, J.; Llobet, E.; Vilanova, X.; Correig, X.; Masana, F.; Rodriguez, A. Microelectron. Reliab. 2000, 40, 807-810.
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