VOLUME 2, NUMBER 12, DECEMBER 2002 © Copyright 2002 by the American Chemical Society
CuO Nanowires Can Be Synthesized by Heating Copper Substrates in Air Xuchuan Jiang, Thurston Herricks, and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received August 14, 2002; Revised Manuscript Received October 11, 2002
ABSTRACT This paper describes a vapor-phase approach to the facial synthesis of cupric oxide (CuO) nanowires supported on the surfaces of various copper substrates that include grids, foils, and wires. A typical procedure simply involved the thermal oxidation of these substrates in air and within the temperature range from 400 to 700 °C. Electron microscopic studies indicated that these nanowires had a controllable diameter in the range of 30−100 nm with lengths of up to 15 µm by varying the temperature and growth time. Electron diffraction and high-resolution TEM studies implied that each CuO nanowire was a bicrystal divided by a (111) twin plane in its middle along the longitudinal axis. A possible mechanism was also proposed to account for the growth of these CuO nanowires.
Cupric oxide (CuO) has been extensively studied because of its close connection to high-Tc superconductors.1 The valence of Cu and its fluctuation are believed to play important roles in determining the superconductivity of various types of cupric compounds.2 Cupric oxide has also been known as a p-type semiconductor that exhibits a narrow band gap (1.2 eV) and a number of other interesting properties.3 For example, monoclinic CuO solid belongs to a particular class of materials known as Mott insulators, whose electronic structures cannot be simply described using conventional band theory.4 Recent studies by several groups indicate that CuO could exist in as many as three different magnetic phases.5 It was a 3D collinear antiferromagnet at temperatures below 213 K. When the temperature was raised, it first became an intermediate noncollinear incommensurate magnetic phase up to 230 K and then acted like a 1D quantum antiferromagnetic material. With regard to its * To whom correspondence should be addressed. E-mail: xia@ chem.washington.edu. 10.1021/nl0257519 CCC: $22.00 Published on Web 10/24/2002
© 2002 American Chemical Society
commercial value, CuO has been widely exploited for use as a powerful heterogeneous catalyst to convert hydrocarbons completely into carbon dioxide and water.6 Cupric oxide is also potentially useful in the fabrication of lithium-copper oxide electrochemical cells, and the relation between the microstructure of CuO solid and its potential as a cathode material has been systematically investigated.7 As it has already been demonstrated for many other semiconductors (e.g., Si, CdSe, and ZnO),8 it is reasonable to expect that the ability to process CuO into nanostructured materials should enrich our understanding of its fundamental properties and enhance its performance in currently existing applications. Cupric oxide can be prepared as nanoparticles of various sizes using a number of methods, with notable examples including mechanical milling of commercial powders,9 activated reactive evaporation of copper,10 and alcohothermal decomposition of copper acetate.11 None of these methods, however, seems to be suitable for the preparation of CuO as
Figure 1. SEM images of CuO nanowires prepared by directly heating copper TEM grids in air at 500 °C for (A-C) 4 h and (D) 2 h. (E, F) SEM images of CuO nanowires that were formed on the surface of a copper wire (0.1 mm in diameter) by heating at 500 °C for 4 h.
nanowires. However, Pfefferkorn et al. (in the 1950s) found that both CuO and Cu2O whiskers could be formed by oxidizing copper substrates at elevated temperature.12 The whiskers were characterized by a relatively short length (100 nm), and the surface coverage of these whiskers was also fairly low. Several groups recently attempted to synthesize CuO nanowires, albeit their efforts have been met with limited success. In 1334
one demonstration, Wang et al. proposed that CuO nanowire might be involved as a byproduct when Cu2O nanowires were formed by reducing copper sulfate with hydrazine in a basic solution.13 In another recent study, Yang et al. observed the formation of polycrystalline nanowires containing both CuO and Cu2O when Cu2S nanowires were oxidized by O2 at elevated temperatures.14 Herein we describe a simple procedure for the synthesis of uniform CuO nanowires with Nano Lett., Vol. 2, No. 12, 2002
Figure 2. (A) TEM image of CuO nanowires prepared by heating a copper grid at 500 °C for 4 h. (B) Electron diffraction pattern taken from a random assembly of these CuO nanowires. (C) TEM image of an individual CuO nanowire showing the twin plane in the middle of this wire (indicated by an arrow). (D) High-resolution TEM image showing the twin boundary of a nanowire. (E) Electron diffraction pattern recorded from an individual CuO nanowire. Indices without subcript t refer to the upper side of the nanowire shown in (C), and indices with subscript t refer to the other side. The e beam was parallel to the [110] axis. These results indicated that each CuO nanowire was a bicrystal: the growth directions were [1h11] and [111], respectively.
controllable diameters in the range of 30-100 nm and with lengths of up to 15 µm. Structural analysis by electron diffraction and high-resolution TEM indicated that each nanowire was a bicrystal divided by a (111) twin plane in the direction parallel to the longitudinal axis. In a typical procedure, the copper substrate was cleaned in an aqueous 1.0 M HCl solution for ∼20 s, followed by repeated rinsing with distilled water. After it had been dried under a N2 gas flow, it was placed in an alumina boat (Al23, Alfa Aesar, MA) and immediately heated to the set-point temperature (at ambient pressure) in a VWR box furnace. We have tested a number of copper substrates: TEM grids (cat. no. 01801, Ted Pella, Redding, CA), foils (99.9% purity, 0.05 mm thick, EM Science, Gibbstown, NJ), and conventional electrical wires (99.9% purity, 0.1 mm in diameter, ARCOR, Northbrook, IL). CuO nanowires of similar quality could be grown on the surfaces of all of these copper substrates. SEM images were obtained using a field-emission microscope (Sirion, FEI, Portland, OR) operated at an acceleration voltage of 5 kV. TEM images were taken with a Philips EM-430 microscope operated at 80 kV. The highresolution TEM image was recorded on a TOPCON 002B Nano Lett., Vol. 2, No. 12, 2002
microscope operated at 200 kV. The X-ray diffraction (XRD) pattern was recorded from a powder sample using a Philips PW-1710 diffractometer (Cu KR radiation, λ ) 1.5406 Å) at a scanning rate of 0.02°/s in 2θ ranging from 10 to 70°. The surfaces of all of the copper substrates were tarnished (when viewed by the naked eye) after they had been treated in air at elevated temperatures. Further examination under an optical or electron microscope indicated the formation of wirelike nanostructures over the entire surfaces of these substrates. Figure 1A-C shows the SEM images of a copper grid after it had been heated in air at 500 °C for 4 h. All nanowires were mainly grown in the planes parallel to the surface of this TEM grid. Although the entire surface of this grid was covered by a high density of nanowires, those protruding from the edges (Figure 1B) appeared to be straighter, much longer, and more uniform in diameter as compared with wires formed on the top surface (Figure 1C). The length of these nanowires could be conveniently controlled by changing the growth time. Figure 1D shows the SEM image of another TEM grid after it had been heated in the box furnace at 500 °C for 2 h. In comparison with the nanowires shown in Figure 1B, a growth rate of ∼3 µm/h 1335
Figure 3. TEM images of CuO nanowires prepared by heating copper grids in air for 4 h at various temperatures: (A) 400, (B) 500, and (C) 600 °C. The corresponding size distributions of these nanowires are shown in graphs D to F. These results suggest that the diameter of the CuO nanowires could be varied in the range of 30 to 100 nm by controlling the reaction temperature.
could be derived. Figure 1E and F shows the SEM images of a copper wire (0.1-mm diameter) after it had been heated in air at 500 °C for 4 h. Similar to that of grid samples, the surface of this microscale wire was also completely covered by a dense array of uniform CuO nanowires. Because of the surface curvature of this substrate, each nanowire was grown in the direction essentially perpendicular to the support. We further characterized the size, structure, and crystallinity of these nanowires using TEM and electron diffraction. The original copper TEM grids could be directly used for some of these studies. Figure 2A shows the TEM image of an array of nanowires protruding from the edges of a copper 1336
grid, indicating the uniformity that could be achieved for these nanowires. Some of these wires appeared to be thicker than they should be as a result of overlapping between wires at different levels. Figure 2B shows the electron microdiffraction pattern recorded from a random assembly of nanowires. All rings could be indexed to the diffraction peaks of monoclinic CuO rather than those of cubic Cu2O, indicating the phase purity of these nanowires. The nanowires could also be removed from the original copper grid (or other substrates) by rinsing with ethanol, redeposited onto a carbon-coated TEM grid, and used for high-resolution TEM studies. Figure 2C shows the TEM image of an individual Nano Lett., Vol. 2, No. 12, 2002
nanowire whose middle is clearly divided by a twin plane along the longitudinal axis. Figure 2D displays a highresolution TEM image, further confirming the bicrystallinity of this nanowire. Each side of this wire was, indeed, a single crystal with a well-defined fringe space pattern, and a twin defect could be observed in the middle. The interplanar spacing for each side was 2.52 and 2.32 Å, respectively. These two values corresponded well with the spacing calculated for {1h11} and {111} planes in monoclinic CuO (cell ) 4.69 Å × 3.43 Å × 5.13 Å, β ) 99.55°).15 Figure 2E shows a diffraction pattern that would be typically observed when the electron beam was focused on an individual nanowire along the [110] direction. The mirrorimage relationship between the two sets of diffraction spots confirmed the formation of a bicrystalline structure within each nanowire. The growth direction for each side of this twined nanowire could be derived from the diffraction spots as [1h11] and [111], respectively. We also investigated the influence of temperature on the growth of CuO nanowires. In these studies, copper foils (∼0.25 cm2 in area) were placed in the furnace and heated for 4 h at different temperatures in the range of 300 to 800 °C. We found that CuO nanowires were formed only in a temperature window between 400 and 700 °C. When the temperature was lower than 400 °C, very few short whiskers were formed, and the surface was essentially coated by small particles. As the substrate temperature was increased beyond 700 °C, the surface was covered by a dense film of micrometer-sized particles. Some of these particles were characterized by well-defined facets, indicating their high crystallinity. For samples prepared between 400 and 700 °C, CuO nanowires were obtained as the major product. It was also found that the diameter of these nanowires had a strong dependence on the temperature. Figure 3 shows the TEM images of three samples that were heated at 400, 500, and 600 °C for 4 h, respectively. These images clearly indicated that the lateral dimension of these nanowires could be reduced from ∼100 to ∼50 and ∼30 nm when the reaction temperature was increased from 400 to 500 and 600 °C, respectively. Two mechanismssvapor-liquid-solid (VLS)16 and vaporsolid (VS)17shave been most commonly used to account for the growth of nanowires in the gas phase. On the basis of our SEM and TEM observations, the VLS mechanism could be excluded because none of our CuO nanowires was terminated in particles. As a result, the VS mechanism seems to be responsible for the growth of CuO nanowires observed in the present study. We note that this mechanism has recently been applied to explain the formation of nanowires from a variety of metal oxides.18 The present procedure for forming cupric oxide nanowires differs from other systems in that a precursor (rather than the direct oxidation of copper) is involved. When copper is oxidized in air, the major product is Cu2O, and CuO is formed slowly only through a second step of oxidation. In this case, Cu2O served as a precursor to CuO. The reactions involved in the entire synthesis can be summarized as the following, with the second one Nano Lett., Vol. 2, No. 12, 2002
Figure 4. XRD pattern of a copper foil (∼0.25 cm2 in area) after it had been heated in air at 500 °C for 4 h. The majority of this copper foil had been converted into Cu2O, with only a small amount of CuO on the surface (in the form of nanowires).
functioning as the rate-determining step for the formation of CuO vapor:19 4Cu + O2 f 2Cu2O
(1)
2Cu2O + O2 f 4CuO
(2)
The slow rate for the formation of CuO ensures a relatively low vapor pressure for this material in the reaction chamber and thus a continuous growth mode and uniform diameter for the CuO nanowires. On the basis of these arguments, the surface of the copper grid shown in Figure 1A should be mainly covered by a dense film of Cu2O, with only a very small amount of CuO in the form of nanowires. This speculation was confirmed by the XRD pattern shown in Figure 4, which was taken from a copper foil (∼0.25 cm2 in area) after it had been heated in air at 500 °C for 4 h. The temperature effect could also be understood by taking into account the dependence of the Gibbs free energy of reaction 2 on temperature. Since the change in entropy (∆S) for reaction 2 has a negative sign, the change in free energy (∆G) for this reaction will change sign (from negative to positive) when the temperature is sufficiently high. At this point, the formation of CuO will be terminated, and thus no nanowires will be observed. On the basis of the standard thermodynamic data from the Handbook,20 this transition temperature was estimated to be around 964 °C. This number agreed reasonably well with the temperature (800 °C) observed in the present study. As the temperature dropped below 400 °C, the formation of CuO became too slow to maintain a sufficiently high vapor pressure for CuO, and thus no nanowire growth would occur on the copper substrate. In summary, we have demonstrated a simple and convenient route to the facial synthesis of uniform CuO nanowires (30-100 nm in diameter) supported on solid substrates. Both TEM and electron diffraction studies indicated that these nanowires were bicrystals, with a (111) twin plane sitting in the middle of each nanowire along the longitudinal axis. These nanowires could be grown up to tens of micrometers in length without branching or entanglement. In addition to their direct use as a heterogeneous catalyst and as a class of 1337
semiconducting nanostructures for device fabrication, these nanowires can also serve as templates from which to prepare nanowires made of other materials (Cu2O, Cu, and Cu2S) by reacting with gases such as H2 and H2S.21
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Acknowledgment. This work has been supported in part by a Career Award from the NSF (DMR-9983893), an AFOSR-DURINT subcontract from SUNY-Buffalo, and a Fellowship from the David and Lucile Packard Foundation. Y.X. is an Alfred P. Sloan Research Fellow (2000-2002) and a Camille Dreyfus Teacher Scholar (2002-2007).
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