# Direct Oxidation Growth of CuO Nanowires from Copper-Containing Substrates

Authors: Benjamin J. Hansen; Ganhua Lu; Junhong Chen
Journal: Journal of Nanomaterials
 (2008-02-05)

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## Abstract

Controlling the growth of semiconducting nanowires with desired properties on a reproducible basis is of particular importance in realizing the next-generation electronic and optoelectronic devices. Here, we investigate the growth of cupric oxide (CuO) nanowires by direct oxidation of copper-containing substrates at500∘C for 150 minutes at various oxygen partial pressures. The substrates considered include a low-purity copper gasket, a high-purity copper foil, compacted CuO and Cu2O thin layers, and layered Cu/CuO and Cu/Cu2O substrates. The morphology, composition, and structure of the product CuO nanowires were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction, and UV-Visible absorption. Selected oxidation processes have been monitored using a thermogravimetric analyzer. The layering structure of the substrate after oxidation was analyzed to elucidate the growth mechanism of CuO nanowires.


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## Body

## 1. Introduction

One-dimensional nanostructures that can efficiently transport electrical
carriers have been an exciting area of research due to their potential applications
in next-generation optoelectronic [1],
electronic [1], bio-
[2], and gas- [3] sensing devices. Research in the controlled growth of semiconducting nanowires
(NWs) is of particular importance in achieving the desired properties on
a reproducible basis. Various growth techniques have been
employed, including the vapor-liquid-solid growth
[4], epitaxial growth
[5], vapor-solid growth [6], wet chemical methods [7],
and electrospinning [8]. For vapor-solid growth, it
has been known since the 1950s that the oxidation of various metals, including
copper, iron, zinc, and tantalum, at intermediate temperatures, results in a
parallel oxide layering with high-density oxide wires/whiskers at the surface [9]. However, only recently with the advent
of advanced characterization and assembly techniques has this phenomenon
revived interest.The simplicity of the vapor-solid technique, together with its
high NW yield and potential for controllability, makes it an attractive avenue for
NW growth.Cupric oxide (CuO) has been extensively studied due to
its application in high Tc superconductors [10] and
as a heterogeneous catalyst [11]. In addition,
bulk CuO with a known band gap of 1.2 eV has an interesting monoclinic crystal structure belonging to the Mott insulator material class whose electronic structure cannot be described by conventional band theory [12].CuO NW arrays have
been recently applied to various applications including gas sensing [3], field emission [13, 14], and photovoltaic devices [7].The vapor-solid oxidation technique has been widely used to grow CuO NWs [6, 14–17] from copper substrates. Recent
studies on CuO NWs have focused on the
influence of temperature, growth time, and oxidative environment on the growth
rate, size distribution, and areal density of CuO
nanowires [6, 14, 15]. It has been shown that the growth time can be used
to control NW length distribution, the growth temperature can be used to control
the NW diameter distribution [6], and the
O2 and
H2O partial pressures strongly affect the
NW areal density [15, 16]. The surface condition of the copper substrate was also shown to
significantly affect the nanowire growth [17].A common problem with the vapor-solid oxidation growth ofCuO NWs from copper substrates is that the mismatching stress between
the substrate and the oxide layer leads to the flaking of the oxide layer and
hence the poor electrical contact between the NW and the substrate.Herein,
we investigate the growth of CuO NWs directly
from copper-containing substrates, such as CuO and Cu2O layers with and
without a Cu substrate. In addition,
the effect of O2 partial pressure on NW
areal density is investigated.The layering of the substrate after oxidation
is analyzed to elucidate the growth mechanism.The resulting CuO NWs on a conducting/semiconducting substrate can be directly used
for electronic applications.


## 2. Experimental Details

Both copper substrates and copper-containing substrates were used for the growth
ofCuO NWs.Copper substrates used include a low-purity
Cu gasket with high surface roughness and a high-purity
Cu foil (5Ns, 0.2 mm thick) with low
surface roughness.Before the NW growth, the Cu
substrates were cleaned in 1 M HCl solution for ~30 seconds, then rinsed in deionized water, and
finally dried in a dry airflow.Copper-containing substrates used include CuO and Cu2O
thin layers that were created by compacting commercial
CuO (3Ns, <74μm) and
Cu2O (2Ns, <74μm) powders under 103 MPa for ~2 minutes.Likewise, CuO
and Cu2O powders were deposited onto
high-purity Cu foils and compacted to create
Cu/CuO and
Cu/Cu2O
layered substrates.CuO NWs were grown by direct oxidation of the
above substrates in pure O2 or
Ar–O2 mixtures.The oxidation processes were normally carried
out inside a Lindberg/Blue Mini-Mite tube furnace.However, to monitor the
oxidation process, a few substrates were oxidized using a TA Instrument SDT
2960 horizontal thermogravimetric analyzer (TGA).During a typical
oxidation procedure, a substrate was exposed to an Ar–O2 gas mixture of 5 lpm with the desired O2 partial pressure, then heated at ~50°C/min
to an isotherm of 500°C and held for 150 minutes.Upon
cooling down, a pure inert gas flow was introduced to prevent the further
oxidation.The O2 partial
pressures were controlled at 10%, 15%, 20%, and 100% (pure O2) by mixing O2 with Ar at appropriate flow rates using mass flow controllers.Following the oxidation process, each sample was analyzed using a Topcon
ABT-32 scanning electron microscope (SEM) equipped with an energy dispersive
X-ray (EDX) spectrometer.SEM images were used to analyze the NW
morphology and oxide layering and the EDX was used to determine the
elemental composition of the sample.The as-produced NW samples were first
scratched off their growing substrates, and then transferred onto copper grids
and analyzed using a Hitachi H 9000 NAR transmission electron microscope
(TEM) for bright field (BF) TEM and high-resolution TEM (HRTEM) imaging and
selected area electron diffraction (SAED).A Scintag XDS 2000 X-ray
diffractometer (XRD) was used for further analyzing the crystal structure of
the samples.An Ocean Optics 2000 UV-Visible spectrometer was used to
obtain the absorption spectrum (reflection mode) of an as-grown
high-density NW array.The reflection from a high-quality mirror was used as
the reference.


## 3. Results and Discussion

Figures1(a) and 1(c) show low-magnification TEM images of nanowires grown in
pure O2 from a high-purity
Cu foil and a low-purity Cu gasket, respectively.The corresponding HRTEM images
of the NWs are shown in Figures 1(b)
and 1(d), respectively.The
visible stripes identified with different contrasts in Figures 1(a) and 1(c) indicate that as-grown nanonwires bear faceted instead of
circular cross-sections.The SAED pattern of the NW in the inset
of Figure 1(a) confirms the
crystallinity of product nanowires and the direction of
diffraction spots is consistent with the fringe direction shown in
the HRTEM image of Figure 1(b).The lattice spacing of the NWs was measured as
0.252 nm, corresponding to the (1¯11) plane of CuO.The direction
of the fringes in Figure 1(b) is
in parallel with the nanowire axis, while the fringes shown in
Figure 1(d) are not parallel to the
wire axis, with a similar angle of 50° reported
in [9].The two observed
fringe directions may be attributed to the difference in
surface orientation or surface roughness of the
corresponding substrates
[18, 19].The direction in which a
crystal grows on a substrate is dependent on the orientation of
the seed crystal.As CuO NWs grow from a Cu foil, which is polycrystalline
and consists of grains with varying crystallographic
orientations, their growth directions would be different from
each other, leading to dissimilar fringes of the NWs
in HRTEM images.Bright field TEM (a) and HRTEM (b) images of a nanowire grown from
a high-purityCu foil with the inset of
(a) being a SAED pattern; (c) BF TEM image of
a nanowire grown from a low-purity
Cu gasket; and (d) HRTEM image of a
nanowire (the inset) shows fringes that are not parallel to
the nanowire axis.
(a)(b)(c)(d)The low-purity copper substrates were oxidized at four differentO2 partial pressures of 10%,
15%, 20%, and 100%.Upon inspection after
oxidation, the surfaces of the copper substrates were tarnished
from light to dark grey with respect to the 10% to 20% O2 partial pressures, whereas the
substrate oxidized in 100% O2
was tarnished almost completely black with only several small
grey areas.The SEM images of the oxidized copper substrates are shown in Figures 2(a)–2(d).Since the NW density varies
throughout the surface of each individual substrate, the images
shown are those representative areas with more uniformly dense NWs
on each substrate.The NW density and the vertical alignment
increase  with increasing O2 partial pressures, while the average
length and diameter of the NWs seem to decrease with decreasing O2 partial pressures.Most
NWs appear to be tapered like a needle or whisker, which may be due
to the high roughness of the substrate and the subsequent merging
of multiple NWs into one NW during the growth.In the case of pure
O2, the areas of highest
NW density were those that tarnished black.And the light to dark
grey samples with 10 to 20% O2 partial pressures correspond to the low to medium NW
density.Since bulk CuO is known to be
grey to black in color and
Cu2O is red, it can be
roughly inferred that the oxide surface and possibly the NWs are CuO.However, in many cases the color
of nanostructures is very different from that of their
corresponding bulk structures due to morphology-induced interference and scattering.Thus, the observed color can only be used as a
rough guide and the actual composition of the surface and the NWs
must be obtained with the aid of advanced characterization
techniques.A UV-Visible absorption spectrum of a high-density NW
array similar to the sample shown in Figure 2(d) is presented in Figure
3.The band gap of the
as-grown NWs was estimated from the absorption spectrum by fitting
a tangential line passing the midpoint of the first absorption
edge (750–940 nm) and reading
out the x-intercept.In this way, a band gap of
approximately 1.2 eV (1030 nm) corresponding to
bulk CuO can be extracted.SEM images of oxidized copper substrates at variousO2 partial pressures: (a) 10%, (b) 15%,
(c) 20%, and (d) 100%.
(a)(b)(c)(d)Figure 3
UV-Visible absorption spectrum of aCuO nanowire array.To further analyze the oxide layering structure, an oxide flake
was carefully removed from a copper substrate oxidized in
pureO2.The SEM image of
the flake is shown in Figure 4(a),
with an amplified figure shown in the inset.Three layers can
be distinguished: a bottom layer that lies directly above the
copper substrate, an intermediate layer that the NWs grow from, and
a top NW layer.The XRD spectrum of an oxidized Cu substrate is shown in Figure 4(b), which indicates the presence of both Cu2O and
CuO.However, no information on
the individual layer composition can be extracted.The
theoretical layering composition (labeled in Figure 4(a)) has been discussed in [8, 9]
with CuO as the NWs and the
intermediate layer and Cu2O
as the bottom layer.The theoretical hypothesis on the NWs has been proved by our experiments.(a) SEM images and (b) XRD spectrum of an oxidized copper
substrate.
(a)(b)To confirm the rest of the layering composition, EDX spectra
were obtained from the top and the bottom surfaces of an oxide
flake similar to that shown in Figure4.In addition, EDX
standards were produced from compacted
CuO (3Ns) and
Cu2O (2Ns) powders.The
resulting spectra and region analysis are presented in Figure 5.The spectra were made up of ~100000
counts, taken at identical parameters of 5 keV
and 10000× magnification with a 60
degree sample tilt.Regions of 0.400–0.600 keV
and 0.620–1.080 keV were created about the OKα and
CuLα peaks, respectively, and the number and the percentage of
gross counts in each region was determined.The relatively
small fraction of oxygen counts detected is due to the low
energy X-rays produced and the subsequent reabsorption upon exiting
the sample material.The results indicate that the percent of
gross counts for oxygen and copper taken from the bottom of the
flake matched that of the standard Cu2O.Likewise, the percent of gross counts for
oxygen and copper taken from the top of the flake matched that of
the standard CuO.Furthermore, a
simple calibration has been carried out to correlate the gross
count ratio of oxygen to copper with the molar ratio of
O to Cu
(or x value of CuOx,
where x equals 0, 0.5 and 1 for
Cu, Cu2O, and CuO, resp.) in
standard reference samples.The resulting x value is
0.99  for the top layer and 0.48  for the bottom layer in our
sample.Our experimental results indicate distinct oxide layers
and directly agree with the theoretical speculation [8, 9] of the
bottom layer of Cu2O and
the intermediate layer of CuO.The
layering structures can be used to interpret the CuO NW growth mechanism as elaborated in [6]: Cu2O acts as a precursor for the second oxidation step
to CuO where the slow
production rate of CuO results in
a localized CuO vapor pressure and a continuous growth mode for CuO NWs by
means of the vapor-solid growth mechanism.EDX spectra and region analysis: (a)CuO standard; (b) top
of the oxide flake; (c) Cu2O standard; and (d) bottom of the oxide flake.
(a)(b)(c)(d)CompactedCuO and Cu2O layers with and without the Cu foil support were also used to grow CuO NWs in pure O2
at 500°C for 150 minutes.The SEM images of
the oxidized substrates are presented in Figure 6.High-density NW growth on the Cu/Cu2O
substrate, medium NW growth on the Cu/CuO substrate, low NW growth on
the Cu2O substrate, and
no NW growth on the CuO substrate have
been observed.The results indicate the importance of
Cu2O to CuO conversion for the growth of CuO NWs as well as the presence of a Cu substrate for high-density NW growth.The importance
of the Cu substrate is possibly in its
role as a Cu ion source and in
establishing a chemical potential gradient for the diffusion
of Cu ions to the substrate surface.In Figure 6(a), the relatively
thick compacted Cu2O layer
with high-density NWs at its surface is seen at the top
side of the Cu substrate and the
familiar layering scheme is observed at the bottom of the
substrate.Also note that the oxide layers of Figures 6(a)–6(c) are uniformly structured with less or without pores, whereas
the presumably unchanged CuO sample
in Figure 6(d) has a porous
structure consistent with compacted powders.SEM images of the substrates after oxidation: (a)Cu/Cu2O, (b) Cu/CuO, (c) Cu2O
thin layer, and (d) CuO thin layer.
The insets show the amplified top surface details of the
corresponding substrates.
(a)(b)(c)(d)To determine if the oxidation took place, theCu2O and CuO thin
layers were processed separately in a TGA.From the TGA results
shown in Figure 7, it is clear that the Cu2O thin
layer oxidized whereas the CuO thin
layer did not.The small decrease in the weight for the CuO thin layer is a consequence of the thermal
expansion of the TGA and can be ignored.XRD spectra were also
obtained from each sample and compared to those of
unprocessed Cu2O and CuO in
Figure 8.For those samples
which oxidized to form CuO
NWs (Cu2O, Cu/Cu2O, Cu/CuO), the XRD
spectra indicate the presence of both CuO and Cu2O, suggesting the
importance of the oxide layering on NW growth.TGA results of the oxidation process from (a)Cu2O thin layer (initial mass:  49.598 mg) and
(b) CuO thin layer (initial mass: 38.061 mg).
(a)(b)XRD spectra of the substrates involvingCu2O (a) and
CuO (b).
(a)(b)


## 4. Conclusion

CuO NWs have been successfully grown
from both copper and copper-containing substrates through
direct oxidation of the substrates in oxidative
environments.The as-grown NWs have a band gap of ~1.2eV
(1030 nm) corresponding to
bulk CuO.Oxygen partial pressure was
found to significantly affect NW growth.The vertical alignment of the NW increases with increasing O2
partial pressures, while the average length and diameter of the
NWs seem to decrease with decreasing O2 partial pressures.Analyses on the
oxidized substrates indicate CuO as the
NWs and the intermediate layer and Cu2O as the bottom layer.The oxidation of
compacted CuO and Cu2O layers with and without a Cu substrate suggests the importance of Cu2O
to CuO conversion for the growth
of CuO NWs, as well as the importance of
a Cu substrate for high-density NW growth.



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*Source: 830474-2008-03-17.xml*