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Optical and magnetic properties of CuO nanowires grown by thermal
oxidation
M. Vila, C. Díaz- Guerra
a)
and J. Piqueras
Departamento de Física de Materiales, Facultad de Físicas, Universidad Complutense de Madrid,
Ciudad Universitaria s/n, E-28040 Madrid, Spain.
a)
e-mail: cdiazgue@fis.ucm.es
ABSTRACT
CuO nanostructures with different morphologies, such as single-crystal nanowires,
nanoribbons and nanorods, have been grown by thermal oxidation of copper in the (380-900)
ºC temperature range. Cathodoluminescence spectra of the nanostructures show a band
peaked at 1.31 eV which is associated to near band gap transitions of CuO. Two additional
bands centred at about 1.23 eV and 1.11 eV, suggested to be due to defects, are observed for
nanostructures grown at high temperatures. The magnetic behaviour of nanowires with
lengths in the range of several microns and diameters of (50-120) nm has been investigated.
Hysteresis loops of the nanowires show ferromagnetic behaviour from 5 K to room
temperature.
P.A.C.S.: 61.46.Km, 75.75.-c, 78.60.Hk, 78.67.-n
Confidential: not for distribution. Submitted to IOP Publishing for peer review 28 January 2010
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1. Introduction
Nanowires and other elongated nanostructures are a subject of wide and increasing
interest due to the variety of their potential applications in nanotechnology. In particular,
intensive research efforts have been devoted to the synthesis and investigation of the physical
and chemical properties of semiconducting oxides such as ZnO, SnO
2
, TiO
2
, In
2
O
3
or, to a
lesser extent, CuO. Cupric oxide (CuO) is a semiconductor often referred as a narrow gap
material with band gap of 1.2 eV e.g. [1,2] and monoclinic structure. Larger band gaps
between 1.6 eV and about 2.1 eV have also been reported [3-5]. This oxide has useful
catalytic properties, e.g ref. 6, and potential applications in lithium-ion batteries, gas sensing
or field emission devices [7-9] and was also investigated because of its relationship with
cuprate high-T
c
superconductors. CuO nanowires have been synthesized by thermal
decomposition of a precursor [10], chemical routes [11] or electrospinning [12]. In addition,
thermal oxidation of copper has been reported by several groups to be an efficient and simple
nanowire growth method [13-17]. In this case, nanowires of different dimensions as well as
larger structures are obtained as a function of the temperature and time of the oxidizing
treatment. Electrical conductivity and field emission of nanowires and nanorods obtained by
thermal oxidation of copper have been previously studied [17,18] but other physical
properties, precisely optical and magnetic properties, have been much less investigated.
Optical behaviour of CuO nanowires has been mainly assessed by absorption techniques,
while luminescence data are scarce and refer to nanowires obtained by other methods [4].
Bulk CuO is an antiferromagnetic material with Néel temperatures reported in the (213-230)
K range. However, surface spins can dominate magnetization in nanostructures because of
uncompensated exchange coupling. This may lead to ferromagnetic-like behaviour at low
temperatures, as reported in hydrothermally synthesized CuO nanorods [19] or in CuO
elongated nanoparticles [20]. In this work, CuO nanowires have been grown by thermal
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oxidation of compacted copper powder at different temperatures. The morphology and
structure of the nanowires have been studied by X-ray diffraction (XRD), scanning electron
microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), while
their chemical composition was assessed by energy dispersive X-ray microanalysis (EDX).
Luminescence was investigated by cathodoluminescence (CL) in the SEM and magnetic
measurements were performed with a superconducting quantum interference device (SQUID).
2. Experimental method
Cu powder of 99.9% purity (Sigma-Aldrich) was used as starting material. The powder
was compacted to form disks of about 2 mm thickness and 7 mm diameter. The samples were
then annealed at 380 ºC, 500 ºC, 600 ºC, 700 ºC or 900 ºC under air flow. Oxidation of copper
takes place during the thermal treatments, leading to the growth of copper oxide
nanostructures on the disk surface. As will be explained below, treatments at 380
o
C yield a
high density of CuO nanowires. In order to study the influence of the annealing time on the
dimensions of the obtained nanowires, the 380 ºC treatments were performed between 3 and
48 hours. The duration of the treatments at higher temperatures (500-900 ºC) was 6 hours.
The structure of the wires was first investigated by XRD and grazing incidence XRD with a
Philips X’Pert PRO diffractometer using CuK
radiation. The morphology of the
nanostructures was investigated by SEM with a Leica 440 or an FEI Inspect S scanning
electron microscope. SEM-CL measurements were carried out at 90 K with an electron beam
energy of 20 keV. The emission was monitored in the visible range (4.0-1.55 eV) by using a
Hamamatsu R428P photon counting device and in the near infrared (IR) range (1.55-0.75 eV)
with a Hamamatsu R5509-43 photomultiplier. A Bruker AXS Quantax spectrometer was used
for EDX measurements. HRTEM investigations were carried out with a field emission JEOL
3000F microscope operating at 300 kV. Two methods were used in order to collect the CuO
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nanowires from the treated disks for HRTEM and magnetic measurements as well. The first
one was to scratch very gently the surface of the disks with a sharp, non-conductive, plastic
tweezer. The second one was to sonicate the disk in butanol, collect the suspension, and wait
until the liquid was evaporated. For HRTEM investigations, drops of the solution containing
the nanostructures were deposited onto carbon coated grids. TEM images and electron
diffraction measurements of the products obtained following both methods showed CuO
nanowires only. For magnetic measurements, 10 to 15 mg of nanowires detached from the
pellets were introduced in a sample capsule of the SQUID avoiding any contact with metallic
tools. Hysteresis loops at 5 K and 300 K were recorded up to applied field values of ± 50 kOe.
3. Results and discussion
SEM images of the Cu disks before thermal treatments show an almost featureless
appearance, as shown in supporting information (see figure S1 available at stacks.iop.org/ …).
After the 380 ºC annealing treatments, the surface of the disks appears uniformly covered by
nanowires whose density and length increase with the treatment time. Many of the wires can
be described as nanoneedles with a pointed tip. Figures 1(a) and 1(b) show the nanowire
distribution in samples annealed at 380 ºC for 3 and 14 hours respectively. In the latter case,
the highest aspect ratio of the nanowires is obtained, with diameters in the range 50-120 nm
and lengths between 3 and 10
µ
m. The density of nanowires, defined as the number of wires
per unit area, was estimated from high-magnification SEM micrographs. For samples treated
for 3 h the nanowires density is ~ 10
8
cm
-2
, while it increases nearly one order of magnitude
for samples treated during 48 h. The morphology and density of the nanowires grown using
the above-mentioned conditions suggests the potential use of these nanostructures as field
emitters. In samples treated at higher temperatures, the nanostructures are wider and most of
them have rod or ribbon shapes rather than the needle-like shape of the wires grown at 380
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ºC. This is shown in the image of the structures grown after the 500 ºC annealing for 6 hours
[Figure 1(c)]. The morphology of the nanostructures grown in the 700-900
0
C temperature
range is also different. Groups of nanocolumns with polygonal sections and rods with
diameters of about 300-500 nm and up to 12 µm long appear grouped on the surface of the
disks [figure 1(d)], although their density is lower than that of the nanowires grown at lower
temperatures.
XRD patterns of the samples treated at 380 ºC show peaks corresponding to Cu, Cu
2
O
and CuO. The peaks of Cu have a much lower relative intensity in the grazing incidence
(GIXRD) patterns, indicating that they are related to the non-oxidised, inner part of the disk.
Figure 2(a) shows the GIXRD pattern of the sample shown in figure 1(b). In the XRD pattern
of the samples prepared at 600 ºC, or at higher temperatures, only Cu
2
O and CuO peaks are
observed, with dominant CuO peaks in grazing incidence diffractograms [figure 2(b)]. XRD
measurements were also carried out in nanowires separated from the treated disks, as shown
in figure 2(c). All the diffraction maxima observed in the corresponding patterns were
indexed to the CuO monoclinic structure (JCPDS card 048-1548). As stated in the
introduction, several authors have reported the growth of CuO nanowires by thermal
oxidation of copper. The mechanism of thermal growth of nanowires in air involves the initial
formation of CuO
2
which acts as precursor of CuO. The XRD observations can be then
explained by the formation of CuO nanowires from an intermediate Cu
2
O layer. The stress at
the CuO-Cu
2
O interface together with diffusion processes are thought to be responsible for
the growth of the CuO nanowires [14]. At moderate temperatures the Cu peaks from the
substrate appear along with the peaks of the oxides while treatments at higher temperatures
lead to a thicker oxide layer so that the patterns consist only of CuO and Cu
2
O peaks. The
previously reported CuO nanowires grown by oxidation have similar morphology and phase
purity to that reported here. In addition, the presence of a low density of wires after oxidation
at temperatures of 600 ºC and higher has been previously observed [14, 15, 21]. However,
