Structural and optical characterization of spinel type cobalt oxide
nanoparticles embedded in amorphous silicon oxide matrix prepared by a
hybrid PVD /PECVD process
Karthika Lakshmi Kolipaka1, Volker Brueser1, Antje Quade1, Jan Schaefer1, Harm Wulff2, Franz Faupel3
1
Leibniz Institute for Plasma Science and Technology, INP Greifswald e.V., Felix-Hausdorff-Str. 2, 17489
Greifswald, Germany; 2Ernst Moritz Arndt University Greifswald, Institute of Physics, Felix-Hausdorff-Str. 6,
17489 Greifswald, Germany; 3University of Kiel, Institute for Materials Science – Multicomponent Materials,
Kaiserstr.2, 24143 Kiel, Germany
Abstract: Composite materials consisting of semiconducting nanoparticles embedded in a dielectric matrix have
attracted a great deal of attention because of their use in catalytic, gas-sensing, optical and magnetic applications.
In this work, a combination of physical vapour deposition (PVD) and plasma enhanced chemical vapour
deposition (PECVD) techniques is employed. Preparation of nanosized spinel type cobalt oxide particles
embedded in an amorphous silicon oxide matrix (Co3O4-SiOx composite) has been carried out by RF magnetron
sputtering of cobalt and simultaneous plasma polymerization of hexamethyldisilazane (HMDSN) and further
calcination. Phase composition studied using X-ray diffraction (XRD) analysis reveals the presence of crystalline
Co3O4 with an average particle size of around 35-45 nm. The bonding nature of the film investigated using X-ray
photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) reveals silicon oxide as the
dominant chemical composition of the matrix. The SEM micrograph depicts the amorphous and porous nature of
the composite surface. The optical properties are similar to that of spinel type cobalt oxide with absorption peaks
around 475 nm and 775 nm. The Tauc’s plot indicates the energy band gap for direct allowed transition as 1.95
eV and 1.38 eV. These obtained values are close to previously reported values.
Keywords: PVD/PECVD, nanocomposites, spinel type cobalt oxide,
hexamethyldisilazane, silicon oxide
1. Introduction
The use of spinel type cobalt oxide is found in
many
applications
such
as
catalysts,
supercapacitors, phosphate ion sensors and CO
sensors [1-4]. Especially cobalt oxide dispersed
in amorphous silica matrix provides interesting
applications such as optical sensors [5]. In the
following contribution we present the
preparation of nano sized spinel type cobalt
oxide embedded in an amorphous silicon oxide
matrix.
2. Experimental
Initially Co – a: SiCNH nanocomposites have
been prepared by the sputtering of cobalt (PVD)
and simultaneous plasma polymerization of
hexamethyldisilazane (HMDSN) (PECVD)
(submitted elsewhere). The process was carried
out in an RF magnetron sputtering system
operating at 13.56 MHz frequency. These Co –
a: SiCNH nanocomposites are calcinated in
ambient air at 500°C for 2h. The calcination
yielded in the formation of nanosized spinel
type cobalt oxide particles embedded in
amorphous silicon oxide matrix.
Phase composition of the composites prepared
on borosilicate glass was investigated using Xray diffraction (XRD) using CuKα radiation (θ2θ Diffractometer Siemens D5000). The
measurements were made in grazing incidence
set at an angle of 0.5° to the sample surface. The
chemical bonding on the composite surface was
studied using X-ray photoelectron spectroscopy
(XPS). X-ray photoelectron spectrophotometer
Axis Ultra, Kratos, Manchester, GB was used to
analyze composites prepared on borosilicate
glass. A monochromatic X-ray radiation of Al
Kα was used. For the wide scan measurement
150 W X-ray radiation was used with pass
energy of 80 eV. For the high resolution
measurement 225W X-ray radiation with pass
energy of 10 eV was used. Charge neutralization
was applied. All the spectra were referenced to
the aliphatic carbon at binding energy of 285.0
eV.
The topography of composites prepared on
aluminum substrate was studied using scanning
electron
microscopy
(SEM)
with
a
backscattered electron detector. Fourier
transform infrared spectra (FTIR) of the
composites prepared on aluminum substrate
were taken using Perkin Elmer spectrometer in
attenuated total reflection (ATR) mode. Optical
absorption measurements of the composites
were
performed
using
a
UV-Vis
spectrophotometer (Analytik Jena) in 1 cm
cuvettes. For this the composites were prepared
on borosilicate glass.
The XRD pattern shows sharp peaks which
correspond to Co3O4. It represents crystalline
Co3O4 with only small broadened profiles of all
measured reflections. Only small broadened
profiles correspond with nearly perfect crystals
(that means only small lattice defects) as well as
large particle sizes. From the pure physical line
profile (Fourier transforms) the mean particle
sizes was calculated to be of 35 nm, from the
Williamson-Hall plot mean particle sizes of 45
nm was obtained. No peaks corresponding to
SiOx are found in the XRD pattern. The FTIR
results discussed further indicate the presence of
SiOx bonds. XRD and FTIR results together
indicate that silicon oxide is not present in
crystalline form.
XPS
Fig 2 gives the survey scan XPS spectrum of the
Co3O4-SiOx composite. The elements cobalt,
oxygen, silicon, and carbon are detected on the
surface of the composite.
3. Results and discussion
XRD
X-ray diffraction pattern obtained for the
Co3O4-SiOx composite is shown in Fig 1.
Figure 2. Survey scan XPS spectrum of Co3O4-SiOx composite.
Figure 1. XRD pattern of Co3O4-SiOx composite.
The highly resolved measured spectra of C 1s,
Si 2p, and Co 2p3/2 peaks are studied further.
Fig 3 depicts the highly resolved measured
spectrum of the Si 2p peak. The binding energy
of the Si 2p peak represents Si(-O)2 and Si(-O)3
bindings[6]. The chemical composition of the
matrix is dominated by SiOx bonds [6, 7] which
is further supported by the FTIR results
discussed below. The highly resolved measured
spectrum of Co 2p3/2 peak shows presence of
cobalt oxides on the surface of the composite.
the presence of amorphous silicon oxide is
confirmed with the FTIR spectrum (as no
silicon oxide bonds were observed in the XRD
results, indicating that no crystalline silicon
oxide is present). Therefore it can be said that
nano sized spinel type cobalt oxide particles are
embedded in the amorphous silicon oxide
matrix.
This
presents
an
interesting
incorporation of magnetic particles inside an
amorphous matrix.
SEM
The SEM micrograph of the Co3O4-SiOx
composite is depicted in Fig 5.
Figure 3. Highly resolved measured Si 2p peak of Co3O4-SiOx
composite.
ATR-FTIR
The ATR-FTIR spectrum of the composite
shows
several
characteristic
bonds
corresponding to silicon oxide. Fig 4 shows the
ATR-FTIR spectrum of the Co3O4-SiOx
composite.
Figure 5. SEM micrograph of Co3O4-SiOx composite. Scale is
show in the picture.
The SEM micrograph in Fig 5 indicates the
porous and amorphous nature of the surface of
Co3O4-SiOx composite. The crystalline Co3O4
nanoparticles can be observed as crystals in
some areas of the micrographs.
Optical measurements
Figure 4. ATR-FTIR spectrum of Co3O4-SiOx composite.
Several peaks in the FTIR spectrum correspond
to silicon oxide bonds. The peaks at 1100 cm-1
and 960 cm-1 correspond to SiO asymmetric
stretching vibrations. The peak at 680 cm-1
corresponds to Si-H bending vibrations. Thus
The optical absorption spectrum of Co3O4 and
Co3O4-SiOx composite is shown in Fig 6. The
absorption spectrum of Co3O4 is included for
comparison. In the absorption spectrum of
Co3O4 peaks are observed around 475 and 775
nm. The same peaks are also observed in the
Co3O4-SiOx composites, but, the peak at 475 nm
is not as significant as in the pure Co3O4. This
can be because of the influence of the silicon
oxide matrix. But, however, the absorption
spectrum is similar to that of pure Co3O4.
4. Conclusion
Spinel type Co3O4 nanoparticles of average size
35-45 nm embedded in amorphous silicon oxide
matrix have been prepared using a hybrid
PVD/PECVD process and further calcination.
The structural, topological, and optical
properties of the Co3O4-SiOx composites were
studied.
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Figure 6. Optical absorption spectra of Co3O4 and Co3O4-SiOx
composites.
The Tauc plot of (αhν) 2 vs hν is shown in Fig 7.
Upon extrapolation of the curve it intercepts the
X-axis at 1.38 eV and 1.95 eV. This gives the
energy band gap for direct allowed transition as
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elsewhere [9]. Thus the crystalline Co3O4
nanoparticles embedded in the amorphous
silicon oxide matrix are characterized with
direct band gap energies of 1.95 eV and 1.38
eV.
Figure 7. Plot of (αhν) 2 vs hν of Co3O4-SiOx composites.
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