Microstructure and optical absorption properties of Au-dispersed CoO thin films

Article abstract of DOI:10.1016/j.jallcom.2011.02.154

Au nanoparticles dispersed cobalt monoxide (CoO) composite films were fabricated via a chemical solution approach combined with a spin-coating processing. Au particles were spherical approximately and uniformly dispersed in the amorphous CoO matrix. The mean diameters of Au particles in the Au/CoO thin films with 40 mol% Au are about 30 nm. The optical absorption peaks due to the surface plasmon resonance (SPR) of Au particles were observed in the UV-vis absorption spectra in the wavelength range of 550-650 nm. The SPR peaks exhibit a red shift and intensify with increasing Au content from 10 to 40 mol%, but show a blue shift and weaken from 40 to 60 mol%. The band gap E_g decreases with increasing Au contents from 10 to 40 mol% but increases by further increasing Au content.

Full text of DOI:10.1016/j.jallcom.2011.02.154

Journal of Alloys and Compounds 509 (2011) 5731–5735
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Microstructure and optical absorption properties of Au-dispersed CoO thin films
∗
Shi-Jing Wang, Bo-Ping Zhang , Li-Ping Yan, Wei Deng
Beijing Key Lab of New Energy Materials and Technology, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Au nanoparticles dispersed cobalt monoxide (CoO) composite films were fabricated via a chemical solu-
tion approach combined with a spin-coating processing. Au particles were spherical approximately and
uniformly dispersed in the amorphous CoO matrix. The mean diameters of Au particles in the Au/CoO
thin films with 40 mol% Au are about 30 nm. The optical absorption peaks due to the surface plasmon
resonance (SPR) of Au particles were observed in the UV–vis absorption spectra in the wavelength range
of 550–650 nm. The SPR peaks exhibit a red shift and intensify with increasing Au content from 10 to
Received 30 September 2010
Received in revised form 23 February 2011
Accepted 27 February 2011
Available online 5 March 2011
Keywords:
Au/CoO
Thin film
Chemical synthesis
Amorphous
Optical properties
4
0 mol%, but show a blue shift and weaken from 40 to 60 mol%. The band gap Eg decreases with increasing
Au contents from 10 to 40 mol% but increases by further increasing Au content.
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
under a reduced pressure and found the catalytic activity of the CoO
powders in the oxidation of CO. Okamoto et al. [17] investigated
the effects of the starting cobalt salt on the cobalt–alumina inter-
Nano-composite films consisting of metal nanoparticles embed-
ded in dielectric matrices have attracted great attention because
of their enhanced nonlinear optical properties [1,2] and possi-
ble applications for all-optical switching devices [3,4]. In the past
decades, the nonlinear optical properties of metal nanoparticles
action modes and cobalt dispersion in CoO/Al O3 catalysts. CoO
2
and it-based materials were also studied in the field of magnetic
materials [18,19]. However, to the best of our knowledge, CoO as
the matrix of the composite nonlinear optical films has not been
reported so far in the nonlinear optical area. In this study, Au-
dispersed CoO composite thin films were successfully prepared by
using a spin-coating method for the first time. The microstructure
and the optical absorption properties of the thin films were inves-
tigated with a special emphasis on the influence of Au contents. An
enhanced nonlinear optical absorption property was found in the
Au-dispersed CoO composite films, whereby it is expected that the
present Au/CoO composite films would have increased potential
applications in the nonlinear optical field.
embedded in various dielectric matrices such as SiO2 [5,6], Al O3
2
[
7], TiO2 [8], NiO [9] and ZnO [10] were much investigated which
is due to a strong surface plasmon resonance (SPR) of metal
nanoparticles in the visible range. The SPR frequency, a signifi-
cant characteristic parameter of the nonlinear optical properties,
depends on the content, size, and shape of the metal particles, as
well as the dielectric properties of metal particles and the sur-
rounding matrix material [11]. Cobalt monoxide (CoO) seems to
be an attractive matrix material due to its relatively wide band gap
(
2.6 eV) [12] and high refractive indices (2.33) [13]. CoO and its
2
.
Experimental
compounds have been investigated in many areas because of their
exceptionally physical and chemical properties. In the area of elec-
tric apparatus, Yu et al. [14] studied the capacitance capacity of CoO
Au/CoO composite films were prepared using a spin-coating technique with
chemical solution method. The precursor solution with a concentration of
a
crystallites prepared by means of developing Co O4 nanoplatelets
0.2 mol/L was prepared by mixing raw materials with CH3OCH2CH2OH (99.989%)
and Co(NO3)2·6H2O (99.0%) supplied by Beijing Beihua Fine Chemical Co., Ltd., China.
The mixture was magnetically stirred for 5 h to obtain the homogeneous solution
and then aged at room temperature for 8 h. HAuCl4·4H2O (99.99%, Beijing Beihua
Fine Chemical Co., Ltd., China) was added into the precursor solution with ratios
of Au/(Au + CoO) = 0, 10, 20, 30, 40, 50 and 60 mol% followed by stirring for 5 h and
aging at room temperature for 8 h. The Au/CoO film was spin coated on a SiO2 glass
substrate with a dimension of 2 cm × 2 cm, which was ultrasonically cleaned in an
acetone solution and dried in the oven in advance. The glass substrate was absorbed
in a circular salver, and the aged Au/CoO solution was dropped uniformly to the
substrate. The substrate was rotated at a low rate of 200–300 rpm for 5 s, and then
at a higher one of 2000–3000 rpm for 15 s. After one coating, the precursor film
3
in the molten salt and Dhage et al. [15] discussed the influence of
an alkali earth (Ba/Sr) element in small quantities on non-linear
electrical properties of SnO ·CoO·Ta O5 ceramic. In the applica-
2
2
tion of heterogeneous catalysis, EI-Shobaky et al. [16] prepared CoO
powders by the thermal decomposition of basic cobalt carbonate
∗ Corresponding author. Tel.: +86 1062334195.
E-mail address: bpzhang@ustb.edu.cn (B.-P. Zhang).
◦
was first pyrolysed at 200 C for 30 s to remove the solvent and a small amount of
0
925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.02.154

5
732
S.-J. Wang et al. / Journal of Alloys and Compounds 509 (2011) 5731–5735
◦
Fig. 2. XPS survey spectrum of Au/CoO thin film with 40 mol% Au annealed at 600 C.
Fig. 1. XRD patterns of Au/CoO thin films with different contents.
Fig. 3 shows the XPS spectra of Co 2p and Au 4f electrons of
◦
Au/CoO thin composite film with 40 mol% Au annealed at 600 C.
water. These sequences of coating and pyrolysis treatments were repeated 2 times
for absorption measurement and 20 times for X-ray diffraction analysis. Then the
pyrolysized sample was annealed at 600 C for 30 min in pure nitrogen (99.995%)
Fig. 3(a) shows two-pairs of peaks centered at the binding energies
of about 780.2 and 796 eV as well as 785.7 and 802.3 eV based on
the Gaussian fitting. As discussed in our previous research [23], the
◦
atmospheres.
former pair of peaks is assigned to the Co2+ 2p3/2 and Co 2p
2+
The phase and crystal structure of all samples were investigated by X-ray
diffraction (XRD: DMAX-RB, Rigaku Inc., Japan) with Cu K␣ radiation (ꢀ = 1.5406 A˚ )
1/2
2
+
electrons and the later one is related to the satellites of the Co
filtered through a Ni foil. The microstructure was observed by transmission electron
microscopy (TEM: JEM-100CXII, JEOL, Japan). The surface chemical state of the pre-
pared films was characterized by X-ray photoelectron spectroscopy (XPS: PHI-5300,
PHI, USA) using Al K␣ radiation (hꢁ = 1486.6 eV) as an X-ray source. The energies of
all spectra were corrected by normalizing with respect to the C 1s peak at 285.0 eV.
The Au 4f and Co 2p spectra were fitted by using the XPS data which were deter-
mined by deconvolution in the program of XPS Peak4.0. The absorption spectra were
measured by UV-visible spectrophotometer (UV-2800, UNICO Instruments Co., Ltd.,
China).
2+
2
p3/2 and Co 2p1/2 electrons in the CoO bond [22,24]. The intense
2+
satellite peaks appear at about 5.5 and 6.3 eV beyond the Co 2p3/2
and Co2+ 2p1/2 peaks. The relative integrated intensities of the satel-
2+
lite structure were about 40% and 84% of the main Co 2p3/2 and
Co 2p1/2 peaks. However, the binding energies of Co 2p3/2 and
Co 2p1/2 here are a little lower than those (Co 2p3/2, 780.5 eV;
Co 2p1/2, 796.3 eV) in our previous results [23] for the pure CoO
2+
2+
2+
2+
2+
3
. Results and discussion
Fig. 1 shows the XRD patterns of Au/CoO thin films with 10, 30
◦
and 50 mol% Au annealed at 600 C for 30 min in N . The result of the
2
pure CoO film is also shown in Fig. 1 for comparison, which indicates
the characteristic peaks of cubic crystalline CoO (PDF#43-1004)
◦
◦
◦
at 36.5 , 42.4 and 61.5 without other phase(s). Such diffraction
peaks of the CoO phase disappear as Au was dispersed, while other
◦
◦
◦
three diffraction peaks at 38.2 , 44.4 , 64.6 appear, which corre-
spond to (1 1 1), (2 0 0), (2 2 0) crystal planes of the cubic Au phase
(
PDF#04-0784). This result reveals that the CoO in the Au/CoO thin
films may be amorphous which was restrained by dispersing Au.
Okumu et al. [20] also reported that the fine dispersion of Ag in TiO2
matrix delayed the crystallization of TiO , which may be due to the
2
fact that the presence of Ag atoms heightens the activation barrier
for crystallization. The amorphous CoO in the Au/CoO thin films
may be attributed to the similar behavior. The diffraction peaks of
Au phase intensify with increasing Au content. The calculated Au
particle sizes by the Scherrer’s equation increase from 25 to 40 nm
when the Au content increases from 10 to 50 mol%.
Fig. 2 shows the XPS survey spectrum of Au/CoO thin film with
◦
4
0 mol% Au which was annealed at 600 C. The Au 4f, Au 4d, Au 4p,
Co 2p, Co 2s, Co LMM, Si 2p, Si 2s, O 1s and O KLL photoelectron
peaks are observed besides Na KLL, Na 1s, C KVV and C 1s peaks.
The appearance of Na peaks is due to the diffusion of Na atoms
existed in the glass substrate into the surface after annealing. The
silicon peak resulted from the SiO2 substrate, and the carbon peak
arose from an overlying contaminant hydrocarbon layer, which was
unavoidable for the XPS analysis. The O peaks centered at binding
energies of about 531 eV belong to the band of Si–O in SiO2 and
Co–O in CoO [21,22], which were hardly distinguishable. No other
impurities are observed.
◦
Fig. 3. XPS spectra of Au/CoO film with 40 mol% Au annealed at 600 C: (a) Co 2p,
and (b) Au 4f.

S.-J. Wang et al. / Journal of Alloys and Compounds 509 (2011) 5731–5735
5733
◦
Fig. 4. TEM micrograph (a) and SAED pattern (b) of Au/CoO thin film with 20 mol% Au annealed at 600 C.
film. Fig. 3(b) manifests two peaks at 84.2 and 87.9 eV based on the
of the SPR peak for the Au/CoO composite films with 40–10 mol%
Au. If the effect of the charge transfer is stronger than that of the
quantum size effect, which leads to the blue shift absorption band
with increasing the particle size for the samples from 40 to 60 mol%
Au. The dispersion threshold of the Au content is near 40 mol%. The
intensity of the SPR peaks strengthen with increasing Au content
from 10 to 40 mol% since more Au particles uniformly dispersed in
the CoO matrix, while that weakening with more Au content may
be due to the aggregation of nanoparticles and unhomogeneous
distribution in the higher Au content.
0
0
Gaussian fitting which refer to Au 4f7 and Au 4f respectively.
/2
5/2
0
They are a little higher than those for metallic gold (Au 4f7/2, 84 eV
21,25] and 83.73 eV [26]; Au 4f5/2, 87.7 eV [21,25]), The slightly
low binding energy of Co 2p3/2 and Co 2p1/2 and high binding
energy of Au 4f7/2 and Au 4f in the Au/CoO composite film may
be attributed to a charge transfer between Co and Au [27]. How-
ever, the Au still keeps in a metallic state and Co in a divalent state
in the composite film, because of the absence of the peaks corre-
0
[
2
+
2+
0
0
5/2
2+
+
sponding to the oxidized Au species (Au 4f7/2, 84.7 eV [28] and
3
+
8
7
5.6 eV [29]; Au 4f7/2, 88.1 eV [25]) and to Co metal (Co0 2p3/2
,
Fig. 5(b) shows theoretical optical absorption spectra of Au/CoO
thin films calculated by Mie theory [34]. The SPR absorption of Au
78.2 eV [21,24]).
Fig. 4 shows the bright-field TEM micrograph (a) and the corre-
sponding selected area electron diffraction (SAED) pattern (b) for
◦
the Au/CoO thin film with 20 mol% Au annealed at 600 C. As shown
in Fig. 4(b), the diffraction rings from inner to outer correspond
to Au crystal faces (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2), respec-
tively. No diffraction spots for CoO crystallites are observed since Au
crystallites restrain the crystallization of CoO in the Au/CoO com-
posite film. The approximately spherical and dark black particles
are considered to be Au particles and the gray matrix belongs to
CoO in Fig. 4(a). The bright-field TEM micrograph and SAED pattern
indicate that Au particles uniformly dispersed in the amorphous
CoO matrix and the mean diameters of particles are about 30 nm,
which are in well agreement with results analyzed in XRD pattern
(
Fig. 1).
Fig. 5(a) shows the optical absorption spectra of the Au-
dispersed CoO thin films with different Au contents annealed at
◦
6
00 C. All samples exhibit an absorption peak in the wavelength
region of 550–650 nm due to the SPR of Au nanoparticles. The SPR
peaks manifest a red shift trend and intensify with increasing Au
content from 10 to 40 mol%, while show a blue shift one and weaken
from 40 to 60 mol%. It is well known that the position of the SPR
peak for small free Au particles manifests blue shift with decreas-
ing particle size (the quantum size effect), which can be explained
by Mie and relevant Mie-like theories [30–32]. However, the blue
shift is also related to another effect of a charge transfer between
metal and oxide. The charge-transfer between metal and oxide in
the sol–gel derived Au/Al O films [33] is corresponded to the blue
2
3
shift of the absorption peaks as the Au particle size grew by raising
◦
annealing temperature from 300 to 800 C. In the present study,
the position of the SPR peak should be a result of the competition
between the quantum size effect and the influence of the charge
transfer. If the quantum size effect is stronger than that of the
charge transfer between Au and Co2 , which may lead to a blue shift
Fig. 5. Optical absorption spectra of Au/CoO thin films with different contents (a)
and theoretical optical absorption spectra of Au/CoO thin films calculated by Mie
theory (b).
+

5
734
S.-J. Wang et al. / Journal of Alloys and Compounds 509 (2011) 5731–5735
widened as the Au content is over 40 mol% as a result of blue shift,
which is in well agreement with the optical absorption spectra.
4. Conclusions
Nanocomposite thin films Au/CoO with different Au contents
were successfully prepared using the spin-coating method by
the chemical solution method and annealed in a pure nitrogen
◦
(
99.995%) atmosphere at 600 C. The approximately spherical Au
particles dispersed in the amorphous CoO matrix grew from 25 to
0 nm with increasing Au contents from 10 to 50 mol%. Au existed
4
in the nature of single metal and CoO kept the oxide state in the
Au/CoO thin composite film. An enhanced optical absorption peaks
due to the surface plasmon resonance of Au particles achieved in
the wavelength range of 550–650 nm. The peaks manifest a red
shift and intensify with increasing Au content from 10 to 40 mol%,
while exhibit the blue shift and weaken from 40 to 60 mol%, which
are different from the theoretical ones of Au/CoO thin films calcu-
lated by Mie theory. The band gap Eg decreases with increasing Au
content from 10 to 40 mol% but increases by further increasing Au
content.
Acknowledgments
Fig. 6. Relationship between (˛hꢁ)2 and hꢁ for Au/CoO thin films with different
contents.
This work was supported by Major State Basic Research Devel-
opment (no. 2007CB613301) and the National Natural Science
Foundation of China (nos. 50842028, 50972012).
nanoclusters in the Au/CoO thin films can be predicted by the well-
known Mie theory.
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