Solvothermal synthesis of CdO and CuO nanocrystals

Article abstract of DOI:10.1016/j.cplett.2004.06.092

Nanocrystals of CdO have been obtained by the decomposition of the cupferron complex in the presence of tri-n-octylphosphine oxide (TOPO) under solvothermal conditions. The precursor:TOPO ratio plays an important role in determining the size of the nanocr

Full text of DOI:10.1016/j.cplett.2004.06.092

Chemical Physics Letters 393 (2004) 493–497
www.elsevier.com/locate/cplett
Solvothermal synthesis of CdO and CuO nanocrystals
a,b,
Moumita Ghosh a,b, C.N.R. Rao
*
a
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre
for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India
b
Received 26 March 2004
Abstract
Nanocrystals of CdO have been obtained by the decomposition of the cupferron complex in the presence of tri-n-octylphosphine
oxide (TOPO) under solvothermal conditions. The precursor:TOPO ratio plays an important role in determining the size of the na-
nocrystals. The nanocrystals have been characterized by electron microscopy, absorption spectroscopy and fluorescence spectrosco-
py, besides X-ray diffraction. The CdO nanocrystals are single crystalline and show evidence for quantum confinement. CuO
nanocrystals could also be prepared by the decomposition of the cupferronate under solvothermal conditions, the particle size being
controlled by the initial precursor concentration.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
nanoparticles by the micro-emulsion method employing
AOT reverse micelles. There is also a report of stearate-
coated CdO nanoparticles of 5–10 nm size range, ob-
tained by the micro-emulsion method starting from an
aqueous solution of a cadmium salt and stearic acid in
xylene [11]. We have developed a new method to prepare
CdO nanoparticles, wherein a cadmium precursor com-
pound, is decomposed under solvothermal [12,13] condi-
tions in presence of a capping agent. For this purpose, we
have employed cadmium cupferronate, Cd(C6H5N2O2)2
as the precursor, and obtained CdO nanoparticles of dif-
ferent sizes by using tri-n-octylphosphine oxide (TOPO)
as the capping agent [14]. We have characterized the
nanoparticles by X-ray diffraction (XRD), electron micr-
oscopy and electronic absorption and emission spectros-
copies. We have also synthesized CuO nanocrystals by
the decomposition of the cupferronate [15] under solvo-
thermal conditions.
CdO is a direct band gap (2.3 eV) semiconductor, with
an indirect band gap of 1.36 eV. Due to its large linear
refractive index (n0 =2.49), it is a promising candidate
for optoelectronics applications and other applications,
including solar cells [1,2], phototransistors [3], photodi-
odes [4], transparent electrodes [5] and gas sensors [6].
Reduction in the dimensionality of such materials from
the three dimensional bulk phase to the zero-dimensional
nanoparticles can lead to enhanced non-linearity, deter-
mined by the quantum size effects and other mesoscopic
effects. Because of these interesting possibilities, there has
been some effort to prepare nanoparticles of CdO. Wu
et al. [7] prepared a nanometer-sized CdO organosol
from an aqueous solution of Cd(NO3)2, a surfactant
and toluene, while Liu et al. [8] synthesized CdO nano-
needles by chemical vapour deposition. CdO nanowires
have been synthesized by decomposing CdCO3 in a
KNO3 salt flux [9]. Zou et al. [10] have prepared CdO
2. Experimental
*
The cupferron complex of Cd(II), Cd(C6H5N2O2)2,
was prepared by the reaction of cadmium acetate with
Corresponding author. Fax: +91-80-846-2766/23622760.
E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.06.092

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M. Ghosh, C.N.R. Rao / Chemical Physics Letters 393 (2004) 493–497
cupferron (N-nitroso-N-phenylhydroxylamine, ammoni-
um salt). In a typical reaction, 1.27 g of AR grade
Cd(CH3COO)2Á2H2O was dissolved in 25 cm3 of water
in a beaker. In another beaker, 1.5 g of cupferron was
solubilised in 60 cm3 of water. The two solutions were
cooled at 0 °C and the cupferron solution slowly added
to the solution of Cd(CH3COO)2 under vigorous stir-
ring. The white precipitate obtained was collected,
washed with 2.5% NH3 solution followed by milli-q wa-
ter, to remove the excess cupferron. The complex was
characterized by chemical analysis was as follows: C,
35.77% found (calculated 37.27%); H, 2.665% (2.59%);
N, 14.09% (14.49%). Thermogravimetric analysis of
the as-prepared Cd(cup)2 in a nitrogen atmosphere,
showed that there was sharp weight loss due to decom-
position around 250 °C.
XRD patterns of the samples were recorded in the
h–2h Bragg–Brentano geometry with a Siemens D5005
diffractometer using Cu Ka(k=0.151418 nm) radiation.
For transmission electron microscope (TEM) studies, a
solution or a dispersion of the sample in a suitable sol-
vent was allowed to evaporate on a carbon-coated Cu
grid. A JEOL (JEM3010) microscope with an accelerat-
ing voltage of 300 kV was used to obtain TEM images.
UV–vis absorption spectra of nanocrystals in toluene
were recorded using a Hitachi U3400 spectrometer.
The reference used was toluene solution of TOPO. Pho-
toluminescence spectra were recorded with a Perkin–El-
mer model LS50B luminescence spectrometer.
3. Results and discussion
In a typical reaction, 0.1 g (0.0258 mmol) of Cd(cup)2
and 0.1 g (0.254 mmol) of TOPO were taken in 48 cm3
toluene and sealed in a Teflon-lined autoclave of 80
cm3 capacity. The mixture was heated at 240 °C for
150 min. A dark reddish brown solid, insoluble in tolu-
ene was obtained as the product. It was washed with tol-
uene for several times, followed by methanol, and dried
at 40 °C for 2 h. This preparation with a Cd(cup)2:TO-
PO ratio of 1:1 yielded one-dimensional structures of
CdO. Different Cd(cup)2:TOPO ratios were employed
to obtain nanocrystals of different sizes. When the ratio
was 1:2 and 1:5, insoluble orange-colored solids were
obtained, and the latter ratio also yielded yellow disper-
sions in toluene. The orange solid contained relatively
bigger nanocrystals (18–30 nm diameter), which was
washed thoroughly with toluene for further characteri-
zation. Nanoparticles were precipitated out from the
yellow organosol by the addition of methanol. The pre-
cipitated nanocrystals which could be redispersed in tol-
uene had diameters in the 3–12 nm range. The size of the
nanocrystals could be changed by varying the relative
concentrations of the reactants. With a 1:5 ratio of
Cd(cup)2:TOPO, we were able to synthesize toluene-sol-
uble CdO nanocrystals, without any insoluble fraction.
In addition to CdO nanocrystals, we have been
able to prepare CuO nanocrystals by decomposing
Cu(cup)2 in toluene under solvothermal conditions.
We have carried out the reactions in the absence of
any capping agent and obtained different sizes of na-
nocrystals by varing the Cu(cup)2 concentration. The
reactions yielded a dark brown organosol which was
stable in toluene and hexane. By the addition of n-oc-
tylamine to the organosol in toluene, the CuO nano-
crystals could be solubulized completely, giving rise
to transparent solutions. In a typical reaction to
obtain 10.5 nm nanocrystals, 0.2 g (0.58 mmol) of
Cu(cup)2 was taken in 48 cm3 toluene (75% filling
fraction) and sealed in a Teflon-lined autoclave of 80
cm3 capacity. The autoclave was heated at 180 °C
for 300 min.
In Fig. 1, we show the XRD patterns of the CdO na-
nocrystals prepared in the presence of TOPO. Pattern
(a) is that of the insoluble orange coloured solid product
obtained with a Cd(cup)2:TOPO ratio of 1:2. The pat-
tern is characteristic of the rock-salt structure with
˚
a=4.695 A (JCPDS card no 05-0640), as established
by Rietveld profile analysis [16]. We show the goodness
of fit as well as the difference profile in Fig. 1a. Based on
the X-ray line broadening, the particle diameter was es-
timated to be 18 nm. In Fig. 1b, we show the XRD pat-
tern of nanocrystals, which are smaller than 18 nm. The
X-ray line widths show them to be around 12 nm in
diameter. In Fig. 2a, we show the TEM image of the na-
nocrystals corresponding to the XRD pattern in Fig. 1b.
The particle size distribution is shown as an inset. The
Fig. 1. Powder XRD patterns of TOPO-capped CdO nanocrystals of
(a) 18 nm (b) 12 nm diameter. Rietveld fit is shown in (a) along with
the difference profile. The vertical lines at the top of the figure are the
expected peak positions.

M. Ghosh, C.N.R. Rao / Chemical Physics Letters 393 (2004) 493–497
495
Fig. 2. (a) TEM image of TOPO-capped CdO nanocrystals of an average diameter of 9.5 nm. The XRD pattern of these particles is shown in Fig. 1b.
Inset shows the size distribution of the nanocrystals. (b) HREM images of CdO nanocrystals.
average particle size from TEM appears to be around
9.5 nm. These particles are single crystalline as revealed
by the high resolution electron microscope image in Fig.
2b. The particles are spherical or elliptical in shape, not
unlike those reported by Dong et al. [11]. The fringe
In Fig. 3a, we compare the electronic absorption
spectra of the TOPO-capped CdO nanocrystals with di-
ameters of (a) ꢀ18 nm, (b) 9.5 2 nm and (c) 4.5 1.5
nm. The 18 nm nanocrystals show an absorption edge
around 2.5 eV (curve (a) in Fig. 3a). Recall that the bulk
sample with an absorption edge of 2.3 eV, show a broad
maxima around 500 nm. The 9.5 nm nanocrystals show
an absorption edge of ꢀ2.8 eV while the 4.5 nm nano-
crystals show an absorption edge of ꢀ3.27 eV, as re-
vealed by spectra (b) and (c) respectively in Fig. 3a.
˚
spacing of 2.68 A observed in the high-resolution elec-
tron microscope image (HREM) corresponds to the sep-
aration between the {111} lattice planes, whereas the
˚
2.336 A spacing corresponds to the separation between
the {200} planes.

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M. Ghosh, C.N.R. Rao / Chemical Physics Letters 393 (2004) 493–497
Fig. 3. (a) Electronic absorption spectra of TOPO-capped CdO nanoparticles with an average diameters of (a) 18 nm (b) 9.5 nm and (c) 4.5 nm . (b)
Photoluminescence spectra of 18 and 4.5 nm CdO nanocrystals at different excitation wavelengths. Curves 1–3 are the photoluminescence spectra of
the 4.5 nm nanocrystals at excitation wavelengths of 430, 480 and 520 nm respectively, 1*, 2* and 3* correspond to the 18 nm particles.
The large blue-shift relative to the bulk CdO observed
with the 4.5 nm nanocrystals is attributed to quantum
confinement. The bulk exciton Bohr radius of CdO is
not available in the literature and it is therefore difficult
to conclude as to which confinement regime the CdO na-
nocrystals belong.
highly crystalline in nature, as revealed by the HREM
˚
image, shown in Fig. 5b. The line spacing of 2.35 A cor-
responds to the separation between {111} planes.
Photoluminescence spectra of CdO have been report-
ed by Wu et al. [7] who suggest that the band at 480 nm
arises from the transition between the conduction and
valence bands. They also report a band at 530 nm due
to near band-gap radiative combination. In Fig. 3b,
we show the photoluminescence spectra of the 18 and
4.5 nm nanocrystals recorded at the excitation wave-
lengths of 430, 480 and 520 nm. The 18 nm nanocrystals
exhibit bands around 540, 560 and 590 nm respectively
at excitation wavelengths of 430, 480 and 520 nm. The
corresponding bands of the 4.5 nm nanocrystals occur
at 495, 540 and 570 nm. We observe the maximum
blue-shift in the case of the lowest wavelength emission
band, which is considered due to transition between va-
lence and the conduction bands.
In Fig. 4 we show the typical XRD patterns of CuO
nanocrystals, obtained by varing the concentration of
Cu(cup)2 in toluene. The patterns (a), (b) and (c) corre-
spond to particle sizes of 10.5, 6 and 4 nm respectively,
as obtained from the X-ray line broadenings. These
XRD patterns give the monoclinic structure of CuO in
agreement with the literature (JCPDS card no 45-0937).
TEM image of CuO nanocrystals with an average diam-
eter of 10 nm is shown in Fig. 5a. The inset in this frame
shows the particle size distribution. The particles are
Fig. 4. XRD patterns of CuO nanocrystals of (a) 10.5 nm, (b) 6 nm
and (c) 4 nm diameter. The vertical lines at the top are the expected
peak positions.

M. Ghosh, C.N.R. Rao / Chemical Physics Letters 393 (2004) 493–497
497
4. Conclusions
CdO and CuO nanocrystals have been prepared suc-
cessfully by a solvothermal route involving the decom-
position of the metal cupferronate in toluene medium.
As most of the transitional metals and some non-transi-
tion metals form complexes with cupferron, this route
can be readily employed to prepare nanocrystals of var-
ious metal oxides soluble in organic solvents. The CdO
and CuO nanocrystals obtained by us are single crystal-
line in nature as revealed by electron microscopy. The
CdO nanocrystals of 4.5 nm diameter show evidence
for quantum confinement.
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