Preparation and characterization of CuI nanorods using Cu(dmg)2 as precursor via water-in-oil (w/o) microemulsions

Article abstract of DOI:10.1016/j.ssc.2006.04.025

CuI nanorods have been firstly prepared by water-in-oil microemulsions using Cu(dmg)₂ as precursor at low temperature as low as 70 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show that the CuI nanorods are pure γ-phase crystals with diameters ranging from 50 to 80 nm and lengths up to 500 nm. UV-vis spectrum reveals that the nanorods exhibit a blue shift and possess of wider band gap energy. Electrochemical impedance spectroscopy (EIS) reveals the nanorods own larger grain boundary resistance. Results of comparative experiments indicate that the rod-like structure of Cu(dmg)₂ crystals leads the CuI crystals growing in one direction especially and the reverse microemulsion system plays a crucial role in making products symmetrical and uniform.

Full text of DOI:10.1016/j.ssc.2006.04.025

Solid State Communications 138 (2006) 526–529
www.elsevier.com/locate/ssc
Preparation and characterization of CuI nanorods using Cu(dmg)2
as precursor via water-in-oil (w/o) microemulsions
a
a
b
Xue-Liang Li a, , Xiao-Yun Zhu , Ti-Lan Duan , Yi-Tai Qian
*
a School of Chemical Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China
b Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry, University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China
Received 22 August 2005; received in revised form 20 January 2006; accepted 15 April 2006 by B.-F. Zhu
Available online 6 May 2006
Abstract
CuI nanorods have been firstly prepared by water-in-oil microemulsions using Cu(dmg)2 as precursor at low temperature as low as 70 8C. X-ray
diffraction (XRD) and transmission electron microscopy (TEM) measurements show that the CuI nanorods are pure g-phase crystals with
diameters ranging from 50 to 80 nm and lengths up to 500 nm. UV–vis spectrum reveals that the nanorods exhibit a blue shift and possess of wider
band gap energy. Electrochemical impedance spectroscopy (EIS) reveals the nanorods own larger grain boundary resistance. Results of
comparative experiments indicate that the rod-like structure of Cu(dmg)2 crystals leads the CuI crystals growing in one direction especially and
the reverse microemulsion system plays a crucial role in making products symmetrical and uniform.
q 2006 Elsevier Ltd. All rights reserved.
PACS: 73.61.Tm; 73.40.Kc; 74.25.Gz; 81.05.Dz; 81.20.Ka
Keywords: A. Nanostructures; B. Crystal growth; C. Grain boundaries; D. Optical properties
1. Introduction
comparison with bulk materials, which may be very useful for
many device applications.
Cuprous iodide is water insoluble solid with three crystal-
line phases a, b and g [1,2]. g-CuI is a p-type semiconductor
with direct band gap [3,4]. Due to its electrical properties, it is
used as a holecollecting agent in dye sensitized solid-state cell
[5], superconductors [6], catalysts for synthesis of organic
compound [7], and effective adsorbents for separating carbon
monoxide from gas mixtures [8].
2. Experimental
All reagents were of analytical grade and used without
further purification.
First, 0.464 g of Dimethylglyoxime (dmgH) and 0.400 g of
Cu(Ac)2$H2O were added into 50 ml of absolute ethanol in
sequence, which was stirred at the room temperature for 30 min
to get brown precipitates Cu(dmg)2. Then the collected
precipitates were dried in air at 60 8C and dispersed in l0 ml
of distilled water. Secondly, 30 ml of cyclohexane, 1 ml of
n-Amylalcohol, 1.511 g of SDS and 2 ml of Cu(dmg)2
suspension were mixed and stirred vigorously to form
microemulsion I. And microemulsion II was made at the
same condition except that Cu(dmg)2 suspension was replaced
by 0.4 mol lK1 KI aqueous solution. Thirdly, Microemulsion I
was added to Microemulsion II with slow stirring and the
mixture was stirred for 2 h, which was manipulated in water
bath at 70 8C. Finally, the product was centrifuged, washed and
vacuum dried in sequence for the following characterizations.
The samples were characterized by powder X-ray diffrac-
tion (XRD) on a D/Max-rB X-ray diffractometer with
There are several methods for preparing CuI, such as
electrodeposition [9], pulsed laser deposition [10], liquid phase
reaction [11] and vacuum evaporation [12]. To the best of our
knowledge, there have been few reports on preparations of CuI
nanoparticals [13,14], and no report on CuI nanorods. In this
paper, we firstly synthesized CuI nanorods by Cu(dmg)2 and KI
in a reverse microemulsion system at 70 8C. The products were
characterized by XRD, TEM, UV and EIS. Results showed
different optical and electrical properties of CuI nanorods in
*
Corresponding author.
E-mail address: xueliangli2005@163.com (X.-L. Li).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.04.025

X.-L. Li et al. / Solid State Communications 138 (2006) 526–529
527
˚
monochromatized Cu Ka (lZ1.5406 A) incident radiation.
XRD patterns were recorded from 20 to 558 (2q) with a
scanning rate of 68/min. The morphologies of the samples were
analyzed by TEM on an H-800 transmission electron
microscope operated at 200 kV. Optical absorption spectrum
was recorded on a Shimadzu UV-2401PC UV–vis recording
spectrophotometer. Impedances were measured on CHI660B
electrochemical workstation in the frequency range of 100 Hz
to 10 mHz at the init E K0.5 V (vs. Hg/HgO (6 M KOH)
electrode).
crystal). So the calculation according XRD pattern is
coincident with the value measured by TEM. The little
disaccord can be attributed to the following reason.
The morphologies of the as-prepared CuI and Cu(dmg)2
crystals were studied by TEM. Fig. 2(a–c) shows that the CuI
crystals are nanorods with diameters ranging from 50 to 80 nm
and lengths up to 500 nm, and this morphology has not been
reported in other papers. The ED pattern in Fig. 2(b) shows the
nanorod is single crystal, which indicates that as-obtained CuI
nanorods are mainly single crystals. Compared to the JCPDS
card, the (220) peak is stronger, revealing the [220] oriented
growth of the CuI nanorods. This new morphology of CuI
might be mainly attributed to the morphology of the precursor
Cu(dmg)2. In Fig. 2(d), we can find that the Cu(dmg)2 crystals
exhibit rod-like structure and this structure might lead the CuI
crystals growing in one direction, especially. To prove this
deduce, CuI prepared by Cu(Ac)2 in the same conditions was
studied by TEM. In Fig. 3(a), the typical morphology of the
sample is triangle flake and no one-dimensional morphology
can be observed.
In the process of EIS measurement, electrodes were
prepared by mechanically pressing (200 kg/cm2) the well-
mixed CuI powders of carbon black and PTFE powders on
titanium flakes (the area of the cathode 10 mm!10 mm). The
composition of the electrodes is CuI, carbon and polytetra-
fluoroethylene (PTFE) with weight ratio of 8.5:1:0.5. A 6 M
solution of KOH in distilled water was used as the electrolyte.
A platinum electrode and an Hg/HgO (6 M KOH) electrode
were used as the counter electrode and reference electrode,
respectively.
The role of reverse microemulsion system can be seen
clearly from two images of Fig. 3. The sample in Fig. 3(b) was
synthesized just in aqueous solution without microemulsions.
The image shows large agglomerates and no single crystal can
be seen apart. The sample in Fig. 3(a) was synthesized in
microemulsion. Compared to Fig. 3(b), most of the crystals in
Fig. 3(a) are thin triangle flakes. So the reverse microemulsion
3. Results and discussion
The XRD pattern of the product is shown in Fig. 1. All
diffraction peaks in the XRD pattern can be perfectly indexed
to pure g-phase CuI (JCPDS, 06-0246). No impurity was
detected, indicating that the as-synthesized product was of high
purity. The average crystalline size of the product can be
estimated by Scherrer equation
D Z ðKlÞ=b cos q
where D is the mean diameter of the nano-particles, K is a
constant (0.89), l is the X-ray wavelength (0.15406 nm in the
present case), b is the corrected X-ray diffraction broadening
(bZBKb, B stands for full width at half maximum and b is the
instrumental line broadening), and q is the Bragg angle of the
X-ray diffraction peak. Calculation made on the strongest peak
at 2qZ25.408 is 79.1 nm. Actually, the size obtained from
XRD pattern broadening is the mean size of single crystal in
the sample. However, the size observed by TEM is the size of
the single particles and the single particles maybe consisted of
several single crystals (in the case that some rods are not single
6000
5000
4000
3000
2000
1000
0
20
25
30
35
40
45
50
55
2 Theta / degree
Fig. 2. (a)–(c): TEM images of the CuI nanorods prepared from Cu(dmg)2 and its
ED pattern, which shows single crystal; (d) TEM image of the Cu(dmg)2 rods.
Fig. 1. XRD pattern of the as-prepared CuI.

528
X.-L. Li et al. / Solid State Communications 138 (2006) 526–529
250
200
150
100
50
CuI naorods
CuI agglomerates
0
0
100
200
300
400
Z'(ohm)
Fig. 3. TEM images of the CuI prepared from Cu(Ac)2 (a) in microemulsions;
(b) without microemulsions.
Fig. 5. Impedance spectra of CuI nanorods prepared as typical process in our
paper and CuI agglomerates prepared from Cu(Ac)2 without microemulsions.
system plays a crucial role in making the as-prepared crystals
dispersed.
semicircle is related to the reaction of the interface between
solution and electrode. And the diameter of the arc indicates
the resistance of electrode material. Because the electrodes are
prepared in the same condition and with the same assistant
materials (carbon, PTFE), so the major factor leading to the
difference of the EIS measurement result is the CuI. Compared
to the agglomerate CuI, the resistance value of nano-CuI is
about twice, which is attributed to discontinuous grain
boundaries and larger grain boundary resistance of the
nanocrystalline [16,17]. In the low-frequency area, a clear
linear part is observed, which is attributed to Warburg
impedance caused by a semi-infinite to finite diffusion
behavior.
Fig. 4 shows the optical absorption spectrum of as-synthe-
sized CuI nanorods. Three strong absorption peaks centered at
220, 278 and 412 nm are shown in the UV spectrum, which is
attributed to band gap absorptions in CuI [3,4]. The optical
band gap Eg can be determined by the following equation
n
ðAhvÞ Z BðhvKEgÞ
where hn is the photo energy, A is absorbency, B is a constant
relative to the material, n is either 2 for direct interband gap or
1/2 for indirect interband gap transitions [15]. The (Ahn)2–hn
curve for the sample is shown in the inset of Fig. 4, which
exhibits the as-obtained CuI nanorods have direct band gap.
Calculated by the extrapolation of above equation, the band
gaps are 2.37, 3.22 and 4.88 eV, respectively. These values are
a little higher than the previously reported values of the band
gap energies of CuI by B. Bouhafs et al. [3], which causes the
blue shifts.
4. Conclusions
CuI nanorods with diameters ranging from 50 to 80 nm and
lengths up to 500 nm have been firstly prepared by the reaction
of Cu(dmg)2 and KI at 70 8C in a reverse microemulsion
system. X-ray diffraction, transmission electron microscopy,
optical absorption spectrum and electrochemical impedance
spectroscopy were used to characterize the samples. It shows
that the rod-like-structure Cu(dmg)2 as precursors can lead the
CuI especially growing in one direction to form one-
dimensional materials and the reverse microemulsion system
plays a crucial role in making products symmetrical and
uniform. Energy bands of the sample show blue shifts
comparing with bulk materials, which are attributed to the
quantum size confinement effect of the nanorods. Impedance
spectrum indicates that the value of CuI nano-particles is larger
than that of CuI agglomerates for the larger grain boundary
resistance of nano-particles.
The impedance spectra (Z00 plotted against Z0) for
as-prepared nano-CuI and agglomerate CuI are shown in
Fig. 5. In the high-frequency region, the impedance of the
2.0
10
E=5.1eV
80
1.8
60
220nm
E=3.18eV
1.6
hv
278nm
40
20
0
412nm
E=2.42eV
1.4
1.2
1.0
0.8
2.0 2..5 3.0
3.5
4.0
4.5 5.0
5.5
6.0
(hv / eV)
Acknowledgements
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm
This paper was supported by Natural Science Foundation of
Anhui Province of China (Grant No. 050440303).The authors
wish to gratefully acknowledge Mr Shupei Tang, Mr Anping
Fig. 4. Optical absorption spectrum and (Ahv)2–hv curve (inset) of
as-synthesized CuI nanorods.

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