Growth and characterization of highly oriented CuBr thin films through room temperature electrochemical route

Article abstract of DOI:10.1016/j.electacta.2008.08.006

A simple electrochemical process has been demonstrated to grow highly oriented copper(I) bromide thin films on indium-doped tin oxide (ITO) glass through reducing CuBr₂ in aqueous solutions at room temperature. The copper(I) bromide thin films grow preferential orientation along the 〈1 1 1〉 crystal axis from the X-ray diffraction patterns. The orientation growth of the CuBr thin films is little affected by the solution pH and applied potentials. The possible mechanism of the orientation growth has been discussed, and the surface energy of different crystal plane of CuBr crystal is believed to play an important role to control the orientation growth of the CuBr thin films. The oriented films exhibit intense free-exciton photoluminescence at room temperature.

Full text of DOI:10.1016/j.electacta.2008.08.006

Electrochimica Acta 54 (2008) 242–246
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Growth and characterization of highly oriented CuBr thin films
through room temperature electrochemical route
a
a,∗
a
b
a,∗∗
He Li , Run Liu , Honglan Kang , Yifang Zheng , Zhude Xu
a
Department of Chemistry, Zhejiang University, Hangzhou 200237, PR China
Breeding Base State Key Laboratory of Green Chemical Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2008
Received in revised form 28 July 2008
Accepted 4 August 2008
Available online 12 August 2008
A simple electrochemical process has been demonstrated to grow highly oriented copper(I) bromide
thin films on indium-doped tin oxide (ITO) glass through reducing CuBr2 in aqueous solutions at room
temperature. The copper(I) bromide thin films grow preferential orientation along the ꢀ1 1 1ꢁ crystal axis
from the X-ray diffraction patterns. The orientation growth of the CuBr thin films is little affected by the
solution pH and applied potentials. The possible mechanism of the orientation growth has been discussed,
and the surface energy of different crystal plane of CuBr crystal is believed to play an important role to
control the orientation growth of the CuBr thin films. The oriented films exhibit intense free-exciton
photoluminescence at room temperature.
Keywords:
CuBr
Electrochemical deposition
Orientation growth
Surface energy
Photoluminescence
©
2008 Elsevier Ltd. All rights reserved.
1
. Introduction
Other solution-phase-based methods have also been explored to
grow nanoscale crystals of CuBr, such as sol–gel [22] and hydrother-
mal synthesis [23,24].
Here, a simple electrochemical deposition process to grow
CuBr thin films by directly reducing CuBr2 in aqueous solution at
room temperature has been demonstrated. The results of X-ray
diffraction show that the electrodeposited CuBr thin films grow
preferential orientation along ꢀ1 1 1ꢁ crystal axis. The highly ori-
ented CuBr crystals show intense free-exciton luminescence at
room temperature.
Copper(I) bromides are of interest because they are good ionic
conductors with very low partial electronic conductivity and wide
band gap semiconductors with unusual photoluminescence (PL).
Copper(I) bromides have been used in organic synthesis [1,2], bat-
teries [3], gas sensors [4–6] and lasers [7–8]. Several methods have
been developed to grow copper(I) bromide thin films based on
vapor-phase techniques, such as radio frequency magnetron sput-
tering [9] and molecular beam epitaxy [10,11]. However, these
methods require high temperature, high vacuum, complex facility,
and rigid experimental procedures and which would greatly hinder
their universal applications.
2. Experimental
Electrodeposition is a well-known solution process that can be
applied in the growth of semiconductor films from aqueous or
nonaqueous solutions [12–14]. The advantages of electrodeposition
compared with other techniques include low process temperature,
low cost and capability of controlling orientations, morphologies
and chirality of the deposited films [15–20]. So far, Knauth and co-
workers have shown that CuBr thin films can be grown through the
Electrochemical experiments were carried out using a potentio-
stat/galvanostat (CHI 601C, CH Instruments, Shanghai, China). The
deposition was performed in a three-electrode cell with a platinum
wire as counter electrode and a saturated calomel electrode (SCE)
as reference electrode. Indium-doped tin oxide (ITO) glass slide
(
≈20 ꢀ/cm) was used as working electrode. Prior to electrodeposi-
tion, the ITO substrate was rinsed with acetone and distilled water
and then cleaned with distilled water in an ultrasonic set for about
−
reaction between Cu and Br [4–6]. Gedanken and his co-workers
obtained the CuBr nano-crystals using sonoelectrochemistry [21].
1
5 min. After that the ITO substrate was dipped in the nitric acid
(10 wt.%) for several seconds and followed by rinsing with distilled
water. The deposition solutions were 20 mM CuBr2 aqueous solu-
tions with an initial pH adjusted to 1–5 using 0.1 M HBr or 0.1 M
NaOH. The deposition temperature was 25 C. The applied depo-
sition potential was varied from 0.05 to −0.4 V versus SCE. Before
∗
Corresponding author. Tel.: +86 571 87953390; fax: +86 571 87951895.
Corresponding author.
E-mail addresses: runliu@zju.edu.cn (R. Liu), pchem@zju.edu.cn (Z. Xu).
◦
∗
∗
0
013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.08.006

H. Li et al. / Electrochimica Acta 54 (2008) 242–246
243
deposition, cyclic voltammetry of 20 mM CuBr2 aqueous solution
was carried out. The solution was deaerated and unstirred. The
working electrode was polycrystal Au sealed in Teflon with expo-
2
sure area of 0.2 cm . The cyclic voltammetry was run at a scan rate
of 10 mV/s.
X-ray diffraction measurements of the films were carried out
◦
◦
using the ꢁ–2ꢁ scan mode (2ꢁ varied from 20 to 80 , Cu K radi-
␣
1
ation (ꢂ = 1.54056 Å)) using a D/max 2550PC apparatus equipped
with thin film optics (Rigaku Corporation, Tokyo, Japan). Data was
◦
collected at a scan step of 0.02 and at conditions of 40 kV and
4
0 mA. FEI SIRION 200 field emission scanning electron micro-
scope (SEM) (FEI Corporation, USA) was used for observation of
the microstructures of the films and the SEM operation voltage
was 5.0 kV. energy dispersive spectroscopy (EDS) was used for the
composition analysis.
Fig. 2. X-ray diffraction ꢁ–2ꢁ scans of the electrodeposits onto ITO glass substrate
(a) and commercial CuBr powders (b). The electrodeposits obtained from 20 mM
Optical transmission spectra were measured on a Shimadzu UV-
CuBr2 aqueous solution and pH of the solution was adjusted to 3 using 0.1 M HBr
and the applied potential was −0.2 V versus SCE.
2
550 spectrophotometer (Shimadzu Corporation, Japan) at a scan
rate of 5 nm/s. Spectra were recorded at room temperature and ITO
glass was used as a reference. The photoluminescence spectra of
the CuBr were measured in air at room temperature using a Hitachi
F-2500 fluorimeter (Hitachi Corporation, Japan) with Xe lamp as
the excitation source. The power (W) of the Xe lamp used as the
exciting source is 150 W. The emission spectra were acquired at
following conditions: an excitation wavelength was set at 325 nm,
the slit width was set 2.5 nm for the emission and 2.5 nm for the
excitation, the scan rate was 15 nm/min, and the photomultiplier
tube voltage was 400 V.
In this work, the applied potentials ranged from 0.05 to −0.4 V
versus SCE versus SCE are chosen to deposit the CuBr.
3.2. Structure characterizations of CuBr thin films
The typical X-ray Bragg–Brentano scan of the electrodeposits
◦
is shown in Fig. 2a. There are one very strong peak at 2ꢁ = 27.1
◦
and one small peak at 2ꢁ = 44.9 . In order to determine the struc-
ture and composition of the electrodeposits, the powder of CuBr
obtained from commercial source was also examined by X-ray
Bragg–Brentano scan and the result is shown in Fig. 2b. There are
3. Results and discussion
◦
◦
◦
four characteristic peaks appeared in Fig. 2b at 27.1 , 44.9 , 53.3 ,
6
3.1. Electrochemical behaviour of CuBr2
◦ ◦
5.5 and 74.2 , respectively. Based on the standard pattern of CuBr
taken from the Joint Committee on Powder Diffraction Standards
(JCPDS) Database (JCPDS no. 82–2118), the XRD patterns in Fig. 2b
can be correspondingly indexed as (1 1 1), (2 2 0), (3 1 1), (4 0 0), and
(3 3 1) of the cubic CuBr, respectively. Compared the XRD patterns of
Fig. 2a to those of b, one can see that the electrodeposited thin films
belong to CuBr with strong preferential orientation along ꢀ1 1 1ꢁ
crystal axis.
The typical CV of 20 mM CuBr2 is shown in Fig. 1. The cathodic
reduction peak a can be attributed to the reduction of Cu2 to Cu+
+
and the potential is about −0.28 V versus SCE. The cathodic reduc-
+
tion peak b can be attributed to the reduction of Cu to Cu and
the potential is about −0.41 V versus SCE. The large anodic peak at
about 0.2 V versus SCE can be attributed to the oxidation of Cu to
+
Cu . Based on the results of CV, it is possible to form stable CuBr
SEM image of the electrodeposited CuBr thin films is shown
in Fig. 3a. Fig. 3a shows that the electrodeposited CuBr thin film
appears as dense film with triangular facet morphology. This is
consistent with the result of XRD that electrodeposited CuBr thin
film grow preferential orientation along the ꢀ1 1 1ꢁ crystal axis. In
order to further confirm that the electrodeposits belong to CuBr, the
EDS was performed for the electrodeposits and is shown in Fig. 3b.
Based on the EDS results, the compositions of the electrodeposits
are mainly composed of Cu and Br and the atom ratio of the Cu
and Br is 49.7:50.3. This again indicates that the electrodeposits are
mainly composed of CuBr.
among certain cathodic potential range. CuBr can be produced by
following reactions:
Cu2 + e → Cu+
+
(1)
(2)
Cu+ + Br− → CuBr
3.3. The effects of pH and applied potentials on orientation
growth of CuBr thin films
In electrocrystallization, the orientation growth usually is
affected by the solution pH and applied potentials. Here the effects
of solution pH and applied potentials have been conducted and
results are shown in Figs. 4 and 5, respectively. One can see that the
electrodeposited CuBr thin films still grow preferential orientation
along the ꢀ1 1 1ꢁ crystal axis under varied pH and applied potentials.
3
.4. Mechanism of orientation growth
Fig. 1. Cyclic voltammetry of 20 mM CuBr2 in aqueous solution and pH of the solu-
tion was adjusted to 3 using 0.1 M HBr. The solution was deaerated and unstirred.
The working electrode was polycrystal Au sealed in Teflon with the exposure area
Normally the preferential orientation of thin films elec-
2
of 0.2 cm . The cyclic voltammetry was run at a scan rate of 10 mV/s.
trodeposited on foreign substrate have two different cases: (1)

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H. Li et al. / Electrochimica Acta 54 (2008) 242–246
Fig. 3. Scanning electron microscopy (SEM) image of the oriented CuBr thin film
on ITO (a) and energy dispersive spectroscopy (EDS) of the electrodeposits (b). The
electrodeposits obtained from 20 mM CuBr2 aqueous solution and pH of the solution
was adjusted to 3 using 0.1 M HBr and the applied potential was −0.2 V versus SCE.
2
The total applied charge was 3 C/cm .
Fig. 6. SEM image (a) and X-ray diffraction ꢁ–2ꢁ scan (b) of initial electrodeposit.
The aqueous solution was consisted of 20 mM CuBr2. The pH of the solution was
adjusted to 3 using 0.1 M HBr. The applied potential was −0.2 V versus SCE and the
deposited time was 50 s.
electrodeposition on a single-crystal or on a poly crystalline sub-
strate. In this situation, if the interaction between the deposits and
substrate is relative strong, the preferred orientation of the elec-
trodeposits is epitaxially influences by the crystalline structure of
the substrate. (2) Electrodeposition on an inert substrate. In this
situation, the epitaxial influence of the substrate is absent and the
preferred orientation of the electrodeposits is determined by the
electrocrystallization conditions or surface energy of the deposits
[
25,26]. Here, the ITO substrate is random oriented polycrystal thin
film and then the ITO substrate should be excluded as a factor to
determine the orientation of the electrodeposited CuBr thin films.
Therefore, the electrocrystallization conditions or surface energy of
the deposits could be considered as factors to determine the orien-
tation of the electrodeposited CuBr thin films. As we have shown
above, the electrocrystallization conditions, such as pH or applied
potentials, have little effects on the growth of preferred orientation.
Besides those, the results of Fig. 6 shows that the CuBr crystals pref-
erentially nucleate along ꢀ1 1 1ꢁ crystal axis on the ITO substrate.
For preferential nuclei, the crystal growth direction is determined
by the surface energy when the interfacial energy between the
substrate and the nuclei can be ignored [27]. Therefore, the ori-
entation growth of the CuBr thin films would be determined by
the lowest surface energy plane of CuBr. As shown in Fig. 7a, a unit
cell structure of the zinc-blended CuBr is made of a face-centered
Fig. 4. X-ray diffraction ꢁ–2ꢁ scans of the electrodeposits onto ITO glass substrate
obtained under different pH. The applied potential was −0.2 V versus SCE and total
2
charge was 2 C/cm , respectively.
+
−
cubic (fcc) Cu sublattice with four Br in tetrahedrally coordinated
sites at (1/4, 1/4, 1/4)-type positions [28]. The structure models
for the (1 0 0), (1 1 0), and (1 1 1) planes are shown in Fig. 7b–d. If
only consider the copper ions, the surface density of atoms in the
2
2
(
1 0 0), (1 1 0), and (1 1 1) planes of the CuBr crystals is 2/a , 1.41/a ,
Fig. 5. X-ray diffraction ꢁ–2ꢁ scans of the electrodeposits onto ITO glass substrate
obtained under different applied potentials. The electrodeposits obtained from
2
and 2.31/a , respectively, where a is the lattice constant of CuBr
crystal. For a face center cubic (fcc) structured metal, the surface
density of atoms for the three lowest index planes is as follows:
2
0 mM CuBr2 aqueous solution and pH of the solution was adjusted to 3 using 0.1 M
HBr and the applied potentials were varied from 0.05 to −0.4 V versus SCE and total
2
applied charge was 2 C/cm , respectively.
n
{
> n
> n
, and the surface energy for the three low-
1 1 1}
{2 0 0}
{1 1 0}

H. Li et al. / Electrochimica Acta 54 (2008) 242–246
245
Fig. 7. (a) Unit cell of CuBr, (b) (1 0 0), (c) (1 1 0), and (d) (1 1 1) planes of CuBr. It shows that the surface density of atoms in the corresponding planes follows
n(1 1 1) > n(2 0 0) > n(1 1 0). In figure, small gray ball is copper ion, big black ball is bromide ion.
est index planes is as follows: ␥
< ␥
< ␥ [29,30]. This
{1 1 0}
displays [10,31]. It would be interesting to study its optical proper-
ties, especially the room temperature stimulated emission. In Fig. 8,
an absorption spectrum of CuBr thin film is shown by a broken line.
The lower and higher energy bands are associated with the Z1,2
and Z3 excitons, respectively [10,31]. The absorption peak energies
of the Z1,2 and Z3 excitons are 2.99 and 3.15 eV, respectively and
they are in agreement with those of CuBr thin films [10]. PL spec-
tra are shown by a solid line. The emission peak energy is 2.94 eV
and a Stoke shift from the Z1,2 absorption band is about 50 meV.
{
1 1 1}
{2 0 0}
rule may also hold for the zinc-blended CuBr. The formation of the
tetrahedral shape and (1 1 1) orientation of CuBr can be ascribed
to the lowest surface energy of the {1 1 1} planes which have the
highest surface density of atoms. For ionic compounds, the surface
charge effects also play a major role in determination their surface
energy.
3.5. Optical properties of CuBr thin films
The luminescence band at 2.94 eV could be associated with Z free-
f
CuBr has very large binding energies (108 eV) and could be
potentially applied in the fabrication of room-temperature blue
emitting devices, field emission displays, and vacuum fluorescent
exciton bound [10,31]. The high intense of the free-exciton PL of
the thin CuBr at room temperature indicates the high-quality CuBr
crystallites with low defects [31].
4. Conclusions
In conclusion, a simple electrochemical deposition process has
been demonstrated to grow highly oriented CuBr thin films onto
ITO glass substrate through reduction of CuBr in aqueous solution
2
at room temperature. The CuBr grow preferential orientation along
the ꢀ1 1 1ꢁ crystal axis from the X-ray diffraction patterns. The sur-
face energy of different crystal plane of CuBr crystals is believed to
play an important role to control the orientation growth of the CuBr
thin films. The oriented film exhibits strong free-exciton lumines-
cence at room temperature. The highly orientated thin films would
be expected to provide more effective electron or ion transportation
than those of random oriented thin films.
Acknowledgements
This work was financially supported by the Zhejiang Provin-
cial Natural Science Foundation of China (Y405131), Scientific
Fig. 8. The absorption and PL spectra of the electrodeposited CuBr thin films on ITO.
A broken line shows the absorption spectra. The solid line shows the PL spectra.

246
H. Li et al. / Electrochimica Acta 54 (2008) 242–246
Research Foundation for Returned Overseas Chinese Scholar of
State Personnel Ministry (G50548) and Scientific Research Founda-
tion for the Doctoral Degree Program of Higher Education of China
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