Electrochemical preparation of photoelectrochemically active CuI thin films from room temperature ionic liquid

Article abstract of DOI:10.1016/j.electacta.2012.01.044

Cuprous iodide (CuI) thin films with photoelectrochemical activity were prepared by anodizing copper wire or copper-electrodeposited tungsten wire in the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-PF₆ RTIL) containing N-butyl-N- methylpyrrolidinium iodide (BMP-I). A copper coating was formed on the tungsten wire by potentiostatic electrodeposition in BMP-dicyanamide (BMP-DCA) RTIL containing copper chloride (CuCl). The CuI films formed using this method were compact, fine-grained and exhibited good adhesion. The characteristic diffraction signals of CuI were observed by powder X-ray diffractometry (XRD). X-ray photoelectron spectroscopy (XPS) also confirmed the formation of a CuI compound semiconductor. The CuI films demonstrated an apparent and stable photocurrent under white light illumination in aqueous solutions and in a RTIL. This method has enabled the electrochemical formation of CuI from a RTIL for the first time, and the first observation of a photocurrent produced from CuI in a RTIL. The coordinating strength of the anions of the RTIL is the key to the successful formation of the CuI thin film. If the coordinating strength of the anions of the RTIL is too strong, no CuI formation is observed.

Full text of DOI:10.1016/j.electacta.2012.01.044

Electrochimica Acta 65 (2012) 204–209
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical preparation of photoelectrochemically active CuI thin films from
room temperature ionic liquid
Hsin-Yi Huang, Da-Jean Chien, Genin-Gary Huang, Po-Yu Chen ,1
∗
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung City 807, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Cuprous iodide (CuI) thin films with photoelectrochemical activity were prepared by anodizing
copper wire or copper-electrodeposited tungsten wire in the room temperature ionic liquid 1-butyl-
Received 27 October 2011
Received in revised form 11 January 2012
Accepted 12 January 2012
Available online 20 January 2012
3
-methylimidazolium hexafluorophosphate (BMI-PF6 RTIL) containing N-butyl-N-methylpyrrolidinium
iodide (BMP-I). A copper coating was formed on the tungsten wire by potentiostatic electrodeposition
in BMP-dicyanamide (BMP-DCA) RTIL containing copper chloride (CuCl). The CuI films formed using this
method were compact, fine-grained and exhibited good adhesion. The characteristic diffraction signals of
CuI were observed by powder X-ray diffractometry (XRD). X-ray photoelectron spectroscopy (XPS) also
confirmed the formation of a CuI compound semiconductor. The CuI films demonstrated an apparent and
stable photocurrent under white light illumination in aqueous solutions and in a RTIL. This method has
enabled the electrochemical formation of CuI from a RTIL for the first time, and the first observation of a
photocurrent produced from CuI in a RTIL. The coordinating strength of the anions of the RTIL is the key
to the successful formation of the CuI thin film. If the coordinating strength of the anions of the RTIL is
too strong, no CuI formation is observed.
Keywords:
Cuprous iodide
Electrodeposition
Anodization
Photocurrent
Compound semiconductor
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
[8] or the electrochemical conversion from Cu to CuI [9], and one
report discussed the conversion of Cu to CuI on Cu(1 1 1) [10].
Room temperature ionic liquids (RTILs) have been recognized
as unique solvent systems and employed for many purposes,
including the electrodeposition of metals, alloys, and semicon-
ductors [11,12] because they have many special properties such
as nonvolatility, (electro)chemical and thermal stability, a wide
temperature range over which they are liquid, and adjustable
physicochemical properties. The electrochemical formation of CuI
has never been carried out in RTILs, however. Herein we report
the electrochemical preparation of CuI thin films converted from
surface Cu by simply anodizing Cu at a constant potential in the
RTIL 1-butyl-3-methylimidazolium hexafluorophosphate (BMI-
Cuprous iodide (CuI) is an important photoelectric semicon-
ductor with a band gap of 3.1 eV and is often used for visible
light-assisted photoelectrochemical and solar energy conversion
systems. CuI is a convenient p-type semiconductor used for con-
structing fully solid-state dye-sensitized photovoltaic cells because
it is optically transparent and has good hole conductivity [1,2].
Several methods have been developed to prepare CuI nanocrys-
tallites [3–5]. Appropriate immobilization of these semiconductor
nanocrystallites on a solid surface is, however, usually required for
practical applications. It would be thus more convenient to directly
form CuI thin films on a surface of interest. The most frequently
used procedures for the direct formation of CuI on a solid surface
involve three key steps developed by Penner [6]. The first step is
the electrodeposition of Cu nanocrystallites on a conductive sur-
PF ) containing N-butyl-N-methylpyrrolidinium iodide (BMP-I).
6
This process can be called iodination. Compared with the published
literature in which direct formation of CuI on an electrode sur-
face was reported [6,8], the method mentioned here involves fewer
steps (the Cu surface can be directly converted to CuI without the
face. Second, the Cu nanocrystallites are oxidized to Cu O. The final
step involves the displacement of the oxide with iodide (from Cu O
2
2
to CuI). Zen and co-workers applied the same procedures in Tris-
transition from Cu to Cu O). In addition, no volatile organic sol-
2
buffer solution, and the formation of Cu O was more controllable
vents such as acetone are required because the RTIL is nonvolatile.
Two types of Cu substrate, Cu wire electrodes and Cu-coated W
wire electrodes, were employed in this study. For the Cu wire elec-
trode, the Cu surface could be directly converted to CuI in one step.
Two steps were essential for the Cu-coated W wire electrode: elec-
trodeposition of Cu and then iodination. The Cu layer coated on the
W wire electrode was formed by electrodeposition at a constant
potential in the RTIL BMP-dicyanamide (BMP-DCA) containing CuCl
2
[
7]. A few studies have reported the direct electrodeposition of CuI
∗ Corresponding author. Tel.: +886 7 3121101x2587; fax: +886 7 3125339.
E-mail address: pyc@kmu.edu.tw (P.-Y. Chen).
ISE member.
1
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2012.01.044

H.-Y. Huang et al. / Electrochimica Acta 65 (2012) 204–209
205
as the source of Cu(I) ions. The obtained CuI thin films were char-
acterized with a scanning electron microscope (SEM) couples with
an energy dispersive spectrometer (EDS) that was employed for
the semi-quantitative elemental analysis. A powder X-ray diffrac-
tometer was used to determine the crystalline structure of the CuI
film and an X-ray photoelectron spectrometer (XPS) was used to
determine the surface state of the CuI films. The CuI thin films
also exhibited a stable photoelectrochemical response in aqueous
media and in an RTIL under the illumination of a halogen light bulb.
2
. Experimental
2.1. Materials and instrumentation
Electrochemical experiments were performed either inside
a glove box (MBRUAN, UNI-LAB B) using a Princeton Applied
Research potentiostat/galvanostat (PAR 263A) or outside a glove
box with a CH Instrument electrochemical analyzer (CHI 660C).
A traditional three-electrode electrochemical cell was employed,
and the detailed configuration has been described previously [13].
A platinum disk electrode (1.6 mm Ø) was used for the voltammet-
ric study in a glove box. For the electrochemical formation of CuI,
Cu wire (Alfa Aesar, 99.9%, 1 mm Ø) or W wire (Alfa Aesar, 99.95%,
Fig. 1. CVs recorded at Pt disk or Cu wire electrode in the solutions indicated in the
−
1
plot. The arrows indicate the initial direction of potential scan. Scan rate: 50 mV s
.
nitrogen atmosphere in a glove box. The obtained CuI films were
mixed and ground with an appropriate amount of dry and pure KBr
salt to form transparent CuI-KBr disks for study by UV–vis spec-
troscopy using UV–vis spectrophotometer (Thermo GENESYS 10S
BioUV-Vis).
0
(
.5 mm Ø), the latter of which was electrodeposited with a Cu layer
Cu/W), was used as the working electrode. A reference electrode
fabricated by immersing a piece of platinum wire in a ferrocene
+
+
(
Fc)/ferrocenium (Fc ) solution (the molar ratio of Fc/Fc = 1) in a
glass tube with a porous Vycor tip was employed. Therefore, the
+
potential was reported with respect to the redox potential of Fc/Fc
3. Results and discussion
for the electrochemical experiments carried out inside a glove box.
A platinum spiral immersed in the IL and separated from the bulk
solution by a porosity E glass frit was used as a counter electrode.
Outside the glove box, an Ag/AgCl (NaCl saturated) reference elec-
trode and a platinum wire counter electrode were used in aqueous
solutions.
3.1. Cyclic voltammetric study
Before formation of the cuprous iodide films (CuI films), evalua-
tion of the behavior of BMP-I in BMI-PF6 RTIL by cyclic voltammetry
was necessary. The cyclic voltammogram (CV) recorded at a plat-
inum disk electrode in BMI-PF6 RTIL containing 100 mM BMP-I is
shown in Fig. 1 (the solid curve). The potential was first scanned at
an initial potential Ei of −0.400 V in the anodic direction, and two
redox couples were observed. The relevant reactions of the two
redox couples are indicated in the figure. The peak potentials of the
two oxidative and the two reductive waves are −0.052 V/0.458 V
and 0.274 V/−0.236 V, respectively. The same voltammetric behav-
The
room
temperature
ionic
liquid
1-butyl-3-
methylimidazolium hexafluorophosphate (BMI-PF6 RTIL) [14]
and N-butyl-N-methylpyrrolidinium iodide (BMP-I) [15] were pre-
pared by following the published procedures. BMP-dicyanamide
(
BMP-DCA) RTIL was also prepared by following the procedure
published in the literature [15].
−
2.2. Formation of CuI thin films
ior of the I ions in BMI-BF4 RTIL (BF4 is the abbreviation for
tetrafluoroborate) has been reported [16].
CuI thin films were prepared in a glove box via conversion of sur-
The dashed curve (E = −1.00 V) in Fig. 1 shows the CV recorded
i
face Cu at a copper wire electrode or a Cu-coated W wire electrode
at a Cu wire electrode in BMI-PF6 containing 100 mM BMP-I. The
electrode potential was initially scanned in the anodic direction.
Apparently, Cu wire can be oxidized in the anodic scan because
two broad oxidative waves were observed at Ep,a1 = −0.536 V
and Ep,a2 = −0.368 V. However, these two oxidative waves do not
demonstrate the typical behavior associated with anodic metal dis-
solution in a RTIL because rounded, rather than sharp oxidative
waves with a steeply increasing current, are observed, indicating
the formation of some insoluble layers. It was found that CuI thin
films could be formed by holding the potential at these oxidative
waves, which involved the production of Cu+ cations (a detailed
discussion follows). Thus, the formation of CuI should result from
+
(
1
Cu/W) by anodization at −0.2 V (vs. Fc/Fc ) in BMI-PF containing
6
00 mM BMP-I. The obtained CuI-coated electrodes are denoted as
CuI/Cu and CuI/W, respectively. The Cu layer coated on a W wire
electrode (Cu/W) was formed by potentiostatic electrodeposition
+
(
5
at −1.9 V vs. Fc/Fc ) from firmly stirred BMP-DCA RTIL containing
0 mM CuCl. The as-prepared CuI films were cleaned by soaking the
CuI/Cu or CuI/W electrodes in acetone to remove residual RTIL and
then dried under nitrogen.
2
.3. Characterization and photoelectrochemical measurements
+
−
The CuI films were characterized using SEM (FEI Quanta 400F
the reaction of Cu + I → CuI.
or Philips XL-40 FEG) and XPS (JEOL JAMP-9500F). The crystalline
structure of the CuI films was analyzed by XRD (Shimadzu Model
XD-D1). The photocurrent produced from the CuI films under the
illumination of a 50 W (Watt) halogen light bulb was studied using
cyclic voltammetry and chronoamperometry (the latter was per-
To provide a comparison, sufficient BMP-I was introduced
into BMI-PF , and the bulk electrolysis of a Cu wire electrode
6
−
was carried out to produce I -coordinated Cu(I) ions (dash-dot
curve in Fig. 1, with an initial potential Ei of −0.450 V). The
applied potential was scanned from the initial potential in the
+
formed at different potential biases (Ebias vs. Ag/AgCl or Fc/Fc )) in
cathodic direction, and two redox couples (c /a1 and c /a ) were
1 2 2
pH 8 phosphate buffer solution (PBS) containing 10 mM KI under
observed. Based on the experimental results in the previous study
[17], the relevant redox reactions for the two redox couples are
an air atmosphere or in BMI-PF containing 30 mM BMP-I under a
6

2
06
H.-Y. Huang et al. / Electrochimica Acta 65 (2012) 204–209
Fig. 3. XRD patterns of (a) Cu foil and CuI/Cu formed from the conversion of Cu foil,
and (b) Cu/W and CuI/W formed from the conversion of Cu/W. Cu/W and CuI film
were prepared by following the procedures indicated in Fig. 2. Insets show the XPS
spectra for the relevant CuI film.
As mentioned above, CuI thin films were converted from the
surface Cu as long as the correct anodizing potential was applied.
Therefore, an applied potential of −0.2 V was employed for the
following experiments in order to convert surface Cu into CuI. In
addition to the applied potential, another key factor for the suc-
cessful formation of CuI is the coordinating strength of the anions
of the RTIL. BMI-PF was employed in this study because the inter-
6
I
−
action between Cu and PF6 is very weak (the anodic dissolution
Fig. 2. SEM micrographs of (a) CuI/Cu formed by anodizing Cu wire at −0.2 V in
BMI-PF6 containing 100 mM BMP-I, (b) Cu/W formed by electrodepositing Cu onto
W wire at −1.9 V in BMP-DCA with 50 mM CuCl, and (c) CuI/W converted from the
Cu/W of (b) by using the same iodination procedure indicated in (a).
of Cu is not feasible in a PF -based RTIL, indicating a very weak
6
−
I
interaction between PF6 and Cu ) compared with other frequently
used anions such as bis((trifluoromethyl)sulfonyl)amide (TFSA)
I
and dicyanamide (DCA) [18]. The interaction between Cu and these
−
two anions (TFSA and DCA) is too strong to allow formation of solid
CuI films because more stable TFSA- or DCA-coordinated Cu(I) com-
plex ions are formed in TFSA- or DCA-based RTILs. The formation
of TFSA- or DCA-coordinated Cu(I) complex ions was confirmed
because copper could be directly anodized and dissolved into TFSA-
and DCA-based RTILs.
assigned as Cu(I) + e ↔ Cu (Ep,c1/Ep,a1 = −0.900/−0.600 (V)) and
−
Cu(II) + e ↔ Cu(I) (E
/E
= −0.145/0.000 (V)), respectively. The
p,c2 p,a2
ꢀ
reductive wave c1 corresponds to the under potential deposition
UPD) of Cu at the Pt surface. No CuI film was formed in this solu-
(
tion no matter what oxidative potential was applied. Thus, the
mechanism mentioned in the literature is not involved in the CuI
formation in this system [8]. In that study, CuI was formed from the
decomposition of CuI32−(CuI3 → CuI + I + 2e ) in acetone. Addi-
2−
−
3.2. Electrochemical formation of CuI thin films and their
characterization
2
tionally, the a wave of the dashed curve overlaps with the a wave
1
1
of the dash-dot curve, indicating that both oxidative waves involve
–
the same reaction (Cu → Cu(I) + e ). The a wave observed in the
As noted above, CuI films can be formed on Cu wire elec-
trode (CuI/Cu) by anodization of the Cu electrode at −0.2 V in
BMI-PF6 containing BMP-I. Fig. 2a shows the SEM micrograph of
2
dashed curve may be due to the formation of a CuI film, leading to
a current fluctuation.

H.-Y. Huang et al. / Electrochimica Acta 65 (2012) 204–209
207
BMP-DCA RTIL was employed for the Cu electrodeposition because
copper halides such as copper chloride (CuCl) have good solubil-
ity in BMP-DCA RTIL due to the strong ligand properties of the DCA
anions. BMP-TFSA RTIL is also appropriate for Cu electrodeposition;
however, the Cu ions, such as Cu(I) ions, must be introduced by
anodic dissolution of copper metal because common copper salts
such as copper halides are insoluble in this RTIL. For BMI-PF RTIL,
6
CuCl is insoluble and anodic dissolution of copper metal is unfea-
−
sible because of the very poor ligand properties of the PF6 ions.
−
Additional Cl ions (ex. from BMI-Cl) have to be introduced to assist
the CuCl dissolution. Electrodeposition of Cu in BMI-PF is feasible
6
but inconvenient.
After the electrodeposition of copper was accomplished at a W
wire electrode (Fig. 2b), the Cu layer coated on the W wire electrode
was converted to CuI (CuI/W) by employing the same iodination
process mentioned in Fig. 2a, and the relevant SEM micrograph is
depicted in Fig. 2c. Compared with Fig. 2a, the CuI film at the CuI/W
electrode has a very different surface morphology, and the crystal-
lites are smaller and more irregular in shape, but the CuI film has
the same good adhesion as the film shown in Fig. 2a. The results
shown in Fig. 2 demonstrate the advantages of preparing CuI thin
films by Cu anodization in RTILs. For certain practical applications,
some type of functional layer may need to be formed on the top of
the CuI film. A fine-grained, rather than a rough, surface of CuI is
essential for this purpose [1]. Therefore, the inhibitor of crystal for-
mation is necessary. It can be observed that the CuI films formed in
this study are compact and fine-grained. Therefore, they are appro-
priate for the formation of a top layer. In addition, preparation of a
compact and smooth Cu coating for use in the formation of CuI thin
films is very easy by electrodeposition in BMP-DCA RTIL, unlike in
Fig. 4. UV–vis spectrum of transparent CuI-KBr disk. The dashed line indicates the
abrupt absorption near 400 nm and the solid line shows the onset absorption near
4
20 nm.
the CuI/Cu electrode. During anodization, the smooth and mirror-
like Cu surface was gradually converted to a white thin layer that
was composed of compact and fine-grained crystalline prisms. To
understand whether the CuI films could be formed on other con-
ductive substrates, a W wire electrode was electrodeposited with a
Cu thin layer (Cu/W) from BMP-DCA RTIL containing 50 mM CuCl,
and the SEM micrograph of this Cu layer is shown in Fig. 2b. Appar-
ently, the electrodeposited Cu layer is very compact and smooth.
Fig. 5. The photovoltammogram of (a) CuI/Cu and the photochronoamperograms of (b) CuI/Cu, and (c) CuI/W at different Ebias in pH 8 PBS containing 10 mM KI. (c) Shows
the photochronoamperogram of CuI/Cu in BMI-PF6 plus 30 mM BMP-I in a glove box.

2
08
H.-Y. Huang et al. / Electrochimica Acta 65 (2012) 204–209
aqueous solutions, where Cu dendrites are always obtained unless
additives such as environmentally harmful thiourea are used.
Although the surface morphologies of the CuI films converted
from different Cu substrates were different to a certain degree, no
noticeable differences could be found in their X-ray diffraction pat-
terns. Fig. 3 shows that both of the CuI films converted from the
different Cu substrates (Cu wire and Cu/W) were crystalline, and
four characteristic diffraction signals of CuI were observed. The rel-
ative intensities of the diffraction peaks observed for the two CuI
films were very similar to those reported in the literature [7]. Aside
from the four apparent CuI signals, other signals observed in Fig. 3
are diffraction signals of the substrates, including copper foil and
tungsten foil.
To further confirm the formation of the CuI films converted from
the different Cu surfaces by iodination in BMI-PF6 RTIL contain-
ing BMP-I, XPS analysis was carried out on the two CuI samples
shown in Figs. 3a and b, and the experimental data are shown in
the relevant insets. The binding energy (BE) of the Cu2p electron
observed for both of the CuI films (CuI/Cu and CuI/W) indicates
that the oxidation state of the surface Cu is +1. The BE of the I3d
electron provides more solid evidence for CuI formation because
its value is nearly identical to the database value for CuI (619 eV)
[
19].
The CuI film obtained by iodination of a copper wire electrode
was carefully scraped off of the electrode and ground with an appro-
priate amount of KBr crystals to form a transparent disk under high
pressure. The UV–vis spectrum of this disk was then acquired to
investigate the electron absorption of the obtained CuI film. The
characteristics of the spectrum (Fig. 4) are very similar to the liter-
ature data. Fig. 4 shows an abrupt absorption near 400 nm (∼3.1 eV
of the CuI energy gap) and an onset absorption near 420 nm [7]. This
behavior is due to the excitation of electrons from the valance band
to the conduction band. The experimental data indicate that the CuI
film prepared in this study has a similar band gap as that of a regular
CuI compound semiconductor. The aforementioned procedure was
employed due to the lack of availability of a good reflective UV–vis
spectrophotometer. The high background absorption (Fig. 4) was
a disadvantage and possibly resulted from the absorption and/or
scattering of KBr.
3.3. Photoelectrochemical examination of the obtained CuI films
CuI is a semiconductor demonstrating photoelectrochemical
properties under visible radiation. Therefore, it is essential to study
the photoelectrochemical properties of the CuI thin films obtained
in this study. By modifying a system reported in the literature [7],
a 0.1 M pH 8 phosphate buffer solution (PBS) containing 10 mM
KI was employed as the electrolyte for studying the photoelec-
trochemical behavior of the CuI films. Cyclic voltammetry and
chronoamperometry were carried out to study the photocurrent
produced from illuminating the CuI films. A 50 W halogen light bulb
was used as the light source for this study. The light bulb was turned
on and off manually. Fig. 5a shows the typical photovoltammogram
of a CuI/Cu electrode. The potential was scanned from the initial
potential of −0.2 V in the cathodic direction and then reversed at
2
Fig. 6. The dependence of photocurrent square (i ) on potential bias (Ebias).
shift of Ebias in the cathodic direction. A similar behavior is observed
for the CuI film at the CuI/W electrode (Fig. 5c), but the stable pho-
toresponses were observed within a narrower Ebias range (from
−0.22 to −0.30 V vs. Ag/AgCl). The photoresponse of the CuI/Cu
−
0.38 V. In order to make the voltammogram more readable, the
anodic scan is shown as a dotted curve. The apparent photocurrents
were observed with the light on and the light off. Fig. 5a also shows
that the two curves (the cathodic and anodic scans) overlap to a
significant degree.
In order to study in more detail the dependence of the pho-
tocurrent on the applied potential, photochronoamperometry was
carried out at a series of potential biases (Ebias). Fig. 5b shows the
typical photoresponses of the CuI film at the CuI/Cu electrode when
different Ebias (from −0.10 to −0.40 V vs. Ag/AgCl) were applied. As
can be seen, the photocurrents are very stable and increased with a
electrode was also studied in BMI-PF RTIL containing 30 mM BMP-
6
I in a glove box (Fig. 5d). A similar behavior as that shown in Fig. 5b
was observed although more significant noise was detected (poorer
mass transport and conductivity in the IL may be the cause of the
noise), and the stable photoresponses were only observed within
+
a very narrow Ebias range (from −0.45 to −0.51 V vs. Fc/Fc ). This
investigation is the first reported study of the photocurrent of CuI
in a RTIL.

H.-Y. Huang et al. / Electrochimica Acta 65 (2012) 204–209
209
The square of the measured photocurrent (i) was plotted as a
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4
[
[
(
Acknowledgement
[
[
This work was supported by the National Science Council of
Taiwan (NSC 98-2113-M-037-007-MY3).