Electrochemistry of copper(I) oxide in the 66.7-33.3 mol % urea-choline chloride room-temperature eutectic melt

Article abstract of DOI:10.1149/1.3377117

The electrochemistry of Cu(I) oxide (Cu₂ O) was examined in the 66.7-33.3% mole fraction (m/o) urea-choline chloride melt. Electrochemical parameters that were measured include the standard heterogeneous rate constant and transfer coefficient of the Cu(I)/Cu(II) reaction and the Cu(I) diffusion coefficient. Data about the density, equivalent conductance, and absolute viscosity of this melt were obtained over the temperature range of 298-353 K. The conductivity and viscosity exhibited the non-Arrhenius behavior typical of glass-forming liquids. Overall, the physicochemical properties of the urea-choline chloride melt are comparable to those of common room-temperature ionic liquids. The electrodeposition of Cu was examined on glassy carbon and platinum electrodes by using potential-step techniques. The critical number of atoms required for the formation of a stable nucleus on glassy carbon was ～0, indicating that active sites on the electrode surface served as critical nuclei. Cu deposits on Ni substrates were dense, nodular, and compact.

Full text of DOI:10.1149/1.3377117

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Journal of The Electrochemical Society, 157 ͑8͒ F96-F103 ͑2010͒
0013-4651/2010/157͑8͒/F96/8/$28.00 © The Electrochemical Society
Electrochemistry of Copper(I) Oxide in the 66.7–33.3 mol %
Urea–Choline Chloride Room-Temperature Eutectic
Melt
Tetsuya Tsuda,a,b,c, Laura E. Boyd,a Susumu Kuwabata,c,d, and
Charles L. Hussey
*
*
a,**,z
aDepartment of Chemistry and Biochemistry, The University of Mississippi, University, Mississippi 38677-
1848, USA
bFrontier Research Base for Global Young Researchers, and cDepartment of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
dJapan Science and Technology Agency, Core Research Evolutional Science and Technology, Kawaguchi,
Saitama 332-0012, Japan
The electrochemistry of Cu͑I͒ oxide ͑Cu2O͒ was examined in the 66.7–33.3% mole fraction ͑m/o͒ urea–choline chloride melt.
Electrochemical parameters that were measured include the standard heterogeneous rate constant and transfer coefficient of the
Cu͑I͒/Cu͑II͒ reaction and the Cu͑I͒ diffusion coefficient. Data about the density, equivalent conductance, and absolute viscosity of
this melt were obtained over the temperature range of 298–353 K. The conductivity and viscosity exhibited the non-Arrhenius
behavior typical of glass-forming liquids. Overall, the physicochemical properties of the urea–choline chloride melt are compa-
rable to those of common room-temperature ionic liquids. The electrodeposition of Cu was examined on glassy carbon and
platinum electrodes by using potential-step techniques. The critical number of atoms required for the formation of a stable nucleus
on glassy carbon was ϳ0, indicating that active sites on the electrode surface served as critical nuclei. Cu deposits on Ni substrates
were dense, nodular, and compact.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3377117͔ All rights reserved.
Manuscript submitted November 23, 2009; revised manuscript received March 10, 2010. Published June 10, 2010.
Although they are liquid at or below room temperature, mixtures
heterogeneous kinetics of the Cu͑II͒/Cu͑I͒ reaction, and, most im-
portantly, the nucleation/growth mechanism for the electrodeposi-
tion of Cu on conductive substrates from solutions of Cu2O in this
DES. To completely analyze the electrochemical data we obtained
during this investigation, it is also necessary to know certain physi-
cal and transport properties of the urea–ChCl system. To this end,
we also present data about the density, equivalent conductance, and
viscosity of this melt system over the temperature range of 298–353
K.
of urea or acetamide with organic or inorganic salts such as NaBr or
choline chloride are not room-temperature ionic liquids ͑RTILs͒ in
the purest sense because they contain an uncharged molecular
component.1-7 However, these deep eutectic solvents ͑DESs͒ as they
are insightfully termed by Abbott et al.8 often possess physico-
chemical properties that are comparable to many RTILs. Further-
more, they can be prepared from commonly available, relatively
nontoxic components at modest expense. Because the cost of pre-
paring these melts is considerably less than that associated with the
preparation of typical RTILs, they are in many cases more attractive
candidates for technological applications. The principal disadvan-
tage of DESs is their relatively low electrical conductivity.
However, this can be improved considerably by the judicious
selection of the salt component. For example, the conductivities
of mixtures of urea with 1-ethyl-3-methylimidazolium chloride
exceed that of the related RTIL, 1-ethyl-3-methylimidazolium
bis͓͑trifluoromethyl͒sulfonyl͔͒imide.9 The acetamide- and urea-
based DES have great possibilities as a new family of green sol-
vents. Like RTILs, they may ultimately lead to new synthetic and
industrial processes. In fact, several important applications based on
these melts have already been proposed.6-13
Many metal oxides, including CuO and Cu2O, dissolve in urea–
choline chloride ͑ChCl͒ DES, but not in common RTILs.14,15 The
former produces a solution of Cu͑II͒, as indicated by the observation
of voltammetric waves that are consistent with the Cu͑II͒/Cu͑I͒ and
Cu͑I͒/Cu electrode reactions.14,15 This unanticipated solvating
power arises from the formation of metal complexes, consisting of
the intact metal oxide and urea. However, a thorough understanding
of the nature of this solvation process is lacking.
In view of its ability to solvate metal oxides, urea–ChCl may
prove to be an important solvent for electroplating. In particular, the
widespread use of copper plating technology in industry and the
potential for the inexpensive nonaqueous electroplating of copper
from urea–ChCl warrants an in-depth investigation of the Cu͑II͒/
Cu͑I͒ and Cu͑I͒/Cu reactions in this DES. Therefore, we present
information about the Cu͑I͒ and Cu͑II͒ diffusion coefficients, the
Experimental
Materials.— Urea ͑Aldrich, ACS reagent, 99.0–100.0%͒ was
pretreated by drying under vacuum ͑1 ϫ 10−3 Torr at 373 K͒. Cho-
line chloride ͑ChCl͒ ͑Aldrich, reagent grade, Ն98%͒ was purified
by precipitation from dry ethanol with ethyl acetate. The 66.7–
33.3% mole fraction ͑m/o͒ urea–ChCl melt, referred to hereafter as
“urea melt,” was prepared by mixing together the appropriate
amount of each component in a capped glass bottle inside a dry
nitrogen-filled glove box equipped with an inert gas purification
system. The bottles were agitated for 24 h at 343 K. The resulting
urea melt was clear and colorless. Following the preparation proce-
dure discussed above, the water content of the melt was checked by
using a Metrohm Karl Fisher model 756 coulometric titration appa-
ratus and was found to be less than 20 ppm ͑␮g g−1͒. Solutions of
Cu͑I͒ were prepared by the direct dissolution of anhydrous copper͑I͒
oxide ͑Aldrich, 99.99 + %͒ in the neat urea melt.
Measurements of physical properties.— Density measurements
were carried out with a 10 mL Pyrex glass pycnometer. Viscosities
were determined with a modified Cannon-Fenske No. 200 viscome-
ter ͑Fischer Scientific͒. The construction of the pycnometer and vis-
cometer as well as the detailed experimental procedures that were
used for these measurements are given in previous papers.16,17 The
pycnometer and the viscometer were calibrated with dry propylene
glycol and the 66.7–33.3 mol % AlCl3ϪEtMeImCl RTIL.18 The
data reported herein are the average values of at least three measure-
ments. Specific conductance measurements were carried out in an
airtight Pyrex glass conductivity cell equipped with Pt flag elec-
trodes. The resistance of the conductivity cell was determined as a
function of frequency with a Stanford Research Systems Model
SR720 LCR meter operated at a drive potential of 0.10 V. The con-
ductivity cell was calibrated with 0.002, 0.01, 0.05, 0.10, and
*
Electrochemical Society Active Member.
Electrochemical Society Fellow.
**
z E-mail: chclh@chem1.olemiss.edu
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0.20 mol L−1 solutions of KCl, according to IUPAC
recommendations.19 Each of the reported specific conductance val-
ues was the average of at least five measurements.
Electrochemical experiments.— Electrochemical
experiments
were carried out with an EG&G PARC model 263A potentiostat/
galvanostat. This instrument was controlled with a desktop computer
utilizing EG&G PARC model 270 software. All electrochemical ex-
periments were carried out in three-electrode cells. Electronic resis-
tance compensation was employed during voltammetric experi-
ments. The working electrodes were
a Pt disk electrode
͑Bioanalytical Systems, MF-2013, 0.020 cm2͒ and a glassy carbon
͑GC͒ disk electrode ͑Bioanalytical Systems, MF-2012,
0.0707 cm2͒. A 0.05 cm diameter Pt wire and a large surface-area
coiled Pt wire ͑Alfa Aesar, 99.95%͒ were used as quasi-reference
and counter electrodes, respectively. The Pt quasi-reference and
counter electrodes were immersed in the neat urea melt and were
separated from the bulk melt by a porosity E glass frit ͑Ace glass͒.
The ferrocenium/ferrocene ͑Fc+/Fc͒ couple was employed as an in-
ternal reference standard for all electrochemical studies. All poten-
tial values are reported with respect to this couple. Because the urea
melt is hygroscopic, electrochemical experiments were carried out
inside the nitrogen-filled glove box described above.
Preparation and characterization of the Cu electro-
deposits.— Nickel rotating disk electrodes were used as the sub-
strates for electrodeposition experiments. The rotation of these elec-
trodes was controlled with a Pine Instruments AFMSRX electrode
rotator. The crystal structure of the electrodeposits was examined by
using a Rigaku International Multi Flex X-ray diffractometer. This
instrument was operated in the ␪-2␪ scan mode and employed
Cu K␣ radiation. The surface morphology of the electrodeposits
was examined with a Hitachi S-2600H or a Keyence VE-9800 scan-
ning electron microscope ͑SEM͒. Energy dispersive X-ray spec-
trometry ͑EDAX͒ was carried out with an EDAX Genesis-XM2. In
addition, elemental analysis of bulk electrodeposits was determined
with a Perkin-Elmer Optima 2100 DV inductively coupled plasma–
optical emission spectrometer ͑ICP–OES͒.
Results and Discussion
Figure 1. ͑a͒ Density of 66.7–33.3 m/o urea–ChCl melt as a function of
temperature; ͑b͒ Arrhenius plots of ͑b͒ the absolute viscosity and ͑᭺͒ the
specific conductivity of the 66.7–33.3 m/o urea–ChCl melt. The solid lines
were calculated from the fitted parameters in Table I.
Physical properties of the urea melt.— The experimental den-
sity data for the 66.7–33.3 m/o urea–ChCl melt exhibited a linear
dependence on the absolute temperature ͑Fig. 1a͒. The linear equa-
tion shown below, where ␳ ͑g cm−3͒ is the density and T is the
absolute temperature, was fitted to these data by using standard re-
gression techniques
␳ = a + bT
͓1͔
k
1
2
␩
ln ␩ =
+
ln T − ln A
͓3͔
␩
The resulting values of a and b are given in Table I along with the
correlation coefficient, ͉R͉, for the fit.
T − T0
Arrhenius plots of the experimental data that were collected for
the specific conductivity, ␴ ͑S cm−1͒, and absolute viscosity, ␩ ͑cP͒,
are shown in Fig. 1b. As observed in the past for many RTILs and
related liquids, these plots are nonlinear. This implies that the urea
melt, like most RTILs, is a glass-forming liquid. In this case, the
transport data can be represented with the three-parameter Vogel–
Tamann–Fulcher ͑VTF͒ equations.20 In these VTF equations, A is a
scaling factor, k is a constant related to the Arrhenius activation
energy for the appropriate transport property, and T0 represents the
ideal glass transition temperature. Equation 2 gives the VTF equa-
tion in natural logarithmic form for the equivalent conductance, ⌳
The experimental data for ␴ were converted to ⌳ by using the ex-
pression ⌳ = ␴M/␳, where M is the equivalent weight of the 66.7
m/o urea melt ͑86.56 g mol−1͒; likewise, the experimental data for
the kinematic viscosity, ␯ ͑cm2 s−1͒, were converted to ␩ by using
the expression ␩ = ␯␳. Equations 2 and 3 shown above were fitted
to the converted experimental data for ⌳ and ␩, and the resulting
parameters are presented in Table I. Figure 2 exhibits linear plots of
−1
ln ⌳ + 1/2 ln T vs ͑T − T0͒
and ln ␩ − 1/2 ln T vs ͑T
−1
− T0͒ that were prepared from the parameters given in Table I,
which verify the applicability of the VTF equations.
The temperature-dependent activation energies for conductance
and viscosity, Ea,⌳ and Ea,␩, can be obtained by partial differentia-
tion of Eq. 2 and 3, respectively, with respect to T,21,22 as shown
below
͑S cm2 mol−1
͒
k⌳
1
2
ln ⌳ = −
−
ln T + ln A⌳
͓2͔
T − T0
ץ
ln ⌳
Rk⌳T2
RT
Equation 3 shows the VTF equation for the absolute viscosity, ␩
͑cP͒, which is the reciprocal of the fluidity, ␩ = 1/␾
Ea,⌳ = RT2
=
−
͓4͔
ͩ ͪ
2
ץ T
͑T − T0͒
2
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Table I. Fitted parameters for the physicochemical properties of the urea melt.
a
b
͉R͉
Density ͑g cm−3
͒
1.3437
T0 ͑K͒
209
T0 ͑K͒
208
−5.24 ϫ 10−4
k⌳ ͑K͒
0.99998
͉R͉
0.99998
͉R͉
0.99999
͉R͉
0.99988
Eq. 1
Eq. 2
Eq. 3
Eq. 6
͓298 Ͻ T ͑K͒ Ͻ 353͔
ln A⌳
8.5180
Equivalent conductance ͑S cm2 mol−1
͓298 Ͻ T ͑K͒ Ͻ 353͔
͒
822.24
k
͑K͒
ln A
␩
␩
Absolute viscosity ͑cP͒
904.60
5.5936
͓303 Ͻ T ͑K͒ Ͻ 353͔
␣
␤
0
0
Kinematic viscosity ͑mm2 s−1
͓303 Ͻ T ͑K͒ Ͻ 353͔
͒
Ϫ4.2458
10.992
these parameters is shown in Fig. 4. It is often difficult to convert
the viscosity data obtained for conventional organic compounds, in-
cluding petroleum fractions, into a linear function,24-26 but in the
urea melt an exact linear dependence on log T can be obtained
because of the negligible vapor pressure of the melt over the tem-
perature range of this study.
ץ
ln ␩
Rk T2
RT
␩
Ea,␩ = − RT2
=
−
͓5͔
ͩ ͪ
2
ץ T
͑T − T0͒
2
The resulting plots of Ea,⌳ and Ea,␩ vs T are shown in Fig. 3. As
expected, the activation energies decrease with increasing T, and the
close correspondence of the two plots suggests that similar configu-
rational changes are involved in these two different transport pro-
cesses. Also shown in this figure are similar plots of Ea,⌳ and Ea,␩ vs
T for an RTIL that exhibits physical and transport properties similar
to those of the urea melt, tri-1-butylmethylammonium bis͓͑trifluo-
romethyl͒sulfonyl͔imide ͑Bu3MeNTf2N͒.23 This comparison shows
that at a given temperature, the values of Ea,⌳ and Ea,␩ for
Bu3MeNTf2N are considerably smaller than that of the urea melt,
especially close to room temperature, indicating smaller activation
barriers for transport in the latter than in the urea melt. However, the
differences between the activation energies for the urea melt and the
RTIL diminish as the temperature is increased, illustrating that the
energetics of the two transport processes for the two different sol-
vents become remarkably similar at higher temperatures.
Reduction of Cu(I) in the urea melt.— Abbott et al.14 reported
that the urea melt can dissolve many metal oxides and proposed that
the facile dissolution of these oxides is a result of the formation of
metal complexes consisting of the metal oxide and urea with the
possible incorporation of chloride as well. In particular, copper͑I͒
oxide, Cu2O, shows a relatively high solubility in the urea melt of
more than 8 ppt; the dissolution rate can be enhanced considerably
by increasing the temperature. Based on these previous
reports,14,15,27 we assumed that upon dissolution in the melt, Cu2O
forms complexes of the general form ͕͓Cu2O·m͑NH2͒2CO͔·nCl−͖n−
and that these complexes are involved in all of the electrochemical
reactions. Henceforth, these complexes are designated simply as
“Cu͑I͒.”
Figure 5a shows cyclic staircase voltammograms ͑CSVs͒ re-
corded at a Pt electrode in the urea melt containing 20.1 mmol L−1
Cu͑I͒. On the negative scan, a single large reduction wave with an
associated oxidation wave and a small prereduction peak at Ϫ0.68 V
corresponding to underpotential deposition ͑UPD͒ are observed
͑Fig. 5a, inset͒. As described below, the major reduction and oxida-
tion waves arise from the deposition and stripping of bulk copper
metal. The charge density attributed to the UPD process was
385.5 ␮C cm−2. The Cu thickness estimated from this value and
the geometrical area of the electrode was equivalent to 1.57 atomic
Because the kinematic viscosity of the urea melt is needed for
the analysis of data produced during the hydrodynamic voltammetry
experiments given below, we also used a convenient empirical ex-
pression to linearize the experimental data for ␯ as a function of T.
This equation was originally reported by Walther24 and modified by
Mehrotra25 and Aboul-Seoud and Moharam26
log log͑␯ + 0.8͒ = ␣0 + ␤0log T
͓6͔
The parameters resulting from fitting this equation to the experimen-
tal data are also collected in Table I, and the line calculated with
Figure 3. Temperature-dependent activation energies for the absolute viscos-
ity ͑b͒ and equivalent conductance ͑᭺͒ for the 66.7–33.3 m/o urea–ChCl
melt and the absolute viscosity ͑᭡͒ and equivalent conductance ͑᭝͒ for the
Bu3MeNTf2N RTIL.
Figure 2. VTF plots of ͑b͒ the absolute viscosity and ͑᭺͒ the equivalent
conductance of the 66.7–33.3 m/o urea–ChCl melt. The solid and dotted
lines show the linear regression results for each data set.
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Figure 4. ͑b͒ Arrhenius plot of the kinematic viscosity of 66.7–33.3 m/o
urea–ChCl melt and ͑᭺͒ variation of the kinematic viscosity according to Eq.
6. The solid line shows the fit of the VTF equation to the experimental data;
the dotted line was calculated from the parameters in Table I resulting from
the fit of Eq. 6 to the experimental data.
layers. Given the typical surface roughness of a carefully polished
metal electrode, this value is reasonably close to one monolayer. A
scan initiated toward positive potentials shows an oxidation wave
and an associated reduction wave arising from the oxidation of Cu͑I͒
to Cu͑II͒ and the reverse of this reaction. This process is discussed
in more detail below.
Figure 5b shows voltammograms recorded at a GC electrode.
The main features of these voltammograms are similar to those seen
at the Pt electrode, except that there is no discernible UPD wave,
and a nucleation loop is apparent when the negative scan is reversed.
This anomalous loop occurs because of the substantial overpotential
Figure 6. ͑a͒ Potential-step current–time transients recorded at a GC disk
electrode in 66.7 m/o urea–ChCl melt containing 20.1 mmol L−1 Cu͑I͒ at
323 K. ͑b͒ Dimensionless current–time transients constructed from the data
in ͑a͒ for ␩op= ͑b͒ Ϫ0.643, ͑᭺͒ Ϫ0.618, ͑᭡͒ Ϫ0.593, and ͑᭝͒ Ϫ0.568 V.
The theoretical transient was fitted to the experimental data by using the
adjustable parameters, ␣ and x, in Eq. 10. ͑c͒ Plot of ln͑AN0͒ as a function of
␩
.
op
required to initiate the first stages of the metal deposition process on
the “foreign” surface of the GC substrate. When the scan is reversed,
metal continues to deposit at more negative potentials because the
electrode surface has been converted to a copper surface. Metal
deposition finally ceases when the potential is scanned positive of
Eeq, which is determined from the intersection point of the anoma-
lous loops with the zero current axis in the CSV.
Chronoamperometric techniques have been employed exten-
sively in RTILs to investigate nucleation/crystallization processes
during the electrodeposition of metals on foreign substrates.7 Thus,
to investigate the nucleation of Cu on GC from the urea melt, we
conducted chronoamperometric experiments in the same solution
used to record the voltammograms in Fig. 5b. The overpotentials
used for these experiments, ␩op, were calculated with the following
equation
␩
= Eapp − Eeq
͓7͔
op
where Eapp is the applied potential. Examples of the resulting chro-
noamperograms are shown in Fig. 6a. As expected from the volta-
mmograms in Fig. 5b, these current–time curves exhibit the features
typically associated with a three-dimensional nucleation rate-
controlled deposition process. The shapes of these transients are
complex and consists of five main elements: ͑i͒ an initial current or
current density due to charging of the electrode double layer, ͑ii͒ a
potential-dependent time delay relevant to the onset of nucleation,
͑iii͒ a rising current density due to nucleation and crystal growth,
Figure 5. CSVs recorded at ͑a͒ platinum and ͑b͒ GC disk electrodes for the
reduction of a 20.1 mmol L−1 solution of Cu͑I͒ in 66.7 m/o urea–ChCl melt
at 323 K. The scan rates were 50 mV s−1. ͑Inset͒ Cyclic voltammograms
recorded at the platinum electrode in the Ϫ0.45 to Ϫ0.85 V potential region;
the scan rates were 5 mV s−1
.
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