Electrodeposition of zinc coatings from the solutions of zinc oxide in imidazolium chloride/urea mixtures

Article abstract of DOI:10.1007/s11426-012-4682-y

To solve the inherent disadvantages in conventional processes for electrodeposition of zinc, it's necessary to develop more high-efficiency and environmentally friendly electrolytes. In this work, it was found that the dissolution of ZnO was remarkably enhanced in some imidazolium chloride by the addition of urea, and the solubility of ZnO in 1:1 [Amim]Cl/urea mixture was as high as 8.35 wt% at 373.2 K. Electrochemical measurements showed that zinc could be readily electrodeposited from the solutions of ZnO. Bright, dense and well adherent zinc coatings with good purity were obtained from 0.6 M solution of ZnO in 1:1 [Amim]Cl/urea at 323.2?343.2 K. It's expected that the solutions of ZnO in imidazolium chloride/urea mixtures have the potential to replace the traditional electrolytes, especially toxic zinc chloride-based ones for zinc electroplating, as well as preparation of zinc materials. Science China Press and Springer-Verlag Berlin Heidelberg 2012.

Full text of DOI:10.1007/s11426-012-4682-y

SCIENCE CHINA
Chemistry
•
·
ARTICLES •
SPECIAL ISSUE · Ionic Liquid and Green Chemistry
August 2012 Vol.55 No.8: 1587–1597
doi: 10.1007/s11426-012-4682-y
Electrodeposition of zinc coatings from the solutions of zinc
oxide in imidazolium chloride/urea mixtures
1
,2
1,2
1
1
ZHENG Yong , DONG Kun , WANG Qian , ZHANG SuoJiang ,
1
,2
1*
ZHANG QinQin & LU XingMei
1
Beijing Key Laboratory of Ionic Liquids Clean Process; State Key Laboratory of Multiphase Complex Systems; Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, China
College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100049, China
2
Received April 18, 2012; accepted April 26, 2012; published online July 6, 2012
To solve the inherent disadvantages in conventional processes for electrodeposition of zinc, it’s necessary to develop more
high-efficiency and environmentally friendly electrolytes. In this work, it was found that the dissolution of ZnO was remarka-
bly enhanced in some imidazolium chloride by the addition of urea, and the solubility of ZnO in 1:1 [Amim]Cl/urea mixture
was as high as 8.35 wt% at 373.2 K. Electrochemical measurements showed that zinc could be readily electrodeposited from
the solutions of ZnO. Bright, dense and well adherent zinc coatings with good purity were obtained from 0.6 M solution of
ZnO in 1:1 [Amim]Cl/urea at 323.2−343.2 K. It’s expected that the solutions of ZnO in imidazolium chloride/urea mixtures
have the potential to replace the traditional electrolytes, especially toxic zinc chloride-based ones for zinc electroplating, as
well as preparation of zinc materials.
electrochemistry, electroplating, metal oxide, zinc deposits, ionic liquids
1
Introduction
trolytes. Therefore, it’s necessary to develop not only novel
techniques, but also more high-efficiency and environmen-
tally friendly electrolytes for the electrodeposition of zinc.
Ionic liquids (ILs), the so-called low-temperature molten
salts are composed entirely of organic cations and organ-
ic/inorganic anions with relatively low melting points
(typically near or below room temperature) [5]. Owing to
their unique structures, ILs exhibit a number of attractive
features, including low volatility, low flammability, broad
electrochemical window and high electrical conductivity
[6, 7]. The study on ILs has gained wide attention and re-
markable progress has been achieved in a wide variety of
research areas over the past two decades [8–10]. Many re-
ports demonstrate that ILs are a class of novel electrolytes
in the electrochemistry, especially in the electrodeposition
of reactive metals [11–13].
Zinc is well known for its anti-corrosive coatings, which
have been widely applied in the electroplating industry [1,
2
]. The conventional processes for preparing zinc coatings
are often conducted by electrodeposition in aqueous solu-
tions and inorganic molten salts. However, these methods
suffer from a series of inherent disadvantages [3, 4]. For
example, the high experimental temperature makes the
deposition difficult to handle in the inorganic salts. The hy-
drogen release generated from aqueous solutions always
results in the hydrogen embrittlement of zinc deposits, low
current efficiency and high energy consumption. Moreover,
the zinc salts and acids used in these methods are unfriendly
to the environment. It’s acknowledged that such problems
are mainly rooted in the natural defects of traditional elec-
ILs have been demonstrated as a promising alternative to
the conventional electrolytes in the electrodeposition of zinc.
*
Corresponding author (email: xmlu@home.ipe.ac.cn)
©
Science China Press and Springer-Verlag Berlin Heidelberg 2012
chem.scichina.com www.springerlink.com

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Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
As early as 1999, Lin et al. [14] first found that chlorozin-
cate based IL was formed by the combination of 1-ethyl-3-
the solutions of ZnO in 1:1 [Amim]Cl/urea were chosen as
model electrolytes and the processes of zinc deposition un-
der different conditions have been explored.
methylimidazolium chloride ([Emim]Cl) and ZnCl
could be successfully electrodeposited from Lewis acidic
Emim]Cl/ZnCl systems in which the molar percentage of
ZnCl was higher than 33%. In the following years, some
chlorozincate-based ILs with different organic cations were
also investigated in this area [15–17]. Although salt/ZnCl
melts were more stable than traditional chloroaluminate ILs
in the presence of water and air, ZnCl was vulnerable to
2
. Zinc
[
2
2
Experimental
2
2
.1 Chemicals
2
All the chemicals used in this work were purchased com-
mercially with analytical grade (purity > 99.5%) from Alfa
Aesar and Sinopharm Chemical Reagent Co., Ltd. Liquid
reagents, including N-methylimidazolium, allyl chloride,
2
hydrolysis and highly toxic. Similarly, many other electro-
lytes prepared from zinc salts may also have such problems
1
-chlorobutane, 2-chloroethanol, acetone and methylene
[18]. In consideration of properties as well as costs, ZnO
chloride were purified by distillation. Choline chloride,
urea, sodium thiocyanate and sodium dicyanamide were
recrystallized twice and dried under vacuum. Zinc oxide
was dried under vacuum for 48 h at 353 K. Prior to use, zinc
plate (99.95%), zinc wire (99.99%), platinum wire (99.95%),
platinum plate (99.95%) and copper foil (99.95%) were
polished with emery paper, cleaned with acetone, treated
with a dilute hydrochloric acid/sulfuric acid mixture, finally
rinsed with deionized water and dried.
can be an ideal alternative to zinc salts. However, ZnO is
hard to dissolve in most traditional ILs besides some
task-specific ones with Brønsted acidic groups [19, 20]. It
should be noted that the active hydrogen protons in these
groups can seriously disrupt the deposition process. There-
fore, the Brønsted acidic ILs are not suitable for electro-
deposition of zinc.
Until 2005, Abbott and his coworkers first found that
ZnO could be dissolved in deep eutectic solvents (DES),
particularly in choline chloride/urea mixture (ChCl/urea)
which was labeled as a new type of ILs [21–23]. What’s
more attracting, zinc might also be electrodeposited from
the resulting solution. The electrolyte system was less sensi-
tive to moisture and air compared with those previously
reported in this field. While these researches open a new
way to our work, some problems still exist and have to be
solved. For example, the DES are generally inferior to im-
idazolium-based ILs in many physiochemical properties,
such as electrical conductivities and chemical stability. The
solubility of ZnO should be further improved. Most of all,
the investigation on the electrodeposition of zinc from the
solutions of ZnO in ILs is very limited and more high-
efficiency electrolytes need exploration.
2.2 Synthesis and characterization
[Amim]Cl
As Zhang et al. [24] described, N-methylimidazolium and
allyl chloride at a molar ratio of 1:1.25 were mixed in a
flask with stirring for 8 h at 353 K. After purification and
drying, the final product was obtained as a yellow liquid.
1
Yield: 96.3%. H NMR: δ = 9.434 (s, 1H), 7.786 (d, 1H),
7
.769 (d, 1H), 6.005 (m, 1H), 5.282 (m, 2H), 4.865 (d, 2H),
and 3.853 (s, 3H) ppm.
[Bmim]Cl
According to Huddleston et al.’s work [25], equal molar
amounts of 1-chlorobutane and N-methylimidazolium were
added to a flask and stirred for 48 h at 343 K. Then the
mixture was extracted by ethyl acetate and dried under
vacuum. The final product was obtained as a white wax-like
In this paper, the dissolution of ZnO in some typical im-
idazolium-baesd salts: 1-allyl-3-methylimidazolium chlo-
ride ([Amim]Cl), 1-butyl-3-methylimidazolium chloride
1
(
[Bmim]Cl),
1-(2-hydroxyethyl)-3-methylimidazolium
solid at room temperature. Yield: 87.4%. H NMR: δ =
chloride ([Hemim]Cl), 1-butyl-3-methylimidazolium dicy-
9
.446 (s, 1H), 7.840 (d, 1H), 7.765 (d, 1H), 4.177 (t, 2H),
3.856 (s, 3H), 1.747 (m, 2H), 1.233 (m, 2H), and 0.873 (m,
H) ppm.
anamide ([Bmim][N(CN) ]) and 1-butyl-3-methylimidazo-
2
lium thiocyanate ([Bmim][SCN]) was studied for the first
time. Experimental results showed that the dissolution of
ZnO in imidazolium chloride could be significantly en-
hanced by the addition of urea. Through electrospray ioni-
sation mass spectrometry (ESI-MS) and IR spectra, the
solvation of ZnO was mainly driven by the formation of
3
[Hemim]Cl
N-Methylimidazolium (0.5 mol) and 2-chloroethanol (0.33
mol) were mixed and stirred at 353 K for 24 h [26]. After
that, the product was extracted by diethyl ether and dried. A
colorless solid was finally obtained at room temperature.
−
complex anion [ZnClO∙urea] . The difference between sol-
1
ubility of ZnO in these systems was also analyzed from the
viewpoint of cation-anion interaction, molar concentration
Yield: 80.7%. H NMR: δ = 9.287 (s, 1H), 7.792 (d, 1H),
7.750 (d, 1H), 5.491 (t, 1H), 4.241 (t, 2H), 3.873 (s, 3H),
and 3.695 (m, 2H) ppm.
−
of urea and Cl . Furthermore, the electrochemical meas-
urements demonstrated that zinc could be readily electro-
deposited from the solutions of ZnO in imidazolium chlo-
ride/urea mixtures. In view of physicochemical properties,
[Bmim][N(CN)
2
]
Followed by the procedure reported in literature [27],

Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
1589
NaN(CN)
2
(0.2 mol) was added into a solution of [Bmim]Cl
900). The maximal error in the solubility measurement is
0.1%. The complete dissolution of ZnO was within 1 h at
each given temperature.
(
0.2 mol) in acetone. The mixture was stirred for 24 h and
filtrated. Then, the resulting filtrate was concentrated by
rotary evaporation and dried. The final product was ob-
tained as a colorless liquid. Yield: 89.4%. H NMR: δ =
1
2.4 The measurement of physicochemical properties
9
3
3
4
.099 (s, 1H), 7.758 (d, 1H), 7.694 (d, 1H), 4.162 (t, 2H),
.849 (s, 3H), 1.769 (m, 2H), 1.265 (m, 2H), and 0.902 (m,
H) ppm. C NMR: δ = 135.79, 122.77, 121.50, 118.37,
The NMR measurements were performed on an av-400
13
MHz spectrometer (Bruker) in DMSO-d
at 298 K. The IR
spectra were recorded on an IR spectroscopy (Nicolet 380)
6
8.38, 35.08, 30.90, 18.35, and 12.30 ppm.
−
1
at 298 K with a resolution of 2 cm . An electrospray ioni-
sation mass spectrometry (ESI-MS, Micromass Q-TOF)
was used to identify the structure and content of ions. The
data of electrical conductivities were obtained through a
conductivity meter (Mettler Toledo FE30) from 298.2 to
373.2 K. The surface morphology and compositional analy-
sis of zinc coatings were examined with an emission scan-
ning electron microscope (SEM, XL30 S-FEG) and energy
dispersive X-ray (EDAX, Oxford INCA300), respectively.
And the X-ray diffraction (XRD) patterns of zinc coatings
were collected on a X’Pert Pro MPD X-ray diffractometer
(PANalytical) with Cu Kα radiation (λ = 0.15405 nm). The
water content in the studied systems was determined by
Karl Fisher titration (751 GPD Titrino).
[
Bmim][SCN]
2
0 g of [Bmim]Cl and 9.6 g of NaSCN were both added
into 30 mL acetone. The mixture was stirred at room tem-
perature for 48 h and then filtered through G4 glass frit.
After the solution was distilled by rotary evaporation, the
residual liquid was diluted with methylene chloride and
filtered again. The filtrate was concentrated by evaporation
and dried under −0.1 MPa at 333 K for 48 h. A reddish liq-
1
uid was finally obtained. Yield: 91.6%. H NMR: δ = 9.083
(
s, 1H), 7.736 (d, 1H), 7.663 (d, 1H), 4.142 (t, 2H), 3.831 (s,
3
H), 1.723 (m, 2H), 1.196 (m, 2H), and 0.826 (m, 3H) ppm.
C NMR: δ = 137.04, 130.29, 124.08, 122.77, 49.15, 36.33,
1.92, 19.33, and 13.78 ppm.
1
3
3
1
:2 ChCl/Urea
2.5 Electrochemical experiments
The IL was synthesized by mixing 1 mole of choline chlo-
ride and 2 mole of urea as Abbott et al. [28] reported. Yield:
All the electrochemical experiments were conducted under
an argon atmosphere in a glove box (MIKROUNA Univer-
sal) where water and oxygen content was both controlled
below 1 ppm. Experiments were performed with an elec-
trochemical workstation (CHI1140A) controlled by
CHI1140A software in a three-electrode cell. In the deter-
mination of cyclic voltammograms, platinum wire working
electrode and platinum plate counter electrode were used.
As for the electrodeposition experiments, the working and
counter electrodes were copper foil and zinc plate, respec-
tively. The reference electrode used was a zinc wire sealed
within a fritted glass tube containing a 0.3 M solution of
ZnO in 1:1 [Amim]Cl/urea. After deposition, all the coat-
ings were washed by absolute alcohol, deionized water and
finally dried in air.
1
9
2
1
9.3%. H NMR: δ = 5.572 (s, 4H), 3.808 (s, 1H), 3.460 (s
H), 3.421 (t, 2H) and 3.129 (s, 9H) ppm. C NMR: δ =
60.46, 67.44, 55.58, 53.66, 53.63, and 53.61 ppm.
1
3
Imidazolium-based salt/urea mixtures
Imidazolium-based salt/urea mixtures with molar ratio of
1
:1 were prepared by adding precise quantity of urea into
each salt, respectively. Each mixture was stirred above the
melting point of imidazolium-based salt until a clear solu-
tion was formed. All the imidazolium-based salt/urea mix-
tures obtained in this work kept stable below 383 K. By
means of Karl Fisher titration, the results of measurement
showed that the water content was below 100 ppm in imid-
azolium-based salt/urea mixtures.
2
.3 The determination of solubilities for ZnO
3
Results and discussion
The method applied in the determination of solubilities was
similar to that in our previous work [29]. ZnO (0.1 wt%
of the salt or salt/urea mixture) was added into dried salt
or salt/urea mixture under dry argon atmosphere. The re-
sulting suspension was heated in an oil bath and the accu-
racy of temperature was ±0.5 K (IKA RET basic). Addi-
tional 0.1 wt% ZnO was added until the solution became
thoroughly clear under a polarization microscope (Olympus
BX51). After ZnO became saturated at a studied tempera-
ture, the solubility of ZnO was determined with a plas-
ma-atomic emission spectrometer (ICP-AES, Dionex ICS-
3
.1 The enhanced dissolution of ZnO in imidazolium
chloride/urea mixtures
In this work, the solubilities of ZnO were first determined in
some typical imdazolium-based salts: [Amim]Cl, [Bmim]Cl,
[Hemim]Cl, [Bmim][SCN] and [Bmim][N(CN) ] owing to
2
their excellent electrochemical properties, high performance
and potential in the dissolution of numerous metal com-
pounds [17, 30–32]. But the experimental results indicated
that ZnO was nearly insoluble in the above salts at 373.2 K.
To our interest, the dissolution of ZnO in imidazolium chlo-

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Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
ride was remarkably enhanced by the addition of urea (see
Table 1). However, no such improvement was observed in
[
2
Bmim][SCN]/urea and [Bmim][N(CN) ]/urea. Among
these systems, the [Amim]Cl/urea mixture with molar ratio
of 1:1 showed the best dissolving capability for ZnO. The
solubility of ZnO in 1:1 [Amim]Cl/urea was as high as 8.35
wt% at 373.2 K, which was much more than that deter-
mined in the 1:2 ChCl/urea mixture (4.02 wt% at 373.2 K)
(see Figure 1). And the solubilities of ZnO in these systems
at 373.2 K decrease in the order 1:1 [Amim]Cl/urea > 1:2
ChCl/urea > 1:1 [Hemim]Cl/urea > 1:1 [Bmim]Cl/urea >
1
:1 [Bmim][SCN]/urea and 1:1 [Bmim][N(CN)
2
]/urea.
It’s evident that the structures of these salts have a sig-
nificant impact on the dissolution behaviors of ZnO in these
systems. To learn more about the solvation mechanism on a
molecular level, the enhanced dissolution of ZnO was fur-
ther investigated by ESI-MS and IR spectra.
Figure 2 ESI-MS spectra of (a) 1:1 [Amim]Cl/urea and (b) 8.35 wt%
ZnO in 1:1 [Amim]Cl/urea in the negative ion mode. The two groups of
−
−
2 2 3
peaks at m/z 193 and 351 correspond to {[Amim]Cl } and {[Amim] Cl } ,
respectively.
According to the measurement of ESI-MS (Figure 2),
only a new group of peaks were observed at m/z 174, 176
and 178 after the dissolution of ZnO in imidazolium chlo-
ride/urea mixtures. This suggests that a complex anion
IR spectra studies were also performed to gain more in-
formation about the complexing pattern of ZnO in the im-
idazolium chloride/urea mixtures. As Figure 3 illustrates,
the dissolution of ZnO in 1:1 [Amim]Cl/urea gives rise to
−
−
[
ZnClO∙urea] was formed among ZnO, urea and Cl anion
as Abbott et al. [21] have reported. But for the suspensions
of ZnO in [Bmim][SCN]/urea and in [Bmim][N(CN) ]/urea,
1
an intensive peak at 2212 cm . The same phenomena were
also seen after the dissolution of ZnO in [Bmim]Cl/urea and
[Hemim]Cl/urea. On the basis of MS data and literature
2
no signals of Zn-containing species were found through MS
experiments. It can be inferred that the smaller volume,
−
1
higher dimensional symmetry and complexing ability of Cl
facilitated the formation of [ZnClO∙urea] .
[22, 33], it’s appropriate to state that the peak at 2212 cm
−
probably derives from the stretching vibration of cumulated
double bond generated between Zn(II) and urea in
−
[
ZnClO∙urea] . Apparently, it’s the oxygen atom of urea
Table 1 Solubilities of ZnO in the imidazolium-based salts and salt/urea
mixtures at 373.2 K

that incorporates directly with Zn(II), while Cl ion plays an
important role in breaking the polar Zn−O bonds.
Solubility (wt% of the solvent)
Salt
−
In general, urea and Cl ion act as two key factors in the
−
in pure salt
< 0.10
in 1:1 salt/urea mixture
formation of complex anion [ZnClO∙urea] , which is con-
[
Amim]Cl
Bmim]Cl
Hemim]Cl
Bmim][SCN]
Bmim][N(CN)
8.35
3.23
firmed as the main driving force for the dissolution of ZnO
in imidazolium chloride/urea mixtures (see eq. (1)). The
solubilities of ZnO in these systems at 373.2 K decrease in
the order 1:1 [Amim]Cl/urea > 1:2 ChCl/urea > 1:1 [Hem-
im]Cl/urea > 1:1 [Bmim]Cl/urea. To our knowledge, the
[
< 0.10
[
< 0.10
3.81
[
< 0.10
< 0.10
< 0.10
[
2
]
< 0.10
Figure 1 Solubilities of ZnO in 1:1 [Amim]Cl/Urea and 1:2 ChCl/urea at
different temperatures.
Figure 3 IR spectra of (a) 1:1 [Amim]Cl/urea and (b) 8.35 wt% ZnO in
1:1 [Amim]Cl/urea.

Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
1591
cation-anion interaction strength in studied salts follows the
order [Amim]Cl < [Hemim]Cl < [Bmim]Cl [29, 34]. Given
the fact that the molar content of Cl ion and urea is much
Pt electrode. And the oxidation peak corresponds to the
stripping process of deposited zinc. From Figure 5, it can
also be found that the reduction potentials of Zn(II) in the
solutions increase in the following order: 0.3 M solution of
ZnO in 1:1 [Amim]Cl/urea < 0.6 M solution of ZnO in 1:1
−
more than that of ZnO in above solutions, it can be inferred
that ZnO is prone to dissolve in the systems with relatively
weak cation-anion interaction, which can effectively pro-
[Amim]Cl/urea
< 1.2 M solution of ZnO in 1:1
−
mote the generation of [ZnClO∙urea] . And it’s expected
[Amim]Cl/urea. This result is consistent with the order of
−
that the enhanced dissolution of ZnO may be achieved in
molar concentration of [ZnClO∙urea] ion in these solutions,
−
−
many other Cl based salts by the addition of urea as well.
which confirms that [ZnClO∙urea] is the dominant exist-
ence form of Zn(II).


ZnO +Cl +urea  [ZnClOurea]
(1)
Meanwhile, additional cyclic voltammograms were car-
ried out with different scan rates to study the electrodeposi-
tion behavior of zinc. The results obtained from 1.2 M solu-
tion of ZnO in 1:1 [Amim]Cl/urea at 303.2 K have been
presented in Figure 6. When the scan rate v increases, the
deposition peak potential φpc and current density ipc of zinc
both shift more negative, but the reduction potentials of
Zn(II) show no significant changes. The inset graph of Fig-
3
.2 The electrodeposition of zinc from solutions of ZnO
in imidazolium chloride/urea mixtures
The enhanced dissolution of ZnO in imidazolium chlo-
ride/urea mixtures lays a good foundation for the low-
temperature electrodeposition of zinc. In addition to the
conclusions in Section 3.1, it’s demonstrated that the
1/2
ure 6 reveals that ipc and v have a good linearity relation-
[
Amim]Cl/urea system displays not only wide liquid range,
but also higher electrical conductivity than [Bmim]Cl/urea,
Hemim]Cl/urea and 1:2 ChCl/urea mixtures when they
[
contain the same molar concentration of ZnO (see Figure 4).
From the perspective of electrodeposition, the solution of
ZnO in [Amim]Cl/urea has more favorable properties and is
worth exploring. Hence, 0.3 M, 0.6 M and 1.2 M solutions
of ZnO in 1:1 [Amim]Cl/urea mixture (298.2 K) were em-
ployed as model electrolytes in the following work.
3
.2.1 Cyclic voltammetry
In order to investigate if zinc could be electrodeposited
from the solutions of ZnO in 1:1 [Amim]Cl/urea system,
cyclic voltammograms were first recorded and analyzed.
Figure 5 shows that a pair of oxidation and reduction peaks
is obtained between −0.5 and −1.5 V in all the three typical
samples. According to the previous research [21], the re-
duction peak is ascribed to the electrodeposition of zinc on
Figure 5 Cyclic voltammograms of the solutions of ZnO in 1:1
[
Amim]Cl/urea on a Pt electrode at 303.2 K. The potential scan rate was
−
1
1
00 mV s .
Figure 6 Cyclic voltammograms of 1.2 M solution of ZnO in 1:1
Figure 4 The electrical conductivities of 0.6 M solution of ZnO in 1:1
[Amim]Cl/urea on a Pt electrode at 303.2 K. The potential scan rate (mV
−
1
[Amim]Cl/urea, 1.2 M solution of ZnO in 1:1 [Amim]Cl/urea and 0.6 M
s ): (a) 20, (b) 40, (c) 60, (d) 80, (e) 100. The inset graph shows the rela-
1/2
solution of ZnO in 1:2 ChCl/urea.
tionship between ipc and v
.

1592
Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
ship in this experiment. Nevertheless, the straight line ob-
tion and progressive nucleation which are suitable for rep-
resenting zinc deposition from imidazolium-based salts [14].
To distinguish these two nucleation models, a common
method is conducted by the comparison between dimen-
sionless experimental current-time transients and dimen-
sionless theoretical transients for each nucleation mecha-
nism. The expressions for instantaneous and progressive
nucleation are given in the following eqs. (2) and (3), re-
spectively [38].
1/2
tained by plotting ipc vs. v does not pass through the origin
as expected. It indicates that the electrodeposition of zinc
from the solution of ZnO in 1:1 [Amim]Cl/urea is influ-
enced by diffusion and kinetic limitations (electron transfer
or surface chemical reaction steps) [35]. Similar conclusion
was also drawn from the other solutions of ZnO in
[Amim]Cl/urea.
3
.2.2 Chronoamperometry
2
1
2
(
i/i ) 1.9542(t/t ) {1 exp[1.2564(t/t )]} (2)
m
m
m
The nucleation/growth process of zinc electrodeposition on
Cu electrodes from the solution of ZnO in [Amim]Cl/urea
mixtures was explored by means of chronoamperometry.
For each chronoamperometric measurement, the potential of
working electrode was stepped from an initial value where
no reduction of Zn(II) would occur to potentials sufficiently
negative to initiate the nucleation/ growth process. A group
of current-time transients gained from different step poten-
tials for the electrodeposition of zinc on Cu electrodes in 1:1
2
1
2
2
(
i/i ) 1.2254(t/t ) {1 exp[2.3367(t/t ) ]} (3)
m
m
m
Before the normalization and plotting of experimental
data, a correction to experimental time was performed for
the potential-dependent induction time t
time axis as t' = t−t and t' = t −t . Figure 8 shows the di-
mensionless plots, (i/i ) vs. (t'/t'
curves from eqs. (2) and (3). It’s apparent that the electro-
deposition of zinc on Cu electrode fits well to the theoretical
models for three-dimensional instantaneous nucleation with
diffusion-controlled growth.
0
, by refining the
0
m
m
0
2
2
m
m
) along with theoretical
[Amim]Cl/urea with 1.2 M ZnO are illustrated in Figure 7.
It can be seen that all the current-time transients are similar
in shape with the well- defined current maxima. Because of
the double layer charging, an abrupt change in current-time
transients appears, especially at −50 mV. The following
increase of current is attributed to the nucleation and sub-
sequent growth of zinc nuclei on Cu electrode surface. After
3
.2.3 Electrodeposition of zinc on copper substrates
Searching for the optimum experimental conditions of zinc
electrodeposition is a key step to the industrial application.
In view of the chemical composition of electrolytes, smooth
zinc coatings and high current efficiency were easier to
achieve under constant potential. Therefore, all the deposi-
tion experiments were conducted by potentiostatic method.
Accordingly, the effects of experimental parameters, such
as temperature (from 323.2 to 373.2 K), reduction potential
from −0.3 to −0.5 V) and concentration of ZnO in 1:1
Amim]Cl/urea mixture on zinc electrodeposition were
investigated. The deposition time and stirring speed in
this work were controlled at 60 min and 300 r min , re-
spectively.
reaching a current maximum i
m m
at a time t , each cur-
rent-time transient starts to decay and diffusion zones of
growing crystallites begin to overlap [36]. Less time is
needed to reach the current maxima when the applied po-
tential is more negative. It reflects that the negative increase
of potential can accelerate the nucleation process.
The electrodeposition of metal on foreign substrates is
usually accompanied with a three-dimensional nuclea-
tion/growth process. Some models have been established to
describe this process in the literature, involving hemispher-
ical diffusion-controlled growth of the nuclei [37]. This
model has two typical limiting cases: instantaneous nuclea-
(
[
−
1
Temperature usually has a significant impact on the sur-
face morphology of metal electrodeposits. In our work, it
was found that shining, smooth and dense zinc coatings
Figure 7 Current-time transients of 1.2 M solution of ZnO in 1:1
Figure 8 Comparison of the dimensionless experimental current-time
transients, derived from the results in Figure 7, with the theoretical curves
for instantaneous nucleation and progressive nucleation.
[Amim]Cl/urea resulting from potential step experiments on Cu electrodes
at 323.2 K.

Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
1593
were obtained in the temperature range of 323.2−343.2 K.
But the coatings turned dark and poorly adherent with the
increase of temperature, especially at 373.2 K. To probe
into the microstructures, the SEM micrographs of zinc
coatings were recorded. The SEM images of the samples
prepared from 0.6 M solution of ZnO in 1:1 [Amim]Cl/urea
at −0.4 V and 323.2−373.2 K are given in Figure 9. It’s ob-
vious that the grain size of zinc crystals increases from
about 0.5 to 5 μm when the temperature rises. The particles
on the coatings prepared at 323.2 K and 343.2 K both have
dense and homogenous spherical structures. However, the
deposits produced at 373.2 K display a rough surface with
thin platelets. For the 0.3 M and 1.2 M solutions of ZnO in
Figure 10 shows the SEM images of zinc obtained from
0.6 M solution of ZnO in 1:1 [Amim]Cl/urea at 343.2 K
with different reduction potentials. The two groups of zinc
coatings have a smaller average grain size compared with
Figure 9(b). Among these three samples, the one produced
at −0.5 V and 343.2 K has the most compact surface, while
larger intergranular crevices are seen in the other two sam-
ples. It can be inferred from above phenomena that the
number of zinc nuclei increases when the reduction poten-
tial becomes more negative. Meanwhile, the growth of nu-
clei could be accelerated by the following increase of cur-
rent density. The surface morphology of coatings is mainly
influenced by these factors at various reduction potentials.
The SEM micrographs of zinc coatings prepared from
another two representative solutions of ZnO in 1:1
[Amim]Cl/urea at 343.2 K and −0.5 V have been presented
in Figure 11. Different from Figure 11(b), the coatings ob-
tained from 0.3 M solution of ZnO in 1:1 [Amim]Cl/urea
are a bit rougher and do not cover the entire surface of cop-
1
:1 [Amim]Cl/urea, the microstructures of zinc change in a
similar way as a function of temperature. Considering the
relationship between electrical conductivity, diffusion speed
and temperature, a higher temperature followed by higher
current density and electrodeposition rate is prone to pro-
mote a less uniform crystal growth.
Figure 9 SEM micrographs of zinc coatings prepared from 0.6 M solution of ZnO in 1:1 [Amim]Cl/urea at −0.4 V. The deposition temperature (K): (a)
23.2, (b) 343.2 and (c) 373.2. On the right are the higher magnification micrographs of zinc coatings.
3

1594
Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
Figure 10 SEM micrographs of zinc coatings prepared from 0.6 M solution of ZnO in 1:1 [Amim]Cl/urea at 343.2 K. The deposition potential (V): (a)
0.3 and (b) −0.5. On the right are the higher magnification micrographs of zinc coatings.
−
per substrate. Based on the results of cyclic voltammetry, it
can be concluded that the reduction potential −0.5 V is not
negative enough to the full growth of zinc nuclei from 0.3
M solution of ZnO in 1:1 [Amim]Cl/urea. By contrast, the
electrodeposition process in 1.2 M solution of ZnO in 1:1
XRD. Figure 12 shows the XRD patterns of two typical
samples which have been described in Figure 9(c) and 10(b).
Based on the standard JCPDS file, all the peaks in Figure 12
are in good agreement with the degree values for zinc. To
describe the preferred orientation, the orientation index IOhkl
of (hkl) plane can be calculated by:
[Amim]Cl/urea at −0.5 V is easy to proceed. Nevertheless,
the resulting deposits are less homogenous and compact
than those shown in Figure 10(b). A possible explanation
for this phenomenon is that the relatively positive deposi-
tion potential and higher viscosity of 1.2 M solution of ZnO
in 1:1 [Amim]Cl/urea lead to the nonuniform distribution of
Ihkl
/
/
Ihkl


IOhkl
=
(4)
Irhkl
Irhkl
where Ihkl is the peak intensity of (hkl) plane for the sample
and Irhkl is the peak intensity of (hkl) plane for the JCPDS
card no. 00-001-1238 [36, 40]. For the rough coatings ob-
tained at 373.2 K and −0.4 V (see Figure 12(a)), the results
of orientation index decrease in the order, IO100 (2.03) >
IO002 (1.60) > IO102 (1.01) > IO201 (0.96) > IO112 (0.87) >
IO101 (0.77) > IO110 (0.72). In comparison, the orientation
index obtained for the bright coatings produced at 343.2 K
and −0.5 V follows the order, IO002 (3.76) > IO100 (0.79) >
IO110 (0.68) > IO112 (0.65) > IO102 (0.64) > IO101 (0.62) >
IO201 (0.27). It can be seen that the preferred orientation
changes to (002) plane for the bright coatings (see Figure
12(b)). Similar relationship between SEM and XRD results
has also been observed for the rest electrodeposited samples
obtained from solutions of ZnO in [Amim]Cl/urea. It’s
proposed that the preferential growth of zinc on copper sub-
strate in the direction (002) can result in bright, smooth and
uniform zinc coatings.
−
current density and [ZnClO∙urea] ions on the surface of
substrates. These findings reveal that an appropriate com-
position of electrolyte is vital to create better zinc coatings.
In general, the most of zinc coatings prepared in this
work were more fine-grained than those reported in litera-
ture under the same experimental conditions [14, 39].
Smoother, denser and more homogenous zinc coatings were
obtained from 0.6 M solution of ZnO in 1:1 [Amim]Cl/urea
at −0.5 V and 323.2−343.2 K. Furthermore, some deposits
with promising functional structures can also be produced
from the solutions of ZnO in [Amim]Cl/urea (see Figure 9(c)).
According to the measurement of MS, IR spectra and ICP-
AES, the composition of electrolytes remained almost un-
changed after electrodeposition. More than 96% current effi-
ciency has been achieved for all the electrodeposition of zinc.
3
.2.4 Crystal structures and composition analysis of zinc
The compositions of zinc coatings were determined by
EDAX in this work. As expected, all the samples displayed
a strong peak for zinc and relatively weak peaks of oxygen
coatings
The crystal structures of zinc coatings were characterized by

Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
1595
Figure 11 SEM micrographs of zinc coatings obtained at 343.2 K and −0.5 V from (a) 0.3 M solution of ZnO in 1:1 [Amim]Cl/urea and (b) 1.2 M solution
of ZnO in 1:1 [Amim]Cl/urea. On the right are the higher magnification micrographs of zinc coatings.
Figure 13 EDAX spectrum for the zinc coatings shown in Figure 10(b).
Figure 12 XRD patterns of the zinc coatings shown in (a) Figure 9(c)
and (b) Figure 10(b).
also be electrodeposited from the solutions of ZnO in
[
Bmim]Cl/urea and [Hemim]Cl/urea mixtures according to
and copper. The copper detected by EDAX may come from
the substrate, while the detected oxygen is probably at-
tributed to the oxidation of zinc. The content of zinc in most
coatings is higher than 95 wt%. For the coatings shown in
Figure 10(b), the content of zinc is as high as 98.2 wt% (see
Figure 13). In fact, the electrodeposits obtained from 0.6 M
solution of ZnO in 1:1 [Amim]Cl/urea at 323.2−343.2 K
and −0.5 V show not only a better surface morphology, but
also preferable purity.
the electrochemical measurements in this work. Unlike
conventional methods, the deposition process is environ-
mentally friendly and easy to handle at mild conditions. It’s
anticipated that hydrogen embrittlement could be reduced
according to this technique. The electrolytes are less sensi-
tive to moisture and can be readily purified and reused.
Meanwhile, the adjustable structure of deposits also makes
it possible to produce some zinc materials.
In summary, our experimental results demonstrate that
the solutions of ZnO in [Amim]Cl/urea, especially 1:1
4
Conclusions
[
Amim]Cl/urea with 0.6 M ZnO display a good perfor-
mance in the electrodeposition of zinc. Similarly, zinc can
The dissolution behavior of ZnO in some typical imidazo-

1596
Zheng Y, et al. Sci China Chem August (2012) Vol.55 No.8
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1
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