Electrochemical synthesis of zinc nanoparticles via a metal-ligand- coordinated vesicle phase

Article abstract of DOI:10.1021/jp9017037

Two salt-free Zn²⁺-ligand-coordinated vesicle phases were prepared from the mixtures of alkyldimethylamine oxide (C_nDMAO, n) 14 and 16, i.e., C₁₄DMAO and C₁₆DMAO) and zinc laurate [(CH₃(CH₂₎₁₀

Full text of DOI:10.1021/jp9017037

J. Phys. Chem. B 2009, 113, 9461–9471
9461
Electrochemical Synthesis of Zinc Nanoparticles via a Metal-Ligand-Coordinated Vesicle
Phase
Yue Gao and Jingcheng Hao*
Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education,
Jinan 250100, P. R. China
ReceiVed: February 24, 2009; ReVised Manuscript ReceiVed: April 28, 2009
2+
Two salt-free Zn -ligand-coordinated vesicle phases were prepared from the mixtures of alkyldimethylamine
oxide (C DMAO, n ) 14 and 16, i.e., C14DMAO and C16DMAO) and zinc laurate [(CH (CH Zn]
n
3
2 2
)10COO)
2
+
in aqueous solution. The two salt-free Zn -ligand-coordinated vesicle phases were subsequently used as
the templating media for fabricating zinc nanoparticles on an indium-doped tin oxide (ITO) electrode via
electrodeposition. Influence of temperature, direct current density, and composition of the surfactant mixtures
on the average diameter and size distribution of Zn nanoparticles was investigated in more detail. The structure
of Zn@ITO was determined by scanning electron microscope and energy-dispersive X-ray spectroscopy
analysis. Monodispersed Zn nanoparticles on ITO electrode (ZnNPs@ITO) with different average diameters
were obtained, and the distribution can be controlled. Finally, ZnO nanostructures on the ITO substrate were
prepared from the as-synthesized Zn@ITO nanoparticles through electrochemical surface oxidation. The
preparation of nanostructured Zn and ZnO particles by our established method may pave the way for a new
templating route from metal-ligand-coordinated vesicles.
Introduction
3
[Al(OA) ], respectively. We have primarily deduced the sche-
matic model of metal ion coordination complex vesicles, in
which the metal ions were actually coordinated to form the
complexes of four chains (either CH chains or CF chains),
leading to a pronounced increase in the geometrical packing
parameter and resulting in a vesicle phase with much better
stability than those of the individual surfactants.1 Electro-
chemistry has been employed as a means of preparing large
numbers of metal nanoparticles,13 and some advantages of
electrochemical methods over chemical ones for synthesizing
small metal particles are the high purity of the particles and the
possibility of a precise particle size control achieved by adjusting
1
Nanoparticle synthesis has attracted much attention as a result
of its optical, electronic, magnetic, and chemical properties and
its subsequent technological potential applications. Many strate-
gies have been employed for synthesizing metal nanoparticles
in aqueous media, including chemical reduction in the presence
2c
2-4
of a stabilizing agent such as polymers or surfactants,
5
6
electrochemistry, sol-gel processes, and so forth. In general,
if metal particles in nanometer-scale or patterned arrays are
wanted, one resorts to various organic assembly templates such
as lyotropic liquid crystals, reverse microemulsions,8 and
7
9
vesicles. Among the examples we have quoted, there are not
14
current density or applied potential. Electrodeposition has also
been coupled with surfactant templating to produce films
so many works reported concerning the preparation of metal
nanoparticles through vesicle-templating media. Faure et al.
prepared onion-type multilayer vesicles using single nonionic
surfactant under shearing, serving as the template and reductant
15
containing ordered structures on solid substrates.
In the system we applied in the current work, a Zn2
surfactant, zinc laurate ([CH (CH Zn), serves as the
+
9
3
2 2
)10COO]
to prepare Ag nanoparticles. However, in most of the examples,
+
10
source of Zn2 ions which are fixedly combined into the
membranes of the vesicles. It is quite different from the tra-
ditional source-template model. By electrodeposition in a simple
two-electrode setup, well-dispersed Zn nanoparticles with self-
patterned array on the ITO substrate were obtained. We
discovered, for the first time, that by varying the experiment
conditions such as temperature, composition of the surfactant
mixture, or the applied current density, the morphology of the
Zn nanoparticles on the ITO substrate can be extremely changed.
Thus, the new route reported here has provided a new way not
only to prepare well-dispersed metal nanoparticles but also to
control their size and shape. The metal reported here is of
interest not only on its own but also for the extraordinary optical
properties of its oxide. ZnO/Zn@ITO with novel morphology
was synthesized from Zn@ITO, which we previously obtained
by electrochemical surface oxidation in alkaline solutions of
inorganic salts such as silver nitrate, zinc acetate, and so forth
are used as the metallic source and mixed with the templating
material (surfactant or polymer) before or after assembly
template formation. It is believed that the metallic source
combines to the assembly template through interaction such as
1
1
electrostatic or dipole-dipole interactions.
Metal ion-ligand-coordinated vesicles, as novel salt-free
surfactant vesicle phases, have attracted much interest in recent
years. As early as 1999, Hoffmann’s group reported the
formation of vesicle phase from the mixture of tetradecyldim-
ethylamine oxide (C14DMAO) and calcium dodecyl sulfate
] for the first time. In our previous work,
Zn , Ba , Ca , and Al coordination complex vesicles were
prepared from mixtures of C14DMAO with zinc, 2,2-dihydro-
1
2a
12b-e
[
Ca(DS)
2
2+
the
2
+
2+
3+
perfluorooctanote [Zn(OOCCH ], calcium oleate [Ca-
OA) ], barium oleate [Ba(OA) ], and aluminum oleate
2
C
6
F
13
)
2
(
2
2
16
amine-alcohol mixtures, and the influences of various experi-
ment conditions on the morphology of the oxide layer were
investigated. This work may reveal a new route for the
*
To whom correspondence should be addressed. E-mail: jhao@sdu.edu.cn.
Fax: +86-531-88366074.
1
0.1021/jp9017037 CCC: $40.75  2009 American Chemical Society
Published on Web 06/23/2009

9
462 J. Phys. Chem. B, Vol. 113, No. 28, 2009
Gao and Hao
Figure 1. Setups for Zn@ITO nanoparticle perparation (A) and ZnO@ITO electrochemical surface oxidation (B).
Figure 2. Phase diagrams and conductivity data of 100 mmol L-
1
C C16DMAO (b) micelle solution with increasing
14DMAO (a) and 100 mmol L-1
amounts of (CH (CH 10COO) Zn at 70.0 ( 0.1 °C.
3
2
)
2
preparation and size control of metal nanoparticles and promise
measured on a conductivity meter DDSJ-308A and a glass
further application of the metal-ligand-coordinated vesicles.
electrode DJS-1C at 70.0 ( 0.1 °C.
Cryogenic Transmission Electron Microscopy (Cryo-
TEM) Observations of Birefringent Lr Phase Sample
Solution. Carbon film grids with a hole size between 1 and 12
mm were used for specimen preparation. A drop of the sample
solution that was thermostatted at 70.0 ( 0.1 °C to equilibrate
for at least four weeks was put on the untreated coated TEM
grid (copper grid, 3.02 mm, 200 mesh). Most of the liquid was
removed with blotting paper, leaving a thin film stretched over
the holes. The specimens were instantly shock frozen by
plunging them into liquid ethane in a temperature-controlled
freezing unit (Zeiss). After the specimens were frozen, the
remaining ethane was removed using blotting paper. The
specimens were inserted into a cryo-transfer holder (Zeiss) and
transferred to a Zeiss CEM 902, equipped with a cryo-stage.
Examinations were carried out at a constant temperature of 90
K. The TEM was operated at an accelerating voltage of 80 kV.
Zero-loss filtered images (∆E ) 0 eV) were taken under low
dose conditions.
Experimental Section
Chemicals. Alkyldimethylamine oxides (C14DMAO and
C16DMAO) were gifts from Clariant AG Gendorf and delivered
as a 25 wt % solution in water. They were crystallized twice
from acetone and characterized by their cmc values and melting
points (mp); C14DMAO: mp ) 130.2-130.5 °C and cmc )
-
4
-1
1
1
.4 × 10 mol L (25 °C), and C16DMAO: cmc ) ∼7.0 ×
-5 -1 17
0
mol L (62 °C). Zinc laurate was purchased from Wako
Co. and was used without further purification. Poly(ethylene
imine) was purchased from Alfa Aesar. All other chemicals used
were analytical grade. The water was triple-distilled.
-
1
Preparation of Birefringent Lr Phase. A 100 mmol L
14DMAO micellar solution was mixed with different amounts
of zinc laurate, [CH (CH 10COO] Zn. The sample solutions
C
3
2
)
2
were prepared at around 70 °C under stirring and then
thermostatted at 70.0 ( 0.1 °C to equilibrate for at least four
weeks. The sample solutions were thermodynamically stable
-
up to a mole fraction xCH (CH ) COO- ) [CH
(CH
)
10COO ]/
Electrochemical Deposition of Zinc Nanoparticles on ITO
Substrate. Electrochemical deposition was performed in a two-
electrode cell. A 1 cm × 1 cm Pt plate was used as the anode,
and a 1 cm × 0.5 cm ITO glass slice was used as the cathode.
3
2 10
3
2
-
[
)
CH
3 2
(CH )
10COO + C14DMAO] ) 0.5. Between xCH (CH ) COO-
3
2 10
0.35 and 0.46, a single transparent birefringent LR phase that
was very stable, turbid, and slightly bluish was observed.
As to the comparison system, the mixture of C16DMAO and
zinc laurate, with the addition of zinc laurate into the 100 mmol
2 3 2 2
ITO glass was pretreated in H O/NH /H O (17:3:1) for 20 min
-
1
for surface hydrophilization and then immersed in 2 mg mL
-
1
L
C
16DMAO micellar solution, a similar phase transition
poly(ethylene imine) for another 20 min, and afterward, in
distilled water for 15 min. The electrolytic solution was basic
process could be observed at T ) 70 °C. Between xCH (CH ) COO-
3
2 10
)
0.3 and 0.39, there was a single transparent biregringent LR
phase.
Conductivity Measurements. The conductivity data of 100
C
14DMAO/[CH
different molar ratios without adding any inorganic salt as
conducting agent. The cell was properly deaerated by N for
3 2 2
(CH )10COO] Zn aqueous solution mixed in
2
mmol L-
1
C
14DMAO or C16DMAO micelle solutions with
10 min before each experiment. The electrolysis was carried
increasing different amounts of [CH
3
(CH 10COO] Zn were
2
)
2
out in the galvanostatic manner at 70 °C. The current density

Electrochemical Synthesis of Zinc Nanoparticles
J. Phys. Chem. B, Vol. 113, No. 28, 2009 9463
Figure 3. Typical birefringent textures of the LR phase for the two systems of C14DMAO/[CH
3
(CH
2
2
)
10COO]
2 2
Zn/H
O with xCH (CH )10COO- ) 0.39
3
2
(
a), xCH (CH ) COO- ) 0.42 (b), and xCH (CH ) COO- ) 0.45 (c), and C16DMAO/[CH
3
(CH
2
)10COO]
2
Zn/H
O with xCH (CH ) COO- ) 0.35 (d), and xCH (CH ) COO-
3
2 10
3
2 10
3
2 10
3
2 10
)
0.39 (e). T ) 70.0 ( 0.1 °C.
trolysis time was chosen depending on the experimental
requirements. The setup for electrochemical surface oxidation
for Zn@ITO to ZnO@ITO is shown in Figure 1B.
Characterization. Polarized microscopy observations of
birefringent LR phase samples were carried out on an AX-
IOSKOP 40/40 FL (Zeiss) microscope. The nanostructures of
Zn@ITO and ZnO@ITO were characterized by a JEOL
JSM6700F field emission scanning electron microscopy.
Results and Discussion
Figure 4. Typical cryo-TEM image of birefringent LR phases for 100
-1
Phase Behavior of C14DMAO/[CH
and C16DMAO/[CH (CH 10COO] Zn/H
14DMAO micellar solution with increasing
(CH 10COO] Zn. The solubility of [CH
Zn is low in water at room temperature. When
(CH
2 2 2
)10COO] Zn/H O
mmol L
at 25.0 ( 0.1 °C.
3 2 2
C16DMAO and [CH (CH )10COO]
Zn (xCH (CH ) COO- ) 0.35)
3
3
2 10
3
2
)
2
2
O Systems. We mixed
-
1
1
00 mmol L
C
and electrolysis time were chosen depending on the experimental
requirements. The setup for electrochemical preparation of
Zn@ITO is shown in Figure 1A.
amounts of [CH
3
2
)
2
3
-
(CH
[CH
2
)
10COO]
2
10COO] Zn was mixed with 100 mmol L
2
-
1
3
(CH
2
)
Electrochemical Surface Oxidation of Zn@ITO into
ZnO@ITO. Following the literature, a mixture of 11.5 mL
C14DMAO micellar solution, the Krafft point of the Zn
1
6
surfactant became lower and the phase behavior was much more
interesting. The samples were prepared at 70 °C and then
thermostatted at 70.0 ( 0.1 °C. The phase diagram of 100 mmol
of H
2
O, 7.5 mL of 2-propanol, 3 mL of diethylamine, 0.5 mL
of H
2
O
2
, and 0.17 g of NaCl was used as the electrolyte. The
-
1
electrolytes were deoxygenated before each experiment. The
electrochemical surface oxidation was carried out in a 50-mL
cell at a temperature of 22 °C with the potential fixed at 1.2 V.
A 1 cm × 1 cm Pt plate served as the anode, and the
as-synthesized Zn@ITO was used as the cathode. The elec-
L
C14DMAO micellar solution mixed with amounts of 100
-1
mmol L is shown in Figure 2a, which includes the conductivity
data of the mixed aqueous solutions. The different phase
boundaries could be determined by the conductivity data. The
samples are thermodynamically stable up to a mole ratio,

9
464 J. Phys. Chem. B, Vol. 113, No. 28, 2009
Gao and Hao
Figure 5. SEM images of the deposition layer of Zn particles on ITO glass surface without (A) and with (B) poly(ethylene imine) pretreatment
from LR phase solution at xCH (CH ) COO- ) 0.39.
3
2 10
x
CH (CH ) COO- ) 0.50. From xCH (CH ) COO- ) 0-0.175, one can
secondary, and tertiary amino groups in the ratio 1:2:1. It is
demonstrated that the metal particles bond to the layer of
polyelectrolyte through amine functionalities, resulting in a much
better nanoparticle coverage on the solid substrate than that on
a bald ITO glass without poly(ethylene imine) modification, as
shown in Figure 5, which was determined by scanning electron
microscope (SEM) observations.
Electrochemical Deposition of Zn Nanoparticles and Size
Control. Self-assembled structures of surfactants have been
extensively studied as templating media for preparing nanopar-
ticles in recent years. Although a general approach to their
fabrication in a precisely controlled manner is not yet available
in the case of synthesizing nanomaterials in surfactant systems,
it is widely accepted that synthesized surfactants have a key
role in determining not only the size but also the shape of the
products. A typical synthesis system consisting of synthesized
3
2 10
3
2 10
note a single transparent solution, which is the L
1
phase.
Between xCH (CH ) COO- ) 0.175 and 0.35, we observe that
3
2 10
macroscopic phases separate into a birefringent LR phase at
the top of the isotropic L phase. After the L /LR region, from
CH (CH ) COO- ) 0.35 to 0.46, we observe a single transparent
1
1
x
3
2 10
birefringent, slightly turbid, and bluish LR phase that is very
stable for more than 1 year at 70.0 ( 0.1 °C. The polarizer
images of a single transparent birefringent, slightly turbid, and
bluish LR phase of a C14DMAO/[CH
3
(CH
2
)
2
10COO]
2
Zn/H
2
O
system with different amounts of [CH
3
(CH
)10COO]
2
Zn are
shown in Figure 3a-c.
We also studied the phase behavior of the system 100 mmol
L-
1
C
16DMAO micellar solution with increasing amounts of
(CH 10COO] Zn. Phase behavior similar to that of the
16DMAO/[CH (CH Zn/H O system was obtained, as
[
CH
C
3
2
)
2
3
2
)
10COO]
2
2
shown in Figure 2b. The polarizer images of the single
2 2
surfactants (e.g., Cd(AOT) or Zn(AOT) ) at the same time as
transparent birefringent, slightly turbid, and bluish LR phase
precursors has been already used to prepare colloidal nano-
crystals.2 It is probable to control nanoparticle size and shape
by simply varying phase behavior of the surfactant system.
(i) Electrochemical Deposition under Different Surfactant
Compositions. We fixed the temperature at 60 °C and selected
2,23
of the C16DMAO/[CH
ferent amounts of [CH
d,e.
Vesicle Structures of Birefringent Lr Phase Sample
Solution. The LR phase samples for the two systems of
14DMAO/[CH (CH Zn/H O and C16DMAO/[CH
CH 10COO] Zn/H O are birefringent, which can be demon-
3
(CH
2
)10COO]
2
Zn/H
2
O system with dif-
3
(CH
2
)
10COO]
2
Zn are shown in Figure
3
surfactant mixtures with different molar ratios, xCH (CH ) COO-
,
3
2 10
C
3
2
)
10COO]
2
2
3
-
as the electrolyte to prepare Zn nanoparticles under otherwise
2
(
2
)
2
2
identical condition. The applied current density is 20 µA/cm ,
strated by the maltese cross texture of the samples, as shown
in Figure 3. The maltese cross texture demonstrates the existence
of lamellar phase.18 The vesicle structures of birefringent LR
phases of a typical sample solution for the system C16DMAO/
and the charge time is fixed to be 5 min. The volume of
surfactant-mixed solution for each experiment is 2 mL. After
electrolysis, a golden metal layer on the ITO surface could be
obviously observed. The as-synthesized Zn@ITO was rinsed
in ethanol and distilled water right afterward under the protection
of nitrogen to clean the surface from attached surfactants. The
microscopic structure of the deposit has been investigated by
SEM (Figure 6) observations subsequently, and the elements
of the deposition layer were determined by EDS (Figure 7).
At xCH (CH ) COO- ) 0.1, the surfactant solution is a transparent
[
3 2 2 2
CH (CH )10COO] Zn/H O were determined by the cryo-TEM
micrographs in Figure 4. The vesicular phase should be triggered
2+
through Zn -ligand complexation between C14DMAO (or
12b-e
3 2 2
C16DMAO) and [CH (CH )10COO] Zn.
The vesicles are not
shielded by excess salts in solution because of an absence of
counterions and there are no additives such as cosurfactant in
the aqueous solutions. Such a vesicle phase has many latent
applications, one of which is to be used to synthesize inorganic
3
2 10
single L
1
phase and the deposited Zn nanoparticles show large
fragments without regular shape. We notice that some of the
large pieces seem to be the assemblies of small particles in
irregular spherical shape. At xCH (CH ) COO- ) 0.35, the surfactant
1
9
nano- or microparticles. We attempted, for the first time to
our knowledge, to synthesize zinc nanoparticles by using
3
2 10
2
+
electrochemical deposition from the studied Zn -ligand-
solution starts to separate into a birefringent LR phase at the
top of the isotropic L phase. The deposition layer shows a
coordinated vesicles as templating media.
1
Surface Pretreatment of ITO by Poly(ethylene imine).
Because the deposition process was performed in aqueous
environment, the ITO glass that serves as the conducting
substrate for deposition has to be pretreated to achieve firm
adhesion to the metal nanoparticles. Following the literature,2
we pretreated the ITO glass by simply immersing it into the
diluted aqueous solution of poly(ethylene imine), a highly
branched water-soluble polymer with a distribution of primary,
mixture of particles with a wide size distribution. As could be
deduced from the histograms in Figure 6, the average diameter
of the particles is 42.0 nm and the standard deviation (SD) is
(20.7 nm. At xCH (CH ) COO- ) 0.39, the surfactant solution is
a single LR phase that is birefringent. One can notice that Zn
nanoparticles have a narrow average size distribution with an
average diameter of 52.6 nm and SD of (12.2 nm. The Zn
nanoparticles bond to the substrate surface with narrower size
3
2 10
0,21

Electrochemical Synthesis of Zinc Nanoparticles
J. Phys. Chem. B, Vol. 113, No. 28, 2009 9465
Figure 6. SEM images of Zn nanoparticles prepared by electrochemical deposition at different molar ratios of the surfactants from the C16DMAO
and [CH (CH 10COO] Zn/H O systems. L
phase solution at xCH (CH ) COO- ) 0.1 (A), L /LR phase solution at xCH (CH ) COO- ) 0.35 and corresponding
particle size distribution (B), LR phase solution at xCH (CH )10COO- ) 0.39 and corresponding particle size distribution (C), and L
3
2
)
2
2
1
3
2
10
1
3
2 10
3
2
1
/precipitates solution
at xCH (CH ) COO- ) 0.5 (D). The ITO surface was pretreated by poly(ethylene imine).
3
2 10
distribution and better coverage. By increasing xCH (CH ) COO-
electrochemically synthesis of comparatively smaller and mono-
dispersed Zn nanoparticles.
(ii) Electrochemical Deposition under Different Tempera-
tures. Temperature plays an important role in the phase
transition of mixed surfactant systems.24 It has been demon-
strated that temperature variation can effectively change the
3
2 10
to 0.5, at which ratio the surfactant solution shows a two-phase
/Lcrystal region, it seems that transformed nanospheres larger
L
1
than 100 nm fuse into continuous bulks. From the above, it can
be primarily deduced that only the single LR phase consisting
of vesicles is the best media as the electrolyte template to the

9
466 J. Phys. Chem. B, Vol. 113, No. 28, 2009
Gao and Hao
interaction between surfactant molecules in the organized
assemblies and thus affect the microscopic structure of the
assembly. The single LR phase solution (xCH (CH ) COO- ) 0.4)
3
2 10
was selected as the electrolyte to investigate temperature effect
on the micromorphology of the deposition Zn nanoparticle layer.
The C14DMAO/(CH (CH )10COO) Zn system is rather temper-
3 2 2
ature-sensitive. When the single LR phase sample solution is
kept at room temperature, the solution will turn into a two-
phase solution with white precipitates floating on top of the
transparent L phase. When the temperature fluctuates within a
1
narrow span around 70 °C (i.e., 50-80 °C), the solution can
remain a single LR phase. A series of electrolysis were done at
5
0, 70, and 80 °C under otherwise identical conditions. The Zn
Figure 7. Typical EDS analysis of Zn nanoparticles prepared by
electrochemical deposition on ITO electrode surface from LR phase
solution at xCH (CH ) COO- ) 0.39.
nanoparticles were determined by SEM observations, which are
shown in Figure 8. It is distinctly shown that when temperature
rises from 50 to 70 °C, the size of deposited particles is reduced.
3
2 10
Figure 8. Zn nanoparticles synthesized by electrochemical deposition from LR phase solution (xCH (CH )10COO- ) 0.40) of a C14DMAO/
3
2
(
3 2 2 2
CH (CH )10COO) Zn/H O system at different temperatures and corresponding particle size distribution. At T ) 50 (A), 70 (B), and 80 °C (C).

Electrochemical Synthesis of Zinc Nanoparticles
J. Phys. Chem. B, Vol. 113, No. 28, 2009 9467
Figure 9. Deposited Zn nanoparticle morphologies from the single LR phase solution (xCH (CH )10COO- ) 0.35) of the C16DMAO/(CH
(CH
system at different temperatures, corresponding birefringent texture, and corresponding particle size distribution. T ) 70 °C (A, B, and C) and T
25 °C (D, E, and F).
2 2
)10COO) Zn
3
2
3
)
From the histograms shown in Figure 8, it is determined that
the average diameters of the particles prepared at 50 and 70 °C
are 57.8 nm with SD of (14.6 and 44.0 nm with SD of (11.6
nm, whereas at 80 °C there are no recognizable spherical
particles yet, leaving only deposited pieces that may result from
surfactant assemblies’ destruction by overheating.
Similar to the studies above, the single LR phase solution of
3 2 2 2
the C16DMAO/(CH (CH )10COO) Zn/H O system was used to
obtain Zn nanoparticles by electrochemical deposition. It is
different from the single LR phase solution of the C14DMAO/
see that the average particle diameter reduces distinctly. From
the histograms, as shown in Figure 9, we can determine that
the average diameters of particles obtained at 25 and 70 °C are
3
1.0 nm with SD of (9.7 nm and 60.4 nm with SD of (11.3
nm.
(iii) Influence of Surfactant Chain Length. The C16DMAO/
(
CH
3
(CH
2
)
10COO)
2
Zn/H
2
O system is selected as the parallel
Zn/H O system. Ac-
to the C14DMAO/(CH
3
(CH
2
)
10COO)
2
2
cording to the phase diagram, as shown in Figure 2, both
solutions with the same composition, xCH (CH )10COO- ) 0.35,
3
2
(
CH
of the C16DMAO/(CH
stable at much lower temperature (i.e., room temperature, ∼25
C). Zn nanoparticles can be synthesized from the single LR
phase solution (xCH (CH ) COO- ) 0.35) of the C16DMAO/
3
(CH
2
)
10COO)
2
Zn/H
2
O system; the single LR phase solution
were used as the electrolyte to synthesize Zn nanoparticles
on ITO electrode surface by electrochemical deposition. The
results, as shown in Figure 10, clearly show that the CH chain
length of the coordinated surfactants extremely influences the
particle size no matter how much the applied current is. Under
the same xCH (CH ) COO- value, Zn nanoparticles obtained from
3
(CH
2
)
10COO)
2
Zn/H O system remains
2
°
3
2 10
(
3 2 2 2
CH (CH )10COO) Zn/H O system within a wider temperature
region, such as at 25 and 70 °C, and the temperature effect on
nanoparticle size is more obvious. Figure 9 shows the SEM
observations of the Zn nanoparticles synthesized by electro-
chemical deposition using the single LR phase solution
3
2 10
the C16DMAO/(CH
smaller than those from the C14DMAO/(CH
O system at two different current densities of 200 and 300
3
(CH
2
)
10COO)
2
Zn/H
2
O system are much
3
2 2
(CH )10COO) Zn/
H
2
2
(
x
CH (CH ) COO- ) 0.35) and the corresponding birefringent
µA/cm . It is determined from the histograms in Figure 10 that
the average diameters of the particles in Figure 10A-D are 59.4
nm with SD of (13.7 nm, 33.2 nm with SD of (9.4 nm, 70.0
3
2 10
texture of the single LR phase solution (xCH (CH ) COO- ) 0.35).
3
2 10
When varying the temperature from 25 to 70 °C, one can clearly

9
468 J. Phys. Chem. B, Vol. 113, No. 28, 2009
Gao and Hao
Figure 10. Deposited Zn nanoparticles from the two systems of C14DMAO/(CH
3
(CH
2
)
10COO)
2
Zn/H
2
O and C16DMAO/(CH
3
(CH
(CH
2
)
)
10COO)
10COO)
2
Zn/
Zn/
H
H
2
O at xCH (CH ) COO- ) 0.35 under different current densities and corresponding particle size distribution (i). C14DMAO/(CH
3
2
2
3
2 10
2
2
2
O, i ) 200 µA/cm (A and C). C16DMAO/(CH
3
(CH
2
)
10COO)
2
Zn/H
2
O, i ) 300 µA/cm (B and D).
nm with SD of (13.9 nm, and 29.0 nm with SD of (8.3 nm,
respectively.
(iW) Influence of the Applied Current Density. To see the
effect of the applied current density upon particle size, we

Electrochemical Synthesis of Zinc Nanoparticles
J. Phys. Chem. B, Vol. 113, No. 28, 2009 9469
Figure 11. Deposited Zn nanoparticles under different applied current densities in the C16DMAO/(CH
0.39 and corresponding particle size distribution. The applied currents (i): 20 (A), 100 (B), 200 (C), and 300 µA/cm (D).
3 2 2 2
(CH )10COO) Zn/H
3 2
O system at xCH (CH )10COO-
2
)
repeated the electrolysis process of the C16DMAO/(CH
CH 10COO) Zn/H
O system with xCH (CH ) COO- ) 0.39 using
2
3
-
deposited Zn particles (Figure 11A). Significantly, from the
histograms and the SEM images in Figure 11B-D, we
determined that the average diameters of the particles prepared
at 100, 200, and 300 µA/cm are 28.6 nm with SD of (8.0 nm,
47 nm with SD of (9.9 nm, and 60.8 nm with SD of (12.6
(
2
)
2
2
3
2 10
current densities of 20, 100, 200, and 300 µA/cm under
2
otherwise identical conditions. We found that, when the applied
2
current density is lower than 100 µA/cm , there are hardly any

9
470 J. Phys. Chem. B, Vol. 113, No. 28, 2009
Gao and Hao
Figure 12. ZnO nanoparticle morphology evolution by galvanization time. Zn nanoparticles (A), after 3 h (B), after 10 h (C), and a larger region
of the ZnO@ITO at lower magnification (D). ZnNPs@ITO was produced from the C16DMAO/(CH (CH 10COO) Zn/H
2
O system at xCH (CH ) COO-
3 2 10
3
2
)
2
2
)
0.39. i ) 150 µA/cm , E ) 1.2 V, and T ) 22 °C.
nm, respectively. Thus, it could be concluded that in electro-
chemical deposition experiment there is a critical current density
to obtain obvious deposition layers, and this value will change
in accord with temperature and surfactant CH chain length given
other conditions.
on the ITO substrate, which was rarely reported before. It was
discovered that by adjusting the temperature, surfactant molar
ratio, applied current density, and the CH chain length of the
applied amine oxide surfactant, we could effectively control the
average diameter and size distribution of the particles. Moreover,
ZnO nanorods on the ITO glass were made from the as-
synthesized ZnNPs@ITO by electrochemical surface oxidation.
Electrochemical Surface Oxidation of ZnNPs@ITO. Al-
though the electrochemical surface oxidation on a Zn cathode
16
has been reported before, there is little work about the surface
oxidation on a ZnNPs@ITO surface which may result in novel
nanostructures. As the ITO glass is used as the conducting
substrate, transparent ZnO@ITO nanostructures with novel
optical characters could be expected. The experiment setup was
a simple two-electrode cell according to the literature,16 and
the as-synthesized ZnNPs@ITO served as the cathode.
Acknowledgment. We thank the NSFC (Grant No. 20625307)
and National Basic Research Program of China (973 Program,
2009CB930103) for financial support.
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