Prussian blue nanowires fabricated by electrodeposition in porous anodic aluminum oxide

Article abstract of DOI:10.1149/1.1775223

Highly ordered prussian blue (PB) nanowire arrays were fabricated successfully for the first time by self-assembling in a nan-oporous anodic aluminum oxide template. The nanowires with different lengths and diameters could be obtained by controlling, respectively, the deposition time and the size of the nanohole in the template. The structure and morphology of the nanowires were characterized by X-ray diffraction, transmission electron microscopy, and selected area electron diffraction. The results showed each of the nanowires was a continuous and preferentially of single crystal with face-centered-cubic structure. The Curie temperature T_c of PB nanowires was lower with respect to that of PB bulk. The growth mechanism of the nanowire is also discussed.

Full text of DOI:10.1149/1.1775223

Journal of The Electrochemical Society, 151 ͑9͒ C567-C570 ͑2004͒
C567
0
013-4651/2004/151͑9͒/C567/4/$7.00 © The Electrochemical Society, Inc.
Prussian Blue Nanowires Fabricated by Electrodeposition
in Porous Anodic Aluminum Oxide
a
a,z
b
b
b
Haiqing Luo, Xingguo Chen, Pingheng Zhou, Huigang Shi, and Desheng Xue
a
b
Department of Chemistry and Key Laboratory for Magnetism and Magnetic Materials of MOE,
Lanzhou University, Lanzhou 730000, China
Highly ordered prussian blue ͑PB͒ nanowire arrays were fabricated successfully for the first time by self-assembling in a nan-
oporous anodic aluminum oxide template. The nanowires with different lengths and diameters could be obtained by controlling,
respectively, the deposition time and the size of the nanohole in the template. The structure and morphology of the nanowires were
characterized by X-ray diffraction, transmission electron microscopy, and selected area electron diffraction. The results showed
each of the nanowires was a continuous and preferentially of single crystal with face-centered-cubic structure. The Curie tem-
perature T of PB nanowires was lower with respect to that of PB bulk. The growth mechanism of the nanowire is also discussed.
c
©
2004 The Electrochemical Society. ͓DOI: 10.1149/1.1775223͔ All rights reserved.
Manuscript submitted June 30, 2003; revised manuscript received January 26, 2004. Available electronically August 11, 2004.
Among the known molecular magnets, prussian blue ͑PB͒
then anodized it for 1-3 h; (iv) removed the oxide film, and anod-
ized the Al sheet for 1 h again. Consequently, a AAO template with
highly ordered pore arrays was obtained.
II III
ϩ
ϩ
ϩ
ϩ
͑
catera-͓MFe Fe ͑CN) ])(M ϭ Li ,Na ,K ,NH ) and related
6
4
cyano-metallate-based coordination polymers offer a range of com-
pounds with unique versatility. The variety of structures and mag-
netic properties of this family of compounds have been extensively
Synthesis of the PB nanowire arrays.—Electrodeposition of the
PB nanowires was carried out using a standard double-electrode
cell. The work electrode ͑a porous alumina template͒ and the
counter electrode ͑pretreated graphite͒ were submerged in a fresh
solution of 0.02 M FeCl , 0.02 M K Fe͑CN) , 0.6 M H BO , and
1
,2
3-5
investigated, and recently reviewed. Although much work has
focused on the relationship between the unit cell structure and mag-
netic properties, relatively few studies have been made on the
growth mechanism, synthesis fashion, and method of controlling the
growing process. These are important aspects in the study of mo-
lecular magnets, because compounds with appropriate magnetic
properties can be acquired by controlling the shape of the magnets.
Recently, different methods mainly in the solution-phase route have
been used to fabricate types of PB such as powder, bulk, thin film,
and nanoparticles.6 Moreover, many studies have also been fo-
cused on the preparation and electrochemical behavior of PB and its
3
3
6
3
3
0.5 M KCl. The electrodeposition progress was kept for 3 min under
1
Hz 13 VAC. After electrodeposition, the resulting oxide layer with
nanowire arrays was removed in a saturated HgCl solution from the
2
remaining aluminum. Then the free-standing nanowires were ob-
tained by dissolving away the aluminum oxide film in a mixed so-
lution of 0.1 M H CrO and 0.1 H PO at room temperature.
-8
2
4
3
4
Instruments.—Surface morphology of the AAO template was
studied using a JSM-5600LV scanning electron microscope ͑SEM͒,
9
-15
analogues.
To our knowledge, no study except our prior work on
16
the PB nanowire has been reported.
X-ray diffraction ͑XRD, Rigaku/Max-2400͒ with Cu K␣ radiation
1
Synthesis of nanowire arrays by means of electrodeposition into
the porous anodic aluminum oxide ͑AAO͒ template is a popular
method. In this method, the nanowire diameter can be determined by
the size of the nanohole, which can be controlled by adjusting the
oxidation voltage in types of anodizing solutions. As a result of
nanosize effect, the nanowire arrays show different characteristics
compared with bulk or other morphologic materials. However, most
of the nanowire arrays prepared by the alternating current ͑ac͒ elec-
trodeposition method are metal and alloys.
was used to identify the phase and analyze the structure of the PB
nanowires. Transmission electron microscopy ͑TEM͒ and selected
area electron diffraction ͑SEAD͒ were performed to probe the mi-
crostructure and morphology of the sample by using a JEO 2000 X
microscope at an operating voltage of 75-100 KV. The magnetic
properties of the PB nanowire arrays were investigated with the
magnetic property measurement system ͑MPMS͒.
Results and Discussion
In this work, according to the electrodepositing fabrication of PB
1
0
Figure 1a-b show the SEM micrographs of different AAO tem-
plates. The pore diameters of 55 and 290 nm were obtained in oxalic
and phosphoric acid solutions under voltages of 40 and 150 V, re-
spectively. The arrays with highly ordered pores are displayed in the
alumina templates. The uniform pores can be seen in the AAO tem-
plates, especially the perfectly hexagonal ordered domains can be
formed over a wide range of pore distances in the AAO template
fabricated at 40 V in oxalic acid solution ͑Fig. 1a͒. It was reported
that the pore diameter and distance were determined by anodic volt-
film by Neef, we first fabricated the PB nanowire arrays by the ac
electrodeposition into the porous AAO template. The possible
growth mechanism of the nanowire is discussed.
Experimental
Fabrication of AAO.—We have fabricated the AAO template by
the following process according to the two-step electrochemical
2
0,21
anodization.17-19
age and a variety of anodic acids.
In this work, the results indi-
First, high purity ͑99.99%͒ aluminum foils were
cate that the AAO of different sizes range from several tens to hun-
dreds of nanometers can be achieved by adjusting the height of the
voltage in types of anodizing solution such as sulfuric, oxalic, and
phosphoric acid.
annealed at 600°C for 6 h. Subsequently, the sample was ultrasoni-
cally degreased in acetone, and cleaned in 5% NaOH at 15°C for 20
min, and a smooth surface was obtained. Then, the aluminum was
electropolished in a 25:75 volume mixture of HClO and C H OH.
4
2
5
Figure 2 shows the TEM image of the PB nanowires with the
average length and diameter of 4 ␮m and 55 nm, which was fabri-
cated in the AAO template anodized at 40 VDC in 0.3 M H C O .
The polished Al samples were anodized at 40 VDC in 2.7 wt % ͑0.3
M͒ H C O at 10°C, which contained the following four steps: ͑i͒
2
2
4
2
2
4
anodized a polished Al sheet for 30 min; (ii) dissolved away the
The results indicate that the diameter of the PB nanowires is con-
sistent with that of the deposited AAO nanoholes. So the uniform
length and diameter of nanowires can be prepared by controlling the
depositing time and the pore diameter of the AAO template.
The XRD patterns of the PB nanowire arrays and the PB bulk are
shown in Fig. 3. The results indicate the nanowire has a face-
oxide film in a mixed solution of 0.2 M H CrO and 0.4 M H PO
2
4
3
4
at 60°C for 5 min; (iii) rinsed the Al sheet with deionized water,
z
E-mail: chenxg@lzu.edu.cn
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C568
Journal of The Electrochemical Society, 151 ͑9͒ C567-C570 ͑2004͒
Figure 3. XRD patterns of PB bulk and alumina template filled with the PB
nanowires, using Cu K␣1 radiation.
nanowire may easily partially dewater by thermal decomposition.
The single crystal turned into polycrystalline, and the SEAD pattern
became diffraction rings, which can be seen in Fig. 4c. The diffrac-
tion rings can be indexed as ͑222͒, ͑400͒, and ͑440͒ reflections from
fcc structure of PB.
The infrared spectrum measured at room temperature in the re-
Ϫ1
Ϫ1
gion from 2000 to 2200 cm shows one peak at 2158 cm , which
II
III
is due to the stretching of CN in the Fe -CN-Fe links. So the
oxidation state of the iron ions in PB nanowire can be expressed as
Fe -CN-Fe , similar to the PB bulk. The field-cooled magnetization
II
III
vs. temperature of PB nanowires at H ϭ 10 Gs shows a T ϭ 4.1 K
c
which is lower with respect to that of PB bulk, T ϭ 5.6 K ͑Fig. 5͒.
c
The susceptibility measurement were carried out from T ϭ 1.9 to
Figure 1. Micrographs of the bottom view of anodic alumna layers. Anod-
ization was conducted in 2.7 wt % ͑0.3 M͒ oxalic acid at 10°C at 40 V ͑a͒,
and 10 wt % phosphoric acid at 150 V ͑b͒. Pore opening was carried out in
Ϫ1
10 K. The linear part of the ␹M vs. temperature curve was fitted to
the Curie-Weiss law ␹M ϭ C/(T-␪). It is concluded that the Weiss
constant ␪ is 3.76 K, which indicates a long ferromagnetic interac-
5
wt % phosphoric acid at 20°C for 10 min.
3
ϩ
tion between the Fe ions through the cyanide bridge and diamag-
2
ϩ
netic Fe ions.
centered-cubic ͑fcc͒ structure, which is similar to the PB bulk. The
peak width of the nanowire arrays is noticeably larger than that of
the bulk, which can be expected from the size effect of nanometer,
the reason is not clearly known.
The SEAD pattern of a PB nanowire shown in Fig. 4a presents
sharp diffraction spots, which can be indexed as ͑040͒, ͑440͒. Figure
In the depositing solution, we selected H BO as a buffer system.
This may assure the stability of the depositing solution at the same
3
3
time, the H BO cannot dissolve the layer of alumina. As there is an
3
3
Al O barrier layer between the porous alumina film and metallic Al
2
3
sheet, in order to make the barrier layer thinner, the voltage was
decreased to 5-7 V by the speed of 2 V per 2 min after the end of the
two-step anodization. Using only the treated AAO by this method,
we can deposit PB nanowires. PB was dissolved quickly in alkalies
such as the NaOH solution, and alumina was dissolved slowly in
acids such as the H SO or HCl solutions. Then we selected the
4b is the dark-field TEM image of the same nanowire. Both indicate
that the nanowire is dense, continuous, and tends to be single crys-
tal. However, when the temperature goes up to 100°C, the crystal
2
4
mixture solution of phosphoric and chromic acids to obtain the pref-
erable freestanding PB nanowires by dissolving away the aluminum
oxide film.
In this work, to get the optimum condition for fabricating the
best PB nanowires, a series of experiments with different elec-
trodeposition conditions was performed. Also high quality deposi-
tion of the PB nanowires was obtained with ac frequency from 1 to
3
Hz and ac voltage from 12 to 13 V. When the frequency or voltage
of the ac was higher than the upper boundary of the optimal range,
there was little or no deposition. It may be seen clearly that H2
emerges from AAO by giving an ac voltage higher than 15 V. We
fabricated the PB nanowires with different lengths by controlling the
electrodeposition time. However, if the deposition time was over 4
min, a thin film on the surface of AAO resulted.
Based on the fabrication of PB film by electrodeposition and the
results of the experiment, the possible primary growth mechanism of
the PB nanowires is discussed as follows. Figure 6 shows the device
of a double electrode system for electrodeposition. During the pro-
cessing of electrodeposition, the electrons come to the bottom of the
nanohole of AAO by tunnelling the thin barrier layer, and
Figure 2. TEM morphology of the PB nanowires.
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Journal of The Electrochemical Society, 151 ͑9͒ C567-C570 ͑2004͒
C569
Figure 5. Field-cooled magnetization vs. temperature curves at H ϭ 10 Gs,
T
thermal variation of the susceptibility of the PB nanowire, ␹M vs. T ͑insert͒.
This may result from the different properties of the metal and PB in
the nature of the PB. The metal or alloy is composed of atoms
combined by a metal bond, and the atom can lose its free electrons
in the outer electron shell easily. However, it needs a certain energy
to overcome the electrostatic attractive forces when the atoms of the
metal and alloy enter solution from the solid phase by losing elec-
trons. During the progress of electrodeposition, an ac of the sine
signal was used. Therefore the electrodeposition of metal or alloy
nanowire is a nonequilibrium process combining deposition and dis-
solution. In a practical sense, the tendency of the deposition speed
for metal and alloy is a little larger than that of the dissolution speed,
which results in some remainders in the nanohole. Then the remain-
ders accumulate constantly and grow into a polycrystalline nano-
wire. However, the growing process of the PB nanowire is different.
3
Ϫ
4Ϫ
First, as soon as ͓Fe͑CN) ]
was reduced to ͓Fe͑CN)6] , the
6
3
ϩ
latter combined with Fe immediately and formed a PB precipitate
3
ϩ
2ϩ
(
Fe 4͓Fe (CN) ͔ •xH O) in the nanoholes. On the other hand,
6
3
2
2
ϩ
the PB has
a
steady structure of fcc, Fe
present in
3
ϩ
2ϩ
Ϫ
Fe ͓Fe (CN)6͔3 is held to CN tightly by a coordination bond.
4
2
ϩ
So it is very difficult for Fe among the PB framework to reenter
the solution by losing an electron. That is, the growth of the PB
nanowire is mainly a continuous deposition process with little dis-
solution, which results in the preferable formation of a single-crystal
nanowire outwards from the nanohole of AAO.
Conclusions
First we have fabricated PB nanowire arrays successfully by ac
electrodeposition into the nanoporous AAO template. The TEM and
SEAD results show that the nanowire is a dense, continuous, and
preferentially single crystal with a structure of fcc. By controlling
the conditions of the fabrication of AAO and the time of elec-
trodeposition, the nanowires with uniform diameter and length were
obtained by self-assembly. Magnetic measurement results indicate
that the Curie temperature of the PB nanowire decreases as the
average number of magnetic interaction neighbors is reduced. It may
Figure 4. ͑a͒ Electron diffraction patterns for a single PB nanowire. ͑b͒
Dark-field image of a single PB nanowire. ͑c͒ Electron diffraction patterns
for a dewatered PB nanowire.
Fe͑CN)6]3 was reduced to ͓Fe͑CN) ] while capturing an elec-
Ϫ
4Ϫ
͓
6
tron. Then ͓Fe͑CN)6]4 combined with Fe in the solution. Sub-
Ϫ
3ϩ
sequently the PB molecule is deposited at the bottom of nanoholes.
1
6
The electrochemical reaction can be formulated as
Fe3 ϩ 3eϪ1 ϩ 3͓Fe͑CN͒6]3Ϫ ϩ xH O
ϩ
4
2
ϩ
→
→
4Fe3 ϩ 3͓Fe͑CN͒6]4Ϫ ϩ xH O
2
Fe4 ͓Fe2ϩ͑CN͒ ͔ •xH O
3ϩ
6
3
2
By the constant tunnelling of electrons, the PB nanowire comes into
being by deposition outwards from the nanohole.
Generally, each of the metal and alloy nanowires prepared by ac
electrodeposition method in AAO is composed of particles of crys-
tals, while the PB nanowire fabricated in this work is a dense, con-
1
6
tinuous, and preferable crystal resulting from the SEAD and TEM.
Figure 6. Device of double-electrode system for electrodeposition.
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C570
Journal of The Electrochemical Society, 151 ͑9͒ C567-C570 ͑2004͒
5. C. G. Go
Int. Ed. Engl., 37, 613 ͑1998͒.
. B. R. E. Wilde, S. N. Ghosh, and B. J. Marshall, Inorg. Chem., 9, 2512 ͑1970͒.
¨
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tages in magnetic, electric, and optical properties et al. Various fur-
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Acknowledgments
1
1
1
1
1
This work was supported by National Natural Science Founda-
tion ͑grant no. A10174027͒ and EYTP of China. X.C. thanks the
Visiting Scholar Foundation of Key Lab for Magnetism and Mag-
netic Material of MOE.
1
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