Size-controlled gold-catalyzed growth of Prussian blue nanopillars

Article abstract of DOI:10.1246/cl.2006.406

Size-dependent nanopillars of Prussian blue were obtained by chemical deposition technique and porous alumina membranes coated with a bottom gold layer. Copyright

Full text of DOI:10.1246/cl.2006.406

406
Chemistry Letters Vol.35, No.4 (2006)
Size-controlled Gold-catalyzed Growth of Prussian Blue Nanopillars
Sailaja Sunkari,1 Sayoko Nagashima,1 Masaki Murata,1 Hiroshi Nishihara,ꢀ1 Yoshitaka Matsui,2
Kazuyuki Nishio,3 and Hideki Masuda2;3
1Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
2Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397
3Kanagawa Academy of Science and Technology, 5-4-30, Nishi Hashimoto, Sagamihara 229-1131
(Received January 10, 2006; CL-060030; E-mail: nisihara@chem.s.u-tokyo.ac.jp)
Size-dependent nanopillars of Prussian blue were obtained
by chemical deposition technique and porous alumina mem-
branes coated with a bottom gold layer.
Anodization for pore widening
(0.3 M H2C2O4, 17 ºC, 40 V, 15
min anodization; 5 wt % H3PO4
30ºC, 40 min-pore widening)
Al sheet
(99.99%)
Anodization
(0.3 M H2C2O4, 17 ºC,
40 V, 12 h )
Dissolution
of porous alumina
(H3PO4 & H2CrO4,
50 ºC, 12 h )
While many coordination compounds exhibit one or two
significant properties confining to a single chemical formula,
Prussian blue (PB), the prototype of all mixed-valent hexacyano-
metalates, AIxBy[C(CN)6]zꢁ (AI = alkali metal ion; B and C =
transition-metal ions) exhibits multifunctionalities like dye pig-
ments, molecular magnetic,1 electrocatalytic,2 electrochromic,3
ion-exchange,4 ion-sensing,5,6 and photomagnetic7 properties.
Hence, even after 300 years of its discovery, PB is being contin-
uously explored for its properties in its various physical and
chemical forms. The studies on PB mainly involved surfaces
electrochemically modified by PB films. Though chemical dep-
osition of PB has been reported previously,8,9 it is not practiced
as a common method. There is a recent report on the magnetic
properties of PB nanowires which have been grown electro-
chemically in a porous alumina template.1
Dissolution
of Al substrate
Removal of barrier layer
(Ar ion milling)
Sputtering of Au
on porous
alumina
Electroplating
of Au layer
Porous alumina
on Au electrode
3D cross sectional
view of porous
membrane
PB deposited Free standing PB nano pllars after
in the nano pores etching porous almina (Ar ion
milling – dry etching; 10 wt %
H3PO4 20 min, 30 ºC – wet etching)
Scheme 1. Schematic illustration of the fabrication of gold-
coated porous alumina membrane and PB deposition within
the membrane following a two-step anodization process.
gold–mica sheet is immersed in 0.01 M HCl solution containing
.
There are several established methodologies towards gener-
ation of size- and shape-dependent nanostructures of interest, of
which anodically oxidized porous alumina template is well ac-
cepted.10,11 Efforts to achieve high aspect ratios still continues,
as properties of nanostructures are highly dependent on the fine
features of the size and shape of the structure generated. In this
communication we report for the first time, the growth of PB
nanorods chemically, in a home made high aspect ratio gold-
coated porous alumina template. Our efforts demonstrate that
nanostructures can be generated chemically by proper modifica-
tion of the porous alumina membranes.
equimolar mixture of K3Fe(CN)6 and FeCl3 6H2O, on which
spontaneous growth of PB film was observed. From these experi-
ments, we found that the presence of gold surface and a solution
pH of 2 were necessary criteria for the spontaneous self deposi-
tion of PB without any external stimuli as current or potential be-
cause the total charge consumed in the reduction of PB estimated
from the cyclic voltammograms was proportional to the immer-
sion time (see Supporting Information, Figure S1). In the ab-
sence of gold surface, PB film formation could not be observed.
Also, when the pH exceeded 3, chemical deposition did not take
place even if the gold–mica sheet was left overnight.
The highly ordered gold-coated porous alumina template
was fabricated by a two-step anodizing process as shown in
Scheme 1.
Following above findings, PB was deposited in the porous
alumina membrane as described. Initially, when a type 1 mem-
brane was used without any pretreatment for deposition for
30 min, nanopillars limited to certain area are obtained. To ob-
tain more wide spread nanopillars, later a type 2 membrane
was used with pretreatment. Prior to deposition for 5 and 15
min, respectively, the template was immersed in water under
vacuum for 1.5 h, immersed in 5 wt % H3PO4 (aq) for 1 min, then
immersed in water under vacuum for 1.5 h, to make the pores
free of any remaining alumina and air gaps. Such pretreated
membrane gave nanopillars spread over all the membrane area.
The nanorods of PB have been grown by chemical deposi-
Anodization of a clean Al sheet ‘‘in 0.3 M oxalic acid at 40 V
for a long period’’ results in a regular pore arrangement at the
bottom side of the oxide film, which was removed by treating
with a mixture of phosphoric and chromic acid mixture. This
surface was once again anodized as before for uniform pore de-
velopment and treated with phosphoric acid to widen the formed
pores. After second anodization, gold was sputtered to provide
electrical contact for further electroplating of gold layer onto
the porous alumina surface. After electroplating of gold layer,
removal of ‘‘bottom alumina layer (barrier layer)’’ results in a
porous alumina membrane ‘‘with opened pores’’ on a gold elec-
trode. This electrode is used for PB deposition, as detailed
below.
.
tion from equimolar solutions of FeCl3 6H2O and K3Fe(CN)6
in the porous alumina template. The addition of K3Fe(CN)6 so-
.
lution to FeCl3 6H2O solution in equal amounts resulted in the
formation of wine red colored solution, from which the nanopil-
lars are deposited. The formation of wine red color upon mixing
of both solutions indicates the formation of a charge-transfer
Prior to deposition within the membrane the spontaneous
growth conditions were optimized by control experiments. A
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.4 (2006)
407
2.0
1.5
1.0
0.5
0.0
-0.5
80
60
40
20
0
(a)
(b)
A
-1
2067.5cm
350
550
750
950
2200
2100
2000
1900
400 nm
200 nm
400 nm
200 nm
100 nm
200 nm
ν/cm-1
λ
/nm
Figure 2. (a) IR spectrum and (b) UV–vis spectrum of PB
nanopillars deposited in gold-coated porous alumina membrane.
standing nanopillars in type 1 membrane after removal of tem-
plate walls. The bulk PB layer covering the membrane surface
can be seen as a thick layer above the pillars. Figures 1c and
1d correspond to nanopillars deposited in type 2 membrane
for 5 min, after removal of the template. Figures 1e and 1f cor-
respond to that of 15 min deposition in type 2 membrane after
removal of the template. In all these cases, we can see the highly
ordered, dense and uniformly spaced nanopillars, standing over
the gold protrusions. Depending on the deposition time, these
nanorods vary in length from ꢂ600{1000 nm, with a diameter
of ꢂ70 nm.
The nature of the formed nanorods is further confirmed by
XPS, micro IR, and micro UV experiments (Figure 2). The value
of Fe2p3=2 from XPS spectra is 709.2 eV, which is similar to that
of bulk PB (see Supporting Inoformation, Figure S2 and Table
S1). The IR spectrum of the nanorods also shows the character-
istic, –CꢃN stretch at 2067 cmꢁ1, which is in accordance with
Figure 1. SEM images of (a) free template, (b) free standing
nanopillars deposited for 30 min, (c) and (d) free standing nano-
pillars deposited for 5 min, and (e) and (f) free standing nanopil-
lars deposited for 15 min of Prussian blue after removal of
porous alumina membrane. The gold rods (ꢂ200 nm long) can
be seen at the bottom as small protrusions.
the reported value of –CꢃN stretch in the IR region, 2068
12
cmꢁ1
.
While, the UV–vis spectrum shows the CT band cen-
complex.
tered about 615 nm, with shoulders at 494 and 765 nm probably
due to interference bands both consistent with the formation of
PB, which is shifted to about 80 nm from the value of bulk
PB, 700 nm. The size effect as a reason of the blue shift is
currently under investigation.
A neutral complex formation from equimolar solutions con-
taining ferric ion and ferricyanide ion was previously reported
by Neff et al.,8 with the formation of BG precipitate slowly,
which contains indefinite amounts of hexacyano ferrate (II)
and (III) ions. Possible mechanism for the formation of PB from
such solutions prior to the precipitation of BG involves the reac-
tions as in eqs 1 and 2.
This work was supported by Grant-in-Aid for Scientic
Research for JSPS Fellows from MEXT, Japan.
Fe3þ þ 1/2H2O ! 1/4O2 þ Hþ þ Fe2þ
þ þ Fe2þ þ [Fe(CN)6]3ꢁ ! KFeFe(CN)6
;
ð1Þ
ð2Þ
References
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Published on the web (Advance View) March 18, 2006; DOI 10.1246/cl.2006.406