Journal of Crystal Growth 294 (2006) 353–357
pH value-dependant growthof a-Fe
2
O
3
hierarchical nanostructures
Chong Jia, Yao Cheng, Feng Bao, Daqin Chen, Yuansheng Wang
Ã
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, China
Received 27 February 2006; received in revised form 27 April 2006; accepted 15 June 2006
Communicated by K. Nakajima
Available online 25 July 2006
Abstract
The controllable synthesis of two kinds of a-Fe
2
O
3
hierarchical nanostructures, i.e., three-dimentional (3D) houseleek-like and two-
dimentional (2D) snowflake-like dendrites were achieved through a simple hydrothermal route by changing pH value. The growth
of a-Fe
2
O
3
dendrites was proceeded by self-assembly through two different modes of oriented attachment (OA): when pHp6, primary
a-Fe
2
O
3
nanoparticles attached preferentially along the six crystallographically equivalent h 1 100i directions, resulting in the formation
of sixfold-symmetric dendrites. While at pHp5, the growth process involved two steps: firstly, primary nanoparticles aggregated to form
round flakes with their up and bottom surfaces parallel to {0 0 0 1} plane. These flakes stacked face-to-face with each other along [0 0 0 1]
direction to construct the single crystalline spindle-like a-Fe
2
O
3
, which then aggregated together at the tips to construct the 3D
houseleek-like dendrites. As far as we know, this is the first time using different modes of OA to realize the morphology control of
hierarchical structures in one reaction system.
r 2006 Elsevier B.V. All rights reserved.
PACS: 61.66.Fn; 61.82.Rx; 81.10.Dn
Keywords: A1. Nanostructures; A2. Hydrothermal crystal growth
1. Introduction
The synthesis of nanophase with controlled shapes,
directional and shape dependent properties is an important
goal of advanced materials chemistry [1–6]. Among the
various tactics used to construct desirable nanostructures,
the oriented attachment (OA) [7] based self-assembly of
nanocrystals should be a successful one adopting the
bottom–up strategy, as has been verified by many examples
over the past few years. Pacholski et al. [8] reported the
formation of high-quality single crystalline ZnO nanorods
through OA of quasi-spherical nanoparticles along c-axis.
The (1 1 1) plane OA of cubic ZnS initial nanocrystals led
to the nanorods or various branched nanostructures [9].
Either length-multiplied 1D nanostructures or 2D crystal
sheets and walls were obtained by self-attachment of
nanorods or nanoribbons through stacking or lateral
lattice fusion [10,11]. Furthermore, the much more com-
plex but ordered 3D architectures could also be obtained
through various OA-based self-assemblies, such as den-
drites [12,13], hollow spheres [14] , hollow octahedrons [15]
and so on. In our previous papers, we have demonstrated
that OA between nanoparticles along specific directions
could lead to the single crystalline dendrites [13], while two-
step OA-based self-assembly constructed the plate-built
cylinders [16]. As diversiform nanostructures could be
acquired through various OA modes, the key to control the
morphology of nanocrystal could be co nverted to the
control of OA modes under this bottom–up self-assembl y
mechanism.
Among a variety of nanostructures, the hierarchical
structures are promising candidates for new functional
nanomaterials. So far, many hierarchical structures of
high-symmetric crystal-system, including cubic PbS [17]
and noble metals [18], hexagonal Fe
2
O
3
[19] and HgS [20] ,
tetragonal tungstate [12] and PbMoO
4
[13], and orthor-
hombic Bi
2
S
3
[21], have been synthesized. However, it is
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doi:10.1016/j.jcrysgro.2006.06.027
Ã
Corresponding author. Tel./fax: +86 591 8370 5402.
E-mail address: yswang@fjirsm.ac.cn (Y. Wang).
still a challenge to control the morphology and understand
the growth mechanism owing to the traditional lack of
understanding of the growth history and shape evolution
process.
a-Fe
2
O
3
(Hematite), the most stable iron oxide under
ambient conditions, is a kind of n-type semiconductor with
a band gap of 2.1 ev [22]. Due to its good stability, high
resistance to corrosion and low cost, a-Fe
2
O
3
is widely used
as catalysts, pigments, gas sensors, and elect rode materials
[23–25]. Stimulated by its potential applications, synthesis
of a-Fe
2
O
3
nanophase with special morphology is a subject
of considerable topical importance [26]. We have success-
fully synthesized the flower-like a-Fe
2
O
3
nanostructures
from nanoparticles through OA of different planes in
solvothermal system before [27]. Herein, we report the
synthesis of a-Fe
2
O
3
hierarchical structures with a series of
novel morphologies in hydrothermal system at low
temperature. By adjusting the pHvalue and thus the
different modes of OA, we realize the control of product
architectures. As we know by literatures, this is the first
time using different modes of OA to realize the morphol-
ogy control in one reaction system.
2. Results and discussion
Fig. 1 shows the XRD patterns of samples obtained from
the solutions with different pH value. The peaks of all
samples are well indexed to the hexagonal a-Fe
2
O
3
with cell
constants a ¼ 0.5038 nm and c ¼ 1.377 nm (JCPDS,
No. 72-0469). No diffraction peaks other than those from
a-Fe
2
O
3
are detected, indicating high purity of a-Fe
2
O
3
samples. It is noticeable that, compared to the standard
pattern, the ð11
20Þ and ð3030Þ diffraction peaks in the
patterns for pHp5 are very strong, while at pHX6 they
become weak oppositely, implying the difference of
preferential growth directions for a-Fe
2
O
3
nanophase
synthesized under different conditions.
The typical FE–SEM images of the as-made samples are
shown in Fig. 2. The pHvalueof the original solution and
the reaction duration for Fig. 2a–f are 3, 4, 5, 5.5, 6, 11,
and 3, 6, 9, 12, 24, 48 h, respectively. At pH ¼ 3, the
products are houseleek-like dendrites sized about 1–2 mm
with several spindle-like ‘‘leaves’’ of several hundred
nanometers in length. When pH is increased to 4, the
‘‘leaves’’ of the dendrites are somewhat slenderized. More
detailed nanostructures appeared from the ‘‘leaves’’ when
the pHvalue further increased to 5, as shown in Fig. 2c,
where all the ‘‘leaves’’ consist of many parallel flakes with
thickness of about 100 nm. The morphology for sample at
pH ¼ 5.5 is somewhat complex (see Fig. 2d), some of the
‘‘leaves’’ evolve from spindle-like to trigonal pyramid-like
with one arris tending flatter than the other two. When the
pH value achieved 6, all the products are 2D snowflake-like
dendrites with sixfold-symmetry, as shown in Fig. 2e. Each
main branch of the dendrites consists of several levels of
sub-branches. With the pHvalue changing from 6 to 11,
the morphology of dendrites does not change too much
except the size changing from 5 to 7 mm to about 15 mm.
Such large snowflake-like dendrites would be unstable: a
large amount of the main branches separate from each
other, as exhibited in Fig. 2f, resulting from the drastic
agitation of boiling solution under hydrothermal condi-
tions.
Fig. 3 demonstrates the TEM photographs of the two
typical products, i.e., 3D houseleek-like and 2D snowflake-
like dendrites with pH ¼ 4 and 6, respectively. Fig. 3b , the
selected area electron diffraction (SAED) pattern recorded
from the squared region in a ‘‘leaf’’ of the hous eleek-like
dendrite in Fig. 3a, indexed to a-Fe
2
O
3
along ½2 1 10 zone
axis, reveals the ‘‘leaf’’ a single crystal with its long axis
along [0 0 0 1] direction. A HRTEM image (Fig. 3c) taken
from the ‘‘leaf’’-tip ofa houseleek-like dendrite presents the
uniform lattice structure, free of detectable crystal defects.
However, the SAED from an entire houseleek-like dendrite
yields complex poly-crystalline pattern, indicating that the
dendrite is fabricated by random aggregation of the
‘‘leaves’’ at the tips. Fig. 3d shows the TEM image of a
sixfold-symmetric snowflake-like dendrite. The SAED
pattern taken from the entire dendrite, shown in Fig. 3e,
reveals it a single crystal with six main branches grown
along the six crystallographically equivalent h1
100i
directions, respectively, as has been reported by Z.L.
Wang’s group previously [19].
To reveal the generation process of the dend rites, time-
dependent experiments were carried out at different
reaction stages in the cases ofpH ¼ 5 and 6, respectively.
For the system with pH ¼ 6, it was found that many
particles sized about 100 nm formed after reacting for 3 h
(Fig. 4a), and these primary particles were confirmed to be
hexagonal Fe
2
O
3
by XRD analysis. When the reaction
lasted to 6 h, these particles aggregated together to
construct the sixfold dendrites with 1–3 mm in size, as
ARTICLE IN PRESS
Fig. 1. The XRD patterns of samples synthesized at different pH values:
pH ¼ 3 for 3 h; pH ¼ 4 for 6 h; pH ¼ 5 for 9 h; pH ¼ 5.5 for 12 h; pH ¼ 6
and 7 for 24 h; pH ¼ 11 for 48 h.
C. Jia et al. / Journal of Crystal Growth 294 (2006) 353–357354
shown in Fig. 4b. Further prolonging the rea ction time to
24 h enabled the evolution from the small dendrites to the
snowflake-like hier archical structures sta ted ab ove (Fig. 2e).
For the syst em with pH ¼ 5, when reacted for 2 h, some
100 nm primary par ticles and round flakes sized 0. 2–0.5 mm
built up by the former were obse rved (Fig. 4c ). It is
noticeable that, as indica ted by the arrow, some of the
flakes hav e stacked fac e-to-face with each other at this sta ge.
The self-assembly of single crystal dendrites and other
hierarchical nanostructures have been widely investigated
in recent years [8–13,16]. In our previous papers, the single
crystal PbMoO
4
dendrites were verified to grow by self-
assembly through OA of nanoparticles sharing a common
crystallographic orientation and joining at the planar
interfaces [13], and the two-step self-assembly process
through OA was found for hexagonal LaF
3
nanophase,
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Fig. 2. FE–SEM images of the samples obtained from the solutions with different pH values and reaction durations: (a) pH ¼ 3 for 3 h; (b) pH ¼ 4 for 6 h;
(c) pH ¼ 5 for 9 h; (d) pH ¼ 5.5 for 12 h; (e) pH ¼ 6 for 24 h; (f) pH ¼ 11 for 48 h. The insets of (c)–(f) present the enlarged images of the products.
Fig. 3. TEM images of the as-made samples: (a) the micrograph of the houseleek-like dendrites; (b) SAED pattern taken from the squared region in 3a; (c)
HRTEM image taken from the ‘‘leaf’’-tip ofa houseleek-like dendrite; (d) and (e) TEM image and its corresponding SAED pattern ofa snowflake-like
dendrite.
C. Jia et al. / Journal of Crystal Growth 294 (2006) 353–357 355
i.e., the primary LaF
3
nanoparticles aggregated together by
coalescence mainly through f10
10g planes to form
nanoplates, which were then stacked face-to-face with each
other along the [0 0 0 1] direction to construct the cylinder-
like single crystals [16]. In the present case, similar to those
of single crystal PbMoO
4
and LaF
3
, the formation of
snowflake-like dendrites and spindle-like a-Fe
2
O
3
could
obviously be ascribed to self-assembly through OA of
primary nanoparticles proceeding in different modes
explained schematically in Fig. 5. At the first stage of
reaction, primary a-Fe
2
O
3
nanoparticles precipitated from
the solution. Further reaction was affected by pHvalue of
the solution: when pHX6, these nanoparticles aggregated
and attached each other along the six crystallographically
equivalent h1
100i directions, just like the case of PbMoO
4
[13], to form snowflake-like single crystal dendrites; while
for solution of pHp5, much more primary nanoparticles
were produced at the first reaction stage due to the faster
reaction velocity (which will be discussed later), and as a
result, instead of the dendrites with six trunks, the round
flakes (regarded as the space-crammed dendrites) with their
up and bottom surfaces parallel to {0 0 0 1} plane were
formed, which then stacked face-to-face with each other
along the [0 0 0 1] direction to build-up the single crystalline
spindle-like a-Fe
2
O
3
, similar to the case of two-step self-
assembly of LaF
3
cylinders [16]. The 3D houseleek-like
morphology was further constructed by the aggregation of
several spindle-like crystals at the tips. Certainly, accom-
panying with self-assembly, the traditional Ostwald ripen-
ing mechanism also acted to form the primary
nanoparticles and smooth the product morphology during
the course of OA. In published literatures, it was a general
situation that only one kind of OA occurs in one reaction
system. Herein, it is notewo rthy that by simply changing
pH value, the a-Fe
2
O
3
dendrites with different morphol-
ogies were formed through different modes of OA in one
reaction system, which may provide a route to access
controlled manufacture of newfangled nanostructures
probably with useful properties.
The chemical reactions concerned in the form ation of
a-Fe
2
O
3
could be proposed below:
½FeðCNÞ
6
3À
aFe
3þ
þ 6CN
À
; (1)
Fe
3þ
þ 3OH
À
a
À
FeOOH þ H
2
O; (2)
2aFeOOHaFe
2
O
3
þ H
2
O, (3)
CN
À
þ H
þ
dHCN: (4)
Among the four equations, Eq. (4) is very important
although it does not participate in the formation of
a-Fe
2
O
3
. Without this reaction, the CN
À
concentration
will increase continuously with the decreasing of Fe
3+
concentration, and the dissociation of [Fe(CN)
6
]
3À
will
thus be strongly restricted, and as a result, the hydro-
thermal reaction will last for only a short duration at the
initial stage. From the above analysis, it could be
concluded that the CN
À
ions play two main roles in the
hydrothermal reaction: ligand of Fe
3+
and reactant of Hþ.
The concentration of Fe
3+
ions is the key factor
determining the velocity of the whole reaction due to
the weak dissociation tendency of [Fe(CN)
6
]
3À
ions
(K
s
¼ 1.0 Â 10
À42
) [19]. As for the hydrolyzation of Fe
3+
ions, among the different factors that can make an effect,
such as the reaction temperature, the reactants’ concentra-
tion, and so on [19], the pHvalue would be the most
significant one. From Eqs. (1) and (4), the concentration
change of Fe
3+
ions brought by the change ofpH value
can be estimated approximately as followe d. With pH
value minus 1, the H
+
concentration increases 10 times,
which leads to the decrease of CN
À
concentration to one-
tenth of before owing to the low ionic constant of HCN
ARTICLE IN PRESS
Fig. 4. FE–SEM images of a-Fe
2
O
3
dendrites obtained under different reaction stages: (a) pH ¼ 6 for 3 h; (b) pH ¼ 6 for 6 h; (c) pH ¼ 5 for 2 h.
Fig. 5. Schematic illustration for the self-assembly of two kinds of a-
Fe
2
O
3
dendrites.
C. Jia et al. / Journal of Crystal Growth 294 (2006) 353–357356
(K
a
¼ 6.2 Â 10
À10
), and in turn, the Fe
3+
concentration
increases about 10
6
times calculated from K
s
of [Fe(CN)
6
]
3À
,
resulting in a tremendous increase of the reaction velocity.
This could exp lain the ch ange of the durat ion for complete
formation of a-Fe
2
O
3
from 48 h (for pH ¼ 11)to3h(for
pH ¼ 3) in our ex periment. Additionally, based on Eq. (4) ,
with increasing or decreasing ofpH value, Eq. (1) moves
toward left or right, respectively, which significantly affects
the supply of Fe
3+
and thus the growth rate of a-Fe
2
O
3
,and
finally re sulting in the different growth modes and product
morphologies.
3. Conclusion
Using a simple hydrothermal route, we realized the
morphology control of a-Fe
2
O
3
dendrites by changing pH
value of the reaction solution. When pHX6, the 2D
snowflake-like dendrites were formed by the self-assembly
of primary a-Fe
2
O
3
nanoparticles through OA preferen-
tially along the six crystallographically equivalent h1
10 0i
directions. While at pHp5, the primary nanoparticles first
aggregated through OA to form round flakes with their up
and bottom surfa ces parallel to {0 0 0 1} plane, which were
then stacked face-to-face with each other along the [0 0 0 1]
direction to build the single crystalline spindle-like a-
Fe
2
O
3
. Finally, the spindle-like crystals were further
aggregated at the tips to construct the 3D houseleek-like
morphology.
4. Experimental procedure
a-Fe
2
O
3
hierarchical structures were synthesized by low-
temperature hydrothermal reaction of the solution contain-
ing 0.015 M K
3
[Fe(CN)
6
] and 0.15 M acetic acid. The pH
value of the solution was adjusted from 3 to 11 using 5 M
ammonia. In a typical experiment, the above-mentioned
solution of 50 mL with different pHvalue was transferred
into a Teflon-sealed autoclave of 70 mL capacity, and
maintained at 140 1 C for a suit able time. After the
autoclave was quickly cooled down to room temperature
by quenching in water, the products with different co lor
(black for pH ¼ 3; brown with different degree for
pH ¼ 4–5.5; and vermeil for pH ¼ 6–11) were filtered off,
repeatedly washed with distilled water and absolute
ethanol, and then dried in air at 50 1C for 4 h.
The morphologies of the samples were observed by the
field emission scanning electron microscope (FE–SEM,
JSM-6700F). The phase and structure were characterized
by X-ray diff raction (XRD, RIGAKU-DMAX2500) with
Cu Ka radiation (l ¼ 0.154056 nm) at a scanning rate of
51/min for 2y ranging from 51 to 851, and the high-
resolution transmission electron microscope (HRTEM,
JEM-2010) operated at 200 kV.
Acknowledgment
This work was supported by the project of Nano-
molecular Functional Materials of Fujian Province
(2005HZ01-1) and the Grants of the Natural Science
Foundation of Fujian (A0320001, Z0513025).
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C. Jia et al. / Journal of Crystal Growth 294 (2006) 353–357 357
. Journal of Crystal Growth 294 (2006) 353–357
pH value-dependant growth of a-Fe
2
O
3
hierarchical nanostructures
Chong Jia, Yao Cheng,. patterns of samples synthesized at different pH values:
pH ¼ 3 for 3 h; pH ¼ 4 for 6 h; pH ¼ 5 for 9 h; pH ¼ 5.5 for 12 h; pH ¼ 6
and 7 for 24 h; pH ¼ 11