Nakajima Available online 25 July 2006 Abstract The controllable synthesis of two kinds of a-Fe2O3hierarchical nanostructures, i.e., three-dimentional 3D houseleek-like and two-dimention
Trang 1Journal of Crystal Growth 294 (2006) 353–357
Chong Jia, Yao Cheng, Feng Bao, Daqin Chen, Yuansheng Wang 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-Fe2O3hierarchical 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-Fe2O3dendrites was proceeded by self-assembly through two different modes of oriented attachment (OA): when pHp6, primary a-Fe2O3nanoparticles attached preferentially along the six crystallographically equivalent h1 1 0 0i 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-Fe2O3, 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
r2006 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 converted to the control of OA modes under this bottom–up self-assembly 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 Fe2O3[19]and HgS[20], tetragonal tungstate [12] and PbMoO4 [13], and orthor-hombic Bi2S3 [21], have been synthesized However, it is
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0022-0248/$ - see front matter r 2006 Elsevier B.V All rights reserved.
doi: 10.1016/j.jcrysgro.2006.06.027
Corresponding author Tel./fax: +86 591 8370 5402.
E-mail address: yswang@fjirsm.ac.cn (Y Wang).
Trang 2still 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-Fe2O3 (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-Fe2O3is widely used
as catalysts, pigments, gas sensors, and electrode materials
[23–25] Stimulated by its potential applications, synthesis
of a-Fe2O3nanophase with special morphology is a subject
of considerable topical importance [26] We have
success-fully synthesized the flower-like a-Fe2O3 nanostructures
from nanoparticles through OA of different planes in
solvothermal system before [27] Herein, we report the
synthesis of a-Fe2O3hierarchical structures with a series of
novel morphologies in hydrothermal system at low
temperature By adjusting the pH value 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 1shows 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-Fe2O3with 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-Fe2O3 are detected, indicating high purity of a-Fe2O3
samples It is noticeable that, compared to the standard
pattern, the ð1 1 2 0Þ and ð3 0 3 0Þ 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-Fe2O3 nanophase synthesized under different conditions
The typical FE–SEM images of the as-made samples are shown inFig 2 The pH value of the original solution and the reaction duration for Fig 2a–fare 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 pH value 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 (seeFig 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 inFig 2e Each main branch of the dendrites consists of several levels of sub-branches With the pH value 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 houseleek-like dendrite inFig 3a, indexed to a-Fe2O3along ½2 1 1 0 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 of a 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 1 0 0i directions, respectively, as has been reported by Z.L Wang’s group previously[19]
To reveal the generation process of the dendrites, time-dependent experiments were carried out at different reaction stages in the cases of pH 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 Fe2O3 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
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.
Trang 3shown in Fig 4b Further prolonging the reaction time to
24 h enabled the evolution from the small dendrites to the
snowflake-like hierarchical structures stated above (Fig 2e)
For the system with pH ¼ 5, when reacted for 2 h, some
100 nm primary particles and round flakes sized 0.2–0.5 mm
built up by the former were observed (Fig 4c) It is
noticeable that, as indicated by the arrow, some of the
flakes have stacked face-to-face with each other at this stage
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 PbMoO4 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 nanophase,
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 of a houseleek-like dendrite; (d) and (e) TEM image and its corresponding SAED pattern of a snowflake-like dendrite.
Trang 4i.e., the primary LaF3nanoparticles aggregated together by
coalescence mainly through f1 0 1 0g 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 PbMoO4 and LaF3, the formation of
snowflake-like dendrites and spindle-like a-Fe2O3 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-Fe2O3nanoparticles precipitated from
the solution Further reaction was affected by pH value of
the solution: when pHX6, these nanoparticles aggregated
and attached each other along the six crystallographically
equivalent h1 1 0 0i directions, just like the case of PbMoO4
[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-Fe2O3, similar to the case of two-step
self-assembly of LaF3 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 noteworthy that by simply changing
pH value, the a-Fe2O3 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 formation of a-Fe2O3could be proposed below:
½FeðCNÞ63aFe3þþ6CN; (1)
Fe3þþ3OHaFeOOH þ H2O; (2)
2aFeOOHaFe2O3þH2O, (3)
CNþHþdHCN: (4) Among the four equations, Eq (4) is very important although it does not participate in the formation of a-Fe2O3 Without this reaction, the CN concentration will increase continuously with the decreasing of Fe3+ concentration, and the dissociation of [Fe(CN)6]3 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
ions play two main roles in the hydrothermal reaction: ligand of Fe3+and reactant of Hþ The concentration of Fe3+ ions is the key factor determining the velocity of the whole reaction due to the weak dissociation tendency of [Fe(CN)6]3 ions (Ks¼1.0 1042) [19] As for the hydrolyzation of Fe3+ ions, among the different factors that can make an effect, such as the reaction temperature, the reactants’ concentra-tion, and so on [19], the pH value would be the most significant one From Eqs (1) and (4), the concentration change of Fe3+ ions brought by the change of pH value can be estimated approximately as followed With pH value minus 1, the H+ concentration increases 10 times, which leads to the decrease of CNconcentration to one-tenth of before owing to the low ionic constant of HCN
Fig 4 FE–SEM images of a-Fe2O3 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-Fe2O3 dendrites.
Trang 5(Ka¼6.2 1010), and in turn, the Fe3+ concentration
increases about 106times calculated from Ksof [Fe(CN)6]3,
resulting in a tremendous increase of the reaction velocity
This could explain the change of the duration for complete
formation of a-Fe2O3from 48 h (for pH ¼ 11) to 3 h (for
pH ¼ 3) in our experiment Additionally, based on Eq (4),
with increasing or decreasing of pH value, Eq (1) moves
toward left or right, respectively, which significantly affects
the supply of Fe3+and thus the growth rate of a-Fe2O3, and
finally resulting in the different growth modes and product
morphologies
3 Conclusion
Using a simple hydrothermal route, we realized the
morphology control of a-Fe2O3dendrites by changing pH
value of the reaction solution When pHX6, the 2D
snowflake-like dendrites were formed by the self-assembly
of primary a-Fe2O3 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 surfaces 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-Fe2O3 Finally, the spindle-like crystals were further
aggregated at the tips to construct the 3D houseleek-like
morphology
4 Experimental procedure
a-Fe2O3hierarchical structures were synthesized by
low-temperature hydrothermal reaction of the solution
contain-ing 0.015 M K3[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 pH value was transferred
into a Teflon-sealed autoclave of 70 mL capacity, and
maintained at 140 1C for a suitable time After the
autoclave was quickly cooled down to room temperature
by quenching in water, the products with different color
(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 diffraction (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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