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View Online / Journal Homepage / Table of Contents for this issue

Controlled growth and magnetic properties of a-Fe2O3 nanocrystals:Octahedra, cuboctahedra and truncated cubes†

Liang Bing Wang,a Le Xin Song,*ab Zheng Dang,a Jie Chen,a Jun Yanga and Jie Zengc

Received 12th December 2011, Accepted 7th March 2012

DOI: 10.1039/c2ce06661k

A series of a-Fe2O3 crystalline materials with fascinating poly-

hedral morphologies, such as octahedral, cuboctahedral and trun-

cated cubic structures, were prepared through a novel solid-phase

sintering process. Our results demonstrated that the structural

transformations among the polyhedra were easily controlled by the

sintering times. This suggested that the Wulff polyhedra trans-

formation could occur in a hexagonal crystal system. Furthermore,

we found that the magnetic properties of the a-Fe2O3 crystals were

associated with their size and shape.

With the prosperity of nanotechnology in the past two decades,

polyhedral theory has largely been developed through experiments.1,2

Polyhedral structures, such as octahedral, cuboctahedral, truncated

cubic and cubic structures, have attracted much attention and have

found wide applications as electrical and optical devices due to their

interesting properties, such as high symmetry, nano-sized dimension

and the existence of critical edges, etc.3 One example is that noble

metal nanocrystals, such as gold, silver and platinum nanopolyhedra,

can be obtained in very small grain sizes (often only below 20 nm),

which exhibit excellent physical and chemical properties.4–6 A great

achievement by Xia and co-workers has been to controllably

synthesize silver nanostructures.7 Furthermore, a controlled struc-

tural transformation between polyhedra is an essential goal of crystal

engineering strategies. Recently, Castleman and co-workers reported

that the morphology of titanium carbide nanoparticles could be

changed from cubic to cuboctahedral by using different methane

concentrations.8 Chen and colleagues revealed a controllable growth

for FePt cuboctahedral, octapodal, truncated cubic, and cubic

nanostructures by means of adjusting the reaction parameters.9

However, although a variety of fascinating features of these structural

transformations may exist, the studies were mostly focused on the

solution-mediated synthesis.10–13 It was necessary, therefore, to

aDepartment of Chemistry, University of Science and Technology of China,Jin Zhai Road 96, Hefei 230026, China. E-mail: solexin@ustc.edu.cn; Fax:+86-551-3601592; Tel: +86-551-3492002bState Key Laboratory of Coordination Chemistry, Nanjing University,Nanjing 210093, ChinacDepartment of Biomedical Engineering, Washington University, St. Louis,Missouri 63130, USA

† Electronic supplementary information (ESI) available: Experimentalsection, SEM, and SQUID. See DOI: 10.1039/c2ce06661k

This journal is ª The Royal Society of Chemistry 2012

develop a novel strategy to construct polyhedral structures upon

solid-phase treatment and to especially address such structural issues.

Hematite (a-Fe2O3) has wide applications in many fields, such as

chemical filters, catalysis aids, and electrode materials in lithium

batteries, owing to its low cost, good performance and high

stability.14–17 Different structures including spheres, rods, wires,

arrays, tubes, belts, disks, rings, dendrites, propellers and flowers have

been elucidated.18–25 Although a-Fe2O3 has been prepared as nano-

crystals with octahedral and cubic shapes,26,27 to the best of our

knowledge, there is no precedent in the literature for the formation of

cuboctahedral and truncated cubic structures of a-Fe2O3. Hence, this

represents the first evidence of these morphologies in a hexagonal

system. It is therefore of great interest to find a way of constructing

such structures and thus to understand the formation mechanisms.

Herein, we conducted a series of critical experiments on this

subject. Typically, an aqueous solution of FeCl3 and b-cyclodextrin

(b-CD)28 was introduced to obtain an aggregate, followed by sin-

tering of the product at specific temperatures for different periods of

time. Samples 1–3, which were determined to be a-Fe2O3, were

obtainedwith sintering at 723K in air for 8 h (sample 1); 16 h (sample

2) and 24 h (sample 3), respectively. It was found that with the

increase in sintering times, the highly ordered crystal structures of the

a-Fe2O3 samples experienced a continuous transformation from

octahedral to cuboctahedral and truncated cubic structures. We have

also proposed a possible mechanism for such a structural trans-

formation. We believe that the present work provides a striking

Fig. 1 XRD patterns of samples 1–3 (A, curves a–c) and FE-SEM

images of samples 1 (B), 2 (C), and 3 (D).

CrystEngComm, 2012, 14, 3355–3358 | 3355

Fig. 2 TEM images and SAED patterns of the octahedral (a, b),

cuboctahedral (c, d) and truncated cube structures (e, f).

Fig. 3 Schematic illustration of the formation of octahedral, cubocta-

hedral and truncated cubic structures.

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example of the close relationship between the sintering conditions and

crystallographies.

Fig. 1A shows theX-ray diffraction (XRD) patterns of the samples

1–3 indicating the formation of a unique a-Fe2O3 structure, since all

the XRD diffraction peak positions and intensities are in good

accordance with the JCPDS reference (89–0596).29 The cell is

hexagonal (space group R�3c) with constants: a ¼ 0.504 nm and

c ¼ 1.377 nm.

Fig. 1B–D shows typical field-emission scanning electron micros-

copy (FE-SEM) images of these three samples. As seen in Fig. 1B,

sample 1 exhibits an octahedral shapewith good uniformity. A higher

magnification (see Fig. S1, ESI†) displays more detail about the

octahedral units consisting of 8 equilateral triangular surfaces and 12

identical edges (with a mean edge length of 250 � 30 nm).

Interestingly, after further sintering for 8 more hours, the size of

these octahedral crystals became bigger (sample 2, with a mean edge

length of 700 � 70 nm), as shown in Fig. 1C. The increase in sizes

implies that sintering times could be applied to control the growth

process of the a-Fe2O3 octahedral particles. In addition, a few well-

ordered cuboctahedral particles were also observed in the same

sample (Fig. S2, ESI†). The mean edge length was determined to be

around 1.2 mm. Note that there are several truncated cubic particles

with an edge length of ca. 350 nm in the bottom left of Figure S2.† It

seems that sintering a-Fe2O3 at 723K for 16 h produced amixture of

polyhedra with different shapes. Although the products were domi-

nated by the bigger octahedra, the structural transformations from

octahedron to cuboctahedron and truncated cube should be

involved.

For sample 3, we found that the major products were truncated

cubes (with a mean edge length of about 500–800 nm, see Fig. 1D)

accompanied by some cuboctahedra (see Figure S3, ESI†). No

octahedral structures were observed. Moreover, it was found in

Figure S3 (ESI†) that the cuboctahedral particles were covered by

small pieces of other particles.We also noticed that the cuboctahedral

structures in sample 3 aremuch bigger andmore equiaxed in shape in

comparison with those in sample 2.

Together, these results not only provide the first example of

cuboctahedral a-Fe2O3 crystals, but also strongly demonstrate that

the crystal structure of the a-Fe2O3 particles has undergone

a continuous transformation from small octahedra to big octahedra,

cuboctahedra and truncated cubes with the increase in sintering

times. Therefore, the sintering time is an essential condition in

controlling the morphologies of the a-Fe2O3 crystals in this

approach.

Studies presented so far on the continuous transformations among

octahedral, cuboctahedral and truncated cubic structures have been

limited to a cubic crystal system.30–35 However, all the a-Fe2O3

crystals reported in this work and elsewhere belong to a hexagonal

crystal system.35 Considering the a-Fe2O3 crystals obtained here

exhibit a typical polyhedron transformation, 36,37 it would be inter-

esting to reveal themechanism of this structural transformation in the

developed polyhedral system.

Fig. 2a, c and e show the transmission electron microscopy (TEM)

images of an octahedral structure (sample 1), a cuboctahedral

structure (sample 2), and a truncated cubic structure (sample 3),

respectively. Further structural characterization of the polyhedra was

carried out by selected-area electron diffraction (SAED) patterns

(Fig. 2b, d and f). These patterns not only reveal that the a-Fe2O3

particles have a single-crystalline structure, but also present

3356 | CrystEngComm, 2012, 14, 3355–3358

[0001] and [2�1�10] zone axis in a hexagonal structure, as shown in

Fig. 2b, d and f. The view directions of the polyhedra are illustrated in

this figure.

A possible formation and transformation mechanism for the

a-Fe2O3 particles was proposed as follows. Some of the b-CD

molecules decomposed under oxygen to form carbon dioxide and

water (eqn (1)), while others were carbonized to produce carbon

particles (eqn (2)).38 The generation and growth of a-Fe2O3 crystal

seeds (eqn (3) and (4)) might to be affected by the in situ carbon

particles produced during synthesis. We used Raman spectra to

investigate the generation of carbon particles during the formation of

the a-Fe2O3 polyhedra. Based on the Raman spectra (Figure S4,

ESI†), we found that the carbon particles are only detectable in a very

early stage (0.5 h) of the sintering process. After sintering for 8 h

(sample 1) or longer (samples 2 and 3), no carbon signals were found.

These observations suggest that the adsorption of the carbon particles

and its capping effect did not play a key role in the generation of

cuboctahedral and truncated cubic a-Fe2O3 crystals. Thanks to the

theoretical studies from other groups on the surface energies of a-

Fe2O3,14,15 the {10�1 2} planes of a-Fe2O3 crystals were found to have

the lowest surface energy. Since the truncated cube of a-Fe2O3 was

mostly encased by such planes, it should be the lowest energy

morphology compared to the octahedron and cuboctahedron struc-

tures. As such, it is not hard to understand the mechanism for the

transformation from octahedra to cuboctahedra and truncated cubes

at an elevated temperature (723 K) due to the thermodynamic

consideration, as shown in Fig. 3. It is also worth noting that, in the

early stage of the reaction, the synthesis is often known as

This journal is ª The Royal Society of Chemistry 2012

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a kinetically controlled process39 and the nanocrystals will take shapes

deviating from the thermodynamically favored one. In this case,

octahedra enclosed by {0001} facets are formed.

C42H70O35 þ 42O2 �����!D42CO2 þ 35H2O (1)

C42H70O35 �����!D42Cþ 35H2O (2)

FeCl3 þ 3H2O �����!DFeðOHÞ3 þ 3HCl (3)

2FeðOHÞ3 �����!D

Fe2O3 þ 3H2O (4)

The effect of the sintering temperatures on the construction of the

a-Fe2O3 crystal structures was also investigated by controlled

experiments. As shown in Figure S5 (ESI†), the a-Fe2O3 particles

obtained at 673 K for 24 h (sample 4) are much smaller (about 200–

300 nm in size) andmore irregular, when compared with sample 3. In

addition, no cuboctahedral and truncated cubic structures were

found, though several octahedral structures appeared. This result

suggests that a lower temperature (50 K) is beneficial for the gener-

ation of small particles, but is highly disadvantageous to the forma-

tion of symmetrical polyhedral structures. In another set of

experiments, we noticed that the a-Fe2O3 crystal obtained at 773 K

for 24 h (sample 5) is composed of a large number of incomplete

polyhedra with a mean edge length of about 1.5–2.0 mm, as shown in

Figure S6, ESI.†

We also investigated the dependence of the magnetic properties of

the five a-Fe2O3 samples on their sizes and shapes using a super-

conducting quantum interference device (SQUID). Fig. 4 shows the

hysteresis loops of samples 1–3, indicating weak ferromagnetic

properties at 300K for this set of samples, which is in agreement with

the observations previously reported.40

The hysteresis loops of samples 4 and 5 (Figure S7, ESI†) nearly

coincide, and they exhibit a lower saturation magnetization (Ms <

0.22 emu g�1) and a lower remanence (Mr < 0.06 emu g�1) compared

to sample 3 (Ms, 0.32 emu g�1 andMr, 0.11 emu g�1). Although all of

the samples have a similar coercivity (Hc, 517Oe), there is a difference

in the shape of the hysteresis loops near the coercive field. This

difference is a reflection of the difference in the rotation of the

magnetic domains in the crystals with different sizes and morphol-

ogies. However, in the cases of samples 1 and 2, there is no saturation

of the magnetization at 300 K in fields up to the maximum applied

magnetic field (30 kOe), and their magnetizations at 30 kOe are 0.83

and 1.35 emu g�1, respectively. The higher magnetizations are

ascribed to a uniform intensity of themagnetization in a thicker crust.

At the same time, the two samples present a small hysteresis loopwith

Fig. 4 Field dependencies of themagnetizations of samples 1–3 at 300K.

This journal is ª The Royal Society of Chemistry 2012

the same coercive forces of 3.7 kOe but different remnant magneti-

zations of 0.23 and 0.35 emu g�1. These results reveal that the size and

shape are two important factors in tuning the magnetic properties of

the a-Fe2O3 crystal materials. This may be related to different

magneto-crystalline anisotropies in different microstructural

phases.41,42

In conclusion, a series of a-Fe2O3 crystalline materials with

different fascinating polyhedral morphologies, such as octahedral,

cuboctahedral and truncated cubic structures, were prepared through

a novel solid-phase sintering process. A mechanism was proposed to

explain the formation and transformation of the polyhedral struc-

tures. Our result suggests that the Wulff’s polyhedral transformation

given in a cubic crystal system can be extended to a hexagonal crystal

system. It also demonstrates that such a structural transformation

between the polyhedral crystals of a-Fe2O3 can be easily controlled

by the sintering times.We believe that these findings will contribute to

the understanding of controlled structural and physical characteris-

tics in nanocrystals.

This project was supported by NSFB (No. J1030412) and NSFC

(No. 21071139).

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