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Cite this: J. Mater. Chem., 2011, 21, 5414

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Solvent-mediated synthesis of magnetic Fe2O3 chestnut-like amorphous-core/g-phase-shell hierarchical nanostructures with strong As(V) removalcapability

Fangzhi Mou,a Jianguo Guan,*a Zhidong Xiao,b Zhigang Sun,a Weidong Shia and Xi-an Fana

Received 1st November 2010, Accepted 10th February 2011

DOI: 10.1039/c0jm03726e

In this paper, a magnetic adsorbent of iron oxide (Fe2O3) with a chestnut-like amorphous-core/g-

phase-shell hierarchical nanostructure (CAHN) has been synthesized in a rationally designed chemical

reaction system, where the decomposition rate of Fe(CO)5 is controlled by the CO generated from the

decomposition of N,N-dimethylformamide under the assistance of the hydrolysis of SnCl2$2H2O. By

utilizing the current chemistry equilibrium dynamics, it is possible to kinetically modulate the

nucleation and subsequent growth of Fe2O3 to form chestnut-like hierarchical nanostructures, in which

single crystalline g-Fe2O3 nanorods, with a diameter of 20 nm and a length of 300 nm, radially grow

from the surfaces of amorphous and porous Fe2O3 sub-microspheres. The detailed possible formation

mechanism of the Fe2O3 CAHNs is proposed according to the experimental results. The as-obtained

Fe2O3 CAHNs show a strong adsorption capability for As(V), with a maximum adsorption capacity of

137.5 mg g�1, because of both the specific surface area, which can be as large as 143.12 m2 g�1, and the

heterogeneous surface properties. Furthermore, their ferromagnetic properties make them easy to

separate from water by magnetic separation. The adsorption process obeys the Freundlich isotherm

model well, but not the Langmuir model, suggesting that a multilayered adsorption occurs on the

surface of the Fe2O3 CAHNs. Our work may shed light on the design and preparation of high

performance 3D hierarchically nanostructured adsorbents.

1. Introduction

Arsenic is considered as a primary highly toxic pollutant in water

sources, and arsenic pollution has attracted increasing atten-

tion.1,2 Long-term exposure may cause lung, bladder, kidney and

skin cancer as well as skin pigmentation changes.3 Because of the

toxicological effects of arsenic, the arsenic maximum contami-

nant level (MCL) in drinking water was recently reduced from 50

to 10 mg L�1 by authorities.4–6 To meet the new MCL arsenic

standard in drinking water, numerous technologies, such as

oxidation,7,8 coagulation,9 sorption,10,11 ion-exchange12 and

reverse osmosis13 have been developed for arsenic removal.

Among them, sorption has some obvious advantages, such as

better performance, easy operation, and lower cost.14

Amorphous hydrous ferric oxide (FeOOH), goethite (a-

FeOOH) and hematite (a-Fe2O3) adsorbents have been widely

studied for removing arsenic fromwater because of their effective

performance, low cost and natural abundance.15–18 However, the

aState Key Laboratory of Advanced Technology for Materials Synthesisand Processing, Wuhan University of Technology, Wuhan, 430070, P. R.China. E-mail: guanjg@whut.edu.cn; Fax: +86-27-87879468; Tel: +86-27-87218832bDepartment of Chemistry, College of Science, Institute of ChemicalBiology, Huazhong Agricultural University, Wuhan, 430070, P. R. China

5414 | J. Mater. Chem., 2011, 21, 5414–5421

as-mentioned iron oxide adsorbents, used in the form of fine

powders, usually made solid/liquid separation and recovery

difficult. Magnetic separation, as a quick and effective technique

for the separation of magnetic particles, can overcome many of

the issues present in filtration, centrifugation or gravitational

separation, and requires much less energy. Hence, magnetic iron

oxide adsorbents, such as maghemite and magnetite, have been

considered to be promising adsorbents because not only can they

be conveniently recovered by magnetic separation technology,

but they also retain desirable adsorptive properties. For example,

magnetic g-Fe2O3 and Fe3O4 nanoparticles with sizes of about

3.8 and 12 nm, respectively, show a significant increase in As(V)

removal capacities compared with large iron-oxide particles or

bulk materials.19–21 However, as the size of a magnetic adsorbent

decreases, its response to an external magnetic field undesirably

decreases, making it increasingly difficult to recover the adsor-

bent after the treatment is completed and therefore magnetic

separation loses ground against other conventional separation

methods.20,22 Magnetic hierarchical nanostructures, which are

constructed with building blocks of nanoparticles,23,24 nano-

plates25 or nanorods26,27 etc., can be regarded as an ideal adsor-

bent for water purification, since usually they not only exhibit

a high specific surface area because of the abundant interparticle

spaces or intraparticle pores resulting from their complex

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structure, but they are also more easily separated and reused

because of their larger size, weaker Brownian motion, and better

magnetic properties compared to nanosized powder adsor-

bents.28–30 However, the 3D flower-like magnetic g-Fe2O3 and

Fe3O4 hierarchical nanostructures have been reported to show

adsorptive capacities for As(V) as low as 4.75 and 4.65 mg g�1 at

pH 4, respectively, because of their relatively low surface areas.31

On the other hand, amorphous nanoparticles of Fe2O3, which

have a larger surface area than the corresponding nanocrystalline

particles, exhibit a superior adsorption performance,32,33 but

suffer from a disadvantage of weak magnetic properties, and the

difficulties of solid/liquid separation and recovery.34 Thus, the

magnetic Fe2O3 amorphous-core/g-phase-shell nanocomposite

structures are expected to achieve both strong As(V) removal

capacities and facile solid/liquid separation because of their

unique core–shell structures and heterogeneous surface proper-

ties, as well as their magnetic properties.

In this paper, to make an adsorbent with a high As(V) removal

capacity and that is easily recoverable in water treatment, we

develop a facile one-step template-free method to synthesize

Fe2O3 chestnut-like amorphous core/g-phase shell hierarchical

nanoarchitectures (CAHNs) by designing a rational chemical

reaction system. A possible growth mechanism for the formation

of Fe2O3 CAHNs is proposed according to the experimental

results. The as-obtained Fe2O3 CAHNs not only show a high

specific surface area and abundant spaces between the radially-

grown nanorods, but they also exhibit an obvious ferromagnetic

property. When they are used as adsorbents to remove arsenic

ions from polluted water, they show remarkable advantages of

a strong As(V) removal capacity and convenient magnetic sepa-

ration. This work provides an effective strategy to design and

fabricate high performance magnetic adsorbents, and the as-

obtained Fe2O3 CAHNs are expected to have a remarkable

potential application in the removal of toxic ions from polluted

water.

2. Experimental

2.1 Preparation of Fe2O3 CAHNs

In a typical synthesis, into a solution containing 80 mL N,N-

dimethylformamide (DMF) and 0.20 mmol SnCl2$2H2O in

a flask at room temperature, 0.5 mL Fe(CO)5 was added drop-

wise while the solution was vigorously stirred with a magnetic

bar. After 30 min of vigorous magnetic stirring, the solution

color changed from light yellow to dark red, indicating the

occurrence of some non-stoichiometric reactions of Fe(CO)5with oxygen in the air or the DMF.35 The as-obtained solution

was then transferred into a 100 mL Teflon-lined stainless steel

autoclave. The autoclave was then sealed and maintained at

200 �C for 8 h. After being cooled to room temperature, the

resulting brown precipitates were isolated by magnetic separa-

tion, repeatedly washed with absolute ethanol several times, and

then dried in a vacuum at 40 �C for 8 h to obtain the as-

synthesized product of Fe2O3 CAHNs. Please take care when

carrying out the above experiment due to the toxicity of Fe(CO)5and SnCl2$2H2O .

To understand the formation mechanism of the Fe2O3

CAHNs, different contrasting experiments were carried out by

This journal is ª The Royal Society of Chemistry 2011

adjusting the reaction conditions, such as the reaction time, the

atmosphere and using different sorts of additives, including

SnCl2$2H2O and hydrochloric acid etc. The comparison exper-

iment that determined the influence of O2 on the structure of the

final products was conducted in the M. Braun Labmaster-130

(Germany) inert gas system.

2.2 Characterization

The phase purity of the products was examined by X-ray

diffraction (XRD) using a Rigaku D/Max-RB diffractometer at

a voltage of 40 kV and a current of 200 mAwith Cu-Ka radiation

(l ¼ 1.5406 �A), with a scanning rate of 4� min�1 in the 2q range

15–80�. A micro-Raman study was performed on the Renishaw

inVia (Britain) laser confocal Raman microscope at room

temperature under an excitation wavelength of 514.5 nm with an

Ar+ laser. The laser power was limited to 0.5 mW to avoid

a possible phase transition during the laser irradiation. The

X-ray photoelectron spectroscopy (XPS) analysis was performed

using a VG Multilab 2000 (USA) system. Scanning Electron

Microscopy (SEM) images and energy dispersive X-ray (EDX)

analysis were obtained using a Hatachi S-4800 (Japan) field-

emission scanning electron microscope. Transmission Electron

Microscopy (TEM) images were captured on a JEM-2100F

instrument at 200.0 kV, and the selected area electron diffraction

(SAED) images were obtained with a camera length of 60 cm.

Magnetic measurements for the samples in powder form were

carried out using a model 4HF vibrating sample magnetometer

(VSM, ADE Co. Ltd, USA) with a maximum magnetic field of

10 kOe. The specific surface area was determined by a multipoint

Brunauer–Emmett–Teller (BET) method using the adsorption

data in the relative pressure (P/P0) range 0.05–0.25. A desorption

isotherm was used to determine the pore size distribution using

the Barrett–Joyner–Halenda method. The nitrogen adsorption

volume at a relative pressure (P/P0) of 0.970 was used to deter-

mine the pore volume and porosity.

2.3 As(V) removal experiments

For the adsorption kinetics of As(V), a solution with As(V)

concentration of 6.96 mg L�1 was firstly prepared using Na3A-

sO4$12H2O as a source of As(V) and the pH was adjusted to 4

with HCl or NaOH. Then, 0.02 g of the adsorbent sample was

added to 50 mL of the above solution with stirring. The adsor-

bents were separated from the solution by magnetic separation

after a specified time, and Inductively Coupled Plasma-Atomic

Emission Spectroscopy (ICPAES, Perkin–Elmer Optima

4300DV) was used to measure the arsenic concentration in the

remaining solutions. To obtain the adsorption isotherm, 0.01 g of

Fe2O3 CAHNs was added to 25 ml of the As(V) aqueous solution

with C0 values of 6.96, 12, 25, 50, 100, 200 and 400 mg L�1 and

this was stirred for 3 h at room temperature (25.0 � 1.0 �C). Thesolid and liquid phases were then magnetically separated and the

arsenic concentration in the remaining solutions was measured

by the ICPAES. The amount of As(V) adsorbed at equilibrium

(qe, mg g�1) was calculated from eqn (1):

qe ¼ ðC0 � CeÞVm

(1)

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Where C0 and Ce represent the concentration of As(V) before and

after removal process, respectively; V is the solution volume and

m the weight of the adsorbent.32

3. Results and discussion

Figs 1A and B show the typical SEM images of the as-obtained

samples. They are obviously composed of chestnut-like nano-

structures with an average diameter of approximately 1.5 mm.

The results of the EDX analysis (the inset of Fig. 1A) confirm

that they mainly consist of iron and oxygen, which agrees with

the chemical composition of Fe2O3, along with a small amount of

carbon impurity, which possibly comes from the disproportion-

ation reaction of CO decomposed mainly by Fe(CO)5. No Sn is

detected by the EDX analysis in the as-prepared samples, sug-

gesting that the by-products containing tin species are removed

by washing and magnetic separation. In order to observe the

internal structure, an ultra-thin section of the as-obtained sample

was fabricated. The TEM image (Fig. 1C) shows that the hier-

archical nanostructure, the ultra-thin section of which forms

a slight crack due to the shear strength generated during the

cutting process, contains a spherical core of about 1 mm in

diameter and nanorods of about 300 nm in length grown radially

from the surface of the core. The high magnification TEM image

Fig. 1 (A, B) The SEM images of the Fe2O3 CAHNs prepared by the

solvothermal method at 200 �C for 8 h, inset of A: the EDX analysis; (C)

the TEM image of an ultra-thin section of the chestnut-like nano-

structure; (D) the magnified TEM image of an ultra-thin section of the

core of the chestnut-like nanostructure, inset: the corresponding SAED

image; (E) the magnified TEM image of g-Fe2O3 nanorods on the surface

of the chestnut-like nanostructure, inset: an HRTEM image taken from

a g-Fe2O3 nanorod.

5416 | J. Mater. Chem., 2011, 21, 5414–5421

(Fig. 1D) reveals that the spherical core consists of inter-

connected nanoparticles and is highly porous. The appearance of

diffuse rings in the SAED pattern (the inset of Fig. 1D) indicates

the amorphous nature of the spherical core. The magnified TEM

image of the nanorods is shown in Fig. 1E, which indicates that

the diameter of the nanorod is about 20 nm. The inset of Fig. 1E

is a high-resolution TEM (HRTEM) image of a nanorod. It

indicates that the nanorod is single crystalline, and the lattice

spacing is measured to be 0.252 nm, which could be assigned to

the (311) plane of cubic g-Fe2O3.

Fig. 2 shows the XRD pattern, Raman spectrum and XPS of

Fe 2p of the as-prepared Fe2O3 CAHNs. It can be seen from

Fig. 2A that the as-obtained sample exhibits broad and weak

XRD peaks, which can be ascribed to its unique amorphous-

core/crystalline-shell structure. Nevertheless, these XRD peaks

match well with the standard XRD patterns of maghemite

(JCPSD Card NO. 19-0629). In Fig. 2B, the Raman peaks at 362

and 701 cm�1 are observed, which are consistent with the Eg and

A1g modes of g-Fe2O3 with an inverse spinel structure.36–38

Figs 2A and B clearly identify the presence of the g-Fe2O3 in our

sample. In order to obtain the oxidation nature of the as-

obtained Fe2O3 CAHNs, we used the heavily ground sample to

measure the XPS spectrum so that the internal amorphous core

Fig. 2 The (A) XRD pattern, (B) Raman spectrum and (C) XPS of Fe

2p of the as-prepared Fe2O3 CAHNs.

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Fig. 4 SEM images of the products obtained (A) under an inert N2

atmosphere instead of air, (B) without SnCl2$2H2O and (C, D) with 0.1

mL 38% HCl instead of 0.20 mmol SnCl2$2H2O in the solution. The

other conditions are the same as those in the typical synthesis.

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was fully exposed to the X-ray and the precise signals of its

oxidation status were available. Fig. 2C indicates that the XPS

main peaks of Fe 2p1/2 and Fe 2p3/2 are found at 724.7 and 711.2

eV, respectively, and a satellite of the Fe 2p3/2 peak can also be

observed at around 719.1 eV. This is consistent with the reported

values observed for Fe3+ compounds.39,40 According to the above

analyses, we can reasonably conclude that the as-prepared

sample is made up of Fe2O3 chestnut-like nanostructures, which

are composed of inner cores formed by aggregated amorphous

iron oxide nanoparticles and outer shells based on maghemite

single crystalline nanorods grown radially from the inner cores.

In order to understand the formation mechanism of the Fe2O3

CAHNs, time-dependent experiments were carried out. As

shown in Fig. 3A, the sample obtained at 200 �C with a reaction

time (t) of 0.5 h is made up of particles with a rough surface

about 350 nm in diameter. The size of the particles obtained at

t¼ 1 h increases to 600 nm, and nanorods begin to appear on the

surface of the particles (Fig. 3B). When t is increased to 2 h

(Fig. 3C), the particles evolve into chestnut-like microspheres

with a diameter of 800 nm and with nanorods grown radially

from the center. The diameter of the nanorods on their surface is

about 12 nm, and the length reaches several hundred nano-

metres. As t increases, the size of the chestnut-like nanostructure

grows gradually and reaches about 1.5 mm at t ¼ 8 h (Fig. 3D).

Meanwhile, the g-Fe2O3 nanorods on the surface become more

compact with increasing t, as shown in the insets of Figs 3C and

D. The morphological evolution with t clearly illustrates that the

sample forms according to a stepwise growth mechanism.

In our protocol, O2 and SnCl2$2H2O in the reaction system

played two key roles in the formation of the Fe2O3 CAHNs.

Fig. 4A shows that the products obtained under a N2 atmosphere

instead of air are mainly microsheets and particles, while Fig. 4B

shows that the products obtained in the absence of SnCl2$2H2O

are microparticles or nanoparticles, together with a few bundles

of nanorods. No chestnut-like hierarchical nanostructure is

found in these two comparative experiments. However, chestnut-

like nanoarchitectures with a widely distributed size can be

formed if SnCl2$2H2O is replaced with a small amount of HCl

solution (Figs 4C and D). In our typical synthesis of Fe2O3

CAHNs, before the system is sealed the precursor Fe(CO)5 has

Fig. 3 SEM images showing the morphological evolution of the sample

with increasing reaction time from (A) 0.5, (B) 1, (C) 2 to (D) 8 h, insets:

the magnified observation of g-Fe2O3 nanorods on the surface.

This journal is ª The Royal Society of Chemistry 2011

been partially transformed into its soluble derivatives containing

Fe(II) via its non-stoichiometric reactions with oxygen in air or

DMF,35 as indicated by the obvious change of the solution color

from light yellow to dark red during the vigorous magnetic

stirring process. The reasonably high yield of the product

suggests that after the solvothermal process the reactant Fe(CO)5is almost decomposed and the oxidants sealed in the autoclave

are enough, although it is too complicated to make the quanti-

tative analysis of the related chemical reaction. However, it is

possible that during the solvothermal reaction there are some

oxidants other than residual air in the autoclave, such as the

dissolved oxygen in the DMF solution, and CO2 produced by the

disproportionation reaction of CO.41–43

Based on the experimental results, we propose that the Fe2O3

CAHNs are formed according to a ‘‘thermal decomposition–

oxidation nucleation–aggregation tip-growth’’ process, which is

schematically shown in Scheme 1. At the beginning of the reac-

tion, Fe(CO)5 in the solution is thermally decomposed into the

metal iron and CO. The iron is immediately oxidized into iron

Scheme 1 A schematic illustration of the formation process of the Fe2O3

CAHNs: (1) the formation of iron by decomposing Fe(CO)5; (2) oxida-

tion of iron into amorphous iron oxide nanoparticles; (3) aggregation of

iron oxide nanoparticles into a porous sphere; (4) tip-growth of maghe-

mite nanorods on the surface of the amorphous iron oxide sphere and

finally (5) the growth into Fe2O3 CAHNs.

J. Mater. Chem., 2011, 21, 5414–5421 | 5417

Fig. 5 The nitrogen adsorption–desorption isotherm for the Fe2O3

CAHNs obtained at 8 h.

Fig. 6 The magnetic hysteresis loop of the Fe2O3 CAHNs obtained

at 8 h.

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oxide by O2 in the reactor, resulting in the formation of iron

oxide nanoparticles, which are amorphous because of the high

reaction rate at this stage. As the amorphous iron oxide nano-

particles have a high surface energy, they easily agglomerate to

form small porous spheres with rough surfaces. Meanwhile, the

DMF gradually degrades into dimethyl ammonia along with CO

at a high reaction temperature (200 �C) in the presence of

SnCl2$2H2O, which generates HCl via a hydrolysis reaction, as

described in the following equations:44–46

SnCl2$2H2O / SnO + 2HCl + H2O

ðCH3Þ2NCOH �!HCl

200 �CðCH3Þ2NHþ CO

It is possible that the amount of CO generated via the

decomposition of DMF and Fe(CO)5 becomes more and more

while the oxygen concentration continuously decreases since the

reaction system is hermetic. As a result, the decomposition rate

of Fe(CO)5 or its derivatives and the formation rate of iron oxide

significantly decreases, leading to the occurrence of a quasi-

equilibrium oxidation reaction of iron. Under this quasi-equi-

librium state, small cubic g-Fe2O3 dendrites grow from the Fe2O3

amorphous spheres and gradually evolve into single crystalline

nanorods following a tip-growth mechanism, which was firstly

proposed to interpret the growth of a-Fe2O3 nanowires.47 In our

protocol, there are many protuberances on the rough surface of

the preformed Fe2O3 amorphous sphere. The heterogeneous

nucleation and anisotropic growth of the g-Fe2O3 nanorods with

(311) facets parallel to its axis direction will selectively occur at

these high-energy protuberances. This tip-growth mechanism

can also be confirmed to some extent by the appearance of

somewhat sharp tips of g-Fe2O3 nanorods on the surface

(Fig. 1D). The growth of g-Fe2O3 nanorods will last ca. 8 h,

finally resulting in the obtained Fe2O3 CAHNs. The crystalline

structure of the nanorods on the surface is of g-Fe2O3 instead of

a-Fe2O3, which is determined by the reductive effect of the

organic solvent (DMF), the released CO and the relatively low

oxidation temperature (200 �C).48,49 The result agrees with the

reported stable phase of g-Fe2O3 in the temperature range 200–

400 �C in air.23,50

As expected from the porous core and hierarchical structure,

the as-obtained Fe2O3 CAHNs manifest a high BET surface area

(S) of 143.12 m2 g�1, which is higher than that of the hierarchical

a-Fe2O3 hollow spheres (12.2 m2 g�1),51 and flower-like a-Fe2O3,

g-Fe2O3 and Fe3O4 nanostructures (34–56 m2 g�1).31 Fig. 5 shows

the nitrogen adsorption and desorption isotherms. Obviously, it

is consistent with the characteristic of a type IV isotherm with

a type H3 hysteresis loop associated with slit-like pores, indi-

cating the presence of mesopores in the size range 2–50 nm.52 In

addition, the observed hysteresis loop shifts to a higher relative

pressure on approaching P/P0 ¼ 1, suggesting the presence of

macropores (>50 nm in size).53 This is also confirmed by the

corresponding pore size distribution, which shows that the Fe2O3

CAHNs possess a bimodal (small and large) pore distribution, as

shown in the inset of Fig. 5. The broad peak ranging from

a mesopore size of about 6 nm up to macropore diameters of

100 nm in size is ascribed to the interspaces between the g-Fe2O3

5418 | J. Mater. Chem., 2011, 21, 5414–5421

nanorods. Another sharp peak in the range 2–6 nm is attributed

to the small pores that exist in the amorphous core. This result is

in good agreement with the TEM observation (Figs 1C and D).

It is favorable for adsorbents/catalysts in water treatment to

possess magnetic properties because they can be conveniently

separated and recovered by magnetic separation technology.

Fig. 6 is the hysteresis loop of the as-obtained Fe2O3 CAHNs.

The non-linear hysteresis loop with a non-zero remnant

magnetization (Mr) and coercivity (Hc) shows the well-

pronounced ferromagnetic property of the Fe2O3 CAHNs. The

saturation magnetization (Ms) is 2.1 emu g�1, which is over two

times higher than that of the amorphous Fe2O3 nanoparticles of

about 5 nm (0.9 emu g�1).54 This may be ascribed to the g-Fe2O3

phase shell of the Fe2O3 CAHNs. The inset of Fig. 6 displays the

magnified region between �200 and 200 Oe. It clearly indicates

the Hc and Mr values are 67.9 Oe and 0.08 emu g�1, respectively.

Because of their high BET surface area and good magnetic

property, we expect that the as-obtained Fe2O3 CAHNs would

have a promising performance in water treatment. Fig. 7A shows

the kinetics of As(V) adsorption onto the as-obtained Fe2O3

CAHNs. It can be seen that 97.4% of the As(V) in the aqueous

solution is removed in 5 min, and all of the As(V) ions in the

solution are almost completely removed in 30 min. After the As

(V) adsorption is over, the as-prepared Fe2O3 adsorbent can be

conveniently separated from the solution in 60 s by a magnet, as

This journal is ª The Royal Society of Chemistry 2011

Fig. 7 (A) The kinetics of As(V) adsorption onto Fe2O3 CAHNs for an

aqueous solution with an initial concentration of 6.96 mg L�1 As(V) and

a pH value of 4. The insets show the removal and recovery of the

dispersed Fe2O3 CAHNs from the aqueous solution at 0, 10, 30 and 60 s

after the application of a 3350 G permanent magnet; (B) The equilibrium

isotherm data of As(V) adsorption at pH ¼ 4 and room temperature, as

well as the non-linear isotherm analysis using both the Langmuir and

Freundlich adsorption equations.

Table 1 The BET surface areas and the maximal As(V) adsorptioncapacities (Qm) of the as-obtained Fe2O3 CAHNs and other magneticadsorbents

Adsorbents S (m2 g�1) Qm (mg g�1) pH Reference

Fe2O3 CAHNs 143 137.5 4 This studyg-Fe2O3 nanoparticles 203 50 3 19g-Fe2O3 flowers 56 4.75 4 31Fe3O4 flowers 34 4.65 4 31Fe3O4 nanoparticles (12 nm) 98.8 46.7 8 21MnFe2O4 nanoparticles 138 90.4 3 55CoFe2O4 nanoparticles 101 73.8 3 55Fe3O4 nanoparticles 102 44.1 3 55

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shown in the insets of Fig. 7A. This suggests that the as-obtained

Fe2O3 CAHNs can quickly and effectively adsorb As(V) ions and

be easily recovered from polluted water.

To evaluate the As(V) adsorption capacity of the as-obtained

Fe2O3 CAHNs, the adsorption isotherm was conducted, as

shown by the dots in Fig. 7B. It can be seen that the amount of As

(V) absorbed on the as-obtained Fe2O3 CAHNs at equilibrium

(qe) increases with increasing Ce, and is not saturated even when

Ce ¼ 345 mg L�1. The maximal As(V) removal capacity (Qm) is as

high as 137.5 mg g�1 within our experimental range (Fig. 7B). A

comparison with Table 1 indicates that this value is much higher

than that of the other magnetic adsorbents for removing As(V)

from polluted water. Furthermore, compared to the conven-

tional nonmagnetic adsorbents, the as-prepared Fe2O3 CAHNs

also show a superior As(V) adsorption capacity. For instance, the

Qm of the activated alumina is 9.20 at pH ¼ 7 while that of the

activated carbon with a different carbon type and ash content

was 2.4–4.9 mg g�1 at pH ¼ 5.56,57 For TiO2 nanoparticles, Qm ¼37.5 mg g�1 at pH ¼ 7.58 It is generally believed that the large

specific surface area is mainly responsible for the strong

adsorption behavior of metal oxides. Table 1 shows that

MnFe2O4 nanoparticles and maghemite nanoparticles exhibit

a much lower Qm than the as-prepared Fe2O3 CAHNs even

though they both have a similar or higher specific surface area

This journal is ª The Royal Society of Chemistry 2011

than the latter.19,55 This reasonably suggests that the high specific

surface area is not the only criterion for the strong As(V)

adsorption capacities, which are sometimes significantly influ-

enced by the surface quality or surface property.55 In our

protocol, the as-obtained Fe2O3 CAHNs obviously have

a heterogeneous surface, formed by the Fe2O3 hierarchical

nanostructure with both an amorphous core and a g-phase shell,

which possibly results in a multilayer As(V) adsorption behavior

and consequently a superior adsorption capacity. But the

nanoparticles of MnFe2O4 and maghemite only have a homoge-

neous surface, which usually shows the monolayer adsorption

behavior of the Langmuir isotherm model.19,55 Thus they show

a weaker adsorption capability for As(V) than the as-obtained

Fe2O3 CAHNs, even though they have a similar or higher surface

area compared to the as-prepared Fe2O3 CAHNs.59

To further understand the adsorption mechanism of the as-

prepared Fe2O3 CAHNs as an adsorbent, both the Langmuir

adsorption model and the Freundlich adsorption model60 were

employed to fit the experimental data, as shown in Fig. 7B. The

detailed Langmuir and Freundlich isotherm parameters are

summarized in Table 2. It is noted that the experimental data

fitted well to the Freundlich adsorption model and not to the

Langmuir adsorption model. The regression coefficient (R2) for

the Freundlich adsorption model reaches 0.993 while for the

Langmuir adsorption model this is as low as 0.847. This suggests

that the As(V) adsorption behavior of the Fe2O3 CAHNs can be

regarded as a multilayer adsorption process, and so the Lang-

muir adsorption model is not suitable for describing it. This is

because the Langmuir isotherm model assumes that homoge-

neous surfaces, in which all sites are energetically equivalent,

adsorb adsorbates only as a monolayer and that there is no

interaction between the adsorbed molecules. In contrast, the

Freundlich isotherm model is an empirical equation based on

multilayer adsorption on heterogeneous surfaces. It assumes that

the stronger binding sites are occupied first by the adsorbates.

The binding strength gradually decreases with an increasing

number of occupied sites.60 Since the as-obtained Fe2O3 CAHNs

consist of porous cores of amorphous Fe2O3 as well as shells of

crystalline g-Fe2O3 nanorods grown radially from the cores, it is

possible that they have heterogeneous surfaces. Specifically, the

surface properties differ in the various sites of the amorphous

core because of the irregular arrangement of atoms, as well as

between the amorphous Fe2O3 core and the crystalline g-Fe2O3

nanorods. Thus, the surface heterogeneities of the Fe2O3

CAHNs lead to different affinities to As(V) at different sites.

J. Mater. Chem., 2011, 21, 5414–5421 | 5419

Table 2 The related parameters of both the Langmuir and Freundlichisotherm models for As(V) equilibrium adsorption on the Fe2O3 CAHNsat pH ¼ 4 and room temperature

Isotherm equationDifferentparameters

Estimatedparameters

Freundlich model qe ¼ KFCe R2 0.993KF 46.86n 5.47

Langmuir model

qe ¼ qmbCe

1þ bCe

R2 0.847qm (mg g�1) 121.7b (mg L�1) 0.172

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Consequently, the As(V) adsorption behavior of the Fe2O3

CAHNs obeys the Freundlich adsorption model well. This

further confirms the assumption that multilayered adsorption

occurs in the Fe2O3 CAHNs.

The Freundlich sorption coefficient (KF) is 46.86 (mg g�1)

(L mg�1)1/n, which is much higher than that of granular ferric

hydroxide (4.45),61 nanocrystalline TiO2 (0.5–0.75),62 iron coated

pottery granules (3.6),63 and tea fungal biomass (10.26).64 This

indicates that the as-obtained Fe2O3 CAHNs has a larger overall

capacity.65,66 It is generally accepted that a value of 0.1 < 1/n #

0.5 represents easy adsorption, a value of 0.5 < 1/n# 1 represents

somewhat difficult adsorption and a value of 1/n > 1 represents

quite difficult adsorption.67 For the removal of As(V) from

polluted water by the Fe2O3 CAHNs, 1/n ¼ 0.183. This indicates

that As(V) ions were easily adsorbed on the as-obtained Fe2O3

CAHNs. Taking into account the strong adsorption capacity,

fast adsorption rate and quick magnetic separation from treated

water, we expect that the Fe2O3 CAHNs developed in the present

study are an efficient magnetic adsorbent for As(V) removal from

aqueous solutions.

4. Conclusions

We developed a one-step template-free method to synthesize

a magnetic adsorbent of Fe2O3 chestnut-like amorphous-core/g-

phase-shell hierarchical nanostructures (CAHNs) in a rationally

designed chemical reaction system. The unique hydrolysis–

decomposition mediated synthesis approach developed here can

kinetically modify the nucleation stage and the subsequent

growth stage of Fe2O3 by utilizing the current chemical equilib-

rium dynamics and will be a valuable addition to the realm of

synthetic strategy regarding complex nanoarchitectures. The as-

obtained Fe2O3 CAHNs show an excellent adsorption capability

for As(V) with a maximum adsorption capacity of 137.5 mg g�1

due to both the specific surface area being as large as 143.12 m2

g�1 and the existence of heterogeneous surfaces. The adsorption

process fits the Freundlich isotherm model well, indicating the

occurrence of multilayered adsorption on the surface of the

Fe2O3 CAHNs. Furthermore, the as-prepared Fe2O3 CAHNs

can be conveniently separated and recovered by magnets because

of their significant ferromagnetism. Thus the as-obtained Fe2O3

CAHNs have a promising application in removing heavy

metallic ions such as As(V) from polluted water. Our work may

shed light on the design and preparation of high performance

5420 | J. Mater. Chem., 2011, 21, 5414–5421

adsorbents of 3D hierarchical nanostructures for the removal of

toxic ions from polluted water.

Acknowledgements

This work was supported by National High-Technology

Research and Development Program of China (No.

2006AA03A209), the Fundamental Research Funds for the

Central Universities (2010-IV-006), the Ministry of Education

(No. NCET-05-0660 and PCSIRT0644), and the China Post-

doctoral Science Foundation (No. 20070420169).

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J. Mater. Chem., 2011, 21, 5414–5421 | 5421