controlled synthesis of α fe2o3 nanorods and its size dependent optical

Controlled synthesis of α-Fe 2 O 3 nanorods and its size-dependent opticalabsorption, electrochemical, and magnetic properties Suyuan Zenga,b, Kaibin Tanga,b,∗, Tanwei Lia aNanomaterial

Controlled synthesis of α-Fe 2 O 3 nanorods and its size-dependent optical

absorption, electrochemical, and magnetic properties

Suyuan Zenga,b, Kaibin Tanga,b,∗, Tanwei Lia

aNanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China,

Hefei, Anhui 230026, People’s Republic of China

bDepartment of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

Received 19 December 2006; accepted 21 March 2007

Available online 10 May 2007

Abstract

Uniform α-Fe2O3nanorods with diameter of about 30 nm and length up to 500 nm were synthesized by a template-free hydrothermal method and a following calcination of the intermediate product in the air at 500◦C for 2 h By carefully tuning the concentration of the reactants, a series of

α-Fe2O3nanorods with gradient in aspect ratios can be obtained The effect of the solvent was also evaluated Based on the experimental facts, the

formation mechanism of this one-dimensional structure was proposed The size-dependent properties of the as-obtained α-Fe2O3nanorods were investigated The optical absorption properties of the samples showed that the band gaps of the samples decreased in the sequence in which the size increased The electrochemical performance of the samples showed that the discharge capacity decreased as the size of the sample increased, which may result from the high surface area and small size The magnetic hysteresis measurements taken at 5 K showed that the coercivities of the samples were related to the aspect ratios of the samples, which may result from the larger shape anisotropy However, the temperature-dependent field cooling magnetization showed that there was no Morin transition in the as-prepared samples, which may result from the surface effect

©2007 Elsevier Inc All rights reserved

Keywords: Hematite; Hydrothermal; Size-dependent; Optical absorption; Lithium ion battery; Magnetism

1 Introduction

One-dimensional (1D) nanostructures, such as nanowires

(NWs), nanorods, and nanotubes (NTs), have attracted

exten-sive attention due to their unique physical and chemical

prop-erties [1–3] These systems, with two restricted dimensions,

not only offer opportunities for investigating the dependence

of electronic transport as well as optical and mechanical

prop-erties on size confinement and dimensionality [4,5], but also

play a crucial role in fields such as data storage [6] and

ad-vanced catalytic and optoelectronic devices[2,7] Developing

new methods for the preparation of nanomaterials as well as the

modification of their size, morphology, and porosity has been

intensively pursued not only for their fundamental scientific

in-terest but also for many technological applications

* Corresponding author Fax: +86 551 360 1791.

E-mail address:kbtang@ustc.edu.cn (K Tang).

Iron oxyhydroxides and iron oxides have been extensively used in the production of pigments, catalysts, gas sensors, netic recording media, and raw materials for hard and soft mag-nets [8–14] Hematite (α-Fe2O3), based on hexagonal close packing of oxygen with iron in 2/3 of the octahedral vacan-cies, is traditionally used as catalyst, pigment, gas sensor, and electrode material[15–18]due to its low cost, high resistance

to corrosion, and environmentally friendly properties Most of these functions depend strongly on the composition and struc-ture of materials In recent years, the synthesis and properties

of the one-dimensional α-Fe2O3nanostructures have attracted

much interest; many one-dimensional α-Fe2O3 nanostructure such as nanorods[19–21], nanowires[22–24], nanobelts[25], and nanotubes [26,27] have been synthesized and used for the investigation of their properties For example, by

oxidiz-ing the surface of the iron substrate, α-Fe2O3nanowires were obtained[22] α-Fe2O3nanowires were also prepared by an an-odic aluminum oxide (AAO) template method[28] Recently,

α-Fe2O3nanotubes and nanorods were selectively synthesized 0021-9797/$ – see front matter © 2007 Elsevier Inc All rights reserved.

doi:10.1016/j.jcis.2007.03.046

In this paper, we propose an easy route for fabricating

α-FeOOH nanorods via a low-temperature hydrothermal method

The α-FeOOH nanorods could be obtained with high yield

(>90%) and good reproducibility By changing the amount of

the reactants, a series of α-FeOOH nanorods with gradients in

aspect ratio can be obtained α-Fe2O3nanorods can be obtained

by calcing the as-obtained α-FeOOH at 500◦C for 2 h at a

heat-ing rate of 1◦C/min, preserving the same rodlike morphology.

The shape-dependent optical absorption, electrochemical, and

magnetic properties are investigated

2 Experimental

2.1 Preparation of α-FeOOH and α-Fe 2 O 3 nanorods

All reagents were analytically pure and used without

fur-ther purification In a typical experiment, 2 mmol FeSO4·7H2O

was added into 40 ml distilled water to form a homogeneous

solution Then 2 mmol anhydrous Na2SO3 was added to the

solution under vigorous magnetic stirring A yellowish

suspen-sion appeared in the solution after several seconds, and the

amount of suspension increased with continuous stirring After

being stirred for 20 min, the slurry was transferred into a 50-ml

Teflon-lined autoclave and maintained at 140◦C for 12 h The

autoclave was then cooled to room temperature naturally The

final yellow solid products were centrifuged and washed with

distilled water and absolute ethanol several times to ensure

to-tal removal of the inorganic ions and then dried at 60◦C under

vacuum for 4 h The α-Fe2O3nanorods were obtained by

heat-ing the as-obtained α-FeOOH nanorods in air at 500◦C for 2 h

at a heating rate of 1◦C/min, preserving the same rodlike

mor-phology

2.2 Sample characterizations

The samples of as-prepared α-FeOOH and α-Fe2O3

nano-structures were characterized by X-ray powder diffraction

(XRD) with a Philips X’Pert Pro Super diffractometer with

CuKα radiation (λ = 1.54178 Å) The transmission electron

microscopy (TEM) images and the selected area diffraction

(SAED) patterns for both α-FeOOH and α-Fe2O3 were

ob-tained on a Hitachi Model H-800 instrument with a tungsten

filament at an accelerating voltage of 200 kV The magnetic

properties of α-Fe2O3were measured using a vibrating

sam-ple magnetometer and a superconducting quantum interference

device The BET tests were determined via a Micromeritics

UV–visible spectrophotometer The magnetic measurements were recorded on a SQUID magnetometer, Quantum Design MPMS

3 Results and discussion

Fig 1a is the XRD pattern of the as-obtained FeOOH nanorods, where all the diffraction peaks can be indexed as

or-thorhombic α-FeOOH with cell constants of a = 0.4592 nm,

b = 0.998 nm, and c = 0.3015 nm, which is consistent with

the reported data (JCPDS Card 81-0464).Fig 1b is the XRD pattern of the product obtained by calcining the as-prepared

α-FeOOH at 500◦C for 2 h, where all the diffraction peaks can be indexed as a hexagonal phase with lattice constants of

a = 0.5013 nm and c = 1.3751 nm, which agrees well with the

literature (JCPDS Card 33-0664)

Fig 2a is the field emission electron microscopy (FESEM)

image of the as-obtained α-FeOOH nanorods, which clearly

demonstrates that the products are composed of large amount

of nanorods These rods, about 30 nm in diameter and length

up to 400 nm, have smooth surfaces along their entire length Fig 2b is the transmission electron microscopy (TEM) image

of a single α-FeOOH nanorod The selected area electron

dif-fraction (SAED) pattern of a single nanorod (inset ofFig 2b) demonstrates the single-crystal nature of the nanorod grown along the[0 0 1] direction.Fig 2c is the FESEM image of the

α-Fe2O3obtained by calcining the α-FeOOH at 500◦C for 2 h,

Fig 1 XRD patterns of (a) as-prepared α-FeOOH nanorods; (b) α-Fe2O3

nanorods obtained by calcing the α-FeOOH nanorods at 500◦C for 2 h.

Fig 2 (a) FESEM image of the as-obtained α-FeOOH nanorods and (b) TEM image of a single α-FeOOH nanorod (inset: SAED pattern of a single α-FeOOH nanorod); (c) FESEM image of the as-obtained α-Fe2O3 nanorods; (d) TEM image of a single α-Fe2O3 nanorod (inset: SAED pattern of a single α-Fe2O3nanorod).

from which we can see that the rodlike morphology perfectly

remained after calcination The SAED pattern of a single

α-Fe2O3nanorod is also taken to verify the growth direction of

the α-Fe2O3(inset ofFig 2d); and the result shows that the

as-obtained α-Fe2O3nanorod is a single crystal grown along the

[0 1 −1 0] direction

The formation of the α-FeOOH nanorods in the solution can

be expressed as follows:

SO2−

4Fe2++ 8OH−+ O2→ FeOOH + 2H2O (2)

As shown above, Fe2+ reacted with the OH− produced by

the hydrolysis of SO2−

3 and O2in the atmosphere, producing the

yellow α-FeOOH suspension When the SO2−

3 hydrolyzed in the water, the pH value of the solution rose uniformly, and this

prevented the occurrence of local supersaturation and

mean-while favored for homogeneous nucleation However, as the

reaction went on, the pH value of the system decreased And

ac-cording to the acid–base surface properties of the metal oxide,

decreasing the pH of the precipitation from the point of zero

charge (PZC) increases the surface charge density by

adsorp-tion of protons and consequently reduces the interfacial tension

of the system[31], which is very important for the formation of

such unique nanostructures

To further understand the role that SO2−

3 played in the synthesis, several experiments involved different amount of

Na2SO3 and other kind of inorganic ions were performed Keeping the amounts of FeSO4 and water constant, the mo-lar ratio between FeSO4 and Na2SO3 varied Fig 3a is the TEM image of the product obtained when the concentration of

SO2−

3 is 0.025 mol L−1, which shows that nanorods with diam-eter about 30 nm and length about 40 nm were obtained As the concentration of the SO2−

3 increases, e.g., 0.075 mol L−1, the product is mainly composed of nanorods with diameter of about 30 nm and length up to 800 nm (Fig 3b), showing that the aspect ratio of the nanorods was tunable However, as the con-centration of SO2−

3 increases further, e.g., 0.1 mol L−1, black powders instead of the yellow product are obtained, which is confirmed to be Fe3O4by the XRD And this can be explained

by the reducing ability of the SO2−

3 Fig 3c is the TEM im-age of the as-obtained Fe3O4, from which it can be seen that the product is composed of hexagonal nanodisks with average size about 50 nm, which may provide a method for the prepa-ration of Fe3O4nanodisks To learn more about the role that

SO2−

3 played in the formation of the one-dimensional structure,

a series of comparative experiments were performed In the case where no Na2SO3 was added, urchin-like nanostructures that was composed of nanoneedles formed (shown asFig 3d) When Cl− is used instead of SO2 − in the reaction, irregular

Fig 3 TEM images of the products under different conditions: (a) prepared in the solution containing 0.05 mol L −1Fe2+and 0.025 mol L−1Na

2 SO3; (b) prepared

in the solution containing 0.05 mol L −1Fe2+and 0.075 mol L−1Na

2 SO3; (c) prepared in the solution containing 0.05 mol L −1Fe2+and 0.1 mol L−1Na

2 SO3; (d) prepared in the solution containing 0.05 mol L −1Fe2+; (e) prepared in the solution containing 0.05 mol L−1Fe2+and 0.05 mol L−1NaCl; (f) prepared in the solution containing 0.05 mol L −1Fe2+and 0.05 mol L−1Na

3 PO4. nanorods as well as nanoparticles obtained (shown asFig 3e),

whereas nanoplates obtained when PO3−

4 is used instead of

SO2−

3 (Fig 3f)

It was believed that the solution method is based on surface

chemistry through changing the interfacial tension to control

the structure and morphology of the products[32] And it has

been reported that by adjusting the interfacial tension of the

reaction system by ethanol, an α-FeOOH nanorod array can

be obtained in the solution[33] Then what the result will be

when ethanol is added into this reaction system? To answer this

question, several experiments that employed mixed solutions of

ethanol and water instead of pure water were performed.Fig 4a

is the TEM image of the product obtained when the solution

is composed of 5 ml ethanol and 35 ml H2O, from which it

can be seen that nanorods with higher aspect ratio are obtained

With a further increase of the amount of ethanol to 10 ml, an

urchin-like nanostructure that is composed of nanorods formed

(Fig 4b) When more ethanol is added, e.g., 20 and 30 ml,

ir-regular nanoparticles and nanorods are obtained (Figs 4c and

4d), which may result from the relative higher concentration of

the reactant compared with that in the water, causing the

reac-tion to be kinetically controlled

To investigate the growth mechanism of such rodlike struc-tures, several experiments that involved intercepting the inter-mediates at different hydrothermal reaction times were per-formed According to the results of these experiments, we be-lieve that the nanorods formed through a RBG (rolling-broken-growth) model, which has been reported in the synthesis of MnO23D nanostructures[34]and CdSe nanorods[35] At the initial stage, a large number of plate structures were obtained (as shown in Fig 5a) The thin flakes tended to curl under elevated temperature and pressure, as shown inFig 5b (after heating for 40 min) As the reaction went on, some thin flakes broke into small nanoneedles (Fig 5d) via a rolling-broken-growth (RGB) process And finally, small nanoneedles would grow into nanorods after heating for 12 h

4 Size-dependent properties of the products

To investigate the size-dependent properties of the α-Fe2O3 nanorods, several samples with gradient in the length have been employed They were synthesized using the method mentioned above They were labeled as S1, S2, and S3, respectively The sizes of the samples were listed inTable 1

Fig 4 TEM images of α-FeOOH obtained when the solution is composed of (a) 5 ml ethanol and 35 ml water; (b) 10 ml ethanol and 30 ml water; (c) 20 ml ethanol

and 20 ml water; (d) 30 ml ethanol and 10 ml water.

Fig 5 TEM images of the α-FeOOH obtained after hydrothermal reaction for (a) 20 min; (b) 40 min; (c) 1 h; (d) 2 h.

Table 1

Names and sizes of the samples employed in the characterization

4.1 Optical absorption properties

The optical absorption properties of samples S1, S2 and S3

were investigated at room temperature by the UV–vis spectra

(Fig 6a) The absorption peaks showed blue shift as the lengths

of the nanorods decrease α-Fe2O3is a n-type semiconductor

and its optical band gap can be obtained by the equation

(3)

(αhν) n = B(hν − Eg),

where α is the absorption coefficient, hν is the photo energy,

B is a constant relative to the material, Egis the band gap, and

nis either 1/2 for an indirect transition or 2 for a direct

transi-tion The (αhν)2∼ hν curves for samples S1, S2, and S3 are

shown inFigs 6b, 6c, and 6d, respectively The band gaps

cal-culated from Eq.(3)are 2.65, 2.60, and 2.45 eV for S1, S2, and

S3, showing an obvious blue shift as the sizes decreased Here,

compared to the reported value of bulk α-Fe2O3(2.2 eV)[36],

the optical absorption band edge of the as-obtained α-Fe2O3

exhibits blue shift with respect to that of the bulk α-Fe2O3

The blue shift could also be attributed to the size effect, which

leads to the broadening of the optical absorption edge It is well known that the semiconductor nanoparticle energy gap in-creases with decrease of the grain size, which leads to a blue shift of the optical absorption edge, and this has been observed

in many semiconductor nanoparticle systems [37–40] Based

on the above considerations, the sequence of the as-obtained

products should be S1 > S2 > S3, which agrees well with our

experimental facts

4.2 Electrochemical properties

It is reported that the lithium intercalation performance is related to the intrinsic crystal structure, where the lithium ions can intercalate into the interlayer, the tunnels, and the holes in the crystal structure [41] α-Fe2O3, based on hexagonal close packing of oxygen with iron in 2/3 of the octahedral vacancies,

is reported[30]to have holes in the first octahedral layer pro-jected along[0 0 1] and [1 0 0], which makes its use in lithium ion batteries possible Here, the electrochemical performance

of the as-prepared α-Fe2O3 samples in the cell configuration

of Li/α-Fe2O3 was evaluated Fig 7 shows the comparative

charge/discharge curves of the α-Fe2O3 samples of S1–S3 in the first cycle The cutoff voltage of samples S1–S3 is about 0.6 V, which is similar to the nanorods [30] and nanoparti-cles reported before[42] The S1 electrode exhibits the highest capacity, 1040 mA h g−1among the three samples The capac-ities of samples S2 and S3 are 1002 and 859 mA h g−1, re-spectively The first discharge capacity possesses the sequence

Fig 6 (a) UV–vis spectra of samples S1–S3; (b), (c), (d) spectrum of samples S1, S2, and S3 obtained by using the energy as abscissa.

Fig 7 First charge–discharge curves of α-Fe2O3samples (S1–S3) at a current density of 0.2 mA cm −2(S1: dashed lines; S2: dotted lines; S3: solid lines).

S1 > S2 > S3, which confirms the sequence in which the sizes

of the sample increase The discharge capacities of the

sam-ples may be related to the size effect of the α-Fe2O3nanorods

Considering the introduction of lithium ions into the holes of

the hematite surface, it is easy to find that the large surface

area is important for the improvement of lithium intercalation performance When the surface area is high, the lithium ion in-tercalation capacity and affinity will be greatly enhanced, since the diffusion lengths of the lithium ions are greatly shortened Then the one with the smallest size and with the highest surface

Fig 8 FC curves for samples (a) S1, (b) S2, and (c) S3 from 300 to 5 K; hysteresis loop for samples (d) S1, (e) S2, and (f) S3 at 5 K.

area is the one that would have the highest discharge

capac-ity Our deduction was further verified by the BET tests The

BET tests show that the surface areas of the three samples were

35.577, 32.000, and 29.303 m2/g for samples S1, S2, and S3,

respectively, which conformed to the discharge capacities of the

three samples

4.3 Magnetic properties

It is of great interest to investigate the magnetic properties of

α-Fe2O3with gradients in aspect ratios Bulk α-Fe2O3, besides

the Néel temperature (T = 960 K), has a first-order magnetic

transition at TM = 263 K, which is called the Morin

transi-tion Below TM, the antiferromagnetically (AF) ordered spins

are oriented along the c-axis, whereas above TM, spins lie AF

in the basal plane of the crystal with a ferromagnetism compo-nent A sharp decrease in magnetization should be observed at

this transition, termed the Morin transition temperature (TM) Figs 8a–8cshow the curves for the temperature dependence of field-cooling (FC) magnetizations from 5 to 300 K, under an applied field of 100 Oe The insets are the corresponding differ-ential FC curves However, the magnetic behaviors for samples S1–S3 were completely different, as shown inFigs 8a–8c: the

FC plots show constant increase and no maximum down to 5 K

possible to observe the intrinsic contribution (AF) A detailed

study is under way

To further understand the magnetic behavior of the

sam-ples, magnetic hysteresis measurements of α-Fe2O3 (samples

S1–S3) were carried out in an applied magnetic field at 5 K,

with the field sweeping from −10 to 10 kOe No saturation

of the magnetization as a function of the field is observed up

to the maximum applied magnetic field in all cases.Figs 8d,

8e, and 8fare the hysteresis loops of samples S1, S2, and S3

at 5 K The coercivity forces of samples S1, S2, and S3 are

67, 146, and 584 Oe, respectively, indicative of soft magnets

The remnant magnetizations of samples S1, S2, and S3 at 5

K are determined to be 0.00007, 0.0024, and 0.039 emu/g It

is reported that the high coercivity may be associated with the

aspect ratio of α-Fe2O3[44], because shape anisotropy would

exert a tremendous effect on the magnetic properties

Symmet-rically shaped nanoparticles, such as spheres, do not have any

net shape anisotropy However, shuttle-like nanoparticles have

shape anisotropy in addition to crystalline anisotropy, which

will increase coercivity α-Fe2O3 nanoparticles with an

aver-age diameter of 3 nm were found to show a coercive force of

50 Oe at 5 K [30] Enhanced anisotropy caused by the

one-dimensional structure induces large magnetic coercivity, where

the magnetic spins are preferentially aligned the long axis and

their reversal to the opposite direction requires higher energies

than for spheres[45] For sample S1, whose shape is very close

to that of the spherical particles, the shape anisotropy is the

low-est among all three samples As the aspect ratio increases, the

shape anisotropy increases Based on the above considerations,

we believe that the sequence can be used to explain the

phe-nomena that we observed in samples S1–S3 at 5 K and at room

temperature

5 Conclusions

An facile route for the preparation of α-Fe2O3nanorods with

a gradient in size was reported By controlling the

concentra-tion of the reactants, the size of the sample can be controlled

The nanorods, with diameters ranging from 20 to 50 nm and

lengths ranging from 50 to 800 nm, were uniform and in high

yield A possible formation mechanism was proposed for this

one-dimensional structure The size-dependent properties of the

samples were investigated The optical absorption properties of

the samples showed that the band gaps of the sample decreased

as the size increased The electrochemical performance of the

samples showed that the discharge capacity decreased as the

Financial support by the National Natural Science Founda-tion of China, the 973 Projects of China, and the Program for New Century Excellent Talents in University (NCET) is grate-fully acknowledged

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