Materials Chemistry and Physics 111 (2008) 438–443

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Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

Facile route to the synthesis of porous �-Fe2O3 nanorods

Saikat Mandal, Axel H.E. Muller ∗

Makromolekulare Chemie II and Bayreuther Zentrum fur Kolloide und Grenzflachen, Universitat Bayreuth, D-95440 Bayreuth, Germany

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Article history:Received 17 September 2007Received in revised form 15 April 2008Accepted 21 April 2008

Keywords:Iron oxideTemplate synthesisPorous materialsMagnetic materials

a b s t r a c t

The requirements of simptrolled morphology continsynthesis of porous hematnanorods with diametershydrothermal method usiafter solvent extraction aremoval of template by sothe generation of pores onFourier transform infraredmicroscopy (TEM & HRTEference device (SQUID) mnanostructure is homogenof the porous �-Fe2O3 nancyl sulphate) plays a key rnew class of inorganic scaimplications in crystal eng

1. Introduction

An important area of research in nanotechnology is the develop-ment of reliable synthesis protocols for nanostructured materialsover a range of chemical compositions, shapes and sizes. Overthe past few years, the synthesis of inorganic nanoscale materialswith special morphologies has been of great interest in mate-rial science [1,2] because the intrinsic properties of nanoscalematerials are mainly determined by their composition, structure,crystallinity, size, and morphology [3]. Compared with nondi-mensional nanoparticles, one-dimensional (1D) nanomaterials aremore interesting because of their potential high technologicalapplications for electronics, photonics, and magnetic materials [4].

In recent years, the preparation of magnetic nanomaterials isunder scrutiny for potential applications in information storage [5],color imaging [6], magnetic refrigeration [7] bioprocessing [8], gassensors [9], ferrofluids [10], and so on. In particular, hematite (�-Fe2O3), the most stable iron oxide, with n-type semiconductingproperties under ambient conditions, is of scientific and techno-logical importance because of its usage in catalysts [11], sensors

∗ Corresponding author.E-mail address: axel.mueller@uni-bayreuth.de (A.H.E. Muller).

0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.matchemphys.2008.04.043

d reliable protocols for the synthesis of anisotropic structures with con-be a major challenge in nanoscience. In this paper we describe the facile

-Fe2O3) nanorods using anionic surfactant as a rod-like template. �-FeOOH0–210 nm and lengths up to 3–5 �m were synthesized in high yield viadium dodecyl sulphate as a template. The porous �-Fe2O3 was obtainedlcining the as-obtained �-FeOOH nanorods at 500 ◦C for 6 h. Even aftert extraction and calcination the shape of the nanorods was intact exceptnanorods. The porous nanorods were analysed by X-ray diffraction (XRD),R) spectroscopy, transmission and high-resolution transmission electroncanning electron microscopy (SEM) and superconducting quantum inter-ements. SEM and TEM images showed that the morphology of hematitein the shape of rods and full of porosity and magnetization measurementss showed weak ferromagnetic behavior. The surfactant SDS (sodium dode-controlling the nucleation and growth of the nanorods and their use as a

s for the synthesis of nanomaterials are salient features of the work withring and nanocomposites design for various applications.

© 2008 Elsevier B.V. All rights reserved.

[12], and lithium-ion batteries [13]. Because of their nontoxicity,low cost, and hue, they are also widely used as polishing mate-rials and roof tiles and for colorants in the pigment and paint

industry [14]. �-Fe2O3 nanorods, nanotubes and nanowires rep-resent a class of 1D magnetic materials, in which carrier motion isrestricted in two directions so that they can exhibit unique behaviorwhich is significantly different from that of the bulk material andexpected essentially to improve photochemical, photophysical, andelectron-transport properties and make it an ideal candidate as aphotocatalyst and as a photoelectrode in solar energy conversionapplications [15]. In addition to the preparation of 1D nanoma-terials, many efforts have been developed for the fabrication ofporous nanostructures with hollow interiors, owing to their spe-cific structure, interesting properties that differ from their solidcounter parts [16–19]. The structural attribute, such as pores ofthe materials can be applied as gas and heavy metal ion adsor-bents, selective separation, support for artificial cells, light fillers,low-dielectric-constant prosthetic materials as well as inorganiccarriers for enzyme immobilization and controlled drug delivery[20–30]. Therefore, by combining the porous 1D nanostructurewith magnetic property, the magnetic porous nanorods can be anideal candidate for the multifunctional nanomaterials such as pho-tonic crystals, host materials, accustic insulation, chemical reactors,biomedical diagnosis agent and targeting drug delivery with MRI

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S. Mandal, A.H.E. Muller / Materials C

capability [31–35]. Many synthesis methods have been developedfor generating 1D �-Fe2O3 nanostructures, such as nanorods [36],nanowires [37], nanobelts [38], and nanotubes [39] using variousmethods such as vapor–solid (VS) reaction [40], vapor–liquid–solidgrowth technique [41], metalorganic chemical vapor deposition(MOCVD) technique [42], sol–gel process [43], hard porous tem-plates [44], �-irradiation method [45]. However, all these reportedmethods either produced solid nanorods, nanowires or hollow nan-otubes but without pores on the wall.

To the best of our knowledge, very few reports on the synthe-sis of porous �-Fe2O3 nanorods have been published to date [46].Owing to their specific characteristics and promising applicationsexploring proper methods for the synthesis of nanoscale porous �-Fe2O3 rods proves to be stimulating and valuable. Therefore, it isimportant to develop the methods to regulate both the pore andparticle morphology in a one-dimensional structure of these mate-rials. Surfactant-assisted methods have been widely used in thepreparation of one-dimensional structure of materials. The surfac-tant plays an important role in determining the morphology of theproducts, as surfactants have proved to be useful and versatile softtemplates that can form different conformations by self assembly

and lead to the formation of different nanostructures. The pres-ence of a rod-like micelle of the surfactant in solution promotedthe formation of one-dimensional rod-like structures.

Herein, we report a new method for the preparation of porous�-Fe2O3 nanorods using rod-like surfactant template and removingthe template by solvent extraction and calcination.

2. Experimental section

A surfactant-assisted synthesis procedure adopted to prepare iron oxidenanorods with a high aspect ratio via hydrothermal process is described in thefollowing sections. All chemicals were analytical grade, purchased from MerckChemicals and used without further purification.

A typical approach employed by us is as follows: 1.28 mmol of FeCl3 and0.04 mmol of FeCl2·4H2O were dissolved in 1.5 mL of purified, deoxygenated waterwith constant magnetic stirring for 10 min. Then this solution was added to 2.5 mL ofa 33 wt.% aqueous solution of SDS (sodium dodecyl sulphate) under N2 atmosphereand followed by vigorous stirring for 2 h. After 2 h stirring, 5 mL of 3M NaOH solu-tion was introduced into the mixed solution under N2 atmosphere with vigorousstirring for 2 h more. After adding the NaOH solution into the reaction mixture abrownish black-colored reaction mixture appears instantaneously. The next step forthe hydrothermal treatment, 2.5 mL of the brownish black-colored reaction mix-ture was transferred into a 25 mL Teflon-lined autoclave and the autoclave wassealed and heated at 120 ◦C for 24 h without shaking or stirring during the heat-

Fig. 1. (A) The XRD patterns recorded from the as-prepared sample (curve 1) and after cand sample after solvent extraction and calcination (curve 2).

try and Physics 111 (2008) 438–443 439

ing period and allowed to cool to room temperature naturally. After the reactionswere completed, the final yellow solid products were centrifuged and washed withdistilled water and absolute ethanol several times, and then dried at 40 ◦C under avacuum for 4 h. The obtained yellow solid products were collected for the follow-ing experiments and characterization. To prepare porous hematite nanostructures,the as-prepared rod-like iron oxide nanostructures were treated/stirred with acidic(hydrochloric)–ethanolic solution at temperature 65 ◦C for 24 h, followed by calci-nation at 500 ◦C with a ramping rate of 5 K min−1 and then maintained at 500 ◦C for6 h. A red-brown precipitate was collected and then washed with distilled water andabsolute ethanol for further characterization. To check the role of surfactant SDS astemplate in the growth process of the colloidal particles, we carried out the sameexperiment without surfactant as control experiment.

Powder X-ray diffraction (XRD) measurements of each sample were performedon a PANalytical X-Pert Pro MRD instrument consisting of a rotating anode genera-tor with a copper target (Cu K� radiation) operating at 45 kV and 40 mA. The XRDpatterns of the samples were recorded in the range from 2� = 20 to 70◦ with a 0.04◦

2� step size and a 100 s count time. Fourier transform infrared (FTIR) spectra ofthe as synthesized and calcined samples were recorded in the diffuse reflectancemode on a Bruker IFS 66 V in the range of 600–4000 cm−1 and at a resolutionof 4 cm−1. The as-synthesized and calcined samples were directly imaged using aLEO 1530 field emission scanning electron microscope (FE-SEM) with a resolutionof 1 nm. Samples for field emission scanning electron microscopy (FE-SEM) wereprepared by solution-casting films onto Si wafers. Samples for TEM (transmissionelectron microscopy) analysis were prepared by placing drops on carbon-coatedcopper TEM grids after dispersing the samples in 2-propanol. The films on the

TEM grids were allowed to stand for 2 min, following which the extra solution wasremoved using a blotting paper and the grid was allowed to dryprior to measure-ment. TEM measurements were performed on a Zeiss CEM 902 Model instrumentoperated at an accelerating voltage at 80 kV. High-resolution transmission electronmicroscopy (HRTEM) measurements were performed on a LEO-922 model instru-ment operated at an accelerating voltage at 200 kV. The magnetic properties of theporous nanorods were examined using SQUID (superconducting quantum interfer-ence device) (Quantum Design, MPMS-7).

3. Results and discussion

Fig. 1A shows the XRD patterns recorded in the 2� range 20–70◦

of the samples before (curve 1) and after calcination (curve 2).Well-defined XRD patterns were observed and all diffraction peakswere perfectly indexed, which are in agreement with the data of�-FeOOH (curve 1) (JCPDS 29-713) and �-Fe2O3 (hematite, curve2) (JCPDS 33-664). The strong and sharp peaks indicate that the�-FeOOH and �-Fe2O3 powders are highly crystalline.

Fig. 1B shows FTIR spectra of as-prepared sample, beforesolvent extraction and calcination (curve 1) and sample after sol-vent extraction and calcination (curve 2) in the spectral region2700–3100 cm−1. The C–H symmetric and antisymmetric stretch-

alcination sample (curve 2). (B) The FTIR spectra of as-prepared sample, (curve 1)

440 S. Mandal, A.H.E. Muller / Materials Chemistry and Physics 111 (2008) 438–443

sample after solvent extraction and calcination, respectively. (C) The TEM image recordeds-obtained �-FeOOH nanorod. (D) The HRTEM image recorded from the porous �-Fe2O3

f one portion of HRTEM image.

9–12 nm sizes on the surface of the nanorods are observed aftercalcination.

The transmission electron microscopy image recorded from theas-prepared sample is shown in Fig. 2C and it clearly shows the

Fig. 2. (A and B) The FE-SEM images recorded from the as-prepared sample and thefrom the as-prepared sample and the inset in (C) shows the HRTEM image of the ananorods sample after calcination and the inset in (D) shows the magnified image o

ing vibration frequencies at 2850 and 2920 cm−1 are clearly seenfor the as-obtained �-FeOOH sample (before solvent extraction andcalcination), whereas after solvent extraction and calcination thosepeaks are absent, which indicates that after solvent extraction andcalcination the surfactant SDS as-template has been removed.

Parts A and B of Fig. 2 show representative field emissionscanning electron microscope (FE-SEM) images recorded froma drop-coated film of the as-prepared sample and the sampleafter solvent extraction and calcination on Si wafers, respec-tively. A densely populated, predominantly rod-like morphologyis observed (Fig. 2A) and the typical diameter and length ofthe rods are estimated to be 170–210 nm and 3–5 �m, respec-tively. The FE-SEM image (Fig. 2A) shows that the surfaces ofthe as-prepared nanorods (before solvent extraction and calci-nation) are very smooth, whereas after removal of the templateby solvent extraction and calcinations, the FE-SEM micrograph(Fig. 2B) reveals the remarkable effect on the macroscopic struc-ture of the nanorods. The presence of nanorods with porous surfaceand typical length ranging from 3–5 �m and diameter of about170–210 nm is observed. Comparing the FE-SEM images recordedbefore (Fig. 2A) and after (Fig. 2B) removal of template, the nanorodsexhibit the porous surface after calcination, while the shape (lengthand diameter) of the nanorods remain almost same in both thecases (before and after template removal). The porous structureis much more clearly seen in the FE-SEM image of the nanorods(Fig. 2B) where the presence of relatively homogeneous pores of

Fig. 3. The TEM image recorded from the as-prepared sample synthesized withoutsurfactant in control experiment.

S. Mandal, A.H.E. Muller / Materials Chemistry and Physics 111 (2008) 438–443 441

set ofetizat

Fig. 4. (A) Magnetic hysteresis loop of the porous �-Fe2O3 nanorods at 300 K. The inmagnetization and the coercivity. (B) Temperature dependence of ZFC and FC magn

rod-like structure with smooth surface. The inset of Fig. 2C showsa HRTEM image of the nanorods. The lattice planes are clearly seenand the interplanar spacing 4.26 A correspond to the (1 0 0) planes,which reveal the crystalline nature of the as-obtained �-FeOOHnanorods (before calcination). Fig. 2D shows the morphology evo-lution that occurred during calcination of the nanorods. It revealsthat the nanorods are full of porosities and it is worth mention-ing that the shape and size of the rods have not changed duringthe calcination and there is no sign of aggregation also. HRTEM isa powerful method for structural analysis on the atomic scale, andthus might provide further insight into the structure of an individ-ual �-Fe2O3 nanorod. The HRTEM image in Fig. 2D shows that the�-Fe2O3 rods have a porous structure with pore diameter in therange of 9–12 nm and the inset of Fig. 2D shows that they are struc-turally uniform with an interplanar spacing of about 3.68 A, whichcorresponds to the (0 1 2) plane of �-Fe2O3.

Fig. 3 shows the representative TEM image recorded froma drop-cast film of the as-prepared sample obtained after thehydrothermal synthesis in control experiment. A densely popu-lated, predominantly irregularly shaped particles are seen that

Scheme 1. Schematic illustration for formation of

this figure shows a magnified view of the hysteresis loop highlighting the residualion of the porous �-Fe2O3 nanorods under an applied magnetic field of 500 Oe.

co-exist with a small percentage of rod-shaped particles. The mor-phologies obtained from the as-prepared sample in the controlexperiment are thus totally different than that obtained from theexperiment in presence of SDS surfactant as template. It is mostlikely that the rod-like micelle of the surfactant helps in the mor-phology selectivity during the growth process.

The magnetic properties of the porous �-Fe2O3 nanorods werefurther investigated using SQUID. To investigate the magneticproperties of the porous �-Fe2O3 nanorods, magnetic hystere-sis measurement was carried out in an applied magnetic field at300 K (room temperature) with the field sweeping from −60 to60 kOe. Fig. 4A shows the hysteresis loop of the porous nanorods.It can be seen that saturation is not reached up to the maximumapplied magnetic field. The inset of this figure shows a magni-fied view of the hysteresis loop recorded for the porous �-Fe2O3nanorods and shows weak ferromagnetic behavior with a rema-nent magnetization of 0.02 emu g−1 and a coercivity of 250 Oe atroom temperature (300 K). The values of the remanent magnetiza-tion and coercivity of these porous-rod-like Fe2O3 nanostructuresare higher than those of rod-like Fe2O3 (without porous) nanostruc-

porous nanorods using rod-like templates.

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tures (2.8 × 10−3 emu g−1 for remanent magnetization and 47 Oe forcoercivity) [47]. It is well known that the magnetization of ferro-magnetic materials is dependent on the morphology and structureof the samples [48]. Fig. 4B shows the curves for the temper-ature dependences of zero-field cooling (ZFC) and field cooling(FC) magnetizations of porous �-Fe2O3 nanorods from 2 to 390 Kunder an applied magnetic field of 500 Oe. It is clearly seen thatthe FC and ZFC magnetization curves split significantly; the ZFCmagnetization decreases sharply, while the FC magnetization risessignificantly. The Morin transition temperature for the porous �-Fe2O3 nanorods (223 K, calculated from differential ZFC curve) islower than that for bulk �-Fe2O3 (263 K), which may be related tothe decrease in diameters for 1-D nanohematite, agreeing with thetheory that TM decreases with decreasing particle size. Because ofnanoscale confinement, nanomaterials can exhibit unusual mag-netic behavior that is quite different from that of conventional bulkmaterials.

In Scheme 1 we illustrate a possible mechanism that couldexplain the formation of the rod-like morphology of iron oxideand the formation of pores during calcination. SDS is known toform cylindrical micelles at high concentration in solution andit is an ionic compound, which ionizes completely in water toform a negatively charged molecule with a long hydrophobic tail.The first step results in a possible electrostatic interaction exist-ing between the added Fe3+/Fe2+ ions and the SDS anion, favoringthe formation of a complex in the precursor. It is expected thaton addition of the hydrolyzing agent (NaOH), very fine particlesof oxide are formed that serve as seeds and are adsorbed on thesurface of the SDS rod-like micelle. Under the hydrothermal pro-cess, as the reaction progresses, the growth proceeds along theactive sites, resulting in the formation of elongated nanostructuresin the form of rods. We believe that because of the electrostaticinteraction of the iron ions and the anionic surfactant SDS, rod-likeconformational inorganic–surfactant composites may form, whichmay serve as templates for the formation of rod-like morpholo-gies. In the last step, the formation of porous hematite was dueto the decomposition of �-FeOOH to �-Fe2O3 during calcinationsin air. It is in agreement with the topotactic reaction [49]. Duringthe process of the calcinations, �-FeOOH decomposes to �-Fe2O3thus we get pure hematite, which is indicated by the XRD pattern(Fig. 1A) also.

4. Conclusions

In conclusion, it has been shown that highly porous iron oxiderod-like structures can be formed using a rod-like micelle astemplate via a hydrothermal process. Under hydrothermal condi-tions, rod-like �-FeOOH nanocomposite and then, after calcinationporous �-Fe2O3 nanorods were obtained. The detailed morphology,crystallinity and magnetic properties of the resulting porous-nanorods were determined using combined SEM, XRD, HRTEM andSQUID measurements. Crystalline porous �-Fe2O3 was obtainedafter heat-treating the as-obtained �-FeOOH nanorods, whichretain the same nanorod morphology, even at 500 ◦C. The proposedmethod has great advantages in large-scale industrial manufactur-ing for a simple hydrothermal process, such as inexpensive rawmaterials, high purity, and a high morphology yield of the products.The surfactant SDS plays a key role in controlling the nucleationand growth of the nanorods, and the possibility of using the ionicsurfactants as rod-like template is exciting. It is our hope that thisconvenient and efficient synthesis route can be applied as a gen-eral method for the preparation of porous 1D nanostructure of othermetal and oxides with possible applications in catalysis and noveloptical materials.

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try and Physics 111 (2008) 438–443

Acknowledgements

SM thanks the Alexander von Humboldt Foundation for aresearch fellowship. We thank Mr. Benjamin Balke (InorganicChemistry, University of Mainz, Germany) and Mr. Ram Sai Yela-manchili (Inorganic Chemistry I, University of Bayreuth, Germany)for the SQUID measurements and providing the hydrothermal reac-tor, respectively.

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