hydrothermal synthesis and characterization of - feooh and fe2o3 uniform nanocrystallines

Herein we demonstrated that under appropriate conditions, rodlike␣-FeOOH goethite and porous fusiform␣-Fe2O3 hematite uniform nanocrystallines could be selectively synthesized in large q

Hydrothermal synthesis and characterization of ␣-FeOOH and

Xiaohe Liua,b, Guanzhou Qiub, Aiguo Yanb, Zhong Wanga, Xingguo Lia,∗

aCollege of Chemistry & Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China

bDepartment of Inorganic Materials, Central South University, Changsha, Hunan 410083, People’s Republic of China

Received 18 April 2006; received in revised form 4 June 2006; accepted 6 June 2006

Available online 24 July 2006

Abstract

Inorganic nanoparticles with controlled size and shape are technologically important due to the strong correlation between these parameters and magnetic, electrical, and catalytic properties Herein we demonstrated that under appropriate conditions, rodlike␣-FeOOH (goethite) and porous fusiform␣-Fe2O3 (hematite) uniform nanocrystallines could be selectively synthesized in large quantities via a facile surfactant sodium dodecyl sulfate (SDS) assisted hydrothermal synthetic route The morphology and structure of the final products were investigated in detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and selected area electron diffraction (SAED) The probable formation mechanism of the␣-FeOOH and ␣-Fe2O3uniform nanoparticles was discussed on the basis of the experimental results

© 2006 Elsevier B.V All rights reserved

Keywords: Hydrothermal; Porous;␣-FeOOH; ␣-Fe2 O3; Nanocrystallines

1 Introduction

Over the past decades, inorganic nanoparticles with

con-trolled size and shape have attracted vast attention because of

their size and shape-dependent properties and great potential

applications Considerable effort has been devoted to the design

and controlled fabrication of inorganic materials with controlled

size and shape Various synthetic methods are continually being

improved to this end Recently a variety of novel shapes such

as tubes[1], rods[2], wire[3], belts[4], prisms[5], and cubes

[6]have been reported through chemical reactions of precursors

at room or slightly elevated temperatures However, with the

growing interest in building advanced materials using nanoscale

building blocks, it remain a challenge to find simple and mild

routes to control the parameters of final products to fine-tune

their properties

Iron oxides have attracted enormous attention owing to their

interesting electrical[7], magnetic[8], and catalytic[9]

proper-ties and wide variety of potential applications in various fields

∗Corresponding author Tel.: +86 10 62765930; fax: +86 10 62765930.

E-mail address:xgli@chem.pku.edu.cn (X Li).

such as electro-optic materials [10], sorbents [11], pigments [12], ion exchangers [13], and magnetic resonance imaging (MRI)[14], particularly in the field of catalysis[15] Amongst the readily available carbon monoxide oxidation catalysts, iron oxide-based materials have been found to be especially attractive candidates as cheap and efficient catalysts [16] Various pro-cedures including wet chemical[17–20], electrochemical[21], thermal decomposition techniques[22], and chemical oxidation

in polymer[23]have been successfully employed for the syn-thesis of iron oxides nanocrystallines As is well known, the properties of iron oxides nanocrystallines sensitively depend on their size and shape In order to improve the functional proper-ties such as catalytic activity, it is significant challenge to control the size and shape of iron oxides nanocrystallines Porous iron oxides nanoparticles may provide some immediate advantages over their solid counterparts because of their relatively low den-sities and large surface areas for their applications In recent years, Oca˜na and co-workers have synthesized uniform iron oxides nanoparticles via aerial oxidation and forced hydroly-sis methods[24], however, the products of such synthesis often involved complicated process or produced in low quantities

In this paper, we demonstrated that rodlike goethite ( ␣-FeOOH) and porous fusiform hematite (␣-Fe2O3) iron oxide 0925-8388/$ – see front matter © 2006 Elsevier B.V All rights reserved.

doi:10.1016/j.jallcom.2006.06.029

uniform nanocrystallines could be selectively synthesized

through a facile hydrothermal synthetic route in large quantities

under mild conditions When only prolonging the hydrothermal

time from 12 to 24 h, the rodlike ␣-FeOOH nanocrystallines

could be transfered to porous fusiform ␣-Fe2O3

nanocrys-tallines In this synthetic system, surfactant sodium dodecyl

sulfate (SDS) was used as a structure-directing agent, and simple

compounds of the hydrated ferrous chloride and sodium

boro-hydride as reactants directly The high yields, simple reaction

apparatus and low reaction temperature give this novel method

a good prospect in the future applications

2 Experimental

All chemicals in this work, such as hydrated ferrous chloride (FeCl2·4H2O),

sodium borohydride (NaBH4), and sodium dodecyl sulfate (SDS) were of

ana-lytical grade, and which were used without further purification.

2.1 Preparation of iron oxides nanocrystallines

In a typical procedure, pure sodium dodecyl sulfate (0.001 mol) and hydrated

ferrous chloride (FeCl2 ·4H2O, 0.001 mol) were firstly dissolved in distilled

water to form a salmon pink micellar solution under vigorous stirring at room

temperature Then, sodium borohydride solution (NaBH4, 0.5 mmol) was added

to the salmon pink solution With the introduction of sodium borohydride

solu-tion, the color of mixed micellar solution turned immediately from salmon pink

to black Next, the mixture was transferred to a Teflon-lined stainless steel

auto-clave of 50 mL capacity Finally, the autoauto-clave was filled with distilled water

up to 75% of the total volume, sealed and maintained at 140 ◦C for 4, 8, 12,

and 24 h, respectively After the heating treatment, the autoclave was allowed to

cool down to room temperature naturally The resulting products were filtered,

washed with distilled water and absolute ethanol, and finally dried in vacuum at

50 ◦C for 6 h.

2.2 Characterization

The rodlike ␣-FeOOH (goethite) and porous fusiform ␣-Fe2O3 (hematite)

nanocrystallines were characterized using various techniques X-ray powder

diffraction (XRD) patterns were obtained on a Bruker D8-advance X-ray

diffrac-tometer with graphite-monochromatized Cu K␣ (λ = 1.54178 ˚A) radiation The

operation voltage and current were kept at 40 kV and 40 mA, respectively TEM

patterns were recorded on a Hitachi Model H-800 transmission electron

micro-scope at an accelerating voltage of 200 kV The samples were dispersed in

absolute ethanol in an ultrasonic bath Then the suspensions were dropped onto

Cu grids coated with amorphous carbon films Selected area electron diffraction

(SAED) was further performed to identify the crystallinity The scanning

elec-tron microscopic (SEM) images were obtained using a LEO 1530 field-emission

scanning electron microscope (FE-SEM), under typical working conditions of

10 kV.

3 Results and discussion

Fig 1shows typical X-ray diffraction (XRD) patterns (2θ

scan) of the goethite (␣-FeOOH) iron oxide nanocrystallines

obtained at 140◦C for different reaction time All the reflections

of the XRD pattern can be finely indexed to an orthorhombic

phase [space group: Pbnm(62)] of␣-FeOOH with cell

param-eters a = 4.64 ˚ A, b = 10.0, c = 3.03 ˚A (JCPDS file Card, No

03-0249) The XRD patterns ofFig 1from a to c correspond to the

goethite (␣-FeOOH) iron oxide nanocrystallines obtained for 4,

8, and 12 h, respectively The XRD pattern (Fig 1a) of sample

shows that the goethite (␣-FeOOH) iron oxide nanocrystallines

Fig 1 The evolution of the XRD patterns of the ␣-FeOOH nanocrystallines obtained at 140 ◦C for different reaction time: (a) 4 h; (b) 8 h; (c) 12 h (* ␣-Fe2 O3).

obtained for 4 h were poorly crystallized With the elongation of reaction time, the XRD patterns of samples at the whole process show that the crystallinities of the samples were continuously improved When the samples maintained for 8 h, the XRD pat-tern of the sample is shown in Fig 1b The main diffraction peaks of ␣-FeOOH are clear observed in the patterns When the reaction time prolonged to 12 h (inFig 1c), the peaks of

␣-FeOOH become slightly weak, along with the weakening of

␣-Fe2O3peaks, which indicates the transition from␣-FeOOH

to␣-Fe2O3 Fig 2 shows the X-ray diffraction pattern of the sample

of hematite (␣-Fe2O3) iron oxide nanocrystallines that were obtained at 140◦C for 24 h The composition could be expressed

as rhombohedral phase␣-Fe2O3[space group:R¯3c(167)] with

lattice constants a = 5.0356, c = 13.7489 (JCPDS file Card, No.

33-0664), since the main diffraction peaks of rhombohedral phase ␣-Fe2O3, 012, 104, 110, 113, 024, 116, 018, 214, and

300, are clearly observed in the patterns No impurity peaks are

Fig 2 The XRD pattern of the porous fusiform ␣-Fe2O3 nanocrystallines obtained at 140 ◦C for 24 h.

Fig 3 (A) TEM image of the sample produced at 140 ◦C for 4 h (B) TEM image of the sample produced at 140◦C for 8 h The insets of (A and B) show the SAED pattern of the rodlike ␣-FeOOH nanocrystallines taken on a mass of ␣-FeOOH nanocrystallines.

observed, indicating the hematite (␣-Fe2O3) iron oxide

success-fully synthesized under current experimental conditions

The size and morphology of the rodlike␣-FeOOH (goethite)

and porous fusiform␣-Fe2O3(hematite) nanocrystallines were

further examined by transmission electron microscopy (TEM)

Fig 3A shows the typical TEM photograph of ␣-FeOOH

nanocrystallines obtained at 140◦C for 4 h through a

surfac-tant SDS assisted hydrothermal synthetic route, and its select

area electron diffraction pattern indicates the sample A is poorly crystallized, being in good agreement with the XRD patterns In contrast to the sample processed for 4 h, the sample prepared

at 140◦C for 8 h is mostly rodlike form, as can be observed more clearly fromFig 3B The inset showing a select area elec-tron diffraction pattern of the rodlike␣-FeOOH nanocrystallines taken on a mass of␣-FeOOH nanorods reveals the satisfactory crystallinity of the sample, which can be indexed to the pure

Fig 4 TEM image of the sample of produced at 140 ◦C for 12 h (B) TEM image of the sample produced at 140◦C for 24 h The inset shows the SAED pattern of the individual fusiform ␣-Fe2 O3 (hematite) nanocrystallines.

Fig 5 (A) Low- and (B) high-magnification SEM images of the fusiform ␣-Fe2 O3 nanocrystallines synthesized with the surfactant SDS process at 140 ◦C for 24 h.

orthorhombic phase of goethite␣-FeOOH When the reaction

time prolonged to 12 h, the previous nanorods appeared

agglom-erate (inFig 4A), which is very critical for the formation of

porous fusiform␣-Fe2O3(hematite) nanocrystallines Fusiform

␣-Fe2O3 (hematite) nanocrystallines were obtained when the

reactants were treated at 140◦C for 24 h, as shown inFig 4B.

TEM observations indicates that about 100% of the products are

fusiform␣-Fe2O3nanocrystallines whose diameter ranges from

about 50 to 70 nm with a length of up to∼200 nm There was an

interesting change in the morphology of the sample of␣-Fe2O3

nanocrystallines With careful observation, the fusiform

␣-Fe2O3nanocrystallines can be made up from many porous

struc-tures The inset is an electron diffraction pattern of an individual

fusiform ␣-Fe2O3 nanocrystallines, which can be indexed to

rhombohedral phase of hematite␣-Fe2O3and exhibits that each

porous fusiform␣-Fe2O3nanocrystalline was a single crystal

Fig 5A and B shows scanning electron microscope images of

a typical sample of fusiform␣-Fe2O3nanocrystallines obtained

at 140◦C for 24 h and indicate the large quantity and good

uniformity␣-Fe2O3nanocrystallines were achieved using this

approach These fusiform ␣-Fe2O3 nanocrystallines have a

mean diameter of 60 nm and length of up to∼200 nm, which

agree well with the TEM results.Fig 5B is a higher

magnifi-cation SEM image obtained from a selected area ofFig 5A

Herein, fusiform␣-Fe2O3 nanocrystallines with many porous

structures can be clearly observed The chemical compositions

of the as-prepared fusiform␣-Fe2O3nanocrystallines have been

investigated by means of EDS Results from EDS spectra (Fig 6)

show that the fusiform␣-Fe2O3nanocrystallines contain Fe and

O, and no contamination elements are detected The atomic ratio

of Fe and O matched their stoichiometries quite well

Associating all those results, the whole reaction to form the

␣-FeOOH and ␣-Fe2O3uniform nanoparticles can be expressed

as the following equation:

4Fe2++ BH4 −+ 3H2O → 4Fe ↓ + H3BO3+ 7H+ (1)

4Fe2++ O2+ 6H2O → 4FeOOH ↓ + 8H+ (3)

Soluble ferrous chlorides will firstly dissociate in water into fer-rous ions, which then react with sodium borohydride to form ultrafine iron particles The reduction of transition metal ions for the production of ultrafine metal particles by BH4 − is a ubiquitous reaction The ultrafine iron particles may be redis-solved under acidic conditions Subsequently, rodlike␣-FeOOH (goethite) nanocrystallines will gradually form under surfactant SDS assisted hydrothermal conditions With the reaction time prolonged, rodlike␣-FeOOH nanocrystallines appear agglom-erate and form fusiform nanocrystallines Finally, the␣-FeOOH

Fig 6 The EDS spectra of the as-prepared porous fusiform ␣-Fe2O3 nanocrys-tallines.

nanocrystallines maybe dehydrate and form porous fusiform

␣-Fe2O3nanocrystallines

4 Conclusion

In summary, we have successfully synthesized the rodlike

goethite (␣-FeOOH) and porous fusiform hematite (␣-Fe2O3)

iron oxide uniform nanocrystallines via a facile surfactant

sodium dodecyl sulfate assisted hydrothermal synthetic route

at mild conditions The influence of reaction time on size and

shape was investigated Results show the hydrothermal time was

dominant When maintained at 140◦C for 4 h, the sample of

goethite (␣-FeOOH) iron oxide is poorly crystallized, however,

the sample of ␣-FeOOH prepared at 140◦C for 8 h is mostly

nanorodlike form with satisfactory crystallinity With

increas-ing the hydrothermal time, goethite (␣-FeOOH) iron oxide

nanorods appeared agglomerate, and a part of the ␣-FeOOH

phase transformed to␣-Fe2O3 phase When the reaction time

prolonged to 24 h, we successfully synthesized porous fusiform

hematite (␣-Fe2O3) iron oxide nanocrystallines with good

uni-formity The synthetic strategy presented here may have a good

prospect in the future application and provide an effective route

to synthesize other metal oxide nanocrystallines Owing to the

excellent physical properties of the iron oxides, it is expected

the rodlike␣-FeOOH (goethite) and porous fusiform ␣-Fe2O3

(hematite) uniform nanocrystallines exhibit some important

applications in, e.g., sensors, magnetic media, catalytic fields,

etc

Acknowledgements

Financial support of this work by National Natural Science

Foundation of China (Grant no 50504017) and Hunan

Provin-cial Natural Science Foundation of China (Grant no 05JJ30104)

is gratefully acknowledged

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