Journal ofMaterials Chemistry A

PAPER

Publ

ishe

d on

21

Dec

embe

r 20

12. D

ownl

oade

d by

Uni

vers

ity o

f L

anca

ster

on

30/1

0/20

14 2

2:49

:09.

View Article OnlineView Journal | View Issue

State Key Laboratory of Coal Conversion

Academy of Sciences, Taiyuan, 030001, C

+86-351-4041153; Tel: +86-351-4063121

† Electronic supplementary information (Epatterns. See DOI: 10.1039/c2ta00987k

Cite this: J. Mater. Chem. A, 2013, 1,3040

Received 5th November 2012Accepted 21st December 2012

DOI: 10.1039/c2ta00987k

www.rsc.org/MaterialsA

3040 | J. Mater. Chem. A, 2013, 1, 30

Hexagonal a-Fe2O3 nanorods bound by high-indexfacets as high-performance electrochemical sensor†

Zhong Liu, Baoliang Lv,* Yao Xu and Dong Wu

Hexagonal a-Fe2O3 nanorods with high-index facets exposed were synthesized using a double ligand-

assisted hydrothermal method (involving PO43� and formamide). The growth process of the synthesized

particles obviously proceeded by two stages: (i) formation of capsule-shaped a-Fe2O3 nanoparticles

along [001] directions and (ii) preferential grow along [112] and/or its equivalent directions to form

hexagonal nanorods. The occurrence of this process can be attributed to the stepwise effect of PO43�

and formamide. First, the shape of the capsule-shaped a-Fe2O3 is mainly regulated by the adsorption of

PO43� on those facets parallel to the c-axis of a-Fe2O3 during the nanocrystal growth because of the

special double coordinated surface hydroxyl on (001) facets, and then the hexagonal structure should

result from the preferential growth along [112] and/or its equivalent directions due to the weak

adsorption of formamide in the Ostwald ripening process. The as-prepared a-Fe2O3 exhibits an excellent

electrochemical sensing capability towards H2O2 due to the exposed high-index facets and more active

sites of low-coordinated atoms that are located on the edges.

1 Introduction

During the last decades, morphological control over inorganiccrystals has attracted intensive attention because of thepromoted physical and chemical properties stemming from thedifferent crystal facets.1,2 As the most stable iron oxide underambient conditions, a-Fe2O3 has been used in the eld ofpigments,3 catalysts,4 magnetic recording media,5 water split-ting,6 anticorrosive agents and gas sensors,7 owing to its lowcost and environmental friendliness. It is well known that theproperties of nanoparticles, especially for those of single crystalstructure, oen depend on their morphologies and surfacemicro-structures, such as exposed facets, surface hydroxylconguration, surface charge situation, and so on.8 Thesesurface situations also adapts for a-Fe2O3 nanoparticles. Theselective control of exposed facets of a-Fe2O3 nanoparticles, andthus the promotion of their chemical reaction efficiency, hasbeen reported by several groups. For example, polyhedrala-Fe2O3 crystals, which are enclosed by high-index (012), (102)and (001) facets, exhibit a good sensor response to ethanolvapor.9 Quasicubic a-Fe2O3 nanoparticles have much higheractivity in CO oxidation than the owerlike structures due to theexclusive exposure of the {110} planes.10 However, the reportedworks mostly focused on particles, as the most popular formdue to their shape anisotropy, the one-dimensional a-Fe2O3

, Institute of Coal Chemistry, Chinese

hina. E-mail: lbl604@sxicc.ac.cn; Fax:

SI) available: Additional XRD and SEM

40–3046

nanoparticles were only reported as capsule-shape, nanotubeand nanoring structures that are covered with a curvedsurface.11,12 To the best of our knowledge, there is no report onone-dimensional a-Fe2O3 nanostructures which are enclosed byspecial or high-index facets.

It is well known that the formation and growth processes ofa-Fe2O3 nanocrystals could be strongly affected by adsorbedligands on different facets. In our previous work, we found thatthe existence of formamide could ensure the exposure of high-index {112} facets and induce the formation of octadecahedrala-Fe2O3 particles.13 Based on that work, we hoped the effect offormamide could be introduced to control the outer surface ofone-dimensional a-Fe2O3 nanoparticles. But we did not want todivide the experimental process into two steps, because once thea-Fe2O3 particles were formed and separated out, the surface ofthese particles would become more perfect and inactive, whichwould make the later modication much more difficult. There-fore, we considered the stepwise inuence of different anions,which is an effective one-stepmethod to control the structures ofnanoparticles. As expected, hexagonal a-Fe2O3 nanorods grownalong the [001] direction and with high-index {112} facetsexposed were obtained by the stepwise inuence of PO4

3� andformamide. These high-index {112} facets exposed nanorodsexhibit outstanding electrochemical sensing capability towardsH2O2 for concentrations ranging from 40 mM to 4.66 mM.

2 Experimental section

The a-Fe2O3 nanorods were prepared by a hydrothermalmethod. In a typical experimental procedure, specic amounts

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

Publ

ishe

d on

21

Dec

embe

r 20

12. D

ownl

oade

d by

Uni

vers

ity o

f L

anca

ster

on

30/1

0/20

14 2

2:49

:09.

View Article Online

of FeCl3, NaH2PO4, and formamide aqueous solutions weremixed together, and then distilled water was added to themixture to keep the nal volume at 80 mL. The concentrationsof FeCl3, NaH2PO4, and formamide were 23 mM, 0.3 mM and2.7 M, respectively. Aer vigorous stirring for 10 min, themixture was transferred into a Teon-lined stainless steelautoclave with a capacity of 100 mL for hydrothermal treatmentat 220 �C for 24 h. Aer the autoclave was cooled to roomtemperature, the precipitate was separated by centrifugation,washed with distilled water and absolute ethanol, and dried at80 �C. In order to understand the roles of these two ligands, weinvestigated their respective effects by varying their concentra-tions while preserving all the other conditions as describedabove. Time-dependent experiments were carried out to inves-tigate the growth process by changing the reaction time to 2 h,4 h, 12 h and 48 h.

X-ray powder diffraction (XRD) patterns of the products wereacquired by a Philips X'Pert PRO SUPER X-ray diffractometerequipped with graphite-monochromatized Cu Ka radiation (l¼1.5418 A). Morphologies of the samples were examined byscanning electron microscopy (SEM, X-650) and high-resolutiontransmission electron microscopy (HRTEM, JEOL-2010). X-rayphotoelectron spectroscopy (XPS) analysis was performed on aPHI 5300� multi-technique system with a Mg Ka X-ray source(Perkin-Elmer Physical Electronics). The preparation of theH2O2

sensor was similar to that described by Li and co-workers exceptthe glassy carbon electrode was modied with different a-Fe2O3

nanorods.14 Amperometric measurements were performed on aCHI760A electrochemical workstation (Shanghai CH Instru-ments, China); a three-compartment electrochemical cell con-tained a platinum plate electrode, a saturated calomel referenceelectrode (SCE), and a bare or modied GC electrode as workingelectrode. H2O2 measurement was carried out at an appliedpotential of�0.5 V (vs. SCE) with successive additions of H2O2 to4.66 mM in pH 7.2 phosphate buffer solution (0.01 M PBS).

3 Results and discussion3.1 Structure and morphology

Fig. 1a shows the XRD pattern of typically prepared nano-crystals. All of the strong diffraction peaks can be indexed to thehexagonal a-Fe2O3 phase (JCPDS 33-0664). The narrow sharppeaks suggest that the a-Fe2O3 products are of high purity andcrystallinity. The morphology of the products was investigated

Fig. 1 (a) XRD pattern and (b) SEM images of the hexagonal a-Fe2O3 nanorods.

This journal is ª The Royal Society of Chemistry 2013

by SEM. Fig. 1b shows a typical large-area SEM image of thesample, indicating the presence of homogeneous, well-shapedhexagonal faceted-nanorods with a length of �150 nm andwidth of �50 nm. The aspect ratio (c/a) of a shape is dened asthe length of the major axis (c) divided by the width of the minoraxis (a). The high-magnication SEM image (inset in Fig. 1b)shows the as-prepared nanocrystals were mainly enclosed by sixsided shapes with rough surfaces which have a length of morethan 100 nm, and an average aspect ratio (c/a) of about 3.

Detailed structural information of the synthesized a-Fe2O3

nanorods was investigated by HRTEM (Fig. 2). Fig. 2a shows theside view of a single particle and Fig. 2b is its correspondingselected area electron diffraction (SAED) pattern. The sharpdiffraction spots reveal the single-crystalline nature of thehexagonal particle, and the diffraction spots can be attributedto (1�12), (�114) and (006) facets and/or their equivalent facetsunder the incident electron beam along the [110] direction.Fig. 2c and d show the tip and side regions in Fig. 2a. In Fig. 2c,three kinds of lattice fringes can be clearly resolved and theirinterplane distances are 0.36, 0.27, 0.23 nm, which can beattributed to the (1�12), (�114) and (006) facets, respectively.

Fig. 2 HRTEM images of the hexagonal a-Fe2O3 nanorods: (a) side-view of asingle particle and (b) its corresponding SAED pattern, (c) the tip and (d) corner ofthe particle in (a); (e) top-view of a single particle and (f) the correspondingcorner.

J. Mater. Chem. A, 2013, 1, 3040–3046 | 3041

Journal of Materials Chemistry A Paper

Publ

ishe

d on

21

Dec

embe

r 20

12. D

ownl

oade

d by

Uni

vers

ity o

f L

anca

ster

on

30/1

0/20

14 2

2:49

:09.

View Article Online

Fig. 2d has the same lattice fringes as Fig. 2c, and the edges ofthe particle are parallel to the (1�12) or other (102) equivalentfacets. Fig. 2e is the top view of a single particle, and Fig. 2f wastaken in the region f of Fig. 2e. In the corner of the particle(Fig. 2f), two kinds of lattice fringes which are parallel to theedges are identied, and their interplane distances are both0.253 nm, which match the (110) and/or its equivalent facets.Based on the above analysis, the side surfaces of the particlesare enclosed by many (112) facets like a pagoda along the [001]direction.

3.2 Condition dependent experiments

To further conrm the growth mechanism, it is important toobtain more information about the formation of the hexagonala-Fe2O3 nanorods by varying the experimental conditions.Therefore, the effects of experimental parameters were investi-gated in detail, mainly investigating the reaction time and theratio of PO4

3� to formamide.3.2.1 Inuence of reaction time. To obtain more detailed

information about the growth process, time-dependent experi-ments were carried out by preserving all the other conditions asin the typical experiment. The representative SEM images of theproducts prepared at different reaction times (2 h, 4 h, 12 h and48 h) are shown in Fig. 3. XRD patterns of the obtained particlesindicate that these particles are all a-Fe2O3 (ESI S1†). When thereaction time is 2 h (Fig. 3a), the product mainly consists ofnanospindles with an aspect ratio (c/a) of about 3. On increasingthe reaction time to 4 h and then 12 h (Fig. 3b and c), thecapsule-shaped nanosticks become shorter and “fatter”successively, and indistinct planes can be observed on thesurface. As the reaction times were prolonged to 24 h and 48 h

Fig. 3 SEM images of the products prepared at different reaction times: (a) 2 h,(b) 4 h, (c) 12 h, and (d) 48 h. [PO4

3�] ¼ 0.3 mM. The concentration of formamideis 2.70 M.

3042 | J. Mater. Chem. A, 2013, 1, 3040–3046

(Fig. 1b and 3d), the capsule-shaped nanosticks disappeared,and monodisperse hexagonal nanorods were formed. As theparticles obtained at 24 h and 48 h have similar morphologies,we can infer the growth process reached its equilibrium aer24 h of reaction. The time-dependent experiments demonstratethat the growth of the hexagonal nanorods is fast rst along the[001] direction, and subsequently becomes slow along otherdirections to form new (112) facets in the recrystallization by theOstwald ripening process.

3.2.2 Inuence of PO43�. The effect of PO4

3� concentrationwas also investigated. Experiments were conducted with a xedconcentration of formamide (2.70 M) but with various amountsof PO4

3�, and the SEM images of these products are shown inFig. 4. Without the addition of PO4

3� (Fig. 4a), the productconsists of octadecahedral a-Fe2O3 particles, and the surface isenclosed mainly by (112) facets.13 When the concentration ofPO4

3� is 0.1 mM (Fig. 4b), the size of the particles is found to bereduced and they tend to grow along the [001] direction. Whenthe PO4

3� concentration reaches 0.2 mM (Fig. 4c), there aresome new surfaces appearing in the middle of the crystals andthe particle growth along the [001] direction become moreobvious. As for 0.3 mM (Fig. 1b) and 0.4 mM (Fig. 4d), the sixsided surfaces dominate the outer surface. We can infer that theanisotropic growth along the [001] direction became faster thanother directions by adding PO4

3� in the crystal growth process.The area of side surface can continuously increase by increasingPO4

3�. These results are mainly attributed to the selectiveadsorption of PO4

3� on the facets parallel to the c-axisrestraining the growth along the radial direction.11

3.2.3 Inuence of formamide. According to our previouswork,13 different concentrations of formamide have differenteffects on the growth of the a-Fe2O3 particle surface. When the

Fig. 4 SEM images of the products prepared at different concentrations ofPO4

3�: (a) 0 mM, (b) 0.1 mM, (c) 0.2 mM, and (d) 0.4 mM. The concentration offormamide is 2.70 M.

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

Publ

ishe

d on

21

Dec

embe

r 20

12. D

ownl

oade

d by

Uni

vers

ity o

f L

anca

ster

on

30/1

0/20

14 2

2:49

:09.

View Article Online

concentration of formamide is 0.2 M without PO43� added, the

obtained a-Fe2O3 particles are of spherical shape with a diam-eter �300 nm (ESI S2†). By increasing the concentration offormamide to more than 0.6 M, we can obtain different octa-decahedral a-Fe2O3 particles with different dominant facetsexposed. With the increasing concentration , the aspect ratio(c/a) of these different octadecahedral a-Fe2O3 particles obvi-ously increased.13

To identify the effect of formamide in the present system,further formamide concentration dependent experiments werecarried out by preserving the concentration of PO4

3� (0.3 mM).Fig. 5a–d show the SEM images of products which wereprepared by varying the concentration of formamide from 0,0.2, 0.6, to 1.3 M, respectively. Without adding formamide, theparticles are of spindle shape with a rough surface (Fig. 5a), andtheir length is near 300 nm. When the concentration of form-amide is 0.2 M, the surface became rough and the aspect ratio(c/a) reduced to 1.0 (Fig. 5b). When the concentration of form-amide reached 0.6 M (Fig. 5c), some of the surface planes can bevaguely observed. When the concentration of formamide wasincreased to 1.3 M (Fig. 5d), the edges and corners were formedon the surface and some planes can be clearly observed. Theparticles in Fig. 5b resemble a core–shell structure. The spindlecore structure is rst formed by PO4

3�, and then the sphericalshell structure is formed by 0.2 M formamide. That is to say, theaspect ratio (c/a) of the products (Fig. 5b) is reduced to about 1.0as a result of the spherical shell structure increasing the radiusof the particles. The morphological changes from nanospindlesto nanoparticles and then to nanorods by the aspect ratio (c/a)varying. This is mainly because the alkalinity of 0.2 M form-amide could accelerate the nucleation rate, but this concen-tration is not enough to adsorb onto a-Fe2O3 and restrain thea-Fe2O3 crystal growth since the effect between formamide and

Fig. 5 SEM images of the products prepared at different concentrations offormamide: (a) 0 M, (b) 0.2 M, (c) 0.6 M, and (d) 1.3 M. [PO4

3�] ¼ 0.3 mM.

This journal is ª The Royal Society of Chemistry 2013

a-Fe2O3 is rather weak in comparison to PO43� (Fig. 5b). When

the concentration of formamide is increased to more than0.6 M, the main effect of formamide would change fromaffecting the nucleation process to selective adsorption on somefacets, and the growth speed of these facets is thus slowed downand the aspect ratio of the particles would accordingly increase.According to our previous work and aforementioned HRTEManalysis,13 the special facets should be (112) and/or its equiva-lent facets. That is to say, the high concentration of formamide(more than 0.6 M) can create new (112) facets when the particleshave fast growth along the [001] direction.

3.3 Formation mechanism of the hexagonal nanorods

To identify the adsorption of PO43� and formamide, XPS anal-

ysis was carried out on the synthesized nanorods, and the resultis shown in Fig. 6. The binding energies were corrected forspecimen charging by referencing the C 1s line to 284.8 eV, andthe insets are the high resolution spectra of N 1s and P 2p. Thepeaks at binding energies of 133.47 eV for P 2p and 399.80 eV forN 1s can be observed, which could be attributed to the surfaceadsorption of PO4

3� and formamide. On the basis of thecondition-dependent experiments, the formation process canbe divided into two steps: nanospindles are rst formed underthe inuence of PO4

3� ions, and nanorods are subsequentlymodied to hexagonal nanorods by formamide. The detailedprocess can be explained as below: in the rst step, compared toformamide, the adsorption ability of PO4

3� ions is muchstronger, and these PO4

3� ions would dominantly inuence thegrowth of a-Fe2O3 particles. It is known that PO4

3� can beadsorbed onto the surface of a-Fe2O3 by forming binuclear andbidentate complexes with two singly coordinated hydroxylgroups.15,16 Pairs of singly coordinated hydroxyl groups arelocated on the (110) and (100) facets, but the surface hydroxylconguration on (001) facets was entirely doubly coordinated.16

As a result, the PO43� ions are anchored on non-(001) facets,

while the adsorption on the (001) facets is very weak because ofthe lack of singly coordinated hydroxyl groups.16,17 Thus, thegrowth of non-(001) facets is restrained while the growth alongthe [001] direction is not obviously affected, and this induces

Fig. 6 XPS spectrum of the synthesized nanorods; the insets are the highresolution spectra of N 1s and P 2p.

J. Mater. Chem. A, 2013, 1, 3040–3046 | 3043

Journal of Materials Chemistry A Paper

Publ

ishe

d on

21

Dec

embe

r 20

12. D

ownl

oade

d by

Uni

vers

ity o

f L

anca

ster

on

30/1

0/20

14 2

2:49

:09.

View Article Online

the appearance of the capsule-shaped structure in the rst 4hours (Fig. 3a and b). With reaction time increasing (Fig. 1b and3d), the area of the particles’ surface increased by their diameterincreasing, and at the same time [Fe(H2PO4)x]

3�x ions areformed under the coordination effect between PO4

3� and Fe3+

ions in the dissolution process.12 The above-mentioned twofactors will weaken the adsorption effect of PO4

3� ions accord-ingly. In contrast, although the adsorption affinity of form-amide is weaker than that of PO4

3� ions,18,19 since theconcentration of formamide is much higher than that of PO4

3�,the morphologies of a-Fe2O3 are gradually inuenced by form-amide in the second stage. The (112) and/or its equivalent facetshave the highest surface energy (1.68 J m�2) of the normalexposed facets,20 and the (112) facets’ structure also has lesssteric hindrance for formamide adsorption with a high densityof exposed Fe3+ in comparison with those normal low-indexfacets, as shown in Fig. 7.13 This results in the selectiveadsorption of formamide on the (112) facets, and leads to therestrained growth of a-Fe2O3 along the [112] direction. With itshigh concentration advantage, the formamide competitivelyadsorbs to the ank of spindle a-Fe2O3 particles, and the PO4

3�

is thus partly desorbed from these surfaces, which indicatesthat the concentration of PO4

3� in solution would accordinglyincrease. According to the references,21 the PO4

3�ions candissolve the a-Fe2O3 particles by coordination with Fe3+ on thetips that are not protected by formamide, and induce at tips ofsome a-Fe2O3 particles. So we obtained hexagonal nanorodswith surface roughness and some at tips which exposed (112)facets. The effects of individual additive and the whole growth

Fig. 7 Crystal structures of (100), (110), (001) and (112) facets of a-Fe2O3.

Fig. 8 The effects of the individual additives of PO43� and formamide, and the gr

3044 | J. Mater. Chem. A, 2013, 1, 3040–3046

process of the synthesized hexagonal nanorods are illustratedin Fig. 8.

3.4 Amperometric detection of hydrogen peroxide with a-Fe2O3

According to the literature,22 a-Fe2O3 is potentially suitable fordetecting hydrogen peroxide in a physiological system (pH 7.2).Because dominant {112} facets are high-index and high surfaceenergy facets,20 the sensing properties of these a-Fe2O3 particlesmight be enhanced. Curves a and b in Fig. 9A show the typicalamperometric responses of the as-prepared a-Fe2O3 hexagonalnanorods and a-Fe2O3 nanorods without high-index facetsexposed (Fig. 5a) modied electrodes to the successive addi-tions of H2O2 at an applied potential of �0.5 V, which is thepotential of the peak of a-Fe2O3 for electrocatalytic reduction forH2O2.22 The calibration curve of amperometric response byhexagonal a-Fe2O3 nanorods is presented in Fig. 9B, whichindicates that the nanorods exhibit excellent biosensor capa-bility towards H2O2 with a linear response for concentrationsranging from 40 mM to 4.66 mM with a correlation coefficient ofR2 ¼ 0.996. The inlays in Fig. 9A and B show magnied repre-sentations of parts of the corresponding lines. The sensordetection sensitivity of the as-prepared hexagonal a-Fe2O3

nanorods modied GC electrode reached 30 mM. The current ofthe hexagonal nanorods modied electrode has a signicantlinear response when the H2O2 concentration increased grad-ually below 660 mM, and a dramatic increase can be found whenthe H2O2 concentration increased up to 1.66 mM. This result

owth process of nanorods.

This journal is ª The Royal Society of Chemistry 2013

Fig. 9 (A) Amperometric response of a-Fe2O3 modified electrode with succes-sive additions of H2O2 to 4.66 mM in phosphate buffer (pH 7.2) at an appliedpotential of �0.5 V (vs. SCE): (a) high-index facets exposed and (b) without high-index facets exposed nanorods (inset: magnified amperometric response with theconcentration from 40 to 160 mM); (B) the calibration plot of amperometricresponse of high-index facets exposed a-Fe2O3 nanorods (inset: magnified cali-bration plot between 40 and 160 mM).

Paper Journal of Materials Chemistry A

Publ

ishe

d on

21

Dec

embe

r 20

12. D

ownl

oade

d by

Uni

vers

ity o

f L

anca

ster

on

30/1

0/20

14 2

2:49

:09.

View Article Online

implied that a H2O2 concentration over this wide range can bedetermined using this curve by current detection. However, thecurrent of a-Fe2O3 nanorods without high-index facets exposeddid not increase when the H2O2 concentration increased to160 mM, and was lower than the current of hexagonal nanorodsmodied electrode when the H2O2 concentration was below4.66 mM. These hexagonal a-Fe2O3 nanorods have a betterdetection effect than the Prussian blue/multiwalled carbonnanotube hybrids and the MnO2 nanomaterials by expansion ofthe detection range,23,24 and they are also better than a chitosan/hematite nanotubes composite lm with increased detectioncurrent.22 Compared to octadecahedral a-Fe2O3 nanoparticles,13

the hexagonal a-Fe2O3 nanorods have a good electroanalyticalability when the concentration of H2O2 increased by more than1 mM each time. The hexagonal a-Fe2O3 nanorods' excellentelectroanalytical ability and the enhanced response may beattributed to the (112) facets which have a lower energy barrierfor H2O2 reduction, and the roughness of the surface which has

This journal is ª The Royal Society of Chemistry 2013

more active sites of low-coordinated atoms located on theedges.14

4 Conclusion

We have synthesized hexagonal single crystal a-Fe2O3 nanorodsby employing a double adsorption-assisted hydrothermalmethod. The cooperative action of PO4

3� and formamide,involving stepwise adsorption and coordination, is a crucialfactor in bringing about the formation of the nanorods. Thesensing capability of the nanorods towards H2O2 was investi-gated, and the response current was enhanced and stable with alinear response over the concentration range of 40 mM to 4.66mM. This work further conrmed the feasibility of the stepwiseinuence of simply adsorbed ligands on the growth behavior ofnanoparticles. It is a promising approach for both the designand modication of inorganic transition metal oxidenanostructures.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (no. 21003147), Natural Science Founda-tion of Shanxi (no. 2011011007-3), Distinguished Young ScholarProject of Institute of Coal Chemistry, Chinese Academy ofSciences (no. 2011SJCRC07) and the State Key Laboratory ofCoal Conversion (SKLCC) in-house project (no. 2011BWZ005).

References

1 J. S. Chen, Y. L. Tan, C. M. Li, Y. L. Cheah, D. Y. Luan,S. Madhavi, F. Y. C. Boey, L. A. Archer and X. W. Lou, J.Am. Chem. Soc., 2010, 132, 6124–6130.

2 Y. N. Xia, T. Yu, D. Y. Kim and H. Zhang, Angew. Chem., Int.Ed., 2011, 50, 2773–2777.

3 J. Wang, W. B. White and J. H. Adair, J. Am. Ceram. Soc., 2005,88, 3449–3454.

4 G. P. Escobar, A. Q. Beroy, M. P. P. Iritia and J. H. Huerta,Chem. Eng. J., 2004, 102, 107–117.

5 R. N. Bhowmik and A. Saravanan, J. Appl. Phys., 2010, 107,053916.

6 M. Frites, Y. A. Shaban and S. U. M. Khan, Int. J. HydrogenEnergy, 2010, 35, 4944–4948.

7 H. Chen, Y. Zhao, M. Yang, J. He, P. K. Chu, J. Zhang andS. Wu, Anal. Chim. Acta, 2010, 659, 266–273.

8 X. L. Mou, X. J. Wei, Y. Li and W. J. Shen, CrystEngComm,2012, 14, 5107–5120.

9 Y. Yang, H. X. Ma, J. Zhuang and X. Wang, Inorg. Chem.,2011, 50, 10143–10151.

10 Y. H. Zheng, Y. Cheng, Y. S. Wang, F. Bao, L. H. Zhou,X. F. Wei, Y. Y. Zhang and Q. Zheng, J. Phys. Chem. B,2006, 110, 3093–3097.

11 C. J. Jia, L. D. Sun, F. Luo, X. D. Han, L. J. Heyderman,Z. G. Yan, C. H. Yan, K. Zheng, Z. Zhang, M. Takano,N. Hayashi, M. Eltschka, M. Klaui, U. Rudiger, T. Kasama,L. Cervera-Gontard, R. E. Dunin-Borkowski, G. Tzvetkovand J. Raabe, J. Am. Chem. Soc., 2008, 130, 16968–16977.

J. Mater. Chem. A, 2013, 1, 3040–3046 | 3045

Journal of Materials Chemistry A Paper

Publ

ishe

d on

21

Dec

embe

r 20

12. D

ownl

oade

d by

Uni

vers

ity o

f L

anca

ster

on

30/1

0/20

14 2

2:49

:09.

View Article Online

12 C.-J. Jia, L.-D. Sun, Z.-G. Yan, L.-P. You, F. Luo, X.-D. Han,Y.-C. Pang, Z. Zhang and C.-H. Yan, Angew. Chem., Int. Ed.,2005, 44, 4328–4333.

13 Z. Liu, B. L. Lv, D. Wu and Y. H. Sun, Eur. J. Inorg. Chem.,2012, 25, 4076–4081.

14 J. Y. Li, S. L. Xiong, J. Pan and Y. T. Qian, J. Phys. Chem. C,2010, 114, 9645–9650.

15 V. Barron and J. Torrent, J. Colloid Interface Sci., 1996, 177,407–410.

16 R. M. Cornell and U. Schwertman, The Iron Oxides: Structure,Properties, Reaction, Occurrences and Use, WILEY-VCH GmbH& Co. KGaA, 2nd edn, 2003, pp. 223–224.

17 X. G. Wang, W. Weiss, S. K. Shaikhutdinov, M. Ritter,M. Petersen, F. Wagner, R. Schlogl and M. Scheffler, Phys.Rev. Lett., 1998, 81, 1038–1041.

3046 | J. Mater. Chem. A, 2013, 1, 3040–3046

18 L. T. Lee and P. Somasundaran, Langmuir, 1989, 5, 854–860.

19 M. J. McGuire, J. Addai-Mensah and K. E. Brernmell, J.Colloid Interface Sci., 2006, 299, 547–555.

20 U. Aschauer, F. Jones, W. Richmond, P. Bowen, A. L. Rohl,G. M. Parkinson and H. Hofman, J. Cryst. Growth, 2008,310, 688–698.

21 B. L. Lv, Y. Xu, D. Wu and Y. H. Sun, CrystEngComm, 2011,13, 7293–7298.

22 J. M. Gong, L. Y. Wang, K. Zhao and D. D. Song, Electrochem.Commun., 2008, 10, 123–126.

23 J. F. Zhai, Y. M. Zhai, D. Wen and S. J. Dong, Electroanalysis,2009, 21, 2207–2212.

24 J. J. Xu, Y. H. Bai, H. Zhang and H. Y. Chen, J. Phys. Chem. C,2008, 112, 18984–18990.

This journal is ª The Royal Society of Chemistry 2013