nd

ihem

ctahgan. Thn tre coe peals

1. Introduction

tructureir potential applications in ca-wasteand naructures remoplatesg holloemplosed insextern

dened hollow structures.

2. Experimental

sium permanganate (KMnO4),formamide and ethanol were all pur-

Powder X-ray diffraction (XRD) patterns were recorded on a

Materials Letters 82 (2012) 237239

Contents lists available at SciVerse ScienceDirect

Materials

j ourna l homepage: www.e lsMn3O4 has been investigated due to its interesting properties anda wide variety of potential applications as a multifunctional material.In this contribution, we report a simple one-pot methods of hollowoctahedrons of Mn3O4 by hydrothermal treatment of KMnO4 in theformamide solution through chemically etching with an intermediatecrystal-templating process. Solid octahedrons are employed as sacri-cial templates. To the best of our knowledge, this is the rst time

PANalytical X'pert PRO diffractometer with Cu K radiation(=1.5425 nm),operating at 40 kV and 40 mA. The morphologiesand size of the products were observed by SEM using JEOL JSM-7500F scanning electronmicroscope and TEMusing a JEOL JEM-2100F.

3. Results and discussionthat Mn3O4 hollow octahedron was synthesiprocess.

Corresponding author. Tel./fax: +86 951 2062004.E-mail address: yang_tl@nxu.edu.cn (T. Yang).

0167-577X/$ see front matter 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.05.070tained. So it is desirablemorphologies of well-

ionized water and alcohol to remove any possible residual reactants.The product was dried in air at ambient condition.low structures, unique properties can be obto develop easy methods to control thely studied and reported because of thtalysis, photonic crystals, light lter,and microvessels for drug deliveryCommon routes to fabricate hollow sta shell of designed materials on variouplates including hard and solid temtemplate-free methods for generatinnanostructures have been developedincluding the nanoscale corrosion-baKirkendall effect [10]. By treating theremoval, energy storagenoscale reactors [16].s involve the growth ofvable or sacricial tem-[79]. Lots of one-potw inorganic micro-andying novel mechanisms,ide-out evacuation andal morphologies of hol-

chased from Sinopharm Chemical Reagent Co. Ltd without furtherpurication. Deionized water further puried by distillation wasused. In a typical procedure, 158 mg of KMnO4 was dissolved in a so-lution of 5 ml of formamide and 10 ml of deionized water under ul-trasonic oscillation. The resultant solution was then transferred andsealed in a 20 ml Teon-lined autoclave, heated at 140 C for 12 h.After the resultant had been maintained at room temperature for acertain period of time, the precipitate were collected through centri-fugation at 3800 rpm for 20 min and washed several times with de-Recently, hollow micro- and nanos es have been extensive- All the reagents used in this study were of analytical grade. Potas-Synthesis of Mn3O4 hollow octahedrons a

Wenhua Wang, Tianlin Yang , Genjiao Yan, Hanyu LKey Laboratory of Energy Resources and Chemical Engineering, College of Chemistry and C

a b s t r a c ta r t i c l e i n f o

Article history:Received 17 April 2012Accepted 20 May 2012Available online 25 May 2012

Keywords:HollowOctahedronHydrothermalEtchingCrystal growthSpectroscopy

Tetragonal Mn3O4 hollow obetween Potassium Permanextra surfactant or templatecroscopy and high-resolutioof the nanostructures. Somformation mechanism of thevolution of the sample revical etching process.zed by a facile chemical

l rights reserved.their possible growth mechanism

ical Engineering, Ningxia University, Yinchuan, 750021, PR China

edrons have been successfully synthesized via a facile hydrothermal reactionate and formamide in distilled water at 140 C for 12 h without the use of anye X-ray diffraction, scanning electron microscopy, transmission electron mi-ansmission electron microscopy have been provided for the characterizationsmparative experiments were carried out at different conditions to study theroducts which has proposed a possible growth mechanism. The morphologythat the hollow octahedrons are formed by the solid octahedron via a chem-

2012 Elsevier B.V. All rights reserved.

Letters

ev ie r .com/ locate /mat le tFig. 1 shows the XRD patterns of the resultant hollow structures.All the diffraction peaks can be readily indexed to the tetragonalphase of Mn3O4 with lattice constants a=b=5.7620 andc=9.4390 (JCPDS 751560), indicating the high purity andcrystallinity.

The morphologies of the products were observed using SEM andTEM. Fig. 2 shows typical SEM images of the Mn3O4 octahedral hollownanostructures. From SEM observations, it can be seen that the as-

corresponding solid octahedrons [12]. At last, nearly all hollow octa-hedrons converge to square rings.

Such feature can be both of thermodynamic and kinetic reasons.The thermodynamic reason can be the strong (and orientation-dependent) adsorption of the reaction products and/or the formationof amorphous surface layers. It has been recently demonstrated thatthe surface and grain boundary adsorption layers in oxides can beas thick as 3 monolayers on free surfaces and up to 10 in grain bound-

10 20 30 40 50 60 70 80

Inte

nsity

(a. u

.)

( 101)

(112) (2

00)

(103)

(211)

(004) (22

0)

(105)

(312)

(303)

(321)

(224)

(314)

(305)

(413)

(332) (42

2)(40

4)

(204)

238 W. Wang et al. / Materials Letters 82 (2012) 237239prepared Mn3O4 hollow octahedrons well inherit the uniform dimen-sions of solid octahedrons with two symmetrical hollow square edgelength of around 200 nm and the surface is relatively smooth. Thewidth of the hollow square is a one-third of the octahedron and halfof the new surface of hollow square. The collapse of these octahe-drons may be caused by rapid mass transport across the walls duringfast dissolution of the Mn3O4 template.

The morphology of Mn3O4 octahedral hollow nanostructures hasalso been further studied by transmission electron microscopy(TEM). Typical TEM images of as-prepared Mn3O4 octahedral hollownanostructures are shown in Fig. 3. A high uniformity of the octahe-drons can be seen from the image, and the inner cavity is clearly re-vealed by the different dark and light contrast between Mn3O4shells and hollow interiors. The wall of the octahedrons is as thin asabout 50 nm in agreement with the above SEM ndings. Fig. 3c isthe corresponding HRTEM image, displaying resolved lattice fringesof (211) planes (d=0.25 nm).

The formation process may be explained as follows: once HCONH2was added into KMnO4 aqueous solution, Mn3O4 nuclei formed im-mediately, which acted as the precursor. The chemical reaction canbe formulated as [11]:

MnO4 HCONH2 H2OMn3O4 CO2 NH4 OH:

The structure formation of the Mn3O4 hollow octahedrons issummarized in Scheme 1. It reveals clearly that the hollow octahe-dron is formed by chemically etching the solid octahedron. Firstly,a solid-structured octahedron which has a solid top angle could be

2/degree

Fig. 1. X-ray diffraction spectra of the as-synthesized hollow octahedrons of Mn3O4.obtained as described in detail in Scheme 1. Then the two top anglesof the octahedron disappear and two symmetrical smooth surfacesappear. As the reaction is going on, the center of the smooth endof the new surfaces appears as a coarse and regular square surface.

Fig. 2. SEM images of Mn3O4 hollow octaWith continuous etching for a longer time, nally, these corrodedoctahedrons eventually transform into the hollow octahedrons with-out two top angles along the two symmetrical coarse square simul-taneously. The etching should start from top angle sites where thereactivity is enhanced as a result of sharp surface curvature. TheMn3O4 octahedrons are formed rst due to the anisotropic growthdriven by chemical potential under hydrothermal condition. Howev-er, it may be not energetically favorable to form solid octahedronsdue to their large top areas of polar metastable surfaces. Instead,the formation of hollow octahedrons without top angles can reducethe top metastable areas and enlarge the lateral areas of the moststable low-index nonpolar surfaces with respect to those of the

Fig. 3. TEM images of Mn3O4 hollow octahedrons (a, b) and the magnied HRTEM isgiven in (c).aries [13]. The amorphous layer also easily forms on the crystallineoxide seeds during wet chemical processes [14]. Of course it is just apossible formation mechanism of the hollow octahedron, the realmechanism still needs further research.

hedrons at different magnications.

4. Conclusions

In summary, tetragonal Mn3O4 hollow octahedrons of manganeseoxide have been synthesized by a hydrothermal procedure betweenKMnO4 and formamide in distilled water without the use of anyextra surfactant or template for the rst time. Various measurementswere used to characterize the structure, morphology of the resultantproducts. The possible formation mechanism of the as-synthesizedhollow Mn3O4 octahedrons were proposed. The evolution of mor-phology shows the clear evidence that the hollow octahedrons areformed by solid octahedron involving in chemical etching process.

References

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[10] Xiong Y, Wiley B, Chen J, et al. Angew Chem Int Ed 2005;44:79137.[11] Ni JP, Lu WC, Zhang LM. J Phys Chem C 2009;113:5460.[12] Luo J, Zhu HT, Fan HM. J Phys Chem C 2008;112:125948.[13] Straumal B, Baretzky B, Mazilkin A, et al. J Eur Ceram Soc 2009;29:196370.[14] Straumal BB, Protasova SG, Mazilkin AA, et al. Mater Lett 2012;71:214.

Scheme 1. Schematic illustration of the synthesis of Mn3O4 hollow octahedrons.

239W. Wang et al. / Materials Letters 82 (2012) 237239Acknowledgments

We are grateful to the NSFC (grants 20962016) and the Key Re-search Project of Education Ministry of China (no. 208159) for nan-cial support.

Synthesis of Mn3O4 hollow octahedrons and their possible growth mechanism1. Introduction2. Experimental3. Results and discussion4. ConclusionsAcknowledgmentsReferences