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Materials Letters 62 (20

Template formation of aluminum–magnesium hydroxidenano-branches on linear poly(ethylene imine)

Tao Xiang a, Lei Zhao a,⁎, Yuanbing Li a, Zhongxing Lei b, Shengli Jin a,Shujing Li a, Yawei Li a, Yonghe Liang a

a Hubei Provence Key Laboratory of Ceramics and Refractories, Wuhan University of Science and Technology, Wuhan 430081, PR Chinab Wugang Refractory CO., LTD., Wuhan, PR China

Received 2 June 2007; accepted 20 September 2007Available online 26 September 2007

Abstract

Linear poly(ethylene imine) (PEI) can self-assemble into fibrous aggregates composed of a crystalline core and a brush-like shell of ethyleneimine (EI) segments, and thus can be used as “templates” to prepare a range of inorganic materials. To further illustrate the feasibility of thistechnique, an investigation of low-temperature formation of nanostructured aluminum–magnesium hydroxide (AMH) on linear PEI “templates” isdescribed, and the corresponding reaction mechanism suggested tentatively. AMH nano-branches were formed after hydrolysis of aluminium andmagnesium lactates on PEI, indicating the site-selective deposition process of AMH on surfaces of PEI. The technique reported in this paper couldbe potentially used to fabricate other nanostructured complex metal oxides.© 2007 Elsevier B.V. All rights reserved.

Keywords: Nanomaterials; Microstructure; Sol–gel preparation; Electron microscopy

1. Introduction

Generation of inorganic materials with interesting nanos-tructures/morphologies using an organic compound as atemplate has attracted great interests during the past decade[1]. Inorganic materials, although exhibiting a great range ofcompositional diversity, often lack structural variety as exhibitedby supermolecular and other organic materials [2]. Although it iseasy to shape inorganic materials at a macroscopic level, it isdifficult to tailor them at a microscopic level.

Natural bio-silicas such as diatoms and sponges are known tohave beautiful complex morphologies ranging in size fromnanometers to micrometers [3,4]. Such intricate hierarchicalstructures grow at neutral pH and ambient temperature.Researches revealed that amine-containing biomolecules andsynthetic polyamine-based molecules could lead to depositionof silica under ambient conditions [5–16]. Nevertheless, sincemost polyamines have poor pre-organizing ability, they are not

⁎ Corresponding author. Tel.: +86 27 68862833; fax: +86 27 68862018.E-mail address: leizhao69@yahoo.com.cn (L. Zhao).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.09.042

so effective in assisting deposition of silica with preciselycontrolled multiple-shapes. To overcome this, a biomimeticapproach using poly(ethylene imine) (PEI) with a linearbackbone has been developed [17–19]. PEI with a linearbackbone can self-assemble into fibrous aggregates in water orwater-based solvents because of its strong crystallizationtendency in aqueous media. One of the interesting aspects ofsuch PEI aggregates is that they can assist deposition of silicananofiber via silicification of tetramethoxysilane under ambientconditions. The possible reason for this might be that PEI canact simultaneously as a template and a catalyst in the depositionprocess of silica.

Scheme 1. Schematic of synthesis process and molecular structure of linear PEI.

Fig. 1. FTIR spectrum of synthesized powder.

Fig. 3. a) Low-magnification and b) high-magnification HRTEM images ofsynthesized powder.

1628 T. Xiang et al. / Materials Letters 62 (2008) 1627–1629

In this work, a biomimetic approach to nanostructured AMHusing linear PEI was investigated and the correspondingmechanism discussed. AMH is a promising precursor for lowtemperature synthesis of magnesium aluminate spinel(MgAl2O4), a high melting material extensively used inrefractories and engineering ceramics industries owing to itsmany excellent properties [20–22]. In addition, another beautyof the present technique is that the functional moieties can beeasily entrapped in AMH sol [23], making it more attractive formany applications.

2. Experimental

Reagent-grade poly(2-ethyl-2-oxazoline) (MW=500,000,MW/Mn=3.1; MW: weight-average molecular weight, Mn:number-average molecular weight), HCl solution, methanol,aluminium lactate and magnesium lactate were used as the mainstarting materials. They were used as received (unless otherwisestated).

Poly(ethylene imine) was prepared by hydrolyzing appro-priate amounts of poly(2-ethyl-2-oxazoline) in 5 M HClsolution [23]. This process, along with the molecular structureof PEI, is shown schematically in Scheme 1.

After 5.0 wt.% PEI methanol solution had been prepared,2.0 wt.% aluminium lactate and magnesium lactate water

Fig. 2. a) High-magnification SEM image of synthesiz

solutions were added respectively to it in volume ratios of 1:1:1,and the mixed solution was stirred for 9 h. AMH precipitatedfrom the solution was washed three times with ethanol andseparated by centrifuge (9000 rpm for 5 min). The resultingpowder was oven-dried for 12 h at 40 °C prior to furthercharacterization.

Microstructural morphologies of synthesized powders wereexamined by a scanning electron microscopy (SEM, XL-30w/

ed powder and b) EDX of branch-shaped phases.

1629T. Xiang et al. / Materials Letters 62 (2008) 1627–1629

TMP) and a high-resolution transmission electron microscopy(HRTEM) (FEI, Tecnai G20). Fourier transform infrared (FTIR)spectra were recorded on a Spectrum One FTIR spectrometer(Perkin-Elmer Cetus Instruments, Norwalk, CT). Energydispersive X-ray (EDX) analysis was also conducted to assistidentification of phases formed in the microstructures.

3. Results and discussion

3.1. FTIR analysis

FTIR spectrum shown in Fig. 1 shows that the main absorptionbands match well with the standard absorption bands of AlOOH andMg(OH)2. The absorption band around ∼3393 cm−1 was attributed to–OH, whereas the bands at 1626, 1405, 1056 and 785 cm−1 and thoseat 734 and 451 cm−1 were attributed to Al–O and Mg–O, respectively.These results indicated that AlOOH and Mg(OH)2 were the mainphases in the synthesized AMH powder.

3.2. Microstructure of synthesized powder

SEM (Fig. 2a), along with EDX (Fig. 2b), shows a homogeneousmicrostructure and the presence of branch-shaped phases containingAl, Mg, O and C. HRTEM (Fig. 3) further reveals that branch-shapedphases were composed of polymer “cores” and AMH “shells”,indicating the site-selective deposition of AMH on the surfaces ofPEI “templates”.

3.3. Formation mechanism

PEI can self-assemble into fibrous aggregates composed of acrystalline core and a brush-like shell of ethylene imine (EI) segments.The free EI (–CH2NCH2–) units (amorphous shell) present on surfacesof PEI branches act as templates, assisting site-selective deposition ofAMH. Aluminium lactate and magnesium lactate hydrolyze rapidlyaccording to reactions (1) and (2) due to the catalytic affect of the –NHunits.

AlðCH3CHOHCOOÞ3 þ 2H2O→AlOðOHÞ þ 3CH3CHOHCOOH ð1Þ

MgðCH3CHOHCOOÞ2 þ 2H2O→MgðOHÞ2 þ 2CH3CHOHCOOH ð2Þ

4. Conclusions

Nano-shaped AMH was prepared under ambient conditionsvia hydrolysis of aluminium lactate and magnesium lactate onself-assembled PEI “templates”. The biomineralization of AMH

have been taken place at the free EI (–CH2NCH2–) units whichon surfaces of PEI aggregates, so the morphologies of theformed AMH have been retained the one of PEI. From theperspective of nanofabrication and application, the templateformation technique illustrated in this work is of greatsignificance, since it not only can be used to preparenanostructured AMH under ambient conditions but also canbe used to assist shape- and structure-tailoring of AMH.

Acknowledgements

The authors would like to thank the Nature ScienceFoundation of Hubei province and the Nature ScienceFoundation of Wuhan University of Science and Technologyfor their financial support.

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