Materials Letters 62 (2008) 4552–4554

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Materials Letters

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Microwave-assisted preparation of calcium sulfate nanowires

Liang Li, Ying-Jie Zhu ⁎, Ming-Guo MaState Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR ChinaGraduate School of the Chinese Academy of Sciences, PR China

⁎ Corresponding author. State Key Laboratory of HigSuperfine Microstructure, Shanghai Institute of CeramicsShanghai 200050, PR China. Tel.: +86 21 52412616; fax:

E-mail address: y.j.zhu@mail.sic.ac.cn (Y.-J. Zhu).

0167-577X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.matlet.2008.08.040

a b s t r a c t

a r t i c l e i n f o

Article history:

We have successfully dev Received 22 May 2008Accepted 22 August 2008Available online 31 August 2008

Keywords:NanomaterialsMicrostructureCalcium sulfateNanowireMicrowave

eloped a new synthetic route for the rapid preparation of calcium sulfatenanowires by thermal transformation of calcium dodecyl sulfate (CDS) in organic solvents of ethylene glycol(EG) and N,N-dimethylformamide (DMF). The products are characterized by X-ray powder diffraction (XRD)and transmission electron microscopy (TEM), and determined to be single-phase CaSO4·0.5H2O consistingof single-crystalline nanowires with aspect ratio up to about 62. In this method, the different types oforganic solvents used have no obvious influences on the morphology, phase, and formation time of theproduct. The microwave heating can remarkably shorten the reaction time compared with conventionalheating methods.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Calcium sulfate abundantly exists in nature and is a very importantindustrial material which is used as food additive, paper-makingmaterial, timbering, medical material, inorganic filler or intensifierin composites, etc. [1–3]. Furthermore, calcium sulfate is aseptic,biocompatible and biodegradable, which is an ideal substitute of bonetransplantation material [4–7], and antibiotic-carrier material [8–10].Calcium sulfate hemihydrate (CaSO4·0.5H2O) and calcium sulfatedihydrate (CaSO4·2H2O) are the most frequently used ones in plentifultypes of calcium sulfate (CaSO4·nH2O, n=0–2). In experiments andclinical uses, it is an easy and common route to obtain CaSO4·2H2O bymixing CaSO4·0.5H2O with water [10–13]. The fabrication andmorphology control of calcium sulfate nanostructures are stillchallenging. Different morphologies of nanosized calcium sulfate,such as nanoparticles [14–17], nanorods [17,18], nanowires (ornanofibers) [15–17,19], nanotubes [17,20], and nanosheets [16], havebeen prepared. Most of them were performed in w/o microemulsionor reverse micelles at room temperature [15,17–19]. Other methodshave also be used to synthesize nanosized calcium sulfate as well, forexample, Sawant et al. obtained calcium sulfate nanoparticles by usingliquid–liquid separation method, a kind of supersaturated precipita-tion method [14]. Gao et al. synthesized calcium sulfate nanotubes by

h Performance Ceramics and, Chinese Academy of Sciences,+86 21 52413122.

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solvothermal method [20]. However, preparation and morphologycontrol of calcium sulfate in organic media or at elevated tempera-tures have been less reported. To the best of our knowledge, therehas been no report on the microwave preparation of calcium sulfatenanostructures.

In this paper, we report a microwave-assisted method for thepreparation of single-crystalline calcium sulfate (CaSO4·0.5H2O)nanowires by using calcium dodecyl sulfate as the precursor.

2. Experimental details

All chemicals were purchased and used as received withoutpurification. Ethylene glycol (EG), N,N-dimethylformamide (DMF) andCaCl2 were analytical grade reagents, and sodium dodecyl sulfate(SDS) was chemical pure. Calcium dodecyl sulfate (CDS) was obtainedby mixing CaCl2 and SDS saturated aqueous solutions. The precipitatewas collected by centrifugation and washed with deionized waterseveral times, then dried in vacuum at 60 °C. In a typical experiment,285 mg CDS was dissolved in 50 mL EG. Then the solution was heatedat 100 °C by themicrowave heating undermagnetic stirring for 15min(sample 1). Sample 2 was prepared by using DMF instead of EGcompared with sample 1. Sample 3 was prepared by oil bath at 100 °Cin EG until it turned white turbid. The products were separated bycentrifugation, washed with absolute alcohol, and dried at 60 °C.

Microwave oven used was a focused single-mode microwavesynthesis system (2.45 GHz, Discover, CEM, USA). X-ray powderdiffraction (XRD) was performed on a Rigaku D/max 2550 V X-raydiffractometer with a high-intensity Cu Kα radiation (λ=1.54178 Å)and a graphite monochromator. Transmission electron microscopy(TEM) micrographs were taken on a JEOL JEM-2100F field emission

Fig. 1. Typical XRD pattern of calcium sulfate nanowires.

Fig. 3. Scheme for the formation of calcium sulfate nanowires.

4553L. Li et al. / Materials Letters 62 (2008) 4552–4554

transmission electron microscope with an accelerating voltage of200 kV.

3. Results and discussion

All as-prepared samples have the similar XRD patterns, as shown in Fig. 1. Allthe reflections can be indexed to a single-crystalline phase of CaSO4·0.5H2O with amonoclinic structure (JCPDS No. 41-0224).

The morphologies of the samples were investigated by TEM (Fig. 2). One can seethat all three samples consisted of nanowires. Selected area electron diffraction (SAED)patterns taken from different positions from an individual nanowire or differentnanowires were essentially the same. Fig. 2c shows a typical single nanowire and itscorresponding SAED pattern (inset of Fig. 2c), indicating the single-crystalline structureof CaSO4·0.5H2O nanowires. Samples 1 and 2 prepared by microwave heating but indifferent organic solvents had similar morphologies (Fig. 2a–c and d,e), indicating thatthe organic solvents of EG and DMF have little influence on themorphology of products.The as-prepared nanowires of the two samples had diameters of about 130 nm andlengths up to 8 μm. The aspect ratios of nanowires were up to about 62.

We also investigated the relationship between heating method and reaction timeby comparing samples 1 and 3. These two samples were prepared both in EG solution at

Fig. 2. TEM micrographs of calcium sulfate nanowires: (a)–(c) sample 1, the ins

100 °C with different heating methods. When microwave heating was used (sample 1),it took only 15 min for the formation of CaSO4·0.5H2O nanowires. In comparison, muchlonger time (60 h) was needed for the formation of CaSO4·0.5H2O nanowires using oilbath heating (sample 3). The results indicate that the reaction time was significantlyinfluenced by heating methods. Herein, microwave-assisted heating method is provedto be advantageous for fast preparation of materials, as previously reported [21].

In this paper, we suppose the possible reaction process in the organic solvent for theformation of CaSO4·0.5H2O nanowires, as shown in Fig. 3.

It is a chain-break decomposition reaction instead of the usual precipitationreaction for the formation of calcium sulfate. The bond between alkane group (dodecyl)and sulfate group in CDS was destroyed by thermal effect and calcium sulfate wasprecipitated in organic medium. The precursor CDS acted as both the reactant andtemplate for the formation of CaSO4·0.5H2O nanowires. The microwave heatingprovides the reaction energy to ensure the reaction take place. Microwave heating isa highly effective heating method, leading to rapid formation of CaSO4·0.5H2Onanowires.

4. Conclusions

In this paper, we have successfully developed a new syntheticroute for the rapid preparation of CaSO4·0.5H2O nanowires bymicrowave-assisted thermal transformation of the calcium dodecylsulfate. This microwave-assisted method does not need any seed,template or other reagents, thus is a simple and fast pathway for large-scale and low-cost production of calcium sulfate nanowires. If theproper sources are chosen, the method demonstrated in this papermay be extended to the synthesis of other 1D nanomaterials.

et shows corresponding SAED pattern; (d) and (e) sample 2; (f) sample 3.

4554 L. Li et al. / Materials Letters 62 (2008) 4552–4554

Acknowledgments

Financial support from the National Natural Science Foundation ofChina (50772124), the Program of Shanghai Subject Chief Scientist(07XD14031), the Key Project for Innovative Research (SCX0606) andDirector Fund of Biomaterials Research Center from Shanghai Instituteof Ceramics is gratefully acknowledged.

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