Facile hard template approach for synthetic hectoritehollow microspheres

Sandesh Y. Sawant a, Radheshyam R. Pawar a,b, Rajesh S. Somani a,b,n, Hari C. Bajaj a,b,nQ1

a Discipline of Inorganic Materials and Catalysis, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR),G.B. Marg, Bhavnagar 364002, Gujarat, Indiab Academy of Scientific and Innovative Research (AcSIR), Council of Scientific and Industrial Research (CSIR), Anusandhan Bhawan, New Delhi 110001, India

a r t i c l e i n f o

Article history:Received 23 January 2014Accepted 11 April 2014

Keywords:ClayCarbon materialsHollow spherePorous materialsDelivery capsule

a b s t r a c t

A facile route for the synthesis of synthetic hectorite hollow microspheres (SHHMS) having a diameter of4–6 mm and wall thickness of 30–50 nm has been demonstrated. Carbon spheres obtained from theautogenic pyrolysis of polypropylene have been utilized as a template for the synthesis of SHHMS. Theformation of the hectorite architecture was confirmed by X-ray diffraction and infrared spectroscopy.The scanning and transmission electron microscopic studies reveal their spherical and hollow nature.The morphology of the obtained product was dependent on the amount of carbon spheres template. Theobtained SHHMS showed the mesoporous textural property with enhanced carbon dioxide adsorptioncapacity (19.2 cm3/g) as compared to that of calcined hectorite (15.3 cm3/g) at 303 K.

& 2014 Published by Elsevier B.V.

1. Introduction

Since past decade, the hollow spheres structure of inorganicmaterials have attracted considerable attention of research com-munity due to their low density and higher surface area comparedto the bulk material [1]. These properties of hollow spheres makeit a potential candidate in the field of controlled drug delivery,cosmetic, filler, catalysis and adsorption [2,3]. Different metal,metal oxides, ceramic, carbon and silicates hollow spheres havebeen synthesized using different methodologies due to theirpotential application in various fields [4,5].

Template mediated synthesis is one of the well-studied and easymethodology for the preparation of hollow structures with desiredshape and size [3]. Recently we have demonstrated the use ofcarbon spheres as template for the synthesis of CuO hollow micro-spheres [6]. Synthetic clay possessing tailored pore structure,unique swelling, intercalation and ion exchange properties, is aversatile and low cost adsorbent [7–9]. The first synthesis ofLaponite hollow microspheres is reported by Caruso et al. [10]using layer by layer deposition on polystyrene template. Thereafter,Bourlinos et al. [11] and Muthusamy et al. [12] synthesized clay

hollow microspheres using spherical anion exchange resin andemulsion technique, respectively. The spray-drying technique isalso an alternative pathway for the synthesis of clay hollow micro-sphere [13]. In this paper, we have demonstrated a simple andconvenient route for the synthesis of synthetic hectorite hollowmicrospheres (SHHMS) using carbon spheres as hard template.

2. Experimental

The carbon spheres utilized as the template for SHHMS wassynthesized by the reported method [6]. The synthesis of hectoritewas carried out with the modification in the reported methodology[14] with reactants in the molar ratios of LiF:MgO:SiO2¼0.266:1.00:1.52. In the first step, the carbon spheres were coated with therequired amount of Mg(OH)2. For this, carbon spheres (0.5 g) weredipped in 10 mL of 1:9 (V/V) methanol–water mixtures and ultra-sonicated for 30 min. The precipitation of magnesium hydroxidewas carried out using MgCl2 �6H2O (0.325 g) and 15 mL of ammoniasolution followed by aging at 80 1C for 3 h. The obtained Mg(OH)2carbon sphere composite was centrifuged and further used for thesynthesis of hectorite without drying. The synthesis of hectorite wascarried out under reflux condition (120 1C for 48 h without stirring)using Mg(OH)2 carbon spheres composite, distilled water (10 mL),LiF (0.0102 g) and Ludox HS-40, a Naþ-stabilized 40% silica sol(DuPont) (Sigma-Aldrich, USA) (0.37 g) as silica and sodium source.Filtration and sufficient water washing followed by drying at 100 1Cfor 8 h resulted into the hectorite coated carbon spheres (SH@CS).

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.04.0670167-577X/& 2014 Published by Elsevier B.V.

n CorrespondingQ2 authors at: Discipline of Inorganic Materials and Catalysis,Central Salt & Marine Chemicals Research Institute, Council of Scientific & IndustrialResearch (CSIR), G.B. Marg, Bhavnagar 364002, Gujarat, India.Tel.: þ91 278 2471793; fax: þ91 278 2567562.

E-mail addresses: rssomani@csmcri.org (R.S. Somani),hcbajaj@csmcri.org (H.C. Bajaj).

Please cite this article as: Sawant SY, et al. Facile hard template approach for synthetic hectorite hollow microspheres. Mater Lett(2014), http://dx.doi.org/10.1016/j.matlet.2014.04.067i

Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎

SHHMS was obtained by the calcination of SH@CS at 500 1C for 2 hat the heating rate of 1 1C/min. The content of carbon spheres wasvaried from 0.5 to 2.0 g and the obtained SHHMS was denoted asSHHMS_0.5 in which the last digits indicates the amount of carbonspheres used.

3. Results and discussion

During the synthesis of SHHMS, each step of the preparationwas monitored by the XRD analysis. In the first step of thesynthesis, the carbon spheres were uniformly coated with mag-nesium hydroxide using the precipitation method. The XRDpattern of Mg(OH)2@CS_0.5 (Fig. 1a) clearly shows the character-istic peaks of magnesium hydroxide at 2θ¼19.51 (001), 37.91 (101),50.51 (102), 58.61 (110) and 62.11 (111) which is also in supportwith the observations made by Wang et al. [15]. The broaddiffraction peak observed at 2θ of 25.31 is the characteristic peakfor the carbon materials due to 002 plane of graphitic arrangement[16]. The characteristic XRD pattern of the SH@CS composite(Fig. 1b) at 2θ¼19.61 (110, 020), 28.3 (004), 35.11 (130, 200),52.91 (150, 240, 310) and 61.01 (060, 330) confirmed the formationof the clay shell on the carbon sphere surface. The calcination ofSH@CS composite completely removes the carbon sphere templatewithout significantly affecting the hectorite architecture (Fig. 1c)and resulting in to the SHHMS. The chemical composition ofSHHMS based on ICP analysis is 1.3% Li2O, 2.7% Na2O, 28.1% MgOand 63.1% SiO2 which corresponds to the ideal hectorite composi-tion: Ex0.66 [Li0.66Mg5.34Si8O20(OH)4], where Ex¼exchangeablemonocation.

The hectorite product obtained using 0.5 g of carbon spheres astemplate shows the formation of hectorite particle with sphericalcages (Fig. S1a) after calcination at 500 1C. Moreover less amountof carbon spheres used as template also resulted into the forma-tion of hectorite particles with spherical cages (Fig. S1a). The XRD

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Fig. 1. XRD pattern of (a) Mg(OH)2@CS_0.5, (b) SH@CS_0.5, (c) SHHMS_0.5,(d) SHHMS_1.0, (e) SHHMS_1.5 and (f) SHHMS_2.0.

Fig. 2. SEM image of (a) SHHMS_1.5, (b) SHHMS_2.0 and TEM images (c, d) of SHHMS_2.0 showing hollow microspheres. Electron diffraction pattern of SHHMS_2.0 is shownin the inset.

S.Y. Sawant et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

Please cite this article as: Sawant SY, et al. Facile hard template approach for synthetic hectorite hollow microspheres. Mater Lett(2014), http://dx.doi.org/10.1016/j.matlet.2014.04.067i

patterns of the obtained products (Fig. 1d, e and f) with varyingamount of carbon spheres template (1.0, 1.5 and 2.0 g) alsoconfirmed the formation of hectorite phase. The SEM images ofsynthetic hectorite (Figs. S1b, 2a and b) clearly depict the effect ofamount of carbon spheres template on its morphology. Withincreasing amount of carbon spheres (from 0.5 to 2.0 g) theformation of hectorite particles (Fig. S1b) results in the formationof hectorite hollow microspheres (Fig. 2a). Finally the use of 2.0 gof carbon spheres (i.e. 10% of hectorite formation on the basis ofhectorite yield) resulted in uniform SHHMS (Fig. 2b). The obtainedproduct also contains the semi-spherical SHHMS due to thebreaking of the spheres (Fig. 2b). The breakage of the spheresmainly observed in case of the relatively larger SHHMS might bedue to their lesser strength.

The TEM analysis (Fig. 2c and d) of SHHMS_2.0 also confirmedthe hollow structure of the obtained hectorite spheres. Theobtained hectorite hollow spheres also contain the hectoritesheets (Fig. 2c), formed side wise or due to the breakage in thehectorite spheres. Fig. 2d shows the TEM image of the individualSHHMS obtained using 2.0 g of carbon spheres as template. Theobtained SHHMS_2.0 (Fig. 2d) was constructed with a very thin(few nanometers) hectorite sheet and also present in the sur-rounding of the spheres. The TEM analysis (Fig. 2d) shows that theobtained SHHMS possesses the diameter of 4–6 mm and wallthickness of 30–50 nm whereas the electron diffraction pattern(inset of Fig. 2d) depicts its polycrystalline/ amorphous nature.

The FT-IR spectrum of the SHHMS_2.0 (Fig. 3a) clearly showsthe Si–O stretching and in-plane bending band at 1028 and468 cm�1 respectively. The bands observed at 3440 and1635 cm�1 were associated with the H–OH stretching and bend-ing vibrations, respectively. The OH stretching vibrations of theMg3(OH) unit shows absorption at 3675 cm�1 whereas the Mg–Obending vibrations gave a low intensity band at 668 cm�1. Thethermal gravimetric analysis (TGA) of hectorite coated carbonspheres (SH@CS_2.0) under oxygen atmosphere (Fig. 3b) showsthe distinct peak in the region of 450–550 1C due to the removal ofcarbon sphere template. The thermal analysis of the SHHMScarried out by differential scanning calorimetry (DSC) and TGA(Fig. S2) showed its stable nature under inert atmosphere withsingle endothermic peak at�100 1C in DSC and no significantweight loss (� 2% up to 600 1C) in TGA, and is in accordance withthe previous literature [8,9]. The band gap of the SHHMS, calcu-lated using diffuse reflectance spectroscopy, was 4.3 eV which is

close to the theoretical value of 4.0 eV indicating its non-conducting nature. The nitrogen sorption studies of all obtainedproducts (Fig. S3) showed the characteristic adsorption isothermof Type IV classified by IUPAC, indicating the mesoporous nature ofSHHMS. The hysteresis loop observed for desorption is associatedwith the filling and emptying of the mesopores by capillarycondensation [17]. The detail textural properties of the SHHMSare given in Table 1. The increase in the surface area was observedwhen the morphology of the synthetic hectorite changesfrom bulk particles to hollow spheres. The SHHMS_2.0 showsthe enhancement in the carbon dioxide adsorption capacity(19.2 cm3/g) (Fig. S4) as compared with the calcined synthetichectorite (15.3 cm3/g) at 303 K [7]. The present SHHMS showedthe mesoporous properties with hollow interior; hence it maypossess the potential application as delivery capsule.

4. Conclusion

A facile hard template approach for the synthesis of SHHMS hasbeen demonstrated. The amount of carbon spheres as templatewith respect to hectorite plays a crucial role in the morphologicalsynthesis of SHHMS. The XRD, FTIR and electron microscopic

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Fig. 3. (a) FT-IR spectrum of the SHHMS_2.0 and TGA spectrum of SH@CS_2.0 under oxygen atmosphere.

Table 1Textural properties of SHHMS prepared using different amount of carbon spherestemplate.

Sample Specific surface area (m2/g) Pore volume(cm3/g)

Pore size (nm)

SBETa SLang

b Smicroc Sext

d VTe Vmicro

f DBETg DBJH

h

SHHMS_0.5 265 330 35 230 0.36 0.01 5.4 6.5SHHMS_1.0 273 350 37 236 0.35 0.02 5.1 6.9SHHMS_1.5 305 380 39 266 0.37 0.02 4.9 6.2SHHMS_2.0 299 368 38 261 0.37 0.02 4.8 6.0

a BET surface area.b Langmuir surface area.c Micropore surface area, calculated using the t-plot method.d External surface area, calculated using the t-plot method.e Single point adsorption total pore volume, obtained at P/P0¼0.9732.f t-plot micropore volume.g Adsorption average pore diameter, obtained from 4 V/A by BET.h BJH desorption average pore width (4 V/A).

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Please cite this article as: Sawant SY, et al. Facile hard template approach for synthetic hectorite hollow microspheres. Mater Lett(2014), http://dx.doi.org/10.1016/j.matlet.2014.04.067i

analysis confirmed the formation of spherical hollow hectoritemicrospheres with diameters of 4–6 mm and wall thickness of30–50 nm. The gas sorption (nitrogen at 77 K and carbon dioxideat 303 K) study reveals their mesoporous nature with enhance-ment in the adsorption capacity. The obtained SHHMS may havepotential application as delivery capsule due to its mesoporoustexture and hollow interior.

Acknowledgment

The authors areQ3 grateful to Council of Scientific and IndustrialResearch (CSIR), New Delhi, India for financial support underNetwork project CSC-0135. Sandesh Sawant and RadheshyamPawar acknowledge CSIR, New Delhi for the award of seniorresearch fellowship.

Appendix A. Supplementary material

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.matlet.2014.04.067.

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Please cite this article as: Sawant SY, et al. Facile hard template approach for synthetic hectorite hollow microspheres. Mater Lett(2014), http://dx.doi.org/10.1016/j.matlet.2014.04.067i