Materials Research Bulletin 44 (2009) 45–50

Controllable synthesis of hierarchical nanostructures of CaWO4 andSrWO4 via a facile low-temperature route

Z. Chen, Q. Gong, J. Zhu, Y.P. Yuan, L.W. Qian, X.F. Qian *

School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China

A R T I C L E I N F O

Article history:

Received 9 January 2008

Received in revised form 3 April 2008

Accepted 9 April 2008

Available online 22 April 2008

Keywords:

A. Nanostructures

B. Chemical synthesis

A B S T R A C T

CaWO4 and SrWO4 nanostructures have been synthesized via a simple microemulsion-mediated route.

With careful control of the fundamental experimental parameters including the concentration of

reactants, the reaction time and the temperature, the products with different morphologies of dumbbell,

coral, rod and dendrite have been obtained, respectively. The possible formation mechanism of these

unique morphologies has been proposed based on surfactant self-assembly under different experimental

conditions. The as-synthesized CaWO4 samples with various morphologies exhibit different photo-

luminescence properties. X-ray powder diffraction, transmission electron microscopy, field-emission

scanning electron microscopy, and luminescence spectroscopy were used to characterize these products.

Crown Copyright � 2008 Published by Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

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1. Introduction

Inorganic nanomaterials of uniform size and shape are ofspecial interest from both theoretical and practical perspectives[1–3]. To systematically manipulate the shapes of inorganiccompounds would greatly benefit various fields, including optics,electronics, catalysis, and medicine [4–6]. The existing methods forsize and shape control generally employ capping agents, e.g.surfactants, ligands, polymers or dendrimers, to confine the crystalgrowth in the nanometer regime [7–9].

In recent years, calcium and strontium tungstates from thescheelite family remain centre of attraction for crystal growers,radiologists, material scientists and physicists due to theirluminescence, thermoluminescence and stimulated Raman scat-tering behavior [10], and have potential application in the field ofphotonics and optoelectronics. For example, calcium tungstate isone of the most widely used phosphors in industrial radiology andmedical diagnosis [11], and it can be employed for a variety ofapplications, e.g. tunable fluorescence and sensor for dark mattersearch [12,13]. Although nanosized CaWO4 and SrWO4 withvarious morphologies have been prepared, such as nanoparticles[14], nanofilms [15], nanopeanuts [16], and bipolar petal [17], tothe best of our knowledge, few works has been reported on thesynthesis of CaWO4 with dumbbell, sandglass or coral shape untilnow.

* Corresponding author. Tel.: +86 21 54743262; fax: +86 21 54741297.

E-mail address: xfqian@sjtu.edu.cn (X.F. Qian).

0025-5408/$ – see front matter . Crown Copyright � 2008 Published by Elsevier Ltd. A

doi:10.1016/j.materresbull.2008.04.008

In this paper, we described the controllable synthesis of CaWO4

and SrWO4 nanomaterials through the reverse micelle/micro-emulsion system, in which the morphologies and the organizedparticles sizes can be readily tuned by adjusting experimentalparameters. The photoluminescence performance of the self-organized crystals was also studied.

2. Experimental

Analytical-grade chemicals cetyltrimethylammonium bromide(CTAB), Na2WO4�2H2O, CaCl2 and SrCl2�6H2O were purchased fromShanghai Chemical Industrial Corporation and used withoutfurther purification. The reaction was carried out in a 50 mL glassjar at a designated temperature.

The synthesis of CaWO4 or SrWO4 nanocrystals was simplyachieved via a microemulsion-mediated route. The detailedreaction conditions and the obtained products are given inTable 1. In a typical procedure, two microemulsion solutionswere prepared by adding 2 mL of 0.2 M CaCl2 and 2 mL of 0.2 MNa2WO4 aqueous solutions to n-octane/CTAB/n-butanol systems(the molar ratio of n-butanol/CTAB is 4.15 M, the molar ratio ofH2O/CTAB is 33 M, and [CTAB] is 0.2 M), respectively, then theywere stirred for 30 min until they became transparent. After that,the two obtained solutions were mixed rapidly and then were setaside at 10 8C for certain time. White products were separated bycentrifugation, washed with deionized water and absolute ethanolseveral times, and then dried at 60 8C for 6 h.

The as-prepared products were characterized by powder X-raydiffraction (XRD, Shimadzu XRD-6000 with Cu Ka radiation,

ll rights reserved.

Table 1Detailed reaction conditions and the obtained products

Sample Formulation detected by XRD Ca (mol/L) T (8C) Time Morphologies

1 CaWO4 0.2 10 5 min Nanoparticles

2 CaWO4 0.2 10 6 h Dumbbell-like nanocrystals

3 CaWO4 0.5 10 6 h Coral-like nanocrystals

4 CaWO4 0.2 25 6 h Nanorods

5 SrWO4 0.2 10 1 h Dendritic nanocrystals

6 SrWO4 0.2 25 1 h Dendritic nanocrystals

a C is the initial concentration of Ca2+ and WO42� ions in the microemulsions.

Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–5046

l = 1.5406 A), field-emission scanning electron microscopy(FESEM, FEI SIRION 200), transmission electron microscopy(TEM, JEOL 2010), and luminescence spectroscopy (PerkinElmerLS50B luminescence spectrometer).

3. Results and discussion

3.1. Detailed structural characterization of the CaWO4 superstructure

with different morphologies

CaWO4 nanocrystals were obtained by the reaction betweenCa2+ and WO4

2� ions solubilized in a cationic surfactant-CTAB-microemulsion system. In Fig. 1a, the XRD pattern clearly shows

Fig. 1. (a) XRD patterns of samples 1–4 (as described in Table 1) and (b) XRD

patterns of SrWO4 (sample 5).

that all the diffraction peaks can be indexed as a pure tetragonalscheelite structure with cell parameters of a = 5.24 A andc = 11.25 A (JCPDS Card No. 77-2233), which confirms that theas-prepared product is CaWO4.

Other samples prepared under different conditions have similarXRD patterns. The peaks of pattern 1 are obviously wider thanthose of the other three patterns implies the smaller size of theobtained nanoparticles (sample 1).

The typical morphology and structure of CaWO4 were furthercharacterized by transmission electron microscopy (TEM) andscanning electron microscopy (FESEM). Fig. 2b–d shows theobtained CaWO4 nanocrystals were in dumbbell-like 3D archi-tectures with highly self-assembled hierarchical and repetitivesuperstructures, rather than the disordered arrangementsreported by the previous studies [14,18]. We can also find thatonly nanoparticles with mean diameters of 20–30 nm wereobtained when the reaction was carried for 5 min at 10 8C inFig. 2a. Though the HRTEM image (Fig. 2e) is not very clear, carefulobservation still reveals that the interplanar spacing of nano-particles is 0.312 nm, corresponding to the (1 1 2) lattice spacingof CaWO4. And the SAED image (inset of e) also confirms thesingle-crystalline nature. As the reaction time was prolonged to6 h, dumbbell-like superstructures of CaWO4 crystals wereobtained. A high-magnification FESEM image clearly reveals thatthe dumbbell superstructure of CaWO4 is about 1.5–2 mm inlength, and the top of the dumbbell is like a cauliflower with about1 mm in diameter (Fig. 2d). These dumbbells are constructed fromthe nanoparticles, in which these highly oriented nanoparticlesconnect with each other to form architectures with recognizableboundaries. The spacing between adjacent lattice fringes alongorientation direction is 0.285 nm, corresponding to the (0 0 4)lattice spacing of CaWO4 (Fig. 2f). The adjacent particles have thesame lattice fringe (d = 0.285 nm) along the same direction,confirming that they have the same orientation. This structure issimilar to the mesocrystal structure.

Reverse micelles or microemulsion systems have been widelyused as ideal media to prepare nanoparticles [19–21]. Water-in-oil(w/o) microemulsion is a transparent and isotropic liquid mediumwith nanosized water pools dispersed in a continuous phase andstabilized by surfactant and co-surfactant molecules at the water/oil interface. These water pools offer ideal microreactors for theformation of nanoparticles. Compared to other synthetic methods,the microemulsion approach has distinct advantages in effectivecontrol over the size and shape of the particles, such as relativelylow reaction temperature, less time consumption, high homo-geneity, and well-crystallized products with definite composition.A schematic diagram of the proposed growth process is shown inFig. 3. When the microemulsion solutions containing Ca2+ andWO4

2�, respectively, were mixed, CaWO4 nucleation and micellarfusion may be concomitant. And nanoparticles with surfactantcoatings are formed in a very short time. The obtainednanoparticles can be regarded as ‘‘spherical core–shell nanopar-ticles’’ with an inorganic core and an organic surfactant shell. Thenthe dumbbell superstructures of CaWO4 were formed via a self-

Fig. 2. Images of CaWO4 nanocrystals formed at 10 8C, with [Ca2+] = [WO42�] = 0.2 M. (a) TEM images of the product formed within 5 min; (b–d) TEM and FESEM images of the

product formed for 6 h; (e) HRTEM image of the product formed within 5 min; inset shows the corresponding ED patterns; (f) HRTEM image of the product formed for 6 h.

Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–50 47

assembly process through the surfactant interactions between thebuilding blocks. At last, prolonging the time leads the products toexperience an Ostwald ripening process.

For comparison, the effect of the initial concentration of Ca2+

and WO42� ions in the microemulsions was examined. When the

concentration of Ca2+ and WO42� ions was increased from 0.2 to

0.5 M, coral-like CaWO4 aggregation was obtained instead of the

dumbbell CaWO4 crystals (Fig. 4a). The ‘‘arms’’ of the coral-like 3Dsuperstructures is in radial structure with about 200–300 nm inlength. The size of the nanoparticle building blocks was about 25–40 nm. The large amount of ‘‘holes’’ appeared on the TEM andFESEM images revealed that the obtained architectures are built bythe original particles. This result also indicated that a higherconcentration of Ca2+ and WO4

2� favored the formation of

Fig. 3. Schematic illustration of the formation of hierarchical CaWO4 at 10 8C, with

[Ca2+] = [WO42�] = 0.2 M.

Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–5048

nanocrystals with higher aspect ratios, and the initial concentra-tion of Ca2+ and WO4

2� ions in microemulsions played a key role inthe final morphology of products.

Further studies indicated that reaction temperature is also animportant influence factor for the shape and size of CaWO4

products. When the reaction was carried out at room temperature,the main morphology of products is in rod shape with lengths inthe range of 2–3 mm (Fig. 4b). The holes on the surfaces of productssuggest that the obtained rods were assembled by the originalparticles. In addition, these obtained rods tend to aggregatetowards flowerlike, organized by two, four or more rods. When thereaction temperature increased, the reaction rate and the reactantdiffusion increased considerably between the microemulsiondroplets, which led to a large size of nanoparticles and uncloseddeposit.

3.2. Optical properties of CaWO4 superstructure with different

morphologies

The photoluminescence performance of the as-synthesizedCaWO4 nanostructures was also studied because of its well-knownphotoluminescent properties. Fig. 5 shows the luminescencespectra of the obtained CaWO4 nanostructure with differentmorphologies, which are recorded from 300 to 460 nm wave-length. The curves 1–4 were obtained from samples 1–4,respectively (as shown in Table 1). It is well known that theintrinsic luminescence of CaWO4 arises from the radioactiverecombination of electrons and holes at WO4

2� sites [22]. Whenexcited at 246 nm, the emission took place at 416 nm for thesesamples.

However, careful observation revealed that the photolumines-cence performances of the obtained CaWO4 nanostructure aredepended on their morphologies and sizes [23,24]. Comparedsample 2 (dumbbell-like shape) with sample 3 (coral-like shape)and sample 4 (nanorods), there is a remarkable change of theabsolute luminescence intensity. This result might be attributed totheir different morphologies and crystalline nature. It is wellknown that higher disposal temperature is benefit to the crystal-linity of products. On the other hand, the reaction rate also hasimportant effect on the crystallinity, and lower reaction rate isfavorable for the crystal growth. As described above, dumbbell-likecrystals (sample 2) were produced at a lower concentration andreaction temperature, which would be benefit for its crystallinitythan others. Moreover, sample 1 has the most absolute lumines-cence intensity. The shapes of the spectrum of the CaWO4

superstructures are very similar to that of the CaWO4 nanopar-ticles, but the fluorescence intensity of CaWO4 superstructures isweaker than that of the nanoparticles. The reason is that theCaWO4 superstructures are formed by the aggregation of thenanoparticles. The different morphologies of CaWO4 have similarspectra because that they are all consisted of the primarynanoparticles. On the other hand, the primary nanoparticles ofthe superstructures can grow via oriented attachment, thefluorescence intensity of the superstructures decrease with theparticles size increasing. The results are well agreed with theproposed growth mechanism of dumbbell-like superstructures ofCaWO4 crystals [25]. As to the exact photoluminescence propertyof CaWO4 crystals need further to study.

3.3. Detailed structural characterization of the SrWO4 superstructure

Similarly, SrWO4 superstructures are also successfully obtainedby the reaction between Sr2+ and WO4

2� in CTAB microemulsions.The morphology of the sample 5 is in dendrite structure. The XRDpattern confirms that the products are pure SrWO4 crystals in a

Fig. 4. (a and b) TEM and SEM images of the sample 3 obtained at 10 8C, with [Ca2+] = [WO42�] = 0.5 M, (c and d) TEM and FESEM images of the sample 4 obtained at room

temperature, with [Ca2+] = [WO42�] = 0.2 M.

Fig. 5. PL spectra of the CaWO4 nanostructures. Curves 1–4 were obtained from

samples 1–4 (as shown in Table 1).

Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–50 49

tetragonal structure with cell parameters of a = 5.41 A andc = 11.95 A, which is according with the JCPDS Card No. 8-490(Fig. 1b). From Fig. 6a and b, the TEM and FESEM images clearlyshows that exclusive dendrite architecture is about 2–3 mm inlength, and the building block is about 40–50 nm. If the reactionwas carried out at room temperature for 1 h, dendrite like SrWO4

superstructure can also be obtained (Fig. 6c and d).

4. Conclusions

In summary, CaWO4 and SrWO4 nanostructures have beensynthesized via a simple microemulsion-mediated route. Withcareful control of the fundamental experimental parametersincluding the concentration of reactants, the reaction time and thereaction temperature, CaWO4 with various morphologies ofdumbbell, coral, rods, and dendrite has been efficiently obtained,respectively. The possible formation mechanism of these uniquemorphologies also has been proposed based on surfactant self-assembly under different experimental conditions. The as-synthesized CaWO4 samples with various morphologies exhibiteddifferent photoluminescence properties. This approach isexpected to provide a new general route for the controlledsynthesis of tungstate luminescence materials in restricteddimensions.

Fig. 6. (a and b) TEM and SEM images of the sample 5 obtained at 10 8C, with [Sr2+] = [WO42�] = 0.2 M, (c and d) TEM and FESEM images of the sample 4 obtained at room

temperature, with [Sr2+] = [WO42�] = 0.2 M.

Z. Chen et al. / Materials Research Bulletin 44 (2009) 45–5050

Acknowledgments

The work described in this paper was supported by the NationalScience Foundation of China (No. 20671061) and the Program forNew Century Excellent Talents of Education Ministry of China.

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