Materials Letters 107 (2013) 329–332

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Materials Letters

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Soheila.

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Room temperature synthesis of tungsten (VI) tri-oxide nanoparticleswith one-pot multi-component reaction in emulsion nanoreactorsstabilized by aerosol-OT

Reza Abazari a,n, Soheila Sanati b

a Department of Chemistry, Faculty of Science, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iranb Department of Chemistry, Payame Noor University, 19395-4697, Tehran, Iran

a r t i c l e i n f o

Article history:Received 20 January 2013Accepted 16 June 2013Available online 22 June 2013

Keywords:Tungsten (VI) tri-oxide nanoparticlesEmulsion solution methodChemical synthesisAerosol-OT surfactant

7X/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.matlet.2013.06.045

esponding author. Tel.: +9826 44260685; fax:ail addresses: Reza_chemist63@yahoo.com (R.sanati@gmail.com (S. Sanati).

a b s t r a c t

Tungsten (VI) tri-oxide (WO3) nanoparticles (NPs) are prepared by emulsion route using dioctylsulfosuccinate sodium salt (aerosol-OT) supermolecular template at room temperature. This studyshows that the aerosol-OT emulsions are suitable for synthesizing WO3 NPs in the absence of any co-surfactant. The field emission scanning electron microscope (FE-SEM), transmission electron microscopy(TEM), X-ray diffraction (XRD), and energy dispersive X-ray (EDAX) analyses have been used tocharacterize the surface morphology, size, structure, phase composition, and proof of the formation ofthe prepared present NPs, respectively. The physicochemical characterizations showed that the phase-pure nano-sized WO3 particles with the aerosol-OT emulsions solution have approximately uniformmorphologies with particles mainly in the range of 5–30 nm and with a monoclinic phase without anyimpurities. The proposed method has the advantage of one pot multicomponent preparation of quitemonodispersed WO3 NPs.

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1. Introduction

In recent years, nanotechnology and nano-science havereceived great attention among the scientific community due tothe new physical and chemical outstanding properties that emergewhen the size of the material becomes nano-sized. In this context,nanostructured oxide semiconductors have been in the focus ofattention as a result of their potential applications in devices suchas gas sensors, solar cells, photo-catalysts, etc. [1,2]. Amongvarious oxide semiconductors, the WO3 is highly interesting andhas widely been studied in view of its numerous interestingstructural and defect properties [3].

Tungsten tri-oxide is an important wide band gap n-typesemiconductor (between 2.2 and 2.8 eV), which includes manydistinctive properties such as electrochromism, photochromism,photo-catalysis, photoluminescence, etc. These characteristicsmake it favorable for a large number of potential applications inthe fields of windows, gas sensors, fuel cell, water splitters, solarcells, batteries, and the like [4–7]. Besides, WO3 is a novelpurificatory ecomaterial which can be suitably used in energyrenewal, energy storage, and environmental cleanup [8]. In all of

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+9821 22853650.Abazari),

these applications, the morphological characteristics of the mate-rials like grain size or shape and structures are of great importanceand depend heavily upon the preparation method. It should benoted that controlling the size of tungsten tri-oxide nanostruc-tures results in the enhancement of visible light photodegradation,which is suitable for indoor photodegradation or when ultravioletradiation is unavailable. In other words, metal oxide nanostruc-tures properties are size-dependent [4,9]. Furthermore, WO3 NPscan be synthesized (grown) in many different shapes, such asnanoporous, nanotubes, nanowires, nanocomposites, nanoparti-cles, nanosheets, etc.

Currently, a variety of physical and chemical techniques existfor the preparation of tungsten tri-oxide nanostructures: sol–gel[10], hydrothermal [11], anodization [12], thermal decomposition[13], spray pyrolysis [14], pulsed electrodeposition [15], etc. Thesemethods, however, do not give the advantage of controllable sizerequired for investigation of the size-dependence of tungsten tri-oxide nanostructures adsorption performance. Moreover, theseprocesses are the complex multistep processes and low yieldenergy-intensive and time-consuming, which limit their use inindustrial applications. Consequently, the development of simpleand effective methods for nanostructures preparation is of prac-tical interest.

A potential alternative method for the synthesis of nanostruc-tures with controllable size, morphology and crystallinity is theemulsion technique. The emulsion was formed using isooctane as

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solvents and aerosol-OT as surfactant. The emulsion droplets act asmicro or nanoreactors. The size of nano-materials can be con-trolled by changing the size of water pools [16,17]. In this context,the present study has sought to produce tungsten tri-oxide NPs byusing the emulsion technique and subsequently characterize themto confirm the formation of nano-sized tungsten tri-oxide. Theimportant advantage offered by our suggested method is perform-ing the process in one step and at room temperature. Thus, this isan economic procedure from both energy and time perspectives.

2. Experimental

Preparation of the WO3 NPs: Fig. 1 illustrates the general sketchfor the preparation process of WO3 NPs highly dispersed using theemulsion method. In two separate 25 mL beakers, two emulsionswith different aqueous phases, one with 0.036 mmol WCl6 and theother one with 0.8 mmol NH4OH were prepared (note that eachemulsion was formed with 0.1 M aerosol-OT concentration and agiven amount of isooctane as oil phase). Then, the emulsioncontaining an aqueous solution of ammonia was added to theemulsion containing an aqueous solution of WCl6. The mixedemulsion was kept at room temperature for 3 days in order toprecipitate. Then, methanol was added to the beaker to makephase separation and to wash the solution for 5 min three times.The final mixture was centrifuged to get samples and it was driedat 60 1C. Crystallization of the product was performed by calcina-tion at 500 1C for 2 h in air. The heating rate was 10 1C/min andnatural cooling was used. Finally, the obtained NPs were char-acterized by FE-SEM, TEM, XRD, and EDAX analyses.

Characterization: FE-SEM image was obtained on a HitachiS-1460 field emission scanning electron microscope using accel-erating voltage of 15 kV. TEM measurement for WO3 NPs wasperformed on a Philips model EM-208S instrument operated at anaccelerating voltage of 100 kV. For TEM analysis, the synthesizedsample was prepared by placing a drop of emulsion solution ontothe carbon coated copper TEM grids and drying it in air at roomtemperature. XRD analysis of the WO3 NPs was carried out using aPhilips diffractometer (Model TM-1800). The scanning angle wasfrom 201 to 601, operating at a voltage of 40 kV applying potential

Fig. 1. Schematic illustration of preparation of th

current of 30 mA. EDAX spectrum was obtained on JEOL JSM-6380LV scanning electron microscope.

3. Results and discussion

The first question relating to the nano-materials are to investigatethe aggregation state, size, shape and morphology. To further studythe detailed structural and morphological characteristics of tungstentri-oxide NPs, was investigated by TEM analysis-addition to FE-SEManalyze. In Fig. 2(a–c), the morphology or shape, size, and particle sizedistribution histogram of WO3 NPs prepared by emulsion method isinvestigated by FE-SEM and TEM analyses, respectively. As can beseen, the product has nearly spherical shape, essentially monodisperseand with some agglomeration (However, few larger loose aggregateswere also observed which may be due to aggregation during thewashing steps). It can be observed from TEM micrograph that theseparticles are with a particle size distribution in the range of 5–30 nm(Fig. 2(c)). As reported in the literature, the spherically monodispersemorphology is an important factor in the low-light scattering at thesurfaces and also in high packing densities [18].

X-ray diffraction is an effective method for investigation of thesolid structure of nano-materials. The strongly diffraction peaksindicated of the XRD pattern shown in Fig. 3(a) suggest theformation of single-phase WO3 NPs synthesized with the highlycrystallize. The sharp diffraction peaks imply good crystallinity ofWO3 NPs. All the diffraction peaks could be indexed with the JointCommittee on Powder Diffraction Standards data of monocliniccrystal structure (JCPDS card no. 75-2072). No diffractions raisedfrom impurities appear in the XRD pattern.

Using the Debye–Scherrer equation, the WO3 NPs mean particlesize (t) is calculated to be 17 nm as follow (the average cellparameter was calculated from (002), (020), (200), (120), (112)and (202) reflections).

t ¼ kλ=β cos θ ð1ÞFig. 3(b) shows the chemical composition of the WO3 NPs

determined by EDAX analysis, which reveal only oxygen and tungstenelements existed in the product with the molar ratio of about 1:3(W/O), which agree well with the XRD result (the presence of signals

e WO3 NPs synthesized in emulsion system.

Fig. 2. FE-SEM image (a), TEM image (b), and their particle size distributionevaluated from TEM observation (c).

Fig. 3. X-ray diffraction spectrum (a), and EDAX analysis of the synthesized WO3

NPs (b).

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of Au and C can be ascribed to the Au and C grid). The absence ofsulfur and sodium atoms indicated that aerosol-OT and sodium saltswere no more present. Therefore, the prepared tungsten tri-oxide NPswere free of impurities under the current synthetic route.

The particle size determined from TEM, SEM, and XRD analysesand also the necessary temperature and time for calcination ofsynthesized pure WO3 nanostructures in the present study have beencompared with some of the other methods reported in the literature(Table 1). The particle size obtained in this study (mean diameter,17 nm) is almost smaller than the values described in the literature forsome of the other techniques. In this regard, according to Table 1,considering the time and temperature required for the particlecalcination and comparison with particle size, it can be concludedthat by the suggested method, pure WO3 nanostructures can beobtained with fairly small particle size, compared to some of theprevious methods in the literature.

In general, all the analyses have consistently shown that thepreparedWO3 NPs by emulsion method were approximately sphericalshape, non-agglomerated, highly crystallized, and also the averageparticle size calculated from the peaks broadening in the XRD patternis consistent with particle size obtained from the TEM micrograph.

4. Conclusion

The present study demonstrates the synthesis of tungsten (VI) tri-oxide NPs within the aerosol-OT emulsions system. Our method forthe preparation of the present NPs has the virtue of simplicity, high

Table 1Comparison of particles size and calcination temperature and time of the WO3 nanostructures determined from TEM, SEM, and XRD analyses in the different methods.

Methods Particle size (TEM or SEM, nm) Crystallite size (XRD, nm) Crystallite temperature (1C) Crystallite time (h) Reference

Sol–gel 20–40 24–40 700 4 [10]Hydrothermal 30–500 37–129 500–800 10 [11]Anodization 50 Not mentioned 450 3 [12]Thermal decomposition 50–200 58–114 550–950 2 [13]Spray pyrolysis 50 144.95 500 6 [14]Pulsed electrodeposition 45–330 Not mentioned 450 6 [15]Sucrose ester microemulsion 10–50 Not mentioned 500 2 [19]Polymeric micelles 4672 Not mentioned 500 4 [20]Pyrolytic decomposition 20–50 Not mentioned 500–600 2 [21]Emulsion 5–30 17 500 2 Present work

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yield, and mild reaction conditions. FE-SEM, TEM, XRD, and EDAXanalyses have been employed to study the particles. Through thismethod, highly crystalline, approximately spherical shape, and well-dispersed WO3 NPs with an average particle size of about 17 nmweresuccessfully synthesized. X-ray diffraction measurement was in agree-ment with data for the monoclinic structure of tungsten tri-oxide NPs.Another advantage of this route is that, the WO3 NPs can besynthesized at room temperature, and the surfactants surroundingthe particles in the solution can easily be removed.

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