Journal of Colloid and Interface Science 330 (2009) 227–236

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier.com/locate/jcis

Synthesis, growth process and photoluminescence properties of SrWO4 powders

J.C. Sczancoski a, L.S. Cavalcante a,∗, M.R. Joya a, J.W.M. Espinosa b, P.S. Pizani a, J.A. Varela b, E. Longo b

a LIEC, Universidade Federal de São Carlos, Departamento de Química e Física, São Carlos, P.O. Box 676, 13565-905, SP, Brazilb IQ, Universidade Estadual Paulista, Araraquara, P.O. Box 355, 14801-907, SP, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 July 2008Accepted 11 October 2008Available online 18 October 2008

Keywords:SrWO4

MicrowaveGrowth processBand gapPhotoluminescence

SrWO4 powders were synthesized by the co-precipitation method and processed in a microwave-hydrothermal (MH) at 140 ◦C for different times. The obtained powders were analyzed by X-raydiffraction (XRD), micro-Raman (MR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy,field-emission gun scanning electron microscopy (FEG-SEM), ultraviolet–visible (UV–vis) absorptionspectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra showed thatthe SrWO4 powders present a scheelite-type tetragonal structure without the presence of deleteriousphases. FT-IR spectra exhibited a high absorption band situated at 831.57 cm−1, which was ascribed tothe W–O antisymmetric stretching vibrations into the [WO4] tetrahedron groups. FEG-SEM micrographssuggested that the processing time is able to influence in the growth process and morphology of SrWO4powders. UV–vis absorption spectra revealed different optical band gap values for these powders. A greenPL emission at room temperature was verified in SrWO4 powders when excited with 488 nm wavelength.

© 2008 Elsevier Inc. All rights reserved.

1. Introduction

At room temperature, the tungstates and molybdates presenta scheelite-type tetragonal structure with general formula ABO4(A = Ba, Ca, Sr, Pb; B = W, Mo) and space group I41/a [1–5]. Inthis type of structure, the B ions are within tetrahedral O-ion cagesand isolated from each other, while the A ions are surrounded byeight oxygens [6,7]. Currently, these materials have been widelyemployed in several industrial applications, such as: optic fiber,humidity sensor, catalysts, scintillation detector, solid-state lasers,photoluminescent devices, photocatalysts, microwave applicationsand more [8–16]. In particular, SrWO4 has attracted considerableattention to the development of new electrooptics devices due toits blue or green luminescence emissions at room temperature[17,18]. Moreover, SrWO4 crystals allow the introduction of dif-ferent lanthanide ions (Er3+, Nd3+, Yb3+, Tm3+), which can beused as matrices for laser active elements with non-linear self-conversion of radiation to a new spectral range [19–23].

Different preparation techniques have been employed to ob-tain this material, mainly including: pulsed laser deposition [24],spray pyrolysis [17], Czochralski [25,26] and polymeric precur-sor method [27]. Generally, these methods require expensiveand sophisticated equipments, high temperatures with long pro-cessing times, expensive precursors and high consumption ofelectric energy [28]. Therefore, it is important to develop new

* Corresponding author. Fax: +55 16 3361 8214.E-mail address: laeciosc@bol.com.br (L.S. Cavalcante).

0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2008.10.034

processing material methods with low costs, environmentallyfriendly and with the possibility of formation of materials on mi-cro and nanoscale with well-defined morphologies. For example,hydrothermal method [29,30], electrochemical method [31–34],simple chemical reaction [35] and chemical solution deposition(CSD) [36]. Morphology-controlled synthesis with well-definedshapes is technologically interesting due to the chemical and phys-ical properties of nano- and microcrystals depend not only on itscomposition, but also on its structure, phase, shape, size and sizedistribution [37–40]. In this case, synthesis methods assisted bymicrowave radiation have received special attention due to its ad-vantages in the formation of materials with different morphologiesand high degree of crystallinity [41].

Recently, Thongten et al. [42] obtained SrWO4 nanoparticlesusing a solvothermal method assisted by cyclic microwave radia-tion. These authors observed that the pH value, microwave powerand prolonged synthesis times are able to influence the structural,morphologic and optical properties of this material. In another re-search, Thongten et al. [43] verified through the cyclic microwaveradiation method that different mol fractions of cetyltrimethylam-monium bromide result in the formation of SrWO4 with differentmorphologies, such as: peach-like, dumb-bells and bundles.

Therefore, in this paper, SrWO4 powders were synthesizedby the co-precipitation method and processed in a microwave-hydrothermal at 140 ◦C for different times (30 min–8 h). The ob-tained powders were analyzed by X-ray diffraction (XRD), micro-Raman (MR) spectroscopy, Fourier transform infrared (FT-IR) spec-troscopy. The growth process was analyzed by field-emission gunscanning electron microscopy (FEG-SEM). The optical properties

228 J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236

were investigated by Ultraviolet–visible (UV–vis) absorption spec-troscopy and photoluminescence (PL) measurements.

2. Materials and methods

2.1. Synthesis and microwave-hydrothermal processing of SrWO4

powders

SrWO4 powders were processed by microwave-hydrothermal(MH) in the presence of polyethylene glycol (PEG). The typicalprocedure is described as follows: 5 × 10−3 mol of tungstic acid(H2WO4) (99% purity, Aldrich), 5 × 10−3 mol of strontium acetate[(CH3CO2)2Sr] (99.5% purity, Aldrich) and 0.1 g of PEG (Mw 200)(99.9% purity, Aldrich) were dissolved in 75 mL of deionized wa-ter. The solution pH was adjusted up to 12 by the addition of5 mL of ammonium hydroxide (NH4OH) (30% in NH3, Synth). Inthe sequence, the aqueous solution was stirred for 20 min in ul-trasound at room temperature. After co-precipitation reaction, thesolution was transferred into a Teflon autoclave, which was sealedand placed into a domestic MH (2.45 GHz, maximum power of800 W). Each MH processing was performed at 140 ◦C for 30 min,1 h, 2 h, 5 h and 8 h, respectively. The heating rate in this sys-tem was fixed at 25 ◦C/min and the pressure into the autoclavewas stabilized at 294 kPa. After microwave-hydrothermal process-ing, the autoclave was cooled at room temperature naturally. Theresulting solution was washed with deionized water several timesto neutralize the solution pH (≈7). Finally, the white precipitateswere collected and dried in a conventional furnace at 75 ◦C forsome hours.

2.2. Characterizations of SrWO4 powders

SrWO4 powders were structurally characterized by XRD us-ing a Rigaku-DMax/2500PC (Japan) with CuKα radiation (λ =1.5406 Å) in the 2θ range from 15◦ to 75◦ with 0.02◦/min. MRspectroscopies were recorded using a T-64000 Jobin-Yvon triplemonochromator coupled to a CCD detector. The spectra were ob-tained using a 514.5 nm wavelength of an argon ion laser, keep-ing its maximum output power at 8 mW. A 100 μm lens wasused to prevent powder overheating. FT-IR spectroscopies wereperformed on a Bruker-Equinox 55 (Germany), using a 30◦ spec-ular reflectance accessory. The powder morphologies were verifiedusing a FEG-SEM (Supra 35-VP, Carl Zeiss, Germany). UV–vis spec-tra were measured using a Cary 5G (USA) equipment in reflec-tion mode. PL measurements of SrWO4 powders were taken witha U1000 Jobin-Yvon double monochromator coupled to a cooledGaAs photomultiplier with a conventional photon counting sys-tem. The 488 nm excitation wavelength of an argon ion laser wasused, keeping its maximum output power at 25 mW. A cylindri-cal lens was used to avoid powder overheating. The slit widthutilized was 100 μm. UV–vis and PL spectra were taken threetimes for each sample in order to ensure the reliability of themeasurements. All measurements were performed at room tem-perature.

3. Results and discussion

3.1. X-ray diffraction analyses

Fig. 1 shows the XRD patterns of SrWO4 powders processed inMH at 140 ◦C for different times.

XRD patterns revealed that the SrWO4 powders can be indexedto the scheelite-type tetragonal structure with space group I41/a,in agreement with the respective JCPDS (Joint Committee on Pow-der Diffraction Standards) card No. 08-0490 [44]. Diffraction peaks

Fig. 1. XRD patterns of SrWO4 powders processed in MH at 140 ◦C for differenttimes. The stars indicate the position of JCPDS card No. 08-0490.

Table 1Lattice parameter values and unit cell volumes of SrWO4 powders processed in MHat 140 ◦C for different times.

Method Tempera-ture(◦C)

Time(min)

a, b latticeparameter(Å)

c latticeparameter(Å)

Unit cellvolume(Å3)

MHa 140 30 5.4065(7) 11.931(6) 348.774(1)

MH 140 60 5.4042(3) 11.927(8) 348.362(2)

MH 140 120 5.4032(7) 11.922(6) 348.084(1)

MH 140 300 5.4130(6) 11.940(6) 349.875(0)

MH 140 480 5.4119(6) 11.933(3) 349.520(4)

JCPDS No. 08-0490 5.4168(0) 11.951(0) 350.660(0)

a MH = microwave-hydrothermal method.

correspondent to the secondary phases were not verified. Accord-ing to the literature [45,46], XRD patterns are able to estimate thedegree of structural order–disorder at long-range in the materi-als. Therefore, the strong and sharp peaks indicate that the SrWO4powders processed in MH are highly crystallized and structurallyordered at long-range. This result shows that the MH system pro-motes the formation of crystalline SrWO4 powders with low heattreatment temperature and reduced processing time than the con-ventional methods [2,5]. The experimental lattice parameters andunit cell volume were calculated using the least square refine-ment from the UnitCell-97 program [47]. These obtained resultsare listed in Table 1.

As it can be seen in Table 1, the lattice parameters of SrWO4powders are in agreement with the values reported in JCPDS card.The small deviations in the lattice parameter values can be mainlyattributed to the effect of microwave radiation in the MH sys-tem. The microwave radiation when interacts with a permanentdipole of a liquid phase results in vibrations on the charged parti-cles [42,48]. The heating promoted by these vibrations contributesto a rapid formation process of SrWO4 phase. As consequence,this mechanism is able to result in distortions and/or strains inthe SrWO4 lattice. In this case, it can be verified through thenon-linear variations on a, b and c lattice parameters and unitcell volume with the processing time in the MH system (Ta-ble 1).

3.2. Micro-Raman analyses

According to Basiev et al. [49], a typical SrWO4 primitive cellis characterized by the [WO4] molecular ionic groups and Sr2+cations. The [WO4] molecular groups with strong covalent bondW–O are peculiar to the tungstates. Due to the weak coupling

J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236 229

Fig. 2. MR spectra in the range from 50 cm−1 to 1000 cm−1 for the SrWO4 powdersprocessed in MH at 140 ◦C for different times.

between the [WO4] molecular groups and the Sr2+ cations, the vi-brational modes observed in Raman spectra of SrWO4 can be clas-sified into two groups, internal and external modes. The internalvibrations correspond to the vibrations within the [WO4] molecu-lar group, considering a stationary mass center. The external vi-brations or lattice phonons are associated to the motion of theSr2+ cations and the rigid molecular unit. The [WO4] tetrahedronspresent a Td-symmetry in the free space. When a [WO4] molecu-lar group is placed into the scheelite structure, its symmetry pointgroup is reduced to S4. The vibration of [WO4] tetrahedrons canbe divided into four internal modes (ν1(A1), ν2(E), ν3(F2) andν5(F2)), one free rotation mode (νfr(F1)) and one translation mode(F2). At room temperature, SrWO4 belongs to symmetry. Thus, theirreducible representation for the symmetry C6

4h at the Γ point ofthe Brillouin zone for the lattice vibrations of SrWO4 crystal in anunit cell can be described by the following equation [25,49]:

Γ = 3Ag + 5Au + 5B g + 3Bu + 5E g + 5Eu . (1)

According to Ling et al. [25], for the symmetry C64h the Raman

and infrared active optical modes at zero wave vector can be de-termined through the following equation:

Γ = 3Ag + 5B g + 5E g + 4Au + 4Eu . (2)

All even vibrations Ag , B g and E g are Raman-active modes,while the odd modes 4Au and 4Eu are active only in infrared fre-quencies.

Fig. 2 shows the MR spectra in the range from 50 cm−1 to1000 cm−1 for the SrWO4 powders processed in MH at 140 ◦C fordifferent times.

The stronger Raman-active vibration modes indicate a stronginteraction between the ions, which mainly arise from the stretch-ing and bending vibrations of the shorter metal–oxygen bondswithin the anionic groups [25]. Only nine Raman-active vibrationmodes were detected in the MR spectra, i.e., three Ag vibrations(921.55, 338.83 and 193.50 cm−1) three B g (839.87, 373.49 and78.98 cm−1) and three E g (800.96, 136.34 and 103.50 cm−1). Theseresults are in agreement with the reported in the literature [50,51].According to Marques et al. [45] and Rosa et al. [46], the Ramanspectra are able to predict the degree of structural order–disorderat short-range in the materials. Therefore, the well-defined Raman-active modes indicate that the SrWO4 powders processed in MHare ordered at short-range (Fig. 2).

FT-IR spectra show the absorption bands of SrWO4 powdersprocessed in MH at 140 ◦C for different times (Supporting infor-mation).

3.3. Field-emission gun scanning electron microscopy analyses

Fig. 3 shows the FEG-SEM micrographs of SrWO4 powders pro-cessed in MH at 140 ◦C for different times.

FEG-SEM micrographs revealed that the SrWO4 powders pro-cessed at 140 ◦C for 30 min in MH system exhibit a large quantityof particles with agglomerate and polydisperse nature (Fig. 3a).As it can be seen in this figure, the random aggregation processbetween the small particles resulted in the formation of a ricegrain-like morphology. We believe that this aggregation processcan be related to the increase of the effective collision rates be-tween the small particles by the microwave radiation [52]. Fig. 3bshows that the processing of SrWO4 powders at 140 ◦C for 1 h ini-tiates the growth process of aggregated particles by coalescence.This behavior can be associated with the influence of PEG-200adsorbed on the surface of these particles. In this case, this poly-mer surfactant is able to promote a separation mechanism of theaggregated particles during the growth process. The increase ofprocessing time for 2 h resulted in the formation of small quasi-octahedrons, as shown in Fig. 3c. This result suggests that thegrowth of the particles contributes to a higher adsorption rate ofPEG on the crystal faces, which promotes interactions betweenthe tails of surface-adsorbed surfactant molecules that possiblyaffect the growth of microcrystals [53]. Fig. 4d shows that theSrWO4 powders processed at 140 ◦C for 5 h resulted in the for-mation of faceted micro-octahedrons along the [110] direction [38,54,55]. It is an indicative that the PEG favors an anisotropic growthcaused by the differences in the surface energies on the differentcrystallographic faces [56]. In the final stage, the long process-ing time intensified the coalescence process between the smallquasi-octahedrons and some faceted micro-octahedrons, resultingin the formation of large faceted micro-octahedrons. These micro-octahedrons can be observed in the SrWO4 powders processed at140 ◦C for 8 h (Fig. 3e). FEG-SEM micrographs also allowed theestimation of the average particle size distribution (height andwidth) of SrWO4 powders processed at 140 ◦C for 8 h through themeasure of approximately 100 particles. In this case, it was ob-served an average particle height distribution from 0.150 μm to0.750 μm and average particle width distribution from 0.125 μm to0.425 μm (Insets in Figs. 3e and 3f). Possibly, the imperfections ordifferences between height and width of these micro-octahedronscan be associated with the influence of PEG during the growthprocess along the [001] direction.

3.4. Growth mechanism for the formation of SrWO4 powders withoctahedron-like morphology

Fig. 4 shows a schematic representation of the synthesis andgrowth mechanism of SrWO4 particles by the MH processing.

Fig. 4a shows the initial formation stage of SrWO4 particlesby the co-precipitation reaction. Firstly, the tungstic acid and thestrontium salt were dissolved in water. The hydrolysis rate ofthis solution was intensified by the addition of 5 mL of NH4OH.As consequence, the electrostatic interaction between Sr2+ andWO2−

4 ions resulted in the formation of SrWO4 particles. This so-lution was stirred for 20 min in ultrasound to accelerate the co-precipitation rate. 0.1 g of PEG was added into this solution to con-trol the kinetics of crystal growth and determine the subsequentmorphology of the products with the processing time [56]. Fig. 4billustrates a schematic representation of a MH system employedin the processing of SrWO4 powders. This MH system was de-veloped through adaptations performed on a domestic microwaveoven (model NN-ST357WRPH, Panasonic) [57], including: temper-ature controller, thermocouple and Teflon autoclave. Fig. 4c showsa schematic representation of the processing of SrWO4 powders in

230 J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236

Fig. 3. FEG-SEM micrographs of SrWO4 powders processed in MH at 140 ◦C for different times. (a) Rice grain-like morphology of SrWO4 powders processed at 30 min,(b) growth of aggregated particles, (c) formation of small quasi-octahedrons, (d) presence of small quasi-octahedrons and faceted micro-octahedrons, (e) and (f) show theformation of large micro-octahedrons. The insets in Figs. 4e–4f show the average particle size distribution (height and width) of SrWO4 micro-octahedrons formed after 8 hof processing.

J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236 231

Fig. 4. Schematic representation of the synthesis and growth mechanism of SrWO4 particles by MH. (a) Co-precipitation reaction, hydrolysis and addition of surfactant(PEG), (b) microwave-hydrothermal system employed in the processing of SrWO4 powders, (c) aggregation of small particles due to the increase of the effective collisionrates by the microwave radiation, (d) formation of a rice grain-like morphology, (e) growth and separation processes of aggregated particles, (f) formation process of smallquasi-octahedrons caused by the absorption of PEG on the crystal faces, (g) simultaneous presence of small quasi-octahedrons and faceted micro-octahedrons after 5 h ofprocessing and (h) large micro-octahedrons grown in the preferential [001] direction.

MH system. Into the Teflon autoclave, the high-frequency electro-magnetic radiation (2.45 GHz) interacts with the permanent dipoleof the liquid phase [48]. This interaction leads to a vibration onthe charged particles or molecules, which result in a rapid heatingof the chemical solution and consequently of SrWO4 particles [42].The increase of system temperature by the microwave radiationleads to a dissociation mechanism of some SrWO4 particles inSr2+ and WO2−

4 ions. However, the thermodynamic conditions andthe electrostatic interaction between these ions favor the recrys-tallization process. Moreover, the microwave radiation is able toaccelerate the solid particles to high velocities, leading to an in-crease of the interparticle collisions and inducing effective fusionat the point of collision [52] (Fig. 4c). We believe that these mech-anisms are responsible for the fast nucleation of SrWO4 seeds and

aggregation of several small particles. Also, during the initial stagesof processing (140 ◦C for 30 min), the fast adsorption and absorp-tion of PEG on the surface of the small particles and the highcollision rates promoted by the microwave radiation favored theformation of a rice grain-like morphology (Fig. 4d—30 min). Theincrease of processing time contributed to a cooperative effect be-tween coalescence process and adsorption of PEG. The coalescencepromotes the growth of aggregated particles and consequently ahigh concentration of PEG can be adsorbed on the surface of theseparticles. When adsorbed, this polymer surfactant leads to a sep-aration process of the aggregated particles and small spaces canbe verified in the rice grain-like morphology of SrWO4 particles(Fig. 4e—1 h). The processing of SrWO4 powders at 140 ◦C for2 h intensify the absorption of PEG on the crystal faces, which

232 J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236

(a) (b)

(c) (d)

(e)

Fig. 5. UV–vis absorbance spectra of SrWO4 powders processed in MH at 140 ◦C for different times.

contributes to the formation process of small quasi-octahedrons(Fig. 4f—2 h). After 5 h of processing, the simultaneous presenceof small quasi-octahedrons and some faceted micro-octahedronsalong the [110] direction [38,54] (Fig. 4g—5 h). Possibly, the PEGinduces an anisotropic growth of these micro-octahedrons. The in-crease of processing time for 8 h promoted the formation of largefaceted micro-octahedrons along the [110] direction due to thegrowth mechanism. In this case, the long processing time con-tributes to the formation of these large faceted micro-octahedrons

through the junctions between small quasi-octahedrons and micro-octahedrons previously formed (Fig. 4h—8 h).

3.5. Ultraviolet–visible absorption spectroscopy analyses

Figs. 5a–5e show the UV–vis absorbance spectra SrWO4 pow-ders processed in MH at 140 ◦C for different times.

The optical band gap energy (Eg) was estimated by the methodproposed by Wood and Tauc [58]. According to these authors the

J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236 233

optical band gap is associated with absorbance and photon energyby the following equation:

hνα ∝ (hν − Eg)1/2,

where α is the absorbance, h is the Planck constant, ν is the fre-quency and Eg is the optical band gap.

In this case, the Eg values of SrWO4 powders were evaluatedextrapolating the linear portion of the curve or tail. The obtainedresults are showed in Figs. 6a–6e and listed in Table 2. This tableshows a comparative between Eg values of SrWO4 obtained in thiswork with those reported in the literature by different methods.

The literature [5,27] has reported that the Eg is associated withthe presence of intermediary energy levels within the band gapof the materials. These energy levels are dependent of the degreeof structural order–disorder in the lattice. Therefore, the increaseof structural organization in the lattice leads to a reduction ofthese intermediary energy levels and consequently increases theEg value. We believe that the Eg also can be related with otherfactors, such as: preparation method, shape (thin film or powder),particle morphology, heat treatment temperature and processingtime [62]. These factors result in different structural defects (oxy-gen vacancies, bond distortions), which are able to promote theformation of intermediary energy levels within the band gap. Prob-ably, the differences verified for the Eg of SrWO4 in Table 2 can beassociated with these factors. Our results indicate that the SrWO4powders processed in MH system are structurally ordered at longand short-range, in agreement with XRD and MR analyses (Figs. 1and 2). In this case, the Eg values are not related to the degreeof structural order–disorder in the lattice. We believe that the Egcan be associated with the formation of intermediary energy levelsdue to the distortions on the [WO4] tetrahedrons during the MHprocessing. These distortions are arising from the vibration process

on the charged particles by the microwave radiation. Also, the Eg

of these powders can be related with the morphology modifica-tions caused by the growth mechanism and/or superficial defectsarising from the high collision rates between the particles by themicrowave radiation. An important factor is that the equation pro-posed by Wood and Tauc [58] no reports the influence of themorphology on Eg of the materials. In this case, the Eg values ob-tained by this equation can be considered qualitative. Therefore,future investigations will be necessary to a better understanding.

Table 2Comparative results between Eg values of SrWO4 obtained in this work with thosereported in the literature by different methods.

Method Shape Tempera-ture (◦C)

Time(h)

Eg

(eV)Ref.[ ]

PPM Powder 600 2 4.50 [5]PPM Powder 700 2 4.70 [5]PPM Thin film 400 2 5.78 [7]PPM Thin film 400 4 4.61 [27]PPM Thin film 600 4 5.76 [27]SP Thin film 700 1 4.90 [17]CSSM Powder 1300 12 3.90 [59]DDFR Crystal 900 2 4.49 [60]CZM Crystal 1000 24 5.08 [61]MH Powder 140 0.50 4.36 [�]MH Powder 140 1 4.49 [�]MH Powder 140 2 4.59 [�]MH Powder 140 5 4.68 [�]MH Powder 140 8 4.43 [�]

Eg = optical band gap energy; Ref. = reference; PPM = polymeric precursormethod; SP = spray pyrolysis; CSSM = conventional solid state method; DDFR =double decomposition flux reaction; CZM = Czochralski method; MH = microwave-hydrothermal; � = this work.

Fig. 6. PL spectra at room temperature of SrWO4 powders processed in MH at 140 ◦C for different times.

234 J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236

3.6. Photoluminescence analyses and model based in distortions

Fig. 6 shows the photoluminescence (PL) spectra at room tem-perature of SrWO4 powders processed in MH at 140 ◦C for differenttimes.

Blasse and Grabmaier [63] reported that the PL emission arisesfrom the radiative return to the ground state, phenomenon that isin concurrence with the non-radiative return to the ground state.In the non-radiative process, the energy of the excited state is usedto excite the vibrations of the host lattice, i.e., heat the lattice. Theradiative emission process occurs more easily if there are trappedholes or trapped electrons within the band gap [27]. PL emissionof tungstates with scheelite-type tetragonal structure is not com-pletely understood. In particular, the literature has reported sev-eral hypotheses to explain the mechanisms responsible by the PLemission of SrWO4. Orhan et al. [27] performed theoretical stud-ies on the PL emission of SrWO4 thin films through structuralorder–disorder. These authors reported the existence of WO3 anddistorted WO4 clusters in the SrWO4 lattice, which are able to in-duce the formation of intermediary energy levels within band gap.These energy levels are composed of oxygen 2p states (near thevalence band) and tungsten 5d states (below the conduction band).In this case, the polarization induced by the symmetry break andthe existence of these localized energy levels are favorable condi-tions for the formation of trapped holes and trapped electrons. Theinfluence of structural order–disorder on the PL properties was alsoreported by Anicete-Santos et al. [5] for SrWO4 powders preparedby the polymeric precursor method and heat treated at 500 ◦C,600 ◦C and 700 ◦C for 2 h. Lou et al. [17] mentioned that the lu-minescence of SrWO4 thin films formed by spray pyrolysis is dueto the intrinsic transitions within the [WO4]2− complex. Thongtemet al. [42,43] attributed the PL emission of SrWO4 powders withthe 1T2 → 1A1 transition of electrons within [WO4] tetrahedron

Table 3Results obtained by the deconvolution of the PL spectra of SrWO4 powders pro-cessed in MH at 140 ◦C for different times.

T a

(◦C)PTb

(h)MEc

(nm)P1 peak P2 peak P3 peak

Center(nm)

Area(%)

Center(nm)

Area(%)

Center(nm)

Area(%)

140 0.5 553.48 523.243 19.37 564.356 43.99 619.966 36.64140 1 555.48 523.243 17.71 564.356 50.48 619.966 31.81140 2 557.36 523.243 17.80 564.356 44.83 619.966 37.37140 5 549.61 523.243 20.91 564.356 49.91 619.966 29.18140 8 552.54 523.243 20.25 564.356 41.26 619.966 38.49

a T = temperature.b PT = processing time.c ME = maximum emission.

groups, which can be treated as excitons. These authors also ver-ified the presence of some shoulders on the PL profile and inter-preted them as extrinsic transitions caused by the defects and/orimpurities in the material. Sun et al. [1] showed that PL emissionof SrWO4 can be modified by the morphology. These authors ob-served that different morphologies contribute to the formation ofsurface defects (roughness) that influence in the PL behavior. Yu etal. [64] verified a green emission in PbWO4 micro-crystals, whichwas attributed to the existence of Frenkel defects structure (oxygenion shifted to the inter-site position with simultaneous creation ofvacancy) in the surface layers.

In Fig. 6, the PL profiles of SrWO4 powders suggest an emissionmechanism by multilevel process i.e., a system in which the relax-ation occurs by means of several paths, involving the participationof several energy states within the band gap of the material [27].Therefore, in order to estimate the contribution of each individ-ual component it was necessary to deconvolute the PL spectra. Thedeconvolution was performed through the PeakFit program (4.05version) [65] using the Voigh area function. As it can be seen in

Fig. 7. Schematic representation of a simple mechanism to explain the PL behavior of SrWO4 powders processed in MH at 140 ◦C for different times. (a) Interaction of themicrowave radiation on the [WO4] tetrahedron groups, (b) three possible distortions on [WO4] tetrahedron groups by the displacement of tungsten atoms along the atomiccoordinates (x, y, and z), (c) proposed model with intermediary energy levels (oxygen 2p and tungsten 5d states) within the band gap, (d, e) recombination of e′–h· pair and(f) PL spectrum of SrWO4 powders processed in MH at 140 ◦C for 2 h.

J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236 235

Figs. 6a–6e, the PL profiles were better adjusted by the addition ofthree peaks. In order to ensure a better analyze of the PL behavior,the peaks were positioned on a same position for all SrWO4 pow-ders. The P1 and P2 peaks localized at 523.243 nm and 564.356 nmcorrespond to the green emission components. The P3 peak situ-ated at 619.966 nm is ascribed to the orange emission component.Table 3 shows the contribution of each individual component (P1,P2 and P3 peaks) on the PL profile.

In this table, it was verified that the maximum PL emissionof SrWO4 powders presents small deviations. Also, small changeswere observed in the area (%) between the three components.These results indicate that the PL emission is probably dependentof the specific atomic arrangements. In this case, the modificationsin the atomic arrangement can be associated with the formationof distortions on the [WO4] tetrahedron groups of SrWO4 powdersby the microwave-radiation.

In Fig. 6f, it was observed that the PL intensity can be influ-enced by the processing time in a MH system. This behavior isnot associated to the band-to-band emission process due to thewavelength’s energy (2.54 eV) to be small than the Eg of SrWO4powders. Probably, the PL of these powders is arising from the con-tribution of different intermediary energy levels within the bandgap. Our results indicate that the SrWO4 powders processed in MHare highly crystalline and structurally ordered at long and short-range, in agreement with XRD patterns (Fig. 1) and MR spectra(Fig. 2). Therefore, PL of these powders is not due to the struc-tural order–disorder in the lattice. As previously described, webelieve that the behavior of this physical property is related tothe influence of microwave radiation on the [WO4] tetrahedrongroups.

Fig. 7 shows a simple mechanism to explain the PL behaviorof SrWO4 powders processed in MH at 140 ◦C for different times.In this figure, the 1 × 1 × 1 unit cell of SrWO4 with I41/a spacewas modeled using the Java Structure Viewer Program (Version1.08lite for Windows) and VRML-View (Version 3.0 for Windows)[66,67].

The literature reports that the tungsten atoms are consideredgood microwave absorbers [68]. Thus, we believe that the interac-tion between microwave radiation and tungsten groups (networkformers) results in a rapid heating of SrWO4 powders and leads toa vibration on the charged particles (Fig. 7a). These factors proba-bly result in a distortion process on the [WO4] tetrahedron groups,favoring the formation of intermediary energy levels within theband gap of this material (Figs. 7b and 7c). These energy levels arecomposed of oxygen 2p states (near the valence band) and tung-sten 5d states (below the conduction band). During the excitationprocess with 488 nm wavelength, some electrons are promotedfrom the oxygen 2p states to tungsten 5d states through the ab-sorption of photons (hν). This mechanism results in the formationof self-trapped excitons (STEs), i.e., trapping of electrons (e′) byholes (h·) (Fig. 7d). The emission process of photons (hν ′) occurswhen an electron localized in a tungsten 5d state decays into anempty oxygen 2p state (Fig. 7e). Consequently, this mechanism isresponsible for the PL emission of SrWO4 powders (Fig. 7f).

This proposed mechanism based on the distortion process of[WO4] tetrahedron groups, consequently can be related with thenon-linear variations on the PL intensity of SrWO4 powders withthe processing time in MH (Fig. 7f). Also, this behavior can be as-sociated with the formation of superficial defects caused by themodifications on the morphology of these powders [69]. These de-fects are arising from rapid heating, high effective collision ratesbetween the small particles and growth processes during the pro-cessing of SrWO4 powders. However, future investigations will benecessary in order to understand the effect of microwave-radiationon the PL properties.

4. Summary

SrWO4 powders were synthesized by the co-precipitationmethod and processed at 140 ◦C for different times in a MH sys-tem. XRD patterns revealed that these powders crystallize in ascheelite-type tetragonal structure with space group I41/a. Thesmall deviations in the lattice parameter values were associatedwith the effect of microwave radiation. MR and FT-IR spectrashowed characteristic modes of these powders. XRD patterns andMR spectra indicated that independent of the processing time ina MH system, SrWO4 powders are ordered at long and short-range. FEG-SEM micrographs showed that the processing time isan important factor in the growth mechanism of SrWO4 pow-ders. These micrographs also indicated that the PEG is a surfac-tant that favors the growth of SrWO4 micro-octahedrons alongthe [001] direction. A possible growth mechanism for the for-mation of SrWO4 micro-octahedrons was proposed. UV–vis ab-sorption spectra showed different optical band gap values, whichwere associated with the presence of intermediary energy levelswithin the band gap. PL behavior was explained through distor-tions on the [WO4] tetrahedron groups by the microwave radia-tion.

Acknowledgments

The authors thank the financial support of the Brazilian re-search financing institutions: CAPES, CNPq and FAPESP. Specialthanks to Prof. Dr. D. Keyson (UFPB) and Dr. D.P. Volanti (UNESP)by the development of the microwave-hydrothermal system.

Supporting information

The online version of this article contains additional supportinginformation.

Please visit DOI: 10.1016/j.jcis.2008.10.034.

References

[1] L. Sun, Q. Guo, X. Wu, S. Luo, W. Pan, K. Huang, J. Lu, L. Ren, M. Cao, C. Hu,J. Phys. Chem. C 111 (2007) 532.

[2] M. Anicete-Santos, R.C. Lima, E. Orhan, M.A.M.A. Maurera, L.G.P. Simões,G. Souza, P.S. Pizani, E.R. Leite, J.A. Varela, E. Longo, J. Comput. Aided Mater.Des. 12 (2005) 111.

[3] G. Jia, C. Tu, J. Li, X. Lu, Z. You, Z. Zhu, B. Wu, J. Appl. Phys. 98 (2005) 0935251.[4] G. Jia, C. Tu, Z. You, J. Li, Z. Zhu, Y. Wang, B. Wu, J. Cryst. Growth 273 (2004)

220.[5] M. Anicete-Santos, F.C. Picon, M.T. Escote, E.R. Leite, P.S. Pizani, J.A. Varela,

E. Longo, Appl. Phys. Lett. 88 (2006) 2119131.[6] G. Zhang, R. Jia, Q. Wu, Mater. Sci. Eng. B 128 (2006) 254.[7] M.A.M.A. Maurera, A.G. Souza, L.E.B. Soledade, F.M. Pontes, E. Longo, E.R. Leite,

J.A. Varela, Mater. Lett. 58 (2004) 727.[8] Y. Zhang, F. Yang, J. Yang, Y. Tang, P. Yuan, Solid State Commun. 133 (2005) 759.[9] J. Yu, L. Qi, B. Cheng, X. Zhao, J. Hazardous Mater. 158 (2008) 301.

[10] J.H. Ryu, J.-W. Yoon, K.B. Shim, Solid State Commun. 133 (2005) 657.[11] J.H. Ryu, J.-W. Yoon, C.S. Lim, W.-C. Oh, K.B. Shim, J. Alloys Compd. 390 (2005)

245.[12] A.P.A. Marques, D.M.A. de Melo, C.A. Paskocimas, P.S. Pizani, M.R. Joya, E.R.

Leite, E. Longo, J. Solid State Chem. 179 (2006) 671.[13] H. Fu, J. Lin, L. Zhang, Y. Zhu, Appl. Catal. A 306 (2006) 58.[14] A.M.E.S. Raj, C. Mallika, O.M. Sreedharan, K.S. Nagaraja, Mater. Lett. 53 (2002)

316.[15] R. Sundaram, K.S. Nagaraja, Mater. Res. Bull. 39 (2004) 581.[16] R.C. Pullar, S. Farrah, N.M. Alford, J. Eur. Ceram. Soc. 27 (2007) 1059.[17] Z. Lou, M. Cocivera, Mater. Res. Bull. 37 (2002) 1573.[18] L. Chen, Y. Gao, Mater. Res. Bull. 42 (2007) 1823.[19] L.I. Ivleva, T.T. Basiev, I.S. Voronina, P.G. Zverev, V.V. Osiko, N.M. Polozkov, Opt.

Mater. 23 (2003) 439.[20] D. Errandonea, C. Tu, G. Jia, I.R. Martín, U.R. Rodrguez-Mendoza, F. Lahoz, M.E.

Torres, V. Lavn, J. Alloys Compd. 451 (2008) 212.[21] J. Sulc, H. Jelnková, T.T. Basiev, M.E. Doroschenko, L.I. Ivleva, V.V. Osiko, P.G.

Zverev, Opt. Mater. 30 (2007) 195.

236 J.C. Sczancoski et al. / Journal of Colloid and Interface Science 330 (2009) 227–236

[22] G. Jia, C. Tu, J. Li, Z. Zhu, Z. You, Y. Wang, B. Wu, J. Alloys Compd. 436 (2007)341.

[23] G. Jia, C. Tu, Z. You, J. Li, Z. Zhu, B. Wu, Solid State Commun. 134 (2005) 583.[24] J.Y. Huang, Q.X. Jia, Thin Solid Films 444 (2003) 95.[25] Z.C. Ling, H.R. Xia, D.G. Ran, F.Q. Liu, S.Q. Sun, J.D. Fan, H.J. Zhang, J.Y. Wang,

L.L. Yu, Chem. Phys. Lett. 426 (2006) 85.[26] J.D. Fan, H.J. Zhang, J.Y. Wang, M.H. Jiang, R.I. Boughton, D.G. Ran, S.Q. Sun, H.R.

Xia, J. Appl. Phys. 100 (2006) 0635131.[27] E. Orhan, M. Anicete-Santos, M.A.M.A. Maurera, F.M. Pontes, C.O. Paiva-Santos,

A.G. Souza, J.A. Varela, P.S. Pizani, E. Longo, Chem. Phys. 312 (2005) 1.[28] Q. Guo, O.J. Kleppa, Thermochim. Acta 288 (1996) 53.[29] Y. Liu, Y. Chu, Mater. Chem. Phys. 92 (2005) 59.[30] F. Zhang, S.-P. Yang, H.-M. Chen, Z.-H. Wang, X.-B. Yu, J. Cryst. Growth 267

(2004) 569.[31] C. Cui, J. Bi, D. Gao, K. Zhao, J. Alloys Compd. 462 (2008) L16.[32] J. Bi, C.-H. Cui, X. Lai, F. Shi, D.-J. Gao, Mater. Res. Bull. 43 (2008) 743.[33] C. Cui, J. Bi, D. Gao, Mater. Lett. 62 (2008) 2222.[34] C. Cui, J. Bi, C. Wu, S. Zhang, D. Gao, Mater. Res. Bull. 43 (2008) 1160.[35] C. Cui, J. Bi, F. Shi, X. Lai, D. Gao, Mater. Lett. 61 (2007) 4525.[36] H. Lei, X. Zhu, Y. Sun, W. Song, J. Cryst. Growth 310 (2008) 789.[37] Z. Luo, H. Li, J. Xia, W. Zhu, J. Guo, B. Zhang, J. Cryst. Growth 300 (2007) 523.[38] Z. Luo, H. Li, J. Xia, W. Zhu, J. Guo, B. Zhang, Mater. Lett. 61 (2007) 1845.[39] J. Yu, H. Yu, H. Guo, M. Li, S. Mann, Small 4 (2008) 87.[40] J. Yu, H. Guo, S.A. Davis, S. Mann, Adv. Funct. Mater. 16 (2006) 2035.[41] J.-H. Lee, C.-K. Kim, S. Katoh, R. Murakami, J. Alloys Compd. 325 (2001) 276.[42] T. Thongtem, A. Phuruangrat, S. Thongtem, Curr. Appl. Phys. 8 (2008) 189.[43] T. Thongtem, S. Kaowphong, S. Thongtem, Appl. Surf. Sci. 254 (2008) 7765.[44] http://www.icdd.com/profile/overview.htm.[45] A.P.A. Marques, F.C. Picon, D.M.A. Melo, P.S. Pizani, E.R. Leite, J.A. Varela,

E. Longo, J. Fluoresc. 18 (2008) 51.[46] I.L.V. Rosa, A.P.A. Marques, M.T.S. Tanaka, D.M.A. Melo, E.R. Leite, E. Longo, J.A.

Varela, J. Fluoresc. 18 (2008) 239.

[47] T.J.B. Holland, S.A.T. Redfern, Miner. Mag. 61 (1997) 65.[48] G.J. Wilson, A.S. Matijasevich, D.R.G. Mitchell, J.C. Schulz, G.D. Will, Lang-

muir 22 (2006) 2016.[49] T.T. Basiev, A.A. Sobol, Y.K. Voronko, P.G. Zverev, Opt. Mater. 15 (2000) 205.[50] X. Zhao, T.L.Y. Cheung, X. Zhang, D.H.L. Ng, J. Yu, J. Am. Ceram. Soc. 89 (2006)

2960.[51] E.-K. Ryu, Y.-D. Huh, Mater. Lett. 62 (2008) 3081.[52] M. Godinho, C. Ribeiro, E. Longo, E.R. Leite, Cryst. Growth Des. 8 (2008) 384.[53] C. Zhang, E. Shen, E. Wang, Z. Kang, L. Gao, C. Hu, L. Xu, Mater. Chem. Phys. 96

(2006) 240.[54] P. Afanasiev, Mater. Lett. 61 (2007) 4622.[55] Z. Luo, H. Li, J. Xia, W. Zhu, J. Guo, B. Zhang, J. Cryst. Growth 300 (2007) 523.[56] J. Geng, Y. Lv, D. Lu, J.-J. Zhu, Nanotechnology 17 (2006) 2614.[57] http://www2.panasonic.com/consumer-electronics/shop/Home-Appliances/.[58] D.L. Wood, J. Tauc, Phys. Rev. B 5 (1972) 3144.[59] H.W. Eng, P.W. Barnes, B.M. Auer, P.M. Woodward, J. Solid State Chem. 175

(2003) 94.[60] S.K. Arora, B. Chudasama, Cryst. Res. Technol. 41 (2006) 1089.[61] R. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea, D. Martínez-García, A. Se-

gura, Eur. Phys. Lett. 83 (2008) 37002.[62] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.A. Varela, P.S. Pizani, E. Longo, Chem.

Eng. J. 140 (2008) 632.[63] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994.[64] J.G. Yu, X.F. Zhan, S.W. Liu, M. Li, S. Mann, D.H.L. Ng, Appl. Phys. A 87 (2007)

113.[65] http://www.systat.com/products/PeakFit/.[66] http://www.jcrystal.com/steffenweber/JAVA/JSV/jsv.html.[67] http://www.km.kongsberg.com/sim.[68] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11

(1999) 882.[69] L.S. Cavalcante, J.C. Sczancoski, R.L. Tranquilin, M.R. Joya, P.S. Pizani, J.A. Varela,

E. Longo, J. Phys. Chem. Solids 69 (2008) 2674.