Synthesis, Characterization, Anisotropic Growth and

Synthesis, Characterization, Anisotropic Growth and Photoluminescence of BaWO4 L. S. Cavalcante,*,† J. C. Sczancoski,† L. F. Lima, Jr.,† J. W. M. Espinosa,‡ P. S. Pizani,† J. A. Varela,‡ and E. Longo‡

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 1002–1012

Departamento de Quı´mica e Fı´sica, UniVersidade Federal de Saõ Carlos, Saõ Carlos, P.O. Box 676, 13565-905, SP, Brazil, and UniVersidade Estadual Paulista, Araraquara, P.O. Box 355, 14801-907, SP, Brazil ReceiVed July 25, 2008; ReVised Manuscript ReceiVed October 7, 2008

ABSTRACT: This paper reports on the synthesis of barium tungstate (BaWO4) powders obtained by the coprecipitation method and processed in a domestic microwave-hydrothermal (MH) at 413 K for different times. These powders were analyzed by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy and fieldemission gun scanning electron microscopy (FEG-SEM). XRD patterns showed that the BaWO4 powders present a scheelite-type tetragonal structure and free of secondary phases. FT-Raman spectra proved the evidence of a tetragonal structure due to W-O stretching vibration into the [WO4] tetrahedron groups. FT-IR spectra revealed a strong shoulder on the ν3 bands in the transmittance spectra of the powders. FEG-SEM micrographs indicated that the powders present an octahedron-like morphology with agglomerated nature and polydisperse particle size distribution. The morphological evolution of BaWO4 powders with the processing time in a MH system was investigated and a possible growth mechanism was proposed. A green photoluminescence (PL) emission at room temperature was verified for the BaWO4 powders when exited by 488 nm wavelength. Finally, a model was proposed to explain this physical property. 1. Introduction In the last years, the literature has reported the preparation of barium tungstate (BaWO4) by different techniques, such as solid-state reaction,1,2 Czochralski crystal growth,3-5 galvanic cell,6 cell electrochemical,7 hydrothermal-electrochemical,8 molten salt synthesis,9 and polymeric precursor method.10,11 However, these methods generally require high reaction temperatures, long processing times and sophisticated equipment. To minimize these factors, researchers have developed and employed new methods and/or chemical routes for the formation of this material, including solvothermal synthesis,12 templatefree precipitation,13 catanionic reverse-micelle,14 sonochemical,15 metathetic room-temperature preparation,16 microemulsionbased route,17 and hydrothermal.18-20 In particular, the conventional hydrothermal (CH) method has received considerable attention because of its interesting advantages, such as: use of solvent environmental friendly, low processing temperatures, and reduced costs with electric energy.21 The hydrothermal process is defined as a material synthesis method in aqueous medium under temperature and pressure conditions. Generally, the powders synthesized by this method are wellcrystallized and easily dispersible in an aqueous medium.22 In contrast, the main drawback is the slow reaction kinetic for any temperature.23 The use of microwave energy in the CH system promoted the development of a new technique able to offer the following advantages: kinetics of the reaction can be enhanced by 1-2 orders of magnitude, formation of materials with different morphologies, rapid heating to treatment temperature saves time and energy and reduced processing times.24,25 Recently, Thongtem et al. 26,27 reported the use of microwave radiation in a solvothermal process to accelerate the formation of tungstates and/or molybdates with scheelite-type structure. * Corresponding author. Tel: 55 16 3361 5215. Fax: 55 16 3351 8350. E-mail: [email protected]. † Universidade Federal de Saõ Carlos. ‡ Universidade Estadual Paulista.

Figure 1. XRD patterns of BaWO4 powders processed in microwavehydrothermal at 413 K for different times. The vertical dashed lines indicate the position and relative intensity of JCPDS card 43-0646.

Photoluminescence (PL) property of tungstates with scheelitetype tetragonal structure and general formula AXO4 (A ) Ca, Sr, Ba, Zn, Pb, Cd and X ) W) has been reported in the literature by several authors.28-33 In particular, BaWO4 is an interesting material because of its potential for the development of solid-state lasers with emissions in specific spectral regions.34 The green PL emission at room temperature of this material has been observed when occurs a structural modification from WO4 (tetrahedral structure) to WO6 (octahedral structure).35 Zhou et al. 36 verified intense blue and weak green PL emissions at room temperature in BaWO4 powders. These authors mentioned that this behavior can be ascribed to the regular lattice and some defect centers relative to oxygen.35,36 In general, many theories and/or hypotheses on the origin of the green PL emission in BaWO4 have been reported in the literature.7,10,30,35-39

10.1021/cg800817x CCC: $40.75  2009 American Chemical Society Published on Web 12/15/2008

Anisotropic Growth and Photoluminescence of BaWO4

Crystal Growth & Design, Vol. 9, No. 2, 2009 1003

Table 1. Comparative Results between the Lattice Parameters and Unit Cell Volume of BaWO4 Obtained in This Work with Those Reported in the Literature by Different Methodsa method HC HC HC MMHC MH MH MH MH MH MH MH MM JCPDS card

T (K)

453 393 413 413 413 413 413 413 413 413

time (min)

avg lattice a ) b (Å)

param c (Å)

unit-cell volume

ref

12.7210 12.7100 12.7107 12.7200 12.6501(9) 12.7128(4) 12.6913(9) 12.6779(3) 12.6962(4) 12.6987(9) 12.7074(5) 12.7041(7) 12.7059

401.0419(7) 400.010(4) 400.003(8) 400.325(1) 395.567(4) 396.109(4) 396.745(5) 397.220(5) 397.290(9) 398.321(1) 398.891(4) 399.180(3) 400.21

10

2880 720 6 12 24 48 96 192 384 768 43-0646

5.6148 5.6100 5.6098 5.6100 5.5919(3) 5.5819(5) 5.5911(5) 5.5975(6) 5.5939(3) 5.6006(1) 5.6027(1) 5.6054(6) 5.6123

17 19 20 b b b b b b b b

a HC ) Hydrothermal conventional, ME ) microemulsion, SSPM ) stepwise solution-phase method, MT ) modified template, MMHC ) microemulsion-mediated hydrothermal conventional; MH ) microwave-hydrothermal, ref ) reference. b This work.

Figure 2. 1 × 1 × 1 unit cell for the BaWO4.

We believe that all explanations are correct; however, the influence of different preparation methods on the PL behavior should have been considered. This factor is able to affect the PL properties because of the modifications in the degree of structural organization of the lattice by the formation of different morphologies and/or defects in surface.40 Therefore, in this paper, we report on the synthesis of BaWO4 powders by the coprecipitation method and processed in a domestic microwave-hydrothermal at 413K for different times. These powders were analyzed by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, field-emission gun scanning electron microscopy (FEG-SEM), and photoluminescence (PL) measurements. A growth mechanism for the BaWO4 powders was proposed. Moreover, PL behavior of these powders

Table 2. Atomic Coordinates Used to Model the BaWO4 Unit Cella atom

site

x

y

z

tungsten barium oxygen

4a 4b 16f

0 0 0.2330

0 0 0.140

0 0.5 0.0820

a

a ) b ) 5.6054 Å and c ) 12.7041 Å.

as a function of processing time was discussed in detail through a proposed mechanism. 2. Experimental Details 2.1. Synthesis and Processing of BaWO4 Powders. BaWO4 powders were synthesized by the coprecipitation method at room temperature and processed in a MH in the presence of polyethylene glycol (PEG). The typical procedure is described as follows: 5 × 10-3 mol of tungstic acid (H2WO4) (99% purity, Aldrich), 5 × 10-3 mol of

1004 Crystal Growth & Design, Vol. 9, No. 2, 2009

Cavalcante et al. laser was used, keeping its maximum output power at 30 mW. A cylindrical lens was used to avoid powder overheating. The slit width utilized was 100 µm. All measurements were performed at room temperature.

3. Results and Discussion

Figure 3. FT-Raman spectra of BaWO4 powders processed in microwave hydrothermal at 413 K for different times: (a) from 6 to 48 min and (b) from 96 to 768 min. The inset shows the [WO4] tetrahedron group with the respective bond angles (R and β). barium nitrate [Ba(NO3)2] (99.5% purity, Aldrich) and 0.1 g of polyethylene glycol (Mw 200) (99.9% purity, Aldrich) were dissolved in 80 mL of deionized water. The solution pH was adjusted up to 10 by the addition of 5 mL of ammonium hydroxide (NH4OH) (30% in NH3, Synth). Afterward, the aqueous solution was stirred for 1.5 h in ultrasound at room temperature. In the sequence, the solution was transferred into a Teflon autoclave, which was sealed and placed inside a domestic MH system (2.45 GHz, maximum power of 800 W). The MH processing was performed at 413K for different times. The heating rate in the MH system was fixed at 298K/min and the pressure into the autoclave was stabilized at 294 kPa. After MH processing, the autoclave was cooled to room temperature naturally. The resulting solution was washed with deionized water several times to neutralize the solution pH (≈ 7) and the white precipitates were finally collected. The obtained powders were dried in a hot plate at 353K for some hours. 2.2. Characterizations of BaWO4 Powders. The obtained powders were structurally characterized by X-ray diffraction (XRD) using a Rigaku-DMax/2500PC (Japan) with Cu KR radiation (λ ) 1.5406 Å) in the 2θ range from 5 to 75° with 0.02°/min. FT Raman spectroscopy was recorded with a Bruker-RFS 100 (Germany). The spectra were obtained using a 1064 nm line of a Nd:YAG laser, keeping its maximum output power at 95 mW. FT-IR spectroscopies were performed in the range from 725 to 925 cm-1, using a Bruker-Equinox 55 spectrometer in transmittance mode. Ultraviolet-visible (UV-vis) spectroscopy was realized with a Cary 5G (USA) equipment. Photoluminescence (PL) spectra were taken using a U1000 Jobin-Yvon double monochromator coupled to a cooled GaAs photomultiplier with a conventional photon counting system. The 488 nm excitation wavelength of an argon ion

3.1. X-ray Diffraction Analyses. Figure 1 shows the XRD patterns of BaWO4 powders processed at 413 K for different times in the MH system. In this figure, XRD patterns revealed that all diffraction peaks of BaWO4 powders can be indexed to the scheelite-type tetragonal structure in agreement with the respective Joint Committee on Powder Diffraction Standards (JCPDS) card 43-0646.41 Any diffraction peaks correspondent to the secondary phases were not verified. Thus, these results indicate that the BaWO4 powders processed in MH are highly crystalline, pure and ordered at long range. The experimental lattice parameters and unit-cell volume of this material were calculated using the least-squares refinement from the UNITCELL-97 program.42 The obtained results are listed in Table 1. These values are in agreement with the reported in the literature 10,17,19,20 and with the respective JCPDS card. Table 1 also presents a comparative between the lattice parameters and unit cell volume results of BaWO4 obtained in this work with those reported in the literature by different methods. In this table, it was verified that the preparation of BaWO4 powders by the MH system leads to a reduction of heat treatment temperature and processing time. The small deviations in the lattice parameters and unit cell volume values can be associated to the distortions in the lattice caused by the strong coupling between microwave radiation and [WO4] tetrahedron groups.43 3.2. Representation of BaWO4 Unit Cell. Figure 2 shows a schematic representation of 1 × 1 × 1 unit cell for the BaWO4 with I41/a space group. This unit cell was modeled through the Java Structure Viewer Program (version 1.08lite for Windows) and VRML-View (version 3.0 for Windows),44,45 using the atomic coordinates listed in Table 2. In this unit cell, the tungsten atoms are coordinated to four oxygen atoms in a tetrahedral configuration.46,47 This tetrahedral configuration is slightly distorted due to the different bond angles between the oxygen atoms (R ) 107.862° and β ) 112.738°).44 The barium atoms are coordinated to eight oxygen atoms localized in the sides and faces of the tetragonal unit cell in a scalenohedra configuration. The crystalline structure presents an ionic character for the Ba2+ cations and covalent for the WO42- anions. The configurations between Ba-O and W-O were highlighted in the unit cell, as shown in Figure 2. 3.3. Fourier Transform Raman Analyses. Panels a and b in Figure 3 show the Raman spectra in the range from 50 to 1000 cm-1 of BaWO4 powders processed at 413 K for different times in MH system. The inset in Figure 3a illustrates a [WO4] tetrahedron group with the respective bond angles (R and β) between the oxygen atoms. Raman-active phonon modes can be employed to estimate the structural order at short-range in the materials. The group theory calculation presents 26 different vibration modes for the BaWO4, which are represented by eq 1.48,49

Γ ) 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu

(1)

where all vibrations (Ag, Bg, and Eg) are Raman-active modes. A and B modes are nondegenerate, whereas the E modes are doubly degenerate. The subscripted g and u for even and odd, respectively, indicate the parity under inversion in centrosym-



Table 3. Comparative Results between the Raman-Active Modes of BaWO4 Obtained in This Work with Those Reported in the Literature by Different Methodsa M

T (K)

t (min)

Bg/

Eg/

Eg[

Bg[

Agf

Egf

Ag1

Bg •

Bg •

Eg9

Eg9

BI†

Ag

ref

CZ CRM CZ MH MH

1273 298 1473 413 413

1440 180 2400 6 768

63 60 63 62 63

74 72 75 74 75

101 98 101 101 102

133

150

344

352

332 332 333

345 344 345

353 354 355

795 790 795 793 794

831 829 831 830 831

926 920.3 925 924 925

49

150 149 150

331 330 332 330 331

332

132 132 133

191 188 191 190 191

52 50 b b

a M ) Method; T ) temperature; t ) time. Assignment modes: / ) [WO4]2- and Ba2+ motions; [ ) νext-external modes; f ) vf.r. (F1) free rotation, 1 ) v2(E); • ) v4 (F2); 9 ) v3(F2); † ) v1(A1). Preparation methods: MH ) microwave-hydrothermal; CZ ) Czocharalski method, CRM ) chemical reaction method. Ref ) references b This work.

metric crystals. One Au and one Eu correspond to zero frequency of acoustic modes, the others are optic modes. The pairs enclosed in parenthesis arise from the motion of the BaWO4 molecule. In materials with scheelite-type structure, the first member (g) is a Raman-active mode and the second member (u) is active only in infrared (IR) frequencies, except for the Bu silent modes that are not IR active. Consequently, we expect 13 zone-center Raman-active modes in BaWO4, as described by eq 2.49,50

Γ ) 3Ag + 5Bg + 5Eg 50,51

(2)

According to the literature, the vibration modes observed in Raman spectra of tungstates are classified into two types, the external and internal modes. The first is called lattice phonon, which corresponds to the motion of Ba2+ cations and the rigid molecular units. The second belongs to the vibration inside the [WO4]2- molecular units, considering a stationary mass center. In free space, [WO4]2- tetrahedrons present a cubic point symmetry Td.50 Its vibrations are composed by four internal modes (ν1(A1), ν2(E1), ν3(F2), and ν4(F2)), one free rotation mode νf.r.(F1) and one translation mode (F2).51 Table 3 shows a comparative between the Raman-modes obtained in this work with those reported in the literature by different methods. As it can be seen in this table, all Raman-active modes of BaWO4 powders obtained in this work are characteristic of a tetragonal structure, in agreement with the literature 49-51 (Table 3). The small deviations in the positions of Raman-active modes can be related with the different preparation methods, average crystal size and/or degree of structural order in the lattice. The reference52 no presents all Raman-active modes observed in this paper. Hence, we believe that probably the BaWO4 presents a small degree of orientation and/or overlap of diffraction peaks with the W substrate. Also, it was observed the formation of a small shoulder on the band situated at 830 cm-1 (dotted circles in panels a and b in Figure 3), which corresponds to the ν3(F2) internal vibration mode.51 We believe that the presence of this small shoulder is caused by the presence of distortions on the [WO4] tetrahedron groups due to the strong interaction between microwave radiation and tungsten atoms during the MH processing.43 3.4. Fourier Transform Infrared Analyses. FT-IR spectra in the range from 750 to 950 cm-1 of BaWO4 powders processed at 413 K for different times in MH system (see the Support information). BaWO4 presents a scheelite-type tetragonal structure in agreement with XRD patterns and FT-Raman spectra (Figures 1 and 3). eq 1 describes that only the ν3(F2) and ν4(F2) modes are IR active.53,54 The ν3 transmittance bands observed at 808 cm-1 and in the range from 887 to 891 cm-1 are attributed to O-W-O antisymmetric stretching vibration into the [WO4] tetrahedron groups.55 These results are in agreement with the reported in the literature.56-58 Possibly, the small variations in the position of the ν3 band are associated with the distortions into these tetrahedron groups by the microwave radiation.

3.5. Field-Emission Gun Scanning Electron Microscopy Micrographs: Morphology and Particle Size Distribution. Figure 4 shows the FEG-SEM micrographs of BaWO4 powders processed at 413 K for 6, 96, and 768 min in MH system. FEG-SEM micrographs showed that the BaWO4 powders processed at 413 K for 6 min exhibit an octahedron-like morphology (images a and b in Figure 4). The high concentration of microparticles revealed an agglomerate nature with polydisperse particle size distribution (width and height). In general, the literature has reported the formation of BaWO4 with octahedron-like morphology without the presence of surfactants and/or templates.9,59,60 In our work, PEG-200 was employed as surfactant-polymeric in synthesis of BaWO4 microoctahedrons. This surfactant promotes the growth of microoctahedrons more faceted. In this case, it was verified that the increase of processing time in a MH system leads to the growth of micro-octahedrons through the junction between two aggregated microparticles. These self-assembled microcrystals can be observed in BaWO4 powders processed at 413K for 96 min (Figure 4c and Figure 4d). The grow process of self-assembled microcrystals occurs preferentially along the [001] direction, as reported in the literature.60,61 The anisotropic growth in this direction can be related with the differences in the surface energies on each face of the crystal due to the influence of PEG200. This surfactant when adsorbed on the surface of the particles with crystal-face-specific promotes interactions between the tails of surface-adsorbed surfactant molecules that possibly affect the growth of microcrystals.62 In Figure 4e, it was observed that the processing of BaWO4 powders at 413 K for 768 min resulted in the growth of self-assembled microcrystals. In this case, this growth process is caused by the coalescence process between the small particles with octahedron-like morphology (inset in Figure 4f). In the final stage, the long processing time intensifies the coalescence process and consequently favors the formation of some large microcrystals selfassembled. Also, it was observed the presence of surface defects (pores/holes) in some micro-octahedrons (see the Support information). Figure 5 shows the average particle size distribution (height and width) of BaWO4 powders processed at 413 K for 6, 96, and 768 min in the MH system. FEG-SEM micrographs allowed to estimate the average particle size distribution of BaWO4 powders through the counting of approximately 100 particles (Figures 5a-f). Figure 5a shows the average particle height distribution in the range from 0.3 to 1.9 µm for the BaWO4 powders processed at 413 K for 6 min. In this figure, 80% of particles with octahedron-like morphology presented an average height from 0.5 to 0.9 µm. Figure 5b shows the average particle width distribution in the range from 0.25 to 0.95 µm for the BaWO4 powders processed at 413 K for 6 min. It was verified that approximately 74% of particles with octahedron-like morphology exhibit an average width from 0.35 to 0.55 µm.


Cavalcante et al.

Figure 4. FEG-SEM micrographs of BaWO4 powders processed in microwave hydrothermal at 413 K for different times. (a, b) BaWO4 powders with octahedron-like morphology formed after 6 min of processing. The inset in (b) shows a schematic representation of micro-octahedrons, specifying the width and the height. (c, d) Formation of micro-octahedrons self-assembled after 96 min of processing. The inset in (d) Illustrates a schematic representation of self-assembled microcrystals. (e, f) Large self-assembled microcrystals after 768 min of processing. The arrows in (e) show the growth process of self-assembled microcrystals along the [001] direction. The arrows in (f) illustrate the growth process of micro-octahedrons up to the formation of large self-assembled microcrystals.

Probably, these imperfections or differences between height and width are caused by influence of the microwave radiation on the initial growth process of the micro-octahedrons.63 BaWO4 powders processed at 431K for 96 min resulted in the presence of self-assembled microcrystals (images c and d in Figure 4), which contributed for the increase of average particle size distribution (width and height). In this case, it

was observed an average particle height distribution from 0.75 to 4.75 µm (Figure 5c) and average particle width distribution from 0.50 to 1.9 µm (Figure 5d). FEG-SEM micrographs of BaWO4 powders processed at 413 K for 768 min revealed the presence of three different types of particles, such as micro-octahedrons, self-assembled microcrystals and large self-assembled microcrystals (images e and f in Figure



Figure 5. Average particle size distribution of BaWO4 powders processed in microwave-hydrothermal at 413 K for different times. (a) average particle width distribution after 6 min of processing, (b) average particle height distribution after 6 min of processing. The inset shows a schematic representation of micro-octahedrons that were counted. (c) Average particle width distribution after 96 min of processing, (d) average particle height distribution after 96 min of processing. The inset shows a schematic representation of micro-octahedrons and self-assembled microcrystals that were counted. (e) Average particle width distribution after 768 min of processing, (f) average particle height distribution after 768 min of processing. The inset shows a schematic representation of micro-octahedrons, self-assembled microcrystals, and large self-assembled microcrystals that were counted.

4). This long processing time promoted an increase of average particle size distribution (height and width) as results of the coalescence process. As consequence, several particles presented an average particle height distribution from 0.95

to 11.85 µm and average particle width distribution from 0.50 to 2.30 µm (images e and f in Figure 5). These results show that the processing time in MH system is an important variable, which is able to influence in the grow process and


Cavalcante et al.

Figure 6. Schematic representation of the synthesis, processing and growth mechanism for the formation of large BaWO4 self-assembled microcrystals. (a) Chemical synthesis; (b) coprecipitation reaction, hydrolysis and addition of surfactant (PEG); (c) microwave-hydrothermal system; (d) increase in the effective collision rate between the small particles by the microwave radiation; (e) growth process of BaWO4 microparticles with octahedronlike morphology to self-assembled microcrystals and (f) growth mechanism of microcrystals as a function of processing time.

consequently, in the average particle size distribution (width and height) of BaWO4 powders. 3.6. Growth Mechanism of BaWO4 Powders with Octahedron-Like Morphology. Figure 6 shows a schematic representation of the stages involved in the synthesis and growth of BaWO4 particles processed in MH system. Figure 6a illustrates the initial synthesis process of BaWO4 particles by the coprecipitation reaction arising from the solubilization process between tungstic acid and barium salt dissolved in water. As can be seen in this figure, the resulting solution was stirred for 1 h in order to accelerate the coprecipitation rate. In Figure 6b, the addition of 5 mL of NH4OH was employed to intensify the hydrolysis rate of the solution. In this case, Ba2+ cations are accept species of electron pairs (Lewis acid), whereas WO42- are electron pair donors (Lewis base). The reaction between these two species Ba2+r:WO42results in a covalent bond. The covalent bond occurs due to the Lewis acid to occupy the lowest molecular orbital (LUMO),

which interacts with the highest molecular orbital (HOMO) of the Lewis base. One-tenth of a gram of PEG was added into this solution to promote an interaction between the small particles preformed. Afterward, the chemical solution was stirred in ultrasound for 30 min and subsequently transferred to the MH system (Figure 6b). Figure 6c illustrates a schematic representation of the MH system employed in the processing of BaWO4 powders. This equipment consists of a series of adaptations performed on a domestic microwave oven (model NN-ST357WRPH, Panasonic).64 Figure 6d shows a schematic representation of the processing of BaWO4 powders in MH system. Inside this system, the high frequency of the microwave radiation interacts with the permanent dipole of the liquid (H2O), which initiates a rapid heating from the resultant molecular rotation. Likewise, permanent or induced dipoles in the dispersed phase cause a rapid heating of the particles.65 Thus, the microwave radiation is able to promote an increase of the



Figure 7. Schematic representation of the hydrothermal cell under microwave radiation, (b) dotted cube shows a distorted [WO4] tetrahedron in the center of BaWO4 unit cell. Possible distortions on the [WO4] tetrahedron groups through displacements on the tungsten atoms along the atomic coordinates (x, y, and z) (c): (0.0;0.0;0.0), (d) (0.05;0.0;0.0), (e) (0.0;0.05;0.0), (f) (0.0;0.0;0.02), (g) (0.03;0.03;0.0), (h) (0.03;0.0;0.02), (i) (0.0;0.03;0.02), (j) interaction of the microwave radiation on the [WO4] tetrahedron groups (k) PL behavior at room temperature of BaWO4 powders processed in microwave-hydrothermal at 413 K for different times.

effective collision rate between the small particles.66 This mechanism favors the coalescence process and leads to a fast nucleation of BaWO4 seeds. The presence of PEG (nonionic surfactant) in the chemical solution probably contributes to the agglomeration process of small particles (micro-octahedrons) because of the interaction between the hydrogen bonding of water with the OH groups of this surfactant.67,68 Figure 6e shows that after 6 min of processing in MH, occurs a fast growth process of BaWO4 microparticles with octahedron-like morphology to self-assembled microcrystals. As previously described, these self-assembled microcrystals are formed by the junction between small aggregated microparticles along the [001] direction (blue arrows in Figure 6e). Possibly, PEG is an important factor for the growth of these microparticles in this preferential direction. In Figure 6f, it was observed that the long processing

time intensify the growth process and results in the formation of large BaWO4 self-assembled microcrystals. In this figure, it was possible to verify the growth mechanism of microoctahedrons as a function of processing time to large selfassembled microcrystals through the increase in the faces along the [001] direction (blue faces in Figure 6f). This proposed growth mechanism indicates that the PEG induced the formation of c-axis-oriented microparticles self-assembled, in agreement with the reported in the literature.69-72 3.7. Photoluminescence Analyses: Distortions and Defects in the BaWO4 Lattice. In particular, PL emission is considered a powerful tool to obtain informations on the electronic structure and degree of structural organization at medium-range of the materials.73 Moreover, this optical property is sensible to the presence of energy levels within the band gap.


Cavalcante et al.

Table 4. Possible Distortions on the [WO4] Tetrahedron Groups through Displacements on the Tungsten Atoms along the Atomic Coordinates (x, y, and z) and Bond Angles (r,β, O, µ, G, and γ) between O-W-O and Variations in the Bond Distances tungsten atom displacements* (x;y;z)

bonds (W-O)

bond distance (Å)

bonds [O-W-O]

bond angles (deg)

(0.0; 0.0; 0.0)

W5-O1 W5-O2 W5-O3 W5-O4

1.7668(1) 1.7668(1) 1.7668(1) 1.7668(1)

(0.05; 0.0; 0.0)

W5-O1 W5-O2 W5-O3 W5-O4

1.8986(5) 1.6687(7) 1.9690(7) 1.5850(6)

(0.0; 0.05; 0.0)

W5-O1 W5-O2 W5-O3 W5-O4

1.8986(5) 1.6687(7) 1.9690(7) 1.5850(6)

W5-O1 W5-O2 W5-O3 W5-O4

1.6466 1.6466 1.9093 1.9093

(0.03; 0.03; 0.0)

W5-O1 W5-O2 W5-O3 W5-O4

1.9569(9) 1.5870(9) 1.8269(6) 1.7352(0)

(0.03; 0.00; 0.02)

W5-O1 W5-O2 W5-O3 W5-O4

1.7740(7) 1.5259(1) 1.8509(4) 1.9794(1)

(0.0; 0.03; 0.02)

W5-O1 W5-O2 W5-O3 W5-O4

1.5785(5) 1.7273(9) 1.8062(6) 1.8062(6)

O1-W5-O2 O1-W5-O3 O1-W5-O4 O2-W5-O3 O2-W5-O4 O3-W5-O4 O1-W5-O2 O1-W5--O3 O1-W5-O4 O2-W5-O3 O2-W5-O4 O3-W5-O4 O1-W5-O2 O1-W5-O3 O1-W5-O4 O2-W5-O3 O2-W5-O4 O3-W5-O4 O1-W5-O2 O1-W5-O3 O1-W5-O4 O2-W5-O3 O2-W5-O4 O3-W5-O4 O1-W5-O2 O1-W5-O3 O1-W5-O4 O2-W5-O3 O2-W5-O4 O3-W5-O4 O1-W5-O2 O1-W5--O3 O1-W5-O4 O2-W5-O3 O2-W5-O4 O3-W5-O4 O1-W5-O2 O1-W5-O3 O1-W5-O4 O2-W5-O3 O2-W5-O4 O3-W5-O4

β ) 112.7384 R ) 107.8627 R ) 107.8627 β ) 112.7384 β ) 112.7384 R ) 107.8627 β ) 110.9593 µ ) 95.1880 F ) 109.8215 φ ) 103.159836 γ ) 122.7395 R ) 111.29243 β ) 110.9593 µ ) 95.1880 F ) 109.8215 φ ) 103.1598 γ ) 122.7395 R ) 111.29243 β ) 126.6009 µ ) 106.6443 F ) 100.7905 φ ) 106.6443 γ ) 106.4243 R ) 106.6444 β ) 111.8083 µ ) 97.9636 F ) 101.1862 φ ) 113.3843 γ ) 118.5046 R ) 111.3447 β ) 125.9781 µ ) 103.9655 F ) 98.9525 φ ) 115.1860 γ ) 108.4505 R ) 100.3173 β ) 125.9781 µ ) 99.0089 F ) 107.8406 φ ) 104.3886 γ ) 114.9320 R ) 100.3592

(0.0; 0.00; 0.02)

a

Atomic coordinates.

In Figure 7, it was proposed as a possible mechanism to explain the PL behavior of BaWO4 powders processed at 413 K for different times in MH system. Figure 7a shows the influence of microwave radiation on the BaWO4 lattice. The tungsten atoms are considered good microwave absorbers.74 In this case, the interaction between microwave radiation and tungsten atoms (network formers) resulted in a rapid heating process on the [WO4] tetrahedron groups. We believe that this mechanism is able to promote the formation of defects and/or distortions on these tetrahedrons. This hypothesis is in agreement with the Raman spectra (Figure 3). Figure 7b presents a slightly distorted [WO4] tetrahedron (dotted cube) in the center of BaWO4 unit cell and with different bond angles between O-W-O (R ) 107.8627° and β ) 112.7384°). The possible distortions on the [WO4] tetrahedron groups by the microwave radiation were simulated through displacements on the tungsten atoms along the atomic coordinates (x, y and z). These displacements are shown in Figure 7c-i and listed in Table 4. Table 4 presents the bond distance values and bond angles between the tungsten and oxygen atoms, using the atomic coordinates (x, y, and z) referents to the distorted [WO4] tetrahedron groups. We believe that these distortions are able to influence in the final response of the PL emission.

In Table 4, the results indicated that the displacement of tungsten atoms occurs by means of six different bond angles (R, β, φ, µ, F, and γ) between O-W-O. Possibly, the variation in these angles is associated with the differences in the bond distances. Figure 7j shows the interaction between the microwave radiation with the [WO4] tetrahedron groups, resulting in distortions and/or defects in the BaWO4 lattice. In Figure 7k, it was observed that the PL spectra are a broadband covering a large part of the visible electromagnetic spectrum, with a maximum emission situated at around 542 nm (green emission). This broad band suggests that the emission process is typical of a multiphonon or multilevel process, i.e., a system in which the relaxation occurs by means of several paths. As can be seen in this figure, the increase in the PL intensity up to 96 min of processing can be related to the reduction of surface defects on the micro-octahedrons/microcrystals and presence of intermediary energy levels within the band gap. After this processing time, the reduction in the PL intensity probably is caused by the reduction of intermediary energy levels within the band gap, increase of average particle size and/or reduction in the distortions on the [WO4] tetrahedron groups. These results are in agreement with the FT-Raman spectra, in which was verified a considerable reduction on the shoulder situated at 830 cm-1 (dotted circles in Figure 3b). Also, it was not verified PL


emission for the BaWO4 powders obtained by the coprecipitation method. Probably, this behavior is caused by the high concentration of surface defects on the particles (see the Support Information). Several hypotheses have been reported in the literature to explain the possible mechanisms responsible by the PL emission of BaWO4. Recently, Lima et al. 10 related that the PL emission at room temperature of BaWO4 powders can be associated with the structural disorder degree in the lattice. However, our results indicate that the BaWO4 powders processed in MH system are highly crystalline and structurally ordered at long and shortrange, as verified through the XRD patterns (Figure 1) and Raman spectra (a and b in Figure 3). Therefore, PL emission of these powders processed in MH is not due to the high degree of structural disorder in the lattice. We believe that the PL emission is caused by the structural defects and/or distortions on the [WO4] tetrahedron groups by the microwave radiation. Moreover, PL response can be related with the growth process of micro-octahedrons, which is able to promote the formation of structural defects in BaWO4 lattice. These factors favor the formation of visible light emission centers responsible by the PL property of this material.75 Our results are in agreement with others works reported in the literature.76-80 4. Conclusions BaWO4 powders were synthesized by the coprecipitation method and processed at 413K for different times in a microwave-hydrothermal system. XRD patterns, FT-Raman and FT-IR spectra showed that these powders crystallize in a scheelite-type tetragonal structure without the presence of deleterious phase. FEG-SEM micrographs revealed that the processing time is able to influence in the growth mechanism of BaWO4 powders. These micrographs also indicated that the PEG is responsible by the anisotropic growth of BaWO4 microparticles along the [001] direction. A possible growth mechanism for the growth of self-assembled BaWO4 microcrystals was discussed in details. PL emission at room temperature of BaWO4 powders exhibited a maximum emission at around 542 nm (green emission). PL behavior was explained through distortions on the [WO4] tetrahedron groups and also due to the structural defects in the BaWO4 lattice. Acknowledgment. The authors are grateful for the financial support from the Brazilian research financing institutions: CAPES, CNPq, and FAPESP. Special thanks to Prof. Dr. D. Keyson and Dr. D.P. Volanti for the development of the domestic microwave hydrothermal system. Supporting Information Available: Additional figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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CG800817X

Synthesis, Characterization, Anisotropic Growth and

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