Morphology and Blue Photoluminescence Emission of PbMoO4

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J. Phys. Chem. C 2009, 113, 5812–5822

Morphology and Blue Photoluminescence Emission of PbMoO4 Processed in Conventional Hydrothermal J. C. Sczancoski,† M. D. R. Bomio,† L. S. Cavalcante,*,‡ M. R. Joya,‡ P. S. Pizani,‡ J. A. Varela,§ E. Longo,§,| M. Siu Li,§ and J. A. Andrés¶ IQ, UniVersidade Estadual Paulista, P.O. Box 355, 14801-907, Araraquara, SP, Brazil, LIEC, Departamento de Quı´mica e Fı´sica, UFSCar, P.O. Box 676, 13565-905, Saõ Carlos, SP, Brazil, Departamento de Bioquı´mica e Tecnologia Quı´mica, UniVersidade Estadual Paulista, P.O. Box 355, 14801-907, Araraquara, SP, Brazil, IFSC, UniVersidade de Saõ Paulo, P.O. Box 369, 13560-970, Saõ Carlos, SP, Brazil, and Departamento de Quı´mica Fı´sica y Analı´tica, UniVersitat Juame I, 12071; Castello, Spain ReceiVed: NoVember 23, 2008; ReVised Manuscript ReceiVed: January 27, 2009

PbMoO4 micro-octahedrons were prepared by the coprecipitation method at room temperature without the presence of surfactants and processed in a conventional hydrothermal at different temperatures (from 60 to 120 °C) for 10 min. These micro-octahedrons were structurally characterized by X-ray diffraction (XRD) and micro-Raman (MR) spectroscopy, and its morphology was investigated by field-emission gun scanning electron microscopy (FEG-SEM). The optical properties were analyzed by ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectra confirmed that the PbMoO4 micro-octahedrons are characterized by a scheelite-type tetragonal structure. FEG-SEM micrographs points out that these structures present a polydisperse particle size distribution in consequence of a predominant growth mechanism via aggregation of particles. In addition, it was observed that the hydrothermal conditions favored a spontaneous formation of micro-octahedrons interconnected along a common crystallographic orientation (oriented-attachment), resulting in self-organized structures. An intense blue PL emission at room temperature was observed in these micro-octahedrons when they were excited with a 350 nm wavelength. The origin of the PL emissions as well as its intensity variations are explained by means of a model based on both distorted [MoO4] and [PbO8] clusters into the lattice. Introduction At room temperature, the molybdates and tungstates with scheelite-type tetragonal structure present a general formula ABO4 (A ) Ca, Sr, Ba, Pb; B ) W, Mo), space group I41/a and symmetry C4h6.1-9 The scheelite-type family of molybdates and tungstates have been investigated extensively with the intention of obtaining new functionalities and behaviors related to a broad range of properties.10 In particular, recent theoretical and experimental studies on the optical properties of lead molybdate (PbMoO4) have been reported in the literature.11-16 This scheelite is a promising material with a wide potential for different industrial applications, such as scintillation detectors,17 birefringent filters,18 fiber optics,19 photoconductivity,20 luminescence,21-24 thermoluminescence25,26 and photocatalysis.27 In the past decades, PbMoO4 has been prepared mainly by solid state reaction28,29 and Czochralski crystal growth.30-33 Nevertheless, the preparation of this material usually requires complex experimental procedures, sophisticated equipment, and rigorous synthesis conditions. Thus, in recent years, new synthesis methods, such as electrochemical reactor,34 chemical route,35 galvanic cell method,36-38 vertically supported liquid membrane,39 solution reaction assisted ball-rotation,40 citrate complex,41,42 chemical reactions,43 and aqueous solution,44 have been * E-mail: [email protected]. † IQ, Universidade Estadual Paulista. ‡ UFSCar. § Universidade de Saõ Paulo. ¶ Universitat Juame I. | Departamento de Bioquı´mica e Tecnologia Quı´mica, Universidade Estadual Paulista.

developed with the intention of minimizing these drawbacks. However, in some of these methods, there still are verified serious problems, mainly including formation of residual organic compounds, polydisperse particle size distribution, and uncontrolled morphology. Therefore, the hydrothermal method has become an effective synthetic route in materials science, drastically increasing the control on the micro/nanometric morphology and orientation.45 In addition, this method is environmentally friendly and depends of the solubility of chemical salts in water under temperature and pressure conditions. The preparation method of PbMoO4 with controlled particle sizes and special morphologies to control and find new properties is an open research line. In principle, few works in the literature have reported the formation of this material with different morphologies under controlled experimental conditions. Recently, Dong and Wu39 reported the synthesis of PbMoO4 nanobelts using a vertically supported liquid membrane system in the presence of ethylenediamine at room temperature. These authors proposed a crystal growth mechanism via oriented attachment and aggregation due to the influence of ethylenediamine (modifier agent) into the system. The growth mechanism known as oriented attachment46-48 is a process in that the adjacent particles are self-assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface.49,50 In this paper, we report the synthesis of PbMoO4 microoctahedrons by the coprecipitation method and processing in a conventional hydrothermal (CH) at different temperatures (60-120 °C) for 10 min. These micro-octahedrons were analyzed by X-ray diffraction (XRD), micro-Raman (MR)

10.1021/jp810294q CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

Morphology, Photoluminescence Emission of PbMoO4

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5813

Figure 1. Flow chart illustrating the experimental procedure employed in the synthesis and processing of PbMoO4 micro-octahedrons.

spectroscopy, ultraviolet-visible (UV-vis) absorption spectroscopy, field-emission gun scanning electron microscopy (FEG-SEM), and photoluminescence (PL) measurements. The main aim is to understand the PL behavior of PbMoO4 microoctahedrons in terms of structural distortions on the [MoO4] and [PbO8] clusters. Experimental procedure Synthesis and Hydrothermal Processing of PbMoO4 Micro-Octahedrons. PbMoO4 micro-octahedrons were synthesized by the coprecipitation method at room temperature and processed in a CH system at different temperatures (from 60 to 120 °C) for 10 min. Surfactants or templates were not used in this chemical synthesis. The typical experimental procedure is described as follows: 5 × 10-3 mol of molybdic acid (H2MoO4) (85% purity, Synth) and 5 × 10-3 mol of lead nitrate [Pb(NO3)2] (99.5% purity, Merck) were dissolved in 75 mL of deionized water. The solution pH was adjusted to 11 by the addition of 5 mL of ammonium hydroxide (NH4OH) (30% in NH3, Synth) to increase the hydrolysis rate. Afterward, this solution was stirred for 30 min in ultrasound to accelerate the coprecipitation rate. In the sequence, this preformed mixture was transferred into a stainless autoclave, which was sealed and placed into a CH system (Nanox Technology S/A, NanoxHydrocell H-100, Brazil).51 The hydrothermal processing was performed at different temperatures in the range from 60 to 120 °C for 10 min using a heating rate of 2 °C/min. After hydrothermal treatment, the autoclave was cooled at room temperature naturally. The resulting solution was washed with deionized water several times to neutralize the solution pH (≈7). Finally, the white precipitates were collected and dried at 70 °C for some hours. Figure 1 shows the typical experimental procedure employed in the synthesis and processing of PbMoO4 microoctahedrons. Characterizations of PbMoO4 Micro-Octahedrons. After CH processing, PbMoO4 micro-octahedrons were structurally

Figure 2. (a) XRD patterns of PbMoO4 micro-octahedrons processed in conventional hydrothermal at different temperatures (60-120 °C) for 10 min and (b) a, b, and c lattice parameters as a function of processing temperature. The vertical bars show the standard mean error.

5814 J. Phys. Chem. C, Vol. 113, No. 14, 2009

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TABLE 1: Comparative Results between the Lattice Parameters and Unit Cell Volume of PbMoO4 Obtained in This Work with Those Reported in the Literature by Different Synthesis Methodsa T method (°C) CZ CZ SSR CH CH CH CH CH JCPDS card

1200 1400 1200 160 60 80 100 120 N°.

t (min)

av lattice, parameters, unit cell a ) b (Å) c (Å) volume

1440 2880 1800 1200 10 10 10 10 44-1486

5.4351 5.4355 5.4360 5.433 5.418 5.421 5.423 5.464 5.433

12.1056 12.108 12.1107 12.11 12.065 12.085 12.088 12.065 12.11

357.603 357.726 357.872 357.456(8) 396.471(3) 396.643(5) 396.584(5) 396.623(6) 357.456(8)

ref 56 57 58 59 this work this work this work this work 52

TABLE 2: Atomic Coordinates Employed To Model the PbMoO4 Unit Cella atoms

site

x

y

z

lead molybdenum oxygen

4b 4a 16f

0 0 0.767 31

0 0 0.140 13

0.5 0 0.081 88

a

a ) b ) 5.464 Å, and c ) 12.065 Å.

T ) temperature, t ) time, CZ ) Czochralski method; SSR ) solid state reaction; CH ) conventional hydrothermal. a

characterized by XRD using a DMax/2500PC diffractometer (Rigaku, Japan) with Cu-KR radiation (λ ) 1.5406 Å) in the 2θ range from 10° to 75° and step size of 0.02°/min. MR measurements were recorded using a T-64000 spectrometer (Jobin-Yvon, France) triple monochromator coupled to a CCD detector. The spectra were performed using a 514.5 nm wavelength of an argon ion laser, keeping its maximum output power at 8 mW. A 100 µm lens was used to prevent sample overheating. The morphologies and particle size distribution were investigated with a Supra 35-VP FEG-SEM (Carl Zeiss, Germany) operated at 6 kV. UV-vis absorption spectra were performed using a Cary 5G (Varian, U.S.A.) equipment in total reflection mode. PL spectra were measured with an Ash Monospec 27 monochromator (Thermal Jarrel, U.S.A.) and a R4446 photomultiplier (Hamamatsu Photonics, U.S.A.). The 350 nm wavelength of a krypton ion laser (Coherent Innova 90 K) was used as excitation source, keeping its maximum output power at 200 mW. All measurements were performed at room temperature. Results and Discussion X-ray Diffraction Analyses. Figure 2a shows the XRD patterns of PbMoO4 micro-octahedrons processed in CH system

Figure 4. MR spectra in the range from 50 to 1000 cm-1 of PbMoO4 micro-octahedrons processed in conventional hydrothermal at different temperatures (60-120 °C) for 10 min.

at different temperatures for 10 min and Figure 2b shows the lattice parameter values as a function of processing temperature. XRD patterns revealed that all diffraction peaks of PbMoO4 micro-octahedrons can be indexed to the scheelite-type tetragonal structure without the presence of secondary phases, in agreement with the respective Joint Committee on Powder Diffraction Standards (JCPDS) card no. 44-1486.52 The relative intensities and sharp diffraction peaks indicated that the PbMoO4 are well-crystallized, suggesting an ordered structure at long range. The lattice parameters were calculated using the leastsquares refinement from the Unitcell-97 program.53 The obtained

Figure 3. PbMoO4 1 × 1 × 1 unit cell illustrating the [MoO4]/[PbO8] clusters.



TABLE 3: Comparative Results between the Raman-Active Modes of PbMoO4 Obtained in This Work and Those Reported in the Literature by Different Synthesis Methods Ma c

CZ SSRc CZc CHc CHc

T (°C)a

t (min)a

Eg, ∇b

Eg, ∇b

Ag, ∇b

Bg, 0b

Eg, fb

Bg, (b

Eg, bb

Eg, 1b

Bg, 1b

Ag, 0b

ref

1200 1300 1250 60 120

1440 720 144 10 10

66

72

102

167

191

71 69 70

78 80 80

107 110 110

171 176 177

193 197 198

317 312 323 324 324

349 347 354 355 357

743 742 748 749 750

764 763 771 773 773

838 864 871 876 876

73 74 75 this work this work

M ) method; T ) temperature; t ) time. b Assignments modes: ∇ ) [MoO4]2- and Pb2+ motions; 0 ) νext, external modes; f ) νfr (F1) free rotation, ( ) ν2(E); b ) ν4(F2); 1 ) ν3(F2); 0 ) ν1(A1). c Preparation methods: CZ ) Czocharalski method, SSR ) solid state reaction, CH ) conventional hydrothermal. a

values are shown in Figure 2b and displayed in Table 1, presenting a good agreement with those reported in the literature.54-59 An analysis of the results presented in Table 1 indicates that the preparation of PbMoO4 microcrystals by means of the CH processing is able to promote a significant reduction in the heat treatment temperature and processing time, as compared to the others’ synthesis methods. The small variations in the lattice parameters and unit cell volumes are indications of small distortions on the PbMoO4 lattice in function of the continuous dissolution and recrystallization process of microcrystals caused by the CH processing conditions.60 Higher a and b lattice parameter values verified for the PbMoO4 microcrystals processed at 120 °C for 10 min can be ascribed to the influence of vapor pressure into the stainless autoclave or strain in the lattice. Representation of PbMoO4 Unit Cell. Figure 3 shows a schematic representation of a PbMoO4 1 × 1 × 1 unit cell with I41/a space group. This unit cell was modeled using the Java Structure Viewer Program (version 1.08lite for Windows) and VRML-View (version 3.0 for Windows)61,62 by means of the atomic coordinates listed in Table 2. In a PbMoO4 unit cell, the Mo atoms are coordinated to four oxygens, and it can be described as [MoO4] clusters, that is, a polyhedron-type with tetrahedral configuration. These [MoO4] clusters are slightly distorted into the unit cell with O-Mo-O bond angles of approximately 107.8° and 112.7°.63,64 On the other hand, it is well-defined in the literature65 that the lead atoms are coordinated to eight oxygens, considered as [PbO8] clusters, forming a sub disphenoid-type polyhedron with scalenohedral configuration.66 The crystal structure presented by the PbMoO4 is characterized by a covalent/ionic character between the Pb-O bonds, whereas the Mo-O bonds present a covalent nature along the [MoO4] clusters. The coordinations of both Pb-O and Mo-O bonds were highlighted outside of the unit cell. Micro-Raman Spectroscopy Analyses. Figure 4 shows the Raman spectra in the range from 50 to 1000 cm-1 of PbMoO4 micro-octahedrons processed in CH system at different temperatures for 10 min. The Raman-active phonon modes can be used to estimate the structural order at short range in the materials. The group theory calculation presents 26 different vibrations for the PbMoO4, which can be represented by eq 1,67,68

Γ ) 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu

(1)

where all vibrations (Ag, Bg, and Eg) are Raman-active, A and B modes are nondegenerate, whereas E modes are doubly degenerate. One g and u correspond to the zero frequency of acoustic modes; the others are optic modes. The pairs of species enclosed in parentheses arise from the motion of PbMoO4

molecules. In materials with scheelite-type structure, the first member of the pairs (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 PbMoO4, as described by eq 2.69,70

Γ ) 3Ag + 5Bg + 5Eg

(2)

According to the literature,71 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 Pb2+ cations and the rigid molecular units. The second belongs to the vibration inside the [MoO4]2- molecular units with the centers of mass stationary. In free space, [MoO4]2- tetrahedrons present a cubic point symmetry Td.71 Its vibrations are composed of four internal modes (ν1(A1), ν2(E1), ν3(F2), and ν4(F2)), one free rotation mode νfr(F1), and one translation mode (F2).72 Table 3 shows a comparatison between the Raman modes obtained in this work and those reported in the literature by different methods. An analysis of the results presented in Table 3 indicated that all Raman-active modes of PbMoO4 micro-octahedrons obtained in this work are characteristic of a tetragonal structure, in agreement with those previously reported in the literature.73-75 The small shifts observed on the positions of Raman modes can be arising from different factors, such as preparation methods, average crystal size, distortions on the O-Mo-O and O-Pb-O bonds, interaction forces between the ions, or degree of structural order in the lattice.76 Moreover, the well-defined active-Raman modes confirm that PbMoO4 are structurally ordered at short-range, independent of processing temperature employed in the CH treatment. Field-Emission Gun Scanning Electron Microscopy Analysis: Morphology and Particle Size Distribution. Figure 5 shows the FEG-SEM micrographs of PbMoO4 micro-octahedrons processed in the CH system at different temperatures for 10 min. FEG-SEM micrographs showed that the PbMoO4 powders processed at 60 °C for 10 min present a large quantity of anisotropic microcrystals with octahedron-like morphology and several nucleation seeds with irregular shapes (Figure 5a). Moreover, it was observed an initial growth process of these morphologies through a self-organization of adjacent microcrystals in a similar crystallographic orientation (“oriented attachment”),77 followed by a subsequent coalescence process (Figure 5b). These morphologies formed by oriented attachment process have been commonly observed for the materials with scheelite-type structure composed of Pb2+ ions48,78 and other ions (Ca2+, Ba2+, and Sr2+).46,47 This behavior can arise from the covalent Pb-O bonds, whereas


Sczancoski et al.

Figure 5. FEG-SEM micrographs of PbMoO4 micro-octahedrons processed in conventional hydrothermal at (a, b) 60, (c, d) 80, (e, f) 100, and (g, h) 120 °C for 10 min.

for X-O bonds (X ) Ca, Ba, Sr), the ionic character appears; therefore, these bonds present directional and radial character,

respectively. As a consequence of these bond processes, the morphologies of materials with scheelite-type structure



Figure 6. Average particle height and width distribution of PbMoO4 micro-octahedrons processed in the conventional hydrothermal at (a, b) 60, (c, d) 80, (e, f) 100, and (g, h) 120 °C for 10 min.

composed of Pb2+ ions tend to be facetted and aligned by “docking” processes involving crystallographic fusion between some faces with high surface energy, creating an extended morphology.50

The processing performed at 80 °C for 10 min contributed to the aggregation between the microparticles, promoting the coalescence of individual micro-octahedrons (Figure 5c) and self-organized micro-octahedrons (Figure 5d and inset). This


Sczancoski et al.

Figure 7. UV-vis absorbance spectra of PbMoO4 micro-octahedrons processed in conventional hydrothermal at (a) 60, (b) 80, (c) 100, and (d) 120 °C for 10 min.

TABLE 4: Comparative Results between Egap Values of PbMoO4 Obtained in This Work and Those Reported in the Literature by Different Synthesis Methods method

temp (°C)

time (min)

Egap (eV)a

ref

CMb LAPWTb CZb NDVMb CHb CHb CHb CHb

900

1800

1000

1440

60 80 100 120

10 10 10 10

2.94 3.62 3.58 4.7 3.14 3.16 3.17 3.19

35 56 87 88 this work this work this work this work

Egap ) optical band gap energy. b CM ) chemical method; LAPWT ) linearized-augmented-plane-wave technique; NDVM ) numerically discrete variational (DV-Xa) method, and CH ) conventional hydrothermal. a

behavior can be verified by the increase in the number of large micro-octahedrons, while the quantity of small micro-octahedrons is decreased. PbMoO4 powders processed at 100 °C for 10 min resulted in the formation of micro-octahedrons with irregular shapes along the different crystallographic planes (Figures 5e, f, and inset). Therefore, this result indicates that, at this temperature, the thermal energy is capable of promoting an intense and continuous nucleation-dissolution-recrystallization mechanism60 during the CH processing. This mechanism is considered highly sensitive to the relative rates of dissolution of the amorphous solid particles and nucleation of the crystalline phase.79,80 As a consequence, this mechanism favored the formation of a high concentration of aggregated microparticles. After CH processing at 120 °C for 10 min, the anisotropic growth of PbMoO4 micro-octahedrons is predominantly controlled by the coalescence process rather than by the oriented attachment mechanism (Figures 5g and h). This behavior can be associated with the role of hydroxide ions in aqueous

medium.81 As can be seen in Figure 5g and h, the coalescence process contributed to the growth of several microparticles and resulted in an imperfectly oriented attachment mechanism between the micro-octahedrons as well as the formation of surface defects on its facets. Figure 6 shows the average particle size distribution (height and width) of PbMoO4 micro-octahedrons processed in the CH system at different temperatures for 10 min. FEG-SEM micrographs were also employed to estimate the average particle size distribution of PbMoO4 micro-octahedrons, making a counting of approximately 100 particles (Figures 6a-h). Figure 6a shows that 79% micro-octahedrons presented an average particle height distribution from 0.5 to 0.9 µm, whereas in Figure 6b, it was verified that 80% microoctahedrons exhibited an average particle width distribution from 0.3 to 0.7 µm. The slight differences or bipyramidal imperfections between height and width of these morphologies can be caused by a faster growth rate along the [001] direction, due to high-energy surface under crystallographic fusion and elimination of the high-energy faces under energy gain in relation to [100] direction that present a low-energy surface. Thus, a crystal growth along [001] is preferred than on the [100] direction.82,83 The CH processing performed at 80 °C for 10 min promoted the aggregation of several microparticles on different crystallographic orientations, favoring the coalescence of PbMoO4 micro-octahedrons.84 Consequently, this crystal growth mechanism resulted in the formation of 68% micro-octahedrons with average particle height distribution in the range from 0.7 to 1.1 µm and 88% micro-octahedrons with average particle width distribution in the range from 0.3 to 0.9 µm (Figure 6c and d). Insets in Figure 6c and d show the different crystallographic planes of PbMoO4 micro-octahedrons simulated in the JCrys



Figure 8. (a) PL spectra at room temperature of PbMoO4 micro-octahedrons processed in the conventional hydrothermal system at different temperatures and (b) possible distortions on both [MoO4] and [PbO8] clusters by means of displacements on the molybdate or lead atoms along the atomic coordinates (x, y, and z).

talSoft 2006 program.85 The increase in processing temperature at 100 °C favored a fast and noncontrolled aggregation process of several microparticles, consequently promoting the formation and anisotropic growth of micro-octahedrons with irregular shapes. This behavior resulted in 84% micro-octahedrons with average particle height distribution in the range from 0.7 to 1.3 µm and 77% micro-octahedrons with average particle width distribution in the range from 0.5 to 0.9 µm (Figure 6f and g). After CH processing at 120 °C for 10 min, the presence of large micro-octahedrons with average height from 0.7 to 1.3 µm (85%) and average width from 0.5 to 1.1 µm (87%) (Figure 4g and 4h) was verified. These results indicate the predominance of the coalescence process via aggregation of particles during the growth of PbMoO4 micro-octahedrons. UV-visible Absorption Spectroscopy Analyses. Figure 7a-d shows the UV-vis absorbance spectra of PbMoO4 microoctahedrons at different temperatures for 10 min. The optical band gap energy (Egap) was calculated by the method proposed by Wood and Tauc.86 According to these

authors, the optical band gap is associated with absorbance and photon energy by the following equation:

hνR ∝ (hν - Egap)n

(3)

where R is the absorbance, h is the Planck constant, ν is the frequency, Egap is the optical band gap, and n is a constant associated with the different types of electronic transitions (n ) 1/2, 2, 3/2, or 3 for direct allowed, indirect allowed, direct forbidden, and indirect forbidden transitions, respectively). According to Lacomba-Perales et al.87 the molybdates and tungstates with scheelite-type tetragonal structure present a direct allowed electronic transition. Thus, in our work, the n ) 1/2 value was adopted as the standard in eq 3. The Egap values of PbMoO4 micro-octahedrons were evaluated extrapolating the linear portion of the curve or tail. The obtained results are shown in Figure 7a-d and listed in Table 4, which


Sczancoski et al.

Figure 9. (a) Wavelength employed in the excitation process of PbMoO4 micro-octahedrons, (b) interaction between wavelength and both [MoO4] and [PbO8] clusters of a PbMoO4 tetragonal structure, (c) proposed wide band model with the presence of intermediary energy levels (oxygen 2p and molybdenum 4d states) within the band gap, (d) formation of STEs, (e) recombination of e′-h• pair, and (f) PL spectrum of PbMoO4 microoctahedrons processed in the conventional hydrothermal system at 60 °C for 10 min.

also shows the Egap values reported in the literature for the PbMoO4 formed by other methods. As can be seen, there is a good agreement between the band gap values obtained in this work and the previous data reported in refs 35 and 89. It is well-established90,91 that the Egap is associated with the presence of intermediary energy levels within the band gap. The presence of these energy levels is dependent on the degree of structural order-disorder in the lattice.92 Therefore, the increase in structural organization leads to a reduction in these intermediary energy levels, increasing the Egap value. In principle, we believe that the Egap also can be related to the other aspects, including preparation method, shape (thin film or powder), morphology, and experimental conditions (heat treatment temperature and processing time).93 It is important to consider that the calculated Egap value in ref 88 is larger; this difference is caused by the presence of Mn3+ ions in PbMoO4. Hence, it leads to an increase in energy levels within the forbidden band region, which may is related to the photochromic effect in the crystal. Photoluminescence Analyses and Model Based on Distortions of the [MoO4] and [PbO8] Clusters in the Lattice. The PL emission process of molybdates is not completely understood yet; therefore, several hypotheses have been reported in the literature to explain the origin of this physical property. Wu et al.83 argued that the 1T2 f 1A1 electronic transitions within the [MoO4] tetrahedron groups are responsible for the blue PL emission in the molybdates. These authors reported that the shoulders verified on the PL profiles arise from extrinsic transitions caused by the defects or impurities (or both) into the material. Loo94,95 explained that the blue luminescence is due to a transition in isolated MoO42- groups, and the green luminescence is because of the superposition of two bands, which is caused by the transfer of an electron occupying an orbital with mainly Pb2+ character to an empty orbital of an adjacent MoO42- group with a predominantly d character. Ryu et al.96 and Yang et al.97 attributed the origin of the PL properties to the morphology, degree of crystallinity, and particle sizes. In addition, Yu et al.98 verified a green emission in PbWO4 microcrystals, which was attributed to the existence of a Frenkel defect structure (oxygen ion shifted to the intersite position with simultaneous creation of vacancy) in the surface layers. Figure 8a, b shows the PL spectrum of PbMoO4 microoctahedrons processed in the CH system at different tempera-

tures for 10 min and possible distortion sets on the [MoO4] and [PbO8] into the lattice. PL spectra present a broad band covering the visible electromagnetic spectra in the range from 400 to 700 nm (Figure 8a). In our work, all PbMoO4 micro-octahedrons obtained by CH processing are structurally ordered at long and short range, in agreement with the XRD and MR analyses (Figure 2a and Figure 4). In this context, the PL emission is not related to the high degree of structural disorder in the lattice. Therefore, the origin and nonlinear variations on the PL intensity can be caused by the modifications on the specific atomic arrangements presented by the [MoO4] and [PbO8] clusters. We can suppose that other different factors can also be involved in this case, such as the aggregation process between particles; high variations on the particle size distribution; and degree of crystallinity, morphology, and surface defects. The possible distortions on the [MoO4] and [PbO8] clusters due to the continuous dissolution and recrystallization mechanism during the crystal growth processes were explained by means of several displacements along the atomic coordinates (x, y, and z), as shown in Figure 8b. The random distortions on the [MoO4] and [PbO8] clusters are able to cause the energy level redistribution within the band gap, promoting fluctuations on the electronic density into the structure. We believe that this behavior plays an important role on the blue PL emission process in the PbMoO4 microcrystals. Wide Band Model Based in Distortions on Both [MoO4] and [PbO8] Clusters. Figure 9 shows a proposed wide band model to explain the PL behavior of PbMoO4 micro-octahedrons by means of random distortions on both [MoO4] and [PbO8] clusters. The wavelength energy used (350 nm ≈ 3.543 eV) is able to excite several electrons localized in different intermediary energy levels within the band gap (Figure 9a). In this context, we believe that the dissolution and recrystallization mechanisms of microcrystals under CH conditions promote a distortion process on the [MoO4] and [PbO8] clusters randomly distributed into the PbMoO4 lattice (Figure 9b). On the basis of this assumption, these distortions probably lead to the formation of intermediary energy levels within the band gap, which are basically composed of oxygen 2p states (above the valence band) and molybdenum 4d states (near the conduction band) (Figure 9c). During the excitation process, some electrons situated in the oxygen 2p states are promoted to molybdenum 4d states by

Morphology, Photoluminescence Emission of PbMoO4 the absorption of photons (hν). Consequently, this mechanism promotes the formation of self-trapped excitons (STEs); i.e., trapping of electrons (e′) by holes (h•) (Figure 9d). The emission ′ process of photons (hν ) occurs when an electron localized in a molybdenum 4d state decays into an empty oxygen 2p state (Figure 9e). Figure 9f shows the PL emission of PbMoO4 microoctahedrons processed in the CH system at 60 °C for 10 min. We postulate that this emission arises from the distortions on the [MoO4] and [PbO8] clusters in the lattice, as mentioned previously in the text. Conclusions In summary, PbMoO4 micro-octahedrons were obtained by the coprecipitation method and processed at different temperatures in the range from 60 to 120 °C for 10 min in a conventional hydrothermal system. XRD patterns and microRaman spectra indicated that the PbMoO4 micro-octahedrons present a scheelite-type tetragonal structure without the presence of secondary phases. These results showed that these structures are well-ordered at long and short range, independent of processing temperature employed in the conventional hydrothermal treatment. FEG-SEM micrographs showed that the processing temperature plays an important role on the growth process of PbMoO4 micro-octahedrons. Moreover, the conventional hydrothermal conditions were able to promote the selforganized PbMoO4 structures via an oriented attachment mechanism along the [001] crystallographic direction. UV-vis absorption spectra showed different optical band gap values, indicating the presence of intermediary energy levels (oxygen 2p and molybdenum 4d states) within the band gap. To explain the PL behavior, a model based on the distortions of both [MoO4] and [PbO8] clusters in the lattices was introduced. It is expected that the conventional hydrothermal method, used here to obtain PbMoO4 micro-octahedrons, a typical example of how subtle changes in the structure of these systems can lead to fundamental changes in these physical properties. Acknowledgment. The authors are thankful for the financial support of the Brazilian research financing institutions: CAPES, CNPq, and FAPESP. Prof. J. Andres acknowledges Ministerio de Educacioń y Cultura of the Spanish Government. Special thank to Nanox Technology S/A for the use of the Hydrocell H-100 equipment. References and Notes (1) Thongtem, T.; Kaowphong, S.; Thongtem, S. Solid State Phenom. 2007, 124-126, 315. (2) Thongtem, T.; Phuruangrat, A.; Thongtem, S. J. Ceram. Proc. Res. 2008, 9, 189. (3) Chen, L.; Gao, Y. Chem. Eng. J. 2007, 131, 181. (4) Cavalcante, L. S.; Sczancoski, J. C.; Tranquilin, R. L.; Joya, M. R.; Pizani, P. S.; Varela, J. A.; Longo, E. J. Phys. Chem. Solids 2008, 69, 2674. (5) Kaowphong, S.; Thongtem, T.; Thongtem, S. Solid State Phenom. 2007, 124-126, 1265. (6) Zhou, G.; Lu¨, M.; Xiu, Z.; Wang, S.; Zhang, H.; Zou, W. J. Cryst. Growth 2005, 276, 116. (7) Liu, S.; Yu, J.; Zhao, X.; Cheng, B. J. Alloys Compd. 2007, 433, 73. (8) Thongtem, T.; Phuruangrat, A.; Thongtem, S. Appl. Surf. Sci. 2008, 254, 7581. (9) Zhao, W.; Song, X.; Chen, G.; Tian, G.; Sun, S. Mater. Lett. 2008, 63, 285. (10) Errandonea, D.; Manjoń, F. J. Prog. Mater. Sci. 2008, 53, 711. (11) Tyagi, M.; Sangeeta, Desai, D. G.; Sabharwal, S. C. J. Lumin. 2008, 128, 22. (12) Piwowarska, D.; Kaczmarek, S. M.; Berkowski, M. J. Non-Cryst. Solids 2008, 354, 4437. (13) Chen, J.; Liu, T.; Cao, D.; Zhao, G. Phys. Stat. Solidi B 2008, 245, 1152.

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Morphology and Blue Photoluminescence Emission of PbMoO4

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