Effect of hydrothermal conditions on the morphology and photoluminescence properties of

Effect of hydrothermal conditions on the morphology and photoluminescence properties of PbMoO4 powders

M.R.D. Bomio*,1, L.S. Cavalcante2, R.L. Tranquilin3, F.V. Motta1, C.A. Paskocimas1, M.S. Li4, J.A. Varela5 and E. Longo2,5 1 DEMat, Universidade Federal do Rio Grande do Norte - UFRN, P.O. Box 1524, 59078-97, Natal, RN, Brazil 2 LIEC, Universidade Federal de São Carlos - UFSCar, P.O. Box 676, 13565-905, São Carlos, SP, Brazil 3 POSMAT, Universidade Estadual Paulista- UNESP, P.O. Box 473, 17033-360, Bauru, SP, Brazil. 4 IFSC, Universidade de São Paulo - USP, P.O. Box 369, 13560-970, São Carlos, SP, Brazil 5 LIEC, Universidade Estadual Paulista- UNESP, P.O. Box 355, 14801-907, Araraquara, SP, Brazil.

In this chapter, we report the synthesis of PbMoO4 micro-octahedrons by conventional hydrothermal (CH) processed at 60, 80, 100 and 120oC for 10 minutes. These micro-octahedrons were structurally characterized by X-ray diffraction (XRD).Their optical properties were investigated by ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) measurements. XRD patterns indicated that these crystals have a scheelite-type tetragonal structure without the presence of deleterious phases. UV-vis absorption measurements evidenced a reduction in optical band gap values (from 3.14 eV to 3.19 eV) with the increase of CH processing temperature. Blue PL emission at room temperature was observed in these micro-octahedrons when they were excited with a 350 nm wavelength. FEG-SEM and TEM 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).

Keywords photoluminescence; lead molybdate; hydrothermal synthesis; Oriented Attachment

1. Introduction

Metal molybdates ((AMoO4; A = Mg, Ca, Bi, Pb, Ba, Mn, La, Ni, Cu, Co) have a great potential for application in various fields such as photoluminescence, microwave applications, optical fibers, scintillator materials, humidity sensors, as hosts for lanthanide activated lasers and catalysis [1-6]. As an important member of this family, lead molybdate PbMoO4 material finds wide applications in several areas including scintillators, acousto-optic modulators/detectors, ion conductors, and so on [7-9]. PbMoO4 crystals can be found naturally in form of a mineral called "Wulfenite", presenting in general a wide range of colors, i.e. from bright orange-red, yellow-orange to brown [10]. This fact, have been verified experimentally and reported by Tyagi et al. [11] with the preparation of PbMoO4 crystals by Czochralski technique with different colorations through stoichiometric/annealing variations. Their optical properties have been extensively studied and many research groups have been analyzed the optical absorption [12, 13], the luminescence properties, [14] and a new investigations been reported as a photocatalyst for the splitting of water [15, 16]. Some computational simulations were also carried out to explain the absorption bands in the transmission region theoretically in perfect, non- stoichiometric and doped PMO crystal [17]. Recently, Fujita et al. have investigated the electronic structure of PbMoO4 by the measurements of polarized reflectivity spectra and X-ray photoemission spectrum, together with theoretical calculation [18, 19]. Molybdates of relatively large bivalent cations (ionic radius > 0.99 Å: Ca, Ba, Pb, Sr) usually exit in the so-called scheelite structure form [20]. PbMoO4 presents space group I41/a, point-group symmetry and Z = 4 [5,6], with two formula units per primitive cell [5,6]. Each of Mo is surrounded by four equivalent O atoms composing the [MoO4]

2- tetrahedral configuration and each divalent metal, Pb, shares corners with eight adjacent O atoms of [PbO8]

2- tetrahedrons. The synthesis of micro and nanoscale inorganic materials with special morphology, size, and hierarchy has attracted considerable attention in the past few decades because of their importance in basic scientific research and potential technological applications [21,23] In this respect, the searching of synthetic routes capable of control over the morphology, crystalline structure, and size of inorganic materials been investigated extensively with the intention of obtaining new functionalities and behaviours related to a broad range of properties [24-28]. The capability to understand and control the assembly process is a fully open question and will be one of the exciting challenges in the near future, with particular attention in recent years to the development of metal oxide nanocrystals with defined sizes and shapes [29-32]. Thanks to the efforts from many research groups, PbMoO4 based materials have been prepared by different procedures: solid state reaction [33, 34], Czochralski crystal growth [35], chemical route [36], galvanic cell method [37], citrate complex [38], sonochemical route [28] microemulsion method [39], hydrothermal route [26,], microwave-assisted synthesis method [27] and coprecipitation method [40]. In this chapter, we report the synthesis of PbMoO4 micro-octahedrons using convectional hydrothermal system without any presence of surfactant. We proposed a crystal growth mechanism via Oriented Attachment and aggregation due to the influence of CH process. For this proposes, we use some microscopic characterization techniques to understand and investigate the growth of PbMoO4 micro-octahedrons.

Current Microscopy Contributions to Advances in Science and Technology (A. Méndez-Vilas, Ed.)

© 2012 FORMATEX 1163

2. Experimental

2.1 Synthesis and hydrothermal processing of PbMoO4 micro-octahedrons

PMO micro-octahedrons were synthesized by the coprecipitation method at room temperature and processed in a convetional hydrothetmal (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 ultrasonic bath 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. 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 50 °C for some hours.

2.2 Characterizations of PbMoO4 micro-octahedrons

After CH processing, PbMoO4 micro-octahedrons were structurally characterized by XRD using a DMax/2500PC diffractometer (Rigaku, Japan) with Cu Kα radiation (λ=1.5406 Å) in the 2θ range from 10° to 75° and step size of 0.02°/min. 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. Particles orientation were investigated using a transmission electron microscope MET-CM200 Philips, USA, operated at 200kV. 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.

3. Results and discussion

3.1 X ray diffraction characterization

X-ray patterns of the PbMoO4 powders processed in CH system at different temperatures for 10 min are presented in Figure 1(A-D). 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[41] The relative intensities and sharp diffraction peaks indicated that the PbMoO4 are well-crystallized, suggesting an ordered structure at long range.

Fig. 1 XRD patterns of PbMoO4 micro-octahedrons processed in conventional hydrothermal at different temperatures (60-120°C) for 10 min

10 20 30 40 50 60 70 80 90

(D)

(C)

(B)

(A) PbMoO4/60oC/10 min

(B) PbMoO4/80oC/10 min

(C) PbMoO4/100oC/10 min

(D) PbMoO4/120oC/10 min

Inte

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(112

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04) (2

00)

(114

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05)

(123

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04)

(220

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(116

) (312

)(2

24)

(400

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08)

(136

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(118

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(008

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21)

(424

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(332

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(415

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(402

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28)

(404

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As Table 1 shows, the lattice parameters of unit cell were calculated using Rietveld refinement for investigated the influence the convectional hydrothermal processing in the unit cell volume and distortion information of [MoO4] and [PbO8] clusters. In the results is possible to observe that the structure of the materials obtained is influenced by hydrothermal processing temperature. However, the tetragonality relation (c/a) decreases with the increases of the processing temperature where can be verify by lattice distortions from the tetragonal structure in PbMoO4 for a pseudo-cubic structure. This behaviour of the decrease of tetragonality can be linked to processing hydrothermal, which provides small distortions in the structures in both clusters [MoO4] and [PbO8] distributed inside the crystal structure of the PbMoO4.

Table 1: Results of lattice parametes of PMO micro-octahedral synthesized by convectional hydrothermal at different temperatures for 10 minutes obtained by the Rietveld method. (SM= Synthesis method; T = temperature; t = processing time ; a, b, c = Lattice parameters, V = Cell Volume; α, β, γ = Cell angles.)

SM T(oC) t(min) a (Å) b (Å) c (Å) b/c c/a V (Å3) α=β=γ (0)

JCPDS nº 44-1486 5,4330 5,4330 12,110 0,4486 2,2290 357,460 90 CH 60 10 5,4390 5,4390 12,1226 0,4487 2,2294 358,615 90 CH 80 10 5,4378 5,4378 12,1226 0,4486 2,2293 358,468 90 CH 100 10 5,4378 5,4378 12,1212 0,4486 2,2291 358,422 90 CH 120 10 5,4389 5,4389 12,1217 0,4487 2,2287 358,575 90

3.2 UV visible absorption and photoluminescence characterizations

In Fig. 2 shows the UV vis absorbance spectra of PbMoO4 micro-octahedrons at 60oC for 10 min (Fig.2(A)) and the optical band gap energy (Egap) evolution as function of the increase of hydrothermal processing temperature (Fig.2(B)). The Egap was calculated by the method proposed by Wood and Tauc [42]. According to these authors, the optical band gap is associated with absorbance and photon energy by the following equation 1.

(1) Where α 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. [43] 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 equation 1. The Egap values of PbMoO4 micro-octahedrons were evaluated extrapolating the linear portion of the curve or tail as can be seen in the Figure 2 (A).

Fig. 2 UV vis absorbance spectra of PbMoO4 micro-octahedrons at 60oC for 10 min (Fig.2(A)) and the optical band gap energy (Egap) evolution as function of the increase of hydrothermal processing temperature (Fig.2(B)). It is well-established [44] 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 [45].



Therefore, the increase in structural organization leads to a reduction in these intermediary energy levels, increasing the Egap value. This behaviour can be seen in the Figure 2(B), where the organization of PbMoO4 micro-octahedral is increase as a function the increases of the processing temperature, consequently the Egap increase. 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 (processing time or temperature). Photoluminescence (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. One of the hypotheses suggested 1T3 →

1A1 electronic transitions within the [MoO4] tetrahedron groups are responsible for the blue PL emission in the molybdates [46]. Another hypothesis is attribute the presence of photoluminescence as intrinsic property of molybdates or due to presence of defects or impurities into the crystal structure. [47] On the other hand, Ryu et al.[48] and Yang et al. [49] attributed the origin of the PL properties to the morphology, degree of crystallinity, and particle sizes In Fig. 3 (A) shows the PL spectrum of PbMoO4 synthesized in conventional hydrothermal at different temperature excited at 350nm (≈ 3,543 eV). It was observed that the energy applied on the materials was enough to promote several electrons located in different intermediate levels within the band gap of the structures of PbMoO4. It is believed that the non-linearity of the photoluminescence intensity (Figure 3 (A)) of material prepared at different temperatures may be related to an intense and continuous nucleation dissolution recrystallization mechanism of the structures [50], during the hydrothermal treatment, thereby promoting distortion in both clusters [MoO4] and [PbO8] distributed within the crystal structure of the PbMoO4, as shown in Table 1. Besides, the distortion of the structures may be attributed as a function of the morphology obtained by the hydrothermal method. As Figure 3(B) shows, an initial growth process of these morphologies through the formation of micro-octahedrons with irregular shapes along the different crystallographic planes of PbMoO4 synthesized at 100oC for 10 minutes, which showed a distinct morphology of other materials obtained in different temperatures. At this temperature, the mechanism of dissolution and recrystallization may have the best experimental conditions (boiling temperature of water), thus creating a large concentration of defects in the structure at intermediate range, thereby providing the possibility a higher excitation quantity of electrons present in the intermediate levels of the material. As a consequence, the FL of this material showed the highest emission compared to other materials, as can be seen in Figure 3(A).

Fig. 3 PL spectra at room temperature of PbMoO4 micro-octahedrons processed in the conventional hydrothermal system at different temperatures (A). FEG-SEM micrographic of formation of micro-octahedrons with irregular shapes along the different crystallographic planes of PbMoO4 synthesized at 100oC for 10 minutes.

3.3 Field-Emission gun scanning electron and transmission electron microscopy characterizations

FEG-SEM micrographs were of fundamental importance to understand the morphological evolution process of PbMoO4 crystals with the variations of the processing temperature. As Figure 4 shows, 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. In this type of synthesis (coprecipitation method) the obtained materials mostly is related to processes of growth of the particles during the precipitation, which are based on nucleation and growth stages [51]. The coprecipitation reactions involve the simultaneous occurrence of these processes, beyond random growth and agglomeration processes [52, 53] Due to experimental difficulties to isolate each of these processes of formation and growth of particles in independent studies, the coprecipitation mechanisms are not fully understood. However, studies in the literature show

350 400 450 500 550 600 650 700 750 800

PbMoO4/60oC/10 min

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that mathematical models the relationship between the critical nucleus size and surface energies may offer certain material growth mechanisms [54] such as Ostwald Ripening mechanism. This mechanism consists in a spontaneous process where particles in a liquid can also grow by a process in which the smaller particles dissolve and the solute precipitates on the larger particles promoting greater stability for the particle. For PbMoO4 powders processed at 60 °C for 10 min, it was observed an initial growth process of these particles through a self-organization of adjacent microcrystals during the hydrothermal processes. In Fig 4 (A) and (B), it is possible to conclude that the principal growth mechanism during the synthesis is the Ostwald Ripening.

Fig. 4 FEG-SEM micrograph of PbMoO4 micro-octahedrons processed in conventional hydrothermal at 60oC for 10 min (A). Low magnification TEM micrographs of PbMoO4 crystals synthesized at 60oC for 10 min showing the growth PbMoO4 structure by Ostwald Ripening mechanism (B). In the late 20th century, Penn and Banfield identified an important crystal growth mechanism in nanoparticles in colloidal dispersions. They called this new mechanism ‘‘Oriented Attachment (OA)’’ or ‘‘oriented aggregation’’ [55-57]. The mechanism of Oriented Attachment was proposed as another significant process for nanostructure growing. The concept involved in this process involves the growth of particles by crystallographic alignment and coalescence of adjacent particles, by eliminating the common interface between the particles, due the presence of a solid–solid interface between the crystals, indicating that the growth process begins only after contact is established between particles [56]. The processing performed at 80 °C for 10 min it was observed the aggregation between the microparticles, promoting the coalescence of individual micro-octahedrons similar as the material processed at 60°C for 10 min (Figure 4 (A) and (B)). This behaviour 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 as shown Figure 3(B). A closer analysis of the results displayed in FEG-SEM micrographs for the material synthesized at 120 °C for 10 min (Figures 5 (A) and (B)), it was observed an initial growth process of these morphologies through a self-organization of adjacent microcrystals in a similar crystallographic orientation. These morphologies formed by Oriented Attachment process have been commonly observed for the materials with scheelite-type structure composed of Pb2+ ions [57] , which tend to be facetted and aligned by “docking” processes involving crystallographic fusion between some faces with high surface energy, creating an extended morphology, as can be illustrated in Figure 5 (A) and (B).

Fig. 5 FEG-SEM micrograph of PbMoO4 micro-octahedrons processed in conventional hydrothermal at 120oC for 10 min (A). Low magnification TEM micrographs of PbMoO4 crystals synthesized at 120oC for 10 min showing the growth PbMoO4 structure by Oriented Attachment mechanism (B).



The temperature-dependent shape evolution process of PbMoO4 crystals suggested that the connected structure formed tend to grow further preferentially along certain crystallographic direction. However, to confirm the growth model suggested in Figure 5 (B) for the material synthesized at 120oC for 10 minutes by CH method, it was necessary to determine if the growth of the crystals occurs in a same plane. The high resolution transmission electron microscopy image (HR-TEM) allowed us to assume that, after collision events during the hydrothermal synthesis, some particles tend to remain attached in compatible crystallographic planes as illustrated in Figure 5 (A) and (B). The HR-TEM image was taken from the connecting region of the adjacent particles (dashed black square in Figure 5 (B)) shows that these particles revealed an oriented aggregation of two particles exactly in the (004) plane, as estimated by the planar spacing of 3.02 Å, being the bonding plane and thus the epitaxial coalescence of the two particles being along the [100] direction, as shows in the Figure 6. This result is in agreement with earlier research reported by Zend et al.[58] and Wang et al.[59]. According to these authors the rate of growth of micro crystal PbMoO4 is much faster along the direction [001] than along [100]. The behaviour also could be also detected by another kind of molybdates, such as BaMoO4 crystals [60].

Fig. 6 High-magnification HR-TEM image showing the growth of a 1D structure from a 0D structure for PbMoO4 micro-octahedrons processed in conventional hydrothermal at 120oC for 10 min by Oriented Attachment mechanism.

4. Conclusion

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 indicated that the PbMoO4 micro-octahedrons present a scheelite-type tetragonal structure without the presence of secondary phases. UV vis absorption spectra showed different optical band gap values, indicating the presence of intermediary energy levels within the band gap and could be related by the processing temperature. The microscopy characterization proved to be a powerful and important tool for the characterization PbMoO4 materials. FEG-SEM micrographs showed that the processing temperature plays an important role on the growth process of PbMoO4 micro-octahedrons having as consequence the improvement of photoluminescence properties for PbMoO4 synthesized at 100oC for 10 minutes due promote distortions of both [MoO4] and [PbO8] clusters in the lattices. TEM micrographs revealed that conventional hydrothermal conditions were able to promote the self-organized PbMoO4 structures via both growth mechanism as Oswald Rippenig as Oriented Attachment. HR-TEM micrographs evidenced that the PbMoO4 micro-octahedrons synthesized at 120oC for 10 minutes are formed by an Oriented Attachment mechanism along the [001] crystallographic direction. 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.

Acknowledgements The support by research financing institutions: RECAM (Rede de Catalisadores Ambientais)/CNPq Processo nº 564913/2010-3 and FAPESP-CEPID 98/14324-8.is gratefully acknowledged.



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