Influence of Particle Size and Morphology - American Chemical Society

Article pubs.acs.org/crystal

Photoluminescence of SrTiO3: Influence of Particle Size and Morphology A. E. Souza,*,†,⊥,# G. T. A. Santos,†,⊥ B. C. Barra,†,⊥ W. D. Macedo, Jr.,†,⊥ S. R. Teixeira,†,⊥,# C. M. Santos,‡ A. M. O. R. Senos,§ L. Amaral,§ and E. Longo∥,⊥ †

Departamento de Física, Química e Biologia, Universidade Estadual Paulista, Presidente Prudente, SP, Brazil Instituto de Tecnologia para o Desenvolvimento − LACTEC, Universidade Federal do Paraná, Curitiba, PR, Brazil § Departamento de Engenharia Cerâmica e do Vidro/CICECO, Universidade de Aveiro, Aveiro, Portugal ∥ Instituto de Química, Universidade Estadual Paulista, Araraquara, SP, Brazil ⊥ Instituto Nacional em Ciências e Tecnologia em Materiais em Nanotecnologia − INCTMN, São Carlos, SP, Brazil ‡

ABSTRACT: SrTiO3 crystalline nanoparticles were prepared using the microwave-assisted hydrothermal method at 140 °C with synthesis times varying from 4 to 160 min. Sample characterization showed that the method is effective in obtaining nanoparticles in a relatively short time, which have the highest photoluminescence emission. The crystalline phase of perovskite-type SrTiO3 is not significantly influenced by synthesis time. However, the SrTiO3 phase is already obtained with a 4 min synthesis time. Also, all samples exhibited photoluminescence at room temperature in the blue-green region, where intensity decreased with increasing synthesis time and particle size. The samples synthesized with the shortest time showed higher photoluminescence emission and smaller particle sizes. The morphology obtained based on FESEM showed cubic nanoparticles with inhomogeneous grain growth at higher temperatures of synthesis in addition to the formation of new architectures.

1. INTRODUCTION Strontium titanate (SrTiO3) has a cubic perovskite structure and is thermically defined at room temperature,1 with a Curie point at a low temperature (−55 °C).2 It exhibits a semiconducting behavior, high dielectric constant, and good thermal stability and also has paraelectric and piezoelectric characteristics, where polarization can be induced by application of an electric field, even without a permanent electric dipole.3,4 This behavior gives it a wide range of applications in microelectronics,5 making it one of the promising materials to be used in tunable microwave devices,6−8 capacitors,3 photocatalysts,9 and light emitters.10 According to the literature, SrTiO3 has characteristics of a semiconductor material with an indirect band gap that varies from 3.2 to 3.4 eV; nevertheless, some works show that this gap can range up to 3.77 eV,10−14 causing an emission of photons in the blue-green region of the spectrum.12 Studies have focused on the effect of particle size or temperature of phase transition between paraelectric and ferroelectric states, and also on the variation of optical and dielectric properties of SrTiO3. It has been observed that SrTiO3 nanoparticles have different properties compared with the bulk compound, such as light emission, which increases in particles on the nanoscale.4 © 2012 American Chemical Society

Furthermore, variations in the structure of SrTiO3 can modify its properties, making it a metallic conductor or even a superconductor at low temperature (less than 90 K).15 Many methods have been used in preparing these types of titanates.3,4,9,10,16,17 An alternative synthetic route is the microwave-assisted hydrothermal method, developed for the preparation of nanoparticles. This method uses low temperatures and short reaction times, due to direct interaction of radiation with water.18−22 The use of microwave radiation linked to the hydrothermal technique shows itself, certainly, as an excellent combination for the preparation of inorganic nanoparticles and nanostructured materials. This method, therefore, represents a large step to a green chemistry approach, using water-soluble salts instead of solvents and the preparation of nanoparticles of high quality.23 The microwave heating has several advantages over conventional heating for chemical synthesis: (1) high heating rate and so, increasing the reaction rate, (2) no direct contact between the heating source and reagents, (3) excellent control of reaction parameters, (4) Received: August 13, 2012 Revised: October 5, 2012 Published: October 8, 2012 5671

dx.doi.org/10.1021/cg301168k | Cryst. Growth Des. 2012, 12, 5671−5679

Crystal Growth & Design

Article

Figure 1. Scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS) images of SrTiO3 samples: (a) 20, (b) 40, and (c) 160 min. Raman spectroscopy was performed using a Renishaw micro-Raman model inVia spectrograph equipped with a Leica microscope (50× objective with ∼1 μm2 spatial resolution) and a CCD detector, with a scan of 100−1400 cm−1, using a 633 nm wavelength of a He−Ne laser. UV visible spectroscopy (UV−vis) was carried out using a Cary 5G UV−vis NIR spectrophotometer in total reflection mode. Two references, Labsphere Certified Reflectance SRS 94-010 and SRD 02-010, were used. The region analyzed was 800−200 nm at a speed of 600 nm/min and a lamp exchange (visible−UV) at 350 nm. PL spectroscopy was performed using a Thermal Jarrell Ash Monospec-27 with a monochromator and a Hamamatsu R955/500 V photomultiplier. The excitation wavelength was 350.7 nm of a krypton-ion laser (Coherent Innova), with an L38 filter and a 0.54 W output power of the laser. The images of scanning electron microscopy (FE-SEM) were obtained using a Zeiss Supra 35 Gemini model, with an in lens detector to enhance resolution. All measurements were performed at room temperature.

selective heating, if the reaction mixture contains compounds of different properties of microwave absorption, (5) high fields, (6) improved selectivity due to low reaction field, (7) improved reproducibility, and (8) automation and high yield synthesis.24,25 Because of its advantages, this method has been used effectively in synthesizing a variety of nanostructured materials, among them, ceramic and metal oxides.26−37 In this study, SrTiO3 (ST) nanoparticles were prepared using the microwave-assisted hydrothermal method with several different times. The ceramic powders were characterized by X-ray diffraction (XRD), ultraviolet−visible absorption spectroscopy (UV−vis), Raman, photoluminescence (PL), and scanning electron microscopy (FE-SEM).

2. EXPERIMENTAL DETAILS 2.1. Synthesis and Processing of SrTiO3 Powders. ST nanoparticles were synthesized using 0.01 mol of SrCl2.6H2O (99%, Synth) dissolved in 20 mL of deionized water in a Teflon cup with a capacity of 110 mL. Under constant mixing and a constant flow of nitrogen gas, 0.01 mol of C12H28TiO4 (97%, Aldrich) was added to the solution, followed by the rapid addition of 50 mL of KOH (85%, Cinética). About 90% of the volume of the Teflon autoclave was completed with deionized water to promote the maximum possible pressure. The cup containing the reactants was placed in the cell reaction, which was sealed and placed in a microwave oven adapted for using microwave radiation of 2.45 GHz and the maximum power of 800 W. The synthetic processing occurred at a heating rate of 140 °C/min up to a temperature of 140 °C. The synthetic reactions were maintained for 4, 10, 20, 40, 80, and 160 min at the final temperature, achieving a maximum pressure of approximately 4 bar, for all times. After the reaction time, the system was cooled to room temperature and the ceramic powder precipitates were washed with deionized water until neutral (pH = 7). Subsequently, the powders obtained were dried at 110 °C for ∼12 h. 2.2. Characterizations of SrTiO3 Powders. The samples synthesized at 20, 40, and 160 min were characterized by scanning electron microscopy with an energy-dispersive detector (SEM-EDS), using a Hitachi SU-70 SEM equipment and carbon ribbon as a substrate. All samples were characterized by X-ray diffraction, using a Shimadzu model XRD-6000 instrument, with Cu Kα1 (λ = 1.5406 Å) and Cu Kα2 (λ = 1.5444 Å) radiation, a voltage of 40 kV, and a current of 30 mA. The scan was done in a range of 2θ angles from 20° to 120°, using a divergence slit of 0.5° and a receiving slit of 0.5° for receiving in continuous scan mode with steps of 0.02° and a scanning speed of 0.2°/min with an accumulation of 6 s/point (totaling 5000 points).

3. RESULTS AND DISCUSSION Initial qualitative results of scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS) for the ST samples synthesized in 20, 40, and 160 min are shown in Figure 1. In most regions of these morphological titanates, there was a homogeneous distribution of the elements Sr and Ti, which can be observed from the quantitative data presented in Table 1. Table 1. Quantitative Data of SEM-EDS Maps of ST Samples atomic concentration (%) titanium strontium oxygen carbon potassium a

20 min

40 min

160 min

14.23 13.62

10.31 10.29 30.35 49.04

16.77 16.21 45.07 21.94

a

29.42 0.41

Not quantified.

Furthermore, the oxygen concentration was found to be higher compared with that of the elements Sr and Ti. This behavior agrees with the SrTiO3 phase stoichiometry, since the amount of oxygen is approximately 3 times the concentration of Ti and Sr atoms. For all samples, a small amount of carbon was identified, which is probably due to the presence of this element (carbon ribbon) in the substrate or the contacts used in the preparation for microscopic analysis, or to the formation of 5672



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The EDS and XRD results show that the hydrothermal method assisted by microwave is effective in obtaining SrTiO3, although a secondary phase (SrCO3) has been formed together with ST. The microwave radiation favors chemical reactions in the hydrothermal process, from the direct coupling between the material and radiation. Thus, an increase in the crystallization kinetics occurs when compared with other conventional methods and makes the synthesis process quicker and economical, saving time and energy.10−12,14,24,25,38 For the SrTiO3, 4 min was enough to obtain the desired phase. The Scherrer equation (eq 1), where k is a constant (here, k = 1), λ is the X-ray wavelength (1.542 Å), β is the width at halfmaximum (fwhm) of the diffraction peak of the plane (1 1 0), and θ is the Bragg angle (2θ = 32.4° peak position), was used to determine the average crystallite size of the powders synthesized (Table 3).

secondary phases present in these samples. In the sample synthesized in 20 min (Figure 1a), a small residual percentage of potassium was also identified, probably resulting from the KOH added during the synthesis process. The samples synthesized in 40 min (Figure 1b) and 160 min (Figure 1c) displayed some regions with a predominance of Sr, although the regions observed clearly showed the SrTiO3 phase formation. The qualitative and quantitative SEM-EDS results are a good initial approximation to check the SrTiO3 phase formation, but they are not conclusive. X-ray diffraction (XRD) was used for the determination and quantification of the phases formed, as shown in Figure 2. There were diffraction peaks of a ST cubic

D=

kλ β cos θ

(1)

The results in Table 3 show that the average crystallite size had a range of ∼13% for the different synthesis times. It is known that crystallites of less than 1 μm are responsible for the broadening and reduction of the intensity peaks of X-ray diffraction. These parameters, from the perspective of longrange order, result in a reduction in the degree of crystallinity. However, it is noted that, although the average size of crystallites is on the order of nanometers, the samples have good crystallinity with long-range order. Also, it appeared that small variations in average crystallite size are little influenced by synthesis time. This result indicates that the mechanism for nucleation and crystal growth is governed by organizing octahedral [TiO6] and cubic-octahedral [SrO12] clusters composing the ST crystalline structure. Once formed, the crystallites grow to a range appropriate for the formation of a small ST single crystal. Thereafter, these small single crystals undergo a process of aggregation to form a polycrystalline particle, whose growth then involves moving the grain boundary. Thus, the variation in synthesis time may be responsible for the growth of the particle (grain), but not crystallite formation. Furthermore, the grain growth of this titanate may be immobilized by the introduction of secondary phases by the fine particles, for example, the phase carbonate present in the sample.22 The optical absorption of light in the UV−visible range was evaluated to estimate the optical band gap of the ST samples. The values were obtained by the Wood and Tauc method,39 according to eq 2, in which h is Planck’s constant, ν is the frequency of the incident photon, and α is the absorption coefficient in the optical gap Eop g , which depends on the transition between bands (0.5 to gap direct or 2 for indirect band gap):

Figure 2. XRD parameters of SrTiO3 samples: (a) 4, (b) 10, (c) 20, (d) 40, (e) 80, and (f) 160 min.

phase with the space group Pm3m (PDF 05-0634) and some peaks of orthorhombic strontium carbonate SrCO3 (PDF 050418) as an additional phase. This secondary phase may have contributed to the mapping containing carbon in SEM-EDS images. The data structure refinement, using the Rietveld method, is shown in Table 2, noting that χ2 represents the goodness-of-fit of refinement parameters, Rwp indicates the success of the refinement, and R-Bragg is the crystallographic model that fits the experimental data. The synthesis time did not influence the formation of the main phase (SrTiO3), whose lattice parameters did not vary significantly. However, the strontium carbonate phase percentage was reduced for the sample synthesized in 20 min (ST20min), which also agreed with the measurement of carbon in the EDS map of this sample (Table 1).

Table 2. Results of the ST Structure Refinement by the Rietveld Method parameters sample

χ2

Rwp (%)

R-Bragg ST (%)

SrCO3 (wt %)

PDF 05-0634 (a = b = c) 3.901 (Å)

ST4min ST10min ST20min ST40min ST80min ST160min

4.173 2.925 2.997 4.611 3.563 3.644

8.61 7.37 7.38 8.90 7.81 7.84

7.99 5.66 5.96 5.57 5.45 5.19

14.68 15.27 8.37 13.97 14.68 14.47

3.925 3.921 3.920 3.922 3.920 3.923

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Table 3. Average Crystallite Size of SrTiO3 Samples average crystallite size (nm) band gap energy (eV) a

4 (min)a

10 (min)a

20 (min)a

40 (min)a

80 (min)a

160 (min)a

average

48 3.6

56 3.4

53 3.4

52 3.5

57 3.4

46 3.5

52 ± 4 3.5 ± 0.1

Time.

hνα ∝ (hν − Egop)m

also be due to the displacement of Ti ions and/or Sr within the clusters [TiO6] and [SrO12], which make up this material.41,42 According to Longo et al.,10 the displacement of the network modifier (Sr) causes an increase in the local disorder when compared with the displacement of forming the network (Ti), in the case of SrTiO3. The greatest disorder occurs when both (forming and network modifier) are shifted. This process leads to a loss of degeneracy in the energy bands of the material to form the intermediate states. Therefore, the intermediate states are generated due to redistribution of charge density around the atoms constituting the lattice, or the movement of the valence bands (VB) and conduction (CB) involving oxygen and titanium atoms, respectively. Several papers have presented theoretical studies on the electronic structure of titanates, where the results, including band mapping, show that VB is derived from the atomic orbitals (2p) of oxygen and is separated by a band gap of CB, which derives from the atomic orbital (3d) of the transition metal Ti. The top of the VB of oxygen is destabilized by the displacement of Ti, that is, by stretching, but without breaking, of the Ti−O bond. Thus, the new states are generated, located within the band gap.10,12,14,39−43 The local distortions that generate interband states behave as an intrinsic defect in this type of perovskite. The absorption in the UV−vis is a useful tool to investigate the local order−disorder degree of these materials, but does not indicate structural disorder with which the localized states are associated. Figure 4 shows the Raman spectrum of the SrTiO3 nanoparticles with a cubic structure, for different times of permanence at 140 °C. The Raman spectrum can detect local distortions of the network and crystallographic defects at the molecular level.22 For cubic systems of the space group Pm3m, the phonons are represented by 3F1u + F2u, and neither represents a Raman-active mode. Wu et al.4 and Lee et al.22

(2)

Plotting (ανh)1/m versus hν near the absorption edge, we obtained a linear fit when the appropriate transition mechanism is assumed, and the intercept with the abscissa shows the transition energy of the optical gap.13,39 According to Van Benthem,40 the direct or indirect nature of the band gap depends on the absorption coefficient (α). In the present study, we observed that the absorption coefficient is low, which features an indirect band gap. The band gap values obtained, shown in Table 3, are close to those described in the literature, which show that ST has an indirect band gap of the order of 3.7 eV.10,12−14 In general, small variations in the optical gap, observed for ordered and disordered samples, can be analyzed by the presence of an exponential optical absorption edge in the low-energy region of the UV−vis spectrum. This tail exponential optical absorption is observed in all UV−vis spectra of ST samples (Figure 3) and is apparently very similar

Figure 3. Absorption spectra in the UV−visible of SrTiO3 samples.

for all, resulting in band-gap energy values without significant changes with synthesis time. The appearance of this absorption edge is controlled by the structural order−disorder degree10 and causes variations in the optical band-gap energy values of titanates. This order−disorder degree is attributed to the existence of oxygen vacancies, defects in the network, impurities, or local distortions, which leads to the existence of additional electronic levels within the forbidden band of the titanate.10 The existence of additional levels to the band gap can

Figure 4. Raman spectroscopy of SrTiO3 samples: (a) 4, (b) 10, (c) 20, (d) 40, (e) 80, and (f) 160 min. 5674



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of the vibrational mode at 727 cm−1 indicates a transverse mode coupling, associated with the cubic phase of this titanate. The band found at 801 cm−1, corresponding to the LO vibrational mode, indicates optical modes of high longitudinal frequency with A1 symmetry for the [TiO6] octahedron. The vibrational modes presented indicate short distance distortions, on the order of the phonon wavelength, in the cubic structure of ST. The increase in this distortion of local symmetry also suggests a noncenter symmetric occupation of the Ti atom in the [TiO6] cluster (no change in coordination - tilt) and may be associated with the presence of impurities or oxygen vacancies, common in perovskites.4,10−12,15,39,42,45 The band found at 1074 cm−1 corresponds to strontium carbonate, as identified by XRD.15,45 The PL phenomenon is considered a powerful tool for obtaining information about the electronic structure and medium-range structural organization of materials.46 On the basis of the X-ray spectrum of the ST sample shown in Figure 2, the titanate was found to have a cubic structure with high symmetry. Thus, no photoluminescent emission should occur in this sample, since this property depends on the structural disorder of the material. However, this notion contradicts the experimental results, as can be observed in Figure 5. The values of the band-gap energy and the Raman-active modes of the ST samples help explain the photoluminescent behavior displayed by these samples. In general, photoluminescent properties are believed to be attributed to the order−disorder degree, such as distortions in the crystal structure, or they can be explained by the electronic states within the band gap due to atomic vacancies, defects, or impurities.47 Thus, photoluminescent emission is considered a typical multilevel and multiphonon process, that is, a system in which relaxation occurs along several paths, involving numerous intermediate states to the band gap, above the valence band, and below the conduction band, as illustrated in Figure 6.10,12 According to literature, PL observed in ST is due to the recombination between electrons and holes trapped in the intermediate states to the band gap, which may have been generated by distorted clusters (and/or complexes, containing oxygen vacancies).10,12,14,15,21,41,47,48 When the electrons localized in these intermediate electronic states recombine

reported in their work that ST at room temperature, with a cubic structure, has second-order Raman-active modes. According to these authors, the first-order Raman modes are symmetrically forbidden in most of these titanates, due to the phonon moment selection rule being zero near the center of the Brillouin zone. In other words, as the ST (cubic) has a center of symmetry, the polarizability of the molecule does not vary during vibration, and therefore, there is no Raman-active mode. According to the authors, a second-order Raman-active mode results from the creation or destruction of two first-order modes. However, many recent studies about SrTiO3 Raman show that vibrational modes can be modified, especially the activation of first-order Raman modes, due to several factors, such as the effect of electrostatic forces, oxygen vacancies, or some external conditions. Thus, each F1u mode is divided into a double degenerate mode E and a nondegenerate A1, whereas the F2u mode is divided into E and B1 modes, which is then active in Raman spectroscopy. In addition, the presence of many long-range electrostatic forces separates the degenerate mode E and the nondegenerate A1 in transverse optical (TO) and longitudinal optical (LO).4,15 Many studies have reported investigations about SrTiO3 Raman modes, between 100 and 1000 cm−1.4,8,22,44 The Raman modes identified for the ST sample are shown in Table 4. Table 4. Raman Vibrational Modes of SrTiO3 Samples vibrational mode

wavenumber (cm−1)

TO2 TO3 TO4 TO LO4 SrCO3

179 270 544 727 801 1074

These results are in agreement with the literature.4,8,15,44 All vibrational modes found, except the band at 1074 cm−1, are first-order Raman modes for SrTiO3. The TO2 and TO4 modes correspond to polar modes, and that of TO3 belongs to nonpolar modes. Thus, the change in polar modes indicates the characteristic polarization of ST nanoparticles. The symmetry

Figure 5. Photoluminescence spectrum of SrTiO3 samples synthesized at different times. 5675



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of the electrons excited by incident radiation energy and holes trapped in the energy levels near the valence band (such as in the diagram in Figure 6). According to the literature,10 violet and blue emission can be associated with shallow or surface defects, whereas green and yellow emission (and also red) is associated with deeper defects.49 Each color, therefore, represents a different type of electronic transition, which depends on a specific structural arrangement and how the clusters are inserted in this arrangement.10 In the work reported by Longo et al.,12 the ST sample, prepared with a polymeric precursor, showed photoluminescence centered in the green region of the visible spectrum, when excited with the same wavelength (350.7 nm) used in our study. These results indicate that both the preparation method and the precursor used have a great influence on this property. Furthermore, it is suggested that additional levels to the band gap materials do not follow a regular pattern. It is also assumed that the radiative recombination can be attributed to the location of these types of defects; that is, the electron−hole recombination can, in fact, follow different paths involving these intermediate levels to the band gap. According to the PL results, shown in Figure 5, the increase in synthesis time caused a decrease in photoluminescent emission, although the band-gap energy did not change significantly (Table 3). As the excitation energy does not change (3.53 eV, 350.7 nm) and the optical band gap does not change significantly, it is likely that the decrease in PL emission

Figure 6. Schematic of the excitation process and photoluminescence emission due to the recombination between electrons and holes.

with holes from the valence band, a radiative recombination occurs, were photoluminescent emission lines are observed. Thus, the photoluminescent emission of an ST sample, excited with a laser of 350.7 nm, is attributed to this type of multiphonon radiative recombination, as observed in the broadband centered in the blue region (∼460 nm) of the spectrum, as shown in Figure 5. In this case, the emission of a photon is derived from the energy difference between a higher electronic state and the ground state, during the recombination

Figure 7. Scanning electron microscopy (FE-SEM) images of the SrTiO3 samples: (a) 4, (b) 10, (c) 20, (d, e) 40, (f) 80, and (g) 160 min. 5676



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Figure 8. Illustrative scheme of the particle growth and its relationship with the photoluminescence of SrTiO3 synthesized at different times.

collide with each other, favoring the coalescence process and consequently crystal growth. This process is repeated for other synthesis times, where the cubes formed tend to be more defined (Figure 7e,f). The sample synthesized in 40 min (Figure 7d) appears to have, among cubic forms, a differentiated morphology, represented by a dodecahedron. The formation of new architectures for the same sample can be related to the thermal instability of the solution caused by the action of microwaves. The poor control of this phenomenon can cause indiscriminate heating during synthesis, with the formation of overheating points (hot spots), fusion, or tensions due to the temperature gradient.53 On the other hand, even from a thermodynamic point of view, we recall the Wulff rule, which states that the equilibrium shape of a crystal corresponds to the minimization of the total surface energy, which varies with the orientation of the crystal. For the growth mechanism, the Wulff rule acts on each face of a crystal. Faces that have high surface energy grow rapidly, have a small surface area, or disappear completely with crystal growth in the final morphology, whereas surfaces with low surface energy will grow slowly and dominate the final phase.2,54 Thus, it is noted that, for a good approximation, these two mechanism types occur simultaneously in the crystal growth of ST, whose final stage is limited by the cubic morphology. With increasing the synthesis time up to 160 min (Figure 7g), the coalescence process is again observed, since bars with a hexagonal straight section are noticed with visible grain boundaries. In addition, there are many agglomerates of spherical nanoparticles, which were apparently formed by the union of nanocubes. This indicates that such nanocubes grow to a certain limit and return to aggregate into a spherical morphology to produce other nanocubes with even greater dimensions. This process repeats throughout the different synthesis times in a cyclical process, maintaining the morphology within a dimensional limit

is related to the higher density of charge carriers or defects in the samples synthesized in shorter times. With the increased synthesis time, the concentration of defects decreases and, consequently, the number of charge carriers (vacancies and electrons trapped) also decreases, reducing the photoluminescence emission of the samples.50−52 It is also important to note that, although the samples showed a secondary phase of carbonates, this did not contribute significantly to the photoluminescence of the samples. Figure 7 shows FE-SEM images of the ST samples. The morphology shown by these materials, in general, enables a configurational arrangement in which the equilibrium shape of the crystal corresponds to energy minimization. Thus, it is possible that, initially, the particles pass through a spherical agglomeration process, but the crystals tend to grow with faceted surfaces with different surface energies, due to the different bonds or atomic density.2 It can be observed that the samples obtained after 4 min of synthesis (Figure 7a) show nanocubes with some regularity, on the order of 90 nm, although some nanoparticle agglomerates appear to have a nearly spherical morphology. These nanoparticles or nanocubes eventually aggregate due to a sufficiently low colloidal stability to the point where two particles are attracted by van der Waals forces.15 However, with increasing the synthesis time up to 10 min, the formation of a cubic morphology with larger dimensions is observed, resulting from the coalescence of smaller nanocubes or coalescence of the agglomerated nanoparticles (Figure 7b,c). During the MAH process, the high-frequency microwave radiation interacts directly with permanent water dipoles in the solution, which initiates a fast heating from molecular rotation. As a result, the microwave radiation is able to promote an effective increase in the rate of collision between the ions, which favors the nucleation process of the SrTiO3 phase. Subsequently, the nucleated particles 5677



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morphology of defined and larger dimensions. With longer times, other architectures are formed, besides cubic morphology. The decrease in photoluminescent emission is associated with the morphology and particle size, in addition to the local order−disorder degree displayed by the samples. Particles with a cubic morphology close to a single crystal and smaller dimensions promote more intense photoluminescent emissions.

probably dominated by the crystallite size, which, as seen in Table 3, does not undergo significant changes. In all samples, it was still possible to identify a porous morphology (spongy), indicating carbonate formation, previously identified by XRD and Raman spectra (Figure 7d, circled). In the work presented by Moreira,15 ST nanoparticles prepared by the MAH method were organized according to a spherical morphology. Assuming that the author used titanium (TiCl4) as the precursor, unlike in our work, we can say that, in addition to the preparation method, the precursors certainly have a significant influence on the final morphology of the material. Considering that the band-gap energy of the ST samples does not vary significantly with synthesis time but that photoluminescent emission decreases, we can affirm that photoluminescent behavior is also directly related to morphology and particle size. A global view of ST morphology shows an evolution in size, from predominantly quite small cubic agglomerates, which acquire other morphologies, such as hexagonal bars with longer synthesis times. An illustrative scheme of the ST particle formation process and growth is shown in Figure 8. In the process of formation, the Sr2+ and TiO32+ ions in aqueous solution are adsorbed chemically assisted by microwave radiation, toward [TiO6] cluster formation. These clusters are grouped together for nucleation of primary crystallites, which subsequently aggregate to form cubic nanoparticles, visible in the FE-SEM images. This initial process is common to all the synthesis times, since the formation of the titanate has already been identified during the initial 4 minutes. The subsequent stage of crystal growth occurs due to the self-organization of the primary particles that are grouped together to form larger particles. For shorter synthesis times, the cubic particles are very small and have larger surface areas, which contribute to increase the photoluminescent emission.4,50 Because the surface effects are more pronounced due to the large ratio between their surface and their volume, the nanocrystalline particles exhibit unique optical properties, which are not observed in bulk crystals.50 When the synthesis time is increased, the particles grow self-assembled, acquiring cubic morphologies with perfect edges, hexagonal bars, and dodecahedrons, helping to decrease photoluminescent emission.55 This behavior is probably due to the minimization of local defects caused by the reduction of interfaces between particles, resulting from coalescence, moving grain boundaries, and increasing particle size. Thus, it is clear that the photoluminescent behavior of ST samples is not only dependent on the ordering degree at the short and medium ranges of samples. There is a mutual influence between photoluminescent emission, morphology, and particle size, which can be controlled by the processing time of the material.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 55 18 3229 5741. Fax: 55 18 3229 5682. E-mail: agda_ [email protected]. Notes

The authors declare no competing financial interest. # Universidade Estadual Paulista, Rua Roberto Simonsen, 305, Presidente Prudente, SP, Brazil.

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ACKNOWLEDGMENTS The authors thank Rorivaldo Camargo for the FE-SEM images; Dr. A. Leyva for his help with English editing of the manuscript; and CMDMC/LIEC, POSMAT, FAPESP, and CAPES for financial support.

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REFERENCES

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4. CONCLUSIONS The results reported in this study show that microwave radiation associated with the hydrothermal process is very efficient for producing SrTiO3 crystalline nanoparticles with a short synthesis time. XRD, Raman, and UV−vis data showed that the reaction time does not influence the formation of the main ST phase. The synthesis time also has no significant influence on the room-temperature photoluminescent emission range of this material. However, the intensity of photoluminescent emission decreases with an increase in time. There is also a contribution to the growth of faceted crystals, which pass through a coalescence process and give rise to a cubic 5678



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