Quantum Mechanics Insight into the Microwave Nucleation of SrTiO3

Article pubs.acs.org/JPCC

Quantum Mechanics Insight into the Microwave Nucleation of SrTiO3 Nanospheres Mário L. Moreira,*,† Valéria M. Longo,‡ Waldir Avansi, Jr.,‡ Mateus M. Ferrer,§ Juan Andrés,∥ Valmor R Mastelaro,⊥ José A. Varela,‡ and Élson Longo‡ †

INCTMN, Instituto de Física e Matemática (IFM), Departamento de Física, Universidade Federal de Pelotas, Campus do Capão do Leão, Caixa Postal, 354, CEP 9601-970, Pelotas, RS, Brazil ‡ INCTMN, Department of Physical Chemistry, Institute of Chemistry, Unesp − Universidade Estadual Paulista, Prof. Francisco Degni Street, s/n°, Quitandinha, Araraquara, SP, 14800-900, Brazil § LIEC, Departamento de Química, UFSCar, Rod. Washington Luiz, km 235, P.O. Box 676, CEP 13565-905, São Carlos, SP, Brazil ∥ Departament de Química Física i Analítica, Universitat Jaume I, Campus de Riu Sec, Castelló E-12080, Spain ⊥ Instituto de Física de São Carlos, USP, P.O. Box 369, 13560-970, São Carlos, SP, Brazil S Supporting Information *

ABSTRACT: An extensive investigation of strontium titanate, SrTiO3 (STO), nanospheres synthesized via a microwaveassisted hydrothermal (MAH) method has been conducted to gain a better insight into thermodynamic, kinetic, and reaction phenomena involved in STO nucleation and crystal growth processes. To this end, quantum-chemical modeling based on the density functional theory and periodic super cell models were done. Several experimental techniques were employed to get a deep characterization of structural and optical features of STO nanospheres. A possible formation mechanism was proposed, based on dehydration of titanium and strontium clusters followed by mesoscale transformation and a selfassembly process along an oriented attachment mechanism resulting in spherical-like shape. Raman and XANES analysis renders a noncentrosymmetric environment for the octahedral titanium, while infrared and first-order Raman modes reveal OH groups which are unsystematically incorporated into uncoordinated superficial sites. These results seem to indicate that the key component is the presence of distorted TiO6 clusters to engender a luminescence property. Analysis of band structure, density of states, and charge map shows that there is a close relationship among local broken symmetry, polarization, and energy split of the 3d orbitals of titanium. The interplay among these electronic and structural features provides necessary conditions to evaluate its luminescent properties under two-energy excitation. properties.8,9 Most of these intriguing properties are, however, often influenced or governed by how its electrons behave, thereby making a fundamental understanding of its electron distribution primarily important. STO is an ideal cubic perovskite structure at room temperature with a space group Pm3m (221).10 The cubic perovskite structure is maintained over a wide temperature range down to 105 K.11 From a structural standpoint, the Ti atom is located in the center of the octahedral site in 6-fold coordination, and the Sr atom at the corners of the cubic cell in 12-fold coordination with oxygen atoms set in the center of the faces makes up a regular cube-octahedron.12 STO is a semiconductor with an indirect band gap usually comprised

1. INTRODUCTION The design and controllable synthesis of structures at different spatial lengths ranging from micrometer to nanometer scales permits control of physical and chemical properties that are very useful for many potential applications such as electronics, optoelectronics, catalysts, pigments, sensors, etc.1−4 The behavior associated with these properties is determined by a series of parameters that may include their size, shape, chemical composition, and morphology, as well as their atomic structure. Perovskite-based materials are very interesting materials due to their potential industrial applications.5−7 Strontium titanate, SrTiO3 (STO), is an important member of the perovskite family which has attracted particular interest for a variety of applications. STO has attracted the broadest attention largely because its physical properties are relevant for the emerging oxide-based electronics, including electrical, dielectric, piezoelectric, semiconducting, superconducting, and magnetic © 2012 American Chemical Society

Received: July 4, 2012 Revised: September 26, 2012 Published: October 12, 2012 24792

dx.doi.org/10.1021/jp306638r | J. Phys. Chem. C 2012, 116, 24792−24808

The Journal of Physical Chemistry C

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between 3.2 and 3.4 eV13,14 which is significantly affected by a reduction in the particle size due to confinement effects.15,16 At lower temperatures, perovskite materials typically undergo a series of phase transitions which involve rotations and tilting of the TiO6 octahedra to produce tetragonal, possibly orthorhombic, and eventually rhombohedral crystal structures.17−21 Electroluminescence is an important property exhibited by STO ceramic, which can be significantly improved by combination with other perovskites such as calcium titanate (CTO) and barium titanate (BTO).22 Structural changes can provide STO with a certain diversity of physical properties, e.g., turning a STO semiconductor system into a typically metallic conductor of about 1.7 × 103 A/cm2 related to a Ti reduction23 or even superconduction at low temperatures (below 90 K).24 The PL of STO has been observed around 540 nm (a broad greenish luminescence) at low temperatures (below 50 K)25 for pure crystals and at room temperature26 for disordered compounds.27 Another recent application to STO particles is scintillation at low temperature in the ultraviolet region if the particles are excited by X-ray ionizing radiation.28 Therefore, STO has received much attention both experimentally and theoretically with respect to its defect chemistry and radiation resistance damage.29−32 To obtain the STO compound, different synthesis methods have been employed such as solid state reactions,33 polymeric precursors,34 laser ablation,35 and hydrothermal synthesis.36 Wet chemical methods clearly offer the highest flexibility in terms of controlling particle size, shape, surface, and structural features while providing high compositional homogeneity, which are crucial parameters in determining physical/chemistry properties. Hydrothermal reactions have advantages in the synthesis of advanced titanate ceramics: (i) pure phases are obtained at temperatures of 240 °C and (ii) precise control over several factors involved in the synthesis such as cation concentration as well as pH and counterions present in the reaction medium.37,38 High temperatures and long treatment times are needed when using hydrothermal methods. The microwave-assisted processing is fast, clean, simple, and often energetically more efficient than conventional heating. Microwave energy can be directly transformed into heat at the bulk of the dielectric materials, where it is absorbed and transferred by different absorption mechanisms such as dipolar relaxations or ionic conduction. Therefore, the use of microwaves as the heating source has created a special subject in this area. The microwave-assisted hydrothermal method (MAH) has attracted much attention in organic reactions as well as in the synthesis of oxide materials, making it possible to obtain materials with new properties39−41 because the microwave is a nonionizing electromagnetic radiation with a higher penetration depth and greatly enhances the rate of nucleation and thereby reduces the synthesis time42 while enabling phase selectivity43 and the control of crystal morphology.44,40,41,45−47 The main advantage of microwave heating is its energy efficiency because power is only applied within the reactive mixture. Precursors and their reaction intermediates are thought to have different dielectric constants along the same reaction paths; these differences can be resolved by selectively coupling to intermediates in their transition states with large advantages over conventional heating methods. Komarneni et al.46,48were pioneers in the study of the microwave effects on the kinetics of crystallization in the hydrothermal synthesis of electronic ceramics. An investigation of the synthesis of inorganic materials

using the hydrothermal method associated with microwave radiation was conducted in 1999 by Rao et al.,49 which supports microwave radiation applications in the preparation of different compounds. In the past few years, researchers revealed that this method of synthesis is able to produce crystals with unusual architecture as characterized by the self-organization.50−54 Recent studies agree that the resulting effect of microwaves on the synthesis of materials is still controversial and poorly understood because the phenomena involved are difficult to clearly define and explain.55 However, several theories have been suggested to elucidate the efficiency of MAH synthesis of materials.56 A key point is the specific capacity of the compound (solvent or reagent) to absorb the microwave radiation energy and convert it into heat.57 Generally, microwave irradiation induces a molecular rotation due to the dipole alignment of water (hydrothermal) with the external oscillating electric field.55,57,58 It is known that the electromagnetic field applies a force on charged particles as a result of particle migration or rotation throughout the solution. These applied forces change direction 2.45 × 109 times per second (2.45 GHz) which corresponds to (but not exactly) the harmonic resonant frequency of dipolar water molecules. A liquid or hydrated ″cluster″ is unable to respond instantaneously to the variable direction of the external oscillating field. Therefore, part of the energy of the incident electromagnetic field is evenly converted into heat inside the reactor.59Major heating mechanisms involved in microwave heating are dipolar polarization or ionic conduction60 due to interactions of dielectric materials (both liquids and solids) with the microwave electromagnetic field. The ability of a material or a reaction medium to convert electromagnetic energy into heat at a given frequency and temperature is related to the loss tangent (tan δ) which itself is a ratio of the dielectric loss (ε″) and dielectric constant (ε′) of the material57,58 Material with a high tan δ couples well with a microwave field and consequently leads to rapid heating (superheating). The parameter tan δ can be classified for hydrothermal systems (water) in the middle range microwave absorption as tan δ = 0.123.57,58 Moreover, other important factors such as the role of OH groups on phase formation and their consequences over final structural characteristics of the compound still need to be addressed. Microwave irradiation has been employed as a heat source in various fields such as in the synthesis of mesoporous silica, in catalysis, and in the synthesis of metal and metal oxide nanoparticles.61−64 Our research group has expanded various efforts to demonstrate that the MAH method is one of the most versatile and low cost approaches to obtain quasicrystalline,52,65 crystalline, micro-, and/or nanoscale particles.50,52,66−68 Compared to standard solvothemal and hydrothermal techniques that rely on heating in conventional furnaces, rapid heating and cooling times in a microwave reactor provide the unique opportunity to take snapshots of the nucleation, crystallization, and morphological evolution of nanostructures that provide similar kinetic information for studies of the growth mechanism by extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD).69 The purpose of the present research is to perform a joint experimental analysis and first-principle calculations on MAH synthesis of STO nanospheres. The paper is organized as follows: In Section 1 are reported the main goals to understand the corresponding nucleation and crystal growth mechanisms, as well as to find a relationship between electronic and 24793



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characterize specific properties of the materials. Both infrared and Raman spectroscopies are directly applied as advanced nondestructive characterization tools used to acquire information on the structural order at short and medium ranges of the compound. Raman spectra were recorded on a RFS/100/S Bruker Fourier transform Raman (FT-Raman) spectrometer with a Nd:YAG laser providing an excitation light at 1064 nm in a spectral resolution of 4 cm−1; the maximum output power was kept at 85 mW. Fourier transformer infrared (FT-IR) spectroscopies were performed in the range from 4000 to 470 cm−1 using a Bruker-Equinox 55 spectrometer in transmittance mode. This knowledge can be used to estimate the crystal potential fluctuations and local atomic arrangement. In addition, currently, it is well recognized that optical characterizations like ultraviolet absorption depend on both structural and electronic properties, including compositional ordering and the presence of impurities and defects. The UV−vis absorption was recorded using a Cary 5G spectrometer in total reflection mode by the integration sphere in the region of 200−800 nm. PL emission depends on electronic excitations and thus is a necessary complement to optical spectroscopies concerning lattice excitations which yield structural information of a completely different character from the information obtained by diffraction-based techniques. PL spectra were collected with a Thermal Jarrel-Ash Monospec 27 monochromator and a Hamamatsu R446 photomultiplier. The 350.7 and 415 nm exciting wavelengths were provided by a krypton ion laser (Coherent Innova) with nominal output power kept at 200 mW, particularly useful as a structural probe in a short-range amorphous compound. XANES spectroscopy provides substantial information about the local order around cations in oxide materials.79−82 Titanium K-edge X-ray absorption spectra were collected at the LNLS (National Synchrotron Light Laboratory) facility using a D04B-XAFS1 beamline. The LNLS storage ring was operated at 1.36 GeV and 160 mA. The Ti K-edge XANES spectra were collected in a transmission mode using a Si(111) channel-cut monochromator. XANES spectra were recorded between 4910 and 5100 eV for the Ti K-edge employing energy steps of 0.3 eV around the edge. To provide good energy reproducibility during the collection of XANES data, the energy calibration of the monochromator was checked while the data were being collected using a Ti metal foil. The size and morphology of the as-obtained samples were determined in a scanning electron microscope Zeiss VP Supra 35 equipped with a field emission gun (FE-SEM) and with a transmission electron microscope (TEM) JEOL JEM2100 URP operating at 200 KV. All measurements were taken at room temperature. 2.3. Computational Method and Periodic Model. Calculations were carried out with the GAUSSIAN 03 program.83 The simulations were performed by using the density functional theory (DFT) at the unrestricted B3LYP level84,85 using standard all-electron 3-21G86 base sets for H, Ti, Sr, and O atoms. Quantum mechanical computations can provide invaluable support for experimental data because many molecules that are not experimentally measured can be evaluated. Full geometry optimizations were carried out for all stationary points. Harmonic frequencies of the optimized structures were calculated to confirm the nature of the minima (zero negative eigenvalues of the Hessian matrix). The simulation was performed using a periodic approximation as implemented in the CRYSTAL06 computer code.87

structural properties associated to photoluminescence emissions as a consequence of its nucleation process. Sections 2 presents the experimental and theoretical procedures. Results and discussion are presented in Section 3, and the Conclusions section closes the paper. Each section is subdivided to favor the comprehension of the paper.

2. EXPERIMENTAL AND THEORETICAL PROCEDURES 2.1. Sample Preparation. Hydrothermal media provide an effective reaction environment for the synthesis of numerous ceramic materials because of the combined effects of solvent, temperature, and pressure on the ionic reaction equilibrium. In addition, this method is environmentally friendly and depends on the solubility of the chemical salts in water.37 On this point, the microwave heating dealing with the hydrothermal environment is a rapidly developing area of research.47,70−74 SrTiO3 samples were synthesized using TiCl4 (99.99%, Aldrich), SrCl2·2H2O (99.9%, Aldrich), and KOH (99%, Merck). Three colorless solutions were prepared: in the first solution, 0.05 mol of the TiCl4 was slowly added to 125 mL of deionized water at approximately 0 °C under vigorous stirring which formed TiO(OH)2 + H+ and Cl−. Similarly, 0.05 mol of SrCl2·2H2O was dissolved in the deionized water. Then, two precursor solutions containing Ti4+ and Sr2+ ions were mixed, homogenized, and shared into five portions of 50 mL, in which 50 mL of the KOH solution (6 M) was added to act as a mineralizer taking the solution to pH = 14. The suspension was loaded into a 110 mL Teflon autoclave which reached 90% of its volume and provided the maximum pressure efficiency to the system.75 The autoclave was sealed and placed into a MAH system which applied 2.45 GHz of microwave radiation (Supporting Information S1) with a maximum output power of 800 W. The heating rate was 140 °C/1 min, and the autoclave was kept at this temperature for 10, 20, 40, 80, and 160 min, denoted STO10, STO20, STO40, STO80, and STO160, respectively, under a pressure of 2.5 bar. Afterward, the autoclave was naturally cooled to room temperature, and the white solid product was washed with deionized water until a neutral pH was reached and then dried at 80 °C for 12 h. 2.2. Characterization Techniques. Different structural techniques were employed to provide detailed information on different time and length scales. Each level has its particular complexity, and the results therefore offer a structural insight into those length scales that determine many important aspects of material phenomenology and their properties. Thus, it is mandatory to use different techniques such as XRD which enables an average depiction of the structure through pattern matching and Rietveld analysis. The corresponding measurements were obtained in a Rigaku DMax 2500PC using Cu Kα1 (λ = 1.5406 Å) and Cu Kα2 (λ = 1. 54434 Å) radiation setup. Data were collected from 20° to 120° in a 2θ range with a 0.5° divergence slit and an 0.3 mm receiving slit using a fixed-time mode with a 0.02° step size and 1 s/point. Rietveld refinements76 were carried out with GSAS software77 which is specially designed to simultaneously refine both structural and microstructural parameters using the least-squares method. The peak profile function was modeled using the convolution of the Thompson−Cox−Hastings pseudo-Voigt (pV-TCH) with the asymmetry function described by Finger et al.78 The background of each pattern was fitted by a polynomial function. Moreover, there is a wide range of sophisticated experimental techniques that can be considered as complementary tools to 24794



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nanoparticles. From a more general note, also the introduction of ions in solution leads to an increased rate of dielectric heating due to Joule heating caused by the mechanism of ionic conduction.49 Ma et al.71 emphasized the use of Cl− ions as a key element for the formation of the BTO tetragonal phase in a hydrothermal environment. The STO crystallization process is expected to take place via hydrolysis−condensation reactions followed by nucleation− growth processes. For such reactions, significant amounts of mineralizer as alkaline agents (KOH) are required.37 In addition, it is well-known that in the hydrothermal synthesis the OH groups play a key role in the formation of perovskitetype oxides as observed previously for the BTO.74 A yield greater than 90% can then be obtained, and this effect can be associated with OH groups acting as catalysts in the reaction, leading to high rates of nucleation.37,95 Alkaline environment must be maintained due to the solubility of strontium and titanium hydroxides as the quantity of the STO compound is continuously increased along the synthesis.96,97 In this case, the low solubility of amorphous strontium and titanium hydroxides is associated with a high nucleation rate in aqueous alkaline medium, increasing the formation process,96−98 which may be described in terms of the following chemical reactions.

Density functional theory (DFT) in conjunction with the Backe’s three-parameter hybrid nonlocal exchange functional84 combined with the Lee−Yang−Parr gradient-corrected correlation functional, B3LYP,85 which has proven to be a very effective tool to deal with the present challenging problem. The parameters controlling the accuracy of the calculation of Coulomb and exchange integrals were set to 10−6 (ITOL1 to ITOL4) and 10−12 (ITOL5), whereas the percent of Fock/ Kohn−Sham matrices mixing was set to 40 (IPMIX = 30). The reciprocal space was sampled according to a regular sublattice determined by the shrinking factor, which was set to 4. We have been particularly confident in employing this functional to study the electronic and structural properties of SrTiO388,89 and CaTiO3.52 STO crystallizes in the cubic perovskite structure (space group Pm3m, Oh symmetry), with three nonequivalent atoms per unit cell. The lattice parameters and internal coordinations were obtained from refined parameters and are listed in Table 2. The atomic centers described by all electron basis sets for STO were: 31(3d)G for Sr,90 86-411(d31)G for Ti,90 and 6-31G* for O (Supporting Information S2). To simulate displacement of Ti atoms in the {001} direction, we used the ATOMDISP option contained in the CRYSTAL program. Titanium atoms were shifted from their previous position in the former unit cell by a (0 0 0.2) Å vector which models the distorted STO network former lattice and produces an asymmetric crystal unit cell. The XcrysDen program91 was used for the band structure drawing design and charge mapping. The vibrational modes analyses and their corresponding frequencies were calculated through numerical second derivatives of the total energies as implemented in the CRYSTAL06 package.92 Lattice constants and internal coordinate data were obtained from Rietveld refinement to better describe the structural distortion derived from experimental data.

SrCl 2·2H 2O(s) + H 2O(l) → Sr 2 +(aq) + 2Cl−(aq) + 3H 2O(l)

(1)

Sr 2 +(aq) + 2Cl−(aq) + 2K+(aq) + 2OH−(aq) → Sr(OH)2 (s) + 2K+(aq) + 2Cl−(aq) TiCl4(aq) + 10H 2O → Ti(OH)−5 + 5H3O+ + 4Cl−

(2) (3)

Dissolution. Precipitation. Hydrolysis - Precipitation. On the basis of experimental and theoretical results, we propose a plausible mechanism for the dehydration reaction of strontium and titanium hydroxides which is illustrated in Figure 1 and detailed by the equation reported in Scheme 1, which in our case happens preferentially if the solution is subjected to microwave radiation.99 The crystallization can be divided into three steps consisting of a liquid phase, a transitional phase, and a crystal phase.100−102 The whole process is illustrated in Figure 1 with two major transition processes corresponding to bulk diffusion and a bonding process which occurs from the solution to solid-phase crystal. The growth units are the titanium and strontium hydroxides, Ti(OH)5 and Sr(OH)2, and the dehydration process occurs though the formation of hydrogen bonds between OH groups of these moieties to form H2O dipolar molecules which start to rotate due to the action of electromagnetic (microwave) radiation. The events occurring in the transition phase are controlled by external factors such as supersaturation and pH. These two factors are dominant in the final STO morphology modification when a growth unit is incorporated into the crystal lattice by a dehydration reaction. Therefore, four different steps can be elucidated from the liquid to the crystal phase: (i) random distribution of Ti(OH)5 and Sr(OH)2 as growth units in the liquid phase; (ii) polarization and orientation of these units in the transition phase; (iii) formation of strong and stable Ti−O−Ti and Sr−O−Sr bonds along dehydration processes and obtaining quasi-crystalline SrTiO3 in the transition phase; and (iv) formation of the final network of SrTiO3 in the crystal phase.

3. RESULTS AND DISCUSSION 3.1. Thermal and Nonthermal Effects Provided by Microwave Radiation on Hydrothermal Synthesis. Microwave application is based on the efficient heating of matter by microwave radiation; meanwhile, if the temperature is raised above 100 °C, it becomes more difficult to heat the solution.58 The mild conditions of MAH synthesis are attributed to rapid and effective interaction between electromagnetic radiation with the permanent dipole moment of water molecules.49,93 Thus, the permanent dipoles of water and potentially induced dipoles in the solution can assist in rapid heating. This type of interaction is linked to the absorption capacity of electromagnetic radiation by the medium as well as its effective conversion of radiation into thermal energy as previously described.57,58 The tan “δ” of water can be increased significantly if ions are introduced into the solution which leads to an increase in the dielectric medium.60,94 KOH was used as a mineralizer agent due to its super saturation during the precipitation process, while strontium and titanium chlorides can be used as precursors due to the desirable presence of Cl− and K+ ions in the solution to improve the dielectric constant of the medium, owing to its high solubility in water.52 As a result, the dipoles or ions in solution can align with the applied electric field which can cause heating by two main ways, i.e., dipolar and ionic conduction.60,94 It is important to note that the ionic conduction mechanism represents a much stronger effect than the dipolar heating in the heat-generating capacity, taking to great consequences for growth 24795



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Figure 1. Schematic nucleation and crystal growth of STO nanospheres.

only after calcinations around at 750 °C,16,103 as already observed for SrZrO3,104 which leads to lattice compression. Irrespective of the technique or conditions, the growth of crystals is governed by general principles involving atom attachment at the growing interfaces. Under hydrothermal conditions, one of the major obstacles to achieving good control over the synthesis of nano- and mesocrystals is separation of the nucleation events from the growth process. Therefore, several considerations will be introduced, and the first consideration is about water viscosity which decreases rapidly when subjected to hydrothermal conditions.105 Even under moderate hydrothermal conditions, the viscosity is still low, so it is reasonable to assume that the mobility of ions and molecules dispersed in a solution is higher under hydrothermal conditions. Thus, an increase in effective collisions between the dehydrated strontium and titanium in a solution52 is expected. At this point, the formation of an agglomerate (disordered assembly) of nanocrystals or even the formation of mesocrystals (mesoscopically structured crystals) may occur as a result of a nanocrystal-based self-assembly process governed by particle− particle and particle−solvent interactions. As the phase formation is related to the effective collision process between dehydrated ions, the concentration of these ions in solution will determine the nucleation and mainly the further growth of crystals.66 3.2. Characterization of Crystalline Phases. XRD patterns of as-obtained samples (see Figure 2) could be indexed as an STO perovskite phase which is a cubic structure under the Pm3m (221) space group in accordance with JCPDS n°. 35-0734. Figure 2 also shows the presence of SrCO3 as an additional phase identified by the peaks at 2θ = 26°, 36°, 44°, and 48°, which match the main diffraction peaks of the orthorhombic phase with a Pmcn (62) space group of strontium carbonate indexed by JCPDS n°. 05-0418. In the Rietveld method, the parameters of both the crystal structure and those structures of STO that are correlated with the physical characteristics of the samples and the instrumental aberrations are refined to obtain the best fit between the observed diffraction pattern and the calculated pattern.106 Rietveld refinement facilitates the structural determinations of the STO crystal lattice parameters through the minimization of the Rwp and χ2 parameters as show in Figure 2. The cubic phase

Scheme 1. Initial Clusters of Strontium and Titanium to Give the Nucleation Process

As illustrated in Scheme 1, the initial complex for titanium is 5-fold coordinate with tetrahedral symmetry. At the moment in which the structure starts to be built by the repetition of the primary titanium 5-fold cluster, strontium hydroxides are introduced to minimize the formation energy of the final structure. It is important to note that oxygen complex vacancies arise if the midrange order is not well established, i.e., if the final structure has random defects at different levels. These kinds of defects are not periodic since the crystal structure is rapidly formed; i.e., only 10 min is enough to achieve STO nanocrystals, so polar domains can be created. It is important to note that it is difficult to separate thermal and nonthermal effects in synthesis by the MAH method. However, nonthermal effects are inherent characteristics of microwaves which promote an increase in the collision efficiency by the mutual orientation of polar molecules and the possible excitement of rotational or vibrational transitions. At this moment, the activation energy is reduced because the heat energy required to dehydrate the hydroxides is lower because the dehydration process is greatly assisted by microwave radiation. Of course, this comes with the cost of incomplete OH group loss during the nucleation (cluster formation) in STO powders. To support this assertion, Table 1 shows the total energy and also the formation energy of each complex with its associated minimal representation of the most stable structure evaluated. Other conformations were also analyzed and different methods considered, although this arrangement was the most coherent way. The annealing hydrothermal process is unable to completely eliminate OH groups on the surface of STO which can be reached

Table 1. Total and Formation Energy of Primary Clusters of Titanium, Strontium, and Their More Stable Associations charge energy ΔE (eV)

Sr(OH)2

Ti(OH)5

[Ti(OH)4]2O

[TiO2(OH)2Sr]2O

0 −3.269,743925

−1 −1.222,719987 −5,660000000

−2 −2.369,407146 −1,6

−2 −4.340,436123 −1,700000000

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distortions as well as compressions and stretches which have special importance in the crystal growth dynamics of general systems.109−111 Then, a nonclassical mechanism proposed for the nucleation and growth of STO nanoparticles in Figure 3 in terms of chemical reactions 1 and 3 and crystal growth is described.

Figure 2. X-ray diffraction patterns of STO depending on the time of synthesis: (a) 10 min, (b) 20 min, (c) 40 min, (d) 80 min, and (e) 160 min.

of STO was obtained with relative success. The relative term applies because of the significant and unwanted presence of strontium carbonate as a secondary phase (quantified in Table 2) as well as a long-range quasi-periodic translational order and long-range orientational order.65 The best convergence and reliability parameters were found for the STO40 sample due to the smallest amount of additional phase among the STO samples. However, a careful observation of the lattice parameter “a” shows there is a downward trend toward the reference value of the ICSD-023076, according to Table 2. The unit cell volume follows the same trend as the unit

Figure 3. Schematic model to illustrate the synthesis and self-assembly of STO as nanospheres.

The mechanism represented by Figure 3 can easily be extended to other compounds such as BaTiO 3 and CaTiO3.52,112 In the second step, the agglomeration process can occur through stabilization of colloidal nanoparticles, which should be so weak that one nanoparticle can attract another only by van der Waals forces. Nevertheless, the flexibility still remains higher to allow the minimum configurational energy,113 avoiding the coalescence process for synthesis times employed, which can be considerably smaller as compared with the conventional hydrothermal method. An analysis of the field emission scanning microscopy (FE-SEM) images (see Figure 4) renders that this process leads to the formation of STO nanostructure aggregates with spherical-like morphology for the STO10 and quasi-cubic assemblies for STO160 samples. Figure 5A shows the TEM images of the STO10 sample. Despite the fact that the apparent morphology of this sample is like a single crystal with a nanospherical-like shape (see Figure 4), HRTEM images of region “A” in Figure 5B reveal that these nanoparticles consist of aggregates of small rounded nanocrystals around 10 nm in diameter. From the analysis of an expanded HRTEM image (see Figure 5c), we are able to determine that the interplanar distance of this nanocrystal is about 0.28 nm, which is related to (110) crystallographic planes of STO perovskite with a cubic structure, as confirmed by XRD data (see Figure 2). As revealed by the parallelism of the lattice fringes (see Figure 5c), the nanocrystals are aligned along the same crystallographic direction which indicates that the particle is like a single crystal. In fact, a lower contrast between the crystallites (see Figure 5A) indicates the existence of nanopores which separate many primary particles. Similar behavior for the growth process of nanostrucutures has also been reported.114,115 According to Calderone et al., small nanocrystal aggregates with nanopores originated from an oriented attachment (a self-assembly process) of small primary nanocrystals.114 Increasing the synthesis time, from the STO10 sample to the STO160 samples, an analysis of the TEM images

Table 2. Benchmarks and Refined Parameters by the Rietveld Method strontium titanate (SrTiO3); Pm3m (221) space group ICSD-023076, cubic; z = 1, a = b = c = 3, 905 Å, α = β = γ = 90°, V = 59, 55 Å3 parameters sample

a (Å)

STO %

SrCO3 %

χ2

Rwp

STO10 STO20 STO40 STO80 STO160

3.925 3.927 3.921 3.917 3.914

87.33 89.44 89.83 88.86 87.91

12.67 10.56 10.16 11.13 12.09

2.15 2.19 2.10 2.26 2.71

6.79 6.88 6.74 7.00 7.60

R-Bragg V(STO) (Å3) 3.34 3.72 2.49 2.50 2.27

60.489 60.564 60.304 60.123 59.993

cell density which ranges from 5.080 mg/cm3 for the STO160 sample to 3.774 mg/cm3 for the STO10 sample, while the reference density value is 5.11 mg/cm3 (ICSD-023076). Thus, based on XRD results and Rietiveld refinements, the STO160 is the most ordered sample. 3.3. Crystal Growth Process. From a thermodynamic viewpoint, the classical model usually referring to the control of crystal form is based on Wullf’s law or Gibbs−Curie− Wulff.107,108 This law suggests that the shape of a crystal is determined by the specific surface energy on each side or facet of the crystal. However, it is well recognized that this thermodynamic argument is particularly unsound for understanding some changes that occur so far from thermodynamic equilibrium as in our case where at least some contour considerations on the Wulff model should be taken into account to best understand the growth of these crystals regarding structural defects and 24797



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Figure 4. Scanning electron microscopy for STO nanospheres synthesized by 10 and 160 min.

Figure 5. High-resolution transmission electron microscopy for STO 10 min, evidencing the characteristic interplanar distance of the cubic STO compound.

used to induce the specific shape. The mediation of this process can be explained in this case by the OH groups adsorbed on the surface of the nanoparticles identified by infrared analysis presented in Figure 7. Crystalline nanoparticles are able to interact by using OH groups to produce the specific design components that organize themselves into desired patterns through an oriented attachment mechanism.114,116 These selfassembled hierarchical superstructures produce a defective single crystal with a spherical or cubic shape which provides promising complex functions117 and direct bridges between nanoscale objects and the macroscale world. 3.4. Infrared Spectroscopy. The vibrational spectrum is indeed an informative source to determine the changes in the structure and composition of the material, which explains why Raman and IR techniques have been extensively employed. Due to the complexity of spectra, quantum-mechanics simulations may help for the detailed assignment of experimental frequencies. Figure 7 shows absorption spectra in the infrared region for STO samples. The low-frequency absorption bands between 450 and 600 cm−1 indicated in Figure 7a correspond to symmetric and asymmetric stretching modes of a Ti−O bond within the TiO6 octahedron cluster. Also, molecular vibrations of the carbonate group are visible between 1442 and 2479 cm−1 as previously described in the XRD analysis.118,119 Moreover, it is very important that the O−H stretching modes between

(see Figure 6A) reveal a similar behavior observed for both samples, i. e., nanostructures with spherical-like morphology and nanopores. Nevertheless, an expanded view of the region, indicated in Figure 6B, shows nanostructures with a nanocubelike shape and numerous nanopores with smaller sizes and lower contrast features in Figure 6B. These nanocubes suggest that with the MAH time treatment increase the morphology evolves from spherical-shaped to cube-shaped. This morphological evolution also was also reported by Calderone et al. for the synthesis of nanostructures obtained by the precipitation method.114 Figure 6C shows that the interplanar distance observed also is about 0.28 nm, which is related to (110) crystallographic planes. An analysis of the region B in Figure 6C shows that the nanopores with a lower contrast are not related to the presence of an amorphous phase which has a remarkable similarity and alignment with the cubic structure of the nanoparticle with the interplanar distances related to (110) crystallographic planes. The single-crystalline nature was confirmed by the fast Fourier transform (FFT) of the region B in Figure 6B related to the [001] zone axis (see Figure 6e). Despite different morphologies, the formation mechanism of as-obtained STO nanostructures follows similar trends. The mechanism corresponds to the predefined interaction between individual small size nanoparticles under the same origin that results in a spontaneous shape. The spontaneous term is adequate in this situation because no surfactant agents were 24798



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Figure 6. High-resolution transmission electron microscopy for STO 160 min, evidencing the mesocrystalline characteristic and the interplanar distance of the cubic STO compound.

Figure 7. Infrared spectroscopy applied to identify the strontium carbonate, OH groups, and Ti−O cluster formation.

2000 and 2500 cm−1 designate the remaining hydrogen bond configurations,99,120 while the weak shoulder at 1600 cm−1 is related to OH bonds and/or pure hydration of particles.121 The band located around 3500 cm−1 corresponds to OH hydration groups.120,122 The asymmetry resulting from the increase in intensity of this band in the low frequency side (3550−3350 cm−1) indicates the formation of hydrogen bonds.123 Hydrogen bonding is usually characterized by (i) broadening of the OH band accompanied by an increase in the absorbance and (ii) frequency shifts to a lower frequency of absorption bands due to ν(OH) stretching vibration. The broad band covering the region amid 3000−3600 cm−1 related to OH

groups and water of hydration expresses a typical asymmetry of hydrogen bonds.121 This result indicates that even after drying the compounds maybe it is possible to detect the remaining hydrogen bonds which are probably located on the perovskite phase surface, but now only as a result of the strong hydration commonly expected for systems synthesized under hydrothermal conditions. 3.5. Raman Spectroscopy. The phenomenon of inelastic light scattering is the usual method to investigate the behavioral changes in the local symmetry of ceramics such as perovskites.124 The establishment of first-order Raman modes for cubic systems elicits much interest to materials science,115 especially for the recognition of inactive Raman features for 24799



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suggest the emergence of polar domains inward of the octahedral site. Polar domains were discussed in section 3.1 in regard to a simulated titanium and strontium initial arrangement in relation to arising of complex oxygen vacancies. According to previous studies,125,126 the activation of transversal and longitudinal TO3 and LO3 (F2u) phonons, respectively (silent modes), and LO4 (A2g)127 indicates distortions on the order of a wavelength phonon in a STO cubic structure.128 Thus, it is plausible to suggest that there is a significant fraction of sites occupied in noncentrosymmetric conditions in the total volume of particles. This situation is illustrated by the inset in Figure 8, where the titanium atom occupies the noncentrosymmetric position in the octahedral site.129 According to Shiratori et al.130,131 on BTO, these bands should have been caused by the lattice defect mode since the evolution of this band is not linked to defects for the other impurity bands. Therefore, one possible origin of this band is the deformation of lattice OH groups, which is particularly feasible because in our synthesis a strong hydrolysis of metal ions is followed by thermal treatment in highly alkaline hydrothermal media. As noted, adsorbed OH− groups at the surface were detected by means of infrared spectroscopy as well as the formation of hydrogen bonds due to the asymmetry of high wavenumber peaks. These vibrational modes resemble the tetragonal structure of the BTO which leads us to believe that the cubic structure lost its inversion symmetry. Thus, the activation of first-order transversal (TO) and longitudinal (LO) Raman modes suggests that the concentration of point defects is significantly higher than in samples prepared by other synthesis methods where no Raman activity was observed. To better understand these experimental results, calculations of the Raman modes have been carried out. While keeping the lattice parameters and internal positions unchanged from the cubic structure, no vibrational modes were considered as active. On the other hand, the displacement of the titanium atom of 0.02 Å in the (001) direction caused longitudinal and transversal vibrations

Figure 8. First-order Raman active modes for cubic STO: (a) 10 min, (b) 20 min, (c) 40 min, (d) 80 min, and (e) 160 min.

Table 3. Simulated and Experimental Modes of Cubic STO Samplesa modes mode mode mode mode mode mode a

(TO2) (TO3) (TO4) (TO) (LO4) SrCO3

P1 P2 P3 P4 P5 P6

ref 115

ref 132

theor.

exp.

175 271 542 --795 1072

171 --537 --790 ---

207 264 510 635 777 ---

179 265 545 724 792 1072

References in table: 115, 132.

cubic systems under ordinary conditions. Raman spectra of our samples are depicted in Figure 8 and show that the local dynamic structure of STO in a cubic structure can be strongly affected by the present synthesis using the MAH method. In particular, strong first-order activity modes at room temperature

Figure 9. Crystal representation of centrosymmetric and noncentrosymetric STO structures, accompanied by the respective band structure and the projected density of states. 24800



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Figure 10. Projected density of states in respect of oxygen 2p orbitals for centrosymmetric and noncentrosymetric STO structures.

which were also observed experimentally (see Table 3). The displacement of 0.02 Å has been selected because it demonstrates better correlations with experimental results. The modes are assigned as follow: 207 cm−1 (O−Sr−O), 264 cm−1 (O−Sr−O) bending mode, 510 cm−1 (Ti−O−Ti) bending mode, 635 cm−1 (Ti−O) stretching mode, and finally 777 cm−1 (Ti−O) stretching mode. A similar result for the BTO cubic/tetragonal transition which was available by an experimental X-ray approach was recently reported.129 A comparison of experimental and theoretical vibrational modes for Raman spectra shows good agreement which is very important because the network parameters and internal coordinates used in the frequency calculation arise from the fit of the diffraction pattern using the Rietveld method. 3.6. Band Structure, Density of States, and Charge Map. First we optimized the lattice constant of the STO cubic cell which was then used in our simulations of the ordered (o) and displaced (d) system. The displaced system was chosen after a series of successive displacements for different atoms.

After this, was conclude that the 0.02 Å into (001) directions is the better representative situation for quasi-crystalline distortion in the STO structure which generates great gradients (∂E/∂x = 0) and optimal positive frequencies (∂2E/∂x2 > 0). The lattice constant deviation from pure STO is rather small when compared with the reference STO-cif used in Rietveld refinements. The optimized parameter was applied to calculated structural parameters using DFT where the diagonalization of the Hessian matrix yields the vibrational modes87 (as discussed above) as a result of their self-values. Other electronic and structural parameters such as orbital distributions which build the band structure as well as the DOS were obtained and depicted in Figure 9. Figure 9 shows band structures for STO160-o (ordered) and STO160-d (displaced) models, respectively. The top of the valence band (VB) is located at the R (0, 1/2, 1/2) point, and the bottom of the conduction band is located at the Γ (0, 0, 0) point; thus, an indirect band gap was obtained for STO160-o. On the other hand, if the titanium atom is displaced 0.2 Å in the (001) direction inward the octahedral cage, it results in a 24801



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Figure 11. Projected density of states of eg and t2g orbials for centrosymmetric and noncentrosymetric STO structures.

evaluated with STO160-o. Note that eg orbital contributions are more significantly energetic since they lie on the imaginary axes of the complex. Therefore, the electromagnetic influence is greater than the influence of the t2g orbitals. We maintain that the periodic displacement performed on Ti ions inward of the octahedral site, i.e., displacement from their centrosymmetric position, results in distortions which resemble a tetragonal lattice. This argument is supported by the above discussion about the vibrational modes obtained for STO; in this case, these modes result in the activation of Raman modes obtained experimentally and previously confirmed by the simulation of distorted systems. Figure 12 shows electron density maps for STO160-o and STO160-d models. The chosen plane in the (110) direction is a vertical plane as indicated in the representation of the double lattice which contains Sr, Ti, and O atoms. An analysis of Figure 12 clearly shows that the bonding between Ti and O has a typical covalent bonding due to hybridization between O (2p) states and Ti (3d) states as previously described in the DOS section. The breaking of the Ti−O bond is visible in Figure 12 for a distorted model, being that the displacement of titanium from its centrosymmetric position when switching from STO160-o and STO160-d results in the deformation of a symmetric structure into two fragments which resemble [TiO6] and [TiO5·Vxo] complex states88 remaining from random defects frizzed into the STO structure as a result of a fast MAH reaction. This structural feature is supported by Scheme 1 where the possible origin for the [TiO5·Vxo] complex cluster as a minority component due to Ti(OH)5 natural formation and a strong formation of distorted

complete new configuration of the VB structure, while the CB is slightly changed. The indirect feature of the band gap was kept, although the kind of indirect transition is altered from R−Γ to M−Γ, M being the (1/2, 1/2, 0) point. Thus, the band gap remains practically unchanged grading from 3.83 eV for an ordered sample to 3.9 eV for a displaced sample. Oxygen 2p contributions are usually more significant in the VB, while in the displaced STO sample the oxygen contributions in the CB are considerable, which means that the displacement of titanium from its centrosymmetric position causes a greater overlap (increase) between the 2p and 3d state. The band population analysis demonstrates the enhanced spatial overlap between O 2p and Ti 3d orbitals which confirms the covalent nature of the Ti−O chemical bonding. Without doubt, the oxygen 2p orbitals are strongly affected by structural distortion in the STO160-d arrangement expressed by DOS projections within the valence band. Exploring in more detail the behavior of the 2p orbital decomposed in px, py, and pz, Figure 10 provide an identifier for specific contributions of each orbital of CB and VB for distorted STO160-d and ordered STO160-o samples. For STO160-o, the contribution for DOS is the same for px, py, and pz orbitals due to the cubic symmetry of the crystal. Nevertheless, for the STO160-d sample, 2p orbitals are wholly reallocated as compared with 2p orbitals of the STO160-o sample. Again, px and py orbitals produce the same kind of contributions between them for total DOS, while pz leads to complete restructuring from displacements performed by titanium atoms. In the case of 3d projections for the total DOS in Figure 11, eg and t2g orbitals are totally changed in the STO160-d if 24802



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orbitals of these elements which also explains why an internal transition of Ti is sensitive to the local symmetry of its cluster [TiO5/TiO6] ratio. The superposition of 2p and 3d orbitals provokes an energy split of the titanium d orbitals into eg (high-energy) and t2g (lower energy) states (see Figure 11). For XANES spectra of STO, at the Ti-pre-edge region, three characteristic peaks are expected (A, B, and C). Peak A corresponds to the decoupling of t2g states of TiO6 octahedra.134 This effect is not relevant for STO due to the low intensity of peak A which indicates a small distortion of the octahedron for t2g orbitals probably because t2g orbitals are located out of bonding directions in the octahedra. Meanwhile, peak B is usually observed around 3 eV above peak A and is also related to the energy split of eg states. As observed in Figure 13, an increase in the intensity of peak B may indicate that the Ti atom is not centrosymmetric as described by Vedrinskii134 and observed in Figure 13. The eg orbitals have the same direction as the bounded O ions in the TiO6 octahedron (see Figure 11). For this reason, t2g orbitals (peak A is absent) are less affected by tetragonal distortion then eg orbitals.135,136 In the case of STO samples, although the XRD analysis indicates the formation of a cubic structure, according to XANES results, the Ti atom is not located exactly in the center of the TiO6 octahedron134 which is in agreement with experimental and simulated Raman vibrational modes (see Figure 13). In this case, the off-center Ti can be related to the slight structural long-range quasi-periodic translational distortions introduced by the fast crystallization and random replacement of oxygen into the surface. The peak B intensity of the Ti pre-edge is slightly affected by the synthesis time as reported in Table 4, by a continuous rise but

Figure 12. Charge density calculated over each atom for displaced and centrosymmetric structures to identify the local polarizations into the clusters.

Table 4. Peak Intensity of Ti Pre-Edge in the STO Matrix Which Indicates the Trend to a Centrosymmetric Position of Ti with Increasing Synthesis Time

TiO6 octahedra was reported. The charge density is modulated by the metal dislocation value, i.e., the distance between the dislocated metal and the oxygen which reduces the electronic interaction as a result of the increased distance between them by 0.02 Å, while the interaction with the oxygen in the direct opposed direction is atomically increased. This asymmetric structure generates polarizations into the STO cubic lattice which can be related to singlet and triplet excited electronic states as a result of cubic STO structure distortions.89 3.7. XANES (X-ray Absorption Near-Edge Structure). The XANES region of the XAFS spectrum is understood to be about 60 eV above the element absorption edge.133 As the titanium metal is linked to six oxygen ions in the cubic perovskite structure, it is certain that there is an overlap between 2p and 3d

sample

B2 intensity

B2 position

ST10 ST20 ST40 ST80 ST160

0.06 0.06 0.06 0.05 0.04

4970.89 4970.81 4970.79 4970.84 4970.79

phase Pm3m Pm3m Pm3m Pm3m Pm3m

cubic cubic cubic cubic cubic

not sufficiently to cause the introduction of real [TiO5/TiO6] relationships. It is noteworthy that the loss of local symmetry on the noncentrosymmetric position of the STO is not necessarily equal to the symmetry loss reported for BaTiO3

Figure 13. X-ray absorption spectroscopy for titanium K-edge and the magnification of pre-edge region to observe the intensification of peak B. 24803



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Figure 14. Light absorption spectroscopy in ultraviolet and visible regions for STO 10 until 160 min.

band gap of 3.69 eV as predicted previously by “ab initio” calculations,27 in good agreement with the experimental results. Theoretical results show enlarged band gaps as a result of optimizations performed for a STO refined structure. The difference between previous calculations of 3.69 eV and our experimental results of 3.7 eV and the result of calculated band gap in the present work (3.83 eV) is only 0.13 eV for an ordered system and 0.19 eV for a distorted lattice. A better understanding of the absorption process requires a direct analysis of the absorbance shape. Note that the scans occur from lower energies to higher energies, so the starting point of the absorbance corresponds to the minimum energy required to promote an outermost electron to the conduction band or intermediate state. However, only few electrons are available with this energy, and as the energy is increased, other electrons become capable of being promoted to these excited states. As can be seen from absorption spectra for STO, the first absorption occurs at about 0.3−0.5 eV below the calculated value of its optical band gap, which indicates several intermediate states arising from distortions in the octahedral cluster (already well characterized by infrared, Raman, and XAFS spectroscopies). 3.9. Photoluminescence Spectroscopy. PL spectroscopy can be applied as a sensitive and nondestructive technique to

by Ravel,136 but the loss produces a similar bias in terms of first-order active Raman modes. Therefore, increasing the synthesis time induces the stabilization of the t2g and eg orbitals of the TiO6 octahedron. Farges et al.137 reported that the increase in intensity and shifts in energy of this peak and the appearance of additional peaks can provide information about the local coordination of the TiO6 cluster. Thus, the displacement in the (001) direction used in our calculations seems to be valid, and the amount of displacement is sufficiently small so that it occurs experimentally. A typical sign of asymmetry in the Ti position for the cubic STO cell strengthens the argument that it is possible to develop ferroelectric materials with Pm3m cubic symmetry. 3.8. Light Absorption Spectroscopy in the Ultraviolet and Visible Region. To understand the effect of structural distortions on STO electronic states distribution, the light absorption between 200 and 800 nm was employed. This range was chosen due to our knowledge regarding the absorbance of STO perovskite at ultraviolet and visible regions. The sample optical band gap (Eg) was calculated by the Wood and Tauc method138 where the calculated value of Eg is related to the absorbance and photon energy. Figure 14 shows optical absorbance spectra of STO. This compound has an indirect 24804



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Figure 15. Photoluminescence emissions of STO samples for excitations at 350 and 415 nm.

the disorder associated with structural distortions at short and medium range as well as Raman and XANES spectroscopies. Meanwhile, broad band emissions are not capable of determining the defect type responsible for structural distortions. Nevertheless, the “ab initio” calculations associated with other experimental techniques render: (i) shallow defects which are situated closer to the conduction and valence bands; (ii) defects related to 3d orbitals of titanium closer to the conduction band and oxygen 2p closer to the valence band; and (iii) an overlapping between 3d and 2p states in both regions.

characterize the extrinsic and/or intrinsic defects of materials.12,139 In our model, we propose that nonradiative energy will be lost by network absorption as an intense vibration or heating.139,140 Moreover, it is expected that the energy supplied is greater than ″ΔE″ minimal, so that after the excitation, some kinetic energy remains in the excited electron−hole pair (exciton). Thus, the self-trapped electrons will be a result of distortions caused by their own photoelectron interaction with phonons and ″Coulomb″ potentials which are responsible for redistributing the density of electronic states within the band gap near the conduction band. If the energy difference between two electronic states is sufficiently large to block its reabsorption by the network, the energy loss occurs by the emission of a photon with a specific wavelength ″λ″. Figure 15 shows a characteristic multiphotonic luminescence process27 because it is composed of many photoelectrons with different wavelengths which originated from different self-trapping states. As the emission profile resembles a Gaussian distribution, the center will be responsible for the primary color of PL. Figure 15 clearly shows that the general shape of the spectrum is a broad band covering almost all visible/ultraviolet spectrum ranges from ∼360 to 680 nm for excitation at 350 nm, which are centered at 465 nm and from ∼520 until 770 nm and for excitation at 415 nm, which are centered at 585 nm. The excitation intensity controls the density of photoexcited electron−hole pairs. Each recombination of electron−hole pairs has a different functional dependence with the density of electronic carriers.141 For example, the number of interface and intermediate states is finite and will be saturated under high excitation. Also, the photoexcited and self-trapped carrier may alter the distribution of the interband states.141 Thus, for comparative purposes, the excitation intensity is kept as 720 mW for all samples. The change of excitation source of 65 nm (350 to 415 nm) produces a shift in emission to the STO samples (the “Stokes shift”). Then, the excitation at 350 nm is at least 0.56 eV above that of the minimum energy required to form the electron− hole pair in STO systems. This assertion is supported by the first absorption detected around 3 eV in UV−visible spectra absorption. On the other hand, the excitation of 415 nm is slightly lower than the minimum necessary energy which indicates that intermediate states should be necessary for this excitation and subsequent PL emission. Again, the effect of the selectivity and inhomogeniety of intermediate states is crucial for PL emission. Due to this strong dependence, we can attribute to PL spectroscopy the ability to provide information about

4. CONCLUSION Understanding STO nucleation using a hydrothermal method assisted by microwave radiation plays a significant role in the preparation of various inorganic materials. In contrast to the classical law, there are also nonclassical pathways that proceed through cluster-based reaction systems. As a consequence, crystallization pathways occur via mesoscopic transformation following (i) random distribution of Ti(OH)5 and Sr(OH)2 as growth units in the liquid phase; (ii) polarization and orientation of these units in the transition phase; (iii) formation of strong and stable Ti−O−Ti and Sr−O−Sr bonds by dehydration processes to obtain quasi-crystalline SrTiO3 in the transition phase; and (iv) finally, promotion of the formation of the final network of SrTiO3 still under nonperfect configuration as modeled by theoretical simulations and supported by experimental results. In this work, we have demonstrated a facile and efficient MAH method to monitor the growth process of SrTiO3 nanocrystals by taking “snapshots” of reaction intermediates using theoretical methods supported by experimental parameters. Structural analyses were conducted through XRD, XANES, Raman, and infrared spectroscopies, which gave clues regarding the local and periodic order−disorder relationships. To investigate the crystal growth process, the FE-SEM was used to analyze mesoscale transformations. To complement experimental results, quantum-chemical modeling based on the DFT and periodic super cell models was conducted. The main conclusions can be sumarized as follows: (i) Although STO was found to preserve its cubic structure and microstructure throughout the synthesis, some disorder was observed during the crystal growth processes which originated from primary titanium and strontium hydroxide clusters; (ii) vibrational infrared and Raman spectroscopy clearly evidences the distortion around octahedral titanium clusters and OH groups adsorbed on the surface; (iii) a network of the STO 24805



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lattice and the link of two adjacent TiO6 octahedra clusters stabilized by strontium hydroxides is achieved by the dehydration process that is activated by the MAH method; (iv) a possible formation mechanism is suggested and discussed to account for experimental and theoretical results based on nucleation followed by mesoscale transformation and a selfassembly process with an oriented attachment mechanism which results in spherical-like shape; and (v) such a growth process of STO nanoparticles along the MAH method should be operative for other perovskite-based materials and will be of great interest in crystal growth and nanochemistry. A careful process to investigate the structural and microstructutal parameters to metal oxides obtained via the MAH method was established in this work.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the support of the Brazilian research financing institutions: CAPES, FAPESP/CEPID 98/14324-0, CNPq, and FAPESP 2009/17752-0. Thanks to Brazilian Synchrotron Light Laboratory (LNLS) user facilities by XAS spectroscopy and Transmission Electron Microscopy analyzes. Juan Andrés acknowledges spatiality Ministerio de Educación y Cultura (project CTQ2009-14541-C02) of the Spanish Government, PROMETEO program (PROMETEO/2009/053) of the ́ Generalitat Valenciana, and Programa de Cooperación Cientifica con Iberoamerica (Brasil), Ministerio de Educación (PHB20090065-PC). They also thank Rorivaldo Camargo and Madalena Tursi for technical contributions.

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