PEROVSKITE NANOPOWDERS: SYNTHESIS ...

In: Nanopowders and Nanocoatings: Production... ISBN: 978-1-60741-940-2 Editor: V. F. Cotler © 2009 Nova Science Publishers, Inc. Section 2.3 is different in text and the TOC, we have followed the TOC for section numbering

Chapter 1

PEROVSKITE NANOPOWDERS: SYNTHESIS, CHARACTERIZATION, PROPERTIES AND APPLICATIONS Xinhua Zhu National Laboratory of Solid State of Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China

ABSTRACT Perovskite materials display a wide spectrum of attractive properties, such as ferroelectricity, piezoelectricity, dielectricity, ferromagnetism, magnetoresistance, and multiferroics, which make them attractive for applications in ferroelectric random access memories, multilayer ceramic capacitors, transducers, sensors and actuators, magnetic random access memories, and the potential new types of multiple-state memories and spintronic devices controlled by electric and magnetic fields. Following a similar trend to the miniaturization as the conventional CMOS (complementary metal oxide semiconductor) devices, the down-sized electronic devices based on perovskite electronic ceramic materials have also been developed. Advances toward nanoscale electronics have increased interest in the field of perovskite nanopowders. Perovskite nanopowders are versatile matrices for generating transition- and rare-earth metal oxides that exhibit a broad spectrum of properties and functions related to the following characteristics: (a) nearly innumerable combinations of metal cations can be accommodated within perovskite structural systems, (b) by reduction/reoxidation processes, nonstoichiometry (i.e., controlled amounts of ordered oxygen vacancies) can be introduced into the structure. In turn, high oxygen ion mobility or modified electronic and magnetic features can be implemented, and (c) the design of composite structural systems containing perovskite building units (perovskite slabs of different thicknesses) allows fine-tuning electronic and magnetic properties. Conventionally, perovskite nanopowders are prepared by solid-state reactions between the corresponding oxides or oxides and carbonates at temperatures above 1000oC. However, the resulting microstructures of perovskite nanopowders obtained from this method, are not suitable for the miniaturization of electronic devices, due to their significant particle agglomeration, poor chemical homogeneity, and coarse large particle sizes. To resolve the problems and to produce homogeneous and stoichiometric perovskite nanopowders, recently wet chemical

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Xinhua Zhu methods have been developed. In this chapter, an overview of the state of art in perovskite nanopowders is presented, which covers their synthesis, characterization, properties and applications. The underlying chemical and physical aspects of the synthesis process of perovskite nanopowders are discussed, focusing on understanding the reaction mechanism and phase transformation from amorphous to the crystalline perovskite phase that occur during the synthesis process. Following a review of the synthesized methods, the microscopic and spectroscopic characterizations of perovskite nanopowders are summarized. And then a wide range of properties and applications of perovskite nanopowders are also addressed. Finally, we provide a perspective on the future outlook of perovskite nanopowders.

Keywords: perovskite nanopowders; synthesis; characterization; properties and applications

1. INTRODUCTION Perovskite materials are one of the most widely investigated functional materials, which have important properties in ferroelectricity, piezoelectricity, dielectricity, ferromagnetism, magnetoresistance, and multiferroics. The perovskite structure is named for the prototype CaTiO3 mineral called perovskite, which is generally metal oxide with the formula ABO3, where B is a small transition metal cation and A is a larger s-, d-, or f-block cation. In a cubic perovskite, the larger cation A resides on the corners of the unit cell, the smaller cation B is in the center of the unit cell, and the oxygen ions (O2-) are on the centers of the faces (Figure 1a) [1]. The perovskite structure can be also built from three-dimensional corner-sharing BO6 octahedra that are connected through B-O-B linkages. The A-site cation fits in the large cavity at the center of eight corner-sharing BO6 octahedra, and the B-site cation resides in the interstitial site of an octahedron of oxygen anions (Figure 1b) [1]. Interestingly, and of technological importance, a variety of compositions crystallizes in the perovskite structure. Typical perovskite materials of technological importance are ferroelectric BaTiO3, PbTiO3, dielectric (Ba,Sr)TiO3, piezoelectric Pb(Zr,Ti)O3, electrostrictive Pb(Mg,Nb)O3, magnetoresistant (La,Ca)MnO3, and multiferroic BiFeO3. They have attracted interest for several decades, with tremendous applications including ferroelectric random access memories, multilayer ceramic capacitors, transducers, sensors and actuators, magnetic random access memories, and the potential new types of multiple-state memories and spintronic devices controlled by electric and magnetic fields [1-8]. The major challenge in manufacturing these materials is the processing of the materials with reliable and reproducible properties [9,10]. Following a similar trend to the miniaturization as the conventional CMOS (complementary metal oxide semiconductor) devices, the down-sized electronic devices based on perovskite electronic ceramic materials have also been developed. Advances toward nanoscale electronics have additionally increased interest in this field of perovskite nanoparticles [11-13]. For example, to develop high volume efficient multilayered ceramic capacitors (MLCCs), the sizes of BaTiO3 particles with high purity and uniform shape used for fabricating the next generation of MLCCs will be lowered down to tens of nanometers. Therefore, synthesis of high-purity, ultra-fine and agglomerate-free perovskite nanopowders with controlled particle size, morphology and stoichiometry, is the critical step in processing of perovskite ceramics with desirable properties. Perovskite nanopowders are versatile matrices for generating transition- and rare-earth metal oxides that exhibit a broad spectrum

Perovskite Nanopowders: Synthesis, Characterization, Properties and Applications

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of properties and functions that are related to the following characteristics: (a) nearly innumerable combinations of metal cations can be accommodated within perovskite structural systems, (b) by reduction / reoxidation processes, nonstoichiometry (i.e., controlled amounts of ordered oxygen vacancies) can be introduced into the structure. In turn, high oxygen ion mobility or modified electronic and magnetic features can be implemented, and (c) the design of composite structural systems containing perovskite building units (perovskite slabs of different thicknesses) allows fine-tuning electronic and magnetic properties.

a

b

Figure 1. (a) an ABO3 perovskite structure in which corner-shared oxygen octahedral extending in three dimensions, and (b) unit cell of ABO3 perovskite structure. Reproduced with permission from [1], Schaak, R. E.; Mallouk, T. E. Perovskites by design: a toolbox of solid-state reactions. Chem Mater. 2002, 14, 1455-1471.Copyright © 2002, American Chemical Society.

The evolution of a method to produce perovskite nanopowders with precise stoichiometry and desired properties is much complex. Conventionally, perovskite nanopowders are prepared by solid-state reactions between the corresponding oxides or oxides and carbonates at temperatures above 1000oC [14,15]. However, the resulting microstructures of perovskite nanopowders obtained from this method are not suitable for the miniaturization of electronic devices, due to their significant particle agglomeration, poor chemical homogeneity, and coarse large particle sizes. To resolve the problems arising from the conventional ceramic techniques and to produce homogeneous and stoichiometric perovskite nanopowders, in recent years, wet-chemical routes have been developed [12,16-18]. They can be better controlled from the molecular precursor to the final material to give highly pure and homogeneous materials, allowing for the low reaction temperatures used. The size and morphology of the particles can be controlled, and metastable phases could be prepared [18]. The objective of this chapter is to provide an overview of the state of art in perovskite nanopowders, which covers their synthesis, characterization, properties and applications. First, we review the synthesized methods for perovskite nanopowders, which include the syntheses using solid, liquid or gas phase precursors. The second section deals with the electron microscopic and spectroscopic tools for characterization of perovskite nanopowders. The microstructural features of perovskite nanopowders revealed by electron microscopes and spectroscopic techniques are addressed. In the context of properties we discuss the unique properties of perovskite nanopowders (e.g., ferroelectric and dielectric, electrical, magnetic, optical, and multiferroic properties), and the size effects for these unique properties are also

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discussed. And then a broad range of applications of perovskite nanopowders is addressed. Finally, we provide a perspective on the future outlook of perovskite nanopowders.

2. SYNTHESIS OF PEROVSKITE NANOPOWDERS Due to the powder size, dimensionality, and composition governing the resultant properties of the nanostructured perovskite materials that are assembled from nanopowders as building blocks to achieve certain desired properties, the synthesis of high-purity, ultra-fine and agglomerate-free perovskite nanopowders with controlled particle size, morphology and stoichiometry is the first and perhaps the most crucial step in processing of perovskite ceramics with desirable properties. The major issues for the synthesis of perovskite nanopowders include: (a) the control of particle size and composition, and (b) the control of the interfaces and distributions of the nanobuilding blocks within the fully formed nanostructured perovskite compounds. Over the past several decades, various methods have been developed to prepared perovskite nanopowders and the related nanostructured perovskite compounds. These various methods include synthesis using solid, liquid or gas phase precursors, which come under physical or chemical processing. In the subsequent sections, some important methods for the preparation of perovskite nanopowders are described.

2.1. Solid-State Reaction Route The solid-state reaction method is the most traditional one for preparing perovskite nanopowders (e.g., BaTiO3, PbTiO3, Pb(Zr,Ti)O3, etc) [14,15]. This process includes weighting starting materials (the corresponding oxides or oxides and carbonates), mixing, milling, and calcining them at elevated temperatures to form the perovskite phase. For example, in synthesis of BaTiO3 nanopowders by solid-state reaction method, the reaction process in air has been proposed to take place in at least three stages and relies on the diffusion of Ba2+ ions into TiO2 [19]. Firstly, BaCO3 reacts with the outer surface region of TiO2 to form a surface layer of BaTiO3 on individual TiO2 grains. Further diffusion of Ba2+ ions into TiO2 necessitates the formation of Ba2TiO4 between the unreacted BaCO3 and the previously formed BaTiO3. After prolonged sintering periods, the intermediate Ba-rich phase Ba2TiO4 reacts with the remaining TiO2 in the core-regions of the TiO2 grains to form BaTiO3. The high temperature calcination produces an agglomerated powder with a coarse particle size which requires additional milling process. However, contamination and other undesirable features during the milling process can create defects in the manufactured products. Furthermore, the more components in the ceramic powders, the more difficult it may be to achieve the desired homogeneity, stoichiometry, and phases. By using nanocrystalline BaCO3 and TiO2 as starting materials, Buscaglia et al. [20] have recently synthesized the perovskite BaTiO3 nanopowders with size of ~ 100 nm and narrow particle size distribution, via a solid-state reaction at calcination temperatures as low as 800oC. The average particle size of powders obtained via this method is essentially determined by the particle size of the used TiO2 because the reaction rate is controlled by the diffusion rate of


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barium ions into the TiO2 lattice [21]. Similar reaction mechanism was also found in the synthesized process of BaZrO3 powders [22]. The morphology of BaZrO3 particles was dependent upon the initial size and shape of the used starting ZrO2 particles. Therefore, fine BaZrO3 powders with particle size of 70-100 nm composing of crystallites of ~ 20-30 nm can be synthesized by using very fine (70-90 nm) starting ZrO2 particles and coarse (~ 1 µm) BaCO3 particles commercially available and calcination at ~ 1000°C. Higher calcination temperatures accelerate the initial stage of reaction but often lead to coarser and moreagglomerated powders.

2.2. Mechanical Milling Method Recently, many perovskite nanoparticles (e.g., BaTiO3, PbTiO3, PbZrO3) have been successfully synthesized by using several mechanical milling methods [23-25]. As viewed from the energy efficiency, the vibro-mill (or vibratory mill or vibro-energy mill) seems to be more attractive than the ball milling [24]. The vibro-milling enjoys several advantages over the conventional ball-milling produces, such as finer particles, narrower size distribution at a faster rate, simple equipment, low cost starting precursors, and large-scale production of nanopowders [23-25]. This implies that the vibro-milling method can be recognized as a powerful method for producing perovskite nanopowders. By choosing proper milling time and the calcination conditions, high purity perovskite nanopowders such as BaTiO3, PbTiO3, PbZrO3 with the smallest particle size of 100 nm, 17 nm, and 31 nm, can be mass-produced, respectively [26]. During the mechanical milling process, the mechano-chemical activation by the heavy milling is the key step, which alters the physicochemical properties of the starting materials and the mechanism of synthesis. Beauger et al. [27] proposed a multi-step reaction model, to describe the formation of perovskite BaTiO3 nanopowders via the mechanical milling process. According to this model, BaTiO3 is easily formed at the surface of TiO2 particles, which also act as the catalysts for BaCO3 decomposition [28]. When the surface BaTiO3 layer is formed by the decomposition of BaCO3 and its reaction with TiO2, the reaction kinetics is governed by the barium and oxygen ion diffusion through this layer into the virgin TiO2 phase. Moreover, it is expected that starting TiO2 with fine particles is very beneficial to acquiring the final BaTiO3 nanopowder due to the increase in the contact area of reactant particles (high reactivity) and their easy decomposition at low temperature. Because of the excess of barium and oxygen ions in the surface layer, a Ba2TiO4 phase is generally formed at the initial stage [29-31]. Homogeneous BaTiO3 powders can be formed gradually by the reaction between Ba2TiO4 and TiO2 via a multi-step reaction process, as schematically shown in Figure 2 [12]. Welham [33] also demonstrated that nanocrystalline BaTiO3 powders with an average particle diameter slightly larger than 10 nm could directly be obtained by high-energy mechanical milling of BaO and TiO2 (rutile) for several days without additional heat treatment. Although the extremely fine particles can be synthesized by this method, such an approach suffers from the disadvantages of quite small batch sizes and very long processing times. In addition, intensive ball-milling process may result in unfavorable contaminations from the milling media.

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Figure 2. Schematic diagram of the multi-step formation mechanism of BaTiO3 by the solid-state reaction of BaCO3 with TiO2. Reproduced with permission from [12], Yoon, D. H.; Lee, B. I. J. Tetragonality of barium titanate powder for a ceramic capacitor application. Ceram Proc Res. 2002, 3, 41- 47. Copyright © 2002, Journal of ceramic Processing Research.

2.3. Wet Chemical Routes Perovskite nanopowders prepared by the conventional solid-state reactions usually suffer from the particles with uncontrolled and irregular morphologies, which result in poor electrical properties of the sintered ceramics. In recent years, various wet chemical methods have been developed to replace the conventional solid-state reactions for the synthesis of perovskited nanopowders. The popular wet chemical methods for the preparation of perovskite nanopowders, include sol-gel method [43-49], alkoxide-hydroxide solprecipitation method [50-54], hydrothermal method [55-63], microwave-hydrothermal [6473], solvothermal syntheses [74-78], glycothermal process method [79-81], spray pyrolysis method [82,83], microemulsion synthesis method [84-87], high-gravity reactive precipitation [88-91] and room-temperature biosynthesis [92,93]. The most important advantages of the wet chemical methods include easy controlling the chemical stoichiometry, producing nanopowders with narrow size distribution, and low crystallization temperature due to the constituents mixed at the quasi-atomic level in a solution system. Due to the wet chemical solution process, a dopant such as paramagnetic ions or rare-earth ions could be readily introduced during the preparation of the precursor solution. In the following subsequent sections, various wet chemical methods uased for preparation of perovskite nanopowders are introduced.

2.3.1. Sol-gel (colloidal) processing Sol-gel process is a popular processing route for the synthesis of perovskite nanopowders (e.g., BaTiO3, PbTiO3, BiFeO3) [43-49]. This process involves the formation of a sol by dissolving the metal aloxide, metal-organic, or metal-inorganic salt precursors in a suitable solvent, subsequent drying of the gel followed by calcination and sintering at high temperature to form perovskite nanopowders. Due to the reacting species homogenized at the atomic level in a sol-gel process, the diffusion distances are considerably reduced compared to a conventional solid-state reaction, therefore, the product can be formed at much lower temperatures. In this process, the selection of starting materials, concentration, pH value, and heat treatment schedule play an important role in affecting the properties of perovskite nanopowders. This has been demonstrated in the case of BaTiO3 perovskite nanopowders


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[94-99]. Barium acetate and titanium isopropoxide are often used as starting materials to synthesize BaTiO3 nanopowders. However, the different rates in the hydrolysis and condensation of Ba and Ti precursors often give rise to the chemical component segregation in the obtained gels. To solve this problem, acetic acid or acetylaceton was often used to control the hydrolysis rate of the Ti precursor, since these complexing agents acts as chelating agents to coordinate with Ti species [100,101]. For the obtained gels, a heat treatment at high-temperature over 600°C is required to remove the unreacted organics and to crystallize the powders. Several steps involves in the transformation from the precursor to the crystalline BaTiO3 nanopowders, including the transformation from the precursor to the amorphous BaTiO3, and then to the threedimensional nucleation of the crystalline BaTiO3 in the amorphous matrix, and finally to the nanocrystal growth of BaTiO3 via a solid-state reaction [102]. To better control the grain size and its distribution, the heat treatment process parameters of the gels (e.g., post-annealing temperature, time and atmosphere, heating rate) must be optimized [102,103]. Normally, higher annealing temperature or longer annealing time can lead to larger grain size of the powders, while slow heating rate and inert annealing atmosphere can inhabit the aggregated behavior of nanopowders in comparison to air or oxygen atmosphere. That was demonstrated in the synthesis of Pb(Zr,Ti)O3 nanopowders [103]. By using these techniques, monodispersed perovskite nanopowders and related nanostructured materials have been successfully fabricated. The particle size can be adjusted from a few nanometers to micrometers via controlling the sold-state polymerization and the heat treatment process [43,96,102]. In the sol-gel routes based on acetate or double alkoxide precursors, however, noncrystalline BaTiO3 precursors are produced at first, and heating to 800oC or above is required to obtain crystalline BaTiO3 particles, which often removes nano-dimensional morphological characteristics of precursors and produces coarser chemically bonded aggregates.

2.3.2. Alkoxide-hydroxide sol-precipitation synthesis Crystalline BaTiO3 nanopowders can be directly synthesized via an alkoxide–hydroxide sol-precipitation process, which was first proposed by Flaschen [50]. This process has been studied extensively to produce crystalline BaTiO3 nanopowders at a low temperature without further calcination at an elevated temperature. This could reduce the manufacturing costs while maintaining better particle characteristics that could be realized by controlling the precipitation processes. Up to date, numerous publications and patents on the alkoxidehydroxide sol-precipitation process of BaTiO3 powders have appeared in the literature [50,51,104-108]. In this process, the hydrolysis and condensation are the key mechanisms of crystal growth. It was recognized that the amount of water and the method of its addition to the reaction system are decisive factors for controlling the size and the shape of precipitates. Many investigations have shown that BaTiO3 nanopowders can be synthesized at low temperature as 80-100oC via alkoxide-hydroxide method by using aqueous alkaline solution as a starting material [104-108]. This can be ascribed to the hydrolysis-condensation reaction occurring instantly upon mixing of aqueous and alcohol solutions. However, the final products are often highly agglomerated, and their morphological characteristics are quite inhomogeneous and undesirable for subsequent powder processing and sintering. By using the solid barium hydroxide octahydrate as starting material, it is possible to control the

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hydrolysis-condensation reaction by using water molecules released in situ as Ba(OH)2⋅8H2O was dissolved in the alcoholic solution. The experimental results have shown that BaTiO3 nanocrystals smaller than 6 nm begin to nucleate at 50oC without forming the intermediate TiO2 anatase, and corner-sharing TiO6 octahedra formed at 60oC. The average size of BaTiO3 precipitates increases up to about 7.5 nm at 80oC, and the BaTiO3 nanopowders show an anomalous lattice expansion with a relatively high tetragonality [52]. Direct formation of the perovskite phase at such low temperatures can be understood from the viewpoint of the coordination chemistry of the transition-metal alkoxide, in which titanium ions are present in unsaturated 4+ oxidation states in tetrahedral coordinations. As the reaction temperature is raised, the hydrolysis and condensation reactions convert them into three-dimensional TiO6 octahedra that share their six corners with other octahedra. The water and hydroxyl ions released from Ba(OH)2⋅8H2O convert tetrahedral Ti-isopropoxide into octahedral Ti(OH)62-, which reacts with Ba2+ to form perovskite BaTiO3 directly.

2.3.3 Decomposition of complex double metal salts Perovskite nanopowders such as BaTiO3 can be prepared by decomposition of complex double metal salts at temperatures above 600oC and up to 1300oC via several intermediate phases [34-36]. In this process, complex double salts of Ba and Ti, such as barium titanyl oxalate BaTiO(C2O4)2⋅4H2O [34,35] or -citrate BaTi(C6H6O7)3⋅6H2O [36], were used as solid precursors. Since both cations are already mixed on an atomic scale in the solid precursor, the thermal treatment leading to the perovskite phase can be performed at much lower temperatures compared with the mixed oxide route. Another advantage, resulting from the intimate mixture, is the possibility to attain very pure and almost exactly stoichiometric compositions under proper synthesis conditions. The reaction mechanism of BaTiO3 nanopowders formed from barium titanyl oxalate has been studied by several techniques. The results suggest that the formation of crystalline BaTiO3 nanopowders includes two steps: (a) the monoclinic crystal structure of the double oxalate initially collapses and converts into an amorphous upon drying, (b) crystallization via the intermediate phases into pseudocubic or tetragonal BaTiO3, depending on the pyrolysis temperature and the dopant content [34-36]. Due to the intimate mixture of barium and titanium ions, the reaction is complete at a much lower temperature in comparison to the conventional solid-state reaction process. Unfortunately, this process does not afford easy control of particle size and interparticle agglomeration: the size and agglomeration of the oxalate are maintained in the decomposed primary barium titanate particles, with a typical average size of approximately 50-250 nm [37,38], depending on the calcination temperature. In addition, the heating rate during calcination is also an effective parameter for controlling the particle size during nonisothermal but rate-controlled thermal decomposition [39]. As an alternative to conventional isothermal calcination, Wada et al. [40,41] proposed a two-step thermal decomposition method using barium titanyl oxalate to synthesize BaTiO3 nanopowders. The first step was carried out at 400oC in flowing O2 for complete dehydration, removal of remaining carbon species, and for preventing the formation of BaCO3 and TiO2. The amorphous phase obtained in the first heating treatment corresponds to the chemical composition of an equimolar mixture of BaCO3 and TiO2. In the second step, upon heat treatment at around 600oC under vacuum, extremely small BaTiO3 crystallites with an


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average particle size of approximately 17 nm were obtained. Particle size can be easily controlled from 17 nm to 100 nm by changing the temperature during the second annealing stage under vacuum [41]. The presence of tetragonality in all of these powders was evidenced by Raman spectroscopy, suggesting that the intrinsic critical size of single crystalline BaTiO3 should be below 17 nm [42].

2.3.4. Hydrothermal routes 2.3.4.1. Hydrothermal process Hydrothermal synthesis involves heating an aqueous suspension of insoluble salts in an autoclave at a moderate temperature and pressure so that the crystallization of a desired phase will take place. The hydrothermal synthesis is a powerful method for the preparation of very fine and homogeneous perovskite powders with a narrow size distribution and spherical morphology. Compared with the routes based on the solid-state reaction or decomposition of the solid precursors, the advantages of hydrothermal crystallization are the reduced energy costs due to the moderate temperatures sufficient for the reaction, less pollution, simplicity in the process equipment, and the enhanced rate of the precipitation reaction. Since there is no necessity for high-temperature calcination in this case, so the additional milling process is eliminated. For an ABO3 perovskite nanopowders, the general hydrothermal reaction can be written as [109] A(OH)s+B(OH)s(dissolution) → A(OH)aq +B(OH)aq (precipitation) → ABO3(s)

(1)

The perovskite BaTiO3 nanopowders have been prepared via hydrothermal process from titanium sources (such as oxide, oxide gels, or metalorganic compounds) and an aqueous solution of Ba(OH)2 with NaOH as a mineralizer. Typically, in a hydrothermal process, this reaction involves the reaction of Ba(OH)2 (or some other strong base with a soluble barium salt) and titanium sources (e.g., titanium alkoxide, titanium oxide, titanium oxide gels, or metalorganic compounds). This process leads to powders with a very high purity and little agglomeration. Furthermore, the low cost and easy handling of the reagents, and the fast reaction rate at low temperatures ensure that deagglomerated powders consisting of small particles with narrow size distribution are readily obtained. Although the optimization of hydrothermal conditions for the preparation of nanosized BaTiO3 has often been a matter of empiricism, much improvement has been achieved in the theoretical understanding of the thermodynamics and kinetics of the process, as well as the mechanisms of particle formation. Based on the standard-state thermodynamical properties of the chemical species involved during hydrothermal synthesis of BaTiO3 (e.g., Gibbs energy of formation, partial molal volumes, heat capacity, and etc.), Lencka and Riman [110-112] calculated the phase diagrams which allow ones to predict the required synthesis parameter (e.g., the ranges of reagent concentrations, pH value, and temperature for maximum yield), guiding the experimental hydrothermal process. To control the growth of BaTiO3 powders with the desired size and particle morphology, reaction mechanisms and thermodynamic modeling for BaTiO3 nanopowder formation during hydrothermal processing have been widely investigated [113-116]. Based on the highresolution transmission electron microscopy (HRTEM) observations on the incompletely and

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fully reacted powders, Pinceloup et al. [113] proposed a dissolution-precipitation model for hydrothermal synthesis of BaTiO3 nanopowders using Ba(OH)2 and TiO2 as precursors. In this model, TiO2 particles are first dissolved to form hydroxytitanium complexes [Ti(OH)n-], and then react with barium ions in the solution to precipitate BaTiO3. On the other hand, Hertl [114] and Hu et al. [115] proposed another in situ heterogeneous transformation model, in which TiO2 particles react initially with the dissolved barium to produce a continuous layer of BaTiO3, and the additional barium must diffuse through this layer and reacts with TiO2 until the supply of TiO2 is exhausted. This model was supported experimentally by the hydrothermal conversion from TiO2 microspheres to nanocrystalline BaTiO3 [115]. Eckert et al. [116] also reported on a mechanism evolution from a dissolution-precipitation process at the early stage of the reaction to an in situ mechanism for the longer reaction times. Recently, Walton et al. [117] investigated the hydrothermal crystallization of BaTiO3 by time-resolved powder neutron diffraction methods in situ, using the newly developed Oxford/ISIS hydrothermal cell. They directly observed that the rapid dissolution of the barium source was followed by dissolution of the titanium source before the onset of crystallization of BaTiO3. These qualitative observations strongly suggest that a homogeneous dissolution-precipitation mechanism dominates in the hydrothermal crystallization of BaTiO3 rather than other possible mechanisms proposed in the literatures [114-116]. These contradictive experimental observations reported previously are probably resulted from the different hydrothermal conditions. The crystalline perovskite phase BaTiO3 can be directly synthesized under hydrothermal conditions, however, the resulting products are usually highly defective in their crystallographic structure [118-123]. The crystal symmetry generally does not correspond to the tetragonal modification that is the thermodynamically stable form for BaTiO3 under normal conditions. Rather a cubic modification is often obtained [118-121,124], although the tetragonal phase may be obtained under certain conditions, such as high processing temperatures and prolonged duration time. Vikanandan et al. [119] suggested that the presence of the metastable cubic phase at room temperature is resulted from the compensation of the residual hydroxyl ions in the oxygen sublattice by cation vacancies. Shi et al. [121] also reported the stabilization of the cubic phase of BaTiO3 synthesized by the hydrothermal method, was caused by surface defects including OH− defects and barium vacancies. Hennings and Schreinemacher [122] reported on the observation of lattice hydroxyls and the effect of their release on the crystallographic recovery in hydrothermal BaTiO3 particles. Norma et al. [125] also reported that in the as-prepared barium titanate nanopaticles with average size of 66 nm, there was a high concentration of the hydroxyl group and barium vacancy, and its crystal structure was assigned to cubic with an expanded lattice by using the Rietveld. It has been found that the structural defects in the BaTiO3 nanopowders synthesized by hydrothermal method are primarily in the form of lattice OH− ions, which are compensated by ''

barium vacancies ( VBa ) created on the surfaces of individual particles to maintain the electro-neutrality [123-129]. The coexistence of high amounts of barium, titanium, and oxygen vacancies provides a rather unstable situation for the BaTiO3 lattice. It is believed that these point defects on the different lattice sites combine and annihilate each other. Therefore, the different charges of the point defects compensate each other to neutrality. As a result, the vanishing vacancies formed upon dehydration are believed to be responsible for the formation


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of intragranular pores (shown in Figure 3a), which partly disappear upon grain growth above 800oC. In the MLCCs ceramics made from the hydrothermal BaTiO3 powders, a strange expansion called “bloating phenomena” (shown in Figure 3b) was observed at the final stage of sintering, which was due to the inherently incorporated hydroxyl ions and protons during the hydrothermal synthetic process [13].

Figure 3. Intragranular porosity of hydrothermal BaTiO3. (a) Powder heat treated for 2 h at 500oC, (b) dielectric X7R ceramics sintered at 1320oC and showing a huge amount of intragranular pores. Reproduced with permission from [13], Pithan, C.; Hennings, D.; Waser, R. Progress in the synthesis of nanocrystalline BaTiO3 powders for MLCC. Int J Appl Ceram Technol. 2005, 2, 1-14. Copyright © 2005, American Ceramic Society.

In summary, hydrothermally synthesized BaTiO3 can show an adverse effect such as bloating in the final stage of the sintering process due to its inherently incorporated hydroxyl ions and protons during the synthetic process, despite the ideal uniform size and spherical particle shape.

2.3.4.2. Solvothermal process Solvothermal synthesis is defined as a hydrothermal reaction that occurs in a non-aqueous solution (e.g., NH3, methanol, ethanol, and n-propanol). In comparison with the hydrothermal processing, solvothermal synthesis has some advantages [76], such as (a) the reaction occurs under mild conditions and gives cubic-phase perovskite powder; and (b) the powders with particle size on the nanometer scale, exhibiting low agglomeration and a narrow particle-size distribution, due to the differences between the solvents. Up to date, several attempts have been made to synthesize superfine BaTiO3 nanopowder by solvothermal synthesis [17,76-78]. Using benzyl alcohol as solvent, BaTiO3 and BaZrO3 nanoparticles were synthesized by solvothermal process at relatively low temperatures of 200-220oC [17]. An assembly of BaTiO3 nanoparticles with an average particle size of 6 nm is shown in Figure 4a. The lack of any surface protecting layers results in some agglomeration of the particles. According to the randomly oriented lattice fringes, the particles have not coalesced. Based on the selected-area electron diffraction (SAED) pattern (see the inset in Figure 4a), the lattice distances measured from the diffraction rings, are in perfect agreement with the cubic and tetragonal modifications of the BaTiO3 perovskite structure. HRTEM images of two isolated particles oriented along the [110] and [111] directions, are shown

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in Figure 4b and Figure 4c, respectively. Figure 4d is a fast Fourier transform (FFT) pattern (equivalent to experimental electron diffraction pattern of the local region) obtained from the HRTEM image shown in Figure 4c, which provides an evidence that the particles are well crystallized in the perovskite structure without the presence of defaults. An overview TEM image for the BaZrO3 nanoparticles is shown in Figure 5a. It is observed that in most cases the primary particles are not isolated, but form wormlike agglomerates with diameters of 2-3 nm and lengths of up to 50 nm. These worms often assemble into larger, ball-like structures. The SAED pattern of such a spherical assembly (the set in Figure 5a) exhibites some broad rings that match with the BaZrO3 structure. Furthermore, the HRTEM image (Figure 5b) shows that the lattice planes of the individual particles in the ball-like structure oriented randomly with respect to each other. The HRTEM pattern of an isolated elongated particle proved the high degree of crystallinity (Figure 5c). This was further confirmed by the FFT pattern of this particle (Figure 5d), which is characteristic for the BaZrO3 structure without structural defaults. Obviously, the particle was aligned along the [111] direction. By using alcohol-based solvents such as ethanol, methanol and n-propanol, nano-sized (~ 20–60 nm) cubic-phase BaTiO3 powders were obtained [76]. However, the tetragonal BaTiO3 nanopowders with sizes of 50-100 nm were synthesized by using EtOH as a solvent [77]. It was found that the particle size was dependent upon the feedstock concentration (e.g., the precursor concentration). With decreasing the particle size from 89 to 58 nm, the amount of the tetragonal phase in the powder was decreased from 85% to 57%, and the cell parameter ratio (c/a) also decreased from 1.0080 to 1.0071. Recently, BaTiO3 nanopowders with sizes down to 5 nm are synthesized by direct reaction between barium hydroxide octahydrate and titanium (IV) tetraisopropoxide under solvothermal conditions (2-methoxyethanol and absolute ethanol, respectively) [78].

2.3.4.3. Glycothermal process The concepts embodied in hydrothermal processing approaches can be extrapolated to non-aqueous systems. By using glycol media (especially 1,4-butanediol solution) instead of water for the hydrothermal reaction, perovskite BaTiO3 nanopowders can be directly synthesized via glycothermal reaction at temperatures lower than that required by the hydrothermal conversion [130]. Glycothermal reaction of metal alkoxide, acetylacetonate, or acetate is a convenient route for the synthesis of crystalline ceramic powders, avoiding the effect of water. It has some novel features different from the conventional hydrothermal technology. First, glycothermal process does not need the mineralizers for the formation of anhydrous crystalline materials in some cases since 1,4-butanediol could act as an oxidizer [130-134]. Therefore, this process prevents the contamination by alkalis and/or halides, which are commonly used in conventional hydrothermal process. Second, glycothermal process significantly reduces the reaction pressure, which is a critical issue for large-scale production. Finally, it is easy to control the size and shape of the synthesized powders in glycol solution without growth-directing agents [134]. The tetragonal BaTiO3 nanoparticles, have been synthesized at temperature as low as 220oC through glycothermal reaction by using Ba(OH)2⋅8H2O and amorphous titanium hydrous gel as precursors and mixture of 1,4butanediol and water as solvent [81]. The glycothermal process, provides a simple low temperature route for producing tetragonal BaTiO3 nanoparticles without alkaline mineralizers, and the molar ratio of Ba/Ti, tetragonality, size and morphology of BaTiO3


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nanoparticles can be controlled by adjusting the reaction conditions (e.g., reaction temperature, volume ratio of 1,4-butandiol/water, the Ba/Ti molar ratio of precursor) [81].

Figure 4. (a) HRTEM image of an assembly of BaTiO3 nanoparticles. The inset is the selected area electron diffraction pattern. (c) and (d) HRTEM images of two isolated particles, and (e) the fast Fourier transform pattern obtained from the HRTEM image shown in Figure c [18]. Reproduced with permission from [18], Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. General soft-chemistry route to perovskites and related materials: synthesis of BaTiO3, BaZrO3, and LiNbO3 nanoparticles. Angew Chem. Int. Edt. 2004, 116, 2320-2323. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 5. (a) TEM image of BaZrO3 nanoparticles. The inset is the selected area electron diffraction pattern. (b) HRTEM image of an assembly of particles, and (c) and (d) HRTEM of isolated particle and the corresponding fast Fourier transform pattern, respectively. Reproduced with permission from [18], Niederberger, M.; Pinna, N.; Polleux, J.; Antonietti, M. General soft-chemistry route to perovskites and related materials: synthesis of BaTiO3, BaZrO3, and LiNbO3 nanoparticles. Angew Chem. Int. Edt. 2004, 116, 2320-2323. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

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2.3.4.3. Microwave-hydrothermal process The microwave-hydrothermal process is often found to be rapid, and has the potential to enhance the crystallization kinetics of hydrothermal process. The term microwavehydrothermal process was coined by Komarneni et al.[135] in 1992, and this process has been used for the rapid synthesis of numerous ceramic oxides, hydroxylated phases, porous materials, and hematite powders [64-73, 136-139]. It offers many distinct advantages over the conventional hydrothermal synthesis, such as cost savings due to rapid kinetics time and energy, rapid internal heating and synthesis of new materials. In the microwave-hydrothermal process, the microwave radiation couples with the material, and the electromagnetic energy is converted into thermal energy, which is absorbed by the material. Therefore, the heat is generated from inside the material, in contrast with conventional autoclave heating methods where the heat is transferred from outside to inside. This internal heat allows very rapid heating to the crystallization temperature, faster kinetics of crystallization by one-to-two orders of magnitude compared to the conventional hydrothermal process, and also saves energy and time. In addition, microwave heating is particularly suitable for perovskite nanopowders because the absorption degree of microwaves by them is much high due to their large dielectric constant and high dielectric loss. Numerous reports have published on synthesis of BaTiO3 nanopowders by microwavehydrothermal process below 200 °C, and these processes were found to be very rapid but they all yielded cubic phase [64-66,140,141]. For example, Khollam et al. [66] obtained submicron-sized BaTiO3 powders (0.1- 0.2µm) at holding time of 30 min. One of the first approaches on the synthesis of the nanosized BaTiO3 powders (about 30 nm) at 30 min, was reported by Jhung et al. [142]. Recently tetragonal BaTiO3 powders are synthesized by microwave-hydrothermal method at typical temperature of 240◦C from hydrous titanium oxide and barium hydroxide, in the absence of chloride ions and alkali metal ions to avoid contaminations. The effects of synthesis conditions, including reaction temperature and time, and reactant composition, on the formation of tetragonal structure and particle size of BaTiO3 powders, have been systematically investigated [143]. The results have shown that the amount of the tetragonal phase and the particle size increased quickly with reaction time, whereas the content of lattice hydroxyl groups decreased. Tetragonal BaTiO3 powder with nearly full tetragonallity (c/a ratio = 1.010) was obtained via the microwave-hydrothermal process performed at 240°C for 20 hours [143]. As the reaction temperature was lowered down to 220◦C, the formation of tetragonal structure and the growth of particles slowed down substantially, showing a critical effect of the reaction temperature on the microwavehydrothermal processing of tetragonal BaTiO3. Higher Ba(OH)2/Ti mole ratio enhanced the formation of tetragonal BaTiO3 and so did higher initial concentration of Ti with fixed Ba(OH)2/Ti ratio. Besides the BaTiO3 nanopowders, Ba1-xSrxTiO3 (x = 0.1-0.4) nanopowders with the average size about 20 nm were also prepared at relatively a short period of time (10 min) via microwave-hydrothermal synthesis [144]. The structure and the average sizes of BST were determined to be in range of 20-50 nm depending on the synthesis time (10-90 min). In conclusion, microwave-hydrothermal process has many advantages over the conventional methods [64-73]. Some of these advantages include, time and energy saving, very rapid heating rates (> 400 K/minute) without damage due to thermal shock, considerably


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reduced processing time and temperature, and fine microstructures. This process is also environmentally friendly.

2.3.5. Spray pyrolysis Spray pyrolysis represents a continuous and single-step preparation method for the production of fine homogeneous oxide powders [145-149]. The process of this method covers the following two steps: (a) the precursor solution (sol), which contains the metal ions dissolved in the desired stoichiometry, is sprayed through a nozzle and suspended in gaseous atmosphere (aerosol generator); (b) the suspended droplets are thermally processed to the product phase by allowing the sol droplets to drift through the heated zone of a furnace. Spray pyrolysis has many variations based on the differences in thermal processing step. Some of them are aerosol decomposition, evaporative decomposition, spray roasting, and spray calcinations [150]. Since the conventional spray pyrolysis results in multiple nanosized crystallites that are virtually inseparable, so they form a three-dimensional network [151-153]. Salt-assisted spray pyrolysis has been developed as a novel route to the preparation of nanoparticles below 100 nm [154,155]. This route requires no further thermal treatment of the product, such as calcination or annealing, because metal salts enhance the crystal growth and the homogeneity of the crystals. Compared with the sol-gel method and related precipitation techniques, the powders produced by salt-assisted spray pyrolysis are less agglomerated with improved crystallinity. Nanomter-sized perovskite particles with excellent compositional homogeneity can be prepared by this method. For example, highly crystalline, dense BaTiO3 nanoparticles were synthesized by using a salt-assisted spray pyrolysis method without the need for postannealing [156]. The particles ranged in size from 30 to 360 nm, depending on the synthesis temperature, with a narrow size distribution. The particle size decreased with decreasing operation temperature. The crystal phase was transformed from tetragonal to cubic at a particle size of about 50 nm at room temperature. Salt-assisted spray pyrolysis process can be used to produce high weight fraction of tetragonal BaTiO3 nanoparticles down to 64 nm in a single step. Nano-sized BaTiO3 particles were also prepared by citric acid-assisted spray pyrolysis [157]. It was found that controlling the spray solution with an organic additive made great differences in the structure and morphology of BaTiO3 particles during the calcination. The citric acid additives prevented phase separation of barium and produced phase-pure BaTiO3 particles at the as-prepared state and enhanced the phase transformability of metastable cubic phase to the tetragonal one during calcination. Tetragonal BaTiO3 nanoparticles with size of ~ 150 nm were successfully obtained by simple ball milling the coarse aggregates prepared from the citric acid-assisted spray pyrolysis and calcination at 1050oC.

2.3.6. Microemulsion synthesis Microemulsion synthesis is defined as an isotropic, thermodynamically stable system constituting the micrometer-sized droplets (micelle) dispersed in an immiscible solvent and an amphiphilic surfactant species on the surface of the micelle [84,85]. The crucial aspect of the microemulsion route is the control of the nanoparticle size through suitable selection and

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addition of a surfactant prior to the hydrolysis of the metal alkoxide sol (reverse micelle of water-in-oil emulsion). The addition of the surfactant molecules creates nanosized domains (nanoreactors) in the range of 0.5 - 10 nm spontaneously, in contrast to the conventional milky macroemulsions, which are only kinetically stabilized and in general prepared by the introduction of mechanical energy. The sizes of nanodomains are only dependent upon the composition of the microemulsion, temperature, and the elastic properties of the separating surfactant film. In particular, for the case of water/oil microemulsion with spherical nanosized aqueous micelles dispersed in an oil matrix, the aqueous droplets can be used as nanoreactors and templates for the preparation of solid nanoparticles. Since the reaction is spatially initiated and confined in the originally aqueous micelles, heterogeneous nucleation and crystal growth may be controlled. This method has been recognized as the most appropriate method for the synthesis of various electroceramic compounds (e.g., piezoelectrics [86], varistors [158], superconducting oxides [159-161], and magnetics [162-165]) in the form of nanopowders. Hempelmann et al. [166-168] demonstrated the preparation of nanoparticles for perovskite type materials such as BaTiO3. Figure 6 shows a high-resolution TEM micrograph of a single BaTiO3 nanoparticle obtained by microemulsion-mediated synthesis in combination with the particle size distribution determined by small angle X-ray scattering [169]. Powder particles well below 10 nm in size may be obtained by this technique. Furthermore, it allows the preparation of stable dispersions of nanopowders for the preparation of thin dielectric layers.

2.3.7. High-gravity reactive precipitation High-gravity reactive precipitation (HGRP) can be described as the reactive precipitation taking place under high-gravitational conditions [89]. For the HGRP synthesis, the key part of the rotating packed bed (RPB, Higee machine) is a packed rotator, which is designed to generate acceleration higher than the gravitational acceleration on the Earth. Three typical kinds of reaction systems are often used in the particle syntheses by the reactive precipitation. These are the liquid-liquid, gas-liquid, and gas-liquid-solid reactant phase systems. Recently uniform BaTiO3 nanoparticles are produced at a low temperature (1128 K. Some of the internal pores were released from the particle’s surface and/or during the grain growth. The presence of the pores affected the density of the BaTiO3 particle. The behavior of the internal pore was observed in situ with increasing temperature on the thermal stage of a TEM device. The results showed that at >1128 K, some pores move out from the particle’s surface during TEM observation. This temperature roughly agrees with the temperature at which the density of BaTiO3 powder sharply increases. During observation with increasing temperature, a thin layer appeared on the particle’s surface at temperature over 573 K and then disappeared at 1193 K.

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Figure 8. Transmission electron microscopy images of (a) as-received BaTiO3 powder (particle size ~ 60 nm) and (b) BaTiO3 powder annealed at 673 K. [180]. Nakano, H.; Urabe, K.; Oikawa, T.; Ikawa, H. Characterization of internal pores in hydrothermally synthesized BaTiO3 particle by transmission electron microscopy. J Am Ceram Soc. 2004, 87, 1594-1597, Copyright © 2004, American Chemical Society.

The hydrothermal BaTiO3 powder with a small particle size are stabilized in a cubic phase at room temperature [42,122,179,180-182], which implies that the distortion of the [TiO6] structure resulting in a cubic-to-tetragonal phase transition as cooled the sample through the Curie temperature is not taken place. A plausible reason is that the small size of the BaTiO3 nanocrystals, which are so small that the structural defects in the particles prevent the completion of the structural transition, leading to high strains within the crystals. The high strains inside the nanoparticles introduced by structural defects (e.g. lattice defects), would make the unit cell distortion (c/a ratio) much smaller than that in the standard BaTiO3. To reveal the high strains in the hydrothermal BaTiO3 nanoparticles by TEM images, Zhu et al. [181] recorded both bright- and dark-field TEM images from the hydrothermal BaTiO3 nanoparticles. Figure 9a is a bright-field TEM image recorded by using a small objective aperture that selects only the (000) central transmitted beam, which shows a narrowdistribution spherical nanoparticles. The dark-field image shown in Figure 9b, was recorded by using a smaller objective aperture that selects the part of the {100} and {110} reflections, as indicated by a circle in Figure 9c. The dark-field image displayed in Figure 9b clearly shows high strains in some BaTiO3 nanoparticles. By using the bright- and dark-field TEM images, Lu et al. [182] also reported several types of TEM contrast variations in an individual BaTiO3 nanocrystal synthesized via hydrothermal method at a temperature of 230°C. It is believed that the different types of variations of TEM contrast indicate the existence of different strains in BaTiO3 nanograins. Therefore, in a TEM image, large strain is indicated by a contrast variation across a particle. If a particle is single crystalline and has no strain, it should be uniform in contrast. However, for a single crystalline particle, if the TEM image shows dark-bright variation in contrast, it is likely to have a high strain within the grain. Strain affects the diffraction behavior of the electrons, resulting in dramatic contrast change. The hydrothermal BaTiO3 nanoparticles exhibit a cubic structure (a high temperature phase) at room temperature, such an abnormal crystallographic phenomenon is closely related to the existence of high strains in these BaTiO3 nanoparticles. The strains introduced by a high


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concentration of lattice defects such as OH- ions and barium vacancies, can make the unit cell distortion (c/a ratio) much smaller compared with that of the standard BaTiO3. As a result, no peak splitting was detected in the XRD patterns of the hydrothermal BaTiO3 powders even though they belong to the tetragonal phase.

Figure 9. (a) Bright-field and (b) dark-field TEM images recorded from the hydrothermal BaTiO3 nanoparticles. (c) An selected area electron diffraction pattern from the BaTiO3 particles showing a perovskite structure. The circle indicates the size and position of the objective aperture used to record the dark-field image displayed in (b). Reproduced with permission from [181], Zhu, X. H.; Zhu, J. M.; Zhou, S. H.; Liu, Z. G.; Ming, N. B. Hydrothermal synthesis of nanocrystalline BaTiO3 particles and structural characterization by high-resolution transmission. J Cryst Growth. 2008, 310, 434-441. Copyright © 2008 Elsevier B.V. All rights reserved.

Figure 10. Schematic diagrams for the formation of (a) STEM and (b) HRTEM images. Reproduced with permission from Dmitri Klenov, Melody Agustin and Susanne Stemmer (proviate communication).

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Generally, TEM is the most powerful and appropriate technique for investigating the characteristics of nanoscale particles, however, for detecting the low elemental concentrations that are typically of environmental interest, in which almost the entire concentration of the trace metal is located in just a few nanoparticles, it is almost impossible to find the nanoparticles that contain the metals of interest. When the elemental distribution is widely scattered, the image contrast in the conventional TEM is minimal except under very high magnification, where only a limited number of particles can be examined. However, as the particles of interest consist of relatively heavy elements, as compared with the matrix material, high-angle annular dark field scanning TEM (HAADF-STEM) is a powerful method for finding these nanoparticles of interest, details are described below.

3.1.3. Scanning transmission electron microscopy (STEM) STEM images (also called as Z-contrast incoherent images, or HAADF images) with atomic-resolution, are formed by using incoherent elastically-scattered electrons, as schematically shown in Figure 10a. Normally, a STEM image is formed by collecting highangle (75-150 mrad) elastically-scattered electrons with an annular dark-field detector (see Figure 10a). Such an annular detector captures a large fraction of the high angle intensity, providing an efficient dark-field imaging mode. The simplicity of STEM image is a direct result of the fact that only electrons scattered through large angles are used to form the image, so that interference effects contribute less to the image. The STEM image is consequently far less sensitive (although not immune) to specimen thickness variations, tilt and defocus. In the STEM operational mode, the electron beam is focused to a very fine spot (as small as 0.1 nm or less). By scanning this fine electron beam in a raster across the specimen and collecting the transmitted or scattered electrons, STEM images can be formed. Since the maximum scattering occurs when the electron probe is centered over a column of atoms, the columns appear bright in the image, whereas little scattering occurs when the probe is centered over a channel between atomic columns, therefore these areas appear dark. In a perfect crystal, atoms with higher atomic number, Z, produce more scattering, so that the intensity of the bright spots in the image can be related to the atomic composition of the corresponding column of atoms in the sample. STEM images can be interpreted more directly in terms of atom types and positions. Unlike conventional TEM, HAADF-STEM is based on imaging the incoherent scattering, and the contrast of the image is not reversed by defocusing above and below the point of “just focus”[185,186]. As the samples of environmental interest that contain nanoparticles with relatively heavy elements as compared with the matrix material, the contrast of HAADFSTEM image is strongly correlated with atomic number and specimen thickness, which is an appropriate method for finding the nanoparticles of interest [187]. An example for this is given in Figure 11, showing a STEM image of Bi-doped Si bulk crystal viewed from [110] direction [188], which reveals the columns containing individual Bi atoms introduced by ion implantation followed by re-crystallization through solid phase epitaxial growth. Single Bi atoms on lattice sites within the crystal are clearly visible. The density of bright spots correlates with the known dose of the doped Bi atoms. Similarly, in the rare-earth metal ionsdoped perovskite nanopowders, it is also possible to identify their substituted positions in the perovskite structure by using high-resolution HAADF-STEM images. In the near future we will see rapid progress in this direction.


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Figure 11. A STEM image of Bi-doped Si sample viewed from [110] direction. Reproduced with permission from [188], Pennycook, S. J.; Lupini, A. R.; Kadavanich, A.; McBride, J. R.; Rosenthal, S. J.; Puetter, R. C.; Yahil, A.; Krivanek, O. L.; Dellby, N.; Nellist, P. D.; Duscher, G.; Wang, L. G.; Pantelides, S. T. Aberration-corrected scanning transmission electron microscopy: the potential for nano-and interface science. Z Metallkd. 2003, 94, 350-357. Copyright © 2003, Carl Hanser Publisher.

3.1.4. High-resolution transmission electron microscopy (HRTEM) High-resolution TEM (HRTEM) images are formed by using nearly parallel electron beam traveling through the sample, and the direct (transmitted) beam and the diffracted beams are allowed to interfere with one another to form a “lattice” image, as schematically shown in Figure 10b. The image process of a HRTEM involves the following three processes: (a) electron scattering in a specimen; (b) formation of diffracted beams at the back focal plane; (c) formation of a high-resolution image at the image plane. HRTEM images are uniquely capable of providing the information about local atomic structures, which are most useful for identifying individual defects in nanocrystals, studying the atomic arrangements at interface between heterostructures. The formation of an HRTEM image is required to use an aperture large enough to include both the transmitted beam and at least one diffraction beam, in which the transmitted (actually, forward-scattered) beam provides a reference phase of the electron wavefront. As a result, HRTEM images in nature, are interference patterns between the forward-scattered and diffracted electron waves from the specimen. For HRTEM image, the great problem is the identification of the atomic species in the image, due to the inversion of image contrast, which is closely related to the specimen thickness, objective lens defocus, and additional interference effects (Fresnel fringes) at the interface between the crystalline substrate and the amorphous dielectric, in non-intuitive ways. Quantitative interpretations of HRTEM images are simple only as the sample is a weak-phase object (WPO) and the microscope is at Scherzer defocus. In this case, the contrast is related directly to the projected potential of the specimen. That means the atom columns appear as dark spots on a bright background, and the darkness of the spots is proportional to the project potential of the

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specimen. However, the WPO approximation is especially difficult to satisfy because the specimen must be thin enough so that the phase change of Bragg-scattered electrons is small relative to the forward-scattered electrons. Actually, only the thin specimens with low atomic numbers are likely to behave as WPOs. However, in most cases, materials have projected potentials strong enough to cause the phases of the Bragg-diffracted beams to change rapidly with the depth in the specimen. Therefore, most specimens do not behave as a WPO beyond a few nanometers in the thickness direction. Due to the ability of revealing the local atomic structures, HRTEM image is the most useful and appropriate technique for identifying structural defects in perovskite nanocrystals. As an example, the microstructural defects such as anti-phase boundaries (APBs) and edge dislocations in hydrothermal BaTiO3 nanoparticles were revealed by HRTEM images at atomic-level [181]. Figure 12a and Figure 12b show the HRTEM images of two individual nanoparticles, respectively. The (100) and (111) lattice fringes are clearly observed in Figures 12a and Figure 12b, respectively. The corresponding Fourier filtered images and the FFT patterns (see insets) of the selected areas marked by boxes in the two HRTEM images, are shown in Figure 12c and Figure 12d, respectively. The Fourier filtered images clearly demonstrates that how the APBs are formed during the particle growth. There are two crystalline regions marked by I and II in the ellipses with fine white line in Figure 12c and Figure 12d. The two crystalline regions in Figure 12c are deviated from each other by a relative displacement of 1/2d100 (d100: the inter-planar distance between two adjacent (100) planes), whereas in Figure 12d, the relative displacement is 1/2d111 (d111: the inter-planar distance between two adjacent (111) planes). During the solid-phase crystallization, the crystalline growth fronts of part I and part II intersect each other. Then, the intersection of growth fronts accommodates the deviation as APBs. The observed APBs near the edge of a BaTiO3 nanoparticle, were formed by the intersection of two crystalline parts with displacement deviation from each other by 1/2d100 or 1/2d111, as revealed by the HRTEM images. Similar conditions were also observed in a crystalline SrBi2Ta2O9 grain [189].

Figure 12. (a) and (b) HRTEM images of two isolated BaTiO3 nanoparticles. (c) and (d) The corresponding Fourier filtered images and the FFT patterns (see insets). Reproduced with permission from [181], Zhu, X. H.; Zhu, J. M.; Zhou, S. H.; Liu, Z. G.; Ming, N. B. Hydrothermal synthesis of nanocrystalline BaTiO3 particles and structural characterization by high-resolution transmission. J Cryst Growth. 2008, 310, 434-441. Copyright © 2008 Elsevier B.V. All rights reserved.


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Figure 13. High-resolution transmission electron microscopy images of the surface structures at the edges of BaTiO3 nanoparticles viewed from the [001] direction. (a) Both a terrace-ledge-kink (TLK) surface structure and small nucleated and triangular islands with two to three atomic layer thickness are observed. (b) and (c) TLK surface structure with both terraces and ledges lying on the {100} planes; only a small amount of ledges lie on the (110) plane. The inset in (b) is a Fourier-filtered image of the corresponding position, which clearly demonstrates two perpendicular sets of (100) and (010) planes. Reproduced with permission from [183], Zhu, X. H.; Wang, J. Y.; Zhang, Z. H.; J. M.; Zhou, S. H.; Liu, Z. G.; Ming, N. B. Atomic-scale characterization of barium titanate powders formed by the hydrothermal process. J Am Ceram Soc. 2008, 91, 1002-1008. Copyright © 2008, American Ceramic Society.

Figure 14. A surface profile HRTEM image of part of a hydrothermal BaTiO3 particle with size of 80 nm. Reproduced with permission from [184], Zhu, X. H.; Zhu, J. M.; Zhou, S. H.; Liu, Z. G.; Ming, N. B.; Hesse, D. BaTiO3 nanocrystals: hydrothermal synthesis and structural characterization. J Cryst Growth. 2005, 283, 553-562. Copyright © 2005 Elsevier B.V. All rights reserved.

A terrace-ledge-kink (TLK) surface structure was also frequently observed at the edges of the hydrothermal BaTiO3 nanoparticles with rough surface morphology, and in most cases the terrace and ledge lie on the {100} planes [183] . The observed TLK surface structure is shown in Figure 13, which can be well interpreted by the theory of periodic bond chains. Small nucleated and triangular BaTiO3 islands with 3 ~ 4 atomic layer highness, and their outside

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surfaces faceted as (100) and (010) planes, were also observed in hydrothermal BaTiO3 nanoparticles, as indicated by arrows in Figure 14 [184]. The rarely-seen {110} surface in the BaTiO3 nanoparticles were found to be reconstructed so that the surface was composed of corners bound by {100} mini-faces like the triangular small islands. Internal defect textures, such as nanoscale multiple (111) twining and complicated (111) intergrowth defects, were also observed in the BaTiO3 nanopowders synthesized by sol-gel and stearic acid-gel (SAG) methods. They were identified as hexagonal-type BaTiO3 structure [190,191]. Complex arrangements of defects lying on the (111) planes were observed in the SAG-derived BaTiO3 nanocrystal with particle size of 10 nm. The density of the small defects was estimated to be on the order of 1027/m3 in the SAG-derived BaTiO3 nanopowders. These high density of defects could result in the cubic phase structure of SAG-derived BaTiO3 powders even with grain size large up to 3.50µm [190].

3.1.5. Spherical-corrected HRTEM/STEM Traditionally, spherical aberration (Cs) of magnetic lenses limits the resolutions of HRTEM and STEM images. In recent years spherical aberration correctors (e.g., hexapole type Cs-correctors proposed by Rose [192]) have been developed to reduce substantially the effective value of Cs of the objective lens. Its main idea is that multipole lenses such as quadrupoles, sexupoles, and octupoles have lens aberrations with different phase shift errors, W(∆k), compared with the short solenoids used for the objective lens. Combining these different functional forms makes it possible to make the overall W(∆k) a more constant function. This is accomplished by placing a set of different lenses along the optical path and tuning their currents. Recently researchers from IBM Thomas J. Watson Research Center and Nion R&D, have taken a step towards reaching the ultimate resolution (sub-angstrom resolution) in an electron microscope [193] by implementing a computer-controlled aberration correction system in a STEM that is less sensitive to the remaining chromatic aberrations. The Cs-corrected STEM mode can provide a sub-angstrom probe with a highbrightness, which offers the prospect of element-selective imaging of single atomic columns using the energy filter. Combined with monochromated HR-EELS, one can further investigate chemistry and electron structure-related properties (e.g., valence state, bonding structure) by single atomic column to column. The Cs-corrected HRTEM mode offers a tunable spherical aberration coefficient from negative to positive values. Properly combining a negative Cs with a positive defocus, at no cost to point resolution, an HRTEM image with bright-contrast of atoms on dark background can be obtained, which can be directly interpreted without image simulation, and light elements such as oxygen atoms and even their vacancies can also be imaged [194-197]. For example, by using the Cs-corrected imaging technique, Jia et al. [197] first performed the atomic-sacle investigations of the electric dipoles near (charged and uncharged) 180° domain walls in thin epitaxial PbZr0.2Ti0.8O3 film sandwiched between two SrTiO3 layers. Figure 15 is an atomic-scale image of the electric dipoles formed by the relative displacements of the Zr/Ti cation columns and the O anion columns in PbZr0.2Ti0.8O3 film, viewed from the [110] direction and recorded under negative spherical-aberration imaging conditions. The local tetragonality c/a and spontaneous polarization inside the domains and across the domain wall were calculated. For the first time, a large difference in atomic details between charged and uncharged domain walls was


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reported. Such breakthrough would improve our ability to see and thoroughly explore the properties of perovskite nanopowders.We can foresee that the new Cs-corrected HRTEM and STEM will benefit perovskite nanopowder materials research in the new era.

Figure 15. Atomic-scale imaging of the electric dipoles formed by the relative displacements of the Zr/Ti cation columns and the O anion columns in the approximately 10-nm-thick PbZr0.2Ti0.8O3 layer −

sandwiched between two SrTiO3 layers. The image was viewed along the [ 110 ] direction and recorded under negative spherical-aberration imaging conditions. The atom columns appear bright on a dark background. The horizontal arrows denote the horizontal interfaces between the PbZr0.2Ti0.8O3 film and the top and the bottom SrTiO3 film layers. The dotted line traces the 180° domain wall between the domain I and domain II. The arrows denoted by ‘PS’ show the directions of the polarization in the 180° domains. Two insets show higher magnifications of the dipoles formed by the displacements of ions in the unit cells. Yellow symbols denote PbO atom columns seen end-on, red symbols for Zr/Ti columns, and blue symbols for oxygen. Reproduced with permission from [197], Jia, C. L.; Mi, S. B.; Urban, K.; Vrejoiu, I.; Alexe, M.; Hesse, D. Atomic-scale study of electric dipoles near charged and uncharged domain walls in ferroelectric films. Nat Mater. 2008, 7, 57-61. Copyright © 2008, Nature Publishing Group.

3.2. Spectroscopic Characterization 3.2.1. X-ray diffraction (XRD) In X-ray diffraction (XRD), a collimated beam of X-rays (wavelength λ: 0.5 -2 Å) is incident on a specimen and is diffracted by the crystalline phases in the specimen according to Bragg’s law (2dsinθ = λ, where d is the spacing between atomic planes in the crystalline

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phase). The intensity of the diffracted X-rays is measured as a function of the diffraction angle 2θ and the specimen’s orientation. As a primary characterization tool for obtaining the critical features such as crystal structure, crystallite size, and strain, X-ray diffraction patterns have been widely used for perovskite nanopowder research [198-200]. Except for single crystalline nanopowders, the randomly oriented crystals in nanopowders cause broadening of the diffraction peaks due to the absence of the total constructive and destructive interferences of X-rays in a finite-sized lattice [201]. This effect becomes more pronounced when the crystallite sizes are in the order of a few nanometers. In addition, inhomogeneous lattice strains and structural faults also lead to the broadening of peaks in X-ray diffraction patterns. Currently, the widely used and simplest method for estimating the crystallite size is based on the Scherrer equation, which can be expressed as [202] 3 d=

Kλ β cos θ

(2)

where d is the crystallite size, λ is the wavelength of the used X-ray, β is the FWHM (full width at half maximum of a diffraction peak), θ is the diffraction angle, and K is a constant close to unity. The major assumptions are that the sample is free of residual strain and has a narrow grain size distribution. As one example, the crystallite size of PbTiO3 nanopowders obtained from combined polymerisation and pyrolysis route, can be determined based on the obvious broadening of the Bragg reflections or, more precisely, from the line shape of diffractions peaks [203]. However, when the contributions due to strains are taken into consideration, the analysis becomes much more complicated.

3.2.2. Extended X-ray absorption fine structure spectroscopy (EXAFS) Extended X-ray absorption fine-structure (EXAFS) measurements, which are oscillations occurring on the high-energy side of an X-ray absorption edge, can be used to identify interatomic distances in materials [204]. An EXAFS experiment involves the irradiation of a sample with a tunable source of monochromatic X-rays from a synchrotron radiation facility. As the X-ray energy is scanned from just below to well above the binding energy of a coreshell electron (e.g., K or L) of a selected element, the X-ray photoabsorption process is monitored. When the energy of the incident X-rays is equal to the electron binding energy, Xray absorption occurs and a steeply rising absorption edge is observed. For energies greater than the binding energy, oscillations of the absorption with incident X-ray energy (i.e., EXAFS) are observed. EXAFS data are characteristic of the structural distribution of atoms in the immediate vicinity of the X-ray absorbing element. The present consensus is that the accuracy of interatomic distance determined by EXAFS is between 0.01 Å and 0.001 Å , depending on the circumstances [205,206]. However, in certain types of study, such as the differential magnetostriction measurements reported by Pettifer et al. [207], direct comparison should be capable of much higher precision. This is especially true when synchrotron radiation is used from a third-generation source, where the fluxes available can be 1013 photons s-1 eV-1. These should be able to produce relative statistical errors in the absorption spectrum of ~10-6 under optimal conditions in a few hours. With such a statistical accuracy, interatomic strains of the order of femtometres and below should be detectable with EXAFS


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[207]. The frequency of the EXAFS is related to the interatomic distance between the absorbing and neighboring atoms. The amplitude of the EXAFS is related to the number, type, and order of neighboring atoms. To probe the local structure in perovskite nanopowders, EXAFS has been proven to be an effective technique. Recently, Frenkel et al. [208] carried out EXAFS study on BaTiO3 particles with different average grain sizes of about 20 nm, 35 nm, and 70 nm prepared by solution-gelation method. The normalized XAFS spectra were obtained by subtracting the background µ 0(k) from the measured absorption coefficient µ(k). The k2 - weighted χ(k) of the samples with different particle size at room temperature, as well as the data with 10µ m particles measured at different temperatures were shown in Figure 16. It was observed that besides a very small difference in amplitude, room temperature EXAFS χ(k) of all the samples with different particle size did not show significant changes. The changes in the amplitude in Figure 16a were larger than the statistical noise between two measurements of the same sample. However, these changes were smaller than those occurred during heating the sample with a 10µm particle size from 80K to 590K (Figure 16b). In this temperature range, the average structure of BaTiO3 exhibits several phase transitions from rhombohedral to orthorhombic to tetragonal to cubic at elevated temperatures. The fact that the changes between the EXAFS signals measured for different particle sizes were smaller than the changes occurred in the sample with a macroscopic particle size at different temperatures, indicated that the local structure of the samples with all the particle sizes measured was essentially the same within the experimental resolution. The magnitude of the Ti atom offcenter displacement did not depend on the particle size. Petkov et al. [209] have recently demonstrated the use of the pair distribution function (PDF) to understand local structure distortions and polar behavior in BaxSr1-xTiO3 (x = 1, 0.5, 0) nanocrystals. They found that locally, refining over the first 15Å, the tetragonal model was the best fit to the experimental PDF; however, over longer distances (15-28 Å), the cubic model was the best fit. Their conclusion was that 5 nm BaTiO3 was on average cubic, but that tetragonal-type distortions in the Ti-O distances are present within the cubic structure.

3.2.3. Energy dispersive X-ray spectroscopy (EDS) Energy dispersive X-ray spectroscopy (EDS) is an analytical technique used for the elemental analysis or chemical characterization of a sample. As a type of spectroscopy, it relies on the investigation of a sample through interactions between electromagnetic radiation and matter, analyzing X-rays emitted by the matter in response to being hit with charged particles. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing X-rays that are characteristic of an element's atomic structure to be identified uniquely from each other. An EDS system setup consists four primary components: the beam source; the solid state X-ray detector usually made from lithium-drifted silicon, Si (Li); the pulse processor; and the analyzer. A detector is used to convert X-ray energy into voltage signals; this information is sent to a pulse processor, which measures the signals and passes them onto an analyzer for data display and analysis.

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Figure 16. k2 - weighted χ(k) for the samples with (a) different particle sizes at 300 K and (b) 10 µm particle size at different temperatures. Reproduced with permission from [208], Frenkel, A. I.; Frey, M. H.; Payne, D. A. XAFS analysis of particle size effect on local structure in BaTiO3. J Synchrotron Rad.1999, 6, 515-517. Copyright © 1999, International Union of Crystallography.

Accuracy of EDS spectrum are affected by many variants. Windows in front of the Si(Li) detector can absorb low-energy X-rays (a.k.a. EDS detectors cannot detect presence of oxygen, carbon, boron, etc.). Differing the over-voltage of the EDS will result in different peak sizes - Raising over-voltage on the SEM will shift the spectrum to the larger energies making higher-energy peaks larger while making lower energy peaks smaller. Also many elements will have overlapping peaks (e.g., Ti Kβ and V Kα, Mn Kβ and Fe Kα). The accuracy of the spectrum can also be affected by the nature of the sample. X-rays can be generated by any atom in the sample that is sufficiently excited by the incoming beam. These X-rays are emitted in any direction, and so may not all escape the sample. The likelihood of an X-ray escaping the specimen, and thus being available to be detected and measured, depends on the energy of the X-ray and the amount and density of material it has to pass through. This can result in reduced accuracy in inhomogeneous and rough samples. The main use of EDS is to accurately determine the composition of the sample under investigation. While the TEM images provide real-time pictures of the size and morphology of the nanopowders, the supplementary EDS analysis provides exact composition of the sample. Several examples [210-213] have demonstrated the use of EDS in analysis of oxide perovskite nanopowders, particularly in the determination of the composition of substituted or nanoparticles composite materials.

3.2.4. Electron energy loss spectroscopy (EELS) Electron energy-loss spectroscopy (EELS) based on electron microscopy is a powerful method for investigating electronic structures of nanometer-scale materials. In which a nearly monochromatic beam of electrons is directed through an ultra-thin specimen, usually in a TEM or STEM electron microscope. As the electron beam propagates through the specimen, it experiences both elastic and inelastic scattering with the constituent atoms, which modifies its energy distribution. Each atomic species in the analyzed volume causes a characteristic change in the energy of the incident beam; the changes are analyzed by means of an electron spectrometer and counted by a suitable detector system. The intensity of the measured signal


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can be used to determine quantitatively the local specimen concentration, the electronic and chemical structure, and the nearest neighbor atomic spacings. The signal in EELS is in the form of ionization edges on a large background. Determinations of chemical concentrations involve a back-ground subtraction to isolate the intensity of the absorption edge (EELS). These isolated intensities are then compared for the different elements in the spectrum, and in many cases are converted into absolute concentrations by use of appropriate constants of proportionality. The accuracy of quantification depends on the reliability of these constants, so significant effort has been devoted to understanding them. More than this, fine details in the EELS spectra can often provide insight into electronic structures. For the state-of-the-art field-emission TEM, an essential feature is its ability to form a nanometer-sized electron probe, which allows for the acquisition of EDS and EELS spectra. This feature for simultaneous structure, composition and bonding information at each location is a powerful combination for understanding the structure and chemistry of perovskite nanopowders. Suzuki et al. [214] have used high energy-resolution EELS (energy resolution ~ 0.2eV) to investigate the electronic structures of BaTiO3 nanocrystals synthesized by chemical vapor deposition. The valence excitation spectra of BaTiO3 nanocrystals with average diameters of 34 nm and 6 nm in an energy range from 2 to 40 eV were shown in Figure 17, demonstrating that the onset energies of spectral intensities were 3.2 eV for 34 nm BaTiO3 nanocrystals and 3.5 eV for 6 nm BaTiO3 nanocrystals. This indicated an increase in the bandgap energy of BaTiO3 with a decrease in crystal sizes. Those onset energies obtained from 90 nm specimen areas showed an excellent agreement with those estimated by previously reported optical measurements. Volume plasmon peaks were observed at 26.5eV in 34 nm BaTiO3 nanocrystals and 25eV for 6 nm BaTiO3 nanocrystals. Dielectric functions of the BaTiO3 nanocrystals derived from loss functions by Kramers-Kronig analysis shows not only an increase in the O 2p → Ti 3d (t2g) transition energy, but also a decrease in the peak energy which corresponds to the O 2p →Ti 3d (eg) transition. These results show that high energyresolution EELS based on TEM, which provides information of electronic structures from specified small specimen areas, is powerful tool not only for the characterization of new materials but also for the basic research of electronic structures of quantum objects.

3.2.5. X-ray photoelectron spectroscopy (XPS) In X-ray photoelectron spectroscopy (XPS) monoenergetic soft X-rays bombard a sample material, causing electrons to be ejected. Identification of the elements present in the sample can be made directly from the kinetic energies of these ejected photoelectrons. On a finer scale it is also possible to identify the chemical state of the elements present from small variations in the determined kinetic energies. The relative concentrations of elements can be determined from the measured photoelectron intensities. For a solid, XPS probes 2-20 atomic layers deep, depending on the material, the energy of the photoelectron concerned, and the angle (with respect to the surface) of the measurement. XPS is one of the important characterization tools for surface chemical analysis, which has the ability to probe the surface to a few atomic layers deep (0.5 - 5 nm) and obtain a semi-quantitative elemental analysis of surfaces without standards. The valence states of the elements that constitute the surfaces can also be deduced from the XPS characterization.

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Figure 17. Valence electron excitation spectra of BaTiO3 nanocrystals (BTNCs) with average particle sizes of 6 and 34 nm in an energy range from 2 to 40 eV ( the local spectra in the energy loss from 1 to 7 eV, seen in inset (c)). The insets (a) and (b) are TEM images of BTNCs with average diameters of 34 nm and 6 nm, respectively.Vertical lines in inset (c) indicated the onsets of spectral intensities. Reproduced with permission from [214], Suzuki, K.; Terauchi, M.; Uemichi, Y.; Kijima, K. High energy-resolution electron energy-loss spectroscopy study of electronic structures of barium titanate nanocrystals. Jpn J Appl Phys. 2005, 44, 7593-7597. Copyright © 2005, the Japan Society of Applied Physics.

Since the surface chemistry of perovskite nanopowders plays an important role in affecting their densification behavior during the sintering process, a knowledge of their surface chemistry is highly necessary [215,216]. In the case of BaTiO3 nanopowders, the surface barium carbonate (BaCO3) formed by reaction of BaTiO3 with atmospheric and/or solvated carbon dioxide (CO2), or stemmed as a residual from the powder synthesis process, can have a significant effect on the sintering behavior of the nanopowders [216,217]. Bu using XPS numerous investigations have been carried out to analyze the surface chemistry and surface phases of a variety of commercial and laboratory-synthesized BaTiO3 nanopowders [216,218-223]. All these investigations revealed a small contribution to the barium photoemission signal which was invariably attributed to the presence of surface BaCO3; however, only in two instances was this barium contribution accompanied by carbon and oxygen signals which could be positively matched with the carbonate [221,223]. None of these studies considered the presence of water adsorbed at the surface of the powders. Bearing these factors in mind, Wegmann et al. [224] characterized the commercial submicrometer BaTiO3 powders by using XPS. Their results showed that the powder particle surfaces were hydrated with physisorbed molecular water and chemisorbed hydroxyl groups. The hydrated surface proved to be stable under the ultra-high vacuum conditions experienced during the XPS experiment and could only be removed by Ar-ion sputtering at 400°C. This behavior suggested that the powder surfaces were thoroughly hydrated under processing conditions experienced during many ceramic forming procedures and that consequently processing aids (e.g., solvents, dispersants and binders) interacted with the adsorbed water layer(s) rather than directly with the ceramic surface. XPS depth profiling by Ar-ion sputtering revealed the powder surfaces to be Ti-rich, confirming the presence of a phase, or phases, to stoichiometrically balance the barium carbonate.


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3.2.6. Infrared (IR) spectroscopy Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels (vibrational modes). These resonant frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, by the associated vibronic coupling. In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. The infrared spectrum of a sample is collected by passing a beam of infrared light through the sample. Examination of the transmitted light reveals how much energy was absorbed at each wavelength. This can be done with a monochromatic beam, which changes in wavelength over time, or by using a Fourier transform instrument to measure all wavelengths at once. From this, a transmittance or absorbance spectrum can be produced, showing at which IR wavelengths the sample absorbs. Analysis of these absorption characteristics reveals details about the molecular structure of the sample. This technique works almost exclusively on samples with covalent bonds. Simple spectra are obtained from samples with few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra. This technique has been used for the characterization of structural defects in perovskite nanopowders. For example, in the hydrothermal BaTiO3 nanopowders, IR spectroscopy recorded from such powders at different temperatures indicated the presence of internal OH- groups [118,119], which was evidenced by an OH- stretching band at 3200-3600 cm-1. The concentration of internal OH- decreased with increasing temperature at which the hydrothermal synthesis was performed. The defect chemistry explaining the incorporation of chemisorbed water into the crystal structure of hydrothermal BaTiO3, was studied by IR spectroscopy using deuterated powders in order to better distinguish chemically bound water from the adsorbed moisture. It has been found that the OH- groups form hydroxide ions on the regular oxygen sites of the perovskite, which are compensated by the formation of acceptor-type metal vacancies such as barium and titanium vacancies [122,123,129]. Both lattice hydroxyl group and lattice vacancies affect the magnitude of the tetragonal distortion of BaTiO3 powders. Although the high concentration of lattice defects in hydrothermal BaTiO3 powders, like lattice hydroxyl group and barium vacancies, do not completely prevent the cubic-to-tetragonal phase transformation as supposed by the “lattice defects” theory, the strains introduced by the lattice defects make the unit cell distortion (c/a ratio) become much smaller than that in the standard BaTiO3. That is the reason why no splitting of diffraction peaks was observed in the XRD patterns of the hydrothermal BaTiO3 powders even though they are in tetragonal phase.

3.2.7. Raman spectroscopy Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. In molecules, a molecular polarizability change, or amount of deformation of the

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electron cloud, with respect to the vibrational coordinate is required for the molecule to exhibit the Raman effect. The amount of the polarizability change will determine the Raman scattering intensity, whereas the Raman shift is equal to the vibrational level that is involved. Typically, a sample is illuminated with a laser beam. Light from the illuminated spot is collected with a lens and sent through a monochromator. Wavelengths close to the laser line, due to elastic Rayleigh scattering, are filtered out while the rest of the collected light is dispersed onto a detector. Raman spectroscopy is the measurement, as a function of wavenumber, of the inelastic light scattering that results from the excitation of vibrations in molecular and crystalline materials. Raman Spectroscopy is sensitive to molecular and crystal structure, which has been extensively used for structure, composition, and phase characterization of materials. They can provide various characteristic vibrational frequencies, such as those associated with lattice defects or surfaces, and derive crucial data on the electronic band structures in solids. As particle sizes in the nanoscale range, new phenomena appear due to the effects of phonon confinement. These include mode wavenumber shifts and line broadening, the appearance of zone boundary phonons, surface phonons, and extremely low wavenumber bands due to excitation of bulk resonances of the particle. One must distinguish between purely nanocrystal effects due to phonon confinement and the increased structural disorder that often accompanies extreme reduction in particle size. Application of Raman spectroscopy as a characterization tool requires careful distinction between modifications of Raman line shape due to disorder in the bulk crystal from modifications due to particle size [225]. Recently, a comprehensively review about Raman spectroscopy of nanomaterials (how Raman spectra related to disorder, particle size and mechanical properties) contributed by Gouadec and Colomban is also available [226]. Up to date, Raman spectroscopy has been widely used to probe the local structure and phase transition of perovskite nanopowders (e.g., BaTiO3 [48,78,182,198,227-229], PbTiO3 [226,230-232], SrTiO3 [233-236]) due to its high sensitivity to the lattice vibrations and dynamics, providing important information about the structure, composition, strain, defects, and phase transitions. Based on the crystallography, there are four triply degenerate optical modes of vibration (3F1u+1F2u) in cubic BaTiO3, which is the Pm3m group. When BaTiO3 transforms into the 4mm group tetragonal phase, each of the F1u modes which are infrared active and Raman inactive splits into modes of symmetry A1+E, while F2u modes which are infrared and Raman inactive split into modes of symmetry B1+E. A1 (non-degenerate), B1 (non-degenerate), and E (doubly degenerate) modes are Raman active. There is further splitting of the vibrational modes because of long-range electrostatic forces associated with lattice ionicity. As a consequence, A1 splits into A1(TO1), A1(TO2), A1(TO3), A1(LO1), A1(LO2), and A1(LO3) and E mode splits into E(TO1), E(TO2), E(TO3), E(LO1), E(LO2), and E(LO3), respectively. The Raman scattering process includes first-order scattering, which obeys the above-stated selective rules and involves one phonon Raman scattering. The second or higher order scattering, which does not obey the above-mentioned selective rules and involves multiphonon Raman scattering, forms combination bands or overtone bands. The wave number of combination bands varies continuously, and that of overtone bands is the multiplicity of the first-order bands. When BaTiO3 undergoes from tetragonal structure into cubic structure, the first-order band should decrease gradually. With respect to the secondorder bands, the case is different and the line frequency cannot be accurately estimated [237]. According to the selection rules, all of the optic modes of BaTiO3 with perfect cubic symmetry should be Raman inactive while the same for the polar tetragonal and orthorhombic


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polymorphous forms should be Raman active [228]. Therefore, in the Raman spectra of the bulk BaTiO3, sharp bands are around 175 cm-1 [A1(TO), E(LO)] and 305 cm-1 [B1, E(TO + LO)] and broad bands around 265 cm-1 [A1(TO)], 520 cm-1 [A1, E(TO)], and 720 cm-1 [A1, E(LO)] are the characteristic peaks of tetragonal phase BaTiO3. Figure 18 shows the crystal structure of as-prepared and heat-treated hydrothermal BaTiO3 powders examined by Raman spectroscopy[238]. The Raman peak at 305 cm−1, the characteristic peak of the tetragonal phase in BaTiO3, is present in all powders, indicating the presence of the tetragonal phase. There is no significant change in the Raman spectra except that there is an intensity jump at 1000oC. This might be related to the migration of lattice defects, because it is believed that 1000oC is the lowest temperature for the multitude migration of lattice defects such as barium and oxygen vacancies [56,129]. Yashima et al. [239] also investigated the size effect on the crystal structure of BaTiO3 nanoparticle with sizes of 40 - 430 nm by Raman spectra along with neutron and high-resolution synchrotron X-ray powder diffraction techniques. They found that the axial ratio c/a of tetragonal BaTiO3 decreased with a decrease in particle size from 430 to 140 nm. Barium titanate particles with a size of 40 nm consisted of (a) tetragonal crystals (83 wt %) with a large cell volume and an axial ratio of unity c/a=1.000(5) and of (b) a hexagonal phase (P63mmc, 17 wt %) with a large unit-cell volume. The ferroelectric phase transitions in nanocrystalline PZT with grain sizes of 60 and 40 nm were investigated by Raman spectra [240]. The results show that the E(TO1) phonon mode of PbTiO3 (“soft” mode) displays a decrease in frequency and an increase in line width with increasing Zr concentration. A discontinuous behavior in the phonon energy for the soft mode occurs at a morphotropic phase boundary (MPB) of x ≈ 0.4 and 0.2 for grain sizes of 60 and 40 nm, respectively, and it can be attributed to a phase transition from ferroelectric tetragonal to ferroelectric rhombohedral phase. The nonzero soft mode frequency near the MPB results from a level repulsion between an additional phonon mode at ≈10 cm-1 and the soft mode. Raman enhanced behavior was observed for the lowest phonon mode with Zr contents in the range of 0.3 to 0.6. The dependence of Raman phonon modes for PbZr0.3Ti0.7O3 upon grain size indicated a grain-size-induced phase transition at about 13 nm [240].

3.2.8. Secondary ion mass spectroscopy (SIMS) Secondary ion mass spectrometry (SIMS) is a technique used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. In the field of surface analysis, SIMS is usually classified into static SIMS and dynamic SIMS [241]. Static SIMS is the process involved in surface atomic monolayer analysis, usually with a pulsed ion beam and a time of flight mass spectrometer, while dynamic SIMS is the process involved in bulk analysis, closely related to the sputtering process, using a DC primary ion beam and a magnetic sector or quadrupole mass spectrometer. In the static SIMS, the bombarded particles with an energy of typical 1-10 keV, are either ions or neutrals. As a result of the interaction of these primary particles with the sample, species are ejected that have become ionized. These ejected species, known as secondary ions, are the analytical signal in SIMS. The use of a low dose of incident particles (typically less than 5 x 1012 atoms/cm2) in static SIMS, is critical to maintain the chemical integrity of the sample surface during analysis. A mass spectrometer sorts the secondary ions with respect to their specific charge-to-mass ratio, thereby providing

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a mass spectrum composed of fragment ions of the various functional groups or compounds on the sample surface. The interpretation of these characteristic fragmentation patterns results in a chemical analysis of the outer few monolayers. The ability to obtain surface chemical information is the key feature distinguishing static SIMS from dynamic SIMS, which profiles rapidly into the sample, destroying the chemical integrity of the sample. In the dynamic SIMS, a solid specimen placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, which are sufficiently energetic to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions (called secondary ions). The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering time (depth). The SIMS depth profile can provide elemental concentrations in the sample as a function the depth, has a great potential in characterizing the concentration profiles of self-organized or consolidated nanostructures. Particularly, SIMS is a powerful analytical technique for determining the elements present in materials, and especially on surfaces, with trace level sensitivity on the order of parts-perbillion (ppb) and sub-nanometer depth resolution and a high spatial resolution (lateral resolution of 5 nm) [242]. A classical SIMS device consists of (a) primary ion gun generating the primary ion beam, (b) a primary ion column, accelerating and focusing the beam onto the sample (and in some devices an opportunity to separate the primary ion species by wien filter or to pulse the beam), (c) high vacuum sample chamber holding the sample and the secondary ion extraction lens, (d) mass analyser separating the ions according to their mass to charge ratio, and (e) ion detection unit [241-245]. SIMS requires a high vacuum with pressures below 10-4 Pa (roughly 10-6 mbar or torr). This is needed to ensure that secondary ions do not collide with background gases on their way to the detector (mean free path), and it also prevents surface contamination by adsorption of background gas particles during measurement. As an example, SIMS was used to investigate the cation diffusion in perovskite oxides based on lanthanum gallates (LaGaO3) doped with strontium on the A site and magnesium on the B site, which exhibit high oxygen-ion conductivity and represent a promising alternative to YSZ (yttria-doped zirconia) as the electrolyte in solid oxide fuel cells [243]. Although cation diffusion in simple perovskites is known to be very slow, there are several important processes that are determined by the slowest moving species, such as sintering or creep. If the cations exhibit different diffusivities, kinetic demixing of the electrolyte [246] can be an additional origin of long term degradation. It is therefore important to obtain data for cation diffusion in La1-xSrxGa1-yMgyO3-(x+y)/2 (LSGM). By means of SIMS cation impurity diffusion of Y, Fe and Cr and cation tracer-diffusion of La, Sr and Mg in La0.1Sr0.1Ga0.1Mg0.1O2.9 were investigated [247]. By combining different modes of SIMS analysis- depth profiling, line scanning and imaging - it was possible to measure diffusion coefficients from about 10-18 cm2 s-1 to 10-6 cm2 s–1and to determine surface exchange coefficients as well. In addition, highresolution SIMS makes it possible to investigate diffusion through space charge layers at surfaces and to distinguish between bulk and grain boundary diffusion in polycrystalline materials, and determine the location of interfaces in solids and their chemical compositional depth profiling [242-244].


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Figure 18. Raman spectra of hydrothermal BaTiO3 powders: (a) as-prepared, and after heat treatment at (b) 400 oC, (c) 600oC, (d) 800 oC, (e) 1000 oC, and (f) 1200oC for one hour. Reproduced with permission from [238], Wei, X. Z.; Li, Y. L. The influence of lattice defects on the crystal structure of hydrothermal BaTiO3 powders. J Ceram Proc Res. 2005, 6, 250-254. Copyright © 2005, Journal of Ceramic Processing Research.

3.2.9. Optical absorption-emission spectroscopy Spectacular changes in optical characteristics of nanocrystalline oxides when compared to those of bulk counterparts have triggered tremendous interest among scientists to understand the basic mechanisms responsible for the fascinating optical absorption-emission, which also helps to examine their potential use in variety of optical applications [248-250]. Optical absorption and emission arises as a result of electronic transitions in solids upon exposure to excitation energies in the range of ~ 102 to 103 kJ/mol that cover the near infrared through visible to ultraviolet. There are various types of optical transition in solids. One type of transition is the promotion of an electron from a localized orbital to a higher energy localized orbital of the same atom (d-d, f-f transitions) or from a localized orbital in one atom to a higher energy localized orbital on an adjacent atom (charge-transfer spectra). Another type of transition can be the promotion of electrons from a localized orbital in one atom to the delocalized energy band (conduction band) of the solid as seen in the case of photoconductive materials. The transition energies associated with these processes differ, thereby requiring different excitation frequencies for obtaining their absorption and emission spectra [251]. Understanding the quantum confinement effect on optical absorption and emission characteristics has been the major objective of optical characterization oxides nanopowders. For example, the visible transitions of Nd3+ ions were found in the neodymium ion-doped perovskite hosts powders (Ca, Ba, Sr)TiO3, which were prepared by wet chemical method. The excitation at the band edge of the host at 335 nm generated an intense red emission at 613 nm with a quantum efficiency of 10.8% for Nd0.005(Ca0.97Ba0.01Sr0.015)TiO3 relative to the commercially used red phosphor [252]. With an increase in the dopant concentration of neodymium ions, the excitation and emission intensities both increased up to 0.5 mol%

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neodymium substitution. There was no shift in the excitation and emission spectra of the samples. The symmetry around the emitting center was more distorted with greater substitution by neodymium ions, resulting in violation of the parity selection rule and thereby producing the red emission. The preliminary photoluminescence properties of polycrystalline powder PbTiO3 were reported by Folkers and Blasse [253]. A broad-band emission in the visible spectral region (also called “green” luminescence) are reported to be universal for ABO3 perovskite-type oxides [254-258]. However, the nature of this wide-band visible luminescence is not well understood, although some mechanisms, such as donor–acceptor recombination [259], transitions in MeO6 complexes [260,261], recombination of electron and hole polarons [262], and charge transfer vibronic exciton [263-266] have been proposed. Recently, Eglitis et al. [267] have performed the quantum chemical calculations and theoretical simulation of the green emission for a PbTiO3 perovskite-type oxides by using the intermediate neglect of differential overlap method combined with the large unit cell periodic defect model. Their results showed that the universal “green” luminescence in the PbTiO3 crystals can be ascribed to the radiative recombination of the self-trapped electrons and holes forming the charge transfer vibronic exciton, rather than due to the electron transitions in a MeO6 complex or donor–acceptor recombination, as intuitively suggested earlier. They also demonstrate that well-parameterized semi-empirical quantum chemical methods could be successfully used for the study of optical properties of modern advanced materials such as oxide perovskites.

3.2.10. Mössbauer spectroscopy Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. In its most common form of Mössbauer absorption spectroscopy, a solid sample is exposed to a beam of gamma radiation, and a detector measures the intensity of the beam that is transmitted through the sample, which will change depending on how many gamma rays are absorbed by the sample. The atoms in the source emitting the gamma rays are the same as the atoms in the sample absorbing them. As can be explained through the Mössbauer effect, a significant fraction of the gamma rays emitted by the atoms in the source do not lose any energy due to recoil and thus have almost the right energy to be absorbed by the target atoms. The gamma-ray energy is varied by accelerating the gamma-ray source through a range of velocities with a linear motor. The relative motion between the source and sample results in an energy shift due to the Doppler effect. In the resulting spectra, gamma-ray intensity is plotted as a function of the source velocity. At velocities corresponding to the resonant energy levels of the sample, some of the gamma-rays are absorbed, resulting in a drop in the measured intensity and a corresponding dip in the spectrum. The number, positions, and intensities of the dips (also called peaks) provide information about the chemical environment of the absorbing nuclei and can be used to characterize the sample. In order to occur Mössbauer absorption of gamma-rays, it is required that the gamma-ray must have the appropriate energy for the nuclear transitions of the atoms being probed, which is almost always achieved by having the same atoms of the same isotope in both the source and the target. Also, the gamma-ray energy should be relatively low, otherwise the system will have a low recoil-free fraction (see Mössbauer effect) resulting in a poor signal-to-noise ratio. Only a handful of elemental isotopes exist for which these criteria are met, so Mössbauer spectroscopy can only be applied to a relatively small group of atoms including:


41

57

Fe, 129I, 119Sn, and 121Sb. Of these, 57Fe is by far the most common element studied using the technique. The Mössbauer spectrum has been extensively used for the characterization of Fe-containing oxides [268]. In particular, the technique provides crucial information about the local order and associated magnetic properties in nanocrystalline ferrites [269-274]. For example, the Mössbauer spectroscopic characterization of Eu-doped or Mn-doped BiFeO3 powders revealed the addition of Eu or Mn in BiFeO3 induced significant modifications in the Mössbauer hyperfine parameters and the magnetic properties of the powders, whereas no significant microstructural or structural changes were observed [270,271].

4. PROPERTIES OF PEROVSKITE NANOPOWDERS 4.1. Ferroelectric and Dielectric Properties In recent years studies on ferroelectric and dielectric properties of perovskite nanopwders have been become a major field of research due to their potential applications in memory devices. It has been well documented that a particle size effect on ferroelectricity exists in many perovskite nanopowders of displacive system such as BaTiO3, PbTiO3, and PbZrO3 [275-283]. That means below the critical particle size, the ferroelectricity disappears. An important motivation for study of the size effects on ferroelectric is to determine the ultimate level to which a device based on such systems can be miniaturized. As a typical example, the size effects on the phase transition of perovskite PbTiO3 nanopowders have been investigated [277-282]. The results show that nanocrystalline PbTiO3 particles with size of 20 - 80 nm exhibit a reduction in tetragonal distortion, ultimately transforming to a cubic phase for smaller particles ( critical size ~ 7 nm at room temperature) [279]. The dielectric constant around the Curie temperature ( Tc) can be expressed by a semi-empirical relation [284,285] 1

ε

−

1

ε max

=

1 (T − Tc )γ A

(3 )

where ε is the dielectric constant, εmax is the maximum dielectric constant value at the transition temperature Tc, A is a constant, and γ is the order of phase transition within the range of 1< γ