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PERSPECTIVE

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Complex perovskite oxide nanocrystals: lowtemperature synthesis and crystal structure Federico A. Rabuffetti and Richard L. Brutchey* This Perspective reviews our recent efforts towards the low-temperature synthesis of complex perovskite oxide ABO3 (A = Sr, Ba; B = Ti, Zr) nanocrystals using the vapor diffusion sol–gel method and the determination of their room-temperature crystal structure. From a synthetic standpoint, emphasis is placed on demonstrating the ability of the vapor diffusion sol–gel approach to yield compositionally complex nanocrystals at low temperatures and atmospheric pressure without the need for postsynthetic heat treatment to achieve a crystalline and phase-pure oxide product. The ability to successfully achieve this is illustrated using Ba1−xSrxTi1−yZryO3 (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) and Eu3+-doped Ba(Ti,Zr)O3 nanocrystals as examples. From the standpoint of the structural analysis, emphasis is placed on highlighting how multiple and complementary spectroscopic techniques that probe atomic correlations in short (≤1 nm), intermediate

Received 8th May 2014, Accepted 18th August 2014 DOI: 10.1039/c4dt01376j www.rsc.org/dalton

(∼1–3 nm), and long (≥3 nm) length scales can be employed to gain insight into the atomic structure of the resulting nanocrystals. Examples that clearly illustrate this strategy of structural characterization are the investigation of the size- and composition-dependence of the structure of polar nanoregions in sub10 nm BaTiO3 and sub-20 nm Ba1−xSrxTiO3 and BaTi1−yZryO3 nanocrystals, and the investigation of the distribution of rare earth dopants in sub-15 nm Eu3+:BaTiO3 nanocrystals.

Department of Chemistry, University of Southern California, Los Angeles, 90089, USA. E-mail: [email protected]

Dr Federico A. Rabuffetti received his B.Sc. (2001) and M.Sc. (2004) in chemistry from Universidad de la Republica (Uruguay). Then, he obtained his Ph.D. in chemistry from Northwestern University (2010) with Prof. Kenneth R. Poeppelmeier and Prof. Peter C. Stair. He is currently a post-doctoral fellow in the group of Prof. Richard L. Brutchey in the department of chemistry at the University of Federico A. Rabuffetti Southern California. He recently accepted a tenure-track Assistant Professor appointment in the Department of Chemistry at Wayne State University. His research interests focus on understanding compositional control of structure–property relationships in functional oxide nanocrystals.

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Prof. Richard L. Brutchey received his B.S. (2000) in chemistry from the University of California, Irvine and his Ph.D. (2005) in chemistry from the University of California, Berkeley. After a post-doctoral fellowship at the University of California, Santa Barbara, he began his independent career in 2007 at the University of Southern California where he is currently an Associate Professor Richard L. Brutchey of Chemistry. The Brutchey group focuses on the design of rational synthetic routes to colloidal inorganic nanocrystals for use in solar energy conversion and energy storage applications. Prof. Brutchey was named a Cottrell Scholar by the Research Corporation for Science Advancement (2010), and was recognized by Chemical Communications as an Emerging Investigator and Dalton Transactions as a New Talent honoree (2012). His work has been highlighted by National Geographic magazine and National Public Radio.

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1.

Introduction

Alkaline-earth perovskite oxide nanocrystals with the formula ABO3 (A = Sr, Ba; B = Ti, Zr) span a wide range of physical phenomena that make them functional materials in the areas of energy storage (as dielectric spacers in capacitors),1 energy conversion (as electrolytes in proton-conducting solid oxide fuel cells),2 heterogeneous catalysis (as supports for noble metal clusters),3 and imaging technologies (as phosphor hosts in field emission displays and as fluorescent biomarkers).4,5 The unit cell of the archetypical ABO3 perovskite structure is shown in Fig. 1 and consists of an assembly of AO12 cuboctahedra and BO6 octahedra. The compositional dependence of the crystal structure is at the origin of the versatility of this family of materials. The synthesis of alkaline-earth perovskite oxide micro- and nanocrystals has been accomplished using a variety of methods that includes: solid state reaction,6–9 molten salt,10,11 sol–gel,12,13 single-step combustion,14,15 flame-spray pyrolysis,16 thermal decomposition of a single-source precursor,17 sol–precipitation,18–20 reverse micelles,21 polymeric precursor,9 and solvothermal synthesis.22–27 These synthetic approaches invariably rely on physical (heat, pressure) and/or chemical (salt, complexing agent, surfactant, mineralizer) agents to achieve a crystalline and phase-pure perovskite product. In the past few years, our group has developed a vapor diffusion sol– gel (VDSG) approach for the synthesis of compositionally complex perovskite oxide nanocrystals under ultrabenign conditions.28–33 From a chemical standpoint, the VDSG method is similar to conventional sol–gel as both approaches are based on the hydrolysis and polycondensation of alkoxide precursors. However, the VDSG method differs from conventional sol–gel in two key aspects: (1) bimetallic rather than monometallic alkoxides are employed as precursors, and (2) water vapor rather than liquid water is employed as the hydrolytic agent (i.e., gas–liquid rather than liquid–liquid hydrolysis). The combination of both features allows the crystallization of perovskite oxide nanocrystals to be achieved at low temperature (25–115 °C), atmospheric pressure, and near-neutral pH, without the need for any postsynthetic heat treatment. The motivation behind the synthetic effort has been to gain compositional control over the nanocrystal structure, particu-

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larly in the context of the potential these perovskite oxide nanocrystals have as high k nanodielectrics.34,35 From this perspective, achieving an accurate description of the nanocrystals’ structure has been a major focus of the research effort in our group.36–39 Special emphasis has been placed on understanding the dependence of the structure of polar nanoregions on the nanocrystal’s size and chemical composition, as these are at the origin of the functionality of the nanocrystals as high k nanodielectrics. Our approach to this fundamental problem was to interrogate the nanocrystal’s atomic arrangement using a series of complementary spectroscopic techniques that probe atomic correlations in short (3 nm) length scales. Among these techniques are X-ray absorption spectroscopy, both near-edge (XANES) and extended fine structure (EXAFS), steady-state spectrofluorometry, Raman spectroscopy, pair distribution function (PDF) analysis of X-ray total scattering, and Rietveld analysis of X-ray diffraction (XRD). This Perspective summarizes our research efforts targeting the low-temperature synthesis of compositionally complex perovskite oxide nanocrystals and the determination of their room-temperature crystal structure. The following discussion is divided in two sections. The first section focuses on the lowtemperature synthesis of perovskite oxide nanocrystals via the VDSG method. The synthesis of two families of nanocrystals is described: (1) Ba1−xSrxTi1−yZryO3 (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) belonging to the BaTiO3–SrTiO3–BaZrO3–SrZrO3 composition space, and (2) rare earth-doped perovskite oxides, specifically Eu3+: BaTiO3 and Eu3+:BaZrO3. This includes a description of the precursors and the apparatus employed, and of the VDSG procedures followed for the preparation of nanocrystals of varying composition. The nanocrystals’ nucleation and growth processes, and their compositional homogeneity and morphology are also described. The second section presents a structural investigation of three families of nanocrystals synthesized via VDSG: (1) sub-10 nm BaTiO3, (2) sub-20 nm, isovalently substituted Ba1−xSrxTiO3 and BaTi1−yZryO3, and (3) sub-15 nm aliovalently substituted Eu3+:BaTiO3. In each case, the fundamental challenges driving the structural investigation are described, and the value of employing the toolkit of spectroscopic and scattering techniques mentioned above to address these challenges is highlighted.

2. Vapor diffusion sol–gel synthesis 2.1.

Fig. 1

The ABO3 perovskite structure.

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Ba1−xSrxTi1−yZryO3 nanocrystals

2.1.1. Precursors. Precursors employed in the VDSG synthesis of Ba1−xSrxTi1−yZryO3 (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) nanocrystals are purchased from Gelest Inc. and consist of double metal alkoxides of formula BaTi(OR)6, SrTi(OR)6, BaZr(OR)6, and SrZr(OR)6, where R = CH2CHCH3OCH3. Molarities range from 0.33 to 0.70 M in a 1 : 3 v/v mixture of n-butanol and 2-methoxypropanol. 2.1.2. Apparatus. The apparatus employed for VDSG is shown in Fig. 2.36 It consists of a 100 mL three-neck flask

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Fig. 2 Apparatus employed for VDSG synthesis. The apparatus has two configurations: (1) precursor injection (A open, C closed, and a septum replaces B), and (2) vapor diffusion (A closed, and B and C open). The arrows depict the flow of nitrogen gas. Adapted from ref. 36 with permission from The Royal Society of Chemistry.

featuring three gas adapters with glass stopcocks (A, B, and C). Adapter A connects the reaction flask to a nitrogen gas line, adapter B connects the reaction flask to a glass bubbler filled with 50 mL of a 0.75 M HCl solution, and adapter C acts as a vent. The inlet of the bubbler is connected to a needle-valve rotameter, which is in turn connected to a nitrogen gas line. The system has two configurations: (1) precursor injection, in which A is open, C is closed, and a septum replaces B; and (2) vapor diffusion, in which A is closed, and B and C are open, thereby allowing nitrogen gas saturated in water vapor to flow over the precursor solution. The vapor flow rate can be controlled via the needle-valve rotameter. The reaction flask can be easily immersed in a temperature-controlled heating bath if temperatures above ambient are required at any point of the synthetic procedure.31 2.1.3. Procedure. Fig. 3a depicts the composition space defined by BaTiO3, SrTiO3, BaZrO3, and SrZrO3 that can be mapped using the VDSG method.33 Hereafter, Ba1−xSrxTi1−yZryO3 nanocrystals are labeled with a four-digit number abcd that indicates their stoichiometry Baa/(a+b)Srb/(a+b)Tic/(c+d )Zrd/(c+d )O3 (e.g., 1151 corresponds to Ba1/2Sr1/2Ti5/6Zr1/6O3). Three different procedures are employed depending on the stoichiometry of the desired product; these are schematically depicted in Fig. 3b and described in detail below. (a) One-step procedure. A one-step procedure consisting of water vapor diffusion at room temperature and atmospheric pressure is employed for the synthesis of BaTiO3, SrTiO3, BaZrO3, and of three-cation Ba1−xSrxTiO3 and BaTi1−yZryO3 nanocrystals. Fig. 4a–g illustrate the VDSG synthesis of BaTiO3 nanocrystals following this procedure. The system is initially set for precursor injection and 2.0 mL of BaTi(OR)6 precursor are transferred via syringe to the reaction flask. The exposed liquid area is ∼3 cm2. Simultaneously, nitrogen gas is bubbled through the 0.75 M HCl solution for 15–30 min. Then, the bubbler is connected to the reaction flask and the system configuration is switched to vapor diffusion (Fig. 4a). An increase in the viscosity of the solution is observed upon continuous flow of water vapor. After ∼8 h, this results in the formation of a fully rigid, monolithic, and crack-free gel (Fig. 4b). The gela-

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Fig. 3 (a) Composition space that can be mapped using the VDSG method. Three-cation phases are located on the edges of the diamond, while four-cation phases are located on composition lines (1), (2), and (3). ○, ●, and ■ symbols correspond to the different synthetic procedures schematically described in (b). (a) is reprinted with permission from Chem. Mater., 2012, 24, 3114–3116. Copyright 2012 American Chemical Society.

tion time can be tuned by adjusting the nitrogen flow rate; gelation times ranging from ∼4 to 8 h are typically employed. A few hours after the formation of the monolithic gel, cracks start to appear along with a clear supernatant (Fig. 4c). Both the cracks and the amount of supernatant grow with time, yielding several small pieces of gel (Fig. 4d and e). The water vapor flow is stopped after 72 h and pieces of the gel are collected (Fig. 4f ). These are washed three times with 5 mL of absolute ethanol, sonicated for 10 min, and centrifuged at 6000 rpm for 20 min. The resulting product is dried under vacuum at room temperature, yielding a fine, off-white powder consisting of BaTiO3 nanocrystals which can be easily redispersed in ethanol (Fig. 4g). In a typical synthesis, yields ranging from 70 to 90% are obtained; this corresponds to masses ranging from 120 to 280 mg, depending on chemical composition. An identical procedure is employed for the synthesis of Ba1−xSrxTiO3 and BaTi1−yZryO3 nanocrystals, the only difference being that a mixture of two bimetallic alkoxides in the desired stoichiometric ratio is employed as the precursor solution. The crystallization of ABO3, Ba1−xSrxTiO3, and BaTi1−yZryO3 nanocrystals from the precursor solution upon continuous flow of water vapor proceeds via hydrolysis and polycondensation according to: ABðORÞ6ðlÞ þ 3H2 OðgÞ ! ABO3ðsÞ þ 6ROHðlÞ

ð1Þ

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ively (see Fig. 3a). This two-step procedure consists of water vapor diffusion at room temperature and atmospheric pressure followed by aging of the resulting gel at 60–80 °C under static nitrogen atmosphere. The crystallization of SrTi1−yZryO3, Ba1−xSrxTi1−xZrxO3, and Ba1−xSrxTi1/3+xZr2/3−xO3 nanocrystals from the precursor solution proceeds according to: ð1  yÞSrTiðORÞ6ðlÞ þ ySrZrðORÞ6ðlÞ þ 3H2 OðgÞ Published on 18 August 2014. Downloaded by University of Utah on 28/09/2014 12:28:06.

! SrTi1y Zry O3ðsÞ þ 6ROHðlÞ ð1  xÞBaZrðORÞ6ðlÞ þ xSrTiðORÞ6ðlÞ þ 3H2 OðgÞ ! Ba1x Srx Tix Zr1x O3ðsÞ þ 6ROHðlÞ

ð4Þ

ð5Þ

  1 2 BaTiðORÞ6ðlÞ þ  x BaZrðORÞ6ðlÞ þ xSrTiðORÞ6ðlÞ þ 3H2 OðgÞ 3 3 ! Ba1x Srx Ti1=3þx Zr2=3x O3ðsÞ þ 6ROHðlÞ

ð6Þ (c) Four-step procedure. A four-step procedure is employed for the synthesis of three-cation Ba1−xSrxZrO3 nanocrystals and of four-cation Ba2/3−xSr1/3+xTixZr1−xO3 nanocrystals lying on composition line (3) (see Fig. 3a). This procedure consists of the following successive steps: (1) water vapor diffusion at room temperature and atmospheric pressure; (2) thermal treatment of the partially hydrolyzed liquid precursor at 115 °C under static nitrogen atmosphere; (3) another step of water vapor diffusion at room temperature and atmospheric pressure, during which gelation occurs; and finally (4) aging of the resulting gel at 115 °C under static nitrogen atmosphere. The crystallization of Ba1−xSrxZrO3 and Ba2/3−xSr1/3+xTixZr1−xO3 nanocrystals proceeds according to: ð1  xÞBaZrðORÞ6ðlÞ þ xSrZrðORÞ6ðlÞ þ 3H2 OðgÞ ! Ba1x Srx ZrO3ðsÞ þ 6ROHðlÞ

ð7Þ

  1 2 SrZrðORÞ6ðlÞ þ  x BaZrðORÞ6ðlÞ þ xSrTiðORÞ6ðlÞ þ 3H2 OðgÞ 3 3 ! Ba2=3x Sr1=3þx Tix Zr1x O3ðsÞ þ 6ROHðlÞ ð8Þ Fig. 4 Synthesis of BaTiO3 nanocrystals via the VDSG method using a 0.5 M BaTi(OR)6 bimetallic alkoxide and water vapor flow as the reagents.

ð1  xÞBaTiðORÞ6ðlÞ þ xSrTiðORÞ6ðlÞ þ 3H2 OðgÞ ! Ba1x Srx TiO3ðsÞ þ 6ROHðlÞ

ð1  yÞBaTiðORÞ6ðlÞ þ yBaZrðORÞ6ðlÞ þ 3H2 OðgÞ ! BaTi1y Zry O3ðsÞ þ 6ROHðlÞ

ð2Þ

ð3Þ

(b) Two-step procedure. A two-step procedure is employed for the synthesis of three-cation SrTi1−yZryO3 nanocrystals, and of four-cation Ba1−xSrxTi1−xZrxO3 and Ba1−xSrxTi1/3+xZr2/3−xO3 nanocrystals lying on composition lines (1) and (2), respect-

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2.1.4. Nucleation and growth. Following the hydrolysis and polycondensation of the precursor solution via ex situ powder XRD and transmission electron microscopy (TEM) provides valuable insight into the nanocrystal nucleation and growth processes.37 Fig. 5a shows a series of XRD patterns of the gel resulting from the hydrolysis and polycondensation of the BaTi(OR)6 bimetallic alkoxide at different times. Diffraction maxima develop upon continuous flow of water vapor, indicating that the gel undergoes an amorphous-to-crystalline transition. According to XRD, the onset of crystallinity appears after 24 h of water vapor diffusion, and no significant changes in the intensity of the diffraction maxima are observed after 36 h, indicating the crystallization is complete at that point. As mentioned earlier, the time required to achieve quantitative crystallization can be tuned by simply changing the nitrogen flow rate through the bubbler; Fig. 5b shows that crystalliza-

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Perspective

Fig. 5 XRD patterns of the gel resulting from the hydrolysis and polycondensation of the BaTi(OR)6 bimetallic alkoxide at different times. Water vapor diffusion times are indicated on the right axis. (a) and (b) correspond to syntheses where the water vapor flow rate is adjusted to obtain gelation times of ∼8 and 4 h, respectively. A quantitative proof of the completion of the amorphous-to-crystalline phase transition after 72 h of water vapor diffusion is given in ref. 37. (a) is reprinted with permission from J. Am. Chem. Soc., 2012, 134, 9475–9487. Copyright 2012 American Chemical Society.

tion can be completed in times as short as 24 h. As-prepared BaTiO3 nanocrystals appear phase pure, with no observable traces of crystalline BaCO3, TiO2, or pyrochlore-type phases. All the diffraction maxima can be indexed to the cubic perovskite ˉm (PDF no. 79-2263) and lattice phase with space group Pm3 33 constant a = 4.0222(4) Å. The nanocrystals readily chemisorb CO2 upon exposure to air, leading to the formation of surface BaCO3. It should be emphasized that both nanocrystal nucleation and growth occur at room temperature and atmospheric pressure, and that no postsynthetic heat treatment is required to achieve a crystalline and phase pure oxide product. Upon moving toward phases lying closer to the BaZrO3–SrZrO3 composition line, however, thermal treatments at low temperatures (i.e., 60–115 °C) are required to prevent phase segregation and to drive the amorphous-to-crystalline transition to completion; hence, the increasing complexity of the synthetic procedure.33 The resulting nanocrystals typically exhibit a weight loss of ∼5–10% upon heating to 600 °C, arising from adsorbed water and chemisorbed alkoxide ligands.37 Fig. 6a–d show a series of TEM images of the gel resulting from the hydrolysis and polycondensation of the BaTi(OR)6 bimetallic alkoxide at different times; the evolution of the nanocrystal size distribution is depicted in Fig. 6e. Samples collected after 12 to 27 h of water vapor diffusion consist of amorphous pieces of the gel within which crystalline nuclei with diameters ranging from 3 to 6 nm are embedded (Fig. 6a and b). The spacing of the lattice fringes observed in these nuclei equals 0.29 nm, corresponding to the {110} crystal planes of the BaTiO3 phase. Continuous flow of water vapor

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Fig. 6 TEM images of the gel resulting from the hydrolysis and polycondensation of the BaTi(OR)6 bimetallic alkoxide after (a) 12, (b) 27, (c) 30, and (d) 72 h of water vapor diffusion. Crystalline nuclei in (a) and (b) are denoted with white arrows and dotted lines. Nanocrystal size distribution histograms and high-resolution images showing lattice fringes corresponding to the {110} crystal planes are given in the insets. (e) Evolution of the nanocrystal size distribution with the water vapor diffusion time; the total number of particles counted is indicated. Reprinted with permission from J. Am. Chem. Soc., 2012, 134, 9475–9487. Copyright 2012 American Chemical Society.

leads to an increase of the volume fraction of the nanocrystalline material relative to the amorphous matrix. Well-defined and discrete BaTiO3 nanocrystals are observed after 30 h of water vapor diffusion (Fig. 6c). In this sample, however, a major fraction of dispersed nanocrystals coexists with a minor

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fraction that appear embedded within fiber-like pieces of the amorphous gel, indicating the amorphous-to-crystalline transition is not complete at this point. This minor fraction disappears upon further diffusion of water vapor to yield discrete, quasispherical nanocrystals exhibiting an average diameter of ∼9 nm (Fig. 6d). 2.1.5. Crystal structure and compositional homogeneity. XRD patterns of Ba1−xSrxTi1−yZryO3 nanocrystals are shown in Fig. 7. All the diffraction maxima can be indexed to the centroˉm (PDF no. 79-2263, symmetric, cubic space group Pm3 74-1296, and 70-3667 for BaTiO3, SrTiO3, and BaZrO3, respectively) with lattice constant values ranging from 3.9080(2) Å for SrTiO3 to 4.1997(5) Å for BaZrO3.33 Diffraction maxima appearing in the pattern of SrZrO3 can be indexed to the orthorhombic space group Pbnm (PDF no. 44-0161) with lattice constants a = 5.7880(10) Å, b = 5.8163(14) Å, and c = 8.1801(14) Å.33 No traces of crystalline secondary phases are observed in any of the patterns, demonstrating that the nanocrystals are phase pure. Their stoichiometry and compositional homogeneity is confirmed by Rietveld analysis.33 Linear fits to the cubic lattice constant a extracted from Rietveld analysis show that substitution of Sr for Ba in the A site and of Zr for Ti in the B site, lead to a monotonic decrease or increase in the unit cell volume, respectively (Fig. 8). The validity of Vegard’s law confirms the formation of true solid solutions with an average homogeneous distribution of the cations in the A and B sites very near to the nominal stoichiometry. More importantly, it demonstrates the ability of the VDSG method to produce compositionally complex Ba1−xSrxTi1−yZryO3 nanocrystals of welldefined stoichiometry by simply changing the ratios of the bimetallic alkoxides in the precursor solution. In particular, it allows the preparation of nanocrystals of zirconium-rich phases that other solution-based approaches have failed to produce.18 2.1.6. Morphology. Representative TEM images of three and four-cation nanocrystals are shown in Fig. 9. These exhibit quasispherical shape with an average diameter ranging from ∼8.44 ± 1.23 to 26.0 ± 2.42 nm, depending on chemical composition.33 High-resolution TEM imaging of individual nanocrystals shows the presence of well-defined lattice fringes corresponding to the {110} crystal planes of the perovskite phase, demonstrating their apparent single crystalline nature. Overall, these results show that the VDSG method is suitable for the preparation of sub-30 nm perovskite oxide nanocrystals of well-defined stoichiometry. 2.2.

Eu3+:BaTiO3 and Eu3+:BaZrO3 nanocrystals

2.2.1. Precursors and procedure. The VDSG method can be extended to the synthesis of rare earth-doped ABO3 nanocrystals. Herein, the preparation of Eu3+-doped BaTiO3 and BaZrO3 nanocrystals is described.32,39 We note, however, that other rare earth ions (e.g., La3+ and Dy3+) can also be incorporated into the perovskite lattice following the same procedure described below.40 The apparatus and procedure are identical to those employed in the synthesis of undoped BaTiO3 nanocrystals. However, in this case the precursor solution consists

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Fig. 7 XRD patterns of Ba1−xSrxTi1−yZryO3 nanocrystals. Substitution levels x and y are indicated on the right axis. Adapted with permission from Chem. Mater., 2012, 24, 3114–3116. Copyright 2012 American Chemical Society.

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Fig. 8 Evolution of the cubic lattice constant as a function of the substitution level in Ba1−xSrxTi1−yZryO3 nanocrystals; the corresponding standard deviations are given in the ESI of ref. 33. Linear fits to the data are shown as dotted lines; values of the residual R2 are indicated. Reproduced with permission from Chem. Mater., 2012, 24, 3114–3116. Copyright 2012 American Chemical Society.

of an Eu(acac)3·nH2O (acac = acetylacetonate) dissolved in the BaTi(OR)6 or BaZr(OR)6 bimetallic alkoxide solution. The crystallization of mEu:BaBO3 (B = Ti, Zr; m = 0–5 mol%) nanocrystals from the precursor solution proceeds according to: mEu3þ þ BaBðORÞ6ðlÞ þ 3H2 OðgÞ ! mEu3þ : BaBO3ðsÞ þ 6ROHðlÞ ð9Þ 2.2.2. Crystal structure. XRD patterns of Eu3+:BaTiO3 and Eu3+:BaZrO3 nanocrystals are shown in Fig. 10. All the diffraction maxima can be indexed to the cubic perovskite phase with ˉm and lattice constant a ∼ 4.01 (BaTiO3) and space group Pm3 4.19 Å (BaZrO3). No segregation of secondary phases such as Eu2O3 or Eu2Zr2O7 is observed; these are often observed in microcrystals and ceramics prepared via high-temperature solid state reaction.41–43 The presence of Eu3+ in the nanocrystalline products is confirmed by inductively coupled plasma atomic emission spectroscopy (ICP–AES), XRD, X-ray total scattering, and steady-state spectrofluorometry. As an example, ICP–AES analysis of mEu3+:BaZrO3 powders yields Eu3+ concentrations of 1.1, 1.8, 2.8, 4.2, and 4.8 mol% for m values of 1, 2, 3, 4, and 5 mol%, respectively, demonstrating an excellent agreement between the actual and nominal doping levels.32 This is particularly remarkable considering the low doping efficiencies typically reported for doped semiconducting nanocrystals.44 2.2.3. Morphology. Representative TEM images of 3 mol% Eu3+:BaTiO3 and Eu3+:BaZrO3 nanocrystals are shown in Fig. 11. These consist of quasispherical particles with a mean diameter of 12.3 ± 2.3 nm (Eu3+:BaTiO3) and 16.4 ± 3.4 nm (Eu3+:BaZrO3). The size distribution of Eu3+:BaTiO3 nanocrystals appears to be nearly independent of the Eu3+ content.39 In contrast, an increase in the mean diameter and polydispersity

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Fig. 9 TEM images of selected three- and four-cation perovskite nanocrystals. High-resolution images showing lattice fringes corresponding to the {110} crystal planes are given in the insets. Corresponding nanocrystal size distribution histograms are shown to the right. Adapted with permission from Chem. Mater., 2012, 24, 3114–3116. Copyright 2012 American Chemical Society.

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Perspective

Fig. 10 XRD patterns of Eu3+-doped BaTiO3 and BaZrO3 nanocrystals. Eu3+ nominal concentrations (mol%) are indicated. Bottom panel is adapted from ref. 32 with permission of Wiley-VCH.

Fig. 11 TEM images of 3 mol% Eu3+:BaTiO3 and Eu3+:BaZrO3. A highresolution image showing lattice fringes corresponding to the {110} crystal planes of BaZrO3 is given in the inset. Corresponding nanocrystal size distribution histograms are shown to the right. Top panel is adapted from ref. 39 with permission from The Royal Society of Chemistry. Bottom panel is adapted from ref. 32 with permission of Wiley-VCH.

of Eu3+:BaZrO3 nanocrystals is observed upon increasing the concentration of Eu3+.32 According to high-resolution TEM imaging, the incorporation of Eu3+ does not alter the apparent single crystalline nature of the nanocrystals.

3. Room temperature crystal structure 3.1.

BaTiO3 nanocrystals

Bulk BaTiO3 has four polymorphs whose crystal structure can be described in terms of their space group symmetry as rhombo-

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hedral (R3m), orthorhombic (Amm2), tetragonal (P4mm), ˉm).45–52 The corresponding phase transitions and cubic (Pm3 occur at −90 °C (rhombohedral-to-orthorhombic), 5 °C (orthorhombic-to-tetragonal), and 120 °C (tetragonal-to-cubic). The rhombohedral, orthorhombic, and tetragonal polymorphs lack an inversion center due to the displacement of the titanium atom from the center of the unit cell. These noncentrosymmetric polymorphs are also ferroelectric; that is, they exhibit a spontaneous electric polarization that can be reversed through the application of an external electric field. In contrast, the centrosymmetric cubic polymorph is paraelectric and behaves as a linear dielectric. From a technological standpoint, the tetragonal polymorph is the most relevant one, as it is a roomtemperature ferroelectric with maximum dielectric permittivity. In this polymorph, the titanium atoms are displaced toward the polar c axis. The temperature dependence of the crystal structure and dielectric behavior is well-established for bulk BaTiO3.49,50 However, the structural phase transitions become increasingly diffuse upon transitioning from the micro- and sub-microscale (>100 nm) to the nanoscale (

Complex perovskite oxide nanocrystals: low-temperature synthesis and crystal structure.

This Perspective reviews our recent efforts towards the low-temperature synthesis of complex perovskite oxide ABO3 (A = Sr, Ba; B = Ti, Zr) nanocrysta...
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