Quasi-Monodisperse Transition-Metal-Doped BaTiO3 (M = Cr, Mn, Fe

Subscriber access provided by University of South Dakota

Article

Quasi-Monodisperse Transition Metal-Doped BaTiO3 (M=Cr, Mn, Fe, Co) Colloidal Nanocrystals with Multiferroic Properties Tommaso Costanzo, John McCracken, Gabriel Caruntu, and Aurelian Rotaru ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01036 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Quasi-Monodisperse Transition Metal-Doped BaTiO3 (M=Cr, Mn, Fe, Co) Colloidal Nanocrystals with Multiferroic Properties

Tommaso Costanzo1,2, John McCracken3, Aurelian Rotaru4 and Gabriel Caruntu*1,2 E-mail: [email protected]

1

Department of Chemistry and Biochemistry, Central Michigan University, 1200 S. Franklin St., Mount Pleasant, MI 48859, USA 2

Science of Advanced Materials Program, Central Michigan University, 1200 S. Franklin St., Mount Pleasant, MI 48859, USA 3

Department of Chemistry, Michigan State University, 578 S. Shaw Ln., East Lansing, MI 48824, USA

4

Department of Electrical Engineering and Computer Science and MANSID Research Center, “Stefan Cel Mare” University, 13, Universitatii St. Suceava, 720229, Romania

1 ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The recent demand of multifunctional materials and devices for advanced applications in energy conversion and data storage resulted into a revival of multiferroics; that is materials characterized by the coexistence of ferromagnetism and ferroelectricity. Despite intense efforts made in the past decade, single phase room temperature multiferroics are yet to be discovered/fabricated. Nanostructured ferroic materials could potentially exhibit multiferroism since a high fraction of their atoms/ions are superficial, thereby altering significantly the properties of the bulk phase. Alternately, a magnetic order can be induced into ferroelectric materials upon aliovalent doping with magnetic ions. Here, we report on the synthesis of aggregate-free single-phase transition metal doped BaTiO3 quasi-monodisperse cuboidal nanocrystals which exhibit multiferroic properties at room temperature and can be suitable for applications in data storage. The proposed synthetic route allows the inclusion of a high concentration of magnetic ions such as Mn+ (M=Cr, Mn, Fe, Co) up to a nominal concentration of 4% without the formation of any secondary phase. The size of the nanocrystals was controlled in a wide range from ~15 nm up to ~70 nm by varying the reaction time from 48 to 144 hours. The presence of unpaired electrons and their magnetic ordering have been probed by electron paramagnetic resonance spectroscopy (EPR), and vibrating sample magnetometer (VSM). Likewise, an acentric structure, associated with the existence of a dielectric polarization, was observed by lattice dynamics analysis and piezoresponse force microscopy (PFM). These results show that high quality titanium-containing perovskite nanocrystals which display multiferroic properties at room temperature can be fabricated via soft solution-based synthetic routes and the properties of these materials can be modulated by changing the size of the nanocrystals and the concentration of the dopant thereby opening the door to the design and study of single phase multiferroic materials.

Keywords: Multiferroic properties, colloidal nanocrystals, transition metal-doped titaniumcontaining perovskites, BaTiO3, piezoresponse force microscopy, magnetic ordering


Page 2 of 33

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION

BaTiO3 is an archetypal ferroelectric perovskite that exhibits polar ordering at room temperature and has been used in many technological applications including data storage, energy conversion, electronics, sensing, catalysis, and nonlinear optics.1–4 In conventional nonvolatile memory devices, binary information is encoded as “0”s and “1”s by using Boolean algebra, which exploits the two extreme orientations (upwards and downwards) of the magnetic or electrical dipoles. With the increasing need for portable storage devices with a high areal density, low-power consumption and fast access times the design of four-state logic memory elements has garnered an increasing interest.5,6 Such multiple-state memories use a special class of materials known as multiferroics7,8 in which the magnetization M and the dielectric polarization P coexist in the same phase.9 This unique ability to present at least two ferroic order parameters makes multiferroics prime candidates not only for data storage, but also in drug delivery, magnetic field sensing, spintronics and microwave technology.10–13 Previous studies focused exclusively on BiFeO3-based perovskite materials, which exhibit multiferroism at room temperature, but the cross coupling between the two order parameters (magnetization and dielectric polarization) is generally weak. Alternately, other groups suggested that ferroelectric perovskites can be amenable to a magnetic order and exhibit multiferroic properties at room temperature upon the aliovalent doping with transition metal ions.14–19 Son and coworkers reported recently on the design of high-density four-state memories by using arrays of 2% (atom) Mn-doped BaTiO3 nanorods obtained by dip pen nanolithography. They found out that these nanorods exhibit a robust ferroelectric and magnetic response at room temperature with a high electrical fatigue resistance and phase shift of the magnetic signal (up to 103 cycles), which shows that transition metal-doped perovskites are strong candidates for implementation into four-level multiferroic memories.5 Similarly, Gao and coworkers found out that the microstructure and strain of YMnO3 films can be tailored by tuning the deposition frequency and, therefore BaTiO3:YMnO3 vertically aligned heteroepitaxial nanocomposite films exhibit multiferroism at room temperature, being suitable for application in data storage, and sensing.20 However, the mechanism through which a magnetic order appears in titanium-based perovskite oxides upon aliovalent doping is complex and there is no full consensus about the factors 3 ACS Paragon Plus Environment


influencing this process. Using the Heisenberg model, Apostolova and coworkers suggested that a magnetic order is induced in Fe-doped BaTiO3 by ferromagnetic super exchange interactions involving the Fe3+ ions occupying different crystallographic sites, as well as an antiferromagnetic exchange coupling between Fe3+ and Fe4+ ions, respectively.14 Rajamani et al. found that the room temperature magnetism observed experimentally in Fe-doped BaTi1-xFexO3 (0.15≤x≤0.5) films obtained by pulse laser deposition is associated with a ferromagnetic interaction between small clusters of Fe ions via a delocalization super exchange mechanism.21 Such small ferromagnetic pockets form upon substitution of the Ti4+ ions with Fe3+/Fe4+ ions in an otherwise paramagnetic or antiferromagnetic spin arrangement. Lin et al.22 and Venkata Ramana et al.23 explained the ferromagnetic behavior of transition metal-doped BaTiO3 structures in terms of the bound magnetic polaron model proposed by Coey and coworkers.24 However, the magnetization values in these materials is much higher than that resulting from the long-range interaction of the magnetic ions and it has been suggested that oxygen vacancies may play an important role in inducing magnetism in these oxides. Shuai and coworkers found recently that the doping of a polycrystalline BaTiO3 thin film with Mn2+ ions to a concentration up to 5% does not necessarily induce a ferromagnetic order in this ferroelectric material.25 Specifically, room temperature magnetic measurements indicated the presence of ferromagnetism only in Mn-doped BaTiO3 films with a high concentration of oxygen vacancies, which indicates that oxygen vacancies play a vital role in inducing the room temperature magnetism in transition metal-doped BaTiO3 materials.25 Although a magnetic order was not observed in their bulk counterparts, from density functional theory (DFT) calculations Mangalam et.al. suggested that nanoparticles of nonmagnetic ferroelectric oxides, such as BaTiO3, can exhibit ferromagnetic properties.10,26 The origin of this unexpected ferromagnetism was ascribed to the presence oxygen vacancies on the surface of the nanoparticles. Thus, two electrons will populate the electronic states with Ti-character at the bottom of the conduction band of the nanostructured oxide thereby inducing a spin polarization and a ferromagnetic coupling. A similar behavior was recently observed by Wang and coworkers in PbTiO3 nanoparticles synthesized by a sol-gel process.27 Such a behavior was confirmed experimentally by the enhancement of the magnetism in these nanoparticles when their size decreased and was explained by a larger surface/volume ratio leading to a larger fraction of oxygen vacancies stabilized on the surface of the nanoparticles.27 Therefore, it is clear that the surface of the 4 ACS Paragon Plus Environment

Page 4 of 33

Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ferroelectric nanoparticles play a crucial role in inducing ferromagnetism in oxide nanostructures. Although many studies have been dedicated to the influence of the surface oxygen vacancies on the magnetic order of ferroelectric oxides, little attention has been paid to the control of the morphology of the nanoparticles in terms of their size and shape. This is presumably due to the lack of a unified synthetic method for titanium-containing perovskite nanoparticles that allows the control of the nanocrystal morphology. Therefore, the wide variety of synthetic approaches used to prepare ferroelectric oxide nanoparticles makes difficult a comparative analysis of the proprieties and their size-dependence of nanoparticles obtained by different synthetic routes. In order to overcome these difficulties, we extended a simple and highly versatile solvothermal approach proposed recently by our group for the synthesis of titanium-based perovskite colloidal nanocrystals28 to doped perovskites, which can exhibit both ferroelectricity and magnetism at room temperature. To demonstrate the versatility of this synthetic approach, we prepared metal doped BaTi1-xMxO3 cuboidal nanocrystals using four different transition metal ions (M=Cr, Mn, Fe, and Co), whose concentration in the perovskite host matrix was varied from 2% to 8%, respectively. We show that regardless of the chemical nature of the transition metal ion, the morphology of the nanocrystals is preserved and can be controlled by varying reactions parameters, such as the polarity of the solvent and the reaction time. The synthesized nanocrystals are quasi-monodisperse and their size can vary from ~10 nm to ~70 nm by simply changing the reaction time, similar to the synthesis of pristine BaTiO3 nanocrystals.28 The proposed synthetic approach can open the door to systematic investigations on the influence of the morphology (size and shape) and surface composition on the multiferroic properties of perovskite nanocrystals. 2. EXPERIMENTAL 2.1. Synthesis Analytical grade reagents, including metal nitrates Ba(NO3)2, titanium butoxide, Ti(OBu)4 97%, manganese nitrate tetrahydrate Mn(NO3)2·4H2O 98%, cobalt nitrate hexahydrate Co(NO3)2·6H2O 97.7%, ethanol, 1-butanol, 1-decanol, NaOH, and oleic acid (90% technical grade) were purchased from Alfa Aesar, while chromium nitrate nonahydrate Cr(NO3)2·9 H2O 98% from EM Science, and ferric nitrate nonahydrate Fe(NO3)3·9 H2O 99.9% from J.T. Baker.



All reagents were used without any further purification. To synthesize BaTiO3 colloidal nanocrystals doped with a particular transition metal ion and a given concentration, syntheses were performed following the synthetic protocol developed previously for the synthesis of BaTiO3 nanocubes.27 A schematic of the synthetic procedure used in this work is presented in Figure 1. In a typical experiment, two aqueous solutions containing 1 mmol Ba(NO3)2 and 12.5 mmol NaOH were mixed with a solution containing (1-x) mmol of Ti(OBu)4, x mmol of the desired transition metal (TM) provided by 0.1 M solution of transition metal nitrate. Subsequent addition of oleic acid, 1-butanol, and 1-decanol yielded a white creamy solution which was stirred vigorously for 5 minutes. The pH of the resultant solution was around 12. The obtained mixture was transferred to a 23 mL Teflon-liner and then placed into a stainless-steel autoclave (Parr Instruments). The autoclave was sealed, and the reaction mixture was heated to 180°C and maintained at this temperature for 48 hours. At the end of the reaction, the autoclave was cooled naturally to room temperature, and the resulting powder was separated by centrifugation to remove the excess of oleic acid. The nanocrystals were then collected, washed with ethanol, and then dispersed in toluene yielding a stable colloidal solution. 2.2. Characterization The morphology of the nanocrystals was characterized with a Hitachi 7700 transmission electron microscope (TEM) equipped with an AMT XR81 CCD high resolution camera. The resolution of the camera was 3296x2472 pixels and the full resolution range was used for all the TEM images presented in this paper. All TEM images were collected with an accelerating voltage of 120 V. The phase purity and the crystal structure of the nanopowders were studied by powder X-ray diffraction (XRD) with a Rigaku Miniflex II X-Ray diffractometer using a Cu source with Kα=1.54059 Å and Kβ=1.54443 Å at 30 kV and 15 mA. Powder XRD data were collected in the angular range 10° ≤ 2θ ≤ 80° in 2θ, with a step size of 0.02°, and a scan rate of 0.1° per minute in a Bragg-Brentano geometry. The refinement of the lattice constant and determination of the crystallite size was carried out using the FullProf suite.29 Specifically, the Thompson, Cox and Hastings (TCH) function and the Le Bail method were used in order to perform a whole pattern profile matching from which lattice constant and crystallite size have been extracted.30,31 The parameters varied during fits were the lattice constant, Gaussian, and Lorenzian broadening parameters of the TCH function. Raman spectroscopy was carried out with a Horiba Xplora One spectrometer equipped with a 532 nm laser and a 100x objective. The 6 ACS Paragon Plus Environment

Page 6 of 33

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectral range considered was from 200 cm-1 to 1000 cm-1. Electron paramagnetic resonance (EPR) data were collected at room temperature on dried powders with a continuous wave spectrometer at the X band frequency (9.8 GHz) with a Bruker Elexsys E680 in a field range that spans from 50 to 650 mT. Dual AC Resonance Tracking piezoresponse force microscopy (DART-PFM) and switching spectroscopy PFM (SS-PFM) were carried out on an MFP-3D system from Asylum Research.32,33 Dielectric spectroscopy was performed using an Agilent 4294A impedance analyzer connected with a custom sample older and temperature controller. VSM data were acquired at room temperature with a Lake Shore 7400 Series vibrating sample magnetometer with a maximum magnetic field of 3.42T and a 10-7 emu noise floor at 10s/point. 3. RESULTS AND DISCUSSION 3.1.Morphology Control of M-Doped BaTiO3 Nanocrystals Figures 2 and 3 show the TEM micrographs of representative samples of Mn-doped BaTiO3 colloidal nanocrystals with a different concentration of the dopant ions and BaTiO3 nanocrystals doped with different transition metals, respectively. The as-synthesized nanocrystals possess a cuboidal shape with a narrow size distribution, the average edge length calculated from all these TEM pictures being 16.4 nm. Furthermore, the size of the nanocrystals calculated from the TEM micrographs (Table 1) was obtained by measuring more than 100 nanocrystals using the Fiji software.34 The minimum number of particles to be measured was determined by assuming a standard distribution and a confidence interval of 95%. Hence, the minimum sample size N can be determined from: 1.96

≥

(1)

where σ is the expected standard deviation and δ the desired error of estimation. To obtain the desired statistical representation of the samples, we have estimated σ by a preliminary measurement on 100 nanocrystals, the value obtained being σ ≈2.3. Additionally, to achieve a precision at a sub-nanometer level, we used δ=0.5, which yielded a value N≈82. The TEM pictures and the data reported in Table 1 strongly indicate that BaTi1-xMxO3 nanocrystals are quasi monodisperse, thereby preserving the cuboidal shape regardless the nature of the transitional metal dopant ion and its concentration in the perovskite host matrix. The narrow size distribution and the retention of oleic acid molecules on the surface of the nanocrystals enable them to self-assemble into monolayers on planar surfaces, which is in good agreement with 7 ACS Paragon Plus Environment


previously reported works.28,35. The crystalline nature of the samples was confirmed by analysis of the selected area electron diffraction pattern (SAED) of the 8% Mn-doped sample, used as a model, which exhibited diffraction rings that can be indexed using the cubic 3 space group (inset of Figure 2d). In Figure S1 a SAED pattern was collected on a few nanocrystals showing only spots, which has confirmed the single crystal nature of the synthesized particles. Our research group previously reported that the size of pristine BaTiO3 nanocrystals can be controlled in the range from 10 to 78 nm by increasing the reaction time from 48 to 120 hours.28 To verify that the same synthetic strategy can be successfully extended to transition metal doped samples, we performed chemical reactions under solvothermal conditions by varying the reaction time using 1% Fe-doped BaTiO3 as a model system. In agreement with the previously reported sizes of pristine BaTiO3 nanocubes,28 the average edge length of the cuboidal BaTi0.99Fe0.01O3 nanocubes was found to vary from 22.3, 46.0 to 68.3 nm, when the reaction time was increased from 96, 120, and 144 hours, respectively (Figure 4a-4c). In Figure 4d is displayed the (110) reflection from the powder X-Ray diffraction pattern of 1% Fe-doped BaTiO3 samples obtained under different reaction times. The full-width at half maximum of this peak was found to progressively increase with increasing the reaction time, corresponding to crystallite sizes of 30.3, 52.6 and 88.1 nm thereby confirming our assumption that the reaction time plays an important role on the size of the resulting colloidal nanocrystals. To verify that the nanocrystals possess a cuboidal shape instead of a plate-like morphology, the sample stage was tilted by 30o during the TEM observation, and the resulting image is presented in Figure 4b.

3.2.Crystal Structure of Transition Metal Doped BaTiO3 Nanocrystals The structural properties of the TM-doped BaTiO3 colloidal nanocrystals were studied by powder X-Ray diffraction (XRD) and Raman spectroscopy. In Figures 5a and 5c are shown the XRD and Raman data for BaTiO3 colloidal nanocrystals doped with different transition metals, while in Figure 5b and 5d is shown the variation of XRD and Raman data as a function of Fe concentration. For this analysis, samples with concentration of 4% (Figures 5a and 5c), and Fe as transition metal in (Figures 5b and 5d) have been used as representative samples. The XRD data were collected after washing the samples with a 1% solution of glacial acetic acid, to remove traces of barium carbonate that generally forms as a secondary phase (with a concentration around 5%) during syntheses performed under air, whereas the Raman data have been collected 8 ACS Paragon Plus Environment

Page 8 of 33

Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on the as-synthesized samples. The XRD patterns match well the standard XRD pattern of cubic BaTiO3 (PDF# 31-074), suggesting the replacement of the titanium ions with transition metal ions and exhibit well defined peaks, indicating that the nanopowders possess a good crystallinity despite the relatively low reaction temperature. As seen in Figure 5a, these reflections were initially indexed in the cubic symmetry (space group no. 221; 3 ). However, as demonstrated by several groups, nanocrystalline BaTiO3 can adopt at room temperature a tetragonal unit cell (space group no. 99; 4

)28,36 whereby the splitting of the peak located at 45o in 2θ is obscured by the line broadening associated with the small size of the crystallites. It is also known that doping BaTiO3 with transition metal, such us Fe, can induce crystallization with hexagonal symmetry (space group no. 194; 6 /

),37 however our XRD data did not show the characteristics reflections (1013) around 26.2o in 2θ and the (2023) around 41.2o in 2θ,

therefore the hexagonal model has been excluded (the absence of the hexagonal phase has also been proven by EPR, and electron diffraction by TEM). To assess which model, between the cubic and tetragonal one, better describes the long-range order in transition metal-doped BaTiO3 cuboidal nanocrystals, the fitting parameters obtained from the whole pattern profile matching fits corresponding to the cubic and tetragonal models were compared. While the results did not show major differences between the fitting parameters by using the cubic and tetragonal models, for the latter model the fit was unstable without converging to a solution, thereby suggesting that the structure refinement from laboratory powder XRD data doesn’t allow an unambiguous distinction between the cubic and tetragonal symmetry in nanoscale titanium-based perovskite oxides. Therefore, the experimental XRD data was fit by using the cubic model, which enabled the refinement of the cell parameter and the estimation of the crystallite size. The refined lattice parameters, along with the ratio between the intensity of the (111) and (200) peaks, the Rwp values and the calculated crystallite sizes are presented in Table 1. It is worth noting that the calculated lattice parameter of pristine BaTiO3 nanocrystals is smaller compared to that of all the other doped samples, indicating the expansion of the perovskite unit cell when Ti4+ ions are replaced by TMn+ ions, respectively. For Fe and Cr-BaTiO3-doped samples, the lattice parameter increases with the concentration of the dopant ion to reach a maximum at dopant concentrations close to 5% followed by a decrease and at higher concentrations of the transition metal ions. The total decrease of the value of the lattice parameter was 0.12 % for the Fe-doped BaTiO3 samples and 0.099% for the Cr9 ACS Paragon Plus Environment


Page 10 of 33

doped BaTiO3 samples, respectively. A similar trend is observed for Co-doped BaTiO3 colloidal nanocrystals; however, in such a case the maximum value remains almost constant when the concentration of the dopant varies from 5% to 8%, whereas in the case of Mn-doped BaTiO3 samples the lattice parameter decreases with increasing the concentration of the dopant ions (Table 1). Such a behavior could be rationalized in terms of the interplay between two different factors; the ionic size of the transition metal dopant ions and the creation of oxygen vacancies in the lattice upon the aliovalent doping of the Ti4+ ions to compensate the unbalanced electrical charges. According to Vegard’s law, the lattice parameter is expected to vary linearly with the size of the dopant cation when Ti4+ ions in BaTiO3 are replaced by transition metal ions. Since both divalent and trivalent first series transition metal ions are larger than the Ti4+ ions in an octahedral coordination environment (Table 2), the lattice parameter of BaTiO3 will increase with increasing the concentration of the dopant ions, as observed in the case of Fe and Cr-doped BaTiO3 nanopowders. When the dopant cations do not replace the ions in the Ti sublattice and a phase segregation occurs, the lattice constant is expected to remain constant regardless of the concentration of the dopant ions in the crystalline host matrix. When an aliovalent substitution occurs in an oxide, the compensation of charges takes which will enable the electric neutrality of the crystal takes place via the formation of oxygen vacancies. However, the role of the oxygen vacancies on the variation of unit cell parameters in oxides is less understood. Using DFT calculations, Aidhy et. al. have shown that the lattice expansion/contraction is dependent on the charge of the oxygen vacancies, as well as the type of oxide material.38 Zhao and coworkers observed an expansion of the volume of BaTiO3 thin films synthesized under a low oxygen partial pressure when the concentration of oxygen vacancies increases.39 Conventionally, oxygen vacancies can be created in oxide materials by either heating the sample under a low oxygen partial pressure or by the aliovalent doping with transition metals. For example, the formation of oxygen vacancies upon doping the TiO2 sublattice of BaTiO3 with a trivalent transition metal can be represented by using the Kröger-Vink formalism: TiO2 TM 2O3  → 2M Ti' + VO•• + 3OO×

(2)

where represents a generic trivalent transition metal (e.g. Fe3+). Equation (2) suggests that the substitution of two Ti4+ ions with two trivalent ions is accompanied by the formation of an oxygen vacancy •• . Thampi and coworkers have recently shown that the aliovalent substitution of Gd3+ ions (r=108 Å) with Ca2+ ions (r= 114 Å) or Zr4+ ions (r= 86 Å) with Sc3+ ions (r= 89 Å) 10 ACS Paragon Plus Environment

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


leads to a decrease of the unit cell value of Gd2Zr2O7, as a result of the creation of anion vacancies,40,41 that will dominate over the decrease of their size and could explain the shrinkage of the lattice parameter at higher concentrations of the transition metal ions.42,43 Additionally, we cannot exclude a partial oxidation of the transition metal ions, which will also induce a decrease of the unit cell parameter, similar to the case of Mn4+ ions formed upon the oxidation of Mn2+ ions in Mn-doped TiO2 nanoparticles (Table 2).44 Such a behavior is in good agreement with ˊ reaction (2) because a higher concentration of should be compensated by an increasing

number of oxygen vacancies (•• ) and reaching the saturation limit of •• will induce the partial

oxidation of the transition metal dopant ions. It is also interesting to note that the inclusion of tetravalent transition metals does not require charge balancing, therefore being favorable. Furthermore, as seen in Table 1 for dopant nominal concentrations higher than 6% the lattice constant remains quasi-constant, thereby suggesting that the solubility limit of the doping has been reached. In Figure S2 are shown the XRD patterns of samples with dopant concentration above 6% showing extra peaks. The presence of extra peaks clearly indicates the presence of a small amount of a secondary phase, which further prove that the saturation limit has been achieved. Unfortunately, our attempts to identify this phase by simple powder XRD analysis have proven unsuccessful, possibly because of the low resolution of the X-Ray diffractometer. More detailed structural studies are currently ongoing and will be published in a forthcoming paper. To further expand our XRD analysis and gain insight into the acentric nature of the unit cell of BaTi1-xMxO3 nanocubes, we have calculated the ratio between the intensity of the (111) and the (200) reflections. This analysis has been previously reported from Hayashi et. al., who used this ratio to define the tetragonality of the samples.45 The (200) and (111) reflections have been selected because for a tetragonal symmetry of the unit cell the former reflection is split in two non-equivalent reflections (200) and (002), whereas the latter is unaffected by phase transitions. Hayashi et. al. reported that in tetragonal titanium-containing perovskite structures this ratio is greater than one, whereas under a dominant cubic phase the ratio should be less than one. As seen in Table 1, the calculated ratio for the biggest M-doped BaTiO3 nanocubes is 1.641 and the splitting of the peak around 45 degrees was visible, whereas for the smaller nanocrystals the values of this ratio are below 1, which suggests the existence of a decreased long-range tetragonality. 11 ACS Paragon Plus Environment


Page 12 of 33

It is worth noting that in Mn and Fe-doped BaTiO3 this ratio increases up to concentration of 6%, and then it suddenly drops. In the case of Cr-doped samples it can be seen in Table 1 that ratio of the (111)/(200) reflections remain almost stable up to 6% and then it suddenly increase, and a similar behavior is observed in Co-doped BaTiO3 with the difference that the increase is seen at concentration of 4%. This rapid variation of the tetragonality of the perovskite unit cell upon increasing the amount of transition metal dopant ions has been ascribed to the formation of secondary phases, reflecting the analysis of the lattice constant variation. The variation of the intensity ratio between (111) and (200) reflections strongly suggest that the transition metal ions replace the Ti4+ ions within the perovskite host crystal. This assumption was confirmed experimentally by energy dispersive X-Ray (EDX) analysis, the experimental results show in Figure S4 clearly indicating the presence of transition metals in the perovskite host structure. These results, along with the absence of a secondary phase for dopant concentrations below 6%, as indicated by the inspection of the powder X-Ray diffraction patterns, strongly suggest that the substitution of the Ti4+ ions by transition metal ions in the perovskite structure of BaTiO3 nanocrystals occurred successfully by the proposed synthetic approach. Although powder XRD analysis allowed us to understand the structural changes occurring in BaTi1-xMxO3 colloidal nanocrystals with different concentrations of the transition metal ions, a clear distinction between the cubic and tetragonal crystal structure was limited due to the peak broadening associated with the small crystallite size which usually obscures the splitting of the peaks due to the existence of a tetragonal disorder within the lattice. Raman spectroscopy has been used as a complementary technique to powder XRD to study the crystal structure of titanium-containing perovskites instead of describing the average, static structure of the crystal, as powder XRD does, the inelastic scattering of photons by a crystal will provide information about its local, dynamic structure, respectively.36,46 In Figure 5c are shown the Raman spectra of different transition metal-doped BaTiO3 colloidal nanocrystals corresponding to a concentration of the dopant ion of 4%, which have been chosen as representative samples. In the cubic 3 (

" !)

space group all

the 15 vibrational degrees of freedom are Raman inactive due to the isotropic distribution of the electrostatic forces around the Ti4+ ions within each individual TiO6 octahedron. Despite the formal absence of active modes in the Raman spectrum of the cubic polymorph, as suggested by symmetry analysis, experimentally two broad bands centered at about 260 and 520 cm-1 have been observed, being attributed others to the presence of a small off-center shift of the titanium 12 ACS Paragon Plus Environment

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atoms.36,46 Similarly, the symmetry analysis of BaTiO3 of the tetragonal BaTiO3 polymorph " (space group 4

($%& ) ) lead to the irreducible representation: Γ = 3)" + +" + 4, in which

all eight modes are Raman active. Furthermore, since long range electrostatic forces induce the splitting in longitudinal optical (LO) and transverse optical (TO) phonons, 15 Raman active modes (3[A1(LO)+A1(TO)], B1, 4[E(LO)+E(TO)]) are expected in the Raman spectrum of tetragonal BaTiO3.47 Based upon temperature-dependent diffraction experiments, Smith et. al. suggested that the tetragonal to cubic phase transition on submicrometric BaTiO3 particles is diffuse, extending over a wide range of temperature and has a mixed displacive and orderdisorder character. Therefore, as the sample is heated above the Curie temperature, the tetragonality of the structure begins to fade away, although small pockets within the crystals still retain an acentric structure, which is reflected into the Raman spectrum as broadened bands at ~260 and 520 cm-1 with a small intensity.36 For this reason, the peaks that are conventionally considered more sensitive to the tetragonal phase are the sharp E(LO+TO), B1 band located around 305 cm-1, and the E(LO), A1(LO) at ~720 cm-1, respectively. It is worth mentioning that regardless of the nature of the dopant ion, the increase of the concentration of the transition metal in the perovskite host crystal is associated with two opposite changes in the intensity of the bands at 305 cm-1 and 720 cm-1 used as reference in this study. Specifically, whereas the intensity of the peak at 305 cm-1 decreases upon increasing the concentration of the dopant ions, the band located at 720 cm-1 becomes more intense and broadens along with shifting to a higher wavenumber, which will result in a peak maximum around 730 cm-1. Figure 5d shows the variation of the Raman spectra upon increasing the Fe concentration from 0% (pristine BaTiO3) to 8% that were chosen as representative samples. Such a behavior has been attributed to the existence of a local stress induced by doping, which yields a local compression of the bond between the oxygen and the TM ions occupying the B sites in the perovskite structure, thereby shifting the E(LO) band to a value of the wavenumber close to 730 cm-1. This assumption is in good agreement with experimental results reported by Chen et.al. in the case of Ce-doped BaTiO3 films.48 Moreover, based upon the strong dependence of the peak at 730 cm-1 on the concentration of the transition metal dopant ions, we have analyzed the ratio of the intensity of the bands at 730 cm-1 and 520 cm-1, respectively. The experimental results are reported in Table 1. Remarkably, this ratio has a value of 0.548 for pristine BaTiO3 that is significantly different from all the other doped samples, for which the 13 ACS Paragon Plus Environment


average is 0.958. Therefore, this analysis emphasizes the variation of the E(LO) band confirming the inclusion of TMs at the B site. In addition to this, the variation of this ratio in each group of doping reveals that for Co and Cr-doped samples there is an increase up to concentration of 6% and then a sudden drop, corroborating the XRD analysis and the formation of secondary phases. However, in the case of Mn and Fe-doped samples the trend of this ratio is less clear, whit an almost linear increase with the exception of the Fe 4% that is much higher. However, we noted that for these two doping types is also visible a small band centered around 640 cm-1, that is typical of the hexagonal phase of BaTiO3.37,49,50 Moreover, the presence and intensity of this band was strongly related to the laser power used during the acquisition of the spectra, which is not uncommon that the local heat generated from the laser can induce structural variation.47 Since, in the XRD, electron diffraction, and EPR data the hexagonal phase has not been detected we concluded that this additional phase is induced from the Raman laser instead of been an intrinsic phase of the nanocrystals. The local formation of the hexagonal phase also explain the strange trend of the ratio between the band at 730 cm-1 and 520 cm-1 in Mn and Fe-doped samples due to the presence varying concentrations of this induced phase. 3.3. Structural and Electron Spin Resonance Properties of Transition Metal-Doped BaTiO3 Nanocrystals The presence and the coordination environment of spin centers introduced by aliovalent doping with first series transition metal ions has been evidenced experimentally by electron paramagnetic resonance (EPR) measurements. As seen in Figure 6, the EPR spectra of BaTi0.96TM0.04O3 (TM=Fe, Cr, and Mn) colloidal nanocrystals, chosen as representative samples, exhibited a very intense signal, centered at -.// ≅ 2 , indicating the presence of unpaired electrons in the perovskite solid host matrix. In contrast, the Co-doped spectrum does not show any intense absorption in the range of the magnetic fields probed. Langhammer and coworkers reported previously that in Co-doped hexagonal BaTiO3 samples cobalt ions are largely present as Co2+ ions, which will lead to a strong absorption at -.// ≅ 4.3, along with a small signal at

-.// ≅ 2.2. Likewise, because Co3+ ions (3d6) in a low spin configuration doesn’t have unpaired

electrons, the corresponding EPR spectrum will not feature any absorption band, whereas the presence of the paramagnetic Co4+ ions is highly improbable and has been usually ruled out.51 The absence of any strong signal in the EPR spectrum of BaTi1-xCoxO3 nanocrystals along with the high similarity between the EPR spectra of these samples and that of pristine BaTiO3 suggest 14 ACS Paragon Plus Environment

Page 14 of 33

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the presence of diamagnetic Co3+ (3d6 ions, low spin) whereas the weak signal at -.// ≅ 2 could

2 be associated to the presence of titanium and oxygen defects; that is •• and 1 , in agreement

with the results reported by Eichel on BaTiO3 synthesized under a reducing atmosphere.52 Interestingly, in the case of the Mn-doped BaTiO3 NCs samples, one can easily observe the typical splitting of the signal into a sextet in the corresponding EPR spectrum, associated with the hyperfine coupling of the unpaired electrons with the nuclear spin of manganese (3 = 5/2), thereby confirming that the Mnn+ ions have been incorporated into the crystalline host structure. Theoretical simulations of the experimental data, shown in Figure S5, indicated that the experimental spectrum is likely associated with the presence of Mn3+ in a mixed tetragonal/cubic BaTiO3 lattice, along with the presence of Mn4+ dimers. The spectra of Fe-doped samples are dominated by an intense peak centered at -.// ≅ 2,

along with a small peak centered at -.// ≅ 4, which has been attributed to the formation of 2 7 2 defects such as 56 , 56 , and [56 – •• ]• .43,52 In Figure S5 are shown the theoretical

simulations carried out on the Fe-doped BaTiO3 samples. The appearance of the theoretical 2 spectrum suggests that [56 – •• ]• defects are absent in the perovskite nanocrystals, which can

be presumably due to a dispersion of the oxygen vacancies within the host crystal. More importantly, the data analysis suggested that the iron ions adopt different oxidation states, corresponding to different spin configurations which are the source of unpaired electrons in the Fe-doped BaTiO3 nanocrystals. Specifically, our calculations indicated that the EPR spectrum is principally associated to the presence of Fe3+ and Fe4+ ions at the Ti4+ sites, with the most important contribution (66%) corresponding to a system with a spin state ; = 1/2 and - =

2.018, ascribed to Fe3+ ions in a low spin (LS) configuration ( t25g eg0 ); whereas both the Fe3+ in a

high spin (HS) configuration ( t23g eg2 ) and the Fe4+ ions have a much smaller contribution, namely 7% and 27%, respectively. The zero-field splitting (ZFS) values for the last two defects where taken from the work on SrTiO3 of Drahus et. al.43 As seen in Figure S5, the Cr-doped spectra showed features like those observed in the Fedoped samples. To interpret this data, the experimental EPR spectra were compared to those reported by Böttcher et. al.53 in which an axial asymmetry related to the hexagonal phase (H2) with -.// ≅ 3.79, has been identified. In the work of Böttcher et. al. the presence of 2 [$> – •• ]• defects was excluded by comparing the experimental values of the zero-field



parameters with the one calculated from the Newman Superposition Model.54 However, unlike the data reported by Böttcher and coworkers, our experimental XRD and Raman results did not suggest the presence of the hexagonal phase in Cr-doped BaTiO3 nanocrystals. Additionally, the simulated EPR spectrum reported in Figure S5, presents a central EPR peak which suggests the presence of Cr3+ ions in a mixed cubic/tetragonal BaTiO3 lattice with a Hamiltonian parameter similar to the one reported from Müller et al.55 The small peak at -.// ≅ 4 was reconstructed using the ZFS parameters reported by Drahus et. al. in Fe-doped SrTiO3, being associated to the 2 presence of [56 – •• ]• defects and Fe4+ions, respectively.43 Taking into account these facts, we 2 hypothesize that the observed signal at -.// ≅ 4 would arise from the presence of [$> – •• ]•

defects, as well as from Cr2+ ions in a high spin configuration ( t23g eg0 ). The experimental EPR results obtained on the M-doped BaTiO3 cuboidal nanoparticles (M= Cr, Mn and Fe) clearly demonstrate that the aliovalent doping of the perovskite structure induce multiple types of complex defects which coexist in the host crystals with some transition metal ions adopting uncommon oxidation states (Mn4+, Fe4+, etc.). Last, but not least, the environment of the dopant ions presents axial ZFS parameters, which indicate the presence of distortions in the nanocrystals. 3.4. Ferroelectric and Magnetic Properties of Transition Metal Doped BaTiO3 Colloidal Nanocrystals To demonstrate the coexistence of the ferromagnetism and ferroelectricity in transition metal-doped BaTiO3 colloidal nanocrystals, we used BaTi1-xFexO3 nanocubes as a model system. Figure 7 shows the dielectric, piezoelectric force microscopy (PFM), and vibrating sample magnetometer (VSM) data of the Fe-doped samples. Dielectric measurements were collected by sputtering Au/Pt electrodes on both side of a powder pellet. The data indicated that the permittivity values decrease with the Fe content in the samples. Interestingly, a broad peak in both the doped and undoped samples was visible in the temperature range from 80 to 100 °C, presumably corresponding to the ferroelectric to paraelectric phase transition. Such experimental evidence of a phase transition suggests that the samples at room temperature possess an intrinsic tetragonal structural distortion, leading to the formation of electrical dipoles associated to the macroscopic ferroelectric behavior of BaTiO3. Moreover, the maximum of the peak corresponds to a lower temperature compared to that of the pristine sample, thereby indicating that the Curie 16 ACS Paragon Plus Environment

Page 16 of 33

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature decreases upon the doping of the BaTiO3 colloidal nanocrystals with iron ions. To probe the ferroelectricity of the samples at the nanoscale level, we employed the DART-PFM method,32 by using the tip as a top electrode on nanocrystals thin film (around 50 nm thick) dropcasted on conductive silicon. In Figure 7d are shown the averaged phase and amplitude signals collected on a region of 6 ? corresponding to 4096 points collected in DART switching spectroscopy mode (DART SS-PFM).33 The DART SS-PFM data clearly show the presence of a hysteretic behavior in the phase signal with a 180o phase shift, as expected from the switching of the dielectric polarization. Likewise, the amplitude signal shows the typical behavior of a ferroelectric material with a butterfly-like shape, furthermore confirming the ferroelectric behavior of the sample. An averaged value of the signals over a wide area was used to confirm that the hysteretic behavior is present over the whole sample and not only at specific location. The ferroelectric nature (i.e. the existence of a switchable intrinsic dielectric polarization) of the samples has been also verified by writing an arbitrary pattern with the PFM tip in contact mode while applying a bias at the tip. Specifically, a square pattern was written using a bias of -10 V inscribed into a larger square pattern, written with a bias voltage of +10 V. As it can be seen in Figure 7b, the phase image clearly shows the two square patterns, indicating the presence of the polarization with an upwards and downwards orientation, which can be reversibly switched with an electrical field. By comparing the topographic image with the PFM phase (Figure 7b), it can be clearly seen that they are completely uncorrelated, confirming that the observed phase shift is not affected by cross-talk effects and is truly an image of the sample polarization. VSM data suggest that both pristine and Fe-doped samples are ferromagnetic and the saturation magnetization (Ms) increases with Fe-doping, in agreement with previously reported works.26,56 In addition, it has been observed that the saturation magnetization of the BaTi0.94Fe0.06O3 sample is the smallest in the M-doped BTO doped series, with a value around 1 ∙ 10A% emu/g, whereas the Ms values of samples with 2% and 4% Fe were 1.5 ∙ 10A% and 2.5∙ 10A% emu/g, respectively. A similar trend in the saturation magnetization in Fe-doped

BaTiO3 has been previously attributed by Yang et. al. to a change in the magnetic order from ferromagnetic to antiferromagnetic, as a result of the decrease of the separation between the magnetic centers at high concentrations of the dopant ions.56 Nonetheless, the presence of a magnetic order (antiferromagnetic and ferromagnetic) in BaTi1-xFexO3 cuboidal nanocrystals demonstrates not only the successful aliovalent doping of the perovskite host crystal, but also the 17 ACS Paragon Plus Environment


coupling between the spin centers. The comparison of the experimental VSM and EPR data suggests the presence of multiple coupling mechanisms. The onset of ferromagnetism in undoped BaTiO3 has been conventionally ascribed to the presence of oxygen vacancies in these crystals, which will consequently lead to the reduction of the diamagnetic Ti4+ to magnetic Ti3+ species, as described in previous works.26,57,58 On the other hand, EPR data suggested the presence of isolated Fe3+ and Fe4+ ions which can interact each other via an oxygen mediated Dzyaloshinskii-Moriya-type interaction, thereby leading to a weak ferromagnetism similar to that described by Kikoin et al. in magnetically diluted dielectrics.59 Further experiments to understand the nature of the magnetic order in transition metal-doped BaTiO3 colloidal nanocrystals are currently in progress and will be reported in a forthcoming paper. However, the existence of oxygen-mediated normal super-exchange coupling mechanisms cannot also be excluded, resulting into an antiferromagnetic coupling. However, the latter is believed to dominate at high doping concentrations, whereby the saturation magnetization is expected to decrease, in good agreement with our XRD analysis that showed the existence of a saturation limit of the oxygen vacancies in the perovskite host crystal. In addition to the super-exchange mechanism, the unidentified extra phases in the 6% Fe sample can also be the reason of the lowered magnetization, and further investigation is underway to elucidate the reason behind the reduced saturation magnetization in BaTi0.94Fe0.06O3. 4. CONCLUSIONS In summary, in this work we demonstrated the versatility of the solvothermal synthesis by showing that this simple synthetic technique is amenable to the rational design of transition metal doped BaTiO3 colloidal nanocrystals with a very good control of the size, shape and internal structure of the nanocrystals. The method allows a direct doping up to nominal concentrations of 4% without any observed segregation of secondary phases. As a direct consequence of the aliovalent doping, additional oxygen vacancies are added into the perovskite structure of BaTiO3, and they have been detected by XRD and EPR data. The inclusion of the transition metals and consequent addition of unpaired electrons was confirmed from room temperature EPR measurements for Mn, Cr, and Fe-doped BaTiO3 from the strong absorption at -.// ≅ 2. The presence of ferromagnetism and ferroelectricity at room temperature was detected from VSM and PFM measurements. These results suggest that BaTiO3 nanocrystals doped with Fe exhibit


Page 18 of 33

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


both ferroelectricity and ferromagnetism at room temperature, which makes them potential candidate for multiferroics materials, and as such, we are further investigating the ferroic/multiferroic properties of these single phase monodisperse nanocrystals. This advancement in the synthesis of multifunctional nanocrystals with tunable chemical identity of the doping ions, doping concentration, and morphology can open the door to their use as building blocks in miniaturized devices like multistate memories, or in catalysis applications, with the possibility of tailoring their performances by choosing the appropriate nanocrystals morphology and doping concentration.

5. ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) through the CAREER grant No. 1434457 and the Central Michigan University through the Office of Research and Sponsored Programs. A.R. acknowledges the CNFIS for financial support through the CNFIS-FDI-2018058 grant and EUFISCDI through the PN-III-P1-1.1-MCD-2018-0098 grant, respectively.



6. BIBLIOGRAPHY (1)

(2) (3)

(4)

(5) (6)

(7) (8) (9) (10)

(11) (12)

(13)

(14)

(15)

(16)

(17)

Lines, M. E.; Glass, A. M. Principles and Applications of Ferroelectrics and Related Materials; Oxford Classic Texts in the Physical Sciences; Oxford University Press: Oxford, 2001. Lott, J.; Xia, C.; Kosnosky, L.; Weder, C.; Shan, J. Terahertz Photonic Crystals Based on Barium Titanate/Polymer Nanocomposites. Adv. Mater. 2008, 20, 3649–3653. Lee, H.-G.; Kim, H.-G. Ceramic Particle Size Dependence of Dielectric and Piezoelectric Properties of Piezoelectric Ceramic‐polymer Composites. J. Appl. Phys. 1990, 67, 2024– 2028. Setter, N.; Damjanovic, D.; Eng, L.; Fox, G.; Gevorgian, S.; Hong, S.; Kingon, A.; Kohlstedt, H.; Park, N. Y.; Stephenson, G. B.; et al. Ferroelectric Thin Films: Review of Materials, Properties, and Applications. J. Appl. Phys. 2006, 100, 051606. Son, J. Y.; Lee, J.-H.; Song, S.; Shin, Y.-H.; Jang, H. M. Four-States Multiferroic Memory Embodied Using Mn-Doped BaTiO3 Nanorods. ACS Nano 2013, 7, 5522–5529. Yau, H. M.; Yan, Z. B.; Chan, N. Y.; Au, K.; Wong, C. M.; Leung, C. W.; Zhang, F. Y.; Gao, X. S.; Dai, J. Y. Low-Field Switching Four-State Nonvolatile Memory Based on Multiferroic Tunnel Junctions. Sci. Rep. 2015, 5, 12826. Fiebig, M. Revival of the Magnetoelectric Effect. J. Phys. Appl. Phys. 2005, 38, R123. Hill, N. A. Why Are There so Few Magnetic Ferroelectrics? J. Phys. Chem. B 2000, 104, 6694–6709. Eerenstein, W.; Mathur, N. D.; Scott, J. F. Multiferroic and Magnetoelectric Materials. Nature 2006, 442, 759–765. Nan, C.-W.; Bichurin, M. I.; Dong, S.; Viehland, D.; Srinivasan, G. Multiferroic Magnetoelectric Composites: Historical Perspective, Status, and Future Directions. J. Appl. Phys. 2008, 103, 031101. Vopson, M. M. Fundamentals of Multiferroic Materials and Their Possible Applications. Crit. Rev. Solid State Mater. Sci. 2015, 40, 223–250. Guduru, R.; Liang, P.; Runowicz, C.; Nair, M.; Atluri, V.; Khizroev, S. Magneto-Electric Nanoparticles to Enable Field-Controlled High-Specificity Drug Delivery to Eradicate Ovarian Cancer Cells. Sci. Rep. 2013, 3, 2953. Nair, M.; Guduru, R.; Liang, P.; Hong, J.; Sagar, V.; Khizroev, S. Externally Controlled On-Demand Release of Anti-HIV Drug Using Magneto-Electric Nanoparticles as Carriers. Nat. Commun. 2013, 4, 1707. Apostolova, I. N.; Apostolov, A. T.; Bahoosh, S. G.; Wesselinowa, J. M. Origin of Ferromagnetism in Transition Metal Doped BaTiO3. J. Appl. Phys. 2013, 113 (20), 203904. Ju, L.; Sabergharesou, T.; Stamplecoskie, K. G.; Hegde, M.; Wang, T.; Combe, N. A.; Wu, H.; Radovanovic, P. V. Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides. J. Am. Chem. Soc. 2012, 134, 1136–1146. Xu, B.; Yin, K. B.; Lin, J.; Xia, Y. D.; Wan, X. G.; Yin, J.; Bai, X. J.; Du, J.; Liu, Z. G. Room-Temperature Ferromagnetism and Ferroelectricity in Fe-Doped BaTiO3. Phys. Rev. B 2009, 79, 134109. Shah, J.; Kotnala, R. K. Induced Magnetism and Magnetoelectric Coupling in Ferroelectric BaTiO3 by Cr-Doping Synthesized by a Facile Chemical Route. J. Mater. Chem. A 2013, 1, 8601–8608. 20 ACS Paragon Plus Environment

Page 20 of 33

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Rajan, S.; Gazzali, P. M. M.; Chandrasekaran, G. Electrical and Magnetic Phase Transition Studies of Fe and Mn Co-Doped BaTiO3. J. Alloys Compd. 2016, 656, 98–109. (19) Verma, K. C.; Kotnala, R. K. Multiferroic Approach for Cr,Mn,Fe,Co,Ni,Cu Substituted BaTiO3 Nanoparticles. Mater. Res. Express 2016, 3, 055006. (20) Gao, X.; Li, L.; Jian, J.; Wang, H.; Fan, M.; Huang, J.; Wang, X.; Wang, H. Vertically Aligned Nanocomposite BaTiO3:YMnO3 Thin Films with Room Temperature Multiferroic Properties toward Nanoscale Memory Devices. ACS Appl. Nano Mater. 2018, 1, 2509. (21) Rajamani, A.; Dionne, G. F.; Bono, D.; Ross, C. A. Faraday Rotation, Ferromagnetism, and Optical Properties in Fe-Doped BaTiO3. J. Appl. Phys. 2005, 98, 063907. (22) Lin, Y.-H.; Yuan, J.; Zhang, S.; Zhang, Y.; Liu, J.; Wang, Y.; Nan, C.-W. Multiferroic Behavior Observed in Highly Orientated Mn-Doped BaTiO3 Thin Films. Appl. Phys. Lett. 2009, 95, 033105. (23) Ramana, E. V.; Yang, S. M.; Jung, R.; Jung, M. H.; Lee, B. W.; Jung, C. U. Ferroelectric and Magnetic Properties of Fe-Doped BaTiO3 Thin Films Grown by the Pulsed Laser Deposition. J. Appl. Phys. 2013, 113, 187219. (24) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Donor Impurity Band Exchange in Dilute Ferromagnetic Oxides. Nat. Mater. 2005, 4, 173–179. (25) Shuai, Y.; Zhou, S.; Bürger, D.; Reuther, H.; Skorupa, I.; John, V.; Helm, M.; Schmidt, H. Decisive Role of Oxygen Vacancy in Ferroelectric versus Ferromagnetic Mn-Doped BaTiO3 Thin Films. J. Appl. Phys. 2011, 109, 084105. (26) Mangalam, R. V. K.; Ray, N.; Waghmare, U. V.; Sundaresan, A.; Rao, C. N. R. Multiferroic Properties of Nanocrystalline BaTiO3. Solid State Commun. 2009, 149, 1–5. (27) Wang, M.; Tan, G.-L.; Zhang, Q. Multiferroic Properties of Nanocrystalline PbTiO3 Ceramics. J. Am. Ceram. Soc. 2010, 93, 2151–2154. (28) Caruntu, D.; Rostamzadeh, T.; Costanzo, T.; Parizi, S. S.; Caruntu, G. Solvothermal Synthesis and Controlled Self-Assembly of Monodisperse Titanium-Based Perovskite Colloidal Nanocrystals. Nanoscale 2015, 7, 12955–12969. (29) Rodríguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction. Phys. B Condens. Matter 1993, 192, 55–69. (30) Thompson, P.; Cox, D. E.; Hastings, J. B. Rietveld Refinement of Debye–Scherrer Synchrotron X-Ray Data from Al2O3. J. Appl. Crystallogr. 1987, 20, 79–83. (31) Bail, A. L. Whole Powder Pattern Decomposition Methods and Applications: A Retrospection. Powder Diffr. 2005, 20, 316–326. (32) Rodriguez, B. J.; Callahan, C.; Kalinin, S. V.; Proksch, R. Dual-Frequency ResonanceTracking Atomic Force Microscopy. Nanotechnology 2007, 18, 475504. (33) Jesse, S.; Baddorf, A. P.; Kalinin, S. V. Switching Spectroscopy Piezoresponse Force Microscopy of Ferroelectric Materials. Appl. Phys. Lett. 2006, 88, 062908. (34) Schindelin, J.; Arganda-Carreras, I.; Frise, E.; Kaynig, V.; Longair, M.; Pietzsch, T.; Preibisch, S.; Rueden, C.; Saalfeld, S.; Schmid, B.; et al. Fiji: An Open-Source Platform for Biological-Image Analysis. Nat. Methods 2012, 9, 676–682. (35) Adireddy, S.; Lin, C.; Cao, B.; Zhou, W.; Caruntu, G. Solution-Based Growth of Monodisperse Cube-Like BaTiO3 Colloidal Nanocrystals. Chem. Mater. 2010, 22, 1946– 1948. (36) Smith, M. B.; Page, K.; Siegrist, T.; Redmond, P. L.; Walter, E. C.; Seshadri, R.; Brus, L. E.; Steigerwald, M. L. Crystal Structure and the Paraelectric-to-Ferroelectric Phase Transition of Nanoscale BaTiO3. J. Am. Chem. Soc. 2008, 130, 6955–6963. 21 ACS Paragon Plus Environment


(37) Zorko, A.; Pregelj, M.; Gomilšek, M.; Jagličić, Z.; Pajić, D.; Telling, M.; Arčon, I.; Mikulska, I.; Valant, M. Strain-Induced Extrinsic High-Temperature Ferromagnetism in the Fe-Doped Hexagonal Barium Titanate. Sci. Rep. 2015, 5, 7703. (38) Aidhy, D. S.; Liu, B.; Zhang, Y.; Weber, W. J. Chemical Expansion Affected Oxygen Vacancy Stability in Different Oxide Structures from First Principles Calculations. Comput. Mater. Sci. 2015, 99, 298–305. (39) Zhao, T.; Chen, F.; Lu, H.; Yang, G.; Chen, Z. Thickness and Oxygen Pressure Dependent Structural Characteristics of BaTiO3 Thin Films Grown by Laser Molecular Beam Epitaxy. J. Appl. Phys. 2000, 87, 7442–7447. (40) Thampi, V. D. S.; Padala, P. R.; Renju, U. A. Influence of Aliovalent Cation Substitution on Structural and Electrical Properties of Gd2(Zr1−xMx)2O7−δ (M = Sc, Y) Systems. RSC Adv. 2015, 5, 88675–88685. (41) Thampi, V.; Padala, P. R.; Radhakrishnan, A. N. Induced Oxygen Vacancies and Their Effect on the Structural and Electrical Properties of a Fluorite-Type CaZrO3–Gd2Zr2O7 System. New J. Chem. 2015, 39, 1469–1476. (42) Azamat, D. V.; Dejneka, A.; Lancok, J.; Trepakov, V. A.; Jastrabik, L.; A. G. Badalyan. EPR Studies of Manganese Centers in SrTiO3: Non-Kramers Mn3+ Ions and Spin-Spin Coupled Mn4+ Dimers. J. Appl. Phys. 2012, 111, 104119. (43) Drahus, M. D.; Jakes, P.; Erdem, E.; Eichel, R.-A. Defect Structure of the Mixed Ionic– Electronic Conducting Sr[Ti,Fe]Ox Solid-Solution System — Change in Iron Oxidation States and Defect Complexation. Solid State Ion. 2011, 184, 47–51. (44) Reddy, M. V.; Sharma, N.; Adams, S.; Rao, R. P.; Peterson, V. K.; Chowdari, B. V. R. Evaluation of Undoped and M-Doped TiO2, Where M = Sn, Fe, Ni/Nb, Zr, V, and Mn, for Lithium-Ion Battery Applications Prepared by the Molten-Salt Method. RSC Adv. 2015, 5, 29535–29544. (45) Hayashi, H.; Nakamura, T.; Ebina, T. In-Situ Raman Spectroscopy of BaTiO3 Particles for Tetragonal–Cubic Transformation. J. Phys. Chem. Solids 2013, 74, 957–962. (46) Gajović, A.; Pleština, J. V.; Žagar, K.; Plodinec, M.; Šturm, S.; Čeh, M. TemperatureDependent Raman Spectroscopy of BaTiO3 Nanorods Synthesized by Using a TemplateAssisted Sol–Gel Procedure. J. Raman Spectrosc. 2013, 44, 412–420. (47) Shiratori, Y.; Pithan, C.; Dornseiffer, J.; Waser, R. Raman Scattering Studies on Nanocrystalline BaTiO3 Part I—Isolated Particles and Aggregates. J. Raman Spectrosc. 2007, 38, 1288–1299. (48) Chen, M. S.; Shen, Z. X.; Tang, S. H.; Shi, W. S.; Cui, D. F.; Chen, Z. H. Stress Effect on Raman Spectra of Ce-Doped BaTiO3 Films. J. Phys. Condens. Matter 2000, 12 (31), 7013. (49) Langhammer, H. T.; Müller, T.; Felgner, K.-H.; Abicht, H.-P. Crystal Structure and Related Properties of Manganese-Doped Barium Titanate Ceramics. J. Am. Ceram. Soc. 2000, 83, 605–611. (50) Nguyen, H. M.; Dang, N. V.; Chuang, P.-Y.; Thanh, T. D.; Hu, C.-W.; Chen, T.-Y.; Lam, V. D.; Lee, C.-H.; Hong, L. V. Tetragonal and Hexagonal Polymorphs of BaTi1−xFexO3−δ Multiferroics Using X-Ray and Raman Analyses. Appl. Phys. Lett. 2011, 99, 202501. (51) Langhammer, H. T.; Böttcher, R.; Müller, T.; Walther, T.; Ebbinghaus, S. G. Defect Properties of Cobalt-Doped Hexagonal Barium Titanate Ceramics. J. Phys. Condens. Matter 2015, 27, 295901.


Page 22 of 33

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(52) Eichel, R. A. Structural and Dynamic Properties of Oxygen Vacancies in Perovskite Oxides—Analysis of Defect Chemistry by Modern Multi-Frequency and Pulsed EPR Techniques. Phys. Chem. Chem. Phys. 2010, 13, 368–384. (53) Böttcher, R.; Erdem, E.; Langhammer, H. T.; Müller, T.; Abicht, H.-P. Incorporation of Chromium into Hexagonal Barium Titanate: An Electron Paramagnetic Resonance Study. J. Phys. Condens. Matter 2005, 17, 2763. (54) Newman, D. J.; Urban, W. Interpretation of S-State Ion E.P.R. Spectra. Adv. Phys. 1975, 24, 793–844. (55) Müller, K. A.; Berlinger, W.; Albers, J. Paramagnetic Resonance and Local Position of Cr3+ in Ferroelectric BaTiO3. Phys. Rev. B 1985, 32, 5837–5844. (56) Yang, L.; Qiu, H.; Pan, L.; Guo, Z.; Xu, M.; Yin, J.; Zhao, X. Magnetic Properties of BaTiO3 and BaTi1−xMxO3 (M=Co, Fe) Nanocrystals by Hydrothermal Method. J. Magn. Magn. Mater. 2014, 350, 1–5. (57) Bahoosh, S. G.; Trimper, S.; Wesselinowa, J. M. Origin of Ferromagnetism in BaTiO3 Nanoparticles. Phys. Status Solidi RRL – Rapid Res. Lett. 2011, 5, 382–384. (58) Sundaresan, A.; Rao, C. N. R. Implications and Consequences of Ferromagnetism Universally Exhibited by Inorganic Nanoparticles. Solid State Commun. 2009, 149, 1197– 1200. (59) Kikoin, K.; Fleurov, V. Superexchange in Dilute Magnetic Dielectrics: Application to (Ti, Co)O2. Phys. Rev. B 2006, 74 (17), 174407. (60) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. Sect. A 1976, 32, 751–767.



Page 24 of 33

Table 1: Results obtained from XRD refinements, dynamic lattice analysis, and TEM micrographs. Errors are given in brackets. Sample

a (Å)

XRD Size (nm)

Rwp

Ratio (111)/(200)

Raman 730/520 cm-1

TEM Size (nm)

TEM size variance

BaTiO3 144 hr Mn 2% Mn 4% Mn 6% Mn 8% Fe 2% Fe 4% Fe 6% Fe 8% Cr 2% Cr 4% Cr 6% Cr 8% Co 2% Co 4% Co 6% Co 8%

4.0165(2) c/a 1.0056 4.01693(5) 4.01664(6) 4.01603(6) 4.01597(5) 4.02073(9) 4.02393(5) 4.01992(6) 4.01849(6) 4.01852(9) 4.01864(9) 4.01554(8) 4.0155(1) 4.01818(6) 4.01931(5) 4.01922(6) 4.01872(6)

17.3 87.0 12.3 13.7 17.3 16.5 11.5 12.3 11.5 13.8 14.0 16.2 16.6 19.0 10.2 13.4 11.3 13.1

14 16.8 16.2 11.5 12.2 13.4 15.1 19.5 13.8 13.6 19.7 20.5 19.7 17.2 13.3 15.5 16.7 14.8

0.807 1.641 0.705 0.719 0.728 0.702 0.779 0.805 0.854 0.775 0.725 0.700 0.709 0.773 0.756 0.699 0.733 0.737

0.548 0.547 0.848 0.883 0.892 0.926 0.890 1.030 0.930 0.958 0.857 0.904 0.988 0.894 0.943 1.082 1.193 1.106

14 68.3 15.7 14.6 14.2 16.6 16.3 16.6 19.5 17.8 16.5 20.5 16.2 17.1 12.4 16.9 15.7 15.0

1.9 14.6 2.2 2.3 1.9 2.1 2.7 1.6 2.6 2.3 2.4 3.2 2.3 2.7 2.0 2.0 2.1 1.7

Table 2: Ionic radii of the hexavalent coordinated transition metals used for doping. Ionic radii are based on R. D. Shannon work.60 Highlighted are the most common oxidation state in each transition metals. Ion

Charge

Ti Mn Mn Mn Mn Mn Fe Fe Fe Fe Fe Cr Cr Cr Cr Co Co

4+ 2+ 2+ 3+ 3+ 4+ 2+ 2+ 3+ 3+ 4+ 2+ 2+ 3+ 4+ 2+ 3+

Spin State

High Spin Low Spin High Spin Low Spin High Spin Low Spin High Spin Low Spin High Spin Low Spin

High Spin Low Spin 24 ACS Paragon Plus Environment

Ionic Radius (Å)

0.605 0.83 0.67 0.645 0.58 0.53 0.78 0.61 0.645 0.55 0.585 0.8 0.73 0.615 0.55 0.745 0.545

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE CAPTIONS

Figure 1: Schematic of the synthetic approach used in this work for the preparation of bare and TM-doped BaTiO3 (TM=Cr, Mn, Fe, Co) colloidal nanocrystals Figure 2: TEM micrographs of Mn-doped nanocrystals at different dopant concentrations. (a) 2%, (b) 4%, (c) 6%, and (d) 8%. SAED pattern in the inset. Figure 3: TEM micrographs of nanocrystals doped with different transition metals but with concentration fixed at 4%. a) Iron, b) Manganese, c) Cobalt, and d) Chromium. Figure 4. Dependence of BaFe0.01Ti0.99O3 nanocrystals’ size to the reaction time. (a) 96 h ;(b) 120 h and (c) 144 h and (d) plot of the most intense peak of the XRD patterns obtained from different reaction times. The inset in (b) is a picture acquired with a stage tilt of 30° in which is visible the cubic shape of the nanocrystals. Figure 5. XRD and Raman data analysis of TM doped BaTiO3 (TM = Cr, Mn, Fe, Co). a) XRD pattern of BaTM0.04Ti0.96O3; b) XRD pattern of BaTi1-xFexO3 (0.02

Quasi-Monodisperse Transition-Metal-Doped BaTiO3 (M = Cr, Mn, Fe

Recommend Documents