Fe-doped SrTiO3 Research Articles

Direct synthesis of undoped/doped SrTiO3 nanoparticles from solution

Directly crystallizing strontium titanate (SrTiO3) nanoparticles from a solution near room temperature poses a challenge due to Ti ions easily form colloids and result in amorphous products. It is also difficult to ensure the uniform incorporation of the dopants into the lattice during synthesis of doped samples. This study successfully achieved this task using the direct synthesis from solution (DSS) method. Both undoped SrTiO3 nanoparticles and Fe-doped one were synthesized in an ethanol solution at the temperatures ranging from 60 to 80 °C, resulting in homogeneous grains with an average size of approximately 15 nm. Despite the solubility limit of Fe3+ in bulk SrTiO3 being around 2.5 %, we successfully synthesized Fe-doped SrTiO3 nanocrystals with high doping level even at a dose of 10 %.The tribocatalytic performance of both undoped and Fe-doped SrTiO3 nanoparticles was evaluated for methylene blue degradation in a solution. Undoped SrTiO3 nanoparticles showed similar catalytic performance to P25. In contrast, Fe-doped SrTiO3 nanoparticles exhibited much higher catalytic performance. This enhanced catalytic performance is attributed to the presence of oxygen vacancies, which are introduced by doping Fe3+ into the SrTiO3 lattice to substitute the Ti ion.

Conductivity and aging behavior of Sr(Ti0.6Fe0.4)1-x O3-δ mixed conductor materials.

Perovskite materials play a significant role in oxygen sensors due to their fascinating electrical and ionic conductivities. The sol-gel technique was employed to prepare various compositions of B-site-deficient Fe-doped SrTiO3 (iron-doped strontium titanate) or Sr(Ti0.6Fe0.4)1-x O3-δ , where x = 0.01, 0.02, and 0.03. The XRD results revealed that the principle crystalline phase of the samples was the cubic perovskite structure. The B-site deficiency improved the ionic and total conductivities of Sr(Ti0.6Fe0.4)1-x O3-δ . A small polaron conduction behavior occurred in the total electrical conductivity. The XPS results showed that the oxygen vacancy value decreased with the rise in the amount of B-site deficiencies. A lower B-site deficiency amount could produce more oxygen vacancies in the lattice but resulted in the ordering of vacancies and then lower ionic conductivity. The aging behavior was caused by the ordering of oxygen vacancies and resulted in a degeneration of electrical features under a long service time. Conversely, augmentation of the B-site deficiency amount inhibited the tendency for the ordering of oxygen vacancies and then promoted the electrical performance under a long usage time. The conduction mechanism of oxygen ions through oxygen vacancies was thoroughly investigated and discussed. The current study presents a feasible approach to ameliorate the physical features of conductors through doping the B-site of the perovskite layer with Fe, which would be a fruitful approach for numerous applications, including oxygen sensors and fuel cells anodes.

Grain growth and segregation in Fe-doped SrTiO3: Experimental evidence for solute drag

In functional ceramics, the impact of dopants on bulk crystals is generally well understood. Their impact on grain boundaries is less well known. The present study investigates the impact of acceptor dopants on grain growth in strontium titanate. Scanning electron microscopy and analytical (scanning) transmission electron microscopy have been used to gain knowledge on Fe segregation behavior, grain sizes, and grain size distributions of SrTiO3. While undoped microstructures show normal grain growth at low temperatures (<1350 °C), doped microstructures evolve bimodally. With increasing acceptor dopant concentration, an increasing population of small grains develops. It is shown that Fe segregates to the grain boundaries due to its negative charge and a positive boundary potential. Thus, the experimental findings seem to be well explained by the theory of solute drag: The diffusion of segregated defects (‘solutes’) at grain boundaries can retard grain boundary migration.

Role of oxygen vacancy on activity of Fe-doped SrTiO3 perovskite bifunctional catalysts for biodiesel production

In this work, the catalytic performance of the mesoporous SrTi1-xFexO3 (x = 0.05, 0.10, 0.15 or 0.20) for biodiesel production from palm oil with a high acid value is investigated. The physicochemical properties of the catalysts are revealed by various characterization methods, including XRD, N2 adsorption-desorption, XPS, Raman, EPR, SEM-EDS, and CO2/NH3-TPD. Fe doping results in many oxygen vacancies in the perovskite structure, which increases the electron density of the active sites on the surface and promotes the polarization of the perovskite structure, thus improving the catalytic activity of the catalyst. The most oxygen vacancies and the best catalytic performance are observed in the SrTi0.85Fe0.15O3 catalyst, where the FAME yield of 97.52% is achieved with the catalyst concentration of 5 wt% and methanol to oil molar ratio of 18:1 at 150 °C for 3 h. Meanwhile, the catalyst maintains a FAME yield of 83.59% for the fourth reused cycle. Also, the catalyst exhibits strong resistance to FFAs, where the FAME yield is still 93.58% even with oleic acid addition of 12 wt% to show the capability in catalyzing the simultaneous esterification and transesterification.

Structural, Electrical and Magnetic Properties of CE and Fe Doped Srtio3

Structural, Electrical and Magnetic Properties of CE and Fe Doped Srtio3

Photochromism of UV-annealed Fe-doped SrTiO3

High-temperature annealing coupled with above bandgap UV illumination is an emerging approach to manipulate defect chemistries and resultant properties of electroceramics. To explore defect-processing-property relationships in these materials, an advanced multiphysics and multiscale model has been developed, which involves (a) high-fidelity first principles simulations of defect energies, (b) grand canonical thermodynamics of defect equilibria, (c) UV-perturbed defect formation energies from Shockley–Read–Hall generation and recombination, and (d) finite-element analyses of electrostatic potential and defect redistribution. Using this model, bottom-up insights into defect mechanisms associated with the UV-induced brown photochromism of Fe-doped SrTiO3 at high temperatures are provided. It is found that UV illumination leads to dissociation of the FeTi-vO complex and reduction in the oxygen vacancy concentration through exchange with the gas reservoir. Changes to these defect populations cause reionization of the FeTi defect from −1 to 0 charge state to maintain charge neutrality. This collectively gives rise to an increased concentration of FeTi0, which is the source of brown chromism. In addition, this model reproduces the experimentally observed electrical resistance degradation of samples annealed in this manner due to the increasing hole concentration in the material with time. The present model itself offers a route to guide and facilitate future efforts in this field.

Effect of Schottky barrier height lowering on resistance degradation of Fe-doped SrTiO3 thin-film capacitor

It is generally believed that the resistance degradation behavior of bulk and thin-film oxide capacitors arises from the oxygen vacancy migration within the oxide and/or the charge injection at the oxide/electrode interface. The magnitude of the degradation in the resistance has been theoretically studied in the literature by solving the electrochemical transport equations while assuming constant Schottky barrier height. The treatment of constant Schottky barrier height in existing models has led to significant underestimation of the resistance degradation. In this work, I incorporated the dependence of Schottky barrier height on the oxygen vacancy concentration at the interface into the existing model to simulate the degradation process in thin-film oxide capacitors. With the consideration of Schottky barrier height lowering from the interface dipole arising from the accumulation of oxygen vacancies at the cathode interface, I found that the leakage current can be increased by more than one order of magnitude, which is more consistent with experimental observations in comparison to the prediction from existing models.

Cation non-stoichiometry in Fe:SrTiO3 thin films and its effect on the electrical conductivity.

The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition. Structural information was obtained by reciprocal space mapping while solution-based inductively-coupled plasma optical emission spectroscopy and positron annihilation lifetime spectroscopy were employed to reveal the cation composition and the predominant point defects of the thin films, respectively. A severe cation non-stoichiometry with Sr vacancies was found in films deposited from stoichiometric targets. The across plane electrical conductivity of such epitaxial films was studied in the temperature range of 250–720 °C by impedance spectroscopy. This revealed a pseudo-intrinsic electronic conductivity despite the substantial Fe acceptor doping, i.e. conductivities being several orders of magnitude lower than expected. Variation of PLD deposition parameters causes some changes of the cation stoichiometry, but the films still have conductivities much lower than expected. Targets with significant Sr excess (in the range of several percent) were employed to improve the cation stoichiometry in the films. The use of 7% Sr-excess targets resulted in near-stoichiometric films with conductivities close to the stoichiometric bulk counterpart. The measurements show that a fine-tuning of the film stoichiometry is required in order to obtain acceptor doped SrTiO3 thin films with bulk-like properties. One can conclude that, although reciprocal space maps give a first hint whether or not cation non-stoichiometry is present, conductivity measurements are more appropriate for assessing SrTiO3 film quality in terms of cation stoichiometry.

Electrochemical drag effect on grain boundary motion in ionic ceramics

The effects of drag imposed by extrinsic ionic species and point defects on the grain boundary motion of ionic polycrystalline ceramics were quantified for the generality of electrical, chemical, or structural driving forces. In the absence of, or for small driving forces, the extended electrochemical grain boundary remains pinned and symmetrically distributed about the structural interface. As the grain boundary begins to move, charged defects accumulate unsymmetrically about the structural grain boundary core. Above the critical driving force for motion, grain boundaries progressively shed individual ionic species, from heavier to lighter, until they display no interfacial electrostatic charge and zero Schottky potential. Ionic p–n junction moving grain boundaries that induce a finite electrostatic potential difference across entire grains are identified for high velocity grains. The developed theory is demonstrated for Fe-doped SrTiO3. The increase in average Fe concentration and grain boundary crystallographic misorientation enhances grain boundary core segregation and results in thick space charge layers, which leads to a stronger drag force that reduces the velocity of the interface. The developed theory sets the stage to assess the effects of externally applied fields such as temperature, electromagnetic fields, and chemical stimuli to control the grain growth for developing textured, oriented microstructures desirable for a wide range of applications.

Induction and control of dielectric relaxation properties in Fe-doped nonstoichiometric SrTiO3 ceramics

The dielectric properties of Fe-doped Ti-rich SrTiO3 ceramics at both A and B sites were investigated. For A site doping, we found one structural phase transition associated with the substitution of smaller Fe ions, and two sets of dielectric relaxations ascribed to oxygen vacancies and hopping conduction between Fe2+ and Fe3+, respectively. Cole-Cole relation shows that both thermally activated dielectric relaxation behaviors mainly originate from the grain boundary. However, for B site doping, they are not observed in the measured temperature range since both the short-range diffusion of oxygen vacancies and electron conduction become the long-range migration, which indicates that the additional conductive channels are opened when Fe ion doping changes from A to B site. The results provide an experimental basis for adjusting dielectric properties in paraelectric materials.

Effect of deposition temperature on ultra-low voltage resistive switching behavior of Fe-doped SrTiO3 films

The effect of deposition temperature on the microstructures and resistive switching properties of Fe-doped SrTiO3 (Fe-STO) films deposited via magnetron sputtering has been investigated. The as-deposited Fe-STO films change from amorphous to polycrystalline when the deposition temperature increases to 600 °C, but 800 °C-deposited Fe-STO films exhibit cracked surface morphologies with Sr-rich nanosheet segregation. Fe-STO films deposited at ≤600 °C exhibit reversible bipolar resistive switching behaviors with ultra-low switching voltages of &amp;lt;±0.6 V, while 450 °C-deposited Fe-STO films retain an ON/OFF resistance ratio of ∼105 after more than 2500 endurance cycles and 600 °C-deposited Fe-STO films exhibit three different resistive switching patterns in sequence. Fe-assisted oxygen-vacancy conductive filaments are responsible for the ultra-low voltage resistive switching behaviors of Fe-STO films.

Direct, spatially resolved observation of defect states with electromigration and degradation of single crystal SrTiO3

Laterally and depth-resolved cathodoluminescence spectroscopy (DRCLS) provided direct, nanoscale measurements of oxygen vacancy and oxygen vacancy complex distributions in undoped and Fe-doped SrTiO3 with high temperature electric field stress associated with dielectric resistance degradation. DRCLS provided direct and spatially resolved observation of oxygen vacancy migration driven by external electric fields from the anode to the cathode in undoped SrTiO3 between laterally separated electrodes, resulting in increased current leakage and lower thermal breakdown strength. DRCLS measurements through planar Pt electrodes after high temperature electric field cycling reveal pronounced oxygen vacancy depletion within the surface space region of the Pt/SrTiO3 Schottky barrier as predicted theoretically. These results provide a direct insight into the transient states impacting the conduction during the electromigration of the oxygen vacancies. The deconvolution of different peaks and their intensity variations relative to the direct bandgap provide methods to gauge the relative defect energetics of these gap states. These data are discussed in relation to providing a tool to further understand conduction in mixed ionic conductors.

Water incorporation into materials with three mobile carriers: Two-fold relaxation of the electromotive force in Fe-doped SrTiO3 and importance of hole trapping

In oxides which exhibit electronic, protonic and oxide ion conductivity, water incorporation can lead to a kinetic build-up of redox gradients. We use Fe-doped SrTiO3, the defect chemistry of which is very well understood to follow the thus generated electromotive force (EMF) as a function of time. In contrast to previous works we include trapping effects in the kinetic description. The agreement between the kinetic modelling and the experimental results is remarkable, given the complexity of the process.

Inhomogeneity and Segregation Effect in the Surface Layer of Fe-Doped SrTiO3 Single Crystals

The effect of Fe doping on SrTiO3 single crystals was investigated in terms of crystal and electronic structure over a wide temperature range in both oxidizing and reducing conditions. The electrical properties were thoroughly studied with a special focus on the resistive switching phenomenon. Contrary to the undoped SrTiO3 crystals, where isolated filaments are responsible for resistive switching, the iron-doped crystals showed stripe-like conducting regions at the nanoscale. The results showed a non-uniform Fe distribution of as-received crystals and the formation of new phases in the surface layer of reduced/oxidized samples. The oxidation procedure led to a separation of Ti(Fe) and Sr, while the reduction resulted in the tendency of Fe to agglomerate and migrate away from the surface as seen from the time of flight mass spectroscopy measurements. Moreover, a clear presence of Fe-rich nano-filament in the reduced sample was found.

Emergent magnetic phase transitions in Fe-doped SrTiO3−δ

In defect engineering, both cation doping and oxygen vacancies play key roles in deciding the properties of oxide, and the utilization of their cooperation has attracted much interest in recent years. Here, we report an emergent magnetic phase transition near 18 K in Fe-doped SrTiO3−δ by utilizing the magnetic interactions between the doped Fe cations and oxygen vacancies. The effects of Fe dopants and oxygen vacancies on the structural and magnetic properties were characterized by a high-resolution X-ray diffraction, Raman spectroscopy, and superconducting quantum interference device. In particular, as the temperature rises across the magnetic phase transition, the coercivity of Fe-SrTiO3−δ decreases from ∼7700 Oe at 2 K to ∼104 Oe at 19 K. Our results of creating emergent magnetic phases with the coeffects of both cation dopants and oxygen vacancies could pave a way to inducing novel quantum states in epitaxial films on Fe-SrTiO3−δ single crystal substrates with the magnetic proximity effect.

Effect on the Chemical, Structural and Magnetic Characteristics on Fe-Doped SrTiO³.

SrTiO³ nanopowders doped with various percentages of Fe (0, 1, 3, and 5 mol%) was synthesized, determining its chemical and structural properties using X-ray diffraction and X-ray photoelectron spectroscopy. The magnetic properties of the samples were studied using hysteresis curves obtained by an alternating gradient magnetometer. Theoretical calculations within the framework of the density functional theory are also performed to support experimental data. The results indicated that Fe incorporated into de SrTiO³ structure induced intrinsic magnetic properties, which increase as the Fe percentage increase. At 5 mol% of Fe in the sample showed a single-domain state, typical of a ferromagnetic material capable for information storage. This latter result would be due to the reduction of oxygen vacancies as more Fe was added and to the presence of Fe2+ and Fe3+ ions reinforced the magnetic behaviour of the compound.

Study on Defects in Fe-Doped SrTiO3 by Positron Annihilation Lifetime Spectroscopy

SrTi1−xFexO3−δ ceramics were prepared using a traditional solid-state reaction method. From X-ray diffraction (XRD) result, we found that the doped Fe3+ dissolved in the lattice, and no secondary phase was observed. Cation vacancies in perovskite oxides were identified via positron annihilation lifetime spectroscopy (PALS) measurements. Undoped and Fe-doped SrTiO3 ceramics and single-crystal SrTiO3 were measured by PALS at room temperature. The results show that the main defects in undoped SrTiO3 ceramics are Ti-related defects, and the isolated Ti vacancy lifetime is about 183.4 ps. With the increase of Fe3+, the concentration of the Ti vacancies decreases accompanied by the appearance of the V″Sr − nV o ∙∙ (defect association of Sr vacancies and multiple O vacancies) vacancy defect complexes.

Electrical properties of Fe-doped SrTiO3 with B-site-deficient for SOFC anodes

Sr(Ti0.6Fe0.4)1-xO3-δ precursors were prepared through the sol–gel method. The samples sintered in air at 1350 °C for 300 min were characterized in terms of conductivity, bulk density, microstructure and phase analysis via AC impedance, scanning electron microscope (SEM), Archimedes’ method and X-ray diffraction (XRD). The total electrical conductivity of Sr(Ti0.6Fe0.4)1-xO3-δ samples can be remarkably improved by B-site-deficient level (x). When x = 0.07, the total electrical conductivity reaches a maximum of 0.3445 S/cm at 800 °C. The microstructure of the sample becomes more porous as the B-site-deficient level increases. The relative density of samples is becoming lower with B-site-deficient level increases. Diffraction peaks without impurities appear in the X-ray diffraction pattern, indicating the formation of a single perovskite structure. In addition, the Sr(Ti0.6Fe0.4)1-xO3-δ/YSZ samples show a great chemical stability and compatibility.

Room temperature multiferroic properties of Fe-doped nonstoichiometric SrTiO3 ceramics at both A and B sites

This letter reports the structural, dielectric, ferroelectric, and magnetic properties of Fe substituted nonstoichiometric STO ceramics. X-ray diffraction and Scanning electron microscope reveal the microstructure of the samples. The room temperature multiferroicity of the Fe-doped Ti-rich STO ceramics is successfully realized by M-H hysteresis loop, P-E hysteresis loop and magnetodielectric measurement. The origins of ferroelectricity may be related to the lattice distortions and antisite-like defects with net local dipoles. The ferromagnetism are mainly contributed by the competitions among the double exchange interactions, super-exchange interactions and the non-linear ferromagnetic exchange coupling. In addition, the ordered spin arrangement of the ferromagnetism is formed by the exchange interaction of Fe atom using first principle simulations.

Detection of Nanoscale Structural Defects in Degraded Fe-Doped SrTiO3 by Ultrafast Photoacoustic Waves

Strontium titanate, SrTiO3, has been intensively investigated as a model material in defect chemistry research. The underlying mechanism of the effects associated with a large variety of defects often requires microstructure imaging. In the present work, the distribution of nanoscale structural defects in electrodegraded Fe-doped SrTiO3 (Fe:STO) single crystals is directly revealed by ultrafast photoacoustic waves. We utilized time-resolved reflectance spectra to nondestructively characterize local structural distortions near the degraded anode and cathode interfaces in both the reduced and oxidized crystals along with transmission electron microscopy to image these defects. We show that an accumulation of oxygen vacancies resulted in significant structural deformations near the degraded cathode interface of the reduced crystal. The defect distribution shows a strong dependence on oxygen vacancy concentration and diffusion within the crystals.