SrTiO3 Samples Research Articles

SrTiO3 nanoparticles and nanofibers: synthesis and comparison of photocatalytic properties

SrTiO3 nanofibers and nanoparticles were respectively synthesized by electrospinning process and sol–gel method. The obtained SrTiO3 photocatalysts were characterized by X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Barrett–Emmett–Teller technique, and spectrophotometer. The X-ray diffraction patterns show that SrTiO3 samples have the pure perovskite-type crystalline phase. The scanning electron microscopy and transmission electron microscopy images indicate that SrTiO3 nanofibers and nanoparticles were respectively obtained by electrospinning process and sol–gel method. SrTiO3 nanofibers show the higher specific surface area and photocurrent than those of SrTiO3 nanoparticles. The photocatalytic activities of SrTiO3 nanofibers and nanoparticles were evaluated by photocatalysis degradation of Rhodamine B and Methyl orange. The SrTiO3 nanofibers exhibit higher photocatalytic activity than SrTiO3 nanoparticles, which is induced by the smaller crystallite size and hierarchical structure of SrTiO3 nanofibers.

Tuning Up or Down the Critical Thickness in LaAlO3 /SrTiO3 through In Situ Deposition of Metal Overlayers.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 and SrTiO3 has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LaAlO3 thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. In this paper, the chemical, electronic, and transport properties of LaAlO3 /SrTiO3 samples capped with different metals grown in a system combining pulsed laser deposition, sputtering, and in situ X-ray photoemission spectroscopy are investigated. The results show that for metals with low work function a q2DES forms at 1-2 uc of LaAlO3 and is accompanied by a partial oxidation of the metal, a phenomenon that affects the q2DES properties and triggers the formation of defects. In contrast, for noble metals, the critical thickness is increased above 4 uc. The results are discussed in terms of a hybrid mechanism that incorporates electrostatic and chemical effects.

Kinetics of the Hydrogen Evolution Reaction at Rh-Doped SrTiO3 Driven By Visible Light: A Study By Photoelectrochemical Impedance Spectroscopy

Because of the gradual exhaustion of carbon-based fuels and the problematic issues associated with their combustion, modern human societies are urged to find alternative, sustainable and renewable energy sources.1 The enormous amount of radiative energy arriving to earth from sun appears as an excellent alternative source of primary energy. In this context, the use of semiconducting materials that can absorb visible light and transform solar radiations into clean fuels is a topic of significant scientific interest. In particular, the hydrogen evolution reaction (HER) that produces hydrogen from earth-abundant water is a widely targeted reaction.2 In electrochemistry, the HER is a multistep process for which a large variety of efficient electrocatalysts, operating in different media and at different pH, have already been developed. But the kinetics of water photo-reduction at photocathodes involves different reaction steps and still requires improvement. In this communication, we report on the kinetics of water photo-reduction using Rh:SrTiO3 as light-absorbing semiconductor. The charge transfer (kt ) and recombination (kr ) rate constants have been determined in order to unravel the heterogeneous mechanism for the HER that prevails at Rh:SrTiO3 / electrolyte interfaces. Rh:SrTiO3 samples were prepared by calcinations of appropriate precursors. The X-ray diffraction pattern of the semiconducting powder confirmed the perovskite structure of SrTiO3 as a pure phase. Though, the presence of Rh as the dopant could not be put in evidence by XRD, an indication that the substitution of Rh4+ atoms by Ti4+ in the crystal lattice did not distort appreciably the structure of the solid. Further characterization was performed by X-ray photoelectron spectroscopy (XPS), and the presence of Rh in the sample was clearly established. The band structure of the photoelectrode was characterized by UV-Vis and Diffuse Reflectance spectroscopies, suggesting a band gap of 2.7 eV as reported before.3 Linear sweep voltammetry at different levels of illumination (λ > 450 nm) showed an increase in cathodic currents upon illumination, which expressed the role of the device as a photocathode. Subsequently, the photoelectrode was characterized using photoelectrochemical impedance spectroscopy (PEIS). The shape of PEIS-diagrams was found to change upon potential cycling. This may be due to either the accumulation of reactive intermediates at the surface of the photoelectrode or to the reduction of Rh4+ to Rh3+ in the material that causes the catalytic activity of the material. PEIS-diagrams then attained a stable shape which permitted to perform a full kinetic analysis. It was observed that kt was independent of light intensity and that kr increased slightly with increasing light power. Conversely, kt increased exponentially while the electrode potential became more negative. Such fact indicated the presence of surface states that caused Fermi level pinning. As a result, an important part of the applied potential dropped in the Helmholtz layer. The Tafel plot of ln kt vs. applied potential exhibited a slope of 93 mV/dec (α = 0.28), which suggested that water discharge at the surface (Volmer reaction, H2O + e- + M = M-H + OH-) was the rate determining step. Overall, a thorough kinetic determination of the HER, photo-assisted by visible light, has been performed and details will be reported during the Meeting.

Nonthermal effects in photostimulated solid state reaction of Mn doped SrTiO3

The effect of a photostimulated solid state reaction was investigated in Mn doped SrTiO3 samples. Light irradiation was performed by either halogen or UV lamps in order to study the effect of the spectral composition, and the results were compared with samples prepared at the same temperatures in a conventional furnace. The obtained samples were studied by X-ray diffraction for structural characterization and by Electron Paramagnetic Resonance, which provides microscopic information about the local environment as well as the valence state of Mn ions. It was found that light irradiation significantly enhances the solid state reaction rate compared to synthesis in the conventional furnace. Moreover, it was observed that UV lamp irradiation is much more effective compared to halogen lamps. This indicates that the absorption of light with energy larger than the materials band gap plays an important role and points towards the nonthermal mechanism of the photostimulated solid state reaction.

STUDY OF STRUCTURAL AND SPECTRAL CHARACTERISTICS OF CRYSTAL Y3Al5O12, Al2O3 AND SrTiO3 DOPED BY 3d- OR 4f- IONS

The subject of our research is a study of the effects of various treatments on the structural and spectral characteristics of oxides (Y3Al5O12, Al2O3 and SrTiO3) belonging to the group of ionic crystals with a wide energy band gap. We have investigated the dielectric and optical characteristics of single laser crystals of Y3Al5O12 (YAG) doped by ions from the group of iron (3d-), Al2O3 single crystal and SrTiO3 crystals doped by 3d and 4f- ions. Dielectric properties of the investigated single crystals were analyzed using dielectric spectroscopy (80 kHz – 15 MHz) in a wide temperature range (50 – 400 K). The comparison of the temperature dependence of the dielectric permittivity (εr) and dielectric loss (tanδ) at different test frequencies for undoped and doped crystals was performed. The optical properties were studied on the basis of the absorption spectra in the UV-Vis region. The experiment results showed that presence of 3d dopants in YAG crystal caused a displacement of the absorption edge and appearance of characteristic peaks. The differences observed in the absorption spectra were probably the result of the 3d ions incorporation on two possible symmetrically different positionings of aluminum ions –tetrahedrical and octahedrical– in the YAG crystal lattice. The dielectric spectroscopy measurements for samples of SrTiO3 doped by 4f ions showed anomalous behaviors of the dielectric permittivity at temperatures around the temperature of the structural phase transition 105 K.

Changes in local surface structure and Sr depletion in Fe-implanted SrTiO3 (001)

Local surface structure of single crystal strontium titanate SrTiO3 (001) samples implanted with Fe in the range of concentrations between 2×1014 to 2×1016Fe/cm2 at 30keV has been investigated. In order to facilitate Fe substitution (doping), implanted samples were annealed in oxygen at 350°C. Sr depletion was observed from the near-surface layers impacted by the ion-implantation process, as revealed by Rutherford Backscattering Spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), X-ray Absorption Near Edge Spectroscopy (XANES), and Atomic Force Microscopy (AFM). Hydrocarbon contaminations on the surface may contribute to the mechanisms of Sr depletion, which have important implications for Sr(Ti1-xFex)O3-δ materials in gas sensing applications.

The synthesis of nano-sized undoped, Bi doped and Bi, Cu co-doped SrTiO3 using two sol–gel methods to enhance the photocatalytic performance for the degradation of dibutyl phthalate under visible light

Two sol–gel methods (the citric acid gel and the Pechini methods) were used for the preparation of nano-sized undoped, Bi-doped and Bi, Cu co-doped SrTiO 3 samples to optimize their properties for the photocatalytic degradation of dibutyl phthalate. The perovskite-like phase was detected for the samples prepared by the citric acid gel method after calcination at 800 °C for 3 h, while in the case of the Pechini method; it was detected after calcination at 800 °C for 9 h. The particle size of the samples prepared by the citric acid gel method is greater than that of the samples prepared by the Pechini method. Cu doping in both methods increased the particle size. Cu doping, Bi doping and Bi, Cu co-doping in both methods shifted the absorption edge to the visible light range as well. The band gap of Bi, Cu co-doped SrTiO 3 is smaller than that of Bi doped SrTiO 3 , which in turn is smaller than that of undoped SrTiO 3 . The highest removal of the total organic carbon (TOC) of DBP was obtained using a Bi, Cu co-doped SrTiO 3 sample prepared by the citric acid gel method. TOC removal of DBP followed pseudo-first order kinetics.

Dielectric response of SrTiO3–SrMg1/3Nb2/3O3 solid solutions in the terahertz–infrared range

The reflection and transmission spectra of ceramic samples of SrTiO3–SrMg1/3Nb2/3O3 solid solutions have been measured in the frequency range of 5–5000 cm–1 and in the temperature range of 5–370 K. Based on these spectra, the spectra of the real e'(ν) and imaginary e''(ν) parts of the complex permittivity e*(ν) have been simulated by the method of dispersion analysis. It has been found that the temperature evolution of the dielectric constant is entirely determined by the behavior of the soft mode.

Conductivity and structure of sub-micrometric SrTiO3–YSZ composites

Sub-micrometric composites of SrTiO3–YSZ (1:1 volume) and samples of SrTiO3 were prepared by high temperature consolidation of precursors obtained by precipitation with NaOH. The structure development and morphology of the precursors were studied by XRD and SEM. The perovskite and fluorite phases in the composites are clearly formed at 600°C with no signs of reaction up to 1100°C; the nominally pure SrTiO3 can be formed at temperatures as low as 400°C. Composites with sub-micrometric grain sizes can be prepared successfully without reaction between the components, although a change in the cell parameter of the SrTiO3 is attributed to the presence of Na.The consolidated composites were studied by impedance spectroscopy between 200 and 400°C and at a fixed temperature of 600°C with a scan in the partial pressure of oxygen. The composites did not exhibit high levels of ionic conductivity in the grain boundary nor the bulk. The conductivity of Na-free composites shows lower levels of conductivity than pure YSZ, while samples with Na showed increased conductivity. The conductivity of SrTiO3 exhibited an enhancement attributed to p-type conductivity, although contributions from protons cannot be disregarded as some Na doping is present.

STUDY OF STRUCTURAL AND SPECTRAL CHARACTERISTICS OF CRYSTAL Y3Al5O12, Al2O3 AND SrTiO3 DOPED BY 3d- OR 4f- IONS

The subject of our research is a study of the effects of various treatments on the structural and spectral characteristics of oxides (Y3Al5O12, Al2O3 and SrTiO3) belonging to the group of ionic crystals with a wide energy band gap. We have investigated the dielectric and optical characteristics of single laser crystals of Y3Al5O12 (YAG) doped by ions from the group of iron (3d-), Al2O3 single crystal and SrTiO3 crystals doped by 3d and 4f- ions. Dielectric properties of the investigated single crystals were analyzed using dielectric spectroscopy (80 kHz – 15 MHz) in a wide temperature range (50 – 400 K). The comparison of the temperature dependence of the dielectric permittivity (er) and dielectric loss (tanδ) at different test frequencies for undoped and doped crystals was performed. The optical properties were studied on the basis of the absorption spectra in the UV-Vis region. The experiment results showed that presence of 3d dopants in YAG crystal caused a displacement of the absorption edge and appearance of characteristic peaks. The differences observed in the absorption spectra were probably the result of the 3d ions incorporation on two possible symmetrically different positionings of aluminum ions –tetrahedrical and octahedrical– in the YAG crystal lattice. The dielectric spectroscopy measurements for samples of SrTiO3 doped by 4f ions showed anomalous behaviors of the dielectric permittivity at temperatures around the temperature of the structural phase transition 105 K.

Large Persistent Photoconductivity in Strontium Titanate at Room Temperature

ABSTRACTStrontium titanate (SrTiO3) is a wide-band-gap semiconductor with a variety of novel properties. In this work, bulk single crystal SrTiO3 samples were heated to 1200°C, resulting in the creation of point defects. These thermally treated samples showed large persistent photoconductivity (PPC) at room temperature. Illumination with sub-gap light (>2.9 eV) caused an increase in free-electron concentration by over two orders of magnitude. After the light is turned off, the conductivity persists at room temperature, with essentially zero decay over several days. The results of electron paramagnetic resonance (EPR) measurements suggest that a point defect is responsible for PPC because the photo-induced response of one of the EPR signals is similar to that seen for the PPC. Due to a large barrier for recapture, the photo-excited electron remains in the conduction band, where it contributes to the conductivity.

Bifunctional-nanotemplate assisted synthesis of nanoporous SrTiO₃ photocatalysts toward efficient degradation of organic pollutant.

Nanoporous SrTiO3 photocatalysts were fabricated via a novel technique, the nanotemplate assisted sol-gel hydrothermal reaction. In the alkaline-environment hydrothermal reaction, the SiO2 nanotemplate not only served as pore generator but also worked on adjusting the local reaction environment around the SrTiO3 nanocrystals. This contributed to a continuous modulation between the surface area and the crystallinity of the photocatalyst. The photocatalytic activities of the nanoporous SrTiO3 samples were evaluated by the degradation of gaseous isopropyl alcohol (IPA). Due to an optimal equilibrium between surface area and crystallinity, a SrTiO3 sample synthesized via adding 40% template (STO-SiO2-40%) showed the highest activity, which achieves 40 and 8 times of enhancement of CO2 evolution in comparison with the sample prepared without template and a commercial nano-SrTiO3, respectively. The photodegradation mechanism of IPA over this sample was also investigated in detail. This synthetic technique is also available to prepare the other nanoporous titanates, such as doped SrTiO3 samples and alkali-metal titanates.

Oxygen vacancy enhanced photocatalytic activity of pervoskite SrTiO(3).

A facile and general method has been developed to fabricate oxygen vacancies on perovskite SrTiO3 (STO) nanocrystals through a controllable solid-state reaction of NaBH4 and SrTiO3 nanocrystals. STO samples with tunable color, oxygen vacancy concentration on nanocrystal surface have been synthesized. TEM results reveal that these STO samples have a crystalline core/amorphous shell structure (SrTiO3@SrTiO3-x). XPS and EPR results disclose that the oxygen vacancy concentration increases with the increase of reaction time and temperature. The concentration of oxygen vacancies calculated from TGA data, could reach 5.07% (atom) in this study. UV-vis spectra and photocatalytic results indicate that oxygen vacancies on STO surface play an important role in influencing the light absorption and photocatalytic performance. However, an excess amount of oxygen vacancies leads to a decrease of photocatalytic performance. The optimal photocatalytic activity for H2 production under UV-vis irradiation is up to 2.2 mmol h(-1) g(-1), which is about 2.3 times than the original SrTiO3, corresponding to 3.28% (atom) of oxygen vacancy concentration.

Correlation between structural and electrical properties in highly porous (Y,Sr)(Ti,Nb)O3−δ SOFC anodes

Abstract In order to find a relationship between structural and electrical properties, niobium and yttrium doped SrTiO3 ceramics were prepared via solid-state reaction. The samples were sintered in hydrogen and air conditions. The samples were also fabricated with a pore-former to obtain highly porous specimens. The electrical properties of Nb-doped SrTiO3 samples and yttrium and niobium co-doped SrTiO3 were compared. The comparable electrical properties were observed and discussed according to previous literature reports. It was noticed that the synthesis in a reducing hydrogen atmosphere can increase the solubility of dopants. Moreover, the samples sintered in air presented lower conductivity level and worse structural properties than the samples sintered in hydrogen. The explanation of obtained results was also suggested and discussed.

Dielectric investigations of solid solutions SrTiO3-KTaO3 and SrTiO3-KNbO3

The temperature dependences of the dielectric constant and dielectric hysteresis loops in ceramic samples of (1 − x)SrTiO3−xKNbO3 and (1 − x)SrTiO3−xKTaO3 (0 ≤ x ≤ 0.3) solid solutions prepared using different heat treatments have been investigated. Phase diagrams of the studied solid solutions have been constructed in the T-x coordinates. It has been shown that, after quenching of samples (spontaneous cooling at room temperature after long-term heating at the sintering temperature of the ceramic samples), the temperature of the induced phase transition increases because of the weakening of random electric fields associated with nonisovalent impurities due to their “frozen” nonequilibrium redistribution. For small concentrations x, strong dielectric relaxation is observed in the temperature range of 150–250 K. A model of relaxing centers, which is based on the local charge compensation of heterovalent impurities, has been proposed.

Persistent Photoconductivity in Bulk Strontium Titanate

ABSTRACTStrontium titanate (SrTiO3) has novel properties, including a large temperature-dependent dielectric constant, and can be doped to make it metallic or even superconducting. The origin of conductivity observed at the SrTiO3/LaAlO3 interface is a topic of intense debate. In the present work, bulk single crystal SrTiO3 samples were heated at 1200°C, with the goal of producing cation vacancies. These thermally treated samples exhibited persistent photoconductivity (PPC) at room temperature. Upon exposure to sub-band-gap light (>2.9 eV), the free-electron density increases by over two orders of magnitude. This enhanced conductivity persists in the dark, at room temperature, for several days with essentially no decay. Light excites an electron from the vacancy to the conduction band, where it remains, due to a large recapture barrier. These observations highlight the importance of defects in determining the electrical properties of oxides and may point toward novel applications.

Electroforming of Fe:STO samples for resistive switching made visible by electrocoloration observed by high resolution optical microscopy

ABSTRACTResistively switching devices have attracted great attention for potential use in future nonvolatile information storage. Among various oxide materials that show resistive switching (RS) behavior, SrTiO3 (STO) is regarded as a model material to study the effect of valence changes accompanying RS in the oxide [1]. In this class of materials, the RS effect is attributed to rely on the migration of oxygen vacancies and an associated valence change in the cation sublattice. To achieve a switchable state, an initial electroforming step is typically required, which is believed to create conductive regions in the insulating material [2]. Under high electrical stress, an oxygen-deficient region, often referred to as the virtual cathode (VC), is formed [3]. The RS occurs across a very short distance between the VC and the anode, allowing for very short switching times. As the electroforming step greatly impacts the device performance and switching variability, its understanding is essential for device optimization. Electroforming is affected by multiple parameters, e.g. voltage, current, temperature, dopant and defect concentrations, ambient gas atmosphere and time. Distinguishing the influence of the particular parameters is a desirable aim and challenging task. Electrocoloration of Fe-doped STO single crystals has proven a valuable means to visualize valence changes of the Fe ions and is thus suitable to study the formation of the VC. Therefore, we performed electrocoloration experiments and used high resolution transmission light optical microscopy to make the redoxprocesses during electroforming visible. The influence of process driving parameters on the evolution of the VC region is studied. The evolution of the VC is interpreted by drift-diffusion simulation of the time evolution of the oxygen vacancy distribution.

Milling-induced phenomena in SrTiO3

The effect of milling was studied for the SrTiO3 samples. We show that by solid state reaction and subsequent milling it is possible to produce SrTiO3 powders with high specific surface area exceeding 10m2/g. However, grinding process induced changes in chemical composition and electronic structure of surface layers of SrTiO3 crystal grains. The changes are discussed in the context of kinetic demixing phenomena and partial amorphization of the surface region.

Electrospun SrTiO3 nanofibers for photocatalytic hydrogen generation

Abstract

Hydrothermal Synthesis and Characterization of Nb<sup>5+</sup>-Doped SrTiO<sub>3</sub> Powders

Nb5+-doped SrTiO3 (STO) particles were synthesized by a simple hydrothermal reaction at 150 °C for 4h. It was found that the main diffraction peaks of Nb5+-doped STO samples with different dosage of dopants shifted gradually to low angles, indicating the little lattice parameter expansions which result from the partial substitution of Ti4+ (0.061 nm) by Nb5+ (0.064 nm) in perovskite structure. From the lattice parameters, the max Nb5+ dopant concentration is about 2 mol%, and the particles obtained were 200 nm-sized and monodispersed.