SrTiO3 Nanocrystals 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.

A mechanistic study on efficient room temperature sonocolloidal obtainment of SrTiO3 nanocrystals

Titanate-based perovskite powders are the most widely used materials for fabrication of multilayered ceramic capacitors and thus are of significant importance. They have been synthesized by two approaches including solid-state and wet chemical processing methods. Synthesis temperature, cost of preparation and quality of synthesized powders are key factors favoring the process. Here, a new ultrasound-assisted precipitation approach is introduced which is able to obtain the quality perovskite nanocrystals such as strontium titanate, barium titanate and so on at room temperature. The obtained powder products were characterized by DTA/TGA, FT-IR, XRD, SEM, EDAX and TEM. The results indicate the room temperature formation of highly pure monosized spherical nanoscale crystals which are loosely agglomerated in nature. It is tried to add the science, mechanism involved in the synthesis and its reaction pattern to the literature along with discussing the cause of low temperature obtainment of the perovskite powder products. The approach described in the current work has the capability to be a general strategy for the synthesis of perovskite materials and essential potentials for large-scale production purposes as it is facile, low temperature and cost-effective.

Harnessing of low-energy IR photons via oxygen-vacancies in SrTiO3 nanocrystals for photocatalytic hydrogen production

The surface-oxygen vacancies were generated to enhance the near-IR light response of the SrTiO3 photocatalyst by heating SrTiO3 in a highly reducing environment. The UV–visible diffuse reflectance spectra of SrTiO3-Ov revealed strong absorption between 400 and 800 nm, while IPCE response was extended beyond 800 nm, confirming the near-IR light response of SrTiO3-Ov. The hydrogen production rates for SrTiO3-Ov at pH 3.0 under full solar spectrum, visible and IR radiation were 100, 25 and 10 μlg−1h−1, respectively. The respective normalized hydrogen with respect to light intensity were found to be 100, 50 and 33 μlg−1h−1 while hydrogen production of pristine SrTiO3 was less than 2 μlg−1h−1 under full solar spectrum. The mid-gap Ov and Ti3+ states in SrTiO3-Ov are key factors in the origin of the near-IR responses of the catalyst and the feasibility of using low-photonic-energy IR light to produce hydrogen by water splitting was demonstrated.

Bio-inspired high-efficiency photosystem by synergistic effects of core-shell structured Au@CdS nanoparticles and their engineered location on {001} facets of SrTiO3 nanocrystals

Natural photosynthesis, which provides a green and high-efficiency energy conversion path by spatial separation of photogenerated carriers through combined actions of molecules ingeniously arranged in an efficient solar nanospace, highlights the importance of rational nanostructure design to realize artificial high-efficiency photosystem. Inspired by these unique features, we constructed a high-efficiency ternary photosystem by selectively decorating the {001} facets of 18-facet SrTiO 3 with Au@CdS photosensitizers via a green photo-assisted method. Benefiting from the dual-facilitated charge carriers transportation in core-shell structured Au@CdS heterojunction and well-faceted 18-facet SrTiO 3 nanocrystal, such a photocatalyst could realize the effective spatial separation of photogenerated electrons and holes. As expected, the 18-facet SrTiO 3 /Au@CdS photocatalyst exhibits superior activity in visible-light-driven photocatalytic hydrogen evolution (4.61 mmol h −1 g −1 ), 166% improvement in comparison with randomly deposited Au@CdS (1.73 mmol h −1 g −1 ). This work offers new insight into the development of green and high-efficiency photocatalytic systems based on the rational nanostructure design by crystal facet engineering.

Enhanced photocatalytic overall water splitting by tuning the relative concentration ratio of bulk defects to surface defects in SrTiO3

Defects such as oxygen vacancies play an important role in the photoactivation processes. However, the roles of surface and bulk defects in photocatalytic water splitting are still ambiguous. This paper would report a new method to introduce surface and bulk defects in SrTiO3 nanocrystals. Melamine was used as a reducing agent which would be carbonized under the N2 atmosphere. During thermal decomposition of melamine, reductive NH3 and C would generate in succession to reduce the SrTiO3 nanocrystals. The relative concentration ratio of bulk defects to surface defects in SrTiO3 nanocrystals would be tuned from 0.78 to 2.55 after reduction treatment. Furthermore, the band structures of surface and bulk defects in SrTiO3 were studied by DFT. The surface oxygen defects would make the band structure shift to a high energy level, while the bulk oxygen defects of SrTiO3 would restore the energy levels of the band structure. Thus, photocatalytic overall water splitting activity would improve significantly by rational relative concentration ratio of bulk defects to surface defects in SrTiO3.

Water Splitting on Multifaceted SrTiO3 Nanocrystals: Calculations of Raman Vibrational Spectrum.

Various photocatalysts are being currently studied with the aim of increasing the photocatalytic efficiency of water splitting for production of hydrogen as a fuel and oxygen as a medical gas. A noticeable increase of hydrogen production was found recently experimentally on the anisotropic faces (facets) of strontium titanate (SrTiO3, STO) nanoparticles. In order to identify optimal sites for water splitting, the first principles calculations of the Raman vibrational spectrum of the bulk and stepped (facet) surface of a thin STO film with adsorbed water derivatives were performed. According to our calculations, the Raman spectrum of a stepped STO surface differs from the bulk spectrum, which agrees with the experimental data. The characteristic vibrational frequencies for the chemisorption of water derivatives on the surface were identified. Moreover, it is also possible to distinguish between differently adsorbed hydrogen atoms of a split water molecule. Our approach helps to select the most efficient (size and shape) perovskite nanoparticles for efficient hydrogen/oxygen photocatalytic production.

Sub-bandgap trap sites for high-density photochemical electron storage in colloidal SrTiO3 nanocrystals.

We report facile and reversible electron storage in colloidal SrTiO3 nanocrystals using photochemical and redox titration methods. A very high electron storage capacity (∼180 e- per 7 nm nanocrystal) is achieved which we attribute to the localized nature of added electrons at sub-bandgap trap sites in these colloidal SrTiO3 nanocrystals. The rate of electron accumulation is also found to be much faster with ethylene glycol as the sacrificial reductant compared to ethanol. This work provides key insight and establishes a kinetic bottleneck in the charge trapping processes.

Water Splitting on Multifaceted SrTiO3 Nanocrystals: Computational Study

Recent experimental findings suggest that strontium titanate SrTiO3 (STO) photocatalytic activity for water splitting could be improved by creating multifaceted nanoparticles. To understand the underlying mechanisms and energetics, the model for faceted nanoparticles was created. The multifaceted nanoparticles’ surface is considered by us as a combination of flat and “stepped” facets. Ab initio calculations of the adsorption of water and oxygen evolution reaction (OER) intermediates were performed. Our findings suggest that the “slope” part of the step showed a natural similarity to the flat surface, whereas the “ridge” part exhibited significantly different adsorption configurations. On the “slope” region, both molecular and dissociative adsorption modes were possible, whereas on the “ridge”, only dissociative adsorption was observed. Water adsorption energies on the “ridge” ( −1.50 eV) were significantly higher than on the “slope” ( −0.76 eV molecular; −0.83 eV dissociative) or flat surface ( −0.79 eV molecular; −1.09 eV dissociative).

Lanthanide dopant stabilized Ti3+ state and supersensitive Ti3+ -based multiparametric luminescent thermometer in SrTiO3:Ln3+ (Ln3+ = Lu3+, La3+, Tb3+) nanocrystals

Herein, we show that the substitution of Sr2+ by trivalent lanthanide ions (Lu3+, La3+, Tb3+) in SrTiO3 nanocrystals stabilizes and enhances Ti3+ near-infrared emission (around 800 nm). This emission occurs from the 6-fold coordinated Ti3+ luminescent centers that appear in the vacancy–Ti3+–O form after lanthanide doping into the Sr2+ site. The strong dependence of the Ti3+ emission on temperature provided means for the tailored chemical engineering of luminescent nanonthermometers able to read the temperature in three ways: from the changes in Ti3+ emission intensity, excited-state lifetime, and from the ratio of Tb3+ and Ti3+ emission intensities. We demonstrated the unprecedented temperature sensitivity of the lifetime-based luminescent thermometer (8.83% K−1) with SrTiO3:Tb3+, along with exceptional repeatability in measurements.

Highly sensitive multiparametric luminescent thermometer for biologically-relevant temperatures based on Mn4+, Ln3+ co-doped SrTiO3 nanocrystals

The constant striving to improve the quality of thermal imaging with the use of luminescent thermometers imposes the search for new materials with better and better thermometric properties. The facility in the modulation of the spectroscopic properties of transition metal doped materials makes them particularly attractive for this type of application. Therefore, in response to this expectation, in this manuscript we report the novel SrTiO3:Mn4+ nanocrystals as a thermographic phosphor of extraordinary thermometric properties in the biologically relevant temperatures. The high relative sensitivity of the SrTiO3:Mn4+ nanocrystals to temperature changes in each of the three temperature readout modes presented in this manuscript, namely 8.67%/K at 313 K in the intensity mode, 3.5%/K at 348 K in the lifetime mode and 5.64%/K at 303 K in the ratiometric mode, make this material one of the most promising candidates for highly sensitive temperature imaging.

Enhanced photocatalytic water splitting with surface defective SrTiO3 nanocrystals

Surface reconstructed SrTiO3 nanocrystals were synthesized by a thermal treatment process in presence of NaBH4 and SrTiO3 nanocrystals. The surface reconstruction of SrTiO3 nanocrystals is attributed to the introduction of surface oxygen vacancies or Ti sites (such as Ti3+ and Ti2+) during the hydrogenation treatment process. The light absorption and the charge transfer ability of SrTiO3 nanocrystals are simultaneously enhanced due to surface oxygen vacancies or Ti sites (such as Ti3+ and Ti2+), which are beneficial to photocatalytic water splitting. Meanwhile, these defects also change the redox potential of the photocatalysts. Since there existed a synergistic effect between the three, the ratio of hydrogen to oxygen production was also regulated.

Grain growth competition during sintering of SrTiO3 nanocrystals: Ordered coalescence of nanocrystals versus conventional mechanism

Grain growth plays a crucial role in developing the microstructures of polycrystalline materials during heating treatment. In this work, SrTiO3 ceramics sintered at different heating rates (10°C/min and 100°C/min) display the distinctly different grain growth behaviors and microstructures between them. The onset temperature of rapid grain growth and mechanical-performance-related fracture mode are apparently different between these two sets of samples. Two grain growth mechanisms are demonstrated to dominate separately the sintering of SrTiO3 nanocrystals at different heating rates, i.e. conventional mechanism of atom-by-atom additions for grain growth at 10°C/min and ordered coalescence of nanocrystals for grain growth at 100°C/min. It is demonstrated that the switch of growth mechanisms is determined by the competition between the activation of grain motions and the formation of steady neck among grains.

Hydrogen evolution with fluorescein-sensitized Pt/SrTiO3 nanocrystal photocatalysts is limited by dye adsorption and regeneration

Photocatalysts are one of several technologies for the conversion of sunlight and water into hydrogen fuel. Here, we show that the photocatalytic activity of fluorescein-sensitized platinum modified SrTiO3 nanocubes for visible light hydrogen evolution from aqueous solutions of methanol, potassium iodide, potassium hexacyanoferrate (II), and triethanolamine is limited by light absorption from the fluorescein and by its self-oxidation, even in the presence of the sacrificial donors. The highest hydrogen evolution rate was observed with Pt/SrTiO3 suspended in 0.5 mM fluorescein at pH 13 with triethanolamine as a sacrificial donor (60 μmol h−1, 0.67 % AQE at 2.64 eV). The other donors (methanol and KI) gave lower H2 rates or no H2 (hexacyanoferrate (II)). This trend can be rationalized with surface photovoltage spectroscopy (SPS) measurements on FL-sensitized SrTiO3 films on indium tin oxide substrates. Electron transfer from the dye to SrTiO3 under 2.75 eV illumination and hole transfer to the sacrificial donors are modulated by the redox potentials of the donors and acceptors. The low activity of hexacyanoferrate appears to be due to e/h recombination, while triethanolamine appears to replace the dye from the SrTiO3 nanocrystals under the conditions of the SPS experiment. Photovoltage values suggest charge transfer is accompanied by irreversible self-oxidation of FL and this is confirmed by transient surface photovoltage measurements. Overall these results suggest that system optimization requires dyes with greater redox stability and stronger binding affinity to the SrTiO3 nanocrystal surface.

(Invited) Ferroelectric Photovoltage Enhancement in Chromium-Doped SrTiO3 nanocrystal Photocatalysts for Hydrogen Evolution

Chromium-doped SrTiO3 nanocrystals of perovskite structure type and 50 nm edge lengths were obtained by hydrothermal synthesis in water from TiO2, Sr(OH)2, and Cr(NO3)3. According to XPS, chromium is present mainly in the +3 oxidation state, and one third is +6. Optical spectroscopy confirms a broad absorption at 2.3-3.0 eV from the Cr states, in addition to the 3.2 eV absorption of the SrTiO3 host. After modification with Pt nanoparticles, SrTiO3:Cr nanocrystals catalyze photochemical H2 evolution from aqueous methanol under visible light illumination and with a an apparent quantum yield of 0.66% at 435 nm. According to surface photovoltage spectroscopy, SrTiO3:Cr nanocrystals are n-type and have an effective band gap of 2.4 eV. Transient illumination experiments reveal an anomalous photovoltage component of up to 6 V that indicates a ferroelectric dipole in the material. The finding of a ferroelectric polarization in SrTiO3:Cr could be significant for improving its photocatalytic properties. Figure 1

The interplay between surface facet and reconstruction on isopropanol conversion over SrTiO3 nanocrystals

Strontium titanate (SrTiO3) is an extensively investigated perovskite for various applications due to its optical, electrical and chemical properties. To gain an in-depth understanding of the active sites involved in heterogeneous catalysis over the broadly used SrTiO3 (STO), we studied a model reaction, isopropanol conversion, on three differently shape-controlled nanocrystals: cube, truncated cube and dodecahedra. SEM, XRD and XPS confirmed the morphology, phase and composition of STO shapes. Low energy ion scattering (LEIS) revealed the occurrence of surface reconstruction over STO shapes during O2 pretreatment at different temperatures. Based on the catalytic activities, scanning transmission electron microscopy images and density functional theory calculations, the step sites on STO derived from surface reconstruction were proposed to be the active sites for isopropanol conversion. This was further confirmed by steady state isotopic kinetic analysis (SSITKA) which demonstrated similar intrinsic turnover frequencies (TOFs) for the differently reconstructed STO shapes. It is concluded that the crystal facets impose an indirect effect on the catalysis of STO via controlling the degrees of surface reconstruction: the less stable STO (1 1 0) facet (dodecahedra) leads to more step sites after reconstruction and hence higher overall reaction rate than the more stable (1 0 0) facet (cube). This work highlights the important interplay between the crystal facet and surface reconstruction in controlling the nature and density of active sties and thus catalysis over complex oxides.

Ferroelectric surface photovoltage enhancement in chromium-doped SrTiO3 nanocrystal photocatalysts for hydrogen evolution

Surface photovoltage spectroscopy discovers abnormally large photovoltage signals possibly associated with a previously unknown ferroelectric effect in Cr-doped SrTiO3 nanocrystals.

Enhanced piezoelectric-induced catalysis of SrTiO3 nanocrystal with well-defined facets under ultrasonic vibration.

Facet engineering of nanocomposite has been confirmed to be an efficient strategy to accelerate their catalytic performances, but to improve their piezoelectric catalytic activities by facet engineering has been seldom reported. Herein, we developed a series of SrTiO3 nanocrystals with exposed {0 0 1} facet, dominant {1 1 0} facet and co-exposed {0 0 1} and {1 1 0} facets, respectively, and firstly revealed its piezoelectric catalytic performance under ultrasonic vibration. Moreover, the relationship between piezoelectric-induced catalytic activity and facet-dependence of SrTiO3 nanocrystal was disclosed for the first time. The SrTiO3 nanocrystal with co-exposed {0 0 1} and {1 1 0} facets exhibited effectively enhanced piezoelectric catalytic activity by degrading Rhodamine B (RhB) under ultrasonic vibration, as compared to that of SrTiO3 nanocrystals with exposed {0 0 1} facet and dominant {1 1 0} facet, respectively. In addition, trapping experiments and active species quantitative experiments confirmed that the co-exposed {0 0 1} and {1 1 0} facets were beneficial to produce O2- and OH with the generation rates of 8.3 and 132.2 μmol g-1 h-1, respectively. The OH radical played a dominant role in piezoelectric catalytic process. Finally, the piezoelectric catalysis mechanism of SrTiO3 surface heterojunction was proposed based on a DFT study. This study presents an in-depth understanding of piezoelectric-induced catalytic of perovskite nanocrystals with exposed well-defined facets.

Influence of the specific surface area and silver crystallite size of mesoporous Ag/SrTiO3 on the selectivity enhancement of ethylene oxide production

ABSTRACTBackgroundEthylene oxide, which is industrially produced by ethylene epoxidation, is an important versatile chemical intermediate, thus its selectivity enhancement can bring substantial economic profits. In this work, the effects of two important, but still debatable, factors, i.e. catalyst specific surface area and active phase crystallite size, on the catalyst selectivity enhancement were investigated. Surfactant assisted sol–gel method with good control ability on the pores structure was selected to produce mesoporous strontium titanate (SrTiO3) nanocrystals with various specific surface area. The SrTiO3 nanocrystals were used as the supports of silver (Ag) for ethylene epoxidation.ResultsThe Ag/SrTiO3 catalysts with various surface areas were obtained by changing the surfactant‐alkoxide ratio and the calcination time. The ethylene oxide selectivity of the prepared catalysts was examined in a laboratory‐scale fixed bed reactor. The best performance was observed at the lowest surfactant–alkoxide ratio of 0.2 and calcination time of 4 h, with the surface area of 1.7 m2/g and Ag crystallite size of 15 nm.ConclusionThe best catalyst selectivity was observed at the lowest surface area and the highest average Ag crystallite size. In this condition, the selectivity of the Ag/SrTiO3 catalyst without any modifier was superior to a commercial Ag/Al2O3 catalyst with 3.7m2/g surface area and alkaline earth metal modifiers. © 2019 Society of Chemical Industry

Constructing SrTiO3-T/CdZnS heterostructure with tunable oxygen vacancies for solar-light-driven photocatalytic hydrogen evolution

Heterostructure plays significant roles in many photocatalytic reactions, as it can provide more efficient route for the charge carrier separation and transferring. Herein, we report a rational design of a new kind of photocatalyst, in which SrTiO3-T/Cd0.5Zn0.5S heterostructure is prepared via a simple hydrothermal method. SrTiO3 with oxygen vacancy is prepared by a controllable solid-state reaction of NaBH4 and SrTiO3 nanocrystals. The oxygen vacancies on the surface of SrTiO3 can be used as electron traps to attract electrons from Cd0.5Zn0.5S and provide proton reduction sites for H2 production during the photocatalysis processing, thereby effectively inhibit the recombination of electron-hole pairs. It has demonstrated that SrTiO3-T/Cd0.5Zn0.5S nanocomposites can achieve the highest photocatalytic hydrogen evolution rate of 25.01 mmol g−1 h−1 under visible light, which is about 2 times than that of the original Cd0.5Zn0.5S.

Ethanol-Quenching Introduced Oxygen Vacancies in Strontium Titanate Surface and the Enhanced Photocatalytic Activity.

Modification of the surface properties of SrTiO3 crystals by regulating the reaction environment in order to improve the photocatalytic activity has been widely studied. However, the development of a facile, effective, and universal method to improve the photocatalytic activity of these crystals remains an enormous challenge. We have developed a simple method to modify the surface environment of SrTiO3 by ethanol quenching, which results in enhanced UV, visible and infrared light absorption and photocatalytic performance. The SrTiO3 nanocrystals were preheated to 800 °C and immediately quenched by submersion in ethanol. X-ray diffraction patterns, electron paramagnetic resonance spectra, and X-ray photoelectron spectra indicated that upon rapid ethanol quenching, the interaction between hot SrTiO3 and ethanol led to the introduction of a high concentration of oxygen vacancies on the surface of the SrTiO3 lattice. Consequently, to maintain the regional charge balance of SrTiO3, Sr2+ could be substituted for Ti4+. Moreover, oxygen vacancies induced localized states into the band gap of the modified SrTiO3 and acted as photoinduced charge traps, thus promoting the photocatalytic activity. The improved photocatalytic performance of the modified SrTiO3 was demonstrated by using it for the decomposition of rhodamine B and production of H2 from water under visible or solar light.