Formation of monolithic SrTiO3-TiO2 ceramic heterostructures by reactive hydrothermal sintering

In a one-pot approach, monolithic SrTiO₃-TiO₂ ceramic heterostructures were obtained using the reactive hydrothermal liquid phase densification (rHLPD). Structural, morphological, and photocatalytic properties of the obtained ceramics were analyzed. The relative density of the formed components reached about 80% with reaction time, temperature, and NaOH concentration variation. It was observed via Rietveld refinement that there was no XRD detectable phase other than TiO₂ and SrTiO₃ in the final structure. The monolithic SrTiO₃-TiO₂ ceramics obtained by hydrothermal reaction at 120 °C for 24 h in 1 M NaOH concentration showed a dielectric constant being around 500, and the dielectric loss was below 0.25 at frequencies higher than 10 kHz. The SrTiO₃-TiO₂ heterostructured monoliths having only 20 vol% total porosity and low specific surface area, demonstrated ∼60% efficiency (in 5 h) in degrading Methylene Blue photo-catalytically.     

One-step synthesis of fine SrTiO3 particles using SrSO4 ore under alkaline hydrothermal conditions

The employment of mineral SrSO₄ crystals and powders for preparing SrTiO₃ compound was investigated, with coexistence of Ti(OH)₄·4.5H₂O gel under hydrothermal conditions, at various temperatures (150–250 °C) for different reaction intervals (0.08–96 h) in KOH solutions with different concentrations. The complete dissolution of the SrSO₄ crystal occurred at 250 °C for 96 h in a 5 M KOH solution, resulting in the synthesis of SrTiO₃ particles with two different shapes (peanut-like and cubic). In contrast, very fine SrTiO₃ pseudospherical particles were crystallized when SrSO₄ powders were employed as precursor. Variations on the SrTiO₃ particle shape and size were found to be caused by the differences in the dissolution rate of the SrSO₄ phase in the alkaline KOH solution. The crystallization of SrTiO₃ particles was achieved by a bulk dissolution–precipitation mechanism of the raw precursors, and this mechanism was further accelerated by increasing the reaction temperature and concentration of the alkaline media. Kinetic data depicted that the activation energy required for the formation of SrTiO₃ powders from the complete consumption of a SrSO₄ single crystal plate under hydrothermal conditions, is 27.9 kJ mol⁻¹. In contrast, when SrSO₄ powders were employed (28–38 μm), the formation of SrTiO₃ powder proceeded very fast even for a short reaction interval of 3 h at 250 °C in a 5 M KOH solution.     

Sacrificial template synthesis of core-shell SrTiO3/TiO2 heterostructured microspheres photocatalyst

The core-shell SrTiO₃/TiO₂ heterostructure was obtained via a combined hydrothermal route and calcination treatment using amorphous spherical TiO₂ as both template and reactant. Adjusting the hydrothermal environments can control the morphology of the post-calcined sample when it is hydrothermally treated at 180 °C/3 h and 200 °C/6 h, respectively. Following the heat treatment at 700 °C/4 h, the obtained powder illustrates the core-shell heterostructure with a hierarchical surface, and the diameter of the microsphere is about 700 nm. This synthesizing route facilitates the formation of a concentration gradient of SrTiO₃ and TiO₂, and subsequently constructs a gradient energy level, which helps the samples exhibited an excellent de-colorize activity over the methylene blue. The possible formation mechanism of core-shell SrTiO₃/TiO₂ heterostructures was proposed to guide the further improvement of their photocatalytic activity.     

Facile synthesis of nanorods of tetragonal barium titanate using ethylene glycol

Tetragonal barium titanate (BaTiO₃) nanorods were synthesized from hydroxide precursor by a hydrothermal/solvothermal method with 10 vol% ethylene glycol as solvent. The hydroxide precursor slurry was prepared by the addition of 10 M NaOH to a mixed solution of BaCl₂ and TiCl₄. When the above aqueous slurry was heated with water only at 200 °C, cubic BaTiO₃ nanocrystals formed, whereas tetragonal BaTiO₃ nanorods were obtained when heated with 10 vol% ethylene glycol. The crystallization of cubic BaTiO₃ via dissolution–reprecipitation of precursor could be suppressed by the addition of ethylene glycol, resulting in the formation of tetragonal BaTiO₃ under hydrothermal treatment at 200 °C.     

Strong tribocatalysis of strontium titanate nanofibers through harvesting friction energy for dye decomposition

Friction is a common clean energy and can be harvested and converted into electricity energy via triboelectricity, which can electrochemically drive dye decomposition in theory. In this work, the tribocatalytic Rhodamine B dye decomposition has been experimentally realized in strontium titanate (SrTiO₃) nanofibers, which are synthesized via a hydrothermal method. In the tribocatalytic dye decomposition process, the friction is exerted in the interface between catalyst surface and a polytetrafluoroethylene (PTFE) Teflon rod setup with the different stirring speed. The RhB dye decomposition ratios of SrTiO₃ nanofibers at these stirring speeds of 200 rpm, 400 rpm, 600 rpm, and 800 rpm are respectively 24.2%, 51.8%, 73.9% and 88.6%, yielding to these reaction rate constants of ～0.0112 h^?1, ～0.0260 h^?1, ～0.0562 h^?1 and ～0.0877 h^?1. The main active species, which play an important role in tribocatalytic process, are the superoxide radicals and holes on basis of the active species quenching experiment results. The excellent tribocatalysis activity makes SrTiO₃ nanofibers potential for application in dye wastewater treatment through utilizing the environmental friction energy.     

Thermodynamics of Nanoscale Calcium and Strontium Titanate Perovskites

The surface enthalpies of nanocrystalline CaTiO₃ and SrTiO₃ perovskites were determined using high‐temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The nanocrystalline samples were synthesized by a hydrothermal method and characterized using powder X‐ray diffraction, FTIR spectroscopy, and Brunauer–Emmett–Teller surface area measurements. The integral heats of water vapor adsorption on the surfaces of nanocrystalline CaTiO₃ and SrTiO₃ are ?78.63 ± 4.71 kJ/mol and ?69.97 ± 4.43 kJ/mol, respectively. The energies of the hydrous and anhydrous surfaces are 2.49 ± 0.12 J/m² and 2.79 ± 0.13 J/m² for CaTiO₃ and 2.55 ± 0.15 J/m² and 2.85 ± 0.15 J/m² for SrTiO₃, respectively. The stability of the perovskite compounds in this study is discussed according to the lattice energy and tolerance factor approach. The energetics of different perovskites suggest that the formation enthalpy becomes more exothermic and surface energy increases with an increase in ionic radius of the “A” site cation (Ca, Sr, and Ba), or with the tolerance factor. PbTiO₃ shows a lower surface energy, weaker water binding, and a less exothermic enthalpy of formation than the alkaline‐earth perovskites.     

Synthesis of La2Mo2O9 powders with nanodomains using polyol procedure

Powders of oxide-ion conductor La₂Mo₂O₉ presenting nanodomains were synthesized by using polyol processes. The influence of different process parameters on the structure and particles morphology was determined. By the classical polyol process, whatever the polyol solvent used (ethylene glycol, diethylene glycol, 1,2-propanediol and tetraethylene glycol) platelet particles with dimensions of few hundred nanometers long and around twenty nanometers thick were observed, as agglomerates of small sub-particles. Only with diethylene glycol, pure La₂Mo₂O₉ was obtained, whereas with microwave assistance, ethylene glycol also leads to pure product. For both ethylene and diethylene glycol solvent, the microwave assisted process resulted in either agglomerates or desert roses conglomerates after heat treatments at 500 °C for 5 min or 2 h. Specific surface areas, ranging between 23 and 36 m² g^?1, have been measured for pure La₂Mo₂O₉ powders.     

Recovery of carbon fibres and production of high quality fuel gas from the chemical recycling of carbon fibre reinforced plastic wastes

A solvolysis process to depolymerize the resin fraction of carbon fibre reinforced plastic waste to recover carbon fibre, followed by hydrothermal gasification of the liquid residual product to produce fuel gas was investigated using batch reactors. The depolymerisation reactions were carried out in ethylene glycol and ethylene glycol/water mixtures at near-critical conditions of the two solvents. With ethylene glycol alone the highest resin removal of 92.1% was achieved at 400 °C. The addition of water to ethylene glycol led to higher resin removals compared to ethylene glycol alone. With an ethylene glycol/water ratio of 5, at 400 °C, resin removal was 97.6%, whereas it was 95.2% when this ratio was 3, at the same temperature. The mechanical properties of the recovered carbon fibre were tested and showed minimal difference in strength compared to the virgin carbon fibre. The product liquid, containing organic resin degradation products was then subjected to catalytic supercritical water gasification at 500 °C and 24 MPa in the presence of NaOH and Ru/Al₂O₃ as catalysts, respectively. Up to 60 mol.% of H₂ gas was produced with NaOH as catalyst, and 53.7 mol.% CH₄ gas was produced in the presence of Ru/Al₂O₃.     

Micellization Behavior of Cationic Gemini Surfactants in Aqueous-Ethylene Glycol Solution

The micellization behavior of gemini surfactants i.e. alkanediyl-α,ω-bis(cetyldimethylammonium bromide) (C₁₆-s-C₁₆,2Br⁻ where s = 3, 4, 10) in 10% (v/v) ethylene glycol solution was investigated by surface tension and conductometric measurements at 300 K. The critical micelle concentration, degree of micellar ionization, surface excess concentration, minimum surface area per molecule of surfactant, surface pressure at the CMC and Gibbs energy of adsorption of the dimeric surfactants have also been determined in the presence of different salts (NaCl, NaBr and NaI). The critical micelle concentration and degree of micellar ionization values decrease significantly in the presence of sodium halides and follows the sequence NaCl < NaBr < NaI. The free energy, enthalpy and entropy of micellization of dimeric surfactants in 10% (v/v) ethylene glycol solution were determined using the temperature dependence of the critical micelle concentration. The standard free energy of micellization was found to be negative in all the cases.     

Effect of annealing temperature in carbon powder on thermoelectric properties of the SrTiO3−δ single crystal in the [111] crystal orientation

SrTiO₃ is a promising thermoelectric material for applications in harsh environments, owing to its excellent thermoelectric (TE) properties and high-temperature stability. The effect of annealing in carbon powders on the TE properties of SrTiO_3?δ in the [111] direction was experimentally investigated and analysed using first-principles calculation. The electrical conductivity of the SrTiO_3?δ single crystals in the [111] direction increases with an increase in annealing temperature in the range of 1573–1673 K, which is opposite to that in the [510] direction as shown in a previous experiment. This is attributed to the different variation trend of carrier concentration. First-principle calculation results show that the carrier concentration in the [111] direction increases with increasing oxygen vacancy concentration, but it is the opposite in the [510] direction. Strong anisotropy of carrier concentration should be caused by the difference in atomic arrangement, making difference in the amount of free electrons that maybe constrained at high oxygen vacancy in different directions. Although the SrTiO_3?δ single crystal annealed at 1573 K shows a lower electrical conductivity, it obtains higher power factor with the maximum value of 2.24 mW m^?1 K^?2 at 323 K, which is predominantly due to its higher effective mass. It also yields the highest ZT value of 0.18 at 873 K owing to the lower thermal conductivity. The results of this study are imperative for the design and development of perovskite TE materials.     

Microstructure and dielectric properties of SrTiO3 ceramics by controlled growth of silica shells on SrTiO3 nanoparticles

SrTiO₃@SiO₂ nanopowder was synthesized via a core-shell nano-scale technique that is known as the Stöber process. The effect of the SiO₂ concentration on microstructure, dielectric response and energy storage properties of SrTiO₃@SiO₂ ceramics was investigated. Transmission electron microscopy (TEM) results confirmed the formation of core–shell nanostructures with controlled shell thicknesses between 2 nm and 13 nm. After increasing SiO₂, a secondary phase with Sr₂TiSi₂O₈ appeared due to inter-diffusion reactions between the SrTiO₃ core and SiO₂ shells during the sintering process. The results show that both breakdown strength and energy density improved apparently. The homogeneous coating of silica on ST cores is considered to dominate the contribution to improved breakdown strength. The composition for SrTiO₃ coated with 2.5 wt% SiO₂ shows the maximum energy storage density (1.2 J/cm³) and a breakdown strength of 310 kV/cm. The former is higher than for pure SrTiO₃ (0.19 J/cm³). Measurements of the dielectric performance indicate that the SrTiO₃@SiO₂ ceramics possess good bias stabilities compared to pure ST ceramics.     

Preparation of porous material from waste bottle glass by hydrothermal treatment

Porous materials were prepared from colored waste glass by hydrothermal treatment with Na₂CO₃ aqueous solution. The resultant specific surface area was approximately 140 m²/g at maximum. Specific surface area increased at first, reached a maximum, and decreased gradually to be constant at approximately 60 m²/g depending on the period of hydrothermal treatment, irrespective of the concentration of Na₂CO₃ aqueous solution. However, the period at which the specific surface area reached maximum shortened with an increase in Na₂CO₃ concentration. On the other hand, the mass of the sample decreased and eventually saturated at approximately 30 mass% of the initial weight during the hydrothermal treatment. Both the dissolution of the mother glass and the formation of crystalline deposits, which were identified as calcite, zeolite-P and analcime, affected the porous structure of the treated samples.     

Influence of particle size and shape on the rate of hydrogen produced by Al-doped SrTiO3 photocatalysts

Photocatalytic hydrogen production rates have been measured from Al-doped SrTiO₃ with a range of controlled shapes and sizes using a high-throughput parallelized and automated photochemical reactor. It is found that the photocatalytic reactivity is influenced by crystal shape and that crystals with a {1 1 0} to {1 0 0} surface area ratio between 1.3 and 1.8 yield more H₂ than crystals with other ratios. Crystals with a {1 1 0}/{1 0 0} surface area ratio of 1.8 generate hydrogen at 550 μmol h⁻¹ g⁻¹ at pH 7, whereas crystals with only {1 0 0} facets exposed generate hydrogen at 300 μmol h⁻¹ g⁻¹ under the same condition. It is likely that the surface area ratio provides the appropriate balance between the photoanodic reaction on the {1 1 0} surface and the photocathodic reaction on the {1 0 0} surface. In the size range of 250–450 nm, larger crystals produce hydrogen at a rate of 400 μmol h⁻¹ g⁻¹ at pH 7, whereas smaller crystals only produce 200 μmol h⁻¹ g⁻¹, suggesting that the larger crystals reduce the rate of electron–hole recombination or back reaction and that the widths of the space charges within the crystal are comparable to the particle radius.     

Synthesis and characterization of 3D ZnO superstructures via a template-free hydrothermal method

The ZnO microrods and flower-like 3D superstructures have been synthesized via a template-free hydrothermal method. The prepared ZnO samples are characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the flower-like 3D ZnO superstructures can be obtained simply by tuning the volume ratio of Zn(OH)₂⁴⁻ precursor solution to water, or the volume ratio of ethylene glycol to water. When the volume ratio of Zn(OH)₄²⁻ precursor solution to water is 1:7, the flower-like 3D ZnO superstructures are obtained, in which the diameter of an individual pencil-like rod is about 1.0 μm, and the tip size is several nanometers. While when the volume ratio of Zn(OH)₄²⁻ precursor solution to ethylene glycol (EG) is 1:1, the flower-like 3D ZnO superstructure consists of hexagonal rods with about 500 nm in diameter. A plausible formation mechanism of flower-like 3D ZnO superstructures is discussed in detail.     

Giant permittivity in Nb-doped SrTiO3 single crystal: Compositional gradient and local structure

Nb-doped SrTiO₃ single crystal exhibited a giant permittivity (>6.5 × 10⁵) with an acceptably low dielectric loss (<10^?1) in a wide temperature range from ?120 to 200 °C, making this material a good candidate for energy storage devices and modern microelectronics components. The mechanisms responsible for the giant permittivity of Nb-doped SrTiO₃ single crystals were studied by means of microstructure characterizations, dielectric measurements, and density-functional theory calculations. A chemical compositional gradient extending from the surfaces was found, forming the internal barrier layer capacitance (IBLC) effect. Polar nanoregions (PNRs) were observed because of local fluctuations in distributions of Nb and oxygen vacancies. While both compositional gradients and local chemistry fluctuations increased polarization of the Nb-doped SrTiO₃ single crystals, the local fluctuations dominated enhanced polarizability. This work suggests that optimizing local structures and chemistries in dielectrics is an effective way to tailor the desired dielectric performance.     

Microwave‐Assisted Hydrothermal Synthesis of Fe2O3‐Sensitized SrTiO3 and its Luminescent Photocatalytic deNOx Activity with CaAl2O4:(Eu,Nd) Assistance

This work focused on the synthesis of high‐activity strontium titanate (SrTiO₃) photocatalysts through an environmentally friendly process. A high energy‐efficient microwave‐assisted hydrothermal method was employed to prepare Fe‐loaded SrTiO₃ that consisted of small particles around 50 nm in diameter. To achieve the effective assistance of long‐lasting fluorescence, the obtained Fe‐loaded SrTiO₃ samples were coupled with CaAl₂O₄:(Eu, Nd). The photocatalytic activities were evaluated by the photo‐decomposition of NO. Fe loaded SrTiO₃ showed high photocatalytic activities not only under visible light irradiation but also in the dark with fluorescence assistance.     

Synthesis of nanostructured magnesium silicate with high surface area and mesoporous structure

Nanostructured magnesium silicate with high BET surface area and mesoporous structure was prepared by a hydrothermal method using polyethylene glycol (PEG) as surfactant and magnesium nitrate and sodium silicate aqueous solution as magnesium and silicate sources, respectively. The effects of different parameters such as hydrothermal treatment, reaction temperature, pH, ethanol/PEG ratio and etc. on the structural properties of the synthesized sample were examined. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption (BET) and scanning electron microscopy (SEM) techniques. The results indicated that hydrothermal treatment increased BET surface area from 290 to 394.2 m²/g and transfer amorphous phase to crystalline. Also, increasing in aging temperature, aging time, pH value and ethanol/PEG ratio did not change surface area by specific procedure, whereas increasing calcination temperature decreased surface area. Furthermore, hydrothermal treatment and increasing in pH value will change hysteresis loop. The highest BET surface area obtained in this paper is 619.8 m²/g.     

Low thermal conductivity of SrTiO3−LaTiO3 and SrTiO3−SrNbO3 thermoelectric oxide solid solutions

Electron-doped SrTiO₃ has been attracting attention as oxide thermoelectric materials, which can convert wasted heat into electricity. The power factor of the electron-doped SrTiO₃, including SrTiO₃-LaTiO₃ and SrTiO₃-SrNbO₃ solid solutions, has been clarified. However, their thermal conductivity (κ) has not been clearly identified thus far. Only a high κ (>12 W m⁻¹ K⁻¹) has been assumed from the electron contribution based on Wiedemann–Franz law. Here, we show that the κ of the electron-doped SrTiO₃ is lower than the assumed κ, and its highest ZT exceeded 0.1 at room temperature. The κ slightly decreased with the carrier concentration (n) when n is below 4 × 10²¹ cm⁻³. In the case of SrTiO₃-SrNbO₃ solid solutions, an upturn in κ was observed when n exceeds 4 × 10²¹ cm⁻³ due to the contribution of conduction electron to the κ. On the other hand, κ decreased in the case of SrTiO₃-LaTiO₃ solid solutions probably due to the lattice distortion, which scatters both electrons and phonons. The highest ZT was 0.11 around n = 1 × 10²¹ cm⁻³. These findings would be useful for the future design of electron-doped SrTiO₃-based thermoelectric materials.     

Effect of Gd and Nb co-substitution on enhancing the thermoelectric power factor of nanostructured SrTiO3

Oxide thermoelectric materials have attracted researchers in recent decade due to their attractive features such as low toxicity, low cost and high chemical robustness. Perovskite based oxide thermoelectric are considered as the promising materials, especially for high temperature thermoelectric applications. In the present work, pure SrTiO₃, Sr_1-xGd_xTiO₃ (0 < x < 0.09) and Sr_1-xGd_xTi_1-yNb_yO₃ were prepared by varying Gd concentration (0 < x < 0.09) using hydrothermal method. The XRD analysis confirmed the high crystalline cubic structured nanocomposite with Gd and Nb substitution. The FESEM images revealed cubic morphology of the particles and the size of the cubes varied with the concentration of the dopant. The chemical compositions of the samples were confirmed by EDX analysis. The binding states and elemental composition of the samples were analyzed by XPS. Both the pure SrTiO₃, Sr_1-xGd_xTiO₃ samples show low electrical resistivity and the co-substituted sample exhibited relatively high resistivity. Seebeck coefficient of the samples increased with Gd concentration. The Gd and Nb co-substituted sample shows relatively higher Seebeck coefficient value compared to Gd substituted samples. The power factor of the nanocomposite were calculated from the obtained Seebeck coefficient and resistivity; Gd and Nb co-substituted sample shows relatively high power factor of 311.7 × 10^?6 Wm^?1K^?2 at 550 K compared to other samples.     

Preparation of spherical and porous strontium titanate particles by hot water and hydrothermal conversion of hydrous titania

Uniform, micron-sized SrTiO₃ particles do not tend to aggregate, and the low surface area of larger particles can be improved by incorporating porous structures, thus offering superior performance for a range of applications. In this study, submicron-to micron-sized SrTiO₃ particles were prepared using hot water or hydrothermal conversion of spherical hydrous titania (TiO₂·nH₂O) and porous hydrous titania. When spherical hydrous titania particles were employed as the starting material, spherical SrTiO₃ particles of hydrous titania were obtained via treatment at 120 °C for 24 h. Similarly, the use of porous hydrous titania particles treated at 90 °C for 48 h resulted in spherical porous SrTiO₃ particles with macropores of porous hydrous titania. These porous SrTiO₃ particles have a specific surface area of ~115 m²/g, which is one of the largest among micron-sized SrTiO₃ particles, thereby making them suitable for use as catalysts or photocatalysts.