Stabilization of Size-Controlled BaTiO3 Nanocubes via Precise Solvothermal Crystal Growth and Their Anomalous Surface Compositional Reconstruction

Promoted Hydride Substitution in BaTiO3 Cubes

We report the synthesis of perovskite oxyhydride BaTiO3-xHx cubes (100-300 nm) using the topochemical hydride reaction of the hydrothermally synthesized oxide. X-ray and neutron diffraction studies revealed that the anion (hydride/oxide) exchange is greatly improved, leading to a maximum hydride content of 0.7, which is higher than previously reported, while no reaction is seen for those with oleic acid on their surface. These results indicate that the well-defined {100} facets, as well as the absence of organic residues, are crucial for promoting hydrogenation. Kissinger analysis on BaTiO2.3H0.7 yielded an activation energy of 165 kJ/mol, much lower than that of BaTiO2.4H0.6 (313 kJ/mol). In contrast to oxides, oxyhydrides with well-defined facets have not been reported before; thus, this study opens new avenues for the rational synthesis of oxyhydride materials with controlled chemical composition and morphology.

Synthesis of BaTiO3-CaTiO3 and BaTiO3-SrTiO3 Core-Shell Nanocubes via the Surface Reconstruction of BaTiO3 Nanocubes

BaTiO3-CaTiO3 and SrTiO3-BaTiO3 core-shell nanocubes were synthesized through the surface reconstruction of BaTiO3 nanocubes, which involved the reaction of titanium oxide with Ca(OH)2, Sr(OH)2, or Sr(OH)2·8H2O in water at 100 °C. The core-shell structure comprised a BaTiO3 nanocube core and a CaTiO3 or SrTiO3 shell. The outermost layer with a perovskite structure also comprised CaTiO3 or SrTiO3, and its thickness was several hundred picometers. The thinnest layer was constructed of only one layer of CaTiO3 or SrTiO3. This is the first presented work on a core-shell nanocube with the outermost layer consisting of only CaTiO3 or SrTiO3 surrounding the BaTiO3 nanocube. The shells of CaTiO3 and SrTiO3 comprise a layer thickness of only one unit cell of ∼0.4 nm (400 pm). Thus, we demonstrate new research on nanocube surfaces on the picometer scale.

Embedding Nano-Piezoelectrics into Heterointerfaces of S-Scheme Heterojunctions for Boosting Photocatalysis and Piezophotocatalysis

Step-scheme (S-scheme) heterojunctions have exhibited great potential in photocatalysis due to their extraordinary light harvesting and high redox capacities. However, inadequate S-scheme recombination of useless carriers in weak redox abilities increases the probability of their recombination with useful ones in strong redox capabilities. Herein, a versatile protocol is demonstrated to overcome this impediment based on the insertion of nano-piezoelectrics into the heterointerfaces of S-scheme heterojunctions. Under light excitation, the piezoelectric inserter promotes interfacial charge transfer and produces additional photocarriers to recombine with useless electrons and holes, ensuring a more thorough separation of powerful ones for CO2 reduction and H2 O oxidation. When introducing extra ultrasonic vibration, a piezoelectric polarization field is established, which allows efficient separation of charges generated by the embedded piezoelectrics and expedites their recombination with weak carriers, further increasing the number of strong ones participating in the redox reactions. Encouraged by the greatly improved charge utilization, significantly enhanced photocatalytic and piezophotocatalytic activities in CH4 , CO, and O2 production are achieved by the designed stacked catalyst. This work highlights the importance in strengthening the necessary charge recombination in S-scheme heterojunctions and presents an efficient and novel strategy to synergize photocatalysis and piezocatalysis for renewable fuels and value-added chemicals production.

A Facile and Eco-Friendly Hydrothermal Synthesis of High Tetragonal Barium Titanate with Uniform and Controllable Particle Size

The preparation of tetragonal barium titanate (BT) powders with uniform and suitable particle sizes is a significant prerequisite for ultra-thin and highly integrated multilayer ceramic capacitors (MLCCs). However, the balance of high tetragonality and controllable particle size remains a challenge, which limits the practical application of BT powders. Herein, the effects of different proportions of hydrothermal medium composition on the hydroxylation process are explored to obtain high tetragonality. The high tetragonality of BT powders under the optimal solvent condition of water:ethanol:ammonia solution of 2:2:1 is around 1.009 and increases with the particle size. Meanwhile, the good uniformity and dispersion of BT powders with particle sizes of 160, 190, 220, and 250 nm benefit from the inhibition of ethanol on the interfacial activity of BT particles (BTPs). The core-shell structure of BTPs is revealed by different lattice fringe spacings of the core and edge and the crystal structure by reconstructed atomic arrangement, which reasonably explains the trend between tetragonality and average particle size. These findings are instructive for the related research on the hydrothermal process of BT powders.

Facile solvothermal synthesis of plate-like submicron NaNbO3 particles

Platy particles of NaNbO3 were successfully prepared by a solvothermal reaction using a methanol/ethanol mixed solvent, in contrast to the formation of cubic NaNbO3 particles from methanol alone.

Optimizing TiO2 through Water-Soluble Ti Complexes as Raw Material for Controlling Particle Size and Distribution of Synthesized BaTiO3 Nanocubes

Barium titanate (BaTiO₃) nanocubes with a narrow particle size distribution were synthesized using a three-step approach. First, a water-soluble Ti complex was synthesized using a hydrolysis method. Next, the titanium dioxide (TiO₂) raw material was synthesized via a hydrothermal method using various water-soluble titanium (Ti) complexes. The TiO₂ exhibited various particle sizes and crystal structures (anatase, rutile, or brookite) depending on the water-soluble Ti complex and the hydrothermal conditions used in its synthesis. Finally, BaTiO₃ nanocubes were subsequently created through a hydrothermal method using the synthesized TiO₂ particles and barium hydroxide octahydrate [Ba(OH)₂·8H₂O] as raw materials. The present study clarifies that the particle size of the BaTiO₃ nanocubes depends on the particle size of the TiO₂ raw material. BaTiO₃ particles with a narrow size distribution were obtained when the TiO₂ particles exhibited a narrow size distribution. We found that the best conditions for the creation of BaTiO₃ nanocubes using TiO₂ involved using lactic acid as a complexing agent, which resulted in a particle size of 166 nm on average. This particle size is consistent with an average of the width of the cubes measured from corner to corner diagonally, which corresponds to a side length of 117 nm. In addition, surface reconstruction of the BaTiO₃ was clarified via electron microscopy observations, identifying the outermost surface as a Ti layer. Electron tomography using high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) confirmed the three-dimensional (3D) structure of the obtained BaTiO₃ nanocubes.