Charge Separation in BaTiO₃ Nanocrystals: Spontaneous Polarization Versus Point Defect Chemistry

Abstract

However, the additional influence of point defects, found within most metal oxide nanoparticles, [9] on the BTO photochemical activity is still lacking. In particular, the creation of point defects under specific synthesis conditions that can act as charge recombination centers has been mostly ignored with these materials systems.Our work intends to fill this gap by studying the influence of ferroelectric effects and point defects on charge separation within various BTO nanoparticles. The photogenerated charge separation yield was quantified by combining O 2 photoadsorption experiments with electron paramagnetic resonance (EPR), using molecular oxygen as an electron scavenger. Compared to the reference TiO 2 anatase nanoparticles of similar size and surface area, BTO nanoparticles annealed at 600 °C have a charge separation yield that is > 10 times higher. This is attributed to the beneficial effect of the perovskite lattice on charge separation. Strikingly, we also report a counter-intuitive decrease in charge separation yield with increasing tetragonal BTO particle size, due to the formation of point defects during the annealing process that acts as recombination centers. [10] Results and DiscussionBaTiO 3 nanoparticles, synthesized via flame spray pyrolysis (FSP, details in the Experimental Section), were subjected to thermal treatment at temperatures ranging from 600 to 900 °C in alternating cycles of continuous pumping (p < 10 −5 mbar) and in an oxygen atmosphere (20 mbar) to remove residual carbon, surface hydroxyls and water (Figure S1, Supporting Information). The resulting nanoparticles are equiaxed, nonfaceted, and crystalline (Figure 1a,b). X-ray diffraction (XRD) patterns (Figure 1c) reveal a cubic crystal structure with broadened diffraction features that depend on the annealing temperature. Annealing at 700, 800, and 900 °C results in a reduction of the diffraction peak widths that indicates crystallite domain size growth.Annealing at 600 °C gives rise to BTO particles with an average crystallite domain size of ≈11 nm, consistent with the average particle size of 13 nm measured via TEM (Figure S1, Supporting Information). Annealing at higher temperatures The fate of photogenerated charges within ferroelectric metal oxides is key for photocatalytic applications. The authors study the contributions of i) tetragonal distortion, responsible for spontaneous polarization, and ii) point defects, on charge separation and recombination within BaTiO 3 (BTO) nanocrystals of cubic and tetragonal structure. Electron paramagnetic resonance (EPR) in combination with O 2 photoadsorption experiments show that BTO nanocrystals annealed at 600 °C have a charge separation yield enhanced by a factor > 10 compared to TiO 2 anatase nanocrystals of similar geometries. This demonstrates for the first time the beneficial effect of the BTO perovskite nanocrystal lattice on charge separation. Strikingly, charge separation is considerably hindered within BTO nanoparticles annealed ≥ 600 °C, due to the formation of Ba-O divacan...

Water-Mediated Conversion of BaTiO3 Nanoparticles into BaCO3 Nanorods in Electrospun Polymer Fibers: Implications for Carbon Capture Applications

“…(Due to broadened diffraction features and based on XRD measurements alone, it is impossible to provide in this case a quantitative phase analysis.) Earlier Raman spectroscopy measurements on comparable materials 30,35 in fact confirmed that both BTO phases, cubic and tetragonal BTO, are present in the BTO VA873 and BTO VA1173 samples. The crystallite domain size was calculated to be around 26 ± 7 nm.…”

“…Values for the corresponding crystallite domain size (12 ± 1 nm) are consistent with the average particle size that was determined from TEM image analysis and particle counting ( d = 17 nm, Table ). The solid’s crystallinity or its absence has a strong impact on the dissolution behavior. It is well-established that due to the generally larger enthalpy associated with the amorphous form, both the corresponding intrinsic dissolution rate and its solubility are generally larger for amorphous solids than those for their crystalline counterparts .…”

Perovskite Oxide as A New Platform for Efficient Electrocatalytic Nitrogen Oxidation

“…Moreover, the EPR experiments (Figure 2e) suggest the introduction of Sr-site defects can induce higher amount of oxygen vacancies in the crystal structures of Sr 0.9 RuO 3 , compared with SrRuO 3 . [27][28][29] To verify whether the oxygen vacancies can promote the adsorption of N 2 molecules, we further carried out N 2 temperature programmed desorption (N 2 -TPD) to further prove the N 2 adsorption of these catalysts (Figure 2f). As showed, relative to SrRuO 3 , the Sr 0.9 RuO 3 sample has higher N 2 -TPD signal and larger peak area, thus indicating its enhanced adsorption of N 2 .…”

Progress of Piezoelectric Semiconductor Nanomaterials in Sonodynamic Cancer Therapy

“…The piezoelectric catalytic performance of perovskite oxide (ABO 3 ) is closely related to its electronic structure, which depends on the unit cell composition of perovskite. Its crystal structure is prone to distortion, making it more practical in the design of piezoelectric materials . When stimulated by external mechanical pressure, Ti 4+ produces displacement, leading to polarization of BTO and continuous separation of electron–hole pairs, and oxidation–reduction reactions occur with surrounding H 2 O and O 2 , generating active oxygen (ROS) substances such as hydroxyl radical (•OH), superoxide anion (•O 2 – ) and singlet oxygen ( 1 O 2 ).…”