Knowledge about the emergence and depletion of point defects in BaTiO3 (BTO) nano-structures during materials processing is key to our understanding of their later activity as components in functional dielectric...

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...

Particle attachment and neck formation inside TiO2 nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO2 nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O2 adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.