Accelerating metal nanoparticle exsolution by exploiting tolerance factor of perovskite stannate.

The octahedral symmetry in ionic crystals can play a critical role in atomic nucleation and migration during solid-solid phase transformation. Similarly, octahedron distortion, which is characterized by Goldschmidt tolerance factor, strongly influences the exsolution kinetics in the perovskite lattice framework during high-temperature annealing. However, a fundamental study on manipulating the exsolution process by octahedron distortion is still lacking. In this study, we accelerate Ni metal exsolution on the surface of perovskite stannates by increasing the [BO6] octahedron distortion in the lattices. Decreasing the A-site ionic radius (rBa2+ = 161 pm → rSr2+ = 144 pm → rCa2+ = 134 pm) increased the density of exsolved Ni nanoparticles by up to 640% (i.e., 47 particles µm-2 of Ba(Sn, Ni)O3 → 304 particles µm-2 of Ca(Sn, Ni)O3) after the identical exsolution process. Based on the theoretical calculation and experimental characterization, the decrease in crystal symmetry by octahedral distortion promoted the Ni exsolution owing to the boosted Ni migration by weakening the bond strength and generating domain boundaries. The findings highlight the importance of octahedral distortion to control atomic migration through the perovskite lattice framework and provide a strategy to tailor the density of uniformly populated nanoparticles in nanocomposite oxides for multifunctional material design.

Reversible Manipulation of Photoconductivity Caused by Surface Oxygen Vacancies in Perovskite Stannates with Ultraviolet Light.

Programmable optoelectronic devices call for the reversible control of the photocarrier recombination process by in-gap states in oxide semiconductors. However, previous approaches to produce oxygen vacancies as a source of in-gap states in oxide semiconductors have hampered the reversible formation of oxygen vacancies and their related phenomena. Here, a new strategy to manipulate the 2D photoconductivity from perovskite stannates is demonstrated by exploiting spatially selective photochemical reaction under ultraviolet illumination at room temperature. Remarkably, the ideal trap-free photocurrent of air-illuminated BaSnO3 (≈200 pA) is reversibly switched into three orders of magnitude higher photocurrent of vacuum-illuminated BaSnO3 (≈335 nA) with persistent photoconductivity depending on ambient oxygen pressure under illumination. Multiple characterizations elucidate that ultraviolet illumination of BaSnO3  under low oxygen pressure induces surface oxygen vacancies as a result of surface photolysis combined with the low oxygen-diffusion coefficient of BaSnO3 ; the concentrated oxygen vacancies are likely to induce a two-step transition of photocurrent response by changing the characteristics of in-gap states from the shallow level to the deep level. These results suggest a novel strategy that uses light-matter interaction in a reversible and spatially confined way to manipulate functionalities related to surface defect states, for the emerging applications using newly discovered oxide semiconductors.

Metal Nanoparticle Exsolution on a Perovskite Stannate Support with High Electrical Conductivity.

In situ exsolution of metal nanoparticles (NPs) is emerging as an alternative technique to deliver thermally stable and evenly dispersed metal NPs, which exhibit excellent adhesion with conducting perovskite oxide supports. Here we provide the first demonstration that Ni metal NPs with high areal density (∼175 µm-2) and fine size (∼38.65 nm) are exsolved from an A-site-deficient perovskite stannate support (La0.2Ba0.7Sn0.9Ni0.1O3-δ (LBSNO)). The NPs are strongly anchored and impart coking resistance, and the Ni-exsolved stannates show exceptionally high electrical conductivity (∼700 S·cm-1). The excellent conductivity is attributed to conduction between delocalized Sn 5s orbitals along with structural improvement toward ABO3 stoichiometry in the stannate support. We also reveal that experimental conditions with strong interaction must be optimized to obtain Ni exsolution without degrading the perovskite stannate framework. Our finding suggests a unique process to induce the formation of metal NPs embedded in stannate with excellent electrical properties.