Study of surface nanoparticle formation by selective phase transition in 3d-transition metal doped $Sr_{0.98}TiO_{3-\delta}$

Abstract

With growing interest in highly active surface, the supported metal nanoparticles hold good promise for chemical and electrochemical reactions owing to their high surface-to-volume ratio and passively unique catalytic activity. Recently, the in-situ growth phenomenon of metal nanoparticles directly from a desired oxide support upon the annealing at reducing atmosphere, known as ‘ex-solution’, has been reported by many researchers in the field of high-temperature catalysis and renewable energy. The ex-solution of metal particles is understood as a result of the phase transition due to the partial reduction of transition metals, which are stable as a cation in an oxide host lattice at oxidizing atmospheres. Compared to conventional nanoparticle synthesis and dispersion techniques, this process is faster, more cost-effective and allows finer and better particle distribution. Furthermore, the sintering of nanoparticles can be prevented by re-oxidation, enhancing the life-time of the supported nanoparticles. Despite extensive research efforts, however, the fundamental mechanisms of the processes remain largely unknown. Accordingly, how to expand choice of applicable materials and how to control particle size and distribution precisely are also unknown, despite the fact that these processes are crucial to render this phenomenon technologically attractive. In this study, we prepared dense thin-films of 3d-transition metal (Mn, Fe, Co, Ni and Cu) doped $Sr_{0.98}TiO_{3-\delta}$ with highly flat surfaces and observed the ex-solved particles on their surfaces depending on the annealing temperature, oxygen partial pressure, and kind of dopant via SEM. $SrTiO_3$ was selected as a model perovskite host due to its superior phase stability in a wide range of temperatures and gas atmospheres, and the thin-film samples were used for reliable and reproducible surface analysis. In addition, we carried out DFT calculation to interpret our experimental observations and identify key descriptors, governing the ex-solution behaviors. The ex-solved nanoparticles decrease size and increase density as annealing temperature increase or oxygen partial pressure decrease. Additionally, the ex-solution trend observed that ex-solution easily occurs even with small thermodynamic driving force in order of Cu, Ni, Co, Fe and Mn. Consequently, I succeeded in explaining these observations with nucleation kinetics and dopant segregation thermodynamics in the absence of coarsening and proposing techniques that can control ex-solution particles extensively.