Epitaxial growth of hexagonal BaNiO3−δ thin films on SrTiO3 (111) substrates

Transition metal oxides containing nickel species in oxidation states higher than 3+ often exhibit high catalytic activity, which makes them promising for applications as advanced electrocatalysts for water splitting and fuel cells. Here, we examine the structure and properties of $\mathrm{BaNi}{\mathrm{O}}_{3}$ (BNO) thin films, containing formally ${\mathrm{Ni}}^{4+}$, grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$ (111) substrates using oxygen-plasma--assisted molecular beam epitaxy. X-ray diffraction and scanning transmission electron microscopy measurements reveal that BNO films have a hexagonal structure with $c$- and $a$-axes of mixed textures showing epitaxial relationships BNO $(0001)\ensuremath{\parallel}\mathrm{SrTi}{\mathrm{O}}_{3}$ (111) and BNO $(10\overline{1}0)\ensuremath{\parallel}\mathrm{SrTi}{\mathrm{O}}_{3}$ (111), respectively. The formation of the $a$-axis texture is dominant due to the smaller lattice mismatch with the substrate. Density functional theory calculations confirm that the hexagonal BNO film with the $a$-axis texture is energetically more favorable than the competing $c$-axis texture. Detailed spectroscopy data analysis indicates that hexagonal BNO films contain mixtures of ${\mathrm{Ni}}^{2+}, {\mathrm{Ni}}^{3+}$, and ${\mathrm{Ni}}^{4+}$ species, with ${\mathrm{Ni}}^{4+}$ being dominant. Our study provides insights into stabilizing ${\mathrm{Ni}}^{4+}$ in complex oxides, which is important for further exploration of the potential of materials containing ${\mathrm{Ni}}^{4+}$.