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 BaNiO3 (BNO) thin films, containing formally Ni4+, grown on SrTiO3 (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) 11 SrTiO3 (111) and BNO (10 (1) over bar0) 11 SrTiO3 (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 Ni2+, Ni3+, and Ni4+ species, with Ni4+ being dominant. Our study provides insights into stabilizing Ni4+ in complex oxides, which is important for further exploration of the potential of materials containing Ni4+.