RESUMO
Perovskite oxide heterostructures have been extensively investigated for their excellent photocatalytic properties. Here, through hybrid density functional theory calculations, we systematically investigate the formation of NaNbO3-NaTaO3 (NBO-NTO) heterostructures. The sequential cations replacement in the superlattices reveals the Nb-Ta ratio range that allows the effective formation of heterostructures, which occurs through a spontaneous polarization mechanism induced by the electrostatic potential discontinuity in the interface. The resulting cation ordering is responsible for the sawtooth-like potential distribution that spatially separates valence and conduction charges and reduces the heterostructure bandgap. The symmetric NBO5/NTO5 junction has the smallest bandgap (2.50 eV) whose transitions are associated with Nb 5dxy orbitals on the interfacial plane. Such a relaxation mechanism provides the heterostructure with anisotropic optical properties and interface absorption peaks closer to the visible light spectrum. The phenomena strongly suggest the use of these heterostructures in photocatalytic reactions, supported by their coherent band-edge alignment with both water splitting and CO2 reforming potentials.
RESUMO
Photocatalysis-assisted water splitting using semiconductor materials greatly depends on the bandgap size and the alignment of band edges relative to the reaction potentials. We used ab initio computational methods to show that the biaxial strain on [100]-oriented orthorhombic NaTaO3 thin films grants the modulation of surface states, favoring either the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER), which basically rules the perovskite photocatalytic performance. Under compression, the outermost TaO6 and TaO4 polyhedra become more distorted, and electrostatic repulsion increases the energy of Ta 5d surface states. As they overcome the O2/H2O potential, they cease to contribute to the OER. At the same time, the H+/H2 remains below the conduction band, leveraging the HER over the OER. The tensile strain lowers the outermost polyhedra distortions, stabilizing both Ta 5d surface and conduction band states, and increasing the charge centered around surface Ta atoms. Consequently, the bands are better aligned with O2/H2O and H+/H2 potentials, which benefits the overall water splitting photocatalysis. Our results evidence that combining facet and strain engineering is an effective way of altering the photocatalytic activity of orthorhombic [100] NaTaO3 thin films.
RESUMO
Ultrathin films of perovskites have attracted considerable attention once they fit in numerous applications. Over the years, controlling and tuning their properties have been attainable when biaxial strain is applied. Through ab initio DFT calculations, (110) ultrathin (Na,K)TaO3 films were submitted to biaxial tensile and compressive strain. Intrinsically, surface Ta shallow states emerge into the bandgap since the (110) cleavage breaks its octahedral symmetry to create TaO4 units. Removal of ligands along the x-y plane stabilizes dx2-y2 orbitals, which decrease in energy due to lower electrostatic repulsion. Such stabilization is maximized when biaxial tensile increases the TaO4 planarity towards a square planar symmetry. Accordingly, the corresponding electronic levels move further into the bandgap. Conversely, compressive biaxial strain intensifies electrostatic repulsion, closing the TaO4 tetrahedra, and surface states move to higher energy zones. The reported strain-driven modulation might be applied in different applications, as photocatalysis, ferroelectricity, and spintronics.
RESUMO
Sodium tantalate nanostructures have been classified among the best materials to conduct photocatalytic reactions. Therefore, understanding the relationship between nanoscale surface phenomena and photocatalytic properties is of fundamental importance. We performed Density Functional Theory calculations to investigate how chemically different facets may affect intrinsic properties of NaTaO3 cubic nanowires. Besides half-metallicity, the NaO-terminated wire relaxes structurally, presenting unoccupied down O 2p levels located above its valence band due to severely reduced coordination of its edges, which may allow it to be applied in spintronics systems. NaTaO-terminated wires have surface TaO4 units that, upon structural reconstruction, become more planar and introduce occupied Ta 5d levels below their conduction band. The emergence of such levels is also related to the overlap of Ta dz2 orbitals from adjacent NaTaO facets. Amongst other properties discussed herein, localized levels may be relevant for photocatalysis not only in terms of intrinsic bandgap engineering but also concerning the alignment with water redox potentials.