Structural and electronic tunability of Ruddlesden–Popper oxyfluorides through nickel–copper substitution in La2Ni1−xCuxO2.5F3 (0 ≤ x ≤ 1)

Abstract

In this contribution we report on the synthesis, structure and optical characterization of Ruddlesden–Popper oxyfluorides La₂Ni_1−xCu_xO_2.5F₃ (0 ≤ x ≤ 1) obtained by topochemical low-temperature fluorination of La₂Ni_1−xCu_xO₄ with polyvinylidene fluoride (PVDF). Our study reveals that the anionic ordering in the tetragonal unit cell of La₂NiO_2.5F₃ persists even at high Cu substitution levels (x = 0.9), with minimal change in unit cell volume. This observation is contrary to expectations based on Jahn–Teller induced unit cell distortions, which were previously reported for the oxides La₂Ni_1−xCu_xO₄, as well as for the closely related oxyfluorides La₂Ni_1−xCu_xO₃F₂. The pure copper-containing compound La₂CuO_2.5F₃ crystallizes in a triclinic version of the same structure, and the symmetry lowering is attributed to the enhanced space requirements of the Jahn–Teller elongated CuO₄F₂ octahedra. The structural investigations based on XRD and ND Rietveld refinements are supported by low-field ¹⁹F MAS NMR experiments. We also report the results of diffuse reflectance UV-Vis measurements, which are complemented by DFT calculations. Here, we demonstrate a strong impact of the Cu substitution on the electronic structure of the oxyfluorides, resulting in band gap energies in the range of 3.4 eV to 1.3 eV, spanning the whole visible spectrum. Notably, first photocatalytic water splitting tests reveal a considerable hydrogen evolution activity for x = 0.2, highlighting the potential of Ruddlesden–Popper oxyfluorides for solar energy applications.