Nanocasting synthesis of mesoporous SnO2 with a tunable ferromagnetic response through Ni loading

Abstract

Undoped and Ni-doped ordered mesoporous SnO₂ powders ([Ni(II)]/[Sn(II)] = 0 : 100, 5 : 95, 15 : 85, 20 : 80) were synthesized by nanocasting from mesoporous KIT-6 silica. The resulting Ni content in the Ni-loaded powders ranged between 1 at% and 9 at%. Successful replication of the silica template was verified by scanning electron microscopies for all samples. Residual silicon content did not surpass 4 at%. X-ray diffraction analyses showed that the powders were nanocrystalline, being the rutile-like phase of SnO₂ the dominant structure. Changes in the lattice constants depending on the Ni content were observed, suggesting that Ni enters the rutile structure of SnO₂ to some extent. No extra phases attributed to Ni were detected in the powders except for the sample synthesized from 20 : 80 [Ni(II)]/[Sn(II)], for which NiO as secondary phase was observed. The oxidation state and spatial distribution of Ni in the powders was investigated by X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) measurements, respectively. For 6 at% and 9 at% Ni content, the presence of Ni²⁺ was established. The corresponding EELS mapping showed that a fraction of Ni (the one not forming part of the rutile phase) tended to accumulate at the pore edges, forming a nanometer-thick NiO layer. Compared to undoped SnO₂, Ni-containing powders exhibit a ferromagnetic response at low and room temperatures. Uncompensated spins at the surface of NiO are likely to contribute, in part, to the observed ferromagnetic properties.