Chemistry, nanostructure and magnetic properties of Co–Ru–B–O nanoalloys

Abstract

In our previous works, Co–B–O and Co–Ru–B–O ultrafine powders with variable Ru content (x_Ru) were studied as catalysts for hydrogen generation through sodium borohydride hydrolysis. These materials have shown a complex nanostructure in which small Co–Ru metallic nanoparticles are embedded in an amorphous matrix formed by Co–Ru–B–O based phases and B₂O₃. Catalytic activity was correlated to nanostructure, surface and bulk composition. However, some questions related to these materials remain unanswered and are studied in this work. Aspects such as: 3D morphology, metal nanoparticle size, chemical and electronic information on the nanoscale (composition and oxidation states), and the study of the formation or not of a Co_xRu_1−x alloy or solid solution are investigated and discussed using XAS (X-ray Absorption Spectroscopy) and Scanning Transmission Electron Microscopy (STEM) techniques. Also magnetic behavior of the series is studied for the first time and the structure–performance relationships discussed. All Co-containing samples exhibited ferromagnetic behavior up to room temperature while the Ru–B–O sample is diamagnetic. For the x_Ru = 0.13 sample, an enhancement in the H_c (coercitive field) and M_s (saturation magnetization) is produced with respect to the monometallic Co–B–O material. However this effect is not observed for samples with higher Ru content. The presence of the Co_xB-rich (cobalt boride) amorphous ferromagnetic matrix, very small metal nanoparticles (Co and Co_xRu_(1−x)) embedded in the matrix, and the antiferromagnetic CoO phase (for the higher Ru content sample, x_Ru = 0.7), explain the magnetic behavior of the series.