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 (xRu) 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 CoxRu1-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 xRu = 0.13 sample, an enhancement in the Hc (coercitive field) and Ms (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 CoxB-rich (cobalt boride) amorphous ferromagnetic matrix, very small metal nanoparticles (Co and CoxRu(1-x)) embedded in the matrix, and the antiferromagnetic CoO phase (for the higher Ru content sample, xRu = 0.7), explain the magnetic behavior of the series.