Synthesis and characterization of plate-like cobalt phosphate particles by forced hydrolysis of a Co(NO3)2–Na4P2O7 solution

Abstract

The production of cobalt phosphate particles was examined using the forced hydrolysis reaction of Co(NO3)2–pyrophosphate (pp: P2O74−) solutions at 100°C for 18 h. The produced particles were characterized by various physicochemical techniques. No particles were produced at higher concentrations of pp or lower concentrations of Co(NO3)2. Plate-like cobalt pyrophosphate particles were produced at all other concentrations of the aforementioned molecules. The width and length of the large plate-like particles ranged from 654 to 1057 nm and from 293 to 571 nm, respectively, while the sizes of the small plate-like particles were ca. 70 nm in width and 45 nm in length. The thicknesses of the small plate-like particles present in two separate samples were estimated to be 16.8 and 16.3 nm from the TEM images. These values are in fair agreement with the calculated d values of 19.6 and 14.4 nm found using Bragg’s equation at XRD peaks of 2θ = 0.45° and 0.62°. The XRD patterns of the small plate-like particles exhibit two small characteristic peaks for Co2P2O7·2H2O, and the large particles produced peaks that were amorphous (broad bands). The concentrations of cobalt and phosphorus of the assayed particles were lower than the theoretical concentrations estimated from the chemical structure of Co2P2O7·2H2O. These lower values are believed to be caused by the lower crystallinity of the particles. The isotherms of H2O adsorption on the small amorphous plate-like particles were step-like at a relative pressure of 0.1–0.2. However, no step-like behavior was observed for other adsorbed molecules such as N2, CCl4, and CH3OH. The observed steps for the amorphous plate-like particles are explained by the swelling of the particles caused by the H2O molecules. The swelling model was further confirmed by the adsorption–desorption measurement of H2O. The adsorption of H2O molecules onto the small plate-like particles was proven to be a reversible reaction.