Phase transformations of (Co Mg1-)3Si2O5(OH)4 phyllosilicate nanoscrolls upon heating in Ar, O2 and H2 containing atmospheres

In the last decades, transition elements doping of phyllosilicates greatly expanded the range of their applications including adsorption, catalysis, and composite materials manufacturing. The preparation of such materials and their subsequent operation often involves continuous thermal treatment. However, information on the thermal behavior of transition metal-doped phyllosilicates is still lacking. Here this gap was filled for the case of synthetic (CoxMg1-x)3Si2O5(OH)4 (x = 0.2, 0.4, 0.6, 0.8 and 1) phyllosilicate nanoscrolls treated in air, Ar and ArH2 gas flows in the 25–1000 °C range. Presence of Co2+, which could be both oxidized and reduced, initiated different phase changes depending on the atmosphere type. Two principal transitions occurred during heating in Ar flow. The first transition was related to the formation of sepiolite-like phase with partial amorphization (after 400 °C). The second transition was the crystallization process of orthosilicate (after 700 °C). These two main processes were accompanied by the appearance of Co3O4 when heated in air atmosphere (at about 300 °C) or the formation of Co0 nanoparticles when heated in ArH2 atmosphere (after 400 °C). The study revealed several morphological features during heat treatment in various gaseous media. First, Co0 nanoparticles oxidized and covered themselves with an oxide shell if they were not stabilized in the phyllosilicate matrix. Second, the metal tended to fill the nanoscrolls inner channel, forming extended nanorods. Finally, double-walled phyllosilicate nanotubes demonstrated exceptionally high thermal stability, conserving their morphology even up to 800 °C.