The physical-chemical phenomena associated with the heat treatment are reflected in the thermogravimetric curve in Fig. 1d. A mass loss of ~17%, between 40 and 420 °C, is attributed to evaporation of water adsorbed physically and chemically by the nanoparticles, as well as to the decomposition of a small portion of organic residues from the synthesis. This result suggests an additional heat treatment at ~400 °C, to remove residual compounds, which was subsequently applied.