In situ microcalorimetry study of ZnFe2O4 nanoparticle formation under solvothermal conditions

Abstract

Solvothermal methods have been widely used to synthesize different kinds of materials. However, only little is known about how precursor solutions react to form solid precipitates via this method. In the present study, in situ microcalorimetry is first used to investigate the formation mechanism under solvothermal conditions, where ZnFe₂O₄ synthesized using a solvothermal method was selected as the model sample. Some novel results are obtained, such as with the experimental temperature increase, (1) the homogeneous solution transforms to a gel containing amorphous Fe₂(C₂H₄O₂)₃ and ZnC₂H₄O₂, and NaNO₃ crystals; (2) the gel dissolves; (3) α-(Fe,Zn)OOH and α-Fe₂O₃ are synthesized; (4) the α-(Fe,Zn)OOH transforms to α-Fe₂O₃; (5) Fe²⁺ is formed at about 159 °C, which acts as a catalyst for the formation of Fe₃O₄; (6) the Fe₃O₄ crystals are synthesized at about 200 °C; (7) the Fe₃O₄ is transformed to the ZnFe₂O₄ with the help of NO₃⁻, and the reaction was kept at 200 °C for about 20 h. This study shows a facile in situ method for the investigation of reaction processes of solvothermal methods.