Fig. 2(a) shows the TEM images of Fe3O4@C@TiO2 micropheres
(S3) obtained by the heat-treatment, in which the typical core–
shell structure can clearly be observed, and the slight decrease of
the thickness during the calcination process may be attributed to
the evaporation of physically and chemically adsorbed water and
the densification of the TiO2 network. The HRTEM image of TiO2
shell after the calcination is displayed in Fig. 2(b)