Fig. 7 presents a series of SEM pictures of the SU6656 synthesized materials. The material recovered directly after the discharge was switched off (i.e. the non-aged A0 sample) shows numerous small particles with sizes of 200–500 nm (Fig. 7a). After ageing for 1 h, solid spheres (500 nm–1 μm in diameter) with small nanorods on their surfaces were obtained (Fig. 7b). As the ageing time was prolonged to 2 h, the spheres became larger (1–2 μm in diameter), and the nanorods on the surfaces grew longer and denser (Fig. 7c). After 3 h, uniform sea-urchin like hollow spheres were formed (Fig. 7d). This result shows that when the iron (II) solution is exposed to the plasma discharge, many iron (hydr) oxide nanoparticles are formed. During the ageing process, these nanoparticles spontaneously aggregated into large spheres to minimize the overall surface energy of the system. Subsequently, short nanorods were grown on the surfaces due to the preference of transition metal oxides for one-dimensional growth ,  and .