fig2

Figure 2. SEM images of S1 (A), S2 (B), S3 (C), S4 (D). TEM images of S1 (E), S2 (F), S3 (G), S4 (H); HRTEM images of S1 (I), S2 (J), S3 (K), S4 (L) and SAED spectrum of S1 (M), S2 (N), S3 (O), S4 (P). SEM: Scanning electron microscopy; TEM: transmission electron microscope; HRTEM: high-resolution transmission electron microscopy.