Scanning Electron Microscopy (SEM) Analysis
The surface morphological characteristics of samples with different thermal treatment were observed using a SU8100 scanning electron microscope (Hitachi High- Technologies Corporation, Tokyo, Japan). The sample powder was stuck on a specimen holder using a double-sided conductive adhesive and then coated with a 10 nm gold film by ion sputtering. An accelerating voltage of 15 kV was used during observation.16 The sample images were obtained at a magnification of 3000 times.