electron microscope (SEM:
S-3000 N, Hitachi, Japan) on gold-coated specimens. Close examination
of the microstructure of the samples was further carried
out by transmission electron microscopy (TEM: JEOL 2100, Japan).
High-resolution X-ray photoelectron spectroscopy (XPS) studies
were performed with a Kratos AXIS Ultra X-ray photoelectron spectrometer.