After that, the above precursor was firstly heated to 800 °C at a heating rate of 5 °C/min, then to 1200 °C at 3 °C/min and maintained at this temperature for 2 h in argon using an alumina tube furnace. Afterwards, the precursor powder was continued to heat from 1200 to 1550 °C at a heating rate of 2 °C/min and kept at this temperature for 2 h. Then, the sample was cooled to room temperature at a cooling rate of 5 °C/min. In the end, gray powder was obtained.