Experimental procedure
ZrCl4 (0.01 mol, analytical grade) and NaBH4 (0.02,
0.04 or 0.08 mol, analytical grade) powders were placed
into a stainless steel autoclave with a quartz liner. The
autoclave was sealed under Ar atmosphere (1 bar) and
heated at 450, 500, 600 or 700 C for 6, 12 or 24 h,
followed by natural cooling to room temperature. The
products in the quartz liner were washed several times
with distilled water and absolute ethanol to remove the
impurities. The final products were vacuum-dried at 60
C for 4 h. Black powder was obtained.
X-ray powder diffraction (XRD) pattern was carried
out on a Rigaku Dmax-cA X-ray diffractometer with
CuKa radiation (k ¼ 1:54178 A). The morphology of
nanocrystalline ZrB2 was observed from transmission
electron microscopy (TEM) images taken with a Hitachi
H-800 transmission electron microscope. X-ray photoelectron
spectra (XPS) were recorded on a VGESCALAB
MKII X-ray photoelectron spectrometer, using
non-monochromatized MgKa X-rays as the excitation
source. The UV–vis absorption spectrum was recorded
on a Shimadzu UV-2410PC UV–vis spectrophotometer.
Thermogravimetric analysis (TGA) was collected with a
Shimadzu-50 thermoanalyzer apparatus under air.