The
formation mechanism of nanoparticles is of great significance
for the controllable synthesis, structural design, and performance
optimization of nanomaterials. In this paper, an economical hydrothermal
method was used to synthesize zinc oxide (ZnO) nanorods. X-ray diffraction,
X-ray absorption fine structure, and small-angle X-ray scattering
techniques were used to probe the structural changes. Scanning electron
microscopy and high-resolution transmission electron microscopy were
used to observe the morphologies of the products. A self-designed
in situ temperature–pressure sample cell was used to control
the hydrothermal conditions. The results demonstrate that an unknown
intermediate phase, Zn(HCO3)2·H2O, was first formed at 50 °C, having a morphology of nanoflakes
with a average thickness of about 35 nm. The intermediate phase Zn(HCO3)2·H2O was determined to have a
monoclinic structure with space group P1211 and the following lattice parameters: a = 11.567
Å, b = 3.410 Å, c = 5.358
Å, β = 96.0011°, and Z = 2. After
a hydrothermal temperature of 140 °C, CO2 and H2O were evaporated from the Zn(HCO3)2·H2O intermediate product and the ZnO nanorods with
a wurtzite structure were formed. The final ZnO nanorods have an average
diameter of about 45 nm and an average length of about 2 μm.
The axial direction of the ZnO nanorods is the [001] crystallographic
direction. By virtue of understanding the formation mechanism, this
work is helpful for the controllable synthesis of ZnO nanoparticles.