1 research outputs found
Effect of Surface Properties on the Microstructure, Thermal, and Colloidal Stability of VB<sub>2</sub> Nanoparticles
Recent
years have seen an increasing research effort focused on nanoscaling
of metal borides, a class of compounds characterized by a variety
of crystal structures and bonding interactions. Despite being subject
to an increasing number of studies in the application field, comprehensive
studies of the size-dependent structural changes of metal borides
are limited. In this work, size-dependent microstructural analysis
of the VB<sub>2</sub> nanocrystals prepared by means of a size-controlled
colloidal solution synthesis is carried out using X-ray powder diffraction.
The contributions of crystallite size and strain to X-ray line broadening
is separated by introducing a modified Williamson–Hall method
taking into account different reflection profile shapes. For average
crystallite sizes smaller than ca. 20 nm, a remarkable increase of
lattice strain is observed together with a significant contraction
of the hexagonal lattice decreasing primarily the cell parameter <i>c</i>. Exemplary density-functional theory calculations support
this trend. The size-dependent lattice contraction of VB<sub>2</sub> nanoparticles is associated with the decrease of the interatomic
boron distances along the <i>c</i>-axis. The larger fraction
of constituent atoms at the surface is formed by boron atoms. Accordingly,
lattice contraction is considered to be a surface effect. The anisotropy
of the size-dependent lattice contraction in VB<sub>2</sub> nanocrystals
is in line with the higher compressibility of its macroscopic bulk
structure along the <i>c</i>-axis revealed by theoretical
calculations of the respective elastic properties. Transmission electron
microscopy indicates that the VB<sub>2</sub> nanocrystals are embedded
in an amorphous matrix. X-ray photoelectron spectroscopy analysis
reveals that this matrix is mainly composed of boric acid, boron oxides,
and vanadium oxides. VB<sub>2</sub> nanocrystals coated with these
oxygen containing amorphous species are stable up to 789 °C as
evidenced by thermal analysis and temperature dependent X-ray diffraction
measurements carried out under Ar atmosphere. Electrokinetic measurement
indicates that the aqueous suspension of VB<sub>2</sub> nanoparticles
with hydroxyl groups on the surface region has a good stability at
neutral and basic pH arising from electrostatic stabilizatio