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Abstract:
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 VB2 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 c.
Exemplary density-functional theory calculations support this trend. The
size-dependent lattice contraction of VB2 nanoparticles is associated
with the decrease of the interatomic boron distances along the c-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 VB2 nanocrystals is in line with the higher compressibility of its
macroscopic bulk structure along the c-axis revealed by theoretical
calculations of the respective elastic properties. Transmission electron
microscopy indicates that the VB2 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. VB2 nanocrystals coated with these oxygen containing amorphous
species are stable up to 789 degrees 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 VB2 nanoparticles with hydroxyl groups on the
surface region has a good stability at neutral and basic pH arising from
electrostatic stabilization
