Effect of Surface Properties on the Microstructure, Thermal, and Colloidal Stability of VB2 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 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 °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