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

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 <i>c</i>. 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 <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 VB₂ 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 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 stabilizatio