2 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
Self-Supporting Hierarchical Porous PtAg Alloy Nanotubular Aerogels as Highly Active and Durable Electrocatalysts
Developing electrocatalysts
with low cost, high activity, and good
durability is urgently demanded for the wide commercialization of
fuel cells. By taking advantage of nanostructure engineering, we fabricated
PtAg nanotubular aerogels (NTAGs) with high electrocatalytic activity
and good durability via a simple galvanic replacement reaction between
the in situ spontaneously gelated Ag hydrogel and the Pt precursor.
The PtAg NTAGs have hierarchical porous network features with primary
networks and pores from the interconnected nanotubes of the aerogel
and secondary networks and pores from the interconnected thin nanowires
on the nanotube surface, and they show very high porosities and large
specific surface areas. Due to the unique structure, the PtAg NTAGs
exhibit greatly enhanced electrocatalytic activity toward formic acid
oxidation, reaching 19 times higher metal-based mass current density
as compared to the commercial Pt black. Furthermore, the PtAg NTAGs
show outstanding structural stability and electrochemical durability
during the electrocatalysis. Noble metal-based NTAGs are promising
candidates for applications in electrocatalysis not only for fuel
cells, but also for other energy-related systems