Core-regenerated vapor-solid growth of hierarchical stem-like VOx nanocrystals on VO2@TiO2 core-shell nanorods: microstructure and mechanism

This paper presents a novel vapor-solid growth approach of stem-like 1D vanadium oxide nanostructures in a core-regenerated fashion on VO2@TiO2 core-shell rods under a very low temperature (400 degrees C). The composition and microstructure of the as-grown nanostructures were analyzed by HRTEM, EDS and XPS, confirming the vapor-solid mechanism. The confinement effect of the shell was analyzed by varying experimental parameters (coating thickness and heating rate), and the shell was found to play a critical role in generating and accumulating vapor with supersaturated concentration for the nanocrystal nucleation. Additionally, PVP was added as a surface modifier to favor the growth, probably by forming carbonaceous/nitrogen gaseous species as carrier media, as deduced by FTIR analysis. By exploring the impact of different heating rates, we found that the mass transfer process was facilitated only using a relatively high heating rate (>1.33 degrees C s(-1)). However, such a growth was still limited only within the early stages (similar to 15 min), because the confined vacuum interface will disappear with the progress of the crystallization of the nanoporous anatase shell. A unique 1D rods to 2D nano-flakes evolution can be achieved by increasing the heating rate from 1.3 degrees C s(-1) to over 8 degrees C s(-1). These findings provide a valuable understanding on the thermodynamics and kinetics during an unique VS based nanocrystal growing process, and also showed a new method to obtain hierarchical heterogeneous nanostructures.