W18O49 Nanowhiskers Decorating SiO2 Nanofibers: Lessons from in-situ SEM/TEM Growth to Large Scale Synthesis and Fundamental Structural Understanding

Tungsten suboxide W18O49 nanowhiskers is a material of great interest due to its potential high-end applications in electronics, near-infrared light shielding, catalysis, and gas sensing. The present study introduces three main approaches for the fundamental understanding of W18O49 nanowhisker growth and structure. Firstly, W¬18O49 nanowhiskers were grown from WO3/a SiO2¬ nanofibers in-situ in a scanning electron microscope (SEM) utilizing a specially designed micro-reactor (uReactor). It was found that irradiation by the electron beam (e-beam) slows the growth kinetics of the W18O49 nanowhisker markedly. Following this, an in-situ¬ TEM study led to some new fundamental understanding of the growth mode of the crystal shear planes in the W¬18O49 nanowhisker and the formation of domain (bundle) structure. High-resolution scanning transmission electron microscopy (HRSTEM) analysis of a cross-sectioned W18O¬49 nanowhisker revealed the well-documented pentagonal Magnéli columns and hexagonal channels characteristics for this phase. Furthermore, a highly crystalline and oriented domain structure and previously unreported mixed structural arrangement of tungsten oxide polyhedrons were analyzed. The tungsten oxide phases found in the cross-section of the W18O49 nanowhisker were analyzed by nanodiffraction and electron energy loss spectroscopy (EELS), which was discussed and compared in the light of theoretical calculations based on the density functional theory (DFT) method. Finally, the knowledge gained from the in-situ SEM and TEM experiments was valorized in developing a multigram synthesis of W18O49/a-SiO2 urchin-like nanofibers in a flow reactor.