On the one hand, the reaction mechanism and the kinetics could be revealed by in-situ TEM experiments. Alternatively, experiments carried out here using in-situ SEM Reactor and under entirely different conditions permitted the visualization of the overall process as a function of the growth parameters. Here, the precise heating profile available by the MEMS chip and the hydrogen pressure tuning within the bell-jar-shaped µReactor made the tool ideal for optimization of the reduction reaction. Figure presents the µReactor with the MEMS chip within SEM used in the present study. Full details of the µReactor with its MEMS chip is provided in the Experimental section. It is important to realize that the pressure inside the Reactor is ~500 Pa (5 Torr), while the vacuum in the SEM chamber is 10 -2 Pa (10 -4 Torr). The following discussion describes the optimization of the W18O49 nanowhiskers growth from the -WO3/a-SiO2 nanofibers in a hydrogen atmosphere and under vacuum at elevated temperatures (up to 900°C) using the µReactor within the SEM. Of the several experiments carried out in the µReactor within the SEM, two are described in detail (Table in Experimental part). The two experiments differed in their reaction temperature and hydrogen pressure. The goal was to study the whole reduction process while observing the morphological changes. Presumably, the even surface formation was caused by tungsten oxide mass transport from the -WO3/a-SiO2 fiber's body into the W18O49 nanowhiskers. Consequently, nanowhiskers grown early-on seeded formation of neighboring whiskers further on during the reaction, thus collectively generating bundles (Figure treaction < 1054 s) on the surface contour of the -WO3/a-SiO2 nanofiber. The emergence of W18O49 nanowhiskers into bundles was reported previously. At this point, the original nanofiber was partially covered with multiple W18O49 nanowhisker forming a bundle, which protruded outside. In addition, surface coarsening of the e-beam irradiated nanofiber was observed (see the arrow in Figure at treaction = 1443 s). This surface coarsening of the nanofibers occurred on scars in the vicinity of the nanowhiskers. The nanowhiskers in the final stage of the reaction were shrunk and in the form of bundles.