In another embodiment of the experiment (Experiment No. 2-see Sec. Supplementary Discussion 2 and its visualization in Video 2), the precursor (-WO3/a-SiO2 nanofibers) was heated in the µReactor without being exposed to hydrogen gas by heating the specimen under high vacuum. In this case, the cap of the reactor was retracted, and the reaction volume was opened to the SEM chamber as shown in Figure .