**2. Materials and Methods**

All the high-purity chemicals used in this experiment were obtained from Sigma Chemical Co, Taiwan. The electrospun Al2O3 microfibers precursor was prepared by the electrospinning method. Typically, the precursor solution was prepared by dissolving aluminum nitrate (Al(NO3)3 9H2O) into 14.4 mL of dimethylformamide (DMF) solvent to make a 0.04 M metal source solution. Then, 2.4 g polyvinylpyrrolidone (Mw = 1,300,000) was mixed into the aforementioned prepared metal source solution followed by constant stirring for 6 h. Finally, a viscous gel-like precursor solution of Al2O3 was obtained. The Al2O3 precursor solution was loaded into a horizontal programmable syringe pump. A schematic image of the fundamental electrospinning process is illustrated in Figure 1. An ordinary electrospinning set-up, a high-voltage source is combined with the metallic needle, which is

connected to a syringe pump. This syringe pump was connected with Teflon tube (length = 125 mm, diameter = 4.2 mm) for conventional electrospinning setup. During the electrospinning process, the precursor solution was placed in a 10 mL syringe equipped with a stainless steel needle (ID = 0.5 mm). A voltage of 20 kV was applied to the stainless steel needle tip, and the collector was fixed at a distance of 16 cm from the needle tip with the flow controlled at 0.02 mL/h. The electrospun Al2O3 precursor was distributed uniformly over the collector to form Al2O3 precursor fibers (Step 1). After the electrospinning, the electrospun Al2O3 precursor fibers were heated at a rate of 5 ◦C/min to the annealing temperature of 600 ◦C in a high-temperature furnace at air atmosphere and then held at that temperature for 2 h, after which Al2O3 nanofibers were formed (Step 2) and the diameter of the Al2O3 nanofibers is <100 nm.

**Figure 1.** Schematic illustration of electrospinning preparation of as-spun fiber.
