2.3.6. TEM Testing

Single titanium dioxide particles were deposited on a 25 mm × 25 mm 1000 ◦C annealed mirrored steel substrate (SUS 304). In this cold-spray process, nitrogen gas was used as the process gas. The distance between the exit of the nozzle and the substrate was fixed at 20 mm. The process traverse speed was 2000 mm/s. Prior to spraying, the substrates were rinsed with acetone. Thin membranes for transmission electron microscopy (TEM, JEOL, Tokyo, Japan) observation were carefully made from deposited TiO2 particles onto annealed 1000 ◦C mirrored steel substrate by the focused ion beam (FIB) milling equipment (FEI Helios Dual Beam 650, FEI, Oregon, USA). Without further sample preparation, electron-transparent membranes were made and investigated by field emission gun (FEG) electron microscopy using a JEOL JEM-2100F FE-TEM with a scanning mode at an applied voltage of 200 kV. The elemental distribution of the membranes was acquired by means of STEM energy-dispersive X-ray spectroscopy (EDX).
