*2.3. Scanning Electron Microscope Test*

The eight groups of prepared samples were cut into strips with a size of <sup>10</sup> × <sup>2</sup> × 2 mm3, and they were placed in liquid nitrogen to freeze for 5 min. Then, the test strips were taken out with tweezers and broken in the middle, and the cross-sections of the samples were treated with gold spraying. Finally, the cross-sections after gold spraying were observed by a scanning electron microscope (Thermo Fisher Scientific, Helios G4 UC, Waltham, MA, USA) to obtain backscattered electron (BSE) images.
