*2.3. Characterization*

The attenuated total reflection infrared (ATR) spectrum was recorded on a Nicolet IS50 Fourier transform infrared (FTIR) spectrometer (Thermo Fisher scientific instruments, Waltham, MA, USA) at resolution of 4 cm−<sup>1</sup> in the range of 400~4000 cm−1. A field emission scanning electron microscopy (SEM, JSM-6360LV, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL-2011, Tokyo, Japan) was employed to inspect the morphology. The surface area and porosity of samples were calculated by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method, respectively

(analysis gas: nitrogen; analysis time: 670 min; thermal delay: 30 s; outgas time: 12 h; bath temperature: 50 ◦C and outgas temperature: 150 ◦C) (Autosorb iQ, Quantachrome, Ashland, Virginia, USA) [29]. The crystallographic analysis of aerogels was performed on X-ray diffractometer (XRD) with Cu-Ka radiation (D/Max-RB diffractometer, Rigaku, Tokyo, Japan) from 5~50◦ by using the dried film of samples. The chemical composition was characterized on a K-Alpha 1063 X-ray photoelectron spectroscope (XPS) with Al Kα X-ray as the excitation source (Thermo Fisher scientific instruments, Waltham, MA, USA).
