Fig. 1. TEM images of samples
(a, b): NHC; (c, d): NHC@RF; (e, f): NHC@SiO₂; (g, h): NHC@RF@SiO₂

Fig. 2. XRD and TR-FTIR pattern of the samples
a: NHC; b: NHC@RF; c: NHC@SiO₂; d: NHC@RF@SiO₂

Fig. 3. Comparison of three infrared testing methods
(a): NHC; (b): NHC@RF; (c): NHC@SiO₂; (d): NHC@RF@SiO₂