Figure 2 Initiation mechanism of photoinitiator.
According to the different absorbance wavelength, PI can be divided into ultraviolet (UV, mainly below 400 nm), visible light (generally in the range of 400 nm to 700 nm) and near-infrared (NIR, usually greater than 700 nm) PI.[20-22] Besides, from the perspective of generated active species, PI can be divided into radical type and cationic type, in which radical type composed of cleavage type (Norrish Ⅰ) and hydrogen abstraction type (Norrish Ⅱ).[15, 23-24] The classification of PI is displayed in Figure 3.