Paper is an important medium for information dissemination and civilization transmission in daily human life (Ma et al., 2018; W. Wang, Xie, He, & Yin, 2014). Traditional paper-making technology is accompanied by pollution (Koutinas et al., 2014; Monte, Fuente, Blanco, & Negro, 2009) and the consumption of forest resources (Kong, Hasanbeigi, & Price, 2016). Its large-scale use will cause serious environmental problems, such as environmental pollution and global warming. Ink-free erasable paper based on color-changing materials provides a new possible way to solve this problem, which attracts people's attention. Erasable paper can exhibit reversible color changes under environmental stimulation of light (Garai, Mallick, & Banerjee, 2016; Shteyner, Srivastava, Chigrinov, Kwok, & Afanasyev, 2013; Yang, Xu, Li, & Gao, 2019; T. Zhang et al., 2018), heat (Chen, Weng, Huang, & Zhang, 2018; Horiguchi, Koshiba, Ueda, Origuchi, & Tsutsui, 2008; Kinashi, Horiguchi, Tsutsui, Ishida, & Ueda, 2010; Lefter et al., 2015), water (Mukhopadhyay, Praveen, Hazra, Maji, & Ajayaghosh, 2015; Sheng et al., 2014; Xi et al., 2018), electricity (Hayes & Feenstra, 2003; Kobayashi, Miura, Nishimura, & Urano, 2008; Mortimer, Dyer, & Reynolds, 2006), magnetism (Ge, Kwon, & Yin, 2010; M. Wang, He, Hu, & Yin, 2013), and mechanical stress (Hu et al., 2016; Mo et al., 2017; L. Wang et al., 2015). It can be used to write and erase information circularly. The emergence and development of erasable papers are expected to significantly reduce traditional paper use, which is significant for environmental protection and sustainable development.
Among all kinds of erasable media, photo-responsive erasable paper has become one of the research hotspots in this field because of its convenience, rapidness, non-contact writing, pollution-free, and other advantages. At present, rewriteable papers based on color change factors of inorganic (Kang et al., 2019; Kim et al., 2016; Li et al., 2018; Nagaoka, Shiratori, & Einaga, 2008; W. Wang et al., 2015; Z. Wang et al., 2010; Yang et al., 2019; Yongqi, 2019), organic (Irie, Fulcaminato, Matsuda, & Kobatake, 2014; Tian, Wang, Li, Zeng, & Chen, 2017; Yonekawa, Mutoh, Kobayashi, & Abe, 2018), and inorganic-organic composite (Escribano et al., 2008; Pardo, Zayat, & Levy, 2011) systems have been developed. Nowadays, most acquired organic photochromic factors fade quickly under visible light, making it difficult to realize long-term information storage. Comparatively, inorganic photochromic materials have the characteristics of high stability, good reversibility, and strong anti-interference, showing excellent industrialization potential.
Common inorganic photochromic materials include silver halide (Kang et al., 2019; Shen, Zhang, Kang, Hao, & Fu, 2019), transition metal oxides (such as TiO2 (W. Wang et al., 2015; S. Zhang et al., 2018), WO3 (Kim et al., 2016) ), and polyoxometalates (POM) (Yang, Guan, & Gao, 2018; Yang et al., 2019; Yongqi, 2019; Zhang, Guo, Zhang, & Hao, 2020). POM has good water solubility and is easy to compound with water-based materials, so it is one of the most widely studied inorganic color-changing materials. Especially molybdates, which have high specific surface and surface energy, multiple active sites, and high selectivity, have attracted the attention of many scholars. Of them, ammonium molybdate [(NH4)6Mo7O24] has the above characteristics and the cost advantage of being cheap and easy to obtain. Under the irradiation of ultraviolet rays, Mo (VI) is reduced to Mo(V) by trapping electrons in ammonium molybdate, showing a photochromic appearance from colorless to blue. Under the condition of rich oxygen, Mo (V) can be oxidized to Mo (VI), and the blue appearance will return to colorless(Coronado & Gomez-Garcia, 1998). In this process, the spatial structure of ammonium molybdate has not changed, and its photochromism is very stable, favored by researchers. For example, Yang et al. (Yang et al., 2018) designed photochromic hydrogel with high sensitivity and durability by combining ammonium molybdate into a polyacrylamide network, which is expected to be applied to flexible visual display and optical storage devices. But its high transparency is not conducive to the reading of printed content. Afterward, Zhang et al. (Zhang et al., 2020)used ammonium molybdate as a photochromic factor to prepare polyacrylamide self-adaptive photochromic organic hydrogel containing antifreeze glycerin and triblock copolymer, which has the characteristics of adjustable transparency, durable thermal performance, and excellent environmental stability. However, at present, the information storage time of hydrogel based on polyacrylamide is short, usually less than 6 hours. Yang et al. (Yang et al., 2019) introduced ammonium molybdate into the polymer film to form a more compact compound than hydrogel, which reduced the chance of contact with oxygen, thus prolonging the color retention time. Still, the dense network structure was not conducive to the erasure of printed information. Subsequently, the research group introduced triglycol to absorb water vapor quickly, loosen its network structure, facilitate oxygen diffusion, and completely fade the film in the humid air. However, it still has some problems, such as low fading efficiency, and the mechanical properties and surface smoothness of paper will be destroyed in the process of fading with the help of water vapor, resulting in adverse effects on its repeated use. Therefore, the realization of optically controlled erase-write of ammonium molybdate film is still an important problem to be solved urgently.
Sodium alginate (SA) (Che, 2017) is a natural linear polysaccharide extracted from brown algae, which has good biodegradability, compatibility, and film-forming property. So, it has become one of the important components of many functional paper materials (WANG Wen-zhuo, 2022). However, the lack of poor toughness has hindered its popularization. Generally, the strength and degradability of paper are changed by adding high molecular materials such as carboxymethyl cellulose. Especially water-soluble polymer polyvinyl alcohol (PVA) (Zhao, Liu, Hui, & Yi, 2016), which has good film-forming properties and adhesive force, can effectively improve functional paper's tensile strength and folding resistance when combined with sodium alginate. At the same time, its biodegradable characteristics make it environmentally friendly. Therefore, the combination of sodium alginate and PVA has become an important choice direction for composite film-forming materials.
In this study, ammonium molybdate was introduced into the composite substrate of PVA and SA, to prepare the photo-erasable film. And the dense film network formed by PVA and SA can prolong the color retention time of ammonium molybdate after discoloration so that the film has the advantage of long-term information storage. At the same time, sodium alginate and polyvinyl alcohol are dissolved in a water solution to realize fading and then regenerated and reused. This approach is straightforward, environmentally friendly, and does not use any organic solvents at any point. This is an environment-friendly closed-loop process of "information writing-dissolution erasing-regeneration writing", and the realization of this method will provide a new strategy for preparing and regenerating optically writable paper.