The films with divergent like a LiCl crystal were obtained in Figure 1 (a1, a2, and a3). The concentration of the film-forming solution was 1.0, 1.5, and 2 wt %, respectively. The DP of cellulose was about 700. It was noticed that the setting of growth temperature at 90 ℃, the crystal size was decreased with the increase of cellulose concentration. Cellulose film retained crystal morphology print after rinsing (Figure 1: b1, b2, and b3). The daisy and chrysanthemum-like crystals appeared on the film's surface when the DP of cellulose was 1400 (Figure 1: a5 and a6), while the size was also different due to the solution concentration variation.
Considering the formation process of the crisp daisy and chrysanthemum-like print, we further investigated the effect of temperature (80-120 ℃) on the morphology of LiCl crystal (Figure S1 and S2). It was observed that the size of the pattern increased first and then decreased in the process of increasing temperature from 80 ℃ to 120 ℃ (Figure S3). Meanwhile, an increasing number of branching was obtained with the increasing temperature (Figure S1 and S2). However, few regular patterns were obtained once the temperature was lowered than 90 ℃ (Figure S1 and S3). It showed that the formation of a perfect crystal needed enough activation energy.
Moreover, the print clarity was improved with the increase of concentration (viscosity) or DP according to the stereo-microscope picture (Figure S1 and S2) and characterization of surface profilometer (Figure 2: E4 and E5, Table S2 and S3). The main reason was that the growing DP and concentration could increase the viscosity of the solution, thus improving the matrix stability, which was conducive to stabilizing the arrangement of LiCl crystal. The higher temperature was beneficial to LiCl crystals' growth and formed a relatively perfect patterned structure under relatively stable substrates.
To further verify the effect of different viscosity (0.031, 2.02, and 19.94 Pa. S) on the morphology of LiCl crystal, the variable cellulose concentrations (1.0, 2.0, and 4.0 wt %) with DP of 300 were used to produce patterned films using the same technique at 100 ℃. While the viscosity was increased, the morphology of LiCl crystals on the cellulose film surface was changed from a divergent pattern (Figure 2: A and A1) to a daisy-like pattern (Figure. 2: B and B1). The pattern became curlier when concentration was increased to 4.0 wt % (Figure. 2: C and C1). It means that the pattern can be regulated by changing the cellulose solution concentration at a specific temperature. Moreover, it was observed that the petal width for daisy or line for divergent and the ridge height were different, although these films had a similar pattern. The characterization of the surface profilometer (Figure 2: E1 and E2, Table S2 and S3) exhibited the width between the neighboring ridge and ridge height at a distance of 1 mm and 2 mm from the center. Furthermore, it was identified that the width between the neighboring ridge and ridge height would continue to increase with increasing distance from the center. This trend was similarly observed with the increase of the cellulose solution viscosity (Figure 2: E4 and Table S2). However, only a little difference was observed on the height ridge and the width between neighboring ridges when the temperature was raised to 100 ℃ (Table S3). We understand that by changing the combination of concentration, temperature, and viscosity, many patterns of different heights and widths could be generated. The crisp pattern was like the unique characteristics of the fingerprint from humans (Figure 2: E3), which may be used for anti-counterfeiting field.
During the experiment, it was observed that various LiCl morphologies could be formed. Besides, their print can be retained on the surface of cellulose film even after rinsing. To further explore the effect of the substrate on LiCl crystals' formation, we first investigated the growth process of its crystal from the evaporation of the pure solvent (DMAc, without cellulose) at different temperatures. As shown in Figure S4, the morphologies of LiCl crystals were almost the same octahedral structure at different temperatures and concentrations of LiCl after evaporation. However, it was evident that the size of the LiCl crystal increased with the increase of temperature and concentration (Figure S4.b). A similar phenomenon was also observed in a previous report while NaCl crystal morphology was produced with the assistance of other compounds such as formamides and urea (Zhang et al. 2011). Since both LiCl and NaCl belong to the cubic crystal system, it explains a similar growth mechanism. The carbonyl oxygen in acetamide had a strong interaction with lithium ions, thus stabilizing the surface of the LiCl crystal (1 1 1), leading to the formation of the octahedral crystal (Figure S4.c). This control experiment also exhibited that the cellulose solution induced some influence on the growth of LiCl crystal.
We further studied the effect of the types of matrix and solvent on the formation of patterns. In contrast, CMC (containing hydroxyl group) and tri-cellulose acetate (little hydroxyl group) were selected as growth matrix for LiCl crystal, where solvents were DMAc and a different solvent, respectively. The morphology evolution of LiCl crystals in LiCl/DMAc and LiCl/water solvent systems was compared and discussed for the CMC matrix. Cubic crystals could also be formed in the CMC matrix regenerated from LiCl/water at 100 ℃, almost the same as pure solvent systems. At the same time, divergent patterns were only produced in the CMC matrix regenerated from LiCl/DMAc, which was similar to that from cellulose substrate (Figure S5: a and b). The formation processing of LiCl crystal could be observed on the surface of CMC films regenerated from solution due to its slower growth rate. It was recorded at 20 s, 60 s, and 120 s for 0.5 wt % (Figure 3: a1, b1, c1) and 2.0 wt % (Figure 3: a2, b2, c2) CMC solution, respectively. The pattern started from cubic LiCl crystals, evolved into the octahedral structure, and finally formed divergent or similar structures. For Tri-cellulose acetate substrates, a similar result was also obtained, although it had no hydroxyl groups.
The above experimental phenomena further revealed that the interaction between LiCl and amide solvent was the prerequisite of forming diversity morphology on a cellulose film. Accordingly, the LiCl crystal growth in the cellulose matrix and its influencing factors was schematically presented in Figure 3 (I, II, III). The solvent evaporated slowly when the temperature was relatively lower (T=70, 80 ℃). The cellulose solution was easy to flow and could not provide a stable matrix for the further growth of LiCl crystals at a low viscosity. Meanwhile, there was not enough activation energy for forming a perfect crystal at a low temperature. Consequently, the LiCl crystals were disseminated as particles in the matrix (Figure 3I). While the solution viscosity was increased, the cellulose solution provided a stable matrix for the growth of the LiCl crystal. The free LiCl molecule released by the evaporation of DMAc began to rearrange and gradually grew along a crystal plane of LiCl to form an octahedral structure. After that, these octahedral LiCl crystals could aggregate and form dendritic LiCl crystals on the film surface (Figure 3II). With the further increase of temperature (T=90, 100, 120 ℃), enough growth activation energy could be provided. Meanwhile, the evaporation rate of the solvent accelerated increased the solution viscosity and stability. Finally, the perfect crystal could be formed, such as divergent, daisy, chrysanthemum, and other morphologies.
The XRD curves of cellulose film with different patterns mainly showed the characterization peaks at 2θ = 12°, 20° (cellulose II) (Figure 4:a) (AlfredD. French. 2020; Xing L et al. 2018). The crystallinity order of film formed at different conditions was as follows: Dendritic film (80 ℃) > Divergent film (90 ℃) > Daisy film (100 ℃) > Film without a pattern (25 ℃). It could be seen that cellulose crystallinity was increasing firstly and then decreasing when the temperature ranged from 25-100 ℃. The daisy-like film (100 ℃) showed higher toughness among the patterned films. The tensile strength of films formed at high temperatures (80, 90, and 100 ℃) was improved, more obviously comparing the film at 25 ℃. The main reason was that LiCl was more easily crystallized due to the solvent evaporation quickly at higher temperatures (Figure S2 and S3). Crystal LiCl could be a nucleating agent during the film formation, which improved the crystallinity of cellulose film and favored the increase of tensile strength.
Interestingly, the pattern on the cellulose film exhibited an excellent anti-counterfeiting function similar to the fingerprint. As shown in Figure S6, the forefinger fingerprint was collected and stored on the phone. The phone locked was readily unlocked by the fingerprint printed on the pattern (Figure S6:c). But it was not unlocked by finger with patterned cellulose film. However, it was not worked after the pattern was collected and stored on the phone (Figure 4c and Video 1). In addition, the anti-counterfeiting function was sustained after one year. In this process, thirty trials were given every month and found all responsive in one second (Figure S7). It is also worth noting that the artificially fingerprinted taggant held up well in harsh conditions such as low (-20 ℃) or high temperature (60 ℃) (Figure S7), RH>65, and exposure time < 1h. The results showed that cellulose film with these patterns could be used as an anti-counterfeiting key like a fingerprint.