*3.2. Formation Mechanism of ATB Samples*

Bamboo is hydrophilic, with plentiful active hydroxyl groups, and the hydroxyl groups in a bamboo substrate can react with certain metal oxides such as ZnO [31], TiO<sup>2</sup> [26],

γ-Fe2O<sup>3</sup> [32], and Cu2O [33]. This method uses the hydrolysis of a solution containing TiF<sup>6</sup> <sup>2</sup><sup>−</sup> in the presence of H3BO<sup>3</sup> as a fluoride scavenger. The fabrication of TiO<sup>2</sup> thin films on the bamboo surface was accomplished by heterogeneous nucleation and homogeneous growth. For the initial heterogeneous nucleation on the bamboo surface, the existence of plentiful R–OH groups as active sites promoted the formation of R–O–Ti linkages between the bamboo surface and TiO<sup>2</sup> particles (Figure 4a).

$$\text{(bamboo)R-OH} + \text{HO-Ti} \rightarrow \text{(bamboo)R-O-Ti} + \text{H}\_2\text{O} \tag{1}$$

The nucleated TiO<sup>2</sup> layer on the bamboo substrate could serve as the seed layer to further boost the homogeneous condensation of the TiO<sup>2</sup> NPs. For the further growth of TiO<sup>2</sup> NPs, the Ti–OH groups present on the surface of previous TiO<sup>2</sup> NPs connected with bamboo could continue to act as the active sites for the subsequent particle growth via olation and oxolation, forming Ti–O–Ti linkage (Figure 4a) [34].

$$\text{Ti-OH} + \text{HO-Ti} \rightarrow \text{Ti-O-Ti} + \text{H}\_2\text{O} \tag{2}$$

From the cross-sectional profile of TiO<sup>2</sup> thin films, columnar crystal growth in the (001) direction can be seen on the bamboo substrate (Figure S4). This columnar morphology is consistent with the XRD measurement, which showed a significantly enhanced peak of (004) reflection (Figure S5). Previous research has demonstrated that the selective adsorption of anions on specific surfaces parallel to the (001) direction can inhibit crystal growth perpendicular to the (001) direction [34]. In our project, different types of anions, such as F−, BO<sup>3</sup> <sup>3</sup>−, BF<sup>4</sup> <sup>−</sup>, and TiF<sup>6</sup> <sup>2</sup>−, were included, which could influence the growth orientation of TiO<sup>2</sup> crystals. Furthermore, the ζ potential of TiO<sup>2</sup> particles obtained using this reaction system was also confirmed to be negative owing to the strong adsorption of anions contained in the solution [34]. The XPS results also supported this standpoint because the presence of F<sup>−</sup> anions on the surface of TB and the F<sup>−</sup> ions on the TiO<sup>2</sup> surface could act as the active sites for the subsequent Ag nanocrystal growth (Figure S6).

In step II, when [Ag(NH3)2] <sup>+</sup> was introduced, positively charged anions were drawn to a negatively charged TiO<sup>2</sup> surface covered by F<sup>−</sup> or OH groups owing to an attractive electrostatic force [35]. The silver mirror reaction generally involves the chemical reduction of the Ag compound into elemental Ag in the solution. The formed Ag subsequently nucleated on the surface of TiO<sup>2</sup> thin films. Figure 4b illustrates the nucleation mechanism of Ag nanocrystals that can be proposed based on SEM observations (Figure 1c–g). TB surfaces provide a certain number of nucleation sites to synthesize Ag nanocrystals. At low-level concentrations of [Ag(NH3)2] + , the nucleation sites are sufficient to deposit Ag nanocrystals. Ag nanocrystals are uniformly deposited on nucleation sites as the concentration of the precursor solution increases. If a high concentration of the precursor solution is provided, the nucleation sites are insufficient for grafting the Ag nanocrystals, resulting in the formation of Ag thin films coated on the surface of TB, as shown in Figure 1c–g.
