*3.1. Structural Investigations*

Figure 1a presents the SEM image of the original bamboo, which comprises numerous sizable parenchyma cells. No other substances were observed on the bamboo surfaces, except the microstructure of the bamboo. After the first step, the TiO<sup>2</sup> thin films with an average thickness of 1.07 µm were self-aggregated by homogeneous TiO<sup>2</sup> NPs on the bamboo surface (Figure 1b,(b1)). Figure 1c–g display the SEM images of the as-prepared samples with different concentrations of AgNO3. As shown in Figure 1c, few Ag NPs appear for an AgNO<sup>3</sup> concentration of 0.005 M. As the AgNO<sup>3</sup> concentration increased from 0.005 to 0.01 M (Figure 1d), the nanosized Ag particles with an average size of ~35 nm were uniformly deposited on the TiO<sup>2</sup> thin films. As shown in Figure 1e, as the AgNO<sup>3</sup> concentration increased to 0.03 M, the particles were self-aggregated together, increasing the average diameters of the Ag NPs accordingly. Most of the uniform Ag NPs gradually vanished, and the particles became denser and even dissolved one another, forming Ag thin films on the TiO<sup>2</sup> surface (Figure 1g). The average thickness of composite thin films is approximately 1.15 µm (Figure 1(g1)). μ μ

**Figure 1.** SEM images of (**a**) unvarnished bamboo, (**b**) TB and its corresponding cross-sectional profile (**b1**), (**c**) ATB-5, (**d**) ATB-10 and the size distribution of Ag nanocrystals (inset), (**e**) ATB-30, (**f**) ATB-50, and (**g**) ATB-200 and its corresponding cross-sectional profile (**g1**). TB: TiO2/bamboo, ATB-*x*: the Ag-NP-decorated TiO2/bamboo samples were denoted as ATB-*x*, with *x* representing the solution concentration (5, 10, 30, 50, and 200 mM) of AgNO<sup>3</sup> as one of the raw materials.

The structure of the ATB-10 sample was further studied by EDS. EDS results confirmed the presence of Ti, Ag, O, F, and C, whereas elemental mapping revealed that the Ti and Ag components were broadly and densely dispersed over the entire sample surface (Figure S2). Figure 2 shows the relative intensity of each element in the EDS spectrum measured along the thickness direction (yellow line). The signals of C, Ti, and Ag at different positions

indicated that the Ag-modified TiO<sup>2</sup> composite thin films were successfully anchored to the bamboo surface.

**Figure 2.** SEM in the line-scanning mode and the element distribution for a cross-sectional profile of ATB-10. ATB-10: Ag/TiO2/bamboo; the solution concentration of AgNO<sup>3</sup> used is 10 mM.

The detailed crystal structures and chemical composition of the as-prepared samples were analyzed by XRD and XPS. As shown in Figure 3a, all samples exhibited similar diffraction peaks at approximately 16◦ , 22◦ , and 35◦ , which can be ascribed to the crystalline cellulose in bamboo. The samples all exhibited a typical anatase TiO<sup>2</sup> phase (JCPDS NO.71-1167), except the original bamboo. Additional diffraction peaks appeared at 38.1◦ , 44.4◦ , and 64.6◦ , which were assigned to the (111), (200), and (220) lattice planes of Ag, respectively [27]. No other characteristic diffraction peaks for impurities were observed in the pattern. However, the Ag diffraction peaks of the ATB-5 (green) sample could not be observed because of the relatively small amount and high dispersion of Ag metal. Notably, increasing the AgNO<sup>3</sup> concentrations from 0.01 to 0.2 M had no discernible effect on the diffraction peak intensity of the Ag metal phase. However, the diffraction peak intensity of crystalline cellulose decreased, suggesting that more Ag NPs were self-aggregated together, forming Ag thin films on the TB surface. This result is consistent with the SEM analysis, which also supported the conjecture of growth mechanism of Ag NPs on the TB surface, as shown in Figure 4b.

Research has previously suggested that only metallic Ag NPs have electron-storage ability. The chemical compositions and valence of Ag were further confirmed by XPS analysis. As shown in Figure S3, the survey spectra of ATB-10 revealed the existence of Ag, O, Ti, F, and C, which was consistent with the EDS results. As shown in Figure 3b, the XPS result of TB shows the core levels of Ti 2p1/2 and Ti 2p3/2 to be approximately at 464.6 and 458.9 eV, respectively, which was assigned to the Ti4+ in anatase TiO2. However, the Ti 2p binding energy of ATB-10 is slightly shifted from 458.9 to 459.2 eV compared with that of TB. This is because the Fermi level of Ag is lower than that of TiO2, so the conduction-band electrons of TiO<sup>2</sup> may be transferred to the Ag deposited on the surface of TiO2, which decreases the outer electron cloud density of Ti ions [28]. Figure 3c shows the high-resolution XPS scans over the Ag 3d peak. The main peaks at 368.5 and 374.5 eV were ascribed to Ag metal, while the binding energies at 367.8 and 373.8 eV were attributed to Ag2O. The two peaks detected at 368.8 and 374.7 eV could be attributed to Ag(NH3)<sup>2</sup> + ions [29]. This observation and XRD analysis results suggested that a small portion of Ag on the NP surface was oxidized to Ag2O during sample drying and handling under

normal ambient conditions, and the amount of Ag2O was too small to be detected by XRD. Many researchers have reported that a small amount of Ag2O on the Ag NP surface could enhance its stability [30].

**Figure 3.** (**a**) XRD patterns from top to bottom are those of original bamboo, ATB-5, ATB-10, ATB-30, ATB-50, and ATB-200. The inset shows a part of the amplification of the XRD pattern (ATB-10). The high-resolution XPS spectra of (**b**) Ti 2p and (**c**) Ag 3d. ATB-*x*: the Ag-NP-decorated TiO2/bamboo samples were denoted as ATB-*x*, with *x* representing the solution concentration (5, 10, 30, 50, and 200 mM) of AgNO<sup>3</sup> as one of the raw materials. **Binding Energy (eV) Binding Energy (eV)**

**Figure 4.** (**a**) Mechanism and process of nanosized Ag-modified TiO thin films anchored to **Figure 4.** (**a**) Mechanism and process of nanosized Ag-modified TiO<sup>2</sup> thin films anchored to the bamboo surface; (**b**) schematic representation of the mineralization model proposed for the deposition of Ag nanocrystals on TiO<sup>2</sup> thin films with low, medium, and high densities of [Ag(NH<sup>3</sup> )2 ] + solution concentration.
