*3.3. Optical Properties*

Figure 5a presents the UV−Vis absorbance spectra of the original bamboo, TB, AB-10, and ATB prepared in the presence of AgNO3: 5, 10, and 30 mM. The original bamboo exhibits strong absorption in the UV region and poor light absorption in the visible-light region from 400 to 800 nm as well as the TB sample. The samples exhibited strong visiblelight absorption after the addition of Ag NPs owing to localized surface plasmon resonance. In other words, they react to visible light. Moreover, the smaller the size of Ag NPs, the greater the intensity of light absorption [36]. When comparing ATB-30 with ATB-10, the intensity of visible-light absorption decreased, implying that the Ag NPs began to grow and agglomerate. The results in Figure 5a are consistent with SEM experimental data. The efficiency of plasmon-mediated electron transfer is dominated by the size of the Ag

NPs, which plays a critical role in determining the reduction potentials of the electrons transferred to the TiO<sup>2</sup> conduction band [37].

**Figure 5.** (**a**) UV–Visible DRS and (**b**) PL spectra (excited wavelength: 300 nm) of the original bamboo, TB, AB-10, and ATB prepared in the presence of AgNO<sup>3</sup> : 5, 10, and 30 mM. 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, and 30 mM) of AgNO<sup>3</sup> as one of the raw materials, AB-10: Ag/ bamboo; the solution concentration of AgNO<sup>3</sup> used is 10 mM.

For semiconductor nanomaterials, the PL spectra are related to the transfer behavior of the photoinduced electrons and holes, so the separation and recombination of photoinduced charge carriers can be reflected. Figure 5b shows the PL spectra of the original bamboo, TB, AB-10, and ATB prepared in the presence of AgNO3: 5, 10, and 30 mM. We discovered that the original bamboo had a much higher PL intensity than other samples. Compared with TB, the intensity of the PL signal for the Ag-decorated samples was much lower, indicating that the deposition of Ag reduced the recombination rate of electrons and holes under light irradiation. The PL intensities of these samples varied in the following order: original bamboo > TB > ATB-5 > AB-10 > ATB-10 ≈ ATB-30. This result could be attributed to the existence of Ag NPs decorated on the TiO<sup>2</sup> thin films, which act as electron trappers to inhibit the recombination of photogenerated electrons and holes and decrease the PL intensity. Generally, the low PL intensity showed a high separation rate of photogenerated electron–hole pairs, resulting in a high photocatalytic activity. Therefore, a lower PL intensity indicates that the ATB samples have higher photocatalytic activities [38].
