###### Abstract
The ultrathin TiO\({}_{2}\) membrane was two atoms thick and Ag or Au NPs were dispersed on the membrane surface, as shown in Figure 1. The particle size ranged from 2 nm-10 nm. The particle size of the decorated Ag was the smallest (ca. 2 nm) when S\({}_{\text{Ag}}\) was prepared by photoreduction with ultraviolet light (Figure 1). Similarly, the particle size of the decorated Au was also the smallest (ca. 2 nm) in S\({}_{\text{Au}}\) prepared by photoreduction with ultraviolet light (Figure 2D). Hydrothermal synthesis at 180 \({}^{\circ}\)C furnished Au particles of ca. 10 nm in size on the TiO\({}_{2}\) membrane. Ag or Au NPs prepared by the photoreduction method were dispersed evenly within the TiO\({}_{2}\) network. However, the NPs aggregated together with hydrothermal synthesis. Generally, Ag and Au NPs were successfully decorated on the TiO\({}_{2}\) membrane. The S\({}_{\text{Ag}}\) and S\({}_{\text{Au}}\) samples exhibited some adsorption in the visible light region as shown in Supplementary Material S1, which could be attributed to the local surface plasmon resonance (LSPR) of the Ag and Au NPs. The LSPR effect on the surface of noble metal NPs is caused by the collective shock of conductive electrons generated by electromagnetic radiation. The size and shape of the noble metal NPs can change the surface density of the electromagnetic field and are thus important factors for the LSPR effect [27]. Obviously, the S\({}_{\text{Ag}}\) sample prepared by photoreduction had a strong absorption in the visible light region. For TiO\({}_{2}\), the absorption in the UV region resulted only from its wide band gap (3.2 eV). After the Ag and Au NPs were introduced, the light response range of TiO\({}_{2}\) clearly extended to the visible region. The Ag and Au NPs on the TiO\({}_{2}\) membrane could transfer free elections generated by the LSPR effect to the valence band of TiO\({}_{2}\) and consequently enhance the photocatalytic activity of TiO\({}_{2}\) under visible light [28; 29; 30]. Figure 1: TEM images of the TiO\({}_{2}\) membrane and those decorated with Ag or Au NPs. (**A**) TiO\({}_{2}\) membrane; (**B**) TiO\({}_{2}\) membrane decorated with Ag NPs by UV irradiation; (**C**) TiO\({}_{2}\) membrane decorated with Ag NPs by hydrothermal synthesis; (**D**) TiO\({}_{2}\) membrane decorated with Au NPs by UV irradiation; (**E**) TiO\({}_{2}\) membrane decorated with Ag NPs by hydrothermal synthesis. Figure 2 shows the photocatalytic degradation of RhB with P25 (TiO\({}_{2}\) particle with size 25 nm) and the decorated TiO\({}_{2}\) membrane. The degradation efficiency of RhB was evaluated by the first order reaction, which is presented in Supplementary Material S2. The catalytic efficiency of photocatalysts was ranked as S\({}_{\text{Ag-UV}}\) > S\({}_{\text{Ag-HY}}\) > S\({}_{\text{Au-UV}}\) > S\({}_{\text{Au-HY}}\) > 5\({}_{\text{O}}\) > P\({}_{\text{P25}}\). When P25 was used as the catalyst, the degradation of RhB under irradiation of visible light was very limited. Nevertheless, the degradation efficiency was improved notably when the TiO\({}_{2}\) membrane was used in the photodegradation reaction. The decorated TiO\({}_{2}\) membrane enhanced the degradation efficiency further. S\({}_{\text{Ag}}\) exhibited higher catalytic efficiency than S\({}_{\text{Au}}\). Besides, S\({}_{\text{Ag}}\)/S\({}_{\text{Au}}\) prepared by UV irradiation exhibited higher degradation efficiency than hydrothermal synthesis. In addition, the recycling experiment of S\({}_{\text{Ag-UV}}\) showed that it could be used over 50 times and that the photocatalytic efficiency of S\({}_{\text{Ag-UV}}\) had not reduced significantly, which is presented in Supplementary Material S3. Mechanistically, in the photocatalytic reaction, photogenerated electrons and holes on the TiO\({}_{2}\) membrane work as the active species to decomposing organic compounds. The existence of Ag and Au NPs on the TiO\({}_{2}\) membrane forms a Schottky junction on the surface of TiO\({}_{2}\), which promotes charge separation and improves the quantum efficiency.