## 3 Results and Discussion  
The fabrication of DSSCs based on freestanding TiO\({}_{2}\) NTAs is shown in Figure 1. The bottom layer was present in the closed-ended freestanding TiO\({}_{2}\) NTAs, but was removed by ion milling in the open-ended TiO\({}_{2}\) NTAs. The DSSCs were fabricated from both the open- and closed-ended freestanding TiO\({}_{2}\) NTAs to compare energy conversion efficiencies. In both cases, the freestanding TiO\({}_{2}\) NTAs were attached to the FTO glass with TiO\({}_{2}\) paste, and Ag NPs were synthesized using UV irradiation (Figure 1a). Carbon materials were synthesized using CVD (Figure 1b). By using the UV irradiation and CVD, Ag NPs and carbon materials were deposited in the channel of highly ordered TiO\({}_{2}\) NTAs without any distortion. The dye (N719) was adsorbed onto both types of freestanding TiO\({}_{2}\) NTAs (Figure 1c). Finally, DSSCs were fabricated by assembling the working electrode (freestanding TiO\({}_{2}\) NTAs on FTO glass) and the counter electrode (Pt on FTO glass), as shown in Figure 1d. FE-SEM images of TiO\({}_{2}\) NTAs are shown in Figure 2. The top view (Figure 2a) shows a pore size of approximately 100 nm after having applied the electrochemical method. The bottom of the TiO\({}_{2}\) NTAs before ion milling (\"closed-ended TiO\({}_{2}\) NTAs\") is shown in Figure 2b, with a total bottom pore size of approximately 100 nm (including that of the wall thickness). However, when the bottom was removed by ion milling to produce the \"open-ended TiO\({}_{2}\) NTAs\" (Figure 2c), the bottom pore was reduced to 30 nm in size, while the wall thickness was approximately 35 nm. An HAADF image of Ag NPs in the channels of TiO\({}_{2}\) NTAs is shown in Figure 2d, and the diameter of Ag NPs was approximately 30 nm. This allowed the Ag NPs to be successfully immobilized inside the channel of TiO\({}_{2}\) NTAs by UV irradiation, and the resulting plasmonic interactions may have affected all the surface areas. A side view of TiO\({}_{2}\) NTAs attached to the FTO glass by TiO\({}_{2}\) paste after being sintered at 450 \({}^{\circ}\)C is shown in Figure 2e. The main role of the TiO\({}_{2}\) paste is to connect the TiO\({}_{2}\) NTAs with the FTO glass surface. The thickness of the TiO\({}_{2}\) film layer was 3 \(\upmu\)m, and the length of the TiO\({}_{2}\) NTAs was approximately 18 \(\upmu\)m.  
Figure 1: Overall scheme of the fabrication of dye-sensitized solar cells (DSSCs), based on freestanding TiO\({}_{2}\) nanotube arrays (TiO\({}_{2}\) NTAs) coated with silver nanoparticles (Ag NPs) and carbon materials. (**a**) Synthesis of Ag NPs in the channel of TiO\({}_{2}\) NTAs, (**b**) deposition of carbon materials, (**c**) dye adsorption, and (**d**) fabrication of the DSSC. Figure 2: Field emission scanning electron microscope (FE-SEM) images of TiO\({}_{2}\) NTAs: (**a**) top view, (**b**) bottom view, (**c**) bottom view after ion milling, (**d**) high-angle annular dark-field (HAADF) image of Ag NPs in the channel of TiO\({}_{2}\) NTAs, and (**e**) side view of TiO\({}_{2}\) NTAs on fluorine-doped tin oxide (FTO) glass. Carbon materials on the TiO\({}_{2}\) NTAs were synthesized by CVD, and Figure 3 shows their structure as confirmed by Raman spectroscopy (TEM images of TiO\({}_{2}\) NTAs were shown in Figure S1). In a previous publication, we reported the optimization of TiO\({}_{2}\) NTAs for DSSCs using carbon materials [25]. The \(B_{1g}\) (397 cm\({}^{-1}\)), \(A_{1g}\) (518 cm\({}^{-1}\)), and \(E_{g}\) (641 cm\({}^{-1}\)) peaks indicated that the TiO\({}_{2}\) NTAs were in the form of anatase TiO\({}_{2}\), as shown in Figure 3a [31]. When carbon materials were synthesized on the TiO\({}_{2}\) NTAs using CVD, the \(G\) band at 1600 cm\({}^{-1}\) represented graphite, while the \(D\) band at 1384 cm\({}^{-1}\) was due to the disorderly network of sp\({}^{2}\) and sp\({}^{3}\) sites in the carbon materials (Figure 3b).