## 3 Results and Discussion  
The Ag/TiO\({}_{2}\)/_p_-Si photocathode is synthesized by using the combination of electron-beam deposition and electrodeposition method. At first, a TiO\({}_{2}\) thin-film was deposited on _p_-Si substrate with 5 nm thickness using an electron-beam evaporator. The obtained TiO\({}_{2}\)/_p_-Si substrate was annealed in a tube furnace at 350 \({}^{\circ}\)C for 30 min under N\({}_{2}\) atmosphere. After that, Ag catalysts are deposited on the TiO\({}_{2}\)/_p_-Si substrate through the electrodeposition method. For the fabrication of R-Ag/TiO\({}_{2}\)/_p_-Si, electrochemical RA treatment was conducted on the Ag/TiO\({}_{2}\)/_p_-Si substrate. A Field-emission scanning electron microscopy (FE-SEM) was conducted to investigate the surface morphology changes induced by the RA treatment. Figure 1b-e shows the SEM images of the fabricated Ag/TiO\({}_{2}\)/_p_-Si photocathode and R-Ag/TiO\({}_{2}\)/_p_-Si photocathode. Figure 1b,c shows the morphology of Ag/TiO\({}_{2}\)/_p_-Si photocathode, which presented Ag particles of several hundred nanometers to micrometers in size have polygonal shapes. After pulsed anodization, the particles are merged into bigger particles randomly shaped (Supplementary Figure S2). However, as shown in Figure 1d,e, the reduction of Ag particles turned the Ag particles to change into the porous-like nanostructures on R-Ag/TiO\({}_{2}\)/_p_-Si photocathode. Compared with the Ag/TiO\({}_{2}\)/_p_-Si photocathode, the surface area of R-Ag/TiO\({}_{2}\)/_p_-Si photocathode was enlarged, and the flat surface of Ag particles was changed into the rough surface. To estimate the change of surface area, the double layer capacitance (C\({}_{\text{dl}}\)) at the solid/liquid interface for R-Ag/TiO\({}_{2}\)/_p_-Si and Ag/TiO\({}_{2}\)/_p_-Si photocathodes was measured by the cyclic voltammetry technique using different scan rates as shown in Figure S3a,b. Supplementary Figure S3c shows that the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode exhibited a higher C\({}_{\rm dl}\) of 0.18 mF cm\({}^{-2}\) than the Ag/TiO\({}_{2}\)/_p_-Si photocathode of 0.018 mF cm\({}^{-2}\), indicating that more active surface area has increased through the RA treatment. To investigate the changes of the crystalline structure after RA treatment, the Ag/TiO\({}_{2}\)/_p_-Si photocathode and the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode were analyzed by X-ray diffraction (XRD). Figure 2a displays the XRD patterns of the Ag and R-Ag catalysts on TiO\({}_{2}\)/_p_-Si photocathodes. XRD patterns for these two photocathodes show 2\(\theta\) reflection peaks at 38.2\({}^{\circ}\), 44.5\({}^{\circ}\), 64.5\({}^{\circ}\), and 77.5\({}^{\circ}\) which all correspond to diffractions along the (111), (200), (220), and (311) planes of the face-centered cubic Ag crystal structure. The Ag/TiO\({}_{2}\)/_p_-Si photocathode has the intense (111) peak, suggesting a preferred crystal orientation. However, XRD patterns for R-Ag/TiO\({}_{2}\)/_p_-Si photocathode became more randomly powder-like Ag reference peaks with relative increase of (200), (220), and (311) peaks compared to the Ag/TiO\({}_{2}\)/_p_-Si photocathode. Therefore, it was confirmed that not only the morphology was changed, but also the crystallinity of the Ag catalysts through RA treatment. Previously, the catalytic activity for electrochemical CO\({}_{2}\) reduction to CO was higher on the (110) plane than on (111) or (100) planes, because of higher density of undercoordinated active sites [31; 40]. For face-centered cubic structures, (110) plane cannot be observable in XRD patterns because the (110) reflection does not occur. Assuming that the amount of the (110) plane exists in proportion to that of the (220) plane, it can be deduced that the amount of the (110) plane is increased in the R-Ag/TiO\({}_{2}\)/_p_-Si photocathode after RA treatment. Therefore, it is expected that the electrochemical reduction of CO\({}_{2}\) efficiency can be enhanced by RA treatment due to an increase of the active sites on (220) planes.