## 1 Introduction  
The uncontrollable increase of CO2 in the atmosphere by the excessive consumption of fossil fuels has become an urgent environmental issue in the world, as it has been considered a primary cause of global warming [1; 2; 3; 4]. Because of this reason, the conversion of CO2 to useful low carbon fuels has attracted considerable research interest [5; 6; 7]. Among the various methods, the photoelectrochemical (PEC) reduction of CO2 has been regarded as a promising strategy because it utilizes solar energy and water to reduce CO2, which mimics natural photosynthesis [8; 9]. Depending on the catalysts and conditions, diverse chemicals, such as carbon monoxide, formic acid, or hydrocarbons can be produced [10; 11; 12; 13; 14; 15]. In aqueous electrolytes, the competitive hydrogen evolution reaction, which produces hydrogen gas, may occur together with the CO\({}_{2}\) reduction reaction. Therefore, syngas (synthesis gas, CO, and H\({}_{2}\) mixtures) can be obtained in aqueous electrolytes, which is a critical C1 feedstock for practical applications, including Fischer-Tropsch synthesis, methanol synthesis, and syngas fermentation [16; 17]. In the PEC system, many kinds of semiconductors, such as Si, oxide, nitride, and sulfide, have been studied for highly efficient PEC CO\({}_{2}\) reduction reactions [8; 18; 19; 20; 21; 22; 23]. Among the semiconductors, _p_-Si is known as one of the most promising materials for photocathodes because of its narrow bandgap of 1.1 eV, earth abundance, and high-level processing technique. However, unfortunately, since the catalytic activity of Si for CO\({}_{2}\) reduction reaction is poor, a high over-potential is needed to reduce CO\({}_{2}\) on Si photocathodes [18; 24]. Therefore, for highly efficient syngas production using an Si photocathode, catalysts that have high catalytic activity for a CO\({}_{2}\) reduction reaction are essential. Metals such as Au and Ag have been known to promote the conversion of CO\({}_{2}\) to CO by electrochemical reduction in aqueous electrolytes [11; 25; 26; 27]. Furthermore, the catalytic activity of these metals varies depending on the size of catalysts, morphology, crystallinity, oxidation states of metal catalysts [11; 28; 29; 30; 31]. [PERSON]'s group first demonstrated the enhancement of catalytic activity for CO\({}_{2}\) reduction reaction of metal catalysts by tuning the nanostructure and the oxidation states of metals through electrochemical reduction of metal oxides in 2012 [11; 32; 33]. The oxide derived Au electrode exhibited remarkable performance, with a very low over-potential of 140 mV and high selectivity for CO production of 98%. However, although Au electrodes have better catalytic activity than Ag electrodes, their application on the industrial scale is difficult because of the high cost of Au metal. For this reason, many kinds of electrochemical treatments on Ag electrodes have been studied as replacement of Au electrodes, such as oxide-derived Ag [34], Ag\({}_{2}\)CO\({}_{3}\)-derived Ag [35; 36], AgCl-derived Ag [37], and AgBr-derived Ag [38]. With these Ag catalysts, efficient and low-cost PEC CO\({}_{2}\) reduction to syngas using Si photocathodes could be realized. However, because Ag catalysts inhibit the light absorption onto Si photocathode, there is a trade-off between the selectivity for CO and the light absorption. Therefore, an appropriate structure for photocathodes is needed [8; 18]. In this study, we demonstrate the enhancement of Ag catalysts by reducing anodic (RA) treatment applied on _p_-Si photocathodes for efficient PEC CO\({}_{2}\) reduction to produce syngas in an aqueous electrolyte. Through the RA treatment, the properties of Ag catalysts, such as morphology, active sites, and electronic structure, are tuned to achieve high catalytic activity for the CO\({}_{2}\) reduction reaction. Furthermore, we propose a new structure for photocathode using SiO\({}_{2}\) patterning, which balances the catalytic activity for CO\({}_{2}\) reduction reaction and light absorption onto the Si photocathode.