Photoelectrochemical (PEC) CO2 reduction offers a promising way to carry out the CO2 sequestration and develop the carbon-neutral technology. Doped Si is by far one of the most technologically important semiconductors with high conductivity and narrow bandgap, potential for CO2 reduction. However, the previous reports on PEC CO2 reduction over Si electrodes were not involved with many nanostructures. Syngas, a kind of chemical feedstocks and a crucial intermediate for hydrocarbon fuels, is highly demanded for industry. Herein, we used a wafer-scale nanoconed Si arrays substrate to load the metallic nanoparticles (Au and Cu). Effective PEC CO2 reduction into syngas was achieved in the aqueous solution with no sacrificial reagents. In contrast, planar Si seldom generated CO under the same reactant environment. Our finding deepens the comprehension of PEC CO2 reduction over the nanostructured materials and gives an inspiration for rational design of the PEC catalysts towards solar-to-chemical conversion.