Large-area Si/TiO2 core/shell nanowire arrays were prepared for the first time by solution approach, followed by annealing in Ar (SiNWs/TiO2-Ar) and air (SiNWs/TiO2-air) atmosphere, respectively. Au nanoparticles were decorated onto the SiNWs/TiO2-Ar samples (SiNWs/TiO2@AuNPs-Ar) to further improve the photoactivity. XRD results showed that the TiO2 shells have an anatase crystalline structure. Photocurrent of solar water splitting was measured by a three electrode setup, in which the Si/TiO2 core/shell nanowire array served as the working electrode. It was found for the first time that Ar ambience annealing is more helpful improving the photocurrent density. The SiNWs/TiO2-Ar sample enhanced the photocurrent by 6.3 times compared to the SiNWs/TiO2-air sample. Furthermore, the SiNWs/TiO2@AuNPs-Ar arrays exhibited a photocurrent of 0.9 mA/cm2, 3.6 times as high as that of its counterpart without Au nanoparticles.