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Graphene nanowalls (GNWs) are wall-like graphene nanosheets that are oriented vertically on a substrate. GNWs have a unique structure and special optoelectronic properties, which enables their use in photodetectors. In this paper, we use plasma-enhanced chemical vapor deposition to directly grow GNWs onto the surface of an n-type lightly doped Si substrate and to optimize the quality of the GNWs by adjusting the growth time and temperature. Furthermore, after the GNWs are lithographically patterned, we use a GNW-Si Schottky structure to develop photodetector arrays which are capable of detecting light from the visible to infrared light spectral range. Throughout the process, GNWs are directly synthesized on a Si substrate without using a catalyst or a transfer step. The process is simple and efficient. Under laser illumination at a wavelength of 792 nm, the highest on/off ratio at zero bias is approximately 105, and the specific detectivity is 7.85 × 106 cm Hz1/2/W. Under a reverse bias of 4 V, the measured responsivity of the detector reaches 1 A/W at room temperature. The device can also produce a light response in the near-infrared band. Upon laser illumination at a wavelength of 1550 nm, the detector shows a responsivity of 12 mA/W at room temperature.