Schottky heterojunctions based on graphene–silicon structures are promising for high-performance photodetectors. However, existing fabrication processes adopt transferred graphene as electrodes, limiting process compatibility and generating pollution because of the metal catalyst. In this report, photodetectors are fabricated using directly grown graphene nanowalls (GNWs) as electrodes. Due to the metal-free growth process, GNWs–Si heterojunctions with an ultralow measured current noise of 3.1 fA Hz^−1/2 are obtained, and the as-prepared photodetectors demonstrate specific detectivities of 5.88 × 10¹³ cm Hz^1/2 W⁻¹ and 2.27 × 10¹⁴ cm Hz^1/2 W⁻¹ based on the measured and calculated noise current, respectively, under ambient conditions. These are among the highest reported values for planar silicon Schottky photodetectors. In addition, an on/off ratio of 2 × 10⁷, time response of 40 μs, cut-off frequency of 8.5 kHz and responsivity of 0.52 A W⁻¹ are simultaneously realized. The ultralow current noise is attributed to the excellent junction quality with a barrier height of 0.69 eV and an ideal factor of 1.18. Furthermore, obvious infrared photoresponse is observed in blackbody tests, and potential applications based on the photo-thermionic effect are discussed.