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Samples of biochar-coupled g-C 3 N 5 /C have been prepared using 3-amino-1,2,4-triazole and peanut oil as precursors to increase the specific surface area of g-C 3 N 5. The results showed that the specific surface area of g-C 3 N 5 /C-1.0 reached 44.7 m2 g−1, and the C/N ratio increased from 0.56 to 1.48. The weakness of its photoluminescence (PL) peak confirmed that g-C 3 N 5 /C-1.0 had the best separation of photogenerated electrons and holes. Furthermore, the fluorescence lifetime of g-C 3 N 5 /C-1.0 is extended to 6.4 ns, compared to 2.9 ns for g-C 3 N 5. Therefore, g-C 3 N 5 /C-1.0 displays superior photocatalytic activity to that of pure g-C 3 N 5 under visible-light irradiation. This is because the large specific surface area leads to increased absorption of visible light and efficient separation of electron–hole pairs. The hydrogen evolution rate of g-C 3 N 5 /C-1.0 is 303.3 μmol g−1 h−1 (cf. 67.5 μmol g−1 h−1 for g-C 3 N 5), and the photodegradation efficiency of rhodamine B (RhB) is as high as 93.7% (cf. 26.1% for g-C 3 N 5). Sample g-C 3 N 5 /C-1.0 also maintained outstanding photocatalytic stability after four cycles. Radical scavenger experiments and ESR tests have shown •O 2 − to be the main reactive species responsible for RhB degradation under visible light. • Biochar-doped g-C 3 N 5 was synthesized by one-step method with peanut oil as biochar source. • Biochar-doped g-C 3 N 5 can improve the specific surface area and light absorption ability. • g-C 3 N 5 /C-1.0 exhibited H 2 evolution of 303.3 μmol g−1 h−1, which was 4.5 times than that of pure g-C 3 N 5. • The photodegradation to rhodamine B of g-C 3 N 5 /C-1.0 can reach 93.7% after 150 min. • The g-C 3 N 5 /C-1.0 has good photocatalytic activity and chemical stability. [ABSTRACT FROM AUTHOR]