The carbon dioxide (CO2) transformation into beneficial fuels or chemicals by photoelectrocatalytic (PeC) reaction has been proved sustainable. As a standalone or component in composites, the graphitic carbon nitride (g-C3N4) catalysts were significantly utilized for PeC approaches. In this research work, as a first instance, a sustainable green approach of simple hydrothermal treatment at various temperatures, such as 90, 120, 150, and 180 °C, was adopted to synthesize g-C3N4 (xgCN) catalysts to enrich their PeC CO2 reduction into methanol activity. The simple hydrothermal treatment achieved increased stacking layer distance of g-C3N4 and fine-tuning of the band gap. The 120gCN catalyst (treated at 120 °C) exhibited a superior current at − 1.0 V of 3.62 mA cm−2 and a 4.39 µmol L−1 h−1 cm−2 methanol generation in PeC CO2 reduction reaction, which was around twofold superior compared to that of PgCN (untreated g-C3N4) catalyst. The excellent efficiency of the 120gCN catalyst was credited to its better production and lesser electron (e−)-hole (h+) pairs recombination rate.