| Summary: | Recently, the engineering of optical bandgaps and morphological properties of graphitic carbon nitride (g-C<sub>3</sub>N<sub>4</sub>) has attracted significant research attention for photoelectrodes and environmental remediation owing to its low-cost synthesis, availability of raw materials, and thermal physical–chemical stability. However, the photoelectrochemical activity of g-C<sub>3</sub>N<sub>4</sub>-based photoelectrodes is considerably poor due to their high electron–hole recombination rate, poor conductivity, low quantum efficiency, and active catalytic sites. Synthesized Ni metal-doped g-C<sub>3</sub>N<sub>4</sub> nanostructures can improve the light absorption property and considerably increase the electron–hole separation and charge transfer kinetics, thereby initiating exceptionally enhanced photoelectrochemical activity under visible-light irradiation. In the present study, Ni dopant material was found to evince a significant effect on the structural, morphological, and optical properties of g-C<sub>3</sub>N<sub>4</sub> nanostructures. The optical bandgap of the synthesized photoelectrodes was varied from 2.53 to 2.18 eV with increasing Ni dopant concentration. The optimized 0.4 mol% Ni-doped g-C<sub>3</sub>N<sub>4</sub> photoelectrode showed a noticeably improved six-fold photocurrent density compared to pure g-C<sub>3</sub>N<sub>4</sub>. The significant improvement in photoanode performance is attributable to the synergistic effects of enriched light absorption, enhanced charge transfer kinetics, photoelectrode/aqueous electrolyte interface, and additional active catalytic sites for photoelectrochemical activity.
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