Display omitted •Calligraphy ink treated with acid introduced oxygen groups into BC, maximizing active sites for H2O2 production.•Hydrothermal synthesis resulted in oxygen-functionalized BC (O-BC/ZnIn2S4), enhancing H2O2 production.•In ZnIn2S4-mediated H2O2 production, we observed a dual phenomenon of production and decomposition.•pH adjustments, especially under basic conditions, led to a ten-fold reduction in ionization, boosting H2O2 production by approximately 14%. Hydrogen peroxide (H2O2), a powerful oxidizing agent, plays a critical role in various sectors including bleaching, wastewater treatment, sterilization, chemical manufacturing, healthcare, energy generation, and environmental cleanup. The conventional method for producing H2O2, known as the anthraquinone process, faces challenges due to its economic inefficiencies and significant safety concerns. This has spurred interest in developing safer and more environmental friendly photocatalysts for H2O2 production. These photocatalysts, when dispersed in water and exposed to sunlight, generate H2O2 by extracting oxygen from the air and hydrogen from water. Our research explores the use of zinc indium sulfide (ZnIn2S4), a material that is photoactive under visible light, which accounts for about 43 % of the solar spectrum. Initially, the pH of the solution was not modified, leading to the ionization of Zn and In. However, upon optimizing the experimental conditions, we observed a substantial reduction in particle ionization and an increase in H2O2 yield. Additionally, by integrating black carbon with oxygen functional groups into the ZnIn2S4 matrix, we achieved a 14 % increase in production efficiency compared to traditional methods.