Precursor-Driven Engineering of g-C3N4/TiO2 Heterojunctions: Influence on Structure, Optical Properties, and Photocatalytic Performance

The precursors that are used to synthesize graphitic carbon nitride (g-C3N4) strongly influence its physicochemical properties. However, their effect on the performance of g-C3N4/TiO2 heterojunctions has received limited attention. In this work, g-C3N4 was synthesized from three commonly used precursors and combined with hydrothermally prepared TiO2 to fabricate MGCN/TiO2 (melamine), UGCN/TiO2 (urea), and TGCN/TiO2 (thiourea) composites under identical conditions. Structural characterization confirmed the coexistence of the constituent phases, while UV–vis diffuse reflectance spectroscopy revealed precursor-dependent optical characteristics and dual absorption transitions in the composites. Zeta potential measurements established the point of zero charge (pHpzc) of 4.28, 4.53, and 5.56 for MGCN/TiO2, UGCN/TiO2, and TGCN/TiO2, respectively. Photoluminescence (PL) analysis revealed the highest emission intensity for MGCN/TiO2 and the lowest for UGCN/TiO2; however, these differences were not interpreted as direct measures of interfacial charge-separation efficiency. The photocatalytic activities of the composites were evaluated through Congo Red (CR) dye degradation under UV irradiation. Among the three composites, TGCN/TiO2 exhibited the best performance, achieving 89.86% removal at 0.50 g/L and pH 9, with an apparent pseudo-first-order rate constant of 8.43 × 10–3 min–1. Radical scavenging experiments showed that superoxide radicals are the dominant reactive species, with hydroxyl radicals and photogenerated holes making additional contributions. The scavenger results and band-edge potentials estimated using the Mulliken electronegativity approach are compatible with a possible S-scheme-type charge-transfer model. These results demonstrate that precursor selection for g-C3N4 is a crucial step, as it affects the structural, optical, and photocatalytic characteristics of g-C3N4/TiO2 heterojunctions.