Precursor-Driven Engineering of g-C3N4/TiO2 Heterojunctions: Influence on Structure, Optical Properties, and Photocatalytic Performance
Maliha Tasnim Kabir, Rudra Kumar Sarker, Fariha Chowdhury, Shahran Ahmed, Md. Nakib Hossen, Shah Md. Masum, Samina Ahmed, Md. Ashraful Islam Molla, Mashrafi Bin MobarakAbstract
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.