Influence of rGO Concentration on the Structural and Optical Properties of ZnO/Zn₂SnO₄/rGO Heterostructure Composites

In this work, a ternary heterostructured semiconductor composite ZnO/Zn₂SnO₄/rGO was successfully synthesized via a hydrothermal method to enhance photocatalytic performance through the synergistic effect of the ZnO/Zn₂SnO₄ heterojunction and the high electrical conductivity of reduced graphene oxide (rGO). The structural, morphological, and chemical properties of the samples were systematically characterized by XRD, FESEM, EDX, FTIR, and XPS. The results confirm the formation of a well-defined ZnO/Zn₂SnO₄ heterostructure with intimate interfacial contact and the successful incorporation of rGO into the composite matrix. XPS analysis reveals strong electronic interactions at the heterointerface, which effectively regulate charge distribution and promote interfacial charge transfer. Photoluminescence spectra show a significant suppression of emission intensity for the ZnO/Zn₂SnO₄/rGO composite, while photocurrent measurements demonstrate a remarkable increase in photocurrent density, indicating efficient separation and transport of photogenerated charge carriers. UV–Vis absorption spectra exhibit overlapping absorption features of ZnO and Zn₂SnO₄, along with an extended absorption edge toward the visible region due to the presence of rGO, resulting in a narrowed optical band gap of the ternary composite. Owing to the synergistic interaction between the ZnO/Zn₂SnO₄ heterostructure and the conductive network of rGO, the ZnO/Zn₂SnO₄/rGO sample with an optimal rGO content of 2.5% exhibits the highest photocatalytic degradation efficiency toward methylene blue, reaching 87.94%. When the rGO content is either lower or higher than the optimal value, the photocatalytic performance decreases due to limited electron transport or excessive surface coverage and light-shielding effects. These findings highlight the importance of optimizing heterostructure architecture and rGO content for the design of efficient photocatalytic materials for environmental remediation.