Hydrothermal synthesis and structural analysis of SnO2–ZnO–rGO nanohybrids for NO2 sensing

This study reports the hydrothermal synthesis and structural–optical characterization of tin dioxide (SnO2), zinc oxide (ZnO), graphene oxide (rGO), and their binary and ternary nanohybrids for potential nitrogen dioxide (NO2) gas sensing at room temperature. Four hybrid systems – SnO2–rGO, ZnO–SnO2, ZnO–rGO, and SnO2–ZnO–rGO – were synthesized under optimized hydrothermal conditions and compared with pristine SnO2 nanorods. Field emission scanning electron microscopy analysis revealed nanorod and nanoparticle morphologies anchored on rGO sheets, confirming effective oxide–carbon integration. X-ray diffraction patterns verified the coexistence of tetragonal SnO2 and hexagonal ZnO phases without impurity formation, with crystallite sizes ranging from ∼4 nm (SnO2) to ∼48 nm (SnO2–ZnO–rGO). Optical bandgap energies, estimated from Tauc plots, varied between 3.35 eV (ZnO) and 6.00 eV (ZnO–rGO), indicating tunable electronic properties arising from heterojunction coupling and rGO incorporation. The ternary SnO2–ZnO–rGO hybrid exhibited a balanced crystallite size (∼48 nm) and stabilized bandgap (∼4.20 eV), suggesting improved charge transport and surface reactivity. The results demonstrate that oxide–oxide heterojunction formation combined with a conductive rGO network provides an effective strategy for engineering nanomaterials suitable for room-temperature NO2 sensing.