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.