Impact of Copper Doping on Nanocrystalline SnO2 2 Thin Films Synthesized by Sol-Gel Coating and Chemical Bath Deposition for Gas Sensor Applications

This study focuses on the preparation of nanocrystalline SnO2 2 and Cu-doped SnO2 2 thin films on glass substrates using sol-gel spin coating and chemical bath deposition (CBD) methods, aimed at gas sensor applications. The films were annealed at 500 degrees C for 60 minutes, and their structural, optical, electrical, and sensing properties were analyzed under varying Cu-doping concentrations (0, 2, 4, and 6 wt.%). X-ray diffraction (XRD) revealed a tetragonal rutile structure for spin-coated films, while CBD films showed both tetragonal rutile and orthorhombic structures. Crystallite sizes decreased with increased Cu doping, ranging from 23 nm to 10.5 nm for spin coating and 22 nm to 8.15 nm for CBD. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) confirmed a reduction in average grain size with higher Cu content. The films demonstrated high light transmittance in the visible spectrum, with a decreasing bandgap energy as Cu concentration increased, from 3.88 eV to 3.63 eV for spin coating and from 3.8 eV to 3.55 eV for CBD. Electrical analysis revealed two activation energies (Ea1 and Ea(2)) that increased with Cu doping. The films were tested for CO gas detection at 50 ppm, with spin-coated films showing maximum sensitivity at 200 degrees C and CBD films at 150 degrees C. These findings suggest that Cu-doped SnO2 films, particularly those prepared via spin coating, are promising candidates for CO gas sensors.