Synergistic effects of vanadium–sodium dual-doping on the structural and CO 2 sensing properties of CuO thin films

Abstract

This study investigates the effects of single and dual dopant engineering on the structural, optical, and CO₂-sensing properties of CuO thin films prepared via a solution-based spin-coating technique. Undoped, V-doped, Na-doped, and V–Na dual-doped films were systematically examined to elucidate the mechanisms underlying performance variation. X-ray diffraction confirmed phase-pure monoclinic CuO in all samples, while progressive peak broadening with doping indicated crystallite refinement. X-ray photoelectron spectroscopy revealed an increasing proportion of defect-related oxygen species, with the dual-doped films exhibiting the highest oxygen-vacancy concentration, attributed to cooperative charge compensation between multivalent V and monovalent Na. Morphological characterization showed a reduction in grain size from approximately 125 nm in the undoped film to about 73–77 nm in the dual-doped films, accompanied by increased surface roughness and grain-boundary density. Optical analyses indicated enhanced transparency and band-gap widening from ∼1.55 eV to ∼1.85 eV, together with decreased extinction coefficients. Electrical measurements demonstrated linear ohmic I–V characteristics and strengthened gas-induced resistance modulation under CO₂ exposure. At 10,200 ppm CO₂, the resistance variation increased from ∼2.3–5.8 GΩ in the undoped film to ∼0.6–9.1 GΩ in the highest dual-doped sample, corresponding to an approximately 2.5-fold enhancement in response and faster response–recovery dynamics. These results indicate that dual doping enables a balanced adjustment of lattice distortion, defect density, and surface accessibility, promoting more effective gas–solid interactions than single-dopant strategies.