Defect-Engineered Al- and Nd-Doped WO3 Films for Enhanced Carbon Monoxide Sensing via Oxygen Vacancy Modulation

Abstract Carbon monoxide (CO) is a highly toxic, colorless, and odorless gas that poses a serious risk to human health, making its reliable detection essential for environmental monitoring and industrial safety. In this work, defect-engineered WO3 films, pure, single-doped, and codoped with Al3+ and Nd3+ were developed to enhance CO sensing at an operating temperature of 200 °C. Synergistic defect engineering was achieved through simultaneous Al and Nd codoping, which enables controlled modulation of oxygen vacancies, charge transport, and surface reactivity. The materials were synthesized via a coprecipitation-assisted ultrasonic method and deposited as films using the Doctor Blade technique. structural, optical, morphological, and electrical characterizations (X-ray diffraction (XRD), Raman, UV–Vis, PL, Scanning electron microscopy (SEM), AFM, and electrical measurements) revealed that Al3+ doping promotes lattice distortion and increased defect density, particularly oxygen vacancies, while Nd3+ induces improved crystallinity and structural relaxation. Gas sensing measurements indicated that all WO3-based sensors responded to CO. Pure WO3 exhibited the highest response (47.37%), followed by WO3–Al (26.31%), WO3–Nd (15.10%), and WO3–AlNd (14.56%) at 200 °C. Notably, all doped WO3 sensors exhibited higher CO responses than the commercial MQ-135 sensor (5.95%) under the same experimental conditions. Among the doped materials, WO3–Al exhibited the superior response, whereas WO3–AlNd showed a balanced combination of response and recovery characteristics, which may be associated with the combined effects of defect modulation and charge transport. Compared with pure WO3, the doped WO3 sensors exhibited faster sensing dynamics, with response and recovery times: 73–31, 27–44, 35–44, and 28–40, respectively. These results highlight the potential of doped WO3 sensors as promising materials for CO sensing applications.