Highly Selective and Reversible Detection of Simulated Breath Hydrogen Sulfide Using Fe‐Doped CuO Hollow Spheres: Enhanced Surface Redox Reaction by Multi‐Valent Catalysts

The precise and reversible detection of hydrogen sulfide (H 2 S) at high humidity condition, a malodorous and harmful volatile sulfur compound, is essential for the self‐assessment of oral diseases, halitosis, and asthma. However, the selective and reversible detection of trace concentrations of H 2 S (≈0.1 ppm) in high humidity conditions (exhaled breath) is challenging because of irreversible H 2 S adsorption/desorption at the surface of chemiresistors. The study reports the synthesis of Fe‐doped CuO hollow spheres as H 2 S gas‐sensing materials via spray pyrolysis. 4 at.% of Fe‐doped CuO hollow spheres exhibit high selectivity (response ratio ≥ 34.4) over interference gas (ethanol, 1 ppm) and reversible sensing characteristics (100% recovery) to 0.1 ppm of H 2 S under high humidity (relative humidity 80%) at 175 °C. The effect of multi‐valent transition metal ion doping into CuO on sensor reversibility is confirmed through the enhancement of recovery kinetics by doping 4 at.% of Ti‐ or Nb ions into CuO sensors. Mechanistic details of these excellent H 2 S sensing characteristics are also investigated by analyzing the redox reactions and the catalytic activity change of the Fe‐doped CuO sensing materials. The selective and reversible detection of H 2 S using the Fe‐doped CuO sensor suggested in this work opens a new possibility for halitosis self‐monitoring.