Oxygen Vacancies Enabled MOF-Derived Tb–SnO₂ Compound for a High-Response, Low Detection Limit, and Humidity-Tolerant Chemiresistive Gas Sensor of Formaldehyde

Tb-doped SnO₂ (represented as Tb–SnO₂) was prepared by calcining Tb-doped Sn MOF synthesized by a solvothermal method. 5% Tb–SnO₂ exhibits excellent selectivity, high response (28.2), and fast response/recovery time (28 s/135 s) toward 50 ppb (volume concentration in parts per billion) formaldehyde (HCHO) at low operating temperature (200 °C). The low detection limit of the HCHO gas sensor is mainly due to the large number of oxygen vacancies in Tb–SnO₂ caused by the charge imbalance between Tb ions and Sn ions during the high-temperature calcination process of Tb-doped Sn MOF. Oxygen vacancies promote the conversion of oxygen molecules into active adsorbed oxygen species, narrow the band gap of semiconductor oxides, and reduce the activation energy of formaldehyde gas-sensing reactions, thereby improving the performance of gas sensors. 5% Tb–SnO₂ gas sensor has strong moisture resistance, with a response value of 209.3 to 10 ppm of HCHO at a high relative humidity of 80%. The moisture resistance mechanism of the material is explained as Tb³⁺/Tb⁴⁺ redox pairs acting as water molecule capture agents, which reduce the occupation of gas-sensing reaction active sites by water molecules on the material surface.