Investigation on acceptor-donor co-doped SnO a capacitive humidity sensor for respiration detection

In this work, we develop a novel capacitive humidity sensor based on Al-Si acceptor-donor co-doped SnO₂ for real-time monitoring of ambient humidity and human respiration. XRD measurements reveal that all samples exhibit a tetragonal rutile phase and the crystallite size of SnO₂ decreases with increasing Al-Si content. The high intensity of the Raman peak at 762 cm^-1 confirms the presence of bridging mode oxygen vacancies in (Al + Si)_0.02Sn_0.98O₂. The EPR results show that the amount of singly ionized oxygen vacancies increases after the introduction of Al-Si. Both types and amounts of oxygen vacancy defects are particularly sensitive to the adsorption of water molecules. Moreover, according to DFT calculations, the contribution of the Si 3s orbital and Al 3s orbital to the band edge verifies the formation of acceptor-donor complexes in Al-Si co-doped SnO₂. The humidity sensing results reveal that the (Al + Si)_0.02Sn_0.98O₂ humidity sensor shows high sensitivity (S = 839), low hysteresis (1.94%) and fast response/recovery times (25 s/5 s). The respiratory intervals during shallow, medium and deep breathing states of (Al + Si)_0.02Sn_0.98O₂ were measured at 2.8 s, 3.8 s and 4.5 s, respectively. The chemical mechanism for the enhancement of humidity sensing performance corresponding to the oxygen vacancy defects induced by Al-Si interplay is proposed.