Giant and controllable humidity sensitivity achieved in (Na plus Nb) co-doped rutile TiO2

The (Na0.25Nb0.75)(x)Ti1-xO2 (NNTO, with x = 0, 0.01, 0.02, and 0.05) powders were prepared by solid-state reaction and used as humidity sensing materials. The humidity sensitive properties of the NNTO based capacitive-type sensors were systematically investigated. Our results revealed that the humidity sensitivity increases with the increasing of doping level. An unprecedented high humidity sensitivity of 13,705 pF/%RH with good repeatability and fast response/recovery time were achieved in the sample with x = 0.05. The point defects of oxygen vacancies, Nb-Ti(center dot) ions, and the secondary phase of NaNbO3 were argued to be responsible for the exciting humidity sensitive properties. Interestingly, the humidity sensitivity can be easily tuned by a small dc bias of 0-4 V. The depression of the humidity-induced Maxwell-Wagner relaxation and the neutralization of the charged point defects due to bias-injected electrons were found to be the cause of the adjustability. This work paves the way for developing capacitive-type sensors with adjustable and excellent performances.