Resistive and capacitive response of nitrogen-dopedTiO2 nanotubes film humidity sensor

Abstract

Dielectric oxides are traditionally used to fabricate resistive surface humidity-sensingdevices, as well as capacitive sandwich-structured sensors. In the present work,relative humidity (RH) sensors were fabricated by employing vertically alignedTiO2 nanotubes array (TNA) film produced using electro-chemical anodization of Tifoil followed by a nitrogen-doping process, simultaneously showing resistiveand capacitive humidity-sensing properties in the range of 11.3–93.6%. Forthe sample formed at optimized experimental conditions, the capacitance (CS) andresistance (RS) of the as-fabricated RH sensors made from nitrogen-dopedTiO2 nanotubes film could be simultaneously obtained. Both the resistive and capacitive sensitivity (KR andKC) of theas-fabricated TiO2 nanotube RH sensors show distinct dependence on the frequency of alternating current(AC) voltage signal and RH. At higher water coverage, water–water interaction will resultin lowering of the water dissociation barrier, leading to an increase of conductance. Withthe increase of RH, the polarization of as-adsorbed water molecules will also occur, causinga sharp increase of capacitance. For an explanation of the frequency response of bothCS andRS, ionic transport, as well as the polarization effect, should be comprehensively considered.The changes of capacitance and resistance at different temperatures are plausibly caused bythermal expansion and surface state modification by adsorption and desorption of oxygenand water.