Preparation And Humidity Sensitivity Of CoTiO₃/TiO₂ Ordered Aligned Nanocrystalline Film

As a binary metallic oxide,cobalt titanate（CoTiO₃）exhibits excellent sensitivity to ethanol,propylene and vapour etc,particularly in:fast response,anti-interference and good long-term stability etc characteristics.However,as a p-type semiconductor sensitive material,due to the influence of the band structure and carrier transmission mechanism,the sensitivity is generally lower than n-type semiconductor.To improve CoTiO₃ humidity sensitivity,the n-type semiconductor TiO₂ is selected to composite a CoTiO₃/TiO₂ heterogeneous film with surface grain arrangement highly ordered.Based on the uniform distribution of p-n heterojunction and oriented growth of nanocrystalline array structure on the film surface,the semiconductor band structure can be changed and the carrier transmission efficiency cam be enhanced greatly.The RF magnetron sputtering and grazing angle deposition technology is used to prepare the CoTiO₃ and CoTiO₃/TiO₂ ordered nanocrystalline film respectively.The X-ray diffraction（XRD）,atomic force microscope（AFM）,transmission electron microscope（TEM）and X-ray photoelectron spectrometer（XPS）are employed to characterize the phase composition,morphology,oriented growth characteristic,defect distribution and chemical valence state of the film.The sensitivity,response/recovery time,detection range and long-term stability of different microstructure films to humidity environment are investigated.The intrinsic relationship between the structure and properties of the films are analyzed to reveal the sensing mechanism of ordered nanocrystalline film with heterojunctions.The main research contents and results are as following:（1）The CoTiO₃ nanocrystalline film is firstly prepared by RF magnetron sputtering.The effect of deposit time,power,air pressure and grazing angle on microstructure and sensing properties of CoTiO₃ film are studied systematically.The experimental results show that the CoTiO₃ films obtained from deposition time 60 min,power 300 W,pressure 0.5 Pa,grazing angle 23°exhibits the best humidity sensing performances.Its surface morphology is similar to rows of hills,the response to 95%RH is up to 4.17,the response and recovery time are 28 s and 48 s respectively.（2）The ordered CoTiO₃/TiO₂ nanocrystalline films were prepared by sputtering TiO₂ on top of the most sensitive CoTiO₃ film.The composite film surface is composed of a large number of nanocrystalline columns which growing along the（104）crystal plane.These nanocrystalline columns are highly ordered and the size is about 122 nm×35 nm.This unique morphology provides a large specific surface area and a great number of active adsorption sites.So the response of the composite film increased by two orders of magnitude,reaching 157.23.And the response and recovery time are 10 s and14 s.The deposition conditions of TiO₂ of this ordered CoTiO₃/TiO₂nanocrystalline films are:deposition time 10 min,power 400 W,pressure 0.5Pa,grazing angle 23°.（3）Ti ion concentration on the surface of ordered CoTiO₃/TiO₂nanocrystalline films was adjusted to further improve its sensitivity.It was found that the sensitivity of the ordered CoTiO₃/TiO₂ nanocrystalline films was changed by a thermal treatment with Ar-H₂ atmosphere at 600℃for 2 h.The response was 111.54,and the response and recovery time are significantly reduced to 2 s and 5 s,showing excellent response/recovery characteristic.The reason why the response decreased and the response/recovery characteristic enhanced is studied.It is believed that the Ar-H₂ gas reduced the film to produce some Ti³⁺,which leads to the decrease of the surface resistance value,and the response decreased as well.Meanwhile,the presence of Ti³⁺means that the oxygen vacancy defects is also existed,so the number of active adsorption sites of water molecules is increased.The adsorption capacity of the film to water is enhanced,showing a faster response and recovery rate.（4）The sensing mechanism of ordered CoTiO₃/TiO₂ nanocrystalline films to humidity is analyzed,including the following aspects:（1）the nanocrystalline columns growing along the c axis provides the larger specific surface area and more active adsorption sites;（2）the hydrophilic properties of the film surface were enhanced by compositing TiO₂;（3）the presence of p-n heterojunction shortens the electron flow path and increase the carrier transport rate;（4）the unique surface morphology induces high RMS value,facilities capillary condensation and Grotthuss chain reaction.