Preparation And Characterization Of Titanium Dioxide?Carbon Nanotube Composite Films

The structures and chemical properties of titanium dioxide prepared by sol-gel have been widely studied in various fields.Recently,TiO₂ films serving as infrared thermal sensitive materials have attracted high attention.However,the low optical absorption intensity and low electrical conductivity of TiO₂ films limit their practical applications.One effective method for improving the optical absorption and conductivity of TiO₂ is to compose them by other materials,such as graphene and carbon nanotubes.TiO₂-CNT composites have attracted more and more attention due to their optimal room temperature resistance and TCR.At present,most of the previous studies on TiO₂-CNT composites were related to the photocatalysis,gas sensors and dye-sensitized solar cells,etc.In contrast,the applications of TiO₂-CNT composites as thermistor material in infrared detectors are still in the development stage.In this thesis,the composite films of TiO₂ and MWCNT were prepared by two different coating techniques,and their surface morphologies,chemical structures,as well as optical,electrical,and bolometric properties were systematically investigated.Firstly,the pure TiO₂ films were prepared by sol-gel method,and the effects of the annealing temperature on the morphologies,crystallinities and optoelectronic properties of the films were studied.Unannealed and annealed TiO₂ films at different annealing temperatures（300 ^℃,400 ^℃,and 500 ^℃）were prepared.The results show that both too high and too low annealing temperatures have negative effects on the films.In contrast,the composite films annealed at 400 ^℃ have a moderate room temperature resistance（1.68×10⁷Ω）,moderate optical absorption intensity and excellent TCR value(-2.59%?K^-1),all of which are desirable for preparing high quality TiO₂ films.Secondly,based on the optimized annealing process for the preparation of pure TiO₂ films,TiO₂-MWCNT composite films were further prepared by sol-gel and mixed spin coating.Meanwhile,the optoelectronic properties of the films are changed with the concentration of MWCNT.The results show that the addition of carbon nanotubes has significant effects on the physical and chemical properties of TiO₂ films.These changes are caused by the interactions between TiO₂ and MWNCT,and reach saturation when the concentration of carbon nanotubes reaches 0.5 wt%.At the same time,adding carbon nanotubes can effectively control the critical parameters of TiO₂ as infrared thermal sensitive materials:optical absorption,room temperature resistance,and TCR.Finally,in order to explore the percolation phenomenon in semiconductor TiO₂,TiO₂-MWCNT composite films were prepared by sol-gel method and spin-spray coating.The concentration of MWCNT is the key factor for achieving excellent performance of the composite films.At and around the percolation threshold of MWCNT（0.25 wt%）,the composite film exhibits sudden changes in both room temperature resistance and TCR.At this MWCNT concentration,the composite film possesses moderate optical absorption,room temperature resistance（2.56×10⁶Ω）,and1/f noise(7.49×10^-13 V²Hz^-1),maximum TCR value(-2.93%·K^-1),and the optimum general thermal parameter(5.35×10¹² K^-1m^3/2).These optimized parameters make the prepared TiO₂-MWCNT films suitable for the applications in uncooled infrared detectors.Particularly,the results presented in this work provide a strong support for confirming the existence of percolation phenomenon in semiconductors.