Controllable Synthesis Of TiO₂Nanotube Arrays And Their Gas Sensing Properties

There are a lot of potential applications for the anodic oxidation TiO2nanotube arrays （TNAs）,for instance, photocatalytic degradation of organic pollutants, dye-sensitized solar cells, photolysisof water into hydrogen, supercapacitors, gas sensing materials and so on, just because of their uniquecharacteristics of highly ordered nanostructures and large specific surface area. This paper focuseson the controllable synthesis of TNAs via anodic oxidation in order to discuss the anodic oxidationreaction mechanism and to explore new and more efficient anodic oxidation processes for thepreparation of high quality TNAs. Then silver （Ag） nanoparticles were achieved for the modificationof the as-prepared TNAs which aims to enhancing the gas sensing properties of the TNAs. FESEM,EDS and Elements SmartMarpping characterization techniques were adopted for the systematicalanalysis of the relationship between the morphology of TNAs and the anodizing process parameters.The following conclusions could be drawn:（1） During the normal anodic oxidation process, titanium surface can not form orderednanotube arrays when the applied voltage is too small （≤10V）; With the gradually increasing ofthe anodization voltage, the thickness of TNAs films increased. However, when the applied voltageincreased to a certain threshold, the film thickness of nanotube length was reduced. As theelectrolyte concentration increases （from0.05mol/L to0.3mol/L）, the outer diameter of thenanotube decreases rapidly. When the electrolyte concentration is0.3mol/L, the outer diametershows the minimum value. With the continued increase of the electrolyte concentration （from0.3mol/L to0.5mol/L）, the outer diameter increased. As the electrolyte concentration increases, theinner diameter of the nanotube also increases firstly and then decreases. The wall thickness ofnanotubes firstly decreases and then increases as the electrolyte concentration changes from0.05mol/L to0.5mol/L.（2） During the rapid anodic oxidation process, TNAs exhibits the quickest growth rate whenn（NH4F）: n（Na₂CO₃）=2:1. And average growth rates of TNAs are quite different at each periods ofthe rapid anodic oxidation. At the initial reaction stage, the average growth rate of TNAs increasesrapidly and the reaction rate reaches its maximum value after30min. The effect of additives on thereaction rate is not very clear when the applied voltage is low. Only after the anodization voltageincreases further, the accelerating effect of additives on the reaction rate could be truly reflected. Therefore, in order to effectively accelerate the growth rate of the, Na₂CO₃additives and the certainvalue of applied voltage are both needed.（3） In order to overcome the surface defects of the normal anodized TNAs films, a two-stepanodization process was developed. And the result of the conventional two-step anodization showsthat when80V+80V anodization voltage was applied, the as-prepared TNAs has better surfacesmoothness than that of the normal anodization. However, there are two inherent defects in theconventional two-step anodized TNAs films. The direction of the first oxidized holes and thesecond-step oxide nanopores are inconsistent with each other. And there are a large number ofmicrocracks in the second oxide layer. In this study, in situ two-step anodic oxidation process wasdeveloped to effectively overcome the inherent defects of the conventional two-step anodizationprocess and large area TNAs film was successfully achieved.（4） Ag nanoparticles modification of TNAs film was prepared by spin-coating-ultravioletlight-reduction of AgNO₃precursors process, which is easily feasible during the experiment. And Agnanoparticles with diameter of about30nm could be distributed onto the surface, the center and thebottom of the TNAs film. The distribution uniformity of Ag nanoparticles is significantly better thanthat of the Ag/TNAs prepared via the pulsed deposition process, the electrochemical depositionprocess and conventional UV-reduction process.（5） The gas sensing properties of the as-prepared TNAs and Ag/TNAs film was systematicallyinvestigated. The results showed that the TNAs prepared by conventional anodizing process and theAg/TNAs films achieved by spin-coating-ultraviolet light-reduction process both have a certain gassensing response at low test temperatures （≤80℃）. And the gas sensing signal of the Agnanoparticles modified TNAs is much better than that of the bare TNAs. Because of the Agnanoparticles loading, Ag nanoparticles modified TNAs showed good gas response-recoverycharacteristics after three cycles test.