Controllable Preparation And Modification Of TiO₂Nanotube Arrays

Anodic oxidation titanium dioxide nanotube arrays as nanostructuresemiconductor compound attracted extensive research interests due to its highlyorder nanostructure, controllable morphology, large specific surface area andfavourable electrical properties, and its potential applications such as gas sensingmaterials, photocatalytic degradation of organic contaminants, dye sensitized solarcells, supercapacitor and so on. In this paper, highly order TiO₂nanotube arrays（TNAs） was prepared by anodization. In order to fabricate TNAs with high qualityand controllable morphology, new and efficient anodic oxidation processes werediscussed. Nitrogen element and Pt nanoparticles were adopted for the doping ormodification of the as-prepared TNAs in order to promote the photocatalytic orgas sensing properties. TNAs （N-TNAs or Pt-TNAs） was characterised by FESEM,XRD, XPS, TEM and EDS techniques. The following conclusions could be drawn:（1） During the conventional anodization process, titanium foil can formwell-aligned nanotube arrays when the potential is in the range from20to70V.With increasing of the anodization voltage, the tube length of TNAs increases.However, when the anodization voltage exceeds to80V, the tube length of TNAsreduces. With increasing the electrolyte concentration （from0.05M to0.2M）, theouter diameter of the TNAs decreases rapidly. When the concentration of NH₄F is0.2M, the outer diameter shows the minimum of147.5nm. Then, the outerdiameter increases with the continued increase of the concentration of NH₄F. Asthe electrolyte concentration increases, the inner diameter and the wall thicknessof the TNAs increases firstly and then decreases.（2） In the glycerol electrolyte, controllable diameters of TNAs are preparedvia controlling the anodization voltage （from10to55V）. Diameter of TNAs hasmonotonous relationship with the anodization voltage, and this linear relationshipcan be expressed as: d=3.625×U.（3） In order to overcome the surface irregularity of the conventional anodizedTNAs, a two-step anodic oxidation method is developed, and the relationship between anodization voltage and morphology is discussed. The results of thetwo-step anodic oxidation show that the as-prepared TNAs has the best surfaceevenness when70V+70V anodization voltage is applied.（4） Nitrogen doped TiO₂nanotube arrays （N-TNAs） are prepared byimmersing TNAs in the ammonia aqueous solution and then annealing in differenttemperatures. The results show no significant effect of annealing on surfacemorphology and microstructure of the TNAs, and the nitrogen doping in TiO₂lattice can decrease temperature of crystal transition from anatase to rutile phase.Compared to the pure TNAs without annealing, the photocurrents increasesignificantly with the annealing of the N-TNAs, and the maximum （0.116mA） canbe observed on the N-TNAs annealed at500℃. Photoelectrochemical experimentsshow that photocurrent and photocatalytic activity of N-TNAs strongly depend onthe annealing temperature, and the N-TNAs annealed at500℃has the best activityto degrade organic pollutants like methyl orange.（5） Anodization and solvothermal reduction method have been developed tofabricate homogeneously dispersive Pt nanoparticles decorated highly order TNAs.Pt nanoparticles with average diameter of20nm are homogeneously distributed onthe tube mouth and wall of TNAs. Pt-TNAs gas sensor exhibits excellent gassensing property. The Pt-TNAs sensor exhibits significant response to ethanol andacetone balanced with air. Furthermore, the fabricated sensor reveals excellentselectivity to ammonia. The experimental results indicate that the sensingmechanism of TNAs-based gas sensor is the effect of the oxygen adsorption anddesorption on the nanotube arrays surface, and the effect of platinum nanoparticleson gas sensing properties is also discussed.