Controllable Growth And Hydrophilic Properties Of TiO₂ Nanorod/Nanotube Films

Superhydrophilic TiO₂ nanofilm is a new functional film with low cost,no pollution,no toxicity and high thermal stability.The special morphology of TiO₂ film surface not only makes it have a large specific surface area,but also increases the roughness of the film surface,which is conducive to improve the surface hydrophilicity of hydrophilic films.The one-dimensional morphology of TiO₂ film surface,such as nanorods and nanotubes,not only has the above advantages,but also has the function of carrier directional transmission,which enables it show excellent superhydrophilic properties under UV illumination.It can be widely used in environmental purification,solar energy conversion,biological materials and other fields.In this work,with Ti foil as titanium source and substrate,TiO₂ nanorod films and TiO₂ nanotube/nanorod films were prepared by acidic hydrothermal method and alkaline hydrothermal method respectively.The morphology,crystal structure,hydrophilic properties and photoelectric properties were characterized and tested.The effects of preparation conditions on the properties of samples were analyzed,and their formation mechanism was discussed.The specific research contents are as follows:TiO₂ nanorod films were prepared by acidic hydrothermal method with 0.6 mol/L HCl as hydrothermal medium and KCl as morphology control agent.By changing the concentration of KCl,the content of Cl^-in hydrothermal medium was controlled,and then the length and growth density of nanorods on the surface of the film were adjusted.It is found that hydrothermal temperature and hydrothermal time had a great influence on the formation and growth of TiO₂ nanorods.Increasing hydrothermal temperature or hydrothermal time was conducive to improve the growth density and uniformity of TiO₂nanorods on the surface of the film;However,the dissolving and consuming process of substrate of Ti foil could be accelerated and there were cracks appearing on the surface when the hydrothermal temperature was excessively high.Moreover,the spacing among nanorods could be shortened and adhesion occured if the hydrothermal time was too long.In terms of heat treatment temperature,it mainly influenced the content and crystal form of TiO₂.The higher the heat treatment temperature was,the better the crystallinity of TiO₂ would be accordingly.Additionally,the mixed crystal structure combined with anatase and rutile phase TiO₂ was easily formed.However,nanorod film layer on the surface of Ti foil substrate fell off when the heat treatment temperature reached 500℃.The results showed that TiO₂ nanorod films with anatase/rutile mixed crystal structure with orderly arrangement,large nanorod length and high growth density could be obtained on the surface of Ti foil when hydrothermal reaction at 180℃for 12 h in the medium of 0.1 mol/L KCl and then heat treatment at 450℃.The sample showed relatively better superhydrophilicity under ultraviolet light,and the contact angle was reduced to 2.1°after illumination for10 min.When the concentration of KCl in acidic hydrothermal medium was excessive high,the substrate of Ti foil would be more susceptible to corrosion,resulting in the pulverization of the substrate after hydrothermal treatment and failure of film formation.Therefore,alkaline hydrothermal method was adopted to prepare the TiO₂ film.The tubular Na₂TiO₃ and K₂Ti₆O₁₃ titanate intermediates were generated after hydrothermal reaction by taking Na OH and KCl as hydrothermal medium and morphology control agent.Additionally,titanates were obtained by ion replacement of HCl solution,and then TiO₂ nanotube/nanorod thin films were obtained by heat treatment.The samples were characterized and the results showed that uniform TiO₂ nanotube/nanorod film could be formed on the surface of Ti foil after hydrothermal reaction in the medium of0.25 mol/L Na OH and 0.15 mol/L KCl at 220℃for 20 h,and then immersion in 1mol/L HCl and heat treatment at 450℃.The contact angle test showed that TiO₂nanotube/nanorod film had excellent superhydrophilic properties and extremely fast droplet spreading speed.When there was no UV illumination,the contact angle of the sample after heat treatment at 450℃was reduced to 0°at 37.5 ms;After 10 min of UV irradiation,it took only 25 ms for the contact angle to reach 0°.It is revealed by the analysis of the formation mechanism of TiO₂nanorod/nanotube film that the synergistic effect of KCl and HCl is able to promote the growth of TiO₂ nanorods along the crystal direction of rutile TiO₂[001]in acidic hydrothermal medium to form TiO₂ nanorod film with good uniformity and high growth density;In alkaline hydrothermal medium,the Na⁺of intermediate Na₂TiO₃ with sheet shape is replaced by K⁺with larger ion radius,resulting in the expansion and curling of nano sheets and the formation of nanotube or nanorod films.