Preparation And Photocatalytic Activity Of Porous Er-TiO₂ Films Under Simulated Visible Light

The TiO₂ photocatalytic technology has the advantages of high photocatalytic activity, no selectivity, complete degradation, no secondary pollutions and make use of the cheap sunlight in the process of degradation of organic pollutants, so it has drawn much attention. But owing to the broad band gap of TiO₂, it can only be activated by ultraviolet light with higher energy. TiO₂ powders are easy to agglomerate, difficult to be separated from the solution after the photocatalytic reaction. In order to resolve the problems mentioned above,modification of TiO₂ powders and the immobilization of TiO₂ powders on some substrates are widely researched. Meanwhile, by fabricating porous structure,the specific surface area of material and the depth of the light reach into the material are enlarged, which leading to the higher photocatalytic activity.In this paper, with span80（rich in hydroxyl） and water-soluble chitosan（rich in hydroxyl and amido） as additive, the coating sol was prepared by the sol-gel method, with this coating sol, a series of porous Er-doping TiO₂ filmswith Er content was 1.0 at% were synthesized by dip/spin coating method on quartz glass substrates. The effect of the preparation parameters, including the number of coating layers, calcination temperature, calcination time and content of the additive in the sol, on the structure and performance of the films was investigated. The films were characterized by X-ray Diffraction pattern（XRD）,Scanning Electron Microscopy（SEM）, Thermal analysis（TG）, and Nitrogen adsorption-desorption method. The photocatalytic degradation reaction of methyl orange（MO） solution was chosen to evaluate photocatalytic performance of the films under simulated visible light. Details are as follows:1. The effect of number of layers. The films prepared with the coating sol containing 0.3 wt% span80, and with four layers were continuous and complete,which had the highest photocatalytic performance. With five layers, the TiO₂ particles in the films were agglomerated, and the quality of the film declined.The films prepared from the coating sol with water-soluble chitosan content of0.04 wt% and three layers were continuous and complete, the films had the highest photocatalytic degradation. The TiO₂ particles in the films with four layers were agglomerate, the quality of the films decreased.2. The effect of calcination temperature. With the increase of the calcination temperature（550 °C-700 °C）, the size of the TiO₂ particles increased quickly, while the photocatalytic activity of the films decreased. The film prepared with 0.3 wt% span80, four coating layers, and calcination temperature of 600 °C had the smallest particles and the highest photocatalytic activity in thefilms prepared with span80. The film prepared with 0.04 wt% water-soluble chitosan, three coating layers, and calcination temperature of 550 °C had the smallest particles and the highest photocatalytic activity in the films prepared with water-soluble chitosan.3. The effect of calcination time. With the increase of the calcination time（0.5 h-3 h）, the crystallinity and the size of TiO₂ particles increased.The film prepared with 0.3 wt% span80, four coating layers, and calcination temperature of 600 °C for 1 h had the highest photocatalytic activity in the films prepared with span80. The film prepared with 0.04 wt% water-soluble chitosan, three coating layers, and calcination temperature of 550 °C for 1 h had the highest photocatalytic activity in the films prepared with water-soluble chitosan.4. The effect of different amount of additive. The films prepared under the conditions that the coating sol contained 0 wt%-0.4 wt% span80, the number of coating layers was four, the calcination temperature was 600 °C, and the calcination time was 1 h. In these films, the one prepared from the coating sol contained 0.3 wt% span80 had the smallest TiO₂ particles, with the size of 13.2nm. This film has the specific surface area of 99 m2g-1, and the highest photocatalytic degradation of MO solution of 55.23%. The films prepared under the conditions that the coating sol contained 0 wt%-0.05 wt% water-soluble chitosan, the number of coating layers was three, the calcination temperature was 550 °C, and the calcination time was 1 h. In these films, the one prepared from the coating sol contained 0.04 wt% water-soluble chitosan had the smallestTiO₂ particles had the smallest TiO₂ particles, with the size of 8.2 nm. This film has the specific surface area of 134 m2g-1, and the highest photocatalytic degradation of MO solution of 64.59%. As additive, water-soluble chitosan was better than span80.