Composite Structural Optimization Of TiO₂Nanorod Array And The Enhancement Of Photocatalytic Degradation Of Gaseous Benzene

With the rapid development of human industrial civilization, more and more heavy chemicals have been utilized for manufacturing of furniture, decoration materials and paints. Among all those materials, benzene is widely used as adhesion agent. However, as carcinogen, benzene is known for its toxicity. It can gradually volatilize under room temperature. When inhaled by human, it is difficult to degrade, causing health problems in a long period of time. For indoor air pollution treatment, TiO2can degrade almost all kinds of organic pollutants like benzene and some inorganic pollutants through photocatalytic procedure. The above characteristics of TiO2endue itself with great research value and application prospect.However, traditional TiO2photocatalysts are mainly in the form of nanoparticles, which can provide highly active electrons and large surface area due to quantum effect. But the aggregation of TiO2nanoparticles can cause dropdown of the surface area and recombination of photo-induced charge carriers in the grain boundary. Moreover, the poor mechanical adhesion of nanoparticles makes them difficult for practical loading, which restrains its application. Compared with nanoparticles, TiO2nanorod array avoids aggregation for better practical loading, which is synthesized by using FTO conductive glass as template through a hydrothermal method. And the single crystalline property of the nanorod is also benefit for separation of the photo-induced charge carriers.In this paper, we successfully regulated the packing density of the TiO2nanorod array by simply changing the concentration of chloride acid in the growth solution. Measurements of XRD and TEM confirms the single crystalline of the TiO2nanorod with high energy (002) facets exposed on top. And the percentage of the exposed (002) facets increases as the packing density increases. However, the photocatalytic degradation of benzene is not enhanced along with the increase of (002) facets. UV-Vis adsorption and photocurrent tests indicates that large and small packing density both are unfavorable for light absorbance. While medially packed TiO2nanorod array has more photo-induced charge carriers due to enhanced light adsorption by light trapping effect, demonstrating the best photocatalytic activity. It suggests that when compared to exposed facets, light adsorption is a much important factor for arrays with ordered structure.Following the investigation of the influence to photocatalytic performance by the intrinsic structure of TiO2nanorod array, we fabricated TiO2nanoparticles onto the TiO2nanorods by a low temperature chemical bath procedure for utilizing the space between the nanorods. Composition with nanoparticles makes the smooth sidewalls of TiO2nanorods become rough, which promotes light absorbance. In addition, the electrons generated by nanoparticles can transfer to FTO glass by using single crystalline nanorod as high speed pathway for enhanced separation from holes. More charge carriers can effectively improve the photocatalytic activity. Whereas, the aggregation of nanoparticles at the bottom of TiO2nanorod array introduces more grain boundaries where photo-induced electrons and holes can recombine with each other.In order to eliminate the aggregation of nanoparticles, we fabricated a layer of TiO2nanofibers on top of the TiO2nanorod array for heterojunction by electrospinning method. But measurement of UV-Vis adsorption shows that this kind of structure is not suitable for light adsorption. What’s worse, the nanofibers themselves are constructed by nanoparticles, which produces large amount of grain boundaries. When electrons transfer to nanorod along the nanofibers, the grain boundaries can act as energy barrier as well as recombination centers which is unfavorable for photocatalysis. And the space between nanorods remains unutilized.Finally, we synthesized a three dimensional (3D) network of heterojunction based on TiO2nanorod array by using two dimensional (2D) BiOCl nanoplates through a solvothermal method, by which the space between nanorod and the outer space on top of nanorod array were both utilized. And no aggregation was found due to the2D structure of BiOCl. Although the UV-Vis adsorption test shows that this3D structure may not favors the light adsorption, but the internal electric field, single crystalline property of BiOCl and the band alignment of TiO2with BiOCl together enhance the transfer of electrons from BiOCl to FTO glass through TiO2nanorod. Moreover, the oxygen vacancies on BiOCl can act as trapping centers for electron to improve the separation of charge carriers. Combined effects of all the factors above cooperate to promote the photocatalytic activity of the BiOCl/TiO23D heterojunction, which remains unchanged after cycles of photcatalytic tests.In conclusion, we firstly optimized the structure of TiO2nanorod array for better photocatalytic activity and revealed that light adsorption is much important factor for arrays with ordered structure. Then based on the structural characteristics of the TiO2nanorod array, our work is focused on the separation of charge carriers then combining OD, ID and2D nanomaterials for construction of3D heterojunction structure, by which we realized the promotion of the photocatalytic performance of TiO2nanorod array step by step. In the end, we synthesized an outstanding photocatalyst with3D network of heterojunction structure. The concepts of the design and researching methods provide new thoughts and reference for synthesis of new nanomaterials with novel photocatalytic activity.