| TiO2 is concerned due to its excellent photocatalyst and photo splitting water properties. But it is limited to application in industry because the low light use ratio. The methods people most study are as follows:First one is to dope some other elements in the TiO2 films such as F and Nb for modification; Second one is prepare composite film combining with other material; The third one is add electrical biasing on the film to separate carriers. These methods mentioned are efficient but complex due to the introduction of other materials and devices. The other novel and efficient way is to modify the micromorphology of the films. The columnar micromorphology attracted a lot attention for its high crystallinity and orientation. But the common methods to deposit TiO2 films with columnar micromorphology are PLD and CVD, which need expensive equipments and vacuum condition. Few reports were about Sol-gel method for its high difficulty to prepare columnar crystals though the method has a lot advantages to deposit films.Based on this, we prepared TiO2 films with columnar micromorphology by sol-gel spin coating process in this paper. First, we did a lot of work on the choice of chelating agents a, the regulation of the parameters related to liquid chemistry and the rational designing of the annealing process, and we deposited the anatase TiO2 films presenting columnar micromorphology whose single coating thickness surpasses 40nm successfully. The result is far larger than that as previously reported. After that, we systematically studied the formation mechanism of the columnar-structure and compared the properties between columnar-structure and granular-structure TiO2 films. The specific results are as follows:(1) The crystallization temperature point of TiO2 films deposited from the sol chelated by acetic acid is lower than those chelated by acetylacetone (AcAc) and diethanol amine (DEA). When the anatase formed, the crystallinity of the films chelated by CH3COOH was better than those chelated by AcAc, and the temperature need to form rutile for CH3COOH is higher than DEA. All the factors make CH3COOH is best choice as chelate agent to prepare compact films. In addition lower PH value and higher H2O/Ti ratio would promote the hydrolysis of tetrabutyl titania. That is benificial to Ti atom existing as Ti-OH. This construction eliminate the amount of -OR and promote the formation of [TiO6] octahedral. Based on the optimum wet chemical parameters, single film thickness, annealing temperature and heating rate were regulated gradually to prepare pure anatase TiO2 films with single layer thickness being 40nm successfully.(2) The necessary condition to prepare TiO2 films with columnar crystals by sol-gel process is to using the multi-coatings method. In addition, the thickness of single layer is thin enough and the temperature rising rate is high enough to promote the hetergeneous nucleation. When the nuclei grow into embryos, thermal injection must be high enough to provide energy for large crystals in the lower layer merger the embryos or the small crystals growing from the embryos in the upper layer. Ultimately the large crystals grow through the layer and present columnar micromorphology. Based on the optimum wet chemical parameters, single film thickness, annealing temperature and heating rate were regulated gradually to prepare pure anatase TiO2 films with single layer thickness being 40nm successfully.(3) Compared to granular-structure TiO2 films, columnar-structure TiO2 films have the properties charactering with the increase of crystalline size resulted in the increase of surface roughness and refractive index in addition to the decrease of the transmittance and energy bandgap. And the photocatalytic performance of columnar-structure TiO2 films is superior to granular-structure TiO2 films due to the fewer grain boundaries. |
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