Preparation, Modification And Photocatalytic Propeties Of TiO₂ Nanomaterials

In the 21st century, environmental pollution has become a serious issue threating human’s daily life along with the improvement of the production and living standard. Nowadays, it has become an enormous challenge for us to solve the environmental problem. To date, extensive researches suggest that semiconductor catalytic has an intriguing prospect in the storage and utilization of solar energy and catalytic conversion technology and organic pollutants treatment. TiO2 has been widely used in photocatalyst, solar cell, antibacterial and self-cleaning materials due to its high stability, low cost, nontoxicity, no secondary pollution and high photocatalytic activity. In particular, the research about photocatalytic degradation of organic pollutants using TiO2 has been attracted much attention. Unfortunately, TiO2, as a semiconductor photocatalyst, has two deadly defects limiting their further application. Firstly, the band gap of anatase TiO2 is 3.2eV, which means it can only absorb the light less than 387.5nm. Secondly, the photo-excited electrons and holes in TiO2 are easily recombined. Therefore, how to widen their absorption spectrum and reduce the recombination of photo-excited electrons and holes have become a hot issue. So far, numerous methods about improving photocatalytic activity of TiO2 have been reported, including ion doping, semiconductor composites, noble metal loading, etc. In recent years, apart from using modification method to improve TiO2 photocatalytic activity, preparing highly active facets dominated TiO2 has been another hot issue. In order to prepare TiO2 with high photocatalytic activity, we have done some works as follows:Graphene oxide and reduced graphene oxide were prepared using the traditional Hummers’ method. Then we prepared composites of graphene and titanate nanotubes by hydrothermal and the samples are sintered to get a composite of graphene and TiO2. The influence of sintering temperature on the morphology of nanotubes and their photocatalytic activity were researched. In addition, the influence of amount of graphene was also investigated. X-ray diffraction (XRD), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy(FTIR) were carried on characterizing their crystallization, morphology and elements states, etc. The results incidate that the main component of composite is titanate without photocatalytic activity, antase TiO2 were formed and crystallization was enhanced through rising of sintering temperature. However, the morphology of nanotubes was destroyed gradually and the large size reduced graphene oxide become small fragment at the process of improving sintering temperature. And the amount of graphene didn’t affect the formation of nanotubes, but the photocatalytic activity was related to its content. The research implied that appropriate graphene could improve their photocatalytic activity, and photocatalytic activity was best when the graphene content was 5% and sintering temperature was 450℃.TiO2 nanosheets dominated with high active crystal facets (001) were prepared by hydrothermal using HF as morphology and facets controlling reagent. Moreover, XRD, TEM and XPS were applied for characterizing their crystallization, morphology and elements states, etc..The influence of dosage of HF on morphology and activity of nanosheets was discussed. When the dosage of HF are 0.4mL,0.6mL,0.8mL, the percentage of (001) facet are 84%,87%and 93%, the length of TiO2 nanosheet are 24nm,31nm and 36nm, respectively. However, the thickness of TiO2 nanosheet are all 6-7nm. Simultaneously, the dosage of HF doesn’t have a big influence in the fluorine content of samples. Rhodamine B were degraded by TiO2 nanosheets,and the results indicate that samples with higher content of (001)facet have better photocatalytic activity. TiO2 nanosheet was washed with NaOH solution and partial fluorine was removed. However, the morphology, phase and the content of (001)facet doesn’t changed after washing with NaOH solution. Methyl orange, rhodamine B and methyl violet were degraded by TiO2 nanosheet and TiO2 nanosheet washed with NaOH solution, the degradation result shows that fluorine is beneficial to the photocatalytic activity of TiO2. This is attributed to fluorine with strong electronegativity can form hydrogen bond with H2O, so a large amount of H2O was absorbed on the sample surface accelerating oxidation of H2O by valence holes. So abundant hydroxyl free radicals generated can improve the photocatalytic activity of TiO2.TiO2/graphene composite was prepared by hydrothermal method, the length of TiO2 nanosheet in composite is approximately dozens of nanometers and phase of sample is anatase TiO2. TiO2 nanosheet are dispersed on graphene,but the agglomeration is serious. It can been seen that the composite prepared has high photocatalytic activity by degradation of methylene blue under visible light.