Study On The Synthesis Of Tio₂ /graphene Composite And Its High Photocatalytic Activity For Reducing Cr(VI)

Hexavalent chromium (Cr(Ⅵ)), which has intensive carcinogenicity and teratogenicity, is a kind of common heavy metal pollutants with high stability in natural environment. As a method commonly used for treatment of Cr(Ⅵ), reducing agents, such as NaHSO₃ or FeSO₄, are used to reduce Cr(Ⅵ) to less harmful Cr(Ⅲ), which is easy to precipitate in alkaline solutions. However, large amount of regent is demanded in this method, and secondary pollution is possible to occur. Hence, it is imminent to develop a new method which is highly efficient and environment-friendly for the treatment of Cr(Ⅵ).Comparing with the chemical agents, TiO₂ is a kind of environment-friendly catalyst, which can supply electrons for reducing Cr(Ⅵ) under the irradiation of ultraviolet light. However, the reduction of Cr(Ⅵ) over neat TiO₂ is not very efficient due to the recombination of the photo-generated electrons and holes. As a novel carbon nanomaterial, graphene is widely applied in catalysis due to its excellent stability and conductivity, and it has been demonstrated that the photocatalytic activity is greatly promoted in TiO₂/graphene composite because the recombination of electrons and holes is efficiently suppressed by graphene. Hence, we hold the view that the effect for the photoreduction of Cr(Ⅵ) can be drastically promoted by using TiO₂/graphene composite, and it has been demonstrated by the experiments. The results are as follows:(1) Graphite oxide was obtained by using Hummers method, followed by ultrasonic stripping in order to obtain graphene oxide (GO) solution. (NH4)2TiF6 was added to GO solution and the mixture was kept at 60℃for 2 h to obtain TiO₂/GO composite(named CA0) with a color of light-brown. Then the solid was calcinated at 300℃for 1h and the dark-gray product (named CA3) was gained.(2) From SEM, XRD and UV-vis diffraction absorbency spectra it is proved that the crystallinity of TiO₂ becomes better, and GO is thermally reduced to graphene during calcination. Zeta potential result shows that the isoelectric point of CA3 is 2.8, which means that the surface of CA3 is positively charged at pH below 2.8, being favorable for the adsorption of negatively charged Cr(Ⅵ) ions. (3) It is demonstrated that CA3 has excellent photocatalytic activity for reducing Cr(Ⅵ). The kinetic constant for the reduction of Cr(Ⅵ) is calculated to be 0.05527min^-1 over CA3, much higher than that of neat TiO₂ (0.01884 min^-1) and CA0 (0.01194 min^-1). We hold the view that the high activity of CA3 is mainly due to better anatase TiO₂ crystallinity during calcination, and the existence of graphene and larger specific surface area in CA3 also has contribution for the activity improvement. It is favorable for the reduction of Cr(Ⅵ) in acidic solution, nevertheless, CA3 has higher activity at pH 6 than TiO₂ and P25 at pH 2, indicating that CA3 has favorable perspective for application. We also found that the photocatalytic activity can be further improved in the presence of some low molecular weight organic acids. We hold the view that the accelerate effect depends on the structure of the added organic acid, in another word, the easier for the organic acid to supply electrons to fill the holes, the more obvious accelerate effect can be observed.