The Regulating Effect Of A Trace Amount Of Water And The Reduction Method Of GO On The Photocatalytic Performance Of TiO₂-GSs Composites

Titanium dioxide (TiO2) has been widely used as photocatalyst, due to itsnon-toxicity, low cost and high chemical stability. However, the high recombinationrate of photogenerated electron-hole pairs results in low photocatalytic efficiency ofTiO2, which hinders its practical application in photocatalysis. Graphene sheets (GSs)has also attracted tremendous attention in the field of photocatalysis, owing to itslarge surface area, favorable adsorption performance and excellent carrier mobility,etc. In this paper, graphene oxide (GO) were prepared by Hummers method usinggraphite as starting materials. A series of TiO2-GSs composites were prepared byalcohothermal method, using GO ethanol suspension, isopropyl titanate (TIP) andabsolute ethanol as raw materials. Photocatalytic performance of TiO2-GSscomposites was investigated by photocatalytic degradation of methyl orange (MO). Itis well known that GSs exhibits good adsorption towards aromatic dye such as MO,which could increase the concentration of MO around the TiO2photocatalyst. GSsalso shows excellent electrical conductivity, which could promote the separation ofphoto-generated electron-hole pairs. Thus, the photocatalytic performance of TiO2was enhanced greatly by depositing in situ TiO2nanoparticles on GSs surface. Inaddition, we found that a trace amount of water in the GO ethanol suspension playsan important regulation role in the crystallinity, particle size, exposed crystal facets ofTiO2in the composite, and thus affects the photocatalytic pefrormance of TiO2-GSscomposites.Based on the above finding, TiO2and TiO2-GSs composites were prepared byalcohothermal method through artificial inclusion of a trace amount of water orproduction of water in situ by decomposing of NH4HCO3during the alcohothermalprocess. The role of a trace amount water in the photocatalytic performance of TiO2and TiO2-GSs composites have been investigated systematically. It was found that theintroduction of a trace amount of water could increase the crystallinity and particlesize of TiO2, and restrain the recombination of photo-generated electron-hole pairs. Inaddition, the formation of {001} crystal plane was related to a trace amount of water introduced though the exact mechanism is not clear. Similarly, the photocatalyticactivity of TiO2was improved greatly through inclusion of ammonium bicarbonateduring the alcohothermal reaction. In addition, during the preparation of TiO2-GSscomposites, the introduction of ammonium bicarbonate has promoted theheterogeneous nucleation of TIP on the surface of GO and the hydrolysis of TIP. As aresult, the loss of the titanium source was reduced clearly and the crystallinity of TiO2was increased obviously. Therefore, the photocatalytic performance of TiO2-GSscomposites was improved significantly.Electrostatic Self-assembly is a common method for the preparation ofcomposites. Normally, GO solution synthesized by Hummers method have a strongnegative surface charge. On the contrary, nano-TiO2sol prepared by the sol-gelmethod could possesses a positive surface charged, as the PH in our synthesis of TiO2nan-sol is far lower than the isoelectric point of TiO2. Thus, TiO2-GO compositescould rapidly be prepared in aqueous media through electrostatic attraction betweenGO and TiO2, then it could be reduced to TiO2-GSs composites through hydrothermalreduction method and hydrazine hydrate reduction method. As a result, the density ofTi-O band and sp2defect in the TiO2-GSs composites reduced by hydrazine hydratereduction method are largest, which is helpful for the separation of photo-generatedelectron-hole pairs. Therefore, the TiO2-GSs composites reduced by hydrazinehydrate showed a higher photocatalytic performance. In addition, the photocatalyticperformance of as-prepared TiO2-GSs would be improved significantly after asolvothermal treatment due to the higher degree of crystallinity.