Synthesis And Photocatalytic Performance Of Functionalized Graphene-Based Titanium Dioxide

Environmental pollution and the lack of natural energy resources are some of the most serious problems.Photocatalytic technology is considered to be a new and effective purification technology for the removal of various organic pollutants.TiO₂semiconductor has attracted intense attention due to its outstanding physical and chemical stability,inexpensiveness and non-toxicity.Nevertheless,the currently available TiO₂ photocatalysts generally suffer from a low photocatalytic efficiency,which significantly has a great effect on the practical applications.Fortunately,the photocatalytic activity of titanium dioxide semiconductor materials can be improved by using electron cocatalysts.Graphene,a two-dimensional monolayer material form of sp² bonded carbon atoms,has been demonstrated to be an excellent support as well as an electron cocatalyst owing to its large specific surface area and high electron mobility.To further modify the graphene as electronic cocatalyst,this paper focuses on the surface functionalization of graphene:?1?the synergistic effect of graphitic N and pyrrolic N on the enhanced photocatalytic performance of nitrogen-doped graphene/TiO₂ nanocomposites;?2?the synergistic effect of cobalt sulfide and graphene for improved TiO₂ photocatalytic activity.The main content and results could be summarized as follows:The functionalization of graphene by nitrogen doping has been found to be an effective electron-cocatalyst to improve the photocatalytic performance of TiO₂.However,the effect of N-bonding configurations?such as graphitic N,pyridinic N and pyrrolic N?on the photocatalytic performance of nitrogen-doped graphene/TiO₂composite?N-rGO/TiO₂?has seldom been investigated.In this study,the different nitrogen sources have been used to prepare the N-rGO/TiO₂ with the aim of obtaining different N-bonding configurations in graphene,and the photocatalytic degradation performance of methyl orange was studied under UV light.It was found that the structural characteristics of N can be greatly influenced by using different nitrogen source precursor for N-rGO.When the NH₃ and CO?NH₂?₂ were used as the N-doping precursors,the resultant of N-rGO photocatalysts mainly showed the pyrrolic N?>70%?and pyridinic N?>10%?.As for the N₂H₄ precursor,the prepared N-rGO primarily exhibited the pyrrolic N?63%?and graphitic N?37%?in graphene.The photocatalytic results indicated that the N-rGO/TiO₂?N₂H₄?displayed the highest photocatalytic activity,which was remarkably larger than that of TiO₂,rGO/TiO₂,N-rGO/TiO₂?NH₃?and N-rGO/TiO₂?CO?NH₂?₂?by a factor of 3.63,2.64,1.61 and1.38,respectively.On the basis of the above results,a synergistic effect of graphitic N and pyrrolic in graphene is proposed to account for the enhanced photocatalytic performance of N-rGO/TiO₂?N₂H₄?,namely,the graphitic-N doped graphene serves as an effective electron-transfer mediator for the photogenerated electrons while the pyrrolic-N doped graphene functions as the oxygen-reduction active site to rapidly promote the following interfacial catalytic reaction.Compared with noble metal cocatalysts,cobalt sulfide is a transition metal sulfides,which is cheap and abundant on earth and has been widely used as electrode material in electrochemistry.However the effect of cobalt sulfide on the photocatalytic performance of semiconductor photocatalysis has seldom been investigated in the field of photocatalysis.In this case,the synergistic effect of cobalt sulfide and graphene on the enhanced photocatalytic activity of TiO₂ was studied.The rGO/TiO₂ composite material was firstly prepared by a hydrothermal method,and then CoS₂ cocatalyst was further loaded on the rGO nanosheets by an impregnation-hydrothermal method to prepare the CoS₂-rGO/TiO₂ photocatalyst.It was found that the dual electron-cocatalyst modified CoS₂-rGO/TiO₂ photocatalyst showed an obviously higher photocatalytic performance than the naked TiO₂ and single-cocatalyst modified photocatalysts?such as CoS₂/TiO₂ and rGO/TiO₂?.Based on these results,a possible synergistic effect mechanism of cobalt sulfide and graphene for the enhancement of TiO₂ photocatalytic activity is proposed,namely,the rGO nanosheets with a large specific surface area and high electron mobility can function as an electron-transfer mediator for the effective transfer of photogenerated electrons from the TiO₂ surface,and the CoS₂ cocatalyst serves as an electron-reduction active site to promote the following interfacial oxygen reduction.