Photocatalytic Performance Of N,S-GQDs/TiO₂ Composites

The increasingly serious environmental problems are behind rapid development of global industry.Industrial waste and organic dyes have caused serious water pollution.Due to low cost,strong oxidation resistance,non-toxic and corrosion resistance,titanium dioxide（TiO₂）has been widely used in the field of photocatalysis.However,the band gap of TiO₂ is 3.0-3.2 e V,which can only absorb UV light of less than 380 nm.Moreover,the electrons in the valance band of TiO₂ which been excited by the light can’t be transferred to the surface efficiently.More excited electrons will be combined with holes.The recombination rate between electrons and holes is high which limits their photocatalytic properties.Graphene quantum dots（GQDs）,which are zero dimensional materials,have a quantum local effect due to the size of the relationship.Therefore,GQDs have superior electrical,optical and magnetic properties.Different from graphene,GQDs can produce the excited electrons under the external conditions owing to the band gap energy.So,GQDs also have the semiconductor property.It has been reported that GQDs exhibit the excellent catalytic performance as a result of their unique electronic band structure.There are some reports about TiO₂/GQDs to improve the overall catalytic performance.At present,improving the synthesis method,reducing the by-products and cost are necessary.Based on the above considerations,we used the low cost P25 as the titanium source.The GQDs were synthesized by the method of “bottom up”.We used citric acid as carbon source,thiourea as nitrogen and sulfur source to synthesize nitrogen and sulfur doped graphene quantum dots（N,S-GQDs）.The main research contents and results in this paper are as follows: 1.TiO₂ nanotubes（TiO₂NT）-reduced graphene oxide（rGO）-N,S co-doped graphene quantum dots（N,S-GQDs）composites were prepared by a facile alkaline hydrothermal reaction and physical stirring process.Firstly,graphite oxide（GO）was prepared by the improved Hummers method.Second,GO and P25 were subjected to hydrothermal reaction according to different mass ratio: 5:95,10:90,15:85 to obtain TiO₂NT+rGO binary composites.Third,the TiO₂NT+10%rGO composite exhibited the best photocatalytic activity during the degradation of methyl orange（MO）under the visible light（l > 400 nm）.Therefore,10%rGO was chosen for the synthesis of the ternary composite.The apparent rate constant of the TiO₂NT+10%rGO+N,S-GQDs composite,exhibiting the highest photodegradation efficiency,is 1.8 times and 16.3 times higher than those of TiO₂NT+10%rGO and pure TiO₂ NT,respectively.2.N,S co-doped commercial TiO₂/N,S co-doped graphene quantum dots（NSTG）composites with band tunability have been synthesized by a facile solvothermal treatment in the presence of thiourea,which acted as a precursor for the dopants.The amount of dopant is changed according to different molar ratio: 1:0.5,1:1,1:3.The band gap of NSTG composites can be tuned by changing the amount of thiourea.Through the XPS VB spectra,the VBs of NST and NSTG composites can be obtained by calculation.The apparent rate constant of NSTG is about 2.4 times and 50.7 times higher than those of NST and P25 through the degradation of methylene blue（MB）under visible light irradiation（λ > 400 nm）.