TiO₂ Activity Enhancement Through The Synergistic Effect Of Carbon Quantum Dots And Photonic Crystals

TiO₂ is one of the most popular photo-catalysts worldwide due to its high activity,high stability,low cost and low toxicity.However,TiO₂ shows very weak visible light absorption and high recombination rate of photo generated electrons and holes?e^--h⁺?,which are adverse to the photocatalytic process.Recent research shows that carbon quantum dots and photonic crystals have positive effect on the light harvesting of TiO₂.However,among these research,either carbon quantum dots or photonic crystals is employed to TiO₂ photo-catalysis separately,not simultaneously.In order to further enhance the photo catalytic performance of TiO₂,this work aims at the enhancement of TiO₂ activity through both carbon quantum dots and photo crystals,that is the synergistic effect of them.The research contents and results are as follows:Firstly,N-doped carbon quantum dots?NCQDs?were synthesized via hydrothermal process.The NCQDs photoluminescence?PL?test showed that the so-called up-conversion photoluminescence?UCPL?phenomenon excited by incoherent monochromatic light is,in fact,just a normal photoluminescence caused by the second-order diffraction light.The real UCPL was occurred only under a pulsed laser excitation.Therefore,NCQDs can not be treated as spectral converter to promote the light harvesting of TiO₂.Instead,due to the visible light absorption ability,NCQDs can still act as a sensitizer to expand the range of light absorption.Secondly,nc-TiO₂ membrane was fabricated by spin-coating method and then sensitized by NCQDs.The effects of sensitization time on the photocatalytic performance of NCQDs/nc-TiO₂ were investigated.The results showed that NCQDs can enhance the photocatalytic performance of nc-TiO₂ by 1.20 and 1.28 fold under visible and UV-Vis light respectively when the sensitization time was 1.5 h.This might be caused by two reasons:one is that NCQDs expand the absorption edge of TiO₂ to visible region.Another reason is that the recombination of e^--h⁺is inhibited by NCQDs.After PL test and analysis,we proposed the reaction mechanisms for NCQDs/nc-TiO₂composite under UV-vis irradiation.Finally,we prepared SiO₂ Opal photonic crystals with different photonic bandgaps?PBG?through vertical deposition technique.Then the NCQDs/nc-TiO₂ membrane was coupled onto the SiO₂ Opal layer to obtain NCQDs/nc-TiO₂/SiO₂ Opal composite membrane.The effects of PBG position on the photocatalytic performance of NCQDs/nc-TiO₂/SiO₂ Opal composite membrane were investigated.Results showed that the photocatalytic kinetic constant of NCQDs/nc-TiO₂/SiO₂ Opal composite membrane reached maximum value when the PBG of SiO₂ Opal was located at 455 nm,which is in the region of the absorption edge of NCQDs/nc-TiO₂.The improvement of photocatalytic activity attributed to the PBG reflection and the slow photon effect of SiO₂ Opal-455 nm.Compared with nc-TiO₂ membrane,the photocatalytic activity of NCQDs/nc-TiO₂/SiO₂ Opal-455 nm composite membrane increased by 3.15 and 2.9fold under visible and UV-Vis light respectively.The results showed that the synergetic effects of NCQDs and SiO₂ Opal photonic crystals can enhance the photocatalytic performance of nc-TiO₂ more efficiently.