A Study on Photoelectrocatalytic Activity and Mechnism of Solar-driven TiO2 Films
Supervisor:zhang yan zong shen fei
Nowadays,two major issues are threatening the human society;one is ecological destruction caused by theenergy extraction and ultization process,and anotheris environmental pollution introduced by industrial development.Therefore,develop of new photoelectrocatalytic materials and new technogies,that can efficient use of solar energy and eco-friendly,to solve the problems of environment pollution and energy crisishave became the hot and focus topicof the global research.In this paper,a solar-driven TiO2 film electrode was prepared on the Ti foil by anodic oxidation.The electrolyte system,oxidation voltage,oxidation time,and calcination temperature and styles were investigated.The controllable preparation and formation mechanism of the solar-driven TiO2film electrode was achieved;Based on this,we studied on the photoelectrocatalytic hydrogen production efficiency and printing and dyeing wastewater degradation efficiency of the prepared TiO2 film electrode;Finally,we studied on the ecotoxicology effects of the prepared TiO2 film electrode,which provided the theoretical and scientific basis for its safe use.The main results and conclusions were summarized as follows:(1)Preparation and formation mechanism of the solar-driven TiO2 film electrodeThe optimal preparation conditions were determined by systematically studying the factors such as electrolyte composition,anodic oxidation voltage and time,calcination temperature,styles and so on.The film prepared by anodic oxidation were amorphous,then turned into anatase phase after annealing at 400 ℃ for 2 h,the mixed phase of anatase and rutile was appeared at 450 ℃,almost all of them turn into rutile phase after 600 ℃.The film was pine-cone like structure,and contained Ti,O,N and C elements,N doped in the anatase lattice of TiO2 as interstitial N,substitutional N,and and molecularly chemisorbed y-N2.The film had excellent optical absorption capacity in the range of 200-2,500 nm.The enhanced visible-infrared absorption was attributed to the N doping and/or Vo and/or the nanoporous pine-cone structure.Electrochemical results showed that the photocurrent density was affected by specific surface area of the film,photoelectron transfer efficiency,thickness of the barrier layer,and light intensity.The formation mechanism of the TiO2 filmwas proposed.First,formation of the anodic oxide layer and pore growth,second,formation of the cracks and TiO2-graphene layers,third,deposition of the TiO2-graphene layers on the cracks,final,formation of the pine-cone structure.(2)Sunlight response mechanism of TiO2 film electrodeFirstly,the doping of N elements and the interfacial structure of mixed crystal generatesa large number of mid-gap statesover valence band maximumof TiO2,and a substantial shift of 1.59 eV.The effective bandgap can be narrowed to 2.74 eV or 1.68 eV.Secondly,Vo creates defect statesof 1.0 to 1.2 eV below the conduction band minimum.The mid-gap states above the valence band maximum and the defect states below the conduction band effectively reduce the bandgap and the energy of the electron transition by light excitation.(3)Photoelectrocatalytic hydrogen generation efficiency and mechanism of the solar-driven TiO2 film electrodeWith the Pt net as the cathode material,NaOH solution as the electrolyte,the photoelectrocatalytic hydrogen production efficiency in UV light reached 331.4 μmol cm-2 h-1.The anodic oxidation process for preparation of TiO2 film electrode,including electrolyte composition,oxidation voltage and time,calcination temperature and style all had great influence on the photoelectrocatalytic hydrogen production efficiency.The experimental results showed that prepared in GR(35 mg)and Mg(NO3)2 · 6H2O(50 ml,100 mg/L)electrolyte at 40 V for 2 min,then calcinated without heating and cooling in muffle oven at 450 ℃ for 2 h exhibited the highest hydrogen generation efficiency.The hydrogen production rates can reach 142.8 and 30.0 μmol cm-2 h-1 under solar and xenon lamps,respectively.What’s more,the TiO2 film electrode showed good stability.The results indicated that the film has the potential to convert solar energy into hydrogen energy.Photoelectrocatalytic hydrogen generation mechanism of solar-driven TiO2 film electrode was proposed.When the TiO2 film electrode is illuminated with sunlight,it produces photo-generated electrons and holes.With the action of bias,the photo-generated electrons are rapidly transferred from the conduction band of TiO2 to theTi substrate and transferred to the cathode Pt network through the external circuit,where protons are reduced to molecular hydrogen.The photo-generated holes left on the photoanode was captured by sacrificial methanol,which effectively promoting the separation of photogenerated electrons and holes,reducing the recombination rate of photo-generated electrons and holes.(4)Photoelectrocatalytic degradation efficiency and mechanism of the solar-driven TiO2 film electrodeThe anodic oxidation process for preparation of TiO2 film electrode,including electrolyte composition,oxidation voltage and time,calcination temperature and style had great influence on the photoelectrocatalytic degradation of MO solution.The experimental results showed that prepared in GR(35 mg)and Mg(NO3)2 · 6H2O(50 ml,100 mg/L)electrolyte at 40 V for 2 min,then calcinated without heating and cooling in muffle oven at 450 °C for 2 h exhibited the highest efficiency of photoelectrocatalytic degradation of MO solution,which up to 99%only 90 min in sunlight,TOC removal rate up to 75%,and the degradation process was in accordance with the first-order kinetics process.In addition,it showed agood stability.For printing and dyeing wastewaterphotoelectrocatalytic degradation,the photoelectrocatalytic degradationwas carried out after flocculation and sedimentation by polymer FeC13 and PAM.After photoelectrocatalytic degradation for 2 h,the removal rates of COD and TOC reached to 90%and 72.8%,respectively.The dye molecules have been mineralized to CO2 and H2O.Photoelectrocatalytic degradation mechanism of solar-driven TiO2 film electrode was proposed.Photo-generated electrons and holes of TiO2 film are produced under sunlight irradiation.Parts of the photo-generated electrons aretransferred to the cathode under the electric field,that suppressing the recombination of the photo-generated electronsand holes.The remaining photo-generated electrons migrate to the surface of TiO2,and react with O2 to produce HO2· and O2.The resulting photo-generated holes react with H2O or OH-adsorbed on the surface to produce OH·.These active groups with strong oxidizing properties react with organic pollutants,and mineralize the organic pollutants into CO2 and H2O.(5)Potential ecological risk assessment of solar-driven TiO2 film electrodeThe potential ecological risk assessment of the TiO2 film electrode was carried out by using rice and zebrafish as the research objects,respectively.Compared with the control group,the dissolved solution of TiO2 film electrode with different areas had no obviously influence on the germination of rice seeds,root length,stem length,adventitious root number,root fresh weight,over ground part weight and root cap ratio of rice seedlings,as well as SOD activity,CAT activity,POD activity,and MDA content of rice seedlings(P>0.05).What’s more,there was no death of zebrafish in 96 hours.It is concluded that the TiO2 film electrode does not dissolve TiO2 nanoparticles or Ti metal during the photoelectrocatalytic treatment,and has no potential effect on plant and animal body.