Adsorption And Photocatalytic Activity Of G-C₃N₄:First-principle Calculation And Experimental Study

Increasingly prominent energy shortage and environment pollution severely threaten human’s manufacture and survival.Semiconductor photocatalysis exhibits great potential in solving these problems because of its application in water splitting,CO₂ reduction and pollutant degradation.Graphitic carbon nitride（g-C₃N₄）is a semiconductor photocatalyst which has visible light response and strong stability.However,the photocatalytic activity of pristine g-C₃N₄ synthesized by facile calcination method is poor.Exploring an effective strategy to improve the photocatalytic activity of g-C₃N₄ is an important research topic.In this dissertation,by employing experimental study and first principle calculation,we explored the isoelectric point of g-C₃N₄,doping effect on g-C₃N₄,g-C₃N₄-based composite photocatalyst and the adsorption of small molecule reactants on g-C₃N₄.The detailed contents were summarized as follows.（1）The zeta potential,isoelectric point and dye adsorption activity of g-C₃N₄were measured.Three g-C₃N₄ samples were synthesized by calcinating melamine,thiourea and urea.In aqueous suspension,the protonation and deprotonation of the amine groups on the surface of g-C₃N₄ resulted in the surface charge on g-C₃N₄.By measuring the zeta potential under various pH conditions,the isoelectric points of the g-C₃N₄ samples were determined to be 5.0,4.4 and 5.1,respectively.The g-C₃N₄samples were negatively charged under neutral condition,therefore they can adsorb methylene blue（cationic dye）and cannot adsorb methyl orange（anionic dye）.（2）g-C₃N₄/Ag₂WO₄ composite photocatalyst with excellent photocatalytic activity of dye degradation was synthesized by an in-situ precipitation method.Dropwise adding Na₂WO₄ solution into g-C₃N₄ suspension containing AgNO₃,we gotβ-Ag₂WO₄ nanoparticle on the surface of g-C₃N₄.The Ag₂WO₄ sample synthesized without the addition of g-C₃N₄ wasα-Ag₂WO₄.Herein,g-C₃N₄ acted as a support for the nucleation and growth ofβ-Ag₂WO₄,and inhibited the phase transformation from metastableβ-Ag₂WO₄ to stableα-Ag₂WO₄.After illumination for 150 mins,the degradation rates of methyl orange were 42%,71%and 95%in the presence of g-C₃N₄,α-Ag₂WO₄andg-C₃N₄/Ag₂WO₄composite,respectively.The g-C₃N₄/Ag₂WO₄ composite had stronger light absorption and higher separation efficiency of photogenerated charge carriers than pure g-C₃N₄.Direct Z-scheme photocatalytic mechanism can simultaneously elucidate the generation of active radicals and the enhanced separation of photogenerated electron–hole pairs.（3）Halogen（F,Cl,Br and I）doped g-C₃N₄ systems were investigated by first principle calculations.The introduction of halogen atoms decreased the band gap,improved the optical absorption and reduced the work function of g-C₃N₄.F atom occupied the valance band and the highest occupied molecular orbital,whereas Cl,Br and I atoms were involved in the conduction band and the lowest unoccupied molecular orbital,which was closely related to the electronegativity of the halogen atoms.（4）The adsorption of CO₂ molecule on tri-s-triazine-based g-C₃N₄ was investigated by first principle calculations.The calculated adsorption energy showed that CO₂ molecule preferred to adsorb at the two-coordinated nitrogen atom.The adsorption of CO₂ molecule resulted in corrugated g-C₃N₄ structure,enlarged band gap and increased work function from 4.65 eV to 5.51 eV.（5）The adsorption of CO₂,O₂,NO and CO molecules on s-triazine-based g-C₃N₄was investigated by first principle calculations.The calculated adsorption energy showed that these gas molecules preferred to adsorb at the open-hollow site encircled by three adjacent s-triazine units.The adsorption of these gas molecules resulted in corrugated g-C₃N₄ structure,enlarged band gap and changed work function from 4.21eV to 5.37 eV,5.01 eV,3.80 eV and 5.33 eV,respectively.