Photocatalytic Activity Of Graphitic Carbon Nitride ?g-C₃N₄? And Its Composites

Energy is the foundation of the sustainable development of human society.However,in the past few decades,the energy shortage and deterioration of living environment are increasingly prominent accompanied by the sharp consumption of fossil fuels in the world and massive increase of released wastes,with accelerating development of modern industrialization.Photocatalytic technology is one of promising clean energy technologies that the solar can be directly absorbed to produce the charge carriers,which then participates a series of redox reactions,realizing the conversion and utilization of clean solar energy,such as converting solar energy into hydrogen energy,reducing greenhouse gas CO₂ into hydrocarbons and purifying wastewater containing organic pollutants.Therefore,photocatalytic technology has shown great application prospect in solving abovementioned energy and pollution problems.As a new type of metal-free photocatalyst,graphitic phase carbon nitride?g-C₃N₄?has a graphite-like structure with tri-s-triazine ring as the structural unit.Moreover,g-C₃N₄ has became one of the hot materials in the field of photocatalysis,because of the advantages of visible light response,stable chemical structure,low preparation cost,high corrosion resistance and unique electronic and energy band structure.However,pure g-C₃N₄ prepared by traditional thermal condensation method usually shows some problems such as incomplete polycondensation of the precursors,uncontrollable microstructure and high recombination rate of photogenerated carriers,which result in low photocatalytic activity and great limition in the wide applications.In order to improve photocatalytic activity,some effective strategies have been taken to increase the solar energy utilization and active sites,such as morphology control,nanocrystallized structure,surface modification and heterojunction construction,as well as improving the kinetic process of photogenerated carrier.The main contents of this study are as follows:Graphitic carbon nitride?g-C₃N₄?was prepared using the melamine as the precursor by the traditional thermal condensation method,which usually encountered many problems,such as uncontrollable morphology,poor crystallinity,high carrier recombination efficiency and high energy consumption in the preparation process.To solve these problems,under acetonitrile-promoted solvothermal conditions,the belt-like carbon nitride with a thickness of¹20 nm was synthesized by co-condensation of guanidine hydrochloride and dicyandiamide at low temperature.It overcomes the problems of high energy consumption and uncontrollable micromorphology in the preparation of g-C₃N₄ by traditional thermal condensation method.This belt-like carbon nitride contains rich functional group defects and high crystal order.The high crystallinity increases the mobility of the charge,while the structural terminals with cyanide and carboxyl groups can promote the absorption of light,electron storage and the binding of CO₂ molecules.The surface defects of crystalline carbon nitride can efficiently reduce vapor phase CO₂ to hydrocarbon fuel and exhibit high selectivity.However,the belt-like carbon nitride still encounters some problems,such as small specific surface area and insufficient adsorption capacity towards CO₂molecules.Therefore,the bulk g-C3N4 was exfoliated into ultrathin structure with thickness of³ nm by thermal exfoliation method in ammonia atmosphere,achieving the two-dimensional g-C₃N₄ nanosheets with high specific surface area.This not only makes its surface amination,but also fully exposes the active site of Lewis base?the lone pair of electrons on nitrogen atom?hidden in the interior of bulk g-C3N4.Therefore,ultrathin g-C₃N₄ nanosheets exhibit high adsorption capacity towards acidic CO₂ molecules.Meanwhile,in this ultra-thin structure,the migration distance of photoelectrons from interior to surface is greatly shortened,which significantly improves the separation,transfer and utilization of photogenerated charge carriers,resulting in the improvement of the photocatalytic activity.Considering that above pure g-C₃N₄ can only absorb part of visible light and the efficiency of separation and transport of photogenerated carriers is still low,this limits the further improvement of its photocatalytic performance.To further improve the photocatalytic activity of g-C₃N₄,a series of g-C₃N₄/polydopamine?g-C₃N₄/PDA?composite photocatalysts were prepared by self-polymerization of dopamine hydrochloride in the dispersion containing g-C3N4 nanosheets.The precursor of dopamine hydrochloride can be in situ condensed into PDA on the surface of g-C₃N₄nanosheets,forming a close?-?*interaction at the interface between g-C3N4 and PDA,which equivalently constructs an electron transfer channel to promote the transfer of photoelectrons from g-C₃N₄ to its surface and captured by the cocatalyst.It is also found that the surface of g-C3N4 modified by PDA could significantly inhibit the natural growth of cocatalyst Pt nanoparticles in the photocatalytic reactions.Generally,the Pt particles with smaller size can provide more unsaturated surface atoms and increase the number of reactive sites,which enhances the photocatalytic hydrogen activity from water splitting.To further improve the separation and transfer efficiency of photogenerated carriers in photocatalysts,the 0D/2D CeO₂ quantum dots/g-C₃N₄ nanosheets Z-scheme heterojunction was constructed by in situ growth of Ce ions on the g-C₃N₄surface into CeO₂ quantum dots with size of 5.5 nm.This nano-Z-type heterojunction not only significantly improves optical absorption ability,but provides more reactive sites.In addition,Z-type charge transfer mechanism in CeO₂ QDs/g-C₃N₄heterojunction not only promotes effective separation and transfer of photogenerated carriers in space,but facilitates photocatalytic oxidation and reduction reactions occured in different positions.Therefore,CeO₂/g-C₃N₄ heterojunction simultaneously exhibits high photocatalytic hydrogen production and antibacterial activity.In addition,the density functional theory?DFT?calculations show that the carbon atoms in the heterojunction and sp² hybrid nitridation atoms are active centers for the photocatalysis of hydrogen production.In comparison with the 0D/2D Z-type heterojunction,the 2D/2D Z-type heterojunction has a larger contact area,which more efficiently promotes the carrier separation and interface reaction.Herein,in the dispersion of containing surfactants and g-C₃N₄ nanosheets,MnO₂ nanosheets were grown in situ on the surface of g-C₃N₄ nanosheets by ion layer adsorption reactions,constructing the two-dimensional/two-dimensional g-C₃N₄/MnO₂ Z-type heterojunction.This two-dimensional heterojunction can not only significantly improve the optical absorption capacity,but also has a large specific surface area and interface area,thus promoting the formation and interface reaction of photogenerated carriers.It is found that the transfer process of photogenerated carriers in heterojunction conforms to the Z-type transfer mechanism,which not only enables photogenerated charge carriers to be effectively separated in space and inhibits their recombination,but also achieves strong redox ability and produces more oxygen-containing active species,achieving high-efficiency photodegradation towards the Rhodamine B and phenol.