| Energy shortage and environmental pollution have become two big challenges in 21st century due to the consumption of fossil fuels and the fast-growing industrial technology.Therefore,developping an environmentally friendly and renewable technology that can be used for producing green energy,and solving energy shortage and environmental problems is needed for the long-term and sustainable development of human society.Among the various proposed tactics,photocatalytic technology,with the advantages of economy,cleanliness,safety and sustainability,is considered as one of the most effective way to solve the energy shortage and environmental problems.In photocatalytic technology,on the hand,solar energy can converts into green chemical energy(eg.H2)directly through the photocatalysis,which is considered to be one of the most promising solutions to solve energy shortage;on the other hand,the photocatalysis can be used to degrade the pollutants in the environment into harmless products such as H2O,CO2 and various salts of inorganic nature through a green chemical process.In recent years,graphitic carbon nitride(g-C3N4)has attracted a great deal of attention from researchers as a novel metal-free polymeric photocatalyst,owing to its fascinating merits,such as low-cost,easy preparation,driven by visible light(the band gap of g-C3N4 is 2.7 eV),nontoxicity,interesting electronic band structure,and excellent chemical and thermal stability.Despite the above advantages,the practical application of pure g-C3N4 is still restricted,mainly because of the high recombination rate of photogenerated electrons and holes(e-/h+),which are counted as charge carriers in photocatalytic process.Hence,the exploration of efficient g-C3N4-based photocatalysts with high charge separation/transfer efficiency and better photocatalytic properties has become a hot topic in the fields of energy and environmental chemistry.In this thesis,several g-C3N4-based photocatalysts have been designed,synthesized and studied.In addition,the reasonable photogenerated electrons and holes separation,transfer and reaction mechanisms have also been proposed.The findings of this paper are carried out as follows:1.With the objectives of enhancing the stability,optical properties and visible-light photocatalytic activity of photocatalysts,we modified oxygen vacancy-rich zinc oxide(Vo-ZnO)with graphitic carbon nitride(g-C3N4).The resulting g-C3N4/Vo-ZnO hybrid photocatalysts showed higher visible-light photocatalytic activity than pure Vo-ZnO and g-C3N4.The hybrid photocatalyst with a g-C3N4 content of 1 wt%exhibited the highest photocatalytic degradation activity under visible-light irradiation(?? 400 nm).In addition,the g-C3N4/Vo-ZnO photocatalyst was not deactivated after five cycles of methyl orange degradation,indicating that it is stable under light irradiation.Finally,a Z-scheme mechanism for the enhanced photocatalytic activity and stability of the g-C3N4/Vo-ZnO hybrid photocatalyst was proposed.The fast charge separation and transport within the g-C3N4/Vo-ZnO hybrid photocatalyst were attributed as the origins of its enhanced photocatalytic performance.2.We develop a strategy to construct a type of g-C3N4-based composite photocatalyst(C3N4/CBV2+),g-C3N4 surface coupled with a viologen redox mediator((1,1'-bis(4-carboxylatobenzyl)-4,4'-bipyridinium)dichloride,denoted as CBV2+)through hydrogen-bonds,for enhanced H2 production from water under visible light irradiation.The CBV2+ molecules not only provide sites for metal particle formation,but also act as an efficient electron transfer mediator to transfer the photoinduced electrons from g-C3N4 to platinum nanoparticles(Pt NPs).The vectorial charge transfer results in an efficient spatial separation of electrons and holes in the C3N4/CBV2+ composite photocatalyst,and facilitates the photogenerated charge carriers for direct photocatalytic water splitting.When 1 wt%CBV2+ is introduced,the hydrogen production rate of C3N4/CBV2+ dramatically increases up to 41.57(?mol h-1,exceeding 85 times over unmodified g-C3N4(only 0.49 ?mol h-1).It is noted that a negligible loss of photocatalytic activity was observed over continuous irradiation of up to 20 h,demonstrating its good stability.The combination of the two emerging functional materials represents a simple but economic and powerful approach for highly effective photocatalytic hydrogen production under visible light irradiation.This study opens a window to rationally develop cost-acceptable materials for more efficient solar energy conversion applications.3.We develop a novel system of g-C3N4/1,1'-Ferrocenedicarboxylic Acid(FcDA)composites for efficient visible light-driven H2 evolution,in which redox mediator FcDA serves as hole-transport molecules,and platinum(Pt)acts as electron sink.FcDA molecules are anchored onto g-C3N4 by hydrogen-bonding interaction between carboxylic groups and amino groups,as well as ?-? interaction between aromatic FcDA and graphitic C3N4.The matched energy-level between g-C3N4 and FcDA facilitates the photogenerated hole transfer from g-C3N4 valence band to the FcDA forming FcDA+ radicals,and photoinduced electrons and holes are efficiently separated.Given the additional charge separation pathway,enhanced hole transfer kinetics and the extremely rapid intermolecular radical reactions,charge recombination are effectively suppressed,therefore,more electrons can be released for hydrogen production.Under optimal experimental conditions,the developed g-C3N4/FcDA composite with 4 wt%FcDA exhibits high water splitting activity with H2 evolution rate up to 77.91 ?mol h-1,which is 371 times higher than that of bare g-C3N4(0.21 ?mol h-1).Moreover,obtained g-C3N4/FcDA photocatalyst displays excellent stability,there is no obvious decrease in H2-production rate after five test cycles.It could be anticipated that our simple modification strategy will offer an avenue to merge the polymeric g-C3N4 photocatalyst with surface organometallic chemistry for high-efficiency solar-to-fuel conversion. |
PDF Full Text Request