Enhancement Of Catalytic Performances For G-C₃N₄ Based Materials

With the development of human society,the problems of environmental pollution and energy shortage are increasingly severe.Fortunately,many research methods have been applied to solve the above problems.Among them,the uses of photocatalysis and electrocatalysis are two effective means not only because of its high effiecient but also its zero emissions and non-pollution.Graphite phase carbonitride(g-C3N4)polymer which has stacked two-dimensional structure is a potential catalyst that has been extensively studied in the field of photo/electrocatalysis,because of its narrow band gap,good redox potential,many surface active sites,cheap,easy to prepare and non-toxic.In the field of photocatalysis,g-C3N4 can realize the photocatalytic decomposition of hydrogen(HER),reduction of CO2 and degradation of pollutants.However,some drawbacks like the extremely fast photogenerated electron hole recombination rate limits its catalytic activity.In electrocatalysis,g-C3N4 has certain ability of oxygen evolution reaction(OER)and oxygen reduction reaction(ORR).However,the high reaction energy barriers of these two reactions lead to the slow kinetics of its electrochemical reaction,which restricts its application.Therefore,it is of great importance to improve the photo/electrocatalytic efficiency of g-C3N4.In our work,based on the density functional theory,the monolayer g-C3N4 was chosen as the research object,and some experiments are carried out to study the photocatalytic efficiency of split water into hydrogen,and to elucidate the catalytic activity of oxygen evolution and oxygen reduction.(1)Semiconductor with large work function could be utilized to adjust the electronic distribution and redox potentials.Base on this idea,a novel t-ZrO2/g-C3N4 composite photocatalyst was prepared by supporting tetragonal ZrO2(t-ZrO2)nanoparticle on graphitic carbon nitride(g-C3N4)film.With the aid of experiment measures and density functional theory simulations,the electronic distribution of g-C3N4 are adjusted by supporting t-ZrO2 due to the transformation of electrons from Zr and O atoms to N atoms in composite sample,which contributes to the improvement of photogenerated carriers mobility and lifetime.Moreover,the composite photocatalyst has stronger reduction ability and more oxidation active sites than g-C3N4 sample and thus exhibits 9.4 and 2.9 times catalytic activity relative to g-C3N4 sample for RhB and TC-H degradation,respectively.Meanwhile,the composite photocatalyst reduces the reaction energy barrier of the HER process and increases the efficiency of photocatalytic decomposition of water to produce hydrogen,increase by 4 times.(2)By molecular doping,the electronic structure of g-C3N4 can be changed and can form new chemical bonds,which cause more efficient separation of photogenerated electron holes(e-/h+),the decrease of the free energy barrier of OER and HER,and thus improve the efficiency of photocatalytic decomposition of water.We investigated the properties of doped monolayer g-C3N4 adsorbing C6H4NO2 using first principle calculations.The obtained results show that,@N1 and@C2 are more stable than other structures,and the newly formed chemical bonds N-C,C-C can change the electronic structure of monolayer g-C3N4,which can cause the changes of conduction band(CB)and valence band(VB)because of relaxing the relevant chemical bonds.In addition,the overpotentials of OER decrease and overpotentials of HER are closer to 0 V in all considered cases.Adsorbing C6H4NO2 molecular can enhance the photocatalytic efficency of water splitting.(3)Although single metal atoms(SMAs)as unique active sites in single-atom catalysts have been extensively investigated,the possible active sites of the hosting catalysts are unfortunately neglected in prior studies.In single-atom catalysts,SMAs can promote the chemical and catalytic activities of host atoms who may act as the second active sites,which results in a significant synergistic effect on the catalytic performance.Using density functional theory calculations,we study the oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)on two different types of active sites,single metal(MO atoms and the neighboring host atoms of several M1/9-C3N4 samples.The contribution of Mi and host atoms to reduce the OER/ORR overpotentials of Fe1,Co1,Ni1,Cu1 and Zn1/g-C3N4,bi-functional electrocatalysts with the OER/ORR overpotentials of 0.50～0.70 V,are investigated.