Constructing Local Charge Environment Of Graphitic Carbon Nitride And Photo/Electrocatalytic Performance Investigation

With the continuous development of global economy,the human society heavily relies on fossil fuels for its energy needs,which results in an ever-increasing emission of carbon dioxide(CO2)into our environment.It is imperative to develop the sustainable energy and reduce CO2 emission in the atmosphere.Besides controlling the CO2 emission,such electroreduction technology is also capable of storing intermittent renewable energy in the form of chemical energies,making the energy supply more flexible.In addition,sustainable energy will occupy a large proportion in the future energy configuration structure.Producing hydrogen from photocatalystic splitting of water is an environmentally-friendly and sustainable pathway.The heterogeneous catalyst is the key component to realize these sustainbale energy conversion systems.An ideal catalyst is expected to demonstrate appropriate adsorption strength for the reaction intermediates and low energy barrier for each electron transfer step.Since catalytic activity is a complex function dictated by many variables such as electronic structure,atomic configuration,and morphology,designing a suitable catalyst for energy catalysis conversion can be very challenging for both laboratory studies and practical applications.Among all the energy conversion technology,researchers usually focus on precious metal as catalyst.To find a cheaper solid catalyst has been a hot topic in the energy chemistry field.Considering that the layered graphite carbon nitride(g-C3N4)is a representative two-dimensional material,and its synthesis and application range are constantly innovating,this thesis mainly focuses on the tuning of the electronic structure and local charge environment of g-C3N4 and investigates the heterogeneous catalysis performance.(1)Efficient and economical photocatalysts for both the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)are required to replace expensive metal-based catalysts used in water splitting devices.Herein,we have developed an inexpensive route to synthesize a carbon-rich graphitic carbon nitride(C-rich g-C3N4)with both nitrogen vacancies and porous structure,which,as a highly efficient photo-induced water splitting catalyst,can meet current demands.The effects of the porous structure,nitrogen vacancies and rich amount of carbon on the electronic band structure and charge transport of g-C3N4 are systematically elucidated.The C-rich g-C3N4 can not only effectively enhance the absorption of visible light,but can also improve the majority carrier mobility and promote photoelectron transport owing to the built-in electric field(BIEF),thus synergistically elongating the diffusion length and lifetime of the photocarriers.Importantly,the metal-free C-rich g-C3N4 photocatalyst demonstrates a higher solar-driven hydrogen production performance,which is over 20.5 times that of pristine g-C3N4,and exhibits an outstanding stability with minimal loss of catalytic activity.Our findings enrich the theory of structure-activity relationship of photocatalyst and provide prinsciple for rational photocatalyst design according to internal electric field.(2)The intrinsic drawbacks of weak visible-light adsorption and poor charge separation efficiency seriously limit g-C3N4's practical applications.Thus,struggling controls over the structure parameters of g-C3N4 to optimize the photoelectrical properties on molecular-level for realizing highly active photocatalysts have attracted a lot of attentions.Herein,a novel isopropanol assisted solvothermal-copolymerization strategy is rationally designed to synthesize a compact O,S co-doping g-C3N4(CNUS)with markedly reinforced ?-?*and n-?*electron transitions.The meliorative structure and energy level configuration result in elevated effects for both visible-light(photon)adsorption and photoinduced carries transfer,and the CNUS exhibits outstanding photocatalytic hydrogen evolution and rhodamine B degradation performance under visible light.The characterization results indicate that the incorporated oxygen and sulfur engineer the local charge between the layered structure,leading to the formation of distrorted structure.As result,the layered-stacking distance of CNUS decreases from 0.328 to 0.322 nm,compared its counterpart(CNU prepared by direct pyrolysis of urea).Importantly,the local charge is enriched by the distorted structure of g-C3N4,which facilitates the rate-limiting separation of photogenerated carriers,and hence improved the visible light photocatalytic efficiency.(3)New approach to activating carbon active sites to facilitate the electroconversion of CO2 through N-vacancy engineering is proposed and confirmed by the density functional theory calculations and experimental results.N-vacancy engineered graphitic C3N4(g-C3N4)is identified as an efficient electrocatalyst to boost CH4 formation owing to the three-coordinating to two-coordinating transition of the carbon atoms that surround N vacancies.The defected g-C3N4(DCN)exhibits a 44%faradaic efficiency with a CH4 partial current density of 14.8 mA/cm2 at-1.27 VRHE in 0.5 M KHCO3 electrolyte,exceeding all the reported carbon-based materials and even being comparative to Cu-based electrocatalysts.Creation of more unsaturated carbon atoms enables the harmonic energy overlap near band gap edge between DCN and*CO,accounting for an improved strength of*CO on DCN,a lower energy barrier and an enhanced CO2RR to form CH4.This strategy holds the promise for tuning atomic configuration to enhance activity of catalyst.(4)Syngas,a CO and H2 mixture mostly generated from non-renewable fossil fuels,is an essential feedstock for production of synfuels and high-value chemicals.Power-to-syngas,that is,the electrochemical conversion of water and carbon dioxide can be regarded as a key-enabling step for a transition of the energy system,which offers additionally features of CO2 valorization and closed carbon cycles.Here we design a single Co atoms anchored g-C3N4 composite to achieve a cheap,stable,selective,and efficient electrocatalyst for tunable syngas production.Series amount of single Co atoms contents are synthesized,and the H2/CO ratio of the produced syngas is tunable from 2.97 to 0.6 by controlling the amount of single Co atoms.DFT studies confirm that the C1 atom nearby the CoN2 is the active site for the CO production and the C2 atom far away from CoN2 is the active center for the H2 production.The carbon atoms in the structure of g-C3N4 is activated by the single atom.This work not only open new horizons for manipulating CO2 electroreduction properties,but also an economically friendly pathway to engineering the single atoms.