Study On The Photogenerated Carrier Transfer Rate Of Graphitic Carbon Nitride In Photocatalytic Hydrogen Production From Water Splitting

Hydrogen energy is an ideal energy source with non-toxic and high calorific value,and the photocatalytic hydrogen production from water splitting technology is an effective way to solve energy shortage and environmental pollution.Because of its simple preparation,abundant raw materials,suitable energy band structure,and photothermal stability,the graphite-phase carbon nitride(g-C3N4)photocatalyst has received extensive attention from researchers.However,its low photogenerated charge separation efficiency and difficulty in migrating to the catalyst surface limit its photocatalytic activity.In this work,to address these drawbacks,g-C3N4 was modified by constructing heterojunctions,doping with potassium ions and introducing nitrogen defects to regulate the migration rate of photogenerated carriers;the photocatalytic hydrogen production rates of all samples were tested.The photogenerated carrier generation,separation and migration as well as the reaction mechanism in the photocatalyst were analyzed by a series of characterizations.The effects of different modification methods on the g-C3N4 energy band structure,PDOS and HOMO-LUMO were explored by using DFT theoretical simulations,and the mechanism of photogenerated carrier migration rate regulation was elaborated in depth.Firstly,hydrothermal citric acid and urea mixed solution was used to prepare carbon quantum dots(CQDs)with surface amino groups.Then,composite photocatalysts with CQDs implanted in g-C3N4 were prepared by thermal polymerization of CQDs and urea self-assembled precursors.To investigate the effect of CQDs on the photogenerated carrier migration and overall photocatalytic performance in g-C3N4,a series of samples with different CQDs doping ratios were prepared(CCN-x).The highest photocatalytic hydrogen production rate of the prepared CCN-10 photocatalyst was achieved when 10 μL CQDs solution was added to the precursor.Under visible light irradiation,the average hydrogen production rate of CCN10 reached 2321 μmol·g-1·h-1,which was 7.4 times of g-C3N4(313 μmol·g-1·h-1).The photocatalytic hydrogen production rate did not show any significant decay during the continuous 25 h reaction.By TEM characterization,CQDs with diameters of 8-10 nm were found to be uniformly doped in g-C3N4.Through UV-vis DRS test light response range.It was found that the visible light absorption of the catalysts was enhanced and the energy band gap was reduced after doping with CQDs,from 2.74 eV for pure gC3N4 to 2.62 eV for CCN-10.By PL and electrochemical tests,it was found that the photogenerated carrier migration rate of the samples increased significantly after doping with CQDs.The energy band gap,DOS and HOMO-LUMO of different models were analyzed using DFT theoretical calculations.It was found that the reduction of the energy band gap was mainly attributed to the sender energy level provided by CQDs,and the change of the photogenerated carrier migration rate was correlated with the change of the N_p orbital of the nitrogen atom at the bridge linkage site.Secondly,potassium ion-doped g-C3N4 photocatalyst(KCN)was synthesized by high-temperature calcination of potassium bromide and urea.When the mass ratio of potassium bromide and urea was 1:500,the hydrogen production rate of KCN reached 460.8 μmol·g-1·h-1,which was 2.1 times of g-C3N4(218.6 μmol·g-1·h-1).The PL and electrochemical tests revealed that the doped potassium ions could modulate the migration rate of photogenerated carriers in g-C3N4.Potassium-doped nitrogendeficient carbon nitride(KCNx)materials were prepared by high-temperature calcination after treatment of KCN with nitric acid solution.The average hydrogen production rate of KCNx under visible light irradiation was 1772.4 μmol·g-1·h-1,which was 8 times of g-C3N4(218.6 μmol·g-1·h-1).After 25 h of continuous photocatalytic reaction,the hydrogen production rate did not show any significant decay.The synergistic effect of potassium ions and nitrogen defects was found by UV-vis DRS characterization to expand the visible light absorption range and narrow the energy band gap from 2.74 eV to 2.16 eV for pure g-C3N4.PL and electrochemical characterization revealed that the photogenerated carriers in the KCNx catalysts migrated at the fastest rate.DFT calculations reveal that the nitrogen defect drives the HOMO-LUMO in gC3N4 to be more off-domain,the charge interaction between layers is enhanced,and potassium ions can act as a bridge between g-C3N4 layers,which together promote the migration of photogenerated carriers.