Research On Structure Adjustment Of Graphitic Carbon Nitride And Its Photocatalytic Water Splitting To Hydrogen Performance

Considering the severe energy crisis and deteriorative environment,exploring green and pollution-free new energy to substitute traditional non-renewable fossil fuel has become a common demand for people to protect the ecological environment and improve the quality of life.Due to the outstanding advantages including facile utilization and low cost,photocatalytic technology has been widely regarded as a method that can effectively alleviate current dilemma.Photocatalysts can achieve a proper solar to chemical energy conversion by adsorbing photons,therefore,researches on photocatalysts are the key factors contributing to this technology.As a metal-free green catalyst,graphitic carbon nitride(g-C3N4)has received extensive attention due to its visible light absorption performance,physical and chemical stability as well as economical production.However,due to factors such as moderate band gap,rapid carrier recombination and small specific surface area,the photocatalytic performance of g-C3N4 was far away from meeting the demands of industrial production.In order to solve these problems,defect engineering,co-catalyst conjunction and molecular cocondensation were applied to strengthen g-C3N4.Relative experimental data have been conducted to explain the reasons of improved performance in the photocatalytic hydrogen production process,and the specific research results are as follows:(1)Concentrated nitric acid can introduce an appropriate amount of oxygen atoms and nitrogen vacancies into the skeleton of g-C3N4.As-generated nitrogen vacancies and oxygen doping atoms caused negative-shift of reduction potential and endowed the catalyst with stronger driving force toward photo-reduction reaction.O atom functioned as the donor or acceptor to improve the electron transport and motion on the conjugated structure due to its greater electronegativity and rich electron properties.While N vacancy destroied the bond equilibrium on the heterocyclic ring thus increased the number of lone pair electrons and providing more active sites.The adjustment of the electronic structure has resulted in rapid transfer of photo-generated carriers over g-C3N4 and motivate carrier to participate in the photocatalytic reaction.The experimental results showed that the photocatalytic hydrogen production rate of modified g-C3N4 was 2.2 mmol·g-1·h-1,almost 2.25 and 3.23 folds to that of blank sample and pure g-C3N4.(2)We synthesized a CNT/Au/g-C3N4 three-phase catalyst material by in situ growing CNT on the g-C3N4 nanosheets.Owing to the splendid electron conductivity,CNT can form a stacking π-π bond with g-C3N4 to realize the vertical separation of electrons and holes,hence reduced the wastage of solar energy.As a result,Au and CNT co-catalysts effectively promoted the transfer of excited carriers and accelerated the catalytic rate of CNT/Au/g-C3N4.The performance of CNT/Au/g-C3N4 was 0.95 mmol·g-1·h-1,which was about three times to that of Au/g-C3N4.Furthermore,the stability of the sample was also verified in the cycle test.(3)1,3,5-Benzenetriamine was applied to form benzene rings decorated g-C3N4 through thermal condensation reaction with urea.The experimental result indicated that benzene was inserted into g-C3N4 to replace the position of the triazine ring without causing much damage to g-C3N4 structure.Also,it can form a π-π conjugated structure with the adjacent heterocyclic ring and therefore changed the intrinsic band structure by reducing band gap,which resulted in expanded response toward visible light.Moreover,the addition of benzene ring can accelerate electron movement and facilitate fast in-plane migration of carriers.The photocatalytic H2 evolution test demonstrated the improved activity of benzene-decorated samples.When the concentration of 1,3,5-triaminobenzene was 0.25 wt%,the optimal hydrogen evolution rate of the sample reached 0.81 mmol·g-1·h-1,which was 2.25 times to its counterpart.