Construction Of Carbon Nitride Nanotube-based Composites And Study Of Photocatalytic Performance

With the rapid development of the economy and industrial technology,the problem of global energy shortage and environmental pollution is becoming more and more serious,which seriously restricts the progress and sustainable development of human society.Nowadays,the energy demand mainly depends on non-renewable fossil energy.However,the consumption of fossil energy will cause damage to ecology and the environment.Therefore,it is urgent to seek clean and renewable new energy.At present,solar energy has attracted wide attention from researchers due to its universality,renewability and inexhaustible supply.Among many solar energy utilization methods,photocatalytic technology can convert solar energy into chemical energy,which has great potential application value in water desorption hydrogen（H₂）,nitrogen fixation and pollutant degradation.It is of great significance to effectively alleviate the ecological environment damage and the shortage of non-renewable energy.Graphitic carbon nitride（CN）has the advantages of non-toxicity,low cost,visible light response,high thermal stability and suitable band gap,and is considered to be a potential catalyst in photocatalytic materials.However,CN has the disadvantages of low crystallinity,small specific surface area,high photogenerated carrier recombination rate and narrow light absorption range,which greatly limits its practical use.In this paper,carbon nitride nanotubes（CNNTs）are used as the research object,and modified by element doping,defect engineering and heterojunction construction to explore the effect on photocatalytic performance and discuss the photocatalytic mechanism.The specific research contents are as follows:（1）Double-defect potassium（K）doping and cyano（-C≡N）sites have been introduced into CN with different morphologies by co-condensation method.It is found that the introduction of appropriate-C≡N defects,the uniform dispersion of K elements and the unique one-dimensional tubular structure improve the conductivity,light capture ability and expose more reactive sites of CN,thereby improving the activity of the photocatalyst.In addition,the K ions inserted into the gaps of the adjacent heptazine ring can be chemically bonded to the N atoms in the heptazine units,which forms K-N site in the framework of CN,and the charge carriers can migrate along the K-N bond acting as the"electron transfer bridge".Under visible light irradiation,compared with undoped bulk CN,CN nanosheets and CNNTs,photocatalysts with K-doped and-C≡N defects showed higher photocatalytic H₂evolution activity.Among them,CNNTs with double defect sites exhibit the best photocatalytic performance(2.11 mmol g^-1h^-1),which is about 2.0 and 40 times higher than that of K-doped bulk CN and bare CNNTs,respectively.The apparent quantum efficiency of 5.28%at 420 nm.（2）Based on the above studies,it can be found that CNNTs have better photocatalytic activity than bulk CN and CN nanosheets.Therefore,it is a feasible strategy to improve the photocatalytic ammonia synthesis performance of CNNTs by non-metallic modification.Sulfur-doped high crystallinity carbon nitride nanotubes（S-CNNTs）were prepared by thermal polymerization of thiourea and melamine precursors.The geometries,morphology,crystallinity,and the doped S content of CNNTs can be tuned with various content of thiourea.The high crystallinity and one-dimensional tubular structure enhance the visible light capture ability and promote the electron migration along the longitudinal direction.The surface N atoms in CNNTs can participate in the ammonia synthesis reaction,whereas the N atoms are firmly stabilized in S-CNNTs-20 because of the formed S-（N）₃coordination.The doping of S element causes the conduction band of the nanotubes to shift to a more negative potential.Doping S impurities result in an upshift of CB to more negative potentials.More importantly,S dopants can trap a large number of electrons,which can be transferred to the antibonding orbital of N₂molecules,resulting in N₂activation.The facilitated N₂activation significantly contributed to the improvement of NH₃generation kinetics.Under optimal addition of 20 wt.%thiourea,the prepared S-CNNTs-20 shows the best photocatalytic performance.The photocatalytic ammonia synthesis rate(0.64 m M g_cat^-1h^-1)is 2.46 times higher than that of pure CNNTs,and has an apparent quantum efficiency of 5.65%at 420 nm.（3）To further improve the adsorption capacity of CNNTs for N₂and photoreduction capability.The Cu_xO/CNNTs heterojunction was prepared by combining copper oxide nanoparticles（Cu_xO NPs）with CNNTs by a simple one-pot calcination method.And the type of heterojunction of the composite material is regulated by changing the calcination gas conditions.Calcining under H₂/Ar（5%H₂）ensured a Z-scheme heterojunction of Cu₂O@Cu O/CNNTs,and core-shell nanostructured Cu₂O@Cu O NPs in particle size of 20-80 nm are firmly anchored along the nanochannels of CNNTs.Calcining under static air conditions led to a Type II heterojunction of Cu O/CNNTs,and larger Cu O NPs of ca.200 nm are on the surface of CNNTs.Compared with Cu O/CNNTs,Cu₂O@Cu O/CNNTs possesses higher N₂chemical adsorption energy,wider visible light absorption range,faster charge carrier transfer rate and stronger photoreduction ability.The effect of the calcination temperature and loading content on the photocatalytic ammonia yield was studied.With the annealing temperature at 400 ℃ and 9 wt%copper percentage,the resulting Z-scheme Cu₂O@Cu O/CNNTs exhibit a nitrogen photofixation rate of 1.38 m M g_cat^-1h^-1,which is about 1.4 and 4.4 times higher than that of Cu O/CNNTs and the bare CNNTs,respectively.The apparent quantum efficiency of 6.28%at 420 nm.