Preparation Of Niobium-Based Nitroxides And Its Photocatalytic Performance Of Water Splitting Into Hydrogen

In recent years,the problem of energy shortage and environmental pollution caused by the burning of fossil fuels has become increasingly serious.Photocatalysis has great application potential for solving environmental energy problems.The development of new photocatalytic materials with high efficiency,long life and low cost is an important prerequisite for promoting the industrial application of photocatalytic technology.Niobium-based nitrogen（oxide）compounds have a band gap structure suitable for Photocatalytic water splitting,and niobium nitride has a good response to visible light.However,its shortcomings such as easy recombination of charge carriers,low separation efficiency,poor photocatalytic activity,and difficulty in the preparation of pure phase compounds,restrict the commercial application process seriously.In view of the above shortcomings of niobium nitride materials,this paper adopts the synergistic strategy of bulk modification and surface modification to develop a new Pd/Co synergistically modified Nb₄N₅ nanocomposite photocatalytic materials,which has significantly improved photocatalytic performance.A controlled synthesis process of Pd/Co synergistically modified Nb₄N₅nano-composite photocatalyst has been established.First,a hydrothermally controlled synthesis process for Nb₂O₅ nanoflowers has been established.The optimized process conditions are mixed acid dosage of 2.15 M HF is 1.6 m L,concentrated HCl is 0.1 m L,hydrothermal temperature of 160℃,hydrothermal time of 3 h,and Nb₂O₅ nanoflowers with large specific surface area and clear edges and corners are obtained.The flowers consist of nanosheets with a thickness of 20-30 nm and a side length of 250-300 nm and nanorods with a diameter of 20-30 nm and a length of 100-200 nm.Secondly,the hydrothermal synthesis and high-temperature nitridation controlled synthesis process of Ti O₂/Nb₄N₅ has been established.The optimized process conditions are in-situ doping of1 wt.%Ti⁴⁺,nitridation temperature of 800℃,and nitridation time of 3 h.A controlled synthesis process of Pd/Co synergistically modified Nb₄N₅ nanoparticles has been established.The optimized process conditions are impregnation and adsorption of 2 wt.%Co³⁺,nitridation temperature of 580℃,nitridation time of 1 h,Oxidation temperature of200℃,Oxidation time of 1 h,photodeposition of 0.8 wt.%Pd²⁺.The photocurrent density of Ti O₂/Nb₄N₅ reaches to 0.035μA·cm^-2,which is 2.5times that of Nb₄N₅ nanoflowers(0.014μA·cm^-2).The separation efficiency of charged carriers is improved significantly.Under simulated sunlight irradiation of 500 W Xenon lamp,the photocatalytic overall water splitting into hydrogen activity of Ti O₂/Nb₄N₅ is109.9μmol·g^-1·h^-1,significantly higher than that of the unmodified Nb₄N₅ sample(87.9μmol·g^-1·h^-1).2～3 nm Ti O₂-QDs modification forms Ti O₂/Nb₄N₅ local hetero-structures,which enhance the light absorption capability from UV to entire visible region,promotes the efficient separation of charged carriers,and increases the specific surface area and micro-mesopores of 2～17 nm.The surface catalytic active sites are increased.All above factors lead to an enhanced photocatalytic activity for overall water-splitting into hydrogen under simulated sunlight irradiation.Pd/Co modified Nb₄N₅ samples increases the number of nanoflowers;Pd/Nb₄N₅,Pd O/Nb₄N₅,Co₂N/Nb₄N₅,Co N/Nb₄N₅,Co₃O₄/Nb₄N₅ local heterointerfaces are formed on the surface;Pd modification promotes the oxidation of Co~0 and Co²⁺to Co³⁺,which is beneficial to enhance the OER activity;the synergistically modified samples show type IV isotherms and H3 type hysteresis loops,and there are irregular slit-like open mesoporous structures with pore sizes concentrated at 4 nm,the specific surface area decreases,which is caused by the deposition of modifiers on the pore structure.Compared with Nb₄N₅,the visible light absorption of the synergistically modified samples decreases,which is due to the shielding effect of the synergistic modification;The increasing order of photocurrent density of the modified samples is Pd-Nb₄N₅4N₅4N₅,The photocurrent density of the synergistically modified sample is increased by 6 times,reaching 0.084μA·cm^-2,but slightly lower than that of Co-Nb₄N₅.Pd helps reduce HER overpotential and provides HER active site;the synergistic effect of Pd O,Co₃O₄,Co₂N and Co N further reduces OER overpotential and provides OER active site.The impedance of Pd/Pd O modified samples decrease significantly,while the impedance of Co₃O₄/Co₂N/Co N modified samples increase.After synergistic modification,Pd modification partially offset the increase in impedance caused by Co modification.Under simulated sunlight irradiation of 500 W Xenon lamp,the photocatalytic overall water splitting into hydrogen activity of Pd/Co-Nb₄N₅ is 164.5μmol·g^-1·h^-1,which is 1.87 times that of unmodified Nb₄N₅(87.9μmol·g^-1·h^-1).Under the irradiation of 420 nm monochromatic light,the apparent quantization efficiency is0.025%,which is 2.27 times that of unmodified Nb₄N₅（0.011%）.The enhanced photocatalytic activity is attributed to the combined effect of the above factors and provides an effective way to further develop high-performance Nb₄N₅nano-photocatalytic materials.