Activation And Conversion Of Transition Metal Oxides And The Photocatalytic Hydrogen Evolution Performance

Photocatalytic decomposition of water for hydrogen evolution is considered to be one of the most potential means to achieve green energy in the future and has attracted wide attention.However,the hydrogen production efficiency of single photocatalysts is low when they are directly used without modification.To improve the photocatalytic hydrogen evolution performance,it is necessary to modify or construct the photocatalyst to obtain higher light absorption performance,photogenerated carrier separation efficiency and more surface reaction active sites.One of the most common methods to improve the performance of photocatalysts is the loading of cocatalysts,which can not only promote the transfer of photogenerated electrons,but also provide active sites for the photocatalytic hydrogen reaction.However,at present,the most efficient and common cocatalysts are still noble metals,such as Pt,whose expensive cost and extremely limited reserves limit their wide application.Therefore,it is urgent to seek efficient and inexpensive materials to replace noble metal cocatalysts.In addition,doping modification and building heterojunction are also very effective means to improve the photocatalytic hydrogen evolution performance of the photocatalysts.Based on this,in the paper,titanium dioxide(Ti O₂)and tantalum pentoxide(Ta₂O₅)transition metal oxide photocatalysts were used as carriers,and the performance of commercial nano-Ti O₂(P25),Ta₂O₅ and tantalum oxynitride(Ta ON)semiconductor photocatalysts for hydrogen evolution were activated by means of loading non-noble metal cocatalysts,morphology control,doping modification and heterojunction construction,respectively.The migration pathways of photogenerated charges and possible catalytic mechanisms were explored by combining various characterization techniques.The specific research contents and main results are as follows:(1)Non-noble metal Co_5.47N/N-r GO as cocatalyst to improved the photocatalytic hydrogen evolution performance of Ti O₂.The nitrogen-doped reduced graphene oxide supported on Co_5.47N(Co_5.47N/N-r GO)cocatalyst was prepared by ammonia nitridation,and it was combined with P25for photocatalytic decomposition of water to hydrogen.The physical and chemical properties of Co_5.47N/N-r GO and P25/Co_5.47N/N-r GO,such as morphology,composition,surface elemental states and light absorption performance,were systematically investigated by combining various characterization methods,and the reasons for the enhanced performance of the P25 composite photocatalysts were also explored.The results showed that the presence of Co_5.47N nanoparticles and N-r GO nanosheets can significantly improve the optical absorption of the composite photocatalysts.The photocurrent and electrochemical impedance test results also confirmed that compared with P25,the P25/Co_5.47N/N-r GO sample had better charge separation efficiency and smaller charge transfer resistance.The synergistic effect of Co_5.47N and N-r GO can promote the rapid transfer of P25 photogenerated electrons to the surface for hydrogen evolution reaction.The optimal hydrogen evolution rate of P25/Co_5.47N/N-r GO was 11.71 mmol h^-1 g^-1 under full-spectrum xenon lamp irradiation with methanol as sacrificial agent,which was equivalent to the performance of P25 supported Pt cocatalyst(11.88 mmol h^-1 g^-1),about 90 and 69 times higher than that of P25and P25/N-r GO,respectively.(2)Non-noble metal ZIF-67 and dia(Co)-based derivatives as cocatalysts to improve the photocatalytic hydrogen evolution performance of Ti O₂.By adjusting the amount of cobalt source and 2-methylimidazole ligand,a series of cobalt-MOF(Co1x)cocatalyst precursors were prepared by one-step liquid phase method,and they were combined with P25 for hydrogen evolution from photolysis water.The results showed that Co1x was composed of ZIF-67 and its isomer dia(Co).Its introduction could significantly improve the catalytic activity of P25,but it needed a certain time for activation.Co16 with poor water stability in P25/Co16 at the beginning of the reaction would first hydrolyze into Co(OH)₂ and Co₃O₄,and then in situ transform into amorphous Co O_x clusters with the size of 2?3 nm under the action of light.The clusters were uniformly loaded on the surface of Ti O₂ nanoparticles,which can act as the reactive sites and promote the separation and migration of photogenerated carriers,thus enhance the hydrogen evolution performance of the photocatalyst.The highest hydrogen evolution rate of 15.73 mmol h^-1 g^-1 was achieved when the content of Co16in the composite photocatalyst was 15 wt%under the irradiation of xenon lamp with methanol as sacrificial agent.The sample also had good hydrogen evolution stability.(3)Morphology design and doping modification to improve the photocatalytic hydrogen evolution performance of Ta₂O₅.The surface fluorinated self-doped Ta₂O₅ nanoshuttles(FTNS)were fabricated by a one-step hydrothermal method.The growth process of FTNS was also discussed based on the phase composition and morphology characteristics of the products at different reaction times,and they were used for the evolution of hydrogen from photohydrolysis water.The results showed that the raw tantalum powder particles gradually transformed into Ta₂O₅nanoshuttles with a length of about 500 nm and a width of about 50 nm under the action of hydrofluoric acid,and the defect energy level and surface fluorine ions were also introduced.The presence of defects(O_v·,Ta⁴⁺)in the sample can enhance the optical absorption of FTNS and also promote the separation and migration of photogenerated carriers.The presence of surface fluoride ions can further inhibit the bulk recombination of carriers by attracting electrons.Both of them together contribute to the enhancement of photocatalytic activity.In addition,compared with the conventional large particle size,the nano-shuttle structure can shorten the transmission distance of photogenerated carriers to the surface,so that they can migrate to the surface more quickly for redox reaction.FTNS prepared by hydrothermal reaction at 180°C for 24 h without any cocatalyst had the optimal hydrogen evolution rate under the irradiation of full-spectrum xenon lamp with methanol as the sacrificial agent,which was 25.9 and 2.5 times higher than that of commercial Ta₂O₅ and fluorine-free Ta₂O₅ nanoshuttles,respectively.The sample also had good hydrogen evolution stability.(4)Construction of ternary heterojunction to improve the photocatalytic hydrogen evolution performance of Ta ON.The Ta ON/Cd S/Zn S multiphase composite heterostructured photocatalyst with good visible light response was successfully fabricated by first nitriding the UV-responsive Ta₂O₅ into visible light-responsive Ta ON using ammonia nitride method,and then loading Cd S and Zn S nanoparticles in situ by solution method.The results showed that the introduction of Cd S can further broaden the light response range of Ta ON.The construction of heterojunction between Ta ON,Cd S and Zn S can not only promote the separation and migration of photogenerated charges,make more photogenerated electrons used for hydrogen production reaction,but also significantly reduce the photocorrosion of Cd S,thereby improving the hydrogen evolution performance and stability of the composite photocatalyst.The best catalytic activity was obtained when the composite photocatalyst prepared with 4 wt%Cd S and 6 wt%Zn S under simulated sunlight irradiation with Na₂S and Na₂SO₃ as sacrificial agent,which was much higher than that of Ta ON and Ta ON/Cd S without Zn S.The stability was also significantly better than that of Cd S and Cd S/Zn S without Ta ON.