Design Of Visible-light Driven Nitride Photocatalysts And Study Of Its Photocatalytic Hydrogen Production

The technology of photocatalytic hydrogen production from water is expected to become one of the best methods for mankind to solve the energy and environmental problems,so it has received widespread attention.The technology of photocatalytic hydrogen production can use clean solar energy to decompose water into hydrogen with high-density energy.Although the photocatalytic hydrogen production technology by semiconductors has made great progress after more than 40years of development,the application of photocatalytic hydrogen production technology in industrial production still faces many problems.The most important thing is the photocatalytic efficiency of the semiconductor photocatalyst is not high enough,and the energy in sunlight cannot be fully utilized.In addition,the high cost of photocatalytic technology is also the main reason to limit its development.Therefore,it is particularly important to modify photocatalytic materials to have high photocatalytic performance and to develop efficient and inexpensive materials for photocatalytic applications.The photocatalytic hydrogen process can be divided into the following steps:?1?the semiconductor photocatalysts absorb the energy of light to generate electron and hole pairs,?2?the photogenerated electrons and holes are separated and transferred,?3?the photogenerated electrons undergo a reduction reaction with the protons on the surface of the photocatalyst to generate hydrogen,and at the same time,the photogenerated holes oxidize the sacrificial agent and are consumed.Improving the efficiency of any step in the above process can effectively improve the photocatalytic hydrogen production performance of semiconductors.Therefore,the more effective methods are as follows:?1?modifying the band structure of the semiconductor by a non-metal element doping method to improve the light absorption performance,that is,improving the efficiency of the first step in the process of photocatalytic hydrogen production from water,?2?improving the separation and transfer process of photo-generated electrons and holes by regulating the structure or building heterojunction,?3?provide more and efficient active sites for the surface reaction of semiconductor photocatalysts through surface regulation and the addition of cocatalysts,etc..Based on this,this thesis takes TiO₂,g-C₃N₄ and other semiconductor photocatalytic materials as carriers,and discusses the doping of non-metallic N or C elements,the construction of Type-II and Z mechanism heterojunctions,and loading non-noble metal cocatalysts,et al.for the effect of the photocatalytic hydrogen production performance of semiconductor materials,and provides theoretical guidance for the development of inexpensive and efficient photocatalysts for photocatalytic hydrogen production from water.The specific research content of the thesis is as follows:?1?An N-doped mesoporous nano-TiO₂(TiO_2-xN_x)photocatalyst with better visible light photocatalytic performance for hydrogen production was obtained by calcining Ti-Si binary oxide in ammonia at 900?and then dissolving SiO₂ by NaOH.The result shows that the incorporation of Si in the Ti-Si binary oxide can effectively inhibit the transformation of TiO₂ from anatase to rutile and the growth of TiO₂ grains at 900?.The mesoporous TiO_2-xN_x photocatalyst has a mixed crystal phase and less than 10 nm of grain size,and the doping of non-metallic N element also makes it have significantly improved visible light absorption performance.Visible light photocatalytic hydrogen production test shows that the mesoporous TiO_2-xN_x photocatalyst with a mixed crystal phase of anatase and rutile and a higher N doping concentration showed the best performance,and the hydrogen production rate reached 15.2?mol/h/g.?2?The MXene-Ti₃C₂ material is easily oxidized to TiO₂ and containing C element,So C-doped TiO₂?C-TiO₂?can be in situ synthesized from MXene.The Type-II C-TiO₂/g-C₃N₄ heterojunction photocatalysts was successfully synthesized by calcining the compound of MXene and g-C₃N₄.We find that MXene can be successfully converted to C-TiO₂ with visible light absorption by calcination at 450?.The results of photocatalytic hydrogen production test shows that when the mass ratio of Ti₃C₂ to g-C₃N₄ is 10 wt%,the prepared C-TiO₂/g-C₃N₄ heterojunction photocatalyst has the best photocatalytic production of up to 1409?mol/h/g.Compared with pure g-C₃N₄ and C-TiO₂,the photocatalytic hydrogen production activity is increased by 8 times and 24 times,respectively.Photocurrent response and electrochemical impedance studies show that C-TiO₂/g-C₃N₄heterojunction photocatalysts have better separation efficiency of photogenerated electrons and holes.The C-TiO₂/g-C₃N₄ heterojunction photocatalyst has excellent visible light photocatalytic hydrogen production performance can be attributed to the improvement of visible light absorption performance of C-TiO₂ and the heterojunction formed between TiO₂ and g-C₃N₄.?3?A Z mechanism WO₃/g-C₃N₄ heterojunction photocatalyst with two-dimensional?2D?nanostructure was synthesized by hydrothermal and calcining methods.Visible light photocatalytic hydrogen production test shows that the 2D WO₃/g-C₃N₄ heterojunction photocatalyst with a WO₃content of 10 wt%has the best photocatalytic performance for hydrogen production from water,with a rate of up to 1853?mol/h/g,which is 6.5 times higher than the pure g-C₃N₄ nanosheets.The enhanced photocatalytic efficiency of the 2D WO₃/g-C₃N₄ heterojunction photocatalyst can be attributed to its 2D nano-layered structure and the Z mechanism heterojunction formed between WO₃ and g-C₃N₄.On the one hand,the 2D nanostructure of WO₃/g-C₃N₄ makes it have a larger specific surface area and shorter transfer path of photogenerated charges,on the other hand,the heterojunction structure formed between WO₃ and g-C₃N₄ can effectively promote the separation of photogenerated electrons and holes,and more photogenerated electrons can take part in the process of photocatalytic hydrogen production.?4?A non-noble Ni-Mo alloy is successfully used as a cocatalyst for g-C₃N₄ photocatalyst,and it shows good photocatalytic hydrogen production performance.For the Ni-Mo/g-C₃N₄ composite photocatalysts,Ni-Mo alloys exist as a solid solution and are tightly combined with g-C₃N₄.In addition,the effects of the amount of Ni-Mo alloy and the ratio of Ni/Mo on its catalytic performance were also studied in detail.The photocatalytic hydrogen production test shows that the Ni-Mo alloy co-catalyst with the optimal ratio can effectively increase the photocatalytic hydrogen production performance of g-C₃N₄ about 37 times,which basically achieves the performance of the noble metal Pt.In addition,the excellent electrical properties of the Ni-Mo alloy made the photocurrent density of the Ni-Mo/g-C₃N₄ composite photocatalyst 25 times higher than that of pure g-C₃N₄,reaching 8.0 mA/cm².It can be seen that the non-noble metal Ni-Mo alloys can be used as a cocatalyst for semiconductor photocatalysts to effectively improve the performance of photocatalytic hydrogen production and can effectively replace the noble metal cocatalysts commonly used today,reducing the cost of photocatalytic technology and promoting photocatalytic technology application in actual production.