Research On Design,synthesis And Photocatalytic Performance Of Organic-Inorganic Hybrid Nanostructured Materials

The demand for energy has exceeded the affordability of ecology and the environment due to the continuous progress of global science and technology,as well as the continuous development of society and economy.The energy crisis and environmental problems have become more and more serious and have become the key issues restricting the sustainable development of human society.Therefore,obtaining renewable clean energy has become more important in society.As a renewable and clean energy,hydrogen has aroused widespread attention which can attributed to the combustion process of hydrogen is not harmful to the environment,because the product is only water during the whole process,which will not produce any harmful substances or release greenhouse gases.Solar energy,as an inexhaustible clean energy source,which can effectively alleviate the current energy shortage problem if it can be fully utilized,because the solar energy can obtain is far exceeds the global annual energy consumption every year.The technology of photocatalytic water splitting,which as a green and sustainable technology can convert solar energy into chemical energy has received much attention from scientific researchers,due to the mild reaction conditions,clean and environmentally friendly.The development of photocatalytic technology is inseparable from the support of photocatalysts,because water cannot absorb and utilize solar energy directly.In recent years,a series of photocatalytic materials have developed and obtained from scientific researchers.Furthermore,they improved the light absorption range and the separation and transfer efficiency of photogenerated electron-hole pairs of photocatalytic materials through various methods.However,the photocatalytic efficiency of current photocatalysts is not satisfactory.Therefore,it has an important strategic significance for solving the global energy crisis of how to construct an efficient photocatalytic system.The composite photocatalysts constructed by the light-absorbing unit and the reaction unit has attracted widespread attention due to its excellent photocatalytic performance.The interface is an extremely important microstructure of composite photocatalysts,that has a crucial impact on the electron transport and catalytic performance of composite materials which serves as a bridge connecting different units.The design strategy for optimizing the composite photocatalysts should be based on the principle of synergistic improvement of light absorption,carrier separation and charge utilization efficiency.It is not only necessary to pay attention to the energy level matching between the components of the composite photocatalysts,but also to the compatibility,stability and the selectivity of the target reaction,in order to control the separation and directional transmission of photogenerated carriers in the light-absorbing unit more rationally and effectively.In this paper,it is aim to construct an efficient composite photocatalysts,and focused on improving the separation efficiency of photogenerated carriers and enhancing the activity of photocatalytic water splitting for hydrogen production.Gradually,this study begins from the adjustment of the morphology and structure of the photocatalysts,in order to increase the active sites of the photocatalytic reaction.And then to the design of the composite photocatalytic materials to improve the separation efficiency of photogenerated carriers and increase the utilization rate of photogenerated electrons in the photocatalytic reaction.At last,the phase conversion of semiconductor materials has controlled to further enhance the separation utilization of photogenerated carriers.The main contents of this paper are as follows:（1）At first,in order to meet the requirements of traditional Ti O₂based photocatalytic materials for photocatalytic performance and morphological structure control,a simple solvothermal and oil bath combined synthesis method was used to prepare Ti O₂@Ni（OH）₂photocatalyst with a three dimensional core-shell heterostructure.The results show that the Ti O₂@Ni（OH）₂ photocatalyst exhibited an excellent hydrogen production rate of photocatalytic water splitting,which was 5.3μmol·g^-1·h^-1,it is nearly 56 and 6.5 times higher than that of pure Ni（OH）₂ and Ti O₂,respectively.The construction of core-shell structure successfully extended the range of photo response to the visible light region for the Ti O₂-based materials,and effectively increased the specific surface area and porosity,which can provide more active sites for the photocatalytic hydrogen evolution reaction.In addition,Ni（OH）₂ as a p-type semiconductor can form p-n type heterostructure with Ti O₂at the interface.The photogenerated carriers will be transferred from Ti O₂ to Ni（OH）₂ due to the internal electric field.Specifically,the photogenerated electrons can be transferred from the CB of Ti O₂ to Ni（OH）₂easily,which can attributed to the potential of Ni²⁺/Ni is slightly lower than the energy level of the conduction band of Ti O₂ and the presence of internal electric field driving force generated by the unique p-n type heterostructure.Based on the above reasons,the purpose of improving the separation efficiency of photogenerated carriers from Ti O₂can be achieved,furthermore,the performance of photocatalytic water splitting hydrogen production also can be improved.（2）TiO₂ as a light absorption unit has great limitations,because of its wide band gap,and it can only absorb a small part of visible light although after modification.Covalent organic frameworks（COFs）have the advantages of unique channel structure,excellent surface area and visible light response.Based on these,a simple and feasible TP-COF/Ti O₂ hybrid material preparation strategy was constructed which is using TP-COF as the light absorption unit.Under the irradiation of visible light,the as-prepared a series of TP-COF/Ti O₂ hybrid materials showed higher photocatalytic performance than pure Ti O₂ and TP-COF.Among them,the TP-COF/Ti O₂=3:1 hybrid material showed the best rate of photocatalytic hydrogen production,which can reach 21.9 mmol·g-1·h-1,it is nearly three times higher than that of pure TP-COF.The significant improvement in photocatalytic activity is not only due to the good band gap structure matching between TP-COF and Ti O₂ nanosheets,but also can attributed to the effective separation and transfer of photogenerated electrons between organic functional groups and semiconductors.Besides that,the Ti O₂ in the TP-COF/Ti O₂=3:1 hybrid material simultaneously exposes the（001）and（101）planes,which can form a surface heterojunction in Ti O₂,and then the photogenerated electrons can be transferred from the（001）plane to the（101）plane,that can further enhance the separation efficiency of photogenerated charges.This strategy provides a new way for the design and synthesis of various high-efficiency visible light-induced organic semiconductor photocatalysts.（3）In order to further reduce the cost of photocatalytic materials,COF/Mo S₂ composite photocatalyst were prepared through the process of electrostatic self-assembly.The photocatalytic hydrogen production activity can reach up to 1.71 mmol·g^-1·h^-1 without any noble metal as cocatalysts under visible light irradiation,which is nearly 35 times higher than that of pure COF.The efficiency of photocatalytic hydrogen production is becoming better with the increase of 1T-Mo S₂ in composite photocatalyst.In this chapter,Mo S₂ with different 1T-Mo S₂ content can prepared by controlling the reaction temperature.And the fact is that,the process of reaction becomes more inadequate with the decreases of reaction temperature,which is resulting in the production of more metastable Mo S₂（1T-Mo S₂）,thereby increasing the content of the metal phase in the as-prepared Mo S₂.The energy level difference between the two conduction bands ensures that the photogenerated electrons from the COF can be effectively transferred to Mo S₂-X,and the similar conduction band positions effectively retain the reduction ability of the photogenerated electrons.Moreover,1T-Mo S₂ has metal properties and many defects compared with 2H-Mo S₂,the excited photoelectrons are more easily transferred to 1T-Mo S₂.An electron-rich environment will be formed in 1T-Mo S₂,which will be consumed by protons to produce hydrogen.That effectively improves the separation efficiency of photogenerated carriers from COF,thereby improving the performance of photocatalytic water splitting to produce hydrogen.