Construction Of TiO₂ Nanotube Multi-homojunction For Photocatalytic Hydrogen Production

As an inexhaustible renewable energy source,solar energy becomes a perfect alternative to the depleting traditional fossil fuels,while clean energy hydrogen has no secondary pollution to the environment during the process of utilization.The conversion of the two energy sources can successfully solve the current environmental energy problems.The emergence of photocatalytic hydrogen generation technology fits the needs of researchers,and photocatalysts as the core of this technology will certainly receive more attention.However,photocatalysts still cannot meet the efficient catalytic activity in the actual reaction process.Among the various modifications of catalysts,the utilization of gradient doping to construct multi-homojunction highlights the great advantage of the modification means.Researchers have found that the use of gradient doping to modify semiconductor materials can usually improve the light absorption efficiency and carrier separation and transport efficiency of catalysts,which has a good modulation effect on photocatalytic activity.TiO₂ is a typical traditional photocatalyst.It is one of the ideal photocatalytic materials owing to its stable chemical properties,non-toxic and harmless,low cost and suitable energy band structure.However,TiO₂ itself has two major shortcomings,one of which is the wide band gap and low efficiency in the utilization of visible light;the other is the high electron-hole recombination,which greatly limits the improvement of photocatalytic performance.On the basis of the above research background,in this thesis the modification of gradient doping was introduced into TiO₂ photocatalyst and the photocatalytic performance of the material was further enhanced by surface loading of noble metals.（1）TiO₂ nanotube array films were prepared based on the anodic oxidation method.The gradient diffusion of Ni in TiO₂ nanotubes was achieved by using electrochemical deposition method and high temperature annealing insulation as the driving force,which led to the fabrication of Ni concentration gradient-doped TiO₂ nanotubes multi-homojunction structure.The successful construction of the multi-homojunction structure was confirmed by the analysis of microscopic morphology,crystal structure and elemental distribution.The catalyst sample with an electrochemical deposition Ni time of 120 s was measured to have the optimal catalytic activity,which could reach a hydrogen precipitation rate of 1.84 mmol g^-1 h^-1 under visible light.The modulation of the internal energy band structure of the catalyst through the gradient doping in multi-homojunction was investigated by a combination of computational and characterization tests.Following that,the role of gradient doping on the carrier transport properties was inspected by a series of photoelectric test systems.Finally,the mechanism of action to enhance the photocatalytic hydrogen production activity was explored.（2）In order to further enhance the catalyst carrier separation and transport efficiency and increase the surface active sites to promote the occurrence of surface reactions on the material,the Ni gradient-doped TiO₂ nanotube multi-homojunction were loaded with different contents of noble metal palladium by photoreduction method utilizing the induced effect of oxygen vacancies introduced by Ni gradient doping.The experimental results showed that the Pd nanoparticles obtained by the photoreduction method were 10-20 nm,which were dispersed on the nanotubes.The photocatalytic hydrogen decomposition experiments revealed that the catalyst exhibited the best catalytic performance with the photoreduction time of 120 s,and the hydrogen decomposition efficiency could reach 5.16 mmol g^-1 h^-1.Then,a series of photoelectrochemical characterization tests were used to explore the effect of Pd content on the photocatalytic performance of TiO₂ nanotube multi-homojunction.The mechanism of photocatalytic reaction of TiO₂ nanotube multi-homojunction modified with noble metal Pd was also thoroughly researched.