Preparation And Photocatalytic Activity Test Of Surface-regulated TiO₂ Nanocrystals

Semiconductor photocatalytic technology is one of the effective ways to efficiently utilize solar energy and control environmental pollution.Titanium dioxide（TiO₂）nanoparticles have become one of the most widely used semiconductor photocatalytic materials due to its chemical stability,low cost,non-toxicity,high photocatalytic activity,lack of secondary pollution,and complete mineralisation.However,the wide band gap and narrow light response range of TiO₂,which can only absorb and utilize the ultraviolet part of sunlight,and has a poor quantum efficiency,all of which significantly limit its practical application.For anatase TiO₂,both theoretical and experimental studies indicated that the{001}facets are much more reactive than the normally exposed{101}facets.Therefore,this thesis focuses on the preparation of anatase TiO₂ that has been enriched with highly active{001}facets in order to increase the transfer efficiency of photogenerated carriers and to obtain high photocatalytic activity TiO₂nanocrystals via structure modification and selective co-catalyst loading.The main research contents and achievements of this thesis are as follows:（1）Preparation of TiO₂ nanocrystals contain highly reactive{001}facets.The synthesis of TiO₂ crystals with various morphologies was carried out using an environmental and fluorine-free approach.Highly reactive{001}exposed anatase TiO₂nanomaterials were prepared by hydrothermal method using potassium titanate nanowire precursors as the titanium source and（NH₄）₂CO₃ as the morphology control agent,and potassium titanate nanowire precursors were treated by washing with water and ethanol to explore the influence of precursor properties on the morphology of TiO₂.The results show that the TiO₂ prepared from the water washing precursor without（NH₄）₂CO₃ is a rod-like structure with the coexistence of{101},{001}and{100}facets,while the TiO₂ prepared from the ethanol washing precursor without（NH₄）₂CO₃ is a typical eight-sided bipyramidal structure with mainly{101}facets.While the morphology changes from an eight-sided bipyramidal to a truncated ten-sided bipyramidal structure after the addition of morphology control agent（NH₄）₂CO₃,with the top and bottom surfaces being{001}facets and the eight isosceles trapezoids on the sides being{101}facets.During the hydrothermal process,（NH₄）₂CO₃ molecules selectively adsorb onto the{001}facets of the anatase TiO₂ nanocrystals,reducing the surface energy and inhibiting crystal growth in the direction perpendicular to the{001}facets,allowing the exposure of the high surface energy{001}facets.Based on the first principle,the growth mechanism of{001}surface induced by（NH₄）₂CO₃ is further explored from the perspective of theoretical calculation,the surface energy of{001}surface in the absence of（NH₄）₂CO₃ molecule adsorption is 0.96 J/m~2,in the presence of（NH₄）₂CO₃ adsorption{001}facets energy drops to0.09 J/m~2 its surface energy decreases the most and becomes the most stable surface,so the adsorption of（NH₄）₂CO₃ molecules can make{001}facets exposed.（2）Study on the photocatalytic activity of TiO₂ nanocrystals containing{001}facets.A series of TiO₂ catalysts were prepared by adjusting the concentration of（NH₄）₂CO₃ to adjust the exposure ratio of{001}facets,and the photocatalytic activity of TiO₂ prepared by crystalline surface modulation was investigated using methylene blue as the target degradation material.It was shown that the exposure ratio of{001}to{101}facets is33:67 when（NH₄）₂CO₃ was added at 0.143 mol/L,which has the optimal photocatalytic activity,and the degradation rate of methylene blue after 15 W UV lamp irradiation 90 min is 88.7%,which is2.36 times the degradation rate of commercial P25.The enhanced photocatalytic activity is attributed to the fact that{101}and{001}facets could form surface heterojunctions,and the formation of heterojunctions could promote the preferential transfer of photogenerated electrons from{001}to{101}facets,while holes were transferred from{101}to{001}facets,thus achieving electron-hole spatial separation,and then inhibit their recombination.（3）To explore the universal applicability of the prepared TiO₂ catalyst to different types of organic pollutants,and further investigate the degradation effect of cationic dye Rhodamine B and anionic dye methyl orange based on TiO₂ catalyst with the highest photocatalytic PT（W）-0.01 N degradation efficiency of MB.The results showed that the series of TiO₂ samples were selective for the degradation of different dyes,and the samples with the best activity to degrade methylene blue were not optimal for the degradation efficiency of rhodamine B and methyl orange.The octave-sided double-cone structure sample mainly exposed to{101}facets（PT（E）-Ti）was the most efficient in photocatalytic degradation of rhodamine B,with a degradation rate of 96.58%after 180 min of illumination,while the degradation rate of rhodamine B of PT（W）-0.01 N was 0.56 times that of PT（E）-Ti,and the degradation efficiency of P25 was only 0.26times that of PT（E）-Ti.When degrading the anionic dye methyl orange under the same conditions,P25 exhibited the optimal photocatalytic activity,the degradation rate after 180 min was 59.20%,and the degradation rate of PT（E）-Ti could reach 80%of P25,while the activity of PT（W）-0.01 N degradation methyl orange was only 0.36 times that of P25.From this analysis,it can be seen that TiO₂ photocatalytic degradation of methylene blue is mainly affected by the separation efficiency of TiO₂ photogenerated electron holes,so PT（W）-0.01 N sample with the highest electron hole separation efficiency has the strongest activity;The degradation of Rhodamine B by TiO₂ is not only affected by the separation efficiency of photogenerated electron holes,but also significantly affected by the specific surface area.Therefore,PT（E）-Ti with relatively excellent electron separation efficiency and specific surface area has the highest activity;The photocatalytic degradation of MO by TiO₂ is mainly affected by the specific surface area,so P25 with the largest specific surface area has the strongest activity.In conclusion,the degradation mechanisms of different dyes are different,and the main factors that determine the performance of the catalyst are also different,which may be the separation efficiency of electron hole pairs or the specific surface area of the catalyst.Increasing the exposure ratio of{001}facets can effectively improve the separation efficiency of electron hole pairs and the reaction activity of TiO₂ catalyst,but at the same time,the specific surface area should be increased to further improve the universal applicability of photocatalysts.（4）Selective deposition of Co₃O₄ on TiO₂ nanocrystals{001}facets and its photocatalytic performance.In order to further improve the photocatalytic activity of the crystal plane regulation of TiO₂,it is proposed to selectively deposit Co₃O₄ on the TiO₂{001}facets to further promote the separation of photogenerated electrons and holes.The degradation effect of rhodamine B were investigated as the target contaminant.The study showed that the optimum ratio of{001}and{101}facets exposure was 1:9 and the optimum deposition of Co₃O₄ was0.018%and its degradation rate is 3.5 times more efficient than P25.P-type semiconductor Co₃O₄ is selectively deposited in the{001}facets of the N-type semiconductor TiO₂ hole-rich,and a P-N heterojunction is formed at the interface,the presence of a heterojunction between{001}and{101}facets can make the photogenerated electrons and holes migrate to the{101}and{001}facets,respectively,while the P-N junction can further transfer the holes of the{001}facets to Co₃O₄,meanwhile the photogenerated electrons tend to migrate in the opposite direction.The synergistic effect of surface heterojunction and P-N junction can effectively promote the separation of photogenerated electrons and holes,thereby enhancing the activity of photodegrading RhB.