Fabrication Of TiO₂ And BiOCl Based Photocatalysts With Highly Exposed （001） Facets And Their Performance For Degradation Of The Organic Pollutants

Photocatalytic technology can effectively convert solar energy into chemical energy and mineralize organic pollutants without secondary pollution,which is a very potential environmental pollution control technology.Its core is photocatalyst.In the process of photocatalytic reaction,the performance of photocatalyst is the key factor to influence the degradation efficiency of pollutants.In recent years,a lot of studies on the TiO₂ and BiOCl photocatalytic materials have been carried out because of their stable chemical properties,non-toxic and high redox potential.However,the practical applications of the two materials were limited to their wide band gap energy（only ultraviolet light can be used）,the high recombination rates of photogenerated electron（e^-）and hole（h⁺）,and the difficulty of recover the powdered catalyst from water,etc.This paper takes TiO₂ and BiOCl as the research objects and prepares the TiO₂ and BiOCl nanosheets with highly exposed（001）facets and a variety of composite photocatalysts by means of crystal regulation,surface modification and Z-type heterostructure construction.Furthermore,it analyzes the phase structures,appearance characteristics,valence bond structures and charge separation efficiency of the prepared photocatalysts by using X-ray diffraction,Raman spectroscopy,scanning electron microscopy and transmission electron microscopy,X-ray energy spectrum,ultraviolet visible diffuse reflection spectroscopy,infrared spectroscopy,photoluminescence spectroscopy and EIS test.Meanwhile it relates the degradation performances and mechanisms of the prepared photocatalysts by selecting p-nitrophenol（p-NP）and tetracycline（TC）organic pollutants as the degradation substrates.The main research contents and results are as follows:A series of TiO₂ nanosheets with different proportion of exposed（001）crystal facets are prepared by solvothermal method.Through characterization and DFT calculation results,the paper states the growth mechanism of TiO2 nanocrystal sheet exposed to high proportion of（001）crystal surface.The relationship between proportion of exposed（001）crystal facets and photocatalytic activity of TiO₂nanosheets is studied by testing the photocatalytic degradation of p-NP.With the increasing proportion of exposed（001）crystal facets,the photocatalytic degradation efficiency of the TiO₂ nanosheet goes up at first and then goes down.When the proportion of exposed（001）crystal facets is 57.1%,the TiO₂ nanosheet（T-1.0HF）has the best photocatalytic activity,and the degradation rate of p-NP is 50.9%after simulated solar irradiation for 90 min.The reason is that the photogenerated e^-is concentrated on the（101）crystal facets of TiO₂ nanosheets,while the photogenerated h⁺is concentrated on the（001）crystal facets.The appropriate proportion of exposed（001）/（101）crystal facets forms crystal face heterojunction on the surface of TiO₂nanosheet,which effectively restricts the recombination of photogenerated e^-and h⁺,and improves the photocatalytic efficiency.In order to further improve the photocatalytic activity of T-1.0HF,the TiO₂/RGO（TR）and TiO₂/g-C₃N₄（TG）composite photocatalysts are prepared by using the modified RGO and g-C₃N₄.Compared with T-1.0HF,the photocatalytic degradation rate of p-NP by TR and TG is improved in different degrees.The mechanisms of photocatalytic degradation of p-NP by TR and TG are obtained through the band gap energy calculation and free radical capture experiments.It is worth noting that the photocatalytic activity of TG is higher than TR,which is due to the Z-type heterojunction that is formed between T-1.0HF and g-C₃N₄ in TG.In addition,taking TR as an example,it is found that the experimental conditions of photocatalytic degradation of p-NP are optimized through the response surface model and when the concentration of p-NP is 9.65 mg/L,the initial p H value is 5.27,and the dosage of TR is 12.45 mg,the degradation efficiency of p-NP is the highest,which reaches 93.5%.In order to solve the problem that powdered photocatalyst is difficult to be separated from water,magnetically separable Ze Fe₂O₄/TiO₂/RGO（ZTR）ternary photocatalysts are synthesized in situ by a solvothermal method.After simulated sunlight irradiation for 60 min and visible light irradiation for 80 min,the degradation rate of p-NP by ZTR reaches more than 90%,and most of the organic carbon in the degraded p-NP is mineralized into CO₂ and H₂O.·O2^-and·OH are the main active species in the process of ZTR photocatalytic degradation of p-NP.The Z-type heterojunction between TiO₂ and Zn Fe₂O₄ is formed which helps to separate photogenerated e^-and h⁺.As the e^-transport medium,RGO further speeds up the transmission of photogenerated carriers.In addition,ZTR is easily separated from the degradation solution under the action of external magnetic field.After five cycles of photocatalytic experiments,ZTR still maintains good photocatalytic activity.On the basis of the above research of TiO₂,this paper takes BiOCl,a broadband semiconductor,as the research object.It prepares highly exposed（001）crystal facets BiOCl nanosheets and Z-type composite photocatalyst of BiOCl/g-C₃N₄（BG）with2D/2D structure by means of crystal plane regulation and heterostructure construction.Compared with BiOCl nanosheets,BG shows excellent photocatalytic activity in the experiment of the degradation of TC,and the degradation rate of TC reaches 97.1%after simulated sunlight irradiation for 60 min.In the process of photocatalytic reaction,the main active species are·O2^-and h⁺,followed by·OH,and no intermediate products,more toxic than TC,are produced.In addition,the cyclic photocatalytic experiments have proved that BG has good stability and repeatability.Above all,this paper takes TiO₂ and BiOCl as the research objects,prepares various composite photocatalysts based on TiO₂ and BiOCl nanosheets with a high proportion of（001）crystal surface exposure by using different modification strategies.It achieves the full utilization of sunlight,rapid separation and transmission of photogenerated carriers,rapid magnetic separation and efficient removal of p-NP and TC.This paper helps to modify the broadband semiconductor materials and lays a good foundation for their further practical application.