Preparation And Photocatalytic Performance Of Novel Sillén Phase Bi-based Layered Materials

Photocatalytic technology is a green and sustainable method of environment pollution treatment and solving the energy crisis problems.As a new type of photocatalyst with a layered structure,Bi-based oxychloride is not only environmentally friendly and strong stability,its suitable energy band width broadens the absorption range of sunlight,and the unique 6s² lone pair electron of Bi element and the low-dimensional structure of the material improves the effective separation of photogenerated carriers and photocatalytic quantum efficiency.Bi-based oxychloride has become a hot research topic in the field of photocatalysis due to its excellent performance in photocatalytic degradation of organic dyes and photolysis of water to produce oxygen.In this paper,the Bi-based oxychloride is to be modified by stoichiometric adjustment,morphology control,surface modification and heterojunction construction.Oxygen-rich Bi-based oxychloride(Bi OCl,Bi₃O₄Cl,Bi₁₂O₁₇Cl₂,Bi₂₄O₃₁Cl₁₀ and Bi₁₂O₁₅Cl₆)are obtained by adjusting the stoichiometric ratio of Bi,O and Cl,and optimizing their electronic structure and light absorption characteristics;Bi OCl is used as the precursor,the flux method（KCl/Na Cl eutectic mixture molten salt）is used to synthesize Bi₂MO₄Cl（M=Gd and Nd）and Bi₄MO₈Cl（M=Nb and Ta）nanosheets;using Na OH solution（adjusting the p H value）to modify the Bi OCl and synthesize（Bi O）₂（OH）_xCl_2-x solid solution,and couple with g-C₃N₄ to construct a heterostructure.The photocatalytic activity of the modified Bi-based oxychloride and the influence of different preparation methods on their morphology and catalytic activity are studied.Each group of as-prepared samples is analyzed by X-ray diffraction（XRD）patterns,field emission scanning electron microscope（FESEM）,transmission electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）,UV-vis diffuse reflectance（UV-vis DRS）and photoluminescence（PL）spectra,etc.The photocatalytic performance of each group of samples is evaluated by degrading organic dyes under sunlight.The photocatalytic performance of Bi₂MO₄Cl（M=Gd and Nd）system is also evaluated by photocatalytic production of O₂.The main research results of this paper are shown as follows:（1）Bi-based oxychloride photocatalysts including Bi OCl,Bi₃O₄Cl,Bi₁₂O₁₇Cl₂,Bi₂₄O₃₁Cl₁₀ and Bi₁₂O₁₅Cl₆ were prepared by a solvothermal（or hydrothermal）method with the controlled amount between Bi（NO₃）₃·5H₂O and NH₄Cl.The photocatalytic activity was evaluated by degrading rhodamine B（Rh B）molecules under visible light irradiation.Among those bismuth-based oxychloride photocatalysts,Bi₁₂O₁₅Cl₆showed the highest Rh B photodegradation activity of 99.85%within 30 min,and its photodegradation rate constant K value was 2.30,5.51,4.31 and 4.72 folds higher than those of Bi OCl,Bi₃O₄Cl,Bi₁₂O₁₇Cl₂ and Bi₂₄O₃₁Cl₁₀,respectively.This result showed that the crystal,band structure and morphology of Bi OCl,Bi₃O₄Cl,Bi₁₂O₁₇Cl₂,Bi₁₂O₁₅Cl₆ and Bi₂₄O₃₁Cl₁₀ had a great influence on the photocatalytic performance.（2）The Bi₂MO₄Cl（M=Gd and Nd）sheets were prepared by a molten salt flux method for the first time.The resulted Bi₂Gd O₄Cl（F-BG）sheets showed the highest photocatalytic activity for photocatalytic O₂ evolution and trypan blue（TB）degradation than Bi₂Nd O₄Cl（F-BN）,Bi₂Gd O₄Cl（S-BG）and Bi₂Nd O₄Cl（S-BN）synthesized by a solid-state method（SSR）.The F-BG sheets showed the larger specific surface areas and abundant pores,which could provide more reactive sites.F-BG sample had the excellent separation and transfer rates of photogenerated charge carriers,leading to the highly photocatalytic performance for both O₂ evolution and TB degradation.All samples had the visible-light-responsive characteristic.Thus,a possible photocatalytic mechanism was proposed based on the radical trapping experiment.The radical trapping experiment revealed hole（h⁺）played a major role,while superoxide radical（·O₂^-）contributed a little to photocatalytic degradation process.（3）Bi₄MO₈Cl（M=Nb and Ta）sheets were successfully prepared by a molten salt method.Bi₄Nb O₈Cl（F-BNC）had the highest photocatalytic activity because its visible light absorption range was wider than that of Bi₄Ta O₈Cl（S-BTC）synthesized by SSR and Bi₄Ta O₈Cl（F-BTC）synthesized by a flux method.Compared with Bi₄Nb O₈Cl（S-BNC）synthesized by SSR,a sheet-like structure was formed,which could provide more active sites for the photocatalytic process,hence increasing the photocatalytic efficiency of F-BNC.The photocatalytic mechanism of F-BNC was analyzed,h⁺played a major role in the photocatalytic degradation of methylene blue（MB）,·O₂^-played a secondary role,while hydroxyl radical（·OH）did not play a role.（4）The layered g-C₃N₄/（Bi O）₂（OH）_xCl_2-x（NBHC）heterojunction composite material was prepared by a hydrothermal method.The two-dimensional/two-dimensional（2D-2D）porous layered Z-type heterojunction could effectively inhibit the recombination of photogenerated electron-hole pairs and improve the separation efficiency.Meanwhile,the NBHC composite material had a larger specific surface area and abundant pores,which could provide more reactive sites for the photocatalytic process.Therefore,NBHC had an excellent photocatalytic performance for pollutant TB degradation.In addition,according to the active species capture experiment and ESR spectra,it was found that the active species·O₂^-played a major role,and·OH contributed to a lesser extent in photocatalytic degradation of TB.A possible Z-scheme photocatalytic mechanism for photocatalytic degradation of TB was proposed.