Study On The Preparation Of BiOX（X=Cl, Br, I）/Bi₂O₂CO₃ And Their Photocatalytic Properties

Photocatalytic technology could utilize the solar energy to excite the semiconduct and form strong oxidizing radicals (·OH, h+ and O2·-), and consequently, achieving the mineralization and complete decomposition of organic pollutants, which have become the international research focus in the toxic and harmful organic treatment. However, traditional TiO2 photocatalyst could be excited by the ultraviolet light because of itself essential property and have the lower quantum efficiency owing to the rapid recombination of photoinduced electrons and holes in TiO2. Therefore, to develop the advanced photocatalytic materials have been one of research topics in the photocatalytic filed. Recently, bismuth-based layered semiconductors have become the research aspect of new photocatalytic material because they exhibited the excellent photocatalytic activity and stabilty under solar light irradiation.In the paper, BiOX(X=Cl,Br,I)/Bi2O2CO3 photocatalysts have been prepared by the hydrolysis method and investigated via the X-ray diffractometry (XRD), scanning electron microscope (SEM), and diffuse reflectance spectra (DRS), and so on. The photocatalytic activities of BiOX(X=Cl,Br,I)/Bi2O2CO3 were evaluated by the degradation of organic compounds under simulant sunlight irradiation. Based on the experiemental and theoretical analysis results, the conclusions are as following:(1) The BiOCl, BiOCl/Bi2O2CO3 composites and Bi2O2CO3 were successfully fabricated by a facile composition-controlled preparation technology at room temperature based on the raw material of Bi(NO3)3·5H2O and HCl. With the amount of Bi2O2CO3 increasing, the absorption spectra of as-prepared photocatalysts were red shifted. When the concentration of NaOH is 0.90 mol/L, the as-prepapred BiOCl/Bi2O2CO3 samples revealed the highest photocatalytic activity and the degradation rate of methyl orange was up to 99% afer 2.5 h under simulant sunlight irradiation, which was superior to the single BiOCl and Bi2O2CO3. Besides, the COD remove rate of 50 mg/L methyl orange was 90% after 8 h, and after four recycle degradation experiment, the photocatalytic activity of BiOCl/Bi2O2CO3 photocatalyst remained 85%. The experiemental results indicated that the BiOCl/Bi2O2CO3 photocatalyst had the excellent photocatalytic activity and stability. Based on the calculated electronic energy band structures using first-principle calculations, the CB energy levels of BiOCl and Bi2O2CO3 are mainly dominated by unoccupied Bi 6p states, and exhibit higher delocalized electron states, making photo-generated electrons on the surface of Bi2O2CO3 easily transfer into CB of BiOCl and leave holes on the VB of Bi2O2CO3. Furthermore, the O 2p states of CO32- slab in Bi2O2CO3 dominate the VBM of Bi2O2CO3, leading to the electronic transition from occupied O 2p states of CO32- slab in VBM to unoccupied Bi 6p states of Bi2O22+ slayer in CBM. Finally, the photo-generated holes concentrated in the surface of Bi2O2CO3 react with OH- and H2O to produce OH-, and electrons scattered in the VB of BiOCl transfer to the surface to react with oxygen to produce O2·-, further oxidizing the organic pollutants. Surface active free radical trapping experiments demonstrate that both O2·- and ·OH played the important roles in the photocatalytic process, and the key active free radical was O2·-Therefore, the enhanced photocatalytic performance of BiOCl/Bi2O2CO3 composites was closely related to the synergistic effect of BiOCl and Bi2O2CO3 on improving the efficient separation of photo-induced electron-hole pairs.(2) The BiOBr、BiOBr/Bi2O2CO3 and Bi2O2CO3 photocatalysts could be prepared via controlling the ratio of Bi3+:CO32-:Br- using a simple low temperature hydrolysis method, where Bi(NO3)3·5H2O, Na2CO3 and NaBr used as raw materials. When the mole ratio of Bi3+:CO32-:Br- were 1:7:1,1:6:2,1:5:3, 1:4:4, BiOBr/Bi2O2CO3 composite photocatalysts with different contents of BiOBr and Bi2O2CO3 were obtained, Bi2O2CO3 and BiOBr photocatalyst could be prepared in the conditions of the ratio values of Bi3+:CO32-:Br- which are 1:6:0 and 1:0:3, respectively. The as-prepapred BiOCl/Bi2O2CO3 samples revealed the higher photocatalytic activity. The degradation rate of methyl orange was 77% afer 2.5 h under simulant sunlight irradiation, and the one of methylene blue was 95% afer 2 h under simulant sunlight irradiation, which were superior to the single BiOCl and Bi2O2CO3.(3) The BiOI, BiOI/Bi2O2CO3 and Bi2O2CO3 photocatalysts were prepared by the various ratio of Bi3+:CO32-:I- by a simple low temperature hydrolysis method, where Bi(NO3)3·5H2O, Na2CO3 and Nal were used as raw materials. The as-prepared samples in the ratio of 1:6:2 for Bi3+:CO32-:I- exhibited the most excellent activity, and the degradation rates of methyl orange, methylene blue and rhodamine B were above 90% after 2 h under simulant sunlight irradiation, which indicating that the BiOI/Bi2O2CO3 photocatalyst had the low selectivity and strong practicability. The key active free radical was O2·- in the photocatalytic process for BiOI/Bi2O2CO3 photocatalyst.