GAO Meiqi 1, LI Song 1, CAI Jiajia 2, QIN Gaowu 1
(1. School of Materials Science and Engineering, Northeastern University, Shenyang 110819, Liaoning, China; 2. School of Energy and Environment,
Anhui University of Technology, Ma'anshan 243002, Anhui, China)
Extended Abstract:[Background and purpose] With the rapid industrialization, dye wastewater pollution has become a serious environmental problem, while photocatalytic technology has been acknowledged to be a highly promising solution, due to its economic, environmental and sustainable characteristics. However, traditional photocatalysts have disadvantages, such as low visible light absorption efficiency and severe recombination of photo-generated charge carriers, thus limiting the photocatalytic degradation performances. Photocatalysts constructed with p-n junction semiconductors can be used to improve the separation efficiency of photo-generated electron-hole pairs and the interface charge transfer efficiency by forming internal electric fields. ZnFe2O4 is an n-type semiconductor with a narrow bandgap and visible light response, while CaFe2O4 is a p-type semiconductor with matched energy levels, which can be utilized to form a p-n junction photocatalysts. Cation exchange method was developed to synthesize new materials by exchanging cations while maintaining the original anion skeleton, which can be used to prepare heterojunctions. Compared with traditional preparation methods, cation exchange method has the advantages of simple processing, low interface defects and large specific surface area. In this study, p-CaFe2O4/n-ZnFe2O4 heterojunction photocatalysts were prepared by using cation exchange method. The ratio of CaFe2O4 to ZnFe2O4 was regulated, while the morphology and structure of the photocatalysts were characterized in detail. Photocatalytic performance of the photocatalysts was evaluated, while the process of p-n junction promoting the separation of photogenerated carriers and the reaction mechanism of the photocatalytic degradation were explained.[Methods] CaFe2O4 powder was prepared by using sol-gel method. Ca(NO3)2·4H2O, Fe(NO3)3·9H2O, C6H8O7·H2O were mixed in proportion to form solution, followed by heating and drying to obtain dry gel. The gel was calcined at 800 ℃ for 4 h and then ball milled for 108 h, with the product to be denoted as CFO. p-CaFe2O4/n-ZnFe2O4 heterojunction photocatalyst was prepared by using cation exchange method. CFO was added to Zn(NO3)2 solution at 150 ℃, for 1 h, 2 h and 3 h, leading to samples with different ratios, denoted as CFO/ZFO-1, CFO/ZFO-2 and CFO/ZFO-3, respectively. The phase composition was examined by using XRD. Microstructure was observed by using SEM. Surface composition was characterized by using XPS. Specific surface area and pore size were measured by using static nitrogen adsorption instruments. Photocatalytic degradation performance was evaluated through photocatalytic degradation of methylene blue.[Results] XRD diffraction peaks of the product matched well with those of CaFe2O4 and ZnFe2O4. TEM and EDS results indicated that the CFO/ZFO exhibited a core-shell structure, with the core consisting of large CaFe2O4 particles and the shell consisting of ZnFe2O4 nanoparticles. The interface was tightly bonded and nearly free of defects. In XPS spectra, there was a slight shift in the Fe 2p peak of CFO/ZFO-2, with respect to CFO, which can be attributed to the interaction between CaFe2O4 and ZnFe2O4, confirming the formation of p-n junction. According to peak area fitting, the concentration of oxygen vacancies increased from 52.38% to 58.45%, after the formation of p-n junction, which is beneficial to the adsorption of target molecules. UV-visible diffuse reflectance spectroscopy results suggested that CFO and CFO/ZFO-2 have strong absorption in the visible region, with an absorption band edge at about 650 nm and a bandgap width of 1.9 eV, showing visible light responsive photocatalytic characteristics. According to the nitrogen isothermal adsorption desorption and pore size distribution curves, after cation exchange reaction, the specific surface area and average pore volume of were significantly increased, with CFO/ZFO-3 having the highest specific surface area (22.26 m2·g–1) and CFO/ZFO-2 having the highest pore volume (0.07 m3·g–1). CFO/ZFO-2 exhibited photocatalytic degradation rate of 83.0% for methylene blue, after 300 min, which was the highest among the three samples. The degradation rate of the p-n junction photocatalyst was significantly higher than that of single-phase CaFe2O4.[Conclusions] p-CaFe2O4/n-ZnFe2O4 heterojunction photocatalysts have been prepared by using the cation exchange method. The ratio of CaFe2O4 and ZnFe2O4 was controlled by adjusting the cation exchange reaction time. This process was simple to operate, had fewer defects at the interface, thus ensuring charge transfer, increasing the specific surface area of the materials and improving the photocatalytic efficiency. It could be extended to heterojunctions of other materials. The p-CaFe2O4/n-ZnFe2O4 heterojunction photocatalyst had higher photocatalytic activity than single-phase CaFe2O4. Among them, CFO/ZFO-2 with 37.10% ZnFe2O4 displayed the highest photocatalytic activity, providing a new choice for the treatment of dye wastewater.
Key words: cation exchange, CaFe2O4; ZnFe2O4; p-n junction; photocatalysis