Flower Globose AgIn₅S₈/AgInS₂-SnIn₄S_{8< /sub> Heterojunction Composites Constructed by Microwave-Assisted Hydrothermal Synthesis for Enhanced Photocatalytic Efficiency Toward Methyl Orange}

Ternary metal sulfides normally have narrow band gap energy levels, leading to fast recombination of photogenerated carriers. The construction of heterojunctions is generally believed to boost the photogenerated carrier efficiency via capturing and transferring electrons, and thus improving the photocatalytic activity. In this paper, different ratios of AgInS2-SnIn4S8 composites were synthesized by a microwave-assisted hydrothermal method, and the molar ratio of SnIn4S8:AgInS2 = 0.5:1 is determined as the optimal ratio based on the experimental results of visible photocatalytic degradation of methyl orange (MO). Furthermore, AgIn5S8/AgInS2-SnIn4S8 composites were successfully prepared by changing the ratio of In, S element, and AgNO3. The component, structural morphology, and surface physicochemical properties of the composites were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflectance absorption spectroscopy, and nitrogen adsorption-desorption tests. The results show that AgInS2 and AgIn5S8 are compounded with SnIn4S8 to form a heterogeneous structure. After the formation of heterogeneous structures, light absorption properties are changed and light absorption range is broadened due to stronger light absorption capacity in AgInS2 and AgIn5S8. As they have matched energy band structures, the electron transfer paths are increased and the e(-)-h(+) lifetime is prolonged, which leads to enhanced photocatalytic performance of AgIn5S8/AgInS2-SnIn4S8 composites. Moreover, the formation of regular and homogeneous flower globose structure under the effect of microwave polarization also has an important contribution to the improvement of their photocatalytic performance. AgIn5S8/AgInS2-SnIn4S8 composites exhibit a certain degradation ability towards MO, and the degradation is almost complete within 180 min under visible light irradiation. Finally, through trapping experiments with the addition of different radical trapping agents and the ESR tests, photocatalytic active species are identified, and thus the possible photocatalytic reaction mechanism of AgIn5S8/AgInS2-SnIn4S8 composite is inferred.