Efficient degradation of cationic dyes by ZnO and ZnO–Fe(III) quantum dots under natural sunlight and UV light

Abstract

Zinc oxide (ZnO) quantum dots (QDs) are very attractive to many applications due to their optical and electronic properties and low toxicity. Among the applications, ZnO has been applied to photocatalysis for degradation or organic pollutants. However, what is still not clear is the function of organic dyes that absorb in the visible region, when they are together with semiconductors known for their best photocatalytic performance in the UV region. Herein, the degradation efficiency of two cationic dyes under natural sunlight irradiation and UV light in the presence of ZnO and ZnO:Fe(III) QDs with the dopant at 0.02, 0.04, and 0.07 mol% was investigated. The QDs were prepared via sol-gel method in ethanol and the photocatalytic assays were carried out in the same environment. Fe(III)-doped ZnO QDs with 0.04 mol% gave the highest apparent rate constants with degradation percentage equal to 84.3% and 66.9%, for methylene blue (MB) and astrazon blue FGGL 300% (AB), respectively, after 7 min under sunlight irradiation. Experiments were carried out with scavengers for hydroxyl (⋅OH) and anion superoxide (O2−⋅) radicals, tert-butanol and 1,4-benzoquinone (1,4-BQ), respectively. MB degradation course did not change in the ethanolic medium, known as ⋅OH scavenger, as well as with an extra amount of tert-butanol. However, the photocatalytic efficiency strongly decreased under the presence of 1,4-BQ. Thus, it is assumed that the MB degradation route in ethanol occurs mainly through the radical species O2−⋅. The study proved to be very sustainable and eco-friendly, being carried out in an ethanolic environment and with natural sunlight. This strategy ensured maximum surface efficiency and photocatalytic activity.