Construction of Isoenergetic Band Alignment between CdS QDs and CaFe2O4@ZnFe2O4 Heterojunction: A Promising Ternary Hybrid toward Norfloxacin Degradation and H2 Energy Production

The increasing demand for clean water triggers advancement in development of nanomaterials for photocatalytic pollutant degradation. Herein, we report for the first time a CdS QDs decorated p-CaFe2O4@n-ZnFe2O4 heterojunction photocatalyst fabricated via a two-step chemical route, i.e. sol–gel auto combustion and precipitation-deposition methods. The prepared photocatalyst proves to be effective toward degradation of the emerging pollutant norfloxacin under solar light irradiation. Systematic analysis including parameters such as pH, interfering ions, and active species was studied in detail. Photoluminescence, photocurrent study, and activity test reveal that 3%CdS QDs/CaFe2O4@ZnFe2O4 has the highest charge separation ability among the prepared composites. Norfloxacin degradation occurs effectively on a ZnFe2O4 surface of CdS QDs/CaFe2O4@ZnFe2O4 through isoenergetic which is evidenced by AFM analysis and a pn-heterojunction based charge transfer mechanism. The modified band gap alignment and isoenergetic electron transfer from CdS QDs to CaFe2O4 photocatalyst are verified by XPS, Mott–Schottky analysis, and AFM studies. Interestingly, with the introduction of CdS QDs into CaFe2O4@ZnFe2O4 hybrid the photocurrent density increases from 5.9 mA/cm2 to 10.39 mA/cm2, which is attributed to isoenergetic charge transfer between CdS QDs and CaFe2O4. The photocatalytic H2 evolution rate of 3% CdS QDs/CaFe2O4@ZnFe2O4 showed 18300 μmol h–1 g–1, which is 1.4 times greater than that of binary hybrid CaFe2O4@ZnFe2O4. Further, 83% photodegradation of 50 ppm norfloxacin has been achieved by the 3% CdS QDs/CaFe2O4@ZnFe2O4 ternary nanocomposite after 90 min irradiation, which is 1.28 times that of CaFe2O4@ZnFe2O4. Upon vigorous analysis, the outstanding photocatalytic performance is greatly attributed to visible light harnessing ability, ample solar light absorption capacity, low photogenerated electron–hole recombination rate, high concentration of photoexcited electron, and excellent photostability.