Smart photocatalytic removal of ammonia through molecular recognition of zinc ferrite/reduced graphene oxide hybrid catalyst under visible-light irradiation

Zinc ferrite loaded with reduced graphene oxide (ZnFe2O4/rGO) and zinc ferrite (ZnFe2O4) catalysts were synthesized via a one-spot method. The catalysts were characterized by X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, Raman spectroscopy, UV-vis diffuse reflectance spectroscopy, surface photovoltage spectroscopy and X-ray photoelectron spectroscopy. Results revealed that the as-synthesized ZnFe2O4/rGO and ZnFe2O4 particles were cubic spinel-type ZnFe2O4 with space group number of Fd3m, and that their average diameters were 7.4 and 7.0 nm, respectively. The photocatalytic results indicated that the ZnFe2O4/rGO hybrid catalyst possesses higher activity than that of the single ZnFe2O4 component under visible-light irradiation. More importantly, the ZnFe2O4/rGO catalyst could recognize ammonia from an organic pollutant-ammonia mixed solution and selectively degrade ammonia and nitrogen gas based on the coordination recognition between Zn cations on ZnFe2O4 and ammonia in solution. Fourier-transform infrared, Raman and X-ray photoelectron spectra confirmed that ammonia was selectively adsorbed on ZnFe2O4 particles. The shifts of Zn 2p3/2 and Zn 2p1/2 binding energies confirmed the coordination between Zn cations and ammonia. The surface photovoltage spectra revealed that the photo-generated holes moved to the surface of ZnFe2O4 particles upon incident visible-light irradiation, and degraded ammonia adsorbed on the catalyst surface. These findings will encourage more investigations of photocatalysis based on coordination recognition.