Uniform Zn2+-Doped BiOI Microspheres Assembled by Ultrathin Nanosheets with Tunable Oxygen Vacancies for Super-Stable Removal of NO

Highly exposed active facets and surface oxygen vacancies (OVs) are beneficial for the photocatalytic removal of various harmful organic compounds. In this study, uniform Zn2+-doped BiOI microspheres, assembled by ultrathin nanosheets with highly exposed {001} facets, with OVs were successfully synthesized for NO removal under visible light irradiation. The phase structure and chemical states are analyzed by means of X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The transmission electron microscopy observations reveal that replacing Bi3+ with Zn2+ can lead to the increased exposure of the {001} facets. X-ray photoelectron and electron paramagnetic resonance spectroscopy results confirm that low-state Zn2+ increases the number of OVs, indicating that an increased number of OVs and a reduced thickness of the nanosheets can enhance the photocatalytic activity for the removal of NO. The photo-oxidative removal efficiency of NO over 3%Zn-BiOI reaches 53.6% and remains highly stable (52.9%) for up to 210 min under visible light irradiation. The calculation of interface adsorption confirms that OVs and Zn2+ can not only offer a donor energy level to extend the solar response range but also act as adsorption sites for low concentration of NO and O2 to optimize the transmission capacity of surface charge carriers. Moreover, this work systematically explains the function of OVs and Zn2+ in the adsorption process of NO. The in situ Fourier transform infrared spectroscopy supports understanding of the photo-oxidative removal mechanisms of NO over Zn-BiOI: (i) the byproducts were bidentate nitrites (bi-NO2–), chelate nitrites (ch-NO2–), and bridging nitrites (br-NO3–) in the dark condition and (ii) the final products were bridging nitrites and bidentate nitrites (bi-NO3–) under visible light irradiation. OVs are found to play an important role both in the dark adsorption and photo-oxidative removal of NO by the 3%Zn-BiOI sample.