Mechanism of MgAl-LDH-induced enhancement in the plasmonic photocatalytic performance over bismuth nanospheres

Abstract

The rapid development of industry, automobiles and manufacturing has emitted an ever-growth amount of the atmospheric gas pollutants to the atmosphere, such as nitric oxide (NO x ), sulfur oxide (SO x ), volatile organic compounds (VOC) and particle matters (PM). NO is deemed as one critical air pollutant as it is the leading origins of several environmental issues, such as the acidic rain, haze, and photochemical smog. To eliminate its environmental risk, it is highly desired to develop one effective, cost-effective and environmental-friendly technology to detect, regulate and remove them. Till now, various technologies have been developed, including the selective physical/chemical adsorption, heterogeneous catalytic reduction/oxidation and photocatalysis, to show high efficiency in separation, conversion and detoxification of NO. Among them, photocatalysis is receiving an ever-increased attention by its high efficiency, low cost and green feature (this technology enables the direct utilization of solar light to trigger the generation of highly active oxidative free radicals for pollutant degradation and mineralization). In the development of photocatalytic technology, the core part is the design and synthesis of highly effective and durable photocatalyst. Plasmonic metal with a unique localized surface plasmon resonance (LSPR) feature have now emerged as one appealing class of photocatalysts with wide applications in environmental remediation and energy conversion. In the plasmonic metal-directed photocatalysis, the surface electrons of plasmonic metal oscillate resonated with the incident photon, giving rise to the generation of hot carriers for catalytic redox reactions. However, most of the current LSPR-directed photocatalysis is limited to the noble metal (Au and Ag). Very recently, the earth-abundant bismuth metal (Bi) was confirmed to show a unique LSPR feature in visible light region, and more importantly, can serve as a direct plasmonic photocatalyst in ppb-level NO removal from a gas flow under UV illumination. However, bare Bi particles do not show enough redox capability as the hot carriers produced via LSPR in Bi metal cannot travel over distances longer than tens of nanometers. In this study, we attempt to introduce the MgAl-layered double hydroxide (MgAl-LDH) to intensify photocatalytic efficiency of the Bi metal. A facile liquid-phase ultrasound-assisted assembly approach was firstly used to synthesize the Bi@MgAl-LDH nanocomposite, in which Bi-NPs are uniformly supported on MgAl-LDH nanosheets. This composite can efficiently and steadily photocatalyze the removal of ppb-level NO from a continuous air flow under ultraviolet light irradiation via the plasmonic effect Bi metal. The removal efficiency reaches 43% for Bi@MgAl-LDH, much higher than the bare Bi-NPs (43%). The combined study of the composite phase/morphology/structure, its optical property, band structure and ESR oxidation radical capture experiments reveal that MgAl-LDH cannot form a heterojunction with Bi-NPs, and the enhanced photocatalysis on Bi@MgAl-LDH originates from the abundant hydroxide ions (OH−) on the surface of MgAl-LDH. These OH− can quickly capture the photoexcited holes to generate ·OH radicals, and simultaneously promote the electron-hole separation by consuming the holes, both of which can raise the utilization efficiency of the photoexcited hot carriers and provide sufficient oxidative radicals for NO oxidation. Additionally, MgAl-LDH has a unique water molecule memory effect, and the OH− that is consumed in photocatalysis can be replenished by adsorbing water from the air, leading to a stable photocatalytic performance. The reaction pathway study of the photocatalytic NO oxidation over Bi@MgAl-LDH by in-situ DRIFTS reveals that NO is completely mineralized into nitrate. The present work should contribute to a deeper understanding of the photocatalytic mechanism and then the design of robust catalysts for air purification.