Enhanced visible light photocatalytic performance and reaction mechanism on Ag/AgCl-LaCO3OH nanorods composite

Abstract

Air pollution has become a focused issue worldwide with the rapid development of industry. As one of the major air pollutants, nitric oxide (NO) could cause serious atmospheric problems, such as acid rain, haze and photochemical smog. Photocatalysis as a green technology has received increasing attention, because it has represented greatly potential application over the late decades. As a novel photocatalyst, LaCO3OH possesses relatively negative conduction band and positive valence band edges, which enbles sufficient potential for photocatalytic air decomposition. But the band gap of LaCO3OH (4.1 eV) allows only a small portion of solar spectrum in the ultraviolet (UV) light region to be absorbed. To extend the light absorption spectra into the visible light region, the combination of narrow band gap Ag/AgCl with the wide band gap LaCO3OH to build heterojunction could utilize abundant solar light. Noble metal nanoparticles (NPs) showed strong visible light absorption due to surface Plasmon resonance effect (SPR). In particular, silver NPs show SPR in the visible region, which has been utilized to develop efficient plasmonic photocatalysts. In this study, a three-component Ag/AgCl-LaCO3OH nanorod photocatalyst has been fabricated successfully through a chemical precipitation treatment and applied to photocatalytic NO removal. The as-prepared samples were characterized by XRD, SEM, TEM, XPS, UV-vis DRS, PL and ESR. The photocatalytic NO oxidation process was monitored by in situ DRIFTS. The results indicated that the visible-light harvesting and the photogenerated carrier separation can be enhanced simultaneously, which could result from introduction of Ag/AgCl into LaCO3OH (LCO) nanorod photocatalyst. Under visible-light irradiation, the photogenerated electron-hole pairs separation is promoted by the plasma resonance (SPR) effect of the metallic Ag0. The hot electrons would be transferred to the defect level endowed by the oxygen vacancies of the LCO. Subsequently, the electron can be transferred to the surface of LCO, captured by O2 to form •O2- radicals, which plays a dominant role in the photocatalytic NO oxidation. In addition, the holes would be transferred to the surface of AgCl, and interact with Cl- to transform Cl0, which can directly participate in the oxidation of NO. Furthermore, Cl0 can be reduced to Cl- and react with Ag+ to forming AgCl, avoiding the photocorrosion of AgCl effectively. Meanwhile, the effect of the mol ratio of Ag/AgCl on the photocatalytic performance was also evaluated. The optimized Ag/AgCl-LCO (mol ratio at 75%) nanorods demonstrated a high NO removal ratio of 54.0%, which far exceeding that of the individual LCO (3.1%) or Ag/AgCl (18.0%) photocatalyst. Moreover, the in situ FT-IR spectra was used to dynamically monitor the intermediate toxic by-products, in combination with the active species, and further reveal the mechanism of photocatalytic NO oxidation from the molecular level. The reaction mechanism can be described as Ag + visible light → e-+h+, e-+O2→ •O2-, h++AgCl→Cl0+Ag+, •O2-+NO→NO3-, Cl0+NO→NO3-+Cl-, NO+2•OH→NO2+H2O, NO2+•OH→NO3-+H+ and Cl- +Ag+→AgCl. The present work could provide perspectives for advancing the photocatalysis efficiency, offer a new insight of the photocatalytic reaction mechanism and promote large-scale environmental decontamination applications of plasmabased semiconductor composites.