Effects of Co, Ni, Cu, and Zn on Photophysical and Photocatalytic Properties of Carbonate Intercalated MII/Cr LDHs for Enhanced Photodegradation of Methyl Orange

To investigate the effects of divalent metal ions on the photophysical and photocatalytic properties of MII/Cr–CO3 layered double hydroxides (LDHs), we varied the MII metal ions (Co, Ni, Cu, and Zn) at a constant MII/Cr atomic ratio of 2:1. The phase structures and textural and optical properties of these catalysts were characterized by powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller (BET) surface area, Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy, and diffuse-reflectance UV–vis (DRUV–vis) spectroscopy. The PXRD measurements demonstrated that all of the as-synthesized LDHs had well-defined layered structures except Ni/Cr–CO3 LDH (LDH2). Optical difference spectra revealed that absorption in the visible region can be attributed to metal-to-metal charge transfer (MMCT) excitation of oxo-bridged bimetallic linkages of MII–O–CrIII in the two neighboring MO6 octahedra. The photocatalytic performances of these catalysts were examined in the degradation of methyl orange (MO) dye under solar light irradiation. The LDH containing cobalt (i.e., LDH1) exhibited the highest photoactivity with 90% MO removal in 3 h under solar light irradiation. The excitation of MII–O–CrIII in solid solution in the visible range was demonstrated to be one of the causes of the enhanced degradation of MO. An MO degradation mechanism over MII/Cr–CO3 LDHs is proposed that is due to the formation of intermediate oxidative species such as hydroxyl radicals and superoxide radicals during the reactions. Moreover, the most active photocatalyst (LDH1) was found to be stable under repeated applications of up to four successive cycles with a nearly constant photocatalytic degradation activity.