Towards understanding photon absorption and emission in MgAl layered double hydroxide

Abstract

The use of layered double hydroxides (LDHs) in advanced photoactive applications requires tailoring their light absorption, the stabilisation of separated charges and the manipulation of charge recombination behaviour. However, understanding of these processes in LDHs is mostly lacking. Here, we dissect the UV-to-mid-infrared absorption and UV-visible luminescence (at 83 K to 298 K) of MgAl-LDH and discuss plausible charge recombination pathways with the aid of Mg(OH)₂, Al(OH)₃ and calcined MgAl-LDH (373 K to 673 K). Excitation of electrons into the conduction band (CB) is accompanied by prominent intragap absorption: excitation of fundamental vibrations and their overtones to 1.7 eV, and a long band tail to the CB edge, from which a bandgap of 3.12 eV was determined. Luminescence spectra reveal strong, broad, overlapping, multi-band emission between 2 eV and 4.5 eV dependent on temperature. Removal of adsorbed water increases luminescence. At 83 K, nearly 50% is emitted above the CB edge—contradicting common behaviour. Luminescence occurs mainly from the band edge at 298 K, or with the removal of hydroxyl- and carbonate groups by calcination. The temperature dependence reveals competition between radiative and non-radiative deactivation pathways, indicating meta-stable CB states and possible polaron formation. Below-band-edge luminescence is believed to be dominated by the effects of vibrational broadening, coordination defects, access to band-tail states, and exciton formation and its binding to defects/Al³⁺ islands in addition to the interlayer carbonate facilitating charge separation.