Phosphorescence by Trapping Defects in Boric Acid Induced by Thermal Processing

The phosphorescence of boric acid (BA, H3BO3) at room temperature is a puzzling phenomenon subject to controversial interpretations although the role of structural defects has not yet been considered. Heat treatments of boric acid cause its transformation into the metaboric phase and amorphous boron oxide (B2O3). The structural changes after thermal processing can create defects that become centers of luminescence and recombination channels in the visible range. In the present work, commercial boric acid is thermally processed at different temperatures. Samples treated between 200 and 400 degrees C exhibit remarkable phosphorescence in the visible range. At approximate to 480 and 528 nm, two distinct phosphorescent emissions occur, associated with trapped charge carriers recombinations identified by thermoluminescence (TL) and electron paramagnetic resonance spectroscopy (EPR). The structural and optical studies suggest that the activation of boric acid phosphorescence after heat treatment is correlated with the presence of defects. The afterglow results from a trapping and detrapping process, which delays the recombination at the active optical centers. Time-dependent density functional study (DFT) of defective BOH molecules and clusters shows the emergence of near UV and blue optical transitions in absorption. These defects trigger the photoluminescence in thermally processed boric acid samples. Boric acid emits phosphorescence in visible range only after dehydration and melting. Defects activate two distinct phosphorescent emissions, with trapping and detrapping process delaying recombination at active optical centers. Time-dependent density functional study reveals near UV and blue optical transitions in absorption due to oxygen vacancies and non-bridging oxygens, triggering photoluminescence.image