Boron Oxide Nanoparticles Exhibiting Defect-Activated Photoluminescence

Engineering emissive defects is key to developing fluorescent or phosphorescent compounds for optics and sensing applications in boron-based materials. The present work reports a bottom-up synthesis of boron oxide nanoparticles directly obtained from crystalline boron. The method does not employ organic solvents or harmful reactants and allows one to obtain pure boron oxide nanoparticles, avoiding the uncontrolled presence of carbon or other contaminants. The nanoparticles are formed through a two-step protocol: ultrasonication in water of bulk boron powder, followed by thermal annealing treatment in air. The first step allows for the formation of nanoparticles containing nonstoichiometric oxo-hydroxy BOH compounds, while the second induces dehydration and further oxidation of the boron species. The annealing temperature controls the number of defects formed during dehydration, and the defects modulate the nanoparticle fluorescence. The defects mainly comprise nonbridging oxygen within trigonal BO₃ units, which have already proved to activate radiative pathways in the BOH system.