pH induced size tuning of Gd2Hf2O7:Eu3+ nanoparticles and its effect on their UV and X-ray excited luminescence

Size tunable nanoparticles (NPs) have played an important role in areas of optoelectmnics, drug delivery, magnetism and many others. Moreover, designing size-tunable NPs without exposing them to high temperature and long time thermal treatment is highly desirable to make agglomerate- and defect-free NPs. In this work, we have designed Gd2Hf2O7:Eu3+ (GHOE) NPs using the molten-salt synthesis (MSS) method with the precursors made from varying precipitant concentrations (correspondingly the pH of the precipitating solution). The obtained NPs have a decreasing size as the pH of the precipitating solution rises and a defect fluorite structure with a large fraction of Eu3+ ions localized at GdO8 distorted scalenohedra, a small fraction residing at Hf4+ site, and the presence of oxygen vacacnies in the vicinity of Eu-Hf'. Maximum photoluminescence and radioluminescence outputs and internal quantum yield have been observed from the GHOE NPs made from 5.0% NH4OH as precipitant due to optimum balance of surface defects and particle clustering. Judd-Ofelt analysis has confirmed that these GHOE NPs have lowest non-radiative transition probability, brightest red emission, largest branching ratio, and highest radiative transition rate. With increasing pH of the precipitating solution, the group symmetry of Eu3+ ions in the GHOE host decreases systematically from D-4 -> C-4v -> C-3v and then saturates, consistent with the pH dependent asymmetry ratio value. Also, the extent of polarizability enhances and the Eu-O bond becomes more covalent as confirmed by the monotonic increase of Omega(2)/Omega(4) ratio. Our work on these optical materials will assist the scientific community to make size-tunable nanoparticles at low synthesis temperature for efficient luminescent devices.