Charge Compensation in Europium-Doped Hafnia Nanoparticles: Solvothermal Synthesis and Colloidal Dispersion

Abstract

Effective charge compensation of europium in hafnium oxide nanoparticles was achieved at low temperature, allowing high doping incorporation (up to 6 at.%) and enhanced luminescence. The efficiency of the incorporation and charge compensation was confirmed by scanning electron microscope energy dispersive X-ray spectroscopy and powder X-ray diffraction measurements. Despite the known polymorphism of hafnium oxide, when doped to a concentration above 3 at.%, only the pure monoclinic phase was observed up to 6 at.% of europium. Furthermore, the low-temperature solvothermal route allowed the direct formation of stable dispersions of the synthesized material over a wide range of concentrations in aqueous media. The dispersions were studied by diffuse light scattering (DLS) to evaluate their quality and by photoluminescence to investigate the incorporation of the dopants into the lattice.

Decoupling of the Crystal Structure and Upconversion in HfO2 Nanocrystals: Emission Chromaticity to Determine the Nanoscale Dopant Distribution

“…It is worth noting that for all compositions, the synthesis led to crystalline nanoparticles without evidence of secondary crystalline or amorphous phases, even before annealing (Figure S2), in line with previous reports on hafnia nanocrystals obtained by solvothermal methods. , Based on that, Raman spectroscopy recorded on samples solely doped with Lu 3+ permitted the identification of the phonon energy distribution in each polymorph. Figure d displays the spectra for both pure m-HfO 2 , presenting vibrational spectra with highest phonon energy at around 800 cm –1 , while c-HfO 2 did not show well-defined Raman modes, with a maximum at 700 cm –1 , similar to what was measured in other works. ,− Concerning the materials composition, the nonaqueous sol–gel synthesis provides very effective doping, with good correspondence between nominal stoichiometry and postsynthetic composition. ,, Indeed, TEM-EDX elemental analysis confirms a good agreement of the RE 3+ concentration with the nominal ones (Figure S3). The EDX mapping also confirms the homogeneous incorporation of all dopants in phase-pure monoclinic and cubic HfO 2 .…”

“…25,41−43 Concerning the materials composition, the nonaqueous sol−gel synthesis provides very effective doping, with good correspondence between nominal stoichiometry and postsynthetic composition. 36,39,44 Indeed, TEM-EDX elemental analysis confirms a good agreement of the RE 3+ concentration with the nominal ones (Figure S3). The EDX mapping also confirms the homogeneous incorporation of all dopants in phase-pure monoclinic and cubic HfO 2 .…”

“…HfO 2 UCNCs codoped by Er 3+ , Yb 3+ , and Lu 3+ were prepared using a microwave-assisted solvothermal reaction from chloride precursors in benzyl alcohol, adapted from previous reports (the details of the synthetic procedure can be found in the Section ). − The successive heat treatment in oxygen at 1000 °C for 10 h promoted crystal growth and coalescence, as testified by PXRD (Figure a) and Scherrer analyses: the initial crystal size of around 2–4 nm yielded final crystal sizes of 20–40 nm irrespective of the composition of the particles. TEM micrographs show that the heat-treated materials consist of loose agglomerates of nanocrystals with diameters in good agreement with the Sherrer analysis (Figure S1).…”

First investigation of the morphological and luminescence properties of HfO2 nanoparticles synthesized by photochemical synthesis

“…6,7 In the literature, most studies on the luminescence properties of hafnia under excitation by optical light and ionizing radiation have been explored in depth on thin films, and pure and doped nanoparticles (NPs). [7][8][9][10][11][12][13][14] In contrast, the research of the spectroscopic properties of HfO 2 single crystals appears only in a limited number of papers. [15][16][17] This discrepancy is related to the complexity of the production of hafnia single crystals due to their high melting point temperature (above 2700 °C).…”

Activation of photosensitive proteins with radioluminescent nanoparticles and X-rays

“…The pure and Ti-doped HfO2 nanoparticles were prepared through a microwave-assisted solvothermal synthesis, as described in reference 148 . Briefly, benzyl alcohol solutions of each precursor (HfCl4 1 M and TiCl4 0.1 M) were prepared as stock solutions.…”