Assessment of the Morphological, Optical, and Photoluminescence Properties of HfO2 Nanoparticles Synthesized by a Sol-Gel Method Assisted by Microwave Irradiation

In this work, we optimized the synthesis of HfO2nanoparticles (NPs) with a nonaqueous sol-gel method assisted bymicrowave heating, with a direct surfactant-free extraction and stabilization in water. To tune the structural, morphological, andphotophysical properties, we explored the influence of reaction time, heating temperature, and type and concentration of a saltprecursor. The controlled size, shape, crystallinity associated with high stability, a good yield of production, and stabilization in waterwithout any surfactant modification of these HfO2NPs open possibilities for future optoelectronic and biomedical applications. Theinvestigation of their optical properties, revealed a high absorption in the UV range and the presence of a large band gap, originatingin transparency at visible wavelengths. Under UV excitation, photoluminescence (PL) shows three emission bands centered at 305,381, and 522 nm and are assigned to the vibronic transition of an excited OH center dot*radical or to a self-trapped exciton, to threefoldoxygen vacanciesVO3with recombination to the valence band, and to defect level, respectively. The presence of oxygen vacanciesassociated with PL properties is particularly attractive for optoelectronic, photocatalysis, scintillator, and UV photosensorapplications. Finally, by changing the nature of the hafnium precursor salt, using hafnium ethoxide or hafnium acetylacetonate, low-crystallized and aggregated NPs were obtained, which requires further investigation