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

Abstract

In this work, we optimized the synthesis of HfO2 nanoparticles (NPs) with a nonaqueous sol–gel method assisted by microwave heating, with a direct surfactant-free extraction and stabilization in water. To tune the structural, morphological, and photophysical properties, we explored the influence of reaction time, heating temperature, and type and concentration of a salt precursor. The controlled size, shape, crystallinity associated with high stability, a good yield of production, and stabilization in water without any surfactant modification of these HfO2 NPs open possibilities for future optoelectronic and biomedical applications. The investigation of their optical properties, revealed a high absorption in the UV range and the presence of a large band gap, originating in 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•* radical or to a self-trapped exciton, to threefold oxygen vacancies V O3 with recombination to the valence band, and to defect level, respectively. The presence of oxygen vacancies associated with PL properties is particularly attractive for optoelectronic, photocatalysis, scintillator, and UV photosensor applications. 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.

Optical and X-ray attenuation properties of hafnium oxide nanoparticles surface functionalized with fucoidan: toward the early diagnosis of atherothrombotic diseases

“…2B and C. This PL em profile was attributed, in a previous paper, 49 to the oxygen vacancies inducing defect levels located deep in the band gap. The PL em spectra for nude NPs were deconvolved with 4 Gaussian fits centered at 305 nm (4.1 eV, band I), 381 nm (3.2 eV, band II), 522 nm (2.4 eV, band III) and 675 nm (1.8 eV, band IV).…”

“…1A) were synthesized using microwave non-aqueous sol–gel synthesis in a two-step process, following a procedure described previously. 49 NPs were surface functionalized with fucoidan, a negatively charged polysaccharide, or with citrate, one of the most used coating agents, using an electrostatic approach for the direct and one-step complexation 54 and taking care of the avidity effect of carboxylate (fucoidan, citrate) and sulfate (fucoidan) groups for the oxide surface. 55 To our knowledge, we reported herein a simple process for the direct and one step complexation of the HfO 2 Np surface.…”

“…Non-coated NPs were synthesized by the reaction of hafnium( iv ) chloride (140 mM with benzyl alcohol (10 ml) at 200 °C for 3 h under microwave irradiation, on a Monowave 300 from Anton Paar (Anton Paar GmbH, Graz, Austria) according to a procedure already described. 49 To coat the NPs with fucoidan, HfO 2 NPs dispersed in water at pH = 2 ([HfO 2 ] ≈ 100 mM) were mixed with various amounts of fucoidan dissolved in water (pH = 7) under stirring for 2 hours, corresponding to a ratio R between 0 and 25, with R = 1000 ([fucoidan]/[HfO 2 ]). The HfO 2 @fucoidan NPs were washed three times by ultracentrifugation (Amicon 100 kDa, Merck Millipore) and suspended in water.…”

“…The Hf concentration was determined by the freeze drying process as described in a previous work 49 and by total reflection X-ray fluorescence spectrometry (TXRF, S2 PICOFOX, Brucker). For TXRF, calibration curves were first obtained using HfO 2 @ fucoidan or HfO 2 @citrate solution (concentration of the stock solution determined after the freeze drying process), Fig.…”

“…In a previous paper, 49 we optimized the synthesis of HfO 2 NPs through a sol–gel method associated with a microwave via the benzyl alcohol route, with direct extraction and stabilization in water without any post-modification treatment. The monoclinic m-HfO 2 NPs, with an ellipsoidal shape (average major and minor axes of 8 and 4 nm, respectively), show strong absorption in the UV range and photoluminescence properties related to the presence of oxygen vacancies.…”