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In the last decade, many efforts have been devoted to the development of smart multifunctional materials. Among them, inorganic nanostructures have gained importance because of their outstanding luminescence properties and their potential applications as new building block materials for the next generation electronics and in a variety of lighting applications. In particular, many recent researches have been addressed on the achievement of progress in the synthesis of nanosized metal oxides. In this field, a significant attention is paid to hafnium dioxide (hafnia or HfO2), which can be employed in optical protective and thermal barrier coatings. Thanks to its mechanical resistance, hafnia finds applications as ceramic, super hard materials and catalysts, or as component in gas sensors and fuel cell electrolytes. Lastly, HfO2 is now evaluated as potential alternative gate dielectric to replace SiO2 in the future generation of electronic nanodevices. Regarding the scintillating properties, the high atomic number Z=72 and the quite high density (9.6 g cm-3) make HfO2 nanocrystals good hosts for phosphor and scintillating applications where a large stopping power for ionizing radiation (X-rays, γ-rays) is required (1). Bulk hafnia is very difficult to grow due to its very high melting point (2774 °C). Actually, HfO2 can be synthetized also in the nanocrystals form to fabricate thin films, optical ceramics and nanocomposite materials (2). Indeed, besides the well-known luminescence of HfO2 nanocrystals activated by the incorporation of rare earth ions, some recent studies evidenced the occurrence of a blue fluorescence from undoped nanocrystals upon UV excitation; remarkably, a broad bluish luminescence appears upon X-ray illumination (RL). These findings suggest the potential of HfO2 nanocrystals as radiation detectors, but the lack of a detailed model that relates their structural and RL properties still hinders the development of efficient nanoscintillators with optimized structure and chemical composition. We studied the RL features of undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale in order to elucidate their origin. Upon X-ray irradiation, the nanocrystals show six emission bands in the near UV/visible spectral range, detectable between 10 K and 300 K. The visible luminescence bands at 2.2 eV, 2.5 eV and 2.8 eV are similar to those detected in our previous PL studies (3), while the UV emission at 4.2 eV and 4.6 eV have been observed for the first time. The excitonic behavior of the UV luminescence is evidenced. The strong increase of the 2.5 eV blue luminescence in annealed samples is likely related to the presence of titanium and it might be used for the design of highly efficient blue scintillating materials. 1. LeLuyer C. et al. (2008), HfO2:X (X = Eu3+, Ce3+, Y3+) Sol Gel Powders for Ultradense Scintillating Materials. J. Phys. Chem. A, 112, pp. 10152-10155. 2. Lange S. et al. (2006), Luminescence of Re-Ions in HfO2 Thin Films and Some Possible Applications. Opt. Mater., 28, pp. 1238-1242. 3. Villa I. et al. (2016), Size-Dependent Luminescence in HfO2 Nanocrystals: Toward White Emission from Intrinsic Surface Defects. Chem. Mater., 28(10), pp. 3245–3253.

High density powder phosphors are of great interest in technological fields like imaging and ionizing radiation detection. The powder form is of choice when the material can hardly be synthesized as bulk single crystal by conventional techniques. This is the case of hafnium oxides having a melting point above 2500 °C. Moreover nanoscale dimensions are an important requirement for fabricating nanocomposites, in nanomedicine, and for the realization of optical ceramics. In this last field materials with cubic structure are foreseen since their isotropic optical response allow the minimization of light scattering at grain interfaces. This work focuses on the synthesis, structural and optical investigation of HfO2 nanoparticles obtained by non-aqueous sol-gel route. In a first investigation, particular attention was paid to doping with europium and with lutetium. Structure and morphology characterization by XRD, TEM/SEM, elemental analysis, and Raman/IR vibrational spectroscopies confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations exceeding a threshold value of nominal 5 mol%, for either Lu3+or Eu3+ [1]. The spectroscopic features of Ti3+ impurities have been recently analyzed by room temperature radio- and photo-luminescence, time resolved luminescence and scintillation experiments. In addition, we have detected an intrinsic blue emission peaking at 2.5 eV and exhibiting a fast photoluminescence decay time of a few nanoseconds. This emission is due to the presence of surface defects; its intensity, as well as that of an additional band peaking at 2.1 eV, can be varied by thermal treatments that lead to surface modifications and variations of particle dimensions. For temperatures between 500 and 650 °C, tuning of the bands intensities induces a white emission under 3.5 eV excitation. The results demonstrate that the control of intrinsic defects is a potential route to design the optical activity of a material at the nanoscale. [1] A. Lauria et al., ACS Nano 7, 7041 (2013).

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Luminescent materials have found a wide variety of applications as phosphors for fluorescent lighting, display devices, X-ray monitoring and imaging, scintillators, and biomedical imaging. Indeed, the research in nanosciencehasbeen especially dedicated to the development of synthesis routes with the ability to synthetizenew material structures for industrial applications [1]. Fervent research in nanostructured materials has resulted in the development of novel synthetic methods to prepare pure and doped luminescent materials, and strategies to control the incorporation of dopant ions in nanoparticles. Hafnium oxide (HfO2) is considered as an attractive material because of its excellent physical and chemical properties, such as its high dielectric constant and insulating characteristics, which allow its application as a dielectric material with relatively high refractive index and wide band gap, as well in the field of optical coatings and next generation metal-oxide semiconductor devices. Moreover in recent investigations, nanometric hafnia when properly doped by rare earths (RE) ions has been also considered for optical applications [2]. Given the importance of HfO2 as host material for RE, its intrinsic optical response is also worth of investigation. In this work, we propose the study of the luminescent properties of undoped HfO2nanoparticles (NPs), achieved through a non-aqueous sol-gel process. The optical properties were studied by photo- and radio-luminescence considering the effect ofthe size and of the crystal phase. A broad composite emission was observed in the visible range, potentially correlated to intrinsic surface defects. A spectral analysis was performed, and the luminescence efficiency was compared to that of standard luminescent materials. Our studies reveal that HfO2 nanopowders may represent an interesting system in both phosphor technology and scintillation applications.

Anatase TiO2 is among the most studied photocatalytic materials for solar energy conversion and environmental cleanup. However, its poor visible light absorption and high facet-dependent performance limits its utilization. In this study chemical substitution (doping) of TiO2 nanoparticles with metal ions (Sb, Cr, or Sb/Nb and Cr/Nb) is presented as an alternative strategy to address both issues simultaneously. Highly crystalline doped and codoped TiO2 nanoparticles were successfully synthesized by a microwave-assisted nonaqueous sol-gel synthesis. The structural and compositional analysis done by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) showed that depending on the doping applied, variations in particles size and morphology were observed. Doped and codoped samples showed improved absorption in the visible range and in comparison to the undoped TiO2 displayed improved photocatalytic (PC) activity. The variations of the PC activity, observed among different samples, are attributed to the effect of doping on (i) particles size/morphology, (ii) optical activity, and (iii) on the surface potential differences for the various crystal facets. We found that Sb-doping in TiO2 diminishes the surface potential difference for {101} reductive and {001} oxidative sites, which makes all crystal surfaces equally attractive to both electrons and holes. Accordingly, in Sb-doped TiO2 nanoparticles the photocatalytic activity is independent of the exposed crystal facets and thus on the particle morphology. This observation also explains the superior PC performance of this material.