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The recent trends in scintillator technologies stimulate research efforts toward the development of novel materials morphologies, such as nanoparticles, able to efficiently convert ionizing radiations into light. For example, scintillating nanoparticles attract great interest in medical oncological therapies. In this work, the structural and morphological properties of HfO2:Ti nanoparticles with Ti concentrations from 0.03 to 10 mol% and subjected to calcination up to 1000 °C are thoroughly characterized; moreover, X-ray photoelectron spectroscopy reveals the incorporation of Ti in both Ti (III) and Ti (IV) chemical states in as prepared samples, while the exclusive presence of Ti(IV) is unambiguously identified in calcined nanoparticles. The optical emission under X-ray excitation evidences an intense Ti (IV)-related luminescence at 2.5 eV in high temperature calcined samples with a few microseconds scintillation lifetime, and efficiency comparable to that of Bi4Ge3O12 reference scintillator. Finally, the competitive role of defects in charge carriers capture is demonstrated by the monotonic increase of the 2.5 eV band during prolonged X-ray irradiation, more evident for nanoparticles with titanium concentration below 1 mol%. HfO2:Ti may also find application in X-ray triggered oncological therapies by using the Ti (IV)-related bright radioluminescence to excite photosensitizer molecules for singlet oxygen generation.

In this work, we investigate the optical properties of hafnium dioxide nanocrystals, upon X-ray irradiation, looking for spectral evolution following thermal treatments in air up to 1000 °C that modify the crystal size as well as their point defect concentrations. Radio-luminescence measurements from 10 K up to room temperature reveal a rich and evolving picture of the optical features. A complete spectral analysis of the broad luminescence spectra reveals the presence of several emission components in the visible and UV regions. The lower energy components peaking at 2.1, 2.5, and 2.9 eV are characterized by a thermal quenching energy of 0.08 eV, while the corresponding value for the UV bands at 4.1 and 4.7 eV is close to 0.23 eV. We tentatively assign the components ranging from 2 to 3 eV to the presence of optically active defects of an intrinsic nature, together with the occurrence of titanium impurities; conversely, the bands at higher energies are likely to be of an excitonic nature. The comparison with previous photo-luminescence studies allows evidencing characteristic differences between the features of luminescence emissions caused by intra-centre excitation and those occurring under ionizing irradiation. Finally, scintillation measurements in the visible range reveal the existence of a fast decay in the nanosecond time scale for the smallest hafnia nanocrystals. This study offers a clear description of HfO2 luminescence characteristics upon excitation by X-rays and can lead to a better comprehension of the structure-property relationship at the nanoscale in metal oxides

Luminescent nanoparticles are researched for their potential impact in medical science, but no materials approved for parenteral use have been available so far. To overcome this issue, we demonstrate that Eu3+-doped hafnium dioxide nanocrystals can be used as non-toxic, highly stable probes for cellular optical imaging and as radiosensitive materials for clinical treatment. Furthermore, viability and biocompatibility tests on artificially stressed cell cultures reveal their ability to buffer reactive oxygen species, proposing an anti-cytotoxic feature interesting for biomedical applications.

Intact spontaneously hypertensive rats (SHR) were studied to assess the effect of prolonged antihypertensive treatment on the elastic behaviour of the external carotid artery. Thirty-week-old SHR received the ACE inhibitor captopril, the ateriolar dilator hydralazine or their vehicle for 6 weeks. These rats were compared to normotensive, vehicle treated WKY rats. The internal diameter of the carotid artery was measured continuously in halothane-anaesthetized rats using an echo-tracking device, and intra-arterial pressure was also monitored continuously, on the controlateral side. Captopril- and hydralazinetreated SHR as well as normotensive controls had similar blood pressure values. No significant shift in the distensibility-pressure curves was observed among vehicle-treated SHR and WKY rats or the SHR which had received captopril or hydralazine. Histological examination of the carotid artery fixed ex vivo with paraformaldehyde showed a significant increase in cross-sectional area in vehicle-treated SHR as compared to their normotensive counterparts. These results therefore suggest that the elastic behaviour of elastic arteries is not necessarily altered by the structural changes developing in response to hypertension

Defect engineering operated on metal oxides by chemical and structural modifications may strongly affect properties suitable for various applications such as photoelectrochemical behavior, charge transport, and luminescence. In this work, we report the tunable optical features observed in undoped monoclinic HfO2 nanocrystals and their dependence on the structural properties of the material at the nanoscale. Transmission electron microscopy together with X-ray diffraction and surface area measurements were used to determine the fine structural modifications, in terms of crystal growth and coalescence of crystalline domains, occurring during a calcination process in the temperature range from 400 to 1000 °C. The fit of the broad optical emission into spectral components, together with time-resolved photoluminescence, allowed us to identify the dual nature of the emission at 2.5 eV, where an ultrafast defect-related intrinsic luminescence (with a decay time of a few nanoseconds) overlaps with a slower emission (decay of several microseconds) due to extrinsic Ti-impurity centers. Moreover, the evolution of intrinsic visible bands during the material transformation was monitored. The relationship between structural parameters uniquely occurring in nanosized materials and the optical properties was investigated and tentatively modeled. The blue emissions at 2.5 and 2.9 eV are clearly related to defects lying at crystal boundaries, while an unprecedented emission at 2.1 eV enables, at relatively low calcination temperatures, the white luminescence of HfO2 under near-UV excitation.

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

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.

Silica–natural rubber nanocomposites were obtained through a novel non-aqueous in situ sol–gel synthesis, producing the amount of water necessary to induce the hydrolysis and condensation of a tetraethoxysilane precursor by esterification of formic acid with ethanol. The method allows the synthesis of low hydrophilic silica nanoparticles with ethoxy groups linked to the silica surface which enable the filler to be more dispersible in the hydrophobic rubber. Thus, high loaded silica composites (75 phr, parts per hundred rubber) were obtained without using any coupling agent. Transmission Electron Microscopy (TEM) showed that the silica nanoparticles are surrounded by rubber layers, which lower the direct interparticle contact in the filler–filler interaction. At the lowest silica loading (up to 30 phr) silica particles are isolated in rubber and only at a large amount of filler (>60 phr) the interparticle distances decrease and a continuous percolative network, connected by thin polymer films, forms throughout the matrix. The dynamic-mechanical properties confirm that the strong reinforcement of the rubber composites is related to the network formation at high loading. Both the improvement of the particle dispersion and the enhancement of the silica loading are peculiar to the non-aqueous synthesis approach, making the method potentially interesting for the production of high-loaded silica–polymer nanocomposites.

In this work a strategy for the control of structure and optical properties of inorganic luminescent oxide-based nanoparticles is presented. The nonaqueous sol-gel route is found to be suitable for the synthesis of hafnia nanoparticles and their doping with rare earths (RE) ions, which gives rise to their luminescence either under UV and X-ray irradiation. Moreover, we have revealed the capability of the technique to achieve the low-temperature stabilization of the cubic phase through the effective incorporation of trivalent RE ions into the crystal lattice. Particular attention has been paid to doping with europium, causing a red luminescence, and with lutetium. Structure and morphology characterization by XRD, TEM/SEM, elemental analysis, and Raman/IR vibrational spectroscopies have confirmed the occurrence of the HfO2 cubic polymorph for dopant concentrations exceeding a threshold value of nominal 5 mol %, for either Lu3+ or Eu3+. The optical properties of the nanopowders were investigated by room temperature radio- and photoluminescence experiments. Specific features of Eu3+ luminescence sensitive to the local crystal field were employed for probing the lattice modifications at the atomic scale. Moreover, we detected an intrinsic blue emission, allowing for a luminescence color switch depending on excitation wavelength in the UV region. We also demonstrate the possibility of changing the emission spectrum by multiple RE doping in minor concentration, while deputing the cubic phase stabilization to a larger concentration of optically inactive Lu3+ ions. The peculiar properties arising from the solvothermal nonaqueous synthesis here used are described through the comparison with thermally treated powders. © 2013 American Chemical Society.

A time-dependent study on the formation of LiFePO 4 with olivine-type structure is presented. The material is synthesized through a non-aqueous route in benzyl alcohol assisted by microwave radiation. The LiFePO 4 forms with an anisotropic morphology of microscale stick-like particles. The detailed structure of these particles and their evolution with reaction time is revealed by transmission electron microscopy; a 3D reconstruction of a particle by electron tomography provides insight into the formation mechanism of these sticks. Without applying a thermal post-annealing treatment or a carbon coating, the electrochemical behavior of the LiFePO 4 microsticks is assessed for the preparation of cathodes in lithium-ion batteries. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Intact spontaneously hypertensive rats (SHR) were studied to assess the effect of prolonged antihypertensive treatment on the elastic behaviour of the external carotid artery. Thirty-week-old SHR received the ACE inhibitor captopril, the ateriolar dilator hydralazine or their vehicle for 6 weeks. These rats were compared to normotensive, vehicle treated WKY rats. The internal diameter of the carotid artery was measured continuously in halothane-anaesthetized rats using an echo-tracking device, and intra-arterial pressure was also monitored continuously, on the controlateral side. Captopril- and hydralazinetreated SHR as well as normotensive controls had similar blood pressure values. No significant shift in the distensibility-pressure curves was observed among vehicle-treated SHR and WKY rats or the SHR which had received captopril or hydralazine. Histological examination of the carotid artery fixed ex vivo with paraformaldehyde showed a significant increase in cross-sectional area in vehicle-treated SHR as compared to their normotensive counterparts. These results therefore suggest that the elastic behaviour of elastic arteries is not necessarily altered by the structural changes developing in response to hypertension