skip to main content

Title: Exploring the optical properties of La 2 Hf 2 O 7 :Pr 3+ nanoparticles under UV and X-ray excitation for potential lighting and scintillating applications
New optical materials with efficient luminescence and scintillation properties have drawn a great deal of attention due to the demand for optoelectronic devices and medical theranostics. Their nanomaterials are expected to reduce the cost while incrementing the efficiency for potential lighting and scintillator applications. In this study, we have developed praseodymium-doped lanthanum hafnate (La 2 Hf 2 O 7 :Pr 3+ ) pyrochlore nanoparticles (NPs) using a combined co-precipitation and relatively low-temperature molten salt synthesis procedure. XRD and Raman investigations confirmed ordered pyrochlore phase for the as-synthesized undoped and Pr 3+ -doped La 2 Hf 2 O 7 NPs. The emission profile displayed the involvement of both the 3 P 0 and 1 D 2 states in the photoluminescence process, however, the intensity of the emission from the 1 D 2 states was found to be higher than that from the 3 P 0 states. This can have a huge implication on the design of novel red phosphors for possible application in solid-state lighting. As a function of the Pr 3+ concentration, we found that the 0.1%Pr 3+ doped La 2 Hf 2 O 7 NPs possessed the strongest emission intensity with a quantum yield of 20.54 ± 0.1%. The more » concentration quenching, in this case, is mainly induced by the cross-relaxation process 3 P 0 + 3 H 4 → 1 D 2 + 3 H 6 . Emission kinetics studies showed that the fast decaying species arise because of the Pr 3+ ions occupying the Hf 4+ sites, whereas the slow decaying species can be attributed to the Pr 3+ ions occupying the La 3+ sites in the pyrochlore structure of La 2 Hf 2 O 7 . X-ray excited luminescence (XEL) showed a strong red-light emission, which showed that the material is a promising scintillator for radiation detection. In addition, the photon counts were found to be much higher when the NPs are exposed to X-rays when compared to ultraviolet light. Altogether, these La 2 Hf 2 O 7 :Pr 3+ NPs have great potential as a good down-conversion phosphor as well as scintillator material. « less
Authors:
; ; ;
Award ID(s):
1710160
Publication Date:
NSF-PAR ID:
10098127
Journal Name:
New Journal of Chemistry
Volume:
42
Issue:
12
Page Range or eLocation-ID:
9381 to 9392
ISSN:
1144-0546
Sponsoring Org:
National Science Foundation
More Like this
  1. Controlled energy transfer has been found to be one of the most effective ways of designing tunable and white photoluminescent phosphors. Utilizing host emission to achieve the same would lead to a new dimension in the design strategy for novel luminescent materials in solid state lighting and display devices. In this work, we have achieved controlled energy transfer by suppressing the host to dopant energy transfer in La 2 Hf 2 O 7 :Eu 3+ nanoparticles (NPs) by co-doping with uranium ions. Uranium acts as a barrier between the oxygen vacancies of the La 2 Hf 2 O 7 host and Eu 3+ doping ions to increase their separation and reduce the non-radiative energy transfer between them. Density functional theory (DFT) calculations of defect formation energy showed that the Eu 3+ dopant occupies the La 3+ site and the uranium ion occupies the Hf 4+ site. Co-doping the La 2 Hf 2 O 7 :Eu 3+ NPs with uranium ions creates negatively charged lanthanum and hafnium vacancies making the system highly electron rich. Formation of cation vacancies is expected to compensate the excess charge in the U and Eu co-doped La 2 Hf 2 O 7 NPs suppressing the formationmore »of oxygen vacancies. This work shows how one can utilize the full color gamut in the La 2 Hf 2 O 7 :Eu 3+ ,U 6+ NPs with blue, green and red emissions from the host, uranium and europium, respectively, to produce near perfect white light emission.« less
  2. The design and development of efficient and stable nuclear waste hosts has drawn intensive interest for long-lived lanthanides and actinides. A detailed investigation of their structure and potential structural evolution are crucial. In this study, we have synthesized lanthanum hafnate La 2 Hf 2 O 7 nanoparticles (NPs) doped with uranium at different concentrations (0–10%) and investigated their structural transition. We have discovered that in our La 2 Hf 2 O 7 :U NPs, the uranium dopants are stabilized at both U 4+ and U 6+ oxidation states in which the U 6+ oxidation state exists in octahedral uranate UO 6 6− form. We also confirmed that the U 4+ ions substituted the Hf 4+ ions with a lifetime of ∼1.0 μs and the UO 6 6− ions resided at the La 3+ sites with a lifetime of ∼9.0 μs. More interestingly, the proportion of the U 4+ ions in the La 2 Hf 2 O 7 :U NPs was higher than that of the UO 6 6− ions at low doping level, but at the doping level higher than 2.5%, the fraction of the UO 6 6− ions was greater than that of the U 4+ ions. Furthermore, wemore »studied the structural phase transformation from order pyrochlore to cotunnite of these La 2 Hf 2 O 7 :U NPs with increasing uranium doping level, and found that ordered pyrochlore phase favors the U 4+ ions whereas disordered cotunnite phase favors the UO 6 6− ions. We further used in situ Raman spectroscopy to confirm the reversible cotunnite to pyrochlore phase transformation of the La 2 Hf 2 O 7 :10%U NPs at 900 °C. Therefore, this work demonstrated the successful development of uranium doped La 2 Hf 2 O 7 NPs and thorough characterization of the fundamental spectra of uranium ions, doping induced phase transformation, and structure–optical property correlation.« less
  3. Desirable phosphors for lighting, scintillation and composite films must have good light absorption properties, high concentration quenching, high quantum efficiency, a narrow color emission, and so forth. In this work, we first show that undoped yttrium hafnate Y 2 Hf 2 O 7 (YHO) nanoparticles (NPs) display dual blue and red bands after excitation using 330 nm light. Based on density functional theory (DFT) calculations, these two emission bands are correlated with the defect states arising in the band-gap region of YHO owing to the presence of neutral and charged oxygen defects. Once doped with Eu 3+ ions (YHOE), the YHO NPs show a bright red emission, a long excited state lifetime and stable color coordinates upon near-UV and X-ray excitation. Concentration quenching is active when Eu 3+ doping reaches 10 mol% with a critical distance of ∼4.43 Å. This phenomenon indicates a high Eu 3+ solubility within the YHO host and the absence of Eu 3+ clusters. More importantly, the optical performance of the YHOE NPs has been further improved by lithium co-doping. The origin of the emission, structural stability, and role of Li + -co-doping are explored both experimentally and theoretically. DFT calculation results demonstrate that Li +more »-co-doping increases the covalent character of the Eu 3+ –O 2− bonding in the EuO 8 polyhedra. Furthermore, the YHOE NPs have been dispersed into polyvinyl alcohol (PVA) to make transparent nanocomposite films, which show strong red emission under excitation at 270 and 393 nm. Overall, we demonstrate that the YHO NPs with Eu 3+ and (Eu 3+ /Li + ) doping have a high emission intensity and quantum efficiency under UV and X-ray excitation, which makes them suitable for use as phosphors, scintillators and transparent films for lighting, imaging and detection applications.« less
  4. Fabrication of highly stable, reversible, and efficient portable sensors for the detection of explosives for safety and security is challenging due to the robustness of the currently available detection tools, limiting their mass deployment to the explosion prone areas. This paper reports a new direction towards the sensing of nitro- and peroxide-based explosives using highly stable rare-earth-doped BaWO 4 nanofibers with remarkable sensitivity and reversibility. BaWO 4 nanofibers doped with Tb 3+ and Eu 3+ ions are fabricated through a sol–gel electrospinning process, and their emission characteristics and application as a fluorescent probe for the sensing of 2-nitrotoluene and H 2 O 2 , explosive taggants representing a broad class of explosives, are studied in detail. Scheelite structured BaWO 4 nanofibers exhibit excellent luminescence characteristics, and the rare-earth ion doping in the polycrystalline BaWO 4 nanofibers is tailored to achieve blue, green, red, and white light emissions. These nanofibers are extremely sensitive to 2-nitrotoluene and H 2 O 2 with rapid response time, and sensitivity is observed within the range of 1–400 ppb and 1–10 ppm, towards 2-nitrotoluene and H 2 O 2 , respectively. The fluorescence quenching of BaWO 4 nanofibers in the presence of 2-nitrotoluene and H 2more »O 2 is exponential with the quenching constants up to 1.73 × 10 6 and 2.73 × 10 4 L mol −1 , respectively, which are significantly higher than those of most of the fluorescent probes based on metal–organic frameworks and conjugated organic materials. After exposing to 2-nitrotoluene, the luminescence of the nanofibers is retained completely upon heating at 120 °C for 10 min and the sensory response is retained as fresh nanofibers, and currently available fluorescent explosive sensors could not achieve such a recovery. The high sensitivity and selectivity of scalable rare-earth-doped BaWO 4 nanofibers provide a new platform for the simultaneous detection of two classes of explosives.« less
  5. Metallothioneins (MTs) are a ubiquitous class of small metal-binding proteins involved in metal homeostasis and detoxification. While known for their high affinity for d 10 metal ions, there is a surprising dearth of thermodynamic data on metals binding to MTs. In this study, Zn 2+ and Cu + binding to mammalian metallothionein-3 (MT-3) were quantified at pH 7.4 by isothermal titration calorimetry (ITC). Zn 2+ binding was measured by chelation titrations of Zn 7 MT-3, while Cu + binding was measured by Zn 2+ displacement from Zn 7 MT-3 with competition from glutathione (GSH). Titrations in multiple buffers enabled a detailed analysis that yielded condition-independent values for the association constant ( K ) and the change in enthalpy (Δ H ) and entropy (Δ S ) for these metal ions binding to MT-3. Zn 2+ was also chelated from the individual α and β domains of MT-3 to quantify the thermodynamics of inter-domain interactions in metal binding. Comparative titrations of Zn 7 MT-2 with Cu + revealed that both MT isoforms have similar Cu + affinities and binding thermodynamics, indicating that Δ H and Δ S are determined primarily by the conserved Cys residues. Inductively coupled plasma mass spectrometry (ICP-MS)more »analysis and low temperature luminescence measurements of Cu-replete samples showed that both proteins form two Cu 4 + –thiolate clusters when Cu + displaces Zn 2+ under physiological conditions. Comparison of the Zn 2+ and Cu + binding thermodynamics reveal that enthalpically-favoured Cu + , which forms Cu 4 + –thiolate clusters, displaces the entropically-favoured Zn 2+ . These results provide a detailed thermodynamic analysis of d 10 metal binding to these thiolate-rich proteins and quantitative support for, as well as molecular insight into, the role that MT-3 plays in the neuronal chemistry of copper.« less