Uranium-Sensitized Luminescence Enhancement in Li₂B₄O₇:U⁶⁺,Eu³⁺ and Potential Application towards Color-Tunable Phosphors and Actinide Sensor

Abstract

Actinide–lanthanide interaction has led to many fascinating results in the past pertaining to applications in the area of energy, health, and the environment. In this work, we utilized UO2 2+ ion codoping to enhance the luminescent properties of the europium ion in a Li2B4O7 (LTB) host. We could achieve 75% energy transfer efficiency, leading to almost doubling of internal quantum yield (IQY) from ∼34 to 64%. The efficacy of energy transfer from a uranyl to a europium ion and enhancement in europium emission was clearly demonstrated using density functional theory calculations, and it proceeds via formation of a UO2–Eu coordination complex. Because of the different extent of UO2 2+ → Eu3+ energy transfer at different europium ion concentrations, color-tunable emission from green–yellow–near-white could be achieved. The optical enhancement has also been explained by detailed electronic structure analysis of LTB in the presence of Eu and U. Here, we also present the feasibility for sensing uranyl (VI) ions through the exploration of a unique Förster resonance energy transfer process to europium(III) using selective excitation of a near-UV 320 nm photon. This work on the LTB:U6+,Eu3+ luminescent phosphor displays the first example demonstrating UO2 2+ → Eu3+ energy-transfer-induced enhancement in IQY and applying the same for designing color-tunable phosphor and an on–off fluorescent sensor for uranyl (VI) ions.

Local Structure and Speciation-Driven UO22+ → Sm3+ Energy Transfer for Enhanced Luminescence in Li2B4O7

“…The quest for red-emitting phosphors to compensate for the red spectral region aroused huge scientific interest in exploring novel Eu 3+ and Sm 3+ -based phosphors. − Sm 3+ phosphors are promising for achieving orange red emissions on near ultraviolet (NUV) and blue excitations . Despite the large progress, the problem of insufficient molar absorption coefficients of Ln 3+ ions in the NUV region due to parity forbidden 4f → 4f transitions needs to be tackled. , The introduction of sensitizer ions in the host matrix is a promising approach to improve the Ln 3+ emission via energy transfer (ET) routes. ,, …”

“…Hence, it would be interesting to investigate the effect of different equatorial coordination of uranyl ions on the photoluminescence (PL) properties of both the Sm 3+ and uranyl ions and energy transfer efficiencies. In our earlier work, we explored the possibilities of sensitizing Eu 3+ by uranyl in LTB, which has been investigated but no correlations of sensitized luminescence with the local structure, coordination, and speciation of dopant and sensitizer have been demonstrated . Generally, the uranyl ion complexation and effect of equatorial oxygens on uranyl luminescence has been studied in solutions while no significant efforts have been made to explore the consequences of speciation of a uranyl ion on its PL properties in solid inorganic phosphor compounds.…”

“…5 Despite the large progress, the problem of insufficient molar absorption coefficients of Ln 3+ ions in the NUV region due to parity forbidden 4f → 4f transitions needs to be tackled. 15,16 The introduction of sensitizer ions in the host matrix is a promising approach to improve the Ln 3+ emission via energy transfer (ET) routes. 15,17,18 Uranyl ions (UO 2 2+ ) are well-known for a strong absorption in the NUV and blue spectral region, and various reports have explored the efficiency of ET in UO 2 2+ → Eu 3+ pair.…”

“…15,16 The introduction of sensitizer ions in the host matrix is a promising approach to improve the Ln 3+ emission via energy transfer (ET) routes. 15,17,18 Uranyl ions (UO 2 2+ ) are well-known for a strong absorption in the NUV and blue spectral region, and various reports have explored the efficiency of ET in UO 2 2+ → Eu 3+ pair. The 5 D 0 energy levels of Eu 3+ emissive states (∼20,000 cm −1 ) which resulted in effective sensitization.…”

“…The 5 D 0 energy levels of Eu 3+ emissive states (∼20,000 cm −1 ) which resulted in effective sensitization. 15,17,19,20 Wang et al have investigated the 4f/5f energy transfer between actinide (uranyl (VI)) and lanthanide (III) centers in europium-incorporated uranyl coordination polymers. 21 A tunable 4f/5f bimodal emission was reported for the first time in heterobimetallic compounds.…”