Creation of near-infrared luminescent phosphors enabled by topotactic reduction of bismuth-activated red-emitting crystals

Abstract

Bismuth-doped luminescent materials have gained significant attention in the past years owing to their huge potential for the applications in telecommunications, biomedicine, and displays. However, the controlled synthesis of these materials, in particular for those luminescing in the near-infrared (NIR), remains a challenging subject of continuous research effort. Herein, we show that the low-temperature topotactic reduction by using Al metal powders as oxygen getters can be adopted as a powerful technique for the conversion of bismuth-doped red-emitting systems into NIR-emitting cousins as a result of the creation of unique crystalline networks. Thorough experimental characterization indicates that the framework oxygen of hosts can be topotactically extracted, thus producing unique metal-oxygen-metal networks in the reduced phases while preserving the crystalline structure of the precursor. Furthermore, X-ray absorption spectroscopy reveals that Bi atoms substitute for both Ba 2+ and P 5+ /B 3+ in BaBPO5 crystals, and subsequent topotactic treatment preferentially changes the local environment of Bi at P 5+ /B 3+ sites, which results in the occurrence of NIR emission owing to the creation of NIR-luminescent, defective Bi-O polyhedra in which Bi bears lower oxidation states with respect to that in the precursor. The site-specific topotactic reduction reaction reported here helps us create peculiar NIR-luminescent Bi-O units, and simultaneously does not seriously affect the red photoluminescence from Bi 2+ situated at the Ba 2+ sites. Given the long-lived, ultrawide NIR emission covering the second biological windows, the phosphors developed here hold great promise for in vivo luminescence and lifetime bioimaging. We anticipate that this low-temperature topotactic reduction strategy can be applied to the development of more novel Bi-doped luminescent materials in various forms that can find a broad range of functional applications.

Recent Advances in Super Broad Infrared Luminescence Bismuth-Doped Crystals

“…In 2016, Liu et al. realized another Bi-doped NIR luminescent phosphate crystal of BaBPO 5 by converting Bi-doped red-emitting systems ( Liu et al., 2016 ). From detailed analysis of X-ray absorption data, the introduced Bi ions should exist mostly in P 5+ and/or B 3+ sites, whereas a minority of them are located in Ba 2+ lattice sites.…”

“…Measurements were performed by monitoring the decay of the emission at 1,164 nm upon 476 nm excitation. Reproduced with permission ( Liu et al., 2016 ). Copyright 2016, The Royal Society of Chemistry.…”

“…Comparing the known radius of possible Bi species and calculating the relative ionic radius difference, they finally agreed that Bi 0 species can be accommodated in Ba 2 P 2 O 7 lattice to feature NIR emission but not in Ca 2 P 2 O 7 or Sr 2 P 2 O 7 . (Liu et al, 2016). From detailed analysis of X-ray absorption data, the introduced Bi ions should exist mostly in P 5+ and/or B 3+ sites, whereas a minority of them are located in Ba 2+ lattice sites.…”

“…Here, another point that should be focused on is that the valence of nearly each substituted cation is lower than 3+ (the most stable valence of Bi in the form of Bi 2 O 3 ), except for Y 3+ in Y 4 GeO 8 (Xu et al, 2011) and P 5+ and B 3+ in BaBPO 5 (Liu et al, 2016). Here, reducing conditions (atmospheres or agents) are usually required during the synthesis process such as topotactic reduction of Bi-activated red-emitting crystals in BaBPO 5 iScience Review using Al powder.…”

Giant enhancement of white light emission from Ca9Ln(PO4)7:Eu2+,Mn2+ (Ln = La, Lu, Gd) phosphors achieved by remote aluminum reduction

“…Topochemical reduction have been used in the development of inorganic functional materials [23][24][25][26][27][28][29][30][31][32], including the luminescent materials [23][24][25]31,32]. Such an effective reaction method enables us to obtain specific metastable phases bearing highly unusual oxidation states and coordination geometries, by using particular reduction agents, such as CaH 2 [23][24][25][26][27][28][29] and elemental Al (that is, the remote Al reduction) [30][31][32]. Compared with CO reduction, the remote Al reduction can avoid the raw materials from being contaminated during the reaction process.…”