Tens of micron-sized unilamellar nanosheets of Y/Eu layered rare-earth hydroxide: efficient exfoliation via fast anion exchange and their self-assembly into oriented oxide film with enhanced photoluminescence

Abstract

Layered rare-earth hydroxide (LRH) crystals of (Y0.95Eu0.05)2(OH)5NO3·nH2O with a lateral size of ∼ 300 μm and a thickness of ∼ 9 μm have been synthesized via a hydrothermal reaction of mixed nitrate solutions in the presence of mineralizer NH4NO3 at 200 °C for 24 h. LRH exhibits the ability to undergo intercalation and anion exchange with DS− (C12H25OSO3−) via hydrothermal treatment. Compared with traditional anion exchange at room temperature, hydrothermal processing not only shortens the anion exchange time from 720 to 24 h but also increases the basal spacing. The arrangements of DS− in the interlayer of LRH are significantly affected by the DS− concentration and reaction temperature, and the basal spacing of the LRH-DS sample in the crystal edge is assumed to be larger than that in the crystal center. A higher DS− concentration and reaction temperature both induce more intercalation of DS− anions into the interlayer gallery, thus yielding a larger basal spacing. Unilamellar nanosheets with a lateral size of ≽60 μm and a thickness of ∼ 1.6 nm can be obtained by delaminating LRH-DS in formamide. The resultant unilamellar nanosheets are single crystalline. Transparent (Y0.95Eu0.05)2O3 phosphor films with a uniform [111] orientation and a layer thickness of ∼ 90 nm were constructed with the nanosheets as building blocks via spin-coating, followed by proper annealing. The oriented oxide film exhibits a strong red emission at 614 nm (the 5D0–7F2 transition of Eu3+), whose intensity is ∼ 2 times that of the powder form owing to the significant exposure of the (222) facets.

Recent progress in layered rare-earth hydroxide (LRH) and its application in luminescence

“…Unilamellar nanosheets with the large lateral size of more than 60 μm and a thickness of only ~1.6 nm (single layer) have then been obtained by delaminating the DS  intercalated LRH in formamide (Figs. 5(b)-5(e)) [25]. The interlayer spacing of LRH is significantly affected by the size of the intercalated anions, and a more weakened interlayer interaction by inserting bigger anions is benificial to exfoliation.…”

“…The projection in the [001] direction for the 251-LRH crystal and in the [111] direction for the derived cubic oxide (Ln 2 O 3 ) present close similarities in terms of the atomic configuration of Ln, and thus the phase transformation from LRH to Ln 2 O 3 occurs quasi-topotactically [19]. In view of these, a number of studies have been focused on the construction of oriented fluorescent films, and the transparent oxide phosphor films of the Y/Eu [19,25], Y/Tb/Eu [27], Y/Gd/Eu [21], and Y/Gd/Dy [29] systems with a uniform [111] orientation, a layer thickness of ~90 nm, and a high transmittance of ≥ 80% have been constructed with the exfoliated and the as-formed nanosheets as building blocks via spin-coating and self-assembly, followed by proper annealing (Fig. 7).…”

“…It is known that layered materials may swell and delaminate into nanosheets in water or an organic solvent under certain conditions. Similar to the procedures applied for LDHs, the 251-LRH crystals can be exfoliated into single-layer nanosheets of ~1.6 nm thick in formamide after exchanging the interlayer anions with dodecylsulfonate (C 12 H 25 OSO 3  , DS  ) [25]. The resultant nanosheets are, however, frequently limited to ~500 nm in lateral size due to the small size of the starting crystals (up to ~5 μm) and cracking of the nanosheets under lengthy (several days) mechanical agitation for exfoliation [26].…”

“…We recently proposed hydrothermal exchange of the interlayer NO 3  for so large LRH crystals, and through kinetics study a "wriggle intercalation" model was proposed for the DS  anions (Fig. 5(a)) [25]. Compared with the traditional anion exchange at room temperature, hydrothermal processing not only shortened the exchange time from 720 to 24 h but also significantly enlarged the basal spacing [25].…”

“…5(a)) [25]. Compared with the traditional anion exchange at room temperature, hydrothermal processing not only shortened the exchange time from 720 to 24 h but also significantly enlarged the basal spacing [25]. Unilamellar nanosheets with the large lateral size of more than 60 μm and a thickness of only ~1.6 nm (single layer) have then been obtained by delaminating the DS  intercalated LRH in formamide (Figs.…”