High‑Efficiency Upconversion Nanophosphors: La0.97RE0.01Yb0.02O2S Derived from Layered Hydroxyl Sulfate Precursors (RE = Ho, Er)

Background

Upconversion (UC) phosphors convert long‑wavelength radiation into short‑wavelength fluorescence and are pivotal for solid‑state lasers, multicolor displays, biomedical imaging, and solar‑cell wavelength conversion. Typically, a sensitizer such as Yb³⁺ absorbs 980‑nm photons, transferring energy to activators (Ho³⁺, Er³⁺, Tm³⁺) with ladder‑like energy levels that enable sequential photon absorption. Lanthanide oxysulfides (RE₂O₂S) combine the low phonon energy (~500 cm⁻¹) of sulfides with the chemical stability of oxides, achieving UC efficiencies comparable to halides while avoiding toxic halide precursors. However, scalable synthesis routes that yield phase‑pure, nanometer‑sized phosphors remain limited. Hydroxyl sulfate layered precursors (RE₂(OH)₄SO₄·2H₂O) can be hydrothermally crystallized and subsequently reduced to RE₂O₂S in hydrogen, offering a green, single‑step conversion with minimal by‑products.

Methods

The starting salts (RE(NO₃)₃·6H₂O, (NH₄)₂SO₄, NH₃·H₂O) were mixed to achieve 2 at.% Yb³⁺ and 1 at.% Ho³⁺ or Er³⁺. After adjusting the solution to pH 9 with ammonia, the mixture was sealed in a 100‑ml Teflon autoclave and hydrothermally treated at 100 °C for 24 h. The resulting hydroxyl sulfate nanoplates were collected, washed, and dried at 70 °C. To convert to oxysulfide, the precursors were annealed in flowing H₂ (200 mL min⁻¹) at 1200 °C for 1 h, heating at 5 °C min⁻¹. Phase purity was confirmed by XRD (Rigaku RINT2200), while particle morphology was examined by FE‑SEM (Hitachi S‑5000). UC spectra were recorded at room temperature with a 978‑nm CW laser (BWT KS3–12322‑105) using a JASCO FP‑6500 fluorospectrophotometer (S/N ≥ 200).

Results and Discussion

XRD patterns of the hydrothermal products match the layered hydroxyl sulfate structure (La₂(OH)₄SO₄·2H₂O), while annealed samples crystallize into hexagonal La₂O₂S (space group P‑3m1). Lattice parameters shrink systematically with the smaller RE³⁺ ions, confirming solid‑solution formation. FE‑SEM images show nanoplates (150–550 nm × 20–30 nm) that fragment into rounded particles after calcination, yielding ~45 nm crystallites.

Under 978‑nm excitation, the Ho³⁺ sample emits strong green (~546 nm), weak red (~658 nm), and dominant NIR (~763 nm) bands, while the Er³⁺ sample shows weak green (~527/549 nm), weak red (~668/672 nm), and strong NIR (~807/858 nm) emission. Chromaticity coordinates for Ho remain stable at (0.30, 0.66), whereas Er shifts from (0.36, 0.61) to (0.32, 0.64) with increasing power, reflecting a higher green‑to‑red ratio.

Log–log plots of emission intensity versus excitation power reveal slopes of ~3 for Ho emissions, indicating a three‑photon upconversion pathway, and slopes of ~2 for Er emissions, indicating a two‑photon process. Energy‑transfer diagrams, adapted from literature, explain the sequential Yb→Ho/Er excitation steps and non‑radiative relaxations that lead to the observed emissions.

Conclusions

La_0.97RE_0.01Yb_0.02O₂S (RE = Ho, Er) upconversion nanophosphors were successfully produced by hydrogen‑induced reduction of hydrothermally grown hydroxyl sulfate nanoplates. The resulting ~45 nm particles exhibit bright UC emission under 978‑nm excitation: a three‑photon green‑NIR cascade for Ho and a two‑photon green‑red‑NIR sequence for Er. The chromaticity of Ho remains fixed, while Er shows a controllable color shift with power. These findings highlight a scalable, environmentally friendly route to high‑performance UC phosphors for optoelectronic and biomedical applications.