Preparation and characterization of Y₂O₃:Bi³⁺,Yb³⁺ nanosheets with wavelength conversion from near-ultraviolet to near-infrared

Highlights: • Y₂O₃:Bi³⁺,Yb³⁺ is a promising candidate for down-converters in solar devices. • Nanosheets were prepared by calcination of hydrothermally-synthesized precursors. • Near-infrared emission from Yb³⁺ were observed under near-ultraviolet excitation. • Precursors calcined at ≤ 800 °C retained square nanosheet morphology. • Photoluminescence properties were improved as calcination temperature increased. - Abstract: Y₂O₃:Bi³⁺,Yb³⁺ nanophosphors, which exhibit near-infrared (NIR) emission under near-ultraviolet (near-UV) excitation, are promising candidates for spectral converters in crystalline silicon solar devices. Herein, Y₂O₃:Bi³⁺,Yb³⁺ nanosheets were prepared by calcination of hydrothermally-synthesized precursors. The effects of the calcination temperature on their properties were investigated. The nanosheets calcined at ≤ 800 °C had sheet-like, square morphologies with average lateral sizes of 210–240 nm, which were smaller than precursor nanosheet with 257 nm attributed to shrinkage through the calcination. The thickness of the nanosheet before and after calcination at 800 °C was ~20 nm. Moreover, the nanosheet had a single-crystal nature. However, at calcination temperatures ≥ 900 °C, the nanosheets lost their morphologies, and transformed into irregular particles. The Y₂O₃:Bi³⁺,Yb³⁺ nanosheets showed visible emission from Bi³⁺ and NIR emission from Yb³⁺ under near-UV excitation. The PL intensity and PL decay time of the NIR emission monotonically increased with increasing calcination temperature. Both increases can be explained by the following effects: (i) removal of H₂O molecules adsorbed on the nanosheet surface, (ii) improvement of crystallinity and decrease of specific surface area, (iii) increase in probability of energy transfer from Bi³⁺ to Yb³⁺, and (iv) oxidation of reduced bismuth during calcination. Furthermore, the photostability of the nanosheets under near-UV excitation was improved by increasing calcination temperature.