The role of 2-ethylhexanoic acid in manipulating the morphology and upconversion of flame-made Y2O3:Yb3+/Ho3+ nanoparticles towards remote temperature sensing

Abstract

Flame spray pyrolysis (FSP) is a potential strategy to scale-up the fabrication of upconversion nanoparticles (UCNPs) from the laboratory to industrial-scale production. The traditional FSP method involves the addition of 2-ethylhexanoic acid (2-EHA) to the precursors to obtain pure homogeneous ultrafine nanoparticles. However, there is still a lack of investigation into how the physical and optical properties are affected by 2-EHA. Here, for the first time, we report the role of 2-EHA in manipulating the morphology, size, and upconversion (UC) properties of Yb³⁺/Ho³⁺ co-doped Y₂O₃ UCNPs. The Y₂O₃:Yb³⁺/Ho³⁺ UCNPs were synthesized by adjusting the molar ratios of 2-EHA to trivalent rare earth ions (RE³⁺) in the precursor solution via the FSP method. The influence on the morphology and upconversion luminescence (UCL) properties of the UCNPs by 2-EHA was studied in detail. As the ratio of 2-EHA to RE³⁺ (2-EHA/RE³⁺) in the precursors increases from 0 to 3, the synthesized UCNPs initially form a mixture of large hollow particles and homogeneous ultrafine nanoparticles at low ratios and then uniform ultrafine nanoparticles at relatively high ratios. The typical green (550 nm) and red (667 nm) UCL reduce as the 2-EHA/RE³⁺ ratio increases. Moreover, the effect of 2-EHA on the temperature sensing behavior of the Y₂O₃:Yb³⁺/Ho³⁺ UCNPs was evaluated via the fluorescence intensity ratio (FIR) method based on the non-thermally coupled ⁵F₄/⁵S₂ and ⁵F₅ levels of Ho³⁺ ions. The highest absolute sensitivity (S_A) and relative sensitivity (S_R) of 6.259% and 0.483% K⁻¹ at 323 K, respectively, were determined and the sensitivity dependence on the 2-EHA/RE³⁺ ratio of the UCNPs was comparatively estimated.