Impact of pH and urea content on size and luminescence of upconverting Y2O3:Yb, Er nanophosphors

Highlights: • The granula size, morphology and upconversion luminescence of Y₂O₃:Yb³⁺, Er³⁺ nanophosphors was tuned via altering the pH values and urea contents. • Steady state and transient spectroscopy along with FTIR analysis are performed to clarify the insight photophysical mechanism. • Nonradiative transition probability modulation due to the OH⁻ group variation could be responsible for improved R/G intensity ratio. - Abstract: The pH value and urea concentration of precursor dramatically impact on the particle size, morphology and upconversion luminescence of Y₂O₃:Yb³⁺ , Er³⁺ nanophosphors. The lower pH leads to the smaller granula size of the final product, for instance, the crystalline size reduces from ∼30 nm to ∼16 nm as pH value varies from 9.3 to 6.7. The intensity of green (2H11/2, 4S3/2 → 4I15/2) and red emission (4F9/2 → 4I15/2) can be effectively and readily tuned by adjusting the pH value and urea content of precursor solution. The sample derived at pH of 6.7 and urea content of 384 mg/mL exhibits the highest red to green ratio, as a result of more surface defects and hydroxyl residues. Besides, the upconversion lifetime investigation further proved that the quenching and nonradiative relaxation from 2H11/2 and 4S3/2 to 4F9/2 caused by the surface defects could be responsible for the relative enhancement of the red emission.