Photocatalytic degradation of methyl orange at different pH values by NaYF4:Yb3+,Tm3+@TiO2 microrod composite photocatalysts under NIR excitation

Abstract

In this work, NaYF₄:Yb³⁺,Tm³⁺@TiO₂ microrod composite photocatalysts were fabricated via a hydrothermal method combined with a two-step wet-chemical coating strategy. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization demonstrated that the NaYF₄:Yb³⁺,Tm³⁺ microrods (∼1.2 µm in length and ∼0.4 µm in width) were uniformly coated with a porous anatase TiO₂ shell. For the optimal sample S3, the thickness of the TiO₂ shell was approximately 10 nm, with the size of a single TiO₂ nanoparticle being about 6 nm. Under excitation with a 980 nm near-infrared (NIR) laser (spot size: 2 cm × 2 cm, total power: 28 W, and effective power density: 7 W cm⁻²), the TiO₂ shell could efficiently absorb the ultraviolet upconversion emission from the NaYF₄:Yb³⁺,Tm³⁺ core, and fluorescence kinetic measurements confirmed that fluorescence resonance energy transfer (FRET) occurs between the core and the shell. Compared with hydrothermally treated NaYF₄:Yb³⁺,Tm³⁺, the fluorescence lifetime of the Tm^{3+ 1}I₆ excited state (at 291 nm emission) in the composite decreased from 128 µs to 100 µs, with a reduction of approximately 21%; in contrast, and the lifetime of the Tm^{3+ 1}G₄ excited state (at 474 nm emission) increased from 413 µs to 456 µs, increasing by about 10%. Photocatalytic degradation experiments with methyl orange (MO, 10 mg L⁻¹, 50 mL) as the target pollutant revealed that the pH value of the solution exerted a significant influence on the degradation efficiency. Under the experimental conditions of 1 mg mL⁻¹ catalyst concentration and 70 W cm⁻² excitation power density (catalyst dosage: 50 mg, room temperature, magnetic stirring at 500 rpm, and 30 min dark adsorption equilibrium), the composite exhibited the optimal photocatalytic performance in an acidic system at pH 2, where the decolorization efficiency of MO was significantly higher than that in the neutral system (pH 6.5) and alkaline system (pH 11). Moreover, the MO decolorization rate decreased sequentially with an increase in the pH value of the solution. This study confirms that the FRET process between NaYF₄:Yb³⁺,Tm³⁺ and TiO₂ is a crucial mechanism for enhancing the NIR-driven photocatalytic activity of composite photocatalysts.