Synthesis, characterization and photocatalytic properties of nanostructured lanthanide doped β-NaYF4/TiO2 composite films

Abstract

The photocatalytic approach is known to be one of the most promising advanced oxidation processes for the tertiary treatment of polluted water. In this paper, β-NaYF₄/TiO₂ composite films have been synthetized through a novel sol-gel/spin-coating approach using a mixture of β-diketonate complexes of Na and Y, and Yb³⁺, Tm³⁺, Gd³⁺, Eu³⁺ as doping ions, together with the TiO₂ P25 nanoparticles. The herein pioneering approach represents an easy, straightforward and industrially appealing method for the fabrication of doped β-NaYF₄/TiO₂ composites. The effect of the doped β-NaYF₄ phase on the photocatalytic activity of TiO₂ for the degradation of methylene blue (MB) has been deeply investigated. In particular, the upconverting TiO₂/β-NaYF₄: 20%Yb, 2% Gd, x% Tm (x = 0.5 and 1%) and the downshifting TiO₂/β-NaYF₄: 10% Eu composite films have been tested on MB degradation both under UV and visible light irradiation. An improvement up to 42.4% in the degradation of MB has been observed for the TiO₂/β-NaYF₄: 10% Eu system after 240 min of UV irradiation.

Figure 1

(a) Scheme of the composite synthesis from the sol–gel formation to the film deposition. (b) XRD patterns of undoped TiO₂/β-NaYF₄ composite and doped with different amounts of doping ions Yb³⁺, Gd³⁺ and Tm³⁺, and Eu³⁺ [deposition temperature: 400 °C; substrate: glass for sample in red and green, and Si for sample in blue].

Figure 2

FE-SEM images of: TiO₂/β-NaYF₄ deposited at 400 °C on Si (100), (a) plan view and (b) cross section; TiO₂/β-NaYF₄: 10% Eu (d). TiO₂/β-NaYF₄: 20% Yb, 2% Gd, 1% Tm deposited at 400 °C on Si (100) (c) plan view and EDX elementary maps (e-l) of Yb, Y, Na, F and Ti elements.

Figure 3

Photo-degradation of an aqueous solution of MB 1.8 × 10^–5 M in the presence of sample 3-UC, namely TiO₂/NaYF₄: Yb³⁺(20%), Gd³⁺(2%), Tm³⁺(0.5%) under visible light irradiation: (a) variation of the UV–Vis absorption spectra of the MB solution in time during the photodegradation (the first spectrum represents the spectrum acquired before the 30 min of dark adsorption and the zero time is exactly at 30 min after dark adsorption); (b) Variation of the concentration of MB in time during the photodegradation.

Figure 4

Photodegradation of an aqueous solution of MB 1.8 × 10^–5 M: (a) variation of the UV–Vis absorption spectra of MB under UV irradiation in the presence of the photocatalyst 4-DS; (b) variation of the concentration of MB in time through photolysis (blue line), photocatalysis with 1-blank (black line) and 4-DS (red line) samples; (c) recycling experiments: variation of the concentration of three solutions of MB (1.8 × 10^–5 M) versus time under UV irradiation in the presence of the same 4-DS sample reused for three times.

Figure 5

Upconversion spectra for the TiO₂/β-NaYF₄: 20% Yb, 2% Gd, 1% Tm (2-UC, black line) and TiO₂/β-NaYF₄: 20% Yb, 2% Gd, 0.5% Tm (3-UC, red line) nanocomposites under 980 nm excitation.

Figure 6

(a) Luminescence excitation spectra (λ_em = 614 nm) and (b) emission spectra (λ_exc = 532 nm) for the TiO₂/NaYF₄: 10% Eu (4-DS sample) nanocomposites: as-prepared (black line); after one photocatalytic cycle in the presence of methylene blue (blue line); after three photocatalytic cycles in the presence of methylene blue (red line). Emission decay curves of the band at 613 nm (c) for the TiO₂/β-NaYF₄: 10% Eu³⁺ nanocomposites: as prepared sample (A); after one photocatalytic cycle (B); after three photocatalytic cycles (C), in the presence of methylene blue. The corresponding fits, using Eq. 2, are indicated by red lines.