Preparation of NaYF4: Gd3+, Yb3+, Tm3+ @ TiO2 and NaYF4: Gd3+, Yb3+, Tm3+ @ TiO2 @ Au nanocomposites and their upconversion and photocatalytic properties under simulated solar light with or without an UV filter

The hexagonal and well-dispersed NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ nanorods with crown-like end were prepared by hydrothermal method. Then TiO 2 and Au were coated on the surface of β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ and β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ @ TiO 2 nanocrystals to form the β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ @ TiO 2 and β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ @ TiO 2 @ Au nanocomposites by two- and three-step synthesis methods respectively. XRD, FESEM and HRTEM results show the presence of hexagonal NaYF 4 , anatase TiO 2 and cubic Au, indicating the formation of core-shell nanostructures of NaYF 4 @ TiO 2 or NaYF 4 @ TiO 2 @ Au. EDS and XPS analyses show the elements of Na, Y, F, Yd, Tm, Gd, O, Ti, and/or Au. Meanwhile, XPS also shows the formation of TiO 2 on β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ nanocrystal and Au 0 on β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ @ TiO 2 nanocomposite respectively. Gd 3+ ion in the NaYF 4 nanocrystal does not affect the upconversion emission peaks and their positions significantly, but influences the photon processes which populate the 1 I 6 , 1 D 2 , 1 G 4 and 3 F 3 emitting levels. In addition, NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ exhibits the strong ultraviolet upconversion emission under the laser excitation wavelength of 698 nm. The β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ @ TiO 2 nanocomposite decomposes 76% of MB, and 77% of RhB within 4.5 h respectively. The upconversion-driven photodegradation rates of β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ @ TiO 2 nanocomposite under infrared and visible light irradiation decompose 42% of MB and 41% of RhB within 4.5 h, and 52% of MB and 42% of RhB within 5 h, respectively, which are more effective than its ultraviolet light-driven photocatalytic activity. Comparing to TiO 2 , the photoactivity of the β-NaYF 4 : Gd 3+ , Yb 3+ , Tm 3+ @ TiO 2 @ Au nanocomposite for MB is enhanced by the irradiation of UV light within 2.5 h.