Visible light driven photocatalysis in chromate(VI)/TiO_2 systems : improving stability of the photocatalyst

Titanium dioxide photosensitization to visible light can be observed in the presence of chromate(VI) ions adsorbed at the surface of TiO 2 . The resulting materials do not show, however, sufficient stability and photostability—chromium(VI) species desorb from the surface and can be reduced to chromium(III). The stability of such systems has been improved by TiO 2 impregnation with fluorochromate(VI), K[CrO 3 F], instead of K 2 Cr 2 O 7 , yielding yellow F-Cr VI @TiO 2 material. Fluorides playing the role of anchors enable stronger adsorption of chromates at titanium dioxide. Also composites of water insoluble chromates(VI) with TiO 2 release significantly lower amounts of Cr VI ions. Such materials have been synthesized accord- ing to two main strategies—the first one involves precipitation of water insoluble chromates in the presence of TiO 2 particles (MCrO 4 @TiO 2 ) while the second strategy comprises of TiO 2 synthesis in the presence of insoluble chromate particles (TiO 2 @MCrO 4 ). Visible light induced photocatalytic activity of synthesized materials was studied in the process of 4-chlorophenol (4-CP) oxidation. A considerable pho- tocatalytic activity was observed in the case of F-Cr VI @TiO 2 and TiO 2 @MCrO 4 type materials (M n + :Ba 2+ , Co 2+ ,Mn 2+ ,Cr 3+ ). Among the composites the highest photoreactivity was shown by TiO 2 @CoCrO 4 (1.7% Cr VI , mol/mol, calcined at 450 ◦ C) and TiO 2 @Cr 2 (CrO 4 ) 3 (5% Cr VI , mol/mol, calcined at 450 ◦ C)

Enhanced UV light emission by core-shell upconverting particles powering up TiO2TiO_2 photocatalysis in near-infrared light

The core-shell NaYb0.99F4:Tm0.01@NaYF4 upconverting particles (UCPs) with a high UV emission to apply in NIR-driven photocatalysis were synthesized. The influence of the Yb3+ doping concentration in NaYxF4:Yb0.99−xTm0.01 core particles, and the role of the NaYF4 shell on the upconversion emission intensity of the UCPs were studied. The absorption of NIR light by the obtained UCPs was maximized by increasing the Yb3+ concentration in the core, reaching the maximum for Y3+-free particles (NaYb0.99F4:Tm0.01). Additionally, covering the NaYb0.99F4:Tm0.01 core with a protective layer of NaYF4 minimized the surface luminescence quenching, which significantly improved the efficiency of upconversion emission. The high intensity of the UV light emitted by the NaYb0.99F4:Tm0.01@NaYF4 under NIR irradiation resulted in a high photocatalytic activity of TiO2 (P25) mixed with the synthesized material