Enhanced near-infrared photocatalysis of NaYF4:Yb, Tm/CdS/TiO2 composites

Abstract

The previous works by our group (Chem. Commun., 2010, 46, 2304–2306; ACS Catal., 2013, 3, 405–412; Phys. Chem. Chem. Phys., 2013, 15, 14681–14688) have reported the near-infrared-driven photocatalysis of broadband semiconductor TiO₂ or ZnO that was combined with upconverting luminescence particles to form a core–shell structure. However, the photocatalytic efficiency is low for this new type of photocatalysts. In this work, NaYF₄:Yb,Tm/CdS/TiO₂ composites for NIR photocatalysis were prepared by linking CdS and TiO₂ nanocrystals on the NaYF₄:Yb,Tm microcrystal surfaces. CdS and TiO₂ were well interacted to form a heterojunction structure. The energy transfer between NaYF₄:Yb,Tm and the semiconductors CdS and TiO₂ was investigated by steady-state and dynamic fluorescence spectroscopy. The photocatalytic activities of the as-prepared composites were evaluated by the degradation of methylene blue in aqueous solution upon NIR irradiation. Significantly, it was found that the united adhesions of CdS and TiO₂ on the NaYF₄:Yb,Tm particle surfaces showed much higher catalytic activities than the individual adhesion of CdS or TiO₂ on the NaYF₄:Yb,Tm surfaces. This was attributed mainly to the effective separation of the photogenerated electron–hole pairs due to the charge transfer across the CdS–TiO₂ interface driven by the band potential difference between them. The presented composite structure of upconverting luminescence materials coupled with narrow/wide semiconductor heterojunctions provides a new model for improved NIR photocatalysis.