Semiconductor photocatalysis is considered as one of promising technologies for the elimination of refractory organic substances. It can use solar light to drive the chemical reaction, which further intensifies its application prospects[1–3]. In the process, powerful photogenerated holes and created hydroxyl radicals are employed to decompose toxic and harmful pollutants. Due to the particularly strong oxidation capacity of these oxidants, most persistent organic compounds can be degraded by photocatalysis. Commonly used photocatalysts, including metal oxides (TiO2 [4], WO3[5]), non-metal compounds (g-C3N4) [6] and perovskite (BiVO4 [7], Bi2WO6 [8]) etc., can only be excited by UV or visible light, leading to low utilization efficiency of solar energy.
Metal organic framworks (MOFs) are organic-inorganic hybrid materials with intramolecular pores formed by metal ion/cluster units connected with an organic bridge-linked ligand by coordination bonds. Recently, photocatalytic degradation of pollutants by MOFs attracted intense attention owing to their exceptional porosity and high tunability in addition to its semiconductor-like performance [9, 10]. The ultrahigh specific surface area of MOFs, typically ranging from 1000 to 10,000 m2/g, provides abundant sites for pollutants to adsorb. The size, geometry, and functionality of the constituents which can be flexibly changed have resulted in more than 20,000 different MOFs being studied until now [11, 12]. Among them, NH2-MIL-125 (Ti) is one of the extensive researched photocatalysts (2.4 eV of band gap) [13]. But until now, the photocatalytic ability of MOFs is not satisfied due to narrow light response and low separation efficiency of photogenerated holes and electrons. The photocatalytic ability of MOFs can be enhanced by constructing heterojunction, in which the separation efficiency of photogenerated charge carriers is enhanced by the built-in field generated at the interfaces of the components [14, 15]. Metal nanoparticles (MNP) embedding is also a popular strategy to elevate the photocatalytic abilities of MOFs. For MNPs (such as Au, Pd, Pt and Ag), the surface plasmon resonance (SPR) of MNPs can expand the light response range to visible light. Furthermore, the Schottky junction, one kind of heterojunction, can be formed to promote the elimination of pollutants [16, 17].
Embedding up-conversion nanoparticles in photocatalysts is another effective method to extend the light response [18, 19]. Up-conversion photoluminescence is considered as an Anti-Stokes photoluminescence, in which up-conversion materials absorb multi low-energy photons to emit one higher energy photon. In the composite, the near infrared (NIR) light can be converted into UV or visible light emissions which can excite the photocatalyst. Thus, the light response can be extended and the exploitation of NIR light by photocatalysis can be achieved. The rare-earth metal doped compounds are the earliestly discovered up-conversion material, which is attributed to the forbidden f-f transition and enough long life of the intermediate energy level. The up-conversion light emitted by these materials has unique optical properties such as high optical stability, long luminescence and narrowband emission. Up to now, the hexagonal phase β-NaYF4 codoped with Yb3+/Er3+ or Yb3+/Tm3+ has been recognized as the most efficient NIR-to-visible up-conversion nanoparticles [20, 21].
In this article, NaYF4: Yb, Tm (NYT), one of efficient converter of NIR to UV and visible light, was embedded in NH2-MIL-125(Ti) (NYT@NH2-MIL-125(Ti)) to boost the full-spectrum photocatalytic performance. Considering that NH2-MIL-125(Ti) shows visible light response, it is reasonable to expect that integrating NYT nanoparticles with NH2-MIL-125(Ti) will achieve a new composite photocatalyst with NIR light response. Rhodamine B (RhB) was chosen as the model pollutant to evaluate the photocatalytic ability. Moreover, the photocatalytic mechanism of NYT@NH2-MIL-125(Ti) was explored.