A study on upconversion UV–vis–NIR responsive photocatalytic activity and mechanisms of hexagonal phase NaYF4:Yb3+,Tm3+@TiO2 core–shell structured photocatalyst
Graphical abstract
Introduction
In recently years, environment contamination, such as water and air pollution, has become a very serious problem all over the world. Photocatalysis is known as a very promising method in solving the problems for it is an environmentally friendly technique [1], [2], [3]. TiO2, as one of the most investigated semiconductor photocatalysts, has been widely studied owing to its low cost, high efficiency, non-toxicity, and environment stability since 1972 [4], [5], [6]. However, the wide band gap of TiO2 restricts its application significantly for it can only be activated by the UV light which only occupies ca. 5% of the solar energy. The visible (Vis) light and near-infrared (NIR) light which possess ca. 48% and ca. 45%, respectively, of the solar energy cannot be used for photocatalysis [7], [8]. Therefore, making sufficient use of the photons with lower energy than the bandgap energy is very essential for the large scale application of TiO2 to solve environment problems in the future.
To date, much effort has been devoted to enhance the light absorption ability of TiO2 by extending its absorption region to the Vis and even NIR range. Several strategies, such as anionic and cationic doping, noble metals deposition, graphene/carbon nanotubes modification, and coupling with other semiconductors, have been used to reach this goal [9], [10], [11], [12], [13], [14]. It is true that the absorption range of TiO2 can be adjusted to the Vis and even NIR region with the help of these methods. However, some problems still existed for the introduced materials may increase the photogenerated electron–hole pair recombination in certain degree [15]. On the other hand, the photocatalytic activity of TiO2 is also affected by the intensity of the irradiated light. Generally, the stronger intensity of the irradiated light, the higher photocatalytic efficiency will be acquired. Based on the already achieved results, converting the lower energy photons to higher energetic ones which can be absorbed by TiO2 is an effective way to further enhance the photocatalytic activity [16], [17], [18], [19], [20].
With the development of materials science, upconversion materials, such as rare earth doped YF3 and NaYF4, have been discovered and studied widely [21], [22], [23]. Compared to YF3, rare earth doped NaYF4 is a more effective upconversion material which can emit bright light, such as green, blue, etc., under NIR light excitation. It is reasonable that the emitted bright fluorescence by rare earth doped NaYF4 can be absorbed by TiO2 for efficient photocatalysis reactions. Ren et al. prepared a composite composed of P25, YF3:Yb3+,Tm3+, and graphene for waste water purification [18]. However, the upconversion ability of YF3:Yb3+,Tm3+ is low and P25 can only absorb the UV light. Xu et al. reported the preparation of N-TiO2/NaYF4:Yb3+,Tm3+ nanocomposite for near infrared-triggered drug release [19]. Tang et al. reported the preparation of NaYF4:Yb3+,Tm3+@TiO2 nanoparticles for methyl blue degradation [8]. However, the NaYF4:Yb3+,Tm3+ reported is nanosized with the cubic phase which has relatively low upconversion ability compared to microsized hexagonal phase NaYF4. On the other hand, the TiO2 and NaYF4 are physically mixed together which cannot transfer the upconverted light to TiO2 sufficiently. More importantly, the reported TiO2 is dominated by the low-reactive {1 0 1} facets and can only absorb the converted light before 500 nm.
In this work, for the first time, we reported the synthesis of core–shell structured photocatalysts composed of microsized hexagonal phase NaYF4:Yb3+,Tm3+ and UV–vis–NIR driven TiO2 nanosheets dominated by the high-reactive {0 0 1} facets. As is well known, the {0 0 1} facets of anatase TiO2 is more reactive than the more thermodynamic stable {1 0 1} facets. More importantly, the microsized hexagonal phase NaYF4:Yb3+,Tm3+ can convert NIR light efficiently to high-energetic UV and blue light which can be totally absorbed by the UV–vis–NIR driven TiO2 nanosheets. A series of control experiments in the degradation of RhB and phenol under the 980 nm laser and simulated sunlight irradiation proves the superiority of the NaYF4@TiO2 core–shell structure than their physical mixture and pure TiO2. The detection of high reactive OH revealed the actual origin of the photocatalytic reaction by the NIR light.
Section snippets
Preparation of TiO2 nanosheets with exposed {0 0 1} facets
The UV–vis–NIR driven TiO2 nanosheets dominated by the {0 0 1} facets were prepared according to the methods reported by our groups before [24]. Typically, 25 ml Ti(OC4H9)4 was added slowly to 15 ml HF aqueous solution with a volume fraction of ca. 16% under continuous stirring at room temperature. The resulted suspension was transferred to a dry Teflon-lined autoclave with a capacity of 60 ml and treated at 240 °C for 24 h. The resulted blue product was washed and collected by centrifugation.
Phase and morphological characterization
The morphology and size of as-synthesized materials were investigated by FESEM and TEM. Fig. 1a displays a typical FESEM image of pure TiO2 nanosheets which preserve an average particle size of ca. 40 nm, and the outline of TiO2 is sheet and square shaped. According to the symmetries of anatase TiO2 and our previous studies, the two square facets can be ascribed to the {0 0 1} facets and the rest eight isosceles trapezoidal surfaces are the {1 0 1} facets [28], [29], [30], [31]. The representative
Conclusions
Upconversion luminescence hexagonal phase NaYF4:Yb3+,Tm3+ microrods were prepared and UV–vis–NIR responsive anatase TiO2 nanosheets with exposed {0 0 1} facets were closely attached to the NaYF4:Yb3+,Tm3+ surface to form a new core–shell structured photocatalyst by chemical modification. The degradation of RhB and phenol with NaYF4@TiO2 upon the 980 nm laser and simulated sunlight irradiation demonstrated the NIR-driven photocatalytic ability. Compared to the physical mixture of NaYF4 and TiO2,
Supporting information
SEM image of the physical mixture of NaYF4 and TiO2 is shown in Fig. S1.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 20901040/B0111), the Key University Science Research Project of Jiangsu Province (No. 10KJA430016), Innovation Foundation for Graduate Students of Jiangsu Province China (CXLX11_0346), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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2022, Journal of Environmental Sciences (China)Citation Excerpt :So far, the key problem of UC photocatalytic materials is stills how to obtain higher UC efficiency based on more optimized composition and structures. At present, the studies of NIR-driven UC photocatalysts are based on the lanthanides doped NaYF4 on account of their high UC efficiency, such as, NaYF4:Yb,Tm@TiO2 (Wang et al., 2014), NaYF4:Yb/Gd/Tm@Bi2WO6 (Anwer and Park, 2019), NaYF4:Yb/Tm-MOFs (Li et al., 2018b), NaYbF4:Tm-BiVO4 (Ullah et al., 2019), which showed excellent photocatalytic performance for the degradation of RhB or Methylene Blue (MB) under NIR light irradiation. Although the size of the synthesized NaYF4 can be synthesized nanocrystals in the UC luminescence field (Chen et al., 2014; Mishra et al., 2019), most of the above composite photocatalytic materials are always at the micron level in, which limits their applications.