A study on upconversion UV–vis–NIR responsive photocatalytic activity and mechanisms of hexagonal phase NaYF₄:Yb³⁺,Tm³⁺@TiO₂ core–shell structured photocatalyst

Highlights

• Upconversion material (NaYF₄:Yb,Tm) is used for photocatalysis.

• UV–vis–NIR driven high-reactive TiO₂ can absorb the upconverted light efficiently.

• Core–shell structure is very important for efficient energy transfer.

• Photocatalytic activity of TiO₂ is enhanced by using the NIR light.

Abstract

A new core–shell structured composite consisting of upconversion hexagonal phase NaYF₄:Yb³⁺,Tm³⁺ (simply named NaYF₄) microrods and UV–vis–NIR driven anatase TiO₂ nanosheets with exposed high-reactive {0 0 1} facets has been prepared and shown to be an advanced NIR and sunlight activated photocatalyst. To understand the nature of NIR-driven photocatalysis of NaYF₄@TiO₂, various analysis methods are conducted. Structure analysis proved that TiO₂ is closely attached on the surface of NaYF₄ and can absorb all the converted NIR light (980 nm laser) for photocatalysis. As a result, the new photocatalyst gives higher photocatalytic activity in decomposing phenol and Rhodamine B (RhB) than their physical mixture and pure TiO₂ under the NIR and simulated sunlight irradiation. High-reactive hydroxyl radicals (OH) analysis confirmed the superiority of the core–shell structure and the significant role of the upconversion material in using the NIR light to improve the photocatalytic activity of as-prepared TiO₂. Finally, a mechanism for NIR driven photocatalysis is proposed which will help to improve the structure design and functionality of new type of photocatalysts.

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.