Blue/green/red colour emitting up-conversion phosphors coupled C-TiO₂ composites with UV, visible and NIR responsive photocatalytic performance

Highlights

• Blue/green/red colours emitting up-conversion phosphor coupled C-TiO₂ were prepared.

• Weak UV, visible LEDs and 980 nm infrared laser were used as exciting sources.

• RhB solution and continuous NO gas were utilized as photocatalytic targets.

• Outstanding photocatalytic ability was presented under different light sources.

• The photonic efficiency of photocatalytic performance was calculated.

Abstract

The composites of C-TiO₂ coupled with blue, green and red of three colours emitting up-conversion phosphors have been prepared by a simple calcination assisted solvothermal method. The composites of the narrow band gap of C-TiO₂ and the unique optical property of up-conversion phosphors enabled to absorb UV, visible and NIR lights simultaneously to proceed photocatalytic reactions effectively. The photocatalytic activities of the composites were evaluated not only for the degradation of RhB solution under NIR light irradiation but also for the destruction of continuous NO gas under the irradiation of UV, visible LEDs and infrared laser. According to the photocatalytic results, the calculated photonic efficiency of infrared light induced photocatalytic activity of samples was much higher than those of UV and visible light responsive ones. Furthermore, the green light emitting up-conversion phosphor coupled C-TiO₂ composite presented the superior photocatalytic performance over blue and red ones.

Introduction

In recent years, air and water contamination has become more and more serious all over the world. Photocatalysis as a green chemistry method for pollution treatment has drawn lots of attention [1], [2], [3], [4]. Although a great amount of novel compounds with various photocatalytic performances have been developed until now [5], [6], [7], TiO₂ is still the most promising photocatalyst due to its chemical stability, environmentally friendly, high oxidative power and low cost [8], [9], [10]. However, it can only be activated by UV light because of the large band gap of ca. 3.2 eV. It is well acknowledged that the UV light just accounts for 5% of solar light, while the residual 45% and 50% of solar light are made up of visible light and near-infrared (NIR) light, respectively [11], [12]. In order to extend the absorption ability of TiO₂ to the visible-light region, many strategies, such as dye sensitizing [13], [14], heterostructure [15], [16], ion doping [17], [18], [19], etc. have been developed. As for NIR light induced photocatalytic utilization, two dominant methods have been proposed. The one way is the doping of TiO₂ with lanthanide ion, which can emit UV or visible light by directly absorbing NIR by the multi-step excitation mechanism. Obregón et al. [20] reported an erbium doped TiO₂ particles prepared by a surfactant free hydrothermal method. The Er-TiO₂ presented nice photoactivities for the degradation of phenol under sun-like excitation, owing to the improved NIR light absorption and NIR–UV up-conversion luminescence contribution induced by Er doping. However, the self-induced up-conversion luminescence by TiO₂ substrate is very poor and not efficient. In this case, the NIR light cannot be effectively used for photocatalysis. The other method is the formation of composite between the high performance up-conversion phosphors (lanthanide-doped NaYF₄, YF₃, Y₂O₃, YOF, etc.) and photocatalyst substrates. For this type of composite, the NIR light cannot be directly utilized for photocatalysis but is first absorbed by the up-conversion phosphor to emit some UV or visible lights, and then the emitted UV or visible lights are reabsorbed by the coexisted photocatalyst to proceed photocatalytic reactions. In order to effectively utilize the emitted lights from up-conversion phosphor for the photocatalysis, one of the key points is the nice overlap of the NIR light excited emission spectrum of the up-conversion phosphor with the absorption spectrum of the photocatalyst. Qin group first studied NIR-responsive photocatalysis of TiO₂ by combining it with YF₃:Yb, Tm or NaYF₄:Yb, Tm [21], [22]. It was reported that the emitted UV light from YF₃:Yb, Tm or NaYF₄:Yb, Tm could be effectively used by TiO₂ for photocatalytic reaction under the irradiation of NIR light. There were also some other reports on NIR light induced composites [23], [24], [25]. Recently, our group [26] also prepared a NaYF₄:Yb, Er/C-TiO₂ composite, which was able to be excited by UV, visible and NIR lights effectively, producing corresponding nice photocatalytic activity due to the excellent absorption capability of C-TiO₂ and outstanding optical property of NaYF₄:Yb, Er up-conversion phosphor. As we all known, different colours emitting up-conversion phosphors can present big difference in the photoluminescence property. In this case, a systematic investigation about the different colours emitting up-conversion phosphors (blue, green, red colours) coupled photocatalyst composites would be meaningful to understand the effect of up-conversion phosphors on the photocatalytic activity of photocatalysts deeply. Nevertheless, to the best of our knowledge, there is still no research about the different colours emitting up-conversion phosphors combined photocatalyst composites.

On the other hand, in order to utilize blue, green, red colour emitting up-conversion phosphors for NIR induced composite photocatalyst, a suitable photocatalyst substrate, which have a nice photocatalytic response in the range of blue, green and red lights, is strongly required. According to our previous results [26], the C doped TiO₂ [27] (C-TiO₂) by replacing the oxygen site with C element in the TiO₂ lattice shows excellent visible light absorption ability up to 800 nm, covering the whole visible light range from blue light to red light. Furthermore, the modified or coupled C-TiO₂ presents the similar excellent photocatalytic performance in the long wavelengths range until 800 nm, indicating its higher efficiency than that of commercial P25. [26], [27] So the C doped TiO₂ would also be a promising photocatalyst substrate for various colours emitting up-conversion phosphors coupled composites.

For the purpose of practical application, a photonic efficiency of energy conversion from NIR light to the photocatalytic performance between up-conversion phosphors and photocatalyst substrate is essential. In addition, it would be better that the photocatalytic testing was carried out not only for pollutant aqueous solution degradation but also toxic gas destruction. Therefore, based on the above mentioned requirements, three types of blue, green, red colour emitting up-conversion phosphors coupled C-TiO₂ composites have been first prepared by a simple calcination assisted solvothermal method in this work, and the photocatalytic performances of them were evaluated for the destruction of NO_x gas and rhodamine B (RhB) solution with the induction of UV, visible and NIR lights, indicating their benefit for practical application in the area of air purification and water cleaning. Besides, the photocatalytic stability and the effect of the power of irradiation light on the photocatalytic activity of composites have also been investigated. More importantly, the apparent photonic efficiency about the photocatalytic ability of the composite photocatalysts under the irradiation of UV, visible and NIR lights were calculated. They are essential for understanding the effect of up-conversions and near infrared light on the photocatalytic performance of semiconductors. The proposed method for NIR light utilization along with those of UV and visible lights by coupling different colours up-conversion phosphors with high performance of C-TiO₂ or other effective substrate probably have promising potential for the dye sensitized solar cell.