Elsevier

Molecular Catalysis

Volume 435, July 2017, Pages 135-143
Molecular Catalysis

Synthesis of n-type TaON microspheres decorated by p-type Ag2O with enhanced visible light photocatalytic activity

https://doi.org/10.1016/j.mcat.2017.03.027Get rights and content

Highlights

  • Porous TaON microspheres were prepared via a simple and mild route.

  • Microspherical Ag2O/TaON pn heterojunctions were constructed.

  • High and stable photocatalytic activity for RhB/MO/4-CP degradation.

  • The pn heterojunction mainly contributes to the high activity of Ag2O/TaON.

Abstract

Constructing pn heterojunction photocatalysts with high performance has attracted great attention. Here, we report a microspherical Ag2O/TaON pn heterojunction as a novel and efficient visible-light-driven (VLD) photocatalyst. Porous TaON microspheres (diameters: 0.8–2 μm) were prepared by a solvothermal precipitation-nitridation method, and then the in situ growth of Ag2O nanoparticles (size: ∼12 nm) on their surfaces was realized by a simple deposition method. All Ag2O/TaON heterojunctions were much more active than pristine TaON or a mechanical mixture of Ag2O and TaON in the degradation of organic pollutants (cationic dye rhodamine B, anionic dye methyl orange and neutral para-chlorophenol) under visible light (λ > 400 nm). Significantly, the highest photocatalytic activity was observed in the heterojunction with a Ag/Ta molar ratio of 0.1/1 (0.1Ag–Ta). The RhB degradation rate constant using 0.1Ag–Ta is approximately 9.7 and 3.1 times higher than those obtained by using TaON microspheres, and the mechanical mixture, respectively. The superior photocatalytic performance of Ag2O/TaON is ascribed to the hierarchical superstructure and efficient charge separation. The mainly involved active species (holes and superoxide radicals) were identified via radical trapping experiments and electron paramagnetic resonance spectroscopy. The results demonstrate that Ag2O/TaON is a potential VLD photocatalyst for wastewater treatment.

Introduction

With the population growth and intensified industrialization, water contamination caused by toxic organic pollutants (such as industrial dyes and phenols) is becoming an increasing issue all over the world [1], [2], [3], [4], [5]. Photocatalysis is an attractive technology for environmental remediation since it can decompose the organic pollutants in water and air [1], [2], [3], [4], [5]. The core of photocatalysis technique is to gain access to efficient and stable photocatalysts. So far, the most famous TiO2 can only respond to ultraviolet (UV) light (less than ∼5% of the solar spectrum), resulting in poor utilization efficiency of solar light. To make full use of solar light, it is urgent to develop efficient visible-light-driven (VLD) catalysts [6].

(Oxy)nitride materials, such as Ge3N4 [7], SrNbO2N [8], LaTiO2N [9], TaON [10], [11], [12], Ta3N5 [13], [14], [15], [16], [17], [18] and GaN:ZnO [19], have stimulated great interest in photocatalysis. Among them, TaON with a proper band gap of ∼2.5 eV and good sunlight harvesting (λ  500 nm) ability, has recently emerged as a promising VLD photocatalyst or photoanode for pollutant degradation or water splitting [10], [11], [12]. However, the low quantum yield, high charge recombination rate, poor photostability and insufficient active sites severely restrain its application and urgently require the modification of TaON to overcome these drawbacks [12].

On the one hand, the proper design of nanostructure can significantly ameliorate the activity of a catalyst [20]. Especially, the hierarchical architecture of a photocatalyst plays an important role in the photocatalytic performance. Generally, the hierarchical catalysts exhibit intriguing physical/chemical properties [2], [3], [20], [21], [22], [23] due to the fact that the hierarchical structure may enhance light harvesting, offer abundant exposed reactive sites, and promote the separation of photogenerated charge carriers. TaON with various 3D nanostructures including nanorod array [24], nanotube array [25], flower-like structure [22], [26] and macroporous structure were developed and exhibited enhanced activity compared with bulk ones. However, most of them suffer from complicated preparation procedure and/or the employment of toxic HF, which limits its application. Thus, it is still challenging to develop a simple and mild route to obtain TaON with a desired 3D hierarchical nanostructure.

On the other hand, the construction of a semiconductor heterojunction can greatly improve the performance of a catalyst [1], [27], [28]. Thus, to improve the activity of TaON, several TaON-based heterojunctions have been fabricated [24], [26], [29], [30], [31], [32], [33], [34], [35], [36], [37]. Notably, existing efforts have been overwhelmingly focused on the development of TaON-based n-n heterojunctions, including PANI/TaON [29], C3N4/TaON [30], ZrO2/TaON [31], WO3/TaON [32], CdS/TaON [33], CoOx/TaON [34], Ta3N5/TaON [35] and AgCl/Ag/TaON [36]. All these heterojunctions exhibited higher photocatalytic and photoelectrochemical (PEC) activity than pure TaON. Notably, in comparison with n-n heterojunction, pn heterojunction is a more effective architecture for the efficient transfer and separation of photogenerated electron–hole pairs due to the existence of a built-in electric field [1], [27]. These advantages play a vital role in improving the photocatalytic activity. However, to the best of our knowledge, there is little work that reports on the construction of TaON-based pn heterojunctions, except for Cu2O/TaON [24] and Bi2O3/TaON [37] heterojunctions. Thus, the exploration of novel TaON-based pn heterojunctions with high performance is intriguing but still underdeveloped.

p-type Ag2O (∼1.3 eV) is an attractive VLD semiconductor for the degradation of organic pollutants due to its high efficiency, good stability and eco-friendliness [38]. Furthermore, Ag2O nanoparticles (NPs) have been employed to construct pn heterojunctions with other n-type oxide semiconductors [39], [40], [41], [42], [43], [44], [45], [46], leading to the enhancement of their photocatalytic activities. However, no Ag2O/(oxy)nitride pn heterojunctions have been developed. Inspired by these facts, the development of hierarchical Ag2O/TaON pn heterojunction with synergistic effect for high performance is appealing and highly anticipated.

Herein, we report the design and preparation of microspherical Ag2O/TaON pn heterojunction composed of porous TaON microspheres and Ag2O nanoparticles via a solvothermal precipitation-nitridation-deposition method. The photocatalytic activities of these heterojunctions were assessed by the degradation of rhodamine B (RhB), methyl orange (MO) and para-chlorophenol (4-CP) under visible light. Furthermore, the photocatalytic mechanism for the improved photocatalytic activity of Ag2O/TaON was well discussed.

Section snippets

Materials

Tantalum chloride (TaCl5), absolute ethanol (CH3CH2OH), silver nitrate (AgNO3) and sodium hydroxide (NaOH) and fructose (C6H12O6) were purchased from Sinopharm Chemical Reagent Co., Ltd (PR China). Rhodamine B (RhB), methyl orange (MO) and para-chlorophenol (4-CP) were purchased from Sigma (America).

Synthesis of TaON

Porous TaON microspheres (denoted as M-TaON) were prepared via a solvothermal precipitation-nitridation method. Typically, TaCl5 (1 mmol, 0.36 g) and fructose (0.25 mmol, 0.045 g) were ultrasonically

Preparation and characterization of catalysts

Microspherical Ag2O/TaON pn heterojunctions were prepared via a solvothermal precipitation-nitridation-deposition method, as illustrated in Fig. 1. These resulting catalysts with different Ag/Ta molar ratios (0.02/1, 0.05/1, 0.1/1 and 0.2/1) were named as 0.02Ag–Ta, 0.05Ag–Ta, 0.1Ag–Ta and 0.2Ag–Ta, respectively.

Firstly, Ta2O5 microsphere (M-Ta2O5) was synthesized via solvothermally treating the ethanol solution containing TaCl5 and fructose. SEM images reveal that M-Ta2O5 shows uniform

Conclusions

In summary, Ag2O/TaON pn heterojunction comprised of porous TaON microspheres and homogeneously distributed Ag2O NPs was prepared via a solvothermal precipitation-nitridation-deposition method. These heterojunctions showed enhanced photocatalytic activity than bare TaON or the mixture of Ag2O and TaON for the degradation of RhB/MO/4-CP. Remarkably, the Ag2O/TaON heterojunction with the Ag/Ta molar of 0.1/1 exhibits the highest activity among the as-prepared catalysts. The exceptional

Acknowledgements

This work was financially supported by the Natural Science Foundation of Zhejiang Province (LQ15E090006), the Research Startup Foundation of Zhejiang Ocean University (Q1447), and the National Natural Science Foundation of China (no. 21377023 and 51602049).

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