Tuning of BixOyCl formation with sonication time during ultrasound-hydrothermal preparation

https://doi.org/10.1016/j.jiec.2020.01.014Get rights and content

Highlights

  • A series of BixOyCl was prepared by an ultrasonic hydrothermal method.

  • Ultrasonic time has a great effect on structure and morphology of BixOyCl.

  • Ultrasonic time also effect on photoelectric and catalytic performance of BixOyCl.

  • U-30 shows the best photocatalytic performance for RhB/CIP of BixOyCl.

  • Suitable oxygen vacancy concentration favors catalytic activity of the photocatalyst.

Abstract

Developing novel and visible-light drive photocatalysts is a hot topic, and bismuth oxychloride is one of the popular photocatalysts. In this paper, a series of bismuth oxychloride were synthesized by ultrasonic-hydrothermal method with different ultrasonic time. By simply changing the ultrasonic time, we can control the morphology, structure, stoichiometry and photoelectric performance of bismuth oxychloride. The possible mechanism for the formation of BixOyCl photocatalysts involved the ultrasonic destruction of chemical bonds and increase of oxygen vacancy concentration, and the influence of ultrasonic on the pH during preparation. The sample of ultrasonic 30 min (U-30) showed the best photocatalytic activity due to the suitable morphology, structure, photoelectric performance, the formation of the heterostructure, and the presence of oxygen vacancies. For the removal of Rhodamine B (/Ciprofloxacin), the reaction rate constant for U-30 was 10.4 (/3.6), 4.9 (/1.6), 6.9 (/4.8), and 9.3 (/2.3) times that of the U-0, U-10, U-20, and U-40, respectively. Photoluminescence spectra showed that the photogenerated electron-hole recombination rate decreased gradually with the ultrasonic time. Further, the possible mechanism of photocatalytic process was investigated. This research provides a green, economical and simple method to modify bismuth oxychloride.

Introduction

With the increase of industrialization and civilization, water pollution has become a difficult challenge facing mankind [1]. Rhodamine B, a conventional contaminant, and Ciprofloxacin, an emerging contaminant, represent the two types of refractory contaminants in water. Their potential accumulation in aquatic organisms and agricultural products (agricultural irrigation) can cause environmental risks and endanger human health [2], [3]. Therefore, the removal of these contaminants from water has a crucial impact on environmental protection.

Photocatalysis has shown great application prospects in the area of environmental purification as an environmentally friendly and low-energy technology [4], [5]. Traditional photocatalysts such as ZnO, TiO2, SnO2 and CdS [6], [7], [8], [9] exhibit the advantages of high photosensitivity and low-cost [10], but have low utilization rate of sunlight, high recombination rate of photogenerated carriers, and are hard to use in practical applications [11]. Therefore, developing novel and visible light drive photocatalysts is necessary.

Recently, bismuth oxychloride-based photocatalysts such as BiOCl, Bi3O4Cl, Bi12O17Cl2, and Bi24O31Cl10 [12], [13], [14], [15] exhibited great potentials in photocatalytic field owing to their good photocatalytic activity and stability, unique layered structure, and excellent photoelectric properties [16]. The structure, morphology, oxygen vacancy, stoichiometric ratio and heterogeneous structure of bismuth oxychloride had important effects on their photocatalytic properties [17], [18], [19], [20], [21]. Exposing more active crystal planes on the surface of photocatalysts by regulating morphology and structure could improve the photocatalyst activity significantly [22]. Appropriate oxygen vacancy concentration could make the photogenerated electron hole pairs separated more facilitated, improving the photocatalytic performance [20]. The construction of semiconductor heterojunction could form a special electrostatic field at the interface of the semiconductor link, which was helpful for the electron–hole pairs separation, and enhanced the catalytic activity and stability of the photocatalyst [23]. So, to obtain better photocatalytic activity and stability, it is necessary to modify the structure, morphology, oxygen defect and heterojunction of bismuth oxychloride-based photocatalysts.

Recently, researchers have modified bismuth oxychloride by various methods. Liu et al. controlled the crystal plane growth and the oxygen vacancy concentration of Bi12O17Cl2 by adding surfactant in solvent thermal method, and obtained a highly efficient photocatalyst [24]. Chakraborty and Kebede constructed a novel WO3-Bi3O4Cl heterojunction in three steps, and the photocatalyst had high photocatalytic activity under visible light [25]. Zheng et al. combined hydro-thermo and calcination to prepare a novel WO3-Bi12O17Cl2 heterojunction material to degrade dye and pharmaceutical, which exhibited high photocatalytic activity under visible light [26]. These modification methods effectively improved the photocatalytic activity of bismuth oxychloride [27], [28], [29]. However, they are complex, expensive, or had secondary pollution. Therefore, it is necessary to modify the bismuth oxychloride photocatalyst in a green, economical and simple method. The introduction of sonication is likely to be achieved this purpose.

Ultrasonic waves can enhance the thermal and mass transfer through cavitation that involves the bubbles’ formation, growth and rupture in the liquid media [30]. Cavitation bubbles release a huge amount of energy during collapse, resulting in local high temperature and pressure environment [31]. That will accelerate the diffusion process of reactants and products [32], promote new solid phase formation [33], affect the size and distribution of particles [34], break chemical bonds [35], cause surface defects of the material, and increase the oxygen vacancy [36]. The tremendous energy produced by the ultrasonic cavitation bubble rupture may also break the chemical bonds of the reaction material to form a new phase or heterojunction structure [38]. Thus, the introduction of sonication will achieve a good effect of modifying bismuth oxychloride, avoiding the secondary pollution problem and making the operation easy. Ultrasonic time is the key factor for all ultrasonic processes. It has important influences on the crystallization, decomposition and recrystallization of the synthesized photocatalyst [37]. The morphology, structure and oxygen vacancy concentration of the photocatalyst may change with the ultrasonic time [35].

In this work, a series of BixOyCl were synthesized by ultrasonic hydrothermal method. Through simply changing the ultrasonic time, we obtained bismuth oxychloride with different morphology, compositions and photoelectric properties. Effects of ultrasonic time on the morphology, structure, composition, photoelectric and photocatalytic activity of the photocatalysts were studied. The potential mechanism of ultrasonic time for tuning bismuth oxychloride was revealed. The findings of this work can provide a green, economical and simple method to modify bismuth oxychloride, and probably a new strategy for tuning the nonstoichiometric ratio of the identical element compound materials such as BiOX (X = I, Br), Mn/CoFe3-xO4, FeCeOx [30], [39], [40].

Section snippets

Materials

Sodium oxalate, benzoquinone and bismuth nitrate pentahydrate were from Xi Ya Chemical Industry Co., Ltd., China. Ethanol and potassium chloride were from Sinopharm Chemical Reagent Co., Ltd., China. Ethanol, tert-butyl alcohol and sodium hydroxide were from Beijing Chemical Works. Rhodamine B and Ciprofloxacin were purchased from Macklin Biochemical Co., Ltd., China. All materials were used without further purification.

Photocatalysts preparation

Typically, 0.298 g of powder KCl was placed in a beaker (200 ml) with 60 ml

XRD analysis

The crystal structure and phase purity of the prepared photocatalysts were first examined using XRD. As shown in Fig. 1, U-0 was well corresponding to Bi12O17Cl2 (ICDD PDF 37-0702), without any additional diffraction peaks; U-(10–30) were corresponding to Bi12O17Cl2 (ICDD PDF 37-0702) and BiOCl (ICDD PDF 06-0249); U-40 was well corresponding to BiOCl (ICDD PDF 06-0249), without any additional diffraction peaks. These results showed that ultrasonic time had a great effect on the phase

Conclusions

In summary, a series of BixOyCl were synthesized by ultrasonic hydrothermal method under different ultrasonic time. The structure, morphology, stoichiometry and photoelectric performance of the products could be tuned by the ultrasonic time. The potential mechanism for the formation of BixOyCl photocatalysts was that ultrasonic cavitation destroyed chemical bonds and increased oxygen vacancy concentration; the effect of ultrasonic on the pH of the preparation reaction solution also contributed

Conflict of interest

The submitted work was not carried out in the presence of any personal, professional or financial relationships that could potentially be construed as a conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51778374) and Shenzhen Technology Innovation Committee (KJYY20180206180737010).

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