Elsevier

Applied Surface Science

Volume 480, 30 June 2019, Pages 395-403
Applied Surface Science

Full length article
Preparation and photocatalytic performance of MWCNTs/BiOCl: Evidence for the superoxide radical participation in the degradation mechanism of phenol

https://doi.org/10.1016/j.apsusc.2019.02.195Get rights and content

Highlights

  • MWCNTs were employed to modify BiOCl.

  • A strong chemical interaction exists between MWCNTs and BiOCl.

  • O2radical dot radicals were formed in the MWCNTs/BiOCl composite system.

  • Photocatalytic activity has been markedly improved.

Abstract

Surface modification is deemed as a reliable strategy to regulate the photocatalytic properties of a bare photocatalyst. In this work, multiwall carbon nanotubes (MWCNTs) decorated BiOCl composites were successfully in-situ fabricated via a hydrothermal routine and studied by a series of methods. The results demonstrate that a strong chemical interaction forms between MWCNTs and BiOCl, supported by data from XRD, Raman and XPS experiments. The photocatalytic activities of the MWCNTs/BiOCl materials were tested under UV light irradiation using phenol as a model molecule. The results reveal that adding relatively low amounts of MWCNTs can ameliorate the photocatalytic activity of BiOCl. Among them, the 3% sample exhibits the best activity towards degradation of phenol. The enhancement in photocatalytic performance can be tightly ascribed to the enhanced separation rate of the photoinduced carriers, in accord with the surface photovoltage spectroscopy (SPS) investigation. MWCNTs facilitate the effective formation of O2radical dot, resulting in rapid decay of phenol over the MWCNTs/BiOCl materials. In view of all the obtained experimental results, a photocatalytic enhancement mechanism for the MWCNTs/BiOCl composite is presented.

Introduction

Semiconductor-based photocatalysis is an environmentally-friendly and effective approach to completely decompose organic pollutants in water and air owing to often innovative, low cost, chemically stable and robust redox capabilities [[1], [2], [3], [4], [5], [6]]. The efficiency of photocatalytic processes tightly correlates with the physical properties of the photocatalysts. Therefore, it is still an ongoing challenge to prepare highly efficient photocatalysts for important large-scale applications such environmental remedy. To address this issue, tremendous photocatalysts have been developed. Among the photocatalysts developed, bismuth-based semiconductor materials have attracted considerable interests and widely used as photocatalysts due to the unique construction, properties and stability [[7], [8], [9], [10], [11]], such as BiOX (X = Cl, Br and I), Bi2S3, Bi2O3, BiWO3, Bi2MO6, Bi2O2CO3, Bi4Ti3O12, BiOIO3, Bi2SiO5 and et al. Among the bismuth-based photocatalysts, bismuth oxychloride (BiOCl) has been considered as one of most prospective photocatalyst.

As a p-type semiconductor, BiOCl possesses layered structure characterized [12,13], the unique open structure and internal static electric fields can greatly reduce the recombination rate of photoinduced carriers [[14], [15], [16]], leading to excellent photocatalytic performance. Indeed, BiOCl displays higher photocatalytic activity than anatase TiO2 or ZnO [17,18]. Nevertheless, the photocatalytic performance of BiOCl is not satisfied to meet the needs for effective future applications because of its too wide band gap, high recombination rate of photo-generated carriers, and sluggish charge carrier migration [[19], [20], [21], [22]]. During photocatalytic processes, the separation dynamics of the photogenerated carriers can significantly determine the photocatalytic efficiency [[23], [24], [25]]. Low separation rate of electron-hole pairs leads to low photocatalytic activity. Consequently, it is crucial to establish approaches to boost the carrier separation efficiency to enable practical use of BiOCl. Up to now, various methods have been employed to promote the separation of photoinduced carriers, such as doping [26], morphology control [27,28], crystal facet exposure [29], surface modification [[30], [31], [32]] and formation of heterojunctions [[33], [34], [35], [36], [37]]. All these strategies can boost the photocatalytic performance of BiOCl by improving the separation efficiency of the photoinduced carriers.

Multiwall carbon nanotubes (MWCNTs) are a most promising class of carbon-based nanomaterials and can be widely applied in many fields owing to their excellent optical, chemical, and electrochemical properties [38]. MWCNTs hold a similar functionality as graphene oxide does. Consequently, MWCNTs can be employed as conducting channels to accelerate the electron transfer [39] and to suppress the recombination rate of photoinduced carriers. Therefore, they are useful in promoting the photocatalytic performance of photocatalysts. In addition, endowing with large specific surface area, MWCNTs are outstanding adsorbents for organic contaminants [40]. At the same time MWCNTs possess numerous active sites for reaction [41], all these virtues are beneficial to the photocatalytic processes of a photocatalyst. In view of these merits, MWCNTs can be used to modify various photocatalysts to tune the photocatalytic performance by facilitating the separation rate of the photoinduced carriers [[42], [43], [44], [45], [46], [47]].

Inspired by this context, MWCNTs were used to promote the photocatalytic performance of BiOCl. The MWCNTs/BiOCl composites were characterized by a series of methods, especially photovoltage spectroscopy (SPS), surface X-ray photoelectron spectroscopy (XPS), Raman and electron spin-resonance (ESR). The SPS results confirm that the separation rate of the photogenerated carriers has been significantly enhanced. The XPS and Raman results prove that a strong chemical interaction forms between MWCNTs and BiOCl, with excellent stability. O2radical dot radicals were detected in the MWCNTs/BiOCl composite photocatalytic system supported by the electro- conductivity of MWCNTs. The photocatalytic ability was tested using phenol as simulated pollutant; the results reveal that all the MWCNTs/BiOCl composites display higher photocatalytic activity than the bare BiOCl. In consideration of all the experimental results obtained, a mechanism for this enhancement is proposed.

Section snippets

Construction of MWCNTs/BiOCl composites

MWCNTs were purchased from Chengdu Institute of Organic Chemistry. The specific surface area of MWCNTs is about 20 m2/g. All other chemicals were analytical grade and provided by Chengdu Kelong Chemical Reagents Factory. In a typical fabrication process of MWCNTs/BiOCl composites, 5 g Bismuth nitrate pentahydrate was added into 60 mL ethylene glycol (EG), afterwards the MWCNTs were dispersed into the Bi3+-EG solution. The suspension system formed was ultrasonically dispersed for 15 min. 10 mL

Characterization of the MWCNTs/BiOCl composite photocatalyst

The effects of MWCNTs on the specific surface area of the photocatalysts are shown in Table 1. As demonstrated in Table 1, the specific surface area of the bare BiOCl is 18.8 m2/g. Compared to the unmodified BiOCl, all the MWCNTs/BiOCl composites display higher specific surface area. The enhanced specific surface area of the MWCNTs/BiOCl composites stems from the presence of MWCNTs in the composites. High specific surface area can supply more active sites for the catalytic reaction, heightening

Conclusions

In this paper, MWCNTs/BiOCl composites have been prepared via a typical hydrothermal route. The results reveal that BiOCl was successfully modified with MWCNTs and a strong chemical interaction forms between BiOCl and MWCNTs, proven by the results from XRD, Raman and XPS. Compared to the reference BiOCl sample, the photocatalytic activities of the MWCNTs/BiOCl composites were enhanced by adding relative low amounts of MWCNTs. Among all the composites, the 3 wt% MWCNTs/BiOCl sample displays the

Acknowledgements

This project was supported financially by National Natural Science Foundation of China (No. 21777168), the program of Science and Technology Department of Sichuan Province (No. 2019YJ0457, No. 2017096), Students Innovation Project of Sichuan Province (No. 201710622065) and the Project of Zigong City (No. 2016HG06, No. 2018YYJC10). CB would like to thank Case Western Reserve University for the support through the Center for Chemical Dynamics.

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