Ternary ZnO/AgI/Ag₂CO₃ nanocomposites: Novel visible-light-driven photocatalysts with excellent activity in degradation of different water pollutants

Highlights

• ZnO/AgI/Ag₂CO₃ nanocomposites were fabricated by an ultrasonic-irradiation method.

• The activity was investigated by photodegradation of four dyes under visible light.

• ZnO/AgI/Ag₂CO₃ (30%) nanocomposite has the best activity under visible light.

• Activity is 19 and 14-folds higher than ZnO/Ag₂CO₃ and ZnO/AgI in degradation of RhB.

Abstract

ZnO/AgI/Ag₂CO₃ nanocomposites with different Ag₂CO₃ contents were fabricated by a facile ultrasonic-irradiation method. The resultant samples were fairly characterized using XRD, EDX, SEM, TEM, UV–vis DRS, FT-IR, and PL techniques to reveal their microstructure, purity, morphology, and spectroscopic properties. Photocatalytic activity of the prepared samples was investigated by photodegradation of four dye pollutants (rhodamine B, methyl orange, methylene blue, and fuchsine) under visible-light irradiation. The photocatalytic experiments in degradation of rhodamine B showed that the ternary ZnO/AgI/Ag₂CO₃ (30%) nanocomposite has an enhanced activity nearly 19 and 14 times higher than those of the binary ZnO/Ag₂CO₃ and ZnO/AgI photocatalysts, respectively. Based on the obtained results, the highly enhanced activity was attributed to generation of more electron-hole pairs under visible-light irradiation and separation of the photogenerated charge carriers due to formation of tandem n-n heterojunctions between counterparts of the nanocomposite. The active species trapping experiments were also examined and it was showed that superoxide ion radicals play a vital role in the photocatalytic degradation reaction. More importantly, the ternary photocatalyst demonstrated good photostability.

Introduction

Nowadays, due to increasing production of various organic compounds in different industries, water contamination has led the major health risks to humans as well as ecological systems, which are growing with a faster rate every year [1]. The wastewaters from these industries are generally released into the environment. Hence, wastewater treatment is an important concern and scientists are looking for proper low-cost technologies to address this concern effectively. Semiconductor-based photocatalytic processes have attracted considerable attention to solve the growing environmental pollution and energy shortages by utilizing abundant sunlight [2], [3], [4]. Zinc oxide (ZnO), with a large band gap of 3.2 eV, is a fascinating photocatalyst due to its low cost, high stability, and environmentally-friendly [5]. However, this photocatalyst can be only excited by ultraviolet or near-ultraviolet irradiations, which remarkably limits its practical applications in large scale; because, the solar radiation contains nearly 5% UV, 43% visible, and 52% infrared irradiations [6]. Hence, designing and preparation of visible-light-driven photocatalysts with considerable activity using ZnO is the major challenge in this research field [5].

One of the most effective strategies to prepare visible-light-driven photocatalysts is combining narrow band gap semiconductors with ZnO [5]. In this regard, silver containing semiconductors with narrow band gaps have attracted more attention due to their photosensitivity for visible light, which could largely use the solar energy, leading to enhanced photocatalytic activity [5], [6], [7]. In the few years ago, many binary ZnO-containing photocatalysts such as ZnO/AgBr, ZnO/Ag₂S, ZnO/Ag₂CO₃, ZnO/Ag₃PO₄, ZnO/AgI, ZnO/Ag₃VO₄, and ZnO/Ag₂CrO₄ have been prepared and their photocatalytic activities under visible-light irradiation investigated [8], [9], [10], [11], [12], [13], [14], [15]. However, the prepared binary photocatalysts have moderate photocatalytic activity under visible-light illumination. Hence, in order to further increase the photocatalytic activity, preparation of ternary photocatalysts by combination of ZnO with two narrow band gap semiconductors have attracted more interests [16], [17], [18]. Recently, we prepared ZnO/AgI nanocomposites by refluxing method and they were used for photocatalytic degradation of an organic pollutant under visible-light irradiation [13], and it was found that the nanocomposite with 0.188 mol fraction of AgI exhibited the best activity relative to the other samples. In order to enhance photocatalytic activity of this nanocomposite, different amounts of Ag₂CO₃, with narrow band gap of 2.30 eV [11], was added to prepare ternary ZnO/AgI/Ag₂CO₃ nanocomposites. It is well known that ZnO, AgI, and Ag₂CO₃ are n-type semiconductors. Therefore, it is expected that by combination of these semiconductors, tandem n-n heterojunctions are formed in the junction area, leading to formation of internal electric field [16], [17], [18], [19], [20]. As we know, the formed electric field can induce spatial separation of photogenerated electron-hole pairs, resulting in highly enhanced photocatalytic activity.

Herein, we report preparation of novel ternary ZnO/AgI/Ag₂CO₃ nanocomposites with different weight percents of Ag₂CO₃, as highly enhanced visible-light-driven photocatalysts. The resultant samples were characterized using X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), Fourier transform-infrared spectroscopy (FT-IR), and photoluminescence (PL) techniques. Photocatalytic activity of the nanocomposites was investigated by visible-light photodegradation of rhodamine B (RhB) and the optimal weight percent of silver carbonate was determined. To reveal ability of the optimal photocatalyst to degrade other dye pollutants, degradations of methylene blue (MB), methyl orange (MO), and fuchsine were also studied. It was demonstrated that ultrasonic-irradiation time and calcination temperature have considerable influence on the degradation reaction. Additionally, the active species during photocatalytic degradation of RhB were also investigated by the addition of different scavengers and it was found that superoxide ion radicals have higher role in the degradation reaction relative to the holes and hydroxyl radicals.