Ag₂C₂O₄ /Ag₃PO₄ composites as efficient photocatalyst for solar light driven degradation of dyes pollutants

Highlights

• The Ag₂C₂O₄/Ag₃PO₄ showed excellent solar light driven photocatalytic activity in BMS reactor.

• The solar light activity of Ag₂C₂O₄/Ag₃PO₄ was much higher than that of pure Ag₂C₂O₄ and Ag₃PO₄.

• The present composite photocatalysts exhibit excellent photo-degradation of AB and EB.

• The underlying photo-degradation mechanism was systematically investigated and discussed.

Abstract

Herein, novel Ag₂C₂O₄/Ag₃PO₄ heterostructured nanocomposites were designed through a simple a co-precipitation method, employing in the photo-degradation of binary mixture (Amido black 10B) AB and (Eosin B) EB under the natural solar light. Their composite properties were described using various analysis such as FE-SEM, EDS, XRD, FTIR, PL and UV–Vis. The Ag₂C₂O₄/Ag₃PO₄ nanocomposite exhibited highly improved photocatalytic activities for binary mixture AB and EB dyes degradation, in comparison to the single Ag₂C₂O₄ and Ag₃PO₄. DRS analysis of Ag₂C₂O₄/Ag₃PO₄ indicates the extension of light absorption in the visible region while PL spectra of these photocatalyst confirm the efficient separation of charge carriers. The correlation among response and factors such as irradiation time, nanocomposite mass, initial AB and EB dyes concentrations were optimized by a central composite design (CCD) combined with response surface methodology (RSM). The enhanced photocatalytic activity of the Ag₂C₂O₄/Ag₃PO₄ was attributed to the enhanced number of electron-hole pairs generated by solar light and the effective separation of the charge carriers owing to the formation of n-n heterojunction Ag₂C₂O₄/Ag₃PO₄.

Introduction

One of the most important goals of water treatment is the elimination inorganic pollutant and the mineralization of organic pollutant to nontoxic inorganic compounds [1]. By increasing global industrialization, most of the traditional contamination purification technologies couldn't meet the needs of environmental management [2]. In the several of decades, semiconductor-based photocatalytic oxidation as a green technique was becoming a promising approach for hydrogen production, CO₂ conversion and degradation of various contaminants [[3], [4], [5]]. The photocatalysis method would generally utilize a powerful oxidizing species such as ^●OH and ^●O⁻₂ radicals generated based on hole and electron, which causes a sequence of reactions afterwards to decompose the macromolecules into smaller and less toxic compounds. In various cases, the macromolecule is entirely mineralized into H₂O, CO₂ and mineral acids [[6], [7], [8]]. Nowadays, the dyes and pigments are extensively used in textile industry, while it in turn threatens aquatic organism ecosystem as well as damage human body [9,10]. Azo-based dyes are the most usual components applied in textile and other related industries. The presence of these dyes in wastewaters prevents the photosynthesis process of aquatic plants by absorbing visible light. Due of their long-term thermal and photo-stability and high water solubility, the removal of azo-based dyes from effluent is extremely difficult [11,12].

Solar light as one kind of promising renewable alternative energy offers a free, clean, non-polluting, stable resource. The wide spectrum of solar light consist of ultraviolet (UV), visible and near-infrared (NIR) that the major driving force for the development of efficient solar light driven photocatalysts [13,14]. Among various photocatalysts, metal oxides are the most suitable substances used for photocatalytic degradation due to high capacity for elimination of toxic contaminants [[15], [16], [17]]. Unfortunately, the wide band gap of most metal oxides such as TiO₂ has limited its widespread application owing to it is activated solely by UV light [18,19]. Typically, different strategies for engineering the band gap of photocatalysts and extending their absorption to the visible light region used such as doping, coupling, and sensitization approaches. Meanwhile, coupling method has been studied deeply to develop efficient photocatalysts, but it still cannot satisfy the requirements for operational conditions [20]. In recent years, many silver-containing photocatalytic substances have received high attention due to their proper band gap and relatively good performance in degradation of contaminant [21,22]. In this regard, various semiconductors such as AgCl, AgBr, AgI, Ag₂O, Ag₂S, Ag₂Se, Ag₂CrO₄, Ag₂CO₃, Ag₂SO₄, Ag₂WO₄, Ag₃PO₄, Ag₃VO₄ and β-AgVO₃ have been used for preparation of various composite photocatalysts such as Ag@AgI–AgCl [23], AgX@Ag₃PO₄ [24], Ag₂O/Ag₂CO₃ [25], AgBr/Ag₂CrO₄ [26], Ag₂WO₄/AgX [27], Ag₃VO₄–AgBr [28], Ag₂S/Ag₃PO₄ [29]. Ag₂C₂O₄ showed highly efficient and stable visible light photocatalytic activity and had a narrow band gap (~3.6–3.8 eV) semiconductor [30,31]. Feng et al. investigated photo-degradation of propylene and acetaldehyde gas by Ag₂C₂O₄/TiO₂ and Ag₂C₂O₄ under visible light irradiation [30]. Abbasi et al. studied degradation of Orange G and Trypan blue by Ag₂C₂O₄/Ag/g-C₃N₄ under solar light irradiation [31]. Silver phosphate (Ag₃PO₄) as one of the efficient photocatalysts is a semiconductor with indirect and direct band gaps (2.36 eV and 2.43 eV, respectively), making it promising as a photocatalyst for decomposition of organic pollutants under natural light [[32], [33], [34], [35]]. However, a main problem associated with Ag₃PO₄ is its poor photochemical stability due to the photocatalyst is irreversibly increased to metallic silver in the lack of an appropriate electron scavenger in aqueous solutions [36]. Degradation of Methylene blue in aqueous solution by graphene-modified nanosized Ag₃PO₄ is studied by Xiang et al. [37]. Also, Bu et al. [38] synthesized a z-Scheme Ag₃PO₄/Ag/WO_3−x photocatalyst and used for degradation of Rodamine B in aqueous solution. Generally, the optimizations of contaminants degradation are performed by one variable at a time (OVAT) method which failure in estimation of interactions among variables that this limitation resolved by experimental design methods such as central composite design (CCD) [39]. These method represented relationship among variables with polynomial equation by reducing the number of experiments, cost and time [40,41].

In the present study, we report on the synthesis, characterization and photocatalytic activity of a new Ag₂C₂O₄/Ag₃PO₄ nanocomposite. The Ag₂C₂O₄/Ag₃PO₄ nanocomposite was prepared through a facile co-precipitation methods and then was characterized by FE-SEM, EDS, XRD, FTIR, PL and UV–Vis. The solar-light driven photocatalytic activity of Ag₂C₂O₄/Ag₃PO₄ nanocomposite was evaluated based on the degradation of AB and EB organic dyes. CCD approach was used for optimization and evaluation of the influence of major variables (pH, irradiation time, dyes concentration, photocatalyst dosage). Furthermore, a possible mechanism for solar-light photoactivity of the nanocomposite for the enhancing photocatalytic efficiency was also proposed based on the active species trapping experiments.