Efficacious separation of electron–hole pairs in CeO₂-Al₂O₃ nanoparticles embedded GO heterojunction for robust visible-light driven dye degradation

Abstract

Herein, we have developed a facile and one pot synthesis of ternary CeO₂-Al₂O₃@GO nanocomposite via wet chemical method. The structural and morphological characteristics of the synthesized nanocomposite was investigated using UV–DRS, FT-IR, XRD, FE-SEM, HR-TEM, EDX and TGA analysis. The CeO₂-Al₂O₃@GO composite was tested for its ability to photocatalytically degrade Rhodamine B (RhB) under visible light illumination. The influence of various operational parameters such as pH, catalyst dosage and initial dye concentration on the photo degradation was investigated in detail. The synthesized CeO₂-Al₂O₃@GO composite shows greater photocatalytic degradation of RhB (99.0%) under visible light irradiation than the raw CeO₂, Al₂O₃, and GO catalysts and any other reported nanocomposite materials. The recyclability results also demonstrate the excellent stability and reusability of the CeO₂-Al₂O₃@GO nanocomposite. This work will be beneficial in the field of industrial and engineering applications in the degradation of organic pollutants. Also, a study of this kind will definitely stimulate many researches in the recently emerging field of solar-driven water splitting.

Introduction

An extensive research on the elimination of hazardous chemical compounds has become worldwide issue for years due to the severe risks to human health and environment [1], [2]. More importantly, synthetic dyes are the major part of our life as they are found in the various products ranging from cosmetics, textile, painting, leather accessories, furniture, pharmaceuticals, denim industries and food stuffs [3]. Especially the dye under consideration, Rhodamine B (RhB) which contains four N-ethyl groups at either side of the xanthene ring, was chosen as the target pollutant. It is highly soluble in water, ethanol, and chemically stable [4]. This dye causes carcinogenicity, toxicity, neurotoxicity and irritation of skin and eyes for humans and animals [5], [6]. Hence the hazardous nature and harmful effects of RhB is necessary to be degraded. The treatment of effluent containing RhB is usually difficult, inconvenient and not cost-effective and generates secondary toxic pollutants during the treatment process [7]. Photocatalytic technology has been used widely in environmental protection because it represents an easy way to utilize the energy of light and is a promising, environmental, and cost-effective method [8], [9]. Semiconductor photocatalysis technique has received considerable attention as an attractive strategy to deal with the removal of dye pollutants [10]. To date, the researchers are very much keen to develop various semiconductor materials such as TiO₂ [11], ZnO [12], SrTiO₃ [13], WO₃ [14], CdS [15], GaN [16], BiVO₄ [17], NaBiO₃ [18], Ag₃PO₄ [19], CeO₂ [20], Bi₂S₃ [21] and Fe₂O₃ [22], for dedyeing of various dyes [23]. There are still some drawbacks such as easy loss of photocatalytic activity, long reaction time, agglomeration and difficulty in recovery of the catalyst materials [24]. Alternatively, the combination of semiconductor with graphene oxide (GO) to form a heterostructure is a feasible route to promote the separation of photogenerated charge carriers [25]. The incorporation of two or more active components onto graphene with excellent particle dispersivity is expected to facilitate wider applications of graphene based nanocomposites. The unique mechanical properties of GO include very large specific surface area, two dimensional monolayer structure, interesting electric, thermal, optical properties and GO exhibits superior performance as catalysts, electrocatalysts and photocatalysts [26]. GO shows extremely high electron mobility, which can be used to reduce h⁺–e⁻ recombination in semiconductor metal nanoparticles [27], [28]. Metal nanoparticles are highly stabilized when supported on ceria and has been attracting a growing attention towards applications [29], [30]. Among other catalysts, cerium dioxide (CeO₂) has received much attention because it is non-toxic, non-photocorrosive and has strong oxidation ability together with high chemical stability. At high temperatures the surface of CeO₂ considerably gets reduced drastically [31]. Many studies have described the processes to enhance charge separation and increase the photocatalytic activity of CeO₂ by many routes, such as metal doping or coupling with other metal oxide materials [32]. The combination of ceria and alumina gives the catalyst better properties compared to the two pure individual oxides [33], [34], [35], [36], [37]. Many studies have demonstrated that most of the metal oxide-covered adsorbents are more efficient than the metal oxide alone. Preparing CeO₂-Al₂O₃ on the surface of GO can still efficiently enhance the photocatalytic performance of CeO₂-Al₂O₃ due to the super high electron migration rate of GO and the hetero junction electric field formed on the interface of GO and CeO₂-Al₂O₃.

Herein, for the first time, a new type of hybrid architecture, CeO₂-Al₂O₃@GO has been successfully synthesized via one-pot wet chemical method. The GO being electron acceptor and CeO₂-Al₂O₃ electron donor make CeO₂-Al₂O₃ to adsorb on the surface of GO sheets via electrostatic interactions, π–π stacking or hydrogen bonding, which promotes photogenerated electron-hole pairs separation, thus improving the photocatalytic efficiency. To accommodate these concerns, a particular adsorption mechanism based on the electrostatic interaction between GO and Ce-Al flakes is considered. The approach was optimized and employed to synthesize CeO₂-Al₂O₃@GO nanocomposite, which exhibits an excellent performance on photocatalytic degradation of RhB under visible light illumination (λ > 554 nm), when compared to pure GO, CeO₂, Al₂O₃ and CeO₂-Al₂O₃ counterparts and other nanocomposite materials reported elsewhere.