Preparation of graphene/TiO₂ composites by nonionic surfactant strategy and their simulated sunlight and visible light photocatalytic activity towards representative aqueous POPs degradation

Abstract

A series of graphene/TiO₂ composites were fabricated using a single-step nonionic surfactant strategy combined with the solvothermal treatment technique. Their phase structure, morphology, porosity, optical absorption property, as well as composition and structure, were characterized. The as-prepared composites were successfully applied to degrade aqueous persistent organic pollutants (POPs) such as rhodamine B, aldicarb, and norfloxacin in simulated sunlight (λ > 320 nm) and visible light (λ > 400 nm) irradiation. The degradation mechanism and kinetics of aqueous POPs were studied in detail. The mineralization of aqueous POPs and the recyclability of the composites were also tested in the same condition.

Introduction

The harmful effect of aqueous persistent organic pollutants (POPs) to human health is increasingly becoming more serious with the development of industry and agriculture [1], [2]. Aqueous POPs are resistant to biodegradation and photolytic degradation because of their stable chemical structure. This resistance has become a serious problem for many researchers [3], [4]. Therefore, alternative and effective treatment processes for aqueous POPs are needed. Photocatalytic oxidation is one of the advanced technologies for aqueous POP removal because of the efficiency in their mineralization of aqueous POPs. In an ideal case, this process produces carbon dioxide, water, and inorganic mineral ions [5]. Among photocatalysts, TiO₂ is most widely used because it is easily available, inexpensive, non-toxic, and has a relatively high chemical stability [6]. However, as a wide band gap semiconductor (3.2 eV), TiO₂ can only absorb the UV region of solar light (3–5%); thus, solar light-assisted TiO₂ has a very low efficiency in terms of photocatalytic process performance [7], [8]. On the other hand, the quantum efficiency of TiO₂ is low because of the fast recombination of photoinduced electron–hole (e⁻–h⁺) pairs [9], [10]. To overcome these drawbacks, studies have focused on designing novel TiO₂-based photocatalysts with a high quantum efficiency and an extended light-response range [11], [12], [13].

Graphene-based semiconductor photocatalysts have attracted increasing attention because of their usefulness in environmental applications [14], [15]. Researchers have developed functionalized graphene-based semiconductor photocatalysts because graphene has an excellent charge carrier mobility at room temperature (200,000 cm² V⁻¹ s⁻¹) [16], [17]. Combining graphene with semiconductor photocatalysts can enhance their photocatalytic performance because their quantum efficiency is enhanced and their band gap energy is decreased [18], [19]. Graphene/TiO₂ composite is one of the most widely studied graphene-based semiconductor photocatalysts because of the advantages of TiO₂ in environmental applications [20], [21]. Compared with TiO₂, graphene/TiO₂ has a higher quantum efficiency and a lower band gap energy [22]. Studies have shown that when TiO₂ is coupled with graphene, its photocatalytic activity is enhanced because the recombination of e⁻–h⁺ pairs is reduced [23], [24]. On the other hand, graphene is nonmetallic. Thus, the atomic orbital hybridization of C and O can form a doping level, which decreases the band gap energy of TiO₂ [25], [26], [27]. However, the photocatalytic activity of graphene/TiO₂ composites is determined to a great extent by its textural and morphological properties. Fabricating graphene/TiO₂ composites with TiO₂ nano-particles that are uniformly dispersed on single-layer graphene nano-sheets and with a large surface area can improve their photocatalytic activity by increasing the number of active site and the accessibility of active sites to the substrate. The nonionic surfactant strategy and the solvothermal treatment technique are suitable methods in preparing graphene/TiO₂ composites with excellent photocatalytic activity. Thus far, no study has prepared graphene/TiO₂ composites with excellent photocatalytic activity by using the nonionic surfactant strategy combined with the solvothermal treatment technique.

Pluronic P123 is a well-known nonionic surfactant that is widely used to prepare ordered mesoporous materials, such as SBA-15 [28], [29], [30]. Introducing P123 in the graphene/TiO₂ preparation process can effectively inhibit the aggregation of TiO₂ nano-particles and graphene nano-sheets [31]. In the graphene/TiO₂ system, the aggregation of TiO₂ nano-particles and graphene nano-sheets enriches more electrons and produces new recombination centers of photogenerated electrons and holes. Thus, the quantum efficiency is reduced [32], [33], [34]. In addition, introducing P123 in the graphene/TiO₂ preparation process can also enhance the surface area of the composite [35], [36], [37]. A large surface area can improve photocatalytic reactivity by reducing the mass transfer resistance of the photocatalytic reaction. Moreover, supramolecular, rather than chemical interaction, occurs between P123 and inorganic precursors, thereby causing P123 to be easily removed via ethanol extraction without damaging the catalyst structure.

The solvothermal treatment technique has been widely employed in nano-material synthesis [38], [39], [40], [41]. Unlike traditional methods, the use of the solvothermal treatment technique produces TiO₂ photocatalysts with high crystallinity, small particle size, and uniform dispersion [42]. More importantly, the C atom in the graphene and the O atom in the TiO₂ can interact on an atomic level at a high temperature and pressure. Thus, the produced photocatalyst demonstrates special electronic transmission properties. On the other hand, as a “green” reduction technology, graphene oxide can be directly reduced to graphene via the solvothermal treatment without using any reductant [43], [44].

To identify a new preparation route for graphene/TiO₂ composites with excellent photocatalytic activity, we fabricated a series of graphene/TiO₂ composites via the single-step nonionic surfactant strategy combined with the solvothermal treatment technique. Dye rhodamine B, pesticide aldicarb, and antibiotic norfloxacin were used as the model compounds to determine the photocatalytic performance of as-prepared graphene/TiO₂ composites in aqueous POPs removal in simulated sunlight and visible light irradiation. The textural properties and electronic structures of the as-prepared graphene/TiO₂ composites were then characterized. These composites were successfully used to degrade aqueous POPs in simulated sunlight and visible light irradiation. The degradation mechanism and kinetics of aqueous POPs were also studied. In addition, the mineralization of aqueous POPs and the recyclability of the composites were also tested in the same condition. The reason for the enhanced photocatalytic activity of the as-prepared graphene/TiO₂ composites was also discussed.