Diffused sunlight driven highly synergistic pathway for complete mineralization of organic contaminants using reduced graphene oxide supported photocatalyst
Graphical abstract
Introduction
Photocatalytic degradation is a promising green route to convert organic pollutants from wastewater into harmless end products [1], [2], [3], [4], [5], [6]. Initially, monometallic semiconducting oxides were exclusively studied for the photocatalytic degradation reactions [7], [8], [9]. By narrowing the band gap energy and thereby increasing absorption toward visible region of light, binary metal oxide composites were found to have better photocatalytic performance compared to single metal oxides, especially when visible light irradiation is used [10], [11], [12]. The major advantage of photocatalytic degradation is that it aims to use solar energy but many systems require ultraviolet light [13]. Nevertheless, the use of solar radiation as an energy source can reduce costs and also benefit the environment. Another major issue associated with photocatalysis is that strong adsorption of pollutants and their degradation products on the photocatalyst surface reduce the performance of catalysts. In addition, in the absence of vigorous stirring, catalyst particles settle down leading to a lower catalytic activity [14].
Ultrasound has also been widely used for chemical reactions, synthesis of nanomaterials, and oxidation of organic pollutants [15], [16], [17]. During ultrasonic irradiation of an aqueous solution, the cleavage of water molecules within cavitation bubbles leads to the formation of H and OH radicals. In oxygen rich solution, hydrogen atoms are converted into HOO radicals, which are also powerful oxidants. The formed OH and HOOradicals are effective oxidizing agents to degrade organic pollutants in aqueous solutions [18]. The shear forces and shock waves generated during cavitation are useful for mass transfer leading to enhanced contact between the pollutant molecules and the catalyst particles. Therefore, ultrasound is successfully used in combination with other oxidation processes such as ozonation, Fenton, and Fenton-like methods [19]. Ultrasound with Fenton-like reagents has shown synergistic effect for the degradation of pentachlorophenol [20], reactive blue 4 [15], methyl tert-butyl ether [21], p-chlorobenzoic acid [22], martius yellow [23] and also for the sonochemical oxidation of arsenic(III) to arsenic(V) [24]. Sonochemical degradation of organic pollutants in aqueous solutions is an effective method. However, the primary products generated are hydrophilic in nature and hence the mineralization of the pollutants and their degradation products is a very slow process.
The combination of both photocatalysis and sonochemical methods may overcome the existing problems of individual methods. A combination of both these advanced oxidation processes (AOPs) seems to enhance the degradation of organic pollutants due to increase in the amount of OH radicals generated during the process [25]. However, most of the combined studies reveal an additive effect when combining photocatalytic reaction with ultrasound. Examples include the degradation of acid orange 8 and acid red 1 in waste water [26], azo dyes in presence of TiO2 catalyst under UV light irradiation [27], [28], and 4-chlorophenol with Bi2O3/TiZrO4 [29].
In this study, methyl orange (MO) dye, which belongs to the class of azo dyes, was chosen as a target contaminants because it is very toxic, mutagenic, and carcinogenic [30], [31], [32]. The sonophotocatalytic degradation of MO with CuO–TiO2/rGO photocatalysts and the mechanism behind the observed synergistic effect under diffused sunlight are discussed in detail. To the best of our knowledge, this is the first study that reports on the significant synergistic effect of ultrasound combined with photocatalytic degradation of organic compounds under diffused sunlight.
Section snippets
Preparation of graphene oxide (GO) and mesoporous TiO2 NPs
Graphene oxide (GO) was prepared from graphite powder by modified Hummers’ method using sodium nitrate, potassium permanganate, and sulphuric acid [33]. Mesoporous TiO2 nanoparticles (NPs) were prepared by ultrasound-assisted method using titanium tetraisopropoxide (TTIP) and glacial acetic acid [34]. In a typical procedure, TTIP (0.032 mol) and glacial acetic acid (0.016 mol) were dissolved in 20 mL of absolute ethanol. The solution was stirred for 1 h and the resulting solution was added drop
Powder X-ray diffraction
X-ray diffraction analysis was carried out to identify the crystal structure and also to confirm the loading of CuO (Fig. 2). The XRD pattern confirmed the formation of only anatase phase of TiO2 with reference to the JCPDS card number 21-1272 [37]. Resemblance of XRD pattern of TiO2 in CuO–TiO2 and CuO–TiO2/rGO verified that no phase change occurred during the loading of CuO NPs. In addition, CuO–TiO2 and CuO–TiO2/rGO showed significant peaks at 35.58, 37.10, 59.13 and 65.38° that correspond
Conclusions
A diffused sunlight driven sonophotocatalytic degradation system was developed for the first time using CuO–TiO2/rGO. XRD and XPS analysis confirmed the formation of CuO and anatase-TiO2 by the simple wet impregnation technique. Uniform decoration of CuO–TiO2 nanocomposites over rGO surface was identified by TEM studies. The presence of a co-catalyst (CuO) narrowed down the effective band gap energy of TiO2 as revealed by UV–vis DRS. Moreover, a phenomenal synergistic effect was experienced by
Acknowledgments
We acknowledge financial support from the SERB (SR/FT/CS-127/2011), DST, New Delhi, India. We also acknowledge Prof. Ick Soo Kim (Division of Frontier Fibers, Institute for Fiber Engineering (IFES), National Shinshu University, Ueda, Japan) for TEM and XPS analysis.
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