Facile fabrication of ZnO nanorods modified with RGO for enhanced photodecomposition of dyes

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Highlights

  • RGO/ZnO-rods composite was obtained by a simple hydrothermal method from GO/ZnO2 microspheres.

  • The photodegradation rate of the RGO/ZnO-rods composite to the dye is higher than 90 % for both MB and MO.

  • The calculated apparent kinetic reaction rate constants (k) of RGO/ZnO is much higher than ZnO, ZnO2 and RGO/ZnO2.

Abstract

Reduced graphene oxide/zinc oxide-rods composites (RGO/ZnO) were prepared through simple hydrothermal treatment of graphene oxide /zinc peroxide (GO/ZnO2) microspheres. The results of TEM, SEM and XRD show that the rod-like structure ZnO were successfully transformed from ZnO2 microspheres after high temperature treatment and combine well with GO. The synthesized RGO/ZnO-rods composites showed better photocatalytic performance as compared to pure ZnO2, ZnO and GO/ZnO2 under UV and visible-light irradiation. The improved photocatalytic performance of RGO/ZnO-rods composites was attributed to the uniform structure and excellent electrical conductivity of RGO, which could reduce the combination rate of the photo electron-hole pairs. After 5 consecutive cycles of testing, the RGO/ZnO-rods composites still have excellent photocatalytic performance, which was found to depend on ZnO content and NaCl concentration but was not affected by pH. The result of ESR experiment confirmed that the superoxide radicals (O2radical dot) and hydroxyl radicals (Oradical dotH) play an important role in the decolorization process of MO. Additionally, the kinetic rate constant (K) and mechanism of RGO/ZnO-rods composites had also been studied.

Introduction

As the amount of pigment discharged increases, toxic and non-biodegradable dyes that penetrate into the water can cause harmful and harmful effects to the environment [1]. As a low-cost technology that can decompose organic pollutants into non-toxic substances, light induced degradation mediated by photocatalytic materials has attracted researcher’s attention because of its green and efficient chemical detoxification of organic pollutants in industrial wastewater [2]. Among the commonly used photocatalysts, zinc oxide (ZnO) is considered as one of the materials with the highest catalytic performance materials because of its green properties, durability and low price [3]. When the band gap energy is the same, ZnO as an n-type semiconductor has a higher absorption range of the solar spectrum, and is considered as an alternative catalyst for TiO2 [4]. It is generally known that the nanostructures of ZnO is an important factor affecting its photocatalytic performance [5]. Compared with other structures such as ZnO nanowires [6], nanofibers [7], nanotubes [8] and nanoneedles [9], ZnO nanorods [10] have attracted much attention due to their advantages of on most substrates, no post-treatment to remove catalyst, more defects and lower crystallinity.

It has been reported that ZnO nanorods can be synthesized with organic zinc compounds through different synthetic methods, such as high temperature chemical vapor transmission [11] and physical vapor deposition [12], anodized aluminum oxide film templates [13], microemulsions [14] and hydrothermal process [15]. But, ZnO rods produced by these synthetic methods are all large in size (length up to 5 μm and diameter up to 200 nm) [16]. And it is generally known that the small rods with small aspect ratio and high oxygen defect concentration have higher photocatalytic activity than large rods [17,18]. Therefore, the synthesis of high-quality ZnO rods by simple and gentle methods is significant and it is still a great challenge for researchers.

It has been proved that the photocatalytic efficiency of ZnO is limited by the fast recombination of photoinduced hole-electron (h+-e) pairs formed under light irradiation and the aggregation of nanoparticles during photocatalysis [19]. Moreover, the photo-corrosion during the light irradiation also has a serious impact on the photocatalytic activity and stability of ZnO [20]. It can be said that preventing hole-electron (h+-e) pairs from recombination is important for improving photocatalytic efficiency. Recently, studies have found that recombination of photogenerated charge carriers can be prevented by combining ZnO with other materials such as semiconductors [21,22], noble metals [23] (indium, gallium and gold) and graphene oxide [24,25]. Among them, graphene oxide (GO) with a two-dimensional sp2 hybrid structure of carbon atoms is generally used as a semiconductor material for coupling zinc oxide through a simple nanocomposite because of its excellent conductivity, adsorption and high surface area [26]. At present, composite materials associating GO or reduced graphene oxide (RGO) to ZnO rods have been developed to increase photocatalytic performance [[27], [28], [29]]. Furthermore, we have successfully synthesized ZnO-NGO composites with high photocatalytic ability by in situ growth of ZnO on aminated grapheme (NGO) [30] in our previous work. Thus, the combination of nanorods ZnO and GO is expected to have excellent absorptivity, conductivity, and controllability, thereby promoting efficient photodegradation of pollutants.

In this work, RGO/ZnO-rods composite with good photocatalytic performance were prepared from GO/ZnO2 microspheres, which were prepared by in-situ generation method using the carboxyl groups on the surface of GO. The use of ZnO2 which can be decomposed into different shapes of zinc oxide makes the preparation method not only simple and mild, but also free of pollutants. The synthesized RGO/ZnO-rods composites were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL) and UV–vis spectrophotometry (UV–vis). The photocatalytic activity of the resulting RGO/ZnO-rods composites were measured by photocatalytic decomposition of Rhodamine B (RhB) and methyl orange (MO) under xenon lamp (300 W) irradition. Moreover, the stability and effect of ZnO content, initial pH and NaCl concentration on the photocatalytic efficiency of RGO/ZnO-rods composites over MO was evaluated. In addition, the kinetic rate constant (K) and mechanism of RGO/ZnO-rods composites had also been evaluated.

Section snippets

Materials

Graphene oxide (GO) was prepared with the method from our previous work [30]. Zinc acetate (C4H6O4Zn) were obtained from Sigma Chemical, Co. Ltd. (Shanghai, China). Rhodamine B (RhB) and Methyl orange (MO) were provided from Aladdin Co. Ltd. (Shanghai, China). All chemicals reagents were used as received without further purification.

Preparation of ZnO2 nanoparticles

ZnO2 nanoparticles were synthesized by the method previously described [31]. Briefly, 25 mL of deionized water containing 2.8 g of KOH was slowly dropped into 25 mL

Synthesis and characterization

The procedure for the preparation of RGO/ZnO-rods composites is illustrated in Scheme1. After the GO was prepared by the Hummer method, the ZnO2 nanoparticles were uniformly grown in situ on the surface. As a novel catalyst, ZnO2 is used for dye degradation due to its ability to generate active oxygen species in the presence of Fe2+ [19]. However, ZnO2 exists in an oxidized form and has a wide band gap, which makes it low in photocatalytic performance when used as a photocatalyst [33]. While,

Conclusion

In conclusion, a rod-shaped RGO/ZnO-rods composites photocatalyst with photocatalytic performance for RhB and MO was prepared by hydrothermal method.

Based on the characterization results, it was concluded that the spherical ZnO2 nanoparticles have been converted into ZnO with a rod structure under hydrothermal conditions. The addition of RGO improved the photocatalytic ability of ZnO by enhancing the dispersibility and suppressing the recombination of photogenerated electron pairs. The

Author contributions

Manman Xie and Dongdong Zhang conducted the experiments, analyzed the data, prepared the figures, and wrote the manuscript presented herein. Yiqing Wang and Yiping Zhao guided the research.

Formatting of funding sources

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially sponsored by State Commission of Science & Technology of China (2016YFC0104100), Nanjing University Graduate Innovation Program (2017CL08) and Program for Innovative Research Team in University of Tianjin (No. TD13-5044).

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