Benxia Li ⇑ , Tongxuan Liu, Yanfen Wang, Zhoufeng Wang

Introduction 

Currently, organic dyes and their effluents have become one of the main sources of water pollution due to the greater demand in industry such as textile, paper, and plastic. These organic dyes contaminate environment by the release of the toxic, cancerogenic, and colored wastewater [1–5]. Most of these dyes escape from traditional wastewater treatment and persist in water because of their high stability against light, temperature, chemicals, and microbial attack [3–5], whereas photocatalytic degradation of the organic pollutants has opened a new door for the elimination of organic dyes in wastewater [6–10]. The photodegradation process utilizes cheaply available semiconductors and leads to complete mineralization of organic compounds to CO2, water, and mineral acids [11]. Thus, semiconductor photocatalysis is deemed to an alternative technique to remove various organic pollutants. Until now, ZnO and TiO2 photocatalysts have been widely used because of its strong oxidizing power, non-toxic nature, and low cost [12– 16]. ZnO is a wide band-gap semiconductor oxide (3.37 eV) with a conduction band edge located at approximately the same level as that of TiO2. More attractively, the electron mobility of ZnO has been proven to be higher than that of TiO2 [17]. However, despite its great potential, the photocatalytic efficiency remains very low because of the fast recombination of the photogenerated electron–hole pairs in the single phase semiconductor. The performance of semiconductor photocatalysts is often enhanced by means of noble metal loading [18–20], ion doping [21,22], and incorporation of electron-accepting materials [23– 25]. These actions are used to extend the light absorption range or suppress the electron–hole recombination. As a rising star of carbon family, graphene has become the focus of considerable interest because of its unique electronic properties and other excellent attributes, such as the large theoretical specific surface area and the high transparency [26–29]. Meanwhile, graphene oxide (GO) is receiving increasing attention because it possesses the similar properties to graphene as well as the special surface structures with the introduced hydroxyl and carboxyl groups for synthesis of GO-containing nanocomposites [30–38]. Particularly, the fabrication of semiconductor/GO composites has attracted substantial research efforts motivated by the desire to improve the photocatalytic efficiency [34–38]. The recent studies have revealed that the composites simultaneously covered three excellent attributes: the increasing adsorptivity of pollutants, extended light absorption range, and efficient charge transportation and separation [25,39], which are the ideal traits of a photocatalyst we have been pursuing for. Therefore, it is believed that anchoring wellorganized ZnO nanostructures on GO sheets can efficiently utilize the combinative merits of ZnO and GO to obtain a photocatalyst with superior performance. This work demonstrated a facile strategy to synthesize ZnO/GO nanocomposite consisting of flower-like ZnO nanoparticles anchored on GO sheets and its use for photocatalytic degradation of organic dye in water under visible light. The composition, morphology, and microstructure of the as-obtained ZnO/GO nanocomposite were characterized. The photocatalytic performance of ZnO/ GO nanocomposite was evaluated by the photodegradation of methylene blue in water and compared with that of pure flowerlike ZnO nanoparticles and GO sheets, to highlight the importance of the anchoring of ZnO nanoparticles on GO sheets for maximum utilization of ZnO photocatalyst and GO as electron collector and transporter in photocatalytic degradation of organic pollutants.

Experimental

Photocatalytic property test The photocatalytic properties of the samples were evaluated by photodegradation of methylene blue (MB) in water under visiblelight irradiation from a 300 W Xe light equipped with a 420 nm cutoff filter (CEL-HXF300/CEL-HXUV300, China). In every experiment, 80 mg of photocatalyst was suspended in 100 mL of a 5.0 × 105 M aqueous solution of MB. Prior to irradiation, the suspension was stirred in the dark for 2 h to achieve an adsorption– desorption equilibrium between the photocatalyst and MB molecules. After that, the solution was exposed to the visible-light irradiation under magnetic stirring. At given time intervals, 3 mL of solutions was sampled for analysis of the MB concentration. The photocatalytic degradation process was monitored using a UV– Vis spectrophotometer (Shimadzu UV2550) to record the characteristic absorption at 665 nm. 3. Results and discussion 3.1. The formation mechanism of ZnO/GO composite In this work, ZnO/GO nanocomposite was fabricated by a twostep aqueous-solution route. GO sheets were firstly prepared using an improved Hummers’ method, and ZnO nanoparticles were then anchored on GO sheets via a facile reaction between Zn2+ and OH ions in aqueous solution. Fig.1 illustrates the fabrication process and formation mechanism of ZnO/GO composite. It has been proved that the surfaces of the chemically exfoliated GO sheets are covered by a large number of hydroxyl, carboxyl, and epoxy groups that are introduced on GO sheets due to oxidation procedures [41,42]. These functional groups can act as anchor sites to enable the subsequent in situ formation of ZnO nanoparticles on GO sheets. The formation of ZnO/GO composite undergoes the following two distinctive stages: (i) when dissolving ZnCl2 into GO suspension, Zn2+ ions will be adsorbed onto the surfaces of GO sheets due to their bonding with the O atoms of the negatively charged oxygen-containing functional groups via electrostatic force. (ii) After the addition of NaOH, the crystal growth units of ZnðOHÞ 2 4 and ZnO2 may combine with the functional groups of GO sheets via intermolecular hydrogen bonds or coordination bonds, acting as anchor sites for ZnO nanoparticles. With heating at 90 C, a large number of ZnO nuclei are formed in a short time due to the hydrolysis reaction of ZnðOHÞ 2 4 . Finally, ZnO/GO nanocomposite is obtained. The in situ formation of ZnO nanoparticles in return caused the exfoliation of the lamellar GO.

(a) The time-dependent absorption spectra of MB solution (5.0 × 10﹣5 mol/L, 100 mL) in the presence of ZnO/GO nanocomposite (80 mg) and under visible-light irradiation, and the inset is the color-change sequence of MB solution during this process. (b) Photodegradation of MB over photocatalyst-free solution (blank), GO sheets, flower-like ZnO particles, ZnO/GO nanocomposite, and annealed ZnO/GO, respectively.

Conclusion 

In conclusion, the ZnO/GO nanocomposite was successfully synthesized via a facile chemical deposition route at low temperature and its use for the photodegradation of organic dye from water under visible light was investigated. The ZnO/GO nanocomposite is composed of flower-like ZnO nanoparticles anchored on graphene-oxide sheets. For the photodegradation of organic dyes from water under visible light, ZnO/GO nanocomposite exhibits much higher photocatalytic efficiency than GO sheets and flower-like ZnO particles. The enhanced photocatalytic performance of ZnO/ GO nanocomposite can be attributed to the efficient photosensitized electron injection and repressed electron recombination due to the electron-transfer process with GO as electron collector and transporter. These features make the ZnO/GO composite an excellent candidate for applications relating to a number of environmental issues. The preparation method may be extended to fabricate more graphene-based composites for a variety of applications, such as catalysts, gas sensors, and nanoelectronic devices