ZnO nanocomposite

Environmental Pollution 249 (2019) 801e811

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Environmental Pollution journal homepage: www.elsevier.com/locate/envpol

Enhanced photocatalytic degradation of methyl orange by porous graphene/ZnO nanocomposite Li Wang, Zhan Li, Jia Chen, Yanni Huang, Haijuan Zhang, Hongdeng Qiu* CAS Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 December 2018 Received in revised form 8 March 2019 Accepted 18 March 2019 Available online 28 March 2019

Degrading aquatic organic pollutants efficiently is very important but strongly relied on the design of photocatalysts. Porous graphene could increase photocatalytic performance of ZnO nanoparticles by promoting the effective charge separation of electron-hole pairs if they can be composited. Herein, porous graphene, ZnO nanoparticles and porous graphene/ZnO nanocomposite were prepared by fine tuning of partial combustion, which graphene oxide imperfectly covered by the layered Zn salt was combusted under muffle furnace within few minutes. Resulting ZnO nanoparticles (32e72 nm) are dispersed uniformly on the surface of graphene sheets, the pore sizes of porous graphene are in the range from ~3 to ~52 nm. The synthesized porous graphene/ZnO nanocomposite was confirmed to show enhanced efficiency under natural sunlight irradiation compared with pure ZnO nanoparticles. Using porous graphene/ZnO nanocomposite, 100% degradation of methyl orange can be achieved within 150 min. The synergetic effect of photocatalysis and adsorption is main reason for excellent MO degradation of PG/ZnO nanocomposite. This work may offer a new route to accurately prepare porous graphene-based nanocomposite and open a door of their applications. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Porous graphene/ZnO nanocomposite Porous graphene ZnO nanoparticles Partial combustion Photocatalytic degradation

1. Introduction Organic dyes are one kind of main sources of the environmental pollutants due to their high toxicity to aquatic creatures and humans (Eskizeybek et al., 2012; Liang et al., 2017). In order to remove these dyes from the polluted water, many methods based on chemical, physical and biological strategies have been employed (Adeleye et al., 2016; Munoz et al., 2009; Subramani and Jacangelo, 2015). Among all methods, the photocatalytic degradation of dyes by photocatalyst is regarded as a higher efficient method because of its economic feasibility and simplicity (Khan and Narula, 2018). In this manner, metal oxide nanoparticles have increasingly gained attention. Among the various oxide semiconductors, ZnO is widely used in photocatalytic degradation due to its distinct electronic structure, non-toxic and environmentally friendly (Abada et al., 2018; Pirhashemi et al., 2018). While photo-generated electron and hole recombination quickly can decrease the photocatalytic activity of ZnO (Asmussen et al., 2009; Ahmad et al., 2013). Thus, it is essential to prevent the charge carrier recombination to

* Corresponding author. E-mail address: [email protected] (H. Qiu). https://doi.org/10.1016/j.envpol.2019.03.071 0269-7491/© 2019 Elsevier Ltd. All rights reserved.

promote the photocatalytic performance of ZnO. Extensive investigations have been proved coupling the photocatalysts with carbonaceous materials could reduce the charge carrier recombination, these carbonaceous materials include graphene, activated carbon and carbon nanotubes (Khalid et al., 2012; Zhang et al., 2018a). Graphene as a two dimensional carbon material, has received extensive interest due to its outstanding properties such as high optical transmittance, excellent mechanical behaviour and good interfacial contact with adsorbents (Chen and Carroll, 2016; Huang et al., 2012; Georgakilas et al., 2012; Kan et al., 2017; Li et al., 2016; Song et al., 2018; Zhu et al., 2010). Ahmad et al., in 2013 have reported that the combination of graphene with ZnO have superior photocatalytic performance because of graphene could facilitates the charge separation in photocatalysis process (Ahmad et al., 2013). Thus, it is possible to improve the photocatalytic performance by modifying the graphene structure. In recent years, porous graphene (PG), as a derivative of graphene, has become the most employed porous material because of its high specific surface area and pores for the electrons/ions transportation or storage (Russo et al., 2013). The porous structures could increase the active sites of materials, moreover, the pores can improve the photocatalytic

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performance by multiscattering of incidence light to increase light utilization (Wang et al., 2015). The existing strategies for synthesizing PG include electron beam or laser irradiation (Koenig et al., 2012), directly heating graphene oxide in air (Han et al., 2014), etching graphene with steam (Han et al., 2014), chemical activation (Zhao et al., 2011), high-energy physical techniques (Celebi et al., 2014) and photocatalytic oxidation (Akhavan and Ghaderi, 2013). These preparation methods are time-consuming and high-cost. Just recently, we reported a combustion synthesis method of PG using the alcohol lamp (Li et al., 2018). On the basis of this finding, in this work, we report a fast and facile way to produce PG with controllable pore size, PG/ZnO nanocomposite and ZnO nanoparticles by muffle furnace heating within few minutes. This method does not require a long time and high cost, and could obtain three materials by the same synthesis process. The photocatalytic performance of PG/ZnO nanocomposite and ZnO nanoparticles were investigated with methyl orange (MO) as simulating contaminant under sunlight irradiation. 2. Materials and methods 2.1. Chemicals Graphite was purchased from Aladdin Bio-Chem Technology Co., LTD. KMnO4, H2SO4 (98 wt%), HCl (37 wt%), H2O2 (30 wt%), Zn(NO3)2$6H2O, MO was purchased from Lanzhou Zhongke Kate Co., Ltd. Distilled water was used in whole experiment process. 2.2. Synthesis of the PG/ZnO nanocomposite, PG and ZnO nanoparticles PG/ZnO nanocomposite, PG and ZnO nanoparticles can be prepared by fine tuning of combustion method as schemed in Scheme 1. First, GO suspension (5 g/L, 1 mL) was added to 3 mL Zn(NO3)2 solution (0.3 g/mL; 1 g/mL; 2 g/mL), and then ultrasonically dispersed for 30 min. Subsequently, the mixed solution was vacuum-filtered with filter paper. After drying, GO covered by layered Zn salts can be tightly packed in filter paper. When the temperature of muffle furnace was reached to the set value, the filter paper loaded the GO covered with a certain amount of Zn salt

was placed in the muffle furnace. During burning, Zn(NO3)2 was decomposed into ZnO nanoparticles, the GO not covered by salts would be oxidized to CO2, forming defects on the surface of graphene. Thus, PG/ZnO nanocomposite was prepared successfully. After washing with HCl, PG with different pore size would be obtained; In addition, through controlling the temperature and time of combustion, ZnO nanoparticles can be obtained after completely burning of GO. 2.3. Material characterizations TGA analysis was performed under oxygen using a synchronous thermal analyzer (STA449F3, Germany) to investigate the thermal properties of materials. XRD was taken by using an X’ Pert Pro (PANalytical, Netherlands) operated to analyze the phase compositions of the products. XPS spectra (ESCALAB 250Xi, USA), Raman spectra (LabRAM HR Evolution, France) and Fourier transform infrared spectrometer (Nexus 870, USA) were also employed. SEM (JSM-6701F, Japan) and TEM (TF20, USA) were used to characterize the morphology and microstructure of materials. N2 adsorptionedesorption isotherms were collected on a surface area and porosity analyzer (ASAP 2010, USA) by the BET method. Ultraviolet-visible (UV-vis) diffuse reflection spectra (DRS) was recorded on a UV-3600 spectrophotometer (Shimadzu, Japan). The photoluminescence spectra (PL) and time-resolved PL spectra (TRPL) were surveyed on an FLS920 spectrophotometer (Edinburgh, UK). The electrochemical measurements were carried out on a CHI660E workstation using three-electrode system, the 0.5M Na2SO4 solution was chose as electrolyte. The 10 mg sample was mixed with 1 mL of ethanol under ultrosonication 30 min for electrochemical analysis. 2.4. Measurement of photocatalytic activities 1.5 mL MO dye solution (13 mg/L) was mixed with certain amount catalysts before sunlight irradiation, the suspension was shaken at 200 rpm in a rotary shaking incubator for 60 min in the dark to reach adsorption-desorption equilibrium. After that, the suspensions were irradiated by natural sunlight in an open atmosphere. After irradiation, the suspensions were centrifuged to

Scheme 1. Mechanism schematic for synthesis of the PG/ZnO nanocomposite, PG and ZnO nanoparticles.

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remove the photocatalyst, we use UV-visible spectrophotometer (PE Lamda35) based on the maximum absorptions of the MO at characteristic wavelength at 470 nm to quantify the concentration of residual MO in the supernatant solutions. The degradation efficiency was evaluated using the following equation:

Degradation ð%Þ ¼

Ci  Ct  100 Ci

where Ci represents the initial MO concentration (mg/L), Ct indicates the MO concentration (mg/L) after a certain irradiation time (t). Photocatalytic mechanism was studied by control experiments with adding different radical scavengers. In this study, we used the tert-butyl alcohol (TBA), benzoquinone (BQ), potassium persulfate (K2S2O8) and ammonium oxalate (AO) as hydroxyl radicals (OH),  þ superoxide radicals (O 2 ), electrons (e ) and holes (h ) scavengers, respectively (Zhang et al., 2013). In order to better compare the effects of different radicals, the degradation percentage of MO dye is reported as Ct/C0. Here, Ct is the MO concentration at each irradiated time interval (t), while C0 is the MO concentration at irradiation time 0 which after the adsorption-desorption equilibrium is reached. Besides, total organic carbon (TOC) of samples was measured to determine the degree of oxidative destruction of organic pollutants.

3. Results and discussion 3.1. Characterization of materials The synthesis condition of PG/ZnO is exactly the same as the PG, so we described it in PG section. ZnO nanoparticles can be obtained after completely burning of GO. As shown in Fig. 1A and B, ZnO nanoparticles are dispersed uniformly on the surface of graphene sheet. The pores of graphene are covered by ZnO nanoparticles with the average size of around 32e72 nm. The morphology of ZnO

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nanoparticles is in good agreement with PG/ZnO nanocomposite (Fig. 1C and D). The size distribution of ZnO nanoparticles is presented in Fig. S1A (Supporting Information). As shown in Fig. S1B, the signal of Zn and O element from the characterization of EDS suggests the composition of ZnO nanoparticles. After washing PG/ZnO nanocomposite with HCl, PG can be obtained. As shown in Fig. 2, the uniform pores of graphene are observed from TEM. As shown in Fig. 2A and B, the effect of temperature and time on the pore size was evaluated. After treatment for 1 min in muffle furnace, with the increase of combustion temperature, the pore sizes of PG at 450  C, 500  C, and 550  C are ~3 nm, ~30 nm, and ~52 nm, respectively. The obtained products did not appear any obvious porous structure when muffle furnace temperature below 400  C. While in 400  C, the pore size and morphology of PG as similar as PG at 450  C. So data of 400  C did not shown in Fig. 2. At 450  C, with the increase of combustion time, the average pore sizes of PG in 1 min, 10 min and 20 min are ~3 nm, ~44 nm, and ~48 nm, respectively. In order to study effect of Zn salt on the porous structure, we using different mass ratio of GO: Zn(NO3)2$6H2O (1: 200, 1: 600 and 1: 1200), after treatment in the muffle furnace for 1 min in 450  C, the average pore sizes for PG is ~3 nm, ~11 nm, ~18 nm, respectively. The corresponding pore size distribution statistics were presented in Fig. S2 (Supporting Information). The combustion process in muffle furnace can obtain the controllable trajectory behavior by rapidly heating and cooling down. For example, treatment condition was set at the condition 450  C for 1 min. At this process, the Zn salt was reduced to ZnO nanoparticles, carbon is oxidized to CO2 for the formation of pores in the area which not covered by Zn salt. As the end of 1 min reaction, we took the product out of the muffle furnace, it will return to room temperature immediately, resulting in the oxidation reaction stopped. The degree of oxidation would be increased by increasing the reaction temperature and time, resulting the pore size of PG increased. For the content of Zn salt, as the concentration increases, the increase of suction force during filtration caused

Fig. 1. TEM (A) and SEM (B) images of PG/ZnO nanocomposite, inset figure in SEM (B) is high magnification SEM image of PG/ZnO nanocomposite; TEM (C) and SEM (D) images of ZnO nanoparticles.

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Fig. 2. TEM images of PG with different pore sizes. A: TEM images of PG prepared under different temperature (450  C, 500  C and 550  C) for 1 min; B: TEM images of PG prepared under different time conditions (1 min, 10 min and 20 min) at 450  C; C: TEM images of PG prepared under different GO: Zn(NO3)2$6H2O ratios (1: 200; 1: 600 and 1: 1200) at 450  C for 1 min.

more volume of solution will be pumped away, resulting in an increase in pore size of PG (Li et al., 2018). The above-mentioned results proved that the muffle furnace combustion could achieve the goal of producing the PG with controllable pore size perfectly. Then we tested the specific surface area of PG by BET method. Considered the large difference between pore sizes of PG at 450  C and 500  C for 1 min, we selected these two samples to conduct the BET analysis. The specific surface area of PG at 450  C and 500  C for 1 min are 648 m2/g and ~1815 m2/g, respectively, which is much higher than thermally reduced GO and PG obtained by other methods (Cao et al., 2015; Li et al., 2018; Wu et al., 2012; Yan et al., 2014; Zhou et al., 2014). As shown in Fig. 3A, it can be noted that the (002) crystal plane for reduced GO at 2q ¼ 26.6 can be observed for PG, which corresponded to the interlayer spacing of 0.33 nm, according to the Bragg equation (Li et al., 2017). There is no characteristic peak corresponding to PG in PG/ZnO nanocomposite, which is probably because of the surface of graphene is covered by ZnO nanoparticles. The diffraction peak of PG is interfered by ZnO nanoparticles. All the characteristic peaks were indexed to the standard data of hexagonal wurtzite ZnO (JCPDS 36e1451). The crystallite size estimated from the XRD peak of pure ZnO nanoparticles using the Scherrer equation (Cullity and Stock, 2014) was ~31 nm. It can be observed that the XRD crystal size of ZnO nanoparticles which is in good

agreement with TEM. FT-IR spectra of PG, PG/ZnO and ZnO are shown in Fig. 3B. Compared with the reported GO, the oxygen-containing groups decreased dramatically, and some of them disappeared completely. These observations confirmed that most oxygen functionalities in PG were removed (Wang et al., 2012). The PG/ZnO nanocomposite represents the same characteristic peak at 439 cm1 with the ZnO which attributed to the formation of Zn-O bond (Gondal et al., 2009; Niu et al., 2003). Raman spectrum could provide more information for the disorderly changes in PG. As shown in Fig. 3C, the Raman spectra of PG and PG/ZnO composite showed similar D and G bands of carbon structure, suggesting that the PG structure is maintained in the PG/ ZnO nanocomposite. The D band indicates the defects and amorphous structure in graphene, while the G band represents the active E2g mode of sp2 carbon atoms (Li et al., 2017). All samples exhibit two peaks at ~1355 and ~1595 cm1. The main dominant peak 433 cm1 (inset figure of Fig. 3C) is the characteristic peak of wurtzite hexagonal ZnO, which represents the Raman active optical phonon mode (Khan, 2010). XPS was used to further analyze the changes in PG functional groups (Fig. 3D). In the XPS C1s spectra of PG, we observed that three main components: 284.8 eV, 286.2 eV and 287.2 eV, which can be corresponded to the C-C bond, C-O bond of epoxy and

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Fig. 3. Characterizations of PG/ZnO, PG and ZnO. (A) XRD; (B) FT-IR spectra; (C) Raman spectra; (D) C1s spectra in XPS.

hydroxyl, and the C¼O from carboxylic acid, respectively (Li et al., 2012). The C1s XPS spectra of PG/ZnO with four main components: C-C, C-O, C¼O and O¼C-OH (289.0 eV) bond. The O¼C-OH bond in PG/ZnO is attributed to the contribution of OH groups bounded to Zn atoms (Liang et al., 2014). The O1s of ZnO and whole spectra of XPS of PG, PG/ZnO and ZnO can be found as shown in Fig. S3 (Supporting Information). TGA curves of PG, PG/ZnO nanocomposite and ZnO nanoparticles were used to gain insight into the stability of the materials (Fig. S4, Supporting Information). In the TGA results, both PG and PG/ZnO show an obvious mass loss over a wide range of temperature, while no change in ZnO confirms the purity and phase of nanoparticles. Through the result of TGA, the PG/ZnO nanocomposite with ~6% weight addition ratio of PG.

natural sunlight irradiation. For further study the optical properties of samples, the band gap energy (Eg) of PG/ZnO and pure ZnO was calculated. Considered the ZnO is a direct band gap semiconductor, so Eg of PG/ZnO and ZnO can be obtained from following equation (Wang et al., 2014):

ðahvÞ2 ¼ A hv  Eg




Here, a represents the absorption coefficient, hv indicates the photon energy, A is a constant and Eg denotes the direct band gap energy. As shown in Fig. 4B, Eg values of PG/ZnO and ZnO are found to be 2.38 and 2.54 eV, respectively. The narrower bad gap of PG/ZnO due to the synergistic interaction between ZnO and PG (Zhang et al., 2010).

3.2. Optical properties 3.3. Photocatalytic performance The optical properties of PG/ZnO and ZnO were determined by the UV-vis diffuse reflectance spectra (DRS) (Fig. 4A). Compared with pure ZnO, it was clear that the adsorption of PG/ZnO nanocomposites in visible light shows stronger response and noticeably red-shifted absorption edge, which reveals that the PG/ZnO nanocomposite has a potential photocatalytic performance under

3.3.1. Photocatalytic degradation of MO under sunlight irradiation The absorption spectrum of photocatalytic degradation of MO dye using PG, PG/ZnO nanocomposites and ZnO catalyst under natural sunlight irradiation for different time intervals were shown in Fig. S5. There are two characteristic absorption peaks of MO

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Fig. 4. (A) UV-vis reflectance spectra (DRS) of the PG/ZnO and ZnO; (B) The corresponding Tauc plots of (ahv)2 versus hv of the PG/ZnO and ZnO.

(270 nm and 470 nm). After reach adsorption-desorption equilibrium, there are no significant changes were observed in MO absorption spectra in the presence of PG under natural sunlight irradiation. After photocatalytic degradation experiment finished, ethanol as the elution was used to remove MO from the adsorbed materials (Fig. S6). We can see that pure PG can only adsorb MO due to the adsorption performance of PG, while PG/ZnO nanocomposites and ZnO nanoparticles have photodegradation performance to MO. Extend of decomposition of the MO dye for different time intervals over PG/ZnO nanocomposite, PG and ZnO nanoparticles were shown in Fig. 5. After reaching adsorption-desorption equilibrium, no significant changes were observed in MO degradation in the presence of photocatalyst under dark conditions. Pure PG only has adsorption performance without photocatalytic performance (Fig. 5A). For the photocatalyst of PG/ZnO (Fig. 5B) and ZnO (Fig. 5C), the degradation efficiency was increased with the contents of photocatalyst increasing, and then the highest efficiency can be obtain 100%. This is because, with the contents of photocatalyst increasing, the number of active sites which available for the photocatalytic reaction would increase (Sedghi and Heidari, 2016). After MO dye was exposed 150 min under the natural sunlight, the degradation efficiency of MO dye is 100% for PG/ZnO nanocomposite, while 210 min for ZnO nanoparticles, which indicated that the PG/ZnO nanocomposite has higher photocatalytic performance than ZnO for the MO decolorization under natural sunlight irradiation. Comparison results of the degradation rate of MO dye as shown in Fig. S7. The introduction of PG in the PG/ZnO nanocomposite not only can effectively reduce the electron-hole recombination, but also the high adsorption of PG increases the adsorption amount of the dye on the catalyst, thereby increasing the photocatalytic rate. The photo image of photocatalytic degradation of MO with PG/ ZnO nanocomposite and ZnO nanoparticles at different irradiation times under natural sunlight irradiation are presented in Fig. S8 (Supporting Information). The photocatalytic degradation process of MO under sunlight should follow a pseudo-first-order reaction (Eskizeybek et al., 2012; Hao et al., 2016), and equation can be expressed using as follows:lnðC0 =Ct Þ ¼ Kapp  t, where Kapp represents apparent reaction rate constant, C0 represents the initial MO concentration (mg/L) at irradiation time 0 min, and Ct represents the MO concentration at the reaction time t (mg/L).

The Kapp values can be calculated from the linear fitting methods of ln(C0/Ct) vs irradiation time (t) (Fig. S9). From Fig. 5D, it could be noted that the photocatalytic activities of the PG/ZnO nanocomposite are higher than that of pure ZnO nanoparticles under natural sunlight irradiation. The obtained results revealed that the rate constant of PG/ZnO and ZnO was 0.035 min1, and 0.023 min1, respectively, which corresponding to the catalyst concentration at 0.5 mg/mL. Furthermore, TOC results were presented in Fig. S10 for examining the mineralization degree of organic pollutants after natural sunlight irradiation. Seen from Fig. S10, it should be clearly observed that 69.2% TOC removal was reached by PG/ZnO within 210 min natural sunlight irradiation, while 44.8% TOC removal for ZnO under same condition. The residual TOC might be related to some small molecular organic acids produced by the catalytic reaction (Zhang et al., 2018a,b). TOC results indicate that PG/ZnO can be used for wastewater treatment and purification due to its high photocatalytic performance. We have also compared the photocatalytic performance of PG/ ZnO nanocomposite with ZnO-based nanocomposites and ZnO nanoparticles prepared by other chemically synthesized methods (Ali et al., 2018; Chen et al., 2013; Hao et al., 2016; Kansal et al., 2007; Karnan and Selvakumar, 2016; Trandafilovi c et al., 2017; Xu et al., 2010; Xu et al., 2016; Zhu et al., 2012) as listed in Table S1. Our findings verified that combine PG with ZnO nanoparticles plays a very important role in promoting the photocatalytic activity of the ZnO composite. 3.3.2. Photocatalytic mechanism Photoluminescence and photoelectrochemical analysis were conducted to further understanding the mechanism of photodegradation reaction. As shown in Fig. 6A, the photocurrent transient response (i-t curves) for the PG/ZnO and ZnO electrodes showed that PG/ZnO has the higher photocurrent response than the pure ZnO, suggesting the remarkably improved separation and migration efficiency of charge carriers (Zhang et al., 2013), which is in agreement with the relative higher photoactivity of PG/ZnO toward MO degradation. The electrochemical impedance spectroscopy (EIS) Nyquist plots of PG/ZnO and ZnO indicated that the PG/ZnO nanocomposite displayed a smaller semicircle than that of the ZnO (Fig. 6B), suggesting that the transfer of charge carriers of PG/ZnO more efficient than ZnO (Gao et al., 2019; Gao et al., 2018a; Meng et al., 2018). The

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Fig. 5. Extend of decomposition of the MO dye with respect to time intervals over PG (A), PG/ZnO nanocomposite (B) and ZnO (C) nanoparticles under natural sunlight irradiation; Apparent rate constants (Kapp) of MO dye degradation for different catalysts concentrations (D). Catalyst concentration: (a): 0.5 mg/mL; (b): 0.25 mg/mL; (c): 0.125 mg/mL; (d): 0.0625 mg/mL. Initial concentration of dyes: 13 mg/L.

steady-state and time-resolved PL analysis, as displayed in Fig. 6C and D. PL spectra under 350 nm excitation wavelength of pure ZnO exhibits a broad band around 428 nm. While compared with ZnO, PL intensity of PG/ZnO showed a sharp decline. There are reports had proved that the reduction in exciton PL intensity is an evidence of effective suppression of recombination of electron-hole pair, which implies the longer lifetime of the photogenerated electrons (Gao et al., 2019; Gao et al., 2018b; Maruthamani et al., 2015). In order to gain more information on the lifetime of photogenerated charge, TRPL measurements were shown in Fig. 6D. Since the lifetime of the sample exceeds the detectable range of the optical system of the test instrument used in this study, so the fitting parameters cannot be given. The PL intensity of PG/ZnO has significantly lower than pure ZnO, while the TRPL intensity of PG/ZnO similar as ZnO. These results indicate the electronic properties of PG are retained in PG/ZnO nanocomposite, so improved optical performance could be predicted in this study. The control experiments with adding different radical scavengers (0.02 M TBA and AO, 0.002 M BQ and K2S2O8) were carried out for reveal the role of radicals and underlying photodegradation mechanism. Fig. 7A and B display the photocatalytic activities of PG/ZnO and ZnO for the MO degradation in the presence of different radical scavengers with exposure to natural sunlight. BQ through electron transfer mechanism to trap O2 (Raja et al., 2005;

Stylidi et al., 2004). When BQ scavenger was added into the reaction system, the photocatalytic activity of PG/ZnO and ZnO was completely inhibited, suggesting that MO photodegradation is driven by the O2 radicals from PG/ZnO and ZnO under natural sunlight irradiation. AO as an effective holes scavenger (Kominami et al., 2001), when AO was added to the reaction system, the rate of degradation was greatly reduced, indicating hole plays an important role in this reaction system. The addition of TBA scavenger for OH also prevents the MO degradation but the inhibition degree is smaller than the cases of BQ and AO added. While the K2S2O8 as electrons scavenger, the degradation rate of MO was increased with K2S2O8 was added to the reaction system. This is because of the S2O28 reacts with electron to produce SO4 which is a strong oxidant. The strong oxidant SO4 could react with neutral molecule water and generates OH (Maruthamani et al., 2015; Rupa et al., 2007a; Rupa et al., 2007b). The reaction equations are as follows:

S2 O8 2 þ e ðCBÞ/SO4  þ SO4 2 SO4  þ H2 O/  OH þ SO4 2 þ Hþ Therefore, the K2S2O8 was added in reaction system could exert a dual function as an electron scavenger and a strong oxidant to increase the degradation rate.

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Fig. 6. (A) i-t curves (B) EIS Nynquist plots, (C) steady-state PL and (D) time-resolved PL (TRPL) spectra of PG/ZnO and ZnO.

Fig. 7. Controlled experiments of MO photodegradation over (A) PG/ZnO and (B) ZnO in the presence of different radical scavengers under natural sunlight irradiation: benzoquinone (BQ), ammonium oxalate (AO), tert-butyl alcohol (TBA) and potassium persulfate (K2S2O8) as superoxide radicals, holes, hydroxyl radicals and electrons scavengers, respectively.

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Fig. 8. The reaction mechanism for enhancement of MO photocatalytic degradation by PG/ZnO nanocomposite.

Based on the above experiments, a possible mechanism for enhancement of MO photocatalytic degradation by PG/ZnO nanocomposite as illustrated in Fig. 8. It can be concluded that the improved photocatalytic performance of PG/ZnO nanocomposite mainly attributed to PG has enhanced adsorption capacity, excellent electrical conductivity, and the good hybrid layer could act as an adsorption active site. Under natural sunlight irradiation, the holes (hþ) were produced in the valence band (VB) due to the electrons (e-) of ZnO in VB were excited to the conduction band (CB). Since PG has excellent electrical conductivity, the photoexcited electrons can be rapidly captured and transferred on the surface of PG, which effectively suppresses the electrons-holes recombination and more charge carriers were left to produce highly reactive substances (O2 and hþ) (Khan and Narula, 2018; Sedghi and Heidari, 2016; Zhang et al., 2009). Then the O2 react with H2O could generate strong oxidant OH to decompose MO dyes. In addition, the surface hydroxyl group (OH-) of ZnO also could trap holes (hþ) to produce OH. Therefore, the PG/ZnO nanocomposite can be considered an ideal and effective material in removing MO dye due to the synergetic effect of photocatalysis and adsorption. 3.3.3. Photocatalytic stability We used five photocatalytic runs for the MO dye to evaluate photocatalytic stability of the PG/ZnO composite and ZnO nanoparticles. As shown in Fig. S11, the photocatalytic activity of the PG/ ZnO nanocomposite and ZnO nanoparticles did not decrease conspicuously. Then after the five successive photocatalytic degradation tests, about 99% and 98% photodegradation efficiency still can be obtained for PG/ZnO nanocomposite and ZnO nanoparticles, respectively, which indicating that the catalysts were quite stable. Moreover, the SEM and XRD test results of PG/ZnO and ZnO after the recycle experiments were showed in Figs. S12 and S13. There was no observable morphological and crystal structures change in the photocatalysts. These results demonstrate that both the PG/ZnO nanocomposites and ZnO have good photocatalytic repeatability and stability over prolonged natural sunlight irradiation.

PG, ZnO and their nanocomposite (PG/ZnO) by fine tuning of partial combustion under muffle furnace within few minutes. After comparison, PG/ZnO nanocomposite exhibited the best photocatalytic performance for the MO degradation under natural sunlight condition. The results demonstrated that PG/ZnO nanocomposite can be an excellent photocatalyst for the purification of environmental organic contaminants. This facile approach may open a door for the preparation of PG, metal oxide and their nanocomposites with promising applications such as catalysis, electrochemistry and energy etc. Associated content Supporting Information Supplementary data related to this article can be found at Elsevier Publications website. Pore size distribution results, whole spectrum of XPS, TGA analysis results, optical absorption spectra and photo image of photocatalytic degradation of MO dye by materials, TOC removal results, comparison results of photocatalytic activity in this study with other studies, photocatalytic stability of materials, SEM and XRD pattern of materials after reaction, can be found in the Supporting Information. Notes The authors declare no conflicting financial interest. Acknowledgements This research was supported by National Science Foundation of China (Nos. 21675164 and 21822407) and the top priority program of “One-Three-Five” Strategic Planning of Lanzhou Institute of Chemical Physics, CAS. Appendix A. Supplementary data

4. Conclusions In conclusion, we report a facile strategy to rapidly synthesize

Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.03.071.

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