Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination

JES-01163; No of Pages 10 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 7 ) XX X–XXX

Available online at www.sciencedirect.com

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Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination Adel A. Ismail1,⁎, M. Faisal2 , Adel Al-Haddad1

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1. Wastewater Treatment and Reclamation Technologies (WTRT), Water Research Center, Kuwait Institute for Scientific Research, Safat 13109, Kuwait 2. Advanced Materials and Nano-Research Centre, Najran University, Najran 11001, Saudi Arabia

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Article history:

Advanced oxidation technologies are a friendly environmental approach for the 16

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Received 2 January 2017

remediation of industrial wastewaters. Here, one pot synthesis of mesoporous WO3 and 17

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Revised 27 April 2017

WO3-graphene oxide (GO) nanocomposites has been performed through the sol–gel 18

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Accepted 2 May 2017

method. Then, platinum (Pt) nanoparticles were deposited onto the WO3 and WO3-GO 19

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Available online xxxx

nanocomposite through photochemical reduction to produce mesoporous Pt/WO3 and Pt/ 20

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Keywords:

clinic and triclinic WO3 phases. Transmission Electron Microscope (TEM) images of Pt/ 22

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Mesoporous

WO3-GO nanocomposites exhibited that WO3 nanoparticles are obviously agglomerated 23

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Pt/WO3 nanocomposites

and the particle sizes of Pt and WO3 are ~10 nm and 20–50 nm, respectively. The 24

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Graphene oxide

mesoporous Pt/WO3 and Pt/WO3-GO nanocomposites were assessed for photocatalytic 25

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Methylene Blue photodegradation

degradation of Methylene Blue (MB) as a probe molecule under visible light illumination. 26

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Visible light

The findings showed that mesoporous Pt/WO3, WO3-GO and Pt/WO3-GO nanocomposites 27

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WO3-GO nanocomposites. X-ray diffraction (XRD) findings exhibit a formation of mono- 21

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exhibited

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photodegradation rates by mesoporous Pt/WO3-GO nanocomposites are 3, 2 and 1.15 29 times greater than those by mesoporous WO3, WO3-GO, and Pt/WO3, respectively. The key 30

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factors of the enhanced photocatalytic performance of Pt/WO3-GO nanocomposites could 31

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WO3 and GO sheets, in addition to the Pt nanoparticles that act as active sites for O2 33 reduction, which suppresses the electron hole pair recombination in the Pt/WO3-GO 34 © 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences.

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Introduction

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To solve the industrial wastewater problems, heterogeneous photocatalysis is one of the most favorable scenarios employing advanced oxidation technology and therefore extreme attention has been paid to it in recent times (Spasiano et al., 2015). However, the disadvantages of a heterogeneous photocatalysis

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nanocomposites.

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be explained by the highly freedom electron transfer through the synergetic effect between 32 Q5

Published by Elsevier B.V.

are minimal photonic efficiency, fast recombination rate of the charge carriers (photogenerated hole–electron), low absorption range, and poisoning of the active sites of photocatalyst (Chowdhury and Balasubramanian, 2014). There are various ways to improve the photocatalysts features and increase the quantum efficiency by adding other semiconductors, precious metals or carbon based

⁎ Corresponding author. E-mail: [email protected] (Adel A. Ismail).

http://dx.doi.org/10.1016/j.jes.2017.05.001 1001-0742 © 2017 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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1. Experimental section

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1.1. Materials

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light illumination. This work demonstrates the findings of our recent investigations on the influence of merging Pt nanoparticles, mesoporous WO3 and GO for the first time as an efficient photocatalyst under visible light illumination to investigate the relationship between synergetic effect of Pt, WO3 and GO on their photocatalytic performance.

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Graphite powder, F-127 the copolymer surfactant EO106-PO70EO106 (EO = \CH2CH2O\ and PO = \CH2(CH3)CHO\) with molecular weight = 12,600 g/mol), hydrogen peroxide solution (30%, w/v) tungstic acid (H2WO4, 99%), C2H5OH, sodium nitrate (NaNO3, 98%), potassium permanganate (KMnO4, 99.5%), chloroplatinic acid (H2PtCl6), HCl, H2SO4 95%–97% and CH3COOH were purchased from Sigma-Aldrich.

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The GO was prepared according to the adaptation of Hummers' methods and the procedure was carried out elsewhere (Hummers and Offeman, 1958; Liu et al., 2010). To homogenously decorate WO3 nanoparticles into the GO sheets, surfactant, WO3, and GO were mixed in one-pot synthesis approach to obtain 80 wt.% WO3–20 wt.% GO nanocomposites. To reduce the possible changeable factors, the molar ratio of the precursors was maintained at 1/0.02/50/2.25/3.75 for WO3/F127/C2H5OH/HCl/ CH3COOH, respectively. In particular, 1.6 g of F127 was dissolved in 30 mL of ethanol by stirring for 60 min and then 2.3 mL of CH3COOH and 0.74 mL of HCl were gradually added while stirring magnetically. The desired amount of H2WO4 and GO were added in the above mixture while stirring for 60 min. Afterward, the mixture was placed in a Petri dish in a humidity chamber at 40°C and a relative humidity of 40%–80% for 12 hr, so that ethanol would evaporate and polymerize of F127 with GO and H2WO4, then the samples were transferred to the oven for drying at 65°C for 24 hr. The as-prepared hybrids were annealed at 450°C in for 4 hr with a heating rate and a cooling rate of 2°C/min to expel the surfactant and produce mesoporous WO3-GO nanocomposites. On the other hand, the mesoporous WO3 was prepared as the same processes of WO3-GO nanocomposite without addition of GO.

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1.2. Preparation of mesoporous WO3 or WO3-GO

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materials (Chowdhury and Balasubramanian, 2014; Gupta et al., 2015; Wang et al., 2013; Ismail et al., 2012) Semiconductor65 graphene nanocomposite is a strategic approach to enhance the 66 photocatalytic activity and hence increase the quantum efficien67 cy (Chowdhury and Balasubramanian, 2014; Gupta et al., 2015; 68 Wang et al., 2013). Graphene has 2D structure material with 69 carbon atoms bonded sp2- which are connected together to 70 obtain hexagonal structures, that each carbon atom is 71 covalently connected with three carbon atoms (Bu et al., 72 2013). It possesses a good acceptor, conductive channel, and 73 photogenerated electrons reservoir, thus noteworthy enhancing 74 the photocatalytic efficiency of semiconductor-graphene nano75 composites (Chowdhury and Balasubramanian, 2014; Li et al., 76 2016). 77 WO3 is very stable in a different environment, is harmless 78 and driven visible-light photocatalysts with 2.7 eV bandgap. It 79 is commonly employed for molecular O2 evolution along with 80 an electron acceptor such as AgNO3, owing to its very low 81 valence band (Abe et al., 2008; Maeda et al., 2010; Zhao and 82 Miyauchi, 2008). However, a rather minimal conduction band 83 has restricted its potential application. Therefore, pure WO3 is 84 not an efficient photocatalyst (Miyauchi, 2008; Bamwenda et 85 al., 1999; Bamwenda and Arakawa, 2001) that does not supply 86 an appropriate potential for O2 reduction potential (E0) [E0 (O2/ 87 Q7 HO2U) = − 0.05 V′ against Normal Hydrogen Electrode (NHE) 0 − 88 Q8 and E (O2/O2U ) = − 0.33 VNHE (Sawyer and Valentine, 1981). 89 Thus, WO3 is not able to scavenge photogenerated electrons to 90 reduce molecular O2, which leads to the fast charge carrier 91 recombination rate and therefore, depress the photonic efficien92 cy. To avoid this problem, Pt nanoparticle doped WO3 shows a 93 high photocatalytic efficiency for the photodegradation of toxic 94 organic pollutants under visible light (Kim et al., 2010), due to the 95 fact that Pt surface expedites the dioxygen multielectron 96 reduction (Abe et al., 2008; Ismail and Bahnemann, 2011), which 97 has a more positive potential for 4-electron reduction and 98 2-electron reduction (David, 1996) than the one-electron 99 Q9 reduction. Pt/WO3 has been synthesized with different morphol100 ogies such as Pt/WO3 nanotube (Zhao and Miyauchi, 2008), 101 macroporous Pt/WO3 (Sadakane et al., 2008), and Pt/WO3 hollow 102 structure (Zhao and Miyauchi, 2009) to promote its potential 103 application. The mesostructure materials possess a high 104 surface area that promotes reactant adsorption and diffusion 105 into the mesopore channels (Villa et al., 2015), indicating a 106 reduction of charge carrier recombination and thus, enhancing 107 photocatalytic performances. Mesoporous WO3 has been wide108 ly synthesized using different techniques in the presence and 109 absence of templates (Teoh et al., 2003; Zhenhai et al., 2013; Yan 110 et al., 2012; Usami et al., 2012; H. Zheng et al., 2016; Y. Zheng et 111 Q10 al., 2016). The extension of charge-carrier lifetime and reduction 112 of electron–hole recombination are considered to be the key 113 factors for enhancing the photocatalytic performance of 114 semiconductor-graphene nanocomposites (Trapalis et al., 115 2016). To the best of my knowledge, the implementation of 116 mesoporous Pt/WO3-graphene (GO) nanocomposites for the 117 photodegradation of organic pollutants has not been reported 118 yet. Thus, our approach here focused on the synthesis of 119 mesoporous WO3-GO and Pt/WO3-GO nanocomposites through 120 a facile one-pot through sol–gel process employing a structure121 directing template and their use for photodegradation of 122 Methylene Blue (MB) as a probe pollutant model under visible 64

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1.3. Preparation of 0.5 wt% Pt/WO3 and 0.5 wt% Pt/WO3-GO 163 Pt was photochemically reduced onto surface and pores of either WO3 or WO3-GO. In typical, 1.0 g of mesoporous WO3-GO nanocomposite was added 200 mL aqueous 1% v/v methanol solution and then added H2PtCl6 solution [2.56 × 10−5 mol] with magnetic stirring. The mixture was subjected to a Philips Hg lamp with ultraviolet (UV) radiation light for 12 hr (the intensity is 2.0 mW/cm2 at 350 nm wavelength). The obtained materials was collected by centrifuging. Then the produced materials were washed three times with acetone and water and finally dried at 110°C overnight to produce 0.5 wt.% Pt/WO3 and 0.5 wt.% Pt/WO3-GO nanocomposites.

Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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Photocatalytic experiment series was carried out to photograde of MB as a model pollutant over the mesoporous WO3, WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites under visible light illumination. Before the illumination, 100 mL MB (0.01 mmol/L) was vigorously stirring for 2 h in the dark in the presence of 0.5 g/L of photocatalyst to determine adsorption equilibrium, thus the consumed MB through adsorption was taken into account. 250 W (250 W lamp, OSRAM Licht AG, Germany) visible lamp with wavelength range 400–700 nm was horizontally maintained above the reactor ~ 20 cm. During experiments, the suspension solution was constantly aerated to provide molecular O2 to expedite the photocatalytic reaction. The MB concentration was determined at interval time at λ = 663 nm, the absorbance coinciding with maximum wavelength of MB by UV–visible spectrophotometer. The photocatalytic efficiency (%) was figured by the following formula:

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Photocatalytic efficiency ¼ ð1−Ct Þ=Co  100 224 223 225 226 227 228 229

where Co and Ct are the initial concentration of MB and the concentration of MB at the illumination time t, respectively. Phoenix 8000 UV-persulfate total organic compound (TOC) Analyzer (Phoenix 8000 UV, Teledyne Technologies Inc., UK) was employed to measure TOC of MB before and after photodegradation experiments.

Fig. 1 displays the XRD pattern of crystal structure of the mesoporous WO3, WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites. The findings indicated that the bare WO3 can be accurately indexed as the monoclinic and a triclinic WO3 modification (Szilagyi et al., 2012). The XRD patterns of the WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites are identical to those of mesoporous WO3 but without the peaks determined to the sheets of GO as a result of successful transformation of GO and its low contents in the final prepared nanocomposites. All of the samples exhibit well crystallinity, suggesting that there is no significant modification in WO3 crystal structure with adding of GO (Li et al., 2016). In addition, no other distinctive peaks for the impurities were assigned, which indicates that the WO3 monoclinic and triclinic were composed without any other phases during the sol–gel synthesis approach. On the other hand, Fig. 1 curves c and d shows the XRD pattern of Pt/WO3 and Pt/WO3-GO nanocomposites. The findings revealed that there are no assigned peaks for Pt nanoparticles owing to its low content in the samples and well dispersed. Fig. 2A illustrates the Raman spectra of the WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites. The three main peaks at 262.4, 701.3 and 802.8 cm−1 are typical characteristics of the WO3 monoclinic phase structure, which matches with the XRD results (Villa et al., 2016; Ismail et al., 2016). Fig. 2A inset exhibits Raman spectra of GO that two peaks were located at 1321 and 1591 cm−1, related to sp3 and sp2 hybridized carbon atoms of hexagonal graphitic structure (D) and sp2 carbon-type structure (G) bands, respectively. The peak intensities of WO3-GO, Pt/WO3 and Pt/ WO3-GO nanocomposites are assigned at 701.3 and 802.8 cm−1, conformable to ν(O\W\O) vibration mode. It is noteworthy that the location of G band is proceeded from 1591 to 1601 cm−1 and the broader D bands shifted from 1321 to 1327 cm−1 in the WO3-GO and Pt/WO3-GO nanocomposites, respectively, indicating a chemical reaction between WO3 and GO (Huang et al., 2013). Fig. 2B shows the FT-IR spectra of GO, WO3-GO, Pt/WO3 and Pt/ WO3-GO samples. Fig. 2Ba of GO revealed that the absorption peak was centered at ~1739 cm−1 as a result of stretching C_O located at the GO sheets of COOH groups edge. The absorption peaks at 1050 and 1229 cm−1 were attributed to the C\O stretching mode and tertiary C\OH, respectively. The wider band at ~3436 cm−1 is explained by stretching vibrations of O\H of H2O molecules surface absorption (Xu et al., 2013, 2016; Bai et al., 2013) and the peak was assigned at ~1617 cm−1 due to the O\H bending. At WO3-GO, Pt/WO3 and Pt/WO3-GO samples, the strong band at ~820 cm−1 is attributed to stretching O\W\O vibration in a WO3 monoclinic phase structure (Guery et al., 1997). It is clearly observed that the peak of stretching O\W\O vibration was assigned in all of the prepared nanocomposites. Also, the absorption peak intensity at 820 cm−1 was modified as stated in the presence of GO, that may be owing to the interaction between GO and WO3 in the prepared nanocomposites. It is noteworthy that the characteristic peak position of WO3 did not shift after adding of GO, suggesting no covalent bond appeared between GO and WO3 in the prepared nanocomposites.

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Mesoporous Pt/WO3-GO nanocomposite was investigated by Transmission Electron Microscope (TEM) at 200 kV with a JEOL (JEM-2100F-UHR, JEOL Ltd., Japan) field-emission instrument fitted with a Gatan GIF 2001 energy filter and a 1k-CCD camera to register electron energy-loss (EEL) spectra combined with energy dispersive X-ray spectroscopy (EDXS) to analyze the elemental compositions. Scanning transmission electron microscope (STEM) was employed to obtain high-angle annular dark-field (HAADF) micrographs. X-ray diffraction (XRD) data were performed by Bruker AXS D4 Endeavor X diffractometer (Bruker AXS D4, Bruker Corporation, US). Bruker Optics IFS66v/s Fourier transforms infrared spectrometer (FT-IR) spectrometer with FRA-106 (Bruker FRA 106, Bruker Corporation, US) Raman attachment was employed to record Raman spectra of the prepared materials. FT-IR spectrum at 400 to 4000 cm− 1 was recorded using Perkin Elmer by mixing KBr with the desired sample (PerkinElmer Inc., US). Photoluminescence (PL) spectra were conducted using fluorescence spectrophotometer F-7000 (F-7000, Hitachi, Japan). The PL intensity was recorded at an excitation wavelength of 315 nm using a xenon lamp. The PL data show the relation between WO3-GO, Pt/WO3 and Pt/WO3-GO photocatalysts and photogenerated electron–hole recombination process. The reflectance spectra were recorded with a Perkin Elmer (Lambda 950, PerkinElmer Inc., US) UV–visible spectrometer at 200–700 nm with a labsphere integrating sphere diffuse reflectance accessory. Then, the bandgap energy of the prepared photocatalysts was derived from the diffuse reflectance spectra.

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Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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WO3 and WO3-GO. The bandgap values are determined with 324 an experimental error range ± 0.1 eV. 325 326

The photocatalytic activities of the mesoporous WO3, WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites for MB photodegradation under visible light illumination are depicted in Fig. 5. The concentration of MB was maintained upon illumination for 2 hr in the absence of photocatalyst, which confirms that there was no photolysis of MB. The equilibrium adsorption in the dark was performed for 2 hr and it was observed that 5%–10% MB concentration was reduced through equilibrium adsorption. The UV–vis spectroscopic absorbance of MB was recorded over the prepared photocatalysts upon illumination as presented in Fig. 5. The distinctive MB absorption peak at λ = 663 nm was used to measure the photocatalytic performance. As demonstrated in Fig. 5, the two absorption bands of MB at λ = 663 and 292 nm were gradually reduced with the boost upon

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The TEM images of the mesoporous Pt/WO3-GO nanocomposite exhibited the agglomerates of WO3 at different particle sizes and the agglomerates consisting of mesoporous WO3 nanoparticles are obvious (Fig. 3a). The particle sizes of the Pt and WO3 are ~ 10 nm and 20–50 nm, respectively (Fig. 3a). The lattice fringe is clearly observed at d = 0.375 nm which corresponds the (020) monoclinic WO3 plane (Fig. 3b). Selected area electron diffraction (SAED) pattern (Fig. 3a and b inset) confirms that a lot of small particles in the analyzed area and the WO3 structure of the material are polycrystalline. The mesoporous Pt/WO3-GO nanocomposite shows pores with different diameters (Fig. 3c and d). Fig. 3c shows the porous of WO3 and the marked area (white circles) is shown at higher magnification (Fig. 3d). The W, Pt and O elements in EDXS analysis was determined. There is a strong signal overlap for the both elements W and Pt (see white arrows). Also, Pt element was detected and its distribution is not homogeneous (Fig. 3e). In the colored summary element maps the chemical composition by different colors can be clearly seen (Fig. 3f). This is an indicator for the material composition, consisting of WO3 nanoparticles (green areas) and Pt nanoparticles (blue area) (Fig. 3f). Diffuse reflectance UV–visible spectra of WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites are depicted in Fig. 4. UV–vis spectra were recorded in the diffuse reflectance mode (R) and converted to the Kubelka–Munk function F(R) to subtract the magnitude of absorption from scattering light. Thus, the bandgap energy was determined from the relation between of the modified Kubelka–Munk function (F(R)E1/2) and the energy of the absorbed light E (Tauc et al., 1966).

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Fig. 1 – XRD (X-ray diffraction) patterns of (a) WO3, (b) WO3-GO, (c) Pt/WO3 and (d) Pt/WO3-GO nanocomposites. GO: graphene oxide; Pt: platinum.

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where h is a constant which is 6.626 × 10−34 J·sec, and v is the frequency of the light in sec−1. The band gaps of the WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites amounted to 2.74, 2.75 and 2.65 eV respectively and the addition of Pt nanoparticles are insignificant for modifying the band gap values of

Fig. 2 – A: Raman spectra of (a) WO3-GO, (b) Pt/WO3 and (c) Pt/WO3-GO, and (inset) bare graphene oxide (GO); B: Fourier transforms infrared spectrometer (FT-IR) spectra of (a) GO, (b) WO3-GO, (c) Pt/WO3 and (d) Pt/WO3-GO samples. GO: graphene oxide; Pt: platinum.

Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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246 nm which suggests the entire photodegradation of aromatic cyclic in MB structures (Mahamoud et al., 2012; Fateh et al., 2011; Park and Choi, 2005). It is noteworthy that MB almost degraded ~95% under visible light illumination within 70 min over Pt/WO3-GO nanocomposite. The relation between C/Co and illumination time was demonstrated in Fig. 6a. The findings revealed that the

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illumination, and almost vanished after 70 min illumination. The absorbance of the main peak of MB at 663 nm, progressively reduced from 0.67 to 0.25 after 70 min illumination over mesoporous WO3 photocatalyst (Fig. 5a). However, mesoporous Pt/WO3-GO nanocomposite was prompted MB decoloration at absorbance values which reduced from 0.67 to 0.05 (Fig. 5b). We could not determine a characteristic band for leuco-MB at

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Fig. 3 – (a) High Resolution Transmission Electron Microscope (HRTEM) images of the mesoporous Pt/WO3-GO nanocomposite; (b) the lattice fringes exhibiting the typical distances WO3 (020) crystallographic plane (3.75 Å); (a inset and b inset) selected area electron diffraction (SAED) pattern; (c and d) mesoporous Pt/WO3-GO nanocomposite showing pores with different diameters; (d) the marked area shown in the micrograph at higher magnification; (e) colored energy dispersive X-ray spectroscopy (EDXS) element maps over all elements consisting of tungsten oxide (green areas) and platinum (blue area) and white circle exhibit different pore diameters; (f) representative EDX spectrum. GO: graphene oxide; Pt: platinum. Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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dominate the restriction of mass transfer by increasing the accessibility of MB near the Pt/WO3-GO nanocomposites surface. The faster MB photodegradation rate over Pt/WO3-GO nanocomposite refers to the further advantages of concentrating the MB near the Pt/WO3-GO nanocomposites surface. It can be documented that WO3 has a wide visible range of solar light. However the use of WO3 for the photodegradation of MB was suppressed as can be seen in the above results (Figs. 5a and 6). The main reason behind this feature is the low conduction band value of WO3, which is additional positive than the potential of oxidation/reduction reaction (Bazarjani et al., 2013). Thus, the photogenerated electron consumption was reduced and the subsequent oxidative degradation of MB by the holes was decreased (Abe et al., 2008; Irie et al., 2008). However, as shown lately for WO3, the photogenerated electrons are comprised in competition of molecular O2 multielectron reduction reactions, which are much positive than using the single-electron approaches (Abe et al., 2008; Irie et al., 2008). These multielectron processes were evidenced to be expedited by a high electron flux that can be acquired through a Pt cocatalyst dispersed on the surface and pores of mesoporous WO3, which conducts as a sink of electrons for molecular O2 multielectron reduction reactions (Abe et al., 2008; Irie et al., 2008). In the case of WO3-GO nanocomposite, electrons in the upper level of valence band (VB) of GO are triggered to the GO conduction band (CB) of upon illumination. The electrons transferred from the CB of GO to the CB of WO3. This generated well separated charge carrier pairs, with likely outstanding photocatalytic efficiency of WO3-GO nanocomposite (Du et al., 2011). The WO3-GO nanocomposite responded to driven visible light owing to the electron transported from GO to the CB of WO3 (Seo et al., 2005; Ng et al., 2010; Ismail et al., 2013). In the case of Pt/WO3-GO nanocomposite, as shown in WO3-GO nanocomposite, the electrons are transported from the CB of GO to the CB of WO3. Subsequently, supposing a Schottky contact through the mesoporous WO3 and Pt nanoparticles, the Pt nanoparticles represent active sites for the molecular O2 reduction generating OU2− radicals due to its pool for photogenerated electrons (Scheme 1). It is known that this latter approach is considered to be the bottleneck in ultimate photocatalysis conversion reactions being the rate determining step owing to its very little thermodynamic driving force. Thus, its expedition through the electron transport catalysis prompted by the Pt nanoparticles boosted the photocatalytic performance (Scheme 1). In general, the enhancement of Pt/WO3-GO nanocomposite is explained by (i) the diminished rate of recombination of charge carrier pairs, (ii) better availability of electrons for reduction reactions o molecular O2, and (iii) generation of a high flux of electrons required for reduction of O2 via multielectron, supporting the efficient hole consumption by the oxidizing agent species. A possible photosensitization mechanism is suggested to elucidate the enhancement of photocatalytic performance of Pt/WO3-GO nanocomposite. As shown in Scheme 1, the electrons in VB of GO were excited to the CB of GO. Thus, electrons can move to the CB of WO3 surface under visible light illumination (Chai et al., 2014; Xiong et al., 2010). The electrons on the VB gain enough energy and transfer in to the CB of WO3, leaving holes on the VB. Then, the Pt nanoparticles

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photocatalytic efficiency of the mesoporous WO3 photocatalyst reached to ~63% under visible light within 70 min illumination, which can be explained by its prompt recombination rate of charge carriers (photogenerated electron– hole). The photocatalytic efficiency of WO3-GO was reached to 82% owing to merging of WO3 and GO. After Pt nanoparticle photoreduction onto the mesoporous WO3 and WO3-GO, the photocatalytic efficiency of mesoporous Pt/WO3 and Pt/ WO3-GO nanocomposites are significantly enhanced to 90% and 94%, respectively. For further evidence, TOC was analyzed to be confirmed that MB could be completely photodegraded. The results exhibited that the photocatalytic efficiency of WO3-GO and Pt/WO3-GO photocatalyst was ~80% and 92%, respectively for mineralization of MB after 6 hr illumination. It is clearly seen that to complete mineralization of MB, the photocatalytic reaction needs more illumination time than 6 hr. These enhancement were attributed to large surface area and the outstanding electronic conductivity of GO, indicating that the photogenerated electrons transfer to the nanocomposites surface, thus suppressing the recombination of charge carriers pairs. Interestingly, GO easily adsorbed MB, which is advantageous to the boosting of photocatalytic efficiency of either WO3-GO or Pt/WO3-GO. Fig. 6b shows the MB photodegradation rates over the mesoporous WO3, Pt/WO3, WO3-GO, and Pt/WO3-GO nanocomposites. The findings indicated that the MB photodegradation rate moves much more quickly over Pt/WO3-GO nanocomposite (8.2 × 10−7 mol/(L·min)) as compared to the mesoporous WO3 (2.73 × 10−7 mol/(L·min)). Interestingly, it can be clearly seen that the MB photodegradation rates by Pt/WO3-GO nanocomposite are greater three times than that by mesoporous WO3, owing to the interaction between GO and WO3 and the more efficient charge carriers transport from WO3 to GO. Also, GO can

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Fig. 4 – Ultraviolet (UV)–vis reflectance spectra of WO3-GO (a), Pt/WO3 (b) and Pt/WO3-GO (c) nanocomposites. Inset, plot of transferred Kubelka-Munk versus energy of the light absorbed of the mesoporous of Pt/WO3-GO nanocomposite. R: diffuse reflectance mode; h: a constant which is 6.626 × 10−34 J·sec; v: the frequency of the light in sec−1; E: the energy of the absorbed light; F(R): (1-R)2/(2R). GO: graphene oxide; Pt: platinum.

Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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subjects photodegradation to yield products according to Eqs. 469 (2)–(5) (Zuo et al., 2014; Le et al., 2012). 470 471

MB  þWO3 →MB˙þ e− CBðWO3  PtÞ

ð1Þ 473 472

WO3  Ptðe− Þ þ O2 →WO3  Pt þ O−• 2

ð2Þ 475 474

MB•þ þ OH− →MB þ•OH

F O

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ð4Þ

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MB•þ þ •OH→products

ð3Þ

450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468

then are represented as active sites for the reduction of molecular O2 generating O2U− radicals due to its pool for photogenerated electrons (Ismail and Bahnemann, 2011; Ismail et al., 2009). On the other hand, the GO can act as an electron acceptor of the WO3 and triggered MB dye, and then efficiently transfers the electrons away from the WO3, prohibiting the charge carrier recombination on the surface of WO3 and hence enhancing the photocatalytic efficiency. A second explanation for the higher photocatalytic activity of Pt/WO3-GO photocatalyst under visible light, photosensitized by MB oxidation which was adsorbed onto the surface and pores of mesoporous Pt/WO3-GO photocatalyst occurred as follows: the excited state of MB (MB*) drives an electron into WO3 conduction band (Eq. (1)) (Scheme 1). The cationic dye radical (MB•+) reacts with adsorbed OH− yielding surface adsorbed •OH radicals. The •OH existing on the surface of WO3-GO accelerated the degradation of MB (Eq. (3)). At the same time, the Pt particles, in contact with the WO3 network, are acting as electron sinks promoting the reduction of O2 onto their surfaces (Eq. (2)) (Scheme 1). The MB dye is then transformed to MB•+ that

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Fig. 5 – (a) Absorbance vs. wavelength as a function of illumination time (t) for the photocatalytic degradation of Methylene Blue (MB) on (a) mesoporous WO3 and (b) Pt/ WO3-GO nanocomposite. Reaction conditions: MB concentration 0.01 mmol/L, photocatalyst loading 0.5 g/L, and volume of MB 100 mL. GO: graphene oxide; Pt: platinum.

Fig. 6 – (a) Photocatalytic degradation of MB in aqueous solution over WO3, WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites under visible light; (b) MB degradation rate of WO3, WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites. Reaction conditions: MB concentration 0.01 mmol/L, photocatalyst loading 0.5 g/L, and volume of MB 100 mL. C and C0: the concentration of MB at the illumination time t and the initial concentration of MB, respectively. GO: graphene oxide; Pt: platinum.

Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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Scheme 1 – Schematic illustration of the proposed mechanism to enhance the photocatalytic efficiency of Pt/WO3-GO nanocomposite as photocatalyst for photodegradation of Methylene Blue (MB), absorption of visible light by either graphene oxide (GO) and the WO3 nanoparticle promotes an electron from the valence band to the conduction band. Pt nanoparticles act as photogenerated electrons sink. Pt: platinum.

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ð5Þ

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The •OH radicals attack the functional group in MB (C\S+_C), which is adsorbed onto the surface of Pt/WO3-GO photocatalyst. Therefore, the first step of degradation of MB can be explained by the cleavage of C\S+ = C bonds. The convert of C\S+_C bond to C\S(_O)\C needs the transformation of the double bond conjugation, which prompts the opening of aromatic ring containing both S and N heteroatoms. (Houas et al., 2001).

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The charge carrier trapping efficiency of WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites can be explained by photoluminescence (PL) spectroscopy as shown in Fig. 7. The findings exhibit a broad emission band at 400–550 nm in the visible light zone. It is thought that the peaks at 453 and 472 nm produce from the electronic transition mediated through the defect levels (oxygen vacancies in the band gap) (H. Zheng et al., 2016; Y. Zheng et al., 2016; Yu et al., 2002). It is clearly seen that the intensity of WO3-GO is drastically decreased after incorporating Pt nanoparticles. It well known the PL emission is produced from the free charge carrier recombination, the lower PL intensity of Pt/WO3-GO nanocomposite display that the it has a lower recombination rate of photo-generated electrons and holes (H. Zheng et al., 2016; Y. Zheng et al., 2016; Chen et al., 2014). The results are in good harmony with the photocatalytic investigations. These findings promote significant of Pt/WO3-GO heterojunction in the reduce of recombination and separation of photogenerated electron–hole pairs.

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3. Conclusions

510 509

In this contribution, one-pot synthesis of mesoporous WO3 and WO3-GO nanocomposite employing the F127 as a template was achieved. Then, Pt was photochemically deposited onto the mesoporous either WO3 or WO3-GO to obtain mesoporous Pt/WO3 and Pt/WO3-GO nanocomposites. The particle sizes of the Pt and WO3 are ~ 10 nm and 20–50 nm, respectively. The photocatalytic activity of mesoporous WO3 increases from 63% to 82% after Pt nanoparticles adding onto mesoporous WO3. On other hand, the photocatalytic activities

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MB•þ þ O−• 2 →products

Fig. 7 – Photoluminescence spectra of WO3-GO, Pt/WO3 and Pt/WO3-GO nanocomposites. GO: graphene oxide; Pt: platinum.

Please cite this article as: Ismail, A.A., et al., Mesoporous WO3-graphene photocatalyst for photocatalytic degradation of Methylene Blue dye under visible light illumination..., J. Environ. Sci. (2017), http://dx.doi.org/10.1016/j.jes.2017.05.001

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of WO3-GO and Pt/WO3-GO nanocomposites reached 90% and 94%, respectively. The superior contact between WO3 and GO sheets expedites the MB photodegradation rate and hence photocatalytic performance upon illumination.

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