rGO Composite by Pickering Emulsion for Photocatalytic Degradation

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Interfacial growth of TiO2-rGO composite by Pickering emulsion for photocatalytic degradation Shenping Zhang, Jian Xu, Jun Hu, Changzheng Cui, and Honglai Liu Langmuir, Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017

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Interfacial growth of TiO2-rGO composite by Pickering emulsion for photocatalytic degradation Shenping Zhang a, Jian Xu b, Jun Hu* a, Changzheng Cui* c, Honglai Liu a.

a. School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China.

b. Shanghai Institute of Measurement and Testing Technology, 1500 Zhang Heng Road, Shanghai, 201203, China

c. Environmental Protection Key Laboratory of Environmental Risk Assessment and Control on Chemical Process, School of Resources and Environmental Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China.

KEYWORDS Pickering emulsion, TiO2-rGO composites, Photocatalytic activity, Tetracycline hydrochloride degradation, 2D sandwich-like monolith. ABSTRACT

A 2D sandwich-like TiO2-rGO composite was fabricated by the Pickering emulsion approach to improve the photocatalytic efficiency. Through an in-situ growth of antase-TiO2 nanoparticles on the interface of O/W type GO Pickering emulsion, TiO2 nanoparticles were closely and densely packed on the surface of well-exfoliated rGO

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sheets, meanwhile, many mesoporous voids acted as the adsorption chamber and microreactor were produced. Evaluated by methylene blue (MB) degradation, its photocatalytic activity was prominent compared with the common TiO2-based photocatalyst, with the rate constants 5 and 3.1 times higher under visible light and xenon lamp, respectively. When we applied it in the photocatalytic degradation of tetracycline hydrochloride (TCH, such as 10 ppm) under the visible light without adding any oxidants, the total removal efficiency was as high as 94% after 40 min. The mechanism of this good photocatalytic efficiency was illustrated by the scavenger trapping tests, which showed that this unique structure of TiO2-rGO composite induced by the Pickering emulsion can significantly enhance the light absorption ability, accelerate the separation rate of electron–hole pairs, increase the adsorption capacity of organic pollutants, and hence improve the photocatalytic efficiency. INTRODUCTION Antibiotics are widely used in concentrated animal feeding operations to treat diseases and improve the growth rate of animals.1,2 The tetracycline antibiotics (TCs) are the most important antibiotic families due to their great therapeutic values.3,4 But 70–90% of the administered dose of TCs are excreted, leading ultimate presence in surface water, ground water, and even treated drinking water.5-9 What’s more, TCs are stable under visible light and difficult to be degraded by biological methods,10 which causes serious environmental problems.11,12 To effectively remove antibiotics becomes a burning issue in the world.


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To date, the photocatalytic degradation is as an efficient technology for removal of antibiotics.13 Among various photocatalysts, TiO2 is regarded as one of the most promising materials.14,15 However, only the ultraviolet (UV) ray consisting of about 5% of the solar spectrum can be absorbed by TiO2 due to the relatively wide band gap (3.20 eV) and high electron–hole recombination rate, which seriously limits its photocatalytic performance and practical applications. Therefore, many TiO2-based composites, such as with metal or nonmetal elements doping and functional groups coupling, have been adopted to enhance the light absorption ability.16-19 Carbon materials are usually used as the support for producing TiO2-based photocatalysts.20,21 Recently, two dimensional graphene and reduced-grapheme oxide (r-GO) sheets have attracted many attentions for their unique structure and properties.22 Sharma et al23 reported the fabrication of TiO2-rGO-CoO photocatalyst by a modified sol-gel method through an in-situ metal doping process. Deepak Kumar et al24 synthesized rGO supported TiO2 nanocomposites by simply mixing anatase-TiO2 nanoparticles with GO ethanol solution under the microwave radiation. Qiu et al25 applied the UVinduced reduction method to prepare TiO2-rGO nanocomposites as the electrode material. However, to produce mesoporous TiO2-rGO composite monolith with specific structures for efficiently enhancing the photocatalytic efficiency still remains great challenges. Pickering emulsion,26 stabilized by the assembly of nanoparticles at oil-water interface, provides a new strategy for producing nanoparticle composites.27-29 With


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oxygen-contained polar groups, amphiphilic GO can acted as a successful stabilizer for the Pickering emulsion.30,31 In our previous work, we found GO sheets could be extremely well exfoliated at the oil-water interface,32,33 which provided large surface area for in-situ interfacial growth of nanoparticles, and hence improved specific properties significantly. Herein, we used Pickering emulsion approach to produce TiO2-rGO composite (Scheme 1). By controlling the hydrolysis rate of tetrabutyl titanate (TBT) through hydrochloric acid, TiO2 nanocrystals successfully grew up at the interface of GO Pickering emulsion to produce a 2D sandwich-like composite monolith. The photocatalytic activity of the obtained mesoporous TiO2-rGO composite were evaluated by the degradation of commercial dye of methylene blue (MB). Then, we selected tetracycline hydrochloride (TCH) as the target antibiotic pollutant to demonstrate their photocatalytic application under visible light, the synergistic effect of adsorption and photodegradation under visible light can significantly promote the removal efficiency of TCH. Finally the photocatalytic mechanism of TiO2-rGO composite were proposed based on the scavenger trapping test for photogenerated radicals and holes.


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Scheme 1. Fabrication of the TiO2-GO composite through the interfacial growth approach induced by GO Pickering emulsion. EXPERIMENTAL SECTION Materials. Tetrabutyl titanate (TBT 98.0%), n-octanol (99.0%), n-butyl alcohol (99.5%) were purchased from Shanghai Lingfeng Chemical Reagent Co, Ltd. Methylene Blue (MB 98.0%), Tetracycline hydrochloride (TCH 99.0%) were purchased from Adamas Reagent Co, Ltd. Natural flake graphite was supplied by Huadong Graphite Processing Factory. All the reagents were used as purchased without further purification. Preparation of graphite oxide. GO was synthesized by a modified Hummers method34 from natural flake graphite. Details can be found in our previous work.33 Preparation of TiO2-rGO composites. For the Pickering emulsion system, the oil phase consisted of n-octanol (4 mL) and nbutyl alcohol (1 mL); whereas the water phase consisted of water (5 mL), GO


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aqueous suspension (5 mL, 2 mg·mL-1) and hydrochloric acid. The oil and water solutions were added together in a 25 mL beaker and emulsified with a B25 emulsifier (B.R.T Co., Ltd.) at a speed of 10000 rpm for 5 min. During the emulsification, TBT was added slowly into the emulsion system. The obtained sepia emulsions was left to stand at room temperature for 24 h to ensure the complete hydrolysis of TBT. Then, the whole slurry was transferred into a 60 ml Teflon-lined stainless steel vessel and kept at 150 oC for 5 h in an oven. After cooling down to room temperature, the black product was washed with ethanol and deionized water, and was dried at 40 oC overnight. Then the calcination was carried out in the muffle roaster at 400 oC or 500 o

C for 1 h under an air atmosphere, with the heating rate of 10 oC·min-1. Pure TiO2,

denoted as p-TiO2 was also prepared for the comparison under similar conditions without adding GO. Moreover, to clearly illustrate the advantages of the Pickering emulsion method, we also prepared TiO2-rGO composite by the conventional method,35,36 which was denoted as TiO2-rGO sol-gel composite; and by the simple mixing method of commercial TiO2 nanoparticles (P25) with GO, which was denoted as P25-rGO mixture to make comparison. Preparation of TiO2-rGO sol-gel composite. In a typical synthesis, the as-synthesized graphene oxide (20 mg) was firstly dispersed in glacial acetic acid (70 mL) and sealed using parafilm, followed by ultrasonication with water bath for 3 h, then tetrabutyl titanate (TBT, 2 mL) was dropwise added into


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the above homogeneous GO suspension with stirring. The mixture was gently stirred for further 10 min and the reaction solution was then transferred into a Teflon-lined stainless steel autoclave. The following procedures were the same as preparation of TiO2-rGO composites as above. Preparation of P25-rGO mixture. 15 mg of GO was put into a solution of 45 mL of deionized water and 15 mL of ethanol under sonication for 1 h to re-exfoliate the GO thoroughly, and 1.5 g P25 nanoparticles was added to the GO suspension. Then the sonication and stirring was employed alternately for 2 h with 30 min for each step until a homogeneous suspension was achieved, which shows a uniform light gray color. The suspension was then poured into a Teflon-lined stainless steel autoclave. The following procedures were the same as preparation of TiO2-rGO composites as above. Characterization of TiO2-rGO composites. The morphology of samples was characterized by scanning electron microscope (SEM) on a JEOL JEM-6360 operated at 15 kV. The porous structure of the samples was characterized on a JEOL JEM-2100 transmission electron microscopy (TEM) operated at 200 kV. Powder X-ray diffraction (XRD) measurements were carried out on a D/Max-2550 VB/PC diffractometer (40 kV, 200 mA), using Cu Kα radiation. Nitrogen adsorption/desorption isotherms analyses were performed at 77 K on a Micromeritics ASAP-3020. All samples were outgassed for 5 h at 100 oC before measurement. The surface areas and the pore size distributions were calculated by the


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Brunauer–Emmett–Teller (BET) and the Barrett Joyner Halenda (BJH) approach, respectively. Thermogravimetric Analysis (TGA) was carried out on a NETZSCH STA-499 to determine the amount of every component. About 5 mg of sample was heated from 30 to 800 oC with a heating rate of 5 oC·min-1 and an air atmosphere with a flow rate of 20 mL·min-1. UV–Visible diffuse reflectance spectra (DRS) were obtained using a CARY 500 spectrophotometer with BaSO4 as the reflectance sample. The Fourier transform infrared (FTIR) spectra of as-prepared samples were analyzed with a NEXUS 470 spectrometer in the range of 4000–400 cm-1 with the KBr pellet technique. The ultra-violet spectrum (UV) is measured by Shimadzu, UV-2550 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000C ESCA spectrometer with an Al Kα radiation. Raman spectra were measured with a Labram HR800 spectrometer with 514 nm laser as the excitation source under ambient conditions. Photocatalytic activity. The photocatalytic activity of samples were measured by the degradation of MB and TCH. Typically, 20 mg of photocatalyst powder was dispersed in 200 mL of 5 ppm MB (or 20 ppm TCH) aqueous solution. The suspension was magnetically stirred in the dark for 1 h to ensure an adsorption/desorption equilibrium. Then the suspension was illuminated by a 300W Xenon lamp (CELHXF300, without optical filter, full spectrum) under stirring. The Xenon lamp was surrounded by the cooling water to maintain the reaction temperature as 30 oC. The pH of solution was adjusted by 0.1 M


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NaOH or 0.1 M HCl. About 4 mL suspension was sampled at regular intervals, and the concentration was monitored by a UV–Vis Spectrophotometer (UV-2550) at λmax = 665 nm for MB (λmax = 360 nm for TCH). When the xenon lamp was buffered by an UV filter, the visible light with wavelength beyond 420 nm was obtained. The blank experiment without photocatalyst and the reference experiment with p-TiO2 were also carried out for comparison. Scavenger Trapping Test for Photogenerated Radicals and Holes. The scavenger trapping experiment for photogenerated radicals and holes was carried out using isopropyl alcohol (IPA, •OH scavenger), p-benzoquinone (BQ, •O2scavenger) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, hole scavenger). For the photogenerated radical trapping experiment, 8 mg IPA and 80 mg TiO2-rGO composite were suspended in 200 mL TCH solution (20 ppm), the following steps were the same as that in the above photocatalytic activity test. For the hole and •O2- trapping experiments, the experimental procedure were similar to the radical one, except that 18.9 mg EDTA-2Na or 5 mg BQ was used instead of IPA. Regeneration and Recyclability. Photocatalytic degradation and regeneration were circularly conducted to check the recyclability of TiO2-rGO composite under visible light, four cycle runs were conducted under the same reaction conditions (catalyst dosage: 0.4 g·L-1, TCH solution volume: 50 mL, solution pH: 5.2). The used TiO2-rGO catalyst was recycled, washed by deionized water and dried at 80 oC for 3 hours before next utilization. The


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degradation capability of the catalyst was analyzed as above photocatalytic activity test. RESULTS AND DISCUSSION Properties of TiO2-rGO Composite. The fabrication process of the TiO2-GO composite through the Pickering emulsion approach was recorded by the optical micrograph (Figure 1). Initially, the droplets of the n-octanol/water Pickering emulsion stabilized by GO sheets showed a smooth interface and a relatively homogeneous size distribution with a diameter about 100 µm (Figure 1a). When TBT was slowly added into the emulsion solution, TBT molecules diffused towards the surface of GO emulsion droplets due to its oilsolubility, and hence the interfacial hydrolysis and nucleation of TiO2 nanoparticle could take place, resulting in the gradual change of the smooth surface into rough one (Figure 1b). However, very fast hydrolysis rate of TBT would usually lead to the condensation of TiO2 particles in bulk water phase. In order to slow down the hydrolysis rate to meet the diffusion rate, and hence the interfacial condensation, HCl and n-butyl alcohol were added into the water phase and oil phase, respectively,37,38 in which HCl partially charged the initially formed titania oligomers to enhance the affinity with the hydrophilic oxygen-containing groups of GO sheet, resulting in a stable growth of TiO2 nanoparticles at the interface of GO emulsion. The pH value adjusted by HCl also showed a shrink effect on the emulsion droplet size (Figure S1). Moreover, too many TBT dosage led to the decrease of the number of emulsion


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droplets, and even worse the collapse of the emulsions (Figure S2). The following hydrothermal process (further hydrolysis and condensation) had greatly improved the dispersity of TiO2 nanoparticles, as well as the Pickering emulsion droplets stabilized by TiO2-GO composite. By optimizing the fabrication conditions, the Pickering emulsion system stabilized by TiO2-GO composite can maintain its stability as long as more than 30 days (Figure S3).

Figure 1. Optical micrographs of (a) GO Pickering emulsions, (b) TiO2-GO composite, with TiO2 nanocrystals growth at the surface of GO Pickering emulsions under the conditions of oil/water volume ratio of 1:2, GO dispersion 2 mg·mL-1 , TBT 0.2 mol·L-1, n-butyl alcohol 0.73 mol·L-1, hydrochloric acid 0.05 mol·L-1. XRD patterns (Figure 2a) showed that the as-synthesized TiO2-rGO composites after the hydrothermal process already exhibited the typical diffraction peaks of anataseTiO2 that 25.5o {101}, 37.9o {004}, 47.9o {200}, 54.8o {105}, 55.1o {211}, and 63.0o {204}, However, the diffraction intensity was relatively low. After the calcination at 400 oC, the intensity of the {101} diffraction peak increased significantly due to the perfect crystallinity. However, further increasing the calcination temperature up to


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500 oC, additional diffraction peaks such as at 27.5o attributed to the {110} of rutileTiO2 appeared. As the photocatalytical efficiency of the anatase-TiO2 is higher than rutile-TiO2,39 all the TiO2-rGO composite samples we discussed below were obtained at the calcination temperature of 400 oC. However, the characteristic peaks of GO can not be observed in all XRD patterns because of the very small amount of GO incorporated in the composite.40 Whereas the Raman spectra of TiO2-rGO composite (Figure 2b) clearly showed the characteristic peaks of both anatase-TiO2 and GO, in which, the peaks at 145 (Eg(1)), 399 (B1g(1)), 637 (Eg(2)) and 516 cm-1 (A1g+B1g(2)) were attributed to the typical modes of anatase-TiO2, and the D-band and G-band of GO at 1349 and 1592 cm−1 shifted a little to 1356 and 1609 cm−1 in TiO2-rGO composite due to the interaction between TiO2 and rGO.41 Moreover, the intensity ratio of D/G bonds increased a little from 0.86 to 0.88 in TiO2-rGO composites, indicating a decrease in the average size of the sp2 domains caused by the hydrothermal reduction.42-44 To clarify the transition of GO to rGO during the hydrothermal process, we compared the XRD patterns (Figure S4a) and FTIR spectra (Figure S4b) of pristine GO before and after the hydrothermal process. The diffraction peaks at 11.4o, related to {002} plane of the pristine GO shifted to 24.7o, which could be indexed as {002} plane of graphene.45 Meanwhile, many strong absorption peaks of various oxygen-containing functional groups in FTIR spectra showed significant decreases in their intensity after the hydrothermal treatment (details were provided in SI). Similarly, XPS spectra of TiO2-rGO and their simulated fitting curves of Ti2p,


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C1s and O1s (Figure S5) also revealed significant decreases of the intensity of the oxygen-containing group bands. Therefore, GO was reduced to r-GO, and TiO2-rGO composite was produced by the hydrothermal reduction. From the TGA analysis (Figure S6), the amount of rGO in the composite was estimated at about 4.21 wt. %.

Figure 2. (a) XRD patterns of TiO2-rGO composites before and after the calcination annealed under different temperatures. (b) Raman spectra of GO, p-TiO2, and TiO2rGO composite. After the calcination, SEM image of the obtained TiO2-rGO composite (Figure 3a) indicated that the emulsion droplets of TiO2-rGO composite were broken into monolith sheets, with dense TiO2 nanoparticles closely aggregated together. The TEM image (Figure 3b) further showed that the monolith was composed of 2D flat sheets of TiO2 nanoparticle aggregations. There were a lot of packing voids among the closely aggregated TiO2 nanoparticles. From the thin edge of the monolith (Figure 3c), well exfoliated rGO sheet can be observed, which provided an evidence of the sandwichlike structure that TiO2 nanoparticles grew up at each side of rGO sheet， with the


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average diameter of about 10 nm. The high resolution TEM image (Figure 3d) revealed the clear lattice fringes with the spacing of 0.35 nm in the obtained TiO2rGO composite, corresponding to the {101} plane of typical perfect anatase-TiO2 crystal. This unique structure produced large amount of mesoporous voids, hence, with relatively larger surface area, more active TiO2 sites exposed, faster transfer of photogenerated electrons, and shorter length of the transport path for molecules, it could be extremely in favor of improving both adsorption and photocatalytic performance.


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Figure 3. Morphlogies of TiO2-rGO composite (a) SEM image of the monlith (b) TEM image of one monlith sheet (c) TEM image of TiO2 nanoparticles supported on exfoliated GO sheets (d) TEM image of TiO2 nanocrystals in TiO2-rGO Compared with p-TiO2, the grey-black powder of TiO2-rGO composite showed a quite different light absorption and emission properties (Figure 4a). With a red shift of the absorption peak and a strong absorption in the visible light range of 400-800 nm in the UV−vis diffuse reflectance spectra, TiO2-rGO composite possessed an enhanced light absorption ability. The absorption edge of TiO2-rGO shifted from λe1=387 nm to λe2=410 nm, accordingly, estimated from the plot of (Ahc/λ)2 versus hc/λ (Figure 4b), the band-gap energy reduced from 3.20 eV of typical anatase-TiO2 to 3.04 eV. Under the UV-light excitation (λ=260 nm) at room temperature, comparing with the broad emission band in the range of 320-510 nm in the PL spectrum (Figure 4c) of p-TiO2, a very low intensity of the emission band of TiO2rGO composite revealed a much lower recombination rate of photo-generated electrons and holes. The improved absorption performance and the promoted separation efficiency of electrons-holes pairs could be attributed to the introduction of rGO and tightly coupled TiO2 nanoparticles on it in the composite, that the interaction between TiO2 and rGO can shift the absorption to red, and a dark carbon composition to effectively reduce the light reflection.46 More importantly, 2D π-conjugated structure of rGO could serve as an effective electron-accepting material and hinder the direct recombination of e-−h+ pairs in TiO2.


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Figure 4. (a) UV-vis diffuse reflectance spectra, (b) estimated band-gap, (c) photoluminescence spectra, and (d) N2 adsorption/desorption isotherms (inset: pore size distribution calculated by the BJH method from the desorption branch) of p-TiO2 and TiO2-rGO composite, respectively Besides the light absorption ability, the specific surface area also played an important role in photocatalytic activity. N2 adsorption/desorption isotherms and the pore size distributions (Figure 4d) revealed a typical IV adsorption isotherm for TiO2-rGO composite, with the onsets of the hysteretic cycle at p/po of 0.4~0.45, suggesting its mesoporous structure. The average size of mesopores in TiO2-rGO composite was about 4.5 nm. As illustrated in Table S1, the BET surface area of GO, p-TiO2, TiO2rGO were 25.28 m2·g-1, 88.64 m2·g-1 and 120 m2·g-1, respectively. Confirmed by the


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TEM images of TiO2-rGO composite (Figure 3b and 3c), by using Pickering emulsion approach, TiO2 nanoparticles densely grew up at the surface of GO sheets, aggregated to form a large amount of mesoporous packing voids, hence effectively enlarged the BET surface area of the composite. The large surface area and suitable mesoporosity would provide more adsorption space and faster transport channels for large organic molecules. More importantly, more active TiO2 sites can be directly exposed to reactants during photocatalytic reaction process. Photocatalytic Activity. The degradation of MB in aqueous solution was selected as the model reaction to evaluate the photocatalytic activity of TiO2-rGO composite. Under the irradiation of a Xenon lamp (300 W), the MB degradation curve (Figure 5a) showed that 100% MB can be degraded by the TiO2-rGO composite in 100 min, while for the same irradiation time, the degradation efficiency was only 86% for p-TiO2. When the irradiation changed into the visible light (>420 nm), the photocatalytic performance of TiO2-rGO composite was still very efficient, that 76% MB can be degraded within 120 min; whereas p-TiO2 showed a much low activity with only 27% MB degradation. The kinetics study revealed that the plotting of logarithm of concentration ratio ln(Ce/C) (Ce is the concentration of MB in solution at adsorption equilibrium) against time t for both p-TiO2 and TiO2-rGO composites showed linear relationships (Figure 5b), suggesting the photocatalytic degradation of MB was a pseudo-first order. From the slopes we can calculate the reaction constants (inset in


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Figure 5b), the value of TiO2-rGO were 5 and 3.1 times high as that of p-TiO2 under the visible light and xenon lamp, respectively. Moreover, comparing with the reported TiO2-carbon-based composites produced by common methods (Table S2), For example, by the physical mixing method,47 the degradation efficiency of graphene wrapped TiO2 composite was of 2.83×10-5 mg(MB)[mg(Cat.)min]-1 under visible light; by a relativly complex two-step sol–gel and solvothermal methods,48 the obtained TiO2/CNSAC composite showed a good degradation efficiency as 4.01×10-4 mg(MB)[mg(Cat.)min]-1 under sunlight irradiation with partial UV light; and by a bulk condensation method,49 C3N4/F-TiO2 composite was produced, with a degradation efficiency of 1.48×10-4 mg(MB)[mg(Cat.)min]-1 under LED light irradiation. The photocatalytic performance of TiO2-rGO composite in this work was preferable, with a lower dosage of photocatalyst and higher efficiency of total MB degradation, that the degradation efficiency was as high 5×10-4 mg(MB)[mg(Cat.)min]-1 under Xenon lamp, and 3.17×10-4 mg(MB)[mg(Cat.)min]1

under visible light. The much higher photocatalytic activity of TiO2-rGO was

attributed to its unique 2D sheet sandwich-like mesoporous structure, that the sandwich intermediate layer of rGO enhanced the visible light absorption, slowed down the recombination rate of photo-induced electrons and holes, and improved MB adsorption through the π-π stacking in mesopores. Thus, through the in-situ interfacial growth of TiO2 nanoparticles on the surface of well exfoliated rGO¸ the Pickering


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emulsion approach was a convenient and successful method for the fabrication of composite photocatalysts.

Figure 5. Comparison of (a) photocatalytic activities and (b) photocatalytic reaction kinetics in the degradation of MB under the xenon lamp and visible light (λ > 420 nm) on TiO2-rGO and p-TiO2, respectively. Application of TCH Photodegradation. Because of the good photocatalytic activity of TiO2-rGO composite, especially, under visible light irradiation, we applied it as the photocatalyst for TCH photodegradation. Generally, there are two steps in a photodegradation, i.e. the adsorption and the degradation. To clearly illustrate each contribution, we conducted TCH photodegradation process into two steps (Figure 6a), before the point of zero, we performed the degradation under dark condition for 60 min, in which the adsorption played the dominant contribution for the TCH concentration change; after that, the visible light irradiation was applied, and the photodegradation was the dominant one. For the low initial TCH concentrations, such as at 10 ppm TCH, the adsorption played


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a so great contribution that the removal efficiency were as high as 38 % under dark. And the following photodegradation was much faster due t o more adsorbed TCH molecules, the total removal efficiency was 94% under visible light for 40 min without adding any oxidants, further prolonging to 120 min, the final removal efficiency were 96.5%. With increasing the initial TCH concentration, both adsorption removal efficiency and photodegradation efficiency decreased. Therefore TiO2-rGO photocatalyst showed better photocatalystic activity in dilute TCH solutions. Moreover, the effects of the dosage of TiO2-rGO composite (Figure S7a) and the interfering ions of Ca2+ and Mg2+ (Figure S7b) on the degradation of TCH were also investigated (details were provided in SI). The low concentration of interfering ions of Ca2+ and Mg2+ (0.01 mol·L-1) showed little side effects on the degradation efficiency of TCH, however, the high concentration of Mg2+ (0.1 mol·L-1) showed a significant decrease of the degradation efficiency due to the chelation between the bivalent metal cations and TCH.50 Since pH can affect the speciation of TCH and the surface charge of TiO2-rGO composite in a similar characteristic,51,52 and the strong electrostatic repulsion between them at either high or low pH values would decrease the adsorption contribution (Figure S8). Since the water is naturally at weak acid or near neutral pH value, at which the good adsorption capacity could promote the photodegradation of TCH. Since the TCH concentration in real sewage outlets of medicine plant is relatively high, the recyclable photoactivity of TiO2-rGO composite was circularly tested under the same operation conditions for 20 ppm TCH


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solution (Figure 6b). After four cycles, the photocatalytic degradation of TCH can still maintain at about 85 % under the visible light irradiation, suggesting its good photoactivity.

Figure 6. (a) Photodegradation of TCH at different initial concentrations (dosage of catalyst, 0.4 g·L-1; pH value, 5.2), (b) Recycling photodegradation of TCH by TiO2rGO composite under visible-light irradiation (dosage of catalyst, 0.4 g·L-1; TCH concentration, 20 ppm; and pH value, 5.2) To illustrate the advantages of TiO2-rGO composite by using Pickering emulsion approach, P25-rGO mixture and TiO2-rGO sol-gel composite were prepared as the comparison. Figure S8a shows the XRD patterns of all the TiO2-rGO photocatalysts, they all exhibited the typical diffraction peaks of anatase-TiO2 that 25.5o {101}, 37.9o {004},

47.9o

{200},

54.8o

{105},

55.1o

{211},

and

63.0o

{204}.

N2

adsorption/desorption isotherms and the pore size distributions of TiO2-rGO, TiO2rGO sol-gel, P25-rGO (Figure S8b) revealed their BET surface area were 120 m2g-1, 98 m2g-1 and 102 m2g-1, respectively. The isotherms of these samples showed a typical


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IV adsorption, with the onsets of the hysteretic cycle at large p/po value, suggesting the mesoporous structure. The average size of mesopores in TiO2-rGO, TiO2-rGO solgel, P25-rGO mixture were about 4.5 nm, 3.5 nm, 3.7 nm, respectively. Figure 7a shows the comparison of the TCH photo-degradation performance on three TiO2-rGO photocatalysts under visible light irradiation. Before the irradiation degradation, the adsorption contributions of TiO2-rGO and TiO2-rGO sol-gel are similar with each other, but P25-rGO mixture is limited. At 120 min, the TCH removal efficiency of TiO2-rGO composite by using Pickering emulsion approach is 89.66%, better than the TCH removal efficiency of TiO2-rGO sol-gel and P25-rGO mixture of 76.88% and 27.04%, respectively. Dynamic studies shown in Figure 7b reveal the rate of TCH degradation of three photocatalysts are all obey the second-order kinetic model. Among them, the TiO2-rGO composite by using Pickering emulsion approach exhibits the greatest photocatalytic rate. The reaction rate constant, denoted as the slope of the straight lines are 0.108, 0.0435 and 0.00385 L·(mol·min)-1 for TiO2-rGO composite, TiO2-rGO sol-gel and P25-rGO mixture respectively, that of TiO2-rGO composite by using Pickering emulsion approach are 2.51 and 27 times high as those of TiO2-rGO sol-gel composite and P25-rGO mixture. By using Pickering emulsion approach, the in-situ growth of TiO2 on GO sheets ensured the seamless connection between them, causing fast transports of e- and h+ of TiO2 induced by visible light irradiation to GO sheets, and effectively resisted their combination. Meanwhile, more active sites of smaller TiO2 nanoparticles can be directly exposed to reactants during photocatalytic


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reaction. Moreover, the relatively larger surface area and mesoporosity provided more adsorption space and faster transport channels for large TCH molecules. Therefore, the Pickering emulsion approach provided a convenient and effective method to prepare photocatalyst.

Figure 7. (a) Comparison of TCH photo-degradation performance on photocatalysts of TiO2-rGO composite, TiO2-rGO sol-gel composite and P25-rGO mixture under visible light irradiation. (b) Kinetic studies of TCH photo-degradation, dots are experimental results, straight lines are the plotting results of the second-order model. (With the initial TCH concentration of 20 ppm, catalyst dosage of 0.4 g·L-1, and pH 5.3) Photocatalytic mechanism. The mechanism of the high photodegradation activity of TiO2-rGO composites, including photo-generated electron-hole separation process and the formation process of free radicals was schematically proposed in Figure 8a. Because of the 2D sandwich-like mesoporous structure of tightly coupling of TiO2 on each side of rGO


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sheets, the visible light absorption threshold was extended to 410 nm, and the band gap of TiO2-rGO composite was decreased to 3.05 eV, which ensured TiO2 can generate the electron-hole pairs under the visible light irradiation. The photogenerated electrons excited from the valance band (VB) to the conduction band (CB) and immediately transferred to rGO sheets due to the in-situ surface growth of TiO2 nanoparticles; while photo-generated holes were left in the VB of TiO2 nanoparticles. Therefore, the recombination of photo-generated electrons and holes was suppressed, and their lifetime was prolonged. Subsequently, the separated electrons reacted with dissolved oxygen and water molecules nearby to form hydroxyl radical (•OH) and peroxyl radical anion (•O2-). Moreover, the mesoporous voids among TiO2 and rGO provided good microreactors for the photodegradation, where the holes, •OH and •O2radicals would be strong oxidant towards the degradation of adsorptive TCH or MB. The photocatalytic mechanism was further verified by the scavenger trapping test by using IPA, EDTA-2Na and BQ as •OH, h+ and •O2- scavenger, respectively. 53 The main oxidative species were detected in the photocatalytic process of TiO2-rGO composite under visible light (Figure 8b). The results evidently showed that the addition of IPA caused almost no changes in the photoactivity of the TiO2-rGO composite, suggesting •OH radicals had few contributions. However, when EDTA2Na or BQ was added to the photocatalytic system, the photocatalytic activity was significantly retarded, revealing that photo-generated holes and •O2- were the main oxidant species in the TiO2-rGO photocatalytic systems under visible light. It was


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worthy to mention again, the mesoporosity and relatively larger surface area of the TiO2-rGO composite also facilitated the adsorption and transportation of THC or MB molecules to enhance the reaction with the active photo-generated holes.

Figure 8. (a) Possible photocatalytic mechanism of TiO2-rGO composite for degradation under the visible light, (b) Verification of mechanism by the trapping tests of radicals and holes. CONCLUSION A 2D sandwich-like mesoporous TiO2-rGO composite monolith were fabricated by the Pickering emulsion approach. By using Pickering emulsion approach, (1) GO sheets were highly exfoliated and extended at the W/O interface, (2) the in-situ growth of TiO2 nanopaticles on GO sheets ensured the seamless connection between them, (3) the size of TiO2 nanopaticles were very small (10 nm) due to the interface confinement effect, (4) the closely aggregation of TiO2 nanopaticles produced mesoporous packing voids with a relatively larger surface area (120 m2·g-1). All of these structure superiorities resulted in the significant enhanced photo-degradation


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performance. The photocatalytic activity of TiO2-rGO was evaluated by MB degradation, in which TiO2-rGO composite showed a high activity that 100% and 76% of MB can be degraded within 100 min under xenon lamp and visible light, while for the same irradiation, for p-TiO2 nanoparticles, the degradation were just 86% and 27%. The MB photocatalytic degradation observed the pseudo-first order kinetic law, the rate constants of TiO2-rGO composite were 5 and 3.1 times higher than that of p-TiO2 nanoparticles under visible light and xenon lamp, respectively. When TiO2-rGO composite was used as the photocatalyst for the degradation of THC, after the optimized operation conditions of pH, the dosage of TiO2-rGO composite, and the interfering ions concentrations, the removal efficiency of 10 ppm THC was as high as 94% under visible light irradiation for 40 min. Based on the results of trapping test for photogenerated radicals and holes, the photocatalytic mechanism of TiO2-rGO composite was proposed. Because of its unique 2D sandwich-like structure, the photocatalytic properties were significantly improved by accelerating the separation of electron–hole pairs, extending the electron life, as well as increasing the specific surface area for good adsorption and fast transportation. In summary, we hope our work had provided a good example for producing TiO2-based composites through the Pickering emulsion approach. In fact we can alternately change different nanoparticles as the Pickering emulsion stabilizers, and produce various assembly composites with high photocatalytic efficiency for pollutant-degradations. ASSOCIATED CONTENT


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Supporting Information. brief description (file type, i.e., PDF) brief description (file type, i.e., PDF) AUTHOR INFORMATION Corresponding Author *(E-mail: [email protected]). * (E-mail: [email protected]). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the National Key Technology Support Program of China (2015BAC04B01) and the Natural Science Foundation of China (Nos. 91334203, 21376074, 21676080). Shanghai Municipal Bureau of Quality and Technical Supervision Foundation of China (No. 2015-03), and the project of FP7-PEOPLE2013-IRSES (PIRSES-GA-2013-612230). REFERENCES (1)

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Table of Contents Pickering emulsion approach and electron transfer of TiO2-rGO composite for efficient TCH degradation under visible light.

Through an in-situ interfacial growth of antase-TiO2 nanoparticles on the surface of wellexfoliated rGO sheets by Pickering emulsion approach, a 2D sheet sandwich-like mesoporous TiO2-rGO composite monolith were fabricate. When TiO2-rGO composite was applied in tetracycline hydrochloride (TCH) degradation, under the visible light without adding any other oxidants, the removal efficiency for 10 ppm THC solution was as high as 94% for 40 min. A large amount of microreactors on the surface of TiO2-rGO can significantly accelerate the separation of electron–hole pairs, prolong the electron life, increase the adsorption capacity of organic substrates, and facilitate the transportation of molecules.

38 ACS Paragon Plus Environment

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Scheme 1. Fabrication of the TiO2-GO composite through the interfacial growth approach induced by GO Pickering emulsion. 49x19mm (300 x 300 DPI)


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Figure 1. Optical micrographs of (a) GO Pickering emulsions, (b) TiO2-GO composite, with TiO2 nanocrystals growth at the surface of GO Pickering emulsions under the conditions of oil/water volume ratio of 1:2, GO dispersion 2 mg/mL, TBT 0.2 mol/L, n-butyl alcohol 0.73 mol/L, hydrochloric acid 0.05mol/L. 33x12mm (300 x 300 DPI)


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Figure 2. (a) XRD patterns of TiO2-rGO composites before and after the calcination annealed under different temperatures. (b) Raman spectra of GO, p-TiO2, and TiO2-rGO composite. 80x33mm (300 x 300 DPI)


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Figure 3. Morphlogies of TiO2-rGO composite (a) SEM image of the monlith (b) TEM image of one monlith sheet (c) TEM image of TiO2 nanoparticles supported on exfoliated GO sheets (d) TEM image of TiO2 nanocrystals in TiO2-rGO 139x131mm (266 x 266 DPI)


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Figure 4. (a) UV-vis diffuse reflectance spectra, (b) estimated band-gap, (c) photoluminescence spectra, and (d) N2 adsorption/desorption isotherms (inset: pore size distribution calculated by the BJH method from the desorption branch) of p-TiO2 and TiO2-rGO composite, respectively 109x85mm (300 x 300 DPI)


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Figure 5. Comparison of (a) photocatalytic activities and (b) photocatalytic reaction kinetics in the degradation of MB under the xenon lamp and visible light (λ > 420 nm) on TiO2-rGO and p-TiO2, respectively. 80x33mm (300 x 300 DPI)


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Figure 6. (a) Photodegradation of TCH at different initial concentrations (dosage of catalyst, 0.4g/L; pH value, 5.2), (b) Recycling photodegradation of TCH by TiO2-rGO composite under visible-light irradiation (dosage of catalyst, 0.4g/L; TCH concentration, 20 ppm; and pH value, 5.2) 78x32mm (300 x 300 DPI)


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Figure 7. (a) Comparison of TCH photo-degradation performance on photocatalysts of TiO2-rGO composite, TiO2-rGO sol-gel composite and P25-rGO mixture under visible light irradiation. (b) Kinetic studies of TCH photo-degradation, dots are experimental results, straight lines are the plotting results of the second-order model. (With the initial TCH concentration of 20 ppm, catalyst dosage of 0.4 g·L-1, and pH 5.3) 37x16mm (600 x 600 DPI)


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Figure 8. (a) Possible photocatalytic mechanism of TiO2-rGO composite for degradation under the visible light, (b) Verification of mechanism by the trapping tests of radicals and holes. 42x18mm (600 x 600 DPI)


rGO Composite by Pickering Emulsion for Photocatalytic Degradation

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