Photocatalytic activity enhancement of anatase–graphene nanocomposite for methylene removal: Degradation and kinetics

    Photocatalytic activity enhancement of anatase–graphene nanocomposite for methylene removal: Degradation and kinetics Mostafa Rezaei, Shiva Salem PII: DOI: Reference:

S1386-1425(16)30246-3 doi: 10.1016/j.saa.2016.04.057 SAA 14424

To appear in: Received date: Revised date: Accepted date:

27 November 2015 9 April 2016 27 April 2016

Please cite this article as: Mostafa Rezaei, Shiva Salem, Photocatalytic activity enhancement of anatase–graphene nanocomposite for methylene removal: Degradation and kinetics, (2016), doi: 10.1016/j.saa.2016.04.057

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Photocatalytic activity enhancement of anatase-graphene nanocomposite for methylene

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removal: degradation and kinetics Mostafa Rezaei, Shiva Salem*

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Faculty of Chemical Engineering, Urmia University of Technology, Urmia, Iran Corresponding author: Tel.: +984413554352, Fax: +984413554184,

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E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract In the present research, the TiO2- graphene nanocomposite was synthesized by an eco-friendly

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method. The blackberry juice was introduced to graphene oxide (GO) as a reducing agent to

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produce the graphene nano-sheets. The nanocomposite of anatase-graphene was developed as a photocatalyst for the degradation of methylene blue, owing to the larger specific surface area and

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synergistic effect of reduced graphene oxide (RGO). The UV spectroscopy measurements

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showed that the prepared nanocomposite exhibited an excellent photocatalytic activity toward the methylene blue degradation. The rate of electron transfer of redox sheets is much

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higher than that observed on GO, indicating the applicability of proposed method for the production of anatase-RGO nanocomposite for treatment of water contaminated by cationic dye.

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The prepared materials were characterized with Fourier transform infrared spectroscopy, X-ray diffraction, Brunauer–Emmett–Teller surface area measurement, scanning electron microscopy

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and transmission electron microscopy. A facile and rapid route was applied for the uniform deposition of anatase nanoparticles on the sheets. The resulting nanocomposite contained nanoparticles with a mean diameter of 10 nm. A mechanism for the photocatalytic activity of nanocomposite was suggested and the degradation reaction obeyed the second-order kinetics. It was concluded that the degradation kinetics is changed due to the reduction of GO in the presence of blackberry juice.

Keywords: Photocatalytic activity; anatase; graphene; anthocyanins; methylene blue; degradation.

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ACCEPTED MANUSCRIPT 1. Introduction The tremendous increase in the use of dyes over the past few decades has eventually resulted

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in the flux of organic substances into the environment which are mostly non-degradable in

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natural condition. Many industries such as paint, textile, plastic, tannery, paper and rubber discharge wastewaters containing anionic and cationic dyes which in tern contaminate the

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natural water and soil. Different techniques are applied for the water and wastewater treatments

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to remove these hazardous materials. These processes can be performed physically as adsorption [1, 2] and/or chemically as photocatalytic reactions [3]. The TiO2 based composites show

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interesting photocatalytic properties and anatase is renowned as a great of importance photocatalyst with the high degradation capacity for degradation of dyes [4]. The evolution of

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hydrogen and oxygen in the presence of TiO2 particles under the sunlight irradiation causes the formation of OH radicals as a result promotes the photocatalytic activity [5]. Therefore, the

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techniques for production nano-sized TiO2 were developed as effective ways to overcome these environmental problems [6].

The photocatalytic efficiency of single phase anatase was seriously limited due to the rapid recombination of photo-generated electrons (e-) and holes (h+) pairs [7]. In order to enhance the photocatalytic activity, many efforts were presented for coupling anatase with other semiconductors to form nanocomposite. The strategy of coupled nanocomposites has been proved to prevent the rapid recombination of photo-generated electrons and holes pairs. The appropriate matching in the photocatalytic systems drives the electrons from one particle to neighbors as a result, electrons and holes separation occurs, steadily. Thus, the interaction between components in the nanocomposite plays an important role in the photocatalytic activity. Graphene is a flat monolayer of carbon atoms packed into a two-dimensional honeycomb

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ACCEPTED MANUSCRIPT lattice [8]. Graphene oxide (GO) and reduced graphene oxide (RGO) are interesting materials for production of TiO2-graphene based photocatalysts [9, 10]. Recently, these materials have found a

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great of attention in the synthesis of photocatalysts due to superior electrical conductivity, high specific surface area and chemical stability. GO has been mainly considered as a precursor for

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cost–effective, large–scale production of TiO2–based photocatalysts. The layers of GO contain

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large amounts of oxygen functional groups, decorating the basal plane and edges of a typical

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graphene sheet [11]. These functional groups are responsible for insulating behavior of GO. The incremental removal of oxygen can move the material to a semiconductor and ultimately to a

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graphene–like material. Although a lot of methods have been developed for the preparation of graphene sheets, the most suitable and efficient approach was the solution based chemical

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reduction of GO to RGO due to low cost and feasibility of production in a controllable condition [12, 13]. For example, hydrazine was used to reduce graphene oxide by stirring in hot water

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under acidic condition [14]. GO and RGO can be readily dispersed in water to yield the stable dispersions by simple sonication. Furthermore, the active sites cause the limitation of nanoparticle growth and improvement in the stability and dispersion of nanoparticles on GO or RGO. The attached nanoparticles are also helpful for electrons and holes separation, maintaining the excellent properties of photocatalyst [15]. From the engineering point of view, a fine control of TiO2-graphene structure is required because the performance of system depends on the reduction state of graphene oxide and TiO2 particle morphology which can be strongly affected by material interfaces and interactions [16]. Various techniques have been developed for fabricating the high reactive TiO2-graphene nanocomposite, which include the hydrothermal [17, 18], direct redox reaction [19], sol–gel [20], solvo-thermal treatment [21], spark plasma [22], ultrasonic spray pyrolysis [23], chemical

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ACCEPTED MANUSCRIPT exfoliation [24], graphitization [25], hydrogels [26] and spin-coating [27]. These approaches have generally produced TiO2-graphene composites with weak interactions among TiO2 and

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graphene or are more complex to apply in practice. In fact, the formation of chemical bonds

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between TiO2 and graphene is the critical point in the performance of photocatalyst. Therefore, the new strategy is urgently required to develop advanced TiO2-graphene nanocomposite. On the

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other hands for improvement of TiO2-graphene nanostructure, it is preferable to use nontoxic

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reagents.

In recent years, plant-mediated biological synthesis of nanoparticles is gaining importance

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due to simplicity and eco-friendliness. Blackberry is a fruit of interest because of its high content of anthocyanins and ellagitannins as well as other phenolic compounds which contribute to its

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high antioxidant capacity. The high antioxidant activity of blackberries is based on their oxygen radical absorbance capacity compared to other fruits [28]. World wide commercial production of

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blackberry is estimated to be approximately 155 thousands tons [29]. Blackberries are mostly consumed fresh but can be processed as juice. North America, Europe, Asia, South America, Oceania Central America and Africa are the main regions for blackberry production. According to the presented introduction, this investigation attempted to synthesize the TiO2RGO nanocomposite for the photocatalytic degradation of methylene blue under sunlight irradiation. Recently, the authors have determined the optimal content of GO in a sol-gel route to fabricate the TiO2-GO nanocomposite as a photocatalyst with high uniformity. The average size and content of TiO2 nanoparticles could be easily controlled by adjusting the appropriate content of GO. It was shown that the formation of TiO2 nanoparticles favors the enhancement of photocatalytic activity. However, it is well known that the reduced graphene oxide usually exhibit the higher photocatalytic activity. Therefore, in this study smaller anatase

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ACCEPTED MANUSCRIPT nanoparticles were uniformly deposited on RGO by modifying the reduction condition of GO in the presence of blackberry juice. This method environmentally features the reduction of GO and

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attachment of TiO2 by eco-friendly technique. The aim of the present work is to increase the

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knowledge on the degradation mechanism of MB by TiO2, TiO2-GO and TiO2-RGO. Finally, a detailed study was presented for MB degradation mechanism by kinetic model.

Materials

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2.1.

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2. Materials and methods

Graphite powder was obtained from Merck Company (104206, purity > 99 wt%, Germany).

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Potassium manganite (223468, purity > 98 wt%) and titaniumbutoxide (24412, purity > 97 wt%) were purchased from Sigma Aldrich (USA). Sulfuric acid (98 wt%), hydrochloric acid (37 wt%)

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and ethanol were supplied by Merck Company. Hydrogen peroxide was a product of Sigma Aldrich. The blackberry juice used for the reduction process was prepared by taking local

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blackberry (Urmia, Iran). Methylene blue (C16H18ClN3S, 115943) corresponding to molecular weight of 319.85 g.mol-1 was obtained from Merck. All chemicals were of guaranteed or analytical grade reagents, commercially available and used without further purification. The water used throughout this work was the deionized grade. 2.2. GO and RGO preparations Graphene oxide was prepared from purified natural graphite by a modified Hummers method [30]. The graphite powder (10.0 g) was added to the concentrated H2SO4 (230 ml) in an ice bath. KMnO4 (30.0 g) was gradually blended under stirring with the obtained suspension. The mixture was stirred at 35 °C for 2 h and then deionized water (150 ml) was slowly added to the mixture, followed by rapid stirring the mixture at 98 °C. The suspension was further diluted with deionized water and the reaction was terminated by blending H2O2 (35 ml) under stirring at room

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ACCEPTED MANUSCRIPT temperature, followed by washing with deionized water several times. The aqueous colloid of GO was prepared by dispersing 0.2 g graphene oxide into 40 ml of

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blackberry juice, resulting in an inhomogeneous dispersion. The suspension was then stirred for 6 h followed by ultra-sonication and centrifugation for 2 h. The obtained material was washed

2.3.

Synthesis of TiO2-graphene nanocomposites

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for several times with deionized water and dried in oven for 24 h at 65 C.

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TiO2-graphene oxide (TGO) and TiO2-reduced graphene oxide (TRGO) nanocomposites were synthesized by a facile, rapid and green process. Firstly, 60 mg of GO or RGO was dispersed in

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60 ml of ethanol by ultra-sonication to form a stable colloids over 3 h at 37 C, and then mixed with 0.67 ml of titaniumbutoxide. The pH of suspensions was adjusted at level of 7 by dropwise

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adding appropriate amounts of hydrochloric acid and ammonia solutions. The products were collected by centrifugation, washed several times with deionized water and then dried in a

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laboratory oven at 80 °C. The samples were achieved after calcination at 400 C over 2 h (TiO2/GO ratio is about 2.6). Following above procedures the pure anatase nanoparticles was synthesized in the absence of GO and RGO. 2.4.

Photocatalytic degradation performance

For studding the photocatalytic degradation performance of TGO and TRGO under sunlight irradiation, the MB solution was prepared by mixing dye with deionized water (3.0 mgl-1) then the photocatalysts (12.0 mg) were added to prepared solution (25 ml). Prior to sunlight degradation, the suspensions were stirred for 30 min in the dark space for adsorptiondesorption equilibrium. The bright blue color of the solutions was gradually vanished, indicating the degradation of cationic dye. The variation of MB concentration with time, 5-90 min, was monitored spectrophotometrically (UV–vis spectrophotometer, T80, PG, Ltd., UK) at a

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ACCEPTED MANUSCRIPT wavelength range of 400-700 nm. The dye degradation is presented as a conversion, 1-(C/C0), where C0 and C are the concentration of MB at dark condition after adsorption-desorption

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equilibrium and the concentration of dye at different irradiation times, respectively. Each set of

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photocatalytic measurements was repeated three times, and the experimental error was found to be within the error bar of ±5%.

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2.5. Characterizations

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Fourier transform infrared spectroscopy (FTIR, Nexus 670, Thermo Nicolet, Germany) was used at room temperature in the range of 400-4000 cm−1 to identify the chemical bands in the

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prepared samples.

The crystalline structures were characterized by X-ray diffraction (XRD) analysis on a

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Brucker X-ray diffractometer (Model D8-Advance, Karlsruhe, Germany) at 40 kV and 30 mA

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with Cu-Kα radiation. The samples were scanned at a rate of 0.02 °s-1 in the 2θ range of 10-70°.

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0.9  cos 

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The apparent crystallite size, D, of TiO2 was determined using the Scherrer formula [7]:

(1)

where β is the breadth of observed diffraction peak at its half-intensity,  is the Bragg angle, and λ is radiation wavelength, 1.5406 nm. The specific surface area of synthesized photocatalysts was measured with the nitrogen absorption isotherms apparatus (PHS 1020, China) according to Brunauer–Emmett–Teller (BET) technique. The samples were degassed at 60 C in a vacuum system overnight before the measurements. The morphology of particles was observed by scanning electron microscopy (SEM, Leo 1430 VP-Germany) on gold coated surfaces. The transmission electron microscopy (TEM, Model CM30, Philips, Nederland) were used for microscopical observation. The sample for 8

ACCEPTED MANUSCRIPT transmission electron microscopy test was prepared by drying a small drop of dispersed particles in the distilled water on 300 mesh copper grid.

Photocatalytic MB degradation

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3.1.

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3. Results ad discussion

In order to demonstrate the photocatalytic ability of TiO2, TGO and TRGO, the degradation

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of MB was examined over 20 min under sunlight irradiation. The dye degradation was measured

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by the diminution of its concentration. Before solar irradiation, the suspension containing dye and photocatalyst was stirred in the dark space to reach the equilibrium adsorption. The

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adsorption and degradation of the dye molecules was negligible in dark space for pure anatase. The degradation was only observed with the simultaneous presence of photocatalyst

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and sunlight. Fig. 1 shows the UV-vis absorption spectra recorded for MB solutions. A strong resonance is clearly observed at 663 nm. This reveals that RGO promotes the photocatalytic

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ability for the removal of MB due to the deposited anatase nanoparticles. The TRGO photocatalyst exhibits the higher photocatalytic activity than those prepared by other routes. The degradation of dye reaches 85 % until 20 min, while TiO2 and TGO degrade approximately 27 and 65 % of MB. TRGO shows superior photocatalytic activity among all studied photocatalysts over 20 min of exposure to sunlight. It is well known that the properties of photocatalysts are sensitively related to electrons and holes separations. This physical characteristic favors contact between the dye and solid surface, removing a larger number of dye molecules. It is evident that the electron transfer influences the efficiency of the photocatalytic process. The diagonal lengths of MB is about 130 Å [31], whereas the particle size of TiO2, GO and RGO are in nano-scale. The electrons in the valence band of anatase can be excited to the conduction band of TiO 2 by sunlight. This reaction leads to positive holes in the valance band. The water molecules

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ACCEPTED MANUSCRIPT can diffuse from the bulk of solution to the pores of the photocatalyst agglomerates and the electrons in the conduction band further react with adsorbed water and oxygen molecules

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to produce hydroxyl and superoxide anions which results in oxidation of MB molecules. 0.7 TiO2

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TGO

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TRGO

0.4

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absorbance

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MB 0.6

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0.2 0.1

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wavelength (nm)

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Fig. 1. UV–vis absorption spectra of MB solution before and after photocatalytic degradation over 20 min under solar irradiation.

Fig. 2 indicates the time dependency of MB degradation in the presence of different photocatalysts in which the initial concentration was changed. Compared to the pure anatase, TGO has a higher photocatalytic activity under solar irradiation. The MB concentration decreases with residence time and the similar behaviors are observed in the degradation of dye at different concentrations. The residence time in which the dye concentration reaches a minimum value, depends on initial concentration and photocatalyst structure. The influence of RGO on the degradation of MB is substantially different than GO, Fig. 3. In general, the amount of MB removed by TRGO increases sharply in all studied concentrations. The reductions of 94, 72 and 61 % in MB concentration are observed in the presence of TGO at different initial concentrations 10

ACCEPTED MANUSCRIPT of 3.0, 5.0 and 8.0 mgl-1, respectively. The MB removal by anatase under the same condition is about 71 %. The results reveal that the removal of MB by TRGO prepared in the presence of

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blackberry juice takes places in two different steps: The first step was found to be rapid, first 20

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min. The second one exhibits a subsequent removal until a constant value is reached, which slows quantitatively an insignificant step, depending on initial concentration of MB. Since there

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is no significant increase on MB degradation at low concentration 40 min is enough for

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purification by TRGO.

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1

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0.6 0.4

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degraded MB.

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TiO2-3.0 mg/l TGO-3.0 mg/l TGO-5.0 mg/l TGO-8.0 mg/l

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Fig. 2. Time dependency of MB degradation in the presence of TiO2 and TGO. The number of MB molecules degraded by TRGO decreases with rise in the initial concentration. Specially, when the concentration is below 5.0 mgl-1, MB can be significantly removed by TRGO and there is a little residue in the solution. At low initial concentration, the number of sites available for degradation is high and consequently the removal process is carried out rapidly. At high concentration, a unit mass of the photocatalyst is exposed to a large number of dye molecules and progressively higher number of MB molecules contacts with the 11

ACCEPTED MANUSCRIPT gradual filling up of appropriate sites in which the radicals are produced. This phenomenon gives a resistance to MB motion as a result, a decrease from 94 to 80 % is observed in dye degradation.

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TRGO- 3.0 mg/l

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degraded MB

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TRGO- 5.0 mg/l TRGO- 8.0 mg/l

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solar irradiation time (min)

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Fig. 3. Time dependency of MB degradation in the presence of TiO2 and TRGO. 3.2. Degradation kinetics

Various photocatalytic processes based on TiO2-graphene compositions have been described by first order kinetic model [7]. In the most of investigations, the concentration was monitored based on the disappearance of initial solution color. The integral of differential rate equation displays the vividly disappearance or decay profile of original compound undergoing photocatalytic degradation. Because, several products with different structures can be formed during photocatalytic reactions, it is more convenient to measure reaction rates based on the disappearance of solution color. The mass law is applied to express the reaction rate which is proportional with concentration. This variation is expressed mathematically as a rate law and the constant of proportionality is termed as a rate constant. There is a reaction leading to trapped electrons, e−, under solar irradiation which subsequently creates the hydroxyl 12

ACCEPTED MANUSCRIPT radical for oxidation of dye. The rate of dye disappearance can be given by differential rate equation known as:

dC  kC n dt

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(2)

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where C is concentration of MB, t is solar irradiation time, k is the rate constant and the power of

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n is the kinetic order. Within the certain time of photocatalytic reaction, only a fraction of dye

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may be degraded. A reaction is first or second order, depending on n value. In order to examine the mechanism of degradation process and photocatalytic reaction, a

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suitable kinetic model is needed to analyze obtained data. The first and second order kinetics have been extensively used to describe the chemical reactions. The conformity between

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experimental data and the model predicted values was expressed by the correlation coefficient. If

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the kinetics is described by first order equation, the change in concentration of the system can be

C  ln    k1t  C0 

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expressed using the following equation:

(3)

where C0 is the concentration of MB at dark condition after adsorption-desorption equilibrium, mgl-1, and k1 is the first order rate constant, min-1. The photo degradation kinetics may be described by a second order equation. By integrating the differential equation and applying the initial condition of t = 0, C = C0, the following equation can be arrangement to obtain a linear form:

1 1   k 2t C C0

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where k2, lmg-1min-1, is rate constant of second-order kinetics. In order to analyze the accuracy of presented kinetic models, the variations of ln(C/C0) versus

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ACCEPTED MANUSCRIPT t are plotted for different photocatalytic conditions. A comparison between the correlation coefficients is presented in Table 1, which indicates the failure of second order kinetics in

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expressing degradation process in the presence of TiO2 and TGO. On the other hand, the

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parameters of second order model were calculated by linearization form of equation at studied concentrations. The slopes and intercepts of plots were used to determine the second-order

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constants for solar degradation in the presence of TRGO. However, the experimental data

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indicate good agreement with theoretical values. The correlation coefficients for the first or second order kinetic models obtained at all the studies concentrations are close to 0.99. The

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results suggest that the degradation processes in the presence of TGO (Fig. 2) and TRGO (Fig. 3) belong to the first and second-order mechanism, respectively.

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The rate constant of dye over TGO is 2.5 times greater than that over anatase. Table 1 also reports the effect of initial MB concentration on the photocatalytic degradation by TGO and

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TRGO composites at 25 °C. The reaction rate decreases with raising the initial concentration. This phenomenon is different in the presence of TGO and TRGO. The employment of RGO might be helpful for the increase of photocatalytic activity, showing a synergistic effect. It is expected that large number of MB cations can be available onto the surface of RGO, providing a higher dye concentration near the anatase nanoparticles and therefore leading to the more efficient contact. Also, the presence of MB cations on the surface of photocatalyst is not significant like that on the surface of RGO as a result, the reaction occurred mainly via the collision of MB and photocatalyst, leading to a slower reaction rate. The degradation rate approximately remains unchanged when initial MB concentration is lower than 5.0 mgl-1. The higher MB concentration on the surface of GO and RGO would lead to filling up the active sites. The prepared composites could not accelerate the electron transfer, causing a decrease in rate

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ACCEPTED MANUSCRIPT constant value. Table 1. The validity and rate constants for solar photocatalytic degradation in the presence of

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prepared photocatalysts Frist and second order kinetics Photocatalyst MB concentration (mgl )

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k2 (lmg-1min-1)

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0.0107

0.934

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0.0752

0.808

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k1 (min-1)

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-1

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0.0186

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3.0

0.0477

5.0

0.0216

0.999

0.0081

0.944

8.0

0.0158

0.986

0.0030

0.982

0.0695

0.737

0.0696

0.987

0.0557

0.855

0.0662

0.988

0.0301

0.723

0.0121

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TRGO

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TiO2

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Structural analyses

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3.3.

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3.3.1. FT-IR spectra

The chemical structure of GO, RGO, anatase, TGO and TRGO characterized by FT-IR are illustrated in Fig 4. The broad band at 3427 cm−1 in the spectra of GO and RGO was related to O-H stretching vibrations which shows that the presence of water physically attached to the surface of samples. The area of this band in the GO spectrum is much smaller than that for RGO spectrum. The rapid adsorption of water from atmosphere is due to the high specific surface area of reduced graphene oxide. The artifacts at 2930 and 2253 cm−1 show the asymmetric stretching vibrations of aliphatic C-H and stretching vibrations of C≡C, respectively. The bands at 1705, 1579, 1404 and 1024 cm−1 in the spectrum of GO are due to the stretching vibrations of C=O, C=C, C-OH and C-O groups. The high intensity of main peaks in GO confirms a large amount of

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ACCEPTED MANUSCRIPT oxygen functional groups after the oxidation process. The peak at 1627 cm−1 is related to the vibrations of adsorbed water molecules. The peaks in the range of 600-800 cm−1, observable at

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the GO and RGO spectra, are related to the =C-H bands. The peaks in the range of 400-600 cm−1

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could be assigned to the C-O and C-C bending vibrations.

Fig. 4. FT-IR spectra of GO, RGO, TiO2, TGO and TRGO. The peaks at 1705 and 2253 cm−1 which are related to stretching vibrations of carboxyl group (C=O), disappeared in the spectrum of RGO. Besides, the significant decrease in the intensity of 16

ACCEPTED MANUSCRIPT peak at 1024 cm-1 indicates the successful reduction of GO, forming RGO in the presence of anthocyanine. Oxygen functional groups completely disappeared in the spectrum of TRGO

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because GO was reduced to RGO. A sharp peak around 525 cm−1 is a characteristic vibration of

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Ti-O-Ti in TiO2-based composites. 3.3.2. XRD patterns

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The XRD patterns of the TiO2 powder calcined at 400 C are shown in Fig. 5 together with

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those for TGO and TRGO. The characteristic peaks of anatase at 2θ of 25.3, 37.8, 48.0, 55.1 and 62.7 are related to the (101), (004), (200), (211) and (204) planes, respectively. The diffraction

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signal assigned to the anatase (101) structure at 25.3 is clearly observed in the all composites.

Fig. 5. XRD patterns of TiO2, TGO and TRGO nanocomposites. 17

ACCEPTED MANUSCRIPT The diffraction signal at 27.5, related to the rutile phase (110), is not observed in the prepared composites. It means that the calcination of composites cannot affect the crystalline

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phase even at temperature of 400 C. Table 2 reports the crystallite size of anatase calculated by

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Scherre’s equation. The diffraction intensity decreases and an increase in width of peaks is

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observable with addition of GO and RGO, which suggests the decrease in crystallite size. A big change in the crystallite size is observed when the synthesis is carried out in the presence of

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RGO.

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Table 2. Crystallite size of anatase and specific surface areas of nanocomposites. Crystallite size (nm)

Specific surface area (m2g-1)

TiO2

17.8

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16.9

61.0

10.4

82.6

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TGO

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Composites

3.3.3. Specific surface area

Although the crystallite size of TiO2 in the absence and presence of GO is approximately the same, a larger specific surface area is obtained when anatase nano particles are decorated on graphene oxide, Table 2. As the crystallite size is decreased by decoration on RGO, the specific surface area grows 4 times in comparison to TiO2 nano powder. As discussed before, oxygen functional groups are almost entirely removed due to antioxidant activity of blackberry juice. It is evident that this change in graphene oxide structure produces empty spaces. As the graphene oxide is reduced, these spaces grow, considerably. This change in number of empty spaces shows an overall positive effect on specific surface area. On the other hands,

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ACCEPTED MANUSCRIPT the reduction of GO by anthocyanine controls the arrangement of anatase crystallites. Finally, the variations in graphene structure and anatase crystallite size promote the

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specific surface area. The product resulting from sol-gel preparation of anatase shows a low

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photocatalytic activity. Indeed, each TiO2 particle acts as a recombination center when e- and h+ pairs are generated under solar irradiation in the absence of GO or RGO. The different

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photocatalytic activity is obtained when TiO2 are decorated on GO and RGO. The very small

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crystallite size, ∼10 nm, implies a high number of crystallites, which act like nanophotochemical radical generator. The effect of anatase distribution on photocatalytic activity of

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TiO2 can be observed in the product resulting from reduction of GO in the presence of blackberry juice. In fact, the low crystallite size combined with extended surface area improves the

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photocatalytic performance. However, the surface area obtained from this new method of preparation is high in comparison to TiO2 and TGO. A larger surface area provides more active

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sites for creating OH radicals, making the photocatalytic process more efficient. 3.3.4. SEM observations

Fig. 6 shows the morphology of TiO2 particles that was prepared by sol-gel method with calcination at 400 C. The high magnification image indicates the presence of nano-crystallites. The smaller particles are agglomerated to form larger particles with non-spherical shape. The prepared powder in basic solution, consists of particles with high agglomeration, 50-200 nm. The randomly packed nanoparticles show many grain boundaries among the particles which causes the low photocatalytic activity.

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Fig. 6. SEM image of TiO2 prepared by sol-gel method.

Fig. 7 represents the SEM images of GO, RGO, TGO, and TRGO. It is well known that

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graphite particles are in the platelet-like crystalline form of carbon. After oxidation and ultra-

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sonication, GO sheets become smaller and transparent, Fig. 7 (a). The reduced graphene oxide

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exhibits typical structure appeared as similar thin sheets, with distinct edges, wrinkled surfaces and folding, Fig. 7 (b). Due to the presence of carbonyl, carboxyle, hydroxyle and epoxy groups GO sheets are thicker than those for RGO. Blackberry juice, which contains anthocyanine, reduces the number of oxygen functional groups, resulting in the thinner sheets. The surfaces of GO sheets were clearly decorated with TiO2 nanoparticles, Fig. 7 (c). The multi-layered RGO are mostly covered by TiO2 nano particles as shown in Fig. 7 (d). The SEM images show that spherical TiO2 nanoparticles are attached on the surface of GO and RGO. The TiO2-RGO hybrid indicates the uniformly sized anatase dispersed on the sheets. The high surface area prevents the agglomeration of TiO2 nanoparticles.

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3.3.5. TEM observations

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Fig. 7. SEM images of GO, RGO, TiO2-GO and TiO2-RGO prepared by sol-gel method.

The TEM images of TiO2, graphene oxide sheet, TGO and TRGO compositions are shown in Fig. 5. The agglomerated nanoparticles of anatase can be seen in TEM image, Fig. 8(a). The size of a micro-crystallite was estimated less than 20 nm. The GO sheet with flat surface is clearly observable in Fig. 8(b). The nano-sized TiO2 particles indicated by the dark color area are not well distributed on the GO sheet as presented in Fig. 8(c). The TiO2 particles are frequently smaller than 20 nm. It also depicts the presence of agglomeration, which change the morphology from spherical shape. Only a small difference is observed in particle size as measured by XRD and TEM. The nano-metrical difference in the obtained values for the particle size of TiO2 is due to the fact that the TEM measurements are based on the difference between the visible grain boundaries, while XRD calculation is based on the extended crystalline region that diffracts X-ray coherently. So, the XRD method has a more stringent criterion and leads to 21

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smaller sizes.

(b)

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Fig. 8. TEM images of (a) TiO2, (b) GO sheet, (c) TGO and (d) TRGO prepared by sol-gel method. The TEM image of TRGO, Fig. 8(d), indicates the formation of nano spherical particles in good agreement with XRD result. The powder consists of nano-metric particles which most of them are smaller than 10 nm. The crystallite size of sample is in the range of 8–10 nm which is significantly smaller than those for TGO. Although both samples calcined at 400 C, the difference in crystallite size can be attributed to decoration on RGO sheets. The anatase 22

ACCEPTED MANUSCRIPT nanoparticles were not significantly formed on GO however, TiO2 nanoparticles were uniformly deposited on RGO. This revealed the necessity of blackberry juice as a reducing agent for

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reduction of oxygen functional groups as well as decoration of nanoparticles. In addition, the

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anatase decoration causes a large change in BET specific surface area. In the present system, the high specific surface area, due to the formation of empty spaces, facilitates the decoration of

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smaller TiO2 particles on RGO, thus resulting in the improved photocatalytic activity.

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3.3.6. MB degradation mechanism

The MB degradation by pure anatase, TGO and TRGO is controlled by three factors. (i)

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Specific surface area of composition which is affected by crystallite size of anatase and surface areas of GO and RGO. (ii) OH radical concentration. (iii) Reaction of OH radicals

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with MB molecules to degrade the dye structure. The third factor is assumed to be very rapid and can be overlooked. The slowest step is electron transfer and limits the

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degradation rate which depends on structural properties of photocatalyst. Fig. 9 describes the mechanism of photocatalytic MB removal under the best degradation condition. With reduction in the presence of blackberry juice the empty spaces grow as a result the specific surface area of RGO rises, considerably. On the other hands, the created empty spaces maintain Ti ions and control the anatase particle size during calcination step. Therefore, the MB degradation reaches 94.0 % within 40 min. The maximum photocatalytic decoloration rate is achieved by TRGO, which is significantly higher than those for pure anatase and TGO. This can be ascribed to the formation of regular structure. The TRGO possess a uniform nano-porous structure, which allows more efficient transport of electron, hence enhancing the efficiency of photocatalyst. Moreover, the nano-spheres allow multiple reflections of solar light within the interior cavity that facilitates more efficient use of sunlight. The improvement of light absorption

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ACCEPTED MANUSCRIPT enhances the transportation of photo-generated electrons and the appropriate distribution of TiO2 nanoparticles are the main reasons for the enhanced photocatalytic activity of TRGO hybrid

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synthesized in the presence of blackberry juice. The electron-hole pairs can initiate oxidation and

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reduction reactions on the surface of TiO2 particles. The electron reacts with adsorbed H2O and produce OH radicals. On the other hand, electrons react with O2 and create the superoxide anion

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radicals. The generated radicals degrade MB molecules and the rate of MB degradation depends

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on electrons-holes separation rate.

The photo-generated electrons play a key role in photocatalytic ability and reaction

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mechanism. The reduction in the electron–hole recombination rate can be controlled by GO and RGO. When anatase is paired with GO and RGO, resulting in the easily transportation of

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electron to the nano-sheets. As a result, the recombination of electron–hole is reduced, significantly. On the other hands, the oxygen functional groups are responsible for

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insulating behavior of GO in comparison to RGO. Therefore, the degradation rate is low in the presence of TGO and obeys first order kinetics. The removal of oxygen can significantly affect the conductivity as a result, the degradation of MB. The superior activity of TRGO may be ascribed to the fact that it has a conductive structure which is more beneficial in enhancing the degradation of dye. In this work, the content of graphene maintained constant and a significant effect was observed on the photocatalytic activity. Because, the electrical resistance of RGO is lower that for GO sheets, such process is energetically favorable and the generated e− and h+ pairs can be efficiently separated, which is regarded as a key factor for the enhancement of photocatalytic activity as a result the second order kinetics governs on degradation.

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Fig. 9. Mechanism of enhanced MB photocatalytic degradation under solar irradiation. 3.3.7. Activity of TiO2 based photocatalysts The present article deals the feasibility of various TiO2 based photocatalysts for MB removal from wastewater. The main aim is to provide a summary of recent information concerning the use of these materials as photocatalysts. Therefore, a list of photocatalysts has been compiled. The reader is strongly encouraged to refer the original research papers for the information on experimental conditions. Although, commercially TiO2 is available, P25 (Degussa), almost GO or RGO nanocomposites were used for the preparation of photocatalysts. P25 is mostly in the anatase form with 20 wt% rutile and is characterized as a relatively good photocatalyst. The degradation ability of each individual composition is determined by the nature of original material and the extent of physical–chemical changes occurring after deposition. The photocatalytic performances of synthesized materials are reported in Table 3 with success for 25

ACCEPTED MANUSCRIPT MB degradation. Since different source of titanium is applied by researchers, it has a variety of photocatalytic properties and thus different performances. Although, the complete photocatalytic

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degradation was reported [22, 33, 34, 40] however, the complex techniques are needed to

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synthesize the photocatalysts. As compared to other studies about the synthesis of TGO and TRGO nanocomposites for MB photocatalytic degradation, the resulting nanocomposite in this

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work indicates significantly higher degradation rate. The higher activities of photocatalysts in

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comparison to P25/GO nanocomposite is due to the excellent electron conductivity of GO or RGO and the chemical bonding between TiO2 and graphene. The source of raw materials, the

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history of preparation and treatment conditions such as temperature and time affect the final properties of products. The method used in this article may also offer an eco-friendly technique

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therefore, there is a need to valorize this green synthesis technique as a potentially inexpensive alternative to the existing commercial photocatalyst. The excellent ability and economic promise

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of TRGO prepared in the presence of blackberry juice exhibits the noticeable performance as reported in Table 3.

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ACCEPTED MANUSCRIPT Table 3. Photocatalytic activity of TGO and TRGO nanocomposites derived from different materials and techniques for degradation of MB. Method

Irradiation time (min)

Performance (%)

Ref.

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Starting material

Spin-coating

UV: 180

55

[27]

TiO2

Hydrothermal

UV: 100

98

[7]

UV: 300

70

[9]

UV: 100

88

[32]

UV: 180

100

[33]

UV: 80

100

[34]

UV: 100

100

[22]

Hydrogel

UV: 55

96

[26]

Sol-gel

UV: 60

80

[35]

Solvo-thermal

UV: 150

70

[36]

Sol-gel

UV: 90

98

[37]

Ti(OCH2CH2CH2CH3)4

Hydrothermal

UV: 30

80

[38]

Ti[OCH(CH3)2]4

Hydrothermal

UV: 50

98

[39]

TiC16H36O4

Sol–gel

UV: 150

100

[40]

TiC16H36O4

Sol–gel

Solar: 40

94

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P25-Degussa Sol-gel

TiCl4

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TiC12H28O4

Sol–gel

TiO2

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Ti(OC3H7)4 Ti(OC3H7)4 Ti(OC3H7)4

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TRGO P25-Degussa

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ACCEPTED MANUSCRIPT 4. Conclusions This article attempted to synthesize the reduced graphene oxide, RGO, by modified

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Hummer’s method and reduction via a rapid and eco-friendly green process in the presence of

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blackberry juice. The results show that oxygen functional groups in GO could be eliminated through stirring in blackberry juice, as confirmed by FTIR analysis. The reduction of GO takes

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place due to the presence of anthocyanins. The synthesized TRGO nanocomposite showed

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excellent photocatalytic activity for degradation of MB. It is found that the enhanced photocatalytic activity is related to the size of TiO2 nanoparticles decorated on RGO, and the

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appropriate control is required to achieve appropriate degradation rate. In addition, a mechanism for the photocatalytic removal of MB by TRGO nanocomposite was suggested to describe the

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synergistic effect of blackberry juice. The potential of TGO and TRGO for the removal of MB from aqueous solution was investigated by changing the irradiation time and MB concentration

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to study the kinetics. The results indicated that anatase and TGO are able to remove dye cations from aqueous solution according to the first order kinetics while in the TRGO system, the dye degradation is controlled by second order kinetics, resulting in an increase in MB degradation from 80 to 94 % over 40 min. The results support a two-step mechanism involved in the removal of MB by TRGO prepared in the presence of blackberry juice. Firstly, a rapid surface complexation of TiO2 nanoparticles, 10 nm, on the RGO and secondly improved conductivity of RGO in comparison to GO. Although, the TGO powder presents lower photocatalytic activity than that for TRGO, but it can be considered as a promising material for the degradation of MB from aqueous solution under solar irradiation. Such a product can be used in the industrial wastewater treatment and the cationic dye degradation under solar irradiation.

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ACCEPTED MANUSCRIPT References [1] L. Karimi, A. Salem, J. Ind. Eng. Chem. 17 (2011) 90–95.

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[2] A. Salem, M. Saghapour, Chemical Papers 70 (2) (2016) 153–163.

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[9] Y. Zhang, C. Pan, J. Mater. Sci. 46 (2011) 2622–2626. [10] J. Li, S. Zhou, G.B. Hong, C.T. Chang, Chem. Eng. J. 219 (2013) 486–491. [11] G. Eda, C. Mattevi, H. Yamaguchi, H. Kim, M. Chhowalla, J. Phys. Chem. 113 (2009) 15768–15771.

[12] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, Chem. Soc. Rev.39 (2010) 228–240. [13] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nanotechnol. 3 (2008) 270–274. [14] K.H. Liao, A. Mittal, S. Bose, C. Leighton, K.A. Mkhoyan, C.W. MacOsko, ACS Nano. 5 (2) (2011)1253–1258. [15] X.Z. Zhou, X. Huang, X.Y. Qi, S.X. Wu, C. Xue, F.Y.C. Boey, Q.Y. Yan, P. Chen, H. Zhang, J. Phys. Chem. C 113 (2009) 10842–10846. [16] W. Yao, Y. Li, D. Yan, M. Ma, Z. He, S. Chai, X. Su, F. Chen, Q. Fu, Chem. Eng. J. 229

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[24] X. Luan, M.T.G. Wing, Y. Wang, Mater. Sci. Semiconductor Process. 30 (2015) 592–598. [25] Y. Wang, R. Shi, J. Lin, Y. Zhu, Appl. Catal. B: Environ. 100 (2010) 179–183. [26] C. Hou, Q. Zhang, Y. Li, H. Wang, J. Hazard. Mater. 205– 206 (2012) 229–235. [27] Y. Zhang, J. Xu, Z. Sun, C. Li, C. Pan, Prog. Natural Sci.: Mater. Int. 21 (2011) 467−471. [28] M.J. Cho, L.R. Howard, R.L. Prior, J.R. Clark, J. Sci. Food Agric. 84 (2004) 1771–1782. [29] L. Kaume, L.R. Howard, L. Devareddy, J. Agric. Food Chem. 60 (2012) 5716–5727. [30] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (6) (1958) 1339–1339. [31] J.C. Santamarina, K.A. Klein, Y.H. Wang, E. Prencke, Canadian Geotechnical J. 39 (2002) 233–241. [32] D. Zhao, G. Sheng, C. Chen, X. Wang, Appl. Catal. B: Environ. 111– 112 (2012) 303– 308. [33] E. Lee, J.Y. Hong, H. Kang, J. Jang, J. Hazard. Mater. 219– 220 (2012) 13–18.

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[40] P.T.N. Nguyen, C. Salim, W. Kurniawan, H. Hinode, Catal. Today 230 (2014) 166–173.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. UV–vis absorption spectra of MB solution before and after photocatalytic degradation

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over 20 min under solar irradiation.

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Fig. 2. Time dependency of MB degradation in the presence of TiO2 and TGO.

Fig. 4. FT-IR spectra of GO, RGO, TGO and TRGO.

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Fig. 3. Time dependency of MB degradation in the presence of TiO2 and TRGO.

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Fig. 5. XRD patterns of TiO2, TGO and TRGO nanocomposites. Fig. 6. SEM image of TiO2 prepared by sol-gel method.

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Fig. 7. SEM images of GO, RGO, TiO2-GO and TiO2-RGO prepared by sol-gel method. Fig. 8. TEM images of (a) TiO2, (b) GO sheet, (c) TGO and (d) TRGO prepared by sol-gel

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Fig. 9. Mechanism of enhanced MB photocatalytic degradation under solar irradiation.

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Figure 1

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0.7 MB 0.6

TiO2 TGO

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absorbance

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450

500

550

600

wavelength (nm)

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650

700

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Figure 2

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TiO2-3.0 mg/l

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degraded MB.

0.8

TGO-3.0 mg/l TGO-5.0 mg/l TGO-8.0 mg/l First order model

0

0

10

20

30

40

50

60

solar irradiation time (min)

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70

80

90

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Figure 3

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TRGO- 3.0 mg/l

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0.4 0.2

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degraded MB

0.8

TRGO- 5.0 mg/l TRGO- 8.0 mg/l second order model

0

0

10

20

30

40

50

60

solar irradiation time (min)

35

70

80

90

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

(b)

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Figure 9

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Table 1. The validity and rate constants for solar photocatalytic degradation in the presence of

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Photocatalyst MB concentration (mgl )

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prepared photocatalysts

R2

0.0186

0.985

0.0477

0.990

0.0216

0.999

0.0158

0.986

3.0

0.0695

5.0 8.0

TGO

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k1 (min-1)

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5.0 8.0

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Kinetics k2 (lmg-1min-1)

R2

0.0107

0.934

0.0752

0.808

0.0081

0.944

0.0030

0.982

0.737

0.0696

0.987

0.0557

0.855

0.0662

0.988

0.0301

0.723

0.0121

0.988

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Table 2. Crystallite size of anatase and specific surface areas of nanocomposites. Crystallite size (nm)

Specific surface area (m2g-1)

TiO2

17.8

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18.7

16.9

61.0

10.4

82.6

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Table 3. Photocatalytic activity of TGO and TRGO nanocomposites derived from different

Method

Irradiation time (min)

Performance (%)

Ref.

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Starting material

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materials and techniques for degradation of MB.

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TGO Spin-coating

UV: 180

55

[27]

TiO2

Hydrothermal

UV: 100

98

[7]

UV: 300

70

[9]

UV: 100

88

[32]

UV: 180

100

[33]

UV: 80

100

[34]

UV: 100

100

[22]

Hydrogel

UV: 55

96

[26]

Sol-gel

UV: 60

80

[35]

Solvo-thermal

UV: 150

70

[36]

Sol-gel

UV: 90

98

[37]

Ti(OCH2CH2CH2CH3)4

Hydrothermal

UV: 30

80

[38]

Ti[OCH(CH3)2]4

Hydrothermal

UV: 50

98

[39]

TiC16H36O4

Sol–gel

UV: 150

100

[40]

TiC16H36O4

Sol–gel

Solar: 40

94

This

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P25- Degussa

P25-Degussa Sol-gel

TiCl4

Sol–gel

TiC12H28O4

Sol–gel

TiO2

Spark plasma

P25-Degussa Ti(OC3H7)4 Ti(OC3H7)4 Ti(OC3H7)4

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights Anatase-graphene nanocomposite was synthesized by an eco-friendly method.

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Blackberry juice was used as a reducing agent to produce the graphene nano-sheets.

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The nanocomposite was applied as a photocatalyst for the degradation of methylene blue. A facile and rapid route was applied for the uniform deposition of anatase on sheets.

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Degradation kinetics in the presence nanocomposite was studied by spectroscopy.

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