TiO2 nanocomposite and their catalytic activity for the reduction of 4-nitrophenol, congo red and methylene blue

TiO2 nanocomposite and their catalytic activity for the reduction of 4-nitrophenol, congo red and methylene blue

Author’s Accepted Manuscript In situ green synthesis of ag nanoparticles on graphene oxide/TiO2 nanocomposite and their catalytic activity for the red...

2MB Sizes 8 Downloads 50 Views

Author’s Accepted Manuscript In situ green synthesis of ag nanoparticles on graphene oxide/TiO2 nanocomposite and their catalytic activity for the reduction of 4-nitrophenol, Congo red and methylene blue Mahmoud Nasrollahzadeh, Monireh Atarod, Babak Jaleh, Mastane Gandomi www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)30032-3 http://dx.doi.org/10.1016/j.ceramint.2016.02.088 CERI12279

To appear in: Ceramics International Received date: 5 February 2016 Revised date: 15 February 2016 Accepted date: 15 February 2016 Cite this article as: Mahmoud Nasrollahzadeh, Monireh Atarod, Babak Jaleh and Mastane Gandomi, In situ green synthesis of ag nanoparticles on graphene oxide/TiO2 nanocomposite and their catalytic activity for the reduction of 4nitrophenol, Congo red and methylene blue, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.02.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

In situ green synthesis of Ag nanoparticles on graphene oxide/TiO2 nanocomposite and their catalytic activity for the reduction of 4-nitrophenol, Congo red and Methylene blue Mahmoud Nasrollahzadeh,*,a,b Monireh Atarod,a Babak Jalehb and Mastane Gandomib a

Department of Chemistry, Faculty of Science, University of Qom, Qom 37185-359, Iran b

Center of Environmental Researches, University of Qom, Qom, Iran

c

Department of Physics, Bu-Ali Sina University, Postal Code 65174, Hamedan, Iran

ABSTRACT The Ag/RGO/TiO2 nanocomposite was synthesized through an environmentally benign, simple, cost efficient, surfactant-free and green method using Euphorbia helioscopia L. leaf extract as a stabilizing and reducing agent. The Euphorbia helioscopia L. leaf extract was used for the reduction of Ag+ ions and GO to Ag NPs and RGO, respectively. The GO/TiO2 and Ag/RGO/TiO2 nanocomposites were characterized by FT-IR, UV-vis, TEM, XRD, SEM, EDS and ICP techniques. The Ag/RGO/TiO2 nanocomposite was highly active for the reduction of 4-nitrophenol (4-NP), Congo red (CR) and Methylene blue (MB) in aqueous media at an ambient temperature. The Ag/RGO/TiO2 nanocomposite was easily separated and recovered from the reaction mixture by centrifugation and reused for several cycles without any significant loss of catalytic activity.

Keywords: Green synthesis, Ag/RGO/TiO2, reduction; Catalytic properties

1. Introduction Dyes and pigments are used in textile, cosmetic, printing, drug and food-processing industries [1]. The dyes used in various industries are often highly toxic to aquatic organisms [1]. Many chemical and physical methods such as active carbon adsorption, dissolved air floatation, biochemical, chemical and microorganisms mediated reductions have been developed for treatment of dye containing effluents [2-4]. Earlier reported methods suffer from certain limitations such as high cost, phase transfer of pollutants, difficult removal of microorganisms from

*

Corresponding author. Tel.: +98 25 32850953; Fax: +98 25 32103595.

E-mail address: [email protected] (M. Nasrollahzadeh). 1

the degraded dye molecules, extreme resistive character of dye to microorganisms, and photolytic stable conditions, still persist in the system which hindered their large scale applications [2-4]. Recently, numbers of metal nanoparticles (NPs) have been extensively exploited for the degradation of dyes due to their large specific surface area, number of active sites, high physical, chemical and thermal stabilities, strong electron transfer abilities and high activity and efficiency [5-6]. The catalytic activity, however, is limited by the agglomeration of metal NPs during reactions. On the other hand, nanoparticles used in the catalytic reactions are small in size, which are impossible to be recovered and the residual metal left in the system confines the reaction. Therefore, because of dyes toxicity and carcinogenic properties, the development of highly efficient, cheap cost, easily recoverable catalyst employed technique for the elimination of these pollutants from the environment is essential. In recent years, the development of heterogeneous catalytic systems via immobilization of metal NPs on a solid support such as titanium dioxide (TiO2) NPs, gum and Fe3O4 has increased [7-9]. Graphene is a one-atom-thick, two-dimensional layer of graphite with carbon atoms arranged in a sixnumbered-ring plane. It is a new form of carbon materials discovered by scientists after fullerene and carbon nanotubes. Ever since its first discovery, graphene has been studied vigorously because of its unique structure and extraordinary electrical, thermal and mechanical properties [10]. Recent achievements on high-throughput of large area graphene highlight its potential for industrial applications because it can be manufactured at a reasonable cost [11]. However, application of materials depends greatly on their intrinsic properties. Hybridization of different materials offers a powerful way to enhance the application of graphene by enabling versatile and tailor-made properties with high performance far beyond those of the individual materials. The perfect structure of graphene shows low chemical reactivity, therefore, graphene oxide (GO), one of the derivatives of graphene that contains a range of reactive oxygen functional groups, is considered to be a good candidate for chemical functionalization. Various inorganic nanoparticles such as Cu, Au, Fe3O4, TiO2, SnO2 and ZnO are used to hybridize with GO or graphene to obtain antibacterial materials, controlled targeted drug carriers, optoelectronic materials, electrode materials and catalytic materials [12-19]. Among the various metal oxide nanoparticles, TiO2 has been investigated by many researcher because of their applications in many fields of chemical engineering, materials engineering, dye-sensitized cells, sensors and in the paint industry [7,20]. TiO2 is widely used as an effective photocatalyst for the photodegradation of organic pollutants in water and air [7,21]. Anatase and rutile are TiO2 polymorphic forms which are relevant in photocatalytic applications [22]. Anatase is usually considered to be more active than rutile. But recent studies suggest that the photocatalytic and 2

photovoltaic properties of TiO2 nanoparticles with mixed phase are better than pure anatase TiO2 [23]. Thus, the combination of TiO2 and GO is promising to simultaneously possess excellent absorptivity, conductivity, and controllability, which could facilitate effective degradation of pollutants. Unique physicochemical properties of Ag NPs are being exploited in various fields such as optics, electronics, biology and catalysis [24,25]. Many chemical and physical techniques have been developed for the synthesis of Ag NPs [26-28]. However, in most cases, the methods have potential hazards to health and environment. In recent years, green synthesis of NPs by using bacteria, fungi, yeast as well as plants have gained greater attention in nanotechnology [29-32]. Biological synthesis does not require the usage of hazardous chemicals, the stabilizing agents, large amounts of energy, high temperature or pressure. Among these biological methods, plant mediated green synthesis of NPs has more advantages over the microbial methods, as it overcomes the tedious processes of culture maintenance along with the extremely reduced reaction time required for preparation of NPs. Unlike chemical and physical methods, biological methods do not generate hazardous waste and the products usually do not need purification. Recently, our research group described the preparation of Ag NPs using extract of Euphorbia helioscopia L. leaves [33]. Ag NPs were characterized by UV-vis, FT-IR, XRD and TEM analysis in our recent work [33]. The previous studies on Euphorbia helioscopia L. leaf (Figure 1) extract revealed the presence of potent antioxidant flavonoid glycosides such as quercetin, quercetin-3-O-glucoside, and 1,2,3-tri-O-galloyl-β-D-Glucose, gallic acid, methyl gallate, pyrogallol, (-)-shikimic acid-4-O-gallate, (-)-shikimic acid-O-gallate, 1-O-galloyl-2,3HHDP-α-D-glucose,

1,3,6-tri-O-galloyl-β-D-glucose,

1,2,3,6-tetra-O-galloyl-B-D-glucose,

resorcinol,

gallicacide-4-O-(6`-O-galloyl)-β-D-glucose [33]. Therefore these potent antioxidants inside the plant extract demonstrate that it can be a potent green source for production of NPs.

Figure 1. Image of the leaves of Euphorbia helioscopia L. Reprinted from ref. 33 with permission. Copyright (2015) Sciencedirect.

3

In continuation of our efforts to develop environmentally friendly synthetic methodologies [34-36], we prepared Ag/RGO/TiO2 nanocomposite via a new, fast, green, facile, simple, cost effective and environment friendly process by Euphorbia helioscopia L. leaf extract as stabilizing and reducing agent for the reduction of Ag+ ions and GO. The catalytic efficiency of Ag/RGO/TiO2 nanocomposite was tested against three organic dyes such as 4-NP, CR and MB. To the best of the author’s knowledge, this is the first biogenic report on the synthesis of Ag/RGO/TiO2 nanocomposite using plant extract. This study opens the door to green synthesis of novel nanocomposites for the application in various fields.

2. Experimental 2.1. Instruments and reagents High-purity chemical reagents were purchased from the Merck and Aldrich chemical companies. All materials were of commercial reagent grade. Titanium dioxide (TiO2, Degussa Company, Germany) powders of 30 nm were used. FT-IR spectra were recorded on a Nicolet 370 FT/IR spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. X-ray diffraction (XRD) measurements were carried out using a Philips powder diffractometer type PW 1373 goniometer (Cu Kα = 1.5406 A˚). The scanning rate was 2º/min in the 2θ range from 10 to 90˚. UV-visible spectral analysis was recorded on a double‐beam spectrophotometer (Hitachi, U‐2900) to ensure the formation of nanoparticles. The shape and size of nanocomposite was identified by transmission electron microscope (TEM) using a Philips EM208 microscope operating at an accelerating voltage of 90 kV. The morphologies of the samples were examined by scanning electron microscopy (SEM) ((Scam MV 2300) Cam type). The chemical composition of the prepared nanostructures was measured by EDS (Energy Dispersive X-ray Spectroscopy) performed in SEM. 2.2. Preparation of the GO The GO was prepared from natural graphite powder by a modified Hummers method [37]. 2.3. Preparation of the graphene oxide/TiO2 nanocomposite Graphene oxide/TiO2 nanocomposite was synthesized with a weight ratio between graphene oxide and TiO 2 at (1:1) by adding 0.5 g of graphene oxide dispersed in 100 ml of water to form graphene oxide suspension by ultrasonication. Then TiO2 powder was dispersed in the above graphene oxide suspension, followed by ultrasonication for 20 min. Then the mixture was stirred at 100°C for 10 h while refluxing and filtered. 2.4. Preparation of Euphorbia helioscopia L. leaf extract 4

The extract was prepared according to our recent work [33]. 2.5. Preparation of Ag NPs using Euphorbia helioscopia L. leaf extract Ag NPs were prepared according to our recent work [33]. 2.6. Green synthesis of the Ag/RGO/TiO2 nanocomposite using Euphorbia helioscopia L. leaf extract 0.05 M AgNO3 was gradually added into prepared GO/TiO2 nanocomposite and magnetically stirred for 1 h. Then, the above extract was added into mixture under vigorous stirring at 80 °C for 5 h. The obtained precipitation was collected, washed with deionized water, dried in vacuum oven at 100 °C for 2 h and used for characterization and investigation of the catalysis. 2.7. Catalytic reduction of 4-NP To an aqueous solution of 4-NP (2.5 mM, 25 mL), the Ag/RGO/TiO2 nanocomposite (7.0 mg) and 25 mL of the newly prepared NaBH4 (0.25 M) were added and the reaction mixture was stirred at room temperature. The UVvisible spectra showed no 4-nitrophenolate ion absorbance peak at 400 nm after 195 s. The rate of reaction was monitored by taking 1.0 mL of the solution, centrifuging them to remove the catalyst and recording the UV-vis spectrum at certain intervals. For the recycling experiment, the catalyst was separated by centrifugation, washed with doubly distilled water and then dried at 100 °C for 2 h for the next cycle. 2.8. Catalytic reduction of CR To an aqueous solution of CR (1.44 × 10-5 M, 25 mL), the Ag/RGO/TiO2 nanocomposite (7.0 mg) and 25 mL of the newly prepared NaBH4 (5.3 × 10-3 M) were added and the reaction mixture was stirred at room temperature. The UV-visible spectra showed no CR absorbance peak at 493 nm after 116 s. The rate of reaction was monitored by taking 1.0 mL of the solution, centrifuging them to remove the catalyst and recording the UV-vis spectrum at certain intervals. For the recycling experiment, the catalyst was separated by centrifugation, washed with doubly distilled water and then dried at 100 °C for 2 h for the next cycle. 2.9. Catalytic reduction of MB To an aqueous solution of MB (3.1 × 10-5 M, 25 mL), the Ag/RGO/TiO2 nanocomposite (5.0 mg) and 25 mL of the newly prepared NaBH4 (5.3 × 10-3 M) were added and the reaction mixture was stirred at room temperature. The UV-visible spectra showed no MB absorbance peak at 663 nm after 4 s. The rate of reaction was monitored by taking 1.0 mL of the solution, centrifuging them to remove the catalyst and recording the UV-vis spectrum at certain intervals. For the recycling experiment, the catalyst was separated by centrifugation, washed with doubly distilled water and then dried at 100 °C for 2 h for the next cycle. 5

3. Results and discussion 3.1. Preparation and characterization of the Ag/RGO/TiO2 nanocomposite The overall procedure for the preparation of the Ag/RGO/TiO 2 nanocomposite contains two steps. Firstly, GO/TiO2 nanocomposite was prepared via a solution-based method, as illustrated in Figure 2. Secondly, Euphorbia helioscopia L. leaf extract was used as a stabilizing and reducing agent to immobilize Ag NPs on the surface of the RGO/TiO2 to form Ag/RGO/TiO2 nanocomposite through reduction Ag+ ions and GO. Here it is noted that Euphorbia helioscopia L. leaf extract plays an important role for Ag NPs immobilization. Graphene oxide

Water Dispersing TiO2 Ultrasound

Reflux

GO/TiO2

Figure 2. Flowchart of preparation of GO/TiO2 nanocomposite.

The synthesized GO/TiO2 nanocomposite was characterized by UV-vis, FT-IR, XRD, SEM and TEM. Figure 3 displays UV-visible absorption spectra of the TiO2 and GO/TiO2 nanocomposite. The enhanced absorption of the GO/TiO2 nanocomposite in the whole visible region can be attributed to the presence of graphene oxide. However, the obvious blue shift of absorption edge in comparison with the TiO 2 is related to the presence of graphene oxide.

Figure 3. UV-visible absorption spectra of pure TiO2 and GO/TiO2 nanocomposite.

6

Figure 4 shows the FT-IR spectra of the pristine GO, TiO2, GO/TiO2 and Ag/RGO/TiO2 nanocomposite. In the FT-IR spectrum for GO, the broad peak centered at 3432 cm-1 is attributed to the O-H stretching vibrations, 1717 cm-1 (stretching vibrations from C=O), 1632 cm-1 (skeletal vibrations from unoxidized graphitic domains) and at 1050 cm-1 (C-O stretching vibrations). FT-IR spectrum of TiO2 is displayed in Figure 4. It should be noted that the characteristic peaks of TiO2 particles are located at around 696 cm-1. The broad peak at 3450 cm-1 corresponds to the stretching motion of the surface hydroxyl or adsorbed water and also the peak at 1636 cm-1 corresponds to the bending vibration of the OH bonds of the adsorbed water. FT-IR spectrum of GO/TiO2 nanocomposite is displayed in Figure 4. Bands at 3431 cm-1 and 1574 cm-1 are ascribed to the O-H stretching and bending vibrations, respectively. While the strong absorption at 517-680 cm-1 is attributed to the stretching vibrations of the Ti-O in TiO2. In the FT-IR spectrum for Ag/RGO/TiO2 nanocomposite, the broad peak centered at 3398 cm-1 is attributed to the O-H stretching vibrations, at 1618 cm-1 (OH bending vibrations), at 1053 cm-1 (C-O stretching vibrations) and 500-640 cm-1 (Ti-O stretching vibrations).

Figure 4. FT-IR spectra of (a) GO, (b) TiO2 and (c) GO/TiO2 and Ag/RGO//TiO2 nanocomposite.

Figure 5 presents the X-ray diffraction pattern of the GO/TiO2 nanocomposite. In Figure 5 characteristic peak of GO could be clearly observed at 2θ = 10.9°. Characteristic peaks of anatase-type (A) TiO2 could be seen in 25.2°, 37.8°, 48.1°, 53.9°, 54.1° and rutile-type (R) TiO2 in 27°, 62.8° and 68.8°. The weight fraction of the rutile and anatase phase in the TiO2 sample, WR and WA, can be estimated from the XRD peak intensities using following formula [38]:

7

wR

1 (1 0.994( Aana Arut )

wA

1 (1 1.26( Arut

(1)

(2)

Aana)

where Aana and Arut represents the X-ray integrated intensities of anatase (101) and rutile (110) diffraction peaks, respectively. The resulting rutile and anatase content in TiO2 and GO/TiO2 nanocomposite is given in Table 1. Anatase to rutile phase transformation is observed.

Figure 5. XRD pattern of GO/TiO2 nanocomposite.

Table 1 Anatase and rutile content in TiO2 and GO/TiO2 nanocomposite. Sample

Content (%) Anatase (101)

Rutile (110)

TiO2

82.5

17.5

GO/TiO2

77.2

22.8

Scanning electron microscope (SEM) images of the graphene oxide and GO/TiO2 nanocomposite are shown in Figure 6. As shown in Figure 6(b), the graphene oxide sheets are covered heavily with TiO2 particles.

8

(a)

(b)

Figure 6. SEM images of (a) graphene oxide and (b) GO/TiO2 nanocomposite.

The TEM image of GO/TiO2 nanocomposite is showed in Figure 7. The light-gray thin films are the GO sheets, and the dark regions on the GO background are due to the presence of TiO 2 particles. It can be clearly seen in Figure 7 that the exfoliated GO sheet was decorated with TiO2 aggregates with average size of below 30 nm. It is observed that the TiO2 particles are nanosized.

Figure 7. TEM image of GO/TiO2 nanocomposite.

Then, Ag NPs was prepared from AgNO3 through biological reduction of Ag+ ions using Euphorbia helioscopia L. leaf extract as a reducing and stabilizing agent. Ag NPs was characterized by UV-vis, FT-IR, XRD and TEM in our recent work [33]. The reduction of the Ag+ ions to Ag(0) using Euphorbia helioscopia L. leaf extract was monitored by the absorption spectra of the synthesized Ag NPs (Figure 8). The maximum absorption of surface plasmon resonance (SPR) band occurred at 440 nm [33].

9

Figure 8. UV-vis spectrum of synthesized Ag NPs using the Euphorbia helioscopia L. leaf extract. Reprinted from ref. 33 with permission. Copyright (2015) Sciencedirect

In the next step, we was prepared the Ag/RGO/TiO2 nanocomposite via the reduction of the Ag+ ions and in situ growth of Ag NPs attached on RGO/TiO2 nanocomposite using Euphorbia helioscopia L. leaf extract as a stabilizing and reducing agent without any surfactant and hazardous chemicals. Milder reaction conditions, easier work up process, use of green solvent, elimination of toxic, expensive and harmful chemicals, and stability of Ag NPs are the main advantages of this environmentally benign and safe protocol. Furthermore, in a safe environment including the non-poisonous chemicals and mild conditions, the GO converted to RGO by Euphorbia helioscopia L. leaf extract which monitored using UV-vis spectroscopy (Figure 9). In the UV-vis spectrum of GO, the main UV signal at λ max 232 nm is characteristic of the aromatic double bonds and the shoulder appeared in 300 nm is for carbonyl absorption which easily demonstrate the π → π* and n → π* transitions, respectively. Moreover, the UV-vis spectrum of the green synthesized RGO shows a red shift signals from 232 nm to 270 nm as a sign of the production of reduced product (Figure 9). Therefore, we thing the structure of synthesized nano catalyst is as Ag/RGO/TiO2 nanocomposite. After completion of the reduction, the separated Ag/RGO/TiO2 nanocomposite was dried and characterized by FT-IR, XRD, FE-SEM, EDS and elemental mapping.

10

Figure 9. UV-visible absorption spectra of GO and RGO.

XRD analysis of the Ag/RGO/TiO2 nanocomposite (Figure 10) show major diffraction peaks at 32.5°, 43.4°, 63.5°, 75.5° and 82.1° (2θ), which can be assigned to (111), (200), (220), (311) and (222) planes of the facecentered cubic (fcc) silver crystals. The structure of the Ag/RGO/TiO2 nanocomposite is analogous to that of Degussa P25 which can be indexed to TiO2 in anatase and rutile phases.

Figure 10. XRD pattern of Ag/RGO/TiO2 nanocomposite.

Figure 11 represents the results of field emission scanning electron micrograms (FESEM) in order to investigate the particle size and morphology of the Ag/RGO/TiO2 nanocomposite. The FESEM of catalyst shows nanoparticles with small sizes.

11

Figure 11. SEM images of the Ag/RGO/TiO2 nanocomposite.

Furthermore, the chemical composition of the Ag/RGO/TiO2 nanocomposite was determined by energy dispersive X-ray spectroscopy (EDS) (Figure 12). It is found that the peaks of C, Ti, O and Ag are observed. Further, the presence of C, Ti, O and Ag was approved with elemental mapping images (Figure 13).

12

2000

AgL 1500

CK TiK 1000

TiL OK ClK AgL 500

TiK ClK

keV

0 0

5

10

Figure 12. EDS spectrum of the Ag/RGO/TiO2 nanocomposite.

Figure 13. EDS elemental mapping of the Ag/RGO/TiO2 nanocomposite.

3.2. Catalytic performance of the Ag/RGO/TiO2 nanocomposite in 4-NP, CR and MB reduction

13

The catalytic activity of the Ag/RGO/TiO2 nanocomposite was evaluated using the reduction of aqueous solutions of 4-NP, CR and MB dyes in the presence of NaBH4 at room temperature (Scheme 1). The progress of the reaction was monitored using UV-visible measurements at regular intervals of time.

As shown in Figure 14 and 15, 4-NP in aqueous medium has a maximum absorption at 317 nm. The peak at 317 nm remains unchanged even for a couple of days in the absence of any catalyst. To evaluate the catalytic activity of the Ag/RGO/TiO2 nanocomposite, 25.0 mL of 4-NP (2.5 mM) was mixed with a freshly prepared aqueous solution of 25.0 mL of NaBH4 (0.25 M) at room temperature, and 7.0 mg of the Ag/RGO/TiO 2 nanocomposite was mixed with the above reaction mixture. The light yellow color of the solution changed to intense yellow when NaBH4 was added into 4-NP solution and showed an absorption peak at about 400 nm due to the formation of 4-nitrophenolate ions in alkaline conditions caused by the addition of NaBH4 and it did not change as time passed. As shown in Figure 14 and 15, when Ag/RGO/TiO2 nanocomposite was added into the reaction system, the peak at 400 nm decreased and a new peak appeared at about 300 nm which corresponded to the formation of 4-aminophenol (4-AP). After about 195 s, the whole peak at 400 nm disappeared and yellow color of the solution was changed to white, which indicated that 4-NP was almost turned to 4-AP.

14

Figure 14. Absorbance spectra of 4-NP before the addition of NaBH4 solution (A); 4-nitrophenolate (B) and 4-AP (C).

Figure 15. UV-vis spectra for reduction of 4-NP catalyzed by Ag/RGO/TiO2 nanocomposite. Conditions: [4-NA] = 2.5×10−3 M; [NaBH4] = 0.25 M; catalyst = 7.0 mg; temperature = 303 K.

To explore the catalytic activity of various catalysts, the reduction of 4-NP to 4-AP with NaBH4 was investigated under various reaction conditions. The results are summarized in Table 2. The catalytic ability of the TiO2 NPs and GO were examined for the reduction of 4-NP to 4-AP. The results indicate that the TiO2 NPs and GO does not work as catalyst for this reaction. Among the catalysts investigated, the Ag/RGO/TiO2 nanocomposite (Table 2, entry 1) showed better catalytic performances. The effect of the amount of NaBH4, as well as catalyst loadings was investigated using Ag/RGO/TiO2 nanocomposite as the catalyst. Different catalyst loadings between 5.0 and 7.0 mg were studied for the reduction of 4-NP (Table 2, entries 1 and 2). Among those, 7.0 mg of catalyst was found to be the best. Then, effect of the concentration of NaBH4 was studied. When 100 equiv. of NaBH4 was used, the best result was achieved (Table 2, entry 1). Since the initial concentration of NaBH4 was very high, it remained essentially constant throughout the reaction. Thus, the reaction was pseudo first-order to 4-NP. 15

The results show that pure Ag NPs could catalyze the reduction of 4-NP to 4-AP, but with slower reaction rates than Ag/RGO/TiO2 nanocomposite. As shown in Table 2, a longer reaction time of 22 minutes was required to achieve the full reduction of 4-NP to 4-AP by NaBH4 using pure Ag NPs. Such observation indicates that the Ag/RGO/TiO2 nanocomposite has higher catalytic activity for the reduction of 4-NP to 4-AP than the Ag NPs alone, which can be attributed to that the TiO2 and RGO supports may play an active part in the catalysis, yielding a synergistic effect. The synergistically enhanced catalytic activity might be explained as the following two points: (1) Ag NPs show a high catalytic activity; (2) RGO/TiO2 nanocomposite provides high surface area and has high adsorption ability toward 4-NP via π-π stacking interactions. Such adsorption provides a relatively high concentration of reactants molecule which is closer to the Ag NPs on RGO/TiO2, leading to highly efficient contact between them. When Ag/RGO/TiO2 nanocomposite is used for catalytic reduction of 4NP, the Ag NPs can transfer electrons from BH4- to 4-NP, which are both absorbed on the catalyst, leading to the production of 4-AP. BH4- as a nucleophile can donate electrons to Ag NPs and 4-NP as an electrophile can capture electrons from the Ag NPs. Finally, the 4-AP desorbs from the supports to create a free surface and the catalytic cycle starts again. Table 2 Effect of various catalysts and amount of NaBH4 for reduction of 4-NP.

a

4-NP (mM, mL)

NaBH4 (Mm, mL)

Catalyst (mg)

Time (s)

2.5, 25

250, 25

Ag/GO/TiO2 (7.0)

195

2.5, 25

250, 25

Ag/GO/TiO2 (5.0)

310

2.5, 25

187.5, 25

Ag/GO/TiO2 (7.0)

347

10, 50 μL

50, 2 mL

Ag NPs (25 μL)

22 min

2.5, 25

250, 25

GO (7.0)

30 mina

2.5, 25

250, 25

TiO2 (7.0)

30 mina

Not completed.

To show the advantage of the Ag/RGO/TiO2 nanocomposite over some of the reported catalysts in the literature, the catalytic role of the Ag/RGO/TiO2 nanocomposite in reduction reaction of 4-NP to 4-AP with NaBH4 was compared with other reported catalyst (Table 3). With an overall look at Table 3, we can say that our method is comparable with other reported methods in term of reaction time.

16

Table 3 Comparison of various catalysts in the reduction of 4-NP to 4-AP with NaBH4. Catalyst

NiFe2O4 NPs Ni NPs

Concentration of

Concentration of

Time

4-NP (mM) [mmol]

NaBH4 (mM) [mmol]

(min)

36 [0.72]

Ref.

1798 [36]

16

39

-4

200 [0.06]

16

40

-3

0.1 [3 × 10 ]

Ni/graphene nanocomposite

0.1 [2 × 10 ]

132 [0.26]

4

41

[email protected] composite

1 [0.01]

92.5 [1.85]

5

42

10 [1 × 10 ]

12

43

Pd-graphene nanohybrid

-4

0.1 [2.9 × 10 ]

-3

-3

Cu3Ni2 bimetallic nanocrystals

0.1 [2 × 10 ]

20 [0.1]

6

44

NiCo2 alloy microstructure

0.1 [1 × 10-3]

60 [0.6]

30

45

60

46

FeNi2 alloy nanostructure

-4

0.1 [1 × 10 ]

60 [0.06]

--5

-3

Au NPs/ionic liquid-graphene

0.025 [3.7 × 10 ]

2.5 [3.8 × 10 ]

7

47

Au-Pd bimetallic NPs/graphene

0.1 [1 × 10-5]

10 [0.01]

3.5

48

Ag/RGO/TiO2 nanocomposite

2.5 [0.05]

250 [6.25]

3.15

This work

In the next step, we also investigated the reduction of CR and MB dyes in the presence of NaBH4 at room temperature by using the Ag/RGO/TiO2 nanocomposite via electron transfer of BH4- ions and the reaction was monitored by recording the time-dependent UV-vis absorption spectra of the mixture using a spectrophotometer. In Figure 16, the absorption peaks at 493 and 663 nm corresponding to CR and MB dyes showed rapid degradation and disappeared after 116 s and immediately, respectively.

Figure 16. UV-vis spectra for reduction of MB (A) and CR (B).

17

A comparison of the reduction of CR and MB dyes between the Ag/RGO/TiO2 nanocomposite and reported catalysts in the literature is shown in Table 4. According to the Table 4, the reduction reaction was carried out by the Ag/RGO/TiO2 nanocomposite in the shortest time. Table 4 Comparison of various catalysts in the reduction of CR and MB dyes with NaBH4. Dye Catalyst Time MB Porous Cu microspheres 8 min SiNWAs-Cu 10 min Au/[email protected] 10 min Ag NPs on silica spheres 7.5 min Aucore-PANIshell 5 min copper nanocrystals 200 s Ag/RGO/TiO2 nanocomposite immediately CR copper nanocrystals 500 s [email protected] 7 min Ag/RGO/TiO2 nanocomposite 116 s

Ref. 49 50 51 52 53 54 This work 54 55 This work

3.3. Recovery and reuse of the Ag/RGO/TiO2 nanocomposite The RGO and TiO2 supports not only enhance the catalytic activity of Ag NPs via a synergistic phenomenon, but also can be easily recovered and recycled several times without any significant loss of catalytic activity. Finally, we studied the reusability of the catalyst in the decolorization of dyes with NaBH4. During the catalytic reaction, the Ag/RGO/TiO2 nanocomposite was removed from the reaction mixture by centrifugation, and the resulting solid mass was washed with water and dried and reused for the next run. The results show that the catalyst efficiency was remained after five reusing of the catalyst. The stability of the reused catalyst was further confirmed by FE-SEM studies. The FE-SEM of recycled catalyst was similar even after the 5th cycle (Figure 17). Also, the amount of the Ag species dissolved into solution caused by leaching of the catalyst was determined by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) analysis of the resulting reaction solution mixture after six times reuse and the results showed that the amount of leached Ag metal from the catalyst is very low.

18

Figure 17. SEM image of recycled Ag/RGO/TiO2 nanocomposite.

4. Conclusions In summary, we have developed an efficient, simple, inexpensive and environmentally benign method for preparation of the Ag/RGO/TiO2 nanocomposite using Euphorbia helioscopia L. leaf extract as a stabilizing and reducing agent. The advantages of this environmentally benign and safe protocol include elimination of toxic, expensive and harmful chemicals, the use of water as solvent, easy-synthesis of the Ag/RGO/TiO2 nanocomposite under environmental-friendly conditions, easy separation of the catalyst and experimental ease. Moreover, the Ag/RGO/TiO2 nanocomposite exhibited high catalytic activity for the reduction of 4-NP, CR and MB in water at room temperature. Furthermore, the catalyst can be recovered and reused for several consecutive runs with no significant loss of catalytic activity.

Acknowledgments We gratefully acknowledge from the Iranian Nano Council and the University of Qom for the support of this work.

References 19

[1]

V. K. Vidhu and D. Philip, Catalytic degradation of organic dyes using biosynthesized silver nanoparticles, Micron 56 (2014) 5462.

[2]

B. Manu, S. Chaudhari, Anaerobic decolorisation of simulated textile wastewater containing azo dyes, Bioresour. Technol. 82 (2002) 225-231.

[3]

R. Patel, S. Suresh, Decolourization of azo dyes using magnesium-palladium system, J. Hazard. Mater. B 137 (2006) 1729-1741.

[4]

L. G. Devi, S. G. Kumar, K. M. Reddy, C. Munikrishnappa, Photo degradation of methyl orange an azo dye by advanced Fenton process using zero valent metallic ion: influence of various reaction parameters and its degradation mechanism, J. Hazard. Mater. 164 (2009) 459-467.

[5]

N. Pradhan, A. Pal, T. Pal, Catalytic Reduction of Aromatic Nitro Compounds by Coinage Metal Nanoparticles, Langmuir, 17 (2001) 1800-1802.

[6]

A.K. Sinha, M. Basu, S. Sarkar, M. Pradhan, T. Pal, Synthesis of gold nanochains via photoactivation technique and their catalytic applications, J. Colloid Interface Sci. 398 (2013) 13-21.

[7]

M. Atarod, M. Nasrollahzadeh, S.M. Sajadi, Euphorbia heterophylla leaf extract mediated green synthesis of Ag/TiO2 nanocomposite and investigation of its excellent catalytic activity for reduction of variety of dyes in water, J. Colloid Interface Sci. 462 (2016) 272-279.

[8]

M. Atarod, M. Nasrollahzadeh, S.M. Sajadi, Green synthesis of a Cu/reduced graphene oxide/Fe3O4 nanocomposite using Euphorbia wallichii leaf extract and its application as a recyclable and heterogeneous catalyst for the reduction of 4-nitrophenol and rhodamine B, RSC Adv. 5 (2015) 91532-91543.

[9]

M. Nasrollahzadeh, Green synthesis and catalytic properties of palladium nanoparticles for the direct reductive amination of aldehydes and hydrogenation of unsaturated ketones, New J. Chem. 38 (2014) 5544-5550.

[10] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666-669. [11] H.L. Guo, X.F. Wang, Q.Y. Qian, F. B. Wang, X. H. Xia, A green approach to the synthesis of graphene nanosheets, ACS Nano 3 (2009) 2653-2659. [12] Williams, B. Seger, P.V. Kamat, TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide, ACS Nano 2 (2008) 1487-1491. [13] X.Y. Yang, X.Y. Zhang, Y.F. Ma, Y. Huang, Y.S. Wang, Y.S. Chen, Superparamagnetic graphene oxide-Fe3O4 nanoparticles hybrid for controlled targeted drug carriers, J. Mater. Chem. 19 (2009) 2710-2714. [14] P. Fakhri, M. Nasrollahzadeh, B. Jaleh, Graphene oxide supported Au nanoparticles as an efficient catalyst for reduction of nitro compounds and Suzuki-Miyaura coupling in water, RSC Adv. 4 (2014) 48691-48697. [15] S.M. Paek, E. Yoo, I. Honma, Enhanced cyclic performance and lithium storage capacity of SnO 2/graphene nanoporous electrodes with three-dimensionally delaminated flexible structure, Nano Lett. 9 (2009) 72-75. [16] P. Fakhri, B. Jaleh, M. Nasrollahzadeh, Synthesis and characterization of copper nanoparticles supported on reduced graphene oxide as a highly active and recyclable catalyst for the synthesis of formamides and primary amines, J. Mol. Catal. A Chem. 383384 (2014) 17-22.

20

[17] M. Nasrollahzadeh, B. Jaleh, A. Jabbari, Synthesis, characterization and catalytic activity of graphene oxide/ZnO nanocomposites, RSC Adv. 4 (2014) 36713-36720. [18] M. Nasrollahzadeh, M. Maham, A. Rostami-Vartooni, M. Bagherzadeh, S.M. Sajadi, Barberry fruit extract assisted in situ green synthesis of Cu nanoparticles supported on a reduced graphene oxide-Fe3O4 nanocomposite as a magnetically separable and reusable catalyst for the O-arylation of phenols with aryl halides under ligand-free conditions, RSC Adv. 5 (2015) 64769-64780. [19] B. Jaleh, A. Jabbari, Evaluation of reduced graphene oxide/ZnO effect on properties of PVDF nanocomposite films, Appl. Surf. Sci. 320 (2014) 339-347. [20] M. Nasrollahzadeh, S.M. Sajadi, Synthesis and characterization of titanium dioxide nanoparticles using Euphorbia heteradena Jaub root extract and evaluation of their stability, Ceram. Int. 41 (2015) 14435-14439. [21] Y.J. Xu, Y. Zhuang, X. Fu, New insight for enhanced photocatalytic activity of TiO2 by doping carbon nanotubes: A case study on degradation of benzene and methyl orange, J. Phys. Chem. C 114 (2010) 2669-2676. [22] B. Jaleh, M.S. Madad, S. Habibi, P. Wanichapichart, M.F. Tabrizi, Evaluation of physico-chemical properties of plasma treated PS-TiO2 nanocomposite film, Surf. Coat. Tech. 206 (2014) 947-950. [23] N. Wetchakun, S. Phanichphant, Effect of temperature on the degree of anatase-rutile transformation in titanium dioxide nanoparticles synthesized by the modified sol-gel method, Curr. Appl. Phys. 8 (2008) 343-346. [24] Y. Shiraishi, N. Toshima, Oxidation of ethylene catalyzed by colloidal dispersions of poly(sodium acrylate)-protected silver nanoclusters, Colloids Surf. A 169 (2000) 59-66. [25] T. Sun, K. Seff, Silver clusters and chemistry in zeolites, Chem. Rev. 94 (1994) 857-870. [26] D. Kim, S. Jeong, J. Moon, Synthesis of silver nanoparticles using the polyol process and the influence of precursor injection, Nanotechnology 17 (2006) 4019-4024. [27] E.M. Egorova, A.A. Revina, Synthesis of metallic nanoparticles in reverse micelles in the presence of quercetin, Colloids Surf. A 168 (2000) 87-96. [28] R. Jin, Y. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zhang, Photoinduced conversion of silver nanospheres to nanoprisms, Science 294 (2001) 1901-1903. [29] E.S. Abdel-Halim, M.H. El-Rafie, S.S. Al-Deyab, Polyacrylamide/guar gum graft copolymer for preparation of silver nanoparticles, Carbohydr. Polym. 85 (2011) 692-697. [30] X. Huang, H. Wu, S. Pu, W. Zhang, X. Liao and B. Shi, One-step room-temperature synthesis of [email protected] core-shell nanoparticles with tunable structure using plant tannin as reductant and stabilizer, Green Chem. 13 (2011) 950-957. [31] S.P. Dubey, M. Lahtinen, M. Sillanpa, Tansy fruit mediated greener synthesis of silver and gold nanoparticles, Process Biochem. 45 (2010) 1065-1071. [32] M. Nasrollahzadeh, S.M. Sajadi, Pd nanoparticles synthesized in situ with the use of Euphorbia granulate leaf extract: Catalytic properties of the resulting particles, J. Colloid Interf. Sci. 462 (2016) 243-251. [33] M. Nasrollahzadeh, S. M. Sajadi, F. Babaei, M. Maham, Euphorbia helioscopia Linn as a green source for synthesis of silver nanoparticles and their optical and catalytic properties, J. Colloid. Interf. Sci. 450 (2015) 374-380.

21

[34] M. Nasrollahzadeh, M. Atarod, S. M. Sajadi, Green synthesis of the Cu/Fe3O4 nanoparticles using Morinda morindoides leaf aqueous extract: A highly efficient magnetically separable catalyst for the reduction of organic dyes in aqueous medium at room temperature, Appl. Sur. Sci. 364 (2016) 636-644. [35] A. Hatamifard, M. Nasrollahzadeh, Green synthesis of a natrolite zeolite/palladium nanocomposite and its application as a reusable catalyst for the reduction of organic dyes in a very short time, J. Lipkowski, RSC Adv. 5 (2015) 91372-91381. [36] M. Atarod, M. Nasrollahzadeh, S.M. Sajadi, Green synthesis of Pd/RGO/Fe3O4 nanocomposite using Withania coagulans leaf extract and its application as magnetically separable and reusable catalyst for the reduction of 4-nitrophenol, J. Colloid Interface Sci. 465 (2016) 249-258. [37] N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V. Buzaneva and A.D. Gorchinskiy, Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations, Chem. Mater. 11 (1999) 771-778. [38] J. Zheng, Z. Liu, X. Liu, X. Yan, D. Li, W. Chu, Facile hydrothermal synthesis and characteristics of B-doped TiO2 hybrid hollow microspheres with higher photo-catalytic activity, J. Alloys Compd. 509 (2011) 3771-3776. [39] A. Goyal, S. Bansal, S. Singhal, Facile reduction of nitrophenols: Comparative catalytic efficiency of MFe 2O4 (M = Ni, Cu, Zn) nano ferrites, Int. J. Hydrogen Energy 39 (2014) 4895-4908. [40] D. Z. Jiang, J. Xie, D. Jiang, X. Wei, M. Chen, Modifiers-assisted formation of nickel nanoparticles and their catalytic application to p-nitrophenol reduction, CrystEngComm 15 (2013) 560-569. [41] Y.-G. Wu, M. Wen, Q.-S. Wu, H. Fang, Ni/graphene nanostructure and its electron-enhanced catalytic action for hydrogenation reaction of nitrophenol, J. Phys. Chem. C 118 (2014) 6307-6313. [42] R. Xu, H. Bi, G. He, J. Zhu, H. Chen, Synthesis of [email protected] composite: A magnetically separable and efficient catalyst for the reduction of 4-nitrophenol, Mat. Res. Bull. 57 (2014) 190-196. [43] Z. Wang, C. Xu, G. Gao, X. Li, Facile synthesis of well-dispersed Pd-graphene nanohybrids and their catalytic properties in 4nitrophenol reduction, RSC Adv. 4 (2014) 13644-13651. [44] B. J. Borah, P. Barali, Surfactant-free synthesis of CuNi nanocrystals and their application for catalytic reduction of 4-nitrophenol, J. Mol. Catal. A: Chem. 390 (2014) 29-36. [45] K.-L. Wu, X.-W. Wei, X.-M. Zhou, D.-H. Wu, X.-W. Liu, Y. Ye, Q. Wang, NiCo2 alloys: Controllable synthesis, magnetic properties, and catalytic applications in reduction of 4-nitrophenol, J. Phys. Chem. C 115 (2011) 16268-16274. [46] K.-L. Wu, R. Yu, X.-W. Wei, Monodispersed FeNi2 alloy nanostructures: solvothermal synthesis, magnetic properties and sizedependent catalytic activity, CrystEngComm 14 (2012) 7626-7632. [47] S. Li, S. Guo, H. Yang, G. Gou, R. Ren, J. Li, Z. Dong, J. Jin, J. Ma, Enhancing catalytic performance of Au catalysts by noncovalent functionalized graphene using functional ionic liquids, J. Hazard. Mat. 270 (2014) 11-17. [48] X. Chen, Z. Cai, X. Chen, M. Oyamac, AuPd bimetallic nanoparticles decorated on graphene nanosheets: their green synthesis, growth mechanism and high catalytic ability in 4-nitrophenol reduction, J. Mater. Chem. A 2 (2014) 5668-5674. [49] Y. Zhang, P. Zhu, L. Chen, G. Li, F. Zhou, D. (Daniel) Lu, R. Sun, F. Zhou and C.-p. Wong, Hierarchical architectures of monodisperse porous Cu microspheres: synthesis, growth mechanism, high-efficiency and recyclable catalytic performance, J. Mater. Chem. A 2 (2014) 11966-11973.

22

[50] X. Yang, H. Zhong, Y. Zhu, H. Jiang, J. Shen, J. Huang and C. Li, Highly efficient reusable catalyst based on silicon nanowire arrays decorated with copper nanoparticles, J. Mater. Chem. A 2 (2014) 9040-9047. [51] Z. Gan, A. Zhao, M. Zhang, W. Tao, H. Guo, Q. Gao, R. Mao and E. Liu, Controlled synthesis of Au-loaded [email protected] composite microspheres with superior SERS detection and catalytic degradation abilities for organic dyes, Dalton Trans. 42 (2013) 85978605. [52] Z.J. Jiang, C.Y. Liu and L.W. Sun, Catalytic properties of silver nanoparticles supported on silica spheres, J. Phys. Chem. B 109 (2005) 1730-1735. [53] S. Dutt, P.F. Siril, V. Sharma and S. Periasamy, Goldcore-polyanilineshell composite nanowires as a substrate for surface enhanced Raman scattering and catalyst for dye reduction, New J. Chem. 39 (2015) 902-908. [54] P. Zhang, Y. Sui, C. Wang, Y. Wang, G. Cui, C. Wang, B. Liu and B. Zou, A one-step green route to synthesize copper nanocrystals and their applications in catalysis and surface enhanced Raman scattering, Nanoscale 6 (2014) 5343-5350. [55] B.K. Ghosh, S. Hazra, B. Naik, N.N. Ghosh, Preparation of Cu nanoparticle loaded SBA-15 and their excellent catalytic activity in reduction of variety of dyes, Powder Technol. 269 (2015) 371-378.

23

Research highlights: 

Green synthesis of Ag/RGO/TiO2 nanocomposite.



Characterization of Ag/RGO/TiO2 nanocomposite by FT-IR, UV-vis, TEM, XRD, SEM, EDS and ICP techniques.



Catalytic reduction of 4-nitrophenol, Congo red and Methylene blue.

24

Graphical Abstract In situ green synthesis of Ag nanoparticles on graphene oxide/TiO2 nanocomposite and their catalytic activity for the reduction of 4-nitrophenol, Congo red and Methylene blue Mahmoud Nasrollahzadeh,* Monireh Atarod, Babak Jaleh and Mastane Gandomi

*

Corresponding author. Tel.: +98 25 32850953; Fax: +98 25 32103595.

E-mail address: [email protected] (M. Nasrollahzadeh). 25