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TiO2 nanocomposite and investigation of its excellent catalytic activity for reduction of variety of dyes in water
Journal of Colloid and Interface Science 462 (2016) 272–279
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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 Monireh Atarod a, Mahmoud Nasrollahzadeh a,⇑, S. Mohammad Sajadi b a b
Department of Chemistry, Faculty of Science, University of Qom, Qom 37185-359, Iran Department of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government, Iraq
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 10 August 2015 Accepted 30 September 2015 Available online 9 October 2015 Keywords: Euphorbia heterophylla Ag/TiO2 nanocomposite NaBH4 Organic dyes Water
a b s t r a c t This work reports a facile and green synthesis of Ag/TiO2 nanocomposite by extract of leaves of Euphorbia heterophylla without any stabilizer or surfactant. The green synthesized Ag/TiO2 nanocomposite was characterized by field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction analysis (XRD) and UV–vis. The Ag/TiO2 nanocomposite was found to be effective catalyst for reduction of various dyes, such as 4-nitrophenol (4-NP), Methyl orange (MO), Congo red (CR) and Methylene blue (MB) in the presence of NaBH4 in water at room temperature. Catalysis reactions were monitored by employing UV–vis spectroscopy. Catalysis reactions followed pseudo-first order rate equation. The catalyst can be recovered and reused several times without significant loss of its catalytic activity. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction The presence of toxic dyes and nitroarene compounds in waste waters is of great concern as these are biologically and chemically ⇑ Corresponding author. E-mail address: [email protected] (M. Nasrollahzadeh). http://dx.doi.org/10.1016/j.jcis.2015.09.073 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
stable; therefore, it is difficult to remove them by natural degradation processes [1,2]. Among them, 4-nitrophenol (4-NP) is a well known toxic and bio-refractory compound which can damage the central nervous system, liver, kidney and blood of humans and animals and may cause various diseases [2]. Degradation of 4-NP to nondangerous product is difficult because of its high stability and low solubility
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in water [3]. Its reduction product, 4-aminophenol (4-AP), is very useful in a wealth of applications that include preparation of analgesic and antipyretic drugs, as a photographic developer, as a corrosion inhibitor, and so on [4]. Therefore, the reduction of 4-NP to 4-AP acquires great importance both environmentally and industrially. Usually reduction of 4-NP to 4-AP is achieved in the presence of metal nanoparticles (MNPs), which can catalyze the reaction by facilitating electron relay from the donor BH4 to the acceptor 4-NP to overcome the kinetic barrier. However, the agglomeration of MNPs is inevitable. An ideal support is needed to decrease nanoparticles agglomeration. To prevent the agglomeration of MNPs and in order to overcome the problems concerning stability, separation, and recovery of MNPs, several inorganic materials such as zeolite, graphene oxide, TiO2 and Fe3O4 have been used as a support for MNPs [5–9]. Among heterogeneous catalysts, TiO2 has been used widely as an efficient support and catalyst in organic reactions due to good chemical and thermal stability, low cost, low toxicity and excellent optical properties, ease of handling, high photocatalytic activity reusability, and benign character [9,10]. There are several problems arising when MNPs are synthesized by chemical methods which include usage of toxic and hazardous reducing agents and organic solvents, which are highly reactive and present potential environmental and biological risks [11,12]. The enormous consumption of energy required to maintain the high temperature and pressure conditions is a great problem associated with physical synthetic methods [13]. Therefore, environmentally benign production methods of MNPs by using various plants extracts, bacteria and fungus are very desirable [14–20]. The biosynthetic techniques for the preparation of MNPs has several advantages over chemical and physical synthetic methods, such as simplicity, cost effectiveness as well as compatibility for biomedical and pharmaceutical applications. Moreover, the use of plant extracts as the biogenic agents potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. To date, there is no report on the biosynthesis of MNPs by utilizing the aqueous extract of Euphorbia heterophylla leaves. Euphorbia plants are widespread in nature ranging from herbs and shrubs to trees in tropical and temperate regions all over the world. The family Euphorbiaceae generally has characteristic milky latex, sticky sap; some are co-carcinogenic, severe skin irritation and toxic to livestock and humans [21]. Euphorbia heterophylla leaf (Fig. 1) is used in traditional medical practices as laxative, antigonorrheal, migraine and wart cures. The plant lattices have been used
Fig. 1. Image of the Euphorbia heterophylla.
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as fish poison, insecticide and ordeal poisons [22,23]. The plant has been reported as a source of phytochemicals such as polyphenolics especially quercetin and diterpenoids with accumulation of antioxidants in the leaves of the genus [24]. Based on our continuous investigations on metal nanoparticle catalyzed reactions [25–31], we wish to report herein an ecofriendly, clean and non-toxic method for the green synthesis of Ag/TiO2 nanocomposite using E. heterophylla leaf extract without any stabilizer or surfactant. In addition, the catalytic activity of Ag/TiO2 nanocomposite for reduction of 4-NP, MO, CR, MB in the presence of NaBH4 in water was studied. More significantly, only a slight loss in the catalyst activity of Ag/TiO2 nanocomposite is seen after several recycles. However, in spite of its ready natural availability, non-toxicity and biological relevance’s, E. heterophylla has never been explored for the green synthesis of Ag/TiO2 nanocomposite. 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. 1H NMR spectra were recorded on a Bruker Avance DRX-400 spectrometer at 400. FT-IR spectra were recorded on a Nicolet 370 FT/IR spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. UV–vis spectral analysis was recorded on a doublebeam spectrophotometer (Hitachi, U-2900) to ensure the formation of nanoparticles. X-ray diffraction (XRD) measurements were carried out using a Philips powder diffractometer type PW 1373 goniometer (Cu Ka = 1.5406 Å). The scanning rate was 2°/min in the 2h range from 10° to 80°. Scanning electron microscopy (SEM) was performed on a Cam scan MV2300. EDS (S3700N) was utilized for chemical analysis of prepared nanostructures. 2.2. Preparation of Euphorbia heterophylla leaf extract 100 G of dried leaves of E. heterophylla was powdered and refluxed at 70 °C with 500 mL of sterile distilled water for 2 h and the mixture was allowed to cool to room temperature. Then, the aqueous extract of E. heterophylla leaves was centrifuged at 6500 rpm and supernatant separated by filtration. 2.3. Preparation of Ag/TiO2 nanocomposite In a typical synthesis of Ag/TiO2 nanocomposite, for further monitoring, first Ag nanoparticles were separately synthesized using the plant extract as following: In a typical synthesis of Ag NPs, 50 mL of E. heterophylla leaf extract was added dropwise to 50 mL of 0.003 M aqueous solution of AgNO3 (99.99%) with constant stirring at 80 °C and the color of the solution was changed from whitish to dark brown during the heating process due to excitation of surface plasmon resonance which indicates the formation of Ag nanoparticles. Reduction of Ag+ to Ag(0) NPs was completed in 20 min (as monitored by UV–vis spectra of the solution). The nanoparticles solution was diluted 10 times with distilled water to avoid the errors due to high optical density of the solution. In a typical synthesis of Ag/TiO2 nanocomposite, 20 mL of the plant extract was added dropwise to 40 mL of a well-mixed solution of AgNO3 (0.003 M) and 1.0 g of Degussa P-25 TiO2 with constant stirring at 60 °C for 2 h. After 20 min the color of the solution changed from yellow to black due to the excitation of surface plasmon resonance which indicates the formation of nanocomposite. Reduction of bimetallic mixture was completed around 45 min
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(as monitored by UV–vis and FT-IR spectra of the solution). Then the colored solution of bimetallic nanoparticles was centrifuged at 7000 rpm for 30 min to completely precipitation of Ag/TiO2 nanocomposite. The obtained precipitation was then washed three times with ethanol, then air dried for 24 h at room temperature. 2.4. Reduction of 4-nitrophenol by Ag/TiO2 nanocomposite Typically, 25 mL of 4-nitrophenol aqueous solution (2.5 mM) was mixed with 7.0 mg of Ag/TiO2 nanocomposite in a beaker, stirring constantly for 2 min. Next, 25 mL of freshly prepared aqueous NaBH4 (0.25 M) was added and the mixture was allowed to stir at room temperature until the deep yellow solution became colorless. The reaction progress was monitored by UV–vis spectroscopy. The concentration of 4-nitrophenol was determined spectrophotometrically at a wavelength of 400 nm. After completion of the reaction, the catalyst was simply separated by brief centrifugation and washed successively with ethanol and water, dried and was used for successive cycles. 2.5. Reduction of methylene blue (MB) by Ag/TiO2 nanocomposite Typically, 5.0 mg of catalyst was added to 25 mL of MB aqueous solution (3.1 10 5). Next, 25 mL of freshly prepared aqueous NaBH4 (5.3 10 3) was added and the mixture was allowed to stir at room temperature. The progress of the conversion reaction was then monitored by recording the time-dependent UV–vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was simply separated from the reaction system by brief centrifugation and washed successively with ethanol and dried for the next cycle. 2.6. Reduction of methyl orange (MO) by Ag/TiO2 nanocomposite Typically, 10.0 mg of catalyst was added to 25 mL of MO aqueous solution (3.0 10 5). Next, 25 mL of freshly prepared aqueous NaBH4 (5.3 10 3) was added and the mixture was allowed to stir at room temperature. The progress of the conversion reaction was then monitored by recording the time-dependent UV–vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was simply separated from the reaction system by brief centrifugation and washed successively with ethanol and dried for the next cycle. 2.7. Reduction of Congo red (CR) by Ag/TiO2 nanocomposite Typically, 5.0 mg of catalyst was added to 25 mL of CR aqueous solution (1.44 10 5). Next, 25 mL of freshly prepared aqueous NaBH4 (5.3 10 3) was added and the mixture was allowed to stir at room temperature. The progress of the conversion reaction was then monitored by recording the time-dependent UV–vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was simply separated from the reaction system by brief centrifugation and washed successively with ethanol and dried for the next cycle.
extracts may act both as reducing agent and stabilizing agents in the synthesis of NPs. 3.1. Characterization of Euphorbia heterophylla leaf extract The UV spectrum of leaf extract of the E. heterophylla (Fig. 2) shows bonds at kmax 360 nm (bond I) due to the transition localized within the ring of cinnamoyl system; whereas the one around 250 nm (bond II) is for absorbance of ring related to the benzoyl system. They are related to the p ? p⁄ transitions and these absorbent bonds demonstrate the presence of polyphenolics as antioxidant source for green synthesis of nanoparticles. The FT-IR analysis was carried out to identify the possible biomolecules responsible for the reduction of Ag NPs and capping them. The FT-IR spectrum of the crude extract, (Fig. 3) depicted some peaks at 3500–3400, 1680, 1460, 1250–1050 cm 1 which represent free OH in molecule and OH group forming hydrogen bonds, carbonyl group (C@O), stretching C@C aromatic ring and CAOH stretching vibrations, respectively. Because of presence the mentioned functional groups inside the structure of polyphenolics, the spectrum can demonstrate the presence of phenolics in the plant leaf extract. The presence of phenolics in the extract could be probably responsible for the reduction of metal ions and formation of nanocomposite. 3.2. Characterization of Ag/TiO2 nanocomposite The stable Ag/TiO2 nanocomposite obtained was fully characterized by UV–vis, XRD, SEM, EDS and FT-IR. The formation of Ag NPs was controlled by UV–vis spectroscopy. Fig. 4 shows the UV–vis spectrum of Ag NPs formation. The reaction was completed after 20 min. Our results showed that the maximum absorbance of green synthesized Ag NPs was at 440 nm due to the surface plasmon absorption of nanosized silver particles. The whitish color of the AgI solution immediately changed into dark brown indicating reduction of Ag+ to Ag0 and formation of Ag NPs as characterized by UV–vis spectrum. The synthesized Ag NPs by the this method are quite stable and no obvious variance in the shape, position and symmetry of the absorption peak is observed even after two month indicating the stability of Ag NPs. Fig. 5 shows the absorption spectrum of Ag/TiO2 nanocomposite due to surface plasmon resonance (SPR) of metallic nanoparticles. At compare with spectra related to Ag NPs, changing the color of the reaction (whitish to dark) and appearance the maxima ranging 250–350 nm indicates the reduction process and formation of nanoparticles. As monitored by UV–vis the synthesized nanoparticles by this method are quite stable with no significant variance in
3. Results and discussion In the present investigation, Ag/TiO2 nanocomposite was synthesized using E. heterophylla leaf extracted solution as a solvent instead of organic solvents. Rapid biosynthesis of Ag/TiO2 nanocomposite using E. heterophylla leaf extract has been investigated which is an easiest, cost efficient, non-toxic, eco-friendly and efficient method for exploiting E. heterophylla leaf. Plant
Fig. 2. UV–vis spectrum of the aqueous extract of the leaves of the Euphorbia heterophylla.
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Fig. 3. FT-IR spectrum of the aqueous extract of the leaves of Euphorbia heterophylla.
Fig. 4. UV–vis spectrum of green synthesized Ag NPs using the Euphorbia heterophylla leaf extract.
Fig. 5. UV–vis spectrum of green synthesized Ag/TiO2 nanocomposite using the aqueous extract of the leaves of Euphorbia heterophylla.
the shape, position and symmetry of the absorption peak even after one month which indicates the stability of product. Furthermore, FT-IR spectrum of Ag/TiO2 nanocomposite is shown in Fig. 6. The appeared bands are lattice vibration modes indicating the functional groups of samples. The broad band around 1000 cm 1 is TiAOATi stretching bond. Also, band at 3480 cm 1 represents the primary AOH stretching of hydroxyl functional group. The band around 1430 cm 1 is generally attribu-
ted to the bending vibration of HAOH groups for TiO2 but after doping of silver on TiO2, the intensity of basic TiO2 peaks hardly changed. Phase investigation of the crystallized product was performed by powder XRD measurements and the powder diffraction pattern of Ag/TiO2 nanocomposite is presented in Fig. 7. The structure of the Ag/TiO2 nanocomposite is analogous to that of Degussa P25 which can be indexed to TiO2 in anatase and rutile phases. The
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Fig. 6. FT-IR spectrum of the green synthesized Ag/TiO2 nanocomposite by Euphorbia heterophylla.
Fig. 7. XRD powder pattern of Ag/TiO2 nanocomposite.
diffraction peaks at 38°, 44.48° are consistent with (1 1 1), (2 0 0) reflections of the metallic silver particles [32]. The X-ray diffraction pattern exhibited face centered cubic (fcc) structure for the metallic silver particles. The pattern was very clean, with no indication of impurities such as silver oxide (Ag2O).
TiK
6000
The elemental composition of the Ag/TiO2 nanocomposite was determined by EDS analysis (Fig. 8). Presence of Ag, Ti and oxygen was confirmed by EDS spectroscopy. FESEM images of green synthesized Ag/TiO2 nanocomposite are shown in Fig. 9, which shows the silver and TiO2 nanoparticles show same diameter (less than 24 nm) and morphology. From FESEM images, it is clear that silver particles are not incorporated in TiO2 lattice, but deposited on the surface of TiO2. 3.3. Reduction of Ag+ to Ag0
5000
4000
TiL OK
3000
AgL AgL
2000 AuM AuM
1000
TiK
AuLl
0 0
5
Fig. 8. EDS spectrum of Ag/TiO2 nanocomposite.
AuL
10
keV
In this work, we have employed E. heterophylla leaf extract as a reducing and stabilizing agent for the synthesis of Ag/TiO2 nanocomposite. The methodology which we have adopted was totally hazard-free, low cost and environment friendly. We speculate that flavonoid and phenolic acids are present in the extract, which may be responsible for synthesis of Ag/TiO2 nanocomposite and its stabilization. It could be due to the presence of hydrogen binding and electrostatic interaction between the bioorganic capping and molecules bound to the Ag NPs. Weaker binding of these biomolecules with nascent Ag nanocrystals could lead to isotropic growth of the crystal and thus formation of spherical nanoparticles. On the basis of above results, a possible mechanism was proposed to the reduction of Ag+ and formation of Ag NPs by the
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Fig. 9. FESEM images of Ag/TiO2 nanocomposite.
E. heterophylla leaf extract. On the basis of this mechanism (Scheme 1), first reaction nucleation, represent the reduction of Ag+ ions into nano zero valent (NZV) metallic particles using flavonoid (FlOH) contents of the plant as green reducing agents (one step one electron oxidation-reduction mechanism to Ag0 and radical, nucleation; rate determining step). After the slow electron transfer step, reaction growth to adsorption may follow. The complexation of the formation of Ag0 atoms with Ag+ ions yield Ag+2 ions and Agn+ dimerize to give yellow sol, Ag2n2+ [33,34]. The adsorption step was further supported by the presence of a faint layer of organic molecules onto the surface of the Ag nanoparticles.
NO2
NH2
OH
OH Ag/TiO2 nanocomposite NaBH4 H2O, r.t.
3.4. Catalytic ability and application of the synthesized Ag/TiO2 nanocomposite for reduction of 4-NP In the present method, the catalytic reduction of 4-NP to 4-4-AP in the presence of NaBH4 in water at room temperature was chosen as a model reaction to evaluate the catalytic activity of Ag/TiO2 nanocomposite (Scheme 2). 4-NP in aqueous medium has a maximum absorption at 317 nm. However, the color of the solution changed to 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 (Scheme 3) in alkaline conditions and it did not change even for 24 h in the absence of catalyst. As shown in Fig. 10, when Ag/TiO2 nanocomposite was added into the solution containing 4-NP and NaBH4, the intensity of the strong absorption peak at 400 nm gradually decreased and a new peak appeared at about 300 nm which corresponded to the formation of 4-AP. After about 120 s, the peak at 400 nm almost disappeared and the color became transparent, which indicated that 4-NP was almost turned to 4-AP.
Scheme 2. The reduction of 4-NP on the surface of Ag/TiO2 nanocomposite.
BH4-
OH
NH2
NO2
NO2
Ag/TiO2
O-
OH
Scheme 3. The reduction reaction for the conversion of 4-NP to 4-AP.
Fig. 10. UV–vis spectrum of reduction of 4-NP.
Scheme 1. Reducing ability of antioxidant phenolics to produce Ag NPs where FIOH and NZV are flavonoid and nano zero valent, respectively.
The completion times for the catalytic reduction of 4-NP to 4-AP reaction in the presence of 50, 75 and 100 equivalents of NaBH4 and varying amounts of Ag/TiO2 nanocomposite are given in
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Table 1. According to data given in Table 1, reaction was not completed in the presence of 50 equivalents of NaBH4 and 5.0 mg of catalyst. In the presence of 75 and 100 equivalents of NaBH4 and 5.0 mg of catalyst, the reaction was completed in 180 s and 150 s, respectively. It was observed that the catalytic efficacy increased with an increase in the amount of NaBH4. Furthermore, in the presence of 100 equivalents of NaBH4 and 7.0 mg of catalyst, complete conversion was achieved within 120 s. The effect of the concentration of catalyst was also studied by carrying out the reduction reactions in the presence of varying amounts of catalyst. The best result was obtained with 100 equivalents of NaBH4 and 7.0 mg of catalyst at room temperature (Table 1). As expected, no target product could be detected in the absence of Ag/TiO2 nanocomposite. The catalytic reduction of 4-NP to 4-AP is an electron transfer process. To our knowledge, Ag NPs supported on the TiO2 can function as the electron mediator between 4-AP (oxidant) and BH4 (reductant), and the electron transfer occurs via the Ag NPs. In the present work, when the catalyst was added to a mixed solution of 4-NP and NaBH4, the reactant molecules are firstly adsorbed on the surface of catalyst via p–p stacking interactions. Such adsorption provides a high concentration of 4-NP near to the Ag NPs on TiO2, leading to highly efficient contact between them. In contrast, without a highly adsorbent TiO2 support, 4-NP must collide with Ag NPs by chance, and remains in contact for the catalysis to proceed. Then, the electron transfer (ET) occurs from the electron donor BH4 to the 4-nitrophenolate ions. The 4-nitrophenolate ions will be reduced to 4-AP at the surface of the metal catalysts and finally the generated product was desorbed from the surface of the catalyst. The absorbance of the reaction solution at 400 nm was measured as a function of time for kinetics investigation. Because NaBH4 is in great excess compared to 4-nitrophenol, its concentration can be regarded as being constant throughout the reaction. Therefore, the reduction of 4-NP to 4-AP can be treated as a pseudo-first-order reaction [35].
Table 2 Completion time for the reduction of MB (MO or CR) using different amounts of Ag/TiO2 nanocomposite.
a
Entry
Dye (M)
1 2 3 4 5 6 7 8 9
CR (1.44 10 CR (1.44 10 CR (1.44 10 MO (3.0 10 MO (3.0 10 MO (3.0 10 MB (3.1 10 MB (3.1 10 MB (3.1 10
NaBH4 (M) 5
) 5 ) 5 ) 5 ) 5 ) 5 ) 5 ) 5 ) 5 )
5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10
3 3 3 3 3 3 3 3 3
Catalyst (mg)
Time
3.0 5.0 10.0 3.0 7.0 10.0 1.0 3.0 5.0
15 mina 30 s 30 s 20 mina 20 mina 9 min 40 mina 165 s 70 s
Not completed.
3.5. Catalytic activity of the synthesized Ag/TiO2 nanocomposite for reduction of MO, CR and MB Dyes and other dyestuffs are the major effluents from the textile industry that cause significant pollution [36]. Most of these dyes are not biodegradable and persist in the environment [37]. Therefore, it is highly desirable to develop an eco-friendly method for reduction these compounds in an aqueous medium. Here, three reactions, the reduction of MO, CR and MB by NaBH4, were chosen as model reactions to evaluate the performance of the as-prepared Ag/TiO2 nanocomposite. The following processes are demonstrated to be involved in this reaction: initially, NaBH4 was adsorbed onto the surface of catalyst to form metal hydride, and then MB (MO or CR) adsorbed onto the surface of catalyst; finally, MB (MO or CR) is reduced and desorbed to create a free space for the reaction to continue. The catalytic reduction of MB (MO or CR) was studied with the different amounts of catalyst, and the results are shown in Table 2.
Table 1 Completion time for the reduction of 4-NP using different amounts of Ag/TiO2 nanocomposite with 50, 75 and 100 equivalents of NaBH4.
a
Entry
Catalyst (mg)
NaBH4 (equivalents)
Time
1 2 3 4 5
5.0 5.0 5.0 7.0 10.0
50 75 100 100 100
40 mina 180 s 150 s 120 s 120 s
Not completed.
Fig. 11. The evolution of the UV–vis spectra of dyes aqueous solution in the presence of Ag/TiO2 nanocomposite, (A) MO; (B) MB and (C) CR.
As shown in Table 2, in the presence of 3.0 mg of catalyst, nearly 92% conversion was achieved in 15 min for reduction of CR (entry 1). The best result was obtained with 5.0, 10.0 and
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5.0 mg of catalyst for reduction of CR, MO and MB, respectively in the presence of NaBH4 (5.3 10 3) at room temperature. The reduction of MO, CR and MB by NaBH4 was monitored spectrophotometrically by following the decrease of absorbance at kmax (kmax (MO) = 465 nm, kmax (CR) = 493 nm and kmax (MB) = 663 nm) with time (Fig. 11). As expected, the catalytic reduction of MO, MB and CR by NaBH4 proceeded successfully. In this catalytic reaction system, since the amount of reductant NaBH4 is excessive compared to the degraded dyes (MO, MB and CR), the reaction rate is roughly independent of the NaBH4 concentration and similar to the reduction of 4-NP, MB (MO or CR) reduction also follows pseudo first-order kinetics with respect to MB (MO or CR) concentration. 3.6. Catalyst recyclability The reusability of the catalysts is one of the advantages of heterogeneous catalysts and makes them useful for industrial applications. The Ag/TiO2 nanocomposite can be easily separated from the reaction mixture by mild centrifugation and washed with distilled water several times for the successive reactions. The recovered catalyst was recycled five times for 100% reduction of 4-nitrophenol. This reusability demonstrates the high stability and turnover of catalyst under operating conditions. Little decrease of efficiency was observed after the 5th cycle. In addition, the catalytic efficiency of the catalyst remained almost constant up to five cycles of operation and the time required for 100% reduction of MO, CR and MB was found to be almost same up to the 5th cycle. 4. Conclusions In conclusion, we show an environmentally friendly method to prepare stable Ag/TiO2 nanocomposite employing E. heterophylla leaf extract without usage of any special capping agents. The flavonoids present in extract of leaves of E. heterophylla act as both reducing and capping/stabilizing agents. The synthesized Ag/TiO2 nanocomposite capped by biomolecules was characterized by SEM, XRD, EDS, FT-IR and UV–vis spectroscopic techniques. The advantages of this biosynthesis method include (i) use of plant extract as an economic and effective alternative, (ii) use of cheap, clean, nontoxic and environmentally benign precursors and (iii) simple procedures without application of toxic reagents or surfactant template. In addition, the catalytic activity of Ag/TiO2 nanocomposite for reduction of variety of dyes in water was also studied. In addition, this methodology offers the competitiveness of recyclability of the catalyst without significant loss of catalytic activity, and the catalyst could be easily recovered and reused several times, thus making this procedure environmentally more acceptable. The synthesized Ag/TiO2 nanocomposite by this
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method is quite stable and can be kept under inert atmosphere for several months. The method is general and may be extended to other MNPs. Acknowledgments We gratefully acknowledge the Iranian Nano Council and the University of Qom for the support of this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
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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 Monireh Atarod a, Mahmoud Nasrollahzadeh a,⇑, S. Mohammad Sajadi b a b
Department of Chemistry, Faculty of Science, University of Qom, Qom 37185-359, Iran Department of Petroleum Geoscience, Faculty of Science, Soran University, PO Box 624, Soran, Kurdistan Regional Government, Iraq
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 10 August 2015 Accepted 30 September 2015 Available online 9 October 2015 Keywords: Euphorbia heterophylla Ag/TiO2 nanocomposite NaBH4 Organic dyes Water
a b s t r a c t This work reports a facile and green synthesis of Ag/TiO2 nanocomposite by extract of leaves of Euphorbia heterophylla without any stabilizer or surfactant. The green synthesized Ag/TiO2 nanocomposite was characterized by field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction analysis (XRD) and UV–vis. The Ag/TiO2 nanocomposite was found to be effective catalyst for reduction of various dyes, such as 4-nitrophenol (4-NP), Methyl orange (MO), Congo red (CR) and Methylene blue (MB) in the presence of NaBH4 in water at room temperature. Catalysis reactions were monitored by employing UV–vis spectroscopy. Catalysis reactions followed pseudo-first order rate equation. The catalyst can be recovered and reused several times without significant loss of its catalytic activity. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction The presence of toxic dyes and nitroarene compounds in waste waters is of great concern as these are biologically and chemically ⇑ Corresponding author. E-mail address: [email protected] (M. Nasrollahzadeh). http://dx.doi.org/10.1016/j.jcis.2015.09.073 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
stable; therefore, it is difficult to remove them by natural degradation processes [1,2]. Among them, 4-nitrophenol (4-NP) is a well known toxic and bio-refractory compound which can damage the central nervous system, liver, kidney and blood of humans and animals and may cause various diseases [2]. Degradation of 4-NP to nondangerous product is difficult because of its high stability and low solubility
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in water [3]. Its reduction product, 4-aminophenol (4-AP), is very useful in a wealth of applications that include preparation of analgesic and antipyretic drugs, as a photographic developer, as a corrosion inhibitor, and so on [4]. Therefore, the reduction of 4-NP to 4-AP acquires great importance both environmentally and industrially. Usually reduction of 4-NP to 4-AP is achieved in the presence of metal nanoparticles (MNPs), which can catalyze the reaction by facilitating electron relay from the donor BH4 to the acceptor 4-NP to overcome the kinetic barrier. However, the agglomeration of MNPs is inevitable. An ideal support is needed to decrease nanoparticles agglomeration. To prevent the agglomeration of MNPs and in order to overcome the problems concerning stability, separation, and recovery of MNPs, several inorganic materials such as zeolite, graphene oxide, TiO2 and Fe3O4 have been used as a support for MNPs [5–9]. Among heterogeneous catalysts, TiO2 has been used widely as an efficient support and catalyst in organic reactions due to good chemical and thermal stability, low cost, low toxicity and excellent optical properties, ease of handling, high photocatalytic activity reusability, and benign character [9,10]. There are several problems arising when MNPs are synthesized by chemical methods which include usage of toxic and hazardous reducing agents and organic solvents, which are highly reactive and present potential environmental and biological risks [11,12]. The enormous consumption of energy required to maintain the high temperature and pressure conditions is a great problem associated with physical synthetic methods [13]. Therefore, environmentally benign production methods of MNPs by using various plants extracts, bacteria and fungus are very desirable [14–20]. The biosynthetic techniques for the preparation of MNPs has several advantages over chemical and physical synthetic methods, such as simplicity, cost effectiveness as well as compatibility for biomedical and pharmaceutical applications. Moreover, the use of plant extracts as the biogenic agents potentially eliminates the elaborate process of cell culture and cell maintenance necessary for the biogenic synthesis of metal nanoparticles using unicellular organisms. To date, there is no report on the biosynthesis of MNPs by utilizing the aqueous extract of Euphorbia heterophylla leaves. Euphorbia plants are widespread in nature ranging from herbs and shrubs to trees in tropical and temperate regions all over the world. The family Euphorbiaceae generally has characteristic milky latex, sticky sap; some are co-carcinogenic, severe skin irritation and toxic to livestock and humans [21]. Euphorbia heterophylla leaf (Fig. 1) is used in traditional medical practices as laxative, antigonorrheal, migraine and wart cures. The plant lattices have been used
Fig. 1. Image of the Euphorbia heterophylla.
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as fish poison, insecticide and ordeal poisons [22,23]. The plant has been reported as a source of phytochemicals such as polyphenolics especially quercetin and diterpenoids with accumulation of antioxidants in the leaves of the genus [24]. Based on our continuous investigations on metal nanoparticle catalyzed reactions [25–31], we wish to report herein an ecofriendly, clean and non-toxic method for the green synthesis of Ag/TiO2 nanocomposite using E. heterophylla leaf extract without any stabilizer or surfactant. In addition, the catalytic activity of Ag/TiO2 nanocomposite for reduction of 4-NP, MO, CR, MB in the presence of NaBH4 in water was studied. More significantly, only a slight loss in the catalyst activity of Ag/TiO2 nanocomposite is seen after several recycles. However, in spite of its ready natural availability, non-toxicity and biological relevance’s, E. heterophylla has never been explored for the green synthesis of Ag/TiO2 nanocomposite. 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. 1H NMR spectra were recorded on a Bruker Avance DRX-400 spectrometer at 400. FT-IR spectra were recorded on a Nicolet 370 FT/IR spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. UV–vis spectral analysis was recorded on a doublebeam spectrophotometer (Hitachi, U-2900) to ensure the formation of nanoparticles. X-ray diffraction (XRD) measurements were carried out using a Philips powder diffractometer type PW 1373 goniometer (Cu Ka = 1.5406 Å). The scanning rate was 2°/min in the 2h range from 10° to 80°. Scanning electron microscopy (SEM) was performed on a Cam scan MV2300. EDS (S3700N) was utilized for chemical analysis of prepared nanostructures. 2.2. Preparation of Euphorbia heterophylla leaf extract 100 G of dried leaves of E. heterophylla was powdered and refluxed at 70 °C with 500 mL of sterile distilled water for 2 h and the mixture was allowed to cool to room temperature. Then, the aqueous extract of E. heterophylla leaves was centrifuged at 6500 rpm and supernatant separated by filtration. 2.3. Preparation of Ag/TiO2 nanocomposite In a typical synthesis of Ag/TiO2 nanocomposite, for further monitoring, first Ag nanoparticles were separately synthesized using the plant extract as following: In a typical synthesis of Ag NPs, 50 mL of E. heterophylla leaf extract was added dropwise to 50 mL of 0.003 M aqueous solution of AgNO3 (99.99%) with constant stirring at 80 °C and the color of the solution was changed from whitish to dark brown during the heating process due to excitation of surface plasmon resonance which indicates the formation of Ag nanoparticles. Reduction of Ag+ to Ag(0) NPs was completed in 20 min (as monitored by UV–vis spectra of the solution). The nanoparticles solution was diluted 10 times with distilled water to avoid the errors due to high optical density of the solution. In a typical synthesis of Ag/TiO2 nanocomposite, 20 mL of the plant extract was added dropwise to 40 mL of a well-mixed solution of AgNO3 (0.003 M) and 1.0 g of Degussa P-25 TiO2 with constant stirring at 60 °C for 2 h. After 20 min the color of the solution changed from yellow to black due to the excitation of surface plasmon resonance which indicates the formation of nanocomposite. Reduction of bimetallic mixture was completed around 45 min
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(as monitored by UV–vis and FT-IR spectra of the solution). Then the colored solution of bimetallic nanoparticles was centrifuged at 7000 rpm for 30 min to completely precipitation of Ag/TiO2 nanocomposite. The obtained precipitation was then washed three times with ethanol, then air dried for 24 h at room temperature. 2.4. Reduction of 4-nitrophenol by Ag/TiO2 nanocomposite Typically, 25 mL of 4-nitrophenol aqueous solution (2.5 mM) was mixed with 7.0 mg of Ag/TiO2 nanocomposite in a beaker, stirring constantly for 2 min. Next, 25 mL of freshly prepared aqueous NaBH4 (0.25 M) was added and the mixture was allowed to stir at room temperature until the deep yellow solution became colorless. The reaction progress was monitored by UV–vis spectroscopy. The concentration of 4-nitrophenol was determined spectrophotometrically at a wavelength of 400 nm. After completion of the reaction, the catalyst was simply separated by brief centrifugation and washed successively with ethanol and water, dried and was used for successive cycles. 2.5. Reduction of methylene blue (MB) by Ag/TiO2 nanocomposite Typically, 5.0 mg of catalyst was added to 25 mL of MB aqueous solution (3.1 10 5). Next, 25 mL of freshly prepared aqueous NaBH4 (5.3 10 3) was added and the mixture was allowed to stir at room temperature. The progress of the conversion reaction was then monitored by recording the time-dependent UV–vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was simply separated from the reaction system by brief centrifugation and washed successively with ethanol and dried for the next cycle. 2.6. Reduction of methyl orange (MO) by Ag/TiO2 nanocomposite Typically, 10.0 mg of catalyst was added to 25 mL of MO aqueous solution (3.0 10 5). Next, 25 mL of freshly prepared aqueous NaBH4 (5.3 10 3) was added and the mixture was allowed to stir at room temperature. The progress of the conversion reaction was then monitored by recording the time-dependent UV–vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was simply separated from the reaction system by brief centrifugation and washed successively with ethanol and dried for the next cycle. 2.7. Reduction of Congo red (CR) by Ag/TiO2 nanocomposite Typically, 5.0 mg of catalyst was added to 25 mL of CR aqueous solution (1.44 10 5). Next, 25 mL of freshly prepared aqueous NaBH4 (5.3 10 3) was added and the mixture was allowed to stir at room temperature. The progress of the conversion reaction was then monitored by recording the time-dependent UV–vis absorption spectra of the mixture using a spectrophotometer. At the end of the reaction, the catalyst was simply separated from the reaction system by brief centrifugation and washed successively with ethanol and dried for the next cycle.
extracts may act both as reducing agent and stabilizing agents in the synthesis of NPs. 3.1. Characterization of Euphorbia heterophylla leaf extract The UV spectrum of leaf extract of the E. heterophylla (Fig. 2) shows bonds at kmax 360 nm (bond I) due to the transition localized within the ring of cinnamoyl system; whereas the one around 250 nm (bond II) is for absorbance of ring related to the benzoyl system. They are related to the p ? p⁄ transitions and these absorbent bonds demonstrate the presence of polyphenolics as antioxidant source for green synthesis of nanoparticles. The FT-IR analysis was carried out to identify the possible biomolecules responsible for the reduction of Ag NPs and capping them. The FT-IR spectrum of the crude extract, (Fig. 3) depicted some peaks at 3500–3400, 1680, 1460, 1250–1050 cm 1 which represent free OH in molecule and OH group forming hydrogen bonds, carbonyl group (C@O), stretching C@C aromatic ring and CAOH stretching vibrations, respectively. Because of presence the mentioned functional groups inside the structure of polyphenolics, the spectrum can demonstrate the presence of phenolics in the plant leaf extract. The presence of phenolics in the extract could be probably responsible for the reduction of metal ions and formation of nanocomposite. 3.2. Characterization of Ag/TiO2 nanocomposite The stable Ag/TiO2 nanocomposite obtained was fully characterized by UV–vis, XRD, SEM, EDS and FT-IR. The formation of Ag NPs was controlled by UV–vis spectroscopy. Fig. 4 shows the UV–vis spectrum of Ag NPs formation. The reaction was completed after 20 min. Our results showed that the maximum absorbance of green synthesized Ag NPs was at 440 nm due to the surface plasmon absorption of nanosized silver particles. The whitish color of the AgI solution immediately changed into dark brown indicating reduction of Ag+ to Ag0 and formation of Ag NPs as characterized by UV–vis spectrum. The synthesized Ag NPs by the this method are quite stable and no obvious variance in the shape, position and symmetry of the absorption peak is observed even after two month indicating the stability of Ag NPs. Fig. 5 shows the absorption spectrum of Ag/TiO2 nanocomposite due to surface plasmon resonance (SPR) of metallic nanoparticles. At compare with spectra related to Ag NPs, changing the color of the reaction (whitish to dark) and appearance the maxima ranging 250–350 nm indicates the reduction process and formation of nanoparticles. As monitored by UV–vis the synthesized nanoparticles by this method are quite stable with no significant variance in
3. Results and discussion In the present investigation, Ag/TiO2 nanocomposite was synthesized using E. heterophylla leaf extracted solution as a solvent instead of organic solvents. Rapid biosynthesis of Ag/TiO2 nanocomposite using E. heterophylla leaf extract has been investigated which is an easiest, cost efficient, non-toxic, eco-friendly and efficient method for exploiting E. heterophylla leaf. Plant
Fig. 2. UV–vis spectrum of the aqueous extract of the leaves of the Euphorbia heterophylla.
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Fig. 3. FT-IR spectrum of the aqueous extract of the leaves of Euphorbia heterophylla.
Fig. 4. UV–vis spectrum of green synthesized Ag NPs using the Euphorbia heterophylla leaf extract.
Fig. 5. UV–vis spectrum of green synthesized Ag/TiO2 nanocomposite using the aqueous extract of the leaves of Euphorbia heterophylla.
the shape, position and symmetry of the absorption peak even after one month which indicates the stability of product. Furthermore, FT-IR spectrum of Ag/TiO2 nanocomposite is shown in Fig. 6. The appeared bands are lattice vibration modes indicating the functional groups of samples. The broad band around 1000 cm 1 is TiAOATi stretching bond. Also, band at 3480 cm 1 represents the primary AOH stretching of hydroxyl functional group. The band around 1430 cm 1 is generally attribu-
ted to the bending vibration of HAOH groups for TiO2 but after doping of silver on TiO2, the intensity of basic TiO2 peaks hardly changed. Phase investigation of the crystallized product was performed by powder XRD measurements and the powder diffraction pattern of Ag/TiO2 nanocomposite is presented in Fig. 7. The structure of the Ag/TiO2 nanocomposite is analogous to that of Degussa P25 which can be indexed to TiO2 in anatase and rutile phases. The
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Fig. 6. FT-IR spectrum of the green synthesized Ag/TiO2 nanocomposite by Euphorbia heterophylla.
Fig. 7. XRD powder pattern of Ag/TiO2 nanocomposite.
diffraction peaks at 38°, 44.48° are consistent with (1 1 1), (2 0 0) reflections of the metallic silver particles [32]. The X-ray diffraction pattern exhibited face centered cubic (fcc) structure for the metallic silver particles. The pattern was very clean, with no indication of impurities such as silver oxide (Ag2O).
TiK
6000
The elemental composition of the Ag/TiO2 nanocomposite was determined by EDS analysis (Fig. 8). Presence of Ag, Ti and oxygen was confirmed by EDS spectroscopy. FESEM images of green synthesized Ag/TiO2 nanocomposite are shown in Fig. 9, which shows the silver and TiO2 nanoparticles show same diameter (less than 24 nm) and morphology. From FESEM images, it is clear that silver particles are not incorporated in TiO2 lattice, but deposited on the surface of TiO2. 3.3. Reduction of Ag+ to Ag0
5000
4000
TiL OK
3000
AgL AgL
2000 AuM AuM
1000
TiK
AuLl
0 0
5
Fig. 8. EDS spectrum of Ag/TiO2 nanocomposite.
AuL
10
keV
In this work, we have employed E. heterophylla leaf extract as a reducing and stabilizing agent for the synthesis of Ag/TiO2 nanocomposite. The methodology which we have adopted was totally hazard-free, low cost and environment friendly. We speculate that flavonoid and phenolic acids are present in the extract, which may be responsible for synthesis of Ag/TiO2 nanocomposite and its stabilization. It could be due to the presence of hydrogen binding and electrostatic interaction between the bioorganic capping and molecules bound to the Ag NPs. Weaker binding of these biomolecules with nascent Ag nanocrystals could lead to isotropic growth of the crystal and thus formation of spherical nanoparticles. On the basis of above results, a possible mechanism was proposed to the reduction of Ag+ and formation of Ag NPs by the
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Fig. 9. FESEM images of Ag/TiO2 nanocomposite.
E. heterophylla leaf extract. On the basis of this mechanism (Scheme 1), first reaction nucleation, represent the reduction of Ag+ ions into nano zero valent (NZV) metallic particles using flavonoid (FlOH) contents of the plant as green reducing agents (one step one electron oxidation-reduction mechanism to Ag0 and radical, nucleation; rate determining step). After the slow electron transfer step, reaction growth to adsorption may follow. The complexation of the formation of Ag0 atoms with Ag+ ions yield Ag+2 ions and Agn+ dimerize to give yellow sol, Ag2n2+ [33,34]. The adsorption step was further supported by the presence of a faint layer of organic molecules onto the surface of the Ag nanoparticles.
NO2
NH2
OH
OH Ag/TiO2 nanocomposite NaBH4 H2O, r.t.
3.4. Catalytic ability and application of the synthesized Ag/TiO2 nanocomposite for reduction of 4-NP In the present method, the catalytic reduction of 4-NP to 4-4-AP in the presence of NaBH4 in water at room temperature was chosen as a model reaction to evaluate the catalytic activity of Ag/TiO2 nanocomposite (Scheme 2). 4-NP in aqueous medium has a maximum absorption at 317 nm. However, the color of the solution changed to 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 (Scheme 3) in alkaline conditions and it did not change even for 24 h in the absence of catalyst. As shown in Fig. 10, when Ag/TiO2 nanocomposite was added into the solution containing 4-NP and NaBH4, the intensity of the strong absorption peak at 400 nm gradually decreased and a new peak appeared at about 300 nm which corresponded to the formation of 4-AP. After about 120 s, the peak at 400 nm almost disappeared and the color became transparent, which indicated that 4-NP was almost turned to 4-AP.
Scheme 2. The reduction of 4-NP on the surface of Ag/TiO2 nanocomposite.
BH4-
OH
NH2
NO2
NO2
Ag/TiO2
O-
OH
Scheme 3. The reduction reaction for the conversion of 4-NP to 4-AP.
Fig. 10. UV–vis spectrum of reduction of 4-NP.
Scheme 1. Reducing ability of antioxidant phenolics to produce Ag NPs where FIOH and NZV are flavonoid and nano zero valent, respectively.
The completion times for the catalytic reduction of 4-NP to 4-AP reaction in the presence of 50, 75 and 100 equivalents of NaBH4 and varying amounts of Ag/TiO2 nanocomposite are given in
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Table 1. According to data given in Table 1, reaction was not completed in the presence of 50 equivalents of NaBH4 and 5.0 mg of catalyst. In the presence of 75 and 100 equivalents of NaBH4 and 5.0 mg of catalyst, the reaction was completed in 180 s and 150 s, respectively. It was observed that the catalytic efficacy increased with an increase in the amount of NaBH4. Furthermore, in the presence of 100 equivalents of NaBH4 and 7.0 mg of catalyst, complete conversion was achieved within 120 s. The effect of the concentration of catalyst was also studied by carrying out the reduction reactions in the presence of varying amounts of catalyst. The best result was obtained with 100 equivalents of NaBH4 and 7.0 mg of catalyst at room temperature (Table 1). As expected, no target product could be detected in the absence of Ag/TiO2 nanocomposite. The catalytic reduction of 4-NP to 4-AP is an electron transfer process. To our knowledge, Ag NPs supported on the TiO2 can function as the electron mediator between 4-AP (oxidant) and BH4 (reductant), and the electron transfer occurs via the Ag NPs. In the present work, when the catalyst was added to a mixed solution of 4-NP and NaBH4, the reactant molecules are firstly adsorbed on the surface of catalyst via p–p stacking interactions. Such adsorption provides a high concentration of 4-NP near to the Ag NPs on TiO2, leading to highly efficient contact between them. In contrast, without a highly adsorbent TiO2 support, 4-NP must collide with Ag NPs by chance, and remains in contact for the catalysis to proceed. Then, the electron transfer (ET) occurs from the electron donor BH4 to the 4-nitrophenolate ions. The 4-nitrophenolate ions will be reduced to 4-AP at the surface of the metal catalysts and finally the generated product was desorbed from the surface of the catalyst. The absorbance of the reaction solution at 400 nm was measured as a function of time for kinetics investigation. Because NaBH4 is in great excess compared to 4-nitrophenol, its concentration can be regarded as being constant throughout the reaction. Therefore, the reduction of 4-NP to 4-AP can be treated as a pseudo-first-order reaction [35].
Table 2 Completion time for the reduction of MB (MO or CR) using different amounts of Ag/TiO2 nanocomposite.
a
Entry
Dye (M)
1 2 3 4 5 6 7 8 9
CR (1.44 10 CR (1.44 10 CR (1.44 10 MO (3.0 10 MO (3.0 10 MO (3.0 10 MB (3.1 10 MB (3.1 10 MB (3.1 10
NaBH4 (M) 5
) 5 ) 5 ) 5 ) 5 ) 5 ) 5 ) 5 ) 5 )
5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10 5.3 10
3 3 3 3 3 3 3 3 3
Catalyst (mg)
Time
3.0 5.0 10.0 3.0 7.0 10.0 1.0 3.0 5.0
15 mina 30 s 30 s 20 mina 20 mina 9 min 40 mina 165 s 70 s
Not completed.
3.5. Catalytic activity of the synthesized Ag/TiO2 nanocomposite for reduction of MO, CR and MB Dyes and other dyestuffs are the major effluents from the textile industry that cause significant pollution [36]. Most of these dyes are not biodegradable and persist in the environment [37]. Therefore, it is highly desirable to develop an eco-friendly method for reduction these compounds in an aqueous medium. Here, three reactions, the reduction of MO, CR and MB by NaBH4, were chosen as model reactions to evaluate the performance of the as-prepared Ag/TiO2 nanocomposite. The following processes are demonstrated to be involved in this reaction: initially, NaBH4 was adsorbed onto the surface of catalyst to form metal hydride, and then MB (MO or CR) adsorbed onto the surface of catalyst; finally, MB (MO or CR) is reduced and desorbed to create a free space for the reaction to continue. The catalytic reduction of MB (MO or CR) was studied with the different amounts of catalyst, and the results are shown in Table 2.
Table 1 Completion time for the reduction of 4-NP using different amounts of Ag/TiO2 nanocomposite with 50, 75 and 100 equivalents of NaBH4.
a
Entry
Catalyst (mg)
NaBH4 (equivalents)
Time
1 2 3 4 5
5.0 5.0 5.0 7.0 10.0
50 75 100 100 100
40 mina 180 s 150 s 120 s 120 s
Not completed.
Fig. 11. The evolution of the UV–vis spectra of dyes aqueous solution in the presence of Ag/TiO2 nanocomposite, (A) MO; (B) MB and (C) CR.
As shown in Table 2, in the presence of 3.0 mg of catalyst, nearly 92% conversion was achieved in 15 min for reduction of CR (entry 1). The best result was obtained with 5.0, 10.0 and
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5.0 mg of catalyst for reduction of CR, MO and MB, respectively in the presence of NaBH4 (5.3 10 3) at room temperature. The reduction of MO, CR and MB by NaBH4 was monitored spectrophotometrically by following the decrease of absorbance at kmax (kmax (MO) = 465 nm, kmax (CR) = 493 nm and kmax (MB) = 663 nm) with time (Fig. 11). As expected, the catalytic reduction of MO, MB and CR by NaBH4 proceeded successfully. In this catalytic reaction system, since the amount of reductant NaBH4 is excessive compared to the degraded dyes (MO, MB and CR), the reaction rate is roughly independent of the NaBH4 concentration and similar to the reduction of 4-NP, MB (MO or CR) reduction also follows pseudo first-order kinetics with respect to MB (MO or CR) concentration. 3.6. Catalyst recyclability The reusability of the catalysts is one of the advantages of heterogeneous catalysts and makes them useful for industrial applications. The Ag/TiO2 nanocomposite can be easily separated from the reaction mixture by mild centrifugation and washed with distilled water several times for the successive reactions. The recovered catalyst was recycled five times for 100% reduction of 4-nitrophenol. This reusability demonstrates the high stability and turnover of catalyst under operating conditions. Little decrease of efficiency was observed after the 5th cycle. In addition, the catalytic efficiency of the catalyst remained almost constant up to five cycles of operation and the time required for 100% reduction of MO, CR and MB was found to be almost same up to the 5th cycle. 4. Conclusions In conclusion, we show an environmentally friendly method to prepare stable Ag/TiO2 nanocomposite employing E. heterophylla leaf extract without usage of any special capping agents. The flavonoids present in extract of leaves of E. heterophylla act as both reducing and capping/stabilizing agents. The synthesized Ag/TiO2 nanocomposite capped by biomolecules was characterized by SEM, XRD, EDS, FT-IR and UV–vis spectroscopic techniques. The advantages of this biosynthesis method include (i) use of plant extract as an economic and effective alternative, (ii) use of cheap, clean, nontoxic and environmentally benign precursors and (iii) simple procedures without application of toxic reagents or surfactant template. In addition, the catalytic activity of Ag/TiO2 nanocomposite for reduction of variety of dyes in water was also studied. In addition, this methodology offers the competitiveness of recyclability of the catalyst without significant loss of catalytic activity, and the catalyst could be easily recovered and reused several times, thus making this procedure environmentally more acceptable. The synthesized Ag/TiO2 nanocomposite by this
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method is quite stable and can be kept under inert atmosphere for several months. The method is general and may be extended to other MNPs. Acknowledgments We gratefully acknowledge the Iranian Nano Council and the University of Qom for the support of this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
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