Received: 30 September 2016
Revised: 17 November 2016
Accepted: 20 November 2016
DOI: 10.1002/aoc.3711
F U L L PA P E R
Iron oxide nanoparticles coated with green tea extract as a novel magnetite reductant and stabilizer sorbent for silver ions: Synthetic application of Fe3O4@green tea/Ag nanoparticles as magnetically separable and reusable nanocatalyst for reduction of 4‐nitrophenol Hojat Veisi | Fatemeh Ghorbani Department of Chemistry, Payame Noor University, Tehran, Iran Correspondence Hojat Veisi, Department of Chemistry, Payame Noor University, Tehran, Iran. Email:
[email protected] Funding information Payame Noor University
Green tea extract having many phenolic hydroxyl and carbonyl functional groups in its molecular framework can be used in the modification of Fe3O4 nanoparticles. Moreover, the feasibility of complexation of polyphenols with silver ions in aqueous solution can improve the surface properties and capacity of the Fe3O4@green tea extract nanoparticles (Fe3O4@GTE NPs) for sorption and reduction of silver ions. Therefore, the novel Fe3O4@GTE NPs nano‐sorbent has potential ability as both reducing and stabilizing agent for immobilization of silver nanoparticles to make a novel magnetic silver nanocatalyst (Fe3O4@GTE/Ag NPs). Inductively coupled plasma analysis, transmission and scanning electron microscopies, energy‐dispersive X‐ray and Fourier transform infrared spectroscopies, and vibrating sample magnetometry were used to characterize the catalyst. Fe3O4@GTE/Ag NPs shows high catalytic activity as a recyclable nanocatalyst for the reduction of 4‐nitrophenol at room temperature. K E Y WO R D S
4‐nitrophenol, green tea, magnetite nanoparticles, reduction, silver
1 | IN T RO D U C T IO N Nanoparticles are of great interest due to their extremely small size and large surface‐to‐volume ratio, which lead to both chemical and physical differences in their properties. Nowadays, most research focuses on the noble metal nanoparticles (such as those of Ag, Au, Pd, Pt) and their various morphologies due to their applicability in the fields of chemistry, biology, physics, material science and medicine.[1] Among the several noble metal nanoparticles, silver nanoparticles have attained special focus in view of their unique properties and applications. In developments of recent years, the use of silver nanoparticles has rapidly increased because of their unusual optical properties, non‐toxicity, their being a safe inorganic antibacterial agent, chemical, electronic, photoelectrochemical, chemical sensing, catalytic, magnetic, antibacterial and biological labelling properties,[2] and their use in drug delivery,[3] food industries,[4] agriculture,[5] Appl Organometal Chem. 2017;31:e3711. https://doi.org/10.1002/aoc.3711
textile industries,[6] water treatment,[7] etc. Generally, the method for silver nanoparticle preparation involves the reduction of silver ions in solution or at high temperature in gaseous environments.[8] However, the reducing reagents may increase the environmental toxicity or biological hazards.[8,9] Moreover, plant‐mediated nanoparticle synthesis[10–16] is preferred because it involves non‐toxic reagents, is cost‐effective, uses environmentally friendly solvents, is a single‐step method for biosynthesis process and is safe for humans and the environment. Nitrophenols are among the most common organic pollutants in industrial and agricultural wastewaters. They can damage the central nervous system, liver, kidney and blood of humans and animals. Degradation of 4‐nitrophenol (4‐NP) to a non‐dangerous product is difficult because of its high stability and low solubility in water.[17,18] Its reduction product, 4‐aminophenol (4‐AP), is very useful and important in many applications that include analgesic and antipyretic
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drugs, photographic developers, corrosion inhibitors, anticorrosion lubricants, and so on.[19] Recently, the development of new nanocatalysts with high catalytic activity for 4‐NP reduction has attracted much attention. For example, Pd–graphene nanohybrids,[20] AuPd nanoparticles–graphene nanosheets,[21] silver supported nanoporous iron oxide microbox hybrids,[22] Au–CeO2@ZrO2 yolk–shell nanoreactors[23] and Ni@Pd core–shell nanoparticle‐modified fibrous silica nanospheres[24] have been synthesized for highly efficient catalytic reduction of 4‐NP by NaBH4. Nanometre‐sized metal oxides are not target‐selective and are unsuitable for samples with complex matrices.[25] Therefore, a suitable coating is essential to overcome such limitations. Also, surface modification stabilizes the nanoparticles and also prevents their oxidation. Magnetic nanoparticles can be efficiently functionalized based on the formation of relatively stable linkers between hydroxyl groups on the nanoparticle surface and suitable anchoring agents such as phosphonic acid and dopamine derivatives.[26] So, green tea extract having many phenolic hydroxyl[27] and carbonyl functional groups[28] in its molecular framework can be used in the modification of magnetic nanoparticles (Scheme 1). Moreover, the feasibility of complexation of polyphenols with polyvalent cations in simple aqueous solutions can improve the surface properties and capacity of the Fe3O4@green tea extract nanoparticles (Fe3O4@GTE NPs) for adsorption and reduction of metal ions. Therefore, the novel Fe3O4@GTE NPs nano‐sorbent has potential ability as both reducing and stabilizing agent for the immobilization of silver nanoparticles to make a novel magnetically separable and reusable catalyst (Fe3O4@GTE/Ag NPs) (Scheme 1).
thoroughly thrice with double‐distilled water before use. An amount of 10 g of the leaves was added to 100 ml of deionized water which was then boiled for 15 min in a water bath. The mixture was then cooled and was filtered through Whatman filter paper no. 1 to obtain aqueous extract. The filtered extract was stored in a refrigerator at 4 °C for further use. For the synthesis of Fe3O4@GTE NPs, in the first step, magnetite nanoparticles (500 mg) were dispersed in 50 ml of water and sonicated for 20 min. Next, the green tea extract was added to the mixture. Afterwards, the solution was stirred for 24 h at room temperature, and the Fe3O4@GTE NPs precipitate obtained was separated by magnetic decantation and washed several times with deionized water. Finally, the Fe3O4@GTE NPs obtained were dried in a vacuum oven at 40 °C for 12 h. 2.2 | Preparation of Fe3O4@GTE/Ag NPs The Fe3O4@GTE NPs (500 mg) were dispersed in deionized water (200 ml) using an ultrasonic bath for 30 min. Subsequently, a solution of AgNO3 (30 mg) in 20 ml of water was added to the dispersion of Fe3O4@GTE NPs and the mixture was stirred for 2 h at room temperature in order to ensure complete reduction of Ag(I) ions in the precursor solution. Then, the Fe3O4@GTE/Ag NPs was separated by magnetic decantation and washed with water and acetone successively to remove the unattached substrates. Scheme 1 depicts the synthetic procedure for Fe3O4@GTE/Ag NPs. The final nanocatalyst was dried in vacuum at 40 °C. The concentration of silver was 0.12 mmol g−1, as determined using inductively coupled plasma (ICP) atomic emission spectrometry.
2 | E XP E R IM E NTA L 2.3 | General procedure for reduction of 4‐NP 2.1 | Preparation of Fe3O4@GTE NPs Leaves of green tea were collected in September 2015 in northern Iran. The fresh green tea leaves were washed
For catalytic testing of Fe3O4@GTE/Ag NPs, reactant solutions of 4‐NP and NaBH4 were freshly prepared in concentrations of 3 mM and 0.3 M, respectively. Subsequently, 1 ml of each solution was added and mixed by magnetic stirring and transferred to a quartz cuvette. Then 2 mg of catalyst was loaded into the cuvette to start the reaction. The intensity of the absorption peak at 400 nm in the UV–visible spectrum was used to monitor the process of the conversion of 4‐NP to 4‐AP.
3 | R E S U LTS AN D D I S CU S S I ON
SCHEME 1
Preparation of Fe3O4@GTE/Ag NPs nanocatalyst
Fourier transform infrared (FT‐IR) spectral analysis was used to determine the surface chemistry of the Fe3O4@GTE/Ag NPs. It was also carried out to recognize the possible biomolecules responsible for synthesis and stabilization of Ag nanoparticles on the surface of the magnetite nanoparticles. In the FT‐IR spectrum (Figure 1), the broad band at 3416 cm−1 can be assigned to stretching of the hydroxyl functional group in alcohol and phenolic compounds and also the O─H bond of
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FIGURE 1
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FT‐IR spectrum of Fe3O4@GTE/Ag NPs
Fe3O4 nanoparticles, while one characteristic peak can be observed at 600 and 572 cm−1, which corresponds to the Fe─O stretching mode of the Fe3O4 lattice. There are several additional peaks in the spectrum belonging to the green tea extract coating. Absorption bands at 1750, 1654 and 1057 cm−1 correspond to stretching vibrations of C═O and C─O. Peaks appearing at 2928 and 2889 cm−1 are characteristic of C─H stretching vibrations. According to these results, FT‐IR analyses suggest that green tea extract is successfully coated on the surface of Fe3O4 nanoparticles. This study substantiates the presence of biopolymer and polyphenols in the sample which coat the Ag nanoparticles as a capping agent and thus stabilizes the Ag nanoparticles. Field emission scanning electron microscopy (SEM) is a primary tool for determining size distribution, particle shape, surface morphology and fundamental physical properties. SEM images of the Fe3O4@GTE/Ag NPs show the successful coating of Fe3O4 nanoparticles with green tea extract and also the formation of uniform spherical Ag nanoparticles on the surfaces (Figure 2). The energy‐dispersive X‐ray spectrum (EDS) confirms the presence of Ag, Fe, C, N and O species in the structure of the material (Figure 3). Furthermore, transmission electron microscopy (TEM) was used to study the morphology of the Fe3O4@GTE/Ag NPs. The particles are found to be very small, spherical in shape and dispersed (Figure 4). It is interesting to note that the particles are surrounded by a thin biomolecular layer, which appears to be responsible for reducing and stabilizing the nanoparticles. Figure 5 shows a representative SEM image and corresponding elemental maps for the synthesized catalyst. It can be seen that Ag nanoparticles are well dispersed in the nanocomposite. The selected area elemental analysis maps reveal the presence of C, Fe and Ag throughout the sample in a homogeneous manner, which confirms the regular uniformity of the prepared sample. The magnetic properties of the Fe3O4@GTE/Ag NPs nanocatalyst containing a magnetite component were studied using vibrating sample magnetometry (VSM) at room temperature. According to our previous work, the magnetic
FIGURE 2
SEM images of Fe3O4@GTE/Ag NPs
saturation value for Fe3O4 is 61.8 emu g−1.[29] The saturation magnetization of the nanocatalyst is determined as 32.8 emu g−1 (Figure 6). The decrease in the saturation magnetization is due to the presence of the green tea extract and Ag nanoparticles on the Fe3O4 surface. In continuation of our research on the synthesis of nanoparticles and their applications as heterogeneous catalysts,[29] in the work reported herein, the reduction of 4‐NP was carried out using NaBH4 as a hydrogen‐producing agent and Fe3O4@GTE/Ag NPs as a nanocatalyst. As the reaction proceeds, the colour of the solution changes gradually from yellow to colourless at room temperature. UV–visible spectra
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FIGURE 3
EDS spectrum of Fe3O4@GTE/Ag NPs
FIGURE 4
TEM image of Fe3O4@GTE/Ag NPs
were recorded at short intervals to monitor the progress of the reaction. The intensity of the absorption peak at 400 nm for 4‐NP was monitored along with time; at the same time, a new absorption peak for 4‐AP appears at 304 nm (Figure 7). The rate of the reduction reaction catalysed by Fe3O4@GTE/Ag NPs was assumed to be independent of the concentration of NaBH4 because this reagent was used in large excess compared to 4‐NP. Therefore, the kinetic data were fitted by a first‐order rate law. A linear relationship between ln(A/A0) and reaction time is obtained for the reduction catalysed by the Fe3O4@GTE/Ag NPs nanocomposite (Figure 7), and the rate constant k is calculated to be 0.067 s−1. The catalyst activity parameter (Ka) of the composite material was also calculated from the ratio of rate constant for the catalyst to the amount of given catalyst added, where Ka = k/m, from which Ka is calculated to be 67 s−1 g−1. The activity parameters used for the comparison of the catalytic activity of Fe3O4@GTE/Ag NPs with those reported for other catalysts employed for the reduction of 4‐NP are listed in Table 1. The catalytic activity of Fe3O4@GTE/Ag NPs, obtained in this study, is 67 s−1 g−1 which is significantly higher than the catalytic activities of the previously reported nanocatalysts (Table 1). So, the prepared nanocatalyst displays superior catalytic activity for the reduction reaction.
SEM image of Fe3O4@GTE/Ag NPs and elemental maps for C, Fe and Ag atoms in the catalyst
FIGURE 5
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TABLE 1
Comparison of catalytic activities for the reduction of 4‐NP k (s−1)
Catalyst
5.19 × 10−3
TAC‐Ag‐10 P(AMPS)‐Ni P(AMPS)‐Co P(AMPS)‐Cu Fe3O4@GTE/Ag NPs
FIGURE 6
Magnetic hysteresis loop of Fe3O4@GTE/Ag NPs
Ka (s−1 g−1)
9.38 × 10 2 × 10 1.72 × 10
−4 −3 −3
67 × 10−3
1.03
[30]
0.019
[31]
0.04
[32]
0.172
[33]
67
This work
groups. On the other hand, the covering shell has good adsorption ability towards the water‐soluble 4‐NP, which could accelerate the process of catalytic reduction. It is suggested that the extract may play an active part in the catalysis, yielding a synergistic effect. The catalytic mechanism for the conversion of 4‐NP into 4‐AP relies on electrons transfer from the BH4− donor to the acceptor 4‐NP through adsorption of the reactant molecules onto the extract surface. The Ag nanoparticles can serve as catalyst to transfer electrons from BH4− to 4‐NP, which are both adsorbed on the catalyst via π–π stacking interactions, leading to the production of amino derivatives. In the present work, when the Fe3O4@GTE/Ag NPs are added to a mixed solution of 4‐NP (oxidant) and NaBH4 (reductant), 4‐nitrophenolate ions (Figure 8) and BH4− are first adsorbed on the surface of the catalyst via physical adsorption and hydrogen bonding. After electron transfer to the Ag nanoparticles, the hydrogen atom forms from the hydride, and then attacks 4‐nitrophenolate ions to reduce them. This electron transfer‐induced hydrogenation of 4‐NP occurs spontaneously at the surface of the metal catalyst. Finally, the generated 4‐AP is desorbed from the surface of the catalyst. The catalyst was separated using an external magnet after the whole reduction process had been monitored, washed several times with water and ethanol successively, and then reused. For practical applications of heterogeneous systems, the level of reusability and the activity of the catalyst are important factors. The catalyst exhibits similar catalytic
FIGURE 8
Reduction of 4‐NP in the presence of NaBH4 and catalyst
FIGURE 9
Recyclability of Fe3O4@GTE/Ag NPs
Plot of ln(A/A0) against reaction time (left) and UV–visible spectra for catalytic reduction of 4‐NP to 4‐AP (right)
FIGURE 7
It should be noted that the functionalized magnetic nanoparticles (Fe3O4@GTE) are not active in the reduction of 4‐NP, with less than 10% conversion observed after 5 h. This indicates the need for the coating of Ag nanoparticles on the surface of the magnetic nanoparticles to create active sites. The enhanced catalytic activity observed for our nanocatalyst is attributed to the many active functional groups, such as C═C and ─OH, that cover the Ag nanoparticles which interact and stabilize the 4‐NP substrate adjacent to the catalytic sites which in turn facilitates the reduction of nitro
Ref.
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performance without significant reduction in the conversion even after nine reaction cycles (Figure 9).
[12] M. Zargar, K. Shameli, G. R. Najafi, F. Farahani, J. Ind. Eng. Chem. 2014, 20, 4169. [13] H. Veisi, R. Ghorbani‐Vaghei, S. Hemmati, M. Haji Aliani, T. Ozturk, Appl. Organometal. Chem. 2015, 29, 26.
4 | C O NC LUS I O N S
[14] A. Saravanakumar, M. Ganesh, J. Jayaprakash, H. Jang, J. Ind. Eng. Chem. 2015, 28, 277.
Magnetite (Fe3O4) nanoparticles were coated with green tea extract to give a novel sorbent as magnetic reducing and capping/stabilizing agent for deposition of silver nanoparticles. ICP analysis, TEM, SEM, EDS, VSM and FT‐IR spectral studies have been used to characterize the prepared nanocatalyst. We have established that the Fe3O4@GTE/Ag NPs catalyst can used for the reduction of 4‐NP to 4‐AP in water at room temperature. Notably, the recoverability of the catalyst using an external magnet with no silver leaching are advantageous characteristics. The catalyst was reused many times with any loss of catalytic activity. The enhanced catalytic activity observed with our nanocatalyst is attributed to the many active functional groups, such as C═C and ─OH, that cover the Ag nanoparticles which interact and stabilize the 4‐NP substrate adjacent to the catalytic sites which in turn facilitates the reduction of nitro groups. Also, the covering shell has the ability to adsorb the water‐soluble 4‐NP, which can accelerate the process of catalytic reduction. It is suggested that the extract may play an active part in the catalysis, yielding a synergistic effect. The results showed that the green approach to synthesizing Ag nanoparticles could be useful for removing toxic pollutants and dyes such as nitroaromatics from the environment. Moreover, the feasibility of complexation of polyphenols on the surface of Fe3O4 nanoparticles with metal ions in aqueous solution can improve the surface properties and capacity of the Fe3O4@GTE NPs for adsorption and reduction of other ions.
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How to cite this article: Veisi H, Ghorbani F. Iron oxide nanoparticles coated with green tea extract as a novel magnetite reductant and stabilizer sorbent for silver ions: Synthetic application of Fe3O4@green tea/Ag nanoparticles as magnetically separable and reusable nanocatalyst for reduction of 4‐nitrophenol. Appl Organometal Chem. 2017;31: e3711. https://doi.org/10.1002/aoc.3711
