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In situ green synthesis of Ag nanoparticles on tea polyphenols-modified graphene and their catalytic reduction activity of 4-nitrophenol
Colloids and Surfaces A: Physicochem. Eng. Aspects 485 (2015) 102–110
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
In situ green synthesis of Ag nanoparticles on tea polyphenols-modified graphene and their catalytic reduction activity of 4-nitrophenol Zhimin Wang ∗ , Cuilian Xu, Xin Li, Zhaohui Liu School of science, Henan Agricultural University, Zhengzhou 450002, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l
• Tea polyphenols were used to reduce
A water dispersible Ag-TPG catalyst with uniformly distributed Ag nanoparticles on the surface of tea polyphenols modified graphene, exhibiting enhanced catalytic activity in 4-nitrophenol reduction, was synthesized and characterized.
and stabilize graphene oxide. • Uniformly distributed Ag nanoparticles could be in situ formed on the tea polyphenols modified surface. • This well dispersed Ag-TPG hybrid exhibited enhanced catalytic activity in 4-nitrophenol reduction.
a r t i c l e
i n f o
Article history: Received 22 May 2015 Received in revised form 24 August 2015 Accepted 4 September 2015 Available online 8 September 2015 Keywords: Graphene Silver nanoparticles Tea polyphenols Green synthesis Catalysis
a b s t r a c t In this work, a complete green synthesis approach to reduced graphene oxide-Ag nanoparticles hybrid by using tea polyphenol as both reducing and stabilizing agent is reported. On the one hand, tea polyphenols can efficiently reduce graphene oxide and adsorb on the surface of the reduced graphene oxide. On the other hand, the surface adsorbed tea polyphenols can further in situ reduce Ag ions to Ag nanoparticles and stabilize them. The structure and physicochemical properties of the resulting nanohybrid were characterized in detail by UV–vis spectroscopy, Fourier transformation infrared spectroscopy, Xray diffraction, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. The obtained results indicated that Ag nanoparticles distributed uniformly on the surface of the functionalized graphene. Meanwhile, the so-formed hybrid material could be well dispersed in water forming a homogeneous dispersion due to the stabilization of tea polyphenols. This nanohybrid, combining the unique catalytic properties of Ag nanoparticles with the excellent adsorption and electron transfer ability of graphene, exhibited enhanced catalytic activity toward the reduction of 4-nitrophenol by NaBH4 . © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene and metal nanoparticles represent two kinds of important components in the current nanoscience and nano-
∗ Corresponding author. Fax: +86 371 6355 8139. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.colsurfa.2015.09.015 0927-7757/© 2015 Elsevier B.V. All rights reserved.
a b s t r a c t
technology [1,2]. Due to their unique structural characters and fascinating properties, graphene and noble metal nanoparticles have been explored extensively for different applications, such as electronic device [3–5], energy conversion and storage [6,7], sensing [8,9], catalytic [10,11], and several others [12–14]. In order to exploit their properties and applications broadly, successful preparation of these materials is an essential prerequisite. Among the documented strategies, chemical reduction has been proved to be
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a promising route for the synthesis of both graphene and metal nanoparticles owing to its facile synthetic nature, controlled, scalable, and reproducible manner [15,16]. In this strategy, there are two major challenges involved in the whole process. The first one is the rational selection of appropriate reagents or method for efficient reduction of graphene oxide (GO) or metal precursor without sacrificing the environment and others, and the second one is to avoid aggregation after reduction of GO or metal precursor owing to the strong van der Waals attractive forces between the reduced moieties. Apart from the single application of graphene and metal nanoparticles, hybridizing the two components and exploring their collective properties has recently aroused wide interests in various areas ranging from bio-applications [17], sensing [18,19], to energy conversion [20,21] and catalysis [22]. In catalytic studies, graphene is often used as a support due to its large surface area (calculated value, 2630 m2 g−1 ) and tunable surface properties [23]. It is believed that efficient capping and protection of catalyst materials, excellent adsorb ability and effective separation and transportation of charge carrier or electron play collective role in activity enhancement of the catalyst [24]. To date, big progress has been made in the synthesis of graphene based hybrids with metal-nanoparticle catalysts and these hybrids showed improved performance in different catalytic reactions [25–31]. However, developing homogeneous composites with a uniform distribution of catalyst and controlling the catalyst loading remains a major problem due to the aggregation effect of graphene and the weak interaction between graphene and the catalyst [23]. To this end, both electrostatic stabilization and chemical functionalization have proven to be useful in suppressing aggregation and improving the adhesion of catalyst with graphene [32]. Nevertheless, the surface-modification process usually suffers from limited scalability, time-consuming and environmental problems. Furthermore, the used reducing agents are always highly toxic, which also limited its practical application to some extents. In this respect, development of green approaches for synthesis of graphene and metal nanoparticle nanohybrids by using environmentally benign materials is highly desirable. Inspired by the reductive and adhesive properties of dopamine, plant polyphenols, which possess similar structure to dopamine, have been widely exploited due to their low cost and bioavailability [28,33]. Among these plant polyphenols, tea polyphenols (TPs) from green tea have recently attracted additional broad interest in various fields ranging from versatile surface coating, nanomaterials and nanocomposites [34–36]. TPs are mainly consisted of epicatechin (EC), epicatechin gallate (ECG), epigallocatechin (EGC) and epigallocatechin gallate (EGCG), in which the EGCG makes up about 50–60% of the total TPs [37]. The multiple pyrogallol and catechol structures of these compounds (Supporting information Fig. S1) make TPs highly water-soluble and excellent antioxidant properties [38]. To date, gold, silver, palladium, and iron nanoparticles have been synthesized and stabilized by using TPs as both reducing and stabilizing agents [39–43]. Moreover, it is also demonstrated that TPs could disperse and stabilize carbon nanotube and boron nitride nanotube due to their strong surface adhesion ability [35,36]. Very recently, TPs have also been shown that they could one-pot reduce and modify GO and form a dispersible TP-graphene composite in water [44]. Taken together, combining the water solubility, low toxicity, and biodegradability with their reductive and adhensive properties, TPs should be an appropriate alternative for green synthesis of metal nanoparticles and graphene hybrid. In view of multiple characters of TPs mentioned above, we herein report a facile green synthetic approach to Ag nanoparticlesgraphene hybrid by using TPs as both reducing and stabilizing agent. At first, TPs can efficiently reduce GO and adsorb on the surface of the reduced GO. Then, the adsorbed TPs can further in situ reduce Ag ions to Ag nanoparticles and stabilize them.
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Because the amounts and distribution of the surface adsorbed TPs are predetermined, well distributed Ag nanoparticles could be in situ synthesized on the surface of TPs modified graphene. Importantly, the resultant Ag-TPG nanohybrid could be well dispersed in water due to the stabilizing function of TPs. Furthermore, it was demonstrated that this water dispersible Ag-TPG nanohybrid could function as an effective catalyst to activate the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of excess NaBH4 following pseudo-first-order kinetics, otherwise unfeasible if only the strong reducing agent NaBH4 was employed. 2. Experimental 2.1. Materials Graphite powder (AP, 325 meshes), AgNO3 and 4-NP were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. The commercial green tea (Xinyang Maojian tea) was purchased from local market and no further treatment was performed before use. All reagents, such as NaNO3 , H2 SO4 , H2 O2 , HCl, KMnO4 , ethanol, NaBH4 , etc, were reagent grade and used as received without further purification. 2.2. Green synthesis of TPs-modified graphene (TPG) The GO was synthesized by oxidizing the graphite powder in a mixture of concentrated sulfuric acid and KMnO4 based on a modified Hummers method as originally presented by Kovtyukhova and colleagues [45,46]. For the tea solution, 5 g of green tea leaf was added in 250 mL of deionized (DI) water and extracted at 80 ◦ C for 30 min under stirring. After extraction, the mixture was centrifuged at 8000 rpm for 30 min. The resultant supernatant was collected, filtered, and then stored at −20 ◦ C before use. The total phenolic content of the aqueous green tea extracts was determined by the Folin–Ciocalteu method using gallic acid as a standard phenolic compound and the result was expressed as mg L−1 gallic acid equivalent (GAE) [47]. The total phenolic content of the prepared tea solution determined by this method is 2835 mg L−1 GAE. We can determine that the extracted polyphenolic compounds from green tea are ca. 14.2 wt%, which is much lower than the average content of TPs (20–30 wt%) in green tea, indicating that the extracted TPs are more soluble in water. For the preparation of TPG, 50 mg GO was added in 100 mL preprepared tea solution and sonicated for 5 min. The reaction mixture was then refluxed at 80 ◦ C with stirring under nitrogen atmosphere for 2 h. The color of the mixture changed from light brown to black, indicating the reduction of graphene oxide, which was stable. After cooling to room temperature, the TPG was isolated by centrifugation. The precipitate was washed repeatedly with DI water in consecutive washing-centrifugation cycles. Ultrasonic treatment was used in every cycle in order to re-disperse the TPG and remove the excess TPs. After washing, the sample was dried at 60 ◦ C in vacuum for 24 h. 2.3. In situ green synthesis of Ag-TPG nanohybrid For the synthesis of Ag-TPG nanohybrid, 0.5 mL 0.1 M of AgNO3 solution was added into 10 mL 0.5 mg mL−1 of TPG aqueous solution and vigorously stirred for 20 min. The reaction mixture was then allowed to settle at room temperature with mild stirring overnight. The resulting slurry was filtered, washed thoroughly with deionized water and then the obtained Ag-TPG hybrid catalyst was dried in a vacuum oven before use. The whole synthesis procedure for the preparation of Ag-TPG nanohybrid was presented in Fig. 1.
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Fig. 1. Schematic process of green reduction of GO and subsequent in situ reduction of Ag ions to form Ag-TPG nanohybrid.
2.4. Materials characterization The experimental process was characterized comprehensively by a variety of techniques. The UV–vis spectra were acquired on a Shimadzu UV-1601PC spectrophotometer. The wavelength was set in the range of 200–800 nm for all the measurements. Transmission electron microscopy (TEM) observations were conducted on a FEI Tecnai G2 F20 microscope with a field-emission gun operating at 200 kV. The sample was prepared by dropping the water solution of Ag-TPG nanohybrid onto copper grids coated with Formvar and carbon film. Wide angle X-ray diffraction (WXRD) measurements were made using a Bruker D8 ADVANCE X-ray diffractometer with Cu K␣ (1.541 Å) radiation (40 kV, 30 mA). Powder samples were mounted on a sample holder and scanned with a step size of 0.01◦ between 2 = 3◦ and 90◦ . Fourier transformation infrared spectroscopy (FTIR) data were obtained using a Nicolet Avator 230 spectrometer. The samples were prepared with KBr. Modification efficiency and the amounts of TPs adsorbed on the graphene were determined using a NETZSCH4 TGA instrument at a 50 mL min−1 flowing rate of nitrogen atmosphere. The temperature was increased from 20 to 900 ◦ C at a rate of 10 ◦ C min−1 . The X-ray photoelectron spectroscopy (XPS) data were obtained with VG Multilab 2000 electron spectrometer from Thermo Scientific using 300 W Al K␣ radiation. The base pressure was about 3 × 10−9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. 2.5. Catalysis study The catalytic reduction reaction of 4-NP was performed in aqueous solution in a standard quartz cell. The overall concentrations of 4-NP and NaBH4 were 0.1 and 10 mM, respectively. In a typical catalytic reaction, 2.9 mL of aqueous solution of 4-NP and 0.1 mL of aqueous NaBH4 solution were mixed together and then purged with nitrogen for 10 min to remove the dissolved oxygen. Next, 2 mL of aqueous Ag-TPG suspensions (0.5 mg mL−1 ) purged with nitrogen was added to the reaction mixture under constant magnetic stirring. Immediately after that, the solution was transferred to a standard quartz cell, and the UV–vis absorption spectra were recorded with a time interval of 120 s in a scanning range of 200–700 nm at ambient temperature. After completion of the reaction the catalyst was recovered by centrifugation. The precipitate was washed repeatedly with DI water in consecutive washing cycles. Ultrasonic treatment was used in every cycle in order to re-disperse the catalyst and remove adsorbed impurities. After washing, the catalyst was used directly for recycling test. After each recycle, the centrifuge supernatant was collected and detected by Atomic absorption spectroscopy to determine the content of Ag metal. 3. Results and discussion 3.1. Green reduction and modification of GO by tea polyphenols As shown in Fig. 2 inset, the as-prepared GO could readily resolve in tea water forming a bright brown transparent solution. The
Fig. 2. UV–vis traces the green reduction of GO to TPG by reflux in tea water at 80 ◦ C under nitrogen atmosphere: (a) GO; (b) tea water; (c) reflux for 30 min; (d) reflux for 1 h and (e) reflux for 2 h. The inset shows the photographs observation of the reduction process.
influence of temperature on the reduction process was investigated. It was found that 80 ◦ C was the appropriate temperature to reduce GO. When the temperature was lower than 80 ◦ C, the reducing effect was weakened considerably as indicated by detecting the characterized colour change from GO to graphene. The conversion of GO to reduced GO was monitored by recording the UV–vis absorption spectra of TPG as a function of time. For detecting the reaction progress, a given amount of reaction solution was extracted in a regular interval. The extracted solution was filtered and washed thoroughly, and then re-dispersed in water to form a dilute solution for UV–vis examination. As shown in Fig. 2, GO shows a peak at 228 nm and a shoulder at 300 nm. As the reaction progressed, the characterized peaks of GO at 228 nm and 300 nm vanished completely, and a new characterized peak belong to the reduced GO established gradually (shown in Supporting information Fig. S2). Prolonging the reaction time, the peak red-shifted from 258 to 272 nm and stabilized there, indicating that the reduction process was completed in 2 h at 80 ◦ C under nitrogen atmosphere. Simultaneously, the colour of the GO solution changed from light brown to black, indicating the alteration of the surface chemical compositions (Fig. 2 inset). It is noteworthy that the TPs also show a small peak cantered at 272 nm, which is just coincided with the characterized peak of the final TPG. Considering that there was no residual peak at this position occurred for the non-complete reduced TPG [Fig. 2(c) and (d)], we conclude that the intense peak of TPG at 272 nm was mainly due to the restored sp2 network of graphene. This result is consistent with the literature [44]. As a control, the pure GO water solution in the absence of TPs was also traced by UV–vis under the same reaction conditions. As shown in Fig. S3, there was no change in UV–vis absorbance happened with the time prolonging, confirming that the reduction action of the TPs. In order to verify the formation of stable TPG suspension, the UV–vis absorption spectra of the TPG water solutions at different
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Fig. 3. UV–vis absorption spectra of the TPG aqueous dispersions under different concentrations. The Inset shows the correlation of absorbance at 272 nm against relative concentration.
Fig. 5. XPS survey scans of C1s region of (a) GO and (b) TPG.
Fig. 4. FT-IR spectra of (a) GO and (b) TPG.
contentions were collected and shown in Fig. 3. It was shown that the characteristic absorbance at the peak of 272 nm enhanced with the increase of concentration, following a good linear relationship (Fig. 3 inset), indicating the good dispersibility of the TPG in water. For investigation of the chemical structural changes from GO to TPG, we performed FT-IR spectroscopic measurements. Fig. 4 shows FT-IR spectra of GO and TPG. After graphite was oxidized into GO, a strong and broad peak was observed in the range of 3129–3440 cm−1 that is attributed to the broad O H coupling vibrations of the C OH group and the O H stretching mode of intercalated water [48]. The characteristic vibration modes of C O (1718 cm−1 ) in carboxylic acid and carbonyl moieties, O H of carboxyl at 1395 cm−1 , and OH of carboxyl groups at 1162 cm−1 , and C O of epoxy or alkoxy at 1064 cm−1 were observed. The peak at 1625 cm−1 is attributed to C C stretching of unoxidized graphitic domains or contribution from the stretching deformation vibration of intercalated water. After the reduction by TPs, as shown in the spectrum of the TPG (Fig. 4b), the C O peak belonged to carboxyl group disappeared completely, and the peak at 1625 cm−1 remained and increased slightly, indicating the efficient reduction. However, the peaks corresponding to the oxygen containing groups almost remain, and the peak intensities increased obviously, which is very different from most of the documental reports about the GO
reduction. This result made us conclude that there were much of oxygen containing groups existed on the surface of TPG. It has been reported that although reduction of GO results in removing most of the surface oxygen, a small amount of hydroxyl and ether groups is always retained in the reduced GO [49]. Furthermore, careful observation provided us some new absorption bands in the fingerprint region as indicated by the arrows in Fig. 4b. These new peaks should originate from the adsorbed TPs. Therefore, we deduce that these oxygen containing groups should originated from two aspects. One is from the residuals of reduced GO, and the other is from the adsorbed hydroxyl-rich TPs. This conclusion was further confirmed by subsequent surface XPS detection and TGA. The XPS survey of C1s in GO and TPG also confirmed the alteration of the chemical compositions. The differences of the XPS spectra between GO and TPG in terms of elemental composition are summarized in Table 1S (Supporting information), which indicates a decrease of oxygen content in TPG compared with that of GO. As shown in Fig. 5a, after oxidation of the graphite into GO, two dominant peaks were observed at 284.7 and 286.8 eV. The peak at 284.7 eV is a characteristic peak for C C/C C bonding of graphite. The other peak at 286.8 eV shows a broad tail to higher binding energy region, which is attributed to emission from the oxidized carbon atoms in the GO. The three fitted peaks at 284.7, 286.8 and 287.9 eV can be assigned to the binding energies of carbon in C C/C C, C O (epoxy/hydroxyls), and C O (carbonyl/ ketone), respectively. For the TPG (Fig. 5b), the characteristic peak for C C/C C bonding of graphite was observed at the same binding energy position and the peak intensity increased obviously compared with the GO spectrum. However, the high binding energy peak for oxidized carbon atoms decreased dramatically in the
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Fig. 6. TGA curves of graphite, GO and TPG.
normalized peak areas. Even so, the peak intensity of the oxygencontaining groups is still higher than that of ordinary chemical reduction, indicating on the one hand the efficient reduction by TPs, on the other hand that much of oxygen groups existed on the surface of TPG although much less than that of GO. This result has also been reflected in the FTIR observation although the peak intensities increased obviously therein. The increase of intensity may ascribe to the non-covalent adsorption of TPs where the infrared absorption can be expressed comprehensively. However for the covalently linked oxygen groups on GO, the infrared absorption may be suppressed to some extent. Further evidence for one-pot reduction and modification of GO was provided by TGA based on the percentage of weight loss. Fig. 6 shows the thermograms of graphite, GO and TPG in the temperature range of 25–800 ◦ C. As demonstrated in Fig. 6, the GO showed a significant weight loss (about 30.8 wt%) in the temperature range of 145–280 ◦ C compared to the pristine graphite, which could be attribute to the decomposition of the surface oxygen-containing groups. The thermal stability of TPG was further examined and compared with that of GO. It was shown that there was almost no significant weight loss below 242 ◦ C (2.71 wt%) for the TPG. When the temperature is higher than 242 ◦ C, a considerable weight loss occurred until 394 ◦ C. The weight loss in this range should be attributed to the decomposition of the adsorbed TPs. Meanwhile, it should be noted that the onset decomposition temperature of TPG was higher than GO about 100 ◦ C. This may be ascribed to the removal of oxygen-containing groups due to the reduction by TPs, which decreased the surface defects and recovered the sp2 carbon network structure to some extent. According to the TGA traces, there were approximately 15 wt% TPs absorbed on the surface of the TPG, which contributed to its well water dispersibility. Notably, the weight loss of TPG is much lower than that of GO, revealing that the amounts of oxygen groups on the surface of GO is much than that of TPG. This result also provides a rational explanation to the conflict between FTIR and XPS. AFM is often employed to determine the thickness and lateral size of graphene. The AFM images and height profiles of GO and RGO were also recorded and shown in Fig. S4. It could be seen that the GO nanosheets show an average thickness of about 1.5 nm, which is consistent with the previously reported thickness for single- or double-layered GO [50]. The average thickness of TPG was found to be about 3.2 nm, which is thicker than that of GO. This increase in thickness is attributed to the adsorption of the TPs on the surface of TPG, which makes TPG well dispersed in aqueous solution.
Fig. 7. UV–vis spectra of GO, TPG and Ag-TPG hybrid in water.
Fig. 8. XRD pattern of (a) GO, (b) tea polyphenols reduced GO and (c) Ag-RGO nanohybrid showing intense peaks indexed to the cubic Ag structure [38◦ (1 1 1), 46◦ (2 0 0), 67◦ (2 2 0), and 77◦ (3 1 1)].
3.2. In situ green synthesis of Ag-TPG nanohybrid Surface adsorbed TPs on TPG could in situ reduce Ag ions to Ag nanoparticles and form homogeneous solution in water. The formation process was traced by using the UV–vis absorption according to the characteristic silver surface plasmon resonance (SPR) absorption band at ca. 420 nm. The typical UV–vis spectra of GO, TPG and Ag-TPG are shown in Fig. 7. Compared to the spectra of GO and TPG, the spectrum of TPG showed a characterized absorption peak of Ag nanoparticles centered at ca. 420 nm, confirming the formation of Ag-TPG nanohybrid. The XRD patterns of GO, TPG and Ag-TPG are shown in Fig. 8. As shown in Fig. 8a, a strong diffraction peak for exfoliated GO at 11.4◦ (0 0 1) with an interlay space (d-spacing) of 0.78 nm is observed. However, this peak vanished in the TPG spectrum confirming the extensive reduction of GO by TPs (Fig. 8b). The reduction of the Ag ions and the formation of Ag nanoparticles are confirmed by the XRD patterns of the resulting Ag-TPG nanohybrid. As shown in Fig.e 8c, four prominent peaks at 2 values of about 38◦ , 46◦ , 67◦ , and 77◦ , corresponding to the characteristic of face centered cubic
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Fig. 9. (a) TEM image of Ag-TPG dispersed in water, (b) corresponding HRTEM image and (c) EDX detection for the Ag-TPG hybrid (the Cu peaks come from the copper grid) and (d) electro-diffraction diagram of selected area for the formed Ag NPs from TEM.
crystalline Ag (JCPDS, Card no. 05-0681), i.e. (1 1 1), (2 0 0), (2 2 0) and (3 1 1) confirmed the success reduction of Ag ions. To further verify the uniform distribution of Ag nanoparticles, the Ag-TPG nanohybrid was characterized by TEM. As shown in Fig. 9a, Ag nanoparticles are uniformly dispersed on the clean graphene with a narrow size distribution. A representative HRTEM image is shown in Fig. 9b where the size of as-synthesized Ag nanoparticles is about 3–5 nm with a narrow distribution. The elemental composition of the synthesized Ag-TPG nanohybrid was further examined by energy dispersive X-ray (EDX) detection. As shown in Fig. 9c, the EDX spectrum showed a strong and typical optical absorption peak at approximately 3 keV, which confirmed the presence of Ag on graphene. The selected-area electron diffraction (SAED) pattern shown in Fig. 9d was obtained by directing the electron beam perpendicular to one of the nanoparticles. The SAED pattern shows obvious ring patterns mixed with several diffraction spots, underlying that the formed Ag nanoparticles were polycrystalline structure. For further investigating the changes of surface element composition upon the in situ reduction of Ag ions, XPS measurement for Ag-TPG was conducted and the C1s survey was shown in Fig. 10a. After in situ reduction of the Ag ions by the adsorbed TPs, the peaks of C C/C C, C O and C O keep almost consistent with that of TPG (Fig. 5b). The XPS data between TPG and Ag-TPG in terms of elemental composition are compared in Table 1S, which indicates that the in situ reduction of Ag ions has almost no effect on the surface oxygen-containing groups. Furthermore, the XPS measurement for
the Ag 3d regions of the Ag-TPG hybrid was also performed to elucidate the oxidation state of the Ag element as shown in Fig. 10b. The peaks at 368.4 and 374.4 eV in the Ag XPS spectrum are attributed to the binding energies of Ag 3d5/2 and Ag 3d3/2 , respectively. These binding energies are consistent with the reported data of reference metallic Ag (Ag0 ) [51], which further proves that the Ag-TPG hybrid has been fabricated successfully. 3.3. Catalytic study Metal nanoparticles-graphene nanohybrid systemes have become highly important in catalysis because of their large surface area, high electronic transport capacity, and extraordinary chemical stability. In this report we have chosen the reduction of 4-NP to 4-AP as a model system to evaluate the catalytic activity of the Ag-TPG catalylst. The catalytic process was monitored by UV–vis spectroscopy. It was seen that an absorption peak of 4-NP undergoes a red shift from 317 to 400 nm immediately upon the addition of aqueous solution of NaBH4 , corresponding to a significant change in solution color from light yellow to yellow-green due to formation of 4-nitrophenolate ion. In the absence of Ag-TPG catalyst, the absorption peak at 400 nm remained unaltered for a long duration, indicating that the NaBH4 itself cannot reduce 4-nitrophenolate ion without a catalyst. In addition, the TPG itself does not show any significant activity for 4-NP reduction (shown in Supporting information Fig. S5), and therefore, it can be regarded as an inert support.
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Fig. 10. (a) C1s and (b) Ag 3d XPS peaks of Ag-TPG nanohybrid.
In the presence of Ag-TPG catalyst and NaBH4 the 4-NP was reduced, and the intensity of the absorption peak at 400 nm decreased gradually with time and after about 12 min it fully disappeared (Fig. 11a). In the meantime, a new absorption peak appeared at 297 nm and increased progressively in intensity. This new peak is attributed to the typical absorption of 4-AP. This result suggests that the catalytic reduction of 4-NP exclusively yielded 4-AP, without any other side products. In the reduction process, the overall concentration of NaBH4 was 10 mM and 4-NP was 0.1 mM. Considering the much higher concentration of NaBH4 compared to that of 4-NP, it is reasonable to assume that the concentration of BH4 − remains constant during the reaction. In this context, pseudo-first-order kinetics could be used to evaluate the kinetic reaction rate of the current catalytic reaction, together with the UV–vis absorption data in Fig. 11a. The absorbance of 4-NP is proportional to its concentration in solution; the absorbance at time t (At ) and time t = 0 (A0 ) are equivalent to the concentration at time t (Ct ) and time t = 0 (C0 ). The rate constant (k) could be determined from the linear plot of ln (Ct /C0 ) versus reduction time in seconds. As expected, a good linear correlation of ln (Ct /C0 ) versus time was obtained as shown in Fig. 11b, whereby a kinetic reaction rate constant k is estimated to be 3.35 × 10−3 s−1 . This value is comparable to that of other metal catalysts for the reduction of 4-NP in the presence of NaBH4 [32]. Furthermore, we have also monitored the cycle stability of the Ag-TPG catalyst by recording the UV–vis absorbance spectra under the same time upon successive cycles. As shown in Fig. S6, the recovered Ag-TPG catalyst exhibited similar catalytic behaviors compared to the fresh one even after five recycles, indicating the stability of the Ag-TPG catalyst. This recyclability is comparable to or better than most of the reported data for the graphene supported metal catalysts [30,33,52]. Even so, there was still a slight and successive decrease of catalytic activity occurred dur-
Fig. 11. (a) UV–vis spectra of 0.1 mM 4-NP with 10 mM NaBH4 in the presence of Ag-TPG as catalyst and (b) plot of ln (Ct /C0 ) against the reaction time for pseudofirst-order reduction kinetics of 4-NP in the presence of excess NaBH4 (10 mM) in aqueous solutions.
ing the recycles of the catalyst. As shown in the inset of Fig. S6, nearly 5% of decrease could be observed after five times recycle. To examine the stability and the possible alteration in morphology or distribution of Ag nanoparticles after five recycles, the catalyst was recovered by simple centrifugation after the reaction. For each recycle, the centrifuge supernatant was collected and analyzed by Atomic Absorption Spectroscopy. It was shown that no silver metal signal during the course of the recycling reaction, confirming the stability of the Ag-TPG catalyst. TEM observation was conducted for the recovered catalyst and shown in Fig. S7. It could be observed that the recovered catalyst almost kept initial distribution state and spherical morphology even after five cycles of reaction. Even so, a slight aggregation of Ag nanoparticles could also be observed as indicated by arrows in the TEM image. This slight aggregation should be responsible for a decrease of catalyst surface area which in turn resulted in the slight and successive decline of catalytic activity as shown in the inset of Fig. S6. Summary In conclusion, a water dispersible Ag-TPG catalyst with uniformly distributed Ag nanoparticles on the surface of TPG was prepared by using a facile in situ green reduction strategy. Due to the dual roles of adsorbed TPs on the surface of graphene, the resulting Ag-TPG hybrid with uniformly distributed Ag nanoparticles could be well dispersed in water forming homogeneous solution.
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The uniform distribution of Ag nanoparticles, high adsorption ability of graphene and the effective electron transfer from graphene to Ag nanoparticles make the Ag-TPG nanohybrid an efficient catalyst in the reduction of 4-NP. This green and novel synthesis route provided a useful platform for the fabrication of graphene based metal nanoparticles heterogeneous catalyst, which might be able to find widespread use in a number of practical catalytic applications.
[20] [21]
[22]
[23]
Acknowledgments Financial support from the Key Science and Technology Foundation of Henan Province (112101110200) and the Science and Technology Foundation of Henan tobacco Corporation (HYKJ201307) is greatly acknowledged.
[24]
[25]
Appendix A. Supplementary data [26]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2015.09. 015.
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
In situ green synthesis of Ag nanoparticles on tea polyphenols-modified graphene and their catalytic reduction activity of 4-nitrophenol Zhimin Wang ∗ , Cuilian Xu, Xin Li, Zhaohui Liu School of science, Henan Agricultural University, Zhengzhou 450002, People’s Republic of China
h i g h l i g h t s
g r a p h i c a l
• Tea polyphenols were used to reduce
A water dispersible Ag-TPG catalyst with uniformly distributed Ag nanoparticles on the surface of tea polyphenols modified graphene, exhibiting enhanced catalytic activity in 4-nitrophenol reduction, was synthesized and characterized.
and stabilize graphene oxide. • Uniformly distributed Ag nanoparticles could be in situ formed on the tea polyphenols modified surface. • This well dispersed Ag-TPG hybrid exhibited enhanced catalytic activity in 4-nitrophenol reduction.
a r t i c l e
i n f o
Article history: Received 22 May 2015 Received in revised form 24 August 2015 Accepted 4 September 2015 Available online 8 September 2015 Keywords: Graphene Silver nanoparticles Tea polyphenols Green synthesis Catalysis
a b s t r a c t In this work, a complete green synthesis approach to reduced graphene oxide-Ag nanoparticles hybrid by using tea polyphenol as both reducing and stabilizing agent is reported. On the one hand, tea polyphenols can efficiently reduce graphene oxide and adsorb on the surface of the reduced graphene oxide. On the other hand, the surface adsorbed tea polyphenols can further in situ reduce Ag ions to Ag nanoparticles and stabilize them. The structure and physicochemical properties of the resulting nanohybrid were characterized in detail by UV–vis spectroscopy, Fourier transformation infrared spectroscopy, Xray diffraction, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. The obtained results indicated that Ag nanoparticles distributed uniformly on the surface of the functionalized graphene. Meanwhile, the so-formed hybrid material could be well dispersed in water forming a homogeneous dispersion due to the stabilization of tea polyphenols. This nanohybrid, combining the unique catalytic properties of Ag nanoparticles with the excellent adsorption and electron transfer ability of graphene, exhibited enhanced catalytic activity toward the reduction of 4-nitrophenol by NaBH4 . © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene and metal nanoparticles represent two kinds of important components in the current nanoscience and nano-
∗ Corresponding author. Fax: +86 371 6355 8139. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.colsurfa.2015.09.015 0927-7757/© 2015 Elsevier B.V. All rights reserved.
a b s t r a c t
technology [1,2]. Due to their unique structural characters and fascinating properties, graphene and noble metal nanoparticles have been explored extensively for different applications, such as electronic device [3–5], energy conversion and storage [6,7], sensing [8,9], catalytic [10,11], and several others [12–14]. In order to exploit their properties and applications broadly, successful preparation of these materials is an essential prerequisite. Among the documented strategies, chemical reduction has been proved to be
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a promising route for the synthesis of both graphene and metal nanoparticles owing to its facile synthetic nature, controlled, scalable, and reproducible manner [15,16]. In this strategy, there are two major challenges involved in the whole process. The first one is the rational selection of appropriate reagents or method for efficient reduction of graphene oxide (GO) or metal precursor without sacrificing the environment and others, and the second one is to avoid aggregation after reduction of GO or metal precursor owing to the strong van der Waals attractive forces between the reduced moieties. Apart from the single application of graphene and metal nanoparticles, hybridizing the two components and exploring their collective properties has recently aroused wide interests in various areas ranging from bio-applications [17], sensing [18,19], to energy conversion [20,21] and catalysis [22]. In catalytic studies, graphene is often used as a support due to its large surface area (calculated value, 2630 m2 g−1 ) and tunable surface properties [23]. It is believed that efficient capping and protection of catalyst materials, excellent adsorb ability and effective separation and transportation of charge carrier or electron play collective role in activity enhancement of the catalyst [24]. To date, big progress has been made in the synthesis of graphene based hybrids with metal-nanoparticle catalysts and these hybrids showed improved performance in different catalytic reactions [25–31]. However, developing homogeneous composites with a uniform distribution of catalyst and controlling the catalyst loading remains a major problem due to the aggregation effect of graphene and the weak interaction between graphene and the catalyst [23]. To this end, both electrostatic stabilization and chemical functionalization have proven to be useful in suppressing aggregation and improving the adhesion of catalyst with graphene [32]. Nevertheless, the surface-modification process usually suffers from limited scalability, time-consuming and environmental problems. Furthermore, the used reducing agents are always highly toxic, which also limited its practical application to some extents. In this respect, development of green approaches for synthesis of graphene and metal nanoparticle nanohybrids by using environmentally benign materials is highly desirable. Inspired by the reductive and adhesive properties of dopamine, plant polyphenols, which possess similar structure to dopamine, have been widely exploited due to their low cost and bioavailability [28,33]. Among these plant polyphenols, tea polyphenols (TPs) from green tea have recently attracted additional broad interest in various fields ranging from versatile surface coating, nanomaterials and nanocomposites [34–36]. TPs are mainly consisted of epicatechin (EC), epicatechin gallate (ECG), epigallocatechin (EGC) and epigallocatechin gallate (EGCG), in which the EGCG makes up about 50–60% of the total TPs [37]. The multiple pyrogallol and catechol structures of these compounds (Supporting information Fig. S1) make TPs highly water-soluble and excellent antioxidant properties [38]. To date, gold, silver, palladium, and iron nanoparticles have been synthesized and stabilized by using TPs as both reducing and stabilizing agents [39–43]. Moreover, it is also demonstrated that TPs could disperse and stabilize carbon nanotube and boron nitride nanotube due to their strong surface adhesion ability [35,36]. Very recently, TPs have also been shown that they could one-pot reduce and modify GO and form a dispersible TP-graphene composite in water [44]. Taken together, combining the water solubility, low toxicity, and biodegradability with their reductive and adhensive properties, TPs should be an appropriate alternative for green synthesis of metal nanoparticles and graphene hybrid. In view of multiple characters of TPs mentioned above, we herein report a facile green synthetic approach to Ag nanoparticlesgraphene hybrid by using TPs as both reducing and stabilizing agent. At first, TPs can efficiently reduce GO and adsorb on the surface of the reduced GO. Then, the adsorbed TPs can further in situ reduce Ag ions to Ag nanoparticles and stabilize them.
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Because the amounts and distribution of the surface adsorbed TPs are predetermined, well distributed Ag nanoparticles could be in situ synthesized on the surface of TPs modified graphene. Importantly, the resultant Ag-TPG nanohybrid could be well dispersed in water due to the stabilizing function of TPs. Furthermore, it was demonstrated that this water dispersible Ag-TPG nanohybrid could function as an effective catalyst to activate the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of excess NaBH4 following pseudo-first-order kinetics, otherwise unfeasible if only the strong reducing agent NaBH4 was employed. 2. Experimental 2.1. Materials Graphite powder (AP, 325 meshes), AgNO3 and 4-NP were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. The commercial green tea (Xinyang Maojian tea) was purchased from local market and no further treatment was performed before use. All reagents, such as NaNO3 , H2 SO4 , H2 O2 , HCl, KMnO4 , ethanol, NaBH4 , etc, were reagent grade and used as received without further purification. 2.2. Green synthesis of TPs-modified graphene (TPG) The GO was synthesized by oxidizing the graphite powder in a mixture of concentrated sulfuric acid and KMnO4 based on a modified Hummers method as originally presented by Kovtyukhova and colleagues [45,46]. For the tea solution, 5 g of green tea leaf was added in 250 mL of deionized (DI) water and extracted at 80 ◦ C for 30 min under stirring. After extraction, the mixture was centrifuged at 8000 rpm for 30 min. The resultant supernatant was collected, filtered, and then stored at −20 ◦ C before use. The total phenolic content of the aqueous green tea extracts was determined by the Folin–Ciocalteu method using gallic acid as a standard phenolic compound and the result was expressed as mg L−1 gallic acid equivalent (GAE) [47]. The total phenolic content of the prepared tea solution determined by this method is 2835 mg L−1 GAE. We can determine that the extracted polyphenolic compounds from green tea are ca. 14.2 wt%, which is much lower than the average content of TPs (20–30 wt%) in green tea, indicating that the extracted TPs are more soluble in water. For the preparation of TPG, 50 mg GO was added in 100 mL preprepared tea solution and sonicated for 5 min. The reaction mixture was then refluxed at 80 ◦ C with stirring under nitrogen atmosphere for 2 h. The color of the mixture changed from light brown to black, indicating the reduction of graphene oxide, which was stable. After cooling to room temperature, the TPG was isolated by centrifugation. The precipitate was washed repeatedly with DI water in consecutive washing-centrifugation cycles. Ultrasonic treatment was used in every cycle in order to re-disperse the TPG and remove the excess TPs. After washing, the sample was dried at 60 ◦ C in vacuum for 24 h. 2.3. In situ green synthesis of Ag-TPG nanohybrid For the synthesis of Ag-TPG nanohybrid, 0.5 mL 0.1 M of AgNO3 solution was added into 10 mL 0.5 mg mL−1 of TPG aqueous solution and vigorously stirred for 20 min. The reaction mixture was then allowed to settle at room temperature with mild stirring overnight. The resulting slurry was filtered, washed thoroughly with deionized water and then the obtained Ag-TPG hybrid catalyst was dried in a vacuum oven before use. The whole synthesis procedure for the preparation of Ag-TPG nanohybrid was presented in Fig. 1.
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Fig. 1. Schematic process of green reduction of GO and subsequent in situ reduction of Ag ions to form Ag-TPG nanohybrid.
2.4. Materials characterization The experimental process was characterized comprehensively by a variety of techniques. The UV–vis spectra were acquired on a Shimadzu UV-1601PC spectrophotometer. The wavelength was set in the range of 200–800 nm for all the measurements. Transmission electron microscopy (TEM) observations were conducted on a FEI Tecnai G2 F20 microscope with a field-emission gun operating at 200 kV. The sample was prepared by dropping the water solution of Ag-TPG nanohybrid onto copper grids coated with Formvar and carbon film. Wide angle X-ray diffraction (WXRD) measurements were made using a Bruker D8 ADVANCE X-ray diffractometer with Cu K␣ (1.541 Å) radiation (40 kV, 30 mA). Powder samples were mounted on a sample holder and scanned with a step size of 0.01◦ between 2 = 3◦ and 90◦ . Fourier transformation infrared spectroscopy (FTIR) data were obtained using a Nicolet Avator 230 spectrometer. The samples were prepared with KBr. Modification efficiency and the amounts of TPs adsorbed on the graphene were determined using a NETZSCH4 TGA instrument at a 50 mL min−1 flowing rate of nitrogen atmosphere. The temperature was increased from 20 to 900 ◦ C at a rate of 10 ◦ C min−1 . The X-ray photoelectron spectroscopy (XPS) data were obtained with VG Multilab 2000 electron spectrometer from Thermo Scientific using 300 W Al K␣ radiation. The base pressure was about 3 × 10−9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. 2.5. Catalysis study The catalytic reduction reaction of 4-NP was performed in aqueous solution in a standard quartz cell. The overall concentrations of 4-NP and NaBH4 were 0.1 and 10 mM, respectively. In a typical catalytic reaction, 2.9 mL of aqueous solution of 4-NP and 0.1 mL of aqueous NaBH4 solution were mixed together and then purged with nitrogen for 10 min to remove the dissolved oxygen. Next, 2 mL of aqueous Ag-TPG suspensions (0.5 mg mL−1 ) purged with nitrogen was added to the reaction mixture under constant magnetic stirring. Immediately after that, the solution was transferred to a standard quartz cell, and the UV–vis absorption spectra were recorded with a time interval of 120 s in a scanning range of 200–700 nm at ambient temperature. After completion of the reaction the catalyst was recovered by centrifugation. The precipitate was washed repeatedly with DI water in consecutive washing cycles. Ultrasonic treatment was used in every cycle in order to re-disperse the catalyst and remove adsorbed impurities. After washing, the catalyst was used directly for recycling test. After each recycle, the centrifuge supernatant was collected and detected by Atomic absorption spectroscopy to determine the content of Ag metal. 3. Results and discussion 3.1. Green reduction and modification of GO by tea polyphenols As shown in Fig. 2 inset, the as-prepared GO could readily resolve in tea water forming a bright brown transparent solution. The
Fig. 2. UV–vis traces the green reduction of GO to TPG by reflux in tea water at 80 ◦ C under nitrogen atmosphere: (a) GO; (b) tea water; (c) reflux for 30 min; (d) reflux for 1 h and (e) reflux for 2 h. The inset shows the photographs observation of the reduction process.
influence of temperature on the reduction process was investigated. It was found that 80 ◦ C was the appropriate temperature to reduce GO. When the temperature was lower than 80 ◦ C, the reducing effect was weakened considerably as indicated by detecting the characterized colour change from GO to graphene. The conversion of GO to reduced GO was monitored by recording the UV–vis absorption spectra of TPG as a function of time. For detecting the reaction progress, a given amount of reaction solution was extracted in a regular interval. The extracted solution was filtered and washed thoroughly, and then re-dispersed in water to form a dilute solution for UV–vis examination. As shown in Fig. 2, GO shows a peak at 228 nm and a shoulder at 300 nm. As the reaction progressed, the characterized peaks of GO at 228 nm and 300 nm vanished completely, and a new characterized peak belong to the reduced GO established gradually (shown in Supporting information Fig. S2). Prolonging the reaction time, the peak red-shifted from 258 to 272 nm and stabilized there, indicating that the reduction process was completed in 2 h at 80 ◦ C under nitrogen atmosphere. Simultaneously, the colour of the GO solution changed from light brown to black, indicating the alteration of the surface chemical compositions (Fig. 2 inset). It is noteworthy that the TPs also show a small peak cantered at 272 nm, which is just coincided with the characterized peak of the final TPG. Considering that there was no residual peak at this position occurred for the non-complete reduced TPG [Fig. 2(c) and (d)], we conclude that the intense peak of TPG at 272 nm was mainly due to the restored sp2 network of graphene. This result is consistent with the literature [44]. As a control, the pure GO water solution in the absence of TPs was also traced by UV–vis under the same reaction conditions. As shown in Fig. S3, there was no change in UV–vis absorbance happened with the time prolonging, confirming that the reduction action of the TPs. In order to verify the formation of stable TPG suspension, the UV–vis absorption spectra of the TPG water solutions at different
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Fig. 3. UV–vis absorption spectra of the TPG aqueous dispersions under different concentrations. The Inset shows the correlation of absorbance at 272 nm against relative concentration.
Fig. 5. XPS survey scans of C1s region of (a) GO and (b) TPG.
Fig. 4. FT-IR spectra of (a) GO and (b) TPG.
contentions were collected and shown in Fig. 3. It was shown that the characteristic absorbance at the peak of 272 nm enhanced with the increase of concentration, following a good linear relationship (Fig. 3 inset), indicating the good dispersibility of the TPG in water. For investigation of the chemical structural changes from GO to TPG, we performed FT-IR spectroscopic measurements. Fig. 4 shows FT-IR spectra of GO and TPG. After graphite was oxidized into GO, a strong and broad peak was observed in the range of 3129–3440 cm−1 that is attributed to the broad O H coupling vibrations of the C OH group and the O H stretching mode of intercalated water [48]. The characteristic vibration modes of C O (1718 cm−1 ) in carboxylic acid and carbonyl moieties, O H of carboxyl at 1395 cm−1 , and OH of carboxyl groups at 1162 cm−1 , and C O of epoxy or alkoxy at 1064 cm−1 were observed. The peak at 1625 cm−1 is attributed to C C stretching of unoxidized graphitic domains or contribution from the stretching deformation vibration of intercalated water. After the reduction by TPs, as shown in the spectrum of the TPG (Fig. 4b), the C O peak belonged to carboxyl group disappeared completely, and the peak at 1625 cm−1 remained and increased slightly, indicating the efficient reduction. However, the peaks corresponding to the oxygen containing groups almost remain, and the peak intensities increased obviously, which is very different from most of the documental reports about the GO
reduction. This result made us conclude that there were much of oxygen containing groups existed on the surface of TPG. It has been reported that although reduction of GO results in removing most of the surface oxygen, a small amount of hydroxyl and ether groups is always retained in the reduced GO [49]. Furthermore, careful observation provided us some new absorption bands in the fingerprint region as indicated by the arrows in Fig. 4b. These new peaks should originate from the adsorbed TPs. Therefore, we deduce that these oxygen containing groups should originated from two aspects. One is from the residuals of reduced GO, and the other is from the adsorbed hydroxyl-rich TPs. This conclusion was further confirmed by subsequent surface XPS detection and TGA. The XPS survey of C1s in GO and TPG also confirmed the alteration of the chemical compositions. The differences of the XPS spectra between GO and TPG in terms of elemental composition are summarized in Table 1S (Supporting information), which indicates a decrease of oxygen content in TPG compared with that of GO. As shown in Fig. 5a, after oxidation of the graphite into GO, two dominant peaks were observed at 284.7 and 286.8 eV. The peak at 284.7 eV is a characteristic peak for C C/C C bonding of graphite. The other peak at 286.8 eV shows a broad tail to higher binding energy region, which is attributed to emission from the oxidized carbon atoms in the GO. The three fitted peaks at 284.7, 286.8 and 287.9 eV can be assigned to the binding energies of carbon in C C/C C, C O (epoxy/hydroxyls), and C O (carbonyl/ ketone), respectively. For the TPG (Fig. 5b), the characteristic peak for C C/C C bonding of graphite was observed at the same binding energy position and the peak intensity increased obviously compared with the GO spectrum. However, the high binding energy peak for oxidized carbon atoms decreased dramatically in the
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Fig. 6. TGA curves of graphite, GO and TPG.
normalized peak areas. Even so, the peak intensity of the oxygencontaining groups is still higher than that of ordinary chemical reduction, indicating on the one hand the efficient reduction by TPs, on the other hand that much of oxygen groups existed on the surface of TPG although much less than that of GO. This result has also been reflected in the FTIR observation although the peak intensities increased obviously therein. The increase of intensity may ascribe to the non-covalent adsorption of TPs where the infrared absorption can be expressed comprehensively. However for the covalently linked oxygen groups on GO, the infrared absorption may be suppressed to some extent. Further evidence for one-pot reduction and modification of GO was provided by TGA based on the percentage of weight loss. Fig. 6 shows the thermograms of graphite, GO and TPG in the temperature range of 25–800 ◦ C. As demonstrated in Fig. 6, the GO showed a significant weight loss (about 30.8 wt%) in the temperature range of 145–280 ◦ C compared to the pristine graphite, which could be attribute to the decomposition of the surface oxygen-containing groups. The thermal stability of TPG was further examined and compared with that of GO. It was shown that there was almost no significant weight loss below 242 ◦ C (2.71 wt%) for the TPG. When the temperature is higher than 242 ◦ C, a considerable weight loss occurred until 394 ◦ C. The weight loss in this range should be attributed to the decomposition of the adsorbed TPs. Meanwhile, it should be noted that the onset decomposition temperature of TPG was higher than GO about 100 ◦ C. This may be ascribed to the removal of oxygen-containing groups due to the reduction by TPs, which decreased the surface defects and recovered the sp2 carbon network structure to some extent. According to the TGA traces, there were approximately 15 wt% TPs absorbed on the surface of the TPG, which contributed to its well water dispersibility. Notably, the weight loss of TPG is much lower than that of GO, revealing that the amounts of oxygen groups on the surface of GO is much than that of TPG. This result also provides a rational explanation to the conflict between FTIR and XPS. AFM is often employed to determine the thickness and lateral size of graphene. The AFM images and height profiles of GO and RGO were also recorded and shown in Fig. S4. It could be seen that the GO nanosheets show an average thickness of about 1.5 nm, which is consistent with the previously reported thickness for single- or double-layered GO [50]. The average thickness of TPG was found to be about 3.2 nm, which is thicker than that of GO. This increase in thickness is attributed to the adsorption of the TPs on the surface of TPG, which makes TPG well dispersed in aqueous solution.
Fig. 7. UV–vis spectra of GO, TPG and Ag-TPG hybrid in water.
Fig. 8. XRD pattern of (a) GO, (b) tea polyphenols reduced GO and (c) Ag-RGO nanohybrid showing intense peaks indexed to the cubic Ag structure [38◦ (1 1 1), 46◦ (2 0 0), 67◦ (2 2 0), and 77◦ (3 1 1)].
3.2. In situ green synthesis of Ag-TPG nanohybrid Surface adsorbed TPs on TPG could in situ reduce Ag ions to Ag nanoparticles and form homogeneous solution in water. The formation process was traced by using the UV–vis absorption according to the characteristic silver surface plasmon resonance (SPR) absorption band at ca. 420 nm. The typical UV–vis spectra of GO, TPG and Ag-TPG are shown in Fig. 7. Compared to the spectra of GO and TPG, the spectrum of TPG showed a characterized absorption peak of Ag nanoparticles centered at ca. 420 nm, confirming the formation of Ag-TPG nanohybrid. The XRD patterns of GO, TPG and Ag-TPG are shown in Fig. 8. As shown in Fig. 8a, a strong diffraction peak for exfoliated GO at 11.4◦ (0 0 1) with an interlay space (d-spacing) of 0.78 nm is observed. However, this peak vanished in the TPG spectrum confirming the extensive reduction of GO by TPs (Fig. 8b). The reduction of the Ag ions and the formation of Ag nanoparticles are confirmed by the XRD patterns of the resulting Ag-TPG nanohybrid. As shown in Fig.e 8c, four prominent peaks at 2 values of about 38◦ , 46◦ , 67◦ , and 77◦ , corresponding to the characteristic of face centered cubic
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Fig. 9. (a) TEM image of Ag-TPG dispersed in water, (b) corresponding HRTEM image and (c) EDX detection for the Ag-TPG hybrid (the Cu peaks come from the copper grid) and (d) electro-diffraction diagram of selected area for the formed Ag NPs from TEM.
crystalline Ag (JCPDS, Card no. 05-0681), i.e. (1 1 1), (2 0 0), (2 2 0) and (3 1 1) confirmed the success reduction of Ag ions. To further verify the uniform distribution of Ag nanoparticles, the Ag-TPG nanohybrid was characterized by TEM. As shown in Fig. 9a, Ag nanoparticles are uniformly dispersed on the clean graphene with a narrow size distribution. A representative HRTEM image is shown in Fig. 9b where the size of as-synthesized Ag nanoparticles is about 3–5 nm with a narrow distribution. The elemental composition of the synthesized Ag-TPG nanohybrid was further examined by energy dispersive X-ray (EDX) detection. As shown in Fig. 9c, the EDX spectrum showed a strong and typical optical absorption peak at approximately 3 keV, which confirmed the presence of Ag on graphene. The selected-area electron diffraction (SAED) pattern shown in Fig. 9d was obtained by directing the electron beam perpendicular to one of the nanoparticles. The SAED pattern shows obvious ring patterns mixed with several diffraction spots, underlying that the formed Ag nanoparticles were polycrystalline structure. For further investigating the changes of surface element composition upon the in situ reduction of Ag ions, XPS measurement for Ag-TPG was conducted and the C1s survey was shown in Fig. 10a. After in situ reduction of the Ag ions by the adsorbed TPs, the peaks of C C/C C, C O and C O keep almost consistent with that of TPG (Fig. 5b). The XPS data between TPG and Ag-TPG in terms of elemental composition are compared in Table 1S, which indicates that the in situ reduction of Ag ions has almost no effect on the surface oxygen-containing groups. Furthermore, the XPS measurement for
the Ag 3d regions of the Ag-TPG hybrid was also performed to elucidate the oxidation state of the Ag element as shown in Fig. 10b. The peaks at 368.4 and 374.4 eV in the Ag XPS spectrum are attributed to the binding energies of Ag 3d5/2 and Ag 3d3/2 , respectively. These binding energies are consistent with the reported data of reference metallic Ag (Ag0 ) [51], which further proves that the Ag-TPG hybrid has been fabricated successfully. 3.3. Catalytic study Metal nanoparticles-graphene nanohybrid systemes have become highly important in catalysis because of their large surface area, high electronic transport capacity, and extraordinary chemical stability. In this report we have chosen the reduction of 4-NP to 4-AP as a model system to evaluate the catalytic activity of the Ag-TPG catalylst. The catalytic process was monitored by UV–vis spectroscopy. It was seen that an absorption peak of 4-NP undergoes a red shift from 317 to 400 nm immediately upon the addition of aqueous solution of NaBH4 , corresponding to a significant change in solution color from light yellow to yellow-green due to formation of 4-nitrophenolate ion. In the absence of Ag-TPG catalyst, the absorption peak at 400 nm remained unaltered for a long duration, indicating that the NaBH4 itself cannot reduce 4-nitrophenolate ion without a catalyst. In addition, the TPG itself does not show any significant activity for 4-NP reduction (shown in Supporting information Fig. S5), and therefore, it can be regarded as an inert support.
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Fig. 10. (a) C1s and (b) Ag 3d XPS peaks of Ag-TPG nanohybrid.
In the presence of Ag-TPG catalyst and NaBH4 the 4-NP was reduced, and the intensity of the absorption peak at 400 nm decreased gradually with time and after about 12 min it fully disappeared (Fig. 11a). In the meantime, a new absorption peak appeared at 297 nm and increased progressively in intensity. This new peak is attributed to the typical absorption of 4-AP. This result suggests that the catalytic reduction of 4-NP exclusively yielded 4-AP, without any other side products. In the reduction process, the overall concentration of NaBH4 was 10 mM and 4-NP was 0.1 mM. Considering the much higher concentration of NaBH4 compared to that of 4-NP, it is reasonable to assume that the concentration of BH4 − remains constant during the reaction. In this context, pseudo-first-order kinetics could be used to evaluate the kinetic reaction rate of the current catalytic reaction, together with the UV–vis absorption data in Fig. 11a. The absorbance of 4-NP is proportional to its concentration in solution; the absorbance at time t (At ) and time t = 0 (A0 ) are equivalent to the concentration at time t (Ct ) and time t = 0 (C0 ). The rate constant (k) could be determined from the linear plot of ln (Ct /C0 ) versus reduction time in seconds. As expected, a good linear correlation of ln (Ct /C0 ) versus time was obtained as shown in Fig. 11b, whereby a kinetic reaction rate constant k is estimated to be 3.35 × 10−3 s−1 . This value is comparable to that of other metal catalysts for the reduction of 4-NP in the presence of NaBH4 [32]. Furthermore, we have also monitored the cycle stability of the Ag-TPG catalyst by recording the UV–vis absorbance spectra under the same time upon successive cycles. As shown in Fig. S6, the recovered Ag-TPG catalyst exhibited similar catalytic behaviors compared to the fresh one even after five recycles, indicating the stability of the Ag-TPG catalyst. This recyclability is comparable to or better than most of the reported data for the graphene supported metal catalysts [30,33,52]. Even so, there was still a slight and successive decrease of catalytic activity occurred dur-
Fig. 11. (a) UV–vis spectra of 0.1 mM 4-NP with 10 mM NaBH4 in the presence of Ag-TPG as catalyst and (b) plot of ln (Ct /C0 ) against the reaction time for pseudofirst-order reduction kinetics of 4-NP in the presence of excess NaBH4 (10 mM) in aqueous solutions.
ing the recycles of the catalyst. As shown in the inset of Fig. S6, nearly 5% of decrease could be observed after five times recycle. To examine the stability and the possible alteration in morphology or distribution of Ag nanoparticles after five recycles, the catalyst was recovered by simple centrifugation after the reaction. For each recycle, the centrifuge supernatant was collected and analyzed by Atomic Absorption Spectroscopy. It was shown that no silver metal signal during the course of the recycling reaction, confirming the stability of the Ag-TPG catalyst. TEM observation was conducted for the recovered catalyst and shown in Fig. S7. It could be observed that the recovered catalyst almost kept initial distribution state and spherical morphology even after five cycles of reaction. Even so, a slight aggregation of Ag nanoparticles could also be observed as indicated by arrows in the TEM image. This slight aggregation should be responsible for a decrease of catalyst surface area which in turn resulted in the slight and successive decline of catalytic activity as shown in the inset of Fig. S6. Summary In conclusion, a water dispersible Ag-TPG catalyst with uniformly distributed Ag nanoparticles on the surface of TPG was prepared by using a facile in situ green reduction strategy. Due to the dual roles of adsorbed TPs on the surface of graphene, the resulting Ag-TPG hybrid with uniformly distributed Ag nanoparticles could be well dispersed in water forming homogeneous solution.
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The uniform distribution of Ag nanoparticles, high adsorption ability of graphene and the effective electron transfer from graphene to Ag nanoparticles make the Ag-TPG nanohybrid an efficient catalyst in the reduction of 4-NP. This green and novel synthesis route provided a useful platform for the fabrication of graphene based metal nanoparticles heterogeneous catalyst, which might be able to find widespread use in a number of practical catalytic applications.
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Acknowledgments Financial support from the Key Science and Technology Foundation of Henan Province (112101110200) and the Science and Technology Foundation of Henan tobacco Corporation (HYKJ201307) is greatly acknowledged.
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Appendix A. Supplementary data [26]
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2015.09. 015.
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