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Reduced Graphene Oxide as a Metal-Free Carbocatalyst for Polymerization of 1-Naphthylamine [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) Reduced graphene oxide (RGO) is used as a carbocatalyst for the electrochemical polymerization and in situ polymerization of 1-naphthylamine (1-NPA). Compared with graphene oxide (GO), RGO shows a much higher heterogeneous catalytic activity, not only on a solid electrode surface, but also in the liquid phase. The catalytic procedure is monitored by cyclic voltammetry (CV) and UV/Vis absorption spectroscopy. Keywords : 1-naphthylamine · carbocatalyst · electrochemistry · graphene · polymerization · reduced graphene oxide Introduction Graphene is the name given to a two-dimensional sheet of sp2-hybridized carbon.[1] Its extended honeycomb network is the basic building block for a long-range p-conjugation structure, which yields extraordinary thermal, mechanical, and electrical properties. The use of graphene-based materials as catalyst supports for the fabrication of novel hybrid materials is a relatively new area with outstanding potential.[2-5] Although graphene-based hybrids possess high catalytic activities, in most cases, the catalytic activity is generated from the supported materials, and not from the carbon. Recently, the concept of "carbocatalyst" was proposed by Bielawski et al. , who identified that graphite oxide (GO) can be used as a powerful catalyst under mild conditions in the liquid phase for various synthetic reactions such as the generation of aldehydes or ketones from various alcohols, alkenes, and alkynes, and so on.[6-15] Furthermore, the authors showed that GO can be used as a catalyst for the preparation of polymeric materials, for example, in the dehydrative polymerization of benzyl alcohol and polymerization of olefins.[16, 17] As a simple and inexpensive catalyst, GO has advantages such as its metal-free reactivity and facile recovery from the reaction media by filtration. Bielawski et al. suggested that the presence of surface-bound oxygen-containing functionalities plays an important role in the observed catalytic properties of GO (GO tends to be highly acidic and strongly oxidizing). They found that no reactivity was displayed if hydrazine-reduced graphene oxide (RGO) or natural flake graphite was used in the above reaction. Subsequently, sulfonated graphene oxide was also demonstrated to be a rapid and water-tolerant carbocatalyst for the high-temperature production of furfural in water and for acid-catalyzed liquid reactions.[18, 19] Surface-area analysis and reaction results suggested that the aryl sulfonic acid groups were the key active sites for the reaction. On the other hand, nitrogen- and boron-doped graphene have been reported recently as highly active catalysts in the absence of any transition metal for the activation of dioxygen in the aerobic oxidation of benzylic compounds, cycloalkanes, and styrene under solvent-free conditions.[20] In addition, the electrocatalytic activity of RGO was proposed by Mu's group.[21] Compared with GO and GO derivatives, it appears that RGO can hardly be used as a catalyst in liquid-phase reactions. Ding Ma et al. first proved that RGO can catalyze the hydrogenation of nitrobenzene at room temperature.[22] Although GO possesses a similar high catalytic activity to RGO, it was reduced in situ by hydrazine hydrate, which was used in the hydrogenation reaction. The true catalyst here is RGO rather than GO. The authors proposed that the unsaturated carbon atoms at the edges and defects of RGO might be catalytic active centers for nitrobenzene. Recently, we compared the catalytic activities of GO and RGO in the radical oxidative polymerization of 3-aminophenylboronic acid to form conductive polymers.[23] Unlike the reaction of small molecules, both GO and RGO showed a catalytic effect on this in situ polymerization. Surprisingly, the results also indicated that RGO has a much higher catalytic activity than GO, not only in the electrochemical polymerization process, but also in the in situ polymerization process. Considering the fact that GO has many more defects than RGO, there must be a totally different reason for the catalytic effect of RGO in the formation of macromolecules. We suggested that both GO and RGO could speed up the polymerization process by concentrating the monomer and oxidant simultaneously on their large surface areas. On the other hand, the unique electronic property of RGO allows it to donate electrons to the monomer to shorten further the formation time of the diimine. To prove the catalytic mechanism of RGO further, we exploited the inherent catalytic properties of RGO to facilitate the polymerization of 1-naphthylamine (1-NPA), with GO used as a reference. 1-NPA is a fused-ring aniline derivative, which is more difficult to polymerize than the pristine aniline owing to the spatial resistance, and the poor processability of poly(1-naphthylamine) (PNPA) restricts its application in the fields of chemical sensors, electrochemical sensors, and corrosion-protective smart coatings. The experimental results indicate that RGO exhibits an excellent catalytic activity for both the electrochemical and in situ polymerizations of 1-NPA. We suppose that our work will be of interest for the future rational design of polymerization systems using RGO as a new catalyst. Results and Discussion The RGO and GO prepared in this work were first characterized by XRD and compared with pristine graphite powder (Figure 1). Graphite powder shows a sharp (002) peak at 26.48 with a typical d-spacing of 3.37 ^. GO exhibits a diffraction peak (002) at a 2q value of 11.368, corresponding to a d-spacing of 7.82 ^, which suggests that the graphite has been successfully oxidized by Hummers' method. The corresponding peak disappears after reduction, indicating the successful formation of RGO. GO exhibited a conductivity of 4.5 ^10^5 Scm^1, and RGO showed an increased conductivity of 1.7 S cm^1. Likewise, elemental analyses revealed the deoxygenation of GO to form RGO. The C/O atomic ratio of RGO was found to be 2.94, which is larger than the value of 1.03 found for the as-prepared GO (Table 1). Figure 2 shows the FTIR absorption spectra of GO and RGO. The spectrum of GO is shifted downward for easy viewing. The spectrum of GO shows the O^H(n(carboxyl)) peak at about 1380 cm^1,andO^H (broad coupling n(O^H)) at about 3410 cm^1 originated from the carboxylic acid group. The characteristic bands associated with C^O stretching at 1050 cm^1 and the stretching vibration of the carboxyl group at 1730 cm^1 are also observed in the spectrum. These characteristics indicate the successful grafting of epoxide and carboxyl groups on the graphite powers to form GO after the oxidation process.[24, 25] The formation of RGO by chemical reduction of GO is evidenced by the disappearance of the stretching vibration of the carboxyl group at 1730 cm^1. The TEM images (Figure 3) of GO and RGO clearly show numerous thin flake structures, which confirming that GO and RGO are not aggregated. First, we explored the catalytic effect of RGO on the electrochemical polymerization of 1-NPA. Figure 4 shows the first five cycles of the electrochemical polymerization of 1-NPA compared for the glassy carbon working electrode (GCE), GO-deposited GCE (GO/GCE), RGO-deposited GCE (RGO/GCE). On the GCE (Figure 4 a), an oxidation peak at 0.792 V appears on the first cycle, which is caused by the oxidation of 1-NPA resulting in the formation of dimer. However, this peak decreases dramatically on the second cycle, and another anodic peak appears at 0.331 V at the same time, which is attributed to the oxidation of the polymer. Figure 4 b shows the electrochemical polymerization of 1-NPA on GO/GCE. An oxidation peak at 0.734 V is seen on the first cycle, which shows a slight negative shift compared with that on the bare GCE. However, after the first cycle, the polymer oxidation peak appears at 0.346 V, which shows a slight positive shift compared with that on the bare GCE. Considering that the peak current is similar to that on the bare GCE, we concluded that although the high surface area of GO/GCE helps increase the polymerization rate, the less conductive property of GO restrains this increase.[23] These two completely opposite effects combine to make the tendency of the polymerization on GO/GCE almost the same as that on the bare GCE. Figure 4 c shows the electrochemical polymerization of 1-NPA on RGO/GCE. An oxidation peak at 0.506 V is observed on the first cycle. In comparison with the GCE, the oxidation peak of 1-NPA on the RGC/GCE shifts to a more negative potential by 0.286 V, with a much larger peak current. Such a large potential shift is strong evidence for the electrocatalytic oxidation of 1-NPA by RGO, which allows the polymerization of 1-NPA at a much lower positive potential compared with that on the GCE. Interestingly, the anodic peak of the polymer occurs at 0.281 V on the second cycle, which is more negative than that on the bare GCE. Subsequently, the peak potential shifts gradually to 0.330 V during the process of electrochemical polymerization. This is perhaps because the rich electronic surface of RGO helps the oxidative polymerization of 1-NPA at a lower potential compared with that on the bare GCE. Upon increasing the number of cycles, the RGO/GCE was gradually covered by PNPA, so the anodic peak shifted gently toward more positive potentials until it reached 0.330 V, which indicated that the surface of the RGO/GCE was covered completely with PNPA. On the other hand, the peak current at 0.281 V apparently increases with an increasing number of cycles on the RGO/GCE. On the fifth cycle, the peak current at about 0.331 V on bare GCE, GO/GCE, and RGO/GCE is 15.93, 12.87, and 796.5 mA, respectively. It is demonstrated that on RGO/GCE, the amount of polymer is 102 times higher than that on the bare GCE. In contrast, a similar amount of polymer is present on the GO/GCE and the bare GCE. Figure 4 d shows the last (25th) cycle of CV on the GCE, GO/ GCE, and RGO/GCE. We can estimate that the peak current for the polymerization in the last (25th) cycle on the RGO/GCE is also about 102 times higher than that on the GCE, which demonstrates clearly that the amount of PNPA deposited on the RGO/GCE was much larger than that on the bare GCE or GO/ GCE. To study the catalytic effect of RGO in the liquid phase, we used UV/Vis absorption spectroscopy to monitor the in situ polymerization processes in the presence of RGO and GO, respectively. The in situ chemical polymerization of 1-NPA was quenched at specific reaction time intervals, and the amounts of unreacted 1-NPA and formed product were determined by UV/Vis spectroscopy. Figure 5 shows the evolution of the UV/Vis spectra recorded at various time intervals during the polymerization of 1-NPA (0.1 g) in HCl solution. In general, the black curves are the monomer absorption spectra without oxidant. The peaks at 279 nm indicate the presence of the monomer. As soon as (NH4)2S2O8 (APS) is added, the in situ polymerization process is triggered. For the polymerization of neat NPA (Figure 5 a), the peak intensity at 279 nm kept the same value during the first 255 min of the polymerization. After 275 min, the monomer peak decreased to half its original intensity, indicating the initiation of the polymerization. After 290 min, with the disappearance of the peak at 279 nm, new peaks at appeared at 300 and 600 nm, which are assigned to the formation of the PNPA backbone (PNPA displays major absorption bands in absolute ethanol at around 300 nm, which are attributed to the p-p* transition, and a localized polaronic transition peak is observed around 600 nm). The intensity of this polymer peak increased gradually up to 315 min polymerization, indicating the propagation of the polymerization process. Upon the addition of a small amount of GO (Figure 5 b) to the polymerization system, although the speed of polymerization increased a little, it still took a relatively long time (235 min) in the initiation step. We suppose that the weak catalytic effect of GO on in situ polymerization is caused by the high surface area of GO, which helps the pre-adsorption of monomers on GO. Upon performing the polymerization in the presence of a small amount of RGO (Figure 5 c), the UV/Vis spectra showed a similar tendency, but were very different from the previous spectra. First, the absorption peak of the monomer vanished more quickly than that of neat 1-NPA. In the presence of RGO, it took only 60 min to decrease to the half its original value, which is almost four times faster than for neat 1-NPA. Furthermore, the propagation process was also shortened dramatically. The polymer appeared much earlier and grew more quickly than with neat 1-NPA. This result suggests that the polymerization efficiency was increased significantly by the addition of RGO to the polymerization solution. The effect of RGO on the rate of in situ polymerization can be determined by considering the influence of RGO on each step of the polymerization. First, RGO significantly speeds up the initiation step : the 1-NPA monomer absorbs on RGO through "p-p" interactions, resulting in a high local concentration of monomer. Secondly, owing to its high electron density, RGO may donate electrons to these monomers to make them more easily oxidized than the free monomers. Thirdly, we propose that cationic intermediates are stabilized through p-interactions akin to a Lewis-acid-type mechanism between oligomers and electron-rich aromatics (RGO). This hypothesis is supported by the similar effect observed for graphite in Friedel-Crafts-type substitution reactions[26] and cleavage reactions of alkyl or aromatic esters using acyl halides.[27] Finally, prealignment of monomers on the RGO surface also facilitates the propagation of the polymerization (Scheme 1). We know that acid has a tremendous influence on the polymerization of 1-NPA, so to determine the catalytic effect of RGO on the polymerization without acid, the same experi- ments were performed for the synthesis of PNPA in the absence of HCl. Figure 6 a-c shows the evolution of the UV/Vis spectra recorded at various time intervals during the course of the polymerization of 1-NPA (0.1 g) without HCl. In neutral conditions, the peaks of the monomer are observed at 242, 279, and 313 nm, and those of the polymer occur at 300 and 600 nm. For the polymerization of pure 1-NPA (Figure 6 a), 1-NPA with GO (Figure 6 b), and 1-NPA with RGO (Figure 6c), the formation times of the polymer are about 190, 105, and 120 min (sky-blue curves), respectively. It seems that GO has a higher catalytic effect than RGO in neutral conditions, which may be because of the good solubility of GO in neutral solution compared with that of RGO. We also examined the UV/Vis spectra after two days of polymerization (Figure 6 d). It is seen that the 600 nm peak shifted to 500 nm two days later ; this change is caused by the oxidation of PNPA. The results show that the intensities of the polymer peaks of PNPA with RGO are higher than those of the other two systems. Thus, although the solubility of RGO in neutral conditions is poor, the polymerization efficiency is increased by the addition of a small amount of RGO, which implies that RGO can also help the growth of the PNPA backbone. Conclusion In this paper we have reported the high active catalytic effect of RGO for the polymerization of 1-NPA at a low catalyst/monomer mass ratio (=0.25 wt %). In contrast to reactions be- tween small organic molecules, we believe that the catalytic mechanism of RGO for polymerization is completely different. First, substrate adsorption, which should facilitate the formation of long-train polymers, is a key step. Second, the electronic effects of the carbon surfaces might improve the catalytic activity of RGO. Therefore, it is very important that these interactions are validated and understood fully. This work, once fully developed, will open up new routes for the applications of RGO. Experimental Section Materials Graphite powder and 1-naphthylamine were purchased from Aldrich and used as received. Other chemicals were used without further purification. All the solutions were prepared using deionized water (18.2 MW). The pH values of solutions were determined with a PHS-3D pH meter. General methods For the identification of the crystalline phase and the extent of surface functionalization of GO and RGO, X-ray diffraction (XRD) patterns and elemental analysis data were collected using a Rigaku D/ MAX 2400 diffractometer and an Elementar Analysensysteme GmbH varioEL cube analyzer, respectively. The conductivities of GO and RGO were also explored with a four-point probe system (ST2253, Suzhou Jingge Electronic Co. , China). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were collected with a TECNAI G2 microscope and a JSM6701F microscope, respectively. FTIR measurements were performed on a Nicolet AVTAR 360 FTIR spectrometer. Electrochemical polymerization was performed with a CHI660D electrochemical workstation (CHI, USA) with a conventional three-electrode system consisting of a glassy carbon working electrode (GCE, 5 mm diameter), a platinum foil counter electrode, and a saturated calomel reference electrode (SCE). The GCE was polished with an alumina slurry of 50 nm diameter on a polishing cloth, and then sonicated in a bath of distilled water for 15 min before use. All the electrochemical experiments were performed in quiescent solution, which was first deoxygenated by bubbling N2 for at least 5 min. In situ polymerization was monitored with a U-2001 UV/Vis spectrophotometer. Preparation of GO and RGO GO nanosheets were prepared from natural graphite powders through a modified Hummers method.[28] In the first step, preoxidized graphite powder was synthesized through the reaction of natural graphite (1 g), sulfuric acid (4 mL), K2S2O8 (0.8 g), and P2O5 (0.8 g); the reaction mixture was maintained at 80 8C for 5 h and the reaction was terminated by adding deionized water (170 mL). This preoxidized graphite powder (600 mg) was further oxidized by sulfuric acid (24 mL) and KMnO4 (3 g), and the reaction mixture was stirred at 35 8C for 2 h. Then, the reaction mixture was maintained at 98 8C for 0.5 h and the reaction was terminated by adding 50 mL deionized water. It was then further treated with H2O2 (30 wt %, 6 mL). The resulting GO solution was filtered and washed several times with deionized water, and metal ions were removed completely with a dialysis membrane over a period of one week, and the product was vacuum-dried overnight at 40 8C. In a typical synthesis, GO (10 mg) was dispersed in deionized water with the aid of ultrasonication to create a 0.1 mg mL^1 dispersion. Hydrazine monohydrate (0.1 mL, 98 %) was then added, and the suspension was heated at 808C for 24 h. Finally, a black RGO dispersion was obtained. The RGO was used for further experiments as soon as possible to avoid its agglomeration. Preparation of GO-modified GCE (GO/GCE) and RGO-modified GCE (RGO/GCE) An aqueous dispersion of pristine GO sheets (0.25 mg mL^1) was achieved through sonication for 30 min. The obtained brown dispersion was then centrifuged for 30 min at 3000 rpm to remove any unexfoliated graphite oxide. A 20 mL aqueous dispersion of GO was dropped on a pretreated GCE and dried at room temperature overnight to form a GO-deposited GCE (GO/GCE). For the reduction of GO, phosphate buffer (0.10m, pH 3.3) was deoxygenated by bubbling N2 for 10 min. Figure 7 shows a linear sweep voltammogram of the GO/GCE in the buffer solution. A reduction peak occurs at ^1.25 V, which is caused by GO reduction. Therefore, the GO/GCE was reduced for 20 min at ^1.25 V in the buffer solution with bubbling N2 to form the RGO-deposited GCE (RGO/GCE),[30] which was used for the electrocatalytic oxidative polymerization of 1-NPA in the acidic solution by using the cyclic voltammetry (CV) method. For the best electrochemical catalytic effect of RGO on the GCE electrode to be obtained, the electrochemical reductive time of GO was explored. Figure 8 shows the last (25th) cycles of the electrochemical polymerization of 1-NPA on RGO/GCE with different electrochemical reductive times. It is clear that the electrochemical catalytic effect of RGO on GCE is best with an electrochemical reductive time of 20 min. Electrochemical polymerization of 1-NPA by cyclic voltammetry Poly(1-naphthylamine) (PNPA) was deposited onto the GCE, GO/ GCE, and RGO/GCE by sweeping the electrochemical potential at ^0.2-1.05 V, ^0.2-1.0 V, and ^0.2-0.65 V, respectively (versus SCE). The scan rate was 100 mV s^1, and the electrolyte contained 1-NPA (10 mm), HCl (0.25 m), and acetonitrile (0.25 m). After the fifth cycle, the potential window was decreased to ^0.1-0.60 V in the subsequent twenty cycles to reduce the possibility of overoxidizing the polymer backbone.[29] In situ polymerization of 1-NPA with and without carbocatalysts The 1-NPA monomer (0.1 g, 0.7 mmol) was dissolved in HCl (25 mL, 1 m) solution, and then GO (or RGO) (1 mL, 0.25 mg mL^1) was added. The solution was bubbled with N2 for 30 min, after which the polymerization was started by adding (NH4)2S2O8 (APS) (5 mL, 0.14 mm). All the experiments were performed under electromagnetic mixing at room temperature. 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D^k^ny, Carbon 2006, 44, 537 - 545. [25] T. Szab^, O. Berkesi, P. Forg^, K. Josepovits, Y. Sanakis, D. Petridis, I. D^k^ny, Chem. Mater. 2006, 18 , 2740 - 2749. [26] G. A. Sereda, V. B. Rajpara, R. L. Slaba, Tetrahedron 2007 , 63, 8351 - 8357. [27] Y. Suzuki, M. Matsushima, M. Kodomari, Chem. Lett. 1998, 319 - 322. [28] Y. Chen, X. Zhang, P. Yu, Y. W. Ma, Chem. Commun. 2009, 4527 - 4529. [29] Y. F. Ma, S. R. Ali, A. S. Dodoo, H. X. He, J. Phys. Chem. B 2006, 110, 16359-16365. [30] B. K. Schmitz, W. B. Euler, J. Electroanal. Chem. 1995, 399, 47 - 52. Received : February 25, 2014 Published online on May 21, 2014 Lin Tan,* Yong-Chen Zhang, Bin Wang, He-Ming Luo, and Hui-Xia Feng*[a] [a] Dr. L. Tan, Y.-C. Zhang, B. Wang, H.-M. Luo, Prof. H.-X. Feng College of Petrochemical Technology Lanzhou university of Technology 287 Langongping Road Lanzhou 730050 (P. R. China) E-mail : [email protected] [email protected] (c) 2014 Blackwell Publishing Ltd. |