Enhanced Electrocatalytic Activity of Copper–Cobalt Nanostructures

Jun 23, 2011 - Raquel Nafria , Aziz Genç , Maria Ibáñez , Jordi Arbiol , Pilar Ramírez de la Piscina ... Jahangeer Ahmed , Tansir Ahamad , Saad M...
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Enhanced Electrocatalytic Activity of CopperCobalt Nanostructures Jahangeer Ahmed,† Aparna Ganguly,† Soumen Saha,† Govind Gupta,‡ Phong Trinh,§ Amos M. Mugweru,*,§ Samuel E. Lofland,^ Kandalam V. Ramanujachary,§ and Ashok K. Ganguli*,† †

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Surface Physics Division, National Physical Laboratory (CSIR), New Delhi, 110012, India § Department of Chemistry and Biochemistry and ^Department of Physics and Astronomy, Rowan University, 201 Mullica Hill Road, Glassboro, New Jersey 08028, United States ‡

bS Supporting Information ABSTRACT: Novel coreshell nanostructures containing Cu and Co have been synthesized using the microemulsion method at 700 C. The core consists of CuCo composite particles, whereas the shell is composed of CuCo alloy particles (shell thickness ∼12 nm). It is to be noted that in bulk CuCo binary system there is practically no miscibility. TEM studies show formation of spherical-shaped nanoparticles of coreshell structures. The composition of the core (CuCo composite) and shell (CuCo alloy) were confirmed by XPS studies. The formation of the CuCo alloy as the shell is mainly driven by surface energy considerations. We have also obtained CuCo nanocomposites (by controlling the concentration of reducing agent) with particle size in the range of 40200 nm. These CuCo nanostructures show ferromagnetic behavior at 4 K. The saturation magnetization of the coreshell (CuCo composite @ CuCo alloy) nanostructure (125 emu/g) is found to be higher than that of pure CuCo nanocomposite or alloy, which may be useful for applications as a soft magnet. Electrochemical studies of these nanocrystalline CuCo particles show higher hydrogen evolution efficiencies (∼5 times) compared to bulk (micrometer-sized) CuCo alloy particles.

1. INTRODUCTION Nanoscale particles of metals, alloys, and coreshell structures have attracted much attention due to their unique electronic, optical, biological, catalytic, and magnetic properties.15 Metallic and bimetallic nanoparticles have applications in drug delivery, in hyperthermia, as electrocatalysts, and in magnetic devices.68 CuCo alloy particles show giant magnetoresistance (GMR) behavior originating mainly from spin-dependent scattering of conduction electrons at the interface of the ferromagnetic particles.911 The microstructural factors controlling GMR effect in granular materials are mainly the size distribution and the volume fraction of the ferromagnetic Co particles embedded in the nonferromagnetic matrix as well as the roughness of the interfaces.12 Ferromagnetic behavior was observed in cobalt-rich bulk CuxCo100x systems (x = 20100) at room temperature while the Cu90Co10 system showed paramagnetic behavior.10 CuCo alloy-based catalysts are selective for higher alcohol (C3 and higher) synthesis from syngas and also in dehydrogenation reactions.13 The electrocatalytic behavior of CuCo, CoNi, FeCo alloy particles for hydrogen and oxygen evolution reaction is also of importance.1416 A number of physical techniques have been used to produce metastable solid solutions of CuCo such as arc melting,11,12 mechanical alloying,10 electrochemical deposition,17 melt-spinning method,1820 and ball milling,21 which show limited solubility of Cu and Co. The coppercobalt binary system is r 2011 American Chemical Society

an immiscible system (no solubility of Cu in Co or Co in Cu) under ambient conditions. The phase diagram of CuCo binary system22 shows that the maximum solid solubility of Co in Cu is 7 wt % at 1112 C, and the solubility decreases sharply with decreasing temperatures becoming negligible below 500 C. Among the known low-temperature chemical routes used for the synthesis of nanoparticles, the microemulsion method (reverse micelle) allows good control of size and homogeneity. Reverse micelles are water in oil microemulsions consisting of nanometer-sized water droplets that are dispersed in an oil medium and stabilized by surfactants. These water droplets are suitable as reaction media for the synthesis of nanoparticles where the size of particles can be controlled by varying the molar ratio of water to surfactant. Earlier, we exploited the microemulsion method for synthesis of Co,23,15 CoNi,15 FeCo,16 and oxide nanoparticles2325 and also designed several coreshell nanostructures.26,27 There are also other reports on synthesis of bimetallic nanoparticles of AuAg,28 AuPd,29 and AuPt30 by microemulsions. In this article, we report the synthesis of novel coreshell nanostructured materials (CuCo composite @ CuCo alloy) and nanocrystalline CuCo composites using the microemulsion method. The formation of CuCo alloy-based shell by this Received: March 14, 2011 Revised: June 17, 2011 Published: June 23, 2011 14526

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method is significant as it gives a possibility to design the entire solid solution of the binary CuCo alloy phase. Electrocatalytic studies (hydrogen evolution reaction) show nearly five times enhancement using the CuCo nanostructures compared to CuCo bulk alloys.

2. EXPERIMENTAL SECTION Cu(NO3)2.3H2O (CDH, 98%), Co(CH3COO)2.4H2O (Qualigens, AR, 98%), NaOH (Qualigens, LR, 97%), N2H4. H2O (Qualigens, 99%), cetyl trimethylammonium bromide (CTAB) (Spectrochem, AR, 99%), 1-butanol (Qualigens, 99.5%), and isooctane (spectrochem, 99%) were used in the synthesis of the coreshell nanoparticles of CuCo nanostructures. These nanoparticles were synthesized by the microemulsion method with CTAB (cetyltrimethyl ammonium bromide) as the surfactant, 1-butanol as the cosurfactant, and isooctane as the oil phase. Four microemulsions with different aqueous phases containing 0.1 M Cu (NO3)2.3H2O, 0.1 M cobalt acetate, 20 M N2H4.H2O, and 0.1 M NaOH were prepared. The weight fractions of various constituents in these microemulsions were as follows: 16.76% of CTAB, 13.90% of 1-butanol, 59.29% of isooctane, and 10.05% of the aqueous phase.15 These microemulsions were mixed slowly on a magnetic stirrer and a green precipitate was obtained. The green precipitate was washed with 1:1 chloroform/methanol mixture and dried in air and subsequently reduced in hydrogen atmosphere at 700 C for 6 h to form the coreshell nanostructures. The same procedure was used for the synthesis of CuCo nanocomposite except that a lower concentration of hydrazine (1.0 M N2H4.H2O) was employed in the initial microemulsion. Powder X-ray diffraction (PXRD) studies were carried out on a Bruker D8 Advance diffractometer with Ni-filtered CuKR radiation with a scan speed of 1 s and scan step of 0.02. TEM studies were carried out with a Technai G2 20 electron microscope operated at 200 kV. The specimens for TEM were prepared by dispersing the powder in acetone by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then drying in air. Cyclic voltammetry (CV) was carried out with a electrochemical workstation (CHI 660c, USA) with ohmic drop 98% compensated. The reference electrode was Ag/AgCl while Pt wire was used as counter electrode. Glassy carbon electrodes (0.07 cm2) used as a working electrode were first polished with 1 μm diamond polishing paste then ultrasonicated in distilled water for 1 min before immobilizing the different materials for hydrogen evolution reactions (HER). Five milligrams of nanoparticle sample was mixed with 2 μL of PVDF. The mixture was placed on glassy carbon electrode and air-dried for about 1 h. The electrode was then placed in the cell containing about 5 mL of 0.5 M KOH solution. For each experiment, freshly prepared electrode and solutions were used. The experiments were carried out at room temperature (2223 C). Cyclic voltammetry was carried out at a scan rate of 40 mV/s with a peak window between 0 and 1.5 V versus Ag/AgCl electrode. Amperometry was carried out for 200 s for each electrode with the potential fixed at 1.4 V versus Ag/AgCl. X-ray photoelectron spectroscopy measurements were performed in an ultrahigh vacuum chamber (P H1257) with base pressure of 4  1010 Torr. The XPS spectrometer was equipped with a high-resolution hemispherical electron energy analyzer (279.4 mm diameter) with 25 meV resolution, and a dual anode

Figure 1. Powder X-ray diffraction pattern of CuCo system (CuCo composite @ CuCo alloy) obtained using 20 M of hydrazine.

Mg/Al KR X-ray source. The source used for this study was the Al (KR) X-ray with excitation energy of 1486.6 eV. The general scan showed very small amount of C and O impurities. Ar+ ions with an energy of 4 keV was used for bombardment for different time periods with simultaneous XPS studies, to determine the elemental composition along the interior of the nanoparticles. Because the incident X-ray beam size was 23 mm in diameter, the results provide an average composition for the nanoparticles. The magnetization studies were carried out at temperatures ranging from 4 to 300 K, in applied fields of up to 5 kOe with a Quantum Design Physical Properties Measurement System.

3. RESULTS AND DISCUSSION The coreshell and composite type of nanoparticles of CuCo were obtained at 700 C from the precursors synthesized using the microemulsion method. PXRD patterns show the green colored powder obtained at room temperature to be amorphous. The IR study confirms that there is no surfactant left after washing with chloroform: methanol mixture (Figure S1 of the Supporting Information). These powders after annealing in hydrogen at 700 C for 6 h turned black in color. PXRD shows formation of a nanocrystalline CuCo alloy (Figure 1) where all reflections could be indexed based on a single phase. From the line broadening studies, the crystallite size was found to be ∼31 nm (Figure S2 of the Supporting Information). TEM studies showed the formation of coreshell nanostructures with a diameter of core ∼80 nm and shell of ∼12 nm, respectively (parts a and b of Figure 2). Part c of Figure 2 shows the EDAX pattern for the coreshell nanostructures confirming the presence of cobalt in the sample. The presence of copper in the sample has also been confirmed from the SEM EDAX given in Figure S3 of the Supporting Information. Coreshell nanoparticles of CuCo appeared to form a regular arrangement of particles (part a of Figure 2), which may be due to magnetic interaction between the particles. Yang et al. have earlier reported nanoscale Cu88Co12 alloy particles with an average size of ∼10 nm from arc melting,12 whereas nanorods of Cu90Co10 alloy (diameter = 200 nm, length = 5 μm) have been reported by electrochemical deposition method using polycarbonate membrane as a template.17 Hydrazine hydrate 14527

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Figure 2. (a, b) TEM and (c) EDAX measurement of CuCo system (CuCo composite @ CuCo alloy).

works as good reducing agent under alkaline conditions as shown by the following reactions: 2Cu2þ þ N2 H4 :H2 O þ 4OH f 2Cu þ N2 þ 5H2 O 2Co2þ þ N2 H4 :H2 O þ 4OH f 2Co þ N2 þ 5H2 O To understand the compositional and structural difference between the core and shell, detailed XPS studies were carried out.

The changes in the relative composition of CuCo alloy and CuCo composite observed in the coreshell nanostructures by removal of the surface layers of the nanoparticles by 4 keV Ar+ ion bombardment provides valuable information regarding the distribution of the nature of CuCo phases along the interior of the nanoparticles. Figure 3 shows the core level XPS spectra of Cu2p3/2 for unsputtered and 2 keVAr+ ion sputtered CuCo alloy nanoparticles. The Cu 2p3/2 core level spectra of unsputtered samples exhibits a broad peak that has been deconvoluted into two peaks at 936.9 eV (peak a1) and 935.2 eV (peak a2). The low energy peak at binding energy of 935.2 eV corresponds to 14528

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Figure 3. XPS spectra of CuCo system (CuCo composite @ CuCo alloy) showing the Cu2p3/2 region after sputtering for different time periods with respect to Cu (ae) for 0, 30, 60, 90, and 120 min; and (fi) with respect to Co after sputtering for 0, 30, 60, 90, and 120 min.

the CuCo alloy phase while the appearance of a small peak at 936.9 eV is a signature of surface layer of CuCo oxide. These results obtained from XPS are in good agreement with those reported elsewhere.31 A significant shift in binding energy was observed as expected for the formation of solid solution of CuCo. The Cu 2p3/2 peaks broadened after 30 min of sputtering which was deconvoluted into three peaks a1, a2, and a3 as shown in part b of Figure 3. The evolution of a3 peak at binding energy of 932.7 eV indicates presence of CuCo composite in the core of the

nanoparticles. On further sputtering the nanoparticles, a1 phase disappears completely and only two peaks (a2 and a3) remained in the Cu core level spectra corresponding to CuCo alloy and CuCo composite (3c, 60 min sputtering). On further sputtering the surface (90 min, part d of Figure 3 and 120 min, and part e of Figure 3), the contribution of the CuCo composite phase (a3) increases and that of the CuCo alloy decrease (a2), which suggests that the core is made up of CuCo composite and covered with CuCo alloy. 14529

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Figure 5. Powder X-ray diffraction patterns of CuCo nanocomposite (obtained using 1 M hydrazine).

Figure 4. Depth profile curve obtained using X-ray photoelectron spectroscopy of (a) coreshell and (b) elemental composition of CuCo nanoparticles.

Similarly when deconvolution of the unsputtered and sputtered sample is carried out we observe that the peak at binding energy of 780.2 eV (b2) that corresponds to CuCo alloy nanoparticles diminishes while the peak at 778.1 eV (peak b3) which corresponds to CuCo composite increases as the sputtering time is increased (parts fi of Figure 3). A detailed discussion on the XPS sputtering and the percentage area covered by various peaks in the deconvoluted spectra of CuCo nanoparticles after different sputtering time has been given in the Supporting Information (Table SII of the Supporting Information). The percentage composition of CuCo alloy and CuCo composite in the coreshell nanoparticles with Ar+ ion depth profiling is shown in part a of Figure 4. Using geometrical consideration and ignoring preferential sputtering and inelastic mean free path of the core electrons, we quantitatively estimate the composition by assuming a radial dependence of the density of each element.32 From the above studies we observe that the shell consists of CuCo alloy, whereas the core comprises of CuCo composite particles and we estimate the shell thickness to be ∼15 nm, which matches closely with TEM studies.

Note that surface energies of cobalt, copper and copper  cobalt alloy are 2.709, 1.934, and 0.75 J m2, respectively.3335 On the basis of the surface energies, we may conclude that coppercobalt alloy would prefer to be on the surface as is observed. The composition was found to be 2:1 for Cu/Co from the depth profile curve (part b of Figure 4). Lowering the hydrazine concentration (1 M) during the synthesis of CuCo nanoparticles leads to formation of a composite phase containing Cu and Co (no coreshell structures). PXRD patterns clearly show the formation of biphasic mixture of copper and cobalt in the nanocomposites (Figure 5). All the reflections in the pattern could be indexed on the basis of face centered cubic cell reported for copper (JCPDS # 851326) and cobalt (JCPDS # 150806). TEM micrographs of these nanocomposites show particles (part a of Figure 6) with size ranging from 40200 nm. High-resolution TEM studies (part b of Figure 6) confirm the presence of copper and the lattice spacing is consistent with that of the (111) plane of Cu. Part c of Figure 6 shows the EDAX analysis of the sample, which confirms the presence of Co in the composite nanoparticle. With 1 M hydrazine, coppercobalt composite nanoparticles are formed. However with a higher concentration (∼20 M) of the reducing agent, coreshell nanostructures were formed with composite (Cu/Co) as core and CuCo alloy as shell. It appears that with higher concentration of the reducing agent, smaller particles of the precursor are formed.36,37 And these smaller sized precursor particles favor the formation of metastable phases (CuCo alloy). It is well-known that some metastable phases can be obtained only by decreasing the size of the material.38 Thus a minor phase of alloy formation takes place. The amorphous precursor (formed at room temperature) contains a mixture of Cu and Co, which on reduction leads to alloy formation when a higher concentration of hydrazine is employed. Annealing the above at high temperature causes the phase separation of the material. It appears that the phase separation is incomplete under our conditions and hence we are left with Cu, Co, and alloy of CuCo and due to surface energy effects the alloy phase migrates preferentially to form a shell on the CuCo composite core. CV was used to characterize the CuCo nanostructures for application as electrode materials. Figure 7 shows cyclic voltammograms of the nanocomposite (CuCo) and coreshell-based nanostructures. Four irreversible peaks were observed for both 14530

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Figure 7. Cyclic voltammograms of CuCo nanocomposite and coreshell nanoparticles on glassy carbon electrode.

Figure 6. (a) TEM and (b) HRTEM corresponding to the (111) plane of copper in the CuCo nanocomposite and (c) TEM EDAX for the nanocomposite.

coreshell and CuCo nanocomposite structures. The peaks may be understood as due to formation of four couples (in basic medium): Cu/Cu+ and Cu+/Cu2+ for copper and Co/Co2+ and Co2+/Co3+. At a potential of 1.2 V, CuCo nanocompositebased cell leads to higher current density (Figure 7) than coreshell nanostructures. However, at about 1.5 V these two materials have almost the same hydrogen evolution efficiency. The nanocomposite may therefore be slightly better for HER activity at lower potentials than 1.4 V. The chronoamperometric method was used to compare the HER properties of the different electrode materials at 1.4 V versus Ag/AgCl electrode. The performance of the electrode material was also determined by the current densities and stability, determined from 10 cycles, with

Figure 8. Chronoamperometric voltammograms of CuCo nanocomposite and coreshell nanoparticles on glassy carbon electrode and at 1.4 V.

each cycle lasting approximately 20 seconds. These two nanostructures as electrode materials were found to be stable for HER during the time of the experiment. Figure 8 shows the chronoamperometric studies comparing the HER for the CuCo nanocomposites and coreshell (CuCo composite @ CuCo alloy) electrode materials. The current generated in each cycle was nearly the same for the two types of nanostructures as electrodes. The minor variation in the currents may indicate slightly different 14531

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particles indicating that the materials were also good electrocatalysts for hydrogen evolution reactions.

’ ASSOCIATED CONTENT

bS

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], Tel: 91-11-26591511, Fax: 91-11-26854715 (A.K.G.); E-mail: [email protected], Tel.: 856-256-5454, Fax: 1-856-256-4478 (A.M.M.).

Figure 9. Hysteresis loops of coreshell (CuCo composite @ CuCo alloy) and composite nanoparticles of CuCo at 4 K.

composition of nanostructures. The nanocomposite shows slightly higher current, which may be due to slightly higher oxidation and reduction rates as a result of higher Cu content than the coreshell nanostructures. The current density (∼15 mA/cm2) for nanocrystalline CuCo particles was found to be much higher (5 times) than the current generated (∼3 mA/cm2) for bulk CuCo alloy.14 The magnetization and the hysteresis loops of both coreshell nanostructures (CuCo composite @ CuCo alloy) and CuCo nanocomposite have been studied at 4 K with an applied magnetic field up to 7 T (Figure 9). The saturation magnetization values of coreshell and composite nanoparticles of CuCo were found to be 124 and 72 emu/g, respectively (Figure 8). Wen et al. have earlier reported the saturation magnetization of 5 emu/g for nanocrystalline CuCo particles (10 nm).39 Note that the reported value of saturation magnetization for bulk CuCo alloy (1:4) is 120 emu/g at 4 K.10 This value decreases with increase in the amount of copper in the CuCo alloy which finally shows paramagnetic behavior for CuCo (9:1) alloy.10 Note that bulk Cu and Co metals are paramagnetic and ferromagnetic, respectively. The coreshell (composite @ alloy) nanostructures prepared by us have higher saturation magnetization value compared to Cu  Co nanocomposite particles and bulk CuCo alloy. The coreshell nanoparticles may be useful as soft magnetic materials due to their high saturation magnetization.

4. CONCLUSIONS The microemulsion method has been used to obtain a novel coreshell nanostructured material (CuCo composite @ CuCo alloy). This method gives the possibilities of obtaining pure metastable alloys which are normally immiscible and thus has scope for stabilizing other binary alloys (e.g., MnNi) which are unstable in the bulk. In addition, by control of the reduction process we have also obtained CuCo composite nanoparticles. The coreshell and composite nanoparticles show ferromagnetic behavior and the saturation magnetization of the coreshell nanoparticles is enhanced significantly compared to the bulk CuCo alloy particles. Electrochemical studies of CuCo nanostructures showed much higher current density (∼15 mA/cm2) compared to bulk CuCo alloy

’ ACKNOWLEDGMENT A.K.G. thanks DST and CSIR, Govt. of India, for financial assistance. S.E.L. acknowledges support of National Science Foundation, MRSEC DMR 0520471. J. Ahmed and A.G. thank CSIR for their senior research fellowship. K.V.R. acknowledges the CP-STIO award from the Department of Science and Technology, Govt. of India. ’ REFERENCES (1) Jo, C.; Lee, J.; Jang, Y. Chem. Mater. 2005, 17, 2667. (2) Mattei, G.; Fernandez, C. D. J.; Mazzoldi, P.; Sada, C.; De, G.; Battaglin, G.; Sangregorio, C.; Gatteschi, D. Chem. Mater. 2002, 14, 3440. (3) Lian, K.; Thorpe, S. J.; Kirk, D. W. Electrochim. Acta 1992, 37, 169. (4) Chen, C. W.; Chen, M. Q.; Serizawa, T.; Akashi, M. Chem. Commun. 1998, 831. (5) Caruso, F.; Fiedler, H.; Haage, K. Colloids Surf., A 2000, 169, 287. (6) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D.: Appl. Phys. 2003, 36, 167. (7) Connolly, J.; St Pierre, T. G.; Rutnakornpituk, M.; Riffle, J. S. J. Phys. D: Appl. Phys. 2004, 37, 2475. (8) Namdeo, M.; Saxena, S.; Tankhiwale, R.; Bajpai, M.; Mohan, Y. M.; Bajpai, S. K. J. Nanosci. Nanotech. 2008, 8, 3247. (9) Wang, W.; Zhu, F.; Weng, J.; Xiao, J.; Lai, W. Appl. Phys. Lett. 1998, 72, 1118. (10) Fan, X.; Mashimo, T.; Huang, X.; Kagayama, T.; Chiba, A.; Koyama, K.; Motokawa, M. Phys. Rev. B 2004, 69, 094432. (11) Wang, W.; Zhu, F.; Lai, W.; Wang, J. Q.; Yang, G.; Zhu, J.; Zhang, Z. J. Phys. D: Appl. Phys. 1999, 32, 1990. (12) Yang, G. Y.; Zhu, J.; Wang, W. D.; Zhang, Z.; Zhu, F. W. Mater. Res. Bull. 2000, 35, 875. (13) Nguyen, T. T.; Zahedi-Niaki, M. H.; Alamdari, H.; Kaliaguine, S. Int. J. Chem. Reactor Eng. 2007, 5, A82. (14) Brossard, L.; Marouis, B. Int. J. Hydro. Energy 1994, 19, 231. (15) Ahmed, J.; Sharma, S.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Colloid Interface Sci. 2009, 336, 814. (16) Ahmed, J.; Kumar, B.; Mugweru, A. M.; Trinh, P.; Ramanujachary, K. V.; Lofland, S. E.; Govind; Ganguli, A. K. J. Phys. Chem. C 2010, 114, 18779. (17) Xue, S.; Cao, C.; Ji, F.; Xu, Y.; Wang, D.; Zhu, H. Mater. Lett. 2005, 59, 3173. (18) Allia, P.; Coy sson, M.; Tiberto, P.; Vinai, F. J. Alloys Compd. 2007, 434, 601. (19) Cezara, J. C.; Tolentino, H. C. N.; Knobel, M. J. Magn. Magn. Mater. 2001, 233, 103. (20) Juarez, G.; Villafuerte, M.; Heluani, S.; Fabietti, L. M.; Urreta, S. E. J. Magn. Magn. Mater. 2008, 320, 22. 14532

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