Nanostructured silver fibers: Facile synthesis based on natural cellulose and application to graphite composite electrode for oxygen reduction

international journal of hydrogen energy 35 (2010) 3258–3262

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Nanostructured silver fibers: Facile synthesis based on natural cellulose and application to graphite composite electrode for oxygen reduction Nafiseh Sharifi a, Fariba Tajabadi b, Nima Taghavinia a,b,* a b

Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588, Iran Physics Department, Sharif University of Technology, Tehran 14588, Iran

article info

abstract

Article history:

The development of cheaper electrocatalysts for fuel cells is an important research area.

Received 24 November 2009

This work proposes a new, simpler and low-cost approach to develop nanostructured silver

Received in revised form

electrocatalysts by using natural cellulose as a template. Silver was deposited by reduction

18 January 2010

of Ag complexes on the surface of cellulose fibers, followed by heat removal of the template

Accepted 30 January 2010

to create self-standing nanostructured silver fibers (NSSFs). X-Ray diffraction (XRD) showed

Available online 2 March 2010

fcc silver phase and X-Ray photoelectron spectroscopy (XPS) demonstrated that the surface was partially oxidized. The morphology of the fibers consisted of 50 nm nanoparticles as

Keywords:

the building blocks, and they possessed a specific surface area of about 25 m2/g, which is

Silver fiber

sufficiently high for electrocatalytic applications. The NSSFs were incorporated in

Cellulose

a graphite composite electrode. The resulting modified electrode displayed a good

Electrocatalyst

electrocatalytic activity for the reduction of dissolved oxygen in basic media. In an

Oxygen reduction

O2-saturated 0.1 M KOH solution, the overpotential to initiate the oxygen reduction reaction reduced and the limiting current increased by increasing the relative amount of silver fibers from 0 to 5 wt%. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Considerable efforts have been devoted to the design and synthesis of one-dimensional nano-materials, such as nanorods [1], nanowires [2], nanotubes [3] and nanofibers [4] due to their unique properties in fabricating electronic [5] and optoelectronic devices [6], biosensors [7], and high surface area electrodes in electrochemistry [8]. Fiber composites can be produced by interfacial polymerization and electrospinning [911]. Template synthesis has been also adopted to prepare one-dimensional materials, using different templates such as nanochannel glass and polymeric membranes and anodized

aluminum substrates [12]. However, their synthesis usually consists of a complicated process. As an alternative, a conformal replication of the morphologies of natural fibrous materials has been used in this study, which offers a relatively low-cost, simple and environmentally safe method for the formation of sophisticated nanostructures [1315]. There have been reports on using natural templates to grow nanostructures of ceramics and ceramic composites [16,17]. Here we demonstrate a simple, facile approach to the deposition of silver nanoparticles (NPs) onto cellulose fibers. Self-standing nanostructured silver fibers (NSSFs) are fabricated by heat removal of the cellulose template, followed by sintering of the

* Corresponding author. Sharif University of Technology, P.O. Box 11155-9161, Tehran 14588, Iran. Tel.: þ98 21 6616 4532; fax: þ98 21 6602 2711. E-mail address: [email protected] (N. Taghavinia). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.01.147

international journal of hydrogen energy 35 (2010) 3258–3262

remaining silver NPs. High electrical conductivity of metals and remarkable electrocatalytic activity in some nanoscale metals due to their large active surface to volume ratio has caused intensive studies on metals as electrode catalysts [8, 1821]. The morphology of the fibers consists of chains of NPs connected to each other which makes NSSF could be practical to increase electrode conductance compared to dispersed NPs when used as electrode modifier. In addition, resulting structure demonstrates high surface area, which enables it as a good candidate for electrocatalytic applications. The oxygen–hydrogen fuel cells belong with the most promising power sources for electric vehicles and some stationary devices. The characteristics of oxygen–hydrogen fuel cells are defined by the slowness of reaction of cathodic reduction of oxygen [22]. Hence, oxygen reduction reaction (ORR) is one of the most important reactions in electrochemistry due to its application in fuel cells [23,24]. In fact, overcoming the large overpotential associated with the ORR is one of the major challenges, which call for the development of high performance cathode catalysts [25,26]. It has been shown that various forms of Ag and Ag-alloy catalysts have comparable overvoltages and similar activities to Pt-alloy for ORR at a much lower cost [18, 2729]. In this study, we used the resulting NSSFs as an ORR electrocatalyst. The NSSFs were mixed with graphite powder and an organic binder for the preparation of a composite electrode.

2.

Experimental

Silver replicas of cellulose fibers were prepared by first chemically depositing silver NPs on the fibers. A typical synthesis is similar to [30]: The starting silver ammonia aqueous solution was made by mixing 5 mM AgNO3 (Acros) solution, 10 mM KOH solution (Wako) and ammonium hydroxide (Guangdong Guanghua, 25%). About 0.2 g of natural cellulose fibers were put in the prepared solution. A solution of 7 mM sucrose (C12H22O11, Merck) was then added to reduce silver on the surface of cellulose fibers. The fibers were then removed from the solution and sonicated in order to remove the remaining silver species and byproducts, and then they were rinsed with water. Silvery fibers were dried at room temperature and were calcined in air at 400  C for 1 h. This removed the cellulose template and produced the final NSSFs. Graphite composite electrode (GCE) was prepared by handmixing PDMS (poly (dimethylsiloxane (Acros))) and graphite powder (Fluka) with 30/70 ratio (w/w). They act as a binder and conductive part of the electrode, respectively. The paste was packed into the cavity of a Teflon tube (1.8 mm diameter). An electrical contact was established via a stainless steel handle. The modified electrode was prepared by mixing NSSFs, PDMS and graphite powder with a ratio of 1:29:70, (w/w/w), and 5:25:70, respectively. The surface morphology and composition were investigated using scanning electron microscopy (SEM) (Philips, XL30) and energy dispersive spectroscopy (EDS) (CamScan, MV2300). Diffuse reflectance spectroscopy (DRS) analysis was performed using AvaSpec-2048TEC spectrometer. The light source was a combination of a 78 W Deuterium lamp and a 5 W Halogen lamp, coupled to the spectrometer using optical

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fibers. X-Ray diffraction (XRD) spectra were measured using a BRUKER (model D4ENDEAVOR) diffractometer with Cu:Ka radiation (l ¼ 0.154178). Surface composition was analyzed by X-Ray photoelectron spectroscopy (XPS) using Al anode of a V.G. Microtech XR3E2 X-ray source and a concentric hemispherical analyzer (Specs model EA10 plus). Photoelectrons were collected at a takeoff angle of 0 from the surface normal. Specific surface area was measured on silver fibers determined by the Brunauer–Emmett–Teller analysis (BET) method from the N2 isotherms collected with a Belsorp mini II BEL system on degassed samples. Voltammetric measurements were performed using an Autolab electrochemical system (PGSTAT302N). The electrochemical cell was assembled with a conventional three-electrode system: an Ag/AgCl/KCl (3 M) reference electrode (Metrohm) and a platinum disk as a counter electrode. Different working electrodes used in this study were a GCE and a GCE modifying with NSSFs. The cell was a one-compartment cell with an internal volume of 10 mL. All experiments were typically conducted at room temperature.

3.

Results and discussion

Fig. 1(a) shows the bunch of silvery fibers after heat treatment which cellulose fibers are omitted from the inner parts of the silver coatings and NSSFs are fabricated, as illustrated in magnified Fig. 1(b) and (c). A homogenous shrinkage in all directions is observed compared to the original cellulose fibers, where the diameter of silver fibers is about 70% smaller than the diameter of cellulose fibers. It seems the formed silver complexes (AgðNH3 Þþ 2 ) in silver ammonia aqueous solution were attracted towards the negatively charged surface of cellulose fibers by electrostatic (i.e. ion–dipole) interactions and homogeneously distributed on the surface of fibers [31]. Therefore, the overall shape of fibers has followed the shape of cellulose fibers and has formed approximately tubular structures. The morphology of the fibers consists of the microfibers (Fig. 1(b)), which are made from randomly connected NPs with approximate diameter of 50 nm as seen in Fig. 1(c). Fig. 1(d) exhibits the DRS of silver fibers where the original absorbance spectrum shows two major absorption bands. The first peak, band A, at around 430 nm is a typical plasmon band of Ag NPs of a few tens of nanometer size. The second peak at around 540 nm, band B, could be attributed to the longitudinal plasmon resonance of rod-shaped silver nanostructures [32] as presented in Fig. 1(b). Specific surface area of the fibers, which was measured using BET analysis of N2 adsorption curves, was found about 25 m2/g. This should be compared to 0.3 m2/g surface area of the starting cellulose fibers due to the high surface area of NPs. Assuming a spherical shape for the NPs, one can find the size of NPs using the value of specific surface area [33]. This is obtained as 23 nm, which is of the same order as observed in the SEM images (Fig. 1(c)). Examining the fibers using X-ray diffractions demonstrated a pure fcc silver phase. The chemical composition of the fibers before and after heat treatment was determined using XPS analysis, which showed the presence of Ag, O and C on the surface of fibers (Fig. 2, curves (a), and (b)). The

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international journal of hydrogen energy 35 (2010) 3258–3262

Fig. 1 – (a, b, and c) SEM images of NSSFs at different magnifications. The arrows in figure (b) show microfibers. In figure (c), the square shows high-resolution image of approximate 50-nm building blocks of NSSFs and (d) DRS of NSSFs after heat removal of cellulose template.

for the zero valence state of silver and AgO, respectively [35]. Hence, the heat treatment of the silvery cellulose fibers at 400  C causes the partial oxidation of the surface of silver fibers. As mentioned, silver replicas of cellulose fibers could have potential applications in high surface area electrodes in electrochemistry as an electrocatalyst; however, their fragility may limit their use. The fragility issue may be overcome if the cellulose is not removed or a composite structure is utilized. The latter was employed and this deficiency was overcome using the organic binder. The surface morphology of the GCE and the electrode modified with 5% silver fibers are shown in Fig. 3. It seems the morphology of the electrode surface has changed due to the presence of the silver fibers, which has caused a more compact morphology. Fig. 4 shows representative linear sweep voltammograms obtained for the reduction of saturated oxygen solution in

existence of C element in XPS, in both spectra, comes from contaminations. In case of silvery cellulose fibers (curve a), parts of the C and O peaks are also related to the presence of C and O elements in cellulose fibers as it can be seen that both the O and C peaks become weaker after heat treatment. Since the cellulose fibers were omitted after heat treatment, an increase in Ag peaks is also observed (curve b). Both the O 1s and C 1s peaks contain shoulders, which indicate that multiple chemical states of carbon and oxygen are present because of the presence of cellulose fibers. Curves c and d show the XPS spectra in the Ag 3d binding energy range. Silver is a metal having anomalous properties in binding energy shifts when being oxidized i.e. the Ag 3d peaks shift to lower binding energy [34]. Compared with the Ag 3d5/2 binding energy observed for silvery cellulose fibers (368.4 eV, curve c), the Ag (3d5/2) peak for silver fibers was shifted to 367.5 eV (curve d). These are the reported values for the binding energy

a

Counts(a.u.)

b

Counts(a.u.)

b

Ag 3d

367.5

Ag 3p3 Ag 3p1 O 1s

C 1s

a

d c 368.4

800

700

600

500

400

Binding Energy (eV)

300

200

378 376 374 372 370 368 366 364

Binding Energy (eV)

Fig. 2 – Survey XPS spectra of silvery fibers (a) before and (b) after heat removal of cellulose template. (c) and (d) are the highresolution counterparts of (a) and (b), respectively.

international journal of hydrogen energy 35 (2010) 3258–3262

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Fig. 3 – SEM images of (a) GCE and (b) modified GCE with 5% NSSFs.

0.1 M KOH on the GCE and GCE modified with 1% and 5% (weight) NSSFs. A comparison of curves (a) with (b), and (c) reveals the electrocatalytic effect of NSSFs towards O2 reduction. For the modified GCEs with 1% and 5% NSSFs, current density increases as compared to the GCE. Current is dependent on the concentration of active sites towards the ORR [36]. By increasing the relative amount of silver fibers from 0 to 5 wt%, the number of silver active sites was increased. This increase in active sites for silver corresponds to a higher number of active sites per surface area of the electrode and two-times augmentation in current density for the modified GCE with 5% NSSFs was observed. A sudden change in slope and a negative current density identifies a sufficient overpotential to initiate the ORR [8]. The mentioned graphs show that the ORR onset potential reduced from 300  5 to 220  5 mV by increasing the relative amount of silver fibers. The GCE exhibits a broad reduction peak with large overpotential (507  5 mV) for the oxygen reduction reaction (ORR) in basic medium. In addition, the overpotential of oxygen reduction on the modified GCE with 5% NSSFs is decreased to 420  5 mV vs. Ag/AgCl. Therefore, the modified GCE with 5% NSSFs

Current Density(μ Α /cm2)

100 50 0 -50 -100 -150

(a) GCE (b) modified GCE with 1% (w) NSSFs (c) modified GCE with 5% (w) NSSFs

(a) (b)

(c)

-250 -600 -400 -200 Potential (mV)

4.

Conclusion

In summary, a procedure consisting of coating cellulose fibers with silver NPs followed by thermal removal of the cellulose template was used to produce self-standing silver fibers. These fibers resemble the morphology of their templates and consist of aggregates of silver NPs. The structures were porous and exhibited a specific surface area of about 25 m2/g, suggesting the approximate diameter of NPs being 23 nm, which is the same order as SEM size. The partial oxidation of the surface of silver fibers is observed after heat treatment at 400  C. Adding 5 wt% NSSFs to graphite improves the oxygen reduction in basic media by increasing the current density and reduction in sufficient overpotential for initiating ORR. NSSFs seem appropriate for applications in high surface area electrodes in electrochemistry such as fuel cells.

references

-200 -300 -800

displays a well-defined peak at lower overpotential, and higher current density for ORR as compared to the GCE, which reveal electrocatlytic effect of NSSFs on ORR. Therefore, modified electrodes show a higher negative current density and/or more positive ORR onset potential, which show better electrocatalytic performance.

0

Fig. 4 – Linear sweep voltammetry graphs of oxygen reduction on (a) GCE, (b) modified GCE with 1 wt% and (c) with 5 wt%. Scan rate is 0.05V/s. (Reference: Ag/AgCl).

[1] Liu W, Zhang Z, Lin H, He W, Ge X, Wang M. Silver nanorods using HEC as a template by g-irradiation technique and absorption dose that changed their nanosize and morphology. Mater Lett 2007;61:1801–4. [2] Sun Y, Yin Y, Mayers BT, Herricks T, Xia Y. Uniform silver nanowires synthesis by reducing AgNO3 with ethylene glycol in the presence of seeds and poly(vinyl pyrrolidone). Chem Mater 2002;14:4736–45. [3] Park JH, Oha SG, Jo BW. Fabrication of silver nanotubes using functionalized silica rod as templates. Mater Chem Phys 2004;87:301–10. [4] Luo K, Dryfe RAW. The formation of silver nanofibres by liquid/liquid interfacial reactions: mechanistic aspects. New J Chem 2009;33:157–63. [5] Wade TL, Hoffer X, Mohammed AD, Dayen JF, Pribat D, Wegrowe JE. Nanoporous alumina wire templates for

3262

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15] [16]

[17]

[18]

[19]

international journal of hydrogen energy 35 (2010) 3258–3262

surrounding-gate nanowire transistors. Nanotechnology 2007;18:125201–4. Willander M, Nur O, Zhao QX, Yang LL, Lorenz M, Cao BQ, et al. Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers. Nanotechnology 2009;20:332001–40. Mohanty P, Yoon I, Kang T, Seo K, Varadwaj KSK, Choi W, et al. Simple vapor-phase synthesis of single-crystalline Ag nanowires and single-nanowire surface-enhanced Raman scattering. J Am Chem Soc 2007;129:9576–7. Kostowskyj MA, Gilliam RJ, Kirk DW, Thorpe S. Silver nanowire catalysts for alkaline fuel cells. Int J Hydrogen Energy 2008;33:5773–8. Jin M, Zhang X, Nishimoto S, Liu Z, Tryk DA, Emeline AV, et al. Light-stimulated composition conversion in TiO2-based nanofibers. J Phys Chem C 2006;111:658–65. Xing S, Zhao G. One-step synthesis of polypyrrole–Ag nanofiber composites in dilute mixed CTAB/SDS aqueous solution. Mater Lett 2007;61:2040–4. Jeon HJ, Kim JS, Kim TG, Kim JH, Yu WR, Youk JH. Preparation of poly(E-caprolactone)-based polyurethane nanofibers containing silver nanoparticles. Appl Surf Sci 2008;254:5886–90. Berchmans S, Nirmal RG, Prabaharan G, Madhu S, Yegnaraman V. Templated synthesis of silver nanowires based on the layer-by-layer assembly of silver with dithiodipropionic acid molecules as spacers. J Colloid Interface Sci 2006;303:604–10. Li XS, Fryxell GE, Wang C, Young J. Templating mesoporous hierarchies in silica thin films using the thermal degradation of cellulose nitrate. Microporous Mesoprorous Mater 2007; 99:308–18. Kemell M, Pore V, Ritala M, Leskela M, Linden M. Atomic layer deposition in nanometer-level replication of cellulosic substances and preparation of photocatalytic TiO2/cellulose composites. J Am Chem Soc 2005;127:14178–9. Shin Y, Exarhos GJ. Template synthesis of porous titania using cellulose nanocrystals. Mater Lett 2007;61:2594–7. Ghanem H, Kormann M, Gerhard H, Popovska N. Processing of biomorphic porous TiO2 ceramics by chemical vapor infiltration and reaction (CVI-R) technique. J Eur Ceram Soc 2007;27:3433–8. Aminian MKh, Taghavinia N, Iraji-zad A, Mahdavi SM, Chavoshi M, Ahmadian S. Highly porous TiO2 nanofibres with a fractal structure. Nanotechnology 2006;17:520–5. Hacker V, Wallnofer E, Baumgartner W, Schaffer T, Besenhard JO, Schrottner H, et al. Carbon nanofiber-based active layers for fuel cell cathodes: preparation and characterization. Elec Comm 2005;7:377–82. Kadirgan F, Kannan AM, Atilan T, Beyhan S, Ozenler SS, Suzer S, et al. Carbon supported nano-sized Pt–Pd and Pt–Co

[20]

[21]

[22]

[23] [24]

[25] [26]

[27]

[28]

[29]

[30]

[31]

[32] [33] [34] [35]

[36]

electrocatalysts for proton exchange membrane fuel cells. Int J Hydrogen Energy 2009;34:9450–60. Rajalakshmi N, Dhathathreyan KS. Nanostructured platinum catalyst layer prepared by pulsed electrodeposition for use in PEM fuel cells. Int J Hydrogen Energy 2008;33:5672–7. MacDonald JP, Gualtieri B, Runga N, Teliz E, Zinola CF. Modification of platinum surfaces by spontaneous deposition: methanol oxidation electrocatalysis. Int J Hydrogen Energy 2008;33:7048–61. Grinberg VA, Kulova TL, Maiorova NA, Dobrokhotova Zh V, Pasynskii AA, Skundin AM, et al. Nanostructured catalysts for cathodes of oxygen–hydrogen fuel cells. Russ J Electrochem 2007;43:75–84. Bockris JO’M. A primer on electrocatalysis. J Serb Chem Soc 2005;70:475–87. Ananth MV, Giridhar VV, Renuga K. Linear sweep voltametry studies on oxygen reduction of some oxides in alkaline electrolytes. Int J Hydrogen Energy 2009;34:658–64. Wang BJ. Recent development of non-platinum catalysts for oxygen reduction reaction. J Power Sources 2005;152:1–15. Ohsaka T, El-Deab MS. Recent research developments in electrochemistry, vol. 7. Transworld, Research Network Publisher India; 2004. Lee HK, Shim JP, Shim MJ, Kim SW, Lee JS. Oxygen reduction behaviour with silver alloy catalyst in alkaline media. Mater Chem Phys 1996;45:238–42. Lima FHB, de Catro JFR, Ticianelli EA. Silver-cobalt bimetallic particles for oxygen reduction in alkaline media. J Power Sources 2006;161:806–12. Lima FHB, Calegaro ML, Ticianelli EA. Electrocatalytic activity of dispersed platinum and silver alloys and manganese oxides for the oxygen reduction in alkaline electrolyte. Russ J Electrochem 2006;42:1283–90. Sharifi N, Taghavinia N. Silver nano-islands on glass fibers using heat segregation method. Mater Chem Phys 2009;113: 63–6. He J, Kunitake T, Watanabe T. Porous and nonporous Ag nanostructures fabricated using cellulose fiber as a template. Chem Commun; 2005:795–6. Dickson RM, Lyon LA. Unidirectional plasmon propagation in metallic nanowires. J Phys Chem B 2000;104:6095–8. Gregg SJ, Sing KSW. Adsorption surface area and porosity. London: Academic; 1982. p35. Weaver JF, Hoflund GB. Surface characterization study of the thermal decomposition of AgO. J Phys Chem 1994;98:8519–24. Hammond JS, Gaarenstroom SW, Winogard N. X-ray photoelectron spectroscopic studies of cadmium and silver oxygen surfaces. Anal Chem 1975;47:2193–9. Hamann CH, Hamnett A, Vielstich W. In: Electrochemistry. 2nd ed. Weinham: Wiley-VCH; 2007.