Solid State Sciences 9 (2007) 27e31www.elsevier.com/locate/ssscie

Macroporous silver monoliths using a simple surfactant

Farid Khan, Muthusamy Eswaramoorthy, C.N.R. Rao*

Chemistry and Physics of Materials Unit and DST unit of Nanoscience, Jawaharlal Nehru Centre for Advanced Scientific Research,

Bangalore 560 064, India

Received 18 August 2006; received in revised form 20 October 2006; accepted 13 November 2006

Available online 16 January 2007

Abstract

An elegant method to synthesize porous silver monoliths using a simple surfactant cum reductant, Triton X-114, as the sacrificial template isdescribed. The gel forming property of the surfactant with silver nitrate is utilized to make the porous framework. The monoliths obtained witha mixture of Triton X-114 and dextran have also been examined. A significant improvement in the pore structure was observed when Triton X-114 was used along with Ludox silica sol, followed by calcination and HF treatment. The presence of interparticle pores in the 20e25 nm rangeon the macroporous silver framework suggests the role of silica spheres in the nanopore formation.� 2006 Elsevier Masson SAS. All rights reserved.

Keywords: Macroporous; Silver sponge; Triton X-114

1. Introduction

Porous metal nanostructures with long-range nanoscaleorder obtained by employing colloidal crystal templateswere described by Velev et al. [1] as early in 1999. Basedon computer simulation, nanoporosity arising from dealloyingof AgeAu alloy was reported a little later by Eriebacher et al.[2] It has been shown that pores are formed by the aggregationof chemically driven noble atoms into two-dimensional clus-ters by phase separation. Nanoporous gold with controlledmulti-modal pore size distribution has been prepared by deal-loying silver from AgeAu alloys by nitric acid treatment [3].The channel width of such a bicontinuous porous gold networkcan be tuned between 5 nm and a few microns by varying thestarting alloy composition. A particularly significant piece ofwork by Mann et al. [4] describes a self-supporting macropo-rous framework of Ag, Au and copper oxide by employingdextran as the template. Magnetic sponges were preparedwith the dextran template by using magnetic nanoparticles inthe place of the metal salt. Highly porous Au beads have

* Corresponding author. Tel.: þ91 80 23653075; fax: þ91 80 22082760.

E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao).

1293-2558/$ - see front matter � 2006 Elsevier Masson SAS. All rights reserve

doi:10.1016/j.solidstatesciences.2006.11.002

been prepared recently by applying emulsion-templated poly-mers as scaffolds [5], while cellulose fiber has been used tofabricate porous Ag nanostructures [6]. Poly(ethyleneimine)hydrogel has also been used as a soft template to make macro-porous silver frameworks [7]. The dome-shaped porous silverframeworks with inner chambers so obtained could bechanged into stair-like films by varying the calcination temper-ature. Porous Ag monoliths can also be prepared in a one-potreaction [8] using silica hydrogel as the template. Whileexploring the ways of preparing macroporous structures andmonoliths of silver by a simple method, we have found thata relatively simple surfactant, Triton X-114, can be effectivelyused for the purpose. Besides using Triton X-114, we haveexamined the macroporous structures obtained by using a com-bination of Triton X-114 with Ludox silica sol and dextran.

2. Experimental

In a typical synthesis, 7.5 g of silver nitrate (Ranbaxy) in7.0 g of water was added to 7.0 g of Triton X-114 (Aldrich)and stirred vigorously. Gelation occurred within 10 min andthe whole mixture got transformed into a semi-solid mass.The product was aged at room temperature for 2 days before

d.

28 F. Khan et al. / Solid State

calcining at 550 �C for 8 h at a heating rate, 1 �C/min in air. Inall the preparation procedures, the same molar ratio of silvernitrate to Triton X-114 was maintained.

To prepare the gel loaded with Ludox (silica sol containing40 wt% silica nanoparticles of diameter in the rage 20e24 nm), 10.8 mL of Ludox sol was added to the above silvernitrateeTriton X-114 solution. A black semi-solid mass gotformed, indicating the reduction of silver ions in the basic me-dium of the Ludox sol. The product was aged for 2 days andcalcined at 750 �C in air. The calcined sample was treated with20% HF for 24 h to dissolve silica. In addition, dextran incor-porated AgNO3eTriton X-114 paste was prepared by mixingthe AgNO3eTriton X-114 solution with dextran (Mr

2 000 000) in the molar ratio 44.1:16.4:0.003:8000 of

Fig. 1. Monolith of silver obtained with Triton X-114 template.

Fig. 2. FESEM image of macroporous silver framework obtained using Triton

X-114 template. Inset shows the magnified image.

AgNO3:Triton X-114:dextran:H2O. The product so obtainedwas dried at room temperature for 2 days and calcined at550 �C for 8 h.

The morphology of the silver monoliths was examined witha field emission scanning electron microscope (FEI Nova-NanoSEM-600). X-ray diffraction (XRD) patterns were re-corded with a Rich-Siefert 3000-TT diffractometer employingCu Ka radiation. Thermogravimetric analysis was carriedout with a Mettler Toledo TGA850 system. The BrunauereEmmetteTeller (BET) surface area for the samples was deter-mined at�196 �C using Quantachrome Autosorb-1C instrument.

3. Results and discussion

Optical micrographs of the porous silver framework mono-liths prepared by using Triton X-114, Triton X-114eLudox

Fig. 3. FESEM image of macroporous silver monolith obtained from Triton X-

114eLudox sol template after HF treatment: (a) low magnification showing

rugged surface and (b) high magnification showing the framework content.

Sciences 9 (2007) 27e31

29F. Khan et al. / Solid State Sciences 9 (2007) 27e31

and Triton X-114edextran templates showed them to havevoids of irregular shapes with sizes varying from 500 mm to1 mm. The micrograph of a typical monolith is known inFig. 1. Porous silver monoliths of various shapes and sizescould be made with all the three templates, by controllingthe initial shape of the gel. The XRD patterns for the Agmonoliths prepared with these templates showed peaks at2.37, 2.05, 1.44, 1.23 and 1.18 A corresponding to the (111),(220), (311) and (222) reflections of silver (JCPDS, JointCommittee on Power Diffraction Standard, 03-0867).

An FESEM image of a silver monolith prepared by usingTriton X-114 as the template is shown in Fig. 2. The image re-veals a porous morphology with a 3-D interconnected

Fig. 4. FESEM image of macroporous silver monolith obtained from Triton

X-114eLudox sol template after HF treatment having smooth framework

surface.

Fig. 5. Low magnification FESEM image of macroporous silver monolith

obtained from Triton X-114edextran template.

network. The pore sizes in the open-framework are well above3 mm. The framework thickness, however, is not uniform andvaries between 5 and 10 mm. We observe silver nanoparticleson the surface as can be seen in the inset of Fig. 2. Open cav-ities on the surfaces of the framework occur due to the bubble-burst process arising from the release of CO2, NO2 and steamduring calcination.

In Fig. 3a, an FESEM image of a silver monolith preparedby using Triton X-114eLudox as the template is shown. Theimage shows a rugged morphology with interconnected poresof a few microns. The pore size is smaller than that obtainedwith Triton X-114 alone as the template. The wall thicknessof the framework is in the range of 1e3 mm, which is smaller

Fig. 6. High magnification FESEM image of macroporous silver monolith

obtained from Triton X-114edextran template.

Fig. 7. TGA curves of as-synthesized (a) AgNO3eTriton X-114 composite, (b)

AgNO3eTriton X-114eLudox composite, and (c) AgNO3eTriton X-114edextran composite.

30 F. Khan et al. / Solid State Sciences 9 (2007) 27e31

Scheme 1. Morphological transformation of silver during calcinations. Yellow circle e silver ions; red circles e silver nanoparticles; blue circles e silica

nanoparticles.

than the thickness observed in monoliths prepared with TritonX-114. A close inspection of the rugged walls reveals that it ismade up of nanoparticles of silver of sizes varying in the 50e100 nm range (Fig. 3b). The interparticle pore size within thewall is in the range of 10e25 nm. The macrporous voids inthese structures have diameters ranging between 500 nm and1 mm. The spherical macroporous voids are likely to beformed by the reduction of the silver ions around the aggre-gates of silica nanoparticles, which are removed by treatmentwith HF. We also encounter regions of smooth silver frame-works with interconnected pores of few hundred nanometersformed by the fusion of bigger particles of sizes greater than500 nm (Fig. 4). EDAX analysis of these structures occasion-ally showed the presence of Si. As the sample was treated ina 20% HF for more than 24 h, followed by repeated washingwith water, it is unlikely that silica nanoparticles exposed tothe surface still remain. It is possible that a few silica particlesare buried inside the Ag framework.

Triton X-114edextran templated silver monoliths alsoshow porous framework structures with the pore sizes rangingfrom 500 nm to few microns. In Fig. 5, the FESEM image ofa typical monolith is shown. The pore sizes in these monolithsare smaller than those found with Triton X-114 alone as thetemplate. Higher magnification images show that the monolithare made up of thin sheets of the porous framework composedof grains of micron-sized silver particles fused in a 2-D ar-rangement. This feature is illustrated in Fig. 6.

The BET surface areas of the silver sponges obtained fromTriton X-114, Triton X-114edextran and Triton X-114eLudox (HF treated) templates were 0.7, 1.0 and 1.9 m2/g, res-pectively. These values are consistent with the distribution ofthe pore sizes, as a decrease in pore size accompanies an in-crease in the surface area. TG analysis of the Ag NO3eTritonX-114 composite shows two stages of weight loss (Fig. 7). Thesharp weight loss observed around 210 �C (26% weight loss)might be attributed to the removal of free Triton X-114

molecules (boiling point 200 �C). The gradual weight loss(42% weight loss) in the 210e400 �C region is due to thedecomposition of the AgNO3eTriton X-114 complex. In thecase of Triton X-114edextran templated silver sponges, thereis a sharp weight loss at 200 �C, from the decomposition ofdextran and Triton X-114. In the case of AgNO3eTriton X-114eLudox composite, the weight loss around 200 �C is sim-ilar to that of the AgNO3eTriton X-114, and there is onlya small weight loss (16%) in the 200e600 �C range indicatingthat nearly 20% silica remains in the network.

4. Conclusions

We have successfully prepared macroporous silver mono-liths by using the surfactant Triton X-114. The use of TritonX-114 as a template has the advantage over the other existingmethods, in that it is inexpensive and environmentally benign.It is to be stated that the formation of macroporous silver byTriton X-114 is nothing to do with the micelle formation asobserved in the conventional mesoporous materials synthesis.Addition of Ludox facilitates the reduction of silver ions evenat room temperature. Reduced silver ions around the aggre-gates of silica nanoparticles present in the AgNO3eTritonX-114 gel matrix gives rise to spherical voids on HF treat-ment. In Scheme 1, we show the morphological changes oc-curring during the process of monolith formation in the caseof Triton X-114 and Triton X-114eLudox templates. Themodified pore structure caused by the introduction of silicaspheres leads to a comparatively higher surface area.

Acknowledgement

One of the authors, (F.K.), thanks Dr. HariSingh GourUniversity, Sagar and the Indian National Science Academyfor enabling him to take up the INSA national fellowship.

31F. Khan et al. / Solid State Sciences 9 (2007) 27e31

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