Materials Chemistry and Physics 91 (2005) 257–260

Ion exchange synthesis of silver vanadates fromorganically templated layered vanadates

S. Sharmaa, M. Panthoferb, M. Jansenb, A. Ramanana,∗a Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India

b Max Planck Institute for Solid State Research, Heisenbergstraße 1, D-70569 Stuttgart, Germany

Received 29 April 2004; received in revised form 2 August 2004; accepted 24 August 2004

Abstract

Ion exchange reactivity of layered vanadates was investigated using the metal ions Na+, K+ and Ag+. Amorphous phases were obtainedwith the alkali metal ions whereas in case of silver, formation of stable�-AgVO3 rods coated with silver nanoparticles was observed. In thispaper, we report a detailed analysis of nanostructured silver vanadate using high-resolution electron microscopy, powder X-ray diffraction(XRD) and thermal analysis.©

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2004 Elsevier B.V. All rights reserved.

eywords:Silver vanadates; Ion exchange; Layered vanadates; Intercalation

. Introduction

Silver vanadium oxides are important materials owing toheir ionic properties and for the use as a cathode mate-ial in lithium ion batteries[1]. Briton and Robinson[2,3]ere the first to report the synthesis of various silver vana-ate phases by the precipitation method. Since then severalroups have reported the formation of silver vanadium ox-

des[4,5]. Among these, AgVO3, AgxV2O5 and Ag2V4O11re the most commonly occurring phases in the solid state

1,6]. In a detailed structural and phase analysis, Fleury etl. [7] established the occurrence of three polymorphs forgVO3, namely �, � and �. Since then further attemptsere made to explore the structural chemistry of these phases

8,9]. Silver vanadium oxide phases are mostly synthesizedsing solid state reactions whereas the soft chemistry routeemains rather unexplored[10]. Znaidi et al.[11] and Kit-aka and co-workers[10,12]have successfully employed theon exchange method for synthesizing silver vanadium ox-des using vanadium oxide xerogels. But such reactions of-en took a very long time to complete from several weeks

to months[12]. Since the ion exchange behaviour of orgically intercalated vanadates and vanadyl phosphates isknown [13–15], we made an attempt to ion exchangetercalated organic cations with various metal ions sucNa+, K+ and Ag+ using hydrothermally synthesized layevanadates as precursors. Whereas reactions with alkalications resulted in the formation of amorphous phasesreaction with Ag+ yielded crystalline silver vanadates. Intestingly, transmission electron microscopy (TEM) analrevealed that the silver vanadates were decorated withmetal particles. To our knowledge such reduced silver mparticles have not been reported elsewhere for the Ag3system, though Zandbergen et al.[15] reported similar morphological features for Ag2− xV4O11 phase using high reslution TEM. Here, we report the ion exchange synthes�-AgVO3, its structural and thermal studies along with phrelations.

2. Experimental details

∗ Corresponding author. Tel.: +91 11 26591507; fax: +91 11 26582277.E-mail address:aramanan@chemistry.iitd.ac.in (A. Ramanan).

Vanadium pentaoxide, vanadium tetraoxide, tetraphenyl-phosphonium bromide, tetraphenylarsonium chloride, tetra-butylphosphonium bromide, tetraethylammonium bromide

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254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserveoi:10.1016/j.matchemphys.2004.08.024

258 S. Sharma et al. / Materials Chemistry and Physics 91 (2005) 257–260

and silver nitrate were obtained from Aldrich and usedwithout further purification. Fourier transformed infra-redspectra were recorded on KBr pellets using a Perkin-Elmer16PC and Nicolet 5DX spectrophotometer. Thermogravimet-ric/differential thermal analyses (DTA) were carried out usingPerkin-Elmer TGA/DTA7 systems on well-ground samplesin air or flowing nitrogen atmosphere with a heating rate of10 K min−1. Scanning electron microscopy (SEM) imageswere taken with the help of a Philips XL 30 ESEM. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8Advance diffractometer using Cu K� radiation. TEM studieswere done on a Philips CM 200 electron microscope.

All layered vanadates were prepared by a method reportedearlier [16]. For further ion exchange reactions the greensolids were stirred with 1 M solution of AgNO3. The progressof ion exchange was clearly evident from the colour change ofthe reaction medium; in all cases we observed colour changesfrom green to yellow during the course of the reaction. After36 h the yellow needle-like solids were separated from thesolution, washed with water and acetone, and dried in air.

3. Results and discussion

The ion exchange reaction of layered vanadates with silveri er noi Dp itha inall e andt per-s otho wasa wereu carryo f sil-v oups,

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Fig. 2. DTA curve for the Ag ion-exchanged product.

confirming that organic cations were completely leached outduring the reaction. In all products the silver-to-vanadiumratio was≈1.0 as established by EDX corresponding to thecomposition AgVO3.

Fig. 3. Evolution of the formation of�-AgVO3 as a function of time (a) 0 h(b) 16 h (c) 36 h and (d) 48 h at 30◦C.

Fig. 4. Indexed powder XRD of�-AgVO3.

ons has resulted in the formation of�-AgVO3. Ion exchangeactions involving alkali metal ions resulted in partial oron exchange.Fig. 1 shows a comparison of powder XRattern of (org)xV2O5 before and after the ion exchange wlkali metal ions. From the figure it is evident that the orig

ayered structure completely collapsed after ion exchanghe product formed is amorphous in nature. Energy disive X-ray analysis (EDAX) indicated the presence of brganic and metal ions and the morphology of the solidslso not homogeneous. Even after several attempts wenable to obtain single phase solids. Hence, we did notut any further investigation of these products. In case oer, ion-exchanged solids showed absence of organic gr

ig. 1. Powder XRD of (a) [(C6H5)4P]0.20V2O5, (b) Na+ ion-exchangend (c) K+ ion-exchanged solids.

S. Sharma et al. / Materials Chemistry and Physics 91 (2005) 257–260 259

Absence of organic moiety from the final product was fur-ther confirmed using thermogravimetric analysis and Fouriertransform infra red spectroscopy. It is well known that�-AgVO3 is quite stable under heat treatment and does not showany phase transformation until its melting point of around470◦C, while metastable�-AgVO3 undergoes a phase trans-formation to�-AgVO3 at 200◦C [9,10]. Considering thesefacts we analysed DTA curves of various solids.Fig. 2showsDTA plots for silver ion-exchanged vanadates. We did notobserve any peak in the lower-temperature region, whichconfirms the absence of an organic moiety. The first en-dothermic peak at 470◦C can be attributed to the melting of�-AgVO3.

3.1. Structural features of AgVO3

Powder XRD analysis was used for structural characteri-zation and phase identification of the materials. The progressof ion exchange reaction was monitored with the help ofpowder XRD. For this purpose the ion-exchanged productswere monitored at different time intervals.Fig. 3shows thatthe parent layered structure of (org)xV2O5 was destroyedwithin a few hours of mixing with silver nitrate. The evo-lution of AgVO3 phase was quite slow. The powder patterncan be unambiguously indexed in the space groupCmwitha= 18.106A, b= 3.5787A andc= 8.043A andβ = 104.44◦.The powder pattern and cell parameters resemble that of�-AgVO3. Fig. 4 shows the indexed powder XRD of the ion-exchanged product.

3.2. Electron microscopic analysis of AgVO3

The morphology of room temperature-synthesised silvervanadate was established using SEM as well as TEM analy-sis.Fig. 5 shows SEM images of ion-exchanged vanadates.

Fig. 5. SEM images of (a) [(C6H5)4As]0.19V2O5 and�-AgVO3 (b), (c).F[

ig. 6. TEM images of layered vanadium oxide precursors: (a)(C6H5)4As]V2O5, (b) [(C6H5)4P]V2O5.

260 S. Sharma et al. / Materials Chemistry and Physics 91 (2005) 257–260

There was a distinct change in the morphology of vanadatesbefore and after ion exchange.Fig. 5a exhibits a flaky natureof the layered vanadates whereasFig. 5b indicates a needle-like morphology of the silver vanadates.Fig. 6 shows TEMimages of the layered vanadates which exhibit rather large-sized rods and sheets of vanadates; on the other hand, Agion-exchanged solids exhibit a fine needle shape morphol-ogy with nanosized clusters on the surface which was laterconfirmed as silver metal particles by spot emission analy-sis (Fig. 7). Due to the small size (2–5 nm) and low volume

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fraction of these silver particles, their presence could not beestablished using powder XRD. It is well known that inter-calation of cations in the vanadium oxide matrix producesmixed-valent layered vanadates. The extent of intercalationin the layered structure is controlled by the reduction of thefully oxidised vanadium state V(V) to reduced vanadium stateV(IV). Such organically intercalated layered vanadates aregood precursors for ion exchange with other metal cationsand have been used for synthesis of reduced vanadium oxidebronzes[13,14].

We expected that the organic cation would be replaced byan equivalent amount of metal ion maintaining the layeredstructure. Contrary to this, alkali metal ions did not undergoany ion exchange reaction. Instead a complete collapse ofthe layered structure with partial or no ion exchange of theorganic moiety was observed.

4. Conclusions

Low-temperature ion exchange synthesis of�-AgVO3was carried out using organically intercalated vanadates asprecursors. In comparison to vanadium oxide xerogels theseprecursors form silver vanadates quite readily under ambientconditions. The formation of silver nanoparticles along with�-AgVO3 nanorods seems to be unprecedented.

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ig. 7. TEM images of�-AgVO3 nanorods with nanosilver sticking on theurface at magnifications of (a) 250k and (b) at 115k.

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