Vanadium pentoxide gels:

Vanadium Pentoxide Gels: I. Structural Study by Electron Diffraction JEAN-JACQUES LEGENDRE l AND JACQUES LIVAGE Laboratoire de Chimie de la MatiOre Condensde, LA 302, ENSCP 11 rue Pierre et Marie Curie 75231 Paris Cddex 05, Paris, France Received July 12, 1982; accepted November 5, 1982 Vanadium pentoxide gels consist of entangled polymeric fibers. Electron diffraction experiments have shown that these fibers are actually arranged as flat ribbons about 103 ~ long, 10z A wide, and 10 ~ thick. A two-dimensional order is observed along these ribbons which does not vary upon swelling. The two-dimensional cell parameters a = 27.0 A and b = 3.6 A suggest that the organization within the fibers is closely related to the lamellar structure of orthorhombic V205. The fiber is made of basic blocks containing 10 vanadium atoms along the a direction. These blocks seem to be linked together by water molecules which are strongly bonded to the structure and cannot be removed without crystallization of the xerogel into orthorhombic V205.

pentoxide gels were made up of entangled fibers (5). Water molecules are chemically and physically bonded to these fibers and a layered structure can be observed by X-ray diffraction (6). Most of the water can be removed by drying the gel at room temperature, leading to the so-called xerogel. This process is reversible and a gel can again be obtained by adding water through a swelling process (7). Moreover, vanadium pentoxide gels exhibit interesting intercalation properties and numerous organic molecules can be intercalated into its layered structure (8). Two sets of X-ray diffraction patterns can be observed. A 001 (in fact, 0k0 in orthorhombic V205 (9)) set corresponding to the one-dimensional interlayer spacing and a hk0 (similarly h01 (9)) one corresponding to the two-dimensional structure of the layers (6). The 001 set is observed on X-ray reflection patterns and the corresponding interlayer spacing varies by steps upon swelling. The hk0 set is observed on X-ray transmission patterns and does not vary when solvent molecules are intercalated. Thus the inner structure of the layers does not seem to be

INTRODUCTION

Transition metal oxide gels and colloids have recently received more attention because of their electronic properties (1). Vanadium pentoxide gels, for instance, exhibit semiconducting properties arising from the hopping of small polarons between V(IV) and V(V) ions (2). Their electrical conductivity reveals to be surprisingly high, about 3 orders of magnitude larger than in the crystallized V205 oxide. Such a high conductivity, together with the fact that gels can easily be deposited as thin layers, could lead to new applications in the field of material science. Patents have recently been taken in order to use vanadium pentoxide gels as antistatic coatings (3) or switching devices (4). No reliable interpretation of the electronic properties of these gels can, however, be pushed forward without an accurate knowledge of their structure. This is why undertaking such a study seemed important; that is the topic of this paper. Previous studies showed that vanadium To whom all correspondence should be addressed. 75

0021-9797/83 $3.00 Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983

Copyright © 1983 by Academic Press, Inc. All rights of reproduction in any form reserved

76

LEGENDRE AND LIVAGE

affected by the swelling or intercalation processes, but up to now, no information has been published about the structure of these layers. This paper presents an electron diffraction study of vanadium pentoxide gels. The domain investigated by such a technique may be much smaller than by X-ray diffraction, allowing a single fiber to be studied. MATERIALS AND METHODS Vanadium pentoxide gels are prepared by polycondensation of vanadic acid. The latter is obtained, free of foreign cations, by an ionexchange technique: a metavanadate solution is allowed to pass through a bed of resin (Dowex SOWX2, 50-100 mesh), according to a procedure described previously (10). A yellow solution of decavanadic acid is first obtained that polymerizes spontaneously upon ageing at room temperature by a polycondensation process (10). A colloidal solution of a gel can be obtained after a few hours depending on the amount of vanadium. A reversible sol-gel transition is observed for vanadium concentrations about 0.1 M (5). High polymeric species are obtained, having an average molecular weight of 106, i.e., containing about 104 vanadium atoms. Some reduction by water on the resin occurs during the preparation and about 1% of the vanadium ions are in the V(IV) oxidation state instead of V(V). Vanadium pentoxide gels loose water upon heating and crystallize around 350°C, leading to orthorhombic V205 (6). Their formula will then be written as V205. nH20 without giving more detail about the chemical nature of these water molecules. Most of the water readily evaporates at room temperature leading to a xerogel with the approximate formula V205" 1.6H20 with a basal interlayer spacing of 11.55/~. Upon heating, one water layer (2.8/~) is removed and a V205.0.5H20 xerogel is obtained around 250°C with a new basal interlayer spacing of 8.75/~ (6). Up to this stage water can again be reintroduced into this xerogel and a gel is obtained through a swelling process during which the interlayer Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983

spacing first increases by steps of 2.8 A. Hydration and dehydration of the vanadium pentoxide physical gel appears to be reversible as long as the last water molecules are not removed from the V205-0.5H20 formula. Electron diffraction studies were performed on a Philips EM 300 electron microscope. The samples were prepared by depositing a drop of a dilute colloidal solution onto a copper grid. Water then readily evaporates and a V205 xerogel is obtained. An initial vanadium concentration of about 10-2 M was necessary to obtain a xerogel layer thin enough for electron microscope observations. RESULTS

Imaging Low magnification images show that a V205 xerogel is made of entangled fibers more than 1000 A long and about 100 A wide (Fig. 1). No noticeable variation of the absorption power is observed either from one fiber to the other, or along a single fiber. This suggests that the thickness of the fiber is almost constant. Observations made on tilted fibers show that their thickness is much smaller than 100 A, presumably around 10 A. The fibers may then be described as long fiat ribbons. It has not been possible to avoid any sample motion under the electron beam, therefore at high magnification good resolution could not be obtained. Due to diffracted intensities being translation invariant, information on the structure has been looked for on diffraction diagrams.

Diffraction Experiments performed on such a xerogel with the incident beam approximately perpendicular to the grid, i.e., to the ribbons, show that they are rather perfectly organized (Fig. 2). Furthermore, such diagrams are similar to those obtained by transmission X-ray diffraction on gels (6) suggesting that the in-

FIG. 1. Electron microscope image o f a V205 xerogel.

FIG. 2. Electron diffraction pattern o f a V 2 O 5 xerogel. T h e electron b e a m is perpendicular to the grid. Journal of Colloid and Interface Science, Vol. 94, No. l, July 1983

2

FIG. 3. (a) Electron diffraction pattern of a V205 xerogel. The electron beam is tilted 30 ° from the grid plane; (1) rings corresponding to the inner organization o f the ribbons; (2) arcs corresponding to the ribbon interorganization. Spots 1, 3, and 4 may be seen with a careful observation of the original photograph. The most intense is spot 3. (b) Scheme of the intersection of the reflecting plane with reciprocal volumes whose reflecting power is strong enough for observation; (1) vector perpendicular to the grid, i.e., direction of the stacking of the ribbons; (2) vectors parallel to the grid. Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983

STRUCTURE

FIG. 4. Fiber diagram from a

O F V205 G E L S , (I)

79

V2O 5 xerogelwhose ribbons are approximatelyparallel.

ternal structure of the fiber is not modified when water is removed. This latter fact allows the results of the present study (performed on a xerogel) to be extended to the gels. The diffraction tings cannot be indexed on the V203 orthorhombic cell basis (9): the organization of the ribbons must be somewhat different. Moreover, a diffraction pattern obtained with an incident beam tilted 30 ° from the grid plane reveals that the rings correspond to a two-dimensional organization: they are only located in a part of the diagram corresponding to the section of a single plane of the reciprocal space by the reflecting plane (Figs. 3a and b). The other spots, located along a direction perpendicular to the tings, correspond to a periodicity of 8.75 A in the stacking of the ribbons. This value is in good agreement with previous X-ray diffraction experiments (6). The absence of any clear localization of the diffuse intensity outside these directions is due to the absence of any

strong correlation between the orientation of the periodically stacked ribbons. Cell Determination Diffraction over a domain where ribbons are approximately parallel yields a "fiber diagram" giving a first approach of the reciprocal space of a single fiber (Fig. 4). However, such a diagram is not accurate enough to allow any reliable indexation, especially if long periods and a monoclinic angle must be taken into account. In order to obtain simpler diagrams suitable for a reliable indexation, diffraction has been performed on a single ribbon. The corresponding diagram is shown on Fig. 5. Notice that continuous tings no longer appear. The diagram exhibits the superposition of several, rather well-defined spot systems having slightly different orientations. Thus one of these systems can be indexed owing to the small extension of the spots. Journal of Colloid and Interface Science, Vol. 94, No. l, July 1983

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LEGENDRE A N D LIVAGE

FIG. 5. Diffraction diagram on a single fiber of V205 xerogel. All the arcs are located on a (1/3.60) ~-1 × (1/27.0) A-I orthogonal lattice. The perfect orthogonality may be verified with the (OA, OB) angle which takes into account the position of well-defined spots (A and B).

One can notice that the lattice is orthogonal. A least square refinement performed from experimental reticular distances gives the fiTABLE I List of the Indexed Diffraction Rings of V205 Xerogel lndexation H

K

N~ o (/~-1)

N,~. (~-1)

Intensity

5 2 3 9 7 14 0

0 1 1 0 1 0 2

0.191 0.285 0.301 0.334 0.388 0.520 0.556

0.185 0.287 0.299 0.334 0.380 0.504 0.555

1 11 7 21 7.5 18.5 23.5

9 16 14

2~ 1J 2

0.651

0.648 0.655 0.760

6 6 7

0.757

Note. The final parameters are obtained by a least square refinement from the experimental radii o f rings (N). b = 3.60 A, a = 27.0 A, ¢/= 90 °. Journal of Colloid and Interface Science, Vol. 94, No. l, July 1983

nal parameters of the two-dimensional cell corresponding to a 0.6% R factor. Results are presented in Table I. The periodicity along the largest dimension of the ribbons (b = 3.6 A) is approximately equal to one of the parameters of orthorhombic V205. However, the periodicity along the perpendicular direction (a = 27.0 A) corresponding to the width of the ribbons is completely different from the other parameters of crystalline V205. Structure

Intensities of the various rings have been measured by microdensitometry from an electron diffraction diagram at the Laboratoire de MicrodensitomOtrie du CNRS. The thickness of the ribbons is small enough to avoid any dynamical correction. This can be checked by comparison with X-ray diffraction intensities.

STRUCTURE OF V205 GELS, (I) A two-dimensional (2D) Patterson function was calculated by Fourier transform of the 2D intensity diagram presented on Table I. It corresponds to the self-correlation function of the electronic density and therefore provides the interatomic vectors in the real structure (11). The analysis o f the Patterson m a p (Fig. 6) suggests that the structure o f the ribbons is close to that of o r t h o r h o m b i c VzOs: nearly all the m a i n peaks correspond to interatomic vectors that can be found in orthorhombic V205. The slight differences observed are probably meaningless according to the rather p o o r accuracy of the experimental data. One defect at least is, however, necessary if the periodicity of crystalline V205 is to be altered (11.51 ~), but a p o o r resolution of experimental data prevents accurate information to be obtained on such a defect. Anyway, its influence remains weak on the diffraction p h e n o m e n o n arising from the whole structure.

DISCUSSION

Electron microscope experiments show that polymeric v a n a d i u m pentoxide gels are m a d e up of entangled fibers. These fibers can be described as long flat ribbons about 1000 long and 100 ~ wide. T h e y exhibit a twodimensional structure defined by the two following cell parameters: a = 27.0 ~ and b = 3.6 ~. This structure appears to be closely related to the a c plane of o r t h o r h o m b i c V205 (9) but some defect must be taken into account in order to explain the difference observed on the a p a r a m e t e r (27.0 ~ instead of 11.51 /~ for VzOs). Electron diffraction experiments alone cannot give m o r e information about the nature of these defects. Thermal analysis on the other hand, indicates that some water (about 0.1H20 per V205) remains strongly b o u n d to the xerogel up to 300°C (6). Its departure occurs just before the exothermic crystallization into orthorhombic V205. These water molecules can therefore be thought to belong to the structure of the layers, which would explain

81

=

8

FIG. 6. Superposition of the experimental Patterson function in the (a, b) plane and interatomic vectors of orthorhombic V205. Half of the period in the a direction is presented (13.5 ~); the period in the b direction corresponds to 3.60 ~. The full circles correspond to experimental Patterson peaks whose intensities are approximately represented by the area of the circles. Large open circles correspond to vanadium-vanadium vectors in orthorhombic VzOs. Small open circles correspond to vanadium-oxygen vectors.

the difference observed with crystalline V205 in which of course, there is no water at all. A rather simple model in good agreement with all these experimental data is suggested on Fig. 7. It consists of orthorhombic V205 blocks linked together. Each o f these blocks contains five V205 groups (i.e., two edgesharing VO5 pyramids linked by corners to similar V205 groups). (a) Strongly bonded water molecules m a y play a role in the linkage of the blocks. Assuming that one water molecule is shared by two blocks, the a m o u n t of water would correspond to V 2 O s ' 0 . 2 H 2 0 , a value in good agreement with previous results. A calculated diffraction diagram based upon such a model (Table II), fits the exper-

5.75~,

a=27.0Z

t

FIG. 7. Model projection of the 2D xerogel organization in the (a, b) plane, a period: 27.0 A, b period: 3.6 A. It consists of five 5.75-A large orthorhombic V205 blocks. Structural discontinuities are indicated by arrows. Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983

82

L E G E N D R E A N D LIVAGE T A B L E II

tO

90

140

rll ....

I....

List of the Estimated Experimental Diffracted Intensities for rather Well-Oriented Fibers. Index 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 1 2 3 4 5 6 7 8 9 10

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 l 1 1 1 1

l~p.

I,~.

0 0 0 0 Weak 0 0 0 Strong Medium 0 0 0 Strong 0 0 0 0 0 0 0 Strong Weak 0 0 0 Medium 0 0 0

0 0 0 1 1 0 0 0 10 5 1 0 0 48 0 0 0 0 3 0 0 7 2 0 0 0 12 0 0 0

Index 11 12 13 14 15 16 17 18 19 0 1 2 3 4 5 6 7 8 9 l0 11 12 13 14 15 16 17 18 19

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

~.p.

I,~.

0 0 0 0 0 Weak 0 0 0 Very strong 0 0 0 0 0 0 0 0 Weak 0 0 0 0 Weak 0 0 0 0 0

0 1 0 0 2 25 10 1 0 74 0 0 0 1 1 0 0 0 10 5 1 0 0 48 0 0 0 0 3

Note. The calculated intensities are based on the structural model suggested in this paper.

imental data quite well: there is a good agreement between observed and calculated weak intensities. The difference for non-zero spots arise from the absence of temperature effect corrections. However, some of these discrepancies, especially the 4 0 spot non-zero calculated intensity, may be assigned to small structural deviations from perfect orthorhombic V205, i.e., small distorsions and bonded water molecules. (b) According to the poor accuracy for intensity measurements, the exact nature of the linkage between the V205 chains cannot be obtained. However, a careful analysis of the Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983

....

I

c

,,, . . . . . h I ................. FIG. 8. (a) Experimental intensities diffracted along the a direction. (b) Calculated intensities diffracted along the a direction. Calculation step: 0.01 ~-1. T h e disordered model consists of four blocks corresponding to the orthorhombic V205 structure. Width of successive blocks: 4, 5, 6, 5, VzO5 groups (each being 5.75 • wide). (c) Limited periodic model similar to the previous one. T h e four blocks are identical: they are built up with five V205 groups as the model in Fig. 7.

diagrams gives evidence for a broadening of the diffraction rings especially in the a direction. This can result from the small width of the fibers (about 100 /~). It may also arise from a statistical fluctuation of the number of the V205 groups which make up the blocks linked together along the width of the ribbons. The experimental diagram in the a direction, shown on Fig. 8, is in good agreement with this interpretation: the restriction of the ribbons width to four V205 blocks (-100 ~), or some fluctuation in the number of V205 chains in the linked blocks do not result in variations of the calculated intensities larger than the errors of the experimental data. A major question remains unanswered concerning the way adjacent blocks are linked together. We assumed that water molecules could be involved in such a linkage. Since diffraction data were collected in the hk0 plane, they cannot provide information on the thickness of the ribbons as well as the vertical organization of the V205 blocks. XRay diffraction experiments, performed in a direction perpendicular to the ribbons show that these blocks actually are not in the same

STRUCTURE OF V205 GELS, (I)

plane; this will be detailed in the following paper (12). The number of vanadium atoms contained in the 2D cell of the xerogel can be directly deduced from the value of a and b parameters. It might be worthy of note that 10 vanadium atoms are thus linked together along the length of a cell. This result could be related to the fact that vanadium pentoxide gels are obtained from the polycondensation of decavanadic acid which also contains 10 vanadium atoms per molecule. We therefore checked that a simple organization of perfect decavanadate ions could not fit the experimental diffraction data (13). REFERENCES 1. Livage, J., and Lernerle, J., Ann. Rev. Mater. Sci. 12, 103 (1982). 2. Bullot, J., Gallais, O., Gauthier, M., and Livage, J., Appl. Phys. Lett. 36, 986 (1980).

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3. Kodak Path6, French Patent BF 2 318 442 (1977) and BF 2429 252 (1979). 4. Bullot, J., and Livage, J., French Patent 81, 13,665 (1981). 5. Gharbi, N., Sanchez, C., Livage, J., Lemerle, J., Nejem, L., and Lefebvre, J., Inorg. Chem. 21, 2758 (1982). 6. Aldebert, P., Baffler, N., Gharbi, N., and Livage, J., Mater. Res. Bull. 16, 669 (1981). 7. Livage, J., Gharbi, N., Leroy, M. C., and Michaud, M., Mater. Res. Bull. 13, 1117 (1978). 8. Aldebert, P., Baffler, N., Gharbi, N., and Livage, J., Mater. Res. Bull. 16, 949 (1981). 9. Backman, H. C., Ahrned, F. R., and Barnes, W. H., Zeit. Krist. 115, 110 (1961). 10. Lemerle, J., Nejem, L., and Lefebvre, J., J. lnorg. Nucl. Chem. 42, 17 (1980). 11. Burger, M. J., "Crystal Structure Analysis," p. 554. Wiley, New York. 12. Legendre, J. J., Aldebert, P., Baffler, N., and Livage, J., J. Colloid lnterface Sci. 94, 84 (1983). 13. Swallow, A. G., Ahmed, F. R., and Barnes, W. H., Acta Cryst. 21, 397 (1966).

Journal of Colloidand InterfaceScience, Vol.94, No. 1, July 1983