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Vanadium pentoxide gels:
Vanadium Pentoxide Gels: II. Structural Study by X-Ray Diffraction JEAN-JACQUES LEGENDRE, l PIERRE ALDEBERT, NOEL BAFFIER, AND JACQUES LIVAGE Laboratoire de Chimie de la Matidre Condens~e LA 302, ENSCP, 11 rue Pierre et Marie Curie 75231 Paris Cedex 05, Paris, France
Received July 12, 1982; accepted N o v e m b e r 5, 1982 Accurate reflection X-ray diffraction m e a s u r e m e n t s performed on V205 gels swelled with various solvents reveal the existence of a c o m m o n modulation function of the Bragg peak intensities. T h e structure of the ribbons, which are the basic solid particles of such gels, is not modified by the a m o u n t or the nature of the solvent (the native m e d i u m H20, DMSO, or propylene carbonate) intercalated between the ribbons. A Patterson function calculated from the diffracted intensities evidences a noncoplanar linkage between the elementary orthorhombie adjacent fibrils constituting the ribbon: 2.8 is the corrugation amplitude of the ribbon. In addition to the most strongly bonded water which seems involved in the linkage of the fibrils, the existence of s o m e strongly bonded interfacial water can be assumed, even in xerogels, from the Patterson function analysis. INTRODUCTION
of ribbons and more precisely on the inner organization of ribbons which were considered as a two-dimensional (2D) structure up to now. Combining this information with the results of Part I (1), we are now able to propose a three-dimensional (3D) structural description of the elementary ribbon.
A first approach ofV205 xerogels has been given in Part I of the present series (1): the ribbon-like structure of this gel has been evidenced by electron diffraction and the structural elements in the organization of the ribbons have been described: they consist of 27-A-wide orthorhombic V205 fibrils linked together side by side. However, since only diffraction data located in the reciprocal plane parallel to the ribbons have been taken into account in the previous work, no information on the organization of the ribbons in a direction perpendicular to this plane is available at present. In this paper we present structural elements of this organization deduced from Xray reflection diffractometry experiments on V205 xerogels, either containing different amounts of water, or intercalated by organic solvent molecules (2, 3). Thus new information is provided on the batch organization
MATERIALS AND METHODS
The study of the organization of the ribbons along their stacking direction needs the preparation of different V205 xerogel samples. In fact the stacking changes noticeably while the ribbon structure remains nearly constant. This is the case with variously hydrated xerogels, in particular the room-temperature xerogel corresponding to the approximate V205-1.6H20 formula and the high-temperature xerogel V205-0.5H20 (4). Such samples can also be prepared owing to the V205 xerogel intercalation properties recently demonstrated (3), particularly with propylene carbonate and DMSO (2). In all
i To w h o m all correspondence should be addressed. 84 0021-9797/83 $3,00 Copyright© 1983by AcademicPress, Inc, All rightsof reproduction in any form reserved.
Journal of Colloid and Interface Science. Vol. 94, No. 1, July 1983
STRUCTURE OF V205 GELS, II
I,I
its~
,~l
jt ,
O.I
• []
l
\. II~I K'
FIG. 1. Representation of properly scaled X-ray diffracted intensities I along the 001 direction as a function of their reciprocal interreticular distance 1/d for various V:O5 gel samples. ([2) V205.1.6H20 xerogel (basal d spacing 11.55/~), (,k) DMSO-V205 intercalate (d = 17.5 /~), and (O) propylene carbonate-V205 intercalate (d = 21.5 A). The obvious common envelope to the various intensities is not drawn; the dotted curve corresponds to the squared Fourier transform of the electronic density projection along the 001 direction obtained by simulation from a V205.1.6H20 xerogel model.
85
calates, the projection of the organization of the ribbons in their basal plane is the same. In other respects the rather sharp 001 Bragg peaks recorded in reflection geometry diagrams reveal an order perpendicular to the platelet: information is thus obtained on the stacking periodicity of the ribbons, the latter depending on the nature and amount of the interfoliar species. Here we present the results of a systematic investigation on several V205 intercalates. It is based on accurate measurements of the intensities of the X-ray diffraction lines, resuiting in the determination of the organization of the ribbons along their stacking direction. RESULTS
cases the native gel spread onto a platelet and dried at room temperature gives rise to a thin film. Solvent intercalation is achieved within few hours with direct immersion of the xerogel into the liquid organic medium. The sample is then investigated by X-ray diffractometry in transmission and reflection geometry. Transmission geometry diagrams are obtained with the incident X-ray beam nearly perpendicular to the platelet; in this way correlations parallel to the platelet can be revealed, i.e., intra-ribbon correlations as far as ribbons are set nearly parallel to this plane as a result of spreading. Reflection geometry diagrams with the incident X-ray beam nearly parallel to the platelet reveal correlations perpendicular to the platelet, i.e., correlations in the stacking of ribbons. In the previous studies mentioned above, it has been pointed out that transmission diagrams are unaffected not only by the V205 gel water content (4) but also by the intercalation of several organic solvents, such as propylene carbonate and DMSO (2). This means that with these selected V205 inter-
X-Ray reflection diffraction patterns of several V205 intercalates have been recorded on a Philips PW 1349 diffractometer (XCu = 1.54 A) and their Bragg peak intensities determined after proper scaling and corrections. Data obtained with the 1.6H20 xerogel, propylene carbonate, and DMSO intercalates appear on Fig. 1. Such a selection seems somewhat restricted at first sight. Several reasons account for this. First of all, the interribbon distances are definitely different for these three samples (11.55, 17.5, and 21.5 A) which minimizes the number of overlapping peaks and results in a comprehensive understanding of the data. Besides, although higher rather well-ordered stages of hydration have been observed, some experimental difficulties still remain unsolved for these nonequilibrium samples, which still impede a quantitative analysis of their diffraction patterns. From the various Bragg peak intensities reported, it is quite obvious that a nearly common envelope exists. This observation is also confirmed by other samples, such as sulfolan and several long chain alkylammonium V205 intercalates. It must also be pointed out that extinctions of Bragg peaks
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
86
LEGENDRE ET AL.
occur around 0.17 and 0.50 ~--1. This has been systematically observed on the reflection patterns of the numerous VzOs gel intercalates prepared (2, 3). The existence of a Bragg intensities envelope common to various V205 gel intercalates gives evidence for the fact that the structure of the ribbons along their stacking direction is, in a first approach, not modified by the intercalated species. This assertion is based upon the fact that Bragg peaks intensities diffracted from periodically repeated identical objects are modulated by the squared Fourier transform of a single object. Therefore, in our case the Bragg peaks envelope is the squared Fourier transform of the electronic density of one ribbon projected on the stacking direction (which is perpendicular to the nearly parallel planes of the ribbons). Since intercalation induces noticeable variation in the projection neither along the stacking direction nor on the perpendicular plane, the inner structure of the ribbon is not modified by the intercalated species. The Bragg peak intensities of a VzOs1.6H20 xerogel, corrected for Lorentz-polarization effect, have been used to calculate a monodimensional Patterson function in order to show the correlations along the stacking direction (Fig. 2). The function exhibits several broad maxima, the most pronounced being located at 2.8 ,~ from the origin. The area of this most important peak, about half of the origin peak, indicates that the structure of a ribbon consists of two nearly identical objects 2.8 A apart in the stacking direction: such a distance is obviously too short to be an interlayer spacing. Since it has previously been demonstrated (1) that the elementary ribbon is built up with 27-A-wide V205 chains linked together, this 2.8-~ shift can only be attributed to a nonplanar linkage between adjacent chains. Thus the elementary layer constituting the V205 gels must be considered as a corrugated ribbon of orthorhombic VzOs (Fig. 3).
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
P
S~
~R
FIG. 2. Patterson function calculated from 001 intensities diffracted by V205.1.6H20 xerogel. The 2.8-A peak arises from the water-free ribbon model shown on Fig. 3. The existence of both secondary maxima can only be explained by the presence of water linked to V205. Indicative intensities of the different Patterson components are given by dotted peaks.
The two other maxima exhibited by the Patterson function (Fig. 2), respectively located around 1.7 and 4.5 ~, cannot be attributed to intraribbon interatomic vectors but more likely to vectors running from vanadium atoms of the ribbon to neighboring molecules, probably water molecules. In order to clear up this problem, we made a simulation of the VzO5" 1.6H20 xerogel diffraction diagram in the stacking direction, based upon the ribbon structural model presented on Fig. 3; with this water-free model, the squared Fourier transform of the electronic density projection along the 001 direction does not fit the experimental data reported
eL ~W a
W
W~
2.8A
FIG. 3. Schematic cross section along the a c plane of the basic V205 xerogel ribbon organization. The ribbon is made up o f several 27-~-wide (a direction) fibrils linked side by side: their length (21000 ~,) is perpendicular to the figure plane (b direction). The links are not represented as far as we have no direct indication on their exact nature and location from our diffraction data. They induce a ribbon corrugation (2.8 ~ amplitude) in the stacking direction (c direction).
S T R U C T U R E OF
on Fig. 1. A rough model including the fight number of water molecules gives a rather good fit as shown by the dotted curve on Fig. 1. In this manner the existence of the two secondary broad Patterson maxima can be explained since the water-free model can give rise to the 2.8-A peak only (Fig. 2). However, it must be pointed out that positions and intensities of these two maxima depend on the location of water molecules. The influence of water on such diffraction data is thus noticeable, but not strong enough to allow an accurate localization of water molecules with Tespect to the V205 ribbon. This explains why we are only able to give an upper limit for the thickness of the elementary ribbon: the value of 8.75 A has been in fact deduced from the interlayer d spacing in the high temperature form of the xerogel (4). An important part of the 0 . 5 H z O / V 2 0 5 which remains at this stage, about 0.3-0.4H20, is strongly bonded to the surface of the water-free ribbon (shown on Fig. 3); the thickness of the latter is obviously less than 8.75 A. DISCUSSION
The above diffraction experiments provide a structural model for the basic VzO5 ribbon but give little information on the part played by water in this structure. Such a contribution is likely to be important and gets clearer in the light of previous results obtained on these gels. The dehydration process of the V205- 1.6HzO xerogel, followed by DTA and GTA, reveals the existence of different kinds of water (4): a strong endothermic peak is observed from room temperature up to 120°C corresponding to the evolution of the water more weakly bonded to the ribbons (about one H20 per VeOs). The interribbon distance correlatively decreases from 11.55 to 8.75 A. Then a small endothermic peak appears between 210 and 270°C while about 0.5H20 per V205 is removed. The weakness of the DTA peak could result from a simultaneous
V205
GELS, II
87
exothermic phenomenon: the shortening of the interribbon distance could indeed induce a partial bonding between adjacent ribbons; of course the almost irreversible character of the xerogel dehydration process in this temperature range supports this hypothesis. The last water departure, about 0.1H20 per V205, occurs around 300°C, giving rise to an endothermic peak, immediately followed by the strong exothermic crystallization peak of orthorhombic V205. The first two kinds of water are not involved in the inner organization of the ribbon: this is confirmed by high-temperature X-ray transmission diffraction patterns which exhibit no noticeable variation up to 300°C (5). This is probably not the case for the last kind of water, even more strongly bonded. The linkage of the 27A-wide elementary V205 fibrils to form the above mentioned corrugated ribbon ideally requires one link for 5 V205 units. The practical number of links is necessarily less since an average ribbon contains only four fibrils and many structural defects probably exist. Thus, assuming one link per molecule of water, the amount of the latter (measured as 0.5HeO for 5 V205 units) is rather consistent with our hypothesis, the accuracy of the determination being taken into account. It must also be pointed out that, around 300°C, when this water is removed, no Bragg peaks are observed on the X-ray reflection diffraction patterns: Bragg peaks of crystalline orthorhombic V205 only appear at 350°C. In the light of our hypothesis, this phenomenon is easily understandable. The departure of the strongly linked water around 300°C splits up the corrugated ribbons into their elementary 27-A-wide V205 fibrils. The explanation for this may be imagined as follows: the Coulombic repulsion forces which give rise to the one-dimension (1D) order, no longer exist between the fibrils when they become neutral V205 particles as a result of the evolution of the water responsible for ribbon splitting. Afterwards a reor-
Journal of Colloid and Interface Science. Vol. 94, No. 1, July 1983
88
LEGENDRE
ganization process takes place in which the elementary fibrils can behave as crystallization germs; when the crystallites become large enough, diffraction patterns of orthorhombic V205 are observed, around 350°C. These considerations tend to show that the third kind of water although present in small amounts plays a determining part in the inner organization of the corrugated ribbon for the basic V205 gel, it is similarly worthy of note that 2.8/~, the corrugation amplitude, corresponds to the size of one water molecule. CONCLUSION
This work emerges on a rather precise structural description of the solid phase constituting V205 gels prepared by polycondensation of decavanadic acid. A previous study has revealed the layered structure of such gels (4). It has been shown in Part I by means of electron diffraction (1) that the basic layer, concerning its 2D average dimensions (1000 × 100 ~) is more likely a ribbon made up of elementary 27 /~-wide fibrils linked side by side. The structure of these fibrils exhibits close resemblances to crystalline V205: as a matter of fact they are built up with the same ordering of V205 pyramidal entities as orthorhombic V205 giving rise to the same 3.6 /~ periodicity along the b axis (Part I, Fig. 7). X-Ray diffraction (Part II) evidenced an outof-plane linkage of the 27 A elementary fibills resulting in a basic corrugated ribbon. The corrugation amplitude value is 2.8 ~, and 8.75 A represents an upper limit for the ribbon thickness as far as this value probably includes some interfacial water bonded to the ribbon. The crosschecking of several observations gives an indication for the linkage of the elementary fibrils inside a ribbon being ensured by strongly bonded water molecules. It must be emphasized that the study of such unusually organized material has involved nonclassical structural determination techniques; consequently, the details of the
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
E T AL.
model proposed are not all reliable to the same degree, some of them requiring further investigations. As a matter of fact, only the well organized part of the gel is taken into account by diffraction techniques; the proportion of the lesser organized part remains unknown: absolute intensity measurements have not been undertaken. Therefore care must be taken when precise correlations between structural characteristics of the gel and the amount of water are stated; they assume either that a poorly organized V205 gel does not exist at all, or that the amount of water is the same in both the well organized and the poorly organized portion of the gel, i.e., that the average amount of water which has been measured corresponds to the well organized gel. This problem will be faced in other experiments aiming at localizing water more precisely by means of complementary diffraction and spectroscopic techniques, mainly neutron diffraction on fully deuterated samples, NMR and ENDOR. This work is in progress. It must be pointed out that a great support to our structural model is already provided by several experimental observations. High polymeric species of such V205 gels are known to contain about 104 vanadium atoms (6). Calculating from our ribbon model (1000 long, 100 A wide) results in a value of the same order. In a forthcoming paper, an experimental evidence for an average ribbon width of about 100 A (corresponding to a few elementary fibrils linked together) will be provided by means of the S.A.N;S. technique. Lastly, this ribbon model emerges on a 2D host structure description of the V205 xerogel which suggests a possible preparation of many V205 intercalates under mild conditions. Beyond, the corrugation of the ribbon seems to be a steric necessity to understand the formation of several intercalates (3). It is thus quite an evidence that such a versatile host structure for intercalation ex-
89
STRUCTURE OF V205 GELS, II
hibits an unusual and complex structure as compared to the classical crystalline 2D host structure such as sheet silicates, graphites, and metal dichalcogenides. It must then be pointed out that in order to be reliable, structural investigation on such imperfectly ordered materials as gels must focus several different techniques on the same problem: electron microscopy, X-ray, electron and neutron diffraction, thermal analysis, and spectroscopy. Only the comparison of several nondependent experimental data can prove to be efficient for confident results about such unusual structural studies. Nevertheless, the results of this first structural approach of the V205 xerogels represent a significant improvement for a better understanding of the properties of these new promising materials.
ACKNOWLEDGMENTS The authors are grateful to A. Quivy for her help in the X-ray diffraction measurements.
REFERENCES 1. Legendre, J. J., and Livage, J., J. Colloid Interface Sci. 94, 75 (1983). 2. Aldebert, P., and Baffler, N., Gharbi, N., and Livage, J., Mater. Res. Bull. 16, 949 (1981). 3. Aldebert, P., Baffler, N., Legendre, J. J., and Livage, J., Rev. Chim. Min. 19, 485 (1982). 4. Aldebert, P., Baffler, N., Gharbi, N., and Livage, J., Mater. Res. Bull. 16, 669 (1981). 5. Aldebert, P., and Baffler, N., unpublished results. 6. Gharbi, N., Sanchez, C., Livage, J., Lemerle, J., Nejem, L., Lefebvre, J., Inorg. Chem. 21, 2758 (1982).
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
Received July 12, 1982; accepted N o v e m b e r 5, 1982 Accurate reflection X-ray diffraction m e a s u r e m e n t s performed on V205 gels swelled with various solvents reveal the existence of a c o m m o n modulation function of the Bragg peak intensities. T h e structure of the ribbons, which are the basic solid particles of such gels, is not modified by the a m o u n t or the nature of the solvent (the native m e d i u m H20, DMSO, or propylene carbonate) intercalated between the ribbons. A Patterson function calculated from the diffracted intensities evidences a noncoplanar linkage between the elementary orthorhombie adjacent fibrils constituting the ribbon: 2.8 is the corrugation amplitude of the ribbon. In addition to the most strongly bonded water which seems involved in the linkage of the fibrils, the existence of s o m e strongly bonded interfacial water can be assumed, even in xerogels, from the Patterson function analysis. INTRODUCTION
of ribbons and more precisely on the inner organization of ribbons which were considered as a two-dimensional (2D) structure up to now. Combining this information with the results of Part I (1), we are now able to propose a three-dimensional (3D) structural description of the elementary ribbon.
A first approach ofV205 xerogels has been given in Part I of the present series (1): the ribbon-like structure of this gel has been evidenced by electron diffraction and the structural elements in the organization of the ribbons have been described: they consist of 27-A-wide orthorhombic V205 fibrils linked together side by side. However, since only diffraction data located in the reciprocal plane parallel to the ribbons have been taken into account in the previous work, no information on the organization of the ribbons in a direction perpendicular to this plane is available at present. In this paper we present structural elements of this organization deduced from Xray reflection diffractometry experiments on V205 xerogels, either containing different amounts of water, or intercalated by organic solvent molecules (2, 3). Thus new information is provided on the batch organization
MATERIALS AND METHODS
The study of the organization of the ribbons along their stacking direction needs the preparation of different V205 xerogel samples. In fact the stacking changes noticeably while the ribbon structure remains nearly constant. This is the case with variously hydrated xerogels, in particular the room-temperature xerogel corresponding to the approximate V205-1.6H20 formula and the high-temperature xerogel V205-0.5H20 (4). Such samples can also be prepared owing to the V205 xerogel intercalation properties recently demonstrated (3), particularly with propylene carbonate and DMSO (2). In all
i To w h o m all correspondence should be addressed. 84 0021-9797/83 $3,00 Copyright© 1983by AcademicPress, Inc, All rightsof reproduction in any form reserved.
Journal of Colloid and Interface Science. Vol. 94, No. 1, July 1983
STRUCTURE OF V205 GELS, II
I,I
its~
,~l
jt ,
O.I
• []
l
\. II~I K'
FIG. 1. Representation of properly scaled X-ray diffracted intensities I along the 001 direction as a function of their reciprocal interreticular distance 1/d for various V:O5 gel samples. ([2) V205.1.6H20 xerogel (basal d spacing 11.55/~), (,k) DMSO-V205 intercalate (d = 17.5 /~), and (O) propylene carbonate-V205 intercalate (d = 21.5 A). The obvious common envelope to the various intensities is not drawn; the dotted curve corresponds to the squared Fourier transform of the electronic density projection along the 001 direction obtained by simulation from a V205.1.6H20 xerogel model.
85
calates, the projection of the organization of the ribbons in their basal plane is the same. In other respects the rather sharp 001 Bragg peaks recorded in reflection geometry diagrams reveal an order perpendicular to the platelet: information is thus obtained on the stacking periodicity of the ribbons, the latter depending on the nature and amount of the interfoliar species. Here we present the results of a systematic investigation on several V205 intercalates. It is based on accurate measurements of the intensities of the X-ray diffraction lines, resuiting in the determination of the organization of the ribbons along their stacking direction. RESULTS
cases the native gel spread onto a platelet and dried at room temperature gives rise to a thin film. Solvent intercalation is achieved within few hours with direct immersion of the xerogel into the liquid organic medium. The sample is then investigated by X-ray diffractometry in transmission and reflection geometry. Transmission geometry diagrams are obtained with the incident X-ray beam nearly perpendicular to the platelet; in this way correlations parallel to the platelet can be revealed, i.e., intra-ribbon correlations as far as ribbons are set nearly parallel to this plane as a result of spreading. Reflection geometry diagrams with the incident X-ray beam nearly parallel to the platelet reveal correlations perpendicular to the platelet, i.e., correlations in the stacking of ribbons. In the previous studies mentioned above, it has been pointed out that transmission diagrams are unaffected not only by the V205 gel water content (4) but also by the intercalation of several organic solvents, such as propylene carbonate and DMSO (2). This means that with these selected V205 inter-
X-Ray reflection diffraction patterns of several V205 intercalates have been recorded on a Philips PW 1349 diffractometer (XCu = 1.54 A) and their Bragg peak intensities determined after proper scaling and corrections. Data obtained with the 1.6H20 xerogel, propylene carbonate, and DMSO intercalates appear on Fig. 1. Such a selection seems somewhat restricted at first sight. Several reasons account for this. First of all, the interribbon distances are definitely different for these three samples (11.55, 17.5, and 21.5 A) which minimizes the number of overlapping peaks and results in a comprehensive understanding of the data. Besides, although higher rather well-ordered stages of hydration have been observed, some experimental difficulties still remain unsolved for these nonequilibrium samples, which still impede a quantitative analysis of their diffraction patterns. From the various Bragg peak intensities reported, it is quite obvious that a nearly common envelope exists. This observation is also confirmed by other samples, such as sulfolan and several long chain alkylammonium V205 intercalates. It must also be pointed out that extinctions of Bragg peaks
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
86
LEGENDRE ET AL.
occur around 0.17 and 0.50 ~--1. This has been systematically observed on the reflection patterns of the numerous VzOs gel intercalates prepared (2, 3). The existence of a Bragg intensities envelope common to various V205 gel intercalates gives evidence for the fact that the structure of the ribbons along their stacking direction is, in a first approach, not modified by the intercalated species. This assertion is based upon the fact that Bragg peaks intensities diffracted from periodically repeated identical objects are modulated by the squared Fourier transform of a single object. Therefore, in our case the Bragg peaks envelope is the squared Fourier transform of the electronic density of one ribbon projected on the stacking direction (which is perpendicular to the nearly parallel planes of the ribbons). Since intercalation induces noticeable variation in the projection neither along the stacking direction nor on the perpendicular plane, the inner structure of the ribbon is not modified by the intercalated species. The Bragg peak intensities of a VzOs1.6H20 xerogel, corrected for Lorentz-polarization effect, have been used to calculate a monodimensional Patterson function in order to show the correlations along the stacking direction (Fig. 2). The function exhibits several broad maxima, the most pronounced being located at 2.8 ,~ from the origin. The area of this most important peak, about half of the origin peak, indicates that the structure of a ribbon consists of two nearly identical objects 2.8 A apart in the stacking direction: such a distance is obviously too short to be an interlayer spacing. Since it has previously been demonstrated (1) that the elementary ribbon is built up with 27-A-wide V205 chains linked together, this 2.8-~ shift can only be attributed to a nonplanar linkage between adjacent chains. Thus the elementary layer constituting the V205 gels must be considered as a corrugated ribbon of orthorhombic VzOs (Fig. 3).
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
P
S~
~R
FIG. 2. Patterson function calculated from 001 intensities diffracted by V205.1.6H20 xerogel. The 2.8-A peak arises from the water-free ribbon model shown on Fig. 3. The existence of both secondary maxima can only be explained by the presence of water linked to V205. Indicative intensities of the different Patterson components are given by dotted peaks.
The two other maxima exhibited by the Patterson function (Fig. 2), respectively located around 1.7 and 4.5 ~, cannot be attributed to intraribbon interatomic vectors but more likely to vectors running from vanadium atoms of the ribbon to neighboring molecules, probably water molecules. In order to clear up this problem, we made a simulation of the VzO5" 1.6H20 xerogel diffraction diagram in the stacking direction, based upon the ribbon structural model presented on Fig. 3; with this water-free model, the squared Fourier transform of the electronic density projection along the 001 direction does not fit the experimental data reported
eL ~W a
W
W~
2.8A
FIG. 3. Schematic cross section along the a c plane of the basic V205 xerogel ribbon organization. The ribbon is made up o f several 27-~-wide (a direction) fibrils linked side by side: their length (21000 ~,) is perpendicular to the figure plane (b direction). The links are not represented as far as we have no direct indication on their exact nature and location from our diffraction data. They induce a ribbon corrugation (2.8 ~ amplitude) in the stacking direction (c direction).
S T R U C T U R E OF
on Fig. 1. A rough model including the fight number of water molecules gives a rather good fit as shown by the dotted curve on Fig. 1. In this manner the existence of the two secondary broad Patterson maxima can be explained since the water-free model can give rise to the 2.8-A peak only (Fig. 2). However, it must be pointed out that positions and intensities of these two maxima depend on the location of water molecules. The influence of water on such diffraction data is thus noticeable, but not strong enough to allow an accurate localization of water molecules with Tespect to the V205 ribbon. This explains why we are only able to give an upper limit for the thickness of the elementary ribbon: the value of 8.75 A has been in fact deduced from the interlayer d spacing in the high temperature form of the xerogel (4). An important part of the 0 . 5 H z O / V 2 0 5 which remains at this stage, about 0.3-0.4H20, is strongly bonded to the surface of the water-free ribbon (shown on Fig. 3); the thickness of the latter is obviously less than 8.75 A. DISCUSSION
The above diffraction experiments provide a structural model for the basic VzO5 ribbon but give little information on the part played by water in this structure. Such a contribution is likely to be important and gets clearer in the light of previous results obtained on these gels. The dehydration process of the V205- 1.6HzO xerogel, followed by DTA and GTA, reveals the existence of different kinds of water (4): a strong endothermic peak is observed from room temperature up to 120°C corresponding to the evolution of the water more weakly bonded to the ribbons (about one H20 per VeOs). The interribbon distance correlatively decreases from 11.55 to 8.75 A. Then a small endothermic peak appears between 210 and 270°C while about 0.5H20 per V205 is removed. The weakness of the DTA peak could result from a simultaneous
V205
GELS, II
87
exothermic phenomenon: the shortening of the interribbon distance could indeed induce a partial bonding between adjacent ribbons; of course the almost irreversible character of the xerogel dehydration process in this temperature range supports this hypothesis. The last water departure, about 0.1H20 per V205, occurs around 300°C, giving rise to an endothermic peak, immediately followed by the strong exothermic crystallization peak of orthorhombic V205. The first two kinds of water are not involved in the inner organization of the ribbon: this is confirmed by high-temperature X-ray transmission diffraction patterns which exhibit no noticeable variation up to 300°C (5). This is probably not the case for the last kind of water, even more strongly bonded. The linkage of the 27A-wide elementary V205 fibrils to form the above mentioned corrugated ribbon ideally requires one link for 5 V205 units. The practical number of links is necessarily less since an average ribbon contains only four fibrils and many structural defects probably exist. Thus, assuming one link per molecule of water, the amount of the latter (measured as 0.5HeO for 5 V205 units) is rather consistent with our hypothesis, the accuracy of the determination being taken into account. It must also be pointed out that, around 300°C, when this water is removed, no Bragg peaks are observed on the X-ray reflection diffraction patterns: Bragg peaks of crystalline orthorhombic V205 only appear at 350°C. In the light of our hypothesis, this phenomenon is easily understandable. The departure of the strongly linked water around 300°C splits up the corrugated ribbons into their elementary 27-A-wide V205 fibrils. The explanation for this may be imagined as follows: the Coulombic repulsion forces which give rise to the one-dimension (1D) order, no longer exist between the fibrils when they become neutral V205 particles as a result of the evolution of the water responsible for ribbon splitting. Afterwards a reor-
Journal of Colloid and Interface Science. Vol. 94, No. 1, July 1983
88
LEGENDRE
ganization process takes place in which the elementary fibrils can behave as crystallization germs; when the crystallites become large enough, diffraction patterns of orthorhombic V205 are observed, around 350°C. These considerations tend to show that the third kind of water although present in small amounts plays a determining part in the inner organization of the corrugated ribbon for the basic V205 gel, it is similarly worthy of note that 2.8/~, the corrugation amplitude, corresponds to the size of one water molecule. CONCLUSION
This work emerges on a rather precise structural description of the solid phase constituting V205 gels prepared by polycondensation of decavanadic acid. A previous study has revealed the layered structure of such gels (4). It has been shown in Part I by means of electron diffraction (1) that the basic layer, concerning its 2D average dimensions (1000 × 100 ~) is more likely a ribbon made up of elementary 27 /~-wide fibrils linked side by side. The structure of these fibrils exhibits close resemblances to crystalline V205: as a matter of fact they are built up with the same ordering of V205 pyramidal entities as orthorhombic V205 giving rise to the same 3.6 /~ periodicity along the b axis (Part I, Fig. 7). X-Ray diffraction (Part II) evidenced an outof-plane linkage of the 27 A elementary fibills resulting in a basic corrugated ribbon. The corrugation amplitude value is 2.8 ~, and 8.75 A represents an upper limit for the ribbon thickness as far as this value probably includes some interfacial water bonded to the ribbon. The crosschecking of several observations gives an indication for the linkage of the elementary fibrils inside a ribbon being ensured by strongly bonded water molecules. It must be emphasized that the study of such unusually organized material has involved nonclassical structural determination techniques; consequently, the details of the
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983
E T AL.
model proposed are not all reliable to the same degree, some of them requiring further investigations. As a matter of fact, only the well organized part of the gel is taken into account by diffraction techniques; the proportion of the lesser organized part remains unknown: absolute intensity measurements have not been undertaken. Therefore care must be taken when precise correlations between structural characteristics of the gel and the amount of water are stated; they assume either that a poorly organized V205 gel does not exist at all, or that the amount of water is the same in both the well organized and the poorly organized portion of the gel, i.e., that the average amount of water which has been measured corresponds to the well organized gel. This problem will be faced in other experiments aiming at localizing water more precisely by means of complementary diffraction and spectroscopic techniques, mainly neutron diffraction on fully deuterated samples, NMR and ENDOR. This work is in progress. It must be pointed out that a great support to our structural model is already provided by several experimental observations. High polymeric species of such V205 gels are known to contain about 104 vanadium atoms (6). Calculating from our ribbon model (1000 long, 100 A wide) results in a value of the same order. In a forthcoming paper, an experimental evidence for an average ribbon width of about 100 A (corresponding to a few elementary fibrils linked together) will be provided by means of the S.A.N;S. technique. Lastly, this ribbon model emerges on a 2D host structure description of the V205 xerogel which suggests a possible preparation of many V205 intercalates under mild conditions. Beyond, the corrugation of the ribbon seems to be a steric necessity to understand the formation of several intercalates (3). It is thus quite an evidence that such a versatile host structure for intercalation ex-
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STRUCTURE OF V205 GELS, II
hibits an unusual and complex structure as compared to the classical crystalline 2D host structure such as sheet silicates, graphites, and metal dichalcogenides. It must then be pointed out that in order to be reliable, structural investigation on such imperfectly ordered materials as gels must focus several different techniques on the same problem: electron microscopy, X-ray, electron and neutron diffraction, thermal analysis, and spectroscopy. Only the comparison of several nondependent experimental data can prove to be efficient for confident results about such unusual structural studies. Nevertheless, the results of this first structural approach of the V205 xerogels represent a significant improvement for a better understanding of the properties of these new promising materials.
ACKNOWLEDGMENTS The authors are grateful to A. Quivy for her help in the X-ray diffraction measurements.
REFERENCES 1. Legendre, J. J., and Livage, J., J. Colloid Interface Sci. 94, 75 (1983). 2. Aldebert, P., and Baffler, N., Gharbi, N., and Livage, J., Mater. Res. Bull. 16, 949 (1981). 3. Aldebert, P., Baffler, N., Legendre, J. J., and Livage, J., Rev. Chim. Min. 19, 485 (1982). 4. Aldebert, P., Baffler, N., Gharbi, N., and Livage, J., Mater. Res. Bull. 16, 669 (1981). 5. Aldebert, P., and Baffler, N., unpublished results. 6. Gharbi, N., Sanchez, C., Livage, J., Lemerle, J., Nejem, L., Lefebvre, J., Inorg. Chem. 21, 2758 (1982).
Journal of Colloid and Interface Science, Vol. 94, No. 1, July 1983