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Synthesis of niobium pentoxide gels
Journal of Non-Crystalline Solids 79 (1986) 383 395 North-Holland, Amsterdam
383
S Y N T H E S I S OF N I O B I U M P E N T O X I D E G E L S C. A L Q U I E R , M.T. V A N D E N B O R R E and M. H E N R Y Lahoratoire Spectrochirnie du Solide, Uniuersiz~ Pierre et Marie Curie. 4, place Jus,~ieu. 7144, 2b ~tage, 75230 Paris Cedex 05, France
Received 14 February 1985
Niobium pentoxide gels can be obtained by the two classical ways already known for silica. namely destabillisation of a sol and alkoxide's hydrolysis. Owing to the lack of reproducibility of the first route and to the high cost of niobium alkoxides, two other processes are proposed. Addition of hydrogen peroxide to the sol greatly improves the destabilisation process, while using chloroalkoxides instead of alkoxides reduces the cost of the precursors, h is thus shown that either monolithic gels or thixotropic gels can be obtained depending on the process used. Upon drying these gels lead to quite different xerogels, that can be either amorphous oxides or even mixed mineralorganic materials.
1. Introduction
The sol-gel synthesis of glasses and ceramics has received significant attention during the last decade [1,2]. It opens new possibilities in the field of material science [3,4]. Most of the systems studied until now have been based o n S i O 2 in order to make glasses. Two routes are usually followed: - The destabilization of colloidal silica. This can be done just by varying the p H of the aqueous solution [5]. This method appears to be quite easy and cheap but not very versatile and therefore its possibilities are rather limited. - Colloidal silica or gels can also be obtained from organometallic precursors, by hydrolysis and polycondensation of metal alkoxides [3,6]. This process is much more versatile. Multicomponent systems can be mixed on a molecular scale leading to a new molecular engineering. Mixed organic inorganic materials can even be synthetized in that way [7]. However, due to the high cost of most alkoxide precursors, this process is limited to a small number of oxides, namely SiO 2 and TiO 2. New developments of the sol-gel process appear nowadays in the field of ceramics. Ultrafine and monodispersed powders can be obtained, allowing improved sintering [8]. New gels based on A1203 or ZrO~ have then been studied. This paper deals with the synthesis of N b 2 0 s gels. Very little information can be found in the literature about such gels. However they could lead to new applications in the field of display devices [9] reversible cathodes [10,11] or ferroelectric ceramics such as LiNbO s [12]. 0022-3093/86/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Experimental
2.1. Niobium pentachloride hydrolysis NbCl 5 is a yellow hygroscopic powder. It was purchased from Fluka (72510) and used as such, without any further purification.
2.1.1. Nb:O~ sols A gelatinous precipitate of hydrated niobium oxide can be obtained by carefully mixing 1.08 g (4 m mol.) of NbC15 into 10 ml of water. The reaction is quite violent and HC1 gas is evolved. Several samples of this voluminous precipitate are then prepared. The first one is left to stand for 24 h until the precipitate completely dissolves giving rise to a clear solution, called "sol 1". Another part of the precipitate is washed with water, several times, without any special care in order to remove most of the CI ions. The gelatinous precipitate then densities and becomes almost insoluble. A very small proportion is dissolved after several days leading to "sol 2". A third part of the gelatinous precipitate is left to stand for 10 min only allowing some deposition at the bottom of the beaker. This precipitate is then very carefully washed with water, avoiding any mechanical disturbance in order not to destroy the colloidal dispersion. It is then left to stand and a clear solution is obtained within 24 h, called "sol 3". The pH of sols 2 and 3 is about 2 while sol 1 remains much more acid.
2.1.2. Gel formation Aqueous colloidal solution can usually be destabillized by adding electrolytes or modifying the pH. Such a procedure cannot be followed here because Nb205 sols appear to be quite sensitive toward electrolytes and flocculation occurs as soon as some foreign ions are introduced into the sol. Destabillization was then obtained by increasing the temperature. Therefore the sols were kept for one hour at 60°C. Let us call t the ageing time of the sol separating the complete peptization of the gelatinous precipitate and the thermal treatment. Different behaviors were observed depending on the nature of the sol. Sol 1 remains quite stable whatever the ageing time might be, unless the destabillization process becomes too severe in which case a precipitate is obtained. - A complete peptization of sol 2 is never observed upon ageing even after several days. A gelatinous precipitate is obtained after the thermal treatment. Gelation occurs with sol 3, but the appearance of the gel greatly depends on the ageing time t. For t = 0, two phases are obtained; a monolithic gel and a clear solution. The gel looks transparent, elastic, non thixotropic and remains stable for months. The gel part becomes more and more important when the ageing time
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increases but some opalescence appears beyond t = 48 h. After one week the whole solution gives rise to a gel that no longer appears monolithic. It is opaque, white and becomes thixotropic. A sol can be obtained just by shaking the beaker. This sol-gel transition is reversible and a gel is formed again through a thermal treatment at 60°C for one hour. All these observations of course depend on the niobium concentration. Below 0.3 M, gelation never occurs while above 0.7 M a clear sol is difficult to obtain. Moreover we noticed that the method of mixing water and NbCI 5 could be very important. Adding water to the NbCI 5 powder for instance, instead of the reverse, very quickly leads to a clear sol, that behaves like sol 1, i.e. gelation never occurs. The purity of the water seems to be very important too, all our experiments were performed with deionised water. Gels have never been obtained with distilled water, only precipitates or colloidal solutions. 2.1.3. Hydrolysis through hydrogen peroxide NbC15 hydrolysis can also be performed with hydrogen peroxide. This process has the advantage of removing most of the chlorine from the gel. Different experimental procedures have been followed. Method I - 0.14 g (0.52 m mol.) of NbC15 are carefully added to 3 ml of a 110 vol. hydrogen peroxide solution (30% H 2 0 2 - P r o l a b o 23619). A violent exothermic reaction occurs, leading to a yellow solution. 7 ml of a 20 vol. hydrogen peroxide solution (6% H 2 0 2 - P r o l a b o 23616) are then added five minutes later. The whole solution is cooled during this time in order to avoid ebullition, till all gas (O2-C12-HC1) stops evolving. A clear yellow solution is thus obtained, the p H of which is close to 4. This solution gives rise to a monolithic yellow gel when left for 12 h at 60°C. Method 2 - A flocculent precipitate is formed, as previously described, by adding NbCI 5 to the water. A 110 vol. hydrogen peroxide solution is then quickly added, in order to avoid any ageing of the precipitate. This one thus dissolves leading to a yellow solution whose pH is about 2. This solution is heated at 60°C in order to remove all gas. It leads to a yellow opaque gel after 12 h. Hydrogen peroxide can also be added after washing and ageing of the precipitate. 24 h later for instance, the reaction appears to be quite gentle and gelation occurs as above. Method 3 - 5 ml of a 110 vol. hydrogen peroxide solution can also be added onto the previous monolithic gel, quickly leading to its dissolution together with some gas evolution. 1 ml of water is then added while the yellow solution is stirred and heated. The viscosity progressively increases. A yellow, opaque and bulky gel is obtained about 10 min later. 2.2. Niobium alkoxide hydrolysis
The precursor is now the niobium pentaethoxide Nb(OEt)5 purchased from Ventron 51119. It is extremely sensitive to water. 2.5 g (7.86 m mol.) of Nb(OEt)5 are carefully dissolved into 500 ml of pure ethanol (RP normapur
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Prolabo 20821). This alcohol is not perfectly dry and still contains some 0.2% of water, (44 m mol.) enough for a complete hydrolysis of all the alkoxo groups. A clear solution is first obtained that quickly becomes opaque, within 10 min at room temperature, leading to a milky sol. This sol remains stable for weeks. It can turn into a gel when the niobium concentration increases, for instance by evaporing the solvent. Gelation occurs for a niobium concentration of about 0.3 M. This gel gives rise to a white powder after drying.
2.3. Niobium chloroalkoxides hydrolysis Chloroalkoxides can easily be obtained by adding NbC15 to an alcohol. Different alcohols have been studied, all of them have been used without any further purification, 8.1 g (30 m mol) of NbC15 being dissolved into 100 ml of alcohol. The reaction is performed in a glove box in order to avoid moisture. It appears to be very exothermic and HC1 gas is evolved. The solution is stirred until the complete dissolution of NbCl 5. It can then be stored in a dry atmosphere without any problem. Hydrolysis is then performed, 24 h later, on 7 ml of the solution. The water is added dropwise under vigorous stiring and gentle heating (30°C) for 10 to 15 min. Let us call h the molar ratio H z O / N b . The behavior towards hydrolysis depends on the alcohol used. With primary alcohol such as ethanol (Prolabo 20821) or methanol (Prolabo 20847), water can be added in excess without any precipitation. The sols are translucent (h = 300 for MeOH and h = 28 for EtOH) and undergo a sol-gel transition together with a quick opacification upon ageing, leading to white gels. When a secondary alcohol is used (Propanol-2 Prolabo 20842), no ageing is needed the white gel setting spontaneously for h = 21. Upon drying (60°C) all these gels lead to fine white powders. The dissolution of NbC15 in a tertiary alcohol (Methyl-2 butanol-2 Prolabo 20802) is difficult, leading to a light brown viscous sol (0.15 M in Nb). A light brown powder is obtained by heating the brown gel at 60°C formed at h = 32. Polyols lead to opaque gels and transparent xerogels. A white glue is obtained for h = 32, with ethanediol (Prolabo 20041), leading to a transparent plastic material after drying. On the other hand a white grease is obtained by diluting a 0.7 M solution of NbC15 in ethanol to 0.3 M with glycerol (Prolabo 24387). Upon prolonged heating (60°C) the gel loses its opacity giving rise to a translucent grease. Non-thixotropy and stability are the main features of these chloroalcogels providing that the niobium concentration is kept between 0.2 M and 0.6 M and that the chloroalkoxide solution is not hydrolyzed immediately after its preparation (24 h delay seems the minimum). We have also checked that water can be added in a straightforward manner without inhibiting the subsequent gelation, the time of setting depending only upon concentration. Another quite important property of these chloroalcogels is their behavior towards UV. Blue colors are obtained from sols and gels while the non-hydro-
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lysed solution becomes purple. A purple color is always obtained with glycerol solution. The larger the time of irradiation, the darker is the color.
3. Discussion
3.1. Hydrogels Insoluble, hydrated niobium pentoxide (niobic acid) can be obtained mainly by two methods. The first one includes the hydrolysis of a water soluble complex of Nb(V) by ammonia, while the second one involves the acidification of an alkali-niobate (V) solution [13]. This precipitate is a white soli& amorphous to X-ray and electron diffraction. Its structure, as well as its water content, is still a much debated question [13,15]. It has been known for a long time [18] that niobium pentoxide hydrosols can be easily produced through washing or centrifugation of the freshly prepared oxide although it is very difficult to obtain a compound free from any electrolyte [13,15,16]. However, the gelation of such hydrosols has been described in only a few cases. Thus using the dialysis technique of Solov'ev et al. [17] to obtain a thixotropic gel starting with a sol containing 3 CI per niobium, the number of chloride ions trapped in the gel being unknown. Hydrosol preparation, by the acidification method, does not seem to be very useful. Most alkali-niobates are not soluble enough and lead to a niobium concentration too low for the sol-gel transition to occur. We have therefore called upon the hydrolysis of niobium pentachloride. In order to avoid the long and tedious process of dialysis, the chlorine ions were removed by careful washing or by adding hydrogen peroxide. The action of water upon NbC15 leads to aqueous solutions of Nb205 and HCI. The state and nature of Nb(V) in aqueous hydrochloric acid has been investigated by spectrophotometry [19], electrodialysis [21] and polarography [22]. Four main species are formed depending on the relative concentrations of H * and C1 ions. In strongly acidic solutions ( > 3 M HC1)INb(OH)2C141 ion is predominant. Decreasing the hydrochloric acid concentration leads to the formation of the neutral complex Nb(OH)2C13 (NbOC13/H20). INb(OH)2C14I
~Nb(OH)2C13 + CI .
If perchloric acid is added at this stage, in order to get a higher concentration of H + ions, a cationic complex is formed: Nb(OH)2C13 + H+ ~ INb(OH)C131 ~+ H20Finally colloid formation is observed below 2M: INb(OH)2C141 + H 2 0 ~ Nb(OH)3C12 + H + + 2C1 . NbC15 being merely dissolved into water at about 0.4 M concentration, the overall chloride concentration remains smaller than 2 M and thus a flocculent
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precipitate of Nb(OH)3C12 a n d / o r Nb(OH)5 is first formed. This precipitate has a quite open texture and therefore peptisation can occur leading to a mixed sol containing small colloidal particles of Nb(OH)3C12 together with Nb(OH)2CI 3 molecules. Owing to the non colloidal nature of the latter, the concentration of colloidal particles in non-washed sols (no. 1) is too small to allow a sol-gel transition. Only precipitates can be obtained through destabilization. We can then understand why the way of mixing water and NbC15 is so important in the hydrosol preparation. Dispersion of NbC15 into water favours Nb(OH)5 a n d / o r Nb(OH)3C12 formation while adding water onto NbC15 favours the non active complex Nb(OH)2C13. This point of view is supported by the work of Kepert et al. [31] showing that as long as water is not in excess, C1- ions remain linked to the niobium atom. If the precipitate is carefully and quickly washed (sol no. 3), Nb(OH)2CI 3 cannot be formed anymore while peptisation still occurs leading to a clear sol where all niobium compounds are in a colloidal form. The open texture of the solid phase is no longer preserved when the precipitate is washed without any care, leading to a more densified structure. As a consequence some competition between peptisation and ageing takes place, a small amount of Nb(V) being peptized while the remainder is transformed into non-reactive hydrated niobium pentoxide. Ageing is a quite general process occurring with numerous hydrated metal oxides; it can be discussed in terms of olation and oxolation reactions [23] as suggested for niobic acid [24]. In this case, polymerisation, with the formation of a primary particle, takes place through intermolecular elimination of water between M ( O H ) , X s _ n units. Owing to the low charge/mass ratio of those particles, flocculation occurs followed by coagulation a n d / o r coalescence if mechanical disturbances are not avoided. Careful washing, keeping the flocculent aspect of the precipitate, allows further peptisation leading to a clear sol containing small colloidal particles. Monolithic and transparent gel can be obtained with such a sol providing ageing has not yet started i.e. the sol must undergo destabilisation immediately after peptisation has stopped. This gel can then be considered as a homogeneous olated polymer where all the bonds are N b - O H - N b bridges. Actually coacervation is observed with a freshly prepared sol; the lower part being more concentrated undergoes some gelation while the upper part remains too dilute to undergo a sol-gel transition. This probably arises from the highly dispersed nature of the sol prepared in this way, a point which has been confirmed by centrifugation experiments. Now, if the sol is aged, Ostwald ripening occurs, decreasing the polydispersity of the sol, smaller particles disappear on behalf of larger ones. Concerning these latter two processes occur: first, particle growth via olation at the outer part of the particle then densification via oxolation ( N b - 0 - N b bridges) inside the particle, the core of which becomes non reactive niobium pentoxide. As a consequence coacervation progressively disappears while opacity (light diffusion) and thixotropy increase as the particles become larger.
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Using distilled water instead of biexchanged water apparently inhibits the gel formation. Chemical analysis of the biexchanged water was then undertaken, showing that distilled water contains less electrolytes than biexchanged water and thus specific adsorption must probably account for the differences observed. However it is quite difficult to determine which ion is determinent, so further study of this point seems usefull. Hydrogen peroxide improves the process, that becomes more reproducible and less sensitive toward the experimental procedure. The following reactions are proved to occur in hydrochloric acid medium [35]. (complex formation: H202 + HCI ~ H202 • HC1 OH oxidation:H202 • HC1 --> H 2 0 ~ ~ H 2 0 + HOC1 CI reduction: HOCI + H 2 0 : ~ H2_~O+ HC1 + 02 dismutation: HOC1 + HCI ~ C12 + H 2 0 , As a result chloride ions are removed through an oxidoreduction process which does not pertube the mechanical equilibrium of the sol, by-products being oxygen and chlorine gas: 3H202 + 2HC1 ~ 4 H 2 0 + 02 + C12. These gases can easily be removed by gentle heating before gelation leaving just water and niobium pentoxide thus avoiding the tedious and delicate steps of washing. Hydrogen peroxide also prevents precipitation of insoluble solids through the formation of peroxo complexes providing that the pH is kept below 3, in which case precipitation of the insoluble 1 : 1 complex occurs [26]. This depolymerisation action of H202 [27] enables us to use distilled water as well as biexchanged water for the hydrolysis reaction and therefore peptisation is not needed anymore. The chemical nature of the peroxocomplex thus formed is not yet elucidated. It may be a peroxocation (INb(OH)4(H202)I + or INbO2(H202) [+) according to Babko et al. [26] and Nabivanets [27] or a peroxoanion INbO2(O-O)[ as stated by Spinner [28]. The yellow color developed on addition of H202 may be due to the formation of peroxoniobic acid [29], but further investigation is needed to confirm this point. As far as gels are concerned, it is known that when hydrogen peroxide is added to a hydrochloric acid solution of niobic acid, a yellow precipitate of niobium perhydroxide Nb205 - H202 • 5H20 is formed [30]. On the other hand peroxo-ortho niobic acid H N b O 4, n H 2 0 can be obtained as a colloidal solution from perniobic acid [18]. In both cases, a yellow and stable amorphous solid is formed upon drying. Further investigations are thus made in order to determine whether such peroxogels are present in our gels. The best gel is obtained when NbC15 is added directly onto H202 which seems quite obvious according to our preceding discussion. This is also the most convenient method of preparation, for washing is not needed. However,
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as shown, hydrogen peroxide can be added at various steps of the process (methods 1-4) without inhibiting the gel formation. The problem previously underlined are thus solved when using H202, however yellow colored gels are obtained and care must be taken in order to avoid bubbles and too much exothermic reaction. 3.2. A lcogels Using a Nb(OR)5 alkoxide instead of NbC15 usually leads to purer gels, the hydrolysis products being ROH instead of HC1. The synthesis of such alkoxides is carried out through dissolution of NbC15 into an alcohol in the presence of ammonia, or by an exchange reaction from another alkoxide such as Nb(OEt)5 [31,32]. These alkoxides form dimers in non complexing solvents as shown by NMR, two octahedral units sharing one edge [33,34]. In complexing medium, depolymerisation occurs leading to the formation of octahedral complexes Nb(OR)5 - (35,36). The flexibility of these alkoxides arises from the labile OR groups that can be replaced by higher alcohols [37], organic esters [38], /3 keto-esters [39,40] /3-diketones [41], glycols [42,43], phenols [43b], oximes or hydroxylamines [44]. Few studies have been performed on the hydrolysis reactions of such compounds. Bradley et al. [45] did not publish their results concerning the hydrolysis of niobium pentaalkoxides, but claimed to have produced from Nb(OEt)5, a crystalline compound Nb8010(OEt)20 [46] whose preparation conditions have unfortunately not been given. Decaniobate anion Nb~002~6 , isostructural to the decavanadate anion, has been proved to occur when Nb(OEt)5 is hydrolysed with tetramethylammonium hydroxide in methanol [47]. Other isopolyniobates (Nb6069 , Nb12037 14- , Nb2406510 ) have been obtained by Fuchs et al. [48] through hydrolysis of Nb(OEt) s with various organic bases at various molar ratios h = H z O / N b . It is difficult to assess if such species are really present in our gels when h > 5.5. Quick opacification of the solution points out that hydrolysis occurs at least partly beyond discrete molecules or anions, but in view of the great reactivity of niobium alkoxide toward water, it is highly probable that the main reaction is: Nb(OEt)5 + 5 H 2 0 ~ Nb(OH)5 + 5EtOH
(1)
and not Nb(OEt)5 + n H 2 0 ~ Nb(OH).(OEt)5 ~ + nEtOH.
(2)
Gelation must proceed from an olation/oxolation process between Nb(OH) 5 molecules with elimination of water rather than polycondensation between hydroxoalkoxoniobates with elimination of alcohol. A detailed study of the hydrolysis of Nb(OEt)5 seems necessary to determine which reaction (1) or (2) is predominant.
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3.3. Chloroalcogels This is a combination of the two previous methods. The halides and oxyhalides of the early transition series and their stability and reactivity in non aqueous media have been reviewed by Walton [49]. It is well known that a solvolysis reaction occurs when NbCI 5 and an alcohol are mixed together leading to the formation of a trichloroalkoxide [14,41,50]. NbC15 + ROH(excess) ~ NbCI 2 (OR)3 + 3HC1.
(3)
These chloroalkoxides have been widely studied from a chemical point of view, the OR group being methanol [51,52], ethanol [53,54], phenol [55], carboxylic acid [50,56], benzoins [57] or quinolinol [58]. In complexing media formation of NbCIz(OR)2L [59-63] and NbCI3(OR)2L [64] occurs, while oxochloroalkoxides NbOCI 2(OR)L, NbOCi(OR)2 L [65,66] are also known. As for the alkoxides, hydrolysis reactions of such compounds have been much less studied [67,68]. Hydrolysis reactions were carried out in six different alcohols and marked differences appeared. These differences can be explained on the basis of the mechanism of the chloroalkoxide formation. Ridge and Todd [69] dealing with SIC14 have studied such a mechanism, where all the chloride ions can be replaced by OR groups just by reacting SiCI 4 and ROH: SiC14 + ROH(excess) -~ Si(OR)4 + 4HC1.
(4)
Two mechanisms appear possible, both involve the initial coordination of a lone pair of electrons from an alcoholic oxygen atom followed by the break of either the hydroxy or alkoxy bond. In both cases, the electron density at the metal atom increases leading to a weakening metal-halogen bond: H+ J O ~ ) N b ( } C 1 --, ~ N b - O R + HC1 -R ~
(5)
I
H R + \ O ~ Nb~-C1 ~ ~ N b - O H + RC1
(6)
Alcohols containing - I inductive groups react in accordance with mechanism (5) while strongly + I inductive groups lead to mechanism (6). Another complication arises from side reactions such as [48]: HCI + ROH ~ R + + H 2 0 + CI
I
R++
C1 --* RC1
(7)
R + H 2 0 ~ H 3 0 + + olefin
Reactions (6) and (7) are favoured when tertiary alcohols (strongly + I inductive effect) are used. In such a case, water is formed in situ allowing hydrolysis and polymerisation to occur. Thus using methyl-2 butanol-2 leads to a viscous (hydrolysis) and lightly coloured (olefin) sol. Such problems do not
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arise with primary or secondary alcohols, while with ethylenglycol or glycerol (strongly-I inductive effect) side reactions are completely avoided. The behaviour of methanol sols can be explained on the basis that chloromethoxides are highly polymeric owing to the relative small size of the methoxy groups with respect to the higher alkoxy groups. In such a case, no coordination position is available for water molecules and depolymerisation must occur first before very fast hydrolysis can proceed. Using polyfunctional alcohols leads to transparent and coherent materials instead of powders after drying. With ethylenglycol two different reactions can occur; an intra molecular reaction: "-. / C 1 0] ~Nb\ + H O - C H z - C H 2 - O H ~ /-~Nb /\ + 2HC1 C1 0
1
(8)
and an intermolecular one (cross-linking): i
Nb-C1 + H O - C H 2 - C H 2 - O H + C I - N b ~ N b - O
I
O-Nb.
(9)
With reaction (9) a mixed mineral organic polymer is obtained without the addition of water while with reaction (8) the same polymer if formed only by adding water: ~] ~Nb\
O-CH2-CH2-OH + H 2 0 ~ ~Nb.. OH
O-CH2-CH2-OH ~Nb\
d
b-Nb~-
+ C 1 - N b ~ - o N b ~/ OH
OH
In both cases monoliths are formed upon drying through cross linking reactions. The same reactions can occur with glycerol, however, owing to the small amount of water added the resulting material looks more organic than mineral (grease). This kind of reaction is quite useful for making new materials and has been applied to the elaboration of contact lenses with Si instead of Nb (7). Photoreduction of Nb(V) to Nb(IV) (blue color) or Nb(III) (purple color) is also observed with these chloroalkoxides. They will be studied in more detail in another paper. 3.4. Relations between the three methods
When NbCI 5 is mixed with alcohol ROH, the NbCI2(OR)3 complex is formed. If we consider water as an alcohol, the R group being a proton, the same complex will be formulated, NbCI2(OH)3. This is just the formulae of the colloid obtained by mixing NbC15 with water. Owing to the strong coordinative properties of the OH groups, this complex NbCI2(OH)3 is not stable as a monomer and olation leads to colloid formation. The same
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phenomenon probably also arises, but to a much lesser extent, when methanol is used instead of water, owing to the relatively small size of the O C H 3 group. When higher alcohols are used, the complex is probably at most a dimer and can be hydrolysed without problems. A quite obvious relation appears between the alkoxide and the chloroalkoxide routes because both precursors have labile alkoxy-groups allowing a good flexibility for the process. As far as purity is concerned, the use of hydrogen peroxide and NbC15 or the use of an alkoxide Nb(OR) 5 are equivalent, for in the two cases N b ( O H ) 5 is generated as an olated polymer leading to purer gels.
4. Conclusion In this work four different ways of preparing niobium pentoxide gels N b 2 0 5, n H 2 0 have been reported. It is possible to obtain gels by the two classical processes known for silica. The use and niobium pentachloride and water is an extension of the " L u d o x " method developed by Phalippou et al. [5]; in both cases a sol formed by peptisation of a solid phase undergoes a destabillisation process leading to the gel. A sol-gel transition is thus proved to occur with niobium pentoxide, the differences with silica arising from the chemical nature of the sol. The great drawback of this method arises from the tedious and delicate procedure and from the lack of reproducibility. We have shown that adding hydrogen peroxide greatly improves the process by removing chloride ions. The other classical method involves niobium alkoxides and can also lead to gels. Purer compounds are thus obtained, however the process is rather limited by the high cost of alkoxides [Nb(OEt)~ being 275 times more expensive than TEOS = Si(OEt)4 ]. The best process, from our point of view is the chloroalkoxide route. This is a good compromise between the flexibility of the alkoxides and the lower cost of the mineral precursor [NbC15 is 15 times less expensive than Nb(OEt)5]. The need for the alkoxide's synthesis is thus avoided. Indeed, it would seem quite strange to make an alkoxide, which will immediately be broken down by water in order to produce the oxide. The versatility of the process arises from the choice of the alcohol. We have shown that quite different xerogels can be obtained according to the precursor used.
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H. Dislich and P. Hinz, J. Non-Cryst. Solids 48 (1982) 11. S.P. Mukherjee, J. Non-Cryst. Solids 42 (1980) 477. B.E. Yoldas, J. Mater. Sci. 12 (1977) 1203. H. Dislich, J. Non-Cryst. Solids 57 (1983) 371. J. Phalippou, M. Prassas and J. Zarzycki, J. Non-Cryst. Solids 48 (1982) 17. S. Sakka and K. Kamiya, J. Non-Cryst. Solids 42 (1980) 403.
394 [7] [8[ [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33} [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53]
C. A lquier et al. / Synthesis of niobium pentoxide gels G. Philipp and H. Schmidt, J. Non-Cryst. Solids 63 (1984) 283. K.S. Mazdiyasni, Ceramics Int. 8 (1982) 42. B. Reichman and A.J. Bard, J. Electrochem. Soc. 127 (1980) 241. B. Reichman and A.J. Bard, J. Electrochem. Soc. 128 (1981) 344. N. Kumagai, K. Tanno, T. Nakajima and N. Watanabe, Electrochim. Acta 28 (1983) 17. Y. Xi, H. McKinstry and L.E. Cross, J. Amer. Ceram. Soc. 66 (1983) 637. F. Fairbrother, The Chemistry of Nb and Ta (Elsevier, Amsterdam, 1967) p. 33. D. Brown, Comprehensive Inorganic Chemistry, ed., J.C. Bailar Jr, Vol. 3 (Pergamon, London, 1973) p. 598. B.K. Sen and A.V. Saha, M.R.B. 16 (1981) 923; 17 (1982) 161,611; 18 (1983) 19. I.M. Gibalo, Analytical Chemistry of the elements - Nb and Ta (Ann Harbor Humphrey, London, 1970) p. 11. S.I. Solov'ev and E.I. Krylov, Russ. J. Inorg. Chem. 3 (1958) 2487. J.W. Mellor, Comprehensive treatise on Inorganic and Theoretical Chemistry Vol. 9 (Longmans Green, London, 1929) p. 856. J.H. Kanzelmeyer, J. Ryan and H. Freund, J. Amer. Chem. Soc. 78 (1956) 3020. D.L. Kepert and R.S. Nyhlhom, J. Chem. Soc. (1965) 2871. B.I. Nabivanets, Russ. J. lnorg. Chem. 9 (1964) 590. D. Cozzi and S. Vivarelli, Z. Anorg. Allg. Chem. 279 (1955) 199. C.L. Rollingson, The Chemistry of Coordination Compounds, ed., J.C. Bailar Jr (Reinhold, New York, 1956) p. 448. M.A. Hughes, J. Less Common Metals 6 (1963) 232. O. Maas and P.G. Hiebert, J. Amer. Chem. Soc. 46 (1924) 290. A.K. Babko, B.I. Nabivanets and I.G. Lukianets, Russ. J. Inorg. Chem. 11 (1966) 671. B.I. Nabivanets, Russ. J. Inorg. Chem. 11 (1966) 1470. B. Spinner, Rev. Chem. Min. 6 (1969) 319. N. Adler and C.F. Hiskey, J. Amer. Chem. Soc. 79 (1957) 1827; 1831; 1834. R.D. Hall and E.F. Smith, J. Amer. Chem. Soc. 27 (1905) 1389. D.C. Bradley, B.N. Chakravarti and W. Wardlaw, J. Chem. Soc. (1956) 2381, 4439. D.C. Bradley, Preparative Inorganic Reactions, ed., W. Jolly, Vol. 2 (Wiley, New York, 1965) p. 169. D.C. Bradley and C.E. Holloway, Offic. Dig. J. Paint Technol. Eng. 37 (1965) 487; J. Chem. Soc. (A) (1968) 219. J.G. Riess and L.G. Hubert-Pfalzgraf, Chimiax 30 (1976) 481. L.G. Hubert-Pfalzgraf and J.G. Riess, J.C.S. Dalton Trans. (1975) 585. L.G. Hubert-Pfalzgraf, lnorg. Chim. Acta 12 (1975) 229. R.C. Mehrotra and P.N. Kapoor, J~ Less Common Metals 10 (1966) 354. R.C. Mehrotra and P.N. Kapoor, J. Less Common Metals 7 (1964) 98. R.C. Mehrotra, R.N. Kapoor, S. Prakash and P.N. Kapoor, Aust. J. Chem. 19 (1966) 2079. R.C. Mehrotra and P.N. Kapoor, J. Less Common Metals 7 (1964) 176. P.N. Kapoor and R.C. Mehrotra, J. Less Common Metals 8 (1965) 339. R.C. Mehrotra and P.N. Kapoor, J. Less Common Metals 8 (1965) 419. Kapoor, R.N., S. Prakash and P.N. Kapoor, Z. Anorg. Allg. Chem. 351 (1967) 219; S. Prakash and R.N. Kapoor, Ind. J. Chem. 5 (1970) 50. R. Bohra, A.K. Rai and R.C. Mehrotra, Ind. J. Chem. 12 (1974) 855. D.C. Bradley, Coordin. Chem. Rev. 2 (1967) 299. D.C. Bradley, M.B. Hursthouse and P.F. Rodesiler, Chem. Commun. (1968) 1112. E.J. Graeber and B. Morosin, Acta Cryst. B33 (1977) 2137. J. Fuchs, K.F. Jahr and G. Heller, Chem. Ber. 96 (1963) 2472. R.A. Walton, Prog. Inorg. Chem. 16 (1972) 1. H. Funk and K. Niederlander, Ber. Deut. Chem. Ges. 61B (1929) 1688. R. Gut, Helv. Chim. Acta 47 (1964) 2262. M. Schonherr and L. Kolditz, Z. Chem. 10 (1970) 72. L. Kolditz and M. Schonherr, Z. Chem. 5 (1965) 349.
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[54] R.C. Mehrotra and P.N. Kapoor, J. Less Common Metals 10 (1966) 348. [55] H. Funk, and Niederlander, Ber. Deut. Chem. Ges. 61B (1928) 249. [56] B. Viard, A. Laarif, F. Theobald and J. Amaudrut, J. Chem. Res. (S) (1983) 252; (M) (1983) 2301. [57] K.C. Malhotra and S.C. Chaudhry, Ind. J. Chem. 14A (1976) 706. [58] M.J. Frazer, B.G. Gillepsie, M. Goldstein and L.I.B. Haines, J. Chem. Soc. (A) (1970) 703. [59] H. Funk, Ber. Deut. Chem. Ges. 67 (1934) 1801. [60] C. Djordjevi6 and V. Katovi6, J. Inorg. Nucl. Chem. 25 (1963) 1099. [61] A. Syamal, D.H. Fricks, D.C. Pantaleo, P.G. King and R.C. Johnson, J. Less Common Metals 19 (1969) 141. [62] K.C. Malhotra, U.K. Banerjee and S.C. Chaudhry, Ind. J. Chem. 16A (1978) 987. [63] R. Gut, H. Buser and E. Schmid, Helv. Chim. Acta 48 (1965) 878. [64] L.G. Hubert-Pfalzgraf, A.A. Pinkerton and J.G. Riess, Inorg. Chem. 17 (1978) 663. [65] B. Kamenar and C.K. Prout, J. Chem. Soc. (A) (1970) 2379. [66] L.G. Hubert-Pfalzgraf and J.G. Riess, Inorg. Chim. Acta 41 (1980) 111; 47 (1980) 7. [67] C. Djordjevi6 and V. Katovi6, Chem. Commun. (1966) 224; J. Less Common Metals 21 (1970) 325; J. Chem. Soc. (A) (1970) 3382. [68] V. Katovi6 and C. Djordjevi6, Inorg. Chem. 9 (1970) 1720. [69] D. Ridge and M. Todd, J. Chem. Soc. (1949) 2637.
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S Y N T H E S I S OF N I O B I U M P E N T O X I D E G E L S C. A L Q U I E R , M.T. V A N D E N B O R R E and M. H E N R Y Lahoratoire Spectrochirnie du Solide, Uniuersiz~ Pierre et Marie Curie. 4, place Jus,~ieu. 7144, 2b ~tage, 75230 Paris Cedex 05, France
Received 14 February 1985
Niobium pentoxide gels can be obtained by the two classical ways already known for silica. namely destabillisation of a sol and alkoxide's hydrolysis. Owing to the lack of reproducibility of the first route and to the high cost of niobium alkoxides, two other processes are proposed. Addition of hydrogen peroxide to the sol greatly improves the destabilisation process, while using chloroalkoxides instead of alkoxides reduces the cost of the precursors, h is thus shown that either monolithic gels or thixotropic gels can be obtained depending on the process used. Upon drying these gels lead to quite different xerogels, that can be either amorphous oxides or even mixed mineralorganic materials.
1. Introduction
The sol-gel synthesis of glasses and ceramics has received significant attention during the last decade [1,2]. It opens new possibilities in the field of material science [3,4]. Most of the systems studied until now have been based o n S i O 2 in order to make glasses. Two routes are usually followed: - The destabilization of colloidal silica. This can be done just by varying the p H of the aqueous solution [5]. This method appears to be quite easy and cheap but not very versatile and therefore its possibilities are rather limited. - Colloidal silica or gels can also be obtained from organometallic precursors, by hydrolysis and polycondensation of metal alkoxides [3,6]. This process is much more versatile. Multicomponent systems can be mixed on a molecular scale leading to a new molecular engineering. Mixed organic inorganic materials can even be synthetized in that way [7]. However, due to the high cost of most alkoxide precursors, this process is limited to a small number of oxides, namely SiO 2 and TiO 2. New developments of the sol-gel process appear nowadays in the field of ceramics. Ultrafine and monodispersed powders can be obtained, allowing improved sintering [8]. New gels based on A1203 or ZrO~ have then been studied. This paper deals with the synthesis of N b 2 0 s gels. Very little information can be found in the literature about such gels. However they could lead to new applications in the field of display devices [9] reversible cathodes [10,11] or ferroelectric ceramics such as LiNbO s [12]. 0022-3093/86/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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2.
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Experimental
2.1. Niobium pentachloride hydrolysis NbCl 5 is a yellow hygroscopic powder. It was purchased from Fluka (72510) and used as such, without any further purification.
2.1.1. Nb:O~ sols A gelatinous precipitate of hydrated niobium oxide can be obtained by carefully mixing 1.08 g (4 m mol.) of NbC15 into 10 ml of water. The reaction is quite violent and HC1 gas is evolved. Several samples of this voluminous precipitate are then prepared. The first one is left to stand for 24 h until the precipitate completely dissolves giving rise to a clear solution, called "sol 1". Another part of the precipitate is washed with water, several times, without any special care in order to remove most of the CI ions. The gelatinous precipitate then densities and becomes almost insoluble. A very small proportion is dissolved after several days leading to "sol 2". A third part of the gelatinous precipitate is left to stand for 10 min only allowing some deposition at the bottom of the beaker. This precipitate is then very carefully washed with water, avoiding any mechanical disturbance in order not to destroy the colloidal dispersion. It is then left to stand and a clear solution is obtained within 24 h, called "sol 3". The pH of sols 2 and 3 is about 2 while sol 1 remains much more acid.
2.1.2. Gel formation Aqueous colloidal solution can usually be destabillized by adding electrolytes or modifying the pH. Such a procedure cannot be followed here because Nb205 sols appear to be quite sensitive toward electrolytes and flocculation occurs as soon as some foreign ions are introduced into the sol. Destabillization was then obtained by increasing the temperature. Therefore the sols were kept for one hour at 60°C. Let us call t the ageing time of the sol separating the complete peptization of the gelatinous precipitate and the thermal treatment. Different behaviors were observed depending on the nature of the sol. Sol 1 remains quite stable whatever the ageing time might be, unless the destabillization process becomes too severe in which case a precipitate is obtained. - A complete peptization of sol 2 is never observed upon ageing even after several days. A gelatinous precipitate is obtained after the thermal treatment. Gelation occurs with sol 3, but the appearance of the gel greatly depends on the ageing time t. For t = 0, two phases are obtained; a monolithic gel and a clear solution. The gel looks transparent, elastic, non thixotropic and remains stable for months. The gel part becomes more and more important when the ageing time
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increases but some opalescence appears beyond t = 48 h. After one week the whole solution gives rise to a gel that no longer appears monolithic. It is opaque, white and becomes thixotropic. A sol can be obtained just by shaking the beaker. This sol-gel transition is reversible and a gel is formed again through a thermal treatment at 60°C for one hour. All these observations of course depend on the niobium concentration. Below 0.3 M, gelation never occurs while above 0.7 M a clear sol is difficult to obtain. Moreover we noticed that the method of mixing water and NbCI 5 could be very important. Adding water to the NbCI 5 powder for instance, instead of the reverse, very quickly leads to a clear sol, that behaves like sol 1, i.e. gelation never occurs. The purity of the water seems to be very important too, all our experiments were performed with deionised water. Gels have never been obtained with distilled water, only precipitates or colloidal solutions. 2.1.3. Hydrolysis through hydrogen peroxide NbC15 hydrolysis can also be performed with hydrogen peroxide. This process has the advantage of removing most of the chlorine from the gel. Different experimental procedures have been followed. Method I - 0.14 g (0.52 m mol.) of NbC15 are carefully added to 3 ml of a 110 vol. hydrogen peroxide solution (30% H 2 0 2 - P r o l a b o 23619). A violent exothermic reaction occurs, leading to a yellow solution. 7 ml of a 20 vol. hydrogen peroxide solution (6% H 2 0 2 - P r o l a b o 23616) are then added five minutes later. The whole solution is cooled during this time in order to avoid ebullition, till all gas (O2-C12-HC1) stops evolving. A clear yellow solution is thus obtained, the p H of which is close to 4. This solution gives rise to a monolithic yellow gel when left for 12 h at 60°C. Method 2 - A flocculent precipitate is formed, as previously described, by adding NbCI 5 to the water. A 110 vol. hydrogen peroxide solution is then quickly added, in order to avoid any ageing of the precipitate. This one thus dissolves leading to a yellow solution whose pH is about 2. This solution is heated at 60°C in order to remove all gas. It leads to a yellow opaque gel after 12 h. Hydrogen peroxide can also be added after washing and ageing of the precipitate. 24 h later for instance, the reaction appears to be quite gentle and gelation occurs as above. Method 3 - 5 ml of a 110 vol. hydrogen peroxide solution can also be added onto the previous monolithic gel, quickly leading to its dissolution together with some gas evolution. 1 ml of water is then added while the yellow solution is stirred and heated. The viscosity progressively increases. A yellow, opaque and bulky gel is obtained about 10 min later. 2.2. Niobium alkoxide hydrolysis
The precursor is now the niobium pentaethoxide Nb(OEt)5 purchased from Ventron 51119. It is extremely sensitive to water. 2.5 g (7.86 m mol.) of Nb(OEt)5 are carefully dissolved into 500 ml of pure ethanol (RP normapur
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Prolabo 20821). This alcohol is not perfectly dry and still contains some 0.2% of water, (44 m mol.) enough for a complete hydrolysis of all the alkoxo groups. A clear solution is first obtained that quickly becomes opaque, within 10 min at room temperature, leading to a milky sol. This sol remains stable for weeks. It can turn into a gel when the niobium concentration increases, for instance by evaporing the solvent. Gelation occurs for a niobium concentration of about 0.3 M. This gel gives rise to a white powder after drying.
2.3. Niobium chloroalkoxides hydrolysis Chloroalkoxides can easily be obtained by adding NbC15 to an alcohol. Different alcohols have been studied, all of them have been used without any further purification, 8.1 g (30 m mol) of NbC15 being dissolved into 100 ml of alcohol. The reaction is performed in a glove box in order to avoid moisture. It appears to be very exothermic and HC1 gas is evolved. The solution is stirred until the complete dissolution of NbCl 5. It can then be stored in a dry atmosphere without any problem. Hydrolysis is then performed, 24 h later, on 7 ml of the solution. The water is added dropwise under vigorous stiring and gentle heating (30°C) for 10 to 15 min. Let us call h the molar ratio H z O / N b . The behavior towards hydrolysis depends on the alcohol used. With primary alcohol such as ethanol (Prolabo 20821) or methanol (Prolabo 20847), water can be added in excess without any precipitation. The sols are translucent (h = 300 for MeOH and h = 28 for EtOH) and undergo a sol-gel transition together with a quick opacification upon ageing, leading to white gels. When a secondary alcohol is used (Propanol-2 Prolabo 20842), no ageing is needed the white gel setting spontaneously for h = 21. Upon drying (60°C) all these gels lead to fine white powders. The dissolution of NbC15 in a tertiary alcohol (Methyl-2 butanol-2 Prolabo 20802) is difficult, leading to a light brown viscous sol (0.15 M in Nb). A light brown powder is obtained by heating the brown gel at 60°C formed at h = 32. Polyols lead to opaque gels and transparent xerogels. A white glue is obtained for h = 32, with ethanediol (Prolabo 20041), leading to a transparent plastic material after drying. On the other hand a white grease is obtained by diluting a 0.7 M solution of NbC15 in ethanol to 0.3 M with glycerol (Prolabo 24387). Upon prolonged heating (60°C) the gel loses its opacity giving rise to a translucent grease. Non-thixotropy and stability are the main features of these chloroalcogels providing that the niobium concentration is kept between 0.2 M and 0.6 M and that the chloroalkoxide solution is not hydrolyzed immediately after its preparation (24 h delay seems the minimum). We have also checked that water can be added in a straightforward manner without inhibiting the subsequent gelation, the time of setting depending only upon concentration. Another quite important property of these chloroalcogels is their behavior towards UV. Blue colors are obtained from sols and gels while the non-hydro-
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lysed solution becomes purple. A purple color is always obtained with glycerol solution. The larger the time of irradiation, the darker is the color.
3. Discussion
3.1. Hydrogels Insoluble, hydrated niobium pentoxide (niobic acid) can be obtained mainly by two methods. The first one includes the hydrolysis of a water soluble complex of Nb(V) by ammonia, while the second one involves the acidification of an alkali-niobate (V) solution [13]. This precipitate is a white soli& amorphous to X-ray and electron diffraction. Its structure, as well as its water content, is still a much debated question [13,15]. It has been known for a long time [18] that niobium pentoxide hydrosols can be easily produced through washing or centrifugation of the freshly prepared oxide although it is very difficult to obtain a compound free from any electrolyte [13,15,16]. However, the gelation of such hydrosols has been described in only a few cases. Thus using the dialysis technique of Solov'ev et al. [17] to obtain a thixotropic gel starting with a sol containing 3 CI per niobium, the number of chloride ions trapped in the gel being unknown. Hydrosol preparation, by the acidification method, does not seem to be very useful. Most alkali-niobates are not soluble enough and lead to a niobium concentration too low for the sol-gel transition to occur. We have therefore called upon the hydrolysis of niobium pentachloride. In order to avoid the long and tedious process of dialysis, the chlorine ions were removed by careful washing or by adding hydrogen peroxide. The action of water upon NbC15 leads to aqueous solutions of Nb205 and HCI. The state and nature of Nb(V) in aqueous hydrochloric acid has been investigated by spectrophotometry [19], electrodialysis [21] and polarography [22]. Four main species are formed depending on the relative concentrations of H * and C1 ions. In strongly acidic solutions ( > 3 M HC1)INb(OH)2C141 ion is predominant. Decreasing the hydrochloric acid concentration leads to the formation of the neutral complex Nb(OH)2C13 (NbOC13/H20). INb(OH)2C14I
~Nb(OH)2C13 + CI .
If perchloric acid is added at this stage, in order to get a higher concentration of H + ions, a cationic complex is formed: Nb(OH)2C13 + H+ ~ INb(OH)C131 ~+ H20Finally colloid formation is observed below 2M: INb(OH)2C141 + H 2 0 ~ Nb(OH)3C12 + H + + 2C1 . NbC15 being merely dissolved into water at about 0.4 M concentration, the overall chloride concentration remains smaller than 2 M and thus a flocculent
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precipitate of Nb(OH)3C12 a n d / o r Nb(OH)5 is first formed. This precipitate has a quite open texture and therefore peptisation can occur leading to a mixed sol containing small colloidal particles of Nb(OH)3C12 together with Nb(OH)2CI 3 molecules. Owing to the non colloidal nature of the latter, the concentration of colloidal particles in non-washed sols (no. 1) is too small to allow a sol-gel transition. Only precipitates can be obtained through destabilization. We can then understand why the way of mixing water and NbC15 is so important in the hydrosol preparation. Dispersion of NbC15 into water favours Nb(OH)5 a n d / o r Nb(OH)3C12 formation while adding water onto NbC15 favours the non active complex Nb(OH)2C13. This point of view is supported by the work of Kepert et al. [31] showing that as long as water is not in excess, C1- ions remain linked to the niobium atom. If the precipitate is carefully and quickly washed (sol no. 3), Nb(OH)2CI 3 cannot be formed anymore while peptisation still occurs leading to a clear sol where all niobium compounds are in a colloidal form. The open texture of the solid phase is no longer preserved when the precipitate is washed without any care, leading to a more densified structure. As a consequence some competition between peptisation and ageing takes place, a small amount of Nb(V) being peptized while the remainder is transformed into non-reactive hydrated niobium pentoxide. Ageing is a quite general process occurring with numerous hydrated metal oxides; it can be discussed in terms of olation and oxolation reactions [23] as suggested for niobic acid [24]. In this case, polymerisation, with the formation of a primary particle, takes place through intermolecular elimination of water between M ( O H ) , X s _ n units. Owing to the low charge/mass ratio of those particles, flocculation occurs followed by coagulation a n d / o r coalescence if mechanical disturbances are not avoided. Careful washing, keeping the flocculent aspect of the precipitate, allows further peptisation leading to a clear sol containing small colloidal particles. Monolithic and transparent gel can be obtained with such a sol providing ageing has not yet started i.e. the sol must undergo destabilisation immediately after peptisation has stopped. This gel can then be considered as a homogeneous olated polymer where all the bonds are N b - O H - N b bridges. Actually coacervation is observed with a freshly prepared sol; the lower part being more concentrated undergoes some gelation while the upper part remains too dilute to undergo a sol-gel transition. This probably arises from the highly dispersed nature of the sol prepared in this way, a point which has been confirmed by centrifugation experiments. Now, if the sol is aged, Ostwald ripening occurs, decreasing the polydispersity of the sol, smaller particles disappear on behalf of larger ones. Concerning these latter two processes occur: first, particle growth via olation at the outer part of the particle then densification via oxolation ( N b - 0 - N b bridges) inside the particle, the core of which becomes non reactive niobium pentoxide. As a consequence coacervation progressively disappears while opacity (light diffusion) and thixotropy increase as the particles become larger.
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Using distilled water instead of biexchanged water apparently inhibits the gel formation. Chemical analysis of the biexchanged water was then undertaken, showing that distilled water contains less electrolytes than biexchanged water and thus specific adsorption must probably account for the differences observed. However it is quite difficult to determine which ion is determinent, so further study of this point seems usefull. Hydrogen peroxide improves the process, that becomes more reproducible and less sensitive toward the experimental procedure. The following reactions are proved to occur in hydrochloric acid medium [35]. (complex formation: H202 + HCI ~ H202 • HC1 OH oxidation:H202 • HC1 --> H 2 0 ~ ~ H 2 0 + HOC1 CI reduction: HOCI + H 2 0 : ~ H2_~O+ HC1 + 02 dismutation: HOC1 + HCI ~ C12 + H 2 0 , As a result chloride ions are removed through an oxidoreduction process which does not pertube the mechanical equilibrium of the sol, by-products being oxygen and chlorine gas: 3H202 + 2HC1 ~ 4 H 2 0 + 02 + C12. These gases can easily be removed by gentle heating before gelation leaving just water and niobium pentoxide thus avoiding the tedious and delicate steps of washing. Hydrogen peroxide also prevents precipitation of insoluble solids through the formation of peroxo complexes providing that the pH is kept below 3, in which case precipitation of the insoluble 1 : 1 complex occurs [26]. This depolymerisation action of H202 [27] enables us to use distilled water as well as biexchanged water for the hydrolysis reaction and therefore peptisation is not needed anymore. The chemical nature of the peroxocomplex thus formed is not yet elucidated. It may be a peroxocation (INb(OH)4(H202)I + or INbO2(H202) [+) according to Babko et al. [26] and Nabivanets [27] or a peroxoanion INbO2(O-O)[ as stated by Spinner [28]. The yellow color developed on addition of H202 may be due to the formation of peroxoniobic acid [29], but further investigation is needed to confirm this point. As far as gels are concerned, it is known that when hydrogen peroxide is added to a hydrochloric acid solution of niobic acid, a yellow precipitate of niobium perhydroxide Nb205 - H202 • 5H20 is formed [30]. On the other hand peroxo-ortho niobic acid H N b O 4, n H 2 0 can be obtained as a colloidal solution from perniobic acid [18]. In both cases, a yellow and stable amorphous solid is formed upon drying. Further investigations are thus made in order to determine whether such peroxogels are present in our gels. The best gel is obtained when NbC15 is added directly onto H202 which seems quite obvious according to our preceding discussion. This is also the most convenient method of preparation, for washing is not needed. However,
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as shown, hydrogen peroxide can be added at various steps of the process (methods 1-4) without inhibiting the gel formation. The problem previously underlined are thus solved when using H202, however yellow colored gels are obtained and care must be taken in order to avoid bubbles and too much exothermic reaction. 3.2. A lcogels Using a Nb(OR)5 alkoxide instead of NbC15 usually leads to purer gels, the hydrolysis products being ROH instead of HC1. The synthesis of such alkoxides is carried out through dissolution of NbC15 into an alcohol in the presence of ammonia, or by an exchange reaction from another alkoxide such as Nb(OEt)5 [31,32]. These alkoxides form dimers in non complexing solvents as shown by NMR, two octahedral units sharing one edge [33,34]. In complexing medium, depolymerisation occurs leading to the formation of octahedral complexes Nb(OR)5 - (35,36). The flexibility of these alkoxides arises from the labile OR groups that can be replaced by higher alcohols [37], organic esters [38], /3 keto-esters [39,40] /3-diketones [41], glycols [42,43], phenols [43b], oximes or hydroxylamines [44]. Few studies have been performed on the hydrolysis reactions of such compounds. Bradley et al. [45] did not publish their results concerning the hydrolysis of niobium pentaalkoxides, but claimed to have produced from Nb(OEt)5, a crystalline compound Nb8010(OEt)20 [46] whose preparation conditions have unfortunately not been given. Decaniobate anion Nb~002~6 , isostructural to the decavanadate anion, has been proved to occur when Nb(OEt)5 is hydrolysed with tetramethylammonium hydroxide in methanol [47]. Other isopolyniobates (Nb6069 , Nb12037 14- , Nb2406510 ) have been obtained by Fuchs et al. [48] through hydrolysis of Nb(OEt) s with various organic bases at various molar ratios h = H z O / N b . It is difficult to assess if such species are really present in our gels when h > 5.5. Quick opacification of the solution points out that hydrolysis occurs at least partly beyond discrete molecules or anions, but in view of the great reactivity of niobium alkoxide toward water, it is highly probable that the main reaction is: Nb(OEt)5 + 5 H 2 0 ~ Nb(OH)5 + 5EtOH
(1)
and not Nb(OEt)5 + n H 2 0 ~ Nb(OH).(OEt)5 ~ + nEtOH.
(2)
Gelation must proceed from an olation/oxolation process between Nb(OH) 5 molecules with elimination of water rather than polycondensation between hydroxoalkoxoniobates with elimination of alcohol. A detailed study of the hydrolysis of Nb(OEt)5 seems necessary to determine which reaction (1) or (2) is predominant.
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3.3. Chloroalcogels This is a combination of the two previous methods. The halides and oxyhalides of the early transition series and their stability and reactivity in non aqueous media have been reviewed by Walton [49]. It is well known that a solvolysis reaction occurs when NbCI 5 and an alcohol are mixed together leading to the formation of a trichloroalkoxide [14,41,50]. NbC15 + ROH(excess) ~ NbCI 2 (OR)3 + 3HC1.
(3)
These chloroalkoxides have been widely studied from a chemical point of view, the OR group being methanol [51,52], ethanol [53,54], phenol [55], carboxylic acid [50,56], benzoins [57] or quinolinol [58]. In complexing media formation of NbCIz(OR)2L [59-63] and NbCI3(OR)2L [64] occurs, while oxochloroalkoxides NbOCI 2(OR)L, NbOCi(OR)2 L [65,66] are also known. As for the alkoxides, hydrolysis reactions of such compounds have been much less studied [67,68]. Hydrolysis reactions were carried out in six different alcohols and marked differences appeared. These differences can be explained on the basis of the mechanism of the chloroalkoxide formation. Ridge and Todd [69] dealing with SIC14 have studied such a mechanism, where all the chloride ions can be replaced by OR groups just by reacting SiCI 4 and ROH: SiC14 + ROH(excess) -~ Si(OR)4 + 4HC1.
(4)
Two mechanisms appear possible, both involve the initial coordination of a lone pair of electrons from an alcoholic oxygen atom followed by the break of either the hydroxy or alkoxy bond. In both cases, the electron density at the metal atom increases leading to a weakening metal-halogen bond: H+ J O ~ ) N b ( } C 1 --, ~ N b - O R + HC1 -R ~
(5)
I
H R + \ O ~ Nb~-C1 ~ ~ N b - O H + RC1
(6)
Alcohols containing - I inductive groups react in accordance with mechanism (5) while strongly + I inductive groups lead to mechanism (6). Another complication arises from side reactions such as [48]: HCI + ROH ~ R + + H 2 0 + CI
I
R++
C1 --* RC1
(7)
R + H 2 0 ~ H 3 0 + + olefin
Reactions (6) and (7) are favoured when tertiary alcohols (strongly + I inductive effect) are used. In such a case, water is formed in situ allowing hydrolysis and polymerisation to occur. Thus using methyl-2 butanol-2 leads to a viscous (hydrolysis) and lightly coloured (olefin) sol. Such problems do not
392
c Alquier et al. / Synthesis of niobium pentoxide gels
arise with primary or secondary alcohols, while with ethylenglycol or glycerol (strongly-I inductive effect) side reactions are completely avoided. The behaviour of methanol sols can be explained on the basis that chloromethoxides are highly polymeric owing to the relative small size of the methoxy groups with respect to the higher alkoxy groups. In such a case, no coordination position is available for water molecules and depolymerisation must occur first before very fast hydrolysis can proceed. Using polyfunctional alcohols leads to transparent and coherent materials instead of powders after drying. With ethylenglycol two different reactions can occur; an intra molecular reaction: "-. / C 1 0] ~Nb\ + H O - C H z - C H 2 - O H ~ /-~Nb /\ + 2HC1 C1 0
1
(8)
and an intermolecular one (cross-linking): i
Nb-C1 + H O - C H 2 - C H 2 - O H + C I - N b ~ N b - O
I
O-Nb.
(9)
With reaction (9) a mixed mineral organic polymer is obtained without the addition of water while with reaction (8) the same polymer if formed only by adding water: ~] ~Nb\
O-CH2-CH2-OH + H 2 0 ~ ~Nb.. OH
O-CH2-CH2-OH ~Nb\
d
b-Nb~-
+ C 1 - N b ~ - o N b ~/ OH
OH
In both cases monoliths are formed upon drying through cross linking reactions. The same reactions can occur with glycerol, however, owing to the small amount of water added the resulting material looks more organic than mineral (grease). This kind of reaction is quite useful for making new materials and has been applied to the elaboration of contact lenses with Si instead of Nb (7). Photoreduction of Nb(V) to Nb(IV) (blue color) or Nb(III) (purple color) is also observed with these chloroalkoxides. They will be studied in more detail in another paper. 3.4. Relations between the three methods
When NbCI 5 is mixed with alcohol ROH, the NbCI2(OR)3 complex is formed. If we consider water as an alcohol, the R group being a proton, the same complex will be formulated, NbCI2(OH)3. This is just the formulae of the colloid obtained by mixing NbC15 with water. Owing to the strong coordinative properties of the OH groups, this complex NbCI2(OH)3 is not stable as a monomer and olation leads to colloid formation. The same
c. A Iquier et al. / Synthesis of niobium pentoxide gels
393
phenomenon probably also arises, but to a much lesser extent, when methanol is used instead of water, owing to the relatively small size of the O C H 3 group. When higher alcohols are used, the complex is probably at most a dimer and can be hydrolysed without problems. A quite obvious relation appears between the alkoxide and the chloroalkoxide routes because both precursors have labile alkoxy-groups allowing a good flexibility for the process. As far as purity is concerned, the use of hydrogen peroxide and NbC15 or the use of an alkoxide Nb(OR) 5 are equivalent, for in the two cases N b ( O H ) 5 is generated as an olated polymer leading to purer gels.
4. Conclusion In this work four different ways of preparing niobium pentoxide gels N b 2 0 5, n H 2 0 have been reported. It is possible to obtain gels by the two classical processes known for silica. The use and niobium pentachloride and water is an extension of the " L u d o x " method developed by Phalippou et al. [5]; in both cases a sol formed by peptisation of a solid phase undergoes a destabillisation process leading to the gel. A sol-gel transition is thus proved to occur with niobium pentoxide, the differences with silica arising from the chemical nature of the sol. The great drawback of this method arises from the tedious and delicate procedure and from the lack of reproducibility. We have shown that adding hydrogen peroxide greatly improves the process by removing chloride ions. The other classical method involves niobium alkoxides and can also lead to gels. Purer compounds are thus obtained, however the process is rather limited by the high cost of alkoxides [Nb(OEt)~ being 275 times more expensive than TEOS = Si(OEt)4 ]. The best process, from our point of view is the chloroalkoxide route. This is a good compromise between the flexibility of the alkoxides and the lower cost of the mineral precursor [NbC15 is 15 times less expensive than Nb(OEt)5]. The need for the alkoxide's synthesis is thus avoided. Indeed, it would seem quite strange to make an alkoxide, which will immediately be broken down by water in order to produce the oxide. The versatility of the process arises from the choice of the alcohol. We have shown that quite different xerogels can be obtained according to the precursor used.
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