Electronic properties of mixed valence oxide gels

Journal of Non-Crystalline Solids 121 (1990) 35-39 North-Holland

35

Section 2. Gel formation and characterization ELECTRONIC

PROPERTIES

OF MIXED VALENCE OXIDE GELS

J. LIVAGE, J.P. J O L I V E T a n d E. T R O N C Spectrochimie du Solide, Universit$ Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France

Transition metal ions exhibit several valence states. Redox reactions occur during the sol-gel synthesis of transition metal oxides. Mixed valence compounds are obtained. Their electronic properties arise from electron exchanges between metal ions in different valence states. Thermally activated electron hopping leads to semiconducting materials. Optical absorption arising from intervalence transfers gives rise to reversible optical switching. They can be used for making electrochromic display devices. Electron and ion transfers can occur at the oxide-water interface. They seem to be typical of mixed valence oxides having a spinel structure and lead to chemical modifications of both the oxide network and the solution.

1. Introduction S o l - g e l synthesis of m e t a l oxides such as silica or a l u m i n a has received a great d e a l o f a t t e n t i o n . G l a s s e s a n d c e r a m i c s are m a d e b y c h e m i c a l p o l y m e r i z a t i o n f r o m m o l e c u l a r precursors. H y d r o x y l a t i o n a n d c o n d e n s a t i o n r e a c t i o n s l e a d to the form a t i o n of sols or gels. D r y i n g a n d d e n s i f i c a t i o n t h e n t r a n s f o r m the gel into m o n o l i t h i c glasses, d e n s e ceramics or fine p o w d e r s [1]. G e l s c a n also be used as such. T h e y are d i p h a s i c materials, m a d e of solvent m o l e c u l e s t r a p p e d inside an o x i d e n e t w o r k . W a t e r a d s o r p t i o n a n d d i s s o c i a t i o n occurs at the o x i d e - s o l v e n t interface l e a d i n g to c h a r g e d particles s u r r o u n d e d b y an a c i d or b a s i c a q u e o u s m e d i u m . O x i d e gels thus exhibit ionic p r o p e r t i e s a n d c a n b e used for m a k i n g m i c r o - i o n i c devices b a s e d o n p r o t o n c o n d u c t i o n or i o n exc h a n g e p r o p e r t i e s [2]. T r a n s i t i o n m e t a l ions are k n o w n to exhibit several valence states so that e l e c t r o n e x c h a n g e r e a c t i o n s can occur. Specific features arise f r o m these reactions so that s o l - g e l synthesis of transition m e t a l oxides differs f r o m the usual s o l - g e l p r o c e s s [3]. This p a p e r r e p o r t s on the electronic p r o p e r t i e s of m i x e d valence o x i d e gels. R e d o x r e a c t i o n s o f m o l e c u l a r p r e c u r s o r s are o f t e n involved in the c h e m i c a l p o l y m e r i z a t i o n process. E l e c t r o n h o p p i n g t h r o u g h the o x i d e n e t w o r k gives rise to semi-

c o n d u c t i n g , o p t i c a l o r m a g n e t i c p r o p e r t i e s . Elect r o n a n d i o n exchanges at the o x i d e - w a t e r interface are s o m e t i m e s observed, m a i n l y with spinel structures.

2. Redox reactions and sol-gel synthesis S o l - g e l c h e m i s t r y is b a s e d on h y d r o x y l a t i o n a n d c o n d e n s a t i o n reactions. T h e s e r e a c t i o n s have b e e n extensively s t u d i e d in the case of silica [4]. T w o r o u t e s are c u r r e n t l y d e s c r i b e d in the literature, d e p e n d i n g on the c h e m i c a l n a t u r e o f m o l e c u lar precursors. T h e m e t a l - o r g a n i c r o u t e is b a s e d o n a l k o x i d e s a n d the i n o r g a n i c r o u t e on a q u e o u s solutions o f m e t a l ions [1]. I n b o t h cases c h e m i c a l r e a c t i o n s involve H ÷ a n d O H - species. R e d o x r e a c t i o n s can also occur d u r i n g the s o l - g e l synthesis of t r a n s i t i o n m e t a l oxides. W h e n d i s s o l v e d in water, t r a n s i t i o n m e t a l cations, M s+, are s o l v a t e d b y d i p o l a r w a t e r molecules. A a e l e c t r o n t r a n s f e r occurs f r o m the b o n d ing 3a t m o l e c u l a r o r b i t a l of the w a t e r m o l e c u l e t o w a r d e m p t y d o r b i t a l s o f the cation. It w e a k e n s the O - H b o n d so that c o o r d i n a t e d w a t e r molecules are m o r e acidic t h a n s o l v e n t w a t e r molecules. D e p r o t o n a t i o n occurs as follows, [ M ( O H 2 ) ~ ] ~+ <--} [ M ( O H ) h ( O H 2 ) ~ _ n ] '~-h>

0022-3093/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

+ h [ U a q ] +,

(1) INVITED LECTURE

36

J. Livage et al. / Electronic properties of mixed valence oxide gels

Z÷ 7 -

MnO~

5-

3 Mn(OH2)~+

~

I

!

!

0

7

14

pH

Fig. 1. Charge-pH diagram giving the chemical nature of hydrolyzed metal cations in aqueous solutions. where n is the coordination number of the metal cation and h the hydrolysis ratio. This equilibrium mainly depends on the charge of the cation and the p H of the solution. A ' c h a r g e - p H ' diagram can be drawn that gives the chemical nature of the precursor (fig. 1). Three domains can be defined corresponding to aquo (h = 0), hydroxo (1 < h < 2 n - 1) or oxo (h = 2 n ) species. Condensation occurs in the hydroxo range. It can be initiated by changing the p H of the solution, i.e. by adding a base or an acid to the aqueous solution [3]. ; Condensation of multivalent ions such as Mn can also be initiated via redox reactions. Manganese exhibits several valence states ranging from Mn(II) to Mn(VII). In an aqueous solution at p H = 7, Mn(II) forms aquo species [Mn(OH2)6] 2÷ while Mn(VII) forms permanganate oxo ions (MnO4)-. Hydroxo species are observed for intermediate oxidation states such as Mn(III) or Mn(IV) (fig. 1). Therefore, it should be possible to initiate hydroxylation and condensation reactions via the reduction o f Mn(VII) or the oxidation of Mn(II). Colloidal oxides (MnO 2, Mn203) are formed upon reduction of [(MnO 4 ) - A ÷ ] solutions (A = H, Li, K, N H 4 . . . . ) with inorganic or organic compounds such as carboxylic acids or polyols [5]. Sol-gel transition occurs when Mn concentration becomes larger than 0.1 mol 1-1. Transparent and stable gels are formed. They give rise to amorphous xerogels upon drying. Crystallization occurs between 300°C and 700°C. Different crystalline

phases are obtained depending on the nature of the counter cation. K + leads to the crystallization of K M n O 2 around 500°C while a spinel phase LiMn204 is obtained around 400°C whith Li + as a counter ion. Oxidation of these powders in sulfuric acid solutions leads to various crystalline MnO 2 phases such as 7 or 8 M n O 2 [6]. Colloidal magnetic iron oxides with a spinel structure can be obtained by adding aqueous mixtures of FeC12 and FeCI 3 (0.1 < F e ( I I ) / F e ( I I I ) < 0.5) to a base solution. It appears that the spinel framework cannot be formed as long as the initial F e ( I I ) / F e ( I I I ) ratio remains smaller than 0.1. In this case different phases such as a or 3' F e O O H are formed. Above 0.1, all iron ions are involved in the spinel phase leading to mixed valence iron deficient oxides ranging between y Fe203 and Fe304 [7]. These precipitates are easily peptized with N ( C H 3 ) n O H or HC104 leading to anionic or cationic sols [8,9]. Fe(II) ions are more or less oxidized during the synthesis. Precipitation from Fe(III) solutions always leads to the formation of hexacoordinated Fe(III) species. When both oxidation states are simultaneously present, crystal field stabilization promotes the formation of octahedral Fe(II) species pushing Fe(III) ions into tetrahedral sites. In the Fe304 lattice all Fe(II) ions are in octahedral sites while Fe(III) ions are equally distributed over tetrahedral and octahedral sites. At room temperature, electron hopping between Fe ions in octhedral sites leads to a mean oxidation state Fe 25+. Once the spinel network is formed, Fe(II) can be oxidized leading to iron deficient phases y Fe203 Fe [Fes/3D1/3]O4 .

3. Small polaron hopping through the oxide network Mixed valence oxide gels contain transition metal ions in different valence states. A strong electron-phonon coupling is usually observed in such oxides. Electrostatic interactions between unpaired electrons and the polar oxide network lead to a polarization of the lattice and a displacement of oxygen ions around the low valence metal ion. A small polaron is formed [10]. The unpaired electron digs its own potential well characterized

J. Livage et al. / Electronic properties of mixed valence oxide gels

by the polaron binding energy Wp that typically ranges around 0.5 eV. Electron transfer occurs via a small polaron hopping process. The hopping rate depends on two parameters: - A phonon term corresponding to the probability for both sites to have the same potential energy. - An electronic term corresponding to the probability for the unpaired electron to tunnel from one site to the other during this coincidence. A general formula for electronic conductivity was proposed by Austin and Mott [11],

A~-B

37

LE

A-B*

opt

bQFig.

3.

o Potential energy diagram of an electron hopping between two metal sites A and B.

o = p ( e 2 / R k T ) C ( 1 - C) e x p ( - 2 a R )

× exp( - W / k T ) ,

(2)

where v is a phonon frequency, R is the hopping distance, C is the concentration of low valent ions and a is the rate of the electronic wave function decay. One of the most striking features of small polaron hopping is that the thermal activation energy, IV, decreases with temperature. A typical conductivity curve is shown in fig. 2 for V205 gels. At high temperature small polaron hopping is activated by an optical multiphonon process. The activation energy is given by W = (Wp + Wa)/2, where Wa corresponds to the potential energy distribution arising from the random disorder of the amorphous gel. The phonon spectrum freezes out

10

7 u It.J

-10

I

I

10

i

2()

IOyT(K")

Fig. 2. Temperature dependence of the electronic conductivity of a vanadium pentoxide xerogel.

as the temperature decreases. Wp drops continuously and an acoustical assisted hopping takes place at very low temperature ( W = Wd/2). Electrical properties of V205 thin films deposited from gels have been extensively studied because of their potential use as antistatic coatings [12] or electrical switching devices [13]. When dried at room temperature, such materials still contain some water and should be described as hydrous oxides V205, nH20. Mixed ionic and electronic conductivity is thus observed [2]. However, electronic conductivity prevails when the water content goes down to n = 0.5. The small polaron model then accounts very well for experimental data. Conductivity does not follow an Arrhenius law. The thermal activation energy decreases with temperature as shown in fig. 2. Electron transfer in mixed valence compounds is often described using a double potential well energy diagram (fig. 3). The electron can be thermally transferred with an activation energy given by the height W = Wp/2 of the potential energy barrier separating both configurations. It can also be optically excited without moving the ions, according to the F r a n c k - C o n d o n principle, with a photon energy hp---4W [14]. Electronic interactions remove the degeneracy at the crossing point of both potential energy curves, leading to two different levels separated by 2J, where J is a transfer integral (fig. 3). d - d overlap usually remains rather small in transition metal oxide gels so that optical absorption typically lies around 1 eV. A broad intervalence band is observed in the red part of the optical spectrum and most mixed

38

J. Livage et al. / Electronic properties of mixed valence oxide gels

valence compounds exhibit a typical blue color [15]. This property can be used for making electrochromic gels [16,17]. Reversible coloration and bleaching can easily be performed in an electrochemical cell as follows: WO 3 + x e - + x L i + ~ LixWO 3. Pure WO 3 is white, while the mixed valence oxide is blue. Amorphous WO 3 thin films can be deposited via the sol-gel process, either from metal-organic precursors (tungsten hexaphenoxide, tungsten ethoxide) or from aqueous solutions of tungstic acid [18]. Other transition metal oxide gels also exhibit electrochromic properties. TiO 2 films made from Ti(OBun)4 turn from white to blue. V205 films made from vanadium oxo-alkoxides turn from yellow to green [17].

4. Electron transfer at the oxide-solution interface

The surface of oxide colloidal particles is solvated by water. Dipolar H20 molecules are adsorbed onto metallic sites giving rise to M - O H 2 species. The potential energy of such solvated sites decreases therefore leading to a localization of the unpaired electrons of the mixed valence oxide network. This was evidenced very clearly for vanadium pentoxide gels. ESR and E N D O R experiments show some hyperfine couplings between unpaired electrons and protons of adsorbed water molecules [19]. This means that unpaired electrons are trapped on solvated vanadium sites rather than in the bulk of the oxide network. Water belongs to the coordination polyhedron of surface V(IV) ions giving rise to hydrated vanadyl species. A similar electron localization process could be invoked in order to explain the behavior of Fe304 colloids in a weakly acidic medium under an oxygen free atmosphere [20]. Around p H = 2, Fe304 colloids exclusively release Fe +÷ ions into the solution. The spinel network progressively transforms into ),-Fe203 without significant structural modification while protons are consumed. The overall reaction corresponds to a ratio H + / Fe ++= 2. Protonation of surface hydroxyl groups F e - O H appears to be the driving force of the process. It lowers the potential energy of surface

iron sites and leads to a localization of charge carriers giving rise to surface Fe(II)-OH~- species. Under such acid conditions, Fe ÷+ ions are hydrolyzed leading to solute [Fe(OH2)6] +÷ species together with the formation of iron vacancies in the octahedral sublattice of the spinel network. Electron localization and the departure of Fe ++ ions leave an extra Fe(III) at the interior of the particle. Electrical neutrality is locally conserved by an outward migration of Fe towards the surface leaving iron vacancies in the spinel lattice. Colloidal particles are thus progressively transformed into 3, Fe203. Peptization occurs when the F e ( I I ) / Fe(III) ratio becomes smaller than 0.15, giving rise to stable cationic sols. Such a transformation is partially reversible. Fe +* ions can be adsorbed onto the surface of y Fe203 colloidal particles at a p H of 8 [21]. Electron transfer and delocalization over octahedral Fe(III) sites of the ),-Fe203 lattice then occurs but no significant iron migration is observed, leaving Fe(III) ions in the adsorbed layer. This process leads to the epitaxial growth of a Fe304 outer layer. It stops when all octahedral sites have an average charge + 2.5. Proton diffusion within the particles should occur simultaneously in order to maintain electrical neutrality. Such reversible electron transfers through the o x i d e - w a t e r interface have been observed in m a n y cases. Ag + ions are adsorbed at the surface of magnetite colloids around p H = 6. A similar electron transfer occurs without significant iron desorption leading to the formation of Ag o particles at the surface of 3, Fe203 colloids. LiMn204 can be obtained via the sol-gel route. It crystallizes at 420°C, a much lower temperature than when the synthesis is made from powders. LiMn204 has a spinel structure; Li + ions are in tetrahedral sites while Mn(III) and Mn(IV) are in octahedral sites. This mixed valence oxide reacts at room temperature with a sulfuric acid solution (pH = 2) leading to the formation of ),-MnO 2. This oxide also exhibits a spinel structure derived from LiMn204 but most of the Li + are removed from tetrahedral sites. Chemical analysis shows that Li* and Mn ++ ions are released in the solution while the oxidation state of Mn in the solid network is close to Mn(IV). Such a topotactic transformation can be explained in the same way

J. Livage et al. / Electronic properties of mixed valence oxide gels

as f o r F e 3 0 4. P r o t o n a t i o n o f m a n g a n e s e s i t e s a t the surface of the particles leads to an electron localization. Disproportionation of Mn(III) into M n ( I I ) a n d M n ( I V ) o c c u r s . M n ÷+ a n d L i ÷ i o n s are removed from the surface by the acid solution. Since the conversion actually does not stop, a model similar to the previous one which allows diffusion of lithium and Mn(III) from the bulk c a n b e i n v o k e d [6]. S u c h a m e c h a n i s m a p p e a r s t o be specific of mixed valence oxides. It seems to be favored by the spinel structure which allows a wide range of stoichiometry.

References [1] B.J.J. Zelinsky and D.R. Uhlmann, J. Phys. Chem. Solids 45 (1984) 1069. [2] J. Livage, P. Barboux, J.C. Badot and N. Baffler, Mater. Res. Soc. Symp. Proc. 121 (1988) 167. [3] J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem. 18 (1988) 259. [4] L.C. Klein, Ann. Rev. Mater. Sci. 15 (1985) 227. [5] J.F. Perez-Benito and E. Brillas, Inorg. Chem. 28 (1989) 390. [6] J.C. Hunter, J. Solid State Chem. 39 (1981) 142.

39

[7] E. Tronc, J.P. Jolivet and R. Massart, Mater. Res. Bull. 17 (1982) 1365. [8] R. Massart, C.R. Acad. Sci. Paris 291C (1980) 1. [9] J.P. Jolivet, R. Massart and J.M. Fruchart, Nouv. J. Chim. 7 (1983) 325. [10] C.H. Chung, J.D. Mackenzie and L. Murawski, Rev. Chim. Min6r. 16 (1979) 308. [11] I.G. Austin and N.F. Mott, Adv. Phys. 18 (1968) 41. [12] C. Sanchez, F. Babonneau, R. Morineau and J. Livage, Philos. Mag. B47 (1983) 279. [13] J. Bullot, O. Gallais, M. Gauthier and J. Livage, Phys. Status Solidi A71 (1982) K1. [14] K.Y. Wong and P.N. Schatz, Prog. Inorg. Chem. 28 (1980) 369. [15] M.B. Robin and P. Day, Adv. Inorg. Chem. Radiochem. 10 (1967) 247. [16] A. Chemseddine, R. Morineau and J. Livage, Solid State Ionics 9-10 (1983) 357. [17] M. Nabavi, S. Doeuff, C. Sanchez and J. Livage, Mater. Sci. Eng. B3 (1989) 203. [18] P. Judeinstein and J. Livage, Mater. Sci. Eng. B3 (1989) 129. [19] P. Barboux, D. Gourier and J. Livage, Coll. Surf. 11 (1984) 119. [20] J.P. Jolivet and E. Tronc, J. Coll. Interf. Sci. 125 (1988) 688. [21] E. Tronc, J.P. Jolivet, R. Massart and J. Lefebvre, J. Chem. Soc. Faraday Trans. I 80 (1984) 2619.