Langmuir 1992,8, 313-319
313
Transformation of Ferric Hydroxide into Spinel by FeI1 Adsorption Elisabeth Tronc,* Philippe Belleville, Jean-Pierre Jolivet, and Jacques Livage Chimie de la Matiere CondensQe (CNRS URA 1466), UniversitQ Pierre et Marie Curie, 75252 Paris Cedex 05, France Received April 18, 1991. In Final Form: July 31, 1991
Products of simultaneous or successive alkalization of FelI1and FelI ions have been investigated at various FeI1/Fe1I1compositions ( x ) between 0 and 0.5. Early materials and their aging behavior have been characterizedby electronmicroscopy, Mossbauer spectroscopy,and kineticsof dissolutionin acidic medium. Both preparation routes yielded very similar features. At x 2 0.4, the spinel was observed from the start. At x 5 0.20, a mixed-valence, short-range ordered material initially formed; it exhibited fast electron hopping at the Mossbauer time scale down to 110 K. This material transformed to goethite at x = 0.05, and exclusively to spinel at x > 0.10. It is shown that conversion to spinel proceeds by dissolutionrecrystallization, and also by solid-state reaction, with electron hopping driving the ordering.
Introduction Among the various iron oxides and oxyhydroxides, spinel oxides play a special role because of their electronic and magnetic properties. The phase ranges from Fe304 (magnetite) to Fe2.6704 (maghemite, yFez03). Its formation in aqueous solutions has been studied extensively in media rich in Fe", in connection with Fe" oxidation, or under the stoichiometric conditions of magnetite (FeY Fe"' = 0.5).14 Far fewer investigations have been devoted to media poor in Fe11.5*6y-Fe203 does not form directly in aqueous medium; it is generally considered to result from the oxidation of stoichiometric or nearly stoichiometric magnetite^.^,^ "Ferric hydroxide" (FH) is the most disordered of the hydrolysis products of Fe"' which are obtained by addition of base at pH 1 5 or heating of acidic solution^.^ It is a very poorly crystallized hydrated oxyhydroxide, made up of a short-range ordered arrangement of Fe(0, OH, OH& octahedra.*tg The structure, locally close to that in goethite (anion stacking sequence AB), probably consists of small units of double chains of edge-sharing octahedra, linked together in a disordered way. This material has been identified with the less ordered form of natural ferrihydrite.1° With time, it generally transforms into goethite (a-FeOOH) and/or hematite (a-FezO3). These compounds form by separate pathways.11J2 Goethite forms by dissolution of FH and reprecipitation of goethite in solution; hematite is formed by a solid-state reaction
* Address correspondence to Dr. Elisabeth Tronc, Chimie de la Matiere CondensBe, T54 E5, UPMC, 4 Place Jussieu, 75252 Paris Cedex 05, France. (1)Misawa, T.; Hashimoto, K.; Shidomaira, S. Corrosion 1974,14,
.". ldl.
(2)Tamaura, Y.;Buduan, P. V.; Katsura, T. J. Chem. SOC.,Dalton Trans. 1981,1807. (3)Tamaura, Y . ; Ito, K.; Katsura, T. J. Chem. Soc., Dalton Trans. 1983,189. (4)Taylor, R. M.; Schwertmann, U. Clay Miner. 1974,IO, 299. (5)Massart, R.;Cabuil, V. J. Chim.Phys. Phys.-Chim.B i d . 1987,84, 967. (6) Cornell, R. M.; Schneider, W. Polyhedron 1989,8,149. (7)Eggleton, R.A.;Fitzpatrick, R. W. Clays Clay Miner. 1988,36,111. (8)Feitknecht,W.; Giovanoli, R.; Michaelis, W.; Miiller, M.Helu. Chim. Acta 1973,56,2847. (9)Combes, J. M.; Manceau, A.; Calas, G.; Bottero, Y. Geochim. Cosmochim. Acta 1989,53,583. (10)Chukhrov, F.V.;Zvyagin, B. B.; Gorshov, A. I.; Yermilova, L. P.; Balashova, V. V. Int. Geol. Reu. 1973,16,1131. (11)Feitknecht, W.;Michaelis, W. Helu. Chim. Acta 1962,26, 212. (12)Schwertmann, U.; Fischer, W. R. Z . Anorg. Allg. Chem. 1966,346, 137.
within FH particles. As these mechanisms are competitive the proportions of goethite and hematite in the system depend on the conditions of the medium (pH, temperature, concentration, foreign ions...). Divalent transition-metal cations (M = Mn, Cu, Ni, Co, Zn), at MI1/Fe1I1concentrations >0.2 roughly, cause FH to transform to a spinel phase by dissolution-reprecipitation.13J4 FelI ions are presumed to have a similar effect.6 In order to investigate the conditions for spinel iron oxide crystallization in FeI1substoichiometric conditions, we have studied the products obtained by coprecipitating mixed FelI1 + xFelI (0.05 5 x 50.5) solutions or precipitating xFelI in the presence of freshly precipitated FeIII. In this paper we present the results concerning the early stages of spinel formation, focusing on the solid precursor.
Experimental Section 1. Materials. Suspensionswere prepared by adding aqueous mixtures of FeCl3 (40cm3,1 M) and FeC12 (2 M, 2 M HC1) in various proportions to a NHa solution (400 cm3,0.6 M, pH 11) under vigorous stirring, at room temperature. The ratio I: = FeI1/FeIr1was varied from 0 to 0.50. A dark brown precipitate instantaneously formed. Samples were taken out as quickly as possible. The time elapsed between the beginning of the synthesis and the first sampling was about 1or 2 min. Such samples are considered representative of the early stage. All stages of the preparation were carried out under nitrogen, using degassed solutions. For agingtimes longer than a day,the solid w a isolated ~ after magnetic settling or centrifuging, washed with degassed distilled water, redispersed in distilled water, and stored under argon. The pH was near 8 and the Fe concentration about 0.1 M. Suspension evolution was followed by taking aliquota at successive time intervals for up to 1month. It was checked that pH variation from 11to 8 brought about by washing and redispersion in water had no influence on suspension aging. Suspensionsat r = 0.15 and 0.50 were also prepared by adding FeI1 after the precipitation of FeIII. 2. Methods. Chemical Analysis. Fell and total Fe concentrations in the suspensions were determined after dissolving the solid in concentrated HC1. Fer1was first titrated potentiometrically by KzCrz07. At the titration end, all Fe was reduced by a SnC12 solution and the mixture was again titrated by Kz-
-
Crz07.
Dissolution Kinetics. The various solid species were characterized by their behavior against dissolution in acidic medium (2 M HC1) according to the procedure described elsewhere.16 (13)Cornell, R.M. Clay Miner. 1988,23,329. (14)Cornell, R.M.;Giovanoli, R. Polyhedron 1988,7, 385.
0143-1463/92/2408-0313$03.00/00 1992 American Chemical Society
314 Langmuir, VoZ.8, No. 1, 1992
Tronc et al.
Dissolution kinetics was followed by reducing Ferr*ions as they were released into solution. KI (0.2 M)was used as the reducing reagent. Formed I 2 was automatically titrated by Na2S203 using a Combi-Titreur 3D Metrohm which maintained the redox potential in the medium a t the equivalent point of Iz/S~03~reduction. By this way, there was no excess I2 in solution and I- concentration was constant during the dissolution. Because of acid excess, H+concentrationwas quasi constant. Data analysis using pseudo-first-order rate laws yielded dissolution rate constants and proportions of the various Fen' species. Acid type and concentration were chosen so that ferric hydroxide dissolves quasi instantaneously whereas crystalline phases dissolve more slowly, at a rate which is mainly determined by their degree of orderingand the size of the particles. The rate constant decreases as ordering is improved and/or particle size is increased. Transmission Electron Microscopy (TEM). Micrographs and diffraction patterns were obtained on a JEOL 100 CXII apparatus. The sampleswere prepared by evaporatingvery dilute ultrasonicated solutions onto carbon-coated grids. d spacings were calibrated using a Au pattern. Mtissbauer Spectrometry. Miissbauerspectra were recorded using a conventional spectrometer (ELSCINT-INEL) with a 57Co/Rhsource, and an OXFORD cryostat. Samples of the early suspensionswere rapidly introduced into a plastic cell and frozen to 77 K by dropping the cell into liquid nitrogen. Velocities were calibrated using an iron foil. Isomer shifts are given relative to metallic iron a t room temperature.
Results 1. Pure Ferric Hydroxide Suspension. TEM micrographs of initial FH show particles of ca. 25-30A (Figure la). As shown by comparing Figure lb,c, the diffraction pattern depends on the hydration state of the material. The pattern of powders (Figure IC)isolated from solution by centrifuging and drying at room temperature is equivalent to the X-ray diffraction (XRD) powder pattern, and typical of ferric hydr~xide:~J~ There are two broad bands at d spacings about 2.6 and 1.5 A, and a weak intermediate band at d 2.3 A; the first ones (indices 10.0 and i1.0, respectively, a0 3 A) result from the local 2D close-packed arrangement of the anions, and the latter (10.1/2, co 2.4 A) is due to random or hexagonal close packing correlations between the l a y e r ~ . ~ - lSamples ~J~ directly obtained from the suspension, without being previously powdered,yielded a somewhatdifferent pattern (Figure lb), with reduced diffuse scattering, and an intermediateband strongerand significantlyshifted ( ~ 2 . 1
A). Under prolonged exposure in the electron beam under focusing conditions, all samples transformed into cubic spinel oxide (punctuated narrow rings at ca. 3, 2.55,2.1, 8.4 A) (Figure ld,e). The 1.7, 1.6, and 1.5 A, a, transformation proceeded more rapidly in samples from the suspension than the powder. The s acing co between the anionic layers is equal to 2.3-2.4 in all iron oxides and oxyhydroxides,17ferrihydrites included.1° Hence, the band at 2.1 A can hardly correspond to the 10.1/2 band. Its position matches that of the 10.2/3 band (CO 2.4 A) which is characteristic of cubic close packing correlations (ABC) between the layers.16 (The second maximum expected in between 2.6 and 1.5 A, about 2.45 A for 10.1/3, is not observed;reduced intensity is consistent with alternating anionic and cationic planes.)
8:
(15) Jolivet,J. P.;Belleville,P.;Tronc,E.; Livage,J. Clays Clay Miner., submitted for publication. (16) Guinier, A. Thborie et Technique de la Radiocristallographie, 3rd ed.; Dunod: Paris, 1964; Chapter 13. (17) Bernal, J. D.; Dasgupta, D. R.; Mackay, A. L. Clay Miner. Bull. 1959,4, 15.
Figure 1. Electron microscopy observations of pure ferric hydroxide: hydrated precipitate (a and b); powder dried a t room temperature (c); effect of electron beam focusing (d and e).
According to Combes et aL9the local structure of ferric hydroxide is not modified by drying. Partial ABC ordering is therefore likely to result from experimental TEM conditions. This is consistent with transformation into spinel under the action of the electron beam, being faster the more hydrated the ferric hydroxide. Referring to the hexagonal axis, the spinel structure is characterized by the hexagonal cell a = 2a0, c = 6 ~ (a 0 = a J d 2 , c = acd3), and ABC stacking sequence of oxygen layers. The parameters are 2 times those of the oxygen cell as a result of cation ordering. Partial ABC ordering indicated by the three-band pattern (2.6,2.1,1.5 A) therefore corresponds to a local structure analogous to a disordered spinel. 2. Influence of the FeI1/Fe1I1 Ratio on the Early Suspension. Electron Microscopy. The suspensions at x 5 0.20 exhibited the same features as the pure FH suspension: particles of ca. 25-30 A (Figure 2a) and a three-band diffraction pattern (2.55, 2.1, 1.5 A) (Figure 2b). When the electron beam was focused on the samples, transformation into spinel proceeded more rapidly than for x = 0. Powders obtained by freeze-drying or centrifuging and drying under reduced pressure yielded the set of punctuated rings typical of spinel ordered materials (Figure 2c); the particle size however remained at ca. 2530 A. A t 0.25 Ix I0.35 a second family of larger particles (ca. 50 A) was observed (Figure 2d). Only this family was present at x > 0.35 (Figure 2e). Diffraction patterns (Figure 2f) are characteristic of spinel particles. Samples obtained from suspensions and powders yielded the same features.
Transformation of Ferric Hydroxide into Spinel
Langmuir, VoZ. 8, No. 1, 1992 315
Figure 2. Electron microscopy observations from early suspensions of various FelI/FelI*compositions ( x ) : x = 0.15 (a and b); powdered (c); x = 0.30 (d); x = 0.50 (e and f).
0
10
20
30 t mfn
Figure 3. Kinetics of dissolution in acidic medium of early suspensionsof various Fe1I/FeJI1compositions. (CO represents the amount of total Fell1, and ct that of FelI1undissolved a t time t.)
Kinetics of Dissolution. At x = 0 and up to 0.20, the very young particles dissolved quasi instantaneously in acidic medium. From x = 0.25 a second kinetic stage was observed (rate constant kl) (Figure 3, Table I). As x was increased, this stage predominated; it became the only stage at x 1 0.40. The value of kl, of the order of 0.15 "in-', is typical of spinel particles with composition FelI/ FelI1= 0.35-0.50, and slightly less than 80 A in size.15 Successive precipitation of FelI1 and FelI ions yielded the same results as coprecipitation regarding TEM examination and dissolution experiments (Table I). Miissbauer investigations showed that FelI ions do not form a
separate phase: They are incorporated in the initial solid or immediately taken up by ferric hydroxide. Over the time scale of the observations, no intermediate phase is detected in spinel formation at x 1 0.40 and the poorly crystallized systems formed at x I0.20 are single-phase. Below, we focus on the properties of this mixed-valence material (FeILFH). 3. Aging of Suspensions at FelI/FelI1I0.20. Suspension evolution was characterized by electron microscopy and dissolution kinetics. The rate constant is very sensitive to particle size and crystallinity, and, to a lesser extent, composition;15 it is thus possible to correlate the species indicated during the dissolution with particle families observed by microscopy. Pure FH suspension aged for 1day showed the same features as the young suspension: The particles were still of ca. 25-30 A, and the dissolution was immediate. After a month, it was all converted into well-crystallized goethite (k = 0.002 min-l). Note that Schwertmann and Murad18 obtained hematite in similar aging conditions (pH 8). This difference may be due to the ferric salt used to precipitate FH, chloride in our case, instead of nitrate; by promoting the dissolution process, adsorbed C1- ions probably favor the formation of goethite. FeI1ions accelerated transformation of the suspension. At x = 0.05, after 2 h of aging, FeILFH represented only 50% of the total FelI1. The rest was in the form of a less reactive species (12 = 1.7 min-l). This species was still made up of particles of ca. 25-30 A, but the structure of these particles exhibited local spinel ordering (four or five diffuse, more or less punctuated, rings for d = 3,2.55,2.1, 1.6, and 1.5 A). The suspension also contained a few acicular crystallites of goethite. After 24 h of aging, FelL FH represented only 9 % of the total FeIII, the proportion of spinel species was practically unchanged (43% ), but ita dissolution rate constant was much lower (k = 0.8 m i d ) , and the proportion of goethite amounted to 48% of the Fe1IX.Goethite was the only constituent after a month. A t x = 0.10.15, after aging for a few minutes the suspensions contained two families of spinel particles in addition to FeILFH particles. The first type (121 = 0.8 min-l) was made up of particles of the same size as FelL FH (25-30 A), and the second type (122 = 0.06 min-l), of much larger particles (ca. 130A in average) (Figure 4a,b). During aging the size of the small spinel particles changed only slightly; that of the large particles considerably increased (up to 350 A after 15 min, 850 A after 4 h, and lo00 A after 24 h) (Figure 4c). Figure 5a shows the evolution of the proportions of the different species as determined from dissolution kinetics. FeILFH rapidly disappears essentially to the profit of the small spinel particles. This species is practically stabilized after l/2 h of aging. Stabilization of the large particles is reached more slowly, approximately within a day. Chemical analysis15of the spinel species separated by fractioning suspensionsshows that the small particles are poor in FelI (FeI1/Fe1I1= 0.05-0.07) and that the large ones are much enriched in FelI (FeI1/Fe1I1 0.30). Precipitation of FelI in the presence of freshly precipitated FelI1practically yielded the same results as coprecipitation. Aging products were the same; only the rate of formation of the spinel species somewhat varied: Formation of the large particles was accelerated; that of the small particles was slowed (Figure 5a). The variation of dissolution rate constants as a function of aging time is given in Figure 5b. The rate constant of each species decreases upon aging: That of the large (18) Schwertmann, U.;Murad, E. Clays Clay Miner. 1983,31, 277.
316 Langmuir, Vol. 8, No. 1, 1992
Tronc et al.
Table I. Characteristics of Dissolution Kinetics of Early Susmnsions as a Function of the FeIr/FeIII Commsition (XI.
imm dissolutn, 96 121, min-1 dissolutn, 96 0
0
0.05
0.10
0.15
0.15
100
100
100
100
100
0.20 100
0.30
0.25 87 0.13
68 0.12
13
32
0.35 35 0.13 65
0.40
0.45
0.50
0.50
0.13 100
0.14
0.19 100
0.16 100
100
Data in italics refer to successive precipitation of Fem and Fen ( x = 0.15 and 0.50), others to coprecipitation.
fast
slow
0
I d 2d (Id
smin 3Smin2h 3h30
t
b ,
Figure 4. Electron microscopy observations from a suspension at FeI1/Fen1= 0.15 a t different aging times (t): diffraction at t = 5 min (a); selected area diffraction a t t = 15 min (b); image of the system a t t = 24 h (c).
particles (k2)rapidly drops during the first hours and then stabilizes; that of the small particles (k1) decreases relatively less, and more gradually. As the rate constant is decreased by improvingcristallinity or increasingparticle size,15the drop in 122 can be related to the particle growth; the decrease in kl with aging time, and also with increasing FeI1 level, can be attributed to structural ordering. 4. Mossbauer Investigation of FeILFH (FeI1/FelIr = 0.15). The initial suspension at x = 0.15 was studied in the frozen state after solidifying the sample by immersion into liquid nitrogen. Reliquefying occurred near 273 K. It made the particles sediment rapidly; the suspension had therefore undergone some modification. As shown by Van der Giessenlgfor ferric hydroxide, this modification takes place during solidification. The freezing process is not fast enough to prevent crystallization of solvent. Dispersion state of the particles is essentially preserved, but the structure of the hydration layer around the particles breaks down. Because of the lack of electrical charges at the particle surface, this structure does not recover upon thawing and the particles aggregate. Solvent crystallization is unlikely to change the internal structure of the particle, but it may affect the atoms at the surface. A preliminary investigation was thus performed in order to investigate the phenomenon. 4.1. Surface Effects Induced by Solidification. Starting with the FeII-FH suspension as frozen to 77 K, spectra were recorded at increasing temperatures. The spectrum area, S, suddenly dropped between 180and 200 K and then regularly decreased (Figure 6). No significant extra line broadeningwas observed,and the variation S(T)/ (19) Van der Giessen, A. A. Philips Res. Rep., Suppl. 1968,12,1.
I
'
01
0 Smin 3 W n 2 h 3 h 3 0
t
l d 24 Sd
lo
Figure 5. Dissolution kinetics of a suspension a t Fen/Fem = 0.15 as a function of aging time: (a) FelI1 partitioning among species characterized by immediate (A),fast (u),and slow ( 0 ) dissolution in the case of simultaneous precipitation, and successive precipitation of FelI1and FeI1(open symbols); (b) rate constant variation for species rapidly (kl) and slowly (k2) dissolved. 450 K
Figure 6. Effect of reheating on the relative area of the M6mbauer spectrum of suspended mixed-valence hydroxide (Fer'/ FelI1= 0.15) initially frozen to 77 K, as a function of reheating temperature (dashed line calculated according to the Debye model).
S(80) remained reversible provided the suspension had not been reliquefied. The spectrum area is proportional to the total resonant absorption which in turn depends on the elastic properties of the medium. Up to 180K the relative absorption follows the Debye model; the system behaves like a fairly rigid lattice, and the Debye temperature, of the order of 450 K, is similar to that found in dried ferric hydroxide.20 Near 190 K the coupling between the particles and the solid (20) Conforto, E.; Rechenberg,H.R.; Jafelicci, M.,Jr. J. Phys. Chem. Solids 1986,47, 1179.
Langmuir, Vol. 8, No. 1, 1992 317
Transformationof Ferric Hydroxide into Spinel I
t
G5t
f
.o0
Ot
5t I :
:
-2
w 0
2
1
v(mm/s)
Figure 7. Mdssbauer spectra at 80 K of a suspension of mixedvalence hydroxide (FeII/FeIrr= 0.15) as frozen (a) and after heat treatment at 255 K (b); pure ferric hydroxide suspension (c).
and FeI1yielded features very similar to those obtained in the case of coprecipitation reported below. There is only one type of FeI1 sites (Dz): All FeI1 ions belong to the same phase. FelI1ions exhibit slightly smaller quadrupole splittings than in pure FH; their environment is therefore somewhat more symmetric. Fe"' coordination is typically octahedral; the presence of tetrahedral sites (isomer shift of 0.3-0.4mm/s) may however be obscured by the distributions of octahedral Fe"' parameters.26 High-TemperatureBehavior. Spectra recorded at various temperatures between 80 and 175 K (Figure 8) exhibit noticeable changes around 105 K. Up to 100 K they are well fitted by the FelI1 component and the FeI1 doublet (Table 11). At 105K there appears a doublet whose parameters are intermediate between those of FeIII and Fe". At 110 K only this doublet is involved in addition to the FeIIIcomponent. The parameters change only little as the temperature further increases. Above 105 K, the isomer shift of the intermediate component is typical of the valence state and the relative area is 2 times that of the FeI1doublet below 105 K. This shows that all FeI1 ions in FeILFH give rise to FeILFelI1 electron exchange. In view of the hyperfine parameters the characteristic time associated with energy variation of the nuclear transitions due to the fluctuating charge is of the order of s.26 Since the system appears static with trapped valencies below 100 K and completely averaged above 110 K, the fluctuation time T is such that s at T I100 K and T > Low-TemperatureBehavior. Spectra recorded below 80 K are shown in Figure 9. A six-line component appears near 50 K. It grows at the expense of the quadrupole pattern as the temperature decreases. At 4 K the spectrum is totally magnetically split; it is nearly symmetric with broad lines. The isomer shift is equal to 0.46 "/e; the hyperfine field distribution is characterized by a field of maximum probability equal to 50.0 T and a half-width of 2.5 T. Such parameters are typical of f e r r i h y d r i t e ~ . ~ ~ , ~ ~ ' ~ ~ No Fe" component is resolved; this may be due to low Fe" concentration and spreading of Fe"' parameters. By analogy with most fine magnetic particle systems, the spectrum evolution suggests a superparamagnetic behavior. The blocking temperature (50%of the absorption in the magnetic and nonmagnetic components) is of the order of 40 K. It is somewhat higher than for ferric hydroxide like systems not treated thermally and containing similarly sized particles (10-30 K).21129J0 This may result from an increase in the magnetization owing to the presence of FeI1 ions.
solvent becomes looser; the particles are still trapped in the solid matrix, but they can move.19~20 This may be due to excitation of vibrations of water molecules. After the suspension had been heat treated above 200 K for a few hours and recooled, the spectra changed irreversibly, particularly regarding FeI1 ion contribution (Figure 7a,b). These spectra, like that of pure FH suspension (Figure 7c), show a broad FelI1 component. This broadening essentially comes from the variety of FeIrl surroundings due to poor crystallinity, it results from distribution of quadrupole splittings and, to a lesser extent, of isomer shifts.21 Two series of fits were carried out, involving either two doublets or a distribution of quadrupole splittings for Fe"' ions;22as both models lead to very similar features, we detail only the results obtained in the latter case (Table 11). Before heat treatment, FelI ions are distributed among two kinds of sites (Dz and Dz'). Both isomer shifts are typical of octahedral coordination; the larger quadrupole splitting of Dz' indicates a larger distortion of corresponding sites. Sites Dz' are converted into sites DZby heat treatment; FelI1sites are practically unaffected. Assuming all iron ions to have the same recoilless fraction, we find a Fe1I/FeI1Icomposition of 0.12, in agreement with chemical analysis (0.15). Sites Dz'may be assigned to surface sites (within a layer 1.7 A thick assuming a particle 30 A in diameter and homogeneous distribution of FeI1 ions). They are most likely created by the process of solidification, solvent crystallization raising stresses at the particle surface which Discussion relax when ice immediately surrounding the particles begins to melt (190 K). Surface FelI1 ions are similarly Under the experimental conditions we used, pure FH affected; because of the electronic configuration (S = 5/z) transformed into goethite. In agreement with previous FelI1ions are much less sensitive to environment variations reporb: this transformation was accelerated by small Fe" than Fe" ions (S = 2), and surface Fe"' ions just undergo amounts ( x = 0.05). It was totally suppressed at FeI1/Fe1I1 a slight reduction in quadrupole splitting. For the same levels above 0.10. Then FH transformed exclusively into reason, the spectrum of pure FH suspension is practically spinel. insensitive to solvent crystallization and the hyperfine The existence of two stable spinel species with very parameters are very similar to those of dried p o ~ d e r s . ~ different ~ ~ ~ ~morphologies ~ ~ ~ and compositions at x = 0.10-0.15 4.2. Characteristicsof FeILFH. All following results implies two distinct formation mechanisms. concern suspensions heat-treated at 255 K overnight. They were perfectly reversible and reproducible. Investigation (25)Cardile, C. M. Clays Clay Miner. 1988, 36, 537. (26)Herber, R. H.; Eckert, H. Phys. Reu. B: Condens. Matter 1985, of a suspension formed by successiveprecipitation of Fe"' 31, 34.
(21)Murad, E.; Bowen, L. H.; Long, G. J.; Quin, T. G. Clay Miner. 1988, 23, 161. (22)Le Caer, G.; Dubois, J. M. J. Phys. E Sci. Instrum. 1979, 12, 1083. (23)Childs, C. W.;Johnston, J. H. Aust. J. Soil Res. 1980, 18, 245. (24)Murad, E.;Schwertmann, U. Am. Mineral. 1980, 65, 1044.
(27)Childs, C. W.;Dickson, D. P. E.; Goodman, B. A.; Lewis, D. G. Aust. J . Soil Res. 1984, 22, 149. (28)Madsen, M. B.; Merup, S.; Koch, C. J. W. Hyperfine Interact. 1986, 27, 329. (29)Coey, J. M. D.; Readman, P. W. Earth Planet. Sci. Lett. 1973,21, 45. (30)Rodmacq, B. J. Phys. Chem. Solids 1984, 45, 1119.
318 Langmuir, Vol. 8, No. 1, 1992
Tronc et al.
Table 11. M6ssbauer Parameters of Purely Ferric (FH) and Mixed-Valence (FeII-FH, FeX1/Fe1I1= 0.16) Hydroxide in Frozen Solution. FelI1 FH FeILFH
a
T,K
IS
80 80
0.45
CQS) 0.72 0.70
0.44
FeI1 QSm
0
0.70 0.60
0.32 0.43
b
(D2) (D2') (D2)
Fei
IS
QS
0.98 1.23 0.98 0.96 0.96
1.86 2.61 1.86 1.82 1.92
IS
QS
A, % FeI* Fei 7.9 3.3 11 10 6
80 0.45 0.69 0.60 0.45 100 0.44 0.66 0.62 0.39 105 0.43 0.64 0.60 0.36 0.58 1.15 12 0.86 22 110 0.41 0.70 0.65 0.39 0.68 120 0.40 0.71 0.65 0.42 0.70 0.83 20 150 0.39 0.72 0.66 0.42 0.68 0.85 21 175 0.38 0.71 0.65 0.42 0.65 0.86 20 0 a and b refer to the suspension as frozen and heat-treated at 255 K overnight, respectively. Isomer shift (IS) and quadrupole splitting (QS) are in millimeters per second. QS distribution for Fen1 ions is characterized by the average ((4s)) and most probable (QS,) values, u (mm/s) is the standard deviation. A is the relative area. Fits involving two Fem doublets instead of QS distribution lead to similar data for FeII and Fei components,and Fe" parameters (IS,QS, A ) such as FH, 80 K (0.45mm/s, 0.97"/a, 33%) (0.44mm/s, 0.57mm/s, 67%); FeLFH (b), 80 K (0.46mm/s, 0.72 mm/s, 60%) (0.43mm/s, 0.39 mm/s, 40%), 150 K (0.40mm/s, 0.83 mm/s, 56%) (0.38mm/s, 0.47 mm/s, 44%).All line widths are ca. 0.4-0.5 mm/s.
5 O
I
f--
7
-i2
't
I :
:
-2
:
tJ
I
I
:
0
2
v(mm/s)
Figure 8. Observed and calculated M6ssbauer spectra of
suspended mixed-valence hydroxide (FeI1/Fe1I1= 0.15, heattreated at 255 K).
The large particles grow by a dissolution-reprecipitation process. These particles have a FeI1/Fe1" composition about 1/3, and there is no indication of a crystalline green rustlike intermediate.6,31 Hence, the complexes that dissolve from FeILFH surface probably have a FeI1/Fe1I1 composition about 1/3 in average, and they reprecipitate as nonstoichiometric magnetite. The small particles have about the same size as the initial FeILFH particles, and they are poor in Fe"; they are likely to be formed by structural rearrangements within Fe"FH particles. TEM and kinetic dissolution data show that, at any x value, spinel ordering rapidly sets in locally within FelL FH particles; the degree of ordering increases with x and with time. This solid-state reaction is in competition with dissolution of complexes from the surface; the FeI1 level rules the kinetics of the two processes and the composition of the complexes. At x = 0.05 spinel order remains very poor, and the soluble species are sufficiently poor in FelI (31) Cornell, R. M.; Schneider,W.; Giovanoli, R. J. Chem. SOC.,Furuday Tram. 1991,87, 869.
-6
0
S v(mm/sl
Figure 9. MBssbauer spectra of suspended mixed-valence hydroxide (FelVFelI1= 0.15, heat-treated at 255 K).
to recrystallize as goethite. Increasing the FelI level accelerates the ordering in FeILFH, thus stabilizing the particles, soluble complexes contain too much FelI for goethite to reprecipitate, and nonstoichiometric magnetite recrystallizes. The dissolution process lowers the FelI/ FelI1 ratio in the particle; it probably goes on as long as soluble species with a FelI/FelI1composition about 1/3can form. Spinel formation by dissolution-reprecipitation is equivalent to what is observed when FH is interacting with various divalent transition-metal cations (M = Mn, Zn, Cu, Co, Ni).13J4 In such cases, however, topotactic conversion of MILFH to spinel does not occur, but structural rearrangements causing conversion to hematite do occur. Thus, topotactic transformation to spinel may be a specific feature of partly reduced FH. As FelI is the only MI1 cation that gives rise to electron transfer with Fe"' at room temperature in spinels, the charge transfer clearly shown by Mossbauer experiments may be the driving force. In pure FH, iron atoms are only coupled via anions. As FelLFelI1 electron transfer is a direct process through overlapping d-orbitals, metal-metal bonding is essential.32 Its presence implies suitable structural rearrangements have occurred. Such rearrangements are made possible by the loose structure of ferric hydroxide and proceed by (32) Shermann, D. M. Phys. Chem. Miner. 1987, 14, 355.
Transformation of Ferric Hydroxide into Spinel
olation-oxolation processes with water e l i m i n a t i ~ n3D .~~ delocalization is likely to produce local cubic close-packed ordering as suggested by electron diffraction. The ordering manifests itself even at low FelI levels ( x = 0.05). FeILFH can form under various situations, but it does not necessarily crystallize into spinel. This is likely to occur only if excess electrons can diffuse within the particle. L-Cysteine interacts with FH and causes partial reduction of the interfacial Fe"' ~ i t e s . ~ aDespite l the production of FeIr ions in a notable amount (FelI/FelI1up to ca. 1) magnetite does not form in the system, neither by dissolution-reprecipitation nor by solid-statereaction. The latter process is probably hindered by lack of electron delocalization, excess electrons remaining trapped at the interfacial Fen sites because of ligand bonding. Conversion of pure FH to spinel under TEM examination seems to be caused by electron injection. Different behaviors of powdered and hydrated samples, transformation of some ferrihydrites,lo~~~ and lack of transformation of others7 point out likely effects of variations in hydration and ordering at short range. The more hydrated and hence the less organized the FH, the more easily it can be reduced and converted into spinel because the rearrangements necessary for electron delocalization are facilitated. Besides, this is supported by lack of interaction between Fe" and partly ordered ferrihydrite as shown by subsequent formation of Fe(OH)2.1° The very high reactivity of the initial FH precipitate makes it outstanding among ferrihydrites. Compared to other FeILFe1I1 mixed-valence systems studied by Miissbauer spectroscopy,Mthe exchange process in Fe'LFH appears remarkable in two respects: On the one hand, the exchange remains rapid on the Mossbauer time scale down to a rather low temperature (-100 K); on the other hand, the valences freeze within a narrow range of temperature ( 10K). At first sight such behavior recalls FesO4 with abrupt valence trapping at the Verwey transition (120 K); a similar process is however very improbable owing to nonstoichiometry and structural disorder.35 Electron transfer is likely to be thermally assisted. The observation of a fully averaged state at 110 K suggests that the activation energy is less than 0.1 eV.32 In such s I7 case one would expect relaxation phenomena I s) to manifest themselves between 110 and 80 K, at least. Fast valence freezing suggests that there is a change in the hopping process around 105 K. Pure FH is an amorphous magnet, of speromagnetic type.29936Below the ordering temperature, the moments within a particle are fixed relative to each other, but randomly oriented. It is a result of antiferromagnetic coupling at short range via bridging oxygen. FeILFelI1 electron transfer through overlapping d-orbitals is facil-
-
(33) Henry, M.; Jolivet, J. P.; Livage, J. Structure Bonding (Berlin), in press. (34) Eckert, H. In Mossbauer Spectroscopy Applied to Inorganic Chemistry; Long, G. J., Grandjean, F.,Eds.; Plenum Press: New York, 1987; Vol. 2, p 125. (35) Ramdani, A.; Steinmetz, J.; Gleitzer, C.; Coey, J. M. D.; Friedt, J. M. J . Phys. Chem. Solids 1987,423,217. (36) Coey, J. M. D. J. Appl. Phys. 1978, 49, 1646.
Langmuir, Vol. 8, No. 1, 1992 319
itated if the system is paramagnetic or if the moments of the two ions are In magnetically ordered FeIL FH, the moments surrounding a mobile electron are likely to be frozen in more or less random orientations, making it much more difficult to hop to a neighboring site.37938 A jump in the activation energy is therefore expected a t the onset of magnetic order. Valence freezing around 105 K might thus be related to freezing of the moments. The magnetic ordering temperature of pure FH ( N 100 K)29J9 supports such an assumption. The activation energy is lower than that generally found in disordered transition-metal o ~ i d e s ,or~ crystalline ~,~~ mineral^;^^^^^ it is comparable to that in nonstoichiometric magnetite.35 High electron mobility down to low temperatures suggests that the structure is made up of an extended arrangement of edge-sharing octahedra and that charge transfer is ~ y m m e t r i c a l .Phonons ~ ~ ~ ~ ~that lower energy barriers are favored by the low reticulation and hydration. Correlative distortions in the surroundings might be responsible for the increase in FelI1quadrupole splittings at the onset of the paramagnetic state (Table 11). Simultaneous and successiveprecipitations of FelI1and Fe" yield similar features. However, the phenomena involved are not exactly similar. In the former case, excess electrons are likely to be homogeneously distributed from the start; in the latter case, they are transferred from adsorbed FelI into ferric hydroxide. Thus, whereas stoichiometric magnetite may form directly by coprecipitation, topotactic formation from ferric hydroxide probably implies a transient state; kinetics effects may be too fast to make it observable. Conclusion Electron transfer between FeIr and Fe"' plays a fundamental role in the formation of spinel iron oxides. It ensures transformation of quasi amorphous ferric hydroxide into spinel by Fe" adsorption even at low Fe" levels. There is immediate formation of a short-range ordered, mixed-valence state which shows fast electron hopping. Electron delocalization brings about local structural rearrangements and drives spinel ordering. Besides this topotactic crystallization of spinel,the transformation can also proceed by dissolution-recrystallization processes. The relative importance of the two pathways depends on the FeI' level in the system.
Acknowledgment. We are grateful to M. Lavergne (CRMP, UniversiSP. et M. Curie) for electron microscopy experiments. Registry No. FeCb, 7705-08-0;FeC12,7758-94-3;Fe304,131761-9; Fe(OH)3, 1309-33-7;goethite, 11115-92-7;iron hydroxide, 11 113-66-9. (37) Mott, N. F.;Davis, E. A. Electronic processes in non-crystalline materials, 2nd ed.; Clarendon Press: Oxford, 1979; Chapter 4. (38) Emin, D. J. Phys. Paris 1980, 41, (25-277. (39) Nunes Filho, E.; Conforto,E.;Rechenberg,H. R. J. Magn. Magn. Mater. 1988, 74, 370. (40) Murawski,L.;Chung,C. H.;Mackenzie,J. D.J.Non-Cryst. Solids 1979. - - . -, .?2. - -, 91. - -. (41) Bullot, J.; Cordier,P.;Gallais, 0.;Gauthier,M.;Livage, J. J . NonCryst. Solids 1984, 68, 123. (42) Amthauer, G.; Rossman, G. R. Phys. Chem. Miner. 1984,11,37.
