1423
Langmuir 1989,5, 1423-1427
Stages in the Evolution of Colloidal Chromium(II1) Oxide? V. Swayambunathan,$ Y. X. Liao,s and D. Meisel*9$ Chemistry Division and Material Sciences Division, Argonne National Laboratory, Argonne, Illinois 60439 Received June 8, 1989. In Final Form: July 31, 1989 Three crystalline stages in the evolution of chromium(II1) oxide particles were identified by transmission electron microscopy and electron diffraction. Equinormal solutions of chromium(II1) salt and potassium hydroxide were found to contain crystalline Cr(OH),-3H20 particles of irregular morphologies after a long aging period a t room temperature. When dispersions of this hydroxide are subjected to heating at 100 "C, larger monodispersed particles of a previously unknown intermediate species were obtained. This crystalline intermediate is proposed to be an oxyhydroxide of the composition CrO(3-x),2(OH)x. Further heating of the solution leads to further growth and phase transformation of the intermediate species into crystalline Cr20, particles of a narrow size distribution.
Introduction Transition-metal oxides and oxyhydroxides are extensively studied in view of their importance in many areas of chemistry such as catalysis, corrosion, coatings, or pigments. These materials can be prepared in the colloidal state of well-defined particles of narrow size distribution, usually by the hydrolysis of the corresponding metal salt. However, the mechanism of formation of such particles and the parameters that can influence their morphology and chemical composition are not well understood. Experimental conditions such as the p H of the solution, reaction temperature, initial concentration of the metal ion, and the nature of the anion can all play an important role in determining the nature of the final product obtained from such hydrolysis reactions. Detailed procedures for preparing the dispersions of a variety of hydrous metal oxides have been developed by MatijeviC and co-workers and were summarized in recent review articles. In this paper, we describe several stages observed in the growth of particles obtained by the forced hydrolysis of chromium(II1) salt solutions. The oxides and oxyhydroxides of this metal, because of their unique magnetic properties, are commonly used in information storage devices. The method of preparation utilized here is similar to that proposed by Garg and MatijeviC? and the crystal structure of the particles is examined. The nature of the hydrolysis products, often labeled nonspecifically as hydrous oxides, was monitored by electron microscopy (TEM) techniques from the early stages of the reaction. Size distribution, morphology, crystal structure, and elemental analysis of the particles were obtained from transmission electron microscopy, electron diffraction, and energy-dispersive X-ray analysis (EDAX) studies. Transformation of t h e initial Cr(OH),.3H20 through a n intermediate phase, proposed to be the oxyhydroxide Cr0(3-x),2(OH)x,to Cr203,all of well-defined crystalline structure, was observed. 's2
Experimental Section The forced hydrolysis was initiated by mixing chrome alum and potassium hydroxide solutions in a 1:l molar ratio at room
'
Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE, under contract NO. W-31-109-ENG-38, * Chemistry Division. 8 Materials Science Division. (1) MatijeviE, E. Ann. Reu. Mater. Sci. 1985, 15, 483. (2) MatijeviC, E. Acc. Chem. Res. 1981, 14, 22. (3) Garg, A.; MatijeviE, E. Langmuir 1988, 4 , 38.
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temperature and then heating the solution to -100 "C as previously described., Chrome alum stock solution (20 mL of a 0.2 M solution) was added to 800 mL of vigorously stirred triply distilled water. This was then treated with 44.1 mL of 9.06 x M KOH stock solution by dropwise addition. The content was made up to a total volume of 1 L ([Cr(III)]= 4.0 mM) with triply distilled water, and the solution was filtered through a O.l-rm Nuclepore membrane. The pH of this solution was measured to be 4.2 at this stage but dropped to 3.5 over a period of 10 days and then remained constant for several months. This starting solution was diluted to 1 mM Cr(II1)with pH 4.1 sulfuric acid, and portions of the diluted (50 mL) mixture were taken in tightly stoppered 250-mL Pyrex bottles to the forced hydrolysis procedure. The solutions were first warmed in a boiling water bath for 3 min with magnetic stirring and then transferred quickly to an oven maintained at 100 "C. After the solutions were heated for varying periods of time, the reaction was quenched in a cold water bath and the content was dialyzed for 24 h against pH 4.1 sulfuric acid by using dialysis membranes of 3500 molecular weight cutoff. The particles thus prepared were deposited on 400-mesh copper grids coated with 50-k, carbon film and examined after drying by TEM, on Philips EM 420 or JEOL lOOCX machines equipped with an EDAX accessory. Long-time heating resulted in precipitation of the particles. In these cases, the solutions were filtered, and the particles present in the filtrate and in the residue were examined separately.
Results and Discussion
It is well-known that the room temperature aging of chromium(II1) salt solutions leads to extensive polymerization of its hydrolysis products. Stunzi and Marty have isolated the first few members (monomer t o hexamer) of the hydrolytic polymers of Cr(II1) by ion-exchange ~hromatography.~ Bell and Matijevie also report the observation of strandlike material which was assigned to a basi * chromium(II1) sulfate They also suggest tht this polymeric compound acts as a precursor to spherical chromium(II1) hydrous oxide particles which were formed on heating solutions of the former. Prior to subjecting the starting solution to forced hydrolysis, we first examined the nature of the particles present in this solution, after aging for 4 months a t room temperature, by TEM. This starting solution was found to contain fairly large particles of irregular shapes and a broad size distribution, as shown in Figure 1. Grain boundaries can be observed in the particles, and they seem to result from cluster-cluster aggregation of smaller spher(4) Stunzi, H.; Marty, W. Znorg. Chem. 1983,22,2145. ( 5 ) Bell, A.; MatijeviE, E. J . Inorg. Nucl. Chem. 1975,37, 907. (6) Bell, A.; Matijevi6, E. J.Phys. Chem. 1974, 78,2621.
0 1989 American Chemical Society
1424 Langmuir, Vol. 5, No.6,I989 6
h
,,*
'
*,%.
Letters rn
~
Figure I. (Left) Transmission electron micrograph of sample obtained on aging an equinormal mixture of chrome alum and potassium hydroxide at rimm temperature. (Right) Electron diffraction pattern of t h e particles identified as the hydroxide Cr(OH),.3H20.
Table 1. Reflections a n d Indices for Various Phases Observed during the Forced nydrolysis of Cr(ll1) intermediate
Cr(OH),.3H,O d . h"
hkl
0.340 (3.340) 101 2.453 (2.437) 002 2.280 (2.321) I 1 1 1.794 (1.791) 210 1 . 6 3 (1.668) 202 1.496 (1.528) 000 1.3'36 (1.326) 220, 203
(CrO(3.,,,9(OH),) CrA d , Ab hkl d.h" hkl 4.229 (4.230) 002 3.597 (0.fiRRI 012 3.118 (0.izn) 2.877 (2.915) 2.529 (2.5111 2.112 (2.1151 1.944 (1.951) 1.690 (1.6571 1.501 (1.4991
110
I11 I12 004 020 032 130
2.666 (2.6fifii 2.4fi3 (2.480) 2.179(2.1:fi1 1.812 (1.8161 1.678 (1.672) 1.457 (1.401)
in4 110 I13 024
I16 300
'Literature valuea in parentheses taken from ref 15. Values in parentheses were calculated assuming hexagonal structure with no = 6.24 and co = 8.46 A.
ical particles. Since the solution was completely transparent, we infer that much of the aggregation occurred during the deposition on the microscope grids. Indeed, dilution of the solutions prior M preparation of the specimen decreases the amount of aggregates formed. Particles of different morphologies and sizes in this sample represent different aggregation numbers. The particles were found IO he crystalline, and electron diffraction analysis revealed the structure of Cr(OH),.3H20. Table I gives the observed interplanar spacings along with the literature values for this phase. Electron diffraction patterns obtained from several areas of the same grid were virtu. ally identical, and no other phase could be observed in specimens from this preparation. Similar observations were made on deposits from samples heated for few minutes. When the starting solution was heated for a short period of time (20 min) at 100 "C, drastic changes in the composition and morphology of the particles were noticed. The sol remains transparent, but no particles of the hydrox-
ide phase could be detected. Electron micrographs (Figure 2) show nearly spherical polygon-shaped particles with a narrow size distribution (see Figure 3). These particles are also crystalline, but analysis of the electron diffraction pattern clearly showed that this material is neither Cr(OH),.3H20 nor any other known form of chromium(II1) oxyhydroxide (a-or 8-CrOOH or Cr,O,). The possibility of formation of y-CrOOH can also be rejected since previous studies on its hydrothermal preparation showed it to be amorphous to X-rays.? High-temperature (20C-300 "C) aging of Cr(OH),.3H20, in the presence or absence of moisture, leads to the formation of trigonal or orthorhombic CrOOH.8.' Yet, the species presently observed is different from any of these previously reported forms of CrOOH. The observed interplanar s p a c ings for this intermediate species are given in Table I. These values were calculated from the diffraction pattern by using a standard sample of gold for calibration. Best fit was obtained for a hexagonal structure with lattice parameters a, = 6.24 and co = 8.46 A. EDAX analysis of this intermediate species showed only chromium and no sulfur, suggesting that it is not a sulfate-containing chromium species which was postulated to be one of the several intermediate species in the complex chemistry of hydrous chromium(II1) oxide sol formation? This, however, only implies that sulfate is not a major component in this phase. There is little doubt that counterions, sulfate in particular, play a major role during the early stages of nucleation, which may profoundly affect the morphology and structure of the final produCt.SBSince this phase is an intermediate phase in the conversion of Cr(OH), to Cr,O, (see below), we propose it to be the ~~
~~
(7) Christensen, A. N.Acta Chem. Scond. A I916,30, 133. (8) Fenerty. J.: Sing. K. S. W.Proc. Eur. Symp. Therm. A d . 1st 1976.304. (9) Giovanoli. R.;Stadelmann, W. Thermoehim. Acto 1913,7,41.
Letters
Langmuir, Vol. 5 , No. 6, 1989 1425
e
. *
-
0.tpm
Figure 2. (Left) Electron micrograph of sample obtained following heating a clicomiiimilll) h y d r m i d r sdutioii for 211 min at 100 OC. (Right) Electron diffraction pattern of the particles labeled as the oxyhyd;oxidr CrO,:, x ~ , 2 1 0 H I x .
DIamster
(A)
Figure 3. Size distribution histogram of the particles in Figure 2.
oxyhydroxide CrO(3,,,,(OH), where the chemical composition (0 < x < 3) is unknown a t this stage. A similar hexagonal phase of iron(II1) oxyhydroxide is known to exist in the "amorphous" hydrate gels of this material" and was recently observed as a crystalline intermediate phase in the growth of hematite." Electron micrographs of the samples obtained by heating the starting solution for 60 min indicates further changes in the structure of the material. The solution becomes turbid on heating for such a long time, and precipitation of the particles occurs on standing over a period of several hours. Figure 4 shows a transmission electron micrograph obtained from this sample. In addition to (10) (a) Okamoto. S.; Sekizaws. H.; Okamoto, S. 1. In Reoctiuity of Solids: Anderson. J. S..Robert, M. W.,Stone. F. S., Eds.: Chapman and Hall: London. 1972: p 341. (b) Towe,K. M.; Bradley, W. F. J. Colloid Interface Sei. 1967,24,384. (e) Van Der Giewen. A. A. J. Inorg. Nuel. Chem. 1 9 6 6 , B . 2155. (11) Mills, G.; Swayambunathan. V.: Patel, R. C.;Meisel. D..to be published.
the presence of a large number of particles of the intermediate species, one can observe larger, spherical particles with diameters in the range of a few tenths of a micrometer. These big particles exhibit some clustering upon precipitation. The electron diffraction pattern of the larger particles, obtained by the convergent beam technique, clearly shows that they are Cr,O,. The observed interplanar distances are given in Table I together with the literature values. It is interesting to note that the particles of the intermediate species in this sample are very nearly of the same sizes as in the 20-min sample, in spite of the longer heating period. However, in this sample they are arranged in a unique ring-type pattern (Figure 4). The reason for this favored arrangement is not clear. Magnetic attraction between individual particles may lead to this ring-type arrangement, although aggregation in this preferred arrangement due to wetting characteristics of the solution on the grid cannot be precluded. At any rate, it appears that on continued heating the particles of the intermediate species aggregate without experiencing any significant change in their individual sizes and undergo a phase transformation process resulting in the formation of larger Cr,O, particles. Upon heating of the starting solution for 120 min, increased turbidity could be observed, and precipitation of the particles occurs rapidly. The precipitated particles were separated by filtration, washed several times with triply distilled water, dried under vacuum, and then studied by TEM. Figure 5 shows the particles present in this powder. Their electron diffraction pattern is virtually identical with that of the large particles seen in the 60-min sample. No particles of the intermediate species could be observed in this specimen. I t is also clear from this electron micrograph that on further heating the size of the individual Cr,O, particles remains simi-
1426 Langmuir, Vol. 5, No. 6,1989
Letters
. -:. ..
.*
C.
. '
r
Figure 4. (Left) Electron micrograph of sample obtained following heating a chrimium(llll hydri,ridr s o l u t i o n for 1 h at 100 OC. Note the ring-shaped arrangement of CrOo.,, ,(OH) particles and the highly aggregated big Cr,O, particles. (Right) Electron diffraction pattern of the particles identified as ihe oxide Cr,O,.
sis of chromium(II1) salt proceeds through the following three stages: Cr(II1) + 3H20+ 30H.- ICr(OH)3-3H20)
-
(hydroxide stage)
+ ((9-x)/2)H20 ICr(OH),~3HZOl lCr0,3-z)~~(OWzl
-
(intermediate oryhydroride atape)
21Cr0~3.x)/2(OH)Il ICrz031+ xH20 (finaloxide stape)
. 0.I"rn
I
Figure 5. Electron micrograph of precipitated particles obtained after heating a chromium(II1) hydroxide solution for 2 h a t 100 "C. These particles are identical with the big particles seen in Figure 4. lar to that observed for the particles aged for only 60 min. The size and shape of these particles are similar to those observed by MatijeviE et al., and we identify them as the oxide rather than the hydroxide." The results detailed above indicate that the hydroly-
The initial chromium(II1) hydroxide may exist as a monomer or in the form of hydroxy-sulfate complexes a t the very early stages. It undergoes polymerization on room temperature aging and eventually produces big particles of irregular morphologies of the trihydroxide hydrate. This observation directly supports the results of Stunzi and Marty, who have studied the polymerization process in great detail.'.l3 The role of sulfate ions and their complexes with Cr(1II) species in the growth process has amply been emphasized?.6 Nevertheless, sulfate does not enter into the crystal lattice of any of these stages. The products of the forced hydrolysis of chromium(II1) salts previously carried out by Matijevii: and co-workers in the presence of sulfate anions were reported t o he amorphous chromium(II1) hydrous oxide particles?.'' Under similar experimental conditions, we positively identify them to he crystalline oxyhydroxides of varying degrees of dehydration. Except for the initial trihydroxide phase, (12) MatijeviE. E.; Lindsay, A. D.: Kratohvil, S.:Jones, M. E.; hson, R. I.: Cayey, N. W. J. Colloid Interface Sei. 1971.36.273, (13) Stunzi, H.; Rotzinger, F. P.: Marty, W. lnorg. Chem. 1984,23, PlMI
(14) Calculated from heats of formation in Standard Potentials in Aqueous Solution; Bard, A. J., Parsons. R., Jordan. J., Fds.; Marcel
Dekker: New York, 1985. (15) X-ray Powder Diffraction Files on Inorganic Phases. International Center for Diffraction Data, Pennsylvania, 1983.
Langmuir 1989,5, 1427-1430 continuous evolution of size could not be detected under any of our experiments. Both the oxyhydroxide and the oxide exist only as well-defined particles of relatively narrow size distribution. It is difficult to envision a ripening growth mechanism that does not lead to a broad size distribution when the two consecutive products coexist. More probable is an aggregation/dehydration mechanism that dominates in these systems. The crucial question of the mechanism of the dehydration of a relatively large particle remains to be answered.
obtained upon aging of the Cr(II1) complexes is the hydroxide, Cr(OH),.3H20, which transforms to a partially dehydrated oxyhydroxide, proposed to be CrO(,-,..,?(OH),.. Further aging a t high temperature leads to the final crystalline oxide, Cr203, phase. It seems, therefore, that the process leading to the final product constitutes a sequence of successive dehydration steps. A better description of the process is perhaps the term forced dehydration, in particular considering the ca. 50 kJ mol-' of free energy required for the net reaction:14 2Cr(OH),
Conclusions Three stages of crystalline particles were identified in the forced hydrolysis of Cr(II1) salts. The first stage
1427
-
Cr203+ 3H20
Registry No. Cr,O,, 1308-38-9.
Specific Heat Is a Useful Indicator of Microstructural Variation in Surfactant Solutions E. Z. Radliliska, S. T. Hyde, and B. W. Ninham' Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, ACT 2601, Australia Received May 17, 1989. In Final Form: August 11,1989 Specific heat measurements in the microemulsion region (L2)and cubic-phase region of the ternary DDAB/water/cyclohexane system are reported. We have distinguished two or possibly three separate cubic phases within the cubic-phase region. The experimental results indicate that the specific heat per surfactant molecule is sensitive to changes in both the surfactant film topology and geometry.
Introduction The issue of microstructure within surfactant solutions is now of central importance to progress in surfactant science. It is rapidly becoming apparent that structures of surfactant interfaces are not confined to the Euclidean forms-spheres, cylinders, and planes. Other more exotic structures are also present, within both random isotropic microemulsions phases and ordered lyotropic mesophases. Furthermore, it seems that the microstructure of surfactant interfaces is subject to transformations, of both geometric and topologic natures, within "single phase" regions of the phase diagram.'-, This apparent diversity of microstructure lends urgency to the development of experimental probes of the geometry of surfactant interfaces. T o date, no single technique has been able to unequivocally establish the details of microstructure, be it small-angle scattering (SAS),4-11 NMR,12*13electrical conductivity and v i ~ c o s i t y ? ' ~ -or ~~ (1) Larsson, K. Nature 1983,304,664. (2) Hyde, S. T.;Andersson, S.; Ericsson, B.; Larsson, K. 2.Kristallogr. 1984,168,213. (3) Andersson, S.; Hyde, S. T.; Larsson, K.; Lidin, S. Chem. Reo. 1988,221. (4) Zemb, T. N.; Hyde, S. T.; Derian, P.-J.; Barnes, I. S.; Ninham, B. W. Phys. Chem. 1987,91,3814. (5) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian, P.-J.; Drifford, M.; Zemb, T. N. J.Phys. Chem. 1988,92,2286. (6) Ninham, B. W.; Barnes, I. S.; Hyde, S. T.; Derian, P.-J.; Zemb, T. N. Europhys. Lett. 1987,4, 561. (7) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian, P.-J.; Drifford, M.; Warr, G. G.; Zemb, T. N. h o g . Colloid Polym. Sci. 1988, 76, 90. ~. (8) Caponetti, E.; Magid, L. J.; Hayter, J. B.; Johnson, J. S. Langmuir 1986,2, 722.
0743-7463189f 2405-1427$01.50f 0
freeze fracture electron mi~roscopy.'~A potentially powerful new tool-calorimetry-is now becoming practical. Enthalpy measurements offer a promising technique to measure the structuring of water," a central problem in this field. However, most phase diagram studies and SAS work have been done a t room temperature, so the structural variation with composition, rather than temperature, is under scrutiny. For that reason, we have attempted to employ the calorimetry technique to track the specific heat of surfactant solutions with composition. In this report, we focus on the specific heat of ternary mixtures of DDAB/water/cyclohexane which form microemulsion and cubic liquid crystalline phases, complementing SAS,"7 conductivity, vis~osity,l~-'~and NMR12,13work. The mixtures exhibit a wide variety of microstructures within these phases, making them ideal for our measurements. For example, bicontinuous micro(9) Porte, G.; Gomati, R.; El Haitamy, 0.; Appell, J.; Marignan, J.
J. Phys. Chem. 1986,90,5746.
(10) Samseth, J.; Chen, S.-H.; Litster, J. D.; Huang, J. S. J. Appl.
Crvntalloer. 1988.21. -. , . ___ --, , 8.15.
(11) Auvray, L. J . Phys. Lett. 1985,46, L-163. (12) Fontell, K.; Jansson, M. Bog. Colloid Polym. Sci. 1988, 76,
1 fiF)
--I.
(13) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, B. W. Acta Chem. Scand. 1986, A40,247. (14) Ninham, B. W.; Chen, S. J.; Evans, D. F. J. Phys. Chem. 1984, 88, 5855. (15) Chen. S. J.: Evans. D. F.: Ninham. B. W.: Mitchell.. D. J.:. Blum. F. D.;Pickup,'S. J.'Phys. Chem.'1986, 90,'842. ' (16) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chem. 1986. 90.2817. ~. ~. (17) Jahn, W.; Strey, R. J. Phys. Chem. 1988, 92, 2294. (18) Casillas, N.; Puig, J. E.; Olayo, R.; Hart, T. J.; Franses, E. I. Langmuir 1989,5, 384. , - - ? - - -
0 1989 American Chemical Society
