Langmuir 1995,11, 3660-3666
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Template-Controlled Synthesis of Superparamagnetic Goethite within Macroporous Polymeric Microspheres FranGoise M. Winnik,**tAndre Morneau,+Ronald F. Ziolo,* Harald D. H. Stover,+and Wen-Hui Lit Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1, and Wilson Center for Research and Technology, Xerox Corporation, Webster, New York 14580 Received May 4, 1995. I n Final Form: June 27, 1995@ A superparamagnetic form of goethite, a-[FeO(OH)],has been obtained within macroporous crosslinked poly(diviny1benzene)microspheres(averagediameter 3.6pm and pore size 50-200 nm) by a chemical process involving (1)sulfonation of the internal and external surfaces of the microspheres, (2) treatment of the sulfonated microspheres with aqueous ferrous chloride solution, and (3) oxidation with hydrogen peroxide, pH 14, 70 “C.Steps 2 and 3 were repeated four times, yielding particles containing up to 19% Fe per total weight of material. TEM micrographs of sectioned microspheres revealed the presence of two forms of goethite: disks 25 nm in diameter and needles 80 nm long and 25 nm wide. Temperature dependent magnetization and susceptibilitydata indicate that the materials are superparamagnetic above 50 K. The calculated magnetic susceptibility for the particles is 3 orders of magnitude larger than that of bulk goethite. Aqueous solutions of ferrous chloride were oxidized under the same conditions (HzOz, pH 14, 70 “C) (1)in the absence of microspheres and (2) in the presence of nonporous polystyrene latex particles carrying surface sulfate groups. Neither situation yielded superparamagnetic goethite.
Introduction A class of composites, the iinanophase”or “nanostructured” materials has garnered interest over the past few years because of their ability to exhibit unusual or enhanced mechanical, optical, magnetic, or other physical properties.lJ These materials are solid substances engineered nearly atom by atom within a well-defined template to produce unique compositions that would not occur were the same reactions carried out in the bulk. A critical obstacle in maintaining a nanoscale material is the tendency of molecular clusters to aggregate to reduce the energy associated with the high ratio of surface to volume. Magnetic oxide particles have been prepared by various chemical reactions in a variety of compartment^,^ including reverse micelle^,^ liposome^,^ membranes,6 polymeric micro sphere^,^ and inorganic templates.s It is often stated that the confinement offered by these systems controls to a great extent the magnetic properties of the resulting materials. The alkaline oxidation of ferrous ions, for example, was carried out under seemingly identical conditions on the macroscopic scale, (1)within synthetic ion-exchange resins by Ziolo et aL9 and (2) within swollen sulfonated cellulose fibers by Raymond et al. lo Within ion-exchange resins the reaction led to a form of y-Fez03, which, unlike most magnetic materials a t room temper~~~~
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Xerox Corp. Abstract published in Advance A C S Abstracts, September 15, 1995. (1)Ashley, S.Mechanical Eng. 1994 (February), 52. (2)Martin, C. R. Acc. Chem. Res. 1996,28, 61. (3)Fendler, J. H. Membrane-mimetic Approach to Advanced Materials; Advances in Polymer Series, 113;Springer-Verlag: Berlin, 1994; pp 172-181. (4)Chen, J. P.;Sorensen, C. M.; Klabunde, K. J.;Hadjipanayis, G. C. J. AppZ. Phys. 1!394,76, 6316.Gobe, M.; Kigiro, K.-N.; Kandori, K.; Kitahara, A. J. Colloid Interface Sei. 1983,93, 293. (5) Mann, S.; Hannington, J. P.; Williams, R. J. P. Nature (London) 1986,324,565. (6)Zhao, X.K.; Herve, P. J.;Fendler, J. H. J. Phys. Chem. 1989,93, 908. (7) Ugelstad, J.; Berge, A,; Ellingsen,T.; Schmid, R.; Nilsen, T.-N.; Mork, P. C.; Stenstad, P.;Hornes, E.; Olsvik, 0.Prog. PoZym. Sei. 1992, 17,87. ( 8 ) Ozeki, S.; Uchiyama, H.; Katada, M. Langmuir 1994, 10, 923. @
0743-746319512411-3660$09.00/0
ature, has a n appreciable degree of optical transparency in the visible region. These materials have potential applications in magnetic toners or inks.l’ When carried out within cellulose fibers, the ferrous ion oxidation led to a mixture of y-FezO3 and FesO4. Intrigued by these results, we set about to probe further the paramount influence of the organic template on the outcome of iron oxide synthesis. We carried out the same ferrous ion oxidation reaction within sulfonated macroporous poly(diviny1benzene) microspheres, chemically similar to the sulfonated ion-exchange resins employed by Ziolo et al. ,9 but smaller in size and with a higher crosslinking density. We used a batch of macroporous microspheres with an average diameter of 3.6pm and pore sizes ranging predominantly from 50-200 nm.12 As prepared, these particles have hydrophobic surfaces. Hence they are not dispersible in aqueous solutions of ferrous salts, the typical reaction media for template-controlled iron oxide preparations. The wetting characteristics of both the internal pore surfaces and the outer surfaces of the microspheres were modified by subsequent sulfonation. Iron oxide particles were incorporated within the sulfonated microspheres by alkaline oxidation of ferrous ions bound to the surface of the microspheres by interaction with the grafted sulfonate groups. The chemical, morphological, and magnetic properties of the new materials are reported here, on the basis of data gathered by chemical analysis, transmission electron microscopy (TEM),superconducting quantum interference device (SQUID) magnetometry, and Mossbauer spectroscopy. The iron oxide formed in the microspheres was identified as goethite (a-FeO(0H))with superparamagnetic ~ r 0 p e r t i e s . l ~ The oxidation of ferrous ions was (9)Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A,; O’Horo, M. P.; Ganguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992,257,219. Vassiliou, J. K.; Mehrotra, V.; Russell, M. W.; Giannelis, E. P.; McMichael, R. D.; Shull, R. D.; Ziolo, R. F. J. AppZ. Phys. 1993, 73,5109. (10)Raymond, L.; Revol, J.-F.; Ryan, D. H.; Marchessault, R. H. Chem. Mater. 1994,6, 249-255. (11)Ziolo, R. F. Xerox Corp., Developer composition containing superparamagnetic polymers. U.S. Patent 4 474 866,1984. (12)Li, W. H.; Stover, H. D. H.; Hamielec, A. E. J . Polym. Sci. Part A: Polym. Chem. 1994,32,2029.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 10, 1995 3661
Synthesis of Superparamagnetic Goethite
diameter pore diameter surface area total pore volume S contentb Fe contentC
Table 1. Physical Properties of the Materials Prepared microsphere latex 3.6 pma 0.64 pm 50-200 nm 257 m2 g-' 0.94 mL g-l 7.49% wlw 0.30% wlw 9.0 x mol g-1 2.34 x loT3mol g-l 3.7-27.6% WIW 9.9-60.1% WIW (0.7-4.9) x mol g-' (1.8-10.8) x mol g-'
comments mercury porosimetry nitrogen adsorption quantitative elemental analysis spectrophotometric analysis
a Broad size distribution (coefficient of variation, 55%). Sulfur concentration calculated on the basis of the weight of virgin particles (microspheres or latices). Range of iron concentrations after one and after five reaction cycles (see the text and Figure l),calculated on the basis of the weight of virgin particles (microspheres or latices).
performed also (1)in the presence ofnonporous polystyrene latex particles carrying surface sulfate groups and (2) in the absence of microspheres, under the same experimental conditions. When latex particles were present in the reaction medium, a n iron oxide coating formed around the particles. This oxide was tentatively identified as ferric trihydroxide, Fe(OH13, and contrary to the oxide prepared in porous microspheres, it did not exhibit superparamagnetic properties. In the absence of the polymeric template, the ferrous ion oxidation led to a complex mixture of iron oxides and hydroxyoxides. These results corroborate the underlying assumption which prompted this work, that by performing the oxidative ferrous ion hydrolysis within functionalized polymeric templates, it is possible to control the chemical composition.
Experimental Section Materials. The preparation of the poly(diviny1benzene) macroporous microspheres was reported by Liet al. 12 They were produced by suspension polymerization of divinylbenzene-55 in the presence of a mixture of toluene and dodecanol used as porogens. The latex particles were prepared by emulsifier-free emulsion polymerization of styrene initiated with potassium persulfate following the procedure reported by Goodwin et ~ 2 1 . Chlorosulfonic acid, sodium hydroxide (NaOH),o-phenanthroline, and aqueous hydrogen peroxide (HzOz, 30% vlv) were obtained from BDH Inc. Ferrous chloride (FeCly4HzO) was purchased from Fisher Scientific. Chemicals were used as received. Reagent grade solvents were employed. Water was deionized with a Barnstead NANOpure water purification system. Instrumentation. Transmission electron microscopy (TEM) was performed with a Phillips CM-12 microscope, operated at 120 kV, equipped with an energy-dispersive X-ray spectroscopy (EDS)microanalysis system (McMaster University, Institute for Materials Research). For TEM analysis, particles were solvent exchanged with ethanol and embedded in a Spurr resin which was cured at 70 "C overnight. Ultrathin sectioning (70 nm in thickness) of the resulting blocks was performed with a RMC MT-7 ultramicrotome equipped with a diamond knife. "he sections were mounted on copper grids. Some sections were coated with carbon before observation, and scanning electron microscopy (SEM) was performed with a ISI-DS130 electron microscope operated at 20 kV. Particles for analysis were coated with gold. X-ray diffraction patterns were measured on a Nicolet XRD instrument using a copper X-ray source. A superconducting quantum interference device (SQUID) Quantum Design magnetometer (McMaster University, Department of Physics and Astronomy) was used for determination of the magnetic properties. The sample (approximately 5 mg) was placed into a #3 gelatin capsule. This was inserted into a cylindrical sample holder (ca. 6 mm in diameter) placed in the sample compartment of the magnetometer. The Mossbauer spectra were recorded at room temperature with a constant-acceleration Mossbauer spectrometer in transmission geometry using a Gbq 57CoRh source (Department of Physics, McGill University, Montreal, Canada). Standards were obtained from Pfizer Pigments Inc. The spectra were fitted using a standard Mossbauer computing fitting program. Neutron activation chemical analyses were (13)Bean, C. P.; Livingston, J. D. J.Appl. Phys. 1959, 30, 120s. (14)Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sei. 1974, 252, 471.
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perfomed in the Centre for Neutron Activation Analysis, Nuclear Research facility, McMaster University, Hamilton, Canada. Quantitative elemental analysis were done by Guelph Chemical Laboratories Ltd., Guelph, Canada. Preparation of Sulfonated Microspheres. A suspension of macroporous poly(diviny1benzene) microspheres (3.0 g) in dichloromethane (100 mL) was degassed in an ultrasonic bath for 10 min. I t was cooled to 0 "C. A solution of chlorosulfonic acid (1.5mL) in dichloromethane (100 mL) was added dropwise to the stirred suspension. At the end of the addition the mixture was stirred for 16 h a t room temperature. The particles were separated by filtration, washed thoroughly with dichloromethane, and dried. The dry powder obtained was washed with water and dried under vacuum at 85 "C. The degree of sulfonation of the particles was determined by elemental sulfur analysis (see Table 1). Preparation of Iron Oxide-Containing Macroporous Microspheres. A suspension of sulfonated microspheres (0.306 g) in aqueous FeClz (15.09 mmol, 50 mL) was stirred vigorously for 12 h. The particles were separated by filtration on a fine fritted-glass funnel. They were washed thoroughly with water until no iron was detected in the eluent (thiocyanide test).15The freshly washed particles were dispersed into 50 mL of water. Aqueous NaOH (150mmol, 40 mL) was added to the suspension. The resultingmixture was heated to 60 "C. M e r the temperature ~ of the mixture reached 60 "C, an aqueous hydrogen peroxide solution (15%, 30 mL) was added dropwise over 15 min. The reaction mixture was kept at 60-70 "C for 1.5 h. The particles were filtered on a fine fritted-glass funnel. They were washed with water until the pH ofthe eluent was neutral.16 Finally, the particles were suspended in ethanol (95%, 5 mL). The suspension was filtered. The beige powder was dried in uucuo at 50 "C for 5 h. Any further iron oxide loading of particles was done by repeating the entire process. To monitor the effect of the initial contact time of the porous microspheres with a ferrous chloride solution, each of ten suspensions of sulfonated macroporous particles (0.030 g) in aqueous FeClz (1.51 mmol, 25 mL) was stirred vigorously for 0.10, 0.25, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0, and 12.0 h, respectively. Each batch was then subjected separately to the entire procedure. The amount of iron in each batch was determined by the o-phenanthroline spectrophotochemical test. Preparation of Iron Oxide-ContainingLatices. A suspension of the latex particles (0.277 g) in aqueous FeClz (15.09 mmol, 50 mL) was stirred vigorously for 12 h. The particles were separated by filtration over a fine fritted-glass funnel. They were washed thoroughly with deionized water until no iron was detected in the eluent. The freshly washed particles were dispersed into 50 mL of water. Aqueous NaOH (150 mmol, 40 mL) was added to the suspension. The resulting mixture was heated to 60 "C. After the temperature of the mixture reached 60 "C, an aqueous hydrogen peroxide solution (15%, 30 mL) was added dropwise over a period of 15 min. The reaction mixture was kept at 60-70 "C for 1.5 h. The particles were separated by filtration over a fine fritted-glass funnel. They were washed thoroughly with water until the pH of the eluent was neutral. They were suspended in ethanol (95%, 5 mL). After about 5 min the suspension was filtered. The recovered latex was dried in (15)Reagent Chemicals, American Chemical Society, Specifications, 6th ed.;American Chemical Society: Washington, DC, 1981;pp 25-26. (16) If dried particles were resuspended in water, the pH of the suspending medium increased rapidly to 10.2, indicating that the washing procedure did not remove all the residual base.
3662 Langmuir, Vol. 11, No. 10, 1995
Winnik et al.
Table 2. Composition of the Microspheres M5= elemental analysis Na Fe S
neutron activation element content % wlw mol g-1 a
Fe 18.8 f 0.8 3.4 x 10-3
3.1 f 0.2 1.4 10-3
15.9 f 0.5 2.8 10-3
3.3 f 0.2 1.0 x 10-3
colorimetrv6 Fe 19.2 f 0.5 3.4 10-3
Modified microspheres obtained after five reaction cycles (see the text). * o-Phenanthroline test. l3
vacuo a t 50 "C for 5 h. Any further treatment of latex was done by the repeating the entire procedure. Ferrous Chloride Oxidation in Solution (Control Experiment). Aqueous NaOH (150mmol, 60 mL) was added to a solution of FeC12 (3.0g, 15.09 mmol) in water (50 mL). The solution was heated to 60 "C. Aqueous hydrogen peroxide (E%, 30 mL) was added dropwise to the solution over a period of 15 min. The reaction mixture was kept a t 60 "C for 1.5 h. The iron oxide was separated by filtration over a fine fritted-glass funnel. The reddish brown precipitate was washed thoroughly, first with water until the pH ofthe eluent was neutral and then with ethanol (95%, 15 mL). It was dried in uucuo a t 50 "C for 5 h. Spectrophotometric Determination of the Iron Incorporated within Microspheresand Latices. A known amount of iron-containing microspheres or latices (cu. 10 mg of dry powder) was treated with concentrated HCl(2 mL) for 30 min a t room temperature. The mixture was filtered on a medium fritted-glass funnel and washed with about 2 mL of water. The eluent was brought to 10 mL by addition of water. The amount of iron in this solution was determined spectrophotochemically using the tris(1,lO-phenanthroline)iron(II)complex test." In order to ascertain the validity of this protocol, a sample of microspheres (M5, obtained after five reaction cycles, see text below) was analyzed by three different techniques: elemental analysis (Fe, S), neutron activation (Fe, Na), and spectrophotometry (Fe). The data from the three sources were in agreement within cu. 10%(Table 2). Unless otherwise specified, the iron contents quoted in the text were determined via colorimetry.
Results Preparation of Magnetic Macroporous Microspheres and Latices. A single batch of microspheres was used throughout this study. They have a number average diameter (D,)of 3.6 pm with a coefficient of variation of about 55%.18 Their internal porosity was determined by a combination of transmission electron microscopy (TEM), mercury porosimetry, and adsorption measurements. l2 Observation by TEM of sectioned resin particles revealed the presence of two groups of pores: (i) small pores of diameter ranging from 50 to 150nm, which contribute the most to the internal surface area, and (ii) intermediate pores approximately 200-500 nm in diameter (Figure 1). The porosity and total surface area of the microspheres were determined by mercury porosimetry and nitrogen adsorption, respectively (Table 2). The amount of surface sulfonic acid groups (2.34x mol g-l, Table 2)exceeds the maximum number of sulfonic acid groups that can be accommodatedon the outer surface of the microspheres. Therefore the internal surfaces have been sulfonated to a large extent. Iron oxide particles were synthesized within the porous network of the sulfonated microspheres by the in-situ procedure described by Ziolo et aL9 Figure 2 illustrates the overall transformation process. The sulfonated microspheres were exposed to an aqueous solution of ferrous chloride to allow the exchange of the sulfonic acid protons for ferrous ions (initial loading in Figure 2). The duration of this initial step, ranging from 6 min to 12 h, had no significant impact on the total iron incorporated within the microspheres (Fe: 4.1 f 0.3% w/w of material), (17) Saywell, L.G.;Cunningham, B. B. Ind. Eng.Chem. 1937,9,67. Mellon, M. G. Ind. Eng. Chen. 1938,IO, 60. (18)The coefficientof variation (cv) is defined as the ratio of the standard deviation of the particle diameter to the number average particle diameter.
1.0 pm Figure 1. TEM micrograph of a microsphere section of before iron oxide incorporation (MI.
indicating a fast saturation of the sulfonate groups. Addition of NaOH (pH 14)to a stirred suspension of the washed ferrous ion-containing particles and subsequent oxidation with hydrogen peroxide (H202,E" = +1.77 V)19 at 60 "C were the steps required to complete the initial loading cycle. The oxidant was added in large excess with respect to Fez+. The amount of iron incorporated within the microspheres was increased significantly by carrying out additional reaction cycles, each starting with reexposure of the microspheres to aqueous FeC12. The total iron content increased from 3.7% w/w after the first cycle to 27.6% w/w after five cycles.20 It is informative to relate the amount of iron oxide formed to the total concentration of the sulfonate groups. The first cycle results in the incorporation of 0.66 x mol Fe/g of microsphere, a value lower than the maximum iron loading attainable (1.17x mol g-l), assuming a 2:l stoichiometric ratio of the S03-/Fe2+ions. After several reaction cycles, the amount of iron exceeded the limit set by the sulfonate concentration, but the iron increments after each cycle mol g-l; (cycle 2,1.18 x mol g-l; cycle 3,1.06 x and cycle 4,1.5 x mol g-l) remained within the limits set by the number of sulfonate sites available. Hence in (19)Day, R.A.; Underwood, A. L. in Quantitative Analysis, 4th ed.; Prentice-Hall,Inc.: Englewood Cliffs, NJ, 1980; p 630. (20)The iron incorporation .in the composites is reported as the amount of iron added to 100 g of either sulfonated particles or latices. This convention was followed throughout the text, unless otherwise specified (see Table 1 for instance).
Synthesis of Superparamagnetic Goethite
Langmuir, Vol. 11, No. 10, 1995 3663 70
I
Microsphereq 0 Latice
60
I
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Macroporous Microspheres
Na+
n
so:
II
10 0
1
2
3
4
5
Number of Loading Cycles Figure 3. Amount of iron (Fen+)incomorated in the nicrospheres and in the latices as a function ofthe number of reaction cycles expressed in percent weight of the virgin material: hatched bars, latex; full bars, microspheres.
!+a+@.;
+ ,
‘SoLAT‘oN
‘S0:Na’ 3
Magnetic Particles
Figure 2. Schematic representation of the process employed in the preparation of the iron oxide-containing materials. Note: the intermediate formation of Fe(OH)2 is postulated on mechanistic grounds, but has not been demonstrated experimentally.
each cycle the sulfonic acid groups are neutralized as sodium sulfonates upon treatment with aqueous sodium hydroxide. The sodium content was confirmed as 2.05 x mol g-l) for particles carried through five cycles. It should be noted that the amount of iron incorporated levels off after about five cycles; presumably at this point the iron oxide particles formed within the microsphere pores prevent the effective exchange of Na+ for Fe2+. A control experiment was performed with microspheres which had not been sulfonated. It was extremely difficult to disperse these in an aqueous ferrous chloride solution. Nonetheless, after one complete loading cycle, a small amount of iron ( -=1%)was detected in the microspheres. The outer surface of the microspheres may offer some possible nucleation sites as a result of the presence of residual methylcellulose, the stabilizer used in the suspension polymerization process. A second series of syntheses was performed using nonporous polystyrene latex particles prepared by emulsifier-free emulsion polymerization initiated with potassium persulfate. It is known that this polymerization process yields particles with sulfate groups attached to their surface. These groups provide electrostatic stabilization to the aqueous latex parti~1es.l~ The amount of sulfur determined by elemental analysis (0.30%w/w, see Table 2) corresponds to the surface charge density (about 10pC cm-2)usually reported for such particles. Some of the sulfate groups initially formed are likely hydrolyzed and oxidized to carboxylic acid groups.21 The latex particles were isolated in dry form. They were dispersed in aqueous ferrous chloride, washed, and subjected to hydrogen peroxide oxidation at pH 14,under conditions identical to those described in the case of the macroporous microspheres. Even though the latex particles carry fewer acid groups, compared to the porous sulfonated microspheres, these are concentrated on the latex surface and act as seeds during the oxidation of the ferrous ions. All of the iron oxide generated during oxidation was adsorbed on the latex surface. After each loading cycle the amount ~
(21) Stone-Masui, J.; Watillon, A. J. Colloid Interface Sci. 1975, 52,479.
of iron oxides deposited on the latex particles was nearly equal to, or even exceeded, that formed within the microspheres (Figure 3). The morphology, composition, and magnetic properties of the iron oxides formed in the two situations are different, as described in the next section. Finally, we performed the oxidation of ferrous chloride in the absence of polymericparticles of any kind, but under otherwise identical conditions. This oxidation is notoriously sensitive to experimental conditions, not only to controllable parameters, such as reagent concentrations, pH, temperature, and time, but also to the inadvertent presence of impurities or even the surface condition and morphology off the reaction In our hands, the ferrous chloride oxidation led to a finely divided brown powder consisting of a mixture of several iron oxides. The powder X-ray diffraction pattern revealed the presence of two crystalline components, the oxyhydroxides goethite (a-FeO(OH))and d-FeO(OH). Various X-ray amorphous iron hydroxide phases, such as the ferric trihydroxide gels Fe(OH)s*nH20,may be present in the mixture as well.23 Even hydroxides which are not truly amorphous, in a crystallographic sense, may give few, if any, diffraction lines, because of their poor crystallinity or ultrafine particle size (
