Langmuir 1988, 4 , 26-31
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Art ic 1es Formation of Monodispersed Pure and Coated Spindle-Type Iron Particlest Tatsuo IshikawaJ and Egon MatijeviC* Department of Chemistry, Clarkson University, Potsdam, New York 13676 Received July 10, 1987. I n Final Form: August 25, 1987 Monodispersed spindle-type colloidal iron was prepared by reducing uniform hematite (a-Fe203)powders in hydrogen atmosphere. The shape and size of iron particles, obtained below 400 "C, were close to those of the precursor, a-Fe203. Silica-coated hematite gave on reduction iron particles of smaller internal crystallite size and larger surface area than pure a-Fe203. The coercivity of reduced solids, which increased with the decrease in crystallite size of iron, ranged from -900 to 1300 Oe and the saturation magnetization from 100 to 140 emu g-l. The coercivity of Co-coated iron particles was -700 Oe and the saturation magnetization 160 emu g-'.
-
Introduction Colloidal iron of high coercivity and large saturation magnetization is essential for high-density recording tapes. The coercivity of particles depends on their shape and size; the maximum value for iron was found a t a diameter of -20 nm.1*2 However, such fine powders are not suitable for manufacturing purposes, because of their low stability. Due to the anisotropy of magnetization, elongated particles are preferable since they allow for proper orientation when dispersed in a film.3,4 Monodispersed acicular particles with the length ranging between 0.1 and 0.5 pm are considered to be most desirable for recording tapes, because of their magnetic and stability proper tie^.^ Anisometric chainlike iron particles have been prepared by electrodeposition6and by treatment of ferrous ions with sodium borohydride in a magnetic field.7 Reduction of iron oxides and oxyhydroxides by gaseous hydrogen8-l0 yielded acicular solids. Since only rather ill-defined precursor cu-Fe20, powder was available, it was not possible to obtain "monodispersed" pure iron particles by the last technique. Recently, spindle-type hematite of varying length and axial ratios was synthesized in our laboratory1' and was then used to prepare corresponding metallic iron powders by reduction with gaseous hydrogen. Surface-modified iron particles were also obtained by pretreating the precursor powders with either silica or cobalt oxide layers. The so-prepared finely dispersed iron samples were characterized by various techniques.
Experimental Section Materials. A. Colloidal Hematite. Hematite particles of various size were synthesized by a modified method described earlier." Ferric hydroxide precipitate was obtained by adding a 1.0 mol dm-3 NaOH solution into 200 cm3of a 0.02 mol dm-3 Fe(N03)3solution until the pH was 9.8. This precipitate was partly redissolved with 20 or 30 cm3of a 1mol dm-3HC1 solution, diluted with 45 cm3of a 0.010 mol dm-3 NaH2P04solution and 'Supported by the IBM Corp., San Jose, CA. * On leave from Osaka University of Education, Osaka, Japan. 0743-7463/88/2404-0026$01.50/0
Table I. Mean Particle Size, Specific Surface Area, and Crystallite Size of a-Fe2O9Samples mean particle axial specific surface crystallite sample length, pm ratio area, mz g-' size (110),nm A 0.49 8 26 [16]" 52 B C D
0.38 0.28
0.21
7 6 5
37 [17] 43 [20] 46 [23]
42 36 36
In brackets are given values calculated by considering spindletype smooth particles. water to 500 cm3,and then aged at 100 "C for 2 days. Samples designated A and B refer to systems to which either 30 or 20 cm3 of a 1 mol dm-3HCl solution was added. Sample C was obtained by mixing 200 cm3of a ferric hydroxide suspension containing 0.030 mol of HC1 with 300 cm3of an 8.3 X lo4 mol dm-3NaH2P04 solution, preheated to 95-100 "C, and finally by aging the mixture at 100 "C for 2 days. Sample D was prepared by adding dropwise 5.4 cm3of a 3.7 mol dm-3 FeC13solution into 1 dm3of a 4.5 X lo" mol dm-3 NaH2P04solution at 95 "C and aging the mixture at 100 "C for 4 days. The resulting precipitates were washed with a 1 mol dm-3 NHIOH solution and doubly distilled water and finally dried under vacuum at 50 "C for 10 h. The silica-coated 01-Fe203particles were prepared by dispersing 0.20 g of an cu-FezO3 powder in 1 dm3 of Na2Si03solution (250 ppm Si) of pH 5.0, adjusted with 1 mol dm-3 HzS04,and aging the mixture at 100 "C for 3 h. The deposition of cobalt oxide on &-Fez03surface was achieved by dispersing 0.20 g of a-Fez03 in 1 dm3 of a 0.010 mol dm-3
(1)Luborsky, F. E.J. Appl. Phys. 1961,32,1715. (2)Luborsky, F.E.; Paine, T. 0. J . Appl. Phys. 1960,31, 685. (3) van der Giessen, A. A. IEEE Trans. Magn., Mag-9 1973, 191. (4) Luborsky, F.E.; Paine, T. 0. J . Appl. Phys. 1960,31, 66s. (5)Sugihara, H.; Tokuoka,Y. Abstracts of Papers; Annual Meeting of the Chemical Society of Japan; 1985,p 1872. (6)Luborsky, F.E.J. Electrochem. SOC.1961,108, 1138. (7)Oppegard, A.L.;Darnell, F. J.; Miller, H. C. J. Appl. Phys. 1961, 32, 1845. (8)van der Giessen, A. A.; Klomp, C. J. IEEE Trans. Magn., Mug-5 1969,317. (9)Suzuki, S.;Omote, Y.; Miya, S.; Yoshida, I.; Ishida, A.; Minegishi, J.; Iwatare, 0.Proc. Int. Conf. Ferrites 1980,556. (10)Ogisu, K.; Takahashi, S. IEEE Trans. Magn., Mag-21. 1985,1489. (11)Ozaki, M.;Kratohvil, S.; MatijeviE, E. J. Colloid Interface Sci. 1984,102, 146.
0 1988 American Chemical Society
Langrnuir, Vol. 4, No. I, I988 27
Pure and Coated Spindle-Type Iron Particles
I
1flU.m
Figure 1. Transmission electron micrographs (TEM) of hematite (u-Fez03)particles prepared as descrihed in the text by procedures designated A, B, C, and D.
CO(CH~COO)~ solution at pH 7.0 and aging the mixture at 100 OC for 3 h.lz B. Reduction of Hematite to Iron. All hematite samples (0.20 g) were first reduced with hydrogen gas at a linear velocity of 16 cm m i d at 300 OC for 1 h Afterward, the process continued with eamplea kept at various temperatures ranging between 300 and 450 OC for an additional 2 h. To prevent oxidation of the resulting fine iron particlea, t h e were treated with ethanol vapor in a nitrogen stream at rmm temperature for 3 h and then stored in t o l ~ e n e ? Before ~ any characterizationprocedure, toluene was gradually evaporated at rmm temperature. Characterization. The Fe, Si, and Co content of the reduction products was determined by atomic absorption spectroscopy with a graphite atomizer (Perkin-Elmer 5000). The crystal structure and crystallite size were assayed by X-ray diffrahmetry (Philips, Norelco) using a Cu Kn line (15mA, 35 kV). The morphology of particles was inspected by transmission electron microscopy (JEOL 1200EX). The particle size distribution histograms were obtained by using a particle size analyzer (Zeias TG73). The BET (12) Sugimoto, T.;MatijeviE, E.J . Inorg. Nul.Chern. 1979.41.165. (13) Asada, S. Nippon Kogaku Koishi 1984,1372.
specific surface areas were measured hy nitrogen adsorption (Quantachrom,Quantasorb). The magnetic properties were determined by a vibrating sample magnetometer and an AC BH meter (Hyst 5000).
Results Properties of Spindle-Typeu-Fe203. Figure 1shows the electron micrographs of the starting hematite particles designated A-D in the Experimental Section. The mean particle length, axial ratio, and specific surface mea of these samples are given in Table I. The crystallite size, calculated hy the Scherrer equation,” is less than the particle length, indicating that these ol-Fe,O, particles are polycrystalline. The specific surface area is larger than the surface area calculated from the particle size assuming a density of 5.3 g m-3. Therefore, hematite prepared as (14) -off, L. V.; Buerger. M. J. The Powder Method in X-Ray Crystallography; McGrsw-Hill: New York, 1985, p 254.
28 Langmuir, Vol. 4, No. I, 1988
Ishikawa and MatijeuiE
a
Figure 2. "EM of particles obtained by reduction of hematite powder (sample A, Figure 1)with hydrogen, a t different temperatures:
(a) 300, (b) 350, (c) 400, and (d) 450 OC.
0.10
3w
354
4w
0.50
4w
TEMPERATURE/ O C Figure 3. Fe content (wt %) in products obtained from a-Fe,O, powders by reduction with hydrogen gas at different temperatures. Hematite precunurrs: m p l e s A (0) and D (0). The triangle gives the calculated percentage of iron in pure magnetite
Figure 4. Mean particle length (0) and internal crystallite size of magnetite (Fe90,, 311) (0) and of iron b F e , 110) ( 0 )ohtained by reduction with hydrogen of hematite powder (sample A) as a function of the reduction temperature.
Langmuir. Val. 4, No. 1, 1988 29
Pure and Coated Spindle-Type Iron Particles
' /Urn
Figure 5. TEM of particles obtained by reduction in hydrogen of silica-coated a-Fe203(sample A) at 400 (a) and 450 OC (b).
REWCED SILICA COATED HEMAlIlE.
=E
A
cowc
PARTICLE LENGTHlpm
Figure 6. Histograms of a-Fe particles obtained by reduction with hydrogen gas at 400 O C of silica-coated hematite powders: sample D (left) and sample A (right). described seems to have pores and/or a rough surface as seen later in Figure 7a. Reduction of a-Fe203.Figure 2 illustrates the reduced solids using sample A as the starting material. Different pictures refer to various reduction temperatures. The particle shape remained essentially unchanged up t o 400 O C , but at 450 O C some sintering is observed. The X-ray analysis of the sample obtained at 300 "C showed dif-
fraction peaks characteristic of magnetite, Fea04,and at 350 O C of Fe304and a-Fe, while above 400 "C only a-iron is detected. The Fe content increases to -98% with the rise in temperature; at 300 OC the content was close to that calculated for pure magnetite (Figure 3). The mean particle length remained essentially unchanged with the reduction temperature. T h e average length of internal crystallites became smaller in magnetite, but larger in iron, as the temperature of hydrogen treatment increased (Figure 4). The specific surface areas of ol-Fe20, decreased with reduction as seen by comparing the corresponding data for sample A in Tables I and 11. Similar results were obtained with other hematite powders. Reduction of Silica-Coated wFe2O3. The electron micrographs of iron particles produced by treating silicacoated a-Fe,03, prepared from sample A, with hydrogen gas at 400 and 450 "C are shown in Figure 5. The particle shape remained unchanged after reduction at 450 O C , in contrast to pure a-Fe20spowder calcined in hydrogen at the same temperature. Table I1 lists the mean particle length, axial ratio, specific surface area, and crystallite size of all iron samples heated at 400 "C. While the mean length and axial ratio did not change, the specific surface area increased and the internal crystallite size decreased on reduction. The histograms of these samples are given in Figure 6. Reduction of Hematite Particles w i t h Deposited Cobalt Oxide. Figure 7b is an electron micrograph of
Table 11. Mean Particle Lsngtb (I), Axial Ratio (R), Smcific Surfaee Area (A,), Crystallite Size ( L ) ,Fe Content, SiOI Content. Coercivity (Ha), Saturation Magnetization (d, and Squareness ( 9 )of Iron Particles Prepared by Reducing Hematite Powders with Hydrogen at 400 "C VSM (CU)' BM (R-P)' VSM (R-P)' . . . . . . prcursor 1, A,, L, Fe, Si% Ha, a,, H., a . , H., a,, sample a-Fe203 rm R m2g-' nm wt 90' wt 90 Oe emug-' S Oe emug-' S Oe emug-' S 130 0.30 0 680 I sample A 0.49 8 18 [26]' 60 95 I1 I11 IV V VI
silica-coated A
0.49 8 57 1231
32
93
1.5
910
140
0.37 1042
115
0.47
972
118
0.45
silicecoatedB 0.38 7 51 f35j 22 93 1.3 1070 110 0.49 1328 108 0.52 1248 112 0.50 silica-coated C 0.29 6 52 [40] 30 93 1.5 930 140 0.42 1136 114 0.49 1058 118 0.49 silica-coated D 0.22 5 65 [43]
