J. Chem. Sac., Faraday Trans. I, 1986,82, 2401-2410 Fourier-transform Infrared Spectroscopy of Colloidal a-, p- and y-Ferric Oxide Hydroxides Tatsuo Ishikawa,* Satomi Nitta and Seiichi Kondo School of Chemistry, Osaka University of Education, 4 4 8 Minamikawahori-cho, Tennoji-ku, Osaka 543, Japan Fourier-transform infrared spectra and surface properties of colloidal a-, 8- and y-ferric oxide hydroxides [Fe(O)OH], which were synthesized and aged in this laboratory, have been studied. The spectrum of the a phase has a strong absorption band at 3150 cm-l and two weak ones at 3485 and 3661 cm-'. That of the B phase has a strong band at 3480 cm-l, a weak band at 3659 cm-l and two very weak bands at 3686 and 3723 cm-l. That of the y phase has a strong band at 3160 cm-l, a weak band at 3624 cm-l and a very weak band at 3528 cm-l.The changes of these bands by the adsorption of heavy water, water and methyl iodide molecules and cupric ions have been studied in detail and the various bands were assigned to various OH groups with respect to the crystal structures of these materials. There are many studies on the properties of colloidal a-, /3- and y-ferric oxide hydroxides, in relation to industrial applications such as starting materials for manufacturing pigments, colour materials and ferromagnetic ferrites and to soil properties and corrosion behaviour. However, their surfaces are still not well characterized. 1.r. spectroscopy was first carried out on the a phase by Russell et al., who identified two absorption bands at 3485 and 3661 cm-l.' Parfitt et al.studied the i.r. spectra of the a phase after adsorption of phosphate and sulphate ions from aqueous solutions and discussed the adsorption sites2* Inouye and coworkers studied the gas-adsorption isotherms of various molecules on the a, p and y phases in relation to the structures of these materiak49 The purpose of this report is to assign the v(0H) band associated with adsorption on the well crystallized a, /3 and y phases to surface sites by means of FTIR spectroscopy and thermodynamic methods. Experimental Materials a-Ferric oxide hydroxide was prepared by adding 1 .O an01 dm-3 sodium hydroxide solution to 0.1 mol dmP3 ferric nitrate solution up to pH 12 at room temperature. The precipitates were aged at pH 12 at room temperature for 10 days, then washed with distilled and deionized water and dried in air at 373 K for 5 h.P-Ferric oxide hydroxide was prepared by boiling 0.1 mol dm-3 ferric chloride solution containing 6 wt % urea. The precipitates were washed with water and dried in air at 373 K for 5 h. y-Ferric oxide hydroxide was obtained by adding hexamethylenetetramine to 0.3 mol dm-3 ferrous chloride solution and oxidizing the resultant ferrous hydroxide suspension with sodium nitrite at 333 K for 45 min. The precipitates were washed with water and dried at 333 K for 10 h. Crystal structures, crystallinity and purity of these samples were examined by means of high-intensity X-ray diffraction (Cu K,, 45 kV, 120 mA) and the absence of residual anion impurities used in the preparations was checked by i.r.All three modifications are non-porous crystalline powders. Their specific surface areas obtained 240 12402 FTIR of Colloidal Ferric Oxide Hydroxides Table 1. Specific surface areas (A,) and particle sizes of a-, j?- and y-ferric oxide hydroxides - -~ mean particle size/nm - sample As/m2 g-' length width thickness a phase 82 310 43 7 p phase 33 260 40 34 y phase I10 160 35 5 cn m W L1 * . . . . . . . I . - . * . . . 3800 3600 3400 3200 3000 2800 3750 3700 3650 w avenumber/ cm -' Fig. 1. The i.r. spectra of a-, /I- and y-ferric oxide hydroxides, treated in U ~ C U O at 348 K for 30 min. (-) a phase; ( . . a ) p phase; (---) y phase. by the nitrogen B.E.T. method are listed in table 1. The particle shapes of the a, p and y phases observed by TEM are close to rectangular thin plates, rectangular rods and rectangular thin plates, respectively.The thickness of these crystals in table 1 was calculated from the densities and specific surface areas of the corresponding materials. Methods Near4.r. spectra were measured mostly in the range 500&2000cm-l with 2cm-l resolution using an FTIR spectrometer (DIGILAB FTS 15E) with a PbSe detector with very high sensitivity in the region of interest. Quartz windows were used in a vacuum cell in which the temperatures of samples were controlled in situ. Samples were pasted on thin glass plates to form a thin layer of material for measuring bands of high absorbance. Otherwise, ordinary disc samples were used for low-absorbance samples. Pressures of water and methyl iodide vapour introduced into the sample cell were measured by a Baratron manometer.lHJH isotope exchange of surface OH groups was carried out by 30 to 40 cycles of adsorption of heavy water on to the sample at 1.33 x lo3 Pa at 298 K for 5 min, followed by desorption. Cupric ions were adsorbed by immersing the samples in a 0.1 mol dm-3 solution of cupric chloride at pH 5 at room temperature and then by drying at the temperatures indicated above for each sample. The amount of cupric ions adsorbed was determined by atomic absorption spectroscopy from the difference in the concentrations of the solutions before and after sample immersion.T. Ishikawa, S . Nitta and S . Kondo Table 2. The influence of adsorption of heavy water, water, methyl iodide and cupric ions on v(0H) bandsa 4OH) adsorption band assignment sample /cm-' D20 H 2 0 CH,I Cu2+ v(0H) 3480 (s) L3723 (vw) 0 00 00 0 00 00 00 0 00 00 X X 00 00 00 00 X X 00 00 00 00 X X X X 00 00 X 00 00 X 00 X X 00 ~- bulk OH surface OH surface OH bulk OH surface OH surface OH surface OH bulk OH ? surface OH a The active, less active snd much less active bands are marked by 00, o and x, respectively.(s), (w) and (vw) indicate strong, weak and very weak bands, respectively. 2403 2 7 3 323 373 4 2 3 4 7 3 323 373 423 47 TlK Fig. 2. The relation between the absorbances of v(0H) bands and temperatures of heating in uacuo. 0,3661 cm-l (aphase); 0,3485 cm-'(aphase); 0,3150 cm-l(aphase); A, 3659 cm-l @phase); A, 3480cm-' (D phase); 0, 3624cm-I ( y phase); W, 3528 cm-l ( y phase); 0, 3160cm-l (Y phase).Results and Discussion 1.r. spectra of a-, 8- and y-ferric oxide hydroxides which were treated in vacuo at 348 K for 30 min are shown in fig. 1. They have various strong and weak absorption bands, together with a very weak set from the 8 phase, as is shown in the right-hand side of this figure with an expanded absorbance scale. The positions of all these bands are listed in table 2 with their relative absorbances marked s, w and vw for strong, weak and very weak bands, respectively. All the intensities of absorption bands of the a and y phases decrease on heating and disappear at ca. 473 K with the accompanying decomposition of these materials to anhydrous oxides,6 as is seen from fig. 2, which shows the change2404 8.C 6.C a T .fl 4 .c 9 2 2.0 0' FTIR of Colloidal Ferric Oxide Hydroxides -1 0.4 I I I I ' J o 5 10 15 20 ageing time/days Fig. 3. The change of the absorbances of v(0H) bands of a-ferric oxide hydroxide by ageing in water. 0, 3661 crn-l; a, 3485 crn-l; 0, 3150 cm-l. I I I 3500 3000 2500 wavenumber/cm-' Fig. 4. The change of the spectrum of y-ferric oxide hydroxide following lH-2H exchange. (-) Before 1H-2H exchange; (---) after 1H-2H exchange (6 times); ( * - - ) after 1H-2H exchange (15 times). of intensity of the weak and strong bands. The intensity of the weak absorption band of the /? phase increases on heating in vacuo up to 423 K and then decreases. This is probably because water strongly adsorbed in the structural channels discussed elsewhere' desorbs on heating. As is seen in fig.3, the intensities of three bands of the a phase increase proportionally with the time of ageing in water at 373 K and are maximal at ca. 10 days. Also, the X-ray diffraction patterns showed similar behaviour. These results indicate that crystals of theT. Ishikawa, S. Nitta and S. Kondo 2405 0 0 10 20 30 40 50 cycles of H-2H exchange Fig. 5. The change of the absorbances of v(0H) bands by 1H-2H exchange. 0, 3150 cm-l (a phase); A, 3480 cm-l (D phase); 0, 3160 cm-l (y phase). Ln 0 3700 3600 3500 w avenum ber/ cm - Fig. 6. The effect of water adsorption on the spectrum of y-ferric oxide hydroxide. (-) Before adsorption; (---) after adsorption at 26.6 Pa at 298 K; (-.-) after adsorption at 106 Pa at 298 K.2406 FTIR of Colloidal Ferric Oxide Hydroxides W N In , p? I 37 00 3600 3500 wavenumber/cm-' Fig.7. The influence of methyl iodide adsorption on the spectrum of y-ferric oxide hydroxide. (-) Before adsorption; (---) after adsorption at 1.33 x lo4 Pa at 298 K ; (..-) after desorption at 323 K. a phase grew sufficiently after 10 days ageing. It was possible to obtain well developed crystals of the and y phases by the methods of preparation used here, with almost no further growth of the crystals by ageing the materials. The behaviour of the absorption bands of the y phase is described below in detail. In fig. 4, both the 3528 and 3624 cm-l bands change rapidly to the two corresponding bands at 2610 and 2670 em-' by 1H-2H exchange, while the strong band at 3160 cm-l changed to a band at 2356 cm-l, but more slowly compared with the former bands.All the ratios of positions (in wavenumbers) of new bands after 1H-2H exchange to those of the original bands are from 1.34 to 1.36, equal to the isotope ratio vOFT/vOD. Hence, these three bands at 3 160, 3528 and 3624 cm-l are without doubt due to OH stretching vibrations. This behaviour is more or less similar to that of the a and p phases. All the weak bands of the a and p phases can be replaced by v(0D) bands very easily, compared with the strong bands, which can be exchanged only very slowly. These results indicate that the OH groups responsible for these weak bands exist on the surface, while the strong bands lie within the bulk structure. Fig. 5 shows the 1H-2H exchange rate of strong bands of each modification. The exchange rate of these modifications is in the order of y $= a > p phases, because OH groups between the layers of the y phase constitute linear hydrogen- bonded chains, so that migration of protons might be easier than in other cases, as is suggested by the crystal structures shown schematically in fig.9, 10 and 11 (later). Gallagher investigated the 1H-3H exchange of OH groups of the a and phases by means of a radioactive tracer method and found that OH groups of the a phase are less exchangeable than those of the a phase,s consistent with the present result. Fig. 6 illustrates the spectra of the y phase before and after water adsorption. The 3624 cm-l band disappeared after adsorbing water vapour and a corresponding perturbed v(0H) band appeared at 3550 crn-l, but, on the other hand, the 3528 cm-l band seemed almost unchanged.T.Ishikawa, S . Nitta and S. Kondo 2407 I I I I I 37 00 3 600 3500 wavenumberlcm-’ Fig. 8. The effect of cupric ion adsorption on the spectrum of y-ferric oxide hydroxide. The amount of adsorbed cupric ion is 5.68 pmol g-l. (-) Before adsorption; (-..) after adsorption. As is seen in fig. 7, the 3624 cm-l band disappeared on introducing methyl iodide gas and a small perturbed band appeared at 3565 cm-l. After evacuation at 323 K, the former band recovered ca. 70430% of its original intensity and the latter band disappeared. This may be because methyl iodide gas has reacted with OH groups to some extent. This reaction was confirmed by measuring the weight change of this material before and after adsorption and desorption of methyl iodide gas.The 3528 and 3 160 cm-l bands showed no change on water or methyl iodide adsorption. The 3661 and 3485 cm-l bands of the a phase and the 3659 cm- band of the p phase disappeared after water and methyl iodide adsorption, but the strong bands of the a and p phases did not change. All of these v(0H) bands showed behaviour similar to those mentioned above on methanol adsorption, with the exception of the a phase. When methanol was adsorbed on this material, not only did the 3485 and 3661 cm-l bands shift to lower wavenumber but also a very small and somewhat broad band appeared at 3640 cm-l which did not disappear on evacuation. When the vacuum system was contaminated by, for instance, vacuum grease, this band appeared more clearly together with a v(CH) band.Therefore, this adsorption site may possibly be some kind of chemically active Lewis acid site which would then be the same site as that adsorbing nitrogen oxide and sulphur dioxide gas,* and as that found by Rochester and T ~ p h a m . ~ $ lo As is seen in fig. 8, the 3624 cm-l band of the y phase almost disappeared on adsorption of cupric ions. This result indicates that the protons of these OH groups are ion-exchanged by cupric ions. The intensity of the 3528 cm-l band was also decreased to some degree by the same procedure, although this band showed no change on water or methyl iodide adsorption. This indicates that this site is located on the surface, but has less hydrogen-bond and dipole interaction with adsorbates than ordinary OH groups do.It is more likely that this is a hydroxide ion site. The results of 1H-2H isotope exchange and adsorption of water, methyl iodide and cupric ions on each band described above are summarized2408 FTIR of Colloidal Ferric Oxide Hydroxides 2 3 4 0 : O @:OH O I F e - - - - : H bond @ : surface OorOH Fig. 9. (a) The crystal structure of a-ferric oxide hydroxide. Double lines show hydrogen bonds. a, b and c are the three crystal axes. (b) The crystal plane (OOl), perpendicular to the predominant plane (010). Dotted lines show hydrogen bonds. bb 0 : O @:OHO:Fe 8 : surface OorOH Fig. 10. (a) The crystal structure of /?-ferric oxide hydroxide. (b) The crystal plane (OOl), perpendicular to the predominant planes [(OlO) and (loo)].T.Ishikawa, S. Nitta and S. Kondo 2409 - - - - : H bond 8 : surface 0 or OH Fig. 11. (a) The crystal structure of y-ferric oxide hydroxide. ( h ) The crystal plane (IOO), perpendicular to the predominant plane (0 10). in table 2, in which the letters 00, o and x show the bands as active, less active and much less active, respectively, for interactions with the molecules or ions mentioned above. The origins of these absorption bands are discussed in some detail on each modification as follows. The predominant crystal planes of the a and y phases, determined by selected-area electron diffraction patterns, are (100) and (0 10) planes, respectively,ll and those of the /? phase are (010) and (100) planes.I2 The fractions of the areas of predominant surfaces to the total surface areas of these particles which can be calculated from the particle sizes and B.E.T.surface areas of these samples, are 84, 93 and 84% for the a, /? and y phases, respectively. Therefore, most of the OH groups which are active to adsorbates are on these surfaces, with those less active being inside the substrates. The crystal structures of the a, p and y phases are schematically illustrated in fig. 9 (a), 10 (a) and 1 1 (a), respectively. The perpendicular sections of the predominant surface planes are also shown in fig. 9(h), 10(b) and 11 (b). The crystal structure of the a phase in fig. 9 (a) consists of strips of condensed octahedra containing ferric ions at each centre and oxygen ions or OH groups at each corner as well. There are channels surrounded by these strips.These strips share 0 atoms on octahedral edges to each other, and the hydrogen bonds are shown by double lines.I4 In fig. 9(b), the a phase can probably have five kinds of OH sites, i.e. 1, 2, 3 and 4 on the surface and site 5 in the channels. The strongly perturbed and intense v(0H) band at 3150 cm-l can be assigned to the bulk OH groups on site 5 , which interact with neighbouring oxygen atoms with hydrogen bonds and whose number is largest compared with other sites, since this band changes to a v(0H) band only slowly and is inactive to adsorption of molecules. The 3485 cm-l band is that of a perturbed v(0H) and can 80 FAR 12410 FTIR of Colloidal Ferric Oxide Hydroxides probably be assigned to OH groups and/or ions on site 1 and/or 3, since these OH groups are coordinated to three ferric ions and bear more negative charge than those which have a higher coordination number.The 3661 cm-l band, which is less perturbed than 3485 cm-' band, may be assigned to OH groups on sites 2 and/or 4 which coordinate to one or two ferric ions and have correspondingly less negative charge. In the crystal structure of the p phase in fig. 10(a),12 octahedra linked to each other form strips parallel to the c-axis. The channels surrounded by these strips are said to contain chloride ions and water molecules alternately in line, with these ions and molecules more or less stable below 423 K.14 The p phase can have five types of OH sites 1, 2, 3, 4 and 5 as is shown in fig. lO(b). The strong 3485 cm-l band can be assigned to hydroxide ions on site 4.This is because the number of sites of this type is the largest. This site lies inside the channels and interacts with water molecules and chloride ions, hence its low wavenumber and also the very slow 1H-2H exchange rate of this band. The OH groups on site 1, which lies on the concave part of the surface, and probably site 5 , which lies inside the empty channels, seem to be responsible for the weak 3659 cm-1 band, i.e. these sites coordinate to three ferric ions and are active to 1H-2H exchange, as well as to adsorbates and cupric ions. The very weak 3686 and 3723 cm-l bands can be assigned to OH groups on site 2 coordinated to two ferric ions and site 3 coordinated to one ferric ion, respectively, in that both of these OH groups are active to 1H-2H exchange, the adsorption of water molecules and the fact that the OH bond of the latter OH group would be more covalent than that of the former site.In the structure of the y phase in fig. 1 1 (a), octahedra constitute two-dimensional layers by sharing edges of each octahedron, with linear chains of hydrogen bonds shown as double lines and dotted lines in fig. 11 ( a ) and (b), respectively, and form crystals of thin ~ 1 a t e s . l ~ Only sites 1 and 2 are present in this structure, in which the number of former sites is much less than that of the latter, as is shown in fig. 11 (b). The strongly perturbed 3160 cm-l band can be assigned to OH groups on site 2 between layers. Site 1 on the surface is the origin of the weak 3624 cm-1 band. Finally, it was not possible to assign the 3528 cm-l band in the present investigation. We thank Dr Yoshiko Nakahara of the Government Industrial Research Institute, Osaka, for her experimental help in transmission electron microscope and X-ray diffraction experiments. References 1 J. D. Russell, R. L. Parfitt, A. R. Fraser and V. C. Farmer, Nature (London), 1974, 248, 220. 2 R. L. Parfitt, J. D. Russell and V. C. Farmer, J. Chem. Soc., Faraday Trans. I, 1976, 72, 1082. 3 R. L. Parfitt and R. S. C. Smart, J. Chem. SOC., Faraduy Trans. 1, 1977,73, 796. 4 K. Kaneko, M. Senzawa, T. Ishikawa and K. Inouye, Bull. Chem. Soc. Jpn, 1975,48, 1764. 5 T. Ishikawa and K. Inouye, Prog. Colloid Polym. Sci., 1983, 68, 152. 6 T. Ishikawa and K. Inouye, Bull. Chem. SOC. Jpn, 1972,45, 2350. 7 T. Ishikawa, K. Kaneko and K. Inouye, Nippon Kagaku Kaishi, 1975, 1635. 8 K. J. Gallagher and D. N. Phillips, Chimia, 1969, 23, 465. 9 C. H. Rochester and S. A. Topham, J. Chem. SOC., Faraday Trans. 1 , 1979, 75, 591. 10 C. H. Rochester and S. A. Topham, J. Chem. Soc., Furaduy Trans. I , 1979, 75, 872. 11 G. W. van Oosterhaut, Acra Crystallogr., 1960, 13, 932. 12 A. L. Mackay, Miner. Mag., 1960, 32, 545. 13 F. J. Ewing, J. Chem. Phys., 1935, 3, 203. 14 T. Ishikawa and K. Inouye, Bull. Chem. SOC. Jpn, 1975, 48, 1580. 15 F. J. Ewing, J. Chem. Phys., 1935, 3, 420. Paper 511557; Received 10th September, 1985