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Effect of HCl additions on forced hydrolysis of FeCl3 solutions
Materials Letters 58 (2004) 2640 – 2645 www.elsevier.com/locate/matlet
Effect of HCl additions on forced hydrolysis of FeCl3 solutions S. Music´ a,*, S. Krehula a, S. Popovic´ b a
Division of Materials Chemistry, Rud-er Bosˇkovic´ Institute, P.O. Box 180, Bijenicˇka cesta 54, HR-10002 Zagreb, Croatia b Department of Physics, Faculty of Science, University of Zagreb, P.O. Box 331, HR-10002 Zagreb, Croatia Received 5 January 2004; accepted 2 April 2004 Available online 25 May 2004
Abstract The effect of HCl additions on the formation of precipitates by hydrolysis of FeCl3 solutions at 90 jC has been investigated using X-ray powder diffraction (XRD), FT-IR spectroscopy and transmission electron microscopy (TEM). In the precipitation systems with 0.1 M FeCl3, the concentration of HCl varied between 0.005 and 0.1 M, whereas the maximum time of hydrolysis was 35 days. At the beginning of the hydrolytic process, the formation of precipitate was strongly suppressed with an increase in HCl concentration. h-FeOOH was found as a single phase in the precipitates for the concentration 0.1 M FeCl3 + 0.01 M HCl after 1 and 3 days of hydrolysis. For the concentrations [HCl] z 0.05 M, the precipitates also contained a small amount of a-FeOOH. FT-IR spectroscopy was proved to be a useful technique in the detection of a-FeOOH. After 35 days of hydrolytic process, a-Fe2O3 was found as the end product of precipitation, whereas some precipitates also contained small amounts or traces of a-FeOOH. The size and shape of the particles strongly depended on the HCl concentration. Forced hydrolysis of 0.01 M FeCl3 + 0.005 M HCl solution for 35 days yielded small and uniform a-Fe2O3 particles ( f 0.08 Am). D 2004 Elsevier B.V. All rights reserved. Keywords: FeCl3 hydrolysis; h-FeOOH; a-Fe2O3; a-FeOOH; XRD; FT-IR; TEM
1. Introduction Iron oxyhydroxides and oxides have found important commercial applications in the production of pigments, catalysts, gas sensors, magnetic recording media, etc. Phase composition, size and shape of the particles must be taken into account in specific applications of iron oxyhydroxides and oxides. Forced hydrolysis of Fe3 + ions in aqueous solutions is a simple method for the preparation of selected iron oxyhydroxides or oxides. The process of forced hydrolysis of Fe3 + ions in aqueous solutions is generally influenced by Fe3 + concentration, pH, temperature and time of hydrolysis. The presence of various cations and anions, as well as the organic polymers, may also influence the chemical, microstructural and physical properties of the resulting precipitate. Forced hydrolysis of aqueous FeCl3 solutions under different physico-chemical conditions was extensively in-
* Corresponding author. E-mail address: [email protected] (S. Music´). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.04.002
vestigated from different standpoints. h-FeOOH (akagane´ ite) is a typical solid product formed by forced hydrolysis of aqueous FeCl3 solutions, and in dependence on physico-chemical conditions, it may transform into aFe2O3 (hematite) [1,2]. The a-Fe2O3 particles of different morphology were obtained by forced hydrolysis of aqueous FeCl3 solutions containing phosphate or hypophosphite ions [3,4]. The effect of surfactants on forced hydrolysis of FeCl3 solutions was investigated [5], and in this work, the most pronounced effect was obtained in the presence of sodium dodecyl sulfate, i.e., the formation of a-FeOOH (goethite) was observed. The effects of various amines [6] and dioxane [7] on the formation of a-Fe2O3 by forced hydrolysis of aqueous FeCl3 solutions were also investigated. In the mixed Fe(NO3)3 + FeCl3 solutions, the phase composition of the solid products obtained by forced hydrolysis at 120 jC was determined by the concentration of the dominant Fe(III) salt [8]. The effect of quinine hydrogen sulfate on the morphology of hFeOOH particles, as well as on the phase composition
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of the solid products obtained by forced hydrolysis of FeCl3 solutions, was shown [9]. Forced hydrolysis of 0.1 M FeCl3 solution at 90 jC in the presence of sodium polyanethol sulphonate yielded spherical h-FeOOH particles of micrometer dimensions [10]. Hydrolysis of aqueous FeCl3 solutions in the presence of hexamethylenetetramine (HMTA) [11,12] or urea [13] at 90 jC was investigated. In these experiments, decomposing HMTA or urea generated OH ions, which made possible a gradual increase in pH. The chemical and microstructural properties of oxide phases precipitated from concentrated FeCl3 solutions were also investigated [14]. The conditions for the preparation of a-Fe2O3 particles of welldefined size and shape from FeCl3 solutions were investigated [15 – 18]. In the present work, we have focused on the forced hydrolysis of FeCl3 aqueous solutions with the aim to obtain more information about the phase composition of precipitated particles and their shapes depending on HCl additions to the precipitation system. These data are important for the researchers investigating influence of anions, cations or polymers on forced hydrolysis of FeCl3 solutions. The present work shows that in specific cases, the effect of HCl addition can be a more important factor in the precipitation process than the presence of various inorganic ions, organic molecules or polymers.
2. Experimental Analytical grade chemicals FeCl36H2O and HCl, supplied by Kemika, and double-distilled water were used. A fresh aqueous FeCl3 solution was prepared before the preparation of precipitation systems C1 to C14. The concentration factor for HCl solutions was F = 1.0000. Experimental conditions for the preparation of the precipitation systems are given in Table 1. Hydrolysis experiments were performed in glass autoclaves in static conditions. After a proper aging time at 90 jC, the precipitates were separated from the mother liquor using an ultraspeed centrifuge (Sorvall RC2-B, operational range up to 20,000 rpm). The precipitates were subsequently washed with double-distilled water and then dried at RT in a vacuum chamber coupled with a corresponding trap and a vacuum pump. pH meter and combined pH electrode (Red Rod pH 0 –14), manufactured by Radiometer Analytical, were used. X-ray powder diffraction (XRD) patterns were taken using an automatic Philips diffractometer, model MPD 1880 (CuKa radiation, graphite monochromator, proportional counter). The FT-IR spectra were recorded using a Perkin-Elmer spectrometer (model 2000, operational range up to f 35 cm 1). The Infrared Data Manager (IRDM) program, also supplied by Perkin-Elmer, was used to process the FT-IR spectra. The samples were pressed into a spectroscopically pure KBr matrix. The particle size and
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Table 1 Experimental conditions for the preparation of samples at 90 jC and the results of XRD phase analysis Sample
[FeCl3] (M)
[HCl] (M)
Time of heating (days)
Phase composition by XRD (approximate molar fraction)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.01 0.01
0.1 0.05 0.01 0.005 0.1 0.05 0.01 0.005 0.1 0.05 0.01 0.005 0.05 0.005
1 1 1 1 3 3 3 3 35 35 35 35 35 35
* A (0.9) + G (0.1) A A * A (0.6) + G (0.3) + H (0.1) A A H + G (traces) H (0.9) + G (0.1) H H H H
Key: A = h-FeOOH, G = a-FeOOH, H = a-Fe2O3. *No precipitate in C1 and C5 systems was noticed.
shape were monitored by a transmission electron microscope (TEM) manufactured by Philips.
3. Results and Discussion The phase composition of the samples, as found by XRD, is given in Table 1. Figs. 1 and 2 show characteristic parts of XRD patterns of selected samples C2, C3, C6 and C7. All samples showed rather sharp or little broadened XRD lines; the broadening depended little on the Miller indices of diffraction lines, indicating anisotropic crystal shapes. The phases present in the samples were identified according to the powder diffraction data contained in the PDF cards [19]: numbers 75-1594 and 34-1266 for hFeOOH, numbers 74-2195 and 29-0713 for a-FeOOH and numbers 89-0599 and 86-0550 for a-Fe2O3. FT-IR spectra of selected samples are shown in Fig. 3. The shape of the spectrum of h-FeOOH phase in sample C2 is not fully developed, whereas all h-FeOOH bands in the spectrum of sample C3 are present. Sample C2 shows IR bands at 420, 485, 635 and 695 cm 1 which could be assigned to h-FeOOH, however, only by taking into account the previous result of XRD measurement (Table 1). Similar conclusion is also valid in the identification of a-FeOOH only on the basis of the band at 891 cm 1. The FT-IR spectrum of sample C6 shows bands which can be assigned to h-FeOOH, a-FeOOH and a-Fe2O3. For example, the bands at 892 and 798 cm 1 are characteristic of a-FeOOH, and they can be assigned to Fe – O –H bending vibrations. Incidentally, FT-IR spectroscopy is a very useful technique in detecting a-FeOOH or g-FeOOH phases present in small fractions. The bands at 695 and 630 cm 1 in sample C6 can be assigned to h-FeOOH. At lower wave numbers, there is a strong overlapping of the IR bands of h-FeOOH, a-FeOOH
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longer times. Forced hydrolysis of FeCl3 solutions was strongly suppressed by the addition of HCl, and Table 1 shows that in the presence of 0.1 M HCl, there were no visible precipitates in the systems C1 and C5 up to 3 days of heating. XRD of the precipitates, produced after 35 days under the same concentration conditions, showed the presence of a-Fe2O3 and traces of a-FeOOH. Forced hydrolysis of the 0.1 M FeCl3 + 0.05 M HCl aqueous solution yielded a small fraction of a-FeOOH after 1 day of heating, whereas after 3 days, a-Fe2O3 was also present. h-FeOOH as a single phase was obtained for concentrations of HCl V 0.01, as proved by XRD and FTIR spectroscopy. Upon heating of FeCl3/HCl solutions, as well as of FeCl3 solutions [10], there is decrease of pH as a consequence of the hydrolytic reactions which consume hydroxyl ions and release protons from H2O molecules. Table 2 shows the change of the pH values, as obtained by monitoring the forced hydrolysis of FeCl3/HCl solutions. At lower concentrations of HCl the greatest pH decrease occurred up to 4 h of the forced hydrolysis. On the other hand, in the precipitation system, 0.1 M FeCl3 + 0.1 M HCl, the pH did not change significantly (pH f 1.07) between 1 and 3 days of the forced hydrolysis. In this time period, the precipitation was not visible. However, after 35 days, the precipitate was present, and pH 0.88 was measured.
Fig. 1. Characteristic parts of XRD patterns at RT of samples C2 and C3. A = akagane´ite, G = goethite.
and a-Fe2O3. The IR bands at 579, 478 and 376 cm 1 can be assigned to a-Fe2O3, taking into account the previous result of XRD measurement. The spectrum of sample C8 corresponds to h-FeOOH as a single phase. The assignations of the IR bands corresponding to h-FeOOH were discussed in our previous work [9]. The FT-IR spectra of samples C13 and C14 can be assigned to a-Fe2O3. Two very weak bands at 892 and 795 cm 1 observed in the FT-IR spectrum of sample C13, due to Fe – O –OH bending vibrations, can be assigned to the traces of a-FeOOH. The FT-IR spectrum of sample C14 showed no presence of a-FeOOH. Iglesias and Serna [20] investigated the IR spectra of aFe2O3 particles of different shapes. The a-Fe2O3 spheres showed IR bands at 575, 485, 385 and 360 cm 1, whereas a-Fe2O3 laths showed IR bands at 650, 525, 440 and 300 cm 1. The present work has unequivocally shown the effect of HCl additions on the phase composition of the precipitates produced by forced hydrolysis of FeCl3 solutions. In the absence of HCl [9], a very fast appearance of h-FeOOH colloids took place by forced hydrolysis of 0.1 M FeCl3 solutions at 90 jC. This is easily visible by the increase in the turbidity of the colloidal suspension for short times of hydrolysis and the sedimentation of h-FeOOH particles for
Fig. 2. Characteristic parts of XRD patterns at RT of samples C6 and C7. A = akagane´ite, G = goethite, H = hematite.
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FeCl3/HCl solutions under the concentration conditions provided in the previous work by Matijevic´ and Scheiner [2]. Bailey et al. [24] proposed a mechanism of the formation of a-Fe2O3 particles of different shapes by forced hydrolysis of FeCl3/HCl solutions. By virtue of that mechanism, (a) double-ellipsoidal particles were obtained by nucleation and growth of a-Fe2O3 on hFeOOH rods, (b) spherical particles were obtained by growth of a-Fe2O3 on partially dissolved h-FeOOH rods which served as template and (c) a-Fe2O3 cubes nucleated on aggregated h-FeOOH rods (raft-like). Atkinson et al. [25] investigated crystal nucleation and growth during hydrolysis of aqueous FeCl3 solutions. It was shown that for partially neutralized FeCl3 solutions (OH/Fe molar ratio = 0 – 2.75), the transformations h-FeOOH ! a-Fe2O3 and h-FeOOH ! a-FeOOH at pH 1– 2 proceeded by way of the dissolution/reprecipitation mechanism, and the precipitation process was promoted by adding seed crystals. Knese et al. [26] used Mo¨ssbauer spectroscopy to investigate the transformation of h-FeOOH to a-Fe2O3 in the suspension at pH 8.5 and 100 jC. a-FeOOH was found as intermediate phase, and it was supposed that the formation of a-Fe2O3 can proceed by way of (a) direct transformation h-FeOOH ! a-Fe2O3, or (b) indirect transformation h-FeOOH ! a-FeOOH ! a-Fe2O3.
Fig. 3. Fourier transform infrared spectra of samples C2, C3, C6, C8, C13 and C14. The spectra were recorded at RT.
In reference literature [1,2,21 –23], no formation of aFeOOH during forced hydrolysis of FeCl3/HCl systems was reported. The formation of a-Fe2O3 particles was considered in terms of the dissolution/reprecipitation mechanism. However, Gotic´ et al. [8] observed the formation of a small fraction of a-FeOOH by forced hydrolysis of 0.2 M FeCl3 + 0.1 M HCl solution at 120 jC for 3 days. Bailey et al. [24] investigated forced hydrolysis of
Table 2 Measured pH values in dependence on the chemical composition of the precipitation systems and heating time Chemical composition
pHa
[FeCl3] (M)
[HCl] (M)
Start
0.1 0.1 0.1 0.1 0.1 0.01 0.01 0.01
– 0.1 0.05 0.01 0.005 – 0.05 0.005
1.86 1.07 1.34 1.72 1.73 2.47 1.43 2.12
4h
1d
3d
35d
1.02 1.25 1.07 0.98
1.01 0.96 0.92 0.91
1.07 0.94 0.87 0.92
0.88
1.32 1.49
1.18 1.48
1.26 1.56
1.38 1.74
0.85 0.95
a pH values were measured at RT after an abrupt cooling of the precipitation systems.
Fig. 4. TEM photographs of samples (a) C2, (b) C3 and (c) C4.
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fraction. The results of this work are in line with the general mechanism, as proposed by Music´ et al. [30] for the hydrolysis of Fe 3 + ions in aqueous solutions of the corresponding chloride, nitrate or sulfate salts. The present work shows that a-FeOOH particles precipitate when a higher concentration of HCl is present at the beginning of the hydrolytic process. Evidently, different complexes of Fe3 + ions are formed in FeCl3 solutions and FeCl3/HCl solutions, and very probably, this factor plays an important role in overall hydrolytic process. Iron(III) chloride complexes are present at high concentrations of HCl, whereas aquo complexes are predominant in FeCl3 solutions without HCl adding. The second factor are very slow kinetics of Fe3 + hydrolysis in highly acidic FeCl3 solutions (precipitation systems C1 and C5 in the present work), which influence the phase composition of the precipitate, as well as the particle size and shape. A possible effect of mixing the precipitation systems, during the forced hydrolysis of FeCl3/HCl aqueous solution should also be investigated in the future. One should bear in mind that FeCl36H2O salts from different vendors may contain ‘‘free’’ HCl which can also influence the overall process of Fe3 + hydrolysis. Fig. 6 shows TEM photographs of samples C9, C12, C13 and C14. Fig. 6a (sample C9) shows a-Fe2O3 particles of different sizes ( f 1 – 4 Am) and shape, whereas Fe2O3 particles of sample C12 (Fig. 6b) also show different sizes ( f 0.75 –1.8 Am) and shape. A great difference in size of aFig. 5. TEM photographs of samples (a) C6 and (b) C8.
TEM photographs of selected samples are shown in Figs. 4 –6. Fig. 4a (sample C2) shows h-FeOOH rods and aFeOOH particle of dendritic nature. The angle between the main body and its dendrite for this type of a-FeOOH particles is 117.5j (calculated value) [27]. Fig. 4b (sample C3) and c (sample C4) shows only h-FeOOH particles. These particles are smaller than those in sample C2, and in sample C4, they have an ellipsoidal shape with good uniformity. Fig. 5 shows TEM photographs of samples C6 and C8. The particles of h-FeOOH (rods), a-FeOOH (stars and dendrites) and a-Fe2O3 (lemon and near spherical shapes) are well visible in Fig. 5a (sample C6). This figure is a selected area TEM image. The inspection of the whole TEM image gives approximately the population of different particles in accordance with XRD phase analysis. Holes and irregular shapes of h-FeOOH and a-FeOOH particles are present, thus indicating the dissolution of these particles. The formation of a-Fe2O3 particles in this precipitation system is directed by the dissolution/reprecipitation mechanism. Furthermore, Wang et al. [28,29] also reported the possibility of the formation of growth centers for a-Fe2O3 particles in the middle of a star-shaped a-FeOOH particle, which is possible due to the crystallographic characteristics of a-Fe2O3 and a-FeOOH crystals. The present work has shown the formation of a-FeOOH at a higher concentration of HCl; however, this phase was in traces or in a small
Fig. 6. TEM photographs of samples (a) C9, (b) C12, (c) C13 and (d) C14.
S. Music´ et al. / Materials Letters 58 (2004) 2640–2645
Fe2O3 particles is observed between samples C13 (Fig. 6c) and C14 (Fig. 6d). The formation of large a-Fe2O3 particles ( f 1.2– 3 Am) is observed for sample C13, whereas in the case of sample C14, uniform and small a-Fe2O3 particles ( f 0.08 Am) are obtained. In upper right corner of Fig. 6d, enlarged a-Fe2O3 particles are shown. Sample C14 did not show any presence of h-FeOOH or a-FeOOH particles. Acknowledgements The authors thank Professor N. Ljubesˇic´ for his valuable assistance in electron microscopy. References [1] E. Matijevic´, J. Colloid Interface Sci. 58 (1977) 374. [2] E. Matijevic´, P. Scheiner, J. Colloid Interface Sci. 63 (1978) 509. [3] M. Ozaki, S. Kratohvil, E. Matijevic´, J. Colloid Interface Sci. 102 (1984) 146. [4] S. Kratohvil, E. Matijevic´, M. Ozaki, Colloid Polym. Sci. 262 (1984) 804. [5] K. Kandori, I. Horii, A. Yasukawa, T. Ishikawa, J. Mater. Sci. 30 (1995) 2145. [6] K. Kandori, A. Yasukawa, T. Ishikawa, J. Colloid Interface Sci. 180 (1996) 446. [7] K. Kandori, Y. Nakamoto, A. Yasukawa, T. Ishikawa, J. Colloid Interface Sci. 202 (1998) 499. [8] M. Gotic´, S. Popovic´, N. Ljubesˇic´, S. Music´, J. Mater. Sci. 29 (1994) 2474. [9] S. Music´, S. Krehula, S. Popovic´, Zˇ. Skoko, Mater. Lett. 57 (2003) 1096.
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[10] S. Music´, M. Gotic´, N. Ljubesˇic´, Mater. Lett. 25 (1995) 69. [11] A. Sˇaric´, K. Nomura, S. Popovic´, N. Ljubesˇic´, S. Music´, Mater. Chem. Phys. 52 (1998) 214. [12] A. Sˇaric´, S. Music´, K. Nomura, S. Popovic´, Mater. Sci. Eng. B 56 (1998) 43. [13] S. Music´, M. Maljkovic´, S. Popovic´, ACH-Models Chem. 136 (1999) 299. [14] S. Music´, G.P. Santana, G. Sˇmit, V.K. Garg, J. Alloys Compd. 278 (1998) 291. [15] N.J. Reeves, S. Mann, J. Chem. Soc., Faraday Trans. 87 (1991) 3875. [16] T. Nakamura, H. Kurokawa, J. Mater. Sci. 30 (1995) 4710. [17] T. Sugimoto, A. Muramatsu, J. Colloid Interface Sci. 184 (1996) 626. [18] T.P. Raming, A.J.A. Winnubst, C.M. Van Kats, A.P. Philipse, J. Colloid Interface Sci. 249 (2002) 346. [19] International Centre for Diffraction Data, Joint Committee on Powder Diffraction Standards, Powder Diffraction File, 1601 Park Lane, Swarthmore, PA 19081, USA. [20] J.E. Iglesias, C.J. Serna, Mineral. Petrogr. Acta 29A (1985) 365. [21] E. Matijevic´, Sˇ. Cimasˇ, Colloid Polym. Sci. 265 (1987) 155. [22] E.K. De Blanco, M.A. Blesa, S.J. Liberman, React. Solids 1 (1986) 189. [23] K. Kandori, S. Tamura, T. Ishikawa, Colloid Polym. Sci. 272 (1994) 812. [24] J.K. Bailey, C.J. Brinker, M.L. Mecartney, J. Colloid Interface Sci. 157 (1993) 1. [25] R.J. Atkinson, A.M. Posner, J.P. Quirk, Clays Clay Miner. 25 (1977) 49. [26] K. Knese, P. Wartewig, C. Michalk, S. Fischer, Hyperfine Interact. 94 (1994) 1999. [27] Y. Maeda, S. Hirono, Jpn. J. Appl. Phys. 20 (1981) 1991. [28] M.K. Wang, V.A. Greenhut, P.H. Hsu, Soil Sci. 132 (1981) 182. [29] M.K. Wang, J. Chin. Agricult. Chem. Soc. 25 (1987) 432. [30] S. Music´, A. Ve´rtes, G.W. Simmons, I. Czako´-Nagy, H. Leidheiser Jr., J. Colloid Interface Sci. 85 (1982) 256.
Effect of HCl additions on forced hydrolysis of FeCl3 solutions S. Music´ a,*, S. Krehula a, S. Popovic´ b a
Division of Materials Chemistry, Rud-er Bosˇkovic´ Institute, P.O. Box 180, Bijenicˇka cesta 54, HR-10002 Zagreb, Croatia b Department of Physics, Faculty of Science, University of Zagreb, P.O. Box 331, HR-10002 Zagreb, Croatia Received 5 January 2004; accepted 2 April 2004 Available online 25 May 2004
Abstract The effect of HCl additions on the formation of precipitates by hydrolysis of FeCl3 solutions at 90 jC has been investigated using X-ray powder diffraction (XRD), FT-IR spectroscopy and transmission electron microscopy (TEM). In the precipitation systems with 0.1 M FeCl3, the concentration of HCl varied between 0.005 and 0.1 M, whereas the maximum time of hydrolysis was 35 days. At the beginning of the hydrolytic process, the formation of precipitate was strongly suppressed with an increase in HCl concentration. h-FeOOH was found as a single phase in the precipitates for the concentration 0.1 M FeCl3 + 0.01 M HCl after 1 and 3 days of hydrolysis. For the concentrations [HCl] z 0.05 M, the precipitates also contained a small amount of a-FeOOH. FT-IR spectroscopy was proved to be a useful technique in the detection of a-FeOOH. After 35 days of hydrolytic process, a-Fe2O3 was found as the end product of precipitation, whereas some precipitates also contained small amounts or traces of a-FeOOH. The size and shape of the particles strongly depended on the HCl concentration. Forced hydrolysis of 0.01 M FeCl3 + 0.005 M HCl solution for 35 days yielded small and uniform a-Fe2O3 particles ( f 0.08 Am). D 2004 Elsevier B.V. All rights reserved. Keywords: FeCl3 hydrolysis; h-FeOOH; a-Fe2O3; a-FeOOH; XRD; FT-IR; TEM
1. Introduction Iron oxyhydroxides and oxides have found important commercial applications in the production of pigments, catalysts, gas sensors, magnetic recording media, etc. Phase composition, size and shape of the particles must be taken into account in specific applications of iron oxyhydroxides and oxides. Forced hydrolysis of Fe3 + ions in aqueous solutions is a simple method for the preparation of selected iron oxyhydroxides or oxides. The process of forced hydrolysis of Fe3 + ions in aqueous solutions is generally influenced by Fe3 + concentration, pH, temperature and time of hydrolysis. The presence of various cations and anions, as well as the organic polymers, may also influence the chemical, microstructural and physical properties of the resulting precipitate. Forced hydrolysis of aqueous FeCl3 solutions under different physico-chemical conditions was extensively in-
* Corresponding author. E-mail address: [email protected] (S. Music´). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.04.002
vestigated from different standpoints. h-FeOOH (akagane´ ite) is a typical solid product formed by forced hydrolysis of aqueous FeCl3 solutions, and in dependence on physico-chemical conditions, it may transform into aFe2O3 (hematite) [1,2]. The a-Fe2O3 particles of different morphology were obtained by forced hydrolysis of aqueous FeCl3 solutions containing phosphate or hypophosphite ions [3,4]. The effect of surfactants on forced hydrolysis of FeCl3 solutions was investigated [5], and in this work, the most pronounced effect was obtained in the presence of sodium dodecyl sulfate, i.e., the formation of a-FeOOH (goethite) was observed. The effects of various amines [6] and dioxane [7] on the formation of a-Fe2O3 by forced hydrolysis of aqueous FeCl3 solutions were also investigated. In the mixed Fe(NO3)3 + FeCl3 solutions, the phase composition of the solid products obtained by forced hydrolysis at 120 jC was determined by the concentration of the dominant Fe(III) salt [8]. The effect of quinine hydrogen sulfate on the morphology of hFeOOH particles, as well as on the phase composition
S. Music´ et al. / Materials Letters 58 (2004) 2640–2645
of the solid products obtained by forced hydrolysis of FeCl3 solutions, was shown [9]. Forced hydrolysis of 0.1 M FeCl3 solution at 90 jC in the presence of sodium polyanethol sulphonate yielded spherical h-FeOOH particles of micrometer dimensions [10]. Hydrolysis of aqueous FeCl3 solutions in the presence of hexamethylenetetramine (HMTA) [11,12] or urea [13] at 90 jC was investigated. In these experiments, decomposing HMTA or urea generated OH ions, which made possible a gradual increase in pH. The chemical and microstructural properties of oxide phases precipitated from concentrated FeCl3 solutions were also investigated [14]. The conditions for the preparation of a-Fe2O3 particles of welldefined size and shape from FeCl3 solutions were investigated [15 – 18]. In the present work, we have focused on the forced hydrolysis of FeCl3 aqueous solutions with the aim to obtain more information about the phase composition of precipitated particles and their shapes depending on HCl additions to the precipitation system. These data are important for the researchers investigating influence of anions, cations or polymers on forced hydrolysis of FeCl3 solutions. The present work shows that in specific cases, the effect of HCl addition can be a more important factor in the precipitation process than the presence of various inorganic ions, organic molecules or polymers.
2. Experimental Analytical grade chemicals FeCl36H2O and HCl, supplied by Kemika, and double-distilled water were used. A fresh aqueous FeCl3 solution was prepared before the preparation of precipitation systems C1 to C14. The concentration factor for HCl solutions was F = 1.0000. Experimental conditions for the preparation of the precipitation systems are given in Table 1. Hydrolysis experiments were performed in glass autoclaves in static conditions. After a proper aging time at 90 jC, the precipitates were separated from the mother liquor using an ultraspeed centrifuge (Sorvall RC2-B, operational range up to 20,000 rpm). The precipitates were subsequently washed with double-distilled water and then dried at RT in a vacuum chamber coupled with a corresponding trap and a vacuum pump. pH meter and combined pH electrode (Red Rod pH 0 –14), manufactured by Radiometer Analytical, were used. X-ray powder diffraction (XRD) patterns were taken using an automatic Philips diffractometer, model MPD 1880 (CuKa radiation, graphite monochromator, proportional counter). The FT-IR spectra were recorded using a Perkin-Elmer spectrometer (model 2000, operational range up to f 35 cm 1). The Infrared Data Manager (IRDM) program, also supplied by Perkin-Elmer, was used to process the FT-IR spectra. The samples were pressed into a spectroscopically pure KBr matrix. The particle size and
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Table 1 Experimental conditions for the preparation of samples at 90 jC and the results of XRD phase analysis Sample
[FeCl3] (M)
[HCl] (M)
Time of heating (days)
Phase composition by XRD (approximate molar fraction)
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14
0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.01 0.01
0.1 0.05 0.01 0.005 0.1 0.05 0.01 0.005 0.1 0.05 0.01 0.005 0.05 0.005
1 1 1 1 3 3 3 3 35 35 35 35 35 35
* A (0.9) + G (0.1) A A * A (0.6) + G (0.3) + H (0.1) A A H + G (traces) H (0.9) + G (0.1) H H H H
Key: A = h-FeOOH, G = a-FeOOH, H = a-Fe2O3. *No precipitate in C1 and C5 systems was noticed.
shape were monitored by a transmission electron microscope (TEM) manufactured by Philips.
3. Results and Discussion The phase composition of the samples, as found by XRD, is given in Table 1. Figs. 1 and 2 show characteristic parts of XRD patterns of selected samples C2, C3, C6 and C7. All samples showed rather sharp or little broadened XRD lines; the broadening depended little on the Miller indices of diffraction lines, indicating anisotropic crystal shapes. The phases present in the samples were identified according to the powder diffraction data contained in the PDF cards [19]: numbers 75-1594 and 34-1266 for hFeOOH, numbers 74-2195 and 29-0713 for a-FeOOH and numbers 89-0599 and 86-0550 for a-Fe2O3. FT-IR spectra of selected samples are shown in Fig. 3. The shape of the spectrum of h-FeOOH phase in sample C2 is not fully developed, whereas all h-FeOOH bands in the spectrum of sample C3 are present. Sample C2 shows IR bands at 420, 485, 635 and 695 cm 1 which could be assigned to h-FeOOH, however, only by taking into account the previous result of XRD measurement (Table 1). Similar conclusion is also valid in the identification of a-FeOOH only on the basis of the band at 891 cm 1. The FT-IR spectrum of sample C6 shows bands which can be assigned to h-FeOOH, a-FeOOH and a-Fe2O3. For example, the bands at 892 and 798 cm 1 are characteristic of a-FeOOH, and they can be assigned to Fe – O –H bending vibrations. Incidentally, FT-IR spectroscopy is a very useful technique in detecting a-FeOOH or g-FeOOH phases present in small fractions. The bands at 695 and 630 cm 1 in sample C6 can be assigned to h-FeOOH. At lower wave numbers, there is a strong overlapping of the IR bands of h-FeOOH, a-FeOOH
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longer times. Forced hydrolysis of FeCl3 solutions was strongly suppressed by the addition of HCl, and Table 1 shows that in the presence of 0.1 M HCl, there were no visible precipitates in the systems C1 and C5 up to 3 days of heating. XRD of the precipitates, produced after 35 days under the same concentration conditions, showed the presence of a-Fe2O3 and traces of a-FeOOH. Forced hydrolysis of the 0.1 M FeCl3 + 0.05 M HCl aqueous solution yielded a small fraction of a-FeOOH after 1 day of heating, whereas after 3 days, a-Fe2O3 was also present. h-FeOOH as a single phase was obtained for concentrations of HCl V 0.01, as proved by XRD and FTIR spectroscopy. Upon heating of FeCl3/HCl solutions, as well as of FeCl3 solutions [10], there is decrease of pH as a consequence of the hydrolytic reactions which consume hydroxyl ions and release protons from H2O molecules. Table 2 shows the change of the pH values, as obtained by monitoring the forced hydrolysis of FeCl3/HCl solutions. At lower concentrations of HCl the greatest pH decrease occurred up to 4 h of the forced hydrolysis. On the other hand, in the precipitation system, 0.1 M FeCl3 + 0.1 M HCl, the pH did not change significantly (pH f 1.07) between 1 and 3 days of the forced hydrolysis. In this time period, the precipitation was not visible. However, after 35 days, the precipitate was present, and pH 0.88 was measured.
Fig. 1. Characteristic parts of XRD patterns at RT of samples C2 and C3. A = akagane´ite, G = goethite.
and a-Fe2O3. The IR bands at 579, 478 and 376 cm 1 can be assigned to a-Fe2O3, taking into account the previous result of XRD measurement. The spectrum of sample C8 corresponds to h-FeOOH as a single phase. The assignations of the IR bands corresponding to h-FeOOH were discussed in our previous work [9]. The FT-IR spectra of samples C13 and C14 can be assigned to a-Fe2O3. Two very weak bands at 892 and 795 cm 1 observed in the FT-IR spectrum of sample C13, due to Fe – O –OH bending vibrations, can be assigned to the traces of a-FeOOH. The FT-IR spectrum of sample C14 showed no presence of a-FeOOH. Iglesias and Serna [20] investigated the IR spectra of aFe2O3 particles of different shapes. The a-Fe2O3 spheres showed IR bands at 575, 485, 385 and 360 cm 1, whereas a-Fe2O3 laths showed IR bands at 650, 525, 440 and 300 cm 1. The present work has unequivocally shown the effect of HCl additions on the phase composition of the precipitates produced by forced hydrolysis of FeCl3 solutions. In the absence of HCl [9], a very fast appearance of h-FeOOH colloids took place by forced hydrolysis of 0.1 M FeCl3 solutions at 90 jC. This is easily visible by the increase in the turbidity of the colloidal suspension for short times of hydrolysis and the sedimentation of h-FeOOH particles for
Fig. 2. Characteristic parts of XRD patterns at RT of samples C6 and C7. A = akagane´ite, G = goethite, H = hematite.
S. Music´ et al. / Materials Letters 58 (2004) 2640–2645
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FeCl3/HCl solutions under the concentration conditions provided in the previous work by Matijevic´ and Scheiner [2]. Bailey et al. [24] proposed a mechanism of the formation of a-Fe2O3 particles of different shapes by forced hydrolysis of FeCl3/HCl solutions. By virtue of that mechanism, (a) double-ellipsoidal particles were obtained by nucleation and growth of a-Fe2O3 on hFeOOH rods, (b) spherical particles were obtained by growth of a-Fe2O3 on partially dissolved h-FeOOH rods which served as template and (c) a-Fe2O3 cubes nucleated on aggregated h-FeOOH rods (raft-like). Atkinson et al. [25] investigated crystal nucleation and growth during hydrolysis of aqueous FeCl3 solutions. It was shown that for partially neutralized FeCl3 solutions (OH/Fe molar ratio = 0 – 2.75), the transformations h-FeOOH ! a-Fe2O3 and h-FeOOH ! a-FeOOH at pH 1– 2 proceeded by way of the dissolution/reprecipitation mechanism, and the precipitation process was promoted by adding seed crystals. Knese et al. [26] used Mo¨ssbauer spectroscopy to investigate the transformation of h-FeOOH to a-Fe2O3 in the suspension at pH 8.5 and 100 jC. a-FeOOH was found as intermediate phase, and it was supposed that the formation of a-Fe2O3 can proceed by way of (a) direct transformation h-FeOOH ! a-Fe2O3, or (b) indirect transformation h-FeOOH ! a-FeOOH ! a-Fe2O3.
Fig. 3. Fourier transform infrared spectra of samples C2, C3, C6, C8, C13 and C14. The spectra were recorded at RT.
In reference literature [1,2,21 –23], no formation of aFeOOH during forced hydrolysis of FeCl3/HCl systems was reported. The formation of a-Fe2O3 particles was considered in terms of the dissolution/reprecipitation mechanism. However, Gotic´ et al. [8] observed the formation of a small fraction of a-FeOOH by forced hydrolysis of 0.2 M FeCl3 + 0.1 M HCl solution at 120 jC for 3 days. Bailey et al. [24] investigated forced hydrolysis of
Table 2 Measured pH values in dependence on the chemical composition of the precipitation systems and heating time Chemical composition
pHa
[FeCl3] (M)
[HCl] (M)
Start
0.1 0.1 0.1 0.1 0.1 0.01 0.01 0.01
– 0.1 0.05 0.01 0.005 – 0.05 0.005
1.86 1.07 1.34 1.72 1.73 2.47 1.43 2.12
4h
1d
3d
35d
1.02 1.25 1.07 0.98
1.01 0.96 0.92 0.91
1.07 0.94 0.87 0.92
0.88
1.32 1.49
1.18 1.48
1.26 1.56
1.38 1.74
0.85 0.95
a pH values were measured at RT after an abrupt cooling of the precipitation systems.
Fig. 4. TEM photographs of samples (a) C2, (b) C3 and (c) C4.
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fraction. The results of this work are in line with the general mechanism, as proposed by Music´ et al. [30] for the hydrolysis of Fe 3 + ions in aqueous solutions of the corresponding chloride, nitrate or sulfate salts. The present work shows that a-FeOOH particles precipitate when a higher concentration of HCl is present at the beginning of the hydrolytic process. Evidently, different complexes of Fe3 + ions are formed in FeCl3 solutions and FeCl3/HCl solutions, and very probably, this factor plays an important role in overall hydrolytic process. Iron(III) chloride complexes are present at high concentrations of HCl, whereas aquo complexes are predominant in FeCl3 solutions without HCl adding. The second factor are very slow kinetics of Fe3 + hydrolysis in highly acidic FeCl3 solutions (precipitation systems C1 and C5 in the present work), which influence the phase composition of the precipitate, as well as the particle size and shape. A possible effect of mixing the precipitation systems, during the forced hydrolysis of FeCl3/HCl aqueous solution should also be investigated in the future. One should bear in mind that FeCl36H2O salts from different vendors may contain ‘‘free’’ HCl which can also influence the overall process of Fe3 + hydrolysis. Fig. 6 shows TEM photographs of samples C9, C12, C13 and C14. Fig. 6a (sample C9) shows a-Fe2O3 particles of different sizes ( f 1 – 4 Am) and shape, whereas Fe2O3 particles of sample C12 (Fig. 6b) also show different sizes ( f 0.75 –1.8 Am) and shape. A great difference in size of aFig. 5. TEM photographs of samples (a) C6 and (b) C8.
TEM photographs of selected samples are shown in Figs. 4 –6. Fig. 4a (sample C2) shows h-FeOOH rods and aFeOOH particle of dendritic nature. The angle between the main body and its dendrite for this type of a-FeOOH particles is 117.5j (calculated value) [27]. Fig. 4b (sample C3) and c (sample C4) shows only h-FeOOH particles. These particles are smaller than those in sample C2, and in sample C4, they have an ellipsoidal shape with good uniformity. Fig. 5 shows TEM photographs of samples C6 and C8. The particles of h-FeOOH (rods), a-FeOOH (stars and dendrites) and a-Fe2O3 (lemon and near spherical shapes) are well visible in Fig. 5a (sample C6). This figure is a selected area TEM image. The inspection of the whole TEM image gives approximately the population of different particles in accordance with XRD phase analysis. Holes and irregular shapes of h-FeOOH and a-FeOOH particles are present, thus indicating the dissolution of these particles. The formation of a-Fe2O3 particles in this precipitation system is directed by the dissolution/reprecipitation mechanism. Furthermore, Wang et al. [28,29] also reported the possibility of the formation of growth centers for a-Fe2O3 particles in the middle of a star-shaped a-FeOOH particle, which is possible due to the crystallographic characteristics of a-Fe2O3 and a-FeOOH crystals. The present work has shown the formation of a-FeOOH at a higher concentration of HCl; however, this phase was in traces or in a small
Fig. 6. TEM photographs of samples (a) C9, (b) C12, (c) C13 and (d) C14.
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Fe2O3 particles is observed between samples C13 (Fig. 6c) and C14 (Fig. 6d). The formation of large a-Fe2O3 particles ( f 1.2– 3 Am) is observed for sample C13, whereas in the case of sample C14, uniform and small a-Fe2O3 particles ( f 0.08 Am) are obtained. In upper right corner of Fig. 6d, enlarged a-Fe2O3 particles are shown. Sample C14 did not show any presence of h-FeOOH or a-FeOOH particles. Acknowledgements The authors thank Professor N. Ljubesˇic´ for his valuable assistance in electron microscopy. References [1] E. Matijevic´, J. Colloid Interface Sci. 58 (1977) 374. [2] E. Matijevic´, P. Scheiner, J. Colloid Interface Sci. 63 (1978) 509. [3] M. Ozaki, S. Kratohvil, E. Matijevic´, J. Colloid Interface Sci. 102 (1984) 146. [4] S. Kratohvil, E. Matijevic´, M. Ozaki, Colloid Polym. Sci. 262 (1984) 804. [5] K. Kandori, I. Horii, A. Yasukawa, T. Ishikawa, J. Mater. Sci. 30 (1995) 2145. [6] K. Kandori, A. Yasukawa, T. Ishikawa, J. Colloid Interface Sci. 180 (1996) 446. [7] K. Kandori, Y. Nakamoto, A. Yasukawa, T. Ishikawa, J. Colloid Interface Sci. 202 (1998) 499. [8] M. Gotic´, S. Popovic´, N. Ljubesˇic´, S. Music´, J. Mater. Sci. 29 (1994) 2474. [9] S. Music´, S. Krehula, S. Popovic´, Zˇ. Skoko, Mater. Lett. 57 (2003) 1096.
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[10] S. Music´, M. Gotic´, N. Ljubesˇic´, Mater. Lett. 25 (1995) 69. [11] A. Sˇaric´, K. Nomura, S. Popovic´, N. Ljubesˇic´, S. Music´, Mater. Chem. Phys. 52 (1998) 214. [12] A. Sˇaric´, S. Music´, K. Nomura, S. Popovic´, Mater. Sci. Eng. B 56 (1998) 43. [13] S. Music´, M. Maljkovic´, S. Popovic´, ACH-Models Chem. 136 (1999) 299. [14] S. Music´, G.P. Santana, G. Sˇmit, V.K. Garg, J. Alloys Compd. 278 (1998) 291. [15] N.J. Reeves, S. Mann, J. Chem. Soc., Faraday Trans. 87 (1991) 3875. [16] T. Nakamura, H. Kurokawa, J. Mater. Sci. 30 (1995) 4710. [17] T. Sugimoto, A. Muramatsu, J. Colloid Interface Sci. 184 (1996) 626. [18] T.P. Raming, A.J.A. Winnubst, C.M. Van Kats, A.P. Philipse, J. Colloid Interface Sci. 249 (2002) 346. [19] International Centre for Diffraction Data, Joint Committee on Powder Diffraction Standards, Powder Diffraction File, 1601 Park Lane, Swarthmore, PA 19081, USA. [20] J.E. Iglesias, C.J. Serna, Mineral. Petrogr. Acta 29A (1985) 365. [21] E. Matijevic´, Sˇ. Cimasˇ, Colloid Polym. Sci. 265 (1987) 155. [22] E.K. De Blanco, M.A. Blesa, S.J. Liberman, React. Solids 1 (1986) 189. [23] K. Kandori, S. Tamura, T. Ishikawa, Colloid Polym. Sci. 272 (1994) 812. [24] J.K. Bailey, C.J. Brinker, M.L. Mecartney, J. Colloid Interface Sci. 157 (1993) 1. [25] R.J. Atkinson, A.M. Posner, J.P. Quirk, Clays Clay Miner. 25 (1977) 49. [26] K. Knese, P. Wartewig, C. Michalk, S. Fischer, Hyperfine Interact. 94 (1994) 1999. [27] Y. Maeda, S. Hirono, Jpn. J. Appl. Phys. 20 (1981) 1991. [28] M.K. Wang, V.A. Greenhut, P.H. Hsu, Soil Sci. 132 (1981) 182. [29] M.K. Wang, J. Chin. Agricult. Chem. Soc. 25 (1987) 432. [30] S. Music´, A. Ve´rtes, G.W. Simmons, I. Czako´-Nagy, H. Leidheiser Jr., J. Colloid Interface Sci. 85 (1982) 256.