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Catalytic synthesis of nanosized hematite particles in solution
Materials Chemistry and Physics 102 (2007) 1–6
Catalytic synthesis of nanosized hematite particles in solution Hui Liu a,b , Yu Wei a,c,∗ , Ping Li a , Yanfeng Zhang a , Yuhan Sun b a
School of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050016, PR China b Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c Key Laboratory of Advanced Film Material of Hebei Province, Hebei Normal University, Shijiazhuang 050016, PR China Received 17 November 2005; received in revised form 17 July 2006; accepted 12 October 2006
Abstract Spherical hematite particles have been quickly synthesized by heating Fe(OH)3 gel with trace Fe(II) in aqueous solution to reflux temperature. The final product is characterized by XRD, TEM, HRTEM, and FT-IR techniques. The influence of various factors on the size and morphology of hematite particles is investigated, respectively. The growth mechanism of hematite particles is also discussed. The results show that both the mechanisms, dissolution/reprecipitation and solid state transformation, work alternately in the transformation process from Fe(OH)3 to hematite particles until the transformation is finished. The magnetism of the product is also studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Catalytic synthesis; Nanosized hematite particles; Mechanism; Dissolution/reprecipitation; Solid state transformation
1. Introduction ␣-Fe2 O3 is a very important multifunctional material, which is extensively used in fine ceramics, pigments, catalysis, light absorption, medicine and so on [1,2]. Recently, it was reported that uniform nanocrystalite ␣-Fe2 O3 particles show better photoelectrochemical response at longer wavelengths in the visible region because it has a narrow band gap than that of TiO2 [2]. Controlling the size and morphology is a matter of considerable importance for the manufacture of hematite particles, as the properties and applications of ultrafine ␣-Fe2 O3 particles depend drastically on the particle size, morphology and monodispersibility [3]. Up to now, various methods have been reported for the preparation of hematite, such as the forced hydrolysis of Fe(III) solution[4–8], gel–sol method [9,10], hydrothermal treatment [3], etc. Although these methods above have their advantages, it is difficult to scale them up because the concentration of salts adopted is low, or the production period is too long, or the preparation processing is complex or they need special equipments. Compared with them, the chemical precipitation method is relatively simple and of low cost as well as
∗ Corresponding author at: School of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050016, PR China. Tel.: +86 311 86268342; fax: +86 311 85893425. E-mail address: [email protected] (Y. Wei).
0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.10.001
easier to control the nanoparticle size. Recently, Fan et al. [11] reported a chemical precipitation method for the preparation of spherical hematite particles. Pure-phase hematite particles can be obtained only when the thermal treatment temperature for the precursor is 500 ◦ C. In the current paper, a simple chemical precipitation system via catalytic phase transformation process was successfully employed to synthesize hematite nanoparticles, based on our previous work [12]. The reaction temperature is the boiling temperature of the hydrothermal system under ambient pressure and the reaction time is not more than 1 h. The experimental results reveal that this method has such advantages as high reactant concentration, very short transformation time as well as less energy intensity, less polluting and leads to high homogeneity. Moreover, the formation mechanism and the magnetism for hematite were also studied. 2. Experimental Analytical grade reagents (iron chloride hexahydrate FeCl3 ·6H2 O and sodium hydroxide NaOH from Tianjin Kaitong Chemical Com.) and distilled water were used in all experiments. The standard condition established for the preparation of hematite particles was as follows. 6.0 M NaOH solution was added dropwise to the constant wellstirred 1.0 M Fe(III) solution (50 ml) in a RT water bath until pH 7 was reached. The precipitation of Fe(OH)3 formed in this system was confirmed to be twoline ferrihydrite by XRD [12]. The pH of final Fe(OH)3 was readjusted to the desired pH 7 with a dilute NaOH solution after addition of trace Fe(II). The volume was adjusted to 100 ml. The suspension stirred with a magnetic stirrer
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was heated to boil and kept refluxing for a certain time. The final product was filtered off, washed with distilled water and then dried. Phase identification was carried out by X-ray diffraction in a Germany Bruker D8-ADVANCE using Cu K␣ radiation. The morphologies of the particles were observed and evaluated by using a Hitachi H-7500 transmission electron microscope or Hitachi S-570 scanning electron microscopy. Lattice images coupled with selected-area electron diffraction were also obtained for individual particle by Hitachi H-9000 high-resolution electron microscopy. The infrared spectrum of the product was recorded by diluting a few milligrams of sample in KBr. The size distribution of the product particles was determined by ZETASIZER-3000HSA . The magnetic data of the solid samples were collected with an American Lakeshore-730 vibrating sample magnetometer.
3. Results and discussion 3.1. The characteristics of as-prepared hematite nanoparticles It has been reported that Fe(II) has obvious catalytic effect on the transformation of Fe(OH)3 gel. This catalytic function can be used to synthesize hematite particles in a very short time. The experimental results have shown that the phase transformation from Fe(OH)3 to hematite can be completed within about 30 min (supporting information of ref. [12]). In order to obtain wellcrystallization hematite particles transformation time should be prolonged to 1 h. The XRD pattern, IR spectrum and TEM image of as-prepared hematite particles are shown in Figs. 1–3, respectively.
Fig. 3. TEM image of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
The result in Fig. 1 reveals that all diffraction peaks are in good agreement with the JCPDS file of hematite (JCPDS 33-0664), which can be indexed as ␣-Fe2 O3 . FT-IR spectrum in Fig. 2 also shows that the typical peaks of Fe–O bond of hematite is at 474.3 and 578.6 cm−1 [13]. The bands at 3359.8 and 1616.2 cm−1 are attributed to water adsorbed on the surface of the product, because the product is dried at RT. Both XRD and IR all indicate that no any other by-product is found. Typical TEM image (Fig. 3) shows that the prepared hematite particles are quasi-spherical. The average size of the hematite particles is about 60–80 nm, which is consistent with the average size obtained by size distribution determination (Fig. 4). Fig. 4 also reveals that the final particles prepared are of characteristic of narrow particle size distribution.
Fig. 1. XRD pattern of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
Fig. 2. FT-IR spectrum of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
Fig. 4. The size distribution of obtained ␣-Fe2 O3 particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
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Fig. 5. HRTEM image (a and b) and ED photo (c) of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
High-resolution electron micrograph of a single hematite particle (Fig. 5(a)) also clearly reveals the composite nature of hematite particle. The surface of the particle is not smooth. However, the crystal lattice on the surface of hematite particle can be seen clearly from Fig. 5(b), which is magnified from the area marked by the white box in Fig. 5(a). The special surface structure of the product leads to a bigger specific surface area, which is probably in favor of its property as a catalyst. For example, when the average size is within 60–80 nm, the specific surface area of as-prepared hematite particles is 31.83 m2 g−1 . We tried to synthesize hematite particles with smooth surface whose particle size is also within 60–80 nm and the specific surface area is only 17.18 m2 g−1 . Moreover, the selected-area electron diffraction (Fig. 5(c)) confirms the crystal structure of hematite particles, giving clear diffraction spots characteristic. The result obtained from ED is consistent with that from XRD. Otherwise, the experimental results show that the product prepared in the current system has good reproducibility. All those results lead us to conclude that this new method, which is entitled the catalytic phase transformation method, can be used to rapidly prepare nanosized hematite particles. 3.2. The size control of hematite nanoparticles The size of hematite nanoparticles depends on both the initial concentration and the pH of Fe(OH)3 . TEM images of the product prepared at different concentrations of Fe(III) and at different pHs are shown in Fig. 6. The results reveal that the size of hematite changes with the initial concentration of Fe(III) at an
invariable pH and nFe(II) /nFe(III) . The particles with diameter ca. 30–50 nm were obtained at CFe(III) = 0.3 M at pH 7 (Fig. 6(a)). When CFe(III) = 0.5 M, the size of the product is about 60–80 nm at pH 7 (Fig. 6(b)). The results also indicate that pH has a great effect on the size of the product. The size of the product obtained at pH 9 is only 30–40 nm at the initial concentration of Fe(OH)3 0.5 M. These results lead us to the conclusion that differentsized hematite nanoparticles can be prepared by regulating the experimental conditions. 3.3. The product obtained by using different iron salts When different iron salts are used as raw materials, nanosized hematite particles can be obtained by the new method (Fig. 7). All the as-prepared hematite particles are sphere shaped. The experimental results show that the transformation rates of Fe(OH)3 gel prepared by various iron salts are different. The transformation rate in FeCl3 system is the fastest and that in Fe2 (SO4 )3 system is the slowest. So, when Fe2 (SO4 )3 is used as raw material, it is necessary to prolong the transformation time appropriately. The above results probably reveal that the species of anion has some influence on the transformation rate and the reason is under the way. 3.4. The effect of additive on the product When trace PO4 3− ion was added into the current system, the experimental result reveals that the spindle particles of hematite were obtained (Fig. 8). The reaction time needs to be prolonged
Fig. 6. TEM images of the hematite particles prepared at different concentrations of Fe(OH)3 and different pHs. nFe(II) /nFe(III) = 0.02: (a) 0.3 M, (b) 0.5 M at pH 7 and (c) 0.5 M at pH 9.
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Fig. 7. TEM images of hematite particles prepared by different iron salts. C = 0.5 M, nFe(II) /nFe(III) = 0.02, pH 7: (a) Fe2 (SO4 )3 and (b) Fe(NO3 )3 .
to about 20 h, which indicates the addition of trace PO4 3− ion restrains the speed of phase transformation of Fe(OH)3 . The influence of PO4 3− ion was explained by preferential adsorption on selected planes [14]. In fact, PO4 3− ion and Fe(II) ion influence the phase transformation of Fe(OH)3 in the current system together. The further research indicates that the cooperating mechanism is rather complicated. The results will be reported elsewhere. 3.5. The magnetization curve of hematite particles Room temperature magnetization curves and TEM images of the as-prepared hematite particles are shown in Figs. 9 and 10, respectively. From Figs. 9 and 10, it can be seen that there are some differences between the two samples. The magnetic hysteresis loops are obtained for the two samples (Fig. 9) prepared with trace Fe(II) (b) or without Fe(II) (a). The remanence and the coercivity of the sample (a) are 56,686 × 10−3 emu g−1
Fig. 9. Magnetization curve of the product obtained in the presence or absence of Fe(II). Determination at RT, C = 0.5 M, pH 7: (a) nFe(II) /nFe(III) = 0 and (b) nFe(II) /nFe(III) = 0.02.
and 0.379 kOe, respectively. For the sample (b) they are 27,043 × 10−3 emu g−1 and 0.580 kOe, respectively. Generally speaking, the magnetization of ferromagnetic materials is very sensitive to the microstructure of a particular sample [15]. Li et al. [16] thought that the difference in the remanence for the samples is related to the morphology of particles and the magnitude of coercivity is inverse proportional to the particle size. In fact, hematite particles prepared in the presence of trace Fe(II) are different from those prepared in the absence of Fe(II) both in morphology and particle size. On the one hand, hematite particles prepared without Fe(II) (Fig. 10(a)) are quasi-cubic and quasi-spherical for the sample prepared with Fe(II) (Fig. 10(b)), which is probably the main reason resulting in the different remanence. On the other hand, the particle size in the sample (a) is more than 100 nm and that in the sample (b) is only 60–80 nm. The result obtained in Fig. 9 consists with Li’s viewpoint [16]. 3.6. Further investigation on the mechanism of the phase transformation from Fe(OH)3 gel to hematite In the previous work [12], it has been confirmed that there are two transformation mechanisms from Fe(OH)3 gel to hematite at pH 5–9 in the presence of Fe(II). One is the catalytic dissolution/reprecipitation from Fe(OH)3 gel to hematite and the other is the catalytic solid state transformation. The results show that FeOH+ can catalyze the dissolution of Fe(OH)3 gel and
Fig. 8. SEM images of ␣-Fe2 O3 particles obtained in the presence of trace PO4 3− ion. CFe(III) = 0.5 M, nFe(II) /nFe(III) = 0.01, pH 5.2 and CNaH2 PO4 = 1.6 × 10−2 M.
Fig. 10. TEM images of hematite particles prepared with Fe(II) and without Fe(II). C = 0.5 M, pH 7: (a) nFe(II) /nFe(III) = 0 and (b) nFe(II) /nFe(III) = 0.02.
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Fig. 11. Changes of the concentration of various species of iron and pH in solution with reaction time. (a) [Fe(OH)3 ]/[Fe(OH)3 ]0 –t, nFe(II) /nFe(III) = 0.02; (b) pH–t, nFe(II) /nFe(III) = 0.02; (c) [FeIII ]sol –t, nFe(II) /nFe(III) = 0.02; (d) [FeIII ]sol –t, nFe(II) /nFe(III) = 0; (e) [FeII ]sol /[FeII ]0 –t, nFe(II) /nFe(III) = 0.02.
Fe(OH)2 can induce the solid state transformation. However, we only discussed the two separate mechanisms and did not further investigate the relationship between them. In order to further understand the relationship between the two transformation mechanisms from Fe(OH)3 gel to hematite in the presence of Fe(II), changes of the concentrations of various species in current system(Ctotal Fe(III) = 0.5 M, pHinitial 7) with the reaction time were determined, respectively. This experiment was carried out under pure N2 (99.999%) to avoid the oxidation of Fe(II). The concentrations of Fe(II) ion and total iron in solution are determined spectrophotometrically by using 1,10phenanthroline [17]. The total iron concentration in solution is determined after reducing iron(III) ion to Fe(II) ion with hydroxylamine. The results are shown in Fig. 11. Obviously, the concentration of Fe(III) in solution in the presence of Fe(II) is from 10 to 100 times as high as that in the absence of Fe(II) (Fig. 11(c and d)). This result reveals that Fe(II) accelerates the dissolution of Fe(OH)3 gel. However, the initial concentration of Fe(OH)3 gel is 0.5 M while the magnitude of the concentration Fe(III) in solution is only 10−4 M. The fact is that the whole transformation from Fe(OH)3 gel to hematite can be completed in a very short time [12]. So it is difficult to image that the whole transformation was completed in a very short time merely by dissolution/reprecipitation. In fact, Fe(OH)3 formed in this system was confirmed to be twoline ferrihydrite by XRD [12]. TEM images show particles ca. 2–3 nm in size [18]. Ferrihydrite and hematite all belong to the hexagonal crystallographic system. The similar crystal structure is the precondition for the solid-state transformation. Based on the above results, we deduce that the two transformation mechanisms should co-exist in the current system. In order to understand this concept, the following sketch (Fig. 12) may be used to show the phase transformation process from Fe(OH)3 gel to hematite particles. According to Figs. 11 and 12, the formation mechanism of hematite in the presence of Fe(II) can be explained as follows. Firstly, those Fe(II) added into the reaction system is adsorbed on the surface of Fe(OH)3 . Fe(II) adsorbed mostly should be FeOH+ in the system of pH 7 according to the report of Иhцкирвeли [19]. The second step is probably electron transfer between
Fig. 12. The sketch of the formation mechanism of hematite particles in the presence of trace Fe(II).
adsorbed FeOH+ and interfacial Fe(III) and this electron transfer is continually repeated. For the sake of convenience, we imagine this electron transfer process proceed batch-by-batch (Fig. 12). Based on the literature data [20], electron transfer is likely to be thermally assisted and the observation of a fully averaged state at 110 K suggests that the activation energy is less than 0.1 eV. So the electron transfer is a very quick process in the current system. When the first batch of electron transfer between Fe(II) and Fe(III) is completed, the original Fe(II) ion is oxidized to be Fe(III) ion and enters into the solution. This is the catalytic dissolution mechanism induced by FeOH+ [12], which can be confirmed by the change of the concentration of Fe(III) ion in solution with the reaction time in Fig. 11(c). At the same time, the original Fe(III) lying on the surface of Fe(OH)3 is reduced to be Fe(II) and subsequently the next batch of electron transfer between newly formed Fe(II) and Fe(III) lying in the second surface occurs. Noticeably, after the first batch of electron transfer, the species of newly formed Fe(II), which is obtained by reducing Fe(III) in Fe(OH)3 , is actually different from that Fe(II) initially adsorbed (FeOH+ ). It is possible that it has the similar structure to Fe(OH)2 . If it is so, the subsequent phase transformation mechanism should be the solid state mechanism induced by Fe(OH)2 [12]. According to the literature data [19], the species of Fe(II) usually change with the pH of the system. That is to say, Fe(II) in form of FeOH+ still may re-form by electron transfer because that the pH of the system decreases with the proceeding of the reaction. Therefore, the catalytic dissolution of Fe(OH)3 still occurs. This concept is confirmed by [Fe3+ ]sol curve (Fig. 11(c)) and pH curve (Fig. 11(b)). Hereafter, the two mechanisms coexist until the whole phase transformation is completed. With the proceeding of the reaction, the pH decreases and Fe(II) desorbs from the gel surface gradually and enters into the solution(Fig. 11(e)). HRTEM image (Fig. 5(a)) and ED pattern (Fig. 5(c)) of hematite particles give another evidence that the two transformation mechanisms coexist in the current system. It is easy to
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find that the surface image of hematite particle (Fig. 5(a)) is not smooth. The whole hematite particle seems to be composed of 10–20 nm tiny particles. However, the peaks in XRD pattern (Fig. 1) are very narrow and sharp. (Their FWHM are 0.232◦ and 0.260◦ for the peak 1 0 4 and 1 1 0.), which indicates that it is a good crystallinity for hematite. ED pattern (Fig. 5(c)) shows that diffraction fringes are diffraction spots and not diffraction rings for nanoparticles, which also confirms the good crystallinity for hematite. So, we propose that hematite particles in the current system form by both dissolution/reprecipitation and solid state transformation. It is possible that Fe(III) ion dissolved into the solution deposits onto the surface of hematite particles formed by solid state transformation. Because the whole phase transformation from Fe(OH)3 gel to hematite is completed in a very short time, the rapid growth rate makes the surface of hematite particles rough. In fact, when we tried to lessen the dissolution of Fe(OH)3 by controlling the experimental process, hematite single crystal particles with smooth surface were obtained The results on this aspect will be reported elsewhere. 4. Conclusion Nanosized hematite (␣-Fe2 O3 ) particles can be synthesized from highly condensed ferric hydroxide gel in the presence of trace Fe(II) in solution in a short time. The size of hematite particles can be controlled by regulating experimental conditions, such as pH or initial concentration of the precursor. The transformation from Fe(OH)3 gel to hematite is triggered by the electron transfer between adsorbed Fe(II) and interfacial Fe(III). The formation of hematite can be explained by two mechanisms—the dissolution/reprecipitation mechanism and the solid state transformation mechanism. The experimental results also show that this synthesis method is of characteristics of simple operation, short reaction time, less energy intensity and less polluting, etc.
Acknowledgment This work was supported by the Chinese Hebei Province Foundation of Natural Science Research (B2006000130, E2006000167). References [1] S. Chatterjee, S. Sarkar, N. Bhattacharyya, J. Photochem. Photobiol. A: Chem. 72 (1993) 183. [2] C. Pulgarin, J. Kiwi, Langmuir 11 (1995) 519. [3] L. Diamandescu, D. Mihaila-Tarabasanu, N. Popescu-Pogrion, A. Totovina, I. Bibicu, Ceram. Int. 25 (1999) 689. [4] E. Matijevi´c, P. Scheiner, J. Colloid Interface Sci. 63 (1978) 509. [5] J.K. Bailey, C.J. Brinker, M.L. Mecartney, J. Colloid Interface Sci. 157 (1993) 1. [6] T. Sugimoto, A. Maramatsu, K. Sakata, D. Shindo, J. Colloid Interface Sci. 158 (1993) 420. [7] M. Oca˜na, M.P. Morales, C.J. Serna, J. Colloid Interface Sci. 171 (1995) 85. [8] T. Sugimoto, A. Maramatsu, J. Colloid Interface Sci. 184 (1996) 626. [9] T. Sugimoto, Y. Wang, H. Itoh, A. Muramatsu, J. Colloid Interface Sci. 134 (1998) 265. [10] H. Itoh, T. Sugimoto, Stud. Surf. Sci. Catal. 132 (2001) 251. [11] H.L. Fan, B.Z. Song, J.H. Liu, Z.Q. Yang, Q.X. Li, Mater. Chem. Phys. 89 (2005) 321. [12] H. Liu, Y. Wei, Y.-H. Sun, J. Mol. Catal. A: Chem. 226 (2005) 135. [13] R.M. Cornell, U. Schwertmann, The Iron Oxides, VCH Publishers, New York, 1996, p.131. [14] N.J. Reeves, S. Mann, J. Chem. Soc. Faraday Trans. 87 (1991) 3875. [15] X. Cao, R. Prozorov, Y. Koltypin, G. Kataby, A. Gedanken, J. Mater. Res. 12 (1997) 402. [16] L. Li, G. Li Jr., R.L. Smith, H. Inomata, Chem. Mater. 12 (2000) 3705. [17] University Compilation Group, Experiment of Analytical Chemistry, High Education Publishing Company, Beijing, 1987, p. 354. [18] J. Zhao, F.E. Huggins, Z. Feng, F. Lu, N. Shah, G.P. Huffman, J. Catal. 143 (1993) 499. [19] Л.И. Иhцкирвeли, И.В. Колоcов, .М. Варшал. Ж. Нeорг. Хим. 20 (1975) 2388. [20] E. Tronc, P. Belleville, J.P. Jolivet, J. Livage, Langmuir 8 (1992) 313.
Catalytic synthesis of nanosized hematite particles in solution Hui Liu a,b , Yu Wei a,c,∗ , Ping Li a , Yanfeng Zhang a , Yuhan Sun b a
School of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050016, PR China b Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c Key Laboratory of Advanced Film Material of Hebei Province, Hebei Normal University, Shijiazhuang 050016, PR China Received 17 November 2005; received in revised form 17 July 2006; accepted 12 October 2006
Abstract Spherical hematite particles have been quickly synthesized by heating Fe(OH)3 gel with trace Fe(II) in aqueous solution to reflux temperature. The final product is characterized by XRD, TEM, HRTEM, and FT-IR techniques. The influence of various factors on the size and morphology of hematite particles is investigated, respectively. The growth mechanism of hematite particles is also discussed. The results show that both the mechanisms, dissolution/reprecipitation and solid state transformation, work alternately in the transformation process from Fe(OH)3 to hematite particles until the transformation is finished. The magnetism of the product is also studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Catalytic synthesis; Nanosized hematite particles; Mechanism; Dissolution/reprecipitation; Solid state transformation
1. Introduction ␣-Fe2 O3 is a very important multifunctional material, which is extensively used in fine ceramics, pigments, catalysis, light absorption, medicine and so on [1,2]. Recently, it was reported that uniform nanocrystalite ␣-Fe2 O3 particles show better photoelectrochemical response at longer wavelengths in the visible region because it has a narrow band gap than that of TiO2 [2]. Controlling the size and morphology is a matter of considerable importance for the manufacture of hematite particles, as the properties and applications of ultrafine ␣-Fe2 O3 particles depend drastically on the particle size, morphology and monodispersibility [3]. Up to now, various methods have been reported for the preparation of hematite, such as the forced hydrolysis of Fe(III) solution[4–8], gel–sol method [9,10], hydrothermal treatment [3], etc. Although these methods above have their advantages, it is difficult to scale them up because the concentration of salts adopted is low, or the production period is too long, or the preparation processing is complex or they need special equipments. Compared with them, the chemical precipitation method is relatively simple and of low cost as well as
∗ Corresponding author at: School of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050016, PR China. Tel.: +86 311 86268342; fax: +86 311 85893425. E-mail address: [email protected] (Y. Wei).
0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.10.001
easier to control the nanoparticle size. Recently, Fan et al. [11] reported a chemical precipitation method for the preparation of spherical hematite particles. Pure-phase hematite particles can be obtained only when the thermal treatment temperature for the precursor is 500 ◦ C. In the current paper, a simple chemical precipitation system via catalytic phase transformation process was successfully employed to synthesize hematite nanoparticles, based on our previous work [12]. The reaction temperature is the boiling temperature of the hydrothermal system under ambient pressure and the reaction time is not more than 1 h. The experimental results reveal that this method has such advantages as high reactant concentration, very short transformation time as well as less energy intensity, less polluting and leads to high homogeneity. Moreover, the formation mechanism and the magnetism for hematite were also studied. 2. Experimental Analytical grade reagents (iron chloride hexahydrate FeCl3 ·6H2 O and sodium hydroxide NaOH from Tianjin Kaitong Chemical Com.) and distilled water were used in all experiments. The standard condition established for the preparation of hematite particles was as follows. 6.0 M NaOH solution was added dropwise to the constant wellstirred 1.0 M Fe(III) solution (50 ml) in a RT water bath until pH 7 was reached. The precipitation of Fe(OH)3 formed in this system was confirmed to be twoline ferrihydrite by XRD [12]. The pH of final Fe(OH)3 was readjusted to the desired pH 7 with a dilute NaOH solution after addition of trace Fe(II). The volume was adjusted to 100 ml. The suspension stirred with a magnetic stirrer
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H. Liu et al. / Materials Chemistry and Physics 102 (2007) 1–6
was heated to boil and kept refluxing for a certain time. The final product was filtered off, washed with distilled water and then dried. Phase identification was carried out by X-ray diffraction in a Germany Bruker D8-ADVANCE using Cu K␣ radiation. The morphologies of the particles were observed and evaluated by using a Hitachi H-7500 transmission electron microscope or Hitachi S-570 scanning electron microscopy. Lattice images coupled with selected-area electron diffraction were also obtained for individual particle by Hitachi H-9000 high-resolution electron microscopy. The infrared spectrum of the product was recorded by diluting a few milligrams of sample in KBr. The size distribution of the product particles was determined by ZETASIZER-3000HSA . The magnetic data of the solid samples were collected with an American Lakeshore-730 vibrating sample magnetometer.
3. Results and discussion 3.1. The characteristics of as-prepared hematite nanoparticles It has been reported that Fe(II) has obvious catalytic effect on the transformation of Fe(OH)3 gel. This catalytic function can be used to synthesize hematite particles in a very short time. The experimental results have shown that the phase transformation from Fe(OH)3 to hematite can be completed within about 30 min (supporting information of ref. [12]). In order to obtain wellcrystallization hematite particles transformation time should be prolonged to 1 h. The XRD pattern, IR spectrum and TEM image of as-prepared hematite particles are shown in Figs. 1–3, respectively.
Fig. 3. TEM image of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
The result in Fig. 1 reveals that all diffraction peaks are in good agreement with the JCPDS file of hematite (JCPDS 33-0664), which can be indexed as ␣-Fe2 O3 . FT-IR spectrum in Fig. 2 also shows that the typical peaks of Fe–O bond of hematite is at 474.3 and 578.6 cm−1 [13]. The bands at 3359.8 and 1616.2 cm−1 are attributed to water adsorbed on the surface of the product, because the product is dried at RT. Both XRD and IR all indicate that no any other by-product is found. Typical TEM image (Fig. 3) shows that the prepared hematite particles are quasi-spherical. The average size of the hematite particles is about 60–80 nm, which is consistent with the average size obtained by size distribution determination (Fig. 4). Fig. 4 also reveals that the final particles prepared are of characteristic of narrow particle size distribution.
Fig. 1. XRD pattern of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
Fig. 2. FT-IR spectrum of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
Fig. 4. The size distribution of obtained ␣-Fe2 O3 particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
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Fig. 5. HRTEM image (a and b) and ED photo (c) of obtained hematite particles. C = 0.5 M, nFe(II) /nFe(III) = 0.02 and pH 7.
High-resolution electron micrograph of a single hematite particle (Fig. 5(a)) also clearly reveals the composite nature of hematite particle. The surface of the particle is not smooth. However, the crystal lattice on the surface of hematite particle can be seen clearly from Fig. 5(b), which is magnified from the area marked by the white box in Fig. 5(a). The special surface structure of the product leads to a bigger specific surface area, which is probably in favor of its property as a catalyst. For example, when the average size is within 60–80 nm, the specific surface area of as-prepared hematite particles is 31.83 m2 g−1 . We tried to synthesize hematite particles with smooth surface whose particle size is also within 60–80 nm and the specific surface area is only 17.18 m2 g−1 . Moreover, the selected-area electron diffraction (Fig. 5(c)) confirms the crystal structure of hematite particles, giving clear diffraction spots characteristic. The result obtained from ED is consistent with that from XRD. Otherwise, the experimental results show that the product prepared in the current system has good reproducibility. All those results lead us to conclude that this new method, which is entitled the catalytic phase transformation method, can be used to rapidly prepare nanosized hematite particles. 3.2. The size control of hematite nanoparticles The size of hematite nanoparticles depends on both the initial concentration and the pH of Fe(OH)3 . TEM images of the product prepared at different concentrations of Fe(III) and at different pHs are shown in Fig. 6. The results reveal that the size of hematite changes with the initial concentration of Fe(III) at an
invariable pH and nFe(II) /nFe(III) . The particles with diameter ca. 30–50 nm were obtained at CFe(III) = 0.3 M at pH 7 (Fig. 6(a)). When CFe(III) = 0.5 M, the size of the product is about 60–80 nm at pH 7 (Fig. 6(b)). The results also indicate that pH has a great effect on the size of the product. The size of the product obtained at pH 9 is only 30–40 nm at the initial concentration of Fe(OH)3 0.5 M. These results lead us to the conclusion that differentsized hematite nanoparticles can be prepared by regulating the experimental conditions. 3.3. The product obtained by using different iron salts When different iron salts are used as raw materials, nanosized hematite particles can be obtained by the new method (Fig. 7). All the as-prepared hematite particles are sphere shaped. The experimental results show that the transformation rates of Fe(OH)3 gel prepared by various iron salts are different. The transformation rate in FeCl3 system is the fastest and that in Fe2 (SO4 )3 system is the slowest. So, when Fe2 (SO4 )3 is used as raw material, it is necessary to prolong the transformation time appropriately. The above results probably reveal that the species of anion has some influence on the transformation rate and the reason is under the way. 3.4. The effect of additive on the product When trace PO4 3− ion was added into the current system, the experimental result reveals that the spindle particles of hematite were obtained (Fig. 8). The reaction time needs to be prolonged
Fig. 6. TEM images of the hematite particles prepared at different concentrations of Fe(OH)3 and different pHs. nFe(II) /nFe(III) = 0.02: (a) 0.3 M, (b) 0.5 M at pH 7 and (c) 0.5 M at pH 9.
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Fig. 7. TEM images of hematite particles prepared by different iron salts. C = 0.5 M, nFe(II) /nFe(III) = 0.02, pH 7: (a) Fe2 (SO4 )3 and (b) Fe(NO3 )3 .
to about 20 h, which indicates the addition of trace PO4 3− ion restrains the speed of phase transformation of Fe(OH)3 . The influence of PO4 3− ion was explained by preferential adsorption on selected planes [14]. In fact, PO4 3− ion and Fe(II) ion influence the phase transformation of Fe(OH)3 in the current system together. The further research indicates that the cooperating mechanism is rather complicated. The results will be reported elsewhere. 3.5. The magnetization curve of hematite particles Room temperature magnetization curves and TEM images of the as-prepared hematite particles are shown in Figs. 9 and 10, respectively. From Figs. 9 and 10, it can be seen that there are some differences between the two samples. The magnetic hysteresis loops are obtained for the two samples (Fig. 9) prepared with trace Fe(II) (b) or without Fe(II) (a). The remanence and the coercivity of the sample (a) are 56,686 × 10−3 emu g−1
Fig. 9. Magnetization curve of the product obtained in the presence or absence of Fe(II). Determination at RT, C = 0.5 M, pH 7: (a) nFe(II) /nFe(III) = 0 and (b) nFe(II) /nFe(III) = 0.02.
and 0.379 kOe, respectively. For the sample (b) they are 27,043 × 10−3 emu g−1 and 0.580 kOe, respectively. Generally speaking, the magnetization of ferromagnetic materials is very sensitive to the microstructure of a particular sample [15]. Li et al. [16] thought that the difference in the remanence for the samples is related to the morphology of particles and the magnitude of coercivity is inverse proportional to the particle size. In fact, hematite particles prepared in the presence of trace Fe(II) are different from those prepared in the absence of Fe(II) both in morphology and particle size. On the one hand, hematite particles prepared without Fe(II) (Fig. 10(a)) are quasi-cubic and quasi-spherical for the sample prepared with Fe(II) (Fig. 10(b)), which is probably the main reason resulting in the different remanence. On the other hand, the particle size in the sample (a) is more than 100 nm and that in the sample (b) is only 60–80 nm. The result obtained in Fig. 9 consists with Li’s viewpoint [16]. 3.6. Further investigation on the mechanism of the phase transformation from Fe(OH)3 gel to hematite In the previous work [12], it has been confirmed that there are two transformation mechanisms from Fe(OH)3 gel to hematite at pH 5–9 in the presence of Fe(II). One is the catalytic dissolution/reprecipitation from Fe(OH)3 gel to hematite and the other is the catalytic solid state transformation. The results show that FeOH+ can catalyze the dissolution of Fe(OH)3 gel and
Fig. 8. SEM images of ␣-Fe2 O3 particles obtained in the presence of trace PO4 3− ion. CFe(III) = 0.5 M, nFe(II) /nFe(III) = 0.01, pH 5.2 and CNaH2 PO4 = 1.6 × 10−2 M.
Fig. 10. TEM images of hematite particles prepared with Fe(II) and without Fe(II). C = 0.5 M, pH 7: (a) nFe(II) /nFe(III) = 0 and (b) nFe(II) /nFe(III) = 0.02.
H. Liu et al. / Materials Chemistry and Physics 102 (2007) 1–6
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Fig. 11. Changes of the concentration of various species of iron and pH in solution with reaction time. (a) [Fe(OH)3 ]/[Fe(OH)3 ]0 –t, nFe(II) /nFe(III) = 0.02; (b) pH–t, nFe(II) /nFe(III) = 0.02; (c) [FeIII ]sol –t, nFe(II) /nFe(III) = 0.02; (d) [FeIII ]sol –t, nFe(II) /nFe(III) = 0; (e) [FeII ]sol /[FeII ]0 –t, nFe(II) /nFe(III) = 0.02.
Fe(OH)2 can induce the solid state transformation. However, we only discussed the two separate mechanisms and did not further investigate the relationship between them. In order to further understand the relationship between the two transformation mechanisms from Fe(OH)3 gel to hematite in the presence of Fe(II), changes of the concentrations of various species in current system(Ctotal Fe(III) = 0.5 M, pHinitial 7) with the reaction time were determined, respectively. This experiment was carried out under pure N2 (99.999%) to avoid the oxidation of Fe(II). The concentrations of Fe(II) ion and total iron in solution are determined spectrophotometrically by using 1,10phenanthroline [17]. The total iron concentration in solution is determined after reducing iron(III) ion to Fe(II) ion with hydroxylamine. The results are shown in Fig. 11. Obviously, the concentration of Fe(III) in solution in the presence of Fe(II) is from 10 to 100 times as high as that in the absence of Fe(II) (Fig. 11(c and d)). This result reveals that Fe(II) accelerates the dissolution of Fe(OH)3 gel. However, the initial concentration of Fe(OH)3 gel is 0.5 M while the magnitude of the concentration Fe(III) in solution is only 10−4 M. The fact is that the whole transformation from Fe(OH)3 gel to hematite can be completed in a very short time [12]. So it is difficult to image that the whole transformation was completed in a very short time merely by dissolution/reprecipitation. In fact, Fe(OH)3 formed in this system was confirmed to be twoline ferrihydrite by XRD [12]. TEM images show particles ca. 2–3 nm in size [18]. Ferrihydrite and hematite all belong to the hexagonal crystallographic system. The similar crystal structure is the precondition for the solid-state transformation. Based on the above results, we deduce that the two transformation mechanisms should co-exist in the current system. In order to understand this concept, the following sketch (Fig. 12) may be used to show the phase transformation process from Fe(OH)3 gel to hematite particles. According to Figs. 11 and 12, the formation mechanism of hematite in the presence of Fe(II) can be explained as follows. Firstly, those Fe(II) added into the reaction system is adsorbed on the surface of Fe(OH)3 . Fe(II) adsorbed mostly should be FeOH+ in the system of pH 7 according to the report of Иhцкирвeли [19]. The second step is probably electron transfer between
Fig. 12. The sketch of the formation mechanism of hematite particles in the presence of trace Fe(II).
adsorbed FeOH+ and interfacial Fe(III) and this electron transfer is continually repeated. For the sake of convenience, we imagine this electron transfer process proceed batch-by-batch (Fig. 12). Based on the literature data [20], electron transfer is likely to be thermally assisted and the observation of a fully averaged state at 110 K suggests that the activation energy is less than 0.1 eV. So the electron transfer is a very quick process in the current system. When the first batch of electron transfer between Fe(II) and Fe(III) is completed, the original Fe(II) ion is oxidized to be Fe(III) ion and enters into the solution. This is the catalytic dissolution mechanism induced by FeOH+ [12], which can be confirmed by the change of the concentration of Fe(III) ion in solution with the reaction time in Fig. 11(c). At the same time, the original Fe(III) lying on the surface of Fe(OH)3 is reduced to be Fe(II) and subsequently the next batch of electron transfer between newly formed Fe(II) and Fe(III) lying in the second surface occurs. Noticeably, after the first batch of electron transfer, the species of newly formed Fe(II), which is obtained by reducing Fe(III) in Fe(OH)3 , is actually different from that Fe(II) initially adsorbed (FeOH+ ). It is possible that it has the similar structure to Fe(OH)2 . If it is so, the subsequent phase transformation mechanism should be the solid state mechanism induced by Fe(OH)2 [12]. According to the literature data [19], the species of Fe(II) usually change with the pH of the system. That is to say, Fe(II) in form of FeOH+ still may re-form by electron transfer because that the pH of the system decreases with the proceeding of the reaction. Therefore, the catalytic dissolution of Fe(OH)3 still occurs. This concept is confirmed by [Fe3+ ]sol curve (Fig. 11(c)) and pH curve (Fig. 11(b)). Hereafter, the two mechanisms coexist until the whole phase transformation is completed. With the proceeding of the reaction, the pH decreases and Fe(II) desorbs from the gel surface gradually and enters into the solution(Fig. 11(e)). HRTEM image (Fig. 5(a)) and ED pattern (Fig. 5(c)) of hematite particles give another evidence that the two transformation mechanisms coexist in the current system. It is easy to
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find that the surface image of hematite particle (Fig. 5(a)) is not smooth. The whole hematite particle seems to be composed of 10–20 nm tiny particles. However, the peaks in XRD pattern (Fig. 1) are very narrow and sharp. (Their FWHM are 0.232◦ and 0.260◦ for the peak 1 0 4 and 1 1 0.), which indicates that it is a good crystallinity for hematite. ED pattern (Fig. 5(c)) shows that diffraction fringes are diffraction spots and not diffraction rings for nanoparticles, which also confirms the good crystallinity for hematite. So, we propose that hematite particles in the current system form by both dissolution/reprecipitation and solid state transformation. It is possible that Fe(III) ion dissolved into the solution deposits onto the surface of hematite particles formed by solid state transformation. Because the whole phase transformation from Fe(OH)3 gel to hematite is completed in a very short time, the rapid growth rate makes the surface of hematite particles rough. In fact, when we tried to lessen the dissolution of Fe(OH)3 by controlling the experimental process, hematite single crystal particles with smooth surface were obtained The results on this aspect will be reported elsewhere. 4. Conclusion Nanosized hematite (␣-Fe2 O3 ) particles can be synthesized from highly condensed ferric hydroxide gel in the presence of trace Fe(II) in solution in a short time. The size of hematite particles can be controlled by regulating experimental conditions, such as pH or initial concentration of the precursor. The transformation from Fe(OH)3 gel to hematite is triggered by the electron transfer between adsorbed Fe(II) and interfacial Fe(III). The formation of hematite can be explained by two mechanisms—the dissolution/reprecipitation mechanism and the solid state transformation mechanism. The experimental results also show that this synthesis method is of characteristics of simple operation, short reaction time, less energy intensity and less polluting, etc.
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