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Preparation and magnetization of hematite nanocrystals with amorphous iron oxide layers by hydrothermal conditions
Materials Research Bulletin 37 (2002) 949±955
Preparation and magnetization of hematite nanocrystals with amorphous iron oxide layers by hydrothermal conditions G.S. Li, R.L. Smith Jr., H. Inomata*, K. Arai Department of Chemical Engineering, Research Center of Supercritical Fluid Technology, Tohoku University, Sendai 980-8579, Japan (Refereed) Received 14 March 2001; accepted 18 January 2002
Abstract Hematite nanocrystals modi®ed with surface layers of amorphous hydrous iron oxides were prepared by hydrothermal conditions in the absence of alkali. The formation temperature was found to be ca. 1308C. When the temperature was lower than 1308C, no product was formed, while above this temperature, the amount of amorphous hydrous iron oxides at the surface of hematite nanocrystals was drastically decreased. The amorphous layers on the hematite nanocrystals obtained at 1308C were determined to be Fe2O31.64H2O. The coercivity for the hematite nanocrystals with modi®ed layers was 0.534 kOe, which is slightly larger than the values for hematite nanocrystals with few agglomerations. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; D. Magnetic properties
1. Introduction Hematite nanocrystals have shown several excellent properties and found a wide ®eld of technological applications including catalysts, pigments, recording medium, and sensors. An agglomerate of hematite particles exhibits a coercivity two times higher than that of commercial hematite [1]. Several synthetic methods have been developed for preparation of hematite nanocrystals including sol±gel, hydrolysis of *
Corresponding author. Tel.: 81-22-217-7283; fax: 81-22-217-7293. E-mail address: [email protected] (H. Inomata).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 6 9 5 - 5
950
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
iron salts, and hydrothermal synthesis [2±4]. Hydrothermal synthesis has shown to be advantageous over other methods in homogeneous nucleation and grain growth of hematite nanocrystals. Preparation conditions such as concentration, reaction temperature and time are the main factors in determining the morphologies and structures of the nanocrystals prepared by hydrothermal conditions. For example, hydrothermal reactions of an aqueous iron nitrate solution at a wide concentration range produce porous hematite nanocrystals (40±80 nm) containing non-intersecting 5±20 nm pores [5], while lowering the concentration to 0.0027± 0.18 M yields some amounts of a second phase that is mainly a-FeOOH (goethite) [6]. Single phase a-Fe2O3 (hematite) can be formed in ferric chloride hydrothermal systems [7]. However, the ferric concentration has to be restricted as low as 0.02± 0.04 M. Beyond this range, second phase b-FeOOH can be readily formed. It is clear that hematite nanocrystals can be obtained by adjusting hydrothermal conditions, but nanocrystals formed in this manner tend to agglomerate. In a previous work, we [8] demonstrated that nanocomposite technology could be an effective solution to this problem with nonmagnetic amorphous silica matrix allowing homogenous grain growth of nanocrystals. It appears to be very important to study the magnetization of nanocrystals and whether they can be modi®ed by addition of magnetic layers. In this work, we report on preparation, grain growth, and magnetization of hematite nanocrystals modi®ed with amorphous hydrous iron oxide layers by hydrothermal conditions. 2. Experimental The samples were prepared by hydrothermal reactions of 0.25 M ferric nitrate solutions in 15 ml stainless steel autoclaves lined by Te¯on (<2508C) and 10 ml stainless steel autoclaves (>2508C). Analytical grade reagent Fe(NO3)39H2O was used as the starting material. The experimental conditions are listed in Table 1. The synthesis process was as follows. An aqueous solution of 0.25 M Fe(NO3)3 was charged into an autoclave and ®lled halfway by volume. Each autoclave was sealed and immersed into the molten salt bath at the desired temperatures for a given period. After the reactions, the autoclaves were cooled in air. The products were washed with distilled water and dried in an oven at 508C. Table 1 Preparation conditions and particle sizes along four crystallographic directions for hematite nanocrystals Condition
D(1 0 4) (nm)
D(1 1 0) (nm)
D(0 2 4) (nm)
D(1 1 6) (nm)
1308C, 1508C, 1958C, 2508C, 4008C,
17.1 21.2 26.6 25.8 25.8
15.6 22.8 26.0 27.8 25.3
23.5 29.6 29.4 35.9 31.1
16.9 24.3 29.2 30.4 28.6
1h 1h 1h 1h 1h
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
951
The structures of the products were characterized by X-ray diffraction on a Ê ). The scan rate was 18/ diffractometer with Mo Ka radiation (lKa1 0:70932 A min. Particle sizes were determined by Scherrer equation. Water content in the samples was detected by TG at a heating rate of 208C/min in air. The morphologies of the particles were observed by transition electron spectroscope (TEM). The magnetic properties were measured by a vibrating-sample magnetometer at room temperature. 3. Results and discussion When the reaction was carried out at a temperature lower than 908C (not shown), no precipitate was obtained. This result is different from those of hydrolysis of ferric nitrate solutions in open systems, where a-FeOOH [6], b-FeOOH [9], or ferrihydrite FeOOH [10] is formed. Fig. 1 shows typical XRD patterns of the products by hydrothermal reactions at temperatures higher than 908C. At a temperature of 1308C, as shown in Fig. 1a, the sample showed strong diffraction peaks matching well with the standard data for a-Fe2O3 (JCPDS 33-664), while two broad peaks centered at ca. 2y of 17 and 278 demonstrated some amounts of amorphous phases. These broad peaks were located closely near to the strong peaks (1 0 4), (1 1 0), (2 1 4), and (3 0 0) for hematite. We assumed that these broad peaks were probably associated with the presence of amorphous hydrous iron oxide, Fe2O3nH2O, on the surface of hematite nanocrystals by forming a nanocomposite. This is indicated by the absence of phase separation for crystalline hematite from amorphous iron oxide and by the great agglomeration of the irregular particles shown in the TEM photograph (Fig. 2). It is reasonable that the amorphous layers could have some strong interactions with the hematite nanocrystals, however, we found that these amorphous layers were unstable. As shown in Fig. 1b and c, the diffraction intensity for the broad peaks was drastically weakened by slightly increasing reaction temperature to 2508C. As described in the following part, higher reaction temperature led to larger grain size. Therefore, the above results indicated that relative content of amorphous layers was decreased with the grain growth of hematite nanocrystals. Grain growth of hematite nanocrystals in the present systems seems to proceed via three steps: (i) homogeneous nucleation of amorphous hydrous iron oxide at a temperature lower than 1308C; (ii) formation of nanocomposite with hematite nanocrystals being modi®ed by surface layers of amorphous hydrous iron oxide; and (iii) dehydration and transformation of amorphous layers into hematite nanocrystals. The transformation process could be estimated from the variation of the particle shape of hematite nanocrystals as calculated from different crystallographic directions [11]: the compatible intensities for diffraction peaks (1 0 4) and (1 1 0) are related to spherical shape of a-Fe2O3 particles, while a large intensity difference for both lines could be indicative of an ellipsoidal particle shape. In the present work, no large intensity difference was observed for these peaks. Particle sizes of the hematite nanocrystals evaluated by Scherrer equation from several typical peaks (1 0 4), (1 1 0), (0 2 4), and (1 1 6) are shown in Table 1. We found that the particle sizes
952
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
Fig. 1. XRD patterns of the products by hydrothermal reactions of 0.25 M Fe(NO3)3 solution at (a) 1308C, (b) 2508C and (c) 4008C.
along these four crystallographic directions were similar at the given temperatures, showing evidence of close to spherical shapes. The hematite nanocrystals obtained at 1308C were smallest, while the broad peaks had a strong intensity in Fig. 1a. With increasing reaction temperature, the particles obtained from the peak (1 0 4) increased from 17.1 nm at 1308C to 26.6 nm at 1958C, and then reached a plateau at 2508C. Similar growth trends have been found for other crystallographic directions. The homogeneous grain growth of hematite nanocrystals could be rationalized in terms of the continuous dehydration and transformation of the amorphous hydrous iron oxide layers, because when the temperature was increased to 1958C, the amorphous layers probably dehydrated while the domains for the hematite nanocrystals were enlarged. Above 1958C, the amorphous layers were almost absent, and therefore, the particle size of nanocrystals did not shown any obvious change. Similar
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
953
Fig. 2. Morphology of the sample obtained at 1308C.
growth processes have been found in the products from nitrate hydrothermal systems [12]. However, as shown in Table 1, when the reaction temperature reached 4008C, the particle size was slightly reduced, which indicated a re-dissolution process at the high temperature acidic hydrothermal conditions. When the reaction systems involve some amounts of alkali species, the phase behavior of nanocrystals changes [13,14]. For example, in the presence of hydrazine, hydrazine reduction of ferric nitrate produces Fe3O4 [13], while in the case of KOH, larger spherical a-Fe2O3 particles are formed [14]. Fig. 3 shows a TG curve of the sample prepared at 1308C. It was observed that the sample lost weight continuously up to 6008C. Generally, the physically adsorbed water is evaporated at ca. 1208C, and dehydration of the lattice hydroxyl groups gives a sharp weight loss over a narrow range around 3308C [15]. Therefore, the relatively wide dehydration process observed in Fig. 3 could be due to the strongly bound water from the amorphous layers on the hematite nanocrystals. The total weight loss was about 15.6%, indicating that the amorphous layers were probably Fe2O31.64H2O. The number of hydroxyl groups in the amorphous layers was estimated to be extremely low as revealed by the absence of characteristic hydroxyl absorptions at ca. 3360 cm 1 in the corresponding infrared spectrum (not shown). This coated structure could be possible since the iron sites were octahedrally coordinated both in the crystalline hematite and in the amorphous state [8,16]. As a result of partial crystallization, the crystalline hematite particles were covered by the amorphous layers with similar chemical compositions.
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G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
Fig. 3. TG curve of the sample obtained at 1308C.
Fig. 4 shows the room temperature magnetization curve of the sample prepared at 1308C. The hysteresis loop did not reach saturation up to the maximum ®eld employed for present measurement system. The remanent magnetization was determined to be 0.046(2) emu/g, which is smaller than 0.10 emu/g for 40 nm trapezoidal a-Fe2O3 [1]. The coercivity was found to be 0.534 kOe, which is slightly larger than the reported values for a-Fe2O3 nanocrystals with few agglomerations [1]. It is known that the remanent magnetization is strongly dependent upon the particle shape, while coercivity is inversely proportional to grain size [8]. The smaller remanent magnetization for the present a-Fe2O3 nanocrystals modi®ed with amorphous surface layers was probably associated with the ®ne spherical shape of hematite nanocrystals, while the slightly larger coercivity was consistent with the relatively small particle size. Even though the magnetic property of the present hematite nanocrystals with amorphous layers did not show much enhancement of the magnetic properties compared with that of agglomeration of hematite nanocrystals [1], the ®nding of
Fig. 4. Magnetization curve of the sample obtained at 1308C.
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
955
this work should help to understand the direct formation of magnetic nanocomposites by hydrothermal conditions. Acknowledgments The TEM analyses were supported by CREST of JST (Japan Science and Technology). References [1] C. Rath, K.K. Sahu, S.D. Kulkarni, S. Anand, S.K. Date, R.P. Das, N.C. Mishra, Appl. Phys. Lett. 75 (1999) 4171. [2] C.V.G. Reddy, K.K. Seela, S.V. Manorama, Int. J. Inorg. Mater. 2 (2000) 301. [3] H. Sesigur, E. Acma, O. Addemir, A. Tekin, Mater. Res. Bull. 31 (1996) 1573. [4] L. Suber, D. Fiorani, P. Imperatori, S. Foglia, A. Montone, R. Zysler, Nanostruct. Mater. 11 (1999) 797. [5] A.A. Burukhin, B.R. Churagulov, N.N. Oleynikov, A.V. Knotko, in: Proceedings of the 6th International Symposium on Hydro. React. and 4th International Conference on Solvo-therm. React., 2P-54, Japan, 2000. [6] E. Matijevic, P. Scheiner, J. Colloid Interface Sci. 63 (1978) 509. [7] M.P. Morales, T.G. Carreno, C.J. Serna, J. Mater. Res. 7 (1992) 2538. [8] L. Li, G. Li, R.L. Smith Jr., H. Inomata, Chem. Mater. 12 (2000) 3705. [9] M. Schuttz, W. Burckhardt, S. Barth, J. Mater. Sci. 34 (1999) 2217. [10] R.A. Eggleton, R.W. Fitzpatrick, Clays Clay Miner. 36 (1988) 111. [11] M. Ocana, M.P. Morales, C.J. Serna, J. Colloid Interface Sci. 212 (1999) 317. [12] M. Hirano, Y. Fukuda, H. Iwata, Y. Hotta, M. Inagaki, J. Am. Ceram. Soc. 83 (2000) 1287. [13] C.V.G. Reddy, K.K. Seela, S.V. Manorama, Int. J. Inorg. Mater. 2 (2000) 301. [14] L. Diamandescu, D.M. Tarabasanu, N.P. Pogrion, A. Totovina, I. Bibicu, Ceram. Int. 25 (1999) 689. [15] G. Li, S. Feng, L. Li, X. Li, W. Jin, Chem. Mater. 9 (1997) 2894. [16] L. Armelao, R. Bertoncello, L. Crociani, G. Depaoli, G. Granozzi, E. Tondello, M. Bettinelli, J. Mater. Chem. 5 (1995) 79.
Preparation and magnetization of hematite nanocrystals with amorphous iron oxide layers by hydrothermal conditions G.S. Li, R.L. Smith Jr., H. Inomata*, K. Arai Department of Chemical Engineering, Research Center of Supercritical Fluid Technology, Tohoku University, Sendai 980-8579, Japan (Refereed) Received 14 March 2001; accepted 18 January 2002
Abstract Hematite nanocrystals modi®ed with surface layers of amorphous hydrous iron oxides were prepared by hydrothermal conditions in the absence of alkali. The formation temperature was found to be ca. 1308C. When the temperature was lower than 1308C, no product was formed, while above this temperature, the amount of amorphous hydrous iron oxides at the surface of hematite nanocrystals was drastically decreased. The amorphous layers on the hematite nanocrystals obtained at 1308C were determined to be Fe2O31.64H2O. The coercivity for the hematite nanocrystals with modi®ed layers was 0.534 kOe, which is slightly larger than the values for hematite nanocrystals with few agglomerations. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; D. Magnetic properties
1. Introduction Hematite nanocrystals have shown several excellent properties and found a wide ®eld of technological applications including catalysts, pigments, recording medium, and sensors. An agglomerate of hematite particles exhibits a coercivity two times higher than that of commercial hematite [1]. Several synthetic methods have been developed for preparation of hematite nanocrystals including sol±gel, hydrolysis of *
Corresponding author. Tel.: 81-22-217-7283; fax: 81-22-217-7293. E-mail address: [email protected] (H. Inomata).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 6 9 5 - 5
950
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
iron salts, and hydrothermal synthesis [2±4]. Hydrothermal synthesis has shown to be advantageous over other methods in homogeneous nucleation and grain growth of hematite nanocrystals. Preparation conditions such as concentration, reaction temperature and time are the main factors in determining the morphologies and structures of the nanocrystals prepared by hydrothermal conditions. For example, hydrothermal reactions of an aqueous iron nitrate solution at a wide concentration range produce porous hematite nanocrystals (40±80 nm) containing non-intersecting 5±20 nm pores [5], while lowering the concentration to 0.0027± 0.18 M yields some amounts of a second phase that is mainly a-FeOOH (goethite) [6]. Single phase a-Fe2O3 (hematite) can be formed in ferric chloride hydrothermal systems [7]. However, the ferric concentration has to be restricted as low as 0.02± 0.04 M. Beyond this range, second phase b-FeOOH can be readily formed. It is clear that hematite nanocrystals can be obtained by adjusting hydrothermal conditions, but nanocrystals formed in this manner tend to agglomerate. In a previous work, we [8] demonstrated that nanocomposite technology could be an effective solution to this problem with nonmagnetic amorphous silica matrix allowing homogenous grain growth of nanocrystals. It appears to be very important to study the magnetization of nanocrystals and whether they can be modi®ed by addition of magnetic layers. In this work, we report on preparation, grain growth, and magnetization of hematite nanocrystals modi®ed with amorphous hydrous iron oxide layers by hydrothermal conditions. 2. Experimental The samples were prepared by hydrothermal reactions of 0.25 M ferric nitrate solutions in 15 ml stainless steel autoclaves lined by Te¯on (<2508C) and 10 ml stainless steel autoclaves (>2508C). Analytical grade reagent Fe(NO3)39H2O was used as the starting material. The experimental conditions are listed in Table 1. The synthesis process was as follows. An aqueous solution of 0.25 M Fe(NO3)3 was charged into an autoclave and ®lled halfway by volume. Each autoclave was sealed and immersed into the molten salt bath at the desired temperatures for a given period. After the reactions, the autoclaves were cooled in air. The products were washed with distilled water and dried in an oven at 508C. Table 1 Preparation conditions and particle sizes along four crystallographic directions for hematite nanocrystals Condition
D(1 0 4) (nm)
D(1 1 0) (nm)
D(0 2 4) (nm)
D(1 1 6) (nm)
1308C, 1508C, 1958C, 2508C, 4008C,
17.1 21.2 26.6 25.8 25.8
15.6 22.8 26.0 27.8 25.3
23.5 29.6 29.4 35.9 31.1
16.9 24.3 29.2 30.4 28.6
1h 1h 1h 1h 1h
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
951
The structures of the products were characterized by X-ray diffraction on a Ê ). The scan rate was 18/ diffractometer with Mo Ka radiation (lKa1 0:70932 A min. Particle sizes were determined by Scherrer equation. Water content in the samples was detected by TG at a heating rate of 208C/min in air. The morphologies of the particles were observed by transition electron spectroscope (TEM). The magnetic properties were measured by a vibrating-sample magnetometer at room temperature. 3. Results and discussion When the reaction was carried out at a temperature lower than 908C (not shown), no precipitate was obtained. This result is different from those of hydrolysis of ferric nitrate solutions in open systems, where a-FeOOH [6], b-FeOOH [9], or ferrihydrite FeOOH [10] is formed. Fig. 1 shows typical XRD patterns of the products by hydrothermal reactions at temperatures higher than 908C. At a temperature of 1308C, as shown in Fig. 1a, the sample showed strong diffraction peaks matching well with the standard data for a-Fe2O3 (JCPDS 33-664), while two broad peaks centered at ca. 2y of 17 and 278 demonstrated some amounts of amorphous phases. These broad peaks were located closely near to the strong peaks (1 0 4), (1 1 0), (2 1 4), and (3 0 0) for hematite. We assumed that these broad peaks were probably associated with the presence of amorphous hydrous iron oxide, Fe2O3nH2O, on the surface of hematite nanocrystals by forming a nanocomposite. This is indicated by the absence of phase separation for crystalline hematite from amorphous iron oxide and by the great agglomeration of the irregular particles shown in the TEM photograph (Fig. 2). It is reasonable that the amorphous layers could have some strong interactions with the hematite nanocrystals, however, we found that these amorphous layers were unstable. As shown in Fig. 1b and c, the diffraction intensity for the broad peaks was drastically weakened by slightly increasing reaction temperature to 2508C. As described in the following part, higher reaction temperature led to larger grain size. Therefore, the above results indicated that relative content of amorphous layers was decreased with the grain growth of hematite nanocrystals. Grain growth of hematite nanocrystals in the present systems seems to proceed via three steps: (i) homogeneous nucleation of amorphous hydrous iron oxide at a temperature lower than 1308C; (ii) formation of nanocomposite with hematite nanocrystals being modi®ed by surface layers of amorphous hydrous iron oxide; and (iii) dehydration and transformation of amorphous layers into hematite nanocrystals. The transformation process could be estimated from the variation of the particle shape of hematite nanocrystals as calculated from different crystallographic directions [11]: the compatible intensities for diffraction peaks (1 0 4) and (1 1 0) are related to spherical shape of a-Fe2O3 particles, while a large intensity difference for both lines could be indicative of an ellipsoidal particle shape. In the present work, no large intensity difference was observed for these peaks. Particle sizes of the hematite nanocrystals evaluated by Scherrer equation from several typical peaks (1 0 4), (1 1 0), (0 2 4), and (1 1 6) are shown in Table 1. We found that the particle sizes
952
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
Fig. 1. XRD patterns of the products by hydrothermal reactions of 0.25 M Fe(NO3)3 solution at (a) 1308C, (b) 2508C and (c) 4008C.
along these four crystallographic directions were similar at the given temperatures, showing evidence of close to spherical shapes. The hematite nanocrystals obtained at 1308C were smallest, while the broad peaks had a strong intensity in Fig. 1a. With increasing reaction temperature, the particles obtained from the peak (1 0 4) increased from 17.1 nm at 1308C to 26.6 nm at 1958C, and then reached a plateau at 2508C. Similar growth trends have been found for other crystallographic directions. The homogeneous grain growth of hematite nanocrystals could be rationalized in terms of the continuous dehydration and transformation of the amorphous hydrous iron oxide layers, because when the temperature was increased to 1958C, the amorphous layers probably dehydrated while the domains for the hematite nanocrystals were enlarged. Above 1958C, the amorphous layers were almost absent, and therefore, the particle size of nanocrystals did not shown any obvious change. Similar
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
953
Fig. 2. Morphology of the sample obtained at 1308C.
growth processes have been found in the products from nitrate hydrothermal systems [12]. However, as shown in Table 1, when the reaction temperature reached 4008C, the particle size was slightly reduced, which indicated a re-dissolution process at the high temperature acidic hydrothermal conditions. When the reaction systems involve some amounts of alkali species, the phase behavior of nanocrystals changes [13,14]. For example, in the presence of hydrazine, hydrazine reduction of ferric nitrate produces Fe3O4 [13], while in the case of KOH, larger spherical a-Fe2O3 particles are formed [14]. Fig. 3 shows a TG curve of the sample prepared at 1308C. It was observed that the sample lost weight continuously up to 6008C. Generally, the physically adsorbed water is evaporated at ca. 1208C, and dehydration of the lattice hydroxyl groups gives a sharp weight loss over a narrow range around 3308C [15]. Therefore, the relatively wide dehydration process observed in Fig. 3 could be due to the strongly bound water from the amorphous layers on the hematite nanocrystals. The total weight loss was about 15.6%, indicating that the amorphous layers were probably Fe2O31.64H2O. The number of hydroxyl groups in the amorphous layers was estimated to be extremely low as revealed by the absence of characteristic hydroxyl absorptions at ca. 3360 cm 1 in the corresponding infrared spectrum (not shown). This coated structure could be possible since the iron sites were octahedrally coordinated both in the crystalline hematite and in the amorphous state [8,16]. As a result of partial crystallization, the crystalline hematite particles were covered by the amorphous layers with similar chemical compositions.
954
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
Fig. 3. TG curve of the sample obtained at 1308C.
Fig. 4 shows the room temperature magnetization curve of the sample prepared at 1308C. The hysteresis loop did not reach saturation up to the maximum ®eld employed for present measurement system. The remanent magnetization was determined to be 0.046(2) emu/g, which is smaller than 0.10 emu/g for 40 nm trapezoidal a-Fe2O3 [1]. The coercivity was found to be 0.534 kOe, which is slightly larger than the reported values for a-Fe2O3 nanocrystals with few agglomerations [1]. It is known that the remanent magnetization is strongly dependent upon the particle shape, while coercivity is inversely proportional to grain size [8]. The smaller remanent magnetization for the present a-Fe2O3 nanocrystals modi®ed with amorphous surface layers was probably associated with the ®ne spherical shape of hematite nanocrystals, while the slightly larger coercivity was consistent with the relatively small particle size. Even though the magnetic property of the present hematite nanocrystals with amorphous layers did not show much enhancement of the magnetic properties compared with that of agglomeration of hematite nanocrystals [1], the ®nding of
Fig. 4. Magnetization curve of the sample obtained at 1308C.
G.S. Li et al. / Materials Research Bulletin 37 (2002) 949±955
955
this work should help to understand the direct formation of magnetic nanocomposites by hydrothermal conditions. Acknowledgments The TEM analyses were supported by CREST of JST (Japan Science and Technology). References [1] C. Rath, K.K. Sahu, S.D. Kulkarni, S. Anand, S.K. Date, R.P. Das, N.C. Mishra, Appl. Phys. Lett. 75 (1999) 4171. [2] C.V.G. Reddy, K.K. Seela, S.V. Manorama, Int. J. Inorg. Mater. 2 (2000) 301. [3] H. Sesigur, E. Acma, O. Addemir, A. Tekin, Mater. Res. Bull. 31 (1996) 1573. [4] L. Suber, D. Fiorani, P. Imperatori, S. Foglia, A. Montone, R. Zysler, Nanostruct. Mater. 11 (1999) 797. [5] A.A. Burukhin, B.R. Churagulov, N.N. Oleynikov, A.V. Knotko, in: Proceedings of the 6th International Symposium on Hydro. React. and 4th International Conference on Solvo-therm. React., 2P-54, Japan, 2000. [6] E. Matijevic, P. Scheiner, J. Colloid Interface Sci. 63 (1978) 509. [7] M.P. Morales, T.G. Carreno, C.J. Serna, J. Mater. Res. 7 (1992) 2538. [8] L. Li, G. Li, R.L. Smith Jr., H. Inomata, Chem. Mater. 12 (2000) 3705. [9] M. Schuttz, W. Burckhardt, S. Barth, J. Mater. Sci. 34 (1999) 2217. [10] R.A. Eggleton, R.W. Fitzpatrick, Clays Clay Miner. 36 (1988) 111. [11] M. Ocana, M.P. Morales, C.J. Serna, J. Colloid Interface Sci. 212 (1999) 317. [12] M. Hirano, Y. Fukuda, H. Iwata, Y. Hotta, M. Inagaki, J. Am. Ceram. Soc. 83 (2000) 1287. [13] C.V.G. Reddy, K.K. Seela, S.V. Manorama, Int. J. Inorg. Mater. 2 (2000) 301. [14] L. Diamandescu, D.M. Tarabasanu, N.P. Pogrion, A. Totovina, I. Bibicu, Ceram. Int. 25 (1999) 689. [15] G. Li, S. Feng, L. Li, X. Li, W. Jin, Chem. Mater. 9 (1997) 2894. [16] L. Armelao, R. Bertoncello, L. Crociani, G. Depaoli, G. Granozzi, E. Tondello, M. Bettinelli, J. Mater. Chem. 5 (1995) 79.