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Phase transformation of β-Fe2O3 hollow nanoparticles
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Materials Letters 62 (2008) 561 – 563 www.elsevier.com/locate/matlet
Phase transformation of β-Fe2O3 hollow nanoparticles Chang-Woo Lee, Sung-Soo Jung, Jai-Sung Lee ⁎ Department of Materials Engineering, Hanyang University, 426-791 Ansan, Korea Received 15 August 2007; accepted 27 August 2007 Available online 4 September 2007
Abstract The phase transformation of β-Fe2O3 with a hollow nanostructure was first investigated by in-situ X-ray diffractometry (XRD) at elevated temperatures. As a starting material, the β-Fe2O3 hollow nanoparticles (HNP) less than 40 nm in size were prepared by chemical vapor condensation (CVC) process. The XRD analysis revealed that the initial β-Fe2O3 phase was transformed to γ-Fe2O3 at 500 °C, and no further transformation from γ-Fe2O3 to α-Fe2O3 was observed up to 800 °C. The experimental findings indicate that the increase of surface area derived from the hollow structure is responsible for the decrease of the transformation temperature and the stability of the γ-Fe2O3 phase at high temperatures. © 2007 Elsevier B.V. All rights reserved. Keywords: Hollow structure; Nanomaterials; β-Fe2O3; Phase transformation; Surface
1. Introduction Since Bonnevie-Svendsen first reported in 1956, much research has focused on revealing the intrinsic properties of βFe2O3 phase in particles or film [1–10]. In particular, the phase transformation of β-Fe2O3 has been studied intensively, because the β-Fe2O3 has been known as an unstable intermediate phase which existed in the evolution to the α-Fe2O3, [2–7]. Thus, such efforts have been made to determine the temporarily existing β-Fe2O3 exactly during the phase transformation or the synthesis of α-Fe2O3. Among the previous research, Gonzalez-Carreno et al.'s report on the β-Fe2O3 explained that the co-existing γ-Fe2O3 hollow particles of 150– 300 nm in size could persist even after a heat-treatment at 900 °C due to the large surface area from hollow structure besides the transformation of β-Fe2O3 to α-Fe2O3 at 500 °C [6]. In this sense, the β-Fe2O3 hollow nanoparticles (HNP) recently reported by the authors can be considered as a suitable candidate in understanding the particle properties relying on
⁎ Corresponding author. Tel.: +82 31 400 5225; fax: +82 31 406 5170. E-mail addresses: [email protected] (C.-W. Lee), [email protected] (S.-S. Jung), [email protected] (J.-S. Lee). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.08.073
surface area [11,12]. Also, some abnormal characteristics, such as a decreased transformation temperature, are predicted from the hollow nanostructure that is 20–40 nm in size, consisting of several polycrystalline shells. From this point of view, the change of the β-Fe2O3 phase at elevated temperatures up to 800 °C was investigated by in-situ XRD measurement and discussed in terms of the surface area effect of a hollow nanostructure on a phase transformation in this study. 2. Experimental β-Fe2O3 HNP were synthesized by a chemical vapor condensation (CVC) process at 800 °C and 400 mbar using a precursor of iron tris acetylacetonate as described in the previous literatures [11,12]. As shown in Fig. 1a, the HNP have a size distribution in the range of 20–40 nm and shell thickness of 3–5 nm. Also, the XRD pattern in Fig. 1b reveals that the HNP consist of only β-Fe2O3 phase [13]. In order to investigate the phase transformation of β-Fe2O3 HNP depending on temperature, the as-received powder of β-Fe2O3 HNP was subjected to in-situ X-ray diffraction (XRD, RU-200B) experiments. A small amount of powder was mounted on a platinum strip, which acted as the sample holder as well as the heating element. The XRD patterns were scanned in the 2θ range 25–45°, with a step size of 0.04° and a rate of 4° min− 1. Heat treatment was carried out up to 800 °C in an Ar atmosphere with a heating rate of 10 °C min− 1, and a
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C.-W. Lee et al. / Materials Letters 62 (2008) 561–563
Fig. 1. (a) TEM micrograph and (b) XRD pattern of β-Fe2O3 hollow nanoparticles (HNP) synthesized at 800 °C and 400 mbar by a chemical vapor condensation process.
measuring time of 20 min at each measuring temperature was employed. 3. Results and discussion Fig. 2 shows the XRD patterns selected at temperatures above 400 °C. From the patterns, it was found that hollow nanoparticles still had β-Fe2O3 as their major crystal phase up to 400 °C, even though the γ-Fe2O3 peak arose at 400 °C. The initial β-Fe2O3 phase was formed at the optimized condition, 800 °C and 400 mbar in a CVC reactor directly from the decomposition of the metal acetylacetonate precursor [11,12]. Thus, the small peak of γ-Fe2O3 in the pattern for the heattreatment at 400 °C was interpreted as evidence of the beginning of the phase transformation of β-Fe2O3. At 500 °C, the β-Fe2O3 peak vanished and, instead, the peak intensity of γ-Fe2O3 increased [14]. As a major phase, the γ-Fe2O3 still remained after the heat-treatment up to 800 °C and there was no evidence of further phase evolution to αFe2O3. Here, an interesting result on the phase transformation of the βFe2O3 is demonstrated. The β-Fe2O3 has been known to be transformed to α-Fe2O3 in the temperature range of 400–600 °C from the previous reports (Table 1) [2,3,5–8]. In those reports, however, α-Fe2O3 was considered just as a final phase recognized after heat-treatment, and no stepwise analysis during the phase evolution of β-Fe2O3 was carried out. Thus, in this study, it is thought that the βFe2O3 was possibly on a transformation to the final α-Fe2O3 phase while passing through the intermediate γ-Fe2O3 phase. Considering the XRD patterns in Fig. 2, it was found that the phase transformation of HNP from β-Fe2O3 to γ-Fe2O3 occurred nearly at 400 °C. Among the previous reports in Table 1, the β-Fe2O3 phase with a grain size of 30–60 nm only had the phase transformation temperature at 400 °C, which was in accordance with the present work [7]. Therefore, it was understood that the smaller grain size of the β-Fe2O3 phase results in a lower transformation temperature, regardless of particle shape. Also, considering the phase transformation of γ-Fe2O3 to α-Fe2O3, which generally occurs from 400–700 °C [6,15–17], it was astonishing that γ-Fe2O3 remained at 800 °C without any sign of phase evolution to α-Fe2O3. For this problem, Gonzalez-Carreno et al.'s study on the phase transformation of 150–300 nm grain-sized γ-Fe2O3 provides us with an inspiring experimental result [6]. In a manner which is similar to the present work, they also confirmed that the γ-Fe2O3 was not transformed to α-Fe2O3 at 900 °C. Thus, the 102 times larger surface
area, at least compared to Gonzalez-Carreno's study, gives us clear evidence for the contribution of the increased surface area of hollow nanostructures below 100 nm to the thermal stability of γ-Fe2O3 at high temperatures. Namely, after the first phase transformation from βFe2O3 to γ-Fe2O3, it is understood that heat energy was mostly consumed to reduce the surface area of hollow particles. Then, additional heat energy from extending heat-treatment time or elevating temperature might be necessary for the next phase transformation step from γ-Fe2O3 to α-Fe2O3. Thus, it is concluded that the tremendous surface area from the hollow nanostructure played a very important role in ruling the phase evolution and phase transformation temperature of the β-Fe2O3 phase. In addition, the above results make us predict that the synthesis of sizecontrolled γ-Fe2O3 nanoparticles will be feasible by using a simple phase transformation of β-Fe2O3 nanoparticles and a tremendous surface area from a hollow structure.
4. Conclusion The effects of extremely increased surface area on the phase transformation behavior of β-Fe2O3 phase with a hollow nanostructure were first explained. The phase transformation of
Fig. 2. In-situ XRD patterns of β-Fe2O3 hollow nanoparticle (HNP) sample selected at 400, 500, 700, and 800 °C during heat-treatment up to 800 °C.
C.-W. Lee et al. / Materials Letters 62 (2008) 561–563 Table 1 Previous reports on the phase transformation of the β-Fe2O3 phase Direction of transformation
Temperature (°C)
Shape
Reference no.
∼ 5000
Polycrystalline thin film Cubic
[2]
500
β→α
∼ 600
β→α
∼ 500
β→α
500
β→α
400
150– 300 30–60 a
β→α β→γ
600 ∼ 400
b4000 a 20–40
a
References
Size (nm)
β→α
500– 1000 –
563
[3]
Polycrystalline thin film Hollow
[5]
Irregular, solid a – Hollow
[7]
[6]
[8] Present work
Shape or size analyzed after heat-treatment or synthesis.
the β-Fe2O3 hollow nanoparticles (HNP) was examined by insitu XRD up to 800 °C. The XRD results showed that the conventionally well-known phase transformation of β-Fe2O3 to α-Fe2O3 was reappraised as a β-Fe2O3 to γ-Fe2O3 transformation by an increase of surface area, typically when the β-Fe2O3 phase is in a hollow nanostructure. Also, it was found that the reduction of grain size of the β-Fe2O3 phase can lower the phase transformation temperature. Acknowledgement This research was supported by the ministry of education and human resources development through the brain Korea 21 (BK 21) program.
[1] Bonnevie-Svendsen, Naturwissenschaften 45 (1958) 542. [2] L. Ben-Dor, E. Fischbein, I. Felner, Z. Kalman, J. Electrochem. Soc. 124 (1997) 451–457. [3] Y. Ikeda, M. Takano, Y. Bando, Bull. Inst. Chem. Res. Kyoto. Univ. 64 (1986) 249–258. [4] E.R. Bauminger, et al., Physica. B. 86–88 (1977) 910–912. [5] L. Ben-Dor, E. Fischbein, Acta. Crystallogr. B32 (1976) 667. [6] T. Gonzalez-Carreno, M.P. Morales, C.J. Serna, J. Mater. Sci. Lett. 13 (1994) 381–382. [7] Y.S. Han, S.M. Yoon, D.K. Kim, Bull. Korean. Chem. Soc. 21 (2000) 1193–1198. [8] R. Zboril, M. Mashlan, D. Krausova, P. Pikal, Hyperfine. Interact. 120 (121) (1999) 497–501. [9] D. Wiarda, G. Weyer, Int. J. Mod. Phys. B. 7 (1993) 353–356. [10] T. Maruyama, T. Kanagawa, J. Electrochem. Soc. 143 (1996) 1675–1677. [11] J.S. Lee, et al., J. Nanopart. Res. 6 (2004) 627–631. [12] C.W. Lee, S.G. Kim, J.S. Lee, Key. Eng. Mater. 317 (318) (2006) 219–222. [13] Powder diffraction file, card 39-0238, JCPDS-ICDD, Swarthmore, PA, 1997. [14] Powder diffraction file, card 39-1346, JCPDS-ICDD, Swarthmore, PA, 1997. [15] G. Ennas, et al., Chem. Mater. 10 (1998) 495–502. [16] S. Chakrabarti, S. Chaudhuri, P.M.G. Nambissan, Phys. Rev. B. 71 (2005) 064105. [17] R. Zboril, M. Mashlan, D. Petridis, Chem. Mater. 14 (2002) 969–982.
Materials Letters 62 (2008) 561 – 563 www.elsevier.com/locate/matlet
Phase transformation of β-Fe2O3 hollow nanoparticles Chang-Woo Lee, Sung-Soo Jung, Jai-Sung Lee ⁎ Department of Materials Engineering, Hanyang University, 426-791 Ansan, Korea Received 15 August 2007; accepted 27 August 2007 Available online 4 September 2007
Abstract The phase transformation of β-Fe2O3 with a hollow nanostructure was first investigated by in-situ X-ray diffractometry (XRD) at elevated temperatures. As a starting material, the β-Fe2O3 hollow nanoparticles (HNP) less than 40 nm in size were prepared by chemical vapor condensation (CVC) process. The XRD analysis revealed that the initial β-Fe2O3 phase was transformed to γ-Fe2O3 at 500 °C, and no further transformation from γ-Fe2O3 to α-Fe2O3 was observed up to 800 °C. The experimental findings indicate that the increase of surface area derived from the hollow structure is responsible for the decrease of the transformation temperature and the stability of the γ-Fe2O3 phase at high temperatures. © 2007 Elsevier B.V. All rights reserved. Keywords: Hollow structure; Nanomaterials; β-Fe2O3; Phase transformation; Surface
1. Introduction Since Bonnevie-Svendsen first reported in 1956, much research has focused on revealing the intrinsic properties of βFe2O3 phase in particles or film [1–10]. In particular, the phase transformation of β-Fe2O3 has been studied intensively, because the β-Fe2O3 has been known as an unstable intermediate phase which existed in the evolution to the α-Fe2O3, [2–7]. Thus, such efforts have been made to determine the temporarily existing β-Fe2O3 exactly during the phase transformation or the synthesis of α-Fe2O3. Among the previous research, Gonzalez-Carreno et al.'s report on the β-Fe2O3 explained that the co-existing γ-Fe2O3 hollow particles of 150– 300 nm in size could persist even after a heat-treatment at 900 °C due to the large surface area from hollow structure besides the transformation of β-Fe2O3 to α-Fe2O3 at 500 °C [6]. In this sense, the β-Fe2O3 hollow nanoparticles (HNP) recently reported by the authors can be considered as a suitable candidate in understanding the particle properties relying on
⁎ Corresponding author. Tel.: +82 31 400 5225; fax: +82 31 406 5170. E-mail addresses: [email protected] (C.-W. Lee), [email protected] (S.-S. Jung), [email protected] (J.-S. Lee). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.08.073
surface area [11,12]. Also, some abnormal characteristics, such as a decreased transformation temperature, are predicted from the hollow nanostructure that is 20–40 nm in size, consisting of several polycrystalline shells. From this point of view, the change of the β-Fe2O3 phase at elevated temperatures up to 800 °C was investigated by in-situ XRD measurement and discussed in terms of the surface area effect of a hollow nanostructure on a phase transformation in this study. 2. Experimental β-Fe2O3 HNP were synthesized by a chemical vapor condensation (CVC) process at 800 °C and 400 mbar using a precursor of iron tris acetylacetonate as described in the previous literatures [11,12]. As shown in Fig. 1a, the HNP have a size distribution in the range of 20–40 nm and shell thickness of 3–5 nm. Also, the XRD pattern in Fig. 1b reveals that the HNP consist of only β-Fe2O3 phase [13]. In order to investigate the phase transformation of β-Fe2O3 HNP depending on temperature, the as-received powder of β-Fe2O3 HNP was subjected to in-situ X-ray diffraction (XRD, RU-200B) experiments. A small amount of powder was mounted on a platinum strip, which acted as the sample holder as well as the heating element. The XRD patterns were scanned in the 2θ range 25–45°, with a step size of 0.04° and a rate of 4° min− 1. Heat treatment was carried out up to 800 °C in an Ar atmosphere with a heating rate of 10 °C min− 1, and a
562
C.-W. Lee et al. / Materials Letters 62 (2008) 561–563
Fig. 1. (a) TEM micrograph and (b) XRD pattern of β-Fe2O3 hollow nanoparticles (HNP) synthesized at 800 °C and 400 mbar by a chemical vapor condensation process.
measuring time of 20 min at each measuring temperature was employed. 3. Results and discussion Fig. 2 shows the XRD patterns selected at temperatures above 400 °C. From the patterns, it was found that hollow nanoparticles still had β-Fe2O3 as their major crystal phase up to 400 °C, even though the γ-Fe2O3 peak arose at 400 °C. The initial β-Fe2O3 phase was formed at the optimized condition, 800 °C and 400 mbar in a CVC reactor directly from the decomposition of the metal acetylacetonate precursor [11,12]. Thus, the small peak of γ-Fe2O3 in the pattern for the heattreatment at 400 °C was interpreted as evidence of the beginning of the phase transformation of β-Fe2O3. At 500 °C, the β-Fe2O3 peak vanished and, instead, the peak intensity of γ-Fe2O3 increased [14]. As a major phase, the γ-Fe2O3 still remained after the heat-treatment up to 800 °C and there was no evidence of further phase evolution to αFe2O3. Here, an interesting result on the phase transformation of the βFe2O3 is demonstrated. The β-Fe2O3 has been known to be transformed to α-Fe2O3 in the temperature range of 400–600 °C from the previous reports (Table 1) [2,3,5–8]. In those reports, however, α-Fe2O3 was considered just as a final phase recognized after heat-treatment, and no stepwise analysis during the phase evolution of β-Fe2O3 was carried out. Thus, in this study, it is thought that the βFe2O3 was possibly on a transformation to the final α-Fe2O3 phase while passing through the intermediate γ-Fe2O3 phase. Considering the XRD patterns in Fig. 2, it was found that the phase transformation of HNP from β-Fe2O3 to γ-Fe2O3 occurred nearly at 400 °C. Among the previous reports in Table 1, the β-Fe2O3 phase with a grain size of 30–60 nm only had the phase transformation temperature at 400 °C, which was in accordance with the present work [7]. Therefore, it was understood that the smaller grain size of the β-Fe2O3 phase results in a lower transformation temperature, regardless of particle shape. Also, considering the phase transformation of γ-Fe2O3 to α-Fe2O3, which generally occurs from 400–700 °C [6,15–17], it was astonishing that γ-Fe2O3 remained at 800 °C without any sign of phase evolution to α-Fe2O3. For this problem, Gonzalez-Carreno et al.'s study on the phase transformation of 150–300 nm grain-sized γ-Fe2O3 provides us with an inspiring experimental result [6]. In a manner which is similar to the present work, they also confirmed that the γ-Fe2O3 was not transformed to α-Fe2O3 at 900 °C. Thus, the 102 times larger surface
area, at least compared to Gonzalez-Carreno's study, gives us clear evidence for the contribution of the increased surface area of hollow nanostructures below 100 nm to the thermal stability of γ-Fe2O3 at high temperatures. Namely, after the first phase transformation from βFe2O3 to γ-Fe2O3, it is understood that heat energy was mostly consumed to reduce the surface area of hollow particles. Then, additional heat energy from extending heat-treatment time or elevating temperature might be necessary for the next phase transformation step from γ-Fe2O3 to α-Fe2O3. Thus, it is concluded that the tremendous surface area from the hollow nanostructure played a very important role in ruling the phase evolution and phase transformation temperature of the β-Fe2O3 phase. In addition, the above results make us predict that the synthesis of sizecontrolled γ-Fe2O3 nanoparticles will be feasible by using a simple phase transformation of β-Fe2O3 nanoparticles and a tremendous surface area from a hollow structure.
4. Conclusion The effects of extremely increased surface area on the phase transformation behavior of β-Fe2O3 phase with a hollow nanostructure were first explained. The phase transformation of
Fig. 2. In-situ XRD patterns of β-Fe2O3 hollow nanoparticle (HNP) sample selected at 400, 500, 700, and 800 °C during heat-treatment up to 800 °C.
C.-W. Lee et al. / Materials Letters 62 (2008) 561–563 Table 1 Previous reports on the phase transformation of the β-Fe2O3 phase Direction of transformation
Temperature (°C)
Shape
Reference no.
∼ 5000
Polycrystalline thin film Cubic
[2]
500
β→α
∼ 600
β→α
∼ 500
β→α
500
β→α
400
150– 300 30–60 a
β→α β→γ
600 ∼ 400
b4000 a 20–40
a
References
Size (nm)
β→α
500– 1000 –
563
[3]
Polycrystalline thin film Hollow
[5]
Irregular, solid a – Hollow
[7]
[6]
[8] Present work
Shape or size analyzed after heat-treatment or synthesis.
the β-Fe2O3 hollow nanoparticles (HNP) was examined by insitu XRD up to 800 °C. The XRD results showed that the conventionally well-known phase transformation of β-Fe2O3 to α-Fe2O3 was reappraised as a β-Fe2O3 to γ-Fe2O3 transformation by an increase of surface area, typically when the β-Fe2O3 phase is in a hollow nanostructure. Also, it was found that the reduction of grain size of the β-Fe2O3 phase can lower the phase transformation temperature. Acknowledgement This research was supported by the ministry of education and human resources development through the brain Korea 21 (BK 21) program.
[1] Bonnevie-Svendsen, Naturwissenschaften 45 (1958) 542. [2] L. Ben-Dor, E. Fischbein, I. Felner, Z. Kalman, J. Electrochem. Soc. 124 (1997) 451–457. [3] Y. Ikeda, M. Takano, Y. Bando, Bull. Inst. Chem. Res. Kyoto. Univ. 64 (1986) 249–258. [4] E.R. Bauminger, et al., Physica. B. 86–88 (1977) 910–912. [5] L. Ben-Dor, E. Fischbein, Acta. Crystallogr. B32 (1976) 667. [6] T. Gonzalez-Carreno, M.P. Morales, C.J. Serna, J. Mater. Sci. Lett. 13 (1994) 381–382. [7] Y.S. Han, S.M. Yoon, D.K. Kim, Bull. Korean. Chem. Soc. 21 (2000) 1193–1198. [8] R. Zboril, M. Mashlan, D. Krausova, P. Pikal, Hyperfine. Interact. 120 (121) (1999) 497–501. [9] D. Wiarda, G. Weyer, Int. J. Mod. Phys. B. 7 (1993) 353–356. [10] T. Maruyama, T. Kanagawa, J. Electrochem. Soc. 143 (1996) 1675–1677. [11] J.S. Lee, et al., J. Nanopart. Res. 6 (2004) 627–631. [12] C.W. Lee, S.G. Kim, J.S. Lee, Key. Eng. Mater. 317 (318) (2006) 219–222. [13] Powder diffraction file, card 39-0238, JCPDS-ICDD, Swarthmore, PA, 1997. [14] Powder diffraction file, card 39-1346, JCPDS-ICDD, Swarthmore, PA, 1997. [15] G. Ennas, et al., Chem. Mater. 10 (1998) 495–502. [16] S. Chakrabarti, S. Chaudhuri, P.M.G. Nambissan, Phys. Rev. B. 71 (2005) 064105. [17] R. Zboril, M. Mashlan, D. Petridis, Chem. Mater. 14 (2002) 969–982.