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Hydrothermal synthesis and characterization of uniform α-FeOOH nanowires in high yield
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Materials Letters 62 (2008) 914 – 917 www.elsevier.com/locate/matlet
Hydrothermal synthesis and characterization of uniform α-FeOOH nanowires in high yield Ping Ou, Gang Xu, Zhaohui Ren, Xiaohong Hou, Gaorong Han ⁎ State Key Lab of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 23 April 2007; accepted 7 July 2007 Available online 13 July 2007
Abstract Large-scale α-FeOOH nanowires with uniform diameter and high aspect ratios were synthesized via hydrothermal reaction at 100 °C. The obtained α-FeOOH nanowire was of diameter of 80 nm and length up to 1.2 μm. X-ray diffraction (XRD) results show that the nanowires are of orthorhombic structure, and Fourier transform infrared (FT-IR) analysis further confirms the formation of orthorhombic phase α-FeOOH. Highresolution TEM (HRTEM) studies indicate the single-crystalline nature of α-FeOOH nanowires with an oriented growth along the [001] axis direction. Based on the results of contrastive experiments, the possible mechanism for hydrothermal synthesis of α-FeOOH nanowires was also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; α-FeOOH nanowires; Crystal structure; Characterization methods
1. Introduction In the last decade the hydrothermal processing has been widely used for preparations of crystalline ceramics as an alternate or new approach because it is essentially less energy intensive, less polluting and leads to high homogeneity and well-crystallized products [1]. Iron oxides are used in a wide variety of applications, including pigments and magnetic media. In most commercial processes, goethite (α-FeOOH) particles can be used as a pigment and are produced as an intermediate in the iron oxide preparations [2–5]. The goethite crystals grow as the form of acicular needles about 1–1.5 μm length with an aspect ratio of 10–15. For magnetic media, the desired product should be of uniform length and same but high aspect ratio. The goethite is then reacted through a series of gas–solid reactions to form γ-Fe2O3. The goethite particle morphology is a critical factor in determining the magnetic properties of the γ-Fe2O3 particles [6]. At the present time, the method which prepared α-FeOOH is air oxidation in industry. Acid conditions and alkaline ⁎ Corresponding author. Tel.: +86 571 87951649; fax: +86 571 87952341. E-mail address: [email protected] (G. Han). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.010
conditions are included in the method. There are two stages in the formation of goethite under alkaline conditions [7]. In step one, ferrous sulfate (or any other ferrous salt) is mixed with excess NaOH (or some other base such as KOH) so that ferrous hydroxide precipitate immediately forms. The amount of excess NaOH used is between 15 and 200% above the stoichiometric requirements. In step two, an oxygen-containing gas is bubbled through the slurry to form α-FeOOH particles for controlling the synthesis conditions. The mechanisms involved in the oxidation reaction and crystal growth are very complex and generally not well understood [8–12]. In many examples of particle formations, the morphology control has been individually met based on many years of experience [11,12]. In this paper, we proposed a facile route for fabricating α-FeOOH nanowires via a low-temperature hydrothermal method. The α-FeOOH nanowires could be obtained with high yield and of good reproducibility. 2. Experimental In the experimental procedure, 4.85 g Fe(NO3)3·9H2O and 2.73 g KOH were dissolved in 10 ml distilled water, respectively, and then, the latter solution was dropped slowly into the latter
P. Ou et al. / Materials Letters 62 (2008) 914–917
915
Fig. 1. XRD pattern of the as-prepared α-FeOOH nanowires.
solution under vigorous stirring. The resulting mixture was loaded into a 50 ml-Telfon-lined autoclave, which was then filled with distilled water up to 80% of the total volume. The autoclave was sealed and maintained at 100 °C for 6 h. After the reaction was completed, the autoclave was cooled to room temperature naturally, and the resulting solid products were filtered off, washed with absolute ethanol and distilled water for several times, and then dried in vacuum at 60 °C for 12 h. The products were characterized by X-ray powder diffraction (XRD) using a Rigaku D/max-RA X-ray diffractometer with CuKa radiation (λ = 1.5406 Å). The product was further investigated using Fourier transform infrared (FT-IR) spectroscopy (AVATAR 360). Field emission scanning electron microscopy (FESEM) images were taken with a JEOL100CX scanning electron microscope. The high-resolution transmission electron microscope (HRTEM) image was taken with a JEM2010 TEM with an accelerating voltage of 200 kV. 3. Results and discussion Fig. 1 shows the XRD pattern of the as-synthesized products. All the diffraction peaks in Fig. 1 can be indexed to pure orthorhombic
Fig. 2. FT-IR spectrum of the as-prepared α-FeOOH nanowires.
Fig. 3. (a) The low-magnification SEM image of the as-prepared α-FeOOH nanowires. (b) The high-magnification SEM image of the as-prepared α-FeOOH nanowires.
structure α-FeOOH with lattice constants of a = 4.608 Å, b = 9.956 Å, c = 3.0215 Å, which are consistent with the values in the standard card (JCPDS no. 29-0713). The strong and sharp diffraction peaks indicate the highly crystalline nature of the α-FeOOH nanowires. No other impurities have been found in the synthesized products. The representative FT-IR spectrum (Fig. 2) of the products indicates the formation of FeOOH since the main absorption bands are in good agreement with that of the standard spectrum of FeOOH [13]. The absorption bands at around 3415 and 637 cm− 1 are the characteristics of OH− 1 ions, and the bands at 1646, 1530, 1386, 1050 and 892 cm− 1 represent the Fe–O vibrational mode in FeOOH. The FT-IR spectrum shows, therefore, that the nanowire products obtained in solutions are predominantly α-FeOOH. The morphology of the as-obtained products were examined by the field emission scanning electron microscopy (FESEM), whereby the solid sample was mounted on a copper slice after ultrasonic dispersion treatment. The low-magnification FESEM image in Fig. 3a shows that the products consist almost entirely of such α-FeOOH 1-D nanostructures, and this indicates the high yield and good uniformity achieved with this approach. The high-magnification SEM image in Fig. 3b reveals that the nanowires with a diameter of 80 nm and length of 1.2 μm are relatively straight. The product was further investigated by high-resolution transmission electron microscopy (HRTEM), whereby the sample was treated by ultrasonic dispersion in ethanol for 30 min and then put onto a copper grid. Fig. 4a shows a representative HRTEM image of an
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P. Ou et al. / Materials Letters 62 (2008) 914–917
nanowire is not crystalline well. This phenomenon was also observed on the flank fringe of α-FeOOH nanowire in Fig. 4c. In the case of oxidation of iron, the first phase to precipitate in hydrolysis of Fe3+ ions is usually ferrihydrite, a poorly crystalline oxide, of rough formula 5Fe2O3, 9H2O [14]. Nevertheless, the exact mechanisms of formation of goethite from the precursor ferrihydrite are not clearly established. Schwertmann et al. have revealed that the pH values of solution play the important role in the process of the transformation of ferrihydrite into goethite [15]. According to Schwertmann's study, in alkaline solutions, the dissolution of ferrihydrite follows the equilibrium Eq. (1). 5Fe2 O3 ; 9H2 OðsÞ þ ð10n−30ÞOH−ðaqÞ
ðn−3Þ−
þ ð66−10nÞH2 O↔10½FeðOHÞn ðH2 OÞð6−nÞ ðaqÞ dð3bn≤6Þ:
ð1Þ
In our experiments, the pH value of solution we measured is around 13. So we think that the value of n is 6 at very alkaline pH and ferrihydrite is dissolved under the complex anions, Fe (OH)3− 6 (n = 6) eventually. So at appropriate temperature, the precursor Fe (OH)3− 6 were condensated by olation and oxolation processes between OH and H2O ligands, which give rise to Fe–OH–Fe and Fe–O–Fe bridges, formed the nucleation of goethite. And then other precursor Fe (OH)3− 6 were aggradated on the nucleation of goethite, lead to the growth of goethite. The fringes of an individual nanowire are not intact, which were observed in Fig. 4b and c, can be explained by the precursor Fe (OH)3− 6 aggradated from in to out in the growth process of goethite. The inconsistent growth rate along different facets can be explained by the “lowest energy” argument resulting in needlike crystals. The nanowires morphology is thermodynamically favored as it allows the extension of the higher energy surfaces with respect to lower ones [16]. This is advantageous for the growth of α-FeOOH nanowires along [001] axis.
4. Conclusions
Fig. 4. (a) HRTEM image and electron diffraction pattern of an individual nanowire. (b) HRTEM image of the vegetal head of an individual nanowire. In panel b, the spot indicated by line arrow shows the initial growth stage on the vegetal head of an individual nanowire. (c) HRTEM image of the flank fringe of an individual nanowire. In panel c, the spot indicated by line arrow shows the initial growth stage on the flank fringe of an individual nanowire.
individual nanowire, clearly revealing that the as-obtained nanowires are structurally uniform and crystalline. The interdistance of lattice planes is approximately 4.2 Å, which can be indexed to the (110) plane. Meanwhile, the electron diffraction pattern (inset in Fig. 4a) can be indexed to the [1–10] zone axis of orthorhombic α-FeOOH, further confirming that the nanowires are crystalline and grow along [001] axis. Fig. 4b shows a typical HRTEM image of the vegetal head of α-FeOOH nanowire, which reveals that the vegetal head of α-FeOOH
In this paper, we report large-yield synthesis of α-FeOOH nanowires with high aspect ratios using Fe(NO3)3·9H2O and KOH as starting reactants under hydrothermal conditions at 100 °C. Structural characterization by XRD revealed that the α-FeOOH product has an orthorhombic structure. Morphology examination by FESEM revealed that the orthorhombic α-FeOOH product has a nanowire structure with diameter of 80 nm and length up to 1.2 μm, and HRTEM and the corresponding electron diffraction investigation showed that the product is of single-crystallinity. The present method is simple, mild, low-cost and large-production, which will be used to synthesize homogeneous nanostructures of other nanowire oxides at relatively low temperature and realize industrial-scale synthesis. Acknowledgment The work is supported by the Chinese National Foundation of Natural Science Research (50452003). References [1] L. Diamandescu, D. Mihă. ilă. i-Tă. ră. bă. isanu, N. Popescu-Pogrion, Mater. Lett. 27 (1996) 253–257. [2] M.P. Sharrock, R.E. Bodnar, J. Appl. Phys. 57 (1985) 3919.
P. Ou et al. / Materials Letters 62 (2008) 914–917 [3] L.M. Bennetch, H.S. Greiner, K.R. Hancock, M. Hoffman, U. S. Patent 3904540, 1975. [4] H. Sesigur, E. Acma, O. Addemir, A. Tekin, Mater. Res. Bull. 31 (1996) 1573. [5] H. Kopke, M. Ohlinger, W. Grau, E. Schoenafinger, H.H. Schneehage, U. S. Patent 4059716, 1977. [6] E.P. Wohlfarth, Ferromagnetic Materials: a Handbook of the Properties of Magnetically Ordered Substances, vol. 2, Elsevier, North-Holland, Amsterdam, 1980. [7] T. Misawa, K. Hashimoto, S. Shimodara, Corros. Sci. 14 (1974) 131. [8] W. Feitknecht, Z. Elektrochem. 63 (1959) 34.
[9] [10] [11] [12] [13] [14] [15] [16]
917
Y. Tamaura, M. Chem. Soc. Dalton Trans. 197 (1981) 1807. W. Feitknecht, P. Schindler, Pure Appl. Chem. 6 (1963) 134. E. Sada, H. Kumazawa, M. Aoyama, Chem. Eng. Commun. 71 (1988) 73. D.L. O'Connor, M.P. Dudukovic, P.A. Ramachandran, Ind. Eng. Chem. Res. 31 (1992) 2516. R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, San Diego, CA, 1971. M. Loan, G.M. Parkinson, W.R. Richmond, Am. Mineral. 90 (2005) 258. U. Schwertmann, H. Stanjek, H.H. Becher, Clay Miner. 39 (2004) 433. B.S.J. Michael, G. Ali, H. Tobias, E.S. Aaron, L. Frank, A.K. Brian, J. Am. Chem. Soc. 125 (2003) 16050.
Materials Letters 62 (2008) 914 – 917 www.elsevier.com/locate/matlet
Hydrothermal synthesis and characterization of uniform α-FeOOH nanowires in high yield Ping Ou, Gang Xu, Zhaohui Ren, Xiaohong Hou, Gaorong Han ⁎ State Key Lab of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 23 April 2007; accepted 7 July 2007 Available online 13 July 2007
Abstract Large-scale α-FeOOH nanowires with uniform diameter and high aspect ratios were synthesized via hydrothermal reaction at 100 °C. The obtained α-FeOOH nanowire was of diameter of 80 nm and length up to 1.2 μm. X-ray diffraction (XRD) results show that the nanowires are of orthorhombic structure, and Fourier transform infrared (FT-IR) analysis further confirms the formation of orthorhombic phase α-FeOOH. Highresolution TEM (HRTEM) studies indicate the single-crystalline nature of α-FeOOH nanowires with an oriented growth along the [001] axis direction. Based on the results of contrastive experiments, the possible mechanism for hydrothermal synthesis of α-FeOOH nanowires was also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; α-FeOOH nanowires; Crystal structure; Characterization methods
1. Introduction In the last decade the hydrothermal processing has been widely used for preparations of crystalline ceramics as an alternate or new approach because it is essentially less energy intensive, less polluting and leads to high homogeneity and well-crystallized products [1]. Iron oxides are used in a wide variety of applications, including pigments and magnetic media. In most commercial processes, goethite (α-FeOOH) particles can be used as a pigment and are produced as an intermediate in the iron oxide preparations [2–5]. The goethite crystals grow as the form of acicular needles about 1–1.5 μm length with an aspect ratio of 10–15. For magnetic media, the desired product should be of uniform length and same but high aspect ratio. The goethite is then reacted through a series of gas–solid reactions to form γ-Fe2O3. The goethite particle morphology is a critical factor in determining the magnetic properties of the γ-Fe2O3 particles [6]. At the present time, the method which prepared α-FeOOH is air oxidation in industry. Acid conditions and alkaline ⁎ Corresponding author. Tel.: +86 571 87951649; fax: +86 571 87952341. E-mail address: [email protected] (G. Han). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.07.010
conditions are included in the method. There are two stages in the formation of goethite under alkaline conditions [7]. In step one, ferrous sulfate (or any other ferrous salt) is mixed with excess NaOH (or some other base such as KOH) so that ferrous hydroxide precipitate immediately forms. The amount of excess NaOH used is between 15 and 200% above the stoichiometric requirements. In step two, an oxygen-containing gas is bubbled through the slurry to form α-FeOOH particles for controlling the synthesis conditions. The mechanisms involved in the oxidation reaction and crystal growth are very complex and generally not well understood [8–12]. In many examples of particle formations, the morphology control has been individually met based on many years of experience [11,12]. In this paper, we proposed a facile route for fabricating α-FeOOH nanowires via a low-temperature hydrothermal method. The α-FeOOH nanowires could be obtained with high yield and of good reproducibility. 2. Experimental In the experimental procedure, 4.85 g Fe(NO3)3·9H2O and 2.73 g KOH were dissolved in 10 ml distilled water, respectively, and then, the latter solution was dropped slowly into the latter
P. Ou et al. / Materials Letters 62 (2008) 914–917
915
Fig. 1. XRD pattern of the as-prepared α-FeOOH nanowires.
solution under vigorous stirring. The resulting mixture was loaded into a 50 ml-Telfon-lined autoclave, which was then filled with distilled water up to 80% of the total volume. The autoclave was sealed and maintained at 100 °C for 6 h. After the reaction was completed, the autoclave was cooled to room temperature naturally, and the resulting solid products were filtered off, washed with absolute ethanol and distilled water for several times, and then dried in vacuum at 60 °C for 12 h. The products were characterized by X-ray powder diffraction (XRD) using a Rigaku D/max-RA X-ray diffractometer with CuKa radiation (λ = 1.5406 Å). The product was further investigated using Fourier transform infrared (FT-IR) spectroscopy (AVATAR 360). Field emission scanning electron microscopy (FESEM) images were taken with a JEOL100CX scanning electron microscope. The high-resolution transmission electron microscope (HRTEM) image was taken with a JEM2010 TEM with an accelerating voltage of 200 kV. 3. Results and discussion Fig. 1 shows the XRD pattern of the as-synthesized products. All the diffraction peaks in Fig. 1 can be indexed to pure orthorhombic
Fig. 2. FT-IR spectrum of the as-prepared α-FeOOH nanowires.
Fig. 3. (a) The low-magnification SEM image of the as-prepared α-FeOOH nanowires. (b) The high-magnification SEM image of the as-prepared α-FeOOH nanowires.
structure α-FeOOH with lattice constants of a = 4.608 Å, b = 9.956 Å, c = 3.0215 Å, which are consistent with the values in the standard card (JCPDS no. 29-0713). The strong and sharp diffraction peaks indicate the highly crystalline nature of the α-FeOOH nanowires. No other impurities have been found in the synthesized products. The representative FT-IR spectrum (Fig. 2) of the products indicates the formation of FeOOH since the main absorption bands are in good agreement with that of the standard spectrum of FeOOH [13]. The absorption bands at around 3415 and 637 cm− 1 are the characteristics of OH− 1 ions, and the bands at 1646, 1530, 1386, 1050 and 892 cm− 1 represent the Fe–O vibrational mode in FeOOH. The FT-IR spectrum shows, therefore, that the nanowire products obtained in solutions are predominantly α-FeOOH. The morphology of the as-obtained products were examined by the field emission scanning electron microscopy (FESEM), whereby the solid sample was mounted on a copper slice after ultrasonic dispersion treatment. The low-magnification FESEM image in Fig. 3a shows that the products consist almost entirely of such α-FeOOH 1-D nanostructures, and this indicates the high yield and good uniformity achieved with this approach. The high-magnification SEM image in Fig. 3b reveals that the nanowires with a diameter of 80 nm and length of 1.2 μm are relatively straight. The product was further investigated by high-resolution transmission electron microscopy (HRTEM), whereby the sample was treated by ultrasonic dispersion in ethanol for 30 min and then put onto a copper grid. Fig. 4a shows a representative HRTEM image of an
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P. Ou et al. / Materials Letters 62 (2008) 914–917
nanowire is not crystalline well. This phenomenon was also observed on the flank fringe of α-FeOOH nanowire in Fig. 4c. In the case of oxidation of iron, the first phase to precipitate in hydrolysis of Fe3+ ions is usually ferrihydrite, a poorly crystalline oxide, of rough formula 5Fe2O3, 9H2O [14]. Nevertheless, the exact mechanisms of formation of goethite from the precursor ferrihydrite are not clearly established. Schwertmann et al. have revealed that the pH values of solution play the important role in the process of the transformation of ferrihydrite into goethite [15]. According to Schwertmann's study, in alkaline solutions, the dissolution of ferrihydrite follows the equilibrium Eq. (1). 5Fe2 O3 ; 9H2 OðsÞ þ ð10n−30ÞOH−ðaqÞ
ðn−3Þ−
þ ð66−10nÞH2 O↔10½FeðOHÞn ðH2 OÞð6−nÞ ðaqÞ dð3bn≤6Þ:
ð1Þ
In our experiments, the pH value of solution we measured is around 13. So we think that the value of n is 6 at very alkaline pH and ferrihydrite is dissolved under the complex anions, Fe (OH)3− 6 (n = 6) eventually. So at appropriate temperature, the precursor Fe (OH)3− 6 were condensated by olation and oxolation processes between OH and H2O ligands, which give rise to Fe–OH–Fe and Fe–O–Fe bridges, formed the nucleation of goethite. And then other precursor Fe (OH)3− 6 were aggradated on the nucleation of goethite, lead to the growth of goethite. The fringes of an individual nanowire are not intact, which were observed in Fig. 4b and c, can be explained by the precursor Fe (OH)3− 6 aggradated from in to out in the growth process of goethite. The inconsistent growth rate along different facets can be explained by the “lowest energy” argument resulting in needlike crystals. The nanowires morphology is thermodynamically favored as it allows the extension of the higher energy surfaces with respect to lower ones [16]. This is advantageous for the growth of α-FeOOH nanowires along [001] axis.
4. Conclusions
Fig. 4. (a) HRTEM image and electron diffraction pattern of an individual nanowire. (b) HRTEM image of the vegetal head of an individual nanowire. In panel b, the spot indicated by line arrow shows the initial growth stage on the vegetal head of an individual nanowire. (c) HRTEM image of the flank fringe of an individual nanowire. In panel c, the spot indicated by line arrow shows the initial growth stage on the flank fringe of an individual nanowire.
individual nanowire, clearly revealing that the as-obtained nanowires are structurally uniform and crystalline. The interdistance of lattice planes is approximately 4.2 Å, which can be indexed to the (110) plane. Meanwhile, the electron diffraction pattern (inset in Fig. 4a) can be indexed to the [1–10] zone axis of orthorhombic α-FeOOH, further confirming that the nanowires are crystalline and grow along [001] axis. Fig. 4b shows a typical HRTEM image of the vegetal head of α-FeOOH nanowire, which reveals that the vegetal head of α-FeOOH
In this paper, we report large-yield synthesis of α-FeOOH nanowires with high aspect ratios using Fe(NO3)3·9H2O and KOH as starting reactants under hydrothermal conditions at 100 °C. Structural characterization by XRD revealed that the α-FeOOH product has an orthorhombic structure. Morphology examination by FESEM revealed that the orthorhombic α-FeOOH product has a nanowire structure with diameter of 80 nm and length up to 1.2 μm, and HRTEM and the corresponding electron diffraction investigation showed that the product is of single-crystallinity. The present method is simple, mild, low-cost and large-production, which will be used to synthesize homogeneous nanostructures of other nanowire oxides at relatively low temperature and realize industrial-scale synthesis. Acknowledgment The work is supported by the Chinese National Foundation of Natural Science Research (50452003). References [1] L. Diamandescu, D. Mihă. ilă. i-Tă. ră. bă. isanu, N. Popescu-Pogrion, Mater. Lett. 27 (1996) 253–257. [2] M.P. Sharrock, R.E. Bodnar, J. Appl. Phys. 57 (1985) 3919.
P. Ou et al. / Materials Letters 62 (2008) 914–917 [3] L.M. Bennetch, H.S. Greiner, K.R. Hancock, M. Hoffman, U. S. Patent 3904540, 1975. [4] H. Sesigur, E. Acma, O. Addemir, A. Tekin, Mater. Res. Bull. 31 (1996) 1573. [5] H. Kopke, M. Ohlinger, W. Grau, E. Schoenafinger, H.H. Schneehage, U. S. Patent 4059716, 1977. [6] E.P. Wohlfarth, Ferromagnetic Materials: a Handbook of the Properties of Magnetically Ordered Substances, vol. 2, Elsevier, North-Holland, Amsterdam, 1980. [7] T. Misawa, K. Hashimoto, S. Shimodara, Corros. Sci. 14 (1974) 131. [8] W. Feitknecht, Z. Elektrochem. 63 (1959) 34.
[9] [10] [11] [12] [13] [14] [15] [16]
917
Y. Tamaura, M. Chem. Soc. Dalton Trans. 197 (1981) 1807. W. Feitknecht, P. Schindler, Pure Appl. Chem. 6 (1963) 134. E. Sada, H. Kumazawa, M. Aoyama, Chem. Eng. Commun. 71 (1988) 73. D.L. O'Connor, M.P. Dudukovic, P.A. Ramachandran, Ind. Eng. Chem. Res. 31 (1992) 2516. R.A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic Press, San Diego, CA, 1971. M. Loan, G.M. Parkinson, W.R. Richmond, Am. Mineral. 90 (2005) 258. U. Schwertmann, H. Stanjek, H.H. Becher, Clay Miner. 39 (2004) 433. B.S.J. Michael, G. Ali, H. Tobias, E.S. Aaron, L. Frank, A.K. Brian, J. Am. Chem. Soc. 125 (2003) 16050.