Synthesis, structure and magnetic properties of non-crystalline ferrihydrite nanoflakes

Journal of Physics and Chemistry of Solids 71 (2010) 1362–1366

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Synthesis, structure and magnetic properties of non-crystalline ferrihydrite nanoflakes M.S. Seehra n,a, V. Singh a, X. Song b, S. Bali c, E.M. Eyring c a

Department of Physics, West Virginia University, Morgantown, WV 26506, USA Department of Mechanical & Aerospace Engineering, West Virginia University, Morgantown, WV 26506, USA c Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA b

a r t i c l e in f o

a b s t r a c t

Article history: Received 4 February 2010 Received in revised form 5 April 2010 Accepted 7 June 2010

Synthesis and characterization of the structural and magnetic properties of a 2-line (2L) ferrihydrite (FHYD) sample based on the composition Fe:Al:Cu ¼ 100:25:5 are reported. Typical of 2L-FHYDs, this sample also yields the two broad lines in X-ray diffraction and triplet in the 1400–1700 cm  1 range in IR spectroscopy. However, in transmission electron microscopy, nanoflakes of about 5–20 nm size but without any hint of diffraction fringes characteristic of crystalline order were observed. Temperature dependence (2–380 K) of the magnetization M vs. applied field H (up to 7 65 kOe) of this noncrystalline ferrihydrite is used to establish a blocking temperature TB C 20 K, Ne´el temperature TN C 365 K and a spin-glass ordering of the surface Fe3 + spins at TS C6 K. These magnitudes of TB and TS are considerably smaller than those of a 5 nm undoped 2L crystalline ferrihydrite with TB ¼ 70 K and TS ¼ 30 K. The fit of the M vs. H data for several T4 TB to a modified Langevin function is shown to collapse onto a universal curve yielding a temperature-independent average magnetic moment mP ¼ 70(5)mB per nanoflake. Analysis of these parameters obtained from the fits of M vs. H data above TB is used to show that the effective average volume of the nanoflakes is about 1/3 that of spherical 5 nm crystalline 2L-FHYD. It is argued that these lower magnitudes of mP, TB, and TS for the nanoflakes result from their smaller effective volume determined here. & 2010 Elsevier Ltd. All rights reserved.

Keywords: A. Non-crystalline ferrihydrite B. Magnetic properties C. Infrared spectroscopy

1. Introduction: Nanocrystalline ferrihydrites (FHYD) with the formula Fe5HO8  4H2O or more generally FeOOH  nH2O occur naturally in Fe-containing waters and sediments and they also have been synthesized in the 4–7 nm sizes [1,2]. Their importance lies in their role in geochemical processes, in their relationship to ferritin (the iron reservoir in living organisms) and in their uses as precursors to various iron oxides and as catalysts and absorbents [1–6]. Depending on the degree of crystallinity and particle size, their X-ray diffraction (XRD) patterns, varying between 2 lines (2L) and 6 lines (6L), are usually indexed on the trigonal unit cell of hematite [6]. Fe3 + ions in the core of undoped 2L-FHYD are antiferromagnetically ordered with Ne´el temperature TN C 350 K [7] and they are suggested to be in octahedral coordination whereas the surface Fe3 + ions are tetrahedrally coordinated [3–6]. This leads to uncompensated surface Fe3 + spins with magnetic moment mP C290 mB per particle for the undoped 2L-FHYD [7,8].

n

Corresponding author. E-mail address: [email protected] (M.S. Seehra).

0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.06.003

In this paper, we report on the magnetic properties of a unique synthetic 2L-FHYD based on the composition Fe:Al:Cu ¼100:25:5 with a blocking temperature TB C20 K as defined by the peak in the ZFC (zero-field-cooled) susceptibility. Typical of 2L-FHYDs, this sample also yields the two broad lines in X-ray diffraction and triplet in the 1400–1700 cm  1 range in IR spectroscopy [9]. However, in transmission electron microscopy (TEM), nanoflakes of about 5–20 nm size consisting of clusters of atoms but without any hint of diffraction fringes characteristic of crystalline order are observed, thus making this sample essentially non-crystalline. This clustering signifies non-uniformity of the thickness of the nanoflakes. The observed flake-like morphology and the noncrystalline nature of this sample likely result from Al and Cu doping. This unique structure of the doped 2L-FHYD has been found to be an excellent catalyst for Fischer–Tropsch synthesis of alternative fuels using syngas (CO+ H2) as the feedstock [10]. Detailed magnetic studies reported here show this sample to have substantially different magnetic properties than those reported for crystalline 2L-FHYD samples [3–9]. These include lower TB ¼20 K and lower magnetic moment mP ¼70 mB/nanoflake compared to TB ¼70 K and mP C290 mB for the 5 nm undoped FHYD [7,8]. The results are explained on the basis of smaller effective volume of the nanoflakes as compared to that of 5 nm

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spherical undoped FHYD. Details of the structural and magnetic properties of this unique 2L-FHYD sample are given below.

2. Synthesis and structural characteristics This sample of 2L-FHYD with the composition Fe:Al:Cu¼ 100:25:5, was precipitated from an aqueous solution containing appropriate amounts of iron, copper and aluminum nitrates after reaction with aqueous sodium carbonate solution. Orange colored precipitates so formed after complete reaction were aged (with stirring) for another 24 h at ambient. This precipitate was filtered, and then washed repeatedly with water followed by drying in air overnight in an oven at 100 1C. Further details on the synthesis and catalytic applications of this material will appear elsewhere [10]. The XRD pattern of the resulting sample using CuKa source ˚ shows the 2-line pattern of Fig. 1. Since the XRD (l ¼1.54185 A) lines are too broad resulting from the overlap of several Bragg peaks [6], a reliable estimate of particle size using the Scherrer broadening is not possible. In the inset of Fig. 1, IR/photo-acoustic spectra (taken with ‘Infinity Gold FTIR system’ by Mattson Instruments) of the sample shows the triplet in the 1400– 1700 cm  1 range characteristic of 2L-FHYD [9]. In undoped 2LFHYD, these bands occur at 1365, 1493 and 1650 cm  1 and they are usually assigned to the bending modes of Fe–O and Fe–OH groups. The slightly increased frequencies of 1397 and 1508 cm  1 for the first two bands observed for this sample may be due to the substitution of lighter Al atoms for Fe. Two micrographs of transmission electron microscopy (TEM) of this sample are shown in Fig. 2. Sizes of the non-agglomerated flakes in Fig. 2 (top) are 7, 8, 13 and 20 nm. More details of one of the nanoflakes, shown in Fig. 2 (bottom) give evidence of non-uniform thickness with clusters of about 5 nm but without any hint of diffraction fringes characteristic of crystalline order. This is in contrast to the previous TEM studies of undoped 2L-FHYD and Mo-doped 2L-FHYD of about 5 nm size in our laboratory where clear evidence for diffraction fringes was reported [8]. Thus, it is concluded that this sample, consisting of thin flakes, lacks any significant crystalline order.

Fig. 2. Shown are TEM micrographs of the flake-like sample. The bar size is 100 nm (5 nm) for the upper(lower) figure.

3. Magnetic properties 15 960 833

Absorbance (a.u.)

3424

1508 1651 1397

10

CO2 *

5

128 scans-PAS 0 4000

3000 2000 -1 Wavenumber (cm )

1000

Fe:Al:Cu = 100:25:5 20

40 60 Two-Theta (deg)

80

100

Fig. 1. X-ray diffraction of the 2L-FHYD sample investigated here with the inset showing the IR spectra of the sample (see text for details).

Measurements of magnetization M of this sample vs. temperature T and magnetic field H were done using a commercial SQUID magnetometer. Temperature variation (2–380 K) of the ZFC and FC(field-cooled) magnetic susceptibility w measured in H¼100 Oe is shown in Fig. 3. At TB ¼20 K, w(ZFC) peaks and for ToTB, distinct bifurcation of w(FC) and w(ZFC) is also observed characteristic of super-paramagnetic blocking at TB. In the inset of Fig. 3, a blow up of the w vs. T data for the higher temperature region shows the onset of distinct bifurcation of the ZFC and FC data beginning near 365 K. This temperature nearly equals the Ne´el temperature TN C350 K of the core Fe3 + spins as measured by the temperature dependence of a magnetic peak in the neutron scattering studies of FeOOD  nD2O nanoparticles [7]. Whether this represents a detection of TN in a 2L-FHYD system using w vs. T needs to be explored in further studies on crystalline 2L-FHYD samples. In antiferromagnets of nm size, the magnetization M for the temperature range of TN 4T4TB is usually fitted to the modified Langevin variation [8,11]: M ¼ Mo LðmP H=kB TÞ þ wa H

ð1Þ

Here L(x)¼coth(x) (1/x) is the Langevin function, mP the magnetic moment per particle, wa the antiferromagnetic susceptibility

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6

1.0

FC

4

H = 100 Oe

0.360

0.8

Field Cooled 0.355 0.350

Zero Field Cooled

0.345 350

3

360

370

380

T (K)

-4

χ (10 emu/g Oe)

5

2

(M - χaH)/Mo

χ (10 emu/g Oe)

0.365

0.6

0.4

ZFC TB = 20 K

0.2

μP = 70(5)μB

1 H = 100 Oe

0.0

0 10

10

100

T (K)

Fig. 3. w vs. T data for 2L-FHYD in H ¼100 Oe. The open symbols are for the FC data and closed symbols are for the ZFC data. The inset shows the data at higher T on an expanded scale.

5

3.0

-3

C = 8.74 x 10 emu K / g Oe -5 χo = 1.15 x 10 emu / g H = 100 Oe

2.0

3

4

1.5

-1

(χ-χo) (10 g Oe / emu)

4

4

χ = χo + (C/T)

2

-1

χ (10 g Oe / emu)

2.5

1.0 1

0.5

ZFC FC

0.0

0

0

50

100

150

200 T (K)

250

300

350

400

Fig. 4. w  1 vs. T plot to obtain wo and to show non-linearity expected from Eq. (2). Plot of (w  wo)  1 vs. T to obtain the Curie constant C is also shown.

(usually determined from the high field regime where M vs. H is almost linear), kB the Boltzmann constant and Mo is the saturation magnetization. For (mPH/kBT){1, Eq. (1) leads to [12]:

w ¼ wo þ ðC=TÞ, C ¼ mP M* =3kB

ð2Þ

Here M* is the magnetization Mo in the limit T-0 K and wo ¼ wa  (C/TN). In Fig. 4, the plot of w  1 vs. T shows the nonlinearity of the variation expected from Eq. (2). Using wo ¼1.15  10  5 emu/g Oe determined from the plot of w vs. (1/T) for the limit (1/T)-0, the plot of (w  wo)  1 vs. T, also shown in Fig. 4, gives the expected linear behavior with C ¼8.74  10  3 emu K/g Oe. Using TN ¼365 K (Fig. 3), wa ¼ wo + (C/TN)¼3.54  10  5 emu/g Oe is obtained. To further check the validity of Eq. (1) for all fields, the plots of M vs. H up to the available H¼65 kOe at different temperatures are shown in the inset of Fig. 5. As Silva et al. [13] have recently emphasized from studies on ferritin, M is not saturated even at H¼65 kOe so that wa ¼3.54  10  5 emu/g Oe determined above is smaller than that determined from the slope of the high field

100 H/T (Oe/K)

1000

Fig. 5. Modified Langevin function fit to the M vs. H data (Eq. (1)) for the 2L-FHYD. The inset shows the M vs. H plots for temperatures above the blocking temperature. The two solid curves bracketing the data points are Langevin fits for mP ¼75 mB and mP ¼ 5 mB.

linear parts of the M vs. H plots at the lower temperatures. However this magnitude of wa nearly equals the susceptibility at 300 and 350 K determined from the observed linear M vs. H plots at all H. At these high temperatures closer to TN, the system is essentially paramagnetic and not super-paramagnetic. For the lower T¼40, 80, 120 and 170 K, the M vs. H data were fitted to Eq. (1) employing the three variable least-squares fit employing ‘‘Sigmaplot 8.0 statistics regression wizard’’ [14]. The three variables Mo, wa and mP were varied using the initial values: Mo ¼10 emu/g, wa ¼3  10  5 emu/g Oe and mP ¼ 75 mB with the additional constraint that mP o400 mB. The choice of initial values of Mo is dictated by the data of M vs. H in Fig. 5 and those for wa and mP dictated by parameters obtained by fit to Eq. (2) described above. As an example, the fit of M vs. H data at 120 K yielded Mo ¼1.69 emu/g, wa ¼ 5.985  10  5 emu/g Oe and mP ¼73.67 mB. From the fits at other temperatures, significant temperature dependence was noticed primarily for Mo and wa. These magnitudes were further fine-tuned to fit the (M waH)/Mo vs. H/T plot for a single mP ¼70 mB. The two solid lines in the plot of Fig. 5 for mP ¼75 mB and mP ¼65 mB, nicely bracket the experimental data at all temperatures. The temperature dependence of Mo and wa resulting from this procedure is shown in Fig. 6, with a temperature independent mP ¼ (7075) mB. Following Eq. (2), M* ¼3kBC/mP. Using mP ¼70 mB, C¼8.74  10  3 emu K/g, density r ¼5.24 g/cm3 of FHYD [7], yields M* ¼5.57 emu/g¼29.2 emu/cm3. Furthermore mP ¼M*V [7] yielding V¼ 22.24 nm3 as the average volume of the particles. Since the particles have flake-like morphology, we approximate them with a disc of diameter D and thickness t yielding V¼ pD2t/4. Since flakes are too thin for thickness measurements even by TEM, assuming t¼1 nm (0.5 nm) yields DC5.3 nm (10.6 nm) as the size of the flake. Some of the smaller flakes shown in the TEM of Fig. 2 are indeed of this size. Because of the severe agglomeration of many of the nanoflakes, a comprehensive size distribution of the flake size could not be determined from the TEM micrographs. Since 2L-FHYD has antiferromagnetic ordering, the source of mP is the uncompensated Fe3 + spins on the A and B sublattices in that mP depends on p¼ nA  nB where nA and nB are number of Fe3 + spins on A and B sublattices and n¼ nA + nB is the total number of spins per particle. Because of broken symmetry on the surface, these uncompensated spins usually occur only on the surface of

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8

2.5

5 2.0 4 1.5

3

1.0

2

μP = 70(5)μB

0.5

1

0.0 0

50

100

150

200

250

T=8K

0.0

-0.5

FC ZFC

-5

6

χa (10 emu/g Oe)

7

3.0 Mo (emu/g)

0.5 M (emu/g)

3.5

0 300

T (K) Fig. 6. Temperature dependence of Mo, and wa obtained from the three variable least-squares fit. These magnitudes are used to obtain the single collapsed curve of Fig. 5. The lines connecting the data point are for visual aid.

the nanoparticles with ordering in the core being bulk like. The Ne´el model [15] predicts p  na where a can be 1/2, 1/3 or 2/3 depending on how the uncompensated spins are distributed. These different cases have been discussed at great length in previous publications on FHYD [7,8]. For our case with V ¼22.24 nm3, n¼824 is determined using the Fe3 + –Fe3 + distance of 3 A˚ in FHYD [3,4]. Each Fe3 + usually has the moment of 5.9 mB yielding p C12 for mP ¼ 70 mB for the number of uncompensated Fe3 + spins per particle. For n ¼824, n1/3 C9 and n1/2 C29 so that the derived mP ¼70 mB is close to pCn1/3 variation for the Ne´el model. The smaller mP ¼ 70 mB determined here for the nanoflakes compared to mP ¼290 mB determined for the nearly spherical FHYD of 5 nm diameter is primarily due to smaller volume of the nanoflakes by a factor of nearly three. The lower TB C20 K observed here for the nanoflakes compared to TB C70 K reported for the undoped near spherical particles of diameter D ¼5 nm [7] can also be explained in terms of the smaller volume of the nanoflakes. Using kBTB ¼KaV/30 [16] where Ka is the anisotropy constant, a factor of 3 reduction in V should lead to a similar reduction in TB assuming the same Ka. A more quantitative comparison is not made because Ka is often size dependent, increasing with decreasing particle size due to additional contribution from surface anisotropy and TB is also affected by interparticle interaction [16]. For ToTB, hysteresis loops up to 765 kOe were measured for the ZFC as well as for the FC case with the sample cooled from 300 K to the measuring temperature in the H¼ 20 kOe. In Fig. 7, we show the hysteresis loops for the two cases at the select temperatures of 2 and 8 K. From such measurements carried out at different temperatures, temperature dependence of the coercivity HC and exchange-bias Heb (loop shift) for the FC case is shown in Fig. 8. Heb ¼0 Oe for T4TS C6 K and for ToTS, HC(FC) 4HC(ZFC). These signatures at TS are characteristics of spin-glass ordering of the surface spins [17–20]. For undoped 2L-FHYD with crystalline size of about 5 nm, TS C30 K with TB C70 K have been reported [8,17]. As noted earlier, the smaller magnitudes of TB and TS in the nanoflakes are largely due to their smaller effective size. In a number of earlier studies of crystalline nanoparticles of 2L-FHYD, a strong signal in the electron magnetic resonance (EMR) spectroscopy attributed to uncompensated surface Fe3 + spins was reported whose temperature dependence followed the behavior observed in other magnetic nanoparticles[21].

T=2K

0.5 M (emu/g)

9

0.0

-0.5

FC ZFC

-3000

-2000

-1000

0 H (Oe)

1000

2000

3000

Fig. 7. Hysteresis loops for the ZFC and FC (at 20 kOe) cases at 2 and 8 K.

1600 Hc (FC)

1200

Hc (ZFC) 800 H (Oe)

4.0

1365

400 0 -400

Heb

-800 0

5

10

15 T (K)

20

25

30

Fig. 8. Temperature dependence of coercivity HC and exchange-bias Heb measured on a sample cooled in zero-field-cooled (ZFC) and field-cooled (FC) in 20 kOe. Heb is negligible for the ZFC sample.

In Si-doped nanocrystalline 2L-FHYD, the width of this EMR signal at room temperature was found to increase from about 700 Oe for the undoped sample to nearly 1100 Oe for 15% Sidoped FHYD [21]. In parallel with this, the crystallinity of the doped samples was found to decrease [9]. In the present noncrystalline sample with even larger Al and Cu doping, only a very weak asymmetric and broad EMR signal is observed. Since EMR in magnetic nanoparticles broadens as temperature is lowered [21], the EMR signal in the present sample became even more difficult to observe with decrease in temperatures. These observations of a very broad EMR line with weak intensity are consistent with structurally disordered system due to doping with Al and Cu.

4. Summary and conclusions In this paper, results on the structural and magnetic properties of a unique synthetic 2L-FHYD nanoflakes without any noticeable crystalline order are reported. The results show that the properties of this system doped with Al and Cu are substantially

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different from those of crystalline 2L-FHYD nanoparticles reported in literature. The lower magnitudes of TB C20 K and TS ¼6 K, and the lower magnitudes of average magnetic moment mP ¼ 70 mB per flake estimated from the data, are considerably smaller than those observed in the nominal 5 nm undoped FHYD (TB C70 K, TS ¼30 K and mP ¼290 mB). These lower magnitudes can be explained in terms of the smaller effective volume of the nanoflakes (determined from the analysis of the data) by a factor of about 1/3 as compared to the volume of the 5 nm spherical FHYD particles. The weaker and broad EMR line observed in this system is consistent with the structural disorder produced by doping with Al and Cu. The unique morphology of this sample combined with its smaller effective volume may be the source of its excellent catalytic properties [10].

Acknowledgements MSS acknowledges the assistance of Mohita Yalamanchi in acquiring the IR spectra. Research support provided by the U.S. Department of Energy (Contract no. DE-FC-26-05NT42456) is gratefully acknowledged. References [1] T.S. Berquo, S.K. Banerjee, R.G. Ford, R.L. Penn, T. Pichler, J. Geophys. Res. 112 (2007) B02102 and references therein. [2] H. Tuysuz, E.L. Salabas, C. Weidenthaler, F. Schuth, J. Am. Chem. Soc. 130 (2008) and references therein.

[3] J. Zhao, Z. Feng, F.E. Huggins, G.P. Huffman, Energy Fuels 8 (1994) 38. [4] J. Zhao, F.E. Huggins, Z. Feng, G.P. Huffman, Clays Clay Miner. 42 (1994) 737. [5] J. Zhao, F.E. Huggins, Z. Feng, G.P. Huffman, Phys. Rev. B 54 (1996) 3403. [6] J.L. Jambor, J.E. Dutrizac, Chem. Rev. 98 (1998) 2549 and references therein. [7] M.S. Seehra, V.S. Babu, A. Manivannan, J.W. Lynn, Phys. Rev. B 61 (2000) 3513. [8] A. Punnoose, T. Phanthavady, M.S. Seehra, N. Shah, G.P. Huffman, Phys. Rev. B 69 (2004) 054425. [9] M.S. Seehra, P. Roy, A. Raman, A. Manivannan, Solid State Commun. 130 (2004) 597. [10] S. Bali, R.D. Ernst, E.M. Eyring, Ultra small synthetic ferrihydrite based catalysts with promoter metals for Fischer–Tropsch synthesis of alternative fuels, Provisional U.S. Patent application filed. [11] S.A. Makhlouf, F.T. Parker, A.E. Berkowitz, Phys. Rev. B 55 (1997) R14717. [12] M.S. Seehra, A. Punnoose, Phys. Rev. B 64 (2001) 132410. [13] N.J.O. Silva, A. Millan, F. Palacio, E. Kampert, V. Zeitler, H. Rakoto, V.S. Amaral, Phys. Rev. B 79 (2009) 104405. [14] For the other temperatures, following magnitudes for Mo (emu/g), wa (10  5 emu/gOe) and mP (mB) were, respectively, obtained in the initial fit: 3.427, 8.116, 65.113 for 40 K; 2.581, 6.733, 68.42 for 80 K; 1.410, 4.979, 67.63 for 170 K; and 1.090, 4.028, 60.061 for 250 K. [15] L. Ne´el, Ann. Geophys. (C.N.R.S.) 5 (1949) 99; L. Ne´el, Adv. Phys. 4 (1955) 191. [16] V. Singh, M.S. Seehra, J. Bonevich, J. Appl. Phys. 105 (2009) 07B518; R. Yanes, O. Chubykalo-Fesenko, H. Kachkachi, D.A. Garanin, R. Evans, R.W. Chantrell, Phys. Rev. B 76 (2007) 064416. [17] A. Punnoose, M.S. Seehra, J. Van Tol, L.C. Brunel, J. Magn. Magn. Mater. 288 (168) (2005). [18] D. Carta, M.F. Casula, A. Corrias, A. Falqui, G. Navarra, G. Pinna, Mater. Chem. Phys. 113 (2009) 349. [19] M. Ali, P. Adie, C.H. Marrows, D. Greig, B.J. Hickey, R.L. Stamps, Nat. Mater. 6 (2007) 70. [20] P. Dutta, M.S. Seehra, S. Thota, J. Kumar, J. Phys.: Condens. Matter 20 (2008) 015218. [21] M.S. Seehra, A. Punnoose, P. Roy, A. Manivannan, IEEE Trans. Magn. 37 (2001) 2207.