Aspartic-Acid-Assisted Hydrothermal Growth and Properties of

Dec 7, 2009 - A biomolecule-assisted hydrothermal route to the fabrication of ... 600 mAh/g in the first cycle and a discharge voltage of 0.92 and 0.7...
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J. Phys. Chem. C 2010, 114, 284–289

Aspartic-Acid-Assisted Hydrothermal Growth and Properties of Magnetite Octahedrons Xiao-Fei Qu,† Gen-Tao Zhou,*,† Qi-Zhi Yao,‡ and Sheng-Quan Fu§ CAS Key Laboratory of Crust-Mantle Materials and EnVironments, School of Earth and Space Sciences; School of Chemistry and Materials; and Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed: September 23, 2009; ReVised Manuscript ReceiVed: NoVember 11, 2009

A biomolecule-assisted hydrothermal route to the fabrication of magnetite (Fe3O4) with uniform microsized and regular octahedral morphology has been successfully developed by use of toxic-free aspartic acid as reducing reagent and FeCl3 · 6H2O as iron source. The as-prepared magnetite was characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and highresolution transmission electron microscopy (HRTEM). Altering different experimental parameters showed that pH of the aspartic acid solution, concentration of aspartic acid, and hydrothermal temperature can significantly influence product phase composition. Furthermore, a series of time-course experiments revealed that growth of regular octahedral magnetite is controlled by the Ostwald ripening process. This biomoleculeassisted route may be expected to be applicable for the fabrication of other transition metal oxides with uniform size and morphology. Besides, magnetic properties of the product were characterized on a vibrating sample magnetometer (VSM). The values of saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) of the magnetite octahedrons are 71.6 emu/g, 9.8 emu/g, and 120 Oe, respectively. The electrochemical performances of the magnetite octahedra exhibit a discharge capacity of ca. 600 mAh/g in the first cycle and a discharge voltage of 0.92 and 0.74 V, respectively. Introduction Among all iron oxides, magnetite (Fe3O4) has long been of scientific and technological interest in the past decades, because of the increasing fundamental understanding of nanomagnetism1 and the promising applications in magnetic data storage,2 ferrofluid,3 sensors,4 spintronics,5 catalysis,6 pigments,7 magnetic resonance imaging contrast enhancement,8 and biomedical fields.9 A variety of synthesis methods have been developed for the preparation of magnetite particles, such as traditional coprecipitation of ferrous and ferric by an alkali,10 reduction of hematite by CO,11 γ-ray irradiation,12 microwave plasma synthesis,13 sonochemical method,14 reverse-phase evaporation in lipid vesicles,15 liquid-foam template method,16 hydrothermal or solvothermal reaction,17 and slow oxidation of ferrous hydroxide,18 etc. However, most of the synthesis techniques frequently involve elaborate technologies (e.g., irradiation and microwave plasma) or toxic initial materials (e.g., hydrazine hydrate, iron carbonyl, and ethylenediamine). Therefore, increasing attention is being paid to facile and eco-friendly synthetic strategies. Recently, biomolecule-assisted route has been proved to be a novel, eco-friendly, and promising method for the formation of various materials, where biomolecules usually serve as structure-directing agents. For example, special proteins extracted from mineralization tissues of marine organisms were introduced to alter the morphology of calcium carbonate by preferentially interacting with certain crystal faces and modifying the overall crystal shape.19 Protein horse spleen ferritin (HSF) was adopted as the template to synthesize iron(III) sulfide-ferritin * Corresponding author. E-mail: [email protected]. † CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences. ‡ School of Chemistry and Materials. § Hefei National Laboratory for Physical Sciences at Microscale.

bioinorganic nanocomposite.20 With the use of bovine serum albumen (BSA) acting as the templating agent, iron (hydro)oxide nanoparticles-protein conjugates was obtained by a sonochemical method.14b Utilizing the protein Mms6 that likely mediates the nucleation and/or growth of mineral phase, Matsunaga’s group synthesized nanosized cuboctahedral magnetite crystals similar to the magnetosomes observed in magnetotactic bacteria.21 Due to specific cylindrical and double helical structure, DNA has also been confirmed to be useful in the assembly of nanoparticles into two- or three-dimensional structures and in the alignment of discrete one-dimensional nanomaterials.22 Several nanowires have been obtained in the presence of DNA, polylysine, or cytochrome c3.23 Furthermore, biomolecules with specific functional groups have been demonstrated to act as not only the structure-directing agents but also the vital initial reactants in the preparation of hierarchical architectures. Komarneni’s group used glutathione (GSH) as the assembling agent and the effective sulfur source to synthesize snowflake-like Bi2S3 nanorods.24 Similarly, porous sponge-like Ni3S2, network-like MnS, pagoda-like PbS, and flowerlike Bi2S3 were fabricated in the addition of L-cysteine, which also functions as the sulfur source and assembling agent.25 Polysaccharide that contains abundant hydroxyl groups, such as starch, has also been used to prepare complicated nanostructures under the hydrothermal synthesis, in which hydroxyl groups serve as the essential reductive reagent.24 Herein, we design a facile synthetic method using aspartic acid as mild reducing reagent to obtain uniform magnetite octahedra. Aspartic acid (Asp) is an R-amino acid with the chemical formula HOOCCH(NH2)CH2COOH. It has been known that the carboxyl group is an electron donating group especially when it appears as anions. For example, various materials, such as CdS and Fe-metal oxides, can be obtained by Ac- as a reductant.26 Besides, the R-amino group in aspartic

10.1021/jp909175s  2010 American Chemical Society Published on Web 12/07/2009

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TABLE 1: Experimental Parameters and Reaction Productsa sample

initial pH of Asp solution (pHAsp)

concn of Asp in suspension (mol/L)

reaction temp (°C)

reaction time (h)

products

DS1-8 DS1-9 DS1-10 DS1-11 DS1-12 DS2-1 DS2-3 DS3-12 DS3-16 DS4-6 DS4-9 DS4-12 DS4-15 DS4-18

8.0 9.0 10.0 11.0 12.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

0.0188 0.0188 0.0188 0.0188 0.0188 0.00376 0.0113 0.0188 0.0188 0.0188 0.0188 0.0188 0.0188 0.0188

200 200 200 200 200 200 200 120 160 200 200 200 200 200

24 24 24 24 24 24 24 24 24 6 9 12 15 18

hematite and akaganeite hematite and akaganeite magnetite magnetite magnetite hematite hematite hematite and akaganeite hematite magnetite magnetite magnetite magnetite magnetite

a

For all samples, the initial concentration of FeCl3 · 6H2O is 0.02 mol/L, and the initial pH of hydrated ferric oxide colloids (pHHFO) is 7.0.

acid may also contribute to reduction because it has been demonstrated that amine-containing compounds can potentially serve as a reducing reagent in the preparation of nanomaterials.27 Compared with other reducing reagents (e.g., hydrazine hydrate and sodium borohydride) in most of hydrothermal syntheses for magnetite, aspartic acid is typically toxic-free, more economical, and eco-friendly. Therefore, this biomoleculeassisted method may provide an eco-friendly opportunity to synthesize other transition metal oxides with uniform size and controllable morphology. Furthermore, we also evaluated magnetic properties and electrochemical performances of the magnetite octahedrons. Experimental Section All chemical reagents were of analytical grade and used as received without any further purification. In a typical synthesis procedure (sample DS1-10, Table 1), 2 mmol of FeCl3 · 6H2O was dissolved in 100 mL of deionized water to form a homogeneous solution, and then NaOH solution was titrated under continuous stirring to obtain hydrated ferric oxides (HFO) colloid. Final pH value of the colloid suspension was controlled at 7.0. Meanwhile, 3.76 mmol of aspartic acid was dissolved in 100 mL of deionized water and adjusted its pH value to 10.0 by titrating NaOH solution. Then, 40 mL of colloid suspension and 40 mL 0.0376 M of aspartic acid solution were transferred into a Teflon-lined stainless steel autoclave, and the final concentration of Asp in the colloid suspension is 0.0188 M. The autoclave was sealed, maintained at 200 °C for 24 h, and then cooled to room temperature naturally. A black precipitate was isolated by decantation and washed several times with deionized water and absolute ethanol. Finally, the well-prepared product was dried in a 40 °C vacuum oven for 12 h. The detailed parameters of the different synthesis runs are also shown in Table 1. The phase of the product was characterized by X-ray powder diffraction (XRD) using an 18 kW advanced X-ray diffractometer with Cu KR radiation (λ ) 1.540 56 Å). The Raman spectrum was taken on a LABRAM-HR Confocal Laser MicroRaman spectrometer using an Ar+ laser with 514.5 nm at room temperature. Scanning electron microscopy imaging was carried out on a JEOL JSM-6700 M field emission scanning electron microscope (FESEM). High-resolution transmission electron microscopy (HRTEM) image and corresponding selected area electron diffraction (SAED) pattern were obtained on a JEM2010F microscope operated at an acceleration voltage of 200 kV. The magnetic properties of the synthesized product were

Figure 1. XRD patterns of the products after different reaction times: (a) 6 h (sample DS4-6, Table 1), (b) 9 h (sample DS4-9), (c) 12 h (sample DS4-12), (d) 15 h (sample DS4-15), (e) 18 h (sample DS418), and (f) 24 h (sample DS1-10).

measured on a BHV-55 vibrating sample magnetometer (VSM) at room temperature. The electrochemical performance of the product was evaluated with a Li metal electrode using a Teflon cell, as described in the literature.28 The positive electrodes were fabricated by pasting slurries of the as-prepared crystallites (85 wt %), carbon black (Super P, 10 wt %), and poly(vinylidene fluoride) (PVDF, 5 wt %) dissolved in N-methylpyrrolidinone (NMP) on Al foil strips by the “doctor blade” technique. The strips were dried at 160 °C for 24 h in a glovebox filled with highly pure N2 gas, pressed under 20 MPa pressure, and kept at 120 °C for 12 h in a vacuum. The electrolyte was 1 M LiPF6 in a 1:1 mixture of ethylene carbonate (EC)/diethyl carbonate (DEC). The separator was Celgard 2500. The cells were assembled in the glovebox filled with highly pure argon gas. The cells were galvanostatically cycled in the 3.0-0.5 V window at a current density of 0.1 mA cm-2. Results and Discussion Figure 1 shows typical XRD patterns of the products after different reaction times (corresponding to the samples in Table 1). All of the reflection peaks (Figure 1a-f) can be well assigned

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2Fe3+ + Fe2+ + 8OH- ) Fe3O4 + 4H2O

Figure 2. Raman spectrum of the magnetite particles (sample DS110, Table 1).

to a spinel structure with the characteristic reflections of iron oxide (magnetite Fe3O4, JCPDS 79-0419, or maghemite γ-Fe2O3, JCPDS 39-1346).29 No diffraction peaks from hematite, iron oxyhydroxides, hydroxides, or other impurities can be found in the products. Notably, when the reaction undergoes a shorter period of time (6 or 9 h), the significant broadening of the reflection peaks occurs, as shown in Figure 1, a and b, indicating that the iron oxide particles obtained at the initial stage may be at a nanosized scale. The mean crystal size estimated by the Scherrer equation is ca. 95 nm based on the measurement of the full width at half-maximum (fwhm) of the (311) peak in Figure 1a. Nevertheless, as hydrothermal time is prolonged over 12 h, all the diffraction peaks (Figure 1c-f) are markedly sharpened, suggesting that the longer reaction times are favorable for the crystallization and growth of the synthesized products. The strong and sharp diffraction peaks of the product obtained after 24 h indicate that the final product is well crystallized. The relative intensity ratios of diffractions I(111)/ I(311) and I(222)/I(311) (Figure 1f) are slightly larger than that of the standard card, which suggest that the structure of final crystals may be {111} oriented. However, it is known that the identification of magnetite and maghemite detected on the ordinary XRD pattern is an obscure work because they have the same structure and the similar lattice parameter a (8.396 Å for magnetite and 8.351 Å for maghemite). In order to identify the exact phase of the product, a further characterization was carried out as well. Raman spectroscopy has been widely used to differentiate iron oxides, especially magnetite and maghemite.17e,30 A representative Raman spectrum of the final product (sample DS110 in Table 1) is presented in Figure 2. The characteristic bands at 667 and 536 cm-1 can be assigned to the A1g and T2g transitions of magnetite, respectively, which are consistent with the values of magnetite reported in the literature.17e,30 The characteristic vibration bands belonging to magnetite can easily be distinguished from characteristic bands of maghemite at 720, 500, and 350 cm-1. Furthermore, no obvious bands of other impurities such as hematite (390, 280, 220 cm-1) or akaganeite (1110, 880 cm-1) can be detected. This further confirms that the black product is a pure magnetite phase. Therefore, the chemical reactions involved in such a synthesis can be described as follows:

HFO(colloid) h Fe3+ + OH3+

Fe

+ Asp f Fe

2+

(1) (2)

(3)

First, hydrated ferric oxide (HFO) colloids gradually dissolve and release Fe3+ ions in the hydrothermal process; meanwhile, partial Fe3+ is reduced to Fe2+ by aspartic acid; then, Fe3+ and Fe2+ ions in solution react to generate magnetite particles. FESEM images in Figure 3 display the size and morphology of the magnetite after 24 h hydrothermal reaction (sample DS110, Table 1). After examining numerous FESEM images of the product, we find that the synthesized magnetite crystals have uniform size and regular octahedral appearance. Figure 3a shows the low-magnification image of the magnetite sample, which indicates that the aspartic-acid-assisted synthetic route can result in a good yield of octahedral particles. The further magnified image in Figure 3b exhibits that the microsized magnetite crystals are as large as ca. 5 µm, and these particles have regular octahedral morphology with sharp-cut edges. An individual magnetite octahedron image is presented in Figure 3c. It is obviously seen that the octahedron has high geometric symmetry and smooth surfaces. Figure 4 shows the HRTEM image of a typical vertex of the octahedron and corresponding SAED pattern. The well-resolved lattice fringes and clear diffraction

Figure 3. FESEM images of the as-prepared magnetite octahedrons after 24 h hydrothermal reaction (sample DS1-10, Table 1): (a) lowmagnification image of large-scale octahedrons; (b) high-magnification image of local octahedrons; and (c) typical high-magnification image of an individual octahedron with smooth surfaces.

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Figure 4. HRTEM image of a vertex on the octahedron and corresponding SAED pattern (inset).

spots suggest that the octahedron is highly crystallized. The interplanar spacing of 0.486 nm corresponds to the {111} lattice planes of magnetite crystal, which is in agreement with the indexed spot. Therefore, the HRTEM and the SAED results further demonstrate that the surface of octahedron is delimitated by {111} crystallographic planes. Combined with the welldeveloped habits of magnetite, it is safely concluded that the related symmetry faces are {111} planes, in agreement with the XRD result (Figure 1f). In previous fabrication of octahedronlike magnetite crystals, toxic initial material, e.g., hydrazine hydrate, sodium borohydride, or ethylenediamine, was usually utilized in the hydrothermal reaction.31 In our route, aspartic acid is introduced to serve as the reducing reagent to obtain uniform magnetite octahedrons. In this regard, an eco-friendly route to the synthesis of uniform particles has successfully been developed. Moreover, our experiments (Table 1) also reveal that phase compositions of the products significantly depend on pH, Asp concentration, and hydrothermal temperature. When initial pHs of the Asp solutions (pHAsp) are less than 10.0, a mixture of hematite (R-Fe2O3) and a small amount of akaganeite (βFeOOH) rather than magnetite is always obtained after 24 h hydrothermal reaction (samples DS1-8 and DS1-9), indicating that Asp cannot reduce Fe3+ to Fe2+. As a result, no magnetite forms. In contrast, the higher initial pHs (ranged from 10 to 12) of the Asp solutions lead to pure phase magnetite (samples DS1-10 to DS1-12, and samples DS4-6 to DS4-18) even though a shorter hydrothermal duration (6 h) is applied (sample DS46). This suggests that the higher pH enhances the reductive ability of aspartic acid. It is probably because under the lower pH conditions some aspartic acid molecules in solution still exist in the form of molecules so that their reductive ability is decreased. Likewise, when the concentrations of the Asp solutions are below 0.0188 M, exclusive hematite phase can be obtained (samples DS2-1 and DS2-3). This indicates that Asp with lower concentrations cannot reduce Fe3+ into Fe2+ even under the 200 °C and 24 h hydrothermal conditions. Besides, reaction temperature also plays a crucial role in the formation of magnetite. At 120 °C, a mixture of hematite and akaganeite (sample DS3-12) is obtained, while at 160 °C hydrothermal reaction produces pure hematite (DS3-16). Nevertheless, until 200 °C the pure phase magnetite can be fabricated (sample DS110). The phase variations of the synthesized products related with temperatures imply that proper high temperature (g200 °C) is favorable to the formation of magnetite.

Figure 5. FE-SEM images of the as-prepared magnetite particles for different reaction times: (a) 6 h (sample DS4-6, Table 1), (b) 9 h (sample DS4-9), (c) 12 h (sample DS4-12), (d) 15 h (sample DS4-15), (e) 18 h (sample DS4-18); and (f) the effort of reaction time on the average dimension of octahedrons.

To understand the growth details of uniform magnetite octahedrons, control experiments (sample DS4-6, -9, -12, -15, -18, Table 1) with different durations of hydrothermal treatments were carried out. FESEM images of the products obtained at different reaction times are shown in Figure 5. A 6 h hydrothermal reaction leads to irregular polyhedral magnetite nanoparticles with the size of ca. 98 nm, as shown in Figure 5a. When the reaction time is prolonged to 9 h, the FESEM image (Figure 5b) reveals that part of the magnetite nanoparticles has grown as large as microscale order and some nanosized octahedron embryos can also be found (see inset in Figure 5b). Furthermore, with increasing reaction time to 12 h, aside from a few magnetite nanoparticles, a large number of octahedra are discernible (Figure 5c). When reaction times are prolonged to 15 and 18 h (Figure 5, d and e), a significant growth of octahedron-like particles can be observed and the mean particle sizes of the octahedra obtained after 15 and 18 h are 0.85 and 4.3 µm, respectively. After 24 h hydrothermal treatment, welldefined regular octahedra of magnetite are obtained (Figure 3). However, it is noteworthy that until 18 h a considerable amount of nanoparticles are accompanied with the octahedra of magnetite (Figure 5b-e). This suggests that the precipitation of nanosized magnetite and the growth of octahedral magnetite occur simultaneously during the hydrothermal treatment. We summarize the effect of reaction times on the average dimensions of octahedrons (Figure 5f), from which it can be noticed that the sizes of octahedrons significantly increase from 15 to 18 h. This probably indicates that at the early stage of magnetite formation Fe3+ reduction and nanosized magnetite precipitation predominate, while at the later stage the growth of octahedral magnetite controlled by Ostwald ripening process predominates. Thus, in such mild growing environment the magnetite crystals prefer to exhibit thermodynamically stable octahedral {111}

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Figure 6. Magnetization hysteresis loops of magnetite octahedrons (sample DS1-10, Table 1) measured at room temperature.

form because the {111} faces are the lowest energetic planes of magnetite crystal. It is well-known that physical and chemical properties of materials significantly depend on their size and shape.17,28,31 The magnetic properties of the microsized magnetite products (sample DS1-10, Table 1) were also investigated at room temperature from -10 000 to 10 000 Oe. In Figure 6 the hysteresis loops of the as-obtained magnetite octahedrons show a proper ferromagnetic behavior with saturation magnetization (Ms) of 71.6 emu/g, and in the inset of Figure 6 an expanded low-field hysteresis reveals that the remanent magnetization (Mr) and coercivity (Hc) are 9.8 emu/g and 120 Oe, respectively. The Ms value is different from that of magnetite nanoparticles or nanorods,17,28,31 and from the theoretical Ms value (ca. 92 emu/g) of bulk magnetite.32 This possibly results from the multiple domains and low anisotropy of magnetite octahedrons. On the one hand, it is suggested by Dunlop that the critical size of the single domain for magnetite particles is 54 nm,33 and thus, the microsized magnetite octahedrons in our case contain more magnetic domains than nanosized magnetite and have different magnetic properties from nanoparticles or nanorods. On the other hand, magnetic properties of materials are significantly influenced by many factors, such as size, structure, shape, surface disorder, etc.17,28,31 The magnetite crystal in our work exhibits the perfect octahedral structure. The low anisotropy, especially easy-magnetization anisotropy, of perfect octahedrons may hinder the saturation magnetization of magnetite particles so that our magnetite octahedrons have a lower Ms value than nanosized magnetite or bulk magnetite. A similar effect of low anisotropy of octahedral magnetite has been proved in the literature.28 Metal oxides with spinel structure have been widely used in the manipulation of Li ion battery because of their low cost, low toxicity, and high electrode potential.34 As a member of the spinel family, the magnetite octahedrons (sample DS1-10, Table 1) were measured for their electrochemical performance. The curve of voltage versus discharge capacity of the magnetite particles in the 3.0-0.5 V window is shown in Figure 7. The magnetite delivers a discharge capacity of ca. 600 mAh/g in the first cycle at a current density of 0.1 mA cm-2, corresponding to the lithium uptake of 5.4 Li per Fe3O4. The value of discharge capacity and the lithium uptake is lower than that reported in the literature,28 possibly due to the declined diffusion coefficient within microsized dense bulk.34a The discharge capacity includes two parts, (i) Fe(II) f Fe(0) and (ii) Fe(III) f Fe(0), which

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Figure 7. Voltage versus discharge capacity curve and cycle performance (inset) for the magnetite octahedrons (sample DS1-10, Table 1)/Li cell in the 3.0-0.5 V window.

are consistent with the two voltage plateaus at 0.92 and 0.74 V, respectively. Similar to the reaction of other transition-metal oxides with lithium,35 the electrochemical performance of magnetite octahedrons can be attributed to intercalation/deintercalation of lithium ions into host structures. Accordingly, in our case, the perfect crystal with smooth surface is of benefit to lithium intercalation/deintercalation. The whole electrochemical reaction can be written as Fe3O4 + xLi+ + xe- T LixFe3O4 T Fe + Li2O. Compared with other magnetic materials in the nano- or submicroscale, the magnetite octahedron electrode possesses a lower voltage. This is possibly because the specificsurface area for the intercalation/deintercalation of lithium ions relatively decreases. We also investigated the cycle performance of the magnetite octahedron electrode (see inset in Figure 7). It can be clearly seen that the capacity dramatically decreases to less than 100 mAh/g after the first discharge. Here it is proposed that a mass of Li2O is formed on the octahedron surface concomitant with the reduction of Fe3O4 during the first discharging process, resulting that the later reaction inside mostly fades. This implies that the size and shape of material can strongly affect the capacity retention of Li battery. Therefore, the electrode material with special structure used in the manipulation of Li battery should be seriously selected. Conclusions In this article, magnetite octohedra can manipulatively be synthesized using an aspartic-acid-assisted route under a mild hydrothermal condition. These magnetite particles as large as ca. 5 µm have regular octahedron morphology. Some experimental factors influencing magnetite precipitation in the hydrothermal process were also investigated. A series of timecourse experiments reveals that at the early stage of magnetite formation Fe3+ reduction and nanosized magnetite formation predominate, while at the later stage Ostwald ripening contributes to the growth of perfect octahedral magnetite. Compared with other synthetic methods, our route is typically toxic-free and environmentally friendly. Therefore, this biomoleculeassisted method can provide a novel opportunity for synthesizing other metal oxides or bioinorganic composites. Moreover, magnetic properties and electrochemical performances of the magnetite octahedrons through this novel biomolecule-inspired method have also been evaluated.

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