Article pubs.acs.org/JPCC
Solvothermal Synthesis of Tunable Electroactive Magnetite Nanorods by Controlling the Side Reaction Haiyan Sun, Bo Chen, Xiuling Jiao, Zhen Jiang, Zhenhua Qin, and Dairong Chen* Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100 P. R. China S Supporting Information *
ABSTRACT: A solvothermal process was designed to synthesize magnetite (Fe3O4) nanorods using iron pentacarbonyl (Fe(CO)5), oleic acid, and hexadecylamine as raw materials. In the preparation process, Fe(CO)5 was first decomposed and oxidized to form FeO. Meanwhile, Fe(CO)5 reacted with oleic acid to form iron oleate. In the system water derived from the reaction between oleic acid and hexadecylamine resulted in the hydrolysis of iron oleate to form the initial Fe3O4 nanorods. In the following process, the dissolution of FeO and decomposition of residual Fe(CO)5 as well as the hydrolysis of iron oleate provided the source for the growth of Fe3O4 nanorods, which led to the enlargement of the particles with time. By adjusting the reaction time or the amount of the added hexadecylamine, the length of uniform nanorods could be tuned from 63 to 140 nm. Furthermore, the as-prepared Fe3O4 nanorods showed excellent performance in electrochemical property and exhibited different magnetic property from spherical nanoparticles and nanoplates for their shape anisotropy.
1. INTRODUCTION Anisotropic nanostructures have attracted widespread attention in the past decades owing to their unique electronic, magnetic, and optical properties in various applications.1−4 In particular, the controlled synthesis of one-dimensional (1-D) metallic and semiconductor nanostructures is one of the research hotspots, as anisotropic morphologies can dramatically influence their physical and chemical properties.5 For example, the gold nanorods exhibit size-normalized optical cross sections that are an order of magnitude higher than those for gold nanospheres with the similar size.6,7 Silver nanorods with different aspect ratios could scatter the color of light from the visible to the near-infrared region due to their anisotropy.8 Co3O4 nanorods predominantly exposed {110} planes presented a highly efficient oxidation catalytic property.9 As an important magnetic material, magnetite was widely applied in energy storage,10 magnetic resonance imaging (MRI),11−13 magnetic data storage,14 ferrofluids,15 catalysis,16 and clinical diagnosis and treatment.17,18 1-D magnetite nanoworms with an elongated assembly of nanoparticles prolonged the blood circulation time, enhanced retention at tumor sites, and improved targeting efficiency compared with spherelike nanoparticles, which demonstrated that the anisotropic magnetite nanoparticles could potentially lead to further advancement in biomedical applications.19 Thus, 1-D nanostructured magnetite with regular and good crystallite morphology is of great importance, for these anisotropic nanomaterials own unique shape-dependent properties and resulting applications.20 In the past decade, many groups paid special attention to the preparation of uniform monodispersed magnetite nanoparticles, © 2012 American Chemical Society
and a series of monodispersed Fe3O4 nanoparticles have been reported for various applications.21,22 However, most preparations focused on the zero-dimensional (0-D) Fe3O4 nanostructures, and a few of studies have reported its two-dimensional (2-D) nanostructures; to the best our knowledge, there are no reports on the synthesis of 1-D magnetite nanostructure, which might be related to its cubic spinel structure. Various chemical routes including coprecipitation,23 microemulsion synthesis,24 sol−gel synthesis,25 sonochemical reaction,26 electrospray synthesis,27 laser pyrolysis,28 solvothermal reaction,29,30 and thermal decomposition have been used to synthesize magnetite nanoparticles.31−35 Among these methods, the thermal decomposition of iron-complex precursors in high-boiling organic solvents containing fatty acids or amines as stabilizing agents is the most popular method for synthesizing high-quality magnetite nanoparticles. In this process, iron oleate and Fe(acac)3 can thermal-decompose and be reduced to form Fe3O4, and fatty acids or amines such as oleic acid and oleylamine can effectively prevent the agglomeration of magnetic nanoparticles and control the size of the nanocrystals with good dispersity.36,37 The thermal decomposition method has achieved great success in controlling the size and dispersity of magnetite nanoparticles, but it cannot modulate the shape of magnetite to produce 1-D nanostructures. Recently, it was reported that 1-D γ-Fe2O3 nanowhisker was prepared by the decomposition of iron oleate complex at 150 Received: December 13, 2011 Revised: February 7, 2012 Published: February 8, 2012 5476
dx.doi.org/10.1021/jp211986a | J. Phys. Chem. C 2012, 116, 5476−5481
The Journal of Physical Chemistry C
Article
°C in 1-octancene.38 However, it is believed that the formation of the product should go through the hydrolysis of iron oleate rather than thermal decomposition because (1) the decomposition of iron oleate to form iron oxide occurs commonly at >300 °C,31−33,35 (2) the thermal decomposition would produce H2 or CO, which reduced Fe3+ to Fe2+ to form Fe3O4 or FeO rather than γ-Fe2O3, (3) the iron oleate precursor should contain trace water, for deionized water was used to wash the sample and it was only dried at room temperature during its preparation. According to above analyses, it is considered that the hydrolysis of iron oleate might result in the formation of 1D nanostructures. If the reduction occurs simultaneously, it is presumed that 1-D Fe3O4 might be formed. Based on this consideration, Fe(CO)5, which easily decomposes and shows reducibility, was selected as iron source. Oleic acid is used as reagent to form iron oleate under solvothermal condition. Water is necessary for the hydrolysis of iron oleate, but the product easily agglomerates for the existence of a large amount of surface hydroxyls if water is added to the system directly, so organic amine, hexadecylamine, is added to release water through its reaction with oleic acid. As a result, the magnetite nanorods with a narrow size distribution are obtained by systematically adjusting the reagent ratio and reaction parameters, and the nanorods exhibit electroactive during electrochemical measurement.
temperature with a maximum magnetic field of 20 kOe. For magnetization measurements, the powder was pressed strongly and fixed in a small cylindrical plastic box. Electrochemical Measurement. Carbon black was used as conductor, and polytetrafluoroethene was used as adhesion agent. Active material (Fe3O4), carbon black, and polytetrafluoroethene with a weight ratio of 80:10:10 were mixed by mortar and pestle. The poly(vinylidene fluoride) was added to the solid mixture to form mash. Then the mash was pressed onto a nickel mesh (ca. 1 cm2) under 10 MPa for 10 min and dried at 80 °C overnight. Electrochemical measurements were tested on a CHI660 electrochemical workstation, and a threeelectrode cell was used. The nickel mesh with Fe3O4 film, a platinum plate, and saturated calomel electrode (SCE) were used as the working electrode, the counter electrode, and the reference electrode, respectively. Na2SO3 solution (1 mol/L) was used as the electrolyte.
3. RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of the product. The positions of the diffraction peaks match well with the cubic magnetite
2. EXPERIMENTAL SECTION Synthesis. All chemicals were analytical grade except hexadecylamine (90%, technical grade) and were used without any further treatment. In a typical preparation of magnetite nanorods, 0.20 g (0.83 mmol) of hexadecylamine and 2.0 mL (6.33 mmol) oleic acid were added to 8.0 mL of n-octanol, and then the mixture was heated to about 50 °C to form a homogeneous liquid with magnetic stirring. After the solution cooled to room temperature, 2.0 mL (13.81 mmol) of Fe(CO)5 was added. The mixture was stirred for a while and transferred into a 15.0 mL of autoclave with a Telfon linear. The autoclave was heated to 200 °C for 6 h. After cooling to room temperature, the precipitate was separated by centrifugation, washed with ethanol three times, and then dried under ambient condition. Characterization. The X-ray diffraction (XRD) patterns of the powder samples were collected at room temperature on a Rigaku D/MAX 2200PC diffractometer with a graphite monochromator and Cu Kα (λ = 0.154 18 nm) radiation. The morphology and microstructure of the products were characterized using a transmission electron microscope (TEM, JEOL JEM-1400) and a high-resolution TEM (HR-TEM, JEOL JEM-2100); before observation, the products were dispersed in cyclohexane. Thermal gravimetric (TG) analysis was carried out on a Netzsch STA449F3 Jupiter thermal gravimetric analyzer at a heating rate of 10.0 °C/min under air and nitrogen atmospheres. The infrared (IR) spectra were examined on a Nicolet 5DX Fourier transform infrared (FT-IR) spectrometer using the KBr pellet technique. Elemental analyses (C, H, N) were performed with a Perkin-Elmer 240 elemental analyzer. The X-ray photoelectron spectrum (XPS) was recorded on a PHI-5300 ESCA spectrometer (Perkin-Elmer) with its energy analyzer working in the pass energy mode at 35.75 eV, and the Al Kα line was used as the excitation source. The binding energy reference was taken at 284.7 eV for the C 1s peak arising from surface hydrocarbons. The hysteresis loops were recorded by using a LDJ9500 vibrating sample magnetometer at room
Figure 1. XRD pattern (a) and XPS spectrum (b) of the typical product.
(JCPDS, No. 65-3107) and can be indexed to (111), (220), (311), (222), (400), (422), (511), (440), and (533) crystal planes of cubic Fe3O4. No peaks of other phases are observed from the pattern, indicating the phase-pure nature of the product. Because of the similarity of the XRD pattern of Fe3O4 and that of γ-Fe2O3, XPS analysis of the product was undertaken to further determine the phase of the product accurately. XPS pattern (Figure 1b) gives the binding energy of Fe 2p. The signals at 711.58 and 725.40 eV correspond to Fe 2p3/2 and Fe2p1/2 levels, which is consistent with the data of Fe3O4 reported in the literature.30 The absence of satellite around 718 eV further demonstrates that the product is pure Fe3O4 without γ-Fe2O3.39 The TEM and HR-TEM images of the product obtained in the typical experiment are shown in Figure 2. From the TEM image, the products display well-defined nanorods with the diameter of ca. 6.5 ± 2 nm and length of ca. 63 ± 5 nm. The nanorods exhibit excellent dispersity in the cyclohexane. Considering the small size and high surface energy of the asprepared Fe3O4, it is presumed that a layer of organics might coat on the nanorods’ surface, which improves the dispersity of the product and prevents the surface Fe2+ from being oxidized to Fe3+. The selected area electron diffraction (SAED) pattern (inset in Figure 2a) exhibits four clear diffraction rings, indicating the good crystallinity of the nanorods. The HRTEM image (Figure 2b) shows continuous parallel lattice fringes with uniform contrast in a single nanorod, revealing its single crystalline nature. The lattice spacings are ca. 0.241 and 5477
dx.doi.org/10.1021/jp211986a | J. Phys. Chem. C 2012, 116, 5476−5481
The Journal of Physical Chemistry C
Article
The first drop (
