Synthesis and Magnetic Properties of Iron Phosphide Nanorods - The

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J. Phys. Chem. C 2010, 114, 4808–4815

Synthesis and Magnetic Properties of Iron Phosphide Nanorods Chieh-Tsung Lo* and Po-Yu Kuo Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan City 70101, Taiwan ReceiVed: October 29, 2009; ReVised Manuscript ReceiVed: February 10, 2010

Iron phosphide nanorods were fabricated by the thermal decomposition of Fe(CO)5 in a solution containing magnetite nanoparticles, trioctylphosphine, and didodecyldimethylammonium bromide at 300 °C under an argon atmosphere. The length of the rods was varied from 30 to 260 nm, whereas the diameter of the rods was ∼8 nm by the multiple injections of Fe(CO)5. It was found that the rod size increased with the number of injections under the constant total injection concentration and reaction time. In addition, the size of the nanorods could be manipulated by changing the reaction time, reaction temperature, and seed concentration. These nanorods, composed of magnetite particles in the core and Fe2P in the shell, exhibit unique magnetic properties. The magnetic properties of nanorods characterized by a superconducting quantum interference device showed that the blocking temperature of the rods increased with rod length and reached a maximum when the length of the rods was ∼60 nm. With the longer rod length, the blocking temperature of the rods decreased with increasing rod size. We believe that our accomplishments to synthesize magnetic nanorods with a core-shell structure in a controlled manner will provide exciting new possibilities on the materials front. Introduction One dimensional magnetic nanomaterials have received tremendous attention because of their potential applications in a variety of disciplines, including data storage media and spindependent electron transport devices.1-5 While the magnetization of magnetic nanostructures strongly depends on their size and anisotropy,6-8 the ability to manipulate the shape and size of nanomaterials is crucial to determine their magnetic properties and achieve the scientific and technological needs. Various strategies have been developed to obtain onedimensional nanorods. The materials of these nanorods consist of metal, metal oxide, and metal chalcogenides, and the shapedependent magnetic, electrical, and optical properties are evaluated. It is well-known that the formation of onedimensional nanorods requires a well-controlled anisotropic crystal growth that can be achieved by the addition of two surfactants with distinct different binding capabilities to the facets.7,9-11 The shape and size of nanorods can be controlled by simply changing the surfactant composition.12 Hyeon et al.10 manipulated the relative binding strength of two surfactants by the reaction temperature. At high temperature, the rapid thermal decomposition of precursors occurs and this kinetically controlled condition induces the growth of nanorods. At low temperature, instead, precursors decompose slowly and the growth of spherical nanoparticles dominates under the thermodynamically controlled condition. Much research has shown that the length of surfactants also plays an important role on the synthesis of nanorods.11,13-15 Herman et al.11 synthesized CdSe nanorods by varying the length of the alkylphosphonic acid ligand. With a decrease of the length of the ligand, nanorods grow more elongated and branched. This is because the longer ligand exhibits steric hindrance that results in the slower rod growth rate, leading to the shorter rods. When the mixtures of two different lengths of ligands are used to prepare nanorods, * To whom correspondence should be addressed. E-mail: tsunglo@ mail.ncku.edu.tw.

the use of a higher molar fraction of the shorter ligand can produce more elongated and branched rods. In addition to the ratio of surfactants, the precursor concentration also affects the formation of nanorods. Cheon et al.16 synthesized CdS nanorods by varying the concentration of precursors. It is observed that higher precursor concentrations can generate nanorods with larger average sizes and relatively monodispersed aspect ratios. Peng et al.17-19 proposed that the shape, size, and size/shape distributions of the nanorods depend strongly on the size of nuclei and the concentration of the remaining monomers after the initial nucleation stage. With the higher monomer concentration, the diffusion of monomers into the nuclei is consumed by the preferred facets of the nanocrystals, resulting in a one-dimensional growth. With the lower monomer concentration, the three-dimensional growth of rods occurs due to the equal sharing of incoming monomers to all the facets. To further tune the length of the nanorods, continuous injection of precursors to increase the monomer concentration is widely used.6-8,20-22 In general, multiple injections of precursors promote the growth of rods along the long axis, while the growth rate of the short axis is inhibited dramatically. Thus, an increase in the aspect ratio of the rods is achieved. Hyeon et al.6,7 synthesized metal phosphide nanorods with continuous injections of precursors using a syringe pump. They found that the injection rate is also crucial to control the length of the nanorods. With the reduction of the injection rate, the time necessary to reach the critical complex concentration for nucleation increases, resulting in nanorods with a lower aspect ratio. In this study, we proposed a method to prepare iron phosphide nanorods with a controlled length. Unlike other one-step methods to produce nanorods, we synthesized nanorods by first preparing nucleation seeds. Further injection of the stock solution comprising precursors and surfactants induced the oriented attachments of particles, leading to the growth of nanorods. The length of the nanorods could be tuned by multiple injections or varying the concentration of the injection products.

10.1021/jp9103239  2010 American Chemical Society Published on Web 03/01/2010

Iron Phosphide Nanorods

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Figure 1. (a) TEM of spherical iron nanoparticles and (b) the corresponding XRD of these particles.

As-prepared nanoparticles were characterized by transmission electron microscopy and X-ray diffraction, and the magnetic properties of these nanorods were analyzed by a superconducting quantum interference device magnetometer. The correlation between the rod size and shape and magnetic properties was discussed. Experimental Methods Sample Preparation. Because of the high hazard nature of Fe(CO)5, it was stored in a refrigerator maintained at -20 °C prior to use. Additionally, all the following manipulations were performed using standard air-free techniques. The synthesis of spherical iron particle seeds was presented elsewhere.23-25 Briefly, 1.0 g of Fe(CO)5 (>97%, Sigma-Aldrich Co.) and 3.0 g of trioctylphosphine oxide (TOPO, 99%, Sigma-Aldrich Co.) were mixed at 70 °C. The resulting solution was added immediately into the preheated TOPO (10.0 g) at 340 °C under an argon atmosphere and aged for 30 min at 320 °C. Asprepared nanoparticles were purified by the addition of excess acetone to remove the residual ions and TOPO. These nanoparticles were redispersed in 10 mL of pyridine and used as nucleation seeds for rod formation. The procedure to prepare nanorods was a modification of the method developed by Park et al.25 A 5.0 g portion of trioctylphosphine (TOP, 90%, Sigma-Aldrich Co.) and 0.5 g of didodecyldimethylammonium bromide (DDAB, 98%, SigmaAldrich Co.) were dissolved in 1 mL of synthesized particle solution. This mixture was heated to 300 °C under an argon atmosphere. The stock solution composed of Fe(CO)5 and TOP was manually injected under argon via a syringe into the mixture, and particles were allowed to grow at 300 °C. In this step, spherical nanoparticles transformed to rod-shaped particles. To obtain the relationship between the number of injections and the size of nanorods, we varied the injection condition to control the length of these particles. The resulting particles were washed several times with hexane and collected by centrifuge for further characterization. Characterization of Nanorods. The length and diameter of magnetic nanorods were characterized by transmission electron microscopy (TEM), operated on JEOL JEM-1400 and Hitachi H7500 electron microscopes. Samples for TEM analysis were prepared by making a drop of particle solution on a copper grid coated with a carbon film. XRD patterns were obtained using a Rigaku RINT-2000 diffractometer. Data was collected in the angle scan from 20° to 80° with a scan rate of 5°/min. The magnetic measurements of the nanorods were performed using a superconducting quantum interface device (SQUID, MPMS XL-7) at National Sun Yat-Sen University in Taiwan. Magne-

tization versus temperature data were collected at temperatures in the range of 5-350 K in the magnetic field of 100 Oe under both field-cooled (FC) and zero-field-cooled (ZFC) conditions. Results and Discussion Synthesis of Magnetic Spherical Particles. Figure 1a shows the spherical nanoparticles used as nucleation seeds for the preparation of magnetic nanorods. The diameter is ∼5.5 ( 0.8 nm. In Figure 1b, the XRD pattern revealed that these particles are composed of iron oxide. However, because magnetite and maghemite have similar XRD patterns, it is difficult to exclude the existence of maghemite in the sample. However, the value of the saturation magnetization of particles suggests that the sample consists of a significant fraction of magnetite.26 Length Control of Magnetic Nanorods. In this study, the synthesis of magnetic nanorods with different aspect ratios was achieved by the use of multiple injections of precursor materials. Conventional studies employed a syringe pump for the continuous delivery of precursor to induce the growth of nanorods.6-8,20-22 Unlike this approach, we injected the same total amount of precursor into the mixture of nucleation seeds and surfactants but varied the number of injections. Figure 2 shows the structure of magnetic nanorods as a function of the number of injections, and the detailed experimental conditions are shown in Table 1. In these reactions, the total amount of injected Fe(CO)5 and TOP and the total reaction time were kept constant. Figure 3 summarizes the diameter, length, and the aspect ratio of these nanorods. The average size and shape distribution were determined by counting at least 100 particles per sample for statistical purposes. It was obtained that both the length (Figure 3a) and the aspect ratio (Figure 3b) of magnetic nanorods were highly dependent on the number of injections. The length of the rods was 36.8 nm for a one time injection and increased to 114.8 nm for eight injections. The diameter of the nanorods, instead, was almost identical to that of the seeds. This result indicates that the synthesis of nanorods can be achieved by the addition of nucleation seeds, and the length of nanorods could be manipulated by multiple injections of the precursor and the surfactant. In Figure 4, the corresponding XRD patterns of those nanorods are depicted. The peak positions were entirely different from those of nucleation seeds, and the patterns were assigned to the (111), (210), and (002) reflections of the hexagonal Fe2P structure.6 The contribution of phosphorus in Fe2P nanorods was from TOP during the reaction. To confirm that the synthesis of nanorods using multiple injections of precursor materials favors the growth of nanorods, we varied the total injected precursor concentration and reaction time, and the detailed experimental conditions are shown in

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Figure 2. TEM images of nanorods prepared by (a) the one-time injection of 0.08 g of Fe(CO)5 and 4.0 g of TOP and reacting for 160 min, (b) the injection of 0.04 g of Fe (CO)5 and 2.0 g of TOP every 80 min for two times, (c) the injection of 0.02 g of Fe(CO)5 and 1.0 g of TOP every 40 min for four times, (d) the injection of 0.013 g of Fe(CO)5 and 0.67 g of TOP every 26.7 min for six times, and (e) the injection of 0.01 g of Fe(CO)5 and 0.5 g of TOP every 20 min for eight times.

TABLE 1: Experimental Conditions for the Synthesis of Nanorodsa

injection times

amount of Fe(CO)5 injected (g/injection)

total amount of Fe(CO)5 injected (g)

amount of TOP injected (g/injection)

total amount of TOP injected (g)

period of injection (min)

total reaction time (min)

1 2 4 6 8

0.08 0.04 0.02 0.013 0.01

0.08 0.08 0.08 0.08 0.08

4.0 2.0 1.0 0.67 0.5

4.0 4.0 4.0 4.0 4.0

80 40 26.7 20

160 160 160 160 160

a

The total amount of Fe(CO)5 and TOP injected was 0.08 and 4.0 g, respectively, and the total reaction time was 160 min.

Table 2. When the total amount of 0.04 g of Fe(CO)5 and 2.0 g of TOP was injected and the total reaction time was 80 min, the length of the nanorods increased monotonically from 27.8 ( 9.2 nm for a single injection, to 34.5 ( 7.3 nm for two injections, to 52.6 ( 9.8 nm for three injections, and to 64.7 ( 15.7 nm for four injections. The aspect ratio of the nanorods varied from 3.7 ( 1.1, 4.9 ( 1.0, and 6.8 ( 1.2 to 8.1 ( 2.3 for a single injection to multiple injections, while the diameter of the nanorods was between 7.0 and 8.0 nm for each sample and did not show the significant difference. From these experiments, we ensure that the preparation of nanorods using multiple injections can efficiently produce nanorods with different lengths, while the diameter of the rods is comparable to that of the nucleation seeds. In the condition of the same total precursor and surfactant concentration during injection, the length of the nanorods increases with the number of injections. However, the length distribution also becomes more polydispersed. We also studied the effect of the total reaction time on the size of the nanorods. Nanorods were prepared by multiple injections with the total amount of 0.04 g of Fe(CO)5 and 2.0 g of TOP injected and the total reaction time of 160 min. This experimental conditions were the same as that to prepare nanorods in Table 2, but the total reaction time was doubled, and the time interval between two consecutive injections also

was doubled. Figure 5 compares the length of the nanorods prepared with different reaction times. It was found that the increase in the reaction time mainly resulted in an increase in the elongation of the nanorods. It has to be pointed out that the diameter of the nanorods also did not show a significant change with time. The increasing length of the nanorods with reaction time can be attributed to the achievement of the complete reaction with time that promotes the growth of nanorods. However, with longer reaction time, the length of the nanorods seems to become more polydispersed. The effect of reaction temperature on the synthesis of nanorods is shown in Figure 6, and the characterization of the resulting particles was summarized in Table 3. In our system, the role of TOP is not only a surfactant but also the solvent to disperse nucleation seeds. While the boiling point of TOP is ∼291 °C, it would be interesting to know how the reaction temperature influences the behavior of TOP and further affects the synthesis of nanorods. When the reaction temperature was at 250 °C, the injection method could not induce the formation of rod-shaped particles (Figure 6a). Instead, the nucleation seeds grew in size from a diameter of 5.5 ( 0.9 to 13.8 ( 2.4 nm. When the temperature increased to 280 °C (Figure 6b) and 285 °C (Figure 6c), rod-shaped particles were obtained with a

Iron Phosphide Nanorods

Figure 3. (a) The length, diameter, and (b) aspect ratio of magnetic nanorods as a function of the number of injections during the synthesis of nanorods. The total amount of Fe(CO)5 and TOP injected was 0.08 and 4.0 g, respectively, and the total reaction time was 160 min.

Figure 4. XRD patterns of magnetic nanorods. The synthetic route to prepare nanorods, from top to bottom, was (a) the one-time injection of 0.08 g of Fe(CO)5 and 4.0 g of TOP and reacted for 160 min, (b) the injection of 0.04 g of Fe (CO)5 and 2.0 g of TOP every 80 min for two times, (c) the injection of 0.02 g of Fe(CO)5 and 1.0 g of TOP every 40 min for four times, (d) the injection of 0.013 g of Fe(CO)5 and 0.67 g of TOP every 26.7 min for six times, and (e) the injection of 0.01 g of Fe(CO)5 and 0.5 g of TOP every 20 min for eight times.

diameter of ∼8 nm and a length of ∼115 nm. However, a large number of spherical particles also existed. When the reaction temperature was close to the boiling point of TOP, as shown in Figure 6d, only nanorods formed. For the reaction temperatures between 300 and 320 °C (Figure 6e-g), well-defined nanorods

J. Phys. Chem. C, Vol. 114, No. 11, 2010 4811 were produced, and the diameter of rods increased slightly from 9.1 to 9.7 nm, and the length of rods grew from ∼170 to ∼223 nm. This result indicates that higher reaction temperature can promote the growth of nanorods. It is known that, when the synthesis of particles is performed at low temperature, this thermodynamically controlled condition favors the slow decomposition of the precursor, and spherical nanoparticles are formed. At high temperature, the reaction is kinetically controlled, which leads to the rapid thermal decomposition. Thus, one-dimensional nanorods are generated.6,10,13,14,27 Although our synthesis procedure involves the addition of nucleation seeds and multiple injections, the results agree well with other research. In Figure 6h, nanorods were prepared at 330 °C, and the resulting particles showed a rough surface. In addition, the diameter of the rods increased significantly to 12.5 nm compared with between 7 and 9 nm in diameter for nanorods prepared at lower temperatures. These nanorods with specific morphologies were presumably due to the intense thermal fluctuations that caused the decomposition of TOP and DDAB28 and, hence, destructed the TOP-particle and DDAB-particle bonding. These results indicate that the reaction temperature is a strong function of the rod growth, and the synthesis of nanorods at an appropriate temperature can manipulate the size of nanorods. Figure 7 shows nanorods prepared with different concentrations of nucleation seeds. In these experiments, ten injections were utilized with 0.02 g of Fe(CO)5 and 0.25 g of TOP at each 20 min interval to prepare nanorods. When the initial seed concentration was 2.5 wt % (Figure 7a), nanorods formed with a diameter of 7.4 ( 1.1 nm and a length of 54.1 ( 6.3 nm in. With an increase in seed concentration to 5.0 wt % (Figure 7b), nanorods grew to 7.5 ( 0.8 nm in diameter and 68.9 ( 8.6 nm in length. The length of the nanorods could be further prolonged to 110.5 ( 22.6 nm (Figure 7c), 159.8 ( 34.8 nm (Figure 7d), and 263.4 ( 58.2 nm (Figure 7e) for 10.0, 20.0, and 30.0 wt % seed concentrations, respectively, while the diameter slightly increased from 7.3 to 8.9 nm with increasing seed concentration. From these results, it was concluded that the length of the nanorods can be manipulated by the seed concentration, and higher seed concentration promotes the growth of nanorods. It has been reported that two possible mechanisms are involved in the formation of nanorods. One is to use a single surfactant to synthesize nanorods.27,29-35 This kind of surfactant, such as DDAB and cetyltrimethylammonium bromide (CTAB), spontaneously organizes into rod-shaped micelles when their concentration reaches a critical micelle concentration. This structure can then be used as a template to promote the growth of nanorods. The other conventional approach is to utilize multiple surfactants in the reaction.6,7,10,20,36,37 If these surfactants exhibit different binding properties to the particles, the inhibiting growth along some crystal faces would lead to the formation of anisotropic particles. In our work, Fe2P nanorods were prepared by multiple injections of Fe(CO)5 precursor and TOP surfactant in the solution containing nucleation seeds, TOP and DDAB. Here, TOP is not only a surfactant but also acts as a solvent to disperse nucleation seeds. In contrast, the role of DDAB on the synthesis of nanorods is not clear. Hence, the preparation of nanorods without the addition of DDAB was performed. It was found that only the spherical particles with a size similar to that of nucleation seeds were prepared under the identical reaction conditions. This indicates that the addition of DDBA is necessary to form rod-shaped particles. As mentioned earlier, DDAB can form micelles and provides as a template to synthesize one-dimensional particles. However, because this micellar structure of DDAB becomes unstable at high temper-

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TABLE 2: Experimental Conditions for the Synthesis of Nanorodsa

injection times

amount of Fe(CO)5 injected (g/injection)

total amount of Fe(CO)5 injected (g)

amount of TOP injected (g/injection)

total amount of TOP injected (g)

period of injection (min)

total reaction time (min)

1 2 3 4

0.04 0.02 0.013 0.01

0.04 0.04 0.04 0.04

2.0 1.0 0.67 0.5

2.0 2.0 2.0 2.0

40 26.7 20

80 80 80 80

a

The total amount of Fe(CO)5 and TOP injected was 0.04 and 2.0 g, respectively, and the total reaction time was 80 min.

Figure 5. Comparison of nanorods prepared with different reaction times. The total amount of Fe(CO)5 and TOP injected was 0.04 and 2.0 g, respectively.

atures,38 the formation of micelles as a template for the growth of nanorods is presumably not possible. We also performed a kinetics study to probe the role of DDAB on the growth of nanorods. Similar to the previous procedure to prepare nanorods, 5.0 g of TOP and 0.5 g of DDAB were dissolved in 1 mL of nucleation seed solution. The stock solution containing 0.02 g of Fe(CO)5 and 0.5 g of TOP was injected into the mixture every 20 min. After each injection, samples were collected and characterized by TEM. After the first injection, only spherical particles were observed. It wasn’t until the third injection that rod-shaped particles were formed with the length of 41.2 ( 5.5 nm. Additionally, these nanorods coexisted with unreacted nucleation seeds. With further injecting the stock solution, nanorods grew from 77.8 ( 18.6 nm after the fourth injection to 131.8 ( 47.6 nm after the fifth injection, and the number of nucleation seeds gradually reduced. It has to be pointed out that, in this kinetics study, the concentration of DDAB was kept constant. If the role of DDAB was to provide a template for particle growth, the size of micelles was supposed to be identical in this study, regardless of the number of injections owing to the same DDAB concentration. Thus, the resulting nanorods should exhibit similar sizes. This assumption of DDAB as a template is apparently not the case to explain our finding, and the growth of nanorods could not follow this mechanism. A previous report proposed that DDAB can strongly bind to the central region of the growing nanoparticles, allowing nanospheres to fuse on the edge to generate uni-directional nanorods.25 On the basis of the characteristics of DDAB and our experimental support, we can conclude that both DDAB and TOP are surfactants in this process, and the cooperative interaction of two surfactants with different binding capabilities enables generating the rod-shaped particles. In addition to DDAB and TOP, the addition of nucleation seeds is also crucial to produce nanorods. We synthesized nanorods without the addition of nucleation seeds, and it resulted in reaction failure, which was evidenced by a lack of color change. Therefore, the mechanism of the formation of nanorods

seems to be the connection of nucleation seeds through the two surfactants with different binding energies under a kinetically controlled high-temperature condition. The decomposed metal complexes were deposited on the surface of nucleation seeds to form a rod-shaped particle. Because DDAB was strongly bound on the central region,25 the continuous connection of nucleation seeds on the edge of the rod and the deposition of metal complexes extend the length of the nanorods, as shown in Figure 8, generating a catenated structure. Previous research demonstrated the use of the multiple injections to continuously deliver precursor solution in a hot surfactant solution to induce the growth of nanorods, and the rods became progressively longer as the injection volume increases.6-8,20 The length of the nanorods was directly proportional to the injection rate of the precursor solution because the lower injection rate allowed more time to reach the critical precursor concentration for nucleation, leading to the formation of shorter rods.7 In our approach, the total precursor concentration kept unchanged, but the precursor concentration in each injection was varied. The nanorods prepared by this method increase in length with an increase in the number of injections or a decrease in precursor concentration in each injection. On the basis of our observation, we proposed the forming process of nanorods as a function of the number of injections. At the lower number of injections (higher precursor concentration in each injection), high growth of nanorods occurs, which causes the isotropic deposition of Fe2P on the nucleation seeds. Nanorods are shorter and more monodispersed. At the higher number of injections (lower precursor concentration in each injection), slow growth of nanorods occurs. This allows the Fe2P to contribute to the connection of nucleation seeds, forming longer rods. To understand the magnetic properties of these nanorods, we investigated the temperature dependence of magnetization using a SQUID magnetometer. Figure 9 shows the M-T curves of nanorods prepared by multiple injections. It was observed that the magnetization of each curve reached a maximum at a certain temperature on the ZFC curves and was determined as the blocking temperature (TB) of these nanorods. In addition, because the ZFC and FC curves overlapped at temperatures higher than the TB of nanorods, this indicates that these nanorods exhibited hysteresis. The blocking temperature of nanorods characterized from M-T curves as a function of rod size is summarized in Table 4. It was obtained that the TB of these nanorods increased with the size of the rods and reached a maximum when the rod length was ∼53 nm. However, when the rod length was larger than 53 nm, TB showed a decrease with increasing rod size. To confirm this behavior that depended on the size of nanorods rather than processing, we synthesized nanorods using different injection conditions. The blocking temperature of these rods was also determined from M-T curves, and the results are shown in Figure 10. The result agrees well with previous results that the TB exhibits a maximum as the rod length is ∼60 nm. It is also confirmed that the behavior

Iron Phosphide Nanorods

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Figure 6. Synthesis of nanorods at different temperatures: (a) 250, (b) 280, (c) 285, (d) 290, (e) 300, (f) 310, (g) 320, and (h) 330 °C. The total amount of Fe(CO)5 and TOP injected was 0.12 and 6.0 g, respectively, and the total reaction time was 240 min.

TABLE 3: Summary of Nanorods Synthesized at Different Temperaturesa reaction temperature (°C)

diameter of particles (nm)

length of particles (nm)

aspect ratio of particles

characteristics of particles

250 280 285 290 300 310 320 330

13.8 ( 2.4 7.8 ( 0.9 8.3 ( 0.6 8.1 ( 1.0 9.1 ( 1.1 9.3 ( 0.9 9.7 ( 1.2 12.5 ( 2.5

13.8 ( 2.4 115.8 ( 22.2 118.6 ( 18.3 114.1 ( 21.2 172.5 ( 44.9 170.3 ( 47.6 228.4 ( 54.3 254.1 ( 68.9

1.0 14.6 ( 3.3 14.4 ( 2.5 14.2 ( 2.8 18.3 ( 4.8 18.3 ( 4.9 23.5 ( 4.6 20.6 ( 5.1

spheres rods and spheres rods and spheres rods rods rods rods rods with a rough surface

a

The total amount of Fe(CO)5 and TOP injected was 0.12 and 6.0 g, respectively, and the total reaction time was 240 min.

of magnetic properties of these nanorods depends strongly on the size of the nanorods and is independent of sample preparation. The relationship between TB and the rod size was given by Stoner and Wohlfarth39

TB )

KV ln(τm /τo)kB

(1)

Here, kB is the Boltzmann constant, V is the volume of the particles, K is the anisotropy constant, τm is the measurement time, and τo is the characteristic constant of particles that is related to the gyromagnetic procession. The natural log of τm/

τo is normally considered as a constant, and the value is approximately 25.8 Thus, eq 1 is rewritten as

TB )

KV 25kB

(2)

In eq 2, TB is proportional to the anisotropy constant and volume of the particles. Simulation results showed that the linear relationship between TB and particle size was pronounced for smaller particles and depended strongly on the interactions between particles.40 Brock et al. suggested that large particles exhibited higher TB than small ones because they were more stable to thermal fluctuations.41 Chen et al. investigated the

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Figure 7. Synthesis of nanorods with different initial seed concentrations: (a) 2.5, (b) 5.0, (c) 10.0, (d) 20.0, and (e) 30.0 wt %. The total amount of Fe(CO)5 and TOP injected was 0.2 and 2.5 g, respectively, and the total reaction time was 200 min.

Figure 8. Connection of nucleation seeds to form Fe2P nanorods.

magnetization of cobalt nanoparticles with a diameter between 1.8 and 4.4 nm and discovered that the anisotropy constant increased as the particle size decreased.42 This was attributed to the core-shell structure of particles, and the shell phase was responsible for the enhanced anisotropy. In our study, nanoparticles with different sizes exhibit a catenated structure, and the TB of these nanorods has a maximum. The different behavior between TB and particle size in this study from eq 2 is attributed to the core-shell structure of our nanorods that consist of Fe3O4 in the core and Fe2P in the shell. Much research has demonstrated that eq 2 fails to explain the magnetic nanorods with a core-shell structure. The nanorods having an oxide or nonmagnetic metal shell significantly affects the magnetic properties of rods because the layer structure causes the shell moments to be incapable of pinning the core moment in moderate applied fields.43,44 In addition, the interparticle interactions between the core and shell materials also influence the value of TB.45-47 Masala et al.48 investigated the magnetic properties of CoFe2O4/ ZnCoFe2O4 and ZnCoFe2O4/CoFe2O4 core/shell particles and found that the behavior of TB of these materials was different from that with a single structure. In addition, the materials with CoFe2O4 in the core and ZnCoFe2O4 in the shell exhibited

Figure 9. M-T curves of nanorods prepared by (a) the one-time injection of 0.08 g of Fe(CO)5 and 4.0 g of TOP and reacting for 160 min, (b) the injection of 0.04 g of Fe (CO)5 and 2.0 g of TOP every 80 min for two times, (c) the injection of 0.02 g of Fe(CO)5 and 1.0 g of TOP every 40 min for four times, (d) the injection of 0.013 g of Fe(CO)5 and 0.67 g of TOP every 26.7 min for six times, and (e) the injection of 0.01 g of Fe(CO)5 and 0.5 g of TOP every 20 min for eight times.

TABLE 4: Magnetic Properties of Nanorods As a Function of Rod Size average length of rods (nm)

average aspect ratio of rods (nm)

blocking temperature (K)

36.8 ( 5.5 38.8 ( 7.3 53.1 ( 9.8 72.2 ( 14.4 114.8 ( 23.9

4.3 ( 0.7 5.4 ( 1.1 6.3 ( 1.3 7.9 ( 1.6 12.9 ( 2.4

100 162 202 151 132

different TB from that with ZnCoFe2O4 in the core and CoFe2O4 in the shell. This unique behavior of magnetic materials enables tuning magnetic properties by varying the composition and the dimensions of both core and shell. In our study, the nanorods with longer lengths exhibit a larger number of Fe3O4 particles, and thus, the interfacial area between Fe3O4 and Fe2P increases. Therefore, the magnetic interaction and the thermal fluctuation of these materials would have influence on the change of TB. When the length of nanorods is below 60 nm, TB increases with

Iron Phosphide Nanorods

Figure 10. TB of nanorods as a function of rod length.

rod length, and this behavior is consistent with the theory in eq 2. This indicates that the particle size is the main factor to affect TB. When the rod size is larger than 60 nm, TB decreases with increasing rod length. This is presumably caused by the interactions between Fe3O4 and Fe2P, and it dominates the contribution of particle size on TB. Conclusions We synthesized Fe2P nanorods via the injection of a Fe(CO)5-TOP complex in a preheated mixture composed of nucleation seeds, TOP, and DDAB. The size of nanorods ranging from 30 to 260 nm in length and ∼8 nm in diameter could be varied by manipulating the injection condition, reaction temperature, reaction time, and the concentration of nucleation seeds. At a constant total precursor concentration and reaction time, the length of the nanorods increases with the number of injections. This is because the lower number of injections contains the higher precursor concentration in each injection, and under this condition, fast growth of nanorods occurs. This causes the isotropic deposition of Fe2P on the nucleation seeds, leading to the short and monodispersed nanorods. In contrast, when the higher number of injections proceeds, the precursor concentration in each injection is low, and nanorods develop under a slow growth rate. This allows Fe2P to contribute to the connection of nucleation seeds, forming longer rods. The blocking temperature of the nanorods depends strongly on the length of the nanorods and reaches a maximum when the rod length is ∼60 nm. This unique behavior of TB was caused by the interparticle interaction between Fe3O4/Fe2P core/shell particles. Acknowledgment. This work is funded by the National Science Council in the Republic of China under Grant No. NSC 97-2221-E-006-024. Supporting Information Available: TEM images of the evolution of nanorods. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tonucci, R. J.; Justus, B. L.; Campillo, A. J.; Ford, C. E. Science 1992, 258, 783–785. (2) Whitney, T. W.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316–1319. (3) Fert, A.; Piraux, L. J. Magn. Magn. Mater. 1999, 200, 338–358. (4) Sellmyer, D. J.; Zheng, M.; Skomski, R. J. Phys.: Condens. Matter 2001, 13, R433–R460. (5) Lu, A.-H.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 1222–1244. (6) Park, J.; Koo, B.; Hwang, Y.; Bae, C.; An, K.; Park, J.-G.; Park, H. M.; Hyeon, T. Angew. Chem., Int. Ed. 2004, 43, 2282–2285.

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