SCIENCE CHINA Chemistry • ARTICLES • · SPECIAL TOPIC · Inorganic Solid State Chemistry and Energy Materials

June 2011 Vol.54 No.6: 923–929 doi: 10.1007/s11426-011-4286-y

Facile shape and size-controlled growth of uniform magnetite and hematite nanocrystals with tunable properties CHEN LiQiao1,2, LIU WeiPing2, CHEN JiaLin2, YANG XianFeng1, LIU Jia1, FU XiongHui1 & WU MingMei1* 1

Key Laboratory of Bioinorganic and Synthetic Chemistry, Ministry of Education; State Key Laboratory of Optoelectronic Materials and Technologies; School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China 2 State Key Lab of Advanced Technology for Comprehensive Utilization of Platinum Metals; Kunming Institute of Precious Metals, Kunming 650106, China Received January 22, 2011; accepted March 28, 2011

Monodispersed magnetite Fe3O4 and hematite -Fe2O3 nanocrystals have been grown in co-solvents of alcohol and water. Either the shape or the size of the nanocrystals could be easily controlled. Both the phases and nanostructures have been characterized by powder X-ray diffraction patterns and electron microscopy. The magnetic and catalytic properties of these products were investigated and compared with each other. The obtained results clearly demonstrate that these iron oxide nanocrystals are soft ferromagnetic at room temperature and -Fe2O3 has a more effective catalytic property on the thermal decomposition of ammonium perchlorate than Fe3O4. Based on the experimental data, it is proposed that the magnetic and catalytic properties of these nanocrystals are dependent not only on the size and shape, but also on the surface structure of the nanocrystals. The nanoplates with significant anisotropic nanostructure demonstrate a highly enhanced performance as compared to nanoparticles. iron oxide, nanocrystals, growth, properties

1

Introduction

Iron oxides, including spinel-structured magnetite (cubic Fe3O4) and hematite (rhombohedral -Fe2O3) nanocrystals, are of growing interest because of their wide fields of application, such as high density storage [1], Li-ion batteries [2, 3], gas sensors [4, 5], drug delivery [6, 7], MRI [8, 9], catalyst [10–13], bioseparation [14] and so on. The study and application of these nanocrystals in many promising fields require that they are monodisperse so that each individual one has nearly identical physical and chemical properties. Tunable properties of nanocrystals are also very important. For example, in the field of biology, the magnetic nanoparticles should have high and tunable magnetic mo*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2011

ment so that they are capable of binding specifically to some biomolecules of interest and able to withstand various physiological conditions [15]. Catalysis is a surface phenomenon, and the catalytic properties of nanoparticles are determined by the nature of nanoparticle surface [13, 16]. Clean surfaces of iron oxide nanocrystals are required to make the active centre of catalysts be effective. Other performances such as magnetic behaviors are also affected by the surface molecules covered on crystals due to spin-canting, and a surface magnetic dead layer [17–19]. Aiming to meet these main challenges in the applications of iron oxide nanocrystals, many research groups have developed various synthetic approaches to nanometer-sized iron oxide particles [15, 20, 21]. However, owing to the intrinsic agglomerating behavior of magnetic nanomaterials, the synthesis of monodisperse iron oxide nanocrystals is usually more difficult than others [22]. Most of the reported chem.scichina.com

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aqueous routes often result in large nanocrystals with poor size-uniform [23, 24]. In order to prevent their agglomeration, high boil point organic solvents such as oleic acid and organic surfactants have been widely used [18, 20, 25, 26]. However, those routes are not only expensive and environment-unfriendly, but also result in the coverage of organic molecules on as-synthesized nanocrystals. Effectively removing those molecules is quite difficult, and they would alter the surface state of nanocrystals and further affect their properties [27, 28]. In addition, those routes usually lead to the yield of spherical nanoparticles in a dimension of several nanometers. They are too small to show strong magnetic properties and their practical applications are limited in many fields [22–24]. In order to effectively address these issues for iron oxide nanocrystals, it is still a challenge and important to develop some facile methods to obtain monodisperse magnetic nanocrystals with reasonable sizes. The controlled synthesis of iron oxide nanomaterials has been a long-time goal for science and industry. In this paper, simple inorganic ferric and ferrous chemicals, FeCl3·6H2O and FeCl2·4H2O as precursors were adopted, and magnetite and hematite nanocrystals were selectively synthesized in co-solvent of alcohol and water without any capping agents. Their magnetic and catalytic properties on thermal decomposition of ammonium perchlorate were also investigated.

2 Experimental section 2.1

Synthesis of iron oxides

Iron trichloride (FeCl3·6H2O, Guangdong Guanghua Chemical Co.), ferrous chloride (FeCl2·4H2O, Guangdong Guanghua Chemical Co.), ethanol (Tianjin Fuyu Chemical Co.), propylene glycol (Tianjin Fuyu Chemical Co.), glycol (Tianjin Fuyu Chemical Co.), and sodium hydroxide pellets (NaOH, Guangdong Guanghua Chemical Co.), and sodium acetate (CH3COONa, Guangdong Guanghua Chemical Co.), all in A.R. grade were used as starting materials without further purification. In a typical synthesis of spinel structured Fe3O4 nanocrystals, 0.273 g of FeCl3·6H2O (1.0 mmol) and 0.10 g (0.50 mmol) of FeCl2·4H2O were dissolved under vigorously magnetic stirring in a mixture of either glycol or propylene glycol (8.0 mL) and water (4.0 mL). Until completely dissolved, 0.160 g (4.0 mmol) of NaOH was added while keeping stirring. The mixture was sealed in a Teflonlined stainless steel autoclave (25 mL) and maintained at 220 °C for 12 h for solvothermal crystallization. The synthesis of -Fe2O3 nanocrystals is as our previous report [29]. 0.273 g of FeCl3·6H2O (1.0 mmol) was dissolved under vigorously magnetic stirring in ethanol (10.0 mL) with 0.7 mL or 2.5 mL water. 0.8 g of sodium acetate was added while stirring. The mixture was sealed in a Teflon-lined stainless steel autoclave (25 mL) and maintained at 180 °C for 12 h for solvothermal crystallization. Following natural

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cooling to ambient temperature, the resulting solid products were washed with distilled water and ethanol several times, respectively, and finally dried at 60 °C for ca. 10 h for characterization. 2.2

Preparation of the samples for catalysis analysis

0.99 g of ammonium perchlorate (AP) and 0.01 g of iron oxide were added into 10 mL of ethanol. After sonication for 30 min, the mixtures were dried at room temperature. As-obtained powders were ground in a mortar with pestle until homogeneous. The powdery mixtures were investigated by TG-DTG to evaluate the catalytic performance of iron oxide on the thermal decomposition of AP. 2.3

Characterizations

The products were characterized by powder X-ray diffraction (pXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD patterns were recorded with a Rigaku D/MAX 2200 VPC diffractometer using Cu K radiation ( = 0.15045 nm) and a graphite monochromator. SEM images were taken with FEI Quanta 400 Thermal FE Environment Scanning Electron Microscope. Samples were gold-coated prior to the SEM analyses. TEM images were prepared on a JEM-2010HR transmission electron microscope operated at an accelerating voltage of 200 kV. TEM samples were prepared by dispersing the powders on holey carbon film supported on copper grids. The magnetic properties of the samples were measured at 300 K on a Quantum Design MPMS XL-7 SQUID magnetometer. Analysis of TG-DTG was performed on a Netzsch STA 409C thermal analyzer, under nitrogen flow of 20 mL/min with a heating rate of 20 °C/min from room temperature to 500 °C.

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Results and discussion

3.1 Structure and shape characterization of iron oxides The products of as-synthesized iron oxides were analyzed using a pXRD technique. The pXRD patterns (Figure 1(a)), which match well with the JCPDS card of No. 89-3854, indicate that the prepared products can be indexed to spinelstructured magnetite Fe3O4 with a lattice parameter of ca. a = 0.8393 nm. No peaks of hematite, metal hydroxides, and other impurities were detected. Herein, two samples were synthesized in co-solvents of dihydric alcohol and water. Sample 1 is from glycol and water while sample 2 is from propylene glycol and water, respectively. The synthesis route of Fe3O4 is similar to that of -Fe2O3 in our previous report [29], where the samples were prepared in co-solvent of monohydric alcohol and water. The two typical samples of the present -Fe2O3 for comparative studies were firstly characterized by powder X-ray diffraction (pXRD). Their

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Figure 1 Powder X-ray diffraction (XRD) patterns of as-obtained products synthesized in co-solvents of alcohol and water. (a) Sample 1 is from the co-solvent of glycol and water, while sample 2 is from the co-solvent of propylene glycol and water. The bars are taken from JCPDS Card No. 89-3854 of magnetite Fe3O4 with spinel structure; (b) sample 3 is from the feedstock with 0.7 mL of water, and sample 4 is from feedstock with 2.5 mL of water. The bars are from JCPDS Card No. 33-0664 of hematite -Fe2O3 with rhombohedral structure.

powder X-ray diffraction patterns (pXRD) are shown in Figure 1(b), which match well with JCPDS card of No.330664 for rhombohedral -Fe2O3, confirming that the two products are hematite without any presence of impurities. This proves that phase-pure Fe3O4 and -Fe2O3 nanocrystals could be readily selectively obtained through a facile route by adopting simple and friendly inorganic ferric and ferrous chemicals as precursors in a similar co-solvent of alcohol and water. The nanostructures of the as-prepared products were further characterized by SEM and TEM (Figure 2). These images obviously show us that the nanocrystals in the four samples are well crystallized and uniformly dispersed. For the Fe3O4 samples synthesized in glycol, the particle sizes are about 32 nm, a little larger than those of 25 nm synthesized in propylene glycol (Figure 2(a), (b)). This suggests that the additions of alcohols have great effects on the growth of Fe3O4. In our previous report [29], the uses of various alcohols could gradually tune the shapes of -Fe2O3 nanocrystals. This is because the {001} lattice plane of -Fe2O3 is polar, and the alcohol as a polar molecular pref-

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Figure 2 SEM and TEM (inset) images of as-obtained products. (a) 32 nm Fe3O4 nanoparticles synthesized in a co-solvent of glycol and water (sample 1); (b) 25 nm Fe3O4 nanoparticles synthesized in a co-solvent of propylene glycol and water (sample 2); (c) -Fe2O3 nanoplates synthesized in a co-solvent of 10 mL of ethanol and 0.7 mL of water (sample 3); (d) -Fe2O3 nanoparticles synthesized in a co-solvent of 10 mL of ethanol and 2.5 mL of water (sample 4).

erentially attached on the {001} facet to limit the growth along <001> direction [29]. However, for cubic Fe3O4, the crystal structure is different. There are no polar surfaces on Fe3O4 nanocrystal. The effect of an alcohol molecule on each facet of Fe3O4 is nearly identical. By using different dihydric alcohol, herein, glycol or propylene glycol with different capping ability, different size of Fe3O4 nanocrystals could be obtained. The SEM and TEM images of two typical samples of -Fe2O3 are also shown in Figure 2(c), (d). The nanoparticles are about 40 nm (Figure 2(d)), and a little larger than that of the as-synthesized Fe3O4 in Figure 2(a), (b). By decreasing the amount of water from 2.5 mL to 0.7 mL, the shape of -Fe2O3 changed into plate-like. This indicates the size-controlled growth of these two iron oxide phases is effective by the above-mentioned strategies. Because of the low boiling points of these used alcohols terminated with OH in the co-solvents, the organic contaminants on the crystal surfaces can be easily removed by slightly heating and/or washing with water and clean crystalline samples can be easily obtained. Therefore, the reaction strategy is successful in the preparation of size- and shape-tunable, monodispersed, and clean iron oxides nanocrystals. Due to these characteristics of iron oxide nanocrystals with sizes in the ranges of 20 and 50 nm, it will be of great interest for investigating size- and shape-tunable physical and chemical performances.

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Magnetic properties of iron oxides

The magnetic properties of iron oxide nanocrystals were conducted at 300 K. Figure 3 shows the hysteresis loops of the samples of Fe3O4. It can be found that both of the samples are ferromagnetic despite small coercivities (Hc) and remanent fields (Mr). The weak ferromagnetic performances might be attributed to small crystal sizes. It has been reported that the size near 20 nm was regarded as the critical one for Fe3O4 for transforming from superparamagnetic to ferromagnetic behavior [15, 30]. Therefore, it is quite important to prepare nanosized nanocrysals but their sizes are more than 20 nm. The saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) for the as-prepared Fe3O4 samples with nanocrystal sizes around 32 nm are 74.2 emu/g, 9.4 emu/g and 107 Oe, respectively. All of the three values are lower than those octahedronal Fe3O4 macrocrystals and bulk magnetite [24, 31]. This is surely attributed to the small particle sizes of the present Fe3O4 nanocrystals [18]. The low coercivities imply that the as-synthesized Fe3O4 nanoparticles are soft magnets. The slight decrease of Ms than that of bulk magnetite (82 emu/g) [31] suggests the as-grown Fe3O4 nanocrystals still possesses high magnetic moment. As the particle size decreases to 25 nm, the value of Ms, Mr and Hc become to be 72.8 emu/g, 10.4 emu/g and 117 Oe. Interestingly, the values of Hc and Mr increase from 107 to 117 Oe, and from 9.4 to 10.4 emu/g respectively as the particle sizes decrease from 32 to 25 nm. The change is different from those reported results for Fe3O4 nanoparticles [19, 28]. However, the values of Ms are decreased as the particle sizes from bulk, 32 nm to 25 nm, showing the magnetic moment of these Fe3O4 samples can be tunable by size change. The magnetic properties of materials are influenced by many factors, not only size, also crystallinity, surface structure, and so on as well [28, 32]. The effect of shape anisot-

Figure 3 Magnetization-hysteresis (M-H) loops of as-prepared Fe3O4 and the enlarged part around the origin (inset) at 300 K. Sample 1(a) and samples 2(b).

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ropy on magnetic property could be confirmed by magnetization-hysteresis (M-H) loops as shown in Figure 4. It could be clearly observed that the coercivity (Hc) value of 298 Oe of -Fe2O3 nanoplates is remarkably greater than that of 52.5 Oe of -Fe2O3 nanoparticles. Correspondingly, the remanent magnetization (Mr) of -Fe2O3 nanoplates, which is 0.059 emu/g, is significantly larger than that of -Fe2O3 nanoparticles, which is 0.017 emu/g. What’s more, the values of the remanent magnetization and coercivity are different from the composite hollow microspheres with a remanent magnetization of 16.278 × 103 emu/g and a coercivity of 784.514 Oe, although they both show soft ferromagnetic behaves [33]. These results unambiguously indicate that the shape anisotropy has a great effect on the magnetic properties. With a considerable change of anisotropy, the magnetic properties have a wide range for tuning. Interestingly, The value of the coercivity is larger than 280 Oe of nanorods with diameters of 30–50 nm and lengths of 500–1100 nm which were synthesized adopting surfactant span80 as the template [34], this shows the as-synthesized nanoplates demonstrate a highly enhanced performance probably due to their clean surfaces. It have been reported that direct syntheses of high-quality, room-temperature ferromagnetic nanoparticles with low coercivities are not easy achieved [18]. Herein, soft magnets of iron oxide are not only successfully obtained, but the magnetic properties can be readily tuned by modifying either size or shape to meet a variety of applications. By the way, the increase of Hc and Mr with the decrease sizes of Fe3O4 particles might be attributed to some anisotropy. From the TEM images of Figure 2(a) and (b), the particles of Fe3O4 are not exactly cubic or spherical but with some irregularity. The increase of surface area to volume ratio and spin canting effects on the surface of magnetic nanocrystals make surface anisotropy of small crystals more

Figure 4 Magnetization-hysteresis (M-H) loops of as-prepared -Fe2O3 samples. The inset is the enlarged ones in the range of 0.5 and 0.5 kOe. Sample 3(a) and sample 4(b).

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obvious [35, 36]. So, the reverse tendency of Hc and Mr as compared to Hs might result from their stronger magnetic anisotropy. The higher aspect ratio of nanocrystals with surfactant-free surface can enhance magnetic properties such as -Fe2O3 nanoplates (Figure 4). 3.3

Catalytic properties of iron oxides

Ammonium perchlorate (AP) has been used as oxidizer in solid rocket propellants and pyrotechnics for decades [37]. The thermal decomposition of ammonium perchlorate has been extensively studied [38–41]. Transition metal oxides, including -Fe2O3 have been reported to be used as burning accelerants of composite solid propellants [42]. However, catalytic performances of Fe3O4 on thermal decomposition of AP are less reported. Herein, we comparatively investigated catalytic behaviors of -Fe2O3 and Fe3O4 nanocrystals on thermal decomposition of AP. Figure 5 shows the TG/DTG curves of thermal decomposition of pure AP and mixtures of AP and 1% Fe3O4. The thermal decomposition of pure AP occurs in two steps in the range of around 313 °C and 402 °C, respectively (Figure 5(a)). These two peaks are denoted as low-temperature decomposition (LTD) and high temperature decomposition peaks (HTD) [43]. The low-temperature decomposition has been proposed to be a heterogeneous process, and solid catalysts would not greatly affect these step reactions [43]. Therefore, after the addition of Fe3O4, the LTD did not change in position as the earlier reports [39, 43]. However, it was found that HTD in the presence of Fe3O4 shifted to lower temperatures, indicating that Fe3O4 could promote the thermal decomposition of AP. The addition of iron oxides lowered the high decomposition temperature of AP by 27 °C for 25 nm, 12 °C for 35 nm samples, respectively. It could be suggested that the size of Fe3O4 crystals is crucial to catalytic performances on thermal decomposition of AP. The Fe3O4 particles with a small size exhibit better catalytic activity than large ones.

Figure 5 TG-DTG curves on AP decomposition of the samples. (a) Pure AP, (b) AP + 1% of 35 nm Fe3O4, and (c) AP + 1% of 25 nm Fe3O4.

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As a comparison, we also further investigated the catalytic performances of -Fe2O3 nanocrystals on thermal decomposition of AP. From Figure 6, it could be found that, for the two samples with the addition of -Fe2O3, the HTD of nanoparticles shifts to 382 °C, and that of nanoplates shifts to 378 °C, respectively. According to the SEM and TEM images of -Fe2O3 (Figure 2(c), (d)), the size of Fe2O3 nanoparticles is about 40 nm, which is larger than that of Fe3O4 particles, but -Fe2O3 particles exhibit better catalytic performances on the decomposition of AP than Fe3O4 ones. Besides size, some other structural parameters such as phase, shape and surface structure are also underlying factors to affect the catalytic properties [36, 39, 40]. The BET values of the present -Fe2O3 nanoplate and nanoparticle are 21.7 and 25.8 m2/g respectively, they are almost close. Due to the high anisotropy of the nanoplates, the surface structure of -Fe2O3 nanocrystal might be an important feature to promote the decomposition of AP. According to the report [13], the external morphology and especially the exposure crystal planes of -Fe2O3 nanocatalyst affect the catalytic activity more significantly than the traditionally accepted factors (such as high BET surface area, hollow structure, etc.) for CO catalytic oxidation. In another report [44], Co3O4 nanorods predominantly exposing their {110} planes show the significantly higher reaction rate for CO oxidation due to the surface richness of active Co3+ sites. Herein, the top-bottom surfaces of nanoplates are enclosed by (001) planes, where the Fe3+ and O2 ions are arranged alternatively parallel to the c-plane [29]. Thus the (001) planes are terminated by either Fe3+ or O2 atoms and these -Fe2O3 nanocrystal surfaces have a net ionic charge [45]. The catalysis decomposition of AP has been proposed that the rate-controlling step is the transfer process of electrons from perchlorate ion to positive holes in p-type semiconducting additives [46, 47]. The high-temperature decomposition process is relevant to the release of interstitial oxygen or lattice oxygen in the presence of metal oxides [48].

Figure 6 TG-DTG curves on AP decomposition of the samples. (a) Pure AP, (b) AP + 1% of -Fe2O3 nanoparticles, and (c) AP + 1% of -Fe2O3 nanoplates.

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Therefore, during catalysis decomposition of AP, the large (001) surfaces of -Fe2O3 nanoplates with a net ionic charge on the surface would facilitate to electron transfers. Thus, the sample of -Fe2O3 nanoplates has the better catalytic performance than that of -Fe2O3 nanoparticles.

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Conclusions

In summary, two types of iron oxide nanocrystals (magnetite Fe3O4 and hematite -Fe2O3 nanocrystals) have been grown in co-solvent of alcohol and water by a facile solvothermal route. No high-boiling-point solvent and strong capping agent were used. The reaction sources were simple inorganic compounds, i.e. FeCl3·6H2O and FeCl2·4H2O. The results show that the method adopting co-solvent of alcohol and water as reaction medium is effective to synthesize monodisperse nanocrystals. These iron oxide nanocrystals exhibit room-temperature soft magnetic properties, and may be used in recording heads, microwave devices and transformers. The addition of iron oxide could decrease the AP decomposition temperature, and -Fe2O3 nanoplates showed enhanced activity over -Fe2O3 and Fe3O4 nanoparticle for the catalysis decomposition. The facile and mild route would be promising as a typical approach for a successful synthesis of phase-pure and well dispersed iron oxide nanocrystals. The crystals with tunable structures would meet specific and varied projects to study and utilize their properties. The facile route, monodisperse crystals and clean samples with tunable shape and size offer a better opportunity for their applications in promising fields.

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20 This work was financially supported by the National Natural Science Foundation of China, Guangdong Province, Guangzhou City and the Ph.D. Programs Foundation of Ministry of Education of China (U0734002, 50872158, 8251027501000010, 2010GN-C011 & 20090171110025).

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