Journal of Colloid and Interface Science 312 (2007) 513–521 www.elsevier.com/locate/jcis Controlled synthesis of α-Fe 2 O 3 nanorods and its size-dependent optical absorption, electrochemical, and magnetic properties Suyuan Zeng a,b , Kaibin Tang a,b,∗ , Tanwei Li a a Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China b Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received 19 December 2006; accepted 21 March 2007 Available online 10 May 2007 Abstract Uniform α-Fe 2 O 3 nanorods with diameter of about 30 nm and length up to 500 nm were synthesized by a template-free hydrothermal method and a following calcination of the intermediate product in the air at 500 ◦ C for 2 h. By carefully tuning the concentration of the reactants, a series of α-Fe 2 O 3 nanorods with gradient in aspect ratios can be obtained. The effect of the solvent was also evaluated. Based on the experimental facts, the formation mechanism of this one-dimensional structure was proposed. The size-dependent properties of the as-obtained α-Fe 2 O 3 nanorods were investigated. The optical absorption properties of the samples showed that the band gaps of the samples decreased in the sequence in which the size increased. The electrochemical performance of the samples showed that the discharge capacity decreased as the size of the sample increased, which may result from the high surface area and small size. The magnetic hysteresis measurements taken at 5 K showed that the coercivities of the samples were related to the aspect ratios of the samples, which may result from the larger shape anisotropy. However, the temperature-dependent field cooling magnetization showed that there was no Morin transition in the as-prepared samples, which may result from the surface effect. © 2007 Elsevier Inc. All rights reserved. Keywords: Hematite; Hydrothermal; Size-dependent; Optical absorption; Lithium ion battery; Magnetism 1. Introduction One-dimensional (1D) nanostructures, such as nanowires (NWs), nanorods, and nanotubes (NTs), have attracted exten- sive attention due to their unique physical and chemical prop- erties [1–3]. These systems, with two restricted dimensions, not only offer opportunities for investigating the dependence of electronic transport as well as optical and mechanical prop- erties on size confinement and dimensionality [4,5], but also play a crucial role in fields such as data storage [6] and ad- vanced catalytic and optoelectronic devices [2,7]. Developing new methods for the preparation of nanomaterials as well as the modification of their size, morphology, and porosity has been intensively pursued not only for their fundamental scientific in- terest but also for many technological applications. * Corresponding author. Fax: +86 551 360 1791. E-mail address: kbtang@ustc.edu.cn (K. Tang). Iron oxyhydroxides and iron oxides have been extensively used in the production of pigments, catalysts, gas sensors, mag- netic recording media, and raw materials for hard and soft mag- nets [8–14]. Hematite (α-Fe 2 O 3 ), based on hexagonal close packing of oxygen with iron in 2/3 of the octahedral vacan- cies, is traditionally used as catalyst, pigment, gas sensor, and electrode material [15–18] due to its low cost, high resistance to corrosion, and environmentally friendly properties. Most of these functions depend strongly on the composition and struc- ture of materials. In recent years, the synthesis and properties of the one-dimensional α-Fe 2 O 3 nanostructures have attracted much interest; many one-dimensional α-Fe 2 O 3 nanostructure such as nanorods [19–21], nanowires [22–24], nanobelts [25], and nanotubes [26,27] have been synthesized and used for the investigation of their properties. For example, by oxidiz- ing the surface of the iron substrate, α-Fe 2 O 3 nanowires were obtained [22]. α-Fe 2 O 3 nanowires were also prepared by an an- odic aluminum oxide (AAO) template method [28]. Recently, α-Fe 2 O 3 nanotubes and nanorods were selectively synthesized 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.03.046 514 S. Zeng et al. / Journal of Colloid and Interface Science 312 (2007) 513–521 through a hydrothermal method using Span80 or L113B as a soft template, and magnetic measurements showed that the magnetic properties were shape-dependent [29]. Xie’s group had also synthesized α-Fe 2 O 3 nanorods with gradients in size and porosity, and the results showed that properties such as the magnetic properties and the electrochemical properties were size-dependent [30]. Nevertheless, it still remains a challenge to develop simple and versatile approaches to synthesize 1D nanostructures of α-Fe 2 O 3 that are easily tunable in size, which will then facilitate our understanding of the shape and size- dependent properties of α-Fe 2 O 3 . In this paper, we propose an easy route for fabricating α- FeOOH nanorods via a low-temperature hydrothermal method. The α-FeOOH nanorods could be obtained with high yield (>90%) and good reproducibility. By changing the amount of the reactants, a series of α-FeOOH nanorods with gradients in aspect ratio can be obtained. α-Fe 2 O 3 nanorods can be obtained by calcing the as-obtained α-FeOOH at 500 ◦ C for 2 h at a heat- ingrateof1 ◦ C/min, preserving the same rodlike morphology. The shape-dependent optical absorption, electrochemical, and magnetic properties are investigated. 2. Experimental 2.1. Preparation of α-FeOOH and α-Fe 2 O 3 nanorods All reagents were analytically pure and used without fur- ther purification. In a typical experiment, 2 mmol FeSO 4 ·7H 2 O was added into 40 ml distilled water to form a homogeneous solution. Then 2 mmol anhydrous Na 2 SO 3 was added to the solution under vigorous magnetic stirring. A yellowish suspen- sion appeared in the solution after several seconds, and the amount of suspension increased with continuous stirring. After being stirred for 20 min, the slurry was transferred into a 50-ml Teflon-lined autoclave and maintained at 140 ◦ C for 12 h. The autoclave was then cooled to room temperature naturally. The final yellow solid products were centrifuged and washed with distilled water and absolute ethanol several times to ensure to- tal removal of the inorganic ions and then dried at 60 ◦ C under vacuum for 4 h. The α-Fe 2 O 3 nanorods were obtained by heat- ing the as-obtained α-FeOOH nanorods in air at 500 ◦ Cfor2h at a heating rate of 1 ◦ C/min, preserving the same rodlike mor- phology. 2.2. Sample characterizations The samples of as-prepared α-FeOOH and α-Fe 2 O 3 nano- structures were characterized by X-ray powder diffraction (XRD) with a Philips X’Pert Pro Super diffractometer with CuKα radiation (λ = 1.54178 Å). The transmission electron microscopy (TEM) images and the selected area diffraction (SAED) patterns for both α-FeOOH and α-Fe 2 O 3 were ob- tained on a Hitachi Model H-800 instrument with a tungsten filament at an accelerating voltage of 200 kV. The magnetic properties of α-Fe 2 O 3 were measured using a vibrating sam- ple magnetometer and a superconducting quantum interference device. The BET tests were determined via a Micromeritics ASAP-2000 nitrogen adsorption apparatus. The performance of the α-Fe 2 O 3 as a cathode was evaluated using a Teflon cell with a lithium metal anode. The cathode was a mixture of α-Fe 2 O 3 , acetylene black, and poly(vinylidene fluoride) with a weight ra- tio of 80/10/10. The electrolyte was 1 M LiPF 6 in a 1:1 mixture of ethylene carbonate/diethyl carbonate, and the separator was Celgard 2500. The cell was assembled in a glove box filled with highly pure argon gas (O 2 and H 2 Olevels<5 ppm). A galvano- static charge/discharge experiment was performed between 3.0 and 0.5 V at a current density of 0.2 mA cm −2 . The ultraviolet and visible spectra were recorded on a JGNA Specord 200 PC UV–visible spectrophotometer. The magnetic measurements were recorded on a SQUID magnetometer, Quantum Design MPMS. 3. Results and discussion Fig. 1a is the XRD pattern of the as-obtained FeOOH nanorods, where all the diffraction peaks can be indexed as or- thorhombic α-FeOOH with cell constants of a = 0.4592 nm, b = 0.998 nm, and c = 0.3015 nm, which is consistent with the reported data (JCPDS Card 81-0464). Fig. 1bistheXRD pattern of the product obtained by calcining the as-prepared α-FeOOH at 500 ◦ C for 2 h, where all the diffraction peaks can be indexed as a hexagonal phase with lattice constants of a = 0.5013 nm and c = 1.3751 nm, which agrees well with the literature (JCPDS Card 33-0664). Fig. 2a is the field emission electron microscopy (FESEM) image of the as-obtained α-FeOOH nanorods, which clearly demonstrates that the products are composed of large amount of nanorods. These rods, about 30 nm in diameter and length up to 400 nm, have smooth surfaces along their entire length. Fig. 2b is the transmission electron microscopy (TEM) image of a single α-FeOOH nanorod. The selected area electron dif- fraction (SAED) pattern of a single nanorod (inset of Fig. 2b) demonstrates the single-crystal nature of the nanorod grown along the [001] direction. Fig. 2c is the FESEM image of the α-Fe 2 O 3 obtained by calcining the α-FeOOH at 500 ◦ Cfor2h, Fig. 1. XRD patterns of (a) as-prepared α-FeOOH nanorods; (b) α-Fe 2 O 3 nanorods obtained by calcing the α-FeOOH nanorods at 500 ◦ Cfor2h. S. Zeng et al. / Journal of Colloid and Interface Science 312 (2007) 513–521 515 Fig. 2. (a) FESEM image of the as-obtained α-FeOOH nanorods and (b) TEM image of a single α-FeOOH nanorod (inset: SAED pattern of a single α-FeOOH nanorod); (c) FESEM image of the as-obtained α-Fe 2 O 3 nanorods; (d) TEM image of a single α-Fe 2 O 3 nanorod (inset: SAED pattern of a single α-Fe 2 O 3 nanorod). from which we can see that the rodlike morphology perfectly remained after calcination. The SAED pattern of a single α- Fe 2 O 3 nanorod is also taken to verify the growth direction of the α-Fe 2 O 3 (inset of Fig. 2d); and the result shows that the as- obtained α-Fe 2 O 3 nanorod is a single crystal grown along the [01−10] direction. Theformationoftheα-FeOOH nanorods in the solution can be expressed as follows: SO 2− 3 + H 2 O → HSO − 3 + OH − ,(1) 4Fe 2+ + 8OH − + O 2 → FeOOH + 2H 2 O. (2) As shown above, Fe 2+ reacted with the OH − produced by the hydrolysis of SO 2− 3 and O 2 in the atmosphere, producing the yellow α-FeOOH suspension. When the SO 2− 3 hydrolyzed in the water, the pH value of the solution rose uniformly, and this prevented the occurrence of local supersaturation and mean- while favored for homogeneous nucleation. However, as the reaction went on, the pH value of the system decreased. And ac- cording to the acid–base surface properties of the metal oxide, decreasing the pH of the precipitation from the point of zero charge (PZC) increases the surface charge density by adsorp- tion of protons and consequently reduces the interfacial tension of the system [31], which is very important for the formation of such unique nanostructures. To further understand the role that SO 2− 3 played in the synthesis, several experiments involved different amount of Na 2 SO 3 and other kind of inorganic ions were performed. Keeping the amounts of FeSO 4 and water constant, the mo- lar ratio between FeSO 4 and Na 2 SO 3 varied. Fig. 3aisthe TEM image of the product obtained when the concentration of SO 2− 3 is 0.025 mol L −1 , which shows that nanorods with diam- eter about 30 nm and length about 40 nm were obtained. As the concentration of the SO 2− 3 increases, e.g., 0.075 mol L −1 , the product is mainly composed of nanorods with diameter of about 30 nm and length up to 800 nm (Fig. 3b), showing that the aspect ratio of the nanorods was tunable. However, as the con- centration of SO 2− 3 increases further, e.g., 0.1 mol L −1 , black powders instead of the yellow product are obtained, which is confirmed to be Fe 3 O 4 by the XRD. And this can be explained by the reducing ability of the SO 2− 3 . Fig. 3c is the TEM im- age of the as-obtained Fe 3 O 4 , from which it can be seen that the product is composed of hexagonal nanodisks with average size about 50 nm, which may provide a method for the prepa- ration of Fe 3 O 4 nanodisks. To learn more about the role that SO 2− 3 played in the formation of the one-dimensional structure, a series of comparative experiments were performed. In the case where no Na 2 SO 3 was added, urchin-like nanostructures that was composed of nanoneedles formed (shown as Fig. 3d). When Cl − is used instead of SO 2− 3 in the reaction, irregular 516 S. Zeng et al. / Journal of Colloid and Interface Science 312 (2007) 513–521 Fig. 3. TEM images of the products under different conditions: (a) prepared in the solution containing 0.05 mol L −1 Fe 2+ and 0.025 mol L −1 Na 2 SO 3 ; (b) prepared in the solution containing 0.05 mol L −1 Fe 2+ and 0.075 mol L −1 Na 2 SO 3 ; (c) prepared in the solution containing 0.05 mol L −1 Fe 2+ and 0.1 mol L −1 Na 2 SO 3 ; (d) prepared in the solution containing 0.05 mol L −1 Fe 2+ ; (e) prepared in the solution containing 0.05 mol L −1 Fe 2+ and 0.05 mol L −1 NaCl; (f) prepared in the solution containing 0.05 mol L −1 Fe 2+ and0.05molL −1 Na 3 PO 4 . nanorods as well as nanoparticles obtained (shown as Fig. 3e), whereas nanoplates obtained when PO 3− 4 is used instead of SO 2− 3 (Fig. 3f). It was believed that the solution method is based on surface chemistry through changing the interfacial tension to control the structure and morphology of the products [32]. And it has been reported that by adjusting the interfacial tension of the reaction system by ethanol, an α-FeOOH nanorod array can be obtained in the solution [33]. Then what the result will be when ethanol is added into this reaction system? To answer this question, several experiments that employed mixed solutions of ethanol and water instead of pure water were performed. Fig. 4a is the TEM image of the product obtained when the solution is composed of 5 ml ethanol and 35 ml H 2 O, from which it can be seen that nanorods with higher aspect ratio are obtained. With a further increase of the amount of ethanol to 10 ml, an urchin-like nanostructure that is composed of nanorods formed (Fig. 4b). When more ethanol is added, e.g., 20 and 30 ml, ir- regular nanoparticles and nanorods are obtained (Figs. 4c and 4d), which may result from the relative higher concentration of the reactant compared with that in the water, causing the reac- tion to be kinetically controlled. To investigate the growth mechanism of such rodlike struc- tures, several experiments that involved intercepting the inter- mediates at different hydrothermal reaction times were per- formed. According to the results of these experiments, we be- lieve that the nanorods formed through a RBG (rolling-broken- growth) model, which has been reported in the synthesis of MnO 2 3D nanostructures [34] and CdSe nanorods [35].Atthe initial stage, a large number of plate structures were obtained (as shown in Fig. 5a). The thin flakes tended to curl under elevated temperature and pressure, as shown in Fig. 5b(after heating for 40 min). As the reaction went on, some thin flakes broke into small nanoneedles (Fig. 5d) via a rolling-broken- growth (RGB) process. And finally, small nanoneedles would grow into nanorods after heating for 12 h. 4. Size-dependent properties of the products To investigate the size-dependent properties of the α-Fe 2 O 3 nanorods, several samples with gradient in the length have been employed. They were synthesized using the method mentioned above. They were labeled as S1, S2, and S3, respectively. The sizes of the samples were listed in Table 1. S. Zeng et al. / Journal of Colloid and Interface Science 312 (2007) 513–521 517 Fig. 4. TEM images of α-FeOOH obtained when the solution is composed of (a) 5 ml ethanol and 35 ml water; (b) 10 ml ethanol and 30 ml water; (c) 20 ml ethanol and 20 ml water; (d) 30 ml ethanol and 10 ml water. Fig. 5. TEM images of the α-FeOOH obtained after hydrothermal reaction for (a) 20 min; (b) 40 min; (c) 1 h; (d) 2 h. Table 1 Names and sizes of the samples employed in the characterization Sample Diameter (nm) Length (nm) S1 20–30 40–50 S2 20–30 400–500 S3 30–40 700–800 4.1. Optical absorption properties The optical absorption properties of samples S1, S2 and S3 were investigated at room temperature by the UV–vis spectra (Fig. 6a). The absorption peaks showed blue shift as the lengths of the nanorods decrease. α-Fe 2 O 3 is a n-type semiconductor and its optical band gap can be obtained by the equation (3)(αhν) n = B(hν − E g ), where α is the absorption coefficient, hν is the photo energy, B is a constant relative to the material, E g is the band gap, and n is either 1/2 for an indirect transition or 2 for a direct transi- tion. The (αhν) 2 ∼ hν curves for samples S1, S2, and S3 are shown in Figs. 6b, 6c, and 6d, respectively. The band gaps cal- culated from Eq. (3) are 2.65, 2.60, and 2.45 eV for S1, S2, and S3, showing an obvious blue shift as the sizes decreased. Here, compared to the reported value of bulk α-Fe 2 O 3 (2.2 eV) [36], the optical absorption band edge of the as-obtained α-Fe 2 O 3 exhibits blue shift with respect to that of the bulk α-Fe 2 O 3 . The blue shift could also be attributed to the size effect, which leads to the broadening of the optical absorption edge. It is well known that the semiconductor nanoparticle energy gap in- creases with decrease of the grain size, which leads to a blue shift of the optical absorption edge, and this has been observed in many semiconductor nanoparticle systems [37–40]. Based on the above considerations, the sequence of the as-obtained products should be S1 > S2 > S3, which agrees well with our experimental facts. 4.2. Electrochemical properties It is reported that the lithium intercalation performance is related to the intrinsic crystal structure, where the lithium ions can intercalate into the interlayer, the tunnels, and the holes in the crystal structure [41]. α-Fe 2 O 3 , based on hexagonal close packing of oxygen with iron in 2/3 of the octahedral vacancies, is reported [30] to have holes in the first octahedral layer pro- jected along [001] and [100], which makes its use in lithium ion batteries possible. Here, the electrochemical performance of the as-prepared α-Fe 2 O 3 samples in the cell configuration of Li/α-Fe 2 O 3 was evaluated. Fig. 7 shows the comparative charge/discharge curves of the α-Fe 2 O 3 samples of S1–S3 in the first cycle. The cutoff voltage of samples S1–S3 is about 0.6 V, which is similar to the nanorods [30] and nanoparti- cles reported before [42]. The S1 electrode exhibits the highest capacity, 1040 mA hg −1 among the three samples. The capac- ities of samples S2 and S3 are 1002 and 859 mA hg −1 ,re- spectively. The first discharge capacity possesses the sequence 518 S. Zeng et al. / Journal of Colloid and Interface Science 312 (2007) 513–521 Fig. 6. (a) UV–vis spectra of samples S1–S3; (b), (c), (d) spectrum of samples S1, S2, and S3 obtained by using the energy as abscissa. Fig. 7. First charge–discharge curves of α-Fe 2 O 3 samples (S1–S3) at a current density of 0.2 mA cm −2 (S1: dashed lines; S2: dotted lines; S3: solid lines). S1 > S2 > S3, which confirms the sequence in which the sizes of the sample increase. The discharge capacities of the sam- ples may be related to the size effect of the α-Fe 2 O 3 nanorods. Considering the introduction of lithium ions into the holes of the hematite surface, it is easy to find that the large surface area is important for the improvement of lithium intercalation performance. When the surface area is high, the lithium ion in- tercalation capacity and affinity will be greatly enhanced, since the diffusion lengths of the lithium ions are greatly shortened. Then the one with the smallest size and with the highest surface S. Zeng et al. / Journal of Colloid and Interface Science 312 (2007) 513–521 519 Fig. 8. FC curves for samples (a) S1, (b) S2, and (c) S3 from 300 to 5 K; hysteresis loop for samples (d) S1, (e) S2, and (f) S3 at 5 K. area is the one that would have the highest discharge capac- ity. Our deduction was further verified by the BET tests. The BET tests show that the surface areas of the three samples were 35.577, 32.000, and 29.303 m 2 /g for samples S1, S2, and S3, respectively, which conformed to the discharge capacities of the three samples. 4.3. Magnetic properties It is of great interest to investigate the magnetic properties of α-Fe 2 O 3 with gradients in aspect ratios. Bulk α-Fe 2 O 3 , besides the Néel temperature (T N = 960 K), has a first-order magnetic transition at T M = 263 K, which is called the Morin transi- tion. Below T M , the antiferromagnetically (AF) ordered spins are oriented along the c-axis, whereas above T M , spins lie AF in the basal plane of the crystal with a ferromagnetism compo- nent. A sharp decrease in magnetization should be observed at this transition, termed the Morin transition temperature (T M ). Figs. 8a–8c show the curves for the temperature dependence of field-cooling (FC) magnetizations from 5 to 300 K, under an applied field of 100 Oe. The insets are the corresponding differ- ential FC curves. However, the magnetic behaviors for samples S1–S3 were completely different, as shown in Figs. 8a–8c:the FC plots show constant increase and no maximum down to 5 K. 520 S. Zeng et al. / Journal of Colloid and Interface Science 312 (2007) 513–521 And this abnormality had also been observed in the α-Fe 2 O 3 nanotubes [29] and mesoporous α-Fe 2 O 3 with disordered walls [43], which has been attributed to the presence of small crys- talline particles in a few regions of the sample. However, as for the samples in our experiments, we believe that the surfaces of nanorods may contribute to the absence of the Morin transition. Regarding the absence of the Morin transition, the shape of the M(T ) curve is not typical of an antiferromagnetic substance ei- ther above or below the spin-reorientation (Morin) transition. And a “dead” surface layer of PM spins (the thickness of the layer increasing as the size of the rod decreases) makes it im- possible to observe the intrinsic contribution (AF). A detailed study is under way. To further understand the magnetic behavior of the sam- ples, magnetic hysteresis measurements of α-Fe 2 O 3 (samples S1–S3) were carried out in an applied magnetic field at 5 K, with the field sweeping from −10 to 10 kOe. No saturation of the magnetization as a function of the field is observed up to the maximum applied magnetic field in all cases. Figs. 8d, 8e, and 8f are the hysteresis loops of samples S1, S2, and S3 at 5 K. The coercivity forces of samples S1, S2, and S3 are 67, 146, and 584 Oe, respectively, indicative of soft magnets. The remnant magnetizations of samples S1, S2, and S3 at 5 K are determined to be 0.00007, 0.0024, and 0.039 emu/g. It is reported that the high coercivity may be associated with the aspect ratio of α-Fe 2 O 3 [44], because shape anisotropy would exert a tremendous effect on the magnetic properties. Symmet- rically shaped nanoparticles, such as spheres, do not have any net shape anisotropy. However, shuttle-like nanoparticles have shape anisotropy in addition to crystalline anisotropy, which will increase coercivity. α-Fe 2 O 3 nanoparticles with an aver- age diameter of 3 nm were found to show a coercive force of 50 Oe at 5 K [30]. Enhanced anisotropy caused by the one- dimensional structure induces large magnetic coercivity, where the magnetic spins are preferentially aligned the long axis and their reversal to the opposite direction requires higher energies than for spheres [45]. For sample S1, whose shape is very close to that of the spherical particles, the shape anisotropy is the low- est among all three samples. As the aspect ratio increases, the shape anisotropy increases. Based on the above considerations, we believe that the sequence can be used to explain the phe- nomena that we observed in samples S1–S3 at 5 K and at room temperature. 5. Conclusions An facile route for the preparation of α-Fe 2 O 3 nanorods with a gradient in size was reported. By controlling the concentra- tion of the reactants, the size of the sample can be controlled. The nanorods, with diameters ranging from 20 to 50 nm and lengths ranging from 50 to 800 nm, were uniform and in high yield. A possible formation mechanism was proposed for this one-dimensional structure. The size-dependent properties of the samples were investigated. The optical absorption properties of the samples showed that the band gaps of the sample decreased as the size increased. The electrochemical performance of the samples showed that the discharge capacity decreased as the size of the sample increased, which may result from the high surface area and small size. The magnetic hysteresis measure- ments taken at 5 K showed that the coercivities of the samples were related to the aspect ratios of the sample, which may result from the larger shape anisotropy. 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