synthesis of hematite (r-fe2o3) nanorods diameter-size and shape effects on their

Synthesis of Hematite (r-Fe 2 O 3 ) Nanorods: Diameter-Size and Shape Effects on Their Applications in Magnetism, Lithium Ion Battery, and Gas Sensors Changzheng Wu, Ping Yin, Xi Zhu, Chuanzi OuYang, and Yi Xie* Department of Nano-materials and Nano-chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science & Technology of China, Hefei, Anhui 230026, China ReceiVed: June 1, 2006; In Final Form: July 16, 2006 We demonstrated in this paper the shape-controlled synthesis of hematite (R-Fe 2 O 3 ) nanostructures with a gradient in the diameters (from less than 20 nm to larger than 300 nm) and surface areas (from 5.9 to 52.3 m 2 /g) through an improved synthetic strategy by adopting a high concentration of inorganic salts and high temperature in the synthesis systems to influence the final products of hematite nanostructures. The benefits of the present work also stem from the first report on the <20-nm-diameter and porous hematite nanorods, as well as a new facile strategy to the less-than-20-nm nanorods, because the less-than-20-nm diameter size meets the vital size domain for magnetization properties in hematite. Note that the porous and nonporous hematite one-dimensional nanostructures with diameter gradients give us the first opportunity to investigate the Morin temperature evolution of nanorod diameter and porosity. Evidently, the magnetic properties for nanorods exhibit differences compared with those for the spherical particle counterparts. Hematite nanorods are strongly dependent on their diameter size and porosity, where the magnetization is not sensitive to the size evolution from submicron particles to the 60-90 nm nanorods, while the magnetic properties change significantly in the case of <20 nm. In other words, for the magnetic properties of nanorods, in a comparable size range, the porous existence could also influence the magnetic behavior. Moreover, applications in formaldehyde (HCHO) gas sensors and lithium batteries for the hematite nanostructures with the diameter/ surface area gradient reveal that the performance of electrochemical and gas-sensor properties strongly depends on the diameter size and Brunauer-Emmett-Teller (BET) surface areas, which is consistent with the crystalline point of view. Thus, this work not only provides the first example of the fabrication of hematite nanostructure sensors for detecting HCHO gas, but also reveals that the surface area or diameter size of hematite nanorods can also influence the lithium intercalation performances. These results give us a guideline for the study of the size-dependent properties for functional materials as well as further applications for magnetic materials, lithium-ion batteries, and gas sensors. 1. Introduction 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 interest but also for many technological applications. Nanoparticles (zero-dimensional (0-D)) and nano- wires/nanorods (one-dimensional (1-D)) with controlled size and shape are of key importance because their electrical, optical, and magnetic properties strongly depend on their size and shape. 1 Hematite (R-Fe 2 O 3 ), the most stable iron oxide, with n-type semiconducting properties under ambient conditions, is of scientific and technological importance because of its usage in catalysts, pigments, magnetic materials, gas sensors, and lithium-ion batteries. 2 Its size and shape effect on corresponding properties has attracted much attention. For example, the Morin transition temperature (T M )ofR-Fe 2 O 3 nanoparticles (0-D) decreases with decreasing spherical particle size according to a 1/d dependence. 3 Additionally, 1-D R-Fe 2 O 3 nanostructures, such as nanorods, 4 nanowires, 5 nanobelts, 6 and nanotubes 7 have also been synthesized and used for investigating their peculiar properties. For example, Woo et al. synthesized R-Fe 2 O 3 nanorods by a sol-gel mediated reaction of ubiquitous Fe 3+ ions in reverse micelles. 8 Zhang et al. managed to grow R-Fe 2 O 3 nanowires out of the oxidized surface of iron substrates. 9 Recently, R-Fe 2 O 3 hollow nanowires with outer diameters of ca. 50 nm have been synthesized through a vacuum-pyrolysis route from β-FeOOH nanowires. 10 Nevertheless, it still remains a challenge to develop simple and versatile approaches to synthesize 1-D nanostructures of R-Fe 2 O 3 with slimmer diam- eters, which will then facilitate our understanding of the shape- and size-dependent properties of R-Fe 2 O 3 . Since the Morin temperature of R-Fe 2 O 3 spherical particles was found to be strongly dependent on particle size and tends to disappear (<5 K) below a diameter of 8-20 nm, 11 their counterpart nanorods/nanowires with a diameter of <20 nm are then significantly necessary for further understanding of the magnetic properties. Currently, because of limited studies on R-Fe 2 O 3 nanorods/nanowires with a diameter of <20 nm and because their <20 nm and porous nanorods have not been obtained so far, their subsequent applications in magnetization fields and investigations on the size-dependent properties of iron oxides are significantly delayed. Herein, we demonstrate the synthesis of R-Fe 2 O 3 nanorods with a gradient in the surface * Corresponding author. Address: Department of Nano-materials and Nano-chemistry. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. Tel: 86-551-3603987. Fax: 86-551-3603987. E- mail: yxie@ustc.edu.cn. 17806 J. Phys. Chem. B 2006, 110, 17806-17812 10.1021/jp0633906 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/19/2006 areas and diameter sizes via an improved strategy: first, by adopting high-concentration salts and high temperature, the as- produced R-orβ-FeOOH nanorods are slimmer than usual in our synthetic system. Then with the influence of inorganic salt ions, the samples pyrolyze at a slower rate, and then the corresponding well-defined hematite nanostructures can be achieved. Note that the hematite crystal structure is a rhombo- hedrally centered hexagonal structure of the corundum type with a closed-packed lattice; there are no tunnels or interlayer spacing for accommodating the inorganic ions any more, as is the case for R-FeOOH or β-FeOOH crystal structures (see Supporting Information). Therefore, the inorganic ions can be easily removed after calcining, and the pure hematite (R-Fe 2 O 3 ) could be obtained after rinsing with water. Here, the hematite morphology with different sizes and shapes could be well controlled by simply choosing different kinds of inorganic salts. This work presents not only a new strategy to produce R-Fe 2 O 3 nanorods with diameters of <20 nm, but also the first report on the synthesis of porous R-Fe 2 O 3 nanorods with diameters of <20 nm. Evidently, the magnetic properties were strongly dependent on the size of their diameter and the porosity in the present work, while the lithium intercalation and HCHO gas sensor properties were significantly dependent on the surface area. Therefore, the present work provides not only the first example of investigating the magnetic property evolution of nanorods/nanowire diameters and porosity, but also the first example of the fabrication of hematite nanostructure sensors for detecting HCHO gas. 2. Experimental Section To prepare FeOOH nanostructure precursors, 50 mL of 0.06 M iron chloride (FeCl 3 ) aqueous solution, with/without the addition of 0.300 mol of inorganic salts (NH 4 Cl, KCl, and Na 2 - SO 4 ) was put in a conical flask and stirred with a magnetic stirrer for 30 min. The homogeneous solution was then transferred into a 60 mL Teflon-lined stainless steel autoclave, sealed, and then heated to 120 °C. After the autoclaves were maintained at 120 °C for 12 h, the resulting yellow product was centrifuged, rinsed with distilled water, and finally dried at 40 °C in a vacuum. The obtained yellow solid products were collected for the following experiments and characterization. To prepare hematite nanostructures, the as-prepared FeOOH nanostructures were heated to 520 °C with a ramping rate of 10 °C min -1 and then maintained at 520 °Cfor8h.The decomposition was performed in air, and the synthetic conditions are summarized in Table 1, where the as-obtained R-Fe 2 O 3 nanostructures have been named S1, S2, S3, and S4. The as- collected R-Fe 2 O 3 products were rinsed with distilled water and finally dried at 40 °C in a vacuum. The samples of as-prepared FeOOH and R-Fe 2 O 3 nanostruc- tures were characterized by X-ray powder diffraction (XRD) with a Philips X’Pert Pro Super diffractometer with Cu KR radiation (λ ) 1.54178 Å). The transmission electron micros- copy (TEM) images for both FeOOH and R-Fe 2 O 3 were obtained on a Hitachi Model H-800 instrument with a tungsten filament at an accelerating voltage of 200 kV. The selected- area electron diffraction patterns and high-resolution transmis- sion electron microscopy (HRTEM) images were obtained on a JEOL-2010 TEM at an acceleration voltage of 200 kV. The porosity and adsorption performance of R-Fe 2 O 3 were deter- mined via a Micromeritics ASAP-2000 nitrogen adsorption apparatus. The magnetic properties of R-Fe 2 O 3 were measured using a vibrating sample magnetometer and superconducting quantum interference device. The performance of the R-Fe 2 O 3 as a cathode was evaluated using a Teflon cell with a lithium metal anode. The cathode was a mixture of β-FeOOH/acetylene black/poly(vinylidene fluoride) with a weight ratio of 85/10/5. 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 glovebox filled with highly pure argon gas (O 2 and H 2 O levels < 5 ppm). A galvanostatic charge/ discharge experiment was performed between 3.0 and 0.5 V at a current density of 0.2 mA cm -2 . Gas sensing measurements were performed with a WS-30A system (Weisheng Instruments Co., Zhengzhou, China), and the system integral error for the WS-30A system was less than (1%. The sensors of the as-prepared samples were fabricated on ceramic tubes with the connection of gold electrodes that were connected by four platinum wires. Here, the sensor structure and the testing principle were similar to that for previous reports. 12 In this case, the mixture of the as-prepared hematite nanomaterials and ethanol was coated as a thin film spanning across the two Au electrodes. After drying at 150 °C for2hinairtoimprove stability, the electrical contact was made through connecting the four platinum wires with the instrument base by silver paste. Before analysis, the sensors- settled chamber was kept under a continuous flow of fresh air for 30 min. During the measurement, the sensors were hosted in a closed plastic tube equipped with appropriate inlets and outlets for gas flow. A given amount of formaldehyde (HCHO) was injected into the chamber by a microinjector. The sensitivity could be measured when the detecting gas was mixed with air homogeneously. Here, the response magnitude, S, is defined as R s(air) /R s(gas) , where R s(air) and R s(gas) are the resistance of the sensor in clean air and in detected gas, respectively. 3. Results and Discussion 3.1. Morphology, Characterization, and Formation Mech- anism of the As-Obtained Hematite Nanostructures. The hematite nanostructures could be originated from the well- controlled FeOOH nanostructure precursors (see Supporting Information), and all the synthetic conditions are summarized in Table 1. Furthermore, the phase and morphology information for the as-obtained products are revealed by Figure 1, where panels c, f, i, and m are the corresponding XRD patterns for panels a-b, d-e, g-h, and j-l, respectively. All the XRD patterns in Figure 1 show characteristics of pure hexagonal R-Fe 2 O 3 (JCPDS card 33-664, a ) 5.035 Å and c ) 13.74 Å). No characteristic peak was observed for other impurities such as β-FeOOH, Fe 3 O 4 , γ-Fe 2 O 3 , and other inorganic ions. As TABLE 1: Synthetic Conditions for Different r-Fe2O3 Nanostructures. R-Fe 2 O 3 sample morphologies average diameter (nm) reaction condition for FeOOH precursors S1 submicron particles 300-500 direct hydrolysis systems S2 nanorods with porosity nanorods: 60-90 porosity: 20-50 FeCl 3 -KCl systems S3 nanorods 5-16 FeCl 3 -Na 2 SO 4 systems S4 nanorods with porosity nanorods: 5-19 porosity: 2-16 FeCl 3 -NH 4 Cl systems Synthesis of Hematite (R-Fe 2 O 3 ) Nanorods J. Phys. Chem. B, Vol. 110, No. 36, 2006 17807 is shown in Figure 1a,b, the appearance of S1 is the hematite submicrometer particles with the diameter range of 300-500 nm, and no hollow structures were found even from the amplified TEM image for the direct hydrolysis system. The calcined products (S2) from the FeCl 3 -KCl system have the regular pores (20-50 nm) distributed along the hematite nanorods with a diameter-size range of 60-90 nm, as shown in Figure 1d,e. The calcining products (S3) obtained in the FeCl 3 -Na 2 SO 4 system were mostly solid nanorods with diam- eters of <15 nm, as shown in Figure 1g. The HRTEM image of a single-crystalline solid nanorod in Figure 1h shows a clear interplanar distance of 0.25 nm, matching well with the d 110 spacing of pure hexagonal hematite. Figure 1j displays that heating the products prepared in the FeCl 3 -NH 4 Cl system yielded the nanoporous crystalline R-Fe 2 O 3 nanostructures (S4) without altering the morphology of the 1-D and even the nanorod bundles, and many holes with sizes of 8-30 nm form in the nanorods (Figure 1k). The TEM image with higher magnification shows that the porous appearance can be clearly observed in all the visible nanorods, with uniform pores of <10 Figure 1. Representative TEM and HRTEM images of the as-obtained R-Fe 2 O 3 nanostructures with different diameter sizes for S1 (a-b), S2 (d-e), S3 (g-h), and S4 (j-l). The corresponding XRD patterns for the samples of S1, S2, S3, and S4 are shown in panels c, f, i, and m, respectively. (n) The magnified (110) peaks for these four samples, where the (110) peak becomes narrower in the sequence of S4-S1, revealing the size evolution. 17808 J. Phys. Chem. B, Vol. 110, No. 36, 2006 Wu et al. nm (Figure 1l). Evidently, the above TEM images were consistent with the analysis of the magnified (110) peak, as shown in Figure 1n, where the (110) peak becomes narrower in the sequence of S4-S1, and the sharper peaks for S1 indicate its good crystallinity and greater grain size than the other three products (S2-S4) whose precursors grew under the control. On the basis of the combination analysis of TEM images and XRD patterns, the as-obtained R-Fe 2 O 3 nanostructures possess diameter- sized gradients in the sequence from S4 to S1 with increasing sizes. As described above, the systems with the addition of inorganic salts retained the morphology of the FeOOH precursors, and regular nanopores formed along the nanorods, while the direct hydrolysis system produced only particles under air conditions. The phenomenon can be explained by the difference in thermalstability behavior based on DrTGA, as shown in Figure 2. From DrTGA curves, one can see that there is a broad exothermal peak with a coexistence of feeble shoulder peaks in the temperature range of 200-400 °C for each of S2-S4, showing that their weight loss is much more lagged. The intensity of the exothermal peak for S1 (Figure 2a) without salt addition in the temperature range of 200-400 °C is evidently stronger than the corresponding peaks for the other three samples with the addition of high-concentration salt ions, indicating that the weight loss is much quicker. Compared with the direct hydrolysis systems (S1), the final products of FeOOH in the other three systems will possess more ions to bind with the tunnel structures or the surface sites of R-orβ-FeOOH crystal, and the total amounts of the residual ions that can be removed in the calcination process 13 are too small to be reflected by the XRD pattern. Thus, the small amount of residual inorganic salt ions seems to be responsible for the formation of hollow structures in our synthetic conditions. In fact, the addition of inorganic salt ions enables the samples to pyrolyze at a slower rate. As for the products of β-FeOOH with the absence of adequate ions in the direct hydrolysis systems, this high-rate pyrolysis process will produce too much energy in a short time to be effectively released from the systems, as indicated by a stronger peak in the curve. Most of the energy was adsorbed in the systems, resulting in the collapse of the nanorods to form larger aggregated particles, as shown in Figure 1a,b, whereas, for the inorganic salt systems, the existence of salt ions binding with the FeOOH precursors impedes the samples to pyrolyze at the higher rates, and the as-produced energy could have enough time to be effectively released from the systems, and the final products of R-Fe 2 O 3 obtained possess the regular pores and the morphology remi- niscent of their precursors (β-FeOOH). In a word, the pyrolysis rates of β-FeOOH seem to be a significantly influencing parameter for the formation of porous R-Fe 2 O 3 and reminiscent of the orientation-ordered nanostructures for R-Fe 2 O 3 in air conditions based on the combined analysis of DrTGA and TEM results. Additionally, it is interesting that when the precursors are prepared in high-concentration Cl - ions, the calcined products have the appearance of porosity (S2 and S4), while in high- concentration SO 4 2- , S3 only has the solid appearance. These results indicate that the existence of large amounts of Cl - ions held in the tunnels in β-FeOOH might favor the appearance of porosity in the calcined products such as S2 and S4. However, the high-concentration large anions such as SO 4 2- , which existed in the surface sites, 14 might favor the formation of a solid morphology for S3. 3.2. BET Surface Areas of r-Fe 2 O 3 Nanostructures. The Brunauer-Emmett-Teller (BET) surface areas of S1 and S2 were found to be 5.9 and 15.8 m 2 /g, respectively, by calculating from the results of N 2 adsorption. The BET values of S3 and S4 show relatively higher surface areas of 32.5 and 52.3 m 2 /g, respectively. Considering the factors that affect specific surface area, we can conclude that, in a comparable size range, it is the pores that increase specific surface area, according to the comparison between S3 and S4, whereas, when there is a wide size discrepancy, it is the size that determines the specific surface area, according to the comparison between S2 and S3. It is worth noting that the as-obtained R-Fe 2 O 3 nanostructures with the gradient in BET surface area and diameter size, provide a fine example to study the size-dependent properties of magnetization, lithium batteries, and gas sensors. 3.3. Magnetic Properties for r-Fe 2 O 3 Nanostructures. Owing to their gradient in the BET surface area and the diameter size, the magnetic behavior of as-obtained R-Fe 2 O 3 nanostruc- tures, which is of importance for practical applications, was investigated for samples S1-S4. Figure 3 shows the curves for the temperature dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetizations from 4 to 300 K, under an applied field of 500 Oe. The insets are the corresponding differential ZFC curves. As for the submicron solid particle sample S1, the FC and ZFC magnetization curves overlap in the entire concerned temperature range, as shown in Figure 3a, displaying the characteristic behavior for R-Fe 2 O 3 with a Morin transition temperature (T M ) of 255 K, which is determined by the sharp peak in the differential ZFC curve (inset in Figure 3a). Normally, bulk hematite has a Morin transition from the low-temperature antiferromagnetic phase to a weakly ferromagnetic phase at 263 K. 15 Here, the T M value for the submicron R-Fe 2 O 3 solid particles is approaching that for bulk hematite. As for the sample with 60-90 nm nanorods with porosities of 20-50 nm (S2), as shown in Figure 3b, the characteristics of the ZFC and FC magnetization curves show the same trend as those of S1, except that these two curves slightly split in the temperature ranges of 4-100 and 260-300 K. Notably, the Morin transition temper- ature of S2 remains 255 K, showing no change compared to that of S1, which indicates that the magnetization is not sensitive to the size evolution from submicron particles to the 60-90 nm nanorods. As for the 5-16 nm nanorods (S3), as shown in Figure 2. DrTGA curves for the samples of the FeOOH precursors obtained by the direct hydrolysis system (a), the FeCl 3 -KCl system (b), the FeCl 3 -Na 2 SO 4 system (c), and the FeCl-NH 4 Cl system (d) at heating rate of 10 °C min -1 , from which the calcining influence parameters for the formation of S1, S2, S3, and S4 could be discussed, respectively. The relation between these system and the final products of S1, S2, S3, and S4 mentioned in this work can be seen in Table 1. Synthesis of Hematite (R-Fe 2 O 3 ) Nanorods J. Phys. Chem. B, Vol. 110, No. 36, 2006 17809 Figure 3c, the FC and ZFC magnetization curves split signifi- cantly; the FC magnetization rises significantly, while the ZFC curve decreases slowly. The split between the FC and ZFC curves reflects the existence of a large size distribution of magnetic units resulting from the decrease in effective size, whose moments block progressively with decreasing tempera- ture. 16 Additionally, the Morin transition temperature for S3 (235 K) is lower than that for S1 and S2, which may be related to the decrease in diameters for 1-D nanohematite, agreeing with the theory that T M decreases with decreasing particle size. Since the temperature of the overlap point is much higher than 100 K, the character of this curve should stem from a pilling center effect, rather than spin glass freezing. 17 As for the sample with 5-19 nm nanorods with porosities of 2-16 nm (S4) in Figure 3d, the FC and ZFC magnetization curves split significantly, and the Morin transition disappears in the concerned temperature range of 4-300 K, indicating that the porous nanorods with diameters less than 20 nm exhibit no Morin transition. In summary, first, although our nanorods are grown prefer- entially along (110) as single crystalline, the wire diameter greatly restrains the maximal volume of the domain. Evidently, T M decreased to 235 K when the diameters are less than 20 nm for S3, while it seems to be insensitive to the size effect in the case of diameters larger than 60 nm (S1 and S2). Second, the experimental results show that the decreasing diameter of nanorods could lead to the split in the ZC and ZFC magnetiza- tion curves, which may result from the decrease in effective size, whose moments block progressively with decreasing temperature. Third, the porosity could also influence the magnetic behavior in a comparable size range. For example, for the case of S3 and S4, the porous product of S4 exhibits no Morin transition in the concerned temperature range of 4-300 K. 3.4 r-Fe 2 O 3 Nanostructures in a Lithium-Ion Battery. It is found 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. 18 As for the hematite crystal structure, each Fe atom is surrounded by six O atoms, whereas each O atom is bound to four Fe atoms in a typical hematite crystal unit. A hematite crystal has a rhombohedrally centered hexagonal structure of the corundum type with a closed-packed lattice in which two-thirds of the octahedral sites are occupied by Fe 3+ ions (see Supporting Information). As seen from the hematite structure along [001], [100], and [110], there are no interlayer spacings and tunnels through the crystal structure (See Sup- porting Information). Upon careful observation of the hematite surface structure, the holes could be observed in the first octahedral layer projected along [001] and [100]; however, the tunnels could not be seen as the layer number was increased, as shown in Figure 4. That is to say, holes existed in the surface hematite crystal, which allowed foreign atoms or molecules to be introduced, for example, Li + ions. When the introduction of lithium ions to the holes in the hematite surface is concerned, it gives us the impression that the lithium intercalation perfor- mance will improve by increasing the surface area or the porosity of the hematite crystals. Therefore, the synthesis of hematite nanocrystals with higher surface area or porosity structures is much needed because of the intercalation capacities and affinities for Li + to the more exposed holes in the hematite Figure 3. Temperature dependence of ZFC and FC magnetization for an applied field of 500 Oe for S1 (a), S2 (b), S3 (c), and S4 (d). Insets are their corresponding differential ZFC curves. 17810 J. Phys. Chem. B, Vol. 110, No. 36, 2006 Wu et al. surface with higher surface area, which could then shorten the diffusion length of lithium ions. 19 Evidently, the electrochemical performance of lithium ions strongly depends on the diameter size and BET surface areas, which agrees with the above considerations. As mentioned above, the sample possesses a surface area with the sequence of S4 > S3 > S2 > S1. The electrochemical performance of the as-prepared hematite R-Fe 2 O 3 samples of S1-S4 in the cell configuration of Li/R-Fe 2 O 3 was evaluated. Figure 5a shows the comparison discharge curves for the concerned four samples of S1-S4 on the first cycle with a cutoff voltage of 0.6 V at a current density of 0.2 mA cm -2 , which is similar to that of the R-Fe 2 O 3 particles. 20 The S4 electrode exhibited a high discharge capacity of 1151 mAh/g, corresponding to 6.8 Li per R-Fe 2 O 3 , while the S3, S2, and S1 electrodes exhibited 1088, 981, and 894 mAh/g, corresponding to 6.5, 5.8, and 5.3 Li per R-Fe 2 O 3 , respectively. According to the results presented above, it is evident that the electrochemical properties of the first discharge capacity possess the sequence of S4 > S3 > S2 > S1, which is consistent with that of the surface areas for the as-obtained R-Fe 2 O 3 nanostructures in this case. 3.5. The r-Fe 2 O 3 Nanostructures in Formaldehyde (HCHO) Gas Sensors. As a toxic chemical component to our health, formaldehyde (HCHO) widely exists in building materials and in the combustion gas of organic materials. Thus, finding a way to fabricate effective sensors for detecting the existence of HCHO is much needed. R-Fe 2 O 3 , an n-type semiconductor with an electrical conductivity highly sensitive to gaseous environ- ments, has been used as a sensor for ethanol and H 2 . 21 Inspired by this, we suspected that the as-obtained R-Fe 2 O 3 with different BET surface areas should also be useful for the fabrication of the HCHO sensors. From the crystalline point of view, there are no interlayer spacings and tunnels through the crystal structure, revealing that increasing the surface area could then produce more activity sites for the HCHO sensors. The gas- sensing characteristics of the as-obtained products from S1 to S4 in response to HCHO are shown in Figure 6, in which the curves are the plot of the gas sensitivity versus HCHO concentration. The gas sensitivity, S g , is defined as R air /R gas , where R air and R gas are the electrical resistances for sensors in air and in gas. 22 Although the sensitivity of all the Fe 2 O 3 Figure 4. Schematic hematite structure projected along either [001] (a) or [100] (b), where holes can be observed in the first octahedral layer. No tunnels can be found as the layer number is increased. Figure 5. First charge-discharge curves of hematite (R-Fe 2 O 3 ) samples (S1-S4) at a current density of 0.2 mA cm -2 . Figure 6. Room-temperature sensor sensitivity to formaldehyde (HCHO) of the as-prepared hematite (R-Fe 2 O 3 ) nanostructures for S1 (a), S2 (b), S3 (c), and S4 (d). Synthesis of Hematite (R-Fe 2 O 3 ) Nanorods J. Phys. Chem. B, Vol. 110, No. 36, 2006 17811 nanostructures (S1-S4) gradually increases with an increase in HCHO gas concentration, as indicated in Figure 6, it can be seen that the sensitivity of the as-obtained Fe 2 O 3 nanostructures follows the sequence S4 > S3 > S2 > S1 under a given HCHO concentration and testing temperature. Notably, this sensitivity sequence is consistent with that for the BET surface area, indicating the sensitivity for the nanostructures is coherent with its corresponding surface area. These results verify the generally accepted opinion that, for R-Fe 2 O 3 -based sensors, the change in resistance is mainly caused by the adsorption and desorption of gas molecules on the surface of the sensing structure. For example, the superior sensing properties for S4 could be due to its porous structure associated with the small grain size, which enables HCHO gas to access more surfaces of the porous- nanorod structures contained in the sensing unit. Therefore, the higher surface area for the R-Fe 2 O 3 nanostructure provides more chances to adsorb and desorb HCHO gas molecules, thus leading to higher sensitivity at room temperature. This will give us a guideline to devise the R-Fe 2 O 3 sensors for detecting the concentration of HCHO gas, which is certainly scientifically and technically interesting. 4. Conclusions In summary, we have described in this paper the shape- controlled synthesis of hematite (R-Fe 2 O 3 ) nanostructures with a gradient in the diameters (from less than 20 nm to larger than 300 nm) and surface areas (from 5.9 to 52.3 m 2 /g) through an improved synthetic strategy. The benefits of the present work also stem from the first report on the porous hematite nanorods with diameters of <20 nm, as well as a new facile strategy to the less-than-20-nm nanorods, because the less-than-20-nm diameter meets the vital size domain for magnetization proper- ties in hematite. Here, the first systematic investigation on the Morin temperature evolution of nanorod/nanowire diameter or porosity found that hematite nanorods are strongly dependent on the diameter size and porosity of the nanorod products. The magnetization is not sensitive to the size evolution from submicron particles to 60-90 nm nanorods, while the magnetic properties change significantly in the case of <20 nm nanorods. In other words, in a comparable size range, the porous existence could also influence the magnetic behavior. Moreover, applica- tions in lithium battery and formaldehyde (HCHO) gas sensors for the hematite nanostructures with diameter/surface area gradients reveal that the performance of the electrochemical and gas-sensor properties strongly depends on the BET surface areas, which can be well understood by the crystalline analysis. Note that this work not only provides the first example of the fabrication of hematite nanostructure sensors for detecting HCHO gas, but also reveals that the nanorod diameter size or porosity can also influence the lithium intercalation perfor- mances. Further work is under way to further study the size- dependent properties for other functional materials as well as further applications for magnetic materials, lithium-ion batteries, and gas sensors. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20321101) and the state key project of fundamental research for nanoma- terials and nanostructures (2005CB623601). Supporting Information Available: Crystal structural analy- sis, synthesis, characterization, and discussion about the forma- tion mechanism for R- and β-FeOOH, as well as the crystal structural analysis for R-Fe 2 O 3 . This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yu, D. B.; Yam, V. W. J. Am. Chem. Soc. 2004, 126, 13200. (2) (a) Huynh, W.; Peng, X. G.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (b) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965. (3) Zysler, R. D. Phys. ReV.B2003, 68, 212408. (4) Vayssieres, L.; Beermann, N.; Lindquist, S E.; Hagfeldt, A. Chem. Mater. 2001, 13, 233. (5) Wang, R. M.; Chen, Y. F.; Fu, Y. 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Nanoparticles (zero-dimensional (0-D)) and nano- wires /nanorods (one-dimensional (1-D)) with controlled size and shape are of key importance