Hydrothermal evolution, optical and electrochemical properties of hierarchical porous hematite nanoarchitectures
© Zhu et al.; licensee Springer. 2013
Received: 16 July 2012
Accepted: 2 December 2012
Published: 2 January 2013
Hollow or porous hematite (α-Fe2O3) nanoarchitectures have emerged as promising crystals in the advanced materials research. In this contribution, hierarchical mesoporous α-Fe2O3 nanoarchitectures with a pod-like shape were synthesized via a room-temperature coprecipitation of FeCl3 and NaOH solutions, followed by a mild hydrothermal treatment (120°C to 210°C, 12.0 h). A formation mechanism based on the hydrothermal evolution was proposed. β-FeOOH fibrils were assembled by the reaction-limited aggregation first, subsequent and in situ conversion led to compact pod-like α-Fe2O3 nanoarchitectures, and finally high-temperature, long-time hydrothermal treatment caused loose pod-like α-Fe2O3 nanoarchitectures via the Ostwald ripening. The as-synthesized α-Fe2O3 nanoarchitectures exhibit good absorbance within visible regions and also exhibit an improved performance for Li-ion storage with good rate performance, which can be attributed to the porous nature of Fe2O3 nanoarchitectures. This provides a facile, environmentally benign, and low-cost synthesis strategy for α-Fe2O3 crystal growth, indicating the as-prepared α-Fe2O3 nanoarchitectures as potential advanced functional materials for energy storage, gas sensors, photoelectrochemical water splitting, and water treatment.
KeywordsHematite Hierarchical nanoarchitectures Hydrothermal Mesoporous Lithium-ion batteries
Three-dimensional hierarchical architectures, or nanoarchitectures, assembled by one-dimensional (1D) nanostructures have attracted extraordinary attention and intensive interests owing to their unique structures and fantastic properties different from those of the monomorph structures [1–5]. Particularly, hierarchical architectures with mesoporous structures have triggered more and more research enthusiasm in recent years for their high surface-to-volume ratio and permeability. Synthesis of mesoporous materials has become a remarkable level in modern materials chemistry . Mesoporous materials are generally synthesized via a soft- or hard-template-aided process, which usually, however, suffers from the removal of templates and resultant structural collapse, although hydrothermal synthesis or treatment has been extensively investigated at various stages with the attempt to improve the hydrothermal stability of the as-synthesized mesoporous products. Consequently, great effort has been made to directly grow mesoporous inorganic materials in the absence of any templates in recent years [7, 8]. Most recently, the hydrothermal method has emerged as a thriving technique for the facile fabrication of the nanoarchitectures [9–12], such as AlOOH cantaloupe , Co(OH)2 and Co3O4 nanocolumns , ZnSe nanoflowers , Ni(OH)2 and NiO microspheres , and even mesoporous SrCO3 microspheres .
As the most stable iron oxide, hematite (α-Fe2O3) has drawn much concern owing to its widespread applications as catalysts, pigments, gas sensors , photoelectrodes [17, 18], starting materials for the synthesis of magnetic iron oxide nanoparticles (NPs) , electrode materials for lithium-ion battery (LIB) [20–26], etc. α-Fe2O3 is considered a promising active lithium intercalation host due to its high theoretical capacity (1,007 mAh·g−1), low cost, and environmental friendliness. In contrast to graphite electrodes, the lithium storage within iron oxides is mainly achieved through the reversible conversion reaction between lithium ions and metal nanocrystals dispersed in a Li2O matrix . Such a process usually causes drastic volume changes (>200%) and severe destruction of the electrode upon electrochemical cycling, especially at a high rate . Particle morphology has been recognized as a key factor influencing the electrochemical performance for lithium storage; thus, hematite nanostructures with different morphologies have been synthesized so as to enhance the electrochemical performance . The mesoporous α-Fe2O3 nanoarchitectures may afford several advantages for LIB application, such as the extended contact area between the active material and the electrolyte as well as the short lithium diffusion length resulting from the thin shell and the hollow space in the central part that buffers the volume expansion during cycling [22, 27, 28].
Up to now, a family of hierarchical α-Fe2O3 architectures (microring , melon-like , columnar , and nanotube  arrays; nanoplatelets ; peanut- , cantaloupe- , or urchin-like  nanoarchitectures, etc.) have been available. Most recently, novel hollow architectures (hollow fibers , hollow particles , hollow microspheres and spindles [37, 38], etc.) and porous nanoarchitectures (nanoporous microscale particles , mesoporous particles [40, 41], nanocrystal clusters , porous nanoflowers , etc.) have emerged as the new highlights in crystal growth. However, hollow or porous hematite nanoarchitectures were generally fabricated via a forced hydrolysis (100°C, 7 to 14 days) reaction , surfactant-assisted solvothermal process [38, 42], and hydrothermal-  or solvothermal-based  or direct  calcination (400°C to 800°C) methods. The reported methodologies exhibited drawbacks such as ultralong time or high energy consumption and potentially environmental malignant. It was still a challenge to directly acquire porous/mesoporous hematite nanoarchitectures via a facile, environmentally benign, and low-cost route.
In our previous work, we developed a hydrothermal synthesis of the porous hematite with a pod-like morphology or short-aspect-ratio ellipsoidal shape (denoted as ‘pod-like’ thereafter) in the presence of H3BO3. However, the process still needed to be optimized, the formation mechanism and the effect of H3BO3 were not clear, and properties and potential applications also needed to be further investigated. In this contribution, we report our newly detailed investigation on the optimization of the process and formation mechanism of the mesoporous nanoarchitectures based on the hydrothermal evolution. In addition, the effect of H3BO3 was discussed, the optical and electrochemical properties of the as-synthesized hematite mesoporous nanoarchitectures as well as nanoparticles were investigated in detail, and the application of the as-synthesized mesoporous hematite nanoarchitectures as anode materials for lithium-ion batteries was also evaluated.
Hydrothermal synthesis of the hierarchical hematite nanoarchitectures
All reagents, such as FeCl3·6H2O, NaOH, and H3BO3, were of analytical grade and used as received without further purification. Monodisperse α-Fe2O3 particles were synthesized via a coprecipitation of FeCl3 and NaOH solutions at room temperature, followed by a facile hydrothermal treatment of the slurry in the presence of H3BO3 as the additive. In a typical procedure, 1.281 g of H3BO3 was poured into 10.1 mL of deionized (DI) water, then 9.3 mL of FeCl3 (1.5 mol·L−1) solution was added, and finally 7.0 mL of NaOH (4 mol·L−1) solution was dropped into the above mixed solution under vigorous magnetic stirring at room temperature, with the molar ratio of FeCl3/H3BO3/NaOH as 2:3:4. After 5 min of stirring, 26.4 mL of the resultant brown slurry was transferred into a Teflon-lined stainless steel autoclave with a capacity of 44 mL. The autoclave was sealed and heated to 90°C to 210°C (heating rate 2°C·min−1) and kept under an isothermal condition for 1.0 to 24.0 h, and then cooled down to room temperature naturally. The product was filtered, washed with DI water for three times, and finally dried at 80°C for 24.0 h for further characterization. To evaluate the effects of the molar ratio of the reactants, the molar ratio of FeCl3/H3BO3/NaOH was altered within the range of 2:(0–3):(2–6), with other conditions unchanged.
Evaluation of the hematite nanoarchitectures as the anode materials for lithium batteries
The electrochemical evaluation of the Fe2O3 NPs and nanoarchitectures as anode materials for lithium-ion batteries were carried out using CR2025 coin-type cells with lithium foil as the counter electrode, microporous polyethylene (Celgard 2400, Charlotte, NC, USA) as the separator, and 1.0 mol·L−1 LiPF6 dissolved in a mixture of ethylene carbonate, dimethyl carbonate, ethylene methyl carbonate (1:1:1, by weight) as the electrolyte. All the assembly processes were conducted in an argon-filled glove box. For preparing working electrodes, a mixed slurry of hematite, carbon black, and polyvinylidene fluoride with a mass ratio of 80:10:10 in N-methyl-2-pyrrolidone solvent was pasted on pure Cu foil with a blade and was dried at 100°C for 12 h under vacuum conditions, followed by pressing at 20 kg·cm−2. The galvanostatic discharge/charge measurements were performed at different current densities in the voltage range of 0.01 to 3.0 V on a Neware battery testing system (Shenzhen, China). The specific capacity was calculated based on the mass of hematite. Cyclic voltammogram measurements were performed on a Solartron Analytical 1470E workstation (Farnborough, UK) at a sweep rate of 0.1 mV·s−1.
The crystal structures of the samples were identified using an X-ray powder diffractometer (XRD; D8-Advance, Bruker, Karlsruhe, Germany) with a Cu Kα radiation (λ = 1.5406 Å) and a fixed power source (40.0 kV, 40.0 mA). The morphology and microstructure of the samples were examined using a field-emission scanning electron microscope (SEM; JSM 7401 F, JEOL, Akishima-shi, Japan) operated at an accelerating voltage of 3.0 kV. The size distribution of the as-synthesized hierarchical architectures was estimated by directly measuring ca. 100 particles from the typical SEM images. The N2 adsorption-desorption isotherms were measured at 77 K using a chemisorption-physisorption analyzer (Autosorb-1-C, Quantachrome, Boynton Beach, FL, USA) after the samples had been outgassed at 300°C for 60 min. The specific surface area was calculated from the adsorption branches within the relative pressure range of 0.10 to 0.31 using the multipoint Brunauer-Emmett-Teller (BET) method, and the pore size distribution was evaluated from the N2 desorption isotherm using the Barrett-Joyner-Halenda method. The optical properties were examined using a UV–vis spectrophotometer (Cary 300, Varian, Palo Alto, CA, USA), with absolute alcohol as the dispersive medium.
Results and discussion
Hematite structures obtained at different molar ratios of the reactants
However, when H3BO3 was introduced into the reaction system, e.g., the molar ratio of FeCl3/H3BO3/NaOH was designed as 2:0.3:4 (Figure 1a4,e,e1) and 2:1.5:4 (Figure 1a5,f,f1), relatively uniform porous pod-like hematite nanoarchitectures were obtained. For the ratio of 2:0.3:4, 90% of the nanoarchitectures have an aspect ratio (ratio of longitudinal length to latitude diameter) within 1.4 to 1.8 (Figure 1e1). For the hematite obtained from a molar ratio of FeCl3/H3BO3/NaOH as 2:1.5:4, 95% of the nanoarchitectures have an aspect ratio within 1.4 to 1.8 (Figure 1f1). Therefore, the introduction of H3BO3 not only preserved the shape of hematite particles, but also improved the morphology uniformity of the nanoarchitectures. This situation was different from that of the formation of peanut-type hematite, which evolved from pseudocubic particles via an ellipsoidal shape with the increasing concentration of the additive such as sulfate or phosphate . On the other hand, compared with those organic surfactant-assisted solvothermal or other solution-based calcination methods, the introduced H3BO3 in the present case could be easily removed via DI water washing and then reused, indicating the environmentally benign characteristic.
Effects of hydrothermal temperature on the hematite product formation
It was worth noting that when treated at a temperature from 90°C to 210°C for 12.0 h, the overall crystallinity of the products became higher (Figure 2a2,a3,a4,a5,a6), and the NPs and cavities within the α-Fe2O3 nanoarchitectures grew larger. The product evolved from compact pod-like nanoarchitectures (Figure 2c,d) to loose (Figure 2e,f) and to looser (Figure 2g,h) pod-like nanoarchitectures. As a matter of fact, with the temperature going up from 120°C to 150°C, to 180°C, and to 210°C, the crystallite size along the  direction, i.e., D104, calculated by the Debye-Scherrer equation also increased from 23.3 to 27.3, to 28.0, and to 31.3 nm, respectively. This was in accordance with the direct observation on the gradual increase in the NP size within the nanoarchitectures (Figure 2d,e,f,g,h), thus accounted for the gradual sharper tendency for the XRD patterns of the corresponding hydrothermal products (Figure 2a3,a4,a5,a6) obtained from 120°C to 210°C. Analogous to those obtained previously (Figure 1c,e,f), the nanoarchitectures obtained at 150°C to 210°C for 12.0 h were speculated to be constituted of 1D assemblies (Figure 2e,f) or NPs (Figure 2g,h).
Determination of the mesoporous structure of the pod-like α-Fe2O3 nanoarchitectures
Mesoporous structures of the α-Fe 2 O 3 synthesized at different temperatures for 12.0 h (FeCl 3 /H 3 BO 3 /NaOH = 2:3:4)
Total pore volume
Average pore diameter
3.9 × 10−2
2.9 × 10−2
2.9 × 10−2
2.1 × 10−2
Comparatively, the looser pod-like nanoarchitecture 3 (Figure 2g, D104 = 28.0 nm) demonstrated a similar adsorbance of N2 (Figure 3c1) whereas with a narrow hysteresis loop at a relative pressure P/P0 of 0.40 to 0.95 and a quasi-bimodal pore diameter distribution (Figure 3c2). Very similarly, the loosest pod-like nanoarchitecture 4 (Figure 2h, D104 = 31.3 nm) exhibited a relatively low adsorbance of N2 (Figure 3d1) with also a narrow hysteresis loop at a relative pressure P/P0 of 0.25 to 0.95 as well as a quasi-bimodal pore diameter distribution (Figure 3d2). It was worth noting that the broad hysteresis loop (Figure 3a1) and relative narrow one (Figure 3b1) were due to the strong and weak capillarity phenomena existing within the compact (Figure 2d) and relatively loose nanoarchitectures (Figure 2e), respectively. Moreover, the characteristic H3-type hysteresis loop (Figure 3b1) indicated the existence of dominant slit pores and channels with a relatively uniform shape and size within the relatively loose pod-like nanoarchitectures (Figure 2e,f). This was in accordance with the SEM observation (Figure 1c) and literature results [45, 46]. The thin hysteresis loops (Figure 3c1,d1) were due to the slight capillarity phenomenon existing within the very loose nanoarchitectures (Figure 2g,h).
As shown in Table 1, with the temperature increasing from 120°C to 150°C, to 180°C, and to 210°C, the corresponding multipoint BET specific surface area of the nanoarchitecture decreased from 21.3 to 5.2, to 2.6, and to 2.0 m2·g−1, respectively. Meanwhile, the total pore volume changed from 3.9 × 10−2 to 2.9 × 10−2, to 2.9 × 10−2, and to 2.1 × 10−2 cm3·g−1, with a roughly decreasing tendency; the average pore diameter changed from 7.3 to 22.1, to 44.7, and to 40.3 nm, with a roughly increasing tendency. Thus, according to the general recognition of the porous materials , nanoarchitectures 3 and 4 were determined as the mesoporous structures, whereas the pore diameters were near the macropores category. As a matter of fact, with the temperature increasing from 120°C to 210°C, the evolution of the BET specific surface area, total pore volume, and average pore diameter of the various-morphology pod-like α-Fe2O3 nanoarchitectures agreed with the variation of the D104 calculated by the Debye-Scherrer equation, also in accordance with the SEM observation (Figure 2d,e,f,g,h).
Evolution of the hydrothermal products during hydrothermal process
Formation mechanism of mesoporous pod-like α-Fe2O3 nanoarchitectures
As known, iron oxyhydroxides (FeOOH) can be crystallized as goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and akaganeite (β-FeOOH), and an environment rich of Cl− was favorable for the formation of β-FeOOH phase . In the present case, a molar ratio of the reactants as FeCl3/H3BO3/NaOH = 2:(0–3):4 led to a surrounding rich of Cl− and thus promoted the formation of β-FeOOH. Tiny β-FeOOH fibrils with poor crystallinity formed at the early stage of the hydrothermal treatment (e.g., 90°C, 12.0 h, Figure 2a1; 105°C, 1.0 to 3.0 h, Figure 4a1,a2) tended to agglomerate with each other owing to the high surface energy, leading to quasi-amorphous agglomerate bulks of irregular shape (Figures 2b and 4b,c). Undoubtedly, the conversion from β-FeOOH to α-Fe2O3 was crucial to the formation of mesoporous pod-like hematite nanoarchitectures. Sugimoto et al. reported a preparation of monodisperse peanut-type α-Fe2O3 particles from condensed ferric hydroxide gel in the presence of sulfate  and found that ellipsoidal hematite turned into a peanut-like shape with the increase in the concentration of sulfate . In the present case, although quasi-spherical α-Fe2O3 NPs were obtained in due case (Figure 1b), the mesoporous hematite nanoarchitectures (Figures 1c,d,e,f and 2d,e,f,g,h) were not directly assembled by those NPs, taking into consideration the remarkable differences of the morphology especially size between the NPs and subunits of nanoarchitectures. It was worth noting that the hydrothermally formed hematite particles exhibited a peanut-like shape at the molar ratio of FeCl3/H3BO3/NaOH as 2:0:2 (Figure 1d) and a pod-like shape at the molar ratio of FeCl3/H3BO3/NaOH as 2:(0–3):4 (Figures 1c,e,f and 2d,e,f,g,h). Moreover, with the content of H3BO3 increasing, the pod-like α-Fe2O3 nanoarchitectures tended to be uniform in size distribution. Consequently, the morphology evolution of the hydrothermally synthesized α-Fe2O3 nanoarchitectures in the presence of boric acid, from a peanut-type to a pod-like shape, was obviously different from that of the peanut-type α-Fe2O3 particles that originated from condensed ferric hydroxide gel in the presence of sulfate .
It is notable, however, that the boric acid played a significant role in the formation of the present mesoporous pod-like α-Fe2O3 nanoarchitectures with uniform morphology and size, confirmed by the above experimental results (Figures 1 and 2). Also, as confirmed to improve the uniformity, the amount of boric acid or molar ratio of FeCl3/H3BO3/NaOH should be tuned within a certain composition range. As known, as a weak acid, H3BO3 could form sodium borate (i.e., borax) after the introduction of NaOH, giving rise to the buffer solution. This could tune the release of hydroxyl ions and further control the mild formation of amorphous Fe(OH)3 gel, leading to subsequent β-FeOOH fibrils with relatively uniform size. This was believed to contribute to the further formation of the peanut-like β-FeOOH/α-Fe2O3 assemblies and ultimate occurrence of the pod-like α-Fe2O3 nanoarchitectures.
Optical absorbance analysis
It was well illustrated that three types of electronic transitions occurred in the optical absorption spectra of Fe3+ substances: (a) the Fe3+ ligand field transition or the d d transitions, (b) the ligand to metal charge-transfer transitions, and (c) the pair excitations resulting from the simultaneous excitations of two neighboring Fe3+ cations that are magnetically coupled. According to [62, 63], the absorption bands near 390 and 430 nm corresponded to the 6A1 → 4E(4G) and 6A1 → 4E, 4A1(4G) ligand field transitions of Fe3+[59, 60]. The observed edge at around 520 to 570 and 600 to 640 nm could be assigned to the 6A1 → 4 T2(4G) ligand field transition of Fe3+. As revealed by Figure 6, the electronic transition for the charge transfer in the wavelength region 380 to 450 nm dominated the optical absorption features of the NPs, while the ligand field transitions in the range of 520 to 640 nm dominated the optical absorption features of the architectures. This indicated that the absorption could be modulated by controlling the size and shape of the hematite, which was quite important for the enhancement of the photoelectrocatalytic activity.
Mesoporous pod-like α-Fe2O3 nanoarchitectures as anode materials for lithium-ion batteries
With lithium ions inserted into the crystal structure of the as-prepared α-Fe2O3, the hexagonal α-Fe2O3 was transformed to cubic Li2Fe2O3. The peak at 0.65 V corresponded to the complete reduction of iron from Fe2+ to Fe0 and the decomposition of electrolyte. A broad anodic peak was recorded in the range of 1.4 to 2.2 V, corresponding to the oxidation of Fe0 to Fe2+ and further to Fe3+[66, 67]. The curve of the subsequent cycle was significantly different from that of the first cycle as only one cathodic peak appeared at about 0.8 V with decreased peak intensity, while the anodic process only showed one broad peak with a little decrease in peak intensity. The irreversible phase transformation during the process of lithium insertion and extraction in the initial cycle was the reason for the difference between the first and second cathodic curves . After the first discharge process, α-Fe2O3 was completely reduced to iron NPs and was dispersed in a Li2O matrix. The decrease of the redox peak intensity implied that the capacity was decreased during cycling.
The charge–discharge curves of the α-Fe2O3 NP (shown in Figure 1b) electrode during the first and second cycles are shown in Figure 7b. In the first discharge curve, there was a weak potential slope located at 1.2 to 1.0 V and an obvious potential plateau at 0.9 to 0.8 V. The capacity obtained above 0.8 V was 780 mAh·g−1 (4.6 mol of Li per mole of α-Fe2O3). After discharging to 0.01 V, the total specific capacity of the as-prepared α-Fe2O3 reached 887 mAh·g−1, corresponding to 5.3 mol of Li per mole of α-Fe2O3. During the second cycle, the discharge curve only showed a slope at 1.0 to 0.8 V, and the capacity was reduced to 824 mAh·g−1. Usually, the slope behavior during the discharge process of metal oxide anode materials was considered to be related with the irreversible formation of a nanocomposite of crystalline grains of metals and amorphous Li2O matrix.
The comparison of the rate as well as cycling performances between Fe2O3 NPs and nanoarchitectures were also conducted, which were obtained by a 12.0-h hydrothermal treatment at 150°C with a molar ratio of FeCl3/H3BO3/NaOH as 2:0:4 (Figure 1b) and 2:3:4 (Figure 2e), respectively. The discharge and charge capacities in the first cycle at a current of 0.1 C were 1,129 and 887 mAh·g−1 for Fe2O3 NPs (Figure 7c) and 1,155 and 827 mAh·g−1 for Fe2O3 nanoarchitectures. For the second cycle, the discharge and charge capacities were 871 and 824 mAh·g−1 for Fe2O3 NPs and 799 and 795 mAh·g−1 for Fe2O3 nanoarchitectures. The Li-ion storage capacitance of the current Fe2O3 NPs/nanoarchitectures reported in this work is higher than that of hematite nanorod (ca. 400 mAh·g−1 at 0.1 C) , nanoflakes , hierarchial mesoporous hematite (ca. 700 mAh·g−1 at 0.1 C) , hollow nanospindles (457 mAh·g−1 at 0.2 mA cm−2) , hollow microspheres (621 mAh·g−1 at 0.2 mA cm−2) , and dendrites (670 mAh·g−1 at 1 mA cm−2) . When the current increased, both the discharge and charge capacities decreased, especially for Fe2O3 NPs (Figure 7c,e). The discharge and charge capacities of Fe2O3 nanoarchitectures were larger than those of Fe2O3 NPs. For instance, when the current rate increased to 2.0 C, the charge and discharge capacities of Fe2O3 nanoarchitectures were 253 and 247 mAh·g−1, while those of Fe2O3 NPs were only 24 and 21 mAh·g−1. This indicated that the Fe2O3 nanoarchitectures presented much improved rate performance for the reason that the porous nature of Fe2O3 nanoarchitectures allow a fast Li-ion diffusion by offering better electrolyte accessibility and also accommodate the volume change of NPs during Li insertion/extraction.
However, similar to many Fe2O3 nanostructures reported in literatures, the α-Fe2O3 nanoarchitectures exhibited a rapid capacity fading within the potential range of 0.01 to 3.0 V, suggesting that the crystalline structure of the electrode materials was destroyed by the insertion/extraction of lithium ions and the electrode decomposed the electrolyte. The Fe2O3 nanoarchitectures presented superior charge/discharge stability to the Fe2O3 NPs, e.g., the charging capacities of Fe2O3 nanoarchitectures (Figure 7f) and NPs (Figure 7d) of the tenth cycle were 503 and 356 mAh·g−1, respectively. This indicated that the mesoporous structure of Fe2O3 nanoarchitectures provided more space for Fe2O3 volume change and avoided severe pulverization. Such an improvement could also be confirmed by the cycling performance of mesoporous hematite , which maintained a good stability attributed from the small Fe2O3 size (ca. 10 nm) and abundant pores. The introduction of conductive carbon into the hematite electrode is an effective way to improve the cycle performance . It is highly expected that the hierarchical Fe2O3 nanoarchitectures with ultrafine Fe2O3 building blocks and interconnected pores afford shorter Li-ion diffusion way, fast diffusion rate, and large-volume changes during the charge/discharge process, which can serve as potential anode materials for Li-ion storage.
Uniform monodisperse hierarchical α-Fe2O3 nanoarchitectures with a pod-like shape have been synthesized via a facile, environmentally benign, and low-cost hydrothermal method (120°C to 210°C, 12.0 h), by using FeCl3·6H2O and NaOH as raw materials in the presence of H3BO3 (molar ratio, FeCl3/H3BO3/NaOH = 2:3:4). The mesoporous α-Fe2O3 nanoarchitectures had a specific surface area of 21.3 to 5.2 m2·g−1 and an average pore diameter of 7.3 to 22.1 nm. The mesoporous α-Fe2O3 nanoarchitectures were formed as follows: the reaction-limited aggregation of β-FeOOH fibrils led to β-FeOOH/α-Fe2O3 peanut-type assembly, which was subsequently and in situ converted into compact pod-like α-Fe2O3 nanoarchitectures and further into loose pod-like α-Fe2O3 nanoarchitectures through a high-temperature, long-time hydrothermal treatment via the Ostwald ripening. Benefiting from their unique structural characteristics, the as-synthesized hierarchical mesoporous pod-like α-Fe2O3 nanoarchitectures exhibited good absorbance and a high specific discharge capacity. Compared with the traditional solid-state monomorph hematite NPs and other complicated porous hematite nanoarchitectures, the as-synthesized hierarchical mesoporous pod-like α-Fe2O3 nanoarchitectures derived from the facile, environmentally benign, and low-cost hydrothermal route can provide an alternative candidate for novel applications in booming fields, such as gas sensors, lithium-ion batteries, photocatalysis, water treatment, and photoelectrochemical water splitting.
This work was supported by the National Natural Science Foundation of China (no. 21276141), the State Key Laboratory of Chemical Engineering, China (no. SKL-ChE-12A05), a project of Shandong Province Higher Educational Science and Technology Program, China (J10LB15), and the Excellent Middle-Aged and Young Scientist Award Foundation of Shandong Province, China (BS2010CL024).
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