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Controlled synthesis of porous FeCO3 microspheres and the conversion to α-Fe2O3 with unconventional morphologywww.elsevier.com/locate/ceramint
Controlled synthesis of porous FeCO3 microspheres and the conversion to α-Fe2O3 with unconventional morphology
Tao Yanga, Zhaohui Huanga, Yangai Liua,n, Minghao Fanga, Xin Ouyangb, Meiling Hua
aSchool of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, PR China bDepartment of Chemical & Materials Engineering, The University of Auckland, Auckland, New Zealand
Received 16 February 2014; received in revised form 8 April 2014; accepted 8 April 2014 Available online 16 April 2014
Abstract
Porous FeCO3 microspheres were synthesized via a facile surfactant- and template-free hydrothermal process. The diameters of FeCO3
microspheres are about 2075 μm. Each FeCO3 microsphere was self-assembled with a number of trilobed wheel-like subunits. The influence of preparation conditions, such as temperature, reaction time and content of urea on the phase composition and morphology were investigated. Based on time-dependent experiments, we proposed the possible formation mechanism for the self-assembled FeCO3 micro-spheres. After calcination at 650 1C, α-Fe2O3 derived from FeCO3 retained the original size and morphology of FeCO3. The prepared α-Fe2O3 with the novel microstructure shows wide potential application as photocatalysts. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Crystal growth; FeCO3; α-Fe2O3; Hydrothermal method
1. Introduction
The design and preparation of iron oxide materials with precise microstructures is currently a hot topic to enhance their applications in catalysis, magnetic storage media and corrosion prevention. As the most stable form of iron oxide, α-Fe2O3 exhibits excellent physicochemical properties, and has been widely used in photo catalysts, rechargeable lithium-ion batteries, electrochemical solar cells, gas sensor, red pigment and field emission fields [1–9]. Over the past decades, extensive studies had been focused on the controllable synthesis of α-Fe2O3 with various structures, such as particles [10–13], cubes [14], rods [15], wires [16], platelets [17], peanuts [18] and spheres [19], and tubes [20]. The selected preparation method significantly affected the obtained structures. Many preparation ways of α-Fe2O3 had been developed, including the hydrothermal approach, the sol–gel process, the gas–solid growth route, chemical precipitation, high-temperature thermal oxidation, etc. However, with the hydrothermal technique, the nano- or micro-structures could be controlled by simply adjusting
10.1016/j.ceramint.2014.04.035 14 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
g author. Tel.: þ86 10 82322186; fax: þ86 10 82322186. ss: [email protected] (Y. Liu).
the reaction time and temperature. The hydrothermal method is widely employed to control the synthesis of iron oxide. α-Fe2O3 is usually fabricated through the thermal decom-
position of FeOOH [21]. The preparation method through the calcination of ferrous carbonate (FeCO3) was seldom reported. α-Fe2O3 can maintain the original morphology of FeCO3
during the conversion because of the topotactic reaction from FeCO3 to α-Fe2O3 [22]. Meanwhile, due to the release of CO2
from FeCO3 during the decomposition, nano-pores would be formed and result in a novel microstructure with relatively large specific surface area. So it is necessary to develop the method to control the size and morphology of FeCO3 and investigate its transformation to α-Fe2O3. FeCO3 with different morphologies, such as microparticles
[23], peanut-like microstructures [17] and microspheres [22,24], have been reported. Liu et al. reported a surfactant- assisted hydrothermal route to prepare FeCO3 microspheres (FCMSs) with the diameters of 70–100 μm. In this paper, monodisperse FCMSs were synthesized via a facile surfactant- and template-free hydrothermal method. This process can effec- tively reduce the unpredictable influence of the toxic products from the surface-adsorbed surfactants and improve the atom
T. Yang et al. / Ceramics International 40 (2014) 11975–1198311976
economy [25]. The α-Fe2O3 with well-defined novel morphol- ogies were obtained by annealing the FCMSs in air at 500 1C for 4 h. To the best of our knowledge, this novel structure of α-Fe2O3 has not been reported.
2. Experimental section
2.1. Materials
Raw materials, ferrous sulphate heptahydrate (FeSO4 7H2O), ascorbic acid (C6H8O6) and urea (CO(NH2)2) were obtained from Beijing Chemistry Regent Company (Beijing, China). All chemical reagents were of analytical grade and utilized as received without further purification.
2.2. Synthesis of FCMSs and α-Fe2O3 microspheres
In the typical preparation procedure of FCMSs, 2 mmol of FeSO4 7H2O and 3 mmol of C6H8O6 were firstly mixed with 80 mL deionized water (DIW). Then, 6 mmol of urea (CO(NH2)2) was introduced into the as-prepared solution under constant stirring for 30 min to form a transparent solution. Then the solution was transferred into a sealed Teflon-lined autoclave with a capacity of 100 mL and treated at the controlled temperature of (16071) 1C for 3 h. After cooling down to room temperature, the precipitate was collected via centrifugation and then washed with DIW and absolute ethanol for several times to obtain the FeCO3
precursor before drying in a vacuum oven at 60 1C for 12 h. In the second step, the α-Fe2O3 microspheres can be produced through the calcination of FCMSs at 500 1C for 4 h in air at a heating rate of 2 1C/min. After the thermal treatment, the oven was cooled down to room temperature, and the calcined samples were then collected for further characterization.
2.3. Material characterization
The crystallinity and phase composition of the products were characterized via X-ray diffraction (XRD) by using CuKα radiation. The morphology of the as-prepared samples was examined by field emission scanning electron microscopy (FESEM, JSM-7001F) with an energy-dispersive X-ray spec- trometer (EDS, Oxford, Link ISIS). UV–visible diffused reflectance spectra of as-annealed α-Fe2O3 powders was recorded on a UV–visible spectrophotometer (Cary 5000, Varian, America), and BaSO4 was utilized as the reflectance standard in the UV–visible diffuse reflectance experiment.
3. Results
The phase composition and crystallinity of the as-obtained specimens were investigated by XRD. All the diffraction peaks of the precursor can be readily indexed to the pure rhombohe- dral structure of FeCO3 with an R-3c space group (a¼4.6935 Å, c¼15.386 Å, JCPDS card #29-0696), as shown in Fig. 1a. No characteristic peak corresponding to Fe3O4, FeOOH, γ-Fe2O3 or other organic impurities were detected.
The micro-morphology of the FeCO3 precursor with different magnifications is shown in Fig. 1b–e. Fig. 1b indicates that the monodisperse FeCO3 microspheres have a relatively narrow size distribution with the diameters of 2075 μm, which is much smaller than that reported in the previous results [22]. As shown in Fig. 1c, many small holes can be observed on the surface and more detailed information is showed in Fig. 1d and e. It is clear that the entire 3D spherical architecture is assembled with substantial trilobed wheel-like subunits with uniform size in the radial direction. The trilobed wheel-like subunits have the diameters of around 210 nm with three symmetrical horns and the center of subunits are convex, as estimated from the magnified top- and side-view of a single architecture of FeCO3. The corresponding EDS spectra (Fig. 1f) reveal that the micro- spheres are composed of C, O and Fe originated from FeCO3.
3.1. The influence of temperature
Fig. 2 shows the FESEM images of the products prepared at different temperatures. The product obtained at 120 1C (Fig. 2a) contains monodisperse microspheres with different diameters (1–18 μm). These microspheres are composed of nanonets or nanoparticles (see Fig. S1a and S1b in the Supplementary information). As the temperature rises to 140 1C or 160 1C, FCMSs with obvious holes and similar diameters (15–20 μm) are formed. The inset in Fig. 2b shows that the spheres are assembled with the interlaced trilobed wheel-like structures. The mean diameter of trilobed wheel-like structures obtained under 140 1C are about 350 nm, which are larger than that of the products prepared at 160 1C (with mean diameter of 210 nm). When the temperature rises to 180 1C, the spheres are broken. XRD pattern of this sample reveals the peaks of Fe3O4
impurities (Fig. S2 in Supplementary information). After the sample is treated at 200 1C, the impurity peaks are stronger indicating that the level of destruction of the spheres is more serious and some lamellar spindle-like microarchitectures are formed (Fig. S1c and S1d in Supplementary information). The results indicate that the higher temperature (180–200 1C) could hinder the preparation of pure FeCO3 phase and undermine the growth of this unique spherical structure.
3.2. The influence of urea content
The FESEM photos of the products with different urea contents are shown in Fig. 3. With the increasing content of urea, the morphology of the samples was transformed from the sphere-like structure to the spindle-like structure. Irregular morphology shown in Fig. 3a is formed under
2 mmol urea addition. But the homogeneous FCMSs can be obtained as urea content is increased (Fig. 3b). As shown in the insets of Fig. 3b, the microspheres (23 μm) are composed of many trilobed wheel-like structures with the mean diameters of about 250 nm. The microspheres (Fig. 3c) obtained under 8 mmol urea show inhomogeneous size (small size of 11 μm; big size of 20 μm) and are composed of irregular trilobed wheel-like structures. Further increase of the urea
Fig. 1. (a) The XRD pattern of the as-obtained FeCO3 microspheres; (b) low-magnification and (c) high-magnification FESEM images of FeCO3 microstructure; (d) the top-view image and (e) the side-view of an individual architecture of FeCO3; (f) the corresponding EDS spectra of FeCO3 microspheres.
T. Yang et al. / Ceramics International 40 (2014) 11975–11983 11977
content (10–60 mmol) results in the destruction of the trilobed wheel-like structures. A spindle-like structure is observed in the sample (see the parts highlighted in red in Fig. 3e and f). Finally the trilobed wheel-like structures disappear and more spindle-like structures emerge when the content rises to 60 mmol.
The urea content can play an important role in determining the phase composition of the samples. According to the corresponding XRD results (Fig. S3 in Supplementary infor- mation), impurity Fe3O4 was formed under the low urea content (2 mmol). The Fe3O4 impurity could be still detected when the content of urea rose to 4 mmol, although the intensity of peaks is extremely low.
3.3. The influence of reaction time
The FESEM images of the products obtained after different reaction time (30 min, 1 h, 1.5 h, and 2 h) are shown in Fig. 4. For the yield is extremely low after 15 min, the results are not collected. The products obtained after 30 min (Fig. 4a and b) possess the round particles with fluffy surface and the diameters of about 1 μm. After heating at 160 1C for 1 h, the morphology transition from round particles to microsphere can be observed (Fig. 4c) and the surface of the microspheres is rough and rugged (Fig. 4d). The product obtained after 1.5 h is composed of the microspheres with the diameter of about 18 μm and no round particle with fluffy surface is remained
Fig. 2. FESEM images of the products prepared by hydrothermal method at different temperature for 3 h with 6 mmol of urea: (a):120 1C; (b): 140 1C; (c): 180 1C; (d): 200 1C.
T. Yang et al. / Ceramics International 40 (2014) 11975–1198311978
(Fig. 4e). The microspheres are assembled with sub-trilobed wheel-like structures with few open holes on the surface (Fig. 4f). After the 2 h reaction, the sub-trilobed wheel-like structures are transformed into trilobed wheel-like structures (Fig. 4g and h), resulting in more obvious holes on the surface. The corresponding XRD patterns indicate that the well- crystallized FeCO3 grains appear at about 1.5 h (Fig. S4 in Supplementary information).
3.4. The preparation of α-Fe2O3
Fig. 5a and b show the XRD patterns and FESEM images of the porous α-Fe2O3 obtained after the 4 h calcination at 500 1C in air, respectively. All the diffraction peaks corresponded to the pure rhombohedral phase of α-Fe2O3 (JCPDS card #33-0664). The FESEM image indicates that the converted α-Fe2O3 crystals retain the pristine morphologies of FeCO3
(Fig. 5b and c). In a certain spectrum range, light can be absorbed to excite
the electrons in a catalyst. Fig. 5d shows the optical absorption of the porous α-Fe2O3 at room temperature and the obvious absorption can be observed at the wavelength shorter than 600 nm. The obtained porous α-Fe2O3 may have the applica- tion potential as photocatalysts in the field photochemistry and environmental protection under the visible light [24].
4. Discussion
Based on the experimental results above, we propose the possible mechanism for the fabrication of FeCO3 microarch- itectures, as shown in Scheme 1. First, CO2 bubbles act as soft templates to guide freshly
formed crystal nuclei to enter an imperfect crystallized FeCO3 round intermediate in the early stage. Driven by the minimization trend of interfacial energy, the round particles are then aggregated [22]. As the structures grow larger, the increased volume allows these round intermedi- ates to be coarsened and surface energy is decreased through the Ostwald ripening process. The thermodynamically unstable smaller structures are dissolved and larger FeCO3
microspheres emerge and continue to adsorb active mono- mers, leading to continuous growth, as shown in Scheme 1. Urea has been used as an effective chemical reagent for the synthesis of highly hierarchical microspheres [26–28]. In this experiment, as a crystal growth modifier, urea plays the critical role in the formation of FCMSs with unconventional morphology. Although urea (CO(NH2)2) can act as the source of hydroxyl
ions and carbonate, it can also bring side effect to the final results, such as the impurity Fe3O4. Urea can release CO2 and NH3 at about 70 1C [29] through Eq. (1). Then, the released NH3 gas is dissolved easily in water solution and increases the
Fig. 3. FESEM images of the products prepared by the hydrothermal method under different urea contents at 160 1C for 3 h: 2 mmol (a); 4 mmol (b); 8 mmol (c); 10 mmol (d); 15 mmol (e); 30 mmol (f); 60 mmol ((g) lower magnification and (h) higher magnification).
T. Yang et al. / Ceramics International 40 (2014) 11975–11983 11979
Fig. 4. FESEM images of the products prepared by the hydrothermal method at 160 1C for different time (6 mmol urea): 30 min (a) lower magnification; (b) higher magnification), 1 h (c) lower magnification; (d) higher magnification), 1.5 h (e) lower magnification; (f) higher magnification) and 2 h (g) lower magnification; (h) higher magnification).
T. Yang et al. / Ceramics International 40 (2014) 11975–1198311980
Fig. 5. (a) The XRD pattern of FMSs samples; (b) low-magnification and (c) high-magnification FESEM images of FCMs microstructure; (d) the UV–vis spectra for FMSs samples.
Scheme 1. The formation mechanism proposed for the fabrication of FeCO3 microspheres.
T. Yang et al. / Ceramics International 40 (2014) 11975–11983 11981
pH value. In addition, the CO2 gas is dissolved in water to form HCO3
and CO3 2 . Due to the higher solubility of NH3
than CO2 and CO2 bubbles act as soft templates in this reaction system, Fe2þ can quickly react with OH to produce Fe(OH)2 suspension when the concentration of CO3
2 is low. A few Fe3þ ions from Fe2þ oxidization during mixing or heating can also react with OH to generate Fe(OH)3. Similar phenom- enon also appears during the synthesis of other transition metal
compounds [30]. The Fe3O4 impurities were resultant from Eqs. (2)–(6).
CO(NH2)2þH2O-2NH3þCO2 (1)
T. Yang et al. / Ceramics International 40 (2014) 11975–1198311982
Fe(OH)3-FeOOHþH2O (5)
2FeOOHþFe(OH)2-Fe3O4þ2H2O (6)
5. Conclusions
Novel FeCO3 microspheres were synthesized by a facile hydrothermal method without any surfactant or template and then α-Fe2O3 spheres were obtained through subsequent calcination of FeCO3 spheres. The reaction temperature and the content of urea played crucial roles in the formation of FeCO3 microspheres. Reaction time was also an important factor of the morphology and the generation of trilobed wheel- like subunits. On the basis of time-dependent experiments, we proposed a two-step growth mechanism for hydrothermally formed FeCO3 microspheres. The whole process is driven by the Ostwald ripening mechanism and the morphology of the product is controlled by surface energy.
Acknowledgments
The authors acknowledge the financial support of the Fundamental Research Funds for the Central Universities (Grant no. 2012067), the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant no. NCET-12-0951), the Science and Technology Innovation Funds for graduate students of China University of Geos- ciences (2012) and the New Star Technology Plan of Beijing (Grant no. 2007A080).
Appendix A. Supplementary material
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ceramint. 2014.04.035.
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