Controlled Synthesis of Porous Feco3 Microspheres and the Conversion

Available online at www.sciencedirect.com CERAMICS INTERNATIONAL

Ceramics International 40 (2014) 11975–11983 www.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 reaction time and temperature. The hydrothermal method is widely employed to control the synthesis of iron oxide. The design and preparation of iron oxide materials with precise α-Fe2O3 is usually fabricated through the thermal decom- microstructures is currently a hot topic to enhance their applications position of FeOOH [21]. The preparation method through the in catalysis, magnetic storage media and corrosion prevention. As calcination of ferrous carbonate (FeCO3) was seldom reported. the most stable form of iron oxide, α-Fe2O3 exhibits excellent α-Fe2O3 can maintain the original morphology of FeCO3 physicochemical properties, and has been widely used in photo during the conversion because of the topotactic reaction from catalysts, rechargeable lithium-ion batteries, electrochemical solar FeCO3 to α-Fe2O3 [22]. Meanwhile, due to the release of CO2 cells, gas sensor, red pigment and field emission fields [1–9].Over from FeCO3 during the decomposition, nano-pores would be the past decades, extensive studies had been focused on the formed and result in a novel microstructure with relatively controllable synthesis of α-Fe2O3 with various structures, such as large specific surface area. So it is necessary to develop the particles [10–13],cubes[14],rods[15],wires[16], platelets [17], method to control the size and morphology of FeCO3 and peanuts [18] and spheres [19], and tubes [20]. The selected investigate its transformation to α-Fe2O3. preparation method significantly affected the obtained structures. FeCO3 with different morphologies, such as microparticles Many preparation ways of α-Fe2O3 had been developed, including [23], peanut-like microstructures [17] and microspheres the hydrothermal approach, the sol–gel process, the gas–solid [22,24], have been reported. Liu et al. reported a surfactant- growth route, chemical precipitation, high-temperature thermal assisted hydrothermal route to prepare FeCO3 microspheres oxidation, etc. However, with the hydrothermal technique, the (FCMSs) with the diameters of 70–100 μm. In this paper, nano- or micro-structures could be controlled by simply adjusting monodisperse FCMSs were synthesized via a facile surfactant- and template-free hydrothermal method. This process can effec- n tively reduce the unpredictable influence of the toxic products Corresponding author. Tel.: þ86 10 82322186; fax: þ86 10 82322186. E-mail address: [email protected] (Y. Liu). from the surface-adsorbed surfactants and improve the atom http://dx.doi.org/10.1016/j.ceramint.2014.04.035 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. 11976 T. Yang et al. / Ceramics International 40 (2014) 11975–11983 economy [25]. The α-Fe2O3 with well-defined novel morphol- The micro-morphology of the FeCO3 precursor with different ogies were obtained by annealing the FCMSs in air at 500 1C magnifications is shown in Fig. 1b–e. Fig. 1b indicates that the for 4 h. To the best of our knowledge, this novel structure of monodisperse FeCO3 microspheres have a relatively narrow size α-Fe2O3 has not been reported. distribution with the diameters of 2075 μm, which is much smaller than that reported in the previous results [22].Asshown 2. Experimental section 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 2.1. Materials that the entire 3D spherical architecture is assembled with substantial trilobed wheel-like subunits with uniform size in the Raw materials, ferrous sulphate heptahydrate (FeSO4 7H2O), radial direction. The trilobed wheel-like subunits have the ascorbic acid (C6H8O6) and urea (CO(NH2)2) were obtained from diameters of around 210 nm with three symmetrical horns and Beijing Chemistry Regent Company (Beijing, China). All chemical the center of subunits are convex, as estimated from the reagents were of analytical grade and utilized as received without magnified top- and side-view of a single architecture of FeCO3. further purification. The corresponding EDS spectra (Fig. 1f) reveal that the microspheres are composed of C, O and Fe originated from FeCO3. 2.2. Synthesis of FCMSs and α-Fe2O3 microspheres 3.1. The influence of temperature In the typical preparation procedure of FCMSs, 2 mmol of FeSO 7H O and 3 mmol of C H O were firstly mixed 4 2 6 8 6 Fig. 2 shows the FESEM images of the products prepared at with 80 mL deionized water (DIW). Then, 6 mmol of urea different temperatures. The product obtained at 120 1C(Fig. 2a) (CO(NH ) ) was introduced into the as-prepared solution 2 2 contains monodisperse microspheres with different diameters under constant stirring for 30 min to form a transparent (1–18 μm). These microspheres are composed of nanonets or solution. Then the solution was transferred into a sealed nanoparticles (see Fig. S1a and S1b in the Supplementary Teflon-lined autoclave with a capacity of 100 mL and treated information). As the temperature rises to 140 1Cor1601C, at the controlled temperature of (16071) 1C for 3 h. After FCMSs with obvious holes and similar diameters (15–20 μm) cooling down to room temperature, the precipitate was are formed. The inset in Fig. 2b shows that the spheres are collected via centrifugation and then washed with DIW and assembled with the interlaced trilobed wheel-like structures. The absolute ethanol for several times to obtain the FeCO 3 mean diameter of trilobed wheel-like structures obtained under precursor before drying in a vacuum oven at 60 1C for 12 h. 140 1C are about 350 nm, which are larger than that of the In the second step, the α-Fe O microspheres can be produced 2 3 products prepared at 160 1C (with mean diameter of 210 nm). through the calcination of FCMSs at 500 1C for 4 h in air at a When the temperature rises to 180 1C, the spheres are broken. heating rate of 2 1C/min. After the thermal treatment, the oven XRD pattern of this sample reveals the peaks of Fe O was cooled down to room temperature, and the calcined 3 4 impurities (Fig. S2 in Supplementary information). After the samples were then collected for further characterization. sample is treated at 200 1C, the impurity peaks are stronger indicating that the level of destruction of the spheres is more 2.3. Material characterization serious and some lamellar spindle-like microarchitectures are formed (Fig. S1c and S1d in Supplementary information). The The crystallinity and phase composition of the products results indicate that the higher temperature (180–200 1C) could were characterized via X-ray diffraction (XRD) by using CuKα hinder the preparation of pure FeCO phase and undermine the radiation. The morphology of the as-prepared samples was 3 growth of this unique spherical structure. 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 3.2. The influence of urea content reflectance spectra of as-annealed α-Fe2O3 powders was recorded on a UV–visible spectrophotometer (Cary 5000, The FESEM photos of the products with different urea Varian, America), and BaSO4 was utilized as the reflectance contents are shown in Fig. 3. With the increasing content of standard in the UV–visible diffuse reflectance experiment. urea, the morphology of the samples was transformed from the sphere-like structure to the spindle-like structure. 3. Results Irregular morphology shown in Fig. 3a is formed under 2 mmol urea addition. But the homogeneous FCMSs can be The phase composition and crystallinity of the as-obtained obtained as urea content is increased (Fig. 3b). As shown in the specimens were investigated by XRD. All the diffraction peaks insets of Fig. 3b, the microspheres (23 μm) are composed of of the precursor can be readily indexed to the pure rhombohe- many trilobed wheel-like structures with the mean diameters of dral structure of FeCO3 with an R-3c space group about 250 nm. The microspheres (Fig. 3c) obtained under (a¼4.6935 Å, c¼15.386 Å, JCPDS card #29-0696), as shown 8 mmol urea show inhomogeneous size (small size of in Fig. 1a. No characteristic peak corresponding to Fe3O4, 11 μm; big size of 20 μm) and are composed of irregular FeOOH, γ-Fe2O3 or other organic impurities were detected. trilobed wheel-like structures. Further increase of the urea T. Yang et al. / Ceramics International 40 (2014) 11975–11983 11977

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. content (10–60 mmol) results in the destruction of the trilobed 3.3. The influence of reaction time wheel-like structures. A spindle-like structure is observed in the sample (see the parts highlighted in red in Fig. 3e and f). The FESEM images of the products obtained after different Finally the trilobed wheel-like structures disappear and more reaction time (30 min, 1 h, 1.5 h, and 2 h) are shown in Fig. 4. spindle-like structures emerge when the content rises to For the yield is extremely low after 15 min, the results are not 60 mmol. collected. The products obtained after 30 min (Fig. 4a and b) The urea content can play an important role in determining possess the round particles with fluffy surface and the the phase composition of the samples. According to the diameters of about 1 μm. After heating at 160 1C for 1 h, the corresponding XRD results (Fig. S3 in Supplementary infor- morphology transition from round particles to microsphere can mation), impurity Fe3O4 was formed under the low urea be observed (Fig. 4c) and the surface of the microspheres is content (2 mmol). The Fe3O4 impurity could be still detected rough and rugged (Fig. 4d). The product obtained after 1.5 h is when the content of urea rose to 4 mmol, although the intensity composed of the microspheres with the diameter of about of peaks is extremely low. 18 μm and no round particle with fluffy surface is remained 11978 T. Yang et al. / Ceramics International 40 (2014) 11975–11983

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.

(Fig. 4e). The microspheres are assembled with sub-trilobed 4. Discussion wheel-like structures with few open holes on the surface (Fig. 4f). After the 2 h reaction, the sub-trilobed wheel-like Based on the experimental results above, we propose the structures are transformed into trilobed wheel-like structures possible mechanism for the fabrication of FeCO3 microarch- (Fig. 4g and h), resulting in more obvious holes on the surface. itectures, as shown in Scheme 1. The corresponding XRD patterns indicate that the well- First, CO2 bubbles act as soft templates to guide freshly crystallized FeCO3 grains appear at about 1.5 h (Fig. S4 in formed crystal nuclei to enter an imperfect crystallized Supplementary information). 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

3.4. The preparation of α-Fe2O3 larger, the increased volume allows these round intermedi- ates to be coarsened and surface energy is decreased through Fig. 5a and b show the XRD patterns and FESEM images of the Ostwald ripening process. The thermodynamically the porous α-Fe2O3 obtained after the 4 h calcination at 500 1C unstable smaller structures are dissolved and larger FeCO3 in air, respectively. All the diffraction peaks corresponded microspheres emerge and continue to adsorb active mono- to the pure rhombohedral phase of α-Fe2O3 (JCPDS card mers, leading to continuous growth, as shown in Scheme 1. #33-0664). The FESEM image indicates that the converted Urea has been used as an effective chemical reagent for the – α-Fe2O3 crystals retain the pristine morphologies of FeCO3 synthesis of highly hierarchical microspheres [26 28].In (Fig. 5b and c). this experiment, as a crystal growth modifier, urea plays the In a certain spectrum range, light can be absorbed to excite critical role in the formation of FCMSs with unconventional the electrons in a catalyst. Fig. 5d shows the optical absorption morphology. of the porous α-Fe2O3 at room temperature and the obvious Although urea (CO(NH2)2) can act as the source of hydroxyl absorption can be observed at the wavelength shorter than ions and carbonate, it can also bring side effect to the final 600 nm. The obtained porous α-Fe2O3 may have the applica- results, such as the impurity Fe3O4. Urea can release CO2 and 1 tion potential as photocatalysts in the field photochemistry and NH3 at about 70 C [29] through Eq. (1). Then, the released environmental protection under the visible light [24]. NH3 gas is dissolved easily in water solution and increases the T. Yang et al. / Ceramics International 40 (2014) 11975–11983 11979

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). 11980 T. Yang et al. / Ceramics International 40 (2014) 11975–11983

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) 11975–11983 11981

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.

pH value. In addition, the CO2 gas is dissolved in water to compounds [30]. The Fe3O4 impurities were resultant from 2 form HCO3 and CO3 . Due to the higher solubility of NH3 Eqs. (2)–(6). than CO2 and CO2 bubbles act as soft templates in this reaction 2 þ CO(NH2)2 þH2O-2NH3þCO2 (1) system, Fe can quickly react with OH to produce Fe(OH)2 2 þ suspension when the concentration of CO3 is low. A few NH3þH2O-NH4 þOH (2) 3 þ 2þ Fe ions from Fe oxidization during mixing or heating can 2þ Fe þ2OH -Fe(OH)2 (3) also react with OH to generate Fe(OH)3. Similar phenom- 3þ enon also appears during the synthesis of other transition metal Fe þ2OH -Fe(OH)3 (4) 11982 T. Yang et al. / Ceramics International 40 (2014) 11975–11983

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