CERAMICSINTERNATIONAL

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(2014) 11975–11983

Ceramics International 40 www.elsevier.com/locate/ceramint

Controlled synthesis of porous FeCO3 microspheres and the conversionto 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 ChinabDepartment 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 2014Available 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 ofpreparation 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. Aftercalcination at 650 1C, α-Fe2O3 derived from FeCO3 retained the original size and morphology of FeCO3. The prepared α-Fe2O3 with the novelmicrostructure 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 precisemicrostructures is currently a hot topic to enhance their applicationsin catalysis, magnetic storage media and corrosion prevention. Asthe most stable form of iron oxide, α-Fe2O3 exhibits excellentphysicochemical properties, and has been widely used in photocatalysts, rechargeable lithium-ion batteries, electrochemical solarcells, gas sensor, red pigment and field emission fields [1–9]. Overthe past decades, extensive studies had been focused on thecontrollable synthesis of α-Fe2O3 with various structures, such asparticles [10–13], cubes [14], rods [15], wires [16], platelets [17],peanuts [18] and spheres [19], and tubes [20]. The selectedpreparation method significantly affected the obtained structures.Many preparation ways of α-Fe2O3 had been developed, includingthe hydrothermal approach, the sol–gel process, the gas–solidgrowth route, chemical precipitation, high-temperature thermaloxidation, etc. However, with the hydrothermal technique, thenano- or micro-structures could be controlled by simply adjusting

10.1016/j.ceramint.2014.04.03514 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ86 10 82322186; fax: þ86 10 82322186.ss: liuyang@cugb.edu.cn (Y. Liu).

the reaction time and temperature. The hydrothermal method iswidely 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 thecalcination of ferrous carbonate (FeCO3) was seldom reported.α-Fe2O3 can maintain the original morphology of FeCO3

during the conversion because of the topotactic reaction fromFeCO3 to α-Fe2O3 [22]. Meanwhile, due to the release of CO2

from FeCO3 during the decomposition, nano-pores would beformed and result in a novel microstructure with relativelylarge specific surface area. So it is necessary to develop themethod to control the size and morphology of FeCO3 andinvestigate 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 productsfrom 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 1Cfor 4 h. To the best of our knowledge, this novel structure ofα-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 fromBeijing Chemistry Regent Company (Beijing, China). All chemicalreagents were of analytical grade and utilized as received withoutfurther purification.

2.2. Synthesis of FCMSs and α-Fe2O3 microspheres

In the typical preparation procedure of FCMSs, 2 mmol ofFeSO4 � 7H2O and 3 mmol of C6H8O6 were firstly mixedwith 80 mL deionized water (DIW). Then, 6 mmol of urea(CO(NH2)2) was introduced into the as-prepared solutionunder constant stirring for 30 min to form a transparentsolution. Then the solution was transferred into a sealedTeflon-lined autoclave with a capacity of 100 mL and treatedat the controlled temperature of (16071) 1C for 3 h. Aftercooling down to room temperature, the precipitate wascollected via centrifugation and then washed with DIW andabsolute 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 producedthrough the calcination of FCMSs at 500 1C for 4 h in air at aheating rate of 2 1C/min. After the thermal treatment, the ovenwas cooled down to room temperature, and the calcinedsamples were then collected for further characterization.

2.3. Material characterization

The crystallinity and phase composition of the productswere characterized via X-ray diffraction (XRD) by using CuKαradiation. The morphology of the as-prepared samples wasexamined by field emission scanning electron microscopy(FESEM, JSM-7001F) with an energy-dispersive X-ray spec-trometer (EDS, Oxford, Link ISIS). UV–visible diffusedreflectance spectra of as-annealed α-Fe2O3 powders wasrecorded on a UV–visible spectrophotometer (Cary 5000,Varian, America), and BaSO4 was utilized as the reflectancestandard in the UV–visible diffuse reflectance experiment.

3. Results

The phase composition and crystallinity of the as-obtainedspecimens were investigated by XRD. All the diffraction peaksof 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 shownin 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 differentmagnifications is shown in Fig. 1b–e. Fig. 1b indicates that themonodisperse FeCO3 microspheres have a relatively narrow sizedistribution with the diameters of 2075 μm, which is muchsmaller than that reported in the previous results [22]. As shownin Fig. 1c, many small holes can be observed on the surface andmore detailed information is showed in Fig. 1d and e. It is clearthat the entire 3D spherical architecture is assembled withsubstantial trilobed wheel-like subunits with uniform size in theradial direction. The trilobed wheel-like subunits have thediameters of around 210 nm with three symmetrical horns andthe center of subunits are convex, as estimated from themagnified 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 atdifferent temperatures. The product obtained at 120 1C (Fig. 2a)contains monodisperse microspheres with different diameters(1–18 μm). These microspheres are composed of nanonets ornanoparticles (see Fig. S1a and S1b in the Supplementaryinformation). 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 areassembled with the interlaced trilobed wheel-like structures. Themean diameter of trilobed wheel-like structures obtained under140 1C are about 350 nm, which are larger than that of theproducts 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 thesample is treated at 200 1C, the impurity peaks are strongerindicating that the level of destruction of the spheres is moreserious and some lamellar spindle-like microarchitectures areformed (Fig. S1c and S1d in Supplementary information). Theresults indicate that the higher temperature (180–200 1C) couldhinder the preparation of pure FeCO3 phase and undermine thegrowth of this unique spherical structure.

3.2. The influence of urea content

The FESEM photos of the products with different ureacontents are shown in Fig. 3. With the increasing content ofurea, the morphology of the samples was transformed from thesphere-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 beobtained as urea content is increased (Fig. 3b). As shown in theinsets of Fig. 3b, the microspheres (�23 μm) are composed ofmany trilobed wheel-like structures with the mean diameters ofabout 250 nm. The microspheres (Fig. 3c) obtained under8 mmol urea show inhomogeneous size (small size of�11 μm; big size of �20 μm) and are composed of irregulartrilobed 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.

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content (10–60 mmol) results in the destruction of the trilobedwheel-like structures. A spindle-like structure is observed inthe sample (see the parts highlighted in red in Fig. 3e and f).Finally the trilobed wheel-like structures disappear and morespindle-like structures emerge when the content rises to60 mmol.

The urea content can play an important role in determiningthe phase composition of the samples. According to thecorresponding XRD results (Fig. S3 in Supplementary infor-mation), impurity Fe3O4 was formed under the low ureacontent (2 mmol). The Fe3O4 impurity could be still detectedwhen the content of urea rose to 4 mmol, although the intensityof peaks is extremely low.

3.3. The influence of reaction time

The FESEM images of the products obtained after differentreaction 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 notcollected. The products obtained after 30 min (Fig. 4a and b)possess the round particles with fluffy surface and thediameters of about 1 μm. After heating at 160 1C for 1 h, themorphology transition from round particles to microsphere canbe observed (Fig. 4c) and the surface of the microspheres isrough and rugged (Fig. 4d). The product obtained after 1.5 h iscomposed of the microspheres with the diameter of about18 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.

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(Fig. 4e). The microspheres are assembled with sub-trilobedwheel-like structures with few open holes on the surface(Fig. 4f). After the 2 h reaction, the sub-trilobed wheel-likestructures 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 inSupplementary information).

3.4. The preparation of α-Fe2O3

Fig. 5a and b show the XRD patterns and FESEM images ofthe porous α-Fe2O3 obtained after the 4 h calcination at 500 1Cin air, respectively. All the diffraction peaks correspondedto the pure rhombohedral phase of α-Fe2O3 (JCPDS card#33-0664). The FESEM image indicates that the convertedα-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 absorptionof the porous α-Fe2O3 at room temperature and the obviousabsorption can be observed at the wavelength shorter than600 nm. The obtained porous α-Fe2O3 may have the applica-tion potential as photocatalysts in the field photochemistry andenvironmental protection under the visible light [24].

4. Discussion

Based on the experimental results above, we propose thepossible 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 crystallizedFeCO3 round intermediate in the early stage. Driven bythe minimization trend of interfacial energy, the roundparticles are then aggregated [22]. As the structures growlarger, the increased volume allows these round intermedi-ates to be coarsened and surface energy is decreased throughthe Ostwald ripening process. The thermodynamicallyunstable 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 thesynthesis of highly hierarchical microspheres [26–28]. Inthis experiment, as a crystal growth modifier, urea plays thecritical role in the formation of FCMSs with unconventionalmorphology.Although urea (CO(NH2)2) can act as the source of hydroxyl

ions and carbonate, it can also bring side effect to the finalresults, such as the impurity Fe3O4. Urea can release CO2 andNH3 at about 70 1C [29] through Eq. (1). Then, the releasedNH3 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) highermagnification), 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 spectrafor FMSs samples.

Scheme 1. The formation mechanism proposed for the fabrication of FeCO3 microspheres.

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pH value. In addition, the CO2 gas is dissolved in water toform HCO3

� and CO32� . Due to the higher solubility of NH3

than CO2 and CO2 bubbles act as soft templates in this reactionsystem, Fe2þ can quickly react with OH� to produce Fe(OH)2suspension when the concentration of CO3

2� is low. A fewFe3þ ions from Fe2þ oxidization during mixing or heating canalso 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 fromEqs. (2)–(6).

CO(NH2)2þH2O-2NH3þCO2 (1)

NH3þH2O-NH4þþOH� (2)

Fe2þ þ2OH�-Fe(OH)2 (3)

Fe3þ þ2OH�-Fe(OH)3 (4)

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 facilehydrothermal method without any surfactant or template andthen α-Fe2O3 spheres were obtained through subsequentcalcination of FeCO3 spheres. The reaction temperature andthe content of urea played crucial roles in the formation ofFeCO3 microspheres. Reaction time was also an importantfactor of the morphology and the generation of trilobed wheel-like subunits. On the basis of time-dependent experiments, weproposed a two-step growth mechanism for hydrothermallyformed FeCO3 microspheres. The whole process is driven bythe Ostwald ripening mechanism and the morphology of theproduct is controlled by surface energy.

Acknowledgments

The authors acknowledge the financial support of theFundamental Research Funds for the Central Universities(Grant no. 2012067), the Program for New Century ExcellentTalents in University of Ministry of Education of China (Grantno. NCET-12-0951), the Science and Technology InnovationFunds 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 foundin the online version at http://dx.doi.org/10.1016/j.ceramint.2014.04.035.

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