Elsevier

Materials Chemistry and Physics

Volume 169, 1 February 2016, Pages 21-27
Materials Chemistry and Physics

Preparation and crystallization of hollow α-Fe2O3 microspheres following the gas-bubble template method

https://doi.org/10.1016/j.matchemphys.2015.11.021Get rights and content

Highlights

  • Formation of hollow hematite microspheres by the gas-bubble template method.

  • This technique does not require hard templates or special conditions of atmosphere.

  • Annealing promotes the transition magnetite to maghemite to hematite.

  • Crystallization of the hematite shells increase with annealing temperature.

Abstract

In this work we report the formation of hollow α-Fe2O3 (hematite) microspheres by the gas-bubble template method. This technique is simple and it does not require hard templates, surfactants, special conditions of atmosphere or complex steps. After reacting Fe(NO3)3.9H2O and citric acid in water by sol–gel, the precursor was annealed in air at different temperatures between 180 and 600 °C. Annealing at 550 and 600 °C generates bubbles on the melt which crystallize and oxidizes to form hematite hollow spheres after quenching. The morphology and crystal evolution are studied by means of X-ray diffraction and scanning electron microscopy. We found that after annealing at 250–400 °C, the sample consist of a mixture of magnetite, maghemite and hematite. Single hematite phase in the form of hollow microspheres is obtained after annealing at 550 and 600 °C. The crystallization and crystal size of the hematite shells increase with annealing temperature. A possible mechanism for hollow sphere formation is presented.

Introduction

The production of hollow microspheres is of current interest due to their promising applications in photonic crystals, encapsulation, drug delivery, catalysis, chemical storage, light fillers and low dielectric constant materials [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. A variety of hollow spheres such as carbide [15], Ni [16], TiO2 [17], NiS [18], Bi2Te3 [19] and ZnO/SnO2 [20] have been successfully fabricated. The most common techniques to produce hollow spheres are based on the use of core organic/inorganic hard templates such as monodispersed silica spheres [21], [22], [23], polymer latex colloids [11], [24], carbon spheres [25] and block copolymers [26], [27] or soft templates, such as emulsion droplets [28], [29], surfactants vesicles [30] and liposome [31]. In general, the template technique involves four major steps (as represented in Fig. 1) [1]: (1) Preparation of the templates; (2) functionalization/modification of the templates surface to achieve favourable surface properties; (3) coating the templates with desired materials or their precursors; and (4) selective removal of the templates in appropriate solvents or calcination to obtain the hollow structures.

The hard template technique is effective for controlling the morphology of the final product. Nevertheless, this technique requires tedious synthetic procedures such as a careful selection of an affine template and a lot of care to prevent the collapse to affecting the quality of the shell during template removal. Some other drawbacks include limited sphere size, quality, purity, cost of production, and low temperature capability of the produced hollow spheres.

Recently different free-template approaches have been developed to produce hollow spheres. Some of these methods are based on Oswald ripening [32], simultaneous blowing and melting hidrogels [33], [34], [35], Kirkendall Effect [36], [37], [38], among others. However the average size of the hollow spheres produced by these methods are usually larger than 10 μm. Furthermore, it is difficult to obtain small microspheres having a narrow particle size distribution, and high purity metal oxide composition. Another less explored method for the production of hollow spheres is ‘the gas-bubble template method’. This method involves the production of gas microbubbles during the chemical preparation of nanoparticles by using selected ligands. It is believed that the nanoparticles cover the surface and become hollow spheres after calcinations at high temperatures [39], [40], [41], [42], [43], [44], [45], [46]. However the exact mechanism for the bubble nucleation and grow is unclear.

Hematite is the most stable iron oxide. It is n-type semiconductor (Eg = 2.2 eV) under ambient conditions and it is easy to synthesize. Due to its magnetic properties, corrosion-resistance, low cost and low toxicity it is widely used in catalysis [47], [48], [49], [50], environmental protection [51], [52], [53], [54], [55], [56], [57], sensors [58], [59], [60], [61], magnetic storage materials [62] and clinic diagnosis and treatment [63]. To date, the preparation of a variety of hematite morphologies such as rhombohedra [64], particles [65], [66], [67], [68], nanocubes [69], [70], rings [71], wires [72], [73], rods [74], [75], fibbers [76], flakes [77], cages [78], airplane-like structures [79] and hierarchical structures [80], [81], [82] have been reported. Recently, some works have reported the production of crystalline hematite hollow spheres through various methods. Some of the approaches are listed in Table 1. Note that most of the existing methods for obtaining the hematite hollow spheres involve templates, surfactants, toxic organic solvents, or complex steps. Among them, the hydrothermal/solvothermal method has some advantage over the rest due to its fast reaction time, effective control of particle shape, and low incorporation of impurities into the products. However, this technique requires of steel pressure vessels or autoclaves during preparation to apply high pressure and thus to achieve the formation of the hollow spheres [88], [89], [90], [91], [92], [93], [94], [95], [96]. In contrast, in this work we report the preparation of hematite hollow spheres by the gas-bubble template technique in which no high pressure or any special conditions of atmosphere are required. Here, the hollow hematite microspheres are formed by annealing sol–gel iron oxide precursor in air. We propose a mechanism for the hollow formation based on the condensation, crystallization and oxidation of bubbles shells at high temperatures. This method is reproducible, simple, cheap, environmental friendly and it allows good control of the size, crystallization and oxidation of the product.

Section snippets

Experimental

Hollow hematite microspheres were produced by a modified gas-bubble template method following annealing in air an iron oxide precursor obtained by sol–gel [98]. For the precursor, 200 ml of colloidal ferric nitrate nine-hydrate (Fe(NO3)3·9H2O) particles and mono hydrated citric acid (C6H8O7·H2O, 0.2 M) were dissolved in 800 ml of de-ionized. The solution was vigorously agitated in a magnetic stirrer at 350 rpm (70 °C) for a period of 48 h to form Fe(OH)3. The citric acid was used as ligand, to

Results and discussion

Fig. 2 shows the X-ray diffraction patterns of the samples after annealing at different temperatures from 180 to 600 °C. Initially, after annealing at 180 °C, the sample consists of an amorphous solid with no preferred reflections in the XRD. After annealing at 250 °C, magnetite (Fe3O4) and maghemite (γ-Fe2O3) coexisting with a small amount of hematite (α-Fe2O3) were found. The first two phases were differentiated in the XRD by following the Kim's method [106], in which the (511) peak around

Conclusions

Hollow hematite microspheres were prepared by the gas-bubble template method. Boiling at high temperatures promotes bubble formation on which crystallites agglomerate, crystallize and oxidize to the hematite phase leading in the formation of hollow microspheres. The size and crystallization of the hematite hollow spheres increases with annealing temperature. After annealing at 550 °C, hollow spheres with mean diameter of 0.889 μm are obtained, whereas after annealing at 600 °C, hollow spheres

Acknowledgements

This work was supported by the Engineering and Physical Science Research Council (EPSRC No. EP/J003638/1). The work in Peru has been supported by the Ministry of Production of Peru through the program “Programa Nacional de Innovación para la Competitividad y Productividad” (Innóvate Perú project No. ECIP-1-P-069-14). The work in Brazil was supported by CNPq (307552/2012-8), CAPES (PNPD-230.007518/2011-11) and FACEPE (APQ-0589-1.05/08).

References (128)

  • R. Pelton

    Adv. Colloid Interf. Sci.

    (2000)
  • G.X. Liu et al.

    J. Solid State Chem.

    (2005)
  • F. Aldinger

    Acta Metall.

    (1974)
  • Y.S. Han et al.

    Chem. Lett.

    (2005)
  • C.L. Jiang et al.

    Nanotechnology

    (2005)
  • J.L. Zhang et al.

    J. Catal.

    (2005)
  • P. Li et al.

    Appl. Catal. B

    (2003)
  • L.C.A. Oliveira et al.

    Water Res.

    (2004)
  • C.H. Lai et al.

    Chemosphere

    (2001)
  • R.C. Wu et al.

    Water Res.

    (2005)
  • R.C. Wu et al.

    Appl. Catal. B

    (2004)
  • F. Herrera et al.

    Appl. Catal. B

    (2001)
  • J.S. Han et al.

    Sens. Actuators B

    (2001)
  • A. Jordan et al.

    J. Magn. Magn. Mater.

    (2001)
  • C.Q. Hu et al.

    Mater. Chem. Phys.

    (2007)
  • S. Komarneni et al.

    Mater. Res. Bull.

    (1992)
  • B. Hou et al.

    Mater. Lett.

    (2006)
  • D.B. Wang et al.

    Mater. Lett.

    (2005)
  • C. Jia et al.

    J. Cryst. Growth

    (2006)
  • L. Sun et al.

    Solid. State Sci.

    (2010)
  • I. Opačak et al.

    Mater. Lett.

    (2010)
  • B.D. Mao et al.

    J. Solid State Chem.

    (2007)
  • S. Ni et al.

    J. Alloys Compd.

    (2009)
  • S. Lian et al.

    Mater. Res. Bull.

    (2006)
  • X.W. Lou et al.

    Adv. Mater.

    (2008)
  • F. Caruso

    Top. Curr. Chem.

    (2003)
  • F. Caruso

    Adv. Mater.

    (2001)
  • W. Schärtl

    Adv. Mater.

    (2000)
  • D. Gan et al.

    J. Am. Chem. Soc.

    (2003)
  • C. Jones et al.

    Macromolecules

    (2003)
  • F. Caruso et al.

    Science

    (1998)
  • F. Caruso

    Chem. Eur. J.

    (2000)
  • Z. Zhong et al.

    Adv. Mater.

    (2000)
  • C.E. Fowler et al.

    J. Mater. Chem.

    (2001)
  • W.R. Zhao et al.

    Adv. Funct. Mater.

    (2006)
  • J. Bertling et al.

    Chem. Eng. Technol.

    (2004)
  • L.F. Xiao et al.

    Chem. Eur. J.

    (2009)
  • C. Li et al.

    Eur. J. Inorg. Chem.

    (2003)
  • J. Bao et al.

    Adv. Mater.

    (2003)
  • F. Caruso et al.

    Adv. Mater.

    (2001)
  • Y. Hu et al.

    Adv. Mater.

    (2003)
  • Y. Jiang et al.

    Chem. Lett.

    (2007)
  • W.W. Wang et al.

    Adv. Funct. Mater.

    (2007)
  • N. Du et al.

    J. Phys. Chem. B

    (2008)
  • N.A. Dhas et al.

    J. Am. Chem. Soc.

    (2005)
  • R.A. Caruso et al.

    Chem. Mater

    (2001)
  • W. Shen et al.

    Chem. Lett.

    (2005)
  • L. Qi et al.

    Adv. Mater.

    (2002)
  • T. Liu et al.

    Langmuir

    (2000)
  • T. Chen et al.

    Adv. Mater.

    (2007)
  • Cited by (15)

    • Hydrothermal synthesis, characterization and thermal stability studies of α-Fe<inf>2</inf>O<inf>3</inf> hollow microspheres

      2022, Advanced Powder Technology
      Citation Excerpt :

      To date, many researchers have reported the synthesis of hematite; however, not many have investigated the fabrication of hematite hollow spheres. Hollow nano/micro-structures of α-Fe2O3 have reportedly been prepared by a variety of synthetic methods, including sol–gel synthesis [13], centrifugal spinning [14], electrospinning [15], wrap–bake–peel processing [6], quasiemulsion-template [16], gas-bubble template [17], hydrothermal [3,10,18], and solvothermal [4,19,20] methods. The hydrothermal method is, by far, the most widely used method for fabricating hollow α-Fe2O3 nano-/micro-structures because of its high yield and simplicity [3].

    • Characterization and magnetic properties of hollow α-Fe<inf>2</inf>O<inf>3</inf> microspheres obtained by sol gel and spray roasting methods

      2019, Journal of Science: Advanced Materials and Devices
      Citation Excerpt :

      According to Özdemir and Dunlop, the values between 0.5 and 0.9 are typical from multidomain hematite particles [74]. Note that in our case, the shells are composed of multiple hematite grains and they should form interacting domains [64]. For the case of the hollow spheres obtained by the spray roaster method, the M(H) loops on the right panels in Fig. 4(b) show clear hysteresis when measured at different temperatures.

    • Recent Applications of Nanometal Oxide Catalysts in Oxidation Reactions

      2019, Advanced Nanomaterials for Catalysis and Energy: Synthesis, Characterization and Applications
    • Recent Applications of Nanometal Oxide Catalysts in Oxidation Reactions

      2018, Advanced Nanomaterials for Catalysis and Energy: Synthesis, Characterization and Applications
    View all citing articles on Scopus
    View full text