Elsevier

Materials Research Bulletin

Volume 47, Issue 9, September 2012, Pages 2513-2517
Materials Research Bulletin

Ammonium mediated hydrothermal synthesis of nanostructured hematite (α-Fe2O3) particles

https://doi.org/10.1016/j.materresbull.2012.05.005Get rights and content

Abstract

Uniform α-Fe2O3 particles of different shapes have been synthesized through hydrothermal process. The additives, the type of Fe(III) salts and reaction conditions in hydrothermal process were thoroughly investigated. The crystalline structure and morphology of the as-synthesized powder have been characterized by using X-ray powder diffraction, scanning electron microscopy and field emission scanning electron microscopy. Rod and ellipsoidal-shaped α-Fe2O3 were obtained with ferric chloride as a precursor, while only irregular-shaped particles were synthesized by using ferric nitrate as precursors in the absence of NH4OH. Direct transformation of micro-rod hematite to ellipsoidal particles with FeCl3 as precursor was also observed by adding NH4OH. It is shown that the nanorod was formed through presumed directional aggregation of rapidly formed nucleus, while the formation of ellipsoidal hematite particles may undergo a nucleation–aggregation–dissolution–recrystallization process in the presence of ammonium.

Highlights

Hydrothermal synthesis of nanostructured hematite (α-Fe2O3) particles. ► NH4OH mediated direct transformation of micro-rod hematite to ellipsoidal particles. ► Ellipsoids formed by controlled aggregation, rather than directional growth.

Introduction

Over the past decade, remarkable progress has been made in the fabrication of quantum dots, quantum wires, layered structures, etc., because of their intriguing size/shape-dependent properties. The morphology of nanocrystals has also been observed to play a crucial effect in determination of their properties [1], [2]. For example, the optical properties of pigment particles are also affected to a large degree by their size and/or shape [3]. Controlled crystal orientation (or shape) is a key factor that determines the anisotropic behavior of certain magnetic materials. However, most of them are semiconductor or metal nanostructures, and a few of them are a magnetic metal oxide system despite their unique nanomagnetism and important technological applications.

Synthetic hematite (α-Fe2O3) has been extensively used in the field of pigments [4], catalysts [5], magnetic materials [6], anticorrosive agents [7] and sensors [8], owning to its low cost, environmental friendliness, and high resistance to corrosion [9]. The synthesis of iron oxides nanocrystals of different size and shape has attracted considerable interesting in recent years. Various α-Fe2O3 nanocrystal building blocks, such as nanoparticles [10], nanorods [11], nanowires [12], nanotubes [13], and nanobelts [14] have been successfully prepared by a variety of methods, including sol–gel process, microemulsion process, forced hydrolysis method, hydrothermal approach, thermal decomposition, spray pyrolysis and chemical precipitation [15]. Nevertheless, it still remains a challenge to develop simple and versatile approaches to synthesize monodisperse α-Fe2O3 crystals with fine shape control.

Hydrothermal synthesis has been a well established and promising approach for preparing controlled inorganic nanocrystals [16]. Hydrothermal processes can initiate nucleation of nanocrystal growth and promote the formation of crystalline products at temperatures substantially lower than those required by conventional solid-state or vapor reactions [17]. Specifically adsorbed anions, such as inorganic phosphate ions [18], organic phosphates [19], citrate and oxalate are effective shape controllers for the formation of hematite particles with complex morphologies and well-defined crystalline features [20]. Hematite colloids have been extensively studied – notably by Matijevic and colleagues [21], many years ago. Depending on the addition of various organic or inorganic additives, the formation processes of hematite particles are usually thought to contain a phase transfer process either from akaganeite (β-FeOOH) to α-Fe2O3 by using FeCl3 [7], [22], or from goethite (α-FeOOH) to α-Fe2O3 by using Fe(NO3)3 [23]. However, detailed structure and morphology evolution of α-Fe2O3 nanostructures is still not fully understood. In this paper, we reported the morphology controllable synthesis of monodisperse α-Fe2O3 nanostructures with rods and ellipsoidal morphologies by hydrothermal method. The synthesis mechanism and shape evolution of the as-prepared α-Fe2O3 particles was discussed. The role of NH4OH in chemical reaction and oriented aggregation was emphasized.

Section snippets

Experimental

All of the chemical reagents were of analytic grade and used without further purification (purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). In a typical experimental procedure, 5 mmol FeCl3.6H2O or Fe(NO3)3, 5 mmol urea and 0.21 mmol NaH2PO4. 2H2O were dissolved in 50 mL distilled water to get an orange solution of 0.10 mol/L of Fe3+ ions. The precursor solution was heated at 90 °C for 3 h and then sealed into a 100 mL Teflon-lined autoclave, followed by hydrothermal treatment at

Structure and morphology

Fig. 1 shows the influence of Fe(III) precursors on the morphology of α-Fe2O3 particles in the presence of NaH2PO4 and NH4OH. It was found that aggregates of a large amount of irregular nanoparticles (Fig. 1a) were precipitated from Fe(NO3)3 solutions when NH4OH was used. However, in the presence of NH4OH, monodispersed α-Fe2O3 ellipsoid nanostructures with mean particle size of 2.2 μm × 0.8 μm can be obtained by using FeCl3 as precursor (Fig. 1b). The TEM and selected area electron diffraction

Conclusions

The morphology can simply be controlled only by adding quantitative amount of NH3·H2O in sovothermal system. When NH3·H2O was not used, nanorods α-Fe2O3 nanophase were obtained. However, ellipsoidal particles self-assembled by nanorods were formed by adding 1 mol/L NH3·H2O. It is assumed that the shape evolution from nanorods to ellipsoid particles was a controlled nucleation–aggregation–dissolution–recrystallization process, in which the NH3·H2O played an important role not only in chemical

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