Synthesis of tuneable porous hematites (α-Fe₂O₃) for gas sensing and lithium storage in lithium ion batteries

Abstract

Tuneable porous α-Fe₂O₃ materials were prepared by using a selective etching method. The structure and morphology of the as-prepared porous hematites have been systematically characterised by X-ray diffraction, field emission scanning electron microscope, and transmission electron microscope. We found that the pore size and pore volume can be controlled by adjusting the etching time during the synthesis process. The porous hematites have been applied for gas sensing and lithium storage in lithium ion cells. The porous α-Fe₂O₃ materials demonstrated a reversible lithium storage capacity of 1269 mAh/g. When used as a sensing material in gas sensors, porous α-Fe₂O₃ exhibited a superior sensitivity towards toxic and flammable gases.

Introduction

Hematite (α-Fe₂O₃) is the most stable iron oxide under ambient conditions with the characteristics of non-toxicity and high resistance to corrosion. It has been intensively investigated for applications in gas sensors [1], [2], rechargeable lithium-ion batteries [3], [4], catalysts [5], magnetic devices [6], and bio-medical fields [7], [8]. The performance of α-Fe₂O₃ strongly depends on the particle size, morphology, and structure. Various α-Fe₂O₃ nanostructures, like one-dimensional nanowires/nanotubes [9], two-dimensional flake/film structures [10], and three-dimensional hollow/porous structures [11], have been synthesised by different methods such as the sol–gel method [12], electrostatic spray deposition [13], hydrothermal treatment [14], and the template method [15]. For gas sensing applications, nanostructures with porous walls or hollow cavities are particularly desirable for improved sensing performance [16]. As anode materials in lithium ion batteries, a porous nanostructure can achieve a high rate specific capacity and good cycling stability. Porous α-Fe₂O₃ nanostructures can be prepared by nanocasting using either hard templates such as porous alumina membranes [17], carbon nanotubes [18], and porous silica [19], or soft templates (surfactants) [20]. Suslick et al. [21] produced an iron/carbon composite by using ultrasound to irradiate a mixture of carbon nano-particles and iron pentacarbonyl in hexadecane, and then obtained hollow hematite particles by elaborately controlling the oxidation of the resulting composite. Xu et al. [22] prepared α-Fe₂O₃ hollow nanospheres by a controlled precipitation of Fe³⁺ with urea in the presence of carbonaceous saccharide nanospheres as hard templates. However, the template-directed synthesis suffers from the disadvantages of low yield and high cost. As an alternative, template-free solution-based synthetic methods have also been reported for the preparation of porous α-Fe₂O₃ nanostructures like nanotubes [23], nanorings [24], and porous nanorods [25], in which the presence of some inorganic salts (NH₄H₂PO₄, Na₂SO₄, Na₂SO₃, NH₄Cl, KCl) are a prerequisite. In addition, the additives and experimental parameters must be carefully selected and controlled. Recently, an inorganic acid etching strategy has been used for the efficient synthesis of 2D transition-metal oxides [26], [27], [28]. However, it is still a challenge to develop a facile approach to synthesis α-Fe₂O₃ with porous or hollow nanostructures.

Herein, we report the synthesis of porous α-Fe₂O₃ via a controlled H₂C₂O₄ etching process. The as-prepared porous α-Fe₂O₃ nanostructures can selectively detect a series of gases, including ethanol, acetone, methanol, butanol, isopropanol and formaldehyde, with a rapid response and high sensitivity. The porous α-Fe₂O₃ also demonstrated a significantly improved lithium storage capacity and cycling stability in lithium ion cells. This is consistent with a generally recognised strategy of using hollow/porous structures to enhance the cycling stability of metal oxide anodes for lithium-ion batteries [29], [30].