Hydrothermal synthesis and visible-light photocatalytic activity of α-Fe₂O₃/TiO₂ composite hollow microspheres

Abstract

Novel α-Fe₂O₃/TiO₂ composite hollow spheres were successfully synthesized by a template-assisted precipitation reaction using urea as a precipitating agent and carbon spheres as templates in a mixed solvent of water and ethanol, and then calcined at 400 °C for 4 h. The as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption–desorption isotherms, and vibrating sample magnetometer. The influence of calcination temperature and the molar ratio of titanium to iron (R) on the photocatalytic activity of the samples was investigated. The results indicated that the composite spheres show magnetic characteristics at room temperature and good photocatalytic activity under visible-light irradiation compare to the single-component α-Fe₂O₃ particles. This method can be further applied to synthesize nanocomposites of magnetic metal oxide and other metal oxide.

Introduction

In recent years, micrometer- and nanometer-sized hollow structures have attracted a great deal of attention because of their low density, high surface area, good surface permeability and large light-harvesting efficiencies [1], [2], [3], [4]. Moreover, these materials can be widely applied in photoelectric devices, catalysis, drug delivery, chromatography separation, and chemical reactors [5], [6], [7], [8], [9]. Various synthetic methods were explored to prepare hollow nanomaterials including Ionic liquids (ILs), self-assembly techniques, hydrothermal techniques, template-assisted techniques, and chemically induced self-transformation [10], [11], [12]. Up to now, template-assisted synthetic method has proved to be the most-applied and most effective route to fabricate inorganic hollow structures.

Titanium dioxide (TiO₂), as one of the most important transition-metal functional oxides, has attracted extensive attention during the past decades for its superior physical and chemical properties and a wide variety of potential use in diverse fields such as solar energy conversion, environmental purification, and water treatment [13], [14], [15], [16]. In particular, TiO₂ hollow naomaterials and nanocomposites have received more and more attention owing to their high photocatalytic activity, chemical stability, low cost and nontoxicity [17], [18]. However, because of its wide band-gap energy (3.02 eV), TiO₂ can only harvest the spectrum with wave lengths in the near-ultraviolet (UV) region shorter than 387 nm, which accounts for merely 4–5% of the solar spectrum. Moreover, TiO₂ follows a relatively high electron–hole recombination rate, which is detrimental to its photoactivity. To solve this issue, different approaches such as transition metal doping, inorganic dye-sensitizing, valuable metal deposition and coupling titania with other semiconductors have been devoted to enhancing the photocatalytic activity of TiO₂ in which the response of the semiconductor was extended toward the visible region [19], [20], [21]. Up to now, it is still a great challenge to effectively immobilize or separate the TiO₂ particles in the photocatalytic system. Magnetic separation provides a very convenient approach for removing and recycling magnetic particles/composites by applying an appropriate magnetic field [22], [23]. Compared to conventional nanopowder photocatalysts, TiO₂ magnetic composites such as Fe₂O₃–TiO₂ or Fe₃O₄–TiO₂ can be regarded as a promising photocatalyst for the environmental purification at the industrial scale as they can be more readily separated from the slurry system by the magnetic separation after photocatalytic reaction and recycled. Very recently, Yu et al. [24] fabricate a hierarchical porous γ-Fe2O3@SiO2@TiO2 composite photocatalyst with superior photocatalytic properties by an effective three-step approach. Mou et al. [25] developed an asymmetric shrinkage approach for the fabrication of magnetic γ-Fe₂O₃/TiO₂ Janus hollow bowls (JHBs) by constructing a precursor solution pair during the solvents evaporation process. Moreover the as-obtained products show an efficient visible-light photocatalytic activity and convenient magnetic separation for water purification.

Herein, novel α-Fe₂O₃/TiO₂ composite hollow spheres are successfully fabricated using carbon spheres prepared from saccharide solution as templates, and their visible-light photocatalytic activity and environment application are carefully investigated. In addition, we also investigated the effects of the molar ratio of iron to titanium (R) on the microstructures and properties, especially on the photocatalytic property.