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A Highly Efficient Chemical Sensor Material for H2S: α‐Fe2O3 Nanotubes Fabricated Using Carbon Nanotube Templates

One-dimensional (1D) nanomaterials, such as nanowires, nanorods, and nanotubes, have been the subject of a growing body of literature because of their intriguing electronic, optical, and mechanical properties. Carbon nanotubes (CNTs) and metal and semiconductor 1D nanostructures synthesized by various protocols have been reported for their promising functions as solar cells, light-emitting diodes, catalysts, etc. Hematite (a-Fe2O3), which is the most stable iron oxide, with n-type semiconducting properties under ambient conditions, has been investigated as a gas sensor, catalyst, and pigment owing to its low cost and high resistance to corrosion. In addition, bulk a-Fe2O3 is well known as an antiferromagnetic material with a Neel temperature of 950 K and a Morin transition temperature of 263 K. The unique magnetic behavior of a-Fe2O3 has been extensively studied in the past decades. [9] In recent years, different 1D a-Fe2O3 nanostructures, such as nanorods, nanowires, nanobelts, and nanotubes have attracted intense interest for their peculiar properties, which originate from a high surface area and low dimensionality. Several methods, such as a template method, a sol–gel strategy, gas–solid reaction techniques, and a hydrothermal approach, have been developed for the synthesis of 1D a-Fe2O3 nanostructures. For example, Woo et al. synthesized a-Fe2O3 nanorods by a sol–gel-meditated reaction of ubiquitous Fe ions in reverse micelles. Zhang and coworkers managed to prepare a-Fe2O3 nanowires by oxygenating pure iron. Chen et al. produced a-Fe2O3 nanotubes through the thermal decomposition reaction of Fe(NO3)3 in air using an anodic alumina membrane as a template at 400 °C. Nevertheless, it still remains a challenge to develop simpler and versatile approaches to synthesize 1D nanostructures of a-Fe2O3. Recently, supercritical fluids (SCFs), characterized by such properties as low viscosity, rapid diffusivity, and absence of surface tension, have stimulated much interest in materials science. Their special physicochemical properties facilitate the transfer of reactants into small spaces, and consequently allow the complete wetting of complex surfaces and poorly wettable substrates. An SCF chemical deposition (SFCD) technique has been explored to fabricate nanocomposites through chemical reactions in SCFs. Herein, we report a novel and simple route to synthesize a-Fe2O3 nanotubes using carbon nanotubes as templates in supercritical (SC) CO2 modified with ethanol. In this method, the CNTs were first fully coated with iron oxide nanoparticles via the thermal decomposition of Fe(NO3)3 in an SC CO2/ ethanol solution at 150 °C. The resulting Fe2O3/CNT composites were then heated in an oxygen atmosphere to remove the CNTs, and a-Fe2O3 nanotubes were subsequently obtained. The as-prepared a-Fe2O3 nanotubes, characterized by a uniform size and shape with a high specific surface area, exhibit superior sensitivity to hydrogen sulfide (H2S) based on the catalytic chemiluminescence (CL). An X-ray diffraction (XRD) analysis was used to determine the chemical composition and crystal structure of the sample. The resolved diffraction peaks shown in Figure 1 can be indexed closely to pure hexagonal a-Fe2O3 with lattice parameters a = 5.0355 A and c = 13.747 A. This matches the standard