Hematite solid and hollow spindles: Selective synthesis and application in gas sensor and photocatalysis
Graphical abstract
Highlights
► The surfactant plays critical roles in controlling the final morphology of hematite. ► High porosity and 3D morphology structures promote its sensing performances. ► The inner surface area of hollow spindles is disabled for photocatalytic degradation.
Introduction
Currently, there is growing interest in the fabrication of nanostructures with desired morphologies and properties. Among the various morphologies of nanostructures, nanostructured hollow spheres are drawing intense research interest not only for their unique properties, but also for their broad range of applications such as in efficient catalysts, drug-delivery carriers, photonic crystals, energy-storage devices, optoelectronic sensors, and gas sensors [1], [2], [3], [4], [5], [6].
Generally, manipulation of hollow materials is performed by template directed synthesis. Representative examples are the layer-by-layer deposition of nanoparticles onto spherical colloids (e.g., polystyrene beads and silica sol) [7], [8], [9] and sacrificial substitutions of metal nanoparticles by those with a higher standard reduction potential [10], [11]. Following these procedures, the templates of colloidal spheres are removed via a timeconsuming treatment (e.g., calcination or dissolution with solvent) to form hollow structure. Template methods have proven to be effective and versatile for the synthesis of a wide array of hollow structures. Nonetheless, disadvantages related to tedious synthetic procedures and low yields have impeded the scale-up production and large-scale applications. In a sense, one-pot template-free synthesis is desirable and preferred. It will be a main task to explore other wet chemical means, aiming at a simple “one-pot” synthetic approach for hollow structures. So, the utilization of some physical phenomena, such as the Kirkendall effect or Ostwald ripening, provides new opportunities for the template-free fabrication of hollow spheres [12], [13], [14].
Among various inorganic materials, hematite (α-Fe2O3), a semiconductor with the band gap of 2.2 eV, is widely used in catalysts [15], pigments [16], sensors [17], [18], [19] and as the raw material for the synthesis of magnetic γ-Fe2O3. In addition to conventional applications, hollow hematite particles might also be used for encapsulation of various guest molecules, and this is likely to lead to novel applications in areas such as functional materials and medicine carrier. Among these fields, much of the interest has been focused on its application in gas sensor and photocatalysis, which will be greatly improved by the hollow structures of the particles [20], [21], [22], [23], [24]. Recently, many groups have synthesized Fe2O3 hollow structures through various methods. Chen and co-workers described a facile route for preparation of submicrometer ferrite/block copolymer hollow spheres [25]. Fe2O3 with ordered mesoporous structure and crystalline walls, was synthesized by use of mesoporous silica as template [26]. Upon calcination, Thomas and co-workers used carbohydrates and metal salts as reaction resources to synthesize various metal oxides sphere [27]. Because of the tedious work in the post treatment of the template, solvothermal and hydrothermal methods were also employed in the preparation of the hollow nanostructures. For example, by a surfactant-assisted solvothermal method, submicrometer-sized hollow hematite particles were successfully prepared [28], [29]. Using dimethylbenzene as solvent, Chen and co-workers had also succeeded in preparing the hematite hollow spindles [30]. Recently, the synthesis of hematite hollow spheres was also reported using by a hydrothermal method that employed surfactant such as cetyltriethylammnonium bromide (CTAB), oxalic acid and poly(ethylene glycol) [16], [31], [32]. However, it still remains a challenge for the selective synthesis among different hollow nanostructures by simply changing one experimental parameter, especially in a facile and environmental friendly way.
Herein we report a simple hydrothermal approach for selective synthesis of porous hollow and solid spindle-like α-Fe2O3 architectures, which require neither complicated techniques nor templates, and the effect of the surfactant CTAB was investigated to understand the formation mechanism of the hematite spindles. Furthermore, the photocatalysis and gas-sensing properties of the spindle-like α-Fe2O3 architectures are also discussed. The abstained nanoporous hollow spindle-like α-Fe2O3 architectures exhibit an excellent gas-sensing sensor signal with test gases.
Section snippets
Synthesis of the samples
To prepare the hematite solid spindles, 2 mmol FeCl3·6H2O was dissolved into 40 ml of deionized water under magnetic stirring. The pH of the solution was adjusted to 3.5 with 1 M NaOH solution. The resulting slurry was then transferred to a 50-ml Teflon-lined autoclave and maintained at 200 °C for 12 h. The autoclave was then cooled down to room temperature naturally. The final red solid products were centrifuged and washed with distilled water and absolute ethanol several times to ensure total
Structure and morphology
Fig. 1 shows XRD patterns of the as-prepared Fe2O3 products. All the diffraction peaks were readily indexed to a pure rhombohedral phase of α-Fe2O3 (JSPDS card no. 89-2810). No other impurities could be detected by XRD. FE-SEM and TEM were employed to investigate the morphologies of the products. Fig. 2 displays the SEM and TEM images of the products. It can be seen that α-Fe2O3 product obtained when CTAB is absent are solid spindle-like structure as shown in Fig. 2a and b. Fig. 2c and d is the
Conclusions
In summary, α-Fe2O3 solid spindles and hollow spindles with size of 2–3 μm were successfully synthesized by a convenient hydrothermal method. Gas sensors were fabricated from the as-synthesized α-Fe2O3 spindles and applied to detecting some reducing gases. Comparative gas sensing tests between gas sensors based on α-Fe2O3 hollow spindles and solid spindles clearly show that the former exhibits more excellent sensing performances, implying a good potential of the hollow porous α-Fe2O3
Acknowledgements
This work has been supported by National Basic Research Program of China (973 Program 2007CB936602 and 2011CB933700), Young College Teachers’ Research Fund Program of Anhui Normal University (Project Nos. 2009xqn70 and 2009xqn71), and Anhui Provincial Natural Science (Project Nos. 10040606Q34 and 090412036).
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