Facile synthesis of α-Fe2O3 nanoparticles for high-performance CO gas sensor
Graphical abstract
Introduction
Metal oxide nanopartices (NPs), which are considered to be some of the most fascinating functional materials have been widely exploited in various important applications [1]. The electrostatic interactions between the positive metallic and negative oxygen ions result in firm and solid ionic bonds. The s-shells of metal oxides are completely filled, so that most of the metal oxides have good thermal and chemical stability. However, their d-shells may be not completely filled, giving them a variety of unique properties that make them potentially of great use in electronic devices. These unique properties include wide bandgaps, high dielectric constants, reactive electronic transitions, optical, and electrochromic characteristics, as well as superconductivity [2], [3]. However, generating new physiochemical properties using the crystallization of metal oxide NPs is highly challenging. The design of such new materials with unexpected physiochemical properties is thus the major motivation in the investigation of new strategies for controlling the crystallization of NPs [4].
Among a long list of metal oxide nanocrystal, iron oxide has been one of the extensively investigated transition metal oxides [5], [6], [7], [8], [9]. Their variable oxidation states, crystal structures, low cost, magnetic properties, and environmental friendly nature are appealing features for researchers [10], [11], [12], [13]. Hematite demonstrated remarkable catalytic properties in oxidative reactions due to the high oxygen ion mobility at the material surface and is thus highly promising for sensor development [14]. These recent studies showed the morphology of α-Fe2O3 have a significant influence on its gas sensing properties. For instance, the α-Fe2O3 thin film showed selective to NO2 [15]. The flowerlike α-Fe2O3 sensor exhibited high performance ethanol sensor [16]. The X-shaped hematite crystals showed excellent sensor performance for CO and H2 gases [17]. The gas sensor based on a single α-Fe2O3 nanowire has fast response and high sensitive to CO [18], and so on. This correlation is attributed to the fact that nanocrystals of different shapes have different facets with different fractions of atoms located at different corners, edges, and different defect sites. Despite these advances, the simple synthesis strategy of desired geometry for iron oxide nanocrystals still needs to be greatly expanded to meet the ever-increasing nanotechnological demand and to clearly understand the interesting phenomenon of shape-dependent properties. Furthermore, few reports exist on the detection of highly toxic CO using α-Fe2O3 nanostructures-based sensor.
In this work, we report a facile approach for synthesis of α-Fe2O3 NPs through a hydrothermal process with subsequent calcinations of the obtained precursor. The resulting sample was used to fabricate gas sensor devices which were then tested for sensitivity to variety of gases. The sensors exhibited excellent performance for detecting CO and thus promising applications.
Section snippets
Materials
All the reagents were of analytical grade and used without further purification. Urea ((NH2)2CO), Ferric nitrate (Fe(NO3)3·9H2O) were purchased from Merck. CTAB was purchased from Aldrich.
Synthesis of α-Fe2O3 nanoparticles
Hematite NPs were synthesized via a facile and scalable hydrothermal method. In a typical experiment, ferric nitrate (4 mmol), CTAB (1.5 g), and urea (30 mmol) were mixed with 70 mL of distilled water under magnetic stirring vigorously at room temperature until the suspension was formed. The suspension was then
Results and discussion
The phase and the crystal structure of the products obtained by calcining the precipitates is determined by monitoring the XRD patterns as shown in Fig. 1. It is found that all the strong and sharp diffraction peaks can be indexed to the profile of α-Fe2O3 with a hexagonal structure. The result is consistent with the values in the literature (JCPDS No: 33-0664). No peaks from other phase are found, suggesting high purity of the as-synthesized α-Fe2O3. The average crystalline size of the α-Fe2O3
Conclusion
In summary, the peanut-like α-Fe2O3 NPs were successfully synthesized through a simple hydrothermal process followed by calcination at 500 °C for 5 h. The sensor based on the α-Fe2O3 nanostructures were studied at different operating temperatures and to various concentrations of different gases (e.g., CO, H2, C2H5OH, and NH3). In comparison with all tested gases, the sensor showed high gas sensing responses, short response and recovery time and selectivity in detecting CO. These results suggest
Acknowledgments
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.06-2014.87. Authors acknowledge to Laboratory of Geology, Geoengineering, Geoenvironment and Climate Change at Vietnam National University, Hanoi for HRTEM characterizations
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