Polyhedral α-Fe2O3 crystals@RGO nanocomposites: Synthesis, characterization, and application in gas sensing
Introduction
The demand for accurate sensors to monitor and control environmental pollution during manufacturing processes has gained increasing interest in recent years [1], [2], [3]. Gas sensors are devices composed of active sensing materials connected to signal transducers. Therefore, the selection and development of potential sensing materials play an important role in designing high-performance gas sensors [4], [5]. Graphene is a unique two-dimensional (2D) carbon nanomaterial comprising of carbon atoms connected by covalent sp2 bonds to form honeycomb-like sheets [6]. This particular architecture gives to graphene many interesting properties, such as high surface-area-to-volume ratio, relevant adsorptivity, outbound chemical/thermostability, and excellent electrical properties [6], [7] and is applied to miniature device (such as conductive switching and bioimaging) [8], [9], catalysis [10], [11], energy storage [12], [13], [14], [15]. In addition, graphene has been fiery studied during the past few years as gas sensing materials [16], [17], [18], [19], [20]. However, low response and poor selectivity of intrinsic graphene limit its use in gas sensing [21].
The modification of graphene is considered as an effective way to improve the sensing performances of graphene-based sensors [17], [18], [19]. Many metal oxides/RGO composites have been reported with combined outstanding properties of both the semiconductor metal oxides and graphene materials to result in enhanced sensing properties thanks to the synergetic effects [4], [17], [22], [23]. For instance, Deng et al. [4] prepared RGO-conjugated Cu2O nanowire mesocrystals to manufacture a highly sensitive sensor toward NO2 at room temperature, surpassing the performance of standalone systems based on Cu2O nanowires networks and RGO sheets. Wang et al. [23] synthesized P-doped ZnO nanosheets@GO nanocomposites with high sensitivities towards acetone, attributed to the unique 2D-2D structure of rigid ZnO and flexible GO. Zhang et al. [19] prepared RGO/SnO2 p-n heterojunction aerogels as efficient 3D sensing frameworks for phenol detection. The aforementioned studies provided strong evidence that defective graphene exhibits distinctive behavior when compared to pristine graphene.
Among the metal oxides in gas sensing properties (ZnO, SnO2, Fe2O3, among others), hematite (α-Fe2O3) is an n-type semiconducting material with a direct band gap (Eg) of 2.1 eV. α-Fe2O3 has widely been applied as gas sensing material due to its friendly environmental character, low-cost, high stability, and elevated resistance to corrosion [19], [23], [24], [25], [26]. However, in spite of these virtues, α-Fe2O3 is still suffering from some limitations in gas sensing, including high operating temperatures (typically > 300 °C), poor selectivity, and low response [27].
As research advances and to overcome the issues related to pure α-Fe2O3, α-Fe2O3 gas sensing materials are gradually improving by combining pure α-Fe2O3 with other metal oxides to form heterojunction, composting with metals to promote the catalytic effects, as well as through hybridizing with carbon nanomaterials to improve conductivity [28], [29], [30], [31], [32], [33], [34], [35]. In this respect, Liang et al. [33] investigated the gas sensing properties of α-Fe2O3@graphene nanocomposites with different graphene contents and found that the content plays a significant role in enhancing the specific surface area, hence the gas response. Liang et al. [34] improved the gas sensing performances of Fe2O3 gas sensors using graphene as sacrificial templates and by setting more open framework structures as gas transmission galleries. Guo et al. [35] used a facile electrospinning method to produce RGO/α-Fe2O3 composite nanofibers with lower resistances due to the unique π-π conjunction structure of RGO that facilitated the electron transfer to α-Fe2O3. The resulting gas sensors based on Fe2O3 and graphene composites showed improvements in terms of response towards the analyte. However, ideal characteristics of a gas sensor should not only be limited to good response but also broader response range from low to high concentration, low optimal operating temperature, quick response and recovery, high stability, and moderate influence on the environment.
In this study, polyhedral α-Fe2O3 crystals@RGO nanocomposites were successfully prepared through dehydration and recrystallization of a hydrothermally synthesized β-FeOOH precursor. The crystal structures and morphologies of the as-prepared nanocomposites were examined by various analytical methods. The polyhedral α-Fe2O3 crystals@RGO nanocomposites were used to fabricate gas sensing devices and tested for detection of various gases, the as-assembled sensors showed high performances towards the detection of acetone, and the relation between the gas sensing properties and structure of the α-Fe2O3@RGO nanocomposites was discussed.
Section snippets
Material
The graphene oxide (GO) (JCGO-95-1-2.6) was purchased from Nanjing Jicang Nano Technology Co., Ltd. (China). Ferric chloride (FeCl3·H2O, 99.0%), ammonium fluoride (NH4F, 99%) and ethanol (CH3CH2OH, ≥99.7%) were from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The distilled water was obtained by our laboratory filtration system. All the reagents were used as received without further purification.
Methods
The GO (5 mg) was first dispersed in 50 mL absolute ethyl alcohols by ultrasonic
Structure and morphology of α-Fe2O3@RGO nanocomposites
The XRD patterns of the as-prepared β-FeOOH@GO, pure α-Fe2O3 and α-Fe2O3@RGO nanocomposites are shown in Fig. 3. The diffraction peaks of β-FeOOH@GO nanocomposite corresponded well with those of typical GO sheets [24], as well as β-FeOOH (JCPDS Card No. 75-1594). This indicated the successful preparation of the precursor. All the prominent diffraction peaks of both pure α-Fe2O3 and α-Fe2O3@RGO were well-indexed in the rhombohedral α-Fe2O3 (JCPDS Card No. 33-0664) [29], with cell parameters of
Conclusions
α-Fe2O3@RGO nanocomposites were synthesized through plain, mild, low-cost and friendly solvothermal method using a precursor obtained by a moderate oil bath for 8 h at 90 °C without the use of surfactant. GO as the host decreased the α-Fe2O3 particle size to less than 200 nm and narrowed the size distribution. The gas sensor based on α-Fe2O3@RGO nanocomposites promoted the specific response to acetone at the optimal operating temperature of 260 °C, which was 20 °C below that of pure α-Fe2O3. In
Acknowledgment
This work is supported by the National Natural Science Foundation of China (No. 51372103).
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