Full Length ArticleEnhanced and selective ammonia detection using In2O3/reduced graphene oxide hybrid nanofibers
Graphical abstract
Introduction
The development of portable, reliable and low-cost gas sensors for applications in various fields is a subject of outmost importance in modern society. Environmental monitoring, industrial production, health care and safety are a few examples where reliable gas sensors are demanded for the quantitative detection of several toxic gases [1], [2], [3], [4]. In particular, ammonia detection and monitoring has received great attention because it is one of the most toxic pollutants and frequently used in many industrial process [5], [6]. Ammonia (NH3) exposure can be harmful to human and animals in concentrations higher than 50 ppm, while 25 ppm of NH3 has been considered the threshold concentration for human exposure during 8 h [7], [8]. Moreover, the detection and quantification of NH3 emission in food has also been used to monitor food spoilage in order to establish safety consumption [9], [10]. Therefore, precise quantitative detection of NH3 is important and requires the development of sensors with good sensitivity, high selectivity, fast response and capability to detect low concentrations of NH3 at room temperature. Inorganic oxides have proved to be a great choice for the detection of toxic gases due to their remarkable attributes as superior stability, low-cost of fabrication and outstanding physical and chemical properties [11], [12], [13].
In2O3, a n-type semiconductor with wide band gap of 3.6 eV, has been widely studied and applied for gas sensing over the last years [14], [15], [16], [17], [18]. Particularly, In2O3 nanofibrous structures can provide advantageous features for gas sensing applications due to their high surface-to-volume ratio, representing more adsorption centres for the gas molecules [19]. Another key feature of In2O3 nanofibers is the interconnected mesoporous hierarchical structure, which enables gas molecules to diffuse and reach the adsorption centres located at the core of the nanofibrous structure [20], [21], [22]. However, In2O3 is also known by its low conductivity at room temperature due to its high potential barrier energy at the grain boundary [23]. An alternative to overcome this drawback is combining In2O3 (n-type) with a p-type semiconductor forming a p-n heterojunction, making possible to decrease the potential barrier energy and consequently, improving the conductivity [22]. Currently, the incorporation of p-type reduced graphene oxide (rGO) into metal oxides is considered an effective route to improve the efficiency of gas-sensing materials because of its enhanced gas adsorption capacity, chemically active defect sites, high conductivity, and outstanding charge-carrier mobility [24], [25], [26], [27].
In this context, here we report the fabrication of an efficient NH3 sensing hybrid material based on In2O3 nanofibers (NFI) functionalized with partially reduced graphene sheets (rGO). The ceramic nanofibrous structure was obtained by electrospinning technique with subsequent calcination at high-temperature. The long-range order, phase composition and crystal structure of the as prepared nanofibers were investigated by X-ray powder diffraction (XRD) and X-ray photo-electron spectroscopy (XPS), while the morphology was evaluated by field-emission gun scanning (FEG-SEM) and transmission electron microscopy (TEM). The NFI structure was combined with rGO and immobilized onto gold interdigitated electrodes (IDEs). The sensing properties of the hybrid material were investigated with impedance spectroscopy measurements by exposing it to various concentrations of ammonia gas (1 up to 60 ppm) at room temperature. The as fabricated sensor exhibited a good sensibility against ammonia gas, which has exceeded the performance of both pristine NFI and rGO. Finally, based on the sensing studies supported by the structural and morphological material characteristics, a possible sensing mechanism was proposed and discussed.
Section snippets
Materials
Indium chloride (InCl3), Poly(vinylpyrrolidone) (PVP, Mw 1,300,000), Monomethylamine (MMA), Trimethylamine (TMA), Triethylamine (TEA), N,N-Dimethylformamide (DMF), Methanol and Ethanol were purchased from Sigma-Aldrich. Acetone and ammonium hydroxide (NH4OH) were obtained from Synth, Brazil. All the chemical reagents were of analytical-grade and used without any further purification.
The gold interdigitated electrodes (IDEs) were fabricated by conventional photolithography using the facility
Structural and morphological properties of NFI-rGO composite
XRD patterns were used to characterize the crystalline structure of GO, rGO, NFI and NFI-rGO (Fig. 1). The XRD pattern of the GO displays a characteristic peak at 10.7°, corresponding to the (0 0 2) planes of stacked GO sheets. Compared to GO, the rGO exhibits a broad diffraction peak at around 25°, confirming the reduction of the oxygen functionalities of GO and suggesting that rGO samples are comprised mainly of few- or single-layer sheets [31], [32]. All the diffraction peaks observed for
Conclusion
Herein, a hybrid sensor for NH3 detection at room temperature has been successfully developed using In2O3 nanofibers assembled with rGO as sensing material. NFI structures were obtained via electrospinning technique followed by a calcination process. The successful formation of In2O3 and the incorporation of rGO were confirmed by XRD, while the nanometric scale of the nanofibers and the rGO characteristic sheets were confirmed by FEG-SEM and HRTEM techniques. Finally, the partial graphene
Acknowledgements
The authors thank the financial support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (grant numbers: 2016/23793-4 , 2017/10582-8 and 2017/12174-4), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (304109/2017-7 and 303.796/2014-6), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) Código de Finaciamento 001 (PNPD20131474 – 33001014004P9), MCTI-SisNano (CNPq/402.287/2013-4) and Rede Agronano (EMBRAPA) from Brazil. The authors
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