Enhancement of 2-butanone sensing properties of SiO2@CoO core-shell structures
Graphical abstract
Introduction
Sensors for the volatile organic compounds (VOCs) detection have been widely studied due to the effects of VOCs exposure on human health, making necessary the air-quality monitoring [1]. Furthermore, different VOCs molecules can be generated by alterations in metabolic pathways, which allow their emissions in body fluids like saliva, urine, and even in the exhaled breath, acting as biomarkers [2]. Different types of cancer can be correlated to certain VOCs levels in the exhaled breath [3]. 2-butanone is an example of a VOC that can be detected in the exhaled breath of patients diagnosed with lung cancer, ovary cancer, and liver cirrhosis [[3], [4], [5]]. Therefore, there is an urgent need for the development of smart sensors that work in real-time selectively and quickly for the non-invasive diagnosis of diseases [2,5,6].
Semiconducting metal oxides (SMOs) can be applied as chemoresistive gas sensors [2,7,8], because they present high sensitivity and selectivity, portability, and low cost [2,[8], [9], [10]]. Among the SMOs, cobalt oxides are p-type SMOs and have a wide area of application, including catalysis [11], lithium-ion battery electrodes [12], solar cells [13], supercapacitors [14], magnetism [15], and gas sensor [11,[14], [15], [16], [17]]. Cobalt (II,III) oxide (Co3O4) has demonstrated huge potential as chemoresistive sensors for the detection of ethanol [18], acetone [19], toluene [20], and other VOCs [21]. Nevertheless, few studies have reported the gas-sensing performance of cobalt (II) oxide (CoO).
The morphology can significantly influence the gas sensing-performance of SMOs. In this sense, different methods have been proposed to create nanostructured SMOs with controlled size and shape, including the use of hard templates [22]. Silica (SiO2) is a hard template widely used because of its well-defined synthesis and obtention of particle size homogeneity [[23], [24], [25]]. Moreover, the application of SiO2 as a support for designing SiO2@SMO core-shell nanostructures not only controls the morphology but also enhances the surface area [26,27], which can consequentially increase the gas-sensing performance.
In this paper, a SiO2@CoO core-shell nanostructure was successfully prepared by the deposition of a Co(II) precursor on the surface of SiO2 spheres, followed by calcination at 350 °C. The SiO2@CoO structure was composed of 2D CoO nanosheets (shell) coating the SiO2 core, resulting in a flower-like morphology. This material showed an outstanding 2-butanone sensing performance, including enhanced sensitivity, selectivity, and short response times. The improved performance is related to the well-controlled morphology with an effective high surface area.
Section snippets
Preparation of SiO2 spheres
The SiO2 spheres were prepared based on the procedure reported by Yu et al. [28]. A mixture containing 15 mL of absolute ethanol (Sigma-Aldrich ≥ 99.5%), 5 mL of deionized water (Millipore Milli-Q, 18.2 MΩ cm at 25 °C), and 0.7 mL of ammonium hydroxide (NH4OH) solution (Sigma-Aldrich, ACS reagent, 28.0–30.0% NH3 basis) was prepared. Then, 0.6 mL of tetraethyl orthosilicate (TEOS) (Sigma-Aldrich, 98%) was added quickly under stirring. The reaction was kept under stirring for 10 h. After that,
Results and discussion
The crystalline structures of the synthesized samples were analyzed by XRD (Fig. 2a). The XRD pattern of SiO2 shows a single broad peak at 2θ of 23°, presenting an amorphous structure. The XRD pattern of the SiO2@CoO core-shell structure exhibits an overlap of the single peak of SiO2 and the peaks at 2θ of 35° and 61°, which were respectively indexed to the (111) and (220) crystal planes of the cubic CoO structure (ICSD 9865). No peaks from Co3O4 or other secondary phases can be noticed. In the
Conclusions
In this paper, we presented the synthesis and VOC-sensing performance of a SiO2@CoO core-shell structure. The tests showed that the SiO2@CoO presented a high VOC-sensing performance in terms of selectivity, sensitivity, and response time. The selectivity tests showed that the SiO2@CoO is selective to 2-butanone with a response almost 2 times higher than that of isopropanol (the VOC with the second-highest response). In terms of sensitivity, the SiO2@CoO material presented a response of 44.7 to
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thank São Paulo Research Foundation – FAPESP (17/01267-1, 17/13230-5, 18/00033-0, 18/01258-5, 18/08271-7, and 16/25267-8), National Council for Scientific and Technological Development – CNPq (308327/2018-7, and PIBITI-48738), and National Council for the Improvement of Higher Education – CAPES. FESEM facility was provided by LMA/IQ/Unesp. We acknowledge the by the Brazilian Center for Research in Energy and Materials (CNPEM) – LNNano/LME (XPS-24622, TEM-C1-25112, and ME-22477) for
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2023, Sensors and Actuators B: ChemicalCitation Excerpt :High butanone concentrations ensure more active β-H atoms in sensing layer, which then significantly promotes their mutual interaction and chemical reaction rate with OA, thereby leading to short response time to butanone vapor [15]. It is faster than those of reported NiS/ZnO spheres (Tres = 10 s) [9], Ce-SnO2 cuboids (Tres = 20 s) [1] and SiO2@CoO core-shell (Tres = 27 s) [41]. Conversely, high butanone concentrations also produce more gaseous products to be desorbed.