Impact of reduced graphene oxide on the ethanol sensing performance of hollow SnO2 nanoparticles under humid atmosphere
Graphical abstract
Introduction
The detection of volatile organic compounds (VOCs) has attracted considerable attention due to its application in environmental and indoor air quality monitoring, as well as in non-invasive disease diagnosis [1], [2]. For these purposes, gas sensors based on metal oxide semiconductors (MOS) are widely used because of their sensibility, low cost, and possibility of scaled-up production [3], [4]. Among the MOS, SnO2, an n-type semiconductor, is one of the most studied material for VOCs monitoring with a great sensitivity toward several gases, including acetone [5], [6], formaldehyde [7], [8], toluene [9], ethanol [10], [11], methanol [12], and so on.
Composites of SnO2 have shown to be a way to improve the sensing performance toward VOCs, for instance, composites with other MOS [13], [14], noble metals [15], [16], and graphene-based materials [2], [17], are reported. The use of reduced graphene oxide (RGO), which is a p-type material, can enhance parameters such as sensor response, recovery rate, stability, sensitivity [17], [18], [19], and selectivity [20], [21], [22], because RGO enhances the electron transport [2] and provides more sites for gas molecules adsorption due to its large specific surface area, defects, sp2-bonded carbon, and residual oxygenated functional groups [22], [23], [24].
For the real application of SnO2 as a gas sensor, it is necessary to consider the influence of humidity since atmospheric air and exhaled breath contain a considerable quantity of water vapor [25], [26]. However, the sensors based on MOS are much sensitive to humidity and, consequently, the response toward an analyte gas significantly decreases under wet conditions [27], [28], [29], [30]. Thus, it is of great importance to developing sensors with less negative effect of humidity for actual applications. Some studies have demonstrated that Ti [31], or Sb-doping [32], and Pd-loading [28], [33], can be effective ways to preserve the stability of the SnO2 gas sensing performance. Nevertheless, to the best of our knowledge, few studies have been conducted to verify the effect of a little amount of RGO on the VOCs detection of SnO2 NPs under controlled humid atmosphere.
In this study, we report a facile one-pot microwave-assisted solvothermal (MAS) synthesis of hollow SnO2 NPs and RGO-SnO2 nanocomposite. The phase composition, morphology, and physical-chemical properties were evaluated, and the first nucleation mechanism of SnO2 was discussed involving olation and oxolation reactions. The obtained samples were tested as VOCs sensors in dry and humid atmosphere with a relative humidity (RH) between 24 and 98%. The findings demonstrated an enhanced ethanol response for the RGO-SnO2 nanocomposite compared with pure hollow SnO2 NPs, which even at a RH of 98% exhibited a high response to 100 ppm of ethanol equal to 43.0.
Section snippets
Synthesis of RGO-SnO2 nanocomposite
Graphite oxide (GO) was used as a precursor of reduced graphene oxide (RGO), and it was prepared by a modified Hummers’ method as described in a previous report [34], [35]. RGO-SnO2 nanocomposite was synthesized by a one-step microwave-assisted solvothermal (MAS) method based on previously reported synthesis [36]. In a typical procedure, a 9.8 mg of GO was dispersed in 30 mL of deionized water (Millipore, 18.2 MΩ cm) using an ultrasonic bath for one hour to promote GO exfoliation. Afterward,
Synthesis and crystal growth
RGO-SnO2 nanocomposite was synthesized by a facile one-pot MAS method, as illustrated in Fig. 1. K2SnO3·3H2O, urea, and GO dispersion were used as precursors in a reaction medium of water and ethylene glycol (EG). The microwave heating at 140 °C for 1 h promoted the formation of hollow SnO2 nanoparticles in a spherical shape (Supplementary information – Fig. S1) concomitantly to GO reduction to RGO. The hollow SnO2 NPs grow in the same way with or without GO in the reaction medium, however in the
Conclusions
In summary, RGO-SnO2 nanocomposite was successfully prepared by a one-step microwave-assisted solvothermal synthesis. In the gas sensing studies, the RGO-SnO2 nanocomposite exhibited an enhanced ethanol sensing performance compared to pure hollow SnO2 NPs in dry and humid atmosphere. When RH was 98%, the RGO-SnO2 nanocomposite showed a sensing response of 43.0–100 ppm of ethanol with a response time of 8 s.
Morphological and structural characterizations confirmed that the applied synthetic method
Acknowledgments
The authors acknowledge the financial support of São Paulo Research Foundation (FAPESP) (grant 2015/05916-9, 2014/17343-0, 2016/04371-1, 2013/23886-4), and National Council for Scientific and Technological Development (CNPQ) (Proc. 444926/2014-3). The XPS facilities were provided by LNNano/CNPEM (Brazilian Nanotechnology National Laboratory, proposal no. 20389). Authors are also thankful to Multiuser Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ/UNICAMP) for Raman analysis. FESEM and
Cecilia de Almeida Zito is a M.Sc. student in Universidade Estadual Paulista. Currently, she is working on the synthesis of materials based on graphene and semiconductors and their application as volatile organic compounds sensors.
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Cecilia de Almeida Zito is a M.Sc. student in Universidade Estadual Paulista. Currently, she is working on the synthesis of materials based on graphene and semiconductors and their application as volatile organic compounds sensors.
Tarcísio Micheli Perfecto is a M.Sc. student in Universidade Estadual Paulista. His current research work involves the application of semiconductors and graphene-based materials as gas sensors at room temperature.
Diogo Paschoalini Volanti received his degree in chemistry from Federal University of São Carlos (UFSCar) and his PhD in chemistry from São Paulo State University (UNESP) in Brazil. He is a member of the faculty of the Department of Chemistry and Environmental Sciences at UNESP in São José do Rio Preto, where he serves as assistant professor of general and inorganic chemistry and director of the Laboratory of Materials for Sustainability (LabMatSus). Dr. Volanti’s current research emphasizes the microwave-assisted hydrothermal synthesis, functional semiconductors nanostructures, gas sensors, graphene, heterogeneous catalysis, photocatalysis, transmission electron microscopy, in situ and operando x-ray absorption spectroscopy.