A comparative study on the gas sensing performance of SnO2 and GO-SnO2 sensor devices.

Using Modified Hummer’s technique, eco-friendly carbon derivative (GO) nanoparticles were obtained from polyethylene terephthalate (PET) precursor. Nanocomposite of GO-SnO2 and undoped SnO2 were synthesized using the coprecipitation method. The as-prepared nanoparticles were subjected to diverse analytical processes employing Transmission electron microscopy (TEM) to study the internal morphological properties of the nanoparticles. Energy dispersive X-ray spectroscopy (EDX) was used to examine elemental quantifications of the nanopowders. Fourier-transform infrared (FTIR) spectroscopy was used to analyze bond structures and functional groups. Dynamic responses of various gas sensor devices to 20 ppm concentrations of methane (CH4) and hydrogen (H2) were investigated as a function of time at room temperature. The GO-SnO2 nanocomposite sensing device demonstrated an ideal detection response with values of 5.00 and 5.08, corresponding to methane and hydrogen analyte gases. The doped SnO2 sensor device outperformed the pure SnO2, accounting for the GO-SnO2 > SnO2 order. Regarding the target gases, the synthesized nanocomposite demonstrated stability and selectivity in the following order of magnitude: H2 > CH4. The GO doping effect was found to have introduced surface defects, increased pores, and enabled more oxygen-active sites to be formed on the sensor device’s surface for dynamic gas sensing response, providing a comparatively enhanced sensor response.


Introduction
Gases are an essential part of built environments irrespective of the level of toxicity [1].Gas sources, both natural and man-made, are embodied in the human environment.Nonetheless, the acceleration of industrialization to satisfy humanity's insatiable wants has led to the atmospheric release of several harmful chemicals, raising the possibility of risks to both human life and the survival of other species [2].The catastrophic potential of these gases is further intensified when depending exclusively on human senses to monitor, measure, and identify the gas type and concentrations.The employment of gas sensors can minimize the inefficiencies of over relying on human sensory.With the numerous limitations of electrochemical, optical, and infrared sensors, semiconducting metal oxide gas sensors have been widely investigated in recent years as better substitutes considering their low toxicity, cost-effectiveness, and tuneable properties towards targeted analyte gases [3].
With the enormous benefits of Chemiresistive gas sensors including fast response dynamics, excellent selectivity, and high sensitivity, they have been widely used to detect several analyte gases.SnO2 has been the most widely used metal oxide semiconductor among the many others, such as ZnO, WO3, TiO2, and SiO2 due to its high sensitivity, neutrality and affordability [4,5].SnO2 detectors, however, possess some drawbacks including low response thresholds to specific analyte gases.These have been linked to potential causes such as grain boundaries and the wider band gap of 3.6 eV.Numerous synthesis approaches, such as noble-metal decorating, doping, morphological research, and combining any of them, are intended to improve the gas detectability properties of SnO2 gas sensors [6,7].Proposed modifications include enhancing the morphological structure of the metal oxide by creating nanostructured materials with larger surface areas and pore volumes, or by impregnating the metal oxides with noble metal nanostructures or metal ions [8].These are two of the most effective methods currently available.The use of dopant materials to rectify defects and manipulate the electrical characteristics of semiconducting metal oxides has been extensively researched.Kumar et al. [9] analyzed the effect of copper doping on the sensitivity of H2S in an article entitled "copper doped SnO2 nanowires as highly sensitive H2S gas sensor.
Recent research has demonstrated the superior charge-carrier transport, and enhanced adsorption capabilities of two-dimensional nanostructures, such as graphene and its derivatives.Despite the high cost of production and the environmental impact of its chemical constituents, many studies have exploited commercial graphene derivatives for sensing enhancing performance.Hydrocarbon materials found in large quantities in municipal solid waste, such as polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), are recyclable plastic sources that pose a threat to the environment [10].Low-cost graphene was produced from these plastics by Kenneth Mensah et al. [11] for use in a variety of environmental applications but yet to be explored in gas sensing operations.Wet synthesis is a low-cost method of incorporating graphene oxide into SnO2 to maximize SnO2 sensitivity to specific analyte gases.However, the preparation technique is crucial to obtaining appropriate nanoparticle characteristics.Coprecipitation was used in the synthesis of nanopowder among the many wet chemical synthesis mechanisms, such as sol-gel, hydrothermal, and others, due to its conventional, economically viable, time-efficient, and added control over the morphological properties [12,13].
This study therefore evaluates the sensitivity performances of fabricated gas sensors toward a few harmful gases to the environment, such as CH4, and H2 gases whose presence at certain thresholds compromise safety.The response pattern and morphological characteristics of SnO2 and GO-SnO2 toward the analyte gases were examined, as well as how these characteristics affected the sensor devices' ability to detect.

GO synthesis.
The obtained PET waste plastic bottles were thoroughly washed, shredded and pyrolyzed in a muffle furnace (ASH AMF 25N) at 700 °C for two hours at a ramping rate of 20 °C min -1 , under N2 gas.After cooling, the burned product was ball milled for 0.5 hours at 360 rpm and stored in a desiccator.Hummer's modified synthesis method was used to oxidize the as-synthesized graphitic material (graphene) in order to add more oxygen molecules to the 2D monolayer.1g of synthesized graphene was added to a 250 ml beaker.The mixture was ice bathed after adding 25 ml of H2SO4. 3 g of KMnO4 were added dropwise, then thoroughly mixed until complete dissolution was achieved.50 ml of distilled water (DW) was introduced to the mixture while maintaining a 100 °C steady temperature for 1 h.Further dilution was performed with 100 ml of DW, and the mixture was cooled to room temperature.10 ml of 33% H2O2 was gently administered to the mixture while slowly stirring until a bright yellowish colour was observed.The supernatant was discarded, precipitate filtered, and residue washed with 5% HCl.The obtained precipitate was ethanol-washed in bath sonicator for particle dispersion and subsequent centrifugation at 8000 rpm for 10 mins. 5 times thorough washing was done to produce pure nanomaterial and increase the pH between 6-7.The yield product was oven-dried at 60 °C for 12 h to obtain the GO powder.

Synthesis of SnO2 nanopowders, sensor device fabrication and testing
Coprecipitation technique was used in synthesizing pure and doped SnO2 nanopowders.0.1 M SnCl2.2H2O was obtained by the dissolution of 2.26 g tin (II) salt precursor in 100 ml DW and homogenized.1.5 M NH4OH was administered gently while stirring to increase the solution's pH to 9. A 50 mg GO dopant was introduced and stirred at 150 rpm over a 70°C heating source for 3 h.The resulting SnO2 and GO-SnO2 nanopowders were filtered, ethanol-washed, sonicated, and centrifuged for 10 min at 8000 rpm to ensure purification.The powders were oven-dried at 60 °C in ambient air after thorough washing.TEM, EDX, and FTIR characterization were performed for all prepared samples. 2 x 2 cm solid-state gas sensor devices were obtained by substrate cutting, washing, bath-sonicating in acetone media, air-drying, and ozone-cleaning.20 wt% of ethanolic suspension with GO, SnO2 and GO-SnO2 was produced.A vacuum spin coater attended 0.1 ml of the respective suspension.The thin films were air-dried and sintered for 5 min at 350 °C.Gold contact electrode was deposited on the thin film via a sputtering process.The fabricated solid-state gas sensor devices were encased inside a homemade gas chamber for CH4 and H2 sensitivity measurement.

FTIR analysis GO, SnO2 and GO-SnO2.
Functional group data for the as-prepared nanostructures with a frequency range of 450 cm -1 to 4000 cm -1 utilizing FTIR analyzer is shown in Figure 1.The morphological features and chemical composition of the as-synthesized nanopowders have a significant impact on the band placement and peak numbers.The SnO2 measured bands at 541 cm -1 and 608 cm -1 , demonstrate Sn-O and O-Sn-O, respectively.The vibrational stretching of the O-H and C=O bonds are attributed for the appearance of bands 3390 cm -1 and 1631 cm -1 , respectively [14].The presence of GO is indicated by bands that are positioned at 1721 cm -1 and 1080 cm -1 , respectively, displaying double C=O and single bond-stretching, C-O.The vibrational stretching that comes from band 1618's graphitization identifies the C=C bond of GO.It is possible to trace the existence of the carboxyl O-H vibrational stretch at peak 3410 cm -1 to GO. Sn-O and O-Sn-O bend stretching was seen in the GO-SnO2 at bands 616 cm -1 and 543 cm -1 .The presence of O-H and C=O bonding is shown by bands at 3471 cm -1 and 1639 cm -1 , respectively.This indicates an easy electron transfer within the doped material for a high sensing response.Analyte gas sensitivity is significantly impacted by the presence of O2-based functional groups since they may result in greater recoveries and reactions [15,16].2a.In Figure 2b, nanoparticles with noticeably finer grain sizes are visible, signifying the effective synthesis of SnO2.In addition, the pore diameters of SnO2 are comparatively lower [17].Wider pore diameters and comparatively bigger grain sizes are seen in Figure 2c.This can be explained by the mesoporous GO (darker region) that is integrated into the SnO2 lattice sites.Improved gas sensitivity is the result of increased porosity in the synthesized materials, which is necessary for the adsorption and desorption mechanism towards analyte gases.Since GO has a lot of O2-based groups, during the nucleation and maturation stages of GO doping, hydroxyl and carboxyl are introduced into the SnO2 for ionic interactions.This gives S 4+ ion deposition more anchor sites [17].Figure 2(d,e,f

Proposed sensing mechanisms.
With ambient oxygen, the gas-sensing apparatus works effectively.Because atmospheric O2 molecules have an exceptionally high electron affinity, as indicated by the following governing chemical equations, these molecules typically adsorbed on the surface of the gas sensing layer in air (1-4) [7,55]. 2 (g) →  2 () (1) ) Eqns. 1-3 demonstrate that the dominance of,  − causes the surface of the gas sensing layers to become bordered by an electron depletion layer (EDL).In the natural environment, a given gas sensor has a higher resistance because of the electron mobility restriction caused by the EDL.However, exposure to the analyte gas breaks the bonds between oxygen and electrons, releasing free electrons and decreasing the EDL thickness for increased conductivity and better gas sensitivity.The presence of GO introduces more improved features such as higher surface pores and active sites on the GO-SnO2 sensor devices.

Gas sensing response.
The sensitivity of gas sensor devices is subject to several operational factors.The fabricated sensor devices were tested for their respective responses and dynamics using a homemade gas chamber.Hydrogen and methane gas were run through the system to measure the respective responses at room temperature.Similar response trends were observed for both analyte gases, which can be ascribed to the electron-rich features of the respective built gas sensing layers hence facilitating electron mobility with minimized electrical resistance.That is, it affirms an n-type built sensor devices with enough electrons in their conduction bands.Comparatively, GO-SnO2 outperformed SnO2 devices given same target gas concentrations.This can be attributed to the doping impact of the mesoporous and electron-rich GO nanoparticles hence the higher dynamic responses as observed in Fig 3 (a,b).Additionally, the absence of GO in SnO2 limits structural defects on its surface hence leading to wider electron depletion layer that restricts free flow of electrons for early detection to be achieved.The optimal gas sensor's (GO-SnO2) operating responses towards 20 ppm H2, and CH4 were 5.08 and 5.00 respectively.

Conclusions
This study investigated the sensing performance of fabricated sensor devices towards methane and hydrogen gases.Nanoparticles of GO, SnO2 and GO-SnO2 were synthesized using Modified Hummer's method and coprecipitation techniques respectively.The optimal sensing performance of GO-SnO2 over SnO2 could be ascribed to the incorporation of active sites, more pores, and structural defects on the surface of GO-SnO2 sensing layer responsible for adsorption.Several analytical techniques such as FTIR, TEM and EDX were employed for ascertaining the functional group and bonds, morphological and elemental quantification investigations into the features of the synthesized materials.For a 20 ppm H2 and CH4 sensitivity test, 5.08 and 5.00 respective responses were recorded.

Figure 3 .
Figure 3. Response curves of gas sensors towards the same concentration of various gases: (a) H2, (b) CH4.