Author: Lam, King Cheong
Title: Room temperature methane gas sensing properties based on graphene related materials
Advisors: Shi, San Qiang (ME)
Degree: Ph.D.
Year: 2017
Subject: Hong Kong Polytechnic University -- Dissertations
Gas detectors -- Materials
Department: Department of Mechanical Engineering
Pages: xviii, 192 pages : color illustrations
Language: English
Abstract: Tin dioxide (SnO₂) is non-toxic, low cost, n-typed with wide band-gap (Eg = 3.6 eV) semi-conductor, have been reported to have high sensitivity and fast response to a number of gases due to the oxygen vacancies on surface after annealing which serves as active sites for gaseous interaction. One dimensional metal oxide nanofibers are promising material in gas sensing due to the high surface area and porosity, which is most efficient in transducing gaseous interactions to electrical signal by directional carrier transport. However, metal oxide gas sensors have drawbacks such as high operating temperature and high power consumption. To improve the conductivity of metal oxides, additive materials such as reduced graphene oxide (RGO) have recently been explored for their advantages of high sensitivity, fast response and good conductivity in gas sensing. The hybridisation of RGO with metal oxides such as SnO₂ has been reported to greatly increase the gas sensitivity by p-n junction effect at much lower operating temperatures. However most operating temperatures of the sensors reported were still higher than room temperature, hence there is still large potential in developing room temperature gas sensing materials with high sensitivity. The objective of this thesis is to investigate room temperature methane gas sensors based on graphene related materials fabricated by drop-drying and electrospinning methods. The first room temperature methane gas sensor was fabricated by a facile drop-drying method from chemical synthesis which incorporate tin dioxide with graphene oxide in situ reduced by four different reducing agents such as D-glucose, sodium borohydride, L-ascorbic acid and hydrazine hydrate. It was found that the reducing power of the reducing agents were in ascending order of D-glucose, sodium borohydride, L-ascorbic acid and hydrazine hydrate. It was found that the methane gas sensitivities of RGO reduced by glucose and ascorbic acid were higher than those by sodium borohydride and hydrazine hydrate due to the residues of glucose/glucono delta-lactone and dehydroascorbic acid, respectively, on the RGO surface, which enlarged the space charge layer and thus improved the response. The resistances of all the heterostructurees with SnO₂ increased due to p-n junction effect. It was found that the sensitivities of the heterostructures of RGO by ascorbic acid and glucose with SnO₂ were the two highest among. The sensitivity of the heterostructure of RGO by ascorbic acid with SnO₂ was found the highest of 76% at 10000 ppm methane concentration at room temperature. The sensitivity of this sensor was not saturated and increased up to 241% at 80000 ppm.
The sensitivity of the heterostructure of RGO by ascorbic acid with SnO₂ was higher than that by glucose which could be due to the synergistic effect between dehydroascorbic acid and SnO₂ deduced from density functional theory calculations. The synergistic effect between dehydroascorbic acid and SnO₂ could be due to the greater charge transfer induced via orbital hybridisation at the carbonyl groups of dehydroascorbic acid, and the more favourable adsorption site atop the in-plane oxygen on SnO2 surface. The second room temperature methane gas sensor was fabricated by a less facile electrospinning method to produce PVA/SnO₂/GO nanofiber, and followed by carbonization under N₂ at different temperatures. The optimal concentration of the pure PVA precursor solution was found at 5 wt% in order to produce PVA nanofibers of uniform and beadless structure with small diameter. PVA/SnO₂/GO nanofibers with different GO concentrations were successfully fabricated at 5 wt% PVA concentration by electrospinning. The nanofibers were uniform, smooth and beadles with smaller diameters as GO content increased. The formation of thinner fiber with more GO content indicating the effect of conductivity was more significant than viscosity with more GO content. The resistances of the composite nanofiber of PVA/SnO₂/GO were successfully decreased by increasing carbonization temperature and GO content while retaining high sensitivity towards methane measured at room temperature. Increasing carbonization temperature would increase relative amount of conductive amorphous carbon produced after carbonization which decreases resistance. While increasing the content of the comparatively more conductive GO would also decrease resistance. The sensitivities of the composite nanofiber of PVA/SnO₂/GO without GO content was determined the lowest which could be due to the thin-skinned layer of amorphous carbon which cover the surface of the core-shell nanofiber structure as the carbon skin layer had shielded SnO₂ from contacting with gases for interaction and also minimized the p-n junction effect by decreasing the contact boundary between the p-typed amorphous carbon and the n-typed SnO₂. The addition of GO in the composite nanofiber completely changed the core-shell structure to porous structures which benefit gaseous interaction by increasing the p-n junction effect and decreasing the shielding effect from the amorphous carbon layer. The sensitivity of the composite nanofiber PVA/SnO₂/GO with GO content of 5 wt% with respect to PVA in precursor solution was found to have the highest sensitivity of 60.5% at carbonization temperature of 550°C towards methane of 1%. The gas sensor was not saturated at high concentration of methane from 6000-10000 ppm which is useful in application of high concentration environment.
Rights: All rights reserved
Access: open access

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