Design,Preparation And Properties Of Humidity Sensor Materials

Humidity sensor detection is widely used in meteorological observation,environmental detection and other fields.At present,humidity sensors are difficult to adapt to the environment of high humidity,high salt and high corrosion over the ocean.Performance parameters of humidity sensors are mainly depended on properties of humidity sensing materials.With rapid development of material science,especially the emergence of new materials,performances of humidity sensors are required more and more.Therefore,development of humidity sensors faces great opportunities and challenges.In order to solve the self-cleaning problems caused by exposure of humidity sensors to atmospheric environment,it is one of important research directions to develop a self-cleaning humidity sensor.It is well known that photocatalytic technology is an environmentally friendly process.Organic pollutants attached during application of a humidity sensor are degraded by photocatalysis.Relationship between microstructure,physical and chemical parameters,and device performance were studied,which laid experimental and theoretical foundation for development and application of self-cleaning functional humidity sensing materials.For application of humidity sensing material in a humidity sensor and self-cleaning function of the device,two kinds of graphene-based humidity sensing materials and a kind of TiO₂-based self-cleaning photocatalytic materials were designed and prepared in this thesis.Relationships between preparation process,physical and chemical parameters,and material properties were investigated.The research contents include the following four aspects:1.Study on preparation and humidity sensitivity of functional materials for sulfophenylazo reduced graphene oxideDiazo coupling reaction and ring opening reaction of epoxy group were used to graft sulfophenylazo groups and sulfanilic acid on rGO.By means of XRD,FT-IR,SEM,BET and Raman,properties of humidity sensing materials were characterized.After grafting of different groups on rGO,the layer spacing was larger than that of rGO.Order of spacing between layers was:rGO-S3>rGO-S2>rGO-S1>rGO>GO.S=O absorption peaks were observed on FT-IR spectra of different humidity sensing materials of the functional rGO,which were not appeared on GO and rGO spectra,indicating that different groups were successfully grafted onto rGO.Raman characterization results showed that,both D peak and G peak of functional rGO were moved to a low wavenumber direction.The intensity ratio of the D band and G band(I_D/I_G)of functionalized rGO was larger than that of rGO,indicating that disorder degree of rGO flakes increased,which was consistent with SEM characterization results.The humidity sensing material grafted with 4-chloro-3-sulfophenylazo groups on rGO exhibited the best humidity sensing performance and the highest sensing response value.2.Preparation and humidity sensing performance of 4-chloro-3-sulfophenylazo groups functionalized reduced graphene oxideReduced graphene oxide grafted by 4-chloro-3-sulfophenylazo groups(rGO-SC)was synthesized by a diazo coupling method.Absorption peaks of S=O,C-Cl and N=N were shown on FT-IR spectra of rGO-SC.N,Cl and S elements on the surface of rGO-SC were confirmed by X-ray photoelectron spectroscopy(XPS),indicating that4-chloro-3-sulfophenylazo group was successfully grafted onto the surface of rGO.Humidity sensors coated with rGO-SC exhibited about three orders of magnitude change of its impedance,a high sensing response value(S=2961.39),short response/recovery time(1.3 s/23.5 s),and a small hysteresis(4.56%).After about 12 months,the response time and recovery time of the humidity sensor coated with rGO-SC were 1.9 s and 27.4 s,respectively.The humidity sensor exhibited good stability and repeatability.Humidity sensing properties were improved by synergistic effect arising between 4-chloro-3-sulfophenylazo group and Cl group on rGO-SC surface.It was found that humidity sensing mechanism rGO-SC was attributed to electronic or ionic conductivity at a low or high relative humidity.3.Preparation and humidity sensitivity of moroxyguanidine modified graphene oxideIn order to solve problems of agglomeration and expansion forGO under high relative humidity,moroxyguanidine modified graphene oxide(GO-G)were synthesized via grafting moroxyguanidine moieties on GO.Agglomeration of GO flakes was inhibited by moroxyguanidine.The 2?of GO-G diffraction peak moved toward to a lower angle and the layer spacing was larger than that of GO.Raman spectra indicated that I_D/I_Gof GO-G(1.029)was greater than that of GO(0.950).Presence of C-N absorption peak was shown in FT-IR spectrum of GO-G.The N element was found to be C-N and pyridine nitrogen which confirmed by XPS,indicating that moroxyguanidine was successfully grafted onto the GO surface.Humidity sensors fabricated by GO-G exhibited four orders of magnitude of its impedance,a high sensing response value(S=24040.77),and a short response/recovery time(2 s/20 s).The N atom in moroxyguanidine on GO-G surface could form hydrogen bonds with H atom in H₂O,which improved humidity sensing properties.Experimental results showed that humidity sensing mechanism of GO-G was attributed to electronic or ionic conductivity at a low or high relative humidity.4.Preparation and photocatalytic performance of TiO₂-based humidity sensor self-cleaning protective layer materialsIn order to solve the organic pollution problem caused by exposure of a humidity sensor to atmospheric environment and improve humidity sensing performance of the humidity sensor,self-cleaning protective layer material of the humidity sensor was designed in this paper.Water molecules could pass through the protective layer material smoothly and were adsorbed on surface of the humidity sensing material.However,organic pollutants adsorbed on surface of the device could be degraded.Different elements doped TiO₂photocatalysts were prepared by sol-gel and precipitation methods.N,Ag co-doped TiO₂exhibited larger overall particle size and water molecules could pass through protective coating material smoothly.The N element in N-doped TiO₂prepared by sol-gel method could not inhibit growth of photocatalyst grain,while the addition of Ag element could inhibit growth of photocatalyst grain.N and Ag co-doping narrowed band gap and significantly improved the ultraviolet and visible absorption performance of the catalyst.Based on phenol degradation model,photocatalytic degradation experiments showed that the photocatalytic efficiency of the photocatalyst prepared by the sol-gel method was significantly higher than that of the photocatalyst prepared by the impregnation method.The efficiency of N,Ag co-doped photocatalyst was 51.3%after ultraviolet visible irradiation for 120 min,and the efficiency was 51.7%after visible light irradiation for 360 min.The UV activity was not affected by the visible light response.