| Formaldehyde widely exists in the indoor decoration materials and clothing fabrics.Excessive formaldehyde is the main reason for Sick Building Syndrome(SBS).If someone always stay in low concentration formaldehyde,his or her life will be threatened by diseases,such as respiratory disease,genic mutation,nasopharynx cancer,brain tumor,pregnancy complications and so on.Those pregnant women and young children are main vulnerable groups,which of course not only brings great pains to the families,but also causes a big burden to the society.In recent years,many methods have been used to detect the contents of formaldehyde,such as spectrophotometry,chromatography,oscilloscopic polarography,electrochemistry and resisitance-type semiconductor detection.However,due to the low energy consumption,high safety coefficient and intelligence,the metal-oxide semiconductor always plays an important role in detecting formaldehyde indoor.From synthesis of different properties of metal-oxide semiconductor,scientists found the particle size and photogenerated electron-hole pairs are the main reason to influence the gas sensor.Just because ZnO includes proper energy gap(3.37 eV)and excellent conductivity,ZnO is used as the main material.We have reduced the ZnO grain size,synthesized the porous hollow ZnO with large specific surface area and constructed the SnO2-ZnO heterostructures to improve the photoelectric properties of the ZnO materials,which can effectively improve the gas sensitivity of the ZnO materials to detect formaldehyde.The content of this dissertation is described as follows in two parts:(1)The porous hollow structure ZnO nanomaterials were successfully prepared by mild oil bath method,which have small grain size,high porosity and large specific surface area.UV-Vis absorption measurements show that porous hollow structure ZnO can effectively utilize UV light.Then,by comparing with the other morphologies of ZnO nanomaterials,porous hollow structure ZnO is more excellent in gas-sensitive response and selectivity to formaldehyde.Combined with the surface photocurrent measurements,the larger specific surface area and porosity of porous hollow structure ZnO not only improve the diffusion and adsorption of formaldehyde on the surface of porous hollow structure ZnO,but also provide more surface active sites to promote the catalytic reaction on the surface of the material,making a large amount of surface active oxygen(-)to capture the electrons back to the material body to form the current.(2)The polyporous SnO2-ZnO were synthesized by a water-bath method.The 3D center-hollow architecture and polyporous surface were also confirmed by field-emission scanning electron microscope and the Brunauer–Emmett–Teller specific surface area.Meanwhile,heterostructure of SnO2-ZnO was observed by transmission electron microscopy in 1% SnO2-ZnO.The results of photoelectric gas-sensing demonstrated that the 1% SnO2-ZnO gas-sensor is not only an excellent response to formaldehyde but also superior sensitivity to low formaldehyde concentration.The response of 1% SnO2-ZnO for the 1 ppm formaldehyde has reached 30% at room temperature,and a detection limit of 0.1 ppm formaldehyde was achieved.We fully discuss the microscopic kinetic of charges by means of the surface photovotage and the transient photovoltage,which shows the 1% SnO2-ZnO has excellent efficient separation and transfer efficiency of photogenerated carriers.What's more,the transient photovoltage spectrum provides convincing evidence that the excess concentration of SnO2 immensely hinder the transfer of electrons so as to efficaciously reduce the sensitivity of SnO2-ZnO for target gas.This result offers a new window to explain the effect of SnO2 concentration on the gas sensing characteristics,and provide more information on the fabricating composite materials for gas sensors. |
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