A Novel Fiber Optic Sensing Technique For Sulfur Dioxide Based On Passivated Porous Silicon

SO₂ is a primary pollutant, which is the major factor leads to acid rain. Acid rain brings tremendous hazard to nature and human being. So monitoring SO₂ in environment is a critical part in pollution prevention. At present, several methods and instruments are available to monitor the concentration of SO₂ via different systems. However, these techniques have their own limits, such as high working temperature or the preparation of electrodes is complex, most of them involve big-sized analysis apparatus or complex test system, it can not monitor SO₂ in-situ. New approaches to the detection and analysis of SO₂ appear to be in urgent need.As novel functional material, porous silicon (PS) electrochemically etched in hydrofluoric acid (HF) solutions has attracted considerable interest because of its strong photoluminescence property under room temperature. The photoluminescence of the etched PS quenched in the SO₂ atmosphere and high concentration of typical atmospheric and industrial waste gases does not evoke a quenching response. Then lots of idea was presented to use PS as the material to detect the concentration of SO₂ in the air.Based on the former work of our team, n-type silicon wafer is used as the start material to prepare porous silicon film by electrochemical anodic oxidation, then a steady photoluminescence PS is obtain by surface passivation. It is expected to get a functional PS material which is anti-oxidized in the air and excellent sensing property to SO₂. The main points of this dissertation are as following:(1) It is summarized that the current studies situation and perspective orient of detecting SO₂, preparation mechanism and applied research status of porous silicon.(2) The optimal condition to prepare the PS for SO₂ detecting is presented: current density 50mA·cm-2, electrolyte HF/C2H5OH=1:1(V/V), oxidized time 15min. Based on the former passivation methods, a new method is presented by immerging the fresh etched PS with inorganic acid and alcohol solution via UV-irradiation for 30min to obtain a stable photoluminescence PS. This method doesn't diminish the sensitivity of PS to SO₂. It showed improved sensitivity and reversibility in response to SO₂. The photoluminescence of the passivated PS is very steady after it was stored in ait for 12monthes.(3)According to the experimental results of porous silicon sensing SO₂ qualitatively and quantitatively, it is discovered that photoluminescence quenching values of porous silicon oxidized by n-type single-crystal silicon wafers correlated with concentration of SO₂ positively. Moreover, researching the selectivity of passivated PS to SO₂ based on the typical pollution source, it shows that the selectivity of the PS is excellent, but the NO2 affects the selectivity only when its concentration is over 100ppm. According to dynamics mechanism of molecule quenching and Stern-Volmer equation, we get the linear response range of PS to detect SO₂ 10¹4000ppm is obtained.This technic applies the novel passivated PS and a mini-size fiber spectrometer to achieve SO₂ sensing process. This system is different from the big-sized analysis apparatus or complex test system. It is a simple, feasible and portable method to monitor SO₂.