Preparation And Gas Sensitive Research Of TiO₂ Nanotube Array Film For The Main Gaseous Pollutants In Pulp And Papermaking Mills

Pulp and papermaking industry is closely related to the development of the national economy and also the main heavy polluter.Despite of the possible environmental and health risks,the analysis of gaseous pollutants in paper mills have been lack of public concerns for a long time,and few researches have been reported.In addition,the situation of industrial pollutant emission tends to very complex due to numerous factors,leading to test the goal is not clear.In order to research the composition of pollution gas from pulp and papermaking industry,and provide fast,simple and accurate measurement method and monitor information for papermaking pollution gases,the main research contents of this thesis are as follows:In this study,the concentrations of four potential gaseous pollutants,volatile organic compounds?VOCs?,formaldehyde?HCHO?,hydrogen sulfide?H2S?,and hydrocarbon compounds?CxHy?,in the ambient air on thirty sampling sites of five pulp and paper mills were detected and compared,firstly.The results were summarized in the following aspects: It was obvious that the situation of gaseous pollution in pulp mill was much more serious than those in paper mills.Being the two common sites in five mills,the wet end of paper machine and the wastewater treatment process,VOCs and H2 S were revealed as the main gaseous pollutants,respectively.Therefore,in order to achieve rapid,convenient,accurate and stable measure of main pollution gas contents in the pulp and paper industry environment air,this paper further carried out the preparation of TiO₂ nanotube array film and researched the performance of the gas sensor based on the film.Through the optimization researches of anodic oxidation,the optimum conditions were obtained: titanium foil with purity at 99.7% and the thickness at 0.01 mm was used as the raw materials,the anodizing was carried out at 30 V for 2 h using a direct-current stabilizer.The growth of the nanotube arrays was obtained using a glycol solution with 0.55 wt% ammonium fluoride and 20 wt% deionized water.After that,the anodized titanium foil was rinsed with deionized water,and dried in air.The samples were annealed at 450? for 2 h in the air.Then the free-standing TiO₂ nanotube array film was successfully synthesized by a one-step anodization method.The characteristic techniques including scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and X-ray diffraction?XRD?were adopted to characterize the morphology and chemical composition of the Ti O₂ nanotube array film.The average inner diameter of the nanotube was around 110 nm with a wall thickness of 16 nm and the tube length was 3.8 ?m.At the same time,different metal ions doped Ti O₂ nanotube array films were prepared by dipping method.Subsequently,gas sensor based on the Ti O₂ nanotube array film was fabricated and its sensing performance toward H2 S was investigated.The results showed that the optimum operating temperature to detect H2 S gas was 300?,the detection range to H2 S gas was 1-50 ppm with the response value of 4.5-26.2,and a good linearity of sensing characteristics could be observed.Meanwhile,the response and recovery time of the sensor to 50 ppm H2 S gas were as low as 22 s and 6 s,respectively.By studying the gas sensitive performances of different kinds of metal ion doping-TiO₂ nanotubes array film,found that when the cobalt ion doping concentration of 0.1 M,the highest sensor sensitivity value was obtained with 199.16 for 50 ppm H2 S at 300?,compared with the pure form,which was increased 6.6 times.Through the analysis results of structural characterization methods,finding that cobalt ions replace titanium ions went into the TiO₂ crystal lattice.Finally,mechanisms of the development of the free-standing TiO₂ nanotube array film and the sensor towards H2 S were discussed.In addition,based on density functional theory,the parameters such as crystal structure,band gap and density of states of the pure and metal ions doped TiO₂ were analyzed.Results showed that the cobalt ion and iron ion doping in the impurity level of the band structure,making the width forbidden band of TiO₂ decreased to 1.418 eV and 1.425 eV from 2.285 eV.Smaller forbidden band width was advantageous to the separation of electronic-hole on the TiO₂,which promoted generation of oxygen ions on the surface of TiO₂.Thus,the reaction efficiency of H2 S and oxygen ions was improved,which effectively promoted the gas-sensing properties of TiO₂ nanotubes array film.