Fabrication And Characterization Of Functional TiO₂ Membranes And Electrodes With Nanostructures And Their Performances

Several intrinsic drawbacks exist in the conventional TiO₂ photocatalytic oxidation process in terms of the application forms of photocatalyst, e.g., powder photocatalyst loss, separation, recovery, and decrease in photocatalytic activity after TiO₂ immobilization, which can be overcome effectively by using photocatalytic membrane reactor (or nanostructured electrodes), directly coupling the photocatalytic oxidation process with the membrane separation technology (or the electrochemical technology). Therefore, TiO₂ composite membranes with both photocatalytic capability and separation performance have been fabricated using different inorganic membranes as substrates by a simple sol-gel technique, and TiO₂ nanostructured film electrodes have been prepared by anodization and hydrothermal reaction methods. The resulting TiO₂ photocatalytic composite membranes and TiO₂ nanostructured film electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-vis diffuse reflectance spectra (DRS) and so forth, and their performances under coupling technologies were investigated and evaluated detailedly. In this dissertation, the following several parts of work have been done:(1) Crack-free TiO₂/ZrO₂ composite membranes on ZrO₂ supports have been successfully fabricated from TiO₂ sols by a simple sol-gel technique. The pore sizes of the prepared TiO₂ composite membrane can be effectively controlled by adjusting TiO₂ sol viscosity, the amount of polyethylene glycol (PEG), and calcination temperature of composite membrane, and thus controlling permeate flux and retention of the resulting TiO₂ composite membrane. The experimental results demonstrate that the retention of TiO₂/ZrO₂ composite membrane (1.0 g PEG 2000, 5 coating cycles) is not high (e.g. the retention of 67% for PEG 20,000), which is due to the larger pore sizes of composite membrane (100-200 nm). By coupling photocatalytic process with membrane separation technology, the removal efficiency of Direct Black 168 (100 mg/L) was improved obviously, being 52% within 400 min, while the values were 20% and 35% with membrane separation alone or photocatalysis alone during the same time, respectively. Lower removal efficiency of dye under coupling technology is ascribed to the larger pore sizes of composite membrane, the shorter hydraulic retention time, and the shortage of the design of reactor.(2) TiO₂/Al₂O₃ composite membranes with both photocatalytic capability and separation performance have been fabricated by simple sol-gel technique. Pore sizes of the resulting composite membrane were mainly controlled by the TiO₂ sol properties and immersion time. XRD patterns indicate that anatase of TiO₂ in composite membrane is dominant phase (86%) and TiO₂ particle size is about 22 nm. In order to obtain high removal efficiency of dye, some operational parameters such as the pressure difference, the initial pH, and air flow were investigated and discussed. By coupling technology, the removal efficiency of Direct Black 168 can reach 84% within 400 min, while the removal efficiencies are only 73% and 64% using photocatalysis alone or membrane separation alone, respectively. In comparison with photocatalysis alone or membrane separation alone, the removal efficiency of dye was improved obviously by coupling technology. The results of TiO₂ composite membrane after being used 6 times by coupling technology show that the pores structure and morphology of the composite membrane have no significant change and damage compared with before being used, and the removal efficiency of dye can be kept at above 65%.(3) Si doped TiO₂/Al₂O₃ composite membranes have been successfully fabricated from SiO₂/TiO₂ sols using porous Al₂O₃ support membranes. XRD patterns confirm that the embedding of amorphous SiO₂ into nanophase TiO₂ matrix helps to increase the thermal stability of TiO₂ which suppresses the phase transformation from anatase to rutile and decrease the size of TiO₂ particles. Moreover, the surface of 20%Si-TiO₂/Al₂O₃ composite membrane with pore sizes of 1.4-10 nm exhibites extremely high affinity for water under UV irradiation with water contact angle decreased from 62°to nearly 5°within 80 min. By coupling membrane separation with photocatalysis technique, the removal efficiency of Direct Black 168 was improved remarkably; being 85% within 100 min, while the values were 66% and 73% with photocatalysis alone or membrane separation alone during the same time, respectively. Similar results can be obtained using sodium dodecylbenzene sulfonate surfactant (SDBS) as test pollutant. Good photocatalytic activity and wettability of composite membrane under UV irradiation help to obtain high permeate flux across the composite membrane. The above results indicate that the Si doped TiO₂/Al₂O₃ composite membrane have the multifunctions of separation, degradation, and improvement of membrane flux in photooxidation of organic contaminants in wastewater.(4) TiO₂ nanostructured film electrodes have been prepared using metal titanium sheet as substrates by anodization and hydrothermal reaction methods. The photoelectrocatalysis reaction resistance (R = k/J_sph+R₀ = R₁+R₀) was measured by simple photoelectrochemical method, and used to express the electron transport characteristics of the nanotubular TiO₂ electrode. The overall resistance was found to consist of a variant (R₁) and an invariant component (R₀). The R₁ was found to be inversely proportional to the saturation photocurrent and depends on the experimental conditions. The proportional constant, k, represents the minimum applied potential bias required to remove 100% of the photogenerated electrons from the photocatalyst layer and was found to be independent of the anodization time. The invariant component of the resistance (R₀) is an inherent property of the semiconductor photocatalyst that represents the sum of ohmic contact impedance at the conducting substrate/TiO₂ interface and crystalline boundaries impedance. The real saturated photocurrent density (J_real-sphd) was found to be independent of R₀ indicating the electron collection efficiency is independent of nanotube length. By controlling hydrothermal reaction conditions, e.g., starting TiO₂ crystalline type, alkali solution concentration, and reaction temperature, three-dimensional network TiO₂ nanotube film and TiO₂ nanobelt array film electrodes can be obtained. The resulting TiO₂ nanostructured electrodes have significant potentials in DSSCs, photoelectrocatalytic oxidation of water, and photoelectrocatalytic oxidation of organic matters. The fabricated TiO₂ nanostructured electrodes can not only solve the issues of powder photocatalyst loss, separation, and recovery, but also improve photocatalytic activity of the electrodes by coupling photocatalysis with electrochemical technology.