Novel methods for preparation of modified 1-dimensional nanomaterials of titanium dioxide for environmental engineering applications

Abstract

Photocatalytic processes of titanium dioxide have been discovered in the early 1980 and till now titanium dioxide nanomaterials are the most popular photocatalysts for scientific researches and industrial applications. In the environmental engineering field, titanium dioxide nanoparticles have been applied in water treatment such as disinfection of drinking water. It is well known that the photocatalytic efficiency of titanium dioxide nanomaterials is correlated with the total surface area and the optical absorption of the materials themself. Original titanium dioxide nanomaterials can only be photo-activated with radiations in the ultra violet (UV) spectrum. As UV radiations make up less than 10% of total solar radiation to the earth's surface, the efficiency of original titanium dioxide nanomaterials is therefore limited under direct sunlight. As a result, extensive studies have been carried out to modify original titanium dioxide nanomaterials to achieve activation in the visible light, and the most popular and effective modification method is doping titanium dioxide with nitrogen. Titanium dioxide can be in various forms and recent developments have allowed the preparation of novel nanostructures of titanium dioxide nanomaterial. Particularly, the 1-Dimension (1-D) structures including nanorod, nanofiber, and nanotube are of great interest. These structures have not only a large surface area but also geometries that may improve the overall efficiency of titanium dioxide nanomaterials as heterogeneous photocatalyst. Moreover, 1-D titanium dioxide nanomaterials under forms such as mats, foils, and films provide better handling and utilization in term of engineering. As a result, 1-D titanium dioxide nanomaterials are considered as very promising catalyst for industrial applications, including in the environmental engineering field. To fully capitalize on the advent of titanium dioxide surface doping and new titanium dioxide structures, modification of 1-D titanium dioxide nanomaterials is being widely studied to enable visible light activation. However, current modification methods, particularly nitrogen doping, still have shortcomings. For instance, nitrogen doping of titanium dioxide nanotubes requires high temperature settings or comes with unwanted damage to the 1-D structure. The primary objective of this doctoral study is to develop novel methods of preparation and modifications of 1-D titanium dioxide nanomaterials for achieving activation with visible light while addressing shortcomings of currently available method. Two types of 1-D titanium dioxide nanomaterials including nanotube and nanofiber are selected as the research targets. The secondary objective is to explore the possibility of applying the as prepared modified 1-D titanium dioxide nanomaterials in the photocatalytic degradation of hydrocarbon contaminants in water with only visible light as the radiation source. Produced water with aromatic hydrocarbon components are chosen as the model with a view to assess the effectiveness and efficiency of modified 1-D titanium dioxide nanomaterials. Moreover, produced water and oily water represent a large portion of wastewater source in the oil and gas production sector. Therefore it is desirable to develop an effective and sustainable treatment technique for produced water reuse. In line with the research objectives, the following research activities have been implemented and accomplished: i) preparation, modification and characterization of visible light activated titanium dioxide nanotubes; ii) preparation, modification and characterization of visible light activated titanium dioxide nanofibers; and iii) assessment of the photocatalytic removal of model aromatic hydrocarbons with the modified 1-D titanium dioxide nanomaterials as prepared in visible light. Titanium dioxide nanotubes were prepared through the popular anodization method with hydroflouride acid and dimethyl sulfoxide (DMSO) as solvent. After the structure being confirmed by SEM, titanium dioxide nanotubes were subjected to treatment by plasma processing techniques with either nitrogen and nitrogen/carbon monoxide as feeding gases. The plasma treated titanium dioxide nanotubes were characterized by SEM, TEM, XRD, and UV-VIS reflection test. These characterization experiments showed that the plasma treated titanium dioxide nanotubes mostly retain the tubular structure. Titanium dioxide tubes are polycrystalline and anatase was found to be the dominant crystalline form within these nanotubes. The mat of plasma treated titanium dioxide nanotubes showed an increased optical absorption in the visible light. In subsequent photocatalytic experiments with methylene blue, the plasma treated titanium dioxide nanotubes exhibited activation with visible light as desired. In the second research task, modified titanium dioxide nanofibers were successfully prepared by a novel method based on electrospinning which enables the in situ nitrogen doping of nanofibers simultaneously. An electrospinning dope was prepared from titanium isopropoxide, DMSO, acetone and polyacrylonitrile (PAN). The application of PAN was hypothesized to improve the stability of annealed titanium dioxide nanofibers and at same time to provide nitrogen for doping of the nanofibers during the preparation process. After being annealed, titanium dioxide/PAN nanofibers were again characterized by SEM, XRD, and UV-VIS reflection test. These characterizations showed that the annealed titanium dioxide/PAN nanofibers still retain the fibrous shape, with titanium dioxide deposited as “nodules” along fibers. This characteristic greatly increased the total surface area of the material. Anatase was the dominant crystalline form and the increased optical absorption in the visible light suggested that nitrogen doping had taken place as hypothesized. Similarly, subsequent photocatalytic experiments with methylene blue confirmed that annealed titanium dioxide/PAN nanofibers exhibited activation with visible light. Lastly, the as prepared modified 1-D titanium dioxide nanomaterials were utilized in degradation experiments of aromatic hydrocarbons including Benzene, Toluene, Ethyl benzene and Xylene (BTEX) in water under visible light. The working solution with BTEX component was designed to resemble the produced water from oil and gas production activities. The removal of BTEX was assessed based on the total organic carbon (TOC). Experimental results showed that the as prepared modified 1-D titanium dioxide were able to degrade BTEX components in water via the photocatalytic processes initiated by visible lights. Comparing to current Best Available Techniques (flotation) and Best Possible Techniques (adsorption), it is assessed that treatment techniques of produced water built upon modified 1-D titanium dioxide nanomaterials is promising with high effectiveness and sustainability. The findings reported in this dissertation have demonstrated the novel methods to prepare visible light-activated 1-D titanium dioxide nanomaterials. Particularly, with the ease in the preparation of N-doped titanium dioxide nanofibers and their high photocatalytic efficiency in visible light as observed, the research results bring up promising opportunities to prepare new types of titanium dioxide nanomaterials which are better suitable for removing harmful contaminants in air and water in environmental engineering applications.