Effect of process parameters on photocatalytic activity of titanium dioxide

Water has become an increasing concern in the last centuries. The rapid industrial growth, the environmental pollution, the depleted water resources and an uncontrolled ground-water deployment have led to a situation where clean and drinkable water is becoming rare. The rapid development of manufacturing technology after the industrial revolution has significantly improved the standards of living but the exploiting of water and its pollution are becoming a factor that is threatening human health and the environment. Pollutants are changing both quantitatively and qualitatively, and the number of chemicals currently in circulation is enormous and continues growing. The wide area of water pollution, its diversification and non-biodegradable contaminants has become a problem that cannot be solved by the natural cleansing cycle. Currently available water treatment technologies such as adsorption or coagulation merely concentrate the pollutants present by transferring them to other phases, but still remain and not being completely “eliminated” or “destroyed”. Other conventional water treatment methods such as sedimentation, filtration, chemical and membrane technologies involve high operating costs and could generate toxic secondary pollutants into the ecosystem. Even chlorination, which is the most commonly and widely used disinfection process, have mutagenic and carcinogenic by-products. These concentrated toxic contaminants are highly redundant and have been concerned worldwide due to the increasing environmental awareness and legislations. These are the reasons that have requested a change in the research path and have led to important development in photocatalysis. In recent years, semiconductor photocatalytic process has shown a great potential as a low-cost, environmental friendly and sustainable water treatment technology to align with the “zero” waste scheme in the water/wastewater industry. The ability of this advanced oxidation technology to remove persistent organic compounds and microorganisms in water has been widely demonstrated. Among these advanced oxidising devices TiO2 has shown to suit all the requirements to be successfully employed in this filed. This dissertation is going to give an overview of the use of titanium dioxide for photocatalytic purpose, starting from the semiconductive characteristics, which makes it so attractive for photoactivated process, and continuing with the production and use of nanostructures. It is clear indeed, that most reactions are kinetically restricted by the low surface area available for reactions and it is easy to understand that a nanostructured surface is able to minimise this limit. In the following chapters then, most of the methods used to produce TiO2 nanotubes are presented with focus on electrochemical anodization as the best approach to produce photocatalytic devices. Electrochemical oxidation reaction of Ti substrate under a specific set of environmental conditions can lead to the creation of nanopores/nanotubes with the unique combination of the highly functional features of TiO2 with a regular and controllable nanoscale geometry (length, tube diameter, and self-ordering). Recent progresses made on anodization parameters and thermal treatments in order to obtain improved nanostructures were exploited together with some new approach based on surface preparation. The photocatalytic efficiency of these samples have been tested using a well-known dye, a solution containing rhodamine B (RhB), which is a waste product of many industries. RhB concentration can be easily monitored measuring the absorbance of the solution and it is a fast method to characterize the samples photoactivity. In laboratory both organic and aqueous electrolytes were used to produce nanotubes The goal was to determine the optimum operation parameters to enhance photo-oxidation efficiency through tube geometry improvements; it has been found that adjusting voltage, time and temperature of anodization it is possible to improve the geometry of the tubes and therefore increase the efficiency of degradation of pollutants. In the same way, performing surface treatments can lead to the growth of nanotubes with better geometrical properties. Eventually, those produced in the organic ones have shown better characteristics. Together with anodization parameters also annealing treatments were exploited; it is known by literature that as formed anodized nanotubes are completely amorphous with the consequent absence of photoactivity. It was observed that an increase in time and temperature of annealing is proportionally related with increase in crystal phases and the formation of anatase and rutile phases during annealing positively influences the nanotubes characteristics with a consequent increase in the material photocatalytic efficiency. Significant improvements were made and, even more important is the knowledge acquired on the relation existing between variables and efficiency.