Photoelectrocatalytic Degradation Of Organic Pollutant And Water Splitting Using Tio₂ Nanotube Arrays

TiO₂-based semiconductor materials have been attracted much attention in fields of organic compounds degradation and water splitting. However, some critical shortcomings limit the practical application of traditional TiO₂ nanoparticles, including poor visible light response, high recombination rate and no effective supporting. Therefore, a kind of self-organised TiO₂ nanotube arrays (TNAs) electrode was prepared by anodization method to overcome such drawbacks. The TNAs own promising photocatalystic properties for their peculiar architecture of that the nanotubular microstructures are perpendicular to the electrically conductive Ti substrate, forming a Schottky-type contact naturally and providing a unidirectional electric channel for the transport of photogenerated electrons. In this paper, the preparation, characterization and formation mechanism of TNAs electrode and its application in organic compound degradation has been studied. On these bases, a highly effective thin-layer photoelectrocatalytic reactor was designed for biomass treatment. Furthermore, CdS-modified TNAs electrode with effective visible light response was applied into biamss treatment and hydrogen production.The surface morphology structure of TNAs was strongly affected by the electrolyte, anodic voltage and reaction period. The experimental results demonstrate that the highly ordered titania nanotubes of approximately 20-200 nm in diameter and 0.5-60μm in length can by fabricated at anodic voltage of 5-60 V in different electrolyte solutions. The as-prepared TNAs, initially amorphous, needed to crystallize in air atmosphere for a certain period at 400℃-600℃. The DRS results indicate that the absorption edge of TNAs increases with the increase in the tube length of TNAs.The formation mechanism of TNAs prepared in HF aqueous electrolyte can be summarized as followed: initially, dense oxide of titania was rapidly formed on the titanium surface, followed by small pore formation. The adjacent small pores were then integrated and become larger pores. At the same time, small tubes were formed. Small tubes were further integrated into larger tubes until the primary tube formation. Finally, the tubular structure was gradually optimized and eventually developed into the highly-ordered TNAs.The highly effective TNAs electrode reveals excellent properties in organic compounds degradation and AO-7, as a typical azo dye, was chosen as the target compound. Moreover, the TNAs electrode possesses excellent stability after 20 repeated PEC processes. The experimental results show that the electrolyte concentration, light intensity, calcinations temperature and initial pH value of the solution were found to be important factors influencing the photoelectrocatalytic (PEC) process of AO-7 degradation.The configuration of PEC reactor is one of important factors determining the degradation efficiency of TiO₂ electrode. Based on the as-prepared TNAs electrode, a novel thin-layer PEC reactor with double side illumination has been developed. Compared with traditional reactor, this novel thin-layer reactor can greatly improve the mass transfer and treatment efficiency of organics at a short time. Even dealing with high concentration organic compounds, the thin-layer reactor still keeps high degradation efficiency. The reactor shows high stability after 20 repeated PEC experiments (93.1 ? 1.3%).Efforts to shift the band gap of TiO₂ via coupling narrow band gap semiconductor photosensitizers (e.g. CdS) have successfully extended its photoresponse to visible light region. In this paper, the CdS quantum dots (¹0 nm in diameter) sensitized TNAs were synthesized by sequential-chemcial bath deposition (S-CBD) method. The CdS incorporated TNAs electrode effectively harvest solar light in the UV as well as the visible light (up to 540 nm) region. The photoelectrical conversion efficiency of CdS/TNAs electrode was ¹1.7 times higher than the pure TNAs electrode. Furthermore, under visible light illumination, the composite photoelectrode generate hydrogen from water containing sulfide ions at a rate of 1.12ml/cm²Â·h.