Self‐Organized TiO2 Nanotube Array Sensor for the Determination of Chemical Oxygen Demand

The principal impetus for the fabrication of functional nanotube materials comes from the promise of discovering unique structure-dependant properties and superior performance that are derived from their intrinsic nanotubular architecture. 1D TiO2 nanotube arrays prepared by the electrochemical anodization of self-organized porous structures on Ti foil have attracted great research interest in recent years owing to their peculiar architecture, remarkable properties, and potential for wide-ranging applications. Uniform TiO2 nanotubes are quite remarkably different in structure from other forms of TiO2, and are highly ordered, high-aspect-ratio structures with nanocrystalline walls perpendicular to electrically conductive Ti substrates, thereby naturally forming a Schottky-type contact. Moreover, these structures can be directly used as electrodes for photoelectric applications since the size of the nanotubes is very precisely controllable. The technological applications of TiO2 nanotube arrays are still at an early stage, but these remarkable structures have already been shown to be very promising for applications in sensing, catalysis, photovoltaics, photoelectrolysis, and nanotemplating. The electrical resistance of the TiO2 nanotubes changes by almost 7 orders of magnitude upon exposure to 1000 ppm H2, [13] the largest ever reported sensitivity of a material to a gas. Furthermore, the H2 evolution rate of TiO2 nanotube arrays has been reported to be 76 mL hw , which is the highest reported H2 generation rate for any oxide system upon photoelectrolysis. TiO2 nanotube arrays have also attracted great interest for enhancing the photocatalytic degradation of various organics, which makes them promising materials for the detection of pollutants. Given the increasing quantities of pollutants that are being dumped into water bodies, environmental monitoring and control have become issues of global concern. Chemical oxygen demand (COD) is one of the most widely used metrics in the field of water-quality analysis in many countries, and is frequently used as an important index for controlling the operation of wastewater treatment plants, wastewater effluent monitoring, and taxation of wastewater pollution. Up till now, several methods have been developed for the determination of accurate COD values, but the development of a rapid, accurate, and environmentally friendly method still remains a formidable challenge. The standard method for COD determination, the K2Cr2O7 method, requires reflux over a long period of time to achieve adequate oxidation and also results in the consumption of several expensive (Ag2SO4), corrosive (H2SO4), and toxic (Hg , Cr2O7 ) chemicals. In an attempt to shorten the time required for analysis, modified K2Cr2O7 methods have been developed based on microwaveor ultrasound-assisted oxidation. Other alternative assays have also been developed such as electrocatalytic determination using PbO2 or Cu sensors in thin-cell reactors, and photocatalytic and photoelectrocatalytic methods based on TiO2 nanomaterial sensors. [18,19] However, all these modified K2Cr2O7 methods are still plagued by the secondary pollution caused by highly toxic Cr(VI) ions, and moreover, the PbO2 sensors pose the risk of the potential release of hazardous Pb during the preparation and disposal of the active material of the sensors. As compared to traditional analytical methods, photoelectrocatalytic approaches are more promising because of the superior oxidative abilities of illuminated TiO2. Furthermore, TiO2 nanomaterials are typically non-toxic, inexpensive, photostable, and environmentally benign. The main drawback of TiO2-based methods in practical applications is the easy recombination of the photogenerated electron/hole pairs in discrete TiO2 nanoparticles and coated nanofilms, which results in low photocatalytic activity. For the determination of COD, this implies a narrow dynamic working range and relatively poor reproducibility. To tackle these problems, here we have combined TiO2 nanotube array sensors that exhibit structure-dependant enhanced photoelectrocatalytic activity with thin-cell reactors that have the advantage of small volume and rapid diffusion rate. A photoelectrocatalytic assay has been developed using these two components to establish a rapid, accurate, and environmentally friendly method for COD determination. The preparation, characterization, and photoelectrocatalytic properties of the TiO2 nanotube array sensors and their application in COD determination are described in detail below. C O M M U N IC A TI O N