Photocatalytic reduction of methylene blue by TiO2 nanotube arrays: effects of TiO2 crystalline phase

TiO2 nanotube arrays were synthesized by anodization of Ti metal sheets followed by thermal annealing at elevated temperatures from 400 to 600 °C. Scanning electron microscopic measurements showed that dense arrays of nanotubes were produced with the inner diameter about 100 nm, wall thickness 35 nm, and length about 10 μm. X-ray diffraction measurements showed that the as-prepared nanotubes were largely amorphous, whereas thermal annealing led to the formation of well-defined anatase crystalline phase. More interestingly, at 470 °C, the brookite crystalline phase also started to emerge, which became better defined at 500 °C and disappeared eventually at higher temperatures, a phenomenon that has not been observed previously in TiO2 nanotube arrays prepared by anodization. The impacts of the TiO2 nanocrystalline structure on the photocatalytic activity were then examined by using the reduction of methylene blue in water as an illustrating example. Upon exposure to UV lights, the visible absorption profiles of methylene blue exhibited apparent diminishment. Based on these spectrophotometric measurements, the corresponding pseudo-first-order rate constant was estimated, and the sample thermally annealed at 500 °C was found to exhibit the highest activity. The strong correlation between the TiO2 crystalline characteristics and photocatalytic performance suggests that the synergistic coupling of the anatase and brookite crystalline domains led to effective charge separation upon photoirradiation and hence improved photocatalytic activity, most probably as a consequence of the vectorial displacement at the nanoscale junctions between these crystalline grains that impeded the dynamics of electron–hole recombination. These results demonstrate the significance of nanoscale engineering in the manipulation of oxide photocatalytic performance

Photocatalytic Pavements

Pavements which have been blended, coated, sprayed, etc., with photocatalytic TiO2 additives have attracted world-wide interest during the past decade-plus period based on their environmentally beneficial abilities to provide reactive (i.e., ‘smog-eating pavement’ plus ‘self-cleaning’) and reflective (i.e., ‘cool pavement’) impacts. The former ‘reactive’ capabilities notably involve a de-polluting property where TiO2 irradiation with UV-A spectrum light is able to oxidatively convert a variety of problematic organic and inorganic pollutants within both atmospheric and aqueous runoff zones. This suite of transportation-generated amenable contaminants notably includes NOX residuals which otherwise represent a serious environmental and human-health challenge within high traffic density, inner-urban highway locations with high-density adjacent resident populations. Multiple laboratory-level photo-reactor studies published over the past several decades have demonstrated this photocatalytic NOX-removal capability, while at the same time scientifically exploring and elucidating key relationships between NOX abatement and various environmental factors (e.g., light wavelength and intensity, ambient relative humidity and surface moisture, pavement temperature, surface soiling impacts, etc.). Field monitoring, albeit in more limited fashion, has provided similarly supportive findings at a number of locations involving not only TiO2-bearing pavements but also locations paved with blocks, pavers, bricks, etc. which have been sprayed or coated with TiO2-enriched admixtures. This chapter, therefore, provides an overview of the related literature covering academic, industrial, patent, and related perspectives and both experimental and full-scale findings. While this existing body of knowledge is substantial, complementary conclusions are also provided regarding recommendations for additional research which appears warranted to pragmatically strengthen the future understanding of TiO2-related pavement performance