The use of controlled periodic illumination with UV LEDs for enhancing photonic efficiency of photocatalytic decomposition processes in water has been investigated using methyl orange as a model compound. The impact of the length of light and dark time periods (TON/TOFF times) on photodegradation and photonic efficiency using a UV LED illuminated photoreactor has been studied. The results have shown an inverse dependency of the photonic efficiency on duty cycle and a very little effect on TON or TOFF time periods, indicating no effect of rate limiting steps through mass diffusion or adsorption/desorption in the reaction. For this reactor, the photonic efficiency under controlled periodic illumination (CPI) matches to that of continuous illumination, for the same average UV light intensities. Furthermore, under CPI conditions, the photonic efficiency is inversely related to the average UV light intensity in the reactor, in the millisecond time regime. This is the first study that has investigated the effect of controlled periodic illumination using UV LED sources. The results not only enhances the understanding of the effect of periodic illumination on photocatalytic processes but also provides a greater insight to the potential of these light sources in photocatalytic reactions.

This review article presents an overview of the application of ultraviolet light-emitting diode (UV LED) sources in heterogeneous photocatalysis within the context of artificial UV sources. The feasibility of UV LEDs as a source of UV irradiation in heterogeneous photocatalysis was first demonstrated almost a decade ago; however, for the most part, photocatalytic experimental set-ups utilise artificial light sources in the form of conventional UV lamps to initiate the desired photocatalytic transformations. A look at all sources of UV irradiation used in heterogeneous photocatalysis is taken with a focus on the growing importance of solid-state lighting devices such as UV LEDs. UV LEDs have higher external quantum efficiency and a lifetime of over 100,000 h; they are small in size and produce directional UV light which can be of the desired wavelength. In recent times, these UV LED sources have become widely applied in heterogeneous photocatalysis studies in the research literature and are fast becoming a viable alternative to conventional UV lamps.

Please cite this article as: Oluwatosin Tokode, Radhakrishna Prabhu, Linda Lawton, Peter K.J.Robertson, Controlled periodic illumination in semiconductor photocatalysis, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem. 2015.12.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.  Experimental and theoretical studies are examined with emphasis on two theoretical models. A Langmuir-Hinshelwood model for incorporating CPI failed to predict photocatalytic rates. A quantum yield model showed good agreement with experimental data. Results from previous as well as some of the most recent studies indicate potential applications of controlled periodic illumination in areas other than just the improvement of the efficiency of the photocatalytic process.

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