Photo‐induced processes in heterogeneous nanosystems. From photoexcitation to interfacial chemical transformations

Abstract

Abstract. This article briefly reviews some of our recent work carried out both from an experimental point of view as well as from a theoretical perspective to gain further understanding of the events that take place in Heterogeneous Photocatalysis. Previously, the multitude of reports from our laboratory and from many others looked at the primary photocatalytic events as involving (a) absorption of light, (b) formation of the free (electrons and holes) and/or trapped charge carriers (Ti 3+ and •OH radicals), and (c) reaction of pre-adsorbed acceptor or donor molecules with the relevant trapped carrier. Our recent work notes that this view is reasonable if the only purpose of photocatalysis is elimination of undesirable environmental pollutants. But when we begin to query how to render a process more efficient, we need to address the primary events following photoexcitation of the photocatalyst, which in most instances has been titanium dioxide (in the anatase form). Owing to the nature of light absorption by TiO 2 we resorted to examining other metal oxides, most of which are dielectric insulators with very large bandgap energies, for example zirconia (ZrO 2 ) and scandia (Sc 2 O 3 ). These dielectrics have provided added information on the photophysical events, many of which are masked by the strong light absorption in titania. Despite some of our recent progress, much remains to be done for a fuller understanding of the events that occur at the surface, which we have often considered to be the greatest and most complex defect in metal oxide particulates.

A surface science perspective on TiO2 photocatalysis

“…Trapped electron detection: Many groups [5,140,186,387,457,499,513,[519][520][521][548][549][550][551][552][553][554][555][556][557][558][559] have used EPR (ESR) to detect and characterize trapped electrons (and holes) on/in TiO 2 , with surface and subsurface trap sites distinguished using chemistry (e.g., O 2 ). The spin state environments allow researchers to differentiate between paramagnetic features associated with Ti (trapped electrons) and O (trapped holes), as well as their populations and reactivities.…”

“…This remarkable level of publication activity reflects the potential for new applications emerging from research in this field. Numerous reviews exist on the subjects of: heterogeneous photochemistry [3][4][5][6][7][8][9][10][11][12], and TiO 2 -based photochemistry [2, that blend in chemistry, physics and engineering perspectives to the field. There are reviews involving the role of TiO 2 -based materials in such subjects as: photocatalytic water splitting and hydrogen production [3,10,[43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60], photoelectrochemistry [58,[61][62][63], dye sensitization and solar energy conversion [46,[64][65][66][67][68][69][70][71][72][73][74]…”

The degradation of phenol in water solution by TiO2 photocatalysis in a helical reactor

“…Oxygen, provided from the air (when the system is stirred), dissolved in the solution scavenges the electron generated, preventing the recombination of electrons and holes. The latter radical species are well known to oxidize a large number of organic substrates (eq.iv), (Horikoshi et al, 2001;Emeline et al, 2001).…”

Photophysical Processes Related to Photoadsorption and Photocatalysis on Wide Band Gap Solids: A Review

“…The Scheme has been firstly presented in [13]. Whereupon it has been used with minor alteration a few times (see, for example, [14]). Nevertheless, it will be briefly discussed below as well.…”

“…In turn, at any rate three adjoining and overlapping spectral regions for photoexcitation [13,15] can be distinguished. There are the following: 1) fundamental absorption bands (hν 1 ) related to band-to-band transitions (type I, Figure 1) in [13,14]. regular crystal bulk (R b ).…”