Nature of Trapped Charge Carriers

Generally it is assumed, that TiIV cations at the surface of the titanium dioxide particle are reduced by the light induced electrons forming Tiin cations [11] which can be considered to be intrinsic surface states localised about 0.1 eV below the conduction band edge, i.e., within the bandgap [12]. An equilibrium between these trapped electrons and free electrons is assumed, but in an acidic medium nearly all electrons are trapped in surface states [11a]. On the basis of their laser flash photolysis measurements Hoffmann and co-workers have extended this mechanistic picture [13]. These authors assume that the CB electrons are trapped in two different Tim sites (reactions (7.4) and (7.5)) 

From the analysis of their experimental results of the investigation of the charge carrier recombination kinetics in titanium dioxide colloidal solutions and in dispersions Serpone et al. and Bowman and co-workers have also assumed the existence of two different traps [5,6]. 

The chemical nature of the trapped holes has not been clearly clarified yet. Older reports assume that the holes are trapped at the titanium dioxide surface in adsorbed hydroxy groups yielding weakly adsorbed hydroxyl radicals (reaction (7.6)) [14,15]. 

Howe and Grdtzel deduced from esr investigations that the hole is trapped in a subsurface oxygen anion (reaction (7.7)) [11a]. 

Other groups are assuming that the trapped hole is an oxygen radical centered at the surface of the titanium dioxide particle, having an energy state lower than the valence band edge of the semiconductor (reaction (7.8)) [lid]. 

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