Influence of the Dopant Concentration on the Photoelectrochemical Behavior of Al-Doped TiO2

In the present study, we explored the effect of Al-dopant concentration within the range of <1.1 wt % on the photoelectrochemical (PEC) activity of an Al-doped TiO2 photoanode. The experimental dependencies of PEC efficiency on Al-dopant concentration indicate that there is an optimal Al concentration of 0.5 wt % corresponding to the highest photoactivity. The analysis of the spectral dependencies of the photocurrent confirms that 0.5 wt % Al provides the highest activity at photoexcitation in both intrinsic and extrinsic absorption spectral range. It was also shown that Al doping does not affect the optical band gap of TiO2. The dependence of PEC activity on Al concentration correlates with the corresponding dependencies of the flat-band potential and work function, indicating the Fermi-level shift toward the conduction band for the Al concentration <0.5 wt % and toward the valence band for the Al concentration >0.5 wt %. Such alteration of the Fermi-level position is explained in terms of alteration of the type of major compensating intrinsic defects from [V-o(center dot center dot)-Ti-Ti '] for the Al concentration <0.5 wt % acting as shallow traps to [Al-Ti'-V-o(center dot center dot)-Al-Ti'](x) for the Al concentration >0.5 wt % acting as deep traps. Transformation of compensating defects from shallow traps, which are ineffective in charge recombination processes, to deep traps, which act as effective recombination centers, is responsible for the optimal dopant concentration, 0.5 wt %, to achieve the higher PEC activity of Al-doped TiO2.