How Insignificant Modifications of Photocatalysts Can Significantly Change their Photocatalytic Activity

Synthetic procedures, including doping, sintering and surface coating, can noticeably affect the physicochemical properties of semiconductors. Introduced changes very often translate into photocatalytic and photoelectrochemical activity alterations. However, in this work we have focused on more subtle treatments, which result in lack of changes observed using XRD, UV-vis, porosimetry, TEM or SEM. We have subjected titanium dioxide (P25, UV100) to a treatment with reducing agents used in procedures of noble metal deposition (citrate, borohydride, and photoreduction), or surface decoration with small amounts of TiO2 by atomic layer deposition (ALD; 10 to 200 deposition cycles), which presumably should be neutral to its activity. Although the “classical” characterization methods did not show any differences between the original and treated samples, spectroelectrochemical (SE-DRS) determination of the density of states (DOS) and catechol adsorption tests revealed a significant influence of such treatments on the photocatalytic activity (photogeneration of HO˙ radicals, water reduction, and herbicide degradation) and photoelectrochemical behaviour of the studied samples. We have shown that the applied slight surface modifications of titanium dioxide (“insignificant” at the first glance) may strongly affect the activity of this material. Such often overlooked effects must be taken into account during a comparative photoactivity analysis of various semiconductors, since an insignificant surface treatment may noticeably influence surface chemistry. We have also demonstrated that SE-DRS can be considered as a useful tool to study these effects, although it can be difficult to correlate a particular treatment with recorded changes in the density of states.