Polyol-derived carbon-modified titania for visible light photocatalysis

Modification of anatase by calcining titanium hydroxide or titania (UVLP) at 250°C in the presence of ethylene glycol and pentaerythritol afforded modified powders TiO2-EG, UVLP-EG, and UVLP-PE, respectively, active in visible light mineralization of 4-chlorophenol. The presence of surface hydroxyl groups is necessary for the modification process, since their removal results in a photocatalyst of much lower activity. When UVLP-EG was calcined at 400°C, the resulting residue is of very low activity suggesting oxidative destruction of an organic photosensitizer. Quasi-Fermi levels of −0.58, −0.53, −0.52, and −0.46 V were observed for TiO2-EG, UVLPcalc, UVLP-EG, and UVLP-PE, respectively. In attempts to desorb the photosensitizer, TiO2-EG, UVLP-EG, and UVLP-PE were treated at 90°C with NaOH as recently performed with the commercial material VLPcom. Whereas the activity of the residual powder was strongly decreased in the case of TiO2-EG, it did not change for UVLP-EG and UVLP-PE. Both the residues, UVLP-EGres and UVLP-PEres, exhibited slightly enhanced mineralization activities as compared to the starting materials. Thus, different from VLPcom and TiO2-EG, no sensitizer but only an organic impurity could be extracted. In agreement with this conclusion, the corresponding extracts had different emission spectra. Similarly, acidic desorption experiments on UVLP-EG again produced a very active residual powder. This inertness of UVLP-EG and UVLP-PE against alkali or acidic treatments suggests that the sensitizer is bound to the titania surface not through an ester bond as proposed for VLPcom.