Crystal Surfaces and Fate of Photogenerated Defects in Shape-Controlled Anatase Nanocrystals: Drawing Useful Relations to Improve the H-2 Yield in Methanol Photosteam Reforming

We comprehensively explored the photocatalytic properties, in H2 production by methanol photosteam reforming, of anatase nanocrystals with nearly rectangular (RC), rhombic (R), and nanobar (NB) shapes having exposed {001}, {101}, and {010} surfaces. The aim was to relate the reactivity both to the type of crystal facets and to the photogenerated defects. The electron spin resonance (ESR) spectra reveal that the amount of Ti3+ (electron traps) is parallel to the H2 evolution rate and becomes a maximum for the RC nanocrystals, which display the highest area of {001} surfaces and the lowest {101} area but also involve a significant area of {010} facets. This points out that the H2 production cannot be related only to the envisaged reducing {101} facets, but that the {010} facets also play a key role. We suggest that the contiguous {001}, {101}, and {010} facets form a highly effective "surface heterojunction" within a RC nanoparticle which drives the electrons photogenerated on {001} facets not just toward the {101} but also to the {010} facets, while the holes are driven toward the {001} facets. This transfer improves the charge separation, thus boosting the photoefficiency of RC nanocrystals compared to that of NB and R nanocrystals. The ESR spectra performed after ultraviolet excitation in the presence of MeOH show the partial annihilation of the Ti3+ features, mainly for highly reactive RC nanocrystals. Because H2 production involves an electron transfer to the proton, a relevant role in H+ photoreduction of the Ti3+ centers present on the exposed {010} and {101} surfaces is suggested. These findings underline the importance of determining the relationship between the photogenerated defects and the exposed crystal surfaces to optimize the photocatalytic properties of anatase nanocrystals. (Figure Presented).