Functionalization of SnO₂ crystals with a covalently-assembled porphyrin monolayer

The functionalization of micro- and nano-sized metal-oxide powders offers many advantages because of their large surface areas and, therefore, the large number of functional molecules that can be grafted onto the grain surfaces. Porphyrin molecules on large band-gap semiconducting metal oxides represent key materials for many different optical and electronic applications. Herein, we have proposed a general two-step procedure for the functionalization of metal-oxide crystals with dye-sensitizers. In particular, we functionalized SnO₂ nanoparticles with a monolayer of the bifunctional trichloro[4-(chloromethyl)phenyl]silane. Then, a monolayer of 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyne was covalently bound to the silanized SnO₂ grains. IR, UV/Vis, and luminescence measurements were used for optical characterization. The measured footprint of the grafted porphyrin molecules indicated total surface coverage of the grains. The surface electronic characterization was performed by using X-ray photoelectron spectroscopy. Emission measurements revealed two strong bands at 664.1 and 721.0 nm that were attributed to the porphyrin monolayer assembled on the surface of the SnO₂ crystals.