Jan 29, 2010
Silatrane anchors sensitizer dye to nanoporous anode
Securing molecules to the surface of metal oxides such as TiO2 and SnO2 is important in various fields including organic photovoltaics, sensors and environmental remediation. Common attachment methods involve carboxylic or phosphonic acids that form ester bonds to the metal oxide surface. However, such bonds are susceptible to hydrolysis at neutral to basic pH in water and in a variety of other solvents. Leaching of the molecule from the surface can cause irreversible damage, limiting the effective lifetime of the material. Methods to form more stable covalent bonds, such as siloxy linkages, have typically required anhydrous conditions and involved complex processes.
Now, researchers at Arizona State University, US, have reported the synthesis of porphyrin- and ruthenium-based dyes containing a remarkably stable trialkoxysilane linker protected with triethanolamine. These stable "silatrane" molecules may be purified using silica gel column chromatography and do not hydrolyse in air. At moderate temperatures the dyes react with metal oxides to form a molecular layer. SEM images of nanoporous SnO2 surfaces with attached dyes show no evidence of the pore blockage that often occurs because of siloxane polymerization.
The study, published in the journal Nanotechnology compares the photoelectrochemical performance of porphyrin and ruthenium dyes bearing either a carboxylic acid or the new amidopropylsilatrane linkage attached to nanoporous semiconductive SnO2 films on conductive glass electrodes (such electrodes use energy from light to drive electrical current production in an electrochemical cell).
Transient spectroscopy was used to determine the rate of electron recombination events that are detrimental to cell performance, revealing that the longer surface linker on the silatrane was able to decrease the recombination rate. The result was an increase in yield of electrical current per absorbed photon of light with the silatrane linkage. Although ruthenium dyes with carboxylic acid linkages readily desorb from metal oxide surfaces in aqueous solutions buffered at neutral pH, the ruthenium-based silatrane dye was strongly bound to SnO2 and able to function with high efficiency in a photoelectrochemical cell under such conditions.
Silatranes have the potential to covalently bind molecules to many metal oxide species, opening the door to applications featuring stably functionalized films or particles. In particular, this method allows the use of water-soluble molecules attached to metal oxides in aqueous solutions over a wide pH range.
About the author
The research was performed by graduate student Bradley Brennan and colleagues in the laboratories of professors Devens Gust, Ana Moore and Thomas A Moore, Department of Chemistry and Biochemistry, Arizona State University. For more details on the team and its work, see related links (on the left hand side of the page).