Infrared spectroscopic study of α-Cyano-4-hydroxycinnamic acid on nanocrystalline tio2 surfaces: anchoring of metal-free organic dyes at photoanodes in dye-sensitized solar cells

Abstract

Adsorption structures of the self-assembled thin films of a-cyano-4-hydroxycinnamic acid (CHCA) anchoring on TiO2 surfaces have been studied by using temperature-dependent diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy. From the presence of the strong v(COO-) band at ̃1390 cm-1 along with the disappearance of the OH bands in the carboxylic acid group in the DRIFT spectra at room temperature,CHCA appeared to adsorb onto TiO2 surfaces as a carboxylate form. The absence of the out-of-plane benzene ring modes of CHCA in the DRIFT spectra suggests a rather vertical orientation of CHCA on TiO2. Above ̃220 °C,CHCA seemed to start to thermally degrade on TiO2 surfaces referring from the disappearance of most vibrational modes in the DRIFT spectra,whereas the v(C = N) bands were found to remain relatively conspicuous as the temperature increased even up to ̃460 °C.