A Spectroscopic and Electrochemical Approach to the Study of the Interactions and Photoinduced Electron Transfer between Catechol and Anatase Nanoparticles in Aqueous Solution

We have combined in situ photoelectrochemical and spectroscopic techniques (Attenuated Total Reflection Infrared, ATR-IR, and Resonance Raman Spectroscopy) for the study of the charge-transfer complex formed upon adsorption of catechol on anatase nanoparticles in contact with aqueous acidic solutions. Vibrational spectroscopies reveal the existence of at least two adsorbate configurations:  catecholate in a chelate configuration and molecularly adsorbed catechol, with apparent values of −12.3 and −10.5 kJ mol-1, respectively. These values are significantly less negative than the values reported for anatase colloidal dispersions. The adsorption of both catechol species on the nanoparticulate anatase thin films follows the Freundlich isotherm. As revealed by resonance Raman spectroscopy, only the adsorbed chelating catecholate forms the charge-transfer complex. The electron transfer from the adsorbate to the anatase nanoparticles has been evidenced by the development of a negative photopotential upon 514.5 or 632.8 nm laser illumination of an anatase nanostructured thin film electrode in contact with a catechol solution. The time evolution of the Raman spectra shows an increasing fluorescence indicating that, upon electron injection, catechol polymerization occurs on the TiO2 surfaces. This conclusion is confirmed by in situ ATR-IR measurements, which show a progressive broadening of the catecholate bands together with the appearance of new signals. This study illustrates the benefits of combining electrochemical, infrared, and Raman techniques for the elucidation of processes occurring at the semiconductor/solution interface. Finally, evidence is given on the different adsorption and reactivity behavior found for suspensions and nanoporous thin films under equivalent experimental conditions.