Visualization and Quantification of the Laser-Induced ART of TiO₂ by Photoexcitation of Adsorbed Dyes

Abstract

Dye-pretreated anatase TiO2 films, commonly used as photoanodes in dye-sensitized solar cells, were utilized as a model system to investigate the laser-induced anatase to rutile phase transformation (ART), using N719 dye, N749 dye, D149 dye, and MC540 dye as photosensitizers. The visible lasers (532 and 785 nm) used for Raman spectroscopy were able to transform pure anatase into rutile at the laser spot when excitation of the dye sensitizer caused an electron injection from the excited state of the dye molecule into the conduction band of TiO2. The three dyes with carboxylic acid anchor groups (N719, N749, and D149 dyes) experienced ART upon dye excitation; diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and Raman spectra validated that these dyes were chemisorbed to the semiconductor surface. The MC540 dye with a sulfonic acid anchor group did not experience ART, and the DRIFTS and Raman spectra were inconclusive about the chemisorption of this dye to TiO2. A TiO2 calibration curve and percent rutile contour plots developed for this project are able to quantify the amount of rutile created at the surface of the samples. These improved chemical images which map rutile concentration help to visualize how ART propagates from the center of the laser spot to the surroundings. Factors such as visible-light absorption and anchor groups that covalently bind to the semiconductor play a key role in effective laser-induced ART.