Optical control of phase transformation in Fe-doped TiO2 nanoparticles

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

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.

Laser-flash-photolysis-spectroscopy: a nondestructive method?

Herein, we report the effect of the laser illumination during the diffuse-reflectance laser-flash-photolysis measurements on the morphological and optical properties of TiO₂ powders. A grey-blue coloration of the TiO₂ nanoparticles has been observed after intense laser illumination. This is explained by the formation of nonreactive trapped electrons accompanied by the release of oxygen atoms from the TiO₂ matrix as detected by means of UV-vis and EPR spectroscopy. Moreover, in the case of the pure anatase sample a phase transition of some TiO₂ nanoparticles located in the inner region from anatase to rutile occurred. It is suggested that these structural changes in TiO₂ are caused by an energy and charge transfer to the TiO₂ lattice.

N3-dye-induced visible laser anatase-to-rutile phase transition on mesoporous TiO2 films

Titanium dioxide has been extensively used in photocatalysis and dye-sensitized solar cells, where control of the anatase-to-rutile phase transformation may allow the realization of more efficient devices exploiting the synergic effects at anatase/rutile interfaces. Thus, a systematic study showing the proof of concept of a dye-induced morphological transition and an anatase-to-rutile transition based on visible laser (532 nm) and nano/micro patterning of mesoporous anatase (Degussa P25 TiO(2)) films is described for the first time using a confocal Raman microscope. At low laser intensities, only the bleaching of the adsorbed N3 dye was observed. However, high enough temperatures to promote melting/densification processes and create a deep hole at the focus and an extensive phase transformation in the surrounding material were achieved using 1s laser pulses of 25-41 mW/cm(2), in resonance with the MLCT band. The dye was shown to play a key role, being responsible for the absorption and efficient conversion of the laser light into heat. As a matter of fact, the dye is photothermally decomposed to amorphous carbon or to gaseous species (CO(x), NO(x), and H(2)O) under a N(2) or O(2) atmosphere, respectively.

One-step synthesis of N- and F-codoped mesoporous TiO2 photocatalysts with high visible light activity

We report on a novel approach to the synthesis of N- and F-codoped mesoporous TiO2 photocatalysts via a reproducible, rapid and single-step combustion method. TiF4 was used as the precursor to provide the source of Ti and F, while urea was used as the fuel as well as the source of the N dopant. The as-synthesized samples were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. The specific surface areas of the samples were determined using a Quantachrome Nova 2200 for the N2 adsorption/desorption under liquid-nitrogen temperature. Our studies show that the fabricated N- and F-codoped TiO2 photocatalysts have mesoporous structure and a very large specific surface area (155.3 m(2) g(-1)) and that the codoping of N and F significantly narrows the TiO2 bandgap energy from 3.2 to 2.45 eV. We further studied the photocatalytic activity of the synthesized N- and F-codoped mesoporous TiO2 through the decomposition of acetic acid, showing that the N- and F-codoped mesoporous TiO2 catalyst fabricated in this study exhibits superb photocatalytic activity and visible light response compared to one of the best commercially available TiO2 photocatalysts, P25.