Photochemistry on rutile TiO2 (110)

    TiO2 (titania) has many important technical applications in surface-related fields, such as heterogeneous catalysis and light-promoted catalysis.  Titania has also been shown to promote solar water-splitting.  A better understanding of how this surface works under different experimental conditions is potentially very useful in similar real-world applications.

    The photooxidation scheme for titania is well known.  Ultraviolet (UV) light can excite electrons from the valence band to the conduction band in titania, forming electron-hole pairs.  Three things can then occur: the electrons can interact with an adsorbed molecule, recombine with the holes, or the holes can interact with the adsorbed molecule.  However, the specific mechanism for reaction is not well known.  The goal of this project is to determine how the surface initiates reaction and learn how the desorbing species partitions energy.  

Figure 1: Photogeneration of electron-hole pairs (step 1) and hole-mediated photooxidation (step 2)

    We can use two different experimental setups to study these reactions.  The first is a 2-laser pump-probe method utilizing time-of-flight (TOF) spectroscopy and 1-photon vacuum-ultraviolet (VUV) ionization.  In this case, a pulsed UV laser (20 Hz) excites the substrate while a second VUV laser ionizes desorbing products.  With this setup, we can analyze desorbing reaction products whose ionization potential is less than 13 electron volts (eV).  The velocity of a desorbing molecule can be obtained by varying the firing times between the two lasers.  Hence, we obtain the translational energy distribution of a desorbing species.

Figure 2: Experimental setup used to detect photoreaction products.

    The second method uses  the same UV excitation laser, but utilizes a quadrupole mass-spectrometer for ionization and dectection. Here, the arrival times are obtained using a multichannel scaler. This setup is necessary for reactions where the products' ionization potential is greater than that of the VUV ionization laser.

    Currently we are studying the photo-oxidation reaction of different organic compounds (ketones), in conjunction with oxygen, to determine the fragmentation pattern of the organic molecule upon photo-oxidation and obtain their kinetic energy distribution. These simple ketones provide model systems where easily controlled surface coverage can be obtained.