Measuring Transient Phenomena in Photoexcited BiVO4 by FTIR

Itamar Benisti , Department of Chemical Engineering, Technion, Haifa, Israel
Yaron Paz, Department Of Chemical Engineering, Technion, Haifa, Israel


Developing of new, highly efficient, photocatalytic materials depends to large extent on better understanding of the physical and chemical phenomena occurring right after photon absorption. Over the years two major techniques have been utilized using pulsed lasers beams for excitation: Time Resolved Microwave Conductivity (TRMC) and UV spectroscopy. While these two complementary methods provide important information on the life times of free carriers they are almost silent with respect to the chemical species involved in the process and with respect to specific loci at which the post-excitation processes occur.

Here we present a third method for studying the excitation of photoactive and, in particular, photocatalytic materials. The method is based on measuring time resolved (5*10-9 sec in resolution) IR spectroscopy of the photocatalytic materials upon excitation with the third harmonic (355 nm) of a Nd:YAG pulsed laser. The time resolved FTIR spectra is obtained by using a “step-scan” configuration, i.e. by recording a temporal signal at a specific location of the moving mirror in the Michelson interferometer, following by altering the position of the mirror and re-measuring the temporal signal upon re-excitation. At the end of the process, an array of data points in the time-distance is obtained, facilitating to perform Fourier Transformation of all data points gathered at a specific time.

The technique is demonstrated by measuring temporal changes in the FTIR spectrum of bismuth vanadate, a stable semiconductor having a bandgap of 2.4 eV, which facilitates visible light activity. This photocatalyst has two phases, monoclinic and tetragonal which differ in their activity and is also known to show faceting-related activity, connected with selective accumulation of charge. Clear differences in the relaxation time of the 740 cm-1, representing the V-O between various species were observed, demonstrating the potential of this technique.