title: Potential-dependent recombination kinetics of photogenerated electrons in n- and p-type GaN photoelectrodes studied by time-resolved IR absorption spectroscopy.
abstract: Recombination kinetics of photogenerated electrons in n-type and p-type GaN photoelectrodes active for H(2) and O(2) evolution, respectively, from water was examined by time-resolved IR absorption (TR-IR) spectroscopy. Illumination of a GaN film with UV pulse (355 nm and 6 ns in duration) gives transient interference spectra in both transmittance and reflection modes. Simulation shows that the interference spectra are caused by photogenerated electrons. We observed that recombination in the microsecond region is greatly affected by the applied potentials, the lifetime becoming longer at negative and positive potentials for n- and p-type GaN electrodes, respectively. There is a good correlation between potential dependence of the steady-state reaction efficiency and that of the number of surviving electrons in the millisecond region. We also performed potential jump measurement to examine the shift in Fermi level by photogenerated charge carriers. In the case of n-type GaN, the electrode potential jumps to the negative side by accumulation of electrons in the bulk. However, in the case of p-type GaN, the electrode potential first jumps to the negative side within 20 μs and gradually shifts to the positive side in a few milliseconds, while the number of charge carriers is constant at >0.2 ms. This two-step process is ascribed to electron transport from the bulk to the surface of GaN, because the electrode potential is sensitive to the number of electrons in the bulk. The results confirm that TR-IR combined with potential jump measurement provides useful information for understanding the behavior of charge carriers in photoelectrochemical systems.
authors.affiliation: Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya, 468-8511, Japan. yamakata@toyota-ti.ac.jp
authors.forename: Akira
authors.initials: A
authors.lastname: Yamakata
authors.forename: Masaaki
authors.initials: M
authors.lastname: Yoshida
authors.forename: Jun
authors.initials: J
authors.lastname: Kubota
authors.forename: Masatoshi
authors.initials: M
authors.lastname: Osawa
authors.forename: Kazunari
authors.initials: K
authors.lastname: Domen