2 research outputs found
Unraveling the Carrier Dynamics of BiVO<sub>4</sub>: A Femtosecond to Microsecond Transient Absorption Study
Bismuth vanadate (BiVO<sub>4</sub>) is a promising semiconductor material for photoelectrochemical
water splitting showing good visible light absorption and a high photochemical
stability. To improve the performance of BiVO<sub>4</sub>, it is of
key importance to understand its photophysics upon light absorption.
Here we study the carrier dynamics of BiVO<sub>4</sub> prepared by
the spray pyrolysis method using broadband transient absorption spectroscopy
(TAS), in thin films as well as in a photoelectrochemical (PEC) cell
under water-splitting conditions. The use of a dual-laser setup consisting
of electronically synchronized Ti:sapphire amplifiers enable us to
measure the femtosecond to microsecond time scales in a single experiment.
On the basis of this data, we propose a model of carrier dynamics
that includes relaxation and trapping rates for electrons and holes.
Hole trapping occurs in multiple phases, with the majority of the
photogenerated holes being trapped with a time constant of 5 ps and
a small fraction of this hole trapping taking place within the instrument
response of 120 fs. The induced absorption band that represents the
trapped holes is modulated by an oscillation of 63 cm<sup>–1</sup>, which is assigned to the coupling of holes to a phonon mode. We
find electrons to undergo a relaxation with a time constant of 40
ps, followed by deeper trapping on the 2.5 ns time scale. On time
scales longer than 10 ns, trap-limited recombination that follows
a power law is found, spanning time scales up to microseconds. Finally,
we observe no spectral or kinetic differences by applying a bias voltage
to the PEC cell, indicating that the effect of a voltage and the charge
transfer processes between BiVO<sub>4</sub> and the electrolyte occurs
on longer time scales. Our results therefore provide new insights
into the carrier dynamics of BiVO<sub>4</sub> and further expand the
application window of TAS as an analytical tool for photoanode materials
Spectroscopic Analysis of a Biomimetic Model of Tyr<sub>Z</sub> Function in PSII
Using
natural photosynthesis as a model, bio-inspired constructs
for fuel generation from sunlight are being developed. Here we report
the synthesis and time-resolved spectroscopic analysis of a molecular
triad in which a porphyrin electron donor is covalently linked to
both a cyanoporphyrin electron acceptor and a benzimidazole–phenol
model for the Tyr<sub>Z</sub>-D<sub>1</sub>His190 pair of PSII. A
dual-laser setup enabled us to record the ultrafast kinetics and long-living
species in a single experiment. From this data, the photophysical
relaxation pathways were elucidated for the triad and reference compounds.
For the triad, quenching of the cyanoporphyrin singlet excited state
lifetime was interpreted as photoinduced electron transfer from the
porphyrin to the excited cyanoporphyrin. In contrast to a previous
study of a related molecule, we were unable to observe subsequent
formation of a long-lived charge separated state involving the benzimidazole–phenol
moiety. The lack of detection of a long-lived charge separated state
is attributed to a change in energetic landscape for charge separation/recombination
due to small differences in structure and solvation of the new triad