6 research outputs found
Aggregation-Induced Expansion of Poly‑(<i>N</i>‑isopropyl acrylamide) Solutions Observed Directly by the Transient Grating Imaging Technique
The anomalous volume expansion of
poly-(<i>N</i>-isopropyl
acrylamide) (PNIPAM) solutions was observed during the thermally induced
polymer phase transition of aqueous solutions having concentrations
in the 3–7 wt % range. The process occurred on a millisecond
time scale, and a laser temperature-jump time-resolved technique was
used to bring about the process. After heating a solution with a pulse
laser exploiting light absorption by dyes added to the solution itself,
a phase transition was observed to take place, and the temporal changes
associated with it were visualized through the transient grating imaging
technique, whereby the solution was heated with a stripe pattern.
We found several processes occurring on a millisecond time scale,
all of which clearly took place after each PNIPAM molecule had collapsed
structurally from a coiled to a globular conformation. During the
so-called demixing process, the globular polymers aggregated with
each other within 10 ms, and suddenly the polymer phase expanded as
aggregation progressed further. After this process, the individual
globular polymers reverted to their coiled conformation via hydration
during the remixing process. We proposed that solution expansion
was caused by the mutual entangling of multiple globular PNIPAM molecules,
instead each globule polymer was separated
Aggregation-Induced Expansion of Poly‑(<i>N</i>‑isopropyl acrylamide) Solutions Observed Directly by the Transient Grating Imaging Technique
The anomalous volume expansion of
poly-(<i>N</i>-isopropyl
acrylamide) (PNIPAM) solutions was observed during the thermally induced
polymer phase transition of aqueous solutions having concentrations
in the 3–7 wt % range. The process occurred on a millisecond
time scale, and a laser temperature-jump time-resolved technique was
used to bring about the process. After heating a solution with a pulse
laser exploiting light absorption by dyes added to the solution itself,
a phase transition was observed to take place, and the temporal changes
associated with it were visualized through the transient grating imaging
technique, whereby the solution was heated with a stripe pattern.
We found several processes occurring on a millisecond time scale,
all of which clearly took place after each PNIPAM molecule had collapsed
structurally from a coiled to a globular conformation. During the
so-called demixing process, the globular polymers aggregated with
each other within 10 ms, and suddenly the polymer phase expanded as
aggregation progressed further. After this process, the individual
globular polymers reverted to their coiled conformation via hydration
during the remixing process. We proposed that solution expansion
was caused by the mutual entangling of multiple globular PNIPAM molecules,
instead each globule polymer was separated
Investigation of Photoexcited Carrier Dynamics in Hematite and the Effect of Surface Modifications by an Advanced Transient Grating Technique
Photoexcited carrier
dynamics in a hematite film with and without
amorphous NiFeO<i><sub>x</sub></i> on the surface was investigated
using the heterodyne transient grating method. We found that two different
electron/hole dynamics took place in the micro- and millisecond time
regions and successfully assigned each component to the decay processes
of electrons and holes trapped at surface states, respectively. It
was also demonstrated that the amorphous NiFeO<i><sub>x</sub></i> coating plays a crucial role in increasing the survival
of the holes at the surface trap states, which was caused by the decrease
in the surface recombination rate
Dependences of the Optical Absorption, Ground State Energy Level, and Interfacial Electron Transfer Dynamics on the Size of CdSe Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Single Crystal Rutile TiO<sub>2</sub>
Quantum
dots (QDs) provide an attractive alternative sensitizer
to organic dyes. However, there have been few reports on QD-sensitized
solar cells (QDSCs) that have photovoltaic conversion efficiencies
exceeding those of dye-sensitized solar cells. This is because of
the lack of fundamental studies of QDs on conventional nanocrystalline
metal oxide electrodes which possess much amount of heterogeneity.
An important first step is an investigation of the dependences of
the optical absorption, the ground state energy level, and the interfacial
electron transfer (IET) on the size of QDs deposited on well characterized
single crystal oxides. The present study focuses on a system of CdSe
QDs adsorbed on the (001), (110), and (111) surfaces of single crystal
rutile-TiO<sub>2</sub>. The optical absorption spectra, characterized
using photoacoustic spectroscopy, were found to be independent of
the surface orientation concerning the optical absorption edge. The
exponential optical absorption tail (Urbach tail) suggests that the
disorder decreases with the increasing size of the QDs and is independent
of the surface orientation. The ground state energy levels of the
QDs were characterized using photoelectron yield spectroscopy. That
on the (001) surface shifts upward, while that on the (110) surface
shifts downward with increasing QD size. That on the (111) surface
is independent of the QD size, indicating the difference of the influence
of the surface orientation on adsorption of the QDs. The IET rate
constant and the relaxation component were characterized. The IET
rate constant was found to decrease as the size of the QDs increases
and depends on the surface orientation, indicating the differences
in the decrease of the free energy change and lower coupling between
the excited state of CdSe QDs and the Ti 3d orbitals in rutile-TiO<sub>2</sub>. The relaxation component increases with increasing QD size
and depends on the surface orientation, correlating with the density
of states in the conduction band of rutile-TiO<sub>2</sub>
Elucidating the Role of Surface Energetics on Charge Separation during Photoelectrochemical Water Splitting
Efficient photoelectrochemical (PEC) water splitting
requires charge
separation and extraction from a photoactive semiconductor. Such a
charge transport process is widely believed to be dictated by the
bulk energetics of the semiconductor. However, its dependence on surface
energetics along the semiconductor/electrolyte interface remains an
open question. Here, we elucidate the influence of surface energetics
on the performance of a well-established Mo-doped BiVO4 photoanode whose surface energetics are regulated by the facet-selective
cocatalyst loading. Surprisingly, photodeposition of RhOx and CoOx cocatalysts
onto the {010} and {110} facets, respectively, strongly enhanced the
charge-separation efficiency, in addition to improving the injection
efficiency for water oxidation. Detailed optoelectrical simulations
confirm that the synergistic enhancement of charge separation originates
from the distinct effects of the cocatalyst loading on the surface
energetics. This insight into the fundamental charge-separation mechanism
in PEC cells provides a perspective for cell design and operation
Air Stable PbSe Colloidal Quantum Dot Heterojunction Solar Cells: Ligand-Dependent Exciton Dissociation, Recombination, Photovoltaic Property, and Stability
We
fabricated the long-term air stable PbSe colloidal quantum dots
(CQDs) based planar heterojunction solar cells (FTO/TiO<sub>2</sub>/PbSe/Au) with relatively larger active area (0.25 cm<sup>2</sup>) using tetrabutylammonium iodide (TBAI, I<sup>–</sup>) as
ligand in solid state ligand-exchange process. For the first time,
we have achieved the whole preparation process of the device in the
ambient atmosphere from PbSe CQDs collection to PbSe colloidal quantum
dot solar cells (CQDSCs) fabrication, then storage and in their following
measurements. Especially, TBAI-treated PbSe CQDSCs exhibited a power
conversion efficiency (PCE) of 3.53% under AM 1.5 G in air, and also
a remarkable long-term stability (more than 90 days) of their storage
in ambient atmosphere has been identified. By contrast, 1,2-ethanedithiol
(EDT), 3-mercaptopropionic acid (MPA) and cetyltrimethylammonium bromide
(CTAB, Br<sup>–</sup>) treated PbSe CQDSCs were further studied.
The ligand-dependent exciton dissociation, recombination, energy level
shift, and air stability of PbSe CQDs treated with these different
ligands were systematically investigated. It was noted that TBAI-treated
PbSe CQDSCs exhibited suppressed recombination, faster charge transfer
rate, and longer carrier lifetimes, which resulted in a higher PCE
and long-term air stability