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>_x</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>_x</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 TiO₂

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-TiO₂. 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-TiO₂. 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-TiO₂

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/TiO₂/PbSe/Au) with relatively larger active area (0.25 cm²) using tetrabutylammonium iodide (TBAI, I^–) 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^–) 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