Quantifying Bulk and Surface Recombination Processes in Nanostructured Water Splitting Photocatalysts via In Situ Ultrafast Spectroscopy

A quantitative description of recombination processes in nanostructured semiconductor photocatalystsone that distinguishes between bulk (charge transport) and surface (chemical reaction) lossesis critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface. This is achieved by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode in a working photoelectrochemical cell. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications that are not perceivable by conventional X-ray diffraction can drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast time scales can limit the overall efficiency even in nanostructures with dimensions smaller than the minority carrier diffusion length. This is particularly true for materials with high carrier concentration, where losses as high as 37% are observed. Our methodology provides a means of evaluating the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity

Plasma-Corona-Processed Nanostructured Coating for Thermoregulative Textiles

A rapid increase in the atmospheric temperature has been reported in recent years worldwide. The lack of proper aid to protect from exposure to the sun during working hours has raised the number of sunburn cases among workers. It is important to promote productive workplaces without compromising safety and health concerns. In the present work, we report the low-temperature plasma (LTP)-assisted tailoring of the surface properties of fabrics to reflect IR radiation from the sun. The LTP technique can be adapted for thermally sensitive materials such as fabrics and textiles due to its lower working temperature range of 30 °C. We have modified various substrates such as commercially available fabric, regular, and boron nitride-incorporated electrospun PET surfaces with tetraethoxy orthosilicate (TEOS) plasma. TEOS plasma treatment can deposit a reactive plasma-polymerized silane nanolayer on the surface of these substrates. The plasma-processed silane nanolayer was systematically characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, Keyence 3D-microscopic imaging, and transmission electron microscopy (TEM). From the SEM and TEM data, the size of the nanoparticles was observed in the range 100–200 nm. The thermal regulation coating thickness was examined with a Keyence 3D imaging technique. The IR reflection potential of the surface was analyzed by using an FLIR thermal imaging system. The data revealed that the plasma-modeled nanosurface shows higher reflective potential toward IR rays, and it seems to be cooler than the unprocessed surface by approximately 15 °C. The stability and efficiency of the plasma-modified electrospun nanolayer in water were satisfactorily examined with SEM and IR imaging. Taken together, these results suggest the excellent potential of plasma processing to develop IR reflective coatings

Role of Defects in the Phase Transition of VO₂ Nanoparticles Probed by Plasmon Resonance Spectroscopy

Defects are known to affect nanoscale phase transitions, but their specific role in the metal-to-insulator transition in VO₂ has remained elusive. By combining plasmon resonance nanospectroscopy with density functional calculations, we correlate decreased phase-transition energy with oxygen vacancies created by strain at grain boundaries. By measuring the degree of metallization in the lithographically defined VO₂ nanoparticles, we find that hysteresis width narrows with increasing size, thus illustrating the potential for domain boundary engineering in phase-changing nanostructures

Exciton Correlations in Intramolecular Singlet Fission

We have synthesized a series of asymmetric pentacene–tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet–triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases