Specific counter-cation effect on the molecular orientation of thiocyanate anions at the aqueous solution interface

Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs2Pb(SCN)2I2(1-x)Br2x as a model, we have revealed how the orientation of SCN- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.

Aqueous Interfaces in Chemical Separations

Chemical separations play a vital role in refinery and reprocessing of critical materials, such as platinum group metals, rare earths, and actinides. The choice of separation system─whether it is liquid-liquid extraction (LLE), sorbents, or membranes─depends on specific needs and applications. In almost all separation processes, the desired metal ions adsorb or transfer across an aqueous interface, such as the solid/liquid interface in sorbents or oil/water interfaces in LLE. Despite these separation technologies being extensively used for decades, our understanding of the molecular-scale mechanisms governing ion adsorption and transport at interfaces remains limited. This knowledge gap presents a significant challenge in meeting the increasing demands for these critical materials due to their growing use in advanced technologies. Fortunately, recent advancements in surface-specific experimental and computational techniques offer promising avenues to bridge this gap and facilitate the development of next-generation separation systems. Interestingly, unanswered questions regarding interfacial phenomena in chemical separations hold great relevance to various fields, including energy storage, geochemistry, and atmospheric chemistry. Therefore, the model interfacial systems developed for studying chemical separations, such as amphiphilic molecules assembled at a solid/water, air/water, or oil/water interface, may have far-reaching implications, extending beyond separations and opening doors to addressing a wide range of scientific inquiries. This perspective discusses recent interfacial studies elucidating amphiphile-ion interactions in chemical separations of metal ions. These studies provide direct, molecular-scale information about solute and solvent behavior at aqueous interfaces, including multivalent and complex ions in highly concentrated solutions, which play key roles in LLE of critical materials.

Thiocyanate Ions Form Antiparallel Populations at the Concentrated Electrolyte/Charged Surfactant Interface

Anions play significant roles in the separation of lanthanides and actinides. The molecular-scale details of how these anions behave at aqueous interfaces are not well understood, especially at high ionic strengths. Here, we describe the interfacial structure of thiocyanate anions at a soft charged interface up to 5 M bulk concentration with combined classical and phase-sensitive vibrational sum frequency generation (PS-VSFG) spectroscopy and molecular dynamics (MD) simulations. At low concentrations thiocyanate ions are mostly oriented with their sulfur end pointing toward the charged surfactants. The VSFG signal reaches a plateau at around 100 mM bulk concentration, followed by significant changes above 1 M. At high concentrations a new thiocyanate population emerges with their sulfur end pointing toward the bulk liquid. The -CN stretch frequency is different for up and down oriented SCN^- ions, indicating different coordination environments. These results provide key molecular-level insights for the interfacial behavior of complex anions in highly concentrated solutions.

In Situ Raman Microdroplet Spectroelectrochemical Investigation of CuSCN Electrodeposited on Different Substrates

Systematic in situ Raman microdroplet spectroelectrochemical (Raman-μSEC) characterization of copper (I) thiocyanate (CuSCN) prepared using electrodeposition from aqueous solution on various substrates (carbon-based, F-doped SnO₂) is presented. CuSCN is a promising solid p-type inorganic semiconductor used in perovskite solar cells as a hole-transporting material. SEM characterization reveals that the CuSCN layers are homogenous with a thickness of ca. 550 nm. Raman spectra of dry CuSCN layers show that the SCN⁻ ion is predominantly bonded in the thiocyanate resonant form to copper through its S−end (Cu−S−C≡N). The double-layer capacitance of the CuSCN layers ranges from 0.3 mF/cm² on the boron-doped diamond to 0.8 mF/cm² on a glass-like carbon. In situ Raman-μSEC shows that, independently of the substrate type, all Raman vibrations from CuSCN and the substrate completely vanish in the potential range from 0 to −0.3 V vs. Ag/AgCl, caused by the formation of a passivation layer. At positive potentials (+0.5 V vs. Ag/AgCl), the bands corresponding to the CuSCN vibrations change their intensities compared to those in the as-prepared, dry layers. The changes concern mainly the Cu−SCN form, showing the dependence of the related vibrations on the substrate type and thus on the local environment modifying the delocalization on the Cu−S bond.