Haneul Jin

Infrared probing of equilibrium and dynamics of metal-selenocyanate ion pairs in N,N-dimethylformamide solutions.

Spectroscopic properties (i.e., peak positions and widths) of vibrational probes are sensitively dependent on their local environments in liquids. Such spectroscopic sensitivities can be utilized for studying the structures and dynamics of a variety of molecular systems. Here, we have studied the ion pairing equilibrium and dynamics of SeCN(-) ion pairs with Li(+) and Mg(2+) cations in N,N-dimethylformamide (DMF). SeCN(-) ion is an excellent vibrational probe for studying ion dynamics in electrolyte solutions, not only because the vibrational lifetime of the CN stretch is substantially long but also because the CN stretch frequency is very sensitive to its local environment. When SeCN(-) ion forms contact ion pairs (CIPs) with Li(+) (Mg(2+)) ion in DMF solutions, the CN stretch frequency is found to be significantly blue-shifted such that free SeCN(-) ion is spectrally well distinguished from Li-SeCN CIP and Mg-SeCN(+) CIP. This fact allows us to study the ion pairing equilibrium between SeCN(-) ion and metal ions as well as the dynamics of metal-SeCN(-) ion pairs. Ion pairing equilibrium between SeCN(-) ion and Li(+) (or Mg(2+)) was studied by temperature-dependent Fourier transform infrared (FTIR) spectroscopy. The formation of CIPs in DMF was found to be entropically favored. Time-resolved IR pump-probe spectroscopy was used to study the vibrational population relaxation and orientational relaxation dynamics. Vibrational lifetimes of free SeCN(-) ion, Li-SeCN CIP, and Mg-SeCN(+) CIP were determined to be 83.6, 72.3, and 55.6 ps, respectively. Orientational relaxation dynamics were found to get slower in the order free SeCN(-) ion, Li-SeCN CIP, and Mg-SeCN(+) CIP. The orientational anisotropy decays of the CIPs, which were well fit by a biexponential function, were explained by two orientational relaxation processes, that is, a restricted (tethered) orientational relaxation of SeCN(-) within the CIPs followed by the overall orientational diffusion of the whole CIPs. The orientational relaxation time constants of Li-SeCN CIP and Mg-SeCN(+) CIP in DMF were twice different but the orientational diffusion radii calculated by the Debye-Stokes-Einstein equation were found to be almost identical within experimental error. The biexponential decay of the orientational anisotropy was analyzed by the wobbling-in-a-cone model. As a vibrational probe, SeCN(-) ion and SeCN group can be potentially used for measuring the molecular dynamics on a relatively long time scale because of their long lifetimes.

Scalable synthesis of djurleite copper sulphide (Cu1.94S) hexagonal nanoplates from a single precursor copper thiocyanate and their photothermal properties

Copper sulphide materials have received great attention due to their low bandgap semiconducting properties. As compared to other chalcogenides, few synthetic examples have been reported, and a simple and scalable synthetic method for preparing size- and shape-controlled copper sulphide nanoparticles is required for potential wide application of these materials. Herein, a facile one pot scalable synthetic route has been developed for preparing highly monodisperse djurleite Cu1.94S hexagonal nanoplates. The thermal decomposition of a single precursor CuSCN was found suitable for preparing a large quantity of highly monodisperse Cu1.94S hexagonal nanoplates; a multi-gram scale product could be obtained in a single step. Under the synthetic scheme developed, the width of Cu1.94S nanoplates with a thickness of ~ 10 nm could be easily tuned from 70 nm to 130 nm. Their optical properties were investigated and their photothermal effect was also studied by photothermal optical coherence reflectometry (PT OCR). Cu1.94S hexagonal nanoplates showed a considerable photothermal effect, which was found to depend on the nanoparticle concentration.

A facet-controlled Rh3Pb2S2 nanocage as an efficient and robust electrocatalyst toward the hydrogen evolution reaction

Highly active and durable electrocatalysts for the hydrogen evolution reaction (HER) may play a pivotal role in commercial success of electrolytic water splitting technology. Among various material classes, binary metal sulphides show a great promise as HER catalysts because of their tunable energy levels conducive to a high catalytic activity and high robustness under harsh operating conditions. On the other hand, facet-controlled nanoparticles with controlled surface energies have gained great recent popularity as active and selective catalysts. However, binary metal sulphide nanoparticles with well-defined facets and high surface areas are very rare. Herein we report the synthesis of a facet-controlled hollow Rh3Pb2S2 nanocage as a new catalytic material and its excellent activity (overpotential: 87.3 mV at 10 mA cm-2) and robustness toward HER under harsh acidic conditions.

One step synthesis of hierarchical dendritic Pt nanostructures with a concave Pt octahedron building unit via simultaneous vertex growth and facet etching

Hierarchical dendritic nanostructures are rare and are usually synthesized by seed mediated epitaxial growth, which requires understanding of the symmetry-directed growth of the epitaxial layer on the facet-controlled nanocrysalline seed. Such a synthetic protocol involves alternation of the reaction conditions, and the one step synthesis of a hierarchical dendritic structure with exact mother to daughter crystal replication is a rarity. Herein we report a convenient one step synthesis of hierarchical dendritic Pt nanostructures with a concave Pt octahedron building unit by adopting reaction conditions in which Pt nanocrystal growth is extremely slow due to the reduction rate slowing the facet-etching process under an oxidizing atmosphere.

An IrRu alloy nanocactus on Cu2−xS@IrSy as a highly efficient bifunctional electrocatalyst toward overall water splitting in acidic electrolytes

Development of highly active and durable bifunctional electrocatalysts for overall water splitting is vital for the economical production of H2 as an alternative energy source. Herein, we report the synthesis of Cu2−xS@IrSy@IrRu nanoparticles (CIS@IrRu NPs), which show excellent catalytic performances for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an acidic electrolyte. Benefiting from the optimal composition of IrRu and the stable IrSy shell, the cactus-like IrRu NPs show high electrocatalytic activity and stability. The cactus-like IrRu NPs exhibit optimal HER and OER performances and high stability at a ratio of Ir/Ru 1.00 : 1.07. In overall water splitting, the CIS@Ir48Ru52 NPs achieve a current density of 10 mA cm−2 at a cell voltage of only 1.47 V in 0.1 M HClO4 electrolyte and show negligible degradation after 100 h of continuous operation in the stability test.