Hailong Chen

Ion clustering in aqueous solutions probed with vibrational energy transfer

Despite prolonged scientific efforts to unravel the hydration structures of ions in water, many open questions remain, in particular concerning the existences and structures of ion clusters in 1∶1 strong electrolyte aqueous solutions. A combined ultrafast 2D IR and pump/probe study through vibrational energy transfers directly observes ion clustering in aqueous solutions of LiSCN, NaSCN, KSCN and CsSCN. In a near saturated KSCN aqueous solution (water/KSCN molar ratio = 2.4/1), 95% of the anions form ion clusters. Diluting the solution results in fewer, smaller, and tighter clusters. Cations have significant effects on cluster formation. A small cation results in smaller and fewer clusters. The vibrational energy transfer method holds promise for studying a wide variety of other fast short-range molecular interactions.

Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures

Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

Nonresonant and Resonant Mode-Specific Intermolecular Vibrational Energy Transfers in Electrolyte Aqueous Solutions

The donor/acceptor energy mismatch and vibrational coupling strength dependences of interionic vibrational energy transfer kinetics in electrolyte aqueous solutions were investigated with ultrafast multiple-dimensional vibrational spectroscopy. An analytical equation derived from the Fermis Golden rule that correlates molecular structural parameters and vibrational energy transfer kinetics was found to be able to describe the intermolecular mode specific vibrational energy transfer. Under the assumption of the dipole-dipole approximation, the distance between anions in the aqueous solutions was obtained from the vibrational energy transfer measurements, confirmed with measurements on the corresponding crystalline samples. The result demonstrates that the mode-specific vibrational energy transfer method holds promise as an angstrom molecular ruler.

Selective Hydrogen Generation from Formic Acid with Well-Defined Complexes of Ruthenium and Phosphorus–Nitrogen PN3-Pincer Ligand

An unsymmetrically protonated PN(3) -pincer complex in which ruthenium is coordinated by one nitrogen and two phosphorus atoms was employed for the selective generation of hydrogen from formic acid. Mechanistic studies suggest that the imine arm participates in the formic acid activation/deprotonation step. A long life time of 150 h with a turnover number over 1 million was achieved.

Vacuum ultraviolet spectroscopic properties of rare earth (RE=Ce,Tb,Eu,Tm,Sm)-doped hexagonal KCaGd(PO4)2 phosphate

Hexagonal KCaGd(PO4)2:RE3+ (RE=Ce,Tb,Eu,Tm,Sm) were synthesized by coprecipitation method and their vacuum ultraviolet–ultraviolet (VUV-UV) spectroscopic properties were investigated. The bands at about 165nm in the VUV excitation spectra are attributed to the host lattice absorptions. For Ce3+-doped samples, the bands at 207, 256, 275, and 320nm are assigned to the 4f-5d transitions of Ce3+ in KCaGd(PO4)2. For Tb3+-doped sample, the bands at 203 and 222nm are related to the 4f-5d spin-allowed transitions. For Eu3+-doped sample, the O2−–Eu3+ charge-transfer band (CTB) at 229nm is observed, and the fine emission spectrum of Eu3+ indicates that Eu3+ ions prefer to occupy Gd3+ or Ca2+ sites in the host lattice. For Tm3+- and Sm3+-doped samples, the O2−–Tm3+ and O2−–Sm3+ CTBs are observed to be at 176 and 186nm, respectively. From the standpoints of the absorption band, color purity, and luminescent intensity, Tb3+-doped KCaGd(PO4)2 is a potential candidate for 172nm excited green plasma display phosphors.

Ion Segregation in Aqueous Solutions

Microscopic structures and dynamics of aqueous salt solutions were investigated with the ultrafast vibrational energy exchange method and anisotropy measurements. In KSCN aqueous solutions of various concentrations, the rotational time constants of SCN(-) anions are proportional to the viscosities of the solutions. However, the reorientation dynamics of the water molecules are only slightly affected by the solution viscosity. With the addition of strongly hydrated F(-) anions, the rotations of both SCN(-) anions and water molecules slow down. With the addition of weakly hydrated I(-) anions, only the rotation of SCN(-) anions slows down with that of water molecules unaffected. Vibrational energy exchange measurements show that the separation among SCN(-) anions decreases with the addition of F(-) and increases with the addition of I(-). The series of experiments clearly demonstrate that both structures and dynamics of ion and water are segregated in the strong electrolyte aqueous solutions.

Thermoelectric properties of YbxEu1−xCd2Sb2

The thermoelectric performance of EuCd(2)Sb(2) and YbCd(2)Sb(2) was improved by mixed cation occupation. The composition, structure, and thermoelectric properties of Yb(x)Eu(1-x)Cd(2)Sb(2) (x=0, 0.5, 0.75, and 1) have been investigated. Polycrystalline samples are prepared by direct reaction of the elements. Thermoelectric properties were investigated after densification of the materials by spark plasma sintering. Yb(x)Eu(1-x)Cd(2)Sb(2) crystallizes in the P3m1 space group. The lattice parameters increase with the europium content. These materials show low electrical resistivity, high Seebeck coefficient, and low thermal conductivity together with high carrier concentration and high carrier mobility. ZT values of 0.88 and 0.97 are obtained for Yb(0.5)Eu(0.5)Cd(2)Sb(2) and Yb(0.75)Eu(0.25)Cd(2)Sb(2) at 650 K, respectively.

Cation Effects on Rotational Dynamics of Anions and Water Molecules in Alkali (Li+, Na+, K+, Cs+) Thiocyanate (SCN-) Aqueous Solutions

Waiting time dependent rotational anisotropies of SCN(-) anions and water molecules in alkali thiocyanate (XSCN, X = Li, Na, K, Cs) aqueous solutions at various concentrations were measured with ultrafast infrared spectroscopy. It was found that cations can significantly affect the reorientational motions of both water molecules and SCN(-) anions. The dynamics are slower in a solution with a smaller cation. The reorientational time constants follow the order of Li(+) > Na(+) > K(+) ~/= Cs(+). The changes of rotational time constants of SCN(-) at various concentrations scale almost linearly with the changes of solution viscosity, but those of water molecules do not. In addition, the concentration-dependent amplitudes of dynamical changes are much more significant in the Li(+) and Na(+) solutions than those in the K(+) and Cs(+) solutions. Further investigations on the systems with the ultrafast vibrational energy exchange method and molecular dynamics simulations provide an explanation for the observations: the observed rotational dynamics are the balanced results of ion clustering and cation/anion/water direct interactions. In all the solutions at high concentrations (>5 M), substantial amounts of ions form clusters. The structural inhomogeneity in the solutions leads to distinct rotational dynamics of water and anions. The strong interactions of Li(+) and Na(+) because of their relatively large charge densities with water molecules and SCN(-) anions, in addition to the likely geometric confinements because of ion clustering, substantially slow down the rotations of SCN(-) anions and water molecules inside the ion clusters. The interactions of K(+) and Cs(+) with water or SCN(-) are much weaker. The rotations of water molecules inside ion clusters of K(+) and Cs(+) solutions are not significantly different from those of other water species so that the experimentally observed rotational relaxation dynamics are only slightly affected by the ion concentrations.

Ultrafast multiple-mode multiple-dimensional vibrational spectroscopy

Ultrafast multiple-dimensional vibrational spectroscopy has been extensively applied to studies of molecular structures and dynamics in condensed phases. Along with the developments of new laser sources and new concepts, increasing improvements and applications of this technique have brought the understanding of molecular systems to a new level. In this review, we first briefly introduce the basic concepts, experimental setups and applications of the technique. The most recent progresses in applying vibrational energy transfers to determine intermolecular distances and vibrational couplings to determine three dimensional molecular conformations with our high power multiple-mode multiple-dimensional vibrational spectroscopy are then introduced in more details.

Coordination Number of Li+ in Nonaqueous Electrolyte Solutions Determined by Molecular Rotational Measurements

The coordination number of Li(+) in acetonitrile solutions was determined by directly measuring the rotational times of solvent molecules bound and unbound to it. The CN stretch of the Li(+) bound and unbound acetonitrile molecules in the same solution has distinct vibrational frequencies (2276 cm(-1) vs 2254 cm(-1)). The frequency difference allows the rotation of each type of acetonitrile molecule to be determined by monitoring the anisotropy decay of each CN stretch vibrational excitation signal. Regardless of the nature of anions and concentrations, the Li(+) coordination number was found to be 4-6 in the LiBF4 (0.2-2 M) and LiPF6 (1-2 M) acetonitrile solutions. However, the dissociation constants of the salt are dependent on the nature of anions. In 1 M LiBF4 solution, 53% of the salt was found to dissociate into Li(+), which is bound by 4-6 solvent molecules. In 1 M LiPF6 solution, 72% of the salt dissociates. 2D IR experiments show that the binding between Li(+) and acetonitrile is very strong. The lifetime of the complex is much longer than 19 ps.