Determination of molecular structures of two dimensional (2D) organic-inorganic hybrid perovskite (OIHPs) nanocrystals at the single-nanocrystal and ensemble levels is essential to understanding the mechanisms responsible for their size-dependent optoelectronic properties and the nanocrystal assembling process, but its detection is still a bit challenge. In this study, we demonstrate for the first time that femtosecond sum frequency generation vibrational spectroscopy can provide a highly sensitive tool for probing the molecular structures of nanocrystals with a size comparable to the Bohr diameter (~10 nm) at the single-nanocrystal level. The SFG signals are monitored using the spectral features of the phenyl group in (R-MBA)2PbBr4 and (R-MBA)2PbI4 nanocrystals (MBA: methyl-benzyl-ammonium). It is found that the SFG spectra exhibit a strong resonant peak at 3067(±3) cm-1 (ν2 mode) and a weak shoulder peak at 3045 (±4) cm-1 (ν7a mode) at the ensemble level, whereas a peak of the ν2 mode and a peak at 3025 (±3) cm-1 (ν20b mode) at the single-nanocrystal level. The nanocrystals at the single-nanocrystal level tend to lie down on the surface. They stand up as the ensemble number and the averaged sizes increase. This finding may provide valuable information on the structural origins for size-dependent photo-physical properties and photoluminescence blinking dynamics in nanocrystals.
In a previous work [{\it J. Chem. Phys.} {\bf 140}, 174105 (2014)], we have shown that a mixed quantum classical (MQC) rate theory can be derived to investigate the quantum tunneling effects in the proton transfer reactions. However, the method is based on the high temperature approximation of the hierarchical equation of motion (HEOM) with the Debye-Drude spectral density, and results in a multi-state Zusman type of equation. We now extend this theory to include quantum effects of the bath degrees of freedom. By writing the full HEOM into a multidimensional partial differential equation in phase space, we can define a new reaction coordinate, and the previous method can be generalized to the full quantum regime. The validity of the new method is demonstrated by using numerical examples including the spin-Boson model, and the double well model for proton transfer reaction. The new method is found to resolve some key problems of the previous theory based on high temperature approximation, including possible numerical instability in long time simulation and wrong rate constant at low temperatures.
The commonly used oxide-supported metal catalysts are usually prepared in aqueous phase, which then often need to undergo calcination before usage. Therefore, the surface hydration and dehydration of oxide supports are critical for the realistic modeling of supported metal catalysts. In this work, by ab initio molecular dynamics (AIMD) simulations, the initial anhydrous monoclinic ZrO2 (-111) surfaces (or formally written as ("1" ̅"11" )) are evaluated within explicit solvents in aqueous phase at mild temperatures. During the simulations, all the two-fold-coordinated O sites will soon be protonated to form the acidic hydroxyls (OLH), remaining the basic hydroxyls (HO*) on Zr. The basic hydroxyls (HO*) can easily diffuse on surfaces via the active proton exchange with the undissociated adsorption waters (H2O*). Within the temperatures ranging from 273K to 373K, in aqueous phase a certain representative equilibrium hydrated m-ZrO2 (-111) surface is obtained with the coverage (θ) of 0.75 on surface Zr atoms. Later, the free energies on the stepwise surface water desorption are calculated by density functional theory (DFT) to mimic the surface dehydration under the mild calcination temperatures lower than 800K. By obtaining the phase diagrams of surface dehydration, the representative partially hydrated m-ZrO2 (-111) surfaces (0.25≤θ<0.75) at various calcination temperatures are illustrated. These hydrated m-ZrO2 (-111) surfaces can be crucial and readily applied for the more realistic modeling of ZrO2 catalysts and ZrO2-supported metal catalysts.
A better understanding of the photophysical processes occurring within organic semiconductors is important for designing and fabricating organic solar cells (OSCs). In this paper, we investigated the triplet‒triplet annihilation process in CuPc thin films with different molecular stacking configurations. The ultrafast transient absorption measurements indicate that the primary annihilation mechanism is one-dimensional exciton diffusion collision destruction. The decay kinetics shows a clearly time-dependent annihilation rate constant with γ ∝ t−1/2. Annihilation rate constants were determined to be γ0 = (2.87 ± 0.02) × 10−20 and (1.42 ± 0.02) × 10−20 cm3·s−1/2 for upright and lying-down configurations, respectively. Compared to the CuPc thin film with an upright configuration, the thin film with a lying-down configuration shows a longer exciton lifetime and a higher absorbance, which are beneficial for OSCs. The results in this paper have important implications on the design and mechanistic understanding of organic optoelectronic devices.
Catalytic hydrolysis of ammonia borane for dehydrogenation is a promising way for generation and storage of hydrogen energy. Catalysts with reduced utilization of costly noble metals while high activity and stability are highly desired. Herein we show that the catalytic activity of the prototypical Pt/SiO2 catalysts towards ammonia borane hydrolysis could be significantly improved by the presence of a layer of Co(OH)2 beneath the supported Pt nanoparticles. By changing the Pt:Co molar ratio in the Pt-Co(OH)2/SiO2 catalysts, the hydrogen generation rates from ammonia borane hydrolysis show a volcano-type curve, with the maximum catalytic activity at the Pt:Co molar ratio of 1:11. The highest turnover frequency value of 829 molH2·molPt−1·min−1 at room temperature outperforms most of the reported Pt-based catalysts, and the apparent activation energy is drastically decreased to 36.2 kJ/mol from 61.6 kJ/mol for Pt/SiO2. The enhanced catalytic performance of Pt-Co(OH)2/SiO2 is attributed to the electrons donation from Co atoms on Co(OH)2 to Pt occurring at the metal-hydroxide interface, which is beneficial for optimizing the oxidation cleavage of the O−H bond of attacked H2O.
The vacuum ultraviolet (VUV) photodissociation of OCS via the F 31Π Rydberg states was investigated in the range of 134–140 nm, by means of the time-sliced velocity map ion imaging technique. The images of S (1D2) products from the CO (X1Σ+) + S (1D2) dissociation channel were acquired at five photolysis wavelengths, corresponding to a series of symmetric stretching vibrational excitations in OCS (F 31Π, v1=0-4). The total translational energy distributions, vibrational populations and angular distributions of CO (X1Σ+, v) coproducts were derived. The analysis of experimental results suggests that the excited OCS molecules dissociate to CO (X1Σ+) and S (1D2) products via non-adiabatic couplings between the upper F 31Π states and the lower-lying states both in the C∞v and Cs symmetry. Furthermore, strong wavelength dependent behavior has been observed: the greatly distinct vibrational populations and angular distributions of CO (X1Σ+, v) products from the lower (v1=0-2) and higher (v1=3,4) vibrational states of the excited OCS (F 31Π, v1) demonstrate that very different mechanisms are involved in the dissociation processes. This study provides evidence for the possible contribution of vibronic coupling and the crucial role of vibronic coupling on the VUV photodissociation dynamics.
A fundamental study on C?C coupling that is the crucial step in the Fischer-Tropsch synthesis (FTS) process to obtain multi-carbon products is of great importance to tailor catalysts and then guide a more promising pathway. It has been demonstrated that the coupling of CO with the metal carbide can represent the early stage in the FTS process, while the related mechanism is elusive. Herein, the reactions of the CuC3H– and CuC3– cluster anions with CO have been studied by using mass spectrometry and theoretical calculations. The experimental results showed that the coupling of CO with the C3H– moiety of CuC3H– can generate the exclusive ion product COC3H–. The reactivity and selectivity of this reaction are greatly higher than that on the reaction of CuC3– with CO, and this H-assisted C?C coupling process was rationalized by theoretical calculations.