Atomistic Insight into Changes in the Vibrational Spectrum of Ionic Liquids under External Electric Field

Abstract

Vibrational spectroscopy is a powerful tool for studying the microstructure of liquids, and anatomizing the nature of the vibrational spectrum (VS) is promising for investigating changes in the properties of liquid structures under external conditions. In this study, molecular dynamics (MD) simulations have been performed to explore changes in the VS of 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) ionic liquid (IL) under an external electric field (EEF) ranging from 0 to 10 V•nm −1 at 350 K. First, the vibrational spectra for [Emim][PF6] IL as well as its cation and anion are separately obtained, and the peaks are strictly assigned. The results demonstrate that the VS calculated by MD simulation can well reproduce the main characteristic peaks in the experimentally measured spectrum. Then, the vibrational spectra of the IL under various EEFs from 0 to 10 V•nm −1 are investigated, and the intrinsic origin of the changes in the vibrational bands (VBs) at 50, 183, 3196, and 3396 cm −1 is analyzed. Our simulation results indicate that the intensities of the VBs at 50 and 183 cm −1 are enhanced. In addition, the VB at 50 cm −1 is redshifted by about 16 cm −1 as the EEF is varied from 0 to 2 V•nm −1 , and the redshift wavenumber increases to 33 cm −1 as the EEF is increased to 3 V•nm −1 and beyond. However, the intensities of the VBs at 3196 and 3396 cm −1 show an obvious decrease. Meanwhile, the VB at 3396 cm −1 is redshifted by about 16 cm −1 when the EEF increases to 3 V•nm −1 , and the redshift increases to 33 cm −1 with an increase in the EEF beyond 4 V•nm −1 . The intensity of the VB at 50 cm −1 increases because of the increase in the total dipole moment of each anion and cation (from 4.34 to 5.46 D), and the redshift is attributed to the decrease in the average interaction energy per ion pair (from −378.7 to −298.0 kJ•mol −1 ) with increasing EEF. The intensity of the VB at 183 cm −1 increases on account of the more consistent orientations for cations in the system with increasing EEF. The VB at 3196 cm −1 weakens visibly because a greater number of hydrogen atoms appear around the carbon atoms on the methyl/ethyl side chains and the vibrations of the corresponding carbon-hydrogen bonds are suppressed under the action of the EEF. Furthermore, the intensity of the VB at 3396 cm −1 decreases due to the decrease in the intermolecular + C-H···F − hydrogen bonds (HBs), while the relaxation effect that is beneficial for the formation of HBs simultaneously exists in the system under the varying EEF, thus causing a redshift of the VB at 3396 cm −1 .

The Collision between g‐C3N4 and QDs in the Fields of Energy and Environment: Synergistic Effects for Efficient Photocatalysis

“…Specifically, cocatalysts can be acted as electron carriers to accept electrons from photogenerated g-C 3 N 4 to their Fermi level, which inhibit the recombination of e − -h + pairs, and then provide abundant active sites to react with absorbed substance (Figure 8A). [25,131,132] Excitingly, some studies have shown that these QDs/g-C 3 N 4 might be a promising alternative to noble metal and avoid high cost. [42,69,97] For instance, Zeng et al adopted a facile strategy to synthesize CoP QDs/ultrafine g-C 3 N 4 fiber (CoP QDs/CNF) photocatalysts.…”

“…[43] The similar transition metal phosphates (like Ni 2 P) in composites with g-C 3 N 4 , efficiently inhibited the recombination of photogenerated e − -h + pairs, which endowed an excellent photocatalytic performance without noble metals. [25,42,52,69] Apart from CQDs or GQDs considered as cocatalysts to enhance charge transfer ability, researchers have taken advantage of their up-conversion properties, which stimulated composites to form more e − -h + pairs. [100,119,120] Recently, a breakthrough study has found that the more uniform and ordered combination of QDs and g-C 3 N 4 endowed better photocatalysis of the composite.…”

“…[49,56,65,70] Interestingly, some QDs/ g-C 3 N 4 with high conductivity can endow photocatalytic performance corresponding to noble metal/g-C 3 N 4 composites with high efficiency. [25,[41][42][43]52,69] The uniform load of QDs is also an effective way to promote H 2 production. [133] The general mechanism of H 2 production is shown in Figure 11D.…”

“…[18,[22][23][24] Since the photocatalytic H 2 evolution of g-C 3 N 4 was first reported in 2009, the research on improving the photocatalytic performance of g-C 3 N 4 -based photocatalysts has gradually become a popular research direction (Figure 2). [18,25,26] Pristine g-C 3 N 4 can be prepared by using the heat treatment of some low-cost nitrogen-rich organic precursors, such as urea, melamine, dicyandiamide, and so on. [27] However, their practical applications have been limited by several shortcomings of pristine g-C 3 N 4 , including low specific surface area, insufficient utilization of visible light (< 460 nm), and rapid recombination of photogenerated electron-hole (e − -h + ) pairs.…”

All-organic covalent organic frameworks/perylene diimide urea polymer S-scheme photocatalyst for boosted H2 generation

“…A promising solution for reducing growing CO2 levels while enhancing energy production is using visible light-driven water splitting for hydrogen generation [1][2][3][4]. Until now, a large number of molecules and inorganic, organic, and heterojunc-tion semiconductor photocatalytic systems have been designed for realizing photocatalytic hydrogen evolution [5][6][7][8][9][10][11][12][13][14][15][16][17]. For photocatalytic H2 evolution (PHE), covalent organic frameworks (COFs) with a large surface area, high crystallinity, and a distinct structure are considered viable alternatives to inorganic semiconductor photocatalysts [18][19][20][21][22].…”