Large Charge Separation Research Articles

Efficiently enhanced degradation for the organic pollutants over GO/BiOI/NiWO4 with Z-scheme heterojunction and DFT studies

In this work, a series GO/BiOI/NiWO4 (n%GBINWO, n=5, 10, 15, 20) were hydrothermally prepared and applied as photocatalysts for the degradation of organic dyes under ambient conditions. The physico-chemical and microstructure of the studied samples were characterized by XRD, SEM, TEM, TG-DTG, XPS, UV-Vis, EIS, and N2-adsorption. Compared with pure NiWO4, the photocatalytic activity of BiOI/NiWO4 and GO/BiOI/NiWO4 were obviously enhanced, and the highest photocatalytic efficiency of 10 %GBINWO was observed to be 98.59 % (methylene blue, MB) under the optimized conditions (catalyst weight of 30 mg, pH of 3, 3 h, 30 °C). The excellent catalytic performance of the 10 %GBINWO was due to the existence of interfacial heterojunction, which endows the ternary composite with the large visible light capacity and high charge separation efficiency. Importantly, after the hybrid model constructed with NiWO4 (-1 1 1) and BiOI (0 0 2) was optimized via DFT calculations, a newly-formed bond of I-Bi-O-M (Ni and W), which promoted the transportation of the photogenerated electrons through newly-formed bridge was observed. Furthermore, the difference in degradation efficiency of various organic pollutants was related to the steric hindrance as well as the molecular structure. Moreover, •O2− was also confirmed as the key active species for 10 %GBINWO catalyzing the degradation process. Besides, the Z-scheme heterojunction between BiOI and NiWO4 was confirmed by TD-DFT calculations.

Robust Red-Absorbing Donor-Acceptor Stenhouse Adduct Photoswitches.

Donor-Acceptor Stenhouse Adduct (DASA), a class of push-pull negative photochrome, has received large interest lately owing to its versatile synthesis, modularity and excellent photoswitching in solutions. From a technological perspective, it is imperative for this class of photoswitches to work robustly in solid state, e. g. thin films. We feature a molecular framework for the optimized design of DASAs by introducing a new thioindoline donor (D3) and assessing its performance against known 2nd generation indoline-based donors. The systematic structure-function investigations suggest that to achieve robust reversible photoswitching, a ground state with low charge separation is desired. DASAs with stronger electron donors and a larger charge separation in the ground state result in a low population of the photothermalstationary state (PTSS) and reduced photostability. The DASA with thioindoline donor (D3A3) seems to be a special case among the donor series as it causes a red shift (ca. 15 nm), however with less polarization of the ground state and marginally better photostability as compared to the unsubstituted 2-methyl indoline (D1A3). We also emphasize the consideration of the key additional factors that can modulate the red-light photoswitching properties of DASA chromophores in polymer thin films, which might not be dominant in homogenous solution state.

High-index facets exposed on metal–organic framework for boosting photocatalytic carbon dioxide reduction

Facets exposed on metal–organic frameworks (MOFs) are considered to be responsible for the efficiency of photocatalytic CO2 conversion. Here, NH2-MIL-125(Ti) with exposed high-index {1 1 2} facets has been investigated in detail for photocatalytic CO2 reduction. Compared with low-index facets, the high-index {1 1 2} facets exhibit superior activity for CO2-to-CO and CO2-to-CH4 conversions, which are 33 and 31 times higher for CO and CH4 productions than the {0 0 1}/{1 1 1} facets. The superior photocatalytic activity is attributed to the large CO2 uptake, efficient photoexcitation, charge separation and migration behaviors of the NH2-MIL-125(Ti) with exposing high-index facets. This study provides an effective strategy to develop MOFs catalysts via facet-engineering for targeted performance.

Interlayer Excitonic Spectra of Vertically Stacked MoSe2/WSe2 Heterobilayers

In article number 2000614, Roland Gillen addresses simulations of the optical spectra of bilayer MoSe2/WSe2 heterostructures using modern theoretical spectroscopy methods. Due to the complex electronic band structure, optical excitation of such heterostructures by incident light (top image on the cover) gives rise to a variety of excitonic resonances in the absorption spectra (middle panel). Here, ab initio simulations reveal contributions from excitons with large interlayer charge separation both below the absorption onset (left side of bottom panel) and as novel ‘interlayer C excitons’ within the band continuum at higher energies (right side of bottom panel).

Photoinduced Rotation of Colloidal Semiconductor Nanocrystals in an Electric Field.

We demonstrate that solution-phase semiconductor nanocrystals (NCs) undergo photoinduced rotation in an external electric field. Present measurements backed by theoretical calculations show that the rotation of colloidal NCs is driven by the excited-state dipole moment, which is counterbalanced by the solvent viscosity drag. Corresponding angular velocities range from 0.5°/ns for cubic CsPbBr3 NCs to 3°/ns for nanoparticles with a large photoinduced charge separation (CdSe/CdS core-shell and dot-in-a-rod NCs). Because of photoinduced rotation, solution-phase semiconductor NCs exhibited an order-of-magnitude increase in the spectral changes caused by the quantum confined Stark effect (QCSE), compared to solid NC assemblies. The enhanced QCSE of colloidal NCs reflected their global alignment in solution, which could be retained in a solid environment by slow crystallization. Overall, we expect that the demonstrated phenomenon of the colloidal nanocrystal rotation in an electric field will open up new avenues for developing electro-optical and voltage-sensitive applications.

Bubbles to Chondrites-II. Chemical fractionations in chondrites

We attempt to develop a possible theory of chemical fractionations in chondrites, that is consistent with various features of chondritic components and current observation of protoplanetary disks (PPD). Combining the 3+2 component fitting calculation that simulates chondrule formation process proposed in paper (I) with additional mixing procedures, we investigate essential causes that made various types of chondrites evolve from the uniform solar system composition, the CI-chondritic composition. Seven chemical types of chondrites (CM, CV, CO, E, LL, L and H) are examined, for which reliable chemical compositions for both bulk chondrites and chondrules therein are known. High vaporization degree of the primordial dust aggregates (dustons) required by the calculation vindicates that the chondrule formation was the driving force for the chemical fractionations in all chondrites examined. Various initial redox states in dustons and different timings of CAIs’ invasion to the chondrule formation zone are identified for different chondrite types. These results, together with a good correlation with the D/H ratios of chondrites measured previously, lead us to the notion that PPD evolved from reducing to oxidizing. We explore the heating mechanism for the chondrule formation and the place it occurred. Only heat source being consistent with our chondrule formation model is lightning discharge. We postulate that large vortices encompassing the snow-line are ideal places for large charge separation to occur between dustons and small ice particles, and that direct strikes on dustons should make them boil for ten seconds and longer and allow a swarm of chondrules released from their surfaces. Chemical fractionations are completed by an aerodynamic separation of dustons from chondrules inside the vortex, in such a way that the dustons fall fast into the vortex center and form a planetesimal immediately, while chondrules with dust mantles fall slow and form a thin veneer on the planetesimal surface. During collisional episodes, the veneers are preferentially fragmented and reassemble themselves by a weak self-gravity to form a rubble-piled chondritic asteroid, i.e. chondrite.

Indium sulfide-based photocatalysts for hydrogen production and water cleaning: a review

Solar illumination is a promising source of primary energy to reduce global warming and to clean polluted waters, thus fostering research of the design of efficient photocatalysts for hydrogen production by water splitting and for contaminant degradation. In particular, photocatalysis by indium sulfide (In2S3) is drawing attention due to its suitable narrow bandgap of 2.0–2.3 eV for visible light harnessing, yet large-scale application of unmodified In2S3 is limited. Here we review the photocatalyst criteria for water splitting, the synthesis and morphological manipulations of In2S3, the synthesis of heterojunctions by coupling semiconductors to increase performance, and doping In2S3. In2S3-based heterojunctions, i.e., traditional type II, all-solid-state, and direct Z-scheme photocatalytic systems show benefits such as larger charge separation, broad solar spectrum absorption, and amended conduction band and valence band edge potentials for maximum pollutant removal and H2 production. The effect of dopant incorporation on electronic modulations of In2S3 is explained by the density functional theory.

Recent Developments of Advanced Ti3+-Self-Doped TiO2 for Efficient Visible-Light-Driven Photocatalysis

Research into the development of efficient semiconductor photocatalytic materials is a promising approach to solving environmental and energy problems worldwide. Among these materials, TiO2 photocatalysts are one of the most commonly used due to their efficient photoactivity, high stability, low cost and environmental friendliness. However, since the UV content of sunlight is less than 5%, the development of visible light-activated TiO2-based photocatalysts is essential to increase the solar energy efficiency. Here, we review recent works on advanced visible light-activated Ti3+-self-doped TiO2 (Ti3+–TiO2) photocatalysts with improved electronic band structures for efficient charge separation. We analyze the different methods used to produce Ti3+–TiO2 photocatalysts, where Ti3+ with a high oxygen defect density can be used for energy production from visible light. We categorize advanced modifications in electronic states of Ti3+–TiO2 by improving their photocatalytic activity. Ti3+–TiO2 photocatalysts with large charge separation and low recombination of photogenerated electrons and holes can be practically applied for energy conversion and advanced oxidation processes in natural environments and deserve significant attention.

Highly Improved Performance of Cotton Air Filters in Particulate Matter Removal by the Incorporation of Metal-Organic Frameworks with Functional Groups Capable of Large Charge Separation.

Currently, air contamination, especially with particulate matters (PMs), is severe in several countries. To increase the efficiency of air filters in PM removal, metal-organic frameworks (MOFs, here, Zr-MOFs, especially with functional groups (FGs) such as -NO2) were coated, after synthesis, onto cotton using covalent bonding for the first time. The removal efficiencies (REs) and quality factors (QFs) of cottons with or without MOFs were in the order: cotton < Zr-MOF/cotton < Zr-MOF-NH2/cotton < Zr-MOF-NH-SO3H/cotton < Zr-MOF-NH3+Cl-/cotton < Zr-MOF-NO2/cotton. This monotonic increase in the PM removal efficiency or QF could be explained with the order of charge separation or developed charges (total, in absolute value: ∼0 to 2.0) on FGs of MOFs. Importantly, Zr-MOF-NO2 coating on cotton showed a very high increase in the performance of cotton in PM removal. QF and RE of Zr-MOF-NO2/cotton were 4.6 times and 6.2 times of the bare cotton, respectively, for PM2.5 removal, even with a very small increase in pressure drop (3 Pa or less) with MOF coating. Based on the research, it can be suggested that coating MOFs on substrates is a promising way to improve the performances of air filters for PM removal, especially when MOFs have FGs that can have large charge separation such as -NO2. This work may pave a way to utilize a functionalized MOF in the effective removal of PMs from air.

UV-Visible Lysine-Glutamate Dimer Excitations in Protein Charge Transfer Spectra: TDDFT Descriptions Using an Optimally Tuned CAM-B3LYP Functional.

Recent reports of distinctive UV-vis absorption profiles for monomeric proteins rich in charged amino acids that span 250-800 nm have opened up a new label-free optical spectral window for probing biomolecular structure and interactions. Combined experimental-computational studies have revealed that such broad absorption profiles of these proteins arise from photoexcited charge transfer (CT) transitions in spatially proximal charged amino acids such as lysine (Lys) and glutamate (Glu). Here, using time-dependent density functional theory (TDDFT) with an optimally tuned CAM-B3LYP functional, we refine the computed UV-vis spectra for Lys-Glu dimers within protein folds and quantify the percentage CT character of the constituent transitions. The optimally tuned functionals are derived through a careful analysis of the CAM-B3LYP parameter space for Lys-Glu dimers as a function of amino-acid conformation and side chain separation. Our studies reveal that the tuned Lys-Glu dimer spectrum spans 150-650 nm and possesses 5 specific types of CT excitations with diverse and large spatial charge separation length scales of 2-10 Å. These include inter-/intra-residue peptide backbone to peptide backbone (BB-CT) excitations spanning 160-210 nm, inter-/intra-residue peptide backbone to side chain (BS-CT) excitations spanning 160-260 nm, and side chain to side chain (SS-CT) excitations, which show the broadest absorption range spanning 260-650 nm.

Photoelectrochemical water oxidation at FTO|WO3@CuWO4 and FTO|WO3@CuWO4|BiVO4 heterojunction systems: An IMPS analysis

Small perturbation techniques such as intensity-modulated photocurrent spectroscopy (IMPS) have become an essential tool to unravel the complex, interrelated processes that govern the charge carrier dynamics in photoelectrochemically active materials for solar water splitting. We have fabricated CuWO4-based photoelectrodes by chemical modification of a WO3 nanorod array as a partially sacrificial template. The electrodes have been characterized by photoelectrochemical techniques including IMPS as a function of the annealing temperature, transforming WO3 either partially or completely to CuWO4. The optical properties illustrate the transformation with the absorbance onset moving from the typical onset for WO3 (about 2.5 eV) to that of CuWO4 (about 2.1 eV). In pure CuWO4 photoelectrodes bulk recombination dominates the photoelectrochemical performance, while for FTO|WO3@CuWO4 heterojunction photoelectrodes much larger charge separation and external quantum efficiencies are obtained. In addition, CuWO4 serves as a protective layer for the WO3 material that is not generally stable in neutral aqueous solutions. The FTO|WO3@CuWO4 heterojunction material was further explored as an underlayer substrate, using a thin BiVO4 film as overlayer. The advantages of this configuration include improved light harvesting as BiVO4 is a direct semiconductor with a much larger absorption coefficient than CuWO4, which is characterized by an indirect gap. It is found that the FTO|WO3@CuWO4 underlayer efficiently extracts the photogenerated electrons from the BiVO4 overlayer, hence, the inclusion of a second heterojunction plays an essential role in improving the charge separation and internal quantum efficiency, minimizing both bulk and surface recombination.

Resonance-Mediated Below-Threshold Delayed Photoemission and Non-Franck-Condon Photodissociation of Cold Oxyallyl Anions.

The photoexcitation of cold oxyallyl anions was studied below the adiabatic detachment threshold at a photon energy of 1.60 eV. Photodetachment was observed through two product channels, delayed electron emission from a long-lived anionic state and dissociative photodetachment via absorption of a second photon. The former produced stable neutral C3 H4 O, while the latter resulted in the concerted elimination of CO+C2 H4 products. The neutral oxyallyl singlet state has a barrier-free route to cyclopropanone as well as zwitterionic character with a large charge separation and dipole moment. The role of long-lived dipole-bound resonances built on the singlet state below the detachment threshold is discussed. These results provide one of the first observations of delayed photoemission in a small cold molecular radical anion, a consequence of the complex electronic structure of the neutral diradical, and provide an example of resonance-mediated control of the photodissociation processes.

Resonance‐Mediated Below‐Threshold Delayed Photoemission and Non‐Franck–Condon Photodissociation of Cold Oxyallyl Anions

AbstractThe photoexcitation of cold oxyallyl anions was studied below the adiabatic detachment threshold at a photon energy of 1.60 eV. Photodetachment was observed through two product channels, delayed electron emission from a long‐lived anionic state and dissociative photodetachment via absorption of a second photon. The former produced stable neutral C3H4O, while the latter resulted in the concerted elimination of CO+C2H4 products. The neutral oxyallyl singlet state has a barrier‐free route to cyclopropanone as well as zwitterionic character with a large charge separation and dipole moment. The role of long‐lived dipole‐bound resonances built on the singlet state below the detachment threshold is discussed. These results provide one of the first observations of delayed photoemission in a small cold molecular radical anion, a consequence of the complex electronic structure of the neutral diradical, and provide an example of resonance‐mediated control of the photodissociation processes.

A theoretical study of styrene and sulfenyl chloride reaction mechanism

The classical description of sulfenyl halides additions to olefins proposes the formation of an episulfonium ion intermediate during the reaction path. However, a careful analysis of experiments performed in the past suggests that there should not be a single mechanistic picture of sulfenyl halide additions to substituted olefins. Our work is the first theoretical study of detailed insights into the mechanism of methylsulfenchloride addition to styrene. The reaction potential energy profile has been calculated by means of the B3LYP functional with the 6-31++G(d, p) basis set. Mixing of the molecular orbital shapes of reaction stationary points has been described. The addition proceeds as a single step reaction from the methylsulfenchloride and styrene reactants via the concerted transition state (TS) to the final product along the reaction path estimated by the calculation of the intrinsic reaction coordinate. The reaction intermediate was not detected by calculations. The polar solvent has a significant effect on the decrease of reaction activation energy barriers. The large charge separation of the S–Cl bond in TS implies that the addition reaction should proceed through the ionic TS.

Photocatalytic degradation of fluoroquinolone antibiotics using ordered mesoporous g-C3N4 under simulated sunlight irradiation: Kinetics, mechanism, and antibacterial activity elimination

The occurrence of fluoroquinolones (FQs) in the ambient environment has raised serious concerns. In this study, the photocatalytic degradation kinetics and mechanism of ciprofloxacin (CIP) was investigated in ordered mesoporous g-C3N4 (ompg-C3N4). Under simulated sunlight irradiation, ompg-C3N4 exhibited a 2.9 fold more rapid reaction for CIP degradation than bulk g-C3N4. This enhancement may be attributed to the large specific surface area and effective charge separation of ompg-C3N4. The eradication of CIP followed the Langmuir–Hinshelwood (L–H) kinetics model, and surface reactions played a significant role during the photocatalysis process. Further study of reactive species (RSs) by both ESR technology and RSs scavenging experiments revealed that the superoxide anion radical (O2−) and photohole (h+) were primarily responsible for the degradation of CIP. Based on the identification of intermediates using liquid chromatography with tandem mass spectrometry (HPLC-MS/MS), and the prediction of reactive sites via Frontier Electron Densities (FEDs), the degradation pathways of CIP were proposed. A comparison of the degradation among FQs revealed that the piperazine moiety showed a dramatic effect on the degradation of FQs during the photocatalysis process. A residual antibiotic activity experiment revealed that ompg-C3N4 provided a very desirable performance for the reduction of antibiotic activity. The sufficient photocatalytic degradation of CIP in ambient water revealed that a sunlight-driven ompg-C3N4 photocatalytic process may be efficiently applied for the remediation of CIP contaminated natural waters.

Non-Fullerene Acceptor-Based Solar Cells: From Structural Design to Interface Charge Separation and Charge Transport.

The development of non-fullerene small molecule as electron acceptors is critical for overcoming the shortcomings of fullerene and its derivatives (such as limited absorption of light, poor morphological stability and high cost). We investigated the electronic and optical properties of the two selected promising non-fullerene acceptors (NFAs), IDIC and IDTBR, and five conjugated donor polymers using quantum-chemical method (QM). Based on the optimized structures of the studied NFAs and the polymers, the ten donor/acceptor (D/A) interfaces were constructed and investigated using QM and Marcus semi-classical model. Firstly, for the two NFAs, IDTBR displays better electron transport capability, better optical absorption ability, and much greater electron mobility than IDIC. Secondly, the configurations of D/A yield the more bathochromic-shifted and broader sunlight absorption spectra than the single moiety. Surprisingly, although IDTBR has better optical properties than IDIC, the IDIC-based interfaces possess better electron injection abilities, optical absorption properties, smaller exciton binding energies and more effective electronic separation than the IDTBR-based interfaces. Finally, all the polymer/IDIC interfaces exhibit large charge separation rate (KCS) (up to 1012–1014 s−1) and low charge recombination rate (KCR) (<106 s−1), which are more likely to result in high power conversion efficiencies (PCEs). From above analysis, it was found that the polymer/IDIC interfaces should display better performance in the utility of bulk-heterojunction solar cells (BHJ OSC) than polymer/IDTBR interfaces.

ZnO nanostructures with different morphology for enhanced photocatalytic activity

ZnO nanomaterials of different morphologies have been synthesized and the effect of morphology on Photocatalytic activity on natural dye has been investigated. Crystalline size and lattice strain of the synthesized particles are determined by XRD analysis and Williamson–Hall (W–H) method respectively. All other important physical parameters such as strain, stress and energy density values are also calculated using W–H analysis using different models such as uniform deformation model, uniform deformation stress model and uniform deformation energy density model. A shift in the peak of FTIR spectrum of ZnO is observed due to morphology effects. The SEM analysis reveals that the synthesized ZnO nanoparticles appear as flake, rod and dot. ZnO quantum dot exhibits higher photocatalytic activity comparing to the other morphologies. Larger surface area, high adsorption rate, large charge separation and the slow recombination of electrons/holes in ZnO dots establish dots as favorable morphology for good photocatalysis. Among the three, ZnO quantum dot shows three-times enhancement in the kinetic rate constants of photocatalysis. The results confirm that availability of specific (active) surface area, photocatalytic potential and quantum confinement of photo-induced carriers differ with morphology.

Exploring more effective polymer donors for the famous non-fullerene acceptor ITIC in organic solar cells by increasing electron-withdrawing ability

Abstract Recently, the non-fullerene acceptor ITIC has attracted a lot of attention in terms of its prominent performance in organic solar cell (OSC). A better match with the donor materials is critical for it, so we on one hand compared the match of two typical donors PBDB-T and PBQ-0F with ITIC trying to find the main difference between them. On the other hand, we designed two donors 1 and 2 based on the same 2D-BDT unit and estimated their cell efficiency aiming to explore more effective donors for ITIC. The molecular dynamics simulation was carried out to get more practical photovoltaic interface, and the density functional theory was employed to estimate the cell efficiency. It is found that PBDB-T not only exhibits higher charge separation efficiency depending on larger charge separation and smaller charge recombination rates than PBQ-0F, but also displays stronger electron-hole coherence from donor to acceptor fragments at its S1 state than PBQ-0F indicating its easier exciton dissociation at excited states. The stronger electron-withdrawing ability of the acceptor fragment in PBDB-T is concluded to make the main contribution to its higher charge separation efficiency, which is further proved by our designed donors 1 and 2. Molecule 2 with stronger electron-withdrawing ability of the acceptor fragment than PBDB-T shows better charge separation performance at the interface than PBDB-T, which again suggests introducing strong electron-withdrawing group in this kind of donors may be a good choice for improving the performance of organic solar cell using ITIC as acceptor.

The closo-Si12C12 molecule from cluster to crystal: Effects of hydrogenation and oligomerization on excited states.

Excited state properties of chain and cyclic oligomers of closo-Si12C12 moieties are calculated using time-dependent density functional theory methods. Ultraviolet, visible, and near-infrared photo-absorption properties are described for oligomers that form by linking closo-Si12C12 monomer moieties through Si-Si bonds. Natural transition orbitals for electron and hole states of stationary-state excitons in oligomers were compared to understand how exciton states are influenced by oligomer structure. Depending on the structure, some prominent excited states have large electron-hole charge separation while others do not; some exhibit exciton delocalization while others do not. With increasing oligomer length, the character of the transition between silicon and carbon regions tends to be maintained. And the extent of exciton delocalization and charge separation for an excitation is strongly influenced by the number and types of Si-Si links between oligomer units. We find that cyclic quadramers have spectroscopy properties akin to those of J-aggregates, including the tendency to collapse oligomer excitation transition energies into a narrow single peak. Hydrogenation influences some excited state distributions and energies. Phase behaviors reveal electron state or hole state equivalence in certain molecules that are differently hydrogenated, illustrating the potential for near-resonant exciton transfer between adjacent donor and acceptor species.

Particle-in-cell simulations of an RF emission mechanism associated with hypervelocity impact plasmas

Radio frequency (RF) emission from hypervelocity impacts has been detected in multiple experiments, but the physical mechanism responsible is not well understood. Plasma is created by these impacts and rapidly expands into the surrounding vacuum; it's been argued that the observed RF emission is associated with a process in the plasma. A recent model proposes that coherent oscillations from a large scale charge separation due to the expansion of the plasma can produce RF. In this paper, we use a discontinuous Galerkin particle-in-cell technique to simulate this process. Initial conditions are drawn from hydrocode impact simulations, and the results are compared to experimental measurements. Under the assumption that there exists a difference in bulk speed of electrons and ions at a certain point in the expansion, we do find that radiation is produced by the plasma.