Electron Acceptor Sites Research Articles

Increased stearic acid grafting density on silica nanoparticles via alumina-activation

Stearic acid (SA) has a low grafting density on silica nanoparticle (NP) surfaces. However, alumina coating on the particle surface effectively increases the surface reactivity with SA, significantly increasing the grafting density of SA. When the alumina coating amount reaches 15 %, the grafting density of SA increases by 350 % compared to that of the reference sample, while the water contact angle reaches 124°. The coated alumina is chemically bonded to the surfaces of the SiO2 particles. After alumina coating, the particle surface is loaded with electron acceptor sites with strong electron-receiving ability, allowing SA to undergo Lewis acid–base reactions with the particle surface.

Polarized Ultrathin BN Induced Dynamic Electron Interactions for Enhancing Acidic Oxygen Evolution

AbstractDeveloping ruthenium‐based heterogeneous catalysts with an efficient and stable interface is essential for enhanced acidic oxygen evolution reaction (OER). Herein, we report a defect‐rich ultrathin boron nitride nanosheet support with relatively independent electron donor and acceptor sites, which serves as an electron reservoir and receiving station for RuO2, realizing the rapid supply and reception of electrons. Through precisely controlling the reaction interface, a low OER overpotential of only 180 mV (at 10 mA cm−2) and long‐term operational stability (350 h) are achieved, suggesting potential practical applications. In situ characterization and theoretical calculations have validated the existence of a localized electronic recycling between RuO2 and ultrathin BN nanosheets (BNNS). The electron‐rich Ru sites accelerate the adsorption of water molecules and the dissociation of intermediates, while the interconnection between the O‐terminal and B‐terminal edge establishes electronic back‐donation, effectively suppressing the over‐oxidation of lattice oxygen. This study provides a new perspective for constructing a stable and highly active catalytic interface.

Polarized Ultrathin BN Induced Dynamic Electron Interactions for Enhancing Acidic Oxygen Evolution.

Developing ruthenium-based heterogeneous catalysts with an efficient and stable interface is essential for enhanced acidic oxygen evolution reaction (OER). Herein, we report a defect-rich ultrathin boron nitride nanosheet support with relatively independent electron donor and acceptor sites, which serves as an electron reservoir and receiving station for RuO2, realizing the rapid supply and reception of electrons. Through precisely controlling the reaction interface, a low OER overpotential of only 180 mV (at 10 mA cm-2) and long-term operational stability (350 h) are achieved, suggesting potential practical applications. In situ characterization and theoretical calculations have validated the existence of a localized electronic recycling between RuO2 and ultrathin BN nanosheets (BNNS). The electron-rich Ru sites accelerate the adsorption of water molecules and the dissociation of intermediates, while the interconnection between the O-terminal and B-terminal edge establishes electronic back-donation, effectively suppressing the over-oxidation of lattice oxygen. This study provides a new perspective for constructing a stable and highly active catalytic interface.

Effects of fused thiophene Π-bridge on the electronic and optical properties of modified theaflavin natural dye

ABSTRACT Natural dye sensitizers are substances that are sensitive to light colour and can be classified into several classes based on their chemical structure. Theaflavin pigments are unexplored natural dyes used as light sensitizers derived from black tea waste. However, the efficiency of natural dye-sensitized solar cells (DSSC) is lower than that of synthetic ones. To increase the efficiency, an amine donor molecule, a thiophene bridge, and a cyanoacrylic acid anchoring group were added. This improvement was achieved by comparing the LHE values, absorption shifts towards the red spectrum, and electronic parameters before and after modification of the theaflavin structure. This study aimed to analyze the effect of a fused thiophene bridge on the colour pigment of theaflavin on the electronic and optical properties of DSSC. TDDFT was employed with the hybrid functional B3LYP and def2-SVP. The calculated results concluded that adding fused thiophene can shift the wavelength with a maximum value of 256 nm and an increase in LHE of 40%. The addition of five thiophenes can shift the absorption wavelength closer to the red spectrum and satisfy the electronic properties of the DSSC, and the number of electron donor and acceptor sites obeying the law of conservation of charge.

Accurate fabrication and orientation of electron acceptor and donor active sites for enhancing photocatalytic overall water splitting

The transfer paths of charge carriers were accurately fabricated and orientated by co-decorating Pt1 and PtNP active sites on supports.

Disclosing the nature of vacancy defects in α-Ag2WO4

Defects at semiconductors with electron acceptor and donor sites govern the electronic and optoelectronic applications due to their unique electronic properties. This work provides deep insight into the nature of defects and the conduction mechanism in α-Ag2WO4. To this aim, a detailed analysis of the results of XRD with Rietveld refinements, FE-SEM images, and measurements of different spectroscopies (impedance, positron annihilation lifetime, and photoluminescence) are carried out on α-Ag2WO4 samples synthesized by a simple co-precipitation method. Two types of vacancy defects: cationic O-vacancies, and anionic Ag or Ag–O vacancy complexes are elucidated with a Schottky p-type potential barrier. The results indicate that the Ag vacancies remain constant during thermal treatment, while an opposite effect is found for the oxygen vacancies. This behavior governs the multifunctional properties of α-Ag2WO4 semiconductors via a tunneling plus thermionic conduction mechanism.

Hydrogen Production by N-Heterocycle Dehydrogenation over Pd Supported on Aerogel-Prepared Mg-Al Oxides

Tetradecahydrophenazine (14HP) is a nitrogen-containing heterocycle compound with a high content of hydrogen that can be released during its dehydrogenation to phenazine (P). The high stability of the 14HP/P pair and relatively low dehydrogenation temperature make 14HP a promising organic hydrogen carrier. This manuscript is devoted to the investigation of hydrogen production by 14HP dehydrogenation over Pd supported on a series of magnesium-aluminum oxides prepared by the aerogel method. This technique made it possible to synthesize catalyst supports characterized by a high surface area and high concentration of surface active sites where active transition metals could be stabilized in a finely dispersed state. The synthesized aerogels had high specific surface areas and pore volumes. A surface area as high as 600 m2/g after calcination at 500 °C was observed for the mixed aerogel with an Mg:Al ratio of 1:4. An increase in the concentration of acidic electron-acceptor sites determined by EPR on the surface of the mixed magnesium-aluminum oxide supports with a high surface area prepared by the aerogel method was found to result in higher hydrogen production due to the faster dehydrogenation of sterically hindered nitrogen-containing tetradecahydrophenazine heterocycles.

Synthesis of Vanadia-Mayenite Nanocomposites and Characterization of Their Structure, Morphology and Surface Sites

Calcium aluminates (CA) with a mayenite structure have attracted a growing interest during the last decades. The present paper reports the preparation of vanadia-mayenite composites performed via an impregnation of pure CA with ammonium vanadate solution. The properties of the prepared materials were explored by a low-temperature nitrogen adsorption/desorption technique, X-ray diffraction analysis, transmission electron microscopy, and spin probe method. As revealed, the addition of vanadium significantly affects the textural properties and the porous structure of mayenite. The blockage of micropores by vanadium species is supposed. The spin probe electron paramagnetic resonance technique based on the adsorption of 1,3,5-trinitrobenzene, phenothiazine, and diphenylamine has been applied to study the active sites on the surface of the composite samples. The results demonstrated an increase in the concentration of weak electron-acceptor sites when the vanadium loading was 10 wt%. X-ray diffraction analysis and transmission electron microscopy studies showed that the composites consist of few phases including mayenite, CaO, and calcium vanadates.

An extension of electron acceptor sites around Thiazolothiazole unit for evaluation of large power conversion efficiency: A theoretical insight

Small organic solar cells containing thiazolothiazole unit as an electron acceptor for solution processed bulk heterojunction (BHJ) small donor-acceptor-donor (D-A-D) type materials have been designed and studied theoretically with state-of-the-art density functional theory and time-dependent density functional theory (TD-DFT) for reliable estimation of their excited state and charge transfer photophysical characteristics for estimating their power conversion efficiencies. The suggested possible synthetic routes with complete reaction information have been also provided for synthesis. The electron acceptor sites around the thiazolothiazole unit have been enlarged by introducing different strong electron withdrawing groups and checked their effects on the voltages (VOC) and fill factor (FF) which are the two main parameters directly influences on power conversion efficiencies. Out of five theoretically studied molecules, the experimental reported data of TT-TTPA (Thiazolothiazole-thiaophene triphenyl amine) has been compared with four designed molecules and concluded that extension of acceptor sites significantly contributed towards the better charge transport properties of electron and hole.

Investigation of the Optical Properties of a Novel Class of Quinoline Derivatives and Their Random Laser Properties Using ZnO Nanoparticles.

Quinoline Schiff bases display potential applications in optoelectronics and laser fields because of their unique optical properties that arise from extensive delocalization of the electron cloud, and a high order of non-linearity. In this context, a new class of conjugated quinoline-derivative viz. N-(quinolin-3-ylmethylene)anilines were synthesized from 2-hydroxyquinoline-3-carbaldehyde in two good yielding steps. The ability of these imines to accept an electron from a donor is denoted by their electron acceptor number and sites, which is calculated using density functional theory (DFT). The optical properties such as FT-IR, Raman, UV-VIS, and EDS spectra were calculated using TD-DFT, which also provided the energy gap, HOMO-LUMO structure. The optical properties of the synthesized imino quinolines were experimentally studied using photoluminescence and absorption spectroscopy. The properties such as Stokes shift and quantum yield were calculated using experimental data. Furthermore, the compound bearing a methyl group on the aryl ring and ZnO nanoparticles (hydrothermally synthesized) were dissolved in toluene, and optically excited with a 355 nm nanosecond laser, which produced a random laser.

Electron Acceptor-Donor Iron Sites in the Iron-Sulfur Cluster of Photosynthetic Electron-Transfer Pathways.

In photosystem I, two electron-transfer pathways via quinones (A1A and A1B) are merged at the iron-sulfur Fe4S4 cluster FX into a single pathway toward the other two Fe4S4 clusters FA and FB. Using a quantum mechanical/molecular mechanical approach, we identify the redox-active Fe sites in the clusters. In FA and FB, the Fe site, which does not belong to the CxxCxxCxxxCP motif, serves as an electron acceptor/donor. FX has two independent electron acceptor Fe sites for A- and B-branch electron transfers, depending on the Asp-B575 protonation state, which causes the A1A-to-FX electron transfer to be uphill and the A1B-to-FX electron transfer to be downhill. The two asymmetric electron-transfer pathways from A1 to FX and the separation of the electron acceptor and donor Fe sites are likely associated with the specific role of FX in merging the two electron transfer pathways into the single pathway.

Free radical scavenging behavior of multidimensional nanomaterials in γ-irradiated epoxy resin and mechanical and thermal performance of γ-irradiated composites

In order to explore the scavenging ability of free radicals in polymers and the derivation mechanism with the different influence of the composition and various structure of nanomaterials, two-dimensional graphene oxide, one-dimensional carbon nanotubes and zero-dimensional carbon black in carbon nanomaterials, and halloysite nanotubes in silicate nanomaterials were selected to carry out a task. The results manifest that the content of free radicals produced by γ-ray irradiation in epoxy resin reduced because of the introduction nanoparticle. Among them, the epoxy resin reinforced by graphene oxide has the least free radical, followed by carbon nanotubes, halloysite nanotubes and carbon black, respectively. The analysis shows that the free radical scavenging capability was significantly different in variety types of nanomaterials. The free radical scavenging ability of carbon nanomaterials is ascribed to its well-organized honeycomb lattice, which is different from the fact that silicate nanomaterials is attributed to its electron acceptor sites. Meanwhile, the stable electron cloud was formed in the carbon nanomaterials, which is more conducive to eliminate the free radicals. The addition of nanomaterials decreases the number of free radicals in the resin, and correspondingly weakens the damage of free radicals for the resin, resulting in improving the thermal stability and mechanical properties of the epoxy resin.

Phase equilibrium modeling for methane solubility in aqueous sodium chloride solutions using an association equation of state

A new formulation for the fraction of electron acceptor sites of methane (CH4) of cubic-plus-association couple with Huron-Vidal (CPA-HV) model is presented, and a combine rule is used to calculate the cross-associating strength of CH4-water (H2O) binary system. Meanwhile, the interaction parameters (Eij, Eji and F) of CPA-HV model are regressed for CH4–H2O binary and CH4–H2O–NaCl ternary systems. The predictions of CPA-HV model are consistent with a lot of binary and ternary experimental data of these systems. Compared with CPA-vdW and SRK-HV models, the accuracy of CPA-HV model is the best for vapor-liquid equilibria of CH4–H2O system. The average absolute relative deviations of the predicted CH4 solubility in H2O and aqueous sodium chloride solutions for CPA-HV model are respectively 5.84% and 6.91%.

Predicting the halogen-n (n = 3-6) synthons to form the "windmill" pattern bonding based on the halogen-bonded interactions.

The "windmill" pattern cyclic halogen polymers (XBr)3 (X = Cl, Br, I) and (BrY)n (n = 3-6, Y = Cl, Br, I) have been investigated using the density functional theory. Due to the anisotropic distribution of its electron density, the halogen atom can form halogen-bonded interactions by functioning as both electron donor sites and electron acceptor sites. For (XBr)3 (X = Cl, Br, I) trimers, the Cl···Cl interaction is the weakest, and the I···I interaction is the strongest. For (BrY)n (n = 3-6, Y = Cl, Br, I), the Br···Br halogen bonds are the strongest in (BrY)4 tetramers. We predict that the iodine-4 synthon may allow creation of a self-assembled island during crystal growth. The angle formed by the electron-depleted sigma-hole, the halogen atom and the electron-rich equatorial belt perpendicular to the bond direction, together with the halogen-bond angle, can be used to explain the geometries and strength of the halogen-bond interactions. © 2018 Wiley Periodicals, Inc.

Silica-coated nanocrystalline TiO2 with improved thermal stability

Deposition of a SiO2 coating on anatase TiO2 nanocrystals is shown to improve their thermal stability. As low as 0.5% Si was shown to preserve the small anatase crystallite size after calcination at 600 °C. Such treatment led to considerable sintering of TiO2 nanocrystals without the silica with the average particle size growth from 9 to 50 nm, surface area decrease from 135 to 22 m2/g and partial anatase conversion to rutile. The phase composition, crystallite size, and surface area of 5%Si-TiO2 samples were largely preserved till the temperatures as high as 800 °C whereas the anatase phase was mostly stably even after calcination at 1000 °C. The phase transformation from anatase to rutile in xerogel TiO2 and TiO2@SiO2 samples apparently did not occur until the crystallites grew larger than the critical size about 50 nm. Electron-acceptor sites capable of ionizing perylene to its radical cations were observed on all samples with anatase crystalline structure. So, the silica shell deposition improves the TiO2 thermal stability without limiting access to the surface active sites.

Stereocontrolled Photoinduced Electron Transfer in Metal-Fullerene Hybrids.

Photoinduced electron transfer in transition-metal complexes linked to a fullerene moiety is of increasing interest. Recently, several stereoisomers of an Ir-complex exhibiting configurational stability at metal center, which does not undergo epimerization have been synthesized (Angew. Chem. Int. Ed. 2017, 56, 2136). The presence of multiple electron donor and acceptor sites located at opposite ends with respect to the metallic center creates the prerequisites for the formation of entirely different charge transfer (CT) states. Here we report the results of quantum mechanical calculations and detailed analysis of excited-state properties for all stereoisomers of the junction. We found that the stereoisomers demonstrate clearly different CT properties by photoexcitation. The found photo-stereospecific effects can be used to design new hybrids with a different type of photoinduced CT state, exhibiting dissimilar activity in photocatalysis.

Dehydrochlorination of 1-Chlorobutane Over Nanocrystalline MgO: The Role of Electron-Acceptor Sites

Nanocrystalline MgO aerogels possess high surface areas and outstanding reactivity with respect to variety of harmful organic substances. The catalytic activity of MgO aerogels in dehydrochlorination of 1-chlorobutane substantially increases with time due to partial MgO conversion to MgCl2. This catalytic activity increase and surface area decrease indicate that more active catalytic sites are formed on the surface during this reaction. The catalytic activity was found to increase significantly during the 1-chlorobutane dehydrochlorination reaction, which is accompanied by the MgO modification with chloride ions. The activation energy of this reaction over partially chlorinated MgO was found to be equal to 135 kJ/mol at T 225 °C. For the first time the concentrations of electron-acceptor sites were measured during a catalytic reaction. No electron-acceptor sites were observed on the surface of initial AP–MgO samples. They appeared only during the reaction. A good correlation was observed between the catalytic activity and the concentration of weak electron-acceptor sites tested using perylene. The obtained results indicate that they may be the active sites accounting for 1-chlorobutane dehydrochlorination in the active state of the catalyst.

Study of MgO transformation into MgF2 in the presence of CF2Cl2•

Alkaline-earth metal oxide aerogels prepared by sol?gel method followed by autoclave drying are nanocrystalline mesoporous materials with high reactivity. Bulk solid-state reaction of MgO aerogels with CF2Cl2 takes place after a long induction period, during which the active sites are accumulated on the surface of the nanoparticles. It was found that vanadium addition has a promoting effect on this reaction accelerating the process of the active sites formation. A method for characterization of electron-acceptor sites by electron spin resonance spectroscopy using perylene as the spin probe was developed. A good correlation was observed between the rate of the CF2Cl2 destructive sorption and the concentration of weak electron-acceptor sites. Simplified models of such sites were suggested. The acid sites on the modified MgO surface were supposed to be originated from separation of the charged fragments resulting in the surface polarization. Uncompensated oxygen substitution for chlorine and/or fluorine ions leads to appearance of Lewis acid sites while HCl/HF chemisorption results in Bronsted acid sites formation.

A first principles investigation of electron transfer between Fe(II) and U(VI) on insulating Al- vs. semiconducting Fe-oxide surfaces via the proximity effect

This study investigates how the intrinsic chemical and electronic properties of mineral surfaces and their associated electron transfer (ET) pathways influence the reduction of U(VI) by surface-associated Fe(II). Density functional theory (DFT), including the Hubbard U correction to the exchange–correlation functional, was used to investigate sorption/redox reactions and ET mechanisms between Fe(II) and U(VI) coadsorbed on isostructural, periodic (001) surfaces of the insulator corundum (α-Al2O3) vs. the semiconductor hematite (α-Fe2O3). Furthermore, the coadsorbed Fe(II) and U(VI) ions are spatially separated from one another on the surfaces (⩾5.9Å) to observe whether electronic-coupling through the semiconducting hematite surface facilitates ET between the adsorbates, a phenomenon known as the proximity effect.The calculations show that the different chemical and electronic properties between the isostructural corundum and hematite (001) surfaces lead to considerably different ET mechanisms between Fe(II) and U(VI). ET on the insulating corundum (001) surface is limited by the adsorbates’ structural configuration. When Fe(II) and U(VI) are spatially separated and do not directly interact with one another (e.g. via an inner-sphere complex), U(VI) reduction by Fe(II) cannot occur as there is no physical pathway enabling ET between the adsorbates. In contrast to the insulating corundum (001) surface, the hematite (001) surface can potentially participate in ET reactions due to the high number of electron acceptor sites from the Fe d-states near the Fermi level at the hematite surface. The adsorption of Fe(II) also introduces d-states near the Fermi level as well as shifts unoccupied d-states of the Fe cations at the hematite surface to lower energies, making the surface more conductive. In turn, electronic coupling through the surface can link the spatially separated adsorbates to one another and provide distinct ET pathways for an electron from Fe(II) to travel through the hematite surface and reach U(VI).The progression and extent of ET occurring on the semiconducting hematite (001) surface via the proximity effect depends on the electronic properties of the surface. ET between the spatially separated U(VI) and Fe(II) occurs most readily when orbitals between the Fe and U adsorbates overlap with those of neighboring O and Fe ions at the hematite surface, as shown by calculations without the Hubbard U correction. Analyses of the spins densities confirm that the U and Fe adsorbates were reduced and oxidized, respectively, (acquiring 0.33 μB and 0.11–0.20 μB, respectively), while Fe cations at the hematite surface were reduced (losing ⩽0.6 μB). If electrons are highly localized, the amount of orbital mixing and electronic coupling through the hematite surface decreases and in turn leads to a lower degree of spin transfer, as predicted by calculations with the Hubbard U correction. Thus, the proximity effect is a potential mechanism on semiconducting surfaces facilitating surface-mediated redox reactions, although its significance varies depending on the electronic properties and subsequent charge-carrying ability of the surface. These results provide insight into ET pathways and mechanisms on insulating Al- and semiconducting Fe oxide surfaces influencing the reduction U(VI) by Fe(II) that may subsequently limit uranium’s transport in the subsurface.

Pyridine adsorption for probing electron-acceptor sites on the surface of titanium oxide with supported silver, copper, and gold ions

The results of investigation of the adsorption kinetics of pyridine (Py) show that the M+z = Ag+, Cu+2, and Au+3 ions supported on the surface of TiO2 (anatase) significantly increase its electron-acceptor properties. Two types of M+z/TiO2 adsorption sites are characterized by the various numbers of the pyridine coordination centers M and their oxidation states. Linear correlations between the rate constants of pyridine adsorption and the ionic radii are observed in the cases of both types of sites. The adsorption stoichiometry of the less active sites is Py-2M, where M = Cu+, Au+, and Ag0. The linear dependence of the antibacterial activity of M+z/TiO2 on the ionic charge z = +1 (for Ag), +2 (for Cu), and +3 (for Au), as well as the ability of bacteria to reduce dissolved silver and gold ions and those supported on TiO2 to colloidal metals, is also found.