Enhancing CO2 reduction through the catalytic effect of a novel silicon haeckelite-inspired 2D material.

We propose a novel 2D material based on silicon haeckelite (Hck), whose structure contains a silicon atom arranged in a periodic pattern of pentagons and heptagons. Stacking the two layers gives rise to a planar geometry of the layers that compose it. This new structure presents a semiconductor character with a band gap of 0.17 eV. Furthermore, we studied CO2 reduction using molecular hydrogen to form formic acid, carbon monoxide, formaldehyde, methanol, and methane. All these have been studied theoretically at the Grimme D3BJ corrected TPSS/def2-SVP level. A massive biflake containing 132 Si atoms was used to model the Hck surface. According to the results, CO2 capture with Hck is a spontaneous step; in contrast, the same process for silicene mono- and bi-flakes studied previously was endergonic. After the capture of CO2, the addition of H2 to the substrate passes through an intermediate containing a Si-H bond. The formation of Si-H intermediates is the origin of the catalytic effect, facilitating H2 dissociation and acting as the hydrogen atom donor for the substrate. These intermediates are transformed by adding hydrogen atoms and losing water molecules, producing formic acid and formaldehyde as the most probable products, with rate-controlling steps of 29.2 and 27 kcal mol-1, whose values were less than those exhibited by the silicene biflake. This means that the silicon haeckelite biflake presents better catalytic activity than the silicene biflake. The results show that the novel 2D silicon hackelite material has remarkable potential for CO2 capture and reduction. The theoretical analysis of this innovative 2D structure provides valuable insights into the potential applications of silicene-based materials.

Catalytic activity of silicene biflakes for CO2 reduction: a theoretical approach.

The use of renewable energy sources to reduce carbon dioxide (CO2) emissions has gained significant attention in recent years. The catalytic reduction of CO2 into value-added products is a promising approach to achieve this goal, and silicene biflakes (2Si) have been identified as potential candidates for this task. In this study, we explored the catalytic activity of these structures using density functional theory calculations. Our results show that the reaction pathway involves the adsorption of CO2 onto the silicene surface, followed by the addition of hydrogen molecules to form products such as formic acid, methanol, methane, carbon monoxide, and formaldehyde. Our proposed mechanism indicates that silicene biflakes exhibit a higher affinity for CO2 than single-layer silicon. We also found that the hydrogenation with H2 occurs by adding one hydrogen atom to the absorbed CO2 and another to the surface of 2Si. Intermediate species are reduced by systematically adding hydrogen atoms and removing water molecules, forming formic acid as the most probable product. The rate-controlling step for this reaction has an energy of 32.9 kcal mol-1. In contrast, the process without a catalyst shows an energy of 74.6 kcal mol-1, suggesting that the silicon bilayer is a structure with outstanding potential to capture and reduce CO2. Our study provides important insights into the fundamental mechanisms underlying the silicene-mediated CO2 reduction and could facilitate the development of more efficient catalysts for this process.

Trifluoromethyl-stabilized 2D nitrogen clusters: a theoretical study.

The stability of 2D all nitrogen clusters containing from 6 to 96 nitrogen atoms, terminated with CF3 groups, has been explored using two computational models: dispersion corrected B3LYP functional and scaled opposite spin Møller-Plesset perturbation theory (SOS-MP2). Single point domain-based local pair natural orbital coupled-cluster theory calculations (DLPNO-CCSD(T)) was used for further energy refinement. All systems were found to be minima, and their stability increases with CF3/N ratio. Larger clusters and anion radicals were not dynamically stable, while some of the cation radicals were found to be minima on potential energy surface. The mechanism of cluster stabilization by CF3 groups is related with interaction of orbitals holding lone electron pairs and antibonding sigma orbitals.

Complexes of Li, Na, and Mg with 2D allotropies of second and third period: a theoretical study.

Complexes of Li, Na, and Mg with graphene, silicene, phosphorene nanoflakes (NFs), and their 2D allotropies have been studied at dispersion corrected TPSS/def-TZVP level of theory. The energy partition analysis of the complexes revealed that for most of the complexes exchange and correlation energies represent dominant contributions to the binding with strong charge transfer from the metal atom to a NF. The exceptions are Mg complexes of graphene and phosphorene NFs where binding is due to dispersion and correlation terms. This difference is also reflected in large Mg-NF distances suggesting weak intermolecular interactions in these complexes. The calculated activation energies for metal hopping are easily achievable at room temperatures for carbon and silicon allotropies. However, they are significantly higher for phosphorus allotropies reaching almost 18 kcal/mol. Generally, activation energies for hopping increase with binding energies for graphene, silicene, and phosphorene NFs. This trend does not observe however for graphene, silicene, and phosphorene 2D allotropies.

In silico modeling: electronic properties of phosphorene monoflakes and biflakes substituted with Al, Si, and S heteroatoms.

This contribution explores the systematic substitution of phosphorene monoflakes (Mfs) and biflakes (Bfs) with aluminum, silicon, and sulfur. These systems were investigated using density functional theory employing the TPSS exchange-correlation functional and complete active space self-consistent field (CASSCF) calculations. Al and Si substitution produces significant structural changes in both Mfs and Bfs compared to S-substituted and pristine systems. However, in Mfs, all heteroatoms generate a decrease in band gap and the ionization potentials (IP), and an increase in electron affinity (EA) in comparison with pristine phosphorene. Al doping improves the hole mobility in the phosphorene monoflake, while Si and S substitutions exhibit a similar behavior on EAs and reorganization energies. For Bfs, the presence of Si-Si and Al-P interlaminar interactions causes structural changes and higher binding energies for Si-Bfs and Al-Bfs. Regarding the electronic properties of Bfs, substitution with Si does not produce significant variations in the band gap. Nevertheless, it conduces the formation of hole transport materials, which does not occur in Si-Mfs. The same is observed for Al systems, whereas no correlation was identified between the doping level and reorganization energies for S complexes. The substitution with Al and S leads to an opposite behavior of the band gap and IP values, while the EA variation is similar. In summary, the nature of heteroatom and the doping degree can modify the semiconductor character and electronic properties of phosphorene mono- and biflakes, whose trends are closely related to the atomic properties considered. Overall, these computational calculations provide significant insights into the study of doped phosphorene materials.

Novel 2D allotropic forms and nanoflakes of silicon, phosphorus, and germanium: a computational study.

The structural variability offered by 2D materials is an essential feature in materials design. Despite its significance, obtaining assemblies with suitable stability remains a challenge. In this work, we theoretically explore novel silicon, phosphorus, and germanium, analogues of haeckelites at hybrid DFT level. Both 2D systems and nanoflakes (NF) have been studied. All materials have been found dynamically stable; Si-, P-, and Ge- analogues of haeckelites were found to be more stable in comparison to the corresponding honeycomb structure than haeckelites in comparison with graphene. All 2D materials showed metallic behavior; however, the difference between inorganic haeckelites and the corresponding honeycomb allotropes is less than that between haeckelites and graphene. Si-, P-, and Ge-, allotropes have much higher electron affinities (EAs) compared to carbon allotropes, while haeckelites have higher EAs than honeycomb structures. Furthermore, Si-, P-, and Ge-structures also exhibit low hopping activation energies for lithium atoms. It makes these materials potentially promising as a component in Li-ion batteries.

Halogen-Doping Effect on the Delithiation and Charge Transfer of (Li2S)10 Cluster.

Previous studies have suggested beneficial effects in lithium-sulfur batteries containing iodide in a sulfur-based cathode or as an electrolyte additive. These effects include preventing electrolyte degradation and improving the cycle stability of Li-S cells. However, little is known regarding the underlying reasons of such performance improvements. In this work, we present a theoretical study of the halogen-doping effect on the delithiation (charge) process on a (Li2S)10 model structure representing a potential final discharge product. It is revealed that the electron polaron is the dominant charge carrier during the charge process, and iodine is a facilitating agent for lithium detachment from the lithium sulfide cluster. However, the graphene support was found as potentially slowing down the ionic transport during the delithiation process due to charge transfer exerted by the support to the doped cluster that may retain the positive ions in the particle.

The electronic structure of van der Waals heterostructures formed by the nanoflakes of black phosphorene with those of graphene and haeckelites: their complexes with Li.

The electronic structure of the van der Waals heterostructures (HSs) of the phosphorene (P) nanoflakes (NFs) with graphene (G) and its allotropy (H1 and H2) NFs, and their complexes with Li have been studied using dispersion-corrected TPSS functional. According to the calculations, the attractive interactions in HSs come from dispersion. It has a relatively small contribution to the binding energy in Li complexes, especially for these forming complexes with G, H1, or H2 NF side. The binding energies between the individual NFs and Li atoms increase in the order G < H1 = H2 = P. The formation of HSs results in a synergetic effect for Li binding energies. This effect is the most notable for phosphorene binding sites; however, it also holds for G, H1, and H2 NFs. The formation of complexes with Li always leads to the almost complete charge transfer from Li to the NFs or HSs. In the case of HSs, the unpaired electron of Li is always located at the carbon NF side independently on the Li binding location. The activation energies of Li hopping for individual NFs are notably higher for P comparing with G, H1, or H2 NFs. The formation of HSs rises slightly the activation energies of Li hopping due to the increase of binding energies in Li-HS complexes. Graphical abstract.

Electronic structure of hybrid pentaheptite carbon nanoflakes containing boron-nitrogen motifs.

The electronic structure of isomeric graphene nanoflakes (NFs) heavily doped with boron and nitrogen atoms has been explored. Dispersion-corrected B3LYP functional has been used for the geometry optimizations. A complete active space method has been used for the energy evaluations. Combined boron and nitrogen doping promotes polyradicalic antiferromagnetic ground states in the NFs and affects the nanoflake geometry. There is a charge transfer from boron to nitrogen atoms which increases with the doping level. This transfer does not involve carbon atoms. Combined doping reduces both the ionization potentials (IPs) and the electron affinities (EAs) of the NFs similar to nitrogen doping alone. Boron does not affect either IPs or EAs being neither n- nor p-type dopant for the isomeric graphene NFs. All hybrid NFs show a tendency to increase the band gaps with doping level, which is promoted by the increment of the bond length alternation with doping. Finally, the hole reorganization energies for the NFs were found to be lower than the electronic ones, positioning the hybrid NF as hole-transporting systems. Graphical Abstract Color coded natural charge differences between charged and neutral states. The excess of positive charge is green for cation radicals and the excess of negative charge is red in anion radicals.

Modeling of silicon- and aluminum-doped phosphorene nanoflakes.

The electronic structure of phosphorene nanoflakes (PNFs) doped with Al and Si has been explored using hybrid functional BHandHlyp/def2-SVP and complete active space (CASSCF) methods. Doping increases the bond length alternation and changes the overall PNF shape. Doping also decreases singlet-triplet splitting in the PNFs. This effect is most notable for Si doping where singlet and triplet states become virtually degenerated. Doping also reduces band gaps and changes the nature of the ground states for Si-doped systems. The ground state of Si-doped PNFs becomes polyradicalic. In general, dopants with even number of valence electrons promote polyradicalic ground state. Doped systems show increased electron affinities (EAs), while the ionization potentials are much less affected. Larger EAs are related with the delocalization of an extra electron over the empty or partially empty 3p orbitals of the dopants. Doping increases the reorganization energies in all cases. Al-doped PNFs are the hole transport materials while Si-doped nanoflakes tend to be electron transport systems. Graphical abstract.

Detection of Multiconfigurational States of Hydrogen-Passivated Silicene Nanoclusters.

Utilizing density functional theory (DFT) and a complete active space self-consistent field (CASSCF) approach,we study the electronic properties of rectangular silicene nano clusters with hydrogen passivated edges denoted by H-SiNCs (nz,na), with nz and na representing the zigzag and armchair directions, respectively. The results show that in the nz direction, the H-SiNCs prefer to be in a singlet (S = 0) ground state for nz > na. However, a transition from a singlet (S = 0) to a triplet (S = 1) ground state is revealed for na > nz. Through the calculated Raman spectrum, the S = 0 and S = 1 ground states can be observed by the E2g (G) and A (D) Raman modes. Furthermore, H-SiNC clusters are shown to have HOMO-LUMO (HL) energy gaps, which decrease as a function of na and nz for S = 0 and S = 1 states. The H-SiNC with a S = 1 ground state can be potentially used for silicene-based spintronic devices.

DFT study of zinc, cadmium, mercury, copper, silver, and gold complexes of 21,23-dioxaporphyrin and one-dimensional arrays of those complexes.

Complexes of 21,23-dioxaporphyrin with neutral Zn, Cd, Hg, Cu, Ag, and Au atoms as well as some one-dimensional arrays of those complexes containing up to ten repeat units were modeled at the PBE/def2-TZVPP level of theory with D3 empirical dispersion correction. The binding energy between the metal atom and the macrocycle was found to vary from 90 kcal/mol for Cu to -14 kcal/mol for Hg. Strong charge transfer from the metal to the macrocycle accompanied complex formation. The complexes were able to form dimers and nanoarrays that were held together mostly by dispersion forces. Different types of dimers were studied: face-to-face (F) and two types of parallel-displaced ones. F dimers were calculated to be the lowest-energy structures for Cu and Ag systems. Nanoarray formation was studied for these complexes. The band gaps (Eg) of the nanoarrays were found to be smaller than 1 eV, and decreased slightly as the number of repeat units in the nanoaggregates increased. The ionization potentials and electron affinities were greatly affected by the number of repeat units due to the delocalization of polarons over the entire nanoarray. The polaron delocalization and the related reorganization energies depended to a considerable extent on the metal present in the complex. For the studied nanoarrays, the reorganization energies for hole and electron transport decreased linearly with 1/n, where n is the number of repeat units in the nanoaggregate; for an infinitely long chain, the reorganization energy was zero for electron transport and 0.03-0.04 eV for hole transport.

Charge transfer complexes of fullerene[60] with porphyrins as molecular rectifiers. A theoretical study.

Molecular diodes based on charge transfer complexes of fullerene[60] with different metalloporphyrins have been modeled. Their current-voltage characteristics and the rectification ratios (RR) were calculated using direct ab initio method at PBE/def2-SVP level of theory with D3 dispersion correction, for voltages ranging from -2 to +2 V. The highest RR of 32.5 was determined for the complex of fullerene[60] with zinc tetraphenylporphyrin at 0.8 V. Other molecular diodes possessed lower RR, however, all complexes showed RR higher than 1 at all bias voltages. The asymmetric evolutions and alignment of the molecular orbitals with the applied bias were found to be essential for generating the molecular diode rectification behavior. Metal nature of metalloporphyrins and the interaction porphyrin-electrode significantly affect RR of molecular diode. Large metal ions like Cd(2+) and Ag(2+) in metalloporphyrins disfavor rectification creating conducting channels in two directions, while smaller ions Zn(2+) and Cu(2+) favor rectification increasing the interaction between gold electrode and porphyrin macrocycle.

Mechanistic aspects of superacid mediated condensation of polyphenols with ketones. Implications for polymer synthesis.

A detailed computational study of possible reaction paths for methanesulfonic and triflic acid mediated polyhydroxyalkylation reaction between resorcinol and trifluoracetone accompanied by cyclodehydration to give 9H-xanthene containing polymers has been carried out at M06-2X/6-311+G level of theory. A cluster solvation model was used for the calculations. The calculations revealed that the most kinetically favorable reaction path involves the cyclodehydration occurring during the polymer forming step. In this case 9H-xanthene formation is promoted by the activated phenyl ring in Wheland intermediate assisting the aromatic nucleophilic substitution of OH group which leads to the cyclization. It has been demonstrated that the inability of methanesulfonic acid to catalyze the formation of 9H-xanthene containing polymers is due to the very high barrier of the rate limiting step of the polymer forming reaction and not the cyclodehydration process.

Zinc-, cadmium-, and mercury-containing one-dimensional tetraphenylporphyrin arrays: a DFT study.

Metal-free, Zn-, Cd-, and Hg-containing one-dimensional tetraphenylporphyrin arrays containing up to eight repeat units were modeled at the PBE/def2-SVP level of theory with D3 empirical dispersion correction. Two different configurations--face to face (F) and parallel displaced (P)--were detected, the latter being the most stable for all types of nanoarrays. According to the calculations, the binding that occurs in nanoarrays is mostly due to dispersion, with binding energies of 33-35 kcal/mol seen for the metal-free nanoarrays and energies of 37-40 kcal/mol for the metal-containing ones. The band gaps, estimated as the S0 → S1 excitation energies and extrapolated to the infinite chain limit using the TD-CAM-B3LYP/def2-SVP model, were close to 2 eV; the band gap size was barely dependent on the nature of the metal and the number of repeat units in the nanoarray. The ionization potentials and electron affinities were greatly influenced by the number of repeat units due to delocalization of polarons across each nanoarray. Polaron delocalization and the related reorganization energies were clearly dependent on the nature of the metal. For the metal-free and Zn-containing nanoarrays, the reorganization energies for hole and electron transport decreased linearly with 1/n, where n is the number of repeat units in the nanoaggregate. The reorganization energies therefore reach zero for an infinitely long chain. These energies for Cd- and Hg-containing nanoarrays were found to be one order of magnitude higher for both hole and electron transport due to the localization of polarons in these nanoarrays.

Multiconfigurational character of the ground states of polycyclic aromatic hydrocarbons. A systematic study.

A systematic study of the electronic structure of polycyclic hydrocarbons from naphthalene to a system containing 80 fused benzene has been carried out. Geometries were optimized for closed shell singlet, open shell singlet, triplet and multiplet states at B3LYP/cc-pVDZ level of theory, D1 (second order Møller-Plesset) and D1 (second-order approximate coupled-cluster) diagnostics have been calculated for studied molecules. Complete active space self-consistent field (10,10)/6-31G(d) single point energy calculations have been carried out for all optimized structures. Multireference character of the ground state becomes important when the number of atoms in the polycyclic hydrocarbon exceeds 40-50. At this point, D1 diagnostics reaches 0.04-0.05 and the squared configuration interaction expansion coefficient for dominant configuration drops to about 0.6. However, only for the three largest systems predominantly polyradicalic ground states have been detected. All other polycyclic hydrocarbons showing significant multiconfigurational character of singlet ground state have only two dominant configurations which are closed shell singlet and doubly excited singlet, respectively. Thus, small polycyclic hydrocarbons have mostly single reference singlet ground state, the medium size systems have notably multireference ground state (singlet or triplet) with only moderate polyradicalic character. The ground state of largest systems is singlet polyradical.

Stabilization of gold nanowires inside nanoaggregates of cyclo[8]thiophene, cyclo[8]selenophene, and cyclo[8]tellurophene: a theoretical study.

The stabilities and electronic properties of gold clusters containing up to six atoms trapped inside cyclo[8]thiophene (CS8), cyclo[8]selenophene (CSe8), and cyclo[8]tellurophene (CTe8) nanoaggregates have been studied using the M06 functional. The 6-31G(d) basis set was used for all atoms except Au and Te, for which the LANL2DZ(d,p) pseudopotential basis set was applied. Single-point energy calculations were performed with the 6-311G(d,p) basis set for all atoms except for Au and Te, for which the cc-TZVP-pp pseudopotential basis set was used. Among the three studied macrocycles, only CS8 and CSe8 were found to be capable of nanoaggregate formation. In the lowest-energy conformer of CTe8, the tellurophene fragments adopt an anti orientation, thus impeding a tubular arrangement of the macrocycles. The formation of gold clusters inside the CS8 and CSe8 nanoaggregates is a thermodynamically favorable process, and could represent a potentially useful method of stabilizing metal nanowires. The binding energy between the nanoaggregate and the gold cluster is always higher for selenium-containing complexes than for sulfur-containing ones because Se has a higher affinity than S for Au in such complexes. Interactions of the gold cluster with the nanoaggregate walls can change the geometry of the most stable isomer for the cluster. The relative energies of different isomers are rather similar, suggesting that they coexist. For nanoaggregates containing Au6 clusters, the cluster geometry when it is inside a nanoaggregate is different from the geometry of the cluster when it is not inside the nanoaggregate, due to the geometric restrictions imposed by the nanoaggregate cavity. The reorganization energy needed to change the geometry leads to lower binding energies for these complexes compared to those of some smaller systems, although the formation of a complex between Au6 and a nanoaggregate with six CS8 or CSe8 macrocycles is still thermodynamically viable.

Reactions of ketones with aromatics in acid media. The effect of trifluoromethyl groups and the acidity media. A theoretical study.

The reactions of acetone, 2,2,2-trifluoroacetone and hexafluoroacetone in methanesulfonic (MSA) and triflic acids (TFSA) with benzene have been studied at M06-2X/6-311+G(d,p) level using cluster-continuum model, where the carbonyl group is explicitly solvated by acid molecules. The introduction of a trifluoromethyl group into the ketone structure reduces the activation energy of the tetrahedral intermediates formation due to an increase of the electrophilicity of the carbonyl group and raises the activation and the reaction energies of the C-O bond cleavage in formed carbinol due to the destabilization of the corresponding carbocation. The introduction of the second trifluoromethyl group inhibits the hydroxyalkylation reaction due to a very strong increase of the reaction and activation energies of the C-O bond cleavage which becomes the rate determining step. The most important catalytic effect of TFSA compared to MSA is not the protonation of the ketone carbonyl, but the reduction of the activation and reaction energies of the carbinol C-O bond cleavage due to better protosolvation properties. Even for TFSA no complete proton transfer to carbonyl oxygen has been observed for free ketones. Therefore, the protonation energies of free ketones cannot be considered as a measure of ketone reactivity in the hydroxyalkylation reaction.

Rectifying behavior of charge transfer complexes of tetrakis(dimethylamino)ethene with acceptor molecules: a theoretical study.

The effect of electric field induced electron transfer on the rectification properties of molecular rectifiers based on charge transfer complexes of tetrakis(dimethylamino)ethane (TDAE) with acceptor molecules was explored. The current-voltage curves and the rectification ratios (RR) for two different molecular rectifiers were obtained using a direct ab initio method at M06/LACVP(d) level of theory in the range from -2 to +2 V. The highest RR of 25.7 was determined for the complex of TDAE with 2-nitropyrene-4,5,9,10-tetraone at 0.5 V (D1), while another rectifier [complex of TDAE with 2,7-dimethyl nitropyrene-4,5,9,10-tetraone (D2)] showed a maximum RR of only 2.9 at 0.3 V. The electric field induced electron transfer occurring in D1 creates a one-way conducting channel consisting of two SOMOs involving the entire D1 complex. In the case of D2, no electron transfer occurs at the applied bias voltages due to the relatively high energy difference between HOMO and LUMO.

Two-photon excited fluorescence of silica nanoparticles loaded with a fluorene-based monomer and its cross-conjugated polymer: their application to cell imaging.

In this work the two-photon activity of nanoparticles obtained from a fluorene monomer (M1) and its cross-conjugated polymer (P1) is reported. Aqueous suspensions of M1 and P1 nanoparticles prepared through the reprecipitation method exhibited maximum two-photon absorption (TPA) cross-sections of 84 and 9860 GM (1 GM = 10(-50) cm(4) s) at 740 nm, respectively, and a fluorescence quantum yield of ~1. Such a two-photon activity was practically equal with respect to that for molecular solutions of M1 and P1. These materials were then successfully encapsulated into silica nanoparticles to provide bio-compatibly. A lung cancer cell line (A549) and a human cervical cancer cell line (HeLa cells) were incubated with our fluorescent silica nanoparticles to carry out two-photon imaging. By means of these studies we demonstrate that optimized nonlinear optical polymers loaded in silica nanoparticles can be used as efficient probes with low cytotoxicity and good photostability for two-photon fluorescence microscopy. To the best of our knowledge, studies concerning polymer-doped silica nanoparticles exhibiting large two-photon activity have not been reported in the literature.