Pentagonal Sites Research Articles

Probing the structural and optical properties of Eu-doped LiAl5O8 using X-ray absorption and hard X-ray excited optical luminescence

X-ray absorption and hard X-ray excited optical luminescence were used to probe the structural and optical properties of LiAl5O8 doped with different Eu concentrations produced by the modified sol-gel method. X-ray diffraction studies revealed that all samples produced have a cubic spinel unit cell of LiAl5O8 and that some impurities of the EuAlO3 phase were observed at concentration limits of 4 and 5 %. Based on the XAS results, they show the stabilization of europium in both the trivalent and bivalent states and reveal that Eu3+ ion can stabilize both in distorted octahedral configurations (non-centrosymmetric) and in the form of pentagonal bipyramid sites. The XEOL spectra were excited at Eu LIII-edge is composed of typical emission peaks attributed to the emission of Eu3+ ions and contaminating ions (Cr3+ and Fe3+). An XEOL luminescence mechanism was also discussed based on the XEOL emission spectra.

The impact of lead on structural, electronic, and vibrational properties of pristine C36 and its boron, nitrogen-dopant C36 on pentagonal and hexagonal rings

In this work, lead's impacts on the structural, electronic, and vibrational characteristics of pure C36 and C36 doped with boron and nitrogen on pentagonal and hexagonal rings are calculated using the DFT method. We provide an in-depth examination of these characteristics and how lead affects them. The adsorption energy, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and electrostatic potentials at the isosurface (EPI) have been studied in detail. Assess the ring dependence of the lead adsorption energy for the hexagonal and pentagonal rings of pristine C36 and B and N impurities at the substitutional site. The adsorption energy for the Pb on pristine C36 is endothermic; however, it is exothermic for the Pb on NB-doped C36 but has different values on the pentagonal and hexagonal rings. The results demonstrate that nitrogen and boron doping significantly increased the Pb atom's adsorption energy when the dopants occupied pentagonal sites. In addition, the bond lengths surrounding the impurity and the bond lengths of Pb atoms placed on pristine C36, BN-doped hexagonal ring, and BN-doped pentagonal ring in C36 are investigated. The electrostatic potential isosurface (EPI) surrounding the BN-doped hexagonal and pentagonal rings of C36 is described. The Mulliken charge transfer analysis demonstrates that the interaction between NB-doped C36 fullerene and lead (Pb) has a substantial effect on the charge transfer process and the reactivity of the doped system towards lead. However, the quantity of charge transmitted depends on the impurity's location on the hexagonal and pentagonal rings. These findings aid in the atomistic understanding of the interaction between a single Pb atom on the surface of C36 and BN-doped C36 at various rings. Our findings not only shed light on the complex interplay between lead and C36 but also offer valuable insights into potential applications in materials engineering and environmental remediation.

Calcium-Decorated Polygon-Graphenes for Hydrogen Storage

We performed first-principles calculations to investigate hydrogen (H2) storage properties of bare and calcium (Ca)-decorated polygon-graphenes, i.e., biphenylene and ψ-graphene monolayers consisting of polygons, from tetragons to octagons. In pristine forms, both biphenylene and ψ-graphene bind H2 weakly. However, upon Ca doping, biphenylene adsorbed up to five H2 molecules regardless of polygonal sites, whereas ψ-graphene anchored up to six and five H2 molecules to pentagonal and heptagonal sites, respectively. In all the cases, the H2 binding energy was ∼0.30 eV, enabling reversible room-temperature H2 storage. The H2 storage capacity can reach ∼6.8 and ∼4.2 wt % for Ca-decorated biphenylene and ψ-graphene, respectively. Using equilibrium thermodynamics, we showed the adsorption and desorption of H2 at 300 and 380 K under ambient pressure, respectively. This clearly indicates that Ca-decorated 2D sp2 carbon sheets with polygons (biphenylene, ψ-graphene) could be used as promising H2 storage materials.

Role of coordination site in governing the structural, electronic and optical properties of Ca-doped strontium barium niobate

The structural, electronic and optical properties of Ca doped SBN60 are studied via first principle calculations based on density functional theory. Influence of Sr site substitution by Ca at square and pentagonal sites has been studied. The bandgap of Ca incorporated SBN is found to depend largely on the coordination of site of doping, where the bandgap increases for the square site substitution and decreases for the pentagonal site substitution. The formation energy calculations reveal that the Ca is more favorable to occupy the pentagonal site which is supported by the UV-Visible spectra study of the thin films of doped and undoped SBN60 grown using PLD technique in which a decrease in the bandgap can be seen in Ca doped SBN. A detailed analysis is provided for the changes in structural parameters upon doping at square and pentagonal sites. Dependence of the optical properties like dielectric constant, refractive index, reflectance and absorption of doped and undoped SBN60 on different polarization directions ‘(100), (010) and (001)’ are reported. The anisotropy in the optical properties can be observed which makes Ca doped SBN a promising material for optoelectronic device applications.

Riemannian Surface on Carbon Anodes Enables Li-Ion Storage at -35 °C.

Since sluggish Li+ desolvation leads to severe capacity degradation of carbon anodes at subzero temperatures, it is urgently desired to modulate electron configurations of surface carbon atoms toward high capacity for Li-ion batteries. Herein, a carbon-based anode material (O-DF) was strategically synthesized to construct the Riemannian surface with a positive curvature, which exhibits a high reversible capacity of 624 mAh g–1 with an 85.9% capacity retention at 0.1 A g–1 as the temperature drops to −20 °C. Even if the temperature drops to −35 °C, the reversible capacity is still effectively retained at 160 mAh g–1 after 200 cycles. Various characterizations and theoretical calculations reveal that the Riemannian surface effectively tunes the low-temperature sluggish Li+ desolvation of the interfacial chemistry via locally accumulated charges of non-coplanar spx (2 < x < 3) hybridized orbitals to reduce the rate-determining step of the energy barrier for the charge-transfer process. Ex-situ measurements further confirm that the spx-hybridized orbitals of the pentagonal defect sites should denote more negative charges to solvated Li+ adsorbed on the Riemannian surface to form stronger Li–C coordinate bonds for Li+ desolvation, which not only enhances Li-adsorption on the curved surface but also results in more Li+ insertion in an extremely cold environment.

Investigation of active sites using solid state 27Al and 31P MAS NMR in ceramic amorphous aluminophosphate materials prepared from different potassium salts of phosphate for the synthesis of diphenyl urea derivatives

Ceramic amorphous aluminophosphate (CAmAlP) catalysts were prepared by precipitation method using different phosphate salts of potassium such as KH2PO4, K4P2O7 and K2HPO4 as well as H3PO4. The prepared materials were characterized by PXRD, FT-IR, XPS, SEM, BET Surface area, NH3-TPD, 27Al NMR and 31P NMR analytical methods. The catalytic activity of the materials was checked in the synthesis of diphenyl urea (DPU) from aniline and diethyl carbonate, under refluxing conditions. Further, the general application of the catalysts was tested using various substituted anilines. The recyclability of the catalysts was also studied. Uncertainties in percentage yields were calculated to check the reproducible surface properties. The P-XRD, BET Surface area and NH3-TPD results indicated that the materials were amorphous with mesoporous texture, surface areas and acidities in the range 200–260 m2/g and 0.4–0.7 mmol/g respectively. 27Al NMR studies revealed that Al is present in three different coordination states such as tetrahedral, pentagonal and octahedral. The relative percentages of these Al sites depends on the type of the potassium precursor phosphate salt used. Both tetrahedral and pentagonal Al sites in conjunction with each other represented catalytically active sites. An increase in the pentagonal sites contributed to additional increments to the catalytic activity of CAmAlP. The catalyst prepared from KH2PO4 was found to be the best and demonstrated 96% DPU yield.

Atomic resolution imaging of cation ordering in niobium\u2013tungsten complex oxides

Energy dispersive X-ray emission imaging at atomic resolution is a powerful tool to solve order–disorder problems in complex metal oxide crystals, supplementing conventional X-ray or neutron diffraction. Here, we use this method, based on scanning transmission electron microscopy, to investigate cation ordering in ternary metal oxides 4Nb2O5·9WO3 and 2Nb2O5·7WO3, which have recently attracted attention as energy storage materials in lithium-ion batteries. Their crystal structures are a tetragonal tungsten bronze-type and its hybrid with a ReO3-type ‘block structure’, respectively. Our study reveals the presence of chemical ordering of metal ions in these materials, which have previously been assumed to be solid-solutions. In particular, we show that the two types of cations, Nb and W, are well ordered in their lattices, and that the Nb ions tend to occupy one third of the pentagonal channel sites. These results demonstrate that atomic resolution X-ray emission imaging is an effective alternative approach for the study of locally ordered crystal structures.

Aromatic penta-linked hydrocarbons in soot nanoparticle formation

A new crosslinking reaction between two pentagonal rings around the periphery of planar pericondensed aromatic molecules is proposed and its impact on soot nanoparticle formation explored. The reaction mechanism was computed, using density functional theory, between an aryl-type σ-radical on a rim-based pentagonal ring attacking another rim-based pentagonal ring. A hydrogen migration allowed for the formation of a double bond forming a planar aromatic penta-linked hydrocarbon (APLH) complex, recently experimentally observed. The clustering of this planar species is compared with a pericondensed polyaromatic hydrocarbon (PCAH) and an aromatic aryl-linked hydrocarbon (AALH) using molecular dynamics and metadynamics. Similar clustering is found for the investigated species compared with a pericondensed structure of similar mass indicating enhanced physical interactions after forming the crosslink. Finally, a further crosslink is possible between the unsaturated pentagonal ring sites forming an aromatic rim-linked hydrocarbon (ARLH) complex of considerable stability. This was confirmed by simulating the stable molecular dynamics of such a complex with on-the-fly quantum forces from a quantum semi-empirical method, revealing possible reactions under flame conditions that might play a role in soot nucleation.

Yttrium-decorated C48B12 as hydrogen storage media: A DFT study

We report a density functional theory calculation dedicated to analyze the behavior of hydrogen adsorption on Yttrium-decorated C48B12. Electron deficient C48B12 is found to promote charge transfer between Y atom and substrate leading to an enhanced local electric field which can significantly improve the hydrogen adsorption. The analysis shows that Y atoms can be individually adsorbed on the pentagonal sites without clustering of the metal atoms, and each Y atom can bind up to six H2. molecules with an average binding energy of −0.46 eV/H2, which is suitable for ambient condition hydrogen storage. The Y atoms are found to trap H2 molecules through well-known “Kubas-type” interaction. Our simulations not only clarify the mechanism of the reaction among C48. B12, Y atoms and H2 molecules, but also predict a promising candidate for hydrogen storage application with high gravimetric density (7.51%).

Ammonia Thermal Treatment toward Topological Defects in Porous Carbon for Enhanced Carbon Dioxide Electroreduction.

Topological defects, with an asymmetric local electronic redistribution, are expected to locally tune the intrinsic catalytic activity of carbon materials. However, it is still challenging to deliberately create high-density homogeneous topological defects in carbon networks due to the high formation energy. Toward this end, an efficient NH3 thermal-treatment strategy is presented for thoroughly removing pyrrolic-N and pyridinic-N dopants from N-enriched porous carbon particles, to create high-density topological defects. The resultant topological defects are systematically investigated by near-edge X-ray absorption fine structure measurements and local density of states analysis, and the defect formation mechanism is revealed by reactive molecular dynamics simulations. Notably, the as-prepared porous carbon materials possess an enhanced electrocatalytic CO2 reduction performance, yielding a current density of 2.84 mA cm-2 with Faradaic efficiency of 95.2% for CO generation. Such a result is among the best performances reported for metal-free CO2 reduction electrocatalysts. Density functional theory calculations suggest that the edge pentagonal sites are the dominating active centers with the lowest free energy (ΔG) for CO2 reduction. This work not only presents deep insights for the defect engineering of carbon-based materials but also improves the understanding of electrocatalytic CO2 reduction on carbon defects.

Ammonia Thermal Treatment Toward Topological Defects in Porous Carbon for Enhanced Carbon Dioxide Electroreduction

The design of specific active sites for the carbon dioxide electro-reduction reaction (CO2RR), is crucial for determining product selectivity and catalysis performance. Here, we present an efficient NH3 thermal treatment for thoroughly removing the pyrrolic-N and pyridinic-N dopants from 3D N-enriched graphene analogue particles, to create high density of topological defects as active sites for CO2RR.[1] Firstly we found that NH3 showed a pronounced effect of eliminating specific N-containing moieties at elevated temperatures, superior to Ar. The identification of topological defects (pentagonal carbon polygon and 585 defect) was investigated by near-edge X-ray absorption fine structure measurements and local density of states analysis, and its formation mechanism was revealed by reactive molecular dynamics simulations.The as-prepared catalysts exhibited excellent performance in flooded half-cell experiments in 0.1 M KHCO3 reaching current densities of 2.8 mA cm-2 and 4.4 mA cm-2 with FECO of 92.7% and 89.2% at -0.6 V and -0.7 V v.s. RHE, respectively at room temperature and atmosphere pressure. These results are among the best performances reported for metal-free catalysts.[2,3,4,5] Density functional theory calculations revealed that the edged pentagonal sites (penta-1 and 585-1 defects) are the dominating active centers with the lowest free energy barriers of CO2RR for CO production.[6,7] References [1] Zhu J.; Huang J.; Mei W. et al, Angew. Chem. Int. Ed. 2019, 58, 3859-3864[2] Wu, J.; Liu, M.; Sharma, P. P. et al, Nano Lett. 2016, 16 (1), 466-70[3] Wu, J.; Yadav, R. M.; Liu, M. et al, ACS Nano 2015, 9 (5), 5364-5371.[4] Daiyan, R.; Tan, X.; Chen, R. et al, Acs Energy Lett. 2018, 3 (9), 2292-2298.[5] Kumar, B.; Asadi, M.; Pisasale, D. et al, Nat. Commun. 2013, 4, 2819.[6] Banhart F., Kotakoski J., Krasheninnikov A.V. ACS Nano, 2011 , 5, 29-41.[7] Dong, Y.,zhang Q.J. et al, Ammonia Thermal Treatment toward Topological Defects in Porous Carbon for Enhanced Carbon Dioxide Electroreduction. (submitted) Figure 1

Effects of A-site cations on the electrical behaviors in (Sr1-xCax)2.1Na0.8Nb5O15 tungsten bronze ferroelectrics

In order to discuss the correlation of composition-structure-performances, it is essential to study the role of Sr/Ca ratio variations on the physical properties in (Sr1-xCax)2.1Na0.8Nb5O15 ceramics with nearly ‘filled’ TB structure. Similar micromorphology of all obtained SCNN ceramics according to the FESEM results was beneficial to reveal the intrinsic structural factors for the electrical performances. Two different dielectric anomalies appeared in the dielectric spectra for all specimens: (1) the high-temperature anomaly corresponded to the ferroelectric phase transition (ferroelectric 4 mm to paraelectric 4/mmm transition); (2) whereas for the low-temperature anomaly, introducing Ca2+ in the Sr2.1Na0.8Nb5O15 composition could force some Na+ ions to occupy bigger pentagonal A2 sites, which could induce the slight distortion of NbO6 octahedron (considered as the subtle structure effects) to further evolve into the structural phase transition from ferroelectric (4 mm)-to-ferroelastic (mm2) (ferroelastic phase transition) at higher Ca2+ contents. The existence of mm2 phase in SCNN ceramics with x > 0.075 could be further confirmed by XRD refinement and Raman results. This finding is helpful for improvement of the fundamental theory and modulation of the performances in tungsten bronze niobates.

Structural investigation of the “tripled-tetragonal-tungsten-bronze” phases Sr2M10−xO27−y (M = Nb, Ta)

Sr2Nb10O27 and Sr2Ta10−xO27−y exhibit the tripled tetragonal-tungsten-bronze (TTTB)-related superstructures. In contrast to Sr2Ta10−xO27−y, where an extended solid solution was found, Sr2Nb10O27 only forms a narrow range of solid solution, Sr1−xNb10O27−z (0 ≤ x ≤ 0.04). Serial X-ray and neutron diffraction studies show that both compounds can be modelled in the orthorhombic Pba2 space group. Lattice parameters obtained from powder X-ray refinements are a = 12.3887(14) Å, b = 37.103(4) Å, c = 3.866(2) Å, for Sr2Ta10−xO27−y, and a = 12.3888(2) Å, b = 36.9839(5) Å, and c = 3.94027(3) Å for Sr2Nb10O27. Neutron Rietveld refinements of both structures revealed the possible reason for the differing solid solution behavior. For Sr2Nb10O27, all Nb sites are fully occupied, whereas in Sr2Ta10−xO27−y, three out of the nine crystallographically unique Ta sites were shown to be partially occupied. Charge adjustment is therefore possible by increasing the Sr concentration, leading to a solid solution series and smaller Ta/Sr ratio as compared to Sr2Nb10O27. In the TTTB-related structures, Nb and Ta ions both occupy octahedral and pentagonal bipyramidal coordination sites. Layers of interconnected corner-shared octahedra were found parallel to the ab plane, with 3-, 4- and 5-fold tunnels found in the structure running parallel to the c-axis. Sr ions partially fill the 4- and 5-fold tunnels.

Reconstruction of the Al13Ru4(010) Approximant Surface Leading to Anisotropic Molecular Adsorption

The atomic structure of the Al13Ru4(010) approximant surface, investigated for annealing temperatures ranging from 873 K to 1143 K, can be described by pentagonal motifs and vacancies. It exhibits an atypical surface reconstruction which manifests itself in real space by well-separated stripes running about 10○ off the [001] direction. Their mutual ordering is drastically improving at high annealing temperature. This is simultaneously accompanied by a filling of the pentagonal hollow sites present in the stripes with atoms or groups of atoms. We provide evidence that these unidirectional features are of true surface origin. The bulk electronic density of states calculated within the density functional theory framework matches the experimental valence band spectra. In addition to step edges, molecular adsorption reveals that stripes are initially the most reactive sites, leading to anisotropic island growth.

Giant fullerene formation through thermal treatment of fullerene soot

Coalescence of fullerenes into giant fullerenes (Cn n > 100) has been observed in the gas phase and inside carbon nanotubes. In this work, we demonstrate the formation of giant fullerenes by heating fullerene soot. Extracting the majority of the magic number fullerenes (C60 and C70) allowed the underlying distribution of fullerenes in the solid state to be measured. Upon heating at 800–1000 °C for 30–60 min under vacuum the mass distribution of fullerenes was found to shift toward larger masses. High resolution electron microscopy was used to compare formation of giant fullerenes by thermal heating and electron beam irradiation, showing the former produced more isolated giant fullerene fragments >1 nm in size. The driving force for coalescence and growth by thermal heating was suggested to be vertex strain at pentagonal sites, providing a route towards the synthesis of fullerenes up to C300.

Ab initio infrared vibrational modes for neutral and charged small fullerenes (C 20 , C 24 , C 26 , C 28 , C 30 and C 60 )

We calculate the infrared (IR) absorption spectra using DFT B3LYP(6-311G) for a range of small closed-cage fullerenes, Cn, n=20, 24, 26, 28, 30 and 60, in both neutral and multiple positive and negative charge states. The results are of use, notably, for direct comparison with observed IR absorption in the interstellar medium. Frequencies fall typically into two ranges, with C-C stretch modes around 1100-1500 cm(-1) (6.7-9.1 μm) and fullerene-specific radial motion associated with under-coordinated carbon at pentagonal sites in the range 600-800 cm(-1) (12.5-16.7 μm). Notably, negatively charged fullerenes show significantly stronger absorption intensities than neutral species. The results suggest that small cage fullerenes, and notably metallic endofullerenes, may be responsible for many of the unassigned interstellar IR spectral lines.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Active site structure and absorption spectrum of channelrhodopsin-2 wild-type and C128T mutant.

In spite of considerable interest, the active site of channelrhodopsin still lacks a detailed atomistic description, the understanding of which could strongly enhance the development of novel optogenetics tools. We present a computational study combining different state-of-the-art techniques, including hybrid quantum mechanics/molecular mechanics schemes and high-level quantum chemical methods, to properly describe the hydrogen-bonding pattern between the retinal chromophore and its counterions in channelrhodopsin-2 Wild-Type and C128T mutant. Especially, we show by extensive ground state dynamics that the active site, containing a glutamic acid (E123) and a water molecule, is highly dynamic, sampling three different hydrogen-bonding patterns. This results in a broad absorption spectrum that is representative of the different structural motifs found. A comparison with bacteriorhodopsin, characterized by a pentagonal hydrogen-bonded active site structure, elucidates their different absorption properties.

Selective Occupancy of Sites by Rare Earths in K&lt;sub&gt;2&lt;/sub&gt;Nd&lt;sub&gt;(1-X)&lt;/sub&gt;Eu&lt;sub&gt;x&lt;/sub&gt;Nb&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;15&lt;/sub&gt; Nanopowders, where X = 0, 0.0025, 0.025, 0.05 and 0.1, Prepared by Modified Polyol Method

Niobates with tetragonal tungsten bronze TTB-type structure have presented great technological potential due to their dielectric, ferroelectric, pyroelectric properties. The preparation by the modified polyol method and structural characterization of K2Nd(1-x)EuxNb5O15 nanopowders, where x = 0; 0.0025; 0.025; 0.05 e 0.1, were investigated. The structural parameters were analyzed as a function of concentration of europium ions in the K2NdNb5O15 host structure using the Rietveld method. From structural parameters was determined the TTB-type structure with tetragonal symmetry, where the pentagonal sites is occupied by K+ and Eu3+ ions and tetragonal sites is occupied only by Nd3+ ions. The addition of europium in the host structure led to a decrease in the lattice parameters, compatible with the increasing degree of distortion of NbO6 polyhedra. The average crystallite size showed values between 18.25 and 26nm.

X-ray standing wave study of Si clusters on a decagonal Al-Co-Ni quasicrystal surface

Quantitative adsorption structure determinations on quasicrystals are scarce because most techniques for measuring surface structures are not well suited to the complex and infinite unit cells of quasicrystals. The normal incidence standing x-ray wave field technique presents a solution to these problems because it can be made inherently surface sensitive and does not involve extensive computational effort. We describe a method for applying this technique to adsorbates on quasicrystals, with specific application to a submonolayer of Si atoms on a decagonal Al-Co-Ni surface. We demonstrate the sensitivity of the technique to both adsorption site and geometry, leading to the conclusion that the Si atoms, which form six-atom pentagonal clusters, have an average height of $1.77\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\AA{}$ above pentagonal hollow sites, with a significant height variation among the Si atoms in the cluster. In particular, the central Si atom sits more deeply than the five surrounding Si atoms, which are, on average, $2.7\phantom{\rule{0.16em}{0ex}}\AA{}$ away from the central Si atom. Although this study was performed on a decagonal quasicrystal that is periodic perpendicular to the surface, we describe how the technique can be applied to cases with no periodicity.

Half-metal behaviour mediated by self-doping of topological line defect combining with adsorption of 3d transition-metal atomic chains in graphene

We present a systematic study of 3d transition-metal (TM) atomic chains adsorbed on a line defect consisting of octagonal and pentagonal carbon rings embedded in a perfect graphene sheet. The results suggest that Sc, Ti, V, Cr and Mn atoms can be spontaneously adsorbed at octagonal sites and form an atomic chain along the line defect in graphene, while Fe, Co and Ni atomic chains are adsorbed at pentagonal sites. Moreover, the Sc, Ti, V, Cr, Mn and Fe chains embedded in line defect of graphene structures show half-metallic characters. These results propose a new route to explore the potential applications of the system in spintronic devices.