Adsorption Of Dimers Research Articles

Clarification of Au dimer adsorption sites on Si (111)-7 × 7 surface by AFM/KPFM

Atomic-scale surface adsorption has been a significant research topic in recent years, with a particular emphasis on the adsorption properties of Au/Si(111)-7 × 7, which are vitally important for pioneering future novel semiconductor devices. Here, we investigated the adsorption of Au dimers on the Si(111)-7 × 7 surface with atomic resolution using non-contact atomic force microscopy and Kelvin probe force microscopy at room temperature. Our results show that the Au dimer adsorbs in the vicinity of the Si rest atoms, exhibiting a distinct localized electron distribution. In density functional theory calculations, three candidate Au dimer adsorption sites have been identified, and the most stable site of Au dimer adsorption aligns with experimental findings. Furthermore, the local electron transfer of Au dimer adsorption has been analyzed, confirming the distribution of electrons around the Au dimer adsorption site. This research reveals that the structure and charge transfer of adsorbed Au dimers on Si(111)-7 × 7 provide insight into the mechanism of the metal-semiconductor system.

Quantum-chemical modeling of sorption interactions of histidine enantiomers with carbon nanotubes

In this study, the elementary act of adsorption of monomers and dimers of histidine enantiomers on a dextrorotatory model CNT-(7,6) chirality from an aqueous solution was studied using quantum chemistry methods to interpret the adsorption isotherm of L- and D-histidine on carbon nanotubes mkNANO MKN-SWCNT S1 and identify the mechanism of sorption sorbent-sorbate interactions. Quantum chemical modelling of the structures was carried out using the GAUSSIAN 09 program by the B3LYP/6-31G(d,p) GD3 method; the influence of the environment was considered using the Tomasi polarization continuum model (PCM). The results of quantum chemical modelling have established a greater number of point interactions of nitrogen and oxygen atoms of the D-isomer with the dextrorotatory CNT, which determines the higher adsorption energy of the D-histidine monomer and dimer on the CNT compared to the L-isomer. It was shown that enantiomers are attached to nanotubes mainly by van der Waals forces and π-π interactions between the imidazole ring of histidine and the carbon nanotube. The contribution of π-π stacking interactions to the adsorption value was assessed using quantum chemistry methods. The formation of 4 different L-amino acid dimers is possible on the surface of CNT: 2L1, 2L2, 2L3, 2L4 and also for the D isomer: 2D1; 2D2; 2D3; 2D4. The results of quantum chemical modelling showed that dimers 2L4 and 2D4 have the highest adsorption energy on CNT. Quantum chemistry methods were also used to calculate the average energy of intermolecular H bonds in seven, eight, and thirteen particle clusters of L-histidine, as well as in seven- and nine-particle clusters of the D-enantiomer. The calculation showed that this value increases with increasing cluster size. This led to a significant contribution of hydrogen bonding to the decrease in the energy of the sorption system during cluster adsorption.

Study on the catalytic hydrodeoxygenation of lignin dimers: Adsorption properties and linkages cleavage

Production of mono-phenols through hydrodeoxygenation is one of the most promising routes for value-added lignin valorization. However, the adsorption characteristic of key intermediates and hydrodeoxygenation mechanism of key linkages in lignin have received inadequate attentions. In this paper, experiments combined with density functional theory calculations were done to explore the adsorption and catalytic HDO mechanism of lignin dimers. It was found that NiFe(111)-Mo2C(001) had a better ability on linkages activation, and showed stronger adsorption on CO containing intermediates, which was favor for further hydrodeoxygenation. Moreover, the calculation results certificated the cleavage of β-O-4 was prior to the hydrodeoxygenation of CO, and the hydrodeoxygenation of β-O-4 included a H· addition to O atom before the C-O cleavage. Finally, the elementary reactions energy barriers were efficiently reduced by NiFe(111)-Mo2C(001) catalyst during the hydrodeoxygenation reactions, which elucidated the superior performance of NiFe catalyst. This work provides a theoretical basis on efficient lignin utilization.

Exploring the Impact of H/D Isotope Effects of Hydrogen Bond Properties in Water Dimer, Trimer, and Water Adsorption on the Ice Ih(0001) Surface

Exploring the Impact of H/D Isotope Effects of Hydrogen Bond Properties in Water Dimer, Trimer, and Water Adsorption on the Ice Ih(0001) Surface

Surface coverage dynamics for reversible dissociative adsorption on finite linear lattices.

Dissociative adsorption onto a surface introduces dynamic correlations between neighboring sites not found in non-dissociative absorption. We study surface coverage dynamics where reversible dissociative adsorption of dimers occurs on a finite linear lattice. We derive analytic expressions for the equilibrium surface coverage as a function of the number of reactive sites, N, and the ratio of the adsorption and desorption rates. Using these results, we characterize the finite size effect on the equilibrium surface coverage. For comparable N's, the finite size effect is significantly larger when N is even than when N is odd. Moreover, as N increases, the size effect decays more slowly in the even case than in the odd case. The finite-size effect becomes significant when adsorption and desorption rates are considerably different. These finite-size effects are related to the number of accessible configurations in a finite system where the odd-even dependence arises from the limited number of accessible configurations in the even case. We confirm our analytical results with kinetic Monte Carlo simulations. We also analyze the surface-diffusion case where adsorbed atoms can hop into neighboring sites. As expected, the odd-even dependence disappears because more configurations are accessible in the even case due to surface diffusion.

Physical properties of a generalized model of multilayer adsorption of dimers.

We investigate the transport properties of a complex porous structure with branched fractal architectures formed due to the gradual deposition of dimers in a model of multilayer adsorption. We thoroughly study the interplay between the orientational anisotropy parameter p_{0} of deposited dimers and the formation of porous structures, as well as its impact on the conductivity of the system, through extensive numerical simulations. By systematically varying the value of p_{0}, several critical and off-critical scaling relations characterizing the behavior of the system are examined. The results demonstrate that the degree of orientational anisotropy of dimers plays a significant role in determining the structural and physical characteristics of the system. We find that the Einstein relation relating to the size scaling of the electrical conductance holds true only in the limiting case of p_{0}→1. Monitoring the fractal dimension of the interface of the multilayer formation for various p_{0} values, we reveal that in a wide range of p_{0}>0.2 interface shows the characteristic of a self-avoiding random walk, compared to the limiting case of p_{0}→0 where it is characterized by the fractal dimension of the backbone of ordinary percolation cluster at criticality. Our results thus can provide useful information about the fundamental mechanisms underlying the formation and behavior of wide varieties of amorphous and disordered systems that are of paramount importance both in science and technology as well as in environmental studies.

DFT study of Ag-X (X = Pd, Pt, Ag) decorated (8,0) single wall carbon nanotube for spintronic applications

The main purpose of the present article is an investigation of the electronic and magnetic properties of (8,0) single-wall carbon nanotube (CNT80) decorated with a diatomic configuration of Ag and X (X = Pd, Pt, and Ag). Density functional theory and the Grimme-D2 approach in spin polarization mode are employed. It is concluded that Ag and X are adsorbed on CNT80 in the dimer form rather than singly atomic adsorption. The spin-distinct electronic band structure and the densities of states are also calculated. The calculations show that Ag-Pd/CNT80 is a bipolar semiconductor with an energy gap of 0.27 eV for spin-up, 0.47 eV for spin-down, and 0.15 eV for spin-flip electrons. Ag-Pt/CNT80 is an anti-ferromagnetic p-type indirect semiconductor with a gap of 0.187 eV for temperatures below 188 K. After 188 K up to around 330 K, this structure converts to an indirect bipolar semiconductor, with an energy gap of 0.31 eV (0.37 eV) for the spin-up (down) channel. Adsorption of Ag dimer onto CNT80 leads to the p-type nonmagnetic semiconductor. As an application of the proposed materials, the absorption of CO as a polar molecule on the Ag-Pd/CNT80 structure is also investigated, and a proposed structure as a spintronic CO sensor is introduced.

Effect of pressure and CO2 dilution on soot formation in laminar inverse coflow flame at conditions close to autothermal reforming

Autothermal reforming (ATR) is an important technology for hydrogen production from natural gas, where soot formation deserves particular attention as it may cause catalyst poisoning. The typical configuration of ATR reactors is that of an inverse diffusion flame (IDF). In this study, soot formation near ATR conditions is investigated experimentally and numerically, focusing on the effects of pressure and CO2 dilution in IDFs with pressure reaching 5 bar. The fuel stream consists of methane diluted with CO2 or N2. The mole fraction of O2 in the oxidant stream is 70%. Polycyclic aromatic hydrocarbons (PAHs) and soot volume fraction are measured by the planar laser-induced fluorescence (PLIF) and planar laser-induced incandesce (PLII) methods. High-fidelity simulations with a detailed soot aerosol model are also performed, and an empirical reactive soot inception model is proposed. A comparison of the results shows that the soot behavior is well predicted by the empirical reactive inception model but not fully captured by the physical inception model based on irreversible dimerization, demonstrating the importance of radical involvement in the soot inception process. Both measurements and predictions show a linear relationship between peak soot volume fraction and pressure, which can be explained by the linear increase of PAH molar concentration with pressure. Simulation analysis indicates that the dimer adsorption and HACA mechanism have a similar quantitative contribution to the soot growth. The CO2 shows stronger suppression effects in soot formation at the investigated pressures compared to N2. The contributions of chemical and thermal effects to soot suppression are numerically analyzed and the results indicate that the chemical effect of CO2 is mainly due to the removal of H radicals through the reaction CO2 + H→CO + OH, providing a soot suppression contribution of two times larger compared to the thermal one.

Cluster approximation applied to multisite-occupancy adsorption: configurational entropy of the adsorbed phase for dimers and trimers on triangular lattices.

The adsorption of dimers and trimers on triangular lattices is studied by combining theoretical modeling and Monte Carlo (MC) simulations. The thermodynamic process is analyzed through the behavior of the configurational entropy per site of the adsorbed phase as a function of the coverage. MC calculations, supplemented by the thermodynamic integration method, are performed in the grand canonical ensemble. The theoretical model used in the present study is called Cluster Approximation (CA), and it is based on exact calculation of states on finite cells. An efficient algorithm allows us to determine the detailed structure of the configuration space for m = l1 × l2 cells. From there, the thermodynamic properties can be obtained. Five systems are investigated, according to the size and shape of the molecule in the adsorbed state: (i) dimers, (ii) linear trimers, (iii) triangular trimers, (iv) 60°-angular trimers and (v) 120°-angular trimers on triangular lattices. Dimer and trimer are the simplest cases of a polyatomic adsorbate, containing all the properties of the multisite-occupancy adsorption and can be used to model several experimental systems. CA solutions are tested by comparison with MC simulations and previous data in the literature. Special interest is devoted to the calculation of the configurational entropy per site in the limit case of θ → 1 (full coverage), where some exact results are available. The theoretical formalism is also applied to model CH4 and CO2 clathrate hydrates. In these systems, a triangular lattice is used to simulate the substrate, and methane(carbon dioxide) molecules can be well represented by triangular(linear) trimers. The good qualitative agreement between simulation and analytical data supports the validity of the CA scheme to predict the behavior of a wide variety of multisite-adsorption models, for which theoretical solutions are very difficult to obtain.

Transport properties in multilayer adsorption of dimers.

In this paper, we study the transport properties (percolation and conductivity) of a two-dimensional structure created by depositing dimers on a one-dimensional substrate where multilayer deposition is allowed. Specifically, we are interested in studying how the mentioned properties vary as a function of the height of the multilayer. The critical parameters of the percolation transition are calculated using finite-size scaling analysis, obtaining the scaling laws for the probability of percolation and the conductivity of the system. To calculate the electrical conductivity of the multilayer, we use the Frank-Lobb algorithm.

Fate of Water Molecules on (11-20) and (1-102) α-Alumina Surfaces: 2D Periodic Self-Consistent Charge-Density Functional Tight-Binding/Molecular Mechanics Molecular Dynamics Study

Numerous studies on the adsorption and dissociation of water molecules on α-alumina surfaces, both experimentally and theoretically, have been presented within decades. However, the dynamical properties of water at the solid–liquid interface are still a challenging problem. To explain the dynamics of the adsorption and dissociation of water molecules on α-alumina surfaces, mainly (11-20) and (1-102) faces, we carried out a two-dimensional periodic self-consistent charge density-functional tight-binding/molecular mechanics molecular dynamics study. In this study, we used the same approach previously applied to the (0001) α-alumina surface. The adsorption of single, dimer, and monolayer water molecules was employed to study the adsorption and dissociation of water molecules at (11-20) and (1-102) α-alumina surfaces. The single water system showed that no significant changes occurred on both surfaces, which indicates that no water dissociation was observed. Dimer water molecules on both surfaces were dissociated to form new OsHw groups following a 1-4 dissociation channel. Monolayer water molecules showed dissociation that occurred on both surfaces. Protonation analysis showed that both surfaces form stable OH groups without attempting to form water molecules. At the 100 ps simulation period, for the (11-20) α-alumina surfaces, up to 1.93 OH groups/nm2 were formed. The (1-102) α-alumina surface showed higher reactivity with a formation rate of up to 5.0 OH groups/nm2. By contrast, 2.1 OH groups/nm2 were formed on the (0001) α-alumina surface. The formation of OH groups on the (11-20) and (1-102) α-alumina surfaces occurred in the equilibration step and continued during the sampling step. In studying these three surfaces using a similar approach, the (1-102) α-alumina surface is the most reactive with water.

Tuning the surface states of TiO2 using Cu5 atomic clusters

If the ability of Cu5 atomic quantum clusters (AQCs) to create states within the bulk gap of TiO2 persists in the presence of oxygen, then the absorption spectrum of the AQC-TiO2 complex could be matched to the peak of the solar spectrum, thereby enhancing its photocatalytic activity under ambient conditions. Here we demonstrate that this is indeed the case, by examining the electronic structure of Cu5 AQCs adsorbed on a rutile (110) TiO2 surface in the presence of oxygen. We find that adsorbed oxygen changes the most stable AQC isomer from a 2-dimensional trapezoidal structure to a 3-dimensional bipyramidal structure. Furthermore, the bipyramidal AQC creates two ferromagnetically coupled polarons in the TiO2. In contrast, the trapezoidal AQC only creates a single polaron. In the presence of oxygen, these polaronic states associated with the bare cluster are joined by further mid-gap energy levels, formed from hybrid states located on copper and oxygen atoms. Dissociated oxygen-dimer adsorption on Cu5 generally leads to significant deformation of the whole cluster, which accounts for the dramatic drop of the total energy compared to molecular oxygen dimer adsorption. The ability of Cu5 AQCs to create states within the bulk gap of TiO2 under ambient conditions demonstrates that the absorption spectrum of the AQC-TiO2 complex is more closely matched to the peak of the solar spectrum, than either the AQC or TiO2 alone.

Probing protein aggregation at buried interfaces: distinguishing between adsorbed protein monomers, dimers, and a monomer-dimer mixture in situ.

Protein adsorption on surfaces greatly impacts many applications such as biomedical materials, anti-biofouling coatings, bio-separation membranes, biosensors, antibody protein drugs etc. For example, protein drug adsorption on the widely used lubricant silicone oil surface may induce protein aggregation and thus affect the protein drug efficacy. It is therefore important to investigate the molecular behavior of proteins at the silicone oil/solution interface. Such an interfacial study is challenging because the targeted interface is buried. By using sum frequency generation vibrational spectroscopy (SFG) with Hamiltonian local mode approximation method analysis, we studied protein adsorption at the silicone oil/protein solution interface in situ in real time, using bovine serum albumin (BSA) as a model. The results showed that the interface was mainly covered by BSA dimers. The deduced BSA dimer orientation on the silicone oil surface from the SFG study can be explained by the surface distribution of certain amino acids. To confirm the BSA dimer adsorption, we treated adsorbed BSA dimer molecules with dithiothreitol (DTT) to dissociate these dimers. SFG studies on adsorbed BSA after the DTT treatment indicated that the silicone oil surface is covered by BSA dimers and BSA monomers in an approximate 6 : 4 ratio. That is to say, about 25% of the adsorbed BSA dimers were converted to monomers after the DTT treatment. Extensive research has been reported in the literature to determine adsorbed protein dimer formation using ex situ experiments, e.g., by washing off the adsorbed proteins from the surface then analyzing the washed-off proteins, which may induce substantial errors in the washing process. Dimerization is a crucial initial step for protein aggregation. This research developed a new methodology to investigate protein aggregation at a solid/liquid (or liquid/liquid) interface in situ in real time using BSA dimer as an example, which will greatly impact many research fields and applications involving interfacial biological molecules.

Electron-phonon interaction in the surface dimer adsorption model

In the scope of the model of surface dimer, formed by the adsorbed atom and below lying substrate surface atom, the problem on the electron-phonon interaction effect on the charge transfer in adsorbed system is considered. The case of adsorption on metal is examined thor oughly. Keywords: adsorbate, substrate, dimer's vibrations, charge transfer.

Surface chemistry considerations of gangue dissolved species in the bastnaesite flotation system

Inefficient flotation of bastnaesite remains a challenge in the production of rare earth elements. This study aimed to investigate the dissolution and adsorption behaviour of species that are commonly released into bastnaesite flotation pulp from Ca/Ba-bearing gangue minerals. The influence and corresponding mechanisms on the bastnaesite mineral surface and collectors, namely sodium oleate (NaOL), were evaluated experimentally based on micro-flotation, zeta potentials, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-Ray photoelectron spectroscopy (XPS) analyses. The flotation recovery of bastnaesite significantly decreased from ∼95% to ∼25%, ∼15%, ∼80%, ∼25% when exposed to calcite, fluorite, barite, and mixed dissolved species, respectively. The zeta potential of bastnaesite was pH sensitive, indicating that H+ and OH- determine the surface potential of bastnaesite. Solution chemistry analyses revealed that the presence of the dissolved species differed at various pH values. In situ ATR-FTIR demonstrated the different effects of the dissolved species from calcite, fluorite, and barite on collector adsorption. The former two dissolved species mainly depressed the chemisorption of the NaOL monomers (RCOO‒), whereas calcite also affected the physical adsorption of the oleic acid molecular dimer (RCOOH·RCOO‒). Moreover, the barite dissolved species only affected the physical adsorption of the NaOL species. The results of XPS analysis revealed that dissolved species from these three gangues could pre-adsorbed onto bastnaesite and affected the interaction with the collector. Density functional theory calculations were employed to provide further theoretical insights into the interactions between the dissolved species from calcite, fluorite, and barite and NaOL.

Spectroscopic Fingerprints of MgCl2/TiCl4 Nanoclusters Determined by Machine Learning and DFT

Understanding the structure and properties of MgCl2/TiCl4 nanoclusters is a key to uncovering the origin of Ziegler–Natta catalysis. In particular, vibrational spectroscopy can sensitively probe the morphology and active species of MgCl2/TiCl4. Here, we determined vibrational spectroscopic fingerprints of 50MgCl2 and 50MgCl2/3TiCl4 which were obtained by nonempirical structure determination based on an evolutionary algorithm and DFT. The adsorption of CO, TiCl4, and Ti2Cl8 dimers was also modeled on each of the coordinatively unsaturated Mg2+ sites available for binding including so-called defect sites, which are likely present at the surface of activated MgCl2 nanocrystals and plausible sites for strong TiCl4 species adsorption. The outcomes of thermodynamical and vibrational analysis were compared to results on ideal surfaces of MgCl2. Vibrational analysis (IR and Raman) on plausible models of TiCl4/ MgCl2 nanoclusters revealed that IR response is useful for distinguishing between the different ways of binding of TiCl4 on different sites of adsorption, whereas Raman response provides a clear fingerprint of supported TiCl4 species.

Probing Monomer and Dimer Adsorption Trends in the MFI Framework.

Porous aluminosilicates such as zeolites are ubiquitous catalysts for the production of high-value and industrially relevant commodity chemicals, including the conversion of hydrocarbons, amines, alcohols, and others. Bimolecular reactions are an important subclass of reactions that can occur on Brønsted acid sites of a zeolite catalyst. Kinetic modeling of these systems at the process scale requires the interaction energetics of reactants and the active sites to be described accurately. It is generally known that adsorption is a coverage-dependent phenomenon, with lower heats of adsorption observed for molecules at higher coverage. However, few studies have systematically investigated the coadsorption of molecules on a single active site, specifically focusing on the strength of interaction of the second adsorbate after the initial adsorption step. In this work, we quantify the unimolecular and bimolecular adsorption energies of varying adsorbates, including paraffins, olefins, alcohols, amines, and noncondensible gases in the acidic and siliceous ZSM-5 frameworks. As a special case, olefin adsorption was examined for physisorption and chemisorption regimes, characterized by π-complex, framework alkoxide and carbenium ion adsorption, respectively. The effects of functional groups and molecular size were quantified, and correlations that relate the adsorption of the second adsorbate identity to that of the first adsorbate are provided.

Dissimilar adsorption of higher-order aggregates compared with monomers and dimers of methylene blue on graphene oxide: An optical spectroscopic perspective

Graphene oxide (GO) is widely used as an adsorbent for organic dyes because of its high surface area and dispersibility. Several studies have investigated the interaction of GO with methylene blue (MB), a model organic dye used in assessing adsorptive efficacy of materials. Such studies establish that MB molecules actively adsorb onto GO nanosheets through a suite of forces, primarily comprising of electrostatic forces, van der Waals forces and π-π interactions. Furthermore, it is largely presumed that all molecular forms of MB—monomers, dimers, and higher-order aggregates, adsorb on GO in an analogous manner, i.e., these studies presume that the relative intensities of these forces are similar for all species. In this study, we show that, although monomers and dimers evince similar adsorptive characteristics with GO, the adsorptive behaviour of higher-order aggregates differs from them. Using spectroscopic insights, we demonstrate a preferential adsorption of monomers and dimers at certain GO concentrations, and vice-versa. We found that this preferential adsorption also depends on the lateral dimensions of GO nanosheets. We posit that this difference is because of an inherent distinction in the ways these molecular species interact with GO—while the monomers and dimers interact through London dispersion and electrostatic forces, the higher-order aggregates largely interact through π-π interactions. These physical insights supplement the present understanding on the fundamental interfacial science between GO and organic dyes.

Emergence of magnetic anisotropy by surface adsorption of transition metal dimers on γ-graphyne framework

In this paper a systematic study is carried out to demonstrate the structural stability and magnetic novelty of adsorbing transition metal (TM) dimers (A-B) on graphyne (GY) surface, GY@A-B. Our research points out that the dimers are strongly adsorbed onto GY due to their large natural pores and the electron affinity of the sp-hybridized carbon atoms. Electronic properties of these dimer-graphyne composite systems are of particular importance as they behave as degenerate semiconductors with partial occupation of states at E F. Furthermore, their remarkable spin polarization (>80%) at Fermi energy (E F) can be of paramount importance in spintronics applications. Most of the GY@A-B structures exhibit large magnetic anisotropies as well as magnetic moments along the out-of-plane direction with respect to the GY surface. Particularly, GY@Co–Ir, GY@Ir–Ir and GY@Ir–Os structures possess positive magnetic anisotropic energies (MAE) of 121 meV, 81 meV and 137 meV, respectively, which are comparable to other well-known TM dimer doped systems. The emergence of high MAE can be understood using the second-order perturbation theory on the basis of the strong spin–orbit coupling (SOC) between the two TMs and the degeneracy of their d-orbitals near E F. A close correspondence between the simulated and the analytical results has been established through our work. Further, a simple estimation shows that, GY@A-B structures have the potential to store data up to 64 PB m−2. These intriguing electronic characteristics along with magnetism suggest GY@A-B to be a promising material for future magnetic storage devices.

Ab-initio study of the effects of charging on the adsorption and diffusion of Au2 on MgO(100)

By using ab-initio calculations, the adsorption and diffusion properties of charged Au dimers (Au2+ and Au2−) on MgO(100) were investigated and compared with those of the charge-neutral dimer (Au20). The most favored adsorption structure of Au2+ on MgO(100) is the upright dimer on a surface O atom in agreement with that of Au20. The surface diffusion of Au2+ on MgO(100) occurs by the leapfrog process with a barrier height of 0.38 eV lower than that of Au20 (0.66 eV). The most stable adsorption structure of Au2− on MgO(100) is the flat-lying dimer on two surface Mg atoms. The surface diffusion of Au2− on MgO(100) can proceed by the hopping and walking processes with a barrier height in the range of 0.05–0.11 eV, which is much lower than the diffusion barriers of Au2+ and Au20. Furthermore, detailed information on the electronic properties of Au20, Au2+, and Au2− are also presented.