Adsorbate-substrate Interaction Research Articles

Strong Substrate-Adsorbate Interactions Direct the Impact of Fluorinated N-Heterocyclic Carbene Monolayers on Au Surface Properties.

Fluorinated self-assembled monolayers (SAMs) have been utilized in a variety of applications such as transistors and optoelectronic devices. However, in most SAMs the fluorinated groups could not be positioned in high proximity to the surface due to steric effects. This limitation hinders the direct analysis of the impact of the fluorination level on surface properties. Herein, fluorinated aromatic N-heterocyclic carbenes (NHCs), with 1-5 fluorine atoms, were self-assembled on a gold substrate. These NHCs enabled the positioning of fluorinated groups in high proximity to the metal surface to identify the influence of the fluorination level on surface properties. Experimental measurements and theoretical calculations identified that all fluorinated NHCs formed SAMs and adopted a flat-lying adsorption configuration while anchored to the metal surface via Au adatom. A higher fluorination level induced a stronger interaction of the fluorinated side groups with the Au surface. The stronger interaction and surface proximity of the fluorinated side groups deteriorated the overall binding energy of the NHC due to the less-optimized adsorption geometry of the carbene carbon. Ultraviolet photoelectron spectroscopy measurements revealed that fluorinated NHC monolayers lowered the surface work function by up to 1 eV and induced an increase of 15-20° in the water contact angle. The impact on surface properties did not vary according to the fluorination level of NHCs, and similar values were measured for NHC with 1-5 fluorine atoms. It is therefore identified that dominant adsorbate-substrate interactions between the fluorinated side groups and the Au surface quenched the distinct impact of the fluorination level on surface functionality.

Defective Diamane: A Superior Sensor for Toxic Gases Capture and Detection with Excellent Selectivity, Sensitivity, and Reversibility at Room Temperature.

The toxic gases emitted from industrial production have caused significant damage to the environment and human health, necessitating efficient gas sensors for their detection and removal. In this work, first-principles calculations are employed to investigate the potential application of diamanes for high-performance toxic gas sensors. The results show that nine gas molecules (CO, CO2, NO, NO2, NH3, SO2, N2, O2, and H2O) are physisorbed on pristine diamane by weak van der Waals interactions. After introducing H/F defects, diamane can effectively capture specific toxic gases (CO, NO, NO2, and SO2) in the presence of interfering gases (N2, O2, and H2O), suggesting excellent selectivity and anti-interference ability. Orbital hybridization and significant charge redistribution between gas molecules and defective diamane dominate the enhanced adsorbate-substrate interactions. More importantly, the high sensitivity and good reversibility of defective diamane for detecting CO, NO, and SO2 molecules enable its reuse as a superior resistance-type gas sensor. Our calculations provide valuable insights into the potential of defective diamane for detecting toxic gases and shed light on the practical application of novel carbon-based materials in the gas-sensing field.

A Blyholder mechanism in the chemisorption of N2O on Ni(111) – studied with Auger-photoelectron coincidence spectroscopy

In heterogeneous catalysis the surface-adsorbate bond strength is critical for the function of the system. Here we study a series consisting of multilayer, bilayer and monolayer N2O on Ni(111) and employ Auger-photoelectron coincidence spectroscopy (APECS) to study the interaction between the molecule and the substrate directly. We observe intensity in the nitrogen Auger spectra that arise from the interaction between molecule and surface (not observed in free molecules) whereas the oxygen spectra are thickness-independent. Since the two nitrogen atoms of N2O are chemically inequivalent we can assign the intensity present in the bilayer and monolayer cases to orbitals centered on the terminal nitrogen which is closest to the Ni(111) surface. Using ab initio, molecular dynamics and solid-state density functional theory calculations we infer a Blyholder model of the surface bond as consisting of donation from the terminal nitrogen lone-pair valence orbital with back-donation from the metal into the unoccupied orbitals on that nitrogen. This coincidence technique can readily be used to study substrate–adsorbate interactions directly with chemical and orbital specificity — this opens up prospects to study fundamental steps of molecular adsorption and heterogeneous catalysis with unprecedented detail.

Ordered mesoporous Fe2Nx electrocatalysts with regulated nitrogen vacancy for oxygen reduction reaction and Zn-air battery

Highly efficient transition-metal electrocatalysts hold great promise for overcoming the sluggish kinetics of the oxygen reduction reaction (ORR), while the dense stacking of active sites within bulk materials constrains electrocatalytic behaviors. Therefore, nano-structure engineering to obtain hierarchal morphology is crucial to enrich the active sites and facilitate the corresponding mass transfer. Here, the three-dimensional interconnected and ordered mesoporous (3DOM) Fe2Nx decorated on TiOy (Fe2Nx @TiOy) is constructed. By introducing nitrogen vacancies, the increased surface area, and active sites boost ORR kinetics, including a high half-wave potential (0.88 V vs reversible hydrogen electrode) and high current density (71 mA cm−2 at 0.8 V) have been reached. The zinc-air battery assembled with Fe2Nx @TiOy catalysts presents a high specific capacity of 809 mAh g−1. Density functional theory analysis and X-ray absorption spectroscopy further confirm the promoter effects of nitrogen vacancies on modulating electronic structure of Fe, through regulating intermediates adsorption/desorption. The shift of its d-band center is also found toward the Fermi energy level, strengthening the adsorbate-substrate interaction. This allows oxygen species to be favorably stabilized onto active sites of Fe2Nx @TiOy.

Adsorbate Formation/Removal and Plasma‐Induced Evolution of Defects in Graphitic Materials

AbstractThe preparation of adsorbate‐free graphene with well‐defined layer numbers is a current challenge in materials and surface science and required to fabricate graphene‐based nanodevices, such as used in nanoelectromechanical systems. One strategy to tailor the layer number is oxygen‐plasma treatment of few‐layer graphene/graphite flakes. However, when graphitic materials are stored in air under ambient conditions, it is almost inevitable that adsorbates deposit on their surfaces. When precisely removing individual graphene layers from graphitic flakes by oxygen‐plasma treatment, the amount and type of adsorbates strongly affect the required plasma‐treatment process and duration. To examine the removal/etching mechanism involved in removing such layers, few‐layer graphene/graphite flakes, with areas of different layer numbers, are stored in ambient air and stepwise exposed to oxygen plasma in a shielded configuration. The flakes are then successively analyzed by multifrequency atomic force microscopy together with Raman spectroscopy, focusing on etching rate, and adsorbate and defect evolution. Combined in‐plane and out‐of‐plane tip–adsorbate–substrate interaction analysis facilitates discrimination of different types of adsorbates (water, polycyclic aromatic hydrocarbons, and linear alkanes) and their formation with time. The results demonstrate the potential regarding the development of an efficient method for cleaning of graphitic surfaces and ablation of individual graphene layers.

Characterizing Molecule-Metal Surface Chemistry with Ab Initio Simulation of X-ray Absorption and Photoemission Spectra.

X-ray photoemission and X-ray absorption spectroscopy are important techniques to characterize chemical bonding at surfaces and are often used to identify the strength and nature of adsorbate-substrate interactions. In this study, we judge the ability of X-ray spectroscopic techniques to identify different regimes of chemical bonding at metal-organic interfaces. To achieve this, we sample different interaction strength regimes in a comprehensive and systematic way by comparing two topological isomers, azulene and naphthalene, adsorbed on three metal substrates with varying reactivity, namely the (111) facets of Ag, Cu, and Pt. Using density functional theory, we simulate core-level binding energies and X-ray absorption spectra of the molecular carbon species. The simulated spectra reveal three distinct characteristics based on the molecule-specific spectral features which we attribute to types of surface chemical bonding with varying strength. We find that weak physisorption only leads to minor changes compared to the gas-phase spectra, weak chemisorption leads to charge transfer and significant spectral changes, and strong chemisorption leads to a loss of the molecule-specific features in the spectra. The classification we provide is aimed at assisting interpretation of experimental X-ray spectra for complex metal-organic interfaces.

Green synthesis of mesoporous Ni-Co layered double hydroxide and its application for removal of 2,4-dinitrophenol from water: A theoretical study complemented by the first principle density functional theory-Monte-Carlo approach

This paper describes the synthesis of a novel mesoporous Ni-Co layered double hydroxide(LDH) and its application for the adsorptive removal of 2,4-dinitrophenol(2,4-DNP) from an aqueous solution. A green synthetic method was used involving a non-aqueous ‘soft chemical' ethanol: acetone mixture in order to avoid intercalation or interlayer contamination by CO2 gas, which is responsible for carbonate formation. The synthesized material was characterized using PXRD, HR-TEM, FESEM, XPS, FT-IR, TGA-DTA, etc. The experimental and model fitting results of adsorption studies showed that equilibrium Freundlich adsorption isotherm and kinetic pseudo-second-order were the best-fitting models. Also, thermodynamic adsorption parameters ∆Ho,∆Soand∆Go were found to be favorable, non-spontaneous, and endothermic. Furthermore, theoretical molecular dynamic simulation (MD) revealed that the nature of adsorbate-substrate (2,4-DNP- Ni-Co LDH) interactions were favorable hydrophobic interactions. DFT-based DMoL3 approach showed that the adsorbate-substrate energy gap (eV) increases with the increase in the interfacial region, explaining the high chemical stability after the adhesion of adsorbate molecules. The regeneration adsorption cycles showed that the Ni-Co LDH acts with an excellent removal affinity towards the 2,4-DNP molecule within 150–180 min from cycles 1–4.

Exploring the Effect of Pd on the Oxygen Reduction Performance of Pt by In Situ Raman Spectroscopy.

Directly monitoring the oxygen reduction reaction (ORR) process in situ is very important to deeply understand the reaction mechanism and is a critical guideline for the design of high-efficiency catalysts, but there is still lack of definite in situ evidence to clarify the effect between adsorbed intermediates and the strain/electronic effect for enhanced ORR performance. Herein, in situ surface-enhanced Raman spectroscopy (SERS) was employed to detect the intermediates during the ORR process on the Au@Pd@Pt core/shell heterogeneous nanoparticles (NPs). Direct spectroscopic evidence of the *OOH intermediate was obtained, and an obvious red shift of the *OOH frequency was identified with the controllable shell thickness of Pd. Detailed experimental characterizations and density functional theory (DFT) calculations demonstrated that such improved ORR activity after inducing Pd into Au@Pt NPs can be attributed to the optimized adsorbate-substrate interaction due to the strain and electronic effect, leading to a higher Pt-O binding energy and a lower O-O binding energy, which was conducive to O-O dissociation and promoted the subsequent reaction. Notably, this work illustrates a relationship between the performance and strain/electronic effect via the intermediate detected by SERS and paves the way for the construction of ORR electrocatalysts with high performance.

Rational design and precise manipulation of nano-catalysts

Nano-catalysis plays a vital role in the chemical transformations and significantly impacts the booming modern chemical industry. The rapid technological enhancements have resulted in serious energy and environmental issues, which are currently spurring the exploration of the novel nano-catalysts in diverse fields. In order to develop the efficient nano-catalysts, it is essential to understand their fundamental physicochemical properties, including the coordination structures of the active centers and substrate-adsorbate interactions. Subsequently, the nano-catalyst design with precise manipulation at the atomic level can be attained. In this account, we have summarized our extensive investigation of the factors impacting nano-catalysis, along with the synthetic strategies developed to prepare the nano-catalysts for applications in electrocatalysis, photocatalysis and thermocatalysis. Finally, a brief conclusion and future research directions on nano-catalysis have also been presented. Focusing on the recent progress in Chen's group, this review summarizes the combined synthetic chemistry, theoretical modeling, and spectroscopy studies of nano-catalysts together with their applications in electrocatalysis, photocatalysis and thermocatalysis.

Highly Ordered N-Heterocyclic Carbene Monolayers on Cu(111).

The benzannulated N-heterocyclic carbene, 1,3-dibenzylbenzimidazolylidene (NHCDBZ) forms large, highly ordered domains when adsorbed on Cu(111) under ultrahigh vacuum conditions. A combination of scanning tunnelling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations reveals that the overlayer consists of vertical benzannulated NHC moieties coordinating to Cu adatoms. Long-range order results from the placement of the two benzyl substituents on opposite sides of the benzimidazole moiety, with their aromatic rings approximately parallel to the surface. The organization of three surface-bound benzyl substituents from three different NHCs into a triangular array controls the formation of a highly ordered Kagome-like surface lattice. By comparison with earlier studies of NHCs on Cu(111), we show that the binding geometry and self-assembly of NHCDBZ are influenced by intermolecular and adsorbate–substrate interactions and facilitated by the flexibility of the methylene linkage between the N-heterocycle and the aromatic wingtip substituents.

Mechanistic insights into chemical reduction of CO2 by reverse water-gas shift reaction on Ru(0001) surface: The water promotion effect

In this work, we systematically illustrate CO2 reduction by H2 on the Ru(0001) surface using periodic DFT calculations with micro-kinetic modeling to explore adsorbate-substrate, adsorbate-solvent, solvent-substrate interactions as well as reaction mechanisms. The effects of adsorbate-substrate interactions, water-mediated protonation kinetics, thermodynamics, and transient potential sweeps on reaction rate and selectivity are also studied. We propose three simple thermodynamic descriptors (CO + O, HCOO, and COOH) that represent the effectiveness of CO selectivity on Ru catalysts. More importantly, we explore the role of water solvation on the CO2 conversion by assessing the H-shuttled (O-H bond formation) and water solvated (C-H bond formation) with various solvation models. To examine the solvation effect, a 6H2O/Ru(0001) water bilayer model is constructed by optimizing six H2O molecules close to the Ru surface, and multiple H-bonds are observed as local-minimum solvation structures among water molecules. Finally, the efficiency of the ruthenium catalyst is expressed by turnover frequencies (TOFs). We hope that these insights will deliver useful guidelines for designing more efficient, earth-abundant electrocatalysts in the future.

Revealing the key role of bonding states in surface chemisorption

It has been widely accepted that the filling of the adsorbate-surface anti-bonding states dominates the variation of adsorption energies among transition metals. However, on the basis of chemical bonding analysis for adsorbate-surface interaction, we find an exception in the adsorption of O, OH, and OOH on nitrogen-coordinated transition metal-doped carbon material. In this material, a weaker (stronger) adsorption is mainly due to the decreasing (increasing) contribution of the filled adsorbate-surface bonding states. This work sheds light on the mechanism of the adsorbate-substrate interaction and suggests that care much be taken in interpreting the surface chemisorption.

In Situ Mapping and Local Negative Uptake Behavior of Adsorbates in Individual Pores of Metal-Organic Frameworks.

Herein, we report the adsorbate behavior in individual local pores of MIL-101, which is a metal-organic framework (MOF) with two heterogeneous mesopores and different metal sites, by combining adsorbate isotherms and in situ crystallography profiles. The in situ mapping shows that the substrate-adsorbate interaction affects the initial adsorption and pore condensation steps. The monolayer adsorption gradient changes greatly depending on the framework metal-adsorbate attraction force. Also, broad inflection points are found in adsorption isotherms, and the initial shape depends on the different metals. Besides, the capillary condensation at a pore draws adsorbates from other local pores. This leads to the local negative uptake behavior in individual pore isotherms. At higher pressure, they move to a larger space, whereas in a relatively low-pressure range the attraction force between the MOF framework and guest molecule influences the amount of rearranged guest molecules. Furthermore, the origin of the characteristic adsorption behavior based on the metals constituting the MOFs and the relative strength of substrate-adsorbate and adsorbate-adsorbate interactions are elucidated through the combined study of electron densities in pores, electron paramagnetic resonance spectroscopy spectra, and density functional theory and Monte Carlo simulations to uncover the previously veiled information on adsorption behavior.

Rationalization of Nonlinear Adsorption Energy-Strain Relations and Brønsted-Evans-Polanyi and Transition State Scaling Relationships under Strain.

Scaling relations play a vital role in high-throughput screening of catalytic materials, and more and more attention is being paid to strain-based regulation of catalytic performance. Here we investigated the variation of several energetics, including adsorption energies in the initial state, transition state, and final state, reaction energies, and energy barriers with strain, by studying CO, BH, NH, CH, and NO adsorption and dissociation on M(111) (M = Cu, Ag, Ni, Pd, or Pt) surfaces. We show that energy barriers, reaction energies, and adsorption energies can vary either linearly or nonlinearly (quadratically) with strain. Systems with stronger adsorbate-substrate interaction and weaker atom-atom interaction in substrates are more likely to exhibit nonlinear relations. The well-known Brønsted-Evans-Polanyi relationships and transition state scaling relationships under strain were also examined, and both of them can be nonlinear under strain, in principle. The observed nonlinear relationships were satisfactorily rationalized with the equations derived from Mechanics of Solids.

Physicochemical Understanding of the Impact of Pore Environment and Species of Adsorbates on Adsorption Behaviour

AbstractFor a better design of adsorbents, it is important to know the intermolecular interaction among adsorbates and host material, leading to improved guest selectivity and uptake capacity. In this study, we demonstrate the influence of the interaction among adsorbates and substrate, controlled by the pore environment and species of adsorbates, on the adsorption behaviour. We report the unique CO2 adsorption behaviour of MOF‐205 due to distinct pore geometry. The precise analysis through gas‐adsorption crystallography with molecular simulation shows that capillary condensation of CO2 in MOF‐205 occurs preferentially in the large dodecahedral pore rather than the small tetrahedral pore, because the interaction of CO2 with MOF‐205 framework is weaker than that among CO2 molecules, while Ar and N2 are sequentially filled into two different pores of MOF‐205 according to their size. Comparison of the materials with different pore environments reveals that the relative strength of the adsorbate–adsorbate and adsorbate–substrate interaction gives rise to different shapes of isotherms.

Physicochemical Understanding of the Impact of Pore Environment and Species of Adsorbates on Adsorption Behaviour.

For a better design of adsorbents, it is important to know the intermolecular interaction among adsorbates and host material, leading to improved guest selectivity and uptake capacity. In this study, we demonstrate the influence of the interaction among adsorbates and substrate, controlled by the pore environment and species of adsorbates, on the adsorption behaviour. We report the unique CO2 adsorption behaviour of MOF-205 due to distinct pore geometry. The precise analysis through gas-adsorption crystallography with molecular simulation shows that capillary condensation of CO2 in MOF-205 occurs preferentially in the large dodecahedral pore rather than the small tetrahedral pore, because the interaction of CO2 with MOF-205 framework is weaker than that among CO2 molecules, while Ar and N2 are sequentially filled into two different pores of MOF-205 according to their size. Comparison of the materials with different pore environments reveals that the relative strength of the adsorbate-adsorbate and adsorbate-substrate interaction gives rise to different shapes of isotherms.

Atomistic Insight into Nitrogen-Terminated Diamond(001) Surfaces by the Adsorption of N, NH, and NH2: A Density Functional Theory Study.

Layer-by-layer construction of diamond devices for spin-sensing calls for the atomistic understanding of the nitrogen species on diamond surfaces. Motivated by recent experiments, we used density functional theory simulations to examine the adsorption of nitrogen species (N, NH, and NH2) on bare and hydrogenated diamond(001) surfaces. On the bare substrate, we find that nitrogen species favor to attack the C═C dimers at low coverages, forming N(ad) and NH(ad) in a bridge configuration and NH2(ad) in a terminal configuration. At increasing coverages up to one full monolayer, the computed adsorption geometries and energetics suggest that the adsorbate-adsorbate interactions are attractive for N(ad), but repulsive for NH(ad) and NH2(ad). On the hydrogenated substrate, the adsorbed nitrogen species are subject to structural modification, as resulted from the weakened adsorbate-substrate interactions. Further, we calculated the vibration of nitrogen species and the 1s core-level shift of surface carbons, providing atomistic insights into the nature of surface bonding. Lastly, we simulated images of representative nitrogen species adsorbed on diamond(001), guiding future work using scanning tunneling microscopy.

Nonintuitive Surface Self-Assembly of Functionalized Molecules on Ag(111).

The fabrication of nanomaterials involves self-ordering processes of functional molecules on inorganic surfaces. To obtain specific molecular arrangements, a common strategy is to equip molecules with functional groups. However, focusing on the functional groups alone does not provide a comprehensive picture. Especially at interfaces, processes that govern self-ordering are complex and involve various physical and chemical effects, often leading to unexpected structures, as we showcase here on the example of a homologous series of quinones on Ag(111). Naively, one could expect that such quinones, which all bear the same functionalization, form similar motifs. In salient contrast, our joint theoretical and experimental study shows that profoundly different structures are formed. Using a machine-learning-based structure search algorithm, we find that this is due to a shift of the balance of three antagonizing driving forces: adsorbate–substrate interactions governing adsorption sites, adsorbate–adsorbate interactions favoring close packing, and steric hindrance inhibiting certain otherwise energetically beneficial molecular arrangements. The theoretical structures show excellent agreement with our experimental characterizations of the organic/inorganic interfaces, both for the unit cell sizes and the orientations of the molecules within. The nonintuitive interplay of similarly important interaction mechanisms will continue to be a challenging aspect for the design of functional interfaces. With a detailed examination of all driving forces, we are, however, still able to devise a design principle for self-assembly of functionalized molecules.

DFT and TD-DFT studies of halogens adsorption on cobalt-doped porphyrin: Effect of the external electric field

DFT and TD-DFT calculations are employed to examine the adsorption of halogen atoms (F, Cl, and Br) and molecules (F2, Cl2, and Br2) on cobalt-doped porphyrin (CoP). The impact of an external static electric field (EF) on the adsorption and the UV–Vis spectra is studied. The adsorption of halogen atoms as well as F2 molecule is chemisorption in the range from −1.14 to −2.41 eV while the adsorption of Cl2 and Br2 is physisorption. The adsorbate–substrate interaction reduces the HOMO-LUMO gap (Eg) to be 1.79–2.72 eV compared to 3.11 eV for CoP. The EF controls the adsorption process where the positive EF improves the corrected adsorption energy (Eadscorr.) while the negative EF attenuates the Eadscorr.. Additionally, the positive EF leads to a decrease in the Eg while, the negative EF increases the Eg. Consequently, for sensing purposes, high sensitivity and low response time are expected for the CoP at the positive EF while negative EF encourages the desorption process and decreases the recovery time. The CoP is optically active with λmax of 494 nm. The UV–Vis spectra analysis shows that the adsorption of halogen atoms causes a blue shift of λmax to be 394, 488, and 491 nm while the adsorption of halogen molecules causes a red shift of λmax to be 892, 846, and 698 nm, respectively. The λmax for adsorbate–substrate complexes is affected by the EF. Our results declare that the cobalt-doped porphyrin may be utilized as a candidate sensor for the considered halogen atoms and molecules.

Uncovering the relationship between the structure and acid-base properties for hyperbranched polyester-polyols self-assembled on carbon surfaces

Abstract The self-assembly of three different generations of hyperbranched polyester-polyol polymers, based on 2,2-bis(methylol)propionic acid units (H20, H30, and H40 Boltorn®), on carbon electrodes was studied. The physico-chemical properties of this family of polymers confined on a surface were explored by electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). Surface pKa of the hyperbranched polymer layers were determined by impedimetric titration by EIS based on the variation in the charge transfer resistance of the electroactive redox probe [Fe(CN)6]4−/[Fe(CN)6]3− at different pH. AFM was used to characterize the surface topography and also its viscoelastic properties by the acquisition of phase images. The relation between structure and acid-base properties can be explained in terms of a rearrangement of the layer due to the effect of intermolecular hydrogen bonds and adsorbate-substrate interactions. In this study, we report a powerful, versatile and yet simple procedure for functionalizing carbon materials surfaces without pre-treatment requirements, which could be useful to generate promising platforms for the development of sensors and drug delivery systems.