Interpretation of x-ray absorption spectroscopy in the presence of surface hybridization

X-ray absorption spectroscopy (XAS) yields direct access to the electronic and geometric structure of hybrid inorganic-organic interfaces formed upon adsorption of complex molecules at metal surfaces. The unambiguous interpretation of corresponding spectra is challenged by the intrinsic geometric flexibility of the adsorbates and the chemical interactions with the interface. Density-functional theory (DFT) calculations of the extended adsorbate-substrate system are an established tool to guide peak assignment in X-ray photoelectron spectroscopy of complex interfaces. We extend this to the simulation and interpretation of XAS data in the context of functional organic molecules on metal surfaces using dispersion-corrected DFT calculations within the transition potential approach. For the prototypical case of 2H-porphine adsorbed on Ag(111) and Cu(111) substrates, we follow the two main effects of the molecule/surface interaction onto the X-ray absorption signatures: (1) the substrate-induced chemical shift of the 1s core levels that dominates in physisorbed systems and (2) the hybridization-induced broadening and loss of distinct resonances that dominate in more chemisorbed systems. Published by AIP Publishing.

Design of Surface-Supported Bio-inspired Networks for CO2 and O2 Activation at Room Temperature

Daniel Eduardo Hurtado Salinas

The structure of a biological system defines its function. Therefore, nature has developed sophisticated systems that catalyze chemical reactions that are vital for life on earth. For instance, the photosynthesis is a biological process that is partially regulated by the metallo-enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase), whose function is to catalyze the capture and transfer of CO2 into organic molecules. This carboxylation reaction occurs at the active site, where a magnesium ion (Mg2+) is bound to organic molecules with carboxylate (COO-) and amine (NH2) functional groups. In the last decades, supramolecular chemistry has proven to be a valid approach to replicate the structural characteristics of these natural metal-organic systems. Metal atoms and organic molecules are used as building blocks to form two-dimensional (2D) metal-organic networks (MONs) on metal surfaces, through a self-assembly process that occurs spontaneously. In these systems the molecules bind to the metal centers and control their oxidation states that, as found in metallo-enzymes, is crucial to perform a catalytic task. Therefore, these supramolecular assemblies show promising features to be explored for heterogeneous catalysis. The main objective of this thesis is to reverse the structure-function equation, that is: instead of finding specific functions of well-known 2D structures for future applications, 2D structures are designed to reproduce the specific functions of well-known bio-systems. Thus, inspired in the structure of the active site of RuBisCO, 2D Mg-based supramolecular assemblies are rationally designed on Cu(100) or Mg(0001) surfaces at room temperature (RT) as a platform for CO2 adsorption. This work is the first study of self-assembly of alkaline earth metal atoms (Mg) with organic molecules ((1,4-benzenedicarboxylic acid (TPA) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT)) on metal surfaces. The first part of this thesis investigates the self-assembly of TAPT molecules. Scanning Tunneling Microscopy (STM) and High Resolution X-ray Photoemission Spectroscopy (HR-XPS) demonstrate that the molecules typically semi-deprotonate to form MONs with the adatoms from the host substrates. The exposure of these structures to CO2 results in the carboxylation of the amino groups. The second part proves that the COOH moieties of TPA molecules fully-deprotonate to form homo-molecular structures. The incorporation of Mg adatoms result in ionic MONs, which reproduce the carboxylate environment of the active site of RuBiSCO. STM, XPS and Density Functional Theory (DFT) analyze the structural changes, the chemical reactions and charge transfer during either the CO2 or O2 adsorption on the Mg2+ centers. The last part focusses on the co-deposition of both molecules to form 2D metallic-hetero-molecular networks with Mg2+ centers. The role of the Mg2+ centers as active sites for molecular adsorption and model systems for gas storage is studied. STM reveals that CO2 partially modifies the network towards a configuration that resembles the active site of RuBisCO. The formation of 3D structures to enhance CO2 adsorption is also evaluated.

EPFL2018

High-Valent Nickel Promoted by Atomically Embedded Copper for Efficient Water Oxidation

Jun Li, Ning Wang, Bo Zhang

Efficient and low-cost electrocatalysts for oxygen evolution reaction (OER), particularly in neutral conditions, are of significant importance for renewable energy technologies such as CO2 reduction and seawater splitting electrolysis. High-valent transition-metal sites have been considered as OER active sites; however, the rational design and construction of these sites remain a big challenge. Here, we report a trimetallic NiFeCu oxyhydroxide electrocatalyst, in which high-valent Ni sites are promoted and stabilized by the atomically embedded Cu, as evidenced by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Through compositional optimization, Ni6Fe1Cu1 catalysts achieved an overpotential of 385 mV at 10 mA cm(-2), a Tafel slope of 164 mV dec(-1), and a stability of 100 h at pH = 7.2. Density function theory calculations demonstrated that the Cu-doping facilitates the formation of high-valent Ni and thus promotes OER electrocatalysis through modulating the d-band center of Ni and reducing the adsorption energy of oxygenated intermediates on the surface of the catalyst. This work paves a promising avenue for the construction of desired high-valent metal OER catalysts by embedding redox inactive metals.

AMER CHEMICAL SOC2020

Self-assembly of multiligand supramolecular architectures at metal surfaces

Alexander Langner

Self-assembly of organic molecules represents an efficient and convenient bottom-up approach for the structural functionalization of surfaces at the nanometer scale. The great potential of assembling supramolecular architectures from organic molecules lies in the vast choice of building blocks that are accessible. This allows to predefine and to direct the molecular organization through the steric and electronic information stored in the molecules. A large variety of homotopic supramolecular structures can be achieved, however limited in complexity. The full realization of highly developed surface architectures for designable chemical functions and physical properties may depend critically on a higher level of complexity, which could be satisfied by utilizing a mixture of different molecular building blocks. Within this thesis, the self-organization of multi-component systems at well-defined metal surfaces is investigated by scanning tunneling microscopy. The mixtures consist of different representatives of linear polyaromatic ligands with carboxylate or pyridyl derived functional groups. The assembly is directed by hydrogen bonding, metal-organic complex formation or a combination of both, i.e. a hierarchical interaction scheme. The aim of this thesis is threefold: First, the homotopic self-assembly of different pyridyl ligands is investigated as a basis for the multi-component systems. One-dimensional chains as well as two dimensional (2D) networks can be formed by copper-pyridyl coordination motifs. The influence of the substrate on these structures will be discussed in detail. Moreover, the self-organization process of multi-components is investigated at the fundamental level. Since functional groups primarily discriminate the different molecular species during the assembly and with that assure a distinguished arrangement, selective bonding schemes are essential for the design of highly ordered supramolecular architectures. Two suitable selective coordination motifs are identified, an iron-carboxylate and a copper-pyridyl complex node. In ligand mixtures, strong preference of one metal species is observed at surfaces, which would not be expected in solution. In addition, redundant mixtures (i.e. ligands of different size but same functional group) serve as model systems to investigate the dynamic self-organization process of modular multicomponent systems, e.g. self-selection or error tolerance, directly with nanometer accuracy. Self-selection is observed if the molecular recognition is steered by a highly reversible coordination bond, while a more robust bonding fosters structural adaption and tolerance to the introduced error (i.e. redundant mixture). These experiments underline the importance of reversibility of the interactions steering the self-assembly process into highly ordered structures. Finally, the extended possibilities due to the multi-component approach for the construction of advanced architectures and structural control are explored: Complex and sophisticated supramolecular networks can be directly synthesized out of simple molecules at the surface. A hierarchical assembly is presented, where a coordination motif defines a primary unit, while hydrogen bonding steers the 2D organization. The secondary level of interaction (hydrogen bonding) can be exclusively addressed, leading to distinct 2D arrangement of the undisturbed primary coordination units. A concept of structural control by rational design is presented, where geometric network parameters can be modified by the replacement of the appropriate molecular species. Due the multi-component nature of the systems, a finer tuning of the structure is enabled (independent substitution of each component) compared to homotopic assemblies. Moreover, a continuation of the multi-component concept is demonstrated, where organizational control is imposed to a homotopic system by adding an additional molecular species. It is demonstrated that a long range order and thermal stability is imposed onto meta-stable Cu-pyridyl coordination chains by cooperative assembly with a carboxylate species.