Close-Packed Arrangements of Flat-On Free-Base Porphyrins Driven by van der Waals Epitaxy

The functionality of low dimensional phases of porphyrins in optical, chemical, electrical, and multimodal combinational devices is strictly related to the control of molecular orientation within the produced solid layers. A promising strategy to drive the growth of adlayers with predictable structural properties relies on the template effect exerted by the substrate. Tetraphenyl porphyrins, being disc-shaped objects, can be adsorbed on a crystal surface by taking on different geometries. An edge-on configuration is adopted when the interactions among molecules overtake those between molecules and substrate, whereas a flat-on configuration is adopted when molecule-substrate interaction is dominant, with the weaker intermolecular interaction driving a close-packed geometry in the adlayer. For this latter reason, square and/or hexagonal lattice symmetries of physisorbed porphyrin layers are disclosed on highly interacting metal substrates such as Au(111). Unfortunately, metal substrates modify the intrinsic properties of porphyrins by suppressing many of their functionalities. To overcome this drawback, here we report the selective growth of porphyrins in a flat-on arrangement on the chiral (110) cleavage surface of the mixed molecular organic crystal formed by 2,5-diketopiperazine and fumaric acid in a 1:1 mole ratio. The energetic advantage ensured by the interaction with the insulating substrate drives the prevalent formation of domains with a square symmetry, which is retained from monolayer to multilayers. However, rare domains with a hexagonal symmetry are revealed and analyzed by high-resolution scanning probe microscopic techniques. The experimental structural analysis performed at the nanoscale, combined with ab initio calculations, allowed us to demonstrate that the molecular architectures we found arise from the simultaneous fulfillment of site adsorption energy maximization driven by peculiar molecular motifs of the selected substrate, close-packing criteria, and epitaxial locking to the substrate surface by weak van der Waals interactions

Optical Anisotropy of Porphyrin Nanocrystals Modified by the Electrochemical Dissolution

Reflectance anisotropy spectroscopy (RAS) coupled to an electrochemical cell represents a powerful tool to correlate changes in the surface optical anisotropy to changes in the electrochemical currents related to electrochemical reactions. The high sensitivity of RAS in the range of the absorption bands of organic systems, such as porphyrins, allows us to directly correlate the variations of the optical anisotropy signal to modifications in the solid-state aggregation of the porphyrin molecules. By combining in situ RAS to electrochemical techniques, we studied the case of vacuum-deposited porphyrin nanocrystals, which have been recently observed dissolving through electrochemical oxidation in diluted sulfuric acid. Specifically, we could identify the first stages of the morphological modifications of the nanocrystals, which we could attribute to the single-electron transfers involved in the oxidation reaction; in this sense, the simultaneous variation of the optical anisotropy with the electron transfer acts as a precursor of the dissolution process of porphyrin nanocrystals

CVD Graphene/Ni Interface Evolution in Sulfuric Electrolyte

Systems comprising single and multilayer graphene deposited on metals and immersed in acid environments have been investigated, with the aim of elucidating the mechanisms involved, for instance, in hydrogen production or metal protection from corrosion. In this work, a relevant system, namely chemical vapor deposited (CVD) multilayer graphene/Ni (MLGr/Ni), is studied when immersed in a diluted sulfuric electrolyte. The MLGr/Ni electrochemical and morphological properties are studied in situ and interpreted in light of the highly oriented pyrolytic graphite (HOPG) electrode behavior, when immersed in the same electrolyte. Following this interpretative framework, the dominant role of the Ni substrate in hydrogen production is clarified

MICROWAVE-ASSISTED BRUCITE AND TALC REACTIONS WITH CO2 AS A PROXY FOR CARBON CAPTURE AND STORAGE BY SERPENTINE

In the last decades many studies have been focusing on Carbon Capture and Storage (CCS) to find a possible remedy to reduce the large increase of anthropogenic carbon dioxide (CO ). Mineral Carbonation (MC) is a potential solution for almost irreversible chemical long-term CCS. It concerns the combination of CaO and MgO with CO forming spontaneously and exothermically dolomite and magnesite. However, kinetic barriers pose sever limitations for the practical exploitation of this reaction. High fractions of MgO are available in silicates such as olivine, orthopyroxene, clinopyroxene and serpentine. To date, data reported that serpentine polymorphs, above all antigorite, is an excellent candidate for fixing the CO as the reaction efficiency is approximately 92% compared to lizardite (40%) and olivine (66%). This is due to the surface reactivity of approximately 18.7 m /g for the dehydrated antigorite compared to10.8 m /g for dehydrated lizardite and 4.6 m /g for olivine. The microwave assisted process for CCS is an innovative technology that can be employed to catalyze the reaction through thermal and non-thermal mechanisms. Some pioneering tests of direct carbonation by microwave hydrothermal equipment have been performed on olivine, lizardite and chrysotile powders [1] but not on antigorite. The structure of serpentine is characterized by corrugated stacked layers of silica and brucite. For this reason, MC involves dissolution of SiO layers, dissolution/dehydration of Mg(OH) layers, and precipitation of magnesium carbonate. To address the chemical response of the single phases, experiments have been performed by both a local microwave-source acting locally on a specific crystal surface and a volume source interacting with an ensemble of grains on synthetic powders and single crystals of pure brucite and talc. In a second step, treatments have been extended to chrysotile, lizardite and antigorite. A characterization of the mechanism and kinetics were performed by scanning probe microscopy on the surface of single crystals phases, supported by Raman spectroscopy and by Scanning and Transmission Electron Microscopy study performed on micro- and nano-sized grains. [1] White, et al. Reaction mechanisms of magnesium silicates with carbon dioxide in microwave fields. Final Report to the U.S. Department ofEnergy, National Energy Technology Laboratory (2004