Statistical Characterization of Heterogeneous Dissolution Rates of Calcite from In situ and Real-Time AFM Imaging

Abstract The evolution of the surface topography of a calcite crystal subject to dissolution is documented through in situ real-time imaging obtained via atomic force microscopy (AFM). The dissolution process takes place by exposing the crystal surface to deionized water. AFM data allow detection of nucleation and expansion of mono- and multilayer rhombic etch pits and are employed to estimate the spreading rate of these structures. Spatially heterogeneous distributions of local dissolution rate are evaluated from the difference between topographic measurements taken at prescribed time intervals. We rest on a stochastic framework of analysis viewing the dissolution rate as a generalized sub-Gaussian (GSG) spatially correlated random process. Our analysis yields: (i) a quantitative assessment of the temporal evolution of the statistics of the dissolution rates as well as their spatial increments; (ii) a characterization of the degree of spatial correlation of dissolution rates and of the way this is linked to the various mechanisms involved in the dissolution process and highlighted through the experimental evidences. Our results indicate that the parameters driving the statistics of the GSG distribution and the spreading rate of the multilayer pits display a similar trend in time, thus suggesting that the evolution of these structures imprints the statistical features of local dissolution rates. Article Highlights We investigate dynamics of dissolution patterns on a calcite crystal in contact with deionized water via AFM imaging Temporal behavior of parameters of our statistical model is consistent with surface pattern evolution A nested model for the spatial correlation of rates embeds multiple mechanisms driving dissolution rate

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

Astronomical silicate nanoparticle analogues produced by pulsed laser ablation on olivine single crystals

Silicate nanoparticles, otherwise referred to as very small grains (VSGs) [1], occur in various astrophysical environments. These grains experience substantial processing (e.g., amorphization) during their lifetime in the diffuse interstellar medium due to events such as grain-grain collisions and irradiation [2]. Moreover, several studies have pointed out that the main building blocks of these silicates are O, Si, Fe, Mg, Al and Ca, all elements that are among the principal constituents of the Earth’s surface [3], thus leading to the name “astronomical silicates”. However, the structure and chemical evolution together with the origin of these grains are still poorly understood and intensively debated [4,5]. The aim of this study is the simulation of space weathering processes on olivine single crystals by liquid phase pulsed laser ablation (LP-PLA). The study of the resulting structure of both the target and the ablated material together with their chemical evolution has been carried out by a multiple technique characterization. In particular, spectroscopy and dynamic light scattering measurements, analyses of the electrostatic properties and reactivity to acids and bases on the obtained colloidal solutions of the ablated nanoproducts have been performed and coupled with highresolution transmission electron microscopy (HR-TEM). Selected olivine target crystals (Fo87) from the São Miguel island (Azores) were analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX). LP-PLA experiments were performed with a Nd:YAG laser focused via a singlet lens onto the surface of the target, which was fixed at the bottom of a polystyrene box filled with 4 ml of deionized water (type 1) to immerge it completely. Laser pulses of 5 ns and 100 mJ simulate the timeframe and energy exchange occurring during grain-grain interstellar collisions [6] and they generate a plasma plume at the crystal/liquid interface. The rapid cooling induced by the confining liquid layer brings about the condensation of the chemical vapor it contains with production of a colloidal solution of nanoparticles. These solutions were analyzed by dynamic light scattering techniques and optical absorption spectroscopy in the range from 200 nm to 1100 nm (6.20 eV - 1.13 eV). Absorption measurements on the colloidal solutions have been compared against reference colloidal solutions dispersed in deionized water (i.e. mesoporous silica [SiO2] nanoparticles, brucite [Mg(OH)2] nanoparticles, aluminum hydroxide [Al(OH)3] nanoparticles, chrysotile [Mg3Si2O5(OH)4] nanotubes, and synthetic forsterite [Mg2SiO4] nanoparticles). Moreover, additional absorption analyses have been carried out as a function of the addition of known aliquots of sulfuric acid and sodium hydroxide solutions. TEM/EDS analyses were then performed on the ablated nanoparticles deposited via electrophoresis on C-coated Cu grids and compositional variations of the ablated target were determined by X-ray photo-emission spectroscopy analyses. The size distribution of LP-PLA synthesized nanoparticles is typically multimodal due to aggregation phenomena. Aggregation is consistent with the measured ζ-potential, which is negative with a relatively low absolute value, within the range 30-50 mV. Nonetheless, a recurrent mode is centered at about 2 nm (hydrodynamic diameter) and it is consistent with the measured size distribution obtained by transmission electron microscopy analysis (average nanoparticles diameter around 3-5 nm). Optical absorption measurements on the ejected material show a main band around 215 nm. This feature is very similar to the “B2 band” reported in several studies on silica glass [7] and ascribed to oxygen vacancies, but its nature is still far to be fully understood. We also found that this feature at 215 nm is very common among both Si and Mg compounds (e.g., Sioxide, Mg-hydroxide, chrysotile). Moreover, additional absorption bands in the range 240-350nm are observed suggesting the formation of new space weathering products as result of the ablation process. Therefore, these results suggest that substantial chemical processing might be expected during space weathering of “typical” interstellar grains into VSGs. Moreover, coupling these experimental results with remote sensing datasets will provide fundamental information about the origin and evolution of these silicate grains

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

Ordered assembling of Co tetra phenyl porphyrin on oxygen-passivated Fe(001): from single to multilayer films

Tetra-phenyl prophyrins (TPP) are an interesting class of organic molecules characterized by a ring structure with a metal ion in their centre. An ordered growth of such molecules can be obtained even on metallic substrates by means of a proper modification of the reactive interface, as we demonstrated for ZnTPP molecules coupled to oxygen-passivated Fe(001) [G. Bussetti et al. Appl. Surf. Sci. 390, 856 (2016)]. More recently, we focused on CoTPP molecules, characterized by a not nil magnetic moment and therefore of potential interest for magnetic applications. As in the ZnTPP case, our results for one monolayer coverage report the formation of an ordered assembly of flat-lying molecules. However, some differences between the two molecular species are observed in the packing scheme and in the degree of electronic interaction with the substrate. With the aim of reaching, also for CoTPP, a comprehensive view of molecular organization on Fe, we complement here our previous investigations by following the growth of the CoTPP film for increasing coverage, showing that an ordered stacking of such molecules is indeed realized at least up to four molecular layers

SILICATE NANOPARTICLES PRODUCED BY LABORATORY SIMULATED SPACE WEATHERING OF OLIVINE SINGLE CRYSTALS

Silicate nanoparticles, otherwise referred to as very small grains (VSGs) [1], occur in the interstellar medium. These grains experience a strong structural modification during their lifetime in the diffuse interstellar medium, due to events such as grain-grain collisions and irradiation. Grain amorphization is one of the major effects, transforming crystalline dust concentrated in star envelopes into amorphous silicate grains populating the interstellar medium [2]. Moreover, several studies have pointed out that the main building blocks of these silicates are O, Si, Fe, Mg, Al and Ca, all elements that are among the principal constituents of the Earth’s surface [3], thus leading to the name “astronomical silicates”. However, the structure and chemical evolution together with the origin of these grains are still poorly understood and intensively debated [4,5]. The aim of this study is the simulation of space weathering processes by liquid phase pulsed laser ablation (LP-PLA) on olivine single crystals. We adopt a multiple technique characterization, taking advantage of optical spectroscopy analyses and high- resolution transmission electron microscopy (HR-TEM), to shed light on the structure and chemical evolution of the ablated material

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

Self-assembly of C60 on a ZnTPP/Fe(001)–p(1 × 1)O substrate: observation of a quasi-freestanding C60 monolayer

Fullerene (C(60)) has been deposited in ultrahigh vacuum on top of a zinc tetraphenylporphyrin (ZnTPP) monolayer self-assembled on a Fe(001)–p(1 × 1)O substrate. The nanoscale morphology and the electronic properties of the C(60)/ZnTPP/Fe(001)–p(1 × 1)O heterostructure have been investigated by scanning tunneling microscopy/spectroscopy and ultraviolet photoemission spectroscopy. C(60) nucleates compact and well-ordered hexagonal domains on top of the ZnTPP buffer layer, suggesting a high surface diffusivity of C(60) and a weak coupling between the overlayer and the substrate. Accordingly, work function measurements reveal a negligible charge transfer at the C(60)/ZnTPP interface. Finally, the difference between the energy of the lowest unoccupied molecular orbital (LUMO) and that of the highest occupied molecular orbital (HOMO) measured on C(60) is about 3.75 eV, a value remarkably higher than those found in fullerene films stabilized directly on metal surfaces. Our results unveil a model system that could be useful in applications in which a quasi-freestanding monolayer of C(60) interfaced with a metallic electrode is required