Nano-protrusions in intercalated graphite: understanding the structural and electronic effects through DFT.

Complex phenomena characterize the intercalation of ions inside stratified crystals. Their comprehension is crucial in view of exploiting the intercalation mechanism to change the transport properties of the crystal or obtaining a fine control of crystal delamination. In particular, the relationship between the concentration and nature of intercalated ions and surface structural modifications of the host stratified crystal is still under debate. Here, we discuss a theoretical effort to provide a rationale for some structural changes observed on the highly oriented pyrolytic graphite (HOPG) surface after electrochemical treatment in perchloric and sulphuric acid solutions. The formation of the so-called nano-protrusions on the basal plane of intercalated graphite was previously observed with scanning tunneling microscopy (STM). In this work, we employed both STM and density functional theory (DFT) simulations to elucidate the physical and chemical mechanisms driving the emergence of these nano-protrusions. The DFT results show that, in a bilayer graphene system, the presence of a single ion can generate a nano-protrusion with 2.49 Å height and 21.27 Å width. In the deformed area, the C-C bond length is stretched by about 2.5% more than the normal graphene bond. These values are of the same dimensional scale as those reported in previous STM experimental results.25 However, the simulated STM images obtained by increasing the amount of intercalated ions per area suggest that the presence of more than one ion is needed for the deformation of the uppermost graphite layer during the early stages of intercalation. In contrast, in a multilayer graphene system, no significant surface deformation is detected when ions are intercalated between the third and fourth layers. Charge analysis indicates an altered distribution of the charges as a consequence of the intercalation. The charge transfer from graphene layers to the intercalated ions results in a surface layer more prone to oxidation.

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.

Application of UV-vis optical spectroscopy in electrochemical processes: case-study of graphite anion intercalation.

The possibility of following electrochemical processes in situ and in real time using optical techniques is important in view of shining a light on the chemical processes at the surface. The interest grows if the optical apparatus is compact and can be employed in industrial quality-check protocols. Here, we show how graphite anion intercalation-an important chemical process to massively produce graphene flakes-can be monitored by a UV-vis spectrometer when the graphite works as an electrode immersed inside the electrochemical cell. Important information on the reversibility or quasi-reversibility of the reaction shows a clear visualization in optical color maps.

Uniaxial Alignment of a Monolayer of Flat-on Free-Base Porphyrins on an Exfoliable Insulating Substrate.

Porphyrins are an extremely valuable class of molecules engaged in a variety of roles spanning from biology to optoelectronics. Manipulation of the chemical and physical properties of the inner cavity of porphyrins has been recognized as crucial for the exploitation of these systems in organic devices, particularly when porphyrins self-organize at the interface with a flat-on orientation of the macrocycle. Such an orientation has been mostly observed on metallic surfaces. Unfortunately, the physical-chemical properties of the molecules result in being largely perturbed due to the molecule-metal interaction. In addition, conducting substrates are unsuited to exploit electrically driven devices based on organic layers. To overcome these issues, we performed a topology-based analysis of insulating organic single crystal structures to identify a surface which (i) ensures easy exfoliation through mechanical methods, (ii) ensures epitaxial match with an overlayer of close-packed flat-on porphyrin molecules, and (iii) displays chirality. The outcome of this work is represented by a unique crystal of mixed 2,5-diketopiperazine and fumaric acid in a 1:1 ratio. We demonstrate that the (110) surface of this crystal fulfills the aforementioned requirements and, thanks to its peculiar subnanometric corrugations, allows one to grow uniaxially aligned monolayers of flat-on porphyrin molecules assembled through van der Waals interactions.

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.

A Versatile Approach to Electrochemical In Situ Ambient-Pressure X-ray Photoelectron Spectroscopy: Application to a Complex Model Catalyst.

We present a new technique for investigating complex model electrocatalysts by means of electrochemical in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). Using a specially designed miniature capillary device, we prepared a three-electrode electrochemical cell in a thin-layer configuration and analyzed the active electrode/electrolyte interface by using "tender" X-ray synchrotron radiation. We demonstrate the potential of this versatile method by investigating a complex model electrocatalyst. Specifically, we monitored the oxidation state of Pd nanoparticles supported on an ordered Co3O4(111) film on Ir(100) in an alkaline electrolyte under potential control. We found that the Pd oxide formed in the in situ experiment differs drastically from the one observed in an ex situ emersion experiment at similar potential. We attribute these differences to the decomposition of a labile palladium oxide/hydroxide species after emersion. Our experiment demonstrates the potential of our approach and the importance of electrochemical in situ AP-XPS for studying complex electrocatalytic interfaces.

Electrochemical scanning probe analysis used as a benchmark for carbon forms quality test.

Carbon forms (graphite, pyrolytic graphite, highly oriented pyrolytic graphite (HOPG), glassy carbon, carbon foam, graphene, buckypaper, etc) are a wide class of materials largely used in technology and energy storage. The huge request of carbon compounds with reliable and tunable physical and chemical properties is tackled by contriving new production protocols and/or compound functionalizations. To achieve these goals, new samples must be tested in a trial-and-error strategy with techniques that provide information in terms of both specimen quality and properties. In this work, we prove that electrochemical scanning probe techniques allow testing the performances of carbon compounds when are used as an electrode inside an electrochemical cell. Comparing the results with a reference sample (namely, HOPG) gives an insight on defects in the specimen structure, performances, and possible applications of the new samples. In this study, we concentrate on traditional carbon forms already employed in many fields versus new recently-developed specimens, in view of possible applications to the field of energy storage.

Blisters on graphite surface: a scanning microwave microscopy investigation.

Scanning microwave microscopy (SMM) is based on the interaction between a sample and an electromagnetic evanescent field, in the microwave frequency range. SMM is usually coupled with a scanning probe microscopy (SPM) technique such as in our case, a scanning tunneling microscope (STM). In this way, the STM tip is used to control the distance between the probe and the sample while acting as an antenna for the microwave field. Thanks to the peculiarity of our home-made setup, the SMM is a suitable method to study blisters formed on HOPG surface as consequence of an electrochemical treatment. Our system has a "broad-band" approach that opens the way to perform local microwave spectroscopy over a broad frequency range. Moreover, microwaves have the ability to penetrate into the sample allowing the sub-surface characterization of materials. The application of the SMM to characterize blisters formed on the HOPG surface provides information on the sub-layer structures.

Vacuum-Deposited Porphyrin Protective Films on Graphite: Electrochemical Atomic Force Microscopy Investigation during Anion Intercalation.

The development of graphene products promotes a renewed interest toward the use of graphite in addition to the historical one for its proven viability as battery electrode. However, when exposed to harsh conditions, the graphite surface ages in ways that still need to be fully characterized. In applications to batteries, to optimize the electrode performances in acid solutions, different surface functionalizations have been studied. Among them, aromatic molecules have been recently proposed. In this communication, we report on the protective effect exerted by a physical-vapor-deposited porphyrin layer. Metal-free tetra-phenyl-porphyrins were deposited on a highly oriented pyrolytic graphite crystal to study the modifications that occur during anion intercalation in graphite. The graphite electrode was plunged in an electrolyte solution of 1 M sulfuric acid and subjected to cyclic voltammetry. The results indicate that blister formation, the characteristic swelling of graphite surface induced by anion intercalation, is significantly perturbed by the porphyrin overlayer; the process is inhibited in those areas where the protective porphyrin film is present. We ascribe the inhibition of the anion intercalation to the protective porphyrin wetting layer.

Filled and empty states of Zn-TPP films deposited on Fe(001)-p(1×1)O.

Zn-tetraphenylporphyrin (Zn-TPP) was deposited on a single layer of metal oxide, namely an Fe(001)-p(1×1)O surface. The filled and empty electronic states were measured by means of UV photoemission and inverse photoemission spectroscopy on a single monolayer and a 20 monolayer thick film. The ionization energy and the electron affinity of the organic film were deduced and the interface dipole was determined and compared with data available in the literature.

Evolution of the graphite surface in phosphoric acid: an AFM and Raman study.

Phosphoric acid is an inorganic acid used for producing graphene sheets by delaminating graphite in (electro-)chemical baths. The observed phenomenology during the electrochemical treatment in phosphoric acid solution is partially different from other acidic solutions, such as sulfuric and perchloric acid solutions, where the graphite surface mainly forms blisters. In fact, the graphite surface is covered by a thin layer of modified (oxidized) material that can be observed when an electrochemical potential is swept in the anodic current regime. We characterize this particular surface evolution by means of a combined electrochemical, atomic force microscopy and Raman spectroscopy investigation.