1,720,994 research outputs found
Blister evolution time invariance at very low electrolyte pH: H 2 SO 4 /graphite system investigated by electrochemical atomic force microscopy
No full textTemporal analysis of blister evolution during anion intercalation in graphite
No full textIn the currently accepted picture, when graphite is immersed and polarized in a diluted sulfuric acid electrolyte, the surface undergoes an invasive process due to the intercalation of solvated sulphate anions inside the crystal. The following evolution of CO, CO2 and O2 promotes the surface swelling and the growth of blisters. Here, we give evidence that the appearance of blisters affects the graphite surface as soon as the oxygen potential is reached, i.e. before the traditionally accepted anion intercalation stage, which instead is demonstrated slowing the blister development. These results suggest a new picture of the solvated anion intercalation in graphite with respect to the current interpretative model
Physical confinement versus adsorption driven reconstruction: The case of sulfate anion interaction with vicinal Cu(111) surfaces
No full textNano-electrochemistry, i.e., the research of the properties of nano-(structured) electrodes and their influence on
electrochemical processes when immersed inside an electrolyte, represents a hot topic in view of applications in
nano-electronics, electro-catalysis and energy storage devices. The role of physical confinement in the electrochemical fabrication and performances of the respective systems have been recently addressed in the context of
metal-organic networks on surfaces, but rarely of nano-structured bare metal surfaces, for instance, regularly
stepped (vicinal) surfaces. In this work we investigate the interplay between physical confinement and adsorbate
induced restructuring by the electrochemical adsorption of sulfate anions on the flat and two distinctly different
vicinal Cu(111) surfaces. Sulfate adsorption on the flat Cu(111) surface is known to create a long-range ordered
Moir ́e-superstructure with lattice parameters in the 2–4 nm range due to an expansion of the topmost layer of
copper atoms with respect to the underlying crystal planes. This restructuring is also observed on a vicinal Cu
(111) surface whose original terrace width is considerably smaller than the lattice vectors of the sulfate induced
Moir ́e-structure. The results clearly indicate not only that the Moir ́e formation lifts the physical confinement
imposed by the initial terrace width, but also shine more light on the Moir ́e formation process itself. Such
adsorbate induced restructuring, of course, depends on the respective adsorbate – electrode combination, but
must, in principle, always be taken into account in order to understand electrochemical processes at nanostructured (and nano-sized) electrode surfaces
Temperature Effects on the HOPG Intercalation Process
Full text linkGraphite intercalation via chemical strategies is a common procedure to delaminate
stratified crystals and obtain a suspension of graphene flakes. The intercalation mechanism at the
molecular level is still under investigation in view of enhancing graphene production and reducing
damage to the original pristine crystal. The latter, in particular, can undergo surface detriment due to
both blister evolution and carbon dissolution. The role of the electrolyte temperature in this process
has never been investigated. Here, by using an in-situ atomic force microscopy (AFM) apparatus,
we explore surface morphology changes after the application of fast cyclic-voltammetries at 343 K,
in view of de-coupling the crystal swelling phenomenon from the other electrochemical processes.
We find that blisters do not evolve as a consequence of the increasing temperature, while the quality of
the graphite surface becomes significantly worse, due to the formation of some adsorbates on possible
defect sites of the electrode surface. Our results suggest that the chemical baths used in graphite
delamination must be carefully monitored in temperature for avoiding undesired electrode detriment
Nano-protrusions in intercalated graphite: understanding the structural and electronic effects through DFT
No full text: 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
Structure and electronic properties of Zn-tetra-phenyl-porphyrin single- and multi-layers films grown on Fe(001)-p(1 × 1)O
No full textThe structure and the electronic properties of thin (1 molecular layer) and thick (20 molecular layers) Zn-tetra-phenyl-porphyrin (ZnTPP) films grown on a single metal oxide (MO) layer, namely Fe(001)-p(1 × 1)O, are shown and discussed. During the first stages of deposition, the ultra-thin MO layer reduces the molecule-substrate interaction enhancing the molecular diffusivity with the respect to other investigated substrates [namely, Si(111), Au(001) and oxygen-free Fe(001)]. On Fe(001)-p(1 × 1)O, ZnTPP molecules form an ordered and stable square-lattice array. The photoemission analysis of the valence bands reveals that all the characteristic features of the molecule are already visible in the 1 monolayer-thick sample spectrum. Similarly, the core level investigation suggests a weak molecule perturbation. The ZnTPP/Fe(001)-p(1 × 1)O interface represents a prototypical system to investigate the organic film adhesion on ultra-thin MO layers and the processes involved during the film growth
Unconventional post-deposition chemical treatment on ultra-thin H2TPP film grown on graphite
No full textApplication of UV-vis optical spectroscopy in electrochemical processes: case-study of graphite anion intercalation
No full textThe 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
Vacuum-Deposited Porphyrin Protective Films on Graphite: Electrochemical Atomic Force Microscopy Investigation during Anion Intercalation
No full textThe 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
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