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Self-Assembled Monolayers of aromatic thiols on gold and copper in aqueous environment: an electrochemical and spectroscopic study
Full text linkThiolate Self-Assembled Monolayers (SAMs) offer the unique opportunity to tailor surface
properties of noble and coinage metal in extremely fine way through a simple bottom-up
approach, making them very attractive in a wide range of technological applications. On
copper surfaces, n-alkanethiolate monolayers have been extensively studied as corrosion
inhibitors in aqueous solutions revealing excellent performances but relatively poor stability,
a problem affecting SAMs in many of them applications. On the other hand, very few studies
have been performed on SAMs constituted by simple aromatic thiols and none of them
assessed their long term stability, although the relatively strong ring interaction should in
principle positively affect the layer durability.
In the present work the protective properties and the long term stability in strongly acidic
solution (H2SO4 0.5 M) of two aromatic SAMs, namely Benzenethiol (BT) and 2-
Naphthalenethiol (2-NT) was compared to that of one long-chain n-alkylic SAM, the 1-
Undecanethiol (1-UT) by means of XPS, Raman spectroscopy, DCA and electrochemical
techniques. The results not only demonstrated the aromatic SAMs to be remarkably stable,
but also highlighted a favourable influence of the aqueous environments on the layers
structures, resulting in a durable enhancement of their protective properties.
Subsequently, the effect of different p-substituent groups (-F, -CH3, -OH, -COOH, -
NHCOCH3) on the performance and the stability of the aromatic SAMs was investigated.
All such molecules, similarly to BT, showed a steep increase of their inhibition efficiency
upon ageing. The rate and the durability of the enhancement resulted mainly dependent from
the polar effect of the tail group; moreover, the hydrophobicity seems to influence the
protective properties of the layers as well.
Finally SAMs of BT and 1-UT on gold, both freshly prepared and aged in ultrapure water,
were characterized electrochemically. Again BT layers showed an improvement of their
structural order following the exposure to aqueous environments, indirectly supporting the
hypothesis of a direct involvement of water molecules in this process. In addition, a new
method to prepare high quality aromatic SAMs on gold has been proposed
A new simple method to heal defects and to improve electrode passivity of aromatic SAMs on gold
No full textIn the present work we report a new, simple method to heal the defects of SAMs constituted by small aromatic thiols, namely benzenethiol (BT) and 2-naphthalenethiol (2-NT), on polycrystalline Au surfaces. The method consists in the alternate immersion of the Au substrate in the thiolic solution and in ultrapure water. The layers obtained with the new protocol were characterized by means of Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS), and were compared with SAMs prepared with the traditional adsorption procedure. The results show that our treatment leads to a remarkable increase of the blocking behavior of the layers, indicating a decrease in density of defective sites, and to an improvement of the SAM stability. On the basis of the obtained results, a possible explanation to this phenomenon is proposed. (C) 2013 Elsevier B.V. All rights reserved
Effect of various terminal groups on long-term protective properties of aromatic SAMs on copper in acidic environment
No full textThe effect of several p-substituent groups (-F, -CH3, -OH, -COOH, -NHCOCH3) on the performances and the stability of aromatic monolayers on copper has been studied. All the results agreed indicating that such aromatic films are very stable and effective as corrosion inhibitors even in harsh conditions (aerated H2SO4 0.5 M). Nevertheless, while XP spectroscopy showed an excellent chemical stability for all the layers regardless the terminal groups, EIS measurements highlighted notably different protective properties and durability for the various SAMs. Such passivation depends not only on the hydrophobicity of the terminal group but, surprisingly, also on the electronic effect of the substituent on the ring as well. In particular, molecules having an electron-withdrawing substituent showed good passivity at first, a steep increase of their protective properties following the exposure to the electrolyte but a relatively poor stability. The presence of an electron-donating substituent, on the other hand, led to relatively poor initial protection, a slow reorganization but an exceptionally long durability. (C) 2013 Elsevier B.V. All rights reserved
Tuning the composition of aromatic binary Self-Assembled Monolayers on copper: An XPS study
No full textIn this study, the XPS characterization of different binary aromatic SAMs on copper is presented. The mixed layers, constituted by benzenethiol (BT) and alternatively 4-fluorobenzenethiol (FBT) or 4-acetamidothiophenol (AA), were prepared by two different methods, namely coadsorption from solution and partial substitution of a preformed BT layer. The overall quality and the surface composition of the different SAMs have been assessed as a function of the solution concentration for the former preparation and of the substitution time for the latter. In addition, the parameters contributing to determine the surface composition of the aromatic mixed layers on copper have been identified. (C) 2014 Elsevier B.V. All rights reserved
Durable Cu corrosion inhibition in acidic solution by SAMs of benzenethiol
No full textThe adsorption of Benzenethiol and 1-Undecanethiol on polycrystalline copper and their ability to inhibit the Cu corrosion process for several days in strongly acidic solution were studied by XPS and EIS. Surprisingly, the SAM of aromatic thiols assured higher protection and higher stability than the SAM of long-chain alkylic thiols in spite of their lower thickness, thanks to a noticeable increase in the charge-transfer resistance observed by EIS after few hours of exposure to the electrolyte. XPS data revealed that the bond with the substrate is more stable in the case of Benzenethiol than with 1-Undecanethiol. This is probably due to the presence of strong ring interactions, which allows the SAM to assume a highly ordered structure. The higher stability of the aromatic SAMs appears to be more important than the layer thickness for substrate passivation. (C) 2011 Elsevier B.V. All rights reserved
A multi-technique approach to the analysis of SAMs of aromatic thiols on copper
No full textThe adsorption of aromatic thiols on Cu and the SAM film stability in acidic solutions have been studied by XPS, contact angle and electrochemical techniques. Three short molecules, benzenethiol (BT), 2-naphthalenethiol (2-NT) and 4-acetamidothiophenol (4-AA), were selected as representatives of aromatic thiols to highlight the effect of aromatic rings and hydrophilic terminal groups on the copper protection. All the three molecules form stable S-Cu bonds as a consequence of their adsorption process on polycrystalline copper. Although none of them provides a full copper passivation, the adsorbed films persist without major degradation on Cu electrodes even after 12 h immersion in 0.5 M sulfuric acid. Comparing the freshly prepared adsorbed films, the larger 2-NT molecule provides a better Cu passivation, but the shorter BT molecule favours a higher surface coverage. The terminal groups of 4-AA are responsible for a higher Cu surface wettability in water, compared to that with SAMs of the other molecules, and allow for an easier charge-transfer to the electrolyte and for a higher electrochemical capacitance. After long enough ageing, however, the 4-AA-based molecular films are able to self-organize and to provide a steadily improving copper passivation. Adlayers of the BT and 2-NT molecules, on the contrary, over a long time tend to protect less and less the Cu substrate, probably because of progressive electrolyte infiltration
Mass transport and charge transfer rates for Co(III)/Co(II) redox couple in a thin-layer cell
No full textRecently, the complex Co(dtb)3 n+ (dtb = 4,4 di tert-butyl-2,2′ bipyridine) in methoxypropionitrile (MPN) solvent has been proposed as an alternative redox mediator in the thin-layer dye sensitized solar cells. The electrochemical properties of this new mediator as a function of temperature were investigated by mean of symmetric golden electrodes thin-layer cell, using three electro-analytical techniques: electrochemical impedance spectroscopy (EIS), slow scan cyclic voltammetry (SCCV) and chronoamperometry (CA). Our study pointed out that, at room temperature, both the electron transfer rate k° = 1.24 10-4 cm s-1 as well as the diffusion coefficient D = 5.85 × 10-7 cm s-1 are rather low. Raising the temperature has a beneficial effect, increasing more than 6 times the standard rate constant of electron transfer and more than 3 times the ionic diffusion coefficient at 80 °C. However, for all the studied temperatures, the slow mass transport of Co(III)/Co(II) species still remains the rate determining step. Viscosity measurements have demonstrated that the ionic mass transport in MPN follows the Stokes' law and the Walden product is constant, in the temperature range investigated. © 2010 Elsevier Ltd. All rights reserved
Enhanced Protective Properties and Structural Order of Self-Assembled Monolayers of Aromatic Thiols on Copper in Contact with Acidic Aqueous Solution
No full textIn the present work we used different techniques to study self-assembled monolayers (SAMs) of two aromatic thiols, namely, benzenethiol (BT) and 2-naphthalenethiol (2-NT), and one alkylic thiol, 1-undecanethiol (1-UT), on polycrystalline copper, comparing their corrosion inhibition efficiency and their stability up to a week in H2SO4 0.5 M. Both electrochemical impedance spectroscopy (EIS) and linear polarization highlighted different aging trends for 1-UT on one side and aromatic thiols on the other. 1-UT was initially the best corrosion inhibitor among the three thiols, as expected from its larger thickness, but it degraded very rapidly. On the contrary, BT and 2-NT showed a noticeable increase of their protective properties during the first hours of exposure to the electrolyte leading to a superior performance over any 1-UT sample. Raman spectroscopy suggested this behavior to be related to an enhancement of the structural order of the aromatic layer. In addition, both XP spectroscopy and electrochemical measurements revealed BT and 2-NT layers to be stabler than 1-UT. In particular, BT layers exposed to H2SO4 0.5 M exhibited better protective properties, with respect to the freshly prepared samples, lasting for over 1 week of aging
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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