Ionic liquid viscosity effects on the functionalization of electrode material through the electroreduction of diazonium

Pt/Polypyrrole Quasi-References Revisited: Robustness and Application in Electrochemical Energy Storage Research

Choosing reference electrodes for nonaqueous electrochemical measurements, especially in energy storage research, is challenging due to lengthy experiments (>1 day), the lack of alternatives to the commonly used Ag/Ag⁺ reference electrode (RE), the introduction of junction potentials, and the possibility of sample contamination. Often, quasi-reference electrodes (QREs) such as Ag wires and Li metal strips are used. However, small changes in electrolyte composition can cause large potential drifts, and their surfaces may be reactive to the solution. Here, we propose an alternative QRE based on polypyrrole electrodeposited on Pt wire (PPyQRE) encased in a glass tube with the open end sealed with commercial frits. While freestanding PPyQRE wires have been reported in the literature, simple encasing of the PPyQRE overcomes the above-mentioned drawbacks of QREs while providing a reliable reference potential that is closer to the performance of an RE. During cyclic voltammetric and bulk electrolysis testing of a redox mediator in solution, the encased PPyQRE exhibited stable reference potentials over multiple charge/discharge cycles with minimal drift (∼5 mV) after ∼2.25 days of operation. We also tested the reliability of our reference during the testing of multilayer graphene Li-ion anodes, which often involve cycling samples at highly reducing potentials (<-3 V vs Fc/Fc⁺) over long durations (>1 day). In the same testing conditions, the Ag/Ag⁺ electrode led to observable Ag deposits on the graphene and large potential drifts (∼50 mV), while the PPyQRE exhibited no measurable drift and revealed changes in voltammetric features that were obscured by reference drift when using Ag/Ag⁺. Minor reference drifts of ∼30 mV over long usage of the PPyQRE (∼2 months) can be addressed by calibration with a ferrocene couple at the end of experiments. These results highlight the advantages of using an encased PPyQRE as a simple and practical reference electrode for electrochemical measurements in the field of nonaqueous energy storage research.

Halogen bonding effect on electrochemical anion oxidation in ionic liquids

Three Ionic liquids (ILs) based on an imidazolium core have been compared and used as solvents for the oxidation of various anions. Electrochemical experiments as well as NMR titrations and X-ray diffraction analyses unambiguously confirm the crucial role of non-covalent halogen bonding on the oxidation potentials and consequently the electrochemical window of the respective ILs.

Tuning the Covering on Gold Surfaces by Grafting Amino-Aryl Films Functionalized with Fe(II) Phthalocyanine: Performance on the Electrocatalysis of Oxygen Reduction

Current selective modification methods, coupled with functionalization through organic or inorganic molecules, are crucial for designing and constructing custom-made molecular materials that act as electroactive interfaces. A versatile method for derivatizing surfaces is through an aryl diazonium salt reduction reaction (DSRR). A prominent feature of this strategy is that it can be carried out on various materials. Using the DSRR, we modified gold surface electrodes with 4-aminebenzene from 4-nitrobenzenediazonium tetrafluoroborate (NBTF), regulating the deposited mass of the aryl film to achieve covering control on the electrode surface. We got different degrees of covering: monolayer, intermediate, and multilayer. Afterwards, the ArNO₂ end groups were electrochemically reduced to ArNH₂ and functionalized with Fe(II)-Phthalocyanine to study the catalytic performance for the oxygen reduction reaction (ORR). The thickness of the electrode covering determines its response in front of ORR. Interestingly, the experimental results showed that an intermediate covering film presents a better electrocatalytic response for ORR, driving the reaction by a four-electron pathway.

Self-limiting covalent modification of carbon surfaces: diazonium chemistry with a twist

The chemistry of carbon surfaces has regained traction in recent years in view of its applicability towards covalent modification of a variety of (2D) materials. A general requisite is the formation of a dense and well-defined monolayer of aryl groups covalently bound to the surface. Given the use of reactive chemistries however, it is often not easy to achieve precise control over the monolayer growth while maintaining high grafting densities. Here we present a straightforward experimental protocol for the fabrication of well-defined covalent monolayers onto the surface of graphite. Using a combination of surface analytical tools, we demonstrate that the ascorbic acid mediated dediazoniation of aryldiazonium salts leads to self-limiting growth of monolayers with high grafting densities. The aryl radicals preferentially attach to the basal plane of the substrate and once the surface is covered with a covalent monolayer, the surface reaction does not proceed further to an appreciable extent. The layer thickness of the covalent films was measured using atomic force microscopy whereas the grafting efficiencies were assessed using Raman spectroscopy. The chemical composition of the grafted films was studied using X-ray photoelectron spectroscopy whereas scanning tunneling microscopy provided nanometer scale insight into the structure of the covalent films. Mechanistic aspects of the process are also discussed. The self-terminating chemistry described here is a new addition to the synthetic armory for covalent modification of materials and sets a strong foundation for achieving precise nanoscale control over the covalent functionalization process.

Long-Range Charge Transport in Diazonium-Based Single-Molecule Junctions

Thin layers of cobalt and ruthenium polypyridyl-oligomers with thicknesses between 2 and 8 nm were deposited on gold by electrochemical reduction of diazonium salts. A scanning tunneling microscope was used to create single-molecule junctions (SMJs). The charge transport properties of the Au-[Co(tpy)₂]_n-Au (n = 1-4) SMJs do not depend markedly on the oligomer length, have an extremely low attenuation factor (β ∼ 0.19 nm^-1), and do not show a thickness-dependent transition between two mechanisms. Resonant charge transport is proposed as the main transport mechanism. The SMJ conductance decreases by 1 order of magnitude upon changing the metal from Co to Ru. In Au-[Ru(tpy)₂]_n-Au and Au-[Ru(bpy)₃]_n-Au SMJs, a charge transport transition from direct tunneling to hopping is evidenced by a break in the length-dependent β-plot. The three different mechanisms observed are a clear molecular signature on transport in SMJs. Most importantly, these results are in good agreement with those obtained on large-area molecular junctions.

Strategies for the Formation of Monolayers From Diazonium Salts: Unconventional Grafting Media, Unconventional Building Blocks

Pioneered by J. Pinson and coll. in 1990s, the reductive grafting of aryldiazonium salts has become a powerful method for surface functionalization. Highly robust interfaces result from this surface attachment, resistant to heat, chemical degradation and ultrasonication. Importantly, this approach can be applied to many materials, ranging from conducting, semi-conducting, oxides to insulating substrates. In addition, either massive, flat surfaces or nanomaterials can be functionalized. The method is easy to process and fast. The grafting process involves the formation of highly reactive aryl radicals able to attack the substrate. However, the generated radicals can also react with already-grafted aryl species, leading to the formation of loosely-packed polyaryl multilayer films, typically of 10-15 nm thick. It is thus highly challenging to control the vertical extension of the deposited layer and to form well-ordered monolayers from aryldiazonium salts. In this mini review, we briefly describe the different strategies that have been developed to prepare well-ordered monolayers. We especially focus on two strategies successfully used in our laboratories, namely the use of unconventional solvents, i.e., room temperature ionic liquids (RTILs), as grafting media and the use of calixarene macrocycles by taking benefit of their pre-organized structure. These strategies give large possibilities for the structuring of interfaces with the widest choice of materials and highlight the potential of aryldiazonium grafting as a competitive alternative to self-assembled monolayers (SAMs) of alkyl thiols.

Robust hydrophobic gold, glass and polypropylene surfaces obtained through a nanometric covalently bound organic layer

The (electro)chemical grafting of a polyfluorinated calix[4]arene on gold, polypropylene and glass is reported. The modified surfaces were characterized by ellipsometry, atomic force microscopy (AFM), and by X-ray photoelectron spectroscopy (XPS). A nanometric, robust and uniform monolayer of covalently surface-bound calix[4]arenes was obtained on the three different materials. For all surfaces, contact angles higher than 110° were recorded, highlighting the hydrophobic character given by this ∼2 nm thin organic monolayer. Remarkably, the contact angle values remained unchanged after 18 months under a laboratory atmosphere. The results presented herein thus present an attractive and sustainable strategy for bringing hydrophobic properties to the interface of a wide range of materials.

The grafting of a polyfluorinated calix[4]arene-tetradiazonium derivative on various surfaces led to the formation of very robust and stable hydrophobic monolayers.

From Langmuir-Blodgett to Grafted Films

A locally organized monolayer film strongly attached to a gold surface is obtained by transfer of a Langmuir-Blodgett (LB) film of octadecylamine (ODA) or alcohol (ODOH) onto a Au surface and simultaneous oxidative electrografting of this film still in contact with the aqueous subphase. As opposed to LB films, these films resist ultrasonication; and unlike electrografted films, they are organized monolayers by construction. They are characterized by AFM (atomic force microscopy), water contact angle, ellipsometry, XPS (X-ray photoelectron spectroscopy), IRRAS (infrared reflection absorption spectroscopy), and GIXD (grazing incidence X-ray diffraction).

Dithienylpyrrole Electrografting on a Surface through the Electroreduction of Diazonium Salts

The control of the interface and the adhesion process are key issues for the development of new application based on electrochromic materials. In this work the functionalization of an electrode’s surface through electroreduction of diazonium generated in situ from 4-(2,5-di-thiophen-2-yl-pyrrol-1-yl)-phenylamine (SNS-An) has been proposed. The synthesis of the aniline derivative SNS-An was performed and the electrografting was investigated by cyclic voltammetry on various electrodes. Then the organic thin film was fully characterized by several techniques and XPS analysis confirms the presence of an organic film based on the chemical composition of the starting monomer and allows an estimation of its thickness confirmed by AFM scratching measurements. Depending on the number of electrodeposition cycles, the thickness varies from 2 nm to 10 nm, which corresponds to a few grafted oligomers. In addition, the grafted film showed a good electrochemical stability depending on the scan rates up to 400 V/s and the electrochemical response of the modified electrode towards several redox probes showed that the attached layer acts as a conductive switch. Therefore, the electrode behaves as a barrier to electron transfer when the standard redox potential of the probe is below the layer switching potential, whereas the layer can be considered as transparent towards the electron transfer for redox probes with a redox potential above it.

Use of calixarenes bearing diazonium groups for the development of robust monolayers with unique tailored properties

Calixarene-based diazonium salts can be easily synthesized in a few steps. This review surveys recent examples that illustrate the key advantages of these highly reactive molecular platforms for surface modification.

Use of Selective Redox Cross-Inhibitors for the Control of Organic Layer Formation Obtained via Diazonium Salt Reduction

The controlled electrochemical deposition of a series of four diazonium salts (4-bromobenzene, 4-iodobenzene, 4-methoxybenzene, and 4-diethylaminobenzene diazonium) on carbon surfaces has been achieved by exploiting the use of three redox mediators: 2,2-diphenyl-1-picrylhydrazyl, chloranil, and dichlone. The efficiency of the method rests on a fast redox cross-reaction in the diffusion layer between the diazonium compound and the reduced form of the selected inhibitor, characterized by an outer-sphere electron transfer. The effect of the inhibitor addition in the deposition solution was characterized using electrochemical techniques, X-ray photoelectron spectroscopy, and atomic force microscopy. Near-monolayers are obtained when the potential of the redox mediator is at least 100 mV lower than the reduction potential of the diazonium salt concerned. A judicious choice of the redox entity can allow, via a fine control of the experimental conditions, to modulate the thickness of organic layers by varying the grafting potential.

Molecular Sieving and Current Rectification Properties of Thin Organic Films

For the purpose of preparing well-organized functional surfaces, carbon and gold substrates were modified using electroreduction of a tetrahedral-shape preorganized tetra-aryldiazonium salt, leading to the deposition of ultrathin organic films. Characterization of the modified surfaces has been performed using cyclic voltammetry, X-ray photoelectron spectroscopy, infrared absorption spectroscopy, ellipsometry, atomic force microscopy, and contact angle measurements. The specific design of the tetra-aryldiazonium salts leads to an intrinsic structuring of the resulting organic films, allowing molecular sieving and current rectification properties toward redox probes in solution.

Advances on Aryldiazonium Salt Chemistry Based Interfacial Fabrication for Sensing Applications

Aryldiazonium salts as coupling agents for surface chemistry have evidenced their wide applications for the development of sensors. Combined with advances in nanomaterials, current trends in sensor science and a variety of particular advantages of aryldiazonium salt chemistry in sensing have driven the aryldiazonium salt-based sensing strategies to grow at an astonishing pace. This review focuses on the advances in the use of aryldiazonium salts for modifying interfaces in sensors and biosensors during the past decade. It will first summarize the current methods for modification of interfaces with aryldiazonium salts, and then discuss the sensing applications of aryldiazonium salts modified on different transducers (bulky solid electrodes, nanomaterials modified bulky solid electrodes, and nanoparticles). Finally, the challenges and perspectives that aryldiazonium salt chemistry is facing in sensing applications are critically discussed.

Controlling Grafting from Aryldiazonium Salts: A Review of Methods for the Preparation of Monolayers

Surface modification by grafting from aryldiazonium salts has been widely studied and applied to many substrates as a simple and versatile method for preparing strongly adherent organic coatings. Unless special precautions or conditions are used, the usual film structure is a loosely packed disordered multilayer; however, over the past decade, attention has been paid to establishing strategies for grafting just a monolayer of modifiers to the surface. To date, four general approaches to monolayer preparation have emerged: use of aryldiazonium ions with cleavable protection groups; use of aryldiazonium ions with steric constraints; grafting in the presence of a radical scavenger; and grafting from ionic liquids. This review describes these approaches, illustrates some of their applications, and highlights the advantages and disadvantages of each.

Electrodeposition of gold nanoparticles on aryl diazonium monolayer functionalized HOPG surfaces

Gold nanoparticle electrodeposition on a modified HOPG surface with a monolayer organic film based on aryl diazonium chemistry has been studied. This organic monolayer is electrochemically grown with the use of 2,2-diphenyl-1-picrylhydrazyl (DPPH), a radical scavenger. The electrodeposition of gold on this modified surface is highly favored resulting in an AuNP surface density comparable to that found on glassy carbon. AuNPs grow only in the areas covered by the organic monolayer leaving free clean HOPG zones. A progressive mechanism for the nucleation and growth is followed giving hemispherical AuNPs, homogeneously distributed on the surface and their sizes can be well controlled by the applied electrodeposition potential. By using AFM, C-AFM and electrochemical measurements with the aid of two redox probes, namely Fe(CN)6(4-)/Fe(CN)6(3-) and dopamine, relevant results about the electrochemical modified surface as well as the gold nanoparticles electrodeposited on them are obtained.

Functionalization of nanomaterials with aryldiazonium salts

This paper reviews the surface modification strategies of a wide range of nanomaterials using aryldiazonium salts. After a brief history of diazonium salts since their discovery by Peter Griess in 1858, we will tackle the surface chemistry using these compounds since the first trials in the 1950s. We will then focus on the modern surface chemistry of aryldiazonium salts for the modification of materials, particularly metallic, semiconductors, metal oxide nanoparticles, carbon-based nanostructures, diamond and clays. The successful modification of sp(2) carbon materials and metals by aryldiazonium salts paved the way to innovative strategies for the attachment of aryl layers to metal oxide nanoparticles and nanodiamonds, and intercalation of clays. Interestingly, diazotized surfaces can easily trap nanoparticles and nanotubes while diazotized nanoparticles can be (electro)chemically reduced on electrode/materials surfaces as molecular compounds. Both strategies provided organized 2D surface assembled nanoparticles. In this review, aryldiazonium salts are highlighted as efficient coupling agents for many types of molecular, macromolecular and nanoparticulate species, therefore ensuring stability to colloids on the one hand, and the construction of composite materials and hybrid systems with robust and durable interfaces/interphases, on the other hand. The last section is dedicated to a selection of patents and industrial products based on aryldiazonium-modified nanomaterials. After nearly 160 years of organic chemistry, diazonium salts have entered a new, long and thriving era for the benefit of materials, colloids, and surface scientists. This tempts us to introduce the terminology of "diazonics" we define as the science and technology of aryldiazonium salt-derived materials.

Sterically controlled functionalization of carbon surfaces with -C6H4CH2X (X = OSO2Me or N3) groups for surface attachment of redox-active molecules

Glassy carbon electrodes were modified by electrochemical reduction of a diazonium molecule ((i)Pr3SiOCH2C6H4N2(+)BF4(-)) featuring a triisopropylsilyl-protected benzylic hydroxyl group. This electrochemical process introduced a monolayer of (i)Pr3SiOCH2C6H4- groups onto the surface of the electrode. The bulky -Si(i)Pr3 protecting group not only prevents the uncontrolled growth of structurally ill-defined and electronically blocking polyphenylene multilayers, but also separates the phenyl groups in the monolayer. Thus, the void spaces between these aryl units should allow a better accommodation of sizable molecules. Removal of the -Si(i)Pr3 protecting groups by (n)Bu4NF exposed the reactive benzylic hydroxyl functionalities that can undergo further transformations to anchor functional molecules. As an example, redox-active ferrocene molecules were grafted onto the modified electrode via a sequence of mesylation, azidation, and copper-catalyzed [3 + 2] cycloaddition reactions. The presence of ferrocenyl groups on the surface was confirmed by X-ray photoelectron spectroscopic and electrochemical studies. The resulting ferrocene-modified glassy carbon electrode exhibits cyclic voltammograms typical of surface-bound redox active species and remarkable electrochemical stability in an acidic aqueous environment.

Ionic and Molecular Liquids: Working Together for Robust Engineering

Because of their outstanding versatility, room-temperature ionic liquids (RTILs) are utilized in an ever increasing number of novel and fascinating applications, making them the Holy Grail of modern materials science. In this Perspective, we address the fundamental research and prospective applications of RTILs in combination with molecular liquids, concentrating on three significant areas: (1) the use of molecular liquids to decrease the viscosity of RTILs; (2) the role of RTIL micelle formation in water and organic solvents; and (3) the ability of RTILs to adsorb pollutant gases. Current achievements are examined, and future directions for the potential uses of RTILs are outlined.