Modification of carbon materials via incorporation of nitrogen has received much attention in recent years due to their performance as electrodes in applications ranging from electroanalysis to electrocatalysis for energy storage technologies. In this work we synthesized nitrogen-incorporated amorphous carbon thin film electrodes (a-C:N) with different degrees of nitrogenation via magnetron sputtering. Electrodes were characterized using a combination of spectroscopic and electrochemical methods, including X-ray photoelectron spectroscopy, ellipsometry, voltammetry, and impedance spectroscopy. Results indicate that low levels of nitrogenation yield carbon materials with narrow optical gaps and semimetallic character. These materials displayed fast electron-transfer kinetics to hexammine ruthenium(II)/(III), an outer-sphere redox couple that is sensitive to electronic properties near the Fermi level in the electrode material. Increasing levels of nitrogenation first decrease the metallic character of the electrodes, increase the impedance to charge transfer and, ultimately, yield materials with optical and electrochemical properties consistent with disordered cluster aggregates rather than amorphous solids. A positive correlation was found between the resistance to charge transfer and the optical gap when using the outer-sphere redox couple. Interestingly, the use of ferrocyanide as a surface-sensitive redox probe resulted in a monotonic increase of the impedance to charge transfer vs nitrogen content. This result suggests that surface chemical effects can dominate the electrochemical response, even when nitrogenation results in enhanced metallic character in carbon electrodes.

Porous manganese carbonate was obtained via solvothermal synthesis using ethanol and urea. The manganese carbonate was subsequently used as a precursor to synthesise mesoporous manganese oxides via thermal treatments at three various temperatures. X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS) results shows that γ-MnO2 is synthesised at 380 and 450 °C while Mn2O3 is produced at the annealing temperature of 575 °C. X-ray absorption spectra show that γ-MnO2 converts completely to Mn2O3 after annealing over the 450–575 °C range. The oxides obtained at 380 °C and 450 °C possess extremely high specific surface area, which is of interest for catalytic applications. The oxides were investigated as electrocatalysts for the oxygen evolution reaction; the oxide prepared at the lowest annealing temperature was found to be the optimum catalyst with an overpotential of 427 ± 10 mV at a current density of 10 mA cm−2, normalised by the geometric area. The improved catalytic activity was related to the presence of defect-rich and highly porous manganese dioxide at the lowest annealing temperature.

Oxygen electrochemistry is at the core of several emerging energy conversion technologies. The role of carbon nanostructures in the electrocatalysis of the oxygen reduction reaction is not well understood. Herein we report an investigation of the role of graphitic edges in oxygen electrochemistry. A new synthetic method was used to create all-carbon model electrode materials with controlled morphology. Electron microscopy results show that synthesized materials possess a high density of graphitic edges. Electrochemical intercalation experiments, however, indicate that the density of electroactive edges does not correlate positively with microscopy results. The materials were then characterized as electrodes for the oxygen reduction reaction in alkaline media. Results suggest that electrochemical determinations of edge and defect density more accurately predict electrocatalytic activity, thus suggesting that in situ characterization techniques are needed to understand the carbon/electrolyte interface.Keywords: defects; electrocatalysis; graphene; graphitic edges; intercalation; ORR; oxygen reduction

•Functionalization of PDMS was successfully achieved using a solution based method.•Long term wettability was achieved via grafting of aryldiazonium glycosides.•Surface bound glycans are recognized at PDMS surfaces by lectins.Polydimethylsiloxane (PDMS) is an extremely important and versatile polymeric material for biomedical and microfluidic devices due to a range of desirable properties. Control of the hydrophilicity of PDMS surfaces is of significant interest due to the potential for developing surfaces with tunable protein adsorption or cell adhesion properties. We report the formation of stable hydrophilic PDMS surfaces by covalent modification with glycans via aryldiazonium chemistry. The PDMS surface was modified by a two step-process including an activation of the PDMS surface, followed by reaction with aryldiazonium glycosides in aqueous solution. The functionalized PDMS was characterized by atomic force microscopy, infrared and X-ray photoelectron spectroscopy, water contact angle measurements and fluorescence microscopy. Our results demonstrate that glycans immobilized via this methodology have the dual function of imparting hydrophilicity and stabilizing the modified surface against hydrophobic recovery. Importantly, the presentation of thus immobilized glycosides makes them available to specific lectin-glycan binding interactions at the polymer-solution interface while, in the absence of specific binding interactions, leads to a reduction in albumin adsorption. This approach provides a novel and efficient route to stable hydrophilic PDMS surfaces with a broad range of applications.

Ultrasonic spray pyrolysis was used in a continuous flow apparatus for the template-free synthesis of iron- and nitrogen-doped porous carbon materials. Solutions of glucose, histidine and Fe(CH3COO)2 were nebulized and pyrolyzed yielding carbon microspheres. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Focused Ion Beam (FIB) milling revealed that microspheres initially possess empty cores and a smooth shell. Further annealing leads to a collapse of this shell, and formation of porous microspheres with high roughness and iron-rich aggregates. X-ray Diffraction (XRD) and Photoelectron Spectroscopy (XPS) were used to investigate bulk and surface chemistry: microspheres were found to undergo graphitization; Fe and Fe3C particles form and become encapsulated within the carbon phase, while the nitrogen present in the precursor solution results in the formation of pyridinic/pyrrolic N-centers. The microspheres were tested as electrocatalysts for the oxygen reduction reaction (ORR) in acidic solution. Polarization curves using a Rotating Disk Electrode (RDE) yielded electrocatalytic behavior, and the number of exchanged electrons n = 3.7 ± 0.2 calculated from Koutecky–Levich plots suggests that direct formation of H2O is the preferred ORR mechanism. These results indicate that this synthetic approach offers a simple and scalable strategy for the preparation of electrode materials for polymer electrolyte membrane fuel cells.

Poly(ether sulfone) membranes (PES) were modified with biologically active monosaccharides and disaccharides using aryldiazonium chemistry as a mild, one-step, surface-modification strategy. We previously proposed the modification of carbon, metals, and alloys with monosaccharides using the same method; herein, we demonstrate modification of PES membranes and the effect of chemisorbed carbohydrate layers on their resistance to biofouling. Glycosylated PES surfaces were characterized using spectroscopic methods and tested against their ability to interact with specific carbohydrate-binding proteins. Galactose-, mannose-, and lactose-modified PES surfaces were exposed to Bovine Serum Albumin (BSA) solutions to assess unspecific protein adsorption in the laboratory and were found to adsorb significantly lower amounts of BSA compared to bare membranes. The ability of molecular carbohydrate layers to impart antifouling properties was further tested in the field via long-term immersive tests at a wastewater treatment plant. A combination of ATP content assays, infrared spectroscopic characterization and He-ion microscopy (HIM) imaging were used to investigate biomass accumulation at membranes. We show that, beyond laboratory applications and in the case of complex aqueous environments that are rich in biomass such as wastewater effluent, we observe significantly lower biofouling at carbohydrate-modified PES than at bare PES membrane surfaces.Keywords: antifouling; biomimetic; carbohydrates; coatings; membranes; poly(ether sulfone)

The optical and catalytic properties of metal nanoparticles have attracted significant attention for applications in a wide variety of fields, thus prompting interest in developing sustainable synthetic strategies that leverage the redox properties of natural compounds or extracts. Here, we investigate the surface chemistry of nanoparticles synthesized using coffee as a biogenic reductant. Building on our previously developed synthetic protocols for the preparation of silver and palladium nanoparticle/carbon composite microspheres, a combination of thermogravimetric and spectroscopic methods was used to characterize the carbon microsphere and nanoparticle surfaces. Infrared reflectance spectroscopy and single particle surface enhanced Raman spectroscopy were used to characterize Pd and Ag metal surfaces, respectively, following synthesis. Strongly adsorbed organic layers were found to be present at metal nanoparticle surfaces after synthesis. The catalytic activity of Pd nanoparticles in hydrogenation reactions was leveraged to study the availability of surface sites, and coffee-synthesized nanomaterials were compared to commercial Pd-based hydrogenation catalysts. Our results demonstrate that biogenic adsorbates block catalytic surface sites and affect nanoparticle functionality. These findings highlight the need for careful analysis of surface chemistry as it relates to the specific applications of nanomaterials produced using greener or more sustainable methods.Keywords: Catalytic activity; Coffee; Nanoparticles; Palladium; Silver; Surface enhanced Raman spectroscopy;

Aryldiazonium cations are widely used to covalently functionalize carbon substrates that display a wide range of composition, from 100% sp2 such as graphite or graphene to 100% sp3 such as diamond and nanodiamond. In this work we investigated the effect that changes in carbon composition have on aryldiazonium adsorption rates and surface reaction mechanism. Quartz crystal microbalance (QCM) was used to investigate the rates of adsorption in situ and in real time at two amorphous carbon substrates, one with high sp2 content (a-C) and one with high sp3 content (a-C:H). A reversible Langmuir adsorption model was found to satisfactorily describe adsorption at a-C:H, yielding an adsorption rate coefficient ka = 3.1 M–1 s–1 and a free energy of adsorption ΔGa = −20.1 kJ mol–1. This model, on the other hand, could not be applied for the interpretation of adsorption curves at a-C. Using electrochemical methods and X-ray photoelectron spectroscopy (XPS), we found that adlayers formed at a-C:H and a-C surfaces differ considerably in composition; in particular, a-C surfaces were found to display higher rates of dediazoniation with respect to a-C:H surfaces. Our findings are interpreted and discussed in the context of current proposed mechanisms for aryldiazonium reactions at surfaces that consist of an adsorption/desorption step followed by a chemisorption via dediazoniation step. Our observations are consistent with proposed mechanisms and strongly suggest that differences in carbon composition result in differences in the relative magnitude of adsorption and chemisorptions rate coefficients.Keywords: adsorption; amorphous carbon; chemisorption; diazonium; QCM;

Carbohydrates are extremely important biomolecules and their immobilization onto solid surfaces is of interest for the development of new biomimetic materials and of new methods for understanding processes in glycobiology. We have developed an efficient surface modification methodology for the functionalization of a range of materials with biologically active carbohydrates based on aryldiazonium chemistry. We describe the synthesis and characterization of carbohydrate reagents, which were subsequently employed for the one-step, solution-based modification of carbon, metals, and alloys with monosaccharides. We used a combination of spectroscopic and nanogravimetric methods to characterize the structure of the carbohydrate layers; we report an average surface coverage of 7.8 × 10–10 mol cm–2 under our experimental conditions. Concanavalin A, a mannose-binding lectin, and Peanut Agglutinin, a galactose-binding lectin, were found to bind from solution to their respective monosaccharide binding partners immobilized at the surface. This result suggests that the spontaneous chemisorption of aryldiazonium monosaccharide precursors leads to the formation of monosaccharide layers that retain the biological recognition specificity of the parent carbohydrate molecule. Finally, we carried out measurements using fluorescently labeled Bovine Serum Albumin (BSA) and found that these carbohydrate coatings reduce unspecific adsorption of this protein at carbon surfaces. These results suggest that aryldiazonium-derived carbohydrate coatings may offer a promising strategy for preventing undesirable protein accumulation onto surfaces.Keywords: carbohydrate; carbon; coatings; diazonium; saccharide;

There has been great interest in synthetic methods that yield supported iron and iron oxide nanoparticles in order to prevent aggregation and improve their transport properties, handling and surface reactivity. In this work we report on the use of electroless deposition methods for the synthesis of carbon-supported iron/iron-oxide (Fe/FeOx) nanoparticles. We have used carbon porous microspheres synthesized via ultraspray pyrolysis as carbon scaffolds for the nucleation and growth of iron nanoparticles. The reported electroless deposition approach results in composite Fe/FeOx/carbon microspheres of narrowly dispersed size. A combination of X-ray powder diffraction (XRD) and X-ray absorption spectroscopies (EXAFS and XANES) was used in order to determine the structure and composition of the Fe/FeOx/carbon microspheres. Microspheres were found to display (14 ± 1)% iron content (w/w), whereby (12 ± 3)% of iron atoms were present as metallic iron and the remaining as maghemite (Fe2O3). Finally, we show that the removal capacity of Fe/FeOx/carbon microspheres for Cr(VI) is (20 ± 2) mg g−1 and that the maximum surface density for Cr adsorbates is (60 ± 6) μg m−2, thus suggesting that these are promising materials for the removal of water pollutants from aqueous solution.

•Natural reductants were used as green electroless deposition reagents.•Room temperature synthesis of supported Ag and Pd nanoparticles was achieved.•Carbon porous microspheres were used as supports.•Synthesis via natural reductants yielded catalytically active nanoparticles.Composite materials are of interest because they can potentially combine the properties of their respective components in a manner that is useful for specific applications. Here, we report on the use of coffee as a low-cost, green reductant for the room temperature formation of catalytically active, supported metal nanoparticles. Specifically, we have leveraged the reduction potential of coffee in order to grow Pd and Ag nanoparticles at the surface of porous carbon microspheres synthesized via ultraspray pyrolysis. The metal nanoparticle-on-carbon microsphere composites were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). To demonstrate the catalytic activity of Pd/C and Ag/C materials, Suzuki coupling reactions and nitroaromatic reduction reactions were employed, respectively.

The chemisorption of aryldiazonium salts is one of the most versatile reactions for the modification of carbon surfaces; in this work we investigated the spontaneous chemisorption of aryldiazonium salts at amorphous carbons of differing graphitic content in order to relate surface reactivity to the valence electronic properties of aryldiazonium cations and carbon surfaces. Two structural isomers that differ by their redox potential were chosen for our studies: 4-nitronaphthalenediazonium tetrafluoroborate (4NND) and 5-nitronaphthalenediazonium tetrafluoroborate (5NND). The adsorption of 4NND and 5NND was studied in situ via attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and ex situ via electrochemistry on two types of graphitic amorphous carbons (a-C), containing 80% and 100% trigonally bonded carbon centers. These two forms of carbon were characterized via electrochemical impedance spectroscopy (EIS), and the more graphitic surface was found to display a heterogeneous charge transfer rate constant 2 orders of magnitude larger than the less graphitic surface. This was consistent with ultraviolet photoelectron spectroscopy (UPS) results showing that the density of occupied states near the Fermi level is higher for the more graphitic substrate. In situ and ex situ studies of adsorption rates show that, on the less graphitic a-C surface, differences in adsorption rate could be explained based on the reduction potentials of the two aryldiazonium cations. However, on the more graphitic surface, we observed no difference in adsorption rates or yields between the two isomers, thus suggesting that spontaneous electron transfer is not rate determining at these surfaces. Gerischer–Marcus theory was used in order to explain the differences in charge transfer rates between the two carbons and to interpret observed differences in aryldiazonium adsorption rates at these substrates. Finally, our results are discussed in light of the current proposed mechanism of aryldiazonium chemisorption.

Amorphous carbon materials find numerous applications in diverse areas ranging from implantable biodevices to electronics and catalysis. The spontaneous grafting of aryldiazonium salts is an important strategy for the modification of these materials, and it is widely used to display a range of functionalities or to provide anchoring groups for further functionalization. We have investigated the spontaneous attachment of 4-nitrobenzenediazonium salts from aqueous solutions onto amorphous carbon materials that differ in their sp2 content, with the aim of understanding to what extent bulk composition affects rates and yields of aryldiazonium adsorption at the carbon/solution interface. Amorphous carbons were deposited in the form of thin films via reactive magnetron sputtering and were characterized using a combination of Raman, infrared, UV–vis, and X-ray photoelectron spectroscopy to determine their sp2 content. Attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) was used to monitor in situ and in real time the aryldiazonium adsorption process at the carbon/solution interface. These measurements demonstrate that rates and yields of adsorption for the same aryldiazonium salt increase nonlinearly vs sp2 concentration. Studies of aryldiazonium salt grafting as a function of time carried out ex situ via cyclic voltammetry showed that the amorphous carbon film with highest sp2 content displays significantly lower grafting yields than glassy carbon, a material with 100% sp2 content. Intercalation experiments using 4-nitrobenzylamine suggest that the difference in relative density of graphitic edge planes exposed at the carbon surface is in excellent agreement with the observed relative grafting yields. We discuss the implications of these results for the development of structure/reactivity relationships that can be leveraged for understanding the surface chemistry of disordered carbon materials.Keywords: amorphous carbon; ATR-FTIR; diazonium; electron transfer; nitrophenyl; p-nitrobenzene diazonium;

Diazonium salts of two nitro-substituted polycyclic aromatic compounds were synthesized and their spontaneous covalent attachment onto amorphous carbon surfaces was studied via electrochemical and spectroscopic techniques. In situ spectroscopic monitoring of the grafting of these compounds at amorphous carbon surfaces via attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) highlighted a marked difference in adsorption rates, which was also evident via ex situ electrochemical analysis. We show that adsorption rate differences cannot be explained based on differences in the solvolysis rates of these two molecules. It was found instead that the relative position of the –N2+ groups with respect to the –NO2 groups affected the reduction potential of the diazonium cations and in turn their adsorption rate at amorphous carbon surfaces. We conclude that differences in the electron density at the carbon atom bound to the diazonium group are responsible for the differences observed in the spontaneous attachment at carbon.

Self-assembled organic layers are an important tool for modifying surfaces in a range of applications in materials science. Covalent modification of metal surfaces with aryldiazonium cations has attracted much attention primarily because this reaction offers a route for spontaneously grafting a variety of aromatic moieties from solution with high yield. We have investigated the kinetics of this process by performing real-time, in situ nanogravimetric measurements. The spontaneous grafting of 4-nitrobenzene diazonium salts onto gold electrodes was studied via quartz crystal microbalance (QCM) from aqueous solutions of the salt at varying concentrations. The concentration dependence of the grafting rate within the first 10 min is best modeled by assuming a reversible adsorption process with free energy comparable to that reported for arylthiols self-assembled on gold. Multilayer formation was observed after extended grafting times and was found to be favored by increasing bulk concentrations of the diazonium salt. Modified gold surfaces were characterized ex situ with cyclic voltammetry, infrared reflection absorbance spectroscopy, and X-ray photoemission spectroscopy. Based on the experimentally determined free energy of adsorption and on the observed grafting rates, we discuss a proposed mechanism for aryldiazonium chemisorption.

The surface of highly ordered pyrolytic graphite (HOPG) has been modified using a new photochemically induced grafting reaction. Thiols have been revealed to behave as privileged substrates for this efficient grafting process. The reaction occurs under extremely mild conditions with visible light and at room temperature. The formation of molecular layers on the graphitic surface has been probed by X-ray photoelectron spectroscopy, cyclic voltammetry, and infrared reflectance absorption spectroscopy. The reaction was investigated in the presence of thiols bearing different terminal groups (−COOH, −OH, −CH(NHCOCH3)COOH, −COOCH2CH3) and in different solvent solutions (DMF, EtOH, CH3CN). Carboxyl and hydroxyl groups as well as the use of acetonitrile as a solvent were found to facilitate the reaction. Our results suggest that the reaction mechanism proceeds via photoinduced electron transfer from the HOPG into the liquid to form highly reactive alkyl radicals able to graft the surface. This type of reactivity of a graphite substrate may be important for general modification strategies of nanotubes and graphene and for new applications of carbon-based materials in photocatalysis.

There has been great interest in synthetic methods that yield supported iron and iron oxide nanoparticles in order to prevent aggregation and improve their transport properties, handling and surface reactivity. In this work we report on the use of electroless deposition methods for the synthesis of carbon-supported iron/iron-oxide (Fe/FeOx) nanoparticles. We have used carbon porous microspheres synthesized via ultraspray pyrolysis as carbon scaffolds for the nucleation and growth of iron nanoparticles. The reported electroless deposition approach results in composite Fe/FeOx/carbon microspheres of narrowly dispersed size. A combination of X-ray powder diffraction (XRD) and X-ray absorption spectroscopies (EXAFS and XANES) was used in order to determine the structure and composition of the Fe/FeOx/carbon microspheres. Microspheres were found to display (14 ± 1)% iron content (w/w), whereby (12 ± 3)% of iron atoms were present as metallic iron and the remaining as maghemite (Fe2O3). Finally, we show that the removal capacity of Fe/FeOx/carbon microspheres for Cr(VI) is (20 ± 2) mg g−1 and that the maximum surface density for Cr adsorbates is (60 ± 6) μg m−2, thus suggesting that these are promising materials for the removal of water pollutants from aqueous solution.

Ultrasonic spray pyrolysis was used in a continuous flow apparatus for the template-free synthesis of iron- and nitrogen-doped porous carbon materials. Solutions of glucose, histidine and Fe(CH3COO)2 were nebulized and pyrolyzed yielding carbon microspheres. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Focused Ion Beam (FIB) milling revealed that microspheres initially possess empty cores and a smooth shell. Further annealing leads to a collapse of this shell, and formation of porous microspheres with high roughness and iron-rich aggregates. X-ray Diffraction (XRD) and Photoelectron Spectroscopy (XPS) were used to investigate bulk and surface chemistry: microspheres were found to undergo graphitization; Fe and Fe3C particles form and become encapsulated within the carbon phase, while the nitrogen present in the precursor solution results in the formation of pyridinic/pyrrolic N-centers. The microspheres were tested as electrocatalysts for the oxygen reduction reaction (ORR) in acidic solution. Polarization curves using a Rotating Disk Electrode (RDE) yielded electrocatalytic behavior, and the number of exchanged electrons n = 3.7 ± 0.2 calculated from Koutecky–Levich plots suggests that direct formation of H2O is the preferred ORR mechanism. These results indicate that this synthetic approach offers a simple and scalable strategy for the preparation of electrode materials for polymer electrolyte membrane fuel cells.