Surfaces, Volume 2, Issue 2 (June 2019) – 16 articles

In many reports, cellulose and nanocellulose have been carbamated using 2,4-toluene diisocyanate (2,4-TDI) to allow the grafting of molecules or polymers onto their surfaces. Such a process usually involves the reaction of the more reactive isocyanate group of TDI (para-NCO) selectively with a hydroxyl group from the cellulose surface, followed by the reaction of the free isocyanate (ortho-NCO) with a desired molecule. After the first step, it is not possible, using elemental analysis, to determine the amount of ortho-NCO on the cellulosic surface, as an ideal para/ortho selectivity is difficult to obtain. This paper presents a simple method for the quantification of ortho-NCOs on the surface of cellulose nanocrystals upon TDI-based carbamation. It relies on the pH increase upon a complete hydrolysis of ortho-NCOs to amine groups using acidified dimethylsulfoxide. The method was found to be accurate and valid for a degree of substitution of up to 20%. Full article

The presence of contaminants in water is a subject of paramount importance nowadays, which can make water improper to human consumption even when these contaminants are present at very low concentrations, causing health issues and economic losses. In this work, we evaluated the performance of nanocomposites based on nylon 6,6/chitosan electrospun nanofibers modified by cellulose nanowhiskers combined with functional materials like silver nanoparticles, gold nanoparticles, and reduced graphene oxide to be used as sensing layers of an electronic tongue (e-tongue) to detect Isoborneol. This compound, found in some plants and essential oils, is used as a natural repellent and also to produce many other chemicals. Additionally, its chemical structure is related to that of 2-methylisoborneol, a critical pollutant in aqueous media. The synergism between the nanomaterials combined with electrospun nanofibers could be verified by the enhancement of the charge transference ability. Additionally, electrical capacitance data measured with the impedimetric e-tongue were treated by Principal Component Analysis (PCA), and revealed the sensing system was able to discriminate samples contaminated with Isoborneol at nanomolar concentrations. Moreover, the electronic tongue system could detect Isoborneol in real water samples under different concentrations. Full article

Hydrogenated nanocrystalline silicon, while being non-charged and non-polar, could be an ideal candidate for the non-covalent and orientation-controlled immobilization of biomolecules thanks to local electric fields around nanocrystals. To that effect, the adsorption of bovine serum albumin on substrates with different densities of nanocrystals, revealed by Raman spectroscopy and X-ray diffraction, was studied using infrared spectroscopy and atomic force microscopy. It was found that the protein–surface interactions followed different mechanisms depending on the nanostructure at the surface: hydrophobic on the non-crystalline part of the surface and electrostatic around the crystalline part. These electrostatic interactions were driven by the electric fields that arose at the junction between crystalline and amorphous structures. These electric fields were found to be strong enough to interact with the amide dipoles, thereby reorienting the adsorbed protein molecules on this part of the surface. Nevertheless, the adsorbed proteins were found to be denatured, which was due to the surface chemistry, and not affected by the nanostructure. Full article

Background: The study of processes in the drying drops of biological fluids (dried drop patterns) and the method of dynamic surface tensiometry can be attributed to integral methods of assessing the state of the organism. Research objective: to establish the relationship between the type of crystallization patterns and the surface tension of human saliva in normal conditions. Methods: 100 volunteers (40 males, 60 females) that were aged 30–59 participated in the study. In all saliva samples, the parameters of dynamic tensiometry, types of crystallization patterns and 11 biochemical parameters were determined. Results: No statistically significant differences in the saliva crystallization patterns were observed, depending on the age and gender characteristics of the volunteers. A negative correlation of the area of the crystallization zone and the surface tension of saliva is shown. When considering the crystallization patterns, their considerable variability was noted; on this basis, the entire studied sample was divided into four clusters by surface tension. Conclusion: In general, the crystallization patterns that are inside the selected groups remain quite heterogeneous. This increases the likelihood of making an incorrect diagnosis when using visual methods to evaluate the crystallization patterns, which significantly limits the use of such diagnostic methods in clinical practice. Full article

Hybrid samples consisting of polymer poly-3(hexylthiophene) (P3HT) and silicon nanoparticles were prepared. It was found that the obtained samples were polymer matrixes with conglomerates of silicon nanoparticles of different sizes (10–10⁴ nm). It was found that, under illumination, the process of nonequilibrium charge carrier separation between the silicon nanoparticles and P3HT with subsequent localization of the hole in the polymer can be successfully detected using electron paramagnetic resonance (EPR) spectroscopy. It was established that the main type of paramagnetic centers in P3HT/silicon nanoparticles are positive polarons in P3HT. For comparison, samples consisting only of polymer and silicon nanoparticles were also investigated by the EPR technique. The polarons in the P3HT and P_b centers in the silicon nanoparticles were observed. The possibility of the conversion of solar energy into electric energy is shown using structures consisting of P3HT polymer and silicon nanoparticles prepared by different methods, including the electrochemical etching of a silicon single crystal in hydrofluoric acid solution and the laser ablation of single-crystal silicon in organic solvents. The results can be useful for solar cell development. Full article

Cubic, octahedral, and rhombic dodecahedral gold nanocrystals enclosed by {100}, {111}, and {110} facets, respectively, were prepared by a seed-mediated growth method at the room temperature. Palladium thin films were coated on these Au nanocrystals by a redox replacement approach to explore their catalytic activities. It is revealed that formic acid and carbon monoxide oxidation in 0.1 M HClO₄ on Au nanocrystals coated with one monolayer (ML) of Pd are facet-dependent and resemble those obtained from corresponding Pd single crystals and Pd films deposited on bulk Au single crystals, suggesting epitaxial growth of Pd overlayers on the Au nanocrystal surfaces. As the Pd film thickness increased, formic acid oxidation current density decreased and the CO oxidation potential moved to more negative. The catalytic activity remained largely unchanged after 3–5 MLs of Pd deposition. The specific adsorption of (bi)sulfate was shown to hinder the formic acid oxidation and the effect decreased with the increasing Pd film thickness. These observations were explained in the framework of the d-band theory. This study highlights the feasibility of engineering high-performance catalysts through deposition of catalytically active metal thin films on facet-controlled inert nanocrystals. Full article

Inspired by nature, tunable wettability has attracted a lot of attention in both academia and industry. Various methods of polymer surface tailoring have been studied to control the changes in wetting behavior. Polymers with a precisely controlled wetting behavior in a specific environment are blessed with a wealth of opportunities and potential applications exploitable in biomaterial engineering. Controlled wetting behavior can be obtained by combining surface chemistry and morphology. Plasma assisted polymer surface modification technique has played a significant part to control surface chemistry and morphology, thus improving the surface wetting properties of polymers in many applications. This review focuses on plasma polymerization and investigations regarding surface chemistry, surface wettability and coating kinetics, as well as coating stability. We begin with a brief overview of plasma polymerization; this includes growth mechanisms of plasma polymerization and influence of plasma parameters. Next, surface wettability and theoretical background structures and chemistry of superhydrophobic and superhydrophilic surfaces are discussed. In this review, a summary is made of recent work on tunable wettability by tailoring surface chemistry with physical appearance (i.e. substrate texture). The formation of smart polymer coatings, which adjust their surface wettability according to outside environment, including, pH, light, electric field and temperature, is also discussed. Finally, the applications of tunable wettability and pH responsiveness of polymer coatings in real life are addressed. This review should be of interest to plasma surface science communality particularly focused controlled wettability of smart polymer surfaces. Full article

The development of catalysts for the oxygen reduction reaction in low-temperature fuel cells depends on efficient and accurate electrochemical characterization methods. Currently, two primary techniques exist: rotating disk electrode (RDE) measurements in half-cells with liquid electrolyte and single cell tests with membrane electrode assemblies (MEAs). While the RDE technique allows for rapid catalyst benchmarking, it is limited to electrode potentials far from operating fuel cells. On the other hand, MEAs can provide direct performance data at realistic conditions but require specialized equipment and large quantities of catalyst, making them less ideal for early-stage development. Using sputtered platinum thin-film electrodes, we show that gas diffusion electrode (GDE) half-cells can be used as an intermediate platform for rapid benchmarking at fuel-cell relevant current densities (~1 A cm⁻²). Furthermore, we demonstrate how different parameters (loading, electrolyte concentration, humidification, and Nafion membrane) influence the performance of unsupported platinum catalysts. The specific activity could be measured independent of the applied loading at potentials down to 0.80 V_RHE reaching a value of 0.72 mA cm⁻² at 0.9 V_RHE in the GDE. By comparison with RDE measurements and Pt/C measurements, we establish the importance of catalyst characterization under realistic reaction conditions. Full article

Ultrasonic machining has been used over a decade to enhance the surface finishing and overall processing characteristics of conventional technologies. The benefits that are usually associated to this approach generate an increasing interest in both academic and industrial fields, especially in the turning operation due to its simple application. In this study, ultrasonic assisted turning is used to study the effect of intermittent tool contact on the surface quality of cast and wrought aluminium alloys. The resulting surface roughness and topography plots were evaluated through a three-dimensional (3D) optical profilometer. Additionally, stereo microscopy and detailed by scanning electron microscopy analyzed chip shape and morphology. The experimental results show that the appropriate use of an ultrasonic intermittent tool can improve the superficial quality up to 82% and reduce the maximum peak height by 59 % for a 0.045 mm/rev feed rate. When the feed rate is increased to 0.18 mm/rev, the surface roughness may be enhanced by 60% and the maximum peak height reduced by 76%. Furthermore, due to the introduction of a distinct cutting mechanism, the traditional chip shape is modified when the ultrasonic tool excitation is applied. A model is suggested to explain the chip growth and the fracture behaviour. Full article

The electro-oxidation of CO on Pt surface is not only fundamentally important in electrochemistry, but also practically important in residential fuel cells for avoiding the poisoning of Pt catalysts by CO. We carried out cyclic voltammetry on Pt(111), (110), (100), (10 10 9), (10 9 8), (10 2 1), (432), and (431) single-crystal surfaces using a three compartment cell to understand the activity and durability towards the electro-oxidation of CO saturated in 0.1 M HClO₄. During the potential cycles between 0.07 and 0.95 V vs. the reversible hydrogen electrode, the current for the electro-oxidation of CO at potentials lower than 0.5 V disappeared, accompanied by surface reconstruction. Among the electrodes, the Pt(100) electrode showed the lowest onset potential of 0.29 V, but the activity abruptly disappeared after one potential cycle; the active sites were extremely unstable. In order to investigate the processes of the deactivation, potential-step measurements were also conducted on Pt(111) in a CO-saturated solution. Repeated cycles of the formations of Pt oxides at a high potential and Pt carbonyl species at a low potential on the surface were proposed as the deactivation process. Full article

Synchrotron-based scanning photoelectron microscopy (SPEM) has opened unique opportunities for exploiting processes occurring at surfaces and interfaces, which control the properties of materials for electrochemical devices, where issues of chemical and morphological complexity at microscopic length scales should be faced and understood. The present article aims to demonstrate the present capabilities of SPEM to explore the surface composition of micro- and nano-structured materials, focusing on cases relevant to electrochemical technologies. We report and discuss a selection of recent results about three different systems, targeting hot topics in the fields of electrochemical energy storage and electrochemical fabrication: (i) an in-depth analysis of Ag-In electrodeposited alloys exhibiting dynamic pattern formation, (ii) the analysis of electrochemical processes at the electrodes of a self-driven solid oxide fuel cell and (iii) an operando characterization of a single-chamber solid oxide fuel cell. The last example has been performed at near-ambient pressure conditions using a unique specially designed setup which extends the traditional capabilities of scanning photoemission microscopes in the ultra-high and high-vacuum regimes to operating conditions that are closer to realistic ones, contributing to overcome the so-called “pressure gap”. Full article

The self-organized formation of nanoscale laser-induced periodic surface structures (LIPSS) is still not fully understood with respect to the dynamics and interplay of contributing complex mechanisms. The transition from randomness to order and the specific role of nano-feedback are of fundamental interest because of their general aspects. In our study, the very first steps of the surface reconfiguration are demonstrated by analyzing the topology of evolving nano-crater maps. The evolution of spatial frequencies and directional arrangement indicate a feedback-driven adaptation of k-vectors to the required excitation conditions of elementary dipoles in the linearly polarized laser field. The time-dependent structure formation was studied by pump-probe diffraction and scattering experiments. The ratio of the contributions of characteristic light patterns enables plasmonic and non-plasmonic mechanisms to be distinguished, which subsequently act at distinctly different time scales. Recently developed multistage models for the dynamics of material modification are confirmed. The influence of accumulation effects is clearly demonstrated by characteristic changes in scattering and diffraction with an increasing number of preceding pulses. It is assumed that the thermal and plasmonic contributions to accumulation are coupled and thus generate spatially variable modifications. Full article

The advances in material science have led to the development of novel and various materials as nanoparticles or thin films. Underpotential deposition (upd) of transition metals appears to be a very sensitive method for probing the surfaces of noble metals, which is a parameter that has an important effect on the activity in heterogeneous catalysis. Underpotential deposition as a surface characterization tool permits researchers to precisely determine the crystallographic orientations of nanoparticles or the real surface area of various surfaces. Among all the work dealing with upd, this review focuses specifically on the main upd systems used to probe surfaces of noble metals in electrocatalysis, from poly‒ and single-crystalline surfaces to nanoparticles. Cu_upd is reported as a tool to determine the active surface area of gold‒ and platinum‒based bimetallic electrode materials. Pb_upd is the most used system to assess the crystallographic orientations on nanoparticles’ surface. In the case of platinum, Bi and Ge adsorptions are singled out for probing (1 1 1) and (1 0 0) facets, respectively. Full article

The modification of the work function of Sn-doped In₂O₃ (ITO) by vacuum adsorption of 4-(Dimethylamino)benzoic acid (4-DMABA) has been studied using in situ photoelectron spectroscopy. Adsorption of 4-DMABA is self-limited with an approximate thickness of a single monolayer. The lowest work function obtained is $2.82\pm 0.1$ eV, enabling electron injection into many organic materials. In order to identify a potential influence of the ITO substrate surface on the final work function, different ITO surface orientations and treatments have been applied. Despite the expected differences in substrate work function and chemical bonding of 4-DMABA to the substrate, no influence of substrate surface orientation is identified. The resulting work function of ITO/4-DMABA substrates can be described by a constant ionization potential of the adsorbed 4-DMABA of $5.00\pm 0.08$ eV, a constant band alignment between ITO and 4-DMABA and a varying Fermi energy in the ITO substrate. This corresponds to the behaviour of a conventional semiconductor heterostructure and deviates from the vacuum level alignment of interfaces between organic compounds. The difference is likely related to a stronger chemical bonding at the ITO/4-DMABA interface compared to the van der Waals bonding at interfaces between organic compounds. Full article

It is increasingly useful to develop bifunctional catalysts for oxygen reduction and oxygen evolution reaction (ORR and OER) for fuel cells, metal-air rechargeable batteries, and unitized regenerative cells. Here, based on the excellent conductivity and stability of ordered mesoporous carbons, and the best ORR and OER activity of Co₃O₄, the composite Co₃O₄/N-HNMK-3 was designed and manufactured by means of a solvothermal method, using ordered N-doped mesoporous carbon (N-HNMK-3) as substrate, and then the bifunctional electrocatalytic performance corresponding to ORR, OER in alkaline media was carefully investigated. The results showed that Co₃O₄/N-HNMK-3 composite, a non-precious metal centered electrocatalyst, displayed excellent ORR performance (activity, selectivity, and stability) close to that of commercial 20 wt.% Pt/C and a promising OER activity near 20 wt.% RuO₂/C. The outstanding bifunctional activities of Co₃O₄/N-HNMK-3 was assessed with the lowest △E value of 0.86 V (E_{OER,10 mA cm⁻²}-E_{ORR,−3 mA cm⁻²}) with respect to the two commercial precious metal-based electrocatalysts. Full article

Blackening belongs to the predominant technological processes in preserving steel surfaces from corrosion by generating a protective magnetite overlayer. In place of the commonly used dipping-procedure into nitrite-containing blackening baths at boiling temperatures that are far above 100 °C, here we describe a more environmentally friendly electrochemical route that operates at temperatures, even below 100 °C. After an investigation of the electrochemical behavior of steel samples in alkaline solutions at various temperatures, the customarily required bath temperature of more than 130 °C could be significantly lowered to about 80 °C by applying a DC voltage that leads to an electrode potential of 0.5−0.6 V vs. Pt. Thus, it was possible to eliminate the use of hazardous sodium nitrite economically and in an optimum way. Electrochemical quantification of the corrosion behavior of steel surfaces that were in contact with 0.1 M KCl solution was carried out by linear sweep voltammetry and by Tafel slope analysis. When comparing these data, even the corrosion rates of conventional blackened surfaces are of the same magnitude as a blank steel surface. This proves that magnetite overlayers represent rather poor protective layers in the absence of additional sealing. Moreover, cyclic voltammetry (CV), atomic force microscopy (AFM), scanning electron microscopy (SEM) and auger electron spectroscopy (AES) characterized the electrochemically blackened steel surfaces. Full article