Surface End Groups Research Articles

LUBRICATION PROPERTIES OF MXenes AND THEIR COMPOSITES: A REVIEW

Friction and wear phenomena are common problems in sliding equipment, so materials with excellent antifriction and wear resistance properties are getting increasingly more attention. Two-dimensional (2D) materials, such as graphene and molybdenum disulfide, have always been the research hotspots on account of their special structures and excellent tribological properties. In 2011, MXene, an important member of the 2D material family, was launched. With excellent properties such as ultra-high strength, rich surface end groups, and relatively low cost, MXenes have great potential as lubricant additives. Although these properties have proved to be the reason why MXenes are impressive, the preparation of MXenes composites and the understanding of their mechanical and tribological mechanisms remain to be further investigated. In addition, the abundant surface end groups of MXenes nanosheets make them relatively easy to be functionalized, which has attracted widespread attention. This review aims to comprehensively understand the tribological properties of MXenes nanolayers and to summarize and sort out recent research progress. First, the preparation of MXenes is reviewed, and then the latest research progress of MXenes in liquid lubrication, solid lubrication, and solid–liquid hybrid systems since the advent of MXenes is summarized. Finally, some current problems are presented and an outlook on the development of MXenes is given.

Micro-Structure and Dielectric Properties of Ti3C2Tx MXene after Annealing Treatment under Inert Gases

At present, the rapid development of electronic devices such as batteries, sensors, capacitors and so on is creating a huge demand for lightweight materials with a designed structure and function. Ti3C2Tx MXene, a lightweight two-dimensional (2D) nanomaterial with excellent electronic properties, has been favored in this field. In this work, Ti3C2Tx MXene was annealed under an inert gas (N2, Ar and CO2) atmosphere to design the crystal structure and interface of the nanosheets, and then the modified nanosheets with specific changes in dielectric properties were obtained. Among them, the key temperature points (100 °C, 300 °C, 500 °C and 800 °C) in the thermogravimetric (TG) test under an air atmosphere were used as the annealing temperature. When annealing under an air atmosphere, with the increase in temperature, the Ti layer gradually oxidized and evaporated, and the original two-dimensional structure was partly destroyed with some of the C atoms reacting with O2 to form CO2. In the inert gas atmosphere, however, the 2D structure is preserved, except that the surface end groups and layer spacing are changed. In addition, some N element doping was introduced into the nanosheets after N2 atmosphere treatment, which changed the original lattice structure. After the Ar atmosphere treatment, some Ti atoms on the surface were oxidized in situ to form TiO2 grains with different crystal forms, which increased the interfacial area. The C-TiO2 structure of the nanosheets was more complete after treatment with the CO2 atmosphere. All the nanosheets after heat treatment with an inert gas atmosphere retained the characteristic morphology of 2D materials, and different changes in the micro-structure caused changes in dielectric properties, thereby meeting the needs of 2D nanomaterials Ti3C2Tx MXene in different scenarios.

MXene by regulating etching conditions enhanced UV resistance of SBS modified asphalt: Evaluating asphalt photo-oxidation and SBS degradation

The pursuit of improving the special properties of modified asphalt using advanced nanomaterials and understanding its mechanism based on material properties is highly attractive topic. In this study, four types of MXene (Ti3C2Tx) (prepared by adjusting the synthesis methods) were incorporated into styrene–butadienestyrene (SBS) block copolymer modified asphalt as ultraviolet (UV) aging resistant modifiers. The UV aging behaviors, such as asphalt photo-oxidation and SBS copolymer degradation, were evaluated by simulating UV aging of modified asphalts using indoor equipment. The chemical and rheological results showed that MXene incorporated modified asphalts had excellent high-temperature performance and rutting resistance. Furthermore, MXene incorporation significantly inhibited asphalt oxidation and SBS copolymer degradation. The underlying mechanism of these intriguing properties was found to be the few-layered structure and -F, -O surface end groups of MXene, which were the key factors responsible for the enhanced anti-UV aging performance of modified asphalt.

(Digital Presentation) A Study on the Poly(Amidoamine) Dendrimer Encapsulated Platinum Nanoparticles for Hydrogen Evolution Reaction in Proton Exchange Membrane Water Electrolyzer

Hydrogen is a non-polluting energy carrier being considered for use in many sectors, including transportation, chemical industry, as well as the energy storage. The main motivation is to reduce utilizing fossil fuels and thus to stop global warming. Hence, speeding up research and development efforts in hydrogen production is critical to ensuring a sufficient share of hydrogen to be used in the envisaged economy sectors in the coming decades. Proton exchange membrane water electrolyzis (PEM WE) is the perspective technology to supply, in combination with the renewable energy sources, so called green hydrogen connected with a zero carbon dioxide emissions. This technology is characterized by high process intensity, good faradaic efficiency, quick response time and the possibility of high-pressure operation [1]. Important aspect, however, represents improvement of its economic competitiveness to the alternative hydrogen production routes. Important aspect represents an effective electrocatalysis of the desired electrode reactions.Number of catalysts are being studied for the hydrogen evolution reaction (HER). However, to the date, Pt-based nanoparticular catalysts are still considered to be a state of the art, robust option for use as a cathode catalyst in PEM WE. But the high cost and limitted stability are still obstacle to use Pt based catalysts in PEM WE effectivelly [1]. Hence, the effort has to be made to improve the stability of Pt nanoparticles and thus to retain its catalytic activity even under reduced loads. Poly(amidoamine) dendrimer (PAMAM) encapsulation of the nanoparticles represents a promising approach to solve this issue. PAMAM works as a template, protecting encapsulated Pt nanoparticles without passivating their surface and preventing them from agglomeration. This approach has been validated by using dendrimer encapsulated Pt nanparticles (Dend-Pt) as an oxygen reduction reaction catalyst in the fuel cells [2]. The PAMAM are monodispersed macromolecules, having a three-dimensional structure. Typically, there are three types of PAMAM, namely cationic (-NH2), anionic (-COONa) and neutral (-OH). Thus, the charge of the surface groups determines the dendrimer type. Based on the size and molecular weight, each type of PAMAM dendrimers has been classified into different generation for example, G1 and G10 are denoted as 1st and 10th generations, respectively [2].The goal of this work is to reduce Pt load required for efficient and stable HER in a PEM WE. PAMAM encapsulated Dend-Pt incorporated into multilayer graphene oxide (MLG) are used for this purpose.At first, the most suitable dendrimer template for Pt encapsulation needs to be identified and an optimum loading improving its HER efficiency in PEM WE determined. Our previous communication has proved the excellent HER activity of the carbon supported Dend-Pt, revealing an important role of the various carbon supports in enhancing conductivity, stability and catalytic activity of the Dend-Pt for HER [3]. Thus, Dend-Pt were synthesized encapsulated within the different types of dendrimer templates with varying surface end groups (-NH2, -COONa and –OH). Different generations of each type of dendrimer were used. The synthesis method consists in a simple chemical reduction in an aqueous solvent at the room temperature. The encapsulated amount of Pt was fixed for the each dendrimer under study. Commercially available MLG was used as a carbon support. The Dend-Pt was incorporated into MLG through chemical covalent linkage using (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide) (EDC) and N-hydroxyl sulfoxuccinimide (NHS) as the cross-linking agents. The amount of the cross-linking agents (EDC and NHS) used was optimised as a the mass ratio of Dend-Pt and MLG with respect to the HER activity of the resulting catalyst. The formation of the zero-valent Dend-Pt was confirmed using UV-Vis absorption and XPS spectroscopy. The structure of Dend-Pt and MLG/Dend-Pt composites were determined using TEM. The loading of the Dend-Pt on MLG surface was determined using the ICP-OES spectrometry. The electrochemical HER catalysis by different catalysts was studied by means of voltammetric methods and electrochemical impedance spectroscopy in an environment of the 0.5 M H2SO4 electrolyte solution. As a proof of concept, the PEM water electrolysis in single-cell setup was perfomed using either a commercial catalyst (40% Pt/C) or Dend-Pt as a cathode catalyst. Iridium dioxide was used as an anode catalyst.AcknowledgementThe authors gratefully acknowledge financial support from “The Grant Agency of the Czech Republic” (GAČR) under project no. 20-21105Y.Reference:[1] M. Prokop, M. Drakselova, K. Bouzek, Curr Opin Electrochem, 20 (2020) 20-27.[2] B. Devadas, A.P. Periasamy, K. Bouzek, Coord. Chem. Rev, 444 (2021) 214062.[3] B. Devadas, T. Imae, Electrochem. Communications, 72 (2016) 135-139. Figure 1

Grain size control of perovskite films based on β-alanine self-assembled monolayers surface treatment

The grain size of the perovskite layer can be tuned by the surface treatment of Ultraviolet ozone (UVO) with β-alanine self-assembled molecular films (SAMs) on the Nickel oxide (NiOx) substrate. The UVO treatment had been used to control the hydroxy (-OH) end-group on NiOx substrate and the condensation reaction of β-alanine SAMs, in which the end-group of the amino group (-NH2) correlates with the nucleation of the perovskite layer. The difference of surface end-group with/without the UVO treatment on the NiOx substrate was revealed by the shift of the oxidation peak of the SAMs layer in the cyclic voltammetry and the electrochemical impedance spectroscopy. It is found that the wettability of NiOx substrate was enhanced by the SAMs treatment time due to the increment of -NH2 end-group and the UVO treatment. Larger grain size of the perovskite layer was achieved on the NiOx substrate with SAMs 15 minutes treatment. Moreover, the optical absorbance of perovskite films in the short wavelength range was enhanced by the assembling time of SAMs. This study provides a method to control the surface morphology of the perovskite active layer in its solar cell device applications.

Identification of main influencing factors on the protein corona composition of PLGA and PLA nanoparticles

Poly(DL-lactic-co-glycolic acid) and poly(DL-lactic acid) are widely used for the preparation of nanoparticles due to favorable characteristics for medical use like biodegradability and controllable degradation behavior. The contact with different media like human plasma or serum leads to the formation of a protein corona that determines the NP’s in vivo processing.In this study, the impact of surface end group identity, matrix polymer hydrophobicity, molecular weight, and incubation medium on the protein corona composition was evaluated. Corona proteins were quantified using Bradford assay, separated by SDS-PAGE, and identified via LC-MS/MS. The acquired data revealed that surface end group identity had the most profound effect on corona composition in both quantitative and qualitative terms. Regarding matrix polymer hydrophobicity, adsorption profiles on NP systems with similar physicochemical characteristics resembled each other. The molecular weight of the matrix polymers proved to impact quantity, but not quality of corona bound proteins. The corona of plasma incubated NP showed adsorption of incubation medium-specific proteins but resembled those of serum incubated NP in terms of protein function, average mass and isoelectric point. Overall, the NP physicochemical properties proved to be easily adjustable determining factors of protein corona formation in physiological environments.

Electromagnetic shielding behavior of heat-treated Ti3C2TX MXene accompanied by structural and phase changes

Ti3C2TX MXenes have great application prospects in the electromagnetic shielding field owing to their unique accordion structure and excellent electrical conductivity. However, understanding the correlation between their thermal stability and electromagnetic shielding performance is still lacking. Additionally, owing to the complex etching environment, some termination groups are present on the surface of MXenes, significantly affecting their thermal and electromagnetic performance. Here, to solve these problems, a heat treatment process was used. In this study, the effect of heat treatment on the removal of these surface adsorption groups was evaluated. Moreover, the evolution of structure, phase, and electromagnetic shielding properties of MXenes with the change in heat treatment temperature was also studied in detail. After heat treatment at 1100 °C, the phase of MXenes changed. Multilayer-TiC, a derivative of MXenes, still exhibited excellent shielding performance, which can be attributed to the elimination of surface end groups, the maintenance of multiple structure, and the increase in content about amorphous C. The total and absorbing shielding effectiveness reached 76.1 dB and 61.3 dB, respectively. Its conductivity was 265 S/m. This study is valuable in studying application prospects of Ti3C2TX MXene in high temperature fields, such as the field of electromagnetic shielding.

Surface Charge Tunability and Size Dependent Luminescence Anisotropy of Aqueous Synthesized ZnS/Dendrimer Nanocomposites

Highly water-soluble, biocompatible, fluorescent, ZnS/Dendrimer nanocomposites have been synthesized, for the first time, with amino-, carboxyl- or hydroxyl-terminated Polyamidoamine dendrimer. Average size of ZnS nanoparticles within the dendrimer matrix was found to be in the range of 2.2-3.1 nm. ZnS nanoparticles with high monodispersity and photoluminescence efficiency was obtained by tuning various experimental parameters. An eight-fold increase in super-saturation of initial reactants caused a sharp focusing of size distribution by 38% and photoluminescence quantum efficiency also increased from 1.3% to 4.2%. ZnS/Dendrimer nanocomposites display constant positive polarized emission with the anisotropy value of 0.2, which makes it a promising candidate for biophysical probe. The anisotropy value was found to increase with increasing particle size of ZnS nanoparticles and also to depend on nature of surface end groups of the dendrimer molecule. Surface charge being an important biological parameter was tuned by varying pH of the reaction medium. Further, it is demonstrated that photoluminescence and surface charge of these nanocomposites are governed by the surface functionality of the dendrimer molecule.

Synthesis of electrochemically active pyrazine based dendrimers

A synthesis of dendrimers with pyrazine as core and m-terphenyl as surface end groups has been achieved. The electrochemical behavior of the dicationic pyrazinium dendrimers was also studied by cyclic voltammetry.

Synthesis and Antibacterial Activity of Dendritic Architectures

Synthesis of dendrimers with pentanone as core unit and m-terphenyl as surface end group has been achieved. The inhibitory activity of pentanone and its dendrimers against human pathogenic bacteria is well-documented.

Synthesis and characterization of axially chiral enantiopure novel dendrimers

The synthesis of enantiopure dendrimers with axially chiral ( S)-1,1′-bi-2-naphthol as a core unit and with m-terphenyl as a hydrophobic surface end group has been achieved. Chiroptical and fluorescence studies indicated the widening of the torsional angle between the two-naphthyl units with increase in the dendritic generation.

Synthesis and Bactericidal Efficacy of Novel Dendrimers

Synthesis of dendrimers with enones as the core unit and m-terphenyl as surface end group has been achieved. All the enones and the dendrimers were antibacterial against Salmonella typhi, ­Salmonella paratyphi and Staphylococcus aureus.

Synthesis of functionalized nanoparticles via copolymerization in microemulsions and surface reactions

Oil-in-water microemulsions of mixtures of styrene and comonomer are easily prepared using titration methods in the presence of nonionic (NPn) or anionic (SDS) surfactants. Functionalized nanoparticles in the 20–30-nm diameter range bearing chloromethyl, active-ester, acid or pyridyl surface end-groups are prepared by polymerization of microemulsions containing mixture of styrene (St) and, respectively, vinylbenzylchloride (VBC), N-acryloyloxysuccinimide (NHA), methacrylic acid (MA) or vinylpyridine (VP). Reactions of nucleophiles on particles bearing either chloromethyl or active-ester surface end-groups, performed directly in the aqueous suspensions, give rise to a wide range of nanoparticles with various functionalities. The main role of the surfactant on such surface reactions is demonstrated and used to improve the reaction yields.

Surface Depletion of End Groups in Amine-Terminated Poly(dimethylsiloxane)

The depletion of alkylamine terminal groups at the vacuum-polymer interface is measured for α,ω-difunctional poly(dimethylsiloxane) oligomers by XPS. The degree of surface end group depletion, within the maximum sampling depth probed (ca. 7 nm), is found to be on the order of 40% for a 960 molecular weight oligomer and decreases slightly with an increase in the oligomer molecular weight. Angle-dependent measurements are applied to determine end-group concentration depth profiles. End-group depletion is largest at the shallowest sampling depths and decays rapidly toward the bulk

Characterization of surface change properties of synthetic microfiltration membranes by means of a streaming current method

By use of an improved streaming current method we characterized the pH dependent surface charge properties of six synthetic microfiltration membranes at the constant electrolyte concentration of 10 −4 mol/l. The observed pH dependence of the surface charge properties of the cellulose acetate and the nylon 66 membranes could be explained by the pH influence on the dissociation and protonation of the surface end groups of membrane materials, while the reaction of the surface modification agents, i.e., the wetting agents, with the electrolytes and the ion adsorption on the membrane surfaces, were the main causes for the surface charge of nonionic polymer membranes (polyvinylidenfluoride and polysulfone membranes) in the solutions which had different pH values.

Dielectric properties of cleaned and monodisperse polystyrene latexes in microwaves

The dielectric behavior (e′, e″) of three well-cleaned monodisperse polystyrene latexes having the same particle size and the same number of chemically-bound surface groups has been studied at a fixed microwave frequency (9.4 GHz), as a function of temperature and surface group (SO 4 − , COO−, OH). A large dielectric relaxation was observed in the sulfate-stabilized latex, which has the most polar surface end-group. The anomalous behavior in the thermal dependence of the hydroxyl and carboxyl-stabilized latexes (the OH latex being more pronounced than the COO− latex) may originate from differences in the experimental conditions used for the preparation of such polymer colloids, or due to the presence of ionic species. On the basis of various dielectric models, the apparent volume fractions of the latexes were calculated. The amount of “bound” water around the latex particle was quantitatively correlated to the polarity of surface end-group (SO 4 − > COO− > OH). The differences between the calculated and actual values were not only a reflection of the thickness of vicinal water, but could also be indicative of the presence of oligomeric species in the suspension's medium (serum) of the latex. The permittivities of hydrated particle and of “bound” water were obtained with a non-linear iterative procedure.

Characterization of particle surface and morphology in vinyl acetate-butyl acrylate emulsion copolymers ? Influence of the copolymerization pathway

Surface characterization was investigated in vinyl acetate (VAc) butyl acrylate (BuA) copolymer latexes of various compositions and prepared with four different emulsion polymerization processes: conventionnal batch, composition-controlled batch, core-shell, emulsifier-free semi-continuous. Surface end-groups (sulfate or carboxylic) titration results were first compared and discussed according to the type of process and as a function of conversion. As previously shown [1], it was confirmed that batch latex particles present a heterogeneous structure with a rich VAc outlayer, as in core-shell particles. As expected, semi-continuous and composition-controlled batch particles exhibit surface end-group characteristics revealing a more homogeneous distribution of both monomers within the particles. These differences in particle morphology were corroborated by analyzing water-polymer interface in these latexes using the soap titration method, with the sodium dodecyl sulfate (SDS) or sodium hexadecyl sulfate (SHS) as emulsifier probes. When the BuA was batch-polymerized onto PVAc seed particles, the estimated surface composition seemed to show that probably phase rearrangement occurs in the particle during the synthesis or upon aging. It was also confirmed that SDS displays an abnormal adsorption due to complexation and solubilization in the rich-VAc shell of the particles.

Characterization of surface endgroups in polystrene latexes

AbstractThe stability against electrolytes for all‐sulfate, all‐carboxyl, and all‐hydroxyl, polystyrene model latexes followed the order, sulfate > carboxyl > hydroxyl when determined from an IR light scattering technique. Two types of hydroxyl endgroups were identified by 13C‐NMR (chemical shift correlations, and model compound comparisons), for the polystyrene model latexes. One type was due to termination of growing chain with an oligomer with a single monomer unit, and the other due to termination of growing chains. The surface hydroxyl groups of an all‐hydroxyl latex were derivatized with hexafluoroacetone, and quantified using 19F‐NMR techniques. Good agreement with the indirect conductometric titration values were obtained.

Room-temperature polymerisation of ethylene on a TiO2(anatase) surface; infrared spectroscopic evidence for an alkylidene–Ti4+polymer end-group and for a ‘hydrogen-bonding’ type of interaction of CH bonds of the polymer chain with the oxide surface

An infrared spectroscopic study of the spontaneous polymerization of ethylene on a sulphate-containing TiO2(anatase) sample shows evidence for an alkylidene surface end-group of the polymer and for substantial perturbation of CH2 groups of the polymer chain by interaction with the oxide surface.