Distribution Of Oxygen Functional Groups Research Articles

High-velocity transverse impact of monolayer graphene oxide by a molecular dynamics study

2D materials have attracted tremendous attention recently due to their unique characteristic under extreme loading conditions such as impact. Owing to its exceptional mechanical properties and low cost of processing, graphene oxide (GO), which is a 2D amorphous material, has been widely utilized as a nanocomposite building block under such loading conditions. To date, extensive researches have been conducted to unravel the chemistry-dependent mechanical properties of GO. Nonetheless, high-velocity impact characterization of single-layer GO received less attention. Therefore, the aforementioned problem is investigated using ReaxFF molecular dynamics simulations in this work. Accordingly, the penetration-resistance of single-layer GO with different oxidation levels under nano-projectile impact is assessed. It is found that (a) as oxidation level increases the onset of complete perforation velocity decreases, (b) penetration-resistance is influenced by hydroxyl-to-epoxide ratio for a fixed oxidation level, and (c) spatial distribution of oxygen functional groups alters the perforation-resistance of a monolayer for the same oxidation level. The findings of this study may provide some insights into the design of surface coatings and novel protective barriers in which GO is used as reinforcement nanofillers.

Effect of the carboxyl functional group at the edges of graphene on the signal sensitivity of dopamine detection

Sensitive and selective detection of dopamine (DA) is important for the early diagnosis of diseases associated with abnormal DA levels. This study investigated the effect of the degree of carboxyl (−COOH) functional groups at the edges of graphene on the signal sensitivity of electrochemical DA detection in the presence of ascorbic acid (AA) and uric acid (UA). We prepared edge carboxylated graphene (ECG) nanoplatelets with a high proportion of oxygen functional groups (26.40%) and heat-treated ECG (HECG) with a low proportion of oxygen functional groups (2.35%), which were used to modify a fluorine-doped tin oxide (FTO) electrodes through the electrospray method. The ECG/FTO electrode showed excellent catalytic activity in separating AA, DA, and UA signals, potentially owing to the strong chemical interactions of these analytes with −COOH groups of ECG. While the HECG/FTO electrode displayed the oxidation signals suppression of AA and UA from those of DA signals with low sensitivity; this can be ascribed to decreased chemical interactions of these analytes with HECG induced by the low proportion of −COOH functional groups. Concurrently, the ECG/FTO sensor showed high sensitivity (ca. 2.11 μA/cm2/μM) and low detection limit (ca. 0.26 μM) for DA detection in the presence of AA and UA, while it was ca. 0.55 μA/cm2/μM and 1.0 μM, respectively, for the HECG/FTO sensor. These results suggest that the dense distribution of oxygen functional groups at the edges of graphene plays a vital role in ensuring high sensitivity and a low limit of DA detection.

Biochar cathode: Reinforcing electro-Fenton pathway against four-electron reduction by controlled carbonization and surface chemistry

Porous biochars have attracted tremendous interests in electrochemical applications. In this study, a family of biochars were prepared from cellulose subject to different carbonization temperatures ranging from 400 to 700 °C, and the biochars were in-situ activated by a molten salt (ZnCl2) to construct a hierarchically porous architecture. The activated porous biochars (ZnBC) were used as a carbocatalyst for electro-Fenton (EF) oxidation of organic contaminants. Results showed that high-temperature carbonization improved the activity of biochar for four-electron oxygen reduction reaction (ORR) due to the rich carbon defects, while the mild-temperature treatment regulated the species and distribution of oxygen functional groups to increase the production of hydrogen peroxide (H2O2) via a selective two-electron ORR pathway. ZnBC-550 was the best cathode material with a high ORR activity without compromise in H2O2 selectivity; a high production rate of H2O2 (796.1 mg/g/h) was attained at −0.25 V vs RHE at pH of 1. Furthermore, Fe(II) addition induced an electro-Fenton system to attain fast decomposition of various organic pollutants at −0.25 V vs RHE (reversible hydrogen electrode) and pH of 3 with a satisfactory mineralization efficiency toward phenolic pollutants. The EF system maintains its excellent stability for 10 cycles. Hydroxyl radicals were identified as the dominant reactive oxygen species based on in situ electron paramagnetic resonance (EPR) analysis and radical quenching tests. This study gains new insights into electrocatalytic H2O2 production over porous biochars and provides a low-cost, robust and high-performance electro-Fenton cathode for wastewater purification.

Nanoscale chemical mapping of oxygen functional groups on graphene oxide using atomic force microscopy-coupled infrared spectroscopy

The unambiguous determination of the chemical functionality over graphene oxide (GO) is important to unleash its potential applications. However, the mapping of oxygen functionalities distribution remains to be unequivocally determined because of highly inhomogeneous non-stoichiometric structures and ultra-thin layers of GO. In this study, we report an experimental observation of the spatial distribution of oxygen functional groups on monolayer and multilayer GO using AFM-IR, atomic force microscopy coupled with infrared spectroscopy. Overcoming conventional IR diffraction limit for several micrometers, the novel AFM-IR reaches high spatial resolution ∼20 nm and could detect IR absorption on ∼1 nm thickness of monolayer GO. With nanoscale chemical mapping, the distribution of different oxygen functional groups is distinguished with AFM-IR over the GO surface. It allows us to observe that these oxygen functional groups prefer to sit on the fold areas, in discrete domains and on the edges of GO, which gave more insights into its chemical nature. The determination of the position of functional groups through precise imaging contributes to our understanding of GO structure-properties relations and paves the way for targeted tethering of polymers, biomaterials, and other nanostructures.

Local plasma activation of PS films with a defined design for biomedical use

In this work, air plasma treatment of polystyrene (PS) films was selectively carried out on a pre-defined pattern by using a micro-plasma needle set-up in an efficient one-step process. Patterns consisting of three vertical lines were selected during the plasma modification process. The space between the lines and the number of treatment repeats were modified and their influence on PS surface characteristics was investigated. The distribution of oxygen functional groups across horizontal sections was shown by performing X-ray photoelectron spectroscopy mapping measurements. These results were compared with water contact angle measurements and showed that increasing the number of treatment repeats leads to an improvement of the wettability and an increase of the percentage of oxygen incorporated in the center of the modified line regions. In addition, the width of the plasma treated lines was found to increase with increasing number of treatment repeats. Physical surface properties were also analyzed on pre-selected locations of the PS surface using atomic force microscopy. Finally, cell tests using human foreskin fibroblasts were carried out to investigate the effect of the plasma activated lines on cell attachment and proliferation. The number of treatment repeats and distance between the lines were found to strongly affect cell adhesion and proliferation. As such, the plasma needle set-up used in this work is able to noticeably improve PS biocompatibility on a pre-determined area by selecting the appropriate plasma operational parameters.

Distribution of oxygen functional groups of graphene oxide obtained from low-temperature atomic layer deposition of titanium oxide

The island-like distribution of the oxygen functional groups of graphene oxide was identified by deposition of TiO2 on the graphene oxide surface using low-temperature atomic layer deposition.

Effects of nitrogen plasma treatment on the surface characteristics of olive stone-based activated carbon

ABSTRACTNitrogen plasma treatment (NPT) of activated carbon (AC) at different conditions was carried out to introduce nitrogen-containing groups onto olive stone-activated carbon (OSAC) surfaces. Textural characteristics of raw and irradiated samples were analyzed by N2 and CO2 adsorption. Surface chemical functional groups were analyzed by X-ray photoelectron spectrometry (XPS) and Fourier Transformed Infrared spectroscopy. The results showed that after NPT, the surface textural properties of irradiated OSAC were slightly damaged, and a gradual decrease in surface area and pore volume was observed during the irradiation. XPS revealed that NPT could change the distribution of oxygen functional groups on the OSAC surface and there were more nitrogen atoms incorporated into the aromatic ring. A tentative explanation for the modification process is proposed. Phenol adsorption was enhanced from 110 mg/g for untreated AC to 635 mg/g for 30-min plasma-treated OSAC.

Atmospheric pressure plasma activation of PP films with a localized μplasma

Abstract Polypropylene (PP) films are locally plasma treated with a single line pattern using a μplasma needle set-up with micro scale accuracy and resolution. The two main plasma treatment conditions, number of treatment repeats and applied power, have been varied and their influence on the surface characteristics has been investigated. X-ray photoelectron spectroscopy (XPS) mapping measurements have been carried out to show the distribution of oxygen functional groups across sections of the plasma treated line patterns. These measurements have been correlated with water contact angle (WCA) results and showed that the created polar oxygenated functionalities are responsible for a sharp local increase in wettability. Increasing power and treatment repeats resulted in an improvement in wetting behavior of PP films and led to wider tracks with higher percentages of incorporated oxygen across the treatment patterns. AFM analysis of a treated straight line also showed that the center of the track line has a higher roughness than the border parts. Increasing power and number of treatment repeats results in a progressive increase of surface roughness and the generation of a granular topography. It has been shown that increasing the power has a more pronounced effect on the PP morphology than the variation of treatment time.

Revisiting Graphene Oxide Chemistry via Spatially-Resolved Electron Energy Loss Spectroscopy

The type and distribution of oxygen functional groups in graphene oxide (GO) and reduced graphene oxide (RGO) remain still a subject of great debate. Local analytic techniques are required to access the chemistry of these materials at a nanometric scale. Electron energy loss spectroscopy in a scanning transmission electron microscope can provide the suitable resolution, but GO and RGO are extremely sensitive to electron irradiation. In this work we employ a dedicated experimental setup to reduce electron illumination below damage limit. GO oxygen maps obtained at a few nanometers scale show separated domains with different oxidation levels. The C/O ratio varies from about 4:1 to 1:1, the latter corresponding to a complete functionalization of the graphene flakes. In RGO the residual oxygen concentrates mostly in regions few tens nanometers wide. Specific energy-loss near-edge structures are observed for different oxidation levels. By combining these findings with first-principles simulations we propose a model for the highly oxidized domains where graphene is fully functionalized by hydroxyl groups forming a 2D-sp$^3$ carbon network analogous to that of graphane.

Reactivity of mesoporous carbon against water – An in-situ XPS study

Functionalized mesoporous carbon catalysts can be used in the acid catalyzed dehydration of fructose to 5-hydroxymethyl furfural (HMF). However, strong deactivation can be observed after preconditioning of the material in the reaction solvent 2-butanol. Surface changes caused by the pretreatment have been studied by XPS. The comparison of the pristine sample and the pretreated carbon sample showed similar distribution of oxygen functional groups by ex-situ XPS, as well as similar behavior during heating in vacuum. However, the addition of water (0.1mbar vapor pressure) and subsequent heating to 130°C exhibited prominent differences in the evolution of the O1s, as well as for the C1s spectra of the two samples. Changes in the surface termination and hydrophobicity of the materials are discussed under the aspect of possible reactions of surface functional groups with the alcoholic solvent and water.

Electrochemical surface oxidation of carbon nanofibers

Carbon nanofiber (CNF) surfaces were functionalized with oxygen-bearing groups through electrochemical oxidation. The electrode was prepared without a binder, allowing easy separation of the functionalized CNFs for subsequent applications. The relationships between the applied potential and the CNF structure with the resulting O/C atomic ratio and the distribution of oxygen functional groups were investigated. Surface groups were identified and characterized by elemental analyses, X-ray photoelectron spectroscopy, micro-attenuated total reflectance FTIR, and cyclic voltammetry. The oxidation of herringbone CNFs was initiated at a relatively low potential at both the anodic and cathodic electrodes, while the O/C atomic ratio remained relatively constant within the range of potentials investigated. The relative concentration of carbonyl and hydroxyl groups increased with increasing potential while the amount of carboxylic groups decreased. The structure of the CNF was important in determining the O/C atomic ratio, which was especially dependent on the spatial arrangement of graphene layers. Tubular CNFs exhibited low O/C atomic ratios while herringbone CNFs, which have a higher surface area, exhibited the largest ratios. The dispersion of the CNFs in water was much more homogeneous following electrochemical oxidation.

Investigating the Influence of Oxygen/Carbon Ratio on Fabrication of Composite Carbon Nanotubes by Self-Assembly Surface Treatment

Through self-assembly monolayer surface treatment, metal oxide nanoparticles uniformly dispersed onto carbon nanotubes (CNTs) surface are investigated. At first, oxidation treatment was performed to increase O/C ratio of CNTs surface at 250°C for 1 hr under an oxygen atmosphere. X-ray photoelectron spectroscopy (XPS) analysis shows that O/C ratio is a increasing function of oxidating time. Distribution of oxygen functional groups on CNTs surface, i.e., carboxyl, carbonyl, phenolic groups, can be identified and deconvoluted by a symmetrical Gauss function. Experiments indicate that heat time for 5hr can produce a greater O/C ratio on CNTs surface. It is observed that carboxyl groups acts an important role to link with metal ions via an ionic interaction, thus, forming a monolayer adsorption on CNTs surface. By heating the treated CNTs, a completely composite nanostructure is thus formed. In the present work, we successfully fabricate three kinds of nanoparticles including SnO2, and RuO2, with an average diameter of 5-10 nm coated on the CNTs.

Analysis of oxygen-functional groups in brown coals

Distribution of oxygen-functional groups (alcohol, phenol, carboxyl, carbonyl, and ether groups) in four brown coals, Australian Yallourn (abbreviated as YL), Indonesian South Banko (SB) and Adaro (AD), and US Beulah Zap (BZ), was evaluated based on chemical analysis and 13C NMR measurement. Concentrations of carbonyl and ether groups were about 20%, based on total oxygen in coal, while the distribution of carboxyl, alcohol, and phenol changed from coal to coal. The evaluation of the amount of reactive ether groups was conducted by treating the sample coals with SiCl 4–NaI, which is able to cleave alkylaryl- and dialkylethers. The results indicate that the sample coals contained reactive ether groups ranging from 0.3 to 1.8 groups per 100 carbon in coal. The relationships between the amounts of oxygen-functional groups and the solubility of coal were also investigated.

Sequential Derivatization and the SIMS Imaging of Coal

Abstract Secondary Ion Mass Spectrometry (SIMS) has been used to examine the low-temperature oxidation of coal, using 18O2 and in conjunction with site-specific reagents in order to identify the surface distribution of oxygen functional groups. This approach leads to some ambiguity in the assignment of functional groups because the inherent complexity of coal chemistry rarely allows any definitive reaction products to be identified. This paper describes results obtained from reaction of coal surfaces with a sequence of reagents designed to remove ambiguous results. This procedure, coupled with SIMS imaging, yields results consistent with an oxidation mechanism which involves the formation of surface peroxides.