Role Of Oxygen Functional Groups Research Articles

Role of oxygen functional groups in the friction of water-lubricated low-index diamond surfaces

Large-scale quantum molecular dynamics simulations unveil eight friction regimes of water-lubricated low-index diamond surfaces. Four of these friction regimes are universal, i.e., they occur on diamond (111), (001), as well as (110). Dry sliding leads to immediate cold welding accompanied by amorphization (regime I). Small amounts of water (less than $8{\mathrm{H}}_{2}\mathrm{O}$ per $\mathrm{n}{\mathrm{m}}^{2})$ can preserve crystallinity and lower friction by localizing shear to interfacial ether groups (regime II). A further increase in water surface density results in passivating hydrogen/hydroxyl layers (regime IV) and finally (for more than $20{\mathrm{H}}_{2}\mathrm{O}$ per $\mathrm{n}{\mathrm{m}}^{2})$ in free water layers between hydrogen/hydroxyl passivated diamond surfaces (regime V). The other four friction regimes are special, i.e., they occur only on certain surfaces. An ultralow friction regime is established by aromatic Pandey surface passivation on diamond (111) surfaces (regime III). On diamond (110) surfaces, regime II coexists with three other regimes: while partial cold welding via C--C bonds (regime VI) or C--O--C bonds (regime VII) leads to frictional shear stresses that are in-between the cold-welding regimes (I and II) and the non-cold-welding regimes (IV and V), the formation of an oxidized carbon monolayer consisting of keto and ether groups results in ultralow friction (regime VIII). Regime VIII is also observed for diamond (001) surfaces. These findings are rationalized by the structural and energetic peculiarities of the different low-index surfaces. Our study provides guidelines for nanoscale control and manipulation of oxygen functional groups on carbon surfaces in boundary lubrication with water or other oxygen-containing lubricants.

Role of oxygen functional groups for structure and dynamics of interfacial water on low rank coal surface: a molecular dynamics simulation

ABSTRACTMolecular dynamics simulations were employed to study the effects of oxygen functional groups for structure and dynamics properties of interfacial water molecules on the subbituminous coal surface. Because of complex composition and structure, the graphite surface modified by hydroxyl, carboxyl and carbonyl groups was used to represent the surface model of subbituminous coal according to XPS results, and the composing proportion for hydroxyl, carbonyl and carboxyl is 25:3:5. The hydration energy with −386.28 kJ/mol means that the adsorption process between water and coal surface is spontaneous. Density profiles for oxygen atoms and hydrogen atoms indicate that the coal surface properties affect the structural and dynamic characteristics of the interfacial water molecules. The interfacial water exhibits much more ordering than bulk water. The results of radial distribution functions, mean square displacement and local self-diffusion coefficient for water molecule related to three oxygen moieties confirmed that the water molecules prefer to absorb with carboxylic groups, and adsorption of water molecules at the hydroxyl and carbonyl is similar.

Selectivity Tuning of Graphene Oxide Based Reliable Gas Sensor Devices by Tailoring the Oxygen Functional Groups: A DFT Study Based Approach

This paper concerns the selectivity tuning of graphene oxide based gas sensor devices, where the role of oxygen functional groups like, carboxyl, carbonyl, hydroxyl (sp2), epoxy, and hydroxyl (sp3) were investigated for physisorption of NO2, NH3, CO, and H2O using first principle calculation (density functional theory) incorporating Atomistix Toolkit (v2015.1). Among the five functional groups under consideration, carboxyl, carbonyl, and hydroxyl (sp2) were considered at the edges on the basal plane, while epoxy and hydroxyl (sp3) groups were considered in the plane perpendicular to that. For all the species, the optimum positions for adsorption, adsorption energy and charge transfer efficiency were investigated for the above two planes. It was found that edge carboxyl group is the most favorable in terms of adsorption energy and charge transfer efficiency targeting NH3, CO, and H2O. On the contrary, for NO2, hydroxyl functionalization was found to be the most efficient one.

Characterization and formation mechanism of pits on diamond {100} face etched by molten potassium nitrite

The morphology of diamond surface etched by molten potassium nitrate was investigated and the formantion mechanism of etched pits was disscussed. The inverted pyramid and square-shaped pits were formed on the {111} face and {100} face of diamond particles, respectively. The size and depth of etching pits on {100} face were apparently smaller than that of {111} face, which suggests {100} face has higher stability due to the role of oxygen functional groups absorb on the structure. At a low etching temperature (600°C), square-shaped pits were firstly formed on {100} face and then gradually changed into octagon shape with the increasing of etching time. The change of shape can be attributed to the configuration of atomic on {100} face of diamond and the change of oxygen partial in molten salt during the etching process. When etch temperature up to 700°C, the edges direction of the preferentially formed square shape pit became an angle of 45°with the edges of later formed square shape pit which indicated that partial oxygen pressure can effect stability of diamond surface and etching direction.

Role of oxygen functional groups in reduced graphene oxide for lubrication

Functionalized and fully characterized graphene-based lubricant additives are potential 2D materials for energy-efficient tribological applications in machine elements, especially at macroscopic contacts. Two different reduced graphene oxide (rGO) derivatives, terminated by hydroxyl and epoxy-hydroxyl groups, were prepared and blended with two different molecular weights of polyethylene glycol (PEG) for tribological investigation. Epoxy-hydroxyl-terminated rGO dispersed in PEG showed significantly smaller values of the friction coefficient. In this condition, PEG chains intercalate between the functionalized graphene sheets, and shear can take place between the PEG and rGO sheets. However, the friction coefficient was unaffected when hydroxyl-terminated rGO was coupled with PEG. This can be explained by the strong coupling between graphene sheets through hydroxyl units, causing the interaction of PEG with the rGO to be non- effective for lubrication. On the other hand, antiwear properties of hydroxyl-terminated rGO were significantly enhanced compared to epoxy-hydroxyl functionalized rGO due to the integrity of graphene sheet clusters.

Carbon-based cathode as an electron donor driving direct bioelectrochemical denitrification in biofilm-electrode reactors: Role of oxygen functional groups

Carbon-based cathodes are widely used in the biofilm-electrode reactor (BER) favoring a direct bioelectrochemical denitrification. However, little is known about how the surface chemistry of carbon affects the formation of bacterial biofilms on the electrode surface, which is fundamental to the functioning of BER for the nitrate removal. In the present study, graphite granules used in the three-dimension BER were oxidized by different methods, such as treatment with HCl, H2O2, HNO3 and HNO3+heat, providing the surface oxygen content as 1.98, 4.10, 7.70 and 11.54 at %, respectively. Likewise, the denitrifying BER equipped with the HNO3+heat-treated cathode exhibited a reaction rate of 27.36 mg NO3−-N L−1 d−1, exceeding 19.96, 14.82 and 13.93 mg NO3−-N L−1 d−1 obtained from the BERs with HNO3, H2O2- and HCl-treated cathodes, respectively. It was revealed that the oxidation treatment led to the increase in the amounts of oxygen functional groups (largely in the form of carboxyl), which were beneficial for the growth of metabolically active bacteria and thus the increased bioelectrochemical reaction kinetics. Bacterial analysis showed that the cathode-driven autotrophic denitrifiers were dominantly represented by Thiobacillus and Sulfurimonas, which displayed the highest abundance on the cathode pretreated with a hot and refluxed HNO3 solution.

Role of oxygen functional groups in graphene oxide for reversible room-temperature NO2 sensing

Reduced graphene oxide (rGO) is one of the promising sensing elements for high-performance chemoresistive sensors because of its remarkable advantages such as high surface-to-volume ratio, outstanding transparency, and flexibility. In addition, the defects on the surface of rGO, including oxygen functional groups, can act as active sites for interaction with gaseous molecules. However, the major drawback of rGO-based sensors is the extremely sluggish and irreversible recovery to the initial state after a sensing event, which makes them incapable of producing repeatable and reliable sensing signals. Here, we show that pristine GO can be used as the active sensing material with reversible and high response to NO2 at room temperature. First-principles calculations, in conjunction with experimental results, reveal the critical role of hydroxyl groups rather than epoxy groups in changing metallic graphene to the semiconducting GO. We show that the adaptive motions of the hydroxyl groups, that is, the rotation of these groups for the adsorption of NO2 molecules and relaxation to the original states during the desorption of NO2 molecules, are responsible for the fast and reversible NO2 sensing behavior of GO. Our work paves the way for realizing high-response, reversible graphene-based room-temperature chemoresistive sensors for further functional convergence.

The role of oxygen functional groups in the adsorption of heteroaromatic nitrogen compounds

A wood-based activated carbon (AC) was oxidized using different oxidants. The resultant adsorbents were applied to adsorb nitrogen (N) containing compounds that appeared in light cycled oil. Appropriate oxidation treatment can increase oxygen functional groups on the surface of AC without much damage to its pore structure. Oxygen functional groups play a key role in enhancing adsorptive selectivity of carbons. Lactone groups can facilitate the selective removal of 1-ring N compounds. Phenolic groups, total CO2-releasing groups and total O groups show an improvement in the adsorption of 2-ring N compounds. Aldehyde groups favor the adsorption of 3-ring and 4-ring N compounds. However, excessive oxidation can result in the collapse of pore structure and closure of pore channels. For instance, the carbon oxidized by a mixture of concentrated H2SO4 and HNO3 has an extremely low adsorption performance.

Mechanism Analysis of Nonvolatile Graphene Oxide Based Reram with Laterally Structured Device

There has been an increase in demand for novel nonvolatile-memory technology for low-power, high speed and high scalability because of development of digital technology. With this background, various types of memory are now enthusiastically developed to overcome the limitations of traditional Si-based memory which is difficulty to miniaturize the device and to advance high performance. Resistive random access memory (ReRAM) is a good candidate for next generation devices due to its high speed, low voltage consumption and high scalability. Recently flexible electronic devices such as wearable electronics and optoelectronics have attracted interest, so flexible ReRAM devices are also required. Metal oxide thin film based memory in ReRAM have hitherto been studied mainly as next generation nonvolatile memory devices and have shown good properties such as retention, endurance, and programming characteristics. However, ReRAM based on metal oxide thin film has limitations of fabrication process for flexible device due to their material properties. ReRAM using graphene oxide (GO) is the most promising candidate for the flexible device due to graphene oxide’s great electrical and mechanical properties. Graphene oxide ReRAM can be fabricated at room temperature on flexible substrates where metal oxides cannot be.Despite of its excellent properties, the resistive switching mechanisms remain unclear. It is generally assumed that filamentary theory and oxygen migration theory are the major mechanisms of resistive switching phenomena. In this work, we demonstrate the resistive switching mechanisms of ReRAM with laterally structured devices, which have two-terminal electrodes in the same plane. The laterally structured graphene oxide ReRAM devices are fabricated by simple spin-coating method on silicon dioxide substrates. The aluminum (Al) electrodes are deposited on spin-coated grapheme oxide layer by thermal evaporation as shown in Figure 1a. These Al/GO/Al lateral devices show memory properties similar to vertical structure as shown in Figure 1b. Conventional memory devices with vertical structure consist of a MIM (metal / insulator / metal) stack are hard to demonstrate the switching properties because the conducting paths are formed inside of the active layer, graphene oxide. In case of lateral structure device, conducting paths are formed on the surface of the active layer, where it’s easily observed the conducting paths directly using TEM or XPS analysis. Graphene oxide sheets have chemically reactive oxygen functional groups such as carboxyl, epoxy and hydrogen groups decorating the edges and the basal plane. It is assumed that these oxygen functional groups acting as the electron trap site influence the performance of the device. With functional group modification which can change the properties of graphene oxide, we can infer the role of oxygen functional groups in switching mechanisms.So, we can demonstrate the mechanism of ReRAM more clearly by lateral memory device.

Role of Oxygen Functional Groups in Carbon Nanotube/Graphene Freestanding Electrodes for High Performance Lithium Batteries

AbstractHierarchical functionalized multiwalled carbon nanotube (MWNT)/graphene structures with thicknesses up to tens of micrometers and relatively high density (>1 g cm−3) are synthesized using vacuum filtration for the positive electrode of lithium batteries. These electrodes, which are self‐standing and free of binder and current collectors, utilize oxygen functional groups for Faradaic reactions in addition to double‐layer charging, which can impart high gravimetric (230 Wh kg−1 at 2.6 kW kg−1) and volumetric (450 Wh L−1 at 5 kW L−1) performance. It is demonstrated that the gravimetric and volumetric capacity, capacitance, and energy density can be tuned by selective removal of oxygen species from as‐prepared functionalized MWNT/graphene structures with heat treatments in H2/Ar, potentially opening new pathways for the design of electrodes with controlled surface chemistry.

Screening biochars for heavy metal retention in soil: Role of oxygen functional groups

Oxygen-containing carboxyl, hydroxyl, and phenolic surface functional groups of soil organic and mineral components play central roles in binding metal ions, and biochar amendment can provide means of increasing these surface ligands in soil. In this study, positive matrix factorization (PMF) was first employed to fingerprint the principal components responsible for the stabilization of heavy metals (Cu, Ni, Cd, Pb) and the release of selected elements (Na, Ca, K, Mg, S, Al, P, Zn) and the pH change in biochar amended soils. The PMF analysis indicated that effective heavy metal stabilization occurred concurrently with the release of Na, Ca, S, K, and Mg originating from soil and biochar, resulting in as much as an order or magnitude greater equilibrium concentrations relative to the soil-only control. In weathered acidic soil, the heavy metal (especially Pb and Cu) stabilization ability of biochar directly correlated with the amount of oxygen functional groups revealed by the O/C ratio, pH pzc, total acidity, and by the 1H NMR analysis. Equilibrium speciation calculation showed minor influence of hydrolysis on the total soluble metal concentration, further suggesting the importance of binding by surface ligands of biochar that is likely to be promoted by biochar-induced pH increase.

Thermolysis of phenethyl phenyl ether. A model of ether linkages in low-rank coal

Currently, an area of interest and frustration for coal chemists has been the direct liquefaction of low rank coal. Although low rank coals are more reactive than bituminous coals, they are more difficult to liquefy and offer lower liquefaction yields under conditions optimized for bituminous coals. Solomon, Serio, and co-workers have shown that: in the pyrolysis and liquefaction of low rank coals, a low temperature cross-linking reaction associated with oxygen functional groups occurs before tar evolution. A variety of pretreatments (demineralization, alkylation, and ion-exchange) have been shown to reduce these retrogressive reactions and increase tar yields, but the actual chemical reactions responsible for these processes have not been defined. In order to gain insight into the thermochemical reactions leading to cross-linking in low rank coal, we have undertaken a study of the pyrolysis of oxygen containing coal model compounds. Solid state NMR studies suggest that the alkyl aryl ether linkage may be present in modest amounts in low rank coal. Therefore, in this paper, we will investigate the thermolysis of phenethyl phenyl ether (PPE) as a model of 0-aryl ether linkages found in low rank coal, lignites, and lignin, an evolutionary precursor of coal. Our results have uncovered a newmore » reaction channel that can account for 25% of the products formed. The impact of reaction conditions, including restricted mass transport, on this new reaction pathway and the role of oxygen functional groups in cross-linking reactions will be investigated.« less