Pristine Graphene Surface Research Articles

Wetting of nanofluids with nanoparticles of opposite surface potentials on pristine CVD graphene

Comparative wettability studies of graphene are conducted for two different nanofluids with opposite surface potentials of +53 mV (45-nm alumina nanoparticles) and −45 mV (28-nm silica nanoparticles), respectively. Aged graphene surface, which has adsorbed abundant hydrocarbon contaminants, shows weak hydrophobicity of about 90° wetting angles for both nanofluids for the tested volume concentration range from 0 to 10 %. For pristine graphene surfaces, however, the contact angle of alumina nanofluids continually increases from 50° to 70° for the same volume concentration increase, but the contact angle of silica nanofluids shows first increase of up to about 1 % concentration and then remains nearly unchanged with further increasing concentration. Since the nanoparticle–graphene interaction at the solid–liquid (SL) interface is expected to be the most crucial in determining the nanofluid wetting angles, the corresponding surface energy $$\gamma_{\text{SL}}$$ is examined from elaboration of $$F_{\text{DLVO}}$$ , the Derjaguin–Landau–Verwey–Overbeek force. The magnitudes of both the repulsive $$F_{\text{DLVO}}$$ on the alumina nanoparticles and the attractive $$F_{\text{DLVO}}$$ on the silica nanoparticles show rapid decreases up to 1 % volume concentration and exhibit slower decreases thereafter. The reduced repulsive $$F_{\text{DLVO}}$$ of the alumina nanoparticle drives the increasing aggregation of nanoparticles on the SL interface with increasing concentration, thus increasing the SL interfacial energy $$\gamma_{\text{SL}}$$ . On the contrary, the reduced attractive $$F_{\text{DLVO}}$$ on the silica nanoparticle retards their aggregation on the SL interface with increasing concentration and slows the increase in $$\gamma_{\text{SL}}$$ , eventually settling on the saturated level of $$\gamma_{\text{SL}}$$ from a certain concentration onwards. These distinctive behaviors of $$\gamma_{\text{SL}}$$ are consistent with the measured contact angles that gradually increase with increasing concentration for the positive surface potential (alumina), but initially increase and then settle for the negative surface potential (silica). This phenomenon strongly supports the critical dependence of nanofluid wetting of pristine graphene on $$F_{\text{DLVO}}$$ in the vicinity of the SL interface.

Adsorption of adenine on the surface of nickel-decorated graphene; A DFT study

In this study the adsorption of adenine molecule on the surface of pristine and Ni-decorated graphenes was theoretically investigated using density functional theory (DFT) to examination the potential of this modified surface to be used as an adsorbent for adenine and related chemicals. Firstly, we tried to find the best place for decoration of Nickel. We found its decoration on hollow site of graphene results the highest stability with releasing energy of ∼−527 kJ/mol. Afterward, we searched to find the side of adenine to be adsorbed. We notified considerable higher values of adsorption energy upon adsorption of adenine in positions P1 and P3 compared to the other possible positions on Ni-decorated graphene whereas there is very weak adsorption on the surface of pristine graphene. We discussed various adsorption configurations by using following analyses: charge transfers, frontier molecular orbital, and density of states (DOSs). As a result, we introduce Ni-decorated graphene as a powerful adsorbent to be used in nano-bio interactions.

Tunable plasmons in few-layer nitrogen-doped graphene nanostructures: A time-dependent density functional theory study

Compared with conventional metal plasmonic materials, surface plasmons in graphene are advantageous in terms of higher confinement, relative low loss, flexible featuring, and good tunability. However, the working frequencies of the pristine graphene (undoped graphene) surface plasmons are located in the terahertz and infrared regions, which limit their applications. Here we show high-frequency plasmons in nitrogen (N)-doped graphene nanostructures investigated by time-dependent density functional theory. We found the optical absorption strength of systems containing two layers to be more than twofold stronger than that of systems with monolayers. The optical absorption strength increases as the interlayer distance increases, and the absorption spectra are red-shifted for impulse excitations polarized in the armchair edge direction $(x$ axis). For microstructures of more than two layers, the optical absorption strength increases as number of layers of the N-doped graphene nanostructures increases. In addition, when the number of layers becomes elevated at low-energy resonances, the absorption spectra are seen to blue-shift. The plasmon energy resonance points are located in the visible and ultraviolet regions. The N-doped graphene provides an effective strategy for nanoscale plasmon devices in the visible and ultraviolet regions, despite their weaker absorption intensities when compared with the pristine graphene.

Adsorption of mercaptopyridine on the surface of Al- and B-doped graphenes: Theoretical study

We used first principles density functional theory (DFT) to study the adsorption of mercaptopyridine molecule (as an important intermediate) on the surface of pristine as well as Al- and B-doped graphene sheets. Different possible configurations have been considered for adsorption on both sheets. The resulted electronic structure of each adsorbed system is noticeably different. Our results revealed very weak adsorption on the surface of pristine graphene while relatively high adsorption was achieved by using Al-doped graphene at proper configuration. The adsorption on the surface of B-doped graphene is considerable at proper configuration but is lower than that of Al-doped graphene. Furthermore we analyzed the parameters of electronic structures, the net charge transfer, and orbital analyses including the HOMO-LUMO distributions and also density of states (DOSs) for all possible configurations. As a result, we introduce Al- and B-doped graphenes as hopeful surfaces for adsorption of MP molecule for different purpose.

The effect of calcium on nitric oxide heterogeneous adsorption on carbon: A first-principles study

The catalytic effect of calcium on nitric oxide (NO) heterogeneous adsorption with carbon was investigated through a first-principles calculation on pristine and calcium decorated graphene models, respectively. Compared with the classical polycyclic carbon model, new graphene computational models with periodical boundary conditions are better in simulating the characteristics of solid phase carbon. The adsorption of a single NO molecule on the pristine graphene surface is physical, but it is dramatically enhanced by the calcium as the absolute value of binding energy Eb increases from 19.34 kJ/mol to 206.02 kJ/mol. In order to investigate the influence of the concentration of NO molecules, the adsorption of clusters containing two and three molecules were examined. On pristine graphene surface, Eb increases with the number of NO molecules, however, on calcium decorated one, Eb demonstrates reverse tendency. This significant difference derives from the distinct mechanisms: van der Waals interaction among the NO molecules plays a crucial role in the adsorption of NO on the pure graphene surface, while the electron transfer from the 4s- and 3d orbitals of calcium to the 2p orbitals of nitrogen and oxygen atoms contributes to the catalytic effect of calcium.

Application of carbon nanostructures toward SO2 and SO3 adsorption: a comparison between pristine graphene and N-doped graphene by DFT calculations

We studied the adsorption of SOx (x = 2,3) molecules on the surface of pristine graphene (PG) and N-doped graphene (NDG) by density functional theory (DFT) calculations at the B3LYP/6-31G(d) level. We used Mulliken and NBO charge analysis to calculate the net charge transfer of adsorbed SOx on pristine and defected graphene systems. Our calculations reveal much higher adsorption energy and higher net charge transfer by using NDG instead of pristine graphene. Furthermore, the density of state (DOS) graphs point to major orbital hybridization between the SOx and NDG, while there is no evidence of hybridization by using pristine graphene. Based on our results, it is found that SO2 and SO3 molecules can be adsorbed on the surface of NDG physically and chemically with adsorption energies (Eads) of −27.5 and 65.2 kJ mol−1 (19.6 and 51.4 kJ mol−1 BSSE), respectively, while low adsorption energies were calculated in the case of using pristine graphene. So we introduced NDG as a sensitive adsorbent/sensor for detection of SO2 and SO3.

Molecular simulation of pH-dependent diffusion, loading, and release of doxorubicin in graphene and graphene oxide drug delivery systems.

In this study, the adsorption of doxorubicin (DOX), an anticancer drug, on pristine graphene (PG) and graphene oxide (GO) nanocarriers with different surface oxygen densities and in an aqueous environment with varying pH levels was investigated using molecular dynamics (MD) simulation. The drug loading and release on the GO nanocarrier was also simulated using pH as the controller mechanism. Overall, the DOX/nanocarrier interactions become stronger as the graphene surface oxygen density increases. Although pH has a negligible effect on the single-molecule drug adsorption on the GO surfaces under acidic and neutral conditions, significantly stronger DOX/nanocarrier interactions occur for the GO nanosheet with a lower surface oxygen density (GO-16, with an O/C ratio of 1 : 6) at basic pH levels. Moreover, the DOX/nanocarrier interactions are greatly weakened in the GO nanosheet with higher surface oxygen density (GO-13, with an O/C ratio of 1 : 3) under basic conditions. These observations are partly attributed to a more favorable geometry of the DOX molecule on the GO-16 surface as opposed to a loosely attached DOX molecule on the edges of the GO-13 nanosheet. When comparing the adsorption kinetics and transport properties of the DOX molecule in different GO systems, the drug diffusion coefficient increases with decreasing pH value (going from basic to neutral to acidic) due to the reduced total water-nanocarrier interactions. The latter observation is an indication of the more facilitated transport of the DOX molecule in an aqueous medium towards the nanocarrier surface at lower pH levels. Finally, we have confirmed the loading and release of the DOX molecules on the GO nanocarrier under neutral (pH = 7) and acidic (pH = 5) conditions, respectively. The former signifies the blood pH level, whereas the latter is reminiscent of the pH of a tumorous cell. The computational results presented in this work reveal the underlying mechanisms of DOX loading and release on PG and GO surfaces, which may be used to design better graphene-based nanocarriers for the DOX delivery and targeting applications.

Density functional study of Al-doped graphene nanostructure towards adsorption of CO, CO2 and H2O

We have studied the structure, adsorption energy, charge transfer and the electronic states of CO, CO2, H2O molecules on the surface of pristine and Al-doped graphene sheets using density functional theory calculations. The adsorption energies have been calculated for the most stable configurations of the molecules on the surface of pristine and Al-doped graphene. Our calculated adsorption energies for the most stable states for CO, CO2 and H2O were −53.8kJmol−1,−102.6kJmol−1, and −120.5kJmol−1 which are correspond to chemisorption process. These results point to the suitability of Al-doped graphene as a powerful adsorbent for practical applications.

Photodegradation of dispersants in colloidal suspensions of pristine graphene

We demonstrate that UV degradation can remove polymeric dispersants from the surface of colloidal pristine graphene. In particular, we investigated the irradiation of polyvinylpyrrolidone (PVP)-dispersed graphene in water; this polymer has been established as a versatile nanosheet dispersant for a range of solvents, and it undergoes photo-oxidative degradation when exposed to UV light. We find that the molecular weight of PVP decreases with irradiation time and subsequently desorbs from the graphene surface. This causes gradual destabilization of graphene and agglomeration in water. The amount of adsorbed PVP decreases by approximately 45% after 4h of irradiation in comparison with the non-irradiated dispersion. At this point, the majority of the stable graphene nanosheets flocculate, likely because of insufficient surface coverage as indicated by thermogravimetric analysis. Graphene aggregates were characterized as a function of irradiation time by optical microscopy, UV–vis spectroscopy, Raman spectroscopy, and conductivity measurements; the data suggest that the agglomerates maintain a graphene-like (rather than graphite-like) structure. The effect is also observed for another graphene dispersant (sapogenin), which suggests that our findings can be generalized to the broader class of photodegradable dispersants.

First principles study of Al-doped graphene as nanostructure adsorbent for NO2 and N2O: DFT calculations

We studied the first principles adsorption phenomena of nitrogen dioxide (NO2) and nitrous oxide (N2O) molecules on the surface of pristine graphene and Al-doped graphene using density functional theory (DFT) calculations. The adsorption energies have been calculated for different possible configurations of the molecules on the surface of pristine and Al-doped graphene. Our calculations reveal that the Al-doped graphene has significant adsorption energy, elevated net charge transferring values and smaller bond distances to gases than that of pristine graphene because of the chemical interaction of the mentioned molecules. Furthermore, the calculated density of states (DOS) show the existing of noteworthy orbital hybridization between NO2 as well as N2O and Al-doped graphene during adsorption process which is proving to strong interaction while there is no evidence for hybridization between the those molecules and the pristine graphene. Our calculated adsorption energies for the most stable states for NO2 and N2O was −62.2kJmol−1 (−48.5kJmol−1 BSSE corrected energy) and −33.9kJmol−1 (−22.7kJmol−1 corrected energy), which are correspond to chemisorption process. These results point to the suitability of Al-doped graphene as a powerful sensor for practical applications.

Modulating the sensing properties of graphene through an eco-friendly metal-decoration process

Herein we investigate the effect of graphene decoration with metal nanoparticles, namely Ag and Pd, starting from pristine graphene (GR). Decoration is directly made on graphene obtained by Liquid Phase Exfoliation of natural graphite in eco-friendly solvents. The nanoparticles are synthesized in situ via chemical reduction from metal salts promoted by microwave treatment, resulting in metal nanoparticles directly stuck onto the pristine graphene surface. Sensing layers based on graphene and metal decorated graphenes were exposed to hydrogen, nitrogen dioxide and ammonia. Pristine graphene showed a marked specificity towards NO2 gas. Palladium nanoparticles enhanced graphene response towards hydrogen, while devices based on graphene/silver nanoparticles were highly specific towards ammonia detection. This eco-friendly and straightforward preparation method of graphene and graphene hybrids demonstrates an effective modulation of the sensing properties of graphene with respect to sensitivity and selectivity.

Adsorption of acetyl halide molecules on the surface of pristine and Al-doped graphene: Ab initio study

We have scrutinized the adsorption energy, electronic structure, natural bond analysis (NBO), density of state (DOS) and global indices for adsorption of acetyl chloride (AC) and acetyl fluoride (AF) on the surface of pristine graphene as well as Al-doped graphene. The adsorption energies have been calculated for the most stable configurations of the molecules on the surface of pristine and Al-doped graphene. According to the calculated parameters, there is very weak physical adsorption of AC and AF on pristine graphene while strong adsorption takes place in the case of Al-doped graphene. The charge transfer from adsorbed molecules to Al-doped graphene surface was confirmed by the natural bond orbital as well as the Mulliken population analysis while there is no charge transfer with pristine graphene. Additionally, the density of states results reveal that orbital hybridization takes place between above-mentioned molecules and Al-doped graphene sheet, whereas there is no hybridization between the molecules and the pristine graphene. Our calculated adsorption energies for the most stable position configurations of AC and AF on Al-doped graphene were −68.8kJmol−1 (−52.6kJmol−1 BSSE corrected energy) and −78.4kJmol−1 (−64.3kJmol−1 BSSE corrected energy) which are correspond to chemisorptions process respectively. These results point to the appropriateness of Al-doped graphene as a powerful adsorbent for practical applications.

Lewis acid-base surface interaction of some boron compounds with N-doped graphene; first principles study

We studied density functional theory (DFT) calculations in terms of energetic and electronic properties toward adsorption of some boron compounds (B(OCH3)3, BF3 and BC13) on the surface of pristine as well as N-doped graphene using WB97XD/6-31 + G(d,p) level of theory. The net charge transfer of mentioned molecules on the surface of pristine and N-doped graphene was calculated with above-mentioned basis set using natural bond orbital and Mulliken charge analysis during complex formation. The computed dipole moment shows when above-mentioned molecules approach to the surface of N-doped graphene, the amount of the dielectric (μD) will change depending on the kind of molecule. Our calculations reveal that N-doped graphene system has much higher adsorption energy, higher net charge transfer value than pristine graphene due to Lewis acid-base interaction. Comparing B(OCH3)3 as an organic boron derivative with boron trihalides (BF3 and BCl3), the Lewis acidity increases in the order of BF3 < BC13< B(OCH3)3 with adsorption energies (Eads) of −8.7, −18.3 and −26.5 kJ/mol (BSSE) respectively, while low adsorption energies were calculated on pristine graphene for mentioned molecules.

Improving Graphene Conductivity through Selective Atomic Layer Deposition

One of the most promising approaches for massive production of transparent conducting electrodes (TCE) is roll-to-roll printing of graphene flakes solution. However, graphene flakes inevitably contain structural defects like grain boundaries and dislocations, together with macroscopic defects such as wrinkles and cracks during transfer. In addition, the printed flakes can’t align with each other perfectly. All these defects and the gaps between detached flakes hinder electrical transport and limit the mechanical strength and reliability of the graphene. Here we report a technique to electrically heal and connect graphene flakes via selective atomic layer deposition (ALD) of conductive materials. We have demonstrated selective growth of ALD aluminum-doped zinc oxide, ruthenium and platinum on pristine graphene surface prepared from exfoliated highly ordered pyrolytic graphite (HOPG). ALD materials nucleate on graphene cracks and edges but exhibit a significant nucleation barrier on intact basal planes due to the chemical inertness from delocalized π-orbitals. We also present a high-throughput fabrication method to create graphene patterns via metal shadow mask and O2 plasma. This model system of patterned CVD graphene islands demonstrates electrical connection of separated graphnene flakes by selective ALD processes. In addition, shadow-mask-patterned CVD graphene field effect transistors (FETs) are used to monitor graphene carrier density and mobility variation after ALD growth. We found considerable carrier density and conductivity increase of a single graphene flake after V2O5 ALD, which can be explained by p-doping of graphene due to the high work function V2O5.

Computational Insights into Oxygen Reduction Reaction and Initial Li2O2 Nucleation on Pristine and N-Doped Graphene in Li–O2 Batteries

Li–O2 batteries have been a subject of extensive studies in the past few decades. However, the oxygen reduction reaction (ORR) mechanism is still unclear on air cathodes and needs to be concretely explored. In this work, by means of density functional theory computations, we systematically investigated the ORR and initial Li2O2 nucleation processes on the surface of pristine and N-doped graphene in Li–O2 batteries. The in-plane pyridinic N-doped graphene is more effective in facilitating the nucleation of Li2O2 clusters than pristine or graphitic N-doped graphene. The overpotential of the rate-controlling step for Li2O2 nucleation decreases with the growth of Li2O2 clusters, and the cluster growth after (Li2O2)2 will follow the process Li → LiO2 → Li2O2 on all considered substrates. Our results should promote the understanding of ORR processes on N-doped graphene catalysts and shed more light on the design and optimization of air cathodes for Li–O2 batteries.

Exploration on the mechanism of DNA adsorption on graphene and graphene oxide via molecular simulations

Several studies have shown that the fundamental principle of biosensors for DNA detection based on graphene oxide (GO) is the preferential adsorption of single-stranded DNA (ssDNA) over double-stranded DNA (dsDNA) on GO. However, the interactions between DNA and GO at the atomic level are poorly understood, and molecular dynamics (MD) simulation can be used to further explore these interactions. In the current study, we performed MD simulations to investigate the dynamic process of both ssDNA and dsDNA adsorption onto GO and pristine graphene (PG). We found that ssDNA was firmly adsorbed and lay flat on the surface of PG and GO, whereas dsDNA was preferentially oriented upright on both surfaces, resulting in weaker adsorption energies. Exploration of the mechanism of DNA segments binding to PG and GO indicated that π–π stacking interaction was the dominant force in the adsorption of DNA segments on PG, while both π–π stacking and hydrogen bonding contributed to the binding affinity between DNA segments and GO. These findings are of significant importance to the understanding of the interactions between DNA and GO, and the optimization of DNA and GO-based biosensors.

Graphene-templated directional growth of an inorganic nanowire.

Assembling inorganic nanomaterials on graphene is of interest in the development of nanodevices and nanocomposite materials, and the ability to align such inorganic nanomaterials on the graphene surface is expected to lead to improved functionalities, as has previously been demonstrated with organic nanomaterials epitaxially aligned on graphitic surfaces. However, because graphene is chemically inert, it is difficult to precisely assemble inorganic nanomaterials on pristine graphene. Previous techniques based on dangling bonds of damaged graphene, intermediate seed materials and vapour-phase deposition at high temperature(,) have only formed randomly oriented or poorly aligned inorganic nanostructures. Here, we show that inorganic nanowires of gold(I) cyanide can grow directly on pristine graphene, aligning themselves with the zigzag lattice directions of the graphene. The nanowires are synthesized through a self-organized growth process in aqueous solution at room temperature, which indicates that the inorganic material spontaneously binds to the pristine graphene surface. First-principles calculations suggest that this assembly originates from lattice matching and π interaction to gold atoms. Using the synthesized nanowires as templates, we also fabricate nanostructures with controlled crystal orientations such as graphene nanoribbons with zigzag-edged directions.

Adsorption of GA module onto graphene and graphene oxide: A molecular dynamics simulation study

Using all-atom molecular dynamics (MD) simulation, we have investigated the adsorption of protein GA module (GA53) onto graphene oxide (GO), compared with similar adsorption onto pristine graphene (PG). We find that: (1) the protein GA53 can be easily and firmly adsorbed onto the surface of GO and PG, but the binding sites are not specific; the main difference is that the secondary structure of GA53 can be well preserved in protein–GO system, while GA53 will partially lose its secondary structure after adsorbed on PG. (2) in protein–GO system, hydroxyl and epoxy groups increase the distance between protein and GO, which weaken their vdW interactions, meanwhile, hydrogen bonds and electrostatic interactions enhance their binding affinity. In protein–PG system, strong vdW interactions between residues of GA53 and PG have destroyed its secondary structure. (3) π–π stacking interactions still exist between aromatic residues and both the basal plane of GO and PG. In comparison with PG, our results suggest that GO presents better biocompatibility to preserve protein secondary structure when simultaneously absorbing protein.

Nanomaterials: A Generic Synthetic Approach to Large‐Scale Pristine‐Graphene/Metal‐Nanoparticles Hybrids (Adv. Funct. Mater. 46/2013)

The homogeneous attachment of well-shaped metal nanoparticles (metal-NPs) on pristine graphene surfaces to construct pristine graphene/metal-NP hybrids is required for applications such as transparent electrodes and conductive composites. On page 5771, G. Meng and co-workers present an environmentally friendly generic synthetic approach to such large-scale hybrids by attaching pre-synthesized metal-NPs (silver nanocubes, yellow) on the pristine graphene surface (green hexagonal grid with purple joints) in ethanol by exploiting van der Waals interactions between the metal-NPs and the pristine graphene.

Impact of Local Curvature and Structural Defects on Graphene–C60 Fullerene Fusion Reaction Barriers

Self-consistent charge density functional tight-binding and density functional theory calculations have been employed to study the energetics of the graphene–C60 fullerene fusion. We show that there is an optimal value of the bond-puckering angle of single-layer graphene-like systems, which facilitates fusion with other low-dimension carbon systems. Specifically, chemical attachment of a C60 fullerene to a single-layer graphene sheet is not feasible from the energetic point of view due to lack of puckering of the pristine graphene surface, but may occur for systems with some surface curvature. The presence of various defects in the graphene surface, including formation of four- and five-membered rings, Stone–Wales defects, or single and double vacancies may create some surface strain leading to formation of reactive sites in graphene, which are susceptive to binding with a fullerene. As an example, we show that a single vacancy in the graphene surface can lead to formation of a stable chemical bond with a...