Plasma-treated Graphene Research Articles

Enhancing durability and oil-water separation efficiency with plasma-treated graphene oxide coated mesh

In recent years, there has been a surge in interest surrounding the fabrication and utilization of superhydrophobic and superhydrophilic surfaces, driven by their exceptional functionalities and properties. Graphene Oxide (GO) has inherent hydrophilicity and underwater superoleophobicity, which has made this material a particularly capable candidate for oil-water separation. Addressing the persistent challenge of efficient oil-water separation in industrial contexts, this study presents the fabrication of a GO-coated stainless steel mesh via a facile dip coating method augmented by an intermediate two-step O2 plasma treatment. The coated meshes were tested with various oil and water mixtures, including neutral, acidic, saline, and hot water, to find separation efficiency and recyclability. Notably, the meshes can achieve excellent separation efficiency of approximately 98.9 % and a superior flux of 11,464 L m−2 h−1 driven by gravity. This is a significant improvement over GO-coated meshes without O2 plasma treatment. Moreover, the plasma-treated meshes exhibit robust long-term durability and chemical stability, maintaining high underwater oil contact angles ≥119° even after extended immersion in diverse pH mediums and salt solutions for 150 days. This work showcases the practical viability of plasma-treated GO-coated meshes for oil-water separation applications and establishes a framework for systematically evaluating their long-term performance in harsh immersion conditions.

Study of dielectric properties of polymer/air plasma treated graphene oxide nanocomposites for electronic applications

In the present study we emphasized on the dielectric properties of Ethylene Vinyl Acetate (EVA) filled with untreated and plasma-treated graphene oxide (GO). The chemical fingerprints of EVA/GO were analysed using FTIR. Decreased ID/IG ratio from 2.99 to 2.75 and 1.60 to 1.16 due to exposed plasma was confirmed by Raman spectroscopy. X-ray diffraction (XRD) disclosed the decreased degree of crystallinity. Electric properties were measured by impedance analyser, dielectric constant was increased upto 11% for untreated GO dispersion and 47% for Air treated Plasma GO dispersed EVA composites. Plasma treatment further improved the exfoliated sites of GO and induced the defects, leading to optimized dielectric properties. Improved dielectric properties of EVA/GO can provide valuable insights into the potential applications in the field of electrical connectors, film capacitors, and pseudo capacitors.

Atomic-scale characterization of defects in oxygen plasma-treated graphene by scanning tunneling microscopy

Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.

Graphene quantum dots induced defect-rich NiFe Prussian blue analogue as an efficient electrocatalyst for oxygen evolution reaction

High energy resource demand has led to the rapid development of hydrogen as a clean fuel through electrolytic water splitting. The exploration of high-performance and cost-effective electrocatalysts for water splitting is a challenging task to obtain renewable and clean energy. However, the sluggish kinetics of oxygen evolution reaction (OER) greatly hindered its application. Herein, a novel oxygen plasma-treated graphene quantum dots embedded Ni-Fe Prussian blue analogue (O-GQD-NiFe PBA) is proposed as a highly active electrocatalysts for OER. Furthermore, the defect induced by GQD can provide an abundant lattice mismatch in the matrix of NiFe PBA, which further facilitates faster electron transport and kinetic performance. After optimization, the as-assembled O-GQD-NiFe PBA exhibits excellent electrocatalytic performance towards OER with a low overpotential of 259 mV for reaching a current density of 10 mA cm−2 and impressive long-term stability for 100 h in an alkaline solution. This work broadens the scope of metal–organic frameworks (MOF) and high-functioning carbon composite as an active material for energy conversion systems.

Plasma-Treated Graphene Surfaces for Trace Dye Detection Using Surface-Enhanced Raman Spectroscopy

Plasma-Treated Graphene Surfaces for Trace Dye Detection Using Surface-Enhanced Raman Spectroscopy

An effective formaldehyde gas sensor based on oxygen-rich three-dimensional graphene

Three-dimensional (3D) graphene with a high specific surface area and excellent electrical conductivity holds extraordinary potential for molecular gas sensing. Gas molecules adsorbed onto graphene serve as electron donors, leading to an increase in conductivity. However, several challenges remain for 3D graphene-based gas sensors, such as slow response and long recovery time. Therefore, research interest remains in the promotion of the sensitivity of molecular gas detection. In this study, we fabricate oxygen plasma-treated 3D graphene for the high-performance gas sensing of formaldehyde. We synthesize large-area, high-quality, 3D graphene over Ni foam by chemical vapor deposition and obtain freestanding 3D graphene foam after Ni etching. We compare three types of strategies—non-treatment, oxygen plasma, and etching in HNO3 solution—for the posttreatment of 3D graphene. Eventually, the strategy for oxygen plasma-treated 3D graphene exceeds expectations, which may highlight the general gas sensing based on chemiresistors.

Quantification of pre- and post-air plasma-treated graphene oxide dispersed polymer blends for high dielectric applications

Polymer nanoblends are in demand for various domestic and industrial applications.

(Invited) Supported Core@Multishell Nanowires and Nanotubes As a Platform for Water Management and Water Energy Harvesting

We will present the one plasma and vacuum reactor methodology for the formation of core@multishell nanowires and nanotubes. This method allows the synthesis of an ample variety of nanoarchitectures combining small-molecules organic nanowires as 1D and 3D soft-templates with the conformal deposition by plasma-assisted methodologies of functional shells including semiconducting oxides as ZnO, TiO2, V2O5, GaOx and Fe2O3 and organic dielectric and semiconducting molecules. A great advantage is high compatibility with the use of different substrates or supports. Hence, NWs and NTs with controlled density, length, and thickness grow from flat processable substrates as TCOs, plasma-treated graphene, laser-treated metals and alloys, metallic meshes, and cellulose filters. In the second part, we will give an overview of the applications of these supported nanostructures for the development of superhydrophobic, anti-icing, and omniphobic surfaces, water-oil separation, photocatalytic water purification, and water splitting. Finally, we will introduce the concept of drop energy harvesting and show the assembly of triboelectric transparent nanogenerators based on semiconducting nanotubes. Acknowledgements. We thank the AEI-MICINN (PID2019-110430GB-C21 and PID2019-109603RA-I0), the Consejería de Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía (PAIDI-2020 through projects US-1263142, ref. AT17-6079, P18-RT-3480), and the EU through cohesion fund and FEDER 2014–2020 programs for financial support. JS-V thanks the University of Seville through the VI PPIT-US and the Ramon y Cajal Spanish National programs. The project leadings to this article have received funding from the EU H2020 program under the grant agreements 851929 (ERC Starting Grant 3DScavengers) and 899352 (FETOPEN-01-2018-2019-2020 - SOUNDofICE).

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases. While preferential n-type doping was observed in graphene domains in nitrogen-containing plasmas, preferential p-type doping was observed at grain boundaries in boron-containing plasmas.

Comparative Studies on Crystallinity, Thermal and Mechanical Properties of Polyketone Grown on Plasma Treated CVD Graphene.

In this work, we report a facile way to control crystalline structures of polyketone (PK) films by combining plasma surface treatment with chemical vapor deposition (CVD) technique. The crystalline structure of PKs grown on plasma-treated graphene and the resulting thermal and mechanical properties were systematically discussed. Every graphene sheet used in this work was produced by CVD method and the production of PKs having different crystallinity were performed on the O2- and N2-doped graphene sheets. It was evident that the CVD-grown graphene sheets acted as the nucleating agents for promoting the crystallization of β-form PK, while suppressing the growth of α-form PK crystals. Regardless of the increase in surface roughness of graphene, surface functionality of the CVD-grown graphene was found to be an important factor in determining the crystalline structure of PK. N2 plasma treatment of the CVD-grown graphene promoted growth of the β-form PK, whereas the O2 plasma treatment of CVD graphene led to transformation of the unoriented β-form PK into the oriented α-form PK. Thus, the resulting thermal and mechanical properties of the PKs were highly dependent on the surface functionality of the CVD graphene. The method of controlling crystalline structure of the PKs suggested in this study, is expected to be very effective in realizing the PK with good processability, heat resistance and mechanical properties.

Selective nitrogen doping of graphene due to preferential healing of plasma-generated defects near grain boundaries

Hyperspectral Raman IMAging (RIMA) is used to study spatially inhomogeneous polycrystalline monolayer graphene films grown by chemical vapor deposition. Based on principal component analysis clustering, distinct regions are differentiated and probed after subsequent exposures to the late afterglow of a microwave nitrogen plasma at a reduced pressure of 6 Torr (800 Pa). The 90 × 90 µm2 RIMA mapping shows differentiation between graphene domains (GDs), grain boundaries (GBs), as well as contaminants adsorbed over and under the graphene layer. Through an analysis of a few relevant band parameters, the mapping further provides a statistical assessment of damage, strain, and doping levels in plasma-treated graphene. It is found that GBs exhibit lower levels of damage and N-incorporation than GDs. The selectivity at GBs is ascribed to (i) a low migration barrier of C adatoms compared to N-adatoms and vacancies and (ii) an anisotropic transport of C adatoms along GBs, which enhances adatom-vacancy recombination at GBs. This preferential self-healing at GBs of plasma-induced damage ensures selective incorporation of N-dopants at plasma-generated defect sites within GDs. This surprising selectivity vanishes, however, as the graphene approaches an amorphous state.

Preferential self-healing at grain boundaries in plasma-treated graphene.

Engineering of defects located in grains or at grain boundaries is central to the development of functional materials. Although there is a surge of interest in the formation, migration and annihilation of defects during ion and plasma irradiation of bulk materials, these processes are rarely assessed in low-dimensional materials and remain mostly unexplored spectroscopically at the micrometre scale due to experimental limitations. Here, we use a hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution to examine plasma-induced damage in a polycrystalline graphene film. Measurements conducted before and after very low-energy (11-13 eV) ion bombardment show defect generation in graphene grains following a zero-dimensional defect curve, whereas domain boundaries tend to develop as one-dimensional defects. Damage generation is slower at grain boundaries than within the grains, a behaviour ascribed to preferential self-healing. This evidence of local defect migration and structural recovery in graphene sheds light on the complexity of chemical and physical processes at the grain boundaries of two-dimensional materials.

Probing plasma-treated graphene using hyperspectral Raman.

Raman spectroscopy provides rich optical signals that can be used, after data analysis, to assess if a graphene layer is pristine, doped, damaged, functionalized, or stressed. The area being probed by a conventional Raman spectrometer is, however, limited to the size of the laser beam (∼1 µm); hence, detailed mapping of inhomogeneities in a graphene sample requires slow and sequential acquisition of a Raman spectrum at each pixel. Studies of physical and chemical processes on polycrystalline and heterogeneous graphene films require more advanced hyperspectral Raman capable of fast imaging at a high spatial resolution over hundreds of microns. Here, we compare the capacity of two different Raman imaging schemes (scanning and global) to probe graphene films modified by a low-pressure plasma treatment and present an analysis method providing assessments of the surface properties at local defects, grain boundaries, and other heterogeneities. By comparing statistically initial and plasma-treated regions of graphene, we highlight the presence of inhomogeneities after plasma treatment linked to the initial state of the graphene surface. These results provided statistical results on the correlation between the graphene initial state and the corresponding graphene-plasma interaction. This work further demonstrates the potential use of global hyperspectral Raman imaging with advanced Raman spectra analysis to study graphene physics and chemistry on a scale of hundreds of microns.

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases.

Rational Design of Photo-Electrochemical Hybrid Devices Based on Graphene and Chlamydomonas reinhardtii Light-Harvesting Proteins

Dye-sensitized solar cells (DSSCs) have been highlighted as the promising alternative to generate clean energy based on low pay-back time materials. These devices have been designed to mimic solar energy conversion processes from photosynthetic organisms (the most efficient energy transduction phenomenon observed in nature) with the aid of low-cost materials. Recently, light-harvesting complexes (LHC) have been proposed as potential dyes in DSSCs based on their higher light-absorption efficiencies as compared to synthetic dyes. In this work, photo-electrochemical hybrid devices were rationally designed by adding for the first time Leu and Lys tags to heterologously expressed light-harvesting proteins from Chlamydomonas reinhardtii, thus allowing their proper orientation and immobilization on graphene electrodes. The light-harvesting complex 4 from C. reinhardtii (LHC4) was initially expressed in Escherichia coli, purified via affinity chromatography and subsequently immobilized on plasma-treated thin-film graphene electrodes. A photocurrent density of 40.30 ± 9.26 μA/cm2 was measured on devices using liquid electrolytes supplemented with a phosphonated viologen to facilitate charge transfer. Our results suggest that a new family of graphene-based thin-film photovoltaic devices can be manufactured from rationally tagged LHC proteins and opens the possibility to further explore fundamental processes of energy transfer for biological components interfaced with synthetic materials.

Plasma-Treated CVD Graphene Gas Sensor Performance in Environmental Condition: The Role of Defects on Sensitivity

In this work, a low-cost resistive gas sensor based on graphene grown by CVD was fabricated and its sensitivity was studied in terms of defect density. CVD graphene was transferred using Polyurethane as sacrifice layer with low contamination and defect-free results. An atmospheric plasma etching system was used to homogeneously induce defects on the sensor’s active area, as investigated through Raman spectroscopy. Device sensing properties were significantly enhanced for greater defect density for both NH3 and NO2. The modified sensors were submitted to different concentrations of both target gas to assess detection limits and overall behavior. It was revealed that defective CVD graphene devices possess sensitivity up to ppm range with linear dependence in the range of values measured. The fabricated sensors presented little to no signal degradation after months of atmospheric exposure.

A Gas Sensing Channel Composited with Pristine and Oxygen Plasma-Treated Graphene.

Oxygen plasma treatment has been reported as an effective way of improving the response of graphene gas sensors. In this work, a gas sensor based on a composite graphene channel with a layer of pristine graphene (G) at the bottom and an oxygen plasma-treated graphene (OP-G) as a covering layer was reported. The OP-G on top provided oxygen functional groups and serves as the gas molecule grippers, while the as-grown graphene beneath serves as a fast carrier transport path. Thus, the composite channel (OP-G/G) demonstrated significantly improved response in NH3 gas sensing tests compared with the pristine G channel. Moreover, the OP-G/G channel showed faster response and recovering process than the OP-G channel. Since this kind of composite channel is fabricated from chemical vapor deposited graphene and patterned with standard photolithography, the device dimension was much smaller than a gas sensor fabricated from reduced graphene oxide and it is favorable for the integration of a large number of sensing units.

Creating defects on graphene basal-plane toward interface optimization of graphene/CuCr composites

In-situ formation of appropriate interfacial carbides by matrix-alloying with carbide-forming elements offers an efficient approach to improve the interfacial bonding of graphene/CuX composites. However, the carbide formation commonly occurs at graphene edge/matrix interface, which is not enough to achieve the sufficient interfacial bonding because the vast majority of graphene/matrix interface is basal-plane/matrix interface rather than edge/matrix interface. To alleviate this limitation, we reported a new design of creating defects on graphene basal-plane (CDGB) to optimize the interface and mechanical properties of graphene/CuCr composites. Plasma treatment was employed to create the structural defects (∼7 nm nanopores) on graphene basal-plane. When incorporating the plasma-treated graphene into the CuCr matrix, the Cr7C3 carbides were found to be in-situ formed at both basal-plane/matrix and edge/matrix interfaces. Ex-situ and in-situ tensile tests both demonstrated that the plasma-treated graphene led to the composite that showed a larger strength enhancement and a higher load transfer capability than untreated counterpart, which was ascribed to the largely improved interfacial bonding contributed by the Cr7C3 formed at basal-plane/matrix interface. This study suggests that the CDBG via plasma treatment affords a feasible solution for the interface optimization of graphene/CuX composites with enhanced mechanical properties.

Temporal-stability of plasma functionalized vertical graphene electrodes for charge storage

Vertical graphene is an emerging material for supercapacitor applications. Yet, its inherent hydrophobic nature restricts the interaction with electrolyte ions, which brings in high demand for developing hydrophilic vertical graphene surfaces. Herein, super-wetting vertical graphene nanosheets are achieved by an in-situ post-deposition oxygen plasma treatment. The plasma-treated vertical graphene nanosheets electrodes found to exhibit a ten-fold enhancement in specific capacitance. However, the super-wetting nature of the plasma-treated vertical graphene nanosheets transformed back to hydrophobic upon exposure to ambient conditions, yet the rate of transformation is governed by the plasma parameters in-turn the type of oxygen functional group attached. Noteworthy, the analogous effect is persisted in supercapacitor performance too. Furthermore, a correlation between the temporal-stability of wetting nature and supercapacitor performance is established; by measuring the charge storage capacity of vertical graphene surfaces with different water contact angle. A preferential shift in dominant oxygen functionalities from carboxyl type to hydroxyl and carbonyl type with an increase in plasma energy is evident. A symmetric coin-type electrochemical capacitor device using super-wetting vertical graphene nanosheets electrodes is fabricated and demonstrated its performance.

Graphene defect engineering for optimizing the interface and mechanical properties of graphene/copper composites

A graphene defect engineering strategy was proposed in this work to tailor the interface and mechanical properties of graphene/Cu composites. Plasma treatment was used to create surface defects (5–10 nm nanopores) on the basal-plane of starting graphene material but without considerably damaging the graphene structure. It was demonstrated that the CuxOy oxides were in situ formed at the defect sites of plasma-treated graphene in the sintering process, which played a bridging role in enhancing the interfacial adhesion of graphene with Cu matrix. Compared to the composites with untreated graphene, the composites with plasma-treated graphene exhibited a higher strength enhancement, and better interface stability in response to thermal cycling, which was ascribed to the CuxOy-coordinated improved interfacial bonding that provided efficient load transfer and thermal stress relaxation. Nevertheless, the overlong plasma treatment could severely damage the graphene structure and result in a reduced strength enhancement. This study suggests that the rational defect engineering of graphene is an efficient approach for optimizing the interface and mechanical properties of graphene/metal composites.