Electrochemical Graphene Research Articles

Ultrasensitive sensing of Androctonus australis hector scorpion venom toxins in biological fluids using an electrochemical graphene quantum dots/nanobody-based platform

Rapid and sensitive detection of low levels of scorpion venom toxins in biological fluids is of tremendous importance for decision-taking in cases of envenomation by scorpions stings. In Tunisia, at least 1200 severe envenomation cases by Androctonus australis hector (Aah) scorpion stings were reported annually. In this work, we report on a novel electrochemical immuno-sandwich to detect the Aah50 toxic fraction within the Aah scorpion venom using the bispecific nanobody format specially designed to highly recognize and neutralize the two most toxic molecules in the AahG50 venom fraction (i.e. AahI and AahII toxins), graphene quantum dots (GQDs) constructed on the surface carbon screen-printed electrodes. Hydroquinone/H2O2/peroxidase system was used to amplify the current in order to achieve the detection of low levels of AahG50. Electrochemical studies revealed a high sensitivity toward the AahG50 with a sensitivity of 18.2 nA mL pg–1 and a picomolar limit of detection as low as 0.55 pg mL–1. The platform exhibits very good metrological performances such as repeatability, reproducibility, selectivity and long storage stability. Matrix effect was found to be insignificant as demonstrated by assays performed in human blood serum and urine.

Low-Power, Electrochemically Tunable Graphene Synapses for Neuromorphic Computing.

Brain-inspired neuromorphic computing has the potential to revolutionize the current computing paradigm with its massive parallelism and potentially low power consumption. However, the existing approaches of using digital complementary metal-oxide-semiconductor devices (with "0" and "1" states) to emulate gradual/analog behaviors in the neural network are energy intensive and unsustainable; furthermore, emerging memristor devices still face challenges such as nonlinearities and large write noise. Here, an electrochemical graphene synapse, where the electrical conductance of graphene is reversibly modulated by the concentration of Li ions between the layers of graphene is presented. This fundamentally different mechanism allows to achieve a good energy efficiency (<500 fJ per switching event), analog tunability (>250 nonvolatile states), good endurance, and retention performances, and a linear and symmetric resistance response. Essential neuronal functions such as excitatory and inhibitory synapses, long-term potentiation and depression, and spike timing dependent plasticity with good repeatability are demonstrated. The scaling study suggests that this simple, two-dimensional synapse is scalable in terms of switching energy and speed.

Synthesis and Transfer of High-Quality Graphene Grown on Al2O3(0001)/Pt(111) Template Wafers

Today, CMOS compatible routes to grow and transfer large graphene sheets are still missing. Chemical vapor deposition (CVD) is the main synthesis technique yielding high-quality wafer-scale graphene. Platinum is expected to be an excellent catalyst to grow monolayer, large scale and high-quality graphene. Moreover, electrochemical graphene transfer methods are demonstrated, which facilitate the reusability of this expensive Pt template. A process window is studied to grow graphene on platinum foils. The influence of growth temperature and time as well as the methane to hydrogen ratio is optimized. Moreover CO2 can be used as a graphene defect etching gas to replace hydrogen, resulting in successful graphene syntheses conditions. It is shown that the polycrystalline nature of the Pt foil makes it extremely difficult to grow a uniform monolayer graphene sheet. It is also demonstrated that a smooth Pt(111) thin film on sapphire (Rrms = 0.2 nm) decreases the amount of graphene nucleation centres and thus strongly reduce the graphene multilayer areas. Therefore, a Pt(111) thin film appears ideally suited to grow millimetre-sized graphene islands in reasonable time frame (< 1 hour growth time). To promote graphene delamination from the platinum growth template, it is of prime importance that an interfacial water (IFW) layer is formed. This IFW layer serves as a pathway for charged ions to intercalate between graphene and its CVD growth substrate. It is demonstrated that only a selection of ions succeed in intercalating and thus in a successful graphene delamination. The ions that are able to intercalate are electro-active outside the electrochemical window of the used solvent. Figure 1

Graphene delamination using 'electrochemical methods': an ion intercalation effect.

The mechanism of graphene delamination from a Pt catalyst growth surface with electrochemical methods is studied. After a water intercalation step, an electrochemical graphene delamination process is done with a variety of different electrolytes. It is shown that (hydrogen or oxygen) bubble formation is not the main driving force to decouple graphene from its catalyst growth substrate. Ion intercalation is identified as the primary component for a fast graphene delamination process from its catalytic growth substrate. When the Pt/graphene sample is negatively charged, cations will intercalate, assuming they do not reduce within the electrochemical window of the solvent. This cation intercalation does result in graphene delamination. In the same way, anions intercalate in positively charged Pt/graphene samples when they do not react within the electrochemical window of the solvent. Furthermore, it is shown that applying a potential is sufficient (current is not needed) to induce ion intercalation and, as a result, graphene delamination. These findings open the door to avoid Na+ or K+ contamination introduced during currently described electrochemical graphene delamination. Alternative electrolytes (i.e. ammonium hydroxide and tetraethylammonium hydroxide) are proposed, due to the absence of alkali contaminants and rapid cation intercalation to delaminate graphene.

The nature of plasmon‐exciton codriven surface catalytic reaction

AbstractIn this communication, we propose physical mechanism of plasmon‐exciton codriven surface catalytic reaction, based on the electrochemical graphene mediated surface‐enhanced Raman scattering spectroscopy. It is found that with the help of graphene, the Fermi Level of the graphene‐roughened Ag electrode hybrid system can be significantly increased, which can efficiently promote the efficiency of surface catalytic reaction compared with that driven by roughened Ag electrode alone.

Electrochemical Exfoliation of Graphite in Aqueous Sodium Halide Electrolytes toward Low Oxygen Content Graphene for Energy and Environmental Applications.

Graphene and graphene-based materials have shown great promise in many technological applications, but their large-scale production and processing by simple and cost-effective means still constitute significant issues in the path of their widespread implementation. Here, we investigate a straightforward method for the preparation of a ready-to-use and low oxygen content graphene material that is based on electrochemical (anodic) delamination of graphite in aqueous medium with sodium halides as the electrolyte. Contrary to previous conflicting reports on the ability of halide anions to act as efficient exfoliating electrolytes in electrochemical graphene exfoliation, we show that proper choice of both graphite electrode (e.g., graphite foil) and sodium halide concentration readily leads to the generation of large quantities of single-/few-layer graphene nanosheets possessing a degree of oxidation (O/C ratio down to ∼0.06) lower than that typical of anodically exfoliated graphenes obtained with commonly used electrolytes. The halide anions are thought to play a role in mitigating the oxidation of the graphene lattice during exfoliation, which is also discussed and rationalized. The as-exfoliated graphene materials exhibited a three-dimensional morphology that was suitable for their practical use without the need to resort to any kind of postproduction processing. When tested as dye adsorbents, they outperformed many previously reported graphene-based materials (e.g., they adsorbed ∼920 mg g-1 for methyl orange) and were useful sorbents for oils and nonpolar organic solvents. Supercapacitor cells assembled directly from the as-exfoliated products delivered energy and power density values (up to 15.3 Wh kg-1 and 3220 W kg-1, respectively) competitive with those of many other graphene-based devices but with the additional advantage of extreme simplicity of preparation.

Giant third-order nonlinearity from low-loss electrochemical graphene oxide film with a high power stability

Giant third-order nonlinear absorption and refraction of electrochemical graphene oxide (EGO) film were investigated in the femtosecond regime using the single beam Z-scan technique. The excellent chemical stability of the EGO film under high-power illumination up to 400 mJ/cm2 is demonstrated, which can be attributed to the low oxidation degree revealed by the optical and Raman spectroscopies. High and broadband linear transmission over 70% has been observed from the visible to the infrared range. The low-loss EGO film with giant third-order nonlinearity, excellent chemical stability, large-scale preparation and flexible integration has a great potential for high-power nonlinear optical applications.

Synthesis of functionalized few-layer graphene through fast electrochemical expansion of graphite

Fast and reproducible production of graphene flaks from graphite is an important process considering the possible industrial applications of graphene. We propose an electrochemical method to produce functionalized few-layer graphenes in minutes, without the need of prolonged ultrasonic treatment. It mainly consists in the cathodic electrochemical expansion of graphite in DMF. Functionalization of graphite is achieved by reducing aryl diazonium salts in organic solution. This simple methodology allows easily and quickly preparation of modified few-layer graphenes. Two different graphite sources were considered in this work for electrochemical graphene flaks production.

Non-destructive electrochemical graphene transfer from reusable thin-film catalysts

We demonstrate an electrochemical method – which we term oxidative decoupling transfer (ODT) – for transferring chemical vapor deposited graphene from physically deposited copper catalyst layers. This copper oxidation-based transfer technique is generally applicable to copper surfaces, and is particularly suitable where the copper is adhered to a substrate such as oxidized silicon. Graphene devices produced via this technique demonstrate 30% higher mobility than similar devices produced by standard catalyst etching techniques. The transferred graphene films cover more than 94% of target substrates – up to 100mm diameter films are demonstrated here – and exhibit a low Raman D:G peak ratio and a homogenous and continuous distribution of sheet conductance mapped by THz time-domain spectroscopy. By applying a fixed potential of −0.4V vs. an Ag/AgCl reference electrode – significantly below the threshold for hydrogen production by electrolysis of water – we avoid the formation of hydrogen bubbles at the graphene–copper interface, preventing delamination of thin sputtered catalyst layers from their supporting substrates. We demonstrate the reuse of the same growth substrate for five growth and transfer cycles and prove that this number is limited by the evaporation of Cu during growth of graphene. This technique therefore enables the repeated use of the highest crystallinity and purity substrates without undue increase in cost.

Electrodeposition of Zinc Oxide on Graphene Tips Electrochemically Exfoliated and O2-Plasma Treated

Zinc oxide (ZnO) presents several applications as piezoelectric transducers, photosensors, solar cells, electrochemical sensors, etc. Electrodeposition of zinc oxide (ZnO) thin films from aqueous solution of zinc nitrate has been deposited in graphenated low cost pencil graphite. The electrochemical graphene production at the tips was performed in a low cost DC source using concentrated sulfonitric solution. The tips were expanded in hydrogen plasma and treated by oxygen plasma to enhance its hydrophilicity. The posterior ZnO electrodeposition was highly efficient and the graphene functionalization contributes to overall electrochemical deposition mechanism.

Evidences of electrochemical graphene functionalization and substrate dependence by Raman and scanning tunneling spectroscopies

Electrochemical functionalization and possible hydrogenation of treated epitaxial graphene samples on 6H-SiC are presented. To attract H+ ions to react with the exposed working cathode, a 10% sulfuric acid electrolyte was used with a Pt counter anode. Functionalization was determined using Raman spectroscopy and measured by a marked increase in I(D)/I(G) ratio and introduction of C-H bond peak at ∼2930 cm−1. There was also a marked increase in fluorescence background, which clearly differentiates functionalization from lattice damage in the graphene. Quantifying the fluorescence, we estimate that H-incorporation as high as 50% was achieved based on results on hydrocarbons, although other functional groups cannot be excluded. We further distinguished these functionalization signatures from lattice damage through measurements on nanocrystalline graphene on a and m plane SiC, which displayed very different surface morphologies and no measureable fluorescence. Finally, we show that the extent of functionalization is strongly substrate dependent by using samples cut from three semi-insulating 6H-SiC substrates with similar resistivity but orientations varying from on-axis (∼0.02°), 0.5° to 1.0° off-axis. This functionalization was found to be thermally reversible at ∼1000 °C. Scanning tunneling spectroscopy indicates the presence of sp3-like localized states not present in the starting graphene, further supporting the assertion that functionalization has occurred.

Evidence of Electrochemical Graphene Functionalization by Raman Spectroscopy

Electrochemical functionalization of treated epitaxial graphene samples on Si-face 6H-SiC are presented in this work. Three semi-insulating 6H-SiC substrates cut from different boules with varying off cut angle (on axis, 0.5° and 1.0° degrees off axis in the [112‾0] direction) were diced into 10mm x 10mm samples and quality EG grown on top. A home-build electrochemical cell was used with current applied though a 10% H2SO4 solution, with a Pt wire and exposed graphene as the anode and cathode respectively. Functionalization was determined using Raman spectroscopy and measured by an increase in D/G ratio, increase in fluorescence background and introduction of C-H bond peak at ~2930 cm-1. Components of the Raman spectra before and after functionalization of all samples used were analyzed to show a substrate dependent effect on functionalization with values such as D/G ratio and normalized fluorescence slope varying between the substrates.