Dhanraj B. Shinde

Fabrication of carbon nanorods and graphene nanoribbons from a metal–organic framework

One- and two-dimensional carbon nanomaterials are attracting considerable attention because of their extraordinary electrical, mechanical and thermal properties, which could lead to a range of important potential applications. Synthetic processes associated with making these materials can be quite complex and also consume large amounts of energy, so a major challenge is to develop simple and efficient methods to produce them. Here, we present a self-templated, catalyst-free strategy for the synthesis of one-dimensional carbon nanorods by morphology-preserved thermal transformation of rod-shaped metal-organic frameworks. The as-synthesized non-hollow (solid) carbon nanorods can be transformed into two- to six-layered graphene nanoribbons through sonochemical treatment followed by chemical activation. The performance of these metal-organic framework-derived carbon nanorods and graphene nanoribbons in supercapacitor electrodes demonstrates that this synthetic approach can produce functionally useful materials. Moreover, this approach is readily scalable and could be used to produce carbon nanorods and graphene nanoribbons on industrial levels.

Electrochemical preparation of luminescent graphene quantum dots from multiwalled carbon nanotubes.

Green luminescent, graphene quantum dots (GQDs) with a uniform size of 3, 5, and 8.2(±0.3) nm in diameter were prepared electrochemically from MWCNTs in propylene carbonate by using LiClO(4) at 90 °C, whereas similar particles of 23(±2) nm were obtained at 30 °C under identical conditions. Both these sets of GQDs displayed a remarkable quantum efficiency of 6.3 and 5.1%, respectively. This method offers a novel strategy to synthesise size-tunable GQDs as evidenced by multiple characterisation techniques like transmission and scanning electron microscopy, atomic force microscopy, Raman spectroscopy and X-ray diffraction (XRD). Photoluminescence of these GQDs can be tailored by size variation through a systematic change in key process parameters, like diameter of carbon nanotube, electric field, concentration of supporting electrolyte and temperature. GQDs are promising candidates for a variety of applications, such as biomarkers, nanoelectronic devices and chemosensors due to their unique features, like high photostability, biocompatibility, nontoxicity and tunable solubility in water.

Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide

Graphene-based membranes demonstrating ultrafast water transport, precise molecular sieving of gas and solvated molecules shows great promise as novel separation platforms; however, scale-up of these membranes to large-areas remains an unresolved problem. Here we demonstrate that the discotic nematic phase of graphene oxide (GO) can be shear aligned to form highly ordered, continuous, thin films of multi-layered GO on a support membrane by an industrially adaptable method to produce large-area membranes (13 × 14 cm2) in <5 s. Pressure driven transport data demonstrate high retention (>90%) for charged and uncharged organic probe molecules with a hydrated radius above 5 Å as well as modest (30–40%) retention of monovalent and divalent salts. The highly ordered graphene sheets in the plane of the membrane make organized channels and enhance the permeability (71±5 l m−2 hr−1 bar−1 for 150±15 nm thick membranes).

Electrochemical Resolution of Multiple Redox Events for Graphene Quantum Dots

Quantum dots: A sequential, single-electron charging process of monodisperse graphene quantum dots (GQDs) encapsulated in a dodecylamine envelope, facilitating a capacitance of a few attofarads is reported. The average GQDs dimensions, as ascertained from high-resolution transmission electron microscopy and atomic force microscopy, of about 3±0.3, 2.6±0.2, and 2.2±0.3 nm control this unprecedented behavior.

Shear Assisted Electrochemical Exfoliation of Graphite to Graphene

The exfoliation characteristics of graphite as a function of applied anodic potential (1-10 V) in combination with shear field (400-74 400 s(-1)) have been studied in a custom-designed microfluidic reactor. Systematic investigation by atomic force microscopy (AFM) indicates that at higher potentials thicker and more fragmented graphene sheets are obtained, while at potentials as low as 1 V, pronounced exfoliation is triggered by the influence of shear. The shear-assisted electrochemical exfoliation process yields large (∼10 μm) graphene flakes with a high proportion of single, bilayer, and trilayer graphene and small ID/IG ratio (0.21-0.32) with only a small contribution from carbon-oxygen species as demonstrated by X-ray photoelectron spectroscopy measurements. This method comprises intercalation of sulfate ions followed by exfoliation using shear induced by a flowing electrolyte. Our findings on the crucial role of hydrodynamics in accentuating the exfoliation efficiency suggest a safer, greener, and more automated method for production of high quality graphene from graphite.

Graphene nanoribbons as prospective field emitter

Field emission characteristics of graphene nanoribbons (GNRs) synthesized by unzipping of multiwall carbon nanotubes using a facile hydrothermal route have been investigated at a base pressure of 1 × 10−8 mbar. The values of turn-on field, required to draw an emission current densities of 1 and 10 μA/cm2, are found to be 2.8 and 5.8 V/μm, respectively, and a maximum emission current density of 500 μA/cm2 has been drawn at an applied field of 9.8 V/μm. The emission current stability of the GNRs emitter was studied at preset values of 1 and 10 μA over a period of 3 h, and is found to be excellent. The field emission results demonstrated herein suggest that GNRs based field emitters can open up many opportunities for their potential utilization as large area field emitters in various vacuum micro-nanoelectronic devices such as flexible field emission displays, portable X-ray, and microwave tubes.

Counter-ion Dependent, Longitudinal Unzipping of Multi-Walled Carbon Nanotubes to Highly Conductive and Transparent Graphene Nanoribbons

Here we report for the first time, a simple hydrothermal approach for the bulk production of highly conductive and transparent graphene nanoribbons (GNRs) using several counter ions from K2SO4, KNO3, KOH and H2SO4 in aqueous media, where, selective intercalation followed by exfoliation gives highly conducting GNRs with over 80% yield. In these experiments, sulfate and nitrate ions act as a co-intercalant along with potassium ions resulting into exfoliation of multi-walled carbon nanotubes (MWCNTs) in an effective manner. The striking similarity of experimental results in KOH and H2SO4 that demonstrates partially damaged MWCNTs, implies that no individual K+, SO42− ion plays a key role in unwrapping of MWCNTs, rather this process is largely effective in the presence of both cations and anions working in a cooperative manner. The GNRs can be used for preparing conductive 16 kΩsq−1, transparent (82%) and flexible thin films using low cost fabrication method.

Hysteresis and charge trapping in graphene quantum dots

We report current hysteresis in response to applied voltage in graphene quantum dots of average diameter 4.5 ± 0.55 nm, synthesized electrochemically using multiwalled carbon nanotubes. In response to step voltages, transient current decay, characteristic of deep and shallow level charge traps with time constants 186 ms and 6 s, is observed. Discharging current transients indicate charge storage of the order of 100 μC. Trap states are believed to arise due to the fast physisorption of external adsorbates, which are found to have a significant effect on charge transport and changes the resistance of the prepared device by an order of 3.

Sequential Electrochemical Unzipping of Single-Walled Carbon Nanotubes to Graphene Ribbons Revealed by in Situ Raman Spectroscopy and Imaging

We report an in situ Raman spectroscopic and microscopic investigation of the electrochemical unzipping of single-walled carbon nanotubes (SWNTs). Observations of the radial breathing modes (RBMs) using Raman spectral mapping reveal that metallic SWNTs are opened up rapidly followed by gradual unzipping of semiconducting SWNTs. Consideration of the resonant Raman scattering theory suggests that two metallic SWNTs with chiralities (10, 4) and (12, 0) get unzipped first at a lower electrode potential (0.36 V) followed by the gradual unzipping of another two metallic tubes, (9, 3) and (10, 1), at a relatively higher potential (1.16 V). The semiconducting SWNTs with chiralities (11, 7) and (12, 5), however, get open up gradually at ±1.66 V. A rapid decrease followed by a subsequent gradual decrease in the metallicity of the SWNT ensemble as revealed from a remarkable variation of the peak width of the G band complies well with the variations of RBM. Cyclic voltammetry also gives direct evidence for unzipping in terms of improved capacitance after oxidation followed by more important removal of oxygen functionalities during the reduction step, as reflected in subtle changes of the morphology confirming the formation of graphene nanoribbons. The density functional-based tight binding calculations show additional dependence of chirality and diameter of nanotubes on the epoxide binding energies, which is in agreement with the Raman spectroscopic results and suggests a possible mechanism of unzipping determined by combined effects of the structural characteristics of SWNTs and applied field.

Electrochemical preparation of nitrogen-doped graphene quantum dots and their size-dependent electrocatalytic activity for oxygen reduction

Here we report a remarkable transformation of nitrogen-doped multiwalled carbon nanotubes (MWCNTs) to size selective nitrogen-doped graphene quantum dots (N-GQDs) by a two-step electrochemical method. The sizes of the N-GQDs strongly depend on the applied anodic potential, moreover increasing potential resulted in a smaller size of N-GQDs. These N-GQDs display many unusual size-dependant optoelectronic (blue emission) and electrocatalytic (oxygen reduction) properties. The presence of N dopants in the carbon framework not only causes faster unzipping of MWCNTs but also provides more low activation energy site for enhancing the electrocatalytic activity for technologically daunting reactions like oxygen reduction. The smaller size of N-GQDs has shown better performance as compared to the large N-GQDs. Interestingly, N-GQDs-3 (size = 2.5±0.3 nm, onset potential = 0.75 V) show a 30-mV higher positive onset potential shift compared to that of N-GQDs-2 (size = 4.7±0.3 nm, onset potential = 0.72 V) and 70 mV than that of N-GQDs-1 (size = 7.2±0.3, onset potential = 0.68 V) for oxygen reduction reaction (ORR) in a liquid phase. These result in the size-dependent electrocatalytic activity of N-GQDs for ORR as illustrated by the smaller sized N-GQDs (2.5±0.3 nm) undoubtedly promising metal-free electrocatalysts for fuel cell applications.