Adsorption Kinetics of WS2 Quantum Dots onto a Polycrystalline Gold Surface.

In this work, we report the adsorption kinetics of electrochemically synthesized WS2 quantum dots (QDs) (ca. 3 nm) onto a polycrystalline gold electrode. The Langmuir adsorption isotherm approach was employed to explore the temperature and adsorbate concentration dependence of the experimentally calculated equilibrium constant of adsorption ( Keq) and the free energy for adsorption (Δ Gads). Subsequently, we extract other thermodynamic parameters, such as adsorption rate constant ( Kads), desorption rate constant ( Kd), the enthalpy of adsorption (Δ Hads), and the entropy of adsorption (Δ Sads). Our findings indicate that Δ Gads is temperature-dependent and ca. -7.64 ± 0.6 kJ/mol, Δ Hads = -43.72 ± 1.7 kJ/mol, and Δ Sads = -0.126 ± 0.017 kJ/(mol K). These investigations on the contribution of the enthalpic and entropic forces to the total free energy of this system underscore the role of entropic forces on the stability of the WS2 QDs monolayer and provide new thermodynamic insights into other transition-metal dichalcogenide quantum dot (TMDQD) monolayers as well.

Co3Fe7/nitrogen-doped graphene nanoribbons as bi-functional electrocatalyst for oxygen reduction and oxygen evolution.

In this work, we report a one-pot solvothermal strategy to synthesize Co3Fe7 incorporated graphene nanoribbons (Co3Fe7). An improved bi-functional electrocatalytic activity over the traditional electrocatalysts is exhibited by the Co3Fe7/nitrogen-doped graphene nanoribbon (NGNR) composite. For instance, this composite Co3Fe7/NGNRs depicted a lower overpotential of 350 mV than NGNRs (380 mV) and IrO2 (450 mV) to sustain 10 mA cm-2 for an oxygen evolution reaction in 1.0 M KOH. Furthermore, during an oxygen reduction reaction, the catalyst exhibited a four-electron pathway and it is interesting to note that its electrocatalytic behavior is on a par with commercial Pt/C. The enhancement in the electrochemical performance can be attributed to the synergistic effect that stems from the electrocatalytically active nitrogen atoms and metal alloy nanoparticles distributed uniformly over the graphene matrix. This unique composition of electrocatalyst is extremely beneficial for practical applications in fuel cells and metal-air batteries due to its high stability and sustained electrochemical activity.

A Single-Step Electrochemical Synthesis of Luminescent WS2 Quantum Dots.

Transition-metal dichalcogenide quantum dots (TMDQDs) with few layers are in the forefront of recent research on tailored 2D layered materials owing to their unique band structure. Such quantum dots (QDs) draw wide interest as potential candidates for components in optoelectronic devices. Although a few attempts towards single step synthesis of MoS2 QDs have been demonstrated, limited methods are available for WS2 QDs. Herein, we demonstrate a one-step electrochemical synthesis of luminescent WS2 QDs from their bulk material. This is achieved by a synergistic effect of perchlorate intercalation in non-aqueous electrolyte and the applied electric field. The average size of the WS2 QDs is 3  ±1 nm (N=102) with few layers. The QDs show a higher photoluminescence (PL) quantum efficiency (5 %) and exhibit an excitation wavelength-dependent photoluminescence. This unprecedented electrochemical avenue offers a strategy to synthesize size tunable WS2 nanostructures, which have been systematically investigated by various characterization techniques such as transmission electron microscopy (TEM), photoluminescence and UV/Vis spectroscopies, and X-ray diffraction (XRD). Time-dependent TEM investigations revealed that time plays a vital role in this electrochemical transformation. This electrochemical transformation provides a facile method to obtain WS2 QDs from their bulk counterpart, which is expected to have a greater impact on the design and development of nanostructures derived from 2D materials.

Facile Green Synthesis of BCN Nanosheets as High-Performance Electrode Material for Electrochemical Energy Storage.

Two-dimensional hexagonal boron carbon nitride (BCN) nanosheets (NSs) were synthesized by new approach in which a mixture of glucose and an adduct of boric acid (H3 BO3 ) and urea (NH2 CONH2 ) is heated at 900 °C. The method is green, scalable and gives a high yield of BCN NSs with average size of about 1 µm and thickness of about 13 nm. Structural characterization of the as-synthesized material was carried out by several techniques, and its energy-storage properties were evaluated electrochemically. The material showed excellent capacitive behaviour with a specific capacitance as high as 244 F g(-1) at a current density of 1 A g(-1) . The material retains up to 96 % of its initial capacity after 3000 cycles at a current density of 5 A g(-1) .

Fractional photo-current dependence of graphene quantum dots prepared from carbon nanotubes.

We report on the photo-conductivity studies of chemically synthesized graphene quantum dots (GQDs) of average size 12 nm obtained by the oxidative acid treatment of MWCNTs. The dependence of photocurrent Iph (Iph = Iill - Idark) on the laser intensity P under a wide range of laser intensities (5 mW ≤ P ≤ 60 mW) shows a fractional power dependence of Iph on light intensity. The temperature dependence (300 K < T < 50 K) of Iph observed in thin films of these GQDs indicates that in the higher temperature region (T > ∼100 K), as the temperature increases, the number of thermally generated carriers increase resulting in increased Iph. At sufficiently low temperatures (T ≤ 100 K), a constant Iph is observed, indicating a constant photo-carrier density. Such a behavior is typically observed in many photoactive disordered semiconductors, which are often used in a variety of applications. We believe that the investigations presented here will enhance our understanding of the photocurrent generation phenomenon in chemically obtained GQDs.

Graphene quantum dots for biosensing and bioimaging.

Graphene Quantum Dots (GQDs) are low dimensional carbon based materials with interesting physical, chemical and biological properties that enable their applications in numerous fields. GQDs possess unique electronic structures that impart special functional attributes such as tunable optical/electrical properties in addition to heteroatom-doping and more importantly a propensity for surface functionalization for applications in biosensing and bioimaging. Herein, we review the recent advancements in the top-down and bottom-up approaches for the synthesis of GQDs. Following this, we present a detailed review of the various surface properties of GQDs and their applications in bioimaging and biosensing. GQDs have been used for fluorescence imaging for visualizing tumours and monitoring the therapeutic responses in addition to magnetic resonance imaging applications. Similarly, the photoluminescence based biosensing applications of GQDs for the detection of hydrogen peroxide, micro RNA, DNA, horse radish peroxidase, heavy metal ions, negatively charged ions, cardiac troponin, etc. are discussed in this review. Finally, we conclude the review with a discussion on future prospects.

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.

Effect of hetero-atom doping on the electrocatalytic properties of graphene quantum dots for oxygen reduction reaction.

Oxygen reduction is an important reaction involved in a diverse variety of energy storage devices and also in many chemical and biological processes. However, the high cost of suitable catalysts like platinum, rhodium, and iridium proves to be a major obstacle for its commercialization. Consequently, many new materials have emerged in recent years such as various forms of carbon, carbides, nitrides, core-shell particles, Mxenes, and transition metal complexes as alternatives to platinum and other noble metals for oxygen reduction reaction (ORR). Among these, Graphene Quantum Dots (GQDs) as metal-free alternatives have captured universal attention, since electrocatalytic properties can be tuned not only by size and functionalization but by heteroatom doping also. We discuss electrocatalytic properties of GQDs (approximate size 3-5 nm) with specific dopants such as N and S focusing on their synergistic effects of co-doping, prepared by solvothermal routes. Cyclic Voltammetry shows benefits of doping as lowering of the onset potentials while steady-state Galvanostatic Tafel polarization measurements show a clear difference in the apparent Tafel slope, along with enhanced exchange current densities, suggesting higher rate constants.

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.

Phosphorene quantum dots: synthesis, properties and catalytic applications.

Phosphorene quantum dots (PQDs) belong to a new class of zero-dimensional functional nanostructures with unique physicochemical and surface properties in comparison with few-layer phosphorene and other 2D analogues. Tunable band gap as a function of number of layers, ease of passivation and high carrier mobility of PQDs have attracted considerable attention in catalysis research due to which spectacular progress has been made in PQD research over the last few years. PQDs are now considered as promising catalytic materials for electrocatalytic water splitting and nitrogen reduction, lithium-sulfur batteries, solar light-driven energy devices and biocatalysis, either in pristine form or as an active component for constructing heterostructures with other 2D materials. In the light of these recent advances, it is worthwhile to review and consolidate PQD research in catalytic applications to understand the challenges ahead and suggest possible solutions. In this review, we systematically summarize various synthetic strategies including ultrasonic and electrochemical exfoliation, solvothermal treatment, blender breaking, milling, crushing and pulsed laser irradiation. Furthermore, the physiochemical properties of PQDs are discussed based on both experimental and theoretical perspectives. The potential applications of PQDs in catalysis with special emphasis on photocatalysis (solar light-driven energy devices) and electrocatalysis (oxygen evolution reactions and hydrogen evolution reactions) -are critically discussed along with the present status, challenges and future perspectives.

Electrochemical unzipping of multi-walled carbon nanotubes for facile synthesis of high-quality graphene nanoribbons.

Here we report a remarkable transformation of carbon nanotubes (CNTs) to nanoribbons composed of a few layers of graphene by a two-step electrochemical approach. This consists of the oxidation of CNTs at controlled potential, followed by reduction to form graphene nanoribbons (GNRs) having smooth edges and fewer defects, as evidenced by multiple characterization techniques, including Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. This type of "unzipping" of CNTs (single-walled, multi-walled) in the presence of an interfacial electric field provides unique advantages with respect to the orientation of CNTs, which might make possible the production of GNRs with controlled widths and fewer defects.

Carbon nanotube-modified sodium dodecyl sulfate-polyacrylamide gel electrophoresis for molecular weight determination of proteins.

The effect of incorporating carbon nanotubes (CNTs) in the gel matrix on the electrophoretic mobility of proteins based on their molecular weight differences was investigated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). More specifically, a reduction in standard deviation in the molecular weight calibration plots by 55% in the case of multiwalled carbon nanotubes (MWCNTs) and by 34% in the case of single-walled carbon nanotubes (SWCNTs) compared with that of pristine polyacrylamide gels was achieved after incorporating an insignificant amount of functionalized CNTs into the gel matrix. A mechanism based on a more uniform pore size distribution in CNT modified polyacrylamide gel matrix is proposed. Furthermore, the impact of SWCNTs and MWCNTs on the mobility of proteins in different molecular weight regimes at a given acrylamide concentration offers a tunable gel matrix in terms of the selection of molecular weight ranges of proteins. The robustness and excellent reproducibility of the CNT-PAGE protocol are expected to have a significant impact on the molecular weight determination of newly isolated proteins.

Competitive wetting of acetonitrile and dichloromethane in comparison to that of water on functionalized carbon nanotube surfaces.

Differential wetting of pristine and ozonized carbon nanotubes has been studied using solvents like acetonitrile and dichloromethane in comparison to the well-known wetting behavior of water. Based on their unique structural and physical properties, functionalized CNT substrates have been used due to the fact that independent variation in molecular as well as electronic properties could be controlled by understanding the wetting of these liquids on carbon nanotubes (CNTs), both pristine as well as ozone treated. The sensitivity of the wetting behavior with respect to molecular interactions has been investigated using contact angle measurements while Raman and XPS studies unravel the differential wetting behavior. Charge-transfer between adsorbed molecules and CNTs has been identified to play a crucial role in determining the interfacial energies of these two liquids, especially in the case of acetonitrile. Ozone treatment has been observed to affect the surface properties of pristine CNTs along with a concomitant change in the wetting dynamics.

Enhanced electrocatalytic performance of functionalized carbon nanotube electrodes for oxygen reduction in proton exchange membrane fuel cells.

Although nitrogen doped CNTs (N-CNTs) are considered as a promising alternative to platinized carbon for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs), the origin of the enhanced ORR activity with N-CNTs is not clear at present. Among several plausible reasons, the exposure of edge plane and creation of impurity band/surface states near the Fermi level are considered as major causes behind the catalytic activity. However, CNTs without nitrogen doping are not known to catalyze the ORR. In this work, we study the ORR activity of functionalized carbon nanotubes with different functional groups, such as sulfonic acid and phosphonic acid, in order to understand the role of surface functionalities in catalyzing the reaction. Functionalized CNTs show significantly enhanced activity towards the ORR, while CNTs without such surface functional groups do not reveal any such special ORR activity. Linear sweep voltammetry experiments with different rotation rates show diffusion controlled limiting current values for functionalized CNTs, and the 'n' values derived from Koutecky-Levich plots are 3.3 and 1.7 for S-MWCNTs and P-MWCNTs, respectively. This work demonstrates the ORR activity of functionalized MWCNTs, which opens up new strategies for electrocatalyst design in PEMFCs.

Correction: Bismuthene nanosheets produced by ionic liquid assisted grinding exfoliation and their use for oxygen reduction reaction.

[This corrects the article DOI: 10.1039/D0RA09763B.].

Bismuthene nanosheets produced by ionic liquid assisted grinding exfoliation and their use for oxygen reduction reaction.

We report the simple synthesis of bismuthene nanosheets (BiNS) by ionic liquid assisted grinding exfoliation, followed by size selection sequential centrifugation steps for the first time. The exfoliation process results in the formation of self-assembled spherule-like superstructures with abundant edge sites, which are able to catalyze the oxygen reduction reaction (ORR) via a two-electron pathway, with a higher efficiency than the bulk Bismuth. We rationalize the enhanced ORR activity of the BiNS to: (i) the presence of 1 dimensional topological edge states, which provide strong conduction channels for electron hopping between the bismuth layers and (ii) the more active role of edge sites in facilitating O2 adsorption and dissociation of O-O bonds compared to the basal plane. The present study provides a pathway for employing 2D topological insulators as a new class of electrocatalysts for clean energy applications.

Ultrafast switching time and third order nonlinear coefficients of microwave treated single walled carbon nanotube suspensions.

Microwave treated water soluble and amide functionalized single walled carbon nanotubes have been investigated using femtosecond degenerate pump-probe and nonlinear transmission experiments. The time resolved differential transmission using 75 femtosecond pulse with the central wavelength of 790 nm shows a bi-exponential ultrafast photo-bleaching with time constants of 160 fs (130 fs) and 920 fs (300 fs) for water soluble (amide functionalized) nanotubes. Open and closed aperture z-scans show saturation absorption and positive (negative) nonlinear refraction for water soluble (amide functionalized) nanotubes. Two photon absorption coefficient, beta0 approximately 250 cm/GW (650 cm/GW) and nonlinear index, gamma approximately 15 cm2/pW (-30 cm2/pW) are obtained from the theoretical fit in the saturation limit to the data for two types of nanotubes.

Solid-state thermal exfoliation of graphite nano-fibers to edge-nitrogenized graphene nanosheets for oxygen reduction reaction.

We demonstrate a simple, single-step and scalable synthesis of edge-nitrogenated graphene nanosheets (E-N-GNS) through thermal exfoliation of graphite platelet nanofibers (GPNF) in the presence of melamine. This material was characterized using different physical characterization techniques which divulges that, the edges are selectively functionalised with pyridinic and pyrrolic type nitrogens leading to the formation of E-N-GNS. Further, the electrocatalytic activity of E-N-GNS towards oxygen reduction reaction (ORR) in alkaline medium was studied using electrochemical techniques to reveal superior electrocatalytic activity of E-N-GNS towards ORR than that of GPNF, perhaps due to the incorporation of N at the edges. The cyclic voltammetry (CV), however, shows a 50 mV and 71 mV positive shift in the onset and peak potentials respectively, which in combination with the rotating ring disk electrode (RRDE) results suggest that ORR follows a direct four electron transfer path on E-N-GNS in comparison with a two electron path on GPNF modified electrodes. This E-N-GNS also shows superior stability for 5000 cycles along with a high methanol-tolerance and durability than that of benchmark ORR electrocatalyst Pt/C (20%) to suggest its potential applications in fuel cells and metal-air batteries.

Role of Structural Distortion in Stabilizing Electrosynthesized Blue-Emitting Phosphorene Quantum Dots.

Luminescent phosphorene quantum dots (PQDs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Herein, we investigate the structural distortion of black phosphorus (BP) under an applied electric field to yield blue luminescent PQDs [average diameter 8 ± 1.5 nm ( N = 60)]. The electrosynthesized PQDs exhibit photoluminescence emission independent of excitation wavelength with 84% quantum efficiency. Structural distortion that occurred during the transformation of BP to PQDs is confirmed by results obtained during transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Further, using first-principles-based density functional theory, calculations on oxygenated and nonoxygenated PQDs augment the experimental observations that an optimum oxygen content maintains the structural integrity of PQDs, above which the structural robustness of PQDs is drastically diminished.

Imaging the stomatal physiology of somatic embryo-derived peanut leaves by scanning electrochemical microscopy.

The stomatal physiology, chlorophyll distribution and photosynthetic activity of somatic embryo (SE)- and seedling-derived peanut plants grown in vitro (test tube-grown) and extra vitrum (soil-grown) are investigated using scanning electrochemical microscopy (SECM). This SECM imaging is performed in two different feedback modes, corresponding to oxygen evolution and chlorophyll distribution. More specifically, the oxygen evolution profiles of the in vitro leaves indicate important differences in leaf anatomy between the SE- and seedling-derived leaves. On the other hand, the chlorophyll distribution images show individual stomata of size ca. 27 +/- 5 microm. Further studies on senescing (aged) leaves reveal interesting voltammograms that vary widely over the stomatal complexes and the surrounding tissues, probably due to the release of electroactive metabolites during chlorophyll breakdown when the leaves turn yellow. Thus, the present investigation could open up new opportunities for characterizing botanical systems using electroanalytical techniques. In addition, it could provide further insights into various areas of current relevance, including signal transduction, cell fate/differentiation and developmental biology.