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.

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.

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.

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.

A single-step, electrochemical synthesis of nitrogen doped blue luminescent phosphorene quantum dots.

Herein, we report a one-step strategy for the electrochemical synthesis of nitrogen doped blue luminescent phosphorene quantum dots (NPQDs) from black phosphorus at room temperature. Nitrogen percentage in NPQDs can be varied by the appropriate choice of solvent and supporting electrolyte. NPQDs [average size 6 ± 1.5 nm (N = 50)] obtained in this work exhibit ca. 88.7% quantum efficiency.

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.

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.

Effect of Dimensionality and Doping in Quasi-"One-Dimensional (1-D)" Nitrogen-Doped Graphene Nanoribbons on the Oxygen Reduction Reaction.

Designing an efficient metal-free electrocatalyst for the oxygen reduction reaction (ORR) is a challenging research theme having enormous practical importance in several renewable energy technologies like fuel cell and metal-air batteries. Here we discuss a cost-effective and commercially viable strategy to develop high-performance nitrogen-doped graphene nanoribbon (N-GNR), which is a quasi-"one-dimensional" analogue of graphene. We have selected the N-GNR system to identify the doping-induced variation in the distribution of active catalytic sites experimentally in graphene-based electrocatalysts. N-GNR exhibits a comparable exchange current density (1.65 × 10-9 vs 2.25 × 10-9 A cm-2), thermodynamic potential (0.80 vs 0.83 V), and smaller Tafel slope (55 vs 60 mV dec-1) with respect to the benchmarking platinum/carbon (Pt/C), and also, more precisely, it goes through a four-electron pathway with low hydrogen peroxide yield. Although the exact mechanism is still not clear, the theme of the work is based on the identification of the possible active sites with the help of experimental evidence like X-ray photoelectron spectroscopy. These results support the assumption that an edge N (pyridinic N)-bonded adjacent C lowers the activation energy barriers of O2 adsorption, predominantly to kinetically facilitate the ORR activity. We hope these results will be helpful in developing more efficient ORR catalysts.

Topotactic transition of α-Co(OH)2 to ß-Co(OH)2 anchored on CoO nanoparticles during electrochemical water oxidation: synergistic electrocatalytic effects.

Herein, we report a single step, anionic surfactant-assisted, low temperature-hydrothermal synthetic strategy of CoO nanoparticles anchored on ß-Co(OH)2 nanosheets which show a low overpotential (295 mV @ 10 mA cm-2) for the oxygen evolution reaction (OER). They also demonstrate much better kinetic parameters compared to the state-of-the-art RuO2. Interestingly, under the OER operational conditions (in alkaline medium), the topotactic transformation of α-Co(OH)2 to a stable Brucite-like ß-Co(OH)2 phase leads to a synergistic interaction between the ß-Co(OH)2 sheets on the CoO nanoparticles for enhancing the OER electrocatalytic 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.

Synthesis of N, F and S co-doped graphene quantum dots.

Graphene quantum dots (GQDs) are a promising category of materials with remarkable size dependent properties like tunable bandgap and photoluminescence along with the possibility of effective chemical functionalization. Doping of GQDs with heteroatoms is an interesting way of regulating their properties. Herein, we report a facile and scalable one-step synthesis of luminescent GQDs, substitutionally co-doped with N, F and S, of ∼2 nm average size by a microwave treatment of multi-walled carbon nanotubes in a customized ionic liquid medium. The use of an ionic liquid coupled with the use of a microwave technique enables not only an ultrafast process for the synthesis of co-doped GQDs, but also provides excellent photoluminescence quantum yield (70%), perhaps due to the interaction of defect clusters and dopants.

The role of the molecular structure of carboxylate-alumoxanes in the enhanced nucleation of polypropylene.

We have reported the use of carboxylate-alumoxanes as efficient nucleating agents for isotactic polypropylene (iPP) with a possible structural correlation to the nucleation efficiency. The unique, butterfly-like structure of carboxylate-alumoxanes correlates well with the nucleation characteristics of iPP and, for the first time, the impact of a thermally induced, crystalline transition of carboxylate-alumoxanes, which alters neither the structural conformation nor the nucleation efficiency of the transformed material, is demonstrated.

Electrochemical synthesis of luminescent MoS2 quantum dots.

Size-controlled synthesis of luminescent quantum dots of MoS2 (≤2 layers) with narrow size distribution, ranging from 2.5 to 6 nm, from their bulk material using a unique electrochemical etching of bulk MoS2 is demonstrated. Excitation-dependent photoluminescence emission is observed in the MoS2 QDs. "As-synthesized" MoS2 QDs also exhibit excellent electrocatalytic activity towards hydrogen evolution reactions.

Electrochemical preparation of vertically aligned, hollow CdSe nanotubes and their p-n junction hybrids with electrodeposited Cu2O.

Vertically aligned, hollow nanotubes of CdSe are grown on fluorine doped tin oxide (FTO) coated glass substrates by ZnO nanowire template-assisted electrodeposition technique, followed by selective removal of the ZnO core using NH4OH. A detailed mechanism of nucleation and anisotropic growth kinetics of nanotubes have been studied by a combination of characterization tools such as chronoamperometry, SEM and TEM. Interestingly, "as grown" CdSe nanotubes (CdSe NTs) on FTO coated glass plates behave as n-type semiconductors exhibiting an excellent photo-response (with a generated photocurrent density value of ∼ 470 µA cm(-2)) while in contact with p-type Cu2O (p-type semiconductor, grown separately on FTO plates) because of the formation of a n-p heterojunction (type II). The observed photoresponse is 3 times higher than that of a similar device prepared with electrodeposited CdSe films (not nanotubes) and Cu2O on FTO. This has been attributed to the hollow 1-D nature of CdSe NTs, which provides enhanced inner and outer surface areas for better absorption of light and also assists faster transport of photogenerated charge carriers.

C@SiNW/TiO2 core-shell nanoarrays with sandwiched carbon passivation layer as high efficiency photoelectrode for water splitting.

One-dimensional heterostructure nanoarrays are efficiently promising as high performance electrodes for photo electrochemical (PEC) water splitting applications, wherein it is highly desirable for the electrode to have a broad light absorption, efficient charge separation and redox properties as well as defect free surface with high area suitable for fast interfacial charge transfer. We present highly active and unique photoelectrode for solar H2 production, consisting of silicon nanowires (SiNWs)/TiO2 core-shell structures. SiNWs are passivated to reduce defect sites and protected against oxidation in air or water by forming very thin carbon layer sandwiched between SiNW and TiO2 surfaces. This carbon layer decreases recombination rates and also enhances the interfacial charge transfer between the silicon and TiO2. A systematic investigation of the role of SiNW length and TiO2 thickness on photocurrent reveals enhanced photocurrent density up to 5.97 mA/cm(2) at 1.0 V vs.NHE by using C@SiNW/TiO2 nanoarrays with photo electrochemical efficiency of 1.17%.

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(+), SO4(2-) 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.