Atomic configuration of nitrogen-doped single-walled carbon nanotubes

Ni-Co bimetallic decorated dodecahedral ZIF as an efficient catalyst for photoelectrochemical degradation of sulfamethoxazole coupled with hydrogen production

In this work, a NiCo bimetallic ZIF (BMZIF) dodecahedron material has been synthesized by the precipitation approach and then used for simultaneously photoelectrocatalytic degradation of sulfamethoxazole (SMX) and hydrogen production. The combination of Ni/Co loading in ZIF structure increased the specific surface area 1484 (m² g^-1) and photocurrent density (0.4 mA cm^-2), which can facilitate the good charge transfer efficiency. In presence of peroxymonosulfate (PMS, 0.1 mM), the complete degradation of SMX (10 mg L^-1) was achieved at initial pH of 7 within 24 min, with the pseudo-first-order rate constants of 0.18 min^-1 and TOC removal efficiency of 85 %. Radical scavenger experiments affirm that ^•OH radicals were the primary oxygen reactive species to drive the SMX degradation. Along with SMX degradation at the anode, the H₂ production was observed at the cathode (140 μmol cm^-2 h^-1), which was 1.5 and 3 times higher than that of Co-ZIF and Ni-ZIF, respectively. The superior catalytic performance of BMZIF was assigned to the distinctive internal structure and synergistic effect between ZIF and Ni/Co bimetals, which improves light absorption and charge conduction efficiency. This study may provide insight into the new way to treat polluted water and simultaneously produce green energy using bimetallic ZIF in a PEC system.

Engineering O-O Species in Boron Nitrous Nanotubes Increases Olefins for Propane Oxidative Dehydrogenation

Boron nitride (BN) has been widely studied as an efficient catalyst for oxidative propane dehydrogenation (OPDH). Oxygen-containing boron species (e.g., BO·, B(OH)_xO_3-x) are generally considered as the active centers in BN for OPDH. Here, we show an effective progressive substitution strategy toward the development of boron-oxygen-nitrogen nanotubes (BONNTs) enriched with O-O species as a highly active, selective, and stable catalyst for OPDH. At 525 °C, an olefin yield of 48.6% is achieved over BONNTs with a propane conversion of 64.4%, 2.8 times that of boron nitrogen nanotubes (BNNTs). Even after reaction for 150 h (475 °C), BONNTs exhibit good olefin yield. Both the B(OH)_xO_3-x and O-O species that coexist in the BONNT catalyst are demonstrated as active centers, which differs from the B(OH)_xO_3-x one in BNNTs. Based on catalytic results, propane and oxygen alternate treatment experiments, and theoretical calculations, the O-O center is more favorable for producing both propylene (C₃₌) and ethylene (C₂₌), which experiences a dehydration pathway and two possible reaction paths with a lower energy barrier to yield olefins, while B(OH)_xO_3-x is mainly responsible for producing few C₃₌.

Effect of the Dopant Configuration on the Electronic Transport Properties of Nitrogen-Doped Carbon Nanotubes

Nitrogen-doped carbon nanotubes (N-CNTs) show promise in several applications related to catalysis and electrochemistry. In particular, N-CNTs with a single nitrogen dopant in the unit cell have been extensively studied computationally, but the structure-property correlations between the relative positions of several nitrogen dopants and the electronic transport properties of N-CNTs have not been systematically investigated with accurate hybrid density functional methods. We use hybrid density functional theory and semiclassical Boltzmann transport theory to systematically investigate the effect of different substitutional nitrogen doping configurations on the electrical conductivity of N-CNTs. Our results indicate significant variation in the electrical conductivity and the relative energies of the different dopant configurations. The findings can be utilized in the optimization of electrical transport properties of N-CNTs.

A Simple Approach to Control the Physical and Chemical Features of Custom-Synthesized N-Doped Carbon Nanotubes and the Extent of Their Network Formation in Polymers: The Importance of Catalyst to Substrate Ratio

This study intends to reveal the significance of the catalyst to substrate ratio (C/S) on the structural and electrical features of the carbon nanotubes and their polymeric nanocomposites. Here, nitrogen-doped carbon nanotube (N-MWNT) was synthesized via a chemical vapor deposition (CVD) method using three ratios (by weight) of iron (Fe) catalyst to aluminum oxide (Al₂O₃) substrate, i.e.,1/9, 1/4, and 2/3, by changing the Fe concentration, i.e., 10, 20, and 40 wt.% Fe. Therefore, the synthesized N-MWNT are labelled as (N-MWNTs)₁₀, (N-MWNTs)₂₀, and (N-MWNTs)₄₀. TEM, XPS, Raman spectroscopy, and TGA characterizations revealed that C/S ratio has a significant impact on the physical and chemical properties of the nanotubes. For instance, by increasing the Fe catalyst from 10 to 40 wt.%, carbon purity increased from 60 to 90 wt.% and the length of the nanotubes increased from 1.2 to 2.6 µm. Interestingly, regarding nanotube morphology, at the highest C/S ratio, the N-MWNTs displayed an open-channel structure, while at the lowest catalyst concentration the nanotubes featured a bamboo-like structure. Afterwards, the network characteristics of the N-MWNTs in a polyvinylidene fluoride (PVDF) matrix were studied using imaging techniques, AC electrical conductivity, and linear and nonlinear rheological measurements. The nanocomposites were prepared via a melt-mixing method at various loadings of the synthesized N-MWNTs. The rheological results confirmed that (N-MWNTs)₁₀, at 0.5–2.0 wt.%, did not form any substantial network through the PVDF matrix, thereby exhibiting an electrically insulative behavior, even at a higher concentration of 3.0 wt.%. Although the optical microscopy, TEM, and rheological results confirmed that both (N-MWNTs)₂₀ and (N-MWNTs)₄₀ established a continuous 3D network within the PVDF matrix, (N-MWNTs)₄₀/PVDF nanocomposites exhibited approximately one order of magnitude higher electrical conductivity. The higher electrical conductivity of (N-MWNTs)₄₀/PVDF nanocomposites is attributed to the intrinsic chemical features of (N-MWNTs)₄₀, such as nitrogen content and nitrogen bonding types.

In Vitro and In Vivo Biocompatibility of Boron/Nitrogen Co-Doped Carbon Nano-Onions

Boron/nitrogen, co-doped, carbon nano-onions (BN-CNOs) have recently shown great promise as catalysts for the oxygen reduction reaction, due to the improved electronic properties imparted by the dopant atoms; however, the interactions of BN-CNOs with biological systems have not yet been explored. In this study, we examined the toxicological profiles of BN-CNOs and oxidized BN-CNOs (oxi-BN-CNOs) in vitro in both healthy and cancer cell lines, as well as on the embryonic stages of zebrafish (Danio rerio) in vivo. The cell viabilities of both cell lines cells were not affected after treatment with different concentrations of both doped CNO derivatives. On the other hand, the analysis of BN-CNOs and oxidized BN-CNO interactions with zebrafish embryos did not report any kind of perturbations, in agreement with the in vitro results. Our results show that both doped CNO derivatives possess a high biocompatibility and biosafety in cells and more complex systems.

Comparative image simulations for phase-plate transmission electron microscopy

Numerous physical phase plates (PP) for phase-contrast enhancement in transmission electron microscopy (TEM) have been proposed and studied with the hole-free or Volta PP having a high impact and interest in recent years. This study is concerned with comparative TEM image simulations considering realistic descriptions of various PP approaches and samples from three different fields of application covering a large range of object sizes. The simulated images provide an illustrative characterization of the typical image appearance and common artifacts of the different PPs and the influence of simulation parameters especially important for PP simulations. A quantitative contrast analysis shows the superior phase-shifting properties of the hole-free phase plate for biological applications and the benefits of adjustable phase plates. The application of PPs in high-resolution TEM imaging, especially of weak-phase objects such as (atomically thin) 2D materials, is shown to increase image interpretability. The software with graphical user interface written and used for the presented simulations is available for free usage.

Metal-Assisted and Solvent-Mediated Synthesis of Two-Dimensional Triazine Structures on Gram Scale

Covalent triazine frameworks are an emerging material class that have shown promising performance for a range of applications. In this work, we report on a metal-assisted and solvent-mediated reaction between calcium carbide and cyanuric chloride, as cheap and commercially available precursors, to synthesize two-dimensional triazine structures (2DTSs). The reaction between the solvent, dimethylformamide, and cyanuric chloride was promoted by calcium carbide and resulted in dimethylamino-s-triazine intermediates, which in turn undergo nucleophilic substitutions. This reaction was directed into two dimensions by calcium ions derived from calcium carbide and induced the formation of 2DTSs. The role of calcium ions to direct the two-dimensionality of the final structure was simulated using DFT and further proven by synthesizing molecular intermediates. The water content of the reaction medium was found to be a crucial factor that affected the structure of the products dramatically. While 2DTSs were obtained under anhydrous conditions, a mixture of graphitic material/2DTSs or only graphitic material (GM) was obtained in aqueous solutions. Due to the straightforward and gram-scale synthesis of 2DTSs, as well as their photothermal and photodynamic properties, they are promising materials for a wide range of future applications, including bacteria and virus incapacitation.

Quantitative Analysis Method for Nitrogen Electron Energy-Loss Near-Edge Structures in Nanocarbons Based on Density Functional Theory Calculations and Linear Regression

Nonmetallic heteroatoms found in carbon nanomaterials act as active sites and exhibit excellent catalytic performance. Owing to structural complexity and the limitations of characterization technology, the identification of active sites in nanocarbon is challenging and controversial. Electron energy-loss spectroscopy is an electron microscope technique with high spatial resolution and a powerful tool for identifying the arrangement of heteroatoms. However, structural information regarding the configuration and distribution of heteroatoms is difficult to obtain using existing analytical methods. Herein, we have developed a method for the quantitative analysis of electron energy-loss near-edge structures to identify accurately nitrogen species in nanocarbon. Based on this approach, the relative amounts of nitrogen species were obtained from linear regression with calculated spectra. The concentration distribution of nanocarbon obtained by this method was consistent with the result of X-ray photoelectron spectroscopy analysis at different depths. Therefore, this fitting method can be used for the quantitative analysis of nitrogen K-edge structures. This provides a new strategy for studying the structure-activity relationships of carbon-based materials and the further design of custom nanocarbon catalysts with high active site densities.

High-Power Microbial Fuel Cells Based on a Carbon-Carbon Composite Air Cathode

Microbial fuel cells (MFCs) can convert organics in wastewater directly to electricity, and improving oxygen reduction reaction (ORR) performance is critical to their development and future applications. Electrocatalytic ORR performance is determined by the intrinsic activity and accessible amounts of active sites. A surface nitrogen-enriched carbon coaxial nanocable (NCCN) is applied as an ORR electrocatalyst and combined with activated carbon (AC) with 80 wt% addition as a carbon-carbon composite air cathode in MFCs. The fully exposed nitrogen active sites of NCCN contribute to the enhanced ORR activity, while the graphitized core affords a rapid pathway for electron transportation. AC serves as a spacer to construct a porous framework with interconnected ion diffusion channels. This cathode thus exhibits a maximum power density of 2090 mW m^-2 , 120% higher than commercial Pt/C electrocatalysts, and also 6% higher than the pure NCCN, indicating a synergistic effect between NCCN and AC. A high-performance NCCN-AC air cathode with a great promise for future MFC applications is reported and an effective strategy to bridge the electrocatalytic performance from nanomaterials to practical devices is presented.

Ping-Pong Energy Transfer in Covalently Linked Porphyrin-MoS2 Architectures

Molybdenum disulfide nanosheets covalently modified with porphyrin were prepared and fully characterized. Neither the porphyrin absorption nor its fluorescence was notably affected by covalent linkage to MoS2 . The use of transient absorption spectroscopy showed that a complex ping-pong energy-transfer mechanism, namely from the porphyrin to MoS2 and back to the porphyrin, operated. This study reveals the potential of transition-metal dichalcogenides in photosensitization processes.

Determination of the Evolution of Heterogeneous Single Metal Atoms and Nanoclusters under Reaction Conditions: Which Are the Working Catalytic Sites?

Identification of active sites in heterogeneous metal catalysts is critical for understanding the reaction mechanism at the molecular level and for designing more efficient catalysts. Because of their structural flexibility, subnanometric metal catalysts, including single atoms and clusters with a few atoms, can exhibit dynamic structural evolution when interacting with substrate molecules, making it difficult to determine the catalytically active sites. In this work, Pt catalysts containing selected types of Pt entities (from single atoms to clusters and nanoparticles) have been prepared, and their evolution has been followed, while they were reacting in a variety of heterogeneous catalytic reactions, including selective hydrogenation reactions, CO oxidation, dehydrogenation of propane, and photocatalytic H₂ evolution reaction. By in situ X-ray absorption spectroscopy, in situ IR spectroscopy, and high-resolution electron microscopy techniques, we will show that some characterization techniques carried out in an inadequate way can introduce confusion on the interpretation of coordination environment of highly dispersed Pt species. Finally, the combination of catalytic reactivity and in situ characterization techniques shows that, depending on the catalyst–reactant interaction and metal–support interaction, singly dispersed metal atoms can rapidly evolve into metal clusters or nanoparticles, being the working active sites for those abovementioned heterogeneous reactions.

Biomass-Derived Carbon: A Value-Added Journey Towards Constructing High-Energy Supercapacitors in an Asymmetric Fashion

Currently, asymmetric supercapacitors (ASCs) produced from supercapacitors (SCs) offer more benefits for energy-storage applications because they display a high operational voltage in aqueous-based electrolytes that may enhance grid storage and zero-power transportation with high energy density in the future. At the same time, the realization of low-cost energy devices through the construction of cheap electrode materials deserves a permanent place in the market once the goals of high energy, extra power, and long cycling stability are achieved. Biomass-derived carbon retrieved from sources such as plants has attracted considerable attention because of the rich abundance, low cost, and environmentally friendliness. In addition, the utilization of porous hierarchical structures has achieved enhanced electrochemical performance with excellent capacitance, outstanding stability, and praiseworthy rate capability. However, issues still persist in procedures used to obtain biomass-derived carbon materials with a high yield and a high degree of carbonization/graphitization, surface functionality, and porous characteristics, wherein the materials are used as electrodes in ASC devices. The present review briefly addresses the need for biomass-derived carbon materials in ASCs, comprehensively categorizes SCs in the context of their historical background, and elucidates the SC mechanism. In addition, influencing factors, such as the pore size distribution, role of surface functional groups, surface area, active-material loading, heteroatom doping, and activation techniques used in the preparation of biomass-derived carbon, have been discussed in detail. Moreover, this review assesses other nanostructured carbon electrodes used in ASCs and advances made in the fabrication of ASCs by using biomass-derived carbon in aqueous electrolytes. Finally, existing challenges and mandatory solutions toward developing cost-effective and high-performance ASCs by using environmentally friendly biomass-derived carbon materials are discussed in detail.

Direct imaging of light-element impurities in graphene reveals triple-coordinated oxygen

Along with hydrogen, carbon, nitrogen and oxygen are the arguably most important elements for organic chemistry. Due to their rich variety of possible bonding configurations, they can form a staggering number of compounds. Here, we present a detailed analysis of nitrogen and oxygen bonding configurations in a defective carbon (graphene) lattice. Using aberration-corrected scanning transmission electron microscopy and single-atom electron energy loss spectroscopy, we directly imaged oxygen atoms in graphene oxide, as well as nitrogen atoms implanted into graphene. The collected data allows us to compare nitrogen and oxygen bonding configurations, showing clear differences between the two elements. As expected, nitrogen forms either two or three bonds with neighboring carbon atoms, with three bonds being the preferred configuration. Oxygen, by contrast, tends to bind with only two carbon atoms. Remarkably, however, triple-coordinated oxygen with three carbon neighbors is also observed, a configuration that is exceedingly rare in organic compounds.

Annular dark field scanning transmission electron microscopy is able to distinguish the contrasts between light elements. Here, the authors directly image the bonding configurations of oxygen and nitrogen atoms in defective graphene, and surprisingly identify instances of unusual triple-coordinated oxygen with three carbon neighbors.

Sonochemical edge functionalisation of molybdenum disulfide

Liquid-phase exfoliation (LPE) has been shown to be capable of producing large quantities of high-quality dispersions suitable for processing into subsequent applications. LPE typically requires surfactants for aqueous dispersions or organic solvents with high boiling point. However, they have major drawbacks such as toxicity, aggregation during solvent evaporation or the presence of residues. Here, dispersions of molybdenum disulfide in acetone are prepared and show much higher concentration and stability than predicted by Hansen parameter analysis. Aiming to understand these enhanced properties, the nanosheets were characterised using UV-visible spectroscopy, zeta potential measurements, atomic force microscopy, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and scanning transmission microscopy combined with spatially-resolved electron energy loss spectroscopy. Also, the performance of the MoS₂ nanosheets exfoliated in acetone was compared to that of those exfoliated in isopropanol as a catalyst for the hydrogen evolution reaction. The conclusion from the chemical characterisation was that MoS₂ nanosheets exfoliated in acetone have an oxygen edge functionalisation, in the form of molybdenum oxides, changing its interaction with solvents and explaining the observed high-quality and stability of the resulting dispersion in a low boiling point solvent. Exfoliation in acetone could potentially be applied as a pretreatment to modify the solubility of MoS₂ by edge functionalisation.

TriQuinoline

The bottom-up synthesis of structurally well-defined motifs of graphitic materials is crucial to understanding their physicochemical properties and to elicit new functions. Herein, we report the design and synthesis of TriQuinoline (TQ) as a molecular model for pyridinic-nitrogen defects in graphene sheets. TQ is a trimer of quinoline units concatenated at the 2- and 8-positions in a head-to-tail fashion, whose structure leads to unusual aromatisation behaviour at the final stage of the synthesis. The central atomic-sized void endows TQ with high proton affinity, which was confirmed empirically and computationally. TQ•H⁺ is a two-dimensional cationic molecule that displays both π-π and CH-π contact modes, culminating in the formation of the ternary complex ([12]cycloparaphenylene(CPP) ⊃ (TQ•H⁺/coronene)) that consists of TQ•H⁺, coronene (flat), and [12]cycloparaphenylene ([12]CPP) (ring). The water-miscibility of TQ•H⁺ allows it to serve as an efficient DNA intercalator for e.g. the inhibition of topoisomerase I activity.

Structural transformations of graphene exposed to nitrogen plasma: quantum chemical molecular dynamics simulations

Nitrogen-doped graphene (N-graphene) has been intensively studied for tailoring the electronic property of the graphene, because different nitrogen configurations influence the electronic properties of N-graphene in different ways. However, atomically precise control of the nitrogen configurations during the doping process remains a challenge in the synthesis of N-graphene. Moreover, additional structural transformations of the graphene carbon network structure as a side-effect of plasma doping are little understood and are as of yet uncontrollable. Therefore, we theoretically investigated the nitrogen doping process of graphene for a range of nitrogen atom incident kinetic energies in nonequilibrium quantum chemical molecular dynamics (QM/MD) simulations. We observed and characterized prominent configurations of N-containing graphene. In analogy to similar, earlier studies of graphene plasma hydrogenation, we observed an Eley-Rideal associative desorption mechanism during the graphene plasma nitrogenation, producing molecular nitrogen. Especially for graphitic-N (Gr-N) and Stone-Wales-defect-N (SW-N) configurations, which are frequently observed in experimental studies, we discovered two typical chemical reaction mechanisms which were well categorized by two key processes: adsorption of primary nitrogen dopant and collision with a secondary nitrogen dopant. We discussed effects of the incident nitrogen energy on the formation mechanism, and propose a method to generate of Gr-N and SW-N configurations selectively by tuning the conditions with respect to the two key formation processes.

Silicon Substitution in Nanotubes and Graphene via Intermittent Vacancies

The chemical and electrical properties of single-walled carbon nanotubes (SWCNTs) and graphene can be modified by the presence of covalently bound impurities. Although this can be achieved by introducing chemical additives during synthesis, it often hinders growth and leads to limited crystallite size and quality. Here, through the simultaneous formation of vacancies with low-energy argon plasma and the thermal activation of adatom diffusion by laser irradiation, silicon impurities are incorporated into the lattice of both materials. After an exposure of ∼1 ion/nm², we find Si-substitution densities of 0.15 nm^-2 in graphene and 0.05 nm^-2 in nanotubes, as revealed by atomically resolved scanning transmission electron microscopy. In good agreement with predictions of Ar irradiation effects in SWCNTs, we find Si incorporated in both mono- and divacancies, with ∼2/3 being of the first type. Controlled inclusion of impurities in the quasi-1D and -2D carbon lattices may prove useful for applications such as gas sensing, and a similar approach might also be used to substitute other elements with migration barriers lower than that of carbon.

An overview of the recent advances in inorganic nanotubes

Advanced nanomaterials play a prominent role in nanoscience and nanotechnology developments, opening new frontiers in these areas. Among these nanomaterials, due to their unique characteristics and enhanced chemical and physical properties, inorganic nanotubes have been considered one of the most interesting nanostructures. In recent years, important progress has been achieved in the production and study of these nanomaterials, including boron nitride, transition metal dichalcogenide nanotubular structures, misfit-based nanotubes and other hybrid/doped nanotubular objects. This review is devoted to the in-depth analysis of recent studies on the synthesis, atomic structures, properties and applications of inorganic nanotubes and related nanostructures. Particular attention is paid to the growth mechanism of these nanomaterials. This is a crucial point for the challenges ahead related to the mass production of high-quality defect-free nanotubes for a variety of applications.

Highly Optimized Nitrogen-Doped MWCNTs through In-Depth Parametric Study Using Design of Experiments

The in-situ nitrogen doping of multiwalled carbon nanotubes via chemical vapor deposition is investigated employing design of experiments (DoE). The establishment of empirical DoE models allowed for the prediction of product features as a function of process conditions in order to systematically synthesize tailor-made nitrogen-doped carbon nanotubes. The high informative content of this approach revealed effects of individual parameters and their interaction with each other. Hence, new valuable insights into the effect of temperature, injection rate, and carrier gas flow on the doping level were obtained which give motivation to approach further theoretical studies on the doping mechanism. Ultimately, competitive nitrogen-doped carbon nanotube features were optimized and yielded promising combinations of achieved doping level, graphitization, and aspect ratios in comparison to present literature values.

Oxygen Evolution Reaction on Nitrogen-Doped Defective Carbon Nanotubes and Graphene

The realization of a hydrogen economy would be facilitated by the discovery of a water-splitting electrocatalyst that is efficient, stable under operating conditions, and composed of earth-abundant elements. Density functional theory simulations within a simple thermodynamic model of the more difficult half-reaction, the anodic oxygen evolution reaction (OER), with a single-walled carbon nanotube as a model catalyst, show that the presence of 0.3-1% nitrogen reduces the required OER overpotential significantly compared to the pristine nanotube. We performed an extensive exploration of systems and active sites with various nitrogen functionalities (graphitic, pyridinic, or pyrrolic) obtained by introducing nitrogen and simple lattice defects (atomic substitutions, vacancies, or Stone-Wales rotations). A number of nitrogen functionalities (graphitic, oxidized pyridinic, and Stone-Wales pyrrolic nitrogen systems) yielded similar low overpotentials near the top of the OER volcano predicted by the scaling relation, which was seen to be closely observed by these systems. The OER mechanism considered was the four-step single-site water nucleophilic attack mechanism. In the active systems, the second or third step, the formation of attached oxo or peroxo moieties, was the potential-determining step of the reaction. The nanotube radius and chirality effects were examined by considering OER in the limit of large radius by studying the analogous graphene-based model systems. They exhibited trends similar to those of the nanotube-based systems but often with reduced reactivity due to weaker attachment of the OER intermediate moieties.

Nitrogen Codoped Unique Carbon with 0.4 nm Ultra-Micropores for Ultrahigh Areal Capacitance Supercapacitors

A full understanding of ion transport in porous carbon electrodes is essential for achieving effective energy storage in their applications as electrochemical supercapacitors. It is generally accepted that pores in the size range below 0.5 nm are inaccessible to electrolyte ions and lower the capacitance of carbon materials. Here, nitrogen-doped carbon with ultra-micropores smaller than 0.4 nm with a narrow size distribution, which represents the first example of electrode materials made entirely from ultra-microporous carbon, is prepared. An in situ electrochemical quartz crystal microbalance technique to study the effects of the ultra-micropores on charge storage in supercapacitors is used. It is found that ultra-micropores smaller than 0.4 nm are accessible to small electrolyte ions, and the area capacitance of obtained sample reaches the ultrahigh value of 330 µF cm^-2 , significantly higher than that of previously reported carbon-based materials. The findings provide a better understanding of the correlation between ultra-micropore structure and capacitance and open new avenues for design and development of carbon materials for the next generation of high energy density supercapacitors.

Feedstock-dependent nitrogen configurations of nitrogen-doped single-walled carbon nanotubes in a CVD process

The modification of nitrogen configurations is a viable way to control the electronic properties of nitrogen-doped single-walled carbon nanotubes (N-doped SWCNTs). N-doped SWCNTs were synthesized by a conventional chemical vapor deposition process with a mixed carbon/nitrogen (C/N) feedstock. While higher feedstock flow rates promote the formation of encapsulated N2 molecules, lower flow rates show a predominance of pyridinic and graphitic nitrogen structures as revealed by X-ray photoemission spectroscopy. Therefore, the nitrogen doping in the sp2 carbon network can be controlled by the flow rate of the C/N feedstock.

Breaking the Nd3+-sensitized upconversion nanoparticles myth about the need of onion-layered structures

Up to now, most strategies to build efficient 800 nm-light responsive upconversion nanoparticles (UCNPs) have included onion-layered structures, in which Nd3+ is confined within the inorganic crystal structure of at least one layer. We report here an easy room-temperature modular preparation of core-shell UCNPs consisting of NaYF4:Yb,Er(Tm)/NaYF4 (UCCS) with Nd3+ anchored at the organic capping by using cucurbituril[7] (CB[7]) as an adhesive. Strikingly, excitation at 800 nm effectively triggers the upconversion emission of UCCS@CB[7]@Nd nanohybrids.

Identifying the Key Role of Pyridinic-N-Co Bonding in Synergistic Electrocatalysis for Reversible ORR/OER

For many regenerative electrochemical energy-conversion systems, hybrid electrocatalysts comprising transition metal (TM) oxides and heteroatom-doped (e.g., nitrogen-doped) carbonaceous materials are promising bifunctional oxygen reduction reaction/oxygen evolution reaction electrocatalysts, whose enhanced electrocatalytic activities are attributed to the synergistic effect originated from the TM-N-C active sites. However, it is still ambiguous which configuration of nitrogen dopants, either pyridinic or pyrrolic N, when bonded to the TM in oxides, predominately contributes to the synergistic effect. Herein, an innovative strategy based on laser irradiation is described to controllably tune the relative concentrations of pyridinic and pyrrolic nitrogen dopants in the hybrid catalyst, i.e., NiCo₂ O₄ NPs/N-doped mesoporous graphene. Comparative studies reveal the dominant role of pyridinic-NCo bonding, instead of pyrrolic-N bonding, in synergistically promoting reversible oxygen electrocatalysis. Moreover, density functional theory calculations provide deep insights into the corresponding synergistic mechanism. The optimized hybrid, NiCo/NLG-270, manifests outstanding reversible oxygen electrocatalytic activities, leading to an overpotential different ΔE among the lowest value for highly efficient bifunctional catalysts. In a practical reversible Zn-air battery, NiCo/NLG-270 exhibits superior charge/discharge performance and long-term durability compared to the noble metal electrocatalysts.

Spectroscopic observation of oxygen dissociation on nitrogen-doped graphene

Carbon nanomaterials’ reactivity towards oxygen is very poor, limiting their potential applications. However, nitrogen doping is an established way to introduce active sites that facilitate interaction with gases. This boosts the materials’ reactivity for bio-/gas sensing and enhances their catalytic performance for the oxygen reduction reaction. Despite this interest, the role of differently bonded nitrogen dopants in the interaction with oxygen is obscured by experimental challenges and has so far resisted clear conclusions. We study the interaction of molecular oxygen with graphene doped via nitrogen plasma by in situ high-resolution synchrotron techniques, supported by density functional theory core level simulations. The interaction leads to oxygen dissociation and the formation of carbon-oxygen single bonds on graphene, along with a band gap opening and a rounding of the Dirac cone. The change of the N 1 s core level signal indicates that graphitic nitrogen is involved in the observed mechanism: the adsorbed oxygen molecule is dissociated and the two O atoms chemisorb with epoxy bonds to the nearest carbon neighbours of the graphitic nitrogen. Our findings help resolve existing controversies and offer compelling new evidence of the ORR pathway.

Robust theoretical modelling of core ionisation edges for quantitative electron energy loss spectroscopy of B- and N-doped graphene

Electron energy loss spectroscopy (EELS) is a powerful tool for understanding the chemical structure of materials down to the atomic level, but challenges remain in accurately and quantitatively modelling the response. We compare comprehensive theoretical density functional theory (DFT) calculations of 1s core-level EEL K-edge spectra of pure, B-doped and N-doped graphene with and without a core-hole to previously published atomic-resolution experimental electron microscopy data. The ground state approximation is found in this specific system to perform consistently better than the frozen core-hole approximation. The impact of including or excluding a core-hole on the resultant theoretical band structures, densities of states, electron densities and EEL spectra were all thoroughly examined and compared. It is concluded that the frozen core-hole approximation exaggerates the effects of the core-hole in graphene and should be discarded in favour of the ground state approximation. These results are interpreted as an indicator of the overriding need for theorists to embrace many-body effects in the pursuit of accuracy in theoretical spectroscopy instead of a system-tailored approach whose approximations are selected empirically.

Incorporating Pyrrolic and Pyridinic Nitrogen into a Porous Carbon made from C60 Molecules to Obtain Superior Energy Storage

Nitrogen-doped porous carbon is obtained by KOH activation of C₆₀ in an ammonia atmosphere. As an anode for Li-ion batteries, it shows a reversible capacity of up to ≈1900 mA h g^-1 at 100 mA g^-1 . Simulations suggest that the superior Li-ion storage may be related to the curvature of the graphenes and the presence of pyrrolic/pyridinic group dopants.

Spectromicroscopy of C60 and azafullerene C59N: Identifying surface adsorbed water

C60 fullerene crystals may serve as important catalysts for interstellar organic chemistry. To explore this possibility, the electronic structures of free-standing powders of C60 and (C59N)2 azafullerenes are characterized using X-ray microscopy with near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy, closely coupled with density functional theory (DFT) calculations. This is supported with X-ray photoelectron spectroscopy (XPS) measurements and associated core-level shift DFT calculations. We compare the oxygen 1s spectra from oxygen impurities in C60 and C59N, and calculate a range of possible oxidized and hydroxylated structures and associated formation barriers. These results allow us to propose a model for the oxygen present in these samples, notably the importance of water surface adsorption and possible ice formation. Water adsorption on C60 crystal surfaces may prove important for astrobiological studies of interstellar amino acid formation.

Simultaneous atomic-resolution electron ptychography and Z-contrast imaging of light and heavy elements in complex nanostructures

The aberration-corrected scanning transmission electron microscope (STEM) has emerged as a key tool for atomic resolution characterization of materials, allowing the use of imaging modes such as Z-contrast and spectroscopic mapping. The STEM has not been regarded as optimal for the phase-contrast imaging necessary for efficient imaging of light materials. Here, recent developments in fast electron detectors and data processing capability is shown to enable electron ptychography, to extend the capability of the STEM by allowing quantitative phase images to be formed simultaneously with incoherent signals. We demonstrate this capability as a practical tool for imaging complex structures containing light and heavy elements, and use it to solve the structure of a beam-sensitive carbon nanostructure. The contrast of the phase image contrast is maximized through the post-acquisition correction of lens aberrations. The compensation of defocus aberrations is also used for the measurement of three-dimensional sample information through post-acquisition optical sectioning.

The use of ptychography with electrons has been limited. Here, Yang et al. demonstrate that the combination of Z-contrast and phase imaging reveals the structure of complex nanomaterials. This practical tool can be used to solve the structure of a beam-sensitive carbon nanostructure at atomic-resolution.

3D Visualization of the Iron Oxidation State in FeO/Fe3O4 Core-Shell Nanocubes from Electron Energy Loss Tomography

The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe(2+) and Fe(3+) ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.

Topological Defects in Metal-Free Nanocarbon for Oxygen Electrocatalysis

A bifunctional graphene catalyst with abundant topological defects is achieved via the carbonization of natural gelatinized sticky rice to probe the underlying oxygen electrocatalytic mechanism. A nitrogen-free configuration with adjacent pentagon and heptagon carbon rings is revealed to exhibit the lowest overpotential for both oxygen reduction and evolution catalysis. The versatile synthetic strategy and novel insights on the activity origin facilitate the development of advanced metal-free carbocatalysts for a wide range of electrocatalytic applications.

A Complete Overhaul of the Electron Energy-Loss Spectroscopy and X-Ray Absorption Spectroscopy Database: eelsdb.eu

The electron energy-loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) database has been completely rewritten, with an improved design, user interface, and a number of new tools. The database is accessible at https://eelsdb.eu/ and can now be used without registration. The submission process has been streamlined to encourage spectrum submissions and the new design gives greater emphasis on contributors' original work by highlighting their papers. With numerous new filters and a powerful search function, it is now simple to explore the database of several hundred EELS and XAS spectra. Interactive plots allow spectra to be overlaid, facilitating online comparison. An application-programming interface has been created, allowing external tools and software to easily access the information held within the database. In addition to the database itself, users can post and manage job adverts and read the latest news and events regarding the EELS and XAS communities. In accordance with the ongoing drive toward open access data increasingly demanded by funding bodies, the database will facilitate open access data sharing of EELS and XAS spectra.

Identifying active sites of CoNC/CNT from pyrolysis of molecularly defined complexes for oxidative esterification and hydrogenation reactions

Cobalt chelate complexes bonded to 2 to 3 nitrogens in the graphene lattice are the active sites for the oxidative esterification of benzyl alcohol by molecular oxygen, the selective reduction of nitrobenzene by hydrogen and the hydrogenated coupling of nitrobenzene and benzaldehyde.

Electronic Structure Modification of Ion Implanted Graphene: The Spectroscopic Signatures of p- and n-Type Doping

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations is used to describe the electronic structure modifications incurred by free-standing graphene through two types of single-atom doping. The N K and C K electron energy loss transitions show the presence of π* bonding states, which are highly localized around the N dopant. In contrast, the B K transition of a single B dopant atom shows an unusual broad asymmetric peak which is the result of delocalized π* states away from the B dopant. The asymmetry of the B K toward higher energies is attributed to highly localized σ* antibonding states. These experimental observations are then interpreted as direct fingerprints of the expected p- and n-type behavior of graphene doped in this fashion, through careful comparison with density functional theory calculations.

Structural and Chemical Dynamics of Pyridinic-Nitrogen Defects in Graphene

High density and controllable nitrogen doping in graphene is a critical issue to realize high performance graphene-based devices. In this paper, we demonstrate an efficient method to selectively produce graphitic-N and pyridinic-N defects in graphene by using the mixture plasma of ozone and nitrogen. The atomic structure, electronic structure, and dynamic behavior of these nitrogen defects are systematically studied at the atomic level by using a scanning transmission electron microscopy. The pyridinic-N exhibits higher chemical activity and tends to trap a series of transition metal atoms (Mg, Al, Ca, Ti, Cr, Mn, and Fe) as individual atoms.

Possibilities and limitations of advanced transmission electron microscopy for carbon-based nanomaterials

A major revolution for electron microscopy in the past decade is the introduction of aberration correction, which enables one to increase both the spatial resolution and the energy resolution to the optical limit. Aberration correction has contributed significantly to the imaging at low operating voltages. This is crucial for carbon-based nanomaterials which are sensitive to electron irradiation. The research of carbon nanomaterials and nanohybrids, in particular the fundamental understanding of defects and interfaces, can now be carried out in unprecedented detail by aberration-corrected transmission electron microscopy (AC-TEM). This review discusses new possibilities and limits of AC-TEM at low voltage, including the structural imaging at atomic resolution, in three dimensions and spectroscopic investigation of chemistry and bonding. In situ TEM of carbon-based nanomaterials is discussed and illustrated through recent reports with particular emphasis on the underlying physics of interactions between electrons and carbon atoms.

SERS Detection of Amyloid Oligomers on Metallorganic-Decorated Plasmonic Beads

Protein misfolded proteins are among the most toxic endogenous species of macromolecules. These chemical entities are responsible for neurodegenerative disorders such as Alzheimer's, Parkinson's, Creutzfeldt-Jakob's and different non-neurophatic amyloidosis. Notably, these oligomers show a combination of marked heterogeneity and low abundance in body fluids, which have prevented a reliable detection by immunological methods so far. Herein we exploit the selectivity of proteins to react with metallic ions and the sensitivity of surface-enhanced Raman spectroscopy (SERS) toward small electronic changes in coordination compounds to design and engineer a reliable optical sensor for protein misfolded oligomers. Our strategy relies on the functionalization of Au nanoparticle-decorated polystyrene beads with an effective metallorganic Raman chemoreceptor, composed by Al(3+) ions coordinated to 4-mercaptobenzoic acid (MBA) with high Raman cross-section, that selectively binds aberrant protein oligomers. The mechanical deformations of the MBA phenyl ring upon complexation with the oligomeric species are registered in its SERS spectrum and can be quantitatively correlated with the concentration of the target biomolecule. The SERS platform used here appears promising for future implementation of diagnostic tools of aberrant species associated with protein deposition diseases, including those with a strong social and economic impact, such as Alzheimer's and Parkinson's diseases.

Nitrogen segregation in nanocarbons

We explore the behaviour of nitrogen doping in carbon nanomaterials, notably graphene, nanotubes, and carbon thin films. This is initially via a brief review of the literature, followed by a series of atomistic density functional calculations. We show that at low concentrations, substitutional nitrogen doping in the sp(2)-C graphenic basal plane is favoured, however once the nitrogen concentration reaches a critical threshold there is a transition towards the formation of the more thermodynamically-favoured nitrogen terminated 'zigzag' type edges. These can occur either via formation of finite patches (polycyclic aromatic azacarbons), strips of sp(2) carbon with zigzag nitrogen edges, or internal nitrogen-terminated hole edges within graphenic planes. This transition to edge formation is especially favoured when the nitrogen can be partially functionalised with, e.g. hydrogen. By comparison with available literature results, notably from electron energy loss spectroscopy and X-ray spectroscopy, the current results suggest that much of the nitrogen believed to be incorporated into carbon nanoobjects is instead likely to be present terminating the edges of carbonaceous impurities attached to nanoobject's surface. By comparison to nitrogen-doped tetrahedrally amorphous carbon, we suggest that this transition at around 10-20% nitrogen concentration and above towards sp(2) coordination via internal nitrogen-terminated edge formation may be a general property of nitrogen-doped carbon materials.

X-ray photoelectron spectroscopy of graphitic carbon nanomaterials doped with heteroatoms

X-ray photoelectron spectroscopy (XPS) is one of the best tools for studying the chemical modification of surfaces, and in particular the distribution and bonding of heteroatom dopants in carbon nanomaterials such as graphene and carbon nanotubes. Although these materials have superb intrinsic properties, these often need to be modified in a controlled way for specific applications. Towards this aim, the most studied dopants are neighbors to carbon in the periodic table, nitrogen and boron, with phosphorus starting to emerge as an interesting new alternative. Hundreds of studies have used XPS for analyzing the concentration and bonding of dopants in various materials. Although the majority of works has concentrated on nitrogen, important work is still ongoing to identify its precise atomic bonding configurations. In general, care should be taken in the preparation of a suitable sample, consideration of the intrinsic photoemission response of the material in question, and the appropriate spectral analysis. If this is not the case, incorrect conclusions can easily be drawn, especially in the assignment of measured binding energies into specific atomic configurations. Starting from the characteristics of pristine materials, this review provides a practical guide for interpreting X-ray photoelectron spectra of doped graphitic carbon nanomaterials, and a reference for their binding energies that are vital for compositional analysis via XPS.

Charge transfer, bonding conditioning and solvation effect in the activation of the oxygen reduction reaction on unclustered graphitic-nitrogen-doped graphene

Under certain conditions and on specific sites the monodentate associative chemisorption of molecular oxygen on graphitic-nitrogen-doped graphene would be favorable.

Stability and spectroscopy of single nitrogen dopants in graphene at elevated temperatures

Single nitrogen (N) dopants in graphene are investigated using atomic-resolution scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). Using an in situ heating holder at 500 °C provided us with clean graphene surfaces, and we demonstrate that isolated N substitutional atoms remain localized and stable in the graphene lattice even during local sp(2) bond reconstruction. The high stability of isolated N dopants enabled us to acquire 2D EELS maps with simultaneous ADF-STEM images to map out the local bonding variations. We show that a substitutional N dopant causes changes in the EELS of the carbon (C) atoms it is directly bonded to. An upshift in the π* peak of the C K-edge EELS of ∼0.5 eV is resolved and supported by density functional theory simulations.