Formation Of Carbon Nanostructures Research Articles

Invited: Non-Precious Metal Electrocatalysts: Accomplishments and Challenges

With increasing demand for energy, the development of the energy storage and conversion technologies has become the focus of an intensive research effort in recent years. Among various technologies, fuel cells, batteries, supercapacitors, and water electrolyzers have been recognized as potentially feasible and efficient devices for portable, stationary, and transportation applications. For both the fuel cells and metal-air batteries, the oxygen reduction reaction (ORR) cathode catalysts play a major role in the device performance characteristics, as determined based on the power output, open circuit voltage, charge-discharge rate, energy efficiency, cycling life (for batteries), etc. Currently, Pt-nanoparticle catalysts, supported on high surface-area carbons, represent the state-of-the-art electrocatalysts for hydrogen oxidation reaction (HOR) and ORR at the polymer electrolyte fuel cell (PEFC) anode and cathode, respectively. However, the prohibitive price and scarcity of Pt have limited its widespread implementation in the PEFC, especially at the cathode, which accounts for approximately 80% of the Pt loading in fuel cells. As a result, non-precious metal catalysts (NPMCs) for the ORR have received more attention in recent years as a possible replacement of precious-metal catalysts.A successful ORR catalyst must combine high activity with good long-term stability – a major challenge in the strongly acidic environment of the PEFC cathode. Since the early work of Jasinski in the 1960s (Nature 201, 1212, 1964), recent breakthroughs in the synthesis of high-performance non-precious metal catalysts (NPMCs) (e.g., Lefèvre et al., Science 324, 71, 2009; Wu et al., Science 332, 443, 2011) make replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility, though the activity and especially durability of the resulting catalysts need to be improved before the technology can become viable.The NPMC performance depends on the selection of precursors, the synthesis chemistry; and especially the catalyst nanostructure. In addition to those, the CNx structures likely play a major role in the performance of ORR active site(s). Apart from possible direct participation in the active site, the transition metal is crucial to in-situ formation of carbon nanostructures (nanotubes, onion-like structures, graphene) by catalyzing the decomposition of the nitrogen/carbon precursor(s) at a high temperatures (800-1000°C). The formation of different carbon and nitrogen-doped carbon nanostructures can be controlled during the synthesis of such NPMCs. The highly-graphitized carbon nanostructures likely serve as a matrix for the formation of the ORR-active groups with improved catalytic activity and durability, containing nitrogen and possibly also metal atoms.Future NPMC synthesis approaches are certain to focus on the precise control of interactions between precursors of the metal and carbon/nitrogen during the heat treatment, with the main purpose being the maximizations of the population of active sites, optimization of nitrogen doping levels, and generation of carbon morphologies capable of hosting active and stable ORR sites. This is evident not only in catalysts developed for PEFCs but also in materials specifically designed for alkaline fuel cells (Chung et al., Nat. Commun. 4:1922 doi:10.1038/ncomms2944, 2013). In the end, however, the much needed progress in ORR electrocatalysis at NPMCs, especially in acid media, will be decided by better understanding of the origin of the NPMC activity and the nature of the active site. That key part of NPMC development will be addressed in this presentation along with the summary of the progress achieved to date and challenges still awaiting non-precious metal electrocatalysis in polymer electrolyte fuel cells. Acknowledgment Financial support from DOE-EERE Fuel Cell Technologies Office and Los Alamos National Laboratory Laboratory-Directed Research and Development (LDRD) Program and is gratefully acknowledged.

Nitrogen-Doped Nanocarbon Supported Pt Electrocatalysts: Oxygen Reduction and Methanol Oxidation

Currently, Pt remains to be the most effective catalysts to catalyze oxygen reduction and methanol oxidation in both acidic and alkaline solution for energy conversion through these direct electrochemical reactions in fuel cell systems. In order to significantly increase the Pt utilization in the catalysts, Pt nanoparticles (NPs) are supported on high-surface-area carbon blacks. However, a fast and significant loss of electrochemically active surface area (ECSA) and thus performance degradation are often observed with the traditional Pt/C catalysts over time. Such phenomena can be mainly ascribed to the corrosion of carbon supports, Ostwald ripening and agglomeration of Pt NPs, and the resulting loss of collective electrocatalytic sites.1 Thus, advanced carbon materials with ordered graphitic structure including carbon nanotubes (CNTs),2,3 carbon nanofibers (CNFs), onion-like carbon,4,5 and graphene6 have been extensively studied as supports in the Pt-based electrocatalysts due to their unique surface structure, high electric conductivity, corrosion resistance and large surface areas. Noteworthy, there has been increasing evidence showing that the electrochemical and physical properties of carbon materials are extremely sensitive to heteroatom (N, B, S and P) doping into the carbon structure.4,5,7 In particular, nitrogen can dope into carbon planes and change their electronic and structural properties. Theoretical studies have shown that nitrogen atoms can be viewed as an n-type carbon dopant that disorders carbon lattices and donates electrons to carbon. Those resulting defects from nitrogen doping may serve as active sites for subsquent Pt doposition or oxygen adsorption, thus strengthening interactions between supports and Pt nanoparticles, and facilitaiting the ORR, respectivley.7 Meanwhile, during the development of non-precious metal catalysts, carbon-based ORR-active M-N-C catalysts (M: Fe or Co),8-11 containing in-situ formation of various carbon nanostructures with nitrogen doping, can be realized by catalyzing the decomposition of the nitrogen/carbon precursor at high temperatures. Importantly, we can control the formation of different nanostructures (e.g., CNT, onion-like carbon, graphene) during the catalyst synthesis through optimizing the transition metals, nitrogen/carbon precursors, and templates.Here, in order to combine Pt and non-precious metal catalysts with an aim of dramatically reducing the amount of Pt and increasing the overall catalytic activity, we are conceiving to develop a hybrid cathode catalyst consisting of Pt nanoparticles and the ORR-active N-doped nanocarbon as supporting materials (Fig. 1). The ORR-active carbon supports with nitrogen doping would not only offer a remarkable support effect by geometrically and electronically modifying the loaded Pt particles, but also provide a large amount of performing non-precious ORR active sites, thereby significantly improving ORR activity and durability of Pt catalysts. In addition, the promotional role of nitrogen doping for carbon supported Pt catalysts for methanol oxidation also will be discussed in this presentation. Fig. 1. Schematic illustration of the formation of bamboo-like nitrogen-doped graphene tubes (N-GT) and Pt/N-GT nanostructures References R. Borup, et al., Chem. Rev. 2007, 107, 3904-3951. G. Wu, Y.-S. Chen,B.-Q. Xu, Electrochem. Commun. 2005, 7, 1237-1243.G. Wu,B.Q. Xu, J Power Sources 2007, 174, 148-158..G. Wu, C.S. Dai, D.L. Wang, D.Y. Li,N. Li, J Mater Chem 2010, 20, 3059-3068.G. Wu, D. Li, C. Dai, D. Wang,N. Li, Langmuir 2008, 24, 3566-3575.C.Z. Zhu,S.J. Dong, Nanoscale 2013, 5, 1753-1767.G. Wu, R. Swaidan, D. Li,N. Li, Electrochim. Acta 2008, 53, 7622-7629. Q. Li, P. Xu, W. Gao, S. Ma, G. Zhang, R. Cao, J. Cho, H.-L. Wang, G. Wu, Adv. Mater. 2014, 26, 1378. G. Wu,P. Zelenay, Acc. Chem. Res. 2013, 46, 1878-1889.G. Wu, K. L. More, C. M. Johnston, P. Zelenay, Science 2011, 332, 443.Q. Li, R. Cao, J. Cho, G. Wu, Adv. Energy Mater.,2014, doi:10.1002/aenm.201301415.

Simultaneous in situ Raman monitoring of the solid and gas phases during the formation and growth of carbon nanostructures inside a cold wall CCVD reactor

Abstract This manuscript reports for the first time, to the best of the authors’ knowledge, the in situ and non-invasive characterization of both solid and gaseous phases involved in a catalytic chemical vapor deposition (CCVD) process applied with the aim to grow carbon nanotubes (CNTs). The here reported characterization comprises the simultaneous in situ analysis of carbon nanostructure formation and growth, the deactivation of the catalyst as well as the in situ monitoring of the carbon source conversion and the accompanying thermal effects in the gas phase. Raman spectroscopy is used as in situ measurement technique and after the CCVD process, as ex situ measurement tool aiming to confirm the in situ measurements performed at harsh conditions inside the reactor (temperatures up to 1000 K). The in situ Raman results have also been compared with post-CCVD SEM and TEM analysis, from which the ambiguous meaning of the D/G-ratio was extracted.

Effect of nitriding/nanostructuration of few layer graphene supported iron-based particles; catalyst in graphene etching and carbon nanofilament growth.

Stable, highly faceted and dispersed iron nitride particles supported on few layer graphene are obtained by ammonia decomposition on iron-based particles at the temperature commonly used for the synthesis of N-doped CNTs and graphene etching. The TEM/EELS analysis reveals nitrogen diffusion in a bulk of the particles. The resulting facet FeNx catalyst exhibits high activity in the etching of graphene, which is assisted by catalyst reorganization. Ammonia decomposition is used for the first time for graphene etching, while the highly faceted catalyst has an impact on the etched channels structures. According to the shape of the active planes of the catalyst, the etching results in sharp "V" channel ends and often "step-like" edges. The FeNx morphology proves previously reported triangularisation of arches in highly N-doped carbon nanotubes. The conditioning of the catalyst by its shaping and nitrogen incorporation is investigated additionally in the carbon nanostructure formation, for decomposition of ethane. The herringbone CNFs, "hollow" bamboo-like CNFs/CNTs or CNTs are effectively observed.

Nanostructure Formation during Amorphous Carbon Films Deposition

In uence of Cu particles for the carbon nanostructures formation during a-C:H lms deposition by plasma enhanced chemical vapor deposition method from pure acetylene gas plasma were analyzed in this work. Silicon wafer and Cu target were simultaneously bombarded by Ar ions for the Cu particles deposition on the silicon before a-C:H lms formation. It was obtained that hydrogenated silicon carbide forms on this defected Si/Cu surface during the rst stage of carbon lm deposition. Structure of a:C-H lms and conditions of nanostructures formation depended on substrate temperature and Cu concentration in the lm, then deposition time was 300 s.

Formation of Carbon Nanostructures During Chemical Demineralization of Indian Coals

Formation of carbon nanostructures of diameters less than 50 nm with an aspect ratio of 100 is achieved during the chemical demineralization of Indian coal. Demineralization was carried out using an aqueous solution of sodium hydroxide at 90°C for 5 hours. Structural analysis using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that removal of the inorganic layer, anisotropically embedded in the carbon layers of the coal, had a strong influence on nanostructure formation. The concentration of alkali and the ratio of solid (coal)-to-liquid (alkali reagent) crucially determine the extent of demineralization and thereby affect the nanostructure formation.

Suspended Carbon Nanowire-Based Structures for Sensor Platforms

In this report, fabrication of monolithic suspended carbon nanowire-based structures including single nanowire and nanomeshes, and their applications as electrochemical/gas/bio-sensors are introduced. The suspended carbon structures are fabricated using the carbon-MEMS process consisting of conventional UV-lithography and pyrolysis which accompanies significant volume shrinkage resulting in the formation of complex suspended carbon nanostructures. The suspended carbon nanowires shows good robustness enough to be processed with photolithography and deposition; this enables patterning of functional materials in sub-micromenter scale without any nanofabrication technologies. The single suspended carbon nanowires of 200 nm diameter can be selectively coated with a thin palladium or ZnO nanowires which showed good gas sensing capability down to 20 ppm hydrogen gas and 50 ppm CO, NOx, H2 gases respectively. Combined with a substrate-bound carbon band electrode, the suspended carbon mesh showed high efficiency of redox cycling enabling highly sensitive sensing of dopamine and glucose.

Hydrogenation-Assisted Graphene Origami and Its Application in Programmable Molecular Mass Uptake, Storage, and Release

The malleable nature of atomically thin graphene makes it a potential candidate material for nanoscale origami, a promising bottom-up nanomanufacturing approach to fabricating nanobuilding blocks of desirable shapes. The success of graphene origami hinges upon precise and facile control of graphene morphology, which still remains as a significant challenge. Inspired by recent progresses on functionalization and patterning of graphene, we demonstrate hydrogenation-assisted graphene origami (HAGO), a feasible and robust approach to enabling the formation of unconventional carbon nanostructures, through systematic molecular dynamics simulations. A unique and desirable feature of HAGO-enabled nanostructures is the programmable tunability of their morphology via an external electric field. In particular, we demonstrate reversible opening and closing of a HAGO-enabled graphene nanocage, a mechanism that is crucial to achieve molecular mass uptake, storage, and release. HAGO holds promise to enable an array of carbon nanostructures of desirable functionalities by design. As an example, we demonstrate HAGO-enabled high-density hydrogen storage with a weighted percentage exceeding the ultimate goal of US Department of Energy.

Effect of Milling Type on Formation of Carbon Nanostructures

In this study, graphite powder was exposed to two different stress types by using the same type of mill in order to understand the formation of different carbon nanostructures. Two types of milling modes were selected as shock-type and shear-type stresses during the milling processes. Both shock-type and shear-type stresses were applied to the graphite powder. Then, milled powders were annealed at high temperatures for different times. The structural changes in the obtained samples were determined by XRD diffraction, SEM, FE-SEM and HR-TEM examinations. New carbon nanostructures were synthesised.

The mechanism of transforming diamond nanowires to carbon nanostructures

The transformation of diamond nanowires (DNWs) with different diameters and geometries upon heating is investigated with density-functional-based tight-binding molecular dynamics. DNWs of 〈100〉 and 〈111〉 oriented cross-section with projected average line density between 7 and 20 atoms Å−1 transform into carbon nanotubes (CNTs) under gradual heating up to 3500–4000 K. DNWs with projected average line density larger than 25 atoms Å−1 transform into double-wall CNTs. The route of transformation into CNTs clearly exhibits three stages, with the intriguing intermediate structural motif of a carbon nanoscroll (CNS). Moreover, the morphology plays an important role in the transformation involving the CNS as one important intermediate motif to form CNTs. When starting with oriented DNWs with a square cross-section consisting of two {111} facets facing each other, one interesting structure with ‘nano-bookshelf’ shape emerges: a number of graphene ‘shelves’ located inside the CNT, bonding to the CNT walls with sp3 hybridized atoms. The nano-bookshelf structures exist in a wide range of temperatures up to 3000 K. The further transformation from nano-bookshelf structures depends on the strength of the joints connecting shelves with CNT walls. Notably, the nano-bookshelf structure can evolve into two end products: one is CNT via the CNS pathway, the other is graphene transformed directly from the nano-bookshelf structure at high temperature. This work sheds light on the microscopic insight of carbon nanostructure formation mechanisms with the featured motifs highlighted in the pathways.

Specific features of the structure and properties of carbon nanocolumns formed by low-temperature chemical vapor deposition

The process of the formation of column-shaped carbon nanostructures by plasma-enhanced chemical vapor deposition is explored. Carbon nanocolumns are formed at 250°C. The structure and properties of the structures are studied by atomic-force microscopy, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The electrical properties of the structures are investigated.

Spontaneous synthesis of carbon nanowalls, nanotubes and nanotips using high flux density plasmas

We have investigated the formation of various carbon nanostructures using extreme plasma fluxes up to four orders of magnitude larger than in conventional plasma-enhanced chemical vapor deposition processing. Carbon nanowalls, multi-wall nanotubes, spherical nanoparticles and nanotips are among the structures detected with electron microscopy methods. Precursor injection or surface pretreatment were not required for the synthesis of the nanostructures. Preliminary experiments with varied plasma composition, sample bias and surface temperature have demonstrated the potential for optimizing the growth of the nanostructures in the current experimental set-up.

Growth of one-dimensional vertically aligned carbon nanostructures on SiC—Catalyst effect

Application of catalyst on SiC wafer resulted in the formation of one-dimensional (1D) vertically aligned carbon nanostructures at low temperature and vacuum values (compared to SiC decomposition) without extra carbon supply into the system. Resulting nanostructures were characterized using scanning electron microscopy and Raman spectroscopy. The effect of catalyst amount is discussed and a mechanism for the 1D carbon nanostructure formation through SiC decomposition in the presence of catalyst is proposed.

Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction

Platinum-based catalysts represent a state of the art in the electrocatalysis of oxygen reduction reaction (ORR) from the point of view of their activity and durability in harnessing the chemical energy via direct electrochemical conversion. However, because platinum is both expensive and scarce, its widespread implementation in such clean energy applications is limited. Recent breakthroughs in the synthesis of high-performance nonprecious metal catalysts (NPMCs) make replacement of Pt in ORR electrocatalysts with earth-abundant elements, such as Fe, Co, N, and C, a realistic possibility. In this Account, we discuss how we can obtain highly promising M-N-C (M: Fe and/or Co) catalysts by simultaneously heat-treating precursors of nitrogen, carbon, and transition metals at 800-1000 °C. The activity and durability of resulting catalysts depend greatly on the selection of precursors and synthesis chemistry. In addition, they correlate quite well with the catalyst nanostructure. While chemists have presented no conclusive description of the active catalytic site for this class of NPMCs, they have developed a designed approach to making active and durable materials, focusing on the catalyst nanostructure. The approach consists of nitrogen doping, in situ carbon graphitization, and the usage of graphitic structures (possibly graphene and graphene oxides) as carbon precursors. Various forms of nitrogen, particularly pyridinic and quaternary, can act as n-type carbon dopants in the M-N-C catalysts, assisting in the formation of disordered carbon nanostructures and donating electrons to the carbon. The CNx structures are likely a crucial part of the ORR active site(s). Noteworthy, the ORR activity is not necessarily governed by the amount of nitrogen, but by how the nitrogen is incorporated into the nanostructures. Apart from the possibility of a direct participation in the active site, the transition metal often plays an important role in the in situ formation of various carbon nanostructures by catalyzing the decomposition of the nitrogen/carbon precursor. We can control the formation of different nanostructures during the synthesis of M-N-C catalysts. For example, in situ formed nitrogen-doped graphene-sheets can only be derived from polyaniline (PANI), probably due to structural similarities between the aromatic structures of PANI and graphene. Highly-graphitized carbon nanostructures may serve as a matrix for the formation of ORR-active groups with improved catalytic activity and durability, containing nitrogen and most probably also metal atoms. In the future, we will likely focus NPMC synthesis approaches on precise control of interactions between precursors of the metal and carbon/nitrogen during the heat treatment. The main purposes will be to maximize the number of active sites, optimize nitrogen doping levels, and generate morphologies capable of hosting active and stable ORR sites.

On the effect of sulphur on the formation of carbon nanotubes and multibranched carbon nanostructures

While sulphur is a ubiquitous element in the field of carbon nanotube (CNT) synthesis, a full understanding of its role is still open to question. A perusal of the literature reveals that it is sometimes referred to as being a "triggering element" for CNT branching [1], a "growth promoter" [2], a means of fragmenting larger-sized catalyst particles to enhance catalytic activity [3], an element which allows controlling both the number of layers in multi-walled CNTs as well as the overall CNT yield [4]. Sulphur has been put forward as an essential means of enhancing nucleation of CNT growth [5] and as having strong effects on the overall morphology of CVD-synthesised carbon nanostructures [6, 7]. In spite of these obviously beneficial effects, sulphur can also be a catalyst poison, depending on its concentration. All these different potential roles are further complicated by the fact that they depend on the synthesis route and the catalyst used. Using highly sensitive investigation tools is vital since even very minute quantities of sulphur may be of critical importance.After a brief overview of the above-mentioned background literature, we will present experimental results obtained for catalytic decomposition of benzene on iron nanoparticles in the presence of CS2. Depending on the exact synthesis conditions, carbon nanoballs, single-walled CNTs, multi-walled CNTs, thick fibres, and/or multibranched nanostructures have been observed. Based on extensive electron microscopy investigations (SEM, TEM, and HRTEM) and on a comprehensive analysis of our CVD process, we propose a phenomenological growth model.

Laser ablation of a silicon target in chloroform: formation of multilayer graphite nanostructures

With the use of high-resolution transmission electron microscopy, selected area electron diffraction and x-ray photoelectron spectroscopy methods of analysis we show that the laser ablation of a Si target in chloroform (CHCl3) by nanosecond UV pulses (40 ns, 355 nm) results in the formation of about 50–80 nm core–shell nanoparticles with a polycrystalline core composed of small (5–10 nm) Si and SiC mono-crystallites, the core being coated by several layers of carbon with the structure of graphite (the shell). In addition, free carbon multilayer nanostructures (carbon nano-onions) are also found in the suspension. On the basis of a comparison with similar laser ablation experiments implemented in carbon tetrachloride (CCl4), where only bare (uncoated) Si nanoparticles are produced, we suggest that a chemical (solvent decomposition giving rise to highly reactive CH-containing radicals) rather than a physical (solvent atomization followed by carbon nanostructure formation) mechanism is responsible for the formation of graphitic shells. The silicon carbonization process found for the case of laser ablation in chloroform may be promising for silicon surface protection and functionalization.

Towards the control of the diameter of individualized single walled carbon nanotubes in CVD process at low temperature

AbstractIn the current work, we show that it is possible to favor the selective growth of single‐walled carbon nanotubes (SWCNTs) with a narrow diameter distribution on supported catalyst particles with a broad size distribution. Carbon nanotubes were grown at 600 °C on silicon substrates. The structure of carbon deposits was controlled by managing the carbon feedstock for adjusting the rate of carbon nanostructures formation on the surface of catalyst particles. Either carbon nanofibers (CNFs) carpets or isolated SWCNTs were obtained. With the fine tune of carbon feedstock, small isolated SWCNTs with a narrow diameter distribution were obtained by limiting the catalytic activity of the largest catalyst particles. HRTEM observations of nanotube embryos have suggested a possible mechanism of multi‐walled carbon nanotubes (MWCNTs) formation that can explain why the growth of MWCNTs with parallel walls seems to be more difficult than SWCNTs or CNFs at low temperature.

Predictive modeling of formation of carbon nanostructures

Modern methods of predictive modeling and progress in understanding the mechanisms of self-assembly of carbon atoms into perfect carbon nanostructures achieved using these methods are described. The concept of multiscale modeling which makes it possible to model the formation of carbon nanostructures based on the first principles and in which the role of experimental data is restricted to verification of the models developed is discussed.

CVD Facility for the Formation of Carbon Nanomaterials on a Space Station Board

A facility for the formation of carbon nanostructures by chemical vapor deposition method on a space station board is presented. Unique solutions for thermal protection, optimal weight and size without using explosive gases and dangerous materials offer an opportunity to use the facility for the formation of carbon nanostructures under complex conditions: large temperature and pressure drops, microgravity and cosmic radiation. Layout of the facility was created and tested on the ground. Varying substrates, gas environment, temperature and pressure inside the chamber, carbon nanotubes and diamond-like coating can be obtained.

Closed network growth of fullerenes

Tremendous advances in nanoscience have been made since the discovery of the fullerenes; however, the formation of these carbon-caged nanomaterials still remains a mystery. Here we reveal that fullerenes self-assemble through a closed network growth mechanism by incorporation of atomic carbon and C(2). The growth processes have been elucidated through experiments that probe direct growth of fullerenes upon exposure to carbon vapour, analysed by state-of-the-art Fourier transform ion cyclotron resonance mass spectrometry. Our results shed new light on the fundamental processes that govern self-assembly of carbon networks, and the processes that we reveal in this study of fullerene growth are likely be involved in the formation of other carbon nanostructures from carbon vapour, such as nanotubes and graphene. Further, the results should be of importance for illuminating astrophysical processes near carbon stars or supernovae that result in C(60) formation throughout the Universe.