(n, m) Distribution of Single-Walled Carbon Nanotubes Grown from a Non-Magnetic Palladium Catalyst

Non-magnetic metal nanoparticles have been previously applied for the growth of single-walled carbon nanotubes (SWNTs). However, the activation mechanisms of non-magnetic metal catalysts and chirality distribution of synthesized SWNTs remain unclear. In this work, the activation mechanisms of non-magnetic metal palladium (Pd) particles supported by the magnesia carrier and thermodynamic stabilities of nucleated SWNTs with different (n, m) are evaluated by theoretical simulations. The electronic metal-support interaction between Pd and magnesia upshifts the d-band center of Pd, which promotes the chemisorption and dissociation of carbon precursor molecules on the Pd surface, making the activation of magnesia-supported non-magnetic Pd catalysts for SWNT growth possible. To verify the theoretical results, a porous magnesia supported Pd catalyst is developed for the bulk synthesis of SWNTs by chemical vapor deposition. The chirality distribution of Pd-grown SWNTs is understood by operating both Pd-SWNT interfacial formation energy and SWNT growth kinetics. This work not only helps to gain new insights into the activation of catalysts for growing SWNTs, but also extends the use of non-magnetic metal catalysts for bulk synthesis of SWNTs.

Kinetics of Carbon Nanotubes and Graphene Growth on Iron and Steel: Evidencing the Mechanisms of Carbon Formation

Carbon formation on steel has recently become an active research area with several important applications, using either carbon nanotubes (CNTs) or graphene structures. The production of vertically aligned CNT (VACNT) forests with combined metals has been explored with important results. Detailed kinetics is the best approach to understand a mechanism. The growth behavior seems complex but can be simplified through the knowledge of the three more common alternative reaction mechanisms/routes. The time required to optimize the production and properties might be reduced. The mechanistic proposal reported in 1971 was better explained recently. The volcano shape Arrhenius plot reported is observed only when Fe, Co, and Ni are used as reaction catalysts. Other metals are catalytically active at higher temperatures, following a different route, which does not require surface catalysis decomposition of the reactive gas. C₂H₂ and low olefins react well, but CH₄ is not reactive via this surface catalysis route. Optimizing production of CNTs, research work is usually based on previous experience, but solid-state science-based studies are available.

Carbon Nanotube Formation on Cr-Doped Ferrite Catalyst during Water Gas Shift Membrane Reaction: Mechanistic Implications and Extended Studies on Dry Gas Conversions

A nanocrystalline chromium-doped ferrite (FeCr) catalyst was shown to coproduce H2 and multiwalled carbon nanotubes (MWCNTs) during water gas shift (WGS) reaction in a H2-permselective zeolite membrane reactor (MR) at reaction pressures of ~20 bar. The FeCr catalyst was further demonstrated in the synthesis of highly crystalline and dimensionally uniform MWCNTs from a dry gas mixture of CO and CH4, which were the apparent sources for MWCNT growth in the WGS MR. In both the WGS MR and dry gas reactions, the operating temperature was 500 °C, which is significantly lower than those commonly used in MWCNT production by chemical vapor deposition (CVD) method from CO, CH4, or any other precursor gases. Extensive ex situ characterizations of the reaction products revealed that the FeCr catalyst remained in partially reduced states of Fe3+/Fe2+ and Cr6+/Cr3+ in WGS membrane reaction while further reduction of Fe2+ to Fe0 occurred in the CO/CH4 dry gas environments. The formation of the metallic Fe nanoparticles or catalyst surface dramatically improved the crystallinity and dimensional uniformity of the MWCNTs from dry gas reaction as compared to that from WGS reaction in the MR. Reaction of the CO/CH4 mixture containing 500 ppmv H2S also resulted in high-quality MWCNTs similar to those from the H2S-free feed gas, demonstrating excellent sulfur tolerance of the FeCr catalyst that is practically meaningful for utilization of biogas and cheap coal-derived syngas.

Contact-Induced Phase Separation of Alloy Catalyst to Promote Carbon Nanotube Growth

In this Letter, using density functional theory based molecular dynamics simulations, we report that contact to a carbon nanotube (CNT) induces phase separation in an alloy catalyst, which promotes CNT growth. During growth of a CNT, the growth front tends to preferentially bond to the more active metal atom in the alloy catalyst, thus triggering a phase separation of the alloy catalyst particle. The accumulation of the active metal stabilizes the open end of the CNT, attracts carbon precursors to rapidly diffuse to the growth front, and avoids catalyst poisoning by preventing the encapsulation of the catalyst. This study resolves a long-term mystery surrounding the higher efficiency of alloy catalysts in CNT growth as compared to a pure metal catalyst and thereby paves the way to a more rational catalyst design for controlled CNT growth.

Modulating Catalytic Activity and Durability of PtFe Alloy Catalysts for Oxygen Reduction Reaction Through Controlled Carbon Shell Formation

Demand on synthetic approaches to high performance electrocatalyst with enhanced durability is increasing for fuel cell applications. In this work, we present a facile synthesis of carbon shell-coated PtFe nanoparticles by using acetylacetonates in metal precursors as carbon sources without an additional polymer coating process for the carbon shell formation. The carbon shell structure is systematically controlled by changing the annealing conditions such as the temperature and gas atmosphere. PtFe catalysts annealed at 700 °C under H2-mixed N2 gas show much higher oxygen reduction reaction (ORR) activity and superior durability compared to a Pt catalyst due to the ultrathin and porous carbon shells. In addition, when increasing the annealing temperature, the carbon shells encapsulating the PtFe nanoparticles improves the durability of the catalysts due to the enhanced crystallinity of the carbon shells. Therefore, it is demonstrated that the developed hybrid catalyst structure with the carbon shells not only allows the access of reactant molecules to the active sites for oxygen reduction reaction but also prevents the agglomeration of metal nanoparticles on carbon supports, even under harsh operating conditions. The proposed synthetic approach and catalyst structure are expected to provide more insights into the development of highly active and durable catalysts for practical fuel cell applications.

One-Step Chemical Vapor Deposition Synthesis of 3D N-doped Carbon Nanotube/N-doped Graphene Hybrid Material on Nickel Foam

3D hybrid nanostructures connecting 1D carbon nanotubes (CNTs) with 2D graphene have attracted more and more attentions due to their excellent chemical, physical and electrical properties. In this study, we firstly report a novel and facile one-step process using template-directed chemical vapor deposition (CVD) to fabricate highly nitrogen doped three-dimensional (3D) N-doped carbon nanotubes/N-doped graphene architecture (N-CNTs/N-graphene). We used nickel foam as substrate, melamine as a single source for both carbon and nitrogen, respectively. The morphology and microstructure were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, isothermal analyses, X-ray photoelectron microscopy and Raman spectra. The obtained 3D N-CNTs/N-graphene exhibits high graphitization, a regular 3D structure and excellent nitrogen doping and good mesoporosity.

Comparison of atomic scale dynamics for the middle and late transition metal nanocatalysts

Catalysis of chemical reactions by nanosized clusters of transition metals holds the key to the provision of sustainable energy and materials. However, the atomistic behaviour of nanocatalysts still remains largely unknown due to uncertainties associated with the highly labile metal nanoclusters changing their structure during the reaction. In this study, we reveal and explore reactions of nm-sized clusters of 14 technologically important metals in carbon nano test tubes using time-series imaging by atomically-resolved transmission electron microscopy (TEM), employing the electron beam simultaneously as an imaging tool and stimulus of the reactions. Defect formation in nanotubes and growth of new structures promoted by metal nanoclusters enable the ranking of the different metals both in order of their bonding with carbon and their catalytic activity, showing significant variation across the Periodic Table of Elements. Metal nanoclusters exhibit complex dynamics shedding light on atomistic workings of nanocatalysts, with key features mirroring heterogeneous catalysis.

The atomistic behaviour of nanocatalysts still remains largely unknown. Here, the authors reveal and explore reactions of nm-sized clusters of 14 technologically important metals in carbon nano test tubes using time-series imaging by atomically-resolved transmission electron microscopy.

Low temperature growth of fully covered single-layer graphene using a CoCu catalyst

A bimetallic CoCu alloy thin-film catalyst is developed that enables the growth of uniform, high-quality graphene at 750 °C in 3 min by chemical vapour deposition. The growth outcome is found to vary significantly as the Cu concentration is varied, with ∼1 at% Cu added to Co yielding complete coverage single-layer graphene growth for the conditions used. The suppression of multilayer formation is attributable to Cu decoration of high reactivity sites on the Co surface which otherwise serve as preferential nucleation sites for multilayer graphene. X-ray photoemission spectroscopy shows that Co and Cu form an alloy at high temperatures, which has a drastically lower carbon solubility, as determined by using the calculated Co-Cu-C ternary phase diagram. Raman spectroscopy confirms the high quality (I_D/I_G < 0.05) and spatial uniformity of the single-layer graphene. The rational design of a bimetallic catalyst highlights the potential of catalyst alloying for producing two-dimensional materials with tailored properties.

A Lamellar Hybrid Assembled from Metal Disulfide Nanowall Arrays Anchored on a Carbon Layer: In Situ Hybridization and Improved Sodium Storage

A lamellar hybrid assembled from metal disulfide (MoS2 , WS2 ) nanowall arrays anchored on nitrogen-doped carbon layers is developed via an in situ hybridization strategy through a synergistic pyrolysis reaction of thiourea and oxometalates. Such a hybrid provides adequate electrical and chemical coupling between the active materials and the carbon substrate, thus realizing a high-efficiency electron-conduction/ion-transportation system and exhibiting excellent sodium-storage properties.

Recent Progress in Obtaining Semiconducting Single-Walled Carbon Nanotubes for Transistor Applications

High purity semiconducting single-walled carbon nanotubes (s-SWCNTs) with a narrow diameter distribution are required for high-performance transistors. Achieving this goal is extremely challenging because the as-grown material contains mixtures of s-SWCNTs and metallic- (m-) SWCNTs with wide diameter distributions, typically inadequate for integrated circuits. Since 2000, numerous ex situ methods have been proposed to improve the purity of the s-SWCNTs. The majority of these techniques fail to maintain the quality and integrity of the s-SWCNTs with a few notable exceptions. Here, the progress in realizing high purity s-SWCNTs in as-grown and post-processed materials is highlighted. A comparison of transistor parameters (such as on/off ratio and field-effect mobility) obtained from test structures establishes the effectiveness of various methods and suggests opportunities for future improvements.

On the Stability and Abundance of Single Walled Carbon Nanotubes

Many nanotechnological applications, using single-walled carbon nanotubes (SWNTs), are only possible with a uniform product. Thus, direct control over the product during chemical vapor deposition (CVD) growth of SWNT is desirable, and much effort has been made towards the ultimate goal of chirality-controlled growth of SWNTs. We have used density functional theory (DFT) to compute the stability of SWNT fragments of all chiralities in the series representing the targeted products for such applications, which we compare to the chiralities of the actual CVD products from all properly analyzed experiments. From this comparison we find that in 84% of the cases the experimental product represents chiralities among the most stable SWNT fragments (within 0.2 eV) from the computations. Our analysis shows that the diameter of the SWNT product is governed by the well-known relation to size of the catalytic nanoparticles, and the specific chirality is normally determined by the product’s relative stability, suggesting thermodynamic control at the early stage of product formation. Based on our findings, we discuss the effect of other experimental parameters on the chirality of the product. Furthermore, we highlight the possibility to produce any tube chirality in the context of recent published work on seeded-controlled growth.

Resilient High Catalytic Performance of Platinum Nanocatalysts with Porous Graphene Envelope

Despite the innumerable developments of nanosized and well dispersed noble metal catalysts, the degradation of metal nanoparticle catalysts has proven to be a significant obstacle for the commercialization of the hydrogen fuel cell. Here, the formation of Pt nanoparticle catalysts with a porous graphene envelope has been achieved using a single step low temperature vaporization process. While these Pt-Gr core-shell nanoparticles possess superior resilience to degradation, it comes at the cost of degraded overall catalyst efficacy. However, it is possible to combat this lower overall performance through inclusion of low concentrations of nitrogen precursor in the initial stage of single-step synthesis, inhibiting the formation of complete graphene shells, as verified by atomic resolution aberration-corrected transmission electron microscopy (AC-TEM) imaging. The resultant porous graphene encapsulated Pt catalysts are found to have both the high peak performance of the bare Pt nanoparticle catalysts and the increased resilience of the fully shielded Pt-Gr core-shells, with the optimal N-doped Pt-Gr yielding a peak efficiency of 87% compared to bare Pt, and maintaining 90% of its catalytic activity after extended potential cycling. The nitrogen treated Pt-Gr core-shells thus act as an effective substitute catalyst for conventional bare Pt nanoparticles, maintaining their catalytic performance over prolonged use.

Insights into carbon nanotube and graphene formation mechanisms from molecular simulations: a review

The discovery of carbon nanotubes (CNTs) and graphene over the last two decades has heralded a new era in physics, chemistry and nanotechnology. During this time, intense efforts have been made towards understanding the atomic-scale mechanisms by which these remarkable nanostructures grow. Molecular simulations have made significant contributions in this regard; indeed, they are responsible for many of the key discoveries and advancements towards this goal. Here we review molecular simulations of CNT and graphene growth, and in doing so we highlight the many invaluable insights gained from molecular simulations into these complex nanoscale self-assembly processes. This review highlights an often-overlooked aspect of CNT and graphene formation-that the two processes, although seldom discussed in the same terms, are in fact remarkably similar. Both can be viewed as a 0D → 1D → 2D transformation, which converts carbon atoms (0D) to polyyne chains (1D) to a complete sp(2)-carbon network (2D). The difference in the final structure (CNT or graphene) is determined only by the curvature of the catalyst and the strength of the carbon-metal interaction. We conclude our review by summarizing the present shortcomings of CNT/graphene growth simulations, and future challenges to this important area.

Kinetics of laser-assisted carbon nanotube growth

Laser-assisted chemical vapour deposition (CVD) growth is an attractive mask-less process for growing locally aligned carbon nanotubes (CNTs) in selected places on temperature sensitive substrates. The nature of the localized process results in fast carbon nanotube growth with high experimental throughput. Here, we report on the detailed investigation of growth kinetics related to physical and chemical process characteristics. Specifically, the growth kinetics is investigated by monitoring the dynamical changes in reflected laser beam intensity during growth. Benefiting from the fast growth and high experimental throughput, we investigate a wide range of experimental conditions and propose several growth regimes. Rate-limiting steps are determined using rate equations linked to the proposed growth regimes, which are further characterized by Raman spectroscopy and Scanning Electron Microscopy (SEM), therefore directly linking growth regimes to the structural quality of the CNTs. Activation energies for the different regimes are found to be in the range of 0.3-0.8 eV.

Communication: Origin of the difference between carbon nanotube armchair and zigzag ends

In this work, we have found that the difference between armchair and zigzag ends of carbon nanotubes (CNTs) does not pertain at close study for individual bonds and thus alternative strategies need to be developed to reach the ultimate goals in selective growth. Based on first-principles simulations, the difference between binding strengths for CNTs of different chirality was investigated using hydrogen dissociation energies at their passivated ends. When all H atoms are removed collectively we find the well-known difference: that armchair bonds are much weaker than zigzag ones, which is typically seen for both CNT ends and graphene edges. However, when individual H atoms are removed we find almost no difference in hydrogen dissociation energies, small difference in bond lengths, which by association means small difference in C-C and M-C binding energies. We show convincingly that the difference in binding energy between armchair and zigzag ends is due to a fragment stabilization effect that is only manifested when all (or several neighbouring) bonds are broken. This is because at armchair ends/edges neighbouring dangling bonds can pair-up to form C≡C triple bonds that constitute a considerable stabilization effect compared to the isolated dangling bonds at zigzag ends/edges. Consequently, in many processes, e.g., catalytic growth where bonds are normally created/broken sequentially, not collectively, the difference between armchair and zigzag ends/edges cannot be used to discriminate growth of one type over the other to achieve chiral selective growth. Strategies are discussed to realize chirality selective growth in the light of the results presented, including addition of C2-fragments to favor armchair tubes.

Hydrogen-induced effects on the CVD growth of high-quality graphene structures

In this work, the hydrogen-induced effects on the CVD growth of high-quality graphene have been systematically studied by regulating the growth parameters mainly related to hydrogen. Experimental results demonstrate that under a high hydrogen flow rate, the competitive etching effect during the growth process is more prominent and even shows macroscopic selectivity. Based on these understandings, the hexagonal graphene domains with diverse edge modalities are controllably synthesized on a large scale by elaborately managing the competitive etching effect of hydrogen that existed during the formation of graphene. This study not only contributes to the understanding of the mechanism of CVD growth, especially the effects of hydrogen used in the system, but also provides a facile method to synthesize high-quality graphene structures with trimmed edge morphologies.