Vertically Aligned Single-walled Carbon Nanotubes Research Articles

Growth and Performance of High-Quality SWCNT Forests on Inconel Foils as Lithium-Ion Battery Anodes.

Large-scale production of vertically aligned single-walled carbon nanotubes (VA-SWCNTs) on metal foils promises to enable technological advancements in many fields, from functional composites to energy storage to thermal interfaces. In this work, we demonstrate growth of high-quality (G/D > 6, average diameters ∼ 2-3 nm, densities > 1012 cm-2) VA-SWCNTs on Inconel metal for use as a lithium-ion battery (LIB) anode. Scale-up of SWCNT growth on Inconel 625 to 100 cm2 exhibits nearly invariant CNT structural properties, even when synthesis is performed near atmospheric pressure, and this robustness is attributed to a growth kinetic regime dominated by the carbon precursor diffusion in the bulk gas mixture. SWCNT forests produced on large-area metal substrates at close to atmospheric pressure possess a combination of structural features that are among the best demonstrated so far in the literature for growth on metal foils. Leveraging these achievements for energy applications, we demonstrate a VA-SWCNT LIB anode with capacity >1200 mAh/g at 1.0C and stable cycling beyond 300 cycles. This robust synthesis of high-quality VA-SWCNTs on metal foils presents a promising route toward mass production of high-performance CNT devices for a broad range of applications.

Osmium catalyzed growth of vertically aligned and small-diameter single-walled carbon nanotubes by alcohol catalytic chemical vapor deposition

We showed that single-walled carbon nanotubes (SWCNTs) could be grown on SiO2/Si substrates by alcohol catalytic chemical vapor deposition (CVD) with an Os catalyst in a cold-wall CVD system. At a growth temperature of 700 °C, web-like SWCNTs were grown with diameters distributed below 0.9 nm. At a growth temperature of 800 °C, the SWCNT density increased and vertically aligned SWCNTs (VA-SWCNTs) 70 nm thick were grown on the SiO2/Si substrates after growth for 30 min. Raman and transmission electron microscope results showed that the diameters of the grown SWCNTs were distributed between 0.65 and 1.0 nm, and were much smaller than those of VA-SWCNTs obtained from conventional catalysts such as Fe and Co. Furthermore, compared with Ir, which has been reported to be a suitable catalyst for growth of small-diameter SWCNTs, we showed that Os could grow SWCNTs with even smaller diameters. Additionally, neither an alumina buffer layer nor cocatalyst were necessary to grow VA-SWCNTs with the Os catalyst. We consider that highly efficient suppression of aggregation and the relatively strong carbon-Os bond strength contributed to growth of small-diameter SWCNTs from the Os catalyst. Our studies are the first to report on Os as a catalyst for SWCNT growth, leading to SWCNTs with diameters less than ~1 nm. Hence, period 6 platinum-group metals are likely to be good catalysts for highly efficient growth of small-diameter SWCNTs.

Effect of growth temperature and ethanol flow rate on synthesis of single-walled carbon nanotube by alcohol catalytic chemical vapor deposition using Ir catalyst in hot-wall reactor

We carried out synthesis of single-walled carbon nanotubes (SWCNTs) in the hot-wall reactor by alcohol catalytic chemical vapor deposition using an Ir as a catalyst. By optimizing the ethanol flow rate, SWCNTs were grown at the growth temperature between 700 °C and 900 °C. Raman results using the four excitation wavelengths of 488, 532, 671 and 785 nm showed that the diameters of most SWCNTs grown from an Ir catalyst were less than 1.0 nm at 700 °C and 800 °C. When the growth temperature was above 800 °C, vertically-aligned SWCNTs were grown, and the thickness reached 1.8 μm after the growth at 900 °C for 60 min under an ethanol flow rate of 500 sccm. However, at 900 °C, the diameters of SWCNTs were enlarged and most of them were distributed above 1 nm. Our result demonstrates that an Ir catalyst is useful to obtain small-diameter SWCNTs on a large scale.

Vertically aligned growth of small-diameter single-walled carbon nanotubes by alcohol catalytic chemical vapor deposition with Ir catalyst

We showed that vertically aligned single-walled carbon nanotubes (VA-SWCNTs) can be grown by alcohol chemical vapor deposition (CVD) with an Ir catalyst in a cold-wall CVD system. VA-SWCNTs were grown on SiO2/Si substrates without any buffer layers under an ethanol pressure of 1 × 10−1 Pa at 800 °C. The SWCNT length increased with increasing growth time. The VA-SWCNT thickness reached almost 5 μm for a growth time of 180 min. Raman and photoluminescence spectroscopies showed that the diameters of the grown SWCNTs were mainly between 0.8 and 1.1 nm; this is much smaller than those of VA-SWCNTs grown with Fe and Co catalysts in previous studies. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectroscopy showed that the Ir catalyst retained the metallic state during SWCNT growth. We suggest that highly efficient suppression of aggregation, an appropriate carbon–Ir bond strength and dissociation of ethanol molecules led to the growth of vertically aligned small-diameter SWCNTs with an Ir catalyst.

Reconfigurable solid-state electrolytes for high performance flexible supercapacitor

To realize high performance and a flexible supercapacitor, it is necessary to address the fundamental issues including low ionic conductivity of solid electrolytes and high interfacial resistance of electrode/electrolyte pairs. Here we present unique solid-state electrolytes by integrating highly engineered nano-porous polyvinyl alcohol (PVA) with super-flat vertically aligned single-walled carbon nanotubes (VA-SWNTs). Highly engineered PVA nano-porous films are fabricated by a generic freeze-thaw process followed by water-miscible solvent treatment in order to create highly controlled nano/microscale pores inside of PVA. Such highly porous PVA films act as both reconfigurable electrolyte template and separator where H3PO4 aqueous solution or ionic liquids can be selectively inserted for a variety of power requirements in flexible electronic applications. Our developed pore formation process is suitable for directly integrating high performance VA-SWNTs electrode as it allows the effective permeation of the polymer electrolyte into nanoscale inter-tube space enabling the easy access and faster transport of ions for higher power capability. This unique entity of reconfigurable electrolyte and nanostructured electrode demonstrates high power and energy densities and remarkable stability after 10,000 charge/discharge cycles.

In situ observation of dewetting-induced deformation of vertically aligned single-walled carbon nanotubes

We investigated dynamical processes of capillary-mediated deformation of vertically aligned single-walled carbon nanotubes (VA-SWCNTs) via in situ observation of their wetting and dewetting behaviors using an environmental scanning electron microscope (ESEM). Three types of wetting behaviors on a VA-SWCNT sample were confirmed, including conical shaped water aggregates, spherical droplets on tips of conical shaped water aggregates, and extensively distributed water layers. While the former two types both resulted in dimples on the VA-SWCNT surface and failed to induce large-scale deformation of VA-SWCNTs, the latter caused the formation of wall-like structures and crack propagation in the VA-SWCNT film during the dewetting process due to directional retraction of vapor-liquid interfaces. This dewetting-induced large-scale deformation that was confirmed by the in situ ESEM observation for the first time represented initial stages of capillary processes, leading to the self-organization of VA-SWCNTs reported in recent literatures. Compared to the previous studies based on ex situ observations of dried samples, our in situ observation successfully captured temporal evolution of the dewetting-induced deformation, facilitating the more precise construction of predictive models of final morphologies of VA-SWCNT films after capillary-mediated densification.

Efficient growth of vertically-aligned single-walled carbon nanotubes combining two unfavorable synthesis conditions

We demonstrate an unexpected recipe for the efficient synthesis of vertically-aligned single-walled carbon nanotubes (SWCNTs). The growth of high-density arrays is achieved by combining two unfavorable conditions: low temperature and mono-metallic Co catalyst, neither of which was previously found to be effective for the synthesis of SWCNT forest. As a result of this new recipe, the as-synthesized SWCNTs have a narrower diameter distribution than those obtained in previous studies, with 85% of the SWCNTs between 1.6 and 2.4 nm. Meanwhile, we utilize our recently established transmission electron microscopy (TEM) technique to characterize this monometallic Co catalyst directly on SiO2. Nanoparticles in a 600 °C reaction are confirmed to be smaller, denser and more uniform than those formed at high temperatures. Our technique further allows to image catalyst at high temperatures, by which Ostwald ripening is visualized at a single particle level. In addition, in situ electron diffraction and imaging confirm that monometallic Co catalyst remains as a solid phase even at 800 °C. The discovery of new growth recipe increases the compatibility of synthesizing SWCNT onto other substrates. The new TEM technique provides experimental evidence for some long-standing hypotheses of catalytic particles and is capable of guiding the design of new catalysts.

Vertically Aligned Single-Walled Carbon Nanotube Growth from Ir Catalysts by Alcohol Gas Source Method

Abstract:We demonstrated that single-walled carbon nanotubes (SWCNTs) grew from Ir catalysts by an alcohol catalytic chemical vapor deposition (ACCVD) method using a gas source-type CVD system. At an ethanol pressure of 1×10−1 Pa at 800°C, vertically aligned SWCNTs (VA-SWCNTs) were grown on SiO2/Si substrates. As the growth time became longer, the VA-SWCNT became thicker, and it reached almost 5 μm for a growth time of 180 min. The Raman spectroscopy results showed that the diameters of the grown SWCNTs were mainly distributed below 1.1 nm, indicating that the SWCNTs grown from Ir catalysts had small diameters compared with those from other metal catalysts.

A Forest of Sub-1.5-nm-wide Single-Walled Carbon Nanotubes over an Engineered Alumina Support

A precise control of the dimension of carbon nanotubes (CNTs) in their vertical array could enable many promising applications in various fields. Here, we demonstrate the growth of vertically aligned, single-walled CNTs (VA-SWCNTs) with diameters in the sub-1.5-nm range (0.98 ± 0.24 nm), by engineering a catalyst support layer of alumina via thermal annealing followed by ion beam treatment. We find out that the ion beam bombardment on the alumina allows the growth of ultra-narrow nanotubes, whereas the thermal annealing promotes the vertical alignment at the expense of enlarged diameters; in an optimal combination, these two effects can cooperate to produce the ultra-narrow VA-SWCNTs. According to micro- and spectroscopic characterizations, ion beam bombardment amorphizes the alumina surface to increase the porosity, defects, and oxygen-laden functional groups on it to inhibit Ostwald ripening of catalytic Fe nanoparticles effectively, while thermal annealing can densify bulk alumina to prevent subsurface diffusion of the catalyst particles. Our findings contribute to the current efforts of precise diameter control of VA-SWCNTs, essential for applications such as membranes and energy storage devices.

Synthesis of subnanometer-diameter vertically aligned single-walled carbon nanotubes with copper-anchored cobalt catalysts.

We synthesize vertically aligned single-walled carbon nanotubes (VA-SWNTs) with subnanometer diameters on quartz (and SiO2/Si) substrates by alcohol CVD using Cu-anchored Co catalysts. The uniform VA-SWNTs with a nanotube diameter of 1 nm are synthesized at a CVD temperature of 800 °C and have a thickness of several tens of μm. The diameter of SWNTs was reduced to 0.75 nm at 650 °C with the G/D ratio maintained above 24. Scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (EDS-STEM) and high angle annular dark field (HAADF-STEM) imaging of the Co/Cu bimetallic catalyst system showed that Co catalysts were captured and anchored by adjacent Cu nanoparticles, and thus were prevented from coalescing into a larger size, which contributed to the small diameter of SWNTs. The correlation between the catalyst size and the SWNT diameter was experimentally clarified. The subnanometer-diameter and high-quality SWNTs are expected to pave the way to replace silicon for next-generation optoelectronic and photovoltaic devices.

Mediated Electron Transfer at Vertically Aligned Single-Walled Carbon Nanotube Electrodes During Detection of DNA Hybridization.

Vertically aligned single-walled carbon nanotube (VASWCNT) assemblies are generated on cysteamine and 2-mercaptoethanol (2-ME)-functionalized gold surfaces through amide bond formation between carboxylic groups generated at the end of acid-shortened single-walled carbon nanotubes (SWCNTs) and amine groups present on the gold surfaces. Atomic force microscopy (AFM) imaging confirms the vertical alignment mode of SWCNT attachment through significant changes in surface roughness compared to bare gold surfaces and the lack of any horizontally aligned SWCNTs present. These SWCNT assemblies are further modified with an amine-terminated single-stranded probe-DNA. Subsequent hybridization of the surface-bound probe-DNA in the presence of complementary strands in solution is followed using impedance measurements in the presence of Fe(CN)63−/4− as the redox probe in solution, which show changes in the interfacial electrochemical properties, specifically the charge-transfer resistance, due to hybridization. In addition, hybridization of the probe-DNA is also compared when it is attached directly to the gold surfaces without any intermediary SWCNTs. Contrary to our expectations, impedance measurements show a decrease in charge-transfer resistance with time due to hybridization with 300 nM complementary DNA in solution with the probe-DNA attached to SWCNTs. In contrast, an increase in charge-transfer resistance is observed with time during hybridization when the probe-DNA is attached directly to the gold surfaces. The decrease in charge-transfer resistance during hybridization in the presence of VASWCNTs indicates an enhancement in the electron transfer process of the redox probe at the VASWCNT-modified electrode. The results suggest that VASWCNTs are acting as mediators of electron transfer, which facilitate the charge transfer of the redox probe at the electrode–solution interface.

Gas–Surface Energy Exchange in Collisions of Helium Atoms with Aligned Single-Walled Carbon Nanotube Arrays

Since gas flows in micro/nano devices are dominated by the interaction of gas molecules and solid surfaces, surface modification technique is one of the critical issues for optimizing the thermal performance of these devices. In this paper, we demonstrate the successful application of vertically aligned single-walled carbon nanotubes (VA-SWNTs) as surface modification material to enhance the energy accommodation of gas molecules on surfaces. The scattering of gas molecules on quartz surfaces covered with VA-SWNTs is investigated by the molecular beam technique. The energy accommodation coefficients of helium, which tend to be small even for rough surfaces because of the large mass mismatch between helium and surface atoms, are close to unity on the modified surfaces. The measurement with the free-standing films further reveals that the high accommodation is attributed to the unique morphology of the films with high porosity, which allows gas molecules to penetrate into the films and realizes a large numbe...

Nanostructuring Ultra-thin Co Films to Active Catalyst Particles for Vertically Aligned Single-walled CNT Growth

In the field of material synthesis using chemically-derived technique, nanostructuring metal catalyst particles towards high quality production of carbon nanotubes (CNTs) has been very attractive. In this work, cobalt (Co) which used as catalyst for vertical growth of CNTs were studied by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Aluminum (Al) films (20 nm) were thermally-oxidized to form aluminum oxide (Al-O) to support 0.5 nm Co catalyst during CNT growth. In growing CNTs by using chemical vapor deposition (CVD) technique, the role and characters of all involving materials are crucial to the growth result. From the Co/Al-O substrate and at 650°C of CVD temperature, 33-μm thick of vertically aligned single-walled CNT (VA-SWCNT) forest has been grown. TEM particle analysis revealed that the Co particles have an average of 3.50 nm experimentally and in principle favored the growth of highly demanded VA-SWCNTs. The as-prepared Co particles are suggested chemically active for the CNT growth. In addition, XPS analysis confirmed the surface chemical state of Co particles prior to the VA-SWCNT growth using ethanol based CVD system.

Heat Conduction Characteristics of Vertically Aligned Single-Walled Carbon Nanotubes Measured by Raman Spectroscopy

In this work, the film thermal conductivity and the film-substrate interfacial thermal contact resistance of vertically-aligned single-walled carbon nanotube (VA-SWNT) films were measured with a proposed method utilizing temperature dependence of Raman spectrum. The proposed method harnesses the excitation laser power of the Raman spectroscopy to heat the VA-SWNT films synthesized on a silicon substrate by alcohol catalytic chemical vapor decomposition (ACCVD) method, and measures film temperature from the Raman spectrum. A relationship between the input laser heat and the measured temperature is modeled with a detailed heat conduction equation, and its numerical solutions were compared with the experimentally measured results to extract the film thermal conductivity and the film-substrate interfacial thermal contact resistance. The method found the thermal conductivity of the VA-SWNT film to be around 2 Wm-1K-1 and the film-substrate interfacial thermal contact resistance to be around 2×10-6 m2KW-1. The obtained film thermal conductivity corresponds to the thermal conductivity equivalent of an individual SWNT of several tens of Wm-1K-1. This value was more than an order of magnitude smaller than the values reported on individual SWNTs.

Chemical vapor deposition-grown vertically aligned single-walled carbon nanotubes length assurance

Chemical vapor deposition is one of the several viable methods for growing vertically aligned single-walled carbon nanotubes (VA-SWNTs). Utilizing cobalt (Co) catalyst supported on multilayer Al/SiO2 and a hydrocarbon feedstock, VA-SWNTs are grown in excess of a millimeter height. To control VA-SWNTs length, one has to use the right combination of process control variables such as hydrocarbon gas flow rate, chamber temperature, and chamber pressure. This paper presents a process meta-model-based full factorial experimental design and analysis to study the yield of tall VA-SWNTs. All of the process variables under the study play a role in influencing VA-SWNTs length; the current study investigates main effects and their interactions. The meta-model-based analysis demonstrates that the hydrocarbon flow rate and the chamber pressure are the most statistically significant control variables that influence the length of VA-SWNTs. In addition, the response surface graph confirms that a higher gas flow rate at lower chamber pressure will consistently yield tall VA-SWNTs. We found that gas flow rate is the most significant of the control variables and only the optimum gas flow rate can ensure the growth of tall VA-SWNTs. We noticed that the interaction of gas flow rate with chamber temperature is also significant to the length of VA-SWNTs grown. All these observations together indicate that the dynamic pressure of the gas in the chamber plays an important role in the assurance of the length of VA-SWNTs. Outcomes of this investigation are beneficial for moving us closer towards producing VA-SWNTs on a mass scale.

Direct growth of vertically aligned single‐walled carbon nanotubes on conducting substrate and its electrochemical performance in ionic liquids

AbstractWe report the fabrication of vertically aligned single‐walled carbon nanotube (VA‐SWCNT) electrodes for a symmetric electrochemical capacitor using a simple, low‐cost ethanol‐based SWCNT growth system. From the CNT direct growth technique, the electrode was easily prepared and consequently assembled with an electrochemically stable ionic liquid as electrolyte. VA‐SWCNTs were directly grown on conducting SUS 310S foils in which the binder material was not incorporated in the capacitor structure. The morphology of as‐grown CNTs was investigated using scanning and transmission electron microscopy. This capacitor demonstrated a gravimetric capacitance of up to 584 F g−1 (at 1 mV s−1 scan rate) and a high‐rate capability, with up to 1000 mV s−1 scan rate being obtained. From the cyclic voltammetric analysis, the capacitance was contributed not only from the ideal double‐layer capacitance, but also from faradaic/redox processes that might have occurred during charge–discharge. Other than the capacitance, the VA‐SWCNT capacitor electrochemical performance was also measured by using frequency response (electrochemical impedance) and charge–discharge analyses.

Chemical Vapor Deposition Growth, Optical, and Thermal Characterization of Vertically Aligned Single-Walled Carbon Nanotubes

Vertically aligned single-walled carbon nanotubes (VA-SWNTs) is expected to be an extra-ordinal material for various optical, electrical, energy, and thermal devices. The recent progress in growth control and characterization techniques will be discussed. The chemical vapor deposition (CVD) growth mechanism of VA-SWNTs is studied based on the in situ growth monitoring by laser absorption during CVD. The growth curves are characterized by an exponential decay of the growth rate from the initial rate determined by ethanol pressure. The initial growth rate and decay of it are discussed with carbon over-coat on metal catalysts and gas phase thermal decomposition of precursor ethanol. For the precisely patterned growth of SWNTs, we recently propose a surface-energy-difference driven selective deposition of catalyst for localized growth of SWNTs. For a self-assembled monolayer (SAM) patterned Si surface, catalyst particles deposit and SWNTs grow only on the hydrophilic regions. The proposed all-liquid-based approach possesses significant advantages in scalability and resolution over state-of-the-art techniques, which we believe can greatly advance the fabrication of nanodevices using high-quality as-grown SWNTs. The optical characterization of the VA-SWNT film using polarized absorption, polarized Raman, and photoluminescence spectroscopy will be discussed. Laser-excitation of a vertically aligned film from top means that each nanotube is excited perpendicular to its axis. Because of this predominant perpendicular excitation, interesting cross-polarized absorption and confusing and practically important Raman features are observed. The extremely high and peculiar thermal conductivity of single-walled carbon nanotubes has been explored by nonequilibrium molecular dynamics simulation approaches. The thermal properties of the vertically aligned film and composite materials are studied by several experimental techniques and Monte Carlo simulations based on molecular dynamics inputs of thermal conductivity and thermal boundary resistance. Current understanding of thermal properties of the film is discussed.

High Electrocatalytic Activity of Vertically Aligned Single-Walled Carbon Nanotubes towards Sulfide Redox Shuttles.

Vertically aligned single-walled carbon nanotubes (VASWCNTs) have been successfully transferred onto transparent conducting oxide glass and implemented as efficient low-cost, platinum-free counter electrode in sulfide –mediated dye-sensitized solar cells (DSCs), featuring notably improved electrocatalytic activity toward thiolate/disulfide redox shuttle over conventional Pt counter electrodes. Impressively, device with VASWCNTs counter electrode demonstrates a high fill factor of 0.68 and power conversion efficiency up to 5.25%, which is significantly higher than 0.56 and 3.49% for that with a conventional Pt electrode. Moreover, VASWCNTs counter electrode produces a charge transfer resistance of only 21.22 Ω towards aqueous polysulfide electrolyte commonly applied in quantum dots-sensitized solar cells (QDSCs), which is several orders of magnitude lower than that of a typical Pt electrode. Therefore, VASWCNTs counter electrodes are believed to be a versatile candidate for further improvement of the power conversion efficiency of other iodine-free redox couple based DSCs and polysulfide electrolyte based QDSCs.

Tailoring 3D Single-Walled Carbon Nanotubes Anchored to Indium Tin Oxide for Natural Cellular Uptake and Intracellular Sensing

The ability to monitor intracellular events in real time is paramount to advancing fundamental biological and clinical science. We present the first demonstration of a direct interface of vertically aligned single-walled carbon nanotubes (VASWCNTs) with eukaryotic cells, RAW 264.7 mouse macrophage cell line. The cells were cultured on indium tin oxide with VASWCNTs. VASWCNTs entered the cells naturally without application of any external force and were shown to sense the intracellular presence of a redox active moiety, methylene blue. The technology developed provides an alluring platform to enable electrochemical study of an intracellular environment.

Vertically Aligned Single-Walled Carbon Nanotubes as Low-cost and High Electrocatalytic Counter Electrode for Dye-Sensitized Solar Cells

A novel dye-sensitized solar cell (DSSC) structure using vertically aligned single-walled carbon nanotubes (VASWCNTs) as the counter electrode has been developed. In this design, the VASWCNTs serve as a stable high surface area and highly active electrocatalytic counter-electrode that could be a promising alternative to the conventional Pt analogue. Utilizing a scalable dry transfer approach to form a VASWCNTs conductive electrode, the DSSCs with various lengths of VASWCNTs were studied. VASWCNTs-DSSC with 34 μm original length was found to be the optimal choice in the present study. The highest conversion efficiencies of VASWCNTs-DSSC achieved 5.5%, which rivals that of the reference Pt DSSC. From the electrochemical impedance spectroscopy analysis, it shows that the new DSSC offers lower interface resistance between the electrolyte and the counter electrode. This reproducible work emphasizes the promise of VASWCNTs as efficient and stable counter electrode materials in DSSC device design, especially taking into account the low-cost merit of this promising material.