Large-diameter Nanotubes Research Articles

Enhanced green hydrogen production via CdS/Cu-doped TiO2 nanotubes: Advancements in photoanode performance

Cadmium Sulfide (CdS) deposited Copper (Cu) doped TiO2 nanotube hybrid photoanode (CdS/Cu-TNT) is developed via electrochemical anodization and subsequent electrochemical deposition for efficient green hydrogen production. Cu-doping of TiO2 Nanotube (TNT) is achieved in one-step anodization, which is more energy efficient than conventional arc melting. Results indicate that CdS/Cu-TNT photoanodes increased carrier density by 9 times and achieved a low bandgap of 2.46 eV which enhancing their suitability as photoanodes. Particle size distribution analysis has shown that Cu doping causes thicker nanotube walls with decreased surface area. CdS is coated over the Cu-doped TNT to enhance the photoelectrochemical properties further. The photocurrent of CdS-deposited TNT is 7.8 times higher than bare TNTs. Raman Spectral Mapping indicates the uniformity of CdS deposition and Cu doping. Photostability experiments reveal excellent performance and switching characteristics for Cu-doped, CdS-deposited, and CdS/Cu-TNT hybrid samples. The hybrid electrode with larger diameter nanotubes shows a 52.39 % increase in the Fill Factor of photocurrent response for CdS/Cu-TNT. Moreover, hydrogen production is significantly enhanced, with CdS/TNT demonstrating a 3.8-fold increase and CdS/Cu-TNT demonstrating a 4.1-fold increase. This research highlights the potential of Cu doping in TiO2 nanotubes for hydrogen generation, offering improvements over the limitations of TiO2 as a photoanode.

Superconductivity in correlated carbon nanotubes under pressure: A Bogoliubov-de Gennes study

In contrast to most microscopic theories of superconductivity based on the reciprocal space, the Bogoliubov-de Gennes (BdG) formalism provides a real-space alternative for addressing inhomogeneous systems. In this article, we study the superconducting states in correlated single-walled carbon nanotubes (SWNTs) with curvature and spin-orbit corrections, as well as the inter-tube interaction through a connecting molecule using an attractive Hubbard model. The results reveal a close relationship between the on-site superconducting gap and the single-electron local density of states. For the limiting case of independent large-diameter nanotubes, the BdG equations can be reduced to the standard Bardeen-Cooper-Schrieffer one with analytical solutions. Moreover, an optimal separation between nanotubes is found, which leads to a maximal superconducting critical temperature. This finding has a remarkable accordance with the experimental data obtained from Buckypapers built of boron doped SWNTs under external pressure.

Morphology Influence on Wettability and Wetting Dynamics of ZnO Nanostructure Arrays

Abstract Changes in nanostructure morphology and size may result in very different surface wettability. In this research, the impact of different morphological parameters on the wetting dynamics of ZnO nanostructured layers is studied. Six different morphologies are chosen to determine the specific wetting processes of ZnO nanostructures: nanoneedles, small diameter rods, large diameter rods, nanotubes, nanoplates, and plain thin films. Wetting dynamics is investigated using conventional sessile drop technique and a novel approach based on electrochemical impedance spectroscopy. The results show that the surface of nanostructured ZnO thin films exhibits both hydrophilic and hydrophobic wetting behaviour, depending on nanostructure form, size, and orientation. ZnO nanostructure arrays are a promising platform for electrochemical and optical sensing in aqueous solutions. The full and effective use of the sensor working surface can be ensured only under the condition of complete wetting of the nanostructured layer. Therefore, it is important to take into account the peculiarities of the wetting process of a specific morphology of nanostructures.

Osteogenic TiO2 composite nano-porous arrays: A favorable platform based on titanium alloys applied in artificial implants

The purpose of the nano-porous TiO2 modification on TC26 (Ti-13 Nb-13Zr) alloy is to settle the issue that the surface inertness of titanium alloys restricts bioactivity and osseointegration of artificial implants. Secondary constant current anodization was conducted to realize the efficient fabrication of the TiO2 composite nano-porous arrays containing the terminal small-diameter nanopores and the upper large-diameter nanotubes. The oxygen bubble mould theory has revealed the anodization mechanism of the TiO2 nano-porous arrays. The anodization parameters and heat treatment conditions are both able to elicit influence on morphology, chemical composition, surface properties and biocompatibility of the as-fabricated samples. The interaction of MC3TC-E1 mouse pre-osteoblasts with a series of TiO2 nano-porous arrays fabricated by primary anodization (PA) and secondary anodization (SA) was investigated by the MTT assay and alkaline phosphatase (ALP) activity assay. The response of the seeded MC3TC-E1 pre-osteoblasts to the as-fabricated samples with various nano-scale dimensions has indicated that appropriate nano-porous morphologies and heat treatment conditions have the capacity to augment cell adhesion, growth, differentiation and functionality. Such fabrication route in this research has provided a promising prospect for development of artificial implants.

ИССЛЕДОВАНИЕ НАНОКОМПОЗИТА НА ОСНОВЕ АЛЮМИНИЯ, УПРОЧНЕННОГО УГЛЕРОДНЫМИ НАНОТРУБКАМИ

aluminum-based nanocomposites reinforced with carbon nanotubes were studied by computational calculations. Numerical simulations were performed within the framework of density functional theory using the plane wave basis set implemented in VASP software package. Non-local van der Waals functional was used to describe. For modeling of aluminum-based composites with large-diameter nanotubes the limiting case of the interface of the graphene/aluminum species was considered. The values of 4.3 meV/Å2 and 18.1 meV/Å2 respectively, were obtained, which are comparable with the van der Waals interaction energies by order. It is also obtained that the critical shear stress value for the defect-free graphene/aluminum system varies from 20 MPa to 70 MPa, depending on the surface type and "armchair" or "zigzag" shear direction. The influence of carbon monovacancies on mechanical properties of the composite was studied. Critical shear stress values of 1500-2000 MPa were obtained for energetically advantageous configurations of composites, depending on defect location, surface type and shear direction. The formation energies of the defects were determined. Finally, the alumina matrix composites with small diameter embedded carbon nanotubes are considered. It is shown that with a decrease in the size of the nanotube its surface curvature is observed and, as a consequence, the binding energy at the interface with the metal matrix is increased. Calculated values of critical stress reach values of the order of ~103 MPa.

Vapor-phase epitaxial re-growth of large diameter single-walled carbon nanotubes

Long single-wall carbon nanotubes (SWCNTs) with a controlled conductivity type or chirality are interesting for fundamental study and are promising in many different technological applications, such as nanoelectronics, optoelectronics, and also upon utilizing them as nanoscale reactors to produce nanomaterials. In this study, the long aligned large diameter SWCNTs and the large diameter nanotube dense networks were synthesized via a vapor-phase epitaxial re-growth method. The nanotubes were re-grown on ST (stable temperature)-cut quartz substrates from short SWCNT seeds using the mixture of ethanol and acetylene as a precursor. The efficient nanotube re-growth was achieved using unsorted SWCNTs with diameters of 1.2–2.0 nm and semiconducting SWCNTs, sorted by an aqueous two-phase extraction method, as seeds. According to our study, the re-grown nanotubes in an array have an average length of 5.5 μm, while the individual re-grown nanotubes can reach up to 20–30 μm. The extensive optical study confirms the preservation of SWCNTs diameter during the re-growth and signifies the high quality of produced nanotubes. We demonstrate the SWCNT chirality selective efficiency of the re-growth, which leads to predominance of the metallic nanotubes.

Charge Transport in Mixed Semiconducting Carbon Nanotube Networks with Tailored Mixing Ratios.

The ability to prepare uniform and dense networks of purely semiconducting single-walled carbon nanotubes (SWNTs) has enabled the design of various (opto-)electronic devices, especially field-effect transistors (FETs) with high carrier mobilities. Further optimization of these SWNT networks is desired to surpass established solution-processable semiconductors. The average diameter and diameter distribution of nanotubes in a dense network were found to influence the overall charge carrier mobility; e.g., networks with a broad range of SWNT diameters show inferior transport properties. Here, we investigate charge transport in FETs with nanotube networks comprising polymer-sorted small diameter (6,5) SWNTs (0.76 nm) and large diameter plasma torch SWNTs (1.17-1.55 nm) in defined mixing ratios. All transistors show balanced ambipolar transport with high on/off current ratios and negligible hysteresis. While the range of bandgaps in these networks creates a highly uneven energy landscape for charge carrier hopping, the extracted hole and electron mobilities vary nonlinearly with the network composition from the lowest mobility (15 cm2 V-1 s-1) for only (6,5) SWNT to the highest mobility (30 cm2 V-1 s-1) for only plasma torch SWNTs. A comparison to numerically simulated network mobilities shows that a superposition of thermally activated hopping across SWNT-SWNT junctions and diameter-dependent intratube transport is required to reproduce the experimental data. These results also emphasize the need for monochiral large diameter nanotubes for maximum carrier mobilities in random networks.

Enhanced Capacitance of TiO2 Nanotubes with a Double-Layer Structure Fabricated in NH4F/H3PO4 Mixed Electrolyte.

TiO2 is an attractive electrode material in fast charging/discharging supercapacitors because of its high specific surface area. However, the low capacitance of TiO2 nanotubes as-anodized in the classical electrolyte restricts their further application in supercapacitors. Here, we study the performances of larger-diameter nanotubes with a double-layer structure fabricated in an NH4F/phosphoric acid (H3PO4) mixed electrolyte. Results show that the double-layer structure increased the specific surface area of nanotubes owing to the cavities between the double layers and the porous structure on walls. After soaking in H3PO4 aqueous solution for 40 min, the nanotubes anodized in the mixed electrolyte containing 6 wt % H3PO4 show a specific capacitance of 13.89 mF cm-2, ∼3.11 times that of the pristine nanotubes in the classical electrolyte. The specific surface area of the soaked nanotubes is up to 113.2 m2 g-1, which is ∼2.94 times that of the pristine nanotubes. The values of specific surface area of the anodized nanotubes and the soaked nanotubes fabricated in the mixed electrolyte containing 6 wt % H3PO4 are roughly equal. It demonstrated that the specific surface area increased mainly due to the double-layer structure. The double-layer structure reveals a new strategy to enhance the specific capacitance of TiO2 nanotubes.

An optimized resistive CNT-based gas sensor with a novel configuration by top electrical contact

The paper proposes a new fabrication method of a resistive gas sensor made of vertically aligned carbon nanotubes (CNT) by the perforated electrode. This new configuration can be also utilized with vertically aligned nanomaterial such as ZnO wires. This method contains two aspects, which are getting top electrical contact from MWCNTs and devising hexagonal pattern holes on the sensor surface to ease the gas flow through the sensor. Reaching ambient molecules to these dense CNT forests is always of the essence; also, the top electrical contact of the CNTs is a fabrication challenge. The high sensitivity of proposed CNT-based sensor is attributed to their collective large porous surface along with etched circular holes located at the apexes and center of a hexagonal pattern on sensor top contact. For optimal sensor performance, the simulation approach by ANSYS-FLUENT software has optimized circular holes diameters and distances. The fabrication process proposes an oxygenated plasma treated CNTs with top electrical contact devised by a certain simple method for the very first time. Top electrical contact has been fabricated possible by the procedure of immersing the CNTs in viscosity liquid and providing electrical contact using conductive paste. Well-aligned multi-walled carbon nanotubes (MWCNT) have been grown on glass substrate placed into DC-PECVD unit bearing approximately temperature of 350 °C. In this paper, the resistivity of carbon nanotubes at different temperatures for the ambient gas and linear sensitivity to oxygen gas has been investigated. The sensors with larger diameter nanotubes are more sensitive. Etching patterned holes with 200 μm diameter enhances the sensitivity up to 50%.

A novel double-layer nanotube structure fabricated in high concentration H3PO4 and fluoride-containing mixed electrolyte without annealing

To increase the specific surface area of nanotubes, we form large diameter nanotubes with double-layer structure by anodizing Ti foils in high concentration H3PO4 and fluoride-containing mixed electrolyte without annealing rather than in pure NH4F electrolyte. This double-layer structure consists of the anions-contaminated layer and the barrier layer. Owing to the addition of PO43− ions into electrolyte, the thickness of the anions-contaminated layer and the diameter of the nanotubes increased. Compared to single-layer structure, this double-layer structure exhibits ∼5.81 times increase in specific surface area of nanotubes because of the multi-hole interface between the two layers for supercapacitors.

Alkane Encapsulation Induces Strain in Small-Diameter Single-Wall Carbon Nanotubes

Encapsulation of linear alkane molecules in the endohedral volumes of small-diameter single-wall carbon nanotubes (SWCNTs) is shown to induce diameter-dependent strain on the hexagonal lattice of carbon atoms composing the tubular structure. For the smallest diameter nanotubes, such as the (6,5), (9,1), (8,3), and (10,0), encapsulation leads to expansive radial strain. This effect is demonstrated through precision measurements of induced shifts in the energy of the intrinsic optical transitions of single-chirality nanotube populations. The effect on the optical transitions from strain is found to exceed that of the effective dielectric medium change when comparing the same SWCNT population filled with an alkane versus those filled with water. This differs from encapsulation of alkanes into larger-diameter nanotubes, for which dielectric effects dominate because of the relative sizes of the guest molecules and the SWCNT cavity. For the SWCNT species examined in this work, the interior cavity diameters are ...

Understanding the Selection Mechanism of the Polymer Wrapping Technique toward Semiconducting Carbon Nanotubes

AbstractNoncovalent functionalization of single‐walled carbon nanotubes (SWNTs) using π‐conjugated polymers has become one of the most effective techniques to select semiconducting SWNTs (s‐SWNTs). Several conjugated polymers are used, but their ability to sort metallic and semiconducting species, as well as the dispersions yields, varies as a function of their chemical structure. Here, three polymers are compared, namely, poly[2,6‐(4,4‐bis‐(2‐dodecyl)‐4H‐cyclopenta[2,1‐b;3,4b′]dithiophene)‐alt‐4,7(2,1,3‐benzothiadiazole)] (P12CPDTBT), poly(9,9‐di‐n‐dodecylfluorenyl‐2,7‐diyl) (PF12), and poly(3‐dodecylthiophene‐2,5‐diyl) (P3DDT) in their ability to select two types of carbon nanotubes comprising small (≈1 nm) and large (≈1.5 nm) diameters. P12CPDTBT is a better dispersant than PF12 for small diameter nanotubes, while both polymers are good dispersants of large diameter nanotubes. However, these dispersions contain metallic species. P3DDT, instead presents the best overall performance regarding the selectivity toward semiconducting species, with a dispersion yield for s‐SWNTs of 15% for small and 21% for large diameter nanotubes. These results are rationalized in terms of electronic and chemical structure showing that: (i) the binding energy is stronger when more alkyl lateral chains adsorb on the nanotube surface; (ii) the binding energy is stronger when the polymer backbone is more flexible; (iii) the purity of the dispersions seems to depend on a strong polymer–nanotube interaction.

A molecular dynamics approach of the role of carbon nanotube diameter on thermal interfacial resistance through vibrational mismatch analysis

Carbon nanotubes (CNT) have been known to increase the heat transfer at the solid-liquid interfaces, but have a limitation due to the thermal interfacial resistance. Vibrational mismatch at the interface leads to this thermal interfacial resistance, which plays an important role in energy transfer at the boundary. Negligible work has been reported on the influence of CNT diameter on the resistance through the vibrational mismatch study. Molecular dynamics simulations have been performed to investigate the effect of single walled armchair CNT diameter on interfacial resistance between CNT and water molecules. This work is an effort to understand the heat transfer phenomenon at the interface of armchair CNT and water molecules. Vibrational mismatch at the interface is quantified by analyzing the vibrational spectra of CNT and water molecules. The thermal interfacial resistance is observed to be relatively higher for the larger diameter nanotube. This is attributed to the higher vibrational mismatch existing for larger diameter CNT due to low overlapping region between vibrational density states of CNT and water molecules. For smaller diameter CNT, the thermal interfacial resistance is low which results in the efficient heat transfer at the interface thus, emphasizing the indispensable role of smaller diameter CNTs in the cooling applications.

(Invited) Modulation of Nanotube Optical Properties By Controlling the Dielectric Environment inside of Single-Wall Carbon Nanotubes

Specific and tunable modification to the optical properties of single-wall carbon nanotubes is demonstrated through the direct encapsulation into the nanotube interior of guest molecules with measured static dielectric constants. Optical measurements on SWCNT populations containing over thirty distinct simple compounds of varying static dielectric from 1.8 to 109 in large diameter nanotubes, and 15 compounds in small diameter nanotubes demonstrate for the first time experimentally the general effect of filler static dielectric on the nanotube optical properties. Comparison to effective medium theory predictions is presented, and guest molecules are identified that lead to effects disobeying the general trend and theory. These results both demonstrate a new degree of exploitable modulation in the optical properties of SWCNTs, and provide a foundation for examining higher order effects in host-guest interactions in well controlled pore size materials.

(Invited) Diameter-Dependent Thermal Defunctionalization of Single-Walled Carbon Nanotubes

Covalent sidewall functionalization of single-walled carbon nanotubes (SWCNTs) is an important tool for tailoring their properties for research purposes and applications. In this study, SWCNT samples were first functionalized by reductive alkylation using metallic lithium and 1-iodododecane in liquid ammonia. Samples of the alkyl-functionalized SWCNTs were then pyrolyzed under an inert atmosphere at selected temperatures between 100 and 500 °C to remove the addends. The extent of defunctionalization was assessed using a combination of thermogravimetric analysis, Raman measurements of the RBM, D, and G bands, absorption spectroscopy of the first- and second-order van Hove peaks, and near-IR fluorescence spectroscopy of (n,m)-specific emission bands. These measurements all indicate a substantial dependence of defunctionalization rate on nanotube diameter, with larger diameter nanotubes showing more facile loss of addends. The effective activation energy for defunctionalization is estimated to be a factor of ~1.44 greater for 0.76 nm diameter nanotubes as compared to those with 1.24 nm diameter. The experimental findings also reveal the quantitative variation with functionalization density of the Raman D/G intensity ratio and the relative near-IR fluorescence intensity. Pyrolyzed samples show spectroscopic properties that are equivalent to those of SWCNTs prior to functionalization. The strong structure-dependence of defunctionalization rate suggests a new approach for diameter-sorting of mixed SWCNT samples on relatively large scales without ultrasonic agitation. In addition, properties of a new reactor designed for large-scale functionalization of SWCNTS will be presented. Figure 1

Doping-dependent G-mode shifts of small diameter semiconducting single-walled carbon nanotubes

We investigate the impact of electrochemical doping on the frequency of the G+-mode of a selection of polymer-sorted, nearly monochiral and semiconducting single-walled carbon nanotubes (SWCNTs) via in-situ Raman spectroscopy of ambipolar electrolyte-gated SWCNT network transistors. The purified semiconducting nanotubes have small diameters from 0.76 nm to 1.39 nm. In contrast to previous experimental data on large diameter nanotubes and current electron-phonon coupling theories, we find a pronounced red-shift (phonon softening) of the G+-mode at low to medium hole concentrations before an upshift is observed for larger hole concentrations. The effect is largest for those nanotubes with the smallest diameters. The pronounced dependence of these frequency shifts on the diameter of the semiconducting SWCNTs suggests that curvature effects start to play a significant role and should not be neglected.

Unzipping of multi-wall carbon nanotubes with different diameter distributions: Effect on few-layer graphene oxide obtention

Few-layer graphene oxide (FLGO) was obtained by chemical unzipping of multi-wall carbon nanotubes (MWCNT) of different diameter distributions. MWCNT were synthesized by catalytic decomposition of methane using Fe‐Mo/MgO catalysts. The variation in the Fe/Mo ratio (1, 2 and 5) was very influential in MWCNT diameter distribution and type of MWCNT obtained, including textural, chemical, structural and morphological characteristics. MWCNT diameter distribution and surface defects content had a profound impact on the characteristics of the resulting FLGO. Thus, MWCNT obtained with the catalyst with a Fe/Mo: 5 and presenting a narrow diameter distribution centered at 8.6±3.3nm led to FLGO maintaining non-oxidized graphite stacking (according to XRD analysis), lower specific surface area and higher thermostability as compared to FLGO obtained from MWCNT showing wider diameter distributions. The presence of more oxygen-containing functionalities and structural defects in large diameter nanotubes promotes the intercalation of species towards the inner layers of the nanotube, resulting in an enhanced MWCNT oxidation and opening into FLGO, what improves both micro- and mesoporosity.

Water in Carbon Nanotubes: The Peculiar Hydrogen Bond Network Revealed by Infrared Spectroscopy.

A groundbreaking discovery in nanofluidics was the observation of the tremendously enhanced water permeability of carbon nanotubes, those iconic objects of nanosciences. The origin of this phenomenon is still a subject of controversy. One of the proposed explanations involves dramatic modifications of the H-bond network of nanoconfined water with respect to that of bulk water. Infrared spectroscopy is an ideal technique to follow modifications of this network through the inter- and intramolecular bonds of water molecules. Here we report the first infrared study of water uptake at controlled vapor pressure in single walled carbon nanotubes with diameters ranging from 0.7 to 2.1 nm. It reveals a predominant contribution of loose H bonds even for fully hydrated states, irrespective of the nanotube size. Our results show that, while the dominating loosely bond signature is attributed to a one-dimensional chain structure for small diameter nanotubes, this feature also results from a water layer with "free" OH (dangling) bonds facing the nanotube wall for larger diameter nanotubes. These experimental findings provide a solid reference for further modeling of water behavior in hydrophobic nanochannels.

Probing the Diameter Limit of Single Walled Carbon Nanotubes in SWCNT: Fullerene Solar Cells

In this work, for the first time, the diameter limit of surfactant wrapped single walled carbon nanotubes (SWCNTs) in SWCNT:C60 solar cells is determined through preparation of monochiral small and large diameter nanotube devices as well as those from polychiral mixtures. Through assignment of the different nanotube chiralities by photoluminescence and optical density measurements a diameter limit yielding 0% internal quantum efficiency (IQE) is determined. This work provides insights into the required net driving energy for SWCNT exciton dissociation onto C60 and establishes a family of (n,m) species which can efficiently be utilized in polymer‐free SWCNT:C60 solar cells. Using this approach the largest diameter nanotube with an IQE > 0% is found to be (8,6) with a diameter of 0.95 nm. Possible strategies to extend this diameter limit are then discussed.

Selective synthesis of large diameter, highly conductive and high density single-walled carbon nanotubes by a thiophene-assisted chemical vapor deposition method on transparent substrates

Selective synthesis of single-walled carbon nanotubes (SWNTs) with controlled properties is an important research topic for SWNT studies. Here we report a thiophene-assisted chemical vapor deposition (CVD) method to directly grow highly conductive SWNT thin films on substrates, including transparent ones. By adding low concentration thiophene into the carbon feedstock (ethanol), the as-prepared carbon nanotubes demonstrate an obvious up-shift in the diameter distribution while the single-walled structure is still retained. In the proposed mechanism, the change in the diameter is sourced from the increase in the carbon yield induced by the sulfur-containing compound. Such SWNTs are found to possess high conductivity with 95% SWNTs demonstrating on/off ratios lower than 100 in transistors. More importantly, it is further demonstrated that this method can be used to directly synthesize dense SWNT networks on transparent substrates which can be utilized as transparent conductive films (TCFs) with very high transparency. Such TCFs can be applied to fabricate a light modulating window as a proof-of-concept. The present work provides important insights into the growth mechanism of SWNTs and great potential for the preparation of TCFs with high scalability, easy operation and low cost.