Sidewalls Of Carbon Nanotubes Research Articles

Catch and release: Gold adsorption and sorbent electrochemical regeneration

Wastewater generated from e-waste leaching is rich in precious metals including gold, silver and platinum. Conventional precipitation and solvent extraction are chemically intensive separations with concerning environmental externalities. Sorbents, in particular carbon nanotubes, have low chemical consumption, and have shown promise for gold adsorption due to their high specific surface area and chemical functionalization potential. However, regenerating sorbents used to adsorb Au is hazardous requiring strong acids. Herein, we delineate the effect of various functional groups on the sidewalls of multiwall carbon nanotubes (MWCNTs) on gold adsorption, and we introduced an acid-free electrochemical technique for Au elution from MWCNTs. Pristine MWCNTs (P-MWCNTs), carboxylic functionalised MWCNTs (COOH-MWCNTs) and amide functionalised MWCNTs (NH2-MWCNTs) were compared for their affinity for Au adsorption from acidic AuCl4−solutions mimicking acidic e-waste leachate. Au adsorption affinity onto MWCNTs followed the order of P-MWCNT > NH2-MWCNT > COOH-MWCNTs. Au elution from Au-saturated MWCNTs was subsequently achieved up to 65%, using acid-free electrochemical desorption in neutral aqueous brine. The Au electro-desorption was shown to have a direct relationship with both the applied current and the mass of the Au adsorbed on the MWCNTs. This study demonstrates enhanced adsorption-based preconcentration of gold and acid-free regeneration of sorbents.

Covalently Bonded Graphene Sheets on Carbon Nanotubes: Direct Growth and Outstanding Properties

AbstractIntegrating 1D carbon nanotubes (CNTs) and 2D graphene with covalent bonds can inherit the outstanding properties of both components and obtain additional advantages. Here, this work reports the preparation of covalently bonded graphene/CNT (G/CNT) structure by a normal chemical vapor deposition method. Specifically, the pre‐synthesized defects on the sidewall of CNTs act as nucleation sites for the growth of graphene sheets to form a branch‐leaf structure. Graphene leaves restrict the sliding and re‐stacking of CNTs, endowing G/CNT hybrid demonstrates excellent anti‐agglomeration properties that are not present in either graphene or CNTs. In addition, the covalently bonded structure and high graphitization degree of graphene sheets and CNTs significantly enhance the comprehensive properties of the G/CNT hybrid material, such as large specific surface area, excellent thermal stability, and high electrical conductivity. Consequently, the microwave absorption properties of G/CNT are significantly enhanced compared with CNTs. This work provides a feasible pathway to synthesize high‐performance covalently bonded G/CNT hybrids.

Voltammetric Evidence of Proton Transport through the Sidewalls of Single-Walled Carbon Nanotubes.

Understanding ion transport in solid materials is crucial in the design of electrochemical devices. Of particular interest in recent years is the study of ion transport across 2-dimensional, atomically thin crystals. In this contribution, we describe the use of a host-guest hybrid redox material based on polyoxometalates (POMs) encapsulated within the internal cavities of single-walled carbon nanotubes (SWNTs) as a model system for exploring ion transport across atomically thin structures. The nanotube sidewall creates a barrier between the redox-active molecules and bulk electrolytes, which can be probed by addressing the redox states of the POMs electrochemically. The electrochemical properties of the {POM}@SWNT system are strongly linked to the nature of the cation in the supporting electrolyte. While acidic electrolytes facilitate rapid, exhaustive, reversible electron transfer and stability during redox cycling, alkaline-salt electrolytes significantly limit redox switching of the encapsulated species. By "plugging" the {POM}@SWNT material with C60-fullerenes, we demonstrate that the primary mode of charge balancing is proton transport through the graphenic lattice of the SWNT sidewalls. Kinetic analysis reveals little kinetic isotope effect on the standard heterogeneous electron transfer rate constant, suggesting that ion transport through the sidewalls is not rate-limiting in our system. The unique capacity of protons and deuterons to travel through graphenic layers unlocks the redox chemistry of nanoconfined redox materials, with significant implications for the use of carbon-coated materials in applications ranging from electrocatalysis to energy storage and beyond.

Antiproliferative Activity of PEG-PEI-SWCNTs against AMJ13 Breast Cancer Cells

Single-walled carbon nanotubes (SWCNTs) involving polyethylene glycol (PEG) and polyethyleneimine (PEI), which contain amines, were produced and analyzed using X-ray diffraction (XRD), UV–vis spectroscopy, Raman spectroscopy (RS), and transmission electron microscopy (TEM) for the spectral and structural analyses of PEG-PEI-SWCNTs. PEG-PEI-SWCNTs demonstrated peaks using UV–vis spectroscopy at 300.98 nm, have a broad peak at 2θ = 23.51, linked to 002 with d-spacing (3.7816), and have crystalline sizes of 8.48 nm. In PEG-PEI-SWCNTs, TEM images demonstrated the accumulation of SWCNTs with PEG-PEI due to an increased main grain size with functionalization to 80.68 nm. The D and G bands in PEG-PEI-SWCNTs moved to 1,292 and 1,586 cm−1, respectively. SWCNTs are observed as a bundle of inhomogeneous, long-curved accumulates containing multitudes of tubes. The PEG-PEI-SWCNTs have a tubular structure that demonstrates particles scattered alongside the carbon nanotube sidewalls, which indicate that PEG and PEI are conjugated to the SWCNTs. Cytotoxicity evaluation of PEG-PEI-SWCNTs (6.25–100 µg/mL) in the breast cancer AMJ13 cell line was conducted for 24 and 72 hr, and the results showed enhanced toxicity to the tumor cells and decreased cytotoxicity against rhabdomyosarcoma (RD) normal cells.

Improving reinforcement properties of CNTs in aluminium matrix composites: a case of surface modification through AlN nano-particle grafting

This paper accentuates a newer approach to modify the surfaces of pristine multiwall carbon nanotubes (CNTs) by two-phase acid treatment, followed by silanization and grafting of functionalized AlN nano-particles onto the CNT surfaces, as a preparatory treatment before reinforcing in liquid metallurgy processed aluminium matrix composites (AMCs) without disrupting their structural integrity. Besides, the surface modified CNTs will have the strong potential to improve their dispersion uniformity, wettability and matrix-reinforcement bonding strength in the AMCs. The AlN nano-particle grafted CNTs were characterized through dispersion stability test, UV-Visible spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, Raman spectroscopy and Thermogravimetric analysis (TGA). Microstructure and elemental analysis were conducted through Scanning electron microscope (SEM) embedded with Energy dispersive X-ray spectroscope (EDS), Field emission scanning electron microscope (FESEM) and Transmission electron microscope (TEM). The aforementioned characteristics of AlN nano-particle grafted CNTs were compared with those of the pristine CNTs, acid treated CNTs and silanized CNTs to demonstrate the effectiveness of AlN nano-particle grafting on the CNT surfaces. Results revealed improvement of dispersion stability, covalent attachment of AlN nano-particles onto CNT surfaces (FTIR spectroscopy), existence of AlN on the CNT sidewalls (XRD analysis) and peaks corresponding to Al and N (EDS). Evidence of AlN on the CNT surfaces was also established through SEM, FESEM, TEM and EDS. Though there was a paltry degradation of CNT surfaces after the acid treatment, it was recovered substantially during silanization, and there was no evidence of tube shortening or fragmentations in SEM of silanized and AlN grafted CNTs.

Adsorption of Peptides onto Carbon Nanotubes Grafted with Poly(ethylene Oxide) Chains: A Molecular Dynamics Simulation Study.

Carbon nanotubes (CNTs) display exceptional properties that predispose them to wide use in technological or biomedical applications. To remove the toxicity of CNTs and to protect them against undesired protein adsorption, coverage of the CNT sidewall with poly(ethylene oxide) (PEO) is often considered. However, controversial results on the antifouling effectiveness of PEO layers have been reported so far. In this work, the interactions of pristine CNT and CNT covered with the PEO chains at different grafting densities with polyglycine, polyserine, and polyvaline are studied using molecular dynamics simulations in vacuum, water, and saline environments. The peptides are adsorbed on CNT in all investigated systems; however, the adsorption strength is reduced in aqueous environments. Save for one case, addition of NaCl at a physiological concentration to water does not appreciably influence the adsorption and structure of the peptides or the grafted PEO layer. It turns out that the flexibility of the peptide backbone allows the peptide to adopt more asymmetric conformations which may be inserted deeper into the grafted PEO layer. Water molecules disrupt the internal hydrogen bonds in the peptides, as well as the hydrogen bonds formed between the peptides and the PEO chains.

Carbon Nanotubes from Synthesis to Picomolar Detection Electrochemical Sensors

Electrochemistry at open ends and sidewalls of carbon nanotubes (CNTs) has been under debate, with opposing viewpoints as to which sites are more electrochemically active. A particular challenge in this field has been the ability to conduct electrochemical studies selectively at the open-ends of CNTs, without measuring contributions from the sidewalls. This talk will discuss the synthesis and assembly of CNTs into electrochemical sensor where open-ended CNTs were employed for electrochemical measurements. The assembly employs drawable CNTs that minimize sample handling and contamination, in the attempt to preserve the pristine nature of CNTs. Highly densified multiwalled carbon nanotube (HD-CNT) fibers were embedded within a polymer matrix protecting the sidewalls and limiting the reactions to the tips of the CNTs. Cyclic voltammetry was employed to examine the electrochemical properties of open-ended CNTs using a conventional bulk electrochemical cell and scanning electrochemical cell microscopy (SECCM). This assemblies have shown to detect extremely low concentrations of Pb2+ in water, neurotransmitters, NADH, furosemide (diuretic drug), and the electrodes were tested with multiple electrochemical techniques.

Exploring the Covalent Doping of Single-Wall Carbon Nanotubes Induced By Photoexcited Hypochlorite

Chemical functionalization of the sidewalls of single-wall carbon nanotubes remains a topic of current interest because it can modify and enhance nanotube optical properties. These modifications can shift SWCNT structure-specific optical transitions further into the near-infrared, which may prove useful for applications in bio-imaging or quantum information processing. Here we present a study on the use of photoexcited hypochlorite ions as a reactant to efficiently produce sparse oxygen-containing defects in SWCNTs. Our studies highlight the formation of new secondary reaction products in the absence of dissolved oxygen. These results in combination with quantum computations provide insight into the structure of the new photoadducts. Additionally, we find that product spectra depend on pH of the reacting system, implying more than one reaction pathway. This finding is supported by measurements of the effects of radical quenchers on product properties. Our results provide improved understanding and control of an important class of nanotube functionalization reactions.

Defect Etching in Carbon Nanotube Walls for Porous Carbon Nanoreactors: Implications for CO2 Sorption and the Hydrosilylation of Phenylacetylene.

A method of pore fabrication in the walls of carbon nanotubes has been developed, leading to porous nanotubes that have been filled with catalysts and utilized in liquid- and gas-phase reactions. Chromium oxide nanoparticles have been utilized as highly effective etchants of carbon nanotube sidewalls. Tuning the thermal profile and loading of this nanoscale oxidant, both of which influence the localized oxidation of the carbon, have allowed the controlled formation of defects and holes with openings of 40–60 nm, penetrating through several layers of the graphitic carbon nanotube sidewall, resulting in templated nanopore propagation. The porous carbon nanotubes have been demonstrated as catalytic nanoreactors, effectively stabilizing catalytic nanoparticles against agglomeration and modulating the reaction environment around active centers. CO2 sorption on ruthenium nanoparticles (RuNPs) inside nanoreactors led to distinctive surface-bound intermediates (such as carbonate species), compared to RuNPs on amorphous carbon. Introducing pores in nanoreactors modulates the strength of absorption of these intermediates, as they bond more strongly on RuNPs in porous nanoreactors as compared to the nanoreactors without pores. In the liquid-phase hydrosilylation of phenylacetylene, the confinement of Rh4(CO)12 catalyst centers within the porous nanoreactors changes the distribution of the products relative to those observed in the absence of the additional pores. These changes have been attributed to the enhanced local concentration of phenylacetylene and the environment in which the catalytic centers reside within the porous carbon host.

Ruthenium‐p‐cymene Complex Side‐Wall Covalently Bonded to Carbon Nanotubes as Efficient Hybrid Transfer Hydrogenation Catalyst

AbstractA half‐sandwich ruthenium‐p‐cymene organometallic complex has been immobilized at Single Walled Carbon Nanotubes (SWNT) sidewalls through a stepwise covalent chemistry protocol. The introduction of amino groups by means of diazonium‐chemistry protocols leads the grafting at the outer walls of the nanotubes. This hybrid material is active in the transfer hydrogenation of ketones to yield alcohols, using as hydrogen source 2‐propanol. SWNT−NH2−Ru presents a broad scope, performing the reaction under aerobic conditions and can be recycled over 9 consecutive reaction runs without losing activity or leaching ruthenium out. Comparison of the activity with related homogeneous catalysts reveals an improved performance due to the covalent bond between the metal and the material, achieving turnover frequencies as high as 192774 h−1.

The Influence of Reaction Time on Non-Covalent Functionalisation of P3HT/MWCNT Nanocomposites.

Non-covalent functionalisation of the carbon nanotube (CNT) sidewall through polymer wrapping is the key strategy for improving well-dispersed CNTs without persistent alteration of their electronic properties. In this work, the effect of reaction time on regioregular poly (3-hexylthiophene-2,5-diyl) (P3HT)-wrapped hydroxylated multi-walled CNT (MWCNT-OH) nanocomposites was investigated. Five different reaction times (24, 48, 72, 96, and 120 h) were conducted at room temperature in order to clearly determine the factors that influenced the quality of wrapped MWCNT-OH. Morphological analysis using Field Emission Scanning Electron Microscopic (FESEM) and High-Resolution Transmission Electron Microscope (HRTEM) analysis showed that P3HT successfully wrapped the MWCNT-OH sidewall, evidenced by the changes in the mean diameter size of the nanocomposites. Results obtained from Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) as well as Thermogravimetric Analysis (TGA) showed a significant effect of the wrapped polymer on the CNT sidewall as the reaction time increased. Overall, the method used during the preparation of P3HT-wrapped MWCNT-OH and the presented results significantly provided a bottom-up approach to determine the effect of different reaction times on polymer wrapping to further expand this material for novel applications, especially chemical sensors.

Highly Selective Adsorption and Desorption of Charged Molecules in Three-Dimensional Networks of Polydopamine-Modified Carbon Nanotube Sponges.

We investigated the selective adsorption and desorption behaviors of charged molecules (calcein, brilliant green, and methylene blue) dissolved in water using polydopamine-modified carbon nanotube (CNT) sponges. Porous CNT sponges (CNTSs) as a scaffold for the selective adsorption and desorption of aqueous molecules were fabricated by using a chemical vapor deposition technique. To improve the hydrophilicity of porous CNTS and to control the adsorption and desorption of aqueous molecules, CNT sidewalls were decorated with a hydrophilic polydopamine layer through noncovalent interactions between CNT sidewalls and polydopamine. After this noncovalent chemical modification, the water contact angle of CNTS was close to 0, and the aqueous solution can rapidly infiltrate the three-dimensional (3D) networks of polydopamine-modified CNTS (Pdop-CNTS). The incorporation of pH-responsive polydopamine in CNTS showed an evident advantage of adsorbing positively charged molecules over a pH range of 10.5-4. In aqueous solutions with pH value of ≤3, Pdop-CNTS selectively adsorbed negatively charged molecules. Aqueous molecules carrying net charges were successfully separated from mixture solutions. Moreover, charged calcein and methylene blue molecules adsorbed on the 3D networks of Pdop-CNTS were selectively desorbed from Pdop-CNTS by tuning the pH value of the desorption solution.

A Novel Carbon Support: Few‐Layered Graphdiyne‐Decorated Carbon Nanotubes Capture Metal Clusters as Effective Metal‐Supported Catalysts

Carbon-supported metal nanocatalysts have received substantial attention for heterogeneous catalysis in industry. Hunting for suitable and impactful carbon supports that have strong interactions with metal nanocatalyst is a matter of great urgency. Herein, a well-designed graphdiyne layer decorated on the carbon nanotubes sidewalls (CNT@GDY) serves as a novel carbon support. This unique hybrid structure effectively traps platinum and palladium atomic clusters (Pt/Pd-ACs) with dimensions of 0.65nm and 1.05nm uniformly and firmly, forming novel carbon-supported metal nanocatalysts (Pt(Pd)-ACs/CNT @ GDY) for efficient hydrogen generation and aromatic nitroreduction, respectively. The Pt-ACs/CNT@GDY can deliver an HER current density of 10mA cm-2 with a small overpotential of 23mV in 0.5 M H2 SO4 , showing a greatly enhanced mass activity, intrinsic activity than the commercial Pt/C catalyst. The Pd-ACs/CNT@GDY also exhibits excellent catalytic activity and a high turnover frequency of 38.0 min-1 for aromatic nitroreduction. The carbon support turns out to possess excellent conductivity, abundant and uniform reactive sites, low redox potential, more negative surface and large specific surface area as well as a strong interaction with ACs, as anticipated in ideal supports, which can be applied in other metal-supported catalysts.

Improved wettability and enhanced ionic transport in highly porous CNT sponge

We investigated the effect of an electric treatment on the wettability of aqueous solution on carbon nanotubes (CNT) and ion transport behaviors in superhydrophobic porous carbon nanotube sponges (CNTS). This electric activation treatment where an electric voltage was applied across highly porous CNT sponge induced an electrowetting effect. This effect significantly reduced interfacial tensions between CNT sidewalls and aqueous liquids. Meanwhile, polar functional groups were also introduced on CNTs. Both electrowetting effect and polar functional groups greatly improved the wettability of aqueous solutions on CNT sidewalls. After the electric treatment, we observed a dramatic increase in the overall rate of ion flow across porous CNT sponges. The formation of solution channels during the electric treatment is responsible for the enhanced ionic transport in porous CNT sponges. The overall rate of ion flow increased with the increases in electric treatment time and voltage. The crucial role of electric treatment parameters in the ion transport provides a new strategy for precisely controlling the ion transport across CNT sponges by tuning electric treatment time or voltage. Importantly, the good wettability of aqueous solution on CNT sidewalls greatly increased the effective surface area of CNT sponges and thus significantly improved the performance of CNTS-based supercapacitors after the electric treatment.

Role of defects in the sensing mechanism of CNTFET gas sensors

In nanotechnology, understanding the effect of interfaces and defects becomes critically important in determining a material's properties and device performance. Many one-dimensional and two-dimensional materials exhibit excellent physical, electrical, thermal, and optical properties, making them highly desirable for a wide array of applications. However, their low dimensionality also means their performance can be susceptible to defects in the material and the interfaces they form with other device components. Carbon nanotubes are often used in sensing applications, typically, in a field-effect transistor configuration (CNTFET). The interface between the contact electrode metal and the nanotube forms a Schottky barrier, which plays an important role in both transistor and sensor characteristics. Modifications to this interface by the environment can modulate the barrier and produce a change in device characteristics. Transistor operation can also be modified by the presence of defects in the carbon nanotube sidewall structure. This paper explores how defects in single-wall carbon nanotubes can affect the sensing mechanism of CNTFET gas sensing devices. Gas exposure measurements were performed on as-grown near pristine (low defect) nanotube devices and compared with plasma irradiated, highly defective nanotube devices. By utilizing selective passivation to isolate structural components that contribute to the sensing mechanism, the study shows that the presence of defects, and their relative densities, has a critical role in gas sensing performance. Recognizing the presence of these defects, even in as-grown nanotubes, can help reconcile seemingly contradictory results in the literature regarding the gas sensing mechanism. This work represents an important step toward understanding the effect of both interfaces and defects for carbon nanotube sensor development.

Boosting the Utilization and Electrochemical Performances of Polyaniline by Forming a Binder-Free Nanoscale Coaxially Coated Polyaniline/Carbon Nanotube/Carbon Fiber Paper Hierarchical 3D Microstructure Composite as a Supercapacitor Electrode.

Nanoscale polyaniline (PANI) is formed on a hierarchical 3D microstructure carbon nanotubes (CNTs)/carbon fiber paper (CFP) substrate via a one-step electrochemical polymerization method. The chemical and structural properties of the binder-free PANI/CNTs/CFP electrode are characterized by field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. The specific capacitance of PANI/CNTs/CFP tested in a symmetric two-electrode system reaches 731.6 mF·cm–2 (1354.7 F·g–1) at a current density of 1 mA·cm–2 (1.8 A·g–1). The symmetric supercapacitor device demonstrates excellent cycling performance up to 10,000 cycles with a capacitance retention of 81.4% at a current density of 1 mA·cm–2 (1.8 A·g–1). The results demonstrate that the binder-free CNTs/CFP composite is a strong backbone for depositing ultrathin PANI layers at a high mass loading. The hierarchical 3D microstructure PANI/CNTs/CFP provides enough space and transporting channels to form an efficient electrode–electrolyte interface for the supercapacitance reaction. The formed nanoscale PANI film coaxially coated on the sidewalls of CNTs enables efficient charge transfer and a shortened diffusion length. Hence, the utilization efficiency and electrochemical performances of PANI are significantly improved. The rational design strategy of a CNT-based binder-free hierarchical 3D microstructure can be used in preparing various advanced energy-storage electrodes for electrochemical energy-storage and conversion systems.

Spatial dependence of the temperature profile along a carbon nanotube during thermal-field emission

An efficient algorithm is described to calculate the spatial dependence of the temperature distribution along a carbon nanotube (CNT) during field emission (FE). The algorithm considers the effects of Joule heating in the CNT and radiative losses from the CNT sidewall and tip. The CNT emission current density and the rate of heat exchange per unit area at the CNT tip due to either Henderson-cooling or Nottingham-heating effects are calculated using recent analytical expressions derived by Jensen [J. Appl. Phys. 126, 065302 (2019)]. The latter are valid in the thermionic and field emission regimes and in the transition region between these two extremes. The temperature dependence of the electrical resistivity ρ(T) and the thermal conductivity κ(T) of the CNT is also included in the model. It is shown that replacing ρ(T) and κ(T) by their spatial averages over the length of the CNT can lead to an overestimate of the value of the external electric field threshold at which thermal runaway of the CNT occurs. These results should be considered when calculating the field emission characteristics of CNT arrays such as from a carbon nanotube fiber whose FE properties are primarily determined by the FE properties of the array of CNTs at the tip of the fiber. Using the new algorithm, the simulation times to calculate the CNT FE characteristics and the spatial temperature distribution are found to be nearly two orders of magnitude faster compared to those required when both the current and energy exchange at the CNT tip are calculated numerically.

Electrochemistry of Controlled-Diameter Carbon-Nanotube Fibers at the Cross Section and Sidewall

Electrochemistry at open ends and sidewalls of carbon nanotubes (CNTs) has been under debate, with opposing viewpoints as to which sites are more electro-chemically active. A particular challenge in this field has been the ability to conduct electrochemical studies at the open ends of CNTs, without measuring contributions from the sidewalls. Among the detailed studies on CNT electrochemistry by various research groups, no reports show a direct comparison between the electrochemistry of open ends and sidewalls. That is to say, electrochemical studies at open ends have always had some contribution from sidewalls. Multiple research groups have reported electrochemistry at CNT sidewalls and close ends, although there are no reports that examine the electron transfer rate at open ends exclusively. In cases where rapid electron transfer at open ends has been reported, this has been on either defective dispersed CNTs at an electroactive surface, or on vertically aligned CNTs which are inherently porous. More interestingly, it was also observed that the electrochemical response at CNT modified electrode surfaces could be dominated by thin layer cell behavior, which arises due to the change in diffusional regime or mass transport. Therefore, in such electrode assemblies, we cannot completely assign the electrochemical response to open ends since sidewalls also contributed significantly to the measured current. In this work, we report on electrochemical measurements taken from the sidewall and the cross-section (i.e., open ends) of highly densified multi-walled carbon nanotube fiber (HD-CNTfs) embedded in a polymer matrix. The cross section of the HD-CNTfs was exposed to reveal only open ends to the electrolyte interface and this platform allowed us to compare electrochemistry at the open ends and sidewalls of HD-CNTfs. Cyclic voltammetry was employed to examine the electrochemical properties of HD-CNTfs using a conventional bulk electrochemical cell and micro-capillary electrochemical measurements. The cross sections and side walls of HD-CNTfs are shown to promote similar electron transfer kinetics with reversible redox behavior. However, side walls of HD-CNTfs also showed large capacitive background current which was found to increase in proportional to CNT fiber diameter. Figure 1

Hybrid Plasma-Liquid Treatment of Carbon Nanotubes for Application in Direct Absorption Solar Thermal Collectors

Awareness of the environmental pollution caused by fossil fuels and the ever-increasing demand for energy has driven research focussed on zero-carbon renewable energy sources. Conscious of this, direct absorption solar thermal collectors are gaining prominence, both for domestic heating and steam generation. Carbon nanotubes (CNTs) have been investigated as an additive to take advantage of the near blackbody absorption and high thermal conductivity to improve system efficiency. Critical to the success of the additive nanoparticles is the stability in the fluid, however, due to their high surface energy CNTs tend to agglomerate which can limit the efficacy of their integration into real-world products. Covalent functionalization of the CNT sidewalls with oxygen-based or nitrogen-based functional groups is one of the most popular techniques to modify the surface energy of the CNTs.1 As an alternative to the harsh chemicals used by traditional chemical functionalisation, plasma-based surface treatments can be used with great efficacy and reduced reaction times.In this work, macroscopic ribbon-like assemblies of carbon nanotubes are functionalized using a simple direct current-based plasma-liquid system. This system utilizes the plasma-generated species in a fluid of 10 %vol ethanol in water, with or without a nitrogen precursor, for the oxygen and/or nitrogen functionalisation of the carbon nanotube assembly. For this treatment, a ribbon-like piece of CNT was used as the anode and a helium plasma discharge triggered by applying 10 mA and 1 - 1.6 kV to act as the cathode. The plasma-generated species are then expected to migrate towards the CNTs and functionalisation the sidewalls.The oxygen content is shown to be increased by between 50 % and 200 % when the treatment solution of 10 %vol ethanol is used in the ribbon electrode configuration. When ethylenediamine is added as a nitrogen precursor, the atomic concentration of nitrogen reaches 23 % in the ribbon electrode configuration, with amine groups, pyrrolic groups and graphitic nitrogen observed in the x-ray photoelectron spectra. This nitrogen content is unmatched in quantity when compared to either a simple soaking procedure or an electrolysis process with a platinum foil replacing the plasma as the counter-electrode. This demonstrates that the plasma either directly or indirectly, by means of plasma-generated species, facilitates and enhances the availability of nitrogen from the ethylenediamine precursor. The potential plasma-induced chemical pathways which lead to the functionalization of the CNTs are also investigated and discussed.In the application of a DASC system, it is found that the covalent functionalisation provided by the plasma-liquid system enhances the stability of the CNT-EG nanofluid, with the oxygen-functionalized samples producing the most stable nanofluids. This functionalization method generates interest for a broader range of applications including mechanically superior advanced composites,2 more sensitive gas sensors,3 enhanced energy storage materials4 or ultra-conductive fibres.5 References (1) Mallakpour, S.; Soltanian, S. Surface Functionalization of Carbon Nanotubes: Fabrication and Applications. RSC Adv. 2016, 6 (111), 109916–109935.(2) Williams, J.; Broughton, W.; Koukoulas, T.; Rahatekar, S. S. Plasma Treatment as a Method for Functionalising and Improving Dispersion of Carbon Nanotubes in Epoxy Resins. J. Mater. Sci. 2013, 48 (3), 1005–1013.(3) Ham, S. W.; Hong, H. P.; Kim, J. H.; Min, S. J.; Min, N. K. Effect of Oxygen Plasma Treatment on Carbon Nanotube-Based Sensors. J. Nanosci. Nanotechnol. 2014, 14 (11), 8476–8481.(4) Hulicova-Jurcakova, D.; Seredych, M.; Lu, G. Q.; Bandosz, T. J. Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon on Its Electrochemical Performance in Supercapacitors. Adv. Funct. Mater. 2009, 19 (3), 438–447.(5) Li, L.; Liu, E.; Shen, H.; Yang, Y.; Huang, Z.; Xiang, X.; Tian, Y. Charge Storage Performance of Doped Carbons Prepared from Polyaniline for Supercapacitors. J. Solid State Electrochem. 2011, 15 (1), 175–182. Figure 1

Trace Ethylene Sensing via Wacker Oxidation.

Ethylene is a dynamic plant hormone, and its temporal monitoring can be used to glean insight into plant health and status. However, the real-time distributed detection of ethylene at trace levels under ambient conditions remains a challenge. We report a single-walled carbon nanotube-based chemiresistor catalyst combination that can detect ppb levels of ethylene in air. Cycling between Pd(II) and Pd(0) via Wacker oxidation with a nitrite cocatalyst imparts response discrimination driven by the chemoselectivity of the chemical transformation. Sensitivity is controlled by a combination of the chemical reaction efficiency and the n-doping strength of the Pd(0) species generated in situ. The covalent functionalization of the carbon nanotube sidewall with pyridyl ligands drastically improves the device sensitivity via enhanced n-doping. The utility of this ethylene sensor is demonstrated in the monitoring of senescence in red carnations and purple lisianthus flowers.