Internal charge transfer in metallicity sorted ferrocene filled carbon nanotube hybrids

Site selective high resolution photoemission and X-ray absorption have revealed the complex interplay between ferrocene and single-walled carbon nanotubes (SWCNTs) upon filling. The use of ultraclean and metallicity sorted nanotubes as starting material and a subsequent ferrocene filling, yields metallicity sorted hybrids, where the details of the internal charge transfer from Fe to the SWCNTs have been distinctly unravelled. An n-type doping of the SWCNTs has been identified, with the peculiarity that purely metallic tubes are prone up to 30% higher doping than their semiconducting counterparts. Concomitantly, the average Fe valency changes from 2 in ferrocene to 2.3 in the ferrocene/semiconducting SWCNT hybrids and to 2.4 in the ferrocene/metallic SWCNT hybrids. The origin of this extra charge transfer has been revealed to be ionic by resonant photoemission, which excludes a finite hybridization between the ferrocene filler and the SWCNTs. This does not only substantiate that the internal charge transfer determines the resulting electronic transport properties in the filled 1D carbon hybrids, but that the metallic or semiconducting character of the encapsulating tubes is crucial towards tailoring and regulating tunable 1D electronic transport properties in these materials that are highly desirable in nanoelectronics.     

Hydrogen sulfide sensing properties of multi walled carbon nanotubes

This paper investigates the effect of functional groups on the hydrogen sulfide sensing properties of multi-walled carbon nanotubes using carboxyl and amide groups and Mo and Pt nanoparticles as decorated precursors in gaseous state at working temperature. Carbon nanotubes were synthesized by the CVD process and decorated with the nano particles; provide higher sensitivity for H₂S gas detection. The MWCNTs were characterized by scanning electron microscopy combined with energy dispersive X-ray (SEM/EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), ATR-IR absorption and Fourier transforms infrared (FT-IR) analyses. The MWCNTs were deposited as a thin film layer between prefabricated gold electrodes on alumina surfaces. The sensitivity of carbon nanotubes was measured for different H₂S gas concentrations and at working temperature. The results showed that the measured electrical conductance of the modified carbon nanotubes with functional groups is modulated by charge transfer with P-type semiconducting characteristics and metal decorated carbon nanotubes exhibit better performances compared to functional groups of carboxyl and amide for H₂S gas monitoring at room temperature.     

Synthesis and characterization of single-walled carbon nanotubes filled with the superionic material SnF2

We report X-ray powder diffraction, high resolution transmission electron microscopy and Raman spectroscopy of the semiconducting single-walled carbon nanotubes filled with the superionic material SnF₂. We first have obtained the Raman spectra of filling SWCNTs using the near-infrared excitation (1064 nm). Our results show that SnF₂ behaves as an electron acceptor with small charge transfer from nanotubes to SnF₂. We interpret the large high-frequency shift of radial breathing modes as due to both charge transfer and the increasing interaction between filling material and nanotubes.     

Production of predominantly semiconducting double-walled carbon nanotubes

The effects of growth conditions, such as methane flow rates and type of substrate on the distribution, structure and properties of nanotubes were examined. A scanning electron microscope equipped with a Raman spectrometer enabled us to obtain critical information about the structure and electrical properties of the nanotubes simultaneously, and it was shown that these were highly dependent on the methane flow rate. At a methane flow rate of 600 cc/min, we primarily obtained double-walled carbon nanotubes having predominantly semiconducting properties. At a higher methane flow rate (700 cc/min), a mixture of single-walled and double-walled carbon nanotubes was created, most of which were semiconducting. At low methane flow rates (300 and 500 cc/min), metallic multi-walled carbon nanotubes were predominated. Carbon nanotubes grown on a quartz substrate were between 4–10 μm in length, whereas those grown on silicon were longer (∼15–20 μm). The primary growth mechanism observed was base growth, although some cap growth did occur. Based on the results of this study, it is now possible to carefully control the synthesis conditions to produce carbon nanotubes that possess specific electrical properties that suit the desired application.     

Comparison of carbon nanotube modified electrodes for photovoltaic devices

Substrates with four different nanotube modifications have been prepared and their electron transport properties measured. Two modification techniques were compared; covalent chemical attachment of both single and multi-walled carbon nanotubes to transparent conductive (fluorine doped tin oxide) glass surfaces and chemical vapour deposition (CVD) growth of both single and multi-walled carbon nanotubes on highly doped conductive silicon wafers. These carbon nanotube modified substrates were investigated using scanning electron microscopy and substrates with nanotubes grown via CVD have a much higher density of nanotubes than substrates prepared using chemical attachment. Raman spectroscopy was used to verify that nanotube growth or attachment was successful. The covalent chemical attachment of nanotubes was found to increase substrate electron transfer substantially compared to that observed for the bare substrate. Nanotube growth also enhanced substrate conductivity but the effect is smaller than that observed for covalent attachment, despite a lower nanotube density in the attachment case. In both modification techniques, attachment and growth, single-walled carbon nanotubes were found to have superior electron transfer properties. Finally, solar cells were constructed from the nanotube modified substrates and the photoresponse from the different substrates was compared showing that chemically attached single-walled nanotubes led to the highest power generation.     

Synthesis of Y-junction single-wall carbon nanotubes

Y-junction single-wall carbon nanotubes (SWNTs) are synthesized using controlled catalysts by chemical vapor deposition. Mo-doped Fe nanoparticles supported by aluminum oxide particles are used as catalysts for growing Y-junction single-wall carbon nanotubes. Distribution of Mo-doped Fe particles plays an important role in Y-junction formation. Transmission electron microscopy confirmed the formation of single-walled structures of Y-junctions with diameters of 2-5 nm. Radial breathing mode peaks in Raman spectra show that our sample has both metallic and semiconducting nanotubes, indicating the possible formation of Y-branching with different electrical properties. The different electrical properties of branch and stem can be utilized in nanoscale three terminal electronic devices. The growth mechanism of Y-junction SWNTs is proposed.     

The photoinduced charge transfer mechanism in aligned and unaligned carbon nanotubes

Using time-resolved reflectivity measurements on unaligned and aligned bundled single-wall carbon nanotubes with a pump energy of 1.55 eV, quasi-resonant with the second Van Hove singularity of semiconducting tubes, a positive sign of the transient reflectivity is detected in unaligned nanotubes. In contrast a negative sign is detected in aligned nanotubes. This discovery addresses a long-standing question showing that in unaligned nanotubes the stronger intertube interactions favor the formation of short-lived free charge carriers in semiconducting tubes. A detailed analysis of the transient reflectivity spectral response shows that the free carriers in the photo-excited state of semiconducting tubes move towards metallic tubes in about 400 fs.     

Formation of single-walled carbon nanotube thin films enriched with semiconducting nanotubes and their application in photoelectrochemical devices

Single-walled carbon nanotube (SWCNT) thin films, containing a high-density of semiconducting nanotubes, were obtained by a gel-centrifugation method. The agarose gel concentration and centrifugation force were optimized to achieve high semiconducting and metallic nanotube separation efficiency at 0.1 wt% agarose gel and 18,000g. The thickness of SWCNT films can be precisely controlled from 65 to 260 nm with adjustable transparency. These SWCNT films were applied in photoelectrochemical devices. Photocurrents generated by semiconducting SWCNT enriched films are 15-35% higher than those by unsorted SWCNT films. This is because of reducing exciton recombination channels as a result of the removal of metallic nanotubes. Thinner films generate higher photocurrents because charge carriers have less chances going in metallic nanotubes for recombination, before they can reach electrodes. Developing more scalable and selective methods for high purity semiconducting SWCNTs is important to further improve the photocurrent generation efficiency by using SWCNT-based photoelectrochemical devices.     

Zn based nanoparticle–carbon nanotube hybrid materials: Interaction and charge transfer

The interaction and charge transfer between Zn-based nanoparticles (NPs) and carbon nanotubes (CNTs) is investigated by X-ray Auger and photoelectron spectroscopy. Charge transfer from Zn NPs toward the CNTs is demonstrated by the presence of an additional feature revealed in the C 1s spectrum of CNTs. This charge transfer was found to vanish after thermal oxidation due to the transformation of the metal Zn into ZnO. Photoluminescence (PL) spectroscopy shows that in the samples ZnO(Nps)/CNTs the PL intensity is quenched.     

Neutralized fluorine radical detection using single-walled carbon nanotube network

It is known that single-walled carbon nanotubes (SWCNTs) can be functionalized by fluorine gas. Here, we report neutralized fluorine radical detection using a matted sheet of SWCNTs, prepared by alternating current dielectrophoresis. Upon exposure to neutralized radicals containing fluorine atoms in a plasma, as model analytes, the conductance of the SWCNT matt showed fast modulation. The transduction mechanism was investigated by electrical transport measurements, X-ray photoelectron spectroscopy and Raman spectroscopy. Metallic nanotubes were shown to react covalently to the near exclusion of semiconducting species. The selectivity was promoted by the curvature-induced strain of the nanotubes. The results open new opportunities for the detection of fluorine radicals at specific locations inside the reaction zone using a simple, miniaturized carbon nanotube network.     

One dimensional nanostructure/nanoparticle composites as photoanodes for dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) show potential as a low cost alternative to silicon solar cells. Power conversion efficiencies exceeding 12% have been achieved for DSCs. Typical DSCs are based on TiO(2) nanoparticle photoanodes, which have numerous grain boundaries, surface defects and trap states as electrons transport from one particle to the other. Such defects and trap states increase back charge transfer (charge recombination) from the photoanode to electrolyte. One dimensional (1D) nanostructures such as nanofibers, nanorods, nanowires, and nanotubes can offer direct and fast electron transport to the electron collecting electrode. However, these 1D nanostructures have a major disadvantage of having insufficient surface area and inefficient dye attachment. To solve this challenge, mixtures of TiO(2) nanoparticles and 1D nanostructures (e.g. nanofibers, nanorods, nanowires, and nanotubes) are used to take advantage of the large surface area of nanoparticles and efficient charge transport of 1D nanostructures. In this article, we review the recent developments in using mixtures of 1D nanostructures and nanoparticles as photoanodes for efficient DSCs. Various randomly oriented and vertically aligned 1D nanostructures and their composites with nanoparticles are discussed. Future increase of efficiency in DSCs using 1D nanostructure/nanoparticle composites will rely on the optimization of diameters of 1D nanostructures, control of ratios of 1D nanostructures and nanoparticles, increase of crystallinity, and reduction of surface defects on the 1D nanostructures. This work will provide guidance for designing and growing appropriate 1D nanostructures, and combining them with nanoparticles at an optimal ratio for efficient DSCs.     

Synthesis and electrochemical properties of CeO2 nanoparticle modified TiO2 nanotube arrays

In this paper, a cerium dioxide (CeO₂) modified titanium dioxide (TiO₂) nanotube array film was fabricated by electrodeposition of CeO₂ nanoparticles onto an anodized TiO₂ nanotube array. The structural investigation by X-ray diffraction, scanning electron microscopy and transmission electron microscopy indicated that the CeO₂ nanoparticles grew uniformly on the walls of the TiO₂ nanotubes. The composite was composed of cubic-phase CeO₂ crystallites and anatase-phase TiO₂ after annealing at 450 °C. The cyclic voltammetry and chronoamperometric charge/discharge measurement results indicated that the CeO₂ modification obviously increased the charge storage capacity of the TiO₂ nanotubes. The charge transfer process at the surface, that is, the pseudocapacitance, was the dominate mechanism of the charge storage in CeO₂-modified TiO₂ nanotubes. The greater number of surface active sites resulting from uniform application of the CeO₂ nanoparticles to the well-aligned TiO₂ nanotubes contributed to the enhancement of the charge storage density.     

Fabrication of graphene/single wall carbon nanotubes/polyaniline composite gels as binder-free electrode materials

A three-dimensional (3D) graphene-based hydrogels system containing one-dimensional (1D) carbon material-single wall carbon nanotubes (SWCNTs) and pseudocapacitor material-polyaniline (PANI) was prepared by combination of cross-linking, reduced and in situ polymerization. The polyaniline nanoparticles were combined with the reduced graphene sheet by π-π conjugation. The as-perpared composite gels could be directly used as electrode materials without binders. Due to the synergistic effect between SWCNTs, graphene sheet and PANI, the graphene/single wall carbon nanotubes/polyaniline (GH/SWCNTs/PANI) composite gel shows the enhanced electrochemical performances. The resultant GH/SWCNTs/PANI gel electroactive material shows a gravimetric specific capacitance of 145.4 F/g at 0.5 A/g and has improved 45% compared with initial graphene hydrogel (GH) at the same current density. And it keeps high retention of 98.8% of the initial capacity after 10,00 charge/discharge cycles at high current density of 10 A/g. The great cycle stability achieved is fundamentally attributed to the support of graphene sheet and single wall carbon nanotubes, which favors stress distribution and charge transfer during the longtime charge/discharge process. The graphene-based hydrogels could be a potential applicant for high rate charge/discharge applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46948.     

Selective laser treatment and laser patterning of metallic and semiconducting nanotubes in single walled carbon nanotube films

Single wall carbon nanotubes (SWCNT) synthesized using mass production methods such as pulsed arc deposition consist of a mixture of metallic and semiconducting nanotubes. In this work, we report on an approach for the selective removal of either metallic or semiconducting SWCNT by a heat-treatment process with cw-lasers and pulsed lasers with specific wavelengths. The results show that using ultraviolet–visible radiation (with wavelengths between 473 nm and 632 nm) it is possible to remove predominantly metallic nanotubes. In contrast, near infrared lasers with 785 nm and 1064 nm wavelengths can be used to remove predominantly the semiconducting nanotubes. Finally, the fabrication of SWCNT films with an anisotropic distribution of metallic and semiconducting nanotubes is demonstrated using a direct laser interference pattering method.     

Sandwich growth of carbon nanotubes

Direct formation of structures that comprise freestanding CNTs connected to two surfaces was, thus far, not possible. In this article we report a novel approach to grow structured, highly oriented carbon nanotubes that are vertically aligned between a substrate and a massive cover. Growth is feasible at pre-determined, e.g., lithographically defined sites on metallic, semiconducting, or glass substrates. A novel, sandwiched catalyst structure and microwave plasma chemical vapor deposition (CVD) led to the formation of freestanding, small diameter carbon nanotubes. Our new technology offers a simple and scalable pathway to create 3D structured nanotube-based two-terminal electronic devices, device arrays, sensors and corresponding electronic circuits.     

Electroless plated gold as a support for carbon nanotube electrodes

A monolayer of -NH₂ terminated 3-aminopropyltriethoxysilane (APS) was self-assembled onto a p-type silicon (1 0 0) substrate. This amine terminated silane monolayer provided an electrostatic point of attachment for citrate stabilised gold colloid nanoparticles, which act as ‘seed’ particles for the electroless deposition of gold, creating an electrolessly deposited gold layer on silicon. A -NH₂ terminated cysteamine monolayer was then deposited onto the gold layer and carbon nanotubes, with high carboxylic acid functionality, were immobilised via a condensation reaction. A redox active molecule ferrocenemethanol was then chemically attached to the immobilised carbon nanotubes. These nanostructures were used as working electrodes in cyclic voltammetry to observe the oxidation and reduction of ferrocene. Important electrochemical parameters such as electrode kinetics, electron transfer rate and surface concentration of the redox active molecules were obtained, providing information on the ability of electroless plated gold surfaces to act as supports for carbon nanotube-based electrodes. This information has also provided insights into the behaviour of vertically aligned carbon nanotubes immobilised on nanoscale gold wires, which have been previously fabricated using atomic force microscopy.     

Organic solar cell materials and active layer designs—improvements with carbon nanotubes: a review

Organic solar cells offer an opportunity to diversify renewable energy sources owing to their low technological cost. They are amenable to large surfaces and can easily be integrated into buildings. It is necessary, however, to improve their energy efficiency and durability for the development of a sustainable technology. In these devices, photovoltaic conversion is based on the separation of photogenerated charges at an interface between electron donor and acceptor materials, which imposes some constraints on the photoactive layer of the cells. In this paper, which includes some of our studies, we address optimization of the active layer: absorption and exciton dissociation steps, the open‐circuit voltage and the active layer morphology. A promising direction proposed to improve the active layer morphology and cell efficiency is the incorporation of highly anisotropic nanoparticles such as carbon nanotubes, which may facilitate charge transport to the electrodes. Dispersion and orientation of the nanotubes in the organic matrix are discussed and we suggest an ideal model polymer solar cell which will maximize performance of the cells by using carbon nanotubes in the active layer. Copyright © 2012 Society of Chemical Industry     

Water‐Processable Multiwalled Nanotube: Phenol‐Induced Reversible Oxidation Process and Ambipolar Charge Transport Property

Due to the fascinating electronic, thermal, and mechanical properties of single‐walled carbon nanotubes (SWCNTs), extensive efforts have been devoted to the development of SWCNT‐based materials. These materials' semiconducting properties and related applications, such as field‐effect transistors (FETs), have been investigated by researchers for many years. However, despite the significant progress achieved, it remains challenging to separate semiconducting and metallic nanotubes from the mixtures of as‐grown SWCNTs. In a few studies, composites of water‐processable phenol formaldehyde resin/multiwalled carbon nanotubes (MWCNTs) have been found to exhibit a quasireversible oxidation process and to behave as semiconductors or field‐effect transistors. This finding has rarely been reported for MWCNTs, and it differs greatly from findings regarding intrinsic semiconductive SWCNTs. Significantly, field‐effect transistors fabricated with MWCNT composites as their semiconductor active layers have shown ambipolar charge transport characteristics. The results provide a high value‐added application pathway for the application of polymer/MWCNTs as the FET materials for electronic devices that offer higher performance at a lower cost.     

Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis

A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells.     

Dramatic increase in the Raman signal of functional groups on carbon nanotube surfaces

Raman spectra of octadecylamine (ODA) molecules covalently bound to the surface of single walled carbon nanotubes were obtained by varying the laser excitation energy from 1.92 to 3.81 eV. A strong dependence of the Raman intensity on the excitation energy was observed and, under specific conditions of resonance, the Raman signal from the amide linkage can become even stronger than the resonant Raman signal of the carbon nanotubes. This result is explained in terms of a chemical enhancement mechanism due to charge transfer between the ODA molecules and the nanotubes. The enhancement factor was up to 100 times higher than the value typically reported for the chemical Surface Enhanced Raman Scattering (SERS) effect. Our results show that carbon nanotubes may contribute to the still controversial understanding of the ‘chemical’ effects to the enhancement in SERS, extending the scope of application of this analytical tool in strategic areas such as biomedicine, catalysis and environmental analysis.