Atomic layer deposition of conformal inorganic nanoscale coatings on three-dimensional natural fiber systems: effect of surface topology on film growth characteristics

Atomic-scale material deposition is utilized to achieve uniform coverage and modification of the surface properties of natural fiber and woven fabric materials, where irregular nanoscale features are embedded in a macroscale interpenetrating fiber network. The complex surface topology of the woven fabric results in significantly different film-growth thickness per ALD cycle as compared to planar surfaces coated using the same process conditions, likely due to reactant adsorption within the fiber starting material, as well as impeded reactant transport out of the fabric system during the purge cycle. Cotton textiles modified with conformal nanoscale Al2O3 are found to show extreme hydrophobic effects, distinctly different from planar surfaces that receive the same coatings. The results highlight key concerns for achieving controlled conformal coatings on complex surfaces and open the possibility for new textile finishing approaches to create novel fabric-based materials with specialized function and performance.     

Aqueous dispersions of single-wall and multiwall carbon nanotubes with designed amphiphilic polycations

Poor solubility of single-walled and multiwalled carbon nanotubes (NTs) in water and organic solvents presents a considerable challenge for their purification and applications. Macromolecules can be convenient solubilizing agents for NTs and a structural element of composite materials for them. Several block copolymers with different chemical functionalities of the side groups were tested for the preparation of aqueous NT dispersions. Poly(N-cetyl-4-vinylpyridinium bromide-co-N-ethyl-4-vinylpyridinium bromide-co-4-vinylpyridine) was found to form exceptionally stable NT dispersions. It is suggested that the efficiency of macromolecular dispersion agents for NT solubilization correlates with the topological and electronic similarity of polymer-NT and NT-NT interactions in the nanotube bundles. Raman spectroscopy and atomic force and transmission electron microcopies data indicate that the polycations are wrapped around NTs forming a uniform coating 1.0-1.5 nm thick. The ability to wind around the NT originates in the hydrophobic attraction of the polymer backbone to the graphene surface and topological matching. Tetraalkylammonium functional groups in the side chains of the macromolecule create a cloud of positive charge around NTs, which makes them hydrophilic. The prepared dispersions could facilitate the processing of the nanotubes into composites with high nanotube loading for electronic materials and sensing. Positive charge on their surface is particularly important for biological and biomedical applications because it strengthens interactions with negatively charged cell membranes. A high degree of spontaneous bundle separation afforded by the polymer coating can also be beneficial for NT sorting.     

Polymer crystallization-driven, periodic patterning on carbon nanotubes

We report herein a unique means to periodically pattern polymeric materials on individual carbon nanotubes (CNTs) using a controlled polymer crystallization method. One-dimensional (1D) CNTs were periodically decorated with polymer lamellar crystals, resulting in nano-hybrid shish-kebab (NHSK) structures. The periodicity of the polymer lamellae varies from 20 to 150 nm. The kebabs are approximately 5-10 nm thick (along CNT direction) with a lateral size of approximately 20 nm to micrometers, which can be readily controlled by varying crystallization conditions. Both polyethylene and Nylon 66 were successfully decorated on single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), as well as vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to "size-dependent soft epitaxy". Because NHSK formation conditions depend on CNT structures, it further provides a unique opportunity for CNT separation. The reported method opens a gateway to periodically patterning polymers and different functional groups on individual CNTs in an ordered and controlled manner, an attractive research field that is yet to be explored.     

Highly controlled coating of biomimetic polydopamine in TiO2 nanotubes

Highly controlled coating of biomimetic polydopamine (PDA) was achieved on titanium dioxide nanotubes (TiO₂ NTs) by exposing TiO₂ NT arrays to a slightly alkaline dopamine solution. The thin films act as photonic sensitizers (enhancing photocurrents and photodegradation) in the visible light range. The PDA coatings can furthermore be used as a platform for decorating the TiO₂ NTs with different co-catalysts and metal nanoparticles (NPs).     

Investigation of the Orientation in Composite Fibers of Polycarbonate with Multiwalled Carbon Nanotubes by Raman Microscopy

The superior association of the inherent good mechanical and electrical properties makes carbon nanotubes (CNT) exceptionally interesting for the production of composite fibers of thermoplastic polymers with CNT. Alignment of the CNT in the polymer fiber is important for improved mechanical properties. Especially the production of fibers makes it necessary to get a controlled orientation and/or alignment of the CNT. We applied transmission electron microscopy (TEM) and polarized Raman microscopy to quantify multiwalled carbon nanotubes (MWNT) orientation, alignment and crystallinity in polycarbonate (PC). The evaluation of the Raman measurements provided an improved alignment orientation of the MWNT in the fibers with increasing take-up velocity during melt spinning and that the crystal structure of the MWNT is not changed through melt spinning.     

Controllable pt nanoparticle deposition on carbon nanotubes as an anode catalyst for direct methanol fuel cells

We report a novel process to prepare well-dispersed Pt nanoparticles on CNTs. Pt nanoparticles, which were modified by the organic molecule triphenylphosphine, were deposited on multiwalled carbon nanotubes by the organic molecule, which acts as a cross linker. By manipulating the relative ratio of Pt nanoparticles and multiwalled carbon nanotubes in solution, Pt/CNT composites with different Pt content were achieved. The so-prepared Pt/CNT composite materials show higher electrocatalytic activity and better tolerance to poisoning species in methanol oxidation than the commercial E-TEK catalyst, which can be ascribed to the high dispersion of Pt nanoparticles on the multiwalled carbon nanotube surface.     

Fabrication of Ag Nanoparticles Embedded in TiO2 Nanotubes: Using Electrospun Nanofibers for Controlling Plasmonic Effects

Composites of electrospun poly(ethylene oxide) (PEO) fibers and silver nanoparticles (Ag NPs) were used as a soft template for coating with TiO₂ by atomic layer deposition (ALD). Whereas the as‐deposited TiO₂ layers on PEO fibers and Ag NPs were completely amorphous, the TiO₂ layers were transformed into polycrystalline TiO₂ nanotubes (NTs) with embedded Ag NPs after calcination. Their plasmonic effect can be controlled by varying the thickness of the dielectric Al₂O₃ spacer between Ag NPs and dye molecules by means of the ALD process. Electronic and spectroscopic analyses demonstrated enhanced photocurrent generation and solar‐cell performance due to the intense electromagnetic field of the dye resulting from the surface plasmon effect of the Ag NPs.     

Directed Assembly of Multi-Walled Nanotubes and Nanoribbons of Amino Acid Amphiphiles Using a Layer-by-Layer Approach

Monodisperse unilamellar nanotubes (NTs) and nanoribbons (NRs) were transformed to multilamellar NRs and NTs in a well-defined fashion. This was done by using a step-wise approach in which self-assembled cationic amino acid amphiphile (AAA) formed the initial NTs or NRs, and added polyanion produced an intermediate coating. Successive addition of cationic AAA formed a covering AAA layer, and by repeating this layer-by-layer (LBL) procedure, multi-walled nanotubes (mwNTs) and nanoribbons were formed. This process was structurally investigated by combining small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM), confirming the multilamellar structure and the precise layer spacing. In this way the controlled formation of multi-walled suprastructures was demonstrated in a simple and reproducible fashion, which allowed to control the charge on the surface of these 1D aggregates. This pathway to 1D colloidal materials is interesting for applications in life science and creating well-defined building blocks in nanotechnology.     

Magnetron sputtering Si interlayer: a protocol to prepare solid phase microextraction coatings on metal-based fiber

Use of metal fibers in solid phase microextraction (SPME) can overcome the fragility drawback of conventional fused-silica ones. However, the surface modification of metal substrates is rather difficult, which largely prevents many mature traditional techniques, such as sol-gel and chemical bonding, being used in fabrication of SPME coating on metal-based fibers. This study demonstrates a protocol to resolve this problem by magnetron sputtering a firm Si interlayer on stainless steel fiber. The Si interlayer was easily modified active group, and attached with a multiwalled carbon nanotubes (MWCNTs) coating using the reported approach. The as-prepared MWCNTs/Si/stainless steel wire fiber not only preserved the excellent SPME behaviors of MWCNTs coatings, but also exhibited a number of advantages including high rigidity, long service life, and good stability at high temperature, in acid and alkali solutions. This new surface modification technique might provide a versatile approach to prepare sorbent coatings on unconfined substrates using traditional methods.     

Carbon nanotubes as efficient catalyst supports for fuel cells with direct ethanol oxidation

Several carbon materials, namely, single-walled nanotubes (CNT1), two-walled nanotubes (CNT2), multiwalled nanotubes (CNT3), and nanofibers (CNF) are synthesized by methane pyrolysis. The resulting nanomaterials are characterized by physical (BET) and electrochemical (charging curves) methods. A catalyst of ethanol electrooxidation PtSn (3: 1, 40 wt % Pt) that involves the mentioned nanomaterials as the supports is synthesized. The catalyst formed on two-walled nanotubes demonstrates the highest activity in ethanol oxidation under model conditions. X-ray diffraction analysis is used in studying the PtSn (3: 1, 40 wt % Pt)/CNT2 catalyst structure. The attained depth of ethanol oxidation is determined by the gas-liquid chromatography. Tests of an ethanol-oxygen fuel cell (FC) with the anodic active layer (AL) based on this catalyst are carried out.     

Fabrication of robust superhydrophobic coatings using PTFE‐MWCNT nanocomposite: Supercritical fluid processing

Fabrication of polymer‐carbon composite nanostructure with good dispersion of each other is critical for the desired application due to the nanostructure flaws, agglomeration, and poor absorption between the 2 materials. Fabrication of superhydrophobic surface coating composites of polytetrafluoroethylene (PTFE) with multiwalled carbon nanotubes (MWCNTs) through supercritical fluid processing was explored in this study. Homogeneity of the composite was characterized by X‐ray diffraction and Raman spectroscopy studies, which reveal that the PTFE and MWCNT are uniform in the composite. Microstructural surface evaluation of field‐emission scanning electron microscope and high‐resolution transmission electron microscope studies display that the coating composite possesses roughness structures and fibrillation of the superhydrophobic surface coating. Superhydrophobic character was evaluated on fiber‐reinforced plastic (FRP) sheets, which showed that the prepared coating composite surface showed self‐cleaning properties with a high water contact angle of 162.7°. The surface wettability was studied by increasing different temperatures (30°C to 300°C) in PTFE‐MWCNT composite, which reveals that the FRP sheets were thermally stable up to 200°C and afterward; they transformed from superhydrophobic to hydrophilic state at 250°C. The superhydrophobic surfaces are thermally stable in extreme environmental conditions, and this technique may be used and extendable for large‐scale applications.     

Electrocatalytic oxidation of ethylene glycol on Pt and Pt-Ru nanoparticles modified multi-walled carbon nanotubes

The synthesis and characterization of catalysts based on nanomaterials, supported on multi-walled carbon nanotubes (CNT) for ethylene glycol (EG) oxidation is investigated. Platinum (Pt) and platinum-ruthenium (Pt-Ru) nanoparticles are deposited on surface-oxidized multi-walled carbon nanotubes [Pt/CNT; Pt-Ru/CNT] by the aqueous solution reduction of the corresponding metal salts with glycerol. The electrocatalytic properties of the modified electrodes for oxidation of ethylene glycol in acidic solution have been studied by cyclic voltammetry (CV), and excellent activity is observed. This may be attributed to the small particle size of the metal nanoparticles, the efficacy of carbon nanotubes acting as good catalyst support and uniform dispersion of nanoparticles on CNT surfaces. The nature of the resulting nanoparticles decorated multiwalled carbon nanotubes are characterized by scanning electron microscopy (SEM) and transmission electron microscopic (TEM) analysis. The cyclic voltammetry response indicates that Pt-Ru/CNT catalyst displays a higher performance than Pt/CNT, which may be due to the efficiency of the nature of Ru species in Pt-Ru systems. The fabricated Pt and Pt-Ru nanoparticles decorated CNT electrodes shows better catalytic performance towards ethylene glycol oxidation than the corresponding nanoparticles decorated carbon electrodes, demonstrating that it is more promising for use in fuel cells.     

Polyester resin and carbon nanotubes based nanocomposite as new-generation coating to prevent biofilm formation

In this article, we have developed protective nanocomposite coatings using unsaturated polyester resin and multiwalled carbon nanotubes showing antimicrobial activity and mechanical durability. Carbon nanotubes retain the mechanical resilience of polyester resin in the nanocomposite, improve its hydrophobic character, and increase the adhesion features of the coating, preventing its stiffness from decreasing due to water absorption or exposure to UV rays. Nanocomposite coating exhibits an appreciable antimicrobial property and a lower level of toxicity compared to pure resin. All these features make this material a good candidate for its use in the field of anti-biofilm coatings.     

Nanoscale Structuring of Surfaces by Using Atomic Layer Deposition

Controlled structuring of surfaces is interesting for a wide variety of areas, including microelectronic device fabrication, optical devices, bio(sensing), (electro‐, photo)catalysis, batteries, solar cells, fuel cells, and sorption. A unique feature of atomic layer deposition (ALD) is the possibility to form conformal uniform coatings on arbitrarily shaped materials with controlled atomic‐scale thickness. In this Minireview, we discuss the potential of ALD for the nanoscale structuring of surfaces, highlighting its versatile application to structuring both planar substrates and powder materials. Recent progress in the application of ALD to porous substrates has even made the nanoscale structuring of high‐surface‐area materials now feasible, thereby enabling novel applications, such as those in the fields of catalysis and alternative energy.     

Carbon Nanotube‐Directed Polytetrafluoroethylene Crystal Growth via Initiated Chemical Vapor Deposition

Initiated chemical vapor deposition (iCVD) has been shown to be suitable for blanketing surfaces with thin polymer coatings of ≈1–2 nm and greater. In this work, iCVD coatings of polytetrafluoroethylene (PTFE) deposited on carbon nanotube (CNT)‐based surfaces show CNT‐templated PTFE single crystal growth. While the coating forms disoriented agglomerates when deposited on an amorphous carbon background, “shish‐kebab” structures are observed when grown on single‐walled carbon nanotubes (SWCNT) as well as CNT buckypaper. It is shown that the shish‐kebab structure is composed of PTFE lamellae arranged with the chain backbones running parallel to the SWCNT axis. This result allows one to control not only the surface chemistry using PTFE but also the coating surface topology.     

The effect of localized lateral growth of multiwalled carbon nanotubes with ammonia plasma post‐treatment

We present horizontally‐oriented multiwalled carbon nanotubes (CNTs) grown by means of thermal chemical vapor deposition. The CNT is across the trenches of the catalytic metals on predefined Ti electrodes. The properties of the lateral multiwalled CNT, following post‐ammonia plasma treatment, are reported. Information about the ammonia plasma treated on the interface structure of CNT is obtained using X‐ray photoelectron spectroscopy. The experimental results show that upon exposure to ammonia, the electrical property of the CNT is found to increase. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of oxyfluorination on electromagnetic interference shielding of polyaniline-coated multi-walled carbon nanotubes

Highly conducting polyaniline (PANi)-coated multi-walled carbon nanotubes (MWCNTs) were prepared by in situ polymerization method for electromagnetic interference shielding. The thickness of the PANi coatings was controlled by the oxyfluorination treatment on the multi-walled carbon nanotubes and analyzed with both SEM and TEM. The oxyfluorination with higher oxygen content produced more hydrophilic functional groups on the surface of multi-walled carbon nanotubes. The functional groups led to the well distribution and coating of PANi on the multi-walled carbon nanotubes resulting in the higher interfacial affinity between them. The uniform coating of PANi on MWCNTs by controlling the oxyfluorination conditions also played a crucial role in increasing the electrical conductivity of nanocomposites. The improved interfacial affinity resulted in the higher electromagnetic interference (EMI) SE of 47.03?dB based on the synergistic combination of the conductive components. The EMI shielding mechanism of PANi on MWCNTs suggested that EMI was mainly shielded by adsorption to avoid secondary EMI.     

Effect of functionalized multiwalled carbon nanotubes on the feasibility of fabrication of composite glass fiber reinforced plastic rebars

The effect of COOH-functionalized multiwalled carbon nanotubes on the temperature–velocity conditions of the fabrication of composite glass fiber reinforced plastic rebars (so-called needletrusion) used in building was studied. EPIKOTE 862 epoxy oligomer (Bisphenol F diglycidyl ether) in combination with EPICUREW curing agent (aromatic diamine) in 100 : 26.4 ratio was used as binder. The use of functionalized multiwalled carbon nanotubes as additives to the unmodified epoxy binder for fabrication of composite materials by needletrusion allows production of high-quality items with the required characteristics without loss in the production capacity. It is preferable to introduce nanotubes in an amount of no more than 0.2 wt %.     

Encapsulation of Homogeneous Catalysts in Mesoporous Materials Using Diffusion‐Limited Atomic Layer Deposition

The heterogenization of homogeneous metal complex catalysts has attracted great attention. The encapsulation of metal complexes into nanochannels of mesoporous materials is achieved by coating metal oxides at/near the pore entrance by diffusion‐limited atomic layer deposition (ALD) to produce a hollow plug. The pore size of the hollow plug is precisely controlled on the sub‐nanometer scale by the number of ALD cycles to fit various metal complexes with different molecular sizes. Typically, Co or Ti complexes are successfully encapsulated into the nanochannels of SBA‐15, SBA‐16, and MCM‐41. The encapsulated Co and Ti catalysts show excellent catalytic activity and reusability in the hydrolytic kinetic resolution of epoxides and asymmetric cyanosilylation of carbonyl compounds, respectively. This ALD‐assisted encapsulation method can be extended to the encapsulation of other homogeneous catalysts into different mesoporous materials for various heterogeneous reactions.     

Composite materials based on polyaniline and multiwalled carbon nanotubes: Morphology and electrochemical behavior

Composites based on polyaniline are prepared via the chemical oxidative polymerization of aniline in the presence of multiwalled carbon nanotubes modified by the sorption of the co-oxidants IrC ₆ ^2? and 2,2′-azino-bis(3-ethyl-benzthiazolin-6-sulfonate). The approach used here, in combination with corresponding conditions of polymerization, ensures the synthesis of composite materials with a high morphological homogeneity of the polymer phase. The study of the electrochemical properties of composites (the reversibility of redox transitions and the stability of capacity parameters) indicates that that they are strongly influenced by the morphological features of the polymer coating. The composite prepared with the use of nanotubes modified by 2,2′-azino-bis(3-ethyl-benzthiazolin-6-sulfonate) possesses better electrochemical characteristics. This effect is associated with a closer to perfect morphology of the polymer coating, a coaxial polyaniline shell highly uniform in thickness along the entire length of nanotubes.