To Dope or Not To Dope: The Effect of Sonicating Single-Wall Carbon Nanotubes in Common Laboratory Solvents on Their Electronic Structure

Single-wall carbon nanotubes (SWCNTs) are commonly dispersed via sonication in a solvent prior to functionalization. We show that solvents such as dichloromethane, chloroform, 1,2-dichloroethane, and o-dichlorobenzene lead to an upward shift in the Raman response of the SWCNTs. We have used o-dichlorobenzene as a model molecule to explain this effect, and an upward shift of 9 cm−1 is observed in the D* band. This blue shift is associated with p-type doping and is triggered only when the nanotubes are sonicated in the solvent. Sonication decomposes the chlorinated solvents, and new species (Cl2 and HCl(g)) are formed. The catalytic Fe nanoparticles inherently present in the nanotubes are etched by chlorine and hydrogen chloride to form iron chlorides during sonication in the solvent. The dopant was identified by X-ray photoelectron spectroscopy. With such knowledge of doping, the choice of solvent becomes crucial for any chemical reaction and can be intentionally tuned to produce SWCNTs films for electronics applications

Ambiguity in the Characterization of Chemically Modified Single-Walled Carbon Nanotubes: A Raman and Ultraviolet−Visible−Near-Infrared Study

Single-walled carbon nanotubes (SWCNTs) sonicated in o-dichlorobenzene and benzyl chloride show anomalous behavior when characterized with a Raman microscope and ultraviolet−visible−near-infrared spectroscopy. SWCNTs treated with the aforementioned solvents lead to a small but distinct increase in the Raman D peak, when irradiated with laser power higher than 0.12 mW/μm2. This can be mistakenly interpreted as covalent functionalization, but we have correlated this increase in the D peak to the charring of polymeric material, which is formed during sonication of the aforementioned solvents. At a temperature estimated to be 280 °C, corresponding to a laser power of 0.31 mW/μm2, the polymers are charred, resulting in an increase in amorphous material. This behavior is in contrast to that of the covalently functionalized SWCNTs, which show a decrease in the D peak as the laser power is increased. These samples also show a depletion in the spectral intensity of the optical absorption spectra of the SWCNTs, which is again a result commonly associated with covalent functionalization. However, by using a washing protocol, we find the Raman and optical spectra of the resulting SWCNTs no longer show features associated with functionalization. Species formed during sonication can drastically affect data interpretation. Our results provide an unambiguous assessment of the cause and effect of wet chemical processing and its impact on characterization

Cause and Consequence of Carbon Nanotube Doping in Water and Aqueous Media

To utilize carbon nanotubes in real-world applications, we have to master their chemistry. At present there is a lack of understanding regarding what happens during basic manipulations, such as doping with acids, forming suspensions by sonication in water with surfactants, or detecting peroxides. We show that sonication of nanotubes in water leads to the in situ formation of molecular oxygen, causing doping, which can be quenched with ethanol. In the presence of the anionic surfactant sodium dodecyl sulfate, oxygen doping is overshadowed by doping due to the sulfate group. Stable suspensions of undoped nanotubes can be created with Triton-X spiked with ethanol. Hydrogen peroxide does not dope, but in high concentrations or in the presence of catalytic iron nanoparticles it decomposes to yield oxygen, which may dope. Hydrochloric acid does not dope, unlike sulfuric acid. Our results clarify the origins of doping while processing carbon nanotubes in water

Investigation of Corrosion-Inhibiting Aniline Oligomer Thin Films on Iron Using Photoelectron Spectroscopy

Polyaniline (PANI) is capable of inhibiting corrosion on iron by inducing the formation of a passive oxide film. The underlying mechanism however, is unknown. We have used photoelectron emission spectromicroscopy of thin films of a model PANI oligomer to investigate its interaction with the iron oxide film covering the iron surface. The oligomer chosen was a phenyl-capped aniline tetramer (PCAT). Thin undoped films of PCAT in its leucoemeraldine form were prepared by physical vapor deposition to obtain films from ∼1 Å to over 10 nm thick. Films were investigated with a photoelectron emission microscope (PEEM) using synchrotron radiation to obtain spatially resolved valence band photoemission spectra. Analysis of PEEM results suggest that PCAT is capable of migrating several micrometers along the substrate surface and causes a decrease in substrate work function wherever present. High-resolution core level and valence band photoelectron spectroscopy using a laboratory-based photon source was used to characterize the substrate and PCAT properties near the PCAT−substrate interface. Characterization of an in situ deposited thin film reveals that the iron substrate exhibits band bending in its oxide and a decrease in work function by 0.5 eV upon adsorption of PCAT

Nature of the Interaction of <i>N</i>,<i>N</i>′‑Diphenyl-1,4-benzoquinonediimine with Iron Oxide Surfaces and Its Mobility on the Same Surfaces

Short chain aniline oligomers are of interest for applications in organic electronics and as corrosion inhibitors for steel, requiring an improved understanding of their interactions with metal oxide films. Here we investigate the interactions of <i>N</i>,<i>N</i>′-diphenyl-1,4-benzoquinonediimine (B2Q1, oxidized form of an aniline dimer) with iron­(III) oxides. B2Q1 transforms into its semiquinone form when interacting with α-Fe₂O₃. The resulting charge transfer between B2Q1 and α-Fe₂O₃ is demonstrated with mid-IR, visible, and Raman spectroscopy. Atomic force microscopy shows the first layer of B2Q1 to be oriented face-on. Thermal analysis also confirms this orientation for submonolayer coverage, whereas molecules start standing up on their edges upon multilayer formation. Thermal analysis shows that the first monolayer of B2Q1 is chemisorbed on the α-Fe₂O₃ surface, and the following multilayers are strongly interacting with each other. The behavior of the oxidized aniline dimer B2Q1 is in stark contrast to its reduced counterpart (DPPD), which also undergoes charge transfer to iron oxide (in opposite direction). B2Q1 interacts more weakly with the surface, causing it to be more mobile. The mobility of B2Q1 provides a clue toward understanding the self-healing behavior of polyaniline corrosion-inhibiting films on steel

Robust Inorganic Membranes from Detachable Ultrathin Tantalum Oxide Films

We report a simple electrochemical method of making individual free-standing and uniform tantalum oxide membranes between 35 and 100 nm thick. These films can be separated, floated on water, and transferred onto various substrates such as Si wafers, glass slides, and TEM grids. Our membranes are mechanically, chemically, and thermally robust, have a high dielectric constant, and a high refractive index, making them potentially useful in sensors, optics, filtration, and catalysis

Roughening of Gold Atomic Steps Induced by Interaction with Tetrahydrofuran

Exposure of a clean gold surface to tetrahydrofuran (THF) under ambient conditions was observed to cause roughening of atomic step edges. This change was followed in situ using a scanning tunneling microscope during the exposure of a gold surface to a controlled stream of THF vapor. THF is a common solvent used in depositing molecules, self-assembled monolayers, and polymer films on surfaces, in electrochemistry, and in chemical reactions. Unlike other solvents, such as methanol, ethanol and diethyl ether, however, we found that THF itself has a profound effect on the surface morphology that needs to be taken into account when reporting on the interactions of solutes with a gold surface. At the same time, this finding may present new opportunities in catalysis or nanostructuring of surfaces

Roughening of Gold Atomic Steps Induced by Interaction with Tetrahydrofuran

Exposure of a clean gold surface to tetrahydrofuran (THF) under ambient conditions was observed to cause roughening of atomic step edges. This change was followed in situ using a scanning tunneling microscope during the exposure of a gold surface to a controlled stream of THF vapor. THF is a common solvent used in depositing molecules, self-assembled monolayers, and polymer films on surfaces, in electrochemistry, and in chemical reactions. Unlike other solvents, such as methanol, ethanol and diethyl ether, however, we found that THF itself has a profound effect on the surface morphology that needs to be taken into account when reporting on the interactions of solutes with a gold surface. At the same time, this finding may present new opportunities in catalysis or nanostructuring of surfaces

Nanopatterning of Transition Metal Surfaces <i>via</i> Electrochemical Dimple Array Formation

Nanoscale surface patterning is of great importance for applications ranging from catalysts to biomaterials. We show the formation of ordered nanoscale dimple arrays on titanium, tungsten, and zirconium during electropolishing, demonstrating versatility of a process previously only reported for tantalum. This is a rare example of an electrochemical pattern formation process that can be translated to other materials. The dimpled surfaces have been characterized with scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy, and electrochemical conditions were optimized for each material. While conditions for titanium and tungsten resemble those for tantalum, zirconium requires a different type of electrolyte. Given the appropriate electropolishing chemistry, formation of these patterns should be possible on any metal surface. The process is very robust on homogeneous surfaces, but sensitive to inhomogeneities in chemical composition, such as in the case of differentially etched alloys. An alternative process for some materials such as platinum is the coating of a dimpled substrate with a thin film of the required material

Long-Range Periodicity in Carbon Nanotube Sidewall Functionalization

Using the Bingel reaction as a model for side-wall functionalization of single-walled carbon nanotubes, we report the discovery of highly regular, long-distance (several nanometer) patterns and examine the conditions for the occurrence of such patterns, possibly due to long-range induced reactivity. Varying periodicities of the patterns have been observed via scanning tunneling microscopy and are attributed to nanotube geometry. Patterns are most prominent on medium heavy functionalized nanotubes and likely tied to a nucleophilic reaction mechanism