Carla Bittencourt

Radio-frequency plasma functionalization of carbon nanotubes surface O2, NH3, and CF4 treatments

Inductive coupled rf-plasma at 13.56 MHz was used to modify multiwalled carbon nanotubes (MWCNTs). This technique can be easily used to tailor the chemical composition of carbon nanotubes by attaching a wide variety of functional groups at their surface: oxygen-, nitrogen-, and fluorine-containing groups have been grafted. The influence of various plasma conditions (power, type of gas, treatment time, pressure, position of the CNT sample inside the chamber) on the functionalization of the MWCNT surface was analyzed by x-ray photoelectron spectroscopy. The results show that for too high oxygen plasma power, chemical etching occurs at the surface of the CNT, thus destroying its structure. On the other hand, for optimal values of the plasma parameters, functional groups (hydroxide, carbonyl, carboxyl, amine, fluorine, etc.) were found to bond to the CNT surface, suggesting that both the concentration and type of the functional groups are in close connection with the plasma conditions. These results were compared to interaction energies predicted by ab initio calculations for different functional groups under consideration, showing that functionalization by oxygen plasma produces mainly functional groups with lower interaction energy.

Gold clusters on oxygen plasma functionalized carbon nanotubes: XPS and TEM studies

Oxygen plasma treated multi-walled carbon nanotubes (MWCNTs) have been decorated with gold nanoclusters by thermal evaporation. Transmission electron microscopy (TEM) shows that the nature and extent of gold coverage can be varied by simultaneously changing the parameters used for the plasma treatment and the gold evaporation time. The evaporated gold clusters on oxygen plasma treated MWCNTs have a more dense distribution than the clusters evaporated on as-synthesized MWCNTs. Analyses of the valence band and the core levels by x-ray photoelectron spectroscopy (XPS) suggest poor charge transfer between the gold clusters and the MWCNTs.

Knitting the Catalytic Pattern of Artificial Photosynthesis to a Hybrid Graphene Nanotexture

The artificial leaf project calls for new materials enabling multielectron catalysis with minimal overpotential, high turnover frequency, and long-term stability. Is graphene a better material than carbon nanotubes to enhance water oxidation catalysis for energy applications? Here we show that functionalized graphene with a tailored distribution of polycationic, quaternized, ammonium pendants provides an sp(2) carbon nanoplatform to anchor a totally inorganic tetraruthenate catalyst, mimicking the oxygen evolving center of natural PSII. The resulting hybrid material displays oxygen evolution at overpotential as low as 300 mV at neutral pH with negligible loss of performance after 4 h testing. This multilayer electroactive asset enhances the turnover frequency by 1 order of magnitude with respect to the isolated catalyst, and provides a definite up-grade of the carbon nanotube material, with a similar surface functionalization. Our innovation is based on a noninvasive, synthetic protocol for graphene functionalization that goes beyond the ill-defined oxidation-reduction methods, allowing a definite control of the surface properties.

Synthesis and Characterization of Boron Azadipyrromethene Single-Wall Carbon Nanotube Electron Donor-Acceptor Conjugates

The preparation of a novel donor-acceptor material, consisting of a red/near-infrared (NIR) absorbing boron azadipyrromethene donor covalently attached to a highly functionalized single-wall carbon nanotube (SWNT) acceptor, which bears great potential in the field of organic photovoltaics, has been demonstrated. Both purification and covalent functionalization of SWNTs have been demonstrated using a number of complementary characterization techniques, including atomic force microscopy, Raman, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared, and NIR-photoluminescence spectroscopy, and a functionalization density of approximately 1 donor molecule per 100 SWNT atoms has been estimated by XPS. The redox behavior of the fluorophore has been investigated by electrochemistry and spectroelectrochemistry as well as by pulse radiolysis. The donor-acceptor properties of the material have been characterized by means of various spectroscopic techniques, such as UV-vis NIR absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and time-resolved transient absorption spectroscopy. Charge transfer from the photoexcited donor to the SWNT acceptor has been confirmed with a radical ion pair state lifetime of about 1.2 ns.

The role of oxygen partial pressure and annealing temperature on the formation of W = O bonds in thin WO3 films

Thin films of tungsten oxide were deposited onto silicon substrates using reactive rf sputtering. The structure of the films is strongly dependent on the conditions of deposition and post-treatment. Important issues are the influences of oxygen pressure during deposition and of annealing temperature. We used x-ray photoelectron spectroscopy to investigate the in-depth composition of the films. The most surface sensitive O 1s core level spectra are made up of two structures, one generated by photoelectrons emitted from oxygen atoms in WO3 (O–W–O) and other at lower energy generated by the photoelectrons emitted from oxygen atoms located at the boundary of the grains (W = O). Using Raman spectroscopy, an increase of the W = O/O–W–O ratio was correlated to an increase in the oxygen partial pressure used during the deposition. A decrease of this ratio was observed while annealing temperature was increased, which was correlated to an increase in the size of the grains that form the films.

Aerosol-Assisted CVD-Grown WO3 Nanoneedles Decorated with Copper Oxide Nanoparticles for the Selective and Humidity-Resilient Detection of H2S

A gas-sensitive hybrid material consisting of Cu2O nanoparticle-decorated WO3 nanoneedles is successfully grown for the first time in a single step via aerosol-assisted chemical vapor deposition. Morphological, structural, and composition analyses show that our method is effective for growing single-crystalline, n-type WO3 nanoneedles decorated with p-type Cu2O nanoparticles at moderate temperatures (i.e., 380 °C), with cost effectiveness and short fabrication times, directly onto microhot plate transducer arrays with the view of obtaining gas sensors. The gas-sensing studies performed show that this hybrid nanomaterial has excellent sensitivity and selectivity to hydrogen sulfide (7-fold increase in response compared with that of pristine WO3 nanoneedles) and a low detection limit (below 300 ppb of H2S), together with unprecedented fast response times (2 s) and high immunity to changes in the background humidity. These superior properties arise because of the multiple p-n heterojunctions created at the nanoscale in our hybrid nanomaterial.

Functionalization of vertically aligned carbon nanotubes

Summary This review focuses and summarizes recent studies on the functionalization of carbon nanotubes oriented perpendicularly to their substrate, so-called vertically aligned carbon nanotubes (VA-CNTs). The intrinsic properties of individual nanotubes make the VA-CNTs ideal candidates for integration in a wide range of devices, and many potential applications have been envisaged. These applications can benefit from the unidirectional alignment of the nanotubes, the large surface area, the high carbon purity, the outstanding electrical conductivity, and the uniformly long length. However, practical uses of VA-CNTs are limited by their surface characteristics, which must be often modified in order to meet the specificity of each particular application. The proposed approaches are based on the chemical modifications of the surface by functionalization (grafting of functional chemical groups, decoration with metal particles or wrapping of polymers) to bring new properties or to improve the interactions between the VA-CNTs and their environment while maintaining the alignment of CNTs.

Highly Ordered Hollow Oxide Nanostructures: The Kirkendall Effect at the Nanoscale

Highly ordered ultra-long oxide nanotubes are fabricated by a simple two-step strategy involving the growth of copper nanowires on nanopatterned template substrates by magnetron sputtering, followed by thermal annealing in air. The formation of such tubular nanostructures is explained according to the nanoscale Kirkendall effect. The concept of this new fabrication route is also extendable to create periodic zero-dimensional hollow nanostructures.

Influence of the doping method on the sensitivity of Pt-doped screen-printed SnO2 sensors

Abstract In this work, we study the influence of the introduction method of Pt atoms on the sensitivity to traces of ethanol of Pt-doped SnO2 sensors. The tin oxide films were obtained by a screen-printing process. Two different methods were employed to introduce Pt atoms on SnO2 films. In the first one, the Pt atoms were added to the screen-printed tin oxide layer by using RF magnetron sputtering and a subsequent thermal treatment. The second method consisted of mixing SnO2 and Pt pastes before the screen-printing process. The different active layers (including un-doped tin oxide) were carefully examined relative to their sensitivity to ethanol at different working temperatures. Sensors prepared by the second method showed sensitivity to ethanol four times higher than one of the sensors prepared by the first method and 12 times higher than un-doped sensors. XPS and scanning electron microscopy (SEM) measurements showed that this behaviour could be associated with the spatial distribution of the doping elements within the tin oxide film. While in Pt-sputtered sensors most of the Pt atoms were found at the surface of the active layer, for the sensors made by mixing Pt and SnO2 pastes, a homogeneous distribution of the Pt atoms was observed. These sensors show high sensitivity and fast response time to ethanol vapours, with a detection limit in the ppb range.

Effects of Oxygen Partial Pressure and Annealing Temperature on the Formation of Sputtered Tungsten Oxide Films

Thin films of tungsten oxide were deposited on silicon substrates using reactive radio frequency sputtering. The structure of the films strongly depends on the conditions of deposition and post-treatment. Important issues are the influences of oxygen pressure during deposition and annealing temperature on the morphology. Atomic force microscopy and scanning electron microscopy revealed that films were formed by grains. The sample deposited with an Ar:O 2 partial pressure ratio of 1:1 showed the highest roughness and the smallest grains when annealed at 350°C. X-ray photoelectron spectroscopy analysis revealed that the films were close to their stoichiometric formulation irrespective the oxygen partial pressure used during film deposition. The number of W=O bonds at the grain boundaries was found to he dependent on the oxygen partial pressure. Analysis by Raman spectroscopy suggested that the structure of the films was monoclinic. On the basis of these results an annealing temperature of 350°C was selected as post-treatment for the fabrication of WO 3 gas sensors. These sensors were highly sensitive, highly selective to ammonia vapors, and moderately responsive to humidity.