Search

Heteroatoms in multiwalled carbon nanotubes modify their chemical reactivity and electronic properties. Nitrogen as a doping element changes the morphology by boosting the surface area, the density of defects, electronic states, and chemically actives sites that improve the electrocatalytic activity and other properties. Nitrogen defects create mainly the pyridinic, pyrrolic, and quaternary-N configurations presenting different localized electronic configurations. This work outlined and studied the distribution and physicochemical properties of N-doped multiwalled carbon nanotubes as a function of the temperature profile along a CVD reactor. X-ray photoelectron spectroscopy demonstrated that the nitrogen content is maximal in the second furnace high-temperature zone, and the pyrrolic-N doping predominate. The ID/IG ratio from Raman spectroscopy increases in this zone, revealing a defective graphitic material. The cyclic voltammetry showed that the specific capacitance increases for pyrrolic-N from the last three-quarters of the reactor due to the augment of active carbon atoms associated with pyrrolic-N defects and the chemical decomposition of nitrogen precursor alongside the reactor. Theoretical calculation on the electronic properties and potential electrostatic surface of carbon nanotubes suggests the surface metallicity when pyridinic-N and quaternary-N are dominant and active sites for possible hydrogen or oxygen binding when pyrrolic-N are prevalent.

The combination of different nanomaterials to achieve particular applications is a topic with an increasing impact on diverse areas of nanotechnology. In particular, the material of La2Zr2O7 (LZO) and NiO could be employed as a source of catalyst for carbon nanotube fabrication. In this work, the Ni/LZO material was utilized to fabricate nitrogen-doped multiwalled carbon nanotube (N-MWCNTs). The N-MWCNTs were produced at 850 °C by the aerosol assisted catalytic chemical vapor deposition method using benzylamine as the sole carbon and nitrogen precursor. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and vibrating sample magnetometry. Before the synthesis of N-MWCNTs, NiO was reduced to Ni to act as a catalyst and LZO as support. N-MWCNTs exhibited the typical feature bamboo shape morphology with small nanoparticles anchored inside and on its surface. The electrochemical studies show interesting results about charge capacity and a hydroquinone/quinone redox process of oxygen functionalities. The magnetic properties of N-MWCNTs showed a ferromagnetic behavior with a saturated magnetization of 43 emu/g at 0.2 T.

In this work, electrochemical sensors were developed for the detection of artificial sweeteners such as D-sorbitol and Maltitol in aqueous solutions. These compounds are classified as polyalcohols and are widely used in the food and beverage industry to replace common sugar. However, their consumption is not currently regulated, and excessive use leads to consequences in the body such as increased blood glucose levels. Graphene oxide (OG) inks were prepared, which were deposited on vitreous carbon (CV) electrodes, followed by enzymatic immobilization. The detection capacity of the biosensors was evaluated applying electrochemical techniques. The biosensor with the best levels of detection, reproducibility and durability for the analytes under study for the detection of D-Sorbitol turned out to be that measured at a working voltage of 0.86 V vs Ag / AgCl / KCl (3 M), depositing 20 μL of OG ink and using a dilution of 0.5 μL / 100 μL of (alcohol oxidase / PBS). In the case of Maltitol the best designed biosensor was worked at 1.06 V vs Ag / AgCl / KCl (3 M), depositing 20 μL of OG ink and using a dilution of 2.0 μL / 100 μL of (alcohol oxidase / phosphate buffer).

The research aims were to construct an air-breathing paper-based microfluidic fuel cell (paper-based μ FC) and to evaluated it with different soft drinks to provide energy for their prospective use in portable devices as an emergency power source. First, in a half-cell configuration, cyclic voltammetry showed that glucose, maltose, and fructose had specific oxidation zones in the presence of platinum-ruthenium on carbon (PtRu/C) when they were individual. Still, when they were mixed, glucose was observed to be oxidized to a greater extent than fructose and maltose. After, when a paper-based μ FC was constructed, PtRu/C and platinum on carbon (Pt/C) were used as anode and cathode, the performance of this μ FC was mostly influenced by the concentration of glucose present in each soft drink, obtaining maximum power densities at room temperature of 0.061, 0.063, 0.060, and 0.073 mW cm − 2 for Coca Cola ® , Pepsi ® , Dr. Pepper ® , and 7up ® , respectively. Interestingly, when the soft drinks were cooled, the performance was increased up to 85%. Furthermore, a four-cell stack μ FC was constructed to demonstrate its usefulness as a possible power supply, obtaining a power density of 0.4 mW cm − 2 , using Coca Cola ® as fuel and air as oxidant. Together, the results of the present study indicate an alternative application of an μ FC using soft drinks as a backup source of energy in emergencies.

We investigated the effects of long-time thermal oxidation (LTO) of nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) at 330 °C in air. By scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, Raman spectroscopy and cyclic voltammetry we characterize the oxidized N-MWCNTs (Ox-NMWCNTs). SEM and TEM characterizations revealed that LTO promoted structural damages driven by Fe-based nanoparticles, producing holey N-MWCNTs. The XRD patterns showed that short times of oxidation process, encapsulated Fe3C, and α-Fe nanoparticles in N-MWCNTs cut or perforate their graphite layers. For LTO (>300 h), Fe3C and α-Fe nanoparticles are transformed into Fe3O2 (hematite structure). Deconvolution analysis of the (0 0 2) crystallographic planes allows us to monitor the graphitization evolution of the Ox-NMWCNTs. XPS characterization demonstrated that the surface composition of Ox-N-MWCNTs is influenced by the LTO, while carbon and nitrogen decrease in percentage, the oxygen increases. We have found that LTO favored the formation of pyrrolic nitrogen doping. The holey N-MWCNTs are a porous material with a moderate surface area. According to the voltammetry studies, when the Ox-N-MWCNTs are acid-treated, the surface area is doubled and presents a double layer capacitance that could be advantageous for energy applications.

Natural catalysts are important for the low-cost and large-scale production of carbon nanostructures. In this study, we used a natural material obtained from thin bands of sedimentary rocks known as Banded Iron Formation (BIF). BIF powders (pristine and ball-milled) were used as catalysts for the large-scale production of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) via an aerosol-assisted catalytic chemical vapor deposition (AACCVD) experiment. The morphology and composition profiles of the BIF powders and N-MWCNT samples were analyzed by scanning and transmission electron microscopies, X-ray diffraction, X-ray photoelectron (XPS) and Raman spectroscopies, and thermogravimetric analysis. XRD characterizations revealed that the BIF powders were mainly composed of quartz and hematite. This result was also confirmed by Raman spectroscopy. The production efficiency of 340% wt./wt. of the N-MWCNTs was obtained by 1 h ball-milled BIF powders. The specific surface area of the N-MWCNTs reached 37.87 m2/g. The type of nitrogen doping, oxygen functional groups, silicon, and carbon species (sp2 and sp3) hosted at the surface of the N-MWCNTs were quantified by XPS. The electrochemical response of electrodes composed of the N-MWCNTs were studied via cyclic voltammetry, electrochemical impedance, and coulombic efficiency determination to assess the possible applications of our synthesized materials as energy storage or sensor systems. The magnetic properties and effects of acid and thermal treatments on the morphology of the N-MWCNTs are also presented.

In this work a new route for preparation of nickel ferrites nanoparticles has been developed. The synthesis is carried out in an electrochemical cell using three electrodes, a sheet of iron was employed as cathode and two sheets of iron and nickel were used as sacrificial anodes. The obtained nanoparticles were washed several times with distilled water, separated magnetically and dried under vacuum with constant temperature for 12 h. The characterization of the nanoparticles was carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Inductively coupled plasma mass spectrometry (ICP–MS). Magnetic measurements were carried out using a vibrating sample magnetometer (VSM). To evaluate the catalytical properties of these nanoparticles against the oxidation of glucose a graphite paste electrode (GPE) was made. The proportions of the nanoparticles in GPE were 5, 10, 20 and 50% in weight. The electrode shows promising properties for its use as catalyst in the glucose oxidation.

We evaluated the performance of glucose/O2 fuel cells, constructed as fluidic chips with controllable fuel flow and comprising a maghemite (γ-Fe2O3) nanoparticle/GOx-coated anode and Pt nanoparticles as the cathode. Maghemite is a biocompatible material that mediates electron transfer during glucose oxidation. The open-circuit voltage of the cell was 0.30 V, with a maximum power density of 30 μW cm−2 at pH 7.4. The voltage cell and power density were compared with those of other microfluidic fuel cells. During continuous operation, the current density of the cell decreased to 50%, likely due to swelling of the enzyme bilayer.