5 research outputs found
Formation of Active Sites for Oxygen Reduction Reactions by Transformation of Nitrogen Functionalities in Nitrogen-Doped Carbon Nanotubes
Heat treating nitrogen-doped multiwalled carbon nanotubes containing up to six different types of nitrogen functionalities transforms particular nitrogen functionalities into other types which are more catalytically active toward oxygen reduction reactions (ORR). In the first stage, the unstable pyrrolic functionalities transform into pyridinic functionalities followed by an immediate transition into quaternary center and valley nitrogen functionalities. By measuring the electrocatalytic oxidation reduction current for the different samples, we achieve information on the catalytic activity connected to each type of nitrogen functionality. Through this, we conclude that quaternary nitrogen valley sites, N-Q<sub>valley</sub>, are the most active sites for ORR in N-CNTs. The number of electrons transferred in the ORR is determined from ring disk electrode and rotating ring disk electrode measurements. Our measurements indicate that the ORR processes proceed by a direct four-electron pathway for the N-Q<sub>valley</sub> and the pyridinic sites while it proceeds by an indirect two-electron pathway <i>via</i> hydrogen peroxide at the N-Q<sub>center</sub> sites. Our study gives both insights on the mechanism of ORR on different nitrogen functionalities in nitrogen-doped carbon nanostructures and it proposes how to treat samples to maximize the catalytic efficiency of such samples
Nitrogen Doping Mechanism in Small Diameter Single-Walled Carbon Nanotubes: Impact on Electronic Properties and Growth Selectivity
Nitrogen doping in carbon nanostructures
has attracted interest
for more than a decade, and recent implementation of such structures
in energy conversion systems has boosted the interest even more. Despite
numerous studies, the structural conformation and stability of nitrogen
functionalities in small diameter single-walled carbon nanotubes (SWNTs),
and the impact of these functionalities on the electronic and mechanical
properties of the SWNTs, are incomplete. Here we report a detailed
study on nitrogen doping in SWNTs with diameters in the range of 0.8–1.0
nm, with well-defined chirality. We show that the introduction of
nitrogen in the carbon framework significantly alters the stability
of certain tubes, opening for the possibility to selectively grow
nitrogen-doped SWNTs with certain chirality and diameter. At low nitrogen
concentration, pyridinic functionalities are readily incorporated
and the tubular structure is well pertained. At higher concentrations,
pyrrolic functionalities are formed, which leads to significant structural
deformation of the nanotubes and hence a stop in growth of crystalline
SWNTs. Raman spectroscopy is an important tool to understand guest
atom doping and electronic charge transfer in SWNTs. By correlating
the influence of defined nitrogen functionalities on the electronic
properties of SWNTs with different chirality, we make precise interpretation
of experimental Raman data. We show that the previous interpretation
of the double-resonance G′-peak in many aspects is wrong and
instead can be well-correlated to the type of nitrogen doping of SWNTs
originating from the p- or n-doping nature of the nitrogen incorporation.
Our results are supported by experimental and theoretical data
Synthesis of Palladium/Helical Carbon Nanofiber Hybrid Nanostructures and Their Application for Hydrogen Peroxide and Glucose Detection
We report on a novel sensing platform
for H<sub>2</sub>O<sub>2</sub> and glucose based on immobilization
of palladium-helical carbon nanofiber (Pd-HCNF) hybrid nanostructures
and glucose oxidase (GOx) with Nafion on a glassy carbon electrode
(GCE). HCNFs were synthesized by a chemical vapor deposition process
on a C<sub>60</sub>-supported Pd catalyst. Pd-HCNF nanocomposites
were prepared by a one-step reduction free method in dimethylformamide
(DMF). The prepared materials were characterized by transmission electron
microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy
(SEM), and Raman spectroscopy. The Nafion/Pd-HCNF/GCE sensor exhibits
excellent electrocatalytic sensitivity toward H<sub>2</sub>O<sub>2</sub> (315 mA M<sup>–1</sup> cm<sup>–2</sup>) as probed
by cyclic voltammetry (CV) and chronoamperometry. We show that Pd-HCNF-modified
electrodes significantly reduce the overpotential and enhance the
electron transfer rate. A linear range from 5.0 μM to 2.1 mM
with a detection limit of 3.0 μM (based on the S/N = 3) and
good reproducibility were obtained. Furthermore, a sensing platform
for glucose was prepared by immobilizing the Pd-HCNFs and glucose
oxidase (GOx) with Nafion on a glassy carbon electrode. The resulting
biosensor exhibits a good response to glucose with a wide linear range
(0.06–6.0 mM) with a detection limit of 0.03 mM and a sensitivity
of 13 mA M<sup>–1</sup> cm<sup>–2</sup>. We show that
small size and homogeneous distribution of the Pd nanoparticles in
combination with good conductivity and large surface area of the
HCNFs lead to a H<sub>2</sub>O<sub>2</sub> and glucose sensing platform
that performs in the top range of the herein reported sensor platforms
Simple Dip-Coating Process for the Synthesis of Small Diameter Single-Walled Carbon Nanotubesî—¸Effect of Catalyst Composition and Catalyst Particle Size on Chirality and Diameter
We report on a dip-coating method to prepare catalyst
particles
(mixture of iron and cobalt) with a controlled diameter distribution
on silicon wafer substrates by changing the solution's concentration
and withdrawal velocity. The size and distribution of the prepared
catalyst particles were analyzed by atomic force microscopy. Carbon
nanotubes were grown by chemical vapor deposition on the substrates
with the prepared catalyst particles. By decreasing the catalyst particle
size to below 10 nm, the growth of carbon nanotubes can be tuned from
few-walled carbon nanotubes, with homogeneous diameter, to highly
pure single-walled carbon nanotubes. Analysis of the Raman radial
breathing modes, using three different Raman excitation wavelengths
(488, 633, and 785 nm), showed a relatively broad diameter distribution
(0.8–1.4 nm) of single-walled carbon nanotubes with different
chiralities. However, by changing the composition of the catalyst
particles while maintaining the growth parameters, the chiralities
of single-walled carbon nanotubes were reduced to mainly four different
types, (12, 1), (12, 0), (8, 5), and (7, 5), accounting for about
70% of all nanotubes
Thermoelectricity Enhanced Electrocatalysis
We
show that thermoelectric materials can function as electrocatalysts
and use thermoelectric voltage generated to initiate and boost electrocatalytic
reactions. The electrocatalytic activity is promoted by the use of
nanostructured thermoelectric materials in a hydrogen evolution reaction
(HER) by the thermoelectricity generated from induced temperature
gradients. This phenomenon is demonstrated using two-dimensional layered
thermoelectric materials Sb<sub>2</sub>Te<sub>3</sub> and Bi<sub>0.5</sub>Sb<sub>1.5</sub>Te<sub>3</sub> where a current density approaching
∼50 mA/cm<sup>2</sup> is produced at zero potential for Bi<sub>0.5</sub>Sb<sub>1.5</sub>Te<sub>3</sub> in the presence of a temperature
gradient of 90 °C. In addition, the turnover frequency reaches
to 2.7 s<sup>–1</sup> at 100 mV under this condition which
was zero in the absence of temperature gradient. This result adds
a new dimension to the properties of thermoelectric materials which
has not been explored before and can be applied in the field of electrocatalysis
and energy generation