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Characterization of nitrogen-doped carbon
nanospheres using electron magnetic
resonance
Jonathan Keartland, Makhosonke Dubazane, Vincent
Marsicano, Nikiwe Kunjuzwa and Neil Coville
DST/NRF Centre of Excellence in Strong Materials
School of Physics and Materials Physics Research Institute, WITS
Molecular Sciences Institute and School of Chemistry, WITS
Introduction
• Carbon nanomaterials are of great scientific and
technological interest at present
• Carbon nanospheres (CNS) were produced using two
separate reactors using different sources of both carbon
and nitrogen
• The CNS were characterized using a range of techniques
• Electron magnetic resonance (EMR) was utilized to
determine the properties of the CNS: in particular to
determine where the nitrogen is, and to estimate the
concentration of paramagnetic nitrogen
Electron Magnetic Resonance
• EMR (also known as ESR and EPR) may be used to
study materials containing free electrons or
paramagnetic defects
Vertical CVD reactor
1. Vertical silica plug flow
reactor
2. Furnace
3. Condenser
4. Delivery cyclone
5. Delivery cyclone
6. Vapourizer
7. Swirled mixer
Sphere Morphology – vertical CVDR
Undoped
TEM
HRTEM
Doped
EMR spectra of undoped
(NK1) and nitrogen-doped
(NK2) carbon nanospheres.
The doped sample shows a
strong narrow paramagnetic
peak super-imposed on a
broad background. The
narrow peak is evidence
that the nitrogen is in
substitutional sites in the
carbon matrix. The broad
background
may
be
ascribed to conduction
electrons.
EMR spectra of three Ndoped CNS obtained using
different nitrogen sources,
normalized to the broad
background.
NK2 – NH3
NK3 – NH4+ ions
NK4 – pyridine
Results broadly agree with
elemental analysis.
Horizontal CVD reactor
Sphere Morphology – horizontal CVDR
Undoped
Doped
EMR spectra of three Ndoped CNS obtained using
mixtures of pyridine (P) and
toluene (T) as sources.
NK7 – 10:90 P:T
NK9 – 90:10 P:T
NK6 – 100:0 P:T
Increase in the nitrogen
content gives a broader and
asymmetric spectrum.
Composite EMR spectra
of three of the N-doped
CNS samples, and the
EMR reference standard
(DPPH) that were used
to determine the g-factor.
The
spectra
were
deconvoluted using a
program written for the
purpose. The first peak
on the left is due to
DPPH in these spectra.
Summary of the results – all samples
Results for the linewidth (ΔB) and the g-factor relative to the
DPPH reference sample (Δg) for all samples
Estimating the paramagnetic nitrogen content
Concentration of paramagnetic nitrogen centres. The relative
concentration is calculated with respect to NK6, while the absolute
concentration is found by comparing the sample to DPPH.
Conclusions
• Undoped and doped CNS have been successfully
produced using two CVD reactors
• The nitrogen is strongly paramagnetic indicating that
the nitrogen is in substitutional sites
• The concentration of paramagnetic nitrogen is no more
than a few percent in the most highly doped sample
• Increasing the nitrogen concentration changes the
character of the EMR spectrum
Acknowledgements
Thanks are due to the following:
• DST/NRF CoE in String Materials, the School of
Physics and the MPRI for financial support
• Physics and Chemistry workshop for technical support
• Administrative support staff of the CoE and the Schools
of Physics and Chemistry