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 Doped TEMHRTEM

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 N- doped CNS obtained using different nitrogen sources, normalized to the broad background. NK2 – NH 3 NK3 – NH 4 + ions NK4 – pyridine Results broadly agree with elemental analysis.

Horizontal CVD reactor

Sphere Morphology – horizontal CVDR Undoped Doped

EMR spectra of three N- doped 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