5 research outputs found

    Deciphering the structural, textural, and electrochemical properties of activated BN-doped spherical carbons

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    In this study, the effect of K2CO3 activation on the structural, textural, and electrochemical properties of carbon spheres (CSs) and boron and nitrogen co-doped carbon spheres (BN-CSs) was evaluated. Activation of the CSs and BN-CSs by K2CO3 resulted in increased specific surface areas and ID/IG ratios. From the X-ray photoelectron spectroscopy (XPS) results, the BN-CSs comprised of 64% pyridinic-N, 24% pyrrolic-N and 7% graphitic-N whereas the activated BN-CSs had 19% pyridinic-N, 40% pyrrolic-N and 22% graphitic-N displaying the effect of activation on the type of N configurations in BN-CSs. A possible BN-co-doping and activation mechanism for the BN-CSs is proposed. Electrochemical analysis of the electrode materials revealed that BN doping, carbon morphology, structure, and porosity played a crucial role in enhancing the capacitive behavior of the CSs. As a proof of concept, a symmetric device comprising the activated BN-CSs displayed a specific power of 800 W kg 1 at a specific current of 1 A g 1 within an operating cell potential of 1.6 V in a 3 M KNO3 electrolyte. The study illustrated for the first time the role of K2CO3 activation in influencing the physical and surface properties of template-free activated BN-CSs as potential electrode materials for energy storage systems.The South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation of South Africa (Grant No. 61056). B.K.M. and B.J.M. would like to thank the University of the Witwatersrand and the DST-NRF Centre of Excellence in Strong Materials (CoESM) for financial support. B.K.M. would also like to acknowledge financial support from the NRF and the University of Pretoria for her postdoctoral fellowship grant.http://www.mdpi.com/journal/nanomaterialsam2019Physic

    Blue- and red-shifts of V2O5 phonons in NH3 environment by in situ Raman spectroscopy

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    A layer of ~30 nm V2O5/100 nm-SiO2 on Si was employed in the in situ Raman spectroscopy in the presence of NH3 effluent from a thermal decomposition of ammonium acetate salt with the salt heated at 100 °C. When the layer is placed at 25 °C, we observe a reversible red-shift of 194 cm−1 V2O5 phonon by 2 cm−1 upon NH3 gas injection to saturation, as well as a reversible blue-shift of the 996 cm−1 by 4 cm−1 upon NH3 injection. However when the sensing layer is placed at 100 °C, the 194 cm−1 remains un-shifted while the 996 cm−1 phonon is red-shifted. There is a decrease/increase in intensity of the 145 cm−1 phonon at 25 °C/100 °C when NH3 interacts with V2O5 surface. Using the traditional and quantitative gas sensor tester system, we find that the V2O5 sensor at 25 °C responds faster than at 100 °C up to 20 ppm of NH3 beyond which it responds faster at 100 °C than at 25 °C. Overall rankings of the NH3 gas sensing features between the two techniques showed that the in situ Raman spectroscopy is faster in response compared with the traditional chemi-resistive tester. Hooke’s law, phonon confinement in ~51 nm globular particles with ~20 nm pore size and physisorption/chemisorption principles have been employed in the explanation of the data presented.A Akande acknowledges CSIR National Centre for Nano- Structured Materials for PhD Studentship position (project number HGER27S and HGER50S) and the National Research Foundation through KIC150917142805 travel grant.http://iopscience.iop.org/0022-3727am2019Physic

    CVD growth of pristine and N-doped graphene films for support of platinum and palladium nanoparticles in electrochemical sensing of dopamine and uric acid

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    A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for degree of Doctor of Philosophy in Chemistry, 2017The synthesis of large-area graphene and nitrogen doped graphene films by atmospheric pressure chemical vapour deposition, for application in electrochemical sensors provides a new platform for developing inexpensive techniques for selective and ultrasensitive detection of electroactive biomolecules. Therefore, optimum growth condition for the synthesis of good quality bilayered graphene films by APCVD technique on a Cu catalyst was developed (10 minutes growth time; 10 sccm of methane). The quality and thickness of the as-synthesized graphene films was further improved by using 3 sccm of hydrogen gas throughout the annealing and growth process. Doping of graphene with nitrogen atoms has been reported to be the most promising technique for modulating the structural and electrochemical properties of as-grown graphene films. A synthesis method for growing nitrogen doped graphene films with high nitrogen content was obtained by in-situ route using 5 sccm of ammonia. Furthermore, both overall nitrogen content (N/C) and configurations in N-doped graphene films were controlled by varying the growth times (i.e. 2, 5, 10, and 20 min). The results indicated that short growth time (2 min) led to N-graphene films that are rich in pyridinic-N and highest N/C value (4.68 %), while longer growth time (20 min) resulted in formation of graphitic-N rich films with N/C value of 2.84 %. Electrocatalytic activity of pristine and Ndoped graphene films as well as pristine and N-graphene-platinum and palladium composites were investigated towards oxidation of dopamine and uric acid. Nitrogen doped based and metal-composite based electrochemical sensors showed better electrocatalytic activity and sensitivity compared to their undoped counterparts. Apart from single atom doping graphene with nitrogen atoms, co-doping with boron and nitrogen atoms was investigated for formation of semiconducting hybrid materials with tunable optical properties. By adjusting the flow rates of methane and the vaporization temperature of boric acid, two types of graphene and hexagonal boron nitride (h-BN) hybrid films were formed. This included novel crystalline hexagonal boron nitride (h-BN) quantum- and nanodots embedded in large-area boron carbon nitride (BCN) films and the atomically thin and quaternary semiconducting hybrid films of boron carbon oxynitride (BCNO).XL201

    Nitrogen-doped hollow carbon spheres as chemical vapour sensors

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    Please read abstract in the article.The NRF, the University of the Witwatersrand and the DST-NRF Centre of Excellence in Strong Materials.http://rsc.li/njc2020-04-12hj2019Physic
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