Colloidal synthesis of MoSe2, ReSe2, and SnSe2 nanomaterials and their carbon nanocomposites for applications in hydrogen evolution reactions
Date
2022
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Abstract
The work herein focuses on the synthesis of MoSe2, ReSe2, and SnSe2 nanostructures using the colloidal method. This work is aimed to show that the colloidal method can be used to produce transition metal dichalcogenide (TMD) nanostructures with excellent catalytic activity toward the hydrogen evolution reaction (HER). Moreover, this work demonstrates that the method can be used to incorporate various techniques that are used to improve the catalytic activity of TMD nanostructures. The catalytic activity of the TMD nanostructures was improved by introducing Se vacancies, chemical doping, increasing exposure of active edge sites, and forming hybrid nanostructures with carbon nanomaterials. The colloidal method was successfully used to synthesize MoSe2, ReSe2, and SnSe2 nanostructures. The MoSe2 and ReSe2 were synthesized using selenium powder as the selenium precursor and oleic acid as the solvent/surfactant. SnSe2 was synthesized using selenium powder as the selenium precursor and olelyamine as the solvent/surfactant. The MoSe2 and ReSe2 nanostructures formed spherical structures flowerlike morphologies that were composed of few-layer nanosheets. The SnSe2 nanostructures formed into 2D nanoplates. The onset potential and overpotential of the MoSe2 nanostructures were recorded as 108 mV and 313 mV respectively. The onset potential and overpotential of the ReSe2 were measured to be 168 mV and 331 mV respectively. These materials were used as a baseline and attempts were made to improve their catalytic activity. A change in the selenium precursor from selenium powder to selenourea was shown to result in the introduction of Se vacancies in the MoSe2. This in turn resulted in improved catalytic activity towards the HER, which was attributed to an increase in the number of active sites provided by the Se vacancies. The onset potential and the overpotential of the SnSe2 nanostructures were measured to be 319 mV and 618 mV respectively. However, the SnSe2 nanoplates were electrochemically activated through H+ intercalation, and the catalytic performance of the nanostructures drastically improved. The electrochemically activated SnSe2 nanoplates exhibited exceptional improvements in catalytic performance with an onset potential of 141 mV and an overpotential of 289 mV. The pristine SnSe2 was used as a baseline and attempts were made to improve the catalytic activity of these materials.
The effect of surface functionalization of the nanostructures on their catalytic activity toward the HER was studied. Three solvents/surfactants commonly used in colloidal synthesis were studied. These were oleylamine (OLA), oleic acid (OA), and trioctylphosphine oxide (TOPO). The surfactants interacted differently with the TMD nanostructures, but a common observation v was made. The use of TOPO instead of OA which was initially used resulted in an improvement of the catalytic activity. The TOPO synthesized ReSe2 nanostructures had a reduced onset potential and overpotential of 73 mV and 171 mV respectively. The TOPO synthesized MoSe2 nanostructures also had a reduced onset potential and overpotential of 297 mV and 193 mV respectively. The TOPO synthesized SnSe2 had improved catalytic activity with an onset potential and overpotential of 229 mV and 569 mV respectively. This improvement in the catalytic activity was attributed to the degree of passivation on the surface of the nanostructures. The computational studies on the ReSe2 nanostructures showed that OA and OLA result in a high degree of passivation of the nanostructure surface compared to the TOPO, this results in the surfactants blocking more of the active sites which negatively impacts the catalytic activity. TOPO is a much bulkier surfactant, which results in a lower degree of surface passivation.
Hybrid nanostructures of the TMDs and carbon nanostructures were produced using colloidal synthesis. Pristine and nitrogen-doped reduced graphene oxide was used for the study. This was done to increase the electrical conductivity of the TMD nanostructures, which would, in turn, result in increased catalytic activity towards the HER. Few-layered ReSe2 nanostructures were grown on rGO (ReSe2-rGO) and N-rGO (ReSe2-N-rGO). The catalytic activity of the ReSe2 improved after incorporating the nanostructures on the carbon nanostructures. The ReSe2-N-rGO had higher catalytic activity than the ReSe2-rGO, this was attributed to the improved electrical conductivity of the N-rGO provided by the nitrogen doping. The onset potential and overpotential of ReSe2-N-rGO were measured to be 115 mV and 218 mV respectively, which were much improved from pristine ReSe2 nanostructures. The SnSe2-NrGO also showed some improvement in the catalytic activity of the nanostructures. However, the MoSe2-N-rGO did not show any improvement and the catalytic activity worsened. This was attributed to the interaction of the MoSe2 with the N-rGO nanosheets, the MoSe2 nanostructures grew independently to the N-rGO nanosheets which were in stark contrast to the ReSe2-N-rGO nanostructures. This limited the interaction of the MoSe2 with the N-rGO and resulted in impaired catalytic activity. Alkali metal doping was used to improve the catalytic activity of SnSe2 nanostructures. Potassium was used to dope the SnSe2 nanostructures. The doping was confirmed using powder X-ray diffraction, X-ray fluorescence spectroscopy, and UV-vis spectroscopy. The potassium doped SnSe2 nanostructure showed an improvement to the catalytic activity compared to the pristine SnSe2 nanostructures. The onset potential and overpotential of the doped nanostructures were measured to be 265 mV and 385 vi mV respectively. This improvement was attributed to the introduction of new active sites on the SnSe2 nanoplates through potassium doping. This was confirmed using the electrochemically active surface area (ECSA) which increased from 8.8 mF/cm2 in the pristine nanostructures to 11.7 mF/cm2 in the doped nanostructures. This work has successfully demonstrated that colloidal synthesis can be used to produce TMDs with excellent catalytic activity. The catalytic activity of some of these materials is comparable to the best catalyst of the same materials reported in the literature that are produced using other methods.
Description
A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, University of the Witwatersrand, 2022
Keywords
MoSe2 nanostructures, ReSe2 nanostructures, Carbon nanocomposites