Colloidal synthesis of NbSe2 nanoflowers and Nb2Se9 nanorods for application as counter electrode materials in dye sensitized solar cells

Kolokoto, Tshwarela
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Solar cells are amongst the most promising technologies for alternative electricity generation due to the abundance of the sun and their reduced impact on the environment. Among many solar cell designs, dye-sensitised solar cells (DSSCs) have the potential to be used at large scale however they have some disadvantages such as low power conversion efficiency (PCE), poor stability and high cost. As such, researchers have investigated various ways of improving on these disadvantages. In this study, we considered the use of niobium selenide nanoparticles as counter-electrodes (CEs) to replace Pt-CEs in DSSCs with the attempt to lower the cost of the device. Pt is an expensive noble metal with high electrocatalytic activity and stability, so a replacement electrode should be lower in cost while maintaining the high electrocatalytic activity and stability. Niobium selenide nanoparticles were of interest due to their unique and tunable properties such as catalytic, electronic, optical and magnetic. NbSe2 is a 2D-layered material and Nb2Se9 is a 1D material made up of chains, each layer or chain is held together by weak van der Waals forces. Therefore, to solve these problems, two important factors needed to be evaluated; the synthetic route of the nanoparticles and the fabrication of the CE. Colloidal synthesis method was used due to its flexibility, allowance for reaction parameter variation and use of low temperatures and reaction times. As such, the following reaction parameters were varied: reaction time, temperature, precursor concentration, capping agent, as well as Nb and Se precursors to investigate the optimum conditions for the synthesis of NbSe2 and Nb2Se9 nanoparticles. NbSe2 nanoflowers were favoured at 320 °C for 120 min in oleylamine (OLA) at a concentration of 0.125 M where NbCl5 was the metal precursor used. The use of selenourea (SU) as a selenium precursor, with the rest of the reaction parameters unchanged, produced rhombohedral NbSe2 nanoflowers while Se and SeO2 produced hexagonal NbSe2 nanoflowers. On the other hand, Nb2Se9 nanorods were formed when the temperature was decreased to 280 °C and the concentration was increased to 0.500 M, hexadecylamine (HDA) or 1-octadecene (ODE), and NbF5 as a metal precursor were used. The use of Se produced monodispersed and crystalline Nb2Se9 triclinic phase nanorods than SU and SeO2. This is a clear indication that each reaction parameter has an effect on the nucleation and growth process of the nanoparticles and in turn on their properties. OLA, HDA and ODE were capping the surface of the nanoparticles. However from both FTIR and NMR spectroscopies evidence of oxidation of the capping agents was observed. The electrocatalytic activity of NbSe2 nanoflowers and Nb2Se9 nanorods were investigated and compared with that of Pt using cyclic voltammetry. Two methods were used to fabricate the CEs i.e. spin-coating and drop-casting method. The spin-casted CEs were more stable and reproducible compared to the drop-casted CEs. Moreover, Nb2Se9 CEs were not stable irrespective of the method of fabrication used. The spin-coated NbSe2 CE had a higher current density and electrochemical double layer capacitance than both Pt and spin-coated Nb2Se9 CEs. This was an indication that NbSe2 nanoflowers have a higher electrocatalytic activity than Pt (because NbSe2 had a current density of 5.88 mA/cm2 whereas Pt was 4.86 mA/cm2) due to high surface area and coverage. As such, it has the potential to produce cost-effective DSSCs by replacing Pt.
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in partial fulfillment for the degree of Master of Science