Synthesis and characterization of Nb2O5 mesostructures with tunable morphology and their application in dye-sensitized solar cells

Mesoporous submicrometric particles of orthorhombic Nb2O5 were prepared by a very simple route consisting in the hydrolysis of niobium ethoxide Nb(OEt)5 in an alcoholic medium containing 1-hexadecylamine as structure-directing agent followed by a hydrothermal treatment. The effects related to the variation of the length of the aliphatic chain of the alcohol solvent (C2 to C4) and the ramification of the alcohol (primary, secondary and tertiary), as well as the reactants to solvent ratio were analyzed in terms of morphology, crystal structure, specific surface area and porosity. The obtained solids, once thermally treated to remove the organics, were thoroughly characterized by powder XRD, UV-Vis spectroscopy (band gap measurement), photovoltage measurements (conduction band edge determination), BET-BJH surface analysis and FESEM. By simply modifying the solvent and reactants/solvent ratio, different morphologies spanning from nearly monodisperse beads to peanut-shaped particles, sintered spheres aggregates, a mix of different morphologies can be achieved. The obtained materials were tested as photoanodes in dye-sensitized solar cells. After the optimization of the thickness of the photoanode and of its chemical treatment in order to improve the inter-particle connections, DSSCs were prepared by using N719 dye and a non-volatile iodide-based electrolyte. The cells were tested by J-V curves under AM 1.5 G illumination and dark, IPCE measurements and electrochemical impedance spectroscopy. A remarkable efficiency value of 3.4% under 1 sun illumination was achieved by employing peanut-shaped particles obtained by using 2-propanol as solvent with the highest used reactant/solvent ratio.