Size-dependent activity of FeSe2 nanosheets for efficient tri-iodide reduction reaction in photovoltaics

Constructing two-dimensional (2D) transition metal selenide with tunable size is of utmost importance to replace traditional noble metal counter electrode (CE) catalyst and display their size-dependent activity for tri-iodide (I3- ) reduction reaction (IRR) in dye-sensitized solar cells (DSSCs), yet it remains challenge. Herein, the theoretical calculation is firstly calculated to uncover the feasibility of FeSe2 as CE catalysts, where the abundant Fe sites at surface acted as the mainly catalytic center for IRR and reinforced interaction between Fe 3d and I 5p endow FeSe2 (111) with excellent catalytic activity for IRR. Inspired by the mentioned inherent merits of FeSe2 and facilitated charge-transfer capability of 2D nanomaterials, the well-defined 2D FeSe2 nanosheets with different size has been precisely synthesized by optimizing reaction temperature via the hot-injection method. As expected, the FeSe2 nanosheets with ca. 210 nm (FeSe2-M) shows higher photovoltaic performance than that of FeSe2-S (ca. 140 nm) and FeSe2-L (ca. 450 nm), proving the size-dependent activity of FeSe2 nanosheets, which can be attributed to the differences in the number of exposed Fe atoms and charge-transfer capability on surface that affected by the size. More importantly, the photovoltaic performance of FeSe2-M is also superior to that of Pt, demonstrating its potential to be used as Pt-free CE catalysts in low-cost DSSCs.