Ni₃Se₄ hollow architectures as catalytic materials for the counter electrodes of dye-sensitized solar cells

Nanoparticles of nickel selenides (Ni₃Se₄) were synthesized with various hollow architectures by a simple, one-step, low temperature hydrothermal process. Using three organic solvents having different alkyl chains, precisely methanol (MeOH), ethanol (EtOH), and propan-1-ol (n-PrOH), the Ni₃Se₄ nanoparticles showed three kinds of morphologies: sea urchins-like, rice-balls-like and strawberry-fruits-like hollow nanostructures, respectively. The solvent polarity shows a trend of MeOH>EtOH>n-PrOH, and the same trend on the surface area and roughness of the corresponding Ni₃Se₄ nanoparticles can be observed. Therefore, we propose a three-step mechanism for Ni₃Se₄ growth: (1) the nucleation based on a selenium core surrounded by free selenium ions; (2) the Ni₃Se₄ formation; (3) the dissolution of selenium core to form the hollow nanoparticles. Accordingly, the first-step is the key state because the higher solvent polarity would cause the more free selenium ions and thereby the larger surface area on Ni₃Se₄ nanoparticles. For the dye-sensitized solar cells (DSSCs), the films of nickel selenide can be utilized as the electrocatalytic counter electrodes (CE) for the redox of I^-/I^₃-. The nickel selenide film obtained using methanol (hereafter urchins-like Ni₃Se₄@MeOH) shows extremely large effective surface area (5.63 times larger than traditional Pt film) and fast redox ability (1.08 times higher than traditional Pt film). The urchins-like Ni₃Se₄@MeOH CE rendered for its DSSC a power conversion efficiency (η) of 8.31%, while the expensive platinum counter electrode (Pt-CE) could bestow for its DSSC only an η of 8.03%. It may be said that nickel selenide is a promising catalytic material to replace the expensive platinum in a DSSC.