Ultrafine sputter-deposited Pt nanoparticles for triiodide reduction in dye-sensitized solar cells: impact of nanoparticle size, crystallinity and surface coverage on catalytic activity

This paper presents a detailed electrochemical impedance spectroscopy and cyclic voltammetry (CV) investigation into the electrocatalytic activity of ultrafine (i.e., smaller than 2 nm) platinum (Pt) nanoparticles generated on a fluorine-doped tin oxide (FTO) surface via room temperature tilted target sputter deposition. In particular, the Pt-decorated FTO electrode surfaces were tested as counter electrode candidates for triiodide (I-3(-)) reduction in dye-sensitized solar cells (DSSCs). We observed a direct correlation between size-dependent Pt nanoparticle crystallinity and the I-3(-) reduction activity underlying DSSC performance. CV analysis confirmed the higher electrocatalytic activities of sputter-deposited crystalline Pt nanoparticles (1-2 nm) compared with either sub-nanometre Pt clusters or a continuous Pt thin film. While the low catalytic activity and DSSC performance of Pt clusters smaller in size than 1 nm is believed to arise from their non-crystalline nature and charge-trapping attributes, we attribute the high catalytic performance of larger Pt nanoparticles in the 1-2 nm regime to their well-defined crystallinity and fast electron transfer kinetics. For DSSC applications, the optimized Pt loading was calculated to be similar to 2.54 x 10(-7) g cm(-2), which corresponds to surface coverage by similar to 1.6 nm sized Pt nanoparticles.