How to Quantify Electrons in Plasmonic Colloidal Metal Oxide Nanocrystals

Distinct from noble metal nanoparticles, doped metal oxide nanocrystals (NCs) exhibit localized surface plasmon resonance (LSPR) in the infrared region that can be tuned by changing the free electron concentration through both synthetic and post-synthetic doping. Redox reagents have commonly been used to post-synthetically modulate the LSPR, but to understand the relationship between the electron transfer processes and the resulting optical changes, it is imperative to quantify electrons in the NCs. Titration and LSPR peak fitting analysis are the most common methods used for quantifying electrons; however, comparison between these methods has previously revealed discrepancies up to an order of magnitude without a clear explanation. Here, we apply these electron quantification techniques concurrently to Sn-doped In2O3 NCs with varying size, doping concentration, and extent of post-synthetic reduction. We find that oxidative titration consistently overestimates the number of electrons per NC, owing to the failure of the assumed stoichiometric equivalence between moles of oxidant added and moles of free electrons extracted from the NCs. The NC characteristics we examine strongly influence the driving force for the oxidation process, affecting the relative agreement between oxidative titration and LSPR fitting; the two methods more closely agree when the electron transfer driving force is larger. Overall, these analyses inform best practices for quantifying electrons in plasmonic semiconductor NCs and reveal how accuracy is affected by NC characteristics.