NCs, upon chemical or electrochemical reduction. The LSPR intensity increases following electron transfer to the NCs with little to no peak shifts, which is unexpected based on the relationship between the plasma frequency and carrier concentration. The modulation of LSPR in Cs:WO3 NCs has been modeled previously as the filling of a shell depleted of conduction electrons near the surface of the material, both when chemically reduced with hydrazine and when electrochemically reduced in non-aqueous electrolytes. The electronic changes we observed using XPS and the changes in the optical properties are consistent with the filling of a depletion layer near the NC surface during electrochemically reduction.