Doped metal oxide nanocrystals (NCs) attract immense attention because of their ability to exhibit a localized surface plasmon resonance (LSPR) that can be tuned extensively across the infrared region of the electromagnetic spectrum. LSPR tunability triggered through compositional and morphological changes during the synthesis (size, shape and doping percentage) is becoming well-established while the principles underlying dynamic, post-synthetic modulation of LSPR are not as well understood. Recent reports have suggested that the presence of a depletion layer on the NC surface may be instrumental in governing the LSPR modulation of doped metal oxide NCs. Here, we employ post-synthetic electron transfer to colloidal Sn-doped In2O3 NCs with varying size and Sn doping concentration to investigate the role of the depletion layer in LSPR modulation. By measuring the maximum change in LSPR frequency after NC reduction, we determine that a large initial volume fraction of the depletion layer in NCs results in a broad modulation of the LSPR energy and intensity. Utilizing a mathematical Drude fitting model, we track the changes in the electron density and the depletion layer volume fraction throughout the chemical doping process, offering fundamental insight into the intrinsic NC response resulting from such electron transfer events. We observe that the maximum change in electron density that can be induced through chemical doping is independent of Sn concentration, and subsequently, the maximum total electron density in the presence of excess reductant is independent of the NC diameter and is dependent only on the as-synthesized Sn doping concentration. This study establishes the central role that surface depletion plays in the electronic changes occurring in the NCs during post-synthetic doping and the results will be instrumental in advancing the understanding of optical and electrical properties of different colloidal plasmonic NCs.

Doped metal oxide nanocrystals (NCs) attract immense attention because of their ability to exhibit a localized surface plasmon resonance (LSPR) that can be tuned extensively across the infrared region of the electromagnetic spectrum. LSPR tunability triggered through compositional and morphological changes during synthesis (size, shape, and doping percentage) is becoming well-established, while the principles underlying dynamic, postsynthetic modulation of LSPR are not as well understood. Recent reports have suggested that the presence of a depletion layer on the NC surface may be instrumental in governing the LSPR modulation of doped metal oxide NCs. Here, we employ postsynthetic electron transfer to colloidal Sn-doped In2O3 NCs with varying sizes and Sn doping concentrations to investigate the role of the depletion layer in LSPR modulation. By measuring the maximum change in the LSPR frequency after NC reduction, we determine that a large initial volume fraction of the depletion layer in NCs results in a broad modulation of the LSPR energy and intensity. Utilizing a mathematical Drude fitting model, we track the changes in the electron density and the depletion-layer volume fraction throughout the chemical doping process, offering fundamental insights into the intrinsic NC response resulting from such electron-transfer events. We observe that the maximum change in electron density that can be induced through chemical doping is independent of Sn concentration, and subsequently, the maximum total electron density in the presence of excess reductant is independent of the NC diameter and is dependent only on the as-synthesized Sn doping concentration. This study establishes the central role that surface depletion plays in the electronic changes occurring in the NCs during postsynthetic doping, and the results will be instrumental in advancing the understanding of optical and electrical properties of different colloidal plasmonic NCs.

Electroreduction of CO2 to formate powered by renewable energy offers an alternative pathway to producing carbon fuels that are traditionally manufactured using fossil fuels. However, achieving simultaneously high partial current density (j HCOO –), high product selectivity (Faradaic efficiency (FEHCOO –)), and low overpotentials (η) remains difficult due to the lack of suitable catalysts. Here, we report the electroreduction of CO2 on Sn-doped indium oxide (ITO) nanocrystal catalysts in an alkaline flow electrolyzer. Colloidally synthesized monodisperse 20 nm ITO nanocrystals (NCs) with various Sn-doping levels (0, 1, 5, 6.5, 8, and 12 atom %) were studied. We find that ITO NC catalysts exhibit a high selectivity for production of HCOO– over CO and H2 (approximately 87% HCOO–, 1–4% CO, and 2–6% H2 at −0.85 V vs RHE), an onset potential for HCOO– as early as −0.21 V vs RHE, and a high partial current density for HCOO– up to 171 mA/cm2 at a cathode potential of −1.08 V vs RHE. The main difference between the catalysts’ performances resides in the onset potential for formate production. The onset of formate production occurred at cell and cathode overpotentials of only −440 and −143 mV, respectively, by the 12% ITO. Analysis of the ITO electrodes before and after electrolysis suggests that no changes in surface composition, crystal structure, or particle size occur under the reduction conditions. Tafel slopes of ITO NC catalysts range from 27 to 52 mV per decade, suggesting that the rate-determining step is likely the proton-coupled electron transfer to CO2 ●–* to form HCOO–*.

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 postsynthetic doping. Redox reagents have commonly been used to postsynthetically 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, a 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 postsynthetic reduction. We find that oxidative titration consistently overestimates the number of electrons per NC, owing to the failure of the assumed stoichiometric equivalents 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 the accuracy is affected by NC characteristics.

The development of electrochromic metal oxide nanocrystals holds promise for improving the sluggish switching kinetics of conventional electrochromic smart windows. Nevertheless, the microscopic processes controlling switching kinetics in nanocrystals may differ from those in traditional bulk materials where ion diffusion following intercalation is often rate limiting. Herein, by systematically investigating the electrochromic response of Sn-doped In 2 O 3 nanoparticles, orthorhombic Nb 2 O 5 nanorods, and monoclinic Nb 12 O 29 nanoplatelets, we elucidate how different charge storage mechanisms, including capacitive charging, surface redox, and intercalation, affect the switching kinetics of electrochromic nanocrystals. The nanocrystals were reduced in both lithium-and tetrabutylammonium-based electrolytes at various potentials to determine which charge storage mechanism governs their electrochromic response, and the optical switching kinetics at a reducing potential were quantified by fitting with an exponential-growth model based on the charging behavior of capacitors. For the surface-dominated capacitive charging and surface redox mechanisms, dual-stage switching kinetics were observed regardless of the materials, switching rapidly at the early stage of reduction and becoming slower over time as charge accumulates in the electric double layer. As for the intercalation mechanism, single-stage switching kinetics controlled by the reaction rate of ion intercalation were observed. By using spectroelectrochemical methods, we demonstrated approaches to define the charge storage mechanisms in electrochromic metal oxide nanocrystals and investigated how these mechanisms affect the switching kinetics of the electrochromic response.