Amorphous transition metal oxides are recognized as leading candidates for electrochromic window coatings that can dynamically modulate solar irradiation and improve building energy efficiency. However, their thin films are normally prepared by energy-intensive sputtering techniques or high-temperature solution methods, which increase manufacturing cost and complexity. Here, we report on a room-temperature solution process to fabricate electrochromic films of niobium oxide glass (NbO) and 'nanocrystal-in-glass' composites (that is, tin-doped indium oxide (ITO) nanocrystals embedded in NbO glass) via acid-catalysed condensation of polyniobate clusters. A combination of X-ray scattering and spectroscopic characterization with complementary simulations reveals that this strategy leads to a unique one-dimensional chain-like NbO structure, which significantly enhances the electrochromic performance, compared to a typical three-dimensional NbO network obtained from conventional high-temperature thermal processing. In addition, we show how self-assembled ITO-in-NbO composite films can be successfully integrated into high-performance flexible electrochromic devices.
The optical extinction coefficients of localized surface plasmon resonance (LSPR) in doped semiconductor nanocrystals (NCs) have intensities determined by the density and damping mechanisms of free charge carriers. We investigate the dependence of the extinction coefficient of tin-doped indium oxide (ITO) NCs on size and dopant concentration and find extinction coefficients as high as 56.6 μm–1 in the near-infrared for 20 nm diameter ITO NCs with 7.5 atomic% Sn. We find ITO NCs to be more efficient infrared light absorbers than metal nanoparticles or molecular dyes. We also find the intensive, volume-normalized extinction coefficient increases significantly with NC doping and NC diameter, but only up to the point of saturation in both cases. We qualitatively analyze trends in LSPR peak position and width to explain the effect of doping and size on extinction.
The optical extinction spectra arising from localized surface plasmon resonance in doped semiconductor nanocrystals (NCs) have intensities and lineshapes determined by free charge carrier concentrations and various mechanisms for damping the oscillation of those free carriers. However, these intrinsic properties are convoluted by heterogeneous broadening when measuring the spectra of ensembles. We reveal that the traditional Drude approximation is not equipped to fit spectra from a heterogeneous ensemble of doped semiconductor NCs and produces fit results that violate Mie scattering theory. The heterogeneous ensemble Drude approximation (HEDA) model rectifies this issue by accounting for ensemble heterogeneity and near-surface depletion. The HEDA model is applied to tin-doped indium oxide NCs for a range of sizes and doping levels, but we expect it to be employed for any isotropic plasmonic particles in the quasistatic regime. It captures individual NC optical properties and their contributions to the ensemble spectra, thereby enabling the analysis of intrinsic NC properties from an ensemble measurement. Quality factors for the average NC in each ensemble are quantified and found to be notably higher than those of the ensemble. Carrier mobility and conductivity derived from the HEDA fits matches that reported in the bulk thin-film literature.
Gelation of colloidal nanocrystals emerged as a strategy to preserve inherent nanoscale properties in multiscale architectures. However, available gelation methods to directly form self-supported nanocrystal networks struggle to reliably control nanoscale optical phenomena such as photoluminescence and localized surface plasmon resonance (LSPR) across nanocrystal systems due to processing variabilities. Here, we report on an alternative gelation method based on physical internanocrystal interactions: short-range depletion attractions balanced by long-range electrostatic repulsions. The latter are established by removing the native organic ligands that passivate tin-doped indium oxide (ITO) nanocrystals while the former are introduced by mixing with small PEG chains. As we incorporate increasing concentrations of PEG, we observe a reentrant phase behavior featuring two favorable gelation windows; the first arises from bridging effects while the second is attributed to depletion attractions according to phase behavior predicted by our unified theoretical model. Our assembled nanocrystals remain discrete within the gel network, based on X-ray scattering and high-resolution transmission electron microscopy. The infrared optical response of the gels is reflective of both the nanocrystal building blocks and the network architecture, being characteristic of ITO nanocrystals' LSPR with coupling interactions between neighboring nanocrystals.
Niobium oxide (Nb 2 O 5 ) is an interesting active material for technologies ranging from catalysis and sensors to energy storage and electrochromic devices owing to its unique optical, electronic, and electrochemical properties. These properties vary between different phases and morphologies in the Nb 2 O 5 system, but systematic studies that correlate properties to phase and morphology are limited by current synthetic methods, which require postsynthetic high temperature treatments and suffer from a lack of direct and precise control over morphology, crystal structure, and stoichiometry. Here, we report a heat-up colloidal synthesis method that produces orthorhombic Nb 2 O 5 nanorods 1 nm in width by 31 nm in length that preferentially grow along the [001] direction. The synthesis is based on aminolysis of niobium oleate in octadecene, and nanorods are formed through three distinct steps: aminolysis-driven formation of niobium oxo clusters, condensation into amorphous Nb 2 O 5 seeds below the reaction temperature (240 °C, under atmospheric pressure), and crystallization and growth of Nb 2 O 5 nanorods. We investigated the electrochromic behavior of nanorod thin films upon Li + intercalation and observed predominantly near-infrared coloration, fast switching kinetics, and durability for at least 500 charge−discharge cycles.
Nanocrystal gelation provides a powerful framework to translate nanoscale properties into bulk materials and to engineer emergent properties through the assembled microstructure. However, many established gelation strategies rely on chemical reactions and specific interactions, e.g., stabilizing ligands or ions on the nanocrystals' surfaces, and are therefore not easily transferrable.Here, we report a general gelation strategy via non-specific and purely entropic depletion attractions applied to three types of metal oxide nanocrystals. The gelation thresholds of two compositionally distinct spherical nanocrystals agree quantitatively, demonstrating the adaptability of the approach for different chemistries. Consistent with theoretical phase behavior predictions, nanocrystal cubes form gels at a lower polymer concentration than nanocrystal spheres, allowing shape to serve as a handle to control gelation. These results suggest that the fundamental underpinnings of depletion-driven assembly, traditionally associated with larger colloidal particles, are also applicable at the nanoscale.
Plasmonic metal oxide nanocrystals are interesting electrochromic materials because they display high modulation of infrared light, fast switching kinetics, and durability. Nanocrystals facilitate solution-based and high-throughput deposition, but typically require handling hazardous nonaqueous solvents and further processing of the as-deposited film with energy-intensive or chemical treatments. We report on a method to produce aqueous dispersions of tin-doped indium oxide (ITO) by refunctionalizing the nanocrystal surface, previously stripped of its native hydrophobic ligands, with a hydrophilic poly(acrylic acid) polymer featuring a low density of methoxy-terminated poly(ethylene oxide) grafts (PAA-mPEO4). To determine conditions favoring the adsorption of PAA-mPEO4 on ITO, we varied the pH and chemical species present in the exchange solution. The extent of polymer wrapping on the nanocrystal surface can be tuned as a function of the pH to prevent aggregation in solution and deposit uniform, smooth, and optical quality spray coated thin films. We demonstrate the utility of polymer-wrapped ITO nanocrystal thin films as an electrochromic material and achieve fast, stable, and reversible near-infrared modulation without the need to remove the polymer after deposition provided that a wrapping density of ∼20% by mass is not exceeded.
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–*.
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