RESUMEN
Nanocrystalline anatase TiO2 is a robust model anode for Li insertion in batteries. The influence of nanocrystal size on the equilibrium potential and kinetics of Li insertion is investigated with in operando spectroelectrochemistry of thin film electrodes. Distinct visible and infrared responses correlate with Li insertion and electron accumulation, respectively, and these optical signals are used to deconvolute bulk Li insertion from other electrochemical responses, such as double-layer capacitance, pseudocapacitance, and electrolyte leakage. Electrochemical titration and phase-field simulations reveal that a difference in surface energies between anatase and lithiated phases of TiO2 systematically tunes the Li-insertion potentials with the particle size. However, the particle size does not affect the kinetics of Li insertion in ensemble electrodes. Rather, the Li-insertion rates depend on the applied overpotential, electrolyte concentration, and initial state of charge. We conclude that Li diffusivity and phase propagation are not rate limiting during Li insertion in TiO2 nanocrystals. Both of these processes occur rapidly once the transformation between the low-Li anatase and high-Li orthorhombic phases begins in a particle. Instead, discontinuous kinetics of Li accumulation in TiO2 particles prior to the phase transformations limits (dis)charging rates. We demonstrate a practical means to deconvolute the nonequilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations, and spectroelectrochemistry.
RESUMEN
Coloration efficiency is an important figure of merit in electrochromic windows. Though it is thought to be an intrinsic material property, we tune optical modulation by effective utilization of ion intercalation sites. Specifically, we enhance the coloration efficiency of m-WO2.72 nanocrystal films by selectively intercalating sodium ions into optically active hexagonal sites. To accurately measure coloration efficiencies, significant degradation during cycling is mitigated by introducing atomic-layer-deposited Al2O3 layers. Galvanostatic spectroscopic measurement shows that the site-selective intercalation of sodium ions in hexagonal tunnels enhances the coloration efficiency compared to a nonselective lithium ion-based electrolyte. Electrochemical rate analysis shows insertion of sodium ions to be capacitive-like, another indication of occupying hexagonal sites. Our results emphasize the importance of different site occupation on spectroelectrochemical properties, which can be used for designing materials and selecting electrolytes for enhanced electrochromic performance. In this context, we suggest sodium ion-based electrolytes hold unrealized potential for tungsten oxide electrochromic applications.
RESUMEN
A synthetic challenge in faceted metal oxide nanocrystals (NCs) is realizing tunable localized surface plasmon resonance (LSPR) near-field response in the infrared (IR). Cube-shaped nanoparticles of noble metals exhibit LSPR spectral tunability limited to visible spectral range. Here, we describe the colloidal synthesis of fluorine, tin codoped indium oxide (F,Sn:In2O3) NC cubes with tunable IR range LSPR for around 10 nm particle sizes. Free carrier concentration is tuned through controlled Sn dopant incorporation, where Sn is an aliovalent n-type dopant in the In2O3 lattice. F shapes the NC morphology into cubes by functioning as a surfactant on the {100} crystallographic facets. Cube shaped F,Sn:In2O3 NCs exhibit narrow, shape-dependent multimodal LSPR due to corner, edge, and face centered modes. Monolayer NC arrays are fabricated through a liquid-air interface assembly, further demonstrating tunable LSPR response as NC film nanocavities that can heighten near-field enhancement (NFE). The tunable F,Sn:In2O3 NC near-field is coupled with PbS quantum dots, via the Purcell effect. The detuning frequency between the nanocavity and exciton is varied, resulting in IR near-field dependent enhanced exciton lifetime decay. LSPR near-field tunability is directly visualized through IR range scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). STEM-EELS mapping of the spatially confined near-field in the F,Sn:In2O3 NC array interparticle gap demonstrates elevated NFE tunability in the arrays.
RESUMEN
Degenerately doped semiconductor nanocrystals (NCs) exhibit strong light-matter interactions due to localized surface plasmon resonance (LSPR) in the near- to mid-infrared region. Besides being readily tuned through dopant concentration introduced during synthesis, this LSPR can also be dynamically modulated by applying an external electrochemical potential. This characteristic makes these materials candidates for electrochromic window applications. Here, using prototypical doped indium oxide NCs as a model system, we find that the extent of electrochemical modulation of LSPR frequency is governed by the depletion width and the extent of inter-NC LSPR coupling, which are indirectly controlled by the dopant density, size, and packing density of the NCs. The depletion layer is a near-surface region with a sharply reduced free carrier population that occurs whenever the surface potential lies below the Fermi level. Changes in the depletion width under applied bias substantially control the spectral modulation of the LSPR of individual NCs and also modify the inter-NC LSPR coupling, which additionally modulates the LSPR absorption on the NC film scale. Here, we show that both of these effects must be considered primary factors in determining the extent of LSPR frequency modulation and that the dominant factor depends on NC size. For a constant doping concentration, depletion effects govern LSPR modulation for smaller NCs, while LSPR coupling is prevalent in larger NCs. Consequently, as the size of the NCs is increased while keeping the doping concentration constant, we observe a reversal in the sign of the LSPR frequency modulation from positive to negative.
RESUMEN
Low-temperature processed mesoporous nanocrystal thin films are platforms for fabricating functional composite thin films on flexible substrates. Using a random arrangement of anisotropic nanocrystals can be a facile solution to generate pores without templates. However, the tendency for anisotropic particles to spontaneously assemble into a compact structure must be overcome. Here, we present a method to achieve random networking of nanorods during solution phase deposition by switching their ligand-stabilized colloidal nature into a charge-stabilized nature by a ligand-stripping chemistry. Ligand-stripped tungsten suboxide (WO2.72) nanorods result in uniform mesoporous thin films owing to repulsive electrostatic forces preventing nanorods from densely packing. Porosity and pore size distribution of thin films are controlled by changing the aspect ratio of the nanorods. This template-free mesoporous structure, achieved without annealing, provides a framework for introducing guest components, therefore enabling our fabrication of inorganic nanocomposite electrochromic films on flexible substrates. Following infilling of niobium polyoxometalate clusters into pores and successive chemical condensation, a WOx-NbOx composite film is produced that selectively controls visible and near-infrared light transmittance without any annealing required. The composite shows rapid switching kinetics and can be stably cycled between optical states over 2000 times. This simple strategy of using anisotropic nanocrystals gives insight into mesoporous thin film fabrication with broader applications for flexible devices.
RESUMEN
Charge carrier mobility in transparent conducting oxide (TCO) films is mainly limited by impurity scattering, grain boundary scattering, and a hopping transport mechanism. We enhanced the mobility in nanocrystal (NC)-based TCO films, exceeding even typical values found in sputtered thin films, by addressing each of these scattering factors. Impurity scattering is diminished by incorporating cerium as a dopant in indium oxide NCs instead of the more typical dopant, tin. Grain boundary scattering is reduced by using large NCs with a size of 21 nm, which nonetheless were sufficiently small to avoid haze due to light scattering. In-filling of the precursor solution followed by annealing results in a NC-based composite film which conducts electrons through metal-like transport at room temperature, readily distinguished by the positive temperature coefficient of resistance. Cerium-doped indium oxide (Ce:In2O3) NC-based composite films achieve a high mobility of 56.0 cm2/V·s, and a low resistivity of 1.25 × 10-3 Ω·cm. The films are transparent to a broad range of visible and near-infrared light from 400 nm to at least 2500 nm wavelength. On the basis of the high conductivity and high transparency of the Ce:In2O3 NC-based composite films, the films are successfully applied as transparent electrodes within an electrochromic device.