Doped tungsten oxide nanocrystals for next generation electrochromic windows

Doped tungsten oxide (WO₃ [subscript -x]) nanocrystals (NCs) have lots of potential for next generation electrochromic windows compared to bulk thin films. The difference lies in intrinsic WO₃ [subscript -x] NC properties of shape and crystalline anisotropy with localized surface plasmon resonance absorption in the shorter wavelength near-infrared range, which can directly affect the major electrochromic performance of switching speed, optical modulation, and cycling stability. This work illustrates how doped WO₃ [subscript -x] NC properties are effectively utilized for enhancing the electrochromic performance. First, how shape anisotropic properties can generate highly porous film is studied using different aspect ratio of WO₂.₇₂ nanorods. By changing the nanorod interaction to electrostatic repulsion from solution ligand-stripping chemistry, highly porous mesoporous thin film from randomly packed nanorods is fabricated. Incorporating guest inorganic materials of niobium polyoxometalate clusters followed by chemical condensation, dual-band modulation of electrochromic films on flexible substrates are demonstrated, tackling cycling stability, and optical modulation issues. Second, how doped semiconductor NCs are effectively used for dynamic Bragg stacks with targeted performance of ‘on and off’ reflectance is studied. Dynamic reflectance tuning can affect the color tuning as well as efficient heat blocking. Judicious NC selection of indium tin oxide and WO₃ [subscript -x] NCs from mechanistic understanding of electrochemical modulation of optical properties, optimization of film processing, and reliable refractive index data from in situ ellipsometry enable accurate Bragg stack optimization from simulation and experimental realization. Third, using monoclinic WO₂.₇₂ nanorods as a model system having different size of three intracrystalline tunnel sites, we study spectroelectrochemical properties with different cation system (lithium, sodium, and tetrabutylammonium ion). In doing so, Al₂O₃ atomic layer deposition is employed to prevent electrolytes degradation and allows to study spectroelectrochemical properties. Na⁺ electrolytes system gives higher coloration efficiency than Li⁺ electrolytes and mainly capacitive charging behavior owing to its occupancy in the hexagonal tunnel sites. The results of these studies suggest general approach to improve electrochromic performance where shape and crystalline anisotropic properties of doped metal oxide nanocrystals can be effectively utilized for impacting spectroelectrochemical properties.Chemical Engineerin

Influence of Crystalline and Shape Anisotropy on Electrochromic Modulation in Doped Semiconductor Nanocrystals

Localized surface plasmon resonance (LSPR) modulation appearing in the near-infrared range in doped semiconductor nanocrystals enriches electrochromic performance. Although crystalline and shape anisotropies influence LSPR spectra, study of their impact on electrochromic modulation are lacking. Here, we study how crystalline anisotropy in hexagonal cesium-doped tungsten oxide nanorods and nanoplatelets affects essential metrics of electrochromic modulation—coloration efficiency (CE) and volumetric capacity—using different sizes of electrolyte cations (tetrabutylammonium, sodium, and lithium) as structurally sensitive electrochemical probes. Nanorod films show higher CE than nanoplatelets in all of electrolytes owing to low effective mass along the crystalline c-axis. When using sodium cations, which diffuse through one-dimensional hexagonal tunnels, electrochemical capacity is significantly greater for platelets than for nanorods. This difference is explained by the hexagonal tunnel sites being more accessible in platelets than in nanorods. Our work sheds light on the role of shape and crystalline anisotropy on charge capacity and CE both of which contribute to overall modulation. </p

Dynamics of Lithium Insertion in Electrochromic Titanium Dioxide Nanocrystal Ensembles

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 Li-insertion from other electrochemical responses, such as double-layer capacitance 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 Li-insertion potentials with particle size. However, particle size does not affect the kinetics of Li-insertion in ensemble electrodes. Rather, Li-insertion rates depend on 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 non-equilibrium charging behavior in nanocrystalline electrodes through a combination of colloidal synthesis, phase field simulations and spectroelectrochemistry.<br /

Template-Free Mesoporous Electrochromic Films on Flexible Substrates from Tungsten Oxide Nanorods

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 (WO_2.72) 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 WO_<i>x</i>–NbO_<i>x</i> 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

Disentangling Photochromism and Electrochromism by Blocking Hole Transfer at the Electrolyte Interface

Disentangling Photochromism and Electrochromism by Blocking Hole Transfer at the Electrolyte Interfac

Solvothermally-synthesized tin-doped indium oxide plasmonic nanocrystals spray-deposited onto glass as near-infrared electrochromic films

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High Mobility in Nanocrystal-Based Transparent Conducting Oxide Thin Films

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