Controlling the Shape Anisotropy of Monoclinic Nb₁₂O₂₉ Nanocrystals Enables Tunable Electrochromic Spectral Range

Abstract

Electrochromic smart windows that modulate the solar transmittance in a wide and selective spectral range can optimize building energy efficiency. However, for conventional materials such as bulk transition metal oxides, the electrochromic spectral range is constrained by their crystal structure with limited tunability. Herein, we report a method to control the shape anisotropy of monoclinic Nb12O29 nanocrystals and obtain a tunable electrochromic spectral range. We demonstrate the synthesis of monoclinic Nb12O29 nanorods (NRs), extending one-dimensionally along the b direction, and monoclinic Nb12O29 nanoplatelets (NPLs), extending two-dimensionally along the b and c directions. Upon electrochemical reduction accompanied by Li insertion, the NR films show increasing absorbance mostly in the near infrared region. In contrast, the NPL films show increasing absorbance in the near infrared region first followed by increasing absorbance in both visible and near infrared regions. To elucidate the influence of shape anisotropy, we used density functional theory to construct the lithiated structures of monoclinic Nb12O29 and in these structures we identified the presence of square planar sites and crystallographic shear sites for Li insertion. By calculating the theoretical spectra of the lithiated structures, we demonstrate that the Li insertion into the square planar sites results in absorption in the near infrared region in both NRs and NPLs due to their extension in the b direction, while the subsequent insertion of Li into the crystallographic shear sites leads to absorption in both visible and near infrared regions, which only occurs in NPLs due to their extension in the c direction.

Site-selective ion intercalation controls spectral response in electrochromic hexagonal tungsten oxide nanocrystals

“…Greater charge localization as a function of lithium-ion intercalation has been used to explain increased visible light electrochromism in niobium oxide NCs as well. 28 This supports our hypothesis that the visible light absorption of Cs:WO3 NCs comes from the localization of charge during the double injection of electrons and cations into the material. The small increase in reduced tungsten species for films reduced in K + and TBA + electrolytes could either be due to a small amount of the reduction of surface tungsten atoms, or it could be the result of final-state effects that become more influential at higher carrier concentrations.…”

Dual-Band Electrochromism: Plasmonic and Polaronic Mechanisms

“…13 In other cases, delocalized electrons may persist even when ions are inserted, depending on the specific interstitial sites occupied. 31,32 A transition from plasmonic to polaronic by varying the potential bias permits spectroscopic control in the visible and NIR regions selectively, leading to a dual-band EC response in a single material. 13,33 On the other hand, additional control over the absorption of the active layer is possible when dual-band response is achieved by making EC composites containing two different EC component materials, e.g., a metal oxide nanocrystal and a conducting polymer [34][35] , two different metal oxides, or two different polymers.…”

Understanding the Role of Charge Storage Mechanisms in the Electrochromic Switching Kinetics of Metal Oxide Nanocrystals

“…Details of this calculation method can be found in previous publications. [9][10][35][36] A crystal visualization program, VESTA, 37 was used to generate the atomistic models for the calculations.…”

“…Nanocrystals of transition metal oxides were synthesized and applied as electrochromic layers to mitigate the influence of Li + intercalation by minimizing diffusion path length. [7][8][9][10][11][12] In this regard, electrochromic coloration mechanisms can differ from the Li + intercalation typically operating in bulk transition metal oxides, with other mechanisms such as capacitive charging and surface redox becoming dominant in some cases. 4,[13][14] Alternatively, plasmonic metal oxide nanocrystals were synthesized with the ability to modulate their transmittance spectrum by capacitive charging only, which circumvents the slow ion diffusion rate in bulk transition metal oxides.…”

“…15 Both approaches rely on the precision of colloidal nanocrystal synthesis that offers better control over crystal structure, crystallite morphology, and electrochromic properties compared to conventional film deposition methods. 9,16 As a consequence of altering the electrochemical mechanisms, in terms of switching kinetics, the microscopic processes limiting the overall rate can be drastically different for nanoscale materials. 17 It is necessary to consider the contributions from both the typical Li + intercalation and other, surface-dominated mechanisms that are more prominent in nanoscale.…”