posted on 2020-07-27, 20:44authored byHsin-Che Lu, Sandeep Ghosh, Naman Katyal, Vikram S. Lakhanpal, Ioana R. Gearba-Dolocan, Graeme Henkelman, Delia J. Milliron
Transition
metal oxide nanocrystals with dual-mode electrochromism
hold promise for smart windows enabling spectrally selective solar
modulation. We have developed the colloidal synthesis of anisotropic
monoclinic Nb12O29 nanoplatelets (NPLs) to investigate
the dual-mode electrochromism of niobium oxide nanocrystals. The precursor
for synthesizing NPLs was prepared by mixing NbCl5 and
oleic acid to form a complex that was subsequently heated to form
an oxide-like structure capped by oleic acid, denoted as niobium oxo
cluster. By initiating the synthesis using niobium oxo clusters, preferred
growth of NPLs over other polymorphs was observed. The structure of
the synthesized NPLs was examined by X-ray diffraction in conjunction
with simulations, revealing that the NPLs are monolayer monoclinic
Nb12O29, thin in the [100] direction and extended
along the b and c directions. Besides
having monolayer thickness, NPLs show decreased intensity of Raman
signal from Nb–O bonds with higher bond order when compared
to bulk monoclinic Nb12O29, as interpreted by
calculations. Progressive electrochemical reduction of NPL films led
to absorbance in the near-infrared region (stage 1) followed by absorbance
in both the visible and near-infrared regions (stage 2), thus exhibiting
dual-mode electrochromism. The mechanisms underlying these two processes
were distinguished electrochemically by cyclic voltammetry to determine
the extent to which ion intercalation limits the kinetics, and by
verifying the presence of localized electrons following ion intercalation
using X-ray photoelectron spectroscopy. Both results support that
the near-infrared absorption results from capacitive charging, and
the onset of visible absorption in the second stage is caused by ion
intercalation.