Diverse Optical Regulations in a Single Electrochromic Layer of V2O5 via an Intrinsic Multiple Phase Transition.

Electrochromic oxides possessing the characteristics of color variation and spectra modulation are desirable for smart windows, displays, and camouflage. Here, it is reported that, upon ion intercalation, a single layer of a model phase-transition material, V2O5, can possess diverse modulation of visible and near-infrared spectra while changing color, via intrinsic multiple phase transitions. Specifically, as the phase transitions from α to δ occur, the band transitions around 2.94 eV (420 nm) weaken, causing a blue shift in the optical absorption edge within the visible region. Simultaneously, new band transitions at 1.21 eV emerge and intensify, leading to a broad optical absorption centered around 1025 nm. As the phase transition progresses from δ to γ, the split-off bands in the γ phase fall below the Fermi level, which leads to a near-infrared transparent nature of γ-LixV2O5. Both absorption edge shifting and dynamically modulating the transmittance in V2O5 are different from other cathodic electrochromic oxides. The color-changing characteristics, together with selective spectral modulation, inspire the realization of multiple working modes for smart windows. Moreover, the optical constants of refractive index (n) and extinction coefficient (k) at various phases for LixV2O5 are also demonstrated. It is anticipated that multiple and reversible phase transitions, which have not yet been realized, will be the key design principle for achieving superior electrochromic devices.