Size-dependent colouration efficiency of electrochromic niobium oxide smart windows

Electrochromic smart windows with electrochromic coatings, have been developed as a key technology for energy-efficient buildings, enabling dynamic control of light and heat transmission in response to external stimuli. To achieve high-performance electrochromic coatings, nanoscale engineering of materials such as niobium pentoxide (Nb 2 O 5 ) is considered as attractive solution to enhance their optical modulation, charge transport dynamics, and switching efficiency. In this study, we report the size-dependent colouration efficiency of Nb 2 O 5 nanostructures synthesized via solvothermal methods using ethanol and benzyl alcohol as solvents and Polyvinylpyrrolidone (PVP) as a surfactant, The prepared materials with particle sizes ranging from 15 nm to 1 μm were deposited onto fluorine-doped tin oxide (FTO) substrates by spray coating to fabricate ∼80 % transparent electrochromic electrodes. These electrodes were assembled into electrochromic devices and evaluated for their charge insertion capacity, switching kinetics, and colouration efficiency. Nb 2 O 5 nanostructures characterized by TEM, SEM, XRD, Raman spectroscopy, and UV–Vis analysis revealed that decreasing particle size improved crystallinity and enhanced electrochromic performance. The 15 nm Nb 2 O 5 films exhibited the highest Li + diffusion coefficient of 5.37 × 10 -14 cm 2 /s and an optical modulation of 19% at 700 nm. Furthermore, they showed switching times with a colouration time of 13 s and bleaching time of 3 s, along with a colouration efficiency (CE) of 5.28 cm 2 /C. In contrast, larger Nb 2 O 5 particles (200 nm) exhibited lower performance. These findings highlight the critical influence of particle size and morphology on the electrochromic properties of Nb 2 O 5 , providing valuable insights into the design of the next-generation smart window coatings through nanostructure control. • Niobium pentoxide (Nb2O5) with different particle size (15 nm to 1 μm) have been synthetized by solvothermal synthesis. • The electrochromic performance of Nb2O5 thin films is strongly influenced by particle size • Results showed that that nanoscale films (∼15 nm) provide the highest coloration efficiency and fastest switching • These results highlight potential development of high-performance Nb2O5-based electrochromic coatings, for scalable smart window technology.