Small polaron induced visible light photochromism in tungsten oxide hydrates- A mechanistic aspect.*

Class of transition metal oxides which act as photochromic materials undergo a reversible color change from native transparent state (characterized by visible and infrared (IR) transmittance) to blue colored state (characterized by the transmission of only visible light and blocking IR). This is due to the intercalation/deintercalation of small cations (e.g.: H⁺) under external stimuli such as sunlight. In this work tungsten oxide hydrates which exhibit reversible photochromism under visible light illumination were prepared by a sol-gel process, in powder and thin film form from a parent sol. The mechanism of coloration is attributed to a two-stage optical phenomenon: (i) formation of photocarriers, leading to the generation of protons which gets intercalated in WO₆ octahedron clusters (ii)  formation of  small polarons, which is evidenced by the absorption of NIR radiation. A small polaron absorption model proposed in the literature has been applied to explain the optical conductivity measurements using spectroscopic ellipsometry. XPS data showed the formation of reduced oxidation states in colored tungsten oxide hydrates, which is a need for small polaron hopping. The optical phonons of tungsten oxide hydrates and structural order were studied by vibrational spectroscopy and XRD. The photochromism observed under solar radiation opens-up cost-effective routes to realize energy saving window glazing.