How does water of crystallization influence the optical properties, band structure and photocatalytic activity of tungsten oxide?

Abstract Both experimental and computational approaches were conducted to elucidate the effect of water of crystallization on optical properties, band structure, and photocatalytic activity of tungsten oxide. WO3·2H2O powder was synthesized at room temperature by the liquid phase deposition (LPD) and, upon annealing, was converted to WO3·2H2O and WO3. The crystallite size was obtained using the Scherrer equation ∼69 nm for WO3·2H2O, ∼91 nm for WO3·H2O, and ∼23 nm for WO3. It was found that the optical band gap energy value for WO3·2H2O (∼ 2.25 eV) is lower than that for WO3·H2O (∼ 2.55 eV) and WO3 (∼ 2.91 eV), and the electron-hole recombination for WO3·2H2O occurs at a lower rate. The computed data showed that water of crystallization does not change the band gap energy by creating new energy states; instead, it alters the crystal structure and hybridization with crystal atoms. The hybridization is stronger for WO3·2H2O compared to WO3·H2O. The computed phonon-induced pure-dephasing times are faster for the hydrated crystals that, as a result, it leads to a long excited-state lifetime. The experimental results confirmed the computed data that WO3·2H2O showed better photocatalytic activity than WO3·H2O and WO3, as well as more stable cyclic performance. Instead, WO3 and WO3·H2O acted as better adsorbents for the elimination of methylene blue. The cyclic performance of the photocatalysts decreased to 87.5, 60, and 50% of the initial performance for WO3·2H2O, WO3·H2O, and WO3, respectively.