Effect of water intercalation into tungsten trioxide structure (WO3·xH2O) (x = 0,1,2): Correlation between structure and photocatalytic performances

Abstract

Tungsten oxide (WO3) is a potential photoanode material for water oxidation, however, its employment is still restricted by its limited light absorption (Eg ∼ 2.75 eV). A particular interest was devoted to tuning its electronic structure and optical bandgap in order to enhance its photocatalytic performance in hydrogen production as well as in advanced oxidation process. One of the promising alternatives for the enhancement of these photocatalytic performances, is the introduction of guest molecules into the tungsten oxide structure. In this paper, the photocatalytic performances of tungsten trioxide were highly improved by the intercalation of water into its structure. The intercalated compounds of chemical formula WO3·xH2O were prepared and characterized by TGA-DSC, PXRD, FT-IR, Raman spectroscopy, SEM-EDS and XPS. The structures of intercalated materials were refined using Rietveld method and the electronic proprieties including bandgap energy, CB and VB edges were estimated using UV–visible reflectance spectroscopy. The obtained results show that the intercalation of water molecules into tungsten oxide structure reduces the gap energy of the WO3 (2.4 eV) to 2.16 eV for WO3∙H2O and to 2.31 eV for WO3∙2H2O. The photocatalytic efficiency of the obtained materials was studied through oxygen evolution reaction tests in the presence of an electron acceptor (Ag+) and the oxidative photodegradation of the orange G dye pollutant. We found that intercalated samples exhibit the highest efficiencies compared with anhydrous samples. These findings are correlated with the increase in the average W-O-W tilt angle in WO3∙2H2O and WO3∙H2O compared to that of WO3. This improvement in the tilt angle induces the shrinkage of the energy bandgap and increases the photocatalytic performance. Furthermore, the stability of the investigated materials was studied as a function of pH, the results reveal that the intercalation of water molecules is beneficial for the chemical stability of the hydrated tungsten oxides. These findings shed light on new alternatives for band-gap engineering and improvement of materials stability through intercalation with water molecules.