Structural and electrochemical properties of carbon ion beam irradiated 12-tungstophosphoric acid

Abstract

In recent years polyoxometalates (POMs) have attracted significant research interest due to versatile properties. These properties are determined by the size, structure and elemental composition of POMs and hence play an important role in their application. In the present study, the ion beam irradiation (10 keV C+ ions, 5 × 1014–2.5 × 1015 ions/cm2) has been utilized for modification of physicochemical properties of 120 nm-thick layer of 12-tungstophosphoric acid (WPA). Scanning electron microscopy analysis of irradiated films showed change of morphology i.e. an increase of WPA grain size with irradiation and coalescence of grains at the highest fluence. This was accompanied by structural changes. Attenuated total reflectance Fourier transform infrared analysis revealed that vibration bands of Keggin anion became less pronounced as fluence increased, while Raman spectra appeared as strongly modified. The effect of irradiation with 1.25 × 1015 ions/cm2 on the structure of WPA was similar to the effect of thermal treatment at 600 °C. Irradiation of WPA led to decrease of the band gap (from 4.07 to 3.92 eV), which was correlated to transformation Keggin anions into a network of WO6 octahedra and PO4 tetrahedra. This is in line with increased number of W=Od bonds observed by UV–Visible diffuse reflectance spectroscopy. Beside transformation to bronzes a reduction of WPA was observed by X-ray Photoelectron Spectroscopy (shift to lower binding energy) and Raman methods, whereas the Raman spectra of irradiated samples were similar to heteropoly blue. The electrochemical properties of irradiated WPA were also assessed. Cyclic voltammetry measurements showed that at up to 1.25 × 1015 ions/cm2 lithiation capacity of WPA increases and activity for hydrogen evolution reaction (HER) improves. The highest fluence caused interconnection of WO6 octahedra, closing of lithiation pathways and decrease in the number of active sites for HER. Our results provide a novel insight into the effects of ion beam irradiation on WPA and demonstrate high potential for tuning of physicochemical properties of POMs that are relevant in wide range of applications.