Effect of microwave treatment on structural characteristics and energy bandgap of electrochemically synthesized hydrated tungsten oxide quantum dots

Abstract

Quantum Dots (QDs) of hydrated tungsten oxide were synthesized via electrolysis using sintered tungsten carbide-6 wt% cobalt (WC - 6Co) scrap as an anode, Ti plate as cathode, and sulfuric acid as an electrolyte at room temperature. The as-synthesized powder was characterized using X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transforms infrared spectroscopy (FTIR), Raman spectroscopy, Electron paramagnetic resonance spectroscopy (EPR), and Ultraviolet-visible spectroscopy (UV - Vis). The XRD analysis confirmed the formation of orthorhombic hydrated tungsten oxide (WO 3 . H 2 O) QDs via electrochemical oxidation of WC. As-synthesized WO 3 . H 2 O QDs were thermally treated using microwave radiation and conventional furnaces at 150 degrees C for 8 min and 45 min, respectively. Thermal treatment of as-synthesized QDs produced partially dehydrated powder consisting of both orthorhombic WO 3 . H 2 O and cubic WO 3 . H 0.5 crystal structures. The TEM analysis showed that the average particle size of QDs was 7.60 nm. Further, an increase in lattice strain was observed on microwave treatment owing to the non-equilibrium phase transformation (i.e., rapid heating) from orthorhombic to cubic crystal structure resulting in the generation of oxygen vacancies. The increase in oxygen vacancy concentrations in QDs on microwave heating was confirmed by XPS, FTIR, EPR, and Raman spectroscopy. The energy bandgaps of as-synthesized and thermally-treated QDs were in the range of 2.4307 - 2.4979 eV. The relatively low energy bandgap of QDs is attributed to the change in crystal structure and increase in the oxygen vacancy concentration. An improved CO gas sensing characteristics of microwave-treated QDs were noted.