Synthesis of Cu-doped WO3 materials with photonic structures for high performance sensors

Abstract

Cu-doped photonic crystal (PC) WO₃ replicas from Morpho butterfly wings have been prepared by using a combined sol–gel templating and calcination method. The exact replications in the Cu-doped PC WO₃ replicas at the micro- and nanoscales were confirmed by scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). A combination of X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), Raman and X-ray photoelectron spectra (XPS) measurements revealed that copper does not form clusters, but is randomly distributed inside the WO₃ matrix lattice. The optical properties of the Cu-doped PC WO₃ replica as well as the Morpho butterfly wing template were investigated by using reflectance spectroscopy, and it was found that the reflected light chromaticity of the Cu-doped PC WO₃ replica was inherited from the PC Morpho butterfly wings. Cu-doped WO₃ replicas without photonic crystal structures (Cu-W replica) were also fabricated in the same way as the Cu-doped PC WO₃ replica but using Euploea mulciber butterfly wings as the template. Chemical sensors fabricated from the Cu-doped PC WO₃ as well as the non-PC Cu-doped WO₃ replicas were tested for a range of gases: (CH₃)₃N (TMA), NH₃, C₂H₅OH, HCHO, CH₃OH, acetone, H₂, CO and NO₂ and they showed a high selectivity for TMA. The sensitivity of the Cu-doped PC WO₃ replica sensors can reach up to 2.0 for a trimethylamine concentration as low as 0.5 ppm at 290 °C. The high sensitivity of the Cu-doped WO₃ replica sensors to TMA is attributed to the catalytic effect of Cu on the reaction between the testing gas and the oxide surface. Furthermore, the Cu-doped PC WO₃ replica sensor is twice as sensitive as the Cu-doped non-PC WO₃ replica to trimethylamine. This may be explained by the photonic crystal structure of the Cu-doped PC WO₃.