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

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.