Fabrication of MMO–TiO2 one-dimensional photonic crystal and its application as a colorimetric sensor

This work reports the fabrication of porous mixed metal oxide (MMO)–TiO2 one-dimensional photonic crystals (1DPCs), which can be used as a colorimetric sensor for the detection and measurement of volatile organic compounds (VOCs) and relative humidity (RH). The 1DPCs were prepared via the alternate deposition of titania and layered double hydroxide (LDH) by the spin-coating technique followed by calcination (which induces the phase transformation of LDH to MMO material). A photonic stop band (PSB) of MMO–TiO2 1DPCs was obtained, which can be tuned throughout the whole visible light region by changing the thickness of either of the composing slabs, as a result of the sufficiently high refractive index contrast between TiO2 and MMO. The 1DPC film as a colorimetric sensor shows significant color variation towards VOCs or RH, due to the change of refractive index induced by the adsorption of volatile gas or water molecule in the mesopores of the MMO–TiO2 structure. In addition, the sensor displays high sensitivity, good stability and reproducibility. Therefore, this work provides a feasible method for the fabrication of 1DPCs based on porous MMO–TiO2 films, which have potential applications as portable, recyclable and visually readable colorimetric sensors.