A hydrogen sensor based on a tungsten oxide inverse opal with low cross-sensitivities to other gases is presented. The position of the photonic band gap is utilized as an optical sensor signal. This optical sensor signal offers the possibility for remote readout and so only the transducer itself has to stay in contact with the gas atmosphere, which should be detected. Metal oxide transducers are mostly stable in harsh environments. However, the packaging technology can cause some stability problems of gas sensors, which can be avoided by remote readout of the metal oxide layer.
The photonic band gap position of a photonic crystal, thus the reflected color, is beside others determined by the refractive index contrast of the consisting materials. If this refractive index contrast changes for example because of a reaction with gas, also the band gap position changes. Based on this principle, the band gap position of a tungsten oxide inverse opal can be utilized as a sensor signal to detect hydrogen, as shown in former work.
The shift of the photonic band gap is caused by the intercalation of hydrogen into the tungsten oxide lattice, the formation of tungsten bronze. The possible back reactions of the tungsten bronze are temperature dependent and now clarified. There are three possible reaction paths for the back reaction: oxidation, decomposition and disproportionation of the tungsten bronze, which only can take place in different temperature regimes. The sensor signal is strongly influenced by the back reaction, which takes place, and so the sensor signal is temperature dependent. This results are presented and conditions for gas measurements are deduced.
The cross-sensitivities of the photonic hydrogen sensor were investigated and only negligible cross-sensitivities to other gases like carbon monoxide, methane and humidity were found. This behavior is in good agreement with the found sensing mechanism of hydrogen intercalation into the tungsten oxide lattice.