Optical hydrogen sensitivity of noble metal–tungsten oxide composite films prepared by sputtering deposition

Composite films composed of noble metals such as Pd, Pt and Au and tungsten oxide (WO3) for optical gas sensing were prepared by simultaneous rf and dc sputtering deposition, and the optical hydrogen sensitivities of these Pd–WO3, Pt–WO3 and Au–WO3 composite films were investigated together with the WO3 film without containing noble metals. At temperatures of 200–250°C, no change in the visible–near IR absorption of the WO3 film without noble metals could be detected by the presence of 1 vol.% H2 in air. The combination of Pd with WO3 film enhanced the detection of H2 in air through an increasing change of absorbance in the whole visible–near IR wavelength region centered at around λ=1000 nm. This increase of absorbance by H2 can be explained by the formation of tungsten bronze (HxWO3), which is enhanced by the dissociation of H2 on Pd. On the other hand, the combination of Pt with WO3 did not make the film sensitive to H2. In the case of the Au–WO3 composite film, the combination of Au made the film sensitive mainly at the plasmon absorption region around visible wavelengths, in addition to the small sensitivity due to the formation of tungsten bronze around near IR wavelengths. The change of absorbance by H2 in the plasmon absorption region, which include both an increasing change at wavelengths around 550 nm and a decreasing change of absorbance at wavelengths around 700 nm, might be because the absorbance by small Au particles in plasmon band was influenced by the change in the dielectric constant of tungsten oxide which surrounded small Au particles.