OPTICAL SENSING PROPERTIES OF WO3 NANOSTRUCTURED THIN FILMS ON SAPPHIRE SUBSTRATE TOWARD HYDROGEN

Many studies have identified tungsten trioxide (WO3) as a promising candidate for optical gas sensing applications. WO3, coated with thin catalytic metals such as Pd, was reported to show a color change from transparent to dark blue upon exposure to oxidizing gas such as hydrogen (H-2). Reliable hydrogen sensor is widely used in medical and energy application area. In this work, WO3 nanostructured thin films were deposited onto sapphire substrates via pulsed laser deposition (PLD) technique by using ArF Excimer laser operating at very short wavelength of 193 nm, the shortest wavelength used in the fabrication of semiconductor oxide thin films. By ablating the target oxides by high energy photons, we could fabricate good crystalline nanostructure thin films. Electron microscopy studies revealed that the uniform and homogeneous WO3 nanostructured films consist of nanorods of about 50 nm sizes. XRD and Raman studies verified good crystalline formation. Absorbance response toward H-2 gas was investigated for a WO3 film coated with 25 angstrom thick palladium (Pd). The Pd/WO3 nanostructured thin films exhibited excellent gasochromic response toward H-2 when measured in the visible-NIR range at 100 degrees C. As low as 0.06% H-2 concentration was clearly sensed. The larger dynamic response was measured at NIR wavelength of 900 nm as compared to the response at visible wavelength of 500 nm. The dynamic response of the films observed in the range of 500-800 nm showed more significant response toward H-2 with low concentrations (0.06%-1%) than the one at single wavelength. As a result, H-2 with very low concentration was able to be sensed reliably in real time. The response and recovery times were found to be < 2 min. The results indicated that the Pd/WO3 nanorod films on sapphire substrates responded to very low H-2 concentration (0.06%) which is well below its lower explosive level threshold (4%).