Effect of processing parameters on anodic nanoporous tungsten oxide film structure and porosity for hydrogen detection

Nanoporous tungsten oxide films were synthesized by an anodic oxidation process in aqueous NaF/HF electrolytes. The tungsten films were deposited by the radio frequency magnetron sputtering method on sapphire substrates, and the anodic oxidation process was conducted in a dual-electrode reaction chamber with graphite electrode. The effects of processing parameters (anodic voltage, time, temperature, and the operation distance) on the morphology and porosity of the synthesized films were investigated experimentally. The samples were characterized by x-ray diffraction and scanning electron microscopy. The results showed that the pore diameter and porosity increased gradually with increasing anodic voltage, whereas the “wall” of the pore was subjected to electric breakdown at 60 V, and the pore diameter and porosity decreased. The pore diameter and porosity showed an early increased and later decreased state as the operation time and distance are increased. The sensitive response in the resistive method is reaction-dominated type and is exhibited as a linear relationship as a function of hydrogen gas concentration. The response toward 500 ppm hydrogen in air is up to 15.1 with a response time of 10 min at 200 °C.