Grain boundaries dependent hydrogen sensitivity in MAO-TiO2 thin films sensors

Anatase TiO2 thin films were fabricated on titanium substrates by micro-arc oxidation (MAO) process and their hydrogen sensing properties were investigated. The obtained films exhibited the maximum H2 gas response at 250 °C but showed no response at the temperature above 300 °C. Electrochemical impedance spectroscopy (EIS) was applied to investigate the gas sensing mechanism. EIS analysis on the as-prepared MAO-TiO2 films with limited porosity reveals that the equivalent electron transfer circuit of grain and grain boundary could be represented electrochemically by a resistor and a constant phase element (CPE) in parallel, respectively, by which a possible surface-controlled gas sensing mechanism is proposed. It is found that temperature dependent grain boundary is a crucial factor governing the gas sensing properties.