Enhancement of sensor response by TiO2 mixing and Au coating on ZnO tetrapod sensor

The sensors based on ZnO tetrapods (T-ZnO) and ZnO tetrapods with TiO2 mixing (T-TiZnO) and Au coating (Au/T-TiZnO) were investigated. The T-ZnO and T-TiZnO were synthesized by a simple thermal oxidation technique. The leg of the tetrapod is about 1–10 μm in length and 160–500 nm in diameter. Au coating was performed by sputtering technique and annealed at 550 °C for 6 h under normal atmosphere. The ethanol sensor response of the T-ZnO, T-TiZnO and Au/T-TiZnO sensors was tested at the operating temperature of 260–360 °C with the ethanol concentration of 50, 100, 200, 500, and 1000 ppm. It was found that at the optimum temperature the Au/T-TiZnO sensor exhibited the highest sensor response and the T-TiZnO sensor exhibited higher sensor response than that of the T-ZnO sensor. The Au/T-TiZnO sensor exhibited the highest sensor response up to 240 at 340 °C under the ethanol concentration of 1000 ppm. The enhancement of the sensor response based on the T-ZnO by TiO2 mixing and Au coating can be explained by alloying effect and metal catalytic effect, respectively. The response enhancement can be explained in terms of electron concentration, n0 in air and reaction rate constant, kEth. The evidence for both effects can be found in higher sensor resistance in air and lower sensor resistance in the ethanol ambient.