Spontaneous gradual accumulation of hexagonally-aligned nano-silica on gold nanoparticles embedded in stabilized zirconia: a pathway from catalytic to NH3-sensing performance.

The present study highlights the influence of nano-impurities on the catalytic/sensing performance of nano-structured Au sensing-electrodes (SEs) housed in a quartz reactor and operated at high temperature over a long period of time. The planar sensor, made from a nano-structured Au-SE on a polished-polycrystalline (pp) yttria-stabilized zirconia (YSZ) substrate exhibited initially negligible electromotive force (emf) response to each of the examined gases (CO, CH(4), C(3)H(8), C(3)H(6), NO(x) and NH(3); 400 ppm each) at 700 °C in the presence of 5 vol.% oxygen and 5 vol.% water vapor. Such a poor emf response was attributed to the excellent gas-phase oxidation/reduction ability of Au nanoparticles embedded in the YSZ substrate at high temperature. The response of the planar sensor made up of nano-structured Au-SE was monitored for about 75 days at 700 °C. As a result of this long-term monitoring, we detected the appearance of highly sensitive and selective NH(3) gas-sensing properties after 45-75 days of sensor operation. Detailed observation of the morphology and composition of the as-fabricated nano-structured Au-SE after 75 days operation at 700 °C revealed the gradual accumulation of hexagonally-aligned SiO(2) nano-impurities on the surface of the Au nanoparticles. The NH(3) sensing mechanism of the YSZ-based sensor using the spontaneously-formed composite (nano-Au + nano-SiO(2))-SE is therefore proposed to be based on a strong acid-base interaction between gaseous NH(3) and SiO(2) nano-impurities, followed by spillover of adsorbed NH(3) towards the nano-Au/pp-YSZ interface.