Influence of Yttria Segregation and Platinum Nanoparticles on the Ionic Conductivity of Yttria-Stabilized Zirconia (YSZ) Films Under Hydrogen Exposure

Abstract

Yttria-stabilized zirconia (YSZ) films have high ionic conductivities over the range 400-800oC which make them useful for solid oxide fuels cells, oxygen sensors, and other sensing applications. Adding catalytic particles to the YSZ surface can induce reactivities and associated conductivity changes for gases other than oxygen. In this work, the surface reactivity towards hydrogen was investigated on thin films with a 8-YSZ composition (8at.% Y2O3 - 92at.% ZrO2) that were deposited onto Langasite (La3Ga5SiO14) substrates using RF-magnetron sputtering in a 5% O2 - 95% Ar gas mixture. Films were grown to thicknesses up to 200 nm using different growth temperatures (30 - 7000C), deposition rates (0.03 - 0.07 nm/s), and substrate bias (-300 - +300 V). To create Pt nanoparticles on the YSZ film surface, Pt was deposited in-situ via e-beam evaporation to equivalent Pt thicknesses of 1 - 10 nm with the YSZ film held at either 25oC or 400oC and then subsequently annealed in an air furnace up to 1000oC to cause Pt agglomeration and nanoparticle formation. To assess steady-state surface reactivity to hydrogen in the range 400 - 600oC after the 1000oC treatments, film ionic conductivities were measured with electrochemical impedance spectroscopy (EIS) using an impedance/gain-phase analyzer after long annealing times in air, in pure N2, or in a 4% H2 - 96% N2 mixture within the furnace tube. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also used to analyze the characteristics of the as-deposited and annealed YSZ and Pt/YSZ films.YSZ films were synthesized with a variety of crystallographic textures, nanomorphology, and composition using the deposition parameters discussed above. Films deposited at 25, 400, or 700oC all have (111) cubic YSZ preferred texture with random in-plane orientation on the Langasite substrate. The smoothest films were produced using a rate of 0.07 nm/s at 400oC. However, films deposited using the other growth conditions contain significant nano-roughness and hillocks generated by film stress as evidenced by Williamson Hall XRD measurements. In films deposited at 25oC there is also a minor (200) grain population in addition to (111). To create Pt nanoparticles on these various film surfaces, Pt was evaporated with the YSZ film surface being held at either 25oC or 400oC. Pt layers (1 - 10 nm) were uniform at 25oC whereas the Pt started to agglomerate during deposition at 4000C.Following deposition, all the YSZ and Pt/YSZ films were annealed in air up to 1000oC and held for 1 hour prior to measuring conductivity and hydrogen reactivity over the range 400-600oC. This 1000oC annealing treatment was performed to (i) promote Pt agglomeration at high temperature to form individual Pt islands, (ii) stabilize the YSZ nanostructure in terms of film stress and grain size, and (iii) obtain high temperature ionic conductivity measurements over a range of temperatures. XPS indicates that post-deposition air annealing causes significant yttria segregation to the YSZ surface region as evidenced by increases in the Y3d/Zr3d photoelectron peak area ratio upon annealing up to 1000oC. This yttria enrichment decreases the film ionic conductivity and decreases the activation energy for ion transport compared to the as-deposited films. EIS experiments performed in air, N2 or 4%H2-N2 between 400-600oC indicate that the YSZ films without Pt that are most sensitive to hydrogen are those deposited at 400-700oC using a rate of 0.03 nm/s which contain rough surfaces, a population of (200) grains, and minimal yttria surface enrichment. The EIS results also show that YSZ films decorated with Pt nanoparticles all exhibit varying baseline conductivities and sensitivities to hydrogen with a magnitude dependent on the Pt nanoparticle distribution as well as film nanostructure. These YSZ and Pt/YSZ films hold promise as hydrogen sensing films that can be incorporated onto a variety of sensing platforms for H2 gas detection and management. Figure 1