Nano Diode-Type Protonic Transport in Hydrated 3 Mol.% Yttria-Stabilized Zirconia (3YSZ) Nanopowders: Humidity Dependence and Isotope Effect (H2O/D2O)

Abstract

In this study, we report the formation of a chemically symmetric nanoscale rectifying junction at the interface between yttria-stabilized zirconia (YSZ, ZrO₂–3 mol.% Y₂O₃) nanoparticles of identical composition but different crystallite sizes. Nanopowders were synthesized via co-precipitation and thermally treated at 400°C, 500°C, and 700°C, yielding crystallite sizes ranging from approximately 7.5 nm to 14 nm, while preserving the cubic fluorite structure and chemical homogeneity. Rectifying contacts were fabricated in three configurations: 400–400, 400–500, and 400–700.Comprehensive structural characterization was carried out using transmission electron microscopy (TEM), x-ray diffraction (XRD), and small-angle x-ray scattering (SAXS). SAXS analysis provided quantitative information on nanoscale domain size, surface roughness, and interparticle correlations, which are critical for understanding interfacial charge transport behavior.Electrical characterization was performed using current–voltage measurements under controlled relative humidity levels (65%, 75%, and 85%) in both H₂O- and D₂O-enriched atmospheres. Junctions formed between nanoparticles with different crystallite sizes (400–500 and 400–700) exhibited pronounced rectifying behavior that intensified with increasing humidity, whereas the symmetric 400–400 junction displayed an approximately ohmic response. Replacement of H₂O with D₂O led to a systematic decrease in current, confirming the dominant contribution of proton-mediated transport within the hydrated interfacial region. Our findings demonstrate that nanoscale size disparity alone, without compositional gradients, can induce diode-like behavior in homogeneous oxide systems. The results highlight the critical role of hydration-assisted interfacial proton conduction and offer a novel strategy for engineering nanoionic rectifiers and environmentally responsive oxide-based electronic devices.