Substrate-Mediated Morphology Control and Its Impact on Hydrogen Sensing in Pd-Based Multicomponent Alloy Thin Films

Pd alloy thin-film hydrogen sensors have attracted considerable attention, but their practical applications are limited by low sensitivity, slow response/recovery kinetics, and poor long-term stability. Although material modification has been extensively employed to improve sensing performance, the substrate effect on thin-film growth and sensing functionality have not yet been systematically investigated. Herein, we systematically investigate the influence of substrate roughness on the hydrogen-sensing performance by depositing identical PdAgCuSnNi multicomponent alloy films on Al2O3, LTCC (low-temperature co-fired ceramic), and SiO2/Si substrates with progressively decreasing surface roughness. Pronounced substrate-dependent variations are observed in key sensing characteristics, including response magnitude, recovery kinetics, baseline stability, and CO interference tolerance. The medium-roughness LTCC-supported sensor exhibits the most balanced overall performance, showing the highest H2 response (5.61%), surpassing those of the Al2O3- (4.20%) and SiO2/Si-supported (4.09%) sensors, together with the fastest recovery (705 s upon removal of 20,000 ppm of H2), the smallest baseline fluctuation (0.016%), and improved selectivity. This substrate effect is primarily attributed to differences in film morphology and interfacial mechanical constraints, which govern hydrogen diffusion and strain accommodation during repeated absorption/desorption cycles. These findings underscore substrate selection as a crucial design parameter for optimizing Pd-based thin-film hydrogen sensors without altering the sensing material itself.