Ultra high response for hydrogen gas on Pd–Au_gr-alloy@ZnO core-shell nanoparticles with Pd–Au gradient composition alloy core

Pd@ZnO core-shell nanoparticles (CS-NPs) have shown relatively low response and selectivity to hydrogen gas due to surface oxidation of the Pd core. To overcome this, Pd@Au@ZnO core-double shell nanoparticles (CDS-NPs) with an added Au shell layer between the Pd core and ZnO shell are fabricated via a two steps hydrothermal route. Each as-prepared Pd@Au@ZnO CDS-NP is composed of a cubic Pd core (particle size of about 13 nm), an inner Au shell layer (approximately 5 nm thick) and an outer ZnO shell layer (approximately 45 nm thick). After calcination at 500 °C, each Pd@Au CS-NP core becomes a gradient composition Pd–Au alloy NP, in which the inner is a Pd-rich alloy and the outer is an Au-rich alloy (that retains the core-shell structure). At an optimal working temperature of 300 °C, the Pd-Au_gr-alloy@ZnO sensor shows a high response (174) toward 100 ppm H₂ gas, which is about 3 and 13 times higher than that demonstrated by Pd@ZnO and pure ZnO sensors, respectively. Moreover, the Pd–Au_gr-alloy@ZnO CS-NPs sensor presents with higher selectivity to hydrogen gas than to other gases. The excellent response and selectivity are the result of a synergistic effect between the catalytic and hydrogen adsorption properties of the gradient composition Pd–Au alloy core.