Hydrogen Sensors Based on 2D Arrays of Self-Assembled Core-Shell Nanoparticles

Au@Pd and Au@Pt core-shell nanoparticles are synthetized with a good control in the precious metal shell thickness. Resulting colloids are assembled as hexagonal close-packed 2D monolayers by using a simple Langmuir-Blodgett method and transferred by dip-coating at the surface of a glass slide supporting interdigitated electrodes. The fabricated resistive sensors show attractive hydrogen sensing performances with reversible responses in extended sensing ranges, a good specificity towards H₂, short response and recovery times.

For Au@Pt based sensors, the dissociative chemisorption of H₂ and O₂ on Pt surface leads to the formation of chemisorbed hydrogen and hydroxyl groups. This surface nature change induces the modification of the scattering of the conduction electrons at both grain surface and intercontacts, tuned by the extent of hydrogen and hydroxyl group coverages. Depending on the Pt shell thickness, scattering at the surface or grain boundaries accounts for the observed conductive or resistive responses, respectively, while the overall sensing behavior is balanced by these two antagonist effects. For Au@Pd based sensors, the specific reactivity of palladium towards hydrogen, leading to the reversible conversion of Pd as PdHx, is originating for the H₂ concentration dependence of the observed either either resistive or conductive responses. In terms of amplitude and sign, these responses strongly depend on the balanced contribution of two antagonist mechanical and electronic effects, promoted by the palladium hydride formation under H₂ atmosphere.

By using the percolation theory and simple data modeling, these Pt and Pd thickness dependent contributions are decorrelated and the sensing mechanisms are described.