Electronic and Catalytic Effects of Single-Atom Pd Additives on the Hydrogen Sensing Properties of Co3O4 Nanoparticle Films.

Atomically dispersed Pd additives significantly enhanced the hydrogen sensing performance of a Co3O4 nanoparticle film, and their electronic along with catalytic roles were comprehensively investigated based on a series of systematic experiments. Aggregates of Co3O4 nanoparticles (approximately 3 nm in size) with homogeneously dispersed Pd additives at concentrations in the range of 1-20% (on a molar basis with respect to Co) were generated in the gas phase via reactive pulsed laser ablation of Co-Pd alloy targets in He/O2 mixtures. The form of the Pd could be modified from single atoms to oxide clusters (1-2 nm), and the effects of these additives on the hydrogen sensing properties of thick films prepared by direct deposition were examined. The highest hydrogen sensing performance was obtained at 5% Pd loading, where single Pd atoms were present at the maximum density. Further Pd loading resulted in the formation of Pd oxide clusters and degraded the sensitivity. X-ray photoelectron spectroscopy and Pd K-edge X-ray absorption spectroscopy showed that single Pd atoms in the Pd4+ state at Co3+ sites on the Co3O4 nanoparticle surfaces donated electrons to the Co3O4 valence band. The greater concentration of free electrons led to an increase in the concentration of ionosorbed oxygen under dry air. Consequently, more ionosorbed oxygen was available for reaction with hydrogen, enhancing sensitivity. In situ X-ray absorption spectroscopy data confirmed that approximately 10% of the single Pd atoms in the Pd4+ state were reduced to Pd2+ during exposure to 1000 ppm H2, implying that a Pd4+ ↔ Pd2+ catalytic redox cycle accelerates the water formation reaction during hydrogen sensing. The present results provide deeper insights and understanding of the effects of noble metal additives on gas sensing, while highlighting the unique role of single-atom additives.