A continuing goal in catalysis research is to engineer the composition and structure of noble metal nanomaterials in order to precisely tune their catalytic activity. Herein, we present proof-of-concept results on the synthesis of supported bimetallic core/shell nanoparticles entirely by atomic layer deposition (ALD). ALD is a novel and scalable method, which can be used to prepare noble-metal catalysts on high surface area support materials. Two properties of ALD of noble metals, namely the Volmer−Weber growth and surfaceselectivity, are exploited to decouple primary island growth from subsequent selective shell growth. This concept is applied to synthesize highly dispersed Pd/Pt and Pt/Pd nanoparticles. Indepth characterization of the nanoparticles provides evidence for the core/shell morphology and for the narrow size distribution. The self-limiting nature of the ALD process allows for independent control of the core and shell dimensions, opening up unique possibilities for precise engineering of metallic nanoparticle properties.

The deposition of Pd and Pt nanoparticles by atomic layer deposition (ALD) has been studied extensively in recent years for the synthesis of nanoparticles for catalysis. For these applications, it is essential to synthesize nanoparticles with well-defined sizes and a high density on large-surface-area supports. Although the potential of ALD for synthesizing active nanocatalysts for various chemical reactions has been demonstrated, insight into how to control the nanoparticle properties (i.e. size, composition) by choosing suitable processing conditions is lacking. Furthermore, there is little understanding of the reaction mechanisms during the nucleation stage of metal ALD. In this work, nanoparticles synthesized with four different ALD processes (two for Pd and two for Pt) were extensively studied by transmission electron spectroscopy. Using these datasets as a starting point, the growth characteristics and reaction mechanisms of Pd and Pt ALD relevant for the synthesis of nanoparticles are discussed. The results reveal that ALD allows for the preparation of particles with control of the particle size, although it is also shown that the particle size distribution is strongly dependent on the processing conditions. Moreover, this paper discusses the opportunities and limitations of the use of ALD in the synthesis of nanocatalysts.

Herein, we report the fabrication of hydrogen gas sensors with enhanced sensitivity and excellent selectivity. The sensor device is based on the strategic combination of ZnO nanowires (NWs) decorated with palladium nanoparticles (Pd NPs) and a molecular sieve metal-organic framework (MOF) nanomembrane (ZIF-8). The Pd NPs permit the sensors to reach maximal signal responses, whereas the ZIF-8 overcoat enables for an excellent selectivity. Three steps were employed for the fabrication: (i) coating of a miniaturized sensor with vapor-grown ZnO NWs, (ii) decoration of these NWs with Pd NPs by atomic layer deposition, and (iii) partial solvothermal conversion of the tuned NWs surface to ZIF-8 nanomembrane. The microstructure and composition investigations of the ZIF-8/Pd/ZnO nanostructured materials confirmed the presence of both metallic Pd NPs and uniform ZIF-8 thin membrane layer. The integration of these nanomaterials within a miniaturized sensor device enabled the assessment of their performance for H detection at concentrations as low as 10 ppm in the presence of various gases such as CH, CH, CHOH, and CHCOCH. Remarkably high-response signals of 3.2, 4.7, and 6.7 ( R/ R) have been measured for H detection at only 10, 30, and 50 ppm, whereas no noticeable response toward other tested gases was detected, thus confirming the excellent H selectivity obtained with such a sensor design. The results obtained showed that the performance of gas sensors toward H gas can be greatly increased by both the addition of Pd NPs and the use of ZIF-8 coating, acting as a molecular sieve membrane. Furthermore, the presented strategy could be extended toward the sensing of other species by a judicious choice of both the metallic NPs and MOF materials with tuned properties for specific molecule detection, thus opening a new avenue for the preparation of highly selective sensing devices.