Nano/subnanometer Pd nanoparticles on oxide supports synthesized by AB-type and low-temperature ABC-type atomic layer deposition: Growth and morphology

The synthesis of uniformly dispersed nano/subnanometer Pd nanoparticles on oxide supports with atomic layer deposition (ALD) has been studied in terms of growth and morphology. In situ quartz crystal microbalance (QCM) measurements showed that AB-type Pd ALD grew more favorably on TiO₂ than on Al₂O₃ at 200 °C by the sequential exposure of Pd(II) hexafluoroacetylacetonate (Pd(hfac)₂) and formalin. The growth rate of AB-type Pd ALD decreased on the Al₂O₃ surface at a lower deposition temperature, and there was negligible growth at 110 °C. However, a new ABC-type Pd ALD, which we developed recently, operates at significantly lower temperature by growing both protected Pd nanoparticles and the support simultaneously. Additionally, these two types of Pd ALD demonstrated very different growth behaviors. Scanning transmission electron microscopy (STEM) studies showed that the size of the Pd nanoparticles could be well controlled by varying AB-type Pd ALD cycles at 200 °C, and low-temperature ABC-type Pd ALD provides a novel way to synthesize highly uniform, ultrafine, supported Pd nanoparticles directly on high-surface-area supports, regardless of loading. Both types of Pd ALD indicate that ALD is a promising technique for synthesizing advanced catalysts with precise control.