2D metal film catalysts can be used as the well-defined model systems to study the quantum size effects (QSEs) in catalysis. The unambiguous identification of the QSEs concept only relies on the accurate control of the catalyst film thickness. Epitaxial growth of metals on semiconductors provides a well-controlled route to prepare 2D metal film catalysts.
In the present work, 2D Al overlayers were epitaxially grown on Si(111)-7Χ7 surfaces and three different Al/Si(111) superstructures were obtained by controlling the nominal thickness of Al: 0.25 ML Al/Si(111) with the phase of α-7Χ7, 0.8 ML Al/Si(111) with the phase of γ-9Χ9, and 16 ML Al with bulk Al structure. The morphology and electronic structure of the Al surfaces were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XPS indicated that the Al-Si interaction follows the order: 0.25 ML Al > 0.8 ML Al > 16 ML Al. Methanol was exposed to the three surfaces to probe the Al surface reactivity. XPS and high resolution electron energy loss spectroscopy (HREELS) results show that the Al overlayers with smaller thickness present higher reactivity to methanol. 0.25 ML Al overlayers favor the dissociation of C-O bonds in CH3OH in comparison to the surface of 16 ML Al film. The Al surface reactivity can be correlated with the electronic structure of the Al surfaces.
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