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Size-selected Pt(n) (n=4,7-10,15) clusters were deposited on TiO(2)(110)-(1x1) surfaces and imaged at atomic resolution using an ultrahigh-vacuum scanning tunneling microscope with a carbon nanotube tip. Clusters smaller than Pt(7) lay flat on the surface with a planar structure and a planar-to-three-dimensional transition occurred at n=8 for Pt(n) clusters on TiO(2). However, both Pt(8) and Pt(9) had two types of geometric structures. The geometric structures depend strongly on the number of atoms in the deposited cluster possibly because of the differences in binding energies in different-sized clusters and different degrees of interaction with the surface. We obtained atomic-resolution images of size-selected clusters on surfaces for the first time, enabling the identification of atomic alignments in the clusters on the surface.

Owing to maximally exposed active sites and a unique electronic structure, atomically thin two-dimensional semiconductor catalysts and atomic-scale metal co-catalysts are beyond their conventional bulk limit, especially for catalysis. Here, we report a two-dimensional titania (2DT) sheet-supported atomic-scale palladium (Pd1/2DT) via a stepwise solvothermal reaction and photochemical reduction approach. Such synthesized 2DT sheets have a layer structure, with a measured optical onset at ∼2.86 eV. An atomically thick 2D structure not only exhibits short-distance migration of photo-induced charge carriers but also provides more anchoring sites to stabilize atomic-scale Pd1 species, thus promoting the separation of charge carriers and stability of Pd1. This Pd1/2DT can catalyze methane conversion to methanol with a selectivity of 94% by using low-energy photons (λ > 420 nm) of solar light. Our work demonstrates that atomically low-dimensional catalysts can result in new and useful catalytic behaviors.

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