Understanding the Nucleation and Growth of Metals on TiO2: Co Compared to Au, Ni, and Pt

The nucleation and growth of Co clusters on vacuum-annealed (reduced) and oxidized TiO2(110) have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density function theory (DFT) calculations. On vacuum-annealed TiO2(110), the Co clusters grow as three-dimensional islands at coverages between 0.02 and 0.25 ML, but the cluster heights range from ∼3 to 5 Å, indicating that the clusters are less than three layers high. In addition to the small cluster sizes, the high nucleation density of the Co clusters and lack of preferential nucleation at the step edges demonstrate that diffusion is slow for Co atoms on the TiO2 surface. In contrast, deposition of other metals such as Au, Ni, and Pt on TiO2 results in larger cluster sizes with a smaller number of nucleation sites and preferential nucleation at step edges. XPS experiments show that Co remains in the metallic state, and there is little reduction of the titania surface by Co. A comparison of the metal−titania binding energies calculated by DFT for Co, Au, Ni, and Pt indicates that stronger metal− titania interactions correspond to lower diffusion rates on the surface, as observed by STM. Furthermore, on oxidized TiO2 surfaces, the diffusion rates of all of the metals decrease, resulting in smaller cluster sizes and higher cluster densities compared to the growth on reduced TiO2. DFT calculations confirm that the metal−titania adsorption energies are higher on the oxidized surfaces, and this is consistent with the lower diffusion rates observed experimentally.