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

Abstract

The nucleation and growth of Co clusters on vacuum-annealed (reduced) and oxidized TiO 2 (110) have been studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density function theory (DFT) calculations. On vacuum-annealed TiO 2 (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 TiO 2 surface. In contrast, deposition of other metals such as Au, Ni, and Pt on TiO 2 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 TiO 2 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 TiO 2. 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.

Epitaxial growth of CeO2(111) film on Ru(0001): Scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study

“…At the same time Galhenage et al reported a larger diffusion coefficient on the reduced TiO 2 surface than that on non-reduced one. 46 They measured a size distribution of the metal clusters formed on vacuum-annealed (reduced) and defect-free TiO 2 (110) surface. On the reduced TiO 2 surface, they found a decrease of the number of clusters and an increased averaged-size of the clusters than on a defect-free TiO 2 surface, which suggests the enhanced diffusion on reduced TiO 2 (110) surface.…”

Atomic scale characterization and surface chemistry of metal modified titanate nanotubes and nanowires

“…29(C)). These two changes can be attributed to the formation of oxides [285,287] and the agglomeration/encapsulation of clusters [288][289][290]. When the sample was treated further in H 2 atmosphere at 600 K for 60 min a small intensity Co 2p 3/2 peak appeared at 779.6 eV indicating the tendency for the reduction of Co species (Fig.…”

Structure and Formation Model of Ag/TiO2 and Au/TiO2 Nanoparticles Synthesized through Ultrasonic Spray Pyrolysis

“…However, when the cluster is anistropic (interfacial energy depends on direction), the shape of the crystal at equilibrium is determined by Wulff construction based on evaluating surface tension as a function of interface orientation [18]. Furthermore, Galhenage et al [19] discussed the relevance of metal cluster growth behaviour on their thermodynamical properties. It was pointed out that, with decreasing metal-oxygen bond strength, the diffusion rates of metal increase, which causes larger agglomerated clusters away from the target thin film growth.…”