Au, Pt, and Au−Pt clusters were grown on TiO2(110) at room temperature and studied by scanning tunneling microscopy. For the same metal coverages, the deposition of pure Pt produces smaller clusters and higher cluster densities compared to pure Au because of the greater mobility of Au on the surface. Heating the surface causes greater sintering of the Au clusters compared to Pt; this behavior is explained by the stronger metal−metal bonds for Pt and the fact that atom detachment is the rate-limiting step in cluster sintering. For the deposition of 0.024 ML of Pt followed by 0.072 ML of Au, bimetallic clusters are formed from the nucleation of Au at existing Pt clusters, whereas the reverse order of deposition results in pure Pt clusters and pure Au clusters coexisting on the surface. The presence of Pt in the bimetallic Pt−Au clusters inhibits sintering, and the average size of the clusters after annealing decreases with increasing Pt composition. Low energy ion scattering experiments demonstrate that the deposition of Au on Pt does not produce core−shell structures with Au on top. Bulk thermodynamics predicts that the cluster surfaces should be pure Au, given that the Au surface free energy is lower than that of Pt, and Au and Pt are immiscible at the compositions studied here. However, surface compositions of the Au−Pt clusters are 10−30% richer in Pt compared to the overall compositions for total coverages of 0.10 ML and 25−75% Pt. These results demonstrate that Au and Pt atoms can intermix at room temperature and the surface properties of Au−Pt nanoclusters are different from those of the bulk. Grazing angle X-ray photoelectron spectroscopy experiments show that annealed Au−Pt clusters are covered by reduced titania. Annealing the Au−Pt clusters to temperatures above 600 K induces encapsulation of the clusters, but the presence of Au at the cluster surface decreases the extent of encapsulation compared to that of pure Pt clusters.
The development of porous well-defined hybrid materials (e.g., metal-organic frameworks or MOFs) will add a new dimension to a wide number of applications ranging from supercapacitors and electrodes to "smart" membranes and thermoelectrics. From this perspective, the understanding and tailoring of the electronic properties of MOFs are key fundamental challenges that could unlock the full potential of these materials. In this work, we focused on the fundamental insights responsible for the electronic properties of three distinct classes of bimetallic systems, MM'-MOFs, MM'-MOFs, and M(ligand-M')-MOFs, in which the second metal (M') incorporation occurs through (i) metal (M) replacement in the framework nodes (type I), (ii) metal node extension (type II), and (iii) metal coordination to the organic ligand (type III), respectively. We employed microwave conductivity, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, pressed-pellet conductivity, and theoretical modeling to shed light on the key factors responsible for the tunability of MOF electronic structures. Experimental prescreening of MOFs was performed based on changes in the density of electronic states near the Fermi edge, which was used as a starting point for further selection of suitable MOFs. As a result, we demonstrated that the tailoring of MOF electronic properties could be performed as a function of metal node engineering, framework topology, and/or the presence of unsaturated metal sites while preserving framework porosity and structural integrity. These studies unveil the possible pathways for transforming the electronic properties of MOFs from insulating to semiconducting, as well as provide a blueprint for the development of hybrid porous materials with desirable electronic structures.
The thermal decomposition of dimethyl methylphosphonate (DMMP) has been studied in ultrahigh vacuum by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) on Ni clusters and films deposited on TiO 2 (110). The four different Ni surfaces under investigation consisted of small Ni clusters (5.0 ( 0.8 nm diameter, 0.9 ( 0.2 nm height) deposited at room temperature and quickly heated to 550 K, large Ni clusters (8.8 ( 1.4 nm diameter, 2.3 ( 0.5 nm height) prepared by annealing to 850 K, a 50 monolayer Ni film deposited at room temperature, and a 50 monolayer Ni film annealed to 850 K. The morphologies of the Ni surfaces were characterized by scanning tunneling microscopy (STM). TPD experiments show that CO and H 2 are the major gaseous products evolved from the decomposition of DMMP on all of the Ni surfaces, and molecular DMMP and methane desorption were also observed. The product yields for CO and H 2 were highest for reactions on the small Ni clusters and unannealed Ni film and lowest for reactions on the large clusters and annealed film. Furthermore, XPS experiments demonstrate that the unannealed Ni surfaces decompose a greater fraction of DMMP at room temperature. The loss of activity for the annealed surfaces is not caused by a reduction in surface area because the annealed surfaces have approximately the same surface area as the small clusters. CO adsorption studies suggest that the loss of activity upon annealing cannot be completely due to a decrease in surface defects, such as step and edge sites, and we propose that a TiO x moiety is responsible for blocking active sites on the annealed Ni surfaces. In comparison to the TiO 2 surface, the small Ni clusters are more chemically active because a greater fraction of DMMP decomposes at room temperature, and the total amount of DMMP decomposition is also higher on the small Ni clusters. Although DMMP decomposes on TiO 2 to produce gaseous methyl radicals, methane, and H 2 , the activity of the substrate surface itself appears to be quenched in the presence of the Ni clusters and films. However, the TiO 2 support plays a significant role in providing a source of oxygen for the recombination of atomic carbon on Ni to form CO, which desorbs above 800 K.
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