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 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.

We have developed an integrated approach that combines synthesis, X-ray photoelectron spectroscopy (XPS) studies, and theoretical calculations for the investigation of active unsaturated metal sites (UMS) in copper-based metal–organic frameworks (MOFs). Specifically, extensive reduction of Cu+2 to Cu+1 at the MOF metal nodes was achieved. Introduction of mixed valence copper sites resulted in significant changes in the valence band structure and an increased density of states near the Fermi edge, thereby altering the electronic properties of the copper-based framework. The development of mixed-valence MOFs also allowed tuning of selective adsorbate binding as a function of the UMS oxidation state. The presented studies could significantly impact the use of MOFs for heterogeneous catalysis and gas purification as well as foreshadow a new avenue for controlling the conductivity of typically insulating MOF materials.

The nucleation, growth, and CO-induced changes in composition for Co–Au bimetallic clusters deposited on TiO2(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. STM experiments show that the mobility of Co atoms on TiO2(110) is significantly lower than of Au atoms; for equivalent or lower coverages of Co, the number of clusters is higher and the average cluster height is smaller than for Au deposition. Consequently, bimetallic clusters are formed by first depositing the less mobile Co atoms, followed by the addition of the more mobile Au atoms. Furthermore, the reverse deposition of Au followed by Co results in clusters of pure Co coexisting with clusters that are Au-rich. For clusters with a total coverage of 0.25 ML, the cluster density increases and average cluster height decreases as the fraction of Co is increased. Annealing to 800 K results in cluster sintering and an increase of ∼3–5 Å in average height for all compositions. LEIS experiments indicate that the surfaces of the bimetallic clusters are 80–100% Au for bulk Au fractions greater than 50%, but Co and Au coexist at the surfaces when there are not enough Au atoms available to completely cover the surfaces of the clusters. After heating to 800 K, pure Co clusters become partially encapsulated by titania, and for bimetallic clusters, the Co is selectively encapsulated at the cluster surface. The desorption of CO from the bimetallic clusters demonstrates that the presence of the CO adsorbate induces diffusion of Co to the cluster surface, but the extent of this diffusion is less than what is observed in the Ni–Au and Pt–Au systems. Density functional theory calculations confirm that for a 50% Co/50% Au bimetallic structure: the surface is predominantly Au in the absence of CO; CO induces diffusion of Co to the cluster surface; and this CO-induced diffusion is less extensive on Co–Au than on the Ni–Au and Pt–Au surfaces.

The present study examines the synthesis of unique Cu nanostructured model catalysts and their catalytic activity toward CO 2 hydrogenation under moderate temperature and pressure reaction conditions. Cu-based nanoparticles (NPs) were synthesized by two chemical deposition methods: (1) 5 nm spherical Cu(OH) 2 NPs deposited on highly oriented pyrolytic graphite (HOPG) by exposing the HOPG substrate to a colloidal solution of copper, and (2) photocatalytic reduction of [Cu(H 2 O) 6 ] 2+ onto a high density of 15 nm TiO 2 NPs grown on HOPG by physical vapor deposition. This photocatalytic reduction results in the deposition of mixed Cu(OH) 2 and Cu 2 O films, while few-nm sized Cu-based NPs are formed on the TiO 2 NPs upon subsequent reduction. The chemistry, structure, and morphology of the resulting samples were characterized using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The thermocatalytic activity for the CO 2 reduction reaction (CO 2 RR) under H 2 was evaluated with synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and temperature-programmed desorption (TPD) experiments. Several intermediates, including CO 2 δ− , HCOO, O−CH 3 , CO 3 2− , CH x , and CO, were observed using AP-XPS. The TiO 2 NPs show activity toward the formation of methanol (CH 3 OH) that occurs mainly through an O−CH 3 intermediate. The TiO 2 NPscore−carbon-shell (TiO 2 @C NPs) shows a clear selectivity toward methane (CH 4 ). The Cu/TiO 2 NPs show, however, an activity toward CO, CH 4 , and CH 3 OH that depends strongly on the percentage of oxygen present on the Cu NPs surface. This study particularly shows the importance played by the TiO 2 NPs for CO 2 adsorption and activation and the Cu NPs for H 2 and CO 2 dissociation. The CO 2 RR mechanisms are discussed on the basis of the intermediate formation and the surface structure and composition.