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.

CO oxidation has been investigated by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on Pt(111), Re films on Pt(111) and a Pt-Re alloy surface. The Pt-Re alloy surface was prepared by annealing Re films on Pt(111) to 1000 K; scanning tunneling microscopy, low energy ion scattering and X-ray photoelectron spectroscopy studies indicate that this treatment resulted in the diffusion of Re into the Pt (111) surface. Under CO oxidation conditions of 500 mTorr O 2 /50 mTorr CO, CO remains on the Pt(111) surface at 450 K, whereas CO desorbs from the Pt-Re alloy surface at lower temperatures. Furthermore, the Pt-Re alloy dissociates oxygen more readily than Pt(111) despite the fact that all of the Re atoms are initially in the subsurface region. Mass spectrometer studies show that the Pt-Re alloy, Re film on Pt andPt (111) all have similar activities for CO oxidation, with the Pt-Re alloy producing ~10% more CO 2 than Pt(111). The Re film is not stable under CO oxidation conditions at temperatures > 450 K due to formation and subsequent sublimation of volatile Re 2 O 7 . However, the Pt-Re alloy surface is more resistant to oxidation and therefore also more stable against Re sublimation.

Generalized-Kohn-Sham (GKS) orbital energies obtained self-consistently from the random phase approximation energy functional with a semicanonical projection (spRPA) were recently shown to rival the accuracy of GW quasiparticle energies for valence ionization potentials. Here, we extend the scope of GKS-spRPA correlated one-particle energies from frontier-orbital ionization to core orbital ionization energies, which are notoriously difficult for GW and other response methods due to strong orbital relaxation effects. For a benchmark consisting of 23 1s core electron binding energies (CEBEs) of second-row elements, chemical shifts estimated from GKS-spRPA one-particle energies yield mean absolute deviations from experiment of 0.2 eV, which are significantly more accurate than the standard GW and comparable to Δ self-consistent field theory without semiempirical adjustment of the energy functional. For small ammonia clusters and cytosine tautomers, GKS-spRPA based chemical shifts capture subtle variations in covalent and noncovalent bonding environments; GKS-spRPA 1s CEBEs for these systems agree with equation-of-motion coupled cluster singles and doubles and ADC(4) results within 0.2–0.3 eV. Two perturbative approximations to GKS-spRPA orbital energies, which reduce the scaling from O(N6) to O(N5) and O(N4), are introduced and tested. We illustrate the application of GKS-spRPA orbital energies to larger systems by using oxygen 1s CEBEs to probe solvation and packing effects in condensed phases of water. GKS-spRPA predicts a lowering of the oxygen 1s CEBE of approximately 1.6–1.7 eV in solid and liquid phases, consistent with liquid-jet X-ray photoelectron spectroscopy and gas phase cluster experiments. The results are rationalized by partitioning GKS-spRPA electron binding energies into static, relaxation, and correlation parts.

The growth and chemical activity of Re, Pt, and Pt−Re bimetallic clusters supported on TiO 2 (110) have been studied. Pure Re clusters interact strongly with the titania support, resulting in the reduction of the titania surface, and the Re clusters also appear to be partially covered by TiO x at Re coverages as high as 13 ML. Bimetallic clusters can be grown from sequential deposition of Pt and Re in either order at high metal coverages (3.7 ML), where the number of initial nucleation sites is large; in contrast, at lower coverages (0.24 ML), pure Re clusters coexist with Pt−Re clusters for Re deposited on Pt due to the higher nucleation density of Re compared with Pt. The surface composition of the high coverage Pt on Re clusters is ∼100% Pt, but the Re on Pt clusters contain both Pt and Re at the surface after diffusion of some fraction of Re atoms in the bulk. The lower surface free energy of Pt compared to Re makes it thermodynamically favorable for Pt to remain at the surface when Pt is deposited on Re, whereas Re atoms deposited on the Pt clusters will diffuse into the clusters. Isotopic labeling experiments that incorporate 18 O into the titania lattice demonstrate that lattice oxygen participates in both CO oxidation on the Pt on Re bimetallic clusters and recombination of carbon and oxygen to form CO on the Re-containing clusters.