Atomic Layer Deposition of Rhenium–Aluminum Oxide Thin Films and ReO_<i>x</i> Incorporation in a Metal–Organic Framework

Methyltrioxorhenium (ReO₃Me) is introduced as the first rhenium atomic layer deposition (ALD) precursor and used to grow rhenium–aluminum oxide thin films in combination with trimethylaluminum (TMA–AlMe₃). The growth rate of the smooth Re–Al oxide films, with general stoichiometry Re_<i>x</i>Al_<i>y</i>O_3<i>x</i>, has been monitored by in situ quartz crystal microbalance (QCM) and ex situ ellipsometry, and found to be 3.2 Å/cycle. X-ray photoelectron spectroscopy (XPS) revealed the mixed valent composition of the film with Re­(III) species being the main component. In addition, ReO₃Me has been successfully used to deposit rhenium oxide in NU-1000, a mesoporous zirconium-based metal–organic framework (MOF). The metalated MOF was found to retain porosity and crystallinity and to be catalytically active for ethene hydrogenation

Postassembly Transformation of a Catalytically Active Composite Material, Pt@ZIF-8, via Solvent-Assisted Linker Exchange

2-Methylimidazolate linkers of Pt@ZIF-8 are exchanged with imidazolate using solvent-assisted linker exchange (SALE) to expand the apertures of the parent material and create Pt@SALEM-2. Characterization of the material before and after SALE was performed. Both materials are active as catalysts for the hydrogenation of 1-octene, whereas the hydrogenation of <i>cis</i>-cyclohexene occurred only with Pt@SALEM-2, consistent with larger apertures for the daughter material. The largest substrate, β-pinene, proved to be unreactive with H₂ when either material was employed as a candidate catalyst, supporting the contention that substrate molecules, for both composites, must traverse the metal–organic framework component in order to reach the catalytic nanoparticles

Bottom-Up Design and Generation of Complex Structures: A New Twist in Reticular Chemistry

The design and subsequent construction of complex structures using simple building blocks represent an interesting challenge in the field of chemistry. In this paper we describe complex nets of the <b>mtn</b> family and their relationship to the most common binary structure in chemistry, MgCu₂. In this bottom-up approach we start by linking simple shapes in space and show the inevitable evolution to highly complex, low density structures

Dual-Function Metal–Organic Framework as a Versatile Catalyst for Detoxifying Chemical Warfare Agent Simulants

The nanocrystals of a porphyrin-based zirconium(IV) metal–organic framework (MOF) are used as a dual-function catalyst for the simultaneous detoxification of two chemical warfare agent simulants at room temperature. Simulants of nerve agent (such as GD, VX) and mustard gas, dimethyl 4-nitrophenyl phosphate and 2-chloroethyl ethyl sulfide, have been hydrolyzed and oxidized, respectively, to nontoxic products <i>via</i> a pair of pathways catalyzed by the same MOF. Phosphotriesterase-like activity of the Zr₆-containing node combined with photoactivity of the porphyrin linker gives rise to a versatile MOF catalyst. In addition, bringing the MOF crystals down to the nanoregime leads to acceleration of the catalysis

Systematic Modulation of Quantum (Electron) Tunneling Behavior by Atomic Layer Deposition on Nanoparticulate SnO₂ and TiO₂ Photoanodes

Ultrathin films of TiO₂, ZrO₂, and Al₂O₃ were conformally created on SnO₂ and TiO₂ photoelectrodes via atomic layer deposition (ALD) to examine their influence upon electron transfer (ET) from the electrodes to a representative molecular receptor, I₃^–. Films thicker than 2 Å engender an exponential decrease in ET time with increasing film thickness, consistent with tunneling theory. Increasing the height of the barrier, as measured by the energy difference between the transferring electron and the bottom of the conduction band of the barrier material, results in steeper exponential drops in tunneling rate or probability. The variations are quantitatively consistent with a simple model of quantum tunneling of electrons through square barriers (i.e., barriers of individually uniform energy height) that are characterized by individually uniform physical thickness. The findings demonstrate that ALD is a remarkably uniform and precise method for modifying electrode surfaces and imply that standard tunneling theory can be used as a quantitative guide to intentionally and predictively modulating rates of ET between molecules and electrodes

Atomically Precise Growth of Catalytically Active Cobalt Sulfide on Flat Surfaces and within a Metal–Organic Framework <i>via</i> Atomic Layer Deposition

Atomic layer deposition (ALD) has been employed as a new synthetic route to thin films of cobalt sulfide on silicon and fluorine-doped tin oxide platforms. The self-limiting nature of the stepwise synthesis is established through growth rate studies at different pulse times and temperatures. Additionally, characterization of the materials by X-ray diffraction and X-ray photoelectron spectroscopy indicates that the crystalline phase of these films has the composition Co₉S₈. The nodes of the metal–organic framework (MOF) <b>NU-1000</b> were then selectively functionalized with cobalt sulfide <i>via</i> ALD in MOFs (AIM). Spectroscopic techniques confirm uniform deposition of cobalt sulfide throughout the crystallites, with no loss in crystallinity or porosity. The resulting material, <b>CoS-AIM</b>, is catalytically active for selective hydrogenation of <i>m</i>-nitrophenol to <i>m</i>-aminophenol, and outperforms the analogous oxide AIM material (<b>CoO-AIM</b>) as well as an amorphous CoS_<i>x</i> reference material. These results reveal AIM to be an effective method of incorporating high surface area and catalytically active cobalt sulfide in metal–organic frameworks

Thermally Enhancing the Surface Areas of Yamamoto-Derived Porous Organic Polymers

Thermal treatment of highly stable porous organic polymers based upon the Yamamoto polymerization of 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene was done. The polymers are shown to be thermally and chemically stable. Upon thermal treatment the polymers are shown to have BET surface areas of ca. 2,000 m²/g and 2,500 m²/g respectively

Toward Inexpensive Photocatalytic Hydrogen Evolution: A Nickel Sulfide Catalyst Supported on a High-Stability Metal–Organic Framework

Few-atom clusters composed of nickel and sulfur have been successfully installed into the Zr­(IV)-based metal–organic framework (MOF) NU-1000 via ALD-like chemistry (ALD = atomic layer deposition). X-ray photoelectron spectroscopy and Raman spectroscopy are used to determine that primarily Ni²⁺ and S^2– sites are deposited within the MOF. In a pH 7 buffered aqueous solution, the porous catalyst is able to produce H₂ gas at a rate of 3.1 mmol g^–1 h^–1 upon UV irradiation, whereas no H₂ is generated by irradiating bare NU-1000. Upon visible light irradiation, little H₂ generation was observed; however, with the addition of an organic dye, rose bengal, NiS-AIM can catalyze the production of H₂ at an enhanced rate of 4.8 mmol g^–1 h^–1. These results indicate that ALD in MOFs (AIM) can engender reactivity within high surface area supports for applications in the solar fuels field

Complete Double Epoxidation of Divinylbenzene Using Mn(porphyrin)-Based Porous Organic Polymers

A series of porphyrin-based porous organic polymers (PPOPs) were synthesized in excellent yields via the Yamamoto–Ullmann couplings of tetrabromo spirobifluorene with several brominated porphyrin monomers. After isolation and demetalation, the metal-free PPOP can be postsynthetically metalated to form a Mn^III–PPOP that is catalytically active toward the selective double-epoxidation of divinylbenzene to divinylbenzene dioxide

Barrier-Layer-Mediated Electron Transfer from Semiconductor Electrodes to Molecules in Solution: Sensitivity of Mechanism to Barrier-Layer Thickness

Electron transfer (ET) phenomena at and near semiconductor/molecule interfaces are of fundamental significance for applications involving liquid-junction photovoltaics, organic photovoltaics, and electrochemical heterogeneous catalysis. To probe mechanisms of electron delivery through barrier layers at such interfaces, we make use of atomic layer deposition to deposit ultrathin films of TiO₂ conformally onto SnO₂ electrodes. In the presence of TiO₂ films (i.e., barrier layers) up to 10 Å thick, electrons are delivered from the electrode to molecules in solution by tunneling through the layers, as evidenced, in part, by an exponential decrease in ET rate with layer thickness. For films thicker than 10 Å, there is little change in ET rate as a function of TiO₂ thickness. To our surprise, thermally annealing a 55 Å layer of TiO₂ on SnO₂ yielded a 10-fold decrease in ET rate compared to that imposed by the as-deposited layer. At applied potentials near the conduction-band edge of SnO₂, and significantly below the band edge of TiO₂, electrochemical impedance spectroscopy with nominally flat, as-deposited TiO₂ indicates the presence of nearly twice the density of electronic states as found with air-annealed samples. These and related observations point to a barrier-layer-thickness-dependent change in the mechanism of electron delivery, from the underlying electrode to solution species, from one based on tunneling to one entailing trap-facilitated hopping. The findings have design implications for the application of interfacial barrier layers to electrochemical and photoelectrochemical problems