Mark D. Allendorf

A model of elementary chemistry and fluid mechanics in the combustion of hydrogen on platinum surfaces

Abstract Using computational methods, we consider the catalyzed combustion of lean hydrogenoxygen mixtures in a stagnation flow over a platinum surface and in a flat-plate boundary layer. The analysis includes elementary chemistry in the gas phase as well as on the surface. The stagnation flow is modeled using a similarity transformation that leads to a one-dimensional boundary-value problem, whereas the flat-plate boundary layer is modeled by the use of the boundary-layer assumption. Predictions of each model are compared with experimental measurements of (a) catalytic ignition and combustion of hydrogenoxygen mixtures at low pressure (100 millitorr) and (b) OH concentration profiles in catalytically supported combustion at atmospheric pressure. The article proposes reaction mechanisms and interprets the catalytic behavior in terms of the chemistry models.

Novel metal–organic framework linkers for light harvesting applications

Metal–organic frameworks (MOFs) are composed of organic linkers and coordinating metals that self-assemble to form a crystalline material with tunable nanoporosity. Their synthetic modularity and inherent long-range order create opportunities for use as new functional electronic materials. Using quantum mechanical computational methodologies we propose novel conjugated organic linkers that are capable of forming the same one-dimensional infinite metal-oxide secondary building units (SBUs) as the well-known IRMOF-74. This structural arrangement allows for the formation of a continuous π–π stacking network that should enable charge transport in fashion analogous to organic semiconductors. The structural and electronic properties (fundamental and optical gaps) of the isolated proposed linkers were modeled using a non-empirically tuned long-range corrected functional that leads to significantly improved results compared with experimental benchmarks. In addition, periodic hybrid density functional calculations were employed to model the extended MOF systems. Our results demonstrate how the electronic properties of MOFs can be readily modified to have favorable orbital alignments with known electron acceptors that should facilitate charge transfer. The predicted properties are in good agreement with experiment (i.e. UV-Vis absorption spectra), demonstrating the power of this computational approach for MOF design.

Connecting structure with function in metal–organic frameworks to design novel photo- and radioluminescent materials

The exemplary structural versatility and permanent porosity of Metal–Organic Frameworks (MOFs) and their consequent potential for breakthroughs in diverse applications have caused these hybrid materials to become the focus of vigorous investigation. These properties also hold significance for applications beyond those traditionally envisioned for microporous materials, such as radiation detection and other luminescence-based sensing applications. In this contribution we demonstrate that luminescence induced by ionizing radiation (also known as scintillation) is common in appropriately designed MOFs and describe how this property can be harnessed to generate novel materials useful for detecting radiation. Through a diverse selection of MOFs, we explore the structural properties of MOFs that give rise to scintillation and photoluminescence in these materials. These results enable us to define a new structure-based hierarchical system for understanding luminescent properties in MOFs. Finally, we describe some performance metrics for MOF-based scintillation counters, such as luminosity and resistance to radiation damage, and discuss how these materials relate to the current state of the art in scintillation counters.

Quantum Monte Carlo simulation of nanoscale MgH2 cluster thermodynamics.

We calculated the desorption energy of MgH(2) clusters using the highly accurate quantum Monte Carlo (QMC) approach, which can provide desorption energies with chemical accuracy (within approximately 1 kcal/mol) and therefore provides a valuable benchmark for such hydrogen-storage simulations. Compared with these QMC results, the most widely used density functional theory (DFT) computations (including a wide range of exchange-correlation functionals) cannot reach a consistent and suitable level of accuracy across the thermodynamically tunable range for MgH(2) clusters. Furthermore, our QMC calculations show that the DFT error depends substantially on cluster size. These results suggest that in simulating metal-hydride systems it is very important to apply accurate methods that go beyond traditional mean-field approaches as a benchmark of their performance for a given material, and QMC is an appealing method to provide such a benchmark due to its high level of accuracy and favorable scaling (N(3)) with the number of electrons.

Ordered metal nanostructure self-assembly using metal–organic frameworks as templates

We demonstrate that nanoporous metal–organic frameworks (MOFs) loaded with silver can serve as templates for ordered nanostructures comprising either silver nanoparticles or nanowires. Exposure to an electron beam breaks down the template, leading to rapid silver coalescence. The geometric and chemical structure of the MOF, as well as the extent of metal loading, determine whether nanoparticles or nanowires are formed and define their size and orientation. Nanowires with diameters as small as 4 nm and aspect ratios >125 can be formed, overcoming the limitations of existing templating methods. This method is relatively simple, compatible with many materials, and proceeds by a distinct template-directed growth mechanism. Since MOFs offer an unprecedented level of synthetic flexibility, combined with highly uniform porosity as a result of their crystalline structure, this approach opens a promising new route for synthesis of self-assembled, ordered nanostructures.

Energy and charge transfer by donor–acceptor pairs confined in a metal–organic framework: a spectroscopic and computational investigation

Molecular organization of donor–acceptor pairs within a metal–organic framework (MOF) offers a new approach to improving energy and charge transfer at donor–acceptor interfaces. Here, the photo-physical effects of infiltrating MOF-177 (ZnO4(BTB)2; BTB = 1,3,5-benzenetribenzoate) with α,ω-dihexylsexithiophene (DH6T) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), representing well-established polymeric and molecular materials used in organic photovoltaics, were probed using UV-visible absorption and luminescence spectroscopies combined with first-principles electronic structure calculations. The energetics of guest molecule infiltration were determined by constructing potential energy curves from self-consistent charge density-functional tight-binding (SCC-DFTB) calculations. These reveal that infiltration is energetically favored and that DH6T and PCBM are strongly bound to MOF-177 by 55 kcal mol−1 and 57 kcal mol−1, respectively. Solution-phase infiltration with PCBM achieved a 22 wt% loading, comparable to those in bulk heterojunction solar cells, but without evidence of phase segregation. DH6T loadings were very light (maximum of ∼1 molecule per 11 unit cells), but this was sufficient to produce significant quenching of the MOF-177 photoluminescence (PL). The coincident appearance of DH6T PL demonstrates that efficient Forster resonance energy transfer (FRET) from the MOF-177 linkers to DH6T occurs. These results show that the MOF is a multifunctional host that not only confines and stabilizes guest molecules, but also plays an active role, serving as a photon antenna that harvests light not efficiently absorbed by a donor molecule (DH6T in this case) and transferring it to guest acceptor molecules. Finally, time-dependent density functional theory (TDDFT) predicts the existence of linker-to-PCBM charge transfer states, suggesting that photoconductivity might be achievable in an appropriately designed guest@MOF system.

The adsorption of H-atoms on polycrystalline β-silicon carbide

The adsorption of atomic hydrogen on a polycrystalline β-silicon carbide (SiC) surface has been studied using temperature-programmed desorption (TPD). Atomic hydrogen adsorbs on this surface, whereas molecular hydrogen does not. Upon heating, the hydrogen recombines and desorbs as H2, producing two peaks in the TPD spectrum at 975 and 1130 K. The unusually broad width of the desorption peaks is attributed to a distribution of surface binding energies for hydrogen atoms. The desorption spectra are fit using a model that assumes two adsorption sites and a Gaussian distribution of binding energies. The average activation energies derived for the two peaks are 63 and 72 kcal/mol.

Spectral- and Pulse-Shape Discrimination in Triplet-Harvesting Plastic Scintillators

In this work, we describe a method to control the relative proportion of prompt and delayed luminosity of organic-based scintillators via direct and exponential emission from an extrinsic triplet state. This approach involves the incorporation of triplet-harvesting heavy metal complexes in plastic scintillator matrices to convert intrinsically non-luminescent host states to highly emissive guest states. Measurements on these plastic scintillators indicate improved light yields over the undoped polymers and the ability to perform neutron/gamma particle-discrimination. A similar extent of molecular-level control is not possible in traditional organic materials due to complex decay kinetics and the absence of spectral information for the delayed triplet-derived emission. The materials described here address these limitations through efficient host-guest triplet harvesting, which enables particle discrimination according to conventional pulse-shape discrimination (PSD) and a previously unreported spectral-shape discrimination (SSD) scheme.

Probing the unusual anion mobility of LiBH4 confined in highly ordered nanoporous carbon frameworks via solid state NMR and quasielastic neutron scattering

Particle size and particle–framework interactions have profound effects on the kinetics, reaction pathways, and even thermodynamics of complex hydrides incorporated in frameworks possessing nanoscale features. Tuning these properties may hold the key to the utilization of complex hydrides in practical applications for hydrogen storage. Using carefully synthesized, highly-ordered, nanoporous carbons (NPCs), we have previously shown quantitative differences in the kinetics and reaction pathways of LiBH4 when incorporated into the frameworks. In this paper, we probe the anion mobility of LiBH4 confined in NPC frameworks by a combination of solid state NMR and quasielastic neutron scattering (QENS) and present some new insights into the nanoconfinement effect. NMR and QENS spectra of LiBH4 confined in a 4 nm pore NPC suggest that the BH4− anions nearer the LiBH4–carbon pore interface exhibit much more rapid translational and reorientational motions compared to those in the LiBH4 interior. Moreover, an overly broadened BH4− torsional vibration band reveals a disorder-induced array of BH4− rotational potentials. XRD results are consistent with a lack of LiBH4 long-range order in the pores. Consistent with differential scanning calorimetry measurements, neither NMR nor QENS detects a clear solid–solid phase transition as observed in the bulk, indicating that borohydride–framework interactions and/or nanosize effects have large roles in confined LiBH4.

Understanding gas-phase reactions in the thermal CVD of hard coatings using computational methods

This article provides a short summary of the theoretical approaches to understanding gas-phase reactions. In particular, the quantum-chemical bond-additivity correction (BAC) method for predicting molecular thermochemistry of gas-phase molecules is described. A brief discussion of the use of RRKM methods for predicting the rates of unimolecular reactions and of ab initio methods for predicting the rates of bimolecular reactions is also presented. Finally, the question of when gas-phase reactions are likely to be important in CVD is discussed. Criteria are proposed for performing a first-order evaluation of the extent of precursor pyrolysis in the CVD of hard coatings. Examples from the CVD of titanium-containing species, boron nitride, and silicon carbide will be used for illustration.