Orientational analysis of atomic pair correlations in nanocrystalline indium oxide thin films.

The application of grazing-incidence total X-ray scattering (GITXS) for pair distribution function (PDF) analysis using >50 keV X-rays from synchrotron light sources has created new opportunities for structural characterization of supported thin films with high resolution. Compared with grazing-incidence wide-angle X-ray scattering, which is only useful for highly ordered materials, GITXS/PDFs expand such analysis to largely disordered or nanostructured materials by examining the atomic pair correlations dependent on the direction relative to the surface of the supporting substrate. A characterization of nanocrystalline In2O3-derived thin films is presented here with in-plane-isotropic and out-of-plane-anisotropic orientational ordering of the atomic structure, each synthesized using different techniques. The atomic orientations of such films are known to vary based on the synthetic conditions. Here, an azimuthal orientational analysis of these films using GITXS with a single incident angle is shown to resolve the markedly different orientations of the atomic structures with respect to the planar support and the different degrees of long-range order, and hence, the terminal surface chemistries. It is anticipated that orientational analysis of GITXS/PDF data will offer opportunities to extend structural analyses of thin films by providing a means to qualitatively determine the major atomic orientation within nanocrystalline and, eventually, non-crystalline films.

Resolving the Heat Generated from ZrO2 Atomic Layer Deposition Surface Reactions.

In situ pyroelectric calorimetry and spectroscopic ellipsometry were used to investigate surface reactions in atomic layer deposition (ALD) of zirconium oxide (ZrO2 ). Calibrated and time-resolved in situ ALD calorimetry provides new insights into the thermodynamics and kinetics of saturating surface reactions for tetrakis(dimethylamino)zirconium(IV) (TDMAZr) and water. The net ALD reaction heat ranged from 0.197 mJ cm-2 at 76 °C to 0.155 mJ cm-2 at 158 °C, corresponding to an average of 4.0 eV/Zr at all temperatures. A temperature dependence for reaction kinetics was not resolved over the range investigated. The temperature dependence of net reaction heat and distribution among metalorganic and oxygen source exposure is attributed to factors including growth rate, equilibrium surface hydroxylation, and the extent of the reaction. ZrO2 -forming surface reactions were investigated computationally using DFT methods to better understand the influence of surface hydration on reaction thermodynamics.

Synthetic Access to a Framework-Stabilized and Fully Sulfided Analogue of an Anderson Polyoxometalate that is Catalytically Competent for Reduction Reactions.

Polyoxometalates (POMs) featuring 7, 12, 18, or more redox-accessible transition metal ions are ubiquitous as selective catalysts, especially for oxidation reactions. The corresponding synthetic and catalytic chemistry of stable, discrete, capping-ligand-free polythiometalates (PTMs), which could be especially attractive for reduction reactions, is much less well developed. Among the challenges are the propensity of PTMs to agglomerate and the tendency for agglomeration to block reactant access of catalyst active sites. Nevertheless, the pervasive presence of transition metal sulfur clusters metalloenzymes or cofactors that catalyze reduction reactions and the justifiable proliferation of studies of two-dimensional (2D) metal-chalcogenides as reduction catalysts point to the promise of well-defined and controllable PTMs as reduction catalysts. Here, we report the fabrication of agglomeration-immune, reactant-accessible, capping-ligand-free CoIIMo6IVS24n- clusters as periodic arrays in a water-stable, hierarchically porous Zr-metal-organic framework (MOF; NU1K) by first installing a disk-like Anderson polyoxometalate, CoIIIMo6VIO24m-, in size-matched micropores where the siting is established via difference electron density (DED) X-ray diffraction (XRD) experiments. Flowing H2S, while heating, reduces molybdenum(VI) ions to Mo(IV) and quantitatively replaces oxygen anions with sulfur anions (S2-, HS-, S22-). DED maps show that MOF-templated POM-to-PTM conversion leaves clusters individually isolated in open-channel-connected micropores. The structure of the immobilized cluster as determined, in part, by X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS) analysis, and pair distribution function (PDF) analysis of total X-ray scattering agrees well with the theoretically simulated structure. PTM@MOF displays both electrocatalytic and photocatalytic competency for hydrogen evolution. Nevertheless, the initially installed PTM appears to be a precatalyst, gaining competency only after the loss of ∼3 to 6 sulfurs and exposure to hydride-forming metal ions.

Resolving the Heat of Trimethylaluminum and Water Atomic Layer Deposition Half-Reactions.

Atomic layer deposition (ALD) is a surface synthesis technique that is characterized by self-limiting reactions between gas-phase precursors and a solid substrate. Although ALD processes have been demonstrated that span the periodic table, a greater understanding of the surface chemistry that affords ALD is necessary to enable greater precision, including area- and site-selective growth. We offer new insight into the thermodynamics and kinetics of the trimethylaluminum (TMA) and H2O ALD half-reactions with calibrated and time-resolved in situ pyroelectric calorimetry. The half-reactions produce 3.46 and 2.76 eV/Al heat, respectively, which is greater than the heat predicted by computational models based on crystalline Al2O3 substrates and closely aligned with the heat predicted by standard heats of formation. The pyroelectric thin-film calorimeter offers submillisecond temporal resolution that uniquely and clearly resolves precursor delivery and reaction kinetics. Both half-reactions are observed to exhibit multiple kinetic rates, with average TMA half-reaction rates at least 2 orders of magnitude faster than the H2O half-reaction kinetics. Comparing the experimental heat with published computational literature and additional first-principles modeling highlights the need to refine our models and mechanistic understanding of even the most ubiquitous ALD reactions.

Selective Hydration of Rutile TiO2 as a Strategy for Site-Selective Atomic Layer Deposition.

The feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO2 facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H2O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO2 single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

Structure-Transport Properties Governing the Interplay in Humidity-Dependent Mixed Ionic and Electronic Conduction of Conjugated Polyelectrolytes.

Polymeric mixed ionic-electronic conductors (MIECs) are of broad interest in the field of energy storage and conversion, optoelectronics, and bioelectronics. A class of polymeric MIECs are conjugated polyelectrolytes (CPEs), which possess a π-conjugated backbone imparting electronic transport characteristics along with side chains composed of a pendant ionic group to allow for ionic transport. Here, our study focuses on the humidity-dependent structure-transport properties of poly[3-(potassium-n-alkanoate) thiophene-2,5-diyl] (P3KnT) CPEs with varied side-chain lengths of n = 4-7. UV-vis spectroscopy along with electronic paramagnetic resonance (EPR) spectroscopy reveals that the infiltration of water leads to a hydrated, self-doped state that allows for electronic transport. The resulting humidity-dependent ionic conductivity (σi) of the thin films shows a monotonic increase with relative humidity (RH) while electronic conductivity (σe) follows a non-monotonic profile. The values of σe continue to rise with increasing RH reaching a local maximum after which σe begins to decrease. P3KnTs with higher n values demonstrate greater resiliency to increasing RH before suffering a decrease in σe. This drop in σe is attributed to two factors. First, disruption of the locally ordered π-stacked domains observed through in situ humidity-dependent grazing incidence wide-angle X-ray scattering (GIWAXS) experiments can account for some of the decrease in σe. A second and more dominant factor is attributed to the swelling of the amorphous domains where electronic transport pathways connecting ordered domains are impeded. P3K7T is most resilient to swelling (based on ellipsometry and water uptake measurements) where sufficient hydration allows for high σi (1.0 × 10-1 S/cm at 95% RH) while not substantially disrupting σe (1.7 × 10-2 S/cm at 85% RH and 8.0 × 10-3 S/cm at 95% RH). Overall, our study highlights the complexity of balancing electronic and ionic transport in hydrated CPEs.

Advanced Materials for Energy-Water Systems: The Central Role of Water/Solid Interfaces in Adsorption, Reactivity, and Transport.

The structure, chemistry, and charge of interfaces between materials and aqueous fluids play a central role in determining properties and performance of numerous water systems. Sensors, membranes, sorbents, and heterogeneous catalysts almost uniformly rely on specific interactions between their surfaces and components dissolved or suspended in the water-and often the water molecules themselves-to detect and mitigate contaminants. Deleterious processes in these systems such as fouling, scaling (inorganic deposits), and corrosion are also governed by interfacial phenomena. Despite the importance of these interfaces, much remains to be learned about their multiscale interactions. Developing a deeper understanding of the molecular- and mesoscale phenomena at water/solid interfaces will be essential to driving innovation to address grand challenges in supplying sufficient fit-for-purpose water in the future. In this Review, we examine the current state of knowledge surrounding adsorption, reactivity, and transport in several key classes of water/solid interfaces, drawing on a synergistic combination of theory, simulation, and experiments, and provide an outlook for prioritizing strategic research directions.

Molecularly Functionalized Electrodes for Efficient Electrochemical Water Remediation.

The development and investigation of materials that leverage unique interfacial effects on electronic structure and redox chemistry are likely to play an outstanding role in advanced technologies for wastewater treatment. Here, the use of surface functionalization of metal oxides with a RuII poly(pyridyl) complex was reported as a way to create hybrid assemblies with optimized electrochemical performance for water remediation, superior to those that could be achieved with the molecular catalyst or metal-oxide electrodes used individually. Mechanistic analysis demonstrated that the molecularly functionalized electrodes could suppress the formation of hydroxyl radicals (i. e., the dominant remediation pathway for bare metal-oxide electrodes), allowing the water remediation to proceed through the highly oxidizing Ru3+ ions in the surface-bound complexes. Furthermore, the underlying metal-oxide substrates played a crucial role in altering the electronic structure and electrochemical properties of the surface-bound catalyst, such that the competing side reaction (i. e., water splitting) was largely inhibited.

Thermal Atomic Layer Deposition of Gold: Mechanistic Insights, Nucleation, and Epitaxy.

An in situ microbalance and infrared spectroscopic study of alternating exposures to Me2Au(S2CNEt2) and ozone illuminates the organometallic chemistry that allows for the thermal atomic layer deposition (ALD) of gold. In situ quartz crystal microbalance (QCM) studies resolve the nucleation delay and island growth of Au on a freshly prepared aluminum oxide surface with single cycle resolution, revealing inhibition for 40 cycles prior to slow nucleation and film coalescence that extends over 300 cycles. In situ infrared spectroscopy informed by first-principles computation provides insight into the surface chemistry of the self-limiting half-reactions, which are consistent with an oxidized Au surface mechanism. X-ray diffraction of ALD-grown gold on silicon, silica, sapphire, and mica reveals consistent out-of-plane oriented crystalline film growth as well as epitaxially directed in-plane orientation on closely lattice-matched mica at a relatively low growth temperature of 180 °C. A more complete understanding of ALD gold nucleation, surface chemistry, and epitaxy will inform the next generation of low-temperature, nanoscale, textured depositions that are applicable to high surface area supports.

Isomerization and Selective Hydrogenation of Propyne: Screening of Metal-Organic Frameworks Modified by Atomic Layer Deposition.

Various metal oxide clusters upward of 8 atoms (Cu, Cd, Co, Fe, Ga, Mn, Mo, Ni, Sn, W, Zn, In, and Al) were incorporated into the pores of the metal-organic framework (MOF) NU-1000 via atomic layer deposition (ALD) and tested via high-throughput screening for catalytic isomerization and selective hydrogenation of propyne. Cu and Co were found to be the most active for propyne hydrogenation to propylene, and synergistic bimetallic combinations of Co and Zn, along with standalone Zn and Cd, were established as the most active for conversion to the isomerized product, propadiene. The combination of Co and Zn in NU-1000 diminished the propensity for full hydrogenation to propane as well as coking compared to its individual components. This study highlights the potential for high-throughput screening to survey monometallic and bimetallic cluster combinations that best affect the efficient transformation of small molecules, while discerning mechanistic differences in isomerization and hydrogenation by different metals.

Resolving the Atomic Structure of Sequential Infiltration Synthesis Derived Inorganic Clusters.

Sequential infiltration synthesis (SIS) is a route to the precision deposition of inorganic solids in analogy to atomic layer deposition but occurs within (vs upon) a soft material template. SIS has enabled exquisite nanoscale morphological complexity in various oxides through selective nucleation in block copolymers templates. However, the earliest stages of SIS growth remain unresolved, including the atomic structure of nuclei and the evolution of local coordination environments, before and after polymer template removal. We employed In K-edge extended X-ray absorption fine structure and atomic pair distribution function analysis of high-energy X-ray scattering to unravel (1) the structural evolution of InOxHy clusters inside a poly(methyl methacrylate) (PMMA) host matrix and (2) the formation of porous In2O3 solids (obtained after annealing) as a function of SIS cycle number. Early SIS cycles result in InOxHy cluster growth with high aspect ratio, followed by the formation of a three-dimensional network with additional SIS cycles. That the atomic structures of the InOxHy clusters can be modeled as multinuclear clusters with bonding patterns related to those in In2O3 and In(OH)3 crystal structures suggests that SIS may be an efficient route to 3D arrays of discrete-atom-number clusters. Annealing the mixed inorganic/polymer films in air removes the PMMA template and consolidates the as-grown clusters into cubic In2O3 nanocrystals with structural details that also depend on SIS cycle number.

Characterizing electronic and atomic structures for amorphous and molecular metal oxide catalysts at functional interfaces by combining soft X-ray spectroscopy and high-energy X-ray scattering.

Amorphous thin film materials and heterogenized molecular catalysts supported on electrode and other functional interfaces are widely investigated as promising catalyst formats for applications in solar and electrochemical fuels catalysis. However the amorphous character of these catalysts and the complexity of the interfacial architectures that merge charge transport properties of electrode and semiconductor supports with discrete sites for multi-step catalysis poses challenges for probing mechanisms that activate and tune sites for catalysis. This minireview discusses advances in soft X-ray spectroscopy and high-energy X-ray scattering that provide opportunities to resolve interfacial electronic and atomic structures, respectively, that are linked to catalysis. This review discusses how these techniques can be partnered with advances in nanostructured interface synthesis for combined soft X-ray spectroscopy and high-energy X-ray scattering analyses of thin film and heterogenized molecular catalysts. These combined approaches enable opportunities for the characterization of both electronic and atomic structures underlying fundamental catalytic function, and that can be applied under conditions relevant to device applications.

The Synthesis Science of Targeted Vapor-Phase Metal-Organic Framework Postmodification.

The postmodification of metal organic frameworks (MOFs) affords exceedingly high surface area materials with precisely installed chemical features, which provide new opportunities for detailed structure-function correlation in the field of catalysis. Here, we significantly expand upon the number of vapor-phase postmodification processes reported to date through screening a library of atomic layer deposition (ALD) precursors, which span metals across the periodic table and which include ligands from four distinct precursor classes. With a large library of precursors and synthesis conditions, we discern trends in the compatibility of precursor classes for well-behaved ALD in MOFs (AIM) and identify challenges and solutions to more precise postsynthetic modification.

The chemical physics of sequential infiltration synthesis-A thermodynamic and kinetic perspective.

Sequential infiltration synthesis (SIS) is an emerging materials growth method by which inorganic metal oxides are nucleated and grown within the free volume of polymers in association with chemical functional groups in the polymer. SIS enables the growth of novel polymer-inorganic hybrid materials, porous inorganic materials, and spatially templated nanoscale devices of relevance to a host of technological applications. Although SIS borrows from the precursors and equipment of atomic layer deposition (ALD), the chemistry and physics of SIS differ in important ways. These differences arise from the permeable three-dimensional distribution of functional groups in polymers in SIS, which contrast to the typically impermeable two-dimensional distribution of active sites on solid surfaces in ALD. In SIS, metal-organic vapor-phase precursors dissolve and diffuse into polymers and interact with these functional groups through reversible complex formation and/or irreversible chemical reactions. In this perspective, we describe the thermodynamics and kinetics of SIS and attempt to disentangle the tightly coupled physical and chemical processes that underlie this method. We discuss the various experimental, computational, and theoretical efforts that provide insight into SIS mechanisms and identify approaches that may fill out current gaps in knowledge and expand the utilization of SIS.

High-Temperature Selective Emitter Design and Materials: Titanium Aluminum Nitride Alloys for Thermophotovoltaics.

The efficiency of a thermophotovoltaic (TPV) system depends critically upon the spectral selectivity and stability of an emitter, which may operate most effectively at temperatures in excess of 1000 °C. We computationally design and experimentally demonstrate a novel selective emitter design based on multilayer nanostructures, robust to off-normal emission angles. A computational search of the material and temperature compatibility space of simple emitter designs motivates new material classes and identifies several promising multilayer nanostructure designs for both TPV absorber and emitter applications. One such structure, comprising a thin (<100 nm) tunable TixAl1-xN (TiAlN) absorber and refractory oxide Bragg reflector is grown on W metal foil. In agreement with simulations, the emitter achieves record spectral efficiency (43.4%) and power density (3.6 W/cm2) for an emitter with at least 1 h of high temperature (>800 °C) operation.

Microfluidic electrochemical cell for in situ structural characterization of amorphous thin-film catalysts using high-energy X-ray scattering.

Porous, high-surface-area electrode architectures are described that allow structural characterization of interfacial amorphous thin films with high spatial resolution under device-relevant functional electrochemical conditions using high-energy X-ray (>50 keV) scattering and pair distribution function (PDF) analysis. Porous electrodes were fabricated from glass-capillary array membranes coated with conformal transparent conductive oxide layers, consisting of either a 40 nm-50 nm crystalline indium tin oxide or a 100 nm-150 nm-thick amorphous indium zinc oxide deposited by atomic layer deposition. These porous electrodes solve the problem of insufficient interaction volumes for catalyst thin films in two-dimensional working electrode designs and provide sufficiently low scattering backgrounds to enable high-resolution signal collection from interfacial thin-film catalysts. For example, PDF measurements were readily obtained with 0.2 Šspatial resolution for amorphous cobalt oxide films with thicknesses down to 60 nm when deposited on a porous electrode with 40 µm-diameter pores. This level of resolution resolves the cobaltate domain size and structure, the presence of defect sites assigned to the domain edges, and the changes in fine structure upon redox state change that are relevant to quantitative structure-function modeling. The results suggest the opportunity to leverage the porous, electrode architectures for PDF analysis of nanometre-scale surface-supported molecular catalysts. In addition, a compact 3D-printed electrochemical cell in a three-electrode configuration is described which is designed to allow for simultaneous X-ray transmission and electrolyte flow through the porous working electrode.

Perturbation of Hydrogen-Bonding Networks over Supported Lipid Bilayers by Poly(allylamine hydrochloride).

Water is vital to many biochemical processes and is necessary for driving fundamental interactions of cell membranes with their external environments, yet it is difficult to probe the membrane/water interface directly and without the use of external labels. Here, we employ vibrational sum frequency generation spectroscopy to understand the role of interfacial water molecules above bilayers formed from zwitterionic (phosphatidylcholine) and anionic (phosphatidylglycerol, PG, and phosphatidylserine, PS) lipids as they are exposed to the common polycation poly(allylamine hydrochloride) (PAH) in 100 mM NaCl. We show that as the concentration of PAH is increased, the interfacial water molecules are irreversibly displaced and find that it requires 10 times more PAH to displace interfacial water molecules from membranes formed from purely zwitterionic lipids when compared to membranes that contain the anionic PG and PS lipids. This outcome is likely due to the difference in (1) the energy with which water molecules are bound to the lipid headgroups, (2) the number of water molecules bound to the headgroups, which is related to the headgroup area, and (3) the electrostatic interactions between the PAH molecules and the negatively charged lipids that are favored when compared to the zwitterionic lipid headgroups. The findings presented here contribute to establishing causal relationships in nanotoxicology and to understanding, controlling, and predicting the initial steps that lead to the lysis of cells exposed to membrane-disrupting polycations or to transfection.

Plasma-Enhanced Atomic Layer Deposition of TiAlN: Compositional and Optoelectronic Tunability.

Titanium nitride (TiN) is a unique refractory plasmonic material, the nanocomposites and alloys of which provide further opportunities to tailor its optical and photonic properties. We prepare TiAlN films of continuously variable compositions through the systematic variation of TiN versus AlN cycle ratio in plasma-enhanced atomic layer deposition (PEALD) and investigate the resulting thin-film composition, crystallinity, and optical properties. The resulting properties of TiAlN films are not simple linear combinations of the TiN and AlN films, which exhibit distinct metallic and dielectric properties, but instead are dramatically influenced by the local chemical environment of neighboring constituents. In situ spectroscopic ellipsometry further enables measurement of the varying optical properties of TiAlN films, which evolve over 10 s of nm of film thickness. The tunable optoelectronic properties of TiAlN films enable durable coatings of variable electrical resistance as well as high-temperature diffusion barriers and optical coatings with application to selective solar absorbers and emitters.

Charge Transfer Dynamics of Phase-Segregated Halide Perovskites: CH3NH3PbCl3 and CH3NH3PbI3 or (C4H9NH3)2(CH3NH3) n-1Pb nI3 n+1 Mixtures.

Lead halide perovskites present a versatile class of solution-processable semiconductors with highly tunable bandgaps that span ultraviolet, visible, and near-infrared portions of the spectrum. We explore phase-separated chloride and iodide lead perovskite mixtures as candidate materials for intermediate band applications in future photovoltaics. X-ray diffraction and scanning electron microscopy reveal that deposition of precursor solutions across the MAPbCl3/MAPbI3 composition space affords quasi-epitaxial cocrystallized films, in which the two perovskites do not alloy but instead remain phase-segregated. First-principle calculations further support the formation of an epitaxial interface and predict energy offsets in the valence band and conduction band edges that could result in intermediate energy absorption. The charge dynamics of variable mixtures of the relatively narrow bandgap (1.57 eV) MAPbI3 perovskite and wide bandgap (3.02 eV) MAPbCl3 are probed to map charge and energy flow direction and kinetics. Time-resolved photoluminescence and transient absorption measurements reveal charge transfer of photoexcited carriers in MAPbCl3 to MAPbI3 in tens of picoseconds. The rate of quenching can be further tuned by replacing MAPbI3 with two-dimensional Ruddlesden-Popper (BA)2(MA) n-1Pb nI3 n+1 ( n = 3, 2, and 1) perovskites, which also remain phase-separated.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts.

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.