Structure-property relationships in metal-organic frameworks for hydrogen storage

Experimental hydrogen isotherms on several metal-organic frameworks (IRMOF-1, IRMOF-3, IRMOF-9, ZIF-7, ZIF-8, ZIF-9, ZIF-11, ZIF-12, ZIF-CoNIm, MIL-101 (Cr), NH2-MIL-101 (Cr), NH2-MIL-101 (Al), UiO-66, UiO-67 and HKUST-1) synthesized in-house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analyzed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF-7 and ZIF-9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of "breathing", a phase transition in which the pore space increases due to adsorption. This work presents novel correlations using the modelled total hydrogen capacities of several MOFs. These capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume capacities of on-board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand.</p

Tin(II) Ureide Complexes:Synthesis, Structural Chemistry and Evaluation as SnO precursors

In an attempt to tailor precursors for application in the deposition of phase pure SnO, we have evaluated a series of tin (1-6) ureide complexes. The complexes were successfully synthesized by employing N,N′-Trialkyl-functionalized ureide ligands, in which features such as stability, volatility, and decomposition could be modified with variation of the substituents on the ureide ligand in an attempt to find the complex with the ideal electronic, steric, or coordinative properties, which determine the fate of the final products. The tin(II) ureide complexes 1-6 were synthesized by direct reaction [Sn{NMe2}2] with aryl and alkyl isocyanates in a 1:2 molar ratio. All the complexes were characterized by NMR spectroscopy as well as elemental analysis and, where applicable, thermogravimetric (TG) analysis. The single-crystal X-ray diffraction studies of 2, 3, 4, and 6 revealed that the complexes crystallize in the monoclinic space group P2(1)/n (2 and 4) or in the triclinic space group P-1 (3 and 6) as monomers. Reaction with phenyl isocyanate results in the formation of the bimetallic species 5, which crystallizes in the triclinic space group P-1, a consequence of incomplete insertion into the Sn-NMe2 bonds, versus mesityl isocyanate, which produces a monomeric double insertion product, 6, under the same conditions, indicating a difference in reactivity between phenyl isocyanate and mesityl isocyanate with respect to insertion into Sn-NMe2 bonds. The metal centers in these complexes are all four-coordinate, displaying either distorted trigonal bipyramidal or trigonal bipyramidal geometries. The steric influence of the imido-ligand substituent has a clear effect on the coordination mode of the ureide ligands, with complexes 2 and 6, which contain the cyclohexyl and mesityl ligands, displaying κ2-O,N coordination modes, whereas κ2-N,N′ coordination modes are observed for the sterically bulkier tert-butyl and adamantyl derivatives, 3 and 4. The thermogravimetric analysis of the complexes 3 and 4 exhibited excellent physicochemical properties with clean single-step curves and low residual masses in their TG analyses suggesting their potential utility of these systems as MOCVD and ALD precursors.</p

Structure-property relationships in metal-organic frameworks for hydrogen storage

Experimental hydrogen isotherms on several metal-organic frameworks (IRMOF-1, IRMOF-3, IRMOF-9, ZIF-7, ZIF-8, ZIF-9, ZIF-11, ZIF-12, ZIF-CoNIm, MIL-101 (Cr), NH2-MIL-101 (Cr), NH2-MIL-101 (Al), UiO-66, UiO-67 and HKUST-1) synthesized in-house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analysed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF-7 and ZIF-9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of “breathing”, a phase transition in which the pore space increases due to adsorption. This work presents novel and more useful correlations using the modelled total hydrogen capacities of several MOFs. These total hydrogen capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume of on-board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand

Comparative Spectroscopic Study Revealing Why the CO2 Electroreduction Selectivity Switches from CO to HCOO– at Cu–Sn- and Cu–In-Based Catalysts

To address the challenge of selectivity toward single products in Cu-catalyzed electrochemical CO2 reduction, one strategy is to incorporate a second metal with the goal of tuning catalytic activity via synergy effects. In particular, catalysts based on Cu modified with post-transition metals (Sn or In) are known to reduce CO2 selectively to either CO or HCOO– depending on their composition. However, it remains unclear exactly which factors induce this switch in reaction pathways and whether these two related bimetal combinations follow similar general structure–activity trends. To investigate these questions systematically, Cu–In and Cu–Sn bimetallic catalysts were synthesized across a range of composition ratios and studied in detail. Compositional and morphological control was achieved via a simple electrochemical synthesis approach. A combination of operando and quasi-in situ spectroscopic techniques, including X-ray photoelectron, X-ray absorption, and Raman spectroscopy, was used to observe the dynamic behaviors of the catalysts’ surface structure, composition, speciation, and local environment during CO2 electrolysis. The two systems exhibited similar selectivity dependency on their surface composition. Cu-rich catalysts produce mainly CO, while Cu-poor catalysts were found to mainly produce HCOO–. Despite these similarities, the speciation of Sn and In at the surface differed from each other and was found to be strongly dependent on the applied potential and the catalyst composition. For Cu-rich compositions optimized for CO production (Cu85In15 and Cu85Sn15), indium was present predominantly in the reduced metallic form (In0), whereas tin mainly existed as an oxidized species (Sn2/4+). Meanwhile, for the HCOO–-selective compositions (Cu25In75 and Cu40Sn60), the indium exclusively exhibited In0 regardless of the applied potential, while the tin was reduced to metallic (Sn0) only at the most negative applied potential, which corresponds to the best HCOO– selectivity. Furthermore, while Cu40Sn60 enhances HCOO– selectivity by inhibiting H2 evolution, Cu25In75 improves the HCOO– selectivity at the expense of CO production. Due to these differences, we contend that identical mechanisms cannot be used to explain the behavior of these two bimetallic systems (Cu–In and Cu–Sn). Operando surface-enhanced Raman spectroscopy measurements provide direct evidence of the local alkalization and its impact on the dynamic transformation of oxidized Cu surface species (Cu2O/CuO) into a mixture of Cu(OH)2 and basic Cu carbonates [Cux(OH)y(CO3)y] rather than metallic Cu under CO2 electrolysis. This study provides unique insights into the origin of the switch in selectivity between CO and HCOO– pathways at Cu bimetallic catalysts and the nature of surface-active sites and key intermediates for both pathways

Polymorph-Selective Deposition of High Purity SnS Thin Films from a Single Source Precursor

Metal chalcogenide thin films have a wide variety of applications and potential uses. Tin­(II) sulfide is one such material which presents a significant challenge with the need for high quality SnS, free of oxide materials (e.g., SnO₂) and higher tin sulfides (e.g., Sn₂S₃ and SnS₂). This problem is compounded further when the target material exhibits a number of polymorphic forms with different optoelectronic properties. Unlike conventional chemical vapor deposition (CVD) and atomic layer deposition (ALD), which rely heavily on having precursors that are volatile, stable, and reactive, the use of aerosol assisted CVD (AA-CVD) negates the need for volatile precursors. We report here, for the first time, the novel and structurally characterized single source precursor (<b>1</b>), (dimethylamido)­(<i>N</i>-phenyl-<i>N</i>′,<i>N</i>′-dimethylthiouriate)­tin­(II) dimer, and its application in the deposition, by AA-CVD, of phase-pure films of SnS. A mechanism for the oxidatively controlled formation of SnS from precursor <b>1</b> is also reported. Significantly, thermal control of the deposition process allows for the unprecedented selective and exclusive formation of either orthorhombic-SnS (α-SnS) or zinc blende-SnS (ZB-SnS) polymorphs. Thin films of α-SnS or ZB-SnS have been deposited onto Mo, fluorine doped tin oxide (FTO), Si, and glass substrates at the optimized deposition temperatures of 375 and 300 °C, respectively. The densely packed polycrystalline thin films have been characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis. These data confirmed the phase purity of the SnS formed. Optical analysis of the α-SnS and ZB-SnS films shows distinctly different optical properties with direct band gaps of 1.34 and 1.78 eV, respectively. Furthermore, photoelectrochemical and external quantum efficiency (EQE) measurements were undertaken to assess the optoelectronic properties of the deposited samples. We also report for the first time the ambipolar properties of the ZB-SnS phase

Nature of Nitrogen Incorporation in BiVO4 Photoanodes through Chemical and Physical Methods

In recent years, BiVO4 has been optimized as a photoanode material to produce photocurrent densities close to its theoretical maximum under AM1.5 solar illumination. Its performance is, therefore, limited by its 2.4 eV bandgap. Herein, nitrogen is incorporated into BiVO4 to shift the valence band position to higher energies and thereby decreases the bandgap. Two different approaches are investigated: modification of the precursors for the spray pyrolysis recipe and post-deposition nitrogen ion implantation. Both methods result in a slight red shift of the BiVO4 bandgap and optical absorption onset. Although previous reports on N-modified BiVO4 assumed individual nitrogen atoms to substitute for oxygen, X-ray photoelectron spectroscopy on the samples reveals the presence of molecular nitrogen (i.e., N-2). Density functional theory calculations confirm the thermodynamic stability of the incorporation and reveal that N-2 coordinates to two vanadium atoms in a bridging configuration. Unfortunately, nitrogen incorporation also results in the formation of a localized state of approximate to 0.1 eV below the conduction band minimum of BiVO4, which suppresses the photoactivity at longer wavelengths. These findings provide important new insights on the nature of nitrogen incorporation into BiVO4 and illustrate the need to find alternative lower-bandgap absorber materials for photoelectrochemical energy conversion applications

Determining Structure-Activity Relationships in Oxide Derived CuSn Catalysts During CO2 Electroreduction Using X-Ray Spectroscopy

The development of earth-abundant catalysts for selective electrochemical CO2 conversion is a central challenge. Cu-Sn bimetallic catalysts can yield selective CO2 reduction toward either CO or formate. This study presents oxide-derived Cu-Sn catalysts tunable for either product and seeks to understand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO2 reduction conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods—X-ray absorption spectroscopy for in situ study, and quasi in situ X-ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu0 under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types: the CO-selective catalysts exhibit a surface Sn content of 13 at. % predominantly present as oxidized Sn, while the formate-selective catalysts display an Sn content of ≈70 at. % consisting of both metallic Sn0 and Sn oxide species. Density functional theory simulations suggest that Snδ+ sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn0 sites hinder H adsorption and promote formate production. This study reveals the complex dependence of catalyst structure, composition, and speciation with electrochemical bias in bimetallic Cu catalysts

Dataset for 'Zn doped Fe2TiO5 photoanodes grown by aerosol-assited chemical vapor deposition'

This dataset includes a range of experimental data collected. In particular, raw data from X-Ray diffraction patterns (XRD), UV- Vis spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), linear sweep voltammetries, photcurrent times curves, IPCE and electrochemical impedance spectroscopy (EIS). Specifics of the methology employed for data collection is described in detail in the experimental part of the paper. Raw data has been labeled following the same methodology as in the paper.Physical Characterisation X- ray diffraction (XRD) patterns were collected from 10 to 60° (2θ) using a Bruker D8 diffractometer with Cu Kα (0.154 nm) radiation. Raman spectroscopy was performed on a Renishaw inVia system using a 532 nm diode-pumped solid-state laser (DPSS) manufactured by Cobolt. A 50x long distance objective was used to focus the laser beam onto the sample. UV-Vis measurements were carried out in a Lambda 950 spectrometer (Perkin Elmer) with an integrating sphere (150 mm InGaAS). The samples were mounted in the center. Diffuse-reflectance UV-Vis measurements were performed in an Agilent Cary 100 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed with a monochromatic Al Kα X-ray-source (1486.74 eV, Specs Focus 500 monochromator). C 1s was used for internal charge correction. Ultraviolet photoelectron spectroscopy (UPS) was carried out with a He I source (E = 21.218 eV) in the same chamber. A hemispherical analyzer (Specs Phoibos 100) was used for both XPS and UPS measurements. The base pressure of the system was ∼10−9 mbar. (Photo)electrochemical characterisation: Photoelectrochemical (PEC) performance of photoanodes was measured under simulated solar light using a WACOM Super Solar Simulator (Model WXS-505-5H, AM 1.5, Class AAA) and an EG&G Princeton Applied Research Potentiostat/Galvanostat (Model 273A). PEC cells were prepared using a three electrode configuration with Pt as the counter electrode, a silver chloride reference electrode (Ag/AgCl-reference electrode, XR300, Radiometer Analytical, EAg/AgCl=0.197 VRHE) and the as-prepared photoanodes as the working electrode. Illumination was directed towards the back of the photoanode (Glass-FTO-sample). 1 M NaOH (pH=13.6) was used as electrolyte. Incident photon-to-current efficiency (IPCE) measurements were performed using an Xe lamp (LOT, LSH302), an Acton Research monochromator (Spectra Pro 2155) and an electronic shutter (Uniblitz LS6). The intensity of the monochromated light was measured by a calibrated photodiode (PD300R-UV, Ophir) just after a clean FTO-glass substrate placed at the working electrode position, in the absence of PEC cell quartz window or PEC cell electrolyte. PEC impedance spectroscopy (PEIS) was carried out under simulated sunlight (AM 1.5G, 100 mW cm-2) using a CompactStat. Potentiostat (Ivium technologies). Measurements were performed in a frequency range from 105 to 0.1 Hz, with an AC voltage amplitude of 10 mV at a potential range of 0.6 to 1.2 VRHE with 0.05V steps, in 1M NaOH. EIS measurements in the dark were also measured to obtain Mott-Schottky plots. These measurements were performed at a fixed frequency of 100 and 1000 Hz.Data for the different technqiues are presented in different documents. Data has been labeled as follows: #_Fe2TiO5, #_Fe2TiO5_Zn, #_Fe2O3 and #_Fe2O3_Zn, where # corresponds to the nomenclature of x and y axis of the plot