Atomic Layer Grown Zinc-Tin Oxide as an Alternative Buffer Layer for Cu2ZnSnS4-Based Thin Film Solar Cells: Influence of Absorber Surface Treatment on Buffer Layer Growth.

Zn1-x Sn x O y (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnO x and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na-CO x species from the CZTS surface, may be useful for the ZTO/CZTS device performance.

Ultrathin wide band gap kesterites.

Kesterite Cu2ZnSnS4 (CZTS), used for thin film solar cells, has a band gap energy around 1.5-1.6 eV with possibilities for further increase through alloying. In some applications for wide band gap solar cells, reduced absorber thickness can be beneficial, to allow partial light transmission. Reduced thickness can also be beneficial to reduce bulk recombination, and so called ultrathin solar cells (<700 nm thick) have been studied for several materials systems. Here, we report performance for CZTS devices down to 250 nm thickness and show that performance loss from thickness reduction is relatively small, partly due to short minority carrier diffusion length. Insertion of thin passivation layers (Al2O3, SiO2 or HfO2) at the Mo/CZTS interface gives improved performance of ultrathin devices, from 4.7% to 5.6% efficiency for best performing cells having 250 nm thick CZTS with Mo as compared to Mo/Al2O3 back contact. The approach of NaF post deposition for making isolating passivation layers conductive is tested for the first time for CZTS and is shown to work. For fabrication of CZTS devices on transparent ITO back contact, the insertion of passivation layers can reduce diffusion of indium into CZTS, but device performance is lower than on Mo back contacts.

Propane Dehydrogenation Catalyzed by Isolated Pt Atoms in ≡SiOZn-OH Nests in Dealuminated Zeolite Beta.

Atomically dispersed noble metal catalysts have drawn wide attention as candidates to replace supported metal clusters and metal nanoparticles. Atomic dispersion can offer unique chemical properties as well as maximum utilization of the expensive metals. Addition of a second metal has been found to help reduce the size of Pt ensembles in bimetallic clusters; however, the stabilization of isolated Pt atoms in small nests of nonprecious metal atoms remains challenging. We now report a novel strategy for the design, synthesis, and characterization of a zeolite-supported propane dehydrogenation catalyst that incorporates predominantly isolated Pt atoms stably bonded within nests of Zn atoms located within the nanoscale pores of dealuminated zeolite Beta. The catalyst is stable in long-term operation and exhibits high activity and high selectivity to propene. Atomic resolution images, bolstered by X-ray absorption spectra, demonstrate predominantly atomic dispersion of the Pt in the nests and, with complementary infrared and nuclear magnetic resonance spectra, determine a structural model of the nested Pt.

Beyond Radical Rebound: Methane Oxidation to Methanol Catalyzed by Iron Species in Metal-Organic Framework Nodes.

Recent work has exploited the ability of metal-organic frameworks (MOFs) to isolate Fe sites that mimic the structures of sites in enzymes that catalyze selective oxidations at low temperatures, opening new pathways for the valorization of underutilized feedstocks such as methane. Questions remain as to whether the radical-rebound mechanism commonly invoked in enzymatic and homogeneous systems also applies in these rigid-framework materials, in which resisting the overoxidation of desired products is a major challenge. We demonstrate that MOFs bearing Fe(II) sites within Fe3-µ3-oxo nodes active for conversion of CH4 + N2O mixtures (368-408 K) require steps beyond the radical-rebound mechanism to protect the desired CH3OH product. Infrared spectra and density functional theory show that CH3OH(g) is stabilized as Fe(III)-OCH3 groups on the MOF via hydrogen atom transfer with Fe(III)-OH groups, eliminating water. Consequently, upon addition of a protonic zeolite in inter- and intrapellet mixtures with the MOF, we observed increases in CH3OH selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, as methanol is protected within the zeolite. We infer from the data that CH3OH(g) is formed via the radical-rebound mechanism on Fe(II) sites but that subsequent transport and dehydration steps are required to protect CH3OH(g) from overoxidation. The results demonstrate that the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, that the selectivity-controlling steps involve both chemical and physical rate phenomena, as well as offering a strategy to mitigate overoxidation in these and similar systems.

Dialing in Catalytic Sites on Metal Organic Framework Nodes: MIL-53(Al) and MIL-68(Al) Probed with Methanol Dehydration Catalysis.

Many metal organic frameworks (MOFs) incorporate metal oxide clusters as nodes. Node sites where linkers are missing can be catalytic sites. We now show how to dial in the number and occupancy of such sites in MIL-53 and MIL-68, which incorporate aluminum-oxide-like nodes. The methods involve modulators used in synthesis and postsynthesis reactions to control the modulator-derived groups on these sites. We illustrate the methods using formic acid as a modulator, giving formate ligands on the sites, and these can be removed to leave µ2-OH groups and open Lewis acid sites. Methanol dehydration was used as a catalytic reaction to probe these sites, with infrared spectra giving evidence of methoxide ligands as reaction intermediates. Control of node surface chemistry opens the door for placement of a variety of ligands on a wide range of metal oxide cluster nodes for dialing in reactivity and catalytic properties of a potentially immense class of structurally well-defined materials.

Atomically Dispersed Metals on Well-Defined Supports including Zeolites and Metal-Organic Frameworks: Structure, Bonding, Reactivity, and Catalysis.

When metals in supported catalysts are atomically dispersed, they are usually cationic and bonded chemically to supports. Investigations of noble metals in this class are growing rapidly, leading to discoveries of catalysts with new properties. Characterization of these materials is challenging because the metal atoms reside on surfaces that are typically nonuniform in composition and structure. We posit that understanding of structures and catalytic properties of these materials is emerging most strongly from investigations of structurally uniform catalysts (metal atoms dispersed on crystalline supports) which can be characterized incisively with atomic-resolution electron microscopy, X-ray absorption spectroscopy, and infrared spectroscopy, bolstered by density functional theory. We assess the literature of such catalysts supported on zeotype materials, metal-organic frameworks, and covalent organic frameworks. Assessing characterization, reactivity, and catalytic performance of catalysts for oxidation, hydrogenation, the water-gas shift reaction, and others, we consider metal-support interactions and ligand effects for various metal-support combinations, evaluating the degree of structural uniformity of exemplary catalysts and summarizing structure-reactivity and structure-catalytic property relationships.

The Surface Chemistry of Metal Oxide Clusters: From Metal-Organic Frameworks to Minerals.

Many metal-organic frameworks (MOFs) incorporate nodes that are small metal oxide clusters. Some of these MOFs are stable at high temperatures, offering good prospects as catalysts-prospects that focus attention on their defect sites and reactivities-all part of a broader subject: the surface chemistry of metal oxide clusters, illustrated here for MOF nodes and for polyoxocations and polyoxoanions. Ligands on MOF defect sites form during synthesis and are central to the understanding and control of MOF reactivity. Reactions of alcohols are illustrative probes of Zr6O8 node defects in UiO-66, characterized by the interconversions of formate, methoxy, hydroxy, and linker carboxylate ligands and by catalysis of alcohol dehydration reactions. We posit that new reactivities of MOF nodes will emerge from incorporation of a wide range of groups on their surfaces and from targeted substitutions of metals within them.

Tuning Catalytic Sites on Zr6O8 Metal-Organic Framework Nodes via Ligand and Defect Chemistry Probed with tert-Butyl Alcohol Dehydration to Isobutylene.

Metal-organic frameworks (MOFs) have drawn wide attention as candidate catalysts, but some essential questions about their nature and performance have barely been addressed. (1) How do OH groups on MOF nodes act as catalytic sites? (2) What are the relationships among these groups, node defects, and MOF stability, and how do reaction conditions influence them? (3) What are the interplays between catalytic properties and transport limitations? To address these questions, we report an experimental and theoretical investigation of the catalytic dehydration of tert-butyl alcohol (TBA) used to probe the activities of OH groups of Zr6O8 nodes in the MOFs UiO-66 and MOF-808, which have different densities of vacancy sites and different pore sizes. The results show that (1) terminal node OH groups are formed as formate and/or acetate ligands present initially on the nodes react with TBA to form esters, (2) these OH groups act as catalytic sites for TBA dehydration to isobutylene, and (3) TBA also reacts to break node-linker bonds to form esters and thereby unzip the MOFs. The small pores of UiO-66 limit the access of TBA and the reaction with the formate/acetate ligands bound within the pores, whereas the larger pores of MOF-808 facilitate transport and favor reaction in the MOF interior. However, after removal of the formate and acetate ligands by reaction with methanol to form esters, interior active sites in UiO-66 become accessible for the reaction of TBA, with the activity depending on the density of defect sites with terminal OH groups. The number of vacancies on the nodes is important in determining a tradeoff between the catalytic activity of a MOF and its resistance to unzipping. Computations at the level of density functional theory show how the terminal OH groups on node vacancies act as Brønsted bases, facilitating TBA dehydration via a carbocation intermediate in an E1 mechanism; the calculations further illuminate the comparable chemistry of the unzipping.

Structure, Dynamics, and Reactivity for Light Alkane Oxidation of Fe(II) Sites Situated in the Nodes of a Metal-Organic Framework.

Metal organic frameworks (MOFs), with their crystalline, porous structures, can be synthesized to incorporate a wide range of catalytically active metals in tailored surroundings. These materials have potential as catalysts for conversion of light alkanes, feedstocks available in large quantities from shale gas that are changing the economics of manufacturing commodity chemicals. Mononuclear high-spin (S = 2) Fe(II) sites situated in the nodes of the MOF MIL-100(Fe) convert propane via dehydrogenation, hydroxylation, and overoxidation pathways in reactions with the atomic oxidant N2O. Pair distribution function analysis, N2 adsorption isotherms, X-ray diffraction patterns, and infrared and Raman spectra confirm the single-phase crystallinity and stability of MIL-100(Fe) under reaction conditions (523 K in vacuo, 378-408 K C3H8 + N2O). Density functional theory (DFT) calculations illustrate a reaction mechanism for the formation of 2-propanol, propylene, and 1-propanol involving the oxidation of Fe(II) to Fe(III) via a high-spin Fe(IV)═O intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange was characterized by in situ X-ray absorption spectroscopy and ex situ Mössbauer spectroscopy. The catalytic relevance of Fe(II) sites and the number of such sites were determined using in situ chemical titrations with NO. N2 and C3H6 production rates were found to be first-order in N2O partial pressure and zero-order in C3H8 partial pressure, consistent with DFT calculations that predict the reaction of Fe(II) with N2O to be rate determining. DFT calculations using a broken symmetry method show that Fe-trimer nodes affecting reaction contain antiferromagnetically coupled iron species, and  highlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (<408 K).

Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports.

Single-site Ir(CO)2 complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)2(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts.

Thermal Stability Limits of Imidazolium Ionic Liquids Immobilized on Metal-Oxides.

Thermal stability limits of 33 imidazolium ionic liquids (ILs) immobilized on three of the most commonly used high surface area metal-oxides, SiO2, γ-Al2O3, and MgO, were investigated. ILs were chosen from a family of 13 cations and 18 anions. Results show that the acidity of C2H of an imidazolium ring is one of the key factors controlling the thermal stability. An increase in C2H bonding strength of ILs leads to an increase in their stability limits accompanied by a decrease in interionic energy. Systematic changes in IL structure, such as changes in electronic structure and size of anion/cation, methylation on C2 site, and substitution of alkyl groups on the imidazolium ring with functional groups have significant effects on thermal stability limits. Furthermore, thermal stability limits of ILs are influenced strongly by acidic character of the metal-oxide surface. Generally, as the point of zero charge (PZC) of the metal-oxide increases from SiO2 to MgO, the interactions of IL and metal-oxide dominate over interionic interactions, and metal-oxide becomes the significant factor controlling the stability limits. However, thermal stability limits of some ILs show the opposite trend, as the chemical activities of the cation functional group or the electron donating properties of the anion alter IL/metal-oxide interactions. Results presented here can help in choosing the most suitable ILs for materials involving ILs supported on metal-oxides, such as for supported ionic liquid membranes (SILM) in separation applications or for solid catalyst with ionic liquid layer (SCILL) and supported ionic liquid phase (SILP) catalysts in catalysis.