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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.

Single-site heterogeneous catalysts receive increasing attention due to their unique catalytic properties and well-defined active sites. We report a combined computational and experimental study on a series of silica-supported metal ion hydrogenation catalysts (i.e., In3+, Ga3+, Zn2+, Mn2+, and Ti4+/SiO2). These catalysts were synthesized, characterized, and evaluated for gas-phase propylene hydrogenation. Computational studies were carried out on active-site structures and reaction mechanisms. An activity–descriptor relationship was established, which correlates a computational quantity (reaction free energy of the metal hydride formation) with the experimental reaction rate, as a function of the metal. Microkinetic modeling provided qualitative kinetic insights into the activity–descriptor relationship. This relationship was used to predict the trend of activities in a variety of M/SiO2 catalysts. These fundamental studies and the developed activity–descriptor relationship open up new opportunities for rational design of hydrogenation catalysts.

Single-site Sc and Y on silica catalysts have been prepared by aqueous and organometallic grafting methods. The former yields Y(III) ions with five bonds at an average bond distance of 2.31 A by X-ray absorption spectroscopy. Although the aqueous synthesis gave single-site Y with low coordination number, these were not catalytic for alkane dehydrogenation or olefin hydrogenation. Single-site Sc(III) and Y(III) species were also prepared by grafting Sc(CH2Si(CH3)3)3(THF)2 and Y(CH2Si(CH3)3)3(THF)2, respectively, and these are catalysts for olefin hydrogenation at temperatures from about 60 to 100 °C; however, they were thermally unstable at higher temperatures necessary for alkane dehydrogenation. The structure of the grafted Y complex was determined by X-ray absorption spectroscopy, IR, and NMR. Grafting led to protonolysis of two of the three CH2Si(CH3)3 ligands. Additionally, there was loss of one THF ligand. The EXAFS indicated that there were four Y–ligand bonds in the surface species, two at 2.16 A a...

We describe the use of a vanadium 3+ precursor for atomic layer deposition (ALD) of thin films that span the common oxidation states of vanadium oxides. Self-limiting surface synthesis of V2O3, VO2, and V2O5 are realized through four distinct reaction mechanisms accessed via judicious choice of oxygen ALD partners. In situ quartz crystal microbalance and quadrupole mass spectrometry were used to study the reaction mechanism of the vanadium precursor with O3, H2O2, H2O/O2, and H2O2/H2. A clear distinction between nonoxidative protic ligand exchange and metal oxidation is demonstrated through sequential surface reactions with different nonmetal precursors. This synergistic effect provides greater control of the resultant metal species in the film, as well as reactive surface species during growth. In an extension of this approach, we introduce oxidation state control through reducing equivalents of H2 gas. When H2 is dosed after H2O2 during growth, amorphous films of VO2 are deposited that are readily cryst...

Despite challenges to control stoichiometry in the vanadium–sulfur system, template-free growth of patronite, VS4, thin films is demonstrated for the first time. A novel atomic layer deposition (ALD) process enables the growth of phase pure films and the study of electrical and vibrational properties of the quasi-one-dimensional (1D) transition metal sulfide. Self-limiting surface chemistry during ALD of VS4 is established via in situ quartz crystal microbalance and quadrupole mass spectrometry between 150 and 200 °C. The V precursor, unconventionally, sheds all organic components in the first half-cycle, while the H2S half-cycle generates the disulfide dimer moiety, S2–2, and oxidizes V3+ to V4+. X-ray analysis establishes VS4 crystallinity and phase purity, as well as a self-limiting growth rate of 0.33 A/cy, modest roughness of 2.4 nm, and expected density of 2.7 g/cm3. Phase pure films enable a new assignment of vibrational modes and corresponding Raman activity of VS4 that is corroborated by density ...

Alkyl-zinc and zinc oxide-type sites were synthesized via atomic layer deposition on high-surface-area silica using an integrated atomic layer deposition-catalysis instrument (I-ALD-CAT). One-cycle ALD experiments using diethylzinc (DEZ) afforded Zn/SiO2 systems that provided key insights into the reactivity and stability of Zn sites as a function of dispersion at the submonolayer level. The I-ALD-CAT tool design allowed for systematic comparison of the reactivity of different grafted zinc sites. Open-shell 16-electron, tricoordinate ethyl zinc-silica sites exhibit higher activity in propane hydrogenation-dehydrogenation compared to 18-electron, tetracoordinate zinc oxide-type centers. Silica surface saturation with Zn(II) sites (∼75% of a monolayer) results in facile zinc agglomeration and catalyst deactivation under reaction conditions. Reduced DEZ dosing coupled with thermal substrate pretreatment techniques (e.g., dehydration under vacuum) resulted in increased Zn dispersion and produced Zn/SiO2 catalysts with improved activity and stability under propylene hydrogenation (200 °C) and propane dehydrogenation (550 °C) conditions.

This study details the surface reaction chemistry relevant to the vapor deposition mechanism of SnO2 thin films by atomic layer deposition. The mechanism was elucidated by combining different spectroscopic studies. Initial nucleation of cyclic N2,N3-di-tert-butylbutane-2,3-diamidotin(II) (1) consists of surface SiOH protonation of ligands as shown by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This SiO2-surface-bound stannylene was further characterized by X-ray absorption (XAS) and resonance Raman spectroscopy. XAS, DRIFTS, and Raman spectroscopy were then used to follow the further reaction of the surface-bound stannylene with different oxygen sources and a second equivalent of 1. It was observed that water does not oxidize the initial surface-bound tin site, and a well-defined, three-coordinate tin(II) species, with two surface oxygen bonds and one coordinated water molecule, was characterized. Treatment of the surface stannylene with protic oxidants such as H2O2 or tBuOOH ful...

Substitutional alloys of several thin film semiconductors have been proposed as intermediate band (IB) materials for use in next-generation photovoltaics, which aim to utilize a larger fraction of the solar spectrum without sacrificing significant photovoltage. We demonstrate a novel approach to IB material growth, namely atomic layer deposition (ALD), to allow unique control over substitutional-dopant location and density. Two new ALD processes for vanadium sulfide incorporation are introduced, one of which incorporates a vanadium(III) amidinate previously untested for ALD. Using this process, we synthesize the first thin film VxIn(2–x)S3 intermediate band semiconductors and further demonstrate that the V:In ratio, and therefore intraband gap density of states, can be finely tuned according to the ALD dosing schedule. Deposition on a crystalline In2S3 underlayer promotes the growth of a tetragonal β-In2S3-like phase VxIn(2–x)S3, which exhibits a distinct sub-band gap absorption peak with onset near 1.1 e...

Dimeric and monomeric supported single-site Fe(II) pre-catalysts on SiO2 have been prepared via organometallic grafting and characterized with advanced spectroscopic techniques. Manipulation of the surface hydroxyl concentration on the support influences monomer/dimer formation. While both pre-catalysts are highly active in liquid-phase hydrogenation, the dimeric pre-catalyst is ∼3× faster than the monomer. Preliminary XAS experiments on the H2-activated samples suggest the active species are isolated Fe(II) sites.

The chemical and electronic interactions of organometallic species with metal oxide support materials are of fundamental importance for the development of new classes of catalytic materials. Chemisorption of Cp*(PMe3)IrMe2 on sulfated alumina (SA) and sulfated zirconia (SZ) led to an unexpected redox mechanism for deuteration of the ancillary Cp* ligand. Evidence for this oxidative mechanism was provided by studying the analogous homogeneous reactivity of the organometallic precursors toward trityl cation ([Ph3C]+), a Lewis acid known to effect formal hydride abstraction by one-electron oxidation followed by hydrogen abstraction. Organometallic deuterium incorporation was found to be correlated with surface sulfate concentration as well as the extent of dehydration under thermal activation conditions of SA and SZ supports. Surface sulfate concentration dependence, in conjunction with a computational study of surface electron affinity, indicates an electron-deficient pyrosulfate species as the redox-active moiety. These results provide further evidence for the ability of sulfated metal oxides to participate in redox chemistry not only toward organometallic complexes but also in the larger context of their application as catalysts for the transformation of light alkanes.

Amorphous silica (SiO2) is commonly used as a support in heterogeneous catalysis. However, because of the structural disorder and temperature-induced change of surface morphology, the structures of silica-supported metal catalysts are difficult to determine. Most studies are primarily focused on understanding the interactions of different types of surface hydroxyl groups with metal ions. In comparison, the effect of siloxane ring size on the structure of silica-supported metal catalysts and how it affects catalytic activity is poorly understood. Here, we have used density functional theory (DFT) calculations to understand the effect of siloxane ring strain on structure and activity of different monomeric Lewis acid metal sites on silica. In particular, we have found that large siloxane rings favor strong dative bonding interaction between metal ion and surface hydroxyls, leading to the formation of high-coordinate metal sites. In comparison, metal–silanol interaction is weak in small siloxane rings, resul...

We report the synthesis, characterization, and catalytic performance for gas phase propane dehydrogenation of single-site Co2+ ions supported on silica. Spectroscopic characterization by resonance Raman, electron paramagnetic resonance, and X-ray near-edge and extended absorption fine structure revealed that tetrahedrally coordinated Co2+ ions are chemisorbed into the trisiloxane rings on the surface of amorphous silica. In situ XAS shows that Co is not oxidized by air nor reduced by hydrogen even at 650 °C. For catalytic propane dehydrogenation, single-site Co2+/SiO2 exhibits selectivities >95% at 550 °C and >90% at 650 °C with stable activity over 24 h. Calculations with hybrid density functional theory support a non-redox mechanism for activation of C–H and H–H bonds by Co2+ similar to that previously reported for single-site Zn2+/SiO2.

Lanthanum strontium chromite based materials are promising candidate for use as electrode components in Intermediate Temperature Proton Conductive Fuel Cell (IT-PCFC)[1]. In this work, (La0.75Sr0.25)0.95CrxFe1-xO3-δ(LSCrF) with x=0.3, 0.4, 0.5, 0.6 and 0.7 perovskite nano-ceramic powder series considered as anode material for IT-PCFCs [2] were focused on due to its structural, physical & chemical and also electrochemical properties corresponding with Cr amount, such as catalytic activities under the operation conditions. The nano-sized LSCrF perovskite powders were systematically synthesized by using sol-gel method[3], to understand whether the increment of Cr amount have any positive or negative effect in terms of catalytic performance, impurity, particle size and surface area. In this regard, all powders were annealed at the different temperatures changing from 500oC to 700oC in order to determine the minimum formation temperature of the powders having different amounts of Cr and Fe in order to enhance smaller particle size with higher surface area. LSCrF nano-powders were structurally characterized by means of X-ray Powder Diffraction (XRD) with Reitveld refinement by using GSAS software [4]. Cr rich samples shows more SrCrO4 impurity phase and bigger crystallite size than Fe-rich samples which leads to poor catalytic performance in the propane dehydrogenation process. Ex-situ X-ray Absorption Spectroscopy study was also performed for Fe (7112 eV) and Cr (5989 eV) K-edge in the fluorescence mode by using a 4-elements Vortex detector. When we looked at the XANES region of both Fe and Cr K edge XAS data to understand the electronic structure of Fe and Cr in the LSCrF powders, Cr K edge XAS data shows a sharp pre-edge peak resulting from SrCrO4 impurity phase having tetrahedral geometry of Cr+6 with 4-fold Oxygen atoms, whereas Fe K-edge XAS data does not show any pre-edge feature due to the octahedral geometry of Fe+3 in LSCrF perovskite structure, as we expect. the C3H8 conversion of Fe rich LSCrF samples (x4 pre-edge intensity and higher surface area is found higher than Cr-rich samples (x>0). Especially, Fe rich LSCr0.4Fe0.6O3 sample having wt.frac.~13.14% SrCrO4 impurity phase considered as better catalyst shows much lower carbon balance than other samples unsurprisingly due to its higher surface area with smaller crystallite size and having more catalytically active SrCrO4 pre-edge intensity proved by XANES analysis. Morphological analysis of the LSCrF powders was done by Scanning Electron Microscopy (SEM/EDX). BET measurements were also performed to calculate their surface area. References: [1] F. Lefebvre-Joud, G. Gauthier, and J. Mougin, “Current status of proton-conducting solid oxide fuel cells development,” J. Appl. Electrochem., vol. 39, no. 4, pp. 535–543, 2009. [2] M. F. Lü et al., “Thermomechanical, transport and anodic properties of perovskite-type (La0.75Sr0.25)0.95CrxFe1-xO3-δ” J. Power Sources, vol. 206, pp. 59–69, 2012. [3] N. P. Bansal and B. Wise, “Sol-gel synthesis of La0.6Sr0.4CoO3-δ and Sm0.5Sr0.5CoO3-δ cathode nanopowders for solid oxide fuel cells,” Ceram. Int., vol. 38, no. 7, pp. 5535–5541, 2012. [4] Larson A C, “General Structure Analysis System (GSAS),” Los Alamos Lab. Rep., vol. 748, pp. 86–748, 1994.

Achieving structurally well-defined catalytic species requires a fundamental understanding of surface chemistry. Detailed structural characterization of the catalyst binding sites in situ, such as single site catalysts on silica supports, is technically challenging or even unattainable. Octadecyltrioxysilane (OTOS) monolayers formed from octadecyltrimethoxysilane (OTMS) at the air-liquid interface after hydrolysis and condensation at low pH were chosen as a model system of surface binding sites in silica-supported Zn(2+) catalysts. We characterize the system by grazing incidence X-ray diffraction, X-ray reflectivity (XR), and X-ray fluorescence spectroscopy (XFS). Previous X-ray and infrared surface studies of OTMS/OTOS films at the air-liquid interface proposed the formation of polymer OTOS structures. According to our analysis, polymer formation is inconsistent with the X-ray observations and structural properties of siloxanes; it is energetically unfavorable and thus highly unlikely. We suggest an alternative mechanism of hydrolysis/condensation in OTMS leading to the formation of structurally allowed cyclic trimers with the six-membered siloxane rings, which explain well both the X-ray and infrared results. XR and XFS consistently demonstrate that tetrahedral [Zn(NH3)4](2+) ions bind to hydroxyl groups of the film at a stoichiometric ratio of OTOS:Zn ∼ 2:1. The high binding affinity of zinc ions to OTOS trimers suggests that the six-membered siloxane rings are binding locations for single site Zn/SiO2 catalysts. Our results show that OTOS monolayers may serve as a platform for studying silica surface chemistry or hydroxyl-mediated reactions.

Thin film solar cells made from earth-abundant, non-toxic materials are needed to replace the current technology that uses Cu(In,Ga)(S,Se)2 and CdTe, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). In this report, pure, stoichiometric, single-phase SnS films were obtained by atomic layer deposition (ALD) using the reaction of bis(N,N′-diisopropylacetamidinato)tin(II) [Sn(MeC(N-iPr)2)2] and hydrogen sulfide (H2S) at low temperatures (100 to 200 °C). The direct optical band gap of SnS is around 1.3 eV and strong optical absorption (α > 104 cm−1) is observed throughout the visible and near-infrared spectral regions. The films are p-type semiconductors with carrier concentration on the order of 1016 cm−3 and hole mobility 0.82–15.3 cm2V−1s−1 in the plane of the films. The electrical properties are anisotropic, with three times higher mobility in the direction through the film, compared to the in-plane direction.

Synchrotron characterization techniques provide some of the most powerful tools for the study of film structure and chemistry. The brilliance and tunability of the Advanced Photon Source allow access to scattering and spectroscopic techniques unavailable with in-house laboratory setups and provide the opportunity to probe various atomic layer deposition (ALD) processes in situ starting at the very first deposition cycle. Here, we present the design and implementation of a portable ALD instrument which possesses a modular reactor scheme that enables simple experimental switchover between various beamlines and characterization techniques. As first examples, we present in situ results for (1) X-ray surface scattering and reflectivity measurements of epitaxial ZnO ALD on sapphire, (2) grazing-incidence small angle scattering of MnO nucleation on silicon, and (3) grazing-incidence X-ray absorption spectroscopy of nucleation-regime Er2O3 ALD on amorphous ALD alumina and single crystalline sapphire.

An integrated atomic layer deposition synthesis-catalysis (I-ALD-CAT) tool was developed. It combines an ALD manifold in-line with a plug-flow reactor system for the synthesis of supported catalytic materials by ALD and immediate evaluation of catalyst reactivity using gas-phase probe reactions. The I-ALD-CAT delivery system consists of 12 different metal ALD precursor channels, 4 oxidizing or reducing agents, and 4 catalytic reaction feeds to either of the two plug-flow reactors. The system can employ reactor pressures and temperatures in the range of 10(-3) to 1 bar and 300-1000 K, respectively. The instrument is also equipped with a gas chromatograph and a mass spectrometer unit for the detection and quantification of volatile species from ALD and catalytic reactions. In this report, we demonstrate the use of the I-ALD-CAT tool for the synthesis of platinum active sites and Al2O3 overcoats, and evaluation of catalyst propylene hydrogenation activity.

In the Buchwald-Hartwig reaction between HIPTBr (HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3 = hexaisopropylterphenyl) and (H2NCH2CH2)3N, it is possible to obtain a 65% isolated yield of (HIPTNHCH2CH2)2NCH2CH2NH2. A second coupling then can be carried out to yield a variety of "hybrid" ligands, (HIPTNHCH2CH2)2NCH2CH2NHAr, where Ar = 3,5-Me2C6H3, 3,5-(CF3)2C6H3, 3,5-(MeO)2C6H3, 3,5-Me2NC5H3, 3,5-Ph2NC5H3, 2,4,6-i-Pr3C6H2, or 2,4,6-Me3C6H2. The hybrid ligands may be attached to Mo to yield [hybrid]MoCl species. From the monochloride species, a variety of other species such as [hybrid]MoN, {[hybrid]MoN2}Na, and {[hybrid]Mo(NH3)}+ can be prepared. [Hybrid]MoN2 species were prepared through oxidation of {[hybrid]MoN2}Na species with ZnCl2, but they could not be isolated. [Hybrid]Mo=N-NH species could be observed as a consequence of the protonation of {[hybrid]MoN2}- species, but they too could not be isolated as a consequence of a facile decomposition to yield dihydrogen and [hybrid]MoN2 species. Attempts to reduce dinitrogen catalytically led to little or no ammonia being formed from dinitrogen. The fact that no ammonia was formed from dinitrogen in the case of Ar = 3,5-Me2C6H3, 3,5-(CF3)2C6H3, or 3,5-(MeO)2C6H3 could be attributed to a rapid decomposition of intermediate [hybrid]Mo=N-NH species in the catalytic reaction, a decomposition that was shown in separate studies to be accelerated dramatically by 2,6-lutidine, the conjugate base of the acid employed in the attempted catalytic reduction. X-ray structures of [(HIPTNHCH2CH2)2NCH2CH2N{3,5-(CF3)2C6H3}]MoCl and [(HIPTNHCH2CH2)2NCH2CH2N(3,5-Me2C6H3)]MoN2}Na(THF)2 are reported.

International audience; Trichlorotris(tetrahydrofuran)molybdenum(III) (MoCl3(THF)3), is a widely used starting material for the synthesis of a broad range of molybdenum compounds. It was first prepared by a carbonyl route, but later the compound was obtained by a reductive procedure in three steps starting from MoCl5. In spite of the various incremental improvements, the three-step procedure remains rather long and requires isolation of both the MoCl4(MeCN)2 and MoCl4(THF)2 intermediates in order to obtain product of sufficient quality. Ethereal solutions of MoCl5 are stable and their conversion to tetrachlorobis(diethyl ether)molybdenum(IV) (MoCl4(OEt2)2) by reducing agents such as norbornene or allyltrimethylsilane has been previously reported. As it turns out, this reduction can be conveniently carried out by metallic tin. This chapter describes both the synthesis and isolation of the MoCl4(OEt2)2 complex and the direct, two-step, single-flask synthesis of MoCl3(THF)3 from MoCl5.

Addition of two equivalents of lithium pyrrolide to Mo(NR)(CHCMe2R')(OTf)2(DME) (OTf = OSO2CF3; R = 2,6-i-Pr2C6H3, 1-adamantyl, or 2,6-Br2-4-MeC6H2; R' = Me or Ph) produces Mo(NR)(CHCMe2R')(NC4H4)2 complexes in good yield. All compounds can be recrystallized readily from toluene or mixtures of pentane and ether and are sensitive to air and moisture. An X-ray structure of a 2,6-diisopropylphenylimido species shows it to be an unsymmetric dimer, {Mo(NAr)(syn-CHCMe2Ph)(η5-NC4H4)(η1-NC4H4)}{Mo(NAr)(syn-CHCMe2Ph)(η1-NC4H4)2}, in which the nitrogen in the η5-pyrrolyl bound to one Mo behaves as a donor to the other Mo. All complexes are fluxional on the NMR time scale at room temperature, with one symmetric species being observed on the NMR time scale at 50 °C in toluene-d8. The dimers react with PMe3 (at Mo) or B(C6F5)3 (at a η5-NC4H4 nitrogen) to give monomeric products in high yield. They also react rapidly with two equivalents of monoalcohols (e.g., Me3COH or (CF3)2MeCOH) or one equivalent of a biphenol or binaphthol to give two equivalents of pyrrole and bisalkoxide or diolate complexes in ~100% yield.