Article pubs.acs.org/cm
Volatile Hexavalent Oxo-amidinate Complexes: Molybdenum and Tungsten Precursors for Atomic Layer Deposition Aidan R. Mouat,† Anil U. Mane,‡ Jeffrey W. Elam,‡ Massimiliano Delferro,*,† Tobin J. Marks,*,† and Peter C. Stair*,†,§ †
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States Energy Systems Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Chemical Sciences & Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡
S Supporting Information *
ABSTRACT: New complexes MoO2(tBuAMD)2 (1) and WO2(tBuAMD)2 (2) (AMD = acetamidinato) are synthesized and fully characterized as precursors for atomic layer deposition (ALD). They contain metal-oxo functionalities not previously utilized in ALD-type growth processes and are fully characterized by 1H and 13C NMR, X-ray diffraction (XRD), Fourier transform infrared, thermogravimetric analysis, single-crystal XRD, and elemental analysis. Guided by quartzcrystal microbalance studies, ALD growth methodologies for both complexes have been developed. Remarkably, these isostructural compounds exhibit dramatic differences in ALD properties. Using 1 and O3, amorphous, ultrathin molybdenum oxynitride (MoON) films are grown on Si(100) wafers. Using 2 and H2O yields amorphous WO3 films on Si(100) wafers that crystallize as WO3 nanowires upon annealing. Although 1/H2O and 2/O3 growth was attempted, effective ALD growth could only be obtained with 1/O3 and 2/H2O, underscoring reactivity differences in these precursors. Film thicknesses, compositions, and optical and electrical parameters are characterized by variable angle spectroscopic ellipsometry, X-ray reflectivity, grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and atomic force microscopy techniques. The hitherto unknown ALD chemistry of group VI metal-oxo compounds lays a foundation for their use in the ALD synthesis of heterogeneous catalysts.
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Atomic layer deposition (ALD) has recently gained interest as a method for the synthesis of heterogeneous catalysts with precise, angstrom-level control over the resulting surface structures.31−46 In a typical binary, A−B-type ALD process,47 a vapor-phase metal precursor (Scheme 1, reaction A) is passed over the surface of an oxide support and allowed to react with surface hydroxyls. Upon consumption of all available surface reactive sites or due to steric crowding, this reaction terminates. In the second cycle, surface hydroxyls are regenerated through an oxygen source or protonolytic agent (e.g., H2O) (Scheme 1). The surface control in an ALD growth sequence permits rational design of the appropriate metal precursor for the selective installation of isolated catalytically active sites. In particular, we envisioned volatile metal−organic complexes bearing intact metal-dioxo functionalities on Mo(VI) and W(VI) centers, as sources for the growth of such centers. While vapor-phase deposition of molybdenum and tungsten oxide
upported, highly dispersed, group VI refractory metal oxide catalysts (Mo, W) are a subject of great interest in heterogeneous catalytic science, particularly as catalysts for commercially relevant transformations1−4 such as hydrocracking,5−7 dehydration,8 hydrodesulfurization (HDS),9 hydrodeoxygenation,10 olefin epoxidation,11 polymerization,12 olefin metathesis,13−15 and electrocatalysis.16,17 The extensive applicability of MoOx and WOx catalysts is a result of their multifunctional role as Brønsted acids, Lewis acids, and/or redox-active centers.1,18,19 While the precise structure of the catalytically active sites is still debated,7 the presence of metaloxo functionalities in either monomeric or polymeric surface species is generally implicated.20,21 The properties of supported MVIOx catalysts change with surface density. Thus, polymeric and oligomeric species exhibit enhanced activity and selectivity,22−24 but at loadings higher than a monolayer (∼4.5−5 M/nm2) crystalline MVIO3 domains begin to form and catalytic activity declines.25,26 In addition to variations in surface density, spectroscopic27 and catalytic studies28,1 indicate that the method of supporting,27,29 pretreatment conditions,7 and reaction conditions30 exert a significant influence on the structure of the supported species and, thus, the overall activity and selectivity. © 2016 American Chemical Society
Received: January 19, 2016 Revised: February 26, 2016 Published: February 26, 2016 1907
DOI: 10.1021/acs.chemmater.6b00248 Chem. Mater. 2016, 28, 1907−1919
Article
Chemistry of Materials
molybdenum oxynitride (MoON) with approximate composition MoO2.48N0.18 while precursor 2 yields ultrathin films of stoichiometric WO3. Such ultrathin films are of interest in a wide variety of applications and frequently exhibit properties different from those of conventional thin films.71 Furthermore, the study of ultrathin films is applicable to ALD catalyst design, as isolated surface structures and sub-monolayer overcoats are frequently of catalytic importance. We characterize our ALD films with variable angle spectroscopic ellipsometry (VASE), atomic force microscope (AFM), X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS).
Scheme 1. Example of an A−B Reaction Sequence for an ALD Growth Process
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films has been realized through ALD and chemical vapor deposition (CVD),48−57 including examples using volatile oxobearing tungsten precursors,55,56 precise control of deposited surface structures has not yet been reported. Previously, Gordon and co-workers58−60 reported that amidinato- and acetamidinato- ligand platforms are viable for volatile metal− organic ALD precursors. We therefore envisioned that bulky acetaminidato- ligands would enable the synthesis of novel hexavalent M(VI) ALD precursors with intact tungstyl and molybdyl functionalities (Scheme 2). In addition to catalytic applications, ALD presents an attractive method for the synthesis of a wide range of devices via conformal deposition of atomically thin films.61 For example, nitrogen-doped molybdenum oxide filmsmolybdenum oxynitrideexhibit superior properties in battery applications.62,63 Thin WO3 films are of interest due to their electrochromic properties, enabling technologies such as “smart windows”,64 or nanostructured,65 nanowire,66 and porous67 films which exhibit properties different than those of traditional morphologies. Here we report the synthesis of two new, hexavalent oxoamidinate ALD precursors, MO2(tBuAMD)2 (M = Mo (1), W (2); tBuAMD = N,N′-di-tert-butylacetamidinate) (Scheme 2). These compounds are synthesized via the reaction of 2 equiv of the lithium salt of N,N′-di-tert-butylcarbodiimide with the corresponding monomeric MO2Cl2(dme) complexes,68−70 as shown in Scheme 2. Both compounds are fully characterized by 1 H and 13C NMR, FT-IR, and single-crystal X-ray diffraction spectroscopies and elemental analysis. Scaleup of the synthetic procedure enabled the synthesis of multigram quantities of both precursors. The volatility and thermal stability of the compounds are confirmed via thermogravimetric analysis (TGA). In quartz-crystal microbalance (QCM) studies, both precursors demonstrate ALD-like growth with no CVD component. The ALD growth procedures developed here for both compounds show that the optimal “B” reagent (Scheme 1) for 1 is O3, while the optimal “B” reagent for 2 is H2O. Using these recipes, we successfully grow ultrathin films (
