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Partially Fluorinated Oxo-Alkoxide Tungsten(VI) Complexes As Erik Jonsson School of Engineering and Computer Science 2014-04 Partially Fluorinated Oxo-Alkoxide Tungsten(VI) Complexes as Precursors for Deposition of WOx Nanomaterials UTD AUTHOR(S): Ashley A. Ellsworth and Amy V. Walker ©2014 The Royal Society of Chemistry. This article may not be further made available or distributed. Find more research and scholarship conducted by the Erik Jonsson School of Engineering and Computer Science here. This document has been made available for free and open access by the Eugene McDermott Library. Contact [email protected] for further information. Dalton Transactions View Article Online PAPER View Journal | View Issue Partially fluorinated oxo-alkoxide tungsten(VI) complexes as precursors for deposition of Cite this: Dalton Trans., 2014, 43, 9226 WOx nanomaterials† Richard O. Bonsu,a Hankook Kim,b Christopher O’Donohue,b Roman Y. Korotkov,c K. Randall McClain,a Khalil A. Abboud,a Ashley A. Ellsworth,d Amy V. Walker,d Timothy J. Andersonb and Lisa McElwee-White*a The partially fluorinated oxo-alkoxide tungsten(VI) complexes WO(OR)4 [4; R = C(CH3)2CF3, 5;R= C(CH3)(CF3)2] have been synthesized as precursors for chemical vapour deposition (CVD) of WOx nano- Received 7th February 2014, crystalline material. Complexes 4 and 5 were prepared by salt metathesis between sodium salts of the Accepted 29th April 2014 fluoroalkoxides and WOCl4. Crystallographic structure analysis allows comparison of the bonding in 4 and DOI: 10.1039/c4dt00407h 5 as the fluorine content of the fluoroalkoxide ligands is varied. Screening of 5 as a CVD precursor by www.rsc.org/dalton mass spectrometry and thermogravimetric analysis was followed by deposition of WOx nanorods. Introduction tivity.2 It is noted that the stoichiometry and crystalline phase is critical in determining the properties of the material.7,8 Tungsten oxide nanostructured materials have been studied CVD has been employed for conformal and large surface 1–3 9–11 for several applications, including gas sensing devices and area growth of WOx films and with appropriate precursors anodic charge injection layers for dye-sensitized solar cells and deposition conditions can enable growth of nanocrystal- 2,4–6 (DSSCs). The interest in WOx nanomaterials for these line materials under moderate reaction conditions. In addition applications is due to its strong and selective response to infra- to the possibility of controlling the properties of CVD materials red (IR) light. Compared to the commonly used TiO2, which through the structure and decomposition chemistry of – responds to ultraviolet radiation (UV), the IR absorption by precursors,12 22 aerosol-assisted chemical vapour deposition 23–25 WOx nanocrystals provides better band gap matching with (AACVD) can be carried out in flexible reaction environ- numerous low lying LUMO infrared absorbing organic dyes ments that are operated in a range of temperatures and press- used in DSSCs.5 Additional investigations have demonstrated ures. With a higher mass transport rate of the precursor, that the photo-responses of WOx nanotubes are modified by the AACVD deposition rate could be several orders of magni- absorption/desorption of small molecules, which facilitates tude higher than in CVD methods that rely on precursor use of this material in sensors.1,2,6 In either application, it is volatilization or other techniques that have been used to 2,3,6,7 observed that an increase in surface area-to-volume ratio of the synthesize WOx. WOx nanocrystals leads to significantly improved photosensi- Tungsten oxide nanostructures can be prepared by CVD from the volatile carbonyl complex W(CO)6, but a carbon nano- 26 tube template or a co-reactant was required. AACVD of WO3 24,27 28 aDepartment of Chemistry, University of Florida, Gainesville, Florida 32611-7200, nanorods from W(OPh)6 and W(CO)6 has also been USA. E-mail: [email protected] reported. In addition, tungsten oxo complexes with ancillary b Department of Chemical Engineering, University of Florida, Gainesville, aminoalkoxide ligands have been used to prepare WOx Florida 32611-6005, USA. E-mail: [email protected] needles by hydrolysis but water is not compatible with some c Arkema Inc., 900 First Ave., King of Prussia, PA 19406, USA applications.29 dDepartment of Materials Science and Engineering RL10, University of Texas at Dallas, 800 W. Campbell Rd, Richardson, Texas 75080, USA Given the limited range of established precursors for WOx †Electronic supplementary information (ESI) available: 1H NMR, 19F NMR and nanostructures, we sought to prepare molecular single source 13C NMR spectra of complexes 4 and 5. Table of bond distances, bond angles, precursors that could be suitable for both conventional CVD atomic coordinates, and equivalent isotropic displacement parameters for 4 and and AACVD. Good candidates would possess adequate vola- 5. DTA measurements of compounds 1, 4 and 5. Thermal behaviours of 1, 4 and tility and reactivity to enable oxide growth by CVD at moderate 5 based on TG/DTA measurements. Relative abundances for positive ion DIPCI 30,31 ffi mass spectra of complexes 4 and 5. CCDC 985672 and 985673. For ESI and crys- temperatures, while being su ciently soluble and stable in tallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00407h suitable solvents for AACVD. Incorporation of trifluoromethyl 9226 | Dalton Trans.,2014,43, 9226–9233 This journal is © The Royal Society of Chemistry 2014 View Article Online Dalton Transactions Paper groups into sterically bulky alkoxide and beta-diketonate temperature and stirred for the next 12 h to obtain a light ligands of CVD precursors has been known to impart brownish solution. The reaction mixture was transferred back – sufficient volatility.32 35 The partially fluorinated alkoxide pre- into the glovebox and the mixture was filtered through a Celite cursor WO(OCH2CF3)4 has been reported for atmospheric pad to yield a yellowish-brown filtrate. At the Schlenk line, 36 ff pressure CVD growth of WOx, but the synthesis was solvent was pulled o under vacuum until a brownish gummy described as problematic and the depositions produced films, residue remained. The residue was dissolved and stirred in in contrast to the nanorod growth reported here. The potential hexane, the solvent was removed and the residue left under precursor compound WO[OC(CH3)2CF3]4 (4) was first men- vacuum overnight. The light brown powder was sublimed tioned as a decomposition product during cross-metathesis between 85–90 °C (400 mTorr) to yield 0.54 g of a yellowish- reactions of the corresponding tungsten alkylidyne and white sublimate (52%). The compound was identified by nitriles.37 We now report an improved synthesis of 4 along comparison to literature data.37 Single crystals suitable for with synthesis and characterization of the related volatile X-ray structure determination were grown from toluene at complex WO[OC(CF3)2CH3]4 (5). Aerosol assisted chemical −3 °C. vapour deposition from 5 results in WOx nanorod growth. Synthesis of WO[OC(CF3)2CH3]4 (5). All reaction protocols were the same as for 4 above, starting with NaOC(CF3)2CH3 (5.85 mmol, 1.19 g) dissolved in 20 mL DME and WOCl4 Experimental details (1.46 mmol, 0.500 g) dissolved in 10 mL DME. The orange colour of WOCl4 immediately turned bluish, then pale yellow General procedures after addition. The reaction was run at room temperature for All reactions were carried out under an atmosphere of dry 12 h to yield a yellow-brown gummy solid, which was sublimed nitrogen using either glovebox or standard Schlenk tech- between 75 and 80 °C (400 mTorr) to yield a pale yellowish 1 niques. All chemicals used were of reagent grade. Methylene sublimate (0.57 g, 48%). H NMR (C6D6, 25 °C): δ 1.52 (s, CH3). 13 δ 1 chloride, diethyl ether, hexane and toluene from Fisher Scien- C NMR (C6D6, 25 °C): 123.75 (q, CF3, JC–F = 287.9 Hz), 2 tific were distilled and stored over molecular sieves (4 Å) for at 87.06 (septet, C(CF3)2CH3 JC–F = 32.3 Hz), 15.76 (s, CH3). 19 least 48 h prior to experiments. Anhydrous 1,2-dimethoxy- FNMR(C6D6,25°C):δ −76.25 (s, C(CF3)2CH3). Anal. calcd for ethane (Sigma-Aldrich), hexamethyldisiloxane (Sigma-Aldrich) WO5C16H12F24: C, 20.78; H, 1.30. Found: C, 20.74; H, 1.44%. and benzene-d6 (Cambridge Isotopes) were stored over 4 Å Crystals suitable for X-ray structure determination were grown molecular sieves for at least 48 h before use. NaH, WCl6 by cooling a sample dissolved in a solvent mixture of toluene (Sigma-Aldrich) and fluorinated tert-butanol (SynQuest labs) and DMSO to −3 °C. 38 were used as received. Compounds WOCl4 (1), NaOC- 32,35 Crystallographic structure determination of 4 and 5 (CH3)2CF3 (2) and NaOCCH3(CF3)2 (3) were synthesized as reported in their respective literature. NMR spectra were X-Ray intensity data were collected at 100 K; 4 on a Bruker recorded on a Varian Mercury 300BB (300 MHz) spectrometer DUO diffractometer and 5 on a Bruker SMART diffractometer using residual protons from deuterated solvents for reference. both using Mo Kα radiation (λ = 0.71073 Å) and an APEXII Elemental analysis was conducted by Complete Analysis Lab- CCD area detector. Raw data frames were read by the program oratory Inc. (Parsippany, NJ). Mass spectrometry was per- SAINT39 and integrated using 3D profiling algorithms. The formed on a Thermo Scientific Trace GC DSQ equipped with resulting data were reduced to produce hkl reflections and direct insertion probe (DIP) using chemical ionization (CI) their intensities and estimated standard deviations. The data with methane as the reagent gas. The DTA/TGA samples were were corrected for Lorentz and polarization effects and − heated 10 °C min 1 to 600 °C.
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