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Title
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Chemical composition of an aqueous oxalato-/citrato- solution as determinant for vanadium oxide phase formation
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Author
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Abstract
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| Aqueous solutions of oxalato- and citrato-VO2+ complexes are prepared, and their ligand exchange reaction is investigated as a function of the amount of citrate present in the aqueous solution via continuous-wave electron paramagnetic resonance (CW EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopy. With a low amount of citrate, monomeric cis-oxalato-VO2+ complexes occur with a distorted square-pyramidal geometry. As the amount of citrate increases, oxalate is gradually exchanged for citrate. This leads to (i) an intermediate situation of monomeric VO2+ complexes with a mix of oxalate/citrate ligands and (ii) a final situation of both monomeric and dimeric complexes with exclusively citrato ligands. The monomeric citrato-VO2+ complexes dominate (abundance > 80%) and are characterized by a 6-fold chelation of the vanadium(IV) ion by 4 RCO2- ligands at the equatorial positions and a H2O/R-OH ligand at the axial position. The different redox stabilities of these complexes, relative to that of dissolved O-2 in the aqueous solution, is analyzed via V-51 NMR. It is shown that the oxidation rate is the highest for the oxalato-VO2+ complexes. In addition, the stability of the VO2+ complexes can be drastically improved by evacuation of the dissolved O-2 from the solution and subsequent storage in a N-2 ambient atmosphere. The vanadium oxide phase formation process, starting with the chemical solution deposition of the aqueous solutions and continuing with subsequent processing in an ambient 0.1% O-2 atmosphere, differs for the two complexes. The oxalato-VO2+ complexes turn into the oxygen-deficient crystalline VO2 B at 400 degrees C, which then turns into crystalline V6O13 at 500 degrees C. In contrast, the citrato-VO2+ complexes form an amorphous film at 400 degrees C that crystallizes into VO2 M-1 and V6O(13) at 500 degrees C. |
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Language
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English
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Source (journal)
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Inorganic chemistry / American Chemical Society. - Easton, Pa
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Publication
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Easton, Pa
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2015
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ISSN
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0020-1669
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DOI
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10.1021/IC5015779
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Volume/pages
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54
:1
(2015)
, p. 69-78
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ISI
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000347506500014
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Pubmed ID
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25517211
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Full text (Publisher's DOI)
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Full text (publisher's version - intranet only)
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