Addition to “On the Charge State of Titanium in Titanium Dioxide”

Addition/Correction pubs.acs.org/JPCL

Addition to “On the Charge State of Titanium in Titanium Dioxide” Daniel Koch and Sergei Manzhos* J. Phys. Chem. Lett. 2017, 8 (7), 1593−1598. DOI: 10.1021/acs.jpclett.7b00313

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e provide here additional context to support the conclusion that we reached1 that “Ti3+” (if one is to use an integer although charge quantization is not required here) is a better description of Ti in TiO2 than “Ti4+”. Raebiger et al.2 showed that the charge on a transition metal atom in a compound is very stable with respect to changes of the formal oxidation state (FOS) due to the so-called negative-feedback charge regulation. In ref 3, charge reporter molecules were used to reach a similar conclusion; specifically for Ti, a charge state of about +3 |e| was estimated.3 Earlier, Christensen and Carter reported significant covalent character of bonds in ZrO2 and concluded that in ZrO2 “Zr is likely to be Zr(II) like” despite the FOS of Zr(IV).4 In ref 1., we directly analyzed valence electron densities to ascertain that one electron charge resides on Ti in TiO2 molecules and solids within less than half of the Ti−O bond length, independently of definitions of common charge assignment schemes. Specifically, valence densities around Ti in TiO2 were compared to those in Ti0...Ti4+ ions. Such ions are directly detectable in plasma,5,6 as opposed to, for example, XPS on solids (often cited as evidence for Ti4+) where changes in charge surrounding the ionic core are detected indirectly. We confirmed that Bader charges provide a reasonable partitioning of space and Bader charges on the order of +2.5 |e| were most reasonable. We note that Bader charges do show full oxidation where it is known to happen (e.g., alkali atoms in semiconductors).7−9 We also note that there is decent agreement between charge densities around Ti in a TiO6 environment measured by X-ray/electron diffraction and computed by DFT.10,11 Phillips fractional ionicity (based on Pauling electronegativity) of a Ti−O bond is 0.6, which implies a significant covalent character (compared, e.g., to 0.9 for LiF). Recent ionic potential models12 that are able to reproduce properties of various TiO2 polymorphs use charges of +2.4 and −1.2 |e| on Ti and O, respectively.13 (Reference 13 reported structures and elastic properties of rutile TiO2, while we confirmed that a potential using these charges can reproduce structures of anatase, rutile, and bronze TiO2.) The “Ti4+” language implies no further oxidation of Ti and no further reduction of O. However, a degree of oxygen reduction is easily seen in ab initio calculations of doped oxides including TiO2,7,14 and oxygen redox has finally been embraced in such systems.15 That it took so long to recognize it15 may have to do with the ubiquitous use of FOS to rationalize the mechanism. Given the above, “Ti4+” following from the ideology of FOS16 appears to be a rather unphysical description. The FOS concept, which itself relies on the ionic approximation17 that clearly does not hold in TiO2, predates modern quantum chemistry18 and the knowledge gained from it, does not seem to be useful here.19 © XXXX American Chemical Society

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +65 6516 4605. Fax: +65 6779 1459. ORCID

Sergei Manzhos: 0000-0001-8172-7903

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REFERENCES

(1) Koch, D.; Manzhos, S. On the Charge State of Titanium in Titanium Dioxide. J. Phys. Chem. Lett. 2017, 8, 1593−1598. (2) Raebiger, H.; Lany, S.; Zunger, A. Charge Self-Regulation upon Changing the Oxidation State of Transition Metals in Insulators. Nature 2008, 453, 763−766. (3) Wolczanski, P. Flipping the Oxidation State Formalism: Charge Distribution in Organometallic Complexes As Reported by Carbon Monoxide. Organometallics 2017, 36, 622−631. (4) Christensen, A.; Carter, E. A. First-principles characterization of a heteroceramic interface: ZrO2(001) deposited on an α-Al2O3(1102) substrate. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 16968− 16983. (5) Khaydarov, R. T.; Beisinbaeva, H. B.; Sabitov, M. M.; Kalal, M.; Berdiyorov, G. R. Effect of Light Gas Atom Inclusions on the Characteristics of Laser-Produced Plasma Ions. Nucl. Fusion 2011, 51, 103041. (6) Abbasi, S. A.; Hussain, M. S.; Ilyas, B.; Rafique, M.; Dogar, A. H.; Qayyum, A. Characterization of Highly Charged Titanium Ions Produced by Nanosecond Pulsed Laser. Laser Part. Beams 2015, 33, 81−86. (7) Legrain, F.; Malyi, O. I.; Manzhos, S. Insertion Energetics of Lithium, Sodium, and Magnesium in Crystalline and Amorphous Titanium Dioxide: a Comparative First-Principles Study. J. Power Sources 2015, 278, 197−202. (8) Malyi, O. I.; Tan, T. L.; Manzhos, S. A Comparative Computational Study of Structures, Diffusion, and Dopant Interactions between Li and Na Insertion into Si. Appl. Phys. Express 2013, 6, 027301. (9) Kim, H.; Kweon, K. E.; Chou, C. Y.; Ekerdt, J. G.; Hwang, G. S. On the Nature and Behavior of Li Atoms in Si: A First Principles Study. J. Phys. Chem. C 2010, 114, 17942−17946. (10) Zheng, J.-C.; Frenkel, A. I.; Wu, L.; Hanson, J.; Ku, W.; Božin, E. S.; Billinge, S. J. L.; Zhu, Y. Nanoscale disorder and local electronic properties of CaCu3Ti4O12: An integrated study of electron, neutron, and X-ray diffraction, X-ray absorption fine structure, and firstprinciples calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 144203. (11) Jiang, B.; Zuo, J. M.; Jiang, N.; O’Keeffe, M.; Spence, J. C. H. Charge density and chemical bonding in rutile, TiO2. Acta Crystallogr., Sect. A: Found. Crystallogr. 2003, 59, 341−350. (12) Catlow, C. R. A.; Stoneham, A. M. Ionicity in Solids. J. Phys. C: Solid State Phys. 1983, 16, 4321−4338. (13) Pedone, A.; Malavasi, G.; Menziani, M. C.; Cormack, A. N.; Segre, U. A New Self-Consistent Empirical Interatomic Potential

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DOI: 10.1021/acs.jpclett.7b01886 J. Phys. Chem. Lett. 2017, 8, 3945−3946

The Journal of Physical Chemistry Letters

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Model for Oxides, Silicates, and Silica-Based Glasses. J. Phys. Chem. B 2006, 110, 11780−11795. (14) Kulish, V.; Koch, D.; Manzhos, S. Ab initio study of Li, Mg and Al insertion into rutile VO2: Fast diffusion and enhanced voltages for multivalent batteries. Phys. Chem. Chem. Phys. 2017, DOI: 10.1039/ C7CP04360K. (15) Seo, D.-H.; Lee, J.; Urban, A.; Malik, R.; Kang, S.; Ceder, G. The Structural and Chemical Origin of the Oxygen Redox Activity in Layered and Cation-Disordered Li-Excess Cathode Materials. Nat. Chem. 2016, 8, 692−697. (16) Walsh, A.; Sokol, A. A.; Buckeridge, J.; Scanlon, D. O.; Catlow, C. R. A. Electron Counting in Solids: Oxidation States, Partial Charges, and Ionicity. J. Phys. Chem. Lett. 2017, 8, 2074−2075. (17) Karen, P.; McArdle, P.; Takats, J. Comprehensive Definition of Oxidation State (IUPAC Recommendations 2016). Pure Appl. Chem. 2016, 88, 831−839. (18) Resta, R. Charge States in Transition. Nature 2008, 453, 735. (19) Koch, D.; Manzhos, S. The Efficacy of Fiction or More on the Charge State of Ti in TiO2 and Formal Oxidation States. arXiv:1707.06851v1 [cond-mat.mtrl-sci].

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DOI: 10.1021/acs.jpclett.7b01886 J. Phys. Chem. Lett. 2017, 8, 3945−3946