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© 2015 Macmillan Publishers Limited. Financial support from the Spanish Ministry of Economy and Competitiveness under projects MAT2012-38045-C04-03 and MAT2012-38045-C04-04 is acknowledged. A.M. also acknowledges project MAT2010-21156-C03-02 for financial support. We thank A. Tejeda for critical discussions. We acknowledge the Spanish Ministry of Economy and Competitiveness and Consejo Superior de Investigaciones Científicas for both financial support under project PIE 201060E013 and provision of synchrotron radiation facilities. We would also like to thank the SpLine beamline staff for their assistance during the SR experiments. GSS and the microscopy effort were supported by the ERC starting Investigator Award, grant #239739 STEMOX. Electron microscopy observations were carried out at the Centro Nacional de Microscopía Electrónica, CNME-UCM. Part of the XRD measurements were performed at the C.A.I. de Difracción de Rayos X-UCM. Computational calculations were performed at the Supercomputing Centre of Galicia (CESGA). Author contributions: O.R.d.l.F. conceived the project and coordinated the research. B.M., I.P. and O.R.d.l.F. prepared the samples and performed the Auger and LEED measurements and the ion beam modifications. B.M., I.P., A.M., J.L.-S, O.R.d.l.F., J.R.-Z., P.F. and G.C. performed the X ray diffraction measurements. G.S.-S. and M.V. performed the STEM measurements. J.I.B. and M.C.M. carried out the Density Functional Theory calculations. All authors wrote and revised the manuscript and extensively discussed the results and their interpretation. Supplementary Information: accompanies this paper at http://www.nature.com/ naturecommunications., A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces., Unión Europea. FP7, Ministerio de Economía y Competitividad (MINECO), España, Consejo Superior de Investigaciones Científicas (CSIC), España, ERC starting Investigator Awards, European Research Council (ERC), Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

En este trabajo hemos estudiado diversas propiedades de la super cie de dos materiales con el n de comprobar la in uencia de la modi caci on inducida mediante bombardeo i onico sobre ellas. Los materiales estudiados son el di oxido de titanio y el rutenio. Las propiedades estudiadas son, para el oxido de titanio, la estructura y la hidro licidad, y para el rutenio, su actividad catal tica. Estas propiedades presentan una gran importancia de cara a algunas de las aplicaciones pr acticas de estos materiales: - La estructura de un material es una caracter stica que de ne muchas de sus propiedades. Por ello, al modi car la estructura super cial estamos modi - cando tambi en su interacci on con el ambiente y sus propiedades intr nsecas, como la resistencia el ectrica. - La hidro licidad es una propiedad relacionada con diversas aplicaciones de los oxidos, entre otros motivos porque estos con frecuencia se forman cuando un metal es introducido en un medio con agua. En el caso del di oxido de titanio, esta propiedad es especialmente interesante debido al comportamiento inusual al incidir luz ultravioleta sobre su super cie, induci endose un estado de superhidro licidad. - La actividad catal tica de la super cie del rutenio est a relacionada directamente con las pilas de combustible, entre otras aplicaciones. Este material muestra una gran diferencia en su comportamiento catal tico en condiciones de ultraalto vac o y en condiciones de trabajo. Es por esto que existe una cierta pol emica sobre cu al es su estado de oxidaci on cuando su actividad catal tica es alta, habiendo tanto estudios que apoyan la opci on de la super cie de rutenio met alico con ox geno quimisorbido como la opci on de la formaci on de un oxido o capa super cial - pre oxido. iv Los resultados obtenidos se resumen en los puntos siguientes: En lo que a la estructura se re ere, hemos determinado mediante diversas t ecnicas de difracci on y microscop a la formaci on de una capa super cial de TiO de estructur, In this work we have studied several surface properties of two di erent materiales in order to con rm the in uence of the modi cation induced by ion bombardment on them. The studied materials are titanium dioxide and ruthenium with varying oxidation degrees. The studied properties are the titanium oxide structure and hydrophilicity, and the ruthenium catalytic activity. These properties have a great importance for some practical applications of these materials: - The structure of a material is a de ning characteristic of many of its properties. Therefore, by modifying the surface structure we are also changing its interaction with the environment and its intrinsic properties, such as electrical resistance. - The hydrophilicity is a property related to various applications of oxides, some of them driven by the fact that these oxides are often formed when a metal is introduced into an aqueous medium. In the case of titanium dioxide, this property is particularly interesting because of its unusual surface behavior when it is illuminated by ultraviolet light, which induces a state of superhydrophilicity. - The catalytic activity of the ruthenium surface is directly related to fuel cells, among other applications. This material shows a great di erence in its catalytic behavior depending on whether it is in ultrahigh vacuum or at working conditions. This fact is the reason why there is some controversy about its oxidation state when its catalytic activity is high, existing studies that support both the oxygen covered metallic surface option and the formation of an oxide surface layer (preoxide) option. The obtained results are summarized in the following points: vii As far as the structure is concerned, we have determined, through various di raction and microscopy techniques, the formation of a surface layer of TiO - rocksalt structure by ion bombarding the surface of TiO2(110) rutile structure. This change in structure is due mainly to the good t between the unit cells o

© 2015 Macmillan Publishers Limited. Financial support from the Spanish Ministry of Economy and Competitiveness under projects MAT2012-38045-C04-03 and MAT2012-38045-C04-04 is acknowledged. A.M. also acknowledges project MAT2010-21156-C03-02 for financial support. We thank A. Tejeda for critical discussions. We acknowledge the Spanish Ministry of Economy and Competitiveness and Consejo Superior de Investigaciones Científicas for both financial support under project PIE 201060E013 and provision of synchrotron radiation facilities. We would also like to thank the SpLine beamline staff for their assistance during the SR experiments. GSS and the microscopy effort were supported by the ERC starting Investigator Award, grant #239739 STEMOX. Electron microscopy observations were carried out at the Centro Nacional de Microscopía Electrónica, CNME-UCM. Part of the XRD measurements were performed at the C.A.I. de Difracción de Rayos X-UCM. Computational calculations were performed at the Supercomputing Centre of Galicia (CESGA). Author contributions: O.R.d.l.F. conceived the project and coordinated the research. B.M., I.P. and O.R.d.l.F. prepared the samples and performed the Auger and LEED measurements and the ion beam modifications. B.M., I.P., A.M., J.L.-S, O.R.d.l.F., J.R.-Z., P.F. and G.C. performed the X ray diffraction measurements. G.S.-S. and M.V. performed the STEM measurements. J.I.B. and M.C.M. carried out the Density Functional Theory calculations. All authors wrote and revised the manuscript and extensively discussed the results and their interpretation. Supplementary Information: accompanies this paper at http://www.nature.com/ naturecommunications., A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces., Unión Europea. FP7, Ministerio de Economía y Competitividad (MINECO), España, Consejo Superior de Investigaciones Científicas (CSIC), España, ERC starting Investigator Awards, European Research Council (ERC), Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub