Intrinsic Nitrogen-doped CVD-grown TiO 2 Thin Films from All-N-coordinated Ti Precursors for Photoelectrochemical Applications** By Sun Ja Kim, Ke Xu, Harish Parala, Radim Beranek, Michal Bledowski, Kirill Sliozberg, Hans-Werner Becker, Detlef Rogalla, Davide Barreca, Chiara Maccato, Cinzia Sada, Wolfgang Schuhmann, Roland A. Fischer, and Anjana Devi* N-doped titanium dioxide (TiO 2 ) thin films are grown on Si(100) and indium tin oxide (ITO)-coated borosilicate glass substrates by metal-organic (MO)CVD. The intrinsic doping of TiO 2 thin films is achieved using all-nitrogen-coordinated Ti precursors in the presence of oxygen. The titanium amide-guanidinate complex, [Ti(NMe 2 ) 3 (guan)] (guan ¼ N,N 0 -diisopropyl-2-dimethyl- amidoguanidinato) has been developed to compensate for the thermal instability of the parent alkylamide [Ti(NMe 2 ) 4 ]. Both of these amide-based compounds are tested and compared as precursors for intrinsically N-doped TiO 2 at various deposition temperatures in the absence of additional N sources. The structure and morphology of TiO 2 thin films are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Rutherford back scattering (RBS), nuclear reaction analysis (NRA), and secondary ion mass spectrometry (SIMS) analyses are performed to determine N content and distribution in the films. The optical and photoelectrochemical properties of TiO 2 thin films on ITO substrates are also examined. N-doped TiO 2 thin films, grown from [Ti(NMe 2 ) 3 (guan)] at 600 8C, exhibit the lowest optical absorption edge (3.0 eV) and the highest visible light photocurrent response. When compared to undoped TiO 2 , while in UV light photoconversion efficiency decreases significantly, the intrinsically N-doped TiO 2 shows enhanced photocurrents under visible light irradiation. Keywords: MOCVD, N-coordinated Ti precursor, N-doping, Photocurrent, TiO 2 thin films 1. Introduction TiO 2 is an important material for many applications, including pigments, UV protection, and dye-sensitized solar cells. [1–3] It has also gained significant attention as an active photocatalyst for water splitting [4] because of its excellent stability, and its band edges straddle the reduction and oxidation potentials of water. [5] Because of its large bandgap (3.0 eV for rutile and 3.2 eV for anatase), however, TiO 2 absorbs only a small fraction (less than 3%) in the UV region of the sunlight reaching the surface of the earth. [6] To broaden the light absorption range of TiO 2 and to improve its harvesting of visible light, one possible method is to dope it with specific elements, [7–10] such as nitrogen. [9,11] N-doped TiO 2 thin films can be grown by several methods, including CVD, reactive magnetron sputtering, pulsed laser deposition, sol-gel, etc. [12–16] Using physical vapor deposi- tion (PVD) methods, nitrogen doping of the TiO 2 films can be extrinsically done with N 2 , [14] NH 3 , or CH 3 CN [15] gas introduction. Titanium tetrachloride (TiCl 4 ) or titanium tetraisopropoxide ([Ti(O i Pr) 4 ]) are conventionally used as precursors for the CVD of TiO 2 films. With these precursors, however, additional N sources, such as NH 3 [17] or N 2 H 4 [18] are required for extrinsic doping to obtain N-doped TiO 2 films. Taking advantage of using a tailored precursor, [19] the incorporation of target elements such as nitrogen into the films can be achieved intrinsically by MOCVD. Accord- ingly, an alternative strategy could be to employ the all- nitrogen-coordinated Ti precursors such as tetrakis(di- methylamido)titanium(IV), [Ti(NMe 2 ) 4 ], which contains DOI: 10.1002/cvde.201206996 Full Paper [*] Prof. A. Devi, S. J. Kim, K. Xu, Dr. H. Parala, Prof. R. Beranek, M. Bledowski, Prof. R. A. Fischer Inorganic Chemistry II, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, 44801 Bochum (Germany) E-mail: anjana.devi@rub.de K. Sliozberg, Prof. W. Schuhmann Analytical Chemistry, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, 44801 Bochum (Germany) Dr. H.-W. Becker, Dr. D. Rogalla Dynamitron Tandem Laboratory of RUBION, Ruhr-University Bochum, 44801 Bochum (Germany) Dr. D. Barreca CNR-ISTM and INSTM, Department of Chemistry, Padova University, 35131 Padova (Italy) Dr. C. Maccato Department of Chemistry and INSTM, Padova University, 35131 Padova (Italy) Dr. C. Sada Department of Physics and Astronomy, Padova University, 35131 Padova (Italy) [**] S. J. K. expresses her appreciation to the National Institute for Inter- national Education of Korea (NIIED) and the Research School of Ruhr-University Bochum (RUB-RS) for providing financial support. The authors thank the Materials Research Department of RUB for supporting this work. R.B. and M.B. are thankful for financial support by the MIWFT-NRW. Chem. Vap. Deposition 2013, 19, 45–52 ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 45