Band Gap Narrowing of Titanium Oxide Semiconductors by Noncompensated Anion-Cation Codoping for Enhanced Visible-Light Photoactivity Wenguang Zhu, 1,2 Xiaofeng Qiu, 3 Violeta Iancu, 2 Xing-Qiu Chen, 1 Hui Pan, 4 Wei Wang, 4 Nada M. Dimitrijevic, 5,6 Tijana Rajh, 5 Harry M. Meyer III, 1 M. Parans Paranthaman, 3 G. M. Stocks, 1 Hanno H. Weitering, 2,1 Baohua Gu, 4 Gyula Eres, 1 and Zhenyu Zhang 1,2 1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 2 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA 3 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 4 Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 5 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA 6 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA (Received 11 September 2009; published 23 November 2009) ‘‘Noncompensated n-p codoping’’ is established as an enabling concept for enhancing the visible-light photoactivity of TiO 2 by narrowing its band gap. The concept embodies two crucial ingredients: The electrostatic attraction within the n-p dopant pair enhances both the thermodynamic and kinetic solubilities, and the noncompensated nature ensures the creation of tunable intermediate bands that effectively narrow the band gap. The concept is demonstrated using first-principles calculations, and is validated by direct measurements of band gap narrowing using scanning tunneling spectroscopy, dramatically redshifted optical absorbance, and enhanced photoactivity manifested by efficient electron-hole separation in the visible-light region. This concept is broadly applicable to the synthesis of other advanced functional materials that demand optimal dopant control. DOI: 10.1103/PhysRevLett.103.226401 PACS numbers: 71.20.Nr, 61.72.S, 84.60.h The development of advanced materials for alternative and sustainable energy applications is an extremely active research area of great visibility and importance. In particu- lar, much effort has been devoted to searching for new types of catalytic materials that can readily split water to generate hydrogen as an environmentally friendly fuel via photolysis using the abundant energy from sunlight. In such efforts of property optimization, one often encounters the challenging need to control precisely the concentration and the location of foreign dopants in a host system. Similar issues are also frequently encountered in other areas, such as control of magnetic dopants in diluted magnetic semiconductors for spintronic applications. TiO 2 is one of the most promising photocatalysts for solar energy utilization and environmental cleanup [1–8]. However, the photoreaction efficiency of TiO 2 is severely limited by its large intrinsic band gap (>3 eV) capable of absorbing only the ultraviolet portion of the solar spectrum [3,5]. A crucial prerequisite for enhancing the solar energy conversion efficiency is to enable TiO 2 to absorb the more abundant visible light by reducing its band gap below 2 eV [5,9]. Since the seminal discovery of Fujishima and Honda [1], numerous attempts have been made to optimize the band gap of TiO 2 by different doping schemes [5,6,10–12]. However, an overwhelming body of the literature reported efforts of trial-and-error nature, lacking a major conceptual breakthrough as the guiding principle. This standing ob- stacle is inherently tied to the fundamental limitations that the thermodynamic solubility in substitutional doping of TiO 2 is extremely low for most dopants, especially for p-type doping [4,6]. As a result, most of the dopants reside at undesirable interstitial sites, which not only compromise the effectiveness of band gap narrowing but also provide numerous recombination centers that are responsible for the loss of photogenerated electron-hole pairs [13,14]. In this Letter, we report a noncompensated n-p co- doping concept to overcome these fundamental limitations. First, the Columbic attraction between the n- and p-type dopants with opposite charge state substantially enhances both the thermodynamic and, in particular, the kinetic solubilities of the dopant pairs in concerted substitutional doping. More profoundly, the noncompensated nature of the n-p pairs consisting, for example, of a single acceptor and a double donor ensures the creation of intermediate electronic bands in the gap region, effectively narrowing the band gap. Controlled creation of such intermediate bands is also highly desirable for solar cell applications [15]. We further show that the position and magnitude of the intermediate bands can be tuned by choosing different combinations and concentrations of the noncompensated n-p pairs. These findings establish the noncompensated n-p codoping concept as a powerful guiding principle in future design of photocatalysts and other functional materials. The concept is first demonstrated quantitatively using first-principles calculations, focusing on the band gap narrowing and the enhanced thermodynamic and kinetic solubilities. These studies predict Cr-N as the preferred PRL 103, 226401 (2009) PHYSICAL REVIEW LETTERS week ending 27 NOVEMBER 2009 0031-9007= 09=103(22)=226401(4) 226401-1 Ó 2009 The American Physical Society