Numerical Investigations into the Tensile Behavior of TiO 2 Nanowires: Structural Deformation, Mechanical Properties, and Size Effects L. Dai, † C. H. Sow, †,‡ C. T. Lim, †,§,| W. C. D. Cheong, ⊥ and V. B. C. Tan* ,†,§ NUS Nanoscience & Nanotechnology InitiatiVe, Department of Physics, Department of Mechanical Engineering, and DiVision of Bioengineering, National UniVersity of Singapore, 117576 Singapore, and Institute of Materials Research and Engineering, 117602 Singapore Received September 8, 2008; Revised Manuscript Received December 23, 2008 ABSTRACT The mechanisms governing the tensile behavior of TiO 2 nanowires were studied by molecular dynamics simulations. Nanowires below a threshold diameter of about 10 Å transformed into a completely disordered structure after thermodynamic equilibration, whereas thicker nanowires retained their crystalline core. Initial elastic tensile deformation was effected by the reconfiguration of surface atoms while larger elongations resulted in continuous cycles of Ti-O bond straightening, bond breakage, inner atomic distortion, and necking until rupture. Nanowires have much better mechanical properties than bulk TiO 2 . Nanowires below the threshold diameter exhibit extraordinarily high stiffness and toughness and are more sensitive to strain rate. Titanium dioxide (TiO 2 ) has a wide range of applications such as in catalysts, photoelectronics, photochemistry, semi- conductors, and surface coatings. TiO 2 nanowires have been extensively studied by material researchers because they are well-suited for many experimental techniques and high- quality TiO 2 crystal materials are commercially available. 1,2 Many experimental works have been reported on the synthesis of nanowires via powder annealing, 3 sol-gel, 4,5 electrospinnning, 6 thermal evaporation, 7,8 etc. The most commonly identified crystal structure in actual TiO 2 nanow- ires is rutile 3,7,8 or anatase. 4,5,9,10 Besides experimental works, there have also been several attempts to formulate interatomic potentials for the compu- tational simulation of TiO 2 nanostructures. Matsui and Akaogi 11 proposed a scheme with a pairwise potential plus electronic charge distribution functions to describe the atomic interactions and successfully produced the geometrical parameters of the most common crystal structures of TiO 2 such as rutile, 12 anatase, 13 brookite, 14 and TiO 2 -II. 15 Swamy and Gale 16 introduced the quantum charge equilibration scheme 17 to the pairwise Morse potential and formed an MS-Q formula. This MS-Q potential was shown to accurately describe TiO 2 crystal geometries and properties. 18-20 How- ever, the MS-Q potential is computationally expensive and consequently its application is limited to small scale (up to hundreds of atoms) crystalline models. 20 Matsui and Akaogi‘s scheme remains the most practical approach for compara- tively large scale computational models and has been used to simulate TiO 2 nanoparticles 20 and nanoclusters. 21,22 Here, we report on the modeling and simulation of TiO 2 nanowires and elucidate the mechanisms associated with the deforma- tion, yield, and ultimate rupture of TiO 2 nanowires. We use the interatomic potential of Matsui and Akaogi, 11 simplified as eq 1, to describe the atomic interactions. In the equation, r ij is the interatomic distance between atoms i and j, q i and q j are the electric charge of the elements, and A, B and C are fitted parameters as shown in Table 1. The Ti-O interaction is considered to predominate material structure and properties as it is much stronger than that of O-O or Ti-Ti. * Corresponding author, mpetanbc@nus.edu.sg. † NUS Nanoscience & Nanotechnology Initiative, National University of Singapore. ‡ Department of Physics, National University of Singapore. § Department of Mechanical Engineering, National University of Sin- gapore. | Division of Bioengineering, National University of Singapore. ⊥ Institute of Materials Research and Engineering. Table 1. Parameter Values for Simplified Matsui and Akaogi’s Potential 11 q Ti (e) q O (e) A (eV) B (Å) C (eV·Å 6 ) 2.196 -1.098 16286 0.194 1215 NANO LETTERS 2009 Vol. 9, No. 2 576-582 10.1021/nl8027284 CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009