Structural Characterization of SiO 2 and Al 2 O 3 Zener-Pinned Nanocrystalline TiO 2 by NMR, XRD and Electron Microscopy Luke A. O’Dell, ² Shelley L.P. Savin, ‡ Alan V. Chadwick, ‡ and Mark E. Smith* ,² Department of Physics, UniVersity of Warwick, CoVentry, CV4 7AL, United Kingdom, and School of Physical Sciences, UniVersity of Kent, Canterbury, Kent, CT2 7NH, United Kingdom ReceiVed: May 23, 2007; In Final Form: July 9, 2007 Nanocrystalline TiO 2 samples were prepared using sol-gel techniques in a pure form and also Zener pinned with either silica or alumina to reduce the growth of the crystallites during the annealing process and to stabilize the anatase phase at high temperatures. These samples were studied using 17 O, 27 Al, and 29 Si nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and electron microscopy. The silica pinning phase was found to successfully restrict nanocrystal growth as well as stabilize the anatase phase at temperatures up to 800 °C. The alumina phase had less of a pinning effect, and it reacted with the TiO 2 to form tialite. 17 O NMR relaxation time measurements on enriched samples showed that the presence of the pinning phases also reduced the activation energy for the oxygen ion diffusion mechanism. 1. Introduction TiO 2 thin films, which are most commonly manufactured using the sol-gel method, 1 are used in applications such as solar cells, lithium batteries, and humidity or oxygen sensors. 2,3 TiO 2 can exist in three different crystalline phases, each consisting of TiO 6 octahedra arranged in different formations, with the sol-gel method tending to produce the anatase phase after heating at 350 °C. 3-5 Depending on crystallite size, distribution, and arrangement, this can transform into the rutile phase at temperatures between 400 and 1000 °C, 6 but usually between 600 and 800 °C in the nanocrystalline case. 3,7 In some cases, the third phase, brookite, is reported to appear as an intermediate between anatase and rutile TiO 2 at temperatures below 500 °C. However, brookite rarely occurs in a pure form, and anatase and rutile are the most common polymorphs. 6 In many applications of TiO 2 , one of these phases, in particular, may be advantageous. For example, the anatase phase has been shown to be more efficient than the rutile phase in photocatalysis, 8,9 and smaller crystal sizes are also advantageous in this application. 10 Anatase nanocrystals have been reported to be most stable at extremely small diameters (below 5-13 nm 11,12 ); however, they have been reported to exist at up to 80 nm. 13 Rutile crystals are known to grow much faster than anatase, 14 so, generally, rutile nanocrystals exhibit larger diameters. 11 To utilize nanocrystalline anatase TiO 2 in applica- tions that require high temperatures, a method is required to prevent the anatase-to-rutile phase transition and restrict the growth of the crystals when the material is heated. Zener pinning 15 has already been shown to restrict the growth of sol- gel prepared nanocrystalline metal oxides during the annealing stage. 16-19 This process involves the addition of a small amount of a second phase, such as silica or alumina, which exists as discrete particles at the nanocrystal interfaces and “pins” the grain boundaries in place by restricting surface diffusion and reducing the radius of curvature of the nanocrystals. In this paper, nanocrystalline TiO 2 samples have been manufactured with either 15% silica or 10% alumina by weight, amounts chosen in light of previous work. 20 These pinning phases have been characterized by 29 Si and 27 Al magic angle spinning (MAS) and nuclear magnetic resonance (NMR) respectively, and the TiO 2 nanocrystals have been studied using X-ray diffraction (XRD) and 17 O NMR to determine what effect the pinning particles have on the TiO 2 phases present at each temperature and on the oxygen ion mobility within the samples. Titania-silica nanocomposites have previously been shown to be more efficient photocatalysts than pure TiO 2 , and Ti- O-Si bonding, which has been studied using 17 O NMR, 21 is already reported to restrict nanocrystal growth. 22 TiO 2 containing 10% silica has been reported to show anatase nanocrystals with diameters of 16 nm after annealing at 500 °C. 22 The presence of highly dispersed alumina has also been shown to stabilize that anatase phase and enhance catalytic properties. 23 A TiO 2 sample containing 20% alumina remained as anatase up to 1100 °C, with the alumina phase existing as γ-Al 2 O 3 at 800 °C, δ-Al 2 O 3 and Al 2 TiO 5 at 900 °C, and R-Al 2 O 3 at 1100 °C. 23 So, Zener pinning nanocrystalline TiO 2 with silica or alumina should not only be advantageous in restricting the growth of the crystals, but the presence of the second phase may also improve the material’s photocatalytic properties. 2. Experimental Details 2.1. Sample Preparation. To prepare the unpinned sol-gel TiO 2 samples, 92.3 mL titanium iso-propoxide (Aldrich Chemi- cal Co.) was added to a 500 mL beaker. Water was added and the solution was stirred until the solution gelled. The white solid was filtered and dried at 80 °C. To manufacture the silica-pinned TiO 2 samples, 83 mL of titanium iso-propoxide and 15 mL of tetraethylorthosilicate (TEOS) were mixed and stirred for 1 h. A 10% v/v solution of ammonia solution in water was prepared and was added dropwise to the alkoxide solution until it gelled. The white solid was then dried at 80 °C. * To whom correspondence should be addressed. E-mail: M.E.Smith.1@ warwick.ac.uk, tel: +44 (0)24 7652 2380, fax: +44 (0)24 7669 2016. ² University of Warwick. ‡ University of Kent. 13740 J. Phys. Chem. C 2007, 111, 13740-13746 10.1021/jp0739871 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/23/2007