Published on Web 10/10/2007 Anatase-TiO2 Nanomaterials: Analysis of Key Parameters Controlling Crystallization Marcos Ferna´ndez-Garcı´a,*,† Carolina Belver,† Jonathan C. Hanson,‡ Xianqin Wang,‡ and Jose´ A. Rodriguez‡ Contribution from the Instituto de Cata´lisis y Petroleoquı´mica, CSIC, C/Marie Curie 2, Cantoblanco, 28049-Madrid, Spain, and Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973 Received June 5, 2007; E-mail: [email protected] Abstract: Nanoparticulated TiO2 materials with anatase structure were synthesized by using a microemulsion method. The structural characteristics of the amorphous solid precursors and their evolution during thermal treatments were studied by using X-ray absorption structure (X-ray absorption near edge structure XANES and extended X-ray absorption fine structure EXAFS), XRD-PDF (X-ray diffraction-pair distribution function), and infrared spectroscopy. Concerning the precursor materials, XANES and EXAFS showed a local order 4+ closely related to that of the anatase structure but containing defective, undercoordinated Ti5c species in 4+ addition to normal Ti6c species. The PDF technique detects differences among samples in the local order (below 1 nm) and showed that primary particle size varies throughout the amorphous precursor series. The physical interpretation of results concerning the amorphous materials and their evolution under thermal treatment gives conclusive evidence that local, intraparticle ordering variations determine the temperature for the onset of the nucleation process and drive the solid behavior through the whole crystallization process. The significance of this result in the context of current crystallization theories of oxide-based nanocrystalline solids is discussed. 1. Introduction decrease of the primary particle size with concomitant potential enhancement of the chemical activity (connected with several Titanium dioxide (TiO2) is one of the most prominent oxide materials for performing various kinds of industrial applications structural and electronic size-related effects) and also of the related to catalysis among which the selective reduction of NOx photochemical/photophysical activities by reduction of light 12 in stationary sources1,2 and photocatalysis for pollutant elimina- scattering. In TiO2 materials, the so-called “quantum-confine- tion3 or organic synthesis4 appear as rather important. Additional ment” or “quantum-size effect” is restricted to very low sizes, applications include its use as a white pigment in paintings,5 as below 10 nm, due to their rather low exciton Bohr radii. This part of photovoltaic devices6 and sensors,7 as a food additive,8 would mean that a significant part of the potential novel in cosmetics9 and as a potential tool in cancer treatment.10 chemical or physical applications needs to be carefully explored 12-14 Experimental approaches to scale down the TiO2 primary in the range of a few nanometers. particle size to the nanometer scale are now actively investigated Anatase, rutile, and brookite are the most common poly- in order to improve its current applications in sensor or catalysis morphs of titania (TiO2). Anatase is the dominant outcome of fields and to reach more advanced ones like its use in the vast majority of liquid-solid and gas-solid transformation- 11 electrochromic devices. As a general result, the nanostructure based preparation methods.15,16 This is a consequence of being induces an increase of surface area by the corresponding a stable polymorph at working temperatures for sizes (e.g., primary particle size) below ca. 15 nm.17,18 However, samples † CSIC. ‡ Brookhaven National Laboratory. often contain some brookite as an impurity (in low percentage) (1) Bosh, H.; Janssen, F. Catal. Today 1988, 2, 369. or, alternatively, mixtures of anatase and rutile, as the presence (2) Forzatti, P. Catal. Today 2000, 62, 51. (3) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahneman, D. W. Chem. ReV. of impurities or phase mixture variables are adjusted or 1995, 95, 69. (4) Maldoti, A.; Molinari, A.; Amadeni, R. Chem. ReV. 2002, 102, 3811. (5) Johnson, R. W.; Thieles, E. S.; French, R. H. Tappi. J. 1997, 80, 233. (12) Ferna´ndez-Garcı´a, M.; Martı´nez-Arias, A.; Hanson J. C.; Rodriguez, J. A. (6) Kalyanasendevan, K.; Gratzel, M. in Optoelectronics Properties of Inorganic Chem. ReV. 2004, 104, 4063. Compounds; Roundhill, D. M., Fackler, J. P., Eds.; Plenum: New York, (13) Zhang, H.-J.; Wang, L.-S. J. Am. Chem. Soc. 2007, 129, 3022. 1999; pp 169-194. (14) Borello, E.; Lamberti, C.; Bordigas, S.; Zecchina, A.; Otero-Arean, C. Appl. (7) Sheveglieri, G., Ed. Gas sensors; Kluwer: Dordrecht, 1992. Phys. Lett. 1997, 71, 2319. (8) Phillips, L. G.; Barbeno, D. M. J. Dairy Sci. 1997, 80, 2726. (15) Synthesis, Properties and Applications of Solid Oxides; Rodrı´guez, J. A., (9) Selhofer, H. Vacuum Thin Films (August, 1999) 15. Ferna´ndez-Garcı´a, M., Eds.; John Wiley: New York, 2007; Chapters 4, 5, (10) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 20. 1,1. (16) Dietbold, U. Surf. Sci. Rep. 2003, 48, 53. (11) Bonhole, P.; Gogniat, E.; Gratzel, M.; Ashrit, P. V. Thin Solid Films 1999, (17) Zhang, H.; Bandfield, J. F. J. Mater. Chem. 1998, 8, 2073. 350, 269. (18) Hu, Y.; Tsai, H. L.; Hung, C. L. Mater Sci. Eng. A 2003, 344, 209. 13604 9 J. AM. CHEM. SOC. 2007, 129, 13604-13612 10.1021/ja074064m CCC: $37.00 © 2007 American Chemical Society Analysis of Key Parameters Controlling Crystallization ARTICLES modulated by changing the preparation conditions (temperature, Table 1. Water/Titanium Molar (R) and Water/Surfactant Molar 12,15,18 (ω) Ratios Used during the Microemulsion Synthesis of precursor concentration, etc.). Ti-containing Solids As the majority of the practical applications of TiO above- 2 sample ω R mentioned are linked to the anatase structure and this phase 12,16,17 T 18 110 stability is in turn related to the primary particle size, Tw 18 110 control of the nanostructure appears as a basic objective in the TA 4.5 220 optimization of the industrial-oriented properties of anatase. TB 3 220 Understanding the crystallization process of anatase from TwB 3 220 amorphous solid materials typically obtained at the initial synthesis step appears of prime importance in this context. In a kinetic mechanism, which is ultimately validated by a multi- more general view, results for titania are also important in that parameter fitting procedure of the experimental data. The they elucidate the general way in which primary particle size assumptions are therefore inherent to the mechanism and are can affect the behavior of oxide materials. only self-consistently proven by fitting; consequently, they may Upon heating, amorphous Ti-containing materials would drive misleading interpretations. transform on anatase.12,15,16,19 Exarhos et al. were the first to In this work we examined the crystallization of anatase from study the kinetics of the corresponding transition of amorphous amorphous titania powders using an essentially assumption-free films supported on silica substrates.19 Under hydrothermal experimental approach. Using bulk and surface structural conditions, several groups gave evidence of the media influence characterization techniques sensitive to both local and long range (pH, presence of ions) on the crystallization mechanism and order, we will prove that the anatase nucleation onset is pointed out that the rate-determining step can be related to the exclusively dependent on amorphous intraparticle structural incorporation of new building units at the surface of the growing characteristics and then that anatase crystallization is essentially anatase crystal (solid-type step) and/or the dissolution of small free of interface interferences. It will be shown that this occurs anatase particles (Ostwald ripening; liquid-type step).20-23 In with samples crystallizing in the 400 to 600 °C temperature other studies, using sol-gel24,25 or microemulsion26 procedures, interval, indicating the invariance of the mechanism with details of the solid-state transformation mechanism leading to temperature. We will also show that the structural characteristics the anatase phase have been reported. Quantitative analysis of of the initial, amorphous powders determine the final morphol- the key kinetic parameters controlling the amorphous titania to ogy of the anatase nanoparticles, inferring in this manner the anatase transformation has been attempted in liquid media under main parameters of the growth step. hydrothermal conditions21 and for solid-solid transformations 2. Experimental Section concerning titania films,19 powders,25,26 or mesostructured27 - systems. The physical characteristics of the transformation (e.g., Preparation of the TiO2 Oxide Precursors. Ti O precursor onset and reaction rate - energy of activation) in air should be materials were prepared using a microemulsion synthetic route by different from those in liquid media as, obviously, dissolution addition of titanium(IV) isopropoxide (Aldrich) to an inverse emulsion steps are critically involved in the latter. The broad range of containing an aqueous phase (50 mL) dispersed in n-heptane (85/10 v/v vs H O; Panreac) and using either Triton X-100 (variable quantity; temperatures where amorphous titania solids transform into 2 Aldrich) or Tween 85 (variable quantity; Aldrich) as surfactant. In the anatase (see below) also tells of a wide range of situations within case of Triton X-100 and following standard recipes, 1-hexanol (105/ the air-assisted transformation occurring in solid materials. In 100 v/v vs surfactant; Aldrich) was utilized as a cosurfactant.30 The fact, crystallization has been considered to be controlled by resulting mixture was vigorously stirred for 24 h, centrifuged, decanted, either surface19,26,27 or interface25 nucleation processes. The rinsed under stirring five consecutive times with methanol (2), water subsequent step, e.g., growth of the anatase particles, is of lesser (2), and acetone (1), in order to eliminate any portion from the organic importance although fine works have been devoted to its study.28 and surfactant media, and dried at 110 °C for 24 h.