8052
J. Phys. Chem. B 1997, 101, 8052-8053
Nanostructured Crystalline TiO2 through Growth Control and Stabilization of Intermediate Structural Building Units T. Moritz, J. Reiss, K. Diesner, D. Su, and A. Chemseddine*,† Hahn-Meitner-Institut, Bereich Physicalische Chemie, Abt. CD, Glienicker Strasse 100, D-14109 Berlin, Germany ReceiVed: February 10, 1997; In Final Form: June 4, 1997X
The present new approach to building nanocrystals of materials with more complex crystal structures is based on the concept of progressive condensation of an intermediate structural unit. A simple method to control the growth of TiO2 nanocrystallites and the formation of nanostructured TiO2-based materials is presented. The method used to form these materials is based on controlling the hydrolysis and polycondensation of titanium alcoxide using organic ligands in order to build and stabilize intermediate building units (slabs). TiO2 particles with different sizes and shapes are obtained, simply by changing the titanium/cation ratio, and exhibit the anatase crystal structure. The small clusters formed condense, leading to well-defined nanocrystallites, which in turn self-assemble into superlattices.
Introduction Control over crystal structure, shape, surface chemistry, and organization of titania nanoparticles has been of interest in various applications, such as in the fabrication of solar cells1 and in photocatalysis.2 The high chemical reactivity of the available precursors such as alcoxides or chloride toward hydrolysis leads generally to a mixture of different polymeric species, gels, or amorphous precipitates. Generally a peptization process is required to convert these polymeric species into crystalline particles. The control over size and shape is difficult during these dissolution-growth processes. In this Letter, we present a new approach to control the growth of titanium dioxide nanocrystallites in the anatase crystal structure, a structure that can be described as a stacking of intermediate building unit (slabs). The present method (Scheme 1) consists of adjusting the hydrolysis conditions to build and stabilize these units using an organic base such as (H3C)4N+‚OH(TMA). This base catalyzes probably a complete hydrolysis of all alcoxide groups. At the same time TMA provides an organic cation to assist and direct the polycondensation process (nucleation and growth), by retaining the intermediate structural units or clusters. These units are well stabilized for a Ti:TMA ratio of about 0.83 and have the property of crystallizing into larger nanocrystallites, which can then self-assemble into superlattices with dimensions greater than micrometer length (Figure 1). An increase in the Ti:TMA (R) ratio leads to their condensation into larger TiO2 particles with the anatase crystal structure. In a typical preparation of particles or superlatices, an 2-propanol solution of titanium isopropoxide (0.6 mL of titanium isopropoxide in 120 mL of 2-propanol) is added to an aqueous solution of tetramethylammonium hydroxide (TMAOH) (0.82 mL in 300 mL of water) in a three-necked flask fitted with a condenser attached. The R values were changed by simply changing the TMAOH concentration of the aqueous solution. Complete precipitation occurs immediately for high R values, but the solution only turns slightly cloudy for low R values. * Corresponding author. † Visiting professor for the winter quarter at the Molecular Design Institute, Department of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA. X Abstract published in AdVance ACS Abstracts, September 15, 1997.
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SCHEME 1: Concept of Progressive Condensation of Intermediate Structural Building Units
Each mixture was then heated until the formed precipitate dissolved. The dissolution time varied from minutes to hours depending on the value of R. The resultant transparent solution was investigated by optical spectroscopy and high-resolution transmission electron microscopy (TEM), and then it was concentrated in a rotary evaporator and deposited on a silicon(100) wafer for powder X-ray diffraction analysis. A representative TEM image of a thin film of the hydrolysis products obtained for R ) 0.83 is shown in Figure 1a. A regular hexagonal array of uniform nanocrystallites is formed over micrometer length scales. The high-magnification image in Figure 1b shows packed hexagonal shaped nanocrystallites forming a superlattice. Electron diffraction confirms the periodicity and the hexagonal structure of the superlattice. The size of the crystallite is about 13 nm, while the spacing between the center of two adjacent nanocrystallites is about 14 nm. The © 1997 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 101, No. 41, 1997 8053
Figure 1. (a, left) Regular hexagonal array of uniform nanocrystallites obtained for R ) 0.83. (b, right) HRTEM showing packed hexagonal shaped nanocrystallites with a diameter of about 13 nm.
figure clearly indicates a two-dimensional ordering in the lateral dimensions of the substrate. The powder X-ray diffraction pattern of these assemblies processed into films shows a series of harmonics at interdistances of 16.56, 8.3, and 5.53 Å, indicating an orientation in the normal direction to the substrate and the formation of a multilayer film. The internal structure of the nanocrystallites is still under investigation. Preliminary results of HRTEM shows crystal anatase slabs packed in an organized manner. In some samples, competing structures are observed. More details on the chemical nature, bonding, and crystal structure will be provided in a full paper using elemental analysis, IR and optical spectroscopies, HRTEM, and X-ray diffraction analysis of this new type of nanostructured materials.3 The morphologies of the polycondensation product observed by HRTEM depend strongly on R. Particles with a triangular prismatic shape with a relatively narrow size distribution are
obtained for R of about 3. High-resolution microscopy clearly shows lattice planes with an interplanar distance of 3.52 Å corresponding to the 101 planes of the anatase crystal structure. Powder X-ray diffraction shows relatively broad anatase Bragg peaks due to the finite size of the particles. A decrease in R subsequently leads to further broadening of these peaks due to a decrease in the size of the inorganic core. This work opens the door to the synthesis and isolation of polytitanate anions on one hand and to a new level of designing nanomaterials with complex crystal structures on the other hand. References and Notes (1) O’Regan, B.; Graetzel, M. Nature 1991, 353, 737. (2) Chemseddine, A.; Boehm, H. P. J. Mol. Catal. 1990, 60, 295. (3) Chemseddine, A.; et al. Manuscript in preparation.
