Synthesis of monodisperse nanoparticles of barium titanate: toward a generalized strategy of oxide nanoparticle synthesis.

The physical properties of materials as they progress from the bulk to the nanoscale regime (1-100 nm) continue to be of immense interest and increasing importance for future technological applications. Nanocrystals display properties generally found to be scientifically different from the bulk material or the atomic or molecular species from which they can be derived. Examples of this phenomenon are manifold.1-3 The study of semiconductor nanocrystal quantum dots is a well-established field, yielding rich, useful ,and application-oriented research.4 A coherent field of study is currently emerging for the systematic examination of nanocrystal oxides with the aim of producing nanoparticles with narrow size distributions and size tunability in the nanoscale regime. Among the important characteristics of nanoparticle systems are facile manipulation and reversible assembly which allow for the possibility of incorporation of nanoparticles into electric, electronic, or optical devices. Such “bottom up” or “self-assembly” approaches are the benchmark of nanotechnology. Ferroelectric materials have been under investigation due to the prospect that the stable polarization states could be used to encode the 1 and 0 of the Boolean algebra that form the basis of memory and logic circuitry (FRAM). The family of complex ferroelectric oxides such as BaTiO3, Pb(Zr,Ti)O3, and (Ba,Sr)TiO3 has far reaching applications in the electronics industry for transducers, actuators, and high-k dielectrics.5 Ferroelectricity is the phenomenon designated to crystals in possession of a spontaneous polarization and hysteresis effects with respect to the dielectric displacement in the presence of an applied electric field. The precise nature of ferroelectricity at the nanoscale, such as critical size dependent suppression in particles and thin films,6 is much debated in the literature.7 Ferroelectric oxide nanoparticles with high anisotropic polarizibility offer the possibility of taking advantage of this dielectric property within the context of nanoscale materials science. Attempts to understand further the finite size effect in ferroelectric oxides led Zhang and co-workers to synthesize free-standing nanoparticle films of Pb(Zr,Ti)O3 (PZT) by a sol-gel method.8 We have synthesized samples of monodisperse nanoparticles of barium titanate, BaTiO3, with diameters ranging from 6 to 12 nm. Barium titanate is a room-temperature displacive ferroelectic in the bulk with a Curie temperature (ferroelectric-paraelectric transition temperature) recorded around 120°. Solution-phase chemical routes can provide the means of producing high purity, crystalline nanoscale materials of a specific and variable size. We propose that sol-gel techniques combined with nanoparticle synthesis methods offer the possibility of a generalized approach to the production of single and complex oxide nanoparticle syntheses. In our procedure for BaTiO3 nanoparticles, a single bimetallic alkoxide molecular precursor is used to ensure the correct stoichiometry of the product. The reactant precursor is highly sensitive to moisture and there is no need for acid/base catalyzed hydrolysis in this case (notwithstanding the generation of ethyl hexanoic acid). The use of alkoxides as precursors for the preparation of metal oxides has been explored with a view to nanostructuring and deposition.9,10 In the synthesis developed here, barium titanium ethyl hexano-isopropoxide (Alfa Aesar), BaTi(O2CC7H15)[OCH(CH3)2]5 (1 mmol), is injected into a mixture of diphenyl ether (Aldrich), (C5H5)2O (50 mL), and stabilizing agent oleic acid (Aldrich), CH3(CH2)7CHdCH(CH2)7CO2H (3 mmol), at 140 °C under argon or nitrogen. The absence of water at this stage prevents the premature hydrolysis of the molecular precursor. Excess 2-propanol is removed under vacuum. The mixture is cooled to 100 °C and a 30 wt % hydrogen peroxide solution (Aldrich) (0.9 mL) is injected through the septum (vigorous exothermic reaction). The solution is maintained in a close system and stirred at 100 °C (under a mild reflux with the remaining water content) over 48 h to promote further hydrolysis and crystallization of the product in an inverse micelle condition. Elemental ICP analysis of the nanoparticle samples extracted at different reaction intervals showed the Ba:Ti ratio to be equimolar. TEM measurements were recorded on a Philips EM 430 (300 kV) for lattice imaging (Figure 1a) and to determine particle size/ distribution by measuring cross diagonals of approximately 100 nanoparticles. Elemental analysis, using energy-dispersive X-ray fluorescence operating in STEM mode, confirmed the presence of Ba, Ti, and O in individual nanoparticles and ensembles. Figure 1b shows that the product consists of regularly shaped nearspherical monodisperse crystalline particles with an average diameter of 8 nm. Controlled evaporation of a hexane dispersion of nanoparticles collected shortly after injection induces selfassembly, as shown in Figure 1c. Figure 1d shows a larger area view of a closs-packed superlattice of 8 nm diameter barium titanate nanoparticles. Strong van der Waals or dipolar interactions drive the formation of the superlattices which can extend over several microns. Crystal structure characterization of the nanoparticles was provided by TEM and XRD. Figure 1a shows a lattice image of a BaTiO3 particle effectively viewed along the [110]c projection. The appearance is of a single crystal domain. The selected area electron diffraction pattern (Figure 1b, inset) can be indexed to cubic symmetry, indicating BaTiO3 to be in the perovskite cubic * To whom correspondence should be addressed. † Columbia University. ‡ IBM T. J. Watson Research Center. (1) Brus, L. E.; Trautman, J. K. Philos. Trans. R. Soc. London Ser. A-Math. Phys. Eng. Sci. 1995, 353, 313-321. (2) Alivisatos, A. P. Science 1996, 271, 933. (3) Heath, J. R. Acc. Chem. Res. 1999, 32, 388. (4) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545-610. (5) Hill, N. A. J. Phys. Chem. B. 2000, 104, 6694-6709. (6) Maruyuma, T.; Saitoh, M.; Skai, I.; Hidaka, T. Appl. Phys. Lett. 1998, 73, 3524-3526. (7) Ghosez, P.; Rabe, K. M. Appl. Phys. Lett. 2000, 76, 2767-2769. (8) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. J. Am. Chem. Soc. 2001, 123, 4344-4345. (9) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613-1614. 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