The Morphology Control of BaTiO 3 Particles Synthesized in Water and a Water/Ethanol Solvent Marjeta Ma cek Kr zmanc, ‡,† Ines Bra cko, ‡ Bojan Budi  c, § and Danilo Suvorov ‡ ‡ Advanced Materials Department, Jo zef Stefan Institute, Ljubljana 1000, Slovenia § Laboratory for Analytical Chemistry, National Institute of Chemistry, Ljubljana 1000, Slovenia The experimental conditions for the growth of shape-controlled BaTiO 3 particles in NaOH and Ba(NO 3 ) 2 aqueous and water/ ethanol solutions using various TiO 2 -containing precursors were studied at 80°C–100°C. The different chemistries and physical characteristics of the precursors resulted in different BaTiO 3 formation rates and morphologies . Nanocrystalline anatase led to irregularly shaped BaTiO 3 particles, whereas star-like, single-crystalline BaTiO 3 particles grew from aerogel TiO 2 and sodium titanate (NT) belts in alkaline aqueous solu- tions. With the addition of ethanol, the star-like BaTiO 3 parti- cles changed to square-like, the size of which decreased with an increase in the ethanol content. The electron microscopy obser- vations supported a dissolution–precipitation mechanism as the primary reaction mechanism for the formation of BaTiO 3 nanocrystals, which further aggregated into single-crystalline star- or square-like particles by oriented attachment. The mod- ification in the water solution with ethanol is believed to influ- ence both the nucleation and aggregation process and consequently influence the particle shape and size. I. Introduction BaTiO 3 has been intensively investigated for a wide range of applications in the field of electroceramics, due to its out- standing dielectric, ferroelectric, and piezoelectric properties. The recent trend for high-performance and miniaturized microelectronic devices has increased the interest in investi- gating low-dimensional BaTiO 3 particles. . In particular, zero- (0-D; nanoparticles), one- (1-D, nanorods), and two- (2D; plates) dimensional BaTiO 3 nanostructures have attracted many attention because of their size- and shape-dependent dielectric and ferroelectric properties. 1 The defined shape of the BaTiO 3 particles also represents an advantage when it comes to assembling the particles in thin films or in three- dimensional (3D) device elements for miniaturized ferroelec- tric devices. The majority of low-temperature synthesis routes produce spherical BaTiO 3 particles with a cubic crystal struc- ture. Elongated and other shape-controlled BaTiO 3 particles were normally prepared by employing a template. 2 However, there is a great deal of interest in the methods that enable the direct creation of BaTiO 3 particles with a defined shape. Several authors showed that other shapes of BaTiO 3 parti- cles, besides spherical, could be obtained using different hydrothermal synthesis conditions. 1,3–8 Bao et al. reported the formation of a variety BaTiO 3 morphologies, which grew from Na 2 Ti 3 O 7 nanostructures in Ba(OH) 2 water solutions in the temperature range from 70°C to 150°C. 1 Depending on the Ba(OH) 2 concentration, the temperature, and the nature of the precursor, the BaTiO 3 particles exhibited the shape of corals, stars, swords, or cubes. Joshi et al. managed to prepare single-crystalline BaTiO 3 nanowires from TiO 2 , Ba(OH) 2 , and ammonia. 3,4,7 These studies dealt mainly with a structural characterization of the BaTiO 3 , although the authors did not provide a detailed explanation of the forma- tion mechanism of this very interesting anisotropic crystal growth. While the thermodynamic stability of BaTiO 3 under hydrothermal conditions is well established, different expla- nations of the mechanism of BaTiO 3 formation exist in the literature. The proposed mechanisms, which are all based on a general nucleation-growth process, are grouped into two major categories: (a) a dissolution–precipitation and (b) an in-situ transformation. According to the dissolution–precipi- tation mechanism, the TiO 2 -containing particles dissolve and form Ti(OH) x 4Àx soluble species, which then react with the barium ions in the solution and precipitate BaTiO 3 . The BaTiO 3 particles can grow on the TiO 2 -containing substrate (heterogeneous nucleation) or form directly in the solution (homogeneous nucleation). 9,10 An in-situ transformation mechanism assumes that barium reacts with the TiO 2 on the surface forming a layer of BaTiO 3 through which additional barium must diffuse until the reaction is completed. W. Hertl proposed that the rate-determining steps (RDSs) in the in-situ transformation mechanism are the reaction between barium and TiO 2 [at high Ba(OH) 2 concentration] and the diffusion of barium species through the product layer [at low Ba(OH) 2 concentration]. 11 It is believed that the product layer slows the velocity of the reaction. A typical in-situ transformation mechanism was observed by Hu et al., who prepared nanocrystalline microspheres of BaTiO 3 , which pre- served the morphology and size of the precursor titania par- ticles. 12 In contrast, Eckert et al. observed that both types of mechanism competed for rate control. 9 A dissolution–precipi- tation process dominated at the beginning of the reaction, whereas an in-situ mechanism prevailed at longer reaction times. The dominance of the dissolution–precipitation mecha- nism for BaTiO 3 formation was also proved by an in-situ time-resolved neutron-diffraction study. 13 We assumed that contradictory experimental observations of the BaTiO 3 formation mechanisms that were reported by different authors resulted from different experimental condi- tions. In addition to the temperature and the pH, the morphology and the crystallinity of the initial TiO 2 -contain- ing precursor (Ti-precursor) is believed to play an important role in determining the prevailing mechanism for BaTiO 3 for- mation and, consequently, define the shape, the crystallinity, and the size of the BaTiO 3 particles. This raises expectations that BaTiO 3 with a controlled shape could be prepared by the selection of the correct initial precursors and experimen- tal conditions. However, an understanding of BaTiO 3 forma- tion from particular precursors is a prerequisite for further tailoring of the BaTiO 3 ’s morphology. M. Parans Paranthaman—contributing editor Manuscript No. 33061. Received April 18, 2013; approved August 6, 2013. † Author to whom correspondence should be addressed. e-mail: marjeta.macek@ijs.si 3401 J. Am. Ceram. Soc., 96 [11] 3401–3409 (2013) DOI: 10.1111/jace.12607 © 2013 The American Ceramic Society J ournal